Experimental Eye Research 133 (2015) 1e2
Contents lists available at ScienceDirect
Experimental Eye Research journal homepage: www.elsevier.com/locate/yexer
Editorial
Introduction to Special Issue on ocular ECM The extracellular matrix (ECM) is a complex, acellular biomaterial that surrounds the cells within all tissues and organs. The ECM has long been recognized as the primary structural component of vertebrate tissues, which defines the shape and mechanical properties necessary for proper organ function. In the eye, the geometry and optical properties of the ECM are critical to the proper refraction and transmission of light that is required for vision. In addition to structural proteins, the ECM consists of a complex mixture of proteoglycans, glycoproteins, enzymes, growth factors and other signaling molecules. Thus, in addition to providing structural support for cells, the ECM plays critical roles in biochemical regulation of cellular differentiation, homeostasis and transformation, our understanding of which is continually evolving. This Special Issue of Experimental Eye Research, starting from the posterior pole and moving anteriorly, presents a series of reviews on both the biomechanical and biochemical roles of the ECM in regulation of ocular development, homeostasis and disease. In the first article, Faissner and coworkers review our current understanding of ECM remodeling and its function during retinal development. In the retina, the ECM forms the milieu surrounding retinal cells, constitutes basement membranes and provides structural as well as mechanical support. In addition, ECM molecules regulate retinal homeostasis and cellular signaling. The authors illustrate how ECM components control axonal growth and guidance of retinal ganglion cells, with a focus on ECM modulation during de- and regeneration processes. In the second article, Al-Ubaidi and Kanan review the role of tyrosine-sulfated proteins in the retina. Tyrosine sulfation has been shown to be necessary for the development of proper retinal structure and function, and the importance of tyrosine sulfation is demonstrated by the evolutionary presence of tyrosyl:protein sulfotransferases, enzymes that catalyze sulfation of tyrosine residues in proteins, and the compensatory abilities of these enzymes. To date, four tyrosine-sulfated proteins have been identified in the retina: fibulin 2, vitronectin, complement factor H (CFH), and opticin. Importantly, vitronectin and CFH regulate the activation of the complement system and are involved in the etiology of some cases of age-related macular degeneration. Additional studies are needed to identify additional tyrosine-sulfated proteins and determine the full impact of sulfation on protein function in retinal homeostasis and disease. Ishikawa and coworkers then review the present knowledge concerning the structure and function of the interphotoreceptor matrix (IPM) under physiological and pathological conditions. The IPM is a highly organized structure with interconnected domains surrounding cone and rod photoreceptor cells and extends throughout the subretinal space. Given that the IPM occupies a strategic interface http://dx.doi.org/10.1016/j.exer.2015.03.007 0014-4835/© 2015 Elsevier Ltd. All rights reserved.
between the neural retina and the retinal pigment epithelium (RPE), it is likely to play an important role in maintaining retinal function by mediating biochemical and biophysical interaction among the photoreceptor cells, RPE, choroidal vasculature, and Müller glia. Recent biochemical studies have revealed that IPM components, including fibulin, matrix metalloproteinases (MMPs), cyclic guanosine monophosphate phosphodiesterase (cGMPP), pigment epithelium-derived factor (PEDF), sialoprotein associated with cones and rods (SPACR), ab-crystallin, and sialoproteoglycan associated with cones and rods (SPACRCAN), play a significant role in the etiology of photoreceptor degeneration. In the fourth article, Bishop and coworkers review the role of the ECM in retinal vascular development and preretinal neovascularization. ECM plays a central role in angiogenesis. ECM degrading enzymes break down the pre-existing vascular basement membrane at an early stage of angiogenesis and subsequently degrade stromal ECM as the new vessels invade into tissues. Conversely, certain ECM components, including collagen, fibronectin and fibrin, are required for endothelial cell migration and tube morphogenesis. In pathological retinal angiogenesis, such as in proliferative diabetic retinopathy, preretinal neovascularization occurs where new blood vessels invade the cortical vitreous gel and these blood vessels require vitreous collagen for their growth. The vitreous is normally anti-angiogenic and contains endogenous ECM inhibitors of angiogenesis, including opticin and thombospondins, as well as ECM fragments such as endostatin. In preretinal neovascularization, the combined anti-angiogenic effects of these molecules are overcome by an excess of growth factors such as vascular endothelial growth factor-A, and new vessels grow into the vitreous with potentially blinding sequelae. Next, Klaassen reviews the role of connective tissue growth factor (CTGF) in the pathogenesis of diabetic retinopathy. CTGF is a secreted protein that modulates the actions of many growth factors and ECM proteins, which leads to tissue reorganization via ECM formation and remodeling, basal lamina thickening, pericyte apoptosis, angiogenesis, wound healing and fibrosis. CTGF contributes to fibrotic responses in diabetic retinopathy, both before clinical manifestations occur in the pre-clinical stage of diabetic retinopathy (PCDR) and in proliferative diabetic retinopathy (PDR), the late clinical stage of the disease. In PCDR, CTGF contributes to thickening of the retinal capillary basal lamina and is involved in loss of pericytes. In PDR, the angio-fibrotic switch is driven by CTGF, in a critical balance with vascular endothelial growth factor. Overall, CTGF could represent a novel therapeutic target in the clinical management of early as well as late stages of diabetic retinopathy. In the sixth article, Roy and coworkers discuss the roles of the vascular basement membrane (BM) and gap junctions in the
2
Editorial / Experimental Eye Research 133 (2015) 1e2
development of diabetic retinopathy. The vascular BM contains ECM proteins that assemble in a highly organized manner to form a supportive substratum for cell attachment, facilitating a myriad of functions that are vital to cell survival and overall retinal homeostasis. The BM provides a microenvironment in which bidirectional signaling through integrins regulates cell attachment, turnover, and functionality. In diabetic retinopathy, thickened vascular BM in the retinal capillaries actively contributes to the development and progression of characteristic changes associated with diabetic retinopathy. High glucose-induced compromised cellecell communication via gap junctions in retinal vascular cells may also disrupt homeostasis in the retinal microenvironment. The next two articles in this series focus on how the ECM impacts the development and progression of glaucoma. First, Downs provides a nontechnical review on optic nerve head (ONH) biomechanics in aging and disease. This review describes both how mechanical forces and the resulting deformations are distributed in the posterior pole and ONH (biomechanics), as well as how the living system responds to those deformations (mechanobiology). The manner in which the ONH responds to IOP elevations as a structural system is discussed, insofar as the acute mechanical response of the lamina cribrosa is confounded with the responses of the peripapillary sclera, prelaminar neural tissues, and retrolaminar optic nerve. Then, a discussion of the biomechanical basis for IOP-driven changes in connective tissues, blood flow, and cellular responses is presented. These biomechanical responses are presented in the context of glaucoma, since ONH biomechanics, aging, and the posterior pole ECM are thought to be centrally involved in the susceptibility, onset and progression of this disease. In the subsequent article, Keller and coworkers review how the trabecular meshwork (TM) ECM regulates outflow resistance and intraocular pressure changes in glaucoma. Most of the outflow resistance in the TM is thought to be from the ECM of the juxtacanalicular region, the deepest portion of the TM, and from the inner wall basement membrane of Schlemm's canal. It is becoming increasingly evident that the extracellular milieu is important in maintaining the integrity of the TM necessary for normal function. Not only have ultrastructural changes been observed in the ECM of the TM in glaucoma, but the stiffness of glaucomatous TM appears to be greater than that of normal tissue. Additionally, TGFb2 has been found to be elevated in the aqueous humor of glaucoma patients and is assumed to be involved in ECM changes deep with the juxtacanalicular region of the TM. Finally, a growing number of mutations have been identified in ECM genes and genes that modulate ECM in humans with glaucoma. Harper and Summers present an overview of the dynamics of scleral ECM remodeling, and its role in normal ocular growth and myopia development. Myopia is a common ocular condition, characterized by excessive elongation of the ocular globe. The elongation of the eye is closely related to the biomechanical properties of the sclera, which in turn are largely dependent on the composition of the scleral ECM. The review discusses the various mechanisms through which the scleral ECM is capable of responding to changes in the visual environment to affect changes in ocular size and refraction, and how these are regulated. Therapies designed to slow the loss of ECM in the human sclera, through inhibition of MMP activity, stimulation of proteoglycan and collagen synthesis, or increasing collagen crosslinking, could potentially be used to slow the progression of myopia. One potential target is choroidal retinaldehyde dehydrogenase 2 (RALDH2), a visually regulated enzyme that, through its synthesis of all-trans-retinoic acid, is a potent regulator of scleral ECM remodeling. The last three articles in this Special Issue concern the cornea. First, Jester and Quantock discuss current methods for studying ocular ECM assembly from the 'nano' to the 'macro' levels of
hierarchical organization. Since collagen is the major structural protein in the eye, the methods presented focus on understanding the molecular assembly of collagen, from the nanometer level using Xray scattering to the millimeter level using second harmonic generated (SHG) signals. Three-dimensional analysis of ECM structure is also discussed, including electron tomography, serial block face scanning electron microscopy (SBF-SEM) and digital image reconstruction. Techniques to detect non-collagenous structural components of the ECM are also presented, and these include immunoelectron microscopy and staining with cationic dyes. Together these approaches are providing new insights into the structural blueprint of the corneal ECM, which can improve our understanding of pathogenic mechanisms underlying ectatic disorders of the cornea and potentially lead to better approaches to corneal tissue engineering. Next, Birk and coworkers provide a review on corneal stroma ECM assembly and collagen fibrillogenesis. In the stroma, homogeneous, small diameter collagen fibrils, regularly packed with a highly ordered hierarchical organization, are essential for function. Corneal collagen fibrillogenesis involves multiple molecules interacting in sequential steps, as well as interactions between keratocytes and stroma matrix components. Collagen V regulates the nucleation of protofibril assembly, thus controlling the number of fibrils and assembly of smaller diameter fibrils in the stroma. The corneal stroma is also enriched in small leucine-rich proteoglycans (SLRPs) that cooperate in a temporal and spatial manner to regulate linear and lateral collagen fibril growth. In addition, the fibril-associated collagens (FACITs) such as collagen XII and collagen XIV have roles in the regulation of fibril packing and inter-lamellar interactions. Keratocytes control the synthesis of ECM components, which in turn interact with the keratocytes to coordinate the regulatory steps into a cohesive process. This finely controlled assembly of the stromal ECM is critical to corneal function, as well as in establishing the appropriate mechanical stability required to maintain corneal shape and curvature. In the final article, Petroll and Miron-Mendoza provide an overview of the biochemical and biophysical factors regulating the mechanical interactions between corneal keratocytes and the stromal ECM. The generation of cellular forces and the application of these physical forces to the ECM play a central role in mediating matrix patterning and remodeling during fundamental processes such as developmental morphogenesis and wound healing. In addition to biochemical factors that can modulate the keratocyte mechanical phenotype, another key player in the regulation of cell-induced ECM patterning is the mechanical state of the ECM itself. The authors first discuss how Rho GTPases regulate the sub-cellular pattern of force generation by corneal keratocytes, and the impact these forces have on the surrounding ECM. They next review how feedback from local matrix structural and mechanical properties can modulate keratocyte phenotype and mechanical activity. Examples of how these biophysical interactions may contribute to clinical outcomes are provided, with particular attention to corneal wound healing. Muayyad R. Al-Ubaidi Centre for Neuroscience, University of Oklahoma College of Medicine, 1100 N Lindsay, Oklahoma City, 73104, OK, USA E-mail address: [email protected]. W. Matthew Petroll* Opthalmology, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, 75390-9057, TX, USA * Corresponding author. E-mail address: [email protected] (W.M. Petroll).
9 March 2015
Contents lists available at ScienceDirect
Experimental Eye Research journal homepage: www.elsevier.com/locate/yexer
Editorial
Introduction to Special Issue on ocular ECM The extracellular matrix (ECM) is a complex, acellular biomaterial that surrounds the cells within all tissues and organs. The ECM has long been recognized as the primary structural component of vertebrate tissues, which defines the shape and mechanical properties necessary for proper organ function. In the eye, the geometry and optical properties of the ECM are critical to the proper refraction and transmission of light that is required for vision. In addition to structural proteins, the ECM consists of a complex mixture of proteoglycans, glycoproteins, enzymes, growth factors and other signaling molecules. Thus, in addition to providing structural support for cells, the ECM plays critical roles in biochemical regulation of cellular differentiation, homeostasis and transformation, our understanding of which is continually evolving. This Special Issue of Experimental Eye Research, starting from the posterior pole and moving anteriorly, presents a series of reviews on both the biomechanical and biochemical roles of the ECM in regulation of ocular development, homeostasis and disease. In the first article, Faissner and coworkers review our current understanding of ECM remodeling and its function during retinal development. In the retina, the ECM forms the milieu surrounding retinal cells, constitutes basement membranes and provides structural as well as mechanical support. In addition, ECM molecules regulate retinal homeostasis and cellular signaling. The authors illustrate how ECM components control axonal growth and guidance of retinal ganglion cells, with a focus on ECM modulation during de- and regeneration processes. In the second article, Al-Ubaidi and Kanan review the role of tyrosine-sulfated proteins in the retina. Tyrosine sulfation has been shown to be necessary for the development of proper retinal structure and function, and the importance of tyrosine sulfation is demonstrated by the evolutionary presence of tyrosyl:protein sulfotransferases, enzymes that catalyze sulfation of tyrosine residues in proteins, and the compensatory abilities of these enzymes. To date, four tyrosine-sulfated proteins have been identified in the retina: fibulin 2, vitronectin, complement factor H (CFH), and opticin. Importantly, vitronectin and CFH regulate the activation of the complement system and are involved in the etiology of some cases of age-related macular degeneration. Additional studies are needed to identify additional tyrosine-sulfated proteins and determine the full impact of sulfation on protein function in retinal homeostasis and disease. Ishikawa and coworkers then review the present knowledge concerning the structure and function of the interphotoreceptor matrix (IPM) under physiological and pathological conditions. The IPM is a highly organized structure with interconnected domains surrounding cone and rod photoreceptor cells and extends throughout the subretinal space. Given that the IPM occupies a strategic interface http://dx.doi.org/10.1016/j.exer.2015.03.007 0014-4835/© 2015 Elsevier Ltd. All rights reserved.
between the neural retina and the retinal pigment epithelium (RPE), it is likely to play an important role in maintaining retinal function by mediating biochemical and biophysical interaction among the photoreceptor cells, RPE, choroidal vasculature, and Müller glia. Recent biochemical studies have revealed that IPM components, including fibulin, matrix metalloproteinases (MMPs), cyclic guanosine monophosphate phosphodiesterase (cGMPP), pigment epithelium-derived factor (PEDF), sialoprotein associated with cones and rods (SPACR), ab-crystallin, and sialoproteoglycan associated with cones and rods (SPACRCAN), play a significant role in the etiology of photoreceptor degeneration. In the fourth article, Bishop and coworkers review the role of the ECM in retinal vascular development and preretinal neovascularization. ECM plays a central role in angiogenesis. ECM degrading enzymes break down the pre-existing vascular basement membrane at an early stage of angiogenesis and subsequently degrade stromal ECM as the new vessels invade into tissues. Conversely, certain ECM components, including collagen, fibronectin and fibrin, are required for endothelial cell migration and tube morphogenesis. In pathological retinal angiogenesis, such as in proliferative diabetic retinopathy, preretinal neovascularization occurs where new blood vessels invade the cortical vitreous gel and these blood vessels require vitreous collagen for their growth. The vitreous is normally anti-angiogenic and contains endogenous ECM inhibitors of angiogenesis, including opticin and thombospondins, as well as ECM fragments such as endostatin. In preretinal neovascularization, the combined anti-angiogenic effects of these molecules are overcome by an excess of growth factors such as vascular endothelial growth factor-A, and new vessels grow into the vitreous with potentially blinding sequelae. Next, Klaassen reviews the role of connective tissue growth factor (CTGF) in the pathogenesis of diabetic retinopathy. CTGF is a secreted protein that modulates the actions of many growth factors and ECM proteins, which leads to tissue reorganization via ECM formation and remodeling, basal lamina thickening, pericyte apoptosis, angiogenesis, wound healing and fibrosis. CTGF contributes to fibrotic responses in diabetic retinopathy, both before clinical manifestations occur in the pre-clinical stage of diabetic retinopathy (PCDR) and in proliferative diabetic retinopathy (PDR), the late clinical stage of the disease. In PCDR, CTGF contributes to thickening of the retinal capillary basal lamina and is involved in loss of pericytes. In PDR, the angio-fibrotic switch is driven by CTGF, in a critical balance with vascular endothelial growth factor. Overall, CTGF could represent a novel therapeutic target in the clinical management of early as well as late stages of diabetic retinopathy. In the sixth article, Roy and coworkers discuss the roles of the vascular basement membrane (BM) and gap junctions in the
2
Editorial / Experimental Eye Research 133 (2015) 1e2
development of diabetic retinopathy. The vascular BM contains ECM proteins that assemble in a highly organized manner to form a supportive substratum for cell attachment, facilitating a myriad of functions that are vital to cell survival and overall retinal homeostasis. The BM provides a microenvironment in which bidirectional signaling through integrins regulates cell attachment, turnover, and functionality. In diabetic retinopathy, thickened vascular BM in the retinal capillaries actively contributes to the development and progression of characteristic changes associated with diabetic retinopathy. High glucose-induced compromised cellecell communication via gap junctions in retinal vascular cells may also disrupt homeostasis in the retinal microenvironment. The next two articles in this series focus on how the ECM impacts the development and progression of glaucoma. First, Downs provides a nontechnical review on optic nerve head (ONH) biomechanics in aging and disease. This review describes both how mechanical forces and the resulting deformations are distributed in the posterior pole and ONH (biomechanics), as well as how the living system responds to those deformations (mechanobiology). The manner in which the ONH responds to IOP elevations as a structural system is discussed, insofar as the acute mechanical response of the lamina cribrosa is confounded with the responses of the peripapillary sclera, prelaminar neural tissues, and retrolaminar optic nerve. Then, a discussion of the biomechanical basis for IOP-driven changes in connective tissues, blood flow, and cellular responses is presented. These biomechanical responses are presented in the context of glaucoma, since ONH biomechanics, aging, and the posterior pole ECM are thought to be centrally involved in the susceptibility, onset and progression of this disease. In the subsequent article, Keller and coworkers review how the trabecular meshwork (TM) ECM regulates outflow resistance and intraocular pressure changes in glaucoma. Most of the outflow resistance in the TM is thought to be from the ECM of the juxtacanalicular region, the deepest portion of the TM, and from the inner wall basement membrane of Schlemm's canal. It is becoming increasingly evident that the extracellular milieu is important in maintaining the integrity of the TM necessary for normal function. Not only have ultrastructural changes been observed in the ECM of the TM in glaucoma, but the stiffness of glaucomatous TM appears to be greater than that of normal tissue. Additionally, TGFb2 has been found to be elevated in the aqueous humor of glaucoma patients and is assumed to be involved in ECM changes deep with the juxtacanalicular region of the TM. Finally, a growing number of mutations have been identified in ECM genes and genes that modulate ECM in humans with glaucoma. Harper and Summers present an overview of the dynamics of scleral ECM remodeling, and its role in normal ocular growth and myopia development. Myopia is a common ocular condition, characterized by excessive elongation of the ocular globe. The elongation of the eye is closely related to the biomechanical properties of the sclera, which in turn are largely dependent on the composition of the scleral ECM. The review discusses the various mechanisms through which the scleral ECM is capable of responding to changes in the visual environment to affect changes in ocular size and refraction, and how these are regulated. Therapies designed to slow the loss of ECM in the human sclera, through inhibition of MMP activity, stimulation of proteoglycan and collagen synthesis, or increasing collagen crosslinking, could potentially be used to slow the progression of myopia. One potential target is choroidal retinaldehyde dehydrogenase 2 (RALDH2), a visually regulated enzyme that, through its synthesis of all-trans-retinoic acid, is a potent regulator of scleral ECM remodeling. The last three articles in this Special Issue concern the cornea. First, Jester and Quantock discuss current methods for studying ocular ECM assembly from the 'nano' to the 'macro' levels of
hierarchical organization. Since collagen is the major structural protein in the eye, the methods presented focus on understanding the molecular assembly of collagen, from the nanometer level using Xray scattering to the millimeter level using second harmonic generated (SHG) signals. Three-dimensional analysis of ECM structure is also discussed, including electron tomography, serial block face scanning electron microscopy (SBF-SEM) and digital image reconstruction. Techniques to detect non-collagenous structural components of the ECM are also presented, and these include immunoelectron microscopy and staining with cationic dyes. Together these approaches are providing new insights into the structural blueprint of the corneal ECM, which can improve our understanding of pathogenic mechanisms underlying ectatic disorders of the cornea and potentially lead to better approaches to corneal tissue engineering. Next, Birk and coworkers provide a review on corneal stroma ECM assembly and collagen fibrillogenesis. In the stroma, homogeneous, small diameter collagen fibrils, regularly packed with a highly ordered hierarchical organization, are essential for function. Corneal collagen fibrillogenesis involves multiple molecules interacting in sequential steps, as well as interactions between keratocytes and stroma matrix components. Collagen V regulates the nucleation of protofibril assembly, thus controlling the number of fibrils and assembly of smaller diameter fibrils in the stroma. The corneal stroma is also enriched in small leucine-rich proteoglycans (SLRPs) that cooperate in a temporal and spatial manner to regulate linear and lateral collagen fibril growth. In addition, the fibril-associated collagens (FACITs) such as collagen XII and collagen XIV have roles in the regulation of fibril packing and inter-lamellar interactions. Keratocytes control the synthesis of ECM components, which in turn interact with the keratocytes to coordinate the regulatory steps into a cohesive process. This finely controlled assembly of the stromal ECM is critical to corneal function, as well as in establishing the appropriate mechanical stability required to maintain corneal shape and curvature. In the final article, Petroll and Miron-Mendoza provide an overview of the biochemical and biophysical factors regulating the mechanical interactions between corneal keratocytes and the stromal ECM. The generation of cellular forces and the application of these physical forces to the ECM play a central role in mediating matrix patterning and remodeling during fundamental processes such as developmental morphogenesis and wound healing. In addition to biochemical factors that can modulate the keratocyte mechanical phenotype, another key player in the regulation of cell-induced ECM patterning is the mechanical state of the ECM itself. The authors first discuss how Rho GTPases regulate the sub-cellular pattern of force generation by corneal keratocytes, and the impact these forces have on the surrounding ECM. They next review how feedback from local matrix structural and mechanical properties can modulate keratocyte phenotype and mechanical activity. Examples of how these biophysical interactions may contribute to clinical outcomes are provided, with particular attention to corneal wound healing. Muayyad R. Al-Ubaidi Centre for Neuroscience, University of Oklahoma College of Medicine, 1100 N Lindsay, Oklahoma City, 73104, OK, USA E-mail address: [email protected]. W. Matthew Petroll* Opthalmology, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, 75390-9057, TX, USA * Corresponding author. E-mail address: [email protected] (W.M. Petroll).
9 March 2015
