Inhibition of beta-cell sodium-calcium exchange enhances glucose-dependent elevations in cytoplasmic calcium and insulin secretion

A multi-layered network model identifies Akt1 as a common modulator of neurodegeneration

The accumulation of misfolded and aggregated proteins is a hallmark of neurodegenerative proteinopathies. Although multiple genetic loci have been associated with specific neurodegenerative diseases (NDs), molecular mechanisms that may have a broader relevance for most or all proteinopathies remain poorly resolved. In this study, we developed a multi-layered network expansion (MLnet) model to predict protein modifiers that are common to a group of diseases and, therefore, may have broader pathophysiological relevance for that group. When applied to the four NDs Alzheimer's disease (AD), Huntington's disease, and spinocerebellar ataxia types 1 and 3, we predicted multiple members of the insulin pathway, including PDK1, Akt1, InR, and sgg (GSK-3β), as common modifiers. We validated these modifiers with the help of four Drosophila ND models. Further evaluation of Akt1 in human cell-based ND models revealed that activation of Akt1 signaling by the small molecule SC79 increased cell viability in all models. Moreover, treatment of AD model mice with SC79 enhanced their long-term memory and ameliorated dysregulated anxiety levels, which are commonly affected in AD patients. These findings validate MLnet as a valuable tool to uncover molecular pathways and proteins involved in the pathophysiology of entire disease groups and identify potential therapeutic targets that have relevance across disease boundaries. MLnet can be used for any group of diseases and is available as a web tool at http://ssbio.cau.ac.kr/software/mlnet.

The context-dependent role of the Na+/Ca2+-exchanger (NCX) in pancreatic stellate cell migration

Pancreatic stellate cells (PSCs) that can co-metastasize with cancer cells shape the tumor microenvironment (TME) in pancreatic ductal adenocarcinoma (PDAC) by producing an excessive amount of extracellular matrix. This leads to a TME characterized by increased tissue pressure, hypoxia, and acidity. Moreover, cells within the tumor secrete growth factors. The stimuli of the TME trigger Ca2+ signaling and cellular Na+ loading. The Na+/Ca2+ exchanger (NCX) connects the cellular Ca2+ and Na+ homeostasis. The NCX is an electrogenic transporter, which shuffles 1 Ca2+ against 3 Na+ ions over the plasma membrane in a forward or reverse mode. Here, we studied how the impact of NCX activity on PSC migration is modulated by cues from the TME. NCX expression was revealed with qPCR and Western blot. [Ca2+]i, [Na+]i, and the cell membrane potential were determined with the fluorescent indicators Fura-2, Asante NaTRIUM Green-2, and DiBAC4(3), respectively. PSC migration was quantified with live-cell imaging. To mimic the TME, PSCs were exposed to hypoxia, pressure, acidic pH (pH 6.6), and PDGF. NCX-dependent signaling was determined with Western blot analyses. PSCs express NCX1.3 and NCX1.9. [Ca2+]i, [Na+]i, and the cell membrane potential are 94.4 nmol/l, 7.4 mmol/l, and - 39.8 mV, respectively. Thus, NCX1 usually operates in the forward (Ca2+ export) mode. NCX1 plays a differential role in translating cues from the TME into an altered migratory behavior. When NCX1 is operating in the forward mode, its inhibition accelerates PSC migration. Thus, NCX1-mediated extrusion of Ca2+ contributes to a slow mode of migration of PSCs.

The role of the Na+/Ca2+-exchanger (NCX) in cancer-associated fibroblasts

Cancer is characterized by uncontrolled growth, invasion, and metastasis. In addition to solid cancer cells, cancer-associated fibroblasts (CAFs) play important roles in cancer pathophysiology. They arise from "healthy" cells but get manipulated by solid cancer cells to supply them and develop a tumor microenvironment (TME) that protects the cancer cells from the immune defense. A wide variety of cell types can differentiate into CAFs, including fibroblasts, endothelial cells, and epithelial cells. Precise Ca²⁺ regulation is essential for each cell including CAFs. The electrogenic Na⁺/Ca²⁺ exchanger (NCX) is one of the ubiquitously expressed regulatory Ca²⁺ transport proteins that rapidly responds to changes of the intracellular ion concentrations. Its transport function is also influenced by the membrane potential and thereby indirectly by the activity of ion channels. NCX transports Ca²⁺ out of the cell (forward mode) or allows its influx (reverse mode), always in exchange for 3 Na⁺ ions that are moved into the opposite direction. In this review, we discuss the functional roles NCX has in CAFs and how these depend on the properties of the TME. NCX activity modifies migration and leads to a reduced proliferation and apoptosis. The effect of the NCX in fibrosis is still largely unknown.

Inhibition of Voltage-Gated Na+ Currents Exerted by KB-R7943 (2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea), an Inhibitor of Na+-Ca2+ Exchanging Process

KB-R7943, an isothiourea derivative, has been recognized as an inhibitor in the reverse mode of the Na⁺-Ca²⁺ exchanging process. This compound was demonstrated to prevent intracellular Na⁺-dependent Ca²⁺ uptake in intact cells; however, it is much less effective at preventing extracellular Na⁺-dependent Ca²⁺ efflux. Therefore, whether or how this compound may produce any perturbations on other types of ionic currents, particularly on voltage-gated Na⁺ current (I_Na), needs to be further studied. In this study, the whole-cell current recordings demonstrated that upon abrupt depolarization in pituitary GH₃ cells, the exposure to KB-R7943 concentration-dependently depressed the transient (I_Na(T)) or late component (I_Na(L)) of I_Na with an IC₅₀ value of 11 or 0.9 μM, respectively. Likewise, the dissociation constant for the KB-R7943-mediated block of I_Na on the basis of a minimum reaction scheme was estimated to be 0.97 μM. The presence of benzamil or amiloride could suppress the I_Na(L) magnitude. The instantaneous window Na⁺ current (I_Na(W)) activated by abrupt ascending ramp voltage (V_ramp) was suppressed by adding KB-R7943; however, subsequent addition of deltamethrin or tefluthrin (Tef) effectively reversed KB-R7943-inhibted I_Na(W). With prolonged duration of depolarizing pulses, the I_Na(L) amplitude became exponentially decreased; moreover, KB-R7943 diminished I_Na(L) magnitude. The resurgent Na⁺ current (I_Na(R)) evoked by a repolarizing V_ramp was also suppressed by adding this compound; moreover, subsequent addition of ranolazine or Tef further diminished or reversed, respectively, its reduction in I_Na(R) magnitude. The persistent Na⁺ current (I_Na(P)) activated by sinusoidal voltage waveform became enhanced by Tef; however, subsequent application of KB-R7943 counteracted Tef-stimulated I_Na(P). The docking prediction reflected that there seem to be molecular interactions of this molecule with the hNa_V1.2 or hNa_V1.7 channels. Collectively, this study highlights evidence showing that KB-R7943 has the propensity to perturb the magnitude and gating kinetics of I_Na (e.g., I_Na(T), I_Na(L), I_Na(W), I_Na(R), and I_Na(P)) and that the Na_V channels appear to be important targets for the in vivo actions of KB-R7943 or other relevant compounds.

Effects of Ion-Transporting Proteins on the Digestive System Under Hypoxia

Hypoxia refers to a state of oxygen limitation, which mainly mediates pathological processes in the human body and participates in the regulation of normal physiological processes. In the hypoxic environment, the main regulator of human body homeostasis is the hypoxia-inducible factor family (HIF). HIF can regulate the expression of many hypoxia-induced genes and then participate in various physiological and pathological processes of the human body. Ion-transporting proteins are extremely important types of proteins. Ion-transporting proteins are distributed on cell membranes or organelles and strictly control the inflow or outflow of ions in cells or organelles. Changes in ions in cells are often closely related to extensive physiological and pathological processes in the human body. Numerous studies have confirmed that hypoxia and its regulatory factors can regulate the transcription and expression of ion-transporting protein-related genes. Under hypoxic stress, the regulation and interaction of ion-transporting proteins by hypoxia often leads to diseases of various human systems and even tumors. Using ion-transporting proteins and hypoxia as targets to explore the mechanism of digestive system diseases and targeted therapy is expected to become a new breakthrough point.

Role of the Intracellular Sodium Homeostasis in Chemotaxis of Activated Murine Neutrophils

The importance of the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in neutrophil function has been intensely studied. However, the role of the intracellular Na⁺ concentration ([Na⁺]_i) which is closely linked to the intracellular Ca²⁺ regulation has been largely overlooked. The [Na⁺]_i is regulated by Na⁺ transport proteins such as the Na⁺/Ca²⁺-exchanger (NCX1), Na⁺/K⁺-ATPase, and Na⁺-permeable, transient receptor potential melastatin 2 (TRPM2) channel. Stimulating with either N-formylmethionine-leucyl-phenylalanine (fMLF) or complement protein C5a causes distinct changes of the [Na⁺]_i. fMLF induces a sustained increase of [Na⁺]_i, surprisingly, reaching higher values in TRPM2^−/− neutrophils. This outcome is unexpected and remains unexplained. In both genotypes, C5a elicits only a transient rise of the [Na⁺]_i. The difference in [Na⁺]_i measured at t = 10 min after stimulation is inversely related to neutrophil chemotaxis. Neutrophil chemotaxis is more efficient in C5a than in an fMLF gradient. Moreover, lowering the extracellular Na⁺ concentration from 140 to 72 mM improves chemotaxis of WT but not of TRPM2^−/− neutrophils. Increasing the [Na⁺]_i by inhibiting the Na⁺/K⁺-ATPase results in disrupted chemotaxis. This is most likely due to the impact of the altered Na⁺ homeostasis and presumably NCX1 function whose expression was shown by means of qPCR and which critically relies on proper extra- to intracellular Na⁺ concentration gradients. Increasing the [Na⁺]_i by a few mmol/l may suffice to switch its transport mode from forward (Ca²⁺-efflux) to reverse (Ca²⁺-influx) mode. The role of NCX1 in neutrophil chemotaxis is corroborated by its blocker, which also causes a complete inhibition of chemotaxis.

Cracking the code of sodium/calcium exchanger (NCX) gating: Old and new complexities surfacing from the deep web of secondary regulations

Cell membranes spatially define gradients that drive the complexity of biological signals. To guarantee movements and exchanges of solutes between compartments, membrane transporters negotiate the passages of ions and other important molecules through lipid bilayers. The Na⁺/Ca²⁺ exchangers (NCXs) in particular play central roles in balancing Na⁺ and Ca²⁺ fluxes across diverse proteolipid borders in all eukaryotic cells, influencing cellular functions and fate by multiple means. To prevent progression from balance to disease, redundant regulatory mechanisms cooperate at multiple levels (transcriptional, translational, and post-translational) and guarantee that the activities of NCXs are finely-tuned to cell homeostatic requirements. When this regulatory network is disturbed by pathological forces, cells may approach the end of life. In this review, we will discuss the main findings, controversies and open questions about regulatory mechanisms that control NCX functions in health and disease.

Calcium Signaling in ß-cell Physiology and Pathology: A Revisit

Pancreatic beta (β) cell dysfunction results in compromised insulin release and, thus, failed regulation of blood glucose levels. This forms the backbone of the development of diabetes mellitus (DM), a disease that affects a significant portion of the global adult population. Physiological calcium (Ca²⁺) signaling has been found to be vital for the proper insulin-releasing function of β-cells. Calcium dysregulation events can have a dramatic effect on the proper functioning of the pancreatic β-cells. The current review discusses the role of calcium signaling in health and disease in pancreatic β-cells and provides an in-depth look into the potential role of alterations in β-cell Ca²⁺ homeostasis and signaling in the development of diabetes and highlights recent work that introduced the current theories on the connection between calcium and the onset of diabetes.

Roles of Na+/Ca2+ exchanger 1 in digestive system physiology and pathophysiology

The Na⁺/Ca²⁺ exchanger (NCX) protein family is a part of the cation/Ca²⁺ exchanger superfamily and participates in the regulation of cellular Ca²⁺ homeostasis. NCX1, the most important subtype in the NCX family, is expressed widely in various organs and tissues in mammals and plays an especially important role in the physiological and pathological processes of nerves and the cardiovascular system. In the past few years, the function of NCX1 in the digestive system has received increasing attention; NCX1 not only participates in the healing process of gastric ulcer and gastric mucosal injury but also mediates the development of digestive cancer, acute pancreatitis, and intestinal absorption. This review aims to explore the roles of NCX1 in digestive system physiology and pathophysiology in order to guide clinical treatments.

Na+/Ca2+ Exchanger a Druggable Target to Promote β-Cell Proliferation and Function

An important feature of type 2 diabetes is a decrease in β-cell mass. Therefore, it is essential to find new approaches to stimulate β-cell proliferation. We have previously shown that heterozygous inactivation of the Na+/Ca2+ exchanger (isoform 1; NCX1), a protein responsible for Ca2+ extrusion from cells, increases β-cell proliferation, mass, and function in mice. Here, we show that Ncx1 inactivation also increases β-cell proliferation in 2-year-old mice and that NCX1 inhibition in adult mice by four small molecules of the benzoxyphenyl family stimulates β-cell proliferation both in vitro and in vivo. NCX1 inhibition by small interfering RNA or small molecules activates the calcineurin/nuclear factor of activated T cells (NFAT) pathway and inhibits apoptosis induced by the immunosuppressors cyclosporine A (CsA) and tacrolimus in insulin-producing cell. Moreover, NCX1 inhibition increases the expression of β-cell-specific genes, such as Ins1, Ins2, and Pdx1, and inactivates/downregulates the tumor suppressors retinoblastoma protein (pRb) and miR-193a and the cell cycle inhibitor p53. Our data show that Na+/Ca2+ exchange is a druggable target to stimulate β-cell function and proliferation. Specific β-cell inhibition of Na+/Ca2+ exchange by phenoxybenzamyl derivatives may represent an innovative approach to promote β-cell regeneration in diabetes and improve the efficiency of pancreatic islet transplantation for the treatment of the disease.

Local and regional control of calcium dynamics in the pancreatic islet

Ca²⁺ is the key intracellular regulator of insulin secretion, acting in the β-cell as the ultimate trigger for exocytosis. In response to high glucose, ATP-sensitive K⁺ channel closure and plasma membrane depolarization engage a sophisticated machinery to drive pulsatile cytosolic Ca²⁺ changes. Voltage-gated Ca²⁺ channels, Ca²⁺ -activated K⁺ channels and Na⁺ /Ca²⁺ exchange all play important roles. The use of targeted Ca²⁺ probes has revealed that during each cytosolic Ca²⁺ pulse, uptake of Ca²⁺ by mitochondria, endoplasmic reticulum (ER), secretory granules and lysosomes fine-tune cytosolic Ca²⁺ dynamics and control organellar function. For example, changes in the expression of the Ca²⁺ -binding protein Sorcin appear to provide a link between ER Ca²⁺ levels and ER stress, affecting β-cell function and survival. Across the islet, intercellular communication between highly interconnected "hubs," which act as pacemaker β-cells, and subservient "followers," ensures efficient insulin secretion. Loss of connectivity is seen after the deletion of genes associated with type 2 diabetes (T2D) and follows metabolic and inflammatory insults that characterize this disease. Hubs, which typically comprise ~1%-10% of total β-cells, are repurposed for their specialized role by expression of high glucokinase (Gck) but lower Pdx1 and Nkx6.1 levels. Single cell-omics are poised to provide a deeper understanding of the nature of these cells and of the networks through which they communicate. New insights into the control of both the intra- and intercellular Ca²⁺ dynamics may thus shed light on T2D pathology and provide novel opportunities for therapy.

The Na+/Ca2+ exchanger and the Plasma Membrane Ca2+-ATPase in β-cell function and diabetes

The rat pancreatic β-cell expresses 6 splice variants of the Plasma Membrane Ca²⁺-ATPase (PMCA) and two splice variants of the Na⁺/Ca²⁺ exchanger 1 (NCX1). In the β-cell Na⁺/Ca²⁺ exchange displays a high capacity, contributes to both Ca²⁺ outflow and influx and participates to the control of insulin release. Gain of function studies show that overexpression of PMCA2 or NCX1 leads to endoplasmic reticulum (ER) Ca²⁺ depletion with subsequent ER stress, decrease in β-cell proliferation and β-cell death by apoptosis. Loss of function studies show, on the contrary, that heterozygous inactivation of NCX1 (Ncx1^+/-) leads to an increase in β-cell function and a 5 fold increase in both β-cell mass and proliferation. The mutation also increases β-cell resistance to hypoxia, and Ncx1^+/- islets show a 2-4 times higher rate of diabetes cure than Ncx1^+/+ islets when transplanted in diabetic animals. Thus, down-regulation of the Na⁺/Ca²⁺ exchanger leads to various changes in β-cell function that are opposite to the major abnormalities seen in diabetes. In addition, the β-cell includes the mutually exclusive exon B in the alternative splicing region of NCX1, which confers a high sensitivity of its NCX splice variants (NCX1.3 & 1.7) to the inhibitory action of compounds like KBR-7943. Heterozygous inactivation of PMCA2 leads to apparented, though not completely similar results.These provide 2 unique models for the prevention and treatment of β-cell dysfunction in diabetes and following islet transplantation.

TRP-Na(+)/Ca(2+) Exchanger Coupling

Na(+)/Ca(2+) exchangers (NCXs) have traditionally been viewed principally as a means of Ca(2+) removal from non-excitable cells. However there has recently been increasing interest in the operation of NCXs in reverse mode acting as a means of eliciting Ca(2+) entry into these cells. Reverse mode exchange requires a significant change in the normal resting transmembrane ion gradients and membrane potential, which has been suggested to occur principally via the coupling of NCXs to localised Na(+) entry through non-selective cation channels such as canonical transient receptor potential (TRPC) channels. Here we review evidence for functional or physical coupling of NCXs to non-selective cation channels, and how this affects NCX activity in non-excitable cells. In particular we focus on the potential role of nanojunctions, where the close apposition of plasma and intracellular membranes may help create the conditions needed for the generation of localised rises in Na(+) concentration that would be required to trigger reverse mode exchange.

Heterozygous inactivation of plasma membrane Ca(2+)-ATPase in mice increases glucose-induced insulin release and beta cell proliferation, mass and viability

AIMS/HYPOTHESIS

Calcium plays an important role in the process of glucose-induced insulin release in pancreatic beta cells. These cells are equipped with a double system responsible for Ca(2+) extrusion--the Na/Ca exchanger (NCX) and the plasma membrane Ca(2+)-ATPase (PMCA). We have shown that heterozygous inactivation of NCX1 in mice increased glucose-induced insulin release and stimulated beta cell proliferation and mass. In the present study, we examined the effects of heterozygous inactivation of the PMCA on beta cell function.

METHODS

Biological and morphological methods (Ca(2+) imaging, Ca(2+) uptake, glucose metabolism, insulin release and immunohistochemistry) were used to assess beta cell function and proliferation in Pmca2 (also known as Atp2b2) heterozygous mice and control littermates ex vivo. Blood glucose and insulin levels were also measured to assess glucose metabolism in vivo.

RESULTS

Pmca (isoform 2) heterozygous inactivation increased intracellular Ca(2+) stores and glucose-induced insulin release. Moreover, increased beta cell proliferation, mass, viability and islet size were observed in Pmca2 heterozygous mice. However, no differences in beta cell glucose metabolism, proinsulin immunostaining and insulin content were observed.

CONCLUSIONS/INTERPRETATION

The present data indicates that inhibition of Ca(2+) extrusion from the beta cell and its subsequent intracellular accumulation stimulates beta cell function, proliferation and mass. This is in agreement with our previous results observed in mice displaying heterozygous inactivation of NCX, and indicates that inhibition of Ca(2+) extrusion mechanisms by small molecules in beta cells may represent a new approach in the treatment of type 1 and type 2 diabetes.

Testing pancreatic islet function at the single cell level by calcium influx with associated marker expression

Studying the response of islet cells to glucose stimulation is important for understanding cell function in healthy and disease states. Most functional assays are performed on whole islets or cell populations, resulting in averaged observations and loss of information at the single cell level. We demonstrate methods to examine calcium fluxing in individual cells of intact islets in response to multiple glucose challenges. Wild-type mouse islets predominantly contained cells that responded to three (out of three) sequential high glucose challenges, whereas cells of diabetic islets (db/db or NOD) responded less frequently or not at all. Imaged islets were also immunostained for endocrine markers to associate the calcium flux profile of individual cells with gene expression. Wild-type mouse islet cells that robustly fluxed calcium expressed β cell markers (INS/NKX6.1), whereas islet cells that inversely fluxed at low glucose expressed α cell markers (GCG). Diabetic mouse islets showed a higher proportion of dysfunctional β cells that responded poorly to glucose challenges. Most of the failed calcium influx responses in β cells were observed in the second and third high glucose challenges, emphasizing the importance of multiple sequential glucose challenges for assessing the full function of islet cells. Human islet cells were also assessed and showed functional α and β cells. This approach to analyze islet responses to multiple glucose challenges in correlation with gene expression assays expands the understanding of β cell function and the diseased state.

Sodium-calcium exchanger 1 regulates epithelial cell migration via calcium-dependent extracellular signal-regulated kinase signaling

Na(+)/Ca(2+) exchanger-1 (NCX1) is a major calcium extrusion mechanism in renal epithelial cells enabling the efflux of one Ca(2+) ion and the influx of three Na(+) ions. The gradient for this exchange activity is provided by Na,K-ATPase, a hetero-oligomer consisting of a catalytic α-subunit and a regulatory β-subunit (Na,K-β) that also functions as a motility and tumor suppressor. We showed earlier that mice with heart-specific ablation (KO) of Na,K-β had a specific reduction in NCX1 protein and were ouabain-insensitive. Here, we demonstrate that Na,K-β associates with NCX1 and regulates its localization to the cell surface. Madin-Darby canine kidney cells with Na,K-β knockdown have reduced NCX1 protein and function accompanied by 2.1-fold increase in free intracellular calcium and a corresponding increase in the rate of cell migration. Increased intracellular calcium up-regulated ERK1/2 via calmodulin-dependent activation of PI3K. Both myosin light chain kinase and Rho-associated kinase acted as mediators of ERK1/2-dependent migration. Restoring NCX1 expression in β-KD cells reduced migration rate and ERK1/2 activation, suggesting that NCX1 functions downstream of Na,K-β in regulating cell migration. In parallel, inhibition of NCX1 by KB-R7943 in Madin-Darby canine kidney cells, LLC-PK1, and human primary renal epithelial cells (HREpiC) increased ERK1/2 activation and cell migration. This increased migration was associated with high myosin light chain phosphorylation by PI3K/ERK-dependent mechanism in HREpiC cells. These data confirm the role of NCX1 activity in regulating renal epithelial cell migration.

Roles of ion transport in control of cell motility

Cell motility is an essential feature of life. It is essential for reproduction, propagation, embryonic development, and healing processes such as wound closure and a successful immune defense. If out of control, cell motility can become life-threatening as, for example, in metastasis or autoimmune diseases. Regardless of whether ciliary/flagellar or amoeboid movement, controlled motility always requires a concerted action of ion channels and transporters, cytoskeletal elements, and signaling cascades. Ion transport across the plasma membrane contributes to cell motility by affecting the membrane potential and voltage-sensitive ion channels, by inducing local volume changes with the help of aquaporins and by modulating cytosolic Ca(2+) and H(+) concentrations. Voltage-sensitive ion channels serve as voltage detectors in electric fields thus enabling galvanotaxis; local swelling facilitates the outgrowth of protrusions at the leading edge while local shrinkage accompanies the retraction of the cell rear; the cytosolic Ca(2+) concentration exerts its main effect on cytoskeletal dynamics via motor proteins such as myosin or dynein; and both, the intracellular and the extracellular H(+) concentration modulate cell migration and adhesion by tuning the activity of enzymes and signaling molecules in the cytosol as well as the activation state of adhesion molecules at the cell surface. In addition to the actual process of ion transport, both, channels and transporters contribute to cell migration by being part of focal adhesion complexes and/or physically interacting with components of the cytoskeleton. The present article provides an overview of how the numerous ion-transport mechanisms contribute to the various modes of cell motility.

2-Aminoadipic acid is a biomarker for diabetes risk

Improvements in metabolite-profiling techniques are providing increased breadth of coverage of the human metabolome and may highlight biomarkers and pathways in common diseases such as diabetes. Using a metabolomics platform that analyzes intermediary organic acids, purines, pyrimidines, and other compounds, we performed a nested case-control study of 188 individuals who developed diabetes and 188 propensity-matched controls from 2,422 normoglycemic participants followed for 12 years in the Framingham Heart Study. The metabolite 2-aminoadipic acid (2-AAA) was most strongly associated with the risk of developing diabetes. Individuals with 2-AAA concentrations in the top quartile had greater than a 4-fold risk of developing diabetes. Levels of 2-AAA were not well correlated with other metabolite biomarkers of diabetes, such as branched chain amino acids and aromatic amino acids, suggesting they report on a distinct pathophysiological pathway. In experimental studies, administration of 2-AAA lowered fasting plasma glucose levels in mice fed both standard chow and high-fat diets. Further, 2-AAA treatment enhanced insulin secretion from a pancreatic β cell line as well as murine and human islets. These data highlight a metabolite not previously associated with diabetes risk that is increased up to 12 years before the onset of overt disease. Our findings suggest that 2-AAA is a marker of diabetes risk and a potential modulator of glucose homeostasis in humans.

Pretreatment of donor islets with the Na(+) /Ca(2+) exchanger inhibitor improves the efficiency of islet transplantation

Pancreatic islet transplantation is an attractive therapy for the treatment of insulin-dependent diabetes mellitus. However, the low efficiency of this procedure necessitating sequential transplantations of islets with the use of 2-3 donors for a single recipient, mainly due to the early loss of transplanted islets, hampers its clinical application. Previously, we have shown in mice that a large amount of HMGB1 is released from islets soon after their transplantation and that this triggers innate immune rejection with activation of DC, NKT cells and neutrophils to produce IFN-γ, ultimately leading to the early loss of transplanted islets. Thus, HMGB1 release plays an initial pivotal role in this process; however, its mechanism remains unclear. Here we demonstrate that release of HMGB1 from transplanted islets is due to hypoxic damage resulting from Ca(2+) influx into β cells through the Na(+) /Ca(2+) exchanger (NCX). Moreover, the hypoxia-induced β cell damage was prevented by pretreatment with an NCX-specific inhibitor prior to transplantation, resulting in protection and long-term survival of transplanted mouse and human islets when grafted into mice. These findings suggest a novel strategy with potentially great impact to improve the efficiency of islet transplantation in clinical settings by targeting donor islets rather than recipients.

Triton X-100 inhibits L-type voltage-operated calcium channels

Triton X-100 (TX-100) is a nonionic detergent frequently used at millimolar concentrations to disrupt cell membranes and solubilize proteins. At low micromolar concentrations, TX-100 has been reported to inhibit the function of potassium channels. Here, we have used electrophysiological and functional techniques to examine the effects of TX-100 on another class of ion channels, L-type voltage-operated calcium channels (VOCCs). TX-100 (30 nmol·L(-1) to 3 μmol·L(-1)) caused reversible concentration-dependent inhibition of recombinant L-type VOCC (CaV 1.2) currents and of native L-type VOCC currents recorded from rat vascular smooth muscle cells and cardiac myocytes, and murine and human pancreatic β-cells. In functional studies, TX-100 (165 nmol·L(-1) to 3.4 μmol·L(-1)) caused concentration-dependent relaxation of rat isolated mesenteric resistance arteries prestimulated with phenylephrine or KCl. This effect was independent of the endothelium. TX-100 (1.6 μmol·L(-1)) inhibited depolarization-induced exocytosis in both murine and human isolated pancreatic β-cells. These data indicate that at concentrations within the nanomolar to low micromolar range, TX-100 significantly inhibits L-type VOCC activity in a number of cell types, an effect paralleled by inhibition of cell functions dependent upon activation of these channels. This inhibition occurs at concentrations below those used to solubilize proteins and may compromise the use of solutions containing TX-100 in bioassays.

Role of ion channels and transporters in cell migration

Cell motility is central to tissue homeostasis in health and disease, and there is hardly any cell in the body that is not motile at a given point in its life cycle. Important physiological processes intimately related to the ability of the respective cells to migrate include embryogenesis, immune defense, angiogenesis, and wound healing. On the other side, migration is associated with life-threatening pathologies such as tumor metastases and atherosclerosis. Research from the last ≈ 15 years revealed that ion channels and transporters are indispensable components of the cellular migration apparatus. After presenting general principles by which transport proteins affect cell migration, we will discuss systematically the role of channels and transporters involved in cell migration.

Na(+)/Ca (2+) exchange and the plasma membrane Ca(2+)-ATPase in β-cell function and diabetes

The rat pancreatic β-cell expresses two splice variants of the Na+/Ca(2+) exchanger 1 (NCX1) and six splice variants of the plasma membrane Ca(2+)-ATPase (PMCA). In the β-cell, Na(+)/Ca(2+) exchange displays a high capacity, contributes to both Ca(2+) outflow and influx and participates to the control of insulin release. Gain of function studies show that overexpression of NCX1 or PMCA2 leads to endoplasmic reticulum (ER) Ca(2+) depletion with subsequent ER stress, decrease in β-cell proliferation and β-cell death by apoptosis. Interestingly, chronic exposure to cytokines or high free fatty acids concentration also induces ER Ca(2+) depletion and β-cell death in diabetes. Loss of function studies shows, on the contrary, that heterozygous inactivation of NCX1 (Ncx1 ( +/- )) leads to an increase in β-cell function (insulin production and release) and a fivefold increase in both β-cell mass and proliferation. The mutation also increases β-cell resistance to hypoxia, and Ncx1 ( +/- ) islets show a four to seven times higher rate of diabetes cure than Ncx1 ( +/+ ) islets when transplanted in diabetic animals. Thus, downregulation of the Na(+)/Ca(2+) exchanger leads to various changes in β-cell function that are opposite to the major abnormalities seen in diabetes. In addition, the β-cell, which is an excitable cell, includes the mutually exclusive exon B in the alternative splicing region of NCX1, which confers a high sensitivity of its NCX splice variants (NCX1.3 & 1.7) to the inhibitory action of compounds like KB-R7943. This provides a unique model for the prevention and treatment of β-cell dysfunction in diabetes and following islet transplantation.

Role of the Na(+)/Ca(2+) exchanger on the development of diabetes mellitus and its chronic complications

Diabetes mellitus (DM) is a serious metabolic disorder with micro- and macrovascular complications that results in significant morbidity and mortality. It is well established that cytosolic Ca(2+) play an important role in controlling insulin secretion in pancreatic β-cells. The Na(+)/Ca(2+) exchanger (NCX), an ion transport protein, is expressed in the plasma membrane of virtually all animal cells. NCX is a reversible carrier that can mediate the transport of Ca(2+) across the plasma membrane in both directions. Therefore, great efforts have been made to identify NCX associated with DM. NCX is expressed in several tissues, and acts in the protection against intracellular calcium overload; in the regulation of insulin secretion by beta cells, and in improving vascular endothelium-dependent relaxation. All these mechanisms are associated with DM pathogenesis and its chronic complications. Therefore, NCX is a candidate protein for the development of these disorders. Only a few studies investigated NCX in relation to chronic complications of diabetes, with inconclusive results.

β-Cell preservation and regeneration in diabetes by modulation of β-cell Ca²⁺ homeostasis

Ca(2+) extrusion from the β-cell is mediated by two processes the Na/Ca exchanger (NCX) and the plasma membrane Ca(2+) -ATPase (PMCA). Gain of function studies show that overexpression of NCX or PMCA leads to endoplasmic reticulum (ER) Ca(2+) depletion with subsequent ER stress, decrease in β-cell proliferation and β-cell death by apoptosis. Interestingly, chronic exposure to cytokines or high free fatty acid concentrations also induce ER Ca(2+) depletion and β-cell death in diabetes. Loss of function studies show, on the contrary, that heterozygous inactivation of NCX1 (Ncx1(+/-)) leads to an increase in β-cell function (insulin production and release), and a fivefold increase in both β-cell mass and proliferation. The mutation also increases β-cell resistance to hypoxia, and Ncx1(+/-) islets show a two to four times higher rate of diabetes cure than Ncx1(+/+) islets when transplanted in diabetic animals. Thus, down-regulation of the Na/Ca exchanger leads to various changes in β-cell function that are opposite to the major abnormalities seen in diabetes. This provides a unique model for the prevention and treatment of β-cell dysfunction in diabetes and following islet transplantation.

Heterozygous inactivation of the Na/Ca exchanger increases glucose-induced insulin release, β-cell proliferation, and mass

OBJECTIVE

We have previously shown that overexpression of the Na-Ca exchanger (NCX1), a protein responsible for Ca(2+) extrusion from cells, increases β-cell programmed cell death (apoptosis) and reduces β-cell proliferation. To further characterize the role of NCX1 in β-cells under in vivo conditions, we developed and characterized mice deficient for NCX1.

RESEARCH DESIGN AND METHODS

Biologic and morphologic methods (Ca(2+) imaging, Ca(2+) uptake, glucose metabolism, insulin release, and point counting morphometry) were used to assess β-cell function in vitro. Blood glucose and insulin levels were measured to assess glucose metabolism and insulin sensitivity in vivo. Islets were transplanted under the kidney capsule to assess their performance to revert diabetes in alloxan-diabetic mice.

RESULTS

Heterozygous inactivation of Ncx1 in mice induced an increase in glucose-induced insulin release, with a major enhancement of its first and second phase. This was paralleled by an increase in β-cell proliferation and mass. The mutation also increased β-cell insulin content, proinsulin immunostaining, glucose-induced Ca(2+) uptake, and β-cell resistance to hypoxia. In addition, Ncx1(+/-) islets showed a two- to four-times higher rate of diabetes cure than Ncx1(+/+) islets when transplanted into diabetic animals.

CONCLUSIONS

Downregulation of the Na/Ca exchanger leads to an increase in β-cell function, proliferation, mass, and resistance to physiologic stress, namely to various changes in β-cell function that are opposite to the major abnormalities seen in type 2 diabetes. This provides a unique model for the prevention and treatment of β-cell dysfunction in type 2 diabetes and after islet transplantation.