In skeletal muscle, both mitochondrial dysfunction and intra-cellular lipid accumulation are characteristic of type 2 diabetes. It is unclear whether lipid accumulation causes cellular dysfunction (lipotoxicity) or if mitochondrial dysfunction leads to a mismatch between uptake and oxidation that secondarily leads to lipid accumulation. Further, athletes have intramuscular lipid accumulation comparable to the obese without mitochondrial dysfunction or insulin resistance, suggesting that muscle activity is important. This proposal describes experiments designed to characterize the time course of the development of obesity, metabolic syndrome and type 2 diabetes in a mouse model (the UCP-dta mouse) that lacks brown adipose tissue, becomes obese and develops insulin resistance. Through careful examination of the time course of the progression of obesity and metabolic syndrome, we ask whether defects in mitochondrial function and gene expression begin before or after appearance of overt symptoms. Further, we ask whether lipid accumulation in muscle precedes the appearance of mitochondrial dysfunction and/or metabolic syndrome. To begin exploration of underlying mechanism, we will measure the expression of critical upstream transcriptional regulators of mitochondrial function, including fatty acid oxidation, during this time course to determine their role in the development of pathology. We will examine the response of muscle to inactivity to understand the role of exercise in maintaining mitochondrial function in relation to lipid overload. These data will better inform our understanding of the events that cause muscle insulin resistance and metabolic dysfunction with obesity and metabolic syndrome.

Public Health Relevance

Modern life provides both easy access to food and also requires relatively little physical activity to acquire food. This combination has led to an epidemic of obesity, which itself leads to a cluster of metabolic disorders, including diabetes. This diabetes, type 2 diabetes, is characterized initially by elevated insulin in the blood, but resistance to its influence in tissues. In skeletal muscle, this means that glucose is unable to enter the cells normally (insulin resistance). However, fats accumulate in the muscle cells as levels of circulating fats in the blood are elevated. One theory for the development of type 2 diabetes is that fat accumulation in muscle cells has a toxic effect, causing the pathology. Another defect seen in diabetic muscle, in addition to insulin resistance, is seen in the ability to use the fuels that do enter the cells. Mitochondria, the power plants of the cell, function abnormally, reducing the capacity to use energy. However, it is unclear if fat accumulates and causes mitochondrial dysfunction, or if another process causes the inability to consume, or burn, the fats that enter the muscle cells, leading to a secondary buildup of fat in muscle cells. This proposal seeks to understand whether fat accumulation in muscle is occurring before the defects in capacity for energy use, thus potentially having a toxic effect, or if buildup of fat follows mitochondrial dysfunction. Differentiating between these two possibilities is important if we are to treat the causes of the disease rather than its consequences.

National Institute of Health (NIH)
National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)
Academic Research Enhancement Awards (AREA) (R15)
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Integrative Physiology of Obesity and Diabetes Study Section (IPOD)
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Laughlin, Maren R
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Miami University Oxford
Schools of Arts and Sciences
United States
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