The roles of acid-sensing ion channel 1a and ovarian cancer G protein-coupled receptor 1 on passive Mg2+ transport across intestinal epithelium-like Caco-2 monolayers

Current opinion on the regulation of small intestinal magnesium absorption

Magnesium (Mg²⁺) has an important role in numerous biological functions, and Mg²⁺ deficiency is associated with several diseases. Therefore, adequate intestinal absorption of Mg²⁺ is vital for health. The small intestine was previously thought to absorb digested Mg²⁺ exclusively through an unregulated paracellular mechanism, which is responsible for approximately 90% of total Mg²⁺ absorption. Recent studies, however, have revealed that the duodenum, jejunum, and ileum absorb Mg²⁺ through both transcellular and paracellular routes. Several regulatory factors of small intestinal Mg²⁺ uptake also have been explored, e.g., parathyroid hormone, fibroblast growth factor-23, apical acidity, proton pump inhibitor, and pH-sensing channel and receptors. The mechanistic factors underlying proton pump inhibitor suppression of small intestinal Mg²⁺, such as magnesiotropic protein dysfunction, higher mucosal bicarbonate secretion, Paneth cell dysfunction, and intestinal inflammation, are currently being explored. The potential role of small intestinal microbiomes in Mg²⁺ absorption has also been proposed. In this article, we reviewed the current knowledge on the mechanisms and regulatory factors of small intestinal Mg²⁺ absorption.

Physiological relevance of proton-activated GPCRs

The detection of H+ concentration variations in the extracellular milieu is accomplished by a series of specialized and non-specialized pH-sensing mechanisms. The proton-activated G protein-coupled receptors (GPCRs) GPR4 (Gpr4), TDAG8 (Gpr65), and OGR1 (Gpr68) form a subfamily of proteins capable of triggering intracellular signaling in response to alterations in extracellular pH around physiological values, i.e., in the range between pH 7.5 and 6.5. Expression of these receptors is widespread for GPR4 and OGR1 with particularly high levels in endothelial cells and vascular smooth muscle cells, respectively, while expression of TDAG8 appears to be more restricted to the immune compartment. These receptors have been linked to several well-studied pH-dependent physiological activities including central control of respiration, renal adaption to changes in acid-base status, secretion of insulin and peripheral responsiveness to insulin, mechanosensation, and cellular chemotaxis. Their role in pathological processes such as the genesis and progression of several inflammatory diseases (asthma, inflammatory bowel disease), and tumor cell metabolism and invasiveness, is increasingly receiving more attention and makes these receptors novel and interesting targets for therapy. In this review, we cover the role of these receptors in physiological processes and will briefly discuss some implications for disease processes.

Acid-Sensing Ion Channel-1a in Articular Chondrocytes and Synovial Fibroblasts: A Novel Therapeutic Target for Rheumatoid Arthritis

Acid-sensing ion channel 1a (ASIC1a) is a member of the extracellular H⁺-activated cation channel family. Emerging evidence has suggested that ASIC1a plays a crucial role in the pathogenesis of rheumatoid arthritis (RA). Specifically, ASIC1a could promote inflammation, synovial hyperplasia, articular cartilage, and bone destruction; these lead to the progression of RA, a chronic autoimmune disease characterized by chronic synovial inflammation and extra-articular lesions. In this review, we provided a brief overview of the molecular properties of ASIC1a, including the basic biological characteristics, tissue and cell distribution, channel blocker, and factors influencing the expression and function, and focused on the potential therapeutic targets of ASIC1a in RA and possible mechanisms of blocking ASIC1a to improve RA symptoms, such as regulation of apoptosis, autophagy, pyroptosis, and necroptosis of articular cartilage, and synovial inflammation and invasion of fibroblast-like cells in synovial tissue.

Factors and Molecular Mechanisms Influencing the Protein Synthesis, Degradation and Membrane Trafficking of ASIC1a

Acid-sensing ion channels (ASICs) are members of the degenerin/epithelial sodium channel superfamily. They are extracellular pH sensors that are activated by protons. Among all ASICs, ASIC1a is one of the most intensively studied isoforms because of its unique ability to be permeable to Ca²⁺. In addition, it is considered to contribute to various pathophysiological conditions. As a membrane proton receptor, the number of ASIC1a present on the cell surface determines its physiological and pathological functions, and this number partially depends on protein synthesis, degradation, and membrane trafficking processes. Recently, several studies have shown that various factors affect these processes. Therefore, this review elucidated the major factors and underlying molecular mechanisms affecting ASIC1a protein expression and membrane trafficking.

The inhibitory role of purinergic P2Y receptor on Mg2+ transport across intestinal epithelium-like Caco-2 monolayer

The mechanism of proton pump inhibitors (PPIs) suppressing intestinal Mg2+ uptake is unknown. The present study aimed to investigate the role of purinergic P2Y receptors in the regulation of Mg2+ absorption in normal and omeprazole-treated intestinal epithelium-like Caco-2 monolayers. Omeprazole suppressed Mg2+ transport across Caco-2 monolayers. An agonist of the P2Y2 receptor, but not the P2Y4 or P2Y6 receptor, suppressed Mg2+ transport across control and omeprazole-treated monolayers. Omeprazole enhanced P2Y2 receptor expression in Caco-2 cells. Forskolin and P2Y2 receptor agonist markedly enhanced apical HCO3- secretion by control and omeprazole-treated monolayers. The P2Y2 receptor agonist suppressed Mg2+ transport and stimulated apical HCO3- secretion through the Gq-protein coupled-phospholipase C (PLC) dependent pathway. Antagonists of cystic fibrosis transmembrane conductance regulator (CFTR) and Na+-HCO3- cotransporter-1 (NBCe1) could nullify the inhibitory effect of P2Y2 receptor agonist on Mg2+ transport across control and omeprazole-treated Caco-2 monolayers. Our results propose an inhibitory role of P2Y2 on intestinal Mg2+ absorption.

Challenges in the Diagnosis of Magnesium Status

Magnesium is a critical mineral in the human body and is involved in ~80% of known metabolic functions. It is currently estimated that 60% of adults do not achieve the average dietary intake (ADI) and 45% of Americans are magnesium deficient, a condition associated with disease states like hypertension, diabetes, and neurological disorders, to name a few. Magnesium deficiency can be attributed to common dietary practices, medications, and farming techniques, along with estimates that the mineral content of vegetables has declined by as much as 80⁻90% in the last 100 years. However, despite this mineral's importance, it is poorly understood from several standpoints, not the least of which is its unique mechanism of absorption and sensitive compartmental handling in the body, making the determination of magnesium status difficult. The reliance on several popular sample assays has contributed to a great deal of confusion in the literature. This review will discuss causes of magnesium deficiency, absorption, handling, and compartmentalization in the body, highlighting the challenges this creates in determining magnesium status in both clinical and research settings.

Involvement of acid-sensing ion channel 1a in gastric carcinoma cell migration and invasion

Acidic microenvironment, particularly acid-sensing ion channel 1a (ASIC1a), has been reported to promote carcinoma cell proliferation as well as migration. In this study, we explored the effect of ASIC1a on migration and invasion of gastric carcinoma (GC). ASIC1a expression levels were examined in paired GC and adjacent normal tissues from 16 patients by immunohistochemistry. Reverse transcription real-time PCR and immunoblotting were conducted to assess the ASIC1a expression levels in the GC cell line AGS after transfection with ASIC1a small hairpin RNA (shRNA). Wound healing and transwell invasion assays were utilized to detect metastasis and invasion following ASIC1a silencing. Tumor formation was used to detect the role of ASIC1a in tumorigenicity in vivo. It was found that ASIC1a expression level was significantly higher in GC tissues showing postoperative metastasis compared with non-metastasis and non-tumor tissues. Moreover, silencing of ASIC1a with shRNA significantly down-regulated ASIC1a expression and reduced GC cell migration and invasion. A moderately acidic extracellular environment inhibited GC cell viability. Furthermore, ASIC1a shRNA caused inhibition of tumorigenicity in vivo. Our study is the first report of attenuating the malignant phenotype of GC in vitro and in vivo by suppressing ASIC1a, and suggests a novel approach to study the relationship between ASICs and GC cell migration and invasion.

Vitamin D metabolism in growing pigs: influence of UVB irradiation and dietary vitamin D supply on calcium homeostasis, its regulation and bone metabolism

The aim of this study was to prove whether pigs are able to synthesize vitamin D (vitD) in the skin and to investigate the influence of ultraviolet irradiation (UVB) on vitD status and calcium (Ca) homeostasis of growing pigs. Thirty-two 11-week-old pigs were kept without access to sunlight and divided into four groups receiving the following treatment in a 2 × 2 factorial design: (i) UVB irradiation or not and (ii) vitD in feed or not. Blood, urine and faeces were sampled every third week. In serum, vitD metabolites, Ca, phosphorus (P), magnesium (Mg) and bone markers were analysed. Digestibility of Ca, P and Mg as well as urinary excretion of these minerals was analysed. After 14 weeks, the animals were slaughtered, and samples of skin, intestines, kidneys and bones (metatarsus) were taken for further analyses: sterols of vitD synthesis in the skin, Ca flux rates in the intestines, expression of genes involved in Ca transport in the intestines and kidneys, bone mineral density (BMD) with the aid of peripheral quantitative computer tomography and bone mineral content by ashing the metatarsus. Irradiated animals showed higher levels of 7-dehydrocholesterol and tachysterol in the skin, higher levels of 25-hydroxycholecalciferol and 1,25-dihydroxycholecalciferol in the serum and higher Ca net flux rates were determined in Ussing chambers. In contrast, the expression of genes involved in Ca transport in the intestines and kidneys was not altered. Similarly, the digestibility of Ca and P as well as the urinary excretion was not affected. With respect to the metatarsus, no differences in mineral contents and BMD were found between groups. At the end of the study, some subclinical signs of beginning vitD 'insufficiency' were observed in the group without access to vitD (represented by higher expression of 1α-hydroxylase in the kidney and increased parathyroid hormone in serum).

Acid-Sensing Ion Channels as Potential Therapeutic Targets in Neurodegeneration and Neuroinflammation

Acid-sensing ion channels (ASICs) are a family of proton-sensing channels that are voltage insensitive, cation selective (mostly permeable to Na⁺), and nonspecifically blocked by amiloride. Derived from 5 genes (ACCN1-5), 7 subunits have been identified, 1a, 1b, 2a, 2b, 3, 4, and 5, that are widely expressed in the peripheral and central nervous system as well as other tissues. Over the years, different studies have shown that activation of these channels is linked to various physiological and pathological processes, such as memory, learning, fear, anxiety, ischemia, and multiple sclerosis to name a few, so their potential as therapeutic targets is increasing. This review focuses on recent advances that have helped us to better understand the role played by ASICs in different pathologies related to neurodegenerative diseases, inflammatory processes, and pain.

Omeprazole suppressed plasma magnesium level and duodenal magnesium absorption in male Sprague-Dawley rats

Hypomagnesemia is the most concerned side effect of proton pump inhibitors (PPIs) in chronic users. However, the mechanism of PPIs-induced systemic Mg²⁺ deficit is currently unclear. The present study aimed to elucidate the direct effect of short-term and long-term PPIs administrations on whole body Mg²⁺ homeostasis and duodenal Mg²⁺ absorption in rats. Mg²⁺ homeostasis was studied by determining the serum Mg²⁺ level, urine and fecal Mg²⁺ excretions, and bone and muscle Mg²⁺ contents. Duodenal Mg²⁺ absorption as well as paracellular charge selectivity were studied. Our result showed that gastric and duodenal pH markedly increased in omeprazole-treated rats. Omeprazole significantly suppressed plasma Mg²⁺ level, urinary Mg²⁺ excretion, and bone and muscle Mg²⁺ content. Thus, omeprazole induced systemic Mg²⁺ deficiency. By using Ussing chamber techniques, it was shown that omeprazole markedly suppressed duodenal Mg²⁺ channel-driven and Mg²⁺ channel-independent Mg²⁺ absorptions and cation selectivity. Inhibitors of mucosal HCO₃^- secretion significantly increased duodenal Mg²⁺ absorption in omeprazole-treated rats. We therefore hypothesized that secreted HCO₃^- in duodenum decreased luminal proton, this impeded duodenal Mg²⁺ absorption. Higher plasma total 25-OH vitamin D, diuresis, and urine PO₄^3- were also demonstrated in hypomagnesemic rats. As a compensatory mechanism for systemic Mg²⁺ deficiency, the expressions of duodenal transient receptor potential melastatin 6 (TRPM6), cyclin M4 (CNNM4), claudin (Cldn)-2, Cldn-7, Cldn-12, and Cldn-15 proteins were enhanced in omeprazole-treated rats. Our findings support the potential role of duodenum on the regulation of Mg²⁺ homeostasis.

Ion channels expression and function are strongly modified in solid tumors and vascular malformations

Background

Several cellular functions relate to ion-channels activity. Physiologically relevant chains of events leading to angiogenesis, cell cycle and different forms of cell death, require transmembrane voltage control. We hypothesized that the unordered angiogenesis occurring in solid cancers and vascular malformations might associate, at least in part, to ion-transport alteration.

Methods

The expression level of several ion-channels was analyzed in human solid tumor biopsies. Expression of 90 genes coding for ion-channels related proteins was investigated within the Oncomine database, in 25 independent patients-datasets referring to five histologically-different solid tumors (namely, bladder cancer, glioblastoma, melanoma, breast invasive-ductal cancer, lung carcinoma), in a total of 3673 patients (674 control-samples and 2999 cancer-samples). Furthermore, the ion-channel activity was directly assessed by measuring in vivo the electrical sympathetic skin responses (SSR) on the skin of 14 patients affected by the flat port-wine stains vascular malformation, i.e., a non-tumor vascular malformation clinical model.

Results

Several ion-channels showed significantly increased expression in tumors (p < 0.0005); nine genes (namely, CACNA1D, FXYD3, FXYD5, HTR3A, KCNE3, KCNE4, KCNN4, CLIC1, TRPM3) showed such significant modification in at least half of datasets investigated for each cancer type. Moreover, in vivo analyses in flat port-wine stains patients showed a significantly reduced SSR in the affected skin as compared to the contralateral healthy skin (p < 0.05), in both latency and amplitude measurements.

Conclusions

All together these data identify ion-channel genes showing significantly modified expression in different tumors and cancer-vessels, and indicate a relevant electrophysiological alteration in human vascular malformations. Such data suggest a possible role and a potential diagnostic application of the ion–electron transport in vascular disorders underlying tumor neo-angiogenesis and vascular malformations.

The Roles of Acidosis in Osteoclast Biology

The adverse effect of acidosis on the skeletal system has been recognized for almost a century. Although the underlying mechanism has not been fully elucidated, it appears that acidosis acts as a general stimulator of osteoclasts derived from bone marrow precursors cells and enhances osteoclastic resorption. Prior work suggests that acidosis plays a significant role in osteoclasts formation and activation via up-regulating various genes responsible for its adhesion, migration, survival and bone matrix degradation. Understanding the role of acidosis in osteoclast biology may lead to development of novel therapeutic approaches for the treatment of diseases related to low bone mass. In this review, we aim to discuss the recent investigations into the effects of acidosis in osteoclast biology and the acid-sensing molecular mechanism.

The pH-sensing receptor OGR1 improves barrier function of epithelial cells and inhibits migration in an acidic environment

The pH-sensing receptor ovarian cancer G protein-coupled receptor 1 (OGR1; GPR68) is expressed in the gut. Inflammatory bowel disease is typically associated with a decrease in local pH, which may lead to altered epithelial barrier function and subsequent gastrointestinal repair involving epithelial cell adhesion and migration. As the mechanisms underlying the response to pH changes are not well understood, we have investigated OGR1-mediated, pH-dependent signaling pathways in intestinal epithelial cells. Caco-2 cells stably overexpressing OGR1 were created and validated as tools to study OGR1 signaling. Barrier function, migration, and proliferation were measured using electric cell-substrate impedance-sensing technology. Localization of the tight junction proteins zonula occludens protein 1 and occludin and the rearrangement of cytoskeletal actin were examined by confocal microscopy. Paracellular permeability and protein and gene expression analysis using DNA microarrays were performed on filter-grown Caco-2 monolayers. We report that an acidic pH shift from pH 7.8 to 6.6 improved barrier function and stimulated reorganization of filamentous actin with prominent basal stress fiber formation. Cell migration and proliferation during in vitro wound healing were inhibited. Gene expression analysis revealed significant upregulation of genes related to cytoskeleton remodeling, cell adhesion, and growth factor signaling. We conclude that acidic extracellular pH can have a signaling function and impact the physiology of intestinal epithelial cells. The deconstruction of OGR1-dependent signaling may aid our understanding of mucosal inflammation mechanisms.