Modulation of beta-catenin and E-cadherin interaction by Vpu increases human immunodeficiency virus type 1 particle release

Cadherin-mediated host-pathogen interactions

Cell adhesion molecules mediate cell-to-cell and cell-to-matrix adhesions and play an immense role in a myriad of physiological processes during the growth and development of a multicellular organism. Cadherins belong to a major group of membrane-bound cell surface proteins that, in coordination with nectins, drive the formation and maintenance of adherens junctions for mediating cell to cell adhesion, cellular communication and signalling. Alongside adhesive function, the involvement of cadherins in mediating host-pathogen interactions has been extensively explored in recent years. In this review, we provide an in-depth understanding of microbial pathogens and their virulence factors that exploit cadherins for their strategical invasion into the host cell. Furthermore, macromolecular interactions involving cadherins and various microbial factors such as secretory toxins and adhesins lead to the disintegration of host cell junctions followed by the entry of the pathogen or triggering downstream signalling pathways responsible for successful invasion of the pathogenic microbes are discussed. Besides providing a comprehensive insight into some of the structural complexes involving cadherins and microbial factors to offer the mechanistic details of host-pathogen interactions, the current review also highlights novel constituents of various cell signalling events such as endocytosis machinery elicited upon microbial infections.

Aedes aegypti lachesin protein binds to the domain III of envelop protein of Dengue virus-2 and inhibits viral replication

Dengue virus (DENV) comprises of four serotypes (DENV-1 to -4) and is medically one of the most important arboviruses (arthropod-borne virus). DENV infection is a major human health burden and is transmitted between humans by the insect vector, Aedes aegypti. Ae. aegypti ingests DENV while feeding on infected humans, which traverses through its gut, haemolymph and salivary glands of the mosquito before being injected into a healthy human. During this process of transmission, DENV must interact with many proteins of the insect vector, which are important for its successful transmission. Our study focused on the identification and characterisation of interacting protein partners in Ae. aegypti to DENV. Since domain III (DIII) of envelope protein (E) is exposed on the virion surface and is involved in virus entry into various cells, we performed phage display library screening against domain III of the envelope protein (EDIII) of DENV-2. A peptide sequence showing similarity to lachesin protein was found interacting with EDIII. The lachesin protein was cloned, heterologously expressed, purified and used for in vitro interaction studies. Lachesin protein interacted with EDIII and also with DENV. Further, lachesin protein was localised in neuronal cells of different organs of Ae. aegypti by confocal microscopy. Blocking of lachesin protein in Ae. aegypti with anti-lachesin antibody resulted in a significant reduction in DENV replication.

The HIV-1 Accessory Protein Vpu Downregulates Peroxisome Biogenesis

People living with HIV can experience accelerated aging and the development of neurological disorders. Recently, we reported that HIV-1 infection results in a dramatic loss of peroxisomes in macrophages and brain tissue. This is significant because (i) peroxisomes are important for the innate immune response and (ii) loss of peroxisome function is associated with cellular aging and neurodegeneration. Accordingly, understanding how HIV-1 infection causes peroxisome depletion may provide clues regarding how the virus establishes persistent infections and, potentially, the development of neurological disorders. Here, we show that the accessory protein Vpu is necessary and sufficient for the induction of microRNAs that target peroxisome biogenesis factors. The ability of Vpu to downregulate peroxisome formation depends on the Wnt/β-catenin pathway. Thus, in addition to revealing a novel mechanism by which HIV-1 uses intracellular signaling pathways to target antiviral signaling platforms (peroxisomes), we have uncovered a previously unknown link between the Wnt/β-catenin pathway and peroxisome homeostasis.

Human immunodeficiency virus type 1 (HIV-1) establishes lifelong infections in humans, a process that relies on its ability to thwart innate and adaptive immune defenses of the host. Recently, we reported that HIV-1 infection results in a dramatic reduction of the cellular peroxisome pool. Peroxisomes are metabolic organelles that also function as signaling platforms in the innate immune response. Here, we show that the HIV-1 accessory protein Vpu is necessary and sufficient for the depletion of cellular peroxisomes during infection. Vpu induces the expression of four microRNAs that target mRNAs encoding proteins required for peroxisome formation and metabolic function. The ability of Vpu to downregulate peroxisomes was found to be dependent upon the Wnt/β-catenin signaling pathway. Given the importance of peroxisomes in innate immune signaling and central nervous system function, the roles of Vpu in dampening antiviral signaling appear to be more diverse than previously realized. Finally, our findings highlight a potential role for Wnt/β-catenin signaling in peroxisome homeostasis through modulating the production of biogenesis factors.

Upsetting the Balance: When Viruses Manipulate Cell Polarity Control

The central importance of cell polarity control is emphasized by the frequency with which it is targeted by many diverse viruses. It is clear that in targeting key polarity control proteins, viruses affect not only host cell polarity, but also influence many cellular processes, including transcription, replication, and innate and acquired immunity. Examination of the interactions of different virus proteins with the cell and its polarity controls during the virus life cycles, and in virally‐induced cell transformation shows ever more clearly how intimately all cellular processes are linked to the control of cell polarity.

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Multiple viruses target cell polarity.

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Viral targeting of polarity frequently occurs through PDZ recognition.

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Biological effects include immune-avoidance, cell proliferation and apoptosis inhibition.

β-Catenin/TCF-4 signaling regulates susceptibility of macrophages and resistance of monocytes to HIV-1 productive infection

Cells of the monocyte/macrophage lineage are an important target for HIV-1 infection. They are often at anatomical sites linked to HIV-1 transmission and are an important vehicle for disseminating HIV-1 throughout the body, including the central nervous system. Monocytes do not support extensive productive HIV-1 replication, but they become more susceptible to HIV-1infection as they differentiate into macrophages. The mechanisms guiding susceptibility of HIV-1 replication in monocytes versus macrophages are not entirely clear. We determined whether endogenous activity of β-catenin signaling impacts differential susceptibility of monocytes and monocyte-derived macrophages (MDMs) to productive HIV-1 replication. We show that monocytes have an approximately 4-fold higher activity of β-catenin signaling than MDMs. Inducing β-catenin in MDMs suppressed HIV-1 replication by 5-fold while inhibiting endogenous β-catenin signaling in monocytes by transfecting with a dominant negative mutant for the downstream effector of β- catenin (TCF-4) promoted productive HIV-1 replication by 6-fold. These findings indicate that β-catenin/TCF-4 is an important pathway for restricted HIV-1 replication in monocytes and plays a significant role in potentiating HIV-1 replication as monocytes differentiate into macrophages. Targeting this pathway may provide a novel strategy to purge the latent reservoir from monocytes/macrophages, especially in sanctuary sites for HIV-1 such as the central nervous system.

Regulation network and expression profiles of Epstein-Barr virus-encoded microRNAs and their potential target host genes in nasopharyngeal carcinomas

Epstein-Barr virus (EBV) is associated with nasopharyngeal carcinoma (NPC) tumorigenesis. However, the mechanism(s) connecting EBV infection and NPC remain unclear. Recently, a new class of EBV microRNAs (miRNAs) has been described. To determine how EBV miRNAs control the expression of host genes, and to understand their potential role in NPC tumorigenesis, we profiled the expression of 44 mature EBV miRNAs and potential host genes in NPC and non-tumor nasopharyngeal epithelial tissues. We found that 40 EBV miRNAs from the BART transcript were highly expressed in NPC. Analysis of potential BART miRNA target genes revealed that 3140 genes and several important pathways might be involved in the carcinogenesis of NPC. A total of 105 genes with potential EBV miRNA binding sites were significantly downregulated, suggesting that EBV miRNAs may regulate these genes and contribute to NPC carcinogenesis. An EBV miRNA and host gene regulation network was generated to provide useful clues for validating of EBV miRNA functions in NPC tumorigenesis.

Human tumour viruses and the deregulation of cell polarity in cancer

The role of cell polarity regulators in the development of cancer has long been an enigma. Despite displaying characteristics of tumour suppressors, the core regulators of polarity are rarely mutated in tumours and there are few data from animal models to suggest that they directly contribute to cancer susceptibility, thus questioning their relevance to human carcinogenesis. However, a body of data from human tumour viruses is now providing compelling evidence of a central role for the perturbation of cell polarity in the development of cancer.

Identification of novel T cell factor 4 (TCF-4) binding sites on the HIV long terminal repeat which associate with TCF-4, β-catenin, and SMAR1 to repress HIV transcription

Molecular regulation of HIV transcription is a multifaceted process dictated in part by the abundance of cellular transcription factors that induce or repress HIV promoter activity. β-Catenin partners with members of the T cell factor (TCF)/LEF transcription factors to regulate gene expression. The interaction between β-catenin and TCF-4 is linked to inhibition of HIV replication in multiple cell types, including lymphocytes and astrocytes. Here, we evaluated the molecular mechanism by which β-catenin/TCF-4 repress HIV replication. We identified for the first time multiple TCF-4 binding sites at -336, -143, +66, and +186 relative to the transcription initiation site on the HIV long terminal repeat (LTR). Two of the sites (-143 and +66) were present in approximately 1/3 of 500 HIV-1 isolates examined. Although all four sites could bind to TCF-4, the strongest association occurred at -143. Deletion and/or mutation of -143, in conjunction with β-catenin or TCF-4 knockdown in cells stably expressing an LTR reporter construct, enhanced basal HIV promoter activity by 5-fold but had no effect on Tat-mediated transactivation of the HIV LTR. We also found that TCF-4, β-catenin, and the nuclear matrix binding protein SMAR1 tether at the -143-nucleotide (nt) site on the HIV LTR to inhibit HIV promoter activity. Collectively, these data indicate that TCF-4 and β-catenin at -143 associate with SMAR1, which likely pulls the HIV DNA segment into the nuclear matrix and away from transcriptional machinery, leading to repression of basal HIV LTR transcription. These studies point to novel avenues for regulation of HIV replication by manipulation of β-catenin signaling within cells.

Role of β-catenin and TCF/LEF family members in transcriptional activity of HIV in astrocytes

The Wnt/β-catenin pathway is involved in diverse cell functions governing development and disease. β-Catenin, a central mediator of this pathway, binds to members of the TCF/LEF family of transcription factors to modulate hundreds of genes. Active Wnt/β-catenin/TCF-4 signaling plays a significant role in repression of HIV-1 replication in multiple cell targets, including astrocytes. To determine the mechanism by which active β-catenin/TCF-4 leads to inhibition of HIV replication, we knocked down β-catenin or TCF/LEF members in primary astrocytes and astrocytomas transiently transfected with an HIV long terminal repeat (LTR)-luciferase reporter that contained an integrated copy of the HIV LTR-luciferase construct. Knockdown of either β-catenin or TCF-4 induced LTR activity by 2- to 3-fold under both the episomal and integrated conditions. This knockdown also increased presence of serine 2-phosphorylated RNA polymerase II (Pol II) on the HIV LTR as well as enhanced its processivity. Knockdown of β-catenin/TCF-4 also impacted tethering of other transcription factors on the HIV promoter. Specifically, knockdown of TCF-4 enhanced binding of C/EBPβ, C/EBPδ, and NF-κB to the HIV LTR, while β-catenin knockdown increased binding of C/EBPβ and C/EBPδ but had no effect on NF-κB. Approximately 150 genes in astrocytes were impacted by β-catenin knockdown, including genes involved in inflammation/immunity, uptake/transport, vesicular transport/exocytosis, apoptosis/cellular stress, and cytoskeleton/trafficking. These findings indicate that modulation of the β-catenin/TCF-4 axis impacts the basal level of HIV transcription in astrocytes, which may drive low level/persistent HIV in astrocytes that can contribute to ongoing neuroinflammation, and this axis also has profound effects on astrocyte biology.

Role of β-catenin/TCF-4 signaling in HIV replication and pathogenesis: insights to informing novel anti-HIV molecular therapeutics

A greater understanding of the interaction between HIV and host signaling pathways that restrict virus production may lead to new methods to purge virus from latent reservoirs and enhance survival/function of cells targeted by HIV. This review highlights the role of the Wnt/β-catenin pathway as a host factor that represses HIV replication in multiple targets, especially those relevant to HIV in the central nervous system.

IFN-gamma mediates enhancement of HIV replication in astrocytes by inducing an antagonist of the beta-catenin pathway (DKK1) in a STAT 3-dependent manner

Typically, IFN-γ is an antiviral cytokine that inhibits the replication of many viruses, including HIV. However, in the CNS, IFN-γ induces HIV-productive replication in astrocytes. Although astrocytes in vitro are refractory to HIV replication, recent in vivo evidence demonstrated that astrocytes are infected by HIV, and their degree of infection is correlated with proximity to activated macrophages/microglia. The ability of IFN-γ to induce HIV replication in astrocytes suggests that the environmental milieu is critical in regulating the permissiveness of astrocytes to HIV infection. We evaluated the mechanism by which IFN-γ relieves restricted HIV replication in astrocytes. We demonstrate that although astrocytes have robust endogenous β-catenin signaling, a pathway that is a potent inhibitor of HIV replication, IFN-γ diminished β-catenin signaling in astrocytes by 40%, as evaluated by both active β-catenin protein expression and β-catenin-mediated T cell factor/lymphoid enhancer reporter (TOPflash) activity. Further, IFN-γ-mediated inhibition of β-catenin signaling was dependent on its ability to induce an antagonist of the β-catenin signaling pathway, Dickkopf-related protein 1, in a STAT 3-dependent manner. Inhibition of STAT3 and Dickkopf-related protein 1 abrogated the ability of IFN-γ to enhance HIV replication in astrocytes. These data demonstrated that IFN-γ induces HIV replication in astrocytes by antagonizing the β-catenin pathway. To our knowledge, this is the first report to point to an intricate cross-talk between IFN-γ signaling and β-catenin signaling that may have biologic and virologic effects on HIV outcome in the CNS, as well as on broader processes where the two pathways interface.