Synaptic localization and presynaptic function of calcium channel β4-subunits in cultured hippocampal neurons

Neurotransmitter release is triggered by the influx of Ca2+ into the presynaptic terminal through voltage gated Ca2+-channels. The shape of the presynaptic Ca2+ signal largely determines the amount of released quanta and thus the size of the synaptic response, Ca2+-channel function is modulated in particular by the auxiliary beta -subunits that interact intracellularly with the pore-forming alpha (1)-subunit, Using retrovirus-mediated gene transfer in cultured hippocampal neurons, we demonstrate that functional GFP-beta (4) constructs colocalize with the synaptic vesicle marker synaptobrevin II and endogenous P/Q-type channels, indicating that beta (4)-subunits are localized to synaptic sites. Costaining with the dendritic marker MAPS revealed that the beta (4)-subunit is transported to dendrites as well as axons, The nonconserved amino- and carboxyl-termini of the beta (2)-subunit were found to target the protein to the synapse. Physiological measurements in autaptic hippocampal neurons infected with green fluorescent protein (GFP)-beta (4) revealed an increase in both excitatory post-synaptic current amplitude and paired pulse facilitation ratio, whereas the GFP-beta (4) mutant, GFP-beta (4)(Delta 50-407), which demonstrated a cytosolic localization pattern, did not alter these synaptic properties. In summary, our data suggest a presynaptic function of the Ca2+-channel beta (4)-subunit in synaptic transmission.