SLC9/NHE gene family, a plasma membrane and organellar family of Na⁺/H⁺ exchangers

pH Homeodynamics and Male Fertility: A Coordinated Regulation of Acid-Based Balance during Sperm Journey to Fertilization

pH homeostasis is crucial for spermatogenesis, sperm maturation, sperm physiological function, and fertilization in mammals. HCO3- and H+ are the most significant factors involved in regulating pH homeostasis in the male reproductive system. Multiple pH-regulating transporters and ion channels localize in the testis, epididymis, and spermatozoa, such as HCO3- transporters (solute carrier family 4 and solute carrier family 26 transporters), carbonic anhydrases, and H+-transport channels and enzymes (e.g., Na+-H+ exchangers, monocarboxylate transporters, H+-ATPases, and voltage-gated proton channels). Hormone-mediated signals impose an influence on the production of some HCO3- or H+ transporters, such as NBCe1, SLC4A2, MCT4, etc. Additionally, ion channels including sperm-specific cationic channels for Ca2+ (CatSper) and K+ (SLO3) are directly or indirectly regulated by pH, exerting specific actions on spermatozoa. The slightly alkaline testicular pH is conducive to spermatogenesis, whereas the epididymis's low HCO3- concentration and acidic lumen are favorable for sperm maturation and storage. Spermatozoa pH increases substantially after being fused with seminal fluid to enhance motility. In the female reproductive tract, sperm are subjected to increasing concentrations of HCO3- in the uterine and fallopian tube, causing a rise in the intracellular pH (pHi) of spermatozoa, leading to hyperpolarization of sperm plasma membranes, capacitation, hyperactivation, acrosome reaction, and ultimately fertilization. The physiological regulation initiated by SLC26A3, SLC26A8, NHA1, sNHE, and CFTR localized in sperm is proven for certain to be involved in male fertility. This review intends to present the key factors and characteristics of pHi regulation in the testes, efferent duct, epididymis, seminal fluid, and female reproductive tract, as well as the associated mechanisms during the sperm journey to fertilization, proposing insights into outstanding subjects and future research trends.

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.

The macrocycle inhibitor landscape of SLC-transporter

In the past years the interest in Solute Carrier Transporters (SLC) has increased due to their potential as drug targets. At the same time, macrocycles demonstrated promising activities as therapeutic agents. However, the overall macrocycle/SLC-transporter interaction landscape has not been fully revealed yet. In this study, we present a statistical analysis of macrocycles with measured activity against SLC-transporter. Using a data mining pipeline based on KNIME retrieved in total 825 bioactivity data points of macrocycles interacting with SLC-transporter. For further analysis of the SLC inhibitor profiles we developed an interactive KNIME workflow as well as an interactive map of the chemical space coverage utilizing parametric t-SNE models. The parametric t-SNE models provide a good discrimination ability among several corresponding SLC subfamilies' targets. The KNIME workflow, the dataset, and the visualization tool are freely available to the community.

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.

Enhancement of intestinal calcium transport by short-chain fatty acids: roles of Na+/H+ exchanger 3 and transient receptor potential vanilloid subfamily 6

Small organic molecules in the intestinal lumen, particularly short-chain fatty acids (SCFAs) and glucose, have long been postulated to enhance calcium absorption. Here, we used 45Ca radioactive tracer to determine calcium fluxes across the rat intestine after exposure to glucose and SCFAs. Confirming previous reports, glucose was found to increase the apical-to-basolateral calcium flux in the cecum. Under apical glucose-free conditions, SCFAs (e.g., butyrate) stimulated the cecal calcium fluxes by approximately twofold, while having no effect on proximal colon. Since SCFAs could be absorbed into the circulation, we further determined whether basolateral SCFA exposure rendered some positive actions. It was found that exposure of duodenum and cecum on the basolateral side to acetate or butyrate increased calcium fluxes. Under butyrate-rich conditions, cecal calcium transport was partially diminished by Na+/H+ exchanger 3 (NHE3) inhibitor (tenapanor) and nonselective transient receptor potential vanilloid subfamily 6 (TRPV6) inhibitor (miconazole). To confirm the contribution of TRPV6 to SCFA-stimulated calcium transport, we synthesized another TRPV6 inhibitor that was demonstrated by in silico molecular docking and molecular dynamics to occlude TRPV6 pore and diminish the glucose- and butyrate-induced calcium fluxes. Therefore, besides corroborating the importance of luminal molecules in calcium absorption, our findings provided foundation for development of more effective calcium-rich nutraceuticals in combination with various absorptive enhancers, e.g., glucose and SCFAs.NEW & NOTEWORTHY Organic molecules in the intestinal lumen, e.g., glucose and short-chain fatty acids (SCFAs), the latter of which are normally produced by microfloral fermentation, can stimulate calcium absorption dependent on transient receptor potential vanilloid subfamily 6 (TRPV6) and Na+/H+ exchanger 3 (NHE3). A selective TRPV6 inhibitor synthesized and demonstrated by in silico docking and molecular dynamics to specifically bind to the pore domain of TRPV6 was used to confirm a significant contribution of this channel. Our findings corroborate physiological significance of nutrients and SCFAs in enhancing calcium absorption.

NHE7 upregulation potentiates the uptake of small extracellular vesicles by enhancing maturation of macropinosomes in hepatocellular carcinoma

BACKGROUND

Small extracellular vesicles (sEVs) mediate intercellular communication that contributes to hepatocellular carcinoma (HCC) progression via multifaceted pathways. The success of cell entry determines the effect of sEV on recipient cells. Here, we aimed to delineate the mechanisms underlying the uptake of sEV in HCC.

METHODS

Macropinocytosis was examined by the ability of cells to internalize dextran and sEV. Macropinocytosis was analyzed in Na(+)/H(+) exchanger 7 (NHE7)-knockdown and -overexpressing cells. The properties of cells were studied using functional assays. pH biosensor was used to evaluate the intracellular and endosomal pH. The expression of NHE7 in patients' liver tissues was examined by immunofluorescent staining. Inducible silencing of NHE7 in established tumors was performed to reveal the therapeutic potential of targeting NHE7.

RESULTS

The data revealed that macropinocytosis controlled the internalization of sEVs and their oncogenic effect on recipient cells. It was found that metastatic HCC cells exhibited the highest efficiency of sEV uptake relative to normal liver cells and non-metastatic HCC cells. Attenuation of macropinocytic activity by 5-(N-ethyl-N-isopropyl)-amiloride (EIPA) limited the entry of sEVs and compromised cell aggressiveness. Mechanistically, we delineated that high level of NHE7, a sodium-hydrogen exchanger, alkalized intracellular pH and acidized endosomal pH, leading to the maturation of macropinosomes. Inducible inhibition of NHE7 in established tumors developed in mice delayed tumor development and suppressed lung metastasis. Clinically, NHE7 expression was upregulated and linked to dismal prognosis of HCC.

CONCLUSIONS

This study advances the understanding that NHE7 enhances sEV uptake by macropinocytosis to promote the malignant properties of HCC cells. Inhibition of sEV uptake via macropinocytosis can be exploited as a treatment alone or in combination with conventional therapeutic approaches for HCC.

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.

Cranberry Proanthocyanidins Mitigate Reflux-Induced Transporter Dysregulation in an Esophageal Adenocarcinoma Model

We recently reported that cranberry proanthocyanidins (C-PACs) inhibit esophageal adenocarcinoma (EAC) by 83% through reversing reflux-induced bacterial, inflammatory and immune-implicated proteins and genes as well as reducing esophageal bile acids, which drive EAC progression. This study investigated whether C-PACs' mitigation of bile reflux-induced transporter dysregulation mechanistically contributes to EAC prevention. RNA was isolated from water-, C-PAC- and reflux-exposed rat esophagi with and without C-PAC treatment. Differential gene expression was determined by means of RNA sequencing and RT-PCR, followed by protein assessments. The literature, coupled with the publicly available Gene Expression Omnibus dataset GSE26886, was used to assess transporter expression levels in normal and EAC patient biopsies for translational relevance. Significant changes in ATP-binding cassette (ABC) transporters implicated in therapeutic resistance in humans (i.e., Abcb1, Abcb4, Abcc1, Abcc3, Abcc4, Abcc6 and Abcc10) and the transport of drugs, xenobiotics, lipids, and bile were altered in the reflux model with C-PACs' mitigating changes. Additionally, C-PACs restored reflux-induced changes in solute carrier (SLC), aquaporin, proton and cation transporters (i.e., Slc2a1, Slc7a11, Slc9a1, Slco2a1 and Atp6v0c). This research supports the suggestion that transporters merit investigation not only for their roles in metabolism and therapeutic resistance, but as targets for cancer prevention and targeting preventive agents in combination with chemotherapeutics.

Targeting NHE6 gene expression identifies lysosome and neurodevelopmental mechanisms in a haploid in vitro cell model

Christianson syndrome (CS) is an X-linked disorder resulting from loss-of-function (LoF) mutations in SLC9A6 encoding the endosomal Na+/H+ exchanger 6 (NHE6). CS presents with developmental delay, seizures, intellectual disability, nonverbal status, postnatal microcephaly, and ataxia. To define transcriptome signatures of NHE6 LoF, we conducted in-depth RNA-sequencing (RNA-seq) analysis on a haploid NHE6 null cell model. CRIPSR/Cas9 genome editing introduced multiple LoF mutations into SLC9A6 in the near haploid human cell line Hap1. Isogenic, paired parental controls were also studied. NHE6 mutant cell lines were confirmed to have intra-endosomal over-acidification as was seen in other NHE6 null cells. RNA-seq analysis was performed by two widely used pipelines: HISAT2-StringTie-DEseq2 and STAR-HTseq-DEseq2. We identified 1056 differentially expressed genes in mutant NHE6 lines, including genes associated with neurodevelopment, synapse function, voltage-dependent calcium channels, and neuronal signaling. Weighted gene co-expression network analysis was then applied and identified a critical module enriched for genes governing lysosome function. By identifying significantly changed gene expression that is associated with lysosomal mechanisms in NHE6-null cells, our analyses suggest that loss of NHE6 function may converge on mechanisms implicated in lysosome-related neurologic disease. Further, this haploid cell model will serve as an important tool for translational science in CS.

GGA1 interacts with the endosomal Na+/H+ Exchanger NHE6 governing localization to the endosome compartment

Mutations in the endosomal Na+/H+ exchanger (NHE6) cause Christianson syndrome (CS), an X-linked neurological disorder. Previous studies have shown that NHE6 functions in regulation of endosome acidification and maturation in neurons. Using yeast two-hybrid screening with the NHE6 carboxyl-terminus as bait, we identify Golgi-associated, Gamma adaptin ear containing, ARF binding protein 1 (GGA1) as an interacting partner for NHE6. We corroborated the NHE6-GGA1 interaction using co-immunoprecipitation (co-IP): using over-expressed constructs in mammalian cells; and co-IP of endogenously-expressed GGA1 and NHE6 from neuroblastoma cells, as well as from mouse brain. We demonstrate that GGA1 interacts with organellar NHEs (NHE6, NHE7 and NHE9) but not with cell-surface localized NHEs (NHE1 and NHE5). By constructing hybrid NHE1/NHE6 exchangers, we demonstrate that the cytoplasmic tail of NHE6 is necessary and sufficient for interactions with GGA1. We demonstrate the co-localization of NHE6 and GGA1 in cultured, primary hippocampal neurons, using super-resolution microscopy. We test the hypothesis that the interaction of NHE6 and GGA1 functions in the localization of NHE6 to the endosome compartment. Using subcellular fractionation experiments, we show that NHE6 is mis-localized in GGA1 knockout cells wherein we find less NHE6 in endosomes but more NHE6 transport to lysosomes, and more Golgi retention of NHE6 with increased exocytosis to the surface plasma membrane. Consistent with NHE6 mis-localization, and Golgi retention, we find the intra-luminal pH in Golgi to be alkalinized. Our study demonstrates a new interaction between NHE6 and GGA1 which functions in the localization of this intra-cellular NHE to the endosome compartment.

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.

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.

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.

Probing Single-Cell Fermentation Fluxes and Exchange Networks via pH-Sensing Hybrid Nanofibers

The homeostatic control of their environment is an essential task of living cells. It has been hypothesized that, when microenvironmental pH inhomogeneities are induced by high cellular metabolic activity, diffusing protons act as signaling molecules, driving the establishment of exchange networks sustained by the cell-to-cell shuttling of overflow products such as lactate. Despite their fundamental role, the extent and dynamics of such networks is largely unknown due to the lack of methods in single-cell flux analysis. In this study, we provide direct experimental characterization of such exchange networks. We devise a method to quantify single-cell fermentation fluxes over time by integrating high-resolution pH microenvironment sensing via ratiometric nanofibers with constraint-based inverse modeling. We apply our method to cell cultures with mixed populations of cancer cells and fibroblasts. We find that the proton trafficking underlying bulk acidification is strongly heterogeneous, with maximal single-cell fluxes exceeding typical values by up to 3 orders of magnitude. In addition, a crossover in time from a networked phase sustained by densely connected "hubs" (corresponding to cells with high activity) to a sparse phase dominated by isolated dipolar motifs (i.e., by pairwise cell-to-cell exchanges) is uncovered, which parallels the time course of bulk acidification. Our method addresses issues ranging from the homeostatic function of proton exchange to the metabolic coupling of cells with different energetic demands, allowing for real-time noninvasive single-cell metabolic flux analysis.

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.

Atypical Macropinocytosis Contributes to Malignant Progression: A Review of Recent Evidence in Endometrioid Endometrial Cancer Cells

Simple Summary

A novel type of macropinocytosis has been identified as a trigger for the malignant progression of endometrial cancer. Transiently reducing epithelial barrier homeostasis leads to macropinocytosis by splitting between adjacent cells in endometrioid endometrial cancer. Macropinocytosis causes morphological changes in well-differentiated to poorly differentiated cancer cells. Inhibition of macropinocytosis promotes a persistent dormant state in the intrinsic KRAS-mutated cancer cell line Sawano. This review focuses on the mechanisms of atypical macropinocytosis and its effects on cellular function, and it describes the physiological processes involved in inducing resting conditions in endometrioid endometrial cancer cells.

Abstract

Macropinocytosis is an essential mechanism for the non-specific uptake of extracellular fluids and solutes. In recent years, additional functions have been identified in macropinocytosis, such as the intracellular introduction pathway of drugs, bacterial and viral infection pathways, and nutritional supplement pathway of cancer cells. However, little is known about the changes in cell function after macropinocytosis. Recently, it has been reported that macropinocytosis is essential for endometrial cancer cells to initiate malignant progression in a dormant state. Macropinocytosis is formed by a temporary split of adjacent bicellular junctions of epithelial sheets, rather than from the apical surface or basal membrane, as a result of the transient reduction of tight junction homeostasis. This novel type of macropinocytosis has been suggested to be associated with the malignant pathology of endometriosis and endometrioid endometrial carcinoma. This review outlines the induction of malignant progression of endometrial cancer cells by macropinocytosis based on a new mechanism and the potential preventive mechanism of its malignant progression.

Cardiac Sodium/Hydrogen Exchanger (NHE11) as a Novel Potential Target for SGLT2i in Heart Failure: A Preliminary Study

Despite the reduction of cardiovascular events, including the risk of death, associated with sodium/glucose cotransporter 2 inhibitors (SGLT2i), their basic action remains unclear. Sodium/hydrogen exchanger (NHE) has been proposed as the mechanism of action, but there are controversies related to its function and expression in heart failure (HF). We hypothesized that sodium transported-related molecules could be altered in HF and modulated through SGLT2i. Transcriptome alterations in genes involved in sodium transport in HF were investigated in human heart samples by RNA-sequencing. NHE11 and NHE1 protein levels were determined by ELISA; the effect of empagliflozin on NHE11 and NHE1 mRNA levels in rats’ left ventricular tissues was studied through RT-qPCR. We highlighted the overexpression of SLC9C2 and SCL9A1 sodium transport genes and the increase of the proteins that encode them (NHE11 and NHE1). NHE11 levels were correlated with left ventricular diameters, so we studied the effect of SGLT2i on its expression, observing that NHE11 mRNA levels were reduced in treated rats. We showed alterations in several sodium transports and reinforced the importance of these channels in HF progression. We described upregulation in NHE11 and NHE1, but only NHE11 correlated with human cardiac dysfunction, and its levels were reduced after treatment with empagliflozin. These results propose NHE11 as a potential target of SGLT2i in cardiac tissue.

Loss of endosomal exchanger NHE6 leads to pathological changes in tau in human neurons

Disruption of endolysosomal and autophagy-lysosomal systems is increasingly implicated in neurodegeneration. Sodium-proton exchanger 6 (NHE6) contributes to the maintenance of proper endosomal pH, and loss-of function mutations in the X-linked NHE6 lead to Christianson syndrome (CS) in males. Neurodegenerative features of CS are increasingly recognized, with postmortem and clinical data implicating a role for tau. We generated cortical neurons from NHE6 knockout (KO) and isogenic wild-type control human induced pluripotent stem cells. We report elevated phosphorylated and sarkosyl-insoluble tau in NHE6 KO neurons. We demonstrate that NHE6 KO leads to lysosomal and autophagy dysfunction involving reduced lysosomal number and protease activity, diminished autophagic flux, and p62 accumulation. Finally, we show that treatment with trehalose or rapamycin, two enhancers of autophagy-lysosomal function, each partially rescue this tau phenotype. We provide insight into the neurodegenerative processes underlying NHE6 loss of function and into the broader role of the endosome-lysosome-autophagy network in neurodegeneration.

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.

Systematic in silico discovery of novel solute carrier-like proteins from proteomes

Solute carrier (SLC) proteins represent the largest superfamily of transmembrane transporters. While many of them play key biological roles, their systematic analysis has been hampered by their functional and structural heterogeneity. Based on available nomenclature systems, we hypothesized that many as yet unidentified SLC transporters exist in the human genome, which await further systematic analysis. Here, we present criteria for defining "SLC-likeness" to curate a set of "SLC-like" protein families from the Transporter Classification Database (TCDB) and Protein families (Pfam) databases. Computational sequence similarity searches surprisingly identified ~120 more proteins in human with potential SLC-like properties compared to previous annotations. Interestingly, several of these have documented transport activity in the scientific literature. To complete the overview of the "SLC-ome", we present an algorithm to classify SLC-like proteins into protein families, investigating their known functions and evolutionary relationships to similar proteins from 6 other clinically relevant experimental organisms, and pinpoint structural orphans. We envision that our work will serve as a stepping stone for future studies of the biological function and the identification of the natural substrates of the many under-explored SLC transporters, as well as for the development of new therapeutic applications, including strategies for personalized medicine and drug delivery.

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.

Na+/H+ Exchanger 1, a Potential Therapeutic Drug Target for Cardiac Hypertrophy and Heart Failure

Cardiac hypertrophy is defined as increased heart mass in response to increased hemodynamic requirements. Long-term cardiac hypertrophy, if not counteracted, will ultimately lead to heart failure. The incidence of heart failure is related to myocardial infarction, which could be salvaged by reperfusion and ultimately invites unfavorable myocardial ischemia-reperfusion injury. The Na⁺/H⁺ exchangers (NHEs) are membrane transporters that exchange one intracellular proton for one extracellular Na⁺. The first discovered NHE isoform, NHE1, is expressed almost ubiquitously in all tissues, especially in the myocardium. During myocardial ischemia-reperfusion, NHE1 catalyzes increased uptake of intracellular Na⁺, which in turn leads to Ca²⁺ overload and subsequently myocardial injury. Numerous preclinical research has shown that NHE1 is involved in cardiac hypertrophy and heart failure, but the exact molecular mechanisms remain elusive. The objective of this review is to demonstrate the potential role of NHE1 in cardiac hypertrophy and heart failure and investigate the underlying mechanisms.

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. In addition, 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.

Effects of inhibitors of SLC9A-type sodium-protein exchangers on Survival Motor Neuron 2 (SMN2) mRNA splicing and expression

Spinal muscular atrophy (SMA) is an autosomal recessive, pediatric-onset disorder caused by the loss of spinal motor neurons thereby leading to muscle atrophy. SMA is caused by the loss of or mutations in the survival motor neuron 1 (SMN1) gene. SMN1 is duplicated in humans to give rise to the paralogous SMN2 gene. This paralog is nearly identical except for a cytosine to thymine (C-to-T) transition within an exonic splicing enhancer (ESE) element within exon 7. As a result, the majority of SMN2 transcripts lack exon 7 (SMNΔ7) which produces a truncated and unstable SMN protein. Since SMN2 copy number is inversely related to disease severity, it is a well-established target for SMA therapeutics development. 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an inhibitor of sodium/proton exchangers (NHEs), has previously been shown to increase exon 7 inclusion and SMN protein levels in SMA cells. In this study, NHE inhibitors were evaluated for their ability to modulate SMN2 expression. EIPA as well as 5-(N,N-hexamethylene)amiloride (HMA) increase exon 7 inclusion in SMN2 splicing reporter lines as well as in SMA fibroblasts. The EIPA-induced exon 7 inclusion occurs via a unique mechanism that does not involve previously identified splicing factors. Transcriptome analysis identified novel targets, including TIA1 and FABP3, for further characterization. EIPA and HMA are more selective at inhibiting the NHE5 isoform, which is expressed in fibroblasts as well as in neuronal cells. These results show that NHE5 inhibition increases SMN2 expression and may be a novel target for therapeutics development. Significance Statement This study demonstrates a molecular mechanism by which inhibitors of the sodium-protein exchanger increase the alternative splicing of SMN2 in spinal muscular atrophy cells. NHE5 selective inhibitors increase the inclusion of full-length SMN2 mRNAs by targeting TIA1 and FABP3 expression, which is distinct from other small molecule regulators of SMN2 alternative splicing. This study provides a novel means to increase full-length SMN2 expression and a novel target for therapeutics development.

A Review on the Role of Bicarbonate and Proton Transporters during Sperm Capacitation in Mammals

Alkalinization of sperm cytosol is essential for plasma membrane hyperpolarization, hyperactivation of motility, and acrosomal exocytosis during sperm capacitation in mammals. The plasma membrane of sperm cells contains different ion channels implicated in the increase of internal pH (pH_i) by favoring either bicarbonate entrance or proton efflux. Bicarbonate transporters belong to the solute carrier families 4 (SLC4) and 26 (SLC26) and are currently grouped into Na⁺/HCO₃⁻ transporters and Cl⁻/HCO₃⁻ exchangers. Na⁺/HCO₃⁻ transporters are reported to be essential for the initial and fast entrance of HCO₃⁻ that triggers sperm capacitation, whereas Cl⁻/HCO₃⁻ exchangers are responsible for the sustained HCO₃⁻ entrance which orchestrates the sequence of changes associated with sperm capacitation. Proton efflux is required for the fast alkalinization of capacitated sperm cells and the activation of pH-dependent proteins; according to the species, this transport can be mediated by Na⁺/H⁺ exchangers (NHE) belonging to the SLC9 family and/or voltage-gated proton channels (HVCN1). Herein, we discuss the involvement of each of these channels in sperm capacitation and the acrosome reaction.

The Less Well-Known Little Brothers: The SLC9B/NHA Sodium Proton Exchanger Subfamily—Structure, Function, Regulation and Potential Drug-Target Approaches

The SLC9 gene family encodes Na+/H+ exchangers (NHEs), a group of membrane transport proteins critically involved in the regulation of cytoplasmic and organellar pH, cell volume, as well as systemic acid-base and volume homeostasis. NHEs of the SLC9A subfamily (NHE 1–9) are well-known for their roles in human physiology and disease. Much less is known about the two members of the SLC9B subfamily, NHA1 and NHA2, which share higher similarity to prokaryotic NHEs than the SLC9A paralogs. NHA2 (also known as SLC9B2) is ubiquitously expressed and has recently been shown to participate in renal blood pressure and electrolyte regulation, insulin secretion and systemic glucose homeostasis. In addition, NHA2 has been proposed to contribute to the pathogenesis of polycystic kidney disease, the most common inherited kidney disease in humans. NHA1 (also known as SLC9B1) is mainly expressed in testis and is important for sperm motility and thus male fertility, but has not been associated with human disease thus far. In this review, we present a summary of the structure, function and regulation of expression of the SLC9B subfamily members, focusing primarily on the better-studied SLC9B paralog, NHA2. Furthermore, we will review the potential of the SLC9B subfamily as drug targets.

The Role of Plasma Membrane Sodium/Hydrogen Exchangers in Gastrointestinal Functions: Proliferation and Differentiation, Fluid/Electrolyte Transport and Barrier Integrity

The five plasma membrane Na⁺/H⁺ exchanger (NHE) isoforms in the gastrointestinal tract are characterized by distinct cellular localization, tissue distribution, inhibitor sensitivities, and physiological regulation. NHE1 (Slc9a1) is ubiquitously expressed along the gastrointestinal tract in the basolateral membrane of enterocytes, but so far, an exclusive role for NHE1 in enterocyte physiology has remained elusive. NHE2 (Slc9a2) and NHE8 (Slc9a8) are apically expressed isoforms with ubiquitous distribution along the colonic crypt axis. They are involved in pH_i regulation of intestinal epithelial cells. Combined use of a knockout mouse model, intestinal organoid technology, and specific inhibitors revealed previously unrecognized actions of NHE2 and NHE8 in enterocyte proliferation and differentiation. NHE3 (Slc9a3), expressed in the apical membrane of differentiated intestinal epithelial cells, functions as the predominant nutrient-independent Na⁺ absorptive mechanism in the gut. The new selective NHE3 inhibitor (Tenapanor) allowed discovery of novel pathophysiological and drug-targetable NHE3 functions in cystic-fibrosis associated intestinal obstructions. NHE4, expressed in the basolateral membrane of parietal cells, is essential for parietal cell integrity and acid secretory function, through its role in cell volume regulation. This review focuses on the expression, regulation and activity of the five plasma membrane Na⁺/H⁺ exchangers in the gastrointestinal tract, emphasizing their role in maintaining intestinal homeostasis, or their impact on disease pathogenesis. We point to major open questions in identifying NHE interacting partners in central cellular pathways and processes and the necessity of determining their physiological role in a system where their endogenous expression/activity is maintained, such as organoids derived from different parts of the gastrointestinal tract.

Acute Stress in Lesser-Spotted Catshark (Scyliorhinus canicula Linnaeus, 1758) Promotes Amino Acid Catabolism and Osmoregulatory Imbalances

Simple Summary

In catsharks (Scyliorhinus canicula), air exposure induces amino acid catabolism altogether with osmoregulatory imbalances. This study describes a novel NHE isoform being expressed in gills that may be involved in ammonium excretion.

Abstract

Acute-stress situations in vertebrates induce a series of physiological responses to cope with the event. While common secondary stress responses include increased catabolism and osmoregulatory imbalances, specific processes depend on the taxa. In this sense, these processes are still largely unknown in ancient vertebrates such as marine elasmobranchs. Thus, we challenged the lesser spotted catshark (Scyliorhinus canicula) to 18 min of air exposure, and monitored their recovery after 0, 5, and 24 h. This study describes amino acid turnover in the liver, white muscle, gills, and rectal gland, and plasma parameters related to energy metabolism and osmoregulatory imbalances. Catsharks rely on white muscle amino acid catabolism to face the energy demand imposed by the stressor, producing NH₄⁺. While some plasma ions (K⁺, Cl⁻ and Ca²⁺) increased in concentration after 18 min of air exposure, returning to basal values after 5 h of recovery, Na⁺ increased after just 5 h of recovery, coinciding with a decrease in plasma NH₄⁺. These changes were accompanied by increased activity of a branchial amiloride-sensitive ATPase. Therefore, we hypothesize that this enzyme may be a Na⁺/H⁺ exchanger (NHE) related to NH₄⁺ excretion. The action of an omeprazole-sensitive ATPase, putatively associated to a H⁺/K⁺-ATPase (HKA), is also affected by these allostatic processes. Some complementary experiments were carried out to delve a little deeper into the possible branchial enzymes sensitive to amiloride, including in vivo and ex vivo approaches, and partial sequencing of a nhe1 in the gills. This study describes the possible presence of an HKA enzyme in the rectal gland, as well as a NHE in the gills, highlighting the importance of understanding the relationship between acute stress and osmoregulation in elasmobranchs.

The Na+/H+ Exchanger 3 in the Intestines and the Proximal Tubule of the Kidney: Localization, Physiological Function, and Key Roles in Angiotensin II-Induced Hypertension

The sodium (Na⁺)/hydrogen (H⁺) exchanger 3 (NHE3) is one of the most important Na⁺/H⁺ antiporters in the small intestines of the gastrointestinal tract and the proximal tubules of the kidney. The roles of NHE3 in the regulation of intracellular pH and acid-base balance have been well established in cellular physiology using in vitro techniques. Localized primarily on the apical membranes in small intestines and proximal tubules, the key action of NHE3 is to facilitate the entry of luminal Na⁺ and the extrusion of intracellular H⁺ from intestinal and proximal tubule tubular epithelial cells. NHE3 is, directly and indirectly, responsible for absorbing the majority of ingested Na⁺ from small and large intestines and reabsorbing >50% of filtered Na⁺ in the proximal tubules of the kidney. However, the roles of NHE3 in the regulation of proximal tubular Na⁺ transport in the integrative physiological settings and its contributions to the basal blood pressure regulation and angiotensin II (Ang II)-induced hypertension have not been well studied previously due to the lack of suitable animal models. Recently, novel genetically modified mouse models with whole-body, kidney-specific, or proximal tubule-specific deletion of NHE3 have been generated by us and others to determine the critical roles and underlying mechanisms of NHE3 in maintaining basal body salt and fluid balance, blood pressure homeostasis, and the development of Ang II-induced hypertension at the whole-body, kidney, or proximal tubule levels. The objective of this invited article is to review, update, and discuss recent findings on the critical roles of intestinal and proximal tubule NHE3 in maintaining basal blood pressure homeostasis and their potential therapeutic implications in the development of angiotensin II (Ang II)-dependent hypertension.

Characteristics and molecular mechanisms through which SGLT2 inhibitors improve metabolic diseases: A mechanism review

Metabolic diseases, such as diabetes, gout and hyperlipidemia are global health challenges. Among them, diabetes has been extensively investigated. Type 2 diabetes mellitus (T2DM), which is characterized by hyperglycemia, is a complex metabolic disease that is associated with various metabolic disorders. The newly developed oral hypoglycemic agent, sodium-glucose cotransporter 2 (SGLT2) inhibitor, has been associated with glucose-lowering effects and it affects metabolism in various ways. However, the potential mechanisms of SGLT2 inhibitors in metabolic diseases have not fully reviewed. Many of the effects beyond glycemic control must be considered off-target effects. Therefore, we reviewed the effects of SGLT2 inhibition on metabolic diseases such as obesity, hypertension, hyperlipidemia, hyperuricemia, fatty liver disease, insulin resistance, osteoporosis and fractures. Moreover, we elucidated their molecular mechanisms to provide a theoretical basis for metabolic disease treatment.

Current Methods to Unravel the Functional Properties of Lysosomal Ion Channels and Transporters

A distinct set of channels and transporters regulates the ion fluxes across the lysosomal membrane. Malfunctioning of these transport proteins and the resulting ionic imbalance is involved in various human diseases, such as lysosomal storage disorders, cancer, as well as metabolic and neurodegenerative diseases. As a consequence, these proteins have stimulated strong interest for their suitability as possible drug targets. A detailed functional characterization of many lysosomal channels and transporters is lacking, mainly due to technical difficulties in applying the standard patch-clamp technique to these small intracellular compartments. In this review, we focus on current methods used to unravel the functional properties of lysosomal ion channels and transporters, stressing their advantages and disadvantages and evaluating their fields of applicability.

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.

The Roles of Solute Carriers in Auditory Function

Solute carriers (SLCs) are important transmembrane transporters with members organized into 65 families. They play crucial roles in transporting many important molecules, such as ions and some metabolites, across the membrane, maintaining cellular homeostasis. SLCs also play important roles in hearing. It has been found that mutations in some SLC members are associated with hearing loss. In this review, we summarize SLC family genes related with hearing dysfunction to reveal the vital roles of these transporters in auditory function. This summary could help us understand the auditory physiology and the mechanisms of hearing loss and further guide future studies of deafness gene identification.

A phylogenetic analysis between humans and D. melanogaster: A repertoire of solute carriers in humans and flies

The solute carrier (SLC) superfamily is the largest group of transporters in humans, with the role to transport solutes across plasma membranes. The SLCs are currently divided into 65 families with 430 members. Here, we performed a detailed mining of the SLC superfamily and the recent annotated family of "atypical" SLCs in human and D. melanogaster using Hidden Markov Models and PSI-BLAST. Our analyses identified 381 protein sequences in D. melanogaster and of those, 55 proteins have not been previously identified in flies. In total, 11 of the 65 human SLC families were found to not be conserved in flies, while a few families are highly conserved, which perhaps reflects the families' functions and roles in cellular pathways. This study provides the first collection of all SLC sequences in D. melanogaster and can serve as a SLC database to be used for classification of SLCs in other phyla.

Physiological Perspectives on Molecular Mechanisms and Regulation of Vesicular Glutamate Transport: Lessons From Calyx of Held Synapses

Accumulation of glutamate, the primary excitatory neurotransmitter in the mammalian central nervous system, into presynaptic synaptic vesicles (SVs) depends upon three vesicular glutamate transporters (VGLUTs). Since VGLUTs are driven by a proton electrochemical gradient across the SV membrane generated by vacuolar-type H⁺-ATPases (V-ATPases), the rate of glutamate transport into SVs, as well as the amount of glutamate in SVs at equilibrium, are influenced by activities of both VGLUTs and V-ATPase. Despite emerging evidence that suggests various factors influencing glutamate transport by VGLUTs in vitro, little has been reported in physiological or pathological contexts to date. Historically, this was partially due to a lack of appropriate methods to monitor glutamate loading into SVs in living synapses. Furthermore, whether or not glutamate refilling of SVs can be rate-limiting for synaptic transmission is not well understood, primarily due to a lack of knowledge concerning the time required for vesicle reuse and refilling during repetitive stimulation. In this review, we first introduce a unique electrophysiological method to monitor glutamate refilling by VGLUTs in a giant model synapse from the calyx of Held in rodent brainstem slices, and we discuss the advantages and limitations of the method. We then introduce the current understanding of factors that potentially alter the amount and rate of glutamate refilling of SVs in this synapse, and discuss open questions from physiological viewpoints.

Receptor Guanylyl Cyclase C and Cyclic GMP in Health and Disease: Perspectives and Therapeutic Opportunities

Receptor Guanylyl Cyclase C (GC-C) was initially characterized as an important regulator of intestinal fluid and ion homeostasis. Recent findings demonstrate that GC-C is also causally linked to intestinal inflammation, dysbiosis, and tumorigenesis. These advances have been fueled in part by identifying mutations or changes in gene expression in GC-C or its ligands, that disrupt the delicate balance of intracellular cGMP levels and are associated with a wide range of clinical phenotypes. In this review, we highlight aspects of the current knowledge of the GC-C signaling pathway in homeostasis and disease, emphasizing recent advances in the field. The review summarizes extra gastrointestinal functions for GC-C signaling, such as appetite control, energy expenditure, visceral nociception, and behavioral processes. Recent research has expanded the homeostatic role of GC-C and implicated it in regulating the ion-microbiome-immune axis, which acts as a mechanistic driver in inflammatory bowel disease. The development of transgenic and knockout mouse models allowed for in-depth studies of GC-C and its relationship to whole-animal physiology. A deeper understanding of the various aspects of GC-C biology and their relationships with pathologies such as inflammatory bowel disease, colorectal cancer, and obesity can be leveraged to devise novel therapeutics.

Sodium hydrogen exchanger (NHE1) palmitoylation and potential functional regulation

AIMS

Determine the effect of palmitoylation on the sodium hydrogen exchanger isoform 1 (NHE1), a member of the SLC9 family.

MAIN METHODS

NHE1 expressed in native rat tissues or in heterologous cells was assessed for palmitoylation by acyl-biotinyl exchange (ABE) and metabolic labeling with [3H]palmitate. Cellular palmitoylation was inhibited using 2-bromopalmitate (2BP) followed by determination of NHE1 palmitoylation status, intracellular pH, stress fiber formation, and cell migration. In addition, NHE1 was activated with LPA treatment followed by determination of NHE1 palmitoylation status and LPA-induced change in intracellular pH was determined in the presence and absence of preincubation with 2BP.

KEY FINDINGS

In this study we demonstrate for the first time that NHE1 is palmitoylated in both cells and rat tissue, and that processes controlled by NHE1 including intracellular pH (pHi), stress fiber formation, and cell migration, are regulated in concert with NHE1 palmitoylation status. Importantly, LPA stimulates NHE1 palmitoylation, and 2BP pretreatment dampens LPA-induced increased pHi which is dependent on the presence of NHE1.

SIGNIFICANCE

Palmitoylation is a reversible lipid modification that regulates an array of critical protein functions including activity, trafficking, membrane microlocalization and protein-protein interactions. Our results suggest that palmitoylation of NHE1 and other control/signaling proteins play a major role in NHE1 regulation that could significantly impact multiple critical cellular functions.

The recycling regulation of sodium-hydrogen exchanger isoform 3(NHE3) in epithelial cells

As the main exchanger of electroneutral NaCl absorption, sodium-hydrogen exchanger isoform 3 (NHE3) circulates in the epithelial brush border (BB) and intracellular compartments in a multi-protein complex. The size of the NHE3 complex changes during rapid regulation events. Recycling regulation of NHE3 in epithelial cells can be roughly divided into three stages. First, when stimulated by Ca²⁺, cGMP, and cAMP-dependent signaling pathways, NHE3 is converted from an immobile complex found at the apical microvilli (MV) into an easily internalized and mobile form that relocates to a compartment near the base of the MV. Second, NHE3 is internalized by clathrin and albumin-dependent pathways into cytoplasmic endosomal compartments, where the complex is reprocessed and reassembled. Finally, NHE3 is translocated from the recycling endosomes (REs) to the apex of epithelial cells, a process that can be stimulated by an increase in sodium-glucose cotransporter 1 (SGLT1) activity, epidermal growth factor receptor (EGFR) signaling, Ca²⁺ signaling, and binding to βPix and SH3 and multiple ankyrin repeat domains 2 (Shank2) proteins. This review describes the molecular steps and protein interactions involved in the recycling movement of NHE3 from the apex of epithelial cells, into vesicles, where it is reprocessed and reassembled, and returned to its original location on the plasma membrane, where it exerts its physiological function.

Rare variant analysis in eczema identifies exonic variants in DUSP1, NOTCH4 and SLC9A4

Previous genome-wide association studies revealed multiple common variants involved in eczema but the role of rare variants remains to be elucidated. Here, we investigate the role of rare variants in eczema susceptibility. We meta-analyze 21 study populations including 20,016 eczema cases and 380,433 controls. Rare variants are imputed with high accuracy using large population-based reference panels. We identify rare exonic variants in DUSP1, NOTCH4, and SLC9A4 to be associated with eczema. In DUSP1 and NOTCH4 missense variants are predicted to impact conserved functional domains. In addition, five novel common variants at SATB1-AS1/KCNH8, TRIB1/LINC00861, ZBTB1, TBX21/OSBPL7, and CSF2RB are discovered. While genes prioritized based on rare variants are significantly up-regulated in the skin, common variants point to immune cell function. Over 20% of the single nucleotide variant-based heritability is attributable to rare and low-frequency variants. The identified rare/low-frequency variants located in functional protein domains point to promising targets for novel therapeutic approaches to eczema.

Genetic studies of eczema to date have mostly explored common genetic variation. Here, the authors perform a large meta-analysis for common and rare variants and discover 8 loci associated with eczema. Over 20% of the heritability of the condition is attributable to rare variants.

Na+/H+-Exchanger Family as Novel Prognostic Biomarkers in Colorectal Cancer

Background

The acidic characteristics of the tumor microenvironment (TME) are attributed to cancer cells' needs of metabolism which produce a large amount of H⁺. In order not to affect its own life activities, it needs to release H⁺ into the intercellular space through an efficient Na⁺/H⁺ exchanger. On account of the intestine whose physiological function is highly dependent on intestinal pH value, NHE family members may play a critical role in the occurrence and development of colorectal cancer (CRC).

Methods

TCGA, GEPIA2, ONCOMINE, UALCAN, STRING, TIMER, Cytoscape, TargetScan, ENCORI, LncBase v.2, DNMIVD, HPA, and CellMiner^TM databases were used in our study.

Results

The mRNA expressions of SLC9A1, SLC9A2, SLC9A3, and SLC9A9 were evidently lower in COAD than in normal samples; however, the mRNA expressions of SLC9A5, SLC9A8, and SLC9B2 were higher. Besides, mRNA expressions of NHE family were extremely associated with clinicopathological features, tumor immune microenvironment and stemness score, DNA methylation, and patient prognosis in COAD. Moreover, we conjectured that NHE family may play a role through MAPK or ErbB signaling pathway according to the results of GO/KEGG enrichment analysis. At last, we found that NHE family members were key factors of various kinds of cancers.

Conclusion

Our study indicated that NHE family represented new diagnostic and therapeutic targets for CRC, which could have important significance for the clinical treatment of CRC.

Halocins, natural antimicrobials of Archaea: Exotic or special or both?

Haloarchaea are adapted to survive under extreme saline conditions by accumulating osmolytes and salts to counteract the high osmotic pressure in their habitats. As a consequence, their proteins have evolved to remain active, or even most active, at very high ionic strength. Halocins are proteinaceous antimicrobial substances that are ribosomally-synthesized by haloarchaea and they provide the producers an advantage in the competition for nutrients and ecological niches. These antimicrobials are stable at high temperature, elevated salt concentrations, and alkaline pH conditions. These properties have endowed them with great potential in diverse biotechnological applications, which involve extreme processing conditions (such as high salt concentrations, high pressure, or high temperatures). They kill target cells by inhibition of Na⁺/H⁺ antiporter in the membrane or modification/disruption of the cell membrane leading to cell lysis. In general, the taxonomy of haloarchaea and their typical phenotypic and genotypic characteristics are well studied; however, information regarding their halocins, especially aspects related to genetics, biosynthetic pathways, mechanism of action, and structure-function relationship is very limited. A few studies have demonstrated the potential applications of halocins in the preservation of salted food products and brine-cured hides in leather industries, protecting the myocardium from ischemia and reperfusion injury, as well as from life-threatening diseases such as cardiac arrest and cancers. In recent years, genome mining has been an essential tool to decipher the genetic basis of halocin biosynthesis. Nevertheless, this is likely the tip of the iceberg as genome analyses have revealed many putative halocins in databases waiting for further investigation. Identification and characterization of this source of halocins may lead to antimicrobials for future therapeutics and/or food preservation. Hence, the present review analyzes different aspects of halocins such as biosynthesis, mechanism of action against target cells, and potential biotechnological applications.

Genome-wide DNA methylation analysis of dogs with high lead exposure living near a lead mining area in Kabwe, Zambia

Lead (Pb) is a heavy metal that has been proven to be toxic to both animals and humans. Genom-wide DNA methylation in domestic dogs exposed to high levels of Pb in Kabwe, Zambia was analyzed in this study. Using next-generation sequencing on samples from 20 domestic dogs (mean blood Pb concentration: 43.6 μg/dL and 7.2 μg/dL in the high and low exposure groups), a digital restriction enzyme analysis of methylation was performed to identify the genomic locations of differentially methylated CpG sites. A validation study on an additional 20 dogs followed (blood Pb concentration: 4.9-29.7 μg/dL). The cluster analysis resolved two broad clusters indicating high and low Pb exposure. The study identified 827 (1.2%) CpG sites with differences in methylation (101 CpG sites were hypermethylated in the low exposure group and 726 were hypermethylated in the high exposure group). The sites corresponded to 26 genes with differentially methylated CpG sites at their promoter regions, including the NGF gene. The methylation of four CpG sites was validated using bisulfite pyrosequencing. The results indicate that aberrant hypermethylation is prevalent in dogs exposed to Pb. The altered DNA methylation of the genes identified in this study contributes to a greater understanding of the epigenetic changes caused by Pb exposure and highlights novel biomarker discoveries across species.

Translocation of LSR from tricellular corners causes macropinocytosis at cell-cell interface as a trigger for breaking out of contact inhibition

Withdrawal from contact inhibition is necessary for epithelial cancer precursor cells to initiate cell growth and motility. Nevertheless, little is understood about the mechanism for the sudden initiation of cell growth under static conditions. We focused on cellular junctions as one region where breaking out of contact inhibition occurs. In well-differentiated endometrial cancer cells, Sawano, the ligand administration for tricellular tight junction protein LSR, which transiently decreased the robust junction property, caused an abrupt increase in cell motility and consequent excessive multilayered cell growth despite being under contact inhibition conditions. We observed that macropinocytosis essentially and temporarily occurred as an antecedent event for the above process at intercellular junctions without disruption of the junction apparatus but not at the apical plasma membrane. Collectively, we concluded that the formation of macropinocytosis, which is derived from tight junction-mediated signaling, was triggered for the initiation of cell growth in static precancerous epithelium.

Protons to Patients: targeting endosomal Na+ /H+ exchangers against COVID-19 and other viral diseases

While there is undeniable evidence to link endosomal acid‐base homeostasis to viral pathogenesis, the lack of druggable molecular targets has hindered translation from bench to bedside. The recent identification of variants in the interferon‐inducible endosomal Na⁺/H⁺ exchanger 9 associated with severe coronavirus disease‐19 (COVID‐19) has brought a shift in the way we envision aberrant endosomal acidification. Is it linked to an increased susceptibility to viral infection or a propensity to develop critical illness? This review summarizes the genetic and cellular evidence linking endosomal Na⁺/H⁺ exchangers and viral diseases to suggest how they can act as a broad‐spectrum modulator of viral infection and downstream pathophysiology. The review also presents novel insights supporting the complex role of endosomal acid‐base homeostasis in viral pathogenesis and discusses the potential causes for negative outcomes of clinical trials utilizing alkalinizing drugs as therapies for COVID‐19. These findings lead to a pathogenic model of viral disease that predicts that nonspecific targeting of endosomal pH might fail, even if administered early on, and suggests that endosomal Na⁺/H⁺ exchangers may regulate key host antiviral defence mechanisms and mediators that act to drive inflammatory organ injury.

Analysis of performance, bone characteristics, and expression of genes involved in the balance of ionic concentrations in broilers subjected to dietary electrolyte balance levels

1. Sodium bicarbonate (NaHCO₃), potassium carbonate (K₂CO₃), and ammonium chloride (NH₄Cl) are commonly used to correct dietary electrolyte balance (DEB) in birds. However, there are many gaps in the knowledge of their effects when used simultaneously. This study investigated the effect of DEB levels on performance, femur bone characteristics and the expression of genes related to the balance of ionic concentrations in broilers at 21 days of age.2. Male Cobb broiler chickens (n = 245), aged 1-21 d, were divided into groups based on a completely randomised design with five DEB levels (110 mEq/kg, 175 mEq/kg, 240 mEq/kg, 305 mEq/kg, and 370 mEq/kg).3. The performance characteristics measured included body weight (BW), body weight gain (BWG), feed intake (FI), feed conversion (FCR) and body weight birds slaughtered (BWS).4. The bone variables assessed in the femur were weight (WE), relative bone weight (RBWE), length (L), width (WI), maximum load supported (MLS), bone-breaking resistance (BR), and Seedor index (SI). In addition, the expression of CHP1, SLC9A1, and SLC24A3 in the livers, intestines and kidneys of birds was evaluated.5. The DEB level of 370 mEq/kg, at Na⁺ content of 0.48%, resulted in the highest averages for L, MLS, and BR of the femur. This DEB level increased the expression of SLC9A1 in the liver and SLC24A3 in the intestine. A 240 mEq/kg DEB level decreased the expression of CHP1 in the liver, while supplementation with 110 mEq/kg increased the expression of SLC24A3 in the kidney.6. In conclusion, 370 mEq/kg DEB improved FCR and increased the mean bone characteristics of the femur (L, MLS, and BR) and the expression of SLC9A1 and SLC24A3 in the liver and intestine, respectively. These findings should be considered in future assessments of the effects of DEB levels on broilers.

Decreased Brain pH and Pathophysiology in Schizophrenia

Postmortem studies reveal that the brain pH in schizophrenia patients is lower than normal. The exact cause of this low pH is unclear, but increased lactate levels due to abnormal energy metabolism appear to be involved. Schizophrenia patients display distinct changes in mitochondria number, morphology, and function, and such changes promote anaerobic glycolysis, elevating lactate levels. pH can affect neuronal activity as H⁺ binds to numerous proteins in the nervous system and alters the structure and function of the bound proteins. There is growing evidence of pH change associated with cognition, emotion, and psychotic behaviors. Brain has delicate pH regulatory mechanisms to maintain normal pH in neurons/glia and extracellular fluid, and a change in these mechanisms can affect, or be affected by, neuronal activities associated with schizophrenia. In this review, we discuss the current understanding of the cause and effect of decreased brain pH in schizophrenia based on postmortem human brains, animal models, and cellular studies. The topic includes the factors causing decreased brain pH in schizophrenia, mitochondria dysfunction leading to altered energy metabolism, and pH effects on the pathophysiology of schizophrenia. We also review the acid/base transporters regulating pH in the nervous system and discuss the potential contribution of the major transporters, sodium hydrogen exchangers (NHEs), and sodium-coupled bicarbonate transporters (NCBTs), to schizophrenia.