Elucidation of the distal convoluted tubule transcriptome identifies new candidate genes involved in renal Mg2+ handling

A distal convoluted tubule-specific isoform of murine SLC41A3 extrudes magnesium

BACKGROUND

The distal convoluted tubule (DCT) plays an indispensable role in magnesium (Mg2+) reabsorption in the kidney. Yet, the extrusion mechanism of Mg2+ has not been identified. The solute carrier 41A3 (SLC41A3) has been suggested to be involved in Mg2+ extrusion, but this has never been conclusively demonstrated.

METHODS

Using available RNA-sequencing data and real-time quantitative PCR, expression of two alternative Slc41a3 transcripts, encoding isoform (Iso) 1 or 2, were assessed in kidney and isolated DCT tubules. HEK293 or HAP1 cells were transfected with plasmids expressing either of the isoforms, followed by 25Mg2+ transport studies. Identification of cis-regulatory elements (CRE) was achieved by combining data from publicly available ATAC sequencing data and luciferase assays.

RESULTS

Gene expression studies revealed a distinct transcript of Slc41a3 in the DCT with an alternative promoter, leading to a protein with a unique N-terminus; SLC41A3-Iso 2. HEK293 cells overexpressing SLC41A3-Iso 2, but not -Iso 1, exhibited 2.7-fold and 1.6-fold higher 25Mg2+ uptake and extrusion, compared to mock, respectively. The transport was independent of Na+, of the Mg2+ channel TRPM7 or of transporters CNNM3 and -4. We identified a CRE accessible in the DCT, ±2.8kb upstream of the transcript. The presence of the CRE increased the Slc41a3-Iso 2 promoter activity 3.8-fold following luciferase assays, indicating the CRE contains an enhancer function.

CONCLUSION

In conclusion, we identified two alternative transcripts of Slc41a3 in mouse. Slc41a3-Iso 2 is enriched within the DCT using specific gene regulatory elements. We speculate that specifically in the DCT, SLC41A3-Iso 2 orchestrates Mg2+ extrusion.

Magnesium Homeostasis: Lessons from Human Genetics

Mg 2+ , the fourth most abundant cation in the body, serves as a cofactor for about 600 cellular enzymes. One third of ingested Mg 2+ is absorbed from the gut through a saturable transcellular process and a concentration-dependent paracellular process. Absorbed Mg 2+ is excreted by the kidney and maintains serum Mg 2+ within a narrow range of 0.7-1.25 mmol/L. The reabsorption of Mg 2+ 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 Mg 2+ 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 Mg 2+ physiology.

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.

Functional characteristics and therapeutic potential of SLC41 transporters

Magnesium (Mg²⁺) plays an important role in various cellular functions such as protein synthesis, DNA stability, energy metabolism, enzyme and channel activities, and muscle contractility. Therefore, intracellular Mg²⁺ concentration is tightly regulated by multiple Mg²⁺ transporters and channels. So far, various candidate genes of Mg²⁺ transporters have been identified, and the research on their structure and function is currently in progress. The Solute Carrier 41 (SLC41) family, which is related to the bacterial Mg²⁺ transporter/channel MgtE, comprises three isoforms of SLC41A1, SLC41A2, and SLC41A3. Based on recent studies, SLC41A1 is thought to mediate Mg²⁺ influx or Na⁺-dependent Mg²⁺ efflux across the plasma membrane, whereas SLC41A2 and SLC41A3 may mediate Mg²⁺ fluxes across either the plasma membrane or organellar membranes. Intriguingly, SLC41A1 variants have been identified in patients with Parkinson's disease (PD) and nephronophthisis-related ciliopathies. Further genetic analyses reveal the association of SLC41A1 polymorphisms with PD risks. This review highlights the recent advances in the understanding of the molecular and functional characteristics of SLC41 family towards its therapeutic and diagnostic applications.

SLC41A1 knockout mice display normal magnesium homeostasis

Transcellular Mg²⁺ reabsorption in the distal convoluted tubule (DCT) of the kidneys plays an important role in maintaining systemic Mg²⁺ homeostasis. SLC41A1 is a Na⁺/Mg²⁺ exchanger that mediates Mg²⁺ efflux from cells and is hypothesized to facilitate basolateral extrusion of Mg²⁺ in the DCT. In this study, we generated a SLC41A1 knockout mouse model to examine the role of SLC41A1 in Mg²⁺ homeostasis. Slc41a1^-/- mice exhibited similar serum and urine Mg²⁺ levels as their wild-type littermates. Dietary restriction of Mg²⁺ resulted in reduced serum Mg²⁺ concentration and urinary Mg²⁺ excretion, which was similar in the wild-type and knockout groups. Expression of genes encoding Mg²⁺ channels and transporters such as transient receptor potential melastatin 6 (Trpm6), transient receptor potential melastatin 7 (Trpm7), cyclin and CBS domain divalent metal cation transport mediator 2 (Cnnm2), and Slc41a3 were unchanged based on genotype. We investigated the potential redundancy of SLC41A1 and its homolog SLC41A3 by generating a double knockout mouse. Although Slc41a3^-/- knockout mice showed significantly reduced serum Mg²⁺ compared with wild-type and Slc41a1^-/- knockout groups, double knockout mice displayed similar serum Mg²⁺ levels as Slc41a3^-/- knockout mice. In conclusion, our data show that SLC41A1 is not involved in the regulation of systemic Mg²⁺ homeostasis in mice. Our data also demonstrate that SLC41A1 does not compensate for the loss of SLC41A3, suggesting different functions of these SLC41 proteins in vivo.NEW & NOTEWORTHY SLC41A1 has been hypothesized to mediate Mg²⁺ extrusion in the distal convoluted tubule and thus regulate Mg²⁺ homeostasis. This study investigated the role of SLC41A1 in Mg²⁺ homeostasis in vivo using a transgenic mouse model. Our results demonstrate that SLC41A1 is not required to maintain normal Mg²⁺ balance in mice. We also show that SLC41A3 is more important than SLC41A1 in regulating systemic Mg²⁺ levels.

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.

The emerging roles and therapeutic potential of cyclin M/CorC family of Mg2+ transporters

Cyclin M (CNNM) and its prokaryotic ortholog CorC belong to a family of proteins that function as Mg²⁺-extruding transporters by stimulating Na⁺/Mg²⁺ exchange, and thereby control intracellular Mg²⁺ levels. The Mg²⁺-extruding function of CNNM is inhibited by the direct binding of an oncogenic protein, phosphatase of regenerating liver (PRL), and this inhibition is responsible for the PRL-driven malignant progression of cancers. Studies with mouse strains deficient for the CNNM gene family revealed the importance of CNNM4 and CNNM2 in maintaining organismal Mg²⁺ homeostasis by participating in intestinal Mg²⁺ absorption and renal reabsorption, respectively. Moreover, CNNM proteins are involved in various diseases, and gene mutations in CNNM2 and CNNM4 cause dominant familial hypomagnesemia and Jalili syndrome, respectively. Genome wide association studies have also revealed the importance of CNNM2 in multiple major diseases, such as hypertension and schizophrenia. Collectively, the molecular and biological characterizations of CNNM/CorC show that they are an intriguing therapeutic target; the current status of drug development targeting these proteins is also discussed.

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.

Magnesium transport in the aglomerular kidney of the Gulf toadfish (Opsanus beta)

Despite having an aglomerular kidney, Gulf toadfish can survive in water ranging from nearly fresh up to 70 parts per thousand salinity. In hyperosmotic environments, the major renal function is to balance the passive Mg²⁺ load from the environment with an equal excretion. However, the molecular transporters involved in Mg²⁺ secretion are poorly understood. We investigated whether environmental MgCl₂ alone or in combination with elevated salinity affected transcriptional regulation of genes classically involved in renal Mg²⁺ secretion (slc41a1, slc41a3, cnnm3) together with three novel genes (trpm6, trpm7, claudin-19) and two isoforms of the Na⁺/K⁺-ATPase α-subunit (nka-α1a, nka-α1b). First, toadfish were acclimated to 5, 9, 35, or 60 ppt water (corresponding to ~ 7, 13, 50 and 108 mmol L^-1 ambient [Mg²⁺], respectively) and sampled at 24 h or 9 days. Next, the impact of elevated ambient [Mg²⁺] was explored by exposing toadfish to control (50 mmol L^-1 Mg²⁺), or elevated [Mg²⁺] (100 mmol L^-1) at a constant salinity for 7 days. Mg²⁺ levels in this experiment corresponded with levels in control and hypersaline conditions in the first experiment. A salinity increase from 5 to 60 ppt stimulated the level of all investigated transcripts in the kidney. In Mg²⁺-exposed fish, we observed a 14-fold increase in the volume of intestinal fluids and elevated plasma osmolality and [Mg²⁺], suggesting osmoregulatory challenges. However, none of the renal gene targets changed expression compared with the control group. We conclude that transcriptional regulation of renal Mg²⁺ transporters is induced by elevated [Mg²⁺] in combination with salinity rather than elevated ambient [Mg²⁺] alone.

Specific disruption of calcineurin-signaling in the distal convoluted tubule impacts the transcriptome and proteome, and causes hypomagnesemia and metabolic acidosis

Adverse effects of calcineurin inhibitors (CNI), such as hypertension, hyperkalemia, acidosis, hypomagnesemia and hypercalciuria, have been linked to dysfunction of the distal convoluted tubule (DCT). To test this, we generated a mouse model with an inducible DCT-specific deletion of the calcineurin regulatory subunit B alpha (CnB1-KO). Three weeks after CnB1 deletion, these mice exhibited hypomagnesemia and acidosis, but no hypertension, hyperkalemia or hypercalciuria. Consistent with the hypomagnesemia, CnB1-KO mice showed a downregulation of proteins implicated in DCT magnesium transport, including TRPM6, CNNM2, SLC41A3 and parvalbumin but expression of calcium channel TRPV5 in the kidney was unchanged. The abundance of the chloride/bicarbonate exchanger pendrin was increased, likely explaining the acidosis. Plasma aldosterone levels, kidney renin expression, abundance of phosphorylated sodium chloride-cotransporter and abundance of the epithelial sodium channel were similar in control and CnB1-KO mice, consistent with a normal sodium balance. Long-term potassium homeostasis was maintained in CnB1-KO mice, but in-vivo and ex-vivo experiments indicated that CnB1 contributes to acute regulation of potassium balance and sodium chloride-cotransporter. Tacrolimus treatment of control and CnB1-KO mice demonstrated that CNI-related hypomagnesemia is linked to impaired calcineurin-signaling in DCT, while hypocalciuria and hyponatremia occur independently of CnB1 in DCT. Transcriptome and proteome analyses of isolated DCTs demonstrated that CnB1 deletion impacts the expression of several DCT-specific proteins and signaling pathways. Thus, our data support a critical role of calcineurin for DCT function and provide novel insights into the pathophysiology of CNI side-effects and involved molecular players in the DCT.

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.

Uromodulin to Osteopontin Ratio in Deceased Donor Urine Is Associated With Kidney Graft Outcomes

BACKGROUND

Deceased-donor kidneys experience extensive injury, activating adaptive and maladaptive pathways therefore impacting graft function. We evaluated urinary donor uromodulin (UMOD) and osteopontin (OPN) in recipient graft outcomes.

METHODS

Primary outcomes: all-cause graft failure (GF) and death-censored GF (dcGF). Secondary outcomes: delayed graft function (DGF) and 6-month estimated glomerular filtration rate (eGFR). We randomly divided our cohort of deceased donors and recipients into training and test datasets. We internally validated associations between donor urine UMOD and OPN at time of procurement, with our primary outcomes. The direction of association between biomarkers and GF contrasted. Subsequently, we evaluated UMOD:OPN ratio with all outcomes. To understand these mechanisms, we examined the effect of UMOD on expression of major histocompatibility complex II in mouse macrophages.

RESULTS

Doubling of UMOD increased dcGF risk (adjusted hazard ratio [aHR], 1.1; 95% confidence interval [CI], 1.02-1.2), whereas OPN decreased dcGF risk (aHR, 0.94; 95% CI, 0.88-1). UMOD:OPN ratio ≤3 strengthened the association, with reduced dcGF risk (aHR, 0.57; 0.41-0.80) with similar associations for GF, and in the test dataset. A ratio ≤3 was also associated with lower DGF (aOR, 0.73; 95% CI, 0.60-0.89) and higher 6-month eGFR (adjusted β coefficient, 3.19; 95% CI, 1.28-5.11). UMOD increased major histocompatibility complex II expression elucidating a possible mechanism behind UMOD's association with GF.

CONCLUSIONS

UMOD:OPN ratio ≤3 was protective, with lower risk of DGF, higher 6-month eGFR, and improved graft survival. This ratio may supplement existing strategies for evaluating kidney quality and allocation decisions regarding deceased-donor kidney transplantation.

Uromodulin: Roles in Health and Disease

Uromodulin, a protein exclusively produced by the kidney, is the most abundant urinary protein in physiological conditions. Already described several decades ago, uromodulin has gained the spotlight in recent years, since the discovery that mutations in its encoding gene UMOD cause a renal Mendelian disease (autosomal dominant tubulointerstitial kidney disease) and that common polymorphisms are associated with multifactorial disorders, such as chronic kidney disease, hypertension, and cardiovascular diseases. Moreover, variations in uromodulin levels in urine and/or blood reflect kidney functioning mass and are of prognostic value for renal function, cardiovascular events, and overall mortality. The clinical relevance of uromodulin reflects its multifunctional nature, playing a role in renal ion transport and immunomodulation, in protection against urinary tract infections and renal stones, and possibly as a systemic antioxidant. Here, we discuss the multifaceted roles of this protein in kidney physiology and its translational relevance.

Modeling Distal Convoluted Tubule (Patho)Physiology: An Overview of Past Developments and an Outlook Toward the Future

The kidneys are essential for maintaining electrolyte homeostasis. Blood electrolyte composition is controlled by active reabsorption and secretion processes in dedicated segments of the kidney tubule. Specifically, the distal convoluted tubule (DCT) and connecting tubule are important for regulating the final excretion of sodium, magnesium, and calcium. Studies unravelling the specific function of these segments have greatly improved our understanding of DCT (patho)physiology. Over the years, experimental models used to study the DCT have changed and the field has advanced from early dissection studies with rats and rabbits to the use of various transgenic mouse models. Developments in dissection techniques and cell culture methods have resulted in immortalized mouse DCT cell lines and made it possible to specifically obtain DCT fragments for ex vivo studies. However, we still do not fully understand the complex (patho)physiology of this segment and there is need for advanced human DCT models. Recently, kidney organoids and tubuloids have emerged as new complex cell models that provide excellent opportunities for physiological studies, disease modeling, drug discovery, and even personalized medicine in the future. This review presents an overview of cell models used to study the DCT and provides an outlook on kidney organoids and tubuloids as model for DCT (patho)physiology. Impact statement This study provides a detailed overview of past and future developments on cell models used to study kidney (patho)physiology and specifically the distal convoluted tubule (DCT) segment. Hereby, we highlight the need for an advanced human cell model of this segment and summarize recent advances in the field of kidney organoids and tubuloids with a focus on DCT properties. The findings reported in this review are significant for future developments toward an advanced human model of the DCT that will help to increase our understanding of DCT (patho)physiology.

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.

A novel distal convoluted tubule-specific Cre-recombinase driven by the NaCl cotransporter gene

Cre-lox technology has revolutionized research in renal physiology by allowing site-specific genetic recombination in individual nephron segments. The distal convoluted tubule (DCT), consisting of distinct early (DCT1) and late (DCT2) segments, plays a central role in Na⁺ and K⁺ homeostasis. The only established Cre line targeting the DCT is Pvalb-Cre, which is limited by noninducibility, activity along DCT1 only, and activity in neurons. Here, we report the characterization of the first Cre line specific to the entire DCT. CRISPR/Cas9 targeting was used to introduce a tamoxifen-inducible IRES-Cre-ERT2 cassette downstream of the coding region of the Slc12a3 gene encoding the NaCl cotransporter (NCC). The resulting Slc12a3-Cre-ERT2 mice were crossed with R26R-YFP reporter mice, which revealed minimal leakiness with 6.3% of NCC-positive cells expressing yellow fluorescent protein (YFP) in the absence of tamoxifen. After tamoxifen injection, YFP expression was observed in 91.2% of NCC-positive cells and only in NCC-positive cells, revealing high recombination efficiency and DCT specificity. Crossing to R26R-TdTomato mice revealed higher leakiness (64.5%), suggesting differential sensitivity of the floxed site. Western blot analysis revealed no differences in abundances of total NCC or the active phosphorylated form of NCC in Slc12a3-Cre-ERT2 mice of either sex compared with controls. Plasma K⁺ and Mg²⁺ concentrations and thiazide-sensitive Na⁺ and K⁺ excretion did not differ in Slc12a3-Cre-ERT2 mice compared with controls when sex matched. These data suggest genetic modification had no obvious effect on NCC function. Slc12a3-Cre-ERT2 mice are the first line generated demonstrating inducible Cre recombinase activity along the entire DCT and will be a useful tool to study DCT function.

Uromodulin in mineral metabolism

PURPOSE OF REVIEW

Uromodulin (UMOD), also known as Tamm-Horsfall protein, is the most abundant protein in human urine. UMOD has multiple functions such as protection against urinary tract infections and nephrolithiasis. This review outlines recent progress made in UMOD's role in renal physiology, tubular transport, and mineral metabolism.

RECENT FINDINGS

UMOD is mostly secreted in the thick ascending limb (TAL) and to a lesser degree in the distal convoluted tubule (DCT). UMOD secretion is regulated by the calcium-sensing receptor. UMOD upregulates ion channels [e.g., renal outer medullary potassium channel, transient receptor potential cation channel subfamily V member 5, and transient receptor potential melastatin 6 (TRPM6)] and cotransporters [e.g., Na,K,2Cl cotransporter (NKCC2) and sodium-chloride cotransporter (NCC)] in the TAL and DCT. Higher serum UMOD concentrations have been associated with higher renal function and preserved renal reserve. Higher serum UMOD has also been linked to a lower risk of cardiovascular disease and diabetes mellitus.

SUMMARY

With better serum UMOD detection assays the extent of different functions for UMOD is still expanding. Urinary UMOD regulates different tubular ion channels and cotransporters. Variations of urinary UMOD secretion can so contribute to common disorders such as hypertension or nephrolithiasis.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

Mg2+ restriction downregulates NCC through NEDD4-2 and prevents its activation by hypokalemia

Hypomagnesemia is associated with reduced kidney function and life-threatening complications and sustains hypokalemia. The distal convoluted tubule (DCT) determines final urinary Mg²⁺ excretion and, via activity of the Na⁺-Cl^- cotransporter (NCC), also plays a key role in K⁺ homeostasis by metering Na⁺ delivery to distal segments. Little is known about the mechanisms by which plasma Mg²⁺ concentration regulates NCC activity and how low-plasma Mg²⁺ concentration and K⁺ concentration interact to modulate NCC activity. To address this, we performed dietary manipulation studies in mice. Compared with normal diet, abundances of total NCC and phosphorylated NCC (pNCC) were lower after short-term (3 days) or long-term (14 days) dietary Mg²⁺ restriction. Altered NCC activation is unlikely to play a role, since we also observed lower total NCC abundance in mice lacking the two NCC-activating kinases, STE20/SPS-1-related proline/alanine-rich kinase and oxidative stress response kinase-1, after Mg²⁺ restriction. The E3 ubiquitin-protein ligase NEDD4-2 regulates NCC abundance during dietary NaCl loading or K⁺ restriction. Mg²⁺ restriction did not lower total NCC abundance in inducible nephron-specific neuronal precursor cell developmentally downregulated 4-2 (NEDD4-2) knockout mice. Total NCC and pNCC abundances were similar after short-term Mg²⁺ or combined Mg²⁺-K⁺ restriction but were dramatically lower compared with a low-K⁺ diet. Therefore, sustained NCC downregulation may serve a mechanism that enhances distal Na⁺ delivery during states of hypomagnesemia, maintaining hypokalemia. Similar results were obtained with long-term Mg²⁺-K⁺ restriction, but, surprisingly, NCC was not activated after long-term K⁺ restriction despite lower plasma K⁺ concentration, suggesting significant differences in distal tubule adaptation to acute or chronic K⁺ restriction.

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.

Magnesium-sensitive upstream ORF controls PRL phosphatase expression to mediate energy metabolism

Phosphatases of regenerating liver (PRL-1, PRL-2, and PRL-3, also known as PTP4A1, PTP4A2, and PTP4A3) control magnesium homeostasis through an association with the CNNM magnesium transport regulators. Although high PRL levels have been linked to cancer progression, regulation of their expression is poorly understood. Here we show that modulating intracellular magnesium levels correlates with a rapid change of PRL expression by a mechanism involving its 5'UTR mRNA region. Mutations or CRISPR-Cas9 targeting of the conserved upstream ORF present in the mRNA leader derepress PRL protein synthesis and attenuate the translational response to magnesium levels. Mechanistically, magnesium depletion reduces intracellular ATP but up-regulates PRL protein expression via activation of the AMPK/mTORC2 pathway, which controls cellular energy status. Hence, altered PRL-2 expression leads to metabolic reprogramming of the cells. These findings uncover a magnesium-sensitive mechanism controlling PRL expression, which plays a role in cellular bioenergetics.

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.

The rise and fall of novel renal magnesium transporters

Body Mg2+ 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 Mg2+ 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 Mg2+ transporters and put their presumed function in context of the renal handling of Mg2+. 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 Mg2+ transporters. TRPM6/7 facilitate Mg2+ influx, SLC41A1 mediates Mg2+ extrusion, and MRS2 and SLC41A3 are implicated in mitochondrial Mg2+ homeostasis. These proteins are highly expressed in the DCT. The function of cyclin M (CNNM) proteins is still under debate. For the other proposed Mg2+ transporters including Mg2+ transporter subtype 1 (MagT1), nonimprinted in Prader-Willi/Angelman syndrome (NIPA), membrane Mg2+ transport (MMgT), Huntingtin-interacting protein 14 (HIP14), and ATP13A4, functional evidence is limited, or functions alternative to Mg2+ transport have been suggested. Additional characterization of their Mg2+ transport proficiency should be provided before further claims about their role as Mg2+ transporter can be made.

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.

Phosphatase of regenerating liver maintains cellular magnesium homeostasis

Phosphatase of regenerating liver (PRL) is highly expressed in malignant cancers and promotes cancer progression. Recent studies have suggested its functional relationship with Mg2+, but the importance and molecular details of this relationship remain unknown. Here, we report that PRL expression is regulated by Mg2+ and PRL protects cells from apoptosis under Mg2+-depleted conditions. When cultured cells were subjected to Mg2+ depletion, endogenous PRL protein levels increased significantly. siRNA-mediated knockdown of endogenous PRL did not significantly affect cell proliferation under normal culture conditions, but it increased cell death after Mg2+ depletion. Imaging analyses with a fluorescent probe for Mg2+ showed that PRL knockdown severely reduced intracellular Mg2+ levels, indicating a role for PRL in maintaining intracellular Mg2+. We also examined the mechanism of augmented expression of PRL proteins and found that PRL mRNA transcription was stimulated by Mg2+ depletion. A series of analyses revealed the activation and the crucial importance of signal transducer and activator of transcription 1 in this process. Collectively, these results implicate PRL in maintaining cellular Mg2+ homeostasis.

Identification of adenylyl cyclase isoforms mediating parathyroid hormone- and calcitonin-stimulated cyclic AMP accumulation in distal tubule cells

Background

The distal convoluted tubule (DCT) is an important nephron site for parathyroid hormone (PTH) and calcitonin regulation of urinary divalent cation excretion. These hormones exert their effects on the DCT in substantial part through activation of adenylyl cyclase (AC); however, it is unknown which AC isoforms are involved.

Methods

To examine this, two mouse DCT cell lines were studied: 209 and D1 cells. AC isoform mRNA expression was analyzed by real-time PCR. Cyclic AMP was measured using enzyme immunoassay.

Results

Calcitonin, but not PTH, stimulated cAMP accumulation in 209 cells, while PTH, but not calcitonin, increased cAMP content in D1 cells. Both cell types expressed AC3, AC4, AC6, AC7, and AC9 mRNA; in both cell types, AC6 mRNA was most abundant, followed by AC9, then AC3 and AC7, with relatively very small amounts of AC4 mRNA. Microdissected mouse DCT had a similar pattern of AC isoform mRNA expression although AC5 mRNA was detected. Individual siRNA knockdown of AC6 and AC9 reduced calcitonin-stimulated cAMP accumulation in 209 cells and PTH-induced cAMP levels in D1 cells. Knockdown of AC3 had no effect on hormonal augmentation of cAMP in either cell line. Surprisingly, knockdown of AC7 increased calcitonin-induced cAMP accumulation in 209 cells as well as PTH-stimulated cAMP content in D1 cells.

Conclusions

Taken together, these findings indicate that AC6 and AC9 mediate calcitonin- and PTH-stimulated cAMP accumulation in DCT cells, while activation of AC7 may paradoxically reduce the stimulatory effects of PTH and calcitonin on cultured DCT cAMP levels.

Uromodulin: from physiology to rare and complex kidney disorders

Uromodulin (also known as Tamm-Horsfall protein) is exclusively produced in the kidney and is the most abundant protein in normal urine. The function of uromodulin remains elusive, but the available data suggest that this protein might regulate salt transport, protect against urinary tract infection and kidney stones, and have roles in kidney injury and innate immunity. Interest in uromodulin was boosted by genetic studies that reported involvement of the UMOD gene, which encodes uromodulin, in a spectrum of rare and common kidney diseases. Rare mutations in UMOD cause autosomal dominant tubulointerstitial kidney disease (ADTKD), which leads to chronic kidney disease (CKD). Moreover, genome-wide association studies have identified common variants in UMOD that are strongly associated with risk of CKD and also with hypertension and kidney stones in the general population. These findings have opened up a new field of kidney research. In this Review we summarize biochemical, physiological, genetic and pathological insights into the roles of uromodulin; the mechanisms by which UMOD mutations cause ADTKD, and the association of common UMOD variants with complex disorders.

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.

HMGCS2 is a key ketogenic enzyme potentially involved in type 1 diabetes with high cardiovascular risk

Diabetes increases the risk of Cardio-vascular disease (CVD). CVD is more prevalent in type 2 diabetes (T2D) than type 1 diabetes (T1D), but the mortality risk is higher in T1D than in T2D. The pathophysiology of CVD in T1D is poorly defined. To learn more about biological pathways that are potentially involved in T1D with cardiac dysfunction, we sought to identify differentially expressed genes in the T1D heart. Our study used T1D mice with severe hyperglycemia along with significant deficits in echocardiographic measurements. Microarray analysis of heart tissue RNA revealed that the T1D mice differentially expressed 10 genes compared to control. Using Ingenuity Pathway Analysis (IPA), we showed that these genes were significantly involved in ketogenesis, cardiovascular disease, apoptosis and other toxicology functions. Of these 10 genes, the 3-Hydroxy-3-Methylglutaryl-CoA Synthase 2 (HMGCS2) was the highest upregulated gene in T1D heart. IPA analysis showed that HMGCS2 was center to many biological networks and pathways. Our data also suggested that apart from heart, the expression of HMGCS2 was also different in kidney and spleen between control and STZ treated mice. In conclusion, The HMGCS2 molecule may potentially be involved in T1D induced cardiac dysfunction.

Identification of SLC41A3 as a novel player in magnesium homeostasis

Regulation of the body Mg(2+) balance takes place in the distal convoluted tubule (DCT), where transcellular reabsorption determines the final urinary Mg(2+) excretion. The basolateral Mg(2+) extrusion mechanism in the DCT is still unknown, but recent findings suggest that SLC41 proteins contribute to Mg(2+) extrusion. The aim of this study was, therefore, to characterize the functional role of SLC41A3 in Mg(2+) homeostasis using the Slc41a3 knockout (Slc41a3(-/-)) mouse. By quantitative PCR analysis it was shown that Slc41a3 is the only SLC41 isoform with enriched expression in the DCT. Interestingly, serum and urine electrolyte determinations demonstrated that Slc41a3(-/-) mice suffer from hypomagnesemia. The intestinal Mg(2+) absorption capacity was measured using the stable (25)Mg(2+) isotope in mice fed a low Mg(2+) diet. (25)Mg(2+) uptake was similar in wildtype (Slc41a3(+/+)) and Slc41a3(-/-) mice, although Slc41a3(-/-) animals exhibited increased intestinal mRNA expression of Mg(2+) transporters Trpm6 and Slc41a1. Remarkably, some of the Slc41a3(-/-) mice developed severe unilateral hydronephrosis. In conclusion, SLC41A3 was established as a new factor for Mg(2+) handling.

P2X6 Knockout Mice Exhibit Normal Electrolyte Homeostasis

ATP-mediated signaling is an important regulator of electrolyte transport in the kidney. The purinergic cation channel P2X6 has been previously localized to the distal convoluted tubule (DCT), a nephron segment important for Mg²⁺ and Na⁺ reabsorption, but its role in ion transport remains unknown. In this study, P2x6 knockout (P2x6^-/-) mice were generated to investigate the role of P2X6 in renal electrolyte transport. The P2x6^-/- animals displayed a normal phenotype and did not differ physiologically from wild type mice. Differences in serum concentration and 24-hrs urine excretion of Na⁺, K⁺, Mg²⁺ and Ca²⁺ were not detected between P2x6^+/+, P2x6^+/- and P2x6^-/- mice. Quantitative PCR was applied to examine potential compensatory changes in renal expression levels of other P2x subunits and electrolyte transporters, including P2x1-5, P2x7, Trpm6, Ncc, Egf, Cldn16, Scnn1, Slc12a3, Slc41a1, Slc41a3, Cnnm2, Kcnj10 and Fxyd2. Additionally, protein levels of P2X2 and P2X4 were assessed in P2x6^+/+ and P2x6^-/- mouse kidneys. However, significant changes in expression were not detected. Furthermore, no compensatory changes in gene expression could be demonstrated in heart material isolated from P2x6^-/- mice. Except for a significant (P<0.05) upregulation of P2x2 in the heart of P2x6^-/- mice compared to the P2x6^+/+ mice. Thus, our data suggests that purinergic signaling via P2X6 is not significantly involved in the regulation of renal electrolyte handling under normal physiological conditions.

Hypomagnesemia in Type 2 Diabetes: A Vicious Circle?

Over the past decades, hypomagnesemia (serum Mg(2+) <0.7 mmol/L) has been strongly associated with type 2 diabetes mellitus (T2DM). Patients with hypomagnesemia show a more rapid disease progression and have an increased risk for diabetes complications. Clinical studies demonstrate that T2DM patients with hypomagnesemia have reduced pancreatic β-cell activity and are more insulin resistant. Moreover, dietary Mg(2+) supplementation for patients with T2DM improves glucose metabolism and insulin sensitivity. Intracellular Mg(2+) regulates glucokinase, KATP channels, and L-type Ca(2+) channels in pancreatic β-cells, preceding insulin secretion. Moreover, insulin receptor autophosphorylation is dependent on intracellular Mg(2+) concentrations, making Mg(2+) a direct factor in the development of insulin resistance. Conversely, insulin is an important regulator of Mg(2+) homeostasis. In the kidney, insulin activates the renal Mg(2+) channel transient receptor potential melastatin type 6 that determines the final urinary Mg(2+) excretion. Consequently, patients with T2DM and hypomagnesemia enter a vicious circle in which hypomagnesemia causes insulin resistance and insulin resistance reduces serum Mg(2+) concentrations. This Perspective provides a systematic overview of the molecular mechanisms underlying the effects of Mg(2+) on insulin secretion and insulin signaling. In addition to providing a review of current knowledge, we provide novel directions for future research and identify previously neglected contributors to hypomagnesemia in T2DM.

Antagonistic Regulation of Parvalbumin Expression and Mitochondrial Calcium Handling Capacity in Renal Epithelial Cells

Parvalbumin (PV) is a cytosolic Ca²⁺-binding protein acting as a slow-onset Ca²⁺ buffer modulating the shape of Ca²⁺ transients in fast-twitch muscles and a subpopulation of neurons. PV is also expressed in non-excitable cells including distal convoluted tubule (DCT) cells of the kidney, where it might act as an intracellular Ca²⁺ shuttle facilitating transcellular Ca²⁺ resorption. In excitable cells, upregulation of mitochondria in “PV-ergic” cells in PV-/- mice appears to be a general hallmark, evidenced in fast-twitch muscles and cerebellar Purkinje cells. Using Gene Chip Arrays and qRT-PCR, we identified differentially expressed genes in the DCT of PV-/- mice. With a focus on genes implicated in mitochondrial Ca²⁺ transport and membrane potential, uncoupling protein 2 (Ucp2), mitocalcin (Efhd1), mitochondrial calcium uptake 1 (Micu1), mitochondrial calcium uniporter (Mcu), mitochondrial calcium uniporter regulator 1 (Mcur1), cytochrome c oxidase subunit 1 (COX1), and ATP synthase subunit β (Atp5b) were found to be up-upregulated. At the protein level, COX1 was increased by 31 ± 7%, while ATP-synthase subunit β was unchanged. This suggested that these mitochondria were better suited to uphold the electrochemical potential across the mitochondrial membrane, necessary for mitochondrial Ca²⁺ uptake. Ectopic expression of PV in PV-negative Madin-Darby canine kidney (MDCK) cells decreased COX1 and concomitantly mitochondrial volume, while ATP synthase subunit β levels remained unaffected. Suppression of PV by shRNA in PV-expressing MDCK cells led subsequently to an increase in COX1 expression. The collapsing of the mitochondrial membrane potential by the uncoupler CCCP occurred at lower concentrations in PV-expressing MDCK cells than in control cells. In support, a reduction of the relative mitochondrial mass was observed in PV-expressing MDCK cells. Deregulation of the cytoplasmic Ca²⁺ buffer PV in kidney cells was counterbalanced in vivo and in vitro by adjusting the relative mitochondrial volume and modifying the mitochondrial protein composition conceivably to increase their Ca²⁺-buffering/sequestration capacity.

Dietary Inulin Fibers Prevent Proton-Pump Inhibitor (PPI)-Induced Hypocalcemia in Mice

Background

Proton-pump inhibitor-induced hypomagnesemia (PPIH) is the most recognized side effect of proton-pump inhibitors (PPIs). Additionally, PPIH is associated with hypocalcemia and hypokalemia. It is hypothesized that PPIs reduce epithelial proton secretion and thereby increase the pH in the colon, which may explain the reduced absorption of and Mg²⁺ and Ca²⁺. Fermentation of dietary oligofructose-enriched inulin fibers by the microflora leads to acidification of the intestinal lumen and by this enhances mineral uptake. This study aimed, therefore, to improve mineral absorption by application of dietary inulin to counteract PPIH.

Methods

Here, C57BL/J6 mice were supplemented with omeprazole and/or inulin. Subsequently, Mg²⁺ and Ca²⁺ homeostasis was assessed by means of serum, urine and fecal electrolyte measurements. Moreover, the mRNA levels of magnesiotropic and calciotropic genes were examined in the large intestine and kidney by real-time PCR.

Results

Treatment with omeprazole significantly reduced serum Mg²⁺ and Ca²⁺ levels. However, concomitant addition of dietary inulin fibers normalized serum Ca²⁺ but not serum Mg²⁺ concentrations. Inulin abolished enhanced expression of Trpv6 and S100g in the colon by omeprazole. Additionally, intestinal and renal mRNA levels of the Trpm6 gene were reduced after inulin intake.

Conclusions

This study suggests that dietary inulin counteracts reduced intestinal Ca²⁺ absorption upon PPI treatment. In contrast, inulin did not increase intestinal absorption of Mg²⁺ sufficiently to recover serum Mg²⁺. The clinical potential of dietary inulin treatment should be the subject of future studies.

Regulation of Mg2+ Reabsorption and Transient Receptor Potential Melastatin Type 6 Activity by cAMP Signaling

The transient receptor potential melastatin type 6 (TRPM6) epithelial Mg(2+) channels participate in transcellular Mg(2+) transport in the kidney and intestine. Previous reports suggested a hormonal cAMP-dependent regulation of Mg(2+) reabsorption in the kidney. The molecular details of this process are, however, unknown. Adenylate cyclase 3 (Adcy3) has been shown to colocalize with the Na(+)/Cl(-) cotransporter, a marker of the distal convoluted segment of the kidney, the principal site of TRPM6 expression. Given the critical role of TRPM6 in Mg(2+) reabsorption, an inducible kidney-specific Adcy3 deletion mouse model was characterized for blood and urinary electrolyte disturbances under a normal--and low--Mg(2+) diet. Increased urinary Mg(2+) wasting and Trpm6 mRNA levels were observed in the urine and kidney of Adcy3-deleted animals compared with wild-type controls. Serum Mg(2+) concentration was significantly lower in Adcy3-deleted animals at day 7 on the low Mg(2+) diet. Using patch clamp electrophysiology, cell surface biotinylation, and total internal reflection fluorescence live cell imaging of transfected HEK293 cells, we demonstrated that cAMP signaling rapidly potentiates TRPM6 activity by promoting TRPM6 accumulation at the plasma membrane and increasing its single-channel conductance. Comparison of electrophysiological data from cells expressing the phosphorylation-deficient S1252A or phosphomimetic S1252D TRPM6 mutants suggests that phosphorylation at this intracellular residue participates in the observed stimulation of channel activity. Altogether, these data support a physiologically relevant magnesiotropic role of cAMP signaling in the kidney by a direct stimulatory action of protein kinase A on the plasma membrane trafficking and function of TRPM6 ion channels.

Genetic loci for serum magnesium among African-Americans and gene-environment interaction at MUC1 and TRPM6 in European-Americans: the Atherosclerosis Risk in Communities (ARIC) study

Background

Low serum magnesium levels have been associated with multiple chronic diseases. The regulation of serum magnesium homeostasis is not well understood. A previous genome-wide association study (GWAS) of European ancestry (EA) populations identified nine loci for serum magnesium. No such study has been conducted in African-Americans, nor has there been an evaluation of the interaction of magnesium-associated SNPs with environmental factors. The goals of this study were to identify genetic loci associated with serum magnesium in an African-American (AA) population using both genome-wide and candidate region interrogation approaches and to evaluate gene-environment interaction for the magnesium-associated variants in both EA and AA populations. We conducted a GWAS of serum magnesium in 2737 AA participants of the Atherosclerosis Risk in Communities (ARIC) Study and interrogated the regions of the nine published candidate loci in these results. Literature search identified the influence of progesterone on MUC1 expression and insulin on TRPM6 expression.

Results

The GWAS approach in African-American participants identified a locus near MUC1 as genome-wide significant (rs2974937, beta = −0.013, p = 6.1x10⁻⁹). The candidate region interrogation approach identified two of the nine loci previously discovered in EA populations as containing SNPs that were significantly associated in African-American participants (SHROOM3 and TRPM6). The index variants at these three loci together explained 2.8 % of the variance in serum magnesium concentration in ARIC African-American participants. On the test of gene-environment interaction in ARIC EA participants, the index variant at MUC1 had 2.5 times stronger association in postmenopausal women with progestin use (beta = −0.028, p = 7.3x10⁻⁵) than in those without any hormone use (beta = −0.011, p = 7.0x10⁻⁸, p for interaction 0.03). At TRPM6, the index variant had 1.6 times stronger association in those with lower fasting insulin levels (<80pmol/L: beta = −0.013, p = 1.6x10⁻⁷; ≥80pmol/L: beta = −0.008, p = 1.8x10⁻², p for interaction 0.03).

Conclusions

We identified three loci that explained 2.8 % of the variance in serum magnesium concentration in ARIC African-American participants. Following-up on functional studies of gene expression identified gene-environment interactions between progestin use and MUC1 and between insulin and TRPM6 on serum magnesium concentration in ARIC European-American participants. These results extend our understanding of the metabolism of serum magnesium.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0219-7) contains supplementary material, which is available to authorized users.

Flavaglines Stimulate Transient Receptor Potential Melastatin Type 6 (TRPM6) Channel Activity

Magnesium (Mg2+) is essential for enzymatic activity, brain function and muscle contraction. Blood Mg2+ concentrations are tightly regulated between 0.7 and 1.1 mM by Mg2+ (re)absorption in kidney and intestine. The apical entry of Mg2+ in (re)absorbing epithelial cells is mediated by the transient receptor potential melastatin type 6 (TRPM6) ion channel. Here, flavaglines are described as a novel class of stimulatory compounds for TRPM6 activity. Flavaglines are a group of natural and synthetic compounds that target the ubiquitously expressed prohibitins and thereby affect cellular signaling. By whole-cell patch clamp analyses, it was demonstrated that nanomolar concentrations of flavaglines increases TRPM6 activity by ∼2 fold. The stimulatory effects were dependent on the presence of the alpha-kinase domain of TRPM6, but did not require its phosphotransferase activity. Interestingly, it was observed that two natural occurring TRPM6 mutants with impaired insulin-sensitivity, TRPM6-p.Val1393Ile and TRPM6-p.Lys1584Glu, are not sensitive to flavagline stimulation. In conclusion, we have identified flavaglines as potent activators of TRPM6 activity. Our results suggest that flavaglines stimulate TRPM6 via the insulin receptor signaling pathway.

Magnesium in man: implications for health and disease

Magnesium (Mg(2+)) is an essential ion to the human body, playing an instrumental role in supporting and sustaining health and life. As the second most abundant intracellular cation after potassium, it is involved in over 600 enzymatic reactions including energy metabolism and protein synthesis. Although Mg(2+) availability has been proven to be disturbed during several clinical situations, serum Mg(2+) values are not generally determined in patients. This review aims to provide an overview of the function of Mg(2+) in human health and disease. In short, Mg(2+) plays an important physiological role particularly in the brain, heart, and skeletal muscles. Moreover, Mg(2+) supplementation has been shown to be beneficial in treatment of, among others, preeclampsia, migraine, depression, coronary artery disease, and asthma. Over the last decade, several hereditary forms of hypomagnesemia have been deciphered, including mutations in transient receptor potential melastatin type 6 (TRPM6), claudin 16, and cyclin M2 (CNNM2). Recently, mutations in Mg(2+) transporter 1 (MagT1) were linked to T-cell deficiency underlining the important role of Mg(2+) in cell viability. Moreover, hypomagnesemia can be the consequence of the use of certain types of drugs, such as diuretics, epidermal growth factor receptor inhibitors, calcineurin inhibitors, and proton pump inhibitors. This review provides an extensive and comprehensive overview of Mg(2+) research over the last few decades, focusing on the regulation of Mg(2+) homeostasis in the intestine, kidney, and bone and disturbances which may result in hypomagnesemia.

Mg2+ homeostasis: the balancing act of TRPM6

PURPOSE OF REVIEW

The tight control of blood magnesium (Mg) levels is of central importance for numerous physiological processes. A persistent low Mg status (hypomagnesemia) is associated with severe health risks and is involved in the pathogenesis of type 2 diabetes mellitus, osteoporosis, asthma, and heart and vascular diseases. The current view has expanded significantly as a result of the identification of novel genes and regulatory pathways involved in hypomagnesemic disorders. This review aims to give an up-to-date overview of transient receptor potential melastatin 6 (TRPM6) regulation and its role in the maintenance of Mg homeostasis.

RECENT FINDINGS

The epithelial Mg channel TRPM6 is considered to be the Mg entry pathway in the distal convoluted tubule of the kidney, where it functions as gatekeeper for controlling the body's Mg balance. Various factors and hormones contribute not only to the function, but also to the dysregulation of TRPM6, which has a substantial impact on renal Mg handling. Recent genetic and molecular studies have further elucidated the signaling processes of epithelial Mg transport, including their effect on the expression and function of TRPM6.

SUMMARY

Knowledge of TRPM6 functioning is of vital importance to decipher its role in Mg handling and will, in particular, provide a molecular basis for achieving a better understanding of Mg mal(re)absorption and hence systemic Mg balance.

CNNM2 mutations cause impaired brain development and seizures in patients with hypomagnesemia

Intellectual disability and seizures are frequently associated with hypomagnesemia and have an important genetic component. However, to find the genetic origin of intellectual disability and seizures often remains challenging because of considerable genetic heterogeneity and clinical variability. In this study, we have identified new mutations in CNNM2 in five families suffering from mental retardation, seizures, and hypomagnesemia. For the first time, a recessive mode of inheritance of CNNM2 mutations was observed. Importantly, patients with recessive CNNM2 mutations suffer from brain malformations and severe intellectual disability. Additionally, three patients with moderate mental disability were shown to carry de novo heterozygous missense mutations in the CNNM2 gene. To elucidate the physiological role of CNNM2 and explain the pathomechanisms of disease, we studied CNNM2 function combining in vitro activity assays and the zebrafish knockdown model system. Using stable Mg²⁺ isotopes, we demonstrated that CNNM2 increases cellular Mg²⁺ uptake in HEK293 cells and that this process occurs through regulation of the Mg²⁺-permeable cation channel TRPM7. In contrast, cells expressing mutated CNNM2 proteins did not show increased Mg²⁺ uptake. Knockdown of cnnm2 isoforms in zebrafish resulted in disturbed brain development including neurodevelopmental impairments such as increased embryonic spontaneous contractions and weak touch-evoked escape behaviour, and reduced body Mg content, indicative of impaired renal Mg²⁺ absorption. These phenotypes were rescued by injection of mammalian wild-type Cnnm2 cRNA, whereas mammalian mutant Cnnm2 cRNA did not improve the zebrafish knockdown phenotypes. We therefore concluded that CNNM2 is fundamental for brain development, neurological functioning and Mg²⁺ homeostasis. By establishing the loss-of-function zebrafish model for CNNM2 genetic disease, we provide a unique system for testing therapeutic drugs targeting CNNM2 and for monitoring their effects on the brain and kidney phenotype.

Mental retardation affects 1–3% of the population and has a strong genetic etiology. Consequently, early identification of the genetic causes of mental retardation is of significant importance in the diagnosis of the disease, as predictor of the progress of the disease and for the determination of treatment. In this study, we identify mutations in the gene encoding for cyclin M2 (CNNM2) to be causative for mental retardation and seizures in patients with hypomagnesemia. Particularly, in patients with a recessive mode of inheritance, the intellectual disability caused by dysfunctional CNNM2 is dramatically severe and is accompanied by severely limited motor skills and brain malformations suggestive of impaired early brain development. Although hypomagnesemia has been associated to several neurological diseases, Mg²⁺ status is not regularly assessed in patients with seizures and mental disability. Our findings establish CNNM2 as an important protein for renal magnesium handling, brain development and neurological functioning, thus explaining the physiology of human disease caused by (dysfunctional) mutations in CNNM2. CNNM2 mutations should be taken into account in patients with seizures and mental disability, specifically in combination with hypomagnesemia.