Shey-Shing Sheu, PhD
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Mitochondria play a central role in numerous fundamental cellular processes ranging from ATP production, Ca2+ homeostasis, reactive oxygen species (ROS) generation, and apoptosis. Disturbances in mitochondrial ATP, Ca2+ and ROS dynamics lead to the pathogenesis of ischemic heart disease, cardiac arrhythmias, heart failure, neurodegenerative diseases, diabetes, and aging. Our long-term research objective is to elucidate cellular and molecular mechanisms by which mitochondria control intracellular Ca2+ and ROS dynamics and translate these mechanisms to the function and dysfunction of the heart.
Current research efforts focus on three projects:
1. 1R01HL122124 “Mitochondria-SR Tethering: Its Role in Cardiac Bioenergetics and Ca2+ Dynamics”
In cardiac muscle cells, uptake of Ca2+ by mitochondria during the excitation-contraction (EC) coupling cycles is important for synchronizing ATP production with the needs of contraction (excitation-bioenergetics (EB) coupling). However, an integrative mechanism to describe the EB coupling is still missing mainly due to the lack of information about the molecular identities of several key proteins involved in this process. Recent ground-breaking studies have shown that mitofusin 2 (Mfn2) is responsible for tethering endoplasmic reticulum (ER) to mitochondria. Moreover, molecular components of the mitochondrial Ca2+ uniporter (mtCU) including the pore unit (MCU) have been uncovered. These progresses open up a new opportunity for applying molecular tools to address several important questions in cardiac muscle cells:
Are mitochondria and SR also tethered together by Mfn2?
How can mtCU, which has been shown to display very low current density in the highly-folded cardiac inner mitochondrial membrane (IMM), react to SR-mediated Ca2+ pulses that oscillate over 500 times per minute in a mouse heart?
Would the disruption of this intimate mitochondria-SR (MITO-SR) Ca2+ transport lead to heart failure (HF)?
We aim to investigate these gap areas in this project. It is of scientific importance and clinical relevance that the present proposal will bring forth the molecular mechanism underlying the cardiac MITO-SR tethering and translate this unique structure to the physiological regulation of mitochondrial Ca2+ influx in bioenergetics and to the pathological implication of energy deficiency and oxidative stress in HF.
2. 2R01HL093671 “Ca2+ and ROS Crosstalk Signaling in Cardiac Mitochondria”
The pivotal role of mitochondrial Ca2+, ROS, and morphology in controlling cell fate is well recognized. It has gained appreciation that Ca2+-dependent redox-sensitive proline-rich tyrosine kinase 2 (Pyk2) functions as a key transducer of stress stimuli involved in pathological cardiac remodeling and the progression of heart failure (HF). We hypothesize that Pyk2 phosphorylates MCU that increases the number of tetrametric channels by oligomerization so that mitochondrial Ca2+ uptake is enhanced. The increases in [Ca2+]m augments ROS generation, which promotes mitochondrial fission. Physiologically, mitochondrial Ca2+ and fission work in concert to increase ATP production efficiently. However, under stress, excessive Pyk2 and MCU activation leads to pathologically high levels of mitochondrial Ca2+, fission, and ROS, which cause prolonged opening of mitochondrial permeability transition pores (mPTP), resulting in cell injury/death and subsequent HF.
To test this hypothesis, we will employ multiple techniques including:
Biochemistry (from in vitro to in situ assays)
Molecular biology (gene knock in or knock out, overexpression, RNA interference)
Cell biology (confocal, fluorescence resonance energy transfer, electron microscopy)
Biophysics (single channel recordings with lipid bilayer or mitoplast), cardiac physiology (echocardiogram)
Phenylephrine infusion mouse model of HF, to obtain experimental results that will lead to mechanistic insights
Since the destruction of mitochondrial Ca2+ homeostasis is a key element for leading to mitochondrial dysfunction-associated clinical phenotypes including heart diseases (e.g. HF), neurodegenerative diseases, metabolic diseases (diabetes), and aging, this research will provide important information regarding the design of novel mitochondria-targeted therapeutic agents for treating these diseases.
3. 1R01HL114760 “Mitochondrial Respiration and Superoxide Production in Healthy and Failing Heart”
Recently, we discovered a transient superoxide production event, the superoxide flash, in individual mitochondria of cardiac myocytes and the myocardium. Preliminary data indicate that the superoxide flash requires intact electron transport chains (ETC) activity, and its frequency is altered by physiological (e.g., changes in heart rate) or pathological (e.g., ischemia and reperfusion) treatments. We hypothesize that the superoxide flash is coupled to stochastic acceleration of ETC activity in single mitochondria and is modulated by key regulators of mitochondrial bioenergetics, including Ca2+, mPTP, and fission/fusion.
We will use state-of-the-art techniques, including targeted superoxide (mt-cpYFP) and H2O2 specific indicators, high resolution confocal imaging, gene transfer and transgenic approaches, and heart failure models to test the overarching hypotheses. If successful, this study would change medicine by making available a sensitive indicator of real-time, in situ determination of mitochondrial function. Considering the extensive involvement of mitochondrial dysfunction and oxidative stress in leading human diseases, including cardiovascular disease, cancer, and diabetes, this work has the potential to broadly impact public health by providing insights into the mechanisms of mitochondrial respiration, ROS production, and oxidative stress.