Regulation of Presynaptic Calcium Channels by Synaptic Proteins

Calcium entry into nerve termini via voltage gated calcium channels is an essential step in neurotransmission. Consequently, second messenger regulation of calcium channel activity modulates synaptic activity. It has been suggested that calcium channels must physically couple to the release machinery, and a physical interaction between a synaptic protein interaction (synprint) site contained within mammalian presynaptic calcium channels and synaptic proteins such as syntaxin 1, SNAP-25, and synaptotagmin has been demonstrated. Interestingly, synaptic calcium channels in invertebrates lack this region. In invertebrates, synaptic transmission is instead dependent on a presynaptic calcium channel splice variant that can physically associate with the adaptor proteins Mint-1 and CASK. We suggest that in the absence of a synprint region, these proteins may localize calcium channels to the synaptic release machinery. The interactions between synaptic proteins and mammalian N-type calcium channels serves to regulate calcium channel activity directly, as well as indirectly by altering second messenger regulation of the channels. This provides for a feedback mechanism that allows the fine-tuning of calcium channel activity during various steps in neurotransmitter release. This does not occur with invertebrate synaptic calcium channel homologs, suggesting that the regulation of calcium channel activity by synaptic proteins is a mechanism unique to vertebrates.