Effects of presynaptic mutations on a postsynaptic Cacna1s calcium channel colocalized with mGluR6 at mouse photoreceptor ribbon synapses.

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 VDCCal subunit Cacnalf, have been shown to altered transmitter release. L-type VDCCal subtype expression in wild-type and mutant mice was examined, to investigate the underlying pathologic mechanism. METHODS. Two antisera against Cacnalf, and a Cacnalf 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 panal antibody colocalized with staining for Cacnalf in wild-type mouse retina. Similarly, in the BassoonΔEx4-5 mouse, residual mislocalized staining for panal and Cacnalf showed colocalization. Unlike the presynaptic location of Cacnalf and panal antibody staining, the skeletal muscle VDCCα1 subunit Cacnals was present postsynaptically at ON-bipolar cell dendrites, where it colocalized with metabotropic glutamate receptor 6 (mGluR6). Surprisingly, Cacnals 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 Ca 2+ 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 Cacnals and Serca2 in ON-bipolar cell dendrites in wild-type animals suggests a putative role in regulation of postsynaptic Ca 2+ flux.