Novel molecular pathways in renal Mg2+ transport: a guided tour along the nephron

The role of TRPM7 in vascular calcification: Comparison between phosphate and uremic toxin

AIMS

Vascular calcification is a common complication in patients with chronic kidney disease and associated with increased morbidity and mortality. The role of TRPM7 in vascular smooth muscle cell (VSMC) transformation during vascular calcification is not clear. We aim to investigate the effects of phosphate and indoxyl sulphate on the expression of TRPM7 and calcification-related molecules in VSMC.

MAIN METHODS

Human aortic smooth muscle cells (HASMC) were treated with phosphate (3.3 mM) or indoxyl sulphate (500 μM and 1000 μM). 2-APB, a channel blocker of TRPM7 was added simultaneously in blocking experiment. Cells were then examined grossly and alizarin red solution was employed for calcification assessment. Lastly, cells were harvested for gene expression and protein abundance analysis.

KEY FINDINGS

Phosphate treatment induced significant increase in BMP2, RUNX2, BMP7, vitamin D receptor (VDR), calcium sensing receptor (CaSR) and TRPM7, but 1-alpha hydroxylase, klotho, DKK1 and sclerostin were not changed. The addition of 2-APB prevented increase of BMP2, RUNX2, BMP7, VDR, CaSR and TRPM7. Indoxyl sulphate treatment was associated with decrease in TRPM7 and DKK1, but increase in RUNX2, BMP2 and VDR were noted. There were no significant alterations in BMP7, CaSR, klotho,1-alpha hydroxylase and sclerostin. Co-treatment with 2-APB reversed the increase in VDR.

SIGNIFICANCE

Both phosphate and indoxyl sulphate induced calcification in VSMC but it was more prominent in phosphate. TRPM7 was upregulated by phosphate but downregulated in indoxyl sulphate treatment. Vascular calcification was reduced by blocking TRPM7 with 2-APB and there was partial anti-calcification effect in indoxyl sulphate.

A novel TRPM6 variant (c.3179T>A) causing familial hypomagnesemia with secondary hypocalcemia

SUMMARY

Familial hypomagnesemia with secondary hypocalcemia (FHSH) is a rare autosomal recessive disorder (OMIM# 602014) characterized by profound hypomagnesemia associated with hypocalcemia. It is caused by mutations in the gene encoding transient receptor potential cation channel member 6 (TRPM6). It usually presents with neurological symptoms in the first months of life. We report a case of a neonate presenting with recurrent seizures and severe hypomagnesemia. The genetic testing revealed a novel variant in the TRPM6 gene. The patient has been treated with high-dose magnesium supplementation, remaining asymptomatic and without neurological sequelae until adulthood. Early diagnosis and treatment are important to prevent irreversible neurological damage.

LEARNING POINTS

Loss-of-function mutations of TRPM6 are associated with FHSH. FHSH should be considered in any child with refractory hypocalcemic seizures, especially in cases with serum magnesium levels as low as 0.2 mM. Normocalcemia and relief of clinical symptoms can be assured by administration of high doses of magnesium. Untreated, the disorder may be fatal or may result in irreversible neurological damage.

Hepatocyte nuclear factor-1β: A regulator of kidney development and cystogenesis

The understanding of the genomics of the renal tissue has gathered a considerable interest and is making rapid progress. The molecular mechanisms as well as the precise function of the associated molecular components toward renal pathophysiology have recently been realized. For the cystic kidney disease, the regulation of gene expression affecting epithelial cells proliferation, apoptosis as well as process of differentiation/de-differentiation represent key molecular targets. For the cystic disorders, molecular targets have been identified, which besides lending heterogeneity to cysts may also provide tools to unravel their functional importance to understand the renal tissue homeostasis. This review focuses on providing comprehensive information about the transcriptional regulatory role of hepatocyte nuclear factor-1β, a homeoprotein, as well as its interacting partners in renal tissue development and pathophysiology.

Nucleotide binding triggers a conformational change of the CBS module of the magnesium transporter CNNM2 from a twisted towards a flat structure

Recent studies suggest CNNM2 (cyclin M2) to be part of the long-sought basolateral Mg2+ extruder at the renal distal convoluted tubule, or its regulator. In the present study, we explore structural features and ligand-binding capacities of the Bateman module of CNNM2 (residues 429-584), an intracellular domain structurally equivalent to the region involved in Mg2+ handling by the bacterial Mg2+ transporter MgtE, and AMP binding by the Mg2+ efflux protein CorC. Additionally, we studied the structural impact of the pathogenic mutation T568I located in this region. Our crystal structures reveal that nucleotides such as AMP, ADP or ATP bind at only one of the two cavities present in CNNM2429-584. Mg2+ favours ATP binding by alleviating the otherwise negative charge repulsion existing between acidic residues and the polyphosphate group of ATP. In crystals CNNM2429-584 forms parallel dimers, commonly referred to as CBS (cystathionine β-synthase) modules. Interestingly, nucleotide binding triggers a conformational change in the CBS module from a twisted towards a flat disc-like structure that mostly affects the structural elements connecting the Bateman module with the transmembrane region. We furthermore show that the T568I mutation, which causes dominant hypomagnesaemia, mimics the structural effect induced by nucleotide binding. The results of the present study suggest that the T568I mutation exerts its pathogenic effect in humans by constraining the conformational equilibrium of the CBS module of CNNM2, which becomes 'locked' in its flat form.

Uriniferous tubule: structural and functional organization

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

New TRPM6 missense mutations linked to hypomagnesemia with secondary hypocalcemia

Despite recent progress in our understanding of renal magnesium (Mg(2+)) handling, the molecular mechanisms accounting for transepithelial Mg(2+) transport are still poorly understood. Mutations in the TRPM6 gene, encoding the epithelial Mg(2+) channel TRPM6 (transient receptor potential melastatin 6), have been proven to be the molecular cause of hypomagnesemia with secondary hypocalcemia (HSH; OMIM 602014). HSH manifests in the newborn period being characterized by very low serum Mg(2+) levels (<0.4 mmol/l) accompanied by low serum calcium (Ca(2+)) concentrations. A proportion of previously described TRPM6 mutations lead to a truncated TRPM6 protein resulting in a complete loss-of-function of the ion channel. In addition, five-point mutations have been previously described. The aim of this study was to complement the current clinical picture by adding the molecular data from five new missense mutations found in five patients with HSH. To this end, patch-clamp analysis and cell surface measurements were performed to assess the effect of the various mutations on TRPM6 channel function. All mutant channels, expressed in HEK293 cells, showed loss-of-function, whereas no severe trafficking impairment to the plasma membrane surface was observed. We conclude that the new TRPM6 missense mutations lead to dysregulated intestinal/renal Mg(2+) (re)absorption as a consequence of loss of TRPM6 channel function.

Ankyrin-3 is a novel binding partner of the voltage-gated potassium channel Kv1.1 implicated in renal magnesium handling

The voltage-gated potassium channel, Kv1.1, was recently identified as a causative gene in isolated dominant hypomagnesemia. The channel is situated in the distal convoluted tubule, where it participates in maintaining a favorable electrical gradient for driving magnesium ion into the cell through the transient receptor potential melastatin 6 channel. Pull-down experiments coupled to mass spectrometry using the carboxy-terminal domain of Kv1.1 as bait were used in mouse kidney lysates. Ankyrin-3 (ANK3) was identified as a binding partner of Kv1.1 and was enriched in isolated distal convoluted tubules as compared to whole kidney. Electrophysiology studies performed in HEK293 cells expressing Kv1.1 showed that ANK3 significantly inhibited Kv1.1-mediated currents (267 compared to 125 pA/pF) for control and ANK3, respectively. Finally, to evaluate a potential role of ANK3 in magnesium handling, the intrarenal abundance of ANK3 was measured in mice fed a low-, normal-, or high-magnesium diet for 10 days. Mice maintained on high dietary magnesium significantly doubled their fractional urinary excretion of magnesium, which coincided with a 1.8-fold increase in the renal expression of ANK3 compared to mice on a normal- or low-magnesium diet. Thus, our observations demonstrate a novel role for ANK3 in modulating the biophysical properties of Kv1.1. Such regulation appears to be particularly important in conditions of high dietary magnesium.

[Magnesium disorders]

Extracellular content in magnesium represents about 1% of total body content, of which plasma magnesium is thus a poor reflect. Hypomagnesaemia is defined by a value lesser than 0.65mmol/L. Its incidence in hospitalized patients ranges between 10 and 15%. Identification of the physiopathology of hypomagnesaemia relies first upon concomitant measurement of plasma and urinary magnesium concentration. Daily magnesium excretion lesser than 1mmol/L or EFMg lesser than 1% sign extra renal origin, due to either low magnesium intake, low intestinal absorption of magnesium or derivation of extracellular magnesium toward bone, such as in bone reparation process after hyperparathyroidism surgery. Daily magnesium excretion higher than 2mmol/L concomitant to hypomagnesaemia indicates native or acquired renal loss of magnesium. Congenital renal and extra-renal losses of magnesium are mainly related to rare monogenic disease, and are inconstantly associated with a renal loss of sodium, potassium and calcium. Recent progress in the genetics of this rare diseases have greatly improved the knowledge about proteins involved in intestinal abortion, renal renal tubular re-absorption of magnesium and its regulations. Hypermagnesemia is a rarer metabolic disorder than hypomagnesemia (about 5% of hospitalized patients). Asymptomatic below 2mmol/L, it progressively alters neuromuscular transmission, autonomic sympathic activity and cardiac conduction, with vital risk above 7mol/L. It is due to acute magnesium input into extracellular volume most often associated with a decrease in glomerular filtration rate, limiting the high physiological ability to excrete magnesium input.