Molecular Basis for Heart Growth Through Hyperplasia and Hypertrophy

Abstract

The heart is an essential organ that facilitates blood flow throughout the body, transporting oxygenated blood from the lungs to the body and in transporting deoxygenated blood from the body to the lungs. Behind each beat is the cardiac sarcomere, a lattice structure containing cytoskeletal filaments and the molecular motors which pull on them. Heart disease is the leading cause of death not only in the United States but globally. Hereditary heart disease, including dilated and hypertrophic cardiomyopathies, can be caused by mutations in sarcomere components, leading them to form aberrantly. Heart attacks, or myocardial infarctions, can also damage the essential cardiac myocytes, which are not regenerative and cannot repair themselves. The cardiac myocyte and its sarcomeres are at the center of how the heart forms and functions, and understanding how these cells form sarcomeres and how they divide is essential for disease treatment. This dissertation investigates the two mechanisms by which the heart grows: growth by cell division (hyperplasia) and growth by individual cell enlargement (hypertrophy). Here, I present a strategy for identifying small molecules that induce cardiac myocyte proliferation and screened library of 429 kinase inhibitors, identifying three small molecules which significantly increase cardiac myocyte proliferation over time. I also demonstrated that cardiac myocytes have a higher proliferative capacity when plated sparsely than when plated in a dense monolayer, and showed that this density-dependent proliferation is regulated by the Hippo pathway. Further, I showed that the proliferative capacity induced upon sparse plating of cardiac myocytes can be increased by combinatorial pharmacological perturbation of the Hippo pathway. The hypertrophy section investigates the roles of adhesive and contractile proteins in formation of sarcomeres, the beating structures of the heart. Here I show that substrate coupling (coupling the cell’s contractile forces to the extracellular matrix) is required for proper sarcomere formation. I further show that myosin II-based force, initially considered to be required for sarcomere formation, is only required for thin filament maturation, with different regulators identified for thick filament formation. These studies contribute to our laboratory’s long-term goal of studying cell biology on a systems level, while also contributing to my long-term goal of developing quantitative measurements of microscopy images and cell biological data.