COVID-19 Usurps Host Regulatory Networks

Abstract

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection causes coronavirus disease 2019 (COVID-19). SARS-CoV-2 binds the angiotensin-converting enzyme 2 (ACE2) on the cell surface and this complex is internalized. ACE2 serves as an endogenous inhibitor of inflammatory signals associated with four major regulator systems: the renin-angiotensin-aldosterone system (RAAS), the complement system, the coagulation cascade, and the kallikrein-kinin system (KKS). Understanding the pathophysiological effects of SARS-CoV-2 on these pathways is needed, particularly given the current lack of proven, effective treatments. The vasoconstrictive, prothrombotic and pro-inflammatory conditions induced by SARS-CoV-2 can be ascribed, at least in part, to the activation of these intersecting physiological networks. Moreover, patients with immune deficiencies, hypertension, diabetes, coronary heart disease, and kidney disease often have altered activation of these pathways, either due to underlying disease or to medications, and may be more susceptible to SARS-CoV-2 infection. Certain characteristic COVID-associated skin, sensory, and central nervous system manifestations may also be linked to viral activation of the RAAS, complement, coagulation, and KKS pathways. Pharmacological interventions that target molecules along these pathways may be useful in mitigating symptoms and preventing organ or tissue damage. While effective anti-viral therapies are critically needed, further study of these pathways may identify effective adjunctive treatments and patients most likely to benefit.

Keywords: COVID-19; SARS-CoV-2; angiotensin II; bradykinin; coagulation; pharmacotherapy; renin-angiotensin-aldosterone system; substance P.

Figure 1

Coronavirus structure. Coronaviruses contain a trimeric spike (S)-protein that mediates attachment to the host receptor, abundant membrane (M)-proteins, envelope (E)-proteins that facilitate assembly and release of the virus, nucleocapsid (N)-proteins that bind positive-strand RNA, and hemagglutinin-esterase (HE)-proteins that bind sialic acids on surface glycoproteins and manifest acetyl-esterase activity.

Figure 2

Characterized SARS-CoV-2 entry. SARS-CoV-2 binds to host cell angiotensin converting enzyme-2 (ACE2). The protease TMPRSS2 is recruited and cleaves ACE2 and activates the S protein for membrane fusion and viral entry.

Figure 3

Proposed model of SARS-CoV-2 pathogenesis. SARS-CoV-2 binds ACE2 on alveolar type I and type II cells, macrophages, neurons, and arterial and venous endothelial cells. Complement, damage-and pathogen-associated molecular pattern ligands (DAMPs, PAMPs), and cytokines prime circulating neutrophils that are recruited in response to IL-8 and IL-6 released from infected cells and mast cells. SARS-CoV-2-induces epithelial apoptosis. Alveolar macrophages remove apoptotic cells through complement independent and dependent mechanisms. Apoptotic cells generate antigens that bind C1q and the interaction may be enhanced by C reactive protein (CRP). The activation of C1q induces C3b deposition for macrophage complement receptor 1 (CR1) binding and phagocytosis. Activated neutrophils produce heparin binding protein (HBP), prostaglandins (PGE2), and extracellular traps (NETs) to capture and kill the virus. NET activity induces a form of cell death called NETosis. Both NETosis and apoptosis generate DAMPs and PAMPs that bind and activate toll-like receptors in promoting inflammation. Mast cells and neutrophils are activated by complement factors and the neuropeptide, substance P (SP), which promotes their degranulation. Mast cells also produce histamine that promotes vasodilation, tryptase involved in complement factor production, and renin in the RAAS. Excessive inflammation, associated with MCP-1-recruited monocytes, promotes the accumulation of fluid, leading to alveolar edema. Platelets are activated by SP and exhibit cross-talk with neutrophil NETs in promoting coagulation. The kallikrein-kinin system is activated by damaged tissue and cells, such as neutrophils. Kallikrein functions as a precursor to bradykinin, activates pro-renin, and cleaves complement C3 and C5 as well as plasminogen. The latter generates plasmin involved in the degradation of fibrin and the formation of D-dimers identified in COVID-19 patient serum. SARS-CoV-2-induced degradation of ACE2 promotes RAAS activity, vasoconstriction, and hypertension. Angiotensin II, various cytokines, and HBP induce the expression of endothelial integrins. In the absence of ACE2, which also binds integrins, the functions of integrins may be dysregulated, promoting inflammation, hypertension, and thromboses. The infection can proceed to acute respiratory distress syndrome (ARDS) and culminates in additional tissues and organs in response to systemic infection.

Figure 4

SARS-CoV-2 in the RAAS pathway. Prostaglandins stimulate the release of pro-renin from juxtaglomerular cells. Pro-renin is cleaved to renin by kallikrein. Renin is also produced by activated mast cells. Renin transforms angiotensinogen into angiotensin I. ACE or mast cell chymase converts angiotensin I into angiotensin II, which binds AT₁R, and stimulates the production of aldosterone and subsequently IL-6. Aldosterone promotes renal distal tubular reabsorption of sodium, increasing blood pressure. In response to intravascular volume expansion, cells in the atrial wall release atrial natriuretic peptide (ANP), which down-regulates angiotensin II activity. Mononuclear leukocyte-derived aspartate decarboxylase (MLDAD) converts the octapeptide angiotensin II to another octapeptide, angiotensin A, which promotes the activation or AT₁R, or may generate the anti-inflammatory heptapeptide (seven amino acid) ligand, alamandine, via ACE2 activity. ACE2 also converts angiotensin II to angiotensin-(1-7), which is a ligand for both the Mas receptor and AT₂R involved in vasodilation and anti-inflammatory responses. Additional aminopeptidases convert angiotensin II into angiotensin III and IV. Angiotensin III can bind Mas and both angiotensin III and substance P activate MGRPRX2 on mast cells. A lack of ACE2 in RAAS due to SARS-CoV-2-induced degradation may suggest a benefit for intervention along the ACE/angiotensin II/AT₁R/aldosterone pathway. These may include renin inhibitors (Renin-i), AT₁R blockers (ARB), ACE inhibitors (ACEi), aldosterone blockers (ALDi), or diuretics.

Figure 5

SARS-CoV-2 in the complement system. Classical, lectin, and alternative are the three pathways in the complement system. Complement components C1, C2, C3, and C4 are present in plasma in inactive forms. In the classical pathway, the C1 component, C1q, recognizes apoptotic cells directly or pathogens indirectly through antibody complexes or associations with pentraxins. In the lectin pathway, mannose-binding lectin (MBL) binds the surface of the pathogen. Serine proteases complex with C1q (C1r/C1s) and MBL (MASP: MBL-associated serine protease), which leads to cleavage of C4 to its fragments (C4b and C4a) and the formation of a C3 and C5 convertase. In the alternative pathway, C3 is spontaneously hydrolyzed and through the activity of factor D forms a C3 convertase and subsequently a C5 convertase. Mast cell tryptase, thrombin or kallikrein can also cleave C3 and C5 whereas renin cleaves only C3. Cleavage fragments from these pathways (e.g. C3a, C5a, C5b) activate immune cell subsets to produce inflammation or coagulation. The terminal product of these pathways, C5b6789, is a membrane attack complex (MAC), that creates a pore in cell membranes by displacing phospholipids. The resulting cell lysis induces inflammatory responses. C1q also acts independent of the complement system and binds its receptor (C1qR) on aggregated platelets and endothelial cells in the promotion of coagulation and angiogenesis, respectively. Tissue and organ damage and excessive inflammation in some COVID-19 patients may indicate that SARS-CoV-2 activates the complement cascade. C1, C3, and C5 inhibitors (i) block factor formation in the complement cascade.

Figure 6

SARS-CoV-2 in coagulation. Primary hemostasis controls platelet aggregation and secondary hemostasis promotes fibrin formation through the clotting cascade, the latter involving intrinsic and extrinsic pathways. In the intrinsic pathway, damage-induced release of endothelial collagen activates factor XII (factor XIIa), which is a reaction that is also involved in the initiation of plasma kallikrein. Factor XIIa acts as a catalyst to activate factor XI to factor XIa. Factor XIa activates factor IX to factor IXa and the latter, acting with factor VIIIa as a cofactor, activates factor X to factor Xa. Tissue injury releases tissue factor into the blood, which activates platelets to induce neutrophil extracellular trap (NET) formation. The components of NETs reciprocally activate platelets and their aggregation. In the extrinsic pathway, tissue factor activates factor VII (factor VIIa), which activates factor X (factor Xa). The common coagulation pathway commences at factor X, with factor Xa, factor Va, calcium and platelet phospholipids forming the prothrombinase complex, which activates prothrombin (factor II) to thrombin (factor IIa). Thrombin cleaves factor V, VIII, factor XIII and fibrinogen. Polymerization of the formed fibrinopeptides produces fibrin, which is covalently cross-linked by FXIIIa to form a stable nascent fibrin clot. These processes can be antagonized by the heparin-dependent activity of antithrombin and plasmin degradation of fibrin into soluble fibrin degradation products (e.g. D-dimer). Plasmin formation is reduced by angiotensin II- or IL-6-induced plasminogen activator inhibitor (PAI)-1, which antagonizes the activity of urokinase plasminogen activator (uPA) and tissue plasminogen activator (tPA). SARS-CoV-2-induced degradation of ACE2 reduces ACE2 cleavage of angiotensin I and II and the anti-inflammatory effects of the ACE2 fragment [Ang-(1-7)], suggesting that inhibitors of RAAS [e.g. ACE inhibitor (ACEi)] may counter the effects of SARS-CoV-2. Mast cells activated by thrombin or complement (C5a) also contribute to coagulation through the preferential production of IL-6 and PAI-1. Drugs that inhibit IL-6 signals (e.g. anti-IL-6 receptor antibodies) may inhibit IL-6-induced PAI-1 production and IL-6 recruitment of neutrophils. Statins also inhibit IL-6 production, the activity of neutrophils and the functions of activated protein C. The use of inhibitors (i) to the clotting cascade such as factor Xa (e.g. apixaban) and factor IIa (e.g. carfilzomib) may impede SARS-CoV-2-induced coagulation responses. Lastly, low molecular weight heparin (LMWH) therapy may promote the effects of anti-thrombin and inhibit the functions of the alarmin, heparin binding protein (HBP), which binds glycosaminoglycan moieties of cell surface proteoglycans and promotes endothelial permeability.

Figure 7

SARS-CoV-2 and kinins. Tissue prekallikrein released from neutrophils and additional cell types is cleaved by cell surface proteases into tissue kallikrein, which promotes the transformation of low molecular weight (LMW) kininogen into kallidin. An aminopeptidase cleaves kallidin and generates bradykinin, which is a product of factor IIa-activated plasma kallikrein and high molecular weight (HMW) kininogen. Kallidin and bradykinin can be cleaved by ACE (kininase II) to form inactive peptides or carboxypeptidase-M (CPM/kininase I), which forms bradykinin 1 receptor (B1R) ligands des-Arg10-kallidin and des-Arg9-bradykinin. ACE2 is the SARS-CoV-2 receptor, suggesting that ACE2 degradation of des-Arg9-bradykinin may be impaired in COVID-19 patients. Both kallidin and bradykinin activate the bradykinin receptor, B2R, which induces the activation of mast cells, the phosphorylation/degradation of IkBα through the 26S proteasome and the release of NF-kB transcription factors involved in the production of cytokines and the prostaglandin-generating enzyme COX-2. Proteasome inhibitors (26Si) antagonize NF-kB activation. B1R and B2R induce a calcium flux that promotes substance P production and the activation of phospholipase A₂ (PLA₂) enzymes. PLA₂ liberates arachidonic acid for COX-2-induced production of prostaglandins which can be blocked with COX-2 inhibitors (COX2i). Prostaglandins induce anti-inflammatory signals. Substance P and bradykinin-induced IL-6 recruit neutrophils into tissues. Substance P also promotes the degranulation of mast cells and neutrophils. Neurokinin-1 (NK1) receptor antagonist (NKi) may block neutrophil recruitment and respiratory burst activity.