Microtubules Direct Axon Growth

Abstract

Two groups investigated mechanisms by which microtubules may be regulated in response to signals that promote axonal movement (see Kalil and Dent). Lee et al. identified the microtubule-associated protein orbit (also known as mast and the ortholog of vertebrate CLASPs) in a genetic screen in Drosophila as interacting with the kinase Abl in the retina. Orbit was implicated in both central and peripheral axon migration in Drosophila and interacted genetically with the Slit-Robo signaling pathway. Analysis of the vertebrate ortholog in Xenopus by visualization of green fluorescent protein (GFP)-tagged CLASP showed that CLASP associated with a subset of microtubules in the growth cone, and these microtubules penetrated into the leading edge of the actin-rich filopodia. Overexpression of GFP-CLASP resulted in microtubule looping and a reduction in the advance of the leading edge of the growth cone. Thus, orbit (or CLASP in vertebrates) appears to be poised between the repellent receptor (robo) and the cytoskeleton to coordinate the movement of microtubules in the growth cone in response to axon guidance cues. Zhou et al. investigated the mechanism by which nerve growth factor (NGF) promoted axon growth in embryonic mouse dorsal root ganglia (DRG) neurons. Various pharmacological treatments and analysis of cells expressing mutants of the signaling components verified that signaling stimulated by NGF proceeded from phosphoinositide 3-kinase (PI3K) to integrin-linked kinase (ILK) to glycogen synthase kinase 3β (GSK-3β) and that this pathway was necessary for NGF-mediated axon elongation. APC (adenomatous polyposis coli), a microtubule-binding protein and target of GSK-3β, was found in axons associated with the cytoskeleton (based on insolubility to detergent extraction) and was visually localized to the tips of growing microtubules. APC localization to the distal ends of axons was lost under conditions that blocked NGF signaling and caused growth cone collapse. Expression of mutants of APC that lacked critical domains for mediating direct or indirect microtubule interactions inhibited axon elongation. Thus, NGF-mediated axon elongation appears to follow a path to APC to control microtubule assembly in the growth cone. K. Kalil, E. W. Dent, Hot +TIPS: Guidance cues signal directly to microtubules. Neuron 42 , 877-883 (2004). [Online Journal] H. Lee, U. Engel, J. Rusch, S. Scherrer, K. Sheard, D. Van Vactor, The microtubule plus end tracking proteins Orbit/MAST/CLASP acts downstream of the tyrosine kinase Abl in mediating axon guidance. Neuron 42 , 913-926 (2004). [Online Journal] F. Q. Zhou, J. Zhou, S. Dedhar, Y.-H. Wu, W. D. Snider, NGF-induced axon growth is mediated by localized inactivation of GSK-3β and functions of the microtubule plus end binding protein APC. Neuron 42 , 897-912 (2004). [Online Journal]