Palandri A, Salvador V R, Wojnacki J, Vivinetto A L, Schnaar R L, Lopez P H H
Laboratorio de Neurobiología, Instituto de Investigación Médica Mercedes y Martin Ferreyra, INIMEC-CONICET-Universidad Nacional de Córdoba, Córdoba, Argentina.
Department of Pharmacology, Johns Hopkins University School of Medicine, Baltimore, MD, USA.
Cell Death Dis. 2015 Sep 3;6(9):e1876. doi: 10.1038/cddis.2015.228.
Myelin-associated glycoprotein (MAG) is a minor constituent of nervous system myelin, selectively expressed on the periaxonal myelin wrap. By engaging multiple axonal receptors, including Nogo-receptors (NgRs), MAG exerts a nurturing and protective effect the axons it ensheaths. Pharmacological activation of NgRs has a modulatory role on p75(NTR)-dependent postnatal apoptosis of motoneurons (MNs). However, it is not clear whether this reflects a physiological role of NgRs in MN development. NgRs are part of a multimeric receptor complex, which includes p75(NTR), Lingo-1 and gangliosides. Upon ligand binding, this multimeric complex activates RhoA/ROCK signaling in a p75(NTR)-dependent manner. The aim of this study was to analyze a possible modulatory role of MAG on MN apoptosis during postnatal development. A time course study showed that Mag-null mice suffer a loss of MNs during the first postnatal week. Also, these mice exhibited increased susceptibility in an animal model of p75(NTR)-dependent MN apoptosis induced by nerve-crush injury, which was prevented by treatment with a soluble form of MAG (MAG-Fc). The protective role of MAG was confirmed in in vitro models of p75(NTR)-dependent MN apoptosis using the MN1 cell line and primary cultures. Lentiviral expression of shRNA sequences targeting NgRs on these cells abolished protection by MAG-Fc. Analysis of RhoA activity using a FRET-based RhoA biosensor showed that MAG-Fc activates RhoA. Pharmacological inhibition of p75(NTR)/RhoA/ROCK pathway, or overexpression of a p75(NTR) mutant unable to activate RhoA, completely blocked MAG-Fc protection against apoptosis. The role of RhoA/ROCK signaling was further confirmed in the nerve-crush model, where pretreatment with ROCK inhibitor Y-27632 blocked the pro-survival effect of MAG-Fc. These findings identify a new protective role of MAG as a modulator of apoptosis of MNs during postnatal development by a mechanism involving the p75(NTR)/RhoA/ROCK signaling pathway. Also, our results highlight the relevance of the nurture/protective effects of myelin on neurons.
髓鞘相关糖蛋白（MAG）是神经系统髓鞘的一种次要成分，选择性地表达于轴突周围的髓鞘膜上。通过与多种轴突受体结合，包括Nogo受体（NgRs），MAG对其包裹的轴突发挥滋养和保护作用。NgRs的药理学激活对运动神经元（MNs）依赖p75（NTR）的出生后凋亡具有调节作用。然而，尚不清楚这是否反映了NgRs在MN发育中的生理作用。NgRs是多聚体受体复合物的一部分，该复合物包括p75（NTR）、Lingo-1和神经节苷脂。配体结合后，这种多聚体复合物以p75（NTR）依赖的方式激活RhoA/ROCK信号通路。本研究的目的是分析MAG在出生后发育过程中对MN凋亡的可能调节作用。一项时间进程研究表明，MAG基因敲除小鼠在出生后的第一周内MNs数量减少。此外，在由神经挤压损伤诱导的依赖p75（NTR）的MN凋亡动物模型中，这些小鼠表现出更高的易感性，而用可溶性形式的MAG（MAG-Fc）治疗可预防这种情况。使用MN1细胞系和原代培养物在依赖p75（NTR）的MN凋亡体外模型中证实了MAG的保护作用。在这些细胞上慢病毒表达靶向NgRs的shRNA序列消除了MAG-Fc的保护作用。使用基于FRET的RhoA生物传感器分析RhoA活性表明，MAG-Fc激活RhoA。对p75（NTR）/RhoA/ROCK通路的药理学抑制，或过表达无法激活RhoA的p75（NTR）突变体，完全阻断了MAG-Fc对凋亡细胞的保护作用。RhoA/ROCK信号通路的作用在神经挤压模型中得到进一步证实，在该模型中，用ROCK抑制剂Y-27632预处理可阻断MAG-Fc的促存活作用。这些发现确定了MAG作为出生后发育过程中MN凋亡调节因子的一种新的保护作用，其机制涉及p75（NTR）/RhoA/ROCK信号通路。此外，我们的结果突出了髓鞘对神经元的滋养/保护作用的相关性。
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