Modeling Ca2+ dynamics of mouse cardiac cells points to a critical role of SERCA's affinity for Ca2+

Endoplasmic Reticulum Calcium Pumps and Tumor Cell Differentiation

Endoplasmic reticulum (ER) calcium homeostasis plays an essential role in cellular calcium signaling, intra-ER protein chaperoning and maturation, as well as in the interaction of the ER with other organelles. Calcium is accumulated in the ER by sarco/endoplasmic reticulum calcium ATPases (SERCA enzymes) that generate by active, ATP-dependent transport, a several thousand-fold calcium ion concentration gradient between the cytosol (low nanomolar) and the ER lumen (high micromolar). SERCA enzymes are coded by three genes that by alternative splicing give rise to several isoforms, which can display isoform-specific calcium transport characteristics. SERCA expression levels and isoenzyme composition vary according to cell type, and this constitutes a mechanism whereby ER calcium homeostasis is adapted to the signaling and metabolic needs of the cell, depending on its phenotype, its state of activation and differentiation. As reviewed here, in several normal epithelial cell types including bronchial, mammary, gastric, colonic and choroid plexus epithelium, as well as in mature cells of hematopoietic origin such as pumps are simultaneously expressed, whereas in corresponding tumors and leukemias SERCA3 expression is selectively down-regulated. SERCA3 expression is restored during the pharmacologically induced differentiation of various cancer and leukemia cell types. SERCA3 is a useful marker for the study of cell differentiation, and the loss of SERCA3 expression constitutes a previously unrecognized example of the remodeling of calcium homeostasis in tumors.

The Golgi Apparatus: Panel Point of Cytosolic Ca(2+) Regulation

The Golgi apparatus (GA), an intermediate organelle of the cell inner membrane system, plays a key role in protein glycosylation and secretion. In recent years, this organelle has been found to act as a vital intracellular Ca(2+) store because different Ca (2+) regulators, such as the inositol-1,4,5-triphosphate receptor, sarco/endoplasmic reticulum Ca(2+) -ATPase and secretory pathway Ca 2+ -ATPase, were demonstrated to localize on their membrane. The mechanisms involved in Ca(2+) release and uptake in the GA have now been established.Here, based on careful backward looking on compartments and patterns in GA Ca (2+) regulation, we review neurological diseases related to GA calcium remodeling and propose a modified cytosolic Ca(2+) adjustment model, in which GA acts as part of the panel point.

The sensitivity of fast muscle contractile function to the major components of the sarcomere Ca(2+)-cycling system

A reaction-diffusion model of a muscle sarcomere was developed to evaluate the sensitivity of force characteristics to diffusion and Ca(2+)-cycling components. The model compared well to experimental force measurements. Diffusion led to Ca(2+) gradients that enhanced maximal force and accelerated relaxation compared to when diffusion was infinitely fast. However, a modest increase in sarcomere length or radius led to a decrease in maximal force. Lowering the Ca(2+) release rate caused a lower maximal force, but increasing the rate led to only modest gains in maximal force while incurring much greater ATP costs associated with reuptake. Greater parvalbumin binding rates decreased maximal force but enhanced relaxation, and this effect was magnified when Ca(2+) uptake rates were lowered as may occur during fatigue. These results show a physiological set of parameters that lead to a functional sarcomere of known dimensions and contractile function, and the effects of parameter variation on muscle function.

Measuring Ca2+-dependent Ca2+-uptake activity in the mouse heart

The apparent Ca(2+) affinity of the isoforms of the sarco/endoplasmic reticulum Ca(2+) ATPase SERCA2 is controlled primarily by two proteins, phospholamban (PLB) and sarcolipin (SLN). The rate of ATP-driven Ca(2+) uptake into sarcoplasmic reticulum (SR)-derived vesicles can be monitored by a technique in which the net uptake of (45)Ca(2+) in the form of an intravesicular calcium oxalate precipitate is recorded. Here, we present details of a modification of such a protocol for determining the apparent Ca(2+) affinity of the Ca(2+) pump, and its control by various regulators, in crude homogenates of mouse heart.

Chronic lead exposure increases blood pressure and myocardial contractility in rats

We investigated the cardiovascular effects of lead exposure, emphasising its direct action on myocardial contractility. Male Wistar rats were sorted randomly into two groups: control (Ct) and treatment with 100 ppm of lead (Pb) in the drinking water. Blood pressure (BP) was measured weekly. At the end of the treatment period, the animals were anaesthetised and haemodynamic parameters and contractility of the left ventricular papillary muscles were recorded. Blood and tissue samples were properly stored for further biochemical investigations. Statistical analyses were considered to be significant at p<0.05. The lead concentrations in the blood reached approximately 13 µg/dL, while the bone was the site of the highest deposition of this metal. BP in the Pb-treated group was higher from the first week of lead exposure and remained at the same level over the next four weeks. Haemodynamic evaluations revealed increases in systolic (Ct: 96±3.79 vs. Pb: 116±1.37 mmHg) and diastolic blood pressure (Ct: 60±2.93 vs. Pb: 70±3.38 mmHg), left ventricular systolic pressure (Ct: 104±5.85 vs. Pb: 120±2.51 mmHg) and heart rate (Ct: 307±10 vs. Pb: 348±16 bpm). Lead treatment did not alter the force and time derivatives of the force of left ventricular papillary muscles that were contracting isometrically. However, our results are suggestive of changes in the kinetics of calcium (Ca⁺⁺) in cardiomyocytes increased transarcolemmal Ca⁺⁺ influx, low Ca⁺⁺ uptake by the sarcoplasmic reticulum and high extrusion by the sarcolemma. Altogether, these results show that despite the increased Ca⁺⁺ influx that was induced by lead exposure, the myocytes had regulatory mechanisms that prevented increases in force, as evidenced in vivo by the increased systolic ventricular pressure.

Cytoplasmic nanojunctions between lysosomes and sarcoplasmic reticulum are required for specific calcium signaling

Herein we demonstrate how nanojunctions between lysosomes and sarcoplasmic reticulum (L-SR junctions) serve to couple lysosomal activation to regenerative, ryanodine receptor-mediated cellular Ca ²⁺ waves. In pulmonary artery smooth muscle cells (PASMCs) it has been proposed that nicotinic acid adenine dinucleotide phosphate (NAADP) triggers increases in cytoplasmic Ca ²⁺ via L-SR junctions, in a manner that requires initial Ca ²⁺ release from lysosomes and subsequent Ca ²⁺-induced Ca ²⁺ release (CICR) via ryanodine receptor (RyR) subtype 3 on the SR membrane proximal to lysosomes. L-SR junction membrane separation has been estimated to be < 400 nm and thus beyond the resolution of light microscopy, which has restricted detailed investigations of the junctional coupling process. The present study utilizes standard and tomographic transmission electron microscopy to provide a thorough ultrastructural characterization of the L-SR junctions in PASMCs. We show that L-SR nanojunctions are prominent features within these cells and estimate that the junctional membrane separation and extension are about 15 nm and 300 nm, respectively. Furthermore, we develop a quantitative model of the L-SR junction using these measurements, prior kinetic and specific Ca ²⁺ signal information as input data. Simulations of NAADP-dependent junctional Ca ²⁺ transients demonstrate that the magnitude of these signals can breach the threshold for CICR via RyR3. By correlation analysis of live cell Ca ²⁺ signals and simulated Ca ²⁺ transients within L-SR junctions, we estimate that “trigger zones” comprising 60–100 junctions are required to confer a signal of similar magnitude. This is compatible with the 110 lysosomes/cell estimated from our ultrastructural observations. Most importantly, our model shows that increasing the L-SR junctional width above 50 nm lowers the magnitude of junctional [Ca ²⁺] such that there is a failure to breach the threshold for CICR via RyR3. L-SR junctions are therefore a pre-requisite for efficient Ca ²⁺signal coupling and may contribute to cellular function in health and disease.

Is the lack of adiponectin associated with increased ER/SR stress and inflammation in the heart?

Objective To study whether there is an association between adiponectin and endoplasmic reticulum/sarcoplasmic reticulum (ERSR) stress. Research design Eleven-month-old male wild-type (WT) and adiponectin knockout (ADKO) mice were placed on chow or high fat diet for 12 weeks. The changes in ER stress and inflammatory genes were determined in the epididymal adipose, as well as heart tissue of adult WT and ADKO mice. To understand the role of ER/SR stress in the regulation of adiponectin, we studied the effect of tunicamycin or palmitate on H9C2 cardiomyoblasts in culture. To demonstrate the protective role of adiponectin, we studied the effect of purified adiponectin on the regulation of ERSR stress genes and inflammation in H9C2 cardiomyoblasts. Results (1) High fat diet increased TNFα in adipose tissue of ADKO mice. (2) ERSR stress genes, HSPa5, ERN1, and GADD34, and inflammation response genes, TNFα and CD68, were increased in heart of ADKO mice. High fat diet did not further increase the effect. (3) Induction of ERSR stress by tunicamycin in H9C2 resulted in the upregulation of ERSR stress response genes along with downregulation of adiponectin, adiponectin receptors 1 and 2, and Serca2A. ER stress was accompanied by down regulation of Iкβα and an increase in HSPa5 proteins. (4) Adiponectin decreased ERSR stress and inflammation response genes and increased Serca2A in to H9C2 cardiomyoblasts. Conclusion The lack of adiponectin is associated with increased ER/SR stress and inflammation in the heart. Adiponectin provides a protective effect by lowering inflammation and ER/SR stress along with increasing Serca2A in H9C2 cells.

Improving cardiac Ca⁺² transport into the sarcoplasmic reticulum in heart failure: lessons from the ubiquitous SERCA2b Ca⁺² pump

As a major Ca2+ pump in the sarcoplasmic reticulum of the cardiomyocyte, SERCA2a (sarcoplasmic/endoplasmic reticulum Ca2+-ATPase 2a) controls the relaxation and contraction of the cardiomyocyte. It is meticulously regulated by adapting its expression levels and affinity for Ca2+ ions to the physiological demand of the heart. Dysregulation of the SERCA2a activity entails poor cardiomyocyte contractility, resulting in heart failure. Conversely, improving cardiac SERCA2a activity, e.g. by boosting its expression level or by increasing its affinity for Ca2+, is a promising strategy to rescue contractile dysfunction of the failing heart. The structures of the related SERCA1a Ca2+ pump and the Na+/K+-ATPase of the plasma membrane exposed the pumping mechanism and conserved domain architecture of these ion pumps. However, how the Ca2+ affinity of SERCA2a is regulated at the molecular level remained unclear. A structural and functional analysis of the closely related SERCA2b Ca2+ pump, i.e. the housekeeping Ca2+ pump found in the endoplasmic reticulum and the only SERCA isoform characterized by a high Ca2+ affinity, aimed to fill this gap. We demonstrated the existence of a novel and highly conserved site on the SERCA2 pump mediating Ca2+ affinity regulation by the unique C-terminus of SERCA2b (2b-tail). It differs from the earlier-described target site of the affinity regulator phospholamban. Targeting this novel site may provide a new approach to improve SERCA2a function in the failing heart. Strikingly, the intramembrane interaction site of the 2b-tail in SERCA2b shares sequence and structural homology with the binding site of the β-subunit on the α Na+/K+-ATPase. Thus P-type ATPases seem to have developed related mechanisms of regulation, and it is a future challenge for us to discover these general principles of P-type regulation.