Mechanisms of COVID: inflammation/cytokine cascade, thrombotic and fibrotic cascade, sepsis and endotheliopathy as key events. Review, text and graphic design by Mercedes BouterThe one question of the COVID-19 pandemic is:
Why are thrombosis and
thromboembolism persistent, in spite of adequate thromboprophylaxis and
anticoagulation?
The answer is: endothelial damage is a major
factor that keeps on driving thrombotic events. Meanwhile, the practice of prophylactic and therapeutic doses of anticoagulation to treat COVID thromboembolism might have actually been inadequate (see "A remaining challenge: hypercoagulability characterizing COVID-19, despite anticoagulation practices", 5 Oktober 2020, "A stubborn complication: the quest for solutions to COVID's thrombosis pandemic", 30 November 2020 and "The quest for solutions to COVID's thrombosis pandemic: endothelial (glycocalyx) dysfunction and mitochondrial dysfunction are starting points", 25 December 2020).
I was baffled to learn that as of January 2021, thromboembolism is still overlooked in COVID treatment trials. Any of the mechanisms of endotheliopathy, inflammation, hypercoagulation and thrombosis should be addressed in order to treat COVID-19 correctly. That is, COVID should be regarded an endothelial and hematologic disease. The basics of COVID-19 are understood, as its underlying mechanisms of endothelial damage and thrombosis, as well as its characteristic of impaired thrombolysis and fibrinolysis are well described since the 20th century. More recent studies, published between 2000-2019 by Gralinski and Levi shed light on urokinase pathways and crosstalks between inflammation and coagulation. In other words: contrary to popular belief (popular claims are mostly false), much is known about the COVID-19 mechanisms and pathologies. Just two main complications are timing and the occurrence of simultaneous cascades: too many events, too little time.
1. From entry of SARS-CoV-2 to COVID-19: how it starts
1.1 TMPRSS2, NRP1, ACE2 and macrophage infiltration;
1.2 PRRs, PAMP, DAMPs, NETs and inflammasome NLRP3;
1.2.1 Hyperactivation of macrophages, neutrophils and NKs incite inflammatory auto-activation;
1.3 Complement system induction of thrombosis and endothelial damage;
2 Dysfunctional endothelial cells (ECs), adhesion moleculed, platelets and OXPHOS;
2.1 Platelet interaction with dysfunctional endothelial cells promotes thrombosis;
2.1.1 Shifting towards a prothrombotic profile (Thromboxane, ADP, P2Y, PAI-1, TF, VWF, low ADAMTS-13);
2.1.2 Hypoxia-inducible factor-1-alpha (HIF-1a): differentiating between inflammatory and anti-inflammatory;
2.1.3 Antiphospholipid antibody formation;
2.2 ADAM17 promotes ACE2 downregulation and subsequent induction of adhesion molecules through TNF-α;
3 Amplification of inflammatory and coagulation cascades promote thrombosis;
3.1 IL-6 promotes adhesion molecules and enhances endothelial permeability through JAK/STAT;
3.2 Endothelial cell activation, amplification of inflammation and thrombosis;
4 Damage to the endothelial glycocalyx, impaired antioxidant activity, sepsis and impaired shear stress;
5 Possible therapeutic targets
1 From entry of SARS-CoV-2 to COVID-19: it starts
1.1 TMPRSS2, NRP1, ACE2 and macrophage infiltration
Looking at a
comprehensive picture of COVID-19, which I have drawn above, COVID
develops along and crossing the lines of inflammation, coagulation,
endotheliopathy and thrombosis. It starts with SARS-CoV-2 binding the
ACE2 receptor, infiltrating macrophages and activating interferon in
epithelial cells in order to upregulate ACE2 to enhance cellular uptake
in cells. ACE2 alone does not explain the coronavirus' success to penetrate cells. While other entry sites might be discovered within the next months, two compontents are certain co-factors of cellular uptake of the coronavirus: TMPRSS2 (the primer) and NRP1. Both components are used to enhance cellular uptake of SARS-CoV-2 through ACE2.
SARS-CoV-2 is not found to directly infect endothelial cells, but does cause endothelial cell dysfunction through direct binding of platelet ACE2, infection of macrophages, megakaryocyte hyperactivation of platelets, activation of PAMP and DAMPs and inflammatory NLRP3 involvement through Toll-like receptor-4 (TLR-4).
1.2 PRRs, PAMP, DAMPs, NETs and inflammasome NLRP3
Upon infection, pattern recognition receptors (PRR), such as Toll-like receptors (TLR) detect viral RNA and Lipopolysaccharide (LPS), pathogen-associated molecular patterns (PAMP). PRR signals interferon regulatory factors (IRF) and NF-kB. PAMPs mediate the breakdown of the endothelial glycocalyx. PAMPs induce Tissue Factor through monocytes. Subsequently, cytokines such as TNF and IL-1β induce Tissue Factor to generate thrombi, activate platelets and fibrotic factors. Activated neutrophils release histones and DNA in neutrophil extracellular traps (NETs). NETs can act as an amplifier for IL-1α-induced endothelial damage (COVID-19 is, in the end, an endothelial disease, European Heart Journal Vol. 41, Issue 32, 21 August 2020, P3038-3044).
In response to cellular damage, damage-associated molecular patterns such as proteins, oxidized phospholipids and DNA are released (DAMPs). DAMPs signal through TLRs, leading to coagulant and inflammatory amplification. DAMPs and PAMPs induce IL-1-autoinduction, a self-amplification loop that induces TNF and IL-6. Being recognized by TLR-4, PAMPs and DAMPs activate the NOD pyrin domain-containing 3 inflammasome (NLRP3). NLRP3 cleaves procaspase-1 into caspase-1, which signals IL-1β and IL-18 release, leading to pyroptosis (inflammatory cell death) (Endothelial activation and dysfunction in COVID-19: from basic mechanisms to potential therapeutic options, Signal Transduction and Targeted Therapy (2020)5:293).
RAS imbalance and ROS further activate the NLRP3 inflammasome. The Bruton Tyrosine Kinase pathway (BTK), which controls macrophages and TLR-mediated activation of the inflammatory NF-kB, maturates IL-1β through NLRP3-mediated endothelial activation (The interplay between inflammatory pathways and COVID-19: a critical review on pathogenesis and therapeutic options, Microbial Pathogenesis 150 (2021): 104673).
1.2.1 Hyperactivation of macrophages, neutrophils and NKs incite inflammatory auto-activation
Hyperactivation of macrophages, neutrophils and Natural Killer Cells (NKs) contribute to the release of DAMPs and PAMPs, inciting auto-activation. Upon detection of DAMPs, TLR9 in endothelial cells can be activated. TLR9 activation is associated with MAPK activation, signaling of NF-kB and activation of ICAM and VCAM adhesion molecules. In addition, PAMPs are involved in activation of the complement system.
1.3 Complement system induction of thrombosis and endothelial damage
As I have discussed in August 2020, the complement system mediates the generation of Reactive Oxygen Species (ROS), activation of NETs, activation of a pro-thombotic, hypercoagulatory profile and endothelial dysfunction through induction of MASP-TAFI, Ultra-Large von Willebrand Factors (ULVWF), PAI-1, IL-1β, IL-8, TM, P-selectin, adhesion molecules MCP-1, E-selectin, ICAM and VCAM. In addition, extensive debris of SARS-CoV-2 complement MAC formations were found in the microvasculature. Thrombosis was accompanied by complement C5b-9 and C4d (Complement associated microvascular injury and thrombosis in the
pathogenesis of severe COVID-19: A report of five cases, Translational
Research, June 2020, Vol. 220).
2 Dysfunctional endothelial cells (ECs), adhesion molecules, platelets and OXPHOS
2.1 Platelet interaction with dysfunctional endothelial cells induces thrombosis
The SARS-CoV-2 Spike protein is found to bind platelet ACE2 to promote thrombosis in COVID (SARS-CoV-2 binds platelet ACE2 to enhance thrombosis in COVID-19, Journal of Hematology & Oncology 2020; 13:120). Inflammation, hypoxia and endothelial cell activation activate platelets and induce platelet apoptosis. Platelet activation and apoptosis induce the release of thrombotic factors/coagulants. Activated platelets enhance P-selectin expression, neutrophil activation, CCL2/3/7 and Interleukins IL-1β, IL-7 and IL-8, Tissue Factor and Hepatocyte Growth Factor. IL-1β increases endothelial cell permeability. Reactive Oxygen Species (ROS) and mitochondrial stress enhance platelet hyperactivation and apoptosis. Platelets bind to neutrophils; formation of Neutrophil Extracellular Traps (NETs) contribute to thrombosis.
2.1.1 Shifting towards a prothrombotic profile (Thromboxane, ADP, P2Y, PAI-1, TF, VWF, low ADAMTS-13)
A progressive kind of thrombocytopenia occurs in COVID-19: platelets are consumed by microclots, then surviving platelets are hyperactivated by megakaryocytes as a means of (over)compensation. Viral damage and damage caused by mechanical ventilation induce platelet activation and apoptosis, further contributing to thrombosis. Platelets react to endothelial cell dysfunction through the release of
prothrombotic Thromboxane, ADP through activation of P2Y, PAI-1 and VEGF. Dysregulation of endothelial cells followed by exposure to P-selectin, Von Willebrand Factor and fibrinogen in its turn activates platelets to express Tissue Factor (TF). VEGF initiates a prothrombotic Tissue Factor amplification loop. While the release of Nitric Oxide (NO) by endothelial cells acts as an antiplatelet agent, NO is impaired as a result of ROS (Innate immunity during SARS-CoV-2: evasion strategies and activation trigger hypoxia and vascular damage, Clinical & Experimental Immunology 2020, 202: 193-209; Thrombocytopathy and endotheliopathy: crucial contributors to COVID-19 thromboinflammation, Nature Reviews Cardiology, 2020).
ADAMTS-13 cleaves Von Willebrand Factor Multimers to modulate thrombotic activity. In COVID-19, VWF antigen (VWF:Ag) levels are markedly increased, while ADAMTS-13 levels are decreased. This VWF:Ag to ADAMTS13 ratio is found to correlate to disease severity, with an overall increase of VWF and decrease of ADAMTS13 in the majority of COVID cases admitted to hospital. An imbalance in VWF:Ag to ADAMTS-13 enhances the hypercoagulable state, platelet hyperactivation and microthrombus formation in COVID-19 (The ADAMTS13-von Willebrand factor axis in COVID-19 patients, Journal of Thrombosis and Haemostasis, 23 November 2020).
2.1.2 Hypoxia-inducible factor-1-alpha (HIF-1a): differentiating between inflammatory and anti-inflammatory
Adenosine Triphosphate (ATP) is the energy supply of cells. Oxidative Phosphorylation (OXPHOS), the Tricarboxylic Acid Cycle (TCA) and glycolysis synthesize ATP; the process is known as "aerobic cellular respiration". During SARS-CoV-2 infection, genes regulating mitochondrial OXPHOS and TCA are downregulated. TCA products citrate, aconitate and fumarate are reported to be depressed in COVID-19 (Metabolic reprogramming and epigenetic changes in vital organs in SARS-CoV-2-induced systemic toxicity, JCI Insight 2021;6(2)).
The function of Hypoxia-inducible factors (HIFs) is to adapt to hypoxic circumstances in order to maintain "normoxic" conditions. In sepsis, macrophage HIF-1α is required to switch from Oxidative Phosphorylation (OXPHOS) to glycolysis (Immunometabolism and Sepsis: A Role for HIF?, Frontiers in Molecular Biosciences Vol. 6, September 2019, Art. 85). HIFs can either induce or reduce an inflammatory state, depending on hypoxic conditions. Prolonged cell stress followed by hypoxia, HIF activation and a decrease in ATP will enhance cell necrosis and inflammation (Hypoxia, HIF-1α and COVID-19: from pathogenic factors to potential therapeutic targets, Acta Pharmacologica Sinica (2020)0:1-8).
The upregulation of VEGF by HIF-1α increases vascular permeability and NET formation. Inflammation, the release of cytokines and thrombotic factors
following SARS-CoV-2 infection contribute to local hypoxia. Low tissue
oxygen levels incite an adaptive response through Hypoxia Inducible
Factors (HIFs) in order to enhance energy and promote angiogenesis, the
generation of new vessels. HIF-1 alpha reduces oxidative stress-induced
apoptosis through relocation to the intermembrane space of mitochondria (HIF-1α protects against oxidative stress by directly targeting mitochondria, Redox Biology 2019 Jul;25).
HIF-1 and HIF-2 regulate endothelial cell adaptation through endothelial migration, growth and differentiation (Primary
endothelial cell-specific regulation of HIF-1 and HIF-2 and their
target gene expression profiles during hypoxia, FASEB Journal 2019 Jul;
33(7): 7929-7941). The release of TNFα causes upregulation of HIF-1α
mRNA. A transition from HIF-1 to HIF-2 marks the shift from acute to
prolonged hypoxia. Accumulation of HIF-1 alpha is hypothesized to occur
due to increased expression of HIF-1 and inhibition of proteasome
degradation under a hypoxic state. HIF-1 alpha may stabilize in
macrophages following activation of Toll-like receptor 4 (TLR4). PAMP,
cell death and DAMP (see 1.2) release upregulate HIF-1 alpha.
Interestingly,
a 2009 study proves that HIF-1α upregulates ACE and downregulates ACE2
in the absence of a true hypoxic state, with significant elevation of
Ang II, a process that was antagonized by telmisartan (Role
of HIF-1α in the regulation of ACE and ACE2 expression in hypoxic human
pulmonary artery smooth muscle cells, Lung Cellular and Molecular
Physiology Vol. 297, Issue 4, October 2009). In addition, HIF-1α is
shown to upregulate ADAM17. This process is hypothesized to exert anti-inflammatory properties. However, due to its highly inflammatory properties under hypoxia and sepsis, HIF-1α contributes to deterioration in COVID-19.
Expression of HIF-1α in alveolar epithelial
cells induces inflammation via the NF-kB pathway and mediates cellular
inflammation through CD4+, CD8+, IL-2 and TNFα (COVID-19-driven
endothelial damage: complement, HIF-1 and ABL2 are potential pathways
of damage and targets for cure, Annals of Hematology 2020 Jun 24:1-7). HIF-1α stimulates glucose
transporters and Lactate Deydrogenase (LDH) and controls the expression
of VEGF (Vascular Endothelial Growth Factor), FOXOa and CXCR4 (a T-cell
receptor). HIF-1 and HIF-2 metabolism is hypothesized to be dedicated to
decreased production of adenosine triphosphate (ATP) through OXPHOS, reducing ROS in the process (Molecular
Basis of "Hypoxic" Signaling, Quiescence, Self-Renewal and
Differentiation in Stem Cells, Anaerobiosis and Stemness 2016, p.
115-141).
When oxidative phosphorylation falls down due to SARS-CoV-2 infection affecting the mitochondria of the host cells, HIF wil come to aid to generate ATP via glycolysis. This, however, is an inadequate way to generate ATP, which will turn out detrimental under prolonged inflammatory and hypoxic circumstances. Aerobic glycolysis even enhances the pathology of COVID. Glycolysis via HIF-1α favors SARS-CoV-2 replication and induces monocyte cytokine production. Through this mechanism, the HIF-1α axis promotes monocyte-driven inhibition of T cell responses and epithelial cell death, worsening the clinical picture (Elevated Glucose Levels Favor SARS-CoV-2 Infection and Monocyte Response through a HIF-1α/Glycolysis-Dependent Axis, Cell Metabolism Vol. 32, Issue 3, September 2020, p437-446). A possible therapeutic target to inhibit HIF-1α-glycolysis in COVID-19 is glycolysis inhibitor 2-deoxyglucose (Diabetes, obesity metabolism and SARS-CoV-2 infection: the end of the beginning, Cell Metabolism 33, March 2021). Furthermore, HIF-1α agonist show a tendency for attracting NETs and phagocyte extracellular traps after stimulation with LPS (Hypoxia, HIF-1α and COVID-19: from pathogenic factors to potential
therapeutic targets, Acta Pharmacologica Sinica (2020)0:1-8).
Thus: the net effect of HIF-1α is complex. While HIFs are indispensable for providing energy to cells under hypoxic circumstances, HIF-1α contributes to inflammation and most likely induces an inadequate T cell response. SARS-CoV-2 downregulates mitochondrial oxidative phosphorylation, which is followed by HIF-induced glycolysis, an inadequate way to generate ATP, which even enhances viral replication in COVID.
2.1.3 Antiphospholipid antibody formation
Upon endothelial dysfunction and Reactive Oxygen Species (ROS) generation following infection with SARS-CoV-2, the beta 2 glycoprotein (β2 GP1) becomes oxidized. The function of non-oxidized β2 GP1 is to control Von Willebrand Factor (vWF) platelet binding to the subendothelium. Under the circumstances of endothelial dysfunction and ROS generation, oxidation of β2 GP1 leads to antiphospholipid antibody formation (aPL). Platelet aggregation occurs through vWF and GPIb. Alpha-granules on platelets release platelet factor to enhance oxidized β2 GP1 and aPL. In addition, granules release ADP, thromboxane and GPIIb, inducing NET formation and fibrotic activity (COVID-19 as a blood clotting disorder masquerading as a respiratory illness: A cerebrovascular perspective and therapeutic implications for stroke thrombectomy, Journal of Neuroimaging 2020;30:555-561).
Antiphospholipid antibodies (aPL) are immunoglobulins, resulting from the interaction of phospholipids with Annexin, prothrombin, cardiolipin. aPL trigger thrombotic activity. Among antiphospholipid antibodies are lupus anticoagulant, anti-cardiolipin antibodies (aCL) and anti-β 2-glycoprotein I antibodies (β2GPI). Notably, antiphospholipid antibodies exert thrombotic/coagulant properties through inhibition of protein C pathways, inhibition of antithrombin and fibrinolysis. Although a brief report observes that aPL were detected in 47% of severely ill COVID patients (Brief Report: Anti-phospholipid antibodies in critically ill patients with COVID-19, Arthritis & Rheumatology 2020 Dec;72:(12)), a recent study shows a low aPL prevalence with regards to COVID thrombosis mechanisms. Moreover, aPL positive patients were not admitted to the ICU (Anti-phospholipid antibodies and immune complexes in COVID-19 patients: a putative role in disease course for anti-annexin-V-antibodies, Clinical Rheumatology 2021 Jan 19:1-7).
Upon a critical reading of two case reports, a conclusion must be drawn that preexisting thrombotic factors such as Factor V Leiden have not been given sufficient attention. Development of aCL antibodies during a septic state cannot be ruled out and the development of aCL independent from thrombotic events in COVID has not been explored (Clinically significant anticardiolipin antibodies associated with COVID-19, Journal of Critical Care 2020 Oct; 59). The presence of high-titer IgG antibody in severe COVID cases has not been proved yet (The coagulopathy, endotheliopathy and vasculitis of COVID-19, Inflammatory Research 2020 Sep 12:1-9; see also "Coagulopathy of COVID-19 and Antiphospholipid antibodies, Journal of Thrombosis and Haemostasis 2020 May 28"). To date, no consecutive studies on the prevalence and developmental course of aPL with regards to COVID coagulopathy and thrombosis have been carried out (Reality Check on Antiphospholipid Antibodies in COVID-19-Associated Coagulopathy, Arthritis & Rheumatology Vol. 73, Issue 1, January 2021).
2.2 ADAM17 promotes ACE2 downregulation and subsequent induction of adhesion molecules through TNF-α
SARS-CoV-2 activates ADAM17, which activates TNF-α and subsequently downregulates ACE2 levels. The ability of ACE2 to increase cardiopulmonaryprotective Angiotensin 1-7 (Ang 1-7) is impaired, resulting in damaging levels of Angiotensin II and increasing levels of Bradykinin. Imbalance of the Renin-Angiotensin/Kinin-Kallikrein-System (RAAS/KKS) leads to permeability of the vasculature ("leaking vessels"), Reactive Oxygen Species (ROS), prostacyclin (PGI2) impairment and nitric oxide impairment. Loss of nitric oxide and prostacyclin results in vasoconstricion, platelet overactivation, upregulation of mitochondrial stress, leukocyte adhesion and impairment of the NRF2 antioxidant function. ACE2 reduces LPS-induced endothelial cell death. ACE2 also induces IL1-β and TNF-α through inhibition of the Janus Kinase (JNK) and NF-kB inflammatory pathways. Loss of ACE2 by activation of ADAM17 and TNF-α promotes adhesion molecule VCAM-1 and chemoattractant MCP-1/CCL2 and enables production of TNF-α, IL-6, adhesion JAM-A and metalloproteases MMP-2 and MMP-9.
ROS and vessel permeability activate calcium, adhesion molecules and the NF-kB pathway, a key contributor to the inflammatory cascade. Ang II and ROS activate inflammatory pathways. The central inflammatory cascade comprises the IL-6/Janus Kinase/STAT pathway, pro-inflammatory interleukins IL-7, IL-8, IL-12 and IL-17, Interferon-gamma (IFN-y), Toll-Like Receptors TLR-3/7/8, TNF-α, IL-1-β, CCL-2/3/7, the MAPK pathway and MIP1a. NOX2 further contributes to generation of ROS. ROS and vascular permeability activate calcium signaling and the NF-kB pathway to promote inflammatory cytokines and adhesion molecules. While TNF-α induces mitochondrial ROS via calcium influx, IFN-y upregulates mitochondrial ROS. Mitrochondrial ROS induces inflammatory cytokines to contribute to the inflammatory cascade.
3 Amplification of inflammatory and coagulation cascades promote thrombosis
3.1 IL-6 promotes adhesion molecules and enhances endothelial permeability through JAK/STAT
IL-6 is known to promote synthesis of coagulation factors fibrinogen, Tissue Factor and factor VIII. IL-6 prompts megakaryocytes to develop platelets and increases vascular permeability through endothelial cell activation of VEGF. IL-6 activates the Janus Kinase/STAT pathway, thereby induces upregulation of adhesion molecules VCAM, ICAM-1, E-selectin, and MCP-1. IL-6 also reduces Nitric Oxide (NO) and increases oxidative stress, enhancing endothelial permeability (Cellular and oxidative mechanisms associated with Interleukin-6 in the vasculature, International Journal of Molecular Science 2017 Dec; 18(12): 2563).
3.2 Endothelial cell activation, amplification of inflammation and thrombosis
In the early stage of COVID-19, C reactive protein (CRP), an acute-phase reactant, increases. CRP is an inflammatory marker, created under control of IL-6 (High-sensitivity C-Reactive Protein, Chapter 18: Immune Function Assessment, in: Textbook of Natural Medicine (Fifth Edition), 2020 P157-165). CRP promotes endothelial cell apoptosis and induces EC activation. In addition, CRP acts on the NF-kB pathway to promote inflammation. When activated by IL-1β and TNF-α, endothelial cells induce pro-thrombotic Von Willebrand Factors (VWF), P-selectin and fibrinogen, thereby attaching platelets. Platelets become activated by endothelial cells. Interacting with endothelial cells, platelets release VEGF, prompting endothelial cells to release Tissue Factor (TF).
Interferon-y activation through T-helper 1 cells (Th1), further stimulates cytokine production by endothelial cells (Endothelial activation and dysfunction in COVID-19: from basic mechanism to potential therapeutic approaches, Signal Transduction and Targeted Therapy 2020:5:293). Beside endothelial cell activation, damage of pericytes accounts for vascular permeability and amplification of the complement system, adding an amplification loop to the thrombotic mechanisms in COVID-19 (Innate immunity during SARS-CoV-2: evasion strategies and activation
trigger hypoxia and vascular damage, Clinical & Experimental
Immunology 2020, 202: 193-209; Thrombocytopathy and endotheliopathy:
crucial contributors to COVID-19 thromboinflammation, Nature Reviews
Cardiology, 2020).
Following SARS-CoV-2 infection, pneumocytes, alveolar cells, infiltrating monocyte-macrophages and neutrophils elicit TNF-α, IL-6, IFN-gamma and MCP-1. Macrophages start the amplification of inflammation, coagulation and thrombosis. Anticoagulant factors are downregulated by activated macrophages; the anticoagulant protein C system is impaired by inflammatory cytokines.More so, CD68+ macrophages are shown to be directly infected by SARS-CoV-2 (Immunity, endothelial injury and complement-induced coagulopathy in COVID-19, Nature Reviews Nephrology 17, 46-64(2021)). Adhesion neutrophils transforming to NETs amplify endothelial damage through IL-1α. Disruption of the endothelial barrier and ROS, IL-1β, TNF-α and NETs promotes Tissue Factor expression.
Tissue Factor activates the coagulation cascade through amplification of Factor VII and X, which enhances thrombin formation. Sepsis with dysfunctional endothelial barrier stimulates autoamplification of coagulation and inflammation through Factor XII and Tissue Factor, activating platelets which in their turn release CD40 and amplify VEGF. Inflammatory amplification increases TF formation, downregulation of thrombomodulin and leukocyte activation. IL-6 increases vascular permeability and promotes endothelial cells to amplify cytokine cascades.
4 Damage to the endothelial glycocalyx, impaired antioxidant activity, sepsis and impaired shear stress
The glycocalyx is a gel-like layer of sialic acid-glycoproteins, heparan sulphate, chondroitin sulphate, hyaluronans (bound to endothelial cells by CD44) and proteoglycans such as syndecans. Glycocalyx sialic acids express Nrf2 in order to exert antioxidant properties counteracting ROS (Glycocalyx sialic acids regulate Nrf2-mediated signaling by fluid shear stress in human endothelial cells, Redox Biology 2021 Jan;38: 101816). The junctions of the endothelial glycocalyx are stabilized by cadherin, claudins and platelet endothelial cell adhesion molecules (PECAM). The fluid layer of the glycocalyx is able to hold antithrombin, albumin and antioxidants. PAMPs damage the integrity of the glycocalyx-endothelial cell barrier, which is observed by shedding of syndecan-1, claudin-5, hyaluronans and syndecan-4 in COVID-19 patients.
Following infection, damage to the endothelial glycocalyx promotes sepsis and coagulopathy, which eventually leads to Disseminated Intravascular Coagulopathy (DIC). PAMPs activate the complement system, induce inflammatory amplification and promote hypercoagulation. Vascular permeability (through ROS, oxidative stress, release of Ang II, impairment of Nitric Oxide, NRF2 loss and PGI2 impairment) dysregulates normal shear stress. Blood flow and shear stress are necessary to maintain the balance between thrombosis/antithrombosis and oxidative/antioxidant activity in order to protect the glycocalyx and endothelial function. Low shear stress and impairment of blood flow shifts endothelial homeostasis towards a thrombotic and sepsis coagulopathy. MMP-9 produced by IL-1β induces the breakdown of the glycocalyx. As a result, an amplification loop of sepsis, glycocalyx shedding, endothelial dysfunction, leaking vasculature and low shear stress increase inflammation and thrombotic activity.
PAMPs activate coagulation through Tissue Factor (TF). Inflammatory cytokines such as IL-6, TNF-α, IL1-β further induce TF. Tissue Factor activates Factor VII, which in its turn activates Factor X with subsequent thrombin and fibrin generation. Activated endothelial cells release P-selectin and VWF to increase TF expression. Low shear stress and loss of shear stress induce red blood cell (erythrocytes) aggregation (The role of endothelial shear stress on haemodynamics, inflammation, coagulation and glycocalyx during sepsis, Journal of Cellular and Molecular Sciences Vol. 24, Issue 21, November 2020, P12258-12271).
5 Possible therapeutic targets
The following therapeutic targets for treatment of COVID-19 comprise inflammatory factor inhibitors, thrombin inhibitors, interleukin inhibitors, Toll Like Receptor expression inhibitors, bradykinin inhibitors and Janus Kinase Pathway inhibitors, Nuclear Factor NF-kB inhibitors, as well as pharmacotherapeutic antioxidants and Vascular Endothelial Growth Factor (VEGF) inhibitors. I also disclosed a few endothelium-specific therapeutic targets I have previously discussed in my messages from November 2020 and December 2020 (A stubborn complication: the quest for solutions to COVID's thrombosis pandemic should highlight the endothelium and glycocalyx); (The quest for solutions to COVID's thrombosis pandemic: endothelial glycocalyx dysfunction and mitochondrial dysfunction are starting points).
IL-1 Anakinra; Rilonacept; Canakinumab;
IL1-beta Ossirene; FR 167653; Flavonoligans; Canakinumab;
IL-6 Sarilumab; Tocilizumab; Ulinastatin; LMT-28; Siltuximab;
TNF-alpha Etanercept; Adalimumab; Infliximab;
Janus Kinase JAK-in-3;
JAK 1 Upadacitinib;
JAK 1/2 Ruxolitinib; Baricitinib; Fedratinib;
JAK 1/2/3 JAK-in-1; Tofacitinib;
RAAS/KKS Icatibant; Fasitibant; Losartan Potassium; Telmisartan; TAPI-1; Noscapine;
HIF TAT-cyclo-CLLFVY TFA; Gramicidin A;
Thrombin Sofigatran Factor IIa inhibitor;
Platelet/P2Y12 Clopidogrel; Ticagrelor; Elinogrel; Prasugrel;
TLR4 Eritoran; Schaftoside; Resatorvid (TAK-242);
Immunomodul. Colchicine; Methotrexate; Cyclosporin; Methylprednisolone;
VEGF Bevacizumab;
VEGFR2 Ramucirumab; SU5408;
NOX & ROS N-acetyl-L-cysteine (NAC); Setanaxib (NOX inhibitor);
VCAM Phellopterin; Gypenoside XLiX; K-7174;
ICAM ICAM-1-in-1; Lifitegrast; A-205804;
NF-kB Muscone; Androgpraholide;
NLRP3 Mulberroside; Muscone; CY-09; INF39;
Caspase-1 Ossirene; Mulberroside;
STAT STAT-3-in-1; STAT-3-in-3
Endothelial (glycocalyx) specific therapeutic options
Vascular leakage-increasing Angiopoietin-2 (Angpt-2) is significantly increased in COVID patients admitted to the ICU for mechanical ventilation, indicating endothelial damage. Tie2 activation is postulated to protect and restore endothelial glycocalyx (eGC) (Tie2
Activation Promotes Protection and Reconstitution of the Endothelial
Glycocalyx in Human Sepsis, Journal of Thrombosis and Haemostasis 2019
Nov;119(11):1827-1838).
Another therapeutic option for endothelial glycocalyx restoration is Sulodexide (SDX) ("Therapeutic
Restoration of Endothelial Glycocalyx in Sepsis, Journal of
Pharmacology and Experimental Therapeutics 2017 Apr; 361(1): 115-121). Recently, recombinant Antithrombin-gamma (AT-γ) is proposed as a therapeutic option for endothelial glycocalyx restoration (Newly developed recombinant antithrombin protects the Endothelial Glycocalyx in an Endotoxin-Induced rat model of sepsis, International Journal of Molecular Sciences, 2021 Jan; 22(1): 176).
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Therapeutic targets for treating COVID: a selection (version January 2021)
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I created this selection of possible therapeutic agents for treating COVID, based on current research
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