Plasmin improves blood-gas barrier function in oedematous lungs by cleaving epithelial sodium channels

Fibrinolytic system and COVID-19: From an innovative view of epithelial ion transport

Lifeways of worldwide people have changed dramatically amid the coronavirus disease 2019 (COVID-19) pandemic, and public health is at stake currently. In the early stage of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, fibrinolytic system is mostly inhibited, which is responsible for the development of hypofibrinolysis, promoting disseminated intravascular coagulation, hyaline membrane formation, and pulmonary edema. Whereas the common feature and risk factor at advanced stage is a large amount of fibrin degradation products, including D-dimer, the characteristic of hyperfibrinolysis. Plasmin can cleave both SARS-CoV-2 spike protein and γ subunit of epithelial sodium channel (ENaC), a critical element to edematous fluid clearance. In this review, we aim to sort out the role of fibrinolytic system in the pathogenesis of COVID-19, as well as provide the possible guidance in current treating methods. In addition, the abnormal regulation of ENaC in the occurrence of SARS-CoV-2 mediated hypofibrinolysis and hyperfibrinolysis are summarized, with the view of proposing an innovative view of epithelial ion transport in preventing the dysfunction of fibrinolytic system during the progress of COVID-19.

Regulation of Epithelial Sodium Transport by SARS-CoV-2 Is Closely Related with Fibrinolytic System-Associated Proteins

Dyspnea and progressive hypoxemia are the main clinical features of patients with coronavirus disease 2019 (COVID-19), which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Pulmonary pathology shows diffuse alveolar damage with edema, hemorrhage, and the deposition of fibrinogens in the alveolar space, which are consistent with the Berlin Acute Respiratory Distress Syndrome Criteria. The epithelial sodium channel (ENaC) is a key channel protein in alveolar ion transport and the rate-limiting step for pulmonary edema fluid clearance, the dysregulation of which is associated with acute lung injury/acute respiratory distress syndrome. The main protein of the fibrinolysis system, plasmin, can bind to the furin site of γ-ENaC and induce it to an activation state, facilitating pulmonary fluid reabsorption. Intriguingly, the unique feature of SARS-CoV-2 from other β-coronaviruses is that the spike protein of the former has the same furin site (RRAR) with ENaC, suggesting that a potential competition exists between SARS-CoV-2 and ENaC for the cleavage by plasmin. Extensive pulmonary microthrombosis caused by disorders of the coagulation and fibrinolysis system has also been seen in COVID-19 patients. To some extent, high plasmin (ogen) is a common risk factor for SARS-CoV-2 infection since an increased cleavage by plasmin accelerates virus invasion. This review elaborates on the closely related relationship between SARS-CoV-2 and ENaC for fibrinolysis system-related proteins, aiming to clarify the regulation of ENaC under SARS-CoV-2 infection and provide a novel reference for the treatment of COVID-19 from the view of sodium transport regulation in the lung epithelium.

PAI-1 regulates AT2-mediated re-alveolarization and ion permeability

Background Acute lung injury is characterized by overwhelmingly elevated PAI-1 in both lung edema fluid and the circulating system. The role of increased PAI-1, encoded by Serpine1 gene, in the regeneration of injured lung epithelium has not been understood completely. This study aimed to investigate the role of Serpine1 in the regulation of alveolar type 2 epithelial cell (AT2) fate in a humanized mouse line carrying diseased mutants (Serpine1Tg). Methods Wild type (wt) and Serpine1Tg AT2 cells were either cultured as monolayers or 3D alveolospheres. Colony forming assay and total surface area of organoids were analyzed. AT1 and AT2 cells in organoids were counted by immunohistochemistry and fluorescence-activated cell sorting (FACS). To test the potential effects of elevated PAI-1 on the permeability in the epithelial monolayers, we digitized the biophysical properties of polarized AT2 monolayers grown at the air-liquid interface. Results A significant reduction in total AT2 cells harvested in Serpine1Tg mice was observed compared with wt controls. AT2 cells harvested from Serpine1Tg mice reduced significantly over the wt controls. Spheroids formed by Serpine1Tg AT2 cells were lesser than wt control. Similarly, the corresponding surface area, a readout of realveolarization of injured epithelium, was markedly reduced in Serpine1Tg organoids. FACS analysis revealed a significant suppression in the number of AT2 cells, in particular, the CD44+ subpopulation, in Serpine1Tg organoids. A lesser ratio of AT1:AT2 cells in Serpine1Tg organoids was observed compared with wt cultures. There was a significant increase in transepithelial resistance but not amiloride inhibition. Conclusions Our study suggests elevated PAI-1 in injured lungs downregulates alveolar epithelial regeneration by reducing the AT2 self-renewal, particularly in the CD44+ cells.

Screening bioactive compounds from Danggui-shaoyao-san for treating sodium retention in nephrotic syndrome using bio-affinity ultrafiltration

ETHNOPHARMACOLOGICAL RELEVANCE

Danggui-shaoyao-san (DSS), a representative formula of Traditional Chinese Medicine (TCM) for promoting blood circulation and diuresis (Huo-Xue-Li-Shui) therapy, has been used to clinically nephrotic syndrome (NS) and relieve nephrotic edema.

AIM OF THE STUDY

To explore the effects and mechanisms of DSS in improving sodium retention and to identify the bioactive compounds from DSS.

MATERIALS AND METHODS

DSS prescriptions were disassembled into Yangxue-Huoxue (YXHX) and Jianpi-Lishui (JPLS). A nephrotic rat model was induced with puromycin aminonucleoside (PAN), and the effects on urinary sodium excretion, urinary plasmin(gen) content, and plasmin activity of DSS, YXHX, and JPLS extracts were assessed. The inhibitory effects on urokinase-type plasminogen activator (uPA) and plasmin activity of extracts were evaluated in vitro. Bio-affinity ultrafiltration and high-performance liquid chromatography quadrupole time-of-flight mass spectrometry (BAU-UPLC-Q/TOF-MS) were used to rapidly screen and qualitatively analyze the uPA/plasmin affinity compounds from DSS extract. Additionally, uPA/plasmin inhibition assays and molecular docking were used to verify the activity and affinity mechanisms of the potential bioactive compounds.

RESULTS

In vivo, DSS, YXHX, and JPLS prevented sodium retention in nephrotic rats. DSS and YXHX treatment decreased urinary plasmin activity but did not alter urinary plasmin(ogen) concentration, and their extracts showed strong uPA and plasmin inhibitory activity in vitro. These results suggested that uPA and plasmin are direct targets of DSS and YXHX in intervening NS sodium retention. Using BAU-UPLC-Q/TOF-MS, gallic acids, methyl gallate, albiflorin, and 1,2,3,4,6-O-pentagalloylglucose (PGG) were screened as uPA or plasmin affinity compounds. Among them, PGG was found to be a uPA and plasmin dual inhibitor, with an IC₅₀ of 6.861 μM against uPA and an IC₅₀ of 149.0 μM against plasmin. The molecular docking results of PGG with uPA and plasmin were consistent with the verification results.

CONCLUSION

Intervening in sodium retention by inhibiting uPA-mediated plasmin generation and plasmin activity in the kidneys could be possible mechanisms for DSS, as indicated by the results in PAN-induced nephrotic rats. We conclude that PGG is a potential bioactive compound responsible for the effect of DSS on natriuresis.

Transmembrane serine protease 2 (TMPRSS2) proteolytically activates the epithelial sodium channel (ENaC) by cleaving the channel's γ-subunit

The epithelial sodium channel (ENaC) is a heterotrimer consisting of α-, β-, and γ-subunits. Channel activation requires proteolytic release of inhibitory tracts from the extracellular domains of α-ENaC and γ-ENaC; however, the proteases involved in the removal of the γ-inhibitory tract remain unclear. In several epithelial tissues, ENaC is coexpressed with the transmembrane serine protease 2 (TMPRSS2). Here, we explored the effect of human TMPRSS2 on human αβγ-ENaC heterologously expressed in Xenopus laevis oocytes. We found that coexpression of TMPRSS2 stimulated ENaC-mediated whole-cell currents by approximately threefold, likely because of an increase in average channel open probability. Furthermore, TMPRSS2-dependent ENaC stimulation was not observed using a catalytically inactive TMPRSS2 mutant and was associated with fully cleaved γ-ENaC in the intracellular and cell surface protein fractions. This stimulatory effect of TMPRSS2 on ENaC was partially preserved when inhibiting its proteolytic activity at the cell surface using aprotinin but was abolished when the γ-inhibitory tract remained attached to its binding site following introduction of two cysteine residues (S155C–Q426C) to form a disulfide bridge. In addition, computer simulations and site-directed mutagenesis experiments indicated that TMPRSS2 can cleave γ-ENaC at sites both proximal and distal to the γ-inhibitory tract. This suggests a dual role of TMPRSS2 in the proteolytic release of the γ-inhibitory tract. Finally, we demonstrated that TMPRSS2 knockdown in cultured human airway epithelial cells (H441) reduced baseline proteolytic activation of endogenously expressed ENaC. Thus, we conclude that TMPRSS2 is likely to contribute to proteolytic ENaC activation in epithelial tissues in vivo.

Targeting Host Cell Proteases to Prevent SARS-CoV-2 Invasion

Severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) has spread worldwide and caused widespread devastation. In the absence of definitive therapy, symptomatic management remains the standard of care. Repurposing of many existing drugs, including several anti-viral drugs, is being attempted to tackle the COVID-19 pandemic. However, most of them have failed to show significant benefit in clinical trials. An attractive approach may be to target host proteases involved in SARS-CoV-2 pathogenesis. The priming of the spike (S) protein of the virus by proteolytic cleavage by the transmembrane serine protease-2 (TMPRSS2) is necessary for the fusion of the virus to the host cell after it binds to its receptor angiotensin converting enzyme-2 (ACE2). There are other proteases with varying spatiotemporal locations that may be important for viral entry and subsequent replication inside the cells, and these include trypsin, furin and cathepsins. In this report, we have discussed the tentative therapeutic role of inhibitors of TMPRSS2, cathepsin, trypsin, furin, plasmin, factor X and elastase in infection caused by SARS-CoV-2. Both available evidence, as well as hypotheses, are discussed, with emphasis on drugs which are approved for other indications such as bromhexine, ammonium chloride, nafamostat, camostat, tranexamic acid, epsilon amino-caproic acid, chloroquine, ulinastatin, aprotinin and anticoagulant drugs. Simultaneously, novel compounds being tested and problems with using these agents are also discussed.

Fibrinolysis influences SARS-CoV-2 infection in ciliated cells

Rapid spread of COVID-19 has caused an unprecedented pandemic worldwide, and an inserted furin site in SARS-CoV-2 spike protein (S) may account for increased transmissibility. Plasmin, and other host proteases, may cleave the furin site of SARS-CoV-2 S protein and γ subunits of epithelial sodium channels (γ ENaC), resulting in an increment in virus infectivity and channel activity. As for the importance of ENaC in the regulation of airway surface and alveolar fluid homeostasis, whether SARS-CoV-2 will share and strengthen the cleavage network with ENaC proteins at the single-cell level is urgently worthy of consideration. To address this issue, we analyzed single-cell RNA sequence (scRNA-seq) datasets, and found the PLAU (encoding urokinase plasminogen activator), SCNN1G (γENaC), and ACE2 (SARS-CoV-2 receptor) were co-expressed in alveolar epithelial, basal, club, and ciliated epithelial cells. The relative expression level of PLAU, TMPRSS2, and ACE2 were significantly upregulated in severe COVID-19 patients and SARS-CoV-2 infected cell lines using Seurat and DESeq2 R packages. Moreover, the increments in PLAU, FURIN, TMPRSS2, and ACE2 were predominately observed in different epithelial cells and leukocytes. Accordingly, SARS-CoV-2 may share and strengthen the ENaC fibrinolytic proteases network in ACE2 positive airway and alveolar epithelial cells, which may expedite virus infusion into the susceptible cells and bring about ENaC associated edematous respiratory condition.

COVID-19-Associated Hyper-Fibrinolysis: Mechanism and Implementations

The emerging novel coronavirus disease (COVID-19), which is caused by the SARS-CoV-2 presents with high infectivity, morbidity and mortality. It presenting a need for immediate understanding of its pathogenicity. Inflammation and coagulation systems are over-activated in COVID-19. SARS-CoV-2 damages endothelial cell and pneumocyte, resulting in hemostatic disorder and ARDS. An influential biomarkers of poor outcome in COVID-19 are high circulating cytokines and D-dimer level. This latter is due to hyper-fibrinolysis and hyper-coagulation. Plasmin is a key player in fibrinolysis and is involved in the cleavage of many viruses envelop proteins, including SARS-CoV. This function is similar to that of TMPRSS2, which underpins the entry of viruses into the host cell. In addition, plasmin is involved in the pathophysiology of ARDS in SARS and promotes secretion of cytokine, such as IL-6 and TNF, from activated macrophages. Here, we suggest an out-of-the-box treatment for alleviating fibrinolysis and the ARDS of COVID-19 patients. This proposed treatment is concomitant administration of an anti-fibrinolytic drug and the anticoagulant.

Osmotic Adaptation by Na+-Dependent Transporters and ACE2: Correlation with Hemostatic Crisis in COVID-19

COVID-19 symptoms, including hypokalemia, hypoalbuminemia, ageusia, neurological dysfunctions, D-dimer production, and multi-organ microthrombosis reach beyond effects attributed to impaired angiotensin-converting enzyme 2 (ACE2) signaling and elevated concentrations of angiotensin II (Ang II). Although both SARS-CoV (Severe Acute Respiratory Syndrome Coronavirus) and SARS-CoV-2 utilize ACE2 for host entry, distinct COVID-19 pathogenesis coincides with the acquisition of a new sequence, which is homologous to the furin cleavage site of the human epithelial Na+ channel (ENaC). This review provides a comprehensive summary of the role of ACE2 in the assembly of Na+-dependent transporters of glucose, imino and neutral amino acids, as well as the functions of ENaC. Data support an osmotic adaptation mechanism in which osmotic and hemostatic instability induced by Ang II-activated ENaC is counterbalanced by an influx of organic osmolytes and Na+ through the ACE2 complex. We propose a paradigm for the two-site attack of SARS-CoV-2 leading to ENaC hyperactivation and inactivation of the ACE2 complex, which collapses cell osmolality and leads to rupture and/or necrotic death of swollen pulmonary, endothelial, and cardiac cells, thrombosis in infected and non-infected tissues, and aberrant sensory and neurological perception in COVID-19 patients. This dual mechanism employed by SARS-CoV-2 calls for combinatorial treatment strategies to address and prevent severe complications of COVID-19.

Functional prediction and comparative population analysis of variants in genes for proteases and innate immunity related to SARS-CoV-2 infection

New coronavirus SARS-CoV-2 is capable to infect humans and cause a novel disease COVID-19. Aiming to understand a host genetic component of COVID-19, we focused on variants in genes encoding proteases and genes involved in innate immunity that could be important for susceptibility and resistance to SARS-CoV-2 infection. Analysis of sequence data of coding regions of FURIN, PLG, PRSS1, TMPRSS11a, MBL2 and OAS1 genes in 143 unrelated individuals from Serbian population identified 22 variants with potential functional effect. In silico analyses (PolyPhen-2, SIFT, MutPred2 and Swiss-Pdb Viewer) predicted that 10 variants could impact the structure and/or function of proteins. These protein-altering variants (p.Gly146Ser in FURIN; p.Arg261His and p.Ala494Val in PLG; p.Asn54Lys in PRSS1; p.Arg52Cys, p.Gly54Asp and p.Gly57Glu in MBL2; p.Arg47Gln, p.Ile99Val and p.Arg130His in OAS1) may have predictive value for inter-individual differences in the response to the SARS-CoV-2 infection. Next, we performed comparative population analysis for the same variants using extracted data from the 1000 Genomes project. Population genetic variability was assessed using delta MAF and Fst statistics. Our study pointed to 7 variants in PLG, TMPRSS11a, MBL2 and OAS1 genes with noticeable divergence in allelic frequencies between populations worldwide. Three of them, all in MBL2 gene, were predicted to be damaging, making them the most promising population-specific markers related to SARS-CoV-2 infection. Comparing allelic frequencies between Serbian and other populations, we found that the highest level of genetic divergence related to selected loci was observed with African, followed by East Asian, Central and South American and South Asian populations. When compared with European populations, the highest divergence was observed with Italian population. In conclusion, we identified 4 variants in genes encoding proteases (FURIN, PLG and PRSS1) and 6 in genes involved in the innate immunity (MBL2 and OAS1) that might be relevant for the host response to SARS-CoV-2 infection.

Why COVID-19 Transmission Is More Efficient and Aggressive Than Viral Transmission in Previous Coronavirus Epidemics?

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is causing a pandemic of coronavirus disease 2019 (COVID-19). The worldwide transmission of COVID-19 from human to human is spreading like wildfire, affecting almost every country in the world. In the past 100 years, the globe did not face a microbial pandemic similar in scale to COVID-19. Taken together, both previous outbreaks of other members of the coronavirus family (severe acute respiratory syndrome (SARS-CoV) and middle east respiratory syndrome (MERS-CoV)) did not produce even 1% of the global harm already inflicted by COVID-19. There are also four other CoVs capable of infecting humans (HCoVs), which circulate continuously in the human population, but their phenotypes are generally mild, and these HCoVs received relatively little attention. These dramatic differences between infection with HCoVs, SARS-CoV, MERS-CoV, and SARS-CoV-2 raise many questions, such as: Why is COVID-19 transmitted so quickly? Is it due to some specific features of the viral structure? Are there some specific human (host) factors? Are there some environmental factors? The aim of this review is to collect and concisely summarize the possible and logical answers to these questions.

Insufficient hyperfibrinolysis in COVID-19: a systematic review of thrombolysis based on meta-analysis and meta-regression

Background How aberrant fibrinolysis influences the clinical progression of COVID-19 presents a clinicopathological dilemma challenging intensivists. To investigate whether abnormal fibrinolysis is a culprit or protector or both, we associated elevated plasma D-dimer with clinical variables to identify a panoramic view of the derangements of fibrinolysis that contribute to the pathogenesis of COVID-19 based on studies available in the literature. Methods We performed this systematic review based on both meta-analysis and meta-regression to compute the correlation of D-dimer at admission with clinical features of COVID-19 patients in retrospective studies or case series. We searched the databases until Aug 18, 2020, with no limitations by language. The first hits were screened, data extracted, and analyzed in duplicate. We did the random-effects meta-analyses and meta-regressions (both univariate and multivariate). D-dimer associated clinical variables and potential mechanisms were schematically reasoned and graphed. Findings Our search identified 42 observational, or retrospective, or case series from six countries (n=14,862 patients) with all races and ages from 1 to 98-year-old. The weighted mean difference of D-dimer was 0.97 μg/mL (95% CI 0.65, 1.29) between relatively mild (or healthy control) and severely affected groups with significant publication bias. Univariate meta-regression identified 58 of 106 clinical variables were associated with plasma D-dimer levels, including 3 demographics, 5 comorbidies, 22 laboratory tests, 18 organ injury biomarkers, 8 severe complications, and 2 outcomes (discharge and death). Of these, 11 readouts were negatively associated with the level of plasma D-dimer. Further, age and gender were confounding factors for the identified D-dimer associated variables. There were 22 variables independently correlated with the D-dimer level, including respiratory rate, dyspnea plasma K+, glucose, SpO2, BUN, bilirubin, ALT, AST, systolic blood pressure, and CK. We thus propose that "insufficient hyperfibrinolysis (fibrinolysis is accelerated but unable to prevent adverse clinical impact for clinical deterioration COVID-19)" as a peculiar mechanism. Interpretation The findings of this meta-analysis- and meta-regression-based systematic review supports elevated D-dimer as an independent predictor for mortality and severe complications. D-dimer-associated clinical variables draw a landscape integrating the aggregate effects of systemically suppressive and locally (i.e., in the lung) hyperactive derangements of fibrinolysis. D-dimer and associated clinical biomarkers and conceptually parameters could be combined for risk stratification, potentially for tracking thrombolytic therapy or alternative interventions.

Elevated Plasmin(ogen) as a Common Risk Factor for COVID-19 Susceptibility

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.

Plasmin improves blood-gas barrier function in oedematous lungs by cleaving epithelial sodium channels

BACKGROUND AND PURPOSE

Lung oedema in association with suppressed fibrinolysis is a hallmark of lung injury. Here, we have tested whether plasmin cleaves epithelial sodium channels (ENaC) to resolve lung oedema fluid.

EXPERIMENTAL APPROACH

Human lungs and airway acid-instilled mice were used for analysing fluid resolution. In silico prediction, mutagenesis, Xenopus oocytes, immunoblotting, voltage clamp, mass spectrometry, and protein docking were combined for identifying plasmin cleavage sites.

KEY RESULTS

Plasmin improved lung fluid resolution in both human lungs ex vivo and injured mice. Plasmin activated αβγENaC channels in oocytes in a time-dependent manner. Deletion of four consensus proteolysis tracts (αΔ432-444, γΔ131-138, γΔ178-193, and γΔ410-422) eliminated plasmin-induced activation significantly. Further, immunoblotting assays identified 7 cleavage sites (K126, R135, K136, R153, K168, R178, K179) for plasmin to trim both furin-cleaved C-terminal fragments and full-length human γENaC proteins. In addition, 9 new sites (R122, R137, R138, K150, K170, R172, R180, K181, K189) in synthesized peptides were found to be cleaved by plasmin. These cleavage sites were located in the finger and the thumb, particularly the GRIP domain of human ENaC 3D model composed of two proteolytic centres for plasmin. Novel uncleaved sites beyond the GRIP domain in both α and γ subunits were identified to interrupt the plasmin cleavage-induced conformational change in ENaC channel complexes. Additionally, plasmin could regulate ENaC activity via the G protein signal.

CONCLUSION AND IMPLICATIONS

Plasmin can cleave ENaC to improve blood-gas exchange by resolving oedema fluid and could be a potent therapy for oedematous lungs.

Molecular Aspects of Co-morbidities in COVID-19 Infection

Coronaviruses are a group of enveloped viruses with single-stranded non-segmented positive-sense RNA genomes. In December 2019, SARS-CoV-2 appeared in China for the first time and quickly spread throughout the world. Although certain medications suggested for other afflictions tend to be potentially effective for curing the infection, there is no approved vaccination or drug available for this virus yet. Comprehension of the disease molecular pathogenesis could provide useful tools for COVID-19 patients in surveillance, prognosis, treatment, vaccine development and therapeutic targeting. The present research aims to summarize the association in COVID-19 patients between molecular dimensions of comorbidities with clinical and preclinical information. Developing an ACE2 inhibitor could be a possible therapeutic target. Plasmin is another possible candidate both in diagnosis and treatment areas. All predicted biomarkers must be validated either through randomized clinical trials or experimental assays before clinical application in patients.