Dynamic changes in the localization of synapse associated proteins during development and differentiation of the mammalian retina
We have examined the developmental distribution and differential localization of presynaptic terminal associated proteins in the mammalian retina. We have used antibodies specific for synaptic vesicle associated proteins Synaptotagmin, Rab 3A, Synaptophysin and Synaptobrevin, and presynaptic terminal membrane associated proteins SNAP-25 and Syntaxin, to characterize their spatio-temporal distribution during retinal differentiation and in the mature retina;The vertebrate retina has a laminar organization consisting of three cellular layers separated by two synaptic layers, the inner piexiform layer and the outer plexiform layer. In general, immunoreactivity for presynaptic terminal associated proteins was first observed in cellular layers, and as differentiation progressed, immunoreactivity was localized to the synaptic or plexiform layers. However, there were distinct differences in the relative intensities and localization of patterns of immunoreactivity for these proteins;This analysis revealed a temporal difference in the onset of detectable immunoreactivity for Synaptotagmin and Rab 3A compared with Synaptophysin and Synaptobrevin, suggesting they may have additional roles in vesicle trafficking during neural development. Immunoreactivity for presynaptic terminal membrane associated protein SNAP-25 is at relatively high levels in cholinergic amacrine cells during their differentiation, but not at maturity. This transient expression of high levels of SNAP-25 may contribute to the functional role these cells play in propagation of spontaneous retinal activity. Spatial and temporal differences in localization of SNARE complex proteins (Synaptophysin, SNAP-25 and Syntaxin), were also observed during development and at maturity. Detectable SNAP-25- and Syntaxin-immunoreactivity preceded Synaptobrevin-immunoreactivity during differentiation of synaptic layers. In addition, immunoreactivity for each protein had a distinct pattern of differential intensities within the inner plexiform or synaptic layer during development and in the mature retina, and differential localization in the OPL;This analysis was the first to systematically characterize the distribution of multiple presynaptic terminal associated proteins, including proteins of the SNARE complex, during development of an organized tissue like the retina. The dynamic, differential patterns of immunoreactivity for these proteins suggests several of them may have additional roles in the development of the nervous system.