The role of astrocytic glycogen metabolism in the regulation of the sleep/wake cycle was suggested decades ago but is still not elucidated. For a long time, glycogen was considered as an energy storage mobilized during high energy states of the brain, like wakefulness, through the activity-dependent release of glycogenolytic neurotransmitters such as noradrenaline (NA), and restored during sleep. However, most of the experimental observations pointed to the absence of brain glycogen depletion after 6 hours of sleep deprivation (SD), a condition favoring glycogenolysis, and therefore failed to confirm the initial assumption. As an alternative, we proposed a "glycogenic" hypothesis that relied on the observation that NA also included long-term glycogen re-synthesis. This involves the stimulation of expression of Protein Targeting to Glycogen (PTG, a glycogen binding subunit of protein phosphatase 1), a major regulator of glycogen synthesis in astrocytes, which may counteract the increased glycogen mobilization induced by SD. To test this hypothesis, we performed both in vitro and in vivo experiments using PTG-KO mice. First, using cultured PTG KO astrocytes we were able to demonstrate that in the absence of PTG i) basal glycogen content was decreased by 80% and ii) NA-induced long term glycogen synthesis was fully abolished. We then went on to characterize the impact of SD in PTG- KO mice on brain glycogen contents and analyzed expression levels of different genes involved in glycogen metabolism. As expected, SD did not affect brain glycogen content in WT mice. However, a 30% increase in PTG mRNA expression was observed. Interestingly, while PTG KO mice displayed an 80% decrease in brain glycogen content under control conditions, no further decrease was observed after SD. Therefore, it is unlikely that PTG contributes to the maintenance of glycogen levels during SD. Gene expression analysis demonstrated that a homolog of PTG, PPP1R6, was also induced by SD suggesting that different mechanisms, in addition to the increase of PTG expression, may be involved in the regulation of glycogen content. Altogether, these results do not support a critical role of PTG in the maintenance of glycogen levels during SD. In addition, this study do not argue in favor of the “glycogenic” hypothesis and points to the involvement of regulators other than PTG that warrant further investigation.
Acknowledgements
This work was supported by grants from Swiss National Science Foundation (FNRS) (grant numbers 31003A_130821 and 310030B_148169) and from the Panacee and Prefargier Foundations to PJM. The PTG knockout mice were supported by NIH grants DK27221 and NS056454 to PJR.
