Investigation of abnormal cardiac function in murine models of hypocontractility and hypercontractility

Abstract

Heart failure has a significant impact on mortality and morbidity. Dilated cardiomyopathy (DCM) is the third most common cause of heart failure and the most common reason for heart transplantation. Familial DCM is known to be caused by mutations in the LMNA gene encoding lamins A and C. New methods to enhance cardiac contractility would be beneficial in the treatment or prevention of heart failure. The focus of this thesis was to evaluate the mechanisms of altered contractility in two mouse models: the LMNA knockout model (homozygous, Lmna-/-; heterozygous, Lmna+/-) generated by targeted deletion of the lmna gene, and the model of enhanced contractility due to cardiac alpha1A-adrenergic receptor (1A-AR) overexpression (A1A1). Previous studies have found altered nuclear-desmin connections in lamin A/C deficient mice. It was proposed that these alterations result in defective force transmission , which leads to DCM. Studies in this thesis have supported this hypothesis. Studies of isolated single cardiomyocytes from mice aged 4-6 weeks demonstrated abnormal cell morphology and contractile dysfunction in Lmna-/- cardiomyocytes, while Lmna+/- cells showed no overt phenotype. Excitation-contraction coupling experiments and forcecalcium studies in skinned fibers excluded altered calcium kinetics as a primary cause of DCM in this model, but there was evidence of reduced sarcomere numbers and reduced sarcomere lengths as a contributor to reduce force generation in Lmna-/- and Lmna+/- mice.

Previous in vivo studies showed that A1A1 mice had enhanced contractility with the absence of hypertrophy. Studies on isolated single cardiomyocytes from A1A1 mice aged 8-12 weeks showed reduced contractility in the absence of 1A-AR stimulation, but an exaggerated response to 1A-AR stimulation. In contrast isolated isovolumic Langendorff perfused A1A1 hearts without 1A-AR stimulation replicated the enhanced contractility observed in vivo. These studies are consistent with down-regulation of contractility due to the hyperactivity of the overexpressed 1A-AR in vivo, which only becomes evident in isolated cells without 1A-AR stimulation due to the loss of functional receptor numbers during isolation. Sufficient spontaneously active 1A-ARs are preserved in the isolated Langendorff heart preparation to ensure maximum contractility driven by increase calcium release.