| The growth cone at the tip of the axon is essential for guiding the axon during development to its appropriate target. Growth cones can be divided into a microtubule-rich central (C-) domain, an actin-rich peripheral (P-) domain and the transition (T-) zone between them. Polarized rearrangement of cytoskeletal elements in the growth cone underlies its turning toward or away from environmental cues.;Growth cone turning requires selective invasion of microtubules from the C-domain into the P-domain in the direction of the cue. Studies to date have attributed this invasion to the dynamic properties of the microtubules. I have tested another idea, specifically that molecular motors involved in organizing microtubules in the mitotic spindle play a critical role in regulating the polarized distribution of microtubules during growth cone turning. This idea builds on observations that the invasion of microtubules into the P-domain is determined by a tug-of-war between forces exerted by cytoplasmic dynein and myosin-II. I hypothesized that kinesin-5, a motor that counterbalances dynein-based forces during mitosis, tips the balance of forces in a polarized fashion within growth cones, to elicit the selective invasion of microtubules into the appropriate side of the growth cone during turning.;I observed that depletion of kinesin-5 or pharmacologic inhibition of kinesin-5 inhibited growth cone turning in response to environmental cues. Kinesin-5 is enriched in the T-zone, where the phospho-kinesin-5 polarizes away from the direction of turning. Experiments with non-phosphorylatable kinesin-5 resulted in increased microtubule invasion and lack of growth cone turning, similar to depletion or inhibition of kinesin-5, indicating that phosphorylation regulates kinesin-5 function in the growth cone. This provides an attractive means by which signaling pathways relevant to growth cone turning could utilize kinesin-5 to polarize the microtubule array. As a test of the feasibility of this idea, I used micro-CALI to locally inactivate kinesin-5 in different domains of the growth cone, and found that local inactivation specifically in the T-zone results in increased invasion of microtubules into the P-domain.;I conclude that kinesin-5 functions in the T-zone of growth cones to regulate growth cone turning and kinesin-5 function is regulated by phosphorylation. |
