Retina cognin (R-cognin) is an intrinsic membrane protein of vertebrate retinal cells which supports tissue-specific cell adhesion and mediates cell type-specific associations during development. As a first step in understanding how R-cognin mediates specific adhesion of retinal cell membranes, we asked if cognin bound to another cognin molecule or to a different macromolecule, a possible cognin-binding protein. To do this, we constructed an affinity column with retinal cell membrane proteins (enriched for cognin) bound to the matrix. Proteins in a detergent extract of retinal cell membranes were exposed to this matrix and those which bound specifically eluted and identified by immunoelectrophoresis. Most prominent among these was a protein with an apparent mass of 64 kDa. The binding of this material to the column was blocked by cognin antibody. To eliminate possible artifacts of molecular interactions in vitro, we sought independent confirmation that 64 kDa protein actually bound R-cognin. Using a modified retina membrane vesicle system, we asked what proteins could be photoaffinity cross-linked to cognin during vesicle aggregation. Cross-linking produced a 114 kDa complex on gels which could be resolved into a 50 kDa (cognin) and a 64 kDa band under reducing conditions. Identification of a 64 kDa protein by independent techniques suggests that cognin promotes association of embryonic chick neural retina cells by binding to this macromolecule or these molecules. Identification of a second component in the mechanism should allow elucidation of cognin's role in mediating cell-cell interactions in developing neural retina.

The histological distribution of R-cognin in chick retinas was determined from embryonic day 8 through 13 wk post-hatching by indirect immunofluorescence using polyclonal anticognin. On embryonic day 8, at a developmental stage without distinct retina layers, most of the cells within the tissue exhibited fluorescence. By embryonic day 12, when the strata of the retina are delineating and initial synapses are beginning to form, R-cognin fluorescence became concentrated in the nascent ganglion cell, and to a lesser extent, inner nuclear layers. By embryonic day 16 (extending through post-hatching day 26), the staining of R-cognin was specific for the ganglion cell and nerve fiber layers. Fluorescence was prominent on the cell somal membranes but also was present on the processes of these cells at the ganglion cell layer-inner plexiform layer interface. Pre-immune serum and anticognin after preabsorption with R-cognin exhibited no fluorescence. The results demonstrated that the known decrease in R-cognin found in the retina during the latter half of embryonic development in the chick is not uniform across the retina, but that R-cognin is preferentially retained on cells within the ganglion cell layer. While cells within the ganglion cell layer also exhibit alpha-bungarotoxin binding, the majority of the latter is found in the inner plexiform layer. Thus, the observations are consistent with a role for R-cognin in the formation or maintenance of functional cell-cell connections within the entire retina prior to developmental day 11, or in retinal ganglion cell layer formation or stability subsequent to embryonic day 11.

The action of R-cognin, a chick neural retina cell recognition glycoprotein, was investigated in vesicles of retina cell membranes. It was found that the aggregation of the vesicles was dependent on membrane proteins, and specifically R-cognin, as vesicle aggregation was inhibited by R-cognin antibody (Fab′). The R-cognin content of the vesicles, and their ability to aggregate, decreased with increasing embryonic age of the tissue. R-cognin mediated aggregation of the vesicles was not dependent on exogenous calcium. Thus, R-cognin was calcium-independent in its membrane linking activity.