Molecular mechanisms of axon guidance and neural connectivity

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7. Project Summary/Abstract This proposal focuses on defining the molecular mechanisms of axon navigation and connectivity. A normal functioning human nervous system requires the interconnection of billions of neurons. Improper formation or maintenance of these connections leads to abnormalities that result in mental diseases and disorders. How are these connections assembled and integrated? Research has revealed that the molecular mechanisms of axon guidance and connectivity are well-conserved between simple and complex animals. Simple animals like flies use many of the same guidance signals as mammals. Therefore, as a step towards understanding how complex nervous systems form and properly function, we have pursued a strategy to determine how the simple model fly nervous system is assembled – where we can also apply high-resolution molecular, genetic, biochemical, cellular, and imaging approaches to solve this problem. Indeed, the goal of my research program is to focus on a group of axons within the simple nervous system of the fly embryo and characterize the molecules and mechanisms that guide them to their targets. In particular, elegant studies from multiple labs have now identified numerous extracellular cues and receptors that guide axons, revealing fundamental mechanisms of how axons form connections. Far less is known, however, of the intracellular signaling pathways and mechanisms that link these guidance cues and their receptors to the control of axon navigation. Likewise, these guidance cues work to either attract or repel axons. Yet, how navigating axons choose between these antagonistic signals when they simultaneously encounter them remains far from clear. To answer these questions, we have focused on one of the largest protein families involved in connectivity, the Semaphorins (Semas) and their Plexin cell-surface receptors. Employing these strategies, our work has uncovered critical new molecules and mechanisms directing guidance/connectivity. Namely, we have discovered MICAL family enzymes, and have found that MICAL and SelR enzymes employ a specific reversible biochemical mechanism to control guidance/connectivity. We have also discovered that precise connectivity occurs via direct links between Semas/Plexins and other guidance molecules including Integrin- mediated adhesive receptors, specific growth factors, cyclic nucleotides, adaptors, small GTPases, serine- threonine and tyrosine kinases, and cytoskeletal assemblers and disassemblers. Now, using these strategies, our preliminary results uncover that these specific molecular pathways that provide connectivity to neurons are spatiotemporally and directly instructed by specific molecular pathways that provide metabolic sustenance to neurons. We therefore hypothesize that specific factors that govern neuronal connectivity are directly, locally, selectively, and instructively controlled by specific factors that govern neuronal metabolism. We propose to test this biomedically significant and innovative hypothesis by employing rigorous molecular, genetic, biochemical, cellular, and imaging approaches and the robust Drosophila high-resolution model system.