Jin Shiying, Martinelli David C, Zheng Xiaobin, Tessier-Lavigne Marc, Fan Chen-Ming
Department of Embryology, Carnegie Institution of Washington, Baltimore, MD 21218; and.
Rockefeller University, New York, NY 10065.
Proc Natl Acad Sci U S A. 2015 Jan 6;112(1):E73-80. doi: 10.1073/pnas.1418629112. Epub 2014 Dec 22.
The myenteric plexus of the enteric nervous system controls the movement of smooth muscles in the gastrointestinal system. They extend their axons between two peripheral smooth muscle layers to form a tubular meshwork arborizing the gut wall. How a tubular axonal meshwork becomes established without invading centrally toward the gut epithelium has not been addressed. We provide evidence here that sonic hedgehog (Shh) secreted from the gut epithelium prevents central projections of enteric axons, thereby forcing their peripheral tubular distribution. Exclusion of enteric central projections by Shh requires its binding partner growth arrest specific gene 1 (Gas1) and its signaling component smoothened (Smo) in enteric neurons. Using enteric neurons differentiated from neurospheres in vitro, we show that enteric axon growth is not inhibited by Shh. Rather, when Shh is presented as a point source, enteric axons turn away from it in a Gas1-dependent manner. Of the Gαi proteins that can couple with Smo, G protein α Z (Gnaz) is found in enteric axons. Knockdown and dominant negative inhibition of Gnaz dampen the axon-repulsive response to Shh, and Gnaz mutant intestines contain centrally projected enteric axons. Together, our data uncover a previously unsuspected mechanism underlying development of centrifugal tubular organization and identify a previously unidentified effector of Shh in axon guidance.
肠神经系统的肌间神经丛控制着胃肠道系统中平滑肌的运动。它们在两层外周平滑肌之间延伸轴突，形成一个贯穿肠壁的管状网状分支结构。管状轴突网络是如何在不向肠上皮中央侵入的情况下形成的，目前尚未得到解答。我们在此提供证据表明，肠上皮分泌的音猬因子（Shh）可阻止肠轴突向中央投射，从而迫使它们在外周呈管状分布。Shh对肠轴突中央投射的排除作用需要其结合伴侣生长抑制特异性基因1（Gas1）及其在肠神经元中的信号传导成分平滑受体（Smo）。利用体外从神经球分化而来的肠神经元，我们发现肠轴突生长不受Shh抑制。相反，当Shh作为点源出现时，肠轴突会以Gas1依赖的方式背离它。在可与Smo偶联的Gαi蛋白中，G蛋白αZ（Gnaz）存在于肠轴突中。对Gnaz的敲低和显性负抑制减弱了轴突对Shh的排斥反应，并且Gnaz突变体肠道中含有向中央投射的肠轴突。总之，我们的数据揭示了一种此前未被怀疑的离心管状组织发育机制，并鉴定出一种此前未被识别的Shh在轴突导向中的效应因子。
