Energy metabolism: transcriptional and epigenetic control

The main focus of our research is to investigate the transcriptional and epigenetic mechanisms responsible for the regulation of oxidative metabolism, either in physiological settings and in metabolic disorders such as diabetes and obesity. Mitochondria play a central role in energy metabolism and their functions are modulated at multiple levels. Several factors regulating mitochondrial function and biogenesis have been described so far. These include mitochondrial transcription factor A (mTFA) that drives replication and transcription of mitochondrial DNA, and the transcriptional coactivator PGC-1 that regulates mitochondrial function and biogenesis in cells and tissues characterized by high oxidative metabolism, e.g. skeletal muscle and brown adipose tissue. Of note, PGC-1 activity can be modulated by nutritional and environmental cues that either regulate its own transcription or affect its acetylation/phosphorylation state. In this respect, sirt-dependent deacetylation of PGC-1 results in its activation and triggers down-stream effects such as a boost of mitochondrial activity or mitochondrial biogenesis. Epigenetics and chromatin dynamics have recently emerged as key contributors in the regulation of gene transcription. Chromatin remodeling is accomplished by means of several post-translational modifications of histone proteins including acetylation, methylation, phosphorylation and ADP-ribosylation. Histone deacetylases (HDACs) participate in the regulation of chromatin acetylation state, and consequently play a critical role in the regulation of gene transcription. By using class specific HDAC inhibitors we could assess that biochemical inhibition of class I HDACs increases mitochondrial biogenesis and oxidative metabolism in C2C12 murine myotubes via upregulation of the coactivator PGC-1, in a HDAC3-dependent manner. Administration of the class I HDAC inhibitor to db/db mice improves the obese and diabetic phenotype, and induces significant metabolic changes in skeletal muscle and in white and brown adipose tissues. We therefore propose that biochemical inhibition of class I HDACs reveals a mitochondrial signature mediated by the transcriptional coactivator Pgc-1 in skeletal muscle and by the Pgc-1/Ppar axis in adipose tissue, leading to insulin sensitizing effect in db/db mice. Another major challenge in mitochondrial biology is the discovery and characterization of new factors and/or regulatory pathways that may complement the action of known regulators, e.g. mTFA, PGC-1, sirtuins. To this end we performed a genome-scale high throughput screening using a gain-of-function mTFA-based approach. Starting from two cDNA libraries consisting of 27,000 genes that account for 70% of known genes, we identified > 400 clones able to induce mTFA promoter activity. The list of positive hits includes genes whose involvement in mitochondrial functions has been already reported. Further characterization of the positive clones revealed that a subset of 131 hits also increase mitochondrial density and function. According to functional analysis by Gene Ontology the main molecular functions associated to the 131 hits are catalytic activity, translation regulator activity, and transcription regulator activity. Ongoing and future work is aimed at the validation and further characterization of the positive hits with the long range goal of identifying new targets for the treatment of metabolic diseases associated to mitochondrial dysfunction. Funded by EU FP6 LSHM-CT2006-037498, Cariplo Foundation 2008.2511, The Armenise-Harvard Foundation and PRIN 2008 ZTN724