Department of Neurobiology, Shanghai Jiaotong University School of Medicine, Shanghai 200025, China.
Brain Res. 2012 Feb 9;1437:1-15. doi: 10.1016/j.brainres.2011.12.008. Epub 2011 Dec 11.
Nogo-A, oligodendrocyte myelin glycoprotein (OMgp) and myelin-associated glycoprotein (MAG) are known as myelin-associated proteins that inhibit axon growth by binding a common receptor, the Nogo66 receptor (NgR). In the CNS, Nogo-A, OMgp and MAG are predominantly expressed by oligodendrocytes. As our previous study revealed that oligodendrocyte progenitor cells (OPCs) did not inhibit neurite outgrowth, it is not clear whether these myelin-associated proteins are expressed in OPCs, and what functions they perform if they are expressed in OPCs. In the present study, with OPCs induced from neural precursor cells (NPCs) derived from rat embryonic spinal cord, and oligodendrocytes differentiated from OPCs, we have observed the expression patterns of Nogo-A, OMgp, MAG and NgR in NPCs, OPCs and oligodendrocytes by immunostaining and western blot assay. We found that Nogo-A could be detected in all tested cells; OMgp could be detected in OPCs and oligodendrocytes, but not in NPCs; MAG was only detected in oligodendrocytes; while NgR could be detected in NPCs and OPCs, but not in oligodendrocytes. These results indicated that the expression pattern of MAG and NgR in OPCs was totally different from that of oligodendrocytes, which might be one of the factors that led to the discrepancy between the two cells in promoting neurite outgrowth. By respectively blocking Nogo-A, OMgp and NgR expressed on OPCs with their corresponding antibodies, we further investigated their roles in the proliferation and differentiation of OPCs, as well as the possible signal pathways involved in. Our results showed that when OPCs were cultured under proliferation condition, blocking Nogo-A, OMgp or NgR did not affect the proliferation of OPCs, but could all significantly prolong their processes. And this effect on OPC processes might involve the phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway. When OPCs were cultured under differentiation condition (containing tri-iodothyronine, T3), blocking Nogo-A, OMgp or NgR could all inhibit the differentiation of OPCs, and this effect might involve the extracellular signal-regulated kinases1/2 (Erk1/2) signaling pathway. These results suggested that under proliferation environment, the functions of Nogo-A, OMgp and NgR expressed in OPCs might be to control the length of processes, thus maintaining the morphology of OPCs. While in differentiation environment, the functions of Nogo-A, OMgp and NgR expressed in OPCs turned to promote the differentiation of OPCs, thus facilitating the maturation of oligodendrocytes. And NgR, as the common receptor for Nogo-A and OMgp, might be the main molecule that mediated these functions in OPCs.
Nogo-A、少突胶质细胞髓鞘糖蛋白（OMgp）和髓鞘相关糖蛋白（MAG）被称为髓鞘相关蛋白，它们通过结合共同的受体——Nogo66 受体（NgR）来抑制轴突生长。在中枢神经系统中，Nogo-A、OMgp 和 MAG 主要由少突胶质细胞表达。由于我们之前的研究表明少突胶质前体细胞（OPCs）不会抑制神经突生长，因此尚不清楚这些髓鞘相关蛋白是否在 OPCs 中表达，如果在 OPCs 中表达，它们具有什么功能。在本研究中，我们利用源自大鼠胚胎脊髓的神经前体细胞（NPCs）诱导的 OPCs 和由 OPCs 分化而来的少突胶质细胞，通过免疫染色和 Western blot 检测，观察了 Nogo-A、OMgp、MAG 和 NgR 在 NPCs、OPCs 和少突胶质细胞中的表达模式。我们发现，Nogo-A 可在所有检测的细胞中被检测到；OMgp 可在 OPCs 和少突胶质细胞中被检测到，但在 NPCs 中未被检测到；MAG 仅在少突胶质细胞中被检测到；而 NgR 可在 NPCs 和 OPCs 中被检测到，但在少突胶质细胞中未被检测到。这些结果表明，MAG 和 NgR 在 OPCs 中的表达模式与少突胶质细胞完全不同，这可能是导致这两种细胞在促进神经突生长方面存在差异的因素之一。通过用相应的抗体分别阻断 OPCs 上表达的 Nogo-A、OMgp 和 NgR，我们进一步研究了它们在 OPCs 增殖和分化中的作用，以及可能涉及的信号通路。我们的结果表明，当 OPCs 在增殖条件下培养时，阻断 Nogo-A、OMgp 或 NgR 并不影响 OPCs 的增殖，但均能显著延长其突起。这种对 OPC 突起的影响可能涉及磷脂酰肌醇 3-激酶（PI3K）/Akt 信号通路。当 OPCs 在分化条件（含三碘甲状腺原氨酸，T3）下培养时，阻断 Nogo-A、OMgp 或 NgR 均可抑制 OPCs 的分化，这种作用可能涉及细胞外信号调节激酶 1/2（Erk1/2）信号通路。这些结果表明，在增殖环境下，OPCs 中表达的 Nogo-A、OMgp 和 NgR 的功能可能是控制突起的长度，从而维持 OPCs 的形态。而在分化环境中，OPCs 中表达的 Nogo-A、OMgp 和 NgR 的功能转变为促进 OPCs 的分化，从而促进少突胶质细胞的成熟。而 NgR 作为 Nogo-A 和 OMgp 的共同受体，可能是介导 OPCs 中这些功能的主要分子。
