M-Cadherin Activates Rac1 GTPase through the Rho-GEF Trio during Myoblast Fusion

Abstract

Cadherins are transmembrane glycoproteins that mediate Ca2؉ -dependent homophilic cell-cell adhesion and play crucial role during skeletal myogenesis. M-cadherin is required for myoblast fusion into myotubes, but its mechanisms of action remain unknown. The goal of this study was to cast some light on the nature of the M-cadherin-mediated signals involved in myoblast fusion into myotubes. We found that the Rac1 GTPase activity is increased at the time of myoblast fusion and it is required for this process. Moreover, we showed that M-cadherin-dependent adhesion activates Rac1 and demonstrated the formation of a multiproteic complex containing M-cadherin, the Rho-GEF Trio, and Rac1 at the onset of myoblast fusion. Interestingly, Trio knockdown efficiently blocked both the increase in Rac1-GTP levels, observed after M-cadherin-dependent contact formation, and myoblast fusion. We conclude that M-cadherin-dependent adhesion can activate Rac1 via the Rho-GEF Trio at the time of myoblast fusion. INTRODUCTIONDuring skeletal muscle development mesodermal precursor cells give rise to committed myoblasts that, after proliferation and migration to the appropriate sites in the embryo, exit the cell cycle, express muscle-specific genes, and fuse into multinucleated myofibers that mature to form multinucleated muscle fibers (Taylor, 2002). Although myoblast fusion is important both during embryonic development and in the maintenance and repair of adult muscles, the mechanisms regulating this process are largely unknown. Myoblast fusion is a multistep process that entails initial recognition and adhesion between myoblasts, their alignment, and finally membrane breakdown and fusion (Doberstein et al., 1997). This process is regulated, at different levels, by a variety of proteins, such as transcription factors or extracellular signaling molecules, including diffusible factors, components of the extracellular matrix, and proteins involved in cell-cell contact (Krauss et al., 2005).In this later group, M-cadherin plays a prominent role. M-cadherin belongs to the cadherin family of Ca 2ϩ -dependent adhesion molecules. Its N-terminal extracellular domain mediates homophilic binding, while the cytoplasmic tail interacts with catenins and is linked to the actin cytoskeleton, thus, coupling the ectodomain interactions to the dynamic intracellular tensile forces (Wheelock and Johnson, 2003b). M-cadherin is found predominantly in developing skeletal muscles and is highly expressed during secondary myogenesis. In mature skeletal muscle, M-cadherin is detectable in satellite cells and on the sarcolemma of myofibers underlying satellite cells (Moore and Walsh, 1993;Rose et al., 1994;Cifuentes-Diaz et al., 1995). M-cadherin is also found at neuromuscular junctions, intramuscular nerves, and in two regions of the CNS, namely the spinal cord and the cerebellum (Cifuentes-Diaz et al., 1996;Bahjaoui-Bouhaddi et al., 1997). M-cadherin-deficient mice do not show defects in skeletal muscle development, probably because of compensation by other cadh...

Small Rho GTPase-mediated actin dynamics at endothelial adherens junctions

“…22 In line with this finding, a rapid increase in Rac1 activity upon VE-cadherin homotypic adhesion was detected biochemically, which was induced using beads coated with the VE-cadherin ectodomain. Thus, as was shown for E-, M-and N-cadherin, [64][65][66][67] we showed that VEcadherin can signal in an outside-in fashion to activate Rac1, providing a bidirectional feedback mechanism. 22 Of note, ongoing local remodeling of cell-cell junctions is observed even in apparently stable endothelial monolayers.…”

Polyelectrolyte Multilayer Films of Controlled Stiffness Modulate Myoblast Cell Differentiation

“…We first followed by phase-contrast microscopy over several days the overall behavior of cells on films cross-linked at different EDC concentrations. During differentiation, C2C12 cells align and fuse together to form myotubes, when cultured on control tissue culture polystyrene surfaces ( Figure 5, J-L), as usually observed [33] (plastic was always used as control surface as C2C12 cells are known to not differentiate optimally on bare glass [10] ). We qualitatively observed cell differentiation, i.e.…”

“…Myoblast differentiation onto the cross-linked (PLL/HA) 12 films was then followed by analysis of the expression of muscle-specific proteins like myogenin and troponin T. For control surfaces, these proteins are known to be expressed after about 1-2 days in DM [33,34] and until at least day 6. Immunoblots of myogenin and troponin T expression at day 3 and day 6 in DM, for cells cultured on EDC30, EDC50 and EDC100 films are shown in Figure 6.…”

“…All image quantifications were performed using Image J software v1.38 (NIH, Bethesda) on cells labeled with antitroponin T or MHC. The fusion index (FI) was determined by dividing the total number of nuclei in myotubes (≥ 2 nuclei) by the total number of nuclei counted [33] . The aspect ratio (AR) was determined by dividing the length of myotubes by their width.…”

The atypical Rac activator Dock180 (Dock1) regulates myoblast fusion in vivo

“…Interestingly, TRIO is a dual GEF for RhoA and Rac1; whether either one or both of these activities are important for muscle development is currently unknown. Recent data in C2C12 myoblasts in vitro suggest a major contribution of TRIO in the fusion process, raising the possibility that other GEFs can compensate for the loss of TRIO in vivo (30). More work will be required to fully appreciate the spatial and temporal activation of Rho GTPases in myoblast fusion.…”