Regulation von Apoptose in Ebolavirus-infizierten Zellen

In the present work the regulation of apoptosis in Ebolavirus-Zaire (ZEBOV)-infected cells was analyzed. Studies with infected monkeys revealed that no apoptosis was detected in ZEBOV-infected cells whereas the number of lymphocytes, not infected by ZEBOV, decreases strongly during infection by apoptosis leading to an impairment if the immune reaction. The secretion of (TNF)-related apoptosis-inducing ligand (TRAIL) from infected cells is in discussion as determining factor but in the present work a reduction in TRAIL-mRNA-levels in ZEBOV-infected cells was detected. Aim of this work was to determine if ZEBOV inhibits the induction of apoptosis in infected cells. It could be shown that infection with ZEBOV does not lead to apoptosis in cultured cells and neither activation of initiator caspases 8 and 9 nor executioner caspase 3 was observed. Infection with ZEBOV does not change the levels of anti-apoptotic protein Bcl-2 and pro-apoptotic protein Bax at mRNA- or protein-level, which regulate apoptosis at the mitochondria. Subsequently ZEBOV-infected cells were exposed to multiple apoptotic stimuli, to activate different apoptosis signaling pathways and therefore to determine a potential inhibitory effect of ZEBOV-infection. However in none of the performed experiments an inhibition of apoptosis by ZEBOV was observed. Neither activation of receptor-induced apoptosis by TRAIL-treatment, nor induction of mitochondrial-dependant apoptosis via Campthotecin-treatment or infection with Vesicular-Stomatitis-Virus were inhibited by ZEBOV-infection. Since inhibition of protein kinase R (PKR) by ZEBOV was already described, it was analyzed if this leads to inhibition of apoptosis. Although ZEBOV inhibits activation of PKR induced by infection with Sendaivirus or transfection of poly-IC, induction of apoptosis was not suppressed. Since it cannot be ruled out that the used stimuli also induce PKR-independent apoptosis signaling, the role of PKR inhibition by ZEBOV in apoptosis regulation could not be clarified definitely. A number of viruses induce survival signaling like PI3K/ Akt-signaling to prevent induction of apoptosis. However at no time in ZEBOV-infection activation of Akt in infected cells was observed. According to this, missing apoptosis in ZEBOV-infected cells can not be explained by induction of survival signaling. Overexpression of the ZEBOV glycoprotein GP in cells leads to induction of apoptosis, which could not be prevented by infection with ZEBOV. This suggests that a balanced viral protein expression is critical for prevention of apoptosis. In summary it could be assessed that infection with ZEBOV does not lead to induction of apoptosis but there is as well no inhibition of apoptosis. In fact it seems that ZEBOV avoids the recognition of pathogenic patterns in infected cells so that virus defense signaling is not induced.