Induction of mutant Sik3Sleepy allele in neurons in late infancy increases sleep need.

Sleep is regulated in a homeostatic manner. Sleep deprivation increases sleep need, which is compensated mainly by increased EEG delta power during non-rapid eye movement sleep (NREMS) and, to a lesser extent, by increased sleep amount. Although genetic factors determine the constitutive level of sleep need and sleep amount in mice and humans, the molecular entity behind sleep need remains unknown. Recently, we found that a gain-of-function Sleepy (Slp) mutation in the salt-inducible kinase 3 (Sik3) gene, which produces the mutant SIK3 (SLP) protein, leads to an increase in NREMS EEG delta power and sleep amount. Since Sik3Slp mice express SIK3 (SLP) in various types of cells in the brain as well as multiple peripheral tissues from the embryonic stage, the cell type and developmental stage responsible for the sleep phenotype in Sik3Slp mice remains to be elucidated. Here, we generated two mouse lines, synapsin1CreERT2 and Sik3ex13flox mice, which enable inducible Cre-mediated, conditional expression of SIK3 (SLP) in neurons upon tamoxifen administration. Administration of tamoxifen to synapsin1CreERT2 mice during late infancy resulted in higher recombination efficiency than administration during adolescence. SIK3 (SLP) expression after late infancy increased NREMS and NREMS delta power in male synapsin1CreERT2; Sik3ex13flox/+ mice. The expression of SIK3 (SLP) after adolescence led to a higher NREMS delta power without a significant change in NREMS amounts. Thus, neuron-specific expression of SIK3 (SLP) after late infancy is sufficient to increase sleep. Significant Statement The propensity to accumulate sleep need during wakefulness and to dissipate it during sleep underlies the homeostatic regulation of sleep. However, little is known about the developmental stage and cell types involved in determining the homeostatic regulation of sleep. Here, we show that Sik3Slp allele induction in mature neurons in late infancy is sufficient to increase NREMS amount and NREMS delta power. SIK3 signaling in neurons constitutes an intracellular mechanism to increase sleep.