Much has been learned about the signaling pathways and networks of proteins that function together to build and modulate synaptic connections. This rich molecular landscape is under the control of multiple classes of regulatory factors. MicroRNA (miR) are versatile posttranscriptional regulators capable of tuning levels of gene expression across a large number of target genes. Through genetic screens in Drosophila, we have discovered that synapse formation and growth are controlled by conserved microRNA genes that orchestrate different stages of synapse development through distinct sets of direct and indirect targets. Having recently created a means of selectively inhibiting the function of any microRNA with spatio-temporal precision in vivo, we are now equipped to survey the functions of all microRNAs in Drosophila in many aspects of neural development, connectivity, behavior, and neurodegeneration. Once this regulatory landscape has been mapped through comprehensive screens in this model organism, it will be possible for us to test the conservation of these mechanisms in mammalian neurons and circuits.
It has become increasingly clear that neuronal connectivity, function and plasticity all rely on the post-transcriptional regulation of gene expression. Recent studies reveal many hundreds of highly localized mRNAs in axons, dendrites and synapses. With an array of techniques, many of which we are developing with and/or learning from our collaborators, we will determine which MicroRNA (miR) are essential for activity-induced remodeling at the synapse. We will use new genetic tools to define the spatial and temporal logic for each miR function. We will then use a state-of-the-art combination of transcriptome sequencing and computational informatics, followed by use of in vivo activity sensors and functional validation, to discover the downstream mechanisms for each miR that intersects our coordinated screens.
Our studies of the LAR receptor phosphatase led us to the discovery that the LAR pathway regulates synaptic growth and the morphogenesis of the active zone – a structure that orchestrates neurotransmitter release at chemical synapses. We have defined factors upstream and downstream of LAR in this context, and the machinery appears to be highly conserved. Upstream, LAR interacts with synaptic heparan sulfate proteoglycans that control distinct aspects of synapse morphogenesis or function. Downstream, LAR associates with the primary synaptic scaffolding molecule Liprin-alpha that is an early and essential component of active zones required in many animal species. Analysis of genetic and proteomic interactions with the LAR-associated non-receptor protein tyrosine kinase Abl led us to many effectors, including the microtubule (MT) plus end interacting protein (TIP) called CLASP. Through additional genetic and proteomic screens we used CLASP to identify other MT+TIPs, including Transforming Acidic Coiled-Coil (TACC) that we have shown to negatively regulate synapse growth. We are currently exploring the roles of TACC in both presynaptic and postsynaptic cells.