Roles of the extracellular matrix in retinal development and maintenance

Repopulation of decellularized retinas with hiPSC-derived retinal pigment epithelial and ocular progenitor cells shows cell engraftment, organization and differentiation

The retinal extracellular matrix (ECM) provides architectural support, adhesion and signal guidance that controls retinal development. Decellularization of the ECM affords great potential to tissue engineering; however, how structural retinal ECM affects in vitro development, differentiation and maturation of ocular cells remains to be elucidated. Here, mouse and porcine retinas were decellularized and the protein profile analyzed. Acellular retinal ECM (arECM) scaffolds were then repopulated with human iPSC-derived retinal pigment epithelial (RPE) cells or ocular progenitor cells (OPC) to assess their integration, proliferation and organization. 3837 and 2612 unique proteins were identified in mouse and porcine arECM, respectively, of which 93 and 116 proteins belong to the matrisome. GO analysis shows that matrisome-related proteins were associated with the extracellular region and cell junction and KEGG pathways related to signalling transduction, nervous and endocrine systems and cell junctions were enriched. Interestingly, mouse and porcine arECMs were successfully repopulated with both RPE and OPC, the latter exhibiting cell lineage-specific clusters. Retinal cells organized into different layers containing well-defined areas with pigmented cells, photoreceptors, Müller glia, astrocytes, and ganglion cells, whereas in other areas, conjunctival/limbal, corneal and lens cells re-arranged in cell-specific self-organized areas. In conclusion, our results demonstrated that decellularization of both mouse and porcine retinas retains common native ECM components that upon cell repopulation could guide similar ocular cell adhesion, migration and organization.

Role of Epithelial-Mesenchymal Transition in Retinal Pigment Epithelium Dysfunction

Retinal pigment epithelial (RPE) cells maintain the health and functional integrity of both photoreceptors and the choroidal vasculature. Loss of RPE differentiation has long been known to play a critical role in numerous retinal diseases, including inherited rod-cone degenerations, inherited macular degeneration, age-related macular degeneration, and proliferative vitreoretinopathy. Recent studies in post-mortem eyes have found upregulation of critical epithelial-mesenchymal transition (EMT) drivers such as TGF-β, Wnt, and Hippo. As RPE cells become less differentiated, they begin to exhibit the defining characteristics of mesenchymal cells, namely, the capacity to migrate and proliferate. A number of preclinical studies, including animal and cell culture experiments, also have shown that RPE cells undergo EMT. Taken together, these data suggest that RPE cells retain the reprogramming capacity to move along a continuum between polarized epithelial cells and mesenchymal cells. We propose that movement along this continuum toward a mesenchymal phenotype be defined as RPE Dysfunction. Potential mechanisms include impaired tight junctions, accumulation of misfolded proteins and dysregulation of several key pathways and molecules, such as TGF-β pathway, Wnt pathway, nicotinamide, microRNA 204/211 and extracellular vesicles. This review synthesizes the evidence implicating EMT of RPE cells in post-mortem eyes, animal studies, primary RPE, iPSC-RPE and ARPE-19 cell lines.

Proteomic Profiling of Retinoblastoma-Derived Exosomes Reveals Potential Biomarkers of Vitreous Seeding

Retinoblastoma (RB) is the most common tumor of the eye in early childhood. Although recent advances in conservative treatment have greatly improved the visual outcome, local tumor control remains difficult in the presence of massive vitreous seeding. Traditional biopsy has long been considered unsafe in RB, due to the risk of extraocular spread. Thus, the identification of new biomarkers is crucial to design safer diagnostic and more effective therapeutic approaches. Exosomes, membrane-derived nanovesicles that are secreted abundantly by aggressive tumor cells and that can be isolated from several biological fluids, represent an interesting alternative for the detection of tumor-associated biomarkers. In this study, we defined the protein signature of exosomes released by RB tumors (RBT) and vitreous seeding (RBVS) primary cell lines by high resolution mass spectrometry. A total of 5666 proteins were identified. Among these, 5223 and 3637 were expressed in exosomes RBT and one RBVS group, respectively. Gene enrichment analysis of exclusively and differentially expressed proteins and network analysis identified in RBVS exosomes upregulated proteins specifically related to invasion and metastasis, such as proteins involved in extracellular matrix (ECM) remodeling and interaction, resistance to anoikis and the metabolism/catabolism of glucose and amino acids.

Scaffolding the retina: the interstitial extracellular matrix during rat retinal development

PURPOSE

To examine the expression of interstitial extracellular matrix components and their role during retinal development.

MATERIAL AND METHODS

Fibronectin (FN), collagen IV (Coll IV) and laminin 5 (Lam 5) expression in rat retinas from developmental stages E17 to adult were studied. In addition, PN5 full-thickness retinas were cultured for 7 days with dispase, which selectively cleaves FN and Coll IV, at either 0.5 U/ml or 5.0 U/ml for 3 or 24h. Eyecups and retinal cultures were examined morphologically using hematoxylin and eosin staining and immunohistochemistry.

RESULTS

Coll IV, Lam 5 and FN were all transiently expressed in the interstitial matrix of the retinal layers during development. The retinal layers in dispase treated explants was severely disturbed in a dose and time dependent manner.

CONCLUSIONS

FN, Lam 5 and Coll IV, are present in the interstitial extracellular matrix during rat retinal development. Enzymatic cleavage of FN and Coll IV early in the lamination process disrupts the retinal layers implicating their pivotal role in this process.

β1 integrin-focal adhesion kinase (FAK) signaling modulates retinal ganglion cell (RGC) survival

Extracellular matrix (ECM) integrity in the central nervous system (CNS) is essential for neuronal homeostasis. Signals from the ECM are transmitted to neurons through integrins, a family of cell surface receptors that mediate cell attachment to ECM. We have previously established a causal link between the activation of the matrix metalloproteinase-9 (MMP-9), degradation of laminin in the ECM of retinal ganglion cells (RGCs), and RGC death in a mouse model of retinal ischemia-reperfusion injury (RIRI). Here we investigated the role of laminin-integrin signaling in RGC survival in vitro, and after ischemia in vivo. In purified primary rat RGCs, stimulation of the β1 integrin receptor with laminin, or agonist antibodies enhanced RGC survival in correlation with activation of β1 integrin’s major downstream regulator, focal adhesion kinase (FAK). Furthermore, β1 integrin binding and FAK activation were required for RGCs’ survival response to laminin. Finally, in vivo after RIRI, we observed an up-regulation of MMP-9, proteolytic degradation of laminin, decreased RGC expression of β1 integrin, FAK and Akt dephosphorylation, and reduced expression of the pro-survival molecule bcl-xL in the period preceding RGC apoptosis. RGC death was prevented, in the context of laminin degradation, by maintaining β1 integrin activation with agonist antibodies. Thus, disruption of homeostatic RGC-laminin interaction and signaling leads to cell death after retinal ischemia, and maintaining integrin activation may be a therapeutic approach to neuroprotection.

Retinal pigment epithelial cells synthesize laminins, including laminin 5, and adhere to them through alpha3- and alpha6-containing integrins

PURPOSE

Retinal diseases are often accompanied by changes in the structure of the multilayered extracellular matrix underlying the retina, Bruch's membrane (BrM). These structural revisions potentially lead to alterations in retinal pigment epithelium (RPE) adhesion, likely via modification of interactions with extracellular matrix (ECM) proteins including laminins in BrM. The purpose of this study was to identify specific laminins in BrM and their receptors in RPE cells.

METHODS

The laminin composition of BrM was determined using biochemical, molecular biological, and immunohistochemical techniques of rat, bovine, and human tissue and cell lines. An adhesion assay was used to test RPE attachment to laminins and the receptors used for this attachment.

RESULTS

BrM contained laminin chains that could form laminin heterotrimers including laminins 1, 5, 10, and 11. RPE cells synthesized these laminin chains in vitro. Therefore, RPE cells may synthesize BrM laminins. The RPE cells preferentially adhered to potential BrM laminins. Although the cells adhered to the BrM component collagen IV, these cells preferentially adhered to laminins. Of the laminins tested, the RPE cells adhered preferentially to laminin 5. The cells interacted with these laminins via specific integrins and attained a different morphology on each laminin. In particular, the RPE cells rapidly attached and flattened on laminin 5.

CONCLUSIONS

BrM contains specific laminins, and RPE cells express integrin receptors for those laminins. The interaction of these specific laminins and integrins most likely leads to differential behavior of RPE cells.

The extracellular matrix component WIF-1 is expressed during, and can modulate, retinal development

We have shown previously that components of the extracellular matrix (ECM) modulate neuronal development. Here, we searched for additional ECM elements that might play roles in retinal histogenesis and identified a secreted glycoprotein that is heavily expressed in the retina. This molecule, named by others Wnt Inhibitory Factor-1 (WIF-1), is expressed during and after the period of rod photoreceptor morphogenesis in the mouse. We show that a potential WIF-1 ligand, Wnt4, as well as a potential Wnt4 receptor, fzd4, and a potential Wnt4 coreceptor, LRP6, are expressed in the region of, and at the time of, rod photoreceptor genesis. WIF-1 and Wnt4 are coexpressed during retinal development and bind to each other; therefore, they are likely to interact during rod production. WIF-1 protein inhibits rod production, and anti-WIF-1 antibodies increase rod production; in contrast, Wnt4 promotes rod production. Together, these data suggest that WIF-1 and Wnt4, both components of the ECM, regulate mammalian photoreceptor development.

Inhibition of protein tyrosine kinase activity disrupts early retinal development

In the present study, we have investigated the role of tyrosine kinase activity during early retinal development in Xenopus laevis. The protein tyrosine kinase (PTK) inhibitors lavendustin A and genistein were used to determine the possible role of tyrosine kinase activity during retinal development in vivo and in vitro. Application of the inhibitors to early embryonic retina disrupted the pattern of lamination in the developing retina. The plexiform layers were severely disorganized or were no longer apparent, and photoreceptor morphogenesis was disrupted. Immunocytochemical analysis verified the presence of focal adhesions in dissociated retinal neuroepithelial cells isolated from St 25 embryos. Application of the PTK inhibitors blocked focal adhesion assembly in these primary cultured cells. To further investigate the regulation of focal adhesions by PTK activity, we examined the effect of lavendustin A on cultured XR1 glial cells. Lavendustin A produced a dose-dependent decrease in the proportion of XR1 cells displaying focal adhesions. Taken together, these results suggest that tyrosine kinase activity is essential for regulating neuroepithelial cell adhesion, migration and morphogenesis during retinal development. Furthermore, the disruption of retinal development may, in part, be due to the inhibition of integrin-mediated signaling.