Partial activation of SIK3 delays the onset of wakefulness and alleviates hypersomnia due to the lack of protein kinase A-phosphorylation site.

Study Objectives: Sleep/wakefulness is regulated by intracellular signaling pathways composed of protein kinases such as salt-inducible kinase 3 (Sik3). Sik3-deficiency in neurons decreases NREM sleep time and electroencephalogram (EEG) delta power during NREM sleep, while Sik3Slp mice lacking a protein kinase A (PKA)-phosphorylation site, S551, show hypersomnia phenotype. In this study, we examined how a phosphomimetic mutation of the 221st threonine residue (T221E), which provides a partial (weak) constitutive activity of the kinase, affects sleep/wakefulness and circadian behavior. We also examined the effect of T221E substitution on the hypersomnia phenotype of Sik3Slp mice.

Methods: We examined the sleep/wake behavior of heterozygous and homozygous Sik3T221E mice and Sik3T221E;Slp mice using EEG and electromyogram (EMG) recording. We also examined the circadian behavior of Sik3T221E mice using a running wheel under the light-dark cycle and constant darkness.

Results: Heterozygous and homozygous Sik3T221E mice showed normal sleep time and sleep homeostatic responses. Homozygous Sik3T221E mice exhibited a delayed onset of wakefulness at the early dark phase and longer circadian periods. Sik3T221E;Slp mice showed decreased NREM sleep time and homeostatic responses compared to Sik3Slp mice.

Conclusion: Our results suggest that the peak onset of wakefulness is sensitive to disturbed kinase activity of SIK3, and the relationship between phosphorylation at T221 and S551 is critical for regulating sleep need.