Progressive renal papillary calcification and ureteral stone formation in mice deficient for Tamm-Horsfall protein

Uromodulin biology

Uromodulin is a kidney-specific glycoprotein which is exclusively produced by the epithelial cells lining the thick ascending limb and early distal convoluted tubule. It is currently recognized as a multifaceted player in kidney physiology and disease, with discrete roles for intracellular, urinary, interstitial and serum uromodulin. Among these, uromodulin modulates renal sodium handling through the regulation of tubular sodium transporters that reabsorb sodium and are targeted by diuretics, such as the loop diuretic-sensitive Na+-K+-2Cl- cotransporter type 2 (NKCC2) and the thiazide-sensitive Na+/Cl- cotransporter (NCC). Given these roles, the contribution of uromodulin to sodium-sensitive hypertension has been proposed. However, recent studies in humans suggest a more complex interaction between dietary sodium intake, uromodulin and blood pressure. This review presents an updated overview of the uromodulin's biology and its various roles, and focuses on the interaction between uromodulin and sodium-sensitive hypertension.

Tamm-Horsfall protein augments neutrophil NETosis during urinary tract infection

Urinary neutrophils are a hallmark of urinary tract infection (UTI), yet the mechanisms governing their activation, function, and efficacy in controlling infection remain incompletely understood. Tamm-Horsfall glycoprotein (THP), the most abundant protein in urine, uses terminal sialic acids to bind an inhibitory receptor and dampen neutrophil inflammatory responses. We hypothesized that neutrophil modulation is an integral part of THP-mediated host protection. In a UTI model, THP-deficient mice showed elevated urinary tract bacterial burdens, increased neutrophil recruitment, and more severe tissue histopathological changes compared to WT mice. Furthermore, THP-deficient mice displayed impaired urinary NETosis during UTI. To investigate the impact of THP on NETosis, we coupled in vitro fluorescence-based NET assays, proteomic analyses, and standard and imaging flow cytometry with peripheral human neutrophils. We found that THP increases proteins involved in respiratory chain, neutrophil granules, and chromatin remodeling pathways, enhances NETosis in an ROS-dependent manner, and drives NET-associated morphologic features including nuclear decondensation. These effects were observed only in the presence of a NETosis stimulus and could not be solely replicated with equivalent levels of sialic acid alone. We conclude that THP is a critical regulator of NETosis in the urinary tract, playing a key role in host defense against UTI.

Tamm-Horsfall protein in humane urine: sex-dependent differences in the excretion and N-glycosylation pattern

Tamm-Horsfall protein (THP) is a highly N-glycosylated protein from epithelial cells of the ascending limb of Henle loop. It is secreted into the urine as part of the innate immune response against uropathogenic pathogens. As women are more likely to suffer from urinary tract infections, biomedical studies were conducted to investigate sex-differences in THP excretion, as well as differences in the THP N-glycosylation pattern. A total of 238 volunteers (92 men, 146 women, 69 with hormonal contraceptives) participated in this study, providing urine samples. Women showed a clear tendency to have higher THP concentration and excretion rates than men (p < 0.16). Regular intake of hormonal contraceptives had no significant influence on urinary THP concentration compared to no regular intake. The individual N-glycosylation pattern of THP in urine samples from randomly selected individuals (10 female, 10 male) was investigated after enzymatic release and MS analysis of the oligosaccharides. Female subjects tended to have an increased proportion of oligomannose type N-glycans and non-fucosylated glycans, whereas men had an increased proportion of fucosylated complex-type glycans. The higher level of oligomannose-type glycans in THP from women might be explained by a self-defence mechanism to overcome the higher infections pressure by the female anatomical properties.

Unveiling the Hidden Power of Uromodulin: A Promising Potential Biomarker for Kidney Diseases

Uromodulin, also known as Tamm-Horsfall protein, represents the predominant urinary protein in healthy individuals. Over the years, studies have revealed compelling associations between urinary and serum concentrations of uromodulin and various parameters, encompassing kidney function, graft survival, cardiovascular disease, glucose metabolism, and overall mortality. Consequently, there has been a growing interest in uromodulin as a novel and effective biomarker with potential applications in diverse clinical settings. Reduced urinary uromodulin levels have been linked to an elevated risk of acute kidney injury (AKI) following cardiac surgery. In the context of chronic kidney disease (CKD) of different etiologies, urinary uromodulin levels tend to decrease significantly and are strongly correlated with variations in estimated glomerular filtration rate. The presence of uromodulin in the serum, attributable to basolateral epithelial cell leakage in the thick ascending limb, has been observed. This serum uromodulin level is closely associated with kidney function and histological severity, suggesting its potential as a biomarker capable of reflecting disease severity across a spectrum of kidney disorders. The UMOD gene has emerged as a prominent locus linked to kidney function parameters and CKD risk within the general population. Extensive research in multiple disciplines has underscored the biological significance of the top UMOD gene variants, which have also been associated with hypertension and kidney stones, thus highlighting the diverse and significant impact of uromodulin on kidney-related conditions. UMOD gene mutations are implicated in uromodulin-associated kidney disease, while polymorphisms in the UMOD gene show a significant association with CKD. In conclusion, uromodulin holds great promise as an informative biomarker, providing valuable insights into kidney function and disease progression in various clinical scenarios. The identification of UMOD gene variants further strengthens its relevance as a potential target for better understanding kidney-related pathologies and devising novel therapeutic strategies. Future investigations into the roles of uromodulin and regulatory mechanisms are likely to yield even more profound implications for kidney disease diagnosis, risk assessment, and management.

Water Loading and Uromodulin Secretion in Healthy Individuals and Idiopathic Calcium Stone Formers

BACKGROUND

Uromodulin is a protein made only by the kidney and released in urine, circulating in polymerizing and nonpolymerizing forms. This protein's multiple functions include inhibition of stone formation in the urine. The physiological determinants of uromodulin production are incompletely understood.

METHODS

We investigated changes in uromodulin levels and key factors governing its production and release in urine and serum. We performed an experiment to determine whether water loading, a common intervention to prevent stone formation, will alter the rate of uromodulin production. During a 2-day period, 17 stone forming participants and 14 control participants were subjected to water loading (day 1) and normal fluid intake (day 2). Uromodulin levels were measured on timed hourly collections in urine and plasma during the period of the study.

RESULTS

Water loading increased urinary uromodulin secretion (33±4 versus 10±4 μ g/min at baseline, P < 0.0001) in stone formers and control participants. Despite high urine volumes, most participants maintained relatively stable urinary uromodulin concentrations. Native Western blots for polymerizing and nonpolymerizing uromodulin suggest that polymerizing uromodulin was the predominant form at higher urinary flow volumes. Urine flow rates and sodium excretion were significant correlates of urinary uromodulin production. Water loading did not affect serum uromodulin levels, which were also not associated with urinary uromodulin.

CONCLUSIONS

Water loading increases the secretion of polymerizing urinary uromodulin. This increased secretion reduces the variability of urinary uromodulin concentrations despite high urine volumes. Serum uromodulin levels were not affected by this treatment.

Proposal for pathogenesis-based treatment options to reduce calcium oxalate stone recurrence

Objective

Prevalence of kidney stone disease continues to increase globally with recurrence rates between 30% and 50% despite technological and scientific advances. Reduction in recurrence would improve patient outcomes and reduce cost and stone morbidities. Our objective was to review results of experimental studies performed to determine the efficacy of readily available compounds that can be used to prevent recurrence.

Methods

All relevant literature up to October 2020, listed in PubMed is reviewed.

Results

Clinical guidelines endorse the use of evidence-based medications, such as alkaline agents and thiazides, to reduce urinary mineral supersaturation and recurrence. However, there may be additional steps during stone pathogenesis where medications could moderate stone risk. Idiopathic calcium oxalate stones grow attached to Randall's plaques or plugs. Results of clinical and experimental studies suggest involvement of reactive oxygen species and oxidative stress in the formation of both the plaques and plugs. The renin-angiotensin-aldosterone system (RAAS), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, mitochondria, and NOD-like receptor pyrin domain containing-3 (NLRP3) inflammasome have all been implicated at specific steps during stone pathogenesis in animal models.

Conclusion

In addition to supersaturation-reducing therapies, the use of anti-oxidants, free radical scavengers, and inhibitors of NADPH oxidase, NLRP3 inflammasome, and RAAS may prove beneficial for stone prevention. Compounds such as statins and angiotensin converting enzyme inhibitors are already in use as therapeutics for hypertension and cardio-vascular disease and have previously shown to reduce calcium oxalate nephrolithiasis in rats. Although clinical evidence for their use in stone prevention in humans is limited, experimental data support they be considered along with standard evidence-based medications and clinical expertise when patients are being counselled for stone prevention.

Kidney function may partially mediated the protective effect of urinary uromodulin on kidney stone

The causal links between urinary uromodulin (uUMOD) and kidney stone disease (KSD) are still not clarified in general population. We assessed their relationships combining 2-sample Mendelian randomization (MR) and multivariable (MVMR) designs among general population of European ancestry. The summary information for uUMOD indexed to creatinine levels (29,315 individuals) and KSD (395,044 individuals) were from 2 independent genome-wide association studies (GWAS). The primary causal effects of exposures on outcomes were evaluated using inverse variance-weighted (IVW) regression model. Multiple sensitivity analyses were also performed. In 2-sample MR, we found that 1-unit higher genetically predicted uUMOD levels were associated with a lower risk of KSD (OR = 0.62; 95% CI 0.55-0.71; P = 2.83E-13). In reverse, we did not find the effect of KSD on uUOMD using IVW (beta = 0.00; 95% CI - 0.06-0.05; P = 0.872) and other sensitivity analyses. In MVMR, uUMOD indexed to creatinine levels were directly associated with the risk of KSD after introducing eGFR, SBP, urinary sodium or all three factors (OR = 0.71; 95% CI 0.64-0.79; P = 1.57E-09). Furthermore, our study supported that the protective effect of uUMOD on KSD may be partially mediated by eGFR (beta = - 0.09; 95% CI - 0.13 to - 0.06; mediation proportion = 20%). Our study supported that the protective effect of genetically predicted higher uUMOD levels on KSD may be partially mediated by eGFR decline, but not via SBP or urinary sodium. uUMOD might be a treatment target in preventing KSD in general population.

Acute kidney injury increases risk of kidney stones-a retrospective propensity score matched cohort study

BACKGROUND

Acute kidney injury (AKI) is common. An episode of AKI may modify the risk of developing kidney stones by potential long-term effects on urine composition. We aimed to investigate the association between AKI and the risk of kidney stone presentations.

METHODS

The retrospective cohort study used patient data (1 January 2008-31 December 2017), from an Australian Local Health District, which included AKI diagnosis, demographics, comorbidities and kidney stone admissions. Time-varying Cox proportional hazards and propensity-matched analysis were used to determine the impact of AKI on the risk of kidney stones. To address possible population inhomogeneity in comparisons between no AKI and hospitalized AKI, sub-group analysis was done comparing inpatient and outpatient AKI versus no AKI, to assess consistency of association with future stones. Sensitivity analysis was undertaken to capture the impact of a known AKI status and AKI severity.

RESULTS

Out of 137 635 patients, 23 001 (17%) had an AKI diagnosis and 2295 (2%) had kidney stone presentations. In the unadjusted analysis, AKI was associated with kidney stones, with AKI used as a time-varying exposure, [hazard ratio (HR) 1.32, 95% confidence interval (CI) 1.16-1.50)]. Both inpatient-AKI (HR 1.19, 95% CI 1.01-1.39) and outpatient-AKI (HR 1.59, 95% CI 1.30-1.94) were significantly associated with future stones compared to no AKI subjects. This association persisted in the adjusted analysis (HR 1.45, 95% CI 1.26-1.66), propensity-matched dataset (HR 1.67, 95% CI 1.40-1.99) and sensitivity analysis. There was a dose-response relationship with higher stages of AKI being associated with a greater risk of kidney stones.

CONCLUSIONS

In a large cohort of patients, AKI is associated with a greater risk of kidney stones, which increases with higher stages of AKI. This association should be examined in other cohorts and populations for verification.

Aqueous extract from Equisetum arvense stimulates the secretion of Tamm-Horsfall protein in human urine after oral intake

BACKGROUND

Within European traditional phytotherapy, extracts from different herbal plants are used for prevention and therapy of uncomplicated urinary tract infections and for flushing out of kidney grits. Besides increased urine flow by slight diuretic effects, also stimulation of Tamm-Horsfall protein (syn. THP, uromodulin) in the distal part of the kidney could explain reduced kidney gravel and anti-virulent activity against uropathogenic E. coli.

PURPOSES

Evaluation of THP-inducing activity of extracts from Equisetum arvense, Levisticum officinalis, Ilex paraguariensis, Juniperus communis, Urtica dioica, and Taraxacum officinale by quantification of THP in urine samples after oral application to humans.

STUDY DESIGN

7 days p.o. application of the test intervention to healthy volunteers (n = 10 per intervention group) and analysis of urine samples at day 1 (untreated control values), and days 3, 6 and 8 on THP content by validated ELISA. Antiadhesive activity of urine samples was monitored by flow cytometry using UPEC strain NU14 against human T24 bladder cells.

RESULTS

An aqueous extract from E. arvense, fully characterized by a specific LC-MS method, induced THP concentration in urine samples significantly during a 7-day p.o. application up to 300%, related to the untreated controls. Ex vivo investigation of the individual and pooled urine samples with elevated THP concentrations showed good correlation to antiadhesive effects against UPEC NU14 to T24 cells. Urine samples of the Equisetum treated volunteers had no effect on the proliferation and on biofilm formation of UPEC NU14. Silica excretion in the urine samples had no correlation to the respective THP levels. Monitoring of electrolyte content in the urine samples indicat ed diuretic effects of the intervention with Equisetum extract. Detailed phytochemical analysis of the Equisetum extract by LC-MS and LC-UV revealed an analytical protocol, which identified > 80 compounds from the extract by MS evaluations and 18 compounds by UV detection. This protocol will provide a valuable tool for future quality control of Equisetum extract.

CONCLUSION

Aqueous extract from E. arvense significantly stimulates THP secretion in urine samples after 7 days of oral intake and inhibits the interplay between UPEC and bladder host cells. This could explain the therapeutic use of this herbal material for urinary tract infections and kidney gravel. Detailed phytochemical analysis of the Equisetum extract by LC-MS and LC-UV revealed an analytical protocol, which identified > 82% of all eluted compounds. This protocol will provide a valuable tool for future quality control of Equisetum extract.

Phenome-Wide Association Study of UMOD Gene Variants and Differential Associations With Clinical Outcomes Across Populations in the Million Veteran Program a Multiethnic Biobank

Introduction

Common variants in the UMOD gene are considered an evolutionary adaptation against urinary tract infections (UTIs) and have been implicated in kidney stone formation, chronic kidney disease (CKD), and hypertension. However, differences in UMOD variant-phenotype associations across population groups are unclear.

Methods

We tested associations between UMOD/PDILT variants and up to 1528 clinical diagnosis codes mapped to phenotype groups in the Million Veteran Program (MVP), using published phenome-wide association study (PheWAS) methodology. Associations were tested using logistic regression adjusted for age, sex, and 10 principal components of ancestry. Bonferroni correction for multiple comparisons was applied.

Results

Among 648,593 veterans, mean (SD) age was 62 (14) years; 9% were female, 19% Black, and 8% Hispanic. In White patients, the rs4293393 UMOD risk variant associated with increased uromodulin was associated with increased odds of CKD (odds ratio [OR]: 1.22, 95% CI: 1.20-1.24, P = 5.90 × 10-111), end-stage kidney disease (OR: 1.17, 95% CI: 1.11-1.24, P = 2.40 × 10-09), and hypertension (OR: 1.03, 95% CI: 1.05-1.05, P = 2.11 × 10-06) and significantly lower odds of UTIs (OR: 0.94, 95% CI: 0.92-0.96, P = 1.21 × 10-10) and kidney calculus (OR: 0.85, 95% CI: 0.83-0.86, P = 4.27 × 10-69). Similar findings were observed across UMOD/PDILT variants. The rs77924615 PDILT variant had stronger associations with acute cystitis in White female (OR: 0.73, 95% CI: 0.59-0.91, P = 4.98 × 10-03) versus male (OR: 0.99, 95% CI: 0.89-1.11, P = 8.80 × 10-01) (P interaction = 0.01) patients. In Black patients, the rs77924615 PDILT variant was significantly associated with pyelonephritis (OR: 0.65, 95% CI: 0.54-0.79, P = 1.05 × 10-05), whereas associations with UMOD promoter variants were attenuated.

Conclusion

Robust associations were observed between UMOD/PDILT variants linked with increased uromodulin expression and lower odds of UTIs and calculus and increased odds of CKD and hypertension. However, these associations varied significantly across ancestry groups and sex.

Oxidized forms of uromodulin promote calcium oxalate crystallization and growth, but not aggregation

Roles of an abundant human urinary protein, uromodulin (UMOD), in kidney stone disease were previously controversial. Recently, we have demonstrated that oxidative modification reverses overall modulatory activity of whole urinary proteins, from inhibition to promotion of calcium oxalate (CaOx) stone-forming processes. We thus hypothesized that oxidation is one of the factors causing those previously controversial UMOD data on stone modulation. Herein, we addressed effects of performic-induced oxidation on CaOx crystal modulatory activity of UMOD. Sequence analyses revealed two EGF-like calcium-binding domains (65th-107th and 108th-149th), two other calcium-binding motifs (65th-92nd and 108th-135th), and three oxalate-binding motifs (199th-207th, 361st-368th and 601st-609th) in UMOD molecule. Analysis of tandem mass spectrometric dataset of whole urinary proteins confirmed marked increases in oxidation, dioxidation and trioxidation of UMOD in the performic-modified urine samples. UMOD was then purified from the normal urine and underwent performic-induced oxidative modification, which was confirmed by Oxyblotting. The oxidized UMOD significantly promoted CaOx crystallization and crystal growth, whereas the unmodified native UMOD inhibited CaOx crystal growth. However, the oxidized UMOD did not affect CaOx crystal aggregation. Therefore, our data indicate that oxidized forms of UMOD promote CaOx crystallization and crystal growth, which are the important processes for CaOx kidney stone formation.

Systemic Effects of Tamm-Horsfall Protein in Kidney Disease

Tamm-Horsfall protein (THP) is produced exclusively by the kidney, where it is released into both the urine and the circulation. Although the primary form of circulating THP is nonpolymerizing, urinary THP exists as a mix of polymerizing and nonpolymerizing forms. Urinary THP has been shown to play roles in such disparate processes as prevention of urinary tract infections and kidney stone formation, along with the regulation of multiple ion channels within the kidney. The generation of THP knockout mouse models has allowed the investigation of these phenomena and shown a prospective role for circulating THP in ischemia-reperfusion acute kidney injury as well as sepsis. Recent studies have suggested that THP is protective in ischemic injury owing to its inhibition of oxidative stress via the calcium channel transient receptor potential cation channel, subfamily M, member 2 t(TRPM2), and protection in sepsis is at least partially due to THP's promotion of macrophage function.

1,25-dihydroxyvitamin D as Predictor of Renal Worsening Function in Chronic Kidney Disease. Results From the PASCaL-1,25D Study

Background

Heterogeneous progression of chronic kidney disease (CKD) toward dialysis advocates improving in renal care management. Diagnosis and staging of CKD relies on estimated glomerular filtration rate (eGFR) and albuminuria. Tubular biomarkers emerged as new predictors of worsening renal function (WRF), due to partial inaccuracy of eGFR and existing WRF in non-proteinuric patients. Active vitamin D is synthesized in renal tubules and participates to mineral adaptation in CKD. Circulating 1,25-dihydroxyvitamin D [1,25(OH)₂D] was poorly investigated as a biomarker of endocrine tubular function and predictor of WRF.

Objective

Investigate capability of 1,25(OH)₂D to predict parathormone (PTH) increase and WRF in CKD stage 3-4.

Methods

PASCaL-1,25D was an observational, prospective, monocentric study. Primary outcomes were absolute and 20% increase in PTH, and WRF defined as 20% reduction in eGFR or dialysis initiation at 6 months.

Results

Seventy-one patients completed follow up. Absolute increase in PTH (1-84) was independently predicted by lower 1,25(OH)₂D levels (p = 0.0134). No association was detected between 1,25(OH)₂D and iPTH increase. Higher 1,25(OH)₂D was associated with reduced risk of WRF at univariate analysis [OR 0.89 (95% CI 0.86-0.93), p = 0.006]. The 1,25(OH)₂D/PTH (1-84) ratio was associated with non-significant 84% risk reduction for WRF [OR 0.16 (95% CI 0.06-0.41), p = 0.05]. Low 1,25(OH)₂D reached 100% sensitivity in predicting WRF in CKD stage 3 (AUC 9.909, p < 0.0001) and non-elderly patients (AUC 0.883, p < 0.0001). Machine learning models retained 1,25(OH)₂D/PTH (1-84) as relevant predictor of WRF together with eGFR and albuminuria. Age influenced interaction between renal and mineral biomarkers.

Conclusion

1,25(OH)₂D deserves attention as biomarker of tubular health, and sensible predictor of WRF on the short run among non-elderly patients affected by stage 3 CKD. The 1,25(OH)₂D/PTH (1-84) ratio may represent a composite biomarker of tubular reserve/endocrine response to the transition from adaptive to maladaptive equilibrium in CKD-MBD.

How Many Cell Types Are in the Kidney and What Do They Do?

The kidney maintains electrolyte, water, and acid-base balance, eliminates foreign and waste compounds, regulates blood pressure, and secretes hormones. There are at least 16 different highly specialized epithelial cell types in the mammalian kidney. The number of specialized endothelial cells, immune cells, and interstitial cell types might even be larger. The concerted interplay between different cell types is critical for kidney function. Traditionally, cells were defined by their function or microscopical morphological appearance. With the advent of new single-cell modalities such as transcriptomics, epigenetics, metabolomics, and proteomics we are entering into a new era of cell type definition. This new technological revolution provides new opportunities to classify cells in the kidney and understand their functions.

Differential biomolecular recognition by synthetic vs. biologically-derived components in the stone-forming process using 3D microfluidics

Calcium phosphate (CaP) biomineralization is the hallmark of extra-skeletal tissue calcification and renal calcium stones. Although such a multistep process starts with CaP crystal formation, the mechanism is still poorly understood due to the complexity of the in vivo system and the lack of a suitable approach to simulate a truly in vivo-like environment. Although endogenous proteins and lipids are engaged with CaP crystals in such a biological process of stone formation, most in vitro studies use synthetic materials that can display differential bioreactivity and molecular recognition by the cellular component. Here, we used our in vitro microfluidic (MF) tubular structure, which is the first completely cylindrical platform, with renal tubular cellular microenvironments closest to the functional human kidney tubule, to understand the precise role of biological components in this process. We systematically evaluated the contribution of synthetic and biological components in the stone-forming process in the presence of dynamic microenvironmental cues that originated due to cellular pathophysiology, which are critical for the nucleation, aggregation, and growth of CaP crystals. Our results show that crystal aggregation and growth were enhanced by immunoglobulin G (IgG), which was further inhibited by etidronic acid due to the chelation of extracellular Ca²⁺. Interestingly, biogenic CaP crystals from mice urine, when applied with cell debris and non-specific protein (bovine serum albumin), exhibited a more discrete crystal growth pattern, compared to exposure to synthetic CaP crystals under similar conditions. Furthermore, proteins found on those calcium crystals from mice urine produced discriminatory effects on crystal-protein attachment. Specifically, such biogenic crystals exhibited enhanced affinity to the proteins inherent to those crystals. More importantly, a physiological comparison of crystal induction in renal tubular cells revealed that biogenic crystals are less effective at producing a sustained rise in cytosolic Ca²⁺ compared to synthetic crystals, suggesting a milder detrimental effect to downstream signaling. Finally, synthetic crystal-internalized cells induced more oxidative stress, inflammation, and cellular damage compared to the biogenic crystal-internalized cells. Together, these results suggest that the intrinsic nature of biogenically derived components are appropriate to generate the molecular recognition needed for spatiotemporal effects and are critical towards understanding the process of kidney stone formation.

Proteomic analysis reveals some common proteins in the kidney stone matrix

Background

Proteins are the most abundant component of kidney stone matrices and their presence may reflect the process of the stone’s formation. Many studies have explored the proteomics of urinary stones and crystals. We sought to comprehensively identify the proteins found in kidney stones and to identify new, reliable biomolecules for use in nephrolithiasis research.

Methods

We conducted bioinformatics research in November 2020 on the proteomics of urinary stones and crystals. We used the ClusterProfiler R package to transform proteins into their corresponding genes and Ensembl IDs. In each study we located where proteomic results intersected to determine the 20 most frequently identified stone matrix proteins. We used the Human Protein Atlas to obtain the biological information of the 20 proteins and conducted Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genome (KEGG) analysis to explore their biological functions. We also performed immunohistochemistry to detect the expression of the top five stone matrix proteins in renal tissue.

Results

We included 19 relevant studies for analysis. We then identified 1,409 proteins in the stone matrix after the duplicates were removed. The 20 most-commonly identified stone matrix proteins were: S100A8, S100A9, uromodulin, albumin, osteopontin, lactotransferrin, vitamin K-dependent protein Z, prothrombin, hemoglobin subunit beta, myeloperoxidase, mannan-binding lectin serine protease 2, lysozyme C, complement C3, serum amyloid P-component, cathepsin G, vitronectin, apolipoprotein A-1, eosinophil cationic protein, fibrinogen alpha chain, and apolipoprotein D. GO and KEGG analysis revealed that these proteins were typically engaged in inflammation and immune response.Immunohistochemistry of the top five stone matrix proteins in renal tissue showed that the expression of S100A8, S100A9, and osteopontin increased, while uromodulin decreased in kidney stone patients. Albumin was rarely expressed in the kidney with no significant difference between healthy controls and kidney stone patients.

Conclusion

Proteomic analysis revealed some common inflammation-related proteins in the kidney stone matrix. The role of these proteins in stone formation should be explored for their potential use as diagnostic biomarkers and therapeutic targets for urolithiasis.

Randall's plaque and calcium oxalate stone formation: role for immunity and inflammation

Idiopathic calcium oxalate (CaOx) stones often develop attached to Randall's plaque present on kidney papillary surfaces. Similar to the plaques formed during vascular calcification, Randall's plaques consist of calcium phosphate crystals mixed with an organic matrix that is rich in proteins, such as inter-α-trypsin inhibitor, as well as lipids, and includes membrane-bound vesicles or exosomes, collagen fibres and other components of the extracellular matrix. Kidney tissue surrounding Randall's plaques is associated with the presence of classically activated, pro-inflammatory macrophages (also termed M1) and downregulation of alternatively activated, anti-inflammatory macrophages (also termed M2). In animal models, crystal deposition in the kidneys has been associated with the production of reactive oxygen species, inflammasome activation and increased expression of molecules implicated in the inflammatory cascade, including osteopontin, matrix Gla protein and fetuin A (also known as α2-HS-glycoprotein). Many of these molecules, including osteopontin and matrix Gla protein, are well known inhibitors of vascular calcification. We propose that conditions of urine supersaturation promote kidney damage by inducing the production of reactive oxygen species and oxidative stress, and that the ensuing inflammatory immune response promotes Randall's plaque initiation and calcium stone formation.

Vasopressin Induces Urinary Uromodulin Secretion By Activating PKA (Protein Kinase A)

[Figure: see text].

Genetic Background but Not Intestinal Microbiota After Co-Housing Determines Hyperoxaluria-Related Nephrocalcinosis in Common Inbred Mouse Strains

Calcium oxalate (CaOx) crystal formation, aggregation and growth is a common cause of kidney stone disease and nephrocalcinosis-related chronic kidney disease (CKD). Genetically modified mouse strains are frequently used as an experimental tool in this context but observed phenotypes may also relate to the genetic background or intestinal microbiota. We hypothesized that the genetic background or intestinal microbiota of mice determine CaOx crystal deposition and thus the outcome of nephrocalcinosis. Indeed, Casp1^-/-, Cybb^-/- or Casp1^-/-/Cybb^-/- knockout mice on a 129/C57BL/6J (B6J) background that were fed an oxalate-rich diet for 14 days did neither encounter intrarenal CaOx crystal deposits nor nephrocalcinosis-related CKD. To test our assumption, we fed C57BL/6N (B6N), 129, B6J and Balb/c mice an oxalate-rich diet for 14 days. Only B6N mice displayed CaOx crystal deposits and developed CKD associated with tubular injury, inflammation and interstitial fibrosis. Intrarenal mRNA expression profiling of 64 known nephrocalcinosis-related genes revealed that healthy B6N mice had lower mRNA levels of uromodulin (Umod) compared to the other three strains. Feeding an oxalate-rich diet caused an increase in uromodulin protein expression and CaOx crystal deposition in the kidney as well as in urinary uromodulin excretion in B6N mice but not 129, B6J and Balb/c mice. However, backcrossing 129 mice on a B6N background resulted in a gradual increase in CaOx crystal deposits from F2 to F7, of which all B6N/129 mice from the 7^th generation developed CaOx-related nephropathy similar to B6N mice. Co-housing experiments tested for a putative role of the intestinal microbiota but B6N co-housed with 129 mice or B6N/129 (3^rd and 6^th generation) mice did not affect nephrocalcinosis. In summary, genetic background but not the intestinal microbiome account for strain-specific crystal formation and, the levels of uromodulin secretion may contribute to this phenomenon. Our results imply that only littermate controls of the identical genetic background strain are appropriate when performing knockout mouse studies in this context, while co-housing is optional.

Uromodulin Isolation and Its N-Glycosylation Analysis by NanoLC-MS/MS

The glycoprotein uromodulin (UMOD) is the most abundant protein in urine, and N-glycans are critical for many biological functions of UMOD. Comprehensive glycan profiling of UMOD provides valuable information to understand the exact mechanisms of glycan-regulated functions. To perform comprehensive glycosylation analysis of UMOD from urine samples with limited volumes, we developed a streamlined workflow that included UMOD isolation from 5 mL of urine from 6 healthy adult donors (3 males and 3 females) and a glycosylation analysis using a highly sensitive and reproducible nanoLC-MS/MS based glycomics approach. In total, 212 N-glycan compositions were identified from the purified UMOD, and 17% were high-mannose glycans, 2% were afucosylated/asialylated, 3% were neutral fucosylated, 28% were sialylated (with no fucose), 46% were fucosylated and sialylated, and 4% were sulfated. We found that isolation of UMOD resulted in a significant decrease in the relative quantity of high-mannose and sulfated glycans with a significant increase of neutral fucosylated glycans in the UMOD-depleted urine relative to the undepleted urine, but depletion had little impact on the sialylated glycans. To our knowledge, this is the first study to perform comprehensive N-glycan profiling of UMOD using nanoLC-MS/MS. This analytical workflow would be very beneficial for studies with limited sample size, such as pediatric studies, and can be applied to larger patient cohorts not only for UMOD interrogation but also for global glycan analysis.

Circulating uromodulin inhibits vascular calcification by interfering with pro-inflammatory cytokine signalling

AIMS

Uromodulin is produced exclusively in the kidney and secreted into both urine and blood. Serum levels of uromodulin are correlated with kidney function and reduced in chronic kidney disease (CKD) patients, but physiological functions of serum uromodulin are still elusive. This study investigated the role of uromodulin in medial vascular calcification, a key factor associated with cardiovascular events and mortality in CKD patients.

METHODS AND RESULTS

Experiments were performed in primary human (HAoSMCs) and mouse (MOVAS) aortic smooth muscle cells, cholecalciferol overload and subtotal nephrectomy mouse models and serum from CKD patients. In three independent cohorts of CKD patients, serum uromodulin concentrations were inversely correlated with serum calcification propensity. Uromodulin supplementation reduced phosphate-induced osteo-/chondrogenic transdifferentiation and calcification of HAoSMCs. In human serum, pro-inflammatory cytokines tumour necrosis factor α (TNFα) and interleukin-1β (IL-1β) co-immunoprecipitated with uromodulin. Uromodulin inhibited TNFα and IL-1β-induced osteo-/chondrogenic signalling and activation of the transcription factor nuclear factor kappa-light-chain-enhancer of activated β cells (NF-kB) as well as phosphate-induced NF-kB-dependent transcriptional activity in HAoSMCs. In vivo, adeno-associated virus (AAV)-mediated overexpression of uromodulin ameliorated vascular calcification in mice with cholecalciferol overload. Conversely, cholecalciferol overload-induced vascular calcification was aggravated in uromodulin-deficient mice. In contrast, uromodulin overexpression failed to reduce vascular calcification during renal failure in mice. Carbamylated uromodulin was detected in serum of CKD patients and uromodulin carbamylation inhibited its anti-calcific properties in vitro.

CONCLUSIONS

Uromodulin counteracts vascular osteo-/chondrogenic transdifferentiation and calcification, at least in part, through interference with cytokine-dependent pro-calcific signalling. In CKD, reduction and carbamylation of uromodulin may contribute to vascular pathology.

Uromodulin: Roles in Health and Disease

Uromodulin, a protein exclusively produced by the kidney, is the most abundant urinary protein in physiological conditions. Already described several decades ago, uromodulin has gained the spotlight in recent years, since the discovery that mutations in its encoding gene UMOD cause a renal Mendelian disease (autosomal dominant tubulointerstitial kidney disease) and that common polymorphisms are associated with multifactorial disorders, such as chronic kidney disease, hypertension, and cardiovascular diseases. Moreover, variations in uromodulin levels in urine and/or blood reflect kidney functioning mass and are of prognostic value for renal function, cardiovascular events, and overall mortality. The clinical relevance of uromodulin reflects its multifunctional nature, playing a role in renal ion transport and immunomodulation, in protection against urinary tract infections and renal stones, and possibly as a systemic antioxidant. Here, we discuss the multifaceted roles of this protein in kidney physiology and its translational relevance.

Extratubular Polymerized Uromodulin Induces Leukocyte Recruitment and Inflammation In Vivo

Uromodulin (UMOD) is produced and secreted by tubular epithelial cells. Secreted UMOD polymerizes (pUMOD) in the tubular lumen, where it regulates salt transport and protects the kidney from bacteria and stone formation. Under various pathological conditions, pUMOD accumulates within the tubular lumen and reaches extratubular sites where it may interact with renal interstitial cells. Here, we investigated the potential of extratubular pUMOD to act as a damage associated molecular pattern (DAMP) molecule thereby creating local inflammation. We found that intrascrotal and intraperitoneal injection of pUMOD induced leukocyte recruitment in vivo and led to TNF-α secretion by F4/80 positive macrophages. Additionally, pUMOD directly affected vascular permeability and increased neutrophil extravasation independent of macrophage-released TNF-α. Interestingly, pUMOD displayed no chemotactic properties on neutrophils, did not directly activate β2 integrins and did not upregulate adhesion molecules on endothelial cells. In obstructed neonatal murine kidneys, we observed extratubular UMOD accumulation in the renal interstitium with tubular atrophy and leukocyte infiltrates. Finally, we found extratubular UMOD deposits associated with peritubular leukocyte infiltration in kidneys from patients with inflammatory kidney diseases. Taken together, we identified extratubular pUMOD as a strong inducer of leukocyte recruitment, underlining its critical role in mounting an inflammatory response in various kidneys pathologies.

Uromodulin (Tamm-Horsfall protein): guardian of urinary and systemic homeostasis

Biology has taught us that a protein as abundantly made and conserved among species as Tamm-Horsfall protein (THP or uromodulin) cannot just be a waste product serving no particular purpose. However, for many researchers, THP is merely a nuisance during urine proteome profiling or exosome purification and for clinicians an enigmatic entity without clear disease implications. Thanks to recent human genetic and correlative studies and animal modeling, we now have a renewed appreciation of this highly prevalent protein in not only guarding urinary homeostasis, but also serving as a critical mediator in systemic inter-organ signaling. Beyond a mere barrier that lines the tubules, or a surrogate for nephron mass, mounting evidence suggests that THP is a multifunctional protein critical for modulating renal ion channel activity, salt/water balance, renal and systemic inflammatory response, intertubular communication, mineral crystallization and bacterial adhesion. Indeed, mutations in THP cause a group of inherited kidney diseases, and altered THP expression is associated with increased risks of urinary tract infection, kidney stone, hypertension, hyperuricemia and acute and chronic kidney diseases. Despite the recent surge of information surrounding THP's physiological functions and disease involvement, our knowledge remains incomplete regarding how THP is normally regulated by external and intrinsic factors, how precisely THP deficiency leads to urinary and systemic pathophysiology and in what clinical settings THP can be used as a theranostic biomarker and a target for modulation to improve patient outcomes.

Uromodulin in mineral metabolism

PURPOSE OF REVIEW

Uromodulin (UMOD), also known as Tamm-Horsfall protein, is the most abundant protein in human urine. UMOD has multiple functions such as protection against urinary tract infections and nephrolithiasis. This review outlines recent progress made in UMOD's role in renal physiology, tubular transport, and mineral metabolism.

RECENT FINDINGS

UMOD is mostly secreted in the thick ascending limb (TAL) and to a lesser degree in the distal convoluted tubule (DCT). UMOD secretion is regulated by the calcium-sensing receptor. UMOD upregulates ion channels [e.g., renal outer medullary potassium channel, transient receptor potential cation channel subfamily V member 5, and transient receptor potential melastatin 6 (TRPM6)] and cotransporters [e.g., Na,K,2Cl cotransporter (NKCC2) and sodium-chloride cotransporter (NCC)] in the TAL and DCT. Higher serum UMOD concentrations have been associated with higher renal function and preserved renal reserve. Higher serum UMOD has also been linked to a lower risk of cardiovascular disease and diabetes mellitus.

SUMMARY

With better serum UMOD detection assays the extent of different functions for UMOD is still expanding. Urinary UMOD regulates different tubular ion channels and cotransporters. Variations of urinary UMOD secretion can so contribute to common disorders such as hypertension or nephrolithiasis.

Animal Models and Renal Biomarkers of Diabetic Nephropathy

Diabetes mellitus (DM) is the first cause of end stage chronic kidney disease (CKD). Animal models of the disease can shed light on the pathogenesis of the diabetic nephropathy (DN) and novel and earlier biomarkers of the condition may help to improve diagnosis and prognosis. This review summarizes the most important features of animal models used in the study of DN and updates the most recent progress in biomarker research.

The Urinary Excretion of Uromodulin is Regulated by the Potassium Channel ROMK

Uromodulin, the most abundant protein in normal urine, is produced by cells lining the thick ascending limb (TAL) of the loop of Henle. Uromodulin regulates the activity of the potassium channel ROMK in TAL cells. Common variants in KCNJ1, the gene encoding ROMK, are associated with urinary levels of uromodulin in population studies. Here, we investigated the functional link between ROMK and uromodulin in Kcnj1 knock-out mouse models, in primary cultures of mouse TAL (mTAL) cells, and in patients with Bartter syndrome due to KCNJ1 mutations. Both global and kidney-specific Kcnj1 knock-out mice showed reduced urinary levels of uromodulin paralleled by increased levels in the kidney, compared to wild-type controls. Pharmacological inhibition and genetic deletion of ROMK in mTAL cells caused a reduction in apical uromodulin excretion, reflected by cellular accumulation. In contrast, NKCC2 inhibition showed no effect on uromodulin processing. Patients with Bartter syndrome type 2 showed reduced urinary uromodulin levels compared to age and gender matched controls. These results demonstrate that ROMK directly regulates processing and release of uromodulin by TAL cells, independently from NKCC2. They support the functional link between transport activity and uromodulin in the TAL, relevant for blood pressure control and urinary concentrating ability.

Association of Serum Uromodulin With ESKD and Kidney Function Decline in the Elderly: The Cardiovascular Health Study

RATIONALE & OBJECTIVE

Uromodulin is released by tubular epithelial cells into the serum and lower levels are associated with more severe interstitial fibrosis and tubular atrophy. Low serum uromodulin (sUMOD) levels are associated with mortality and cardiovascular disease. However, little is known about the association of sUMOD levels with long-term kidney outcomes in older adults, a population with a high prevalence of interstitial fibrosis and tubular atrophy.

STUDY DESIGN

Case-cohort study and case-control study.

SETTING & PARTICIPANTS

Random subcohort (n=933) and additional cases of end-stage kidney disease (ESKD) and kidney function decline (≥30% decline in estimated glomerular filtration rate [eGFR]) during follow-up of the Cardiovascular Health Study (CHS).

PREDICTOR

sUMOD level.

OUTCOMES

ESKD (n=14) from the random subcohort and all additional ESKD cases from outside the random subcohort (n=39) during follow-up (10 years, case-cohort study); kidney function decline of≥30% eGFR at 9 years of follow-up in individuals with repeated eGFR assessments from the random subcohort (n=56) and additional cases (n=123). 224 participants from the random subcohort served as controls (case-control study).

ANALYTICAL APPROACH

Modified multivariable Cox regression for ESKD and multivariable logistic regression for kidney function decline. Both analyses adjusted for demographics, eGFR, urinary albumin-creatinine ratio, and other kidney disease progression risk factors.

RESULTS

Mean age of the random subcohort was 78 years, 40% were men, 15% were black. Mean sUMOD level was 127±64ng/mL and eGFR was 63±19mL/min/1.73m2. In multivariable analysis, each 1-SD higher sUMOD level was associated with 63% lower risk for ESKD (HR, 0.37; 95% CI, 0.14-0.95). In demographic-adjusted analyses of kidney function decline, each 1-SD higher sUMOD level was associated with 25% lower odds of kidney function decline (OR, 0.75; 95% CI, 0.60-0.95); after multivariable adjustment, the association was attenuated and no longer significant (OR, 0.88; 95% CI, 0.68-1.14).

LIMITATIONS

Possibility of survival bias in the kidney function decline analysis.

CONCLUSIONS

Higher sUMOD levels may identify elderly persons at reduced risk for ESKD.

TRPV5 in renal tubular calcium handling and its potential relevance for nephrolithiasis

Nephrolithiasis or renal stone disease is an increasingly common problem, and its relatively high recurrence rate demands better treatment options. The majority of patients with nephrolithiasis have stones that contain calcium (Ca²⁺), which develop upon "supersaturation" of the urine with insoluble Ca²⁺ salts; hence processes that influence the delivery and renal handling of Ca²⁺ may influence stone formation. Idiopathic hypercalciuria is indeed frequently observed in patients with kidney stones that contain Ca²⁺. Genetic screens of nephrolithiasis determinants have identified an increasing number of gene candidates, most of which are involved in renal Ca²⁺ handling. This review provides an outline of the current knowledge regarding genetics of nephrolithiasis and will mainly focus on the epithelial Ca²⁺ channel transient receptor potential vanilloid 5 (TRPV5), an important player in Ca²⁺ homeostasis. Being a member of the TRP family of ion channels, TRPV5 is currently part of a revolution in structural biology. Recent technological breakthroughs in the cryo-electron microscopy field, combined with improvements in biochemical sample preparation, have resulted in high-resolution 3-dimensional structural models of integral membrane proteins, including TRPV5. These models currently are being used to explore the proteins' structure-function relationship, elucidate the molecular mechanisms of channel regulation, and study the putative effects of disease variants. Combined with other multidisciplinary approaches, this approach may open an avenue toward better understanding of the pathophysiological mechanisms involved in hypercalciuria and stone formation, and ultimately it may facilitate prevention of stone recurrence through the development of effective drugs.

Uromodulin deficiency alters tubular injury and interstitial inflammation but not fibrosis in experimental obstructive nephropathy

Human GWAS and Mendelian genetic studies have linked polymorphic variants and mutations in the human uromodulin gene (UMOD) with chronic kidney disease. The primary function of this kidney‐specific and secreted protein remains elusive. This study investigated whether UMOD deficiency modified responses to unilateral ureteral obstruction (UUO)‐induced kidney injury. Kidneys harvested from groups of wild‐type (UMOD+/+) and knockout (UMOD−/−) male mice (n = 7–10 each) were studied on days 7, 14, and 21. Compared to sham kidneys, UMOD protein levels increased 9–13x after UUO and were associated with increased urinary UMOD levels. Kidney KIM‐1 protein levels were higher in the UMOD−/− groups at all time‐points (4–14x). The UMOD−/− groups also had higher KIM‐1 kidney‐to‐urine relative ratios (5–35x). In vitro studies using KIM‐1 expressing 769‐P cells showed lower KIM‐1 levels in the presence of UMOD protein. Levels of proapoptotic genes and the epithelial cell apoptotic protein marker M30 were significantly lower in the UMOD−/− groups. Both M30 and KIM‐1 colocalized with intraluminal UMOD protein deposits. Interstitial inflammation was less intense in the UMOD−/− groups. Renal fibrosis severity (kidney collagen mRNA and protein) was similar in both genotypic groups on days 7, 14, and 21. Our findings suggest a role for UMOD‐dependent inhibition of KIM‐1 expression and its apoptotic cell scavenging responses during chronic obstruction‐associated tubular injury.

Nephrolithiasis secondary to inherited defects in the thick ascending loop of henle and connecting tubules

Twin and genealogy studies suggest a strong genetic component of nephrolithiasis. Likewise, urinary traits associated with renal stone formation were found to be highly heritable, even after adjustment for demographic, anthropometric and dietary covariates. Recent high-throughput sequencing projects of phenotypically well-defined cohorts of stone formers and large genome-wide association studies led to the discovery of many new genes associated with kidney stones. The spectrum ranges from infrequent but highly penetrant variants (mutations) causing mendelian forms of nephrolithiasis (monogenic traits) to common but phenotypically mild variants associated with nephrolithiasis (polygenic traits). About two-thirds of the genes currently known to be associated with nephrolithiasis code for membrane proteins or enzymes involved in renal tubular transport. The thick ascending limb of Henle and connecting tubules are of paramount importance for renal water and electrolyte handling, urinary concentration and maintenance of acid-base homeostasis. In most instances, pathogenic variants in genes involved in thick ascending limb of Henle and connecting tubule function result in phenotypically severe disease, frequently accompanied by nephrocalcinosis with progressive CKD and to a variable degree by nephrolithiasis. The aim of this article is to review the current knowledge on kidney stone disease associated with inherited defects in the thick ascending loop of Henle and the connecting tubules. We also highlight recent advances in the field of kidney stone genetics that have implications beyond rare disease, offering new insights into the most common type of kidney stone disease, i.e., idiopathic calcium stone disease.

Uromodulin-related autosomal-dominant tubulointerstitial kidney disease-pathogenetic insights based on a case

Uromodulin-related autosomal-dominant tubulointerstitial kidney disease (ADTKD-UMOD) is a rare monogenic disorder that is characterized by tubulointerstitial fibrosis and progression of kidney function loss, and may progress to end-stage renal disease. It is usually accompanied by hyperuricaemia and gout. Mutations in the uromodulin gene (UMOD) resulting in malfunctioning of UMOD are known to be the cause of ADTKD-UMOD, which is assumed to be an endoplasmatic reticulum (ER) storage disease. As a case vignette, we report a 29-year-old female with a suspicious family history of chronic kidney disease presenting with progressive loss of renal function, hyperuricaemia and frequent urinary tract infections. Urinary tract infections and pyelonephritides may represent a clinical feature of uromodulin malfunction as it plays a protective role against urinary tract infections despite only sporadic data on this topic. ADTKD-UMOD was diagnosed after genetic testing revealing a missense mutation in the UMOD gene. Light microscopy showed excessive tubular interstitial fibrosis and tubular atrophy together with signs of glomerular sclerosis. Electron microscopic findings could identify electron dense storage deposits in the ER of tubular epithelial cells of the thick ascending loop. Immunohistological staining with KDEL (lysine, aspartic acid, glutamic acid, leucine) showed positivity in the tubular cells, which likely represents ER expansion upon accumulation of misfolded UMOD which could trigger the unfolded protein response and ER stress. This review highlights pathophysiological mechanisms that are subject to ADTKD-UMOD.

Development of a two-stage model system to investigate the mineralization mechanisms involved in idiopathic stone formation: stage 2 in vivo studies of stone growth on biomimetic Randall's plaque

Idiopathic stone formers often form calcium oxalate (CaOx) stones that are attached to calcium phosphate (CaP) deposits in the renal tissue, known as Randall's plaques (RP). Plaques are suggested to originate in the renal tubular basement membrane and spread into the interstitial regions where collagen fibrils and vesicles become mineralized; if the epithelium is breached, the RP becomes overgrown with CaOx upon exposure to urine. We have developed a two-stage model system of CaP-CaOx composite stones, consisting of Stage (1) CaP mineralized plaque, followed by Stage (2) CaOx overgrowth into a stone. In our first paper in this series (Stage 1), osteopontin (and polyaspartate) were found to induce a non-classical mineralization of porcine kidney tissues, producing features that resemble RP. For the Stage 2 studies presented here, biomimetic RPs from Stage 1 were implanted into the bladders of rats. Hyperoxaluria was induced with ethylene glycol for comparison to controls (water). After 4 weeks, rats were sacrificed and the implants were analyzed using electron microscopy and X-ray microanalyses. Differences in crystal phase and morphologies based upon the macromolecules present in the biomimetic plaques suggest that the plaques have the capacity to modulate the crystallization reactions. As expected, mineral overgrowths on the implants switched from CaP (water) to CaOx (hyperoxaluric). The CaOx crystals were aggregated and mixed with organic material from the biomimetic RP, along with some amorphous and spherulitic CaOx near the "stone" surfaces, which seemed to have become compact and organized towards the periphery. This system was successful at inducing "stones" more similar to human idiopathic kidney stones than other published models.

Uromodulin regulates renal magnesium homeostasis through the ion channel transient receptor potential melastatin 6 (TRPM6)

Up to 15% of the population have mild to moderate chronic hypomagnesemia, which is associated with type 2 diabetes mellitus, hypertension, metabolic syndrome, and chronic kidney disease. The kidney is the key organ for magnesium homeostasis, but our understanding of renal magnesium regulation is very limited. Uromodulin (UMOD) is the most abundant urinary protein in humans, and here we report that UMOD has a role in renal magnesium homeostasis. Umod-knockout (Umod ^-/-) mice excreted more urinary magnesium than WT mice and displayed up-regulation of genes promoting magnesium absorption. The majority of magnesium is absorbed in the thick ascending limb. However, both mouse strains responded similarly to the diuretic agent furosemide, indicating appropriate function of the thick ascending limb in the Umod ^-/- mice. Magnesium absorption is fine-tuned in the distal convoluted tubule (DCT) via the apical magnesium channel transient receptor potential melastatin 6 (TRPM6). We observed decreased apical Trpm6 staining in the DCT of Umod ^-/- mice. Applying biotinylation assays and whole-cell patch-clamp recordings, we found that UMOD enhances TRPM6 cell-surface abundance and current density from the extracellular space. UMOD physically interacted with TRPM6 and thereby impaired dynamin-dependent TRPM6 endocytosis. WT mice fed a low-magnesium diet had an increased urinary UMOD secretion compared with the same mice on a regular diet. Our results suggest that increased urinary UMOD secretion in low-magnesium states reduces TRPM6 endocytosis and thereby up-regulates TRPM6 cell-surface abundance to defend against further urinary magnesium losses.

Fibulin-7, a heparin binding matricellular protein, promotes renal tubular calcification in mice

Ectopic calcification occurs during development of chronic kidney disease and has a negative impact on long-term prognosis. The precise molecular mechanism and prevention strategies, however, are not established. Fibulin-7 (Fbln7) is a matricellular protein structurally similar to elastogenic short fibulins, shown to bind dental mesenchymal cells and heparin. Here, we report that Fbln7 is highly expressed in renal tubular epithelium in the adult kidney and mediates renal calcification in mice. In vitro analysis revealed that Fbln7 bound heparin at the N-terminal coiled-coil domain. In Fbln7-expressing CHO-K1 cells, exogenous heparin increased the release of Fbln7 into conditioned media in a dose-dependent manner. This heparin-induced Fbln7 release was abrogated in CHO-745 cells lacking heparan sulfate proteoglycan or in CHO-K1 cells expressing the Fbln7 mutant lacking the N-terminal coiled-coil domain, suggesting that Fbln7 was tethered to pericellular matrix via this domain. Interestingly, Fbln7 knockout (Fbln7^-/-) mice were protected from renal tubular calcification induced by high phosphate diet. Mechanistically, Fbln7 bound artificial calcium phosphate particles (aCPP) implicated in calcification and renal inflammation. Binding was decreased significantly in Fbln7^-/- primary kidney cells relative to wild-type cells. Further, overexpression of Fbln7 increased binding to aCPP. Addition of heparin reduced binding between aCPP and wild-type cells to levels of Fbln7^-/- cells. Taken together, our study suggests that Fbln7 is a local mediator of calcium deposition and that releasing Fbln7 from the cell surface by heparin/heparin derivatives or Fbln7 inhibitory antibodies may provide a novel strategy to prevent ectopic calcification in vivo.

Tamm-Horsfall Protein is a Potent Immunomodulatory Molecule and a Disease Biomarker in the Urinary System

Tamm–Horsfall protein (THP), or uromodulin (UMOD), is an 80–90-kDa phosphatidylinositol-anchored glycoprotein produced exclusively by the renal tubular cells in the thick ascending limb of the loop of Henle. Physiologically, THP is implicated in renal countercurrent gradient formation, sodium homeostasis, blood pressure regulation, and a defense molecule against infections in the urinary system. Investigations have also revealed that THP is an effective binding ligand for serum albumin, immunoglobulin G light chains, complement components C1 and C1q, interleukin (IL)-1β, IL-6, IL-8, tumor necrosis factor (TNF)-α, and interferon-γ through its carbohydrate side chains for maintaining circulatory and renal immune homeostasis. Thus, THP can be regarded as part of the innate immune system. UMOD mutations play crucial roles in congenital urolithiasis, hereditary hyperuricemia/gout, and medullary cystic kidney diseases. Recent investigations have focused on the immunomodulatory effects of THP on immune cells and on THP as a disease biomarker of acute and chronic kidney diseases. Our studies have suggested that normal urinary THP, through its epidermal growth factor (EGF)-like domains, binds to the surface-expressed EGF-like receptors, cathepsin G, or lactoferrin to enhance polymorphonuclear leukocyte phagocytosis, proinflammatory cytokine production by monocytes/macrophages, and lymphocyte proliferation by activating the Rho family and mitogen-activated protein kinase signaling pathways. Furthermore, our data support both an intact protein core structure and carbohydrate side chains are important for the different protein-binding capacities of THP. Prospectively, parts of the whole THP molecule may be used for anti-TNF-α therapy in inflammatory diseases, autoantibody-depleting therapy in autoimmune disorders, and immune intensification in immunocompromised hosts.

Tamm-Horsfall protein/uromodulin deficiency elicits tubular compensatory responses leading to hypertension and hyperuricemia

Expression of Tamm-Horsfall protein (THP or uromodulin) is highly restricted to the kidney thick ascending limb (TAL) of loop of Henle. Despite the unique location and recent association of THP gene mutations with hereditary uromodulin-associated kidney disease and THP single nucleotide polymorphisms with chronic kidney disease and hypertension, the physiological function(s) of THP and its pathological involvement remain incompletely understood. By studying age-dependent changes of THP knockout (KO) mice, we show here that young KO mice had significant salt and water wasting but were partially responsive to furosemide, due to decreased luminal translocation of Na-K-Cl cotransporter 2 (NKCC2) in the TAL. Aged THP KO mice were, however, markedly oliguric and unresponsive to furosemide, and their NKCC2 was localized primarily in the cytoplasm as evidenced by lipid raft floatation assay, cell fractionation, and confocal and immunoelectron microscopy. These aged KO mice responded to metolazone and acetazolamide, known to target distal and proximal tubules, respectively. They also had marked upregulation of renin in juxtaglomerular apparatus and serum, and they were hypertensive. Finally, the aged THP KO mice had significant upregulation of Na-coupled urate transporters Slc5a8 and Slc22a12 as well as sodium-hydrogen exchanger 3 (NHE3) in the proximal tubule and elevated serum uric acid and allantoin. Collectively, our results suggest that THP deficiency can cause progressive disturbances in renal functions via initially NKCC2 dysfunction and later compensatory responses, resulting in prolonged activation of the renin-angiotensin-aldosterone axis and hyperuricemia.

Anatomically-specific intratubular and interstitial biominerals in the human renal medullo-papillary complex

Limited information exists on the anatomically-specific early stage events leading to clinically detectable mineral aggregates in the renal papilla. In this study, quantitative multiscale correlative maps of structural, elemental and biochemical properties of whole medullo-papillary complexes from human kidneys were developed. Correlative maps of properties specific to the uriniferous and vascular tubules using high-resolution X-ray computed tomography, scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy, and immunolocalization of noncollagenous proteins (NCPs) along with their association with anatomy specific biominerals were obtained. Results illustrated that intratubular spherical aggregates primarily form at the proximal regions distant from the papillary tip while interstitial spherical and fibrillar aggregates are distally located near the papillary tip. Biominerals at the papillary tip were closely localized with 10 to 50 μm diameter vasa recta immunolocalized for CD31 inside the medullo-papillary complex. Abundant NCPs known to regulate bone mineralization were localized within nanoparticles, forming early pathologic mineralized regions of the complex. Based on the physical association between vascular and urothelial tubules, results from light and electron microscopy techniques suggested that these NCPs could be delivered from vasculature to prompt calcification of the interstitial regions or they might be synthesized from local vascular smooth muscle cells after transdifferentiation into osteoblast-like phenotypes. In addition, results provided insights into the plausible temporal events that link the anatomically specific intratubular mineral aggregates with the interstitial biomineralization processes within the functional unit of the kidney.

Association of urinary uromodulin with kidney function decline and mortality: the health ABC study

Background: Urine uromodulin (uUMOD) is a protein secreted by the kidney tubule. Recent studies have suggested that higher uUMOD may be associated with improved kidney and mortality outcomes. Methods: Using a case-cohort design, we evaluated the association between baseline uUMOD levels and ≥ 30% estimated glomerular filtration rate (eGFR) decline, incident chronic kidney disease (CKD), rapid kidney function decline, and mortality using standard and modified Cox proportional hazards regression. Results: The median value of uUMOD was 25.8 µg/mL, mean age of participants was 74 years, 48% were women, and 39% were black. Persons with higher uUMOD had lower prevalence of diabetes and coronary artery disease (CAD), and had lower systolic blood pressure. Persons with higher uUMOD also had higher eGFR, lower urinary albumin to creatinine ratio (ACR), and lower C-reactive protein (CRP). There was no association of uUMOD with > 30% eGFR decline. In comparison to those in the lowest quartile of uUMOD, those in the highest quartile had a significantly (53%) lower risk of incident CKD (CI 73%, 18%) and a 51% lower risk of rapid kidney function decline (CI 76%, 1%) after multivariable adjustment. Higher uUMOD was associated with lower risk of mortality in demographic adjusted models, but not after multivariable adjustment. Conclusion: Higher levels of uUMOD are associated with lower risk of incident CKD and rapid kidney function decline. Additional studies are needed in the general population and in persons with advanced CKD to confirm these findings.

Uromodulin: from physiology to rare and complex kidney disorders

Uromodulin (also known as Tamm-Horsfall protein) is exclusively produced in the kidney and is the most abundant protein in normal urine. The function of uromodulin remains elusive, but the available data suggest that this protein might regulate salt transport, protect against urinary tract infection and kidney stones, and have roles in kidney injury and innate immunity. Interest in uromodulin was boosted by genetic studies that reported involvement of the UMOD gene, which encodes uromodulin, in a spectrum of rare and common kidney diseases. Rare mutations in UMOD cause autosomal dominant tubulointerstitial kidney disease (ADTKD), which leads to chronic kidney disease (CKD). Moreover, genome-wide association studies have identified common variants in UMOD that are strongly associated with risk of CKD and also with hypertension and kidney stones in the general population. These findings have opened up a new field of kidney research. In this Review we summarize biochemical, physiological, genetic and pathological insights into the roles of uromodulin; the mechanisms by which UMOD mutations cause ADTKD, and the association of common UMOD variants with complex disorders.

Development of next generation sequencing panel for UMOD and association with kidney disease

Chronic kidney disease (CKD) has a prevalence of approximately 10% in adult populations. CKD can progress to end-stage renal disease (ESRD) and this is usually fatal unless some form of renal replacement therapy (chronic dialysis or renal transplantation) is provided. There is an inherited predisposition to CKD with several genetic risk markers now identified. The UMOD gene has been associated with CKD of varying aetiologies. An AmpliSeq next generation sequencing panel was developed to facilitate comprehensive sequencing of the UMOD gene, covering exonic and regulatory regions. SNPs and CpG sites in the genomic region encompassing UMOD were evaluated for association with CKD in two studies; the UK Wellcome Trust Case-Control 3 Renal Transplant Dysfunction Study (n = 1088) and UK-ROI GENIE GWAS (n = 1726). A technological comparison of two Ion Torrent machines revealed 100% allele call concordance between S5 XL™ and PGM™ machines. One SNP (rs183962941), located in a non-coding region of UMOD, was nominally associated with ESRD (p = 0.008). No association was identified between UMOD variants and estimated glomerular filtration rate. Analysis of methylation data for over 480,000 CpG sites revealed differential methylation patterns within UMOD, the most significant of these was cg03140788 p = 3.7 x 10-10.

A mouse model for inherited renal fibrosis associated with endoplasmic reticulum stress

Renal fibrosis is a common feature of renal failure resulting from multiple etiologies, including diabetic nephropathy, hypertension and inherited renal disorders. However, the mechanisms of renal fibrosis are incompletely understood and we therefore explored these by establishing a mouse model for a renal tubular disorder, referred to as autosomal dominant tubulointerstitial kidney disease (ADTKD) due to missense uromodulin (UMOD) mutations (ADTKD-UMOD). ADTKD-UMOD, which is associated with retention of mutant uromodulin in the endoplasmic reticulum (ER) of renal thick ascending limb cells, is characterized by hyperuricemia, interstitial fibrosis, inflammation and renal failure, and we used targeted homologous recombination to generate a knock-in mouse model with an ADTKD-causing missense cysteine to arginine uromodulin mutation (C125R). Heterozygous and homozygous mutant mice developed reduced uric acid excretion, renal fibrosis, immune cell infiltration and progressive renal failure, with decreased maturation and excretion of uromodulin, due to its retention in the ER. The ER stress marker 78 kDa glucose-regulated protein (GRP78) was elevated in cells expressing mutant uromodulin in heterozygous and homozygous mutant mice, and this was accompanied, both in vivo and ex vivo, by upregulation of two unfolded protein response pathways in primary thick ascending limb cells from homozygous mutant mice. However, this did not lead to an increase in apoptosis in vivo. Thus, we have developed a novel mouse model for renal fibrosis, which will be a valuable resource to decipher the mechanisms linking uromodulin mutations with ER stress and renal fibrosis.

Summary: A mouse model for renal fibrosis caused by uromodulin mutations reveals roles for ER stress and the unfolded protein response.

Ex Vivo Removal of Stones in Donor Kidneys by Flexible Ureteroscopy Prior to Renal Transplantation: A Case Report

Patient: Male, 33

Final Diagnosis: Donation after cardiac death (DCD)

Symptoms: None

Medication:—

Clinical Procedure: Ex vivo removal of stones in donor kidneys by flexible ureteroscop

Specialty: Transplantology

How do stones form? Is unification of theories on stone formation possible?

There are two basic pathways for formation of calcium based kidney stones. Most idiopathic calcium oxalate (CaOx) stones are formed in association with sub-epithelial plaques of calcium phosphate (CaP), known as Randall's plaques, on renal papillary surfaces. Crystal formation and retention within the terminal collecting ducts, the ducts of Bellini, leading to the formation of Randall's plugs, is the other pathway. Both pathways require supersaturation leading to crystallization, regulated by various crystallization modulators produced in response to changing urinary conditions. High supersaturation, as a result of a variety of genetic and environmental factors, leads to crystallization in the terminal collecting ducts, eventually plugging their openings into the renal pelvis. Stasis behind the plugs may lead to the formation of attached or unattached stones in the tubular lumen. Deposition of crystals on the plug surface facing the pelvic or tubular urine may result in stone formation on the Randall's plugs. Kidneys of idiopathic stone formers may be subjected to oxidative stress as a result of increased urinary excretion of calcium/oxalate/phosphate and/or decrease in the production of functional crystallization inhibitors or in relation to co-morbidities such as hypertension, atherosclerosis, or acute kidney injury. We have proposed that production of reactive oxygen species (ROS) causes dedifferentiation of epithelial/endothelial cells into osteoblast type cells and deposition of CaP in the basement membrane of renal tubules or vessels. Growth, aggregation and melding of CaP crystals leads to the formation of plaque which grows by further calcification of interstitial collagen and membranous vesicles. Plaque becomes exposed to pelvic urine once the covering papillary epithelium is breached. Surface layers of CaP are replaced by CaOx through direct transformation or demineralization of CaP and mineralization of CaOx. Alternatively, or in addition, CaOx crystals nucleate directly on the plaque surface. Stone growth may also depend upon supersaturation in the pelvic urine, triggering further nucleation, growth and aggregation.

Histological aspects of the "fixed-particle" model of stone formation: animal studies

Crystallization by itself is not harmful as long as the crystals are not retained in the kidneys and are allowed to pass freely down the renal tubules to be excreted in the urine. A number of theories have been proposed, and studies performed, to determine the mechanisms involved in crystal retention within the kidneys. It has been suggested that urinary transit through the nephron is too fast for crystals to grow large enough to be retained. Thus, free particle mechanism alone cannot lead to stone formation, and there must be a mechanism for crystal fixation within the kidneys. Animal model studies suggest that crystal retention is possible through both the free- and fixed-particle mechanisms. Crystal-cell interaction leads to pathological changes which promote crystal attachment to either epithelial cells or their basement membrane. Alternatively, crystals aggregate and produce large enough particles to block the tubules particularly at sites, where urinary flow is affected because of changes in the luminal diameter of the tubule. Crystal deposits plugging the openings of the ducts of Bellini may be the result of such a phenomenon. Intratubular crystals translocating to renal interstitium may produce osteogenic changes in the epithelial or endothelial cells resulting in the formation of the Randall's plaques. Thus, fixation appears to be either through the formation of Randall's plugs, crystal plugs clogging the openings of the ducts of Bellini or sub-epithelial crystal deposits, and the Randall's plaques.

Mucin-1 Increases Renal TRPV5 Activity In Vitro, and Urinary Level Associates with Calcium Nephrolithiasis in Patients

Hypercalciuria is a major risk factor for nephrolithiasis. We previously reported that Uromodulin (UMOD) protects against nephrolithiasis by upregulating the renal calcium channel TRPV5. This channel is crucial for calcium reabsorption in the distal convoluted tubule (DCT). Recently, mutations in the gene encoding Mucin-1 (MUC1) were found to cause autosomal dominant tubulointerstitial kidney disease, the same disease caused by UMOD mutations. Because of the similarities between UMOD and MUC1 regarding associated disease phenotype, protein structure, and function as a cellular barrier, we examined whether urinary MUC1 also enhances TRPV5 channel activity and protects against nephrolithiasis. We established a semiquantitative assay for detecting MUC1 in human urine and found that, compared with controls (n=12), patients (n=12) with hypercalciuric nephrolithiasis had significantly decreased levels of urinary MUC1. Immunofluorescence showed MUC1 in the thick ascending limb, DCT, and collecting duct. Applying whole-cell patch-clamp recording of HEK cells, we found that wild-type but not disease mutant MUC1 increased TRPV5 activity by impairing dynamin-2- and caveolin-1-mediated endocytosis of TRPV5. Coimmunoprecipitation confirmed a physical interaction between TRPV5 and MUC1. However, MUC1 did not increase the activity of N-glycan-deficient TRPV5. MUC1 is characterized by variable number tandem repeats (VNTRs) that bind the lectin galectin-3; galectin-3 siRNA but not galectin-1 siRNA prevented MUC1-induced upregulation of TRPV5 activity. Additionally, MUC1 lacking VNTRs did not increase TRPV5 activity. Our results suggest that MUC1 forms a lattice with the N-glycan of TRPV5 via galectin-3, which impairs TRPV5 endocytosis and increases urinary calcium reabsorption.

A structured interdomain linker directs self-polymerization of human uromodulin

Uromodulin (UMOD)/Tamm-Horsfall protein, the most abundant human urinary protein, plays a key role in chronic kidney diseases and is a promising therapeutic target for hypertension. Via its bipartite zona pellucida module (ZP-N/ZP-C), UMOD forms extracellular filaments that regulate kidney electrolyte balance and innate immunity, as well as protect against renal stones. Moreover, salt-dependent aggregation of UMOD filaments in the urine generates a soluble molecular net that captures uropathogenic bacteria and facilitates their clearance. Despite the functional importance of its homopolymers, no structural information is available on UMOD and how it self-assembles into filaments. Here, we report the crystal structures of polymerization regions of human UMOD and mouse ZP2, an essential sperm receptor protein that is structurally related to UMOD but forms heteropolymers. The structure of UMOD reveals that an extensive hydrophobic interface mediates ZP-N domain homodimerization. This arrangement is required for filament formation and is directed by an ordered ZP-N/ZP-C linker that is not observed in ZP2 but is conserved in the sequence of deafness/Crohn's disease-associated homopolymeric glycoproteins α-tectorin (TECTA) and glycoprotein 2 (GP2). Our data provide an example of how interdomain linker plasticity can modulate the function of structurally similar multidomain proteins. Moreover, the architecture of UMOD rationalizes numerous pathogenic mutations in both UMOD and TECTA genes.

The serine protease hepsin mediates urinary secretion and polymerisation of Zona Pellucida domain protein uromodulin

Uromodulin is the most abundant protein in the urine. It is exclusively produced by renal epithelial cells and it plays key roles in kidney function and disease. Uromodulin mainly exerts its function as an extracellular matrix whose assembly depends on a conserved, specific proteolytic cleavage leading to conformational activation of a Zona Pellucida (ZP) polymerisation domain. Through a comprehensive approach, including extensive characterisation of uromodulin processing in cellular models and in specific knock-out mice, we demonstrate that the membrane-bound serine protease hepsin is the enzyme responsible for the physiological cleavage of uromodulin. Our findings define a key aspect of uromodulin biology and identify the first in vivo substrate of hepsin. The identification of hepsin as the first protease involved in the release of a ZP domain protein is likely relevant for other members of this protein family, including several extracellular proteins, as egg coat proteins and inner ear tectorins.

DOI:http://dx.doi.org/10.7554/eLife.08887.001

Several proteins in humans and other animals contain a region called a 'zona pellucida domain'. This domain enables these proteins to associate with each other and form long filaments. Uromodulin is one such protein that was first identified more than fifty years ago. This protein is known to play a role in human diseases such as hypertension and kidney failure, but uromodulin’s biological purpose still remains elusive.

Uromodulin is only made in the kidney and it is the most abundant protein in the urine of healthy individuals. Uromodulin also contains a so-called 'external hydrophobic patch' that must be removed before the zona pellucida domain can start to form filaments. This hydrophobic patch is removed when uromodulin is cut by an unknown enzyme; this cutting releases the rest of the uromodulin protein from the surface of the cells that line the kidney into the urine.

Brunati et al. have now tested a panel of candidate enzymes and identified that one called hepsin is able to cut uromodulin. Hepsin is embedded in the cell membrane of the cells that line the kidney. When the level of hepsin was artificially reduced in cells grown in the laboratory, uromodulin remained anchored to the cell surface, its processing was altered and it did not form filaments.

Brunati et al. next analysed mice in which the gene encoding hepsin had been deleted. While these animals did not have any major defects in their internal organs, they had much lower levels of uromodulin in their urine. Furthermore, this residual urinary protein was not cut properly and it did not assemble into filaments. Thus, these findings reveal that hepsin is the enzyme that is responsible for releasing uromodulin in the urine. This discovery could be exploited to alter the levels of uromodulin release, and further studies using mice lacking hepsin may also help to understand uromodulin’s biological role. Finally, it will be important to understand if hepsin, or a similar enzyme, is also responsible for the release of other proteins containing the zona pellucida domain.

DOI:http://dx.doi.org/10.7554/eLife.08887.002