Calcium Channel Beta Subunits and SCA6-Type Calcium Channel Alpha Subunits C-Termini Regulate Targeting and Function of Presynaptic Calcium Channels in Hippocampal Neurons

Ca²⁺ channel β subunits determine the transport and physiological properties of high voltage activated Ca²⁺ channel complexes, and the poly-glutamination within the C-terminus (CT) of the P/Q-type Ca²⁺ channel α subunit is linked with Spinocerebellar ataxia type 6 (SCA6). In the first part of this study we analyzed the distribution of the Ca_vβ subunit family members in hippocampal neurons and correlated their synaptic distribution with their involvement in transmitter release. We found that exogenously expressed Ca_vβ_4b and Ca_vβ_2a subunits distribute in clusters and localize to synapses, while Ca_vβ_1b and Ca_vβ₃ are homogenously distributed. According to their localization Ca_vβ_2a and 4b subunits modulate the synaptic plasticity of autaptic hippocampal neurons, i.e. Ca_vβ_2a induces depression, while Ca_vβ_4b induces paired-pulse facilitation followed by synaptic depression during longer stimuli trains. The induction of paired-pulse facilitation by Ca_vβ_4b is correlated with a reduction in the release probability and cooperativity of the transmitter release. These results suggest that Ca_vβ subunits determine the gating properties of the presynaptic Ca²⁺ channels within the presynaptic terminal in a subunit specific manner and may be involved in the organization of the Ca²⁺ channel relative to the release machinery.

We also examined whether the poly-glutamination of P/Q-type channel CT can increase the channel stability and induce a gradual accumulation of a CT degradation product. We demonstrated that the poly-glutaminated CT degradation product distributes in cytoplasmic aggregates in cultured neurons as found in SCA6 patients and has drastic physiological effects on synaptic function and synapse assembly. Our results show that the CTs induce a change in the Ca²⁺ dependence of transmitter release correlated with a reduced vesicular release probability, which causes synaptic depression during repetitive high frequency stimulations. The CT containing the SCA6 mutation also caused an increase in the number of synapses. Our results predict that CT degradation products derived from the P/Q-type channels in SCA6 patients would reduce synaptic strength in each synaptic terminal, but increased the overall synapse formation per neuron. The increased synapse formation may give a mechanistic explanation for the survival of nucleo-olivary pathways in SCA6, which is not observed in other SCAs.