Renal effects of Tamm-Horsfall protein (uromodulin) deficiency in mice

Associations of Urinary Uromodulin with Clinical Characteristics and Markers of Tubular Function in the General Population

BACKGROUND AND OBJECTIVES

Allelic variants in UMOD, the gene coding for uromodulin, are associated with rare tubulointerstitial kidney disorders and risk of CKD and hypertension in the general population. The factors associated with uromodulin excretion in the normal population remain largely unknown, and were therefore explored in this study.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Urinary uromodulin excretion was measured using a validated ELISA in two population-based cohorts that included more than 6500 individuals. The Swiss Kidney Project on Genes in Hypertension study (SKIPOGH) included 817 adults (mean age±SD, 45±17 years) who underwent renal ultrasonography and performed a 24-hour urine collection. The Cohorte Lausannoise study included 5706 adults (mean age, 53±11 years) with fresh spot morning urine samples. We calculated eGFRs using the CKD-Epidemiology Collaboration formula and by 24-hour creatinine clearance.

RESULTS

In both studies, positive associations were found between uromodulin and urinary sodium, chloride, and potassium excretion and osmolality. In SKIPOGH, 24-hour uromodulin excretion (median, 41 [interquartile range, 29-57] mg/24 h) was positively associated with kidney length and volume and with creatinine excretion and urine volume. It was negatively associated with age and diabetes. Both spot uromodulin concentration and 24-hour uromodulin excretion were linearly and positively associated (multivariate analyses) with eGFR<90 ml/min per 1.73 m(2).

CONCLUSION

Age, creatinine excretion, diabetes, and urinary volume are independent clinical correlates of urinary uromodulin excretion. The associations of uromodulin excretion with markers of tubular functions and kidney dimensions suggest that it may reflect tubule activity in the general population.

Chronic hypoxia-inducible transcription factor-2 activation stably transforms juxtaglomerular renin cells into fibroblast-like cells in vivo

On the basis of previous observations that deletion of the von Hippel-Lindau protein (pVHL) in juxtaglomerular (JG) cells of the kidney suppresses renin and induces erythropoietin expression, this study aimed to characterize the events underlying this striking change of hormone expression. We found that renin cell-specific deletion of pVHL in mice leads to a phenotype switch in JG cells, from a cuboid and multiple vesicle-containing form into a flat and elongated form without vesicles. This shift of cell phenotype was accompanied by the disappearance of marker proteins for renin cells (e.g., aldo-keto reductase family 1, member 7 and connexin 40) and by the appearance of markers of fibroblast-like cells (e.g., collagen I, ecto-5'-nucleotidase, and PDGF receptor-β). Furthermore, hypoxia-inducible transcription factor-2α (HIF-2α) protein constitutively accumulated in these transformed cells. Codeletion of pVHL and HIF-2α in JG cells completely prevented the phenotypic changes. Similar to renin expression in normal JG cells, angiotensin II negatively regulated erythropoietin expression in the transformed cells. In summary, chronic activation of HIF-2 in renal JG cells leads to a reprogramming of the cells into fibroblast-like cells resembling native erythropoietin-producing cells located in the tubulointerstitium.

The signaling pathway of uromodulin and its role in kidney diseases

The uromodulin (UMOD) is a glycoprotein expressed exclusively by renal tubular cells lining the thick ascending limb of the loop of Henle. UMOD acts as a regulatory protein in health and in various conditions. For kidney diseases, its role remains elusive. On one hand, UMOD plays a role in binding and excretion of various potentially injurious products from the tubular fluid. On the other hand, chronic kidney disease is associated with higher serum levels of UMOD. Signaling pathways might be very important in the pathogenesis of kidney diseases. We performed this review to provide a relatively complete signaling pathway flowchart for UMOD to the investigators who were interested in the role of UMOD in the pathogenesis of kidney diseases. Here, we reviewed the signal transduction pathway of UMOD and its role in the pathogenesis of kidney diseases.

Validation of Uromodulin as a Candidate Gene for Human Essential Hypertension

A recent genome-wide association study identified a locus on chromosome 16 in the promoter region of the uromodulin ( UMOD ) gene that is associated with hypertension. Here, we examined the hypertension signal with functional studies in Umod knockout (KO) mice. Systolic blood pressure was significantly lower in KO versus wild-type (WT) mice under basal conditions (KO: 116.6±0.3 mm Hg versus WT: 136.2±0.4 mm Hg; P <0.0001). Administration of 2% NaCl did not alter systolic blood pressure in KO mice, whereas it increased in WT mice by ≈33%, P <0.001. The average 24-hour urinary sodium excretion in the KO was greater than that of WT mice ( P <0.001). Chronic renal function curves demonstrate a leftward shift in KO mice, suggesting that the relationship between UMOD and blood pressure is affected by sodium. Creatinine clearance was increased during salt loading with 2% NaCl in the KO mice, leading to augmented filtered Na + excretion and further Na + loss. The difference in sodium uptake that exists between WT and KO strains was explored at the molecular level. Urinary tumor necrosis factor-α levels were significantly higher in KO mice compared with WT mice ( P <0.0001). Stimulation of primary thick ascending limb of the loop of Henle cells with exogenous tumor necrosis factor-α caused a reduction in NKCC2A expression ( P <0.001) with a concurrent rise in the levels of UMOD mRNA ( P <0.001). Collectively, we demonstrate that UMOD regulates sodium uptake in the thick ascending limb of the loop of Henle by modulating the effect of tumor necrosis factor-α on NKCC2A expression, making UMOD an important determinant of blood pressure control.

Common noncoding UMOD gene variants induce salt-sensitive hypertension and kidney damage by increasing uromodulin expression

Hypertension and chronic kidney disease (CKD) are complex traits representing major global health problems. Multiple genome-wide association studies have identified common variants in the promoter of the UMOD gene, which encodes uromodulin, the major protein secreted in normal urine, that cause independent susceptibility to CKD and hypertension. Despite compelling genetic evidence for the association between UMOD risk variants and disease susceptibility in the general population, the underlying biological mechanism is not understood. Here, we demonstrate that UMOD risk variants increased UMOD expression in vitro and in vivo. Uromodulin overexpression in transgenic mice led to salt-sensitive hypertension and to the presence of age-dependent renal lesions similar to those observed in elderly individuals homozygous for UMOD promoter risk variants. The link between uromodulin and hypertension is due to activation of the renal sodium cotransporter NKCC2. We demonstrated the relevance of this mechanism in humans by showing that pharmacological inhibition of NKCC2 was more effective in lowering blood pressure in hypertensive patients who are homozygous for UMOD promoter risk variants than in other hypertensive patients. Our findings link genetic susceptibility to hypertension and CKD to the level of uromodulin expression and uromodulin's effect on salt reabsorption in the kidney. These findings point to uromodulin as a therapeutic target for lowering blood pressure and preserving renal function.

Uriniferous tubule: structural and functional organization

The uriniferous tubule is divided into the proximal tubule, the intermediate (thin) tubule, the distal tubule and the collecting duct. The present chapter is based on the chapters by Maunsbach and Christensen on the proximal tubule, and by Kaissling and Kriz on the distal tubule and collecting duct in the 1992 edition of the Handbook of Physiology, Renal Physiology. It describes the fine structure (light and electron microscopy) of the entire mammalian uriniferous tubule, mainly in rats, mice, and rabbits. The structural data are complemented by recent data on the location of the major transport- and transport-regulating proteins, revealed by morphological means(immunohistochemistry, immunofluorescence, and/or mRNA in situ hybridization). The structural differences along the uriniferous tubule strictly coincide with the distribution of the major luminal and basolateral transport proteins and receptors and both together provide the basis for the subdivision of the uriniferous tubule into functional subunits. Data on structural adaptation to defined functional changes in vivo and to genetical alterations of specified proteins involved in transepithelial transport importantly deepen our comprehension of the correlation of structure and function in the kidney, of the role of each segment or cell type in the overall renal function,and our understanding of renal pathophysiology.

Common noncoding UMOD gene variants induce salt-sensitive hypertension and kidney damage by increasing uromodulin expression

Hypertension and chronic kidney disease (CKD) are complex traits representing major global health problems. Multiple genome-wide association studies have identified common variants in the promoter of the UMOD gene, which encodes uromodulin, the major protein secreted in normal urine, that cause independent susceptibility to CKD and hypertension. Despite compelling genetic evidence for the association between UMOD risk variants and disease susceptibility in the general population, the underlying biological mechanism is not understood. Here, we demonstrate that UMOD risk variants increased UMOD expression in vitro and in vivo. Uromodulin overexpression in transgenic mice led to salt-sensitive hypertension and to the presence of age-dependent renal lesions similar to those observed in elderly individuals homozygous for UMOD promoter risk variants. The link between uromodulin and hypertension is due to activation of the renal sodium cotransporter NKCC2. We demonstrated the relevance of this mechanism in humans by showing that pharmacological inhibition of NKCC2 was more effective in lowering blood pressure in hypertensive patients who are homozygous for UMOD promoter risk variants than in other hypertensive patients. Our findings link genetic susceptibility to hypertension and CKD to the level of uromodulin expression and uromodulin's effect on salt reabsorption in the kidney. These findings point to uromodulin as a therapeutic target for lowering blood pressure and preserving renal function.

Standardized, systemic phenotypic analysis of Umod(C93F) and Umod(A227T) mutant mice

Uromodulin-associated kidney disease (UAKD) summarizes different clinical features of an autosomal dominant heritable disease syndrome in humans with a proven uromodulin (UMOD) mutation involved. It is often characterized by hyperuricemia, gout, alteration of urine concentrating ability, as well as a variable rate of disease progression inconstantly leading to renal failure and histological alterations of the kidneys. We recently established the two Umod mutant mouse lines Umod(C93F) and Umod(A227T) on the C3H inbred genetic background both showing kidney defects analogous to those found in human UAKD patients. In addition, disease symptoms were revealed that were not yet described in other published mouse models of UAKD. To examine if further organ systems and/or metabolic pathways are affected by Umod mutations as primary or secondary effects, we describe a standardized, systemic phenotypic analysis of the two mutant mouse lines Umod(A227T) and Umod(C93F) in the German Mouse Clinic. Different genotypes as well as different ages were tested. Beside the already published changes in body weight, body composition and bone metabolism, the influence of the Umod mutation on energy metabolism was confirmed. Hematological analysis revealed a moderate microcytic and erythropenic anemia in older Umod mutant mice. Data of the other analyses in 7-10 month-old mutant mice showed single small additional effects.

Type of uromodulin mutation and allelic status influence onset and severity of uromodulin-associated kidney disease in mice

Uromodulin-associated kidney disease (UAKD) is a dominant heritable renal disease in humans which is caused by mutations in the uromodulin (UMOD) gene and characterized by heterogeneous clinical appearance. To get insights into possible causes of this heterogeneity of UAKD, we describe the new mutant mouse line Umod(C93F), leading to disruption of a putative disulfide bond which is also absent in a known human UMOD mutation, and compare the phenotype of this new mouse line with the recently published mouse line Umod(A227T). In both mutant mouse lines, which were both bred on the C3H background, the Umod mutations cause a gain-of-toxic function due to a maturation defect of the mutant uromodulin leading to a dysfunction of thick ascending limb of Henle's loop (TALH) cells of the kidney. Umod mutant mice exhibit increased plasma urea and Cystatin levels, impaired urinary concentration ability, reduced fractional excretion of uric acid and nephropathological alterations including uromodulin retention in TALH cells, interstitial fibrosis and inflammatory cell infiltrations, tubular atrophy and occasional glomerulo- und tubulocystic changes, a phenotype highly similar to UAKD in humans. The maturation defect of mutant uromodulin leads to the accumulation of immature uromodulin in the endoplasmic reticulum (ER) and to ER hyperplasia. Further, this study was able to demonstrate for the first time in vivo that the severity of the uromodulin maturation defect as well as onset and speed of progression of renal dysfunction and morphological alterations are strongly dependent on the particular Umod mutation itself and the zygosity status.

Association between genotype and phenotype in uromodulin-associated kidney disease

BACKGROUND AND OBJECTIVES

Uromodulin-associated kidney disease (UAKD) is an autosomal dominant disease caused by uromodulin (UMOD) gene mutations. This study explored genotype-phenotype correlations by examining the relationship between the type of UMOD mutation and the age at onset of ESRD.

DESIGN, SETTING, PARTICIPANTS & MEASUREMENTS

Extensive bibliographic research was used to ascertain patient-level data of all patients with UAKD published up to October 2011. Data included sex; ages at onset of hyperuricemia, gout, and ESRD; and UMOD genotype. Kaplan-Meier analysis and Cox proportional hazards models fitted with shared gamma frailty terms to adjust for within-family correlations were used to model time to event.

RESULTS

Thirty-one peer-reviewed publications reporting on 202 patients from 74 families with 59 different UMOD mutations were included. Median ages at onset of hyperuricemia, gout, and ESRD were 24, 40, and 56 years, respectively. Men developed gout and ESRD significantly earlier than did women (age at ESRD was 50 years for men and 60 for women; P=0.04, shared frailty model). Median ages at ESRD development were lowest with Cys77Tyr (37.5 years) and highest with Gln316Pro (65.5 years) UMOD mutations. Onset of ESRD was significantly earlier with UMOD mutations located within the epidermal growth factor domains 2 and 3 (range, 45-52 years; P<0.01 and 0.04, respectively) compared with the cysteine-rich domains (range, 60-65 years; by shared frailty model).

CONCLUSIONS

The UMOD genotype is related to the clinical phenotype of UAKD. This finding may assist in counseling of patients.

Deletion of von Hippel-Lindau protein converts renin-producing cells into erythropoietin-producing cells

States of low perfusion pressure of the kidney associate with hyperplasia or expansion of renin-producing cells, but it is unknown whether hypoxia-triggered genes contribute to these changes. Here, we stabilized hypoxia-inducible transcription factors (HIFs) in mice by conditionally deleting their negative regulator, Vhl, using the Cre/loxP system with renin-1d promoter-driven Cre expression. Vhl (−/−(REN)) mice were viable and had normal BP. Deletion of Vhl resulted in constitutive accumulation of HIF-2α in afferent arterioles and glomerular cells and HIF-1α in collecting duct cells of the adult kidney. The preglomerular vascular tree developed normally, but far fewer renin-expressing cells were present, with more than 70% of glomeruli not containing renin cells at the typical juxtaglomerular position. Moreover, these mice had an attenuated expansion of renin-producing cells in response to a low-salt diet combined with an ACE inhibitor. However, renin-producing cells of Vhl (−/−(REN)) mice expressed the erythropoietin gene, and they were markedly polycythemic. Taken together, these results suggest that hypoxia-inducible genes, regulated by VHL, are essential for normal development and physiologic adaptation of renin-producing cells. In addition, deletion of Vhl shifts the phenotype of juxtaglomerular cells from a renin- to erythropoietin-secreting cell type, presumably in response to HIF-2 accumulation.

Annexin A1 modulates macula densa function by inhibiting cyclooxygenase 2

Annexin A1 (ANXA1) exerts anti-inflammatory effects through multiple mechanisms including inhibition of prostaglandin synthesis. Once secreted, ANXA1 can bind to G protein-coupled formyl peptide receptors (Fpr) and activate diverse cellular signaling pathways. ANXA1 is known to be expressed in cells of the juxtaglomerular apparatus, but its relation to the expression of cyclooxygenase 2 (COX-2) in thick ascending limb and macula densa cells has not been elucidated. We hypothesized that ANXA1 regulates the biosynthesis of COX-2. ANXA1 abundance in rat kidney macula densa was extensively colocalized with COX-2 (95%). Furosemide, an established stimulus for COX-2 induction, caused enhanced expression of both ANXA1 and COX-2 with maintained colocalization (99%). In ANXA1-deficient mice, COX-2-positive cells were more numerous than in control mice (+107%; normalized to glomerular number; P < 0.05) and renin expression was increased (+566%; normalized to glomerular number; P < 0.05). Cultured macula densa cells transfected with full-length rat ANXA1 revealed downregulation of COX-2 mRNA (−59%; P < 0.05). Similarly, treatment with dexamethasone suppressed COX-2 mRNA in the cells (−49%; P < 0.05), while inducing ANXA1 mRNA (+56%; P < 0.05) and ANXA1 protein secretion. Inhibition of the ANXA-1 receptor Fpr1 with cyclosporin H blunted the effect of dexamethasone on COX-2 expression. These data show that ANXA1 exerts an inhibitory effect on COX-2 expression in the macula densa. ANXA1 may be a novel intrinsic modulator of renal juxtaglomerular regulation by inhibition of PGE2 synthesis.

Genetics and hypertension: is it time to change my practice?

Recent advances in genotyping technology and in particular a number of large-scale genome-wide association studies have helped to unravel the genetic basis of hypertension. Although our knowledge is still far from being complete it is important to ask how genetic findings could be translated to clinical practice. In a first step we summarize the strategies to dissect the genetics of hypertension from candidate gene studies to genome-wide association studies and recent sequencing experiments. The greatest hope in this context is the development of new drugs that are based on newly discovered pathophysiological principles. We describe examples where significant therapeutic effects are achieved with agents targeting pathways that contribute only small amounts to the genetic variability of a phenotype. There are good reasons to believe that new drugs will be developed based on genetic data in hypertension. We also highlight the potential for pharmacogenetics and risk stratification. The former is not currently supported by a larger body of evidence, but well designed studies are under way. The latter needs to follow the same principles for evaluation of other novel biomarkers of cardiovascular risk and is unlikely to influence clinical practice in the next few years.

Tamm-Horsfall protein regulates circulating and renal cytokines by affecting glomerular filtration rate and acting as a urinary cytokine trap

Although few organ systems play a more important role than the kidneys in cytokine catabolism, the mechanism(s) regulating this pivotal physiological function and how its deficiency affects systemic cytokine homeostasis remain unclear. Here we show that elimination of Tamm-Horsfall protein (THP) expression from mouse kidneys caused a marked elevation of circulating IFN-γ, IL1α, TNF-α, IL6, CXCL1, and IL13. Accompanying this were enlarged spleens with prominent white-pulp macrophage infiltration. Lipopolysaccharide (LPS) exacerbated the increase of serum cytokines without a corresponding increase in their urinary excretion in THP knock-out (KO) mice. This, along with the rise of serum cystatin C and the reduced inulin and creatinine clearance from the circulation, suggested that diminished glomerular filtration may contribute to reduced cytokine clearance in THP KO mice both at the baseline and under stress. Unlike wild-type mice where renal and urinary cytokines formed specific in vivo complexes with THP, this "trapping" effect was absent in THP KO mice, thus explaining why cytokine signaling pathways were activated in renal epithelial cells in such mice. Our study provides new evidence implicating an important role of THP in influencing cytokine clearance and acting as a decoy receptor for urinary cytokines. Based on these and other data, we present a unifying model that underscores the role of THP as a major regulator of renal and systemic immunity.

Na+-K+-2Cl- cotransporter type 2 trafficking and activity: the role of interacting proteins

The central role of Na+-K+-2Cl- cotransporter type 2 (NKCC2) in vectorial transepithelial salt reabsorption in thick ascending limb cells from Henle's loop in the kidney is evidenced by the effects of loop diuretics, the pharmacological inhibitors of NKCC2, that are amongst the most powerful antihypertensive drugs available to date. Moreover, genetic mutations of the NKCC2 encoding gene resulting in impaired apical targeting and function of NKCC2 transporter give rise to a pathological phenotype known as type I Bartter syndrome, characterised by a severe volume depletion, hypokalaemia and metabolic alkalosis with high prenatal mortality. On the contrary, excessive NKCC2 activity has been linked with inherited hypertension in humans and in rodent models. Interestingly, in animal models of hypertension, NKCC2 upregulation is achieved by post-translational mechanisms underlining the need to analyse the molecular mechanisms involved in the regulation of NKCC2 trafficking and activity to gain insights in the pathogenesis of hypertension.

Uromodulin in kidney injury: an instigator, bystander, or protector?

Uromodulin, also known as Tamm-Horsfall protein, is a glycoprotein expressed exclusively by renal tubular cells lining the thick ascending limb of the loop of Henle. Although the physiologic functions of this protein remain elusive, significant progress has been made during the last decade that highlights the importance of uromodulin in the pathophysiology of various diseases, such as medullary cystic kidney disease, urinary tract infections, and nephrolithiasis. Meanwhile, there is renewed interest in the role of uromodulin in kidney injury, both acute and chronic. In this article, we review the existing evidence that supports a role for uromodulin in acute kidney injury, chronic kidney disease, and renal inflammation. Contrary to the conventional view of uromodulin as an instigator in kidney injury, new data from uromodulin knockout mice show a protective role for this protein in acute kidney injury, possibly through downregulating interstitial inflammation. In chronic kidney disease, uromodulin excretion, when adjusted for kidney function, is increased; the significance of this is unclear. Although it has been suggested that uromodulin exacerbates progressive kidney injury, we propose that the elevation in uromodulin secretion is instead reactive to injury and reflects an increase of uromodulin in the renal parenchyma, where it slows the injury process.

[Familial juvenile hyperuricemic nephropathy]

Familial juvenile hyperuricemic nephropathy is a rare autosomal dominant disease. It is characterized by abnormal handling of urate responsible for hyperuricaemia often complicated of gouty arthritis. Renal failure is due to tubulointerstitial nephritis. Ultrasonography sometimes finds renal cysts of variable size and number. Renal histology, although not specific, shows interstitial fibrosis, atrophic tubules, sometimes enlarged and with irregular membrane thickening. Renal failure progresses to end stage between 30 and 60 years of age. Allopurinol treatment is recommended at the early stages of the disease, its efficacy on slowing down the progression of the disease is however not proven. There is genetic heterogeneity in familial juvenile hyperuricemic nephropathy. Uromodulin encoding Tamm-Horsfall protein is the only gene to date identified, responsible in less than half of the families. The described mutations most often concern a cystein and are clustering in exon 4. These mutations result in abnormal retention of the protein in endoplasmic reticulum of Henle loop cells and in reduction of its urinary excretion. The pathophysiology of the disease is however still dubious. Indeed, Tamm-Horsfall protein functions are not well known (anti-infectious role, cristallisation inhibition, immunomodulating role). Knock-out mice do not develop renal phenotype but are more prone to E. coli urinary infections. Uromodulin gene mutations have also been described in medullary cystic kidney disease, an autosomal dominant tubulointerstitial nephropathy, considered at first as a distinct disorder. Genetic progress allowed us to consider familial juvenile hyperuricemic nephropathy and medullary cystic kidney disease as the two facets of a same disease, we should call uromodulin associated kidney diseases. At least two other genes have been implicated in similar clinical presentation: TCF2 and the gene encoding renin.

Exosomes and the kidney: prospects for diagnosis and therapy of renal diseases

Exosomes are 40-100 nm membrane vesicles secreted into the extracellular space by numerous cell types. These structures can be isolated from body fluids including urine and plasma. Exosomes contain proteins, mRNAs, miRNAs, and signaling molecules that reflect the physiological state of their cells of origin and consequently provide a rich source of potential biomarker molecules. Aside from diagnostic uses, exosome-mediated transfer of proteins, mRNAs, miRNAs, and signaling molecules offer the promise that they may be used for therapeutic purposes. In this review, we integrate new knowledge about exosomes from outside the field of nephrology with recent progress by renal researchers in order to provide a basis for speculation about how the study of exosomes may affect the fields of nephrology and renal physiology in the next few years.

Diagnostic value, clinical utility and pathogenic significance of reactivity to the molecular targets of Crohn's disease specific-pancreatic autoantibodies

Pancreatic autoantibodies (PAB) giving characteristic staining patterns of the exocrine pancreas by indirect immunoflourescence appear to be specific markers of Crohn's disease (CrD), being present in approximately 30% of patients with CrD and in less than 5% of patients with ulcerative colitis (UC). Some studies have suggested that PAB are associated with specific disease phenotypes and that their detection may be of clinical significance. Thorough investigation of the role of PAB in the immunopathogenesis of inflammatory bowel diseases (IBD) has been hampered due to the lack of identity of their antigenic targets. The recent identification of the pancreatic zymogen granule protein 2 (GP2) as the major target of PAB has led to the development of an enzyme immunoassay that helps determine the diagnostic and clinical relevance of antigen-specific immune responses. Recent studies have demonstrated that GP2 is expressed on the apical surface of intestinal membranous cells of the follicle-associated epithelium, and is essential for host-microbial interaction and the initiation of bacteria-specific mucosal immune responses. This review critically discusses recent reports investigating the diagnostic and clinical utility of GP2-specific autoantibody responses in patients with IBD. Hints towards a better understanding of the immunogenicity of GP2 are also provided.

Molecular regulation of NKCC2 in the thick ascending limb

The kidney plays an essential role in blood pressure regulation by controlling short-term and long-term NaCl and water balance. The thick ascending limb of the loop of Henle (TAL) reabsorbs 25-30% of the NaCl filtered by the glomeruli in a process mediated by the apical Na(+)-K(+)-2Cl(-) cotransporter NKCC2, which allows Na(+) and Cl(-) entry from the tubule lumen into TAL cells. In humans, mutations in the gene coding for NKCC2 result in decreased or absent activity characterized by severe salt and volume loss and decreased blood pressure (Bartter syndrome type 1). Opposite to Bartter's syndrome, enhanced NaCl absorption by the TAL is associated with human hypertension and animal models of salt-sensitive hypertension. TAL NaCl reabsorption is subject to exquisite control by hormones like vasopressin, parathyroid, glucagon, and adrenergic agonists (epinephrine and norepinephrine) that stimulate NaCl reabsorption. Atrial natriuretic peptides or autacoids like nitric oxide and prostaglandins inhibit NaCl reabsorption, promoting salt excretion. In general, the mechanism by which hormones control NaCl reabsorption is mediated directly or indirectly by altering the activity of NKCC2 in the TAL. Despite the importance of NKCC2 in renal physiology, the molecular mechanisms by which hormones, autacoids, physical factors, and intracellular ions regulate NKCC2 activity are largely unknown. During the last 5 years, it has become apparent that at least three molecular mechanisms determine NKCC2 activity. As such, membrane trafficking, phosphorylation, and protein-protein interactions have recently been described in TALs and heterologous expression systems as mechanisms that modulate NKCC2 activity. The focus of this review is to summarize recent data regarding NKCC2 regulation and discuss their potential implications in physiological control of TAL function, renal physiology, and blood pressure regulation.

Activation of the bumetanide-sensitive Na+,K+,2Cl- cotransporter (NKCC2) is facilitated by Tamm-Horsfall protein in a chloride-sensitive manner

Active transport of NaCl across thick ascending limb (TAL) epithelium is accomplished by Na(+),K(+),2Cl(-) cotransporter (NKCC2). The activity of NKCC2 is determined by vasopressin (AVP) or intracellular chloride concentration and includes its amino-terminal phosphorylation. Co-expressed Tamm-Horsfall protein (THP) has been proposed to interact with NKCC2. We hypothesized that THP modulates NKCC2 activity in TAL. THP-deficient mice (THP(-/-)) showed an increased abundance of intracellular NKCC2 located in subapical vesicles (+47% compared with wild type (WT) mice), whereas base-line phosphorylation of NKCC2 was significantly decreased (-49% compared with WT mice), suggesting reduced activity of the transporter in the absence of THP. Cultured TAL cells with low endogenous THP levels and low base-line phosphorylation of NKCC2 displayed sharp increases in NKCC2 phosphorylation (+38%) along with a significant change of intracellular chloride concentration upon transfection with THP. In NKCC2-expressing frog oocytes, co-injection with THP cRNA significantly enhanced the activation of NKCC2 under low chloride hypotonic stress (+112% versus +235%). Short term (30 min) stimulation of the vasopressin V2 receptor pathway by V2 receptor agonist (deamino-cis-D-Arg vasopressin) resulted in enhanced NKCC2 phosphorylation in WT mice and cultured TAL cells transfected with THP, whereas in the absence of THP, NKCC2 phosphorylation upon deamino-cis-D-Arg vasopressin was blunted in both systems. Attenuated effects of furosemide along with functional and structural adaptation of the distal convoluted tubule in THP(-/-) mice supported the notion that NaCl reabsorption was impaired in TAL lacking THP. In summary, these results are compatible with a permissive role for THP in the modulation of NKCC2-dependent TAL salt reabsorptive function.

The rediscovery of uromodulin (Tamm-Horsfall protein): from tubulointerstitial nephropathy to chronic kidney disease

Uromodulin (Tamm-Horsfall protein) is the most abundant protein excreted in the urine under physiological conditions. It is exclusively produced in the kidney and secreted into the urine via proteolytic cleavage. Its biological function is still not fully understood. Uromodulin has been linked to water/electrolyte balance and to kidney innate immunity. Also, studies in knockout mice demonstrated that it has a protective role against urinary tract infections and renal stone formation. Mutations in the gene encoding uromodulin lead to rare autosomal dominant diseases, collectively referred to as uromodulin-associated kidney diseases. They are characterized by progressive tubulointerstitial damage, impaired urinary concentrating ability, hyperuricemia, renal cysts, and progressive renal failure. Novel in vivo studies point at intracellular accumulation of mutant uromodulin as a key primary event in the disease pathogenesis. Recently, genome-wide association studies identified uromodulin as a risk factor for chronic kidney disease (CKD) and hypertension, and suggested that the level of uromodulin in the urine could represent a useful biomarker for the development of CKD. In this review, we summarize these recent investigations, ranging from invalidation studies in mouse to Mendelian disorders and genome-wide associations, which led to a rediscovery of uromodulin and boosted the scientific and clinical interest for this long discovered molecule.

Tamm-Horsfall glycoprotein interacts with renal outer medullary potassium channel ROMK2 and regulates its function

Tamm-Horsfall glycoprotein (THGP) or Uromodulin is a membrane protein exclusively expressed along the thick ascending limb (TAL) and early distal convoluted tubule (DCT) of the nephron. Mutations in the THGP encoding gene result in Familial Juvenile Hyperuricemic Nephropathy (FJHN), Medullary Cystic Kidney Disease type 2 (MCKD-2), and Glomerulocystic Kidney Disease (GCKD). The physicochemical and biological properties of THGP have been studied extensively, but its physiological function in the TAL remains obscure. We performed yeast two-hybrid screening employing a human kidney cDNA library and identified THGP as a potential interaction partner of the renal outer medullary potassium channel (ROMK2), a key player in the process of salt reabsorption along the TAL. Functional analysis by electrophysiological techniques in Xenopus oocytes showed a strong increase in ROMK current amplitudes when co-expressed with THGP. The effect of THGP was specific for ROMK2 and did not influence current amplitudes upon co-expression with Kir2.x, inward rectifier potassium channels related to ROMK. Single channel conductance and open probability of ROMK2 were not altered by co-expression of THGP, which instead increased surface expression of ROMK2 as determined by patch clamp analysis and luminometric surface quantification, respectively. Despite preserved interaction with ROMK2, disease-causing THGP mutants failed to increase its current amplitude and surface expression. THGP(-/-) mice exhibited increased ROMK accumulation in intracellular vesicular compartments when compared with WT animals. Therefore, THGP modulation of ROMK function confers a new role of THGP on renal ion transport and may contribute to salt wasting observed in FJHN/MCKD-2/GCKD patients.

Advances in membranous vesicle and exosome proteomics improving biological understanding and biomarker discovery

Exosomes are membranous vesicles released by cells in extracellular fluids: they have been found and analyzed in blood, urine, amniotic fluid, breast milk, seminal fluid, saliva and malignant effusions, besides conditioned media from different cell lines. Several recent papers show that exosome proteomes of different origin include both a common set of membrane and cytosolic proteins, and specific subsets of proteins, likely correlated to cell-type associated functions. This is particularly interesting in relation to their possible involvement in human diseases. The knowledge of exosome proteomics can help not only in understanding their biological roles but also in supplying new biomarkers to be searched for in patients' fluids. This review offers an overview of technical and analytical issues in exosome proteomics, and it highlights the significance of proteomic studies in terms of biological and clinical usefulness.

Tamm-Horsfall protein-deficient thick ascending limbs promote injury to neighboring S3 segments in an MIP-2-dependent mechanism

Tamm-Horsfall protein (THP) is a glycoprotein expressed exclusively in thick ascending limbs (TAL) of the kidney. We recently described a novel protective role of THP against acute kidney injury (AKI) via downregulation of inflammation in the outer medulla. Our current study investigates the mechanistic relationships among the status of THP, inflammation, and tubular injury. Using an ischemia-reperfusion model in wild-type and THP-/- mice, we demonstrate that it is the S3 proximal segments but not the THP-deficient TAL that are the main targets of tubular injury during AKI. The injured S3 segments that are surrounded by neutrophils in THP-/- mice have marked overexpression of neutrophil chemoattractant MIP-2 compared with wild-type counterparts. Neutralizing macrophage inflammatory protein-2 (MIP-2) antibody rescues S3 segments from injury, decreases neutrophil infiltration, and improves kidney function in THP-/- mice. Furthermore, using immunofluorescence volumetric imaging of wild-type mouse kidneys, we show that ischemia alters the intracellular translocation of THP in the TAL cells by partially shifting it from its default apical surface domain to the basolateral domain, the latter being contiguous to the basolateral surface of S3 segments. Concomitant with this is the upregulation, in the basolateral surface of S3 segments, of the scavenger receptor SRB-1, a putative receptor for THP. We conclude that TAL affects the susceptibility of S3 segments to injury at least in part by regulating MIP-2 expression in a THP-dependent manner. Our findings raise the interesting possibility of a direct role of basolaterally released THP on regulating inflammation in S3 segments.

Involvement of urinary proteins in the rat strain difference in sensitivity to ethylene glycol-induced renal toxicity

Ethylene glycol (EG) exposure is a common model for kidney stones, because animals accumulate calcium oxalate monohydrate (COM) in kidneys. Wistar rats are more sensitive to EG than Fischer 344 (F344) rats, with greater COM deposition in kidneys. The mechanisms by which COM accumulates differently among strains are poorly understood. Urinary proteins inhibit COM adhesion to renal cells, which could alter COM deposition in kidneys. We hypothesize that COM accumulates more in Wistar rat kidneys because of lower levels of inhibitory proteins in urine. Wistar and F344 rats were treated with 0.75% EG in drinking water for 8 wk. Twenty-four-hour urine was collected every 2 wk for analysis of urinary proteins. Similar studies were conducted for 2 wk using 2% hydroxyproline (HP) as an alternative oxalate source. Total urinary protein was higher in F344 than Wistar rats at all times. Tamm-Horsfall protein was not different between strains. Osteopontin (OPN) levels in Wistar urine and kidney tissue were higher and were further increased by EG treatment. This increase in OPN occurred before renal COM accumulation. Untreated F344 rats showed greater CD45 and ED-1 staining in kidneys than untreated Wistars; in contrast, EG treatment increased CD45 and ED-1 staining in Wistars more than in F344 rats, indicating macrophage infiltration. This increase occurred in parallel with the increase in OPN and before COM accumulation. Like EG, HP induced markedly greater oxalate concentrations in the plasma and urine of Wistar rats compared with F344 rats. These results suggest that OPN upregulation and macrophage infiltration do not completely protect against COM accumulation and may be a response to crystal retention. Because the two oxalate precursors, EG and HP, produced similar elevations of oxalate, the strain difference in COM accumulation may result more so from metabolic differences between strains than from differences in urinary proteins or inflammatory responses.

Association of variants at UMOD with chronic kidney disease and kidney stones-role of age and comorbid diseases

Chronic kidney disease (CKD) is a worldwide public health problem that is associated with substantial morbidity and mortality. To search for sequence variants that associate with CKD, we conducted a genome-wide association study (GWAS) that included a total of 3,203 Icelandic cases and 38,782 controls. We observed an association between CKD and a variant with 80% population frequency, rs4293393-T, positioned next to the UMOD gene (GeneID: 7369) on chromosome 16p12 (OR = 1.25, P = 4.1×10⁻¹⁰). This gene encodes uromodulin (Tamm-Horsfall protein), the most abundant protein in mammalian urine. The variant also associates significantly with serum creatinine concentration (SCr) in Icelandic subjects (N = 24,635, P = 1.3×10⁻²³) but not in a smaller set of healthy Dutch controls (N = 1,819, P = 0.39). Our findings validate the association between the UMOD variant and both CKD and SCr recently discovered in a large GWAS. In the Icelandic dataset, we demonstrate that the effect on SCr increases substantially with both age (P = 3.0×10⁻¹⁷) and number of comorbid diseases (P = 0.008). The association with CKD is also stronger in the older age groups. These results suggest that the UMOD variant may influence the adaptation of the kidney to age-related risk factors of kidney disease such as hypertension and diabetes. The variant also associates with serum urea (P = 1.0×10⁻⁶), uric acid (P = 0.0064), and suggestively with gout. In contrast to CKD, the UMOD variant confers protection against kidney stones when studied in 3,617 Icelandic and Dutch kidney stone cases and 43,201 controls (OR = 0.88, P = 5.7×10⁻⁵).

Chronic kidney disease (CKD) is a common condition that is associated with substantial morbidity and mortality and has been recognized as a major public health problem worldwide. Common causes of CKD include hypertension, diabetes, and inflammatory disorders. Previous studies have shown a significant genetic contribution to kidney disease and a recent genome-wide association study yielded a variant in the UMOD gene that affects the risk of CKD. Here, we replicate the association between UMOD and CKD in an independent analysis. We also demonstrate for the first time an interaction between the UMOD variant and age that suggests that this variant may adversely affect the aging kidney and its adaptation to age-related risk factors of kidney disease, such as hypertension and diabetes. Furthermore, we show that the UMOD variant that affects risk of CKD also provides protection against kidney stone disease.

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

Mammalian urine contains a range of macromolecule proteins that play critical roles in renal stone formation, among which Tamm-Horsfall protein (THP) is by far the most abundant. While THP is a potent inhibitor of crystal aggregation in vitro and its ablation in vivo predisposes one of the two existing mouse models to spontaneous intrarenal calcium crystallization, key controversies remain regarding the role of THP in nephrolithiasis. By carrying out a long-range follow-up of more than 250 THP-null mice and their wild-type controls, we demonstrate here that renal calcification is a highly consistent phenotype of the THP-null mice that is age and partially gene dosage dependent, but is gender and genetic background independent. Renal calcification in THP-null mice is progressive, and by 15 mo over 85% of all the THP-null mice develop spontaneous intrarenal crystals. The crystals consist primarily of calcium phosphate in the form of hydroxyapatite, are located more frequently in the interstitial space of the renal papillae than intratubularly, particularly in older animals, and lack accompanying inflammatory cell infiltration. The interstitial deposits of hydroxyapatite observed in THP-null mice bear strong resemblances to the renal crystals found in human kidneys bearing idiopathic calcium oxalate stones. Compared with 24-h urine from the wild-type mice, that of THP-null mice is supersaturated with brushite (calcium phosphate), a stone precursor, and has reduced urinary excretion of citrate, a stone inhibitor. While less frequent than renal calcinosis, renal pelvic and ureteral stones and hydronephrosis occur in the aged THP-null mice. These results provide direct in vivo evidence indicating that normal THP plays an important role in defending the urinary system against calcification and suggest that reduced expression and/or decreased function of THP could contribute to nephrolithiasis.

Unravelling the genetic basis of renal diseases; from single gene to multifactorial disorders

Chronic kidney disease is common with up to 5% of the adult population reported to have an estimated glomerular filtration rate of < 60 ml/min/1.73 m(2). A large number of pathogenic mutations have been identified that are responsible for 'single gene' renal disorders, such as autosomal dominant polycystic kidney disease and X-linked Alport syndrome. These single gene disorders account for < 15% of the burden of end-stage renal disease that requires dialysis or kidney transplantation. It has proved more difficult to identify the genetic susceptibility underlying common, complex, multifactorial kidney conditions, such as diabetic nephropathy and hypertensive nephrosclerosis. This review describes success to date and explores strategies currently employed in defining the genetic basis for a number of renal disorders. The complementary use of linkage studies, candidate gene and genome-wide association analyses are described and a collation of renal genetic resources highlighted.

Tamm-Horsfall protein and urinary exosome isolation

Urinary exosomes have been proposed as starting material for discovery of protein biomarkers of kidney disease. Current protocols for their isolation use a two-step differential centrifugation process. Due to their low density, exosomes are expected to remain in the low-speed (17,000 x g) supernatant and to sediment only when the sample is spun at high speed (200,000 x g). Analysis using western blot and electron microscopy found that urinary exosomes are also present in the low-speed pellet entrapped by polymeric Tamm-Horsfall protein, thus diminishing the procedure's reproducibility. Here we show that addition of dithiothreitol to the low-speed pellet disrupted the polymeric network, presumably by reduction of disulfide bonds linking the monomers. This modification shifted the exosomal proteins from the low- to the high-speed pellet. Also, by shifting the Tamm-Horsfall protein to the high-speed pellet, the use of dithiothreitol makes it feasible to use Tamm-Horsfall protein to normalize excretion rates of exosomal proteins in spot urines. We tested this by western blot, and found that there was a high degree of correlation between exosomal proteins and Tamm-Horsfall protein in the high-speed pellet. Since the yield of exosomes by differential centrifugation can be increased by chemical reduction, Tamm-Horsfall protein may be a suitable normalizing variable for urinary exosome studies when quantitative urine collections are not practical.

Novel missense mutation of uromodulin in mice causes renal dysfunction with alterations in urea handling, energy, and bone metabolism

Uromodulin-associated kidney disease is a heritable renal disease in humans caused by mutations in the uromodulin ( UMOD) gene. The pathogenesis of the disease is mostly unknown. In this study, we describe a novel chemically induced mutant mouse line termed UmodA227T exhibiting impaired renal function. The A227T amino acid exchange may impair uromodulin trafficking, leading to dysfunction of thick ascending limb cells of Henle's loop of the kidney. As a consequence, homozygous mutant mice display azotemia, impaired urine concentration ability, reduced fractional excretion of uric acid, and a selective defect in concentrating urea. Osteopenia in mutant mice is presumably a result of chronic hypercalciuria. In addition, body composition, lipid, and energy metabolism are indirectly affected in heterozygous and homozygous mutant UmodA227T mice, manifesting in reduced body weight, fat mass, and metabolic rate as well as reduced blood cholesterol, triglycerides, and nonesterified fatty acids. In conclusion, UmodA227T might act as a gain-of-toxic-function mutation. Therefore, the UmodA227T mouse line provides novel insights into consequences of disturbed uromodulin excretion regarding renal dysfunction as well as bone, energy, and lipid metabolism.

Multiple loci associated with indices of renal function and chronic kidney disease

Chronic kidney disease (CKD) has a heritable component and is an important global public health problem because of its high prevalence and morbidity. We conducted genome-wide association studies (GWAS) to identify susceptibility loci for glomerular filtration rate, estimated by serum creatinine (eGFRcrea) and cystatin C (eGFRcys), and CKD (eGFRcrea < 60 ml/min/1.73 m(2)) in European-ancestry participants of four population-based cohorts (ARIC, CHS, FHS, RS; n = 19,877; 2,388 CKD cases), and tested for replication in 21,466 participants (1,932 CKD cases). We identified significant SNP associations (P < 5 × 10(-8)) with CKD at the UMOD locus, with eGFRcrea at UMOD, SHROOM3 and GATM-SPATA5L1, and with eGFRcys at CST and STC1. UMOD encodes the most common protein in human urine, Tamm-Horsfall protein, and rare mutations in UMOD cause mendelian forms of kidney disease. Our findings provide new insights into CKD pathogenesis and underscore the importance of common genetic variants influencing renal function and disease.

The C-terminal cytoplasmic domain of human proEGF is a negative modulator of body and organ weights in transgenic mice

We generated transgenic mice to study the in vivo role of the cytoplasmic domain of human proEGF (proEGFcyt). Post-pubertal proEGFcyt transgenic (tg) mice displayed an up to 15% reduction in body weight, including smaller kidney and brain weights as compared to control littermates. Renal histology, gene expression profiles, and functional parameters were normal. In both sexes, serum levels of IGFBP-3 were reduced. Circulating IGF-I/IGF-II levels were unchanged. Histomorphological analysis revealed isolated foci of liver necrosis specific to proEGFcyt tg mice. In conclusion, we identified proEGF cytoplasmic domain as a novel modulator of whole body and organ-specific growth in mice.

Obstructive Protein Cast Nephropathy in Cynomolgus Monkeys Treated with Small Organic Molecules

We have observed a renal toxicity consistent with an obstructive protein cast nephropathy in cynomolgus macaques but not in other species treated with different therapeutic candidates having a common carboxylic acid moiety, suggesting a species-specific sensitivity. Here, we present renal toxicity findings consistent with a protein cast nephropathy in a 2–week safety study in cynomolgus monkeys. Light microscopic changes consisted of intratubular cast formation, tubular dilatation, interstitial inflammation, and expansion of the medullary interstitium. Tubular cast material was identified as Tamm-Horsfall protein (THP) and, on ultrastructure, crystalloid material was present in vacuoles of tubular epithelium. It is hypothesized that microcrystal formation in the urinary tubular spaces induces aggregation of THP protein and cast formation in monkeys. Drug-induced obstructive nephropathy is not identified as a major problem in humans; thus, the clinical relevance of the above findings in monkeys is not clear.

Decreased urinary concentration of Tamm-Horsfall protein is associated with development of renal failure and cardiovascular death within 20 years in type 1 but not in type 2 diabetic patients

OBJECTIVE

The first changes in the diabetic kidney are glycogen deposits in the epithelial cells of the thick ascending limb of Henle. These cells produce Tamm-Horsfall protein (THP). Is low excretion of THP associated with the development of renal insufficiency or cardiovascular disease?

MATERIAL AND METHODS

Urine samples were collected at baseline in patients with type 1 (n = 131) and type 2 (n = 108) diabetes who were followed for a mean of 14 years (range 1-20 years) and 4.5 years (range 1-15 years), respectively.

RESULTS

Twenty percent of type 1 and 54% of type 2 diabetic patients died and 24% and 29%, respectively developed uraemia. A decreased urinary concentration of THP (u-THP) was associated with an eight-fold increased risk of renal failure and cardiovascular death in type 1 but not in type 2 diabetic patients, irrespective of the degree of albuminuria and glycosylated haemoglobin and blood pressure levels. There were no differences in the degrees of albuminuria, serum creatinine or u-THP between the two types of diabetic patients at baseline. Low u-THP occurred in 8% and 9% of normoalbuminuric type 1 and type 2 diabetic patients, respectively.

CONCLUSION

A decreased u-THP was associated with an eight-fold increased risk of cardiovascular death and uraemia in type 1 but not in type 2 diabetic patients.

Localization of phosphatidylinositol phosphate kinase IIgamma in kidney to a membrane trafficking compartment within specialized cells of the nephron

PIP4Ks (type II phosphatidylinositol 4-phosphate kinases) are phosphatidylinositol 5-phosphate (PtdIns5P) 4-kinases, believed primarily to regulate cellular PtdIns5P levels. In this study, we investigated the expression, localization, and associated biological activity of the least-studied PIP4K isoform, PIP4Kγ. Quantitative RT-PCR and in situ hybridization revealed that compared with PIP4Kα and PIP4Kβ, PIP4Kγ is expressed at exceptionally high levels in the kidney, especially the cortex and outer medulla. A specific antibody was raised to PIP4Kγ, and immunohistochemistry with this and with antibodies to specific kidney cell markers showed a restricted expression, primarily distributed in epithelial cells in the thick ascending limb and in the intercalated cells of the collecting duct. In these cells, PIP4Kγ had a vesicular appearance, and transfection of kidney cell lines revealed a partial Golgi localization (primarily the matrix of the cis-Golgi) with an additional presence in an unidentified vesicular compartment. In contrast to PIP4Kα, bacterially expressed recombinant PIP4Kγ was completely inactive but did have the ability to associate with active PIP4Kα in vitro. Overall our data suggest that PIP4Kγ may have a function in the regulation of vesicular transport in specialized kidney epithelial cells.

Tamm-Horsfall protein protects the kidney from ischemic injury by decreasing inflammation and altering TLR4 expression

Tamm-Horsfall protein (THP) is a glycoprotein with unclear functions expressed exclusively in thick ascending limbs (TAL) of the kidney. Its role in ischemic acute kidney injury is uncertain, with previous data suggesting a possible negative effect by enhancing cast formation and promoting inflammation. Using a recently characterized THP knockout mouse (THP-/-), we investigated the role of THP in renal ischemia-reperfusion injury (IRI). In wild-type mice (THP+/+), THP expression was increased by injury. THP-/- mice developed more functional and histological renal damage after IRI compared with THP+/+. THP-/- kidneys showed more inflammation and tubular necrosis. Cast formation correlated with the severity of injury and was independent of THP presence. THP absence was associated with a more necrotic, rather than apoptotic, phenotype of cell death. The outer medulla was predominantly affected, where significant interstitial neutrophil infiltration was detected in proximity to injured S3 proximal tubular segments and TAL. This coincided with an enhanced expression of the innate immunity receptor Toll-like receptor 4 (TLR4) in S3 segments of THP-/- compared with THP+/+ mice. Specifically, a basolateral S3 expression of TLR4 was more evident in THP-/- kidneys compared with a more apical distribution in THP+/+. Such basolateral location for TLR4 allows a greater interaction with proinflammatory ligands present in the interstitium during ischemia. In conclusion, we are showing a completely novel role for a very old protein in the setting of renal injury. Our data suggest that THP stabilizes the outer medulla in the face of injury by decreasing inflammation, possibly through an effect on TLR4.

Proteomic techniques identify urine proteins that differentiate patients with interstitial cystitis from asymptomatic control subjects

OBJECTIVE

The purpose of this study was to identify differences in urine proteins between patients with interstitial cystitis (IC) and asymptomatic control (AC) subjects with the use of proteomic techniques.

STUDY DESIGN

Nine patients with IC and their age-, race-, and sex-matched AC subjects volunteered a urine specimen. Urine proteins were separated with the use of 2-dimensional polyacrylamide gels. Differing proteins underwent digestion and matrix-assisted laser desorption ionization-time of flight mass spectrometry. Computer-assisted data analysis was used to identify the corresponding protein. Differences in urine protein responses between patients with IC and AC subjects were evaluated by the Mann-Whitney U test to account for the nonnormal frequency distribution of the parameter estimate or chi-square when data were bimodal.

RESULTS

Four proteins differed significantly between patients with IC and AC subjects. The AC subjects had a greater concentration of a uromodulin (P = .019) and two kininogens (P = .023, .046). The patients with IC had a greater concentration of inter-alpha-trypsin inhibitor heavy chain H4 (P = .019).

CONCLUSION

These urine protein isoforms may be biomarkers for IC.

Control of renal uric acid excretion and gout

PURPOSE OF REVIEW

Impaired renal uric acid excretion is the major mechanism of hyperuricemia in patients with primary gout. This review highlights recent advances in the knowledge of normal mechanisms of renal uric acid handling and derangement of these mechanisms in uric acid underexcretion.

RECENT FINDINGS

The discovery of URAT1 has facilitated identification of other molecules potentially involved in uric acid transport in the renal tubules. Some of these molecules show gender differential expression in animal experiments. Sodium-dependent monocarboxylate cotransporters have been shown to transport lactate and butyrate, and may have roles in hyperuricemia associated with diabetic ketoacidosis and alcohol ingestion. Certain polymorphisms in SLC22A12 may be associated with the development of hyperuricemia or gout, although confirmation is needed. Mechanisms of hyperuricemia associated with uric acid underexcretion in patients with familial juvenile hyperuricemic nephropathy also remain to be clarified. Distal tubular salt wasting and compensatory upregulation of the resorption of sodium and uric acid in the proximal tubule may explain the hyperuricemia associated with this disorder.

SUMMARY

Much progress has been made in understanding the mechanisms of renal uric acid handling. Elucidation of the mechanisms of hyperuricemia in patients with familial juvenile hyperuricemic nephropathy will shed light on the function of uromodulin, functional impairment of which eventually results in diminished uric acid excretion.

Mouse models of polycystic kidney disease

Polycystic kidney disease (PKD) is a diverse group of human monogenic lethal conditions inherited as autosomal dominant (AD) or recessive (AR) traits. Recent development of genetically engineered mouse models of ADPKD, ARPKD, and nephronophthisis/medullary cystic disease (NPHP) are providing additional insights into the molecular mechanisms governing of these disease processes as well as the developmental differentiation of the normal kidney. Genotypic and phenotypic mouse models are discussed and provide evidence for the fundamental involvement of cell-matrix, cell-cell, and primary cilia-lumen interactions, as well as epithelial proliferation, apoptosis, and polarization. Structure/function relationships between the PKD1, PKD2, PKHD1, and NPHP genes and proteins support the notion of a regulatory multiprotein cystic complex with a mechanosensory function that integrates signals from the extracellular environment. The plethora of intracellular signaling cascades that can impact renal cystic development suggest an exquisitely sensitive requirement for integrated downstream transduction and provide potential targets for therapeutic intervention. Appropriate genocopy models that faithfully recapitulate the phenotypic characteristics of the disease will be invaluable tools to analyze the effects of modifier genes and small molecule inhibitor therapies.

Renal calcinosis and stone formation in mice lacking osteopontin, Tamm-Horsfall protein, or both

Although often supersaturated with mineral salts such as calcium phosphate and calcium oxalate, normal urine possesses an innate ability to keep them from forming harmful crystals. This inhibitory activity has been attributed to the presence of urinary macromolecules, although controversies abound regarding their role, or lack thereof, in preventing renal mineralization. Here, we show that 10% of the mice lacking osteopontin (OPN) and 14.3% of the mice lacking Tamm-Horsfall protein (THP) spontaneously form interstitial deposits of calcium phosphate within the renal papillae, events never seen in wild-type mice. Lack of both proteins causes renal crystallization in 39.3% of the double-null mice. Urinalysis revealed elevated concentrations of urine phosphorus and brushite (calcium phosphate) supersaturation in THP-null and OPN/THP-double null mice, suggesting that impaired phosphorus handling may be linked to interstitial papillary calcinosis in THP- but not in OPN-null mice. In contrast, experimentally induced hyperoxaluria provokes widespread intratubular calcium oxalate crystallization and stone formation in OPN/THP-double null mice, while completely sparing the wild-type controls. Whole urine from OPN-, THP-, or double-null mice all possessed a dramatically reduced ability to inhibit the adhesion of calcium oxalate monohydrate crystals to renal epithelial cells. These data establish OPN and THP as powerful and functionally synergistic inhibitors of calcium phosphate and calcium oxalate crystallization in vivo and suggest that defects in either molecule may contribute to renal calcinosis and stone formation, an exceedingly common condition that afflicts up to 12% males and 5% females.

Mouse models and the urinary concentrating mechanism in the new millennium

Our understanding of urinary concentrating and diluting mechanisms at the end of the 20th century was based largely on data from renal micropuncture studies, isolated perfused tubule studies, tissue analysis studies and anatomical studies, combined with mathematical modeling. Despite extensive data, several key questions remained to be answered. With the advent of the 21st century, a new approach, transgenic and knockout mouse technology, is providing critical new information about urinary concentrating processes. The central goal of this review is to summarize findings in transgenic and knockout mice pertinent to our understanding of the urinary concentrating mechanism, focusing chiefly on mice in which expression of specific renal transporters or receptors has been deleted. These include the major renal water channels (aquaporins), urea transporters, ion transporters and channels (NHE3, NKCC2, NCC, ENaC, ROMK, ClC-K1), G protein-coupled receptors (type 2 vasopressin receptor, prostaglandin receptors, endothelin receptors, angiotensin II receptors), and signaling molecules. These studies shed new light on several key questions concerning the urinary concentrating mechanism including: 1) elucidation of the role of water absorption from the descending limb of Henle in countercurrent multiplication, 2) an evaluation of the feasibility of the passive model of Kokko-Rector and Stephenson, 3) explication of the role of inner medullary collecting duct urea transport in water conservation, 4) an evaluation of the role of tubuloglomerular feedback in maintenance of appropriate distal delivery rates for effective regulation of urinary water excretion, and 5) elucidation of the importance of water reabsorption in the connecting tubule versus the collecting duct for maintenance of water balance.

Sepsis induces an increase in thick ascending limb Cox-2 that is TLR4 dependent

Cyclooxygenase-2 (Cox-2) is an inducible enzyme responsible for the formation of inflammatory prostanoids such as prostaglandins and thromboxane. Its role in the pathophysiology of inflammatory states like sepsis is increasingly recognized. Recently, we demonstrated that sepsis upregulates the endotoxin receptor Toll-like receptor 4 (TLR4) in rat kidney. Because Cox-2 is one of the downstream products of TLR4 activation, we hypothesized that sepsis-induced changes in renal Cox-2 expression are TLR4 dependent. Indeed, we show that in Sprague-Dawley rats, cecal ligation and puncture (a sepsis model) increases Cox-2 expression in cortical and medullary thick ascending loops (cTAL and mTAL, respectively) as well as inner medullary collecting ducts. These are all sites of increased TLR4 expression during sepsis. To determine the actual dependence on TLR4, we measured Cox-2 expression in wild-type and mutant mice which harbor a TLR4 gene deletion (TLR4-/-). In wild-type mice, sepsis increased Cox-2 expression in proximal tubules, cTAL, and mTAL. In contrast, septic TLR4-/- mice showed no significant increase in cTAL or mTAL Cox-2 expression. Furthermore, renin was absent from juxtaglomerular cells of TLR4-/- mice. We conclude that the dependence of sepsis-induced renal Cox-2 expression on TLR4 is tubule specific. The TLR4-dependent Cox-2 expression is mostly restricted to cortical and medullary thick ascending loops of Henle that characteristically express and secrete Tamm-Horsfall protein.

Combined renal tubular acidosis and diabetes insipidus in hematological disease

BACKGROUND

A 39-year-old male with multiple myeloma was admitted for treatment with melphalan and autologous stem cell reinfusion. He presented with hypokalemia and hyperchloremic non-anion-gap metabolic acidosis with a high urinary pH. He also had hypomagnesemia, hypophosphatemia, hypouricemia, proteinuria and glucosuria. The patient subsequently developed polyuria with a low urine osmolality, hypernatremia and, finally, acute renal failure.

INVESTIGATIONS

Physical examination, blood and urine analyses, kidney biopsy and tonicity balance.

DIAGNOSIS

Fanconi syndrome with proximal (type II) renal tubular acidosis caused by myeloma kidney. Renal tubular acidosis was complicated by probable nephrogenic diabetes insipidus and acute renal failure.

MANAGEMENT

Potassium supplementation, sodium bicarbonate therapy, intravenous fluid therapy and dialysis.