Most of us sleep 7-8 h per night, and if we are deprived of sleep our performance suffers greatly; however, a few do well with just 3-4 h of sleep-a trait that seems to run in families. Determining which genes underlie this phenotype could shed light on the mechanisms and functions of sleep. To do so, we performed mutagenesis in Drosophila melanogaster, because flies also sleep for many hours and, when sleep deprived, show sleep rebound and performance impairments. By screening 9,000 mutant lines, we found minisleep (mns), a line that sleeps for one-third of the wild-type amount. We show that mns flies perform normally in a number of tasks, have preserved sleep homeostasis, but are not impaired by sleep deprivation. We then show that mns flies carry a point mutation in a conserved domain of the Shaker gene. Moreover, after crossing out genetic modifiers accumulated over many generations, other Shaker alleles also become short sleepers and fail to complement the mns phenotype. Finally, we show that short-sleeping Shaker flies have a reduced lifespan. Shaker, which encodes a voltage-dependent potassium channel controlling membrane repolarization and transmitter release, may thus regulate sleep need or efficiency.

The functions of sleep remain elusive, but a strong link exists between sleep need and neuronal plasticity. We tested the hypothesis that plastic processes during wake lead to a net increase in synaptic strength, and sleep is necessary for synaptic renormalization. We found that, in 3 Drosophila neuronal circuits, synapse size or number increases after a few hours of wake and decreases only if flies are allowed to sleep. A richer wake experience resulted in both larger synaptic growth and greater sleep need. Finally, we demonstrate that the gene Fmr1 (fragile X mental retardation 1) plays an important role in sleep-dependent synaptic renormalization.

Calcium imaging with protein-based indicators1,2 is widely used to follow neural activity in intact nervous systems, but current protein sensors report neural activity at timescales much slower than electrical signalling and are limited by trade-offs between sensitivity and kinetics. Here we used large-scale screening and structure-guided mutagenesis to develop and optimize several fast and sensitive GCaMP-type indicators3–8. The resulting ‘jGCaMP8’ sensors, based on the calcium-binding protein calmodulin and a fragment of endothelial nitric oxide synthase, have ultra-fast kinetics (half-rise times of 2 ms) and the highest sensitivity for neural activity reported for a protein-based calcium sensor. jGCaMP8 sensors will allow tracking of large populations of neurons on timescales relevant to neural computation.

In mammals, sleep is thought to be important for health, cognition, and memory. Fruit flies share most features of mammalian sleep, and a recent study found that Drosophila lines carrying loss-of-function mutations in Shaker (Sh) are short sleeping, suggesting that the Sh current plays a major role in regulating daily sleep amount. The Sh current is potentiated by a ␤ modulatory subunit coded by Hyperkinetic (Hk). Here, we demonstrate that severe loss-of-function mutations of Hk reduce sleep and do so primarily by affecting the Sh current. Moreover, we prove, using a transgenic approach, that a wild-type copy of Hk is sufficient to restore normal sleep. Furthermore, we show that short-sleeping Hk mutant lines have a memory deficit, whereas flies carrying a weaker hypomorphic Hk allele have normal sleep and normal memory. By comparing six short-sleeping Sh lines with two normal sleeping ones, we also found that only alleles that reduce sleep also impair memory. These data identify a gene, Hk, which is necessary to maintain normal sleep, and provide genetic evidence that short sleep and poor memory are linked.Key words: Drosophila; sleep; learning; memory; Shaker; hyperkinetic IntroductionSleep is thought to be important for health, cognition, and memory (Horne, 1988;Bonnet and Arand, 1997;Durmer and Dinges, 2005). Many features of sleep are shared between mammals and fruit flies. As in mammals, sleep in Drosophila consists of long periods of behavioral immobility with increased arousal threshold (Hendricks et al., 2000;Shaw et al., 2000), is associated with changes in brain electrical activity (Nitz and Tononi, 2002) and gene expression (Cirelli et al., 2005a;Zimmerman et al., 2006), is reduced by caffeine and stimulants (Shaw et al., 2000;Hendricks et al., 2003a;Andretic et al., 2005), and becomes fragmented with aging (Koh et al., 2006). In both mammals and flies, sleep is homeostatically regulated, because its duration and intensity increase with the duration of previous waking (Huber et al., 2004), and sleep deprivation (SD) results in reduced performance (Huber et al., 2004).In a recent study, we found that Sh mns flies, which carry a point mutation in a conserved Shaker (Sh) domain, sleep only 3-4 h/d, whereas their wild-type controls sleep 8 -14 h/d (Cirelli et al., 2005b). After crossing out genetic modifiers accumulated over many generations, we found that other Sh alleles become short sleepers and fail to complement the short sleeping Sh mns phenotype, suggesting that the Sh current is responsible for the short sleeping phenotype. The Sh locus encodes the ␣ subunit of a tetrameric potassium channel that passes a voltage-activated fastinactivating (I A ) current (Schwarz et al., 1988). Sh orthologs (K v ) occur in vertebrates (Littleton and Ganetzky, 2000) and, in both mammals and flies, play a major role in the control of membrane repolarization and transmitter release (Schwarz et al., 1988). Hyperkinetic (Hk) encodes a ␤ subunit that binds to each ␣ subunit in the Sh tetramer ( Fig. 1), and its presenc...