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

Abstract

In Sleepy (Sik3Slp) or Sik3S551A mice, deletion or mutation of inhibitory phosphorylation site serine551 (S551) 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 or quality. 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-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. Statement of SignificancePrevious studies have identified SIK3 as a key sleep regulatory kinase, of which gain-of-function mutations markedly increase sleep quantity and intensity in Sleepy mice. Multiple SLEEPY isoforms are expressed in mouse brain neurons, but their respective contributions to the hypersomnia phenotypes of Sleepy mice remain unclear. Here, we identified a new short isoform of SLP/SIK3 and examined sleep phenotypes resulted from specific expression of SLEEPY-short or long isoform. Our results indicate that both long and short isoforms of SLEEPY kinase contribute critically to increases of sleep quantity and intensity in Sleepy mice. Future studies are needed to identify downstream substrates of SLEEPY kinase that mediate regulation of sleep quantity/intensity, which should facilitate development of new treatments for human sleep disorders.