(Philadelphia, PA) - When things go wrong, cells turn to built-in safety mechanisms for survival. One of those mechanisms involves calcium uptake by mitochondria, the energy-producing powerhouses of cells. Long a mystery, new research by scientists at the Temple University School of Medicine (TUSM) Center for Translational Research shows exactly how mitochondria handle damaging excess calcium from the intracellular environment, and how problems with calcium regulation can lead to vascular damage.
"Mitochondrial calcium regulation is essential for cell survival," explained senior investigator Muniswamy Madesh, PhD, Assistant Professor in the Center for Translational Research and the Department of Biochemistry at TUSM. "But the calcium uptake mechanism of mitochondria has been unknown."
In the late 1970s, researchers discovered a mitochondrial calcium influx "set point," a point at which calcium levels become high enough in the cytoplasm (intracellular fluid) to trigger calcium uptake into mitochondria. The set point was determined to be about 3 μM. Dr. Madesh and colleagues previously discovered that below the set point, a protein now known as MICU1 works to suppress calcium influx.
Dr. Madesh's new paper, which appears this week in the journal Cell Reports, is the result of a concentrated effort to identify and describe specific interactions of MICU1. His team began by establishing a novel protein flux dynamics assay, which allowed the researchers to see where MICU1 interactions take place within mitochondria. They then introduced mutations into different regions of the MICU1 protein and investigated how the mutations affected interactions that regulate mitochondrial calcium influx.
In their protein flux experiments in cells, the team discovered that MICU1 is located in the interior region of the mitochondrion. They also identified the specific regions of MICU1 that determine binding with the uniporter that transports calcium into the mitochondrion.
To characterize the physiological relevance of MICU1, the researchers conducted experiments in mice in which MICU1 was silenced. They found that reduced MICU1 activity resulted in prolonged calcium uptake, chronic oxidative stress, and vascular dysfunction. It also diminished the ability of endothelial cells, which form the inner lining of blood vessels, to migrate, a process necessary for the formation of new blood vessels.
The new work sheds light on ways in which calcium and mitochondrial dysfunction contribute to cell and vascular damage, leading to new opportunities for the discovery of therapies capable of preventing cell injury. According to Madesh, "If we can slow down calcium uptake and protect mitochondria, we may be able to keep mitochondrial energy levels up."
The findings have implications for other research being conducted at Temple's Center for Translational Medicine, where there is particular interest in oxidative damage sustained from conditions such as ischemic reperfusion (when blood flow resumes following a temporary pause, such as during a heart attack).
"Calcium overload and oxidative stress are implicated in cardiovascular and neurodegenerative diseases, aging, and metabolic syndrome," Madesh said. "Calcium overload and oxidative stress is a common feature in disease. It happens all the time."
Also contributing to the work were Nicholas E. Hoffman, Santhanam Shamugapriya, Sudarsan Rajan, Karthik Mallilankaraman, Rajesh Kumar Gandhirajan, Krishnalatha Sreekrishnanilayam, Kalimuthusamy Natarajaseenivasan, and Sandhya Vallem in the Department of Biochemistry and Center for Translational Medicine at TUSM; Xueqian Zhang and Ronald J. Vagnozzi in the Center for Translational Medicine at Temple; Harish C. Chandramoorthy in the Department of Biochemistry and Center for Translational Medicine at Temple and the Stem Cell Unit and Department of Clinical Biochemistry at King Khalid University; Lukas M. Ferrer in the Department of Surgery at Temple; Thomas Force and Joseph Y. Cheung in the Center for Translational Medicine and the Department of Medicine at Temple; and Eric T. Choi in the Department of Surgery and the Cardiovascular Research Center at TUSM.
The research was supported in part by National Institutes of Health grants HL086699, HL109920 and 1S10RR027327-01.
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Temple Health refers to the health, education and research activities carried out by the affiliates of Temple University Health System and by Temple University School of Medicine.
Temple University Health System (TUHS) is a $1.4 billion academic health system dedicated to providing access to quality patient care and supporting excellence in medical education and research. The Health System consists of Temple University Hospital (TUH), ranked among the "Best Hospitals" in the region by U.S. News & World Report; TUH-Episcopal Campus; TUH-Northeastern Campus; Fox Chase Cancer Center, an NCI-designated comprehensive cancer center; Jeanes Hospital, a community-based hospital offering medical, surgical and emergency services; Temple Transport Team, a ground and air-ambulance company; and Temple Physicians, Inc., a network of community-based specialty and primary-care physician practices. TUHS is affiliated with Temple University School of Medicine.
Temple University School of Medicine (TUSM), established in 1901, is one of the nation's leading medical schools. Each year, the School of Medicine educates approximately 840 medical students and 140 graduate students. Based on its level of funding from the National Institutes of Health, Temple University School of Medicine is the second-highest ranked medical school in Philadelphia and the third-highest in the Commonwealth of Pennsylvania. According to U.S. News & World Report, TUSM is among the top 10 most applied-to medical schools in the nation.