Katsuyama Yu, Okada Toshiaki, Matsumoto Jun, Ohtsuka Yukio, Terashima Toshio, Okamura Yasushi
Molecular Neurobiology Group, Neuroscience Research Institute, AIST Tsukuba Central 6-12, Tsukuba, Ibaraki 305-8566, Japan.
Dev Biol. 2005 Feb 15;278(2):310-22. doi: 10.1016/j.ydbio.2004.11.007.
In the tadpole larvae of the ascidian Halocynthia roretzi, six motor neurons, Moto-A, -B, and -C (a pair of each), are localized proximal to the caudal neural tube and show distinct morphology and innervation patterns. To gain insights into early mechanisms underlying differentiation of individual motor neurons, we have isolated an ascidian homologue of Islet, a LIM type homeobox gene. Earliest expression of Islet was detected in a pair of bilateral blastomeres on the dorsal edge of the late gastrula. At the neurula stage, this expression began to disappear and more posterior cells started to express Islet. Compared to expression of a series of motor neuron genes, it was confirmed that early Islet-positive blastomeres are the common precursors of Moto-A and -B, and late Islet-positive cells in the posterior neural tube are the precursors of Moto-C. Overexpression of Islet induced ectopic expression of motor neuron markers, suggesting that Islet is capable of regulating motor neuron differentiation. Since early expression of Islet colocalizes with that of HrBMPb, the ascidian homologue of BMP2/4, we tested a role of BMP in specification of the motor neuron fate. Overexpression of HrBMPb led to expansion of Lim and Islet expression toward the central area of the neural plate, and microinjection of mRNA coding for a dominant-negative BMP receptor weakened the expression of these genes. Our results suggest that determination of the ascidian motor neuron fate takes place at late gastrula stage and local BMP signaling may play a role in this step.
在海鞘（Halocynthia roretzi）的蝌蚪幼虫中，六个运动神经元，即Moto - A、- B和 - C（各一对），位于尾神经管近端，呈现出明显的形态和神经支配模式。为了深入了解单个运动神经元分化的早期机制，我们分离出了一种LIM型同源框基因Islet的海鞘同源物。最早在晚期原肠胚背侧边缘的一对双侧卵裂球中检测到Islet的表达。在神经胚阶段，这种表达开始消失，更多后部细胞开始表达Islet。与一系列运动神经元基因的表达相比，证实早期Islet阳性卵裂球是Moto - A和 - B的共同前体，而后部神经管中晚期Islet阳性细胞是Moto - C的前体。Islet的过表达诱导了运动神经元标志物的异位表达，表明Islet能够调节运动神经元的分化。由于Islet的早期表达与BMP2/4的海鞘同源物HrBMPb的表达共定位，我们测试了BMP在运动神经元命运特化中的作用。HrBMPb的过表达导致Lim和Islet的表达向神经板中央区域扩展，而注射编码显性负性BMP受体的mRNA会减弱这些基因的表达。我们的结果表明，海鞘运动神经元命运的决定发生在晚期原肠胚阶段，局部BMP信号可能在此步骤中发挥作用。
