Specific presynaptic functions require distinct Drosophila Cav2 splice isoforms

Abstract

The multiplicity of neural circuits that accommodate the sheer infinite number of computations conducted by brains requires diverse synapse and neuron types. At the vertebrate presynaptic active zone functional diversity can be achieved by the expression of different voltage gated calcium channels of the Ca v 2 family. In fact, release probability and other aspects of presynaptic function are tuned by different combinations of Ca v 2.1, Ca v 2.2, and Ca v 2.3 channels. By contrast, most invertebrate genomes contain only one Ca v 2 gene. The one Drosophila Ca v 2 homolog, cacophony, localizes to presynaptic active zones to induce synaptic vesicle release. We hypothesize that Drosophila Ca v 2 functional diversity is enhanced by two specific exon pairs that are mutually exclusively spliced and not conserved in vertebrates, one in the voltage sensor and one in the intracellular loop containing the binding site(s) for Caβ and G-protein βγ subunits. We test our hypothesis by combining opto- and electrophysiological with neuroanatomical approaches at a fast glutamatergic model synapse, the Drosophila larval neuromuscular junction. We find that alternative splicing in the voltage sensor affects channel activation voltage and is imperative for normal synapse function. Only the isoform with the higher activation voltage localizes to the presynaptic active zone and mediates evoked release. Removal of this Ca v 2 splice isoforms renders fast glutamatergic synapses non-functional. The By contrast, alternative splicing at the other alternative exon does not affect Ca v 2 presynaptic expression, but it tunes multiple aspects of presynaptic function. While expression of one exon yields normal transmission, expression of the other exon reduces channel number in the active zone and thus release probability. It also affects short term plasticity and abolishes presynaptic homeostatic plasticity. Thus, in Drosophila alternative splicing provides a mechanism to regulate different aspects of presynaptic functions with only one Ca v 2 gene.