Extracellular Muscle Myosin II Promotes Sensory Axon Formation.
Myosin II is an intracellular force-generating enzyme with no known extracellular action. In the course of experiments involving trituration loading of skeletal myosin II into embryonic sensory neurons we observed that extracellular application of myosin II to neurons resulted in a robust increase in the number of axons initiated by each neuron, but did not alter the rate of axon extension. Substratum bound myosin II in the presence of laminin was sufficient to elicit increases in axon formation. However, in the absence of laminin, extracellular myosin II alone was not sufficient to promote axon formation, although it allowed neuron survival in the presence of neurotrophin. Myosin II promoted the attachment of neurons to the substratum in the absence or presence of laminin. In addition to promoting the initiation of axons, extracellular myosin II also increased the frequency of axon collateral branching. Finally, extracellular myosin II did not affect growth cone collapse in response to semaphorin-IIIA, but attenuated the inhibitory action of chondroitin sulfate proteoglycans on axon extension. Surprisingly, these results demonstrate that extracellular myosin II promotes attachment of neurons and increases axon formation and branching. The potential significance of these observations is discussed in the context of myosin II release from injured muscle and a previous demonstration of extracellular myosin II association with the extracellular matrix.