Our laboratory investigates nuclear receptor involvement in developmental, physiologic and pathophysiologic regulation of muscle metabolism.
The nuclear receptor (NR) superfamily of ligand responsive transcription factors are important regulators of cellular metabolism. As a group, they present an exciting frontier for the study and therapeutic targeting of metabolism in health and disease. The role of the Estrogen-related Receptor (ERR) group of NRs in skeletal muscle energy metabolism is a major focus of studies in the laboratory. ERRs regulate metabolic gene targets by direct binding and transactivation or repression of important energy metabolic genes. ERRs are involved in regulating myocyte mitochondrial capacity (biogenesis and electron transport/oxphos enzyme gene expression) and glucose versus fatty acid substrate selection. In addition to their essential role in directing target gene regulation with the PGC-1 coactivators, ERRs have PGC-1-independent regulatory roles in muscle and cancer.
Combining biochemical, metabolic, molecular mechanism, histochemical, and genetic approaches the focus of our laboratory is to characterize energy metabolic regulation using various models:
Cell-based analysis of pathways involved in energy regulation. Primary adult and myogenic cell culture models are used to characterize ERRa and ERRg target gene regulation using chromatin immunoprecipitation and gene expression profiling. We employ reporter gene and protein-protein interaction assays to identify transcription factors/cofactors involved in regulating muscle energy metabolism. These studies inform the design of molecular tools (i.e. dominant negative or selective mutants), the identification of novel interacting proteins and characterization of ERR regulation by ligands or signaling pathways.
We assess cellular metabolism using Seahorse flux analyzer to investigate novel molecular targets of the ERR pathway. Several ERRa targets are being investigated to determine the mechanism involved altering cancer cell growth, insulin signaling and glucose uptake and to investigate whether they play a role in other ERRa expressing cells.
Transgenic mouse models to determine in vivo function and relevance of regulatory pathways. Mouse models in which ERRs are conditionally overexpressed or deleted are being used to determine in vivo regulatory function in skeletal muscle. The muscle-specific ERRa and ERRg knockout and ERRa overexpressing mice are being used to investigate the role of these receptor isoforms in the development of insulin resistance in response to high fat diet, exercise mediated adaptations and muscle regeneration.
Using comparative models to investigate regulatory pathways involved in mediating metabolic plasticity. In collaboration with colleagues at Miami University in Ohio we are characterizing the role of PPARs in regulating lipid metabolism in skeletal muscle, liver, and adipose tissue throughout the annual cycle in migratory birds. We used RNA-seq in skeletal muscle and adipose to identify transcripts expressed in these tissues that may be involved in fatty acid uptake and metabolism. The PPAR isoforms have the same tissue expression pattern in birds and mammals (PPARα, liver, skeletal muscle; PPARδ, skeletal muscle; PPARγ, adipose), and expression is upregulated in these target tissues prior to migration. The aims of the studies are to characterize PPAR isoform expression and function in adipose and flight muscle across the annual cycle and to determine whether PPAR activation is necessary or sufficient for pre-migratory adipose and muscle metabolic plasticity. These comparative studies have relevance to understanding evolutionary selected adaptive mechanisms by which skeletal muscle maintains or restores insulin sensitivity in the context of lipid overload, a pathologic metabolic state in mammalian systems.
The long-term goal of my research program is to understand the gene regulatory and upstream signaling programs involved in msucle energy metabolism and growth. Muscle accounts for 40% of body mass and approximately 30% of resting metabolism in adults. The mechanisms that regulate muscle metabolism in health and disease have important implications for whole body energetic and glucose homeostasis. The ATP generating capacity and selection of fats or glucose as the preferred substrate for energy generation are largely determined at the level of the mitochondria but also involve regulation of substrate uptake. In cardiac and skeletal muscle metabolic pathways undergo dramatic shifts during development and maturation, and are altered in response to physiologic stimuli, such as exercise and fasting. Metabolic dysregulation in muscle that occurs in obesity and insulin resistance is an important contributor to the pathophysiology of human disease. Understanding the molecular mechanisms regulating energy metabolism in skeletal muscle will advance our understanding of the causes of metabolic dysregulation in obesity, diabetes, heart failure and aging.