Abstract
The L-type voltage-gated calcium channels (L-VGCCs) are activated under high depolarization voltages. They are vital for diverse biological events, including cell excitability, differentiation, and synaptic transmission. In retinal photoreceptors, L-VGCCs are responsible for neurotransmitter release and are under circadian influences. However, the mechanism of L-VGCC regulation in photoreceptors is not fully understood. Here, we show that retinoschisin, a highly conserved extracellular protein, interacts with the L-VGCCalpha1D subunit and regulates its activities in a circadian manner. Mutations in the gene encoding retinoschisin (RS1) cause retinal disorganization that leads to early onset of macular degeneration. Since ion channel activities can be modulated through interactions with extracellular proteins, disruption of these interactions can alter physiology and be the root cause of disease states. Co-immunoprecipitation and mammalian two-hybrid assays showed that retinoschisin and the N-terminal fragment of the L-VGCCalpha1 subunit physically interacted with one another. The expression and secretion of retinoschisin are under circadian regulation with a peak at night and nadir during the day. Inhibition of L-type VGCCs decreased membrane-bound retinoschisin at night. Overexpression of a missense RS1 mutant gene, R141G, into chicken cone photoreceptors caused a decrease of L-type VGCC currents at night. Our findings demonstrate a novel bidirectional relationship between an ion channel and an extracellular protein; L-type VGCCs regulate the circadian rhythm of retinoschisin secretion, whereas secreted retinoschisin feeds back to regulate L-type VGCCs. Therefore, physical interactions between L-VGCCalpha1 subunits and retinoschisin play an important role in the membrane retention of L-VGCCalpha1 subunits and photoreceptor-bipolar synaptic transmission.