An acute respiratory disease, caused by a novel coronavirus (SARS-CoV-2, previously known as 2019-nCoV), the coronavirus disease 2019 (COVID-19) has spread throughout China and received worldwide attention. On 30 January 2020, World Health Organization (WHO) officially declared the COVID-19 epidemic as a public health emergency of international concern. The emergence of SARS-CoV-2, since the severe acute respiratory syndrome coronavirus (SARS-CoV) in 2002 and Middle East respiratory syndrome coronavirus (MERS-CoV) in 2012, marked the third introduction of a highly pathogenic and large-scale epidemic coronavirus into the human population in the twenty-first century. As of 1 March 2020, a total of 87,137 confirmed cases globally, 79,968 confirmed in China and 7169 outside of China, with 2977 deaths (3.4%) had been reported by WHO. Meanwhile, several independent research groups have identified that SARS-CoV-2 belongs to β-coronavirus, with highly identical genome to bat coronavirus, pointing to bat as the natural host. The novel coronavirus uses the same receptor, angiotensin-converting enzyme 2 (ACE2) as that for SARS-CoV, and mainly spreads through the respiratory tract. Importantly, increasingly evidence showed sustained human-to-human transmission, along with many exported cases across the globe. The clinical symptoms of COVID-19 patients include fever, cough, fatigue and a small population of patients appeared gastrointestinal infection symptoms. The elderly and people with underlying diseases are susceptible to infection and prone to serious outcomes, which may be associated with acute respiratory distress syndrome (ARDS) and cytokine storm. Currently, there are few specific antiviral strategies, but several potent candidates of antivirals and repurposed drugs are under urgent investigation. In this review, we summarized the latest research progress of the epidemiology, pathogenesis, and clinical characteristics of COVID-19, and discussed the current treatment and scientific advancements to combat the epidemic novel coronavirus.

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The origin, transmission and clinical therapies on coronavirus disease 2019 (COVID-19) outbreak - an update on the status.

The presence of DNA in the cytoplasm of mammalian cells is a danger signal that triggers host immune responses such as the production of type I interferons. Cytosolic DNA induces interferons through the production of cyclic guanosine monophosphate–adenosine monophosphate (cyclic GMP-AMP, or cGAMP), which binds to and activates the adaptor protein STING. Through biochemical fractionation and quantitative mass spectrometry, we identified a cGAMP synthase (cGAS), which belongs to the nucleotidyltransferase family. Overexpression of cGAS activated the transcription factor IRF3 and induced interferon-β in a STING-dependent manner. Knockdown of cGAS inhibited IRF3 activation and interferon-β induction by DNA transfection or DNA virus infection. cGAS bound to DNA in the cytoplasm and catalyzed cGAMP synthesis. These results indicate that cGAS is a cytosolic DNA sensor that induces interferons by producing the second messenger cGAMP.

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Cyclic GMP-AMP Synthase is a Cytosolic DNA Sensor that Activates the Type-I Interferon Pathway

Interferon-stimulated gene (ISG) products take on a number of diverse roles. Collectively, they are highly effective at resisting and controlling pathogens. In this review, we begin by introducing interferon (IFN) and the JAK-STAT signaling pathway to highlight features that impact ISG production. Next, we describe ways in which ISGs both enhance innate pathogen-sensing capabilities and negatively regulate signaling through the JAK-STAT pathway. Several ISGs that directly inhibit virus infection are described with an emphasis on those that impact early and late stages of the virus life cycle. Finally, we describe ongoing efforts to identify and characterize antiviral ISGs, and we provide a forward-looking perspective on the ISG landscape.

Interferon-Stimulated Genes: A Complex Web of Host Defenses

Cytosolic DNA induces type I interferons and other cytokines that are important for antimicrobial defense but can also result in autoimmunity. This DNA signaling pathway requires the adaptor protein STING and the transcription factor IRF3, but the mechanism of DNA sensing is unclear. We found that mammalian cytosolic extracts synthesized cyclic guanosine monophosphate–adenosine monophosphate (cyclic GMP-AMP, or cGAMP) in vitro from adenosine triphosphate and guanosine triphosphate in the presence of DNA but not RNA. DNA transfection or DNA virus infection of mammalian cells also triggered cGAMP production. cGAMP bound to STING, leading to the activation of IRF3 and induction of interferon-β. Thus, cGAMP functions as an endogenous second messenger in metazoans and triggers interferon production in response to cytosolic DNA.

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Cyclic-GMP-AMP Is An Endogenous Second Messenger in Innate Immune Signaling by Cytosolic DNA

It is increasingly understood that autophagy is an ancient defence mechanism that has become incorporated into numerous immunological pathways. As discussed in this Review, its immunological roles include the elimination of microorganisms, the control of inflammation, the regulation of antigen presentation and lymphocyte homeostasis, and the secretion of immune mediators.

Autophagy in infection, inflammation and immunity

During virus infection, the adaptor proteins MAVS and STING transduce signals from the cytosolic nucleic acid sensors RIG-I and cGAS, respectively, to induce type I interferons (IFNs) and other antiviral molecules. Here we show that MAVS and STING harbor two conserved serine and threonine clusters that are phosphorylated by the kinases IKK and/or TBK1 in response to stimulation. Phosphorylated MAVS and STING then bind to a positively charged surface of interferon regulatory factor 3 (IRF3) and thereby recruit IRF3 for its phosphorylation and activation by TBK1. We further show that TRIF, an adaptor protein in Toll-like receptor signaling, activates IRF3 through a similar phosphorylation-dependent mechanism. These results reveal that phosphorylation of innate adaptor proteins is an essential and conserved mechanism that selectively recruits IRF3 to activate the type I IFN pathway.

https://science.sciencemag.org/content/sci/early/2015/01/28/science.aaa2630.full.pdf

Phosphorylation of innate immune adaptor proteins MAVS, STING, and TRIF induces IRF3 activation

The innate immune system utilizes pattern-recognition receptors (PRRs) to detect the invasion of pathogens and initiate host antimicrobial responses such as the production of type I interferons and proinflammatory cytokines. Nucleic acids, which are essential genetic information carriers for all living organisms including viral, bacterial, and eukaryotic pathogens, are major structures detected by the innate immune system. However, inappropriate detection of self nucleic acids can result in autoimmune diseases. PRRs that recognize nucleic acids in cells include several endosomal members of the Toll-like receptor family and several cytosolic sensors for DNA and RNA. Here, we review the recent advances in understanding the mechanism of nucleic acid sensing and signaling in the cytosol of mammalian cells as well as the emerging role of cytosolic nucleic acids in autoimmunity.

Innate Immune Sensing and Signaling of Cytosolic Nucleic Acids

Invasion of microbial DNA into the cytoplasm of animal cells triggers a cascade of host immune reactions that help clear the infection; however, self DNA in the cytoplasm can cause autoimmune diseases. Biochemical approaches led to the identification of cyclic guanosine monophosphate-adenosine monophosphate (cGAMP) synthase (cGAS) as a cytosolic DNA sensor that triggers innate immune responses. Here, we show that cells from cGAS-deficient (cGas(-/-)) mice, including fibroblasts, macrophages, and dendritic cells, failed to produce type I interferons and other cytokines in response to DNA transfection or DNA virus infection. cGas(-/-) mice were more susceptible to lethal infection with herpes simplex virus 1 (HSV1) than wild-type mice. We also show that cGAMP is an adjuvant that boosts antigen-specific T cell activation and antibody production in mice.

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Jiaxi Wu

Papers

Cyclic GMP-AMP Synthase is a Cytosolic DNA Sensor that Activates the Type-I Interferon Pathway

Cyclic-GMP-AMP Is An Endogenous Second Messenger in Innate Immune Signaling by Cytosolic DNA

Phosphorylation of innate immune adaptor proteins MAVS, STING, and TRIF induces IRF3 activation

Innate Immune Sensing and Signaling of Cytosolic Nucleic Acids

Pivotal Roles of cGAS-cGAMP Signaling in Antiviral Defense and Immune Adjuvant Effects