Sleep is vital and universal, but the underlying mechanisms that drive and control it remain elusive. In essence, the neural control of sleep requires that sleep need is sensed during waking and discharged during sleep. In Drosophila, sleep deprivation leads to the accumulation of reactive oxygen species (ROS) in the mitochondria of sleep-control neurons projecting to the dorsal fan-shaped body (dFB). This internal representation of sleep need is then translated into sleep via increased excitability of these neurons by the redox-sensitive β-subunit of the voltage-gated potassium channel Shaker.
The unknown signalling cascades transducing sleep pressure (tracked by ROS levels) to sleep (via increased excitability) must thus entail rearrangements of the mitochondrial machinery.
To obtain a comprehensive view of the cellular and molecular arsenal operating within dFB neurons, we characterised transcriptomes of single cells isolated from brains of rested and sleep-deprived flies.
Sleep deprivation selectively upregulated genes encoding mitochondrial proteins and was accompanied by reversible morphological changes indicating organelle fragmentation.
Likewise, artificially inducing mitochondrial fragmentation or fusion in dFB neurons affected their electrical properties and sleep in opposing ways: hyperfused mitochondria increased neural excitability and sleep duration, while fragmented mitochondria led to the opposite changes. Since mitochondrial dynamics reflect and influence ATP levels, we measured the cellular ATP content and found that it increased with sleep drive. Conversely, tuning the ATP content by dissipating the mitochondrial proton-motive force specifically in dFB neurons diminished sleep. Moreover, addition of ATP in neurons with fragmented mitochondria rescued their blunted excitability.
Since mitochondrial dynamics and ATP are linked to ROS production, our study suggests a causal and bidirectional link between cellular bioenergetics and excitability of sleep-control neurons. Our first single-cell transcriptome of an animal’s sleep-control neurons in different conditions of sleep pressure will help to elucidate the physiological variables that are linked to the essential function of sleep.