We thank the Cell and Tissue Imaging Facility of the Institut du Fer &#224; Moulin, where all image acquisition and analysis have been performed. This work has been supported in part by the Centre National de la Recherche Scientifique, the Institut National de la Sant&#233; et de la Recherche M&#233;dicale, the Sorbonne Universit&#233; Sciences, and by grants from Sorbonne Universit&#233;s-Pierre et Marie Curie University (Emergence-UPMC program 2011/2014), the Fondation pour la Recherche sur le Cerveau, the Fondation de France, the Fondation pour la Recherche M&#233;dicale &#34;Equipe FRM DEQ2014039529&#34;, the French Ministry of Research (Agence Nationale pour la Recherche ANR-17-CE16-0008 and the Investissements d&#39;Avenir programme &#34;Bio-Psy Labex&#34; ANR-11-IDEX-0004-02) and a Collaborative Research in Computational Neuroscience program, National Science Foundation/French National Agency for Research (number : 1515686). All the authors are affiliated to research groups which are members of the Paris School of Neuroscience (ENP) and of the Bio-Psy Labex. F.E. is a Ph.D. student affiliated with Sorbonne Universit&#233;, Coll&#232;ge Doctoral, F-75005 Paris, France, and is funded by the Bio-Psy Labex. V.M. is a post-doctoral fellow funded by the Collaborative Research in Computational Neuroscience program, National Science Foundation/French National Agency for Research (number: 1515686). The authors thank Marta Kolodziejczak who participated in the initiation of the project.

All experiments were approved by the local ethical committee (Darwin Committee, agreements #1170 and #10921).
1. Preparation of Glass Micropipettes for the Local Application of Compounds
<ol>
	<li>Prepare pipettes from borosilicate thin-wall glass capillaries with an electrode puller. Adjust the parameters to obtain pipettes with a 4 - 5 &#181;m diameter at their extremity. Figure 2D shows one pipette in brightfield at low magnification.</li>
</ol>
<p class="jov...

<ol>
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W., Bisht, K., Tremblay, M. -. &. #. 2. 0. 0. ;. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microglia+across+the+lifespan:+from+origin+to+function+in+brain+development,+plasticity+and+cognition.">Microglia across the lifespan: from origin to function in brain development, plasticity and cognition.</a> The Journal of Physiology. , (2016).</li><li>Davalos, 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=ATP+mediates+rapid+microglial+response+to+local+brain+injury+in+vivo.">ATP mediates rapid microglial response to local brain injury in vivo.</a> Nature Neuroscience. 8 (6), 752-758 (2005).</li><li>Nimmerjahn, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Resting+Microglial+Cells+Are+Highly+Dynamic+Surveillants+of+Brain+Parenchyma+in+Vivo.">Resting Microglial Cells Are Highly Dynamic Surveillants of Brain Parenchyma in Vivo.</a> Science. 308 (5726), 1314-1318 (2005).</li><li>Dissing-Olesen, 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=Activation+of+neuronal+NMDA+receptors+triggers+transient+ATP-mediated+microglial+process+outgrowth.">Activation of neuronal NMDA receptors triggers transient ATP-mediated microglial process outgrowth.</a> The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 34 (32), 10511-10527 (2014).</li><li>Gyoneva, S., Traynelis, S. 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V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Altered+morphological+dynamics+of+activated+microglia+after+induction+of+status+epilepticus.">Altered morphological dynamics of activated microglia after induction of status epilepticus.</a> Journal of Neuroinflammation. 12, 202 (2015).</li><li>Abiega, O., 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=Neuronal+Hyperactivity+Disturbs+ATP+Microgradients,+Impairs+Microglial+Motility,+and+Reduces+Phagocytic+Receptor+Expression+Triggering+Apoptosis/Microglial+Phagocytosis+Uncoupling.">Neuronal Hyperactivity Disturbs ATP Microgradients, Impairs Microglial Motility, and Reduces Phagocytic Receptor Expression Triggering Apoptosis/Microglial Phagocytosis Uncoupling.</a> PLoS Biology. 14 (5), e1002466 (2016).</li><li>Madry, 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=Microglial+Ramification,+Surveillance,+and+Interleukin-1&#946;+Release+Are+Regulated+by+the+Two-Pore+Domain+K+Channel+THIK-1.">Microglial Ramification, Surveillance, and Interleukin-1&#946; Release Are Regulated by the Two-Pore Domain K+Channel THIK-1.</a> Neuron. 97 (2), 299-312 (2018).</li><li>Honda, 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=Extracellular+ATP+or+ADP+induce+chemotaxis+of+cultured+microglia+through+Gi/o-coupled+P2Y+receptors.">Extracellular ATP or ADP induce chemotaxis of cultured microglia through Gi/o-coupled P2Y receptors.</a> The Journal of Neuroscience: The Official Journal of the Society for Neuroscience. 21 (6), 1975-1982 (2001).</li><li>Haynes, S. E., 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=The+P2Y12+receptor+regulates+microglial+activation+by+extracellular+nucleotides.">The P2Y12 receptor regulates microglial activation by extracellular nucleotides.</a> Nature Neuroscience. 9 (12), 1512-1519 (2006).</li><li>Wu, L. -. J., Vadakkan, K. 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N., 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=Microglia+Promote+Learning-Dependent+Synapse+Formation+through+Brain-Derived+Neurotrophic+Factor.">Microglia Promote Learning-Dependent Synapse Formation through Brain-Derived Neurotrophic Factor.</a> Cell. 155 (7), 1596-1609 (2013).</li><li>Wu, Y., Dissing-Olesen, L., Macvicar, B. 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The authors have nothing to disclose.

By maintaining, unlike in dissociated or organotypic slice culture, a structural integrity with limited network adjustments, acute brain slices allow researchers to study microglia in their physiological environment. However, one of the major limitations is the fact that the slicing procedure creates injuries that can rapidly compromise the viability of neurons, particularly in the adult brain. As microglia are particularly reactive to cell damage, it is important to limit neuronal cell death as much as possible to prese...

Microglial cells are the brain&#39;s resident macrophages and play a role in both physiological and pathological conditions1,2. They have a highly branched morphology and are constantly extending and retracting their processes3,4. This &#34;scanning&#34; behavior is believed to be related and necessary to the survey of their surroundings. The morphological plasticity of microglia is expressed in three modes. First, some compounds rapidly modulate microglial morphology: the addition of ATP5,6 or NMDA5,7 in the medium bathing acute brain slices increases the complexity of microglial ramifications, whereas norepinephrine decreases it6. These effects either are directly mediated by microglial receptors (for ATP and norepinephrine) or require an ATP release from neurons (for NMDA). Second, the growth and retraction speed of microglial processes, called motility or &#34;surveillance&#34;, can be affected by extracellular factors8, homeostasis disruptions9,10, or mutations9,10,11. Third, in addition to these isotropic changes of morphology and motility, microglia have the capacity to extend their processes directionally toward a pipette delivering ATP3,5,12,13,14, in culture, in acute brain slices or in vivo, or delivering 5-HT in acute brain slices15. Such oriented growth of microglial processes, also called directional motility, was first described as a response to a local laser lesion3,4. Thus, physiologically, it may be related to the response to injury or required for targeting microglial processes toward synapses or brain regions requiring pruning during development15,16, or in physiological17,18,19 or pathological situations9,18,19,20 in adulthood. The three types of morphological changes rely on different intracellular mechanisms11,13,20, and one given compound does not necessarily modulate all of them (e.g., NMDA, which acts indirectly on microglia, has an effect on morphology but does not induce directional motility5,7). Therefore, when aiming to characterize the effect of a compound, a mutation or a pathology on microglia, it is important to characterize the three components of their morphological plasticity. Here, we describe a method to study the directional growth of microglial processes toward a local source of compound, which is, here, ATP or 5-HT.
There are several models to study microglia processes&#39; attraction: primary cultures in 3D environment6,18,19, acute brain slices6,13,15,&#160;and in vivo imaging3,13. The in vivo approach is the best to preserve the physiological state of microglia. However, intravital imaging of deep regions requires complex surgical procedures and, therefore, it is often limited to superficial cortical layers. The use of microglia primary culture is the easiest technique to test a large number of conditions with a limited number of animals. Nevertheless, it is impossible to obtain the same cell morphology as in vivo, and cells lose their physiological interactions with neurons and astrocytes. Acute brain slices represent a compromise between these two approaches. This model allows researchers to study brain structures which are otherwise difficult to reach and to image with high resolution in vivo, and to investigate slices from neonatal stages, whereas transcranial microscopy is mostly performed at adulthood. Finally, it makes it possible to observe in real-time the effects of local drug application, and to repeat experiments while using a limited number of animals. Nonetheless, an issue with acute brain slices is the limited time (a few hours) during which the cells remain alive, notably for slices from mice older than two weeks, and the potential change of microglia morphology over time21,22.
Here, we describe a protocol to prepare acute brain slices of young and adult Cx3cr1GFP/+ mice up to two months old, with the preservation of microglia morphology and motility for several hours. We, then, describe how to use these slices to study the attraction of microglial processes toward compounds like ATP or 5-HT.

<table>
	<tbody>
		<tr>
			<td>for pipettes preparation</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Clark Borosilicate Thin Wall Capillaries</td>
			<td>Harvard Apparatus</td>
			<td>30-0065</td>
			<td>Borosilicate Thin Wall without Filament, 1.5 mm OD, 1.17 mm ID, 75 mm L , Pkg. of 225</td>
		</tr>
		<tr>
			<td>DMZ Universal Puller</td>
			<td>Zeitz Instrumente</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>for solutions</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Calcium Chloride dihydrate (CaCl2)</td>
			<td>Sigma</td>
			<td>C5080</td>
			<td></td>
		</tr>
		<tr>
			<td>Choline Chloride</td>
			<td>Sigma</td>
			<td>C7527</td>
			<td></td>
		</tr>
		<tr>
			<td>D-(+)-Glucose</td>
			<td>Sigma</td>
			<td>G8270</td>
			<td></td>
		</tr>
		<tr>
			<td>L-Ascorbic acid</td>
			<td>Sigma</td>
			<td>A5960</td>
			<td></td>
		</tr>
		<tr>
			<td>Magnesium Chloride solution 1M (MgCl2)</td>
			<td>Sigma</td>
			<td>63020</td>
			<td></td>
		</tr>
		<tr>
			<td>Potassium chloride SigmaUltra &#62;99,0% (KCl)</td>
			<td>Sigma</td>
			<td>P9333</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium bicarbonate (NaHCO3)</td>
			<td>Sigma</td>
			<td>S5761</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium Chloride (NaCl)</td>
			<td>Sigma</td>
			<td>S5886</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium phosphate monobasic</td>
			<td>Sigma</td>
			<td>S5011</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium pyruvate</td>
			<td>Sigma</td>
			<td>P2256</td>
			<td></td>
		</tr>
		<tr>
			<td>Ultrapure water</td>
			<td>MilliQ</td>
			<td></td>
			<td>for all the solutions</td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>for slice preparation</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>2x 200 mL crystalizing dishes</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>80 mL Pyrex beaker</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Antlia-3C Digital Peristaltic pump</td>
			<td>DD Biolab</td>
			<td>178961</td>
			<td>For mice perfusion and 2-photon chamber perfusion (aCSF)</td>
		</tr>
		<tr>
			<td>Carbogen 5% CO2/95% O2</td>
			<td>Air Liquide France Industrie</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Dolethal</td>
			<td>Vetoquinol</td>
			<td>Dolethal 50 mg/mL</td>
			<td></td>
		</tr>
		<tr>
			<td>Filter papers (Whatman)</td>
			<td>Sigma</td>
			<td>WHA1001042</td>
			<td>Whatman qualitative filter paper, Grade 1 (Pore size: 11&#181;M)</td>
		</tr>
		<tr>
			<td>Fine Scissors - Sharp</td>
			<td>Fine Science Tools</td>
			<td>14060-60</td>
			<td></td>
		</tr>
		<tr>
			<td>Food box 10 cm diameter, 8 cm Height</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>glue (ethyl cyanoacrylate)</td>
			<td>Loctite</td>
			<td>super glue 3 power flex</td>
			<td></td>
		</tr>
		<tr>
			<td>Hippocampal Tool (spatula)</td>
			<td>Fine Science Tools</td>
			<td>10099-15</td>
			<td>The largest extremity has to be angled at 90 &#176;</td>
		</tr>
		<tr>
			<td>Ice</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Iris Forceps (curved)</td>
			<td>Moria</td>
			<td>MC31</td>
			<td></td>
		</tr>
		<tr>
			<td>Lens cleaning tissue</td>
			<td>THOR LABS</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Nylon mesh strainer</td>
			<td></td>
			<td></td>
			<td>diameter 7 cm</td>
		</tr>
		<tr>
			<td>Razor blades</td>
			<td>Electron Microscopy Sciences</td>
			<td>72000</td>
			<td>For the slicer</td>
		</tr>
		<tr>
			<td>scalpel blade</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Slice interface holder</td>
			<td></td>
			<td></td>
			<td>home-made, the file for 3D printing is provided in Supplemental Material</td>
		</tr>
		<tr>
			<td>Surgical Scissors - Sharp</td>
			<td>Fine Science Tools</td>
			<td>14002-14</td>
			<td></td>
		</tr>
		<tr>
			<td>Vibrating slicer</td>
			<td>Thermo Scientific</td>
			<td>720-2709</td>
			<td>Model: HM 650V (Vibrating blade microtome)</td>
		</tr>
		<tr>
			<td>Water bath</td>
			<td></td>
			<td></td>
			<td>Set at 32&#176;C (first recovery step)</td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>for slice imaging</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>&#215; 25 0.95 NA water-immersion objective</td>
			<td>Leica Microsystems (Germany)</td>
			<td>HCX Irapo</td>
			<td></td>
		</tr>
		<tr>
			<td>2-photon MP5 upright microscope with resonant scanners (8 kHz) and two HyD Hybrid detectors</td>
			<td>Leica Microsystems (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Antlia-3C Digital Peristaltic pump</td>
			<td>DD Biolab</td>
			<td>178961</td>
			<td>For 2-photon chamber perfusion with aCSF</td>
		</tr>
		<tr>
			<td>Carbogen 5% CO2/95% O2</td>
			<td>Air Liquide France Industrie</td>
			<td>I1501L50R2A001</td>
			<td></td>
		</tr>
		<tr>
			<td>Chameleon Ultra2 Ti:sapphire laser</td>
			<td>Coherent (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>disposable transfer pipettes , wide mouth</td>
			<td>ThermoFischer scientific</td>
			<td>for example : 232-11</td>
			<td>5.8ml with fin tip, but we cut it (approx 7cm) to have a 4 mm diameter mouth</td>
		</tr>
		<tr>
			<td>emission filter SP680</td>
			<td>Leica Microsystems (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>fluorescent cube containing a 525/50 emission filter and a 560 dichroic filter (for fluorescence collection)</td>
			<td>Leica Microsystems (Germany)</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>glass beaker with 50 mL of ACSF to maintain constant perfusion of the slice</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Heating system</td>
			<td>Warner Instrument Corporation</td>
			<td>Automatic Heater Controller TC-324B</td>
			<td>to maintain perfusion solution at 32&#176;C</td>
		</tr>
		<tr>
			<td>perfusion chamber</td>
			<td></td>
			<td></td>
			<td>home-made, the file for 3D printing is provided in Supplemental Material</td>
		</tr>
		<tr>
			<td>slice holder (&#34;harp&#34;)</td>
			<td></td>
			<td></td>
			<td>home made : hairpin made of platinum with the two branches joined by parallel nylon threads</td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>for slice stimulation </td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Adenosine 5&#8242;-triphosphate disodium salt hydrate (ATP)</td>
			<td>Sigma</td>
			<td>A-26209</td>
			<td>to be prepared ex-temporaneously : 1mg/ml (3mM) stock solution prepared the day of the experiment, kept at 4&#176;C (a few hours) and diluted just before use</td>
		</tr>
		<tr>
			<td>Fluorescein (optional)</td>
			<td>Sigma</td>
			<td>F-6377</td>
			<td>use at 1 &#181;M final</td>
		</tr>
		<tr>
			<td>Micromanipulator</td>
			<td>Luigs and Neumann</td>
			<td>SM7</td>
			<td>connected to the micropipette holde</td>
		</tr>
		<tr>
			<td>Micropipette holder</td>
			<td></td>
			<td></td>
			<td>same as for eletrophysiology</td>
		</tr>
		<tr>
			<td>Serotonin hydrochloride</td>
			<td>Sigma</td>
			<td>H-9523</td>
			<td>aliquots of 50mM stock solution in H20 kept at -20&#176;C. 500&#181;M solution prepared the day of the experiment.</td>
		</tr>
		<tr>
			<td>Syringe 5mL (without needle)</td>
			<td>Terumo medical products</td>
			<td>SS+05S1</td>
			<td></td>
		</tr>
		<tr>
			<td>Transparent tubing</td>
			<td>Fischer Scientific</td>
			<td>11750105</td>
			<td>Saint Gobain Performance Plastics&#8482; Tygon&#8482; E-3603 Non-DEHP Tubing</td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>for image analysis</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Fiji</td>
			<td></td>
			<td>https://fiji.sc</td>
			<td>Schindelin, J. et al Nat. Methods (2012) doi 10.1038</td>
		</tr>
		<tr>
			<td>Icy</td>
			<td>Institut Pasteur</td>
			<td>http://icy.bioimageanalysis.org</td>
			<td>de Chaumont, F. et al. Nat. Methods (2012)</td>
		</tr>
		<tr>
			<td>Name</td>
			<td>Company</td>
			<td>Catalog Number</td>
			<td>Comments</td>
		</tr>
		<tr>
			<td>mice</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>CX3CR1-GFP mice</td>
			<td>Jung et al, 2000</td>
			<td></td>
			<td>male or females, P3 to 2 months-old ; we have backcrossed these mice on 129sv background.</td>
		</tr>
		<tr>
			<td>CX3CR1creER-YFP mice</td>
			<td>Parkhurst et al 2013</td>
			<td></td>
			<td>male or females, P3 to 2 months-old ; we have backcrossed these mice on 129sv background.</td>
		</tr>
	</tbody>
</table>

two-photon imaging of microglial processes' attraction toward atp or serotonin in acute brain slices

Microglial cells are resident innate immune cells of the brain that constantly scan their environment with their long processes and, upon disruption of homeostasis, undergo rapid morphological changes. For example, a laser lesion induces in a few minutes an oriented growth of microglial processes, also called &#34;directional motility&#34;, toward the site of injury. A similar effect can be obtained by delivering locally ATP or serotonin (5-hydroxytryptamine [5-HT]). In this article, we describe a protocol to induce a directional growth of microglial processes toward a local application of ATP or 5-HT in acute brain slices of young and adult mice and to image this attraction over time by multiphoton microscopy. A simple method of quantification with free and open-source image analysis software is proposed. A challenge that still characterizes acute brain slices is the limited time, decreasing with age, during which the cells remain in a physiological state. This protocol, thus, highlights some technical improvements (medium, air-liquid interface chamber, imaging chamber with a double perfusion) aimed at optimizing the viability of microglial cells over several hours, especially in slices from adult mice.

This protocol describes a method to induce, observe, and quantify the oriented growth of microglial processes toward a locally applied compound, for example, ATP or 5-HT, in acute brain slices from young or adult (at least up to two-month-old) mice. Among the factors that contribute to maintaining brain slices from adult animals in a healthy state for several hours is the use of two tools designed to optimize cell survival at two steps of the protocol. First, the interface slice holder in...

Watch this Scientific Journal Video about Two-photon Imaging of Microglial Processes' Attraction Toward ATP or Serotonin in Acute Brain Slices at JoVE.com

Two-photon Imaging of Microglial Processes&#039; Attraction Toward ATP or Serotonin in Acute Brain Slices

Microglia, the resident immune cells of the brain, respond quickly with morphological changes to modifications of their environment. This protocol describes how to use two-photon microscopy to study the attraction of microglial processes toward serotonin or ATP in acute brain slices of mice.

Two-photon Imaging of Microglial Processes' Attraction Toward ATP or Serotonin in Acute Brain Slices

Microglial cells are resident innate immune cells of the brain that constantly scan their environment with their long processes and, upon disruption of ...

To fully characterize a microglial phenotype, it's important to address both basal and directional motility. This protocol can be used to test the directional motility of microglia by two-photon imaging in mice brain slices, using customized 3D print interface and perfusion chambers. Demonstrating the procedure will be Fanny Etienne, a Ph.D.student, and Vincenzo Mastriolia, a post-doctoral researcher, both from our laboratories.At least 30 minutes before the dissection, start bubbling 70 milliliters of choline artificial cerebrospinal fluid, or choline aCSF, on ice. Add 150 milliliters of choline aCSF at 32 degrees Celsius, with carbogen. Place a 200 milliliter crystallizing dish with a bar magnet into a sealed food box and add 200 milliliters of aCSF to the dish.Place a 3D printed interface slice holder on top of the dish and remove excess fluid from the crystallizing dish, until only a thin film of solution covering the mesh of the interface slide holder remains. Add a few millimeters of aCSF at the bottom of the food box and start bubbling the fluid with carbogen. Then, close the sealed box while maintaining constant carbogenation to create a humidified, 95%oxygen, 5%carbon dioxide-rich interface chamber.After harvesting, place the brain onto a piece of aCSF-soaked filter paper and dissect out the region of interest, according to the preferred angle of slicing. For coronal slices, position and glue the caudal face of the brain onto a 10 centimeter Petri dish attached to the cutting block of a vibrating slicer, and position the block in the reservoir chamber of the vibrating slicer within a large chamber filled with ice. Fill the dish with ice-cold choline aCSF and use the slicer to obtain 300-micrometer thick brain slices, using a four millimeter diameter disposable transfer pipette after every pass of the blade to collect each slice.Let each slice recover in the 32 degree Celsius choline aCSF for about 10 minutes after it is obtained, before transferring the slices onto pieces of lens cleaning paper, topped with a drop of choline aCSF. Then aspirate the excess choline aCSF and use a spatula to transfer the slices onto the mesh of the interface chamber, allowing the slices to recover in this environment for at least 30 minutes. 30 minutes before starting the recording, connect the peristaltic pump to a customized recording chamber with top and bottom perfusion for optimizing the oxygenation and viability of the tissue slices, and clean the entire perfusion system with 50 milliliters of ultrapure water.At the end of the cleaning cycle, start the perfusion of the recording chamber with 50 milliliters of aCSF in a glass beaker under constant carbogenation, and use a disposable, wide-mouth transfer pipette to transfer the first slice to be imaged to the beaker to remove the lens paper. When the section has sunk to the bottom of the beaker, transfer the section to the recording chamber and position the slice holder onto the slice to minimize movement induced by the perfusion flow. Use bright-field illumination to target the brain region of interest under a five to 10x magnification.Then, use a 25x objective with a 0.35x water immersion lens to adjust the position of the viewing field. Use fluorescence illumination to locate the fluorescent microglial cells and backfill the pipette with 10 microliters of aCSF containing the compound of interest at its final concentration. Point the tip downward with gentle shaking to remove any air bubbles trapped in the tip and mount the filled pipette into a pipette holder connected to a five milliliter syringe with the plunger positioned at the five milliliter position and mounted onto a three-axis micromanipulator.Lower the pipette gently toward the slice, controlling and adjusting the objective at the same time, until the pipette tip lightly touches the surface of the slice. Now tune the laser and switch the microscope to the multiphoton mode. Make sure that chamber is screened from any light source and switch on the non-descanned detectors.Set the gain and use a lookup table with a color-coded upper limit to avoid saturating the pixels in the image. Then, start recording for a total duration of at least 30 minutes, slowly depressing the syringe plunger from the five to one milliliter position, after five minutes, over a period of five seconds, to apply the compound to the section. For image analysis, first perform z-projection and drift correction with ImageJ on the file of interest.Then, open the modified file in Icy and draw a circular, 35-micrometer diameter region of interest, centered over the injection site. Run the movie again with the ROI, to ensure that it is well-positioned. Then, use the region of interest intensity evolution plugin to measure the mean intensity over time in the region of interest, and save the results as a xls file.This protocol allows the responses induced by different compounds, like ATP or serotonin, to be measured. Although the injection of ATP elicits an increase of fluorescence, after most ATP injections, there is an immediate, slight decrease in fluorescence due to tissue distortion that comes back after a few minutes. The size of the region of interest also impacts the quantification, as increasing the diameter reduces the variability among the experiments, but decreases the accuracy and magnitude of the detected response.Assessment of the microglial response at a specific time point can be useful for the statistical comparison of different compounds, or to test the effects of specific antagonists added to the perfusion solution. To quantify the motility in three dimensions, know that you may have to change the z-step interval and the sampling rate of the acquisition to fit within the analysis requirements.

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Research

JoVE Journal

Neuroscience

9.7K Views.  Institut National de la Santé et de la Recherche Médicale (INSERM), UMR-S 1270 Paris, France. To fully characterize a microglial phenotype, it's important to address both basal and directional motility. This protocol can be used to test the directional motility of microglia by two-photon imaging in mice brain slices, using customized 3D print interface and perfusion chambers. Demonstrating the procedure will be Fanny Etienne, a Ph.D.student, and Vincenzo Mastriolia, a post-doctoral researcher, both from our laboratories.At least 30 minutes before the dissection, start bubbling ...