Post-transcriptional regulation of energy usage: glucose and lipid metabolism

National Institute On Aging

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Abstract

The studies in this Project focus on understanding the RNA-binding proteins (RBPs) and noncoding (nc)RNAs that influence energy metabolism, since the processes that generate energy become impaired with aging. In particular, we have studied the regulation of insulin production, adipogenesis, and autophagy by RBPs and ncRNAs. Glucose homeostasis. With rising appreciation that glucose metabolism is extensively regulated at the post-transcriptional level, we recently identified the HuD as an RBP that associated with the 5UTR of the mRNA encoding insulin and repressed insulin translation. On the same topic of insulin, we obtained evidence that the microRNA miR-196b induced the production of Insulin 2 via the 5'UTR, likely by competing with HuD for binding the same (Panda et al., PLoS ONE 2014). Other energy metabolism. During this review period, we also discovered that the mammalian RNA-binding protein AUF1 (AU-binding factor 1) binds to numerous mRNAs that encode muscle-specific factors. We thus investigated the function of AUF1 in skeletal muscle differentiation. In mouse C2C12 myocytes, where AUF1 levels rise at the onset of myogenesis and remain elevated throughout myocyte differentiation into myotubes, RNP immunoprecipitation (RIP) analysis indicated that AUF1 binds prominently to Mef2c (myocyte enhancer factor 2c) mRNA, which encodes the key myogenic transcription factor MEF2C. By performing mRNA half-life measurements and polysome distribution analysis, we found that AUF1 associated with the 3' untranslated region (UTR) of Mef2c mRNA and promoted MEF2C translation without affecting Mef2c mRNA stability. In addition, AUF1 promoted Mef2c gene transcription via a lesser-known role of AUF1 in transcriptional regulation. Importantly, lowering AUF1 delayed myogenesis, while ectopically restoring MEF2C expression levels partially rescued the impairment of myogenesis seen after reducing AUF1 levels. We propose that MEF2C is a key effector of the myogenesis program promoted by AUF1 (Panda et al., Mol Cell Biol 2014). Other RBPs involved in myogenesis include HuR and KSRP, which influence the half-life of the Nucleophosmin mRNA and thereby influence muscle fiber formation (Cammas et al., Nature Communications, 2014). Mitochondria. Over the past twelve months, we have made important progress towards understanding the role and metabolism of long noncoding (lnc)RNA in mitochondrial function. Some mitochondrial lncRNAs are encoded by nuclear DNA, but the mechanisms that mediate their transport to mitochondria are poorly characterized. One such nuclear DNA-encoded lncRNA, RMRP, was found to be the target of RBPs HuR and GRSF1, which associated with RMRP and mobilized it to mitochondria. In cultured human cells, HuR bound RMRP in the nucleus and exported it to the cytosol; subsequently, GRSF1 facilitated the translocation of RMRP through the mitochondrial inner membrane and increased the mitochondrial abundance of RMRP. Accordingly, silencing GRSF1 impaired the import of RMRP into mitochondria and lowered oxygen consumption rates. Our findings delineate a mechanism whereby RBPs mediate the transport of nuclear DNA-encoded lncRNAs into the mitochondria. This work was reviewed in a top-tier journal and we are currently revising it for publication.