Okada Toshiaki, Katsuyama You, Ono Fumihito, Okamura Yasushi
Molecular Neurobiology Group, Neuroscience Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 6th, Higashi 1-1-1, Ibaraki, Tsukuba, 305-8566, Japan.
Dev Biol. 2002 Apr 15;244(2):278-92. doi: 10.1006/dbio.2002.0585.
The generation of distinct classes of motor neurons underlies the development of complex motile behavior in all animals and is well characterized in chordates. Recent molecular studies indicate that the ascidian larval central nervous system (CNS) exhibits anteroposterior regionalization similar to that seen in the vertebrate CNS. To extend the understanding about the diversity of motor neurons in the ascidian larva, we have identified the number, position, and projection of individual motor neurons in Halocynthia roretzi, using a green fluorescent protein under the control of a neuron-specific promoter. Three pairs of motor neurons, each with a distinct shape and innervation pattern, were identified along the anteroposterior axis of the neural tube: the anterior and posterior pairs extend their axons toward dorsal muscle cells, whereas the middle pair project their axons toward ventral muscle. Overexpression of a dominant-negative form of a potassium channel in these cells resulted in paralysis on the injected side, thus these cells must constitute the major population of motor neurons responsible for swimming behavior. Lim class homeobox genes have been known as candidate genes that determine subtypes of motor neurons. Therefore, the expression pattern of Hrlim, which is a Lim class homeobox gene, was examined in the motor neuron precursors. All three motor neurons expressed Hrlim at the tailbud stage, although each down-regulated Hrlim at a different time. Misexpression of Hrlim in the epidermal lineage led to ectopic expression of TuNa2, a putative voltage-gated channel gene normally expressed predominantly in the three pairs of motor neurons. Hrlim may control membrane excitability of motor neurons by regulating ion channel gene expression.
不同类型运动神经元的产生是所有动物复杂运动行为发育的基础，在脊索动物中已有充分研究。最近的分子研究表明，海鞘幼虫的中枢神经系统（CNS）呈现出与脊椎动物中枢神经系统相似的前后区域化。为了进一步了解海鞘幼虫运动神经元的多样性，我们利用神经元特异性启动子控制下的绿色荧光蛋白，确定了罗氏海鞘中单个运动神经元的数量、位置和投射。沿着神经管的前后轴确定了三对运动神经元，每对具有独特的形状和支配模式：前后两对将轴突伸向背侧肌肉细胞，而中间一对将轴突伸向腹侧肌肉。在这些细胞中过表达钾通道的显性负性形式导致注射侧麻痹，因此这些细胞必定构成负责游泳行为的主要运动神经元群体。Lim类同源框基因一直被认为是决定运动神经元亚型的候选基因。因此，我们检测了Lim类同源框基因Hrlim在运动神经元前体中的表达模式。所有三个运动神经元在尾芽期均表达Hrlim，尽管每个神经元在不同时间下调Hrlim。在表皮谱系中错误表达Hrlim导致TuNa2的异位表达，TuNa2是一种假定的电压门控通道基因，通常主要在三对运动神经元中表达。Hrlim可能通过调节离子通道基因表达来控制运动神经元的膜兴奋性。
