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Hyperuricemia is often encountered as glomerular filtration rate decreased. It is associated with a more rapid decline of the renal function, but causality has not been demonstrated. Recent studies showed that treatment of hyperuricemia did not affect the progression in chronic kidney disease (CKD) patients. Thus, treatment with hypouricemic drugs of patients suffering of CKD and displaying asymptomatic hyperuricemia is not recommended. However, patients with CKD present often with acute flairs of gout, which might be difficult to treat. Therapeutic options are discussed in this article.Une hyperuricémie apparaît précocement en cas de diminution de la filtration glomérulaire et de maladie rénale chronique. Elle est associée à un déclin plus rapide de la fonction rénale, mais un lien de causalité n’a pas été démontré. Plusieurs études récentes n’ont pas montré d’effet bénéfique d’un traitement hypo-uricémiant sur l’évolution de la fonction rénale. Ainsi, en cas d’hyper uricémie asymptomatique chez un patient souffrant de maladie rénale chronique, un traitement hypo-uricémiant n’est pas indiqué. Cependant, les patients souffrant de maladie rénale chronique font plus fréquemment des crises de goutte, et leur prise en charge est complexe car la maladie est à la fois plus résistante au traitement et les options thérapeutiques sont limitées. Celles-ci sont revues dans cet article.

The objective of the present study was to theoretically investigate the mechanisms underlying uric acid transport in the proximal tubule (PT) of rat kidneys, and their modulation by factors, including Na+, parathyroid hormone, ANG II, and Na+-glucose cotransporter-2 inhibitors. To that end, we incorporated the transport of uric acid and its conjugate anion urate in our mathematical model of water and solute transport in the rat PT. The model accounts for parallel urate reabsorption and secretion pathways on apical and basolateral membranes and their coupling to lactate and α-ketoglutarate transport. Model results agree with experimental findings at the segment level. Net reabsorption of urate by the rat PT is predicted to be ~70% of the filtered load, with a rate of urate removal from the lumen that is 50% higher than the rate of urate secretion. The model suggests that apical URAT1 deletion significantly reduces net urate reabsorption across the PT, whereas ATP-binding cassette subfamily G member 2 dysfunction affects it only slightly. Inactivation of basolateral glucose transporter-9 raises fractional urate excretion above 100%, as observed in patients with renal familial hypouricemia. Furthermore, our results suggest that reducing Na+ reabsorption across Na+/H+ exchangers or Na+-glucose cotransporters augments net urate reabsorption. The model predicts that parathyroid hormone reduces urate excretion, whereas ANG II increases it. In conclusion, we have developed the first model of uric acid transport in the rat PT; this model provides a framework to gain greater insight into the numerous solutes and coupling mechanisms that affect the renal handing of uric acid.

Inflammatory bowel diseases (IBD) including ulcerative colitis and Crohn's disease are diseases with impaired epithelial barrier function. We aimed to investigate whether mutated prostasin and thus, reduced colonic epithelial sodium channel activity predisposes to develop an experimentally dextran sodium sulfate (DSS)-induced colitis. Wildtype, heterozygous (fr/+), and homozygous (fr/fr) prostasin-mutant rats were treated 7 days with DSS followed by 7 days of recovery and analyzed with respect to histology, clinicopathological parameters, inflammatory marker mRNA transcript expression, and sodium transporter protein expression. In this study, a more detailed analysis on rat fr/fr colons revealed reduced numbers of crypt and goblet cells, and local angiodysplasia, as compared with heterozygous (fr/+) and wildtype littermates. Following 2% DSS treatment for 7 days followed by 7 days recovery, fr/fr animals lost body weight, and reached maximal diarrhea score and highest disease activity after only 3 days, and strongly increased cytokine levels. The histology score significantly increased in all groups, but fr/fr colons further displayed pronounced histological alterations with near absence of goblet cells, rearrangement of the lamina propria, and presence of neutrophils, eosinophils, and macrophages. Additionally, fr/fr colons showed ulcerations and edemas that were absent in fr/+ and wildtype littermates. Following recovery, fr/fr rats reached, although significantly delayed, near-normal diarrhea score and disease activity, but exhibited severe architectural remodeling, despite unchanged sodium transporter protein expression. In summary, our results demonstrate a protective role of colonic prostasin expression against experimental colitis, and thus represent a susceptibility gene in the development of inflammatory bowel disease.

Calcium is a key component of the bone mineral hydroxyapatite. During osteoclast-mediated bone resorption, hydroxyapatite is dissolved and significant quantities of calcium are released. Several calcium transport systems have previously been identified in osteoclasts, including members of the sodium/calcium exchanger (NCX) family. Expression pattern and physiological role of NCX isoforms in osteoclasts, however, remain largely unknown at the moment. Our data indicate that all three NCX isoforms (NCX1, NCX2, and NCX3) are present in murine osteoclasts. RANKL-induced differentiation of murine osteoclast precursors into mature osteoclasts significantly attenuated the expression of NCX1, while NCX2 and NCX3 expressions were largely unaffected. To study the role of NCX1 during osteoclast differentiation and bone resorption, we crossed mice with exon 11 of the NCX1 gene flanked by loxP sites with cathepsin K-Cre transgenic mice. Mature osteoclasts derived from transgenic mice exhibited an 80-90% reduction of NCX1 protein. In vitro studies indicate that NCX1 is dispensable for osteoclast differentiation, but NCX1-deficient osteoclasts exhibited increased resorptive activity. In line with these in vitro findings, mice with an osteoclast-targeted deletion of the NCX1 gene locus displayed an age-dependent loss of bone mass. Thus, in summary, our data reveal NCX1 as a regulator of osteoclast-mediated bone resorption.

Mutations of the KCNJ2 gene encoding the potassium channel Kir2.1 were previously shown to cause Andersen's syndrome (AS), a multisystem disease manifesting with developmental abnormalities, cardiac arrhythmias and periodic paralyses. We conducted a search for KCNJ2 mutations among 188 unrelated patients suspected to have long QT syndrome (LQTS). The screening was performed by denaturing high-performance liquid chromatography (dHPLC) and DNA sequencing. Two novel mutations of the KCNJ2 gene were detected: a missense threonine to alanine mutation (T75A) in the N-terminal region (family 1) and an in-frame deletion of two amino acids (DeltaFQ163-164) in the M2 transmembrane region (family 2). In addition, a previously described silent polymorphism C1146T was detected. In family 1, some of the affected family members had a history of periodic muscle weakness characteristic of AS, but no dysmorphic features. The mean QTc interval of the affected members were 444 +/- 24 ms (family 1, n=7) and 456 +/- 8 ms (family 2, n=2). The mutations affect functionally important regions of the KCNJ2 channel protein: upon injection of the Xenopus oocytes with the wild type and mutant KCNJ2 constructs, the channel proteins were correctly synthesized and localized to the cell surface, but no measurable inward K(+) current could be detected for the mutant KCNJ2 constructs. In conclusion, we report two novel loss-of-function mutations of the KCNJ2 channel, affecting different domains of the channel protein. Mutations of the KCNJ2 gene should be considered in genetic subclassification of LQTS patients, even in the absence of overt manifestations of AS.

Defects in the astrocytic membrane protein MLC1, the adhesion molecule GlialCAM or the chloride channel ClC-2 underlie human leukoencephalopathies. Whereas GlialCAM binds ClC-2 and MLC1, and modifies ClC-2 currents in vitro, no functional connections between MLC1 and ClC-2 are known. Here we investigate this by generating loss-of-function Glialcam and Mlc1 mouse models manifesting myelin vacuolization. We find that ClC-2 is unnecessary for MLC1 and GlialCAM localization in brain, whereas GlialCAM is important for targeting MLC1 and ClC-2 to specialized glial domains in vivo and for modifying ClC-2's biophysical properties specifically in oligodendrocytes (OLs), the cells chiefly affected by vacuolization. Unexpectedly, MLC1 is crucial for proper localization of GlialCAM and ClC-2, and for changing ClC-2 currents. Our data unmask an unforeseen functional relationship between MLC1 and ClC-2 in vivo, which is probably mediated by GlialCAM, and suggest that ClC-2 participates in the pathogenesis of megalencephalic leukoencephalopathy with subcortical cysts.

ClC-2 is a broadly expressed plasma membrane chloride channel that is modulated by voltage, cell swelling, and pH. A human mutation leading to a heterozygous loss of ClC-2 has previously been reported to be associated with epilepsy, whereas the disruption of Clcn2 in mice led to testicular and retinal degeneration. We now show that the white matter of the brain and spinal cord of ClC-2 knock-out mice developed widespread vacuolation that progressed with age. Fluid-filled spaces appeared between myelin sheaths of the central but not the peripheral nervous system. Neuronal morphology, in contrast, seemed normal. Except for the previously reported blindness, neurological deficits were mild and included a decreased conduction velocity in neurons of the central auditory pathway. The heterozygous loss of ClC-2 had no detectable functional or morphological consequences. Neither heterozygous nor homozygous ClC-2 knock-out mice had lowered seizure thresholds. Sequencing of a large collection of human DNA and electrophysiological analysis showed that several ClC-2 sequence abnormalities previously found in patients with epilepsy most likely represent innocuous polymorphisms.

Liddle's syndrome is a monogenic form of hypertension caused by mutations in the PY motif of the COOH terminus of β- and γ-epithelial Na+channel (ENaC) subunits. These mutations lead to retention of active channels at the cell surface. Because of the critical role of this PY motif in the stability of ENaCs at the cell surface, we have investigated its contribution to the ENaC response to aldosterone and vasopressin. Mutants of the PY motif in β- and γ-ENaC subunits (β-Y618A, β-P616L, β-R564stop, and γ-K570stop) were stably expressed by retroviral gene transfer in a renal cortical collecting duct cell line (mpkCCDcl4), and transepithelial Na+transport was assessed by measurements of the benzamil-sensitive short-circuit current ( Isc). Cells that express ENaC mutants of the PY motif showed a five- to sixfold higher basal Isccompared with control cells and responded to stimulation by aldosterone (10-6M) or vasopressin (10-9M) with a further increase in Isc. The rates of the initial increases in Iscafter aldosterone or vasopressin stimulation were comparable in cells transduced with wild-type and mutant ENaCs, but reversal of the effects of aldosterone and vasopressin was slower in cells that expressed the ENaC mutants. The conserved sensitivity of ENaC mutants to stimulation by aldosterone and vasopressin together with the prolonged activity at the cell surface likely contribute to the increased Na+absorption in the distal nephron of patients with Liddle's syndrome.

The epithelial Na(+) channel (ENaC), located in the apical membrane of tight epithelia, allows vectorial Na(+) absorption. The amiloride-sensitive ENaC is highly selective for Na(+) and Li(+) ions. There is growing evidence that the short stretch of amino acid residues (preM2) preceding the putative second transmembrane domain M2 forms the outer channel pore with the amiloride binding site and the narrow ion-selective region of the pore. We have shown previously that mutations of the alphaS589 residue in the preM2 segment change the ion selectivity, making the channel permeant to K(+) ions. To understand the molecular basis of this important change in ionic selectivity, we have substituted alphaS589 with amino acids of different sizes and physicochemical properties. Here, we show that the molecular cutoff of the channel pore for inorganic and organic cations increases with the size of the amino acid residue at position alpha589, indicating that alphaS589 mutations enlarge the pore at the selectivity filter. Mutants with an increased permeability to large cations show a decrease in the ENaC unitary conductance of small cations such as Na(+) and Li(+). These findings demonstrate the critical role of the pore size at the alphaS589 residue for the selectivity properties of ENaC. Our data are consistent with the main chain carbonyl oxygens of the alphaS589 residues lining the channel pore at the selectivity filter with their side chain pointing away from the pore lumen. We propose that the alphaS589 side chain is oriented toward the subunit-subunit interface and that substitution of alphaS589 by larger residues increases the pore diameter by adding extra volume at the subunit-subunit interface.

In order to identify the zoogeographic origin of shrews (genus Crocidura) living on the oceanic island of Sulawesi, 15 taxa from Southeast Asia and 1 from Europe were examined for sequence variation in a segment (617 bp) of the mitochondrial cytochrome b gene. The null hypothesis of a monophyletic origin of all Sulawesian shrews was investigated by a phylogenetic reconstruction using maximum parsimony. According to a parametric bootstrap which simulated sequence evolution for these taxa, the null hypothesis could be rejected as highly unlikely (P0.01). Therefore, the molecular phylogeny strongly suggests that overwater colonization of Sulawesi by shrews succeeded on at least two occasions. The first, relatively ancient wave of colonizers radiated and gave rise to a surprizingly diverse assemblage of at least five species which now coexist in perfect sympatry on Sulawesi. The second wave, of more recent origin, gave rise to Crocidura nigripes, a species which retained close genetic affinities with other Malay shrews.

Tight control of extracellular and intracellular inorganic phosphate (Pi) levels is critical to most biochemical and physiologic processes. Urinary Pi is freely filtered at the kidney glomerulus and is reabsorbed in the renal tubule by the action of the apical sodium -dependent phosphate transporters, NaPi-11a/NaPi-11c/Pit2. However, the molecular identity of the protein(s) participating in the basolateral Pi efflux remains unknown. Evidence has suggested that xenotropic and polytropic retroviral receptor 1 (XPR1) might be involved in this process. Here, we show that conditional inactivation of Xpr1 in the renal tubule in mice resulted in impaired renal Pi reabsorption. Analysis of Pi transport in primary cultures of proximal tubular cells or in freshly isolated renal tubules revealed that this Xpr1 deficiency significantly affected Pi efflux. Further, mice with conditional inactivation of Xpr1 in the renal tubule exhibited generalized proximal tubular dysfunction indicative of Fanconi syndrome, characterized by glycosuria, aminoaciduria, calciuria, and albuminuria. Dramatic alterations in the renal transcriptome, including a significant reduction in NaPi-lla/NaPi-llc expression, accompanied these functional changes. Additionally, Xpr1-deficient mice developed hypophosphatemic rickets secondary to renal dysfunction. These results identify XPR1 as a major regulator of Pi homeostasis and as a potential therapeutic target in bone and kidney disorders.