49. Regulation of PTEN function and structural stability in hibernating thirteen-lined ground squirrels

Abstract

For many small mammals, survival over the winter months is a serious challenge because of low environmental temperatures and limited food availability. The solution for many species, such as thirteen-lined ground squirrels (Ictidomys tridecemlineatus), is hibernation, an altered physiological state characterized by metabolic rate depression to achieve major energy savings, and a coordinated depression of non-essential ATP-expensive functions such as protein synthesis. This study examines the regulation of PTEN phosphatase, a negative regulator of the insulin receptor network, over the torpor–arousal cycle of hibernation in the skeletal muscle of I. tridecemlineatus. Immunoblots showed that the ratio of non-phosphorylated PTEN (Ser380,Thr382/383) to total PTEN levels were significantly elevated in (by 1.4-fold) during late torpor compared to euthermic controls; this was coupled with a significant decrease in Km for PIP3 (by 59%) in late torpor. Pulse-proteolysis analysis of purified PTEN protein showed a decrease in structural stability of PTEN during late torpor compared to euthermic controls (decreased I50 urea by 21%). Furthermore, the increase in PTEN activity observed was correlated with a decrease in the ratio of phosphorylated PDK-1(Ser241) to total PDK-1 in late torpor, suggesting a downstream effect of PTEN activation during torpor. Transcriptional analysis showed that mRNA expression of both PTEN and PDK-1 was unchanged during the course of the hibernation cycle. Overall, the results indicate that post-translational modifications, more specifically phosphorylation, play a crucial role in regulating the enzymatic activity and half-life of PTEN, a major regulator of the insulin signaling pathway, during torpor. Funded by NSERC Canada. For more information go to: www.carleton.ca/∼kbstorey.