During development, neurons grow structures called axons and dendrites that are specialized for carrying signals. The number of these structures and their shapes give each neuron a particular appearance, and allow neurons to form specific connections with each other that result in the nervous system's ability to function properly. Currently, the ways in which particular combinations of molecules regulate axon and dendrite morphology in the developing nervous system of mammals are not well understood. Previous scientific work has found a family of molecules called Semaphorins that control both axon and dendrite morphology by binding to molecules on the outer membrane of nerve cells (called receptors). This project will investigate what happens inside developing nerve cells after Semaphorin molecules bind to their receptors, in order to provide invaluable new details about the molecular control systems inside cells that ultimately determine the proper formation of neuronal connections and signaling functions. In addition to this scientific goal, this project will integrate research, mentorship and education for minority high school, undergraduate and graduate students through their joint involvement in STEM enrichment opportunities. Due to a unique collaboration between the principal investigator and a scientist at the Weizmann Institute of Science in Israel, an exchange program will also be established to provide female scientists with broader exposure and training experiences at both the national and international level. Data generated from this study will be disseminated in the form of presentations at national/international scientific meetings, and will be published in peer-reviewed journals that can be accessed by other scientists and the general public. New reagents and resources generated from this study will be freely shared with the broader scientific community and will be sent to archiving databases and repositories.
Neurons exhibit diverse morphologies, and developmentally acquire the unique axonal and dendritic arborizations necessary for their specific functions. Cell surface receptors are known to control this complex process, mainly by eliciting attractive/permissive or repulsive/inhibitory cellular responses in distinct neuronal populations. However, the intracellular mechanisms underlying these multi-functional responses to receptor-initiated signaling events are still poorly understood. The Plexin-A4 receptor that binds the Semaphorin-3A ligand is known to collapse growth cones on the axons of sensory neurons, while it promotes dendrite formation in cortical neurons. The specific aims of the present project are to 1) elucidate the mechanistic logic underlying these disparate functions of the Plexin-A4 receptor; and 2) to determine whether the Plexin-A4 cytoplasmic domains activate similar or different intracellular signaling cascades during axon collapse and dendrite formation. Novel mouse genetic tools generated for this project will be used to determine which signaling cytoplasmic domains of Plexin-A4 are required for these disparate functions. One candidate Plexin-A4 downstream interactor that may play key roles in both axon growth cone collapse and dendrite elaboration will be examined following semaphorin activation of Plexin-A4 signaling in sensory versus cortical neurons. High-throughput, morphology based siRNA screening will be performed to identify additional novel downstream components of the Plexin-A4 signaling cascade that regulate dendrite and growth cone behaviors. The results will provide key mechanistic insights about how neurons establish their unique cellular morphologies, explain how Plexin-A4 regulates axon and dendrite behavior, and identify novel molecules in the Semaphorin-3A signaling pathway that impact the formation and organization of neuronal connections during nervous system development.
