Biswarup Ghosh

Calcium signaling phenomena in heart diseases: a perspective.

Ca2+ is a major intracellular messenger and nature has evolved multiple mechanisms to regulate free intracellular (Ca2+)i level in situ. The Ca2+ signal inducing contraction in cardiac muscle originates from two sources. Ca2+ enters the cell through voltage dependent Ca2+ channels. This Ca2+ binds to and activates Ca2+ release channels (ryanodine receptors) of the sarcoplasmic reticulum (SR) through a Ca2+ induced Ca2+ release (CICR) process. Entry of Ca2+ with each contraction requires an equal amount of Ca2+ extrusion within a single heartbeat to maintain Ca2+ homeostasis and to ensure relaxation. Cardiac Ca2+ extrusion mechanisms are mainly contributed by Na+/Ca2+ exchanger and ATP dependent Ca2+ pump (Ca2+-ATPase). These transport systems are important determinants of (Ca2+)i level and cardiac contractility. Altered intracellular Ca2+ handling importantly contributes to impaired contractility in heart failure. Chronic hyperactivity of the β-adrenergic signaling pathway results in PKA-hyperphosphorylation of the cardiac RyR/intracellular Ca2+ release channels. Numerous signaling molecules have been implicated in the development of hypertrophy and failure, including the β-adrenergic receptor, protein kinase C, Gq, and the down stream effectors such as mitogen activated protein kinases pathways, and the Ca2+ regulated phosphatase calcineurin. A number of signaling pathways have now been identified that may be key regulators of changes in myocardial structure and function in response to mutations in structural components of the cardiomyocytes. Myocardial structure and signal transduction are now merging into a common field of research that will lead to a more complete understanding of the molecular mechanisms that underlie heart diseases. Recent progress in molecular cardiology makes it possible to envision a new therapeutic approach to heart failure (HF), targeting key molecules involved in intracellular Ca2+ handling such as RyR, SERCA2a, and PLN. Controlling these molecular functions by different agents have been found to be beneficial in some experimental conditions.

Matrix Metalloproteinase‐2‐Mediated Inhibition of Na + ‐Dependent Ca2 + Uptake by Superoxide Radicals (O2. ‐ ) in Microsomes of Pulmonary Smooth Muscle

Treatment of microsomes (preferably enriched with endoplasmic reticulum) isolated from bovine pulmonary artery smooth muscle tissue with the O2. ‐‐generating system (hypoxanthine (HPX) plus xanthine oxidase (XO)), markedly stimulated matrix metalloproteinase‐2 (MMP‐2) activity and also enhanced Ca2 + ATPase activity and ATP‐dependent Ca2 + uptake. Pretreatment with superoxide dismutase (SOD) and tissue inhibitor of metalloproteinase (TIMP‐2) (50 μg ml‐1), preserved the increase in MMP‐2 activity, Ca2 + ATPase activity and also ATP‐dependent Ca2 + uptake in the microsomes. In contrast, Na +‐dependent Ca2 + uptake in the microsomes was found to be inhibited by the O2. ‐‐generating system. Additionally, O2. ‐ ‐induced inhibition of Na + ‐dependent Ca2 + uptake was reversed by SOD and TIMP‐2 (50 μg ml‐1). Electron microscopy revealed that treatment with the O2. ‐‐generating system did not cause any noticeable damage to the microsomes. O2. ‐ ‐induced changes in MMP‐2 activity, ATP‐dependent Ca2 + uptake and Na +‐dependent Ca2 + uptake, were not reversed upon pretreatment of the microsomes with a low dose (5 μg ml‐1) of TIMP‐2 which, on the contrary, reversed MMP‐2 (1 μg ml‐1)‐mediated alteration on these parameters. The inhibition of Na +‐dependent Ca2 + uptake by O2. ‐ and MMP‐2, overpowered the stimulation of ATP‐dependent Ca2 + uptake in the microsomes. Treatment of TIMP‐2 (5 μg ml‐1) with the O2. ‐‐generating system abolished the inhibitory effect of TIMP‐2 (5 μg ml‐1) on MMP‐2 (1 μg ml‐1) (measured by 14C‐gelatin degradation). Overall, the present study suggests that O2. ‐ inactivated TIMP‐2, the ambient inhibitor of MMP‐2, leading to activation of the ambient proteinase, MMP‐2, which subsequently stimulated Ca2 + ATPase activity and ATP‐dependent Ca2 + uptake, but inhibited Na +‐dependent Ca2 + uptake, resulting in a marked decrease in Ca2 + uptake in the smooth muscle microsomes. IUBMB Life, 56: 267‐276, 2004

Role of MMP-2 in inhibiting Na+ dependent Ca2+ uptake by H2O2 in microsomes isolated from pulmonary smooth muscle

Treatment of microsomes (preferentially enriched with endoplasmic reticulum) isolated from bovine pulmonary artery smooth muscle tissue with H2O2 (1 mM) markedly stimulated matrix metalloproteinase activity and also inhibited Na+ dependent Ca2+ uptake. Electron micrograph revealed that H2O2 (1 mM) does not cause any damage to the microsomes. MMP-2 and TIMP-2 were determined to be the ambient protease and corresponding antiprotease of the microsomes. Pretreatment with vitamin E (1 mM) and TIMP-2 (50 μg/ml) reversed the effect produced by H2O2 (1 mM) on Na+ dependent Ca2+ uptake in the microsomes. However, H2O2 (1 mM) caused changes in MMP-2 activity and Na+ dependent Ca2+ uptake were not reversed upon pretreatment of the microsomes with a low concentration of 5 μg/ml of TIMP-2 which otherwise reversed MMP-2 (1 μg/ml) mediated increase in 14C-gelatin degradation and inhibition of Na+ dependent Ca2+ uptake. Combined treatment of the microsomes with a low dose of MMP-2 (0.5 μg/ml) and H2O2 (0.5 mM) inhibited Na+ dependent Ca2+ uptake in the microsomes compared to the respective low dose of either of them. Direct treatment of TIMP-2 (5 μg/ml) with H2O2 (1 mM) abolished the inhibitory effect of the inhibitor on 14C-gelatinolytic activity elicited by 1 μg/ml of MMP-2. Thus, one of the mechanisms by which H2O2 activates MMP-2 could be due to inactivation of TIMP-2 by the oxidant. The resulting activation of MMP-2 subsequently inhibits Na+ dependent Ca2+ uptake in the microsomes. (Mol Cell Biochem 270: 79–87, 2005)