Background
Human herpesvirus 6 (HHV-6) is a T-lymphtropic and neurotropic virus that can infect various types of cells. Sequential studies reported that apoptosis of glia and neurons induced by HHV-6 might act a potential trigger for some central nervous system (CNS) diseases. HHV-6 is involved in the pathogenesis of encephalitis, multiple sclerosis (MS) and fatigue syndrome. However, the mechanisms responsible for the apoptosis of infected CNS cells induced by HHV-6 are poorly understood. In this study, we investigated the cell death processes of primary human fetal astrocytes (PHFAs) during productive HHV-6A infection and the underlying mechanisms.
Results
HHV-6A can cause productive infection in primary human fetal astrocytes. Annexin V-PI staining and electron microscopic analysis indicated that HHV-6A was an inducer of apoptosis. The cell death was associated with activation of caspase-3 and cleavage of poly (ADP-ribose) polymerase (PARP), which is known to be an important substrate for activated caspase-3. Caspase-8 and -9 were also significantly activated in HHV-6A-infected cells. Moreover, HHV-6A infection led to Bax up-regulation and Bcl-2 down-regulation. HHV-6A infection increased the release of Smac/Diablo, AIF and cytochrome c from mitochondria to cytosol, which induced apoptosis via the caspase-dependent and -independent pathways. In addition, we also found that anti-apoptotic factors such as IAPs and NF-κB decreased in HHV-6A infected PHFAs.
Conclusion
This is the first demonstration of caspase-dependent and -independent apoptosis in HHV-6A-infected glial cells. These findings would be helpful in understanding the mechanisms of CNS diseases caused by HHV-6.
Keywords: Apoptosis; Human herpesvirus 6A; Primary human fetal astrocyte; Caspase
Background
Human herpesvirus 6 (HHV-6), a member of the beta herpesvirus family, is a T-lymphotropic virus and the causal agent of exanthema subitum. In recent studies, HHV-6 has been detected in numerous central nervous system (CNS) diseases including encephalitis, multiple sclerosis, temporal lobe epilepsy and glioma. These findings suggest that HHV-6 may be associated with some CNS diseases. In vitro, HHV-6 has been shown to infect human glial cells (microglia, oligodendrocytes and astrocytes) and induce apoptosis. However, the molecular mechanisms of apoptosis induced by HHV-6 in glial cells are not fully understood as yet.
Apoptosis, a programmed suicide death of cells, which is characterized by chromatin condensation, DNA fragmentation, membrane blebbing, and cell shrinkage, can occur through the intrinsic and extrinsic casepase pathways. Caspases, a family of cysteine proteases, regulate the initiation and the final execution of apoptosis in receptor-mediated and mitochondria-mediated pathways. In the receptor-mediated pathway, caspase-8 is the initiator caspase that can directly activate the final executioner caspase-3. In the mitochondria-mediated pathway, mitochondria release several pro-apoptotic factors including cytochrome c, Smac/Diablo, and apoptosis-inducing factor (AIF) into the cytosol. Cytosolic cytochrome c binds with apoptotic protease activating factor 1 (APAF1) to produce active caspase-9 and subsequently active caspase-3 for caspase-dependent apoptosis. Samc/Diablo is an antagonistic protein for inhibitor of apoptosis proteins (IAPs), promotes apoptosis along with cytochrome c by activating caspases. Mitochondria-mediated apoptosis may also occur in caspase-independently way after mitochondrial release of AIF that is translocated to the nucleus for induction of chromatin condensation and DNA fragmentation.
In the present study, we investigated the effect and molecular mechanism of HHV-6A inducing apoptosis in primary human fetal astrocytes (PHFAs). We found that HHV-6A induced apoptosis in PHFAs through both caspase-dependent and -independent apoptotic pathways. In addition, our finding also demonstrated that HHV-6A could promote cell death by suppressing IAPs and NF-κB-mediated anti-apoptosis pathways. To our knowledge, this is the first demonstration of the mechanisms of apoptosis induced by HHV-6A in astrocytes.