MAP1B coordinates microtubule and actin filament remodeling in adult mouse Schwann cell tips and DRG neuron growth cones

Abstract

We previously described the function of MAP1B in both turning and branching of regenerating neurites. Our results suggested implication of MAP1B in coupling of actin and microtubule movements, a hypothesis investigated here using DRG neurons and Schwann cells (SCs), which also transiently express MAP1B.

Cell motility and cytoskeletal rearrangements were assessed before and after addition of lysophosphatidic acid (LPA), an extracellular signaling phospholipid triggering changes in actin distribution and cell morphology. First, we show that MAP1B is required for SC migration in vitro, extending our previous work on its function in growth cone motility. Second, LPA stimulation induces drastic retraction of processes from MAP1B-expressing cells in a two-step process: actin contraction, which is followed by microtubule backfolding. More importantly, we provide evidence that MAP1B is required for microtubule backfolding, thereby unravelling an important molecular mechanism implicated in coupling the movements of actin and microtubules during process retraction of neural cells.

Introduction

Over the years, numerous studies have demonstrated that movement of cells requires a precise coordination between the major cytoskeletal components, i.e. actin filaments and microtubules (MTs), and their associated proteins (Rodriguez et al., 2003). However, the mechanisms underlying this coordination remain unclear. Several hypotheses have been suggested, involving regulation by the Rho-GTPases family of signaling molecules, equilibrium between motor-driven forces, or a direct structural connection between actin and microtubules (for review see Dehmelt and Halpain, 2004, Myers et al., 2006, Rodriguez et al., 2003).

The common theme of these hypotheses is the involvement of cytoskeleton-associated proteins that control and regulate actin as well as microtubule polymerization and dynamics. Expression of a particular set of microtubule and actin associated protein, in specific combinations, is thus likely to influence cytoskeletal coordination in a specific cell type (Dent and Gertler, 2003).

One of these proteins is MAP1B, a microtubule-associated protein that has been described as being mainly neuronal. Its expression is associated with axon growth during development (Riederer, 2007), and with axonal regeneration and plasticity in the adult (Nothias et al., 1996, Nothias et al., 1997, Soares et al., 2005, Soares et al., 2002).

In a previous study, we provided evidence that MAP1B regulates turning and branching of neurites regenerating from adult mice DRG neurons (Bouquet et al., 2004), two processes requiring a precise coordination of actin and microtubules (Challacombe et al., 1996, Dent and Kalil, 2001, Gordon-Weeks, 1993, Lin and Forscher, 1993, Tanaka and Kirschner, 1995, Tanaka and Kirschner, 1991). Since MAP1B has been shown to bind both MTs and actin (Noiges et al., 2002, Pedrotti and Islam, 1996, Togel et al., 1998), we hypothesized that MAP1B's fundamental role in growth cone motility is to couple the responses of actin and MTs to guidance cues.

Besides its neuronal expression, MAP1B is also transiently expressed in premyelinating cells (Fischer et al., 1990, Ma et al., 1999, Vouyiouklis and Brophy, 1993) and is also reinduced in adult reactive SCs, which de-differentiate and proliferate after a peripheral lesion and migrate into the lesion site (for review, see Komiyama and Suzuki, 1992, LeBlanc and Poduslo, 1990, Mirsky and Jessen, 1996, Oaklander and Spencer, 1988). Thus, expression of MAP1B in both SCs and neurons coincides with cell and axon motility. Interestingly, the cellular processes developed by cultured SCs are “capped” by a structure closely resembling a neuronal growth cone (Bridgman and Dailey, 1989, Small, 1985).

The present study was undertaken to determine whether MAP1B was also required for motility of Schwann cells. Furthermore, we hypothesized that MAP1B is necessary for coupling of actin and microtubule movements in the neural cell types where it is expressed. Thus, to investigate the mechanisms of MAP1B function, we triggered motility of adult mice DRG neurons and Schwann cells with lisophosphatidic acid (LPA), an extracellular signaling phospholipid that produces actin rearrangement and induces changes in the morphology and motility of both SCs and neurons (Barber et al., 2004, Fukushima et al., 2002a, Fukushima et al., 2002b, Fukushima et al., 1998, Fukushima and Morita, 2006, Fukushima et al., 2000, Moolenaar, 1995, Moolenaar et al., 2004, Ridley and Hall, 1992, Sayas et al., 2002, Sayas et al., 1999, Tigyi et al., 1996a, Tigyi et al., 1996b, Weiner et al., 2001, Yuan et al., 2003).

We demonstrate that, in both adult DRG neurons and in SCs, LPA treatment induces the retraction of processes in a myosin- and MAP1B-dependent manner. We finally provide evidence for an MAP1B-dependent mechanism linking the movements of F-actin and microtubules that is common to neuronal growth cones and SC tips.