title: Inhibition of Rac GTPase triggers a c-Jun- and Bim-dependent mitochondrial apoptotic cascade in cerebellar granule neurons.
abstract: 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.
authors.affiliation: Research Service, Veterans Affairs Medical Center, Denver, Colorado 80220, USA.
authors.forename: Shoshona S
authors.initials: SS
authors.lastname: Le
authors.forename: F Alexandra
authors.initials: FA
authors.lastname: Loucks
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Udo
authors.forename: Sarah
authors.initials: S
authors.lastname: Richardson-Burns
authors.forename: Reid A
authors.initials: RA
authors.lastname: Phelps
authors.forename: Ron J
authors.initials: RJ
authors.lastname: Bouchard
authors.forename: Holger
authors.initials: H
authors.lastname: Barth
authors.forename: Klaus
authors.initials: K
authors.lastname: Aktories
authors.forename: Kenneth L
authors.initials: KL
authors.lastname: Tyler
authors.forename: Eric R
authors.initials: ER
authors.lastname: Kandel
authors.forename: Kim A
authors.initials: KA
authors.lastname: Heidenreich
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Linseman
mesh_terms: D000818:Animals; D017209:Apoptosis / Q000187:drug effects / Q000502:physiology*; D051017:Apoptosis Regulatory Proteins; D001426:Bacterial Proteins / Q000494:pharmacology; D001427:Bacterial Toxins / Q000494:pharmacology; D000072224:Bcl-2-Like Protein 11; D002352:Carrier Proteins / Q000502:physiology*; D020169:Caspases / Q000378:metabolism; D002448:Cell Adhesion / Q000502:physiology; D002470:Cell Survival / Q000502:physiology; D002478:Cells, Cultured; D002531:Cerebellum / Q000502:physiology*; D045304:Cytochromes c / Q000378:metabolism; D004789:Enzyme Activation / Q000187:drug effects; D005799:Genes, Dominant; D016023:Integrins / Q000502:physiology; D048031:JNK Mitogen-Activated Protein Kinases / Q000502:physiology; D008565:Membrane Proteins / Q000502:physiology*; D008928:Mitochondria / Q000502:physiology*; D009474:Neurons / Q000502:physiology; D011518:Proto-Oncogene Proteins / Q000502:physiology*; D016755:Proto-Oncogene Proteins c-jun / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D020744:rac GTP-Binding Proteins / Q000378:metabolism; D020830:rac1 GTP-Binding Protein / Q000235:genetics / Q000494:pharmacology / Q000502:physiology*; D020741:rho GTP-Binding Proteins / Q000037:antagonists & inhibitors
chemical_list: D051017:Apoptosis Regulatory Proteins; D001426:Bacterial Proteins; D001427:Bacterial Toxins; D000072224:Bcl-2-Like Protein 11; C000606778:Bcl2l11 protein, rat; D002352:Carrier Proteins; D016023:Integrins; D008565:Membrane Proteins; D011518:Proto-Oncogene Proteins; D016755:Proto-Oncogene Proteins c-jun; C031815:toxB protein, Clostridium difficile; D045304:Cytochromes c; D048031:JNK Mitogen-Activated Protein Kinases; D020169:Caspases; D020744:rac GTP-Binding Proteins; D020830:rac1 GTP-Binding Protein; D020741:rho GTP-Binding Proteins