Circadian rhythms: To Sync or Not To Sync

Summary:

Using real-time imaging of circadian gene expression, a new study reveals how a light pulse briefly desynchronizes clock neurons in the fly brain before they settle into a new, synchronized daily rhythm.

Since the first transatlantic flight, people have complained of feeling poorly for days following travel across time zones. Chronodisruption (a mismatch between an organism’s daily rhythms and local time) was identified as the prime culprit; yet 96 years later there is no reliable cure for jet lag or other related disruptions like shift-work and seasonal affective disorders (SAD). An evolutionarily ancient system present in perhaps all unicellular and multicellular organisms, the circadian circuit integrates environment input like light to synchronize and coordinate daily physiological and behavioral rhythms. This circuit must be robust enough to anticipate reliable events like sunrise and mealtimes but flexible enough to adapt to seasonal changes like photoperiod. In this issue of Current Biology, Roberts et al.[1] find evidence for flexibility in the circadian system of flies. Light that shifts daily rhythms in behavior also transiently reduces synchrony among neurons in the circadian circuit. Potentially, this flexibility could be harnessed and amplified to ‘cure’ jet lag, shift work, and SAD.

The circadian circuit in animals depends on the synchronization of endogenously rhythmic neurons[2, 3]. Unsynchronized, these clock neurons fail to produce a rhythm robust enough to drive daily behaviors. The identification of a mammalian neuropeptide (VIP) and its fly homologue (PDF) as necessary for the intercellular synchronization of circadian rhythms seemed to solidify the stance that synchrony is beneficial. Deficiencies in PDF or its receptor, VIP or its receptor resulted in a dramatic loss of synchrony among cells and, behaviorally, weak intrinsic rhythms and a big advance in the time of daily activity onset in a light-dark cycle (that is, the mutants behave like larks) [4, 5]. The case seemed settled – synchrony within the circadian circuit benefits circadian rhythms much like synchronized contraction within the heart pacemaker is necessary for a healthy beating heart. However, the reality is much more nuanced.

Recent experiments and models have questioned whether desynchrony is always pathological. For example, synchrony among circadian cells changes with seasons [6, 7]. Furthermore, the circadian system has a weak spot, ominously termed the “singularity,” where a single light pulse, applied at the right time with the right intensity, yields arrhythmicity. Two papers engineered light sensitivity into fibroblasts and found that the arrhythmicity could be explained largely as reduced synchrony among circadian cells[8, 9]. This may seem like a terrible flaw in the design of a system, but perhaps not. A recent paper found that addition of sufficient levels of VIP can shift some cells more than others resulting in transient desynchrony [10]. This was termed “phase tumbling” because it was reminiscent of apparent random-walk movements of bacteria as they seek a distant food source. The researchers tested whether phase tumbling before a trip could reduce jetlag (Fig. 1). They found that injection of VIP into the brain allowed mice to entrain significantly faster to an 8-h advanced light schedule (i.e. an eastward trip across 8 time zones [10]. This led to the hypothesis that reducing synchrony can allow a network of coupled oscillators, like the cells of the circadian system, to accelerate entrainment. Does the circadian system normally take advantage of phase tumbling?

Figure 1.

The synchronized brain. Reliable daily changes in gene expression, excitability, hormone release and many other physiological events depend on synchrony among circadian cells in the brain. In a steady-state environment, these cells remain synchronized to each other with a stable phase relationship (A). Roberts et al. show that a light pulse delivered to the fly brain transiently shuffles the alignment of circadian cells (B). This relatively brief “phase retuning” or “phase tumbling” may be useful in alleviating jetlag or problems associated with shift work. Note how, prior to a shift (dark grey bar) in the light-dark schedule (yellow and grey bars), addition of a brief stimulus like a light pulse (yellow rectangle) results in reduced synchrony (colored diamonds show the daily peak for each cell) and then fewer days until the system is entrained (compare the red arrows in C and D).

Using real-time whole brain bioluminescent imaging of clock gene expression, Roberts et al. show that circadian cells in the fly brain transiently desynchronize in response to light. They refer to this as “phase retuning” because, after light reduces the synchrony among neurons, the cells appear to resynchronize with stronger rhythms and greater phase shifted synchrony than was seen before the light pulse. This highlights a major advantage of studying the fly brain. Whereas mammals lack extraretinal photoreceptors, the fly brain responds to light in vitro. Many of the clock cells express photopigments so that more naturalistic stimuli can be used to stimulate this circadian system in a dish.[1]

In both flies and mammals, the ‘master’ circadian circuit is a relatively small population of neurons. These clock neurons have been identified for their essential role in regulating circadian behavior. In flies and mice, clock neurons can be categorized into subgroups based on neuropeptide content, response to environmental inputs and timing of circadian activities. Roberts et al. show, for example, that, like cells within the mammalian suprachiasmatic nucleus (SCN), groups of clock neurons in the fly brain differ in the timing of their Period gene expression[11]. In the isolated fly brain, a transgenic luciferase-based reporter reveals reliable daily waves of Period transcription across distinct subsets of clock neurons. This suggests a network of heterogeneous oscillators that, when allowed to communicate with each other, can regulate outputs to specific times of day. In addition, most, but not all clock neurons in flies, are directly light sensitive (e.g. the DN3 neurons lack the photopigment, Cryptochrome). Roberts et al. noted that some cells known to respond directly to light tended to shift more and resychronize faster than other cells. Thus, by watching the dynamics of resynchronization, researchers are beginning to infer the network wiring of circadian oscillators in the brain.

Roberts et al. provide compelling evidence that in response to a light pulse, the circadian circuit desynchronizes to resynchronize in a heterogeneous but consistent manner. This leads to the hypothesis that desychrony is an intrinsic and useful feature of the circadian circuit. However, key questions remain unanswered. What accounts for the differing responses among single cells? Does phase retuning change synaptic strengths? Roberts et al. like to refer to the “new state of strengthened synchrony” following a light pulse. This transient state should be contrasted with changes induced by weeks of gradually changing photoperiod. Does phase retuning occur regardless of the time and intensity of the light? Roberts et al. tested the effects of a 15-minute light pulse (approximately twice as bright as office illumination) delivered during the late night. Addition of VIP, for example, dose-dependently tumbles the phases of SCN cells and accelerates re-entrainment, independent of when it is applied [10]. Finally, how does synchrony within the circadian circuit translate to behaviors as diverse as sleep/wake, fasting/feeding, and mood? Once we understand the intricacies of circadian circuit entrainment, brief pre-treatments like phase retuning could work to realign the circadian circuit in people suffering from jet lag, shift work, and seasonal affective disorder. Travelers may desynchronize to synchronize.