Patients with hypertension, diabetes, coronary heart disease, cerebrovascular illness, chronic obstructive pulmonary disease, and kidney dysfunction have worse clinical outcomes when infected with SARS-CoV-2, for unknown reasons. The purpose of this review is to summarize the evidence for the existence of elevated plasmin(ogen) in COVID-19 patients with these comorbid conditions. Plasmin, and other proteases, may cleave a newly inserted furin site in the S protein of SARS-CoV-2, extracellularly, which increases its infectivity and virulence. Hyperfibrinolysis associated with plasmin leads to elevated D-dimer in severe patients. The plasmin(ogen) system may prove a promising therapeutic target for combating COVID-19.
Elevated plasmin(ogen) is a common feature in people with underlying medical conditions, including hypertension, diabetes, cardiovascular disease, cerebrovascular disease, and chronic renal illness, who are susceptible to SARS-CoV-2 infection.
Plasmin enhances the virulence and infectivity of SARS-CoV-2 virus by cleaving its spike proteins.
Extremely increased D-dimer in COVID-19 patients results from plasmin-associated hyperactive fibrinolysis.
D-dimer and viral load are independent risk factors of disease severity and mortality.
Antiproteases targeting plasmin(ogen) may be a promising approach to combat COVID-19.
Cleavage of Coronavirus by Plasmin and Other Host Proteases
Single-cell profiling of human lung tissues (the LGEA portal: https://research.cchmc.org/pbge/lunggens/mainportal.html) reveals that furin is predominately expressed in human alveolar type II (AT2) cells in the respiratory system, while plasminogen, kallikrein, and trypsin are expressed in both airway and alveolar type I and II epithelial cells. Plasminogen is also expressed in endothelial cells. Cytosolic furin is enriched in the Golgi apparatus. The possibility for non-furin proteases to cleave viral envelope proteins is supported by the evidence that in furin-defective LoVo cells, the cleavage-dependent process of HIV gp160 is as efficient as in normal cell lines . We have demonstrated that plasmin is capable of cleaving furin sites in the γ subunit of human epithelial sodium channels (ENaC).
The S protein of coronaviruses may be cleaved by plasmin, trypsin, cathepsins, elastase, and TMPRSS family members; cleavage of S protein may mediate enhancement of virus entry into bronchial epithelial cells. Plasmin also cleaves the S proteins of SARS-CoV in vitro. In addition, HCoV-HKU1 S proteins are cleaved by kallikrein in the S1/S2 region and mediate the entry of HCoV-HKU1 to nonpermissive rhabdomyosarcoma cells . The clinical relevance of non-furin cleavage remains unknown due to the paucity of in vivo evidence for the role of plasmin cleavage of SARS-CoV. Also, it remains to be demonstrated that the envelope proteins of SARS-CoV-2 strain are cleaved by plasm.
The cleavage of influenza virus by plasmin is well characterized. Proteolysis of influenza HA proteins enables fusion with the host endosome. Acidification of the endosome promotes viral membrane fusion and activates the M2 ion channel, which pumps protons (H1) into the interior of the viral core to initiate uncoating of the M1 protein. Nuclear replication occurs, and viral gene products are transported to the plasma membrane for assembly. The fibrinolytic zymogen plasminogen (activated by urokinase or tissue-like plasminogen activator to generate plasmin) has been shown to cleave the influenza HA proteins. The HA cleavage site of A/WSN/1933 H1N1 influenza virus governs virus spread in a plasmin-dependent manner. Mini-plasmin, a plasmin fragment, is distributed predominantly in the epithelial cells of the bronchioles and potentiates the replication of both plasmin-sensitive and plasmin-insensitive influenza A virus strains, suggesting a pivotal role of plasmin in the spread and pathogenicity of the influenza virus. Additionally, kallikreins cleave and activate HA of the influenza virus H1, H2, and H3 subtypes. Similar to coronavirus and influenza viruses, plasmin, trypsin, thrombin, and furin enhance RSV-induced cytopathology. Local fibrin and clot formation are implicated in host defense against influenza virus infections, thus plasminogen may affect lung injury and repair by interfering with these processes.
Protease cleavage may enhance or decrease the activities of various proteins. For example, prostasin increases the activity (60–80%) of ENaC, whereas TMPRSS2 markedly decreases ENaC function and protein levels. In neural tissues, brain-derived neurotrophic factor precursor (proBDNF) is cleaved either intracellularly by furin-like proteases or extracellularly by plasmin or matrix metalloproteinases. However, plasmin, but not related proteases, cleaves proBDNF furin sites extracellularly. The inhibitory effects of TMPRSS2 could be corrected by serine protease inhibitors, such as camostat mesylate that has been approved for clinical use in Japan. The beneficial effects of camostat mesylate, an antiprotease, may be partially due to the inhibition of plasmin .
PLASMIN(OGEN) IN ARDS
Fibrinolysis in COVID-19
In comparison with patients with mild COVID-19 (such as those who did not require ICU stays, did not develop ARDS or pneumonia, and who survived), patients with severe COVID-19 have higher comorbidities, including 56% for hypertension, 21% for heart diseases, 18% for diabetes, 12% for cerebrovascular diseases, and 7% for cancer (TABLE 1). Some patients have more than one, even up to five preexisting conditions. Multivariate regression further links hypertension with increased incidence and fatality. Hyperfibrinolysis, reflected by elevated serum D-dimer levels, was present in 97% of COVID-19 patients at admission and increased further in all patients before death (TABLE 2). FDPs were significantly increased as well. This is accompanied by a prolonged prothrombin time particularly in non-survivors. Platelet counts were decreased significantly in severe and dead patients. 71.4% of non-survivors meet the criteria of the International Society on Thrombosis and Hemostasis (ISTH) for DIC, suggesting the coexistence of coagulation activation and hyperfibrinolysis in patients with severe COVID-19 infection. In contrast, D-dimer levels decreased to control levels in survivors or non-ARDS patients.
The mortality rate of patients with COVID-19 who did not develop ARDS is 9 versus 49% for those who did develop ARDS (45). Of note, ARDS/respiratory failure remains the leading cause of death (70%), followed by sepsis/MOF (28%), heart failure (15%), hemorrhage (6%), and renal failure (4%) (TABLE 3). Coagulation/hemorrhage ranks among the top three leading causes of death. Furthermore, multivariate regression analysis identifies D-dimer and age as independent risk factors for mortality (TABLE 4)These findings suggest that the normalization of hyperactive fibrinolysis may be a therapeutic target.
CLINICAL RELEVANCE AND PERSPECTIVE
The cleavage of the new furin sites in the S protein of SARS-CoV-2 virus by plasmin and other proteases may enhance its infectivity by expediting entry, fusion, duplication, and release in respiratory cells. Elevated plasmin(ogen) levels are a common feature in COVID-19 patients with underlying medical conditions. The elevated plasmin(ogen) could be an independent factor for risk stratification of patients with COVID-19. Measurements of plasmin(ogen) levels and its enzymatic activity may be important biomarkers of disease severity in addition to resultant D-dimer. The administration of antiproteases to suppress plasmin activity in the respiratory system may prevent, or at least decrease, SARS-CoV-2 entry into respiratory cells and improve the clinical outcome of patients with COVID-19. As demonstrated in vitro, a serine protease inhibitor for TMPRSS2 blocks SARS-CoV-2 S protein-driven entry into cells (30). Clinical trials conducted in China are testing various protease inhibitors (29). Currently there are no proper animal models of COVID-19 with underlying medical conditions to test new therapeutic agents. Healthy mice and monkeys infected with SARS-CoV-2 develop either mild lung injury or show no symptoms of disease (8). It remains to be seen whether mice and monkeys with preexisting comorbid conditions and higher plasmin levels develop COVID-19 when infected with SARS-CoV-2. Targeting hyperfibrinolysis with a broad spectrum or specific anti-plasmin compounds may prove to be a promising strategy for improving the clinical outcome of patients with comorbid conditions.
Reference & Source information: https://journals.physiology.org/
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