Control of sleep by a network of cell cycle genes

Abstract

Sleep is essential for health and cognition, but the molecular and neural mechanisms of sleep regulation are not well understood. We recently reported the identification of TARANIS (TARA) as a sleep-promoting factor that acts in a previously unknown arousal center in Drosophila. tara mutants exhibit a dose-dependent reduction in sleep amount of up to ∼60%. TARA and its mammalian homologs, the Trip-Br (Transcriptional Regulators Interacting with PHD zinc fingers and/or Bromodomains) family of proteins, are primarily known as transcriptional coregulators involved in cell cycle progression, and contain a conserved Cyclin-A (CycA) binding homology domain. We found that tara and CycA synergistically promote sleep, and CycA levels are reduced in tara mutants. Additional data demonstrated that Cyclin-dependent kinase 1 (Cdk1) antagonizes tara and CycA to promote wakefulness. Moreover, we identified a subset of CycA expressing neurons in the pars lateralis, a brain region proposed to be analogous to the mammalian hypothalamus, as an arousal center. In this Extra View article, we report further characterization of tara mutants and provide an extended discussion of our findings and future directions within the framework of a working model, in which a network of cell cycle genes, tara, CycA, and Cdk1, interact in an arousal center to regulate sleep.

Keywords: Cdk1; CycA; Drosophila; TARANIS; behavior; cell cycle genes; pars lateralis; sleep.

Figure 1.

Behavioral phenotypes of tara mutants. (A) Percentage of control and tara flies (n=31−64) that were awakened by a 1 sec pulse of 100 lux light delivered at ZT16. Only flies that were asleep prior to the light pulse are included. (B) Survivorship curves of female control and tara^1/s132 flies (n = 66−118). (C) Percentage of control, tara^1/+, tara^s132, and tara^1/s132 flies (n = 37−50) that crossed a 10 cm mark within 10 sec against gravity. (D) Percentage of control and tara^1/s132 flies (n = 46−68) that chose food with 25 mM sucrose, in the absence or presence of 3 mM quinine, over 5 mM sucrose. Control and tara flies showed an equivalent preference for a higher concentration of sugar and an equivalent avoidance of bitter tasting quinine. Mean ± SEM is shown. *p < 0.05, **p < 0.01, ***p < 0.001, Chi-square test (A, D), log rank test (B), and one-way ANOVA followed by Dunnett post hoc test relative to control flies (C).

Figure 2.

tara and E2f1 appear not to interact for sleep regulation. (A) Sleep profile of background control (white circles), E2f1^EY14408/+ (gray diamonds), tara^e01264/s132 (black triangles), and E2f1^EY14408/+, tara^e01264/s132 (open red squares) female flies (n=17−21) in 30 min bins. The white and black bars below the X-axis represent 12 h light and 12 h dark periods, respectively. (B) Total daily sleep amount for the same genotypes indicated in (A). (C) Total daily sleep of the indicated genotypes (n = 16 for all genotypes). Mean ± SEM is shown. ns: not significant, 2-way ANOVA followed by Tukey post hoc test (B, C).

Figure 3.

Working model of how TARA promotes sleep. TARA upregulates CycA, which negatively regulates Cdk1 activity in the PL neurons. We propose that increased activity of Cdk1 leads to an increase in the excitability of PL neurons, which promotes wakefulness and feeding. We further speculate that TARA levels and/or activity are modulated by sleep debt and satiety. The two diagrams depict PL neurons when flies have high sleep debt and satiety (left) and when they have low sleep debt and satiety (right), respectively. The mammalian counterparts are indicated in parentheses, and broken lines represent connections that need further investigation.

Figure 4.

TARA protein levels do not cycle in circadian pacemaker neurons. (A) Immunostaining of TARA::GFP in male fly brains on the 3^rd day in DD. We used transgenic flies that carry an artificial exon encoding GFP inserted into an intron of tara in the genome and therefore are expected to produce endogenous levels of TARA protein fused to GFP. Brains were dissected at indicated circadian times (CT) and stained for GFP (green) and PDF (red), which was used to identify small ventral lateral neurons (sLN_vs), the pacemaker neurons in DD. Scale bar 10µm. (B) Quantification of TARA::GFP signal in sLN_vs. Data from 11−16 brain hemispheres are presented. Mean ± SEM is shown. ns: not significant, 2-way ANOVA followed by Tukey post hoc test (B).