Tang S, Qiu J, Nikulina E, Filbin M T
Biology Department, Hunter College, 695 Park Avenue, New York, New York, 10021, USA.
Mol Cell Neurosci. 2001 Sep;18(3):259-69. doi: 10.1006/mcne.2001.1020.
The adult, mammalian CNS does not regenerate after injury largely because of a glial scar and inhibitors of regeneration in myelin. To date, two myelin inhibitors, myelin-associated glycoprotein (MAG) and Nogo, both transmembrane proteins, have been identified. No secreted inhibitors of regeneration have been described. However, a proteolytic fragment of MAG (dMAG), consisting of the entire extracellular domain, is readily released from myelin and is found in vivo. Here, we show, first, that a soluble, chimeric form of MAG (MAG-Fc), when secreted from CHO cells in a collagen gel and hence in the absence of a fixed substrate, inhibits/deflects neurite outgrowth from P6 dorsal root ganglion (DRG) neurons. This inhibition was blocked when a MAG monoclonal antibody was included in the gel and a control chimera sialoadhesin-Fc (Sn-Fc), which, like MAG, binds neurons in a sialic acid-dependent manner but does not inhibit axonal growth, had no effect. Using the same assay system we showed that factors secreted from damaged white matter inhibited/deflected neurite outgrowth. This inhibition was neutralized when a MAG monoclonal antibody was included in the gel and there was no inhibition when white matter from a MAG knockout mouse was used. Factors secreted from damaged white matter from wild-type mice had no effect on neurite outgrowth from E18 DRG neurons. These results show that factors secreted from damaged white matter inhibit axonal regeneration and that the majority of inhibitory activity can be accounted for by dMAG. Thus, released dMAG is likely to play an important role in preventing regeneration, immediately after injury before the glial scar forms.
成年哺乳动物的中枢神经系统在损伤后无法再生，这主要是由于胶质瘢痕以及髓鞘中的再生抑制因子。迄今为止，已鉴定出两种髓鞘抑制因子，即髓鞘相关糖蛋白（MAG）和Nogo，它们均为跨膜蛋白。尚未有分泌型再生抑制因子的相关报道。然而，MAG的一个蛋白水解片段（dMAG），由整个细胞外结构域组成，很容易从髓鞘中释放出来，并在体内被发现。在此，我们首先表明，一种可溶性的、嵌合形式的MAG（MAG-Fc），当从CHO细胞分泌到胶原凝胶中，即在没有固定底物的情况下，会抑制/使P6背根神经节（DRG）神经元的神经突生长发生偏向。当凝胶中加入MAG单克隆抗体时，这种抑制作用被阻断，而对照嵌合体唾液酸粘附素-Fc（Sn-Fc），它与MAG一样，以唾液酸依赖的方式结合神经元，但不抑制轴突生长，没有产生影响。使用相同的检测系统，我们发现受损白质分泌的因子会抑制/使神经突生长发生偏向。当凝胶中加入MAG单克隆抗体时，这种抑制作用被中和，而使用MAG基因敲除小鼠的白质时则没有抑制作用。野生型小鼠受损白质分泌的因子对E18 DRG神经元的神经突生长没有影响。这些结果表明，受损白质分泌的因子会抑制轴突再生，并且大部分抑制活性可由dMAG解释。因此，释放的dMAG可能在损伤后胶质瘢痕形成之前立即阻止再生过程中发挥重要作用。
