Sodium/calcium exchanger: influence of metabolic regulation on ion carrier interactions

Structural dynamics of Na+ and Ca2+ interactions with full-size mammalian NCX

Cytosolic Ca2+ and Na+ allosterically regulate Na+/Ca2+ exchanger (NCX) proteins to vary the NCX-mediated Ca2+ entry/exit rates in diverse cell types. To resolve the structure-based dynamic mechanisms underlying the ion-dependent allosteric regulation in mammalian NCXs, we analyze the apo, Ca2+, and Na+-bound species of the brain NCX1.4 variant using hydrogen-deuterium exchange mass spectrometry (HDX-MS) and molecular dynamics (MD) simulations. Ca2+ binding to the cytosolic regulatory domains (CBD1 and CBD2) rigidifies the intracellular regulatory loop (5L6) and promotes its interaction with the membrane domains. Either Na+ or Ca2+ stabilizes the intracellular portions of transmembrane helices TM3, TM4, TM9, TM10, and their connecting loops (3L4 and 9L10), thereby exposing previously unappreciated regulatory sites. Ca2+ or Na+ also rigidifies the palmitoylation domain (TMH2), and neighboring TM1/TM6 bundle, thereby uncovering a structural entity for modulating the ion transport rates. The present analysis provides new structure-dynamic clues underlying the regulatory diversity among tissue-specific NCX variants.

Neuronal and astrocyte NCX isoform/splice variants: How do they participate in Na+ and Ca2+ signalling?

NCX1, NCX2, and NCX3 gene isoforms and their splice variants are characteristically expressed in different regions of the brain. The tissue-specific splice variants of NCX1-3 isoforms show specific expression profiles in neurons and astrocytes, whereas the relevant NCX isoform/splice variants exhibit diverse allosteric modes of Na+- and Ca2+-dependent regulation. In general, overexpression of NCX1-3 genes leads to neuroprotective effects, whereas their ablation gains the opposite results. At this end, the partial contributions of NCX isoform/splice variants to neuroprotective effects remain unresolved. The glutamate-dependent Na+ entry generates Na+ transients (in response to neuronal cell activities), whereas the Na+-driven Ca2+ entry (through the reverse NCX mode) raises Ca2+ transients. This special mode of signal coupling translates Na+ transients into the Ca2+ signals while being a part of synaptic neurotransmission. This mechanism is of general interest since disease-related conditions (ischemia, metabolic stress, and stroke among many others) trigger Na+ and Ca2+ overload with deadly outcomes of downstream apoptosis and excitotoxicity. The recently discovered mechanisms of NCX allosteric regulation indicate that some NCX variants might play a critical role in the dynamic coupling of Na+-driven Ca2+ entry. In contrast, the others are less important or even could be dangerous under altered conditions (e.g., metabolic stress). This working hypothesis can be tested by applying advanced experimental approaches and highly focused computational simulations. This may allow the development of structure-based blockers/activators that can selectively modulate predefined NCX variants to lessen the life-threatening outcomes of excitotoxicity, ischemia, apoptosis, metabolic deprivation, brain injury, and stroke.

Light-Induced Nanoscale Deformation in Azobenzene Thin Film Triggers Rapid Intracellular Ca2+ Increase via Mechanosensitive Cation Channels

Epithelial cells are in continuous dynamic biochemical and physical interaction with their extracellular environment. Ultimately, this interplay guides fundamental physiological processes. In these interactions, cells generate fast local and global transients of Ca2+ ions, which act as key intracellular messengers. However, the mechanical triggers initiating these responses have remained unclear. Light-responsive materials offer intriguing possibilities to dynamically modify the physical niche of the cells. Here, a light-sensitive azobenzene-based glassy material that can be micropatterned with visible light to undergo spatiotemporally controlled deformations is used. Real-time monitoring of consequential rapid intracellular Ca2+ signals reveals that the mechanosensitive cation channel Piezo1 has a major role in generating the Ca2+ transients after nanoscale mechanical deformation of the cell culture substrate. Furthermore, the studies indicate that Piezo1 preferably responds to shear deformation at the cell-material interphase rather than to absolute topographical change of the substrate. Finally, the experimentally verified computational model suggests that Na+ entering alongside Ca2+ through the mechanosensitive cation channels modulates the duration of Ca2+ transients, influencing differently the directly stimulated cells and their neighbors. This highlights the complexity of mechanical signaling in multicellular systems. These results give mechanistic understanding on how cells respond to rapid nanoscale material dynamics and deformations.

Structural mechanisms of the human cardiac sodium-calcium exchanger NCX1

Na+/Ca2+ exchangers (NCX) transport Ca2+ in or out of cells in exchange for Na+. They are ubiquitously expressed and play an essential role in maintaining cytosolic Ca2+ homeostasis. Although extensively studied, little is known about the global structural arrangement of eukaryotic NCXs and the structural mechanisms underlying their regulation by various cellular cues including cytosolic Na+ and Ca2+. Here we present the cryo-EM structures of human cardiac NCX1 in both inactivated and activated states, elucidating key structural elements important for NCX ion exchange function and its modulation by cytosolic Ca2+ and Na+. We demonstrate that the interactions between the ion-transporting transmembrane (TM) domain and the cytosolic regulatory domain define the activity of NCX. In the inward-facing state with low cytosolic [Ca2+], a TM-associated four-stranded β-hub mediates a tight packing between the TM and cytosolic domains, resulting in the formation of a stable inactivation assembly that blocks the TM movement required for ion exchange function. Ca2+ binding to the cytosolic second Ca2+-binding domain (CBD2) disrupts this inactivation assembly which releases its constraint on the TM domain, yielding an active exchanger. Thus, the current NCX1 structures provide an essential framework for the mechanistic understanding of the ion transport and cellular regulation of NCX family proteins.

Interactions between calcium regulatory pathways and mechanosensitive channels in airways

INTRODUCTION

Asthma is a chronic lung disease influenced by environmental and inflammatory triggers and involving complex signaling pathways across resident airway cells such as epithelium, airway smooth muscle, fibroblasts, and immune cells. While our understanding of asthma pathophysiology is continually progressing, there is a growing realization that cellular microdomains play critical roles in mediating signaling relevant to asthma in the context of contractility and remodeling. Mechanosensitive pathways are increasingly recognized as important to microdomain signaling, with Piezo and transient receptor protein (TRP) channels at the plasma membrane considered important for converting mechanical stimuli into cellular behavior. Given their ion channel properties, particularly Ca2+ conduction, a question becomes whether and how mechanosensitive channels contribute to Ca2+ microdomains in airway cells relevant to asthma.

AREAS COVERED

Mechanosensitive TRP and Piezo channels regulate key Ca2+ regulatory proteins such as store operated calcium entry (SOCE) involving STIM and Orai channels, and sarcoendoplasmic (SR) mechanisms such as IP3 receptor channels (IP3Rs), and SR Ca2+ ATPase (SERCA) that are important in asthma pathophysiology including airway hyperreactivity and remodeling.

EXPERT OPINION

Physical and/or functional interactions between Ca2+ regulatory proteins and mechanosensitive channels such as TRP and Piezo can toward understanding asthma pathophysiology and identifying novel therapeutic approaches.

Estrogenic Modulation of Ionic Channels, Pumps and Exchangers in Airway Smooth Muscle

To preserve ionic homeostasis (primarily Ca²⁺, K⁺, Na⁺, and Cl^-), in the airway smooth muscle (ASM) numerous transporters (channels, exchangers, and pumps) regulate the influx and efflux of these ions. Many of intracellular processes depend on continuous ionic permeation, including exocytosis, contraction, metabolism, transcription, fecundation, proliferation, and apoptosis. These mechanisms are precisely regulated, for instance, through hormonal activity. The lipophilic nature of steroidal hormones allows their free transit into the cell where, in most cases, they occupy their cognate receptor to generate genomic actions. In the sense, estrogens can stimulate development, proliferation, migration, and survival of target cells, including in lung physiology. Non-genomic actions on the other hand do not imply estrogen's intracellular receptor occupation, nor do they initiate transcription and are mostly immediate to the stimulus. Among estrogen's non genomic responses regulation of calcium homeostasis and contraction and relaxation processes play paramount roles in ASM. On the other hand, disruption of calcium homeostasis has been closely associated with some ASM pathological mechanism. Thus, this paper intends to summarize the effects of estrogen on ionic handling proteins in ASM. The considerable diversity, range and power of estrogens regulates ionic homeostasis through genomic and non-genomic mechanisms.

Structure-Based Function and Regulation of NCX Variants: Updates and Challenges

The plasma-membrane homeostasis Na⁺/Ca²⁺ exchangers (NCXs) mediate Ca²⁺ extrusion/entry to dynamically shape Ca²⁺ signaling/in biological systems ranging from bacteria to humans. The NCX gene orthologs, isoforms, and their splice variants are expressed in a tissue-specific manner and exhibit nearly 10⁴-fold differences in the transport rates and regulatory specificities to match the cell-specific requirements. Selective pharmacological targeting of NCX variants could benefit many clinical applications, although this intervention remains challenging, mainly because a full-size structure of eukaryotic NCX is unavailable. The crystal structure of the archaeal NCX_Mj, in conjunction with biophysical, computational, and functional analyses, provided a breakthrough in resolving the ion transport mechanisms. However, NCX_Mj (whose size is nearly three times smaller than that of mammalian NCXs) cannot serve as a structure-dynamic model for imitating high transport rates and regulatory modules possessed by eukaryotic NCXs. The crystal structures of isolated regulatory domains (obtained from eukaryotic NCXs) and their biophysical analyses by SAXS, NMR, FRET, and HDX-MS approaches revealed structure-based variances of regulatory modules. Despite these achievements, it remains unclear how multi-domain interactions can decode and integrate diverse allosteric signals, thereby yielding distinct regulatory outcomes in a given ortholog/isoform/splice variant. This article summarizes the relevant issues from the perspective of future developments.

Sodium-calcium exchanger-3 regulates pain "wind-up": From human psychophysics to spinal mechanisms

Repeated application of noxious stimuli leads to a progressively increased pain perception; this temporal summation is enhanced in and predictive of clinical pain disorders. Its electrophysiological correlate is "wind-up," in which dorsal horn spinal neurons increase their response to repeated nociceptor stimulation. To understand the genetic basis of temporal summation, we undertook a GWAS of wind-up in healthy human volunteers and found significant association with SLC8A3 encoding sodium-calcium exchanger type 3 (NCX3). NCX3 was expressed in mouse dorsal horn neurons, and mice lacking NCX3 showed normal, acute pain but hypersensitivity to the second phase of the formalin test and chronic constriction injury. Dorsal horn neurons lacking NCX3 showed increased intracellular calcium following repetitive stimulation, slowed calcium clearance, and increased wind-up. Moreover, virally mediated enhanced spinal expression of NCX3 reduced central sensitization. Our study highlights Ca2+ efflux as a pathway underlying temporal summation and persistent pain, which may be amenable to therapeutic targeting.

Control of Ca2+ and metabolic homeostasis by the Na+/Ca2+ exchangers (NCXs) in health and disease

Spatial and temporal control of calcium (Ca²⁺) levels is essential for the background rhythms and responses of living cells to environmental stimuli. Whatever other regulators a given cellular activity may have, localized and wider scale Ca²⁺ events (sparks, transients, and waves) are hierarchical determinants of fundamental processes such as cell contraction, excitability, growth, metabolism and survival. Different cell types express specific channels, pumps and exchangers to efficiently generate and adapt Ca²⁺ patterns to cell requirements. The Na⁺/Ca²⁺ exchangers (NCXs) in particular contribute to Ca²⁺ homeostasis by buffering intracellular Ca²⁺ loads according to the electrochemical gradients of substrate ions - i.e., Ca²⁺ and sodium (Na⁺) - and under a dynamic control of redundant regulatory processes. An interesting feature of NCX emerges from the strict relationship that connects transporter activity with cell metabolism: on the one hand NCX operates under constant control of ATP-dependent regulatory processes, on the other hand the ion fluxes generated through NCX provide mechanistic support for the Na⁺-driven uptake of glutamate and Ca²⁺ influx to fuel mitochondrial respiration. Proof of concept evidence highlights therapeutic potential of preserving a timed and balanced NCX activity in a growing rate of diseases (including excitability, neurodegenerative, and proliferative disorders) because of an improved ability of stressed cells to safely maintain ion gradients and mitochondrial bioenergetics. Here, we will summarize and review recent works that have focused on the pathophysiological roles of NCXs in balancing the two-way relationship between Ca²⁺ signals and metabolism.

Identification of quantitative trait nucleotides and candidate genes for tuber yield and mosaic virus tolerance in an elite population of white guinea yam (Dioscorea rotundata) using genome-wide association scan

BACKGROUND

Improvement of tuber yield and tolerance to viruses are priority objectives in white Guinea yam breeding programs. However, phenotypic selection for these traits is quite challenging due to phenotypic plasticity and cumbersome screening of phenotypic-induced variations. This study assessed quantitative trait nucleotides (QTNs) and the underlying candidate genes related to tuber yield per plant (TYP) and yam mosaic virus (YMV) tolerance in a panel of 406 white Guinea yam (Dioscorea rotundata) breeding lines using a genome-wide association study (GWAS).

RESULTS

Population structure analysis using 5,581 SNPs differentiated the 406 genotypes into seven distinct sub-groups based delta K. Marker-trait association (MTA) analysis using the multi-locus linear model (mrMLM) identified seventeen QTN regions significant for TYP and five for YMV with various effects. The seveteen QTNs were detected on nine chromosomes, while the five QTNs were identified on five chromosomes. We identified variants responsible for predicting higher yield and low virus severity scores in the breeding panel through the marker-effect prediction. Gene annotation for the significant SNP loci identified several essential putative genes associated with the growth and development of tuber yield and those that code for tolerance to mosaic virus.

CONCLUSION

Application of different multi-locus models of GWAS identified 22 QTNs. Our results provide valuable insight for marker validation and deployment for tuber yield and mosaic virus tolerance in white yam breeding. The information on SNP variants and genes from the present study would fast-track the application of genomics-informed selection decisions in breeding white Guinea yam for rapid introgression of the targeted traits through markers validation.

Proton-modulated interactions of ions with transport sites of prokaryotic and eukaryotic NCX prototypes

The cytosolic pH decline from 7.2 to 6.9 results in 90% inactivation of mammalian Na⁺/Ca²⁺ exchangers (NCXs) due to protons interactions with regulatory and transport domains ("proton block"). Remarkably, the pH titration curves of mammalian and prokaryotic NCXs significantly differ, even after excluding the allosteric effects through regulatory domains. This is fascinating since "only" three (out of twelve) ion-coordinating residues (T50S, E213D, and D240N) differ between the archaeal NCX_Mj and mammalian NCXs although they contain either three or two carboxylates, respectively. To resolve the underlying mechanisms of pH-dependent regulation, the ion-coordinating residues of NCX_Mj were mutated to imitate the ion ligation arrays of mammalian NCXs; the mutational effects were tested on the ion binding/transport by using ion-flux assays and two-dimensional infrared (2D IR) spectroscopy. Our analyses revealed that two deprotonated carboxylates ligate 3Na⁺ or 1Ca²⁺ in NCX prototypes with three or two carboxylates. The Na⁺/Ca²⁺ exchange rates of NCX_Mj reach saturation at pH 5.0, whereas the Na⁺/Ca²⁺ exchange rates of the cardiac NCX1.1 gradually increase even at alkaline pHs. The T50S replacement in NCX_Mj "recapitulates" the pH titration curves of mammalian NCX by instigating an alkaline shift. Proteolytic shaving of regulatory CBD domains activates NCX1.1, although the normalized pH-titration curves are comparable in trypsin treated and untreated NCX1.1. Thus, the T50S-dependent alkaline shift sets a dynamic range for "proton block" function at physiological pH, whereas the CBDs (and other regulatory modes) modulate incremental changes in the transport rates rather than affect the shape of pH dependent curves.

New Insights into Multiple Sclerosis Mechanisms: Lipids on the Track to Control Inflammation and Neurodegeneration

Multiple sclerosis (MS) is a central nervous system disease with complex pathogenesis, including two main processes: immune-mediated inflammatory demyelination and progressive degeneration with axonal loss. Despite recent progress in our understanding and management of MS, availability of sensitive and specific biomarkers for these both processes, as well as neuroprotective therapeutic options targeted at progressive phase of disease, are still being sought. Given their abundance in the myelin sheath, lipids are believed to play a central role in underlying immunopathogenesis in MS and seem to be a promising subject of investigation in this field. On the basis of our previous research and a review of the literature, we discuss the current understanding of lipid-related mechanisms involved in active relapse, remission, and progression of MS. These insights highlight potential usefulness of lipid markers in prediction or monitoring the course of MS, particularly in its progressive stage, still insufficiently addressed. Furthermore, they raise hope for new, effective, and stage-specific treatment options, involving lipids as targets or carriers of therapeutic agents.

Calcium Dyshomeostasis in Alzheimer's Disease Pathogenesis

Alzheimer’s disease (AD) is the most common age-related neurodegenerative disorder that is characterized by amyloid β-protein deposition in senile plaques, neurofibrillary tangles consisting of abnormally phosphorylated tau protein, and neuronal loss leading to cognitive decline and dementia. Despite extensive research, the exact mechanisms underlying AD remain unknown and effective treatment is not available. Many hypotheses have been proposed to explain AD pathophysiology; however, there is general consensus that the abnormal aggregation of the amyloid β peptide (Aβ) is the initial event triggering a pathogenic cascade of degenerating events in cholinergic neurons. The dysregulation of calcium homeostasis has been studied considerably to clarify the mechanisms of neurodegeneration induced by Aβ. Intracellular calcium acts as a second messenger and plays a key role in the regulation of neuronal functions, such as neural growth and differentiation, action potential, and synaptic plasticity. The calcium hypothesis of AD posits that activation of the amyloidogenic pathway affects neuronal Ca²⁺ homeostasis and the mechanisms responsible for learning and memory. Aβ can disrupt Ca²⁺ signaling through several mechanisms, by increasing the influx of Ca²⁺ from the extracellular space and by activating its release from intracellular stores. Here, we review the different molecular mechanisms and receptors involved in calcium dysregulation in AD and possible therapeutic strategies for improving the treatment.

Palmitoylation: A new mechanism for control of NCX1 function

A paper by Gök et al., identified Zinc Finger DHHC-Type Palmitoyltransferase 5 (zDHHC5) and Acyl-Protein Transferase 1 (APT1) as the enzymes responsible for the dynamic palmitoylation of NCX1. Palmitoylation occurs at the cell surface and increases the affinity of NCX1 for lipid rafts. Additionally, they discovered that palmitoylation controls the affinity of NCX1 for exchange inhibitory protein (XIP) and regulates intracellular calcium concentration. These findings provide new insights into endogenous control of NCX1 function and will drive future investigations directed at understanding its full potential.

Molecular Mean-Field Theory of Ionic Solutions: A Poisson-Nernst-Planck-Bikerman Model

We have developed a molecular mean-field theory—fourth-order Poisson–Nernst–Planck–Bikerman theory—for modeling ionic and water flows in biological ion channels by treating ions and water molecules of any volume and shape with interstitial voids, polarization of water, and ion-ion and ion-water correlations. The theory can also be used to study thermodynamic and electrokinetic properties of electrolyte solutions in batteries, fuel cells, nanopores, porous media including cement, geothermal brines, the oceanic system, etc. The theory can compute electric and steric energies from all atoms in a protein and all ions and water molecules in a channel pore while keeping electrolyte solutions in the extra- and intracellular baths as a continuum dielectric medium with complex properties that mimic experimental data. The theory has been verified with experiments and molecular dynamics data from the gramicidin A channel, L-type calcium channel, potassium channel, and sodium/calcium exchanger with real structures from the Protein Data Bank. It was also verified with the experimental or Monte Carlo data of electric double-layer differential capacitance and ion activities in aqueous electrolyte solutions. We give an in-depth review of the literature about the most novel properties of the theory, namely Fermi distributions of water and ions as classical particles with excluded volumes and dynamic correlations that depend on salt concentration, composition, temperature, pressure, far-field boundary conditions etc. in a complex and complicated way as reported in a wide range of experiments. The dynamic correlations are self-consistent output functions from a fourth-order differential operator that describes ion-ion and ion-water correlations, the dielectric response (permittivity) of ionic solutions, and the polarization of water molecules with a single correlation length parameter.

Additive Effect of Resveratrol on Astrocyte Swelling Post-exposure to Ammonia, Ischemia and Trauma In Vitro

Swelling of astrocytes represents a major component of the brain edema associated with many neurological conditions, including acute hepatic encephalopathy (AHE), traumatic brain injury (TBI) and ischemia. It has previously been reported that exposure of cultured astrocytes to ammonia (a factor strongly implicated in the pathogenesis of AHE), oxygen/glucose deprivation, or to direct mechanical trauma results in an increase in cell swelling. Since dietary polyphenols have been shown to exert a protective effect against cell injury, we examined whether resveratrol (RSV, 3,5,4'-trihydroxy-trans-stilbene, a stilbenoid phenol), has a protective effect on astrocyte swelling following its exposure to ammonia, oxygen-glucose deprivation (OGD), or trauma in vitro. Ammonia increased astrocyte swelling, and pre- or post-treatment of astrocytes with 10 and 25 µM RSV displayed an additive effect, while 5 µM did not prevent the effect of ammonia. However, pre-treatment of astrocytes with 25 µM RSV slightly, but significantly, reduced the trauma-induced astrocyte swelling at earlier time points (3 h), while post-treatment had no significant effect on the trauma-induced cell swelling at the 3 h time point. Instead, pre- or post-treatment of astrocytes with 25 µM RSV had an additive effect on trauma-induced astrocyte swelling. Further, pre- or post-treatment of astrocytes with 5 or 10 µM RSV had no significant effect on trauma-induced astrocyte swelling. When 5 or 10 µM RSV were added prior to, or during the process of OGD, as well as post-OGD, it caused a slight, but not statistically significant decline in cell swelling. However, when 25 µM RSV was added during the process of OGD, as well as after the cells were returned to normal condition (90 min period), such treatment showed an additive effect on the OGD-induced astrocyte swelling. Noteworthy, a higher concentration of RSV (25 µM) exhibited an additive effect on levels of phosphorylated forms of ERK1/2, and p38^MAPK, as well as an increased activity of the Na⁺-K⁺-Cl^- co-transporter-1 (NKCC1), factors known to induce astrocytes swelling, when the cells were treated with ammonia or after trauma or ischemia. Further, inhibition of ERK1/2, and p38^MAPK diminished the RSV-induced exacerbation of cell swelling post-ammonia, trauma and OGD treatment. These findings strongly suggest that treatment of cultured astrocytes with RSV enhanced the ammonia, ischemia and trauma-induced cell swelling, likely through the exacerbation of intercellular signaling kinases and ion transporters. Accordingly, caution should be exercised when using RSV for the treatment of these neurological conditions, especially when brain edema is also suspected.

Differentiation of human colon tissue in culture: Effects of calcium on trans-epithelial electrical resistance and tissue cohesive properties

Background and aims

Human colonoid cultures maintained under low-calcium (0.25 mM) conditions undergo differentiation spontaneously and, concomitantly, express a high level of tight junction proteins, but not desmosomal proteins. When calcium is included to a final concentration of 1.5–3.0 mM (provided either as a single agent or as a combination of calcium and additional minerals), there is little change in tight junction protein expression but a strong up-regulation of desmosomal proteins and an increase in desmosome formation. The aim of this study was to assess the functional consequences of calcium-mediated differences in barrier protein expression.

Methods

Human colonoid-derived epithelial cells were interrogated in transwell culture under low- or high-calcium conditions for monolayer integrity and ion permeability by measuring trans-epithelial electrical resistance (TEER) across the confluent monolayer. Colonoid cohesiveness was assessed in parallel.

Results

TEER values were high in the low-calcium environment but increased in response to calcium. In addition, colonoid cohesiveness increased substantially with calcium supplementation. In both assays, the response to multi-mineral intervention was greater than the response to calcium alone. Consistent with these findings, several components of tight junctions were expressed at 0.25 mM calcium but these did not increase substantially with supplementation. Cadherin-17 and desmoglein-2, in contrast, were weakly-expressed under low calcium conditions but increased with intervention.

Conclusions

These findings indicate that low ambient calcium levels are sufficient to support the formation of a permeability barrier in the colonic epithelium. Higher calcium levels promote tissue cohesion and enhance barrier function. These findings may help explain how an adequate calcium intake contributes to colonic health by improving barrier function, even though there is little change in colonic histological features over a wide range of calcium intake levels.

Na+/K+-ATPase α isoform deficiency results in distinct spreading depolarization phenotypes

Compromised Na⁺/K⁺-ATPase function is associated with the occurrence of spreading depolarization (SD). Mutations in ATP1A2, the gene encoding the α2 isoform of the Na⁺/K⁺-ATPase, were identified in patients with familial hemiplegic migraine type 2 (FHM2), a Mendelian model disease for SD. This suggests a distinct role for the α2 isoform in modulating SD susceptibility and raises questions about underlying mechanisms including the roles of other Na⁺/K⁺-ATPase α isoforms. Here, we investigated the effects of genetic ablation and pharmacological inhibition of α1, α2, and α3 on SD using heterozygous knock-out mice. We found that only α2 heterozygous mice displayed higher SD susceptibility when challenged with prolonged extracellular high potassium concentration ([K⁺]_o), a pronounced post SD oligemia and higher SD speed in-vivo. By contrast, under physiological [K⁺]_o, α2 heterozygous mice showed similar SD susceptibility compared to wild-type littermates. Deficiency of α3 resulted in increased resistance against electrically induced SD in-vivo, whereas α1 deficiency did not affect SD. The results support important roles of the α2 isoform in SD. Moreover, they suggest that specific experimental conditions can be necessary to reveal an inherent SD phenotype by driving a (meta-) stable system into decompensation, reminiscent of the episodic nature of SDs in various diseases.

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.

Live Mitochondrial or Cytosolic Calcium Imaging Using Genetically-encoded Cameleon Indicator in Mammalian Cells

Calcium (Ca²⁺) imaging aims at investigating the dynamic changes in live cells of its concentration ([Ca²⁺]) in different pathophysiological conditions. Ca²⁺ is an ubiquitous and versatile intracellular signal that modulates a large variety of cellular functions thanks to a cell type-specific toolkit and a complex subcellular compartmentalization. Many Ca²⁺ sensors are presently available (chemical and genetically encoded) that can be specifically targeted to different cellular compartments. Using these probes, it is now possible to monitor Ca²⁺ dynamics of living cells not only in the cytosol but also within specific organelles. The choice of a specific sensor depends on the experimental design and the spatial and temporal resolution required. Here we describe the use of novel Förster resonance energy transfer (FRET)-based fluorescent Ca²⁺ probes to dynamically and quantitatively monitor the changes in cytosolic and mitochondrial [Ca²⁺] in a variety of cell types and experimental conditions. FRET-based sensors have the enormous advantage of being ratiometric, a feature that makes them particularly suitable for quantitative and in vivo applications.

Intracellular Calcium Dysregulation by the Alzheimer's Disease-Linked Protein Presenilin 2

Alzheimer's disease (AD) is the most common form of dementia. Even though most AD cases are sporadic, a small percentage is familial due to autosomal dominant mutations in amyloid precursor protein (APP), presenilin-1 (PSEN1), and presenilin-2 (PSEN2) genes. AD mutations contribute to the generation of toxic amyloid β (Aβ) peptides and the formation of cerebral plaques, leading to the formulation of the amyloid cascade hypothesis for AD pathogenesis. Many drugs have been developed to inhibit this pathway but all these approaches currently failed, raising the need to find additional pathogenic mechanisms. Alterations in cellular calcium (Ca2+) signaling have also been reported as causative of neurodegeneration. Interestingly, Aβ peptides, mutated presenilin-1 (PS1), and presenilin-2 (PS2) variously lead to modifications in Ca2+ homeostasis. In this contribution, we focus on PS2, summarizing how AD-linked PS2 mutants alter multiple Ca2+ pathways and the functional consequences of this Ca2+ dysregulation in AD pathogenesis.

Sodium-Calcium Exchangers of the SLC8 Family in Oligodendrocytes: Functional Properties in Health and Disease

The solute carrier 8 (SLC8) family of sodium-calcium exchangers (NCXs) functions as an essential regulatory system that couples opposite fluxes of sodium and calcium ions across plasmalemmal membranes. NCXs, thereby, play key roles in maintaining an ion homeostasis that preserves cellular integrity. Hence, alterations in NCX expression and regulation have been found to lead to ionic imbalances that are often associated with intracellular calcium overload and cell death. On the other hand, intracellular calcium has been identified as a key driver for a multitude of downstream signaling events that are crucial for proper functioning of biological systems, thus highlighting the need for a tightly controlled balance. In the CNS, NCXs have been primarily characterized in the context of synaptic transmission and ischemic brain damage. However, a much broader picture is emerging. NCXs are expressed by virtually all cells of the CNS including oligodendrocytes (OLGs), the cells that generate the myelin sheath. With a growing appreciation of dynamic calcium signals in OLGs, NCXs are becoming increasingly recognized for their crucial roles in shaping OLG function under both physiological and pathophysiological conditions. In order to provide a current update, this review focuses on the importance of NCXs in cells of the OLG lineage. More specifically, it provides a brief introduction into plasmalemmal NCXs and their modes of activity, and it discusses the roles of OLG expressed NCXs in regulating CNS myelination and in contributing to CNS pathologies associated with detrimental effects on OLG lineage cells.

Astrocytic modulation of potassium under seizures

The contribution of an impaired astrocytic K⁺ regulation system to epileptic neuronal hyperexcitability has been increasingly recognized in the last decade. A defective K⁺ regulation leads to an elevated extracellular K⁺ concentration ([K⁺]_o). When [K⁺]_o reaches peaks of 10-12 mM, it is strongly associated with seizure initiation during hypersynchronous neuronal activities. On the other hand, reactive astrocytes during a seizure attack restrict influx of K⁺ across the membrane both passively and actively. In addition to decreased K⁺ buffering, aberrant Ca²⁺ signaling and declined glutamate transport have also been observed in astrogliosis in epileptic specimens, precipitating an increased neuronal discharge and induction of seizures. This review aims to provide an overview of experimental findings that implicated astrocytic modulation of extracellular K⁺ in the mechanism of epileptogenesis.

Mitochondrial Calcium Uptake Is Instrumental to Alternative Macrophage Polarization and Phagocytic Activity

Macrophages are highly plastic and dynamic cells that exert much of their function through phagocytosis. Phagocytosis depends on a coordinated, finely tuned, and compartmentalized regulation of calcium concentrations. We examined the role of mitochondrial calcium uptake and mitochondrial calcium uniporter (MCU) in macrophage polarization and function. In primary cultures of human monocyte-derived macrophages, calcium uptake in mitochondria was instrumental for alternative (M2) macrophage polarization. Mitochondrial calcium uniporter inhibition with KB-R7943 or MCU knockdown, which prevented mitochondrial calcium uptake, reduced M2 polarization, while not affecting classical (M1) polarization. Challenging macrophages with E. coli fragments induced spikes of mitochondrial calcium concentrations, which were prevented by MCU inhibition or silencing. In addition, mitochondria remodelled in M2 macrophages during phagocytosis, especially close to sites of E. coli internalization. Remarkably, inhibition or knockdown of MCU significantly reduced the phagocytic capacity of M2 macrophages. KB-R7943, which also inhibits the membrane sodium/calcium exchanger and Complex I, reduced mitochondria energization and cellular ATP levels, but such effects were not observed with MCU silencing. Therefore, phagocytosis inhibition by MCU knockdown depended on the impaired mitochondrial calcium buffering rather than changes in mitochondrial and cellular energy status. These data uncover a new role for MCU in alternative macrophage polarization and phagocytic activity.

Quantitative Proteomic Analysis to Understand the Mechanisms of Zinc Oxide Nanoparticle Toxicity to Daphnia pulex (Crustacea: Daphniidae): Comparing with Bulk Zinc Oxide and Zinc Salt

The widespread use of zinc oxide nanoparticles (ZnO NPs) has resulted in their release to the environment. There has been concern about the ecotoxicity of ZnO NPs, but little is known about their toxic mechanisms. In the present study, we conducted acute toxicity tests to show that ZnO NPs are more toxic to the freshwater crustacean Daphnia pulex compared to bulk ZnO or ZnSO₄·7H₂O. To provide an integrated and quantitative insights into the toxicity of ZnO NPs, we conducted isobaric tags for relative and absolute quantitation (iTRAQ) proteomic analysis, which detected 262, 331, and 360 differentially expressed proteins (DEPs) in D. pulex exposed to ZnO NPs, bulk ZnO, and ZnSO₄·7H₂O, respectively. Among the DEPs, 224 were shared among the three treatments. These proteins were related to energy metabolism, oxidative stress, and endoplasmic reticulum stress. The three forms of Zn all caused D. pulex to downregulate Chitinase expression, disrupt Ca²⁺ homeostasis, and reduce expression of digestive enzymes. Nevertheless, 29 proteins were expressed only in the ZnO NP treatment. In particular, histone (H3) and ribosomal proteins (L13) were obviously influenced under ZnO NP treatment. However, increased expression levels of h3 and l13 genes were not induced only in ZnO NP treatment, they were sensitive to Zn ions under the same exposure concentration. These results indicate that the three zinc substances have a similar mode of action and that released zinc ions are the main contributor to ZnO NP toxicity to D. pulex under a low concentration. Further investigation is needed to clarify whether a small proportion of DEPs or higher bioavailability cause ZnO NPs to be more toxic compared to bulk ZnO or ionic zinc.

Emerging Roles in the Biogenesis of Cytochrome c Oxidase for Members of the Mitochondrial Carrier Family

The mitochondrial carrier family (MCF) is a group of transport proteins that are mostly localized to the inner mitochondrial membrane where they facilitate the movement of various solutes across the membrane. Although these carriers represent potential targets for therapeutic application and are repeatedly associated with human disease, research on the MCF has not progressed commensurate to their physiologic and pathophysiologic importance. Many of the 53 MCF members in humans are orphans and lack known transport substrates. Even for the relatively well-studied members of this family, such as the ADP/ATP carrier and the uncoupling protein, there exist fundamental gaps in our understanding of their biological roles including a clear rationale for the existence of multiple isoforms. Here, we briefly review this important family of mitochondrial carriers, provide a few salient examples of their diverse metabolic roles and disease associations, and then focus on an emerging link between several distinct MCF members, including the ADP/ATP carrier, and cytochrome c oxidase biogenesis. As the ADP/ATP carrier is regarded as the paradigm of the entire MCF, its newly established role in regulating translation of the mitochondrial genome highlights that we still have a lot to learn about these metabolite transporters.

Maintenance of intracellular Ca2+ basal concentration in airway smooth muscle (Review)

In airway smooth muscle, the intracellular basal Ca²⁺ concentration [_b(Ca²⁺)_i] must be tightly regulated by several mechanisms in order to maintain a proper airway patency. The _b[Ca²⁺]_i is efficiently regulated by sarcoplasmic reticulum Ca²⁺-ATPase 2b, plasma membrane Ca²⁺-ATPase 1 or 4 and by the Na⁺/Ca²⁺ exchanger. Membranal Ca²⁺ channels, including the L-type voltage dependent Ca²⁺ channel (L-VDCC), T-type voltage dependent Ca²⁺ channel (T-VDCC) and transient receptor potential canonical 3 (TRPC3), appear to be constitutively active under basal conditions via the action of different signaling pathways, and are responsible for Ca²⁺ influx to maintain _b[Ca²⁺]_i. The two types of voltage-dependent Ca²⁺ channels (L- and T-type) are modulated by phosphorylation processes mediated by mitogen-activated protein kinase kinase (MEK) and extracellular-signal-regulated kinase 1 and 2 (ERK1/2). The MEK/ERK signaling pathway can be activated by G-protein-coupled receptors through the α_q subunit when the endogenous ligand (i.e., acetylcholine, histamine, leukotrienes, etc.) is present under basal conditions. It may also be stimulated when receptor tyrosine kinases are occupied by the appropriate ligand (cytokines, growth factors, etc.). ERK1/2 phosphorylates L-VDCC on Ser⁴⁹⁶ of the β₂ subunit and Ser¹⁹²⁸ of the α₁ subunit, decreasing or increasing the channel activity, respectively, and enabling it to switch between an open and closed state. T-VDCC is also probably phosphorylated by ERK1/2, although further research is required to identify the phosphorylation sites. TRPC3 is directly activated by diacylglycerol produced by phospholipase C (PLC_β or _γ). Constitutive inositol 1,4,5-trisphosphate production induces the release of Ca²⁺ from the sarcoplasmic reticulum through inositol triphosphate receptor 1. This ion induces Ca²⁺-induced Ca²⁺ release through the ryanodine receptor 2 (designated as Ca²⁺ ‘sparks’). Therefore, several Ca²⁺ handling mechanisms are finely tuned to regulate basal intracellular Ca²⁺ concentrations. It is conceivable that alterations in any of these processes may render airway smooth muscle susceptible to develop hyperresponsiveness that is observed in ailments such as asthma.

Calcium- and ATP-dependent regulation of Na/Ca exchange function in BHK cells: Comparison of NCX1 and NCX3 exchangers

Na⁺/Ca²⁺ exchangers (NCX) mediate bidirectional Ca²⁺ fluxes across cell membranes and contribute to Ca²⁺ homeostasis in many cell types. Exchangers are regulated by gating reactions that depend on Na⁺ and Ca²⁺ binding to transport and regulatory sites. A Na⁺_i-dependent inactivation is prominent in all isoforms, whereas Ca²⁺_i-dependent regulation varies among isoforms. Here we characterize new details of NCX operation and describe differences and similarities between NCX3 and NCX1 regulation by intracellular Ca²⁺ and ATP. To compare isoforms, we employed BHK cells expressing NCX3 or NCX1 constitutively and exchange activity was analysed in whole-cell and excised patch recordings under "zero-trans" conditions (i.e., with only one transported ion species on each side). Using BHK cells with low cytoplasmic Ca²⁺ buffering, outward (reverse) currents, reflecting Ca²⁺ influx, are activated by applying extracellular Ca²⁺ (Ca_o) in the presence of Na⁺ on the cytoplasmic side. When firstly activated, peak outward NCX3 currents rapidly decay over seconds and then typically develop a secondary transient peak with slower kinetics, until Ca_o removal abolishes all outward current. The delayed rise of outward current is the signature of an activating process since peak outward NCX3 currents elicited at subsequent Ca_o bouts remain stimulated for minutes and slower decline towards a non-zero level during continued Ca_o application. Secondary transient peaks and current stimulation are suppressed by increasing the intracellular Ca²⁺ buffer capacity or by replacing cytoplasmic ATP with the analogues AMP-PNP or ATPγS. In BHK cells expressing NCX1, outward currents activated under identical settings decay to a steady-state level during single Ca_o application and are significantly larger, causing strong and long-lived run down of subsequent outward currents. NCX1 current run down is not prevented by increasing cytoplasmic Ca²⁺ buffering but secondary transient peaks in the outward current profile can be resolved in the presence of ATP. Finally, inward currents recorded in patches excised from NCX3-expressing cells reveal a proteolysis-sensitive, Ca-dependent inactivation process that is unusual for NCX1 forward activity. Together, our results suggest that NCX function is regulated more richly than appreciated heretofore, possibly including processes that are lost in excised membrane patches.

Calcium-Induced Differentiation of Human Colon Adenomas in Colonoid Culture: Calcium Alone versus Calcium with Additional Trace Elements

Previous murine studies have demonstrated that dietary Aquamin, a calcium-rich, multi-mineral natural product, suppressed colon polyp formation and transition to invasive tumors more effectively than calcium alone when provided over the lifespan of the animals. In the current study, we compared calcium alone to Aquamin for modulation of growth and differentiation in human colon adenomas in colonoid culture. Colonoids established from normal colonic tissue were examined in parallel. Both calcium alone at 1.5 mmol/L and Aquamin (provided at 1.5 mmol/L calcium) fostered differentiation in the adenoma colonoid cultures as compared with control (calcium at 0.15 mmol/L). When Aquamin was provided at an amount delivering 0.15 mmol/L calcium, adenoma differentiation also occurred, but was not as complete. Characteristic of colonoids undergoing differentiation was a reduction in the number of small, highly proliferative buds and their replacement by fewer but larger buds with smoother surface. Proliferation marker (Ki67) expression was reduced and markers of differentiation (CK20 and occludin) were increased along with E-cadherin translocalization to the cell surface. Additional proteins associated with differentiation/growth control [including histone-1 family members, certain keratins, NF2 (merlin), olfactomedin-4 and metallothioneins] were altered as assessed by proteomics. Immunohistologic expression of NF2 was higher with Aquamin as compared with calcium at either concentration. These findings support the conclusions that (i) calcium (1.5 mmol/L) has the capacity to modulate growth and differentiation in large human colon adenomas and (ii) Aquamin delivering 0.15 mmol/L calcium has effects on proliferation and differentiation not observed when calcium is used alone at this concentration. Cancer Prev Res; 11(7); 413-28. ©2018 AACR.

Lowered dietary phosphorus affects intestinal and renal gene expression to maintain mineral homeostasis with immunomodulatory implications in weaned piglets

BACKGROUND

In monogastric animals, phosphorus (P) homeostasis is maintained by regulating intestinal absorption, bone mobilization, and renal excretion. Since P is a non-renewable resource, a shortage is imminent due to widespread over-usage in the farming and animal husbandry industries. As a consequence, P efficiency should be improved in pig production. We sought to characterize the transcriptional response in re-/absorbing and excreting tissues in pigs to diets varying in calcium: phosphorus ratios. Weaned piglets were assigned to one of three groups fed diets varying in digestible P content for a period of five weeks. Gene expression profiles were analyzed in jejunum, colon, and kidney.

RESULTS

Transcriptome analysis revealed that reduced dietary P intake affects gene expression in jejunum and kidney, but not in colon. The regulation of mineral homeostasis was reflected via altered mRNA abundances of CYP24A1, CYP27A1, TRPM6, SPP1, and VDR in jejunum and kidney. Moreover, lowered abundances of transcripts associated with the classical complement system pathway were observed in the jejunum. In kidney, shifted transcripts were involved in phospholipase C, calcium signaling, and NFAT signaling, which may have immunomodulatory implications.

CONCLUSIONS

Our results revealed local transcriptional consequences of variable P intake in intestinal and renal tissues. The adaptive responses are the result of organismal efforts to maintain systemic mineral homeostasis while modulating immune features at local tissue sites. Therefore, the deviation from the currently recommended dietary P supply must be carefully considered, as the endogenous mechanisms that respond to low P diets may impact important adaptive immune responses.

A novel mechanism underlies atrazine toxicity in quails (Coturnix Coturnix coturnix): triggering ionic disorder via disruption of ATPases

The widely used atrazine has been reported to exhibit extensive ecological hazards. Due to the biological accumulation, atrazine elicits widespread toxic effects on different organisms. However, true proof for the mechanism of atrazine-induced toxicity is lacking. To determine the potential mechanism by which atrazine exerted toxic effects, quails were treated with atrazine (0, 50, 250 and 500 mg/kg) by gavage administration for 45 days. Atrazine significantly increased the histological alterations and serum creatine kinase, lactate dehydrogenase and choline esterase levels. A marked disorder in ionic (Na+, K+, Ca2+ and Mg2+)contents and the decrease of ATPases (Na+-K+-ATPase, Ca2+-ATPase, Mg2+-ATPase and Ca2+-Mg2+-ATPase) activities were observed in the heart and liver of atrazine-exposed quails. Of note, it was also observed that atrazine suppressed the transcription of Na+, K+ transfer associated genes (Na+-K+-ATPase subunits) and Ca2+ transfer associated genes (Ca2+-ATPase subunits, solute carriers) in heart and liver. In conclusion, atrazine induced cardiac and hepatic damage via causing the ionic disorder, triggering the transcription of the ion transporters and leading the histopathological and functional alternations in the heart and liver of quails. This study demonstrated atrazine significantly induced the ionic disorder via decreasing the ATPases activities and disturbing the transcription of the ion transporters.

High-Sodium Diet Has Opposing Effects on Mean Arterial Blood Pressure and Cerebral Perfusion in a Transgenic Mouse Model of Alzheimer's Disease

BACKGROUND

Cerebral ionic homeostasis impairment, especially Ca2+, has been observed in Alzheimer's disease (AD) and also with hypertension. Hypertension and AD both have been implicated in impaired cerebral autoregulation. However, the relationship between the ionic homeostasis impairment in AD and hypertension and cerebral blood flow (CBF) autoregulation is not clear.

OBJECTIVE

To test the hypothesis that a high-salt diet regimen influences the accumulation of amyloid-β (Aβand CBF) and CBF, exacerbates cognitive decline, and increases the propensity to AD.

METHODS

Double transgenic mice harboring the amyloid-β protein precursor (APPswe), and presenilin-1 (PSEN1) along with control littermates, 2 months of age at initiation of special diet, were divided into 4 groups: Group A, APP/PS1 and Group B, controls fed a high-sodium (4.00%) chow diet for 3 months; Group C, APP/PS1 and Group D, controls fed a low-sodium (0.08%) regular chow diet for 3 months. Mean arterial blood pressure (MAP) and CBF were measured noninvasively using the tail MAP measurement device and magnetic resonance imaging, respectively. Aβ plaques numbers in the cortex and hippocampus of APP/PS1 were quantified.

RESULTS

In contrary to controls, APP/PS1 mice fed a high-salt diet did not show markedly elevated mean systolic and diastolic blood pressure (134±4.8 compared with 162±2.8 mmHg, and 114±5.0 compared with 137±20 mmHg, p< 0.0001). However, a high-salt diet increased CBF in both APP/PS1 and controls and did not alter the cerebral tissue integrity. Aβ plaques were significantly reduced in the cortex and hippocampus of mice fed a high-salt diet.

CONCLUSION

These data suggest that a high-salt diet differently affects MAP and CBF in APP/PS1 mice and controls.

Effects of dietary supplementation of arginine-silicate-inositol complex on absorption and metabolism of calcium of laying hens

The effects of supplementation of arginine-silicate-inositol complex (ASI; 49.5–8.2–25 g/kg, respectively) to laying hens were investigated with respect to eggshell quality, calcium (Ca) balance, and expression of duodenal proteins related to Ca metabolism (calbindin and tight junction proteins). A total of 360 laying hens, 25 weeks old, were divided into 3 groups consisting of 6 replicate of cages, 20 birds per cage. The groups were fed a basal diet and the basal diet supplemented with 500 or 1000 mg ASI complex per kilogram for 90 days. Data were analyzed by ANCOVA using data during the first week of the adaptation period as covariates. As the ASI complex supplementation level increased, there were increases in feed intake (P < 0.0001), egg production (P < 0.001), egg weight (P < 0.0001) and eggshell weight (P < 0.001) weight, and shell thickness (P < 0.001) and decreases in feed conversion ratio and cracked egg percentage (P < 0.0001 for both). Concentrations of serum osteocalcin (P < 0.0001), vitamin D (P < 0.0001), calcium (P < 0.001), phosphorus (P < 0.001), and alkaline phosphatase (P < 0.008) as well as amounts of calcium retention (P < 0.0001) and eggshell calcium deposition (P < 0.001), and Ca balance (P < 0.0001) increased, whereas amount of calcium excretion (P < 0.001) decreased linearly in a dose-dependent manner. The ASI complex supplementation increased expressions of calcium transporters (calbindin-D28k, N sodium-calcium exchanger, plasma membrane calcium ATPase, and vitamin D receptor) and tight junction proteins (zonula occludens-1 and occludin) in the duodenum in a linear fashion (P < 0.0001 for all). In conclusion, provision of dietary ASI complex to laying hens during the peak laying period improved eggshell quality through improving calcium utilization as reflected by upregulation of genes related to the calcium metabolism. Further studies are needed to elucidate the contribution of each of the ASI complex ingredients.

Dynamic distinctions in the Na+/Ca2+ exchanger adopting the inward- and outward-facing conformational states

Na⁺/Ca²⁺ exchanger (NCX) proteins operate through the alternating access mechanism, where the ion-binding pocket is exposed in succession either to the extracellular or the intracellular face of the membrane. The archaeal NCX_Mj (Methanococcus jannaschii NCX) system was used to resolve the backbone dynamics in the inward-facing (IF) and outward-facing (OF) states by analyzing purified preparations of apo- and ion-bound forms of NCX_Mj-WT and its mutant, NCX_Mj-5L6-8. First, the exposure of extracellular and cytosolic vestibules to the bulk phase was evaluated as the reactivity of single cysteine mutants to a fluorescent probe, verifying that NCX_Mj-WT and NCX_Mj-5L6-8 preferentially adopt the OF and IF states, respectively. Next, hydrogen-deuterium exchange-mass spectrometry (HDX-MS) was employed to analyze the backbone dynamics profiles in proteins, preferentially adopting the OF (WT) and IF (5L6-8) states either in the presence or absence of ions. Characteristic differences in the backbone dynamics were identified between apo NCX_Mj-WT and NCX_Mj-5L6-8, thereby underscoring specific conformational patterns owned by the OF and IF states. Saturating concentrations of Na⁺ or Ca²⁺ specifically modify HDX patterns, revealing that the ion-bound/occluded states are much more stable (rigid) in the OF than in the IF state. Conformational differences observed in the ion-occluded OF and IF states can account for diversifying the ion-release dynamics and apparent affinity (K_m ) at opposite sides of the membrane, where specific structure-dynamic elements can effectively match the rates of bidirectional ion movements at physiological ion concentrations.

Cellular Na+ handling mechanisms involved in airway smooth muscle contraction (Review)

A decrease in bronchial diameter is designated as bronchoconstriction (BC) and impedes the flow of air through the airway. Asthma is characterized by inflammation of the airways, reversible BC and nonspecific hyperreactivity. These last two symptoms are dependent on airway smooth muscle. Stimuli that trigger contraction can be characterized as chemical (neurotransmitters, cytokines and terpenoids) and physical (volume inspired, air pressure). Both stimuli activate signaling pathways by acting on membrane proteins and facilitating the passage of ions through the membrane, generating a voltage change and a subsequent depolarization. Na⁺ plays an important role in preserving the resting membrane potential; this ion is extracted from the cells by the Na⁺/K⁺ ATPase (NKA) or introduced into the cytoplasm by the Na⁺/Ca²⁺ exchanger (NCX). During depolarization, Na⁺ appears to accumulate in specific regions beneath the plasma membrane, generating local concentration gradients which determine the handling of Ca²⁺. At rest, the smooth muscle has a basal tone that is preserved by the continuous adjustment of intracytoplasmic concentrations of Ca²⁺ and Na⁺. At homeostasis, the Na⁺ concentration is primarily dependent on three structures: the NKA, the NCX and non-specific cation channels (NSCC). These three structures, their functions and the available evidence of the probable role of Na⁺ in asthma are described in the present review.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Hyponatraemia-induced rhabdomyolysis complicated by anuric acute kidney injury: a renal replacement conundrum

Hyponatraemia-induced rhabdomyolysis is a rare, but reported phenomenon, particularly in patients with chronic schizophrenia on depot antipsychotics prone to psychogenic polydipsia. To the best of our knowledge, there are no reported cases of hyponatraemia-induced rhabdomyolysis complicated by oligo-anuric acute kidney injury (AKI) requiring continuous renal replacement therapy (CRRT). The initiation of CRRT is complicated in severe hyponatraemia, predominantly due to the need to avoid rapid changes in tonicity associated with rapid changes in sodium. We report a case of severe hyponatraemia (104 mmol/L) complicated by oligo-anuric rhabdomyolysis-induced AKI and our management of the renal prescription.

Paclitaxel-induced increase in NCX activity in subpopulations of nociceptive afferents: A protective mechanism against chemotherapy-induced peripheral neuropathy?

We recently demonstrated, in a rat model of chemotherapy-induced peripheral neuropathy (CIPN), that there is a significant decrease in the duration of the depolarization-evoked Ca(2+) transient in isolated somata of putative nociceptive afferents innervating the glabrous skin of the hindpaw, but no change in transient magnitude or the resting concentration of intracellular Ca(2+) ([Ca(2+)]i). Because the Na(+)-Ca(2+) exchanger (NCX) only contributes to the regulation of the duration of the evoked Ca(2+) transient, in putative nociceptive dorsal root ganglion (DRG) neurons, we hypothesized that an increase in NCX activity underlies the CIPN-induced change in this subpopulation of neurons. Acutely dissociated retrogradely labeled sensory neurons from naïve, vehicle-, and paclitaxel-treated rats were studied with fura-2 based Ca(2+) imaging. There was no difference in the relative level of NCX activity between glabrous neurons from paclitaxel-treated or control rats. However, in contrast to the relatively large and long lasting Ca(2+) transients needed to evoke NCX activity in neurons from naïve rats, there was evidence of resting NCX activity in glabrous neurons from both vehicle- and paclitaxel-treated rats. More interestingly, there was a paclitaxel-induced increase in NCX activity in putative nociceptive neurons innervating the thigh, neurons in which there is no evidence of a change in the depolarization-induced Ca(2+) transient, or a body site in which there was a change in nociceptive threshold. Furthermore, while the majority of NCX activity in glabrous neurons is sensitive to the NCX3-preferring blocker KB-R7943, the increase in NCX activity in thigh neurons was resistant to KB-R7943 but sensitive to the NCX1-preferring blocker SEA0400. These results suggest that a mechanism(s) other than NCX underlies the paclitaxel-induced decrease in the duration of the evoked Ca(2+) transient in putative nociceptive glabrous skin neurons. However, the compensatory response to paclitaxel observed may also explain why only subpopulations of sensory neurons are impacted by paclitaxel, raising the intriguing possibility that CIPN is due to the failure of injured neurons to appropriately compensate for the deleterious consequences of this compound.

Mechanism of extracellular ion exchange and binding-site occlusion in a sodium/calcium exchanger

Na⁺/Ca²⁺ exchangers utilize the Na⁺ electrochemical gradient across the plasma membrane to extrude intracellular Ca²⁺, and play a central role in Ca²⁺ homeostasis. Here, we elucidate their mechanisms of extracellular ion recognition and exchange through a structural analysis of the exchanger from Methanococcus jannaschii (NCX_Mj) bound to Na⁺, Ca²⁺ or Sr²⁺ in various occupancies and in an apo state. This analysis defines the binding mode and relative affinity of these ions, establishes the structural basis for the anticipated 3Na⁺:1Ca²⁺ exchange stoichiometry, and reveals the conformational changes at the onset of the alternating-access transport mechanism. An independent analysis of the dynamics and conformational free-energy landscape of NCX_Mj in different ion-occupancy states, based on enhanced-sampling molecular-dynamics simulations, demonstrates that the crystal structures reflect mechanistically relevant, interconverting conformations. These calculations also reveal the mechanism by which the outward-to-inward transition is controlled by the ion-occupancy state, thereby explaining the emergence of strictly-coupled Na⁺/Ca²⁺ antiport.

Cardiac-specific miRNA in cardiogenesis, heart function, and cardiac pathology (with focus on myocardial infarction)

Cardiac miRNAs (miR-1, miR133a, miR-208a/b, and miR-499) are abundantly expressed in the myocardium. They play a central role in cardiogenesis, heart function and pathology. While miR-1 and miR-133a predominantly control early stages of cardiogenesis supporting commitment of cardiac-specific muscle lineage from embryonic stem cells and mesodermal precursors, miR-208 and miR-499 are involved in the late cardiogenic stages mediating differentiation of cardioblasts to cardiomyocytes and fast/slow muscle fiber specification. In the heart, miR-1/133a control cardiac conductance and automaticity by regulating all phases of the cardiac action potential. miR-208/499 located in introns of the heavy chain myosin genes regulate expression of sarcomeric contractile proteins. In cardiac pathology including myocardial infarction (MI), expression of cardiac miRNAs is markedly altered that leads to deleterious effects associated with heart wounding, arrhythmia, increased apoptosis, fibrosis, hypertrophy, and tissue remodeling. In acute MI, circulating levels of cardiac miRNAs are significantly elevated making them to be a promising diagnostic marker for early diagnosis of acute MI. Great cardiospecific capacity of these miRNAs is very helpful for enhancing regenerative properties and survival of stem cell and cardiac progenitor transplants and for reprogramming of mature non-cardiac cells to cardiomyocytes.

Pharmacological evidence that potentiation of plasmalemmal Ca(2+)-extrusion is functionally coupled to inhibition of SR Ca(2+)-ATPases in vascular smooth muscle cells

Cyclopiazonic acid (CPA), a specific inhibitor of sarcoplasmic reticulum (SR) Ca(2+)-ATPases, causes slowly developing and subsequently diminishing characteristic contractions in vascular smooth muscle, and the second application of CPA has incompletely repeatable effects, depending on the vessel type. The objective of the present study was to examine the mechanisms underlying the significant decrease of CPA-induced contractions upon the second application. A pharmacological intervention of Ca(2+) extrusion process as a strategy was performed to modulate vasoconstrictor effects of CPA in rat aortic ring preparations. CPA-induced contractions, expressed as percentages of the contractions induced by KCl (80 mM), were significantly decreased from 44.1 ± 5.7 to 7.6 ± 1.8 % (P < 0.001) upon the second application. The contractions, however, were completely repeatable in the presence of vanadate, an inhibitor of ATPases, but not of ouabain, an inhibitor of Na(+)-pumps. Strikingly, CPA-induced contractions were sustained and completely repeatable in Na(+)-free and low Na(+) medium. Furthermore, we found that the contractions were completely repeatable in the presence of 2',4'-dichlorobenzamil, an inhibitor of the forward mode of Na(+)/Ca(2+) exchangers, but not of KBR7943, an inhibitor of the reverse mode of Na(+)/Ca(2+) exchangers. Our findings indicate that CPA by inducing a transient rise in cytosolic Ca(2+) level causes a long-lasting upregulation of plasma membrane (PM) Ca(2+) extruders and thus leads to a diminished contraction upon its second application in blood vessels. This suggests that there is a functional coupling between PM Ca(2+) extruders and SR Ca(2+)-ATPases in rat aortic smooth muscle cells.

Computational approaches for classification and prediction of P-type ATPase substrate specificity in Arabidopsis

As an extended gamut of integral membrane (extrinsic) proteins, and based on their transporting specificities, P-type ATPases include five subfamilies in Arabidopsis, inter alia, P4ATPases (phospholipid-transporting ATPase), P3AATPases (plasma membrane H(+) pumps), P2A and P2BATPases (Ca(2+) pumps) and P1B ATPases (heavy metal pumps). Although, many different computational methods have been developed to predict substrate specificity of unknown proteins, further investigation needs to improve the efficiency and performance of the predicators. In this study, various attribute weighting and supervised clustering algorithms were employed to identify the main amino acid composition attributes, which can influence the substrate specificity of ATPase pumps, classify protein pumps and predict the substrate specificity of uncharacterized ATPase pumps. The results of this study indicate that both non-reduced coefficients pertaining to absorption and Cys extinction within 280 nm, the frequencies of hydrogen, Ala, Val, carbon, hydrophilic residues, the counts of Val, Asn, Ser, Arg, Phe, Tyr, hydrophilic residues, Phe-Phe, Ala-Ile, Phe-Leu, Val-Ala and length are specified as the most important amino acid attributes through applying the whole attribute weighting models. Here, learning algorithms engineered in a predictive machine (Naive Bays) is proposed to foresee the Q9LVV1 and O22180 substrate specificities (P-type ATPase like proteins) with 100 % prediction confidence. For the first time, our analysis demonstrated promising application of bioinformatics algorithms in classifying ATPases pumps. Moreover, we suggest the predictive systems that can assist towards the prediction of the substrate specificity of any new ATPase pumps with the maximum possible prediction confidence.

Genome-wide investigation and expression analysis of Sodium/Calcium exchanger gene family in rice and Arabidopsis

BACKGROUND

The Na(+)/Ca(2+) Exchanger (NCX) protein family is a member of the Cation/Ca(2+) exchanger superfamily and its members play important roles in cellular Ca(2+) homeostasis. While the functions of NCX family of proteins is well understood in humans, not much is known about the total complement of Na(+)/Ca(2+) exchangers in plants and their role in various physiological and developmental processes. In the present study, we have identified all the NCX proteins encoded in the genomes of rice and Arabidopsis and studied their phylogeny, domain architecture and expression profiles across different tissues, at various developmental stages and under stress conditions.

RESULTS

Through whole genome investigation, we identified twenty-two NCX proteins encoded by fifteen genes in rice and sixteen NCX proteins encoded by thirteen genes in Arabidopsis. Based on phylogenetic reconstruction, these could be classified into five clades, members of most of which were found to possess distinct domain architecture. Expression profiling of the identified NCX genes using publicly available MPSS and microarray data showed differential expression patterns under abiotic stresses, and at various development stages. In rice, OsNCX1, OsNCX8, OsNCX9 and OsNCX15 were found to be highly expressed in all the plant parts and various developmental stages. qRT-PCR based expression analysis revealed that OsNCX3, OsNCX10 and OsNCX15 were highly induced by salt and dehydration stress. Besides, expression profiling showed differential regulation of rice NCX genes in response to calcium and EGTA. Interestingly, expression of none of the NCX genes was found to be co-regulated by NaCl and calcium.

CONCLUSIONS

Together, our results present insights into the potential role of NCX family of proteins in abiotic stresses and development. Findings of the present investigation should serve as a starting point for future studies aiming functional characterization of plant NCX family proteins.

Trafficking of Na+/Ca2+ exchanger to the site of persistent inflammation in nociceptive afferents

Persistent inflammation results in an increase in the amplitude and duration of depolarization-evoked Ca(2+) transients in putative nociceptive afferents. Previous data indicated that these changes were the result of neither increased neuronal excitability nor an increase in the amplitude of depolarization. Subsequent data also ruled out an increase in voltage-gated Ca(2+) currents and recruitment of Ca(2+)-induced Ca(2+) release. Parametric studies indicated that the inflammation-induced increase in the duration of the evoked Ca(2+) transient required a relatively large and long-lasting increase in the concentration of intracellular Ca(2+) implicating the Na(+)/Ca(2+) exchanger (NCX), a major Ca(2+) extrusion mechanism activated with high intracellular Ca(2+) loads. The contribution of NCX to the inflammation-induced increase in the evoked Ca(2+) transient in rat sensory neurons was tested using fura-2 AM imaging and electrophysiological recordings. Changes in NCX expression and protein were assessed with real-time PCR and Western blot analysis, respectively. An inflammation-induced decrease in NCX activity was observed in a subpopulation of putative nociceptive neurons innervating the site of inflammation. The time course of the decrease in NCX activity paralleled that of the inflammation-induced changes in nociceptive behavior. The change in NCX3 in the cell body was associated with a decrease in NCX3 protein in the ganglia, an increase in the peripheral nerve (sciatic) yet no change in the central root. This single response to inflammation is associated with changes in at least three different segments of the primary afferent, all of which are likely to contribute to the dynamic response to persistent inflammation.

Genetics of Human and Canine Dilated Cardiomyopathy

Cardiovascular disease is a leading cause of death in both humans and dogs. Dilated cardiomyopathy (DCM) accounts for a large number of these cases, reported to be the third most common form of cardiac disease in humans and the second most common in dogs. In human studies of DCM there are more than 50 genetic loci associated with the disease. Despite canine DCM having similar disease progression to human DCM studies into the genetic basis of canine DCM lag far behind those of human DCM. In this review the aetiology, epidemiology, and clinical characteristics of canine DCM are examined, along with highlighting possible different subtypes of canine DCM and their potential relevance to human DCM. Finally the current position of genetic research into canine and human DCM, including the genetic loci, is identified and the reasons many studies may have failed to find a genetic association with canine DCM are reviewed.

The TRPC channel blocker SKF 96365 inhibits glioblastoma cell growth by enhancing reverse mode of the Na(+) /Ca(2+) exchanger and increasing intracellular Ca(2+)

BACKGROUND AND PURPOSE

SKF 96365 is well known for its suppressing effect on human glioblastoma growth by inhibiting pre-activated transient receptor potential canonical (TRPC) channels and Ca(2+) influx. The effect of SKF 96363 on glioblastoma cells, however, may be multifaceted and this possibility has been largely ignored.

EXPERIMENTAL APPROACH

The effects of SKF 96365 on cell cycle and cell viability of cultured human glioblastoma cells were characterized. Western blot, Ca(2+) imaging and patch clamp recordings were used to delineate cell death mechanisms. siRNA gene knockdown provided additional evidence.

KEY RESULTS

SKF 96365 repressed glioblastoma cell growth via increasing intracellular Ca(2+) ([Ca(2+) ]i ) irrespective of whether TRPC channels were blocked or not. The effect of SKF 96365 primarily resulted from enhanced reverse operation of the Na(+) /Ca(2+) exchanger (NCX) with an EC50 of 9.79 μM. SKF 96365 arrested the glioblastoma cells in the S and G2 phases and activated p38-MAPK and JNK, which were all prevented by the Ca(2+) chelator BAPTA-AM or EGTA. The expression of NCX in glioblastoma cells was significantly higher than in normal human astrocytes. Knockdown of the NCX1 isoforms diminished the effect of SKF 96365 on glioblastoma cells.

CONCLUSIONS AND IMPLICATIONS

At the same concentration, SKF 96365 blocks TRPC channels and enhances the reverse mode of the NCX causing [Ca(2+) ]i accumulation and cytotoxicity. This finding suggests an alternative pharmacological mechanism of SKF 96365. It also indicates that modulation of the NCX is an effective method to disrupt Ca(2+) homeostasis and suppress human glioblastoma cells.