UBC Theses and Dissertations
Intracellular mechanisms underlying growth cone collapse To, Kenneth Chi-Wan
During the course of development, expression of attractive and inhibitory guidance cues play a pivotal role in the pathfinding decisions of a growing neuron. In addition, injury-induced recapitulation of their expression, particularly inhibitory cues, likely influences the course of axonal regeneration, thus providing a rationale for the intense focus in this area of research. Though significant progress has been made, it remains poorly described what signaling cascades, and in what combination, are involved during inhibitory-cue induced growth cone collapse. Therefore, to further understand why neurons are repelled or inhibited by certain cues, the aim of this thesis is to identify the underlying intracellular mechanisms regulating growth cone collapse induced by inhibitory cues. Using a novel anti-invasive compound called Motuporamine C(MotC), I have characterized in chapter 2 its effects as a regulator of neuronal outgrowth. I found that MotC was a robust stimulator of growth cone collapse leading to a cessation of neurite growth. This was partially mediated through Rho-ROCK signaling, a pathway involved in regulating actin dynamics. Based on this partial response, I hypothesized that other signal transduction pathways were involved. I addressed this in chapter 3 by identifying calcium-activated calpain, a protease well-characterized in playing a role in adhesion regulation, was also activated during MotC-induced growth cone collapse. Furthermore, I show that concurrent inhibition of both Rho-ROCK and calpain pathways are necessary for maximum attenuation of the MotC-mediated collapse response. Since these results were identified using an organic molecule not endogenously expressed in vertebrate organisms, I hypothesized in chapter 4 that similar pathways would be activated in response to a physiological in vivo guidance cue. Using the inhibitory cue Semaphorin 5B (Sema5B), I found in addition to the activation of calpain, the phosphatase calcineurin was also involved in mediating Sema5B-induced growth cone collapse. Moreover, it is the combination of calpain- and calcineurin-mediated pathways that is required for evoking maximal growth cone collapse and that cross-talk between these two effector molecules occurs. These results are of particular interest since previously it was proposed by Gomez and Zheng (2006) that calpain and calcineurin signaling cascades were parallel pathways. Taken together, my findings show that different inhibitory cues activate multiple intracellular pathways that appear to impinge on different aspects of the intracellular machinery regulating motility. The combinatorial activation of these pathways is necessary for mediating maximal growth cone collapsing effects. Moreover, the elucidation of common signaling cascades between inhibitory cues to induce growth cone collapse may eventually provide novel targets for the development of new therapeutic strategies to promote functional recovery following neuronal injury.
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