Name: human microglia-like cells: differentiation from induced pluripotent stem cells and in vitro live-cell phagocytosis assay using human synaptosomes
Uploaded: 2022-08-18T14:30:00
Description: Watch this Scientific Journal Video about Human Microglia-like Cells: Differentiation from Induced Pluripotent Stem Cells and In Vitro Live-cell Phagocytosis Assay using Human Synaptosomes at JoVE.com

The authors thank Michael Ward for providing the WTC11 hNIL iPSC line for motor neuron differentiation and the Jackson Laboratories for supplying the KOLF2.1J WT clone B03 iPSC line used for microglia differentiation. We also thank Dorothy Schafer for her support during the implementation of the protocols, Anthony Giampetruzzi and John Landers for their help with the live-cell imaging system as well as Hayden Gadd for his technical contributions during revisions and Jonathan Jung for his collaboration in this study. This work was supported by the Dan and Diane Riccio Fund for Neuroscience from UMASS Chan Medical School and the Angel Fund, Inc.

NOTE: All the reagents used in this protocol must be sterile, and all the steps must be performed in a biosafety cabinet under sterile conditions. All the iPSC lines, as well as maintenance and differentiation media, are described in the Table of Materials. The microglia differentiation method illustrated below is based on previously published protocols7,8,12 with new modifications described herein.
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The differentiation protocol described here provides an efficient method to obtain iPSC-derived microglia-like cells in ~6-8 weeks with high purity and in a sufficient yield to perform immunofluorescence experiments and other assays that require a higher number of cells. This protocol has yielded up to 1 &#215; 106 iMGs in 1 week, which allows for protein and RNA extraction and corresponding downstream analyses (e.g., RNASeq, qRT-PCR, western blot, mass spectrometry). That said, a limitation of this protocol i...

Microglia are the resident immune cells in the central nervous system (CNS) and play a crucial role in developing the CNS. Microglia are also important in the adult brain for maintaining homeostasis and actively responding to trauma and disease processes. Cumulative evidence shows that microglia are key contributors to the pathogenesis of multiple neurodevelopmental and neurodegenerative diseases1,2. Although current knowledge about microglial biology has been predominately derived from mouse models, recent studies have elucidated important differences between murine and human microglia, underscoring the need for developing technologies to study the genetics and biological functions of human microglia3,4. Isolation of microglia from dissected primary tissue can severely modify microglia properties5, potentially confounding results acquired with such cells. The overall goal of this method is to differentiate human iPSCs into iMGs, thereby providing a cell culture system to study human microglia under basal conditions. Furthermore, a phagocytosis assay using a fully human model system is included herein as a means to study the functionality of iMGs, both as a quality control measure and to assess iMG dysfunction in the context of disease.
Multiple protocols for microglia differentiation from iPSCs have recently emerged in the literature6,7,8,9,10. Potential disadvantages of some protocols include extended or long periods of differentiation, the addition of multiple growth factors, and/or complex experimental procedures6,9,10. Here, a &#34;user-friendly&#34; differentiation method is demonstrated that recapitulates aspects of microglia ontogeny through differentiation of iPSCs into precursor cells termed primitive macrophage precursors (PMPs)7,11. PMPs are generated as described previously, with some optimizations presented herein12. The PMPs mimic MYB-independent yolk-sac-derived macrophages, which give rise to microglia during embryonic development by invading the brain before blood-brain barrier closure13. To terminally differentiate PMPs into iMGs, we used a fast and simplified monoculture method based on protocols by Haenseler et al. and Brownjohn et al., with some modifications to generate an efficient microglia differentiation method in which iMGs robustly express microglia-enriched markers7,8. This differentiation method can be reproduced in laboratories with expertise in the culture of iPSCs and with research goals aiming to study microglia biology using a human model system.
iPSC-derived microglia represent a biologically relevant source of human microglia for in vitro experimentation and are an important tool to investigate microglial canonical functions, including phagocytosis. Microglia are the professional phagocytes of the brain and CNS, where they clear cell debris, aggregated proteins, and degraded myelin14. Microglia also function in synaptic remodeling by engulfing synapses and in the defense against external infections through phagocytosis of pathogens15,16. In this protocol, phagocytosis by iMGs is assessed using human synaptosomes as material for iMG engulfment. To this end, a description for isolating synaptosomes derived from human i3LMNs is described. The i3LMN-derived human synaptosomes are labeled with a pH-sensitive dye that allows for quantification of synaptosomes localized within acidic compartments during phagosome processing and degradation in vitro. A phagocytosis assay using live-cell microscopy is shown for monitoring the dynamic process of microglia engulfment in real-time. This functional assay establishes a basis to investigate possible defects in microglial phagocytosis in health and disease using a complete human system.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Antibodies for immunofluorescence analysis</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>anti-IBA1 rabbit antibody</td>
			<td>Wako Chemical USA</td>
			<td>NC9288364</td>
			<td>1:350 dilution</td>
		</tr>
		<tr>
			<td>anti-P2RY12 rabbit antibody</td>
			<td>Sigma-Aldrich</td>
			<td>HPA014518</td>
			<td>1:50 dilution</td>
		</tr>
		<tr>
			<td>anti-TMEM119 rabbit antibody</td>
			<td>Sigma-Aldrich</td>
			<td>HPA051870</td>
			<td>1:100 dilution</td>
		</tr>
		<tr>
			<td>Antibodies for Western blot analysis</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>anti-&#946;-Tubulin rabbit antibody</td>
			<td>Abcam</td>
			<td>ab6046</td>
			<td>1:500 dilution</td>
		</tr>
		<tr>
			<td>anti-Synaptophysin (SYP) rabbit antibody</td>
			<td>Abclonal</td>
			<td>A6344</td>
			<td>1:1,000 dilution</td>
		</tr>
		<tr>
			<td>anti-PSD95 mouse antibody</td>
			<td>Millipore</td>
			<td>MAB1596</td>
			<td>1:500 dilution</td>
		</tr>
		<tr>
			<td>Borate buffer components</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Boric acid (100 mM)</td>
			<td>Sigma</td>
			<td>B6768</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium bicarbonate (NaHCO3) BioXtra</td>
			<td>Sigma-Aldrich</td>
			<td>S6297-250G</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium chloride (75 mM)</td>
			<td>Sigma</td>
			<td>&#160;S7653</td>
			<td></td>
		</tr>
		<tr>
			<td>Sodium tetraborate (25 mM)</td>
			<td>Sigma</td>
			<td>221732</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell culture materials</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>6-well plates</td>
			<td>Greiner Bio-One</td>
			<td>657160</td>
			<td></td>
		</tr>
		<tr>
			<td>40 &#956;m Cell Strainers</td>
			<td>&#160;Falcon</td>
			<td>352340</td>
			<td></td>
		</tr>
		<tr>
			<td>100 mm x 20 mm Tissue Culture Treated</td>
			<td>CELLTREAT</td>
			<td>229620</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell Lifter, Double End, Flat Blade &#38; Narrow Blade, Sterile</td>
			<td>CELLTREAT</td>
			<td>229305</td>
			<td></td>
		</tr>
		<tr>
			<td>low adherence round-bottom 96-well plate</td>
			<td>Corning</td>
			<td>7007</td>
			<td></td>
		</tr>
		<tr>
			<td>Primaria 24-well Flat Bottom Surface Modified Multiwell Cell Culture Plate</td>
			<td>Corning</td>
			<td>353847,</td>
			<td></td>
		</tr>
		<tr>
			<td>Primaria 6-well Cell Clear Flat Bottom Surface-Modified Multiwell Culture Plate</td>
			<td>Corning</td>
			<td>353846</td>
			<td></td>
		</tr>
		<tr>
			<td>Primaria 96-well Clear Flat Bottom Microplate</td>
			<td>Corning</td>
			<td>353872</td>
			<td></td>
		</tr>
		<tr>
			<td>Cell dissociation reagents</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Accutase</td>
			<td>&#160;Corning</td>
			<td>25058CI</td>
			<td>dissociation reagents used for lower motor neuron differentiation</td>
		</tr>
		<tr>
			<td>TrypLE reagent</td>
			<td>Life Technologies</td>
			<td>12-605-010</td>
			<td>dissociation reagents used for microglia differentiation</td>
		</tr>
		<tr>
			<td>UltraPure 0.5 M EDTA, pH 8.0</td>
			<td>Invitrogen</td>
			<td>15575020</td>
			<td></td>
		</tr>
		<tr>
			<td>Coating reagents for cell culture</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Matrigel GFR Membrane Matrix</td>
			<td>Corning&#8482;</td>
			<td>354230</td>
			<td>Referred as to extracellular matrix coating reagent</td>
		</tr>
		<tr>
			<td>CellAdhere Laminin-521</td>
			<td>STEMCELL Technology</td>
			<td>77004</td>
			<td>Referred as to laminin 521</td>
		</tr>
		<tr>
			<td>Poly-D-Lysine</td>
			<td>Sigma</td>
			<td>P7405</td>
			<td>Reconstitute to 0.1 mg/mL in borate buffer</td>
		</tr>
		<tr>
			<td>Poly-L-Ornithine</td>
			<td>Sigma</td>
			<td>&#160;P3655</td>
			<td>Reconstitute to 1 mg/mL in borate buffer</td>
		</tr>
		<tr>
			<td>Components of iPSC media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>&#160;mTeSR Plus Kit</td>
			<td>STEMCELL Technology</td>
			<td>100-0276</td>
			<td>To prepare iPSC media mixed the components to 1x</td>
		</tr>
		<tr>
			<td>Components of EB media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>BMP-4</td>
			<td>Fisher Scientific</td>
			<td>PHC9534</td>
			<td>final concentration 50 ng/mL</td>
		</tr>
		<tr>
			<td>iPSC media</td>
			<td></td>
			<td></td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>ROCK inhibitor Y27632</td>
			<td>Fisher Scientific</td>
			<td>BD 562822</td>
			<td>final concentration 10 &#181;M</td>
		</tr>
		<tr>
			<td>SCF</td>
			<td>PeproTech</td>
			<td>300-07</td>
			<td>final concentration 20 ng/mL</td>
		</tr>
		<tr>
			<td>VEGF</td>
			<td>PeproTech</td>
			<td>100-20A</td>
			<td>final concentration 50 ng/mL</td>
		</tr>
		<tr>
			<td>Components of PMP base media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>GlutaMAX</td>
			<td>Gibco</td>
			<td>35050061</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin (10,000 U/mL)</td>
			<td>Gibco</td>
			<td>15140122</td>
			<td>final concentration 100 U/mL</td>
		</tr>
		<tr>
			<td>X-VIVO 15</td>
			<td>Lonza</td>
			<td>12001-988</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>Components of PMP complete media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>55 mM 2-mercaptoethanol</td>
			<td>Gibco</td>
			<td>21985023</td>
			<td>final concentration 55 &#181;M</td>
		</tr>
		<tr>
			<td>IL-3</td>
			<td>PeproTech</td>
			<td>200-03</td>
			<td>final concentration 25 ng/mL</td>
		</tr>
		<tr>
			<td>M-CSF</td>
			<td>PeproTech</td>
			<td>300-25</td>
			<td>final concentration 100 ng/mL</td>
		</tr>
		<tr>
			<td>PMP base media</td>
			<td></td>
			<td></td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>Components of iMG base media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Advanced DMEM/F12</td>
			<td>Gibco</td>
			<td>12634010</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>GlutaMAX</td>
			<td>Gibco</td>
			<td>35050061</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>N2 supplement, 100x</td>
			<td>Gibco</td>
			<td>17502-048</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>Penicillin-Streptomycin (10,000 U/mL)</td>
			<td>Gibco</td>
			<td>15140122</td>
			<td>final concentration 100 U/mL</td>
		</tr>
		<tr>
			<td>Components of iMG complete media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>55 mM 2-mercaptoethanol</td>
			<td>Gibco</td>
			<td>21985023</td>
			<td>final concentration 55 &#181;M</td>
		</tr>
		<tr>
			<td>IL-34</td>
			<td>PeproTech or Biologend</td>
			<td>200-34 or 577904</td>
			<td>final concentration 100 ng/mL</td>
		</tr>
		<tr>
			<td>iMG base media</td>
			<td></td>
			<td></td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>M-CSF</td>
			<td>PeproTech</td>
			<td>300-25</td>
			<td>final concentration 5 ng/mL</td>
		</tr>
		<tr>
			<td>TGF-&#946;</td>
			<td>PeproTech</td>
			<td>100-21</td>
			<td>final concentration 50 ng/mL</td>
		</tr>
		<tr>
			<td>Components of Induction base media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>DMEM/F12 with HEPES</td>
			<td>Gibco</td>
			<td>11330032</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>GlutaMAX</td>
			<td>Gibco</td>
			<td>35050061</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>N2 supplement, 100x</td>
			<td>Gibco</td>
			<td>17502-048</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>Non-essential amino acids (NEAA), 100x</td>
			<td>Gibco</td>
			<td>11140050</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>Components of Complete induction media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Compound E</td>
			<td>Calbiochem</td>
			<td>565790</td>
			<td>final concentration 0.2 &#956;M and reconstitute stock reagent to 2 mM in 1:1 ethanol and DMSO</td>
		</tr>
		<tr>
			<td>Doxycycline</td>
			<td>Sigma</td>
			<td>D9891</td>
			<td>final concentration 2 &#956;g/mL and reconstitute stock reagent to 2 mg/mL in DPBS</td>
		</tr>
		<tr>
			<td>Induction base media</td>
			<td></td>
			<td></td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>ROCK inhibitor Y27632</td>
			<td>Fisher Scientific</td>
			<td>BD 562822</td>
			<td>final concentration 10 &#956;M</td>
		</tr>
		<tr>
			<td>Components of Neuron media</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>B-27 Plus Neuronal Culture System</td>
			<td>Gibco</td>
			<td>A3653401</td>
			<td>final concentration 1x for media and suplemment</td>
		</tr>
		<tr>
			<td>GlutaMAX</td>
			<td>Gibco</td>
			<td>35050061</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>N2 supplement, 100x</td>
			<td>Gibco</td>
			<td>17502-048</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>Non-essential amino acids (NEAA), 100x</td>
			<td>Gibco</td>
			<td>11140050</td>
			<td>final concentration 1x</td>
		</tr>
		<tr>
			<td>iPSC lines used in this study</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>KOLF2.1J: WT clone B03</td>
			<td>The Jackson Laboratories</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>WTC11 hNIL</td>
			<td>National Institute of Health</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Synaptosome isolation reagents</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>BCA Protein Assay Kit</td>
			<td>Thermo Scientific Pierce</td>
			<td>23227</td>
			<td></td>
		</tr>
		<tr>
			<td>dimethyl sulfoxide (DMSO)</td>
			<td>Sigma</td>
			<td>D2650</td>
			<td></td>
		</tr>
		<tr>
			<td>Syn-PER Synaptic Protein Extraction Reagent</td>
			<td>Thermo Scientific</td>
			<td>87793</td>
			<td>Referred as to cell lysis reagent for isolation of synaptosomes</td>
		</tr>
		<tr>
			<td>Phagocytosis assay dyes</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>NucBlue Live Ready reagent</td>
			<td>Invitrogen</td>
			<td>&#160;R37605</td>
			<td></td>
		</tr>
		<tr>
			<td>pHrodo Red, succinimidyl ester</td>
			<td>ThermoFisher Scientific</td>
			<td>&#160;P36600</td>
			<td>Referred as to pH-sensitive dye</td>
		</tr>
		<tr>
			<td>Other cell-culture reagents</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Trypan Blue, 0.4% Solution</td>
			<td>AMRESCO INC</td>
			<td>K940-100ML</td>
			<td></td>
		</tr>
		<tr>
			<td>Bovine serum albumin (BSA)</td>
			<td>Sigma</td>
			<td>22144-77-0</td>
			<td></td>
		</tr>
		<tr>
			<td>BrdU</td>
			<td>Sigma</td>
			<td>B9285</td>
			<td>Reconstitute to 40 mM in sterile water</td>
		</tr>
		<tr>
			<td>Cytochalasin D</td>
			<td>Sigma</td>
			<td></td>
			<td>final concentration 10 &#181;M</td>
		</tr>
		<tr>
			<td>DPBS with Calcium and magnesium</td>
			<td>Corning</td>
			<td>21-030-CV</td>
			<td></td>
		</tr>
		<tr>
			<td>DPBS without calcium and magnesium</td>
			<td>Corning</td>
			<td>21-031-CV</td>
			<td>Referred as to DPBS</td>
		</tr>
		<tr>
			<td>KnockOut&#160; DMEM/F-12</td>
			<td>Gibco</td>
			<td>12660012</td>
			<td>Referred as to DMEM-F12 optimized for growth of human embryonic and induced pluripotent stem cells</td>
		</tr>
		<tr>
			<td>Laminin Mouse Protein, Natural</td>
			<td>Gibco</td>
			<td>23017015</td>
			<td>Referred as to laminin</td>
		</tr>
		<tr>
			<td>Software and Equipment</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Centrifuge</td>
			<td>Eppendorf</td>
			<td>Model 5810R</td>
			<td></td>
		</tr>
		<tr>
			<td>Cytation 5 live cell imaging reader</td>
			<td>Biotek</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Gen5 Microplate Reader and Imager Software</td>
			<td>Biotek</td>
			<td>version 3.03</td>
			<td></td>
		</tr>
		<tr>
			<td>Multi-Therm Heat-Shake</td>
			<td>Benchmark</td>
			<td></td>
			<td>refer as tube shaker</td>
		</tr>
		<tr>
			<td>Water sonicator</td>
			<td>Elma</td>
			<td>Mode Transsonic 310</td>
			<td></td>
		</tr>
	</tbody>
</table>

human microglia-like cells: differentiation from induced pluripotent stem cells and in vitro live-cell phagocytosis assay using human synaptosomes

Microglia are the resident immune cells of myeloid origin that maintain homeostasis in the brain microenvironment and have become a key player in multiple neurological diseases. Studying human microglia in health and disease represents a challenge due to the extremely limited supply of human cells. Induced pluripotent stem cells (iPSCs) derived from human individuals can be used to circumvent this barrier. Here, it is demonstrated how to differentiate human iPSCs into microglia-like cells (iMGs) for in vitro experimentation. These iMGs exhibit the expected and physiological properties of microglia, including microglia-like morphology, expression of proper markers, and active phagocytosis. Additionally, documentation for isolating and labeling synaptosome substrates derived from human iPSC-derived lower motor neurons (i3LMNs) is provided. A live-cell, longitudinal imaging assay is used to monitor engulfment of human synaptosomes labeled with a pH-sensitive dye, allowing for investigations of iMG's phagocytic capacity. The protocols described herein are broadly applicable to different fields that are investigating human microglia biology and the contribution of microglia to disease.

To generate iMGs using this protocol, it is important to start with undifferentiated iPSCs that show compact colony morphology with well-defined edges (Figure 2A). Dissociated iPSCs maintained as described in the EB formation section will form spherical aggregates, termed EBs, which will grow in size until day 4 of differentiation (Figure 2B). Once the EBs are collected and plated in the appropriate conditions for PMP generation, they attach to the Matrigel-coat...

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Human Microglia-like Cells: Differentiation from Induced Pluripotent Stem Cells and In Vitro Live-cell Phagocytosis Assay using Human Synaptosomes

This protocol describes the differentiation process of human induced pluripotent stem cells (iPSCs) into microglia-like cells for in vitro experimentation. We also include a detailed procedure for generating human synaptosomes from iPSC-derived lower motor neurons that can be used as a substrate for in vitro phagocytosis assays using live-cell imaging systems.

Human Microglia-like Cells: Differentiation from Induced Pluripotent Stem Cells and In Vitro Live-cell Phagocytosis Assay using Human Synaptosomes

Microglia are the resident immune cells of myeloid origin that maintain homeostasis in the brain microenvironment and have become a key player in multiple ...

iPS-derived microglia represent a biologically relevant source of human microglia for in vitro experimentation, and are an important tool to investigate microglia biology in health and disease. We present a simple microglia differentiation protocol that requires minimal use of growth factors and achieves high purity and yield. Also, we demonstrate a fully humanized phagocytosis assay.iPSC-derived microglia-like cells are very sensitive to media evaporation. You should avoid using the wells in the corner of the cell culture plate, and fill all empty wells with water to improve survival. Once Induced Pluripotent Stem Cells, or iPSCs, have reached 80%confluency, dissociate the colonies by washing with one milliliter of DPBS, and adding one milliliter of a dissociation reagent for two minutes at 37 degrees Celsius.Dislodge the colonies using a cell lifter by scraping multiple times to create a single cell suspension. Collect the suspension, and transfer them to a 15-milliliter conical tube containing nine milliliters of DPBS. Then centrifuge the cells at 500 times g for one minute, remove the supernatant, and resuspend in one milliliter of the Embryoid Body, or EB, medium.Take 10 microliters of cells, and dilute one to two with trypan blue. Count the cells on a hemocytometer, and based on the cell count, dilute the cell stock to a final dilution of 10, 000 cells per 100 microliters. For plating cells, add 100 microliters of the diluted cells per well into a low adherence round-bottom 96-well plate.Centrifuge the plate at 125 times g for three minutes, and incubate at 37 degrees Celsius and 5%carbon dioxide for four days. On day two, using a multi-channel pipette, replace the old half-EB medium with a fresh medium. To generate Primitive Macrophage Precursors, or PMPs, coat the wells of a six-well plate by adding one milliliter of ice cold Matrigel coating solution.Then incubate the plate at 37 degrees Celsius and 5%carbon dioxide for two hours, or overnight. On day four of EB differentiation, using a one-milliliter pipette tip, transfer the EBs to the Matrigel-coated wells. Pipette up and down to dislodge the EBs from the well.Then hold the plate tilted to allow the EBs to settle down at the edge of the well. Once all the EBs have settled down, gently pipette and replace the old medium, while keeping the EB at the edge of the well with three milliliters of freshly prepared PMP complete medium. Evenly distribute the cells in the wells by manually shuffling the plate side to side and back to front.Then incubate the plate for seven days to allow the EBs to attach to the bottom of the well. After seven days, inspect the EBs under a light field microscope at four times magnification to ensure that they are attached to the bottom of the wells. Perform a half medium change, and on day 21, replace the complete medium with three milliliters of PMP complete medium.On day 28, look for round cells referred to as PMPs in the supernatant. Then collect the medium containing the PMPs using a 10-milliliter pipette and automatic pipetter without disturbing the EBs. Transfer the PMPs in the medium to a 15-milliliter conical tube.Centrifuge the collected PMPs at 200 times g for four minutes and aspirate the supernatant before re-suspending them in one to two milliliters of microglia-like cells, or IMG basal medium. Then count the cells on the hemocytometer as described previously. Centrifuge the rest of the cells, and dilute the PMPs to the desired concentration.Then plate the cells at a density of 10 to the fifth cells per square centimeter on cell culture-treated plates using freshly prepared IMG complete medium. For live cell phagocytosis assay, plate 20 to 30 times 10 to the fourth PMPs into a 96-well plate and 100 microliters of IMG complete medium and follow the differentiation process for 10 days. On the day of the assay, remove 40 microliters of medium per well, and add 10 microliters of the nuclear staining solution.Incubate the plate for two hours. Thaw the labeled synaptosomes on ice, and gently sonicate using a water sonicator for one minute. Again, immediately place the synaptosomes on ice.Dilute the labeled synaptosomes in IMG complete medium at one microliter of synaptosomes per 50 microliters of medium. For negative control, prepare 60 micromolar Cytochalasin D in IMG complete medium to inhibit actin polymerization, and thus phagocytosis. Then add 10 microliters of this solution to each well for a final concentration of 10 micromolar, and incubate for 30 minutes.Remove the plate from the incubator, and incubate at 10 degrees Celsius for 10 minutes. Maintain the plate on ice, and add 50 microliters of medium containing synaptosomes. Centrifuge the plate at 270 times g for three minutes at 10 degrees Celsius, and maintain the plate on ice until imaging acquisition.Insert the plate into a live cell imaging reader, and select the wells to be analyzed. Then select a 20X objective lens. Next, adjust the focus, Light Emitting Diode, or LED, intensity, integration time, and gain of the bright-field and blue channels.The synaptosome fluorescence should be negligible at the initial time point. Select the number of individual tiles to be acquired in a montage per well. Acquire 16 tiles at the center of the well, imaging approximately 5%of the total well area.Set the temperature to 37 degrees Celsius, and the desired time interval for imaging. Next, open the analysis software. Then open the experiment that contains the images, and click on the Data Reduction icon.In the menu, pick Imaging Stitching under Imaging Processing to create a complete image from the four by four individual tiles in the montage with the parameters described in the text manuscript. Define an intensity threshold using the DAPI and RFP channels for these images. Open an image, and click on Analyze.Under Analysis, select Cellular Analysis, and under Detection Channel, pick a stitched image either in the DAPI, or RFP channels. Next, go to the Primary Mask and Count tab, and establish a threshold value and object size values that properly select the cell nuclei in the DAPI channel, or the synaptosome signal in the RFP channel. To count the number of nuclei, go to the Data Reduction menu, and under Image Analysis, select Cellular Analysis.Again, go to the Primary Mask and Count tab, and under Channel, select the DAPI stitched images, and use the parameters described in the text manuscript. Next, go to the Calculated Metrics tab, and select Cell Count. Then to obtain the area of the synaptosome signal, go to the Data Reduction menu, and under Analysis, select Cellular Analysis.Again, go to the Primary Mask and Count tab, and under Channel, select RFP stitched images, and use the parameters detailed in the text manuscript. Next, go to the Calculated Metrics tab, and select Object Sum Area. In the Data Reduction tab, click on OK to allow the software to analyze all the acquired images.Once the images are analyzed, export the Object Sum Area and Cell Count values for each time point. Then divide the Object Sum Area by the Cell Count to calculate the normalized area per time point. If comparing multiple treatments, or genotypes, calculate the phagocytosis index using the given equation.Undifferentiated iPSCs show compact colony morphology with well-defined edges. Dissociated iPSCs formed spherical aggregates termed EBs, and grow in size until day four of differentiation. When the EBs are plated for PMP generation, they attach to the Matrigel-coated plates, and a layer of cells spreads and surrounds the spherical aggregates.On day 28, round cells with a large cytoplasm to nucleus ratio appear in the suspension. It is strongly recommended to culture iPSCs in Laminin 521-coated plates instead of Matrigel, because of the higher PMP yield in Laminin 521-coated plates. After the exposure of PMPs to IMG medium, cells acquire a microglia-like morphology with a small cytoplasm in the presence of elongated processes.Further, microglia identity was verified by immunofluorescent staining. Typically, more than 95%of the cells express IB1 and approximately 90%of the cells express P2RY12 and TMEM119. Western blot analysis confirmed that iPSC derived lower motor neurons for human synaptosomes'expressed synaptic markers, synaptophysin and postsynaptic density protein 95.In control compared to the initial time point, by 10 hours, the red fluorescent signal was robust, as most of the synaptosomes had been engulfed, and were localized to acidic intracellular compartments. In Cytochalasin D-treated IMGs, a strong reduction of the red signal for zero and 10 hours indicated that the detected signal results from phagocytic events. The area of engulfed synaptosomes increased with time until 16 hours.However, the area of red signal from Cytochalasin D-treated cells did not increase with time. iPS-derived Microglia like cells can be used for various applications, including phagocytosis, and inflammatory response in the study of neurodevelopmental and neurodegenerative diseases.

human-microglia-like-cells-differentiation-from-induced-pluripotent

Research

JoVE Journal

Neuroscience

Live-cell Imaging of Sensory Organ Precursor Cells in Intact Drosophila Pupae

Since the discovery of Green Fluorescent Protein (GFP), there has been a revolutionary change in the use of live-cell imaging as a tool for understanding fundamental biological mechanisms. Striking progress has been particularly evident in Drosophila, whose extensive toolkit of mutants and transgenic lines provides a convenient model to study evolutionarily-conserved developmental and cell biological mechanisms. We are interested in understanding the mechanisms that control cell fate specification in the adult peripheral nervous system (PNS) in Drosophila. Bristles that cover the head, thorax, abdomen, legs and wings of the adult fly are individual mechanosensory organs, and have been studied as a model system for understanding mechanisms of Notch-dependent cell fate decisions. Sensory organ precursor (SOP) cells of the microchaetes (or small bristles), are distributed throughout the epithelium of the pupal thorax, and are specified during the first 12 hours after the onset of pupariation. After specification, the SOP cells begin to divide, segregating the cell fate determinant Numb to one daughter cell during mitosis. Numb functions as a cell-autonomous inhibitor of the Notch signaling pathway.
Here, we show a method to follow protein dynamics in SOP cell and its progeny within the intact pupal thorax using a combination of tissue-specific Gal4 drivers and GFP-tagged fusion proteins 1,2.This technique has the advantage over fixed tissue or cultured explants because it allows us to follow the entire development of an organ from specification of the neural precursor to growth and terminal differentiation of the organ. We can therefore directly correlate changes in cell behavior to changes in terminal differentiation. Moreover, we can combine the live imaging technique with mosaic analysis with a repressible cell marker (MARCM) system to assess the dynamics of tagged proteins in mitotic SOPs under mutant or wildtype conditions. Using this technique, we and others have revealed novel insights into regulation of asymmetric cell division and the control of Notch signaling activation in SOP cells (examples include references 1-6,7 ,8).

Live-cell Imaging of Sensory Organ Precursor Cells in Intact Drosophila Pupae

In this video, we describe a method for live cell imaging of asymmetrically dividing sensory organ progenitor cells and epidermal cells in intact Drosophila pupae

Since the discovery of Green Fluorescent Protein (GFP), there has been a revolutionary change in the use of live-cell imaging as a tool for understanding ...

Efficient Derivation of Human Neuronal Progenitors and Neurons from Pluripotent Human Embryonic Stem Cells with Small Molecule Induction

There is a large unfulfilled need for a clinically-suitable human neuronal cell source for repair or regeneration of the damaged central nervous system (CNS) structure and circuitry in today's healthcare industry. Cell-based therapies hold great promise to restore the lost nerve tissue and function for CNS disorders. However, cell therapies based on CNS-derived neural stem cells have encountered supply restriction and difficulty to use in the clinical setting due to their limited expansion ability in culture and failing plasticity after extensive passaging1-3. Despite some beneficial outcomes, the CNS-derived human neural stem cells (hNSCs) appear to exert their therapeutic effects primarily by their non-neuronal progenies through producing trophic and neuroprotective molecules to rescue the endogenous cells1-3. Alternatively, pluripotent human embryonic stem cells (hESCs) proffer cures for a wide range of neurological disorders by supplying the diversity of human neuronal cell types in the developing CNS for regeneration1,4-7. However, how to channel the wide differentiation potential of pluripotent hESCs efficiently and predictably to a desired phenotype has been a major challenge for both developmental study and clinical translation. Conventional approaches rely on multi-lineage inclination of pluripotent cells through spontaneous germ layer differentiation, resulting in inefficient and uncontrollable lineage-commitment that is often followed by phenotypic heterogeneity and instability, hence, a high risk of tumorigenicity7-10. In addition, undefined foreign/animal biological supplements and/or feeders that have typically been used for the isolation, expansion, and differentiation of hESCs may make direct use of such cell-specialized grafts in patients problematic11-13. To overcome these obstacles, we have resolved the elements of a defined culture system necessary and sufficient for sustaining the epiblast pluripotence of hESCs, serving as a platform for de novo derivation of clinically-suitable hESCs and effectively directing such hESCs uniformly towards clinically-relevant lineages by small molecules14 (please see a schematic in Fig. 1). Retinoic acid (RA) does not induce neuronal differentiation of undifferentiated hESCs maintained on feeders1, 14. And unlike mouse ESCs, treating hESC-differentiated embryoid bodies (EBs) only slightly increases the low yield of neurons1, 14, 15. However, after screening a variety of small molecules and growth factors, we found that such defined conditions rendered retinoic acid (RA) sufficient to induce the specification of neuroectoderm direct from pluripotent hESCs that further progressed to neuroblasts that generated human neuronal progenitors and neurons in the developing CNS with high efficiency (Fig. 2). We defined conditions for induction of neuroblasts direct from pluripotent hESCs without an intervening multi-lineage embryoid body stage, enabling well-controlled efficient derivation of a large supply of human neuronal cells across the spectrum of developmental stages for cell-based therapeutics.

We have established a protocol for induction of neuroblasts direct from pluripotent human embryonic stem cells maintained under defined conditions with small molecules, which enables derivation of a large supply of human neuronal progenitors and neuronal cell types in the developing CNS for neural repair.

There is a large unfulfilled need for a clinically-suitable human neuronal cell source for repair or regeneration of the damaged central nervous system ...

The Specification of Telencephalic Glutamatergic Neurons from Human Pluripotent Stem Cells

Here, a stepwise procedure for efficiently generating telencephalic glutamatergic neurons from human pluripotent stem cells (PSCs) has been described. The differentiation process is initiated by breaking the human PSCs into clumps which round up to form aggregates when the cells are placed in a suspension culture. The aggregates are then grown in hESC medium from days 1-4 to allow for spontaneous differentiation. During this time, the cells have the capacity to become any of the three germ layers. From days 5-8, the cells are placed in a neural induction medium to push them into the neural lineage. Around day 8, the cells are allowed to attach onto 6 well plates and differentiate during which time the neuroepithelial cells form. These neuroepithelial cells can be isolated at day 17. The cells can then be kept as neurospheres until they are ready to be plated onto coverslips. Using a basic medium without any caudalizing factors, neuroepithelial cells are specified into telencephalic precursors, which can then be further differentiated into dorsal telencephalic progenitors and glutamatergic neurons efficiently. Overall, our system provides a tool to generate human glutamatergic neurons for researchers to study the development of these neurons and the diseases which affect them.

This procedure yields telencephalic neurons by going through checkpoints which are similar to those observed during human development. The cells are allowed to spontaneously differentiate, are exposed to factors which push them towards the neural lineage, are isolated, and are plated onto coverslips to allow for terminal differentiation and maturation.

Here, a stepwise procedure for efficiently generating telencephalic glutamatergic neurons from human pluripotent stem cells (PSCs) has been described. The ...

Feeder-free Derivation of Neural Crest Progenitor Cells from Human Pluripotent Stem Cells

Human pluripotent stem cells (hPSCs) have great potential for studying human embryonic development, for modeling human diseases in the dish and as a source of transplantable cells for regenerative applications after disease or accidents. Neural crest (NC) cells are the precursors for a large variety of adult somatic cells, such as cells from the peripheral nervous system and glia, melanocytes and mesenchymal cells. They are a valuable source of cells to study aspects of human embryonic development, including cell fate specification and migration. Further differentiation of NC progenitor cells into terminally differentiated cell types offers the possibility to model human diseases in vitro, investigate disease mechanisms and generate cells for regenerative medicine. This article presents the adaptation of a currently available in vitro differentiation protocol for the derivation of NC cells from hPSCs. This new protocol requires 18 days of differentiation, is feeder-free, easily scalable and highly reproducible among human embryonic stem cell (hESC) lines as well as human induced pluripotent stem cell (hiPSC) lines. Both old and new protocols yield NC cells of equal identity.

Neural crest (NC) cells derived from human pluripotent stem cells (hPSC) have great potential for modeling human development and disease and for cell replacement therapies. Here, a feeder-free adaptation of the currently widely used in vitro differentiation protocol for the derivation of NC cells from hPSCs is presented.

Human pluripotent stem cells (hPSCs) have great potential for studying human embryonic development, for modeling human diseases in the dish and as a ...

Directed Dopaminergic Neuron Differentiation from Human Pluripotent Stem Cells

Dopaminergic (DA) neurons in the substantia nigra pars compacta (also known as A9 DA neurons) are the specific cell type that is lost in Parkinson&#8217;s disease (PD). There is great interest in deriving A9 DA neurons from human pluripotent stem cells (hPSCs) for regenerative cell replacement therapy for PD. During neural development, A9 DA neurons originate from the floor plate (FP) precursors located at the ventral midline of the central nervous system. Here, we optimized the culture conditions for the stepwise differentiation of hPSCs to A9 DA neurons, which mimics embryonic DA neuron development. In our protocol, we first describe the efficient generation of FP precursor cells from hPSCs using a small molecule method, and then convert the FP cells to A9 DA neurons, which could be maintained in vitro for several months. This efficient, repeatable and controllable protocol works well in human embryonic stem cells (hESCs) and human induced pluripotent stem cells (hiPSCs) from normal persons and PD patients, in which one could derive A9 DA neurons to perform in vitro disease modeling and drug screening and in vivo cell transplantation therapy for PD.

We, based on knowledge from developmental biology and published research, developed an optimized protocol to efficiently generate A9 midbrain dopaminergic neurons from both human embryonic stem cells and human induced pluripotent stem cells, which would be useful for disease modeling and cell replacement therapy for Parkinson&#8217;s disease.

Dopaminergic (DA) neurons in the substantia nigra pars compacta  (also known as A9 DA neurons) are the specific cell type that is lost in ...

Conversion of Human Induced Pluripotent Stem Cells (iPSCs) into Functional Spinal and Cranial Motor Neurons Using PiggyBac Vectors

We describe here a method to obtain functional spinal and cranial motor neurons from human induced pluripotent stem cells (iPSCs). Direct conversion into motor neuron is obtained by ectopic expression of alternative modules of transcription factors, namely Ngn2, Isl1 and Lhx3 (NIL) or Ngn2, Isl1 and Phox2a (NIP). NIL and NIP specify, respectively, spinal and cranial motor neuron identity. Our protocol starts with the generation of modified iPSC lines in which NIL or NIP are stably integrated in the genome via a piggyBac transposon vector. Expression of the transgenes is then induced by doxycycline and leads, in 5 days, to the conversion of iPSCs into MN progenitors. Subsequent maturation, for 7 days, leads to homogeneous populations of spinal or cranial MNs. Our method holds several advantages over previous protocols: it is extremely rapid and simplified; it does not require viral infection or further MN isolation; it allows generating different MN subpopulations (spinal and cranial) with a remarkable degree of maturation, as demonstrated by the ability to fire trains of action potentials. Moreover, a large number of motor neurons can be obtained without purification from mixed populations. iPSC-derived spinal and cranial motor neurons can be used for in vitro modeling of Amyotrophic Lateral Sclerosis and other neurodegenerative diseases of the motor neuron. Homogeneous motor neuron populations might represent an important resource for cell type specific drug screenings.

Conversion of Human Induced Pluripotent Stem Cells iPSCs into Functional Spinal and Cranial Motor Neurons Using PiggyBac Vectors

This protocol allows rapid and efficient conversion of induced pluripotent stem cells into motor neurons with a spinal or cranial identity, by ectopic expression of transcription factors from inducible piggyBac vectors.

We describe here a method to obtain functional spinal and cranial motor neurons from human induced pluripotent stem cells (iPSCs). Direct conversion into ...

Neuronal Differentiation from Mouse Embryonic Stem Cells In vitro

The neural differentiation of mouse embryonic stem cells (mESCs) is a potential tool for elucidating the key mechanisms involved in neurogenesis and potentially aid in regenerative medicine. Here, we established an efficient and low cost method for neuronal differentiation from mESCs in vitro, using the strategy of combinatorial screening. Under the conditions defined here, the 2-day embryoid body formation + 6-day retinoic acid induction protocol permits fast and efficient differentiation from mESCs into neural precursor cells (NPCs), as seen by the formation of well-stacked and neurite-like A2lox and 129 derivatives that are Nestin positive. The healthy state of embryoid bodies and the timepoint at which retinoic acid (RA) is applied, as well as the RA concentrations, are critical in the process. In the subsequent differentiation from NPCs into neurons, N2B27 medium II (supplemented by Neurobasal medium) could better support the long term maintenance and maturation of neuronal cells. The presented method is highly efficiency, low cost and easy to operate, and can be a powerful tool for neurobiology and developmental biology research.

Here, we established a low cost and easy to operate method that directs fast and efficient differentiation from embryonic stem cells into neurons. This method is suitable for popularization among laboratories and can be a useful tool for neurological research.

The neural differentiation of mouse embryonic stem cells (mESCs) is a potential tool for elucidating the key mechanisms involved in neurogenesis and ...

Derivation, Expansion, Cryopreservation and Characterization of Brain Microvascular Endothelial Cells from Human Induced Pluripotent Stem Cells

Brain microvascular endothelial cells (BMECs) can be differentiated from human induced pluripotent stem cells (iPSCs) to develop ex vivo cellular models for studying blood-brain barrier (BBB) function. This modified protocol provides detailed steps to derive, expand, and cryopreserve BMECs from human iPSCs using a different donor and reagents than those reported in previous protocols. iPSCs are treated with essential 6 medium for 4 days, followed by 2 days of human endothelial serum-free culture medium supplemented with basic fibroblast growth factor, retinoic acid, and B27 supplement. At day 6, cells are sub-cultured onto a collagen/fibronectin matrix for 2 days. Immunocytochemistry is performed at day 8 for BMEC marker analysis using CLDN5, OCLN, TJP1, PECAM1, and SLC2A1. Western blotting is performed to confirm BMEC marker expression, and absence of SOX17, an endodermal marker. Angiogenic potential is demonstrated with a sprouting assay. Trans-endothelial electrical resistance (TEER) is measured using chopstick electrodes and voltohmmeter starting at day 7. Efflux transporter activity for ATP binding cassette subfamily B member 1 and ATP binding cassette subfamily C member 1 is measured using a multi-plate reader at day 8. Successful derivation of BMECs is confirmed by the presence of relevant cell markers, low levels of SOX17, angiogenic potential, transporter activity, and TEER values ~2000 &#937; x cm2. BMECs are expanded until day 10 before passaging onto freshly coated collagen/fibronectin plates or cryopreserved. This protocol demonstrates that iPSC-derived BMECs can be expanded and passaged at least once. However, lower TEER values and poorer localization of BMEC markers was observed after cryopreservation. BMECs can be utilized in co-culture experiments with other cell types (neurons, glia, pericytes), in three-dimensional brain models (organ-chip and hydrogel), for vascularization of brain organoids, and for studying BBB dysfunction in neuropsychiatric disorders.

This protocol details an adapted method to derive, expand, and cryopreserve brain microvascular endothelial cells obtained by differentiating human induced pluripotent stem cells, and to study blood brain barrier properties in an ex vivo model.

Brain microvascular endothelial cells (BMECs) can be differentiated from human induced pluripotent stem cells (iPSCs) to develop ex vivo cellular models ...

Shuttle Box Assay as an Associative Learning Tool for Cognitive Assessment in Learning and Memory Studies using Adult Zebrafish

Cognitive deficits, including impaired learning and memory, are a primary symptom of various developmental and age-related neurodegenerative diseases and traumatic brain injury (TBI). Zebrafish are an important neuroscience model due to their transparency during development and robust regenerative capabilities following neurotrauma. While various cognitive tests exist in zebrafish, most of the cognitive assessments that are rapid examine non-associative learning. At the same time, associative-learning assays often require multiple days or weeks. Here, we describe a rapid associative-learning test that utilizes an adverse stimulus (electric shock) and requires minimal preparation time. The shuttle box assay, presented here, is simple, ideal for novice investigators, and requires minimal equipment. We demonstrate that, following TBI, this shuttle box test reproducibly assesses cognitive deficit and recovery from young to old zebrafish. Additionally, the assay is adaptable to examine either immediate or delayed memory. We demonstrate that both a single TBI and repeated TBI events negatively affect learning and immediate memory but not delayed memory. We, therefore, conclude that the shuttle box assay reproducibly tracks the progression and recovery of cognitive impairment.

Learning and memory are potent metrics in studying either developmental, disease-dependent, or environmentally induced cognitive impairments. Most cognitive assessments require specialized equipment and extensive time commitments. However, the shuttle box assay is an associative learning tool that utilizes a conventional gel box for rapid and reliable assessment of adult zebrafish cognition.

Cognitive deficits, including impaired learning and memory, are a primary symptom of various developmental and age-related neurodegenerative diseases and ...

Flow Cytometric Analysis of Multiple Mitochondrial Parameters in Human Induced Pluripotent Stem Cells and Their Neural and Glial Derivatives

Mitochondria are important in the pathophysiology of many neurodegenerative diseases. Changes in mitochondrial volume, mitochondrial membrane potential (MMP), mitochondrial production of reactive oxygen species (ROS), and mitochondrial DNA (mtDNA) copy number are often features of these processes. This report details a novel flow cytometry-based approach to measure multiple mitochondrial parameters in different cell types, including human induced pluripotent stem cells (iPSCs) and iPSC-derived neural and glial cells. This flow-based strategy uses live cells to measure mitochondrial volume, MMP, and ROS levels, as well as fixed cells to estimate components of the mitochondrial respiratory chain (MRC) and mtDNA-associated proteins such as mitochondrial transcription factor A (TFAM). 
By co-staining with fluorescent reporters, including MitoTracker Green (MTG), tetramethylrhodamine ethyl ester (TMRE), and MitoSox Red, changes in mitochondrial volume, MMP, and mitochondrial ROS can be quantified and related to mitochondrial content. Double staining with antibodies against MRC complex subunits and translocase of outer mitochondrial membrane 20 (TOMM20) permits the assessment of MRC subunit expression. As the amount of TFAM is proportional to mtDNA copy number, the measurement of TFAM per TOMM20 gives an indirect measurement of mtDNA per mitochondrial volume. The entire protocol can be carried out within 2-3 h. Importantly, these protocols allow the measurement of mitochondrial parameters, both at the total level and the specific level per mitochondrial volume, using flow cytometry.

This study reports a novel approach to measure multiple mitochondrial functional parameters based on flow cytometry and double staining with two fluorescent reporters or antibodies to detect changes in mitochondrial volume, mitochondrial membrane potential, reactive oxygen species level, mitochondrial respiratory chain composition, and mitochondrial DNA.

Mitochondria are important in the pathophysiology of many neurodegenerative diseases. Changes in mitochondrial volume, mitochondrial membrane potential ...