Mechanisms for light-dependent activation of circadian clock neurons in Drosophila

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Life on our planet is temporally organized due to the daily changes of light and temperature caused by the Earth’s rotation around its own axis. As a consequence organisms exposed to these environmental fluctuations evolved daily (circadian) timers, which allow them to adjust their behavioral activities and physiology to external time. Circadian clocks are endogenous molecular timers that control behavior, as for example the daily sleep/wake cycle. While sleep is considered as essential for the performance and survival of animals, it also bears risks. For example, during sleep animals are more prone to predation and sleep also restricts foraging time. To minimize these risks, systems are in place that allow the organism to quickly wake up and perform in dangerous situations. Their function is to lower the ‘arousal threshold’, which is normally high during sleep. In the fruit fly Drosophila, a special group of ‘arousal neurons’ has been identified in the central brain. These neurons, 4 on each side of the brain, belong to the circadian clock network, consisting of 150 neurons that are responsible for regulating daily fly activity. These arousal neurons have features that clearly distinguish them from the other clock neurons, as they for example do not contain an endogenous circadian oscillator. Nevertheless, they are connected to the other clock neurons and deliver light information to them. The arousal neurons are able to at least temporarily overwrite the behavioral program instructed by the other clock neurons.In this proposal, we want to understand the molecular details of how the arousal neurons are activated by light and thereby arouse the animal. Light perceived by the visual system and by the blue-light photoreceptor Cryptochrome can activate the arousal neurons. This light activation affects the QUASIMODO (QSM) protein, which is attached to the outside of the arousal neuron membrane. Interestingly QSM interacts with a Chloride (Cl-) transporter protein (NKCC) that, when active, leads to Cl- influx into the cell. In other insects and in the mammalian circadian clock, the balance between NKCC and its ‘antagonist’ KCC (transports Cl- out of the cell) determines if GABA acts as an exciting or inhibiting neurotransmitter. We hypothesize that QSM could control light arousal in Drosophila by inhibiting NKCC function in the dark, so that GABA inhibits the arousal neurons in darkness. During the light, QSM will be rapidly cleaved off the membrane, leading to a release of NKCC inhibition, so that GABA now excites the neuron. This hypothesis can explain how a presumably enduring signal (GABA) can quickly change its polarity in response to a sudden change in the environment (light), and thereby lower the arousal threshold of the animal. We propose an array of molecular and behavioral experiments to test our hypothesis.