Chadaram Srinivas Rao, Laskowski Michael B, Madison Roger D
WWAMI Medical Education Program and Department of Biological Sciences, University of Idaho, Moscow, Idaho 83843, USA.
J Neurosci. 2007 Dec 19;27(51):13938-48. doi: 10.1523/JNEUROSCI.3055-07.2007.
Spinal motor pools project to target muscles forming distinct rostrocaudal topographic maps during development and regeneration. To define the mechanisms underlying these neuromuscular maps we studied the preferential outgrowth of embryonic spinal cord neurites on muscle membranes from different axial positions and explored the role of ephrin A ligands. We found all five ephrin As (EphAs) expressed in serratus anterior, gluteus maximus and diaphragm muscles. In the diaphragm, four of the five ephrin As are expressed as a caudal to rostral gradient. When ephrin A function is disrupted in muscle membranes by deletion of glycosyl-phosphatidylinositol anchored ephrin A ligands with phosphatidylinositol-specific phospholipase C enzyme treatment or by blocking of ephrin A ligands with EphA fusion proteins, or by genetic manipulation leading to ephrin A2/A5 mutant mice, the spinal cord neurites loose their preference for the membranes of corresponding axial position; suggesting a significant role for ephrins in topographic choices made by growing motor neurons. To closely approximate topographic choices presented to embryonic neurites in vivo, neurites within the phrenic motor pool were challenged to make outgrowth choices on membranes of their normal target, the diaphragm muscle. We observed that neurites from rostral cervical segments (C1 and C2) prefer to grow on rostral diaphragm membranes; caudal cervical neurites (C6-C8) choose caudal diaphragm membranes; a transition of positional preference occurs at C4 and this ability is lost in ephrin A2/A5 mutant mice. These results demonstrate for the first time topographical outgrowth of axons from within a motor pool onto a single target muscle in vitro.
脊髓运动神经元池在发育和再生过程中投射到目标肌肉，形成独特的 rostrocaudal 地形图。为了确定这些神经肌肉图谱背后的机制，我们研究了胚胎脊髓神经突在来自不同轴向位置的肌肉膜上的优先生长，并探讨了 ephrin A 配体的作用。我们发现所有五种 ephrin A（EphAs）在锯肌、臀大肌和膈肌中均有表达。在膈肌中，五种 ephrin A 中的四种以从尾到头的梯度表达。当通过用磷脂酰肌醇特异性磷脂酶 C 酶处理缺失糖基磷脂酰肌醇锚定的 ephrin A 配体、或用 EphA 融合蛋白阻断 ephrin A 配体、或通过基因操作导致 ephrin A2/A5 突变小鼠，使肌肉膜中的 ephrin A 功能受到破坏时，脊髓神经突失去了对相应轴向位置膜的偏好；这表明 ephrins 在运动神经元生长的地形选择中起重要作用。为了更接近体内呈现给胚胎神经突的地形选择，膈运动神经元池内的神经突被挑战在其正常目标膈肌的膜上做出生长选择。我们观察到，来自颈段头端（C1 和 C2）的神经突更喜欢在膈肌头端膜上生长；颈段尾端神经突（C6 - C8）选择膈肌尾端膜；在 C4 处发生位置偏好的转变，并且这种能力在 ephrin A2/A5 突变小鼠中丧失。这些结果首次证明了运动神经元池内的轴突在体外向单个目标肌肉的地形生长。
