Axon extension in the fast and slow lanes: Substratum‐dependent engagement of myosin II functions

Abstract

Axon extension involves the coordinated regulation of the neuronal cytoskeleton. Actin filaments drive protrusion of filopodia and lamellipodia while microtubules invade the growth cone, thereby providing structural support for the nascent axon. Furthermore, in order for axons to extend the growth cone must attach to the substratum. Previous work indicates that myosin II activity inhibits the advance of microtubules into the periphery of growth cones, and myosin II has also been implicated in mediating integrin-dependent cell attachment. However, it is not clear how the functions of myosin II in regulating substratum attachment and microtubule advance are integrated during axon extension. We report that inhibition of myosin II function decreases the rate of axon extension on laminin, but surprisingly promotes extension rate on polylysine. The differential effects of myosin II inhibition on axon extension rate are attributable to myosin II having the primary function of mediating substratum attachment on laminin, but not on polylysine. Conversely, on polylysine the primary function of myosin II is to inhibit microtubule advance into growth cones. Thus, the substratum determines the role of myosin II in axon extension by controlling the functions of myosin II that contribute to extension.

Engineering neuronal growth cones to promote axon regeneration over inhibitory molecules

“…On laminin, blebbistatin has been shown to reduce, rather than to promote axon growth of peripheral neurons (28,39). The seemingly apparent discrepancy between previous studies (28, Representative images of axons at the permissive-inhibitory border.…”

“…39) and ours might be due to differences in the substratum. In our study, neurons were cultured on polylysine (100 μg/mL) plus 5-10 μg/mL of laminin, whereas other studies applied a higher concentration of laminin (25 μg/mL) without polylysine (39) or with a less adhesive substrate, polyornithine (28). The final outcome of NMII inhibition on axon growth over permissive substrates might depend on the adhesiveness of the substrates, but further studies are required to address this issue.…”

“…Although blebbistatin has been shown to promote MT protrusion toward growth cone periphery on permissive substrates (39), how it regulates growth cone cytoskeletal structures and subsequent axon growth on CNS inhibitors, to our knowledge, has never been examined. On the basis of the reorganization of the actin structure and the reduction in F-actin levels in the growth cone, it is likely that the axon growthpromoting effect of NMII inhibition on inhibitory substrates occurs via increased MT extension toward the growth cone leading edge, which occurs as consequence of the attenuation of retrograde actin flow and the loss of actin arc structure (Fig.…”

The Scaffold Protein POSH Regulates Axon Outgrowth

“…Myosin II has been implicated as a key player in process outgrowth through its ability to regulate process extension, retraction, adhesion, and detachment (Wylie and Chantler, 2001;Brown and Bridgman, 2003;Chantler and Wylie, 2003;Medeiros et al, 2006;Even-Ram et al, 2007;Ketschek et al, 2007). Therefore, RNAi was used to determine whether a reduction of myosin II function would alter POSH RNAi or Shroom RNAi process length relative to control cortical neurons.…”

Micropatterned ECM substrates reveal complementary contribution of low and high affinity ligands to neurite outgrowth

“…Increased branching of neurites, essentially observed on LN spots, should be related to this relatively high stiffness of the distal segment, necessary to counteract the forces involved in the extension of filopodia establishing an equilibrium with point contacts at their tip [see also, Steketee and Tosney, 2002]. Attachment to a suitable substrate and maturation of adhesion sites appear indispensable for extending neurites to be able to exert forces that necessitate well-coordinated cytoskeletal dynamics [Jay, 2001;Brown and Bridgman, 2003;Turney and Bridgman, 2005;Woo and Gomez, 2006;Ketschek et al, 2007;Schaefer et al, 2008].…”