Relationship of exercise capacity and left ventricular dimensions in patients with a normal ejection fraction. An exploratory study.

Objectives: Extreme endurance exercise is known to be associated with an enlargement of the left ventricular (LV) chamber, whereas inactivity results in inverse changes. It is unknown if these dimensional relationships exist in patients.

Methods: We analyzed the relationship of exercise capacity and LV dimension in a cohort of sequential patients with a normal ejection fraction undergoing stress echocardiography.

Partial downregulation of junctin enhances cardiac calcium cycling without eliciting ventricular arrhythmias in mice.

Human failing hearts exhibit significant decreases in junctin expression levels with almost nondetectable levels, which may be associated with premature death, induced by lethal cardiac arrhythmias, based on mouse models. However, the specific contribution of junctin to the delayed afterdepolarizations has been difficult to delineate in the phase of increased Na(+)-Ca(2+) exchanger activity accompanying junctin ablation. Thus we characterized the heterozygous junctin-deficient hearts, which expressed 54% of junctin levels and similar increases in Na(+)-Ca(2+) exchanger activity, as the null model.

Sarcoplasmic reticulum calcium overloading in junctin deficiency enhances cardiac contractility but increases ventricular automaticity.

Background: Abnormal sarcoplasmic reticulum calcium (Ca) cycling is increasingly recognized as an important mechanism for increased ventricular automaticity that leads to lethal ventricular arrhythmias. Previous studies have linked lethal familial arrhythmogenic disorders to mutations in the ryanodine receptor and calsequestrin genes, which interact with junctin and triadin to form a macromolecular Ca-signaling complex. The essential physiological effects of junctin and its potential regulatory roles in sarcoplasmic reticulum Ca cycling and Ca-dependent cardiac functions, such as myocyte contractility and automaticity, are unknown.

Junctin is a prominent regulator of contractility in cardiomyocytes.

Junctin is one of the components of the ryanodine receptor Ca release channel complex in sarcoplasmic reticulum. To determine the role of acute alteration of junctin protein levels on cardiomyocyte contractility, we used adenoviral-mediated gene transfer techniques in adult rat cardiomyocytes. Acute downregulation of junctin by 40% resulted in significant increases in cell shortening, rate of contraction (+dL/dt), and rate of relaxation (-dL/dt).

Phospholamban as a therapeutic modality in heart failure.

Increases in diastolic Ca2+ and impaired relaxation in failing hearts have been suggested to reflect the deteriorated function of the sarcoplasmic reticulum Ca-ATPase (SERCA2), whose activity is regulated by phospholamban (PLN). PLN is a reversible inhibitor of SERCA2's Ca2+ affinity and cardiac contractility. Studies in genetically altered mouse models have demonstrated that the levels and the degree of PLN phosphorylation are critical in modulating basal Ca2+ handling and contractility.

The presence of Lys27 instead of Asn27 in human phospholamban promotes sarcoplasmic reticulum Ca2+-ATPase superinhibition and cardiac remodeling.

Background: Phospholamban (PLN) is an inhibitor of the Ca2+ affinity of sarcoplasmic reticulum (SR) Ca2+-ATPase (SERCA2). The amino acid sequence of PLN is highly conserved, and although all species contain asparagine (Asn), human PLN is unique in containing lysine (Lys) at amino acid 27.

Methods And Results: Human PLN was introduced in the null background.

Role of phospholamban phosphorylation on Thr17 in cardiac physiological and pathological conditions.

The sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a) is under the control of a closely associated SR protein named phospholamban (PLN). Dephosphorylated PLN inhibits the SR Ca(2+) pump, whereas phosphorylation of PLN, at either Ser(16) by PKA or Thr(17) by calmodulin-dependent protein kinase II (CaMKII), reverses this inhibition, thus increasing SERCA2a activity and the rate of Ca(2+) uptake by the SR. This would in turn lead to an increase in the velocity of relaxation, SR Ca(2+) load, and myocardial contractility.

Hsp20 and its cardioprotection.

The small heat shock protein Hsp20, also referred to as P20/HspB6, is expressed in the brain, stomach, liver, lung, kidney, blood, smooth muscle, skeletal muscle, and cardiac tissue. Although Hsp20 is not heat-inducible, several cellular signaling pathways appear to regulate its biologic functions. In recent years, tremendous advances have been made in elucidating the significance of Hsp20 in smooth muscle and its potential benefits on coronary vasculature.

Novel cardioprotective role of a small heat-shock protein, Hsp20, against ischemia/reperfusion injury.

Background: Heat-shock proteins (Hsps) have been shown to render cardioprotection from stress-induced injury; however, little is known about the role of another small heat-shock protein, Hsp20, which regulates activities of vasodilation and platelet aggregation, in cardioprotection against ischemia injury. We recently reported that increased expression of Hsp20 in cardiomyocytes was associated with improved contraction and protection against beta-agonist-induced apoptosis.

Methods And Results: To investigate whether overexpression of Hsp20 exerts protective effects in both ex vivo and in vivo ischemia/reperfusion (I/R) injury, we generated a transgenic (TG) mouse model with cardiac-specific overexpression of Hsp20 (10-fold).

Proteomic analysis of hyperdynamic mouse hearts with enhanced sarcoplasmic reticulum calcium cycling.

Depressed sarcoplasmic reticulum (SR) Ca-cycling is a hallmark of human and experimental heart failure. Strategies to improve this impairment by either increasing SERCA2a levels or decreasing phospholamban (PLN) activity have been suggested as promising therapeutic targets. Indeed, ablation of PLN gene in mice was associated with greatly enhanced cardiac Ca-cycling and performance.

Threonine-17 phosphorylation of phospholamban: a key determinant of frequency-dependent increase of cardiac contractility.

Multiple studies have shown that phospholamban (PLN) plays a key role in regulation of frequency-dependent increase of cardiac contraction, a hallmark of the contractile reserve in myocardium. However, the mechanisms underlying this relationship remain elusive. Phosphorylation of PLN occurs on residues: serine-16 (Ser(16)) and threonine-17 (Thr(17)) in vivo.

Small heat-shock protein Hsp20 phosphorylation inhibits beta-agonist-induced cardiac apoptosis.

Activation of the sympathetic nervous system is a common compensatory feature in heart failure, but sustained beta-adrenergic activation induces cardiomyocyte death, leading to cardiac remodeling and dysfunction. In mouse cardiomyocytes, we recently reported that prolonged exposure to beta-agonists is associated with transient increases in expression and phosphorylation of a small heat-shock protein, Hsp20. To determine the functional significance of Hsp20, we overexpressed this protein and its constitutively phosphorylated (S16D) or nonphosphorylated (S16A) mutant in adult rat cardiomyocytes.

Overexpression of phospholamban in slow-twitch skeletal muscle is associated with depressed contractile function and muscle remodeling.

The relative amount of sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) and its crucial inhibitor phospholamban (PLN) are closely regulated and play a pivotal role in maintaining muscle function. The functional importance of PLN has been intensively investigated in cardiac muscle. However, little is known about the role of PLN in the slow-twitch skeletal muscle, which expresses a significantly lower level of PLN but a similar level of SERCA2a compared with cardiac muscle.

Phosphoproteome analysis of cardiomyocytes subjected to beta-adrenergic stimulation: identification and characterization of a cardiac heat shock protein p20.

Posttranslational modification of target substrates underlies biological processes through activation/inactivation of signaling cascades. To concurrently identify the phosphoprotein substrates associated with cardiac beta-adrenergic signaling, the mouse myocyte phosphoproteome was analyzed using 2-D gel electrophoresis in combination with 32P autoradiography. Phosphoprotein spots, detected by silver staining, were identified using MALDI-TOF mass spectrometry in conjunction with computer-assisted protein spot matching.

Chronic SR Ca2+-ATPase inhibition causes adaptive changes in cellular Ca2+ transport.

Phospholamban, the critical regulator of the cardiac SERCA2a Ca2+ affinity, is phosphorylated at Ser16 and Thr17 during beta-adrenergic stimulation (eg, isoproterenol). To assess the impact of nonphosphorylatable phospholamban, a S16A, T17A double-mutant (DM) was introduced into phospholamban knockout mouse hearts. Transgenic lines expressing DM phospholamban at levels similar to wild types (WT) were identified.

From mouse to man: understanding heart failure through genetically altered mouse models.

Human heart failure, a complex disease with heterogeneous etiologies, remains one of the most life-threatening diseases known. Identification of "candidate genes" and molecular and biochemical mediators of cardiac hypertrophy and failure has been vigorously pursued to dissect the pathogenesis and signaling pathways of this disease. With the availability of murine cardiac-specific promoters, transgenesis and gene targeting technologies have revolutionized the field of cardiac research.

Combined phospholamban ablation and SERCA1a overexpression result in a new hyperdynamic cardiac state.

Objective: Phospholamban ablation or ectopic expression of SERCA1a in the heart results in significant increases in cardiac contractile parameters. The aim of the present study was to determine whether a combination of these two genetic manipulations may lead to further augmentation of cardiac function.

Methods: Transgenic mice with cardiac specific overexpression of SERCA1a were mated with phospholamban deficient mice to generate a model with SERCA1a overexpression in the phospholamban null background (SERCA1(OE)/PLB(KO)).

Functional interplay between dual site phospholambam phosphorylation: insights from genetically altered mouse models.

Dephosphorylated phospholamban (PLB) is an inhibitor of the affinity of the sarcoplasmic reticulum (SR) Ca2+ pump (SERCA2) for Ca2+. Phosphorylation of PLB relieves its inhibitory effects on SERCA2, with subsequent acceleration of Ca2+ transport into the SR lumen, which has been suggested to underlie the positive inotropic and lusitropic actions of beta-adrenergic agonists in the mammalian heart. PLB can be phosphorylated at Ser16 by cAMP-dependent protein kinase (PKA) and Thr17 by Ca2+-calmodulin-dependent protein kinase (CaMKII) during beta-agonist stimulation.

Enhanced myocyte contractility and Ca2+ handling in a calcineurin transgenic model of heart failure.

Objective: Impaired myocyte Ca2+ handling is a common characteristic of failing hearts and increases in calcineurin activity, a Ca2+-sensitive phosphatase, have been implicated in heart failure phenotype. Transgenic mice with cardiac-specific expression of an active form of calcineurin display depressed function, hypertrophy and heart failure. We examined whether defects in cardiomyocyte Ca2+ handling properties contribute to the impaired cardiac function in calcineurin transgenic mice.