Salome Murinello is supported by American Diabetes Association grant #1-16-PDF-072. This work was supported by grants to Martin Friedlander from the National Institutes of Health (National Eye Institute EY11254 and EY22025) and the Lowy Medical Research Institute.

All animals were treated in accordance with the ethical guidelines established by the Scripps Research Institute.
1. Preparation of Materials for Injection
<ol>
	<li>Sterilize a 33 G needle and syringe: disassemble and autoclave at 115&#160; &#959;C. Prepare needles for injection by rising in sterile phosphate buffered saline (PBS).</li>
	<li>Thaw fluorescently labeled particles at room temperature for 5 - 10 min. Prepare particle solution for injection by reconstituting to a 50 mg/ml solution in ster...

<ol>
	<li>Gomez-Nicola, D., Perry, V. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microglial+dynamics+and+role+in+the+healthy+and+diseased+brain:+a+paradigm+of+functional+plasticity.">Microglial dynamics and role in the healthy and diseased brain: a paradigm of functional plasticity.</a> Neuroscientist. 21, 169-184 (2015).</li><li>Tremblay, M. E., Lowery, R. L., Majewska, A. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microglial+interactions+with+synapses+are+modulated+by+visual+experience.">Microglial interactions with synapses are modulated by visual experience.</a> PLoS Biol. 8, e1000527 (2010).</li><li>Schafer, D. P., 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+sculpt+postnatal+neural+circuits+in+an+activity+and+complement-dependent+manner.">Microglia sculpt postnatal neural circuits in an activity and complement-dependent manner.</a> Neuron. 74, 691-705 (2012).</li><li>Sierra, A., 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+shape+adult+hippocampal+neurogenesis+through+apoptosis-coupled+phagocytosis.">Microglia shape adult hippocampal neurogenesis through apoptosis-coupled phagocytosis.</a> Cell Stem Cell. 7, 483-495 (2010).</li><li>Sierra, A., Abiega, O., Shahraz, A., Neumann, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Janus-faced+microglia:+beneficial+and+detrimental+consequences+of+microglial+phagocytosis.">Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis.</a> Front Cell Neurosci. 7 (6), (2013).</li><li>Chan, G., 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=CD33+modulates+TREM2:+convergence+of+Alzheimer+loci.">CD33 modulates TREM2: convergence of Alzheimer loci.</a> Nat Neurosci. 18, 1556-1558 (2015).</li><li>Murinello, S., Mullins, R. F., Lotery, A. J., Perry, V. H., Teeling, J. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fcgamma+receptor+upregulation+is+associated+with+immune+complex+inflammation+in+the+mouse+retina+and+early+age-related+macular+degeneration.">Fcgamma receptor upregulation is associated with immune complex inflammation in the mouse retina and early age-related macular degeneration.</a> Invest Ophthalmol Vis Sci. 55, 247-258 (2014).</li><li>Ransohoff, R. M., Perry, V. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microglial+physiology:+unique+stimuli,+specialized+responses.">Microglial physiology: unique stimuli, specialized responses.</a> Annu Rev Immunol. 27, 119-145 (2009).</li><li>Derecki, N., Cronk, J., Kipnis, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Assay+of+phagocytic+function+in+primary+murine+microglia.">Assay of phagocytic function in primary murine microglia.</a> Protoc Exchange. , (2012).</li><li>Fricker, M., Oliva-Martin, M. J., Brown, G. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Primary+phagocytosis+of+viable+neurons+by+microglia+activated+with+LPS+or+Abeta+is+dependent+on+calreticulin/LRP+phagocytic+signalling.">Primary phagocytosis of viable neurons by microglia activated with LPS or Abeta is dependent on calreticulin/LRP phagocytic signalling.</a> J Neuroinflammation. 9, 196 (2012).</li><li>Koenigsknecht-Talboo, J., Landreth, G. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microglial+phagocytosis+induced+by+fibrillar+beta-amyloid+and+IgGs+are+differentially+regulated+by+proinflammatory+cytokines.">Microglial phagocytosis induced by fibrillar beta-amyloid and IgGs are differentially regulated by proinflammatory cytokines.</a> J Neurosci. 25, 8240-8249 (2005).</li><li>Neher, J. J., 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=Inhibition+of+microglial+phagocytosis+is+sufficient+to+prevent+inflammatory+neuronal+death.">Inhibition of microglial phagocytosis is sufficient to prevent inflammatory neuronal death.</a> J Immunol. 186, 4973-4983 (2011).</li><li>Fricker, 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=MFG-E8+mediates+primary+phagocytosis+of+viable+neurons+during+neuroinflammation.">MFG-E8 mediates primary phagocytosis of viable neurons during neuroinflammation.</a> J Neurosci. 32, 2657-2666 (2012).</li><li>Mildner, A., 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=Distinct+and+non-redundant+roles+of+microglia+and+myeloid+subsets+in+mouse+models+of+Alzheimer's+disease.">Distinct and non-redundant roles of microglia and myeloid subsets in mouse models of Alzheimer's disease.</a> J Neurosci. 31, 11159-11171 (2011).</li><li>Neher, J. J., 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=Phagocytosis+executes+delayed+neuronal+death+after+focal+brain+ischemia.">Phagocytosis executes delayed neuronal death after focal brain ischemia.</a> Proc Natl Acad Sci U S A. 110, E4098-E4107 (2013).</li><li>Perego, C., Fumagalli, S., De Simoni, M. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Temporal+pattern+of+expression+and+colocalization+of+microglia/macrophage+phenotype+markers+following+brain+ischemic+injury+in+mice.">Temporal pattern of expression and colocalization of microglia/macrophage phenotype markers following brain ischemic injury in mice.</a> J Neuroinflammation. 8, 174-174 (2011).</li><li>Preissler, J., 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=Altered+microglial+phagocytosis+in+GPR34-deficient+mice.">Altered microglial phagocytosis in GPR34-deficient mice.</a> Glia. 63, 206-215 (2015).</li><li>Hughes, M. M., Field, R. H., Perry, V. H., Murray, C. L., Cunningham, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Microglia+in+the+degenerating+brain+are+capable+of+phagocytosis+of+beads+and+of+apoptotic+cells,+but+do+not+efficiently+remove+PrP(Sc),+even+upon+LPS+stimulation.">Microglia in the degenerating brain are capable of phagocytosis of beads and of apoptotic cells, but do not efficiently remove PrP(Sc), even upon LPS stimulation.</a> Glia. 58, 2017-2030 (2010).</li><li>Westenskow, P. 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=Using+flow+cytometry+to+compare+the+dynamics+of+photoreceptor+outer+segment+phagocytosis+in+iPS-derived+RPE+cells.">Using flow cytometry to compare the dynamics of photoreceptor outer segment phagocytosis in iPS-derived RPE cells.</a> Invest Ophthalmol Vis Sci. 53, 6282-6290 (2012).</li><li>London, A., Benhar, I., Schwartz, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+retina+as+a+window+to+the+brain-from+eye+research+to+CNS+disorders.">The retina as a window to the brain-from eye research to CNS disorders.</a> Nat Rev Neurol. 9, 44-53 (2013).</li><li>Fritzenwanger, M., Jung, C., Goebel, B., Lauten, A., Figulla, H. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impact+of+short-term+systemic+hypoxia+on+phagocytosis,+cytokine+production,+and+transcription+factor+activation+in+peripheral+blood+cells.">Impact of short-term systemic hypoxia on phagocytosis, cytokine production, and transcription factor activation in peripheral blood cells.</a> Mediators Inflamm. , 429501 (2011).</li><li>Ragsdale, R. L., Grasso, R. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+improved+spectrofluorometric+assay+for+quantitating+yeast+phagocytosis+in+cultures+of+murine+peritoneal+macrophages.">An improved spectrofluorometric assay for quantitating yeast phagocytosis in cultures of murine peritoneal macrophages.</a> J Immunol Methods. 123, 259-267 (1989).</li><li>Stokes, L., Surprenant, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dynamic+regulation+of+the+P2X4+receptor+in+alveolar+macrophages+by+phagocytosis+and+classical+activation.">Dynamic regulation of the P2X4 receptor in alveolar macrophages by phagocytosis and classical activation.</a> Eur J Immunol. 39, 986-995 (2009).</li><li>Kotani, 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=Intraoperative+modulation+of+alveolar+macrophage+function+during+isoflurane+and+propofol+anesthesia.">Intraoperative modulation of alveolar macrophage function during isoflurane and propofol anesthesia.</a> Anesthesiology. 89, 1125-1132 (1998).</li><li>Xu, X., Feng, J., Zuo, Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Isoflurane+preconditioning+reduces+the+rat+NR8383+macrophage+injury+induced+by+lipopolysaccharide+and+interferon+gamma.">Isoflurane preconditioning reduces the rat NR8383 macrophage injury induced by lipopolysaccharide and interferon gamma.</a> Anesthesiology. 108, 643-650 (2008).</li><li>abd-el-Basset, E., Fedoroff, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+bacterial+wall+lipopolysaccharide+(LPS)+on+morphology,+motility,+and+cytoskeletal+organization+of+microglia+in+cultures.">Effect of bacterial wall lipopolysaccharide (LPS) on morphology, motility, and cytoskeletal organization of microglia in cultures.</a> J Neurosci Res. 41, 222-237 (1995).</li><li>Menon, V., Thomas, R., Ghale, A. R., Reinhard, C., Pruszak, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flow+cytometry+protocols+for+surface+and+intracellular+antigen+analyses+of+neural+cell+types.">Flow cytometry protocols for surface and intracellular antigen analyses of neural cell types.</a> J Vis Exp. , (2014).</li><li>Jackson, W. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Selective+vulnerability+to+neurodegenerative+disease:+the+curious+case+of+Prion+Protein.">Selective vulnerability to neurodegenerative disease: the curious case of Prion Protein.</a> Dis Model Mech. 7, 21-29 (2014).</li><li>O'Koren, E. G., Mathew, R., Saban, D. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fate+mapping+reveals+that+microglia+and+recruited+monocyte-derived+macrophages+are+definitively+distinguishable+by+phenotype+in+the+retina.">Fate mapping reveals that microglia and recruited monocyte-derived macrophages are definitively distinguishable by phenotype in the retina.</a> Sci Rep. 6, 20636 (2016).</li><li>Usui, Y., 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=Angiogenesis+and+Eye+Disease.">Angiogenesis and Eye Disease.</a> Annu Rev Vision. 1, 155-184 (2015).</li></ol>

There are three critical steps in this method: (1) intravitreal injection of fluorescently labeled particles; (2) harvesting and preparation of retinal tissue; and (3) flow cytometric analysis. We recommend that researchers practice intravitreal injections prior to performing the method we present here. Albino mice (e.g., BALB/c) and a colored solution (e.g., fluorescently labeled particles) can be used for easy visualization of the needle and injected solution. Intravitreal injections are challenging a...

The overall goal of this method is to accurately assess and quantify in vivo microglial phagocytosis. Microglia are the tissue resident macrophages of the central nervous system (CNS). They perform a variety of functions to ensure maintenance of tissue homeostasis. These include immune surveillance, secretion of neurotrophic factors and, of pivotal importance, phagocytosis1. Microglial phagocytosis is key in several important events during development of the brain and retina, such as phagocytosis of irrelevant synapses (synaptic pruning) and removal of apoptotic neurons2-4. Furthermore, microglial phagocytosis of damaged or apoptotic neurons, cellular debris, and invading microbes has been shown to be essential for maintaining CNS homeostasis through adulthood5. Finally, microglial phagocytosis has been implicated in the pathogenesis of several neurodegenerative diseases, including Alzheimer&#39;s disease and age-related macular degeneration, where it has been suggested that defective or insufficient phagocytic capacity may contribute to the build-up of amyloid-&#946; (A&#946;) plaques and drusen, respectively6,7.
Microglial function is tightly regulated by their microenvironment, notably by soluble factors such as tumor-growth factor &#946; or cell-cell interactions. Neurons constitutively express several cell surface ligands, such as CD200 and CX3CL1, while microglia exclusively express the respective receptors CD200R and CX3CR1. These receptors contain immunoreceptor tyrosine-based inhibition motifs (ITIMs) in their intracellular portion. These inhibitor receptors are critical for preventing the over-stimulation of microglia, which can contribute to neuroinflammation. Thus, under normal physiological conditions, cell-cell interactions between neurons and microglia keep microglia in a quiescent state. During tissue injury, however, neurons can down-regulate expression of these ligands, removing their inhibitory effect on microglia activation. Microglial function (including phagocytosis) is thus tightly linked to their microenvironment8. Nevertheless, to date, there are no standardized assays to study microglia phagocytosis in a physiological context or in a way that fully replicates their CNS microenvironment.
Several assays have been developed to measure phagocytic activity of microglia in vitro, where primary microglia or microglia cell lines are cultured with target cells (e.g., apoptotic neurons) or fluorescently labeled beads. Target uptake is then assessed using fluorescent imaging microscopy or flow cytometry9-12. These assays allow testing of how pharmacological or genetic manipulation may affect microglial phagocytosis and, while informative, fail to fully replicate the complex in vivo environment. Indirect methods for examining microglial phagocytosis in vivo have been reported: these are accomplished by staining of molecules thought to be involved in phagocytosis (e.g., CD68), assessing physical proximity of microglia and targets for phagocytosis (e.g., compromised neurons or synaptic elements), or by immunohistochemical detection of phagocytic targets within microglial cells (e.g., A&#946;)13-17. Two studies have used more direct approaches to assess microglia phagocytosis in vivo. Hughes and colleagues have used imaging techniques to measure microglial uptake of beads delivered via the intracranial route18. Sierra et al. developed a refined method to quantitatively assess microglia phagocytosis of apoptotic cells using complex imaging techniques4. However, these methods involve complicated protocols for tissue preparation, sectioning, imaging, and analysis. We have previously used flow cytometric analysis to assess phagocytosis of photoreceptor outer segments by retinal pigmented epithelium (RPE) cells in culture19. Here, we describe a protocol to rapidly assess uptake of fluorescently labeled particles by retinal microglia as a quantitative measure of in vivo microglia phagocytosis.
The protocol we describe here allows for reliable and quantitative measurement of retinal microglial phagocytosis in just under six hours in three critical steps: (1) intravitreal delivery of fluorescently labeled particles, (2) harvest and preparation of retinal tissue, and (3) flow cytometric analysis. The method we have developed is a robust method to assess microglial phagocytosis in the retina, and it can be successfully used to test how various compounds or genetic manipulation alter this key microglial function in physiological settings. As a specialized area of the CNS, the retina is an easily accessible model system to study microglia function20. While this method was developed in the eye, we believe it can be useful for all neuroscientists investigating microglia phagocytic function.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Stereomicroscope</td>
			<td>Nikon</td>
			<td>Discontinued</td>
			<td></td>
		</tr>
		<tr>
			<td>Hamilton&#160;syringe, 600 series</td>
			<td>Sigma</td>
			<td>26702</td>
			<td></td>
		</tr>
		<tr>
			<td>33 gauge, Small Hub RN NDL, 0.5 in, point style 4 - 12o</td>
			<td>Hamilton</td>
			<td>7803-05</td>
			<td></td>
		</tr>
		<tr>
			<td>Zymosan A (S. cerevisiae) BioParticles, Alexa Fluor 488 conjugate</td>
			<td>ThermoFisher Scientific</td>
			<td>Z-23373</td>
			<td>Prepare immediately before injection</td>
		</tr>
		<tr>
			<td>DPBS</td>
			<td>Corning</td>
			<td>21-030-CV</td>
			<td></td>
		</tr>
		<tr>
			<td>Dumont #5/45 Forceps</td>
			<td>Fine Science Tools</td>
			<td>11251-35</td>
			<td>Need two</td>
		</tr>
		<tr>
			<td>Dumont #5SF Forceps</td>
			<td>Fine Science Tools</td>
			<td>11252-00</td>
			<td></td>
		</tr>
		<tr>
			<td>Vannas Spring Scissors - 3mm Cutting Edge</td>
			<td>Fine Science Tools</td>
			<td>15000-10</td>
			<td>Curved</td>
		</tr>
		<tr>
			<td>Neural Tissue Dissociation Kit &#8211; Postnatal Neurons</td>
			<td>Miltenyi Biotec</td>
			<td>130-094-802</td>
			<td></td>
		</tr>
		<tr>
			<td>5 mL Polystyrene Round-bottom Tube</td>
			<td>Falcon</td>
			<td>352054</td>
			<td></td>
		</tr>
		<tr>
			<td>96 well U-bottom plate</td>
			<td>Falcon</td>
			<td>353077</td>
			<td></td>
		</tr>
		<tr>
			<td>Stain Buffer (BSA)</td>
			<td>BD Biosciences</td>
			<td>554657</td>
			<td></td>
		</tr>
		<tr>
			<td>CD11b-BV650 Antibody</td>
			<td>BioLegend</td>
			<td>101259</td>
			<td></td>
		</tr>
		<tr>
			<td>Ly6C-APC-Cy7</td>
			<td>BioLegend</td>
			<td>128025</td>
			<td></td>
		</tr>
		<tr>
			<td>Ly6G-PE-Cy7</td>
			<td>BioLegend</td>
			<td>127617</td>
			<td></td>
		</tr>
		<tr>
			<td>Propidium Iodide</td>
			<td>BD Biosciences</td>
			<td>556463</td>
			<td></td>
		</tr>
		<tr>
			<td>Purified anti-mouse CD16/32 Antibody</td>
			<td>BioLegend</td>
			<td>101301</td>
			<td></td>
		</tr>
	</tbody>
</table>

assessing retinal microglial phagocytic function in vivo using a flow cytometry-based assay

Microglia are the tissue resident macrophages of the central nervous system (CNS) and they perform a variety of functions that support CNS homeostasis, including phagocytosis of damaged synapses or cells, debris, and/or invading pathogens. Impaired phagocytic function has been implicated in the pathogenesis of diseases such as Alzheimer's and age-related macular degeneration, where amyloid-&#946; plaque and drusen accumulate, respectively. Despite its importance, microglial phagocytosis has been challenging to assess in vivo. Here, we describe a simple, yet robust, technique for precisely monitoring and quantifying the in vivo phagocytic potential of retinal microglia. Previous methods have relied on immunohistochemical staining and imaging techniques. Our method uses flow cytometry to measure microglial uptake of fluorescently labeled particles after intravitreal delivery to the eye in live rodents. This method replaces conventional practices that involve laborious tissue sectioning, immunostaining, and imaging, allowing for more precise quantification of microglia phagocytic function in just under six hours. This procedure can also be adapted to test how various compounds alter microglial phagocytosis in physiological settings. While this technique was developed in the eye, its use is not limited to vision research.

Here we describe a method to rapidly and reliably quantify the number of phagocytic retinal microglia in a physiological setting using flow cytometric analysis (Figure 2). This method can be adapted to test the effect of compounds and/or genetic manipulation on the phagocytic capacity of microglia (Figures 3A, 3B). It can also be used in young (10 - 20 days postnatal) or adult mice (Figure 3C). Varying doses of lipopolysaccharide...

Watch this Scientific Journal Video about Assessing Retinal Microglial Phagocytic Function In Vivo Using a Flow Cytometry-based Assay at JoVE.com

Assessing Retinal Microglial Phagocytic Function In Vivo Using a Flow Cytometry-based Assay

Microglial phagocytosis is critical for the maintenance of tissue homeostasis and inadequate phagocytic function has been implicated in pathology. However, assessing microglia function in vivo is technically challenging. We have developed a simple but robust technique for precisely monitoring and quantifying the phagocytic potential of microglia in a physiological setting.

Assessing Retinal Microglial Phagocytic Function In Vivo Using a Flow Cytometry-based Assay

Microglia are the tissue resident macrophages of the central nervous system (CNS) and they perform a variety of functions that support CNS homeostasis, ...

The overall goal of this procedure is to assess the phagocytic function of retinal microglia in vivo. This method can help answer key questions in the ophthalmology field, such as whether certain compounds alter microglia phagocytic function in a physiological setting. The main advantage of this technique is that it uses flow cytometry, allowing a fast and precise quantitative analysis of microglial phagocytosis.Helping me to demonstrate the procedure will be Susumu Sakimoto, a postdoc from my laboratory. Begin by loading a 33-gauge needle and syringe with 0.5 microliters of fluorescently labeled particle solution. Then place an anesthetized mouse sideways on soft material under a surgical microscope and confirm the appropriate level of sedation by a lack of response to toe pinch.Next, use forceps to carefully apply pressure around one eyelid, so that the eye ball pops slightly out of the socket. Then grasp the head with two fingers just above the ear and by the animal's jaw, and gently stretch the skin parallel to the eyelids to keep the eye slightly bulged out of the socket. Intravitreal injections are challenging, and if not performed correctly will lead to biased and variable results.To reduce trauma to the eye, hold the animal in a stable position with minimal head movement. To puncture the eyeball, gently grasp the mouse with one hand. Then locate the corneal limbus where the cornea and sclera connect, which is visible as a gray circle in pigmented mice.Holding the syringe in the other hand, insert the needle into the limbus. Then retract the needle slightly to expel a small volume of vitreous fluid, and slowly depress the plunger to inject the particles. When all of the beads have been delivered, slowly withdraw the syringe to avoid reflux of the injected material and apply moisturizing drops to keep the eye hydrated once the eye has retracted back into place.Then place the animal on a heating pad in its own cage with monitoring until it is fully recovered. Three hours after intravitreal injection, use 45 degree angle forceps to gently press against the eyelid to proptose the eyeball. Position the forceps behind the eyeball and pull.Then transfer the eyeball to the dry area of a petri dish, containing a small volume of PBS with calcium and magnesium under a dissecting microscope. And use one tip of a superfine forceps to perforate the eye in the corneal limbus. Next, holding the eyeball with fine 45-degree angled forceps, use spring scissors to cut around the corneal limbus until roughly half of the circumference is cut.Transfer the eyeball into PBS and use a second pair of fine 45-degree angled forceps to tear the cornea and sclera apart. The lens and retina will come out intact. Ensure the lens and retina are separated and transfer the retina to a 5.4 milliliter polystyrene test tube, containing two milliliters of PBS with calcium and magnesium.To obtain a single cell suspension of the retinal cells, use a neuronal tissue dissociation kit according to the manufacturer's instructions and resuspend the cells in 200 microliters of staining buffer. Then transfer the sample to one well of a 96-well U-bottom plate. After centrifugation, invert the plate to discard the supernatant and block the FC receptors with 25 microliters of stain buffer containing CD16, CD32 antibody per well for five minutes at room temperature.Next label the cells with the antibodies of interest for 15 minutes at room temperature in the dark. Then pellet the cells and follow this by a wash in 200 microliters of fresh staining buffer per well. Now resuspend the pellets in 200 microliters of stain buffer and viability dye and transfer the samples to 1.2 milliliter microtiter tubes.Then wash the wells with an additional 100 microliters of staining buffer and viability dye and pool the washes with the corresponding samples for analysis by flow cytometry. This method can be used in 10 to 20-day old postnatal or adult mice and can be adapted to test the effect of compounds and/or the genetic manipulation of microglial phagocytosis. For example, in this experiment after intraperitoneal challenge with varying doses of LPS, a 1.42 milligram per kilogram dose of LPS was determined to induce a statistically significant increase in the percentage of phagocytic microglia, compared to vehicle challenged controls.Once mastered, this technique can be completed in six hours if it is performed properly. Following this procedure, other methods like cell sorting followed by qPCR or proteomic analysis, can be used to answer further questions about the differences between phagocytic and non phagocytic microglia in the retina. In developing this technique, it's our expectation that we can now make it possible for vision and neuro scientists to further explore microglial phagocytic function in situ and in a physiologically relevant context.

assessing-retinal-microglial-phagocytic-function-vivo-using-flow

Research

JoVE Journal

Immunology and Infection

9.5K visualizações.  The Scripps Research Institute. The overall goal of this procedure is to assess the phagocytic function of retinal microglia in vivo. This method can help answer key questions in the ophthalmology field, such as whether certain compounds alter microglia phagocytic function in a physiological setting. The main advantage of this technique is that it uses flow cytometry, allowing a fast and precise quantitative analysis of microglial phagocytosis.Helping me to demonstrate the procedure will be Susumu Sakimoto, a postdoc...