Myosin reins in neurites

Myosin II pulls growth cones in the right direction, as shown by Stephen Turney and Paul Bridgman (Washington University, St. Louis, MO).Growing neurons in the developing embryo are directed by guidance cues such as laminin-1 (LN1), which steer the extension of neurite growth cones. Bridgman had previously noticed that neuronal growth cones contain high levels of myosin II. As this motor generates force on the cytoskeleton, he figured it might be involved in turning neurites in response to guidance cues.

Such was the case for LN1, as shown by the growth of neurites at borders between LN1 and polyornithine substrates. Normally, growing neurites rapidly retreat from polyornithine and turn back into the laminin surface. But when myosin II activity was inhibited, the neurites ignored the change in substrate and grew over polyornithine.

Turning depended on the activation of integrins—the LN1 receptors. The subsequent activation of focal adhesion kinases might activate or recruit myosin II. On polyornithine, both myosin II and focal complexes are randomly distributed. On LN1, however, myosin IIB concentrated in the transitional domain of the growth cone—intermingled with or just behind the new front of focal complexes. Myosin placement in relation to adhesion sites might pull neurites toward more LN1 and away from unwanted substrates.

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