Synaptic protein expression by regenerating adult photoreceptors

Regeneration of functionally normal synapses is required for functional recovery after degenerative central nervous system insults and requires proper expression and targeting of presynaptic proteins by regenerating neurons. The reconstitution of presynaptic terminals by regenerating adult neurons is poorly understood, however. We examined the intrinsic ability of regenerating adult retinal photoreceptors to reconstitute properly differentiated presynaptic terminals in the absence of target contact. The expression and localization of vesicle-associated membrane protein (VAMP), synaptic vesicle protein 2 (SV2), synaptophysin, synapsin I, and synaptosomal-associated protein of 25 kDa (SNAP-25) was assessed immunocytochemically. Photoreceptor terminals in the intact retina contain VAMP, SV2, synaptophysin, and SNAP-25, but not synapsin I. Isolated, regenerating adult photoreceptors intrinsically expressed the proper complement of synaptic vesicle proteins in the absence of target contact: VAMP, SV2, and synaptophysin were present at all stages of regenerative growth; synapsin I was never expressed. At early stages of regenerative growth, VAMP, SV2, and synaptophysin were diffusely localized in the cell, with prominent VAMP labeling distributed along the plasma membrane. SV2 and synaptophysin rapidly localized to regenerated terminals, but VAMP accumulated much more slowly, indicating that these proteins are trafficked independently. In contrast, labeling for SNAP-25, which is associated with the presynaptic plasma membrane, was undetectable in regenerating photoreceptors, suggesting that SNAP-25 expression is target-regulated. Thus, regenerating photoreceptors can intrinsically regulate the expression of the proper set of synaptic vesicle proteins. Proper expression of other presynaptic proteins, such as SNAP-25, and proper subcellular localization of synaptic proteins such as VAMP, however, may require extrinsic cues such as target contact.