Synaptogenesis is perturbed in Crx-/- photoreceptors
The Crx-/- mouse demonstrates the most severe abnormality of photoreceptor synapses reported to date. The peripherin-/- mouse develops a normal complement of photoreceptor terminals which then degenerate as the photoreceptors are lost [30]. Also, similarly in rhodopsin (Rho) and cyclic nucleotide-gated channel, alpha-3 (CNGA3) double knockout mice (Rho-/-, CNGA3-/-), synapses are reported to form normally [49]. These observations demonstrate that photoreceptor synaptogenesis can occur in the absence of outer segment formation. In keeping with this observation is the fact that some electroretinogram activity is present in peripherin-/- mice, suggesting that minimal phototransduction is present in these mice, enough to drive activity at the photoreceptor synapse. In vitro studies wherein synapse elements are formed in the absence of proper outer segment development and, therefore, possible absence of light-dependent photoreceptor activity, have indicated the independence of phototransduction and synapse formation, at least for the initial stages [50,51]. These data then suggest that the fact that the Crx-/- photoreceptors do not have proper synaptic endings is not due to a lack of outer segment formation. A more likely explanation is that Crx plays a role in photoreceptor synapse formation, perhaps by regulating directly, or indirectly, important genes in this process. Using immunohistochemistry, we examined the expression of common pre-synaptic terminal proteins, including KIF3a, SV2, and synaptophysin, and were unable to observe qualitative differences between Crx-/- and control tissue at P14 (data not shown). Examination of their RNA levels by SAGE showed no significant difference for all 3 genes, though very few tags were recovered from these genes and thus the analysis of RNA levels may not be significant [35]. However, since other genes expressed in photoreceptors were significantly altered in their expression level in the Crx-/- mouse, there are many candidates that could be important for photoreceptor morphogenesis. Tags from three genes from proteins expressed in photoreceptor terminals were found to be decreased in a statistically significant fashion, namely the HGF-regulated tyrosine kinase substrate, the CRIPT protein, and synaptotagmin 1 (Blackshaw and Cepko, unpublished data). An example of a gene that was increased in the Crx-/- retina is HRG4 (a homologue of the C. elegans Unc119 gene) (Blackshaw and Cepko, unpublished data) which encodes a component of the ribbon synapse [33]. The fact that it is upregulated might indicate a response to the lack of proper terminal structures. Several other genes encoding putative cytoskeletal elements also were increased (e.g. microtubule associated protein 4) or decreased (e.g. cofilin 1) in the Crx-/- retina, with P values of <.005. It is not known whether any of these genes are involved in building or regulating synaptic structures, but they are now genes that might lead to a better understanding of the construction and function of the relatively unique structure of the ribbon synapse.
Abnormal photoreceptor terminal formation was noted in a study that examined retinal development in the laminin beta2 chain-deficient mouse [52]. Several pathologies were noted in these mice. First, laminin beta2 chain-deficient mice displayed abnormal outer segment elongation, but a more mild phenotype than that of the Crx-/- mice; the outer segments were reduced by 50% in length. Also photoreceptor terminals were perturbed in laminin beta2 mutants, but again the phenotype was more subtle then that of Crx-/- mice. The outer plexiform layer of the beta2-deficient retinas demonstrated only 7% normal invaginating synapses, while the remainder had various pathologies, including floating synaptic ribbons, as seen here. The mechanistic relationship of these two molecules, if any, in photoreceptor morphogenesis is unknown to date. The mRNA for laminin beta2 was not detected in the SAGE study of the relative RNA levels in Crx-/- and Crx+/+ and thus we cannot comment on whether the levels of RNA for laminin beta2 were altered.