Bravo-Ambrosio Arlene, Kaprielian Zaven
Dominick P. Purpura Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
Int J Mol Sci. 2011;12(12):8539-61. doi: 10.3390/ijms12128539. Epub 2011 Nov 29.
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.
生物体的生存以及适应不断变化的环境在很大程度上依赖于运动回路的功能。运动神经元（MNs）是中枢神经系统（CNS）神经元中独特的一类，它们将轴突投射出中枢神经系统。一旦进入外周，运动轴突会沿着高度定型的轨迹行进，通常离它们的细胞体有相当远的距离，以支配合适的肌肉靶点。寻路运动轴突做出的一个关键决定是通过背侧还是腹侧运动出口点（MEPs）离开中枢神经系统。与在理解调节MN亚型的特化和肢体肌肉的神经支配机制方面取得的重大进展相比，关于MN轴突如何投射出中枢神经系统的了解却非常少。然而，主要在果蝇中的少数研究已经确定了运动轴突离开脊髓所需的转录因子，在某些情况下还确定了候选的下游效应分子。值得注意的是，在脊椎动物中，被称为边界帽（BC）细胞的特殊神经嵴细胞衍生物预先形成并划定了运动出口点。然而，令人惊讶的是，MN轴突离开并不需要BC细胞，而是限制MN细胞体不会沿着它们的轴突异位迁移出中枢神经系统。在这里，我们描述了已在果蝇和脊椎动物中研究运动轴突离开的少量研究，并讨论了我们对引导运动轴突离开中枢神经系统机制的零碎认识。
