Most animals have daily rhythms of activity: some are awake during the day and asleep at night, whilst others are more active at night, or during the twilight hours around dawn and dusk. These cycles of activity are driven by an internal body clock, which is reset in response to external cues, like light and temperature, and which keeps the animal in sync with the day–night cycle.

The fruit fly Drosophila has daily—or circadian—rhythms of behavior, which are controlled by a network of genes that are switched ‘on’ or ‘off’ at different times in every 24-hr period. These circadian genes encode various proteins, including PERIOD and TIMELESS. The levels of these two proteins increase during the day and into the night, until they reach a point at which they cause their own genes to be switched off. PERIOD and TIMELESS are then destroyed each morning, and the cycle begins anew. Most of these same proteins perform similar functions in mammals.

In the fly brain, two groups of neurons express these key proteins and control the timings of activity or movement. One group, called M cells, regulates activity in the morning and also produces a small molecule called PDF. Another group, called E cells, controls evening activity, but is less well-defined.

Since M cells can maintain circadian rhythms even in total darkness, these cells were also considered key ‘pacemaker neurons’. However, Guo et al. now challenge this view by identifying five E cells that are the major source of circadian activity. Blocking the release of signaling molecules from these neurons severely disrupted movement in both the morning and the evening. The E cells are also critical for timekeeping under a normal day–night cycle.

Guo et al. found that the rhythm of the E cells is reset when the M cell neurons fire, which causes a release of PDF molecules. Further, PDF molecules reset the E cells by causing the degradation of the TIMELESS protein—which is similar to the effect of light, although light cause TIMELESS to be degraded via a different biochemical pathway.

Guo et al. suggest that under normal light–dark conditions the E cells are important for driving the flies' activity as well as for overall timekeeping. The M cells, instead, appear to function primarily to integrate information about light and reset the E cell clock. Challenges moving forward will include understanding other ways in which the firing of neurons can affect timekeeping, as well as looking if there any differences between the five E cells.

Animals use endogenous circadian pacemakers to control their physiology and behavior with roughly 24-hr periodicity (Bass and Takahashi, 2010; Thut et al., 2012). Intracellular timekeeping mechanisms include transcriptional feedback loops, which involve many key genes in Drosophila. They include period (per), timeless (tim), clock (clk), cycle (cyc), and doubletime (dbt). Coordination of their encoded protein activities (period, PER; timeless, TIM; clock, CLK; cycle, CYC: and doubletime, DBT) contributes to the intracellular cycling of the molecular pacemaker, which is similar between flies and mammals (Dubruille and Emery, 2008; Menet and Rosbash, 2011). PER and TIM concentrations increase during the day, and they eventually negatively regulate their own transcription. Biochemical oscillations in the head occur in part through a direct interaction of PER and TIM with the positive transcription factor CLK: CYC (Dubruille and Emery, 2008; Menet and Rosbash, 2011). They also require the photoreceptor cryptochrome (CRY) as well as a cycling light:dark (LD) environment, that is, RNA and protein oscillations damp rapidly in constant darkness (Stanewsky et al., 1998). The central brain is probably different as its molecular and behavioral rhythms persist in constant darkness and is CRY-independent (Stanewsky et al., 1998).

Nonetheless, CRY is expressed within many of the 75 pairs of central brain circadian neurons and is necessary for an important biological feature of circadian rhythms, namely, light-mediated phase adjustment or phase-shifting (Emery et al., 1998). There is good evidence in favor of a cell-autonomous view of Drosophila phase-shifting: light penetrates the thin insect cuticle (Fogle et al., 2011) and causes a CRY conformational change within circadian neurons (Ozturk et al., 2011). The altered CRY molecules recruit the E3 ligase JETLAG (JET) to TIM (Koh et al., 2006; Peschel et al., 2009). Premature TIM degradation then causes phase advances in the late night, whereas inappropriate TIM degradation during the TIM accumulating phase in the early night causes phase delays.

Two groups of central brain circadian neurons appear particularly important for behavioral rhythms. The 4 PDF-expressing small ventrolateral neurons (sLNvs) dictate morning activity as well as the rhythmicity in constant darkness (Renn et al., 1999; Blanchardon et al., 2001; Stoleru et al., 2004). This latter feature, free running locomotor activity rhythms, has caused the s-LNvs to be considered the major fly pacemaker neurons. A less well-defined set of cells directs evening activity. These neurons (E cells) also dictate circadian behavior in constant light and probably include the 6 LNds and the PDF-negative 5^th s-LNv (Picot et al., 2007).

We identify here five E neurons as a major source of circadian and behavioral rhythmicity as well as locomotor activity, of which the 2 CRY+ E neurons are most important. The phase of these cells is shifted by the M cell firing and requires PDF as well as the PDF receptor within these five E cells. Moreover, brief firing of M cells at different times of day generates a phase response curve (PRC), which resembles a proper light-mediated PRC. Brief M cell firing also resembles light by causing rapid TIM degradation within downstream circadian neurons, but firing-mediated phase-shifting is CRY-independent and exploits the E3 ligase components CUL-3 to degrade TIM, at least in the early night-delay zone. Our findings suggest that E cells are very important for timekeeping under light–dark conditions whereas an important function of PDF cells is for light-mediated phase adjustment, that is, to integrate light information and appropriately fire. This degrades TIM within E cells, which modulates the phase of E cell oscillations and behavioral rhythms.

Diurnal behavior in Drosophila is currently best explained by a dual-oscillator model, which emphasizes the M cells as master pacemakers and the E cells as secondary slave oscillators. Here we provide a modified configuration with a more prominent role for 5 Dv-E cells (Figure 9). This view is based on several new and unanticipated results. First, the 5 Dv-E cells are not only responsible for the evening anticipation peak and its timing, but they also contribute to the morning peak, free running rhythms and total activity. Otherwise put, these cells appear to play a key role in all aspects of circadian rhythms and locomotor activity. Second, E cell resetting can occur in a non cell-autonomous manner, through M cell neuron firing, PDF release and E cell PDFR activation. Third, TIM level changes within the 5 Dv-E cells in response to M cell firing likely make a major contribution to the resulting phase changes, at least in the early night when phase delays occur, and therefore make a major contribution to circadian timekeeping under more normal LD conditions.

The classical cell-autonomous model of Drosophila phase-shifting has long contrasted with the well-established view of phase-shifting in the mammalian SCN. In this system, firing from optic track activates SCN NMDA receptors, which is necessary and sufficient for light-mediated phase shifts. Importantly, the contribution of PDF cell firing and PDF to Drosophila phase-shifting brings this system closer to that of mammals. Because many aspects of Vasoactive intestinal polypeptide (VIP) function in mammals resemble those of PDF in flies (Vosko et al., 2007), it will be interesting to know if VIP cell firing can mimic light and phase-shift mammalian rhythms.

An important contribution of the E cell TIM level change to phase delays is consistent with previous work showing that a full light pulse at ZT15 induces rapid and potent TIM degradation in all 6 LNds as well as reduced TIM degradation in response to a light pulse of decreased intensity (Tang et al., 2010). This conclusion is also consistent with more recent data from Emery et al. (Lamba et al., 2014). Our data here also point to a more important role of the LNds rather than the M cells: the LNds show more TIM degradation in response to firing and a bigger response to the CUL-3 knockdown, which correlates with the decrease in phase delay caused by the knockdown (Figure 8).

This previous work also indicates that there is no observed TIM degradation in the PDF-positive M cells after a light pulse (Tang et al., 2010). This is consistent with older work showing that cry rescue in non M cell circadian neurons (with Tim-GAL4/Pdf-GAL80) provides a robust light mediated PRC (Stoleru et al., 2007), suggesting that canonical CRY-JET-mediated TIM degradation within M cells is unnecessary to convey light information to the brain clock. Our data extend this picture to the more dorsal circadian DNs by showing that TIM degradation within this circadian subpopulation is probably also not essential, at least to phase delays (Figure 8B,C) (Lamba et al., 2014). Although these data do not exclude a more inclusive view, our results indicate that acute PDF cell activation causes TIM degradation within the 5 Dv-E cells and that this degradation correlates with the magnitude of the phase delay. We note however that PDF cell activation may also function in other ways, for example by chronically affecting PER levels (Li et al., 2014) or by activating other downstream neurons (Seluzicki et al., 2014).

CUL-3 is the first ubiquitin ligase component other than JET to connect with phase-shifting. Importantly, CUL-3 has been shown to participate in light-independent TIM degradation within the cytoplasm (Grima et al., 2012). This is where TIM is localized during its accumulating phase in the early night (see Figure 6A), which perfectly matches the preferential role of CUL-3 in promoting delay zone phase-shifts.

However, the CUL-3 RNAi effects are incomplete. The new non cell-autonomous pathway described here may therefore cooperate with the more traditional cell-autonomous light-CRY-JET pathway to effect phase shifting. As the latter is essential for light-mediated phase-shifting, another non-exclusive possibility is that the two pathways are in series rather than in parallel, that is, that the light-CRY-JET pathway is upstream of neuronal firing and that the RNAi effects reflect incomplete knockdowns. This notion fits with recent studies suggesting that the light-CRY interaction promotes neuronal firing (Fogle et al., 2011).

At least 2 and probably 3 LNds are CRY-negative by antibody staining criteria even after many days of extended incubation in DD (Yoshii et al., 2008). As all 6 LNds respond similarly to a full light pulse and to neuronal firing, this indicates that TIM degradation within the CRY-negative LNds is non cell-autonomous and probably requires communication from M cells and/or from other neurons like their CRY-positive LNd neighbors (Lamba et al., 2014). If the CRY-negative LNds are also PDFR-negative (Im et al., 2011), other neuropeptides, neurotransmitters or perhaps even gap junctions may participate in the transfer of light or firing information upstream of TIM degradation.

Surprisingly, output from the 5 Dv-E cells controls a substantial fraction of evening activity as well as overall locomotor activity (Figure 2). These conclusions are also based on the phenotypes of Kir as well as dTrpA1 expression under Dv-E GAL4 control (Figure 2B and data not shown). As Dv-E GAL4/UAS-TNT flies have significantly impaired activity in DD as well as in LD (Figure 2A), the residual locomotor activity may be clock-independent. However, these flies still manifest an evening anticipation peak under LD conditions (Figure 2A), suggesting incomplete suppression by TNT or a role of other circadian activity neurons. As the circadian activity-promoting subset of circadian neurons expresses CRY and PDFR and the Dv-E GAL4 driver only labels 1 CRY positive LNd (Figure 2), a simple explanation is that one or more of the other 2 CRY⁺ LNds also promotes locomotor activity.

The Dv-E cells also contribute to morning activity. This is because rescue of PDFR expression only in the Dv-E cells is sufficient to rescue the morning anticipation peak (Figure 5C). In addition, Dv-E cell output is necessary for a robust morning peak (Figure 2A,B). Yet previous results connect the M cells to morning activity (Choi et al., 2012). These cells also communicate via PDF with the DN1p dorsal neurons, which also impact the morning anticipation peak (Zhang et al., 2010a, 2010b). Although all of these considerations suggest that redundant circuits downstream of the M cells underlie the morning anticipation peak, only output from the Dv-E cells has been shown to be necessary for morning activity, and there is evidence that even the M cells are not necessary for the morning activity peak (Sheeba et al., 2010).

These PDF+ M cells have been considered the master pacemakers based on their critical role in keeping time in constant darkness. However, their firing-resetting properties, the identification of the Dv-E cells as major activity neurons even in DD and the likely importance of TIM degradation within these cells for phase adjustment suggest a different reason for the strong molecular rhythms of the PDF neurons in DD and their contribution to behavioral rhythmicity under these conditions. These features likely reflect the important role of the PDF neurons as light-sensitive cells, directly through CRY and indirectly as post-synaptic targets of other light-sensitive cells such as photoreceptor cells within the eye and dorsal brain (Malpel et al., 2002; Yoshii et al., 2008 and unpublished data). We therefore suggest that a major function of the PDF neurons is to integrate light information and use it to phase-adjust the Dv-E cells, which are important pacemakers under LD conditions (Figure 9). Phase is a more important parameter than period under these more ‘natural’ conditions, and expression of period-altering mutants under Dv-E control appropriately alters the phase of the major evening peak but fails to do so under PDF control (Figure 1D and data not shown).

Lastly, our data offer a simple mechanistic explanation for a long-standing difference between peripheral and central circadian oscillators in Drosophila. The amplitude of peripheral oscillators is dependent on light as they damp rapidly after transfer of flies to constant darkness. Fly peripheral oscillators are also dependent on the photoreceptor CRY (Stanewsky et al., 1998; Malpel et al., 2002). In contrast, the circadian brain network and resultant rhythmic behavior persist in constant darkness and in the absence of CRY (Emery et al., 1998, 2000). Our work here can explain this difference in a simple way: neuronal firing replaces the function of light and CRY and even acts at the same step in the circadian cycle, namely, to promote TIM degradation within circadian neurons. We suggest that this step is intrinsically weak and that TIM degradation is normally potentiated every day, by light and/or by firing. This ensures the maintenance of robust rhythms of brain and behavior without compromising the light- and firing-sensitivity needed for phase adjustment.

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Author details

1. Fang Guo

   1. Department of Biology, Brandeis University, Waltham, United States
   2. National Center for Behavioral Genomics, Brandeis University, Waltham, United States
   3. Howard Hughes Medical Institute, Brandeis University, Waltham, United States

   Contribution

   FG, Conception and design, Acquisition of data, Analysis and interpretation of data, Drafting or revising the article

   Competing interests

   The authors declare that no competing interests exist.

2. Isadora Cerullo

   Howard Hughes Medical Institute, Brandeis University, Waltham, United States

   Present address

   Penn Medicine (University of Pennsylvania Health System), Philadelphia, United States

   Contribution

   IC, Acquisition of data, Analysis and interpretation of data

   Competing interests

   The authors declare that no competing interests exist.

3. Xiao Chen

   1. Department of Biology, Brandeis University, Waltham, United States
   2. National Center for Behavioral Genomics, Brandeis University, Waltham, United States
   3. Howard Hughes Medical Institute, Brandeis University, Waltham, United States

   Contribution

   XC, Acquisition of data, Analysis and interpretation of data

   Competing interests

   The authors declare that no competing interests exist.

4. Michael Rosbash

   1. Department of Biology, Brandeis University, Waltham, United States
   2. National Center for Behavioral Genomics, Brandeis University, Waltham, United States
   3. Howard Hughes Medical Institute, Brandeis University, Waltham, United States

   Contribution

   MR, Conception and design, Analysis and interpretation of data, Drafting or revising the article

   For correspondence

   rosbash@brandeis.edu

   Competing interests

   The authors declare that no competing interests exist.

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