Loss of the Human Cytomegalovirus US16 Protein Abrogates Virus Entry into Endothelial and Epithelial Cells by Reducing the Virion Content of the Pentamer

Ligands and receptors in human cytomegalovirus entry: Current therapies and new directions

The demand for human cytomegalovirus (HCMV) vaccines was first raised by a committee convened during the 1990s. A comprehensive investigation into the mechanism of viral infection supports the prioritization of developing drugs or vaccines that specifically target receptors and ligands involved in the infection process. As primary targets for neutralizing antibodies to combat HCMV, viral ligands (trimer, pentamer, and glycoprotein B) have crucial roles and exhibit substantial antiviral potential, which could be exploited for breakthroughs in antiviral research.

The US21 viroporin of human cytomegalovirus stimulates cell migration and adhesion

The human cytomegalovirus (HCMV) US12 gene family contributes to virus-host interactions by regulating the virus' cell tropism and its evasion of host innate immune responses. US21, one of the 10 US12 genes (US12-US21), is a descendant of a captured cellular transmembrane BAX inhibitor motif-containing gene. It encodes a 7TMD endoplasmic reticulum (ER)-resident viroporin (pUS21) capable of reducing the Ca2+ content of ER stores, which, in turn, protects cells against apoptosis. Since regulation of Ca2+ homeostasis affects a broad range of cellular responses, including cell motility, we investigated whether pUS21 might also interfere with this cytobiological consequence of Ca2+ signaling. Indeed, deletion of the US21 gene impaired the ability of HCMV-infected cells to migrate, whereas expression of US21 protein stimulated cell migration and adhesion, as well as focal adhesion (FA) dynamics, in a way that depended on its ability to manipulate ER Ca2+ content. Mechanistic studies revealed pUS21-mediated cell migration to involve calpain 2 activation since its inhibition prevented the viroporin's effects on cell motility. Pertinently, pUS21 expression stimulated a store-operated Ca2+ entry (SOCE) mechanism that may determine the activation of calpain 2 by promoting Ca2+ entry. Furthermore, pUS21 was observed to interact with talin-1, a calpain 2 substrate, and crucial protein component of FA complexes. A functional consequence of this interaction was confirmed by talin-1 knockdown, which abrogated the pUS21-mediated increase in cell migration. Together, these results indicate the US21-encoded viroporin to be a viral regulator of cell adhesion and migration in the context of HCMV infection. IMPORTANCE Human cytomegalovirus (HCMV) is an opportunistic pathogen that owes part of its success to the capture, duplication, and tuning of cellular genes to generate modern viral proteins which promote infection and persistence in the host by interfering with many cell biochemical and physiological pathways. The US21 viral protein provides an example of this evolutionary strategy: it is a cellular-derived calcium channel that manipulates intracellular calcium homeostasis to confer edges to HCMV replication. Here, we report on the characterization of a novel function of the US21 protein as a viral regulator of cell migration and adhesion through mechanisms involving its calcium channel activity. Characterization of HCMV multifunctional regulatory proteins, like US21, supports the better understanding of viral pathogenesis and may open avenues for the design of new antiviral strategies that exploit their functions.

Insights into the Transcriptome of Human Cytomegalovirus: A Comprehensive Review

Human cytomegalovirus (HCMV) is a widespread pathogen that poses significant risks to immunocompromised individuals. Its genome spans over 230 kbp and potentially encodes over 200 open-reading frames. The HCMV transcriptome consists of various types of RNAs, including messenger RNAs (mRNAs), long non-coding RNAs (lncRNAs), circular RNAs (circRNAs), and microRNAs (miRNAs), with emerging insights into their biological functions. HCMV mRNAs are involved in crucial viral processes, such as viral replication, transcription, and translation regulation, as well as immune modulation and other effects on host cells. Additionally, four lncRNAs (RNA1.2, RNA2.7, RNA4.9, and RNA5.0) have been identified in HCMV, which play important roles in lytic replication like bypassing acute antiviral responses, promoting cell movement and viral spread, and maintaining HCMV latency. CircRNAs have gained attention for their important and diverse biological functions, including association with different diseases, acting as microRNA sponges, regulating parental gene expression, and serving as translation templates. Remarkably, HCMV encodes miRNAs which play critical roles in silencing human genes and other functions. This review gives an overview of human cytomegalovirus and current research on the HCMV transcriptome during lytic and latent infection.

HCMV-encoded viral protein US12 promotes autophagy by inducing autophagy flux

The human cytomegalovirus (HCMV)-encoded US12 gene family is a group of ten predicted seven-transmembrane domain proteins that are structurally similar to G-protein-coupled receptors or transmembrane Bax inhibitor-1 motif-containing proteins; however, the roles of US12 family proteins in virus-host interactions remain to be discovered. Here, we suggest a new function of the US12 protein in regulating cellular autophagy. US12 is predominantly located to the lysosome and interacts with the lysosomal membrane protein 2 (LAMP2). A liquid chromatography-mass spectrometry (MS)/MS-based targeted proteomics analysis shows that US12 is tightly correlated with autophagy. US12 induces autophagy via upregulating ULK1 phosphorylation and subsequent LC3-II conversion, thereby accelerating autophagic flux. Moreover, HeLa cells overexpressing US12 displays intense LC3-specific staining and autolysosome formation even under nutrient-sufficient conditions. Furthermore, the physical interaction of p62/SQSTM1 with US12 is involved in the resistance to the degradation of p62/SQSTM1 by autophagy, despite the induction of both autolysosome formation and autophagic flux. Although the effect of US12 expression in HCMV infection on autophagy remains undetermined, these findings provide new insights into the viral drivers of host autophagy during HCMV evolution and pathogenesis.

Neutralization Epitopes in Trimer and Pentamer Complexes Recognized by Potent Cytomegalovirus-Neutralizing Human Monoclonal Antibodies

Human cytomegalovirus (HCMV) infects 36% to almost 100% of adults and causes severe complications only in immunocompromised individuals. HCMV viral surface trimeric (gH/gL/gO) and pentameric (gH/gL/UL128/UL130/UL131A) complexes play important roles in HCMV infection and tropism. Here, we isolated and identified a total of four neutralizing monoclonal antibodies (MAbs) derived from HCMV-seropositive blood donors. Based on their reactivity to HCMV trimer and pentamer, these MAbs can be divided into two groups. MAbs PC0012, PC0014, and PC0035 in group 1 bind both trimer and pentamer and neutralize CMV by interfering with the postattachment steps of CMV entering into cells. These three antibodies recognize antigenic epitopes clustered in a similar area, which are overlapped by the epitope recognized by the known neutralizing antibody MSL-109. MAb PC0034 in group 2 binds only to pentamer and neutralizes CMV by blocking the binding of pentamer to cells. Epitope mapping using pentamer mutants showed that amino acid T94 of the subunit UL128 and K27 of UL131A on the pentamer are key epitope-associated residues recognized by PC0034. This study provides new evidence and insight information on the importance of the development of the CMV pentamer as a CMV vaccine. In addition, these newly identified potent CMV MAbs can be attractive candidates for development as antibody therapeutics for the prevention and treatment of HCMV infection. IMPORTANCE The majority of the global population is infected with HCMV, but severe complications occur only in immunocompromised individuals. In addition, CMV infection is a major cause of birth defects in newborns. Currently, there are still no approved prophylactic vaccines or therapeutic monoclonal antibodies (MAbs) for clinical use against HCMV infection. This study identified and characterized a panel of four neutralizing MAbs targeting the HCMV pentamer complex with specific aims to identify a key protein(s) and antigenic epitopes in the HCMV pentamer complex. The study also explored the mechanism by which these newly identified antibodies neutralize HCMV in order to design better HCMV vaccines focusing on the pentamer and to provide attractive candidates for the development of effective cocktail therapeutics for the prevention and treatment of HCMV infection.

Intermittent bulk release of human cytomegalovirus

Human Cytomegalovirus (HCMV) can infect a variety of cell types by using virions of varying glycoprotein compositions. It is still unclear how this diversity is generated, but spatio-temporally separated envelopment and egress pathways might play a role. So far, one egress pathway has been described in which HCMV particles are individually enveloped into small vesicles and are subsequently exocytosed continuously. However, some studies have also found enveloped virus particles inside multivesicular structures but could not link them to productive egress or degradation pathways. We used a novel 3D-CLEM workflow allowing us to investigate these structures in HCMV morphogenesis and egress at high spatio-temporal resolution. We found that multiple envelopment events occurred at individual vesicles leading to multiviral bodies (MViBs), which subsequently traversed the cytoplasm to release virions as intermittent bulk pulses at the plasma membrane to form extracellular virus accumulations (EVAs). Our data support the existence of a novel bona fide HCMV egress pathway, which opens the gate to evaluate divergent egress pathways in generating virion diversity.

Deciphering the Potential Coding of Human Cytomegalovirus: New Predicted Transmembrane Proteome

CMV is a major cause of morbidity and mortality in immunocompromised individuals that will benefit from the availability of a vaccine. Despite the efforts made during the last decade, no CMV vaccine is available. An ideal CMV vaccine should elicit a broad immune response against multiple viral antigens including proteins involved in virus-cell interaction and entry. However, the therapeutic use of neutralizing antibodies targeting glycoproteins involved in viral entry achieved only partial protection against infection. In this scenario, a better understanding of the CMV proteome potentially involved in viral entry may provide novel candidates to include in new potential vaccine design. In this study, we aimed to explore the CMV genome to identify proteins with putative transmembrane domains to identify new potential viral envelope proteins. We have performed in silico analysis using the genome sequences of nine different CMV strains to predict the transmembrane domains of the encoded proteins. We have identified 77 proteins with transmembrane domains, 39 of which were present in all the strains and were highly conserved. Among the core proteins, 17 of them such as UL10, UL139 or US33A have no ascribed function and may be good candidates for further mechanistic studies.

Mutagenesis of Human Cytomegalovirus Glycoprotein L Disproportionately Disrupts gH/gL/gO over gH/gL/pUL128-131

Cell-free and cell-to-cell spread of herpesviruses involves a core fusion apparatus comprised of the fusion protein glycoprotein B (gB) and the regulatory factor gH/gL. The human cytomegalovirus (HCMV) gH/gL/gO and gH/gL/pUL128-131 facilitate spread in different cell types. The gO and pUL128-131 components bind distinct receptors, but how the gH/gL portions of the complexes functionally compare is not understood. We previously characterized a panel of gL mutants by transient expression and showed that many were impaired for gH/gL-gB-dependent cell-cell fusion but were still able to form gH/gL/pUL128-131 and induce receptor interference. Here, the gL mutants were engineered into the HCMV BAC clones TB40/e-BAC4 (TB), TR, and Merlin (ME), which differ in their utilization of the two complexes for entry and spread. Several of the gL mutations disproportionately impacted gH/gL/gO-dependent entry and spread over gH/gL/pUL128-131 processes. The effects of some mutants could be explained by impaired gH/gL/gO assembly, but other mutants impacted gH/gL/gO function. Soluble gH/gL/gO containing the L201 mutant failed to block HCMV infection despite unimpaired binding to PDGFRα, indicating the existence of other important gH/gL/gO receptors. Another mutant (L139) enhanced the gH/gL/gO-dependent cell-free spread of TR, suggesting a "hyperactive" gH/gL/gO. Recently published crystallography and cryo-electron microscopy studies suggest structural conservation of the gH/gL underlying gH/gL/gO and gH/gL/pUL128-131. However, our data suggest important differences in the gH/gL of the two complexes and support a model in which gH/gL/gO can provide an activation signal for gB. IMPORTANCE The endemic betaherpesvirus HCMV circulates in human populations as a complex mixture of genetically distinct variants, establishes lifelong persistent infections, and causes significant disease in neonates and immunocompromised adults. This study capitalizes on our recent characterizations of three genetically distinct HCMV BAC clones to discern the functions of the envelope glycoprotein complexes gH/gL/gO and gH/gL/pUL128-13, which are promising vaccine targets that share the herpesvirus core fusion apparatus component, gH/gL. Mutations in the shared gL subunit disproportionally affected gH/gL/gO, demonstrating mechanistic differences between the two complexes, and may provide a basis for more refined evaluations of neutralizing antibodies.

The Human Cytomegalovirus Protein UL116 Interacts with the Viral Endoplasmic-Reticulum-Resident Glycoprotein UL148 and Promotes the Incorporation of gH/gL Complexes into Virions

Heterodimers of glycoproteins H (gH) and L (gL) comprise a basal element of the viral membrane fusion machinery conserved across herpesviruses. In human cytomegalovirus (HCMV), the glycoprotein UL116 assembles onto gH at a position similar to that occupied by gL, forming a heterodimer that is incorporated into virions. Here, we show that UL116 promotes the expression of gH/gL complexes and is required for the efficient production of infectious cell-free virions. UL116-null mutants show a 10-fold defect in production of infectious cell-free virions from infected fibroblasts and epithelial cells. This defect is accompanied by reduced expression of two disulfide-linked gH/gL complexes that play crucial roles in viral entry: the heterotrimer of gH/gL with glycoprotein O (gO) and the pentameric complex of gH/gL with UL128, UL130, and UL131. Kifunensine, a mannosidase inhibitor that interferes with endoplasmic reticulum (ER)-associated degradation (ERAD) of terminally misfolded glycoproteins, restored levels of gH, gL, and gO in UL116-null-infected cells, indicating that constituents of HCMV gH complexes are unstable in the absence of UL116. Further, we find that gH/UL116 complexes are abundant in virions, since a major gH species not covalently linked to other glycoproteins, which has long been observed in the literature, is detected from wild-type but not UL116-null virions. Interestingly, UL116 coimmunoprecipitates with UL148, a viral ER-resident glycoprotein that attenuates ERAD of gO, and we observe elevated levels of UL116 in UL148-null virions. Collectively, our findings argue that UL116 is a chaperone for gH that supports the assembly, maturation, and incorporation of gH/gL complexes into virions. IMPORTANCE HCMV is a betaherpesvirus that causes dangerous opportunistic infections in immunocompromised patients as well as in the immune-naive fetus and preterm infants. The potential of the virus to enter new host cells is governed in large part by two alternative viral glycoprotein H (gH)/glycoprotein L (gL) complexes that play important roles in entry: gH/gL/gO and gH/gL/UL128-131. A recently identified virion gH complex, comprised of gH bound to UL116, adds a new layer of complexity to the mechanisms that contribute to HCMV infectivity. Here, we show that UL116 promotes the expression of gH/gL complexes and that UL116 interacts with the viral ER-resident glycoprotein UL148, a factor that supports the expression of gH/gL/gO. Overall, our results suggest that UL116 is a chaperone for gH. These findings have important implications for understanding HCMV cell tropism as well as for the development of vaccines against the virus.

The Human Cytomegalovirus UL116 Glycoprotein Is a Chaperone to Control gH-Based Complexes Levels on Virions

Human cytomegalovirus (HCMV) relies in large part upon the viral membrane fusion glycoprotein B and two alternative gH/gL complexes, gH/gL/gO (Trimer) and gH/gL/UL128/UL130/UL131A (Pentamer) to enter into cells. The relative amounts of Trimer and Pentamer vary among HCMV strains and contribute to differences in cell tropism. Although the viral ER resident protein UL148 has been shown to interact with gH to facilitate gO incorporation, the mechanisms that favor the assembly and maturation of one complex over another remain poorly understood. HCMV virions also contain an alternative non-disulfide bound heterodimer comprised of gH and UL116 whose function remains unknown. Here, we show that disruption of HCMV gene UL116 causes infectivity defects of ∼10-fold relative to wild-type virus and leads to reduced expression of both gH/gL complexes in virions. Furthermore, gH that is not covalently bound to other viral glycoproteins, which are readily detected in wild-type HCMV virions, become undetectable in the absence of UL116 suggesting that the gH/UL116 complex is abundant in virions. We find evidence that UL116 and UL148 interact during infection indicating that the two proteins might cooperate to regulate the abundance of HCMV gH complexes. Altogether, these results are consistent with a role of UL116 as a chaperone for gH during the assembly and maturation of gH complexes in infected cells.

Cytomegalovirus Strain TB40/E Restrictions and Adaptations to Growth in ARPE-19 Epithelial Cells

Despite displaying broad tropism in vivo, human cytomegalovirus (CMV) contained in bodily fluids replicates inefficiently in most cultured cell types except fibroblasts. As propagation in fibroblasts leads to the accumulation of genomic changes, a number of strains were generated by serial passaging on endothelial cells. One of these, TB40/E, was shown to contain a mixture of genetically distinct virus variants, and to retain tropism for fibroblasts, endothelial and epithelial cells. Cloning of an endotheliotropic subpopulation produced the TB40-BAC4 variant, extensively used in CMV tropism studies. Because TB40-BAC4 represents only one of the different variants comprising TB40/E, we generated a series of epithelial-cell adapted stocks derived from a TB40/E mixed stock, rather than from TB40-BAC4. Within two passages on ARPE-19 cells, virus populations were produced with the ability to enter and initiate replication with similar efficiencies in both epithelial cells and fibroblasts. Although the ability to release progeny also increased, cell-free virus yields from ARPE-19 cells remained consistently two to three-logs lower than from fibroblasts, hinting at the existence of a post-entry and post-genome synthesis block in epithelial cells. Multinucleated syncytia also rapidly appeared exclusively in ARPE-19 cell cultures, where their numbers and dimensions increased with virus passage. Irrespective of the number of infected nuclei comprising each syncytium, however, only one cytoplasmic virion assembly compartment was consistently observed, leading us to speculate that improvements in entry efficiency associated with ARPE-19 cell adaptation lead to the development of syncytia, which may negatively affect progeny release by limiting the amount of resources available to maturing virions.

The role of human cytomegalovirus in atherosclerosis: a systematic review

Atherosclerosis is a progressive vascular disease with increasing morbidity and mortality year by year in modern society. Human cytomegalovirus (HCMV) infection is closely associated with the development of atherosclerosis. HCMV infection may accelerate graft atherosclerosis and the development of transplant vasculopathy in organ transplantation. However, our current understanding of HCMV-associated atherosclerosis remains limited and is mainly based on clinical observations. The underlying mechanism of the involvement of HCMV infection in atherogenesis remains unclear. Here, we summarized current knowledge regarding the multiple influences of HCMV on a diverse range of infected cells, including vascular endothelial cells, vascular smooth muscle cells, monocytes, macrophages, and T cells. In addition, we described potential HCMV-induced molecular mechanisms, such as oxidative stress, endoplasmic reticulum stress, autophagy, lipid metabolism, and miRNA regulation, which are involved in the development of HCMV-associated atherogenesis. Gaining an improved understanding of these mechanisms will facilitate the development of novel and effective therapeutic strategies for the treatment of HCMV-related cardiovascular disease.

Distribution of the CMV glycoprotein gH/gL/gO and gH/gL/pUL128/pUL130/pUL131A complex variants and associated clinical manifestations in infants infected congenitally or postnatally

Human cytomegalovirus (CMV) is a major cause of morbidity in fetuses following intrauterine infection. The glycoprotein (g) envelope trimeric gH/gL/gO and pentameric gH/gL/pUL128/pUL130/pUL131A complexes are required for CMV entry into fibroblasts and endothelial/epithelial cells, respectively, and both are targets for neutralizing antibodies. The role of sequence variability among viral strains in the outcome of congenital CMV infection is controversial. Variation in the CMV UL75 gene encoding glycoprotein H (gH), the UL115 (gL), the UL74 (gO), and the UL128 locus (UL128L) encoding three structural proteins (pUL128, pUL130, and pUL131A) was determined in 82 newborns with congenital CMV infection and 113 infants with postnatal or unproven congenital CMV infection. Genotyping was performed by sequencing analysis of PCR-amplified fragments and the PCR-restriction fragment length polymorphism (RFLP) method, and the viral load was measured by quantitative real-time PCR. The obtained results demonstrated that (1) different CMV variants and mixed CMV infections can be detected in newborns infected congenitally; (2) the gH1 genotype, UL130 variant 6, and UL131A variant 1 were associated with some signs/symptoms within cohort of pediatric patients, mainly consisting of infants with symptomatic CMV infection. The results revealed that pUL130, pUL131A, and gH polymorphisms seemed to be associated with the outcome of CMV infection in infants.

Entry of betaherpesviruses

In this chapter, we present an overview on betaherpesvirus entry, with a focus on human cytomegalovirus, human herpesvirus 6A and human herpesvirus 6B. Human cytomegalovirus (HCMV) is a complex human pathogen with a genome of 235kb encoding more than 200 genes. It infects a broad range of cell types by switching its viral ligand on the virion, using the trimer gH/gL/gO for infection of fibroblasts and the pentamer gH/gL/UL128/UL130/UL131 for infection of other cells such as epithelial and endothelial cells, leading to membrane fusion mediated by the fusion protein gB. Adding to this scenario, however, accumulating data reveal the actual complexity in the viral entry process of HCMV with an intricate interplay among viral and host factors. Key novel findings include the identification of entry receptors platelet-derived growth factor-α receptor (PDGFRα) and Netropilin-2 (Nrp2) for trimer and pentamer, respectively, the determination of atomic structures of the fusion protein gB and the pentamer, and the in situ visualization of the state and arrangement of functional glycoproteins on virion. This is covered in the first part of this review. The second part focusses on HHV-6 which is a T lymphotropic virus categorized as two distinct virus species, HHV-6A and HHV-6B based on differences in epidemiological, biological, and immunological aspects, although homology of their entire genome sequences is nearly 90%. HHV-6B is a causative agent of exanthema subitum (ES), but the role of HHV-6A is unknown. HHV-6B reactivation occasionally causes encephalitis in patients with hematopoietic stem cell transplant. The HHV-6 specific envelope glycoprotein complex, gH/gL/gQ1/gQ2 is a viral ligand for the entry receptor. Recently, each virus has been found to recognize a different cellular receptor, CD46 for HHV 6A amd CD134 for HHV 6B. These findings show that distinct receptor recognition differing between both viruses could explain their different pathogenesis.

Pathogen at the Gates: Human Cytomegalovirus Entry and Cell Tropism

The past few years have brought substantial progress toward understanding how human cytomegalovirus (HCMV) enters the remarkably wide spectrum of cell types and tissues that it infects. Neuropilin-2 and platelet-derived growth factor receptor alpha (PDGFRα) were identified as receptors, respectively, for the trimeric and pentameric glycoprotein H/glycoprotein L (gH/gL) complexes that in large part govern HCMV cell tropism, while CD90 and CD147 were also found to play roles during entry. X-ray crystal structures for the proximal viral fusogen, glycoprotein B (gB), and for the pentameric gH/gL complex (pentamer) have been solved. A novel virion gH complex consisting of gH bound to UL116 instead of gL was described, and findings supporting the existence of a stable complex between gH/gL and gB were reported. Additional work indicates that the pentamer promotes a mode of cell-associated spread that resists antibody neutralization, as opposed to the trimeric gH/gL complex (trimer), which appears to be broadly required for the infectivity of cell-free virions. Finally, viral factors such as UL148 and US16 were identified that can influence the incorporation of the alternative gH/gL complexes into virions. We will review these advances and their implications for understanding HCMV entry and cell tropism.

Human cytomegalovirus US21 protein is a viroporin that modulates calcium homeostasis and protects cells against apoptosis

The human cytomegalovirus (HCMV) US12 gene family comprises a set of 10 contiguous genes (US12 to US21) with emerging roles in the regulation of virus cell tropism, virion composition, and immunoevasion. Of all of the US12 gene products, pUS21 shows the highest level of identity with two cellular transmembrane BAX inhibitor motif-containing (TMBIM) proteins: Bax inhibitor-1 and Golgi anti-apoptotic protein, both of which are involved in the regulation of cellular Ca²⁺ homeostasis and adaptive cell responses to stress conditions. Here, we report the US21 protein to be a viral-encoded ion channel that regulates intracellular Ca²⁺ homeostasis and protects cells against apoptosis. Indeed, we show pUS21 to be a 7TMD protein expressed with late kinetics that accumulates in ER-derived vesicles. Deletion or inactivation of the US21 gene resulted in reduced HCMV growth, even in fibroblasts, due to reduced gene expression. Ratiometric fluorescence imaging assays revealed that expression of pUS21 reduces the Ca²⁺ content of intracellular ER stores. An increase in cell resistance to intrinsic apoptosis was then observed as an important cytobiological consequence of the pUS21-mediated alteration of intracellular Ca²⁺ homeostasis. Moreover, a single point mutation in the putative pore of pUS21 impaired the reduction of ER Ca²⁺ concentration and attenuated the antiapoptotic activity of pUS21wt, supporting a functional link with its ability to manipulate Ca²⁺ homeostasis. Together, these results suggest pUS21 of HCMV constitutes a TMBIM-derived viroporin that may contribute to HCMV's overall strategy to counteract apoptosis in infected cells.

Human Cytomegalovirus Tropism Modulator UL148 Interacts with SEL1L, a Cellular Factor That Governs Endoplasmic Reticulum-Associated Degradation of the Viral Envelope Glycoprotein gO

UL148 is a viral endoplasmic reticulum (ER)-resident glycoprotein that contributes to human cytomegalovirus (HCMV) cell tropism. The influence of UL148 on tropism correlates with its potential to promote the expression of glycoprotein O (gO), a viral envelope glycoprotein that participates in a heterotrimeric complex with glycoproteins H and L that is required for infectivity. In an effort to gain insight into the mechanism, we used mass spectrometry to identify proteins that coimmunoprecipitate from infected cells with UL148. This approach led us to identify an interaction between UL148 and SEL1L, a factor that plays key roles in ER-associated degradation (ERAD). In pulse-chase experiments, gO was less stable in cells infected with UL148-null mutant HCMV than during wild-type infection, suggesting a potential functional relevance for the interaction with SEL1L. To investigate whether UL148 regulates gO abundance by influencing ERAD, small interfering RNA (siRNA) silencing of either SEL1L or its partner, Hrd1, was carried out in the context of infection. Knockdown of these ERAD factors strongly enhanced levels of gO but not other viral glycoproteins, and the effect was amplified in the presence of UL148. Furthermore, pharmacological inhibition of ERAD showed similar results. Silencing of SEL1L during infection also stabilized an interaction of gO with the ER lectin OS-9, which likewise suggests that gO is an ERAD substrate. Taken together, our results identify an intriguing interaction of UL148 with the ERAD machinery and demonstrate that gO behaves as a constitutive ERAD substrate during infection. These findings have implications for understanding the regulation of HCMV cell tropism.IMPORTANCE Viral glycoproteins in large part determine the cell types that an enveloped virus can infect and hence play crucial roles in transmission and pathogenesis. The glycoprotein H/L heterodimer (gH/gL) is part of the conserved membrane fusion machinery that all herpesviruses use to enter cells. In human cytomegalovirus (HCMV), gH/gL participates in alternative complexes in virions, one of which is a trimer of gH/gL with glycoprotein O (gO). Here, we show that gO is constitutively degraded during infection by the endoplasmic reticulum-associated degradation (ERAD) pathway and that UL148, a viral factor that regulates HCMV cell tropism, interacts with the ERAD machinery and slows gO decay. Since gO is required for cell-free virus to enter new host cells but dispensable for cell-associated spread that resists antibody neutralization, our findings imply that the posttranslational instability of a viral glycoprotein provides a basis for viral mechanisms to modulate tropism and spread.

Betaherpesvirus Virion Assembly and Egress

Virions are the vehicle for cell-to-cell and host-to-host transmission of viruses. Virions need to be assembled reliably and efficiently, be released from infected cells, survive in the extracellular environment during transmission, recognize and then trigger entry of appropriate target cells, and disassemble in an orderly manner during initiation of a new infection. The betaherpesvirus subfamily includes four human herpesviruses (human cytomegalovirus and human herpesviruses 6A, 6B, and 7), as well as viruses that are the basis of important animal models of infection and immunity. Similar to other herpesviruses, betaherpesvirus virions consist of four main parts (in order from the inside): the genome, capsid, tegument, and envelope. Betaherpesvirus genomes are dsDNA and range in length from ~145 to 240 kb. Virion capsids (or nucleocapsids) are geometrically well-defined vessels that contain one copy of the dsDNA viral genome. The tegument is a collection of several thousand protein and RNA molecules packed into the space between the envelope and the capsid for delivery and immediate activity upon cellular entry at the initiation of an infection. Betaherpesvirus envelopes consist of lipid bilayers studded with virus-encoded glycoproteins; they protect the virion during transmission and mediate virion entry during initiation of new infections. Here, we summarize the mechanisms of betaherpesvirus virion assembly, including how infection modifies, reprograms, hijacks, and otherwise manipulates cellular processes and pathways to produce virion components, assemble the parts into infectious virions, and then transport the nascent virions to the extracellular environment for transmission.

The pentameric complex of human Cytomegalovirus: cell tropism, virus dissemination, immune response and vaccine development

Between the 1980s and 1990s, three assays were developed for diagnosis of human cytomegalovirus (HCMV) infections: leuko (L)-antigenemia, l-viremia and l-DNAemia, detecting viral protein pp65, infectious virus and viral DNA, respectively, in circulating leukocytes Repeated initial attempts to reproduce the three assays in vitro using laboratory-adapted strains and infected cell cultures were consistently unsuccessful. Results were totally reversed when wild-type HCMV strains were used to infect either fibroblasts or endothelial cells. Careful analysis and sequencing of plaque-purified viruses from recent clinical isolates drew attention to the ULb' region of the HCMV genome. Using bacterial artificial chromosome technology, it was shown by both gain-of-function and loss-of-function experiments that UL131-128 genes are indispensable for virus growth in endothelial cells and virus transfer to leukocytes. In addition, a number of clinical isolates passaged in human fibroblasts had lost both properties (leuko-tropism and endothelial cell-tropism) when displaying a mutation in the UL131-128 locus (referred to as UL128L). In the following years, it was shown that pUL128L was complexed with gH and gL to form the pentameric complex (PC), which is required to infect endothelial, epithelial and myeloid cells. The immune response to PC was studied extensively, particularly its humoral component, showing that the great majority of the neutralizing antibody response is directed to PC. Although anti-HCMV antibodies may act with other mechanisms than mere neutralizing activity, these findings definitely favour their protective activity, thus paving the way to the development of a potentially protective HCMV vaccine.