Transit and integration of extracellular mitochondria in human heart cells

Abstract

Tissue ischemia adversely affects the function of mitochondria, which results in impairment of oxidative phosphorylation and compromised recovery of the affected organ. The impact of ischemia on mitochondrial function has been most extensively studied in the heart because of the morbidity and mortality associated with injury to this organ. Because conventional methods to preserve cell viability and function following an ischemic injury are limited in their efficacy, we developed a unique approach to protect the heart by transplanting respiration-competent mitochondria isolated from a non-ischemic tissue to the ischemic region. Our experiments in animals have shown that transplantation of isolated mitochondria to injured heart tissue leads to decreases in cell death, increases in energy production, and improvements in contractile function. We also discovered that exogenously-derived mitochondria injected or perfused into ischemic hearts were readily internalized by cardiac cells through actin-dependent endocytosis. Here, we describe the use of three-dimensional super-resolution microscopy and transmission electron microscopy to determine the intracellular fate of exogenous mitochondria in non-dividing human iPS-derived cardiomyocytes and dividing primary human cardiac fibroblasts. We show isolated mitochondria are internalised in human cardiac cells within minutes and then transported to endosomes and lysosomes. The majority of exogenous mitochondria escape from these compartments and fuse with the endogenous mitochondrial network, while some organelles are degraded through hydrolysis. Understanding this process may guide the development of treatments directed at replacing or augmenting impaired mitochondria in ischemic tissues and provide new options to rejuvenate dysfunctional mitochondria in a wide range of human diseases and disorders.