Abstract
Purpose :
In rod photoreceptors, several phototransduction components display light-dependent translocation between cellular compartments. Notably, the G protein transducin translocates from rod outer segments (OS) to inner segments (IS)/spherules in bright light. Recently Frmpd1 was identified as a rod and rod-bipolar specific protein in the retina. Interestingly, Frmpd2 is shown to interact with Lrit1 in cone photoreceptor inner segments to modulate synaptic transmission from cones to ON-bipolar cells under varying background light intensities. We thus questioned whether Frmpd1 together with Gpsm2, a G-protein modulator, are involved in rod to rod-bipolar cell synaptic transmission, and have a role in transducin transport.
Methods :
Knockout mice of Frmpd1 and Gpsm2 were used to examine the effects of their loss in mediating transducin localization and synaptic signaling at the rod synapse. To induce transducin translocation, dark adapted mice were light adapted for 45 minutes. The localization of transducin, Frmpd1 and Gpsm2 in dark conditions and following translocation, as well as their molecular interactions, was assessed using immunofluorescence and co-immunoprecipitation, respectively. Electroretinogram and whole cell patch-clamp recordings were used to record light evoked responses from rods and rod bipolar cells.
Results :
Following bright light adaptation, transducin translocated to the IS and to the synaptic terminal. Frmpd1 and Gpsm2 were found to co-localize in the inner segment, were they interacted with transducin. Tranducin translocation in the transgenic mice was also found to be heavily delayed, indicating that both Frmpd1 and Gpsm2 are necessary for the transport of transducin back from the IS to the OS. Following light-adaptation the rods of Frmpd1 and Gpsm2 mice displayed normal desensitization of the flash response. However, the light adapted rod bipolar cells were desensitized by more ~2 fold compared to WT mice.
Conclusions :
Our molecular analysis indicates that both Frmpd1 and Gpsm2 are involved in the transport of transducin back to the OS following translocation. The reduced sensitivity of rod bipolar cells in Frmpd1 and Gpsm2 KO mice suggests they contribute to the fine-tuning of rod synaptic signaling in bright light conditions, possibly increasing the dynamic range of rod bipolar cell signaling.
This is a 2020 ARVO Annual Meeting abstract.