Baculoviruses and apoptosis

Apoptosis is a conserved mechanism of programmed cell death that is essential for development and homeostasis of animals. In addition, apoptosis is an effective strategy used by vertebrate and invertebrate hosts to counter viral infection, since the induction of programmed cell death in virus-infected cells efficiently prevents viral amplification and spreading. Curiously, apoptosis has not been involved in antiviral defences in Drosophila so far. This may reflect the limited number of viruses that have been tested. Indeed, apoptosis has mostly been associated as an antiviral defence against DNA virus infections, and these viruses have not yet been identified in Drosophila.

Insect DNA viruses, such as baculoviruses, have evolved methods to bypass this antiviral defence by expressing suppressors of apoptosis. A characteristic example of such a suppressor is the cas-pase inhibitor p35, encoded by the baculovirus AcMNPV. Expression of p35 protein in infected cells prevents apoptosis, increases virus titre in the insect tissues and allows successful infection of the host (Clarke and Clem, 2003). Infectivity of p35 mutant viruses is greatly reduced, thus revealing the important role of apoptosis as an antiviral defence mechanism in insects. Subsequent efforts to identify other suppressors of apoptosis led to the identification of the family of inhibitors of apoptosis (IAP) (Srinivasula and Ashwell, 2008). Unlike p35, baculoviral IAPs have cellular orthologues, which interact with caspases and inhibit them. Of note, the Drosophila genome encodes two IAP molecules, DIAP1 and DIAP2, one of which (DIAP2) is a component of the Imd pathway.

Microarray and proteomic analysis suggest that apoptosis could be involved in antiviral defences in flies. For example, the gene encoding the caspase Damm appears to be upregulated following DCV infection in flies (Dostert et al,, 2005). On the other hand, experiments in tissue culture Drosophila Schneider's DL1 cells revealed that FHV infection induces expression of croquemort (involved in the uptake of apoptotic bodies) and represses expression of DIAP1 (Go et al, 2006). Interestingly, FHV was indeed shown to trigger apoptosis in DL1 cells. FHV inhibits host-cell protein synthesis, which leads to the rapid depletion of the short-lived DIAP1 protein, and activation of the caspases DRONC and DrICE, leading to apoptosis. Importantly, however, apoptosis seems to be of little consequence for FHV multiplication, which questions the importance of programmed cell death in the control of FHV infection (Settles and Friesen, 2008).

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