Enter the kill zone: initiation of death signaling during virus entry

Caspase-mediated cleavage of nucleocapsid protein of a protease-independent porcine epidemic diarrhea virus strain

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Infection of PEDV 8aa in Vero cells leads to apoptotic cell death.

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Caspase 6 or 7 can cleave PEDV 8aa N protein at the late stage of the replication.

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The caspase-mediated cleavage occurs between D424 and G425 near C-terminal of N protein.

Porcine epidemic diarrhea virus (PEDV) infection in neonatal piglets can cause up to 100% mortality, resulting in significant economic loss in the swine industry. Like other coronaviruses, PEDV N protein is a nucleocapsid protein and abundantly presents at all stages of infection. Previously, we reported that the N protein of trypsin-independent PEDV 8aa is cleaved during virus replication. In this study, we further investigated the nature of N protein cleavage using various methods including protease cleavage assays with or without various inhibitors and mutagenesis study. We found that PEDV 8aa infection in Vero cells leads to apoptotic cell death, and caspase 6 or 7 can cleave PEDV 8aa N protein at the late stage of the replication. The caspase-mediated cleavage occurs between D⁴²⁴ and G⁴²⁵ near the C-terminal of N protein. We also report that both cleaved and uncleaved N proteins are exclusively localized in the cytoplasm of PEDV infected cells.

An Orchestra of Reovirus Receptors: Still Searching for the Conductor

Viruses are constantly engaged in a molecular arms race with the host, where efficient and tactical use of cellular receptors benefits critical steps in infection. Receptor use dictates initiation, establishment, and spread of viral infection to new tissues and hosts. Mammalian orthoreoviruses (reoviruses) are pervasive pathogens that use multiple receptors to overcome protective host barriers to disseminate from sites of initial infection and cause disease in young mammals. In particular, reovirus invades the central nervous system (CNS) with serotype-dependent tropism and disease. A single viral gene, encoding the attachment protein σ1, segregates with distinct patterns of CNS injury. Despite the identification and characterization of several reovirus receptors, host factors that dictate tropism via interaction with σ1 remain undefined. Here, we summarize the state of the reovirus receptor field and discuss open questions toward understanding how the reovirus attachment protein dictates CNS tropism.

Investigations of Pro- and Anti-Apoptotic Factors Affecting African Swine Fever Virus Replication and Pathogenesis

African swine fever virus (ASFV) is a large DNA virus that replicates predominantly in the cell cytoplasm and is the only member of the Asfarviridae family. The virus causes an acute haemorrhagic fever, African swine fever (ASF), in domestic pigs and wild boar resulting in the death of most infected animals. Apoptosis is induced at an early stage during virus entry or uncoating. However, ASFV encodes anti-apoptotic proteins which facilitate production of progeny virions. These anti-apoptotic proteins include A179L, a Bcl-2 family member; A224L, an inhibitor of apoptosis proteins (IAP) family member; EP153R a C-type lectin; and DP71L. The latter acts by inhibiting activation of the stress activated pro-apoptotic pathways pro-apoptotic pathways. The mechanisms by which these proteins act is summarised. ASF disease is characterised by massive apoptosis of uninfected lymphocytes which reduces the effectiveness of the immune response, contributing to virus pathogenesis. Mechanisms by which this apoptosis is induced are discussed.

Concepts and mechanisms underlying chemotherapy induced immunogenic cell death: impact on clinical studies and considerations for combined therapies

Chemotherapy has historically been thought to induce cancer cell death in an immunogenically silent manner. However, recent studies have demonstrated that therapeutic outcomes with specific chemotherapeutic agents (e.g. anthracyclines) correlate strongly with their ability to induce a process of immunogenic cell death (ICD) in cancer cells. This process generates a series of signals that stimulate the immune system to recognize and clear tumor cells. Extensive studies have revealed that chemotherapy-induced ICD occurs via the exposure/release of calreticulin (CALR), ATP, chemokine (C–X–C motif) ligand 10 (CXCL10) and high mobility group box 1 (HMGB1). This review provides an in-depth look into the concepts and mechanisms underlying CALR exposure, activation of the Toll-like receptor 3/IFN/CXCL10 axis, and the release of ATP and HMGB1 from dying cancer cells. Factors that influence the impact of ICD in clinical studies and the design of therapies combining chemotherapy with immunotherapy are also discussed.

Innate immunity to adenovirus

Human adenoviruses are the most widely used vectors in gene medicine, with applications ranging from oncolytic therapies to vaccinations, but adenovirus vectors are not without side effects. In addition, natural adenoviruses pose severe risks for immunocompromised people, yet infections are usually mild and self-limiting in immunocompetent individuals. Here we describe how adenoviruses are recognized by the host innate defense system during entry and replication in immune and nonimmune cells. Innate defense protects the host and represents a major barrier to using adenoviruses as therapeutic interventions in humans. Innate response against adenoviruses involves intrinsic factors present at constant levels, and innate factors mounted by the host cell upon viral challenge. These factors exert antiviral effects by directly binding to viruses or viral components, or shield the virus, for example, soluble factors, such as blood clotting components, the complement system, preexisting immunoglobulins, or defensins. In addition, Toll-like receptors and lectins in the plasma membrane and endosomes are intrinsic factors against adenoviruses. Important innate factors restricting adenovirus in the cytosol are tripartite motif-containing proteins, nucleotide-binding oligomerization domain-like inflammatory receptors, and DNA sensors triggering interferon, such as DEAD (Asp-Glu-Ala-Asp) box polypeptide 41 and cyclic guanosine monophosphate-adenosine monophosphate synthase. Adenovirus tunes the function of antiviral autophagy, and counters innate defense by virtue of its early proteins E1A, E1B, E3, and E4 and two virus-associated noncoding RNAs VA-I and VA-II. We conclude by discussing strategies to engineer adenovirus vectors with attenuated innate responses and enhanced delivery features.

Coxsackievirus A16 infection triggers apoptosis in RD cells by inducing ER stress

Coxsackievirus A16 (CA16) infection, which is responsible for hand, foot and mouth disease (HFMD), has become a common health problem in Asia due to the prevalence of the virus. Thus, it is important to understand the pathogenesis of CA16 infection. Viruses that induce endoplasmic reticulum (ER) stress are confronted with the unfolded protein response (UPR), which may lead to apoptotic cell death and influence viral replication. In this study, we found that CA16 infection could induce apoptosis and ER stress in RD cells. Interestingly, apoptosis via the activation of caspase-3, -8 and -9 in the extrinsic or intrinsic apoptotic pathways in RD cells was inhibited by 4-phenyl butyric acid (4PBA), a chemical chaperone that reduces ER stress. These results suggest that CA16 infection leads to ER stress, which in turn results in prolonged ER stress-induced apoptosis. This study provides a new basis for understanding CA16 infection and host responses.

Bovine adenovirus 3 core protein precursor pVII localizes to mitochondria, and modulates ATP synthesis, mitochondrial Ca2+ and mitochondrial membrane potential

Viruses modulate the functions of mitochondria by translocating viral proteins to the mitochondria. Subcellular fractionation and sensitivity to proteinase K/Triton X-100 treatment of mitochondrial fractions of bovine adenovirus (BAdV)-3-infected/transfected cells suggested that core protein pVII localizes to the mitochondria and contains a functional mitochondrial localization signal. Moreover, mitochondrial localization of BAdV-3 pVII appears to help in the retention of mitochondrial Ca(2+), inducing a significant increase in the levels of ATP and maintaining the mitochondrial membrane potential (MMP) in transfected cells. In contrast, mitochondrial localization of BAdV-3 pVII has no significant effect on the levels of cytoplasmic Ca(2+) and reactive oxygen species production in the transfected cells. Consistent with these results, expression of pVII in transfected cells treated with staurosporine decreased significantly the activation of caspase-3. Our results suggested that BAdV-3 pVII localizes to mitochondria, and interferes with apoptosis by inhibiting loss of the MMP and by increasing mitochondrial Ca(2+) and ATP production.

Implication of Hsc70, PDI and integrin αvβ3 involvement during entry of the murine rotavirus ECwt into small-intestinal villi of suckling mice

In the present study, a homologous rotavirus, ECwt, infecting small intestinal villi isolated from ICR and BALB/c mice were used as a model for identifying cell-surface molecules involved in rotavirus entry. Small-intestinal villi were treated with anti-Hsc70, anti-PDI, anti-integrin β3 or anti-ERp57 antibodies or their corresponding F(ab')2 fragments before inoculation with rotavirus ECwt, RRV or Wa. Pretreatment of villi decreased virus infectivity by about 50-100 % depending of the rotavirus strain, antibody structure and detection assay used. Similar results were obtained by treating viral inocula with purified proteins Hsc70, PDI or integrin β3 before inoculation of untreated villi. Rotavirus infection of villi proved to be sensitive to membrane-impermeant thiol/disulfide inhibitors such as DTNB and bacitracin, suggesting the involvement of a redox reaction in infection. The present results suggest that PDI, Hsc70 and integrin β3 are used by both homologous and heterologous rotaviruses during infection of isolated mouse villi.

Competitive cell interactions in cancer: a cellular tug of war

Within tissues, cells sense differences in fitness levels and this can lead to fitter cells eliminating less fit, albeit viable, cells via competitive cell interactions. The involvement of several cancer-related genes in this phenomenon has drawn attention to a potential connection between competitive cell interactions and cancer. Indeed, initial studies found that tumor-promoting genes can turn cells into 'supercompetitors', able to kill normal cells around them. However, more recently it has been observed that cells harboring certain cancer-promoting mutations can be eliminated by surrounding normal cells, suggesting that competitive cell interactions could also have a tumor-suppressive role. These findings suggest a new view whereby tumor and host cells engage in a bidirectional tug of war, the outcome of which may have a profound impact on disease progression.

Role of apoptosis in disease

Since the initial description of apoptosis, a number of different forms of cell death have been described. In this review we will focus on classic caspase-dependent apoptosis and its variations that contribute to diseases. Over fifty years of research have clarified molecular mechanisms involved in apoptotic signaling as well and shown that alterations of these pathways lead to human diseases. Indeed both reduced and increased apoptosis can result in pathology. More recently these findings have led to the development of therapeutic approaches based on regulation of apoptosis, some of which are in clinical trials or have entered medical practice.

Recent progress in studies of arterivirus- and coronavirus-host interactions

Animal coronaviruses, such as infectious bronchitis virus (IBV), and arteriviruses, such as porcine reproductive and respiratory syndrome virus (PRRSV), are able to manifest highly contagious infections in their specific native hosts, thereby arising in critical economic damage to animal industries. This review discusses recent progress in studies of virus-host interactions during animal and human coronavirus and arterivirus infections, with emphasis on IBV-host cell interactions. These interactions may be directly involved in viral replication or lead to the alteration of certain signaling pathways, such as cell stress response and innate immunity, to facilitate viral replication and pathogenesis.

Beneficial actions of melatonin in the management of viral infections: a new use for this "molecular handyman"?

Melatonin (N‐acetyl‐5‐methoxytryptamine) is a multifunctional signaling molecule that has a variety of important functions. Numerous clinical trials have examined the therapeutic usefulness of melatonin in different fields of medicine. Clinical trials have shown that melatonin is efficient in preventing cell damage under acute (sepsis, asphyxia in newborns) and chronic states (metabolic and neurodegenerative diseases, cancer, inflammation, aging). The beneficial effects of melatonin can be explained by its properties as a potent antioxidant and antioxidant enzyme inducer, a regulator of apoptosis and a stimulator of immune functions. These effects support the use of melatonin in viral infections, which are often associated with inflammatory injury and increases in oxidative stress. In fact, melatonin has been used recently to treat several viral infections, which are summarized in this review. The role of melatonin in infections is also discussed herein. Copyright © 2012 John Wiley & Sons, Ltd.

Caspase cleavage of viral proteins, another way for viruses to make the best of apoptosis

Viral infection constitutes an unwanted intrusion that needs to be eradicated by host cells. On one hand, one of the first protective barriers set up to prevent viral replication, spread or persistence involves the induction of apoptotic cell death that aims to limit the availability of the cellular components for viral amplification. On the other hand, while they completely depend on the host molecular machinery, viruses also need to evade the cellular responses that are meant to destroy them. The existence of numerous antiapoptotic products within the viral kingdom proves that apoptosis constitutes a major threat that should better be bypassed. Among the different strategies developed to deal with apoptosis, one is based on what viruses do best: backfiring the cell on itself. Several unrelated viruses have been described to take advantage of apoptosis induction by expressing proteins targeted by caspases, the key effectors of apoptotic cell death. Caspase cleavage of these proteins results in various consequences, from logical apoptosis inhibition to more surprising enhancement or attenuation of viral replication. The present review aims at discussing the characterization and relevance of this post-translational modification that adds a new complexity in the already intricate host-apoptosis-virus triangle.