Regulation of Synapse Size and Function by pre-mRNA Splicing

Pre-mRNA alternative splicing is an important mechanism for the generation and regulation of synaptic protein diversity, but few factors governing this process have been identified. From a screen for Drosophila mutants with aberrant synaptic development, we identified beag, a mutant with fewer synaptic boutons and decreased neurotransmitter release. Beag encodes a spliceosomal protein similar to splicing factors in humans and C. elegans. Another novel gene encoding a spliceosomal protein, dsmu1, has similar synaptic phenotypes as beag and the two genes function in the same genetic pathway to regulate synaptic growth. We find that both beag and dsmu1 mutants have isoform specific changes in synaptic levels of Fasciclin II (FasII), the Drosophila ortholog of Neural Cell Adhesion Molecule (NCAM). NCAM and many other synaptic cell adhesion molecules have multiple splice isoforms that play distinct roles in synapse development and function. FasII has three major splice isoforms, one of which is attached to the plasma membrane via a glycophosphatidylinositol (GPI) linkage (FasII-GPI) and two of which are transmembrane proteins that vary in their extracellular domains (FasII-TMA and FasII-TMB). FasII-TMA and FasII-TMB are necessary and sufficient for normal NMJ development, while FasII-GPI is not. fasII and beag genetically interact and we show that restoration of FasII-TMA can rescue synaptic morphology in beag mutants while expression of other isoforms cannot. We further demonstrate that this FasII isoform has unique functions in synaptic development independent of trans-synaptic adhesion. Beag and dsmu1 mutants demonstrate an essential role for these novel splicing factors in the regulation of synapse development and function.