Sleep is a whole–organism phenomenon accompanied by global changes in neural activity. We previously identified SLEEPLESS (SSS) as a novel glycosylphosphatidyl–inositol–anchored protein required for sleep in Drosophila. Here, we demonstrate a critical role for SSS in regulating the sleep–modulating potassium channel, Shaker. SSS and Shaker exhibit similar expression patterns in the brain and specifically affect each other’s expression levels. sss mutants exhibit altered Shaker localization, reduced Shaker current density, and slower Shaker current kinetics. Transgenic expression of sss in sss mutants rescues defects in Shaker expression and activity cell–autonomously and also suggests that SSS functions in wake–promoting, cholinergic neurons. Importantly, in heterologous cells, SSS accelerates kinetics of Shaker currents and can be co–immunoprecipitated with Shaker, suggesting that SSS interacts with Shaker and modulates its activity. SSS is predicted to belong to the Ly–6/neurotoxin superfamily, suggesting a novel mechanism for regulation of neuronal excitability by endogenous toxin–like molecules.

Summary Sleep:wake cycles break down with age, but the causes of this degeneration are not clear. Using a Drosophila model we addressed the contribution of circadian mechanisms to this aged-induced deterioration. We found that in old flies free-running circadian rhythms (behavioral rhythms assayed in constant darkness) have a longer period and an unstable phase before they eventually degenerate. Surprisingly, rhythms are weaker in light:dark cycles and the circadian-regulated morning peak of activity is diminished under these conditions. On a molecular level, aging results in reduced amplitude of circadian clock gene expression in peripheral tissues. However, oscillations of the clock protein PERIOD (PER) are robust and synchronized among different clock neurons, even in very old, arrhythmic flies. To improve rhythms in old flies, we manipulated environmental conditions, which can have direct effects on behavior, and also tested a role for molecules that act downstream of the clock. Coupling temperature cycles with a light:dark schedule or reducing expression of protein kinase A (PKA) improved behavioral rhythms and consolidated sleep. Our data demonstrate that a robust molecular time-keeping mechanism persists in the central pacemaker of aged flies, and reducing PKA can strengthen behavioral rhythms.

The central circadian clock in mammals is housed in the brain and is based on cyclic transcription and translation of clock proteins. How the central clock regulates peripheral organ function is unclear. However, cyclic expression of circadian genes in peripheral tissues is well established, suggesting that these tissues have their own endogenous oscillators. Reproduction is a process influenced by circadian rhythms in many organisms, thus making the testis an attractive model for studying clock function in peripheral organs. However, results addressing cyclic expression of clock genes in the mammalian testis are inconsistent. To resolve this issue, RNA was extracted from testes of mice at various times of day. Expression of the circadian clock genes mPer1, mPer2, Bmal1, Clock, mCry1, and Npas2 was constant at all times. Immunohistochemical localization of mPER1 and CLOCK proteins revealed restricted expression only in cells at specific developmental stages of spermatogenesis. For mPER1, these stages are the spermatogonia and the condensing spermatids. In contrast, CLOCK expression was restricted to round spermatids, specifically within the developing acrosome. Expression of mPER1 and CLOCK was constant at all times of day. These results suggest that clock proteins have noncircadian functions in spermatogenesis. Noncircadian expression of clock genes was also found in the thymus, which, like the testis, is composed primarily of differentiating cells. We propose that cyclic expression of clock genes is suspended during cellular differentiation.

The Drosophila PAR domain protein 1 (Pdp1) gene encodes a transcription factor with multiple functions. One isoform, PDP1 , was proposed to be an essential activator of the core clock gene, Clock (Clk). However, a central clock function for PDP1 was recently disputed, and genetic analysis has been difficult due to developmental lethality of Pdp1-null mutants. Here we report the discovery of a mutation that specifically disrupts the Pdp1 isoform. Homozygous Pdp1 mutants are viable and exhibit arrhythmic circadian behavior in constant darkness and also in the presence of light:dark cycles. Importantly, the mutants show diminished expression of CLK and PERIOD (PER) in the central clock cells. In addition, expression of PDF (pigment-dispersing factor) is reduced in a subset of the central clock cells. Loss of Pdp1 also alters the phosphorylation status of the CLK protein and disrupts cyclic expression of a per-luciferase reporter in peripheral clocks under free-running conditions. Transgenic expression of PDP1 in clock neurons of Pdp1 mutants can restore rhythmic circadian behavior. However, transgenic expression of CLK in these mutants rescues the expression of PER in the central clock, but fails to restore behavioral rhythms, suggesting that PDP1 has effects outside the core molecular clock. Together, these data support a model in which PDP1 functions in the central circadian oscillator as well as in the output pathway.