Mechanisms underlying mitochondrial function and biogenesis : implications for type 2 diabetes mellitus and obesity

Abstract

The ample supply of food, in conjunction with a sedentary lifestyle and certain genetic risk factors contribute to the rise in obesity, insulin resistance and type 2 diabetes mellitus. Reduced mitochondrial capacity for oxidative metabolism has been implicated as one possible cause of insulin resistance in several tissues; such as liver and skeletal muscle. The adipose-derived hormone leptin and the metabolic sensor 5’-AMP-activated protein kinase, are two key regulators that modulate intracellular fuel handling. The aim of this thesis is to investigate the effects of these metabolic signals on tissue-specific mitochondrial respiration and biogenesis.

The aim of study I was to investigate the role of the AMPK γ3 subunit in determining mitochondrial function in glycolytic skeletal muscle. The AMPK signaling axis is a metabolic switch regulated by the intracellular energy charge. A single-nucleotide mutation (R225Q) in the AMPKγ3 subunit causes elevated basal enzyme activity. Transgenic expression in mice (Tg-AMPKγ3R225Q) increased expression of regulators and mediators of substrate oxidation, as well as components of mitochondrial dynamics and electron transport. In summary, this single nucleotide mutation is associated with mitochondrial biogenesis, concomitant with increased expression of transcription factors that regulate mitochondrial proteins.

The focus of study II was to characterize tissue-specific mitochondrial function in permeabilized tissue from lean and leptin receptor-deficient obese db/db mice. Respiratory capacity in oxidative soleus muscle was similar between genotypes, except for decreased complex II function in db/db mice. Oxidative function in glycolytic EDL muscle was higher in db/db mice than in lean littermates; likely as a result of increased mitochondrial biogenesis. Maximal respiratory capacity in liver from db/db mice was blunted, concomitant with increased mitochondrial fission. In summary, mitochondrial respiratory performance is controlled by tissue-specific mechanisms and is not uniformly altered in obesity.

The aim of study III was to determine tissue-specific mitochondrial respiration in obese leptin-deficient ob/ob mice, and lean littermates, following treatment with leptin or saline. Oxidative capacity in soleus muscle was unaffected in saline- and leptin-treated ob/ob mice, whereas maximal electron transport capacity was increased with obesity in EDL muscle. Regulation of transcription and mitochondrial fission in EDL was altered in saline-treated ob/ob mice, and only partially normalized with leptin repletion. In liver, maximal respiratory capacity and mediators of lipid oxidation were reduced with in saline- and leptin-treated ob/ob mice; while leptin treatment normalized indicators of mitochondrial stress.

In conclusion, mitochondrial respiratory function is a dynamic process that is tightly regulated to meet the energy needs of the cell. Despite profound alterations in whole-body or intracellular energy sensing, mitochondrial adaptation can occur and respiratory adaptations are comparatively modest. This highlights the need to target several pathways of metabolic regulation to modulate mitochondrial function to improve systemic homeostasis.