Molecular
architecture
PETER
D. YURCHENCO’
Department
of Pathology,
Robert
of basement
AND JOHANNES
C. SCHITTNY
Wood Johnson
School,
Medical
Basement
membranes
are
with
support,
functions.
The
molecular
are
created
through
tween
unique
tomers.
Type
IV
architectures
collagen
glycoprotein,
to
type
heparan
form
laminin
IV
sulfate
basement
membrane laminin
function in response
molecular
occur
tions
in
such
heparin
P. D.;
SCHITTNY,
ment
membranes.
Key Words: tactin
nidogen
net-
and C.
IV
basement
and two.
interA large
for charge-
itself
collagen
tissues,
through
J
membrane
needs. may
variaand
the
macromolecules
sulfate.
USA
1577-1590;
collagen
of base1990.
laminin
en-
MEMBRANES ARE SPECIALIzED extracellular matrices found in nearly all multicellular animal species and are produced by epithelial cells, endothelial cells, and many mesenchymal cells. The sheet-like structures, which appear early in development, serve as supports for cells and cell layers. Continuous basement membranes act as passive selective molecular sieves between tissue compartments (1). Basement membranes also act to impede the passage of cells and there appear to be specific mechanisms that permit inflammatory cells and metastatic tumor cells (2) to focally degrade matrix and emigrate across these barriers. In recent BASEMENT
© FASEB
be
important
for
a morphologically
labile
struc-
ture. These extracellular matrices, while sharing many of the same structural elements, exhibit variations in their compositional ratios and in the presence of protomeric isoforms: these variations probably contribute to architectural and functional heterogeneity.
-YIJRCHENCO,
architecture
proteoglycan
0892-6636/90/0004-1577/$01.50.
08854,
dent on the binding of cell receptors to matrix determinants (3). The morphology of a typical basement membrane, such as that found under many epithelia and endotheha, is a thin sheet-like structure sandwiched between a cell layer and a thick collagenous stroma (Fig. 1). In epithelia the lamina densa is connected to stroma through anchoring fibrils (4). Variations of this classical morphology can be found in several locations. Skeletal muscle is surrounded by basement membrane and possesses small differentiated patches at the neuromuscular junction, important in signal transduction. The corneal endothelial basement membrane (Descemet’s) is thick, lacks a lamina lucida, and contains collagenaseresistant hexagonal lattices. Basement membranes can change in structure and shape in the course of development. Newly formed capillary bud basement membranes, for example, are initially discontinuous, rich in laminin, and lack type IV collagen (5), features that may
The
be regu-
through
substitutions,
4:
its
in development,
matrices
Molecular
a
oligomers of structure
assembly
exogenous
in the
through
and
physiological
heparan
FASEB
the
dimers
isoform
J.
center
bind
of these
of
sulfated
Heterogeneity
or after primary influence
as
type
in different
compositions,
modifying
can
to different
architecture
lated during
polymer
important
chains.
and
a fourterminal-
is firmly anchored
to form
and
its
bridging
and
glycosaminoglycan and
form a coval-
Laminin,
a second
sieving,
interaction
bind
NH2-terminal,
through
proteoglycan,
molecular
and
using
near
collagen,
dependent core-protein
pro-
a dumbbell-shaped
binds
with
be-
proteoglycan
framework.
Entactin/nidogen,
acts
and
matrices
interactions
self-assembles
interactions
work,
regulatory
of these
binding
chains,
polygonal
glycoprotein,
domain
cell
and lateral association,
stabilized
armed
extracellular
and
glycoprotein
COOH-terminal, ently
specialized
sieving, specific
New Jersey
years it has become apparent that basement membranes act as solid-phase regulators of cell attachment, growth, and differentiation: these functions are depen-
ABSTRACT
matrices
Piscataway,
membranes
BASEMENT
MEMBRANE
SELF-ASSEMBLY
Basement membranes are formed from glycoprotein and proteoglycan protomers, which interact with each other to produce defined supramolecular assemblies. We can discern two orders of structure within these matrices: the molecular structure of each component and the supramolecular architecture that results from specific interactions between protomers. Functions of support and sieving are clearly dependent on architecture whereas cell-matrix interactions, primarily dependent on the presence of site-specific determinants on protomers, can be modified by architecture (6-8). The assembly of basement membrane from its components appears, to a large extent, to be one of mass actiondriven “self-assembly?’ These interactions involve pro-
‘To whom correspondence should be sent, at: Department of Pathology, UMDNJ-Robert W. Johnson Medical School, 675 Hoes Lane, Piscataway, NJ 08854, USA.
1577
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
tomers binding to themselves oligomers and polymers, and form heterologous complexes.
to produce homologous binding to each other to The process of assembly
S
Nd
iS
II
I
I
C
I.
i
Figure
I
2. Structural
model
I
IOO
I
of type
I5Ores
IV collagen
monomer.
This
col-
lagen monomer is formed by the parallel associative alignment of two al(IV) chains and one a2(IV) chain (10). The molecule is shown with a signal peptide (5), HN2-terminal cysteine-rich domain (7S), main triple helical (TH) domain, and COOH-terminal globular domain (Nd). The black bars, lines, and boxes (between 7S and NC1) indicate the locations and relative sizes of interruptions of the Gly-Xaa-Yaa sequence in the two chains. Dots indicate sites of pepsin cleavage, C, the cysteine regions intra/interchain stabilization, and K, sites of lysine-derived cross-links. A unique intrachain disulfide-stabilized loop is present in the cr2(IV) chain near its center. Diagram redrawn from the work of Kuhn and coworkers (10).
is complex, involving both well as covalent stabilizations collagen,
laminin,
reversible interactions and cross-links. Type
entactin/nidogen,
and
teoglycans are major and unique ment membranes. Of these, type
nm account
for
the
greatest
several
components IV collagen
mass
in the
as IV pro-
of baseand lamiEngelbreth-
HoIm-Swarm (EHS)2 (9) and other basement membranes. Both weak and strong interactions can be identified between matrix components, and there may be a coupling of these interactions such that the strong ones drive protomers to a high local concentration, allowing weaker interactions (KD’S possibly below 10 tM)
to
operate
and
contribute
to
final
structure.
A
theoretical advantage of weak associations is to permit reshuffling of bonds affecting structure to meet different functional needs. Much of the work on basement membrane architecture has derived from interactions studied in vitro using components isolated from the EHS tumor. In some cases it has been possible to verify such tion
structural
information
of molecular
TYPE
1ii;.
Figure
1. Basement
membrane
morphology.
Electron
micrographs
of chick corneal epithelial basement membrane illustrating relationships of superficial epithelial cell layer and deep stromal matrix. A) Scanning electron micrograph of basement membrane (BM) surface (2 days after hatching). Note overlying epithelial cell layer (CL) which has partially separated from membrane. B) Scanning electron micrograph of basement membrane surface (6 day embryo). The underlying stromal (St) collagen fibers can be seen just beneath a hole in the membrane surface. C) Transmission electron micrograph of a basement membrane (12-day embryo) sandwiched between cell and stroma. It is divided into laminin lucida (11), laminin densa (Id), and lamina fibro-reticularis (If), the latter layer traversed by anchoring fibers and not always observed. Electron micrographs provided by Dr. David Birk (Department of
Pathology,
1578
Robert
Vol. 4
Wood Johnson
April
1990
Medical
School).
direct
visualiza-
in situ.
IV COLLAGEN
Type IV collagen
_-;,
through
architecture
structure
The collagen monomer (Fig. 2), derived from three polypeptide chains [al(IV)2a2(IV)], is a flexible threadlike molecule measuring about 400 nm in contour length and possessing a distinctive globular domain (NCI) at its COOH terminus (Fig. 2 and Fig. 3). Unlike the interstitial collagens, the COOH-terminal and NH2-terminal cessed before
al(IV) coincide
domains self-assembly.
and a2(IV) to produce
are
not proteolytically proInterruptions in the human
triple helical domains generally 26 irregularly spaced sites that im-
2Abbreviations: EHS, Engelbreth-HoIm-Swarm; SPARC, secreted protein acidic and rich in cysteine; BM, basement membrane; St, stromal, E, elastase; P. pepsin; CL, cell layer; TH, triple helical; CB, cyanogen bromide.
The FASEB Journal
YURCHENCO
AND
SCHITTNY
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
tion may occur as an trolled redox conditions.
intracellular
Second, four NH2-terminal (16-20) to form a 28-nm, main (Fig. 3C). Assembly
Figure
Low
3. angle
Electron micrographs Pt/C rotary shadowed
of type replicas.
IV
collagen
B) Dimers C) NH2-terminal
bound through COOH-terminal interactions (20). (7S) interactions (arrows) observed with collagen from which the COOH-terminal globules have been proteolytically removed. Monomer substrate (1), dimeric (2), and trimeric (3) intermediates and completed tetramer (4) are shown (17). D) Lateral associations between collagen dimers form a polygonal network (20). Examples of branching strands are indicated with arrows.
increased
flexibility
to the
whole
dimeric
ontiparaltel tetrameric
molecule
antiparatlel
al
aggregation
(10).
of the locations of many of these interdifferent species (3, 11) argue for their
functional importance. The NH2-terminal 30-nm segment is referred to as the 7S domain (12) and homologous
con-
ends bind to each other end-overlapped (7S) doproceeds through anti-
aggregation
part
under
complexes. monomers
A) Collagen
(20).
The invariance ruptions among
event
COOH-terminal
segments
of
the
three
chains
B#{176}
cc
(227-229 amino acid residues), which each possess six cysteines, form a disulfide-stabilized globular domain (13). Cells have been found to bind triple helical but not NH2-terminal
(7S) or COOH-terminal
globular
domains
(3). Based on analysis of minor COOH-terminal globular peptides extracted from the glomerular basement membrane, it has been proposed that basement membrane collagen can also possess variant a3 and a4 chains (14). One of these peptides specifically reacts with Goodpasture’s antisera (14). The structural significance of these specialized chains remains to be determined.
Type IV collagen
self-assembly
Using three types gen can assemble ment membrane terminal (12, 13,
globules 15). Comparison
disulfide-linked generated by finding
of interactions (Fig. 3), type IV collainto a stable three-dimensional basenetwork. First, pairs of COOHunite to form linear dimers (Fig. 3B)
that
the
of dimeric
and
monomeric
a1-globular domain peptide CNBr digestion led to the same
CNBr
fragments
were
fragments surprising disulfide
bonded (15). From this it was concluded that the dimeric globular domain is formed by complete disulfide exchange (Fig. 4A) between corresponding cysteines of the two monomeric domains. Reoxidation in vitro of reductively cleaved globular domain (NC1) leads to the formation of a dimeric product in the presence of glutathione and urea (13). In more physiological buffers in the absence of sulihydryl reagents, intact monomers do not readily convert to dimers, suggesting that this reac-
BASEMENT
MEMBRANE
MOLECULAR
ARCHITECTURE
Figure 4. Mechanisms of type IV collagen carboxyland aminoterminal interactions. A) Each noncollagenous COOH-terminal end is divided into two homologous subdomains (I and II), each composed of a pair of disulfide-stabilized loops. The disulfide exchanges that occur between a corresponding pair of loops of one collagen monomer and another are depicted (CB = cyanogen bromide peptides). Reprinted with permission from ref 15. B) Schematic representations of azimuthal orientation of the four NH2-terminal domain triple helices that form 7S domain. Double lines indicate hydrophobic surfaces that interact through pairs of antiparallel overlaps. N and C indicated directional orientation of molecules. Reprinted with permission from ref 19. C) Diagram of 7S covalent overlap interactions between antiparallel pairs of al(IV) NH2-termjnaj domains. Vertical lines show positions of hydrophobic interactions while -SS- indicates disulfide cross-links and -KK#{247}-marks putative lysine/hydroxylysine-derived crosslinks. Illustration redrawn from Glanville et al. (18).
1579
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
1580
Vol. 4
April
1990
The FASEB Journal
YURCHENCO
AND
SCHIUNY
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
parallel dimeric and trimeric intermediates (Fig. 3C) formed initially through cooperative noncovalent interactions (17, 19). A hydrophobic self-interacting edge on each NH2-terminal end has been identified from the primary sequence: the four ends maximize hydrophobic contact (Fig. 4B), resuhting in antiparallel interactions of correct axial (about 30 residue overlap) and azimuthal orientation (19). This orientation in turn limits the complex to tetramer size and places cysteine and lysine/hydroxylysine residues in correct position on the corresponding chains to form disulfide and nonreducible cross-links (Fig. 4C) (18, 19). Third, type IV collagen dimers self-interact through lateral (side-by-side) associations (20-22). This interaction, first identified in vitro (20), is characterized by a thermally reversible and concentration-dependent assembly in which extensive irregular networks form (Fig. 3D and Fig 5). The irregular polygonal geometry
of the network argues for more than a single allowed spatial relationship of one chain segment to its partners. Self-assembly requires the presence of dimeric collagen COOH-terminal globular domains (20, 23), and purified globules will both inhibit polymerization and bind collagen at various sites along the chain (23). Recently the existence of laterally associated networks has been confirmed in tissue basement membranes (Fig. 5) of the human amnion (21) and EHS tumor (22). The collagenous network was visualized in the electron microscope as an extensive irregular polygonal network possessing integral globular domains, the same shape and size as those of purified collagen dimers. Lateral associations were noted between monomolecular filaments with the formation of branching strands of variable but narrow diameters (Fig. 5C, F). The locations of the 7S domains were seldom obvious, either in reconstituted polymers or in basement membranes, and may not have been recognized because of superimposed lateral associations. A remarkable feature of the EHS and amniotic networks was the presence of supramolecular helices of monomolecular filaments in the network (Fig. 5D, G, H). The similarity between reconstituted and tissue basement membrane collagen networks is evidence that the information for assembly
is encoded in the collagen molecules themselves and that the network is a widespread supramolecular architecture of basement membrane collagen. Although covalent bonding of mammalian collagenous networks appears to occur only at the aminoand carboxylterminal regions, the loops formed by these ends would be expected to irreversibly entrap and stabilize helically
Figure 5. Electron micrographs freeze-dried basement membrane
wrapped, laterally associated filaments. Furthermore, such helices would probably only form where there are free ends prior to 7S formation. Recently Drosophila type IV collagen has been characterized (11, 24). It has only al(IV) chains with a similar molecular morphology to mammalian collagen and assembles into disulfide-linked dimers and higher oligomers. The collagen thread domain possesses nine cysteines whose spacing, unlike that in mammals, could permit many opportunities for disulfide bonding between molecules in both parallel and antiparallel arrays, and which may lead to a disulfide-stabilized network. LAMININ
Laminin
structure
Laminin membranes,
(Fig.
-850
short arms (-37 nm) (25). Each short arm generally (in rotary shadowed replicas, Fig. 7a) possesses a pair of globular domains and the long arm a larger globule at its end (25): a third globule is sometimes visualized in one of the short arms (26). Laminin is a complex of three genetically different polypeptide chains, an A chain (-400 kDa) and two smaller B chains (- 200 kDa each) (3). These three chains have been completely sequenced (27-29) and the predicted and
kDa)
6), a major component of basement is a flexible four-armed glycoprotein (MR
one
consisting
long
arm
of three
(- 77 nm)
domain structure is in good tron-microscopic and physical tail elsewhere (26-29), there
agreement with the elecdata. As discussed in deis homology between the
short-arm separated
globular domains, and these by EGF-like cysteine-rich repeats.
terminal long arm
moieties through
of all three triple-coiled
domains are The COOH-
chains join to form the a-helices, and the chains
are held together by disulfide bridges at the vertex and near the large long-arm globule. Laminin has a fairly high carbohydrate content (12-15%), with nine forms of N-linked oligosaccharides, mostly of the complex variety (30). The function of this carbohydrate is not well understood although suppression of glycosylation, while decreasing secretion, has not been found to adversely affect disulfide bonding between subunits or to alter heparin binding (31). The different structural domains of laminin serve different architectural and cellinteractive Many
functions. types of cells
interact
found to dramatically influence ing, growth, and differentiation
of type IV collagen laterally associated network. Gallery (21, 22) shown contrast-reversed. A) Amniotic basement
with
laminin,
which
is
cell attachment, spread(3). These effects have
of high angle, single-direction membrane network following
Pt/C replicas of depletion of non-
collagenous components with guanidine. Arrows indicate globular domains: large arrowheads point into lateral associations at branch points; small arrowhead indicated edge of carbon support sheath. B) Reconstituted network of purified type IV collagen dimers. Thin arrows indicate branching; thicker arrows indicate globules. C) Section of polygonal amniotic network revealing lateral joining of monomeric filaments (1-3) to form thicker strand. Complex branch point indicated with long arrow, located above globule (horizontal arrow). D) Detail of panel C revealing supercoil formation (diagonal arrows) below globular domain. E) Monomeric filament has metal-coated diameter of 2.5 nm (1.5 nm + 1 nm deposited metal). F) Fusion of two monomeric filaments. C, H) Supercoil formation in network of EHS matrix.
BASEMENT
MEMBRANE
MOLECULAR
ARCH ITECTURE
1581
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
A Vi
/
Entactin
nidogen
there are cies that ment and laminin,
:
:
El’ (self-assembly,
V
IVb:
)
C&l-bifldiflg .
--
-
Bi E4 (self
ssem
:lllb
1iVa
-
.i
S
ElO I ) Y
II
B2 P1 (cell b.ndung)
-
chain,
-
;
designated
synaptic
laminin,
has
been
detected
in the synaptic cleft of the neuromuscular junction (37). At present, the binding function of these variant forms of laminin is not well understood, although they are likely to provide differential cell signaling or alter molecular architecture.
#{149}l
E8 (cell binding) :
:
variant forms of laminin within a given speare expressed at different stages of developin different tissue locations. Schwannoma cell for example, possesses only three arms in a Y shape and lacks an identifiable A chain (34). However, the presence of a large globular domain at the end of the long arm argues for the presence of a third truncated chain with a similar COOH-terminal region (-200 kDa) In kidney development the primitive mesenchyme expresses laminin B chain mRNA before A chain mRNA (35), a transition important for tubule formation. A variant of laminin called merosin (36) has a nearly identical four-armed morphology but a different A chain primary structure (40% homology to A chain in the EHS tumor). The insolubility of this laminm, like mouse tumor laminin, is dependent on divalent cation-dependent interactions. A laminin-like B
S
G
‘:‘
.
‘
Laminin
.
,P1
at
branes newly
-heIix .
Figure 6. Diagram of laminin molecule reveals relationship of A, Bl, and B2 chains that form an asymmetrical four-armed molecule, The three short arms are formed by the NH2-terminal moieties of the three chains, which then unite in a coiled-coiled ct-helix long arm. The three chains are disulfide-linked at two sites. Globular domains of the short arms are separated by consecutive cysteine-rich EGF-like repeats. The large A chain-derived globular (G) domain at the end of the long arm is further subdivided into five disulfidestabilized loops. Sequence-defined domains marked with roman numerals and G; proteolytically derived (E, elastase; P, pepsin) domains (3, 26, 40) indicated with dashed lines. Model based on sequence work of Yamada and co-workers (27-29) and structural work of Engel and co-workers (25, 26).
been one of the most intensely investigated aspects of extracellular matrix biology, and we will not attempt to cover this important aspect of basement membrane function here. Two distinct geographical areas of lamiing
near site
cells important
with high for cell
the intersection near
the
end
affinity and regulation:
of the cross of the
long
arm
are proposed short-arm
(fragment (fragment
as beregions
P1) and
a
E8).
that lack collagen formed capillary
matrix (38)) laminin may provide the framework. Laminin will aggregate in vitro into large polymers in a temperature-, time-, and concentration-dependent manner (39). Aggregation exhibits both concentration and thermal reversibility, and there is a critical concentration for polymerization of about 60 nM reflecting cooperative nucleation-
isoforms
Four-armed laminin of strikingly similar A and B chain structure has been found in basement membranes of diverse species (26), including insects (32, 33). On the other hand it is becoming increasingly apparent that 1582
Vol. 4
April
1990
.
.
.
propagation
particular, assembly dependent dependent been
type
assembly.
Divalent
cation,
calcium
in
is required for polymerization, and selfcan be separated into an initial temperatureoligomer-forming step followed by a calciumpolymer-forming step (39). Recently (40) it shown
that
half-maximal
aggregation
is
achieved at 10-15 jiM calcium (10-fold higher concentrations are required if magnesium is used). However, only 1-3 of the 16 calcium ions that can a bind a single laminin molecular are of sufficient affinity to account for polymerization. It is thought that activation of polymerization tional changes
is a result in laminin
of higher
affinity
termined
although
in fragment Laminin
interactions
(e.g., in embryogenesis (3), in buds (5), and in the M1536-B3
cell extracellular only polymer
has
nm bind
and calcium
For the remainder of our discussion of this glycoprotein we shall focus on basement membrane-forming interactions of classical four-arm laminin. Like type IV collagen, laminin forms polymers. In basement mem-
E3 (he nfl binding, se f-assembly) c s-r
self-assembly
of calcium-induced
calcium at least
conforma-
(41). At present the exact sites interaction have not been deone
of these
sites
is present
El’.
Terminal domain interactions are essential for laminm self-assembly into polymers (39, 41, 43-46). Electron microscopy of aggregated laminin reveals (39) the ends of short and long arms to be attached in dimers and oligomers (Fig. 7). Fragment E4 of laminin (NH2terminal
The FASEB Journal
short-arm
globule
and
adjacent
YURCHENCO
stem
AND
of
B!
SCHIUNY
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
It
Figure
7. Laminin
monomers interactions gregates,
chain)
and
its dimers
and
oligomers.
(row a) and its self-associated dimers between short arms and long arms which appear collapsed, is uncertain.
will
inhibit
laminin
Electron
(b), oligomers can be identified
aggregation
and
bind to laminin in a temperatureand dependent manner (45). Fragment El’ (which B2 and A chain short-arm globules) inhibits polymerization
(45)
and
self-interacts
micrographs
(41)
of low angle
rotary
shadowed
Pt/C
replicas
(67)
of laminin
(c), and polymers in dimers
fragment P! (which lacks these globules) will riot. Although fragment E4 binds fragment El’, it will not bind itself: thus the interactions between short arms are between heterologous chains (46). As intact short arms are both necessary and sufficient for aggregation, related
directly
calciumpossesses laminin whereas
(d). Interpretive drawings shown on right side. End-to-end and oligomers. However, molecular organization of larger ag-
a
Figure 8. Three-dimensional architecture of laminin polymer gel. Laminin/entactin (3.5 mg/mI) was heat-gelled in isotonic Tris-buffer (pH 7.4), flash-frozen, fractured, freeze-etched, and replicated with Pt/C at high angle using a Balzers BAF500K. Stereo pair is shown contrast reversed. Porous polymeric lattice with many short 30-40 nm segments joining at vertices. Some thickening of delicate network due
to salt deposition.
BASEMENT
MEMBRANE
Appearance
MOLECULAR
is consistent
with
ARCHITECTURE
end-associated
polymer
(43).
1583
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
the long-arm globules also appear to contribute to assembly (39, 42, 44). What molecular architecture is formed in threedimensional space? In considering models, we have started with the assumption that although laminin appears as a moderately flexible cross in 2-dimensional glycerol spreads, in unrestricted space it will more likely maximize the separation of its four arms on tetrahedral coordinates. In the EHS basement membrane, laminin is present at about 13 jiM, a concentration in which the center of each molecule would be separated from the center of each neighbor by about 50 nm in all three axes. As the short and long arms extend out from their center 37 and 77 nm, respectively, this would be well above the minimal distance for the ends of the arms of laminin to touch each other. In examining polymer space-filling models, it becomes evident that the valence of binding for each arm (i.e., the number of laminin arms that can bind at a single terminal arm domain)
is important
for
determining
three-dimensional
possibility
that
cytoskeleton,
laminin
architectures
are
needs
to be considered.
ENTACTIN/NIDOGEN
Entactin/nidogen
and
(47)
structure nidogen
and
(47),
co-workers
macromolecule
are
(Fig.
(48),
discovered
different
9)
(49-51).
by Chung
for the same
names
This
glycoprotein
(-150 kDa; 1217 amino acid residues) contains about 5% carbohydrate (3), is sulfated at tyrosine residues 262 and 267 (51), is shaped like a dumbbell (52), is highly susceptible to proteolysis (49), and has both matrix and cell binding activity (3). The 17-nm long rod connecting the NH2- and COOH-terminal globules
is
composed
repeats,
and
repeats
27KU. 1mm
variant
formed in different basement membranes as a consequence of interactions with neighboring components, in a manner analogous to the variations in actin
Entactin
architecture. These valence numbers remain to be determined but probably exceed one. Such a model is supported by replicas of freeze-etched laminin polymers (Fig. 8) in which a quasi-regular geometric architecture composed of many 35- to 40-nm struts meeting at vertices is observed. We propose that laminin polymers are important architectural features of basement membranes: the molecular density of laminin in tissue is high and would permit arm contacts between all neighboring
a
molecules in three dimensions; laminin exhibits similar gelation properties alone or in the presence of other matrix components; laminin-rich basement membranes that lack collagen exist in tissues and are made by tumors; laminin/entactin is selectively solubilized from several basement membranes with EDTA and EGTA. However, unlike the case with type IV collagen, we have not yet directly visualized and studied laminin architecture within tissue basement membranes. The
the
in
of
there
five
are
globules
Entactin/nidogen,
consecutive
two (50,
cysteine-rich
additional
cysteine-rich
51).
laminin,
and
type
IV collagen
interactions
Nd-lOO Th Nd-80
Nd-130
) NH2
a
\
Nd-40
b
Th
I a r5S-SSI
d
t R&D
t lyr-504
‘
/
typically
tion with laminin
(KD
mediated
/
i(
Entactin/nidogen
COOH
I\,j EGF-like
through
exists
1-10
=
the
nM)
proximal ley A) Diagram
of nidogen correlated with its amino acid sequence. Subdomains of morphological domains include seven EGF-like repeats. Positions of cleavage by endogenous proteases (Nd) and thrombin (Th), and sites of tyrosinc sulfation are noted. Reproduced with permission from ref 51. B, C) Rotary shadowed replicas of laminin/nidogen complexes (B) and isolated nidogen (C). Bar equals 100 nm. Reproduced with permission from ref (52).
1584
Vol. 4
April
1990
fairly tight associa(52). This binding is
COOH-terminal
globule
of the
dumbbell (3) and the central domain (probably domain III of the Bi chain) of laminin (52). Extraction of the EHS tumor and leech ganglions in physiological buffers containing chelating agent releases most laminin and entactin/nidogen as a near-equimolar complex which can then be dissociated in 2 M guanidine-HC1 (3). The COOH-terminal globule of entactin/nidogen has also been found to bind type IV collagen at triple helical regions clustering principally about 80 nm away from the COOH-terminal globule (53). Thus entactin/nidogen can act as a bridge between the two major basement membrane proteins. There is also morphological evidence that the ends of laminin teract with the type IV collagen
Figure 9. Entactin/nidogen.
in
et
short
arms chain
to the COOH-terminal al.
and
(53) long
have arms
found are
found
can directly about 140
globule that
even
innm
(54). Aumailthough
laminin
with
collagen,
associated
once entactin/nidogen is extracted from laminin in guanidine, significant laminin interactions with collagen cannot be detected. One interpretation of the data is that entactin/nidogen bridging is the major (highest affinity) interaction and that direct laminin-collagen interactions represent an example of coupled low-/high-
The FASEB Journal
YURCHENCO
AND
SCHIUNY
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
affinity interactions. In evaluating models of laminin and type IV heteropolymers, it becomes evident that one network may become enmeshed with another without actual bonding (see model illustration), and that a function of specific laminin-collagen interactions could be to orient and restrain the two polymers in space with respect to each other. PROTEOGLYCANS
AND
GLYCOSAMINOGLYCANS
Heparan
sulfate
proteoglycans
Heparan sulfate proteoglycans are a class of macromolecules characterized by a protein core covalently bound to heparan sulfate chains. The polyanionic chains confer charge-dependent selective filtration properties to basement membranes (1). Proteoglycans of different core size and with different heparan sulfate chain size and sulfation can be found in basement membranes, and have been characterized (9, 55, 56). One high-density heparan sulfate proteoglycan (130 kDa), possessing a small core and four short polysaccharide chains, is loosely bound to the EHS matrix (9) and found to weakly bind laminin, fibronectin, and type IV collagen. Its tissue distribution and physiological role are unclear. The low-density heparan sulfate proteoglycan, firmly bound to the EHS basement membrane and requiring chaotropic buffers for extraction (55, 56), is present in many basement membranes including renal glomerulus (57). It has an elongated core subdivided into a tandem array of globular domains (56) with intervening connectors: several heparan sulfate chains extend from one end of the core (Fig. 10). The core (MR = 400-450 kDa), a single polypeptide stabilized by intrachain disulfides, is highly sensitive to proteolysis (58), and it has been proposed that there is a normal physiological processing to smaller forms
(57);
however,
artifactual
proteolysis
Proteoglycan The
chaotropic
strongly
Heparin-
conditions that
there
required are
for
high-affinity
extraction binding
in-
and
heparan
sulfate
interactions
These sulfate-substituted uronic acid/N-acetylglucosamine repeating units can possess multiple biological functions including anticoagulation, filtration, and the regulation of cell behavior. Heparins have a higher degree of sulfation than heparan sulfates, and the two can be regarded as two ends of a compositional spectrum. Heparins represent an important secretory product of bone marrow-derived cells whereas heparan sulfates are found both on cell surfaces and within basement membranes. Functions of these glycosaminoglycans are dependent on the degree and sites of sulfation (61). the
Both heparin and heparan sulfate bind laminin (3), former with greater avidity. However, although
there is evidence for the existence of high-affinity, longarm heparin binding (3, 30, 42), and also for lowaffinity, short-arm versy has developed
heparin over
the
binding (62, significance
63), and
controsites of
weaker interactions (3, 42, 62, 63). Heparin binds to intact laminin/entactin complex with a high affinity dissociation constant of 0.1 jiM (42). Low affinity-heparin interactions with the short arms, which cannot be measured below 3 jiM heparin (42), are observed at high concentrations of heparin (63). It has also been found that heparin (64) and heparan sulfate (9) interact with type IV collagen. By rotary shadow decoration (64),
Figure 10. Low-density heparan sulfate proteoglycan. Electron micrograph of low angle Pt/C rotary shadowed replica. Bar equals 100 nm. Note the three thread-like heparan sulfate chains extending from one pole of the elongated and thicker core protein (58).
suggest
teractions between proteoglycan and basement membrane. Core binding sites, however, may have been lost or altered due to partial denaturation during purification. Although putative proteoglycan-anchoring binding sites have yet to be elucidated, there is in vitro evidence for other types of core interaction. First, the core can self-assemble into dimers and oligomers (58). When low-density proteoglycan or isolated core is incubated in buffers of physiological pH and ionic strength, it will form larger structures that can be analyzed by electron microscopy and sedimentation. In rotary shadowed Pt/C replicas, the binding region is found at one end of the core at the pole opposite the heparan sulfate linkage site, and dimers appear as double-length structures. Limited trypsin proteolysis cleaves the core and the moiety possessing the chains loses its ability to self-assemble (58). Second, proteoglycan and core protein promote epithelial cell attachment and 26-, 36-, and 38-kDa cell surface proteins that bind the core have been identified (60).
during
purification cannot be excluded. Two cDNA clones have provided amino acid sequence for about 40% of the mouse low density proteoglycan core (59). One region consists of a pair of globular domains bounded and connected by EGF-like cysteine-rich repeats similar to the short-arm structure in laminin while another region consists of disulfide-bonded loops similar to the neural cell adhesion glycoprotein.
core interactions
the
principal
site has
been
mapped
to the
globular
(NC1) domain with two lower affinity interactions in the collagen chain in regions 100 and 300 nm from the globule (64, 65). Finally, heparan sulfate chains within the basement membrane act as repositories for basic fibroblast growth factor. This growth factor can be released from extracellular matrix with heparitinase or heparin in an active form and binds endothelial cell extracellular matrix with an apparent KD of 620 nM (66). 1585
BASEMENT MEMBRANE MOLECULAR ARCHITECTURE m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
tardation characteristics. For example, in acute inflammation one effect of mast cell-released heparin adjacent to the postcapillary venule could be to transiently alter basement membrane sieving through such architectural modifications.
Chondroitin TVPE
VII
COLLAGEN
Members
mAb- 161 mAb- Vii
OTHER
pAb-Vil
11. Model
of relationship
between
type VII
collagen
and
an
anchoring fibril. Type VII procollagen molecules dimerize by overlapping at their NH2-terminal domains, perhaps with the subsequent processing of the NH2-terminal globular domain (+). These dimers then condense to form anchoring fibrils. The multidomain COOH-terminal globules in turn bind both to the lamina densa and the stromal anchoring plaques. In the detail of the COOHterminal region (bottom of figure), sites of epitopes recognized by monoclonal antibodies are indicated by the arrow and brackets. Reproduced with permission from ref 4.
Heparin modulation polymerization
of laminin
and collagen
IV
We have found that heparin can regulate laminin polymerization (42, 67) and resulting molecular architecture. The effect is biphasic in which a sharp augmentation of polymerization occurs at low (
architecture
PETER
D. YURCHENCO’
Department
of Pathology,
Robert
of basement
AND JOHANNES
C. SCHITTNY
Wood Johnson
School,
Medical
Basement
membranes
are
with
support,
functions.
The
molecular
are
created
through
tween
unique
tomers.
Type
IV
architectures
collagen
glycoprotein,
to
type
heparan
form
laminin
IV
sulfate
basement
membrane laminin
function in response
molecular
occur
tions
in
such
heparin
P. D.;
SCHITTNY,
ment
membranes.
Key Words: tactin
nidogen
net-
and C.
IV
basement
and two.
interA large
for charge-
itself
collagen
tissues,
through
J
membrane
needs. may
variaand
the
macromolecules
sulfate.
USA
1577-1590;
collagen
of base1990.
laminin
en-
MEMBRANES ARE SPECIALIzED extracellular matrices found in nearly all multicellular animal species and are produced by epithelial cells, endothelial cells, and many mesenchymal cells. The sheet-like structures, which appear early in development, serve as supports for cells and cell layers. Continuous basement membranes act as passive selective molecular sieves between tissue compartments (1). Basement membranes also act to impede the passage of cells and there appear to be specific mechanisms that permit inflammatory cells and metastatic tumor cells (2) to focally degrade matrix and emigrate across these barriers. In recent BASEMENT
© FASEB
be
important
for
a morphologically
labile
struc-
ture. These extracellular matrices, while sharing many of the same structural elements, exhibit variations in their compositional ratios and in the presence of protomeric isoforms: these variations probably contribute to architectural and functional heterogeneity.
-YIJRCHENCO,
architecture
proteoglycan
0892-6636/90/0004-1577/$01.50.
08854,
dent on the binding of cell receptors to matrix determinants (3). The morphology of a typical basement membrane, such as that found under many epithelia and endotheha, is a thin sheet-like structure sandwiched between a cell layer and a thick collagenous stroma (Fig. 1). In epithelia the lamina densa is connected to stroma through anchoring fibrils (4). Variations of this classical morphology can be found in several locations. Skeletal muscle is surrounded by basement membrane and possesses small differentiated patches at the neuromuscular junction, important in signal transduction. The corneal endothelial basement membrane (Descemet’s) is thick, lacks a lamina lucida, and contains collagenaseresistant hexagonal lattices. Basement membranes can change in structure and shape in the course of development. Newly formed capillary bud basement membranes, for example, are initially discontinuous, rich in laminin, and lack type IV collagen (5), features that may
The
be regu-
through
substitutions,
4:
its
in development,
matrices
Molecular
a
oligomers of structure
assembly
exogenous
in the
through
and
physiological
heparan
FASEB
the
dimers
isoform
J.
center
bind
of these
of
sulfated
Heterogeneity
or after primary influence
as
type
in different
compositions,
modifying
can
to different
architecture
lated during
polymer
important
chains.
and
a fourterminal-
is firmly anchored
to form
and
its
bridging
and
glycosaminoglycan and
form a coval-
Laminin,
a second
sieving,
interaction
bind
NH2-terminal,
through
proteoglycan,
molecular
and
using
near
collagen,
dependent core-protein
pro-
a dumbbell-shaped
binds
with
be-
proteoglycan
framework.
Entactin/nidogen,
acts
and
matrices
interactions
self-assembles
interactions
work,
regulatory
of these
binding
chains,
polygonal
glycoprotein,
domain
cell
and lateral association,
stabilized
armed
extracellular
and
glycoprotein
COOH-terminal, ently
specialized
sieving, specific
New Jersey
years it has become apparent that basement membranes act as solid-phase regulators of cell attachment, growth, and differentiation: these functions are depen-
ABSTRACT
matrices
Piscataway,
membranes
BASEMENT
MEMBRANE
SELF-ASSEMBLY
Basement membranes are formed from glycoprotein and proteoglycan protomers, which interact with each other to produce defined supramolecular assemblies. We can discern two orders of structure within these matrices: the molecular structure of each component and the supramolecular architecture that results from specific interactions between protomers. Functions of support and sieving are clearly dependent on architecture whereas cell-matrix interactions, primarily dependent on the presence of site-specific determinants on protomers, can be modified by architecture (6-8). The assembly of basement membrane from its components appears, to a large extent, to be one of mass actiondriven “self-assembly?’ These interactions involve pro-
‘To whom correspondence should be sent, at: Department of Pathology, UMDNJ-Robert W. Johnson Medical School, 675 Hoes Lane, Piscataway, NJ 08854, USA.
1577
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
tomers binding to themselves oligomers and polymers, and form heterologous complexes.
to produce homologous binding to each other to The process of assembly
S
Nd
iS
II
I
I
C
I.
i
Figure
I
2. Structural
model
I
IOO
I
of type
I5Ores
IV collagen
monomer.
This
col-
lagen monomer is formed by the parallel associative alignment of two al(IV) chains and one a2(IV) chain (10). The molecule is shown with a signal peptide (5), HN2-terminal cysteine-rich domain (7S), main triple helical (TH) domain, and COOH-terminal globular domain (Nd). The black bars, lines, and boxes (between 7S and NC1) indicate the locations and relative sizes of interruptions of the Gly-Xaa-Yaa sequence in the two chains. Dots indicate sites of pepsin cleavage, C, the cysteine regions intra/interchain stabilization, and K, sites of lysine-derived cross-links. A unique intrachain disulfide-stabilized loop is present in the cr2(IV) chain near its center. Diagram redrawn from the work of Kuhn and coworkers (10).
is complex, involving both well as covalent stabilizations collagen,
laminin,
reversible interactions and cross-links. Type
entactin/nidogen,
and
teoglycans are major and unique ment membranes. Of these, type
nm account
for
the
greatest
several
components IV collagen
mass
in the
as IV pro-
of baseand lamiEngelbreth-
HoIm-Swarm (EHS)2 (9) and other basement membranes. Both weak and strong interactions can be identified between matrix components, and there may be a coupling of these interactions such that the strong ones drive protomers to a high local concentration, allowing weaker interactions (KD’S possibly below 10 tM)
to
operate
and
contribute
to
final
structure.
A
theoretical advantage of weak associations is to permit reshuffling of bonds affecting structure to meet different functional needs. Much of the work on basement membrane architecture has derived from interactions studied in vitro using components isolated from the EHS tumor. In some cases it has been possible to verify such tion
structural
information
of molecular
TYPE
1ii;.
Figure
1. Basement
membrane
morphology.
Electron
micrographs
of chick corneal epithelial basement membrane illustrating relationships of superficial epithelial cell layer and deep stromal matrix. A) Scanning electron micrograph of basement membrane (BM) surface (2 days after hatching). Note overlying epithelial cell layer (CL) which has partially separated from membrane. B) Scanning electron micrograph of basement membrane surface (6 day embryo). The underlying stromal (St) collagen fibers can be seen just beneath a hole in the membrane surface. C) Transmission electron micrograph of a basement membrane (12-day embryo) sandwiched between cell and stroma. It is divided into laminin lucida (11), laminin densa (Id), and lamina fibro-reticularis (If), the latter layer traversed by anchoring fibers and not always observed. Electron micrographs provided by Dr. David Birk (Department of
Pathology,
1578
Robert
Vol. 4
Wood Johnson
April
1990
Medical
School).
direct
visualiza-
in situ.
IV COLLAGEN
Type IV collagen
_-;,
through
architecture
structure
The collagen monomer (Fig. 2), derived from three polypeptide chains [al(IV)2a2(IV)], is a flexible threadlike molecule measuring about 400 nm in contour length and possessing a distinctive globular domain (NCI) at its COOH terminus (Fig. 2 and Fig. 3). Unlike the interstitial collagens, the COOH-terminal and NH2-terminal cessed before
al(IV) coincide
domains self-assembly.
and a2(IV) to produce
are
not proteolytically proInterruptions in the human
triple helical domains generally 26 irregularly spaced sites that im-
2Abbreviations: EHS, Engelbreth-HoIm-Swarm; SPARC, secreted protein acidic and rich in cysteine; BM, basement membrane; St, stromal, E, elastase; P. pepsin; CL, cell layer; TH, triple helical; CB, cyanogen bromide.
The FASEB Journal
YURCHENCO
AND
SCHITTNY
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
tion may occur as an trolled redox conditions.
intracellular
Second, four NH2-terminal (16-20) to form a 28-nm, main (Fig. 3C). Assembly
Figure
Low
3. angle
Electron micrographs Pt/C rotary shadowed
of type replicas.
IV
collagen
B) Dimers C) NH2-terminal
bound through COOH-terminal interactions (20). (7S) interactions (arrows) observed with collagen from which the COOH-terminal globules have been proteolytically removed. Monomer substrate (1), dimeric (2), and trimeric (3) intermediates and completed tetramer (4) are shown (17). D) Lateral associations between collagen dimers form a polygonal network (20). Examples of branching strands are indicated with arrows.
increased
flexibility
to the
whole
dimeric
ontiparaltel tetrameric
molecule
antiparatlel
al
aggregation
(10).
of the locations of many of these interdifferent species (3, 11) argue for their
functional importance. The NH2-terminal 30-nm segment is referred to as the 7S domain (12) and homologous
con-
ends bind to each other end-overlapped (7S) doproceeds through anti-
aggregation
part
under
complexes. monomers
A) Collagen
(20).
The invariance ruptions among
event
COOH-terminal
segments
of
the
three
chains
B#{176}
cc
(227-229 amino acid residues), which each possess six cysteines, form a disulfide-stabilized globular domain (13). Cells have been found to bind triple helical but not NH2-terminal
(7S) or COOH-terminal
globular
domains
(3). Based on analysis of minor COOH-terminal globular peptides extracted from the glomerular basement membrane, it has been proposed that basement membrane collagen can also possess variant a3 and a4 chains (14). One of these peptides specifically reacts with Goodpasture’s antisera (14). The structural significance of these specialized chains remains to be determined.
Type IV collagen
self-assembly
Using three types gen can assemble ment membrane terminal (12, 13,
globules 15). Comparison
disulfide-linked generated by finding
of interactions (Fig. 3), type IV collainto a stable three-dimensional basenetwork. First, pairs of COOHunite to form linear dimers (Fig. 3B)
that
the
of dimeric
and
monomeric
a1-globular domain peptide CNBr digestion led to the same
CNBr
fragments
were
fragments surprising disulfide
bonded (15). From this it was concluded that the dimeric globular domain is formed by complete disulfide exchange (Fig. 4A) between corresponding cysteines of the two monomeric domains. Reoxidation in vitro of reductively cleaved globular domain (NC1) leads to the formation of a dimeric product in the presence of glutathione and urea (13). In more physiological buffers in the absence of sulihydryl reagents, intact monomers do not readily convert to dimers, suggesting that this reac-
BASEMENT
MEMBRANE
MOLECULAR
ARCHITECTURE
Figure 4. Mechanisms of type IV collagen carboxyland aminoterminal interactions. A) Each noncollagenous COOH-terminal end is divided into two homologous subdomains (I and II), each composed of a pair of disulfide-stabilized loops. The disulfide exchanges that occur between a corresponding pair of loops of one collagen monomer and another are depicted (CB = cyanogen bromide peptides). Reprinted with permission from ref 15. B) Schematic representations of azimuthal orientation of the four NH2-terminal domain triple helices that form 7S domain. Double lines indicate hydrophobic surfaces that interact through pairs of antiparallel overlaps. N and C indicated directional orientation of molecules. Reprinted with permission from ref 19. C) Diagram of 7S covalent overlap interactions between antiparallel pairs of al(IV) NH2-termjnaj domains. Vertical lines show positions of hydrophobic interactions while -SS- indicates disulfide cross-links and -KK#{247}-marks putative lysine/hydroxylysine-derived crosslinks. Illustration redrawn from Glanville et al. (18).
1579
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
1580
Vol. 4
April
1990
The FASEB Journal
YURCHENCO
AND
SCHIUNY
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
parallel dimeric and trimeric intermediates (Fig. 3C) formed initially through cooperative noncovalent interactions (17, 19). A hydrophobic self-interacting edge on each NH2-terminal end has been identified from the primary sequence: the four ends maximize hydrophobic contact (Fig. 4B), resuhting in antiparallel interactions of correct axial (about 30 residue overlap) and azimuthal orientation (19). This orientation in turn limits the complex to tetramer size and places cysteine and lysine/hydroxylysine residues in correct position on the corresponding chains to form disulfide and nonreducible cross-links (Fig. 4C) (18, 19). Third, type IV collagen dimers self-interact through lateral (side-by-side) associations (20-22). This interaction, first identified in vitro (20), is characterized by a thermally reversible and concentration-dependent assembly in which extensive irregular networks form (Fig. 3D and Fig 5). The irregular polygonal geometry
of the network argues for more than a single allowed spatial relationship of one chain segment to its partners. Self-assembly requires the presence of dimeric collagen COOH-terminal globular domains (20, 23), and purified globules will both inhibit polymerization and bind collagen at various sites along the chain (23). Recently the existence of laterally associated networks has been confirmed in tissue basement membranes (Fig. 5) of the human amnion (21) and EHS tumor (22). The collagenous network was visualized in the electron microscope as an extensive irregular polygonal network possessing integral globular domains, the same shape and size as those of purified collagen dimers. Lateral associations were noted between monomolecular filaments with the formation of branching strands of variable but narrow diameters (Fig. 5C, F). The locations of the 7S domains were seldom obvious, either in reconstituted polymers or in basement membranes, and may not have been recognized because of superimposed lateral associations. A remarkable feature of the EHS and amniotic networks was the presence of supramolecular helices of monomolecular filaments in the network (Fig. 5D, G, H). The similarity between reconstituted and tissue basement membrane collagen networks is evidence that the information for assembly
is encoded in the collagen molecules themselves and that the network is a widespread supramolecular architecture of basement membrane collagen. Although covalent bonding of mammalian collagenous networks appears to occur only at the aminoand carboxylterminal regions, the loops formed by these ends would be expected to irreversibly entrap and stabilize helically
Figure 5. Electron micrographs freeze-dried basement membrane
wrapped, laterally associated filaments. Furthermore, such helices would probably only form where there are free ends prior to 7S formation. Recently Drosophila type IV collagen has been characterized (11, 24). It has only al(IV) chains with a similar molecular morphology to mammalian collagen and assembles into disulfide-linked dimers and higher oligomers. The collagen thread domain possesses nine cysteines whose spacing, unlike that in mammals, could permit many opportunities for disulfide bonding between molecules in both parallel and antiparallel arrays, and which may lead to a disulfide-stabilized network. LAMININ
Laminin
structure
Laminin membranes,
(Fig.
-850
short arms (-37 nm) (25). Each short arm generally (in rotary shadowed replicas, Fig. 7a) possesses a pair of globular domains and the long arm a larger globule at its end (25): a third globule is sometimes visualized in one of the short arms (26). Laminin is a complex of three genetically different polypeptide chains, an A chain (-400 kDa) and two smaller B chains (- 200 kDa each) (3). These three chains have been completely sequenced (27-29) and the predicted and
kDa)
6), a major component of basement is a flexible four-armed glycoprotein (MR
one
consisting
long
arm
of three
(- 77 nm)
domain structure is in good tron-microscopic and physical tail elsewhere (26-29), there
agreement with the elecdata. As discussed in deis homology between the
short-arm separated
globular domains, and these by EGF-like cysteine-rich repeats.
terminal long arm
moieties through
of all three triple-coiled
domains are The COOH-
chains join to form the a-helices, and the chains
are held together by disulfide bridges at the vertex and near the large long-arm globule. Laminin has a fairly high carbohydrate content (12-15%), with nine forms of N-linked oligosaccharides, mostly of the complex variety (30). The function of this carbohydrate is not well understood although suppression of glycosylation, while decreasing secretion, has not been found to adversely affect disulfide bonding between subunits or to alter heparin binding (31). The different structural domains of laminin serve different architectural and cellinteractive Many
functions. types of cells
interact
found to dramatically influence ing, growth, and differentiation
of type IV collagen laterally associated network. Gallery (21, 22) shown contrast-reversed. A) Amniotic basement
with
laminin,
which
is
cell attachment, spread(3). These effects have
of high angle, single-direction membrane network following
Pt/C replicas of depletion of non-
collagenous components with guanidine. Arrows indicate globular domains: large arrowheads point into lateral associations at branch points; small arrowhead indicated edge of carbon support sheath. B) Reconstituted network of purified type IV collagen dimers. Thin arrows indicate branching; thicker arrows indicate globules. C) Section of polygonal amniotic network revealing lateral joining of monomeric filaments (1-3) to form thicker strand. Complex branch point indicated with long arrow, located above globule (horizontal arrow). D) Detail of panel C revealing supercoil formation (diagonal arrows) below globular domain. E) Monomeric filament has metal-coated diameter of 2.5 nm (1.5 nm + 1 nm deposited metal). F) Fusion of two monomeric filaments. C, H) Supercoil formation in network of EHS matrix.
BASEMENT
MEMBRANE
MOLECULAR
ARCH ITECTURE
1581
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
A Vi
/
Entactin
nidogen
there are cies that ment and laminin,
:
:
El’ (self-assembly,
V
IVb:
)
C&l-bifldiflg .
--
-
Bi E4 (self
ssem
:lllb
1iVa
-
.i
S
ElO I ) Y
II
B2 P1 (cell b.ndung)
-
chain,
-
;
designated
synaptic
laminin,
has
been
detected
in the synaptic cleft of the neuromuscular junction (37). At present, the binding function of these variant forms of laminin is not well understood, although they are likely to provide differential cell signaling or alter molecular architecture.
#{149}l
E8 (cell binding) :
:
variant forms of laminin within a given speare expressed at different stages of developin different tissue locations. Schwannoma cell for example, possesses only three arms in a Y shape and lacks an identifiable A chain (34). However, the presence of a large globular domain at the end of the long arm argues for the presence of a third truncated chain with a similar COOH-terminal region (-200 kDa) In kidney development the primitive mesenchyme expresses laminin B chain mRNA before A chain mRNA (35), a transition important for tubule formation. A variant of laminin called merosin (36) has a nearly identical four-armed morphology but a different A chain primary structure (40% homology to A chain in the EHS tumor). The insolubility of this laminm, like mouse tumor laminin, is dependent on divalent cation-dependent interactions. A laminin-like B
S
G
‘:‘
.
‘
Laminin
.
,P1
at
branes newly
-heIix .
Figure 6. Diagram of laminin molecule reveals relationship of A, Bl, and B2 chains that form an asymmetrical four-armed molecule, The three short arms are formed by the NH2-terminal moieties of the three chains, which then unite in a coiled-coiled ct-helix long arm. The three chains are disulfide-linked at two sites. Globular domains of the short arms are separated by consecutive cysteine-rich EGF-like repeats. The large A chain-derived globular (G) domain at the end of the long arm is further subdivided into five disulfidestabilized loops. Sequence-defined domains marked with roman numerals and G; proteolytically derived (E, elastase; P, pepsin) domains (3, 26, 40) indicated with dashed lines. Model based on sequence work of Yamada and co-workers (27-29) and structural work of Engel and co-workers (25, 26).
been one of the most intensely investigated aspects of extracellular matrix biology, and we will not attempt to cover this important aspect of basement membrane function here. Two distinct geographical areas of lamiing
near site
cells important
with high for cell
the intersection near
the
end
affinity and regulation:
of the cross of the
long
arm
are proposed short-arm
(fragment (fragment
as beregions
P1) and
a
E8).
that lack collagen formed capillary
matrix (38)) laminin may provide the framework. Laminin will aggregate in vitro into large polymers in a temperature-, time-, and concentration-dependent manner (39). Aggregation exhibits both concentration and thermal reversibility, and there is a critical concentration for polymerization of about 60 nM reflecting cooperative nucleation-
isoforms
Four-armed laminin of strikingly similar A and B chain structure has been found in basement membranes of diverse species (26), including insects (32, 33). On the other hand it is becoming increasingly apparent that 1582
Vol. 4
April
1990
.
.
.
propagation
particular, assembly dependent dependent been
type
assembly.
Divalent
cation,
calcium
in
is required for polymerization, and selfcan be separated into an initial temperatureoligomer-forming step followed by a calciumpolymer-forming step (39). Recently (40) it shown
that
half-maximal
aggregation
is
achieved at 10-15 jiM calcium (10-fold higher concentrations are required if magnesium is used). However, only 1-3 of the 16 calcium ions that can a bind a single laminin molecular are of sufficient affinity to account for polymerization. It is thought that activation of polymerization tional changes
is a result in laminin
of higher
affinity
termined
although
in fragment Laminin
interactions
(e.g., in embryogenesis (3), in buds (5), and in the M1536-B3
cell extracellular only polymer
has
nm bind
and calcium
For the remainder of our discussion of this glycoprotein we shall focus on basement membrane-forming interactions of classical four-arm laminin. Like type IV collagen, laminin forms polymers. In basement mem-
E3 (he nfl binding, se f-assembly) c s-r
self-assembly
of calcium-induced
calcium at least
conforma-
(41). At present the exact sites interaction have not been deone
of these
sites
is present
El’.
Terminal domain interactions are essential for laminm self-assembly into polymers (39, 41, 43-46). Electron microscopy of aggregated laminin reveals (39) the ends of short and long arms to be attached in dimers and oligomers (Fig. 7). Fragment E4 of laminin (NH2terminal
The FASEB Journal
short-arm
globule
and
adjacent
YURCHENCO
stem
AND
of
B!
SCHIUNY
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
It
Figure
7. Laminin
monomers interactions gregates,
chain)
and
its dimers
and
oligomers.
(row a) and its self-associated dimers between short arms and long arms which appear collapsed, is uncertain.
will
inhibit
laminin
Electron
(b), oligomers can be identified
aggregation
and
bind to laminin in a temperatureand dependent manner (45). Fragment El’ (which B2 and A chain short-arm globules) inhibits polymerization
(45)
and
self-interacts
micrographs
(41)
of low angle
rotary
shadowed
Pt/C
replicas
(67)
of laminin
(c), and polymers in dimers
fragment P! (which lacks these globules) will riot. Although fragment E4 binds fragment El’, it will not bind itself: thus the interactions between short arms are between heterologous chains (46). As intact short arms are both necessary and sufficient for aggregation, related
directly
calciumpossesses laminin whereas
(d). Interpretive drawings shown on right side. End-to-end and oligomers. However, molecular organization of larger ag-
a
Figure 8. Three-dimensional architecture of laminin polymer gel. Laminin/entactin (3.5 mg/mI) was heat-gelled in isotonic Tris-buffer (pH 7.4), flash-frozen, fractured, freeze-etched, and replicated with Pt/C at high angle using a Balzers BAF500K. Stereo pair is shown contrast reversed. Porous polymeric lattice with many short 30-40 nm segments joining at vertices. Some thickening of delicate network due
to salt deposition.
BASEMENT
MEMBRANE
Appearance
MOLECULAR
is consistent
with
ARCHITECTURE
end-associated
polymer
(43).
1583
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
the long-arm globules also appear to contribute to assembly (39, 42, 44). What molecular architecture is formed in threedimensional space? In considering models, we have started with the assumption that although laminin appears as a moderately flexible cross in 2-dimensional glycerol spreads, in unrestricted space it will more likely maximize the separation of its four arms on tetrahedral coordinates. In the EHS basement membrane, laminin is present at about 13 jiM, a concentration in which the center of each molecule would be separated from the center of each neighbor by about 50 nm in all three axes. As the short and long arms extend out from their center 37 and 77 nm, respectively, this would be well above the minimal distance for the ends of the arms of laminin to touch each other. In examining polymer space-filling models, it becomes evident that the valence of binding for each arm (i.e., the number of laminin arms that can bind at a single terminal arm domain)
is important
for
determining
three-dimensional
possibility
that
cytoskeleton,
laminin
architectures
are
needs
to be considered.
ENTACTIN/NIDOGEN
Entactin/nidogen
and
(47)
structure nidogen
and
(47),
co-workers
macromolecule
are
(Fig.
(48),
discovered
different
9)
(49-51).
by Chung
for the same
names
This
glycoprotein
(-150 kDa; 1217 amino acid residues) contains about 5% carbohydrate (3), is sulfated at tyrosine residues 262 and 267 (51), is shaped like a dumbbell (52), is highly susceptible to proteolysis (49), and has both matrix and cell binding activity (3). The 17-nm long rod connecting the NH2- and COOH-terminal globules
is
composed
repeats,
and
repeats
27KU. 1mm
variant
formed in different basement membranes as a consequence of interactions with neighboring components, in a manner analogous to the variations in actin
Entactin
architecture. These valence numbers remain to be determined but probably exceed one. Such a model is supported by replicas of freeze-etched laminin polymers (Fig. 8) in which a quasi-regular geometric architecture composed of many 35- to 40-nm struts meeting at vertices is observed. We propose that laminin polymers are important architectural features of basement membranes: the molecular density of laminin in tissue is high and would permit arm contacts between all neighboring
a
molecules in three dimensions; laminin exhibits similar gelation properties alone or in the presence of other matrix components; laminin-rich basement membranes that lack collagen exist in tissues and are made by tumors; laminin/entactin is selectively solubilized from several basement membranes with EDTA and EGTA. However, unlike the case with type IV collagen, we have not yet directly visualized and studied laminin architecture within tissue basement membranes. The
the
in
of
there
five
are
globules
Entactin/nidogen,
consecutive
two (50,
cysteine-rich
additional
cysteine-rich
51).
laminin,
and
type
IV collagen
interactions
Nd-lOO Th Nd-80
Nd-130
) NH2
a
\
Nd-40
b
Th
I a r5S-SSI
d
t R&D
t lyr-504
‘
/
typically
tion with laminin
(KD
mediated
/
i(
Entactin/nidogen
COOH
I\,j EGF-like
through
exists
1-10
=
the
nM)
proximal ley A) Diagram
of nidogen correlated with its amino acid sequence. Subdomains of morphological domains include seven EGF-like repeats. Positions of cleavage by endogenous proteases (Nd) and thrombin (Th), and sites of tyrosinc sulfation are noted. Reproduced with permission from ref 51. B, C) Rotary shadowed replicas of laminin/nidogen complexes (B) and isolated nidogen (C). Bar equals 100 nm. Reproduced with permission from ref (52).
1584
Vol. 4
April
1990
fairly tight associa(52). This binding is
COOH-terminal
globule
of the
dumbbell (3) and the central domain (probably domain III of the Bi chain) of laminin (52). Extraction of the EHS tumor and leech ganglions in physiological buffers containing chelating agent releases most laminin and entactin/nidogen as a near-equimolar complex which can then be dissociated in 2 M guanidine-HC1 (3). The COOH-terminal globule of entactin/nidogen has also been found to bind type IV collagen at triple helical regions clustering principally about 80 nm away from the COOH-terminal globule (53). Thus entactin/nidogen can act as a bridge between the two major basement membrane proteins. There is also morphological evidence that the ends of laminin teract with the type IV collagen
Figure 9. Entactin/nidogen.
in
et
short
arms chain
to the COOH-terminal al.
and
(53) long
have arms
found are
found
can directly about 140
globule that
even
innm
(54). Aumailthough
laminin
with
collagen,
associated
once entactin/nidogen is extracted from laminin in guanidine, significant laminin interactions with collagen cannot be detected. One interpretation of the data is that entactin/nidogen bridging is the major (highest affinity) interaction and that direct laminin-collagen interactions represent an example of coupled low-/high-
The FASEB Journal
YURCHENCO
AND
SCHIUNY
m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
affinity interactions. In evaluating models of laminin and type IV heteropolymers, it becomes evident that one network may become enmeshed with another without actual bonding (see model illustration), and that a function of specific laminin-collagen interactions could be to orient and restrain the two polymers in space with respect to each other. PROTEOGLYCANS
AND
GLYCOSAMINOGLYCANS
Heparan
sulfate
proteoglycans
Heparan sulfate proteoglycans are a class of macromolecules characterized by a protein core covalently bound to heparan sulfate chains. The polyanionic chains confer charge-dependent selective filtration properties to basement membranes (1). Proteoglycans of different core size and with different heparan sulfate chain size and sulfation can be found in basement membranes, and have been characterized (9, 55, 56). One high-density heparan sulfate proteoglycan (130 kDa), possessing a small core and four short polysaccharide chains, is loosely bound to the EHS matrix (9) and found to weakly bind laminin, fibronectin, and type IV collagen. Its tissue distribution and physiological role are unclear. The low-density heparan sulfate proteoglycan, firmly bound to the EHS basement membrane and requiring chaotropic buffers for extraction (55, 56), is present in many basement membranes including renal glomerulus (57). It has an elongated core subdivided into a tandem array of globular domains (56) with intervening connectors: several heparan sulfate chains extend from one end of the core (Fig. 10). The core (MR = 400-450 kDa), a single polypeptide stabilized by intrachain disulfides, is highly sensitive to proteolysis (58), and it has been proposed that there is a normal physiological processing to smaller forms
(57);
however,
artifactual
proteolysis
Proteoglycan The
chaotropic
strongly
Heparin-
conditions that
there
required are
for
high-affinity
extraction binding
in-
and
heparan
sulfate
interactions
These sulfate-substituted uronic acid/N-acetylglucosamine repeating units can possess multiple biological functions including anticoagulation, filtration, and the regulation of cell behavior. Heparins have a higher degree of sulfation than heparan sulfates, and the two can be regarded as two ends of a compositional spectrum. Heparins represent an important secretory product of bone marrow-derived cells whereas heparan sulfates are found both on cell surfaces and within basement membranes. Functions of these glycosaminoglycans are dependent on the degree and sites of sulfation (61). the
Both heparin and heparan sulfate bind laminin (3), former with greater avidity. However, although
there is evidence for the existence of high-affinity, longarm heparin binding (3, 30, 42), and also for lowaffinity, short-arm versy has developed
heparin over
the
binding (62, significance
63), and
controsites of
weaker interactions (3, 42, 62, 63). Heparin binds to intact laminin/entactin complex with a high affinity dissociation constant of 0.1 jiM (42). Low affinity-heparin interactions with the short arms, which cannot be measured below 3 jiM heparin (42), are observed at high concentrations of heparin (63). It has also been found that heparin (64) and heparan sulfate (9) interact with type IV collagen. By rotary shadow decoration (64),
Figure 10. Low-density heparan sulfate proteoglycan. Electron micrograph of low angle Pt/C rotary shadowed replica. Bar equals 100 nm. Note the three thread-like heparan sulfate chains extending from one pole of the elongated and thicker core protein (58).
suggest
teractions between proteoglycan and basement membrane. Core binding sites, however, may have been lost or altered due to partial denaturation during purification. Although putative proteoglycan-anchoring binding sites have yet to be elucidated, there is in vitro evidence for other types of core interaction. First, the core can self-assemble into dimers and oligomers (58). When low-density proteoglycan or isolated core is incubated in buffers of physiological pH and ionic strength, it will form larger structures that can be analyzed by electron microscopy and sedimentation. In rotary shadowed Pt/C replicas, the binding region is found at one end of the core at the pole opposite the heparan sulfate linkage site, and dimers appear as double-length structures. Limited trypsin proteolysis cleaves the core and the moiety possessing the chains loses its ability to self-assemble (58). Second, proteoglycan and core protein promote epithelial cell attachment and 26-, 36-, and 38-kDa cell surface proteins that bind the core have been identified (60).
during
purification cannot be excluded. Two cDNA clones have provided amino acid sequence for about 40% of the mouse low density proteoglycan core (59). One region consists of a pair of globular domains bounded and connected by EGF-like cysteine-rich repeats similar to the short-arm structure in laminin while another region consists of disulfide-bonded loops similar to the neural cell adhesion glycoprotein.
core interactions
the
principal
site has
been
mapped
to the
globular
(NC1) domain with two lower affinity interactions in the collagen chain in regions 100 and 300 nm from the globule (64, 65). Finally, heparan sulfate chains within the basement membrane act as repositories for basic fibroblast growth factor. This growth factor can be released from extracellular matrix with heparitinase or heparin in an active form and binds endothelial cell extracellular matrix with an apparent KD of 620 nM (66). 1585
BASEMENT MEMBRANE MOLECULAR ARCHITECTURE m www.fasebj.org by Kaohsiung Medical University Library (163.15.154.53) on August 15, 2018. The FASEB Journal Vol. ${article.issue.getVolume()}, No. ${article.issue.getIssueNumber()
tardation characteristics. For example, in acute inflammation one effect of mast cell-released heparin adjacent to the postcapillary venule could be to transiently alter basement membrane sieving through such architectural modifications.
Chondroitin TVPE
VII
COLLAGEN
Members
mAb- 161 mAb- Vii
OTHER
pAb-Vil
11. Model
of relationship
between
type VII
collagen
and
an
anchoring fibril. Type VII procollagen molecules dimerize by overlapping at their NH2-terminal domains, perhaps with the subsequent processing of the NH2-terminal globular domain (+). These dimers then condense to form anchoring fibrils. The multidomain COOH-terminal globules in turn bind both to the lamina densa and the stromal anchoring plaques. In the detail of the COOHterminal region (bottom of figure), sites of epitopes recognized by monoclonal antibodies are indicated by the arrow and brackets. Reproduced with permission from ref 4.
Heparin modulation polymerization
of laminin
and collagen
IV
We have found that heparin can regulate laminin polymerization (42, 67) and resulting molecular architecture. The effect is biphasic in which a sharp augmentation of polymerization occurs at low (
