Protein mediated regulation of the NHE1 isoform of the Na + /H + exchanger in renal cells. A regulatory role of Hsp90 and AKT kinase

Shift of conventional paradigm of heart failure treatment: from angiotensin receptor neprilysin inhibitor to sodium-glucose co-transporter 2 inhibitors?

Current clinical guidelines for heart failure (HF) contain a brand new therapeutic strategy for HF with reduced ejection fraction (HFrEF), which is based on neurohumoral modulation through the use of angiotensin receptor neprilysin inhibitors. There is a large body of evidence for the fact that sodium-glucose co-transporter 2 inhibitors may significantly improve all-cause mortality, cardiovascular mortality and hospitalization for HF in patients with HFrEF who received renin-angiotensin system blockers including angiotensin receptor neprilysin inhibitors, β-blockers and mineralocorticoid receptor antagonists. The review discusses that sodium-glucose co-transporter 2 inhibitors have a wide spectrum of favorable molecular effects and contribute to tissue protection, improving survival in HFrEF patients.

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

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

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

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

Amino acid transporter SLC6A14 depends on heat shock protein HSP90 in trafficking to the cell surface

Plasma membrane transporter SLC6A14 transports all neutral and basic amino acids in a Na/Cl - dependent way and it is up-regulated in many types of cancer. Mass spectrometry analysis of overexpressed SLC6A14-associated proteins identified, among others, the presence of cytosolic heat shock proteins (HSPs) and co-chaperones. We detected co-localization of overexpressed and native SLC6A14 with HSP90-beta and HSP70 (HSPA14). Proximity ligation assay confirmed a direct interaction of overexpressed SLC6A14 with both HSPs. Treatment with radicicol and VER155008, specific inhibitors of HSP90 and HSP70, respectively, attenuated these interactions and strongly reduced transporter presence at the cell surface, what resulted from the diminished level of the total transporter protein. Distortion of SLC6A14 proper folding by both HSPs inhibitors directed the transporter towards endoplasmic reticulum-associated degradation pathway, a process reversed by the proteasome inhibitor - bortezomib. As demonstrated in an in vitro ATPase assay of recombinant purified HSP90-beta, the peptides corresponding to C-terminal amino acid sequence following the last transmembrane domain of SLC6A14 affected the HSP90-beta activity. These results indicate that a plasma membrane protein folding can be controlled not only by chaperones in the endoplasmic reticulum, but also those localized in the cytosol.

Structural and Functional Changes in the Na+/H+ Exchanger Isoform 1, Induced by Erk1/2 Phosphorylation

The human Na⁺/H⁺ exchanger isoform 1 (NHE1) is a plasma membrane transport protein that plays an important role in pH regulation in mammalian cells. Because of the generation of protons by intermediary metabolism as well as the negative membrane potential, protons accumulate within the cytosol. Extracellular signal-regulated kinase (ERK)-mediated regulation of NHE1 is important in several human pathologies including in the myocardium in heart disease, as well as in breast cancer as a trigger for growth and metastasis. NHE1 has a N-terminal, a 500 amino acid membrane domain, and a C-terminal 315 amino acid cytosolic domain. The C-terminal domain regulates the membrane domain and its effects on transport are modified by protein binding and phosphorylation. Here, we discuss the physiological regulation of NHE1 by ERK, with an emphasis on the critical effects on structure and function. ERK binds directly to the cytosolic domain at specific binding domains. ERK also phosphorylates NHE1 directly at multiple sites, which enhance NHE1 activity with subsequent downstream physiological effects. The NHE1 cytosolic regulatory tail possesses both ordered and disordered regions, and the disordered regions are stabilized by ERK-mediated phosphorylation at a phosphorylation motif. Overall, ERK pathway mediated phosphorylation modulates the NHE1 tail, and affects the activity, structure, and function of this membrane protein.