| Summary: | <p>Though universal and highly conserved, the biological function of sleep remains largely
mysterious. One approach to solving this mystery is to study the neural mechanisms that
regulate sleep and waking. There are two such mechanisms: the circadian clock, which
synchronizes sleep with periodic changes in the external environment, and the sleep
homeostat, which responds to (still largely unknown) internal changes that require sleep
to reset. Understanding this second, homeostatic control system will likely offer
important clues to the biological function of sleep.</p>
<p>In the fruit fly, Drosophila melanogaster, a homeostatic sleep switch has been
discovered in a cluster of two dozen neurons with projections to the dorsal fan-shaped
body (dFB) in the central brain. The electrical activity of these cells induces sleep and is
antagonistically modulated by two potassium currents: Shaker-based A-type currents
promote sleep, whereas Sandman-based leak currents inhibit sleep. The potassium
conductances are, in turn, regulated by Rho GTPase activating proteins such as
Crossveinless-c (Cv-c). How this molecular machinery logs a fly’s sleep history and
responds by altering the electrical properties of the dFB neuronal membrane is currently
unknown.</p>
<p>In this study, two molecular mechanisms were identified that signal changes in
sleep drive within dFB neurons. The first mechanism involves Rho GTPases and their
regulators. Catalytically ‘dead’ Cv-c was found to be unable to rescue the short-sleeping
phenotype of cv-c mutants, implicating the GTPase cycle of a small G protein of the Rho
family in sleep regulation. Through behavioural screens, Rho1 was identified as a
candidate Cv-c substrate, and Rac1 as a Rho1 antagonist. Targeted ablations of Rho1 and
Rac1 in the dFB neurons of cv-c mutants rescued and exacerbated sleep deficits,
respectively. Thus, active Rac1 and Rho1 promote sleep and wakefulness in a mutually
antagonistic fashion.</p>
<p>The second mechanism involves the action of oxidation by-products on
Hyperkinetic (Hk), a beta subunit of the potassium channel Shaker whose function in dFB
neurons is important for sleep. Hk is a functional aldo-keto reductase and senses cellular
redox levels. This redox sensing capacity is required to restore sleep in short-sleeping Hk<sup>1</sup>
mutants. Mitochondrial respiration is a primary source of reactive oxygen species (ROS).
In vivo measurements of mitochondrial ROS in sleep deprived flies revealed that sleep
pressure correlated with ROS concentrations in dFB neuron dendrites. Elevating ROS in
dFB neurons by overexpressing mutant superoxide dismutase (Sod1) increased sleep,
whereas overexpressing wild type Sod1, which neutralizes ROS, decreased sleep. These
effects were occluded in the absence of functional Hk, which implicates it as the site of
redox modulation of membrane excitability. Thus, ROS acts as a potential link between
energy metabolism, oxidation, and sleep.</p>
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