Domain-specific functions of LRIT3 in synaptic assembly and retinal signal transmission.

LRIT3 is a leucine-rich repeat (LRR) protein that is expressed in the retina, and its absence causes complete congenital stationary night blindness (cCSNB), a genetically diverse disorder characterized by impaired low-light vision, myopia, and nystagmus. LRIT3 is expressed in rod and cone photoreceptors, and it transsynaptically organizes the assembly of the glutamate signaling complex, the signalplex, on depolarizing bipolar cells (DBCs). LRIT3 is a single-pass membrane protein with extracellular LRR, IG, and FN3 domains. We express domain deletion constructs using rAAV and examine the impact on LRIT3 trafficking, as well as the structural and functional recovery of the signalplex in DBCs. We show the LRR domain may be required for trafficking LRIT3 to the synapse in cones, but not rods, and it is needed for reassembly and function of the rod BC signalplex. The IG domain is required for the localization of TRPM1 to the signalplex and thus its function. The FN3 domain is not necessary for either DBC signalplex assembly or function. Our data demonstrate that the LRR and IG domains of LRIT3 are crucial for TRPM1 localization and retinal function, and that restoring Nyctalopin localization to the DBC signalplex alone is insufficient to restore TRPM1 expression. Based on our findings, we propose a model in which the LRR domain transsynaptically binds with Nyctalopin, while the IG domain interacts with TRPM1.