Siesta-time is in the genes.

In hot climates, as Noel Coward remarked, only “mad dogs and Englishmen go out in the midday sun,” while the locals seem hardwired to take an afternoon siesta. Temperature also affects activity elsewhere in the animal kingdom. One example is garter snakes, which are active at night in warm temperatures, by day at cold temperatures, and show activity around dawn and dusk (with a siesta in between) at intermediate temperatures. How does temperature regulate these changes in behavior?Daily changes in activity are controlled by the circadian clock. In the best-studied cases, these clocks are based on gene products that oscillate to produce a self-sustaining negative feedback loop (reviewed by Dunlap 1999xDunlap, J.C. Cell. 1999; 96: 271–290Abstract | Full Text | Full Text PDF | PubMed | Scopus (1780)See all ReferencesDunlap 1999). There are instances of temperature affecting clock gene products themselves. One example is the Neurospora oscillating clock protein, FREQUENCY (FRQ). Translation initiation is regulated by temperature to change the ratio of two different forms of FRQ, each of which on its own has the ability to keep time over a different temperature range (Liu et al. 1997xLiu, Y., Garceau, N.Y., Loros, J.J., and Dunlap, J.C. Cell. 1997; 89: 477–486Abstract | Full Text | Full Text PDF | PubMed | Scopus (173)See all ReferencesLiu et al. 1997). In a second example, Drosophila collected from the wild display a polymorphism in a threonine–glycine dipepetide repeat near the middle of the PERIOD (PER) clock protein, and these subtly different PER forms are better suited to withstand the ranges of temperature found in their environments (Sawyer et al. 1997xSawyer, L.A., Hennessy, J.M., Peixoto, A.A., Rosato, E., Parkinson, H., Costa, R., and Kyriacou, C.P. Science. 1997; 21: 17–20See all ReferencesSawyer et al. 1997).The latter study suggested a relationship between PER protein and temperature regulation. Now, in this issue of Neuron, Majercak et al. 1999xMajercak, J., Sidote, D., Hardin, P.E., and Edery, I. Neuron. 1999; 24: 219–230Abstract | Full Text | Full Text PDF | PubMed | Scopus (207)See all ReferencesMajercak et al. 1999 find a connection between per RNA, temperature, and siesta-time. per RNA cycles with a peak once every 24 hr. The authors show that per RNA accumulates to higher levels and at an earlier time in each cycle at 18°C compared to 29°C. Furthermore, they find that this correlates with increased splicing of the intron closest to the 3′ end of the per 3′ UTR at 18°C. This is a rapid response: a shift down in temperature increases splicing of this intron, leading to higher per RNA levels within an hour.Majercak et al. 1999xMajercak, J., Sidote, D., Hardin, P.E., and Edery, I. Neuron. 1999; 24: 219–230Abstract | Full Text | Full Text PDF | PubMed | Scopus (207)See all ReferencesMajercak et al. 1999 then tested the simple idea that per RNA lacking this intron would mimic the 18°C increased amplitude per RNA cycles. They tested flies transformed with either per DNA already missing the 3′ intron or with per DNA in which splicing is prevented. Surprisingly, in both cases the flies failed to show the 18°C increase in per RNA. In contrast, a wild-type per transgene behaved like the endogenous gene. Therefore, they concluded that the act of splicing itself, rather than the structure of the RNA, is the primary determinant of different per RNA levels at different temperatures. There are precedents for this: in cell culture, splicing of the 3′-most intron can stimulate 3′ end formation, thereby increasing the production of mature mRNA (Nesic et al. 1993xNesic, D., Cheng, J., and Maquat, L.E. Mol. Cell. Biol. 1993; 13: 3359–3369PubMedSee all ReferencesNesic et al. 1993). It will be interesting to find out whether splicing is generally temperature dependent in Drosophila, or whether this is specific to the per 3′ intron.The consequence of an earlier and higher per RNA peak at 18°C is earlier PER protein accumulation. Strikingly, the early accumulation of PER at low temperatures is correlated with changes in the timing of locomotor activity. Unlike the bursts of activity observed at 25°C centered around dawn and dusk, flies at 18°C have a single broad daytime peak of activity, with no siesta. There was previous evidence that the per gene can regulate the timing of fly activity from experiments comparing different fly species. The daily activity of D. melanogaster and D. pseudoobscura at 25°C differs, with D. pseudoobscura active primarily at dusk. Petersen et al. 1988xPetersen, G., Hall, J.C., and Rosbash, M. EMBO J. 1988; 7: 3939–3947PubMedSee all ReferencesPetersen et al. 1988 showed that this was due to the different per genes, since transforming the melanogaster fly with the pseudoobscura per gene resulted in pseudoobscura-like dusk-active animals.Aside from temperature, the other major environmental stimulus that alters the circadian behavior of flies is light. TIMELESS (TIM), the protein product of a second clock gene, binds and stabilizes PER in the cytoplasm, and the PER–TIM complex then translocates to the nucleus. It is well established that light degrades TIM (reviewed by Young 1998xYoung, M.W. Annu. Rev. Biochem. 1998; 67: 135–152Crossref | PubMed | Scopus (147)See all ReferencesYoung 1998). PER–TIM complex formation is delayed if flies are exposed to light while the complex is accumulating because TIM is degraded and PER becomes unstable in the absence of TIM. Majercak et al. 1999xMajercak, J., Sidote, D., Hardin, P.E., and Edery, I. Neuron. 1999; 24: 219–230Abstract | Full Text | Full Text PDF | PubMed | Scopus (207)See all ReferencesMajercak et al. 1999 show that at 18°C a longer period of daylight delays both PER accumulation and evening activity. Therefore, the fly clock can read light and temperature independently, integrating these cues by regulating the timing of accumulation and overall levels of the PER–TIM complex. This integration is summarized in the figure. It makes seasonal sense because the combination of more per RNA in the cold, and less TIM degradation in shorter days, reinforces early PER–TIM accumulation in winter, precluding a siesta. Majercak et al. 1999xMajercak, J., Sidote, D., Hardin, P.E., and Edery, I. Neuron. 1999; 24: 219–230Abstract | Full Text | Full Text PDF | PubMed | Scopus (207)See all ReferencesMajercak et al. 1999 also show that tim RNA levels respond to both light and temperature, but since the mechanism is unclear, we only include small arrows.So how does the early accumulation of the PER–TIM complex prevent a siesta? This is the key unanswered question highlighted by these experiments. It is well established that when PER and TIM translocate to the nucleus, they negatively regulate transcription of their own genes (reviewed by Hardin 1998xHardin, P.E. Curr. Opin. Neurobiol. 1998; 8: 642–647Crossref | PubMed | Scopus (22)See all ReferencesHardin 1998). It is likely that they also regulate additional clock-controlled genes, which may link the clock to output pathways that determine behavior. Indeed, Jin et al. 1999xJin, X., Shearman, L.P., Weaver, D.R., Zylka, M.J., De Vries, G.J., and Reppert, S.M. Cell. 1999; 96: 57–68Abstract | Full Text | Full Text PDF | PubMed | Scopus (633)See all ReferencesJin et al. 1999 have shown that mammalian PER and TIM can directly regulate vasopressin gene expression in the mammalian pacemaker. Therefore, changes in the levels of the PER–TIM complex could change the magnitude and duration of the responses of downstream genes. Additionally, there is evidence for posttranscriptional regulation of clock-controlled genes (So and Rosbash 1997xSo, W.V. and Rosbash, M. EMBO J. 1997; 16: 7146–7155Crossref | PubMedSee all ReferencesSo and Rosbash 1997), and this may involve the PER–TIM complex as suggested by the studies of Suri et al. 1999xSuri, V., Lanjuin, A., and Rosbash, M. EMBO J. 1999; 18: 675–686Crossref | PubMed | Scopus (39)See all ReferencesSuri et al. 1999. Clearly, the identification of additional PER–TIM targets will help us understand how the molecular cycles of the clock are transformed into behavioral signals. In summary, the finding that temperature regulates clock genes means that the decision to take a siesta, at least for Drosophila, really is hardwired into the genes.Figure 101Model of How the Drosophila Clock Integrates Temperature and Light to Regulate Clock Gene Products and Behaviorper and tim RNAs are transcribed, and their protein products interact and translocate back to the nucleus where they repress the transcription of their own genes. This gives rise to rhythmic cycles of per and tim transcription. Cold increases PER production and darkness decreases TIM degradation. Thus, in the winter, siestas are precluded.View Large Image | View Hi-Res Image | Download PowerPoint Slide