Our research is devoted to understanding the regulation of neuronal connectivity and synaptogenesis. We combine genetic approaches with molecular biology, sophisticated imaging, and behavioral assays to understand the cell biological mechanisms necessary to build and maintain functional neural circuits.

Neuronal Morphogenesis and Cytoskeletal Dynamics

Developing neurons face a complicated and dynamic embryonic landscape with many types of developmental information. Initial work in our lab was focused on the axon guidance functions of multiple conserved neuronal receptors, including the receptor protein tyrosine phosphatase LAR. We have defined factors upstream and downstream of LAR, and the machinery appears to be highly conserved, including key signaling molecules such as the Abelson (Abl) family of non-receptor tyrosine kinases. Using genetic approaches, we find a combination of cytoskeletal regulatory proteins that coordinate actin and microtubule dynamics are required to sculpt neuronal arbors during development. For example, we identified the microtubule plus-tip interacting protein (MT+TIP) CLASP as a protein required for Abl function in vivo. We have used genetic and proteomic tools to define a network of functional partners for CLASP suggesting coordination of the two major polymer systems.

The Formation and Growth of Functional Synaptic Connections

After axons reach their appropriate destinations in the brain, they must construct a specialized cellular junction or synapse in order to communicate with its target cell in a functional circuit. 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 activity is mediated by a pathway linking the phosphatase cytoskeletal remodeling. In addition, we find that this pathway is under the regulation of genes linked to human mental retardation, suggesting a molecular model for disorders of cognitive dysfunction.

MicroRNA Regulation of Synapse Specificity and Morphogenesis

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. MicroRNAs 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 many 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.