Besides promoting morning activity, the sLNvs have an additional and crucial function. They keep brain pacemaker

neurons coherently synchronized and can thus maintain circadian behavioral rhythms even if flies are under constant conditions (Lin et al., 2004; Renn et al., 1999; Yoshii et al., 2009b). They perform this remarkable task by secreting SRT1720 chemical structure the neuropeptide PDF (Renn et al., 1999). The receptor for PDF (PDFR) is broadly expressed in circadian neurons (Hyun et al., 2005; Im and Taghert, 2010; Lear et al., 2005; Lear et al., 2009; Mertens et al., 2005). If PDF or PDFR is missing, flies become rapidly arrhythmic in constant darkness (DD), and in Pdf0 flies, circadian neurons are desynchronized in DD ( Hyun et al., 2005; Lear et al., 2005; Lin et al., 2004; Mertens et al., 2005; Renn et al., 1999; Yoshii

et al., 2009b). These phenotypes are remarkably similar to those seen in mice lacking either the neuropeptide Vasoactive Intestinal Polypeptide (VIP) or its receptor (called either VIPR or VPAC2) ( Aton et al., 2005), which are both expressed in the brain pacemaker structure of the mammalian brain: the Suprachiasmatic Nucleus (SCN). It is interesting that VPAC2 and PDFR are not just functional homologs but actually share considerable sequence similarities ( Helfrich-Förster, 2005). The neural mechanisms by which coherent circadian behavior is generated are thus well conserved in the animal PAK6 kingdom. INK1197 Beside arrhythmicity in DD, mutations in Drosophila PDF or its receptor have other characteristic consequences under LD conditions: the

morning peak of activity is severely reduced, and the phase of the evening peak is advanced ( Hyun et al., 2005; Lear et al., 2005; Mertens et al., 2005; Renn et al., 1999). This reflects the importance of the sLNvs in the control of morning activity and their ability to determine the phase of circadian molecular rhythms in other circadian neurons. PDFR belongs to the class II G-Protein coupled receptor (GPCR) family. Solid evidence indicates that it is positively coupled to cyclic AMP (cAMP) signaling ( Choi et al., 2012; Duvall and Taghert, 2012; Mertens et al., 2005; Shafer et al., 2008). However, the proteins participating in the PDFR signaling pathway only begin to be identified, with Gsα and the adenylate cyclase AC3 playing an important role in the sLNvs ( Choi et al., 2012; Duvall and Taghert, 2012). Gene expression can be modulated by small RNA molecules called microRNAs (miRNAs) (Bartel, 2004). They are generated by an enzymatic cascade from precursor RNAs (Liu and Paroo, 2010). After being transcribed, pri-miRNAs are cleaved in Drosophila by PASHA and DROSHA into pre-miRNAs, which are processed into mature miRNAs by DICER1 (DCR1) and LOQUASCIOUS (LOQS).