][]EVIEWS 6 Coulson, A., Sulston, J., Brenner, S. and Karn, J. (1986) Proc. Natl Acad. Sci. USA 83, 7821-7825 7 Saunders, R.D.C. et al. (1989) Nucleic Acids Res. 17, 9027-9037 8 Sulston, J. et al. (1988) CABIOS 4, 125-132 9 Sulston, J., Mallett, F., Durbin, R. and Horsnell, T. (1989) CABIOS 5, 101-106 10 Siden-Kiamos, 1. et al. (1990) Nucleic Acids Res. 18, 6261-6270 11 Sorsa, V. (1988) Chromosome Maps of Drosophila (Vols 1, 2), CRC Press 12 Bender, W., Spierer, P. and Hogness, D.S. (1983)/. Mol. Biol. 168, 17-33 13 Garza, D., Ajioka, J,W., Burke, D.T. and Hartl. D.L. (1989) Science 246, 6414546 14 Olson, M., Hood, L., Cantor, C. and Botstein, D. (1989) Science 245, 1434-1435
O r g a n i s m s use a variety of mechanisms to move, including flagellar or ciliary beating, and pseudopodial extensions that are characteristic of cell types ranging from amoebae to leukocytes. Here, we focus on models for this latter type of cell migration. Extension of pseudopodia at the leading edge of a cell coincides with retraction of the rear of the cell, resulting in net forward movement. The mechanism by which cells perform this type of movement has been a subject of debate for many years, and remains controversial. Despite differences in the details of cell movement in different cell types, the underlying mechanisms are probably similar in most migrating cells. Although adhesion will not be discussed, the ability of cells to adhere to their substratum in a regulated fashion is clearly critical to all models of cell migrationS-3. The purpose of this article is first to outline some of the major models that have been proposed to explain cell locomotion, and then to describe recent studies performed with the amoeba Dictyostelium d i s c o i d e u m , in which molecular genetics has been used to dissect the mechanisms of cell locomotion.
Models for celllocomotion Early observations of cytoplasmic streaming in the giant amoebae led to models in which a transition of cytoplasm from a more solid or 'gel' state to a more liquid or 'sol' state is coupled to cortical contraction to drive the cell forward 4. In the most basic version of this model, the central cytoplasm, or endoplasm, of the cell exists in a sol state, and the periphery, or ectoplasm, exists in a gel state. Contraction at the rear of the cell squeezes the sol material forward, where it is converted to gel at the newly formed leading peripheral zone. Ectoplasm at the rear of the cell is continuously converted to sol, providing new solated cytoplasm to be squeezed to the front. The presence of a stiff cortical layer, or cytoskeleton, and a more fluid endoplasm in virtually all eukaryotic migratory cells has led many cell biologists to hypothesize that this cycling of cytoplasm may be a key event that drives cell locomotion (see Ref. 5 for a detailed view of this type of 'solationcontraction' model). A wealth of studies has demonstrated that assembly and disassembly of the
F . ~ KAEATOS IS IN THE INSTITUTE OF MOLECULAR BIOLOGY AND t i BIOTECHNOLOGY, RESEARCH CENTER OF CRETE, HERAKLION, 711 10 ~ CRETE, GREECE AND THE DEPARTMENT OF CELLULAR AND i DEVELOPMENTAL BIOLOGY, HARVARD UNIVERSITY, 16 D M N I T Y AVE, ] CAMBPdDG~ MA 02138~ USA," C Louzs Is IN THE INSTITUTE OF]
MOLECUtARBmzoar AND BIOTECnNOLOGY,RESEARChCENTEROr CRET~ HERAKLION,711 10 CRET~ GREECE;C SAVAKISrS IN rnE INyrrr~TE or MOLECULARBIOLOGYAND BmTECnNOLOGY, RESEARCH CENTER OF CRETE, HERAKLION, 711 l O CRETE, GREECE; D.M. GLOVER : IS IN THE DEPARTMENT OF BIOCHEMISTRY, UNIVERSITY OF DUNDEE, [
DUNDEEDDI 4HN, UK; M. ASHSURNERIS m THE DEPARTMENTOF i I GENETIC~ CAMBRIDGE UNIVERSITY, OOWNING STREET, CAMBRIL)GE, i UK," A~. LINK IS IN THE DEPARTME,N~f OF GENETICS, HAR~XRD MEDICAL SCHOOL, 2 5 SHATI'UCK ST, BOSTON, M A 02115, USA; i L SID$N-KIAMOS IS IN THE INSTITUTE OF MOLECULAR BIOLOGY AND i BIOTECHNOLOGY, RESEARCH CENTER OF CRETE, HERAKLION, 711 10 CRETE, GREECE; AND R . D . C SAU:~DER5 15 l~N FHE DEPARTME3,1 o k BIOCHEMISTRY, UNIVERSITY OF DUNDEE, DUNDEE D D I 4HN, UK. [
Molecular genetics of cell migration: Dictyostelium as a model system THOMAS T. EGELHOFFAND JAMES A. SPUDICH A central unresolved issue in modern cell biology concerns bow eukaryotic cell migration is achiever Although the underlying mechanics o f cell locomotion appear similar in cells ranging from amoebae to leukocytes, the organisms that have been historically studied have not been amenable to the techniques of modern molecular genetics. The recent development of high-efficiency gene targeting technology f o r DictyosteUum discoideum, coupled with the classic cell migration behavior o f this organism, offers an opportunity to resolve many of the controversial issues concerning cell locomotion. cytoskeletal protein actin is responsible for gel-sol transitions in migratory cells. Complex regulatory events involving cellular messengers and actin-binding proteins appear to control the in t,it,o properties of actin, allowing gel-sol transitions to bc coupk:d ~; other cellular events and to extracellular signals ¢> The immunolocalization of the contractile protei1~ myosin to the posterior cortex of migrating Dictyosteliurn cells has provided support for gel-sol transition models of cell motility by offering a source of contraction at the rear of the celV. This conventional myosin (also known as myosin II) is a two-headed molecule similar in structure to muscle mw)sin. It assembles into bipolar filaments that are thought to be the active functional unit in vivo. This myo.qn has been written into several models for cell locomotion over the years. More recently another class of myosins. known as the myosin I class, has been found re> bc ubiquitous in non-muscle cells s. These single-headed molecular motors have the conserved globular myosin head containing actin-based contractile activity, t>ut lack the (z-helical coiled-coil tail of conxcnti{>nai myosin that allows dimerization and bipolar filament
T1G MAY1991 VOL. 7 "~'O. 5 "elt~)l El,
O r g a n i s m s use a variety of mechanisms to move, including flagellar or ciliary beating, and pseudopodial extensions that are characteristic of cell types ranging from amoebae to leukocytes. Here, we focus on models for this latter type of cell migration. Extension of pseudopodia at the leading edge of a cell coincides with retraction of the rear of the cell, resulting in net forward movement. The mechanism by which cells perform this type of movement has been a subject of debate for many years, and remains controversial. Despite differences in the details of cell movement in different cell types, the underlying mechanisms are probably similar in most migrating cells. Although adhesion will not be discussed, the ability of cells to adhere to their substratum in a regulated fashion is clearly critical to all models of cell migrationS-3. The purpose of this article is first to outline some of the major models that have been proposed to explain cell locomotion, and then to describe recent studies performed with the amoeba Dictyostelium d i s c o i d e u m , in which molecular genetics has been used to dissect the mechanisms of cell locomotion.
Models for celllocomotion Early observations of cytoplasmic streaming in the giant amoebae led to models in which a transition of cytoplasm from a more solid or 'gel' state to a more liquid or 'sol' state is coupled to cortical contraction to drive the cell forward 4. In the most basic version of this model, the central cytoplasm, or endoplasm, of the cell exists in a sol state, and the periphery, or ectoplasm, exists in a gel state. Contraction at the rear of the cell squeezes the sol material forward, where it is converted to gel at the newly formed leading peripheral zone. Ectoplasm at the rear of the cell is continuously converted to sol, providing new solated cytoplasm to be squeezed to the front. The presence of a stiff cortical layer, or cytoskeleton, and a more fluid endoplasm in virtually all eukaryotic migratory cells has led many cell biologists to hypothesize that this cycling of cytoplasm may be a key event that drives cell locomotion (see Ref. 5 for a detailed view of this type of 'solationcontraction' model). A wealth of studies has demonstrated that assembly and disassembly of the
F . ~ KAEATOS IS IN THE INSTITUTE OF MOLECULAR BIOLOGY AND t i BIOTECHNOLOGY, RESEARCH CENTER OF CRETE, HERAKLION, 711 10 ~ CRETE, GREECE AND THE DEPARTMENT OF CELLULAR AND i DEVELOPMENTAL BIOLOGY, HARVARD UNIVERSITY, 16 D M N I T Y AVE, ] CAMBPdDG~ MA 02138~ USA," C Louzs Is IN THE INSTITUTE OF]
MOLECUtARBmzoar AND BIOTECnNOLOGY,RESEARChCENTEROr CRET~ HERAKLION,711 10 CRET~ GREECE;C SAVAKISrS IN rnE INyrrr~TE or MOLECULARBIOLOGYAND BmTECnNOLOGY, RESEARCH CENTER OF CRETE, HERAKLION, 711 l O CRETE, GREECE; D.M. GLOVER : IS IN THE DEPARTMENT OF BIOCHEMISTRY, UNIVERSITY OF DUNDEE, [
DUNDEEDDI 4HN, UK; M. ASHSURNERIS m THE DEPARTMENTOF i I GENETIC~ CAMBRIDGE UNIVERSITY, OOWNING STREET, CAMBRIL)GE, i UK," A~. LINK IS IN THE DEPARTME,N~f OF GENETICS, HAR~XRD MEDICAL SCHOOL, 2 5 SHATI'UCK ST, BOSTON, M A 02115, USA; i L SID$N-KIAMOS IS IN THE INSTITUTE OF MOLECULAR BIOLOGY AND i BIOTECHNOLOGY, RESEARCH CENTER OF CRETE, HERAKLION, 711 10 CRETE, GREECE; AND R . D . C SAU:~DER5 15 l~N FHE DEPARTME3,1 o k BIOCHEMISTRY, UNIVERSITY OF DUNDEE, DUNDEE D D I 4HN, UK. [
Molecular genetics of cell migration: Dictyostelium as a model system THOMAS T. EGELHOFFAND JAMES A. SPUDICH A central unresolved issue in modern cell biology concerns bow eukaryotic cell migration is achiever Although the underlying mechanics o f cell locomotion appear similar in cells ranging from amoebae to leukocytes, the organisms that have been historically studied have not been amenable to the techniques of modern molecular genetics. The recent development of high-efficiency gene targeting technology f o r DictyosteUum discoideum, coupled with the classic cell migration behavior o f this organism, offers an opportunity to resolve many of the controversial issues concerning cell locomotion. cytoskeletal protein actin is responsible for gel-sol transitions in migratory cells. Complex regulatory events involving cellular messengers and actin-binding proteins appear to control the in t,it,o properties of actin, allowing gel-sol transitions to bc coupk:d ~; other cellular events and to extracellular signals ¢> The immunolocalization of the contractile protei1~ myosin to the posterior cortex of migrating Dictyosteliurn cells has provided support for gel-sol transition models of cell motility by offering a source of contraction at the rear of the celV. This conventional myosin (also known as myosin II) is a two-headed molecule similar in structure to muscle mw)sin. It assembles into bipolar filaments that are thought to be the active functional unit in vivo. This myo.qn has been written into several models for cell locomotion over the years. More recently another class of myosins. known as the myosin I class, has been found re> bc ubiquitous in non-muscle cells s. These single-headed molecular motors have the conserved globular myosin head containing actin-based contractile activity, t>ut lack the (z-helical coiled-coil tail of conxcnti{>nai myosin that allows dimerization and bipolar filament
T1G MAY1991 VOL. 7 "~'O. 5 "elt~)l El,
