Figure 2. The circadian clock network is conserved in flies and mammals.

Nematodes appear to use non-conserved mechanism to generate circadian rhythms. Light inputs generate oscillations on gene expression and locomotion, among other variables, and PDF and melatonin might be mediators of enviromental cues. In higher species, the circadian clock is constituted by the CPN in flies and the SCN in mammals. In Drosophila there are four clock genes, which interact in a negative feedback loop: Clock (Clk), cycle (cyc, an ortholog of BMAL1 in mammals), timeless (tim), and period (per). CLK and CYC activate and regulate expression levels of per and tim This leads to periodic increase in the levels of PER and TIM proteins, which accumulate in cell nuclei, and repress CLK/ CYC activators, causing suppression of per and tim transcription. PDF is expressed in a subset of CPNs and affects circadian period. Increased nutrient sensing via dTOR signaling lengthens the circadian period through S6K and GSK3, which also affects TIM. In mammals, CLOCK and BMAL1 are transcriptional activators that bind the enhancer sequences of mPer (Per 1, Per 2 and Per 3) and mCry (Cryptochrome; Cry1 and Cry2). PER and CRY proteins accumulate in the cytoplasm and translocate to the nucleus to inhibit CLOCK:BMAL1. The CLOCK/BMAL1 complex also activates transcription of the nuclear receptors REV-ERBα/β and RORα/β/δ and γ. REV-ERB acts as negative regulators of ROR. ROR and REV-ERB proteins compete for binding sites in the promoter region of Bmal1 and regulate its transcription. SIRT1 interacts with CLOCK and deacetylates BMAL1 and PER. AMPK modulated levels of CRY1 and PER and regulates SIRT1. mTOR is also regulated in a circadian manner in the mouse SCN. VIP is expressed in a subset of neurons of SCN and affects the circadian period. These interactions ensure temporal homeostasis during development and reproduction.