The growth cone of developing axons guides axon extension through the extracellular matrix (ECM) by sensing gradients of environmental guidance cues that initiate attractive or repulsive steering. Chemotactic growth cone guidance is also important in the context of nervous system injury, as factors released from the breakdown of myelin may act as chemorepellents and inhibit axon elongation, thereby preventing functional recovery. Understanding the molecular mechanisms that mediate growth cone guidance could provide important insights for developing strategies to enhance regeneration after injury or neurodegenerative disease. Cytoplasmic Ca2+ signals mediate the action of many guidance cues, but the link between surface receptor activation and Ca2+ signaling is largely unknown. Likewise, an understanding of the cellular processes underlying growth cone chemotaxis remains incomplete. Current models rely heavily on cytoskeletal rearrangements, but in vivo studies have demonstrated that regulated adhesion to the ECM is also critical for proper guidance. The goal of the proposed research is to define the transduction mechanisms underlying the chemotactic guidance of axonal growth cones. Specifically, we aim to define the intracellular signals that mediate growth cone detection of extracellular guidance cues, the interactions between early signal transduction pathways, and the regulation of downstream effector processes that control the direction of axon extension. Our preliminary findings have led us to establish a CENTRAL HYPOTHESIS that growth cone detection of guidance cues is mediated by polarized phosphoinositide 3-kinase (PI3K) and Akt signaling at the surface membrane, which triggers local Ca2+ signals and stimulates endocytic and exocytic machinery to redistribute receptors for ECM and guidance cues asymmetrically at the growth cone surface and initiate chemotactic guidance. The proposal is organized into four interrelated specific aims that will define the following: first, the role of PI3K/Akt signaling in mediating growth cone chemotaxis;second, how PI3K/Akt signaling activates Ca2+ guidance signals in the growth cone;third, how PI3K/Akt and Ca2+ signaling regulate vesicle dynamics during growth cone turning;and fourth, how PI3K/Akt and Ca2+ signaling regulate trafficking of integrin and guidance receptors during growth cone turning. This study will provide novel insights into the early signals that mediate the detection of guidance cues, the amplification of guidance signals, and the regulation of cellular machinery that controls membrane dynamics and the redistribution of surface receptors during chemotactic growth cone guidance.
In the developing nervous system the growing tip of nerve cells extends through a complex environmental matrix to the appropriate target cells by sensing gradients of guidance cues that initiate attractive or repulsive steering. This guidance is also important in the context of nervous system injury, as factors released from the breakdown of myelin may act as repellents and inhibit elongation, thereby preventing functional recovery. The goal of this research is to define signals that mediate the detection of guidance cues and determine how these signals regulate cellular processes to control the direction of extension. The findings will contribute to our understanding of the development of the nervous system and provide insights into potential therapeutic approaches for promoting regeneration after neurodegenerative disease or injury.
