Coordination between Differentially Regulated Circadian Clocks Generates Rhythmic Behavior

Abstract

Specialized groups of neurons in the brain are key mediators of circadian rhythms, receiving daily environmental cues and communicating those signals to other tissues in the organism for entrainment and to organize circadian physiology. In Drosophila, the "circadian clock" is housed in seven neuronal clusters, which are defined by their expression of the main circadian proteins, Period, Timeless, Clock, and Cycle. These clusters are distributed across the fly brain and are thereby subject to the respective environments associated with their anatomical locations. While these core components are universally expressed in all neurons of the circadian network, additional regulatory proteins that act on these components are differentially expressed, giving rise to "local clocks" within the network that nonetheless converge to regulate coherent behavioral rhythms. In this review, we describe the communication between the neurons of the circadian network and the molecular differences within neurons of this network. We focus on differences in protein-expression patterns and discuss how such variation can impart functional differences in each local clock. Finally, we summarize our current understanding of how communication within the circadian network intersects with intracellular biochemical mechanisms to ultimately specify behavioral rhythms. We propose that additional efforts are required to identify regulatory mechanisms within each neuronal cluster to understand the molecular basis of circadian behavior.

Figure 1.

A schematic of the anatomical structure of the fly brain circadian clock neuronal network. (A) Location and names of the seven neuronal clusters that make up the circadian neuronal network. (B) Structure of the ommatidia that make up the fly retina, displaying the expression of rhodopsins (Rh) in receptor cells (R). Two of the receptor cells within R1-6 are removed from the schematic to reveal R7 and R8 for clarity. (C) The relative position of the receptor cells and H-B eyelet with respect to the LNvs in the brain. The positive and negative effects of acetylcholine (ACh) and histamine (His) are shown in gray. (D) The expression pattern of neuropeptides in each of the circadian neuronal clusters. (E) The expression pattern of receptors in each of the circadian neuronal cluster. (F) The sources and the targets of pigment-dispersing factor (PDF), neuropeptide F (NPF), and short neuropeptide F (sNPF). (+) and (−) symbols represent the functional effect of the neuropeptides in nonspecified motor neurons. (G) The expression pattern of CK2α and CRY in each neuronal cluster. (H) A schematic of peak expression of indicated proteins. Arrows represent the time of peak expression and are color-coded.

Figure 2.

Model of the negative feedback loop of Drosophila melanogaster. (A) The CLK/CYC activator complex binds to target promoters to activate transcription of target genes, including per, tim, and cwo. PER and TIM assemble in the cytoplasm to form the repressor complex, which is transported into the nucleus through the activity of SGG and CK2 and binds to the activator complex on the DNA. CWO binding to CLK-binding regions on the DNA coincides with repressor complex-mediated removal of the activator complex, serving as a competitive inhibitor. CK2-mediated TIM degradation leads to DBT-mediated PER degradation and release of the activator complex, closing the negative feedback loop. (B) The CLK/CYC activator complex promotes the expression of PDP1ε and VRI. PDP1ε promotes the expression of clk and cyc, while VRI inhibits it.

Figure 3.

Summary of morning and evening anticipation advances and delays. The indicated gene is overexpressed in morning cells (LNvs; M), evening cells (LNds, fifth s-LNv, DNs; E), or both. The advance or delay of the anticipation peaks in the first 24 h of constant darkness is depicted by the arrow. Presence of absence of the circles indicates the presence or absence of an anticipation peak. The DBT data (Yao et al. 2016), SGG data (Stoleru et al. 2005), CK2β RNAi data (Lear et al. 2005b; Zhang et al. 2010), and Timekeeper (Tik) data (Smith et al. 2008) are summarized from the indicated sources. *Changes in anticipation were inferred from data in the indicated source. **Lack of change in morning anticipation is inferred from incomplete data that coincides with wild-type flies in the indicated source.