Restoration of myocardial β-adrenergic receptor signaling after left ventricular assist device support

ObjectiveLeft ventricular assist device support for patients with chronic heart failure can significantly improve β-adrenergic receptor signaling, which is likely critical to myocardial recovery. The mechanism underlying the restoration of β-adrenergic receptor signaling is unclear. This study investigates our hypothesis that restoration of cardiac β-adrenergic receptor signaling by left ventricular assist devices results from inhibition of the G protein–coupled receptor kinase-2, a G protein–coupled receptor kinase that specifically phosphorylates and desensitizes agonist-occupied β-adrenergic receptors.MethodsLeft ventricular β-adrenergic receptor signaling was assessed in biopsy specimens taken from patients with chronic heart failure (n = 12) at the time of left ventricular assist device implantation (heart failure group) and again at the time of heart transplantation (left ventricular assist device group). Signaling was also studied in left ventricular biopsy specimens from nonfailing control (n = 8) hearts (nonfailing control group). Signaling was assessed by measuring sarcolemmal membrane β-adrenergic receptor density, adenylyl cyclase activity, G protein expression, and G protein–coupled receptor kinase-2 expression and activity.ResultsLeft ventricular β-adrenergic receptor signaling was severely decreased in the heart failure group versus that seen in the nonfailing control group, as demonstrated by adenylyl cyclase activity. G protein–coupled receptor kinase-2 expression and activity was increased 3-fold in the heart failure group versus that seen in the nonfailing control group. After left ventricular assist device support, β-adrenergic receptor signaling was restored to levels similar to those seen in the nonfailing control group. G protein–coupled receptor kinase-2 expression and activity were markedly diminished after left ventricular assist device support compared with that seen in the heart failure group and were not different from that seen in the nonfailing control group.ConclusionIn chronic heart failure left ventricular assist device support leads to restoration of cardiac β-adrenergic receptor signaling. The primary mechanism appears to be diminished myocardial G protein–coupled receptor kinase-2 activity. This demonstrates the potentially beneficial effects of G protein–coupled receptor kinase-2 inhibition on β-adrenergic receptor signaling in heart failure and might represent a novel therapeutic strategy for this disease process

Molecular and functional characterization of a novel cardiac-specific human tropomyosin isoform.

BACKGROUND: Tropomyosin (TM), an essential actin-binding protein, is central to the control of calcium-regulated striated muscle contraction. Although TPM1alpha (also called alpha-TM) is the predominant TM isoform in human hearts, the precise TM isoform composition remains unclear. METHODS AND RESULTS: In this study, we quantified for the first time the levels of striated muscle TM isoforms in human heart, including a novel isoform called TPM1kappa. By developing a TPM1kappa-specific antibody, we found that the TPM1kappa protein is expressed and incorporated into organized myofibrils in hearts and that its level is increased in human dilated cardiomyopathy and heart failure. To investigate the role of TPM1kappa in sarcomeric function, we generated transgenic mice overexpressing cardiac-specific TPM1kappa. Incorporation of increased levels of TPM1kappa protein in myofilaments leads to dilated cardiomyopathy. Physiological alterations include decreased fractional shortening, systolic and diastolic dysfunction, and decreased myofilament calcium sensitivity with no change in maximum developed tension. Additional biophysical studies demonstrate less structural stability and weaker actin-binding affinity of TPM1kappa compared with TPM1alpha. CONCLUSIONS: This functional analysis of TPM1kappa provides a possible mechanism for the consequences of the TM isoform switch observed in dilated cardiomyopathy and heart failure patients