Phosphorylation of microtubule-associated protein IB and axonal growth

Phosphorylated MAP1B is induced in central sprouting of primary afferents in response to peripheral injury but not in response to rhizotomy

A peripheral nerve lesion induces sprouting of primary afferents from dorsal root ganglion (DRG) neurons into lamina II of the dorsal horn. Modifications of the environment in consequence to the axotomy provide an extrinsic stimulus. A potential neuron-intrinsic factor that may permit axonal sprouting is microtubule-associated protein 1B (MAP1B) in a specific phosphorylated form (MAP1B-P), restricted to growing or regenerating axons. We show here that both in rat and mouse, a sciatic nerve cut is rapidly followed by the appearance of MAP1B-P expression in lamina II, increasing to a maximum between 8 and 15 days, and diminishing after three months. Evidence is provided that sprouting and induction of MAP1B-P expression after peripheral injury are phenomena concerning essentially myelinated axons. This is in accordance with in situ hybridization data showing especially high MAP1B-mRNA levels in large size DRG neurons that give rise to myelinated fibers. We then employed a second lesion model, multiple rhizotomy with one spared root. In this case, unmyelinated CGRP expressing fibers do indeed sprout, but coexpression of MAP1B-P and CGRP is never observed in lamina II. Finally, because a characteristic of myelinated fibers is their high content in neurofilament protein heavy subunit (NF-H), we used NF-H-LacZ transgenic mice to verify that MAP1B-P induction and central sprouting were not affected by perturbing the axonal organization of neurofilaments. We conclude that MAP1B-P is well suited as a rapidly expressed, axon-intrinsic marker associated with plasticity of myelinated fibers.

Dishevelled-1 regulates microtubule stability: a new function mediated by glycogen synthase kinase-3beta

Dishevelled has been implicated in the regulation of cell fate decisions, cell polarity, and neuronal function. However, the mechanism of Dishevelled action remains poorly understood. Here we examine the cellular localization and function of the mouse Dishevelled protein, DVL-1. Endogenous DVL-1 colocalizes with axonal microtubules and sediments with brain microtubules. Expression of DVL-1 protects stable microtubules from depolymerization by nocodazole in both dividing cells and differentiated neuroblastoma cells. Deletion analyses reveal that the PDZ domain, but not the DEP domain, of DVL-1 is required for microtubule stabilization. The microtubule stabilizing function of DVL-1 is mimicked by lithium-mediated inhibition of glycogen synthase kinase-3beta (GSK-3beta) and blocked by expression of GSK-3beta. These findings suggest that DVL-1, through GSK-3beta, can regulate microtubule dynamics. This new function of DVL-1 in controlling microtubule stability may have important implications for Dishevelled proteins in regulating cell polarity.