Plasmin in urine from patients with type 2 diabetes and treatment-resistant hypertension activates ENaC in vitro

Influence of proteolytic cleavage of ENaC's γ subunit upon Na+ and K+ handling

The epithelial Na+ channel (ENaC) γ subunit is essential for homeostasis of Na+, K+, and body fluid. Dual γ subunit cleavage before and after a short inhibitory tract allows dissociation of this tract, increasing channel open probability (PO), in vitro. Cleavage proximal to the tract occurs at a furin recognition sequence (143RKRR146, in the mouse γ subunit). Loss of furin-mediated cleavage prevents in vitro activation of the channel by proteolysis at distal sites. We hypothesized that 143RKRR146 mutation to 143QQQQ146 (γQ4) in 129/Sv mice would reduce ENaC PO, impair flow-stimulated flux of Na+ (JNa) and K+ (JK) in perfused collecting ducts, reduce colonic amiloride-sensitive short-circuit current (ISC), and impair Na+, K+, and body fluid homeostasis. Immunoblot of γQ4/Q4 mouse kidney lysates confirmed loss of a band consistent in size with the furin-cleaved proteolytic fragment. However, γQ4/Q4 male mice on a low Na+ diet did not exhibit altered ENaC PO or flow-induced JNa, though flow-induced JK modestly decreased. Colonic amiloride-sensitive ISC in γQ4/Q4 mice was not altered. γQ4/Q4 males, but not females, exhibited mildly impaired fluid volume conservation when challenged with a low Na+ diet. Blood Na+ and K+ were unchanged on a regular, low Na+, or high K+ diet. These findings suggest that biochemical evidence of γ subunit cleavage should not be used in isolation to evaluate ENaC activity. Furthermore, factors independent of γ subunit cleavage modulate channel PO and the influence of ENaC on Na+, K+, and fluid volume homeostasis in 129/Sv mice, in vivo.NEW & NOTEWORTHY The epithelial Na+ channel (ENaC) is activated in vitro by post-translational proteolysis. In vivo, low Na+ or high K+ diets enhance ENaC proteolysis, and proteolysis is hypothesized to contribute to channel activation in these settings. Using a mouse expressing ENaC with disruption of a key proteolytic cleavage site, this study demonstrates that impaired proteolytic activation of ENaC's γ subunit has little impact upon channel open probability or the ability of mice to adapt to low Na+ or high K+ diets.

Amiloride Reduces Urokinase/Plasminogen-Driven Intratubular Complement Activation in Glomerular Proteinuria

SIGNIFICANCE STATEMENT

Proteinuria predicts accelerated decline in kidney function in CKD. The pathologic mechanisms are not well known, but aberrantly filtered proteins with enzymatic activity might be involved. The urokinase-type plasminogen activator (uPA)-plasminogen cascade activates complement and generates C3a and C5a in vitro / ex vivo in urine from healthy persons when exogenous, inactive, plasminogen, and complement factors are added. Amiloride inhibits uPA and attenuates complement activation in vitro and in vivo . In conditional podocin knockout (KO) mice with severe proteinuria, blocking of uPA with monoclonal antibodies significantly reduces the urine excretion of C3a and C5a and lowers tissue NLRP3-inflammasome protein without major changes in early fibrosis markers. This mechanism provides a link to proinflammatory signaling in proteinuria with possible long-term consequences for kidney function.

BACKGROUND

Persistent proteinuria is associated with tubular interstitial inflammation and predicts progressive kidney injury. In proteinuria, plasminogen is aberrantly filtered and activated by urokinase-type plasminogen activator (uPA), which promotes kidney fibrosis. We hypothesized that plasmin activates filtered complement factors C3 and C5 directly in tubular fluid, generating anaphylatoxins, and that this is attenuated by amiloride, an off-target uPA inhibitor.

METHODS

Purified C3, C5, plasminogen, urokinase, and urine from healthy humans were used for in vitro / ex vivo studies. Complement activation was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, immunoblotting, and ELISA. Urine and plasma from patients with diabetic nephropathy treated with high-dose amiloride and from mice with proteinuria (podocin knockout [KO]) treated with amiloride or inhibitory anti-uPA antibodies were analyzed.

RESULTS

The combination of uPA and plasminogen generated anaphylatoxins C3a and C5a from intact C3 and C5 and was inhibited by amiloride. Addition of exogenous plasminogen was sufficient for urine from healthy humans to activate complement. Conditional podocin KO in mice led to severe proteinuria and C3a and C5a urine excretion, which was attenuated reversibly by amiloride treatment for 4 days and reduced by >50% by inhibitory anti-uPA antibodies without altering proteinuria. NOD-, LRR- and pyrin domain-containing protein 3-inflammasome protein was reduced with no concomitant effect on fibrosis. In patients with diabetic nephropathy, amiloride reduced urinary excretion of C3dg and sC5b-9 significantly.

CONCLUSIONS

In conditions with proteinuria, uPA-plasmin generates anaphylatoxins in tubular fluid and promotes downstream complement activation sensitive to amiloride. This mechanism links proteinuria to intratubular proinflammatory signaling. In perspective, amiloride could exert reno-protective effects beyond natriuresis and BP reduction.

CLINICAL TRIAL REGISTRY NAME AND REGISTRATION NUMBER

Increased Activity of a Renal Salt Transporter (ENaC) in Diabetic Kidney Disease, NCT01918488 and Increased Activity of ENaC in Proteinuric Kidney Transplant Recipients, NCT03036748 .

Proteolytic Cleavage of the ENaC γ Subunit - Impact Upon Na + and K + Handling

The ENaC gamma subunit is essential for homeostasis of Na + , K + , and body fluid. Dual subunit cleavage before and after a short inhibitory tract allows dissociation of this tract, increasing channel open probability (P O ), in vitro . Cleavage proximal to the tract occurs at a furin recognition sequence ( 143 RKRR 146 in mouse). Loss of furin-mediated cleavage prevents in vitro activation of the channel by proteolysis at distal sites. We hypothesized that 143 RKRR 146 mutation to 143 QQQQ 146 ( Q4 ) in 129/Sv mice would reduce ENaC P O , impair flow-stimulated flux of Na + (J Na ) and K + (J K ) in perfused collecting ducts, reduce colonic amiloride-sensitive short circuit current (I SC ), and impair Na + , K + , and body fluid homeostasis. Immunoblot of Q4/Q4 mouse kidney lysates confirmed loss of a band consistent in size with the furin-cleaved proteolytic fragment. However, Q4/Q4 male mice on a low Na + diet did not exhibit altered ENaC P O or flow-induced J Na , though flow-induced J K modestly decreased. Colonic amiloride-sensitive I SC in Q4/Q4 mice was not altered. Q4/Q4 males, but not females, exhibited mildly impaired fluid volume conservation when challenged with a low Na + diet. Blood Na + and K + were unchanged on a regular, low Na + , or high K + diet. These findings suggest that biochemical evidence of gamma subunit cleavage should not be used in isolation to evaluate ENaC activity. Further, factors independent of gamma subunit cleavage modulate channel P O and the influence of ENaC on Na + , K + , and fluid volume homeostasis in 129/Sv mice, in vivo .

Oral Furosemide and Hydrochlorothiazide/Amiloride versus Intravenous Furosemide for the Treatment of Resistant Nephrotic Syndrome

BACKGROUND

Data on diuretic treatment in nephrotic syndrome (NS) are scarce. Our goal was to assess the non-inferiority of the combined oral diuretics (furosemide/hydrochlorothiazide/amiloride) compared to intravenous (i.v.) furosemide in patients with NS and resistant edema.

METHODS

We conducted a prospective randomized trial on 22 patients with resistant nephrotic edema (RNE), defined as hypervolemia and a FENa < 0.2%. Based on a computer-generated 1:1 randomization, we assigned patients to receive either intravenous furosemide (40 mg bolus and then continuous administration of 5 mg/h) or oral furosemide (40 mg/day) and hydrochlorothiazide/amiloride (50/5 mg/day) for a period of 5 days. Clinical and laboratory measurements were performed daily. Hydration status was assessed by bioimpedance on day 1 and at the end of day 5 after treatment initiation. The primary endpoint was weight change from baseline to day 5. Secondary endpoints were hydration status change measured by bioimpedance and safety outcomes (low blood pressure, severe electrolyte disturbances, acute kidney injury and worsening hypervolemia).

RESULTS

Primary endpoint analysis showed that after 5 days of treatment, there was a significant difference in weight change from baseline between groups [adjusted mean difference: -3.33 kg (95% CI: -6.34 to -0.31), p = 0.03], with a higher mean weight change in the oral diuretic treatment group [-7.10 kg (95% CI: -18.30 to -4.30) vs. -4.55 kg (95%CI: -6.73 to -2.36)]. Secondary endpoint analysis showed that there was no significant difference between groups regarding hydration status change [adjusted mean difference: -0.05 L (95% CI: -2.6 to 2.6), p = 0.96], with a mean hydration status change in the oral diuretic treatment group of -4.71 L (95% CI: -6.87 to -2.54) and -3.91 L (95% CI: -5.69 to -2.13) in the i.v. diuretic treatment group. We observed a significant decrease in adjusted mean serum sodium of -2.15 mmol/L [(95% CI: -4.25 to -0.05), p = 0.04]), favored by the combined oral diuretic treatment [-2.70 mmol/L (95% CI: -4.89 to -0.50) vs. -0.10 mmol/L (95%CI: -1.30 to 1.10)]. No statistically significant difference was observed between the two groups in terms of adverse events.

CONCLUSIONS

A combination of oral diuretics based on furosemide, amiloride and hydrochlorothiazide is non-inferior to i.v. furosemide in weight control of patients with RNE and a similar safety profile.

A Serine Protease Inhibitor, Camostat Mesilate, Suppresses Urinary Plasmin Activity and Alleviates Hypertension and Podocyte Injury in Dahl Salt-Sensitive Rats

In proteinuric renal diseases, the serine protease (SP) plasmin activates the epithelial sodium channel (ENaC) by cleaving its γ subunit. We previously demonstrated that a high-salt (HS) diet provoked hypertension and proteinuria in Dahl salt-sensitive (DS) rats, accompanied by γENaC activation, which were attenuated by camostat mesilate (CM), an SP inhibitor. However, the effects of CM on plasmin activity in DS rats remain unclear. In this study, we investigated the effects of CM on plasmin activity, ENaC activation, and podocyte injury in DS rats. The DS rats were divided into the control diet, HS diet (8.0% NaCl), and HS+CM diet (0.1% CM) groups. After weekly blood pressure measurement and 24-h urine collection, the rats were sacrificed at 5 weeks. The HS group exhibited hypertension, massive proteinuria, increased urinary plasmin, and γENaC activation; CM treatment suppressed these changes. CM prevented plasmin(ogen) attachment to podocytes and mitigated podocyte injury by reducing the number of apoptotic glomerular cells, inhibiting protease-activated receptor-1 activation, and suppressing inflammatory and fibrotic cytokine expression. Our findings highlight the detrimental role of urinary plasmin in the pathogenesis of salt-sensitive hypertension and glomerular injury. Targeting plasmin with SP inhibitors, such as CM, may be a promising therapeutic approach for these conditions.

Amiloride evokes significant natriuresis and weight loss in kidney transplant recipients with and without albuminuria

Albuminuria in kidney transplant recipients (KTRs) is associated with hypertension and aberrant glomerular filtration of serine proteases that may proteolytically activate the epithelial Na+ channel (ENaC). The present nonrandomized, pharmacodynamic intervention study aimed to investigate if inhibition of ENaC increases Na+ excretion and reduces extracellular volume in KTRs dependent on the presence of albuminuria. KTRs with and without albuminuria (albumin-to-creatinine ratio > 300 mg/g, n = 7, and <30 mg/g, n = 7, respectively) were included and ingested a diet with fixed Na+ content (150 mmol/day) for 5 days. On the last day, amiloride at 10 mg was administered twice. Body weight, 24-h urine electrolyte excretion, body water content, and ambulatory blood pressure as well as plasma renin, angiotensin II, and aldosterone concentrations were determined before and after amiloride. Amiloride led to a significant decrease in body weight, increase in 24-h urinary Na+ excretion, and decrease in 24-h urinary K+ excretion in both groups. Urine output increased in the nonalbuminuric group only. There was no change in plasma renin, aldosterone, and angiotensin II concentrations after amiloride, whereas a significant decrease in nocturnal systolic blood pressure and increase in 24-h urine aldosterone excretion was observed in albuminuric KTRs only. There was a significant correlation between 24-h urinary albumin excretion and amiloride-induced 24-h urinary Na+ excretion. In conclusion, ENaC activity contributes to Na+ and water retention in KTRs with and without albuminuria. ENaC is a relevant pharmacological target in KTRs; however, larger and long-term studies are needed to evaluate whether the magnitude of this effect depends on the presence of albuminuria.NEW & NOTEWORTHY Amiloride has a significant natriuretic effect in kidney transplant recipients (KTRs) that relates to urinary albumin excretion. The epithelial Na+ channel may be a relevant direct pharmacological target to counter Na+ retention and hypertension in KTRs. Epithelial Na+ channel blockers should be further investigated as a mean to mitigate Na+ and water retention and to potentially obtain optimal blood pressure control in KTRs.

The serine protease plasmin plays detrimental roles in epithelial sodium channel activation and podocyte injury in Dahl salt-sensitive rats

Salt-sensitive hypertension is associated with poor clinical outcomes. The epithelial sodium channel (ENaC) in the kidney plays pivotal roles in sodium reabsorption and blood pressure regulation, in which its γ subunit is activated by extracellular serine proteases. In proteinuric nephropathies, plasmin filtered through injured glomeruli reportedly activates γENaC in the distal nephron and causes podocyte injury. We previously reported that Dahl salt-sensitive (DS) rats fed a high-salt (HS) diet developed hypertension and proteinuria along with γENaC activation and that a synthetic serine protease inhibitor, camostat mesilate, mitigated these changes. However, the role of plasmin in DS rats remained unclear. In this study, we evaluated the relationship between plasmin and hypertension as well as podocyte injury and the effects of plasmin inhibitors in DS rats. Five-week-old DS rats were divided into normal-salt diet, HS diet, and HS+plasmin inhibitor (either tranexamic acid [TA] or synthetic plasmin inhibitor YO-2) groups. After blood pressure measurement and 24 h urine collection over 5 weeks, rats were sacrificed for biochemical analyses. The HS group displayed severe hypertension and proteinuria together with activation of plasmin in urine and γENaC in the kidney, which was significantly attenuated by YO-2 but not TA. YO-2 inhibited the attachment of plasmin(ogen) to podocytes and alleviated podocyte injury by inhibiting apoptosis and inflammatory/profibrotic cytokines. YO-2 also suppressed upregulation of protease-activated receptor-1 and phosphorylated ERK1/2. These results indicate an important role of plasmin in the development of salt-sensitive hypertension and related podocyte injury, suggesting plasmin inhibition as a potential therapeutic strategy.

Interaction of Severe Acute Respiratory Syndrome Coronavirus 2 and Diabetes

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is causing a worldwide epidemic. It spreads very fast and hits people of all ages, especially patients with underlying diseases such as diabetes. In this review, we focus on the influences of diabetes on the outcome of SARS-CoV-2 infection and the involved mechanisms including lung dysfunction, immune disorder, abnormal expression of angiotensin-converting enzyme 2 (ACE2), overactivation of mechanistic target of rapamycin (mTOR) signaling pathway, and increased furin level. On the other hand, SARS-CoV-2 may trigger the development of diabetes. It causes the damage of pancreatic β cells, which is probably mediated by ACE2 protein in the islets. Furthermore, SARS-CoV-2 may aggravate insulin resistance through attacking other metabolic organs. Of note, certain anti-diabetic drugs (OADs), such as peroxisome proliferator-activated receptor γ (PPARγ) activator and glucagon-like peptide 1 receptor (GLP-1R) agonist, have been shown to upregulate ACE2 in animal models, which may increase the risk of SARS-CoV-2 infection. However, Metformin, as a first-line medicine for the treatment of type 2 diabetes mellitus (T2DM), may be a potential drug benefiting diabetic patients with SARS-CoV-2 infection, probably via a suppression of mTOR signaling together with its anti-inflammatory and anti-fibrosis function in lung. Remarkably, another kind of OADs, dipeptidyl Peptidase 4 (DPP4) inhibitor, may also exert beneficial effects in this respect, probably via a prevention of SARS-CoV-2 binding to cells. Thus, it is of significant to identify appropriate OADs for the treatment of diabetes in the context of SARS-CoV-2 infections.

Plasminogenuria is associated with podocyte injury, edema, and kidney dysfunction in incident glomerular disease

Urinary plasminogen/plasmin, or plasmin (ogen) uria, has been demonstrated in proteinuric patients and exposure of cultured podocytes to plasminogen results in injury via oxidative stress pathways. A causative role for plasmin (ogen) as a "second hit" in kidney disease progression has yet to have been demonstrated in vivo. Additionally, association between plasmin (ogen) uria and kidney function in glomerular diseases remains unclear. We performed comparative studies in a puromycin aminonucleoside (PAN) nephropathy rat model treated with amiloride, an inhibitor of plasminogen activation, and measured changes in plasmin (ogen) uria. In a glomerular disease biorepository cohort (n = 128), we measured time-of-biopsy albuminuria, proteinuria, and plasmin (ogen) uria for correlations with kidney outcomes. In cultured human podocytes, plasminogen treatment was associated with decreased focal adhesion marker expression with rescue by amiloride. Increased glomerular plasmin (ogen) was found in PAN rats and focal segmental glomerulosclerosis (FSGS) patients. PAN nephropathy was associated with increases in plasmin (ogen) uria and proteinuria. Amiloride was protective against PAN-induced glomerular injury, reducing CD36 scavenger receptor expression and oxidative stress. In patients, we found associations between plasmin (ogen) uria and edema status as well as eGFR. Our study demonstrates a role for plasmin (ogen)-induced podocyte injury in the PAN nephropathy model, with amiloride having podocyte-protective properties. In one of the largest glomerular disease cohorts to study plasminogen, we validated previous findings while suggesting a potentially novel relationship between plasmin (ogen) uria and estimated glomerular filtration rate (eGFR). Together, these findings suggest a role for plasmin (ogen) in mediating glomerular injury and as a viable targetable biomarker for podocyte-sparing treatments.

Plasminogen Deficiency and Amiloride Mitigate Angiotensin II-Induced Hypertension in Type 1 Diabetic Mice Suggesting Effects Through the Epithelial Sodium Channel

Background Diabetic nephropathy is a common diabetes mellitus complication associated with hypertension, proteinuria, and excretion of urinary plasmin that activates the epithelial sodium channel, ENaC, in vitro. Here we hypothesized that the deletion of plasminogen and amiloride treatment protect against hypertension in diabetes mellitus. Methods and Results Male plasminogen knockout (plasminogen-deficient [Plg^-/-]) and wild-type mice were rendered diabetic with streptozotocin. Arterial blood pressure was recorded continuously by indwelling catheters before and during 10 days of angiotensin II infusion (ANGII; 30-60 ng/kg per minute). The effect of amiloride infusion (2 mg/kg per day, 4 days) was tested in wild-type, diabetic ANGII-treated mice. Streptozotocin increased plasma and urine glucose concentrations and 24-hour urine albumin and plasminogen excretion. Diabetic Plg^-/- mice displayed larger baseline albuminuria and absence of urine plasminogen. Baseline mean arterial blood pressure did not differ between groups. Although ANGII elevated blood pressure in wild-type, diabetic wild-type, and Plg^-/- control mice, ANGII did not change blood pressure in diabetic Plg^-/- mice. Compared with ANGII infusion alone, wild-type ANGII-infused diabetic mice showed blood pressure reduction upon amiloride treatment. There was no difference in plasma renin, ANGII, aldosterone, tissue prorenin receptor, renal inflammation, and fibrosis between groups. Urine from wild-type mice evoked larger amiloride-sensitive current than urine from Plg^-/- mice with or without diabetes mellitus. Full-length γ-ENaC and α-ENaC subunit abundances were not changed in kidney homogenates, but the 70 kDa γ-ENaC cleavage product was increased in diabetic versus nondiabetic mice. Conclusions Plasmin promotes hypertension in diabetes mellitus with albuminuria likely through the epithelial sodium channel.

Attributes of dysgeusia and anosmia of coronavirus disease 2019 (COVID-19) in hospitalized patients

OBJECTIVES

While chemosensory dysfunctions, dysgeusia and anosmia/hyposmia, are recognized as distinctive symptoms of COVID-19, their temporality of presentation and association with the patient age, gender, disease severity, and comorbidities has been sparsely studied. Hence, we evaluated the latter associations of chemosensory dysfunction, in hospitalized COVID-19 patients in the United Arab Emirates (UAE).

MATERIALS AND METHODS

Information on chemosensory dysfunction and history of chronic systemic comorbidities, if any, was obtained from 149 COVID-19 patients in an infectious disease hospital in UAE, using their medical records, as well as from a face-to-face questionnaire survey. Additionally, a modified SNOT-22 questionnaire that measures disease-specific quality of life in patients with upper respiratory tract affections was also administered.

RESULTS

Chemosensory dysfunction was reported by 94.6% of the cohort, and anosmia with dysgeusia was significantly more in males than females with severe COVID-19. Males with moderate COVID-19 and systemic comorbidities were more likely to present with chemosensory dysfunction in comparison with females. SNOT-22 questionnaire revealed that nasal blockage and runny nose were more prevalent in mild/moderate, than in the severe, state of COVID-19.

CONCLUSION

Our data confirm the commonality of chemosensory dysfunction during COVID-19 progression, and the significantly more pronounced combined dysfunction in males with severe COVID-19, and comorbidities.

Salt-sensitive hypertension in chronic kidney disease: distal tubular mechanisms

Chronic kidney disease (CKD) causes salt-sensitive hypertension that is often resistant to treatment and contributes to the progression of kidney injury and cardiovascular disease. A better understanding of the mechanisms contributing to salt-sensitive hypertension in CKD is essential to improve these outcomes. This review critically explores these mechanisms by focusing on how CKD affects distal nephron Na+ reabsorption. CKD causes glomerulotubular imbalance with reduced proximal Na+ reabsorption and increased distal Na+ delivery and reabsorption. Aldosterone secretion further contributes to distal Na+ reabsorption in CKD and is not only mediated by renin and K+ but also by metabolic acidosis, endothelin-1, and vasopressin. CKD also activates the intrarenal renin-angiotensin system, generating intratubular angiotensin II to promote distal Na+ reabsorption. High dietary Na+ intake in CKD contributes to Na+ retention by aldosterone-independent activation of the mineralocorticoid receptor mediated through Rac1. High dietary Na+ also produces an inflammatory response mediated by T helper 17 cells and cytokines increasing distal Na+ transport. CKD is often accompanied by proteinuria, which contains plasmin capable of activating the epithelial Na+ channel. Thus, CKD causes both local and systemic changes that together promote distal nephron Na+ reabsorption and salt-sensitive hypertension. Future studies should address remaining knowledge gaps, including the relative contribution of each mechanism, the influence of sex, differences between stages and etiologies of CKD, and the clinical relevance of experimentally identified mechanisms. Several pathways offer opportunities for intervention, including with dietary Na+ reduction, distal diuretics, renin-angiotensin system inhibitors, mineralocorticoid receptor antagonists, and K+ or H+ binders.

Association of ACE2 receptor and ACEIs/ARBs with disease severity in COVID-19

Coronavirus disease 2019 (COVID-19) is found to be associated with various comorbidities which include cardiovascular diseases, hypertension, and diabetes. The impaired regulation of renin-angiotensin-aldosterone system (RAAS) has been seen in COVID-19 patients, but whether RAAS inhibitors, such as angiotensin-converting enzyme inhibitors (ACEIs) and angiotensin II type 1 receptor blockers (ARBs), are responsible for worsening of clinical conditions remains unknown. Herein, we review the role of angiotensin-converting enzyme 2 (ACE2) expression in disease progression, its association with comorbidities and COVID-19, and summarize the clinical evidence for several potential directions for future research work on ACEIs/ARBs in COVID-19 patients.

SARS-CoV-2 receptor ACE2-dependent implications on the cardiovascular system: From basic science to clinical implications

The current COVID-19 pandemic started several months ago and is still exponentially growing in most parts of the world - this is the most recent and alarming update. COVID-19 requires the collaboration of nearly 200 countries to curb the spread of SARS-CoV-2 while gaining time to explore and improve treatment options especially for cardiovascular disease (CVD) and immunocompromised patients, who appear to be at high-risk to die from cardiopulmonary failure. Currently unanswered questions are why elderly people, particularly those with pre-existing comorbidities seem to exhibit higher mortality rates after SARS-CoV-2 infection and whether intensive care becomes indispensable for these patients to prevent multi-organ failure and sudden death. To face these challenges, we here summarize the molecular insights into viral infection mechanisms and implications for cardiovascular disease. Since the infection starts in the upper respiratory system, first flu-like symptoms develop that spread throughout the body. The wide range of affected organs is presumably based on the common expression of the major SARS-CoV-2 entry-receptor angiotensin-converting enzyme 2 (ACE2). Physiologically, ACE2 degrades angiotensin II, the master regulator of the renin-angiotensin-aldosterone system (RAAS), thereby converting it into vasodilatory molecules, which have well-documented cardio-protective effects. Thus, RAAS inhibitors, which may increase the expression levels of ACE2, are commonly used for the treatment of hypertension and CVD. This, and the fact that SARS-CoV-2 hijacks ACE2 for cell-entry, have spurred controversial discussions on the role of ACE2 in COVID-19 patients. In this review, we highlight the state-of-the-art knowledge on SARS-CoV-2-dependent mechanisms and the potential interaction with ACE2 expression and cell surface localization. We aim to provide a list of potential treatment options and a better understanding of why CVD is a high risk factor for COVID-19 susceptibility and further discuss the acute as well as long-term cardiac consequences.

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.

Urokinase-type plasminogen activator contributes to amiloride-sensitive sodium retention in nephrotic range glomerular proteinuria in mice

AIM

Activation of sodium reabsorption by urinary proteases has been implicated in sodium retention associated with nephrotic syndrome. The study was designed to test the hypothesis that nephrotic proteinuria in mice after conditional deletion of podocin leads to urokinase-dependent, amiloride-sensitive plasmin-mediated sodium and water retention.

METHODS

Ten days after podocin knockout, urine and faeces were collected for 10 days in metabolic cages and analysed for electrolytes, plasminogen, protease activity and ability to activate γENaC by patch clamp and western blot. Mice were treated with amiloride (2.5 mg kg^-1 for 2 days and 10 mg kg^-1 for 2 days) or an anti-urokinase-type plasminogen activator (uPA) targeting antibody (120 mg kg^-1 /24 h) and compared to controls.

RESULTS

Twelve days after deletion, podocin-deficient mice developed significant protein and albuminuria associated with increased body wt, ascites, sodium accumulation and suppressed plasma renin. This was associated with increased urinary excretion of plasmin and plasminogen that correlated with albumin excretion, urine protease activity co-migrating with active plasmin, and the ability of urine to induce an amiloride-sensitive inward current in M1 cells in vitro. Amiloride treatment in podocin-deficient mice resulted in weight loss, increased sodium excretion, normalization of sodium balance and prevention of the activation of plasminogen to plasmin in urine in a reversible way. Administration of uPA targeting antibody abolished urine activation of plasminogen, attenuated sodium accumulation and prevented cleavage of γENaC.

CONCLUSIONS

Nephrotic range glomerular proteinuria leads to urokinase-dependent intratubular plasminogen activation and γENaC cleavage which contribute to sodium accumulation.

Regulating ENaC's gate

Epithelial Na⁺ channels (ENaCs) are members of a family of cation channels that function as sensors of the extracellular environment. ENaCs are activated by specific proteases in the biosynthetic pathway and at the cell surface and remove embedded inhibitory tracts, which allows channels to transition to higher open-probability states. Resolved structures of ENaC and an acid-sensing ion channel revealed highly organized extracellular regions. Within the periphery of ENaC subunits are unique domains formed by antiparallel β-strands containing the inhibitory tracts and protease cleavage sites. ENaCs are inhibited by Na⁺ binding to specific extracellular site(s), which promotes channel transition to a lower open-probability state. Specific inositol phospholipids and channel modification by Cys-palmitoylation enhance channel open probability. How these regulatory factors interact in a concerted manner to influence channel open probability is an important question that has not been resolved. These various factors are reviewed, and the impact of specific factors on human disorders is discussed.

Proteasuria-The impact of active urinary proteases on sodium retention in nephrotic syndrome

Sodium retention and extracellular volume expansion are typical features of patients with nephrotic syndrome. In recent years, from in vitro data, endoluminal activation of the epithelial sodium channel (ENaC) by aberrantly filtered serine proteases has been proposed as an underlying mechanism. Recently, this concept was supported in vivo in nephrotic mice that were protected from proteolytic ENaC activation and sodium retention by the use of aprotinin for the pharmacological inhibition of urinary serine protease activity. These and other findings from studies in both rodents and humans highlight the impact of active proteases in the urine, or proteasuria, on ENaC-mediated sodium retention and edema formation in nephrotic syndrome. Targeting proteasuria could become a therapeutic approach to treat patients with nephrotic syndrome. However, pathophysiologically relevant proteases remain to be identified. In this review, we introduce the concept of proteasuria to explain tubular sodium avidity and conclude that proteasuria can be considered as a key mechanism of sodium retention in patients with nephrotic syndrome.

New insights regarding epithelial Na+ channel regulation and its role in the kidney, immune system and vasculature

PURPOSE OF REVIEW

This review describes recent findings regarding the epithelial Na channel (ENaC) and its roles in physiologic and pathophysiologic states. We discuss new insights regarding ENaC's structure, its regulation by various factors, its potential role in hypertension and nephrotic syndrome, and its roles in the immune system and vasculature.

RECENT FINDINGS

A recently resolved structure of ENaC provides clues regarding mechanisms of ENaC activation by proteases. The use of amiloride in nephrotic syndrome, and associated complications are discussed. ENaC is expressed in dendritic cells and contributes to immune system activation and increases in blood pressure in response to NaCl. ENaC is expressed in endothelial ENaC and has a role in regulating vascular tone.

SUMMARY

New findings have emerged regarding ENaC and its role in the kidney, immune system, and vasculature.

Ion channels and transporters in diabetic kidney disease

Type 1 and 2 diabetes mellitus are major medical epidemics affecting millions of patients worldwide. Diabetes mellitus is the leading cause of diabetic kidney disease (DKD), which is the most common cause of end-stage renal disease (ESRD). DKD is associated with significant changes in renal hemodynamics and electrolyte transport. Alterations in renal ion transport triggered by pathophysiological conditions in diabetes can exacerbate hypertension, accelerate renal injury, and are integral to the development of DKD. Renal ion transporters and electrolyte homeostasis play a fundamental role in functional changes and injury to the kidney during DKD. With the large number of ion transporters involved in DKD, understanding the roles of individual transporters as well as the complex cascades through which they interact is essential in the development of effective treatments for patients suffering from this disease. This chapter aims to gather current knowledge of the major renal ion transporters with altered expression and activity under diabetic conditions, and provide a comprehensive overview of their interactions and collective functions in DKD.

A Novel Mechanism of Renal Microcirculation Regulation: Connecting Tubule-Glomerular Feedback

PURPOSE OF REVIEW

In this review, we summarized the current knowledge of connecting tubule-glomerular feedback (CTGF), a novel mechanism of renal microcirculation regulation that integrates sodium handling in the connecting tubule (CNT) with kidney hemodynamics.

RECENT FINDINGS

Connecting tubule-glomerular feedback is a crosstalk communication between the CNT and the afferent arteriole (Af-Art), initiated by sodium chloride through the epithelial sodium channel (ENaC). High sodium in the CNT induces Af-Art vasodilation, increasing glomerular pressure and the glomerular filtration rate and favoring sodium excretion. CTGF antagonized and reset tubuloglomerular feedback and thus increased sodium excretion. CTGF is absent in spontaneous hypertensive rats and is overactivated in Dahl salt-sensitive rats. CTGF is also modulated by angiotensin II and aldosterone. CTGF is a feedback mechanism that integrates sodium handling in the CNT with glomerular hemodynamics. Lack of CTGF could promote hypertension, and CTGF overactivation may favor glomerular damage and proteinuria. More studies are needed to explore the alterations in renal microcirculation and the role of these alterations in the genesis of hypertension and glomerular damage in animals and humans.

KEY POINTS

• CTGF is a vasodilator mechanism that regulates afferent arteriole resistance. • CTGF is absent in spontaneous hypertensive rats and overactivated in Dahl salt-sensitive rats. • CTGF in excess may promote glomerular damage and proteinuria, while the absence may participate in sodium retention and hypertension.

The Absence of the ACE N-Domain Decreases Renal Inflammation and Facilitates Sodium Excretion during Diabetic Kidney Disease

BACKGROUND

Recent evidence emphasizes the critical role of inflammation in the development of diabetic nephropathy. Angiotensin-converting enzyme (ACE) plays an active role in regulating the renal inflammatory response associated with diabetes. Studies have also shown that ACE has roles in inflammation and the immune response that are independent of angiotensin II. ACE's two catalytically independent domains, the N- and C-domains, can process a variety of substrates other than angiotensin I.

METHODS

To examine the relative contributions of each ACE domain to the sodium retentive state, renal inflammation, and renal injury associated with diabetic kidney disease, we used streptozotocin to induce diabetes in wild-type mice and in genetic mouse models lacking either a functional ACE N-domain (NKO mice) or C-domain (CKO mice).

RESULTS

In response to a saline challenge, diabetic NKO mice excreted 32% more urinary sodium compared with diabetic wild-type or CKO mice. Diabetic NKO mice also exhibited 55% less renal epithelial sodium channel cleavage (a marker of channel activity), 55% less renal IL-1β, 53% less renal TNF-α, and 53% less albuminuria than diabetic wild-type mice. This protective phenotype was not associated with changes in renal angiotensin II levels. Further, we present evidence that the anti-inflammatory tetrapeptide N-acetyl-seryl-asparyl-lysyl-proline (AcSDKP), an ACE N-domain-specific substrate that accumulates in the urine of NKO mice, mediates the beneficial effects observed in the NKO.

CONCLUSIONS

These data indicate that increasing AcSDKP by blocking the ACE N-domain facilitates sodium excretion and ameliorates diabetic kidney disease independent of intrarenal angiotensin II regulation.

Urinary Plasmin(ogen) as a Prognostic Factor for Hypertension

Introduction

Plasmin and its precursor, plasminogen, are detectable in urine from patients with glomerular disease. Urinary plasmin(ogen) levels correlate with blood pressure (BP) and may contribute to renal Na⁺ retention by activating the epithelial Na⁺ channel (ENaC). In a longitudinal nested-cohort study, we asked whether urinary plasmin(ogen) levels predict subsequent increase in BP, incident hypertension, or mortality in subjects with type I diabetes, who often develop proteinuria.

Methods

The Pittsburgh Epidemiology of Diabetes Complications (EDC) study followed up type I diabetic subjects for 25 years. Urine specimens from 70 subjects with a spectrum of baseline urinary albumin levels were examined. Outcomes included increased BP after 2 years (≥1 SD over baseline systolic or diastolic BP, examined via logistic regression), 25-year incident hypertension (≥140/90 mm Hg or initiating BP-lowering medications), and all-cause or cardiovascular mortality, examined using Cox regression.

Results

Subjects experiencing a 2-year increase in BP had higher baseline urinary plasmin(ogen)/creatinine levels (uPl/Cr) than other subjects (P = 0.04); the difference in baseline urinary albumin/creatinine levels (uAlb/Cr) was similar (P = 0.07). Baseline uPl/Cr was associated with increased 25-year hypertension incidence (hazard ratio = 2.05, P = 0.001), all-cause mortality (HR = 2.05, P = 0.01) and cardiovascular mortality (HR = 3.30, P = 0.005), although not independent of uAlb/Cr.

Conclusion

This is the first long-term prospective study addressing clinical outcomes associated with increased urinary plasmin(ogen). Findings are consistent with a role for plasmin(ogen) in promoting increased BP, but also demonstrate the difficulty in distinguishing effects due to plasmin(ogen) from those of albuminuria.

Albuminuria in kidney transplant recipients is associated with increased urinary serine proteases and activation of the epithelial sodium channel

Albuminuria predicts adverse renal outcome in kidney transplant recipients. The present study addressed the hypothesis that albuminuria is associated with increased urine serine proteases with the ability to activate the epithelial sodium channel (ENaC) and with greater extracellular volume and higher blood pressure. In a cross-sectional design, kidney transplant recipients with ( n = 18) and without ( n = 19) albuminuria were included for office blood pressure measurements, estimation of volume status by bioimpedance, and collection of spot urine and plasma samples. Urine was analyzed for serine proteases and for the ability to activate ENaC current in vitro. Urine exosome protein was immunoblotted for prostasin and γ-ENaC protein. In the present study, it was found that, compared with nonalbuminuria (8.8 mg/g creatinine), albuminuric (1,722 mg/g creatinine) kidney transplant recipients had a higher systolic and diastolic blood pressure, despite receiving significantly more antihypertensives, and a greater urinary total plasminogen, active plasmin, active urokinase-type plasminogen activator, and prostasin protein abundance, which correlated significantly with u-albumin. Fluid overload correlated with systolic blood pressure, urinary albumin/creatinine, and plasminogen/creatinine. Urine from albuminuric kidney transplant recipients evoked a greater amiloride- and aprotinin-sensitive inward current in single collecting duct cells (murine cell line M1). γENaC subunits at 50 and 75 kDa showed increased abundance in urine exosomes from albuminuric kidney transplant recipients when compared with controls. These findings show that albuminuria in kidney transplant recipients is associated with hypertension, ability of urine to proteolytically activate ENaC current, and increased abundance of γENaC. ENaC activity could contribute to hypertension and adverse outcome in posttransplant proteinuria.

Urine albumin is a superior predictor of preeclampsia compared to urine plasminogen in type I diabetes patients

Pregnant women with type I diabetes mellitus (T1DM) are at increased risk of developing preeclampsia (PE). Plasminogen is aberrantly filtrated from plasma into tubular fluid in PE patients and activated to plasmin. Plasmin activates the epithelial sodium channel in the collecting ducts potentially causing impaired sodium excretion, suppression of the renin-angiotensin-aldosterone system, and hypertension in PE. The objective of the study was to test whether urinary total plasmin(ogen)/creatinine ratio and plasma concentration of aldosterone were better predictors of PE in pregnant women with T1DM compared with urine albumin and haemoglobin A_1C. The design was a longitudinal observational study of 88 pregnant T1DM patients at 2 Danish centers. Spot urine- and blood samples were collected at gestational weeks 12, 20, 28, 32, and 36. U-plasmin(ogen)/creatinine ratio increased during pregnancy. In gestational week 36, the ratio was significantly increased in the T1DM patients developing PE (P < .05). P-aldosterone was significantly increased in gestational week 20 in the group developing PE (P < .05). U-albumin/creatinine ratio was significantly increased and predicted PE at all tested gestational ages. U-albumin/creatinine ratio had a stronger association with the development of PE compared to u-total plasmin(ogen)/creatinine ratio and p-aldosterone. The positive association between u-total plasmin(ogen) and development of PE late in pregnancy is compatible with involvement in PE pathophysiology. The significance of albumin in urine emphasizes the importance of preventing renal complications when planning pregnancy in patients with type I diabetes.

Epithelial Na+ Channel Regulation by Extracellular and Intracellular Factors

Epithelial Na⁺ channels (ENaCs) are members of the ENaC/degenerin family of ion channels that evolved to respond to extracellular factors. In addition to being expressed in the distal aspects of the nephron, where ENaCs couple the absorption of filtered Na⁺ to K⁺ secretion, these channels are found in other epithelia as well as nonepithelial tissues. This review addresses mechanisms by which ENaC activity is regulated by extracellular factors, including proteases, Na⁺, and shear stress. It also addresses other factors, including acidic phospholipids and modification of ENaC cytoplasmic cysteine residues by palmitoylation, which enhance channel activity by altering interactions of the channel with the plasma membrane.

Significant natriuretic and antihypertensive action of the epithelial sodium channel blocker amiloride in diabetic patients with and without nephropathy

OBJECTIVE

Diabetic nephropathy is associated with aberrant glomerular filtration of serine proteases. The study was designed to test the hypothesis that the epithelial sodium channel is activated proteolytically by urine plasmin in diabetic nephropathy and mediates renal sodium retention.

METHODS

In an open-label intervention study on type 1 diabetes patients on standardized NaCl intake (200 mmol/day) with (n = 15) and without diabetic nephropathy (control, n = 12), urinary Na excretion in response to oral amiloride (20 or 40 mg/day for 2 days) was compared.

RESULTS

A total of 27 patients completed the study and nine diabetic nephropathy and eight control study participants were compliant (24-h urine Na excretion of 200 mmol ± 30%). Amiloride increased significantly total and fractional Na excretion in both groups. Total natriuresis and weight loss were significantly larger in the control group compared with diabetic nephropathy at day 1 of amiloride, whereas fractional Na excretion did not differ. Amiloride intervention increased plasma renin concentration only in diabetic nephropathy group; it reduced SBP in both groups, whereas DBP was reduced in diabetic nephropathy group only. Albuminuria was reduced significantly by amiloride in diabetic nephropathy group. Urine total amiloride concentration was not different between groups (12 ± 1 and 16 ± 1 μmol/l, respectively). Urine total plasminogen and active plasmin were reduced after amiloride in diabetic nephropathy.

CONCLUSION

Amiloride increased renal Na excretion, reduced blood pressure, albuminuria, and total and active plasmin in urine. It is concluded that epithelial sodium channel is an attractive target to attain blood pressure control in long-term type I diabetes with no enhanced activity associated with nephropathy.

Trial of Amiloride in Type 2 Diabetes with Proteinuria

Introduction

Renal sodium (Na⁺) retention and extracellular fluid volume expansion are hallmarks of nephrotic syndrome, which occurs even in the absence of activation of hormones that stimulate renal Na⁺ transporters. Plasmin-dependent activation of the epithelial Na⁺ channel has been proposed to have a role in renal Na⁺ retention in the setting of nephrotic syndrome. We hypothesized that the epithelial Na⁺ channel inhibitor amiloride would be an effective therapeutic agent in inducing a natriuresis and lowering blood pressure in individuals with macroscopic proteinuria.

Methods

We conducted a pilot double-blind randomized cross-over study comparing the effects of daily administration of either oral amiloride or hydrochlorothiazide to patients with type 2 diabetes and macroscopic proteinuria. Safety and efficacy were assessed by monitoring systolic blood pressure, kidney function, adherence, weight, urinary Na⁺ excretion, and serum electrolytes. Nine subjects were enrolled in the trial.

Results

No significant difference in systolic blood pressure or weight was seen between subjects receiving hydrochlorothiazide and those receiving amiloride (P ≥ 0.15). Amiloride induced differences in serum potassium (P < 0.001), with a 0.88 ± 0.30 mmol/l greater acute increase observed. Two subjects developed acute kidney injury and hyperkalemia when treated with amiloride. Four subjects had readily detectable levels of urinary plasminogen plus plasmin, and 5 did not. Changes in systolic blood pressure in response to amiloride did not differ between individuals with versus those without detectable urinary plasminogen plus plasmin.

Discussion

In summary, among patients with type 2 diabetes, normal renal function, and proteinuria, there were reductions in systolic blood pressure in groups treated with hydrochlorothiazide or amiloride. Acute kidney injury and severe hyperkalemia were safety concerns with amiloride.

Diuretic Resistance

Diuretic resistance is defined as a failure to achieve the therapeutically desired reduction in edema despite a full dose of diuretic. The causes of diuretic resistance include poor adherence to drug therapy or dietary sodium restriction, pharmacokinetic issues, and compensatory increases in sodium reabsorption in nephron sites that are not blocked by the diuretic. To illustrate the pathophysiology and management of diuretic resistance, we describe a patient with nephrotic syndrome. This patient presented with generalized pitting edema and weight gain despite the use of oral loop diuretics. Nephrotic syndrome may cause mucosal edema of the intestine, limiting the absorption of diuretics. In addition, the patient's kidney function had deteriorated, impairing the tubular secretion of diuretics. He was admitted for intravenous loop diuretic treatment. However, this was ineffective, likely due to compensatory sodium reabsorption by other tubular segments. The combination of loop diuretics with triamterene, a blocker of the epithelial sodium channel, effectively reduced body weight and edema. Recent data suggest that plasmin in nephrotic urine can activate the epithelial sodium channel, potentially contributing to the diuretic resistance in this patient. This case is used to illustrate and review the mechanisms of, and possible interventions for, diuretic resistance.

The function and regulation of acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC): IUPHAR Review 19

Acid-sensing ion channels (ASICs) and the epithelial Na(+) channel (ENaC) are both members of the ENaC/degenerin family of amiloride-sensitive Na(+) channels. ASICs act as proton sensors in the nervous system where they contribute, besides other roles, to fear behaviour, learning and pain sensation. ENaC mediates Na(+) reabsorption across epithelia of the distal kidney and colon and of the airways. ENaC is a clinically used drug target in the context of hypertension and cystic fibrosis, while ASIC is an interesting potential target. Following a brief introduction, here we will review selected aspects of ASIC and ENaC function. We discuss the origin and nature of pH changes in the brain and the involvement of ASICs in synaptic signalling. We expose how in the peripheral nervous system, ASICs cover together with other ion channels a wide pH range as proton sensors. We introduce the mechanisms of aldosterone-dependent ENaC regulation and the evidence for an aldosterone-independent control of ENaC activity, such as regulation by dietary K(+) . We then provide an overview of the regulation of ENaC by proteases, a topic of increasing interest over the past few years. In spite of the profound differences in the physiological and pathological roles of ASICs and ENaC, these channels share many basic functional and structural properties. It is likely that further research will identify physiological contexts in which ASICs and ENaC have similar or overlapping roles.

Podocyte injury: the role of proteinuria, urinary plasminogen, and oxidative stress

Podocytes are the key target for injury in proteinuric glomerular diseases that result in podocyte loss, progressive focal segmental glomerular sclerosis (FSGS), and renal failure. Current evidence suggests that the initiation of podocyte injury and associated proteinuria can be separated from factors that drive and maintain these pathogenic processes leading to FSGS. In nephrotic urine aberrant glomerular filtration of plasminogen (Plg) is activated to the biologically active serine protease plasmin by urokinase-type plasminogen activator (uPA). In vivo inhibition of uPA mitigates Plg activation and development of FSGS in several proteinuric models of renal disease including 5/6 nephrectomy. Here, we show that Plg is markedly increased in the urine in two murine models of proteinuric kidney disease associated with podocyte injury: Tg26 HIV-associated nephropathy and the Cd2ap^-/- model of FSGS. We show that human podocytes express uPA and three Plg receptors: uPAR, tPA, and Plg-RKT. We demonstrate that Plg treatment of podocytes specifically upregulates NADPH oxidase isoforms NOX2/NOX4 and increases production of mitochondrial-dependent superoxide anion (O₂^-) that promotes endothelin-1 synthesis. Plg via O₂^- also promotes expression of the B scavenger receptor CD36 and subsequent increased intracellular cholesterol uptake resulting in podocyte apoptosis. Taken together, our findings suggest that following disruption of the glomerular filtration barrier at the onset of proteinuric disease, podocytes are exposed to Plg resulting in further injury mediated by oxidative stress. We suggest that chronic exposure to Plg could serve as a "second hit" in glomerular disease and that Plg is potentially an attractive target for therapeutic intervention.

Association of Plasminuria with Overhydration in Patients with CKD

BACKGROUND AND OBJECTIVES

Hypervolemia is a common feature of patients with CKD and associated with hypertension. Recent work has shown stimulation of sodium retention by urinary plasmin during nephrotic syndrome. However, it is unclear whether plasminuria plays a role in patients with stable CKD and non-nephrotic proteinuria.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In this cross-sectional study, we analyzed the fluid status of 171 patients with CKD consecutively presenting to our outpatient clinic from 2012 to 2013 using bioimpedance spectroscopy (Body Composition Monitor [BCM]; Fresenius Medical Care, Germany) and its associations to the urinary excretion of plasminogen and plasmin from a spot urine sample. Two-electrode voltage clamp measurements were performed in Xenopus laevis oocytes expressing human epithelial sodium channel to investigate whether plasmin in concentrations found in urine can activate the channel.

RESULTS

Overhydration >5% and overhydration >10% of the extracellular volume were found in 29% and 17% of the patients, respectively, and overhydration was associated with edema, hypertension, higher stages of CKD, and proteinuria. Proteinuria was the strongest independent predictor for overhydration (+0.58 L/1.73 m(2) per 10-fold increase; P<0.001). Urinary excretion of plasmin(ogen) quantified by ELISA correlated strongly with proteinuria (r=0.87) and overhydration (r=0.47). Using a chromogenic substrate, active plasmin was found in 44% of patients and correlated with proteinuria and overhydration. Estimated urinary plasmin concentrations were in a range sufficient to activate epithelial sodium channel currents in vitro. In multivariable analysis, urinary excretion of plasmin(ogen) was associated with overhydration similar to proteinuria.

CONCLUSIONS

Hypervolemia in patients with CKD is strongly associated with proteinuria, even in the non-nephrotic range. Protein-rich urine contains high amounts of plasminogen and active plasmin, rendering plasminuria as a possible link between proteinuria and hypervolemia.

Amiloride lowers blood pressure and attenuates urine plasminogen activation in patients with treatment-resistant hypertension

In conditions with albuminuria, plasminogen is aberrantly filtered across the glomerular barrier and activated along the tubular system to plasmin. In the collecting duct, plasmin activates epithelial sodium channels (ENaC) proteolytically. Hyperactivity of ENaC could link microalbuminuria/proteinuria to resistant hypertension. Amiloride, an ENaC inhibitor, inhibits urokinase-type plasminogen activator. We hypothesized that amiloride (1) reduces blood pressure (BP); (2) attenuates plasminogen-to-plasmin activation; and (3) inhibits urine urokinase-type plasminogen activator in patients with resistant hypertension and type 2 diabetes mellitus (T2DM).In an open-label, non-randomized, 8-week intervention study, a cohort (n = 80) of patients with resistant hypertension and T2DM were included. Amiloride (5 mg/d) was added to previous triple antihypertensive treatment (including a diuretic and an inhibitor of the renin-angiotensin-aldosterone system) and increased to 10 mg if BP control was not achieved at 4 weeks. Complete dataset for urine analysis was available in 60 patients. Systolic and diastolic BP measured by ambulatory BP monitoring and office monitoring were significantly reduced. Average daytime BP was reduced by 6.3/3.0 mm Hg. Seven of 80 cases (9%) discontinued amiloride due to hyperkalemia >5.5 mol/L, the most frequent adverse event. Urinary plasmin(ogen) and albumin excretions were significantly reduced after amiloride treatment (P < .0001). Urokinase activity was detectable in macroalbuminuric urine, with a tendency toward reduction in activity after amiloride treatment. Amiloride lowers BP, urine plasminogen excretion and activation, and albumin/creatinine ratio, and is a relevant add-on medication for the treatment of resistant hypertension in patients with T2DM and microalbuminuria.

Sodium retention and volume expansion in nephrotic syndrome: implications for hypertension

Sodium retention is a major clinical feature of nephrotic syndrome. The mechanisms responsible for sodium retention in this setting have been a subject of debate for years. Excessive sodium retention occurs in some individuals with nephrotic syndrome in the absence of activation of the renin-angiotensin-aldosterone system, suggesting an intrinsic defect in sodium excretion by the kidney. Recent studies have provided new insights regarding mechanisms by which sodium transporters are activated by factors present in nephrotic urine. These mechanisms likely have a role in the development of hypertension in nephrotic syndrome, where hypertension may be difficult to control, and provide new therapeutic options for the management of blood pressure and edema in the setting of nephrotic syndrome.

Diabetic nephropathy is associated with increased urine excretion of proteases plasmin, prostasin and urokinase and activation of amiloride-sensitive current in collecting duct cells

BACKGROUND

Diabetic nephropathy (DN) is associated with hypertension, expanded extracellular volume and impaired renal Na(+) excretion. It was hypothesized that aberrant glomerular filtration of serine proteases in DN causes proteolytic activation of the epithelial sodium channel (ENaC) in the kidney by excision of an inhibitory peptide tract from the γ subunit.

METHODS

In a cross-sectional design, urine, plasma and clinical data were collected from type 1 diabetic patients with DN (n = 19) and matched normoalbuminuric type 1 diabetics (controls, n = 20). Urine was examined for proteases by western immunoblotting, patch clamp and ELISA. Urine exosomes were isolated to elucidate potential cleavage of γENaC by a monoclonal antibody directed against the 'inhibitory' peptide tract.

RESULTS

Compared with control, DN patients displayed significantly higher blood pressure and urinary excretion of plasmin(ogen), prostasin and urokinase that correlated directly with urine albumin. Western blotting confirmed plasmin, prostasin and urokinase in urine from the DN group predominantly. Urine from DN evoked a significantly larger amiloride-sensitive inward current in single collecting duct cells compared with controls. Immunoblotting of urine exosomes showed aquaporin 2 in all patient samples. Exosomes displayed a virtual absence of intact γENaC while moieties compatible with cleavage by furin only, were shown in both groups. Proteolytic cleavage by the extracellular serine proteases plasmin or prostasin was observed in DN samples predominantly.

CONCLUSION

DN is associated with increased urinary excretion of plasmin, prostasin and urokinase and proteolytic activation of ENaC that might contribute to impaired renal Na(+) excretion and hypertension.

Urinary serine proteases and activation of ENaC in kidney--implications for physiological renal salt handling and hypertensive disorders with albuminuria

Serine proteases, both soluble and cell-attached, can activate the epithelial sodium channel (ENaC) proteolytically through release of a putative 43-mer inhibitory tract from the ectodomain of the γ-subunit. ENaC controls renal Na(+) excretion and loss-of-function mutations lead to low blood pressure, while gain-of-function mutations lead to impaired Na(+) excretion, hypertension, and hypokalemia. We review an emerging pathophysiological concept that aberrant glomerular filtration of plasma proteases, e.g., plasmin, prostasin, and kallikrein, contributes to proteolytic activation of ENaC, both in acute conditions with proteinuria, like nephrotic syndrome and preeclampsia, and in chronic diseases, such as diabetes with microalbuminuria. A vast literature on renin-angiotensin-aldosterone system and volume homeostasis from the last four decades show a number of common characteristics for conditions with albuminuria compatible with impaired renal Na(+) excretion: hypertension and volume retention is secondary to proteinuria in, e.g., preeclampsia and nephrotic syndrome; plasma concentrations of renin, angiotensin II, and aldosterone are frequently suppressed in proteinuric conditions, e.g., preeclampsia and diabetic nephropathy; blood pressure is salt-sensitive in conditions with microalbuminuria/proteinuria; and extracellular volume is expanded, plasma atrial natriuretic peptide (ANP) concentration is increased, and diuretics, like amiloride and spironolactone, are effective blood pressure-reducing add-ons. Active plasmin in urine has been demonstrated in diabetes, preeclampsia, and nephrosis. Urine from these patients activates, plasmin-dependently, amiloride-sensitive inward current in vitro. The concept predicts that patients with albuminuria may benefit particularly from reduced salt intake with RAS blockers; that distally acting diuretics, in particular amiloride, are warranted in low-renin/albuminuric conditions; and that urine serine proteases and their activators may be pharmacological targets.