Date of Degree
PhD (Doctor of Philosophy)
Molecular Physiology and Biophysics
Mark E. Anderson
A long held hypothesis in mitochondrial biology holds that increases in mitochondrial Ca2+ levels stimulate the activity of matrix dehydrogenases that catalyze production of NADH and eventually donate electrons to electron transport in order to increase ATP formation. At the same time, mitochondrial Ca2+ overload is a deleterious event leading to opening of the mitochondrial permeability transition pore, increasing reactive oxygen species and initiating pathways that contribute to cell death. These fundamental hypotheses are best studied in the heart because of the critical energy supply-demand relationship in myocardium, but were untestable in vivo until the discovery of the mitochondrial Ca2+ uniporter (MCU). The molecular identity of the MCU pore forming subunit was recently discovered, which allowed me to study a transgenic mouse with myocardial delimited expression of a dominant negative MCU.
My lab developed mice with myocardial-delimited transgenic expression of a dominant negative MCU to test these fundamental hypotheses and to determine how MCU controls physiological and pathological stress responses in vivo, ex vivo, and in situ. My studies provide new, unanticipated information that contributes to our understanding the relationship between mitochondrial Ca2+, oxygen utilization, cardiac pacemaking and pathologic stress responses in heart. Here, I show that mice with myocardial-targeted MCU inhibition have hearts with surprisingly high oxygen consumption rates due to elevated cytoplasmic Ca2+ in response to physiological stress. Loss of MCU effectively preserved inner mitochondrial membrane potential and prevented an oxidative burst thought to drive myocardial injury and death, but nevertheless failed to protect myocardium from ischemia-reperfusion injury. Increases in oxygen consumption, elevation in cytoplasmic Ca2+ and transcriptional reprogramming mitigate the protective actions of MCU inhibition in vivo. Mice with myocardial selective MCU inhibition have a reduced response to isoproterenol-induced heart rate increase but have normal baseline heart rates. My studies provide novel insight into how MCU contributes to myocardial Ca2+ homeostasis, metabolism, and transcription leading to surprising actions on physiological and pathophysiological responses in heart.
Heart disease is the most common cause of death in the western world. The heart requires large amounts of energy to pump oxygen rich blood to peripheral organs. This energy is supplied by organelles called mitochondria, colloquially known as the powerhouse of the cell. Ca2+ entry into mitochondria has long been hypothesized to increase the ability of mitochondria to produce energy. This hypothesis was untestable in living animals until 2011 when the identity of the mitochondrial calcium uniporter (MCU) was discovered. MCU forms the pore for Ca2+ entry into mitochondria. I studied mice expressing a mutated form of MCU (DN-MCU) only in the heart. I hypothesized that the DN-MCU expressing mice would be unable to respond to physiological stresses. Additionally, mitochondrial Ca2+ overload can trigger maladaptive responses that lead to heart injury and cell death. I hypothesized that DN-MCU mice would show protection from cell death in response to a pathologic condition known to increase mitochondrial Ca2+. I found that DN-MCU mouse hearts had mitochondria with lower Ca2+ than normal mice, lower enzyme activity of Ca2+ sensitive enzymes and normal heart function at baseline. Electrically stimulated DN-MCU hearts had higher oxygen consumption when paced because of higher Ca2+ outside of mitochondria, suggesting cardiac inefficiency. Despite lack of Ca2+ entry into mitochondria, DN-MCU hearts were not protected from cell death after ischemia-reperfusion injury, a model known to involve mitochondrial Ca2+ overload. These data suggest that MCU inhibition alters responses to physiological stress, but doesn’t protect hearts from cell death.
publicabstract, Calcium, Heart, Ischemia-Reperfusion, MCU, Mitochondria, Myocardium
Copyright 2016 Tyler Paul Rasmussen