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

Genetic drivers of age-related changes in urinary magnesium excretion

Although age-dependent alterations in urinary magnesium (Mg2+) excretion have been described, the underlying mechanism remains elusive. As heritability significantly contributes to variations in urinary Mg2+ excretion, we measured urinary Mg2+ excretion at different ages in a cohort of genetically variable Diversity Outbred (DO) mice. Compared with animals aged 6 mo, an increase in Mg2+ excretion was observed at 12 and 18 mo. Quantitative trait locus (QTL) analysis revealed an association of a locus on chromosome 10 with Mg2+ excretion at 6 mo of age, with Oit3 (encoding oncoprotein-induced transcript 3; OIT3) as our primary candidate gene. To study the possible role of OIT3 in renal Mg2+ handling, we generated and characterized Oit3 knockout (Oit3-/-) mice. Although a slightly lower serum Mg2+ concentration was present in male Oit3-/- mice, this effect was not observed in female Oit3-/- mice. In addition, urinary Mg2+ excretion and the expression of renal magnesiotropic genes were unaltered in Oit3-/- mice. For animals aged 12 and 18 mo, QTL analysis revealed an association with a locus on chromosome 19, which contains the gene encoding TRPM6, a known Mg2+ channel involved in renal Mg2+ reabsorption. Comparison with RNA sequencing (RNA-Seq) data revealed that Trpm6 mRNA expression is inversely correlated with the QTL effect, implying that TRPM6 may be involved in age-dependent changes in urinary Mg2+ excretion in mice. In conclusion, we show here that variants in Oit3 and Trpm6 are associated with urinary Mg2+ excretion at distinct periods of life, although OIT3 is unlikely to affect renal Mg2+ handling.NEW & NOTEWORTHY Aging increased urinary magnesium (Mg2+) excretion in mice. We show here that variation in Oit3, a candidate gene for the locus associated with Mg2+ excretion in young mice, is unlikely to be involved as knockout of Oit3 did not affect Mg2+ excretion. Differences in the expression of the renal Mg2+ channel TRPM6 may contribute to the variation in urinary Mg2+ excretion in older mice.

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.

Hypomagnesaemia with varying degrees of extrarenal symptoms as a consequence of heterozygous CNNM2 variants

Variants in the CNNM2 gene are causative for hypomagnesaemia, seizures and intellectual disability, although the phenotypes can be variable. This study aims to understand the genotype-phenotype relationship in affected individuals with CNNM2 variants by phenotypic, functional and structural analysis of new as well as previously reported variants. This results in the identification of seven variants that significantly affect CNNM2-mediated Mg2+ transport. Pathogenicity of these variants is further supported by structural modelling, which predicts CNNM2 structure to be affected by all of them. Strikingly, seizures and intellectual disability are absent in 4 out of 7 cases, indicating these phenotypes are caused either by specific CNNM2 variant only or by additional risk factors. Moreover, in line with sporadic observations from previous reports, CNNM2 variants might be associated with disturbances in parathyroid hormone and Ca2+ homeostasis.

Mitochondrial Dysfunction in Kidney Tubulopathies

Mitochondria play a key role in kidney physiology and pathology. They produce ATP to fuel energy-demanding water and solute reabsorption processes along the nephron. Moreover, mitochondria contribute to cellular health by the regulation of autophagy, (oxidative) stress responses, and apoptosis. Mitochondrial abundance is particularly high in cortical segments, including proximal and distal convoluted tubules. Dysfunction of the mitochondria has been described for tubulopathies such as Fanconi, Gitelman, and Bartter-like syndromes and renal tubular acidosis. In addition, mitochondrial cytopathies often affect renal (tubular) tissues, such as in Kearns-Sayre and Leigh syndromes. Nevertheless, the mechanisms by which mitochondrial dysfunction results in renal tubular diseases are only scarcely being explored. This review provides an overview of mitochondrial dysfunction in the development and progression of kidney tubulopathies. Furthermore, it emphasizes the need for further mechanistic investigations to identify links between mitochondrial function and renal electrolyte reabsorption.

New insights into the structure and function of CNNM proteins

Magnesium (Mg2+ ) is the most abundant divalent cation in cells and plays key roles in almost all biological processes. CBS-pair domain divalent metal cation transport mediators (CNNMs) are a newly characterized class of Mg2+ transporters present throughout biology. Originally discovered in bacteria, there are four CNNM proteins in humans, which are involved in divalent cation transport, genetic diseases, and cancer. Eukaryotic CNNMs are composed of four domains: an extracellular domain, a transmembrane domain, a cystathionine-β-synthase (CBS)-pair domain, and a cyclic nucleotide-binding homology domain. The transmembrane and CBS-pair core are the defining features of CNNM proteins with over 20 000 protein sequences known from over 8000 species. Here, we review the structural and functional studies of eukaryotic and prokaryotic CNNMs that underlie our understanding of their regulation and mechanism of ion transport. Recent structures of prokaryotic CNNMs confirm the transmembrane domain mediates ion transport with the CBS-pair domain likely playing a regulatory role through binding divalent cations. Studies of mammalian CNNMs have identified new binding partners. These advances are driving progress in understanding this deeply conserved and widespread family of ion transporters.

Integrative analyses highlight functional regulatory variants associated with neuropsychiatric diseases

Noncoding variants of presumed regulatory function contribute to the heritability of neuropsychiatric disease. A total of 2,221 noncoding variants connected to risk for ten neuropsychiatric disorders, including autism spectrum disorder, attention deficit hyperactivity disorder, bipolar disorder, borderline personality disorder, major depression, generalized anxiety disorder, panic disorder, post-traumatic stress disorder, obsessive-compulsive disorder and schizophrenia, were studied in developing human neural cells. Integrating epigenomic and transcriptomic data with massively parallel reporter assays identified differentially-active single-nucleotide variants (daSNVs) in specific neural cell types. Expression-gene mapping, network analyses and chromatin looping nominated candidate disease-relevant target genes modulated by these daSNVs. Follow-up integration of daSNV gene editing with clinical cohort analyses suggested that magnesium transport dysfunction may increase neuropsychiatric disease risk and indicated that common genetic pathomechanisms may mediate specific symptoms that are shared across multiple neuropsychiatric diseases.

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.

Discovery and ranking of the most robust prognostic biomarkers in serous ovarian cancer

Progress in ovarian cancer treatment lags behind other tumor types. With diagnosis usually at an advanced stage, there is a high demand for reliable prognostic biomarkers capable of the selection of effective chemo- and targeted therapies. Our goal was to establish a large-scale transcriptomic database and use it to uncover and rank survival-associated genes. Ovarian cancer cohorts with transcriptome-level gene expression data and clinical follow-up were identified from public repositories. All samples were normalized and entered into an integrated database. Cox univariate survival analysis was performed for all genes and was followed by multivariate analysis for selected genes involving clinical and pathological variables. False discovery rate was computed for multiple hypothesis testing and a 1% cutoff was used to determine statistical significance. The complete integrated database comprises 1816 samples from 17 datasets. Altogether, 2468 genes were correlated to progression-free survival (PFS), and 704 genes were correlated with overall survival (OS). The most significant genes were WBP1L, ASAP3, CNNM2, and NCAPH2 for progression-free survival and CSE1L, NUAK1, ALPK2, and SHKBP1 for overall survival. Genes significant for PFS were also preferentially significant for predicting OS as well. All data including HR and p values as well as the used cutoff values for all genes for both PFS and OS are provided to enable the ranking of future biomarker candidates across all genes. Our results help to prioritize genes and to neglect those which are most likely to fail in studies aiming to establish new clinically useful biomarkers and therapeutic targets in serous ovarian cancer.

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.

The p.Pro482Ala Variant in the CNNM2 Gene Causes Severe Hypomagnesemia Amenable to Treatment with Spironolactone

Renal hypomagnesemia syndromes involving CNNM2 protein pathogenic variants are associated with variable degrees of neurocognitive dysfunction and hypomagnesemia. Here, we report a family with a novel CNNM2 p.Pro482Ala variant, presenting with overt hypomagnesemia and mild neurological involvement (autosomal dominant renal hypomagnesemia 6, HOMG6, MIM# 613882). Using a bioinformatics approach, we showed that the p.Pro482Ala amino acid substitution causes a 3D conformational change in CNNM2 structure in the cystathionin beta synthase (CBS) domain and the carboxy-terminal protein segment. A novel finding was that aldosterone inhibition with spironolactone helped to alleviate hypomagnesemia and symptoms in the proband.

FAM111A is dispensable for electrolyte homeostasis in mice

Autosomal dominant mutations in FAM111A are causative for Kenny-Caffey syndrome type 2. Patients with Kenny-Caffey syndrome suffer from severe growth retardation, skeletal dysplasia, hypoparathyroidism, hypocalcaemia, hyperphosphataemia and hypomagnesaemia. While recent studies have reported FAM111A to function in antiviral response and DNA replication, its role in regulating electrolyte homeostasis remains unknown. In this study, we assessed the role of FAM111A in the regulation of serum electrolyte balance using a Fam111a knockout (Fam111a-/-) C57BL/6 N mouse model. Fam111a-/- mice displayed normal weight and serum parathyroid hormone (PTH) concentration and exhibited unaltered magnesium, calcium and phosphate levels in serum and 24-hour urine. Expression of calciotropic (including Cabp28k, Trpv5, Klotho and Cyp24a1), magnesiotropic (including Trpm6, Trpm7, Cnnm2 and Cnnm4) and phosphotropic (Slc20a1, Slc20a2, Slc34a1 and Slc34a3) genes in the kidneys, duodenum and colon were not affected by Fam111a depletion. Only Slc34a2 expression was significantly upregulated in the duodenum, but not in the colon. Analysis of femurs showed unaffected bone morphology and density in Fam111a-/- mice. Kidney and parathyroid histology were also normal in Fam111a-/- mice. In conclusion, our study is the first to characterise the function of FAM111A in vivo and we report that mice lacking FAM111A exhibit normal electrolyte homeostasis on a standard diet.

Rare hypomagnesemia, seizures, and mental retardation in a 4-month-old patient caused by novel CNNM2 mutation Tyr189Cys: Genetic analysis and review

Background

Hypomagnesemia, seizures, and mental retardation (HSMR) syndrome is a rare genetic disease. Presently, only 24 cases have been reported and the clinical features of the disease are yet to be fully described, thereby making diagnosis challenging.

Methods

Trio‐whole‐exome sequencing was used for the patient and her parents, and the structure of the variant protein was analyzed by molecular dynamics. Finally, the characteristics of HSMR were summarized by reviewing the previous literature.

Results

The main disease manifestations in the patient were seizures, liver function damage, hypomagnesemia, atrial septal defect, and sinus arrhythmia. A novel mutation in CNNM2 (c.566A>G/p.Tyr189Cys) was identified by genetic detection. The parents were wild type, and the mutation was rated as pathogenic by American College of Medical Genetics and Genomics guidelines. Ab initio modeling and molecular dynamics simulation show that the mutation destroys the surrounding hydrogen bonds, which may reduce the local stability of the protein structure. In the previous literature, only 24 children with HSMR have been reported, mainly manifested as hypomagnesemia, mental retardation, seizures, and language and motor impairment.

Conclusion

We have reported the second case of HSMR in the Chinese population, which further expands the phenotypic spectrum of congenital heart disease and the variation spectrum of CNNM2.

CNNM proteins selectively bind to the TRPM7 channel to stimulate divalent cation entry into cells

Magnesium is essential for cellular life, but how it is homeostatically controlled still remains poorly understood. Here, we report that members of CNNM family, which have been controversially implicated in both cellular Mg²⁺ influx and efflux, selectively bind to the TRPM7 channel to stimulate divalent cation entry into cells. Coexpression of CNNMs with the channel markedly increased uptake of divalent cations, which is prevented by an inactivating mutation to the channel’s pore. Knockout (KO) of TRPM7 in cells or application of the TRPM7 channel inhibitor NS8593 also interfered with CNNM-stimulated divalent cation uptake. Conversely, KO of CNNM3 and CNNM4 in HEK-293 cells significantly reduced TRPM7-mediated divalent cation entry, without affecting TRPM7 protein expression or its cell surface levels. Furthermore, we found that cellular overexpression of phosphatases of regenerating liver (PRLs), known CNNMs binding partners, stimulated TRPM7-dependent divalent cation entry and that CNNMs were required for this activity. Whole-cell electrophysiological recordings demonstrated that deletion of CNNM3 and CNNM4 from HEK-293 cells interfered with heterologously expressed and native TRPM7 channel function. We conclude that CNNMs employ the TRPM7 channel to mediate divalent cation influx and that CNNMs also possess separate TRPM7-independent Mg²⁺ efflux activities that contribute to CNNMs’ control of cellular Mg²⁺ homeostasis.

Magnesium is essential for cellular life, but how is it homeostatically controlled? This study shows that proteins of the CNNM family bind to the TRPM7 channel to stimulate divalent cation entry into cells, independent of their function in regulating magnesium ion efflux.

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.

Magnesium Deficiency Alters Expression of Genes Critical for Muscle Magnesium Homeostasis and Physiology in Mice

Chronic Mg²⁺ deficiency is the underlying cause of a broad range of health dysfunctions. As 25% of body Mg²⁺ is located in the skeletal muscle, Mg²⁺ transport and homeostasis systems (MgTHs) in the muscle are critical for whole-body Mg²⁺ homeostasis. In the present study, we assessed whether Mg²⁺ deficiency alters muscle fiber characteristics and major pathways regulating muscle physiology. C57BL/6J mice received either a control, mildly, or severely Mg²⁺-deficient diet (0.1%; 0.01%; and 0.003% Mg²⁺ wt/wt, respectively) for 14 days. Mg²⁺ deficiency slightly decreased body weight gain and muscle Mg²⁺ concentrations but was not associated with detectable variations in gastrocnemius muscle weight, fiber morphometry, and capillarization. Nonetheless, muscles exhibited decreased expression of several MgTHs (MagT1, CNNM2, CNNM4, and TRPM6). Moreover, TaqMan low-density array (TLDA) analyses further revealed that, before the emergence of major muscle dysfunctions, even a mild Mg²⁺ deficiency was sufficient to alter the expression of genes critical for muscle physiology, including energy metabolism, muscle regeneration, proteostasis, mitochondrial dynamics, and excitation–contraction coupling.