Impaired clock output by altered connectivity in the circadian network

Abstract

Substantial progress has been made in elucidating the molecular processes that impart a temporal control to physiology and behavior in most eukaryotes. In Drosophila, dorsal and ventral neuronal networks act in concert to convey rhythmicity. Recently, the hierarchical organization among the different circadian clusters has been addressed, but how molecular oscillations translate into rhythmic behavior remains unclear. The small ventral lateral neurons can synchronize certain dorsal oscillators likely through the release of pigment dispersing factor (PDF), a neuropeptide central to the control of rhythmic rest-activity cycles. In the present study, we have taken advantage of flies exhibiting a distinctive arrhythmic phenotype due to mutation of the potassium channel slowpoke (slo) to examine the relevance of specific neuronal populations involved in the circadian control of behavior. We show that altered neuronal function associated with the null mutation specifically impaired PDF accumulation in the dorsal protocerebrum and, in turn, desynchronized molecular oscillations in the dorsal clusters. However, molecular oscillations in the small ventral lateral neurons are properly running in the null mutant, indicating that slo is acting downstream of these core pacemaker cells, most likely in the output pathway. Surprisingly, disrupted PDF signaling by slo dysfunction directly affects the structure of the underlying circuit. Our observations demonstrate that subtle structural changes within the circadian network are responsible for behavioral arrhythmicity.circadian circuitry ͉ Drosophila ͉ pigment dispersing factor ͉ potassium channels ͉ slowpoke R hythmicity in rest-activity cycles in Drosophila is under control of the circadian clock, which is based on selfsustaining, cell-autonomous transcriptional negative feedback loops. These feedback loops ultimately give rise to rhythms in the abundance, phosphorylation state, and nuclear localization of key intracellular proteins, such as period (PER) and timeless (TIM) (1). To date, several neuronal clusters have been shown to include a molecular oscillator. The one best understood encompasses the small ventral lateral neurons (LNvs), comprised of five cells, of which four rhythmically release the neuropeptide pigment dispersing factor (PDF) at their dorsal terminals. Other oscillators within the fly brain include the dorsal lateral neurons (LNds) together with the dorsal neurons (DN1-3) (2). Ablation of all LNvs by overexpression of proapoptotic genes, as well as null mutations on the pdf gene or its receptor, cause behavioral arrhythmicity a few days upon transfer to constant conditions (3-6) likely through the gradual loss of synchronization among the components of the small LNv cluster (7).The question of how the intracellular molecular oscillations taking place within specific neuronal clusters ultimately drive rhythmic locomotor activity has only recently been approached in Drosophila (7-11). Molecular oscillations must be somehow transduced into neuronal function to...

The Neuropeptide Pigment-Dispersing Factor Adjusts Period and Phase ofDrosophila's Clock

“…This multiphasic SCN organization accounts for many properties of circadian organization, including the ability of the SCN to produce multiple-phase peak rhythms in vitro and in vivo Mrugala et al, 2000;Schwartz and Meijer, 2004) as well as to trigger various physiological output rhythms, and to respond to seasonal variations in the photoperiod by altering the coupling of its constituent oscillators (Pittendrigh and Daan, 1976). The same seems to apply for Drosophila (Grima et al, 2004;Shafer et al, 2004;Stoleru et al, 2004Stoleru et al, , 2005Bachleitner et al, 2007;Fernández et al, 2007;Murad et al, 2007). We show here that Pdf 01 mutants are unable to adapt their activity pattern to long photoperiods, indicating that PDF plays a major role in the seasonal adaptation of the clock.…”

Effect of photoperiod on clock gene expression and subcellular distribution of PERIOD in the circadian clock neurons of the blow fly Protophormia terraenovae

“…In D. melanogaster, PER nuclear localization has been well investigated in LN vS (Fernández et al 2007;Shafer et al 2002Shafer et al , 2004Smith et al 2008). In the LN v s, PER enters the nucleus from midnight and disappears gradually as lightsoff approaches.…”

A histone modification identifies a DNA element controllingsloBK channel gene expression in muscle

“…This phenotype is thought to arise because slo -encoded BK channels are functionally important in a neuronal output from the central circadian pacemaker (Fernandez et al, 2007). We compared the circadian rhythmicity of slo Δ55b homozygotes to wild type animals and to animals homozygous for the slo 4 null allele under free-running conditions (Figure 8).…”