The development of the nervous system requires the proper differentiation, migration and morphogenesis of neurons. The morphogenesis of individual neurons and the assembly of the trillions of neuronal connections that compose the human nervous system occurs through guided extension of axons and dendrites. The long-term objective of our research is to better understand the intracellular signaling cascades and effector mechanisms that are responsible for axon outgrowth and guidance in the developing brain. For this we must understand how nerve growth cones detect, integrate and respond to soluble, as well as cell- and substratum-associated guidance molecules in their environment. Mutations in genes involved in the detection and transduction of axon guidance information into directed neurite outgrowth are likely responsible for many deficits in cognitive function, including autisms, dyslexias, psychological disorders and mental retardations. Axon extension proceeds through a sequential process that involves leading edge membrane protrusion driven by actin polymerization, followed by adhesion and protrusion stabilization. New protrusions that do not adhere are retracted, as do existing protrusions that de-adhere. While extensive research has focused on the signals that control membrane protrusion and retraction, surprisingly little is known about the regulation of adhesion. Stabilization of growth cone protrusions to extracellular matrix (ECM) ligands occurs at specialized adhesion sites called point contacts. Point contacts are macromolecular complexes, containing both structure and signaling proteins, which link the cytoskeleton to the ECM through transmembrane integrin receptors. Our research is focused on understanding the molecular signaling events that control point contact assembly, maturation and disassembly and how axon guidance cues influence these processes to control axon pathfinding. Importantly, our evidence suggests that growth promoting axon guidance cues stimulate point contact assembly and turnover, while inhibitory cues slow the assembly of new point contacts and reduce turnover. We hypothesize that guidance of axons to their proper targets and stabilization of synaptic contacts requires modulation of integrin-dependent point contacts. We will test this hypothesis using a variety of approaches and model systems in three specific aims.
In Aim 1, we will examine the role of Focal Adhesion Kinase (FAK) in the control of adhesion dynamics, veil protrusion and phosphotyrosine signaling at filopodial tips in response to axon guidance cues.
In Aim 2, we will examine the role of p21-Activated Kinase (PAK) proteins in the regulation of integrin-dependent adhesion, cellular protrusion and axon outgrowth. Finally, in Aim 3, we will determine the role of adhesion site dynamics in axon guidance at several choice point in both Xenopus and zebrafish embryos.

Public Health Relevance

The development of a functional nervous system requires accurate guidance of axons and dendrites to their target locations and establishment of synaptic connections. This proposal is focused on understanding how axon guidance cues regulate axon outgrowth by modulating integrin-dependent adhesions. As a number of cognitive disorders result from improper axon pathfinding, understanding the molecular basis for normal neural development is essential for designing therapeutic interventions.

National Institute of Health (NIH)
National Institute of Neurological Disorders and Stroke (NINDS)
Research Project (R01)
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Neurodifferentiation, Plasticity, and Regeneration Study Section (NDPR)
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Riddle, Robert D
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University of Wisconsin Madison
Anatomy/Cell Biology
Schools of Medicine
United States
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Santiago-Medina, Miguel; Gregus, Kelly A; Nichol, Robert H et al. (2015) Regulation of ECM degradation and axon guidance by growth cone invadosomes. Development 142:486-96
Doers, Matthew E; Musser, Michael T; Nichol, Robert et al. (2014) iPSC-derived forebrain neurons from FXS individuals show defects in initial neurite outgrowth. Stem Cells Dev 23:1777-87
Gomez, Timothy M; Letourneau, Paul C (2014) Actin dynamics in growth cone motility and navigation. J Neurochem 129:221-34
Santiago-Medina, Miguel; Gregus, Kelly A; Gomez, Timothy M (2013) PAK-PIX interactions regulate adhesion dynamics and membrane protrusion to control neurite outgrowth. J Cell Sci 126:1122-33
Kerstein, Patrick C; Jacques-Fricke, Bridget T; Rengifo, Juliana et al. (2013) Mechanosensitive TRPC1 channels promote calpain proteolysis of talin to regulate spinal axon outgrowth. J Neurosci 33:273-85
Saengsawang, Witchuda; Taylor, Kendra L; Lumbard, Derek C et al. (2013) CIP4 coordinates with phospholipids and actin-associated proteins to localize to the protruding edge and produce actin ribs and veils. J Cell Sci 126:2411-23
Myers, Jonathan P; Robles, Estuardo; Ducharme-Smith, Allison et al. (2012) Focal adhesion kinase modulates Cdc42 activity downstream of positive and negative axon guidance cues. J Cell Sci 125:2918-29
Santiago-Medina, Miguel; Myers, Jonathan P; Gomez, Timothy M (2012) Imaging adhesion and signaling dynamics in Xenopus laevis growth cones. Dev Neurobiol 72:585-99
Myers, Jonathan P; Gomez, Timothy M (2011) Focal adhesion kinase promotes integrin adhesion dynamics necessary for chemotropic turning of nerve growth cones. J Neurosci 31:13585-95
Gomez, Timothy M (2011) Pioneering studies on the mechanisms of neuronal morphogenesis. Dev Neurobiol 71:780-4

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