Splicing of the period Gene 3′-Terminal Intron Is Regulated by Light, Circadian Clock Factors, and Phospholipase C

Abstract

The daily timing of circadian (Х24-h) controlled activity in many animals exhibits seasonal adjustments, responding to changes in photoperiod (day length) and temperature. In Drosophila melanogaster, splicing of an intron in the 3 untranslated region of the period (per) mRNA is enhanced at cold temperatures, leading to more rapid daily increases in per transcript levels and earlier "evening" activity. Here we show that daily fluctuations in the splicing of this intron (herein referred to as dmpi8) are regulated by the clock in a manner that depends on the photoperiod (day length) and temperature. Shortening the photoperiod enhances dmpi8 splicing and advances its cycle, whereas the amplitude of the clock-regulated daytime decline in splicing increases as temperatures rise. This suggests that at elevated temperatures the clock has a more pronounced role in maintaining low splicing during the day, a mechanism that likely minimizes the deleterious effects of daytime heat on the flies by favoring nocturnal activity during warm days. Light also has acute inhibitory effects, rapidly decreasing the proportion of dmpi8-spliced per transcript, a response that does not require a functional clock. Our results identify a novel nonphotic role for phospholipase C (no-receptor-potential-A [norpA]) in the temperature regulation of dmpi8 splicing.Circadian rhythms are driven by cellular oscillators known as clocks or pacemakers and are an important aspect of the temporal organization observed in a wide range of organisms from bacteria to humans (11,12). These clocks exhibit free-running (self-sustaining) periods of Х24 h in the absence of environmental cues. Nonetheless, an important adaptive feature of circadian oscillators is that they are synchronized (entrained) by daily changes in environmental modalities, most notably visible light and ambient temperature.Light is almost certainly the predominant entraining agent in nature. Under natural conditions the light-dark (LD) cycle aligns the phases of clocks and evokes daily adjustments in the approximately 24-h endogenous periods of these oscillators such that they precisely match the 24-h solar day. The duration of day length (photoperiod) can modify the temporal alignment between a circadian rhythm and local time (22). A physiologically relevant advantage of this inherent flexibility of clocks is that the daily distributions of physiological and behavioral rhythms are not rigidly locked to local time but can be adjusted for seasonal changes in day length.Despite the obvious importance of photoperiod, ambient temperature is also a key environmental modality regulating the timing of circadian rhythms (50). This makes intuitive sense, because in temperate latitudes seasonal changes in day length are also accompanied by predictable changes in average daily temperatures. Thus, circadian clocks play an important role in endowing organisms with the ability to anticipate daily and seasonal changes in environmental conditions, resulting in physiological and behavioral rhythms that occu...

Even a stopped clock tells the right time twice a day: circadian timekeeping in Drosophila

“…This response is controlled by the splicing of an intron within the 3′ UTR of per [57]. Regulation of per splicing integrates seasonal information, as it responds to both temperature and light [57][58][59]. At low temperatures, or under the short photoperiods associated with colder days, per splicing levels are increased at least in photoreceptor cells where this phenomenon has largely been studied.…”

“…This leads to earlier processing of the per transcript and earlier accumulation of PER protein, resulting in an earlier phase of evening activity, and so reducing the mid-day siesta. This allows the behaviour of a fly to be fine-tuned to any given day across the seasons [57][58][59]. Rescue of per 01 mutants with a per transgene where the intron cannot be spliced fails to rescue this adaptive response [57].…”

“…Information regulating per splicing is received through a signalling pathway involving the NorpA Phospholipase-C [58,59]. Furthermore, NorpA has subsequently been shown to be a general factor involved in temperature entrainment of behaviour, as NorpA mutants fail to entrain behavioural rhythms to temperature cycles in LL.…”

Synchronization of theDrosophilaCircadian Clock by Temperature Cycles

“…During short photoperiods and cold temperatures, splicing is enhanced, leading to an earlier rise of per mRNA and protein, which is likely responsible for the earlier behavioral activity peak under cold and short-day conditions (Majercak et al 1999). In norpA mutants, splicing of the per intron occurs in the "cold" mode, even if the flies are kept under warm and long-day conditions (Collins et al 2004;Majercak et al 2004). Levels of the spliced per transcript are also high in DD, indicating that the role of norpA in the regulation of per splicing is independent of light.…”

“…This effect is enhanced by short and long photoperiods, respectively (Majercak et al 1999), which suggested that the combined response to temperature and photoperiod allows the fly to adjust its behavior to the different seasons of the year. More recent work has revealed that this adaptation is mediated via a splicing event in the last intron of the per gene, which ultimately results in an earlier accumulation of Per protein in short and cold days versus long and warm days (Collins et al 2004;Majercak et al 2004; also see below).…”