Cell death suppressors encoded by cytomegalovirus

Human Cytomegalovirus Promotes Survival of Infected Monocytes via a Distinct Temporal Regulation of Cellular Bcl-2 Family Proteins

Monocytes play a key role in the hematogenous dissemination of human cytomegalovirus (HCMV) to target organ systems. To infect monocytes and reprogram them to deliver infectious virus, HCMV must overcome biological obstacles, including the short life span of monocytes and their antiviral proapoptotic response to infection. We have shown that virally induced upregulation of cellular Mcl-1 promotes early survival of HCMV-infected monocytes, allowing cells to overcome an early apoptotic checkpoint at around 48 h postinfection (hpi). Here, we demonstrate an HCMV-dependent shift from Mcl-1 as the primary antiapoptotic player to the related protein, Bcl-2, later during infection. Bcl-2 was upregulated in HCMV-infected monocytes beginning at 48 hpi. Treatment with the Bcl-2 antagonist ABT-199 only reduced the prosurvival effects of HCMV in target monocytes beginning at 48 hpi, suggesting that Mcl-1 controls survival prior to 48 hpi, while Bcl-2 promotes survival after 48 hpi. Although Bcl-2 was upregulated following viral binding/signaling through cellular integrins (compared to Mcl-1, which is upregulated through binding/activation of epidermal growth factor receptor [EGFR]), it functioned similarly to Mcl-1, adopting the early role of Mcl-1 in preventing caspase-3 cleavage/activation. This distinct, HCMV-induced shift from Mcl-1 to Bcl-2 occurs in response to a cellular upregulation of proapoptotic Bax, as small interfering RNA (siRNA)-mediated knockdown of Bax reduced the upregulation of Bcl-2 in infected monocytes and rescued the cells from the apoptotic effects of Bcl-2 inhibition. Our data demonstrate a distinct survival strategy whereby HCMV induces a biphasic regulation of cellular Bcl-2 proteins to promote host cell survival, leading to viral dissemination and the establishment of persistent HCMV infection.

IMPORTANCE

Hematogenous dissemination of HCMV via infected monocytes is a crucial component of the viral survival strategy and is required for the establishment of persistent infection and for viral spread to additional hosts. Our system of infected primary human blood monocytes provides us with an opportunity to answer specific questions about viral spread and persistence in in vivo-relevant myeloid cells that cannot be addressed with the more traditionally used replication-permissive cells. Our goal in examining the mechanisms whereby HCMV reprograms infected monocytes to promote viral dissemination is to uncover new targets for therapeutic intervention that would disrupt key viral survival and persistence strategies. Because of this important role in maintaining survival of HCMV-infected monocytes, our new data on the role of Bcl-2 regulation during viral infection represents a promising molecular target for mitigating viral spread and persistence.

The human cytomegalovirus protein UL37 exon 1 associates with internal lipid rafts

The human cytomegalovirus (HCMV) protein UL37 exon 1 (pUL37x1), also known as viral mitochondrion-localized inhibitor of apoptosis (vMIA), sequentially traffics from the endoplasmic reticulum (ER) through mitochondrion-associated membranes (MAMs) to the outer mitochondrial membrane (OMM), where it robustly inhibits apoptosis. Here, we report the association of pUL37x1/vMIA with internal lipid rafts (LRs) in the ER/MAM. The MAM, which serves as a site for lipid transfer and calcium signaling to mitochondria, is enriched in detergent-resistant membrane (DRM)-forming lipids, including cholesterol and ceramide, which are found in lower concentrations in the bulk ER. Sigma 1 receptor (Sig-1R), a MAM chaperone affecting calcium signaling to mitochondria, is anchored in the MAM by its LR association. Because of its trafficking through the MAM and partial colocalization with Sig-1R, we tested whether pUL37x1/vMIA associates with MAM LRs. Extraction with methyl-β-cyclodextrin (MβCD) removed pUL37x1/vMIA from lysed but not intact cells, indicating its association with internal LRs. Furthermore, the isolation of DRMs from purified intracellular organelles independently verified the localization of pUL37x1/vMIA within ER/MAM LRs. However, pUL37x1/vMIA was not detected in DRMs from mitochondria. pUL37x1/vMIA associated with LRs during all temporal phases of HCMV infection, indicating the likely importance of this location for HCMV growth. Although detected during its sequential trafficking to the OMM, the pUL37x1/vMIA LR association was independent of its mitochondrial targeting signals. Rather, it was dependent upon cholesterol binding. These studies suggest a conserved ability of UL37 proteins to interact with cholesterol and LRs, which is functionally distinguishable from their sequential trafficking to mitochondria.

Latency and reactivation of human cytomegalovirus

Human cytomegalovirus (HCMV) persists as a subclinical, lifelong infection in the normal human host, maintained at least in part by its carriage in the absence of detectable infectious virus – the hallmark of latent infection. Reactivation from latency in immunocompromised individuals, in contrast, often results in serious disease. Latency and reactivation are defining characteristics of the herpesviruses and key to understanding their biology. However, the precise cellular sites in which HCMV is carried and the mechanisms regulating its latency and reactivation during natural infection remain poorly understood. This review will detail our current knowledge of where HCMV is carried in healthy individuals, which viral genes are expressed upon carriage of the virus and what effect this has on cellular gene expression. It will also address the accumulating evidence suggesting that reactivation of HCMV from latency appears to be linked intrinsically to the differentiation status of the myeloid cell, and how the cellular mechanisms that normally control host gene expression play a critical role in the differential regulation of viral gene expression during latency and reactivation.

Cell death suppression by cytomegaloviruses

Cytomegaloviruses (CMVs), a subset of betaherpesviruses, employ multiple strategies to suppress apoptosis in infected cells and thus to delay their death. Human cytomegalovirus (HCMV) encodes at least two proteins that directly interfere with the apoptotic signaling pathways, viral inhibitor of caspase-8-induced apoptosis vICA (pUL36), and mitochondria-localized inhibitor of apoptosis vMIA (pUL37 x 1). vICA associates with pro-caspase-8 and appears to block its recruitment to the death-inducing signaling complex (DISC), a step preceding caspase-8 activation. vMIA binds and sequesters Bax at mitochondria, and interferes with BH3-only-death-factor/Bax-complex-mediated permeabilization of mitochondria. vMIA does not seem to either interact with Bak, a close structural and functional homologue of Bax, or to suppress Bak-mediated permeabilization of mitochondria and Bak-mediated apoptosis. All sequenced betaherpesviruses, including CMVs, encode close homologues of vICA, and those vICA homologues that have been tested, were found to be functional cell death suppressors. Overt sequence homologues of vMIA were found only in the genomes of primate CMVs, but recent observations made with murine CMV (MCMV) indicate that non-primate CMVs may also encode a cell death suppressor functionally resembling vMIA. The exact physiological roles and relative contributions of vMIA and vICA in suppressing death of CMV-infected cells in vivo have not been elucidated. There is strong evidence that the cell death suppressing function of vMIA is indispensable, and that vICA is dispensable for replication of HCMV. In addition to suppressed caspase-8 activation and sequestered Bax, CMV-infected cells display several other phenomena, less well characterized, that may diminish, directly or indirectly the extent of cell death.