Non-canonical Phototransduction Mediates Synchronization of the Drosophila melanogaster Circadian Clock and Retinal Light Responses.

The daily light-dark cycles represent a key signal for synchronizing circadian clocks. Both insects and mammals possess dedicated "circadian" photoreceptors but also utilize the visual system for clock resetting. In Drosophila, circadian clock resetting is achieved by the blue-light photoreceptor cryptochrome (CRY), which is expressed within subsets of the brain clock neurons. In addition, rhodopsin-expressing photoreceptor cells contribute to light synchronization. Light resets the molecular clock by CRY-dependent degradation of the clock protein Timeless (TIM), although in specific subsets of key circadian pacemaker neurons, including the small ventral lateral neurons (s-LNvs), TIM and Period (PER) oscillations can be synchronized by light independent of CRY and canonical visual Rhodopsin phototransduction. Here, we show that at least three of the seven Drosophila rhodopsins can utilize an alternative transduction mechanism involving the same α-subunit of the heterotrimeric G protein operating in canonical visual phototransduction (Gq). Surprisingly, in mutants lacking the canonical phospholipase C-β (PLC-β) encoded by the no receptor potential A (norpA) gene, we uncovered a novel transduction pathway using a different PLC-β encoded by the Plc21C gene. This novel pathway is important for behavioral clock resetting to semi-natural light-dark cycles and mediates light-dependent molecular synchronization within the s-LNv clock neurons. The same pathway appears to be responsible for norpA-independent light responses in the compound eye. We show that Rhodopsin 5 (Rh5) and Rh6, present in the R8 subset of retinal photoreceptor cells, drive both the long-term circadian and rapid light responses in the eye.

Ogueta M ，Hardie RC ，Stanewsky R 《-》

Four of the six Drosophila rhodopsin-expressing photoreceptors can mediate circadian entrainment in low light.

Light is the major stimulus for the synchronization of circadian clocks with day-night cycles. The light-driven entrainment of the clock that controls rest-activity rhythms in Drosophila relies on different photoreceptive molecules. Cryptochrome (CRY) is expressed in most brain clock neurons, whereas six different rhodopsins (RH) are present in the light-sensing organs. The compound eye includes outer photoreceptors that express RH1 and inner photoreceptors that each express one of the four rhodopsins RH3-RH6. RH6 is also expressed in the extraretinal Hofbauer-Buchner eyelet, whereas RH2 is only found in the ocelli. In low light, the synchronization of behavioral rhythms relies on either CRY or the canonical rhodopsin phototransduction pathway, which requires the phospholipase C-β encoded by norpA (no receptor potential A). We used norpA(P24) cry(02) double mutants that are circadianly blind in low light and restored NORPA function in each of the six types of photoreceptors, defined as expressing a particular rhodopsin. We first show that the NORPA pathway is less efficient than CRY for synchronizing rest-activity rhythms with delayed light-dark cycles but is important for proper phasing, whereas the two light-sensing pathways can mediate efficient adjustments to phase advances. Four of the six rhodopsin-expressing photoreceptors can mediate circadian entrainment, and all are more efficient for advancing than for delaying the behavioral clock. In contrast, neither RH5-expressing retinal photoreceptors nor RH2-expressing ocellar photoreceptors are sufficient to mediate synchronization through the NORPA pathway. Our results thus reveal different contributions of rhodopsin-expressing photoreceptors and suggest the existence of several circuits for rhodopsin-dependent circadian entrainment. J. Comp. Neurol. 524:2828-2844, 2016. © 2016 Wiley Periodicals, Inc.

Saint-Charles A ，Michard-Vanhée C ，Alejevski F ，Chélot E ，Boivin A ，Rouyer F ... -  《-》

Light Sampling via Throttled Visual Phototransduction Robustly Synchronizes the Drosophila Circadian Clock.

The daily changes of light and dark exemplify a prominent cue for the synchronization of circadian clocks with the environment. The match between external and internal time is crucial for the fitness of organisms, and desynchronization has been linked to numerous physical and mental health problems. Organisms therefore developed complex and not fully understood mechanisms to synchronize their circadian clock to light. In mammals and in Drosophila, both the visual system and non-image-forming photoreceptors contribute to circadian clock resetting. In Drosophila, light-dependent degradation of the clock protein TIMELESS by the blue light photoreceptor Cryptochrome is considered the main mechanism for clock synchronization, although the visual system also contributes. To better understand the visual system contribution, we generated a genetic variant exhibiting extremely slow phototransduction kinetics, yet normal sensitivity. In this variant, the visual system is able to contribute its full share to circadian clock entrainment, both with regard to behavioral and molecular light synchronization. This function depends on an alternative phospholipase C-β enzyme, encoded by PLC21C, presumably playing a dedicated role in clock resetting. We show that this pathway requires the ubiquitin ligase CULLIN-3, possibly mediating CRY-independent degradation of TIMELESS during light:dark cycles. Our results suggest that the PLC21C-mediated contribution to circadian clock entrainment operates on a drastically slower timescale compared with fast, norpA-dependent visual phototransduction. Our findings are therefore consistent with the general idea that the visual system samples light over prolonged periods of time (h) in order to reliably synchronize their internal clocks with the external time.

Ogueta M ，Hardie RC ，Stanewsky R 《-》

Circadian photoreception in Drosophila: functions of cryptochrome in peripheral and central clocks.

Ivanchenko M ，Stanewsky R ，Giebultowicz JM 《JOURNAL OF BIOLOGICAL RHYTHMS》

Rhodopsin 5- and Rhodopsin 6-mediated clock synchronization in Drosophila melanogaster is independent of retinal phospholipase C-β signaling.

Szular J ，Sehadova H ，Gentile C ，Szabo G ，Chou WH ，Britt SG ，Stanewsky R ... -  《-》