Functional consequences of brain glycogen deficiency on the sleep-wake cycle regulation in PTG-KO mice

Abstract

Introduction: In the CNS, glycogen is mainly localized in astrocytes where its levels are linked to neuronal activity. Astrocytic glycogen synthesis is regulated by glycogen synthase (GS) activity that is positively controlled by protein targeting to glycogen (PTG) expression levels. Although the role of glycogen in sleep/wake regulation is still poorly understood, we have previously demonstrated that, following a 6 hour gentle sleep deprivation (GSD), PTG mRNA expression and GS activity increased in the brain in mice while glycogen levels were paradoxically maintained and not affected. In order to gain further insight on the role of PTG in this process, we studied the sleep/wake cycle parameters in PTG knockout (PTG-KO) mice under baseline conditions and after a 6 hour GSD. Glycogen levels as well as mRNAs expression of genes related to energy metabolism were also determined in several brain areas. Materials and methods: Adult male C57BL/6J (WT) and PTG-KO mice were sleep-recorded under baseline conditions (24 h recordings, 12 h light/dark cycle) and following 6 hours GSD from ZT00 to ZT06. Vigilance states were visually scored (4 s temporal window). Spectral analysis of the EEG signal was performed using a discrete Fourier transformation. Glycogen measurements and gene expression analysis were assessed using a biochemical assay and quantitative RT-PCR respectively, on separate cohorts in WT vs PTG-KO mice at the end of the 6 hours GSD or in control animals (CTL) in different brain structures. Results: Quantitative analysis of the sleep/wake cycle under baseline conditions did not reveal major differences between the WT and the PTG-KO mice. However, during the dark period, the PTG-KO mice showed a significant increase in the number of wake and slow wave sleep episodes (respectively +26.5±8% and +26.1±8%; p< 0.05) together with a significant shortening in their duration (-21.6±7.2% and -14.3±2.8%; p< 0.01). No such quantitative changes were observed during paradoxical sleep (PS). However, the spectral analysis of PS indicated that there was a significant increase of the spectral power between 7 and 8.5 Hz in PTG-KO compared to WT mice. As expected, SD did not affect brain glycogen content in WT mice even though a 20 to 90% increase in PTG mRNA expression was measured depending on the brain structure analyzed. PTG KO mice displayed an 80% decrease in brain glycogen content compared to WT under control conditions with no further decrease after GSD. Conclusions: Although, it is unlikely that PTG contributes to the maintenance of glycogen levels during SD, the deletion of its gene resulted in EEG modifications of the theta band during the PS under baseline conditions and the absence of a significant PS rebound after GSD. The results provide the first evidence for a role of PTG in sleep and wakefulness, specifically in the regulation of PS, which warrants further investigation.