Vivek J. Kadambi

Cardiac-specific overexpression of phospholamban alters calcium kinetics and resultant cardiomyocyte mechanics in transgenic mice.

Phospholamban is the regulator of the cardiac sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity and an important modulator of basal contractility in the heart. To determine whether all the SR Ca(2+)-ATPase enzymes are subject to regulation by phospholamban in vivo, transgenic mice were generated which overexpressed phospholamban in the heart, driven by the cardiac-specific alpha-myosin heavy chain promoter. Quantitative immunoblotting revealed a twofold increase in the phospholamban protein levels in transgenic hearts compared to wild type littermate hearts. The transgenic mice showed no phenotypic alterations and no changes in heart/body weight, heart/lung weight, and cardiomyocyte size. Isolated unloaded cardiac myocytes from transgenic mice exhibited diminished shortening fraction (63%) and decreased rates of shortening (64%) and relengthening (55%) compared to wild type (100%) cardiomyocytes. The decreases in contractile parameters of transgenic cardiomyocytes reflected decreases in the amplitude (83%) of the Ca2+ signal and prolongation (131%) in the time for decay of the Ca2+ signal, which was associated with a decrease in the apparent affinity of the SR Ca(2+)-ATPase for Ca2+ (56%), compared to wild type (100%) cardiomyocytes. In vivo analysis of left ventricular systolic function using M mode and pulsed-wave Doppler echocardiography revealed decreases in fractional shortening (79%) and the normalized mean velocity of circumferential shortening (67%) in transgenic mice compared to wild type (100%) mice. The differences in contractile parameters and Ca2+ kinetics in transgenic cardiomyocytes and the depressed left ventricular systolic function in transgenic mice were abolished upon isoproterenol stimulation. These findings indicate that a fraction of the Ca(2+)-ATPases in native SR is not under regulation by phospholamban. Expression of additional phospholamban molecules results in: (a) inhibition of SR Ca2+ transport; (b) decreases in systolic Ca2+ levels and contractile parameters in ventricular myocytes; and (c) depression of basal left ventricular systolic function in vivo.

Cardiac-specific Overexpression of Mouse Cardiac Calsequestrin Is Associated with Depressed Cardiovascular Function and Hypertrophy in Transgenic Mice

Calsequestrin is a high capacity Ca2+-binding protein in the sarcoplasmic reticulum (SR) lumen. To elucidate the functional role of calsequestrin in vivo, transgenic mice were generated that overexpressed mouse cardiac calsequestrin in the heart. Overexpression (20-fold) of calsequestrin was associated with cardiac hypertrophy and induction of a fetal gene expression program. Isolated transgenic cardiomyocytes exhibited diminished shortening fraction (46%), shortening rate (60%), and relengthening rate (60%). The Ca2+ transient amplitude was also depressed (45%), although the SR Ca2+storage capacity was augmented, as suggested by caffeine application studies. These alterations were associated with a decrease in L-type Ca2+ current density and prolongation of this channel’s inactivation kinetics without changes in Na+-Ca2+ exchanger current density. Furthermore, there were increases in protein levels of SR Ca2+-ATPase, phospholamban, and calreticulin and decreases in FKBP12, without alterations in ryanodine receptor, junctin, and triadin levels in transgenic hearts. Left ventricular function analysis in Langendorff perfused hearts and closed-chest anesthetized mice also indicated depressed rates of contraction and relaxation of transgenic hearts. These findings suggest that calsequestrin overexpression is associated with increases in SR Ca2+ capacity, but decreases in Ca2+-induced SR Ca2+ release, leading to depressed contractility in the mammalian heart.

Heart block, ventricular tachycardia, and sudden death in ACE2 transgenic mice with downregulated connexins

Angiotensin converting enzyme related carboxypeptidase (ACE2) is a recently discovered homolog of angiotensin converting enzyme with tissue-restricted expression, including heart, and the capacity to cleave angiotensin peptides. We tested the hypothesis that cardiac ACE2 activity contributes to features of ventricular remodeling associated with the renin-angiotensin system by generating transgenic mice with increased cardiac ACE2 expression. These animals had a high incidence of sudden death that correlated with transgene expression levels. Detailed electrophysiology revealed severe, progressive conduction and rhythm disturbances with sustained ventricular tachycardia and terminal ventricular fibrillation. The gap junction proteins connexin40 and connexin43 were downregulated in the transgenic hearts, indicating that ACE2-mediated gap junction remodeling may account for the observed electrophysiologic disturbances. Spontaneous downregulation of the ACE2 transgene in surviving older animals correlated with restoration of nearly normal conduction, rhythm, and connexin expression.

Interactions between phospholamban and β-adrenergic drive may lead to cardiomyopathy and early mortality

Background —Relieving the inhibition of sarcoplasmic reticular function by phospholamban is a major target of &bgr;-adrenergic stimulation. Chronic &bgr;-adrenergic receptor activity has been suggested to be detrimental, on the basis of transgenic overexpression of the receptor or its signaling effectors. However, it is not known whether physiological levels of sympathetic tone, in the absence of preexisting heart failure, are similarly detrimental. Methods and Results —Transgenic mice overexpressing phospholamban at 4-fold normal levels were generated, and at 3 months, they exhibited mildly depressed ventricular contractility without heart failure. As expected, transgenic cardiomyocyte mechanics and calcium kinetics were depressed, but isoproterenol reversed the inhibitory effects of phospholamban on these parameters. In vivo cardiac function was substantially depressed by propranolol administration, suggesting enhanced sympathetic tone. Indeed, plasma norepinephrine levels and the phosphorylation status of phospholamban were elevated, reflecting increased adrenergic drive in transgenic hearts. On aging, the chronic enhancement of adrenergic tone was associated with a desensitization of adenylyl cyclase (which intensified the inhibitory effects of phospholamban), the development of overt heart failure, and a premature mortality. Conclusions —The unique interaction between phospholamban and increased adrenergic drive, elucidated herein, provides the first evidence that compensatory increases in catecholamine stimulation can, even in the absence of preexisting heart failure, be a primary causative factor in the development of cardiomyopathy and early mortality.

Differential regulation of p38 mitogen-activated protein kinase mediates gender-dependent catecholamine-induced hypertrophy

OBJECTIVE Exogenous catecholamine exposure has been associated with p38 mitogen-activated protein kinase (MAPK) and cardiac hypertrophy. In this study, we investigated the regulation of p38 MAPK in cardiac remodeling elicited by endogenous adrenergic mechanisms. METHODS Transgenic male and female mice with fourfold phospholamban (PLB) overexpression exhibited enhanced circulating norepinephrine (NE), as a physiological compensatory mechanism to attenuate PLBs inhibitory effects. This enhanced noradrenergic state resulted in left ventricular hypertrophy/dilatation and depressed function. RESULTS Male transgenics exhibited ventricular hypertrophy and mortality at 15 months, concurrent with cardiac p38 MAPK activation. Female transgenics, despite similar contractile dysfunction, displayed a temporal delay in p38 activation, hypertrophy, and mortality (22 months), which was associated with sustained cardiac levels of MAP Kinase Phosphatase-1 (MKP-1), a potent inhibitor of p38. At 22 months, decreases in cardiac MKP-1 were accompanied by increased levels of p38 activation. In vitro studies indicated that preincubation with 17-beta-estradiol induced high MKP-1 levels, which precluded NE-induced p38 activation. CONCLUSION These findings suggest that norepinephrine-induced hypertrophy is linked closely with p38 MAP kinase activation, which can be endogenously modulated through estrogen-responsive regulation of MKP-1 expression.

Mouse phospholamban gene expression during development in vivo and in vitro.

To establish a murine model that may allow for definition of the precise role of phospholamban in myocardial contractility through selective perturbations in the phospholamban gene, we initiated studies on the role of phospholamban in the murine heart. Intact beating hearts were perfused in the absence or presence of isoproterenol, and quantitative measurements of cardiac performance were obtained. Isoproterenol stimulation was associated with increases in the affinity of the sarcoplasmic reticulum Ca2+ pump for Ca2+ that were due to phospholamban phosphorylation. To assess the regulation of phospholamban gene expression during murine development, Northern blot and polymerase chain reaction analyses were used. Phospholamban mRNA was first detected in murine embryos on the ninth day of development (the time when the cardiac tube begins to contract). In murine embryoid bodies, which have been shown to recapitulate several aspects of cardiogenesis, phospholamban mRNA was detected on the seventh day (the time when spontaneous contractions are first observed). Only those embryoid bodies that exhibited contractions expressed phospholamban transcripts, and these were accompanied by expression of the protein, as revealed by immunofluorescence microscopy. Sequence analysis of the cDNA encoding phospholamban in embryoid bodies indicated complete homology to that in adult hearts. The deduced amino acid sequence of murine phospholamban was identical to rabbit cardiac phospholamban but different from dog cardiac and human cardiac phospholamban by one amino acid. These data suggest that phospholamban, the regulator of the Ca(2+)-ATPase in cardiac sarcoplasmic reticulum, is present very early in murine cardiogenesis in utero and in vitro, and this may constitute an important determinant for proper development of myocardial contractility.

Antibody drug conjugates - Trojan horses in the war on cancer.

Antibody drug conjugates (ADCs) consist of an antibody attached to a cytotoxic drug by means of a linker. ADCs provide a way to couple the specificity of a monoclonal antibody (mAb) to the cytotoxicity of a small-molecule drug and, therefore, are promising new therapies for cancer. ADCs are prodrugs that are inactive in circulation but exert their cytotoxicity upon binding to the target cancer cell. Earlier unsuccessful attempts to generate ADCs with therapeutic value have emphasized the important role each component plays in determining the efficacy and safety of the final ADC. Scientific advances in engineering antibodies for maximum efficacy as anticancer agents, identification of highly cytotoxic molecules, and generation of linkers with increased stability in circulation have all contributed to the development of the many ADCs that are currently in clinical trials. This review discusses parameters that guide the selection of the components of an ADC to increase its therapeutic window, provides a brief look at ADCs currently in clinical trials, and discusses future challenges in this field.

Monomeric Phospholamban Overexpression in Transgenic Mouse Hearts

Phospholamban, a prominent modulator of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase activity and basal contractility in the mammalian heart, has been proposed to form pentamers in native SR membranes. However, the monomeric form of phospholamban, which is associated with mutating Cys41 to Phe41, was shown to be as effective as pentameric phospholamban in inhibiting Ca2+ transport in expression systems. To determine whether this monomeric form of phospholamban is also functional in vivo, we generated transgenic mice with cardiac-specific overexpression of the mutant (Cys41-->Phe41) phospholamban. Quantitative immunoblotting indicated a 2-fold increase in the cardiac phospholamban protein levels compared with wild-type controls, with approximately equal to 50% of phospholamban migrating as monomers and approximately 50% as pentamers upon SDS-PAGE. The mutant-phospholamban transgenic hearts were analyzed in parallel with transgenic hearts overexpressing (2-fold) wild-type phospholamban, which migrated as pentamers upon SDS-PAGE. SR Ca(2+)-uptake assays revealed that the EC50 values for Ca2+ were as follows: 0.32 +/- 0.01 mumol/L in hearts overexpressing monomeric phospholamban, 0.49 +/- 0.05 mumol/L in hearts overexpressing wild-type phospholamban, and 0.26 +/- 0.01 mumol/L in wild-type control mouse hearts. Analysis of cardiomyocyte mechanics and Ca2+ kinetics indicated that the inhibitory effects of mutant-phospholamban overexpression (mt) were less pronounced than those of wild-type phospholamban overexpression (ov) as assessed by depression of the following: (1) shortening fraction (25% mt versus 45% ov), (2) rates of shortening (27% mt versus 48% ov), (3) rates of relengthening (25% mt versus 50% ov) (4) amplitude of the Ca2+ signal (21% mt versus 40% ov), and (5) time for decay of the Ca2+ signal (25% mt versus 106% ov) compared with control (100%) myocytes. The differences in basal cardiac, myocyte mechanics and Ca2+ transients among the animal groups overexpressing monomeric or wild-type phospholamban and wild-type control mice were abolished upon isoproterenol stimulation. These findings suggest that pentameric assembly of phospholamban is important for mediating its optimal regulatory effects on myocardial contractility in vivo.

Hypertrophy and functional alterations in hyperdynamic phospholamban-knockout mouse hearts under chronic aortic stenosis.

OBJECTIVE To determine whether the hyperdynamic phospholamban-knockout hearts are capable of withstanding a chronic aortic stenosis. METHODS The transverse section of the aorta was banded in phospholamban-knockout and their isogenic wild-type mice, which were followed with echocardiography in parallel, along with sham-operated mice, before and at 2.5, 5 and 10 weeks after surgery. RESULTS Cardiac decompensation was evidenced by the presence of lung congestion in some banded knockouts and wild-types, giving rise to a subset of non-failing and failing hearts within each group. The incidence of heart failure was not genotype-dependent but rather associated with higher heart rates before surgery. The development of left ventricular hypertrophy was similar between knockouts and wild-types and longitudinal assessment of end-diastolic dimension indicated progressive increases after banding, with a greater dilation in failing mice. Fractional shortening was reduced in failing knockouts and wild-types to a similar degree, with an earlier onset in the knockouts. In addition, fractional shortening was decreased in non-failing knockouts but not wild-types. Ejection times shortened after aortic banding particularly for failing hearts. Assessment of the SR Ca(2+)-ATPase protein levels indicated similar downregulation for failing knockouts and wild-types, while the phospholamban levels were not significantly altered in wild-types. CONCLUSION The hyperdynamic phospholamban-knockout hearts are able to compensate against a sustained aortic stenosis similar to wild-types.

Phospholamban Is Present in Endothelial Cells and Modulates Endothelium-Dependent Relaxation: Evidence From Phospholamban Gene-Ablated Mice

Vascular endothelial cells regulate vascular smooth muscle tone through Ca2+-dependent production and release of vasoactive molecules. Phospholamban (PLB) is a 24- to 27-kDa phosphoprotein that modulates activity of the sarco(endo)plasmic reticulum Ca2+ ATPase (SERCA). Expression of PLB is reportedly limited to cardiac, slow-twitch skeletal and smooth muscle in which PLB is an important regulator of [Ca2+]i and contractility in these muscles. In the present study, we report the existence of PLB in the vascular endothelium, a nonmuscle tissue, and provide functional data on PLB regulation of vascular contractility through its actions in the endothelium. Endothelium-dependent relaxation to acetylcholine was attenuated in aorta of PLB-deficient (PLB-KO) mice compared with wild-type (WT) controls. This effect was not due to actions of nitric oxide on the smooth muscle, because sodium nitroprusside-mediated relaxation in either denuded or endothelium-intact aortas was unaffected by PLB ablation. Relative to denuded vessels, relaxation to forskolin was enhanced in WT endothelium-intact aortas. The endothelium-dependent component of this relaxation was attenuated in PLB-KO aortas. To investigate whether these changes were due to PLB, WT mouse aorta endothelial cells were isolated. Both reverse transcriptase-polymerase chain reaction and Western blot analyses revealed the presence of PLB in endothelial cells, which were shown to be >98% pure by diI-acetylated LDL uptake and nuclear counterstaining. These data indicate that PLB is present and modulates vascular function as a result of its actions in endothelial cells. The presence of PLB in endothelial cells opens new fields for investigation of Ca2+ regulatory pathways in nonmuscle cells and for modulation of endothelial-vascular interactions.