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 trans-synaptically 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. The mechanism by which LRIT3 controls postsynaptic receptor organization remains unknown. We address this by using rAAV to express deletion constructs in LRIT3 knockout retinas and examining LRIT3 trafficking, as well as the structural and functional recovery of the signalplex in DBCs. We show the LRR domain is required for trafficking LRIT3 to the synapse in cones, but not rod photoreceptors, although it is needed for reassembly and function of the rod BC signalplex. Neither the IG nor the FN3 domain is needed for synaptic localization of LRIT3. However, the IG domain is required for the localization of TRPM1 to the signalplex and thus function. The FN3 domain is not necessary for either DBC signalplex assembly or function. Our data demonstrates that the LRR and IG domains of LRIT3 are crucial for TRPM1 localization and retinal function, with the LRR domain playing a key role in the differential function of LRIT3 at rod and cone synapses. Notably, our results show 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 trans-synaptically binds with Nyctalopin, while the IG domain interacts with TRPM1.