Effects of Presynaptic Mutations on a Postsynaptic Cacna1s Calcium Channel Colocalized with mGluR6 at Mouse Photoreceptor Ribbon Synapses

Abstract

PURPOSE: Photoreceptor ribbon synapses translate light-dependent changes of membrane potential into graded transmitter release via L-type voltage-dependent calcium channel (VDCC) activity. Functional abnormalities (e.g., a reduced electroretinogram b-wave), arising from mutations of presynaptic proteins, such as Bassoon and the VDCCα1 subunit Cacna1f, have been shown to altered transmitter release. L-type VDCCα1 subtype expression in wild-type and mutant mice was examined, to investigate the underlying pathologic mechanism. METHODS: Two antisera against Cacna1f, and a Cacna1f mouse mutant (Cacna1fΔEx14-17) were generated. Immunocytochemistry for L-type VDCCα1 subunits and additional synaptic marker proteins was performed in wild-type, BassoonΔEx4-5 and Cacna1fΔEx14-17 mice. RESULTS: Active zone staining at photoreceptor ribbon synapses with a panα1 antibody colocalized with staining for Cacna1f in wild-type mouse retina. Similarly, in the BassoonΔEx4-5 mouse, residual mislocalized staining for panα1 and Cacna1f showed colocalization. Unlike the presynaptic location of Cacna1f and panα1 antibody staining, the skeletal muscle VDCCα1 subunit Cacna1s was present postsynaptically at ONbipolar cell dendrites, where it colocalized with metabotropic glutamate receptor 6 (mGluR6). Surprisingly, Cacna1s labeling was severely downregulated in the BassoonΔEx4-5 and Cacna1fΔEx14-17 mutants. Subsequent analyses revealed severely reduced ON-bipolar cell dendritic expression of the sarcoplasmic reticulum Ca²⁺ ATPase Serca2 in both mouse mutants and of mGluR6 in the Cacna1fΔEx14-17 mutant. CONCLUSIONS: Presynaptic mutations leading to reduced photoreceptor- to-bipolar cell signaling are associated with disturbances in protein expression within postsynaptic dendrites. Moreover, detection of Cacna1s and Serca2 in ON-bipolar cell dendrites in wild-type animals suggests a putative role in regulation of postsynaptic Ca²⁺ flux.