UC San Diego

The main purpose of this dissertation was to identify and characterize how both internal and external remodeling of the cardiomyocyte affects contractility with age. Not only can substructures within the cardiomyocyte, i.e. the cytoskeleton, remodel with age affecting longitudinal force transduction, but external structures within the basement membrane can remodel, affecting lateral force transduction. Both inherently change how cardiomyocytes sense and respond to their environment. Specifically, changes to either means of mechanosensing can lead to pressure or volume overload induced hypertrophy of cardiomyocytes resulting in heart failure. Recent work has been aimed at distinguishing which age-related molecular changes help sustain function over time (physiological aging) versus those that directly lead to impairment in cardiac function (pathological aging). In terms of many organ structures, aging has typically been thought of as the accumulation of damage or insults over time, but in the heart, where regeneration is minimal, these age-related changes may help cardiomyocytes compensate for loss in numbers as well as increased stress with age. A complete understanding of cardiac molecular remodeling is imperative if we are to understand the complex nature of age-related cardiac decline to design better treatments or utilize inherent beneficial mechanisms to improve cardiac function with age.

For this PhD dissertation, I began searching for conserved components of cardiac aging in Drosophila melanogaster, a rapidly aging and genetically homologous organism with a simple heart structure. I identified two substructures, basement membrane and cytoskeleton that in flies recapitulated age-related remodeling observed in rats and monkeys. First, I examined basement membrane remodeling which has experienced very little study outside of development. Using the Drosophila genetic toolbox, we employed a UAS-GAL4 system to knockdown basement membrane genes Laminin and Collagen IV that experienced upregulation with age in other models, and were overexpressed in our wildtype fly line that exhibits diminished cardiac function. These studies proved for the first time, the direct link between basement membrane remodeling and cardiac function, and showed that downregulating Laminin and Collagen IV in a genetic background in which they are overexpressed correlates with an improvement in contractility, fractional shortening, and lifespan.

I also identified age-related Vinculin upregulation and recruitment to intercalated discs and costameres in cardiomyocytes as a conserved phenotype between flies, rats, and monkeys. By employing Drosophila melanogaster, to further interrogate the mechanical impact this upregulation was having in the heart, I overexpressed Vinculin in the fly hearts using the Gal4-UAS system and performed Myosin Heavy Chain knockdown with Vinculin overexpression rescue experiments. These data revealed that Vinculin overexpression correlated with significant improvements in contractile velocities, fractional shortening, and relative power produced. By performing transmission electron microscopy on these hearts, I showed that the myofibrils in hearts overexpressing Vinculin experience improved crystallinity which could explain the improved contraction observed. I also examined what effect improved cardiac output in flies overexpressing Vinculin had on organismal metabolism and healthspan since these changes impacted a highly metabolic organ integral to nutrient delivery. Through whole fly mass spectrometry, we found significant age-related accumulation of metabolites in flies overexpressing Vinculin in the heart in addition to improved organismal fitness with age. These data suggest that a cardiac specific cytoskeletal perturbation can significantly impact systemic metabolic homeostasis and alter age-related metabolism improving healthspan as well as lifespan. Taken together this research points to the effectiveness of the Drosophila model to study the mechanics of cardiac aging and to find conserved mechanisms that either help or hinder cardiac function so that we may target these to promote healthier aging.