CRTC Potentiates Light-Independent timeless Transcription to Sustain Circadian Rhythms in Drosophila

Abstract

Light is one of the strongest environmental time cues for entraining endogenous circadian rhythms. Emerging evidence suggests that CREB-regulated transcription co-activator 1 (CRTC1) is a key player in this pathway, stimulating light-induced Period1 (Per1) transcription in mammalian clocks. Here, we demonstrate a light-independent role of Drosophila CRTC in timely activating circadian transcription to sustain free-running circadian behaviors. Genomic deletion of the crtc locus causes long but poor locomotor rhythms in constant darkness (DD). Overexpression or RNA interference-mediated depletion of CRTC in circadian pacemaker neurons similarly impairs the DD rhythmicity, indicating that Drosophila circadian clocks are sensitive to the dosage of CRTC. The crtc null mutation delays the overall phase of circadian gene expression yet crtc effects on the oscillating levels of TIMELESS (TIM) proteins are most evident in the clock neurons. In fact, CRTC overexpression enhances CLOCK/CYCLE-activated transcription of tim but not per in clock-less S2 cells whereas CRTC depletion suppresses it. Consistently, TIM overexpression partially but significantly rescues the behavioral rhythmicity in crtc mutants. Taken together, our data demonstrate that Drosophila CRTC potentiates tim transcription to coordinate molecular rhythms with circadian behaviors, implicating CRTC-dependent clock mechanisms have co-evolved with selective clock targets among different species.