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.

The kidney releases a nonpolymerizing form of uromodulin in the urine and circulation that retains the external hydrophobic patch domain

Uromodulin [Tamm-Horsfall protein (THP)] is a glycoprotein uniquely produced in the kidney. It is released by cells of the thick ascending limbs apically in the urine and basolaterally in the renal interstitium and systemic circulation. Processing of mature urinary THP, which polymerizes into supramolecular filaments, requires cleavage of an external hydrophobic patch (EHP) at the COOH-terminus. However, THP in the circulation is not polymerized, and it remains unclear if nonaggregated forms of THP exist natively in the urine. We propose that an alternative processing path, which retains the EHP domain, can lead to a nonpolymerizing form of THP. We generated an antibody that specifically recognizes THP with retained EHP (THP + EHP) and established its presence in the urine in a nonpolymerized native state. Proteomic characterization of urinary THP + EHP revealed its COOH-terminus ending at F617. In the human kidney, THP + EHP was detected in thick ascending limb cells and less strongly in the renal parenchyma. Using immunoprecipitation followed by proteomic sequencing and immunoblot analysis, we then demonstrated that serum THP has also retained EHP. In a small cohort of patients at risk for acute kidney injury, admission urinary THP + EHP was significantly lower in patients who subsequently developed acute kidney injury during hospitalization. Our findings uncover novel insights into uromodulin biology by establishing the presence of an alternative path for cellular processing, which could explain the release of nonpolymerizing THP in the circulation. Larger studies are needed to establish the utility of urinary THP + EHP as a sensitive biomarker of kidney health and susceptibility to injury.NEW & NOTEWORTHY In this work, we discovered and characterized a novel form of uromodulin that does not polymerize because it retains an external hydrophobic patch at the COOH-terminus. These findings establish an alternative form of cellular processing of this protein and elucidate new aspects of its biology. We also provide evidence suggesting that measuring urinary nonpolymerizing uromodulin could be a promising assay to assess the risk of acute kidney injury.

Cryo-EM structure of native human uromodulin, a zona pellucida module polymer

Assembly of extracellular filaments and matrices mediating fundamental biological processes such as morphogenesis, hearing, fertilization, and antibacterial defense is driven by a ubiquitous polymerization module known as zona pellucida (ZP) "domain". Despite the conservation of this element from hydra to humans, no detailed information is available on the filamentous conformation of any ZP module protein. Here, we report a cryo-electron microscopy study of uromodulin (UMOD)/Tamm-Horsfall protein, the most abundant protein in human urine and an archetypal ZP module-containing molecule, in its mature homopolymeric state. UMOD forms a one-start helix with an unprecedented 180-degree twist between subunits enfolded by interdomain linkers that have completely reorganized as a result of propeptide dissociation. Lateral interaction between filaments in the urine generates sheets exposing a checkerboard of binding sites to capture uropathogenic bacteria, and UMOD-based models of heteromeric vertebrate egg coat filaments identify a common sperm-binding region at the interface between subunits.

The cryo-EM structure of the human uromodulin filament core reveals a unique assembly mechanism

The glycoprotein uromodulin (UMOD) is the most abundant protein in human urine and forms filamentous homopolymers that encapsulate and aggregate uropathogens, promoting pathogen clearance by urine excretion. Despite its critical role in the innate immune response against urinary tract infections, the structural basis and mechanism of UMOD polymerization remained unknown. Here, we present the cryo-EM structure of the UMOD filament core at 3.5 Å resolution, comprised of the bipartite zona pellucida (ZP) module in a helical arrangement with a rise of ~65 Å and a twist of ~180°. The immunoglobulin-like ZPN and ZPC subdomains of each monomer are separated by a long linker that interacts with the preceding ZPC and following ZPN subdomains by β-sheet complementation. The unique filament architecture suggests an assembly mechanism in which subunit incorporation could be synchronized with proteolytic cleavage of the C-terminal pro-peptide that anchors assembly-incompetent UMOD precursors to the membrane.

Management of Tamm-Horsfall Protein for Reliable Urinary Analytics

PURPOSE

Urinary extracellular vesicles (uEVs) are a novel source of biomarkers. However, urinary Tamm-Horsfall Protein (THP; uromodulin) interferes with all vesicle isolation attempts, precipitates with normal urinary proteins, thus, representing an unwanted "contaminant" in urinary assays. Thus, the aim is to develop a simple method to manage THP efficiently.

EXPERIMENTAL DESIGN

The uEVs are isolated by hydrostatic filtration dialysis (HFD) and treated with a defined solution of urea to optimize release of uEVs from sample. Presence of uEVs is confirmed by transmission electron microscopy, Western blotting, and proteomic profiling in MS.

RESULTS

Using HFD with urea treatment for uEV isolation reduces sample complexity to a great extent. The novel simplified uEV isolation protocol allows comprehensive vesicle proteomics analysis and should be part of any urine analytics to release all sample constituents from THP trap.

CONCLUSIONS AND CLINICAL RELEVANCE

The method brings a quick and easy protocol for THP management during uEV isolation, providing major benefits for comprehensive sample analytics.

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.

Functional analysis of UMOD gene and its effect on inflammatory cytokines in serum of essential hypertension patients

OBJECTIVE

The study aimed to investigate the function of uromodulin (UMOD) gene and its effect on inflammatory cytokines in serum of essential hypertension patients.

METHODS

The online database and software of computer were used for bioinformatics analysis on UMOD gene as well as the structure and function of its encoding proteins. Moreover, radioimmunoassay and enzyme linked immunosorbent assay was adopted to validate the content of urine UMOD protein of essential hypertension patients and their serum inflammatory cytokines.

RESULTS

As an alkaline and hydrophilic protein, UMOD has no transmembrane region, but it does have a signal peptide sequence. It is mainly located extracellularly, belonging to a secreted protein, whose secondary structure was based mainly on Random coil which account for 58.44%. According to function prediction, it is found that the UMOD protein has stress response which may be participate in the inflammatory reaction. It has been observed from the experiment which was designed on the basis of the correlation between inflammation reaction and essential hypertension that the content of urine UMOD protein of essential hypertension patients who is in stage I was (28.71 ± 10.53) mg/24 h and when compared with the control group's content (30.15 ± 14.10 mg/24 h), the difference was not obviously; The content of urine UMOD protein of essential hypertension patients who's in stage II and III was (18.24 ± 6.12) mg/24 h and (9.43 ± 3.16) mg/24 h, respectively, which were obviously lower than that of the control group (P<0.01). Additionally, the serum inflammatory cytokines, such as TNF-α, IL-6 and IL1-α content of essential hypertension patients were all markedly higher than that of control group (P<0.05).

CONCLUSION

For essential hypertension patients, there's a close relationship between the expression level of UMOD gene and inflammatory cytokines, which were manifested as the negative correlation between the level of the gene's expression and inflammatory cytokines. That has certain reference value to realize the targeted treatment for essential hypertension through regulated blood pressure conversely in the view of expression level of inflammatory cytokines.

[Kidney diseases associated with uromodulin (Tamm-Horsfall protein)]

Uromodulin is the most abundant protein excreted in the urine under physiological conditions. It is exclusively expressed in the kidney by epithelial cells lining the thick ascending limb of Henles loop. It is mainly localized at the apical plasma membrane of tubular cells and released through a proteolytic cleavage. Although its function is still elusive it is proposed to have a protective role against urinary tract infection and kidney stone formation, in ion transport and in kidney innate immunity. Mutations in the gene UMOD encoding uromodulin lead to rare autosomal dominant diseases, collectively referred to as uromodulin-associated kidney disease, that are characterized by progressive tubulo-interstitial damage, impaired urinary concentrating ability, hyperuricemia, and progressive renal failure. Recently, genome-wide association studies identified uromodulin as a risk factor for chronic kidney disease and hypertension. Risk variants in the UMOD gene are common in all studied populations and are associated with higher expression and urinary level of the protein.

EGF receptor-dependent mechanism may be involved in the Tamm-Horsfall glycoprotein-enhanced PMN phagocytosis via activating Rho family and MAPK signaling pathway

Our previous studies showed that urinary Tamm–Horsfall glycoprotein (THP) potently enhanced polymorphonuclear neutrophil (PMN) phagocytosis. However, the domain structure(s), signaling pathway and the intracellular events responsible for THP-enhanced PMN phagocytosis remain to be elucidated. THP was purified from normal human urine. The human promyelocytic leukemia cell line HL-60 was induced to differentiate into PMNs by all-trans retinoid acid. Pretreatment with different MAPK and PI3K inhibitors was used to delineate signaling pathways in THP-enhanced PMN phagocytosis. Phosphorylation of molecules responsible for PMN phagocytosis induced by bacterial lipopolysaccharide (LPS), THP, or human recombinant epidermal growth factor (EGF) was evaluated by western blot. A p38 MAPK inhibitor, SB203580, effectively inhibited both spontaneous and LPS- and THP-induced PMN phagocytosis. Both THP and LPS enhanced the expression of the Rho family proteins Cdc42 and Rac that may lead to F-actin re-arrangement. Further studies suggested that THP and EGF enhance PMN and differentiated HL-60 cell phagocytosis in a similar pattern. Furthermore, the EGF receptor inhibitor GW2974 significantly suppressed THP- and EGF-enhanced PMN phagocytosis and p38 and ERK1/2 phosphorylation in differentiated HL-60 cells. We conclude that EGF receptor-dependent signaling may be involved in THP-enhanced PMN phagocytosis by activating Rho family and MAP kinase.

The binding affinity and molecular basis of the structure-binding relationship between urinary Tamm-Horsfall glycoprotein and tumor necrosis factor-α

In a previous study we noted significant THP binding to TNF-α, but did not explore the molecular basis of the structure-binding relationship. In this study, we used lectin-binding ELISA to assess the carbohydrate compositions of THP, BSA, IgG, TNF-α, and IFN-g. We identified β(1,4)-N-acetylglucosamine oligomers (GlcNAc) and GlcNAc/branched mannose in BSA, IgG, TNF-α, and THP, but not in IFN-g. These carbohydrate moieties mediated binding with THP. Small amounts of Siaα(2,3)Gal/ GalNAc, Sia(2,6)Gal/GalNAc, and mannose residues were also present in THP and TNF-α. Binding affinity (K(d)) between THP and TNF-α by Scatchard plot analysis was 1.4-1.7 × 10⁻⁶ M, lower than antigen-antibody or ligand-receptor binding affinities. To elucidate the structure-binding relationship of THP-TNF-α, THP was digested with neuraminidase, β-galactosidase, O-sialoglycoprotein endopeptidase, carboxypeptidase Y, or proteinase K. β-galactosidase increased binding capacity of THP for TNF-α. Monosaccharide inhibition suggested that α-methyl-D-mannoside, GlcNAc, and GalNAc, but not sialic acid, suppress THP-TNF-α binding as detected by ELISA. We conclude that sugar-lectin and sugar-protein interactions between cognate sites in THP and TNF-α mediate their binding.

Uromodulin exclusion list improves urinary exosomal protein identification

Advances in mass spectrometry (MS) have encouraged interest in its deployment in urine biomarker studies, but success has been limited. Urine exosomes have been proposed as an ideal source of biomarkers for renal disease. However, the abundant urinary protein, uromodulin, cofractionates with exosomes during isolation and represents a practical contaminant that limits MS sensitivity. Uromodulin depletion has been attempted but is labor- and time-intensive and may remove important protein biomarkers. We describe the application of an exclusion list (ExL) of uromodulin-related peptide ions, coupled with high-sensitivity mass spectrometric analysis, to increase the depth of coverage of the urinary exosomal proteome. Urine exosomal protein samples from healthy volunteers were subjected to tandem MS and abundant uromodulin peptides identified. Samples were run for a second time, while excluding these uromodulin peptides from fragmentation to allow identification of peptides from lower-abundance proteins. Uromodulin exclusion was performed in addition to dynamic exclusion. Results from these two procedures revealed 222 distinct proteins from conventional analysis, compared with 254 proteins after uromodulin exclusion, of which 188 were common to both methods. By unmasking a previously unidentified protein set, adding the ExL increased overall protein identifications by 29.7% to a total of 288 proteins. A fixed ExL, used in combination with conventional methods, effectively increases the depth of urinary exosomal proteins identified by MS, reducing the need for uromodulin depletion.

Calcium oxalate monohydrate aggregation induced by aggregation of desialylated Tamm-Horsfall protein

Tamm-Horsfall protein (THP) is thought to protect against calcium oxalate monohydrate (COM) stone formation by inhibiting COM aggregation. Several studies reported that stone formers produce THP with reduced levels of glycosylation, particularly sialic acid levels, which leads to reduced negative charge. In this study, normal THP was treated with neuraminidase to remove sialic acid residues, confirmed by an isoelectric point shift to higher pH. COM aggregation assays revealed that desialylated THP (ds-THP) promoted COM aggregation, while normal THP inhibited aggregation. The appearance of protein aggregates in solutions at ds-THP concentrations ≥1 μg/mL in 150 mM NaCl correlated with COM aggregation promotion, implying that ds-THP aggregation induced COM aggregation. The aggregation-promoting effect of the ds-THP was independent of pH above its isoelectric point, but was substantially reduced at low ionic strength, where protein aggregation was much reduced. COM aggregation promotion was maximized at a ds-THP to COM mass ratio of ~0.025, which can be explained by a model wherein partial COM surface coverage by ds-THP aggregates promotes crystal aggregation by bridging opposing COM surfaces, whereas higher surface coverage leads to repulsion between adsorbed ds-THP aggregates. Thus, desialylation of THP apparently abrogates a normal defensive action of THP by inducing protein aggregation, and subsequently COM aggregation, a condition that favors kidney stone formation.