Na+/Ca2+ exchangers: three mammalian gene families control Ca2+ transport

Abstract

Mammalian Na+/Ca2+ exchangers are members of three branches of a much larger family of transport proteins [the CaCA (Ca2+/cation antiporter) superfamily] whose main role is to provide control of Ca2+ flux across the plasma membranes or intracellular compartments. Since cytosolic levels of Ca2+ are much lower than those found extracellularly or in sequestered stores, the major function of Na+/Ca2+ exchangers is to extrude Ca2+ from the cytoplasm. The exchangers are, however, fully reversible and thus, under special conditions of subcellular localization and compartmentalized ion gradients, Na+/Ca2+ exchangers may allow Ca2+ entry and may play more specialized roles in Ca2+ movement between compartments. The NCX (Na+/Ca2+ exchanger) [SLC (solute carrier) 8] branch of Na+/Ca2+ exchangers comprises three members: NCX1 has been most extensively studied, and is broadly expressed with particular abundance in heart, brain and kidney, NCX2 is expressed in brain, and NCX3 is expressed in brain and skeletal muscle. The NCX proteins subserve a variety of roles, depending upon the site of expression. These include cardiac excitation-contraction coupling, neuronal signalling and Ca2+ reabsorption in the kidney. The NCKX (Na2+/Ca2+-K+ exchanger) (SLC24) branch of Na+/Ca2+ exchangers transport K+ and Ca2+ in exchange for Na+, and comprises five members: NCKX1 is expressed in retinal rod photoreceptors, NCKX2 is expressed in cone photoreceptors and in neurons throughout the brain, NCKX3 and NCKX4 are abundant in brain, but have a broader tissue distribution, and NCKX5 is expressed in skin, retinal epithelium and brain. The NCKX proteins probably play a particularly prominent role in regulating Ca2+ flux in environments which experience wide and frequent fluctuations in Na+ concentration. Until recently, the range of functions that NCKX proteins play was generally underappreciated. This situation is now changing rapidly as evidence emerges for roles including photoreceptor adaptation, synaptic plasticity and skin pigmentation. The CCX (Ca2+/cation exchanger) branch has only one mammalian member, NCKX6 or NCLX (Na+/Ca2+-Li+ exchanger), whose physiological function remains unclear, despite a broad pattern of expression.

Expression and function of Na+/Ca2+ exchangers 1 and 3 in human macrophages and monocytes

“…Current knowledge indicates that the overall functional and pharmacological properties of the three exchangers are similar [7,32]. Therefore, under certain circumstances, the loss of function of one exchanger may be compensated by the other.…”

“…39: 1405-1418 DOI 10.1002 Leukocyte signaling 1405 described as the result of alternative splicing of the three genes [9][10][11]. NCX1 is largely predominant in mammalian tissues such as heart, brain, and kidney while the expression of NCX2 and NCX3 appears to be limited to the brain and skeletal muscle [7]. NCX is a key element regulating intracellular Ca 21 concentrations ([Ca 21 ] i ) in many excitable cells, including myocardiocytes, retinal rod photoreceptors, smooth muscle cells, and neurons [3].…”

“…Three isoforms of NCX, NCX1, NCX2, and NCX3 have been cloned in mammals, outlining a multigene family whose members share a similar molecular structure [7]. NCX is modeled with nine transmembrane segments involved in ion transport and a long central cytoplasmic loop that has regulatory functions [8].…”

“…However, truncated forms lacking certain transmembrane segments, but still possessing NCX activity, have also been described as the result of alternative splicing of the three genes [9][10][11]. NCX1 is largely predominant in mammalian tissues such as heart, brain, and kidney while the expression of NCX2 and NCX3 appears to be limited to the brain and skeletal muscle [7].…”

“…However, in some cases it can contribute to the Ca 21 influx into the cells by working in the reverse mode (i.e. coupling Ca 21 influx with Na 1 efflux) [4][5][6].Three isoforms of NCX, NCX1, NCX2, and NCX3 have been cloned in mammals, outlining a multigene family whose members share a similar molecular structure [7]. NCX is modeled with nine transmembrane segments involved in ion transport and a long central cytoplasmic loop that has regulatory functions [8].…”