Unpuzzling COVID-19: tissue-related signaling pathways associated with SARS-CoV-2 infection and transmission

Abstract

The highly infective coronavirus disease 19 (COVID-19) is caused by a novel strain of coronaviruses - the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) - discovered in December 2019 in the city of Wuhan (Hubei Province, China). Remarkably, COVID-19 has rapidly spread across all continents and turned into a public health emergency, which was ultimately declared as a pandemic by the World Health Organization (WHO) in early 2020. SARS-CoV-2 presents similar aspects to other members of the coronavirus family, mainly regarding its genome, protein structure and intracellular mechanisms, that may translate into mild (or even asymptomatic) to severe infectious conditions. Although the mechanistic features underlying the COVID-19 progression have not been fully clarified, current evidence have suggested that SARS-CoV-2 may primarily behave as other β-coronavirus members. To better understand the development and transmission of COVID-19, unveiling the signaling pathways that may be impacted by SARS-CoV-2 infection, at the molecular and cellular levels, is of crucial importance. In this review, we present the main aspects related to the origin, classification, etiology and clinical impact of SARS-CoV-2. Specifically, here we describe the potential mechanisms of cellular interaction and signaling pathways, elicited by functional receptors, in major targeted tissues/organs from the respiratory, gastrointestinal (GI), cardiovascular, renal, and nervous systems. Furthermore, the potential involvement of these signaling pathways in evoking the onset and progression of COVID-19 symptoms in these organ systems are presently discussed. A brief description of future perspectives related to potential COVID-19 treatments is also highlighted.

Keywords: COVID-19; Coronavirus; SARS-CoV-2; signaling pathway.

Figure 1. Putative tissues/organs infected by SARS-CoV-2 and related COVID-19 symptoms

(1) Viral entry depends on the binding of spike (S) viral envelope protein to ACE2 in the host cell surface. TMPRSS2, together with other proteinases, processes S protein and allows viral endocytosis [28]. (2) The digestive system, heart, kidneys, respiratory system and peripheral neurons are tissues/cells where SARS-CoV-2 might infect, generating different symptoms observable in COVID-19 patients [31,32,90,142,196,250–252]. (3) Upon tissue infection, neurons that innervate those tissues could potentially be invaded by SARS-CoV-2 and infect the CNS by trans-synaptic route exchange (via peripheral nerves), thus promoting an interneuronal transfer of SARS-CoV-2, similar to other coronaviruses [183,184]. Nevertheless, other potential routes for CNS infection have also been hypothesized [166,176].

Figure 2. Canonical ACE2 pathway links multiple organ damage in COVID-19

SARS-CoV-2 infection down-regulates ACE2 expression and leads to the production of pro-inflammatory mediators, such as IL-6 [35]. Angiotensin-I (Ang-I) is converted into Ang-II by the ACE in the extracellular space. ACE2 is able to further cleave Ang-II to Ang(1-7), which binds MasR receptors on the cell surface and promotes anti-inflammatory, vasodilation and anti-fibrotic effects [35]. Since ACE2 is down-regulated during viral infection, this event will lead to the accumulation of Ang-II and binding to AT1R receptors on cellular membrane. AT1R signals through JAK-STAT and induces fibrosis, pro-inflammatory gene expression and vasoconstriction [48,251]. Multiple organs express ACE2 and are target for SARS-CoV-2. As they lose ACE2-mediated protection, Ang-II signaling contributes to the pathological findings observed in COVID-19 patients, such as disseminated coagulopathy and acute tissue damage [91].

Figure 3. Signaling pathways involved in COVID-19 pathophysiology

Toll-like receptors (TLRs) 3 and TLR 7/8 recognize SARS-CoV-2 RNA and initiate the inflammatory cascade via type I and type II IFN gene expression and NF-κB nuclear translocation [98,107]. Via NF-κB, the expression of multiple pro-inflammatory genes is stimulated, including pro-IL-1β, pro-IL-18, TNF and IL-6 [62–64]. The virus is also recognized by cytoplasmic NLRP3, which forms, together with ASC and caspase-1 (Casp-1), the inflammasome complex that will cleave and release mature forms of IL-1β and IL-18 [122]. The cytokines IL-1β, IL-18 and TNF bind to specific receptors and promote further NF-κB nuclear translocation and phosphorylation of p38 MAPK, which will lead to great expression of pro-inflammatory cytokines and chemokines [69,135]. IL-6, an important player in COVID-19, binds IL-6R and gp130 receptors to activate JAK/STAT-3 pathway and then contribute to the CRS observed in COVID-19 patients [88].

Figure 4. Possible mechanisms that explain viral persistence and disease severity

(1) Upon viral infection, cells increase the secretion of IL-6, which will paracrinally induce the expression of SOCS-1 via STAT-3 transcription factors [88]. (2) SOCS-1 hampers IFN antiviral signaling, via STAT-1 DE phosphorylation. The inhibition of expression of IFN genes leads to poor antiviral defense, and, as a consequence, reduced T-bet transcription, leading to defective Th1 polarization of CD4⁺ T cells [86,87,110,253]. (3) Upon viral entry, p38 is phosphorylated and, thereafter, perform downstream phosphorylation of target transcription factors, such as CREB, ATF-1 and AP-1. This effect leads to increased inflammatory gene expression and pro-survival gene expression, which may prolong the viral permanence in the infected cell [136]. (4) Upon recognition of viral particles and LPS from bacteria, TLR4-mediated signaling may lead to increased deleterious inflammation via NF-kB recruitment and p38 phosphorylation [105,106]. TLR4 polymorphism might explain the differential susceptibility to ARDS in COVID-19 patients [106].

Figure 5. Potential drug candidates for COVID-19 treatment

(1) Commercially available angiotensin-II receptor antagonists, such as losartan and derivates, could act by mitigating the deleterious effects of AT1R activation in COVID-19 [229]. Activation of such signaling cascade results in RAS up-regulation and leads to pro-inflammatory and pro-fibrotic effects in infected tissues [43–45]. (2) Blockage of RAS can prevent tissue damage in COVID-19 patients [229]. (3) The use of ACE inhibitors may also contribute to decrease of the response of RAS system [229]. (4) Inhibitors of serine protease such as TMPRSS2 may prevent the cleavage of S1 and S2 domains in the viral spike (S) protein, decreasing the ability of SARS-CoV-2 to infect cells [28]. Interleukin-6 (IL-6) is one of the major cytokines involved in COVID-19 progression, which leads to CRS and tissue damage due to severe inflammation [51]. The use of commercially available molecules, such as the monoclonal antibody against the IL-6 receptor, (5) Tocilizumab [242] and (6) Baricitinib [246,247], an inhibitor of the JAK/STAT pathway, can halt inflammation and mitigate deleterious effects related to IL-6. (7) Upon viral entry and denudation, viral RNA is released and translated into immature precursor proteins by the host ribosome machinery. These precursor proteins are processed by viral proteases and then form the mature replication complex RNA-dependent RNA polymerase (RdRp), which enables viral RNA replication. Part of the expanded RNA is translated into structural proteins, such as envelope, spike and nucleocapsid that will form and release novel virions [232,233]. The specific RdRp mediated RNA replication can be inhibited by selective drugs, such as Remdesivir [235] and Favipiravir [239], rendering the virus unable to propagate and, consequently, halting the infection.