Extracellular Matrix: The Medium Can Affect the Message

A CG Conjunctival Implant

Results of this study by Hsu et al. (p. 2404) indicate that implantation of a porous collagen-glycosaminoglycan (CG) matrix into wounds of the rabbit conjunctiva drastically reduce contraction and prevent the formation of subconjunctival scar. The CG matrix has many potential applications in clinical situations involving conjunctival reconstruction. The findings are also important relative to understanding the mechanisms of wound healing in the conjunctiva. The data indicate that contractile cells (myofibroblasts) are present in conjunctival wounds and may be involved in wound contraction and scar formation. 

Osteonectin/SPARC in Retina

Osteonectin/SPARC, a secreted protein with broad functions in dividing and proliferating cells, is abundantly expressed in monkey retina. Rodríguez et al. (p. 2438) observe there is an 8- to 10-fold greater expression of osteonectin/SPARC mRNA in macular versus peripheral RPE, and the protein is localized to the outer plexiform layer (OPL). Higher mRNA expression in the macular RPE may be due to greater metabolic rate and protein turnover or to other unrelated macula-specific or enriched function(s). Localization to the OPL suggests a novel function for osteonectin/SPARC perhaps related to maintenance of the synaptic junctions or as an extracellular calcium modulator via its EF-hand domain. 

Basement Membrane in Corneal Epithelium

The physiological significance of the basement membrane in corneal epithelial wound healing was investigated by observing changes in the localization of four adhesion proteins of intercellular junctions (connexin43 for gap junctions, desmoglein 1 or 2 for desmosomes, E-cadherin for adherens junctions, and occludin for tight junctions), and of the basement membrane component laminin-1 after excimer laser photoablation of the rat cornea. Suzuki et al. (p. 2495) found that re-expression of connexin43 and desmoglein 1 or 2 coincided with the reappearance of laminin-1, whereas the expression of E-cadherin and occludin was apparent regardless of the absence or presence of laminin-1. Re-establishment of the basement membrane might therefore be required for reformation of gap junctions and desmosomes during wound repair in the corneal epithelium. 

Amniotic Membrane and Corneal Reconstruction

Corneal epithelial cells can be cultivated for ocular surface reconstruction, and amniotic membrane has been proposed as a potential substrate. Koizumi et al. (p. 2506) found that the membrane removed of its own epithelial cells promotes better corneal epithelial cell colonization than does intact amniotic membrane. Also, cells from the limbus colonize the membrane more readily than those from the central cornea, and stratify nicely. Intact amniotic membrane is beneficial when spread on bare sclera to inhibit conjunctival overgrowth; however, denuded amniotic membrane is preferred for the ex vivo expansion of limbal epithelial cells in preparation for transplantation. 

Heparin and LEDGF Activity

Heparin stabilizes and potentiates the functional activity of lens epithelium-derived growth factor (LEDGF), a novel growth and survival factor originally isolated from human lens epithelium. LEDGF is known to upregulate the expression of Hsp27 and αB-crystallin in lens epithelial cells and enhance the resistance of cells to oxidative stress. Fatma et al. (p. 2648) observe that heparin stabilizes LEDGF against proteolytic degradation by trypsin and chymotrypsin, pH stress, and heat-inactivation, and it plays an important, but as yet undefined, role in the translocation of LEDGF from the extracellular space to the nucleus. LEDGF is a new member of the heparin-requiring growth factors. 

Osteonectin/SPARC and RPE

The study by Magee et al. (p. 2707) demonstrates that human retinal pigment epithelial (HRPE) cells produce and secrete the matricellular protein osteonectin SPARC (secreted protein, acidic, and rich in cysteine) in a cell density dependent way. This synthesis is not affected by the differentiating agents retinoic and butyric acids. Although the counteradhesive properties of SPARC implicate it in cell proliferation, migration, and differentiation, the findings suggest alternative or additional roles for HRPE-derived SPARC. Since SPARC may regulate angiogenesis, HRPE-derived SPARC might play a role in vascularization at the chorioretinal interface and the vascular abnormalities of age-related macular degeneration. 

A Novel Mucinlike Glycoprotein in Retina

Uehara et al. (p. 2759) have isolated the cDNA for the core protein of a mucinlike glycoprotein (MLGAPC) from bovine retina. The study shows by in situ hybridization and immunocytochemistry that MLGAPC localizes to both rods and cones, being present in the interphotoreceptor matrix and the photoreceptor synaptic layer. The authors propose that MLGAPC may be involved in similar physiological functions in the extracellular matrices of the subretinal space and the synaptic cleft, providing a structure suitable for both intercellular adhesion and the exchange of biologically active molecules.