Meta-Biol

• Pathways

Coronaviruses (CoVs) are positive-sense RNA viruses that replicate in the interior of double membrane vesicles (DMV) in the cytoplasm of infected cells (Stertz S et al. 2007; Knoops K et al. 2008). The viral replication and transcription are facilitated by virus-encoded non-structural proteins (SARS-CoV-1 nsp1–nsp16) that assemble to form a DMV-bound replication-transcription complex (RTC). The replication strategy of CoVs can generate both single-stranded RNA (ssRNA) and double-stranded RNA (dsRNA) species, that may act as pathogen-associated molecular patterns (PAMPs) recognized by pattern recognition receptor (PRR) such as toll-like receptor 7 (TLR7) and TLR8, antiviral innate immune response receptor RIG-I (also known as DEAD box protein 58, DDX58) and interferon-induced helicase C domain-containing protein 1 (IFIH1, also known as MDA5) (Cervantes-Barragan L et al. 2007; Chen Y et al. 2009, 2011; Daffis S et al. 2010; Li Y et al. 2013). The activated PRRs trigger signaling pathways to produce type I and type III interferons IFNs and proinflammatory mediators that perform antiviral functions. This Reactome module describes the mechanisms underlying PRR-mediated sensing of the severe acute respiratory syndrome coronavirus type 1 (SARS-CoV-1) infection. First, endosomal recognition of viral ssRNA occurs by means of TLR7 and TLR8 which detect GU-rich ssRNA sequences. Specifically, GU-rich ssRNA oligonucleotides derived from SARS-CoV-1 stimulated mononuclear phagocytes to release considerable levels of pro‑inflammatory cytokines TNF‑a, IL‑6 and IL‑12 via TLR7 and TLR8 (Li Y et al. 2013). Second, SARS-CoV-1 dsRNA replication intermediates can be recognized by cytoplasmic receptors DDX58 and IFIH1 which bind to mitochondrial antiviral-signaling protein (MAVS, IPS-1) to induce the IFN-mediated antiviral response. In addition, the module shows an antiviral function of interferon-induced protein with tetratricopeptide repeats 1 (IFIT1) that directly binds and sequesters viral single-stranded uncapped 5′-ppp RNA and cap-0 RNA (Daffis S et al. 2010). This module also describes several strategies developed by SARS-CoV-1 to evade or alter host immunity, including escaping innate immune sensors, inhibiting IFN production and signaling, and evading antiviral function of IFN stimulated gene (ISG) products. For example, viral dsRNA replication intermediates derived from SARS‑CoV‑1 were shown to associate with RTC bound to double membrane vesicles, which protected viral RNA from sensing by DDX58 or IFIH1 (Stertz S et al. 2007; Knoops K et al. 2008). Further, SARS-CoV-1 encodes nsp14 and nsp16 which possess guanine-N7-methyltransferase activity and 2’-O-methyl-transferase activity respectively (Chen Y et al. 2009, 2011). SARS-CoV-1 nsp14 generates 5' cap-0 viral RNA (m7GpppN, guanine N7-methylated) and nsp16 further methylates cap-0 viral RNA. These viral RNA modifications mimic the 5'-cap structure of host mRNAs allowing the virus to efficiently evade recognition by cytosolic DDX58 and IFIH1 (Chen Y et al. 2009, 2011; Daffis S et al. 2010). The nsp16-mediated ribose 2′-O-methylation of viral RNA also blocks the antiviral function of IFIT1 complexes (Menachery VD et al. 2014). Further, the uridylate‐specific endoribonuclease (EndoU) activity of viral nsp15 degrades viral RNA to hide it from innate immune sensors (Bhardwaj K et al. 2006; Ricagno S et al. 2006). Moreover, SARS-CoV-1 encodes several proteins that directly bind to host targets associated with SARS‑CoV‑1 infection and cytokine production (Frieman M et al. 2009; Hu Y et al. 2017; Kopecky-Bromberg SA et al. 2007; Lindner H et al. 2005; Siu KL et al. 2009). This Reactome module describes several such binding events and their consequences. For example, as a de-ubiquitinating enzyme, viral nsp3 binds to and removes polyubiquitin chains of signaling proteins such as TRAF3, TRAF6, STING, IkBA, and IRF3 thereby modulating the formation of signaling complexes and the activation of IRF3/7 and NFkappaB (Sun L et al. 2012; Chen X et al. 2014; Li SW et al. 2016). This inhibits IFN production downstream of TLR7/8, DDX58, IFIH1, MAVS and STING signaling pathways. Binding of SARS-CoV-1 nucleocapsid (N) protein to E3 ubiquitin ligase TRIM25 inhibits TRIM25-mediated DDX58 ubiquitination and DDX58-mediated signaling pathway (Hu Y et al. 2017). Next, SARS‑CoV‑1 membrane (M) protein targets IBK1/IKBKE and TRAF3 to prevent the formation of the TRAF3:TANK:TBK1/IKBKE complex and thereby inhibits TBK1/IKBKE‑dependent activation of IRF3/IRF7 transcription factors downstream of DDX58, IFIH1 and adaptor MAVS (Siu KL et al. 2009; 2014). The ion channel activities of open reading frame 3a (orf3a or 3a) and E contribute to activation of the NLRP3 inflammasome leading to highly inflammatory pyroptotic cell death (Nieto‑Torres JL et al. 2015; Chen IY et al. 2019; Yue Y et al. 2018). Viral 3a promoted the NLRP3-mediated formation of PYCARD (ASC) speck by interaction with both TRAF3 and PYCARD (ASC) (Siu KL et al. 2019). Binding of 3a to caspase-1 (CASP1) enhanced CASP1-mediated cleavage of interleukin 1 beta (IL‑1β) downstream of the NLRP3 inflammasome pathway (Yue Y et al. 2018). Like 3a, SARS-CoV-1 8b was found to bind to NLRP3 activating the NLRP3 inflammasome and triggering IL‑1β release (Shi CS et al. 2019). 8b was also shown to bind IRF3, inhibiting subsequent IRF3 dimerization (Wong et al. 2018). At the plasma membrane, binding of SARS-CoV-1 7a to host BST2 disrupts the antiviral tethering function of BST2 which restricts the release of diverse mammalian enveloped viruses (Taylor JK et al. 2015). SARS-CoV-1 9b (orf9b) inhibits the MAVS-mediated production of type I IFNs by targeting TOMM70 on the mitochondria (Jiang HW et al. 2020). SARS-CoV-1 6 (orf6) inhibits the IFN signaling pathway by tethering karyopherins KPNA2 and KPNB1 to the endoplasmic reticulum (ER)/Golgi intermediate compartment (ERGIC) and thus blocking the KPNA1:KPNB1-dependent nuclear import of STAT1 (Frieman M et al. 2007). Binding of SARS-CoV-1 nsp1 to peptidyl-prolyl isomerases (PPIases) and calcipressin-3 (RCAN3) significantly activates the cyclophilin A/NFAT pathway, ultimately enhancing the induction of the IL-2 promoter (Pfefferle et al, 2011; Law et al, 2007). At last, SARS‑CoV‑1 3b, after translocating to the nucleus, binds to transcription factor RUNX1 and increases its promoting activity (Varshney et al, 2012).

Infectious diseases are ones due to the presence of pathogenic microbial agents in human host cells. Processes annotated in this category include bacterial, viral and parasitic infection pathways.

Bacterial infection pathways currently include some metabolic processes mediated by intracellular Mycobacterium tuberculosis, the actions of clostridial, anthrax, and diphtheria toxins, and the entry of Listeria monocytogenes into human cells.

Viral infection pathways currently include the life cycles of SARS-CoV viruses, influenza virus, HIV (human immunodeficiency virus), and human cytomegalovirus (HCMV).

Parasitic infection pathways currently include Leishmania infection-related pathways.

Fungal infection pathways and prion diseases have not been annotated.

Biological processes are captured in Reactome by identifying the molecules (DNA, RNA, protein, small molecules) involved in them and describing the details of their interactions. From this molecular viewpoint, human disease pathways have three mechanistic causes: the inclusion of microbially-expressed proteins, altered functions of human proteins, or changed expression levels of otherwise functionally normal human proteins.

The first group encompasses the infectious diseases such as influenza, tuberculosis and HIV infection. The second group involves human proteins modified either by a mutation or by an abnormal post-translational event that produces an aberrant protein with a novel function. Examples include somatic mutations of EGFR and FGFR (epidermal and fibroblast growth factor receptor) genes, which encode constitutively active receptors that signal even in the absence of their ligands, or the somatic mutation of IDH1 (isocitrate dehydrogenase 1) that leads to an enzyme active on 2-oxoglutarate rather than isocitrate, or the abnormal protein aggregations of amyloidosis which lead to diseases such as Alzheimer's.

Infectious diseases are represented in Reactome as microbial-human protein interactions and the consequent events. The existence of variant proteins and their association with disease-specific biological processes is represented by inclusion of the modified protein in a new or variant reaction, an extension to the 'normal' pathway. Diseases which result from proteins performing their normal functions but at abnormal rates can also be captured, though less directly. Many mutant alleles encode proteins that retain their normal functions but have abnormal stabilities or catalytic efficiencies, leading to normal reactions that proceed to abnormal extents. The phenotypes of such diseases can be revealed when pathway annotations are combined with expression or rate data from other sources.

Depending on the biological pathway/process immediately affected by disease-causing gene variants, non-infectious diseases in Reactome are organized into diseases of signal transduction by growth factore receptors and second messengers, diseases of mitotic cell cycle, diseases of cellular response to stress, diseases of programmed cell death, diseases of DNA repair, disorders of transmembrane transporters, diseases of metabolism, diseases of immune system, diseases of neuronal system, disorders of developmental biology, disorders of extracellular matrix organization, and diseases of hemostatis.