Urinary Plasmin Activates Collecting Duct ENaC Current in Preeclampsia

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.

Urinary Plasminogen as a Marker of Disease Progression in Human Glomerular Disease

RATIONALE & OBJECTIVE

Glomerular disorders have a highly variable clinical course, and biomarkers that reflect the molecular mechanisms underlying their progression are needed. Based on our previous work identifying plasminogen as a direct cause of podocyte injury, we designed this study to test the association between urine plasmin(ogen) (ie, plasmin and its precursor plasminogen) and end-stage kidney disease (ESKD).

STUDY DESIGN

Multicenter cohort study.

SETTING & PARTICIPANTS

1,010 patients enrolled in the CureGN Cohort with biopsy-proven glomerular disease (focal segmental glomerulosclerosis, membranous nephropathy, and immunoglobulin A nephropathy).

PREDICTORS

The main predictor was urine plasmin(ogen) at baseline. Levels were measured by an electrochemiluminescent immunoassay developed de novo. Traditional clinical and analytical characteristics were used for adjustment. The ratio of urine plasmin(ogen)/expected plasmin(ogen) was evaluated as a predictor in a separate model.

OUTCOME

Progression to ESKD.

ANALYTICAL APPROACH

Cox regression was used to examine the association between urinary plasmin(ogen) and time to ESKD. Urinary markers were log2 transformed to approximate normal distribution and normalized to urinary creatinine (Log2uPlasminogen/cr, Log2 urinary protein/cr [UPCR]). Expected plasmin(ogen) was calculated by multiple linear regression.

RESULTS

Adjusted Log2uPlasminogen/cr was significantly associated with ESKD (HR per doubling Log2 uPlasminogen/cr 1.31 [95% CI, 1.22-1.40], P<0.001). Comparison of the predictive performance of the models including Log2 uPlasminogen/cr, Log2 UPCR, or both markers showed the plasmin(ogen) model superiority. The ratio of measured/expected urine plasmin(ogen) was independently associated with ESKD: HR, 0.41 (95% CI, 0.22-0.77) if ratio<0.8 and HR 2.42 (95% CI, 1.54-3.78) if ratio>1.1 (compared with ratio between 0.8 and 1.1).

LIMITATIONS

Single plasmin(ogen) determination does not allow for the study of changes over time. The use of a cohort of mostly white patients and the restriction to patients with 3 glomerular disorders limits the external validity of our analysis.

CONCLUSIONS

Urinary plasmin(ogen) and the ratio of measured/expected plasmin(ogen) are independently associated with ESKD in a cohort of patients with glomerular disease. Taken together with our previous experimental findings, urinary plasmin(ogen) could be a useful biomarker in prognostic decision making and a target for the development of novel therapies in patients with proteinuria and glomerular disease.

PLAIN-LANGUAGE SUMMARY

Glomerular diseases are an important cause of morbidity and mortality in patients of all ages. Knowing the individual risk of progression to dialysis or transplantation would help to plan the follow-up and treatment of these patients. Our work studies the usefulness of urinary plasminogen as a marker of progression in this context, since previous studies indicate that plasminogen may be involved in the mechanisms responsible for the progression of these disorders. Our work in a sample of 1,010 patients with glomerular disease demonstrates that urinary plasminogen (as well as the ratio of measured to expected plasminogen) is associated with the risk of progression to end-stage kidney disease. Urine plasminogen exhibited good performance and, if further validated, could enable risk stratification for timely interventions in patients with proteinuria and glomerular disease.

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 .

The dilemma of sodium intake in preeclampsia: beneficial or detrimental?

Preeclampsia (PE) is a disorder involving de novo development of hypertension plus end organ damage after 20 weeks of gestation. PE is considered to be a heterogeneous disease. There are 2 main types of PE: early-onset (<34 weeks of gestation), which is considered to be a placental disorder and is associated with vasoconstriction, low cardiac output, and placental hypoperfusion and organ damage due to decreased microcirculation to maternal organs; and late-onset PE, which is primarily a disorder of pregnant women with obesity, diabetes, and/or cardiovascular abnormalities. In late-onset PE, there is avid sodium reabsorption by the maternal kidneys, causing hypervolemia and increased cardiac output, along with vasodilatation causing venous congestion of organs. Although PE has been a well-known disease for a long time, it is interesting to note that there is no specific sodium (salt) intake recommendation for these patients. This may be due to the fact that studies since as far back as the 1900s have shown conflicting results, and the reasons for the inconsistent findings have not been fully explained; furthermore, the type of PE in these studies was not specifically defined. Some studies suggest that sodium restriction may be detrimental in early-onset PE, but may be feasible in late-onset PE. To explore this paradox, the current review explains the hemodynamic factors involved in these 2 types of PE, summarizes the findings of the current studies, and highlights the knowledge gaps and the research needed to determine whether increase or restriction of salt or sodium intake is beneficial in different types of PE.

Proteolytic Activation of the Epithelial Sodium Channel (ENaC): Its Mechanisms and Implications

Epithelial sodium channel (ENaC) are integral to maintaining salt and water homeostasis in various biological tissues, including the kidney, lung, and colon. They enable the selective reabsorption of sodium ions, which is a process critical for controlling blood pressure, electrolyte balance, and overall fluid volume. ENaC activity is finely controlled through proteolytic activation, a process wherein specific enzymes, or proteases, cleave ENaC subunits, resulting in channel activation and increased sodium reabsorption. This regulatory mechanism plays a pivotal role in adapting sodium transport to different physiological conditions. In this review article, we provide an in-depth exploration of the role of proteolytic activation in regulating ENaC activity. We elucidate the involvement of various proteases, including furin-like convertases, cysteine, and serine proteases, and detail the precise cleavage sites and regulatory mechanisms underlying ENaC activation by these proteases. We also discuss the physiological implications of proteolytic ENaC activation, focusing on its involvement in blood pressure regulation, pulmonary function, and intestinal sodium absorption. Understanding the mechanisms and consequences of ENaC proteolytic activation provides valuable insights into the pathophysiology of various diseases, including hypertension, pulmonary disorders, and various gastrointestinal conditions. Moreover, we discuss the potential therapeutic avenues that emerge from understanding these mechanisms, offering new possibilities for managing diseases associated with ENaC dysfunction. In summary, this review provides a comprehensive discussion of the intricate interplay between proteases and ENaC, emphasizing the significance of proteolytic activation in maintaining sodium and fluid balance in both health and disease.

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.

Urine excretion of C3dg and sC5b-9 coincide with proteinuria and development of preeclampsia in pregnant women with type-1 diabetes

OBJECTIVE

Pregnant women with type-1 diabetes have an increased risk of preeclampsia with kidney injury and cardiovascular complications. Urine excretion of plasmin and soluble membrane attack complex (sC5b-9) is elevated in severe preeclampsia. We hypothesized a coupling between these events and that active plasmin promotes intratubular complement activation and membrane deposition.

METHODS

Stored urine and plasma samples from pregnant women with type-1 diabetes (n = 88) collected at gestational weeks 12, 20, 28, 32, 36 and 38 were used. In the cohort, 14 women developed preeclampsia and were compared with 16 nonpreeclampsia controls.

RESULTS

Urine C3dg and sC5b-9-associated C9 neoantigen/creatinine ratios increased and were significantly higher in women who developed preeclampsia. Plasma concentrations did not change with gestation. Urine plasmin(ogen) correlated to urine C3dg (r = 0.51, P < 0.001) and C9 neoantigen (r = 0.68, P < 0.001); urine albumin correlated to C3dg (r = 0.44, P < 0.001) and C9 (r = 0.59, P < 0.001). Membrane-associated C3dg and C9 neoantigen was detected in urinary extracellular vesicles from patients but not controls at 36 weeks. Receiver operating characteristic curves showed that C3dg and C9 neoantigen were inferior to albumin as predictive biomarkers for preeclampsia.

CONCLUSION

In preeclampsia, urinary excretion of activated complement relates significantly to albuminuria and to plasmin(ogen) but not to activation in plasma. Intratubular complement activation in preeclampsia is a postfiltration event tightly related to proteinuria/plasminogenuria and a possible mechanistic link to cellular damage and kidney injury.

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.

Novel inhibitors and activity-based probes targeting serine proteases

Serine proteases play varied and manifold roles in important biological, physiological, and pathological processes. These include viral, bacterial, and parasitic infection, allergic sensitization, tumor invasion, and metastasis. The use of activity-based profiling has been foundational in pinpointing the precise roles of serine proteases across this myriad of processes. A broad range of serine protease-targeted activity-based probe (ABP) chemotypes have been developed and we have recently introduced biotinylated and "clickable" peptides containing P1 N-alkyl glycine arginine N-hydroxy succinimidyl (NHS) carbamates as ABPs for detection/profiling of trypsin-like serine proteases. This present study provides synthetic details for the preparation of additional examples of this ABP chemotype, which function as potent irreversible inhibitors of their respective target serine protease. We describe their use for the activity-based profiling of a broad range of serine proteases including trypsin, the trypsin-like protease plasmin, chymotrypsin, cathepsin G, and neutrophil elastase (NE), including the profiling of the latter protease in clinical samples obtained from patients with cystic fibrosis.

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.

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.

Normal-range urinary albumin excretion associates with blood pressure and renal electrolyte handling in pregnancy

Albuminuria in the pathological range is a significant predictor of preeclampsia. In healthy persons, high normal urinary albumin predicts a later incidence of hypertension and is associated with salt sensitivity of blood pressure. We hypothesized that in pregnancy urinary albumin in the normal range associates with blood pressure through activation of distal Na⁺ reabsorption and renal salt retention by plasma factors cofiltered with albumin. We analyzed 24-h urine collections and plasma samples from gestational week 29 of 560 pregnant women from the Odense Child Cohort, a Danish population-based cohort. Plasma and urinary aldosterone were measured by ELISA. Plasma and urinary Na⁺, K⁺, Cl^-, and creatinine were also determined. Predictive values of urinary albumin were assessed by linear mixed, multiple, and Cox regression analyses. Primary outcomes were blood pressure and renal electrolyte handling. Twenty-four-hour urinary albumin excretion at gestational week 29 associated with gestational blood pressure trajectory, with adjusted β coefficients (95% confidence intervals) for each 10-fold increase in urinary albumin as follows: 5.71 (1.60 to 9.81) mmHg for systolic blood pressure and 4.39 (1.41 to 7.38) mmHg for diastolic blood pressure. Urinary albumin was inversely associated with fractional excretion rates of Na⁺, K⁺, and Cl^-, with adjusted β coefficients (95% confidence intervals) for each 10-fold increase in urine albumin as follows: -0.25 (-0.35 to -0.14), -5.06 (-6.81 to -3.30), and -0.28 (-0.41 to -0.15), respectively. In conclusion, at gestational week 29, urinary albumin excretion in the normal range associated with blood pressure and renal electrolyte handling independent of potential confounders.

Vascular Cardio-Oncology: Vascular Endothelial Growth Factor inhibitors and hypertension

Since the formation of new blood vessels is essential for tumour growth and metastatic spread, inhibition of angiogenesis by targeting the vascular endothelial growth factor (VEGF) pathway is an effective strategy for various types of cancer, most importantly renal cell carcinoma, thyroid cancer, and hepatocellular carcinoma. However, VEGF inhibitors have serious side effects, most importantly hypertension and nephropathy. In case of fulminant hypertension, this may only be handled by lowering the dosage since the blood pressure rise is proportional to the amount of VEGF inhibition. These effects pathophysiologically and clinically resemble the most severe complication of pregnancy, preeclampsia, in which case an insufficient placenta leads to a rise in sFlt-1 levels causing a decrease in VEGF availability. Due to this overlap, studies in preeclampsia may provide important information for VEGF inhibitor-induced toxicity and vice versa. In both VEGF inhibitor-induced toxicity and preeclampsia, endothelin (ET)-1 appears to be a pivotal player. In this review, after briefly summarizing the anticancer effects, we discuss the mechanisms that potentially underlie the unwanted effects of VEGF inhibitors, focusing on ET-1, nitric oxide and oxidative stress, the renin-angiotensin-aldosterone system, and rarefaction. Given the salt sensitivity of this phenomenon, as well as the beneficial effects of aspirin in preeclampsia and cancer, we next provide novel treatment options for VEGF inhibitor-induced toxicity, including salt restriction, ET receptor blockade, and cyclo-oxygenase inhibition, in addition to classical antihypertensive and renoprotective drugs. We conclude with the recommendation of therapeutic drug monitoring to improve patient outcome.

Urokinase-type plasminogen activator (uPA) is not essential for epithelial sodium channel (ENaC)-mediated sodium retention in experimental nephrotic syndrome

AIM

In nephrotic syndrome, aberrantly filtered plasminogen (plg) is converted to active plasmin by tubular urokinase-type plasminogen activator (uPA) and thought to lead to sodium retention by proteolytic activation of the epithelial sodium channel (ENaC). This concept predicts that uPA is an important factor for sodium retention and that inhibition of uPA might be protective in nephrotic syndrome.

METHODS

Activation of amiloride-sensitive currents by uPA and plg were studied in Xenopus laevis oocytes expressing murine ENaC. In doxorubicin-induced nephrotic mice, uPA was inhibited pharmacologically by amiloride and genetically by the use of uPA-deficient mice (uPA^-/- ).

RESULTS

Experiments in Xenopus laevis oocytes expressing murine ENaC confirmed proteolytic ENaC activation by a combination of plg and uPA which stimulated amiloride-sensitive currents with concomitant cleavage of the ENaC γ-subunit at the cell surface. Treatment of nephrotic wild-type mice with amiloride inhibited urinary uPA activity, prevented urinary plasmin formation and sodium retention. In nephrotic mice lacking uPA (uPA^-/- ), urinary plasmin formation from plg was suppressed and urinary uPA activity absent. However, in nephrotic uPA^-/- mice, sodium retention was not reduced compared to nephrotic uPA^+/+ mice. Amiloride prevented sodium retention in nephrotic uPA^-/- mice which confirmed the critical role of ENaC in sodium retention.

CONCLUSION

uPA is responsible for the conversion of aberrantly filtered plasminogen to plasmin in the tubular lumen in vivo. However, uPA-dependent plasmin generation is not essential for ENaC-mediated sodium retention in experimental nephrotic syndrome.

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.

Is urinary excretion of plasminogen associated with development of pre-eclampsia? An observational, explorative case-control study

OBJECTIVES

Pre-eclampsia (PE) is characterised by renal glomerular endotheliosis and injury to the glomerular filtration barrier with proteinuria. Patients with PE display aberrant filtration of the plasma proenzyme plasminogen which is activated, in the tubular fluid, to plasmin. Plasmin may activate the epithelial sodium channel and cause impaired sodium excretion and contribute to hypertension. An explorative study was conducted to test the association between urinary total plasminogen/plasmin and the development of PE. A positive association was hypothesised.

DESIGN

An observational, explorative, nested case-control study of healthy pregnant women.

SETTINGS

A Danish County hospital. Samples were collected between 2001 and 2004.

PARTICIPANTS

1631 healthy pregnant women participated. Urine samples were collected longitudinally six times during pregnancy. 30 developed PE (cases) and were compared with 146 randomly selected healthy pregnant women (controls).

PRIMARY OUTCOME

The association between total plasminogen/plasmin excreted in the urine and PE development is expressed by ORs. Total urinary excretion of plasminogen/plasmin was defined by the urine plasminogen-plasmin/creatinine ratio.

SECONDARY OUTCOME

The association between urine (u)-albumin/creatinine ratio, u-aldosterone/creatinine ratio and PE development is expressed by ORs. The correlation between urinary (u-) plasmin and u-aldosterone concentration is expressed as a correlation coefficient.

RESULTS

The development of PE in late pregnancy was associated with increased levels of the urine plasminogen-plasmin/creatinine ratio (OR=2.35; 95% CI: 1.12 to 4.93; p<0.05).U-aldosterone/creatinine ratio did not predict PE at any time. U-albumin/creatinine ratio was positively associated with the development of PE from gestational week 33 (OR=14.04; 95% CI: 2.56 to 76.97; p<0.01) and in week 33-35 (OR=14.15; 95% CI: 3.44 to 58.09; p<0.001) and after gestational week 36, respectively.

CONCLUSION

Aberrant filtration of plasminogen may contribute to the pathophysiological features of impaired sodium excretion and hypertension associated with PE late in pregnancy. However, increased urinary albumin levels reveal stronger associations with PE development compared with urinary plasminogen levels.

Food-derived bioactive peptides and their role in ameliorating hypertension and associated cardiovascular diseases

Non-communicable diseases including cardiovascular diseases (CVDs) and associated metabolic disorders are responsible for nearly 40 million deaths globally per year. Hypertension or high blood pressure (BP) is one of the primary reasons for the development of CVDs. A healthy nutritional strategy complementing with physical activity can substantially reduce high BP and prevent the occurrence of CVD-associated morbidity and mortality. Bioactive peptides currently are the next wave of the promising bench to clinic options for potential targeting chronic and acute health issues including hypertension. Peptides demonstrating anti-inflammatory, anti-oxidant, and angiotensin-converting enzyme-I inhibitory activity are widely studied for the amelioration of hypertension and associated CVDs. Isolating these potent bioactive peptides from different food sources is a promising endeavor toward nutraceutical based dietary management and prevention of hypertension. Understanding the pathophysiology of hypertension and the action mechanisms of the bioactive peptides would complement in designing and characterizing more potent peptides and suitable comprehensive dietary plans for the prevention of hypertension and associated CVDs.

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.

Pre-eclampsia is associated with altered expression of the renal sodium transporters NKCC2, NCC and ENaC in urinary extracellular vesicles

Pre-eclampsia is a hypertensive disorder of pregnancy characterised by hypertension and sodium retention by the kidneys. To identify changes in sodium uptake proteins in the tubules of the distal nephron, we studied their expression in urinary extracellular vesicles or exosomes (uEVs). Urine was collected from women with pre-eclampsia or during normal pregnancy, and from healthy non-pregnant controls. uEVs were isolated by centrifugation and analyzed by Western blot. Expression, proteolytic cleavage and phosphorylation was determined by densitometric analysis normalized to the exosome marker CD9. Results showed a significant increase in phosphorylation of the activating S130 site in NKCC2, the drug target for frusemide, in women with pre-eclampsia compared with normal pregnant women. Phosphorylation of the activating sites T101/105 in NKCC2 was similar but the activating T60 site in NCC, the drug target for thiazide diuretics, showed significantly less phosphorylation in pre-eclampsia compared with normal pregnancy. Expression of the larger forms of the α subunit of ENaC, the drug target for amiloride, was significantly greater in pre-eclampsia, with more fragmentation of theγ subunit. The differences observed are predicted to increase the activity of NKCC2 and ENaC while reducing that of NCC. This will increase sodium reabsorption, and so contribute to hypertension in pre-eclampsia.

Hydronephrosis is associated with elevated plasmin in urine in pediatric patients and rats and changes in NCC and γ-ENaC abundance in rat kidney

Obstruction of urine flow at the level of the pelvo-ureteric junction (UPJO) and subsequent development of hydronephrosis is one of the most common congenital renal malformations. UPJO is associated with development of salt-sensitive hypertension, which is set by the obstructed kidney, and with a stimulated renin-angiotensin-aldosterone system (RAAS) in rodent models. This study aimed at investigating the hypothesis that 1) in pediatric patients with UPJO the RAAS is activated before surgical relief of the obstruction; 2) in rats with UPJO the RAAS activation is reflected by increased abundance of renal aldosterone-stimulated Na transporters; and 3) the injured UPJO kidney allows aberrant filtration of plasminogen, leading to proteolytic activation of the epithelial Na channel γ-subunit (γ-ENaC). Hydronephrosis resulting from UPJO in pediatric patients and rats was associated with increased urinary plasminogen-to-creatinine ratio. In pediatric patients, plasma renin, angiotensin II, urine and plasma aldosterone, and urine soluble prorenin receptor did not differ significantly before or after surgery, or compared with controls. Increased plasmin-to-plasminogen ratio was seen in UPJO rats. Intact γ-ENaC abundance was not changed in UPJO kidney, whereas low-molecular cleavage product abundance increased. The Na-Cl cotransporter displayed significantly lower abundance in the UPJO kidney compared with the nonobstructed contralateral kidney. The Na-K-ATPase α-subunit was unaltered. Treatment with an angiotensin-converting enzyme inhibitor (8 days, captopril) significantly lowered blood pressure in UPJO rats. It is concluded that the RAAS contributes to hypertension following partial obstruction of urine flow at the pelvo-ureteric junction with potential contribution from proteolytic activation of ENaC.

Renin-Angiotensin-Aldosterone Profiles in Pregnant Women With Chronic Hypertension

Pregnant women with chronic hypertension are at risk for increased blood pressure and superimposed preeclampsia (SPE) in late pregnancy. Alterations in the renin-aldosterone system are a feature of normal pregnancy; however, their role in chronic hypertension with and without SPE is less clear. We performed a prospective, longitudinal trial of 108 women with chronic hypertension and measured plasma renin activity (PRA), 24-hour urine sodium, urine potassium, and urine aldosterone (Ualdo) at 12, 20, 28, and 36 weeks and postpartum. SPE developed in 34% of pregnancies. PRA was lower in women who developed SPE at weeks 28 (5.99 versus 6.22 ng/mL per hour; P<0.001) and 36 (5.71 versus 7.74 ng/mL per hour; P=0.002). Ualdo was lower in women with SPE compared with those without SPE at 28 weeks (59.6 versus 81.3 μg/d; P=0.039). Mean arterial pressure was inversely related to both PRA (r=-0.23; P<0.0001) and Ualdo (r=-0.11; P=0.029). PRA and Ualdo were positively associated with each other (r=0.5327; P<0.0001) after adjusting for urine potassium, urine sodium, serum potassium, and mean arterial pressure. PRA and Ualdo were lower in women of black race compared with other racial groups (P<0.001). Our results demonstrate that in women with chronic hypertension PRA and Ualdo increase in early pregnancy and subsequently decrease in women who develop SPE. These findings are consistent with the hypothesis that sodium retention may contribute to the elevation in blood pressure in SPE.

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.

The impact of salt intake during and after pregnancy

Although high blood pressure before pregnancy is associated with a risk of gestational hypertension and preeclampsia, no convincing evidence has been produced to show that dietary salt reduction helps in the prevention and treatment of hypertension during pregnancy. Thus the current guidelines do not recommend a sodium restriction during pregnancy to prevent gestational hypertension and the development of preeclampsia. However, the long-term impact of hypertensive disorders of pregnancy for life-threatening diseases later in life is a critical issue. Gestational hypertension could contribute to the risk of developing hypertension later in life, and recent studies have suggested that gestational hypertension and preeclampsia are linked to cardiovascular complications. In this article, we provide an overview of the current perspectives on the salt intake of pregnant women and consider both the short-term influence and the impact beyond the perinatal period.

Management of Hypertension in Patients with Chronic Kidney Disease

Hypertension is the leading factor in the global burden of disease. It is the predominant modifiable risk factor for stroke, heart disease, and kidney failure. Chronic kidney disease (CKD) is both a common cause and sequel of uncontrolled hypertension. The pathophysiology of CKD-associated hypertension is complex and multi-factorial. This paper reviews the key pathogenic mechanisms of CKD-associated hypertension, the importance of standardized blood pressure (BP) measurement in establishing the diagnosis and management plus the significance of ambulatory BP monitoring for assessment of diurnal BP variation commonly seen in CKD. The optimal BP target in CKD remains a matter of discussion despite recent clinical trials. Medical therapy can be difficult and challenging. In addition to lifestyle modification and dietary salt restriction, treatment may need to be individualized based on co-morbidities. Combination of antihypertensive drugs, including appropriate diuretic choice and dose, is of great significance in hypertension management in CKD.

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.

Pregnancy as a critical window for blood pressure regulation in mother and child: programming and reprogramming

Pregnancy is a critical time for long-term blood pressure regulation in both mother and child. Pregnancies complicated by placental insufficiency, resulting in pre-eclampsia and intrauterine growth restriction, are associated with a threefold increased risk of the mother to develop hypertension later in life. In addition, these complications create an adverse intrauterine environment, which programmes the foetus and the second generation to develop hypertension in adult life. Female offspring born to a pregnancy complicated by placental insufficiency are at risk for pregnancy complications during their own pregnancies as well, resulting in a vicious circle with programmed risk for hypertension passing from generation to generation. Here, we review the epidemiology and mechanisms leading to the altered programming of blood pressure trajectories after pregnancies complicated by placental insufficiency. Although the underlying mechanisms leading to hypertension remain the subject of investigation, several abnormalities in angiotensin sensitivity, sodium handling, sympathetic activity, endothelial function and metabolic pathways are found in the mother after exposure to placental insufficiency. In the child, epigenetic modifications and disrupted organ development play a crucial role in programming of hypertension. We emphasize that pregnancy can be viewed as a window of opportunity to improve long-term cardiovascular health of both mother and child, and outline potential gains expected of improved preconceptional, perinatal and post-natal care to reduce the development of hypertension and the burden of cardiovascular disease later in life. Perinatal therapies aimed at reprogramming hypertension are a promising strategy to break the vicious circle of intergenerational programming of hypertension.

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.

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.

Hypertension in Chronic Kidney Disease

Hypertension, a global public health problem, is currently the leading factor in the global burden of disease. It is the major modifiable risk factor for heart disease, stroke and kidney failure. Chronic kidney disease (CKD) is both a common cause of hypertension and CKD is also a complication of uncontrolled hypertension. The interaction between hypertension and CKD is complex and increases the risk of adverse cardiovascular and cerebrovascular outcomes. This is particularly significant in the setting of resistant hypertension commonly seen in patient with CKD. The pathophysiology of CKD associated hypertension is multi-factorial with different mechanisms contributing to hypertension. These pathogenic mechanisms include sodium dysregulation, increased sympathetic nervous system and alterations in renin angiotensin aldosterone system activity. Standardized blood pressure (BP) measurement is essential in establishing the diagnosis and management of hypertension in CKD. Use of ambulatory blood pressure monitoring provides an additional assessment of diurnal variation in BP commonly seen in CKD patients. The optimal BP target in the treatment of hypertension in general and CKD population remains a matter of debate and controversial despite recent guidelines and clinical trial data. Medical therapy of patients with CKD associated hypertension can be difficult and challenging. Additional evaluation by a hypertension specialist may be required in the setting of treatment resistant hypertension by excluding pseudo-resistance and treatable secondary causes. Treatment with a combination of antihypertensive drugs, including appropriate diuretic choice, based on estimated glomerular filtration rate, is a key component of hypertension management in CKD patients. In addition to drug treatment non-pharmacological approaches including life style modification, most important of which is dietary salt restriction, should be included in the management of hypertension in CKD patients.

Role of tubulointerstitial plasmin in the progression of IgA nephropathy

BACKGROUND

Plasmin has recently been reported to be associated with renal fibrosis in experimental models, but its role in human renal diseases is unclear.

METHODS

Fifty-seven patients with IgA nephropathy (IgAN) were evaluated retrospectively. Plasmin in their renal biopsy tissues was assessed by in situ zymography using a plasmin-sensitive synthetic peptide, and the relationships between patients' histologic or clinical parameters and their renal plasmin activity [assessed semiquantitatively by calculating the positively stained percentage of the total tubulointerstitial (TI) area] were evaluated.

RESULTS

Plasmin activity was observed almost exclusively in the TI space (mainly in the interstitium and partly in the tubular epithelial cells) and was significantly stronger in patients with TI lesion (tubular atrophy/interstitial fibrosis and tubulointerstitial inflammation) than in those without TI lesion. It was significantly and positively correlated with the global glomerulosclerosis rate and significantly and negatively correlated with estimated glomerular filtration rate not only at the time of renal biopsy but also at the end of the follow-up period. Double stainings for plasmin activity and inflammatory cells, cytokeratin, or α-smooth muscle actin (α-SMA) in selected patients revealed TI infiltration of inflammatory cells, attenuated tubular epithelial expression of cytokeratin, and augmented interstitial expression of α-SMA close to upregulated plasmin activity in the TI space.

CONCLUSIONS

These data suggest that TI plasmin is associated with TI inflammation leading to renal fibrosis, and can cause the decline in renal function seen in patients with IgAN. Reducing plasmin in situ may therefore be a promising therapeutic approach slowing renal fibrogenesis and improving renal function.

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.

Association studies suggest a key role for endothelin-1 in the pathogenesis of preeclampsia and the accompanying renin-angiotensin-aldosterone system suppression

Women with preeclampsia display low renin-angiotensin-aldosterone system activity and a high antiangiogenic state, the latter characterized by high levels of soluble Fms-like tyrosine kinase (sFlt)-1 and reduced placental growth factor levels. To investigate whether renin-angiotensin-aldosterone system suppression in preeclampsia is because of this disturbed angiogenic balance, we measured mean arterial pressure, creatinine, endothelin-1 (ET-1), and renin-angiotensin-aldosterone system components in pregnant women with a high (≥85; n=38) or low (<85; n=65) soluble Fms-like tyrosine kinase-1/placental growth factor ratio. Plasma ET-1 levels were increased in women with a high ratio, whereas their plasma renin activity and plasma concentrations of renin, angiotensinogen, and aldosterone were decreased. Plasma renin activity-aldosterone relationships were identical in both the groups. Multiple regression analysis revealed that plasma renin concentration correlated independently with mean arterial pressure and plasma ET-1. Plasma ET-1 correlated positively with soluble Fms-like tyrosine kinase-1 and negatively with plasma renin concentration, and urinary protein correlated with plasma ET-1 and mean arterial pressure. Despite the lower plasma levels of renin and angiotensinogen in the high-ratio group, their urinary levels of these components were elevated. Correction for albumin revealed that this was because of increased glomerular filtration. Subcutaneous arteries obtained from patients with preeclampsia displayed an enhanced, AT2 receptor-mediated response to angiotensin II. In conclusion, a high antiangiogenic state associates with ET-1 activation, which together with the increased mean arterial pressure may underlie the parallel reductions in renin and aldosterone in preeclampsia. Because ET-1 also was a major determinant of urinary protein, our data reveal a key role for ET-1 in the pathogenesis of preeclampsia. Finally, the enhanced angiotensin responsiveness in preeclampsia involves constrictor AT2 receptors.

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

BACKGROUND

Aberrant filtration of plasminogen from plasma and subsequent activation to plasmin in the urinary space may activate proteolytically the epithelial sodium channel, ENaC. In conditions with chronic albuminuria, this may cause hypertension. It was hypothesized that patients with type 2 diabetes mellitus (T2DM) and treatment-resistant hypertension excrete plasmin(ogen) in urine in proportion to albumin and that plasmin confers to urine the ability to activate ENaC.

METHOD

Patients (n = 113) with T2DM and resistant hypertension, defined as systolic blood pressure (SBP) more than 130 mmHg and/or diastolic blood pressure (DBP) more than 80 mmHg despite use of at least three drugs with one diuretic and one renin-angiotensin system inhibitor, were included. Urine was analyzed for albumin, creatinine, plasmin(ogen), protease activity, and ability to activate inward current in single collecting duct cells.

RESULTS

Mean ambulatory SBP/DBP was 143 ± 1/77 ± 0.7 mmHg; HbA1c 7.35%; and eGFR 81.0 ml/min per 1.73 m (geometric means). Patients with microalbuminuria (39%) and macroalbuminuria (13%) displayed significantly elevated levels of urinary plasmin(ogen) normalized to urine creatinine compared with patients with normal excretion of albumin (48%). Urinary plasminogen correlated significantly to urine albumin. Western immunoblotting and gelatine zymography confirmed active plasmin in urine samples from patients with microalbuminuria and macroalbuminuria. Single collecting duct cells displayed significantly increased, amiloride-sensitive, inward current when superfused with urine from albuminuric patients compared with patients with normal albumin excretion. Urinary plasminogen/creatinine ratio correlated significantly with 24-h ambulatory blood pressure.

CONCLUSION

Aberrant presence of plasmin in preurine may inappropriately activate ENaC in patients with type 2 diabetes and microalbuminuria. This may contribute to treatment-resistant hypertension.

A long isoform of the epithelial sodium channel alpha subunit forms a highly active channel

A long isoform of the human Epithelial Sodium Channel (ENaC) α subunit has been identified, but little data exist regarding the properties or regulation of channels formed by α728. The baseline whole cell conductance of oocytes expressing trimeric α728βγ channels was 898.1±277.2 and 49.59±13.2 µS in low and high sodium solutions, respectively, and was 11 and 2 fold higher than the conductances of α669βγ in same solutions. α728βγ channels were also 2 to 5 fold less sensitive to activation by the serine proteases subtilisin and trypsin than α669βγ in low and high Na+ conditions. The long isoform exhibited lower levels of full length and cleaved protein at the plasma membrane and a rightward shifted sensitivity to inhibition by increases of [Na+]i. Both channels displayed similar single channel conductances of 4 pS, and both were activated to a similar extent by reducing temperature, altogether indicating that activation of baseline conductance of α728βγ was likely mediated by enhanced channel activity or open probability. Expression of α728 in native kidneys was validated in human urinary exosomes. These data demonstrate that the long isoform of αENaC forms the structural basis of a channel with different activity and regulation, which may not be easily distinguishable in native tissue, but may underlie sodium hyperabsorption and salt sensitive differences in humans.

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.

The epithelial sodium channel γ-subunit is processed proteolytically in human kidney

The epithelial sodium channel (ENaC) of the kidney is necessary for extracellular volume homeostasis and normal arterial BP. Activity of ENaC is enhanced by proteolytic cleavage of the γ-subunit and putative release of a 43-amino acid inhibitory tract from the γ-subunit ectodomain. We hypothesized that proteolytic processing of γENaC occurs in the human kidney under physiologic conditions and that proteinuria contributes to aberrant proteolytic activation. Here, we used monoclonal antibodies (mAbs) with specificity to the human 43-mer inhibitory tract (N and C termini, mAbinhibit, and mAb4C11) and the neoepitope generated after proteolytic cleavage at the prostasin/kallikrein cleavage site (K181-V182 and mAbprostasin) to examine human nephrectomy specimens. By immunoblotting, kidney cortex homogenate from patients treated with angiotensin II type 1 receptor antagonists (n=6) or angiotensin-converting enzyme inhibitors (n=6) exhibited no significant difference in the amount of full-length or furin-cleaved γENaC or the furin-cleaved-to-full-length ratio of γENaC compared with homogenate from patients on no medication (n=5). Patients treated with diuretics (n=4) displayed higher abundance of full-length and furin-cleaved γENaC, with no significant change in the furin-cleaved-to-full-length γENaC ratio. In patients with proteinuria (n=6), the inhibitory tract was detected only in full-length γENaC by mAbinhibit. Prostasin/kallikrein-cleaved γENaC was detected consistently only in tissue from patients with proteinuria and observed in collecting ducts. In conclusion, human kidney γENaC is subject to proteolytic cleavage, yielding fragments compatible with furin cleavage, and proteinuria is associated with cleavage at the putative prostasin/kallikrein site and removal of the inhibitory tract within γENaC.