Gill ionocyte remodeling mediates blood pH regulation in rockfish (Sebastes diploproa) exposed to environmentally relevant hypercapnia.

Marine fishes excrete excess H+ using basolateral Na+/K+-ATPase (NKA) and apical Na+/H+-exchanger 3 (NHE3) in gill ionocytes. However, the mechanisms that regulate H+ excretion during exposure to environmentally relevant hypercapnia (ERH) remain poorly understood. Here, we explored transcriptomic, proteomic, and cellular responses in gills of juvenile splitnose rockfish (Sebastes diploproa) exposed to three days of ERH conditions (pH ~7.5; ~1,600 µatm pCO2). Blood pH was fully regulated at ~7.75 despite a lack of significant changes in gill (1) mRNAs coding for proteins involved in blood acid-base regulation, (2) total NKA and NHE3 protein abundance, and (3) ionocyte density. However, ERH-exposed rockfish demonstrated increased NKA and NHE3 abundance on the ionocyte plasma membrane coupled with wider apical membranes and greater extension of apical microvilli. The observed gill ionocyte remodeling is consistent with enhanced H+ excretion that maintains blood pH homeostasis during exposure to ERH and does not necessitate changes at the expression or translation levels. These mechanisms of phenotypic plasticity may allow fishes to regulate blood pH during environmentally relevant acid-base challenges, and thus have important implications for both understanding how organisms respond to climate change and for selecting appropriate metrics to evaluate its impact on marine ecosystems.

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.

Revisiting the association of sudden infant death syndrome (SIDS) with polymorphisms of NHE3 and IL13.

OBJECTIVES: Disturbances of the central nervous system and immune system are thought to play a role in sudden infant death syndrome (SIDS). Dysregulated expression of sodium (Na+)/hydrogen (H+) exchanger 3 (NHE3) in the brainstem and of interleukin 13 (IL13) in the lungs has been observed in SIDS. An association of single-nucleotide polymorphisms (SNPs) in NHE3 and IL13 with SIDS has been proposed, but controversial results were reported. Therefore, there is a need to revisit the association of SNPs in NHE3 and IL13 with SIDS. METHODS: Genotyping of rs71597645 (G1131A) and rs2247114 (C2405T) in NHE3 and rs20541 (+ 4464A/G) in IL13 was performed in 201 SIDS cases and 338 controls. A meta-analysis was performed after merging our data with previously published data (all from European populations). RESULTS: Polymorphisms rs2247114 (NHE3) and rs20541 (IL13) were significantly associated with SIDS overall and in multiple subgroups, but no association was found for rs71597645 (NHE3). After combining our data with previously published data, a fixed-effect meta-analysis showed that rs2247114 in NHE3 retained a significant association with SIDS under a recessive model (OR 2.78, 95%CI 1.53 to 5.06; p = 0.0008). CONCLUSION: Our findings suggest an association of NHE3 variant rs2247114 (C2405T), though not rs71597645 (NHE3), with SIDS. A potential role of rs20541 (IL13) still has to be elucidated. Especially NHE3 seems to be an interesting topic for future SIDS research.

SLC9A5 promotes tumor growth and cell motility via ACOX1-mediated peroxisomal fatty acid oxidation.

Growing evidence suggests a strong association between decreased lipid catabolism and the development of cancer. Solute carrier family 9 member A5 (SLC9A5) plays a regulatory role in colorectal function. However, the specific involvement of SLC9A5 in colorectal cancer (CRC) remains unclear, as well as its potential connection to lipid catabolism. We found that SLC9A5 exhibited significantly higher expression in CRC tumor tissues compared to adjacent paratumor tissues, as confirmed through analysis of the TCGA database and validation on a CRC tissue chip using IHC. Furthermore, in vitro experiments showed that knockdown of SLC9A5 resulted in suppressed cell proliferation, migration, and invasion. Then we performed bioinformatics analysis and found that SLC9A5 was significantly enriched in peroxisomal fatty acid oxidation (FAO) pathway and negatively correlated with its first rate-limiting enzyme acyl-CoA oxidases (ACOX). Interestingly, the expression of ACOX1, as well as FAO process indicated by changes in very long chain fatty acid levels, were enhanced upon SLC9A5 knockdown in CRC cells. Moreover, the attenuated tumor growth, migration, invasion, and increased FAO observed after SLC9A5 knockdown could be reversed by simultaneous knockdown of both SLC9A5 and ACOX1. In summary, these findings reveal the oncogenic role of SLC9A5 in CRC, particularly in relation to ACOX1-mediated peroxidation, and might serve as a promising therapeutic target for inhibiting the progression of colorectal cancer.

SLC9A2, suppressing by the transcription suppressor ETS1, restrains growth and invasion of osteosarcoma via inhibition of aerobic glycolysis.

Recent studies have shown that Solute Carrier Family 9 Member A2 (SLC9A2) could serve as a biomarker for cancer. However, its mechanism of action in osteosarcoma (OS) was still unclear. In this study, the data sets GSE154530 and GSE99671 were downloaded from the Gene Expression Omnibus (GEO) database, and 31 differentially expressed genes (DEGs) related to methylation were screened by bioinformatics analysis tools. Subsequently, SLC9A2 was screened as a candidate gene from DEGs, which was significantly downregulated in OS. CCK-8, transwell, western blotting and Seahorse XFe24 Cell Metabolic Analyzer assays demonstrated that overexpression of SLC9A2 could constrain OS cell proliferation, invasion, and aerobic glycolysis. Dual-luciferase reporter gene assay and chromatin immunoprecipitation (ChIP) assays indicated ETS proto-oncogene 1 (ETS1) was a transcription suppressor of SLC9A2, and overexpression of ETS1 could promote methylation levels in specific regions of the SLC9A2 promoter. ETS1 could promote the proliferation, invasion, and aerobic glycolysis ability of OS cells, as well as tumor growth in vivo by inhibiting the expression of SLC9A2. In addition, SLC9A2, suppressing by ETS1, restrains growth and invasion of OS via inhibition of aerobic glycolysis. Thus, SLC9A2 can function as a key inhibitory factor in the aerobic glycolysis to inhibit proliferation and invasion of OS. This indicated that SLC9A2 has a potential targeted therapeutic effect on OS.

The sodium proton exchanger NHE9 regulates phagosome maturation and bactericidal activity in macrophages.

Acidification of phagosomes is essential for the bactericidal activity of macrophages. Targeting machinery that regulates pH within the phagosomes is a prominent strategy employed by various pathogens that have emerged as major threats to public health. Nascent phagosomes acquire the machinery for pH regulation through a graded maturation process involving fusion with endolysosomes. Meticulous coordination between proton pumping and leakage mechanisms is crucial for maintaining optimal pH within the phagosome. However, relative to mechanisms involved in acidifying the phagosome lumen, little is known about proton leakage pathways in this organelle. Sodium proton transporter NHE9 is a known proton leakage pathway located on the endosomes. As phagosomes acquire proteins through fusions with endosomes during maturation, NHE9 seemed a promising candidate for regulating proton fluxes on the phagosome. Here, using genetic and biophysical approaches, we show NHE9 is an important proton leakage pathway associated with the maturing phagosome. NHE9 is highly expressed in immune cells, specifically macrophages; however, NHE9 expression is strongly downregulated upon bacterial infection. We show that compensatory ectopic NHE9 expression hinders the directed motion of phagosomes along microtubules and promotes early detachment from the microtubule tracks. As a result, these phagosomes have shorter run lengths and are not successful in reaching the lysosome. In accordance with this observation, we demonstrate that NHE9 expression levels negatively correlate with bacterial survival. Together, our findings show that NHE9 regulates lumenal pH to affect phagosome maturation, and consequently, microbicidal activity in macrophages.

A novel perspective on the evolutionary loss of plasma-accessible carbonic anhydrase at the teleost gill.

The gills of most teleost fishes lack plasma-accessible carbonic anhydrase (paCA) that could participate in CO2 excretion. We tested the prevailing hypothesis that paCA would interfere with red blood cell (RBC) intracellular pH regulation by ß-adrenergic sodium-proton exchangers (ß-NHE) that protect pH-sensitive haemoglobin-oxygen (Hb-O2) binding during an acidosis. In an open system that mimics the gills, ß-NHE activity increased Hb-O2 saturation during a respiratory acidosis in the presence or absence of paCA, whereas the effect was abolished by NHE inhibition. However, in a closed system that mimics the tissue capillaries, paCA disrupted the protective effects of ß-NHE activity on Hb-O2 binding. The gills are an open system, where CO2 generated by paCA can diffuse out and is not available to acidifying the RBCs. Therefore, branchial paCA in teleosts may not interfere with RBC pH regulation by ß-NHEs, and other explanations for the evolutionary loss of the enzyme must be considered.

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.

Downregulation of SLC9A8 Promotes Epithelial-Mesenchymal Transition and Metastasis in Colorectal Cancer Cells via the IL6-JAK1/STAT3 Signaling Pathway.

BACKGROUND: SLC9A8 has been shown to be involved in mucus layer formation, intestinal mucosal integrity, and hyperproliferation of colitis-associated tumor development. However, its effects on the epithelial-mesenchymal transition (EMT) and the metastasis of colorectal cancer (CRC) remain unknown. AIMS: To explore whether SLC9A8 participates in EMT and the metastasis of CRC. METHODS: Western blotting and immunohistochemistry were performed to evaluate the expression of SLC9A8 in CRC patients. At the cellular level, the effect of SLC9A8 on proliferation, migration, and invasion was measured using cell viability analysis, flow cytometry analysis, and Transwell assays. Mouse tumor xenograft and metastasis models were established to analyze whether knockdown of SLC9A8 increased tumor volume, tumor weight, and metastasis. Moreover, whether downregulated expression of SLC9A8 promotes EMT via activation of the IL6-JAK1-STAT3 signaling pathway was investigated. RESULTS: SLC9A8 protein was downregulated in CRC tissues, and this downregulation was significantly associated with tumor size, lymph node status, pTNM stage, and poor prognosis. SLC9A8 overexpression markedly suppressed cell proliferation, migration, and invasion. Downregulation of SLC9A8 promoted CRC cell proliferation, migration, and invasion. Moreover, knockdown of SLC9A8 also increased tumor volume, tumor weight, and metastasis in vivo. Meanwhile, downregulation of SLC9A8 significantly promoted the in vitro migration of CRC cells via EMT by activating the IL6-JAK1/STAT3 signaling pathway. CONCLUSIONS: Downregulation of SLC9A8 plays an important role in EMT and metastasis of CRC progression and may become a new potential therapeutic target for the treatment of CRC.

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.

Human Colonoid-Myofibroblast Coculture for Study of Apical Na+/H+ Exchangers of the Lower Cryptal Neck Region.

Cation and anion transport in the colonocyte apical membrane is highly spatially organized along the cryptal axis. Because of lack of experimental accessibility, information about the functionality of ion transporters in the colonocyte apical membrane in the lower part of the crypt is scarce. The aim of this study was to establish an in vitro model of the colonic lower crypt compartment, which expresses the transit amplifying/progenitor (TA/PE) cells, with accessibility of the apical membrane for functional study of lower crypt-expressed Na+/H+ exchangers (NHEs). Colonic crypts and myofibroblasts were isolated from human transverse colonic biopsies, expanded as three-dimensional (3D) colonoids and myofibroblast monolayers, and characterized. Filter-grown colonic myofibroblast-colonic epithelial cell (CM-CE) cocultures (myofibroblasts on the bottom of the transwell and colonocytes on the filter) were established. The expression pattern for ion transport/junctional/stem cell markers of the CM-CE monolayers was compared with that of nondifferentiated (EM) and differentiated (DM) colonoid monolayers. Fluorometric pHi measurements were performed to characterize apical NHEs. CM-CE cocultures displayed a rapid increase in transepithelial electrical resistance (TEER), paralleled by downregulation of claudin-2. They maintained proliferative activity and an expression pattern resembling TA/PE cells. The CM-CE monolayers displayed high apical Na+/H+ exchange activity, mediated to >80% by NHE2. Human colonoid-myofibroblast cocultures allow the study of ion transporters that are expressed in the apical membrane of the nondifferentiated colonocytes of the cryptal neck region. The NHE2 isoform is the predominant apical Na+/H+ exchanger in this epithelial compartment.

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.

The HVCN1 voltage-gated proton channel contributes to pH regulation in canine ventricular myocytes.

Regulation of intracellular pH (pHi ) in cardiomyocytes is crucial for cardiac function; however, currently known mechanisms for direct or indirect extrusion of acid from cardiomyocytes seem insufficient for energetically efficient extrusion of the massive H+ loads generated under in vivo conditions. In cardiomyocytes, voltage-sensitive H+ channel activity mediated by the HVCN1 proton channel would be a highly efficient means of disposing of H+ , while avoiding Na+ loading, as occurs during direct acid extrusion via Na+ /H+ exchange or indirect acid extrusion via Na+ -HCO3- cotransport. PCR and immunoblotting demonstrated expression of HVCN1 mRNA and protein in canine heart. Patch clamp analysis of canine ventricular myocytes revealed a voltage-gated H+ current that was highly H+ -selective. The current was blocked by external Zn2+ and the HVCN1 blocker 5-chloro-2-guanidinobenzimidazole. Both the gating and Zn2+ blockade of the current were strongly influenced by the pH gradient across the membrane. All characteristics of the observed current were consistent with the known hallmarks of HVCN1-mediated H+ current. Inhibition of HVCN1 and the NHE1 Na+ /H+ exchanger, singly and in combination, showed that either mechanism is largely sufficient to maintain pHi in beating cardiomyocytes, but that inhibition of both activities causes rapid acidification. These results show that HVCN1 is expressed in canine ventricular myocytes and provides a major H+ extrusion activity, with a capacity similar to that of NHE1. In the beating heart in vivo, this activity would allow Na+ -independent extrusion of H+ during each action potential and, when functionally coupled with anion transport mechanisms, could facilitate transport-mediated CO2 disposal. KEY POINTS: Intracellular pH (pHi ) regulation is crucial for cardiac function, as acidification depresses contractility and causes arrhythmias. H+ ions are generated in cardiomyocytes from metabolic processes and particularly from CO2 hydration, which has been shown to facilitate CO2 venting from mitochondria. Currently, the NHE1 Na+ /H+ exchanger is viewed as the dominant H+ extrusion mechanism in cardiac muscle. We show that the HVCN1 voltage-gated proton channel is present and functional in canine ventricular myocytes, and that HVCN1 and NHE1 both contribute to pHi regulation. HVCN1 provides an energetically efficient mechanism of H+ extrusion that would not cause Na+ loading, which can cause pathology, and that could contribute to transport-mediated CO2 disposal. These results provide a major advance in our understanding of pHi regulation in cardiac muscle.

Christianson syndrome: A novel splicing variant of SLC9A6 causes exon skipping in a Chinese boy and a literature review.

BACKGROUND: Variants in the endosomal solute carrier family 9 member A6 (SLC9A6)/(Na+ ,K+ )/H+ exchanger 6 (NHE6) gene have been linked to epilepsy, speech loss, truncal ataxia, hyperkinesia, and postnatal microcephaly. METHODS: In the present study, we evaluated genetic alterations in a 3-year-old Chinese boy displayed features of epilepsy, psychomotor retardation, microcephaly, low body weight, difficulty in feeding, excessive movement, attention loss, ataxia, and cerebellar atrophy and his healthy family using WES method. The identified variant was further confirmed by Sanger sequencing method. Finally, minigene assays were used to verify whether the novel SLC9A6 intronic variant influenced the normal splicing of mRNA. RESULTS: We identified a novel hemizygous splicing variant [NM_001042537.1: c.1463-1G>A] in SLC9A6 by trio-based exome sequencing. The minigene expression in vitro confirmed the splicing variant altered a consensus splice acceptor site of SLC9A6 intron 11, resulting in skipping over exon 12. CONCLUSIONS: Our finding extends the catalog of pathogenic intronic variants affecting SLC9A6 pre-mRNA splicing and provides a basis for the genetic diagnosis of CS.

Physiological and Molecular Function of the Sodium/Hydrogen Exchanger NHA2 (SLC9B2).

NHA2, also known as SLC9B2, is an orphan intracellular Na+/H+ exchanger (NHE) that has been associated with arterial hypertension and diabetes mellitus in humans. The objective of this NCCR TransCure project was to define the physiological and molecular function of NHA2, to develop a high resolution kinetic transport assay for NHA2 and to identify specific and potent compounds targeting NHA2. In this review, we summarize the results of this highly interdisciplinary and interfaculty effort, led by the groups of Proffs. Jean-Louis Reymond, Christoph von Ballmoos and Daniel Fuster.

Donor Splice Site Variant in SLC9A6 Causes Christianson Syndrome in a Lithuanian Family: A Case Report.

Background and Objectives: The pathogenic variants of SLC9A6 are a known cause of a rare, X-linked neurological disorder called Christianson syndrome (CS). The main characteristics of CS are developmental delay, intellectual disability, and neurological findings. This study investigated the genetic basis and explored the molecular changes that led to CS in two male siblings presenting with intellectual disability, epilepsy, behavioural problems, gastrointestinal dysfunction, poor height, and weight gain. Materials and Methods: Next-generation sequencing of a tetrad was applied to identify the DNA changes and Sanger sequencing of proband's cDNA was used to evaluate the impact of a splice site variant on mRNA structure. Bioinformatical tools were used to investigate SLC9A6 protein structure changes. Results: Sequencing and bioinformatical analysis revealed a novel donor splice site variant (NC_000023.11(NM_001042537.1):c.899 + 1G > A) that leads to a frameshift and a premature stop codon. Protein structure modelling showed that the truncated protein is unlikely to form any functionally relevant SLC9A6 dimers. Conclusions: Molecular and bioinformatical analysis revealed the impact of a novel donor splice site variant in the SLC9A6 gene that leads to truncated and functionally disrupted protein causing the phenotype of CS in the affected individuals.

In vivo crystals reveal critical features of the interaction between cystic fibrosis transmembrane conductance regulator (CFTR) and the PDZ2 domain of Na+/H+ exchange cofactor NHERF1.

Crystallization of recombinant proteins has been fundamental to our understanding of protein function, dysfunction, and molecular recognition. However, this information has often been gleaned under extremely nonphysiological protein, salt, and H+ concentrations. Here, we describe the development of a robust Inka1-Box (iBox)-PAK4cat system that spontaneously crystallizes in several mammalian cell types. The semi-quantitative assay described here allows the measurement of in vivo protein-protein interactions using a novel GFP-linked reporter system that produces fluorescent readouts from protein crystals. We combined this assay with in vitro X-ray crystallography and molecular dynamics studies to characterize the molecular determinants of the interaction between the PDZ2 domain of Na+/H+ exchange regulatory cofactor NHE-RF1 (NHERF1) and cystic fibrosis transmembrane conductance regulator (CFTR), a protein complex pertinent to the genetic disease cystic fibrosis. These experiments revealed the crystal structure of the extended PDZ domain of NHERF1 and indicated, contrary to what has been previously reported, that residue selection at positions -1 and -3 of the PDZ-binding motif influences the affinity and specificity of the NHERF1 PDZ2-CFTR interaction. Our results suggest that this system could be utilized to screen additional protein-protein interactions, provided they can be accommodated within the spacious iBox-PAK4cat lattice.

Adrenergically induced translocation of red blood cell ß-adrenergic sodium-proton exchangers has ecological relevance for hypoxic and hypercapnic white seabass.

White seabass (Atractoscion nobilis) increasingly experience periods of low oxygen (O2; hypoxia) and high carbon dioxide (CO2, hypercapnia) due to climate change and eutrophication of the coastal waters of California. Hemoglobin (Hb) is the principal O2 carrier in the blood and in many teleost fishes Hb-O2 binding is compromised at low pH; however, the red blood cells (RBC) of some species regulate intracellular pH with adrenergically stimulated sodium-proton-exchangers (ß-NHEs). We hypothesized that RBC ß-NHEs in white seabass are an important mechanism that can protect the blood O2-carrying capacity during hypoxia and hypercapnia. We determined the O2-binding characteristics of white seabass blood, the cellular and subcellular response of RBCs to adrenergic stimulation, and quantified the protective effect of ß-NHE activity on Hb-O2 saturation. White seabass had typical teleost Hb characteristics, with a moderate O2 affinity (Po2 at half-saturation; P50 2.9 kPa) that was highly pH-sensitive (Bohr coefficient -0.92; Root effect 52%). Novel findings from super-resolution microscopy revealed ß-NHE protein in vesicle-like structures and its translocation into the membrane after adrenergic stimulation. Microscopy data were corroborated by molecular and phylogenetic results and a functional characterization of ß-NHE activity. The activation of RBC ß-NHEs increased Hb-O2 saturation by ∼8% in normoxic hypercapnia and by up to ∼20% in hypoxic normocapnia. Our results provide novel insight into the cellular mechanism of adrenergic RBC stimulation within an ecologically relevant context. ß-NHE activity in white seabass has great potential to protect arterial O2 transport during hypoxia and hypercapnia but is less effective during combinations of these stressors.

The sodium/proton exchanger SLC9C1 (sNHE) is essential for human sperm motility and fertility.

Asthenozoospermia, defined by the absence or reduction of sperm motility, constitutes the most frequent cause of human male infertility. This pathological condition is caused by morphological and/or functional defects of the sperm flagellum, which preclude proper sperm progression. While in the last decade many causal genes were identified for asthenozoospermia associated with severe sperm flagellar defects, the causes of purely functional asthenozoospermia are still poorly defined. We describe here the case of an infertile man, displaying asthenozoospermia without major morphological flagellar anomalies and carrying a homozygous splicing mutation in SLC9C1 (sNHE), which we identified by whole-exome sequencing. SLC9C1 encodes a sperm-specific sodium/proton exchanger, which in mouse regulates pH homeostasis and interacts with the soluble adenylyl cyclase (sAC), a key regulator of the signalling pathways involved in sperm motility and capacitation. We demonstrate by means of RT-PCR, immunodetection and immunofluorescence assays on patient's semen samples that the homozygous splicing mutation (c.2748 + 2 T > C) leads to in-frame exon skipping resulting in a deletion in the cyclic nucleotide-binding domain of the protein. Our work shows that in human, similar to mouse, SLC9C1 is required for sperm motility. Overall, we establish a homozygous truncating mutation in SLC9C1 as a novel cause of human asthenozoospermia and infertility.

Role of Genetic Mutations of the Na+/H+ Exchanger Isoform 1, in Human Disease and Protein Targeting and Activity.

The mammalian Na+/H+ exchanger isoform one (NHE1) is a plasma membrane protein that is ubiquitously present in human cells. It functions to regulate intracellular pH removing an intracellular proton in exchange for one extracellular sodium and is involved in heart disease and in promoting metastasis in cancer. It is made of a 500 amino acid membrane domain plus a 315 amino acid, regulatory cytosolic tail. The membrane domain is thought to have 12 transmembrane segments and a large membrane-associated extracellular loop. Early studies demonstrated that in mice, disruption of the NHE1 gene results in locomotor ataxia and a phenotype of slow-wave epilepsy. Defects included a progressive neuronal degeneration. Growth and reproductive ability were also reduced. Recent studies have identified human autosomal homozygous recessive mutations in the NHE1 gene (SLC9A1) that result in impaired development, ataxia and other severe defects, and explain the cause of the human disease Lichtenstein-Knorr syndrome. Other human mutations have been identified that are stop codon polymorphisms. These cause short non-functional NHE1 proteins, while other genetic polymorphisms in the NHE1 gene cause impaired expression of the NHE1 protein, reduced activity, enhanced protein degradation or altered kinetic activation of the protein. Since NHE1 plays a key role in many human physiological functions and in human disease, genetic polymorphisms of the protein that significantly alter its function and are likely play significant roles in varying human phenotypes and be involved in disease.