Longitudinal study of urinary excretion of phosphate, calcium, and uric acid in mutant NHERF-1 null mice

The molecular sociology of NHERF1 PDZ proteins controlling renal hormone-regulated phosphate transport

Parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF23) control extracellular phosphate levels by regulating renal NPT2A-mediated phosphate transport by a process requiring the PDZ scaffold protein NHERF1. NHERF1 possesses two PDZ domains, PDZ1 and PDZ2, with identical core-binding GYGF motifs explicitly recognizing distinct binding partners that play different and specific roles in hormone-regulated phosphate transport. The interaction of PDZ1 and the carboxy-terminal PDZ-binding motif of NPT2A (C-TRL) is required for basal phosphate transport. PDZ2 is a regulatory domain that scaffolds multiple biological targets, including kinases and phosphatases involved in FGF23 and PTH signaling. FGF23 and PTH trigger disassembly of the NHERF1-NPT2A complex through reversible hormone-stimulated phosphorylation with ensuing NPT2A sequestration, down-regulation, and cessation of phosphate absorption. In the absence of NHERF1-NPT2A interaction, inhibition of FGF23 or PTH signaling results in disordered phosphate homeostasis and phosphate wasting. Additional studies are crucial to elucidate how NHERF1 spatiotemporally coordinates cellular partners to regulate extracellular phosphate levels.

Mutations in an unrecognized internal NPT2A PDZ motif disrupt phosphate transport and cause congenital hypophosphatemia

The Na+-dependent phosphate cotransporter-2A (NPT2A, SLC34A1) is a primary regulator of extracellular phosphate homeostasis. Its most prominent structural element is a carboxy-terminal PDZ ligand that binds Na+/H+ Exchanger Regulatory Factor-1 (NHERF1, SLC9A3R1). NHERF1, a multidomain PDZ protein, establishes NPT2A membrane localization and is required for hormone-inhibitable phosphate transport. NPT2A also possesses an uncharacterized internal PDZ ligand. Two recent clinical reports describe congenital hypophosphatemia in children harboring Arg495His or Arg495Cys variants within the internal PDZ motif. The wild-type internal 494TRL496 PDZ ligand binds NHERF1 PDZ2, which we consider a regulatory domain. Ablating the internal PDZ ligand with a 494AAA496 substitution blocked hormone-inhibitable phosphate transport. Complementary approaches, including CRISPR/Cas9 technology, site-directed mutagenesis, confocal microscopy, and modeling, showed that NPT2A Arg495His or Arg495Cys variants do not support PTH or FGF23 action on phosphate transport. Coimmunoprecipitation experiments indicate that both variants bind NHERF1 similarly to WT NPT2A. However, in contrast with WT NPT2A, NPT2A Arg495His, or Arg495Cys variants remain at the apical membrane and are not internalized in response to PTH. We predict that Cys or His substitution of the charged Arg495 changes the electrostatics, preventing phosphorylation of the upstream Thr494, interfering with phosphate uptake in response to hormone action, and inhibiting NPT2A trafficking. We advance a model wherein the carboxy-terminal PDZ ligand defines apical localization NPT2A, while the internal PDZ ligand is essential for hormone-triggered phosphate transport.

Mutations in an unrecognized internal NPT2A PDZ motif disrupt phosphate transport causing congenital hypophosphatemia

The Na ⁺ -dependent phosphate cotransporter-2A (NPT2A, SLC34A1) is a primary regulator of extracellular phosphate homeostasis. Its most prominent structural element is a carboxy-terminal PDZ ligand that binds Na ⁺ /H ⁺ Exchanger Regulatory Factor-1 (NHERF1, SLC9A3R1). NHERF1, a multidomain PDZ protein,establishes NPT2A membrane localization and is required for hormone-sensitive phosphate transport. NPT2A also possesses an uncharacterized internal PDZ ligand. Two recent clinical reports describe congenital hypophosphatemia in children harboring Arg ⁴⁹⁵ His or Arg ⁴⁹⁵ Cys variants within the internal PDZ motif. The wild-type internal ⁴⁹⁴ TRL ⁴⁹⁶ PDZ ligand binds NHERF1 PDZ2, which we consider a regulatory domain. Ablating the internal PDZ ligand with a ⁴⁹⁴ AAA ⁴⁹⁶ substitution blocked hormone-sensitive phosphate transport. Complementary approaches, including CRISPR/Cas9 technology, site-directed mutagenesis, confocal microscopy, and modeling, showed that NPT2A Arg ⁴⁹⁵ His or Arg ⁴⁹⁵ Cys variants do not support PTH or FGF23 action on phosphate transport. Coimmunoprecipitation experiments indicate that both variants bind NHERF1 similarly to WT NPT2A. However, in contrast to WT NPT2A, NPT2A Arg ⁴⁹⁵ His or Arg ⁴⁹⁵ Cys variants remain at the apical membrane and are not internalized in response to PTH. We predict that Cys or His substitution of the charged Arg ⁴⁹⁵ changes the electrostatics, preventing phosphorylation of the upstream Thr ⁴⁹⁴ , interfering with phosphate uptake in response to hormone action, and inhibiting NPT2A trafficking. We advance a model wherein the carboxyterminal PDZ ligand defines apical localization NPT2A, while the internal PDZ ligand is essential for hormone-triggered phosphate transport.

Lack of xanthine dehydrogenase leads to a remarkable renal decline in a novel hypouricemic rat model

Uric acid (UA) is the final metabolite in purine catabolism in humans. Previous studies have shown that the dysregulation of UA homeostasis is detrimental to cardiovascular and kidney health. The Xdh gene encodes for the Xanthine Oxidoreductase enzyme group, responsible for producing UA. To explore how hypouricemia can lead to kidney damage, we created a rat model with the genetic ablation of the Xdh gene on the Dahl salt-sensitive rat background (SS^Xdh−/−). SS^Xdh−/− rats lacked UA and exhibited impairment in growth and survival. This model showed severe kidney injury with increased interstitial fibrosis, glomerular damage, crystal formation, and an inability to control electrolyte balance. Using a multi-omics approach, we highlighted that lack of Xdh leads to increased oxidative stress, renal cell proliferation, and inflammation. Our data reveal that the absence of Xdh leads to kidney damage and functional decline by the accumulation of purine metabolites in the kidney and increased oxidative stress.

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A novel rat model of hypouricemia was created by the gene ablation of the Xdh gene

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The SS^Xdh-/- rat showed a failure to thrive, kidney injury, and functional decline

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Multi-omics revealed increased inflammation and oxidative stress in SS^Xdh-/- rats

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.

Antiurolithic effects of medicinal plants: results of in vivo studies in rat models of calcium oxalate nephrolithiasis-a systematic review

Urolithiasis is one of the oldest diseases affecting humans, while plants are one of our oldest companions providing food, shelter, and medicine. In spite of substantial progress in understanding the pathophysiological mechanisms, treatment options are still limited, often expensive for common people in most parts of the world. As a result, there is a great interest in herbal remedies for the treatment of urinary stone disease as an alternative or adjunct therapy. Numerous in vivo and in vitro studies have been carried out to understand the efficacy of herbs in reducing stone formation. We adopted PRISMA guidelines and systematically reviewed PubMed/Medline for the literature, reporting results of various herbal products on in vivo models of nephrolithiasis/urolithiasis. The Medical Subject Heading Terms (Mesh term) "Urolithiasis" was used with Boolean operator "AND" and other related Mesh Unique terms to search all the available records (July 2019). A total of 163 original articles on in vivo experiments were retrieved from PubMed indexed with the (MeshTerm) "Urolithiasis" AND "Complementary Therapies/Alternative Medicine, "Urolithiasis" AND "Plant Extracts" and "Urolithiasis" AND "Traditional Medicine". Most of the studies used ethylene glycol (EG) to induce hyperoxaluria and nephrolithiasis in rats. A variety of extraction methods including aqueous, alcoholic, hydro-alcoholic of various plant parts ranging from root bark to fruits and seeds, or a combination thereof, were utilized. All the investigations did not study all aspects of nephrolithiasis making it difficult to compare the efficacy of various treatments. Changes in the lithogenic factors and a reduction in calcium oxalate (CaOx) crystal deposition in the kidneys were, however, considered favorable outcomes of the various treatments. Less than 10% of the studies examined antioxidant and diuretic activities of the herbal treatments and concluded that their antiurolithic activities were a result of antioxidant, anti-inflammatory, and/or diuretic effects of the treatments.

NHERF1 Loss Upregulates Enzymes of the Pentose Phosphate Pathway in Kidney Cortex

(1) Background: We previously showed Na/H exchange regulatory factor 1 (NHERF1) loss resulted in increased susceptibility to cisplatin nephrotoxicity. NHERF1-deficient cultured proximal tubule cells and proximal tubules from NHERF1 knockout (KO) mice exhibit altered mitochondrial protein expression and poor survival. We hypothesized that NHERF1 loss results in changes in metabolic pathways and/or mitochondrial dysfunction, leading to increased sensitivity to cisplatin nephrotoxicity. (2) Methods: Two to 4-month-old male wildtype (WT) and KO mice were treated with vehicle or cisplatin (20 mg/kg dose IP). After 72 h, kidney cortex homogenates were utilized for metabolic enzyme activities. Non-treated kidneys were used to isolate mitochondria for mitochondrial respiration via the Seahorse XF24 analyzer. Non-treated kidneys were also used for LC-MS analysis to evaluate kidney ATP abundance, and electron microscopy (EM) was utilized to evaluate mitochondrial morphology and number. (3) Results: KO mouse kidneys exhibit significant increases in malic enzyme and glucose-6 phosphate dehydrogenase activity under baseline conditions but in no other gluconeogenic or glycolytic enzymes. NHERF1 loss does not decrease kidney ATP content. Mitochondrial morphology, number, and area appeared normal. Isolated mitochondria function was similar between WT and KO. Conclusions: KO kidneys experience a shift in metabolism to the pentose phosphate pathway, which may sensitize them to the oxidative stress imposed by cisplatin.

Phosphaturia in kidney stone formers: Still an enigma

Calcium kidney stones are common worldwide. Most are idiopathic and composed of calcium oxalate. Calcium phosphate is present in around 80% and may initiate stone formation. Stone production is multifactorial with a polygenic genetic contribution. Phosphaturia is found frequently among stone formers but until recently received scant attention. This review examines possible mechanisms for the phosphaturia and its relevance to stone formation from a wide angle. There is a striking lack of clinical data. Phosphaturia is associated, but not correlated, with hypercalciuria, increased 1,25 dihydroxy-vitamin D [1,25 (OH)₂D], and sometimes evidence of disturbances in proximal renal tubular function. Phosphate reabsorption in the proximal renal tubules requires tightly regulated interaction of many proteins. Paracellular flow through intercellular tight junctions is the major route of phosphate absorption from the intestine and can be reduced therapeutically in hyperphosphatemic patients. In monogenic defects stones develop when phosphaturia is associated with hypercalciuria, generally explained by increased 1,25 (OH)₂D production in response to hypophosphatemia. Calcification does not occur in disorders with increased FGF23 when phosphaturia occurs in isolation and 1,25 (OH)₂D is suppressed. Candidate gene studies have identified mutations in the phosphate transporters, but in few individuals. One genome-wide study identified a polymorphism of the phosphate transporter gene SLC34A4 associated with stones. Others did not find mutations obviously linked to phosphate reabsorption. Future genetic studies should have a wide trawl and should focus initially on groups of patients with clearly defined phenotypes. The global data should be pooled.

Emerging Roles of Diacylglycerol-Sensitive TRPC4/5 Channels

Transient receptor potential classical or canonical 4 (TRPC4) and TRPC5 channels are members of the classical or canonical transient receptor potential (TRPC) channel family of non-selective cation channels. TRPC4 and TRPC5 channels are widely accepted as receptor-operated cation channels that are activated in a phospholipase C-dependent manner, following the G_q/11 protein-coupled receptor activation. However, their precise activation mechanism has remained largely elusive for a long time, as the TRPC4 and TRPC5 channels were considered as being insensitive to the second messenger diacylglycerol (DAG) in contrast to the other TRPC channels. Recent findings indicate that the C-terminal interactions with the scaffolding proteins Na⁺/H⁺ exchanger regulatory factor 1 and 2 (NHERF1 and NHERF2) dynamically regulate the DAG sensitivity of the TRPC4 and TRPC5 channels. Interestingly, the C-terminal NHERF binding suppresses, while the dissociation of NHERF enables, the DAG sensitivity of the TRPC4 and TRPC5 channels. This leads to the assumption that all of the TRPC channels are DAG sensitive. The identification of the regulatory function of the NHERF proteins in the TRPC4/5-NHERF protein complex offers a new starting point to get deeper insights into the molecular basis of TRPC channel activation. Future studies will have to unravel the physiological and pathophysiological functions of this multi-protein channel complex.

Evidence for a role of PDZ domain-containing proteins to mediate hypophosphatemia in calcium stone formers

Background

Hypophosphatemia (HYP) is common among calcium stone formers (SFs) and in rare cases is associated with mutations in sodium-phosphate cotransporters or in Na+/H+ exchanger regulatory factor 1 (NHERF1), but the majority of cases are unexplained. We hypothesized that reduced sodium-phosphate cotransporter activity mediated via NHERF1 or a similar PDZ domain-containing protein, causes HYP. If so, other transport activities controlled by NHERF1, such as NHE3 and URAT1, might be reduced in HYP.

Methods

To test this idea, we analyzed two large but separate sets of 24-h urine samples and paired serums of 2700 SFs from the University of Chicago and 11 073 SFs from Litholink, a national laboratory. Patients were divided into quintiles based on serum phosphate.

Results

Males were more common in the lowest phosphate tiles in both datasets. Phosphate excretion did not vary across the quintiles, excluding diet as a cause of HYP. Tubule maximum (Tm) phosphate per unit glomerular filtration rate decreased and fractional excretion increased with decreasing phosphate quintiles, indicating reduced tubule phosphate reabsorption was responsible for HYP. Urine pH and serum chloride increased with decreasing serum phosphate, suggesting a coordinate change in NHE3 activity. Serum uric acid and Tm uric acid decreased significantly with decreasing serum phosphate, while uric acid excretion did not vary. Conclusion. HYP in SFs results from decreased tubule phosphate reabsorption and, being associated with related changes in other proximal tubule transporters, may arise from alterations in or signaling to PDZ-containing proteins.

In vitro and in vivo models for the study of urolithiasis

Preclinical animal research has greatly contributed and will continue to contribute in our understanding of various disease states and provided methods for more understanding of disease states and designs to test novel pharmaco-therapeutic interventions against these diseases. For urolithiasis, scientists have developed numerous in vitro and in vivo models that attempt to replicate human urolithiasis. In this review, I have explained in vitro and in vivo models that are more common, affordable, and easy to replicate. In the in vitro models, I have focused on the CaOx crystallization models and in the in vivo models, hyperoxaluric rat model has been explained along with other available option such as Knockout (KO) mice and fly models. Each model has been explained stepwise along with its pros and cons.

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.

Animal models of urinary stone disease

The etiology of stone disease remains unknown despite the major technological advances in the treatment of urinary calculi. Clinically, urologists have relied on 24-h urine collections for the last 30-40 years to help direct medical therapy in hopes of reducing stone recurrence; yet little progress has been made in preventing stone disease. As such, there is an urgent need to develop reliable animal models to study the pathogenesis of stone formation and to assess novel interventions. A variety of vertebrate and invertebrate models have been used to help understand stone pathogenesis. Genetic knockout and exogenous induction models are described. Surrogates for an endpoint of stone formation have been urinary crystals on histologic examination and/or urinalyses. Other models are able to actually develop true stones. It is through these animal models that real breakthroughs in the management of urinary stone disease will become a reality.

PTH and Vitamin D

PTH and Vitamin D are two major regulators of mineral metabolism. They play critical roles in the maintenance of calcium and phosphate homeostasis as well as the development and maintenance of bone health. PTH and Vitamin D form a tightly controlled feedback cycle, PTH being a major stimulator of vitamin D synthesis in the kidney while vitamin D exerts negative feedback on PTH secretion. The major function of PTH and major physiologic regulator is circulating ionized calcium. The effects of PTH on gut, kidney, and bone serve to maintain serum calcium within a tight range. PTH has a reciprocal effect on phosphate metabolism. In contrast, vitamin D has a stimulatory effect on both calcium and phosphate homeostasis, playing a key role in providing adequate mineral for normal bone formation. Both hormones act in concert with the more recently discovered FGF23 and klotho, hormones involved predominantly in phosphate metabolism, which also participate in this closely knit feedback circuit. Of great interest are recent studies demonstrating effects of both PTH and vitamin D on the cardiovascular system. Hyperparathyroidism and vitamin D deficiency have been implicated in a variety of cardiovascular disorders including hypertension, atherosclerosis, vascular calcification, and kidney failure. Both hormones have direct effects on the endothelium, heart, and other vascular structures. How these effects of PTH and vitamin D interface with the regulation of bone formation are the subject of intense investigation.

Sorting nexin 27 couples PTHR trafficking to retromer for signal regulation in osteoblasts during bone growth

The parathyroid hormone 1 receptor (PTHR) is central to the process of bone formation and remodeling. PTHR signaling requires receptor internalization into endosomes, which is then terminated by recycling or degradation. Here we show that sorting nexin 27 (SNX27) functions as an adaptor that couples PTHR to the retromer trafficking complex. SNX27 binds directly to the C-terminal PDZ-binding motif of PTHR, wiring it to retromer for endosomal sorting. The structure of SNX27 bound to the PTHR motif reveals a high-affinity interface involving conserved electrostatic interactions. Mechanistically, depletion of SNX27 or retromer augments intracellular PTHR signaling in endosomes. Osteoblasts genetically lacking SNX27 show similar disruptions in PTHR signaling and greatly reduced capacity for bone mineralization, contributing to profound skeletal deficits in SNX27-knockout mice. Taken together, our data support a critical role for SNX27-retromer mediated transport of PTHR in normal bone development.

Unified theory on the pathogenesis of Randall's plaques and plugs

Kidney stones develop attached to sub-epithelial plaques of calcium phosphate (CaP) crystals (termed Randall's plaque) and/or form as a result of occlusion of the openings of the Ducts of Bellini by stone-forming crystals (Randall's plugs). These plaques and plugs eventually extrude into the urinary space, acting as a nidus for crystal overgrowth and stone formation. To better understand these regulatory mechanisms and the pathophysiology of idiopathic calcium stone disease, this review provides in-depth descriptions of the morphology and potential origins of these plaques and plugs, summarizes existing animal models of renal papillary interstitial deposits, and describes factors that are believed to regulate plaque formation and calcium overgrowth. Based on evidence provided within this review and from the vascular calcification literature, we propose a "unified" theory of plaque formation-one similar to pathological biomineralization observed elsewhere in the body. Abnormal urinary conditions (hypercalciuria, hyperoxaluria, and hypocitraturia), renal stress or trauma, and perhaps even the normal aging process lead to transformation of renal epithelial cells into an osteoblastic phenotype. With this de-differentiation comes an increased production of bone-specific proteins (i.e., osteopontin), a reduction in crystallization inhibitors (such as fetuin and matrix Gla protein), and creation of matrix vesicles, which support nucleation of CaP crystals. These small deposits promote aggregation and calcification of surrounding collagen. Mineralization continues by calcification of membranous cellular degradation products and other fibers until the plaque reaches the papillary epithelium. Through the activity of matrix metalloproteinases or perhaps by brute physical force produced by the large sub-epithelial crystalline mass, the surface is breached and further stone growth occurs by organic matrix-associated nucleation of CaOx or by the transformation of the outer layer of CaP crystals into CaOx crystals. Should this theory hold true, developing an understanding of the cellular mechanisms involved in progression of a small, basic interstitial plaque to that of an expanding, penetrating plaque could assist in the development of new therapies for stone prevention.

Interstitial calcinosis in renal papillae of genetically engineered mouse models: relation to Randall's plaques

Genetically engineered mouse models (GEMMs) have been highly instrumental in elucidating gene functions and molecular pathogenesis of human diseases, although their use in studying kidney stone formation or nephrolithiasis remains relatively limited. This review intends to provide an overview of several knockout mouse models that develop interstitial calcinosis in the renal papillae. Included herein are mice deficient for Tamm-Horsfall protein (THP; also named uromodulin), osteopontin (OPN), both THP and OPN, Na(+)-phosphate cotransporter Type II (Npt2a) and Na(+)/H(+) exchanger regulatory factor (NHERF-1). The baseline information of each protein is summarized, along with key morphological features of the interstitial calcium deposits in mice lacking these proteins. Attempts are made to correlate the papillary interstitial deposits found in GEMMs with Randall's plaques, the latter considered precursors of idiopathic calcium stones in patients. The pathophysiology that underlies the renal calcinosis in the knockout mice is also discussed wherever information is available. Not all the knockout models are allocated equal space because some are more extensively characterized than others. Despite the inroads already made, the exact physiological underpinning, origin, evolution and fate of the papillary interstitial calcinosis in the GEMMs remain incompletely defined. Greater investigative efforts are warranted to pin down the precise role of the papillary interstitial calcinosis in nephrolithiasis using the existing models. Additionally, more sophisticated, second-generation GEMMs that allow gene inactivation in a time-controlled manner and "compound mice" that bear several genetic alterations are urgently needed, in light of mounting evidence that nephrolithiasis is a multifactorial, multi-stage and polygenic disease.

Footprints of directional selection in wild Atlantic salmon populations: evidence for parasite-driven evolution?

Mechanisms of host-parasite co-adaptation have long been of interest in evolutionary biology; however, determining the genetic basis of parasite resistance has been challenging. Current advances in genome technologies provide new opportunities for obtaining a genome-scale view of the action of parasite-driven natural selection in wild populations and thus facilitate the search for specific genomic regions underlying inter-population differences in pathogen response. European populations of Atlantic salmon (Salmo salar L.) exhibit natural variance in susceptibility levels to the ectoparasite Gyrodactylus salaris Malmberg 1957, ranging from resistance to extreme susceptibility, and are therefore a good model for studying the evolution of virulence and resistance. However, distinguishing the molecular signatures of genetic drift and environment-associated selection in small populations such as land-locked Atlantic salmon populations presents a challenge, specifically in the search for pathogen-driven selection. We used a novel genome-scan analysis approach that enabled us to i) identify signals of selection in salmon populations affected by varying levels of genetic drift and ii) separate potentially selected loci into the categories of pathogen (G. salaris)-driven selection and selection acting upon other environmental characteristics. A total of 4631 single nucleotide polymorphisms (SNPs) were screened in Atlantic salmon from 12 different northern European populations. We identified three genomic regions potentially affected by parasite-driven selection, as well as three regions presumably affected by salinity-driven directional selection. Functional annotation of candidate SNPs is consistent with the role of the detected genomic regions in immune defence and, implicitly, in osmoregulation. These results provide new insights into the genetic basis of pathogen susceptibility in Atlantic salmon and will enable future searches for the specific genes involved.

Male hypercalciuric stone formers with low renal calcium reabsorption

AIMS

Hypercalciuria is a common poorly understood abnormality among stone formers. We aimed to identify hypercalciuric male stone formers with a primary defect in renal calcium reabsorption and to look for associated risk factors.

METHODS

A retrospective cross-sectional database study of 623 male idiopathic calcium stone formers with normal plasma ultrafilterable calcium levels attending the Southampton stone clinic. Filtered calcium was estimated from plasma ultrafilterable calcium (60% of total plasma calcium) and 24 h creatinine clearance. Reabsorbed calcium was the difference between filtered and excreted calcium.

RESULTS

276 men had hypercalciuria (urine calcium >7.50 mmol/24 h); 347 had normocalciuria. Hypercalciuric men filtered more calcium than normocalciuric men: median values 247 and 227 mmol/24 h, but the ranges overlapped (175-371 and 153-316 mmol/24 h). However, across the entire filtration range, hypercalciuric men reabsorbed less of the filtered calcium. Among the hypercalciuric men, we noticed differences between those with high and low filtration. We therefore compared data for hypercalciuric men in the highest and lowest filtration quintiles (n=55). Men with high filtration were younger at their first stone episode and had significantly higher plasma ultrafilterable calcium and calcium reabsorption, urinary calcium, oxalate, urate and creatinine excretion and creatinine clearance. 35% with high filtration and 40% with low filtration had recurrent stones; 27% and 20%, respectively, had an affected first-degree relative.

CONCLUSIONS

Hypercalciuric men reabsorbed proportionately less filtered calcium than normocalciuric men. Among hypercalciuric men, the risks for stones were higher in those with a high than a low filtered calcium load and presentation was earlier.

A priori and a posteriori approaches for finding genes of evolutionary interest in non-model species: osmoregulatory genes in the kidney transcriptome of the desert rodent Dipodomys spectabilis (banner-tailed kangaroo rat)

One common goal in evolutionary biology is the identification of genes underlying adaptive traits of evolutionary interest. Recently next-generation sequencing techniques have greatly facilitated such evolutionary studies in species otherwise depauperate of genomic resources. Kangaroo rats (Dipodomys sp.) serve as exemplars of adaptation in that they inhabit extremely arid environments, yet require no drinking water because of ultra-efficient kidney function and osmoregulation. As a basis for identifying water conservation genes in kangaroo rats, we conducted a priori bioinformatics searches in model rodents (Mus musculus and Rattus norvegicus) to identify candidate genes with known or suspected osmoregulatory function. We then obtained 446,758 reads via 454 pyrosequencing to characterize genes expressed in the kidney of banner-tailed kangaroo rats (Dipodomys spectabilis). We also determined candidates a posteriori by identifying genes that were overexpressed in the kidney. The kangaroo rat sequences revealed nine different a priori candidate genes predicted from our Mus and Rattus searches, as well as 32 a posteriori candidate genes that were overexpressed in kidney. Mutations in two of these genes, Slc12a1 and Slc12a3, cause human renal diseases that result in the inability to concentrate urine. These genes are likely key determinants of physiological water conservation in desert rodents.

Regulation of G protein-coupled receptor function by Na+/H+ exchange regulatory factors

Many G protein-coupled receptors (GPCR) exert patterns of cell-specific signaling and function. Mounting evidence now supports the view that cytoplasmic adapter proteins contribute critically to this behavior. Adapter proteins recognize highly conserved motifs such as those for Src homology 3 (SH3), phosphotyrosine-binding (PTB), and postsynaptic density 95/discs-large/zona occludens (PDZ) docking sequences in candidate GPCRs. Here we review the behavior of the Na+/H+ exchange regulatory factor (NHERF) family of PDZ adapter proteins on GPCR signalling, trafficking, and function. Structural determinants of NHERF proteins that allow them to recognize targeted GPCRs are considered. NHERF1 and NHERF2 are capable also of modifying the assembled complex of accessory proteins such as β-arrestins, which have been implicated in regulating GPCR signaling. In addition, NHERF1 and NHERF2 modulate GPCR signaling by altering the G protein to which the receptor binds or affect other regulatory proteins that affect GTPase activity, protein kinase A, phospholipase C, or modify downstream signaling events. Small molecules targeting the site of NHERF1-GPCR interaction are being developed and may become important and selective drug candidates.

Ultrastructural investigation of crystal deposits in Npt2a knockout mice: are they similar to human Randall's plaques?

PURPOSE

Idiopathic Ca oxalate stones may develop with attachment to renal interstitial Ca phosphate deposits (Randall's plaques). Sodium phosphate cotransporter (Npt2a) null mice have hypercalciuria and hyperphosphaturia, and produce tubular and interstitial Ca phosphate deposits. To determine whether this mouse is suitable for Randall's plaque investigations we chronologically studied Ca phosphate deposit sites, structure and composition.

MATERIALS AND METHODS

The kidneys of Npt2a null mice 2 days to 1 year old were examined by light, scanning and transmission electron microscopy. Electron diffraction and energy dispersive x-ray microanalyses were done to determine mineral composition.

RESULTS

Poorly crystalline, biological apatite deposits were seen in collecting duct lumina. Deposits consisted of aggregates approximately 5 μm in diameter appearing as microspheres of concentrically organized needle or plate-like, matrix rich crystals. Epithelium/crystal interfaces were filled with membrane bound vesicles. Some tubules were completely occluded by crystals and occasionally lost the epithelium while crystals moved into the interstitium.

CONCLUSIONS

Ca phosphate crystals formed in the tubular lumina and were organized as microspheres. The aggregation of Ca phosphate crystals produced nuclei, which grew by adding crystals at the periphery. They eventually became large enough to occlude the tubular lumina and obliterate the tubular epithelium, leading to the relocation of microliths into the interstitium. The pathogenesis of interstitial deposits in Npt2a null mice appears different from that proposed for Randall's plaques. Since Npt2a null mice purge the renal crystal deposits, these mice may serve as a model in which to investigate the elimination of crystal deposits in children and adults with nephrocalcinosis.

Experimentally induced hyperoxaluria in MCP-1 null mice

Experimental animal model studies suggest that calcium oxalate (CaOx) crystal deposition in the kidneys is associated with the development of oxidative stress, epithelial injury and inflammation. There is increased production of inflammatory molecules including osteopontin (OPN), monocyte chemoattractant protein-1 (MCP-1) and various subunits of inter-alpha-inhibitor such as bikunin. What does the increased production of such molecules suggest? Is it a cause or consequence of crystal deposition? We hypothesized that over-expression and increased production of MCP-1 is a result of the interaction between renal epithelial cells and CaOx crystals after their deposition in the renal tubules. We induced hyperoxaluria in MCP-1 null as well as wild type mice and examined pathological changes in their kidneys and urine. Both wild type and MCP-1 null male mice became hyperoxaluric and demonstrated CaOx crystalluria. Neither of them developed crystal deposits in their kidneys. Both showed some morphological changes in their renal proximal tubules. Significant pathological changes such as cell death and increased urinary excretion of LDH were not seen. Results suggest that at least in mice (1) Increase in oxalate and decrease in citrate excretion can lead to CaOx crystalluria but not CaOx nephrolithiasis; (2) MCP-1 does not play a role in crystal retention within the kidneys; (3) Expression of OPN and MCP-1 is not increased in the kidneys in the absence of crystal deposition; (4) Crystal deposition is necessary for significant pathological changes and movement of monocytes and macrophages into the interstitium.

Experimental induction of calcium oxalate nephrolithiasis in mice

PURPOSE

The availability of various transgenic and knockout mice provides an excellent opportunity to better understand the pathophysiology of calcium oxalate stone disease. However, attempts to produce calcium oxalate nephrolithiasis in mice have not been successful. We hypothesized that calcium oxalate nephrolithiasis in mice requires increasing urine calcium and oxalate excretion, and experimentally induced hyperoxaluria alone is not sufficient. To provide evidence we induced hyperoxaluria by administering hyperoxaluria inducing agents in normocalciuric and hypercalciuric mice, and investigating various aspects of nephrolithiasis.

MATERIALS AND METHODS

We administered ethylene glycol, glyoxylate or hydroxyl proline via diet in male and female normocalciuric B6 mice, and in hypercalciuric sodium phosphate co-transporter type 2 a -/- mice for 4 weeks. We collected 24-hour urine samples on days 0, 3, 7, 14, 21 and 28, and analyzed them for pH, creatinine, lactate dehydrogenase calcium and oxalate. Kidneys were examined using light microscopy. Urine was examined for crystals using light and scanning electron microscopy.

RESULTS

Hypercalciuric mice on hydroxyl proline did not tolerate treatment and were sacrificed before 28 days. All mice on ethylene glycol, glyoxylate or hydroxyl proline became hyperoxaluric and showed calcium oxalate crystalluria. No female, normocalciuric or hypercalciuric mice showed renal calcium oxalate crystal deposits. Calcium oxalate nephrolithiasis developed in all mice on glyoxylate and in some on ethylene glycol. In all mice the kidneys showed epithelial injury. Male mice particularly on glyoxylate had more renal injury and inflammatory cell migration into the interstitium around the crystal deposits.

CONCLUSIONS

Results confirm that hyperoxaluria induction alone is not sufficient to create calcium oxalate nephrolithiasis in mice. Hypercalciuria is also required. Kidneys in male mice are more prone to injury than those in female mice and are susceptible to calcium oxalate crystal deposition. Perhaps epithelial injury promotes crystal retention. Thus, calcium oxalate nephrolithiasis in mice is gender dependent, and requires hypercalciuria and hyperoxaluria.

Role of NHERF and scaffolding proteins in proximal tubule transport

Eukaryotic cells coordinate specific responses to hormones and growth factors by spatial and temporal organization of "signaling components." Through the formation of multiprotein complexes, cells are able to generate "signaling components" that transduce hormone signals through proteins, such as PSD-95/Dlg/ZO-1(PDZ)-containing proteins that associate by stable and dynamic interactions. The PDZ homology domain is a common protein interaction domain in eukaryotes and with greater than 500 PDZ domains identified, it is the most abundant protein interaction domain in eukaryotic cells. The NHERF (sodium hydrogen exchanger regulatory factor) proteins are PDZ domain-containing proteins that play an important role in maintaining and regulating cell function. NHERF-1 was initially identified as a brush border membrane-associated phosphoprotein essential for the cAMP/PKA-induced inhibition of the sodium hydrogen exchanger isoform 3 (NHE3). Mouse, rabbit and human renal proximal tubules also express NHERF-2 (E3KARP), a structurally related protein, which in model cell systems also binds NHE3 and mediates its inhibition by cAMP. PDZK1 (NHERF-3) and IKEPP (NHERF-4) were later identified and found to have similar homology domains, leading to their recent reclassification. Although studies have revealed similar binding partners and overlapping functions for the NHERF proteins, it is clear that there is a significant amount of specificity between them. This review focuses primarily on NHERF-1, as the prototypical PDZ protein and will give a brief summary of its role in phosphate transport and the development of some forms of nephrolithiasis.

Phosphaturia as a promising predictor of recurrent stone formation in patients with urolithiasis

Purpose

Recent studies have suggested that renal phosphate leakage and the associated phosphaturia are significant underlying causes of calcium urolithiasis. The aims of this study were to assess whether phosphaturia relates to urinary metabolic abnormalities and recurrent stone formation.

Materials and Methods

A database of patient histories and urine chemistries was analyzed for 1,068 consecutive stone formers (SFs) and 106 normal controls. Urine values for phosphaturia that were higher than 95% of the normal control values were defined as indicating hyperphosphaturia, and the effect of phosphaturia on urinary metabolites and stone recurrence was determined. Of these patients, 247 patients (23.1%) who had been followed up for more than 36 months or had a recurrence of stones during follow-up (median, 46.0 months; range, 5-151) were included in the analyses for stone recurrence.

Results

Of the SFs, 19.9% (212/1,068) had increased urinary phosphate excretion. SFs with hyperphosphaturia had a greater urinary volume and higher levels of calcium, uric acid, oxalate, and citrate than did SFs with normophosphaturia. A multivariate Cox regression model, stratified by stone episodes, revealed that hyperphosphaturia was an independent predictor of recurrent stone formation in first-time SFs (hazard ratio [HR]: 2.122; 95% confidence interval [CI]: 1.100-4.097; p=0.025). No association was detected between hyperphosphaturia and recurrent stone formation in recurrent SFs. Kaplan-Meier curves showed identical results.

Conclusions

This study demonstrates that hyperphosphaturia is closely associated with urinary metabolic abnormalities. Furthermore, hyperphosphaturia is a significant risk factor for stone recurrence in first-time SFs.

NHERF1 mutations and responsiveness of renal parathyroid hormone

Impaired renal phosphate reabsorption, as measured by dividing the tubular maximal reabsorption of phosphate by the glomerular filtration rate (TmP/GFR), increases the risks of nephrolithiasis and bone demineralization. Data from animal models suggest that sodium-hydrogen exchanger regulatory factor 1 (NHERF1) controls renal phosphate transport. We sequenced the NHERF1 gene in 158 patients, 94 of whom had either nephrolithiasis or bone demineralization. We identified three distinct mutations in seven patients with a low TmP/GFR value. No patients with normal TmP/GFR values had mutations. The mutants expressed in cultured renal cells increased the generation of cyclic AMP (cAMP) by parathyroid hormone (PTH) and inhibited phosphate transport. These NHERF1 mutations suggest a previously unrecognized cause of renal phosphate loss in humans.

Urine electrolyte, mineral, and protein excretion in NHERF-2 and NHERF-1 null mice

The adaptor proteins sodium/hydrogen exchanger regulatory factor (NHERF)-1 and NHERF-2 have overlapping tissue distribution in renal cells and overlapping specificity in their binding to renal transporters and other proteins. To compare the kidney-specific differences in the function of these adaptor proteins, NHERF-1 and NHERF-2 null mice were compared with wild-type control mice. In NHERF-2 null mice, the renal proximal tubule abundance and distribution of NHERF-1 and NHERF-3 were not different from those in wild-type animals. The glomerular expression of podocalyxin and ZO-1 also did not differ. NHERF-1 null mice had increased urinary excretion of phosphate, calcium, and uric acid compared with wild-type control and NHERF-2 null mice. Because of the association between NHERF-2 and podocalyxin in glomeruli and ClC-5 in the renal proximal tubule, the urinary excretion of protein was determined. There were no differences in the urinary excretion of protein or low-molecular-weight proteins between wild-type control, NHERF-1(-/-), and NHERF-2(-/-) mice. These studies indicate that the increased urinary excretion of phosphate and uric acid are specific to NHERF-1 null mice and highlight the fact that predictions about the role of adaptor proteins such as the NHERF proteins obtained from studies of model cell systems must be confirmed in whole animals.

Interactions of the growth-related, type IIc renal sodium/phosphate cotransporter with PDZ proteins

Despite similar molecular structures, the growth-related sodium/phosphate cotransporter NaPiIIc is regulated differently than the main NaPiIIa phosphate transporter. Using two-hybrid systems and immunoprecipitation, we identified several proteins that interact with NaPiIIc that might account for this differential regulation. NaPiIIc interacted with the PDZ domain-containing sodium-hydrogen exchange-regulating factor (NHERF) 1 and NHERF3 through novel binding motifs in its C terminus. NaPiIIc from brush-border membranes coprecipitated with both NHERF1 and NHERF3, with more NHERF3 co-precipitated in rats fed a low-phosphorus diet. NaPiIIc colocalizes with both NHERF1 and NHERF3 in brush-border membranes of rats fed either a low- or high-phosphorus diet. When mouse NaPiIIc was transfected into opossum kidney cells, it was localized mainly in apical microvilli and the trans-Golgi. Both confocal and total internal reflection microscopy show that NaPiIIc colocalizes with NHERF1 and NHERF3 in the apical microvilli, and this was not altered by truncation of the last three amino acids of NaPiIIc. Interactions of NaPiIIc with NHERF1 and NHERF3 were modulated by the membrane-associated 17 kDa protein (MAP17) similarly to NaPiIIa, but only the MAP17-NaPiIIc-NHERF3 complexes were internalized to the trans-Golgi. Our study shows that NaPiIIc interacts with a limited number of PDZ domain proteins, and the mechanisms and consequences of such interactions differ from those of NaPiIIa.

Regulatory Binding Partners and Complexes of NHE3

NHE3 is the brush-border (BB) Na+/H+exchanger of small intestine, colon, and renal proximal tubule which is involved in large amounts of neutral Na+absorption. NHE3 is a highly regulated transporter, being both stimulated and inhibited by signaling that mimics the postprandial state. It also undergoes downregulation in diarrheal diseases as well as changes in renal disorders. For this regulation, NHE3 exists in large, multiprotein complexes in which it associates with at least nine other proteins. This review deals with short-term regulation of NHE3 and the identity and function of its recognized interacting partners and the multiprotein complexes in which NHE3 functions.

Sodium-Hydrogen Exchanger Regulatory Factor-1 Interacts with Mouse Urate Transporter 1 to Regulate Renal Proximal Tubule Uric Acid Transport

Sodium-hydrogen exchanger regulatory factor-1-deficient (NHERF-1(-/-)) mice demonstrate increases in the urinary excretion of phosphate, calcium, and uric acid associated with interstitial deposition of calcium in the papilla of the kidney. These studies examine the role of NHERF-1 in the tubular reabsorption of uric acid and regulation of mouse urate transporter 1 (mURAT1), a newly described transporter that is responsible for the renal tubular reabsorption of uric acid. In primary cultures of mouse renal proximal tubule cells, uric acid uptake was significantly lower in NHERF-1(-/-) cells compared with wild-type cells over a large range of uric acid concentrations in the media. Western immunoblotting revealed a 56 +/- 6% decrease in the brush border membrane (BBM) expression of mURAT1 in NHERF-1(-/-) compared with wild-type control kidneys (P < 0.05). Confocal microscopy confirmed the reduced apical membrane expression of mURAT1 in NHERF-1(-/-) kidneys and demonstrated mislocalization of mURAT1 to intracellular vesicular structures. Para-aminohippurate significantly inhibited uric acid uptake in wild-type cells (41 +/- 2%) compared with NHERF-1(-/-) cells (8.2 +/- 3%). Infection of NHERF-1(-/-) cells with adenovirus-green fluorescence protein-NHERF-1 resulted in significantly higher rates of uric acid transport (15.4 +/- 1.1 pmol/microg protein per 30 min) compared with null cells that were infected with control adenovirus-green fluorescence protein (7.9 +/- 0.3) and restoration of the inhibitory effect of para-aminohippurate (% inhibition 34 +/- 4%). These findings indicate that NHERF-1 exerts a significant effect on the renal tubular reabsorption of uric acid in the mouse by modulating the BBM abundance of mURAT1 and possibly other BBM uric acid transporters.

Physiology of epithelial Ca2+ and Mg2+ transport

Ca2+ and Mg2+ are essential ions in a wide variety of cellular processes and form a major constituent of bone. It is, therefore, essential that the balance of these ions is strictly maintained. In the last decade, major breakthrough discoveries have vastly expanded our knowledge of the mechanisms underlying epithelial Ca2+ and Mg2+ transport. The genetic defects underlying various disorders with altered Ca2+ and/or Mg2+ handling have been determined. Recently, this yielded the molecular identification of TRPM6 as the gatekeeper of epithelial Mg2+ transport. Furthermore, expression cloning strategies have elucidated two novel members of the transient receptor potential family, TRPV5 and TRPV6, as pivotal ion channels determining transcellular Ca2+ transport. These two channels are regulated by a variety of factors, some historically strongly linked to Ca2+ homeostasis, others identified in a more serendipitous manner. Herein we review the processes of epithelial Ca2+ and Mg2+ transport, the molecular mechanisms involved, and the various forms of regulation.

Proximal tubular handling of phosphate: A molecular perspective

Members of the SLC34 gene family of solute carriers encode for three Na+-dependent phosphate (P i) cotransporter proteins, two of which (NaPi-IIa/SLC34A1 and NaPi-IIc/SLC34A3) control renal reabsorption of P i in the proximal tubule of mammals, whereas NaPi-IIb/SCLC34A2 mediates P i transport in organs other than the kidney. The P i transport mechanism has been extensively studied in heterologous expression systems and structure-function studies have begun to reveal the intricacies of the transport cycle at the molecular level using techniques such as cysteine scanning mutagenesis, and voltage clamp fluorometry. Moreover, sequence differences between the three types of cotransporters have been exploited to obtain information about the molecular determinants of hormonal sensitivity and electrogenicity. Renal handling of P i is regulated by hormonal and non-hormonal factors. Changes in urinary excretion of P i are almost invariably mirrored by changes in the apical expression of NaPi-IIa and NaPi-IIc in proximal tubules. Therefore, understanding the mechanisms that control the apical expression of NaPi-IIa and NaPi-IIc as well as their functional properties is critical to understanding how an organism achieves P i homeostasis.