Genetic and Molecular Analysis of a Novel Insulin Regulatory Protein

Scripps Florida, Jupiter FL

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Abstract

PROJECT SUMMARY The molecular mechanisms that underpin the development of insulin resistance with age remain poorly defined, yet a better understanding of this process is critical, given the fact that obesity, metabolic disease and diabetes have reached epidemic status in the US. We discovered that alternative splicing of the insulin receptor in humans leads to truncated isoforms that have the potential to inhibit insulin signaling, by acting as decoy receptors or soluble binding proteins. Changes in the frequency of alternative splicing events could provide a novel mechanism by which insulin resistance and metabolic disorders develop. Determination of the physiological significance of these new isoforms is critical in establishing their role in the pathophysiology of insulin resistance with age. However, such studies in mammals are expensive, time consuming and risky. In contrast, the nematode C. elegans provides an economical system in which to rapidly determine their physiological relevance. To this end, we discovered a truncated isoform of the insulin receptor in the worm, termed DAF-2B, that is directly analogous to mammalian IR-C and IR-D. This discovery provides an opportunity to rapidly and economically define the physiological role and regulation of truncated insulin receptors in a genetically tractable organism, prior to initiating studies in mammals. Using a novel splicing reporter strain that permits visualization of differential splicing between the full length and truncated daf-2 isoforms in vivo, we find that daf-2b expression varies across tissues and through development, suggesting that these transcripts are subject to specific regulatory mechanisms. Importantly, over-expression of the daf-2b cDNA in worms produces phenotypes consistent with attenuated insulin signaling, including increased nuclear localization of the FOXO transcription factor DAF-16 and extended lifespan. Conversely, preliminary analysis of a daf-2b deletion strain, generated using CRISPR, indicates that loss of daf-2b confers insulin sensitivity. Finally, using our daf-2b reporter strain, we identified specific splicing factors that alter the expression of daf-2b, including one whose activity may be directly regulated by insulin signaling and Akt phosphorylation. Exploration of the mechanism by which splicing factor activity regulates the expression of truncated IR will identify novel points of regulation and intervention in this pathway. We hypothesize that aberrant or mis-regulated expression of truncated IR-C and IR-D isoforms in mammals could be causally involved in the pathogenesis of insulin resistance, diabetes and other forms of metabolic disease. Thus, determination of the physiological role and regulation of truncated insulin receptor isoforms in worms will provide a solid foundation from which to establish studies of equivalent isoforms in mammals.