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Item does not contain fulltext, BACKGROUND: Gitelman syndrome is a salt-losing tubulopathy characterized by hypokalemic alkalosis and hypomagnesemia. It is caused by homozygous recessive or compound heterozygous pathogenic variants in SLC12A3 , which encodes the Na + -Cl - cotransporter (NCC). In up to 10% of patients with Gitelman syndrome, current genetic techniques detect only one specific pathogenic variant. This study aimed to identify a second pathogenic variant in introns, splice sites, or promoters to increase the diagnostic yield. METHODS: Long-read sequencing of SLC12A3 was performed in 67 DNA samples from individuals with suspected Gitelman syndrome in whom a single likely pathogenic or pathogenic variant was previously detected. In addition, we sequenced DNA samples from 28 individuals with one variant of uncertain significance or no candidate variant. Midigene splice assays assessed the pathogenicity of novel intronic variants. RESULTS: A second likely pathogenic/pathogenic variant was identified in 45 (67%) patients. Those with two likely pathogenic/pathogenic variants had a more severe electrolyte phenotype than other patients. Of the 45 patients, 16 had intronic variants outside of canonic splice sites (nine variants, mostly deep intronic, six novel), whereas 29 patients had an exonic variant or canonic splice site variant. Midigene splice assays of the previously known c.1670-191C>T variant and intronic candidate variants demonstrated aberrant splicing patterns. CONCLUSION: Intronic pathogenic variants explain an important part of the missing heritability in Gitelman syndrome. Long-read sequencing should be considered in diagnostic workflows for Gitelman syndrome.

Radboud University, 25 mei 2023, Promotor : Bindels, R.J.M. Co-promotores : Baaij, J.H.F. de, Deinum, J., Contains fulltext : 292281.pdf (Publisher’s version ) (Closed access)

Item does not contain fulltext, SIGNIFICANCE STATEMENT: Several recent studies identified mitochondrial mutations in patients with Gitelman or Fanconi syndrome. Mitochondrial cytopathies are generally not considered in the diagnostic workup of patients with electrolyte disorders. In this systematic review, we investigated the presence of electrolyte disorders in patients with mitochondrial cytopathies to determine the relevance of mitochondrial mutation screening in this population. Our analysis demonstrates that electrolyte disorders are commonly reported in mitochondrial cytopathies, often as presenting symptoms. Consequently, more clinical attention should be raised for mitochondrial disease as cause for disturbances in electrolyte homeostasis. Further prospective cohort studies are required to determine the exact prevalence of electrolyte disorders in mitochondrial cytopathies. BACKGROUND: Electrolyte reabsorption in the kidney has a high energy demand. Proximal and distal tubular epithelial cells have a high mitochondrial density for energy release. Recently, electrolyte disorders have been reported as the primary presentation of some mitochondrial cytopathies. However, the prevalence and the pathophysiology of electrolyte disturbances in mitochondrial disease are unknown. Therefore, we systematically investigated electrolyte disorders in patients with mitochondrial cytopathies. METHODS: We searched PubMed, Embase, and Google Scholar for articles on genetically confirmed mitochondrial disease in patients for whom at least one electrolyte is reported. Patients with a known second genetic anomaly were excluded. We evaluated 214 case series and reports (362 patients) as well as nine observational studies. Joanna Briggs Institute criteria were used to evaluate the quality of included studies. RESULTS: Of 362 reported patients, 289 had an electrolyte disorder, with it being the presenting or main symptom in 38 patients. The average number of different electrolyte abnormalities per patient ranged f

Item does not contain fulltext, Dietary fibers have been shown to increase the intestinal absorption of calcium (Ca(2+)) and magnesium (Mg(2+)). However, the mechanisms that explain the enhanced electrolyte absorption remain unknown. Therefore, this study aims to investigate the short-term and long-term effects of 5% (w/w) sodium butyrate (Na-butyrate), an important end-metabolite of bacterial fermentation of dietary fibers, on Ca(2+) and Mg(2+) homeostasis in mice. Serum Ca(2+) levels were only significantly increased in mice treated with Na-butyrate for 1 day. This was associated with a twofold increase in the mRNA expression levels of Trpv6 in the proximal and distal colon. Contrary, Na-butyrate did not affect serum Mg(2+) concentrations at either of the intervention periods. However, we observed a reduction in urinary Mg(2+) excretion, although not significantly, after 1 day of treatment. A significant reduction of 2.5-fold in urinary Mg(2+) excretion was observed after 14 days of treatment. Indeed, 14-day Na-butyrate supplementation increased colonic Trpm7 expression by 1.2-fold compared to control mice. In conclusion, short-term Na-butyrate supplementation increases serum Ca(2+) levels in mice. This was associated with increased mRNA expression levels of Trpv6 in the colon, suggesting that Na-butyrate regulates the expression of genes involved in active intestinal Ca(2+) absorption.

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Contains fulltext : 285278.pdf (Publisher’s version ) (Open Access)

Contains fulltext : 283350.pdf (Publisher’s version ) (Closed access), Magnesium (Mg(2+)) is the most prevalent divalent intracellular cation. As co-factor in many enzymatic reactions, Mg(2+) is essential for protein synthesis, energy production, and DNA stability. Disturbances in intracellular Mg(2+) concentrations, therefore, unequivocally result in delayed cell growth and metabolic defects. To maintain physiological Mg(2+) levels, all organisms rely on balanced Mg(2+) influx and efflux via Mg(2+) channels and transporters. This review compares the structure and the function of prokaryotic Mg(2+) transporters and their eukaryotic counterparts. In prokaryotes, cellular Mg(2+) homeostasis is orchestrated via the CorA, MgtA/B, MgtE, and CorB/C Mg(2+) transporters. For CorA, MgtE, and CorB/C, the motifs that form the selectivity pore are conserved during evolution. These findings suggest that CNNM proteins, the vertebrate orthologues of CorB/C, also have Mg(2+) transport capacity. Whereas CorA and CorB/C proteins share the gross quaternary structure and functional properties with their respective orthologues, the MgtE channel only shares the selectivity pore with SLC41 Na(+)/Mg(2+) transporters. In eukaryotes, TRPM6 and TRPM7 Mg(2+) channels provide an additional Mg(2+) transport mechanism, consisting of a fusion of channel with a kinase. The unique features these TRP channels allow the integration of hormonal, cellular, and transcriptional regulatory pathways that determine their Mg(2+) transport capacity. Our review demonstrates that understanding the structure and function of prokaryotic magnesiotropic proteins aids in our basic understanding of Mg(2+) transport.

Radboud University, 05 oktober 2022, Promotor : Bindels, R.J.M. Co-promotor : Baaij, J.H.F. de, Contains fulltext : 281783.pdf (Publisher’s version ) (Closed access)

Contains fulltext : 232627.pdf (Publisher’s version ) (Open Access), Purinergic signaling regulates several renal physiological and pathophysiological processes. Extracellular vesicles (EVs) are nanoparticles released by most cell types, which, in non-renal tissues, modulate purinergic signaling. The aim of this study was to investigate the effect of EVs from renal proximal tubule (HK2) and collecting duct cells (HCD) on intra- and intersegment modulation of extracellular ATP levels, the underlying molecular mechanisms, and the impact on the expression of the alpha subunit of the epithelial sodium channel (αENaC). HK2 cells were exposed to HK2 EVs, while HCD cells were exposed to HK2 and HCD EVs. Extracellular ATP levels and αENaC expression were measured by chemiluminescence and qRT-PCR, respectively. ATPases in EV populations were identified by mass spectrometry. The effect of aldosterone was assessed using EVs from aldosterone-treated cells and urinary EVs (uEVs) from primary aldosteronism (PA) patients. HK2 EVs downregulated ectonucleoside-triphosphate-diphosphohydrolase-1 (ENTPD1) expression, increased extracellular ATP and downregulated αENaC expression in HCD cells. ENTPD1 downregulation could be attributed to increased miR-205-3p and miR-505 levels. Conversely, HCD EVs decreased extracellular ATP levels and upregulated αENaC expression in HCD cells, probably due to enrichment of 14-3-3 isoforms with ATPase activity. Pretreatment of donor cells with aldosterone or exposure to uEVs from PA patients enhanced the effects on extracellular ATP and αENaC expression. We demonstrated inter- and intrasegment modulation of renal purinergic signaling by EVs. Our findings postulate EVs as carriers of information along the renal tubules, whereby processes affecting EV release and/or cargo may impact on purinergically regulated processes.

Contains fulltext : 229882.pdf (publisher's version ) (Open Access), Hypomagnesaemia is a common feature of renal Na(+) wasting disorders such as Gitelman and EAST/SeSAME syndrome. These genetic defects specifically affect Na(+) reabsorption in the distal convoluted tubule, where Mg(2+) reabsorption is tightly regulated. Apical uptake via TRPM6 Mg(2+) channels and basolateral Mg(2+) extrusion via a putative Na(+) -Mg(2+) exchanger determines Mg(2+) reabsorption in the distal convoluted tubule. However, the mechanisms that explain the high incidence of hypomagnesaemia in patients with Na(+) wasting disorders of the distal convoluted tubule are largely unknown. In this review, we describe three potential mechanisms by which Mg(2+) reabsorption in the distal convoluted tubule is linked to Na(+) reabsorption. First, decreased activity of the thiazide-sensitive Na(+) /Cl(-) cotransporter (NCC) results in shortening of the segment, reducing the Mg(2+) reabsorption capacity. Second, the activity of TRPM6 and NCC are determined by common regulatory pathways. Secondary effects of NCC dysregulation such as hormonal imbalance, therefore, might disturb TRPM6 expression. Third, the basolateral membrane potential, maintained by the K(+) permeability and Na(+) -K(+) -ATPase activity, provides the driving force for Na(+) and Mg(2+) extrusion. Depolarisation of the basolateral membrane potential in Na(+) wasting disorders of the distal convoluted tubule may therefore lead to reduced activity of the putative Na(+) -Mg(2+) exchanger SLC41A1. Elucidating the interconnections between Mg(2+) and Na(+) transport in the distal convoluted tubule is hampered by the currently available models. Our analysis indicates that the coupling of Na(+) and Mg(2+) reabsorption may be multifactorial and that advanced experimental models are required to study the molecular mechanisms.

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Item does not contain fulltext, BACKGROUND: Genetic loss of function of AGT (angiotensinogen), REN (renin), ACE (angiotensin-converting enzyme), or AGTR1 (type-1 angiotensin II receptor) leads to renal tubular dysgenesis (RTD). This syndrome is almost invariably lethal. Most surviving patients reach stage 5 chronic kidney disease at a young age. METHODS: Here, we report a 28-year-old male with a homozygous truncating mutation in AGTR1 (p.Arg216*), who survived the perinatal period with a mildly impaired kidney function. In contrast to classic RTD, kidney biopsy showed proximal tubules that were mostly normal. During the subsequent three decades, we observed evidence of both tubular dysfunction (hyperkalemia, metabolic acidosis, salt-wasting and a urinary concentrating defect) and glomerular dysfunction (reduced glomerular filtration rate, currently ~30 mL/min/1.73 m(2), accompanied by proteinuria). To investigate the recurrent and severe hyperkalemia, we performed a patient-tailored functional test and showed that high doses of fludrocortisone induced renal potassium excretion by 155%. Furthermore, fludrocortisone lowered renal sodium excretion by 39%, which would have a mitigating effect on salt-wasting. In addition, urinary pH decreased in response to fludrocortisone. Opposite effects on urinary potassium and pH occurred with administration of amiloride, further supporting the notion that a collecting duct is present and able to react to fludrocortisone. CONCLUSIONS: This report provides living proof that even truncating loss-of-function mutations in AGTR1 are compatible with life and relatively good GFR and provides evidence for the prescription of fludrocortisone to treat hyperkalemia and salt-wasting in such patients.

01 april 2021, Contains fulltext : 238837.pdf (Publisher’s version ) (Open Access), Hypomagnesemia, seizures, and intellectual disability (HSMR) syndrome is a rare disorder caused by mutations in the cyclin M2 (CNNM2) gene. Due to the limited number of cases, extensive phenotype analyses of these patients have not been performed, hindering early recognition of patients. In this study, we established the largest cohort of HSMR to date, aiming to improve recognition and diagnosis of this complex disorder. Eleven novel variants in CNNM2 were identified in nine single sporadic cases and in two families with suspected HSMR syndrome. (25) Mg(2+) uptake assays demonstrated loss-of-function in seven out of nine variants in CNNM2. Interestingly, the pathogenic mutations resulted in decreased plasma membrane expression. The phenotype of those affected by pathogenic CNNM2 mutations was compared with five previously reported cases of HSMR. All patients suffered from hypomagnesemia (0.44-0.72 mmol/L), which could not be fully corrected by Mg(2+) supplementation. The majority of patients (77%) experienced generalized seizures and exhibited mild to moderate intellectual disability and speech delay. Moreover, severe obesity was present in most patients (89%). Our data establish hypomagnesemia, seizures, intellectual disability, and obesity as hallmarks of HSMR syndrome. The assessment of these major features offers a straightforward tool for the clinical diagnosis of HSMR.

Radboud University, 02 juni 2021, Promotores : Hoenderop, J.G.J., Bindels, R.J.M. Co-promotores : Baaij, J.H.F. de, Wijst, J.A.J. van der, Contains fulltext : 233371 .pdf (Publisher’s version ) (Open Access)

1 juni 2020, Contains fulltext : 220968.pdf (Publisher’s version ) (Closed access), The kidney is a remarkable organ that accomplishes the challenge of removing waste from the body and simultaneously regulating electrolyte and water balance. Pro-urine flows through the nephron in a highly dynamic manner and adjustment of the reabsorption rates of water and ions to the variable tubular flow is required for electrolyte homeostasis. Renal epithelial cells sense the tubular flow by mechanosensation. Interest in this phenomenon has increased in the past decade since the acknowledgement of primary cilia as antennae that sense renal tubular flow. However, the significance of tubular flow sensing for electrolyte handling is largely unknown. Signal transduction pathways regulating flow-sensitive physiological responses involve calcium, purinergic and nitric oxide signalling, and are considered to have an important role in renal electrolyte handling. Given that mechanosensation of tubular flow is an integral role of the nephron, defective tubular flow sensing is probably involved in renal disease. Studies investigating tubular flow and electrolyte transport differ in their methodology, subsequently hampering translational validity. This Review provides the basis for understanding electrolyte disorders originating from altered tubular flow sensing as a result of pathological conditions.

1 januari 2020, Contains fulltext : 219905.pdf (publisher's version ) (Open Access), Calcineurin inhibitors (CNIs) are immunosuppressive drugs used to prevent graft rejection after organ transplant. Common side effects include renal magnesium wasting and hypomagnesemia, which may contribute to new-onset diabetes mellitus, and hypercalciuria, which may contribute to post-transplant osteoporosis. Previous work suggested that CNIs reduce the abundance of key divalent cation transport proteins, expressed along the distal convoluted tubule, causing renal magnesium and calcium wasting. It has not been clear, however, whether these effects are specific for the distal convoluted tubule, and whether these represent off-target toxic drug effects, or result from inhibition of calcineurin. The CNI tacrolimus can inhibit calcineurin only when it binds with the immunophilin, FKBP12; we previously generated mice in which FKBP12 could be deleted along the nephron, to test whether calcineurin inhibition is involved, these mice are normal at baseline. Here, we confirmed that tacrolimus-treated control mice developed hypomagnesemia and urinary calcium wasting, with decreased protein and mRNA abundance of key magnesium and calcium transport proteins (NCX-1 and Calbindin-D28k ). However, qPCR also showed decreased mRNA expression of NCX-1 and Calbindin-D28k , and TRPM6. In contrast, KS-FKBP12(-/-) mice treated with tacrolimus were completely protected from these effects. These results indicate that tacrolimus affects calcium and magnesium transport along the distal convoluted tubule and strongly suggests that inhibition of the phosphatase, calcineurin, is directly involved.

Radboud University, 11 februari 2020, Promotores : Bindels, R.J.M., Peters, Dorien J.M. Co-promotor : Arjona, F.J., Contains fulltext : 215968.pdf (publisher's version ) (Open Access)

Contains fulltext : 220948.pdf (publisher's version ) (Open Access), Extracellular vesicles (EV) are nanosized particles released by a large variety of cells. They carry molecules such as proteins, RNA and lipids. While urinary EVs have been longer studied as a source of biomarkers for renal and non-renal disorders, research on EVs as regulatory players of renal physiological and pathological processes has experienced an outbreak recently in the past decade. In general, the microenvironment and (patho)physiological state of the donor cells affect the cargo of the EVs released, which then determines the effect of these EVs once they reach a target cell. For instance, EVs released by renal epithelial cells modulate the expression and function of water and solute transporting proteins in other cells. Also, EVs have been demonstrated to regulate renal organogenesis and blood flow. Furthermore, a dual role of EVs promoting, but also counteracting, disease has also been reported. EVs released by renal tubular cells can reach fibroblasts, monocytes, macrophages, T cells and natural killer cells, thus influencing the pathogenesis and progression of renal disorders like acute kidney injury and fibrosis, nephrolithiasis, renal transplant rejection and renal cancer, among others. On the contrary, EVs may also exert a cytoprotective role upon renal damage and promote recovery of renal function. In the current review, a systematic summary of the key studies from the past 5 years addressing the role of EVs in the modulation of renal physiological and pathophysiological processes is provided, highlighting open questions and discussing the potential of future research.

Contains fulltext : 219079.pdf (Publisher’s version ) (Open Access)

Contains fulltext : 208665.pdf (publisher's version ) (Closed access), Proton pump inhibitors (PPIs) are used by millions of patients for the treatment of stomach acid-reflux diseases. Although PPIs are generally considered safe, about 13% of the users develop hypomagnesemia. Despite rising attention for this issue, the underlying mechanism is still unknown. Here, we examine whether the gut microbiome is involved in the development of PPI-induced hypomagnesemia in wild-type C57BL/6J mice. After 4 wk of treatment under normal or low dietary Mg(2+) availability, omeprazole significantly reduced serum Mg(2+) levels only in mice on a low-Mg(2+) diet without affecting the mRNA expression of colonic or renal Mg(2+) transporters. Overall, 16S rRNA gene sequencing revealed a lower gut microbial diversity in omeprazole-treated mice. Omeprazole induced a shift in microbial composition, which was associated with a 3- and 2-fold increase in the abundance of Lactobacillus and Bifidobacterium, respectively. To examine the metabolic consequences of these microbial alterations, the colonic composition of organic acids was evaluated. Low dietary Mg(2+) intake, independent of omeprazole treatment, resulted in a 10-fold increase in formate levels. Together, these results imply that both omeprazole treatment and low dietary Mg(2+) intake disturb the gut internal milieu and may pose a risk for the malabsorption of Mg(2+) in the colon.-Gommers, L. M. M., Ederveen, T. H. A., van der Wijst, J., Overmars-Bos, C., Kortman, G. A. M., Boekhorst, J., Bindels, R. J. M., de Baaij, J. H. F., Hoenderop, J. G. J. Low gut microbiota diversity and dietary magnesium intake are associated with the development of PPI-induced hypomagnesemia.

Contains fulltext : 201059.pdf (publisher's version ) (Open Access)

Contains fulltext : 203333.pdf (publisher's version ) (Open Access), BACKGROUND: Inhibition of the Na+/glucose co-transporter 2 is a new therapeutic strategy for diabetes. It is unclear how proximal loss of Na+ (and glucose) affects the subsequent Na+ transporters in the proximal tubule (PT), thick ascending limb of loop of Henle (TAL), distal convoluted tubule (DCT) and collecting duct (CD). METHODS: Mice on a high fat diet were administered 3 doses streptozotocin 6 days prior to oral dapagliflozin administration or vehicle for 18 days. A control group of lean mice were also included. Body weight and glucose were recorded at regular intervals during treatment. Renal Na+ transporters expression in nephron segments were analyzed by RT-qPCR and Western blot. RESULTS: Dapagliflozin treatment resulted in a significant reduction in body weight and blood glucose compared to vehicle-treated controls. mRNA results showed that Na+-hydrogen antiporter 3 (NHE3), Na+/phosphate cotransporter (NaPi-2a) and epithelial Na+ channel expression was increased, Ncx1, ENaCbeta and ENaCgamma expression declined (p all < 0.05), respectively, in dapagliflozin-treated mice when compared with saline vehicle mice. Na-K-2Cl cotransporters and Na-Cl cotransporter mRNA expression was not affected by dapagliflozin treatment. Na+/K+-ATPase (Atp1b1) expression was also increased significantly by dapagliflozin treatment, but it did not affect Atp1a1 and glucose transporter 2 expression. Western blot analysis showed that NaPi-2a, NHE3 and ATP1b1 expression was upregulated in dapagliflozin-treated diabetic mice when compared with saline vehicle mice (p < 0.05). CONCLUSION: Our findings suggest that dapagliflozin treatment augments compensatory changes in the renal PT in diabetic mice.

Contains fulltext : 201333.pdf (publisher's version ) (Open Access)

Contains fulltext : 202864.pdf (publisher's version ) (Open Access)

Contains fulltext : 202614.pdf (publisher's version ) (Closed access)

Contains fulltext : 207187pub.pdf (publisher's version ) (Closed access), The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Contains fulltext : 204238.pdf (publisher's version ) (Open Access)

Contains fulltext : 204286.pdf (publisher's version ) (Closed access), Hypomagnesemia (blood Mg(2+) concentration <0.7 mM) is a common electrolyte disorder in patients with type 2 diabetes (T2D), but the etiology remains largely unknown. In patients with T2D, reduced blood Mg(2+) levels are associated with an increased decline in renal function, independent of glycemic control and hypertension. To study the underlying mechanism of this phenomenon, we investigated the renal effects of hypomagnesemia in high-fat-diet (HFD)-fed mice. In mice fed a low dietary Mg(2+), the HFD resulted in severe hypomagnesemia within 4 wk. Renal or intestinal Mg(2+) wasting was not observed after 16 wk on the diets. Despite the absence of urinary or fecal Mg(2+) loss, the HFD induced a reduction in the mRNA expression transient receptor potential melastatin type 6 in both the kidney and colon. mRNA expression of distal convoluted tubule (DCT)-specific genes was down-regulated by the LowMg-HFD, indicating atrophy of the DCT. The low dietary Mg(2+) resulted in severe HFD-induced proximal tubule phospholipidosis, which was absent in mice on a NormalMg-HFD. This was accompanied by albuminuria, moderate renal damage, and alterations in renal energy metabolism, including enhanced gluconeogenesis and cholesterol synthesis. In conclusion, this study shows that hypomagnesemia is a consequence of diet-induced obesity and insulin resistance. Moreover, hypomagnesemia induces major structural changes in the diabetic kidney, including proximal tubular phospholipidosis, providing a novel mechanism for the increased renal decline in patients with hypomagnesemic T2D.-Kurstjens, S., Smeets, B., Overmars-Bos, C., Dijkman, H. B., den Braanker, D. J. W., de Bel, T., Bindels, R. J. M., Tack, C. J. J., Hoenderop, J. G. J., de Baaij, J. H. F. Renal phospholipidosis and impaired magnesium handling in high-fat-diet-fed mice.

Radboud University, 15 februari 2017, Promotores : Bindels, R.J.M., Hoenderop, J.G.J., Hoorn, E.J. Co-promotor : Wijst, J.A.J. van der, Contains fulltext : 182809.pdf (publisher's version ) (Open Access)

Contains fulltext : 196722.pdf (publisher's version ) (Open Access), Up to 15% of the population have mild to moderate chronic hypomagnesemia, which is associated with type 2 diabetes mellitus, hypertension, metabolic syndrome, and chronic kidney disease. The kidney is the key organ for magnesium homeostasis, but our understanding of renal magnesium regulation is very limited. Uromodulin (UMOD) is the most abundant urinary protein in humans, and here we report that UMOD has a role in renal magnesium homeostasis. Umod-knockout (Umod(-/-) ) mice excreted more urinary magnesium than wild-type (WT) and displayed up-regulation of genes promoting magnesium absorption. The majority of magnesium is absorbed in the thick ascending limb. However, both mouse strains responded similarly to the diuretic agent furosemide, indicating appropriate function of the thick ascending limb in the Umod(-/-) mice. Magnesium absorption is fine-tuned in the distal convoluted tubule (DCT) via the apical magnesium channel transient receptor potential melastatin 6 (TRPM6). We observed decreased apical Trpm6 staining in the DCT of Umod(-/-) mice. Applying biotinylation assays and whole-cell patch-clamp recordings we found that UMOD enhances TRPM6 cell-surface abundance and current density from the extracellular space. UMOD physically interacted with TRPM6 and thereby impaired dynamin-dependent TRPM6 endocytosis. WT mice fed a low-magnesium diet had an increased urinary Umod secretion compared with the same mice on a regular diet. Our results suggest that increased urinary UMOD secretion in low-magnesium states reduces TRPM6 endocytosis and thereby up-regulates TRPM6 cell-surface abundance to defend against further urinary magnesium losses.

Contains fulltext : 193490.pdf (publisher's version ) (Closed access), Body Mg(2+) balance is finely regulated in the distal convoluted tubule (DCT), where a tight interplay among transcellular reabsorption, mitochondrial exchange, and basolateral extrusion takes place. In the last decades, several research groups have aimed to identify the molecular players in these processes. A multitude of proteins have been proposed to function as Mg(2+) transporter in eukaryotes based on phylogenetic analysis, differential gene expression, and overexpression studies. However, functional evidence for many of these proteins is lacking. The aim of this review is, therefore, to critically reconsider all putative Mg(2+) transporters and put their presumed function in context of the renal handling of Mg(2+). Sufficient experimental evidence exists to acknowledge transient receptor potential melastatin (TRPM) 6 and TRPM7, solute carrier family 41 (SLC41) A1 and SLC41A3, and mitochondrial RNA splicing 2 (MRS2) as Mg(2+) transporters. TRPM6/7 facilitate Mg(2+) influx, SLC41A1 mediates Mg(2+) extrusion, and MRS2 and SLC41A3 are implicated in mitochondrial Mg(2+) homeostasis. These proteins are highly expressed in the DCT. The function of cyclin M (CNNM) proteins is still under debate. For the other proposed Mg(2+) transporters including Mg(2+) transporter subtype 1 (MagT1), nonimprinted in Prader-Willi/Angelman syndrome (NIPA), membrane Mg(2+) transport (MMgT), Huntingtin-interacting protein 14 (HIP14), and ATP13A4, functional evidence is limited, or functions alternative to Mg(2+) transport have been suggested. Additional characterization of their Mg(2+) transport proficiency should be provided before further claims about their role as Mg(2+) transporter can be made.

Contains fulltext : 194433.pdf (publisher's version ) (Open Access)

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1 januari 2018, Contains fulltext : 184156.pdf (publisher's version ) (Closed access), Magnesium (Mg(2+)) homeostasis is critical for metabolism. However, the genetic determinants of the renal handling of Mg(2+), which is crucial for Mg(2+) homeostasis, and the potential influence on metabolic traits in the general population are unknown. We obtained plasma and urine parameters from 9099 individuals from seven cohorts, and conducted a genome-wide meta-analysis of Mg(2+) homeostasis. We identified two loci associated with urinary magnesium (uMg), rs3824347 (P=4.4x10(-13)) near TRPM6, which encodes an epithelial Mg(2+) channel, and rs35929 (P=2.1x10(-11)), a variant of ARL15, which encodes a GTP-binding protein. Together, these loci account for 2.3% of the variation in 24-hour uMg excretion. In human kidney cells, ARL15 regulated TRPM6-mediated currents. In zebrafish, dietary Mg(2+) regulated the expression of the highly conserved ARL15 ortholog arl15b, and arl15b knockdown resulted in renal Mg(2+) wasting and metabolic disturbances. Finally, ARL15 rs35929 modified the association of uMg with fasting insulin and fat mass in a general population. In conclusion, this combined observational and experimental approach uncovered a gene-environment interaction linking Mg(2+) deficiency to insulin resistance and obesity.

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Radboud University, 12 september 2018, Promotores : Hoenderop, J.G.J., Bindels, R.J.M. Co-promotor : Baaij, J.H.F. de, Contains fulltext : 194292.pdf (publisher's version ) (Open Access)

01 maart 2018, Item does not contain fulltext

Contains fulltext : 193539.pdf (publisher's version ) (Closed access), The distal convoluted tubule (DCT) of the kidney plays an important role in blood pressure regulation by modulating Na(+) reabsorption via the Na(+)-Cl(-) cotransporter (NCC). A diet containing high salt (NaCl) and low K(+) activates NCC, thereby causing Na(+) retention and a rise in blood pressure. Since high blood pressure, hypertension, is associated with changes in serum calcium (Ca(2+)) and magnesium (Mg(2+)) levels, we hypothesized that dietary Na(+) and K(+) intake affects Ca(2+) and Mg(2+) transport in the DCT. Therefore, the present study aimed to investigate the effect of a high-Na(+)/low-K(+) diet on renal Ca(2+) and Mg(2+) handling. Mice were divided in four groups and fed a normal-Na(+)/normal-K(+), normal-Na(+)/low-K(+), high-Na(+)/normal-K(+), or high-Na(+)/low-K(+) diet for 4 days. Serum and urine were collected for electrolyte and hormone analysis. Gene and protein expression of electrolyte transporters were assessed in kidney and intestine by qPCR and immunoblotting. Whereas Mg(2+) homeostasis was not affected, the mice had elevated urinary Ca(2+) and phosphate (Pi) excretion upon high Na(+) intake, as well as significantly lower serum Ca(2+) levels in the high-Na(+)/low-K(+) group. Alterations in the gene and protein expression of players involved in Ca(2+) and Pi transport indicate that reabsorption in the proximal tubular and TAL is affected, while inducing a compensatory response in the DCT. These effects may contribute to the negative health impact of a high-salt diet, including kidney stone formation, chronic kidney disease, and loss of bone mineral density.

Contains fulltext : 194769.pdf (Publisher’s version ) (Closed access)