Myosin II Activity Facilitates Microtubule Bundling in the Neuronal Growth Cone Neck

Abstract

SUMMARY The cell biological processes underlying axon growth and guidance are still not well understood. An outstanding question is how a new segment of the axon shaft is formed in the wake of neuronal growth cone advance. For this to occur, the highly dynamic, splayed-out microtubule (MT) arrays characteristic of the growth cone must be consolidated (bundled together) to form the core of the axon shaft. MT-associated proteins stabilize bundled MTs, but how individual MTs are brought together for initial bundling is unknown. Here, we show that laterally moving actin arcs, which are myosin II-driven contractile structures, interact with growing MTs and transport them from the sides of the growth cone into the central domain. Upon Myosin II inhibition, the movement of actin filaments and MTs immediately stopped and MTs unbundled. Thus, Myosin II-dependent compressive force is necessary for normal MT bundling in the growth cone neck.

Cytoplasmic Linker Proteins Regulate Neuronal Polarization through Microtubule and Growth Cone Dynamics

“…In particular, CLIPs positively regulated growth cone dynamics and restrained actin arc formation in the transition zone of the growth cone. It was hypothesized that protruding microtubules within the growth cone may function as a "pushing force" for the axon to grow, whereas the actin cytoskeleton may restrict the growth of the protruding microtubules (Burnette et al, 2008;Lee and Suter, 2008;. In support of this hypothesis, we found that depleting the actin cytoskeleton or inhibiting the actin arc contractility in DN-CLIP-transfected neurons enabled microtubules to protrude into the peripheral zone of the growth cone.…”

“…6 E, F ). However, in DN-CLIP-expressing neurons, actin arcs, which are known to prevent microtubule protrusion in the peripheral growth cone (Burnette et al, 2008;Lee and Suter, 2008;Lowery and Van Vactor, 2009), surrounded the central domain of the growth cone perpendicular to the filopodia projection axis (Fig. 6 D; supplemental Movie 3, available at www.…”

“…Actin arcs prevent microtubules from protruding to the peripheral region of the growth cone by myosin II-mediated contractility (Burnette et al, 2008;Lee and Suter, 2008;Schaefer et al, 2008;Lowery and Van Vactor, 2009). We therefore hypothesized that a disassembly of the actin cytoskeleton or an inhibition of the myosin II-mediated actin arc compression could also cause microtubules to protrude further distally into the growth cone, even if CLIPs are nonfunctional.…”

“…Myosin II-based retrograde transport of actin filaments toward the center of the growth cone results in the generation of actin arcs (Schaefer et al, 2002) that prevent microtubules from invading the peripheral region (Burnette et al, 2008;Lee and Suter, 2008;Schaefer et al, 2008;Lowery and Van Vactor, 2009). During active growth phases, actin arcs reorient toward the growth direction, creating an F-actin-free corridor into which microtubules protrude (Lee and Suter, 2008).…”

The contractome – a systems view of actomyosin contractility in non-muscle cells

“…The mechanical properties and shape of the new daughter cell are determined by its ability to contract and maintain tension in its cortical cytoskeleton (Bischofs et al, 2008;Martens and Radmacher, 2008;Schafer and Radmacher, 2005). In addition, the dynamic properties of the actin cytoskeleton and its organization into a variety of actin-rich structures, such as lamellipodia and stress fibers, are also affected by contractility (Hirata et al, 2008;Hotulainen and Lappalainen, 2006;Medeiros et al, 2006;Svitkina et al, 1997;Wilson et al, 2010;Yang et al, 2012), which also affect the movement and bundling of microtubules (Burnette et al, 2008;Schaefer et al, 2008).…”