Molecular dynamics of photoreceptor synapse formation in the developing chick retina

The cellular and molecular mechanisms underlying photoreceptor synaptogenesis are poorly understood. Furthermore, a detailed picture of the molecular composition of photoreceptor synapses, or their subtypes, is not yet available, nor do we know what differences, if any, exist among those subtypes. To address these questions, we investigated temporal and spatial patterns of expression and assembly of photoreceptor presynaptic components during chick embryo retinal development and early posthatched life by using reverse transcriptase polymerase chain reaction (RT-PCR), dissociated retinal cells, laser-capture microdissection (LCM), immunocytochemistry and confocal microscopy. Immunocytochemistry in tissue sections and dissociated cells showed many similarities and few differences in the synaptic composition of rods and cone subtypes, which, however, were found to project to different strata within the outer plexiform layer. A striking finding was the precise timetable of expression of synaptic genes and proteins during synaptogenesis. Although mRNAs for some synaptic molecules appeared as early as embryonic day (ED) 5-8 (the time of inner retina synaptogenesis), others were undetectable before the time of onset of photoreceptor synaptogenesis on ED13, including CAST, rim2, synapsin-2, syntaxin-3, synaptotagmin, glutamate receptors -1, -4, and -5, homer-1 and -2, and tenascin-R. Most synaptic proteins in photoreceptors followed a similar sequence of expression: they were negative or weakly positive before ED13, appeared in inner segments between ED13 and ED15, became subsequently detectable in perinuclear and axonal regions, and by ED18 were assembled into synaptic terminals and became undetectable in the inner segments. The identity of the signals that regulate the coordinated expression of these synaptic components remains to be investigated.