SORLA Expression in Synaptic Plexiform Layers of Mouse Retina

Sorting protein-related receptor containing LDLR class A repeats (SORLA; also known as LR11) exerts intraneuronal trafficking functions in the central nervous system. Recently, involvement of SORLA in retinogenesis was proposed, but no studies have examined yet in detail the expression pattern of this sorting receptor in the retina. Here, we provide a spatio-temporal characterization of SORL1 mRNA and its translational product SORLA in the postnatal mouse retina. Using stereological analysis, we confirmed previous studies showing that receptor depletion in knockout mice significantly reduces the number of cells in the inner nuclear layer (INL), suggesting that functional SORLA expression is essential for the development of this retinal strata. qPCR and Western blot analyses showed that SORL1/SORLA expression peaks at postnatal day 15, just after eye opening. Interestingly, we found that transcripts are somatically located in several neuronal populations residing in the INL and the ganglion cell layer, whereas SORLA protein is also present in the synaptic plexiform layers. In line with receptor expression in dendritic terminals, we found delayed stratification of the inner plexiform layer in knockout mice, indicating an involvement of SORLA in neuronal connectivity. Altogether, these data suggest a novel role of SORLA in synaptogenesis. Receptor dysfunctions may be implicated in morphological and functional impairments of retinal inner layer formation associated with eye disorders.