PDF shapes the architecture of a key circadian pacemaker circuit

GOROSTIZA, EA; CERIANI, MF

LI

Congreso; Neurobiology of Drosophila; 2011

Cold Spring Harbor Laboratories

The neuropeptide pigment-dispersing factor (PDF) is a key component among the signals that synchronize and support molecular oscillations within the circadian pacemaker groups in Drosophila. It is expressed in only one group of neurons, the lateral ventral neurons (LNvs). This group contains 2 types of neurons, the small-LNvs (sLNvs) and the large-LNvs (lLNvs). The sLNvs project to the dorsal protocerebrum, a region where most of the somas of other relevant circadian clusters are located, and are indispensable for maintaining behavioral rhythmicity under constant conditions. PDF immunoreactivity cycles at the level of these dorsal terminals, and it has been proposed that its rhythmic release sets the phase of most of the remaining clock neurons. We have previously shown that the complexity of the dorsal arborizations from the sLNvs change throughout the day under light-dark cycles and under free running conditions, and proposed it could represent an additional mechanism to encode and transmit time of day information.In an effort to understand this phenomenon and its relationship with PDF levels (and/or function) we evaluated the degree of structural plasticity in a PDF null mutant. Surprisingly, in pdf01 null brains the axonal projections of 1 or 2 sLNvs fail to recognize their targets and continue growing towards the posterior optic tract , without crossing the middle line. To identify the time window in which PDF signaling is required for the proper establishment of the circuit architecture we acutely downregulated PDF levels at defined stages during development.  To do so we employed an inducible version of the GAL4/UAS system termed GeneSwitch to activate transcription of specific RNAis only in the presence of the inducer. We discovered that PDF is necessary during larval stage 1 for the correct development of the sLNv projections. This miss-routing phenotype is also present in a PDF-receptor (PDFR) mutant. Therefore, we investigated where PDFR is necessary employing specific RNAis and different GAL4-drivers. We found that loss of PDFR in Tim+PDF- neurons is sufficient to cause the defect.Finally, we investigated the signaling pathway involved in this phenomenon. Expression of constitutively active receptors of the BMP signaling pathway such as TKV, in a PDFR-mutant context, prevents the miss-routing phenotype. We conclude that PDF has a non-circadian role during sLNvs-development by which the pre- and postsynapses communicate with each other, likely coordinating the correct target recognition.