Differentiation-dependent susceptibility of human muscle cells to Zika virus infection

Muscle cells are potential targets of many arboviruses, such as Ross River, Dengue, Sindbis, and chikungunya viruses, that may be involved in the physiopathological course of the infection. During the recent outbreak of Zika virus (ZIKV), myalgia was one of the most frequently reported symptoms. We investigated the susceptibility of human muscle cells to ZIKV infection. Using an in vitro model of human primary myoblasts that can be differentiated into myotubes, we found that myoblasts can be productively infected by ZIKV. In contrast, myotubes were shown to be resistant to ZIKV infection, suggesting a differentiation-dependent susceptibility. Infection was accompanied by a caspase-independent cytopathic effect, associated with paraptosis-like cytoplasmic vacuolization. Proteomic profiling was performed 24h and 48h post-infection in cells infected with two different isolates. Proteome changes indicate that ZIKV infection induces an upregulation of proteins involved in the activation of the Interferon type I pathway, and a downregulation of protein synthesis. This work constitutes the first observation of primary human muscle cells susceptibility to ZIKV infection, and differentiation-dependent restriction of infection from myoblasts to myotubes. Since myoblasts constitute the reservoir of stem cells involved in reparation/regeneration in muscle tissue, the infection of muscle cells and the viral-induced alterations observed here could have consequences in ZIKV infection pathogenesis.

Muscle cells are potential targets of many arboviruses, such as Ross River, Dengue, Sindbis, and chikungunya viruses, and may be involved in the disease manifestation. During the recent outbreak of Zika virus (ZIKV), myalgia was one of the most frequently reported symptoms. We investigated the susceptibility of human muscle cells to ZIKV infection. Using an in vitro model of human muscle stem cells (myoblasts) that can be differentiated into differentiated muscle cells (myotubes), we found that myoblasts can be infected by ZIKV. In contrast, myotubes were shown to be resistant to ZIKV infection. Infection induced the death of infected cells. Protein levels 24h and 48h post-infection indicate that ZIKV infection induces an upregulation of proteins involved in the activation of the Interferon type I pathway, and a downregulation of protein synthesis. This work constitutes the first observation of primary human muscle cells susceptibility to ZIKV infection, muscle stem cells being susceptible while differentiated muscle cells are resistant. Since myoblasts constitute the reservoir of stem cells involved in reparation/regeneration in muscle tissue, the infection of muscle cells and the viral-induced alterations observed here could have consequences during ZIKV infection.

EV71 3C protease induces apoptosis by cleavage of hnRNP A1 to promote apaf-1 translation

Enterovirus 71 (EV71) induces apoptosis to promote viral particle release. Earlier work showed that EV71 utilizes its 3C protease to induce apoptosis in a caspase-3-dependent pathway, though the mechanism is unknown. However, work from Vagner, Holcik and colleagues showed that host protein heterogeneous ribonucleoprotein A1 (hnRNP A1) binds the IRES of cellular apoptotic peptidase activating factor 1 (apaf-1) mRNA to repress its translation. In this work, we show that apaf-1 expression is essential for EV71-induced apoptosis. EV71 infection or ectopic expression of 3C protease cleaves hnRNP A1, which abolishes its binding to the apaf-1 IRES. This allows IRES-dependent synthesis of apaf-1, activation of caspase-3, and apoptosis. Thus, we reveal a novel mechanism that EV71 utilizes for virus release via a 3C protease-hnRNP A1-apaf-1-caspase-3-apoptosis axis.

The 3C protease of enterovirus A71 counteracts the activity of host zinc-finger antiviral protein (ZAP)

Enterovirus A71 (EV-A71) is a positive-strand RNA virus that causes hand-foot-mouth disease and neurological complications in children and infants. Although the underlying mechanisms remain to be further defined, impaired immunity is thought to play an important role. The host zinc-finger antiviral protein (ZAP), an IFN-stimulated gene product, has been reported to specifically inhibit the replication of certain viruses. However, whether ZAP restricts the infection of enteroviruses remains unknown. Here, we report that EV-A71 infection upregulates ZAP mRNA in RD and HeLa cells. Moreover, ZAP overexpression rendered 293 T cells resistant to EV-A71 infection, whereas siRNA-mediated depletion of endogenous ZAP enhanced EV-A71 infection. The EV-A71 infection stimulated site-specific proteolysis of two ZAP isoforms, leading to the accumulation of a 40 kDa N-terminal ZAP fragment in virus-infected cells. We further revealed that the 3C protease (3Cpro) of EV-A71 mediates ZAP cleavage, which requires protease activity. Furthermore, ZAP variants with single amino acid substitutions at Gln-369 were resistant to 3Cpro cleavage, implying that Gln-369 is the sole cleavage site in ZAP. Moreover, although ZAP overexpression inhibited EV-A71 replication, the cleaved fragments did not show this effect. Our results indicate that an equilibrium between ZAP and enterovirus 3Cpro controls viral infection. The findings in this study suggest that viral 3Cpro mediated ZAP cleavage may represent a mechanism to escape host antiviral responses.

Oxymatrine provides protection against Coxsackievirus B3-induced myocarditis in BALB/c mice

Oxymatrine is the primary pharmacological component of Sophora flavescens Ait. In the present study, we investigated the protective effect of oxymatrine against Coxsackievirus B3-induced myocarditis in mice. Coxsackievirus B3-infected HeLa cells were treated with oxymatrine and the viral titer, as well as the degree of cellular proliferation were determined. Additionally, BALB/c mice were infected with Coxsackievirus B3 and received differing concentrations of oxymatrine. On days 5 and 12 following treatment, mice were sacrificed, and serum lactate dehydrogenase, creatine kinase-MB isozyme, and tumor necrosis factor-α levels were quantified. The heart index and degree of myocardial tissue inflammation were also assessed. On day 5, the Coxsackievirus B3 TCID₅₀ values of the heart tissue, and the expression of NTR, IFN-γ, and TNF-α genes in the myocardial tissue were measured. Our results showed that oxymatrine exhibits potent antiviral effects on Coxsackievirus B3 as 50% inhibition was achieved at a concentration as low as 0.238 mg/mL. Oxymatrine markedly reduced the viral titer and inhibited cardiac myocyte pathology exhibited in viral myocarditis. Furthermore, oxymatrine treatment reduced the expression of Coxsackievirus B3 NTR and mouse TNF-α genes compared to the controls. Therefore, our findings indicate that oxymatrine is a promising potent antiviral agent against Coxsackievirus B3-induced myocarditis.

Control of the negative IRES trans-acting factor KHSRP by ubiquitination

Cells and viruses can utilize internal ribosome entry sites (IRES) to drive translation when cap-dependent translation is inhibited by stress or viral factors. IRES trans-acting factors (ITAFs) are known to participate in such cap-independent translation, but there are gaps in the understanding as to how ITAFs, particularly negative ITAFs, regulate IRES-driven translation. This study found that Lys109, Lys121 and Lys122 represent critical ubiquitination sites for far upstream element-binding protein 2 (KHSRP, also known as KH-type splicing regulatory protein or FBP2), a negative ITAF. Mutations at these sites subsequently reduced KHSRP ubiquitination and abolished its inhibitory effect on IRES-driven translation. We further found that interaction between the Kelch domain of Kelch-like protein 12 (KLHL12) and the C-terminal domain of KHSRP contributed to KHSRP ubiquitination, leading to downregulation of enterovirus IRES-mediated translation in infected cells and increased competition against other positive ITAFs. Together, these results show that ubiquitination can exert control over IRES-driven translation via modification of ITAFs, and to the best of our knowledge, this is the first description of such a regulatory mechanism for IRES-dependent translation.

Antiviral and myocyte protective effects of murine interferon-β and -α2in coxsackievirus B3-induced myocarditis and epicarditis in Balb/c mice

The present study tested the hypothesis that murine (m)IFN-β or mIFN-α2can eliminate cardiac viral load and protect cardiomyocytes from injury in animals infected with coxsackievirus B3 (CVB3). CVB3-inoculated male Balb/c mice exhibited signs of illness, including lethargy, progressive weight loss, and death (10% on day 3 and 100% on day 8). Cardiac viral load was high [4,277 ± 1,009 plaque-forming units and 25 ± 5 copies CVB3/hypoxanthine guanine phosphoribosyl transferase 1 mRNA] on day 4. The cardiac tissue exhibited severe inflammatory infiltration and myocyte damage with an average myocarditis integrated pathology score of 2.1 ± 0.2 on day 7. Most of the mice infected with CVB3 also developed epicarditis, and 55% had intraventricular thrombi present. Treatment with mIFN-β [2.5 to 10 million international units (MIU)/kg] dose-dependently improved the general health status in CVB3-inoculated mice, as evidenced by reduction in weight loss, prevention of death, elimination of cardiac viral load, protection of myocytes from injury, decrease in inflammatory cell infiltration, and attenuation of intraventricular thrombus formation. Treatment with 10 MIU/kg mIFN-α2resulted in a similar level of efficacy as that induced by 5 MIU/kg mIFN-β, with the exception that mIFN-α2did not reduce cardiac CVB3 mRNA. However, mIFN-α2, but not any dose group of mIFN-β, significantly attenuated CVB3-induced epicarditis. These data demonstrate antiviral effects for both mIFN-β and mIFN-α2, which lead to protection of the mice from CVB3-induced myocarditis. However, the potential mechanisms leading to a differential host response for the two isoforms of mIFN remain to be elucidated.

Innate Defense Mechanism Against Virus Infection Within the Cardiac Myocyte Requiring gp130-STAT3 Signaling

Background— Little is known about innate immune mechanisms within the cardiac myocyte that determine susceptibility to enterovirus infection, an important cause of myocarditis and subsequent heart failure. Although interferon (IFN) generally plays a key role in innate immunity, ablation of IFN receptors has little or no effect on acute coxsackievirus B3 infection in the heart. Interestingly, gp130-cytokine–mediated stimulation of neonatal ventricular myocytes has a cytoprotective effect against virus infection in culture that can be inhibited by suppressors of cytokine signaling (SOCS)-3, a physiological inhibitor of gp130 signaling that does not affect IFN signaling. Therefore, we hypothesized that inhibition of gp130 signaling by SOCS3 would change cardiac myocyte susceptibility to virus infection without affecting IFN signaling.

Methods and Results— We generated cardiac-specific SOCS3 transgenic mice. Despite an intact IFN-mediated antiviral response in adult transgenic myocytes, there was a marked increase in susceptibility to viral infection in the SOCS3 transgenic mouse hearts. This indicated the presence of IFN-independent innate defense mechanisms within the cardiac myocyte. Subsequently, we demonstrated that cardiac-specific gp130-knockout mice also had increased susceptibility to viral infection. Furthermore, we demonstrated that the gp130-mediated increase in survival of infected myocytes occurred through a signal transducers and activators of transcription-3–dependent mechanism that did not affect viral replication. This was accompanied by a persistent expression of full-length dystrophin after coxsackievirus B3 infection. In addition, we found that both SOCS3 transgenic and gp130-deficient mice had a decrease in α-sarcoglycan.

Conclusions— SOCS3-mediated regulation of gp130 signaling can affect susceptibility to viral infection in the heart. Increased cardiac cell survival through gp130–signal transducers and activators of transcription-3 signaling appears to play an important role in preserving nondividing cardiac myocytes until specific immune responses begin to clear the virus.

Fatal myocardial necrosis caused by Staphylococcus lugdunensis and cytomegalovirus in a patient with scleroderma

A 42-year-old woman developed a rapidly progressing fatal heart failure. At the autopsy extensive necrosis of the myocardium was seen, with an almost complete absence of inflammatory cells and the presence of bacterial structures identified as Staphylococcus lugdunensis by PCR. In addition, the cytomegalovirus genome was found to be located inside the cardiomyocytes.

Novel role for integrin-linked kinase in modulation of coxsackievirus B3 replication and virus-induced cardiomyocyte injury

Viral myocarditis is a major cause of sudden cardiac death in children and young adults. Among viruses, coxsackievirus B3 (CVB3) is the most common agent for myocarditis. Recently, more consideration has been given to the role of signaling pathways in pathogenesis of enteroviral myocarditis, providing new platform for identifying a new potential therapeutic target for this, so far, incurable disease. Previously, we reported on the role of the protein kinase-B/Akt in CVB3 replication and virus-induced cell injury. Here, we report on regulation of virus-induced Akt activation by the integrin-linked kinase in infected mouse cardiomyocytes and HeLa cells. This study also presents the first observation that inhibition of ILK in CVB3-infected cells significantly improves the viability of infected cells, while blocking viral replication and virus release. Complementary experiments using a constitutively active form of Akt1 revealed that the observed protective effect of ILK inhibition is dependent on the associated downregulation of virus-induced Akt activation. To our knowledge, this is the first report of such beneficial effects of ILK inhibition in a viral infection model and conveys new insights in our efforts to characterize a novel therapeutic target for treatment of enteroviral myocarditis.

Enhancement of apoptosis via an extrinsic factor, TNF-alpha, in cells infected with cytopathic bovine viral diarrhea virus

Isolates of bovine viral diarrhea virus (BVDV) are divided into cytopathic (cp) and noncytopathic (ncp) biotypes according to their effect on cultured cells. Calves persistently infected with ncp BVDV are known to develop lethal mucosal disease (MD) after superinfection by cp BVDV. Although the UV-irradiated supernatant of cp BVDV-infected cells has been reported to have no capacity to induce cell death, we found that it could enhance cell death through apoptosis. Up-regulation of tumor necrosis factor alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS) mRNAs was detected specifically in cp BVDV-infected primary cell cultures. Suppression of TNF-alpha via antisense oligonucleotide transfection or incubation with a polyclonal antibody against TNF-alpha resulted in attenuation of apoptosis induced by cp BVDV, suggesting that TNF-alpha participates in apoptosis execution. Although TNF-alpha is one of the iNOS-inducible factors, the iNOS up-regulation was not regulated by TNF-alpha. And iNOS was revealed to serve as anti-apoptotic factor, contrary to our expectation. In addition, the expression level of both TNF-alpha and iNOS mRNAs in the ncp BVDV-infected cells was kept lower than that in the mock-infected cells, suggesting that ncp BVDV reduced or interfered with the factor triggering the expression of both mRNAs. These characteristic mRNA transcriptions would help to explain why BVDV acts differently in cells as well as in vivo, depending on its biotype. To elucidate viral factors inducing TNF-alpha and iNOS may be critical to understand the mechanism of MD development, which closely correlates with cp BVDV-induced apoptosis.

Adeno-associated virus terminal repeat (TR) mutant generates self-complementary vectors to overcome the rate-limiting step to transduction in vivo

An important limitation of recombinant adeno-associated virus (rAAV) vector efficiency is the requirement of hostcell-mediated synthesis of double-stranded DNA from the single-stranded genome. We have bypassed this step in a specialized self-complementary rAAV (scAAV) vector, by utilizing the tendency of AAV to package DNA dimers when the replicating genome is half the length of the wild type (wt). To produce these vectors efficiently, we have deleted the terminal resolution site (trs) from one rAAV TR, preventing the initiation of replication at the mutated end. These constructs generate single-stranded, inverted repeat genomes, with a wt TR at each end, and a mutated TR in the middle. After uncoating, the viral DNA folds through intramolecular base pairing within the mutant TR, which then proceeds through the genome to form a double-stranded molecule. We have used the scAAV to investigate barriers to rAAV transduction in the mouse liver, muscle and brain. In each tissue, scAAV was characterized by faster onset of gene expression and higher transduction efficiency. This study confirms earlier predictions that complementary-strand DNA synthesis is the primary barrier to rAAV-2 transduction. The scAAV is unaffected by this barrier, and provides an extremely efficient vector for gene transfer into many types of cells in vivo.

Coxsackie B viruses use multiple receptors to infect human cardiac cells

Viral myocarditis is an inflammatory disease of the heart muscle that can be fatal. The primary viruses that have been linked to myocarditis and dilated cardiomyopathy are the human enteroviruses. The most common viruses associated with this disease are the Coxsackie B viruses and in particular Coxsackievirus B3 and Coxsackievirus B5. Early events in viral infection include attachment of the virus onto cell surface receptors. Even though, CD55 and Coxsackievirus adenovirus receptor protein (CAR) have been identified as receptors for Coxsackievirus B3, the exact mechanisms that Coxsackievirus B3 and B5 use to infect the cardiac muscle are not yet known. In this study, attempts were made to inhibit Coxsackievirus B3 and Coxsackievirus B5 infectivity of cardiac cells by using CAR and CD55 specific antibodies. The results show that these antibodies could not completely inhibit Coxsackievirus B3 and Coxsackievirus B5 binding or infectivity. Furthermore five new proteins have been identified that are used by Coxsackieviruses for binding to cardiac tissue and are distinct from CAR or CD55, leading us to believe that these viruses may use a different set of receptors for infection of cardiac muscle.

Role of CD1d in coxsackievirus B3-induced myocarditis

The myocarditic (H3) variant of Coxsackievirus B3 (CVB3) causes severe myocarditis in BALB/c mice and BALB/c mice lacking the invariant J alpha 281 gene, but minimal disease in BALB/c CD1d(-/-) animals. This indicates that CD1d expression is important in this disease but does not involve the invariant NKT cell often associated with CD1d-restricted immunity. The H3 variant of the virus increases CD1d expression in vitro in neonatal cardiac myocytes whereas a nonmyocarditic (H310A1) variant does not. V gamma 4(+) T cells show increased activation in both H3-infected BALB/c and J alpha 281(-/-) mice compared with CD1d(-/-) animals. The activated BALB/c V gamma 4(+) T cells from H3-infected mice kill H3-infected BALB/c myocytes and cytotoxicity is blocked with anti-CD1d but not with anti-MHC class I (K(d)/D(d)) or class II (IA/IE) mAbs. In contrast, H3 virus-infected CD1d(-/-) myocytes are not killed. These studies demonstrate that CD1d expression is essential for pathogenicity of CVB3-induced myocarditis, that CD1d expression is increased early after infection in vivo in CD1d(+) mice infected with the myocarditic but not with the nonmyocarditic CVB3 variant, and that V gamma 4(+) T cells, which are known to promote myocarditis susceptibility, appear to recognize CD1d expressed by CVB3-infected myocytes.