Mechanisms of metabolic rate depression: following nature's lead

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The objective of my research is to discover the molecular principles and mechanisms of regulatory control that underlie the phenomenon of metabolic arrest in nature. To survive severe environmental challenges such as subzero temperatures, oxygen deprivation or arid conditions, many animal species use metabolic rate depression (MRD) to enter states of torpor or dormancy. My lab analyzes the biochemistry involved. Using molecular tools and vertebrate models of hibernation (ground squirrels), freeze tolerance (wood frogs), anoxia tolerance (turtles) and estivation (African clawed frogs) (and selected invertebrates too), we explore the adaptations of enzymes and proteins, the mechanisms that suppress and reprioritize ATP-expensive cell functions, and the changes in cell signalling and gene expression that both control MRD and engineer long term cell preservation and life extension in hypometabolic states. Goals for continuing and proposed work over the next five years address new concepts in the regulation of hypometabolism. (A) TRANSCRIPTIONAL SUPPRESSION: Our recent work on hibernation and anoxia tolerance showed involvement of epigenetic mechanisms in the global silencing of gene transcription during hypometabolism; these include DNA methylation and posttranslational modifications (PTMs) of histone proteins (altered acetylation, phosphorylation). This new insight will be developed with a comprehensive analysis of the pattern and extent of epigenetic modifications across hypometabolic systems coupled with quantification of changes in the activities and regulation of the enzymes involved (e.g. histone acetylases & deacetylases, DNA methyltransferases). Studies will also analyze the role of SUMOylation (a PTM prominent in the nucleus of hibernating ground squirrels) in inhibiting the action of transcription factors (TFs) and thereby helping to suppress of global gene expression during MRD. (B) MICRO-RNA: My lab provided the first evidence for microRNA action as a crucial mechanism of post-transcriptional control over mRNA transcripts, aiding both suppression of ATP-expensive translation and providing for storage of transcripts during MRD. Our continuing studies will investigate microRNA action as a principle of hypometabolism across animal systems and focus on the actions of selected microRNAs in controlling the synthesis of specific TFs and other proteins. (C) SIGNALING & GENE/PROTEIN EXPRESSION: Our recent work showed targeted roles for selected signalling pathways and TFs in suppression of ATP-expensive functions (e.g. protein synthesis, cell cycle) or up-regulation of cytoprotection (e.g. antioxidants, chaperones) during MRD. Proposed studies analyze the reorganization of other metabolic functions during hypometabolism (e.g. anti-apoptosis and autophagy) and the involvement of TFs (e.g. p53, STATs, SMADs) that respond to stress. (D) ENZYME REGULATION: Studies will explore new concepts in enzyme control identified by our work: e.g. phosphorylation control of dehydrogenase enzymes, acetylation as a PTM affecting enzyme activity and regulation. Kinetic, bioinformatic, modelling and stability (using our novel differential scanning fluorimetry method) studies will be integrated to understand enzyme/protein adaptation in hypometabolic systems. Overall, the research will make major contributions to understanding the universal biochemical mechanisms of hypometabolism in nature, mechanisms that not only define the winter survival strategies of many Canadian animals but also have crucial potential applications for biomedicine such as to the development of inducible torpor as a medical intervention strategy.