Role of the mitochondrial signaling pathway in murine coronavirus-induced oligodendrocyte apoptosis.
A previous study demonstrated that infection of rat oligodendrocytes by mouse hepatitis virus (MHV) resulted in apoptosis, which is caspase dependent (Y. Liu, Y. Cai, and X. Zhang, J. Virol. 77:11952-11963, 2003). Here we determined the involvement of the mitochondrial pathway in MHV-induced oligodendrocyte apoptosis. We found that caspase-9 activity was 12-fold higher in virus-infected cells than in mock-infected cells at 24 h postinfection (p.i.). Pretreatment of cells with a caspase-9 inhibitor completely blocked caspase-9 activation and partially inhibited the apoptosis mediated by MHV infection. Analyses of cytochrome c release further revealed an activation of the mitochondrial apoptotic pathway. Stable overexpression of the two antiapoptotic proteins Bcl-2 and Bcl-xL significantly, though only partially, blocked apoptosis, suggesting that activation of the mitochondrial pathway is partially responsible for the apoptosis. To identify upstream signals, we determined caspase-8 activity, cleavage of Bid, and expression of Bax and Bad by Western blotting. We found a drastic increase in caspase-8 activity and cleavage of Bid at 24 h p.i. in virus-infected cells, suggesting that Bid may serve as a messenger to relay the signals from caspase-8 to mitochondria. However, treatment with a caspase-8 inhibitor only slightly blocked cytochrome c release from the mitochondria. Furthermore, we found that Bax but not Bad was significantly increased at 12 h p.i. in cells infected with both live and UV-inactivated viruses and that Bax activation was partially blocked by treatment with the caspase-8 inhibitor. These results thus establish the involvement of the mitochondrial pathway in MHV-induced oligodendrocyte apoptosis.
Liu
Yin
Y
Department of Microbiology and Immunology, University of Arkansas for Medical Sciences, 4301 W. Markham St., Slot 511, Little Rock, AR 72205-7199, USA.
Pu
Yinghui
Y
Zhang
Xuming
X
D000818:Animals; D017209:Apoptosis; D051023:BH3 Interacting Domain Death Agonist Protein / Q000378:metabolism*; D061945:Caspase Inhibitors; D020169:Caspases / Q000378:metabolism*; D002460:Cell Line; D004789:Enzyme Activation; D047908:Intracellular Signaling Peptides and Proteins; D008928:Mitochondria / Q000502:physiology*; D006517:Murine hepatitis virus / Q000472:pathogenicity* / Q000502:physiology; D009836:Oligodendroglia / Q000473:pathology* / Q000821:virology; D015398:Signal Transduction / Q000502:physiology
D051023:BH3 Interacting Domain Death Agonist Protein; C494815:Bid protein, mouse; D061945:Caspase Inhibitors; D047908:Intracellular Signaling Peptides and Proteins; D020169:Caspases