Regulation of sleep quantity and intensity by long and short isoforms of SLEEPY kinase

In Sleepy (Sik3Slp) or Sik3S551A mice, deletion or mutation of inhibitory phosphorylation site serine551 from salt-inducible kinase 3 (SIK3) markedly increases daily non-rapid eye movement sleep (NREMS) amount, accompanied with constitutively elevated NREMS delta power density-a measure of sleep intensity. Multiple SLP/SIK3 isoforms are expressed in mouse brain neurons, however, their respective roles in sleep regulation remain to be elucidated. Here, we identified a new and most abundant short isoform of SLP/SIK3 and examined sleep phenotypes resulted from isoform-specific expression of SLP-short (S) and long (L) isoforms. Adeno-associated virus (AAV)-mediated adult brain chimeric (ABC)-expression of SLP-S in neurons, but not in astrocytes, significantly and constitutively elevates NREMS delta power, whereas slightly increases NREMS amount. The ability of SLP-S to regulate sleep quantity/intensity is abrogated by kinase-inactivating mutations, suggesting that the sleep-promoting activity of SLP-S is dependent on its kinase activity. In Sik3S551A-L knock-in mice, isoform-specific expression of SIK3S551A-L (or SLP-L) significantly increases NREMS amount with a modest effect on NREMS delta power. ABC-expression of SLP-S complements the sleep phenotypes of heterozygous Sik3S551A-L mice by further increasing NREMS amount and NREMS delta power to levels of Sik3Slp or Sik3S551A mice. Taken together, these results indicate that both SLP-L and SLP-S isoforms contribute critically to the increases of sleep quantity and intensity in Sik3Slp or Sik3S551A mice.