μ-Calpain-mediated deregulation of cardiac, brain, and kidney NCX1 splice variants

The Cardiac Na+ -Ca2+ Exchanger: From Structure to Function

Ca²⁺ homeostasis is essential for cell function and survival. As such, the cytosolic Ca²⁺ concentration is tightly controlled by a wide number of specialized Ca²⁺ handling proteins. One among them is the Na⁺ -Ca²⁺ exchanger (NCX), a ubiquitous plasma membrane transporter that exploits the electrochemical gradient of Na⁺ to drive Ca²⁺ out of the cell, against its concentration gradient. In this critical role, this secondary transporter guides vital physiological processes such as Ca²⁺ homeostasis, muscle contraction, bone formation, and memory to name a few. Herein, we review the progress made in recent years about the structure of the mammalian NCX and how it relates to function. Particular emphasis will be given to the mammalian cardiac isoform, NCX1.1, due to the extensive studies conducted on this protein. Given the degree of conservation among the eukaryotic exchangers, the information highlighted herein will provide a foundation for our understanding of this transporter family. We will discuss gene structure, alternative splicing, topology, regulatory mechanisms, and NCX's functional role on cardiac physiology. Throughout this article, we will attempt to highlight important milestones in the field and controversial topics where future studies are required. © 2021 American Physiological Society. Compr Physiol 12:1-37, 2021.

Cracking the code of sodium/calcium exchanger (NCX) gating: Old and new complexities surfacing from the deep web of secondary regulations

Cell membranes spatially define gradients that drive the complexity of biological signals. To guarantee movements and exchanges of solutes between compartments, membrane transporters negotiate the passages of ions and other important molecules through lipid bilayers. The Na⁺/Ca²⁺ exchangers (NCXs) in particular play central roles in balancing Na⁺ and Ca²⁺ fluxes across diverse proteolipid borders in all eukaryotic cells, influencing cellular functions and fate by multiple means. To prevent progression from balance to disease, redundant regulatory mechanisms cooperate at multiple levels (transcriptional, translational, and post-translational) and guarantee that the activities of NCXs are finely-tuned to cell homeostatic requirements. When this regulatory network is disturbed by pathological forces, cells may approach the end of life. In this review, we will discuss the main findings, controversies and open questions about regulatory mechanisms that control NCX functions in health and disease.

Modulation of the cardiac Na+-Ca2+ exchanger by cytoplasmic protons: Molecular mechanisms and physiological implications

A precise temporal and spatial control of intracellular Ca²⁺ concentration is essential for a coordinated contraction of the heart. Following contraction, cardiac cells need to rapidly remove intracellular Ca²⁺ to allow for relaxation. This task is performed by two transporters: the plasma membrane Na⁺-Ca²⁺ exchanger (NCX) and the sarcoplasmic reticulum (SR) Ca²⁺-ATPase (SERCA). NCX extrudes Ca²⁺ from the cell, balancing the Ca²⁺entering the cytoplasm during systole through L-type Ca²⁺ channels. In parallel, following SR Ca²⁺ release, SERCA activity replenishes the SR, reuptaking Ca²⁺ from the cytoplasm. The activity of the mammalian exchanger is fine-tuned by numerous ionic allosteric regulatory mechanisms. Micromolar concentrations of cytoplasmic Ca²⁺ potentiate NCX activity, while an increase in intracellular Na⁺ levels inhibits NCX via a mechanism known as Na⁺-dependent inactivation. Protons are also powerful inhibitors of NCX activity. By regulating NCX activity, Ca²⁺, Na⁺ and H⁺ couple cell metabolism to Ca²⁺ homeostasis and therefore cardiac contractility. This review summarizes the recent progress towards the understanding of the molecular mechanisms underlying the ionic regulation of the cardiac NCX with special emphasis on pH modulation and its physiological impact on the heart.

Molecular basis of calpain cleavage and inactivation of the sodium-calcium exchanger 1 in heart failure

Cardiac sodium (Na(+))-calcium (Ca(2+)) exchanger 1 (NCX1) is central to the maintenance of normal Ca(2+) homeostasis and contraction. Studies indicate that the Ca(2+)-activated protease calpain cleaves NCX1. We hypothesized that calpain is an important regulator of NCX1 in response to pressure overload and aimed to identify molecular mechanisms and functional consequences of calpain binding and cleavage of NCX1 in the heart. NCX1 full-length protein and a 75-kDa NCX1 fragment along with calpain were up-regulated in aortic stenosis patients and rats with heart failure. Patients with coronary artery disease and sham-operated rats were used as controls. Calpain co-localized, co-fractionated, and co-immunoprecipitated with NCX1 in rat cardiomyocytes and left ventricle lysate. Immunoprecipitations, pull-down experiments, and extensive use of peptide arrays indicated that calpain domain III anchored to the first Ca(2+) binding domain in NCX1, whereas the calpain catalytic region bound to the catenin-like domain in NCX1. The use of bioinformatics, mutational analyses, a substrate competitor peptide, and a specific NCX1-Met(369) antibody identified a novel calpain cleavage site at Met(369). Engineering NCX1-Met(369) into a tobacco etch virus protease cleavage site revealed that specific cleavage at Met(369) inhibited NCX1 activity (both forward and reverse mode). Finally, a short peptide fragment containing the NCX1-Met(369) cleavage site was modeled into the narrow active cleft of human calpain. Inhibition of NCX1 activity, such as we have observed here following calpain-induced NCX1 cleavage, might be beneficial in pathophysiological conditions where increased NCX1 activity contributes to cardiac dysfunction.