Membrane alterations during chick neural retina development: a freeze-fracture study

Gap-Junction Proteins in Retinal Development: New Roles for the “Nexus”

Gap-junction channels, the cytoplasmic proteins that associate with them, and the transcriptional networks that regulate them are increasingly being viewed as critical communications hubs for cell signaling in health and disease. As a result, the term “nexus,” which was the original structural name for these focal intercellular links, is coming back into use with new proteomic and transcriptomic meanings. The retina is better understood than any other part of the vertebrate central nervous system in respect of its developmental patterning, its diverse neuronal types and circuits, and the emergence of its definitive structure-function correlations. Thus, studies of the junctional and nonjunctional nexus roles of gap-junction proteins in coordinating retinal development should throw useful light on cell signaling in other developing nervous tissues.

The arm-repeat protein NPRAP (neurojungin) is a constituent of the plaques of the outer limiting zone in the retina, defining a novel type of adhering junction

In the retina, special plaque-bearing adhering junctions are aligned to form a planar system (the "outer limiting zone," OLZ) of heterotypic connections between the photoreceptor cells and the surrounding glial cells ("Müller cells"), together with homotypic junctions. In the plaques of these junctions, which contain N-cadherin-and possibly also related cadherins-we have identified, by immunolocalization techniques, a recently discovered neural tissue-specific protein, neurojungin, a member of the plakoglobin/armadillo protein family. In these plaques we have also detected other adherens plaque proteins, such as alpha- and beta-catenin, protein p120, and vinculin, as well as proteins known as constituents of tight junction plaques, such as symplekin and protein ZO-1, and the desmosomal plaque protein plakophilin 2. This unusual combination of proteins and the demonstrated absence of plakoglobin define the OLZ junctions as a new and distinct category of adhering junction, which probably has special architectural functions.

Intramembranous particle distribution and filipin binding in dysplastic canine retina

We examined dysplastic canine retina for changes in intramembranous particle (IMP) density and filipin binding to sterols. Differences in IMP density were identified in incipiently dysplastic fetal retina and also in the degree of filipin binding near the onset of the dysplastic process. The data suggest that there are temporal differences in IMPs and filipin-sterol complexes that may be related to the formation of retinal folds and disorganized dysplastic retina proliferation.

The external limiting membrane in developing normal and dysplastic canine retina

Development of the external limiting membrane of the retina in English springer spaniel fetuses and neonates with incipient stages of retinal dysplasia and in normal mongrel fetuses and neonates was examined using transmission electron microscopy and the freeze-fracture technique. The external limiting membrane of the retina of normal canine fetuses was composed of zonulae adherentes in which there were focal areas of apparent apposition between adjacent ventricular cell membranes. Freeze-fracture examination revealed gap junctions within the external limiting membrane that were composed of 8-10 nm P-face particles in aggregates of variable size and shape. These junctions corresponded to areas of focal membrane apposition. Tight junction strands were also identified in deep E-face grooves which were located perpendicular to the external limiting membrane. At day 46 of gestation in incipiently dysplastic English springer spaniel fetal eyes, there was a marked decrease in the size and area occupied by gap junctions within the external limiting membrane as compared with retinas of mongrel control fetuses at the same age and of other age groups. This apparent loss of gap junctions, coincident with the incipient onset of histologically detectable dysplastic events in the sensory retina, may contribute to the morphogenesis of the defect.

Glycoprotein differences among cells of the 14-day embryonic chick neural retina

In order to test the hypothesis that the progressive layering and differentiation of cell types during the development of the neural retina are associated with cell surface alterations we have separated distinct cell populations from the 14-day embryonic chick retina. Cells of these populations have been shown to differ in associative behavior and intramembrane particle content. We now report that these cells differ in cell surface glycoproteins. Proteins were labeled with two different extrinsic labels and one metabolic label. We used enzymatic transfer to galactose from UDP-gal to cellular acceptors, and borotritide reduction after galactose oxidation as extrinsic labels. Glucosamine incorporation was used as the metabolic label. In all these cases, we were able to identify bands on electrophoretic gels which were unique to individual populations.

Cells isolated from the embryonic neural retina differ in behavior in vitro and membrane structure

Several subpopulations of cells were isolated from trypsin-dissociated embryonic (14 days) chick retinas. The cells of each subpopulation differed in associative behavior measured by cell aggregation and stationary culture assays and in glycoproteins that contain glucosamine. Freeze-fracture analysis showed that these populations also differed in intramembrane particle content.