Cathepsin B increases ENaC activity leading to hypertension early in nephrotic syndrome

Cathepsin B induces kidney diseases through different types of programmed cell death

Cathepsin B (CTSB), a key cysteine protease, plays essential roles in physiological and pathological processes. As research progresses, interest in how CTSB triggers different types of programmed cell death (PCD) to induce the onset and development of diseases is increasing. Several recent studies suggest that different types of PCD mediated by CTSB play key roles in kidney diseases. In this review, we outline the fundamental mechanisms by which CTSB triggers different types of PCD in several kidney diseases and discuss the function of CTSB in various segments of the kidney. Moreover, we explore the possibilities and prospects of using CTSB as a therapeutic target for kidney diseases.

Membranous Nephropathy

Membranous nephropathy is a glomerular disease that may be caused by exogenous risk factors in genetically predisposed individuals (primary MN) or may be associated with other autoimmune diseases, drug exposure, or cytotoxic agents (secondary MN). Primary membranous nephropathy (PMN) is an autoimmune disease in which antigens-mainly the phospholipase A2 receptor-are located in the podocytes and are targeted by circulating antibodies, leading to in situ formation of immune complexes that activate the complement system. Clinically, the disease is characterized by nephrotic syndrome (NS) and associated complications. The outcome of PMN can vary, but untreated patients with NS may progress to end-stage kidney disease (ESKD) in 35-40% of cases within 10 years. Treatment primarily aims to prevent NS complications and progression to ESKD. The most commonly used immunosuppressive drugs are rituximab, corticosteroids, cyclophosphamide, and calcineurin inhibitors. Most patients may experience an improvement of proteinuria, which can sometimes be followed by NS relapse. Fewer than 50% of patients with PMN achieve complete and stable remission. In addition to immunosuppressive therapy, antiproteinuric, anti-lipemic, and anticoagulant medicaments are often required.

Uncovering the molecular networks of ferroptosis in the pathogenesis of type 2 diabetes and its complications: a multi-omics investigation

BACKGROUND

Diabetes is a multi-factorial disorder and related complications constitute one of the principal causes of global mortality and disability. The role of ferroptosis in diabetes and its complications is intricate and significant. This study endeavors to disclose the role of ferroptosis in the aforementioned diseases from multiple perspectives through multi-omics.

METHODS

We performed genetic correlation analyses via the Linkage Disequilibrium Score and High-Definition Likelihood approaches for type 2 diabetes (T2D) and its complications. The data concerning the expression of ferroptosis-related genes (FRGs) were obtained from the meta-analysis of studies on gene expression and protein abundance. Mendelian randomization analyses and cross-validation were implemented using the discovery cohort, replication cohort, and imaging genomics cohort of T2D and its complications. Moreover, we conducted colocalization analyses on T2D and tissue-specific single-cell RNA sequencing investigations on the complications to complement the results.

RESULTS

Genetic association analysis indicated that the selected datasets could be incorporated into a secondary analysis of T2D complications. In the primary analysis, six FRGs (CDKN1A, ENO3, FURIN, RARRES2, TYRO3, and YTHDC2) were found to be positively associated with T2D risk. Conversely, eight FRGs (ARNTL, CAMKK2, CTSB, FADS2, KDM5A, MEG3, SREBF1, and STAT3) were inversely associated with T2D risk. The 14 FRGs were included in the secondary analysis. Within the FRGs, which received full support from both the discovery and replication cohorts, and were further validated by imaging genomics, higher levels of CDKN1A were positively associated with DKD risk. Higher levels of CAMKK2 and KDM5A were associated with a decreased risk of DKD. For DCM, higher levels of CTSB were positively associated with DCM risk. And genetically predicted higher levels of ARNTL and SREBF1 were associated with a decreased risk of NAFLD. Finally, we validated the tissue-specific expression of each complication with scRNA-seq datasets.

CONCLUSIONS

This study identified FRGs in relation to T2D and its complications, which may enhance the understanding of the pathogenic mechanisms of their development. Meanwhile, it offers cross-validation for imaging genomics and further indicates the direction for non-invasive diagnosis.

Mycobacterium avium inhibits protein kinase C and MARCKS phosphorylation in human cystic fibrosis and non-cystic fibrosis cells

In cystic fibrosis (CF), there is abnormal translocation and function of the cystic fibrosis transmembrane conductance regulator (CFTR) and an upregulation of the epithelial sodium channel (ENaC). This leads to hyperabsorption of sodium and fluid from the airway, dehydrated mucus, and an increased risk of respiratory infections. In this study, we performed a proteomic assessment of differentially regulated proteins from CF and non-CF small airway epithelial cells (SAEC) that are sensitive to Mycobacterium avium. CF SAEC and normal non-CF SAEC were infected with M. avium before the cells were harvested for protein. Protein kinase C (PKC) activity was greater in the CF cells compared to the non-CF cells, but the activity was significantly attenuated in both cell types after infection with M. avium compared to vehicle. Western blot and densitometric analysis showed a significant increase in cathepsin B protein expression in M. avium infected CF cells. Myristoylated alanine rich C-kinase substrate (MARCKS) protein was one of several differentially expressed proteins between the groups that was identified by mass spectrometry-based proteomics. Total MARCKS protein expression was greater in CF cells compared to non-CF cells. Phosphorylation of MARCKS at serine 163 was also greater in CF cells compared to non-CF cells after treating both groups of cells with M. avium. Taken together, MARCKS protein is upregulated in CF cells and there is decreased phosphorylation of the protein due to a decrease in PKC activity and presumably increased cathepsin B mediated proteolysis of the protein after M. avium infection.

Cathepsin B in cardiovascular disease: Underlying mechanisms and therapeutic strategies

Cathepsin B (CTSB) is a member of the cysteine protease family, primarily responsible for degrading unnecessary organelles and proteins within the acidic milieu of lysosomes to facilitate recycling. Recent research has revealed that CTSB plays a multifaceted role beyond its function as a proteolytic enzyme in lysosomes. Importantly, recent data suggest that CTSB has significant impacts on different cardiac pathological conditions, such as atherosclerosis (AS), myocardial infarction, hypertension, heart failure and cardiomyopathy. Especially in the context of AS, preclinical models and clinical sample imaging data indicate that the cathepsin activity-based probe can reliably image CTSB activity in foam cells and atherosclerotic plaques; concurrently, it allows synchronous diagnostic and therapeutic interventions. However, our knowledge of CTSB in cardiovascular disease is still in the early stage. This paper aims to provide a comprehensive review of the significance of CTSB in cardiovascular physiology and pathology, with the objective of laying a theoretical groundwork for the development of drugs targeting CTSB.

Enrichment of Bioactive Lipids in Urinary Extracellular Vesicles and Evidence of Apoptosis in Kidneys of Hypertensive Diabetic Cathepsin B Knockout Mice after Streptozotocin Treatment

Cathepsin B (CtsB) is a ubiquitously expressed cysteine protease that plays important roles in health and disease. Urinary extracellular vesicles (uEVs) are released from cells associated with urinary organs. The antibiotic streptozotocin (STZ) is known to induce pancreatic islet beta cell destruction, diabetic nephropathy, and hypertension. We hypothesized that streptozotocin-induced diabetic kidney disease and hypertension result in the release of bioactive lipids from kidney cells that induce oxidative stress and renal cell death. Lipidomics was performed on uEVs isolated from CtsB knockout mice treated with or without STZ, and their kidneys were used to investigate changes in proteins associated with cell death. Lysophosphatidylethanolamine (LPE) (18:1), lysophosphatidylserine (LPS) (22:6), and lysophosphatidylglycerol (LPG) (22:5) were among the bioactive lipids enriched in uEVs from CtsB knockout mice treated with STZ compared to untreated CtsB mice (n = 3 uEV preparations per group). Anti-oxidant programming was activated in the kidneys of the CtsB knockout mice treated with STZ, as indicated by increased expression of glutathione peroxidase 4 (GPX4) and the cystine/glutamate antiporter SLC7A11 (XCT) (n = 4 mice per group), which was supported by a higher reactivity to 4-hydroxy-2-nonenal (4-HNE), a marker for oxidative stress (n = 3 mice per group). Apoptosis but not ferroptosis was the ongoing form of cell death in these kidneys as cleaved caspase-3 levels were significantly elevated in the STZ-treated CtsB knockout mice (n = 4 mice per group). There were no appreciable differences in the pro-ferroptosis enzyme acyl-CoA synthetase long-chain family member 4 (ACSL4) or the inflammatory marker CD93 in the kidneys (n = 3 mice per group), which further supports apoptosis as the prevalent mechanism of pathology. These data suggest that STZ treatment leads to oxidative stress, inducing apoptotic injury in the kidneys during the development of diabetic kidney disease and hypertension.

Complement Activation in Nephrotic Glomerular Diseases

The nephrotic syndrome holds significant clinical importance and is characterized by a substantial protein loss in the urine. Damage to the glomerular basement membrane or podocytes frequently underlies renal protein loss. There is an increasing belief in the involvement of the complement system, a part of the innate immune system, in these conditions. Understanding the interactions between the complement system and glomerular structures continually evolves, challenging the traditional view of the blood-urine barrier as a passive filter. Clinical studies suggest that a precise inhibition of the complement system at various points may soon become feasible. However, a thorough understanding of current knowledge is imperative for planning future therapies in nephrotic glomerular diseases such as membranous glomerulopathy, membranoproliferative glomerulonephritis, lupus nephritis, focal segmental glomerulosclerosis, and minimal change disease. This review provides an overview of the complement system, its interactions with glomerular structures, and insights into specific glomerular diseases exhibiting a nephrotic course. Additionally, we explore new diagnostic tools and future therapeutic approaches.

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.

Tempol treatment normalizes membrane expression of epithelial transport proteins in the kidney of salt-loaded hypertensive diabetic db/db mice

OBJECTIVE

Hypertension exacerbates the progression and severity of diabetic kidney disease. In this study, we addressed the hypothesis that tempol acts at multiple segments of the nephron to normalize the abundance of sodium coupled epithelial transport proteins in the luminal plasma membrane to mitigate high blood pressure in salt-loaded hypertensive diabetic db/db mice.

METHODS

Soluble and membrane fractions from freshly homogenized kidney cortex tissue samples were resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and probed for specific proteins by Western blotting. Western blotting for specific urinary extracellular vesicle (uEV) markers and nanoparticle tracking analysis was performed to characterize each uEV preparation from each group. A one-way ANOVA was performed to determine statistical significance between three groups (hypertensive diabetic db/db mice treated with vehicle, hypertensive diabetic db/db mice treated with tempol, and wild-type mice).

RESULTS

Tempol treatment reduced systolic blood pressure in hypertensive diabetic db/db mice compared to db/db mice that received vehicle. We observed attenuated membrane protein expression of the sodium hydrogen exchanger 3 (NHE3), sodium potassium chloride co-transporter (NKCC2), sodium chloride cotransporter (NCC), and epithelial sodium channel (ENaC) in the kidney of salt-loaded hypertensive diabetic db/db mice infused with tempol by osmotic minipump for 5 days compared to hypertensive diabetic db/db mice infused with vehicle. Also, the infusion of tempol in hypertensive diabetic db/db mice reduced the augmented protein expression of protein kinase c (PKC) epsilon observed in the vehicle treated hypertensive diabetic db/db kidney when compared to the healthy wild-type kidney. The amount of uEV and their size profiles were comparable between the three groups.

CONCLUSIONS

This study demonstrates that tempol down-regulates epithelial transport mechanisms in each segment of the nephron and normalizes salt-induced high blood pressure in diabetic animals presumably in a PKC dependent manner.

Nephrotic syndrome: pathophysiology and consequences

In patients with kidney disease, nephrotic syndrome can lead to several complications including progressive kidney dysfunction. Proteinuria may lead to the formation of cellular or fibrous crescents with reciprocal development of rapidly progressive glomerulonephritis or focal glomerulosclerosis. Proteinuria may also cause overload and dysfunction of tubular epithelial cells, eventually resulting in tubular atrophy and interstitial fibrosis. Hypoalbuminemia is usually associated with increased risk of mortality and kidney dysfunction. Dyslipidemia may increase the risk of atherosclerotic complications, cause podocyte dysfunction and contribute to vascular thrombosis. Urinary loss of anticoagulants and overproduction of coagulation factors may facilitate a hypercoagulable state. Edema, hypogammaglobulinemia, loss of complement factors, and immunosuppressive therapy can favor infection. Treatment of these complications may reduce their impact on the severity of NS. Nephrotic syndrome is a kidney disorder that can worsen the quality of life and increase the risk of kidney disease progression.

Integration of genetic fine-mapping and multi-omics data reveals candidate effector genes for hypertension

Genome-wide association studies of blood pressure (BP) have identified >1,000 loci, but the effector genes and biological pathways at these loci are mostly unknown. Using published association summary statistics, we conducted annotation-informed fine-mapping incorporating tissue-specific chromatin segmentation and colocalization to identify causal variants and candidate effector genes for systolic BP, diastolic BP, and pulse pressure. We observed 532 distinct signals associated with ≥2 BP traits and 84 with all three. For >20% of signals, a single variant accounted for >75% posterior probability, 65 were missense variants in known (SLC39A8, ADRB2, and DBH) and previously unreported BP candidate genes (NRIP1 and MMP14). In disease-relevant tissues, we colocalized >80 and >400 distinct signals for each BP trait with cis-eQTLs and regulatory regions from promoter capture Hi-C, respectively. Integrating mouse, human disorder, gene expression and tissue abundance data, and literature review, we provide consolidated evidence for 436 BP candidate genes for future functional validation and discover several potential drug targets.

The Role of Cathepsin B in Pathophysiologies of Non-tumor and Tumor tissues: A Systematic Review

Cathepsin B (CTSB), a lysosomal cysteine protease, plays an important role in human physiology and pathology. CTSB is associated with various human diseases, and its expression level and activity are closely related to disease progression and severity. Physiologically, CTSB is integrated into almost all lysosome-related processes, including protein turnover, degradation, and lysosome-mediated cell death. CTSB can lead to the development of various pathological processes through degradation and remodeling of the extracellular matrix. During tumor development and progression, CTSB has two opposing effects. Its pro-apoptotic properties reduce malignancy, while its proteolytic enzymatic activity promotes invasion and metastasis, thereby inducing malignancy. Here, we discuss the roles of CTSB in tumor and non-tumor disease pathophysiologies. We conclude that targeting the activity or expression of CTSB may be important for treating tumor and non-tumor diseases.

Decreased MARCKS Protein Expression in Kidney Cortex Membrane Fractions of Cathepsin B Knockout Mice Is Associated with Reduced Lysophosphatidylcholine and Protein Kinase C Activity

Cathpesin B is a multi-functional protease that plays numerous roles in physiology and pathophysiology. We hypothesized that actin cytoskeleton proteins that are substrates of cathepsin B, various lipids, and kinases that are regulated by lipids would be down-regulated in the kidney of cathepsin B knockout mice. Here, we show by Western blot and densitometric analysis that the expression and proteolysis of the actin cytoskeleton proteins myristoylated alanine-rich C-kinase substrate (MARCKS) and spectrin are significantly reduced in kidney cortex membrane fractions of cathepsin B knockout mice compared to C57B6 wild-type control mice. Lipidomic results show that specific lipids are increased while other lipids, including lysophosphatidylcholine (LPC) species LPC (16:0), LPC (18:0), LPC (18:1), and LPC (18:2), are significantly decreased in membrane fractions of the kidney cortex from Cathepsin B null mice. Protein Kinase C (PKC) activity is significantly lower in the kidney cortex of cathepsin B knockout mice compared to wild-type mice, while calcium/calmodulin-dependent protein kinase II (CaMKII) activity and phospholipase D (PLD) activity are comparable between the two groups. Together, these results provide the first evidence of altered actin cytoskeleton organization, membrane lipid composition, and PKC activity in the kidneys of mice lacking cathepsin B.

Metformin Alleviates Diabetes-Associated Hypertension by Attenuating the Renal Epithelial Sodium Channel

Diabetic nephropathy is the primary cause of morbidity in type 2 diabetes mellitus (T2DM) patients. New data indicate that hypertension, a common comorbidity in T2DM, can worsen outcomes of diabetic nephropathy. While metformin is a commonly prescribed drug for treating type 2 diabetes, its blood pressure regulating ability is not well documented. The aim of this study was to investigate the effect of metformin on normalizing blood pressure in salt-loaded hypertensive diabetic db/db mice. Sixteen-week-old male and female diabetic db/db mice were individually placed in metabolic cages and then randomized to a control vehicle (saline) or metformin treatment group. We evaluated the blood pressure reducing ability of metformin in salt-induced hypertension and progression of nephropathy in db/db mice. We observed that metformin- normalized systolic blood pressure in hypertensive diabetic mice. Mechanistically, metformin treatment reduced renal cathepsin B expression. Low cathepsin B expression was associated with reduced expression and activity of the epithelial sodium channel (ENaC), sodium retention, and thus control of hypertension. In addition, we identified that urinary extracellular vesicles (EVs) from the diabetic mice are enriched in cathepsin B. Compared to treatment with urinary EVs of vehicle-treated hypertensive diabetic mice, the amiloride-sensitive transepithelial current was significantly attenuated upon exposure of renal collecting duct cells to urinary EVs isolated from metformin-treated db/db mice or cathepsin B knockout mice. Collectively, our study identifies a novel blood pressure reducing role of metformin in diabetic nephropathy by regulating the cathepsin B-ENaC axis.

Human Alpha-1 Antitrypsin Attenuates ENaC and MARCKS and Lowers Blood Pressure in Hypertensive Diabetic db/db Mice

Hypertension may develop before or after the onset of diabetes and it is known to increase the risk of developing diabetic nephropathy. Alpha-1 antitrypsin (AAT) is a multi-functional protein with beneficial effects in various diseases but its role in reducing blood pressure in the diabetic kidney has not been thoroughly studied. Like blood pressure, epithelial sodium channels (ENaC) and its adaptor protein myristoylated alanine-rich C-kinase substrate (MARCKS) are regulated by circadian rhythms. Our hypothesis is that administration of human AAT (hAAT) reduces blood pressure in hypertensive diabetic mice by attenuating membrane expression of ENaC and its association with the actin cytoskeleton. First, we show hAAT administration results in reduced blood pressure in diabetic db/db mice compared to vehicle treatment in both the inactive and active cycles. Western blotting and immunohistochemistry analyses showed a reduction of ENaC and the actin cytoskeleton protein, MARCKS in the kidneys of diabetic db/db mice treated with hAAT compared to vehicle. hAAT treatment resulted in elevated amounts of extracellular vesicles present in the urine of diabetic db/db mice compared to vehicle treatment both in the inactive and active cycles. Multiple hexosylceramides, among other lipid classes increased in urinary EVs released from hAAT treated hypertensive diabetic mice compared to vehicle treated mice. Taken together, these data suggest hAAT treatment could normalize blood pressure in the diabetic kidney in a mechanism involving attenuation of renal ENaC and MARCKS protein expression and possibly ceramide metabolism to hexosylceramide in kidney cells.

PIP2 Interacts Electrostatically with MARCKS-like Protein-1 and ENaC in Renal Epithelial Cells

We examined the interaction of a membrane-associated protein, MARCKS-like Protein-1 (MLP-1), and an ion channel, Epithelial Sodium Channel (ENaC), with the anionic lipid, phosphatidylinositol 4, 5-bisphosphate (PIP₂). We found that PIP₂ strongly activates ENaC in excised, inside-out patches with a half-activating concentration of 21 ± 1.17 µM. We have identified 2 PIP₂ binding sites in the N-terminus of ENaC β and γ with a high concentration of basic residues. Normal channel activity requires MLP-1's strongly positively charged effector domain to electrostatically sequester most of the membrane PIP₂ and increase the local concentration of PIP₂. Our previous data showed that ENaC covalently binds MLP-1 so PIP₂ bound to MLP-1 would be near PIP₂ binding sites on the cytosolic N terminal regions of ENaC. We have modified the charge structure of the PIP₂ -binding domains of MLP-1 and ENaC and showed that the changes affect membrane localization and ENaC activity in a way consistent with electrostatic theory.

Role of ion channels in the mechanism of proteinuria (Review)

Proteinuria is a common clinical manifestation of kidney diseases, such as glomerulonephritis, nephrotic syndrome, immunoglobulin A nephropathy and diabetic nephropathy. Therefore, proteinuria is considered to be a risk factor for renal dysfunction. Furthermore, proteinuria is also significantly associated with the progression of kidney diseases and increased mortality. Its occurrence is closely associated with damage to the structure of the glomerular filtration membrane. An impaired glomerular filtration membrane can affect the selective filtration function of the kidneys; therefore, several macromolecular substances, such as proteins, may pass through the filtration membrane and promote the manifestation of proteinuria. It has been reported that ion channels play a significant role in the mechanisms underlying proteinuria. Ion channel mutations or other dysfunctions have been implicated in several diseases, therefore ion channels could be used as major therapeutic targets. The mechanisms underlying the action of ion channels and ion transporters in proteinuria have been overlooked in the literature, despite their importance in identifying novel targets for treating proteinuria and delaying the progression of kidney diseases. The current review article focused on the four key ion channel groups, namely Na⁺, Ca²⁺, Cl^- and K⁺ ion channels and the associated ion transporters.

Mannan-binding lectin serine protease-2 (MASP-2) in human kidney and its relevance for proteolytic activation of the epithelial sodium channel

Proteolytic activation of the renal epithelial sodium channel (ENaC) is increased by aldosterone. The aldosterone-sensitive protease remains unidentified. In humans, elevated circulating aldosterone is associated with increased urinary extracellular vesicle (uEVs) excretion of mannan-binding lectin associated serine protease-2 (MASP-2). We hypothesized that MASP-2 is a physiologically relevant ENaC-activating protease. It was confirmed that MASP2 mRNA is abundantly present in liver but not in human and mouse kidneys. Aldosterone-stimulation of murine cortical colleting duct (mCCD) cells did not induce MASP-2 mRNA. In human kidney collecting duct, MASP-2 protein was detected in AQP2-negative/ATP6VB1-positive intercalated cells suggestive of MASP2 protein uptake. Plasma concentration of full-length MASP-2 and the short splice variant MAp19 were not changed in a cross-over intervention study in healthy humans with low (70 mmol/day) versus high (250 mmol/day) Na+ intake despite changes in aldosterone. The ratio of MAp19/MASP-2 in plasma was significantly increased with a high Na+ diet and the ratio correlated with changes in aldosterone and fractional Na+ excretion. MASP-2 was not detected in crude urine or in uEVs. MASP2 activated an amiloride-sensitive current when co-expressed with ENaC in Xenopus oocytes, but not when added to the bath solution. In monolayers of collecting duct M1 cells, MASP2 expression did not increase amiloride-sensitive current and in HEK293 cells, MASP-2 did not affect γENaC cleavage. MASP-2 is neither expressed nor co-localized and co-regulated with ENaC in the human kidney or in urine after low Na+ intake. MASP-2 does not mediate physiological ENaC cleavage in low salt/high aldosterone settings.

Rodent models to study sodium retention in experimental nephrotic syndrome

Sodium retention and edema are hallmarks of nephrotic syndrome (NS). Different experimental rodent models have been established for simulating NS, however, not all of them feature sodium retention which requires proteinuria to exceed a certain threshold. In rats, puromycin aminonucleoside nephrosis (PAN) is a classic NS model introduced in 1955 that was adopted as doxorubicin-induced nephropathy (DIN) in 129S1/SvImJ mice. In recent years, mice with inducible podocin deletion (Nphs2^Δipod ) or podocyte apoptosis (POD-ATTAC) have been developed. In these models, sodium retention is thought to be caused by activation of the epithelial sodium channel (ENaC) in the distal nephron through aberrantly filtered serine proteases or proteasuria. Strikingly, rodent NS models follow an identical chronological time course after the development of proteinuria featuring sodium retention within days and spontaneous reversal thereafter. In DIN and Nphs2^Δipod mice, inhibition of ENaC by amiloride or urinary serine protease activity by aprotinin prevents sodium retention, opening up new and promising therapeutic approaches that could be translated into the treatment of nephrotic patients. However, the essential serine protease(s) responsible for ENaC activation is (are) still unknown. With the use of nephrotic rodent models, there is the possibility that this (these) will be identified in the future. This review summarizes the various rodent models used to study experimental nephrotic syndrome and the insights gained from these models with regard to the pathophysiology of sodium retention.

ENaC activation by proteases

Proteases are fundamental for a plethora of biological processes, including signalling and tissue remodelling, and dysregulated proteolytic activity can result in pathogenesis. In this review, we focus on a subclass of membrane‐bound and soluble proteases that are defined as channel‐activating proteases (CAPs), since they induce Na⁺ ion transport through an autocrine mechanism when co‐expressed with the highly amiloride‐sensitive epithelial sodium channel (ENaC) in Xenopus oocytes. These experiments first identified CAP1 (channel‐activating protease 1, prostasin) followed by CAP2 (channel‐activating protease 2, TMPRSS4) and CAP3 (channel‐activating protease 3, matriptase) as in vitro mediators of ENaC current. Since then, more serine‐, cysteine‐ and metalloproteases were confirmed as in vitro CAPs that potentially cleave and regulate ENaC, and thus this nomenclature was not further followed, but is accepted as functional term or alias. The precise mechanism of ENaC modulation by proteases has not been fully elucidated. Studies in organ‐specific protease knockout models revealed evidence for their role in increasing ENaC activity, although the proteases responsible for ENaC activation are yet to be identified. We summarize recent findings in animal models of these CAPs with respect to their implication in ENaC activation. We discuss the consequences of dysregulated CAPs underlying epithelial phenotypes in pathophysiological conditions, and the role of selected protease inhibitors. We believe that these proteases may present interesting therapeutic targets for diseases with aberrant sodium homoeostasis.

Behind every smile there's teeth: Cathepsin B's function in health and disease with a kidney view

Cathepsin B (CatB) is a very abundant lysosomal protease with endo- and carboxydipeptidase activities and even ligase features. In this review, we will provide a general characterization of CatB and describe structure, structure-derived properties and location-dependent proteolytic actions. We depict CatB action within lysosome and its important roles in lysosomal biogenesis, lysosomal homeostasis and autophagy rendering this protease a key player in orchestrating lysosomal functions. Lysosomal leakage and subsequent escape of CatB into the cytosol lead to harmful actions, e.g. the role in activating the NLPR3 inflammasome, affecting immune responses and cell death. The second focus of this review addresses CatB functions in the kidney, i.e. the glomerulus, the proximal tubule and collecting duct with strong emphasis of its role in pathology of the respective segment. Finally, observations regarding CatB functions that need to be considered in cell culture will be discussed. In conclusion, CatB a physiologically important molecule may, upon aberrant expression in different cellular context, become a harmful player effectively showing its teeth behind its smile.

Transgenic Mice Overexpressing Human Alpha-1 Antitrypsin Exhibit Low Blood Pressure and Altered Epithelial Transport Mechanisms in the Inactive and Active Cycles

Human alpha-1 antitrypsin (hAAT) is a versatile protease inhibitor, but little is known about its targets in the aldosterone-sensitive distal nephron and its role in electrolyte balance and blood pressure control. We analyzed urinary electrolytes, osmolality, and blood pressure from hAAT transgenic (hAAT-Tg) mice and C57B/6 wild-type control mice maintained on either a normal salt or high salt diet. Urinary sodium, potassium, and chloride concentrations as well as urinary osmolality were lower in hAAT-Tg mice maintained on a high salt diet during both the active and inactive cycles. hAAT-Tg mice showed a lower systolic blood pressure compared to C57B6 mice when maintained on a normal salt diet but this was not observed when they were maintained on a high salt diet. Cathepsin B protein activity was less in hAAT-Tg mice compared to wild-type controls. Protein expression of the alpha subunit of the sodium epithelial channel (ENaC) alpha was also reduced in the hAAT-Tg mice. Natriuretic peptide receptor C (NPRC) protein expression in membrane fractions of the kidney cortex was reduced while circulating levels of atrial natriuretic peptide (ANP) were greater in hAAT-Tg mice compared to wild-type controls. This study characterizes the electrolyte and blood pressure phenotype of hAAT-Tg mice during the inactive and active cycles and investigates the mechanism by which ENaC activation is inhibited in part by a mechanism involving decreased cathepsin B activity and increased ANP levels in the systemic circulation.

Experimental nephrotic syndrome leads to proteolytic activation of the epithelial Na+ channel in the mouse kidney

Proteolytic activation of the renal epithelial Na⁺ channel (ENaC) involves cleavage events in its α- and γ-subunits and is thought to mediate Na⁺ retention in nephrotic syndrome (NS). However, the detection of proteolytically processed ENaC in kidney tissue from nephrotic mice has been elusive so far. We used a refined Western blot technique to reliably discriminate full-length α-ENaC and γ-ENaC and their cleavage products after proteolysis at their proximal and distal cleavage sites (designated from the NH₂-terminus), respectively. Proteolytic ENaC activation was investigated in kidneys from mice with experimental NS induced by doxorubicin or inducible podocin deficiency with or without treatment with the serine protease inhibitor aprotinin. Nephrotic mice developed Na⁺ retention and increased expression of fragments of α-ENaC and γ-ENaC cleaved at both the proximal cleavage site and, more prominently, the distal cleavage site, respectively. Treatment with aprotinin but not with the mineralocorticoid receptor antagonist canrenoate prevented Na⁺ retention and upregulation of the cleavage products in nephrotic mice. Increased expression of cleavage products of α-ENaC and γ-ENaC was similarly found in healthy mice treated with a low-salt diet, sensitive to mineralocorticoid receptor blockade. In human nephrectomy specimens, γ-ENaC was found in the full-length form and predominantly cleaved at its distal cleavage site. In conclusion, murine experimental NS leads to aprotinin-sensitive proteolytic activation of ENaC at both proximal and, more prominently, distal cleavage sites of its α- and γ-subunit, most likely by urinary serine protease activity or proteasuria.NEW & NOTEWORTHY This study demonstrates that murine experimental nephrotic syndrome leads to aprotinin-sensitive proteolytic activation of the epithelial Na⁺ channel at both the α- and γ-subunit, most likely by urinary serine protease activity or proteasuria.

Increased endothelial sodium channel activity by extracellular vesicles in human aortic endothelial cells: Putative role of MLP1 and bioactive lipids

Extracellular vesicles (EVs) contain biological molecules and are secreted by cells into the extracellular milieu. The endothelial sodium channel (EnNaC) plays an important role in modulating endothelial cell stiffness. We hypothesized EVs secreted from human aortic endothelial cells (hAoEC) positively regulate EnNaC in an autocrine dependent manner. A comprehensive lipidomic analysis using targeted mass spectrometry was performed on multiple preparations of EVs isolated from the conditioned media of hAoEC or complete growth media of these cells. Cultured hAoEC challenged with EVs isolated from the conditioned media of these cells resulted in an increase in EnNaC activity when compared to the same concentration of media derived EVs or vehicle alone. EVs isolated from the conditioned media of hAoEC but not human fibroblast cells were enriched in MARCKS Like Protein 1 (MLP1). The pharmacological inhibition of the negative regulator of MLP1, protein kinase C, in cultured hAoEC resulted in an increase in EV size and release compared to vehicle or pharmacological inhibition of protein kinase D. The MLP1 enriched EVs increased the density of actin filaments in cultured hAoEC compared to EVs isolated from human fibroblast cells lacking MLP1. We quantified 141 lipids from glycerolipids, glycerophospholipids, and sphingolipids in conditioned media EVs that represented twice the number found in control media EVs. The concentrations of sphingomyelin, lysophosphatidylcholine and phosphatidylethanolamine were higher in conditioned media EVs. These results provide the first evidence for EnNaC regulation in hAoEC by EVs and provide insight into a possible mechanism involving MLP1, unsaturated lipids, and bioactive lipids.

Proteases, Mucus, and Mucosal Immunity in Chronic Lung Disease

Dysregulated protease activity has long been implicated in the pathogenesis of chronic lung diseases and especially in conditions that display mucus obstruction, such as chronic obstructive pulmonary disease, cystic fibrosis, and non-cystic fibrosis bronchiectasis. However, our appreciation of the roles of proteases in various aspects of such diseases continues to grow. Patients with muco-obstructive lung disease experience progressive spirals of inflammation, mucostasis, airway infection and lung function decline. Some therapies exist for the treatment of these symptoms, but they are unable to halt disease progression and patients may benefit from novel adjunct therapies. In this review, we highlight how proteases act as multifunctional enzymes that are vital for normal airway homeostasis but, when their activity becomes immoderate, also directly contribute to airway dysfunction, and impair the processes that could resolve disease. We focus on how proteases regulate the state of mucus at the airway surface, impair mucociliary clearance and ultimately, promote mucostasis. We discuss how, in parallel, proteases are able to promote an inflammatory environment in the airways by mediating proinflammatory signalling, compromising host defence mechanisms and perpetuating their own proteolytic activity causing structural lung damage. Finally, we discuss some possible reasons for the clinical inefficacy of protease inhibitors to date and propose that, especially in a combination therapy approach, proteases represent attractive therapeutic targets for muco-obstructive lung diseases.

Plasminogen deficiency does not prevent sodium retention in a genetic mouse model of experimental nephrotic syndrome

AIM

Sodium retention is the hallmark of nephrotic syndrome (NS) and mediated by the proteolytic activation of the epithelial sodium channel (ENaC) by aberrantly filtered serine proteases. Plasmin is highly abundant in nephrotic urine and has been proposed to be the principal serine protease responsible for ENaC activation in NS. However, a proof of the essential role of plasmin in experimental NS is lacking.

METHODS

We used a genetic mouse model of NS based on an inducible podocin knockout (Bl6-Nphs2^tm3.1Antc *Tg(Nphs1-rtTA*3G)^8Jhm *Tg(tetO-cre)^1Jaw or nphs2^Δipod ). These mice were crossed with plasminogen deficient mice (Bl6-Plg^tm1Jld or plg^-/- ) to generate double knockout mice (nphs2^Δipod *plg^-/- ). NS was induced after oral doxycycline treatment for 14 days and mice were followed for subsequent 14 days.

RESULTS

Uninduced nphs2^Δipod *plg^-/- mice had normal kidney function and sodium handling. After induction, proteinuria increased similarly in both nphs2^Δipod *plg^+/+ and nphs2^Δipod *plg^-/- mice. Western blot revealed the urinary excretion of plasminogen and plasmin in nphs2^Δipod *plg^+/+ mice which were absent in nphs2^Δipod *plg^-/- mice. After the onset of proteinuria, amiloride-sensitive natriuresis was increased compared to the uninduced state in both genotypes. Subsequently, urinary sodium excretion dropped in both genotypes leading to an increase in body weight and development of ascites. Treatment with the serine protease inhibitor aprotinin prevented sodium retention in both genotypes.

CONCLUSIONS

This study shows that mice lacking urinary plasminogen are not protected from ENaC-mediated sodium retention in experimental NS. This points to an essential role of other urinary serine proteases in the absence of plasminogen.

Proteasuria in nephrotic syndrome-quantification and proteomic profiling

Nephrotic syndrome is characterized by urinary excretion of plasma proteases or proteasuria. There is a lack of data on the quantity, activity status and identity of these aberrantly filtered proteases. We established a fluorescence-based substrate assay to quantify protease activity in urine samples from healthy and nephrotic humans and mice. Protease class activity was determined after addition of specific inhibitors. Individual proteases were identified by tandem mass spectrometry (MS/MS). In spot urine samples from 10 patients with acute nephrotic syndrome of various etiology, urinary protease activity was significantly increased compared to that of healthy persons (753 ± 178 vs. 244 ± 65 relative units, p < 0.05). The corresponding proteases were highly sensitive to inhibition by the serine protease inhibitors AEBSF (reduction by 85 ± 6% and 72 ± 8%, respectively) and aprotinin (83 ± 9% vs. 25 ± 6%, p < 0.05). MS/MS of all urinary proteins or after AEBSF purification showed that most of them were active serine proteases from the coagulation and complement cascade. These findings were recapitulated in mice, pointing to a similar pathophysiology. In conclusion, nephrotic syndrome leads to increased urinary excretion of active plasma proteases which can be termed proteasuria. Serine proteases account for the vast majority of urinary protease activity in health and nephrotic syndrome. SIGNIFICANCE STATEMENT: In this study, we found that nephrotic urine samples of humans and mice have a significantly increased protease activity compared to healthy urine samples, using a universal pentapeptide substrate library. This was driven by increased excretion of aprotinin-sensitive serine proteases. With tandem mass spectrometry, we provide a comprehensive and systematic overview of all urinary proteases or the "urine proteasome". We identified renally expressed proteases in health and addition of proteases from the coagulation and complement cascade in the nephrotic state. These results set the basis to study the role of urinary proteases at both health and nephrotic syndrome to find diagnostic markers of renal disease as well as possible therapeutic targets.

Cysteine cathepsins as therapeutic targets in inflammatory diseases

Introduction: Cysteine cathepsins are involved in the development and progression of numerous inflammation-associated diseases such as cancer, arthritis, bone and immune disorders. Consequently, there is a drive to progress research efforts focused on cathepsin use in diagnostics and as therapeutic targets in disease.Areas covered: This review discusses the potential of cysteine cathepsins as therapeutic targets in inflammation-associated diseases and recent advances in preclinical and clinical research. We describe direct targeting of cathepsins for treatment purposes and their indirect use in diagnostics.Expert opinion: The targeting of cysteine cathepsins has not translated into the clinic; this failure is attributed to off- and on-target side effects and/or the lack of companion biomarkers. This field now embraces developments in diagnostic imaging, the activation of prodrugs and antibody-drug conjugates for targeted drug delivery. The future lies in improved molecular tools and therapeutic concepts that will find a wide spectrum of uses in diagnostic and therapeutic applications.

Peptidomic Analysis of Urine from Youths with Early Type 1 Diabetes Reveals Novel Bioactivity of Uromodulin Peptides In Vitro

Chronic hyperglycemia is known to disrupt the proteolytic milieu, initiating compensatory and maladaptive pathways in the diabetic kidney. Such changes in intrarenal proteolysis are captured by the urinary peptidome. To elucidate the early kidney response to chronic hyperglycemia, we conducted a peptidomic investigation into urines from otherwise healthy youths with type 1 diabetes and their non-diabetic peers using unbiased and targeted mass spectrometry-based techniques. This cross-sectional study included two separate cohorts for the discovery (n = 30) and internal validation (n = 30) of differential peptide excretion. Peptide bioactivity was predicted using PeptideRanker and subsequently verified in vitro Proteasix and the Nephroseq database were used to identify putative proteases responsible for peptide generation and examine their expression in diabetic nephropathy. A total of 6550 urinary peptides were identified in the discovery analysis. We further examined the subset of 162 peptides, which were quantified across all thirty samples. Of the 15 differentially excreted peptides (p < 0.05), seven derived from a C-terminal region (589SGSVIDQSRVLNLGPITRK607) of uromodulin, a kidney-specific protein. Increased excretion of five uromodulin peptides was replicated in the validation cohort using parallel reaction monitoring (p < 0.05). One of the validated peptides (SGSVIDQSRVLNLGPI) activated NFκB and AP-1 signaling, stimulated cytokine release, and enhanced neutrophil migration in vitro. In silico analyses highlighted several potential proteases such as hepsin, meprin A, and cathepsin B to be responsible for generating these peptides. In summary, we identified a urinary signature of uromodulin peptides associated with early type 1 diabetes before clinical manifestations of kidney disease and discovered novel bioactivity of uromodulin peptides in vitro Our present findings lay the groundwork for future studies to validate peptide excretion in larger and broader populations, to investigate the role of bioactive uromodulin peptides in high glucose conditions, and to examine proteases that cleave uromodulin.

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.

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.

Cathepsin B increases ENaC activity leading to hypertension early in nephrotic syndrome

The NPHS2 gene, encoding the slit diaphragm protein podocin, accounts for genetic and sporadic forms of nephrotic syndrome (NS). Patients with NS often present symptoms of volume retention, such as oedema formation or hypertension. The primary dysregulation in sodium handling involves an inappropriate activation of the epithelial sodium channel, ENaC. Plasma proteases in a proteinuria‐dependent fashion have been made responsible; however, referring to the timeline of symptoms occurring and underlying mechanisms, contradictory results have been published. Characterizing the mouse model of podocyte inactivation of NPHS2 (Nphs2^∆pod) with respect to volume handling and proteinuria revealed that sodium retention, hypertension and gross proteinuria appeared sequentially in a chronological order. Detailed analysis of Nphs2^∆pod during early sodium retention, revealed increased expression of full‐length ENaC subunits and αENaC cleavage product with concomitant increase in ENaC activity as tested by amiloride application, and augmented collecting duct Na⁺/K⁺‐ATPase expression. Urinary proteolytic activity was increased and several proteases were identified by mass spectrometry including cathepsin B, which was found to process αENaC. Renal expression levels of precursor and active cathepsin B were increased and could be localized to glomeruli and intercalated cells. Inhibition of cathepsin B prevented hypertension. With the appearance of gross proteinuria, plasmin occurs in the urine and additional cleavage of γENaC is encountered. In conclusion, characterizing the volume handling of Nphs2^∆pod revealed early sodium retention occurring independent to aberrantly filtered plasma proteases. As an underlying mechanism cathepsin B induced αENaC processing leading to augmented channel activity and hypertension was identified.