EGF increases TRPM6 activity and surface expression

Sodium/Glucose Cotransporter 2 Inhibitors and Magnesium Homeostasis: A Review

Magnesium (Mg2+), also known as "the forgotten ion," is the second most abundant intracellular cation and is essential in a broad range of intracellular physiological and biochemical reactions. Its deficiency, hypomagnesemia (Mg2+<1.8mg/dL), is a prevalent condition and routinely poses challenges in its management in clinical practice. Sodium/glucose cotransporter 2 (SGLT2) inhibitors have emerged as a new class of drugs with treating hypomagnesemia as their unique extraglycemic benefit. The beneficial effect of SGLT2 inhibitors on magnesium balance in patients with diabetes with or without hypomagnesemia has been noted as a class effect in recent meta-analysis data from randomized clinical trials. Some reports have demonstrated their role in treating refractory hypomagnesemia in patients with or without diabetes. Moreover, studies on animal models have attempted to illustrate the effect of SGLT2 inhibitors on Mg2+homeostasis. In this review, we discuss the current evidence and possible pathophysiological mechanisms, and we provide directions for further research. We conclude by suggesting the effect of SGLT2 inhibitors on Mg2+homeostasis is a class effect, with certain patients gaining significant benefits. Further studies are needed to examine whether SGLT2 inhibitors can become a desperately needed novel class of medicines in treating hypomagnesemia.

Enriched Single-Nucleus RNA-Sequencing Reveals Unique Attributes of Distal Convoluted Tubule Cells

SIGNIFICANCE STATEMENT

High-resolution single-nucleus RNA-sequencing data indicate a clear separation between primary sites of calcium and magnesium handling within distal convoluted tubule (DCT). Both DCT1 and DCT2 express Slc12a3, but these subsegments serve distinctive functions, with more abundant magnesium-handling genes along DCT1 and more calcium-handling genes along DCT2. The data also provide insight into the plasticity of the distal nephron-collecting duct junction, formed from cells of separate embryonic origins. By focusing/changing gradients of gene expression, the DCT can morph into different physiological cell states on demand.

BACKGROUND

The distal convoluted tubule (DCT) comprises two subsegments, DCT1 and DCT2, with different functional and molecular characteristics. The functional and molecular distinction between these segments, however, has been controversial.

METHODS

To understand the heterogeneity within the DCT population with better clarity, we enriched for DCT nuclei by using a mouse line combining "Isolation of Nuclei Tagged in specific Cell Types" and sodium chloride cotransporter-driven inducible Cre recombinase. We sorted the fluorescently labeled DCT nuclei using Fluorescence-Activated Nucleus Sorting and performed single-nucleus transcriptomics.

RESULTS

Among 25,183 DCT cells, 75% were from DCT1 and 25% were from DCT2. In addition, there was a small population (<1%) enriched in proliferation-related genes, such as Top2a , Cenpp , and Mki67 . Although both DCT1 and DCT2 expressed sodium chloride cotransporter, magnesium transport genes were predominantly expressed along DCT1, whereas calcium, electrogenic sodium, and potassium transport genes were more abundant along DCT2. The transition between these two segments was gradual, with a transitional zone in which DCT1 and DCT2 cells were interspersed. The expression of the homeobox genes by DCT cells suggests that they develop along different trajectories.

CONCLUSIONS

Transcriptomic analysis of an enriched rare cell population using a genetically targeted approach clarifies the function and classification of distal cells. The DCT segment is short, can be separated into two subsegments that serve distinct functions, and is speculated to derive from different origins during development.

Drug-induced glomerular diseases

Drug-induced kidney diseases represent a wide range of diseases that are responsible for a significant proportion of all acute kidney injuries and chronic kidney diseases. In the present review, we focused on drug-induced glomerular diseases, more precisely podocytopathies - minimal change diseases (MCD), focal segmental glomerulosclerosis (FSGS) - and membranous nephropathies (MN), from a physiological and a pharmacological point of view. The glomerular filtration barrier is composed of podocytes that form foot processes tightly connected and directly in contact with the basal membrane and surrounding capillaries. The common clinical feature of these diseases is represented by the loss of the ability of the filtration barrier to retain large proteins, leading to massive proteinuria and nephrotic syndrome. Drugs such as non-steroidal anti-inflammatory drugs (NSAIDs), D-penicillamine, tiopronin, trace elements, bisphosphonate, and interferons have been historically associated with the occurrence of MCD, FSGS, and MN. In the last ten years, the development of new anti-cancer agents, including tyrosine kinase inhibitors and immune checkpoint inhibitors, and research into their renal adverse effects highlighted these issues and have improved our comprehension of these diseases.

Metal profiling in coronary ischemia-reperfusion injury: Implications for KEAP1/NRF2 regulated redox signaling

Coronary ischemia-reperfusion (IR) injury results from a blockage of blood supply to the heart followed by restoration of perfusion, leading to oxidative stress induced pathological processes. Nuclear factor erythroid 2-related factor 2 (NRF2), a master antioxidant transcription factor, plays a key role in regulating redox signaling. Over the past decades, the field of metallomics has provided novel insights into the mechanism of pro-oxidant and antioxidant pathological processes. Both redox-active (e.g. Fe and Cu) and redox-inert (e.g. Zn and Mg) metals play unique roles in establishing redox balance under IR injury. Notably, Zn protects against oxidative stress in coronary IR injury by serving as a cofactor of antioxidant enzymes such as superoxide dismutase [Cu-Zn] (SOD1) and proteins such as metallothionein (MT) and KEAP1/NRF2 mediated antioxidant defenses. An increase in labile Zn2+ inhibits proteasomal degradation and ubiquitination of NRF2 by modifying KEAP1 and glycogen synthase kinase 3β (GSK3β) conformations. Fe and Cu catalyse the formation of reactive oxygen species via the Fenton reaction and also serve as cofactors of antioxidant enzymes and can activate NRF2 antioxidant signaling. We review the evidence that Zn and redox-active metals Fe and Cu affect redox signaling in coronary cells during IR and the mechanisms by which oxidative stress influences cellular metal content. In view of the unique double-edged characteristics of metals, we aim to bridge the role of metals and NRF2 regulated redox signaling to antioxidant defenses in IR injury, with a long-term aim of informing the design and application of novel therapeutics.

Renal diseases that course with hypomagnesemia. Comments on a new hereditary hypomagnesemic tubulopathy

Renal diseases associated with hypomagnesemia are a complex and diverse group of tubulopathies caused by mutations in genes encoding proteins that are expressed in the thick ascending limb of the loop of Henle and in the distal convoluted tubule. In this paper, we review the initial description, the clinical expressiveness and etiology of four of the first hypomagnesemic tubulopathies described: type 3 Bartter and Gitelman diseases, Autosomal recessive hypomagnesemia with secondary hypocalcemia and Familial hypomagnesemia with hypercalciuria and nephrocalcinosis. The basic biochemical patterns observed in renal tubular hypomagnesemias and the modalities of transport and interaction that occur between the transporters involved in the reabsorption of magnesium in the distal convoluted tubule are described below. Finally, the recent report of a new renal disease with hypomagnesemia, type 2 hypomagnesemia with secondary hypocalcemia caused by reduced TRPM7 channel activity is described.

Risk Factors of Cetuximab-Induced Hypomagnesemia and the Effect of Magnesium Prophylaxis in Patients with Head and Neck Cancer: A Retrospective Study

Hypomagnesemia is a characteristic adverse event of cetuximab in patients with head and neck cancer (HNC). However, there is limited information about its prevalence, risk factors, and preventive strategies. This study aimed to investigate the risk factors of hypomagnesemia and examine the preventive effects of prophylactic magnesium (Mg) administration. We initially investigated HNC patients treated with cetuximab between 2013 and 2019. Our institute started prophylactic Mg treatment (20-mEq Mg sulfate administration before cetuximab) in practice during this period. We retrospectively assess the preventive efficacy by comparing patients before and after its implementation. In total, 109 patients were included. In 60 patients without prophylaxis, all-grade and grade ≥2 hypomagnesemia at 3 months occurred in 61.7 and 15.0% of patients. The incidence of hypomagnesemia was not affected by regimens and concomitant medications. In 49 patients treated with prophylactic Mg treatment, there was no significant decrease in the cumulative incidence of hypomagnesemia. However, the preventive Mg treatment eliminated the need for additional Mg repletion to maintain Mg levels in patients treated with paclitaxel + cetuximab. A risk factor in patients without prophylaxis was a low Mg level at pre-treatment (≤2.0 mg/dL) (odds ratio: 6.03, 95% confidence interval: 1.78-20.4, p = 0.004), whereas that in patients with prophylaxis was the number of cetuximab doses (≥10) (odds ratio: 5.50, 95% confidence interval: 1.52-19.87, p = 0.009). In conclusion, a low pre-treatment Mg level was the only risk factor that could be avoided by prophylactic Mg administration. This preventive intervention is recommended for managing cetuximab-induced hypomagnesemia.

Magnesium and Liver Metabolism Through the Lifespan

Within the organism, the liver is the main organ responsible for metabolic homeostasis and xenobiotic transformation. To maintain an adequate liver weight-to-bodyweight ratio, this organ has an extraordinary regenerative capacity and is able to respond to an acute insult or partial hepatectomy. Maintenance of hepatic homeostasis is crucial for the proper functioning of the liver, and in this context, adequate nutrition with macro- and micronutrient intake is mandatory. Among all known macro-minerals, magnesium has a key role in energy metabolism and in metabolic and signaling pathways that maintain liver function and physiology throughout its life span. In the present review, the cation is reported as a potential key molecule during embryogenesis, liver regeneration, and aging. The exact role of the cation during liver formation and regeneration is not fully understood due to its unclear role in the activation and inhibition of those processes, and further research in a developmental context is needed. As individuals age, they may develop hypomagnesemia, a condition that aggravates the characteristic alterations. Additionally, risk of developing liver pathologies increases with age, and hypomagnesemia may be a contributing factor. Therefore, magnesium loss must be prevented by adequate intake of magnesium-rich foods such as seeds, nuts, spinach, or rice to prevent age-related hepatic alterations and contribute to the maintenance of hepatic homeostasis. Since magnesium-rich sources include a variety of foods, a varied and balanced diet can meet both macronutrient and micronutrient needs.

Magnesium reabsorption in the kidney

Mg²⁺ is essential for many cellular and physiological processes, including muscle contraction, neuronal activity, and metabolism. Consequently, the blood Mg²⁺ concentration is tightly regulated by balanced intestinal Mg²⁺ absorption, renal Mg²⁺ excretion, and Mg²⁺ storage in bone and soft tissues. In recent years, the development of novel transgenic animal models and identification of Mendelian disorders has advanced our current insight in the molecular mechanisms of Mg²⁺ reabsorption in the kidney. In the proximal tubule, Mg²⁺ reabsorption is dependent on paracellular permeability by claudin-2/12. In the thick ascending limb of Henle's loop, the claudin-16/19 complex provides a cation-selective pore for paracellular Mg²⁺ reabsorption. The paracellular Mg²⁺ reabsorption in this segment is regulated by the Ca²⁺-sensing receptor, parathyroid hormone, and mechanistic target of rapamycin (mTOR) signaling. In the distal convoluted tubule, the fine tuning of Mg²⁺ reabsorption takes place by transcellular Mg²⁺ reabsorption via transient receptor potential melastatin-like types 6 and 7 (TRPM6/TRPM7) divalent cation channels. Activity of TRPM6/TRPM7 is dependent on hormonal regulation, metabolic activity, and interacting proteins. Basolateral Mg²⁺ extrusion is still poorly understood but is probably dependent on the Na⁺ gradient. Cyclin M2 and SLC41A3 are the main candidates to act as Na⁺/Mg²⁺ exchangers. Consequently, disturbances of basolateral Na⁺/K⁺ transport indirectly result in impaired renal Mg²⁺ reabsorption in the distal convoluted tubule. Altogether, this review aims to provide an overview of the molecular mechanisms of Mg²⁺ reabsorption in the kidney, specifically focusing on transgenic mouse models and human hereditary diseases.

Cardiovascular toxicity of tyrosine kinase inhibitors during cancer treatment: Potential involvement of TRPM7

Receptor tyrosine kinases (RTKs) are a class of membrane spanning cell-surface receptors that transmit extracellular signals through the membrane to trigger diverse intracellular signaling through tyrosine kinases (TKs), and play important role in cancer development. Therapeutic approaches targeting RTKs such as vascular endothelial growth factor receptor (VEGFR), epidermal growth factor receptor (EGFR), and platelet-derived growth factor receptor (PDGFR), and TKs, such as c-Src, ABL, JAK, are widely used to treat human cancers. Despite favorable benefits in cancer treatment that prolong survival, these tyrosine kinase inhibitors (TKIs) and monoclonal antibodies targeting RTKs are also accompanied by adverse effects, including cardiovascular toxicity. Mechanisms underlying TKI-induced cardiovascular toxicity remain unclear. The transient receptor potential melastatin-subfamily member 7 (TRPM7) is a ubiquitously expressed chanzyme consisting of a membrane-based ion channel and intracellular α-kinase. TRPM7 is a cation channel that regulates transmembrane Mg²⁺ and Ca²⁺ and is involved in a variety of (patho)physiological processes in the cardiovascular system, contributing to hypertension, cardiac fibrosis, inflammation, and atrial arrhythmias. Of importance, we and others demonstrated significant cross-talk between TRPM7, RTKs, and TK signaling in different cell types including vascular smooth muscle cells (VSMCs), which might be a link between TKIs and their cardiovascular effects. In this review, we summarize the implications of RTK inhibitors (RTKIs) and TKIs in cardiovascular toxicities during anti-cancer treatment, with a focus on the potential role of TRPM7/Mg²⁺ as a mediator of RTKI/TKI-induced cardiovascular toxicity. We also describe the important role of TRPM7 in cancer development and cardiovascular diseases, and the interaction between TRPM7 and RTKs, providing insights for possible mechanisms underlying cardiovascular disease in cancer patients treated with RTKI/TKIs.

Magnesium Homeostasis: Lessons from Human Genetics

Mg2+, the fourth most abundant cation in the body, serves as a co-factor for about 600 cellular enzymes. One third of ingested Mg2+ is absorbed from the gut through a saturable transcellular process and a concentration-dependent paracellular process. Absorbed Mg2+ is excreted by the kidney and maintains serum Mg2+ within a narrow range of 0.7 to 1.25 mmol/L. The reabsorption of Mg2+ by the nephron is characterized by paracellular transport in the proximal tubule and thick ascending limb. The nature of the transport pathways in the gut epithelia and thick ascending limb has emerged from an understanding of the molecular mechanisms responsible for rare monogenetic disorders presenting with clinical hypomagnesemia. These human disorders due to loss-of function mutations, in concert with mouse models have led to a deeper understanding of Mg2+ transport in the gut and renal tubule. This review focuses on the nature of the transporters and channels revealed by human and mouse genetics and how they are integrated into an understanding of human Mg2+ physiology.

Hypomagnesemia and Cardiovascular Risk in Type 2 Diabetes

Hypomagnesemia is tenfold more common in individuals with type 2 diabetes (T2D), compared to the healthy population. Factors that are involved in this high prevalence are low Mg2+ intake, gut microbiome composition, medication use and presumably genetics. Hypomagnesemia is associated with insulin resistance, which subsequently increases the risk to develop T2D or deteriorates glycaemic control in existing diabetes. Mg2+ supplementation decreases T2D associated features like dyslipidaemia and inflammation; which are important risk factors for cardiovascular disease (CVD). Epidemiological studies have shown an inverse association between serum Mg2+ and the risk to develop heart failure (HF), atrial fibrillation (AF) and microvascular disease in T2D. The potential protective effect of Mg2+ on HF and AF may be explained by reduced oxidative stress, fibrosis and electrical remodeling in the heart. In microvascular disease, Mg2+ reduces the detrimental effects of hyperglycemia and improves endothelial dysfunction. Though, clinical studies assessing the effect of long-term Mg2+ supplementation on CVD incidents are lacking and gaps remain on how Mg2+ may reduce CVD risk in T2D. Despite the high prevalence of hypomagnesemia in people with T2D, routine screening of Mg2+ deficiency to provide Mg2+ supplementation when needed is not implemented in clinical care as sufficient clinical evidence is lacking. In conclusion, hypomagnesemia is common in people with T2D and is both involved as cause, probably through molecular mechanisms leading to insulin resistance, and consequence and is prospectively associated with development of HF, AF and microvascular complications. Whether long-term supplementation of Mg2+ is beneficial, however, remains to be determined.

Structural and functional comparison of magnesium transporters throughout evolution

Magnesium (Mg²⁺) is the most prevalent divalent intracellular cation. As co-factor in many enzymatic reactions, Mg²⁺ is essential for protein synthesis, energy production, and DNA stability. Disturbances in intracellular Mg²⁺ concentrations, therefore, unequivocally result in delayed cell growth and metabolic defects. To maintain physiological Mg²⁺ levels, all organisms rely on balanced Mg²⁺ influx and efflux via Mg²⁺ channels and transporters. This review compares the structure and the function of prokaryotic Mg²⁺ transporters and their eukaryotic counterparts. In prokaryotes, cellular Mg²⁺ homeostasis is orchestrated via the CorA, MgtA/B, MgtE, and CorB/C Mg²⁺ 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 Mg²⁺ 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⁺/Mg²⁺ transporters. In eukaryotes, TRPM6 and TRPM7 Mg²⁺ channels provide an additional Mg²⁺ 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 Mg²⁺ transport capacity. Our review demonstrates that understanding the structure and function of prokaryotic magnesiotropic proteins aids in our basic understanding of Mg²⁺ transport.

Possible role for rare TRPM7 variants in patients with hypomagnesaemia with secondary hypocalcaemia

BACKGROUND

Hypomagnesaemia with secondary hypocal-caemia (HSH) is a rare autosomal recessive disorder caused by pathogenic variants in TRPM6, encoding the channel-kinase transient receptor potential melastatin type 6. Patients have very low serum magnesium (Mg2+) levels and suffer from muscle cramps and seizures. Despite genetic testing, a subgroup of HSH patients remains without a diagnosis.

METHODS

In this study, two families with an HSH phenotype but negative for TRPM6 pathogenic variants were subjected to whole exome sequencing. Using a complementary combination of biochemical and functional analyses in overexpression systems and patient-derived fibroblasts, the effect of the TRPM7-identified variants on Mg2+ transport was examined.

RESULTS

For the first time, variants in TRPM7 were identified in two families as a potential cause for hereditary HSH. Patients suffer from seizures and muscle cramps due to magnesium deficiency and episodes of hypocalcaemia. In the first family, a splice site variant caused the incorporation of intron 1 sequences into the TRPM7 messenger RNA and generated a premature stop codon. As a consequence, patient-derived fibroblasts exhibit decreased cell growth. In the second family, a heterozygous missense variant in the pore domain resulted in decreased TRPM7 channel activity.

CONCLUSIONS

We establish TRPM7 as a prime candidate gene for autosomal dominant hypomagnesaemia and secondary hypocalcaemia. Screening of unresolved patients with hypocalcaemia and secondary hypocalcaemia may further establish TRPM7 pathogenic variants as a novel Mendelian disorder.

Urinary Epidermal Growth Factor: A Promising "Next Generation" Biomarker in Kidney Disease

BACKGROUND

The epidermal growth factor (EGF) is a globular protein that is generated in the kidney, especially in the loop of Henle and the distal convoluted tubule. While EGF is nonexistent or hardly detectable in plasma, it is present in normal people's urine. Until now, risk stratification and chronic kidney disease (CKD) diagnosis have relied on estimated glomerular filtration rate (eGFR) and urine albumin/creatinine ratio (uACR), both of which reflect glomerular function or impairment. Tubular dysfunction, on the other hand, may also be associated with renal failure.

SUMMARY

Because decreased urine EGF (uEGF) indicates tubular atrophy and interstitial fibrosis, this biomarker, together with eGFR and uACR, may be employed in the general population for risk assessment and diagnosis of CKD. uEGF levels have been shown to correlate with intrarenal EGF mRNA expression and have been found to decrease in a variety of glomerular and non-glomerular kidney disorders.

KEY MESSAGE

uEGF, uEGF/creatinine, or uEGF/monocyte chemotactic peptide-1 are possible "new generation" biomarkers linked to a variety of kidney diseases that deserve further investigation as a single biomarker or as part of a multi-biomarker panel.

Transcriptional Control of Trpm6 by the Nuclear Receptor FXR

Farnesoid x receptor (FXR) is a nuclear bile acid receptor that belongs to the nuclear receptor superfamily. It plays an essential role in bile acid biosynthesis, lipid and glucose metabolism, liver regeneration, and vertical sleeve gastrectomy. A loss of the FXR gene or dysregulations of FXR-mediated gene expression are associated with the development of progressive familial intrahepatic cholestasis, tumorigenesis, inflammation, and diabetes mellitus. Magnesium ion (Mg²⁺) is essential for mammalian physiology. Over 600 enzymes are dependent on Mg²⁺ for their activity. Here, we show that the Trpm6 gene encoding a Mg²⁺ channel is a direct FXR target gene in the intestinal epithelial cells of mice. FXR expressed in the intestinal epithelial cells is absolutely required for sustaining a basal expression of intestinal Trpm6 that can be robustly induced by the treatment of GW4064, a synthetic FXR agonist. Analysis of FXR ChIP-seq data revealed that intron regions of Trpm6 contain two prominent FXR binding peaks. Among them, the proximal peak from the transcription start site contains a functional inverted repeat 1 (IR1) response element that directly binds to the FXR-RXRα heterodimer. Based on these results, we proposed that an intestinal FXR-TRPM6 axis may link a bile acid signaling to Mg²⁺ homeostasis.

Hereditary kidney diseases associated with hypomagnesemia

In the kidney, a set of proteins expressed in the epithelial cells of the thick ascending loop of Henle and the distal convoluted tubule directly or indirectly play important roles in the regulation of serum magnesium levels. Magnesium reabsorption in the thick ascending loop of Henle occurs through a passive paracellular pathway, while in the distal convoluted tubule, the final magnesium concentration is established through an active transcellular pathway. The players involved in magnesium reabsorption include proteins with diverse functions including tight junction proteins, cation and anion channels, sodium chloride cotransporter, calcium-sensing receptor, epidermal growth factor, cyclin M2, sodium potassium adenosine triphosphatase subunits, transcription factors, a serine protease, and proteins involved in mitochondrial function. Mutations in the genes that encode these proteins impair their function and cause different rare diseases associated with hypomagnesemia, which may lead to muscle cramps, fatigue, epileptic seizures, intellectual disability, cardiac arrhythmias, and chronic kidney disease. The purpose of this review is to describe the clinical and genetic characteristics of these hereditary kidney diseases and the current research findings on the pathophysiological basis of these diseases.

Clinical and Genetic Approach to Renal Hypomagnesemia

Magnesium (Mg²⁺) is an important intracellular cation and essential to maintain cell function including cell proliferation, immunity, cellular energy metabolism, protein and nucleic acid synthesis, and regulation of ion channels. Consequences of hypomagnesemia affecting multiple organs can be in overt or subtle presentations. Besides detailed history and complete physical examination, the assessment of urinary Mg²⁺ excretion is help to differentiate renal from extra-renal (gastrointestinal, tissue sequestration, and shifting) causes of hypomagnesemia. Renal hypomagnesemia can be caused by an increased glomerular filtration and impaired reabsorption in proximal tubular cells, thick ascending limb of the loop of Henle or distal convoluted tubules. A combination of renal Mg²⁺ wasting, familial history, age of onset, associated features, and exclusion of acquired etiologies point to inherited forms of renal hypomagnesemia. Based on clinical phenotypes, its definite genetic diagnosis can be simply grouped into specific, uncertain, and unknown gene mutations with a priority of genetic approach methods. An unequivocal molecular diagnosis could allow for prediction of clinical outcome, providing genetic counseling, avoiding unnecessary studies or interventions, and possibly uncovering the pathogenic mechanism. Given numerous identified genes responsible for Mg²⁺ transport in renal hypomagnesemia over the past two decades, several potential and specific molecular and cellular therapeutic strategies to correct hypomagnesemia are promising.

mTOR-Activating Mutations in RRAGD Are Causative for Kidney Tubulopathy and Cardiomyopathy

BACKGROUND

Over the last decade, advances in genetic techniques have resulted in the identification of rare hereditary disorders of renal magnesium and salt handling. Nevertheless, approximately 20% of all patients with tubulopathy lack a genetic diagnosis.

METHODS

We performed whole-exome and -genome sequencing of a patient cohort with a novel, inherited, salt-losing tubulopathy; hypomagnesemia; and dilated cardiomyopathy. We also conducted subsequent in vitro functional analyses of identified variants of RRAGD, a gene that encodes a small Rag guanosine triphosphatase (GTPase).

RESULTS

In eight children from unrelated families with a tubulopathy characterized by hypomagnesemia, hypokalemia, salt wasting, and nephrocalcinosis, we identified heterozygous missense variants in RRAGD that mostly occurred de novo. Six of these patients also had dilated cardiomyopathy and three underwent heart transplantation. We identified a heterozygous variant in RRAGD that segregated with the phenotype in eight members of a large family with similar kidney manifestations. The GTPase RagD, encoded by RRAGD, plays a role in mediating amino acid signaling to the mechanistic target of rapamycin complex 1 (mTORC1). RagD expression along the mammalian nephron included the thick ascending limb and the distal convoluted tubule. The identified RRAGD variants were shown to induce a constitutive activation of mTOR signaling in vitro.

CONCLUSIONS

Our findings establish a novel disease, which we call autosomal dominant kidney hypomagnesemia (ADKH-RRAGD), that combines an electrolyte-losing tubulopathy and dilated cardiomyopathy. The condition is caused by variants in the RRAGD gene, which encodes Rag GTPase D; these variants lead to an activation of mTOR signaling, suggesting a critical role of Rag GTPase D for renal electrolyte handling and cardiac function.

Diagnostic Dilemma in an Adolescent Girl with an Eating Disorder, Intellectual Disability, and Hypomagnesemia

Neurological disorders, including seizures, migraine, depression, and intellectual disability, are frequently associated with hypomagnesemia. Specifically, magnesium (Mg2+) channel transient receptor potential melastatin (TRPM) 6 and TRPM7 are essential for brain function and development. Both channels are also localized in renal and intestinal epithelia and are crucial for Mg2+(re)absorption. Cyclin M2 (CNNM2) is located on the basolateral side of the distal convoluted tubule. In addition, it plays a role in the maintenance of plasma Mg2+ levels along with TRPM6, which is present at the apical level. The CNNM2 gene is crucial for renal magnesium handling, brain development, and neurological functioning. Here, we identified a novel mutation in the CNNM2 gene causing a cognitive delay in a girl with hypomagnesemia. We suggest testing for CNNM2 mutation in patients with neurological impairment and hypomagnesemia.

Molecular Mechanisms of Renal Magnesium Reabsorption

Magnesium is an essential cofactor in many cellular processes, and aberrations in magnesium homeostasis can have life-threatening consequences. The kidney plays a central role in maintaining serum magnesium within a narrow range (0.70-1.10 mmol/L). Along the proximal tubule and thick ascending limb, magnesium reabsorption occurs via paracellular pathways. Members of the claudin family form the magnesium pores in these segments, and also regulate magnesium reabsorption by adjusting the transepithelial voltage that drives it. Along the distal convoluted tubule transcellular reabsorption via heteromeric TRPM6/7 channels predominates, although paracellular reabsorption may also occur. In this segment, the NaCl cotransporter plays a critical role in determining transcellular magnesium reabsorption. Although the general machinery involved in renal magnesium reabsorption has been identified by studying genetic forms of magnesium imbalance, the mechanisms regulating it are poorly understood. This review discusses pathways of renal magnesium reabsorption by different segments of the nephron, emphasizing newer findings that provide insight into regulatory process, and outlining critical unanswered questions.

Genetic and drug-induced hypomagnesemia: different cause, same mechanism

Magnesium (Mg2+) plays an essential role in many biological processes. Mg2+ deficiency is therefore associated with a wide range of clinical effects including muscle cramps, fatigue, seizures and arrhythmias. To maintain sufficient Mg2+ levels, (re)absorption of Mg2+ in the intestine and kidney is tightly regulated. Genetic defects that disturb Mg2+ uptake pathways, as well as drugs interfering with Mg2+ (re)absorption cause hypomagnesemia. The aim of this review is to provide an overview of the molecular mechanisms underlying genetic and drug-induced Mg2+ deficiencies. This leads to the identification of four main mechanisms that are affected by hypomagnesemia-causing mutations or drugs: luminal transient receptor potential melastatin type 6/7-mediated Mg2+ uptake, paracellular Mg2+ reabsorption in the thick ascending limb of Henle's loop, structural integrity of the distal convoluted tubule and Na+-dependent Mg2+ extrusion driven by the Na+/K+-ATPase. Our analysis demonstrates that genetic and drug-induced causes of hypomagnesemia share common molecular mechanisms. Targeting these shared pathways can lead to novel treatment options for patients with hypomagnesemia.

Cyclin M2 (CNNM2) knockout mice show mild hypomagnesaemia and developmental defects

Patients with mutations in Cyclin M2 (CNNM2) suffer from hypomagnesaemia, seizures, and intellectual disability. Although the molecular function of CNNM2 is under debate, the protein is considered essential for renal Mg²⁺ reabsorption. Here, we used a Cnnm2 knock out mouse model, generated by CRISPR/Cas9 technology, to assess the role of CNNM2 in Mg²⁺ homeostasis. Breeding Cnnm2^+/- mice resulted in a Mendelian distribution at embryonic day 18. Nevertheless, only four Cnnm2^-/- pups were born alive. The Cnnm2^-/- pups had a significantly lower serum Mg²⁺ concentration compared to wildtype littermates. Subsequently, adult Cnnm2^+/- mice were fed with low, control, or high Mg²⁺ diets for two weeks. Adult Cnnm2^+/- mice showed mild hypomagnesaemia compared to Cnnm2^+/+ mice and increased serum Ca²⁺ levels, independent of dietary Mg²⁺ intake. Faecal analysis displayed increased Mg²⁺ and Ca²⁺ excretion in the Cnnm2^+/- mice. Transcriptional profiling of Trpm6, Trpm7, and Slc41a1 in kidneys and colon did not reveal effects based on genotype. Microcomputed tomography analysis of the femurs demonstrated equal bone morphology and density. In conclusion, CNNM2 is vital for embryonic development and Mg²⁺ homeostasis. Our data suggest a previously undescribed role of CNNM2 in the intestine, which may contribute to the Mg²⁺ deficiency in mice and patients.

Mechanisms coupling sodium and magnesium reabsorption in the distal convoluted tubule of the kidney

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 Mg²⁺ reabsorption is tightly regulated. Apical uptake via TRPM6 Mg²⁺ channels and basolateral Mg²⁺ extrusion via a putative Na⁺ -Mg²⁺ exchanger determines Mg²⁺ 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 Mg²⁺ 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 Mg²⁺ 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 Mg²⁺ 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⁺ -Mg²⁺ exchanger SLC41A1. Elucidating the interconnections between Mg²⁺ and Na⁺ transport in the distal convoluted tubule is hampered by the currently available models. Our analysis indicates that the coupling of Na⁺ and Mg²⁺ reabsorption may be multifactorial and that advanced experimental models are required to study the molecular mechanisms.

Mg2+ Transporters in Digestive Cancers

Despite magnesium (Mg²⁺) representing the second most abundant cation in the cell, its role in cellular physiology and pathology is far from being elucidated. Mg²⁺ homeostasis is regulated by Mg²⁺ transporters including Mitochondrial RNA Splicing Protein 2 (MRS2), Transient Receptor Potential Cation Channel Subfamily M, Member 6/7 (TRPM6/7), Magnesium Transporter 1 (MAGT1), Solute Carrier Family 41 Member 1 (SCL41A1), and Cyclin and CBS Domain Divalent Metal Cation Transport Mediator (CNNM) proteins. Recent data show that Mg²⁺ transporters may regulate several cancer cell hallmarks. In this review, we describe the expression of Mg²⁺ transporters in digestive cancers, the most common and deadliest malignancies worldwide. Moreover, Mg²⁺ transporters’ expression, correlation and impact on patient overall and disease-free survival is analyzed using Genotype Tissue Expression (GTEx) and The Cancer Genome Atlas (TCGA) datasets. Finally, we discuss the role of these Mg²⁺ transporters in the regulation of cancer cell fates and oncogenic signaling pathways.

Transient receptor potential channel regulation by growth factors

Calcium (Ca²⁺) is one of the most universal secondary messengers, owing its success to the immense concentration gradient across the plasma membrane. Dysregulation of Ca²⁺ homeostasis can result in severe cell dysfunction, thereby initiating several pathologies like tumorigenesis and fibrosis. Transient receptor potential (TRP) channels represent a superfamily of Ca²⁺-permeable ion channels that convey diverse physical and chemical stimuli into a physiological signal. Their broad expression pattern and gating promiscuity support their potential involvement in the cellular response to an altering environment. Growth factors (GF) are essential biochemical messengers that contribute to these environmental changes. Since Ca²⁺ is essential in GF signaling, altering TRP channel expression or function could be a valid strategy for GF to exert their effect onto their target. In this review, a comprehensive understanding of the current knowledge regarding the activation and/or modulation of TRP channels by GF is presented.

TRPM Channels in Human Diseases

The transient receptor potential melastatin (TRPM) subfamily belongs to the TRP cation channels family. Since the first cloning of TRPM1 in 1989, tremendous progress has been made in identifying novel members of the TRPM subfamily and their functions. The TRPM subfamily is composed of eight members consisting of four six-transmembrane domain subunits, resulting in homomeric or heteromeric channels. From a structural point of view, based on the homology sequence of the coiled-coil in the C-terminus, the eight TRPM members are clustered into four groups: TRPM1/M3, M2/M8, M4/M5 and M6/M7. TRPM subfamily members have been involved in several physiological functions. However, they are also linked to diverse pathophysiological human processes. Alterations in the expression and function of TRPM subfamily ion channels might generate several human diseases including cardiovascular and neurodegenerative alterations, organ dysfunction, cancer and many other channelopathies. These effects position them as remarkable putative targets for novel diagnostic strategies, drug design and therapeutic approaches. Here, we review the current knowledge about the main characteristics of all members of the TRPM family, focusing on their actions in human diseases.

Deletion of the transcription factor Prox-1 specifically in the renal distal convoluted tubule causes hypomagnesemia via reduced expression of TRPM6 and NCC

The renal distal convoluted tubule (DCT) is critical for the fine-tuning of urinary ion excretion and the control of blood pressure. Ion transport along the DCT is tightly controlled by posttranscriptional mechanisms including a complex interplay of kinases, phosphatases, and ubiquitin ligases. Previous work identified the transcription factor Prox-1 as a gene significantly enriched in the DCT of adult mice. To test if Prox-1 contributes to the transcriptional regulation of DCT function and structure, we developed a novel mouse model (NCC^cre:Prox-1^flox/flox) for an inducible deletion of Prox-1 specifically in the DCT. The deletion of Prox-1 had no obvious impact on DCT structure and growth independent whether the deletion was achieved in newborn or adult mice. Furthermore, DCT-specific Prox-1 deficiency did not alter DCT-proliferation in response to loop diuretic treatment. Likewise, the DCT-specific deletion of Prox-1 did not cause other gross phenotypic abnormalities. Body weight, urinary volume, Na⁺ and K⁺ excretion as well as plasma Na⁺, K⁺, and aldosterone levels were similar in Prox-1_DCT^KO and Prox-1_DCT^Ctrl mice. However, Prox-1_DCT^KO mice exhibited a significant hypomagnesemia with a profound downregulation of the DCT-specific apical Mg²⁺ channel TRPM6 and the NaCl cotransporter (NCC) at both mRNA and protein levels. The expression of other proteins involved in distal tubule Mg²⁺ and Na⁺ handling was not affected. Thus, Prox-1 is a DCT-enriched transcription factor that does not control DCT growth but contributes to the molecular control of DCT-dependent Mg²⁺ homeostasis in the adult kidney.

NaCl cotransporter activity and Mg2+ handling by the distal convoluted tubule

The genetic disease Gitelman syndrome, knockout mice, and pharmacological blockade with thiazide diuretics have revealed that reduced activity of the NaCl cotransporter (NCC) promotes renal Mg²⁺ wasting. NCC is expressed along the distal convoluted tubule (DCT), and its activity determines Mg²⁺ entry into DCT cells through transient receptor potential channel subfamily M member 6 (TRPM6). Several other genetic forms of hypomagnesemia lower the drive for Mg²⁺ entry by inhibiting activity of basolateral Na⁺-K⁺-ATPase, and reduced NCC activity may do the same. Lower intracellular Mg²⁺ may promote further Mg²⁺ loss by directly decreasing activity of Na⁺-K⁺-ATPase. Lower intracellular Mg²⁺ may also lower Na⁺-K⁺-ATPase indirectly by downregulating NCC. Lower NCC activity also induces atrophy of DCT cells, decreasing the available number of TRPM6 channels. Conversely, a mouse model with increased NCC activity was recently shown to display normal Mg²⁺ handling. Moreover, recent studies have identified calcineurin and uromodulin (UMOD) as regulators of both NCC and Mg²⁺ handling by the DCT. Calcineurin inhibitors paradoxically cause hypomagnesemia in a state of NCC activation, but this may be related to direct effects on TRPM6 gene expression. In Umod^-/- mice, the cause of hypomagnesemia may be partly due to both decreased NCC expression and lower TRPM6 expression on the cell surface. This mini-review discusses these new findings and the possible role of altered Na⁺ flux through NCC and ultimately Na⁺-K⁺-ATPase in Mg²⁺ reabsorption by the DCT.

Magnesium Absorption in Intestinal Cells: Evidence of Cross-Talk between EGF and TRPM6 and Novel Implications for Cetuximab Therapy

Hypomagnesemia is very commonly observed in cancer patients, most frequently in association with therapy with cetuximab (CTX), a monoclonal antibody targeting the epithelial growth factor receptor (EGFR). CTX-induced hypomagnesemia has been ascribed to renal magnesium (Mg) wasting. Here, we sought to clarify whether CTX may also influence intestinal Mg absorption and if Mg supplementation may interfere with CTX activity. We used human colon carcinoma CaCo-2 cells as an in vitro model to study the mechanisms underlying Mg transport and CTX activity. Our findings demonstrate that TRPM6 is the key channel that mediates Mg influx in intestinal cells and that EGF stimulates such influx; consequently, CTX downregulates TRPM6-mediated Mg influx by interfering with EGF signaling. Moreover, we show that Mg supplementation does not modify either the CTX IC50 or CTX-dependent inhibition of ERK1/2 phosphorylation. Our results suggest that reduced Mg absorption in the intestine may contribute to the severe hypomagnesemia that occurs in CTX-treated patients, and Mg supplementation may represent a safe and effective nutritional intervention to restore Mg status without impairing the CTX efficacy.

Epidermal growth factor signaling through transient receptor potential melastatin 7 cation channel regulates vascular smooth muscle cell function

OBJECTIVE

Transient receptor potential (TRP) melastatin 7 (TRPM7) cation channel, a dual-function ion channel/protein kinase, regulates vascular smooth muscle cell (VSMC) Mg2+ homeostasis and mitogenic signaling. Mechanisms regulating vascular growth effects of TRPM7 are unclear, but epidermal growth factor (EGF) may be important because it is a magnesiotropic hormone involved in cellular Mg2+ regulation and VSMC proliferation. Here we sought to determine whether TRPM7 is a downstream target of EGF in VSMCs and if EGF receptor (EGFR) through TRPM7 influences VSMC function. Approach and results: Studies were performed in primary culture VSMCs from rats and humans and vascular tissue from mice deficient in TRPM7 (TRPM7+/Δkinase and TRPM7R/R). EGF increased expression and phosphorylation of TRPM7 and stimulated Mg2+ influx in VSMCs, responses that were attenuated by gefitinib (EGFR inhibitor) and NS8593 (TRPM7 inhibitor). Co-immunoprecipitation (IP) studies, proximity ligation assay (PLA) and live-cell imaging demonstrated interaction of EGFR and TRPM7, which was enhanced by EGF. PP2 (c-Src inhibitor) decreased EGF-induced TRPM7 activation and prevented EGFR-TRPM7 association. EGF-stimulated migration and proliferation of VSMCs were inhibited by gefitinib, PP2, NS8593 and PD98059 (ERK1/2 inhibitor). Phosphorylation of EGFR and ERK1/2 was reduced in VSMCs from TRPM7+/Δkinase mice, which exhibited reduced aortic wall thickness and decreased expression of PCNA and Notch 3, findings recapitulated in TRPM7R/R mice.

CONCLUSIONS

We show that EGFR directly interacts with TRPM7 through c-Src-dependent processes. Functionally these phenomena regulate [Mg2+]i homeostasis, ERK1/2 signaling and VSMC function. Our findings define a novel signaling cascade linking EGF/EGFR and TRPM7, important in vascular homeostasis.

Metformin regulates TRPM6, a potential explanation for magnesium imbalance in type 2 diabetes patients

Metformin therapy is associated with lower serum magnesium (Mg²⁺) levels in type 2 diabetes patients. The TRPM6 channel determines the fine-tuning of Mg²⁺ (re)absorption in intestine and kidney. Therefore, we aimed to investigate the short- and long-term effects of metformin on TRPM6. Patch clamp recordings and biotinylation assays were performed upon 1 h of incubation with metformin in TRPM6-transfected HEK293 cells. Additionally, 24 h of treatment of mDCT15 kidney and hCaco-2 colon cells with metformin was applied to measure the effects on endogenous TRPM6 expression by quantitative real-time PCR. To assess Mg²⁺ absorption, ²⁵Mg²⁺ uptake measurements were performed using inductively coupled plasma mass spectrometry. Short-term effects of metformin significantly increased TRPM6 activity and its cell surface trafficking. In contrast, long-term effects significantly decreased TRPM6 mRNA expression and ²⁵Mg²⁺ uptake. Metformin lowered TRPM6 mRNA levels independently of insulin- and AMPK-mediated pathways. Moreover, in type 2 diabetes patients, metformin therapy was associated with lower plasma Mg²⁺ concentrations and fractional excretion of Mg²⁺. Thereby, short-term metformin treatment increases TRPM6 activity explained by enhanced cell surface expression. Conversely, long-term metformin treatment results in downregulation of TRPM6 gene expression in intestine and kidney cells. This long-term effect translated in an inverse correlation between metformin and plasma Mg²⁺ concentration in type 2 diabetes patients.

Learning Physiology From Inherited Kidney Disorders

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.

TRPM7, Magnesium, and Signaling

The transient receptor potential melastatin-subfamily member 7 (TRPM7) is a ubiquitously expressed chanzyme that possesses an ion channel permeable to the divalent cations Mg2+, Ca2+, and Zn2+, and an α-kinase that phosphorylates downstream substrates. TRPM7 and its homologue TRPM6 have been implicated in a variety of cellular functions and is critically associated with intracellular signaling, including receptor tyrosine kinase (RTK)-mediated pathways. Emerging evidence indicates that growth factors, such as EGF and VEGF, signal through their RTKs, which regulate activity of TRPM6 and TRPM7. TRPM6 is primarily an epithelial-associated channel, while TRPM7 is more ubiquitous. In this review we focus on TRPM7 and its association with growth factors, RTKs, and downstream kinase signaling. We also highlight how interplay between TRPM7, Mg2+ and signaling kinases influences cell function in physiological and pathological conditions, such as cancer and preeclampsia.

Cyanidin Increases the Expression of Mg2+ Transport Carriers Mediated by the Activation of PPARα in Colonic Epithelial MCE301 Cells

Mg²⁺ deficiency may be involved in lifestyle-related diseases, including hypertension, cardiovascular diseases, and diabetes mellitus. Dietary Mg²⁺ is absorbed in the intestine mediated through transcellular and paracellular pathways. However, there is little research into what factors upregulate Mg²⁺ absorption. We searched for food constituents that can increase the expression levels of Mg²⁺ transport carriers using mouse colonic epithelial MCE301 cells. Cyanidin, an anthocyanidin found in black beans and berries, increased the mRNA levels of Mg²⁺ transport carriers including transient receptor potential melastatin 6 (TRPM6) channel and cyclin M4 (CNNM4). The cyanidin-induced elevation of Mg²⁺ transport carriers was blocked by GW6471, a peroxisome proliferator-activated receptor α (PPARα) inhibitor, but not by PPARγ, PPARδ, and protein kinase A inhibitors. Cyanidin-3-glucoside showed similar results to cyanidin. Cyanidin increased the protein levels of TRPM6 and CNNM4, which were distributed in the apical and lateral membranes, respectively. The nuclear localization of PPARα and reporter activities of Mg²⁺ transport carriers were increased by cyanidin, which were inhibited by GW6471. The cyanidin-induced elevation of reporter activity was suppressed by a mutation in a PPAR-response element. Fluorescence measurements using KMG-20, an Mg²⁺ indicator, showed that Mg²⁺ influx and efflux from the cells were enhanced by cyanidin, and which were inhibited by GW6471. Furthermore, cyanidin increased paracellular Mg²⁺ flux without affecting transepithelial electrical resistance. We suggest that cyanidin increases intestinal Mg²⁺ absorption mediated by the elevation of TRPM6 and CNNM4 expression, and may constitute a phytochemical that can improve Mg²⁺ deficiency.

Renal phospholipidosis and impaired magnesium handling in high-fat-diet-fed mice

Hypomagnesemia (blood Mg²⁺ 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 Mg²⁺ 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 Mg²⁺, the HFD resulted in severe hypomagnesemia within 4 wk. Renal or intestinal Mg²⁺ wasting was not observed after 16 wk on the diets. Despite the absence of urinary or fecal Mg²⁺ 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 Mg²⁺ 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.

Inhibition of non-receptor tyrosine kinase Src induces phosphoserine 256-independent aquaporin-2 membrane accumulation

KEY POINTS

Aquaporin-2 (AQP2) is crucial for water homeostasis, and vasopressin (VP) induces AQP2 membrane trafficking by increasing intracellular cAMP, activating PKA and causing phosphorylation of AQP2 at serine 256, 264 and 269 residues and dephosphorylation of serine 261 residue on the AQP2 C-terminus. It is thought that serine 256 is the master regulator of AQP2 trafficking, and its phosphorylation has to precede the change of phosphorylation state of other serine residues. We found that Src inhibition causes serine 256-independent AQP2 membrane trafficking and induces phosphorylation of serine 269 independently of serine 256. This targeted phosphorylation of serine 269 is important for Src inhibition-induced AQP2 membrane accumulation; without serine 269, Src inhibition exerts no effect on AQP2 trafficking. This result helps us better understand the independent pathways that can target different AQP2 residues, and design new strategies to induce or sustain AQP2 membrane expression when VP signalling is defective.

ABSTRACT

Aquaporin-2 (AQP2) is essential for water homeostasis. Upon stimulation by vasopressin, AQP2 is phosphorylated at serine 256 (S256), S264 and S269, and dephosphorylated at S261. It is thought that S256 is the master regulator of AQP2 trafficking and membrane accumulation, and that its phosphorylation has to precede phosphorylation of other serine residues. In this study, we found that VP reduces Src kinase phosphorylation: by suppressing Src using the inhibitor dasatinib and siRNA, we could increase AQP2 membrane accumulation in cultured AQP2-expressing cells and in kidney collecting duct principal cells. Src inhibition increased exocytosis and inhibited clathrin-mediated endocytosis of AQP2, but exerted its effect in a cAMP, PKA and S256 phosphorylation (pS256)-independent manner. Despite the lack of S256 phosphorylation, dasatinib increased phosphorylation of S269, even in S256A mutant cells in which S256 phosphorylation cannot occur. To confirm the importance of pS269 in AQP2 re-distribution, we expressed an AQP2 S269A mutant in LLC-PK1 cells, and found that dasatinib no longer induced AQP2 membrane accumulation. In conclusion, Src inhibition causes phosphorylation of S269 independently of pS256, and induces AQP2 membrane accumulation by inhibiting clathrin-mediated endocytosis and increasing exocytosis. We conclude that S269 can be phosphorylated without pS256, and pS269 alone is important for AQP2 apical membrane accumulation under some conditions. These data increase our understanding of the independent pathways that can phosphorylate different residues in the AQP2 C-terminus, and suggest new strategies to target distinct AQP2 serine residues to induce membrane expression of this water channel when VP signalling is defective.

Diabetes-induced hypomagnesemia is not modulated by metformin treatment in mice

Approximately 30% of patients with type 2 diabetes mellitus (T2D) have hypomagnesemia (blood magnesium (Mg²⁺) concentration <0.7 mmol/L). In T2D patients, treatment with metformin is associated with reduced blood Mg²⁺ levels. To investigate how T2D and metformin affect Mg²⁺ homeostasis db/m and db/db mice were treated with metformin or placebo. Mice were housed in metabolic cages to measure food and water intake, and to collect urine and feces. Serum and urinary Mg²⁺ concentrations were determined and mRNA expression of magnesiotropic genes was determined in kidney and distal colon using RT-qPCR. Db/db mice had significantly lower serum Mg²⁺ levels than db/m mice. Mild hypermagnesuria was observed in the db/db mice at two weeks, but not at four weeks. Metformin-treatment had no effect on the serum Mg²⁺ concentration and on the urinary Mg²⁺ excretion. Both in kidney and distal colon of db/db mice, there was a compensatory upregulation in the mRNA expression of magnesiotropic genes, such as transient receptor potential melastatin 6 (Trpm6), whereas metformin treatment did not affect gene expression levels. In conclusion, we show that T2D causes hypomagnesemia and that metformin treatment has no effect on Mg²⁺ homeostasis in mice.

Exome sequencing identifies a novel frameshift variant causing hypomagnesemia with secondary hypocalcemia

Hypomagnesemia with secondary hypocalcemia is a rare autosomal-recessive disorder characterized by intense hypomagnesemia associated with hypocalcemia (HSH). Mutations in the TRPM6 gene, encoding the epithelial Mg2+ channel TRPM6, have been proven to be the molecular cause of this disease. This study identified causal mutations in a 2-month-old male patient of hypomagnesemia from a consanguineous marriage. Biochemical analyses indicated the diagnosis of HSH due to primary gastrointestinal loss of magnesium. Whole exome sequencing of the trio (i.e. proband and both parents) was carried out with mean coverage of > 150×. ANNOVAR was used to annotate functional consequences of genetic variation from exome sequencing data. After variant filtering and annotation, a number of single nucleotide variants (SNVs) and 2 bp deletion at exon26:c.4402_4403delCT in TRPM6 gene were identified. This deletion which resulted in a novel frameshift mutation in exon 26 of this gene was confirmed by Sanger sequencing. With these investigations in hand, the patient was managed with magnesium sulphate. The patient remained asymptomatic and was developmentally and neurologically normal till his last follow up.

Epidermal growth factor and its influencing variables in healthy children and adults

Background & objective

Epidermal growth factor (EGF) stimulates cell proliferation and differentiation after binding to its receptor. Next to its role in magnesium homeostasis, EGF disturbances have been described in oncology, diabetes and autism spectrum disorders. The aim of this study was to determine EGF serum and urine values for both healthy children and adults. Next, we investigated the relation between several variables and urinary and serum EGF concentrations.

Methods

Both healthy adults (n = 50) and children (n = 78) were included. Serum and urinary EGF concentrations were measured with ELISA technology.

Results

Serum EGF was inversely correlated with age (r = —0.873; p<0.001) and positively correlated with serum magnesium (r = 0.597; p<0.001). The urinary EGF was also inversely correlated with age (r = -0.855; p<0.001). In adults and children older than 13 years of age, the urinary EGF significantly differed between sexes (p = 0.001). Urinary EGF was positively correlated with serum magnesium (r = 0.583; p<0.001) and creatinine clearance (r = 0.524; p<0.001) and negatively correlated with the fractional excretion of magnesium (r = 0.248; p = 0.014). In a multivariate model, age and serum magnesium remained independently related to serum EGF while age, serum EGF and serum magnesium remained independently related to urinary EGF.

Conclusions

This study provides valuable insights in urinary and serum EGF patterns in healthy subjects. By systematically correcting EGF for body surface, significant correlations with age, gender and magnesium were observed.

SLC41A1 is essential for magnesium homeostasis in vivo

Solute carrier family 41 member A1 (SLC41A1) has been suggested to mediate magnesium (Mg²⁺) transport by several in vitro studies. However, the physiological function of SLC41A1 remains to be elucidated. In this study, cellular Mg²⁺ transport assays combined with zebrafish slc41a1 knockdown experiments were performed to disclose SLC41A1 function and its physiological relevance. The gene slc41a1 is ubiquitously expressed in zebrafish tissues and is regulated by water and dietary Mg²⁺ availability. Knockdown of slc41a1 in zebrafish larvae grown in a Mg²⁺-free medium resulted in a unique phenotype characterized by a decrease in zebrafish Mg content. This decrease shows that SLC41A1 is required to maintain Mg²⁺ balance and its dysfunction results in renal Mg²⁺ wasting in zebrafish larvae. Importantly, the Mg content of the larvae is rescued when mouse SLC41A1 is expressed in slc41a1-knockdown zebrafish. Conversely, expression of mammalian SLC41A1-p.Asp262Ala, harboring a mutation in the ion-conducting SLC41A1 pore, did not reverse the renal Mg²⁺ wasting. ²⁵Mg²⁺ transport assays in human embryonic kidney 293 (HEK293) cells overexpressing SLC41A1 demonstrated that SLC41A1 mediates cellular Mg²⁺ extrusion independently of sodium (Na⁺). In contrast, SLC41A1-p.Asp262Ala expressing HEK293 cells displayed similar Mg²⁺ extrusion activities than control (mock) cells. In polarized Madin-Darby canine kidney cells, SLC41A1 localized to the basolateral cell membrane. Our results demonstrate that SLC41A1 facilitates renal Mg²⁺ reabsorption in the zebrafish model. Furthermore, our data suggest that SLC41A1 mediates both Mg²⁺ uptake and extrusion.

Uromodulin regulates renal magnesium homeostasis through the ion channel transient receptor potential melastatin 6 (TRPM6)

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 WT mice 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.

Role of Magnesium Deficiency in Promoting Atherosclerosis, Endothelial Dysfunction, and Arterial Stiffening as Risk Factors for Hypertension

Arterial hypertension is a disease with a complex pathogenesis. Despite considerable knowledge about this socially significant disease, the role of magnesium deficiency (MgD) as a risk factor is not fully understood. Magnesium is a natural calcium antagonist. It potentiates the production of local vasodilator mediators (prostacyclin and nitric oxide) and alters vascular responses to a variety of vasoactive substances (endothelin-1, angiotensin II, and catecholamines). MgD stimulates the production of aldosterone and potentiates vascular inflammatory response, while expression/activity of various antioxidant enzymes (glutathione peroxidase, superoxide dismutase, and catalase) and the levels of important antioxidants (vitamin C, vitamin E, and selenium) are decreased. Magnesium balances the effects of catecholamines in acute and chronic stress. MgD may be associated with the development of insulin resistance, hyperglycemia, and changes in lipid metabolism, which enhance atherosclerotic changes and arterial stiffness. Magnesium regulates collagen and elastin turnover in the vascular wall and matrix metalloproteinase activity. Magnesium helps to protect the elastic fibers from calcium deposition and maintains the elasticity of the vessels. Considering the numerous positive effects on a number of mechanisms related to arterial hypertension, consuming a healthy diet that provides the recommended amount of magnesium can be an appropriate strategy for helping control blood pressure.

Longitudinal Study of the Role of Epidermal Growth Factor on the Fractional Excretion of Magnesium in Children: Effect of Calcineurin Inhibitors

BACKGROUND

It was shown in animal models and adults that the epidermal growth factor (EGF) is involved in the pathophysiology of calcineurin inhibitor (CNI) induced renal magnesium loss. In children, however, the exact mechanism remains unclear, which was set as the purpose of the present study.

METHODS

Children with nephrotic syndrome and renal transplant children treated with CNI (n = 50) and non-CNI treated children (n = 46) were included in this study. Urine and serum samples were collected at three time points to determine magnesium, creatinine, and EGF. The magnesium intake was calculated from a food frequency questionnaire.

RESULTS

Serum Mg²⁺ and urinary EGF/creatinine were significantly lower in the CNI treated children, with significantly more CNI-treated children developing hypomagnesaemia. In the latter patients, the fractional excretion of magnesium (FE Mg²⁺) was significantly higher. Urinary EGF, age, renal function, and serum magnesium were independent predictors of the FE Mg²⁺. Only 29% of the children reached the recommended daily intake of magnesium. The magnesium intake did not differ between hypomagnesemic and normomagnesemic patients and was not a predictor of the FE Mg²⁺.

CONCLUSIONS

In CNI-treated children who developed hypomagnesemia, the FE Mg²⁺ was increased. The urinary EGF concentration, age, and renal function are independent predictors of the FE Mg²⁺.

Magnesium Handling in the Kidney

Magnesium is a divalent cation that fills essential roles as regulator and cofactor in a variety of biological pathways, and maintenance of magnesium balance is vital to human health. The kidney, in concert with the intestine, has an important role in maintaining magnesium homeostasis. Although micropuncture and microperfusion studies in the mammalian nephron have shone a light on magnesium handling in the various nephron segments, much of what we know about the protein mediators of magnesium handling in the kidney have come from more recent genetic studies. In the proximal tubule and thick ascending limb, magnesium reabsorption is believed to occur primarily through the paracellular shunt pathway, which ultimately depends on the electrochemical gradient setup by active sodium reabsorption. In the distal convoluted tubule, magnesium transport is transcellular, although magnesium reabsorption also appears to be related to active sodium reabsorption in this segment. In addition, evidence suggests that magnesium transport is highly regulated, although a specific hormonal regulator of extracellular magnesium has yet to be identified.

Magnesium Balance in Chronic and End-Stage Kidney Disease

This article explores the effects of CKD and end-stage kidney disease on magnesium balance. In CKD, there is decreased glomerular filtration of magnesium. Decreased tubular reabsorption can compensate to a degree, but once CKD stage 4 is reached there is a tendency toward hypermagnesemia. In dialysis, magnesium balance is dependent on the constituents of the dialysate that the blood is exposed to. The concentration of dialysate magnesium is just one of the factors that need to be considered. During transplantation, there are particular effects of immunosuppressants that can affect the magnesium balance and need to be considered by the clinician.

Common single nucleotide polymorphisms in transient receptor potential melastatin type 6 increase the risk for proton pump inhibitor-induced hypomagnesemia: a case-control study

OBJECTIVE

Proton pump inhibitors (PPIs) are effective drugs for the treatment of gastric acid-related disorders. Serious adverse events are rare for PPIs, but recent data suggest that PPIs cause hypomagnesemia. The aim of this study was to estimate the frequency of PPI-induced hypomagnesemia and to define the risk factors for its development.

MATERIALS AND METHODS

A total of 133 chronic users of PPIs were enrolled and patients were distinguished on the basis of their serum Mg concentrations. Common single nucleotide polymorphisms (SNPs) in the candidate gene, transient receptor potential melastatin type 6 (TRPM6), were screened.

RESULTS

Seventeen out of 133 patients had PPI-induced hypomagnesemia. The duration of PPI use was longer in those with hypomagnesemia (7.7 vs. 5.2 years). Two common SNPs in TRPM6 (rs3750425 and rs2274924) increased the risk for PPI-induced hypomagnesemia by 5.8-fold.

CONCLUSION

We found hypomagnesemia in 13% of PPI users. SNPs in TRPM6 drive the risk of developing hypomagnesemia during chronic PPI use.

Genetics of Magnesium Disorders

Background

Magnesium (Mg²⁺), the second most abundant cation in the cell, is woven into a multitude of cellular functions. Dysmagnesemia is associated with multiple diseases and, when severe, can be life-threatening.

Summary

This review discusses Mg²⁺ homeostasis and function with specific focus on renal Mg²⁺ handling. Intrarenal channels and transporters related to Mg²⁺ absorption are discussed. Unraveling the rare genetic diseases with manifestations of dysmagnesemia has greatly increased our understanding of the complex and intricate regulatory network in the kidney, specifically, functions of tight junction proteins including claudin-14, -16, -19, and -10; apical ion channels including: TRPM6, K_v1.1, and ROMK; small regulatory proteins including AC3 and ANK3; and basolateral proteins including EGF receptor, γ-subunit (FXYD2) of Na-K-ATPase, K_ir4.1, CaSR, CNNM2, and SLC41A. Although our understanding of Mg²⁺ handling of the kidney has expanded considerably in the last two decades, many questions remain. Future studies are needed to elucidate a multitude of unknown aspects of Mg²⁺ handling in the kidney.

Key Message

Understanding rare and genetic diseases of Mg²⁺ dysregulation has expanded our knowledge and furthers the development of strategies for preventing and managing dysmagnesemia.