A rhodopsin in the brain functions in circadian photoentrainment in Drosophila

Abstract

Animals partition their daily activity rhythms through their internal circadian clocks, which are synchronized by oscillating day-night cycles of light. The fruit fly, Drosophila melanogaster, senses day/night cycles in part through rhodopsin-dependent light reception in the compound eye, and photoreceptor cells in the Hofbauer-Buchner (H-B) eyelet1. However, a more significant light entrainment pathway is mediated in central pacemaker neurons in the brain. The Drosophila circadian clock is extremely light sensitive. However, the only known light sensor in pacemaker neurons, the flavoprotein, cryptochrome (Cry)2,3, responds only to high levels of light in vitro4. These observations indicate the existence of an additional light-sensing pathway in fly pacemaker neurons5. Here, we identified an uncharacterized rhodopsin, Rh7, which functions in circadian light entrainment through circadian pacemaker neurons in the brain. The pacemaker neurons respond to violet light, which was dependent on Rh7. While loss of either cry or rh7 caused minor affects on photoentrainment, the defects in the double mutant were profound. The circadian photoresponse to constant light was impaired in the rh7 mutant, especially under dim light. The demonstration that Rh7 functions in circadian pacemaker neurons represents the first role for an opsin in the central brain.

Coordination between Differentially Regulated Circadian Clocks Generates Rhythmic Behavior

“…Consistent with this model, removal of the CRY CTT promotes CRY-TIM binding without the need for light activation, leading to weaker entrainment in flies lacking the CTT (Busza et al 2004). Despite a weakened ability to entrain to light, cry mutants lacking the CTT can be entrained under low light conditions (Dissel et al 2004), likely through the CRY-independent visual system, or the deep brain photoreceptor Rh7 as described above (Rieger et al 2003;Schlichting et al 2014;Ni et al 2017). …”

“…Although dim light is thought to be mediated through a CRY-independent mechanism (Bachleitner et al 2007), suggesting morning anticipation under these conditions arises from retinal activity, recent evidence suggests that CRY may also respond to prolonged exposure to dim light in the brain (Vinayak et al 2013). The recently discovered Rh7 also responds to dim light (Ni et al 2017), but how it coordinates with the retinal response is yet to be determined. In a retinal response, either the retina can also communicate to other parts of the CCNN in ways that have not yet been determined, or retinal communication through the LNvs does not require that the LNvs express a functional oscillator.…”

“…However, the eye does not contribute to the circadian clock through CRY-dependent light input (Emery et al 2000b;Bachleitner et al 2007;Yoshii et al 2015), despite expressing CRY (Yoshii et al 2008), suggesting that these two light pathways converge independently to synchronize the circadian clock to the environment. More recently, a deep brain rhodopsin, Rh7, was demonstrated to be involved in light entrainment of pacemaker neurons (Ni et al 2017). Undoubtedly, future work will uncover the molecular mechanisms by which Rh7 informs the circadian clock.…”

Unconventional Roles of Opsins

“…Unexpectedly, this opsin is expressed in the central brain in a subset of circadian pacemaker neurons (Figure 4 a ) (Ni et al 2017). Despite this unusual cellular localization, Rh7 is a light sensor as it can substitute for Rh1 in the eyes and confers light sensitivity to tissue culture cells in the purple range.…”

Ex vivo Whole-cell Recordings in Adult Drosophila Brain

“…Although the process is invasive and may cause damage to the brain, intact neurons and functional circuits can be persevered and maintained for up to one hour after skillful dissection. Labs have used whole brain dissection and whole-cell recordings to characterize the electrical properties of circadian neurons (Sheeba et al , 2008b), uncover the electrical cellular mechanisms responsible for sleep and arousal (Sheeba et al , 2008a), discover a new light-sensing pathway in the brain (Ni et al , 2017), determine the mechanism of action for a common pesticide (Qiao et al , 2014), find a memory suppressor miRNA that regulates an autism susceptibility gene (Guven-Ozkan et al , 2016), and describe synaptic dysfunction in a model of Parkinson’s Disease (Sun et al , 2016). …”