Transcriptional regulation of Na+/H+ exchanger expression in the intact mouse

SLC9 Gene Family: Function, Expression, and Regulation

The Slc9 family of Na⁺ /H⁺ exchangers (NHEs) plays a critical role in electroneutral exchange of Na⁺ and H⁺ in the mammalian intestine as well as other absorptive and secretory epithelia of digestive organs. These transport proteins contribute to the transepithelial Na⁺ and water absorption, intracellular pH and cellular volume regulation as well as the electrolyte, acid-base, and fluid volume homeostasis at the systemic level. They also influence the function of other membrane transport mechanisms, affect cellular proliferation and apoptosis as well as cell migration, adherence to the extracellular matrix, and tissue repair. Additionally, they modulate the extracellular milieu to facilitate other nutrient absorption and to regulate the intestinal microbial microenvironment. Na⁺ /H⁺ exchange is inhibited in selected gastrointestinal diseases, either by intrinsic factors (e.g., bile acids, inflammatory mediators) or infectious agents and associated bacterial toxins. Disrupted NHE activity may contribute not only to local and systemic electrolyte imbalance but also to the disease severity via multiple mechanisms. In this review, we describe the cation proton antiporter superfamily of Na⁺ /H⁺ exchangers with a particular emphasis on the eight SLC9A isoforms found in the digestive tract, followed by a more integrative description in their roles in each of the digestive organs. We discuss regulatory mechanisms that determine the function of Na⁺ /H⁺ exchangers as pertinent to the digestive tract, their regulation in pathological states of the digestive organs, and reciprocally, the contribution of dysregulated Na⁺ /H⁺ exchange to the disease pathogenesis and progression. © 2018 American Physiological Society. Compr Physiol 8:555-583, 2018.

Mammalian Glutamyl Aminopeptidase Genes (ENPEP) and Proteins: Comparative Studies of a Major Contributor to Arterial Hypertension

Glutamyl aminopeptidase (ENPEP) is a member of the M1 family of endopeptidases which are mammalian type II integral membrane zinc-containing endopeptidases. ENPEP is involved in the catabolic pathway of the renin-angiotensin system forming angiotensin III, which participates in blood pressure regulation and blood vessel formation. Comparative ENPEP amino acid sequences and structures and ENPEP gene locations were examined using data from several mammalian genome projects. Mammalian ENPEP sequences shared 71-98% identities. Five N-glycosylation sites were conserved for all mammalian ENPEP proteins examined although 9-18 sites were observed, in each case. Sequence alignments, key amino acid residues and predicted secondary and tertiary structures were also studied, including transmembrane and cytoplasmic sequences and active site residues. Highest levels of human ENPEP expression were observed in the terminal ileum of the small intestine and in the kidney cortex. Mammalian ENPEP genes contained 20 coding exons. The human ENPEP gene promoter and first coding exon contained a CpG island (CpG27) and at least 6 transcription factor binding sites, whereas the 3'-UTR region contained 7 miRNA target sites, which may contribute to the regulation of ENPEP gene expression in tissues of the body. Phylogenetic analyses examined the relationships of mammalian ENPEP genes and proteins, including primate, other eutherian, marsupial and monotreme sources, using chicken ENPEP as a primordial sequence for comparative purposes.

Simulated diabetic ketoacidosis therapy in vitro elicits brain cell swelling via sodium-hydrogen exchange and anion transport

A common complication of type 1 diabetes mellitus is diabetic ketoacidosis (DKA), a state of severe insulin deficiency. A potentially harmful consequence of DKA therapy in children is cerebral edema (DKA-CE); however, the mechanisms of therapy-induced DKA-CE are unknown. Our aims were to identify the DKA treatment factors and membrane mechanisms that might contribute specifically to brain cell swelling. To this end, DKA was induced in juvenile mice with the administration of the pancreatic toxins streptozocin and alloxan. Brain slices were prepared and exposed to DKA-like conditions in vitro. Cell volume changes were imaged in response to simulated DKA therapy. Our experiments showed that cell swelling was elicited with isolated DKA treatment components, including alkalinization, insulin/alkalinization, and rapid reductions in osmolality. Methyl-isobutyl-amiloride, a nonselective inhibitor of sodium-hydrogen exchangers (NHEs), reduced cell swelling in brain slices elicited with simulated DKA therapy (in vitro) and decreased brain water content in juvenile DKA mice administered insulin and rehydration therapy (in vivo). Specific pharmacological inhibition of the NHE1 isoform with cariporide also inhibited cell swelling, but only in the presence of the anion transport (AT) inhibitor 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid. DKA did not alter brain NHE1 isoform expression, suggesting that the cell swelling attributed to the NHE1 was activity dependent. In conclusion, our data raise the possibility that brain cell swelling can be elicited by DKA treatment factors and that it is mediated by NHEs and/or coactivation of NHE1 and AT.

Na+/H+ exchanger 1 inhibition reverses manifestation of peripheral diabetic neuropathy in type 1 diabetic rats

Evidence for an important role for Na(+)/H(+) exchangers in diabetic complications is emerging. The aim of this study was to evaluate whether Na(+)/H(+) exchanger 1 inhibition reverses experimental peripheral diabetic neuropathy. Control and streptozotocin-diabetic rats were treated with the specific Na(+)/H(+) exchanger 1 inhibitor cariporide for 4 wk after 12 wk without treatment. Neuropathy end points included sciatic motor and sensory nerve conduction velocities, endoneurial nutritive blood flow, vascular reactivity of epineurial arterioles, thermal nociception, tactile allodynia, and intraepidermal nerve fiber density. Advanced glycation end product and markers of oxidative stress, including nitrated protein levels in sciatic nerve, were evaluated by Western blot. Rats with 12-wk duration of diabetes developed motor and sensory nerve conduction deficits, thermal hypoalgesia, tactile allodynia, and intraepidermal nerve fiber loss. All these changes, including impairment of nerve blood flow and vascular reactivity of epineurial arterioles, were partially reversed by 4 wk of cariporide treatment. Na(+)/H(+) exchanger 1 inhibition was also associated with reduction of diabetes-induced accumulation of advanced glycation endproduct, oxidative stress, and nitrated proteins in sciatic nerve. In conclusion, these findings support an important role for Na(+)/H(+) exchanger 1 in functional, structural, and biochemical manifestations of peripheral diabetic neuropathy and provide the rationale for development of Na(+)/H(+) exchanger 1 inhibitors for treatment of diabetic vascular and neural complications.

Sgk1 sensitivity of Na(+)/H(+) exchanger activity and cardiac remodeling following pressure overload

Sustained increase of cardiac workload is known to trigger cardiac remodeling with eventual development of cardiac failure. Compelling evidence points to a critical role of enhanced cardiac Na(+)/H(+) exchanger (NHE1) activity in the underlying pathophysiology. The signaling triggering up-regulation of NHE1 remained, however, ill defined. The present study explored the involvement of the serum- and glucocorticoid-inducible kinase Sgk1 in cardiac remodeling due to transverse aortic constriction (TAC). To this end, experiments were performed in gene targeted mice lacking functional Sgk1 (sgk1 (-/-)) and their wild-type controls (sgk1 (+/+)). Transcript levels have been determined by RT-PCR, cytosolic pH (pH( i )) utilizing 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein (BCECF) fluorescence, Na(+)/H(+) exchanger activity by the Na(+)-dependent realkalinization after an ammonium pulse, ejection fraction (%) utilizing cardiac cine magnetic resonance imaging and cardiac glucose uptake by PET imaging. As a result, TAC increased the mRNA expression of Sgk1 in sgk1 (+/+) mice, paralleled by an increase in Nhe1 transcript levels as well as Na(+)/H(+) exchanger activity, all effects virtually abrogated in sgk1 (-/-) mice. In sgk1 (+/+) mice, TAC induced a decrease in Pgc1a mRNA expression, while Spp1 mRNA expression was increased, both effects diminished in the sgk1 (-/-) mice. TAC was followed by a significant increase of heart and lung weight in sgk1 (+/+) mice, an effect significantly blunted in sgk1 (-/-) mice. TAC increased the transcript levels of Anp and Bnp, effects again significantly blunted in sgk1 (-/-) mice. TAC increased transcript levels of Collagen I and III as well as Ctgf mRNA and CTGF protein abundance, effects significantly blunted in sgk1 (-/-) mice. TAC further decreased the ejection fraction in sgk1 (+/+) mice, an effect again attenuated in sgk1 (-/-) mice. Also, cardiac FDG-glucose uptake was increased to a larger extent in sgk1 (+/+) mice than in sgk1 (-/-) mice after TAC. These observations point to an important role for SGK1 in cardiac remodeling and development of heart failure following an excessive work load.

Na+/H+ exchanger isoform 1 facilitates cardiomyocyte embryonic stem cell differentiation

Embryonic stem cells provide one potential source of cardiomyocytes for cardiac transplantation; however, after differentiation of stem cells in vitro, cardiomyocytes usually account for only a minority of cells present. To gain insights into improving cardiomyocyte development from stem cells, we examined the role of the Na(+)/H(+) exchanger isoform 1 (NHE1) in cardiomyocyte differentiation. NHE1 protein and message levels were induced by treatment of CGR8 cells to form embryoid bodies and cardiomyocytes. The NHE1 protein was present on the cell surface and NHE1 inhibitor-sensitive activity was detected. Inhibition of NHE1 activity during differentiation of CGR8 cells prevented cardiomyocyte differentiation as indicated by decreased message for transcription factors Nkx2-5 and Tbx5 and decreased levels of alpha-myosin heavy chain protein. Increased expression of NHE1 from an adenoviral vector facilitated cardiomyocyte differentiation. Similar results were found with cardiomyocyte differentiation of P19 embryonal carcinoma cells. CGR8 cells were treated to induce differentiation, but when differentiation was inhibited by dispersing the EBs, myocardial development was inhibited. The results demonstrate that NHE1 activity is important in facilitating stem cell differentiation to cardiomyocyte lineage. Elevated NHE1 expression appears to be triggered as part of the process that facilitates cardiomyocyte development.

Molecular biology of the myocardial Na+/H+ exchanger

The mammalian Na(+)/H(+) exchanger is a pH regulatory membrane protein that uses the sodium gradient to translocate one intracellular proton in exchange for one extracellular sodium. There are nine isoforms of the protein with varying tissue and cellular distribution, some isoforms are predominantly intracellular. In the myocardium, the Na(+)/H(+) exchanger type 1 isoform (NHE1) is the only plasma membrane isoform present in significant quantities. It plays an important role during ischemia/reperfusion damage to the myocardium and has recently been implicated in myocardial hypertrophy. The NHE1 gene is made from 12 exons and a differentially spliced version mediates Na(+)/Li(+) exchange. The NHE1 promoter is regulated by several transcription factors. In the myocardium, transcription factors both proximal and distal to the start site affect expression, including AP-2 and a thyroid responsive element. Recently, reactive oxygen species have also been shown to be important regulators of the NHE1 promoter. Structural and functional analysis of the NHE1 protein has shown that transmembrane segments IV, VII and IX are important in ion transport and susceptibility to pharmacological inhibition. NHE1 protein and mRNA levels are elevated by cardiac ischemia/reperfusion, hypertrophy and acidosis. Understanding the mechanism by which NHE1 mediates transport and its regulation of expression will give novel insights into its contributions in cardiovascular disease.

The cardiac Na-H exchanger: a key downstream mediator for the cellular hypertrophic effects of paracrine, autocrine and hormonal factors

The major mechanism by which the heart cell regulates intracellular pH is the Na+–H+exchanger (NHE) with the NHE-1 isoform as the primary cardiac subtype. Although NHE-1 has been implicated in mediating ischemic injury, more recent evidence implicates the antiporter as a key mediator of hypertrophy, which is produced by various autocrine, paracrine and hormonal factors such as endothelin-1, angiotensin II, and α1adrenoceptor agonists. These agonists activate the antiporter via phosphorylation-dependent processes. NHE-1 inhibition is likely conducive to attenuating the remodelling process after myocardial infarction. These effects probably occur independently of infarct size reduction and involve attenuation of subsequent postinfarction heart failure. As such, inhibitors of NHE offer substantial promise for clinical development that will attenuate acute responses to myocardial postinfarction and chronic pos t infarction, which evolve toward heart failure. The regulation of NHE-1 is discussed as is its potential role in mediating cardiomyocyte hypertrophy.Key words: NHE-1, cardiac hypertrophy, heart failure, myocardial remodelling.

Regulation of Expression of the Na+/H+ Exchanger by Thyroid Hormone

The Na+/H+ exchanger is a pH regulatory protein with a ubiquitous distribution in eukaryotic cells. Several isoforms of the Na+/H+ exchanger are known. The first isoform to be characterized and cloned, NHE1, is present on the plasma membrane of cells and functions to remove one intracellular proton in exchange for one extracellular sodium ion. It is involved in pH regulation, cell growth, differentiation, and cell migration. NHE1 is also involved in the cycle of damage that occurs in the heart with ischemic heart disease. Recent studies have shown that the Na+/H+ exchanger is regulated in response to thyroid hormone. Reduction in circulating thyroid hormone levels reduces the amount of both protein and mRNA of NHE1. Conversely, an elevation of thyroid hormone levels has the opposite effects. Transcriptional regulation of NHE1 expression has been demonstrated. The NHE1 promoter contains a TR alpha(1) binding site located between -841 to -800 bp. This element responds positively to TR alpha(1). This regulation of the NHE1 promoter by thyroid hormone is proposed to be responsible for postnatal changes in expression of the Na+/H+ exchanger.