We thank Michael Rosbash for his mentorship and support during the initial phase of the development of this method. This work was funded by the JST PRESTO program, the Swiss National Science Foundation (31003A_149893 and 31003A_169548), the European Research Council (ERC-StG-311194), Novartis Foundation for Medical-Biomedical Research (13A39) and the University of Geneva.

1. Preparation of Stock Solutions Under a Culture Hood
<ol>
	<li>Prepare 400 mL of 1x Schneider Active Medium (SAM) optimized for ex vivo culture of larval brains (modified from reference24,25) (Table 1). Aliquot to 5 mL and flash freeze in liquid nitrogen (LN2), store at - 80 &#176;C.</li>
	<li>Prepare 1x Dissecting Saline Solution (DSS) by diluting 10x stock solution (modified from reference26) (<st...

<ol>
	<li>Sheeba, V., Kaneko, M., Sharma, V. K., Holmes, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Drosophila+circadian+pacemaker+circuit:+Pas+De+Deux+or+Tarantella?.">The Drosophila circadian pacemaker circuit: Pas De Deux or Tarantella?.</a> Crit Rev Biochem Mol Biol. 43 (1), 37-61 (2008).</li><li>Granados-Fuentes, D., Herzog, E. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+clock+shop:+coupled+circadian+oscillators.">The clock shop: coupled circadian oscillators.</a> Exp Neurol. 243, 21-27 (2013).</li><li>Zhang, Y., Emery, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Chapter+15+-+Molecular+and+neural+control+of+insects+circadian+rhythms.">Chapter 15 - Molecular and neural control of insects circadian rhythms.</a> Insect molecular biology and biochemistry. , 513-551 (2012).</li><li>Hardin, P. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+genetic+analysis+of+circadian+timekeeping+in+Drosophila.">Molecular genetic analysis of circadian timekeeping in Drosophila.</a> Adv Genet. 74, 141-173 (2011).</li><li>Yoshii, T., Rieger, D., Helfrich-Forster, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Two+clocks+in+the+brain:+an+update+of+the+morning+and+evening+oscillator+model+in+Drosophila.">Two clocks in the brain: an update of the morning and evening oscillator model in Drosophila.</a> Prog Brain Res. 199, 59-82 (2012).</li><li>Malpel, S., Klarsfeld, A., Rouyer, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Circadian+synchronization+and+rhythmicity+in+larval+photoperception-defective+mutants+of+Drosophila.">Circadian synchronization and rhythmicity in larval photoperception-defective mutants of Drosophila.</a> J Biol Rhythms. 19 (1), 10-21 (2004).</li><li>Mazzoni, E. O., Desplan, C., Blau, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Circadian+pacemaker+neurons+transmit+and+modulate+visual+information+to+control+a+rapid+behavioral+response.">Circadian pacemaker neurons transmit and modulate visual information to control a rapid behavioral response.</a> Neuron. 45 (2), 293-300 (2005).</li><li>Sabado, V., Vienne, L., Nunes, J. M., Rosbash, M., Nagoshi, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fluorescence+circadian+imaging+reveals+a+PDF-dependent+transcriptional+regulation+of+the+Drosophila+molecular+clock.">Fluorescence circadian imaging reveals a PDF-dependent transcriptional regulation of the Drosophila molecular clock.</a> Sci Rep. 7, 41560 (2017).</li><li>Hyun, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Drosophila+GPCR+Han+is+a+receptor+for+the+circadian+clock+neuropeptide+PDF.">Drosophila GPCR Han is a receptor for the circadian clock neuropeptide PDF.</a> Neuron. 48 (2), 267-278 (2005).</li><li>Lear, B. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+G+protein-coupled+receptor,+groom-of-PDF,+is+required+for+PDF+neuron+action+in+circadian+behavior.">A G protein-coupled receptor, groom-of-PDF, is required for PDF neuron action in circadian behavior.</a> Neuron. 48 (2), 221-227 (2005).</li><li>Mertens, I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=PDF+receptor+signaling+in+Drosophila+contributes+to+both+circadian+and+geotactic+behaviors.">PDF receptor signaling in Drosophila contributes to both circadian and geotactic behaviors.</a> Neuron. 48 (2), 213-219 (2005).</li><li>Ayaz, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Axonal+injury+and+regeneration+in+the+adult+brain+of+Drosophila.">Axonal injury and regeneration in the adult brain of Drosophila.</a> J Neurosci. 28 (23), 6010-6021 (2008).</li><li>Prithviraj, R., Trunova, S., Giniger, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ex+vivo+culturing+of+whole,+developing+Drosophila+brains.">Ex vivo culturing of whole, developing Drosophila brains.</a> J Vis Exp. (65), (2012).</li><li>Siller, K. H., Serr, M., Steward, R., Hays, T. S., Doe, C. Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Live+imaging+of+Drosophila+brain+neuroblasts+reveals+a+role+for+Lis1/dynactin+in+spindle+assembly+and+mitotic+checkpoint+control.">Live imaging of Drosophila brain neuroblasts reveals a role for Lis1/dynactin in spindle assembly and mitotic checkpoint control.</a> Mol Biol Cell. 16 (11), 5127-5140 (2005).</li><li>Zschatzsch, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Regulation+of+branching+dynamics+by+axon-intrinsic+asymmetries+in+Tyrosine+Kinase+Receptor+signaling.">Regulation of branching dynamics by axon-intrinsic asymmetries in Tyrosine Kinase Receptor signaling.</a> Elife. 3, e01699 (2014).</li><li>Cabernard, C., Doe, C. Q. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Live+imaging+of+neuroblast+lineages+within+intact+larval+brains+in+Drosophila.">Live imaging of neuroblast lineages within intact larval brains in Drosophila.</a> Cold Spring Harb Protoc. 2013 (10), 970-977 (2013).</li><li>Januschke, J., Gonzalez, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+interphase+microtubule+aster+is+a+determinant+of+asymmetric+division+orientation+in+Drosophila+neuroblasts.">The interphase microtubule aster is a determinant of asymmetric division orientation in Drosophila neuroblasts.</a> The Journal of Cell Biology. 188 (5), 693-706 (2010).</li><li>Williamson, W. R., Hiesinger, P. 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(37), (2010).</li><li>Roberts, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Light+evokes+rapid+circadian+network+oscillator+desynchrony+followed+by+gradual+phase+retuning+of+synchrony.">Light evokes rapid circadian network oscillator desynchrony followed by gradual phase retuning of synchrony.</a> Curr Biol. 25 (7), 858-867 (2015).</li><li>Sehadova, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Temperature+entrainment+of+Drosophila's+circadian+clock+involves+the+gene+nocte+and+signaling+from+peripheral+sensory+tissues+to+the+brain.">Temperature entrainment of Drosophila's circadian clock involves the gene nocte and signaling from peripheral sensory tissues to the brain.</a> Neuron. 64 (2), 251-266 (2009).</li><li>Sellix, M. T., Currie, J., Menaker, M., Wijnen, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fluorescence/luminescence+circadian+imaging+of+complex+tissues+at+single-cell+resolution.">Fluorescence/luminescence circadian imaging of complex tissues at single-cell resolution.</a> J Biol Rhythms. 25 (3), 228-232 (2010).</li><li>Liang, X., Holy, T. E., Taghert, P. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synchronous+Drosophila+circadian+pacemakers+display+nonsynchronous+Ca(2)(+)+rhythms+in+vivo.">Synchronous Drosophila circadian pacemakers display nonsynchronous Ca(2)(+) rhythms in vivo.</a> Science. 351 (6276), 976-981 (2016).</li><li>Liang, X., Holy, T. E., Taghert, P. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+Series+of+Suppressive+Signals+within+the+Drosophila+Circadian+Neural+Circuit+Generates+Sequential+Daily+Outputs.">A Series of Suppressive Signals within the Drosophila Circadian Neural Circuit Generates Sequential Daily Outputs.</a> Neuron. , (2017).</li><li>Kuppers-Munther, B., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+new+culturing+strategy+optimises+Drosophila+primary+cell+cultures+for+structural+and+functional+analyses.">A new culturing strategy optimises Drosophila primary cell cultures for structural and functional analyses.</a> Dev Biol. 269 (2), 459-478 (2004).</li><li>Schneider, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Differentiation+of+Larval+Drosophila+Eye-Antennal+Discs+in+Vitro.">Differentiation of Larval Drosophila Eye-Antennal Discs in Vitro.</a> J Exp Zool. 156, 91-103 (1964).</li><li>Jiang, S. A., Campusano, J. M., Su, H., O'Dowd, D. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Drosophila+mushroom+body+Kenyon+cells+generate+spontaneous+calcium+transients+mediated+by+PLTX-sensitive+calcium+channels.">Drosophila mushroom body Kenyon cells generate spontaneous calcium transients mediated by PLTX-sensitive calcium channels.</a> J Neurophysiol. 94 (1), 491-500 (2005).</li><li>Hafer, N., Schedl, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dissection+of+larval+CNS+in+Drosophila+melanogaster.">Dissection of larval CNS in Drosophila melanogaster.</a> J Vis Exp. 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The authors have nothing to disclose.

Here we described the method for long-term fluorescence time-lapse microscopy of cultured larval brains. The success of this type of experiments depends on multiple factors, such as the health of the culture, method for immobilization of the brain explant, fluorescence intensity and signal-to-noise ratio of the reporter, temporal and spatial resolution, and accessibility to the explant. These factors can be mutually exclusive. For example, increasing time-lapse frequency affects the health of the culture, and the immobil...

Circadian clocks help organisms to adapt to the periodic environmental changes generated by the 24 h rotation of the Earth. The interlocked transcriptional-translational feedback loops commonly underlie the molecular machinery of circadian clocks across species1. The circadian pacemaker circuit composed of clock-containing neurons integrates time-of-day information conveyed by the environmental cues, such as light/dark (LD) and temperature cycles, to orchestrate the rhythms of a plethora of daily physiological and behavioral processes2,3. The coordination of molecular rhythms with neuronal inputs and outputs is critically important for the operation of the circadian circuit but remains only partially understood.
In Drosophila, at the core of the molecular clock, the CLOCK/CYCLE (CLK/CYC) heterodimer activates the transcription of period (per) and timeless (tim). PER and TIM form a complex and enter the nucleus, where they inhibit transcriptional activity of CLK/CYC and consequently their own transcription. Post-transcriptional and post-translational regulations cause delays between CLK/CYC-mediated transcription and repression by PER/TIM, ensuring the generation of circa 24-h molecular oscillations1,3,4. About 150 neurons containing these molecular clocks form a circuit to control circadian behavior of adult flies5. A much simpler yet fully functional circadian circuit consisting of 3 groups of clock neurons - 5 ventral Lateral Neurons (LNvs; 4 PDF-positive LNvs and one PDF-negative LNv, see below), 2 Dorsal Neuron 1s (DN1s) and 2 Dorsal Neuron 2s (DN2s) - is present in the larval brain6,7.
The simple larval circadian circuit offers an excellent model to study the interactions between inter-neuronal communication and the molecular clockwork. Using our newly developed fluorescent reporter PER-TDT, which mimics the levels of PER protein and its subcellular location, we sought to characterize the dynamics of the molecular clockwork in different clock neuron subgroups in the larval circadian circuit8. Furthermore, knowing the key role of the neuropeptide pigment-dispersing factor (PDF) produced by 4 LNvs in regulating circadian rhythms at the neuronal level9,10,11, we wanted to examine the direct effect of PDF on the molecular clocks. To this end, we developed a method to monitor circadian gene expression rhythms in larval brain explant over multiple days by time-lapse confocal microscopy. The protocol was also adapted for pharmacological assays to test the effect of PDF or other compounds on the level of PER-TDT. Thus, the adaptation consists of making the brain explant culture accessible to drug application, increasing temporal resolution and imaging for a shorter duration.
Ex vivo culture of Drosophila brains of different developmental stages have been previously established12,13,14,15,16,17,18. Whereas these protocols have been used for imaging various biological phenomena, some of them are not compatible with imaging at single-cell resolution or do not support the culture for more than several hours. Alternative methods to perform long-term live imaging of circadian neurons in Drosophila include bioluminescence imaging of molecular rhythms19,20,21 and fluorescence imaging of calcium indicator with light sheet microscopy22,23. Although bioluminescence imaging can achieve higher temporal resolution and light sheet microscopy can be adaptable for in vivo imaging, they are limited on the spatial resolution and require specialized microscope systems.
The method described here is tailored to visualize fluorescent signals in the whole brain culture at single-cell resolution over multiple days. This facile and versatile method could be adapted to image cultured adult fly brains and for pharmacological experiments to study many different problems in Drosophila neurobiology.

<table>
	<tbody>
		<tr>
			<td>KH2PO4</td>
			<td>Sigma-Aldrich</td>
			<td>P5655</td>
			<td>I am not sure they are exactly the same ones we have in the lab. I chose &#34;suitable for insect cell culture&#34; whenever available</td>
		</tr>
		<tr>
			<td>CaCl2</td>
			<td>Sigma-Aldrich</td>
			<td>C7902</td>
			<td></td>
		</tr>
		<tr>
			<td>MgSO4.7H2O</td>
			<td>Sigma-Aldrich</td>
			<td>230391</td>
			<td></td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>Sigma-Aldrich</td>
			<td>S5886</td>
			<td></td>
		</tr>
		<tr>
			<td>NaHCO3</td>
			<td>Sigma-Aldrich</td>
			<td>S5761</td>
			<td></td>
		</tr>
		<tr>
			<td>D-(+) Glucose</td>
			<td>Sigma-Aldrich</td>
			<td>G7021</td>
			<td></td>
		</tr>
		<tr>
			<td>Yeast extract</td>
			<td>Sigma-Aldrich</td>
			<td>Y1000</td>
			<td></td>
		</tr>
		<tr>
			<td>Insulin</td>
			<td>Sigma-Aldrich</td>
			<td>I0516-5ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin</td>
			<td>Sigma-Aldrich</td>
			<td>P4333</td>
			<td></td>
		</tr>
		<tr>
			<td>BIS-TRIS</td>
			<td>Sigma-Aldrich</td>
			<td>B4429</td>
			<td></td>
		</tr>
		<tr>
			<td>L-(&#8722;)-Malic acid</td>
			<td>Sigma-Aldrich</td>
			<td>M7397</td>
			<td></td>
		</tr>
		<tr>
			<td>D-(+)-Trehalose dihydrate</td>
			<td>Sigma-Aldrich</td>
			<td>T0167</td>
			<td></td>
		</tr>
		<tr>
			<td>Succinic acid</td>
			<td>Sigma-Aldrich</td>
			<td>S9512</td>
			<td></td>
		</tr>
		<tr>
			<td>Fumaric acid</td>
			<td>Sigma-Aldrich</td>
			<td>F8509</td>
			<td></td>
		</tr>
		<tr>
			<td>&#945;-Ketoglutaric acid</td>
			<td>Sigma-Aldrich</td>
			<td>K1128</td>
			<td></td>
		</tr>
		<tr>
			<td>Non-heat-inactivated, Foetal Calf Serum (FCS) Mycoplasma and Virus screened</td>
			<td>BioConcept Ltd. Amimed</td>
			<td>2-01F30-I</td>
			<td></td>
		</tr>
		<tr>
			<td>HEPES-KOH, pH 7.4</td>
			<td>E&#38;K Scientific Products</td>
			<td>EK-654011</td>
			<td></td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>Sigma-Aldrich</td>
			<td>P5405</td>
			<td></td>
		</tr>
		<tr>
			<td>NaH2PO4</td>
			<td>Sigma-Aldrich</td>
			<td>S5011</td>
			<td></td>
		</tr>
		<tr>
			<td>Sucrose</td>
			<td>Sigma-Aldrich</td>
			<td>S7903</td>
			<td></td>
		</tr>
		<tr>
			<td>Fibrinogen from bovine plasma</td>
			<td>Calbiochem (Merck)</td>
			<td>341573-1GM</td>
			<td>CAUTION: Harmful by inhalation, in contact with skin and if swallowed. Manipulate under laminar flow</td>
		</tr>
		<tr>
			<td>Thrombin from bovine plasma</td>
			<td>Sigma-Aldrich</td>
			<td>T9549</td>
			<td>CAUTION: Health Hazard, use gloves</td>
		</tr>
		<tr>
			<td>PDF, NH2-NSELINSLLSLPKNMNDA-OH</td>
			<td>Chi Scientific</td>
			<td>custom made</td>
			<td></td>
		</tr>
		<tr>
			<td>Vaccum grease</td>
			<td>Sigma-Aldrich</td>
			<td>18405</td>
			<td></td>
		</tr>
		<tr>
			<td>35 mm Dish, No. 1.5 Coverslip, 20 mm Glass Diameter, Uncoated</td>
			<td>MatTek</td>
			<td>P35G-1.5-20-C</td>
			<td></td>
		</tr>
		<tr>
			<td>Corning Falcon Easy-Grip Tissue Culture Dishes</td>
			<td>fisherscientific</td>
			<td>08-772A</td>
			<td></td>
		</tr>
		<tr>
			<td>Sterile 500 mL Steritop-GP 33 mm threaded bottle top filter, 0.22 &#956;m</td>
			<td>Millipore</td>
			<td>SCGPS05RE</td>
			<td></td>
		</tr>
		<tr>
			<td>Polytetrafluoroethylene (PTFE) film</td>
			<td>Dupont</td>
			<td>200A Teflon FEP Film</td>
			<td></td>
		</tr>
		<tr>
			<td>Millex-HV Syringe Filter Unit, 0.45&#160;&#181;m, PVDF, 33&#160;mm, gamma sterilized</td>
			<td>Millipore</td>
			<td>SLHV033RS</td>
			<td></td>
		</tr>
		<tr>
			<td>Millex-GV Syringe Filter Unit, 0.22&#160;&#181;m, PVDF, 33&#160;mm, gamma sterilized</td>
			<td>Millipore</td>
			<td>SLGV033RS</td>
			<td></td>
		</tr>
		<tr>
			<td>Three-well glass dissection dish</td>
			<td>Any company</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Fine forceps, size 5, Dumont</td>
			<td>Fine Science Tools</td>
			<td>11254-20</td>
			<td></td>
		</tr>
		<tr>
			<td>Tandem scanner inverted TCS SP5 confocal microscope, with resonant scanner and HyD photo-multiplier detectors</td>
			<td>Leica microsystem CMS GmbH</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Temperature control chamber</td>
			<td>Life Imaging Services</td>
			<td>The CUBE &#38; BOX temperature control system, custom designed</td>
			<td></td>
		</tr>
		<tr>
			<td>Stage-top humidity controller</td>
			<td>Life Imaging Services</td>
			<td>custom made</td>
			<td></td>
		</tr>
		<tr>
			<td>Water Immersion Micro Dispenser: dispenser, extended micro-pump MP6 series and Autoimmersion Objective Controller software</td>
			<td>Leica microsystem CMS GmbH</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>SUM-stack creation and 3D correction drift plugin</td>
			<td>ImageJ software</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>10x iterative deconvolution</td>
			<td>AutoQuant and Imaris software</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

single-cell resolution fluorescence live imaging of drosophila circadian clocks in larval brain culture

The circadian pacemaker circuit orchestrates rhythmic behavioral and physiological outputs coordinated with environmental cues, such as day/night cycles. The molecular clock within each pacemaker neuron generates circadian rhythms in gene expression, which underlie the rhythmic neuronal functions essential to the operation of the circuit. Investigation of the properties of the individual molecular oscillators in different subclasses of pacemaker neurons and their interaction with neuronal signaling yields a better understanding of the circadian pacemaker circuit. Here, we present a time-lapse fluorescent microscopy approach developed to monitor the molecular clockwork in clock neurons of cultured Drosophila larval brain. This method allows the multi-day recording of the rhythms of genetically encoded fluorescent circadian reporters at single-cell resolution. This setup can be combined with pharmacological manipulations to closely analyze real-time response of the molecular clock to various compounds. Beyond circadian rhythms, this multipurpose method in combination with powerful Drosophila genetic techniques offers the possibility to study diverse neuronal or molecular processes in live brain tissue.

Here, we show the representative results of the long-term recording of a circadian fluorescent reporter in ex vivo larval brain culture, and the live imaging results of PDF bath application on reporter expression.
Non-wandering L3 larvae expressing the molecular clock reporter PER-TDT and UAS-mCD8::YFP driven by a clock neuron driver Clk(856)-gal4 (Figure 1C) were entrained in LD. ...

Watch this Scientific Journal Video about Single-cell Resolution Fluorescence Live Imaging of Drosophila Circadian Clocks in Larval Brain Culture at JoVE.com

Single-cell Resolution Fluorescence Live Imaging of Drosophila Circadian Clocks in Larval Brain Culture

The goal of this protocol is to establish ex vivo Drosophila larval brain culture optimized to monitor circadian molecular rhythms with long-term fluorescence time-lapse imaging. The application of this method to pharmacological assays is also discussed.

Single-cell Resolution Fluorescence Live Imaging of Drosophila Circadian Clocks in Larval Brain Culture

The circadian pacemaker circuit orchestrates rhythmic behavioral and physiological outputs coordinated with environmental cues, such as day/night cycles. ...

The overall goal of this procedure is to culture Drosophila larval brains for multi-day time-lapse fluorescence microscopy. This method can help answer key questions in the field of Drosophila neurogenetics, such as gene expression changes that occur in specific neuronal populations. The main advantage of this technique is that it enables the imaging of fluorescent probes at single cell resolution in live brain over the course of hours to days.When preparing the reagents for this procedure, work under a culture hood. Both an aliquot of SAM medium and of thrombin must be thawed to begin. Set aside the thrombin.Take a 2.5 milliliter aliquot of SAM, and put the remaining SAM on ice and set it aside. Then put penicillin streptomycin in the aliquoted SAM and keep it at room temperature. For long-term imaging experiments, prepare an imagining chamber, apply autoclaved vacuum grease to the plastic regions around the well of a glass bottom dish.Then sterilize the greased dish with UV light under the hood. Now, mix an aliquot of SAM into the fibrinogen and transfer the mixture to 37 degrees Celsius, where it won't polymerize for two hours. For the dissection, clean the forceps in two three-well glass dissection dishes with 70%ethanol.Then load four wells with 400 microliters of DSS, and one well with 400 microliters of SAM. Keep the dissection dishes on a chilled plate. From a vial of propagating flies, scoop out some medium containing non-wandering third instar larvae, and transfer up to seven with forceps to a well of cold DSS.Then, pass the larvae through three of the four DSS-loaded wells to clean them off. Leave them in the final well until dissected. Use the fourth well of DSS to dissect the larval brains with a pair of clean fine forceps.Use the well established inside-out method. With proficiency each larvae can be processed in about 30 seconds. After each dissection, use a 20 microliter pipet tip pre-rinsed with DSS to transfer the brain in about three microliters of DSS to the SAM filled well.Prepare up to seven brains for the embedding procedure. After transferring the brain into working solution, dispense the brain and medium onto the lid of a sterile petri dish. Then, add another 3.5 microliters of SAM with antibiotics, and add 3.5 microliters of warm SAM with fibrinogen.It is important to maintain the SAM medium with fibrinogen at 37 degrees throughout the procedure. Next, mix the solution around the brain by gently pipetting with the same pipet tip and aspirate two microliters of the solution to deposit on a coverslip for a glass bottom dish. Then add 0.8 microliters of thrombin to the droplet and mix very quickly to initialize the polymerization which will appear white.Then, use forceps to gently pick the brain and set it onto the polymerizing fibrin matrix. I think the thrombin, after the brain was transferred to the droplet, it's not recommended, as it complicates a lot of the procedure. Orient the brain as needed, and very gently push the brain as close to the coverslip as possible.Then pick a layer of polymerized fibrin and fold it on top of the brain using small and gentle movements that do not detach the matrix. Continue to fold layers of polymerized fibrin over the brain from different parts of the clot to stabilize the brain. Then deposit 20 microliters of SAM with antibiotics over the embedded brain to halt the thrombin activity.Using this procedure, proceed with embedding five to six brains on a glass bottom dish. For long-term imaging, immerse the embedded brains in 600 microliters of SAM with antibiotics. Then lay a piece of PTFE membrane over the medium and stick the edges of the membrane to the greased parts of the dish.For short assays, fill the dish with two milliliters of SAM with antibiotics, immersing the brains and omit the use of the PTFE membrane. Using the described procedures, brains expressing a molecular clock reporter driven by a clock neuron driver, were entrained in light-dark then prepared as explant cultures. The long-term cultures were set up during the light phase, just prior to dark.A SUM-stack containing the neuron of interest was created in the mean fluorescence intensity of the area corresponding to the neuron was measured after background subtraction. To determine rhythmicity of the reporter expression, both maximum entropy spectral analysis and manual inspection were used. Next, larvae of the same genotype were dissected either one hour after subjective dawn, or one hour after subjective dusk.The brains were imaged twice within an hour to get a baseline. Then, PDF or vehicle was added to the culture medium and the brains were imaged hourly for six hours. In brains harvested shortly after dark, the PER-TDT expression profile in LNvs showed a time of day-dependent increase in fluorescence intensity upon PDF addition.This also occurred in the DN1s and to a lesser extent on the DN2s. After watching this video, you should have a good understanding of how to make ex vivo brain culture from Drosophila larvae using a 3D matrix of fibrin. Once mastered, the whole procedure takes about 30 minutes, from dissection to embedding.Prior to starting the dissections, prepare the imaging setup so the brain can be imaged immediately after embedding. Following this procedure, adult Drosophila brains can be also cultured for fluorescence time-lapse imaging in order to answer additional questions.

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JoVE Journal

Neuroscience

7.4K visualizações.  University of Geneva. The overall goal of this procedure is to culture Drosophila larval brains for multi-day time-lapse fluorescence microscopy. This method can help answer key questions in the field of Drosophila neurogenetics, such as gene expression changes that occur in specific neuronal populations. The main advantage of this technique is that it enables the imaging of fluorescent probes at single cell resolution in live brain over the course of hours to days.When preparing the reagents for this proce...