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Crossing the Border: Molecular Control of Motor Axon Exit
Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, USA
Department of Pathology, Albert Einstein College of Medicine, Bronx, NY 10461, USA
* Author to whom correspondence should be addressed.
Received: 17 October 2011; in revised form: 5 November 2011 / Accepted: 8 November 2011 / Published: 29 November 2011
Abstract: Living organisms heavily rely on the function of motor circuits for their survival and for adapting to ever-changing environments. Unique among central nervous system (CNS) neurons, motor neurons (MNs) project their axons out of the CNS. Once in the periphery, motor axons navigate along highly stereotyped trajectories, often at considerable distances from their cell bodies, to innervate appropriate muscle targets. A key decision made by pathfinding motor axons is whether to exit the CNS through dorsal or ventral motor exit points (MEPs). In contrast to the major advances made in understanding the mechanisms that regulate the specification of MN subtypes and the innervation of limb muscles, remarkably little is known about how MN axons project out of the CNS. Nevertheless, a limited number of studies, mainly in Drosophila, have identified transcription factors, and in some cases candidate downstream effector molecules, that are required for motor axons to exit the spinal cord. Notably, specialized neural crest cell derivatives, referred to as Boundary Cap (BC) cells, pre-figure and demarcate MEPs in vertebrates. Surprisingly, however, BC cells are not required for MN axon exit, but rather restrict MN cell bodies from ectopically migrating along their axons out of the CNS. Here, we describe the small set of studies that have addressed motor axon exit in Drosophila and vertebrates, and discuss our fragmentary knowledge of the mechanisms, which guide motor axons out of the CNS.
Keywords: motor axon exit; axon pathfinding; spinal cord; motor exit point; dorsally-exiting motor neuron; ventrally-exiting motor neuron; Nkx2.9; Robo; Slit
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Bravo-Ambrosio, A.; Kaprielian, Z. Crossing the Border: Molecular Control of Motor Axon Exit. Int. J. Mol. Sci. 2011, 12, 8539-8561.
Bravo-Ambrosio A, Kaprielian Z. Crossing the Border: Molecular Control of Motor Axon Exit. International Journal of Molecular Sciences. 2011; 12(12):8539-8561.
Bravo-Ambrosio, Arlene; Kaprielian, Zaven. 2011. "Crossing the Border: Molecular Control of Motor Axon Exit." Int. J. Mol. Sci. 12, no. 12: 8539-8561.