Le Shoshona S, Loucks F Alexandra, Udo Hiroshi, Richardson-Burns Sarah, Phelps Reid A, Bouchard Ron J, Barth Holger, Aktories Klaus, Tyler Kenneth L, Kandel Eric R, Heidenreich Kim A, Linseman Daniel A
Research Service, Veterans Affairs Medical Center, Denver, Colorado 80220, USA.
J Neurochem. 2005 Aug;94(4):1025-39. doi: 10.1111/j.1471-4159.2005.03252.x.
Rho GTPases are key transducers of integrin/extracellular matrix and growth factor signaling. Although integrin-mediated adhesion and trophic support suppress neuronal apoptosis, the role of Rho GTPases in neuronal survival is unclear. Here, we have identified Rac as a critical pro-survival GTPase in cerebellar granule neurons (CGNs) and elucidated a death pathway triggered by its inactivation. GTP-loading of Rac1 was maintained in CGNs by integrin-mediated (RGD-dependent) cell attachment and trophic support. Clostridium difficile toxin B (ToxB), a specific Rho family inhibitor, induced a selective caspase-mediated degradation of Rac1 without affecting RhoA or Cdc42 protein levels. Both ToxB and dominant-negative N17Rac1 elicited CGN apoptosis, characterized by cytochrome c release and activation of caspase-9 and -3, whereas dominant-negative N19RhoA or N17Cdc42 did not cause significant cell death. ToxB stimulated mitochondrial translocation and conformational activation of Bax, c-Jun activation, and induction of the BH3-only protein Bim. Similarly, c-Jun activation and Bim induction were observed with N17Rac1. A c-jun N-terminal protein kinase (JNK)/p38 inhibitor, SB203580, and a JNK-specific inhibitor, SP600125, significantly decreased ToxB-induced Bim expression and blunted each subsequent step of the apoptotic cascade. These results indicate that Rac acts downstream of integrins and growth factors to promote neuronal survival by repressing c-Jun/Bim-mediated mitochondrial apoptosis.
Rho GTP酶是整合素/细胞外基质和生长因子信号传导的关键转导分子。尽管整合素介导的黏附作用和营养支持可抑制神经元凋亡，但Rho GTP酶在神经元存活中的作用尚不清楚。在此，我们已确定Rac是小脑颗粒神经元（CGN）中一种关键的促存活GTP酶，并阐明了由其失活引发的死亡途径。通过整合素介导的（RGD依赖性）细胞黏附和营养支持，Rac1的GTP负载在CGN中得以维持。艰难梭菌毒素B（ToxB）是一种特异性的Rho家族抑制剂，可诱导Rac1的选择性半胱天冬酶介导的降解，而不影响RhoA或Cdc42蛋白水平。ToxB和显性负性N17Rac1均可引发CGN凋亡，其特征为细胞色素c释放以及半胱天冬酶-9和-3的激活，而显性负性N19RhoA或N17Cdc42不会导致明显的细胞死亡。ToxB刺激Bax的线粒体易位和构象激活、c-Jun激活以及仅含BH3结构域蛋白Bim的诱导。同样，N17Rac1也可观察到c-Jun激活和Bim诱导。一种c-Jun N端蛋白激酶（JNK）/p38抑制剂SB203580和一种JNK特异性抑制剂SP600125可显著降低ToxB诱导的Bim表达，并使凋亡级联反应的后续各步骤减弱。这些结果表明，Rac在整合素和生长因子的下游发挥作用，通过抑制c-Jun/Bim介导的线粒体凋亡来促进神经元存活。
