cytochrome-c-t and Rotavirus-Infections

Viruses have evolved to encode multifunctional proteins to control the intricate cellular signaling pathways by using very few viral proteins. Rotavirus is known to express six nonstructural and six structural proteins. Among them, NSP4 is the enterotoxin, known to disrupt cellular Ca(2+) homeostasis by translocating to endoplasmic reticulum. In this study, we have observed translocation of NSP4 to mitochondria resulting in dissipation of mitochondrial membrane potential during virus infection and NSP4 overexpression. Furthermore, transfection of the N- and C-terminal truncated NSP4 mutants followed by analyzing NSP4 localization by immunofluorescence microscopy identified the 61-83-amino acid region as the shortest mitochondrial targeting signal. NSP4 exerts its proapoptotic effect by interacting with mitochondrial proteins adenine nucleotide translocator and voltage-dependent anion channel, resulting in dissipation of mitochondrial potential, release of cytochrome c from mitochondria, and caspase activation. During early infection, apoptosis activation by NSP4 was inhibited by the activation of cellular survival pathways (PI3K/AKT), because PI3K inhibitor results in early induction of apoptosis. However, in the presence of both PI3K inhibitor and NSP4 siRNA, apoptosis was delayed suggesting that the early apoptotic signal is initiated by NSP4 expression. This proapoptotic function of NSP4 is balanced by another virus-encoded protein, NSP1, which is implicated in PI3K/AKT activation because overexpression of both NSP4 and NSP1 in cells resulted in reduced apoptosis compared with only NSP4-expressing cells. Overall, this study reports on the mechanism by which enterotoxin NSP4 exerts cytotoxicity and the mechanism by which virus counteracts it at the early stage for efficient infection.

Topics: Animals; Apoptosis; Calcium; Caspases; Cytochromes c; Enterotoxins; Enzyme Activation; Gene Expression Regulation, Viral; Glycoproteins; Haplorhini; HeLa Cells; Humans; Mice; Mice, Inbred BALB C; Mitochondria; Mitochondrial ADP, ATP Translocases; Mutation; Phosphatidylinositol 3-Kinases; Proto-Oncogene Proteins c-akt; Rotavirus; Rotavirus Infections; Toxins, Biological; Viral Nonstructural Proteins

Rotaviruses are the leading cause of infantile viral gastroenteritis worldwide. Mature enterocytes of the small intestine infected by rotavirus undergo apoptosis, and their replacement by less differentiated dividing cells probably leads to defective absorptive function of the intestinal epithelium, which, in turn, contributes to osmotic diarrhea and rotavirus pathogenesis. Here we show that infection of MA104 cells by the simian rhesus rotavirus strain RRV induced caspase-3 activation, DNA fragmentation, and cleavage of poly(ADP-ribose) polymerase; all three phenomena are features of apoptosis. RRV induced the release of cytochrome c from mitochondria to the cytosol, indicating that the mitochondrial apoptotic pathway was activated. RRV infection of MA104 cells activated Bax, a proapoptotic member of the Bcl-2 family, as revealed by its conformational change. Most importantly, Bax-specific small interfering RNAs partially inhibited cytochrome c release in RRV-infected cells. Thus, mitochondrial dysfunction induced by rotavirus is Bax dependent. Apoptosis presumably leads to impaired intestinal functions, so our findings contribute to improving our understanding of rotavirus pathogenesis at the cellular level.

Topics: Animals; Apoptosis; bcl-2-Associated X Protein; Blotting, Western; Caspase 3; Cell Line; Cytochromes c; DNA Fragmentation; Enzyme Activation; Enzyme-Linked Immunosorbent Assay; Immunoprecipitation; Macaca mulatta; Mitochondria; Poly(ADP-ribose) Polymerases; RNA, Small Interfering; Rotavirus; Rotavirus Infections; Signal Transduction