We have characterized a light-input pathway regulating Drosophila clock neuron excitability. The molecular clock drives rhythmic electrical excitability of clock neurons, and we show that the recently discovered light-input factor Quasimodo (Qsm) regulates this variation, presumably via an Na A ll organisms are subject to predictable but drastic daily environmental changes caused by the earth's rotation around the sun. It is critical for the fitness and well-being of an individual to anticipate these changes, and this anticipation is done by circadian timekeeping systems (clocks). These regulate changes in behavior, physiology, and metabolism to ensure they occur at certain times during the day, thereby adapting the organism to its environment (1). The circadian system consists of three elements: the circadian clock to keep time, inputs that allow entrainment, and outputs that influence physiology and behavior (2). Like a normal clock, circadian clocks run at a steady pace (24 h) and can be reset. In nature this environmental synchronization is done via daily light and temperature cycles, food intake, and social interactions (3).In Drosophila the central clock comprises 75 neuron pairs grouped into identifiable clusters that subserve different circadian functions (Fig. 1A). The molecular basis of the circadian clock is remarkably conserved from Drosophila to mammals (4). This intracellular molecular clock drives clock neurons to express circadian rhythms in electrical excitability, including variation in membrane potential and spike firing. Clock neurons are depolarized and fire more during the day than at night, and circadian changes in the expression of clock-controlled genes encoding membrane proteins such as ion channels and transporters likely contribute to these rhythms (5-8). Such cyclical variations in activity play a critical role in synchronizing different clock neurons and conveying circadian signals to other parts of the nervous system and body (9, 10). Furthermore, they provide positive feedback to the molecular clock, which can dampen rapidly without such feedback (7,11,12).Light resets the circadian clock every morning to synchronize the clock to the environment via Timeless (Tim) degradation after activation of the blue-light photoreceptor Cryptochrome (Cry), Quasimodo (Qsm), and potentially also visual photoreceptors (13-17). Qsm acts either independently or downstream of Cry and also is able to affect clock protein stability in Qsmnegative neurons by an unknown non-cell-autonomous mechanism (16). Recently Cry has been shown to regulate clock neuron excitability via the redox sensor of the Hyperkinetic voltage-gated potassium (K V )-β subunit (Hk) (18,19), and here we ask if Qsm affects the clock neurons in a similar way.Membrane potential is important for control of circadian behavior, and manipulation of Shaw and the Narrow Abdomen (NA) channels, both of which are expressed and function within clock neurons influence neuronal electrical activity, the circadian clock, and clockcontrolled behavi...

Many animals use Earth’s magnetic field (also known as the geomagnetic field) for navigation1. The favoured mechanism for magnetosensitivity involves a blue-light-activated electron-transfer reaction between flavin adenine dinucleotide (FAD) and a chain of tryptophan residues within the photoreceptor protein CRYPTOCHROME (CRY). The spin-state of the resultant radical pair, and therefore the concentration of CRY in its active state, is influenced by the geomagnetic field2. However, the canonical CRY-centric radical-pair mechanism does not explain many physiological and behavioural observations2–8. Here, using electrophysiology and behavioural analyses, we assay magnetic-field responses at the single-neuron and organismal levels. We show that the 52 C-terminal amino acid residues of Drosophila melanogaster CRY, lacking the canonical FAD-binding domain and tryptophan chain, are sufficient to facilitate magnetoreception. We also show that increasing intracellular FAD potentiates both blue-light-induced and magnetic-field-dependent effects on the activity mediated by the C terminus. High levels of FAD alone are sufficient to cause blue-light neuronal sensitivity and, notably, the potentiation of this response in the co-presence of a magnetic field. These results reveal the essential components of a primary magnetoreceptor in flies, providing strong evidence that non-canonical (that is, non-CRY-dependent) radical pairs can elicit magnetic-field responses in cells.

SummaryMany animals use the Earth’s magnetic field (geoMF) for navigation1. The favored mechanism for magnetosensitivity involves a blue-light (BL) activated electron transfer reaction between flavin adenine dinucleotide (FAD) and a chain of tryptophan (Trp) residues within the photoreceptor protein, CRYPTOCHROME (CRY). The spin-state of the resultant radical pair (RP) and hence the concentration of CRY in its active state is influenced by the geoMF2. The canonical CRY-centric radical pair mechanism (RPM) does not, however, explain many physiological and behavioural observations2–8. Here, using electrophysiology and behavioural analyses, we assay magnetic field (MF) responses at single neuron and organismal level. We show that the 52 C-terminal (CT) amino acids of CRY, which are missing the FAD binding domain and the Trp chain, are sufficient to facilitate magnetoreception. We also show that increasing intracellular FAD potentiates both BL-induced and MF-dependent effects on the activity mediated by the CT. Additionally, high levels of FAD alone are sufficient to cause BL neuronal sensitivity and, remarkably, potentiation of this response in the co-presence of a MF. These unexpected results reveal the essential components of a primary magnetoreceptor in flies, providing strong evidence that non-canonical (i.e., non-CRY-dependent) RPs can elicit MF responses in cells.