cDNA cloning and expression of the cardiac Na+/Ca2+ exchanger from Mozambique tilapia (Oreochromis mossambicus) reveal a teleost membrane transporter with mammalian temperature dependence

Effects of change in temperature on the cardiac contractility of broad-snouted caiman (Caiman latirostris) during digestion

In many reptiles, digestion has been associated with the selection of higher body temperatures, the so-called post-prandial thermophilic response. This study aimed to investigate the excitation-contraction (E-C) coupling in postprandial broad-snouted caimans (Caiman latirostris) in response to acute warming within a preferred body temperature range of crocodiles. Isometric preparations subjected to a temperature transition from 25°C to 30°C were used to investigate myocardial contractility of postprandial caimans, that is, 48 h after the animals ingested a rodent meal corresponding to 15% of body mass. The caiman heart exhibits a negative force-frequency relationship that is independent of the temperature. At 25°C, cardiac muscle was able to maintain a constant force up to 36 bpm, above which it decreased significantly, reaching minimum values at the highest frequency of 84 bpm. Moreover, E-C coupling is predominantly dependent on transsarcolemmal Ca²⁺ transport denoted by the lack of significant ryanodine effects on force generation. On the contrary, ventricular strips at 30°C were able to sustain the cardiac contractility at higher pacing frequencies (from 12 to 144 bpm) due to an important role of Na⁺ /Ca²⁺ exchanger in Ca²⁺ cycling, as indicated by the decay of the post-rest contraction, and a significant contribution of the sarcoplasmic reticulum above 72 bpm. Our results demonstrated that the myocardium of postprandial caimans exhibits a significant degree of thermal plasticity of E-C coupling during acute warming. Therefore, myocardial contractility can be maximized when postprandial broad-snouted caimans select higher body temperatures (preferred temperature zone) following feeding.

A Na+/Ca2+ exchanger-like protein (AtNCL) involved in salt stress in Arabidopsis

Calcium ions (Ca(2+)) play a crucial role in many key physiological processes; thus, the maintenance of Ca(2+) homeostasis is of primary importance. Na(+)/Ca(2+) exchangers (NCXs) play an important role in Ca(2+) homeostasis in animal excitable cells. Bioinformatic analysis of the Arabidopsis genome suggested the existence of a putative NCX gene, Arabidopsis NCX-like (AtNCL), encoding a protein with an NCX-like structure and different from Ca(2+)/H(+) exchangers and Na(+)/H(+) exchangers previously identified in plant. AtNCL was identified to localize in the Arabidopsis cell membrane fraction, have the ability of binding Ca(2+), and possess NCX-like activity in a heterologous expression system of cultured mammalian CHO-K1 cells. AtNCL is broadly expressed in Arabidopsis, and abiotic stresses stimulated its transcript expression. Loss-of-function atncl mutants were less sensitive to salt stress than wild-type or AtNCL transgenic overexpression lines. In addition, the total calcium content in whole atncl mutant seedlings was higher than that in wild type by atomic absorption spectroscopy. The level of free Ca(2+) in the cytosol and Ca(2+) flux at the root tips of atncl mutant plants, as detected using transgenic aequorin and a scanning ion-selective electrode, required a longer recovery time following NaCl stress compared with that in wild type. All of these data suggest that AtNCL encodes a Na(+)/Ca(2+) exchanger-like protein that participates in the maintenance of Ca(2+) homeostasis in Arabidopsis. AtNCL may represent a new type of Ca(2+) transporter in higher plants.

Structural and dynamic aspects of Ca2+ and Mg2+ binding of the regulatory domains of the Na+/Ca2+ exchanger

Intracellular Ca2+ regulates the activity of the NCX (Na+/Ca2+ exchanger) through binding to the cytosolic CBD (Ca2+-binding domain) 1 and CBD2. In vitro studies of the structure and dynamics of CBD1 and CBD2, as well as studies of their kinetics and thermodynamics of Ca2+ binding, greatly enhanced our understanding of NCX regulation. We describe the fold of the CBDs in relation to other known structures and review Ca2+ binding of the different CBD variants from a structural perspective. We also report on new findings concerning Mg2+ binding to the CBDs and finally we discuss recent results on CBD1–CBD2 interdomain interactions.

The plasma membrane Ca²+ ATPase and the plasma membrane sodium calcium exchanger cooperate in the regulation of cell calcium

Calcium is an ambivalent signal: it is essential for the correct functioning of cell life, but may also become dangerous to it. The plasma membrane Ca(2+) ATPase (PMCA) and the plasma membrane Na(+)/Ca(2+) exchanger (NCX) are the two mechanisms responsible for Ca(2+) extrusion. The NCX has low Ca(2+) affinity but high capacity for Ca(2+) transport, whereas the PMCA has a high Ca(2+) affinity but low transport capacity for it. Thus, traditionally, the PMCA pump has been attributed a housekeeping role in maintaining cytosolic Ca(2+), and the NCX the dynamic role of counteracting large cytosolic Ca(2+) variations (especially in excitable cells). This view of the roles of the two Ca(2+) extrusion systems has been recently revised, as the specific functional properties of the numerous PMCA isoforms and splicing variants suggests that they may have evolved to cover both the basal Ca(2+) regulation (in the 100 nM range) and the Ca(2+) transients generated by cell stimulation (in the μM range).

Characterization of zebrafish (Danio rerio) NCX4: a novel NCX with distinct electrophysiological properties

Members of the Na+/Ca2+ exchanger (NCX) family are important regulators of cytosolic Ca2+ in myriad tissues and are highly conserved across a wide range of species. Three distinct NCX genes and numerous splice variants exist in mammals, many of which have been characterized in a variety of heterologous expression systems. Recently, however, we discovered a fourth NCX gene (NCX4), which is found exclusively in teleost, amphibian, and reptilian genomes. Zebrafish (Danio rerio) NCX4a encodes for a protein of 939 amino acids and shows a high degree of identity with known NCXs. Although knockdown of NCX4a activity in zebrafish embryos has been shown to alter left-right patterning, it has not been demonstrated that NCX4a functions as a NCX. In this study, we 1) demonstrated, for the first time, that this gene encodes for a novel NCX; 2) characterized the tissue distribution of zebrafish NCX4a; and 3) evaluated its kinetic and transport properties. While ubiquitously expressed, the highest levels of NCX4a expression occurred in the brain and eyes. NCX4a exhibits modest levels of Na+-dependent inactivation and requires much higher levels of regulatory Ca2+ to activate outward exchange currents. NCX4a also exhibited extremely fast recovery from Na+-dependent inactivation of outward currents, faster than any previously characterized wild-type exchanger. While this result suggests that the Na+-dependent inactive state of NCX4a is far less stable than in other NCX family members, this exchanger was still strongly inhibited by 2 microM exchanger inhibitory peptide. We demonstrated that a new putative member of the NCX gene family, NCX4a, encodes for a NCX with unique functional properties. These data will be useful in understanding the role that NCX4a plays in embryological development as well as in the adult, where it is expressed ubiquitously.

Gene structure evolution of the Na+-Ca2+ exchanger (NCX) family

Background

The Na⁺-Ca²⁺ exchanger (NCX) is an important regulator of cytosolic Ca²⁺ levels. Many of its structural features are highly conserved across a wide range of species. Invertebrates have a single NCX gene, whereas vertebrate species have multiple NCX genes as a result of at least two duplication events. To examine the molecular evolution of NCX genes and understand the role of duplicated genes in the evolution of the vertebrate NCX gene family, we carried out phylogenetic analyses of NCX genes and compared NCX gene structures from sequenced genomes and individual clones.

Results

A single NCX in invertebrates and the protochordate Ciona, and the presence of at least four NCX genes in the genomes of teleosts, an amphibian, and a reptile suggest that a four member gene family arose in a basal vertebrate. Extensive examination of mammalian and avian genomes and synteny analysis argue that NCX4 may be lost in these lineages. Duplicates for NCX1, NCX2, and NCX4 were found in all sequenced teleost genomes. The presence of seven genes encoding NCX homologs may provide teleosts with the functional specialization analogous to the alternate splicing strategy seen with the three NCX mammalian homologs.

Conclusion

We have demonstrated that NCX4 is present in teleost, amphibian and reptilian species but has been secondarily and independently lost in mammals and birds. Comparative studies on conserved vertebrate homologs have provided a possible evolutionary route taken by gene duplicates subfunctionalization by minimizing homolog number.

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

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.

Effects of temperature and calcium availability on cardiac contractility in Synbranchus marmoratus, a neotropical teleost

An isometric muscle preparation was used to investigate the importance of the ventricular sarcoplasmic reticulum (SR) and extracellular Ca2+ (1.25 up to 11.25 mM) to force generation at 25 degrees C (acclimation temperature), 15 and 35 degrees C. The post-rest tension and force-frequency relationship were conducted with and without 10 microM ryanodine in the bathing medium. Increments in extracellular Ca2+ resulted in increases in twitch force development only at 35 degrees C. A significant post-rest potentiation was recorded for the control preparations at 25 degrees C (100% to 119.8+/-4.1%). However, this post-rest potentiation was inhibited by ryanodine only at 25 degrees C (100% to 97.6+/-1.5%). At 35 degrees C, force remained unchanged in the control preparations, but a significant post-rest decay was recorded in the presence of ryanodine (100% to 76.6+/-4.6%) while at 15 degrees C, ryanodine was not able to preventing the post-rest potentiation observed in the control preparations. The increases in the imposed contraction frequency caused a decline of the force at 25 and 35 degrees C and ryanodine decreased significantly peak tension at both temperatures. The findings suggest a high or medium calcium turnover, possibly related to the presence of a functional SR, whose functionality is diminished when temperature is decreased.

Sarcolemmal ion currents and sarcoplasmic reticulum Ca2+content in ventricular myocytes from the cold stenothermic fish, the burbot(Lota lota)

The burbot (Lota lota) is a cold stenothermic fish species whose heart is adapted to function in the cold. In this study we use whole-cell voltage-clamp techniques to characterize the electrophysiological properties of burbot ventricular myocytes and to test the hypothesis that changes in membrane currents and intracellular Ca2+ cycling associated cold-acclimation in other fish species are routine for stenothermic cold-adapted species. Experiments were performed at 4°C, which is the body temperature of burbot for most of the year, and after myocytes were acutely warmed to 11°C, which is in the upper range of temperatures experienced by burbot in nature. Results on K+ channels support our hypothesis as the relative density of K-channel conductances in the burbot heart are similar to those found for cold-acclimated cold-active fish species. IK1 conductance was small (39.2±5.4 pS pF-1 at 4°C and 71.4±1.7 pS pF-1 at 11°C)and IKr was large (199±27 pS pF-1 at 4°C and 320.3±8 pS pF-1 at 11°C) in burbot ventricular myocytes. We found high Na+-Ca2+ exchange(NCX) activity (35.9±6.3 pS pF-1 at 4°C and 58.6±8.4 pS pF-1 at 11°C between -40 and 20 mV),suggesting that it may be the primary pathway for sarcolemmal (SL)Ca2+ influx in this species. In contrast, the density(ICa, 0.81±0.13 pA pF-1 at 4°C, and 1.35±0.18 pA pF-1 at 11°C) and the charge(QCa, 0.24±0.043 pC pF-1 at 4°C and 0.21±0.034 pC pF-1 at 11°C) carried by the l-type Ca2+ current was small. Our results on sarcolemmal ion currents in burbot ventricular myocytes suggest that cold stenothermy and compensative cold-acclimation involve many of the same subcellular adaptations that culminate in enhanced excitability in the cold.