The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Genes for endosomal pH regulators NHE6 and NHE9 are dysregulated in the substantia nigra in Parkinson's disease

Endosomal acid base balance functions as a master orchestrator within the cell, engaging with many cellular pathways to maintain homeostasis. Mutations in the endosomal pH regulator Na+/H+ exchanger NHE6 may disrupt this delicate balancing act and cause monogenic Parkinsonism. Here, gene expression studies in post-mortem substantia nigra of Parkinson's disease (PD) patients and normal controls were performed to investigate whether NHE6 represents a pathophysiological link between monogenic and sporadic PD. The substantia nigra in PD displayed down-regulation of NHE6, coincident with a loss of expression of several SNARE signalling pathway members, suggesting impaired membrane fusion and vesicle-recycling. Increased abundance of related NHE9 was also identified in the parkinsonian nigra that could reflect compensatory changes or be a consequence of neuronal dysfunction. The current model suggests the possibility that neurons expressing low levels of NHE6 are more susceptible to injury in PD, potentially directly contributing to the loss of nigral dopaminergic neurons and the genesis of the disease. These results have important implications for disease-modifying therapies as they suggest that endosomal pH correctors, including epigenetic modifiers that regulate NHE6 expression, may be beneficial for PD. Thus, aberrant endosomal acidification in the nigrostriatal pathway is a possible unifying pathomechanism in both monogenic and sporadic PD, with implications for understanding and treating this disorder. Replication of these observations in the post-mortem brains of Alzheimer's disease and frontotemporal dementia patients supports a model of conserved mechanisms underlying injury and death of neurons.

Na+/H+-exchange inhibition by cariporide is compensated via Na+,HCO3--cotransport and has no net growth consequences for ErbB2-driven breast carcinomas

Defense against intracellular acidification of breast cancer tissue depends on net acid extrusion via Na+,HCO3--cotransporter NBCn1/Slc4a7 and Na+/H+-exchanger NHE1/Slc9a1. NBCn1 is increasingly recognized as breast cancer susceptibility protein and promising therapeutic target, whereas evidence for targeting NHE1 is discordant. Currently, selective small molecule inhibitors exist against NHE1 but not NBCn1. Cellular assays-with some discrepancies-link NHE1 activity to proliferation, migration, and invasion; and disrupted NHE1 expression can reduce triple-negative breast cancer growth. Studies on human breast cancer tissue associate high NHE1 expression with reduced metastasis and-in some molecular subtypes-improved patient survival. Here, we evaluate Na+/H+-exchange and therapeutic potential of the NHE1 inhibitor cariporide/HOE-642 in murine ErbB2-driven breast cancer. Ex vivo, cariporide inhibits net acid extrusion in breast cancer tissue (IC50 = 0.18 μM) and causes small decreases in steady-state intracellular pH (pHi). In vivo, we deliver cariporide orally, by osmotic minipumps, and by intra- and peritumoral injections to address the low oral bioavailability and fast metabolism. Prolonged cariporide administration in vivo upregulates NBCn1 expression, shifts pHi regulation towards CO2/HCO3--dependent mechanisms, and shows no net effect on the growth rate of ErbB2-driven primary breast carcinomas. Cariporide also does not influence proliferation markers in breast cancer tissue. Oral, but not parenteral, cariporide elevates serum glucose by ∼1.5 mM. In conclusion, acute administration of cariporide ex vivo powerfully inhibits net acid extrusion from breast cancer tissue but lowers steady-state pHi minimally. Prolonged cariporide administration in vivo is compensated via NBCn1 and we observe no discernible effect on growth of ErbB2-driven breast carcinomas.

Impaired hippocampal plasticity associated with loss of recycling endosomal SLC9A6/NHE6 is ameliorated by the TrkB agonist 7,8-dihydroxyflavone

Proper maintenance of intracellular vesicular pH is essential for cargo trafficking during synaptic function and plasticity. Mutations in the SLC9A6 gene encoding the recycling endosomal pH regulator (Na+, K+)/H+ exchanger isoform 6 (NHE6) are causal for Christianson syndrome (CS), a severe form of X-linked intellectual disability. NHE6 expression is also downregulated in other neurodevelopmental and neurodegenerative disorders, such as autism spectrum disorder and Alzheimer's disease, suggesting its dysfunction could contribute more broadly to the pathophysiology of other neurological conditions. To understand how ablation of NHE6 function leads to severe learning impairments, we assessed synaptic structure, function, and cellular mechanisms of learning in a novel line of Nhe6 knockout (KO) mice expressing a plasma membrane-tethered green fluorescent protein within hippocampal neurons. We uncovered significant reductions in dendritic spines density, AMPA receptor (AMPAR) expression, and AMPAR-mediated neurotransmission in CA1 pyramidal neurons. The neurons also failed to undergo functional and structural enhancement during long-term potentiation (LTP). Significantly, the selective TrkB agonist 7,8-dihydroxyflavone restored spine density as well as functional and structural LTP in KO neurons. TrkB activation thus may act as a potential clinical intervention to ameliorate cognitive deficits in CS and other neurodegenerative disorders.

The Role of Cornichons in the Biogenesis and Functioning of Monovalent-Cation Transport Systems

Monovalent-cation homeostasis, crucial for all living cells, is ensured by the activity of various types of ion transport systems located either in the plasma membrane or in the membranes of organelles. A key prerequisite for the functioning of ion-transporting proteins is their proper trafficking to the target membrane. The cornichon family of COPII cargo receptors is highly conserved in eukaryotic cells. By simultaneously binding their cargoes and a COPII-coat subunit, cornichons promote the incorporation of cargo proteins into the COPII vesicles and, consequently, the efficient trafficking of cargoes via the secretory pathway. In this review, we summarize current knowledge about cornichon proteins (CNIH/Erv14), with an emphasis on yeast and mammalian cornichons and their role in monovalent-cation homeostasis. Saccharomyces cerevisiae cornichon Erv14 serves as a cargo receptor of a large portion of plasma-membrane proteins, including several monovalent-cation transporters. By promoting the proper targeting of at least three housekeeping ion transport systems, Na+, K+/H+ antiporter Nha1, K+ importer Trk1 and K+ channel Tok1, Erv14 appears to play a complex role in the maintenance of alkali-metal-cation homeostasis. Despite their connection to serious human diseases, the repertoire of identified cargoes of mammalian cornichons is much more limited. The majority of current information is about the structure and functioning of CNIH2 and CNIH3 as auxiliary subunits of AMPAR multi-protein complexes. Based on their unique properties and easy genetic manipulation, we propose yeast cells to be a useful tool for uncovering a broader spectrum of human cornichons´ cargoes.

Acidic preconditioning induced intracellular acid adaptation to protect renal injury via dynamic phosphorylation of focal adhesion kinase-dependent activation of sodium hydrogen exchanger 1

BACKGROUND

Disruptions in intracellular pH (pHi) homeostasis, causing deviations from the physiological range, can damage renal epithelial cells. However, the existence of an adaptive mechanism to restore pHi to normalcy remains unclear. Early research identified H+ as a critical mediator of ischemic preconditioning (IPC), leading to the concept of acidic preconditioning (AP). This concept proposes that short-term, repetitive acidic stimulation can enhance a cell's capacity to withstand subsequent adverse stress. While AP has demonstrated protective effects in various ischemia-reperfusion (I/R) injury models, its application in kidney injury remains largely unexplored.

METHODS

An AP model was established in human kidney (HK2) cells by treating them with an acidic medium for 12 h, followed by a recovery period with a normal medium for 6 h. To induce hypoxia/reoxygenation (H/R) injury, HK2 cells were subjected to hypoxia for 24 h and reoxygenation for 1 h. In vivo, a mouse model of IPC was established by clamping the bilateral renal pedicles for 15 min, followed by reperfusion for 4 days. Conversely, the I/R model involved clamping the bilateral renal pedicles for 35 min and reperfusion for 24 h. Western blotting was employed to evaluate the expression levels of cleaved caspase 3, cleaved caspase 9, NHE1, KIM1, FAK, and NOX4. A pH-sensitive fluorescent probe was used to measure pHi, while a Hemin/CNF microelectrode monitored kidney tissue pH. Immunofluorescence staining was performed to visualize the localization of NHE1, NOX4, and FAK, along with the actin cytoskeleton structure in HK2 cells. Cell adhesion and scratch assays were conducted to assess cell motility.

RESULTS

Our findings demonstrated that AP could effectively mitigate H/R injury in HK2 cells. This protective effect and the maintenance of pHi homeostasis by AP involved the upregulation of Na+/H+ exchanger 1 (NHE1) expression and activity. The activity of NHE1 was regulated by dynamic changes in pHi-dependent phosphorylation of Focal Adhesion Kinase (FAK) at Y397. This process was associated with NOX4-mediated reactive oxygen species (ROS) production. Furthermore, AP induced the co-localization of FAK, NOX4, and NHE1 in focal adhesions, promoting cytoskeletal remodeling and enhancing cell adhesion and migration capabilities.

CONCLUSIONS

This study provides compelling evidence that AP maintains pHi homeostasis and promotes cytoskeletal remodeling through FAK/NOX4/NHE1 signaling. This signaling pathway ultimately contributes to alleviated H/R injury in HK2 cells.

A DNA nanodevice for mapping sodium at single-organelle resolution

Cellular sodium ion (Na+) homeostasis is integral to organism physiology. Our current understanding of Na+ homeostasis is largely limited to Na+ transport at the plasma membrane. Organelles may also contribute to Na+ homeostasis; however, the direction of Na+ flow across organelle membranes is unknown because organellar Na+ cannot be imaged. Here we report a pH-independent, organelle-targetable, ratiometric probe that reports lumenal Na+. It is a DNA nanodevice containing a Na+-sensitive fluorophore, a reference dye and an organelle-targeting domain. By measuring Na+ at single endosome resolution in mammalian cells and Caenorhabditis elegans, we discovered that lumenal Na+ levels in each stage of the endolysosomal pathway exceed cytosolic levels and decrease as endosomes mature. Further, we find that lysosomal Na+ levels in nematodes are modulated by the Na+/H+ exchanger NHX-5 in response to salt stress. The ability to image subcellular Na+ will unveil mechanisms of Na+ homeostasis at an increased level of cellular detail.

Structure and mechanism of the K+/H+ exchanger KefC

Intracellular potassium (K+) homeostasis is fundamental to cell viability. In addition to channels, K+ levels are maintained by various ion transporters. One major family is the proton-driven K+ efflux transporters, which in gram-negative bacteria is important for detoxification and in plants is critical for efficient photosynthesis and growth. Despite their importance, the structure and molecular basis for K+-selectivity is poorly understood. Here, we report ~3.1 Å resolution cryo-EM structures of the Escherichia coli glutathione (GSH)-gated K+ efflux transporter KefC in complex with AMP, AMP/GSH and an ion-binding variant. KefC forms a homodimer similar to the inward-facing conformation of Na+/H+ antiporter NapA. By structural assignment of a coordinated K+ ion, MD simulations, and SSM-based electrophysiology, we demonstrate how ion-binding in KefC is adapted for binding a dehydrated K+ ion. KefC harbors C-terminal regulator of K+ conductance (RCK) domains, as present in some bacterial K+-ion channels. The domain-swapped helices in the RCK domains bind AMP and GSH and they inhibit transport by directly interacting with the ion-transporter module. Taken together, we propose that KefC is activated by detachment of the RCK domains and that ion selectivity exploits the biophysical properties likewise adapted by K+-ion-channels.

Ectopic expression of BmeryCA in Bombyx mori increases silk yield and mechanical properties by altering the pH of posterior silk gland

The silk glands are the specialized tissue where silk protein synthesis, secretion, and conformational transitions take place, with pH playing a critical role in both silk protein synthesis and fiber formation. In the present study, we have identified erythrocyte carbonic anhydrase (BmeryCA) belonging to the α-CA class in the silk gland, which is a Zn2+ dependent metalloenzyme capable of efficiently and reversibly catalyzing the hydrated reaction of CO2 to HCO3-, thus participating in the regulation of acid-base balance. Multiple sequence alignments revealed that the active site of BmeryCA was highly conserved. Tissue expression profiling showed that BmeryCA had relatively high expression levels in hemolymph and epidermis but is barely expressed in the posterior silk gland (PSG). By specifically overexpressing BmeryCA in the PSG, we generated transgenic silkworms. Ion-selective microelectrode (ISM) measurements demonstrated that specifically overexpression of BmeryCA in the PSG led to a shift in pH from weakly alkaline to slightly neutral conditions. Moreover, the resultant PSG-specific BmeryCA overexpression mutant strain displayed a significant increase in both silk yield and silk fiber mechanical properties. Our research provided new insights into enhancing silk yield and improving the mechanical properties of silk fibers.

Cytosolic and Acrosomal pH Regulation in Mammalian Sperm

As in most cells, intracellular pH regulation is fundamental for sperm physiology. Key sperm functions like swimming, maturation, and a unique exocytotic process, the acrosome reaction, necessary for gamete fusion, are deeply influenced by pH. Sperm pH regulation, both intracellularly and within organelles such as the acrosome, requires a coordinated interplay of various transporters and channels, ensuring that this cell is primed for fertilization. Consistent with the pivotal importance of pH regulation in mammalian sperm physiology, several of its unique transporters are dependent on cytosolic pH. Examples include the Ca2+ channel CatSper and the K+ channel Slo3. The absence of these channels leads to male infertility. This review outlines the main transport elements involved in pH regulation, including cytosolic and acrosomal pH, that participate in these complex functions. We present a glimpse of how these transporters are regulated and how distinct sets of them are orchestrated to allow sperm to fertilize the egg. Much research is needed to begin to envision the complete set of players and the choreography of how cytosolic and organellar pH are regulated in each sperm function.

An Endosomal Acid-Regulatory Feedback System Rewires Cytosolic cAMP Metabolism and Drives Tumor Progression

Although suppressed cAMP levels have been linked to cancer for nearly five decades, the molecular basis remains uncertain. Here, we identify endosomal pH as a novel regulator of cytosolic cAMP homeostasis and a promoter of transformed phenotypic traits in colorectal cancer. Combining experiments and computational analysis, we show that the Na+/H+ exchanger NHE9 contributes to proton leak and causes luminal alkalinization, which induces resting [Ca2+], and in consequence, represses cAMP levels, creating a feedback loop that echoes nutrient deprivation or hypoxia. Higher NHE9 expression in cancer epithelia is associated with a hybrid epithelial-mesenchymal (E/M) state, poor prognosis, tumor budding, and invasive growth in vitro and in vivo. These findings point to NHE9-mediated cAMP suppression as a pseudostarvation-induced invasion state and potential therapeutic vulnerability in colorectal cancer. Our observations lay the groundwork for future research into the complexities of endosome-driven metabolic reprogramming and phenotype switching and the biology of cancer progression.

IMPLICATIONS

Endosomal pH regulator NHE9 actively controls cytosolic Ca2+ levels to downregulate the adenylate cyclase-cAMP system, enabling colorectal cancer cells to acquire hybrid E/M characteristics and promoting metastatic progression.

Cross-talk between zinc and calcium regulates ion transport: A role for the zinc receptor, ZnR/GPR39

Zinc is essential for many physiological functions, with a major role in digestive system, skin health, and learning and memory. On the cellular level, zinc is involved in cell proliferation and cell death. A selective zinc sensing receptor, ZnR/GPR39 is a Gq-coupled receptor that acts via the inositol trisphosphate pathway to release intracellular Ca2+. The ZnR/GPR39 serves as a mediator between extracellular changes in Zn2+ concentration and cellular Ca2+ signalling. This signalling pathway regulates ion transporters activity and thereby controls the formation of transepithelial gradients or neuronal membrane potential, which play a fundamental role in the physiological function of these tissues. This review focuses on the role of Ca2+ signalling, and specifically ZnR/GPR39, with respect to the regulation of the Na+/H+ exchanger, NHE1, and of the K+/Cl- cotransporters, KCC1-3, and also describes the physiological implications of this regulation.

New functions and roles of the Na+-H+-exchanger NHE3

The sodium/proton exchanger isoform 3 (NHE3) is expressed in the intestine and the kidney, where it contributes to hydrogen secretion and sodium (re)absorption. The roles of this transporter have been studied by the use of the respective knockout mice and by using pharmacological inhibitors. Whole-body NHE3 knockout mice suffer from a high mortality rate (with only ∼30% of mice surviving into adulthood), and based on the expression of NHE3 in both intestine and kidney, some conclusions that were originally derived were based on this rather complex phenotype. In the last decade, more refined models have been developed that added temporal and spatial control of NHE3 expression. For example, novel mouse models have been developed with a knockout of NHE3 in intestinal epithelial cells, tubule/collecting duct of the kidney, proximal tubule of the kidney, and thick ascending limb of the kidney. These refined models have significantly contributed to our understanding of the role of NHE3 in a tissue/cell type-specific manner. In addition, tenapanor was developed, which is a non-absorbable, intestine-specific NHE3 inhibitor. In rat and human studies, tenapanor lowered intestinal Pi uptake and was effective in lowering plasma Pi levels in patients on hemodialysis. Of note, diarrhea is seen as a side effect of tenapanor (with its indication for the treatment of constipation) and in intestine-specific NHE3 knockout mice; however, effects on plasma Pi were not supported by this mouse model which showed enhanced and not reduced intestinal Pi uptake. Further studies indicated that the gut microbiome in mice lacking intestinal NHE3 resembles an intestinal environment favoring the competitive advantage of inflammophilic over anti-inflammatory species, something similar seen in patients with inflammatory bowel disease. This review will highlight recent developments and summarize newly gained insight from these refined models.

Acid-base transporters in the context of tumor heterogeneity

The copious metabolic acid production and -extrusion by cancer cells render poorly vascularized regions of solid tumors highly acidic. A growing list of proton - and bicarbonate transporters has been suggested to contribute to net acid extrusion from cancer cells, and/or been shown to be dysregulated and favor malignant development in various cancers. The great majority of these roles have been studied at the level of the cancer cells. However, recent advances in understanding of the cellular and physicochemical heterogeneity of solid tumors both enable and necessitate a reexamination of the regulation and roles of acid-base transporters in such malignancies. This review will briefly summarize the state-of-the-art, with a focus on the SLC9A and SLC4A families, for which most evidence is available. This is followed by a discussion of key concepts and open questions arising from recent insights and of the challenges that need to be tackled to address them. Finally, opportunities and challenges in therapeutic targeting of the acid-base transportome in cancers will be addressed.

The crossing of two unwound transmembrane regions that is the hallmark of the NhaA structural fold is critical for antiporter activity

Cell pH and Na+ homeostasis requires Na+/H+ antiporters. The crystal structure of NhaA, the main Escherichia coli Na+/H+ antiporter, revealed a unique NhaA structural fold shared by prokaryotic and eukaryotic membrane proteins. Out of the 12 NhaA transmembrane segments (TMs), TMs III-V and X-XII are topologically inverted repeats with unwound TMs IV and XI forming the X shape characterizing the NhaA fold. We show that intramolecular cross-linking under oxidizing conditions of a NhaA mutant with two Cys replacements across the crossing (D133C-T340C) inhibits antiporter activity and impairs NhaA-dependent cell growth in high-salts. The affinity purified D133C-T340C protein binds Li+ (the Na+ surrogate substrate of NhaA) under reducing conditions. The cross-linking traps the antiporter in an outward-facing conformation, blocking the antiport cycle. As many secondary transporters are found to share the NhaA fold, including some involved in human diseases, our data have importance for both basic and clinical research.

The Hydrophilic C-terminus of Yeast Plasma-membrane Na+/H+ Antiporters Impacts Their Ability to Transport K. J Mol Biol 2024;436:168443. [PMID: 38211892 DOI: 10.1016/j.jmb.2024.168443] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Yeast plasma-membrane Na+/H+ antiporters (Nha/Sod) ensure the optimal intracellular level of alkali-metal cations and protons in cells. They are predicted to consist of 13 transmembrane segments (TMSs) and a large hydrophilic C-terminal cytoplasmic part with seven conserved domains. The substrate specificity, specifically the ability to recognize and transport K+ cations in addition to Na+ and Li+, differs among homologs. In this work, we reveal that the composition of the C-terminus impacts the ability of antiporters to transport particular cations. In the osmotolerant yeast Zygosaccharomyces rouxii, the Sod2-22 antiporter only efficiently exports Na+ and Li+, but not K+. The introduction of a negative charge or removal of a positive charge in one of the C-terminal conserved regions (C3) enabled ZrSod2-22 to transport K+. The same mutations rescued the low level of activity and purely Li+ specificity of ZrSod2-22 with the A179T mutation in TMS6, suggesting a possible interaction between this TMS and the C-terminus. The truncation or replacement of the C-terminal part of ZrSod2-22 with the C-terminus of a K+-transporting Nha/Sod antiporter (Saccharomyces cerevisiae Nha1 or Z. rouxii Nha1) also resulted in an antiporter with the capacity to export K+. In addition, in ScNha1, the replacement of three positively charged arginine residues 539-541 in the C3 region with alanine caused its inability to provide cells with tolerance to Li+. All our results demonstrate that the physiological functions of yeast Nha/Sod antiporters, either in salt tolerance or in K+ homeostasis, depend on the composition of their C-terminal parts.

Ion and lipid orchestration of secondary active transport

Transporting small molecules across cell membranes is an essential process in cell physiology. Many structurally diverse, secondary active transporters harness transmembrane electrochemical gradients of ions to power the uptake or efflux of nutrients, signalling molecules, drugs and other ions across cell membranes. Transporters reside in lipid bilayers on the interface between two aqueous compartments, where they are energized and regulated by symported, antiported and allosteric ions on both sides of the membrane and the membrane bilayer itself. Here we outline the mechanisms by which transporters couple ion and solute fluxes and discuss how structural and mechanistic variations enable them to meet specific physiological needs and adapt to environmental conditions. We then consider how general bilayer properties and specific lipid binding modulate transporter activity. Together, ion gradients and lipid properties ensure the effective transport, regulation and distribution of small molecules across cell membranes.

Common as well as unique methylation-sensitive DNA regulatory elements in three mammalian SLC9C1 genes

The SLC9C1 gene (which encodes the NHE10 protein) is essential for male fertility in both mice and humans, however the epigenetic mechanisms regulating its testis/sperm-specific gene expression have yet to be studied. Here we identify and characterize DNA regulatory elements of the SLC9C1 gene across three mammalian species: mouse, rat, and human. First, in silico analysis of these mammalian SLC9C1 genes identified a CpG island located upstream of the transcription start site in the same relative position in all three genes. Further analysis reveals that this CpG island behaves differently, with respect to gene regulatory activity, in the mouse SLC9C1 gene than it does in the rat and human SLC9C1 gene. The mouse SLC9C1 CpG island displays strong promoter activity by itself and seems to have a stronger gene regulatory effect than either the rat or human SLC9C1 CpG islands. While the function of the upstream SLC9C1 CpG island may be divergent across the three studied species, it appears that the promoters of these three mammalian SLC9C1 genes share similar DNA methylation-sensitive regulatory mechanisms. All three SLC9C1 promoter regions are differentially methylated in lung and testis, being more hypermethylated in lung relative to the testis, and DNA sequence alignments provide strong evidence of primary sequence conservation. Luciferase assays reveal that in vitro methylation of constructs containing different elements of the three SLC9C1 genes largely exhibit methylation-sensitive promoter activity (reduced promoter activity when methylated) in both HEK 293 and GC-1spg cells. In total, our data suggest that the DNA methylation-sensitive elements of the mouse, rat, and human SLC9C1 promoters are largely conserved, while the upstream SLC9C1 CpG island common to all three species seems to perform a different function in mouse than it does in rat and human. This work provides evidence that while homologous genes can all be regulated by DNA methylation-dependent epigenetic mechanisms, the location of the specific cis-regulatory elements responsible for this regulation can differ across species.

Ion Transport and Inhibitor Binding by Human NHE1: Insights from Molecular Dynamics Simulations and Free Energy Calculations

The human Na+/H+ exchanger (NHE1) plays a crucial role in maintaining intracellular pH by regulating the electroneutral exchange of a single intracellular H+ for one extracellular Na+ across the plasma membrane. Understanding the molecular mechanisms governing ion transport and the binding of inhibitors is of importance in the development of anticancer therapeutics targeting NHE1. In this context, we performed molecular dynamics (MD) simulations based on the recent cryo-electron microscopy (cryo-EM) structures of outward- and inward-facing conformations of NHE1. These simulations allowed us to explore the dynamics of the protein, examine the ion-translocation pore, and confirm that Asp267 is the ion-binding residue. Our free energy calculations did not show a significant difference between Na+ and K+ binding at the ion-binding site. Consequently, Na+ over K+ selectivity cannot be solely explained by differences in ion binding. Our MD simulations involving NHE1 inhibitors (cariporide and amiloride analogues) maintained stable interactions with Asp267 and Glu346. Our study highlights the importance of the salt bridge between the positively charged acylguanidine moiety and Asp267, which appears to play a role in the competitive inhibitory mechanism for this class of inhibitors. Our computational study provides a detailed mechanistic interpretation of experimental data and serves the basis of future structure-based inhibitor design.

Sex differences in blood pressure regulation and hypertension: renal, hemodynamic, and hormonal mechanisms

The teleology of sex differences has been argued since at least as early as Aristotle's controversial Generation of Animals more than 300 years BC, which reflects the sex bias of the time to contemporary readers. Although the question "why are the sexes different" remains a topic of debate in the present day in metaphysics, the recent emphasis on sex comparison in research studies has led to the question "how are the sexes different" being addressed in health science through numerous observational studies in both health and disease susceptibility, including blood pressure regulation and hypertension. These efforts have resulted in better understanding of differences in males and females at the molecular level that partially explain their differences in vascular function and renal sodium handling and hence blood pressure and the consequential cardiovascular and kidney disease risks in hypertension. This review focuses on clinical studies comparing differences between men and women in blood pressure over the life span and response to dietary sodium and highlights experimental models investigating sexual dimorphism in the renin-angiotensin-aldosterone, vascular, sympathetic nervous, and immune systems, endothelin, the major renal sodium transporters/exchangers/channels, and the impact of sex hormones on these systems in blood pressure homeostasis. Understanding the mechanisms governing sex differences in blood pressure regulation could guide novel therapeutic approaches in a sex-specific manner to lower cardiovascular risks in hypertension and advance personalized medicine.

Functional analysis of two SLC9A6 frameshift variants in lymphoblastoid cells from patients with Christianson syndrome

BACKGROUND

Christianson syndrome (CS) is caused by mutations in SLC9A6 and is characterized by global developmental delay, epilepsy, hyperkinesis, ataxia, microcephaly, and behavioral disorder. However, the molecular mechanism by which these SLC9A6 mutations cause CS in humans is not entirely understood, and there is no objective method to determine the pathogenicity of single SLC9A6 variants.

METHODS

Trio-based whole exome sequencing (WES) was carried out on two individuals with suspicion of CS. qRT-PCR, western blot analysis, filipin staining, lysosomal enzymatic assays, and electron microscopy examination, using EBV-LCLs established from the two patients, were performed.

RESULTS

Trio-based WES identified a hemizygous SLC9A6 c.1560dupT, p.T521Yfs*23 variant in proband 1 and a hemizygous SLC9A6 c.608delA, p.H203Lfs*10 variant in proband 2. Both children exhibited typical phenotypes associated with CS. Expression analysis in EBV-LCLs derived from the two patients showed a significant decrease in mRNA levels and no detectable normal NHE6 protein. EBV-LCLs showed a statistically significant increase in unesterified cholesterol in patient 1, but only non-significant increase in patient 2 when stained with filipin. Activities of lysosomal enzymes (β-hexosaminidase A, β-hexosaminidase A + B, β-galactosidase, galactocerebrosidase, arylsulfatase A) of EBV-LCLs did not significantly differ between the two patients and six controls. Importantly, by electron microscopy we detected an accumulation of lamellated membrane structures, deformed mitochondria, and lipid droplets in the patients' EBV-LCLs.

CONCLUSIONS

The SLC9A6 p.T521Yfs*23 and p.H203Lfs*10 variants in our patients result in loss of NHE6. Alterations of mitochondria and lipid metabolism may play a role in the pathogenesis of CS. Moreover, the combination of filipin staining with electron microscopy examination of patient lymphoblastoid cells can serve as a useful complementary diagnostic method for CS.

A pharmacokinetic evaluation of tenapanor for the treatment of irritable bowel syndrome with constipation: an update of the literature

INTRODUCTION

Tenapanor is the latest addition to the second-line pharmacotherapeutic options for the treatment of irritable bowel syndrome with constipation. It is a first-in-class inhibitor of type 3 sodium/hydrogen exchanger (NHE3), characterized by very low oral absorption. Its pharmacological properties are discussed here based on the latest literature.

AREAS COVERED

A general description of tenapanor is provided, highlighting those pharmacokinetic and pharmacodynamic characteristics of the drug which may be of major importance for tolerability and safety. This description is associated with a summary and analysis of currently available toxicological data.

EXPERT OPINION

Plasma concentrations of free tenapanor after oral administration are well below the half maximal inhibitory concentration for NHE3, so that systemic effects of the drug are minimal. Therefore, the action of tenapanor is limited to NHE3 located on the apical membrane of enterocytes. The consequent reduction in intestinal sodium absorption increases the intraluminal content by osmosis, which in turn enhances the propulsive activity of the colon. Diarrhea is the most frequent adverse effect of tenapanor. Increased fecal sodium and water excretion do not appear to expose patients to short- and long-term hydro-electrolyte imbalances.

Structure and electromechanical coupling of a voltage-gated Na+/H+ exchanger

Voltage-sensing domains control the activation of voltage-gated ion channels, with a few exceptions1. One such exception is the sperm-specific Na+/H+ exchanger SLC9C1, which is the only known transporter to be regulated by voltage-sensing domains2-5. After hyperpolarization of sperm flagella, SLC9C1 becomes active, causing pH alkalinization and CatSper Ca2+ channel activation, which drives chemotaxis2,6. SLC9C1 activation is further regulated by cAMP2,7, which is produced by soluble adenyl cyclase (sAC). SLC9C1 is therefore an essential component of the pH-sAC-cAMP signalling pathway in metazoa8,9, required for sperm motility and fertilization4. Despite its importance, the molecular basis of SLC9C1 voltage activation is unclear. Here we report cryo-electron microscopy (cryo-EM) structures of sea urchin SLC9C1 in detergent and nanodiscs. We show that the voltage-sensing domains are positioned in an unusual configuration, sandwiching each side of the SLC9C1 homodimer. The S4 segment is very long, 90 Å in length, and connects the voltage-sensing domains to the cytoplasmic cyclic-nucleotide-binding domains. The S4 segment is in the up configuration-the inactive state of SLC9C1. Consistently, although a negatively charged cavity is accessible for Na+ to bind to the ion-transporting domains of SLC9C1, an intracellular helix connected to S4 restricts their movement. On the basis of the differences in the cryo-EM structure of SLC9C1 in the presence of cAMP, we propose that, upon hyperpolarization, the S4 segment moves down, removing this constriction and enabling Na+/H+ exchange.

Structures of a sperm-specific solute carrier gated by voltage and cAMP

The newly characterized sperm-specific Na+/H+ exchanger stands out by its unique tripartite domain composition1,2. It unites a classical solute carrier unit with regulatory domains usually found in ion channels, namely, a voltage-sensing domain and a cyclic-nucleotide binding domain1,3, which makes it a mechanistic chimera and a secondary-active transporter activated strictly by membrane voltage. Our structures of the sea urchin SpSLC9C1 in the absence and presence of ligands reveal the overall domain arrangement and new structural coupling elements. They allow us to propose a gating model, where movements in the voltage sensor indirectly cause the release of the exchanging unit from a locked state through long-distance allosteric effects transmitted by the newly characterized coupling helices. We further propose that modulation by its ligand cyclic AMP occurs by means of disruption of the cytosolic dimer interface, which lowers the energy barrier for S4 movements in the voltage-sensing domain. As SLC9C1 members have been shown to be essential for male fertility, including in mammals2,4,5, our structure represents a potential new platform for the development of new on-demand contraceptives.

Architecture and rearrangements of a sperm-specific Na+/H+ exchanger

The sperm-specific sodium hydrogen exchanger, SLC9C1, underlies hyperpolarization and cyclic nucleotide stimulated proton fluxes across sperm membranes and regulates their hyperactivated motility. SLC9C1 is the first known instance of an ion transporter that uses a canonical voltage-sensing domain (VSD) and an evolutionarily conserved cyclic nucleotide binding domain (CNBD) to influence the dynamics of its ion-exchange domain (ED). The structural organization of this 'tripartite transporter' and the mechanisms whereby it integrates physical (membrane voltage) and chemical (cyclic nucleotide) cues are unknown. In this study, we use single particle cryo-electron microscopy to determine structures of a metazoan SLC9C1 in different conformational states. We find that the three structural domains are uniquely organized around a distinct ring-shaped scaffold that we call the 'allosteric ring domain' or ARD. The ARD undergoes coupled proton-dependent rearrangements with the ED and acts as a 'signaling hub' enabling allosteric communication between the key functional modules of sp9C1. We demonstrate that binding of cAMP causes large conformational changes in the cytoplasmic domains and disrupts key ARD-linked interfaces. We propose that these structural changes rescue the transmembrane domains from an auto-inhibited state and facilitate their functional dynamics. Our study provides a structural framework to understand and further probe electrochemical linkage in SLC9C1.

Na+/H+ Exchangers (NHEs) in Mammalian Sperm: Essential Contributors to Male Fertility

Na+/H+ exchangers (NHEs) are known to be important regulators of pH in multiple intracellular compartments of eukaryotic cells. Sperm function is especially dependent on changes in pH and thus it has been postulated that NHEs play important roles in regulating the intracellular pH of these cells. For example, in order to achieve fertilization, mature sperm must maintain a basal pH in the male reproductive tract and then alkalize in response to specific signals in the female reproductive tract during the capacitation process. Eight NHE isoforms are expressed in mammalian testis/sperm: NHE1, NHE3, NHE5, NHE8, NHA1, NHA2, NHE10, and NHE11. These NHE isoforms are expressed at varying times during spermatogenesis and localize to different subcellular structures in developing and mature sperm where they contribute to multiple aspects of sperm physiology and male fertility including proper sperm development/morphogenesis, motility, capacitation, and the acrosome reaction. Previous work has provided evidence for NHE3, NHE8, NHA1, NHA2, and NHE10 being critical for male fertility in mice and NHE10 has recently been shown to be essential for male fertility in humans. In this article we review what is known about each NHE isoform expressed in mammalian sperm and discuss the physiological significance of each NHE isoform with respect to male fertility.

Structural and functional implications of SLC13A3 and SLC9A6 mutations: an in silico approach to understanding intellectual disability

BACKGROUND

Intellectual disability (ID) is a condition that varies widely in both its clinical presentation and its genetic underpinnings. It significantly impacts patients' learning capacities and lowers their IQ below 70. The solute carrier (SLC) family is the most abundant class of transmembrane transporters and is responsible for the translocation of various substances across cell membranes, including nutrients, ions, metabolites, and medicines. The SLC13A3 gene encodes a plasma membrane-localized Na+/dicarboxylate cotransporter 3 (NaDC3) primarily expressed in the kidney, astrocytes, and the choroid plexus. In addition to three Na + ions, it brings four to six carbon dicarboxylates into the cytosol. Recently, it was discovered that patients with acute reversible leukoencephalopathy and a-ketoglutarate accumulation (ARLIAK) carry pathogenic mutations in the SLC13A3 gene, and the X-linked neurodevelopmental condition Christianson Syndrome is caused by mutations in the SLC9A6 gene, which encodes the recycling endosomal alkali cation/proton exchanger NHE6, also called sodium-hydrogen exchanger-6. As a result, there are severe impairments in the patient's mental capacity, physical skills, and adaptive behavior.

METHODS AND RESULTS

Two Pakistani families (A and B) with autosomal recessive and X-linked intellectual disorders were clinically evaluated, and two novel disease-causing variants in the SLC13A3 gene (NM 022829.5) and the SLC9A6 gene (NM 001042537.2) were identified using whole exome sequencing. Family-A segregated a novel homozygous missense variant (c.1478 C > T; p. Pro493Leu) in the exon-11 of the SLC13A3 gene. At the same time, family-B segregated a novel missense variant (c.1342G > A; p.Gly448Arg) in the exon-10 of the SLC9A6 gene. By integrating computational approaches, our findings provided insights into the molecular mechanisms underlying the development of ID in individuals with SLC13A3 and SLC9A6 mutations.

CONCLUSION

We have utilized in-silico tools in the current study to examine the deleterious effects of the identified variants, which carry the potential to understand the genotype-phenotype relationships in neurodevelopmental disorders.

Thermodynamically consistent determination of free energies and rates in kinetic cycle models

Kinetic and thermodynamic models of biological systems are commonly used to connect microscopic features to system function in a bottom-up multiscale approach. The parameters of such models-free energy differences for equilibrium properties and in general rates for equilibrium and out-of-equilibrium observables-have to be measured by different experiments or calculated from multiple computer simulations. All such parameters necessarily come with uncertainties so that when they are naively combined in a full model of the process of interest, they will generally violate fundamental statistical mechanical equalities, namely detailed balance and an equality of forward/backward rate products in cycles due to Hill. If left uncorrected, such models can produce arbitrary outputs that are physically inconsistent. Here, we develop a maximum likelihood approach (named multibind) based on the so-called potential graph to combine kinetic or thermodynamic measurements to yield state-resolved models that are thermodynamically consistent while being most consistent with the provided data and their uncertainties. We demonstrate the approach with two theoretical models, a generic two-proton binding site and a simplified model of a sodium/proton antiporter. We also describe an algorithm to use the multibind approach to solve the inverse problem of determining microscopic quantities from macroscopic measurements and, as an example, we predict the microscopic p K a values and protonation states of a small organic molecule from 1D NMR data. The multibind approach is applicable to any thermodynamic or kinetic model that describes a system as transitions between well-defined states with associated free energy differences or rates between these states. A Python package multibind, which implements the approach described here, is made publicly available under the MIT Open Source license.

Control of intracellular pH and bicarbonate by CO2 diffusion into human sperm

The reaction of CO2 with H2O to form bicarbonate (HCO3-) and H+ controls sperm motility and fertilization via HCO3--stimulated cAMP synthesis. A complex network of signaling proteins participates in this reaction. Here, we identify key players that regulate intracellular pH (pHi) and HCO3- in human sperm by quantitative mass spectrometry (MS) and kinetic patch-clamp fluorometry. The resting pHi is set by amiloride-sensitive Na+/H+ exchange. The sperm-specific putative Na+/H+ exchanger SLC9C1, unlike its sea urchin homologue, is not gated by voltage or cAMP. Transporters and channels implied in HCO3- transport are not detected, and may be present at copy numbers < 10 molecules/sperm cell. Instead, HCO3- is produced by diffusion of CO2 into cells and readjustment of the CO2/HCO3-/H+ equilibrium. The proton channel Hv1 may serve as a unidirectional valve that blunts the acidification ensuing from HCO3- synthesis. This work provides a new framework for the study of male infertility.

Palmitoylation of solute carriers

Post-translational modifications are an important mechanism in the regulation of protein expression, function, and degradation. Well-known post-translational modifications are phosphorylation, glycosylation, and ubiquitination. However, lipid modifications, including myristoylation, prenylation, and palmitoylation, are poorly studied. Since the early 2000s, researchers have become more interested in lipid modifications, especially palmitoylation. The number of articles in PubMed increased from about 350 between 2000 and 2005 to more than 600 annually during the past ten years. S-palmitoylation, where the 16-carbon saturated (C16:0) palmitic acid is added to free cysteine residues of proteins, is a reversible protein modification that can affect the expression, membrane localization, and function of the modified proteins. Various diseases like Huntington's and Alzheimer's disease have been linked to changes in protein palmitoylation. In humans, the addition of palmitic acid is mediated by 23 palmitoyl acyltransferases, also called DHHC proteins. The modification can be reversed by a few thioesterases or hydrolases. Numerous soluble and membrane-attached proteins are known to be palmitoylated, but among the approximately 400 solute carriers that are classified in 66 families, only 15 found in 8 families have so far been documented to be palmitoylated. Among the best-characterized transporters are the glucose transporters GLUT1 (SLC2A1) and GLUT4 (SLC2A4), the three monoamine transporters norepinephrine transporter (NET; SLC6A2), dopamine transporter (DAT; SLC6A3), and serotonin transporter (SERT; SLC6A4), and the sodium-calcium exchanger NCX1 (SLC8A1). While there is evidence from recent proteomics experiments that numerous solute carriers are palmitoylated, no details beyond the 15 transporters covered in this review are available.

Common as well as unique methylation-sensitive DNA regulatory elements in three mammalian SLC9C1 genes

The SLC9C1 gene (which encodes the NHE10 protein) is essential for male fertility in both mice and humans, however the epigenetic mechanisms regulating its testis/sperm-specific gene expression have yet to be studied. Here we identify and characterize DNA regulatory elements of the SLC9C1 gene across three mammalian species: mouse, rat, and human. First, in silico analysis of these mammalian SLC9C1 genes identified a CpG island located upstream of the transcription start site in the same relative position in all three genes. Further analysis reveals that this CpG island behaves differently, with respect to gene regulatory activity, in the mouse SLC9C1 gene than it does in the rat and human SLC9C1 gene. The mouse SLC9C1 CpG island displays strong promoter activity by itself and seems to have a stronger gene regulatory effect than either the rat or human SLC9C1 CpG islands. While the function of the upstream SLC9C1 CpG island may be divergent across the three studied species, it appears that the promoters of these three mammalian SLC9C1 genes share similar DNA methylation-sensitive regulatory mechanisms. All three SLC9C1 promoter regions are differentially methylated in lung and testis, being more hypermethylated in lung relative to the testis, and DNA sequence alignments provide strong evidence of primary sequence conservation. Luciferase assays reveal that in vitro methylation of constructs containing different elements of the three SLC9C1 genes largely exhibit methylation-sensitive promoter activity (reduced promoter activity when methylated) in both HEK 293 and GC-1spg cells. In total, our data suggest that the DNA methylation-sensitive elements of the mouse, rat, and human SLC9C1 promoters are largely conserved, while the upstream SLC9C1 CpG island common to all three species seems to perform a different function in mouse than it does in rat and human. This work provides evidence that while homologous genes can all be regulated by DNA methylation-dependent epigenetic mechanisms, the location of the specific cis-regulatory elements responsible for this regulation can differ across species.

Biological fractionation of lithium isotopes by cellular Na+/H+ exchangers unravels fundamental transport mechanisms

Lithium (Li) has a wide range of uses in science, medicine, and industry, but its isotopy is underexplored, except in nuclear science and in geoscience. ⁶Li and ⁷Li isotopic ratio exhibits the second largest variation on earth's surface and constitutes a widely used tool for reconstructing past oceans and climates. As large variations have been measured in mammalian organs, plants or marine species, and as ⁶Li elicits stronger effects than natural Li (∼95% ⁷Li), a central issue is the identification and quantification of biological influence of Li isotopes distribution. We show that membrane ion channels and Na⁺-Li⁺/H⁺ exchangers (NHEs) fractionate Li isotopes. This systematic ⁶Li enrichment is driven by membrane potential for channels, and by intracellular pH for NHEs, where it displays cooperativity, a hallmark of dimeric transport. Evidencing that transport proteins discriminate between isotopes differing by one neutron opens new avenues for transport mechanisms, Li physiology, and paleoenvironments.

Neutrophil metabolomics in severe COVID-19 reveal GAPDH as a suppressor of neutrophil extracellular trap formation

Severe COVID-19 is characterized by an increase in the number and changes in the function of innate immune cells including neutrophils. However, it is not known how the metabolome of immune cells changes in patients with COVID-19. To address these questions, we analyzed the metabolome of neutrophils from patients with severe or mild COVID-19 and healthy controls. We identified widespread dysregulation of neutrophil metabolism with disease progression including in amino acid, redox, and central carbon metabolism. Metabolic changes in neutrophils from patients with severe COVID-19 were consistent with reduced activity of the glycolytic enzyme GAPDH. Inhibition of GAPDH blocked glycolysis and promoted pentose phosphate pathway activity but blunted the neutrophil respiratory burst. Inhibition of GAPDH was sufficient to cause neutrophil extracellular trap (NET) formation which required neutrophil elastase activity. GAPDH inhibition increased neutrophil pH, and blocking this increase prevented cell death and NET formation. These findings indicate that neutrophils in severe COVID-19 have an aberrant metabolism which can contribute to their dysfunction. Our work also shows that NET formation, a pathogenic feature of many inflammatory diseases, is actively suppressed in neutrophils by a cell-intrinsic mechanism controlled by GAPDH.

Cation/proton antiporters: novel structure-driven pharmaceutical opportunities

Cation/proton antiporters (CPAs) regulate cells' salt concentration and pH. Their malfunction is associated with a range of human pathologies, yet only a handful of CPA-targeting therapeutics are presently in clinical development. Here, we discuss how recently published mammalian protein structures and emerging computational technologies may help to bridge this gap.

The SLC9C2 Gene Product (Na+/H+ Exchanger Isoform 11; NHE11) Is a Testis-Specific Protein Localized to the Head of Mature Mammalian Sperm

Na+/H+ exchangers (NHEs) are a family of ion transporters that regulate the pH of various cell compartments across an array of cell types. In eukaryotes, NHEs are encoded by the SLC9 gene family comprising 13 genes. SLC9C2, which encodes the NHE11 protein, is the only one of the SLC9 genes that is essentially uncharacterized. Here, we show that SLC9C2 exhibits testis/sperm-restricted expression in rats and humans, akin to its paralog SLC9C1 (NHE10). Similar to NHE10, NHE11 is predicted to contain an NHE domain, a voltage sensing domain, and finally an intracellular cyclic nucleotide binding domain. An immunofluorescence analysis of testis sections reveals that NHE11 localizes with developing acrosomal granules in spermiogenic cells in both rat and human testes. Most interestingly, NHE11 localizes to the sperm head, likely the plasma membrane overlaying the acrosome, in mature sperm from rats and humans. Therefore, NHE11 is the only known NHE to localize to the acrosomal region of the head in mature sperm cells. The physiological role of NHE11 has yet to be demonstrated but its predicted functional domains and unique localization suggests that it could modulate intracellular pH of the sperm head in response to changes in membrane potential and cyclic nucleotide concentrations that are a result of sperm capacitation events. If NHE11 is shown to be important for male fertility, it will be an attractive target for male contraceptive drugs due to its exclusive testis/sperm-specific expression.

Inhibition of NHE1 transport activity and gene transcription in DRG neurons in oxaliplatin-induced painful peripheral neurotoxicity

Oxaliplatin (OHP)-induced peripheral neurotoxicity (OIPN), one of the major dose-limiting side effects of colorectal cancer treatment, is characterized by both acute and chronic syndromes. Acute exposure to low dose OHP on dorsal root ganglion (DRG) neurons is able to induce an increase in intracellular calcium and proton concentration, thus influencing ion channels activity and neuronal excitability. The Na⁺/H⁺ exchanger isoform-1 (NHE1) is a plasma membrane protein that plays a pivotal role in intracellular pH (pH_i) homeostasis in many cell types, including nociceptors. Here we show that OHP has early effects on NHE1 activity in cultured mouse DRG neurons: the mean rate of pH_i recovery was strongly reduced compared to vehicle-treated controls, reaching levels similar to those obtained in the presence of cariporide (Car), a specific NHE1 antagonist. The effect of OHP on NHE1 activity was sensitive to FK506, a specific calcineurin (CaN) inhibitor. Lastly, molecular analyses revealed transcriptional downregulation of NHE1 both in vitro, in mouse primary DRG neurons, and in vivo, in an OIPN rat model. Altogether, these data suggest that OHP-induced intracellular acidification of DRG neurons largely depends on CaN-mediated NHE1 inhibition, revealing new mechanisms that OHP could exert to alter neuronal excitability, and providing novel druggable targets.

Bicarbonate transporter SLC4A7 promotes EMT and metastasis of HNSCC by activating the PI3K/AKT/mTOR signaling pathway

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide. Currently, therapeutic modalities such as surgery, chemotherapy, radiotherapy, and immunotherapy are being used to treat HNSCC. However, the treatment outcomes of most patients are dismal because they are already in middle or advanced stage by the time of diagnosis and poorly responsive to treatments. It is therefore of great interest to clarify mechanisms that contribute to the metastasis of cells to identify possible targets for therapy. In this study, we identified the Na⁺ -coupled bicarbonate transporter, SLC4A7, play essential roles in the metastasis of HNSCC. Our results showed that the relative expression of SLC4A7 messenger RNA was highly expressed in HNSCCs samples from TCGA, and compared with precancerous cells of human oral mucosa (DOK), SLC4A7 was highly expressed in HNSCC cell lines. In vitro and in vivo experiments showed that dysregulation of SLC4A7 had minor influence on the proliferation of HNSCC but impacted HNSCC's migration and invasion. Meanwhile, SLC4A7 could promote epithelial-mesenchymal transition (EMT) in HNSCC. RNA-seq, KEGG pathway enrichment analysis and Western blot further revealed that downregulation of SLC4A7 in HNSCC cells inhibited the PI3K/AKT pathway. These findings were further validated via rescue experiments using a small molecule inhibitor of PI3K/mTOR (GDC-0980). Our findings suggest that SLC4A7 promotes EMT and metastasis of HNSCC through the PI3K/AKT/mTOR signaling pathway, which may be a valuable predictive biomarker and potential therapeutic target in HNSCC.

Altered distribution and localization of organellar Na+/H+ exchangers in postmortem schizophrenia dorsolateral prefrontal cortex

Schizophrenia is a complex and multifactorial disorder associated with altered neurotransmission as well as numerous signaling pathway and protein trafficking disruptions. The pH of intracellular organelles involved in protein trafficking is tightly regulated and impacts their functioning. The SLC9A family of Na⁺/H⁺ exchangers (NHEs) plays a fundamental role in cellular and intracellular pH homeostasis. Four organellar NHE isoforms (NHE6-NHE9) are targeted to intracellular organelles involved in protein trafficking. Increased interactions between organellar NHEs and receptor of activated protein C kinase 1 (RACK1) can lead to redistribution of NHEs to the plasma membrane and hyperacidification of target organelles. Given their role in organelle pH regulation, altered expression and/or localization of organellar NHEs could be an underlying cellular mechanism contributing to abnormal intracellular trafficking and disrupted neurotransmitter systems in schizophrenia. We thus characterized organellar NHE expression, co-immunoprecipitation with RACK1, and Triton X-114 (TX-114) phase partitioning in dorsolateral prefrontal cortex of 25 schizophrenia and 25 comparison subjects by Western blot analysis. In schizophrenia after controlling for subject age at time of death, postmortem interval, tissue pH, and sex, there was significantly decreased total expression of NHE8, decreased co-immunoprecipitation of NHE8 (64%) and NHE9 (56%) with RACK1, and increased TX-114 detergent phase partitioning of NHE6 (283%), NHE9 (75%), and RACK1 (367%). Importantly, none of these dependent measures was significantly impacted when comparing those in the schizophrenia group on antipsychotics to those off of antipsychotics for at least 6 weeks at their time of death and none of these same proteins were affected in rats chronically treated with haloperidol. In summary, we characterized organellar NHE expression and distribution in schizophrenia DLPFC and identified abnormalities that could represent a novel mechanism contributing to disruptions in protein trafficking and neurotransmission in schizophrenia.

Organization, dynamics and mechanoregulation of integrin-mediated cell-ECM adhesions

The ability of animal cells to sense, adhere to and remodel their local extracellular matrix (ECM) is central to control of cell shape, mechanical responsiveness, motility and signalling, and hence to development, tissue formation, wound healing and the immune response. Cell-ECM interactions occur at various specialized, multi-protein adhesion complexes that serve to physically link the ECM to the cytoskeleton and the intracellular signalling apparatus. This occurs predominantly via clustered transmembrane receptors of the integrin family. Here we review how the interplay of mechanical forces, biochemical signalling and molecular self-organization determines the composition, organization, mechanosensitivity and dynamics of these adhesions. Progress in the identification of core multi-protein modules within the adhesions and characterization of rearrangements of their components in response to force, together with advanced imaging approaches, has improved understanding of adhesion maturation and turnover and the relationships between adhesion structures and functions. Perturbations of adhesion contribute to a broad range of diseases and to age-related dysfunction, thus an improved understanding of their molecular nature may facilitate therapeutic intervention in these conditions.

Severe acute respiratory syndrome coronavirus 2 may cause liver injury via Na+/H+ exchanger

The liver has many significant functions, such as detoxification, the urea cycle, gluconeogenesis, and protein synthesis. Systemic diseases, hypoxia, infections, drugs, and toxins can easily affect the liver, which is extremely sensitive to injury. Systemic infection of severe acute respiratory syndrome coronavirus 2 can cause liver damage. The primary regulator of intracellular pH in the liver is the Na⁺/H⁺ exchanger (NHE). Physiologically, NHE protects hepatocytes from apoptosis by making the intracellular pH alkaline. Severe acute respiratory syndrome coronavirus 2 increases local angiotensin II levels by binding to angiotensin-converting enzyme 2. In severe cases of coronavirus disease 2019, high angi-otensin II levels may cause NHE overstimulation and lipid accumulation in the liver. NHE overstimulation can lead to hepatocyte death. NHE overstimulation may trigger a cytokine storm by increasing proinflammatory cytokines in the liver. Since the release of proinflammatory cytokines such as interleukin-6 increases with NHE activation, the virus may indirectly cause an increase in fibrinogen and D-dimer levels. NHE overstimulation may cause thrombotic events and systemic damage by increasing fibrinogen levels and cytokine release. Also, NHE overstimulation causes an increase in the urea cycle while inhibiting vitamin D synthesis and gluconeogenesis in the liver. Increasing NHE3 activity leads to Na⁺ loading, which impairs the containment and fluidity of bile acid. NHE overstimulation can change the gut microbiota composition by disrupting the structure and fluidity of bile acid, thus triggering systemic damage. Unlike other tissues, tumor necrosis factor-alpha and angiotensin II decrease NHE3 activity in the intestine. Thus, increased luminal Na⁺ leads to diarrhea and cytokine release. Severe acute respiratory syndrome coronavirus 2-induced local and systemic damage can be improved by preventing virus-induced NHE overstimulation in the liver.

Targeted Mitochondrial Drugs for Treatment of Ischemia-Reperfusion Injury

Ischemia-reperfusion injury is a complex hemodynamic pathology that is a leading cause of death worldwide and occurs in many body organs. Numerous studies have shown that mitochondria play an important role in the occurrence mechanism of ischemia-reperfusion injury and that mitochondrial structural abnormalities and dysfunction lead to the disruption of the homeostasis of the whole mitochondria. At this time, mitochondria are not just sub-organelles to produce ATP but also important targets for regulating ischemia-reperfusion injury; therefore, drugs targeting mitochondria can serve as a new strategy to treat ischemia-reperfusion injury. Based on this view, in this review, we discuss potential therapeutic agents for both mitochondrial structural abnormalities and mitochondrial dysfunction, highlighting the application and prospects of targeted mitochondrial drugs in the treatment of ischemia-reperfusion injury, and try to provide new ideas for the clinical treatment of the ischemia-reperfusion injury.

Allosteric links between the hydrophilic N-terminus and transmembrane core of human Na+ /H+ antiporter NHA2

The human Na+ /H+ antiporter NHA2 (SLC9B2) transports Na+ or Li+ across the plasma membrane in exchange for protons, and is implicated in various pathologies. It is a 537 amino acids protein with an 82 residues long hydrophilic cytoplasmic N-terminus followed by a transmembrane part comprising 14 transmembrane helices. We optimized the functional expression of HsNHA2 in the plasma membrane of a salt-sensitive Saccharomyces cerevisiae strain and characterized in vivo a set of mutated or truncated versions of HsNHA2 in terms of their substrate specificity, transport activity, localization, and protein stability. We identified a highly conserved proline 246, located in the core of the protein, as being crucial for ion selectivity. The replacement of P246 with serine or threonine resulted in antiporters with altered substrate specificity that were not only highly active at acidic pH 4.0 (like the native antiporter), but also at neutral pH. P246T/S versions also exhibited increased resistance to the HsNHA2-specific inhibitor phloretin. We experimentally proved that a putative salt bridge between E215 and R432 is important for antiporter function, but also structural integrity. Truncations of the first 50-70 residues of the N-terminus doubled the transport activity of HsNHA2, while changes in the charge at positions E47, E56, K57, or K58 decreased the antiporter's transport activity. Thus, the hydrophilic N-terminal part of the protein appears to allosterically auto-inhibit cation transport of HsNHA2. Our data also show this in vivo approach to be useful for a rapid screening of SNP's effect on HsNHA2 activity.

Stress-induced reduction of Na+/H+ exchanger isoform 1 promotes maladaptation of neuroplasticity and exacerbates depressive behaviors

Major depression disorder (MDD) is a neuropsychiatric disorder characterized by abnormal neuronal activity in specific brain regions. A factor that is crucial in maintaining normal neuronal functioning is intracellular pH (pHi) homeostasis. In this study, we show that chronic stress, which induces depression-like behaviors in animal models, down-regulates the expression of the hippocampal Na⁺/H⁺ exchanger isoform 1, NHE1, a major determinant of pHi in neurons. Knockdown of NHE1 in CA1 hippocampal pyramidal neurons leads to intracellular acidification, promotes dendritic spine loss, lowers excitatory synaptic transmission, and enhances the susceptibility to stress exposure in rats. Moreover, E3 ubiquitin ligase cullin4A may promote ubiquitination and degradation of NHE1 to induce these effects of an unbalanced pHi on synaptic processes. Electrophysiological data further suggest that the abnormal excitability of hippocampal neurons caused by maladaptation of neuroplasticity may be involved in the pathogenesis of this disease. These findings elucidate a mechanism for pHi homeostasis alteration as related to MDD.

Crystal structure of the Na+/H+ antiporter NhaA at active pH reveals the mechanistic basis for pH sensing

The strict exchange of protons for sodium ions across cell membranes by Na^+/H⁺ exchangers is a fundamental mechanism for cell homeostasis. At active pH, Na⁺/H⁺ exchange can be modelled as competition between H⁺ and Na⁺ to an ion-binding site, harbouring either one or two aspartic-acid residues. Nevertheless, extensive analysis on the model Na⁺/H⁺ antiporter NhaA from Escherichia coli, has shown that residues on the cytoplasmic surface, termed the pH sensor, shifts the pH at which NhaA becomes active. It was unclear how to incorporate the pH senor model into an alternating-access mechanism based on the NhaA structure at inactive pH 4. Here, we report the crystal structure of NhaA at active pH 6.5, and to an improved resolution of 2.2 Å. We show that at pH 6.5, residues in the pH sensor rearrange to form new salt-bridge interactions involving key histidine residues that widen the inward-facing cavity. What we now refer to as a pH gate, triggers a conformational change that enables water and Na⁺ to access the ion-binding site, as supported by molecular dynamics (MD) simulations. Our work highlights a unique, channel-like switch prior to substrate translocation in a secondary-active transporter.

Ion transfer mechanisms in Mrp-type antiporters from high resolution cryoEM and molecular dynamics simulations

Multiple resistance and pH adaptation (Mrp) cation/proton antiporters are essential for growth of a variety of halophilic and alkaliphilic bacteria under stress conditions. Mrp-type antiporters are closely related to the membrane domain of respiratory complex I. We determined the structure of the Mrp antiporter from Bacillus pseudofirmus by electron cryo-microscopy at 2.2 Å resolution. The structure resolves more than 99% of the sidechains of the seven membrane subunits MrpA to MrpG plus 360 water molecules, including ~70 in putative ion translocation pathways. Molecular dynamics simulations based on the high-resolution structure revealed details of the antiport mechanism. We find that switching the position of a histidine residue between three hydrated pathways in the MrpA subunit is critical for proton transfer that drives gated trans-membrane sodium translocation. Several lines of evidence indicate that the same histidine-switch mechanism operates in respiratory complex I.

The Physiological Function and Potential Role of the Ubiquitous Na+/H+ Exchanger Isoform 8 (NHE8): An Overview Data

The Na⁺/H⁺ exchanger transporters (NHE) play an important role in various biologic processes including Na⁺ absorption, intracellular pH homeostasis, cell volume regulation, proliferation, and apoptosis. The wide expression pattern and cellular localization of NHEs make these proteins pivotal players in virtually all human tissues and organs. In addition, recent studies suggest that NHEs may be one of the primeval transport protein forms in the history of life. Among the different isoforms, the most well-characterized NHEs are the Na⁺/H⁺ exchanger isoform 1 (NHE1) and Na⁺/H⁺ exchanger isoform 3 (NHE3). However, Na⁺/H⁺ exchanger isoform 8 (NHE8) has been receiving attention based on its recent discoveries in the gastrointestinal tract. In this review, we will discuss what is known about the physiological function and potential role of NHE8 in the main organ systems, including useful overviews that could inspire new studies on this multifaceted protein.

Prokaryotic Na+/H+ Exchangers—Transport Mechanism and Essential Residues

Na⁺/H⁺ exchangers are essential for Na⁺ and pH homeostasis in all organisms. Human Na⁺/H⁺ exchangers are of high medical interest, and insights into their structure and function are aided by the investigation of prokaryotic homologues. Most prokaryotic Na⁺/H⁺ exchangers belong to either the Cation/Proton Antiporter (CPA) superfamily, the Ion Transport (IT) superfamily, or the Na⁺-translocating Mrp transporter superfamily. Several structures have been solved so far for CPA and Mrp members, but none for the IT members. NhaA from E. coli has served as the prototype of Na⁺/H⁺ exchangers due to the high amount of structural and functional data available. Recent structures from other CPA exchangers, together with diverse functional information, have allowed elucidation of some common working principles shared by Na⁺/H⁺ exchangers from different families, such as the type of residues involved in the substrate binding and even a simple mechanism sufficient to explain the pH regulation in the CPA and IT superfamilies. Here, we review several aspects of prokaryotic Na⁺/H⁺ exchanger structure and function, discussing the similarities and differences between different transporters, with a focus on the CPA and IT exchangers. We also discuss the proposed transport mechanisms for Na⁺/H⁺ exchangers that explain their highly pH-regulated activity profile.

The genome-wide identification and adaptive evolution of slc9 genes in Leuciscus waleckii under extremely alkaline conditions

The solute carrier family 9 (slc9) genes, especially slc9a isoform coding proteins contribute to electroneutral countertransport of H⁺ for Na⁺ across the plasmalemmal and organellar membranes, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. These functional properties determine a potential basis for organisms to challenge stressful conditions. However, these well-done researches have been reported more in mammals. Thus, in this study, a total of eleven slc9 genes were identified from the latest version genome of L. waleckii, a cyprinid fish that could tolerate extremely alkaline environments (pH 9.6). The evolutionary footprint of slc9 genes was uncovered via the analysis of copy numbers, gene structure, motif composition, chromosome location and phylogenetic relationship. More importantly, there were two SNPs located on 5' UTR and three non-synonymous mutations in the coding region of the slc9a3.2 gene by comparing freshwater with alkaline water populations attached to resequencing technology. Slc9a3.2 gene was a statistically significant low expression in gill tissue with extremely alkaline pressure. Generally, slc9 gene family in L. waleckii was highly conserved. Several important SNPs with high Fst values were identified where non-synonymous mutations occurred between freshwater and alkaline water populations, and they may play an important role in specific functional differentiation. Slc9 genes had clear tissue expression preferences and were involved in abiotic stress response, indicating their roles in physiological function and strong self-regulating capacity. Our insight into the genetic variations that take place in the individual genes under extreme conditions could provide a feasible example for studying specific molecular mechanisms based on genomic data with increasing environmental stress.

Identification of Intestinal NaCl Absorptive-Anion Secretory Cells: Potential Functional Significance

Use of human enteroids studied in the undifferentiated and differentiated state that mimic the intestinal crypt and villus, respectively, has allowed studies of multiple enterocyte populations, including a large population of enterocytes that are transitioning from the crypt to the villus. This population expresses NHE3, DRA, and CFTR, representing a combination of Na absorptive and anion secretory functions. In this cell population, these three transporters physically interact, which affects their baseline and regulated activities. A study of this cell population and differentiated Caco-2 cells transduced with NHE3 and endogenously expressing DRA and CFTR has allowed an understanding of previous studies in which cAMP seemed to stimulate and inhibit DRA at the same time. Understanding the contributions of these cells to overall intestinal transport function as part of the fasting and post-prandial state and their contribution to the pathophysiology of diarrheal diseases and some conditions with constipation will allow new approaches to drug development.