Regulation of skeletal muscle and kidney metabolism by the PGC-1 family of transcriptional coactivators

Metabolism is key for life and involves the interplay of anabolic and catabolic reactions within a cell to meet the required energy needs. Each cell, tissue and organ possesses a unique metabolic profile that all contribute to systemic energy homeostasis and disturbances at any level profoundly affect whole body metabolism. The complex integration of metabolism takes place at different levels including adjustments in gene transcription. Over the last 20 years, transcriptional coregulators have emerged as important players in the regulation of gene expression and the field is constantly expanding identifying new coregulator proteins and their metabolic functions. The peroxisome proliferator-activated receptor γ coactivator-1 (PGC-1) family includes the three family members termed PGC-1α, PGC-1β and PGC-related coactivator. They have been implicated to play important roles in oxidative metabolism and mitochondrial homeostasis in a variety of different tissues including skeletal muscle and kidney. Thus, during the course of this thesis, we studied the physiological and pathophysiological effects of PGC-1α- and PGC-1β-specific ablation in these two organs in four different projects.
In the first project, we assessed the role of PGC-1α in skeletal muscle in response to chronic ketogenic diet feeding. Ketogenic diets have gained more and more attention as therapeutic strategies in the treatment of metabolic diseases and other pathological disorders. However, the mode of action is still poorly understood, particularly upon chronic administration. Next to liver, brain, and heart, skeletal muscle is one of the main players involved in the regulation of physiological and pathophysiological ketosis. Thus, we studied the effects of 12 weeks of ketogenic diet feeding in wildtype (WT) and PGC-1α muscle-specific knockout mice (PGC-1α MKO). Importantly, muscle PGC-1α was essential to increase oxygen consumption and transcript levels of genes involved in fatty acid oxidation as well as to maintain exercise performance upon ketogenic diet feeding. Therefore, we elucidated a new role for muscle PGC-1α in the regulation of physiological adaptations to chronic ketogenic diet administration.
In the second project, we studied the PGC-1α-dependent transcriptional changes in skeletal muscle upon acute bouts of exercise and chronic exercise training. Skeletal muscle is a highly plastic organ with an enormous capacity to adapt its metabolism to different energy needs. Interestingly, many of these metabolic changes, especially in response to exercise, are known to be mediated by PGC-1α. Thus, we performed acute time-course and chronic exercise experiments with WT and PGC-1α MKO mice and defined the PGC-1α-dependent and -independent transcriptional changes. Thereby, we identified the WT time-course-specific and acute core exercise responses and could demonstrate that PGC-1α is substantially involved in the regulation of these adaptations in skeletal muscle. Furthermore, while the acute exercise response involved many transcriptional changes, chronic exercise training exerted only minor adaptations in gene expression levels. Thus, we elucidated new important aspects of PGC-1α in the regulation of skeletal muscle exercise physiology.
The third project was aimed at determining the role of PGC-1β in skeletal muscle in response to fasting. Skeletal muscle constitutes the largest protein reservoir of the body and its catabolism is the main source of amino acids for hepatic gluconeogenesis during energy deprived conditions. Thus, skeletal muscle emerges as one of the key players in the whole body response to fasting, yet, the complex regulation of skeletal muscle metabolism upon energy deprivation is still poorly understood. Thus, we evaluated the involvement of PGC-1β in the control of fasting-induced skeletal muscle adaptations in WT and PGC-1β MKO mice. Interestingly, 24 h of fasting induced only a partial muscle mass loss in PGC-1β MKO animals, which was characterized by reduced myostatin mRNA levels, a blunted induction of atrophy markers gene expression and absent activation of AMP-dependent and cAMP-dependent protein kinases in comparison to WT animals. Furthermore, PGC-1β MKO mice exhibited increased transcriptional activity of the nuclear factor of activated T-cells, cytoplasmic 1 (Nfatc1) and showed elevated PGC-1α expression levels. Thus, our data suggest that PGC-1β might inhibit Nfatc1 transcriptional activity during fasting-induced muscle atrophy. These data shed new light on the complex regulation of skeletal muscle metabolism under energy deprived conditions and revealed PGC-1β as an important player in the control of fasting.
The fourth project of this thesis assessed the function of PGC-1α in podocyte and kidney metabolism. Glomerular filtration is the first step in urine production and involves different types of cells including podocytes, which are part of the glomerular filtration barrier that contributes to the prevention of protein loss from the primary filtrate. Mitochondrial dysfunction has been implicated to trigger podocyte injury, which eventually progresses to the development of chronic kidney disease. However, mitochondrial function and its contribution to podocyte disorders are still poorly understood. Thus, we studied the role of PGC-1α in podocyte metabolism under basal and stress-induced conditions in WT and PGC-1α podocyte-specific knockout mice (PGC-1α PKO). The mild increase in glomerular basement membrane thickness in PGC-1α PKO animals did not result in any functional deficits and young and aged PGC-1α PKO mice showed unchanged kidney and podocyte function under basal and stress-induced conditions in comparison to WT animals. Therefore, we concluded that PGC-1α is not mandatory for normal podocyte function in-vivo.
In summary, this thesis describes new aspects of PGC-1α and PGC-1β in the regulation of skeletal muscle and kidney metabolism. Moreover, we identified new molecular pathways and mechanisms by which these two coactivators exert their biological functions. Finally, our results might serve as cornerstone in the development of future therapeutic strategies for the treatment of metabolic disorders and other disease conditions.