Name: simultaneous measurement of mitochondrial calcium and mitochondrial membrane potential in live cells by fluorescent microscopy
Uploaded: 2017-01-24T14:00:00
Description: Watch this Scientific Journal Video about Simultaneous Measurement of Mitochondrial Calcium and Mitochondrial Membrane Potential in Live Cells by Fluorescent Microscopy at JoVE.com

We thank Dr Kirstin Elgass and Dr Sarah Creed from Monash Micro Imaging for technical assistance, and the Wellcome Trust and Medical Research Council UK for financial support. MMcK is supported the Australian Research Council Future Fellowship Scheme (FT120100459), the William Buckland Foundation, The Australian Mitochondrial Disease Foundation (AMDF), The Hudson Institute of Medical Research and Monash University. This work was supported by the Victorian Government Operational Infrastructure Support Scheme.

1. Preparation of Cells
<ol>
	<li>Grow cells on 10 cm cell culture dishes or 75 cm2 flasks in culture media [10 ml Dulbecco&#39;s Modified Eagles Medium (DMEM) supplemented with 5% (v/v) fetal bovine serum (FBS) and 1x penicillin/streptomycin (p/s)] at 37 &#176;C/5% CO2.</li>
	<li>To harvest cells, remove media by aspiration, then wash with 5 ml 1x phosphate buffered saline (1x PBS). Remove 1x PBS by aspiration, then add 1.5 ml 0.25% (w/v) Trypsin/0.25% (w/v) ethylenediaminetetraacetic acid (EDTA)...

<ol>
	<li>Szabadkai, G., Duchen, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondria:+the+hub+of+cellular+Ca2++signaling.">Mitochondria: the hub of cellular Ca2+ signaling.</a> Physiology. 23, 84-94 (2008).</li><li>Jacobson, J., Duchen, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interplay+between+mitochondria+and+cellular+calcium+signalling.">Interplay between mitochondria and cellular calcium signalling.</a> Mol. Cell. Biochem. 256-257, 209-218 (2004).</li><li>Bhosale, G., Sharpe, J. A., Sundier, S. Y., Duchen, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calcium+signaling+as+a+mediator+of+cell+energy+demand+and+a+trigger+to+cell.">Calcium signaling as a mediator of cell energy demand and a trigger to cell.</a> Ann. N. Y. Acad. Sci. 1350, 107-116 (2015).</li><li>Duchen, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondria+calcium-dependent+neuronal+death+and+neurodegenerative+disease.">Mitochondria calcium-dependent neuronal death and neurodegenerative disease.</a> Pflugers Arch. 464, 111-121 (2012).</li><li>Drummond, R. M., Mix, T. C., Tuft, R. A., Walsh, J. V., Fay, F. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondrial+Ca2++homeostasis+during+Ca2++influx+and+Ca2++release+in+gastric+myocytes+from+Bufo+marinus.">Mitochondrial Ca2+ homeostasis during Ca2+ influx and Ca2+ release in gastric myocytes from Bufo marinus.</a> J. Physiol. 522, 375-390 (2000).</li><li>Hajnoczky, G., Robb-Gaspers, L. D., Seitz, M. B., Thomas, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Decoding+of+cytosolic+calcium+oscillations+in+the+mitochondria.">Decoding of cytosolic calcium oscillations in the mitochondria.</a> Cell. 82, 415-424 (1995).</li><li>Davidson, S. M., Duchen, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+mitochondrial+calcium+signalling+with+fluorescent+probes+and+single+or+two+photon+confocal+microscopy.">Imaging mitochondrial calcium signalling with fluorescent probes and single or two photon confocal microscopy.</a> Methods Mol. Biol. 810, 219-234 (2012).</li><li>Pozzan, T., Rudolf, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+of+mitochondrial+calcium+in+vivo.">Measurements of mitochondrial calcium in vivo.</a> Biochim. Biophys. Acta. 1787, 1317-1323 (2009).</li><li>Rizzuto, R., Simpson, A. W., Brini, M., Pozzan, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+changes+of+mitochondrial+Ca2++revealed+by+specifically+targeted+recombinant+aequorin.">Rapid changes of mitochondrial Ca2+ revealed by specifically targeted recombinant aequorin.</a> Nature. 358, 325-327 (1992).</li><li>Pitter, J. G., Maechler, P., Wollheim, C. B., Spat, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondria+respond+to+Ca2++already+in+the+submicromolar+range:+correlation+with+redox+state.">Mitochondria respond to Ca2+ already in the submicromolar range: correlation with redox state.</a> Cell Calcium. 31, 97-104 (2002).</li><li>Schoenmakers, T. J., Visser, G. J., Flik, G., Theuvenet, A. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=CHELATOR:+an+improved+method+for+computing+metal+ion+concentrations+in+physiological+solutions.">CHELATOR: an improved method for computing metal ion concentrations in physiological solutions.</a> BioTechniques. 12, 870-879 (1992).</li><li>McKenzie, M., Duchen, M. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Impaired+Cellular+Bioenergetics+Causes+Mitochondrial+Calcium+Handling+Defects+in+MT-ND5+Mutant+Cybrids.">Impaired Cellular Bioenergetics Causes Mitochondrial Calcium Handling Defects in MT-ND5 Mutant Cybrids.</a> PLoS One. 11, e0154371 (2016).</li><li>Berridge, M. J., Lipp, P., Bootman, M. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+versatility+and+universality+of+calcium+signalling.">The versatility and universality of calcium signalling.</a> Nat. Rev. Mol. Cell Biol. 1, 11-21 (2000).</li><li>Homolya, L., Hollo, Z., Germann, U. A., Pastan, I., Gottesman, M. M., Sarkadi, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fluorescent+cellular+indicators+are+extruded+by+the+multidrug+resistance+protein.">Fluorescent cellular indicators are extruded by the multidrug resistance protein.</a> J. Biol. Chem. 268, 21493-21496 (1993).</li><li>Fujimoto, K., Chen, Y., Polonsky, K. S., Dorn, G. W. . 2. n. d. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Targeting+cyclophilin+D+and+the+mitochondrial+permeability+transition+enhances+beta-cell+survival+and+prevents+diabetes+in+Pdx1+deficiency.">Targeting cyclophilin D and the mitochondrial permeability transition enhances beta-cell survival and prevents diabetes in Pdx1 deficiency.</a> Proc. Natl. Acad. Sci. U.S.A. 107, 10214-10219 (2010).</li><li>Rao, V. K., Carlson, E. A., Yan, S. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondrial+permeability+transition+pore+is+a+potential+drug+target+for+neurodegeneration.">Mitochondrial permeability transition pore is a potential drug target for neurodegeneration.</a> Biochim. Biophys. Acta. 1842, 1267-1272 (2014).</li></ol>

Calcium plays a critical role in many cell processes, including muscle contraction, neuronal signaling and cell proliferation13. Increases in cell calcium concentrations are often associated with energy demand, with calcium able to directly stimulate mitochondrial oxidative phosphorylation to raise ATP generation3. It is therefore essential that we have the ability to effectively monitor mitochondrial calcium accumulation and to be able to compare how this function is affec...

Mitochondria play an important role in regulating intracellular Ca2+ concentration by acting as local Ca2+ buffers1. Ca2+ enters the mitochondria via the Ca2+ uniporter, a process driven by the electrochemical gradient that exists across the mitochondrial inner membrane (&#916;&#936;m)2. Once inside the mitochondrial matrix, Ca2+ can activate oxidative phosphorylation by stimulating three rate-limiting dehydrogenases of the citric acid cycle3. This stimulation maintains &#916;&#936;m, which is temporarily dissipated as the positive calcium ions cross the mitochondrial inner membrane into the mitochondrial matrix. If the Ca2+ concentration within the mitochondria becomes very high, mitochondrial permeability transition can be initiated, resulting in the dissipation of &#916;&#936;m, the cessation of oxidative phosphorylation and the induction of cell death signaling pathways4.
The important role that mitochondria play in the spatial buffering of cellular calcium makes the accurate monitoring of mitochondrial calcium critical. Various methods have been established to monitor mitochondrial calcium, including the use of rhodamine based dyes. One such dye, Rhod-2, AM, is quite effective at partitioning to the mitochondria to measure mitochondrial Ca2+ levels5,6. However, care must be used as some dye will accumulate in other organelles, such as liposomes, or remain in the cell cytosol. Nevertheless, downstream analyses can be employed to distinguish these signals from those from the mitochondria7.
Another technique to monitor mitochondrial calcium utilizes fluorescent reporter constructs8. The benefit of these genetically encoded probes is that they can be specifically targeted to the mitochondria by using endogenous N-terminal peptides, for example the N-terminal targeting signal of human COX subunit VIII. This system has been employed to generate a mitochondrial-targeted aequorin probe which has proved extremely useful for investigating mitochondrial calcium signaling9. The main drawback of these genetically encoded probes is that they need to be introduced into the cells by transient expression (which is not feasible for certain cell types and can produce variable results) or by creating stable expression systems (which is time consuming).
To circumvent the problems outlined above, we have developed a new protocol to measure mitochondrial Ca2+ and &#916;&#936;m simultaneously. This protocol is based on a previously described method that adds exogenous calcium to permeabilized cells10. Our protocol has three main advantages over other methods: firstly, we use Fluo-4, AM and TMRM to monitor mitochondrial Ca2+ and &#916;&#936;m, two dyes that have very distinct spectral properties; secondly, the cells are permeabilized so that the Fluo-4 signal is only detecting mitochondrial Ca2+ and not Ca2+ localized to other organelles or the cell cytosol; and thirdly, the use of Fluo-4 to detect mitochondrial Ca2+ allows for fast and simple cell staining, negating any cell transfection or transformation issues that exist if using genetically encoded probes.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Dulbecco&#39;s Modified Eagle Medium (DMEM)</td>
			<td>ThermoFisher</td>
			<td>10566016</td>
		</tr>
		<tr>
			<td>fetal bovine serum (FBS)</td>
			<td>ThermoFisher</td>
			<td>16000044</td>
		</tr>
		<tr>
			<td>1x phosphate buffered saline (PBS)</td>
			<td>ThermoFisher</td>
			<td>10010023</td>
		</tr>
		<tr>
			<td>100x penicillin/streptomycin (p/s)</td>
			<td>ThermoFisher</td>
			<td>15140122</td>
		</tr>
		<tr>
			<td>0.25% Trypsin / 0.25% EDTA</td>
			<td>ThermoFisher</td>
			<td>25200056</td>
		</tr>
		<tr>
			<td>8-well chambered coverslip</td>
			<td>ibidi</td>
			<td>80826</td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>Sigma-Aldrich</td>
			<td>793566</td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>Sigma-Aldrich</td>
			<td>P9541&#160;</td>
		</tr>
		<tr>
			<td>MgSO4</td>
			<td>Sigma-Aldrich</td>
			<td>746452</td>
		</tr>
		<tr>
			<td>KH2PO4</td>
			<td>Sigma-Aldrich</td>
			<td>795488</td>
		</tr>
		<tr>
			<td>D-glucose</td>
			<td>Sigma-Aldrich</td>
			<td>G8270&#160;</td>
		</tr>
		<tr>
			<td>CaCl2</td>
			<td>Sigma-Aldrich</td>
			<td>746495</td>
		</tr>
		<tr>
			<td>HEPES</td>
			<td>Sigma-Aldrich</td>
			<td>H3375&#160;</td>
		</tr>
		<tr>
			<td>MgCl2</td>
			<td>Sigma-Aldrich</td>
			<td>M2670&#160;</td>
		</tr>
		<tr>
			<td>EGTA</td>
			<td>Sigma-Aldrich</td>
			<td>E4378&#160;</td>
		</tr>
		<tr>
			<td>HEDTA</td>
			<td>Sigma-Aldrich</td>
			<td>H8126&#160;</td>
		</tr>
		<tr>
			<td>malate</td>
			<td>Sigma-Aldrich</td>
			<td>M1000</td>
		</tr>
		<tr>
			<td>glutamate</td>
			<td>Sigma-Aldrich</td>
			<td>G1626</td>
		</tr>
		<tr>
			<td>ADP</td>
			<td>Sigma-Aldrich</td>
			<td>A5285</td>
		</tr>
		<tr>
			<td>Ca2+ free Hank&#8217;s buffered salt solution (HBSS)&#160;</td>
			<td>ThermoFisher</td>
			<td>14175-095</td>
		</tr>
		<tr>
			<td>tetramethylrhodamine, methyl ester, perchlorate (TMRM)</td>
			<td>ThermoFisher</td>
			<td>T668</td>
		</tr>
		<tr>
			<td>Verapamil</td>
			<td>Sigma-Aldrich</td>
			<td>V4629</td>
		</tr>
		<tr>
			<td>Fluo-4 acetoxymethyl ester (Fluo-4, AM)</td>
			<td>ThermoFisher</td>
			<td>F14201</td>
		</tr>
		<tr>
			<td>dimethyl sulfoxide (DMSO)</td>
			<td>ThermoFisher</td>
			<td>D12345</td>
		</tr>
		<tr>
			<td>carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP)</td>
			<td>Sigma-Aldrich</td>
			<td>C2920</td>
		</tr>
		<tr>
			<td>digitonin</td>
			<td>Sigma-Aldrich</td>
			<td>D141</td>
		</tr>
		<tr>
			<td>thapsigargin</td>
			<td>Sigma-Aldrich</td>
			<td>T9033</td>
		</tr>
		<tr>
			<td>Pluronic F-127&#160;</td>
			<td>ThermoFisher</td>
			<td>P3000MP&#160;</td>
		</tr>
		<tr>
			<td>hemacytometer</td>
			<td>VWR</td>
			<td>631-0925</td>
		</tr>
		<tr>
			<td>10 cm cell culture dishes</td>
			<td>Corning</td>
			<td>COR430167</td>
		</tr>
		<tr>
			<td>75 cm2 cell culture flasks</td>
			<td>Corning</td>
			<td>COR430641</td>
		</tr>
	</tbody>
</table>

simultaneous measurement of mitochondrial calcium and mitochondrial membrane potential in live cells by fluorescent microscopy

Apart from their essential role in generating ATP, mitochondria also act as local calcium (Ca2+) buffers to tightly regulate intracellular Ca2+ concentration. To do this, mitochondria utilize the electrochemical potential across their inner membrane (&#916;&#936;m) to sequester Ca2+. The influx of Ca2+ into the mitochondria stimulates three rate-limiting dehydrogenases of the citric acid cycle, increasing electron transfer through the oxidative phosphorylation (OXPHOS) complexes. This stimulation maintains &#916;&#936;m, which is temporarily dissipated as the positive calcium ions cross the mitochondrial inner membrane into the mitochondrial matrix.
We describe here a method for simultaneously measuring mitochondria Ca2+ uptake and &#916;&#936;m in live cells using confocal microscopy. By permeabilizing the cells, mitochondrial Ca2+ can be measured using the fluorescent Ca2+ indicator Fluo-4, AM, with measurement of &#916;&#936;m using the fluorescent dye tetramethylrhodamine, methyl ester, perchlorate (TMRM). The benefit of this system is that there is very little spectral overlap between the fluorescent dyes, allowing accurate measurement of mitochondrial Ca2+ and &#916;&#936;m simultaneously. Using the sequential addition of Ca2+ aliquots, mitochondrial Ca2+ uptake can be monitored, and the concentration at which Ca2+ induces mitochondrial membrane permeability transition and the loss of &#916;&#936;m determined.

We have used this protocol to examine the effects of an MT-ND5 mutation on the ability of 143B cell mitochondria to buffer increases in calcium12. In the example shown here, control 143B cells were loaded with TMRM and Fluo-4, AM before permeabilization with digitonin. After 5 min of imaging, eight sequential additions of a 1:100 dilution of 40 mM exogenous CaCl2 were made, with the final free Ca2+ ion concentration [Ca2+] ...

Watch this Scientific Journal Video about Simultaneous Measurement of Mitochondrial Calcium and Mitochondrial Membrane Potential in Live Cells by Fluorescent Microscopy at JoVE.com

Simultaneous Measurement of Mitochondrial Calcium and Mitochondrial Membrane Potential in Live Cells by Fluorescent Microscopy

Mitochondria can utilize the electrochemical potential across their inner membrane (&#916;&#936;m) to sequester calcium (Ca2+), allowing them to shape cytosolic Ca2+ signaling within the cell. We describe a method for simultaneously measuring mitochondria Ca2+ uptake and &#916;&#936;m in live cells using fluorescent dyes and confocal microscopy.

Apart from their essential role in generating ATP, mitochondria also act as local calcium (Ca2+) buffers to tightly regulate intracellular Ca2+ ...

The overall goal of this procedure is to provide a simple method for simultaneously measuring mitochondrial calcium uptake and mitochondrial membrane potential in live cells using confocal microscopy. This method can help to answer key questions in the mitochondrial field, such as how mitochondria act as local calcium stores to tightly regulate intracellular calcium concentration. The main advantage of this technique is that both mitochondrial calcium and mitochondrial membrane potential can be specifically measured simultaneously in life cells without the need for genetic manipulation.Though this method can provide insight into calcium regulation in mitochondrial disease pathology, it can also be used to study other newer degenerative conditions, when urinal calcium regulation in crucial. In this procedure, grow the cells on ten centimeter cell culture dishes, or 75 square centimeter flasks, in culture media at 37 degrees Celsius for 24 hours. To harvest the cells, remove media by aspiration, and wash the cells with five milliliters of PBS.Afterward, remove PBS by aspiration. Then add 1.5 milliliters of Trypsin EDTA, and incubate the cells at 37 degrees Celsius for 2 minutes. Next tap the dish gently to remove the cells.Resuspend them in five milliliters of culture media. Afterward, count the cells with a hemocytometer. The day before confocal imaging, plate the cells at 60 to 70 percent confluency in dishes or chambered cover slips.Then incubate the cells overnight at 37 degrees Celsius to allow the cells to attach and recover. In this procedure, prepare the record staining solution with TMRM, Fluo 4 AM, and surfactant. Remove the culture media and wash the cells with 100 microliters of PBS.Next remove PBS and incubate the cells in 100 microliters of record staining solution for 45 minutes at room temperature. Following that, remove the record staining solution and wash the cells in 100 microliters of calcium-free HBSS to remove excess Fluo 4 AM and TMRM. Then prepare the intracellular imagining solution with digitonin, TMRM, and thapsigargin.Next remove the calcium-free HPSS and add 300 microliters of IM imaging solution to the cells. Leave the cells to equilibrate at room temperature for ten minutes before imaging with the calcium chloride additions. Use a low laser power of approximately 5 percent to minimize any photo damage to the cells.Use the setting on the microscope lasers for the excitation and emission spectra of TMRM and Fluo 4 AM.Use a low laser power of approximately five percent with high gain to minimize photo damage of the cells. Next set the microscope to scan images every twenty five seconds. Scan the cells for ten minutes to establish the base line readings for TMRM and Fluo 4 signals.Then pause the image scanning and add 3 microliters of 40 millimolar calcium chloride stock solution directly to the cells in 300 microliters of IM imaging solution. Mix gently with a pipette. When adding calcium solution to the cells, be careful not to move the dish on the microscope stage.Continue image scanning for approximately four minutes. Then repeat calcium chloride additions every four minutes until the desired TMRM or Fluo 4 signals are reached, usually around eight to ten additions. Subsequently add a 1 to 100 dilution of one millimolar FCCP directly to the cells to dissipate the inner membrane potential.Then image the cells for a further five minutes before the experiment is finished. Once the imaging is complete, determine the intensity of the Fluo 4 and TMRM signals using the image analysis software. To do so, open the image, click on the selection tool and select a region of interest that contains mitochondria.Multiple regions of interest can be selected for measurement by opening Analyze, Tools, ROI Manager. Click Add to include the selected ROI for intensity measurement. Repeat previous steps until all ROIs have been added to the ROI manager.Measure the average fluorescent intensity of the TMRM and Fluo 4 signals for each region of interest by selecting the more multi measure function. Make sure that the Measure All Slices is selected, and that One Row Per Slice is deselected. Then click Okay to obtain a table of mean fluorescent intensity of the TMRM and Fluo 4 signals for each ROI at each time point.Then combine the data to calculate the average intensity and standard deviation for the TMRM and Fluo 4 signals. In this experiment, 143B cells were preloaded with Fluo 4 AM and TMRM before permeabilization with digitonin. Following each calcium chloride addition, the TMRM signal decreases as the influx of calcium ions into the mitochondria dissipates the mitochondrial membrane potential.A small membrane potential is evident following permeability transition as the addition of 10 micromolar FCCP still induces a collapse of membrane potential. Here the mitochondrial calcium concentration, indicated by Fluo 4 signal, begins to increase following the second calcium chloride addition when the free calcium in the media reaches 0.32 micromolar. Each subsequent calcium chloride addition caused a progressive increase in mitochondrial calcium.Shown here are the images of simultaneous measurements of membrane potential in mitochondrial calcium. The red signal indicates TMRM and the green signal indicates Fluo 4. Once mastered, this technique can be done in approximately two hours if performed properly.After watching this video, you should have a good understanding of how to measure mitochondrial calcium levels and mitochondrial membrane potential simultaneously on confocal microscopy using the addition of exogenous calcium to the permeabilized cells. Don't forget that working with FCCP and digitonin can be extremely hazardous, and precautions such as wearing PPE should always been taking when performing this procedure.

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Research

JoVE Journal

Biology

Fluorescence-based Measurement of Store-operated Calcium Entry in Live Cells: from Cultured Cancer Cell to Skeletal Muscle Fiber

Store operated Ca2+ entry (SOCE), earlier termed capacitative Ca2+ entry, is a tightly regulated mechanism for influx of extracellular Ca2+ into cells to replenish depleted endoplasmic reticulum (ER) or sarcoplasmic reticulum (SR) Ca2+ stores1,2. Since Ca2+ is a ubiquitous second messenger, it is not surprising to see that SOCE plays important roles in a variety of cellular processes, including proliferation, apoptosis, gene transcription and motility. Due to its wide occurrence in nearly all cell types, including epithelial cells and skeletal muscles, this pathway has received great interest3,4. However, the heterogeneity of SOCE characteristics in different cell types and the physiological function are still not clear5-7.
The functional channel properties of SOCE can be revealed by patch-clamp studies, whereas a large body of knowledge about this pathway has been gained by fluorescence-based intracellular Ca2+ measurements because of its convenience and feasibility for high-throughput screening. The objective of this report is to summarize a few fluorescence-based methods to measure the activation of SOCE in monolayer cells, suspended cells and muscle fibers5,8-10. The most commonly used of these fluorescence methods is to directly monitor the dynamics of intracellular Ca2+ using the ratio of F340nm and F380nm (510 nm for emission wavelength) of the ratiometric Ca2+ indicator Fura-2. To isolate the activity of unidirectional SOCE from intracellular Ca2+ release and Ca2+ extrusion, a Mn2+ quenching assay is frequently used. Mn2+ is known to be able to permeate into cells via SOCE while it is impervious to the surface membrane extrusion processes or to ER uptake by Ca2+ pumps due to its very high affinity with Fura-2. As a result, the quenching of Fura-2 fluorescence induced by the entry of extracellular Mn2+ into the cells represents a measurement of activity of SOCE9. Ratiometric measurement and the Mn+2 quenching assays can be performed on a cuvette-based spectrofluorometer in a cell population mode or in a microscope-based system to visualize single cells. The advantage of single cell measurements is that individual cells subjected to gene manipulations can be selected using GFP or RFP reporters, allowing studies in genetically modified or mutated cells. The spatiotemporal characteristics of SOCE in structurally specialized skeletal muscle can be achieved in skinned muscle fibers by simultaneously monitoring the fluorescence of two low affinity Ca2+ indicators targeted to specific compartments of the muscle fiber, such as Fluo-5N in the SR and Rhod-5N in the transverse tubules9,11,12.

The extent of store-operated Ca2+ entry (SOCE) can be monitored using fluorescent Ca2+ indicators. Mn2+ quenching of such indicators assays SOCE in cultured cells and skeletal muscle fibers. A technique allowing spatial and temporal resolution of SOCE by confocal imaging of mechanically skinned muscle fibers is also described.

Store operated Ca2+ entry (SOCE), earlier termed capacitative Ca2+ entry, is a tightly regulated mechanism for influx of extracellular Ca2+ into cells to ...

Intravital Microscopy for Imaging Subcellular Structures in Live Mice Expressing Fluorescent Proteins

Here we describe a procedure to image subcellular structures in live rodents that is based on the use of confocal intravital microscopy. As a model organ, we use the salivary glands of live mice since they provide several advantages. First, they can be easily exposed to enable access to the optics, and stabilized to facilitate the reduction of the motion artifacts due to heartbeat and respiration. This significantly facilitates imaging and tracking small subcellular structures. Second, most of the cell populations of the salivary glands are accessible from the surface of the organ. This permits the use of confocal microscopy that has a higher spatial resolution than other techniques that have been used for in vivo imaging, such as two-photon microscopy. Finally, salivary glands can be easily manipulated pharmacologically and genetically, thus providing a robust system to investigate biological processes at a molecular level.
In this study we focus on a protocol designed to follow the kinetics of the exocytosis of secretory granules in acinar cells and the dynamics of the apical plasma membrane where the secretory granules fuse upon stimulation of the beta-adrenergic receptors. Specifically, we used a transgenic mouse that co-expresses cytosolic GFP and a membrane-targeted peptide fused with the fluorescent protein tandem-Tomato. However, the procedures that we used to stabilize and image the salivary glands can be extended to other mouse models and coupled to other approaches to label in vivo cellular components, enabling the visualization of various subcellular structures, such as endosomes, lysosomes, mitochondria, and the actin cytoskeleton.

Intravital microscopy is a powerful tool that enables imaging various biological processes in live animals. In this article, we present a detailed method for imaging the dynamics of subcellular structures, such as the secretory granules, in the salivary glands of live mice.

Here we describe a procedure to image subcellular structures in live rodents that is based on the use of confocal intravital microscopy. As a model organ, ...

Simultaneous pH Measurement in Endocytic and Cytosolic Compartments in Living Cells using Confocal Microscopy

Intracellular pH is tightly regulated and differences in pH between the cytoplasm and organelles have been reported1. Regulation of cellular pH is crucial for homeostatic control of physiological processes that include: protein, DNA and RNA synthesis, vesicular trafficking, cell growth and cell division. Alterations in cellular pH homeostasis can lead to detrimental functional changes and promote progression of various diseases2. Various methods are available for measuring intracellular pH but very few of these allow simultaneous measurement of pH in the cytoplasm and in organelles. Here, we describe in detail a rapid and accurate method for the simultaneous measurement of cytoplasmic and organellar pH by using confocal microscopy on living cells3. This goal is achieved with the use of two pH-sensing ratiometric dyes that possess selective cellular compartment partitioning. For instance, SNARF-1 is compartmentalized inside the cytoplasm whereas HPTS is compartmentalized inside endosomal/lysosomal organelles. Although HPTS is commonly used as a cytoplasmic pH indicator, this dye can specifically label vesicles along the endosomal-lysosomal pathway after being taken up by pinocytosis3,4. Using these pH-sensing probes, it is possible to simultaneously measure pH within the endocytic and cytoplasmic compartments. The optimal excitation wavelength of HPTS varies depending on the pH while for SNARF-1, it is the optimal emission wavelength that varies. Following loading with SNARF-1 and HPTS, cells are cultured in different pH-calibrated solutions to construct a pH standard curve for each probe. Cell imaging by confocal microscopy allows elimination of artifacts and background noise. Because of the spectral properties of HPTS, this probe is better suited for measurement of the mildly acidic endosomal compartment or to demonstrate alkalinization of the endosomal/lysosomal organelles. This method simplifies data analysis, improves accuracy of pH measurements and can be used to address fundamental questions related to pH modulation during cell responses to external challenges.

Several methods are available for measuring intracellular pH but very few of these allow simultaneous measurement of cytoplasmic and organellar pH. Here, we describe in detail a rapid and accurate methodology to simultaneously measure cytoplasmic and vesicular pH by ratiometric imaging of living cells.

Intracellular pH is tightly regulated and differences in pH between the cytoplasm and organelles have been reported1. Regulation of cellular pH is crucial ...

In Vivo 4-Dimensional Tracking of Hematopoietic Stem and Progenitor Cells in Adult Mouse Calvarial Bone Marrow

Through a delicate balance between quiescence and proliferation, self renewal and production of differentiated progeny, hematopoietic stem cells (HSCs) maintain the turnover of all mature blood cell lineages. The coordination of the complex signals leading to specific HSC fates relies upon the interaction between HSCs and the intricate bone marrow microenvironment, which is still poorly understood[1-2].
We describe how by combining a newly developed specimen holder for stable animal positioning with multi-step confocal and two-photon in vivo imaging techniques, it is possible to obtain high-resolution 3D stacks containing HSPCs and their surrounding niches and to monitor them over time through multi-point time-lapse imaging. High definition imaging allows detecting ex vivo labeled hematopoietic stem and progenitor cells (HSPCs) residing within the bone marrow. Moreover, multi-point time-lapse 3D imaging, obtained with faster acquisition settings, provides accurate information about HSPC movement and the reciprocal interactions between HSPCs and stroma cells.
Tracking of HSPCs in relation to GFP positive osteoblastic cells is shown as an exemplary application of this method. This technique can be utilized to track any appropriately labeled hematopoietic or stromal cell of interest within the mouse calvarium bone marrow space.

In Vivo 4-Dimensional Tracking of Hematopoietic Stem and Progenitor Cells in Adult Mouse Calvarial Bone Marrow

The nature of the interactions between hematopoietic stem and progenitor cells (HSPCs) and bone marrow niches is poorly understood. Custom hardware modifications and a multi-step acquisition protocol allow the use of two-photon and confocal microscopy to image ex vivo labeled HSPCs homed within bone marrow areas, tracking interactions and movement.

Through a delicate balance between quiescence and proliferation, self renewal and production of differentiated progeny, hematopoietic stem cells (HSCs) ...

Simultaneous Mapping and Quantitation of Ribonucleotides in Human Mitochondrial DNA

Established approaches to estimate the number of ribonucleotides present in a genome are limited to the quantitation of incorporated ribonucleotides using short synthetic DNA fragments or plasmids as templates and then extrapolating the results to the whole genome. Alternatively, the number of ribonucleotides present in a genome may be estimated using alkaline gels or Southern blots. More recent in vivo approaches employ Next-generation sequencing allowing genome-wide mapping of ribonucleotides, providing the position and identity of embedded ribonucleotides. However, they do not allow quantitation of the number of ribonucleotides which are incorporated into a genome. Here we describe how to simultaneously map and quantitate the number of ribonucleotides which are incorporated into human mitochondrial DNA in vivo by Next-generation sequencing. We use highly intact DNA and introduce sequence specific double strand breaks by digesting it with an endonuclease, subsequently hydrolyzing incorporated ribonucleotides with alkali. The generated ends are ligated with adapters and these ends are sequenced on a Next-generation sequencing machine. The absolute number of ribonucleotides can be calculated as the number of reads outside the recognition site per average number of reads at the recognition site for the sequence specific endonuclease. This protocol may also be utilized to map and quantitate free nicks in DNA and allows adaption to map other DNA lesions that can be processed to 5&#180;-OH ends or 5&#180;-phosphate ends. Furthermore, this method can be applied to any organism, given that a suitable reference genome is available. This protocol therefore provides an important tool to study DNA replication, 5&#180;-end processing, DNA damage, and DNA repair.

Here we describe a method amenable to simultaneously quantitate and genome-wide map ribonucleotides in highly intact DNA at single-nucleotide resolution, combining enzymatic cleavage of genomic DNA with its alkaline hydrolysis and subsequent 5&#180;-end sequencing.

Established approaches to estimate the number of ribonucleotides present in a genome are limited to the quantitation of incorporated ribonucleotides using ...

Simultaneous Measurements of Intracellular Calcium and Membrane Potential in Freshly Isolated and Intact Mouse Cerebral Endothelium

Cerebral arteries and their respective microcirculation deliver oxygen and nutrients to the brain via blood flow regulation. Endothelial cells line the lumen of blood vessels and command changes in vascular diameter as needed to meet the metabolic demand of neurons. Primary endothelial-dependent signaling pathways of hyperpolarization of membrane potential (Vm) and nitric oxide typically operate in parallel to mediate vasodilation and thereby increase blood flow. Although integral to coordinating vasodilation over several millimeters of vascular length, components of endothelium-derived hyperpolarization (EDH) have been historically difficult to measure. These components of EDH entail intracellular Ca2+ [Ca2+]i increases and subsequent activation of small- and intermediate conductance Ca2+-activated K+ (SKCa/IKCa) channels.
Here, we present a simplified illustration of the isolation of fresh endothelium from mouse cerebral arteries; simultaneous measurements of endothelial [Ca2+]i and Vm using Fura-2 photometry and intracellular sharp electrodes, respectively; and a continuous superfusion of salt solutions and pharmacological agents under physiological conditions (pH 7.4, 37 &#176;C). Posterior cerebral arteries from the Circle of Willis are removed free of the posterior communicating and the basilar arteries. Enzymatic digestion of cleaned posterior cerebral arterial segments and subsequent trituration facilitates removal of adventitia, perivascular nerves, and smooth muscle cells. Resulting posterior cerebral arterial endothelial &#34;tubes&#34; are then secured under a microscope and examined using a camera, photomultiplier tube, and one to two electrometers while under continuous superfusion. Collectively, this method can simultaneously measure changes in endothelial [Ca2+]i and Vm in discrete cellular locations, in addition to the spreading of EDH through gap junctions up to millimeter distances along the intact endothelium. This method is expected to yield a high-throughput analysis of the cerebral endothelial functions underlying mechanisms of blood flow regulation in the normal and diseased brain.

Demonstrated here are protocols for (1) freshly isolating intact cerebral endothelial "tubes" and (2) simultaneous measurements of endothelial calcium and membrane potential during endothelium-derived hyperpolarization. Further, these methods allow for pharmacological tuning of endothelial cell calcium and electrical signaling as individual or interactive experimental variables.

Cerebral arteries and their respective microcirculation deliver oxygen and nutrients to the brain via blood flow regulation. Endothelial cells line the ...

Improving the Accuracy of Flow Cytometric Assessment of Mitochondrial Membrane Potential in Hematopoietic Stem and Progenitor Cells Through the Inhibition of Efflux Pumps

As cellular metabolism is a key regulator of hematopoietic stem cell (HSC) self-renewal, the various roles played by the mitochondria in hematopoietic homeostasis have been extensively studied by HSC researchers. Mitochondrial activity levels are reflected in their membrane potentials (&#916;&#936;m), which can be measured by cell-permeant cationic dyes such as TMRM (tetramethylrhodamine, methyl ester). The ability of efflux pumps to extrude these dyes from cells can limit their usefulness, however. The resulting measurement bias is particularly critical when assessing HSCs, as xenobiotic transporters exhibit higher levels of expression and activity in HSCs than in differentiated cells. Here, we describe a protocol utilizing Verapamil, an efflux pump inhibitor, to accurately measure &#916;&#936;m across multiple bone marrow populations. The resulting inhibition of pump activity is shown to increase TMRM intensity in hematopoietic stem and progenitor cells (HSPCs), while leaving it relatively unchanged in mature fractions. This highlights the close attention to dye-efflux activity that is required when &#916;&#936;m-dependent dyes are used, and as written and visualized, this protocol can be used to accurately compare either different populations within the bone marrow, or the same population across different experimental models.

Xenobiotic efflux pumps are highly active in hematopoietic stem and progenitor cells (HSPCs) and cause extrusion of TMRM, a mitochondrial membrane potential fluorescent dye. Here, we present a protocol to accurately measure mitochondrial membrane potential in HSPCs by TMRM in the presence of Verapamil, an efflux pump inhibitor.

As cellular metabolism is a key regulator of hematopoietic stem cell (HSC) self-renewal, the various roles played by the mitochondria in hematopoietic ...

Measurement of Mitochondrial Mass and Membrane Potential in Hematopoietic Stem Cells and T-cells by Flow Cytometry

A fine balance of quiescence, self-renewal, and differentiation is key to preserve the hematopoietic stem cell (HSC) pool and maintain lifelong production of all mature blood cells. In recent years cellular metabolism has emerged as a crucial regulator of HSC function and fate. We have previously demonstrated that modulation of mitochondrial metabolism influences HSC fate. Specifically, by chemically uncoupling the electron transport chain we were able to maintain HSC function in culture conditions that normally induce rapid differentiation. However, limiting HSC numbers often precludes the use of standard assays to measure HSC metabolism and therefore predict their function. Here, we report a simple flow cytometry assay that allows reliable measurement of mitochondrial membrane potential and mitochondrial mass in scarce cells such as HSCs. We discuss the isolation of HSCs from mouse bone marrow and measurement of mitochondrial mass and membrane potential post ex vivo culture. As an example, we show the modulation of these parameters in HSCs via treatment with a metabolic modulator. Moreover, we extend the application of this methodology on human peripheral blood-derived T cells and human tumor infiltrating lymphocytes (TILs), showing dramatic differences in their mitochondrial profiles, possibly reflecting different T cell functionality. We believe this assay can be employed in screenings to identify modulators of mitochondrial metabolism in various cell types in different contexts.

Here we describe a reliable method to measure mitochondrial mass and membrane potential in ex vivo cultured hematopoietic stem cells and T cells.

A fine balance of quiescence, self-renewal, and differentiation is key to preserve the hematopoietic stem cell (HSC) pool and maintain lifelong production ...

Quantification of Mitochondrial Membrane Potential and Superoxide Levels

Fluorescence-Based Quantification of Mitochondrial Membrane Potential and Superoxide Levels Using Live Imaging in HeLa Cells

Mitochondria are dynamic organelles critical for metabolic homeostasis by controlling energy production via ATP synthesis. To support cellular metabolism, various mitochondrial quality control mechanisms cooperate to maintain a healthy mitochondrial network. One such pathway is mitophagy, where PTEN-induced kinase 1 (PINK1) and Parkin phospho-ubiquitination of damaged mitochondria facilitate autophagosome sequestration and subsequent removal from the cell via lysosome fusion. Mitophagy is important for cellular homeostasis, and mutations in Parkin are linked to Parkinson's disease (PD). Due to these findings, there has been a significant emphasis on investigating mitochondrial damage and turnover to understand the molecular mechanisms and dynamics of mitochondrial quality control. Here, live-cell imaging was used to visualize the mitochondrial network of HeLa cells, to quantify the mitochondrial membrane potential and superoxide levels following treatment with carbonyl cyanide m-chlorophenyl hydrazone (CCCP), a mitochondrial uncoupling agent. In addition, a PD-linked mutation of Parkin (ParkinT240R) that inhibits Parkin-dependent mitophagy was expressed to determine how mutant expression impacts the mitochondrial network compared to cells expressing wild-type Parkin. The protocol outlined here describes a simple workflow using fluorescence-based approaches to quantify mitochondrial membrane potential and superoxide levels effectively.

Author Spotlight: Fluorescence-Based Quantification of Mitochondrial Membrane Potential and Superoxide Levels Using Live Imaging in HeLa Cells  

This technique describes an effective workflow to visualize and quantitatively measure mitochondrial membrane potential and superoxide levels within HeLa cells using fluorescence-based live imaging.

Mitochondria are dynamic organelles critical for metabolic homeostasis by controlling energy production via ATP synthesis. To support cellular metabolism, ...

Imaging of Mitochondrial Peroxide in Living Yeast Cells Using mtHyper7

Imaging of mtHyPer7, a Ratiometric Biosensor for Mitochondrial Peroxide, in Living Yeast Cells

Mitochondrial dysfunction, or functional alteration, is found in many diseases and conditions, including neurodegenerative and musculoskeletal disorders, cancer, and normal aging. Here, an approach is described to assess mitochondrial function in living yeast cells at cellular and subcellular resolutions using a genetically encoded, minimally invasive, ratiometric biosensor. The biosensor, mitochondria-targeted HyPer7 (mtHyPer7), detects hydrogen peroxide (H2O2) in mitochondria. It consists of a mitochondrial signal sequence fused to a circularly permuted fluorescent protein and the H2O2-responsive domain of a bacterial OxyR protein. The biosensor is generated and integrated into the yeast genome using a CRISPR-Cas9 marker-free system, for&#160;more consistent expression compared to plasmid-borne constructs.
mtHyPer7 is quantitatively targeted to mitochondria, has no detectable effect on yeast growth rate or mitochondrial morphology, and provides a quantitative readout for mitochondrial H2O2 under normal growth conditions and upon exposure to oxidative stress. This protocol explains how to optimize imaging conditions using a spinning-disk confocal microscope system and perform quantitative analysis using freely available software. These tools make it possible to collect rich spatiotemporal information on mitochondria both within cells and among cells in a population. Moreover, the workflow described here can be used to validate other biosensors.

Author Spotlight: Advancing Mitochondrial Research - mtHyper7 Biosensor for Subcellular Analysis 

Hydrogen peroxide (H2O2) is both a source of oxidative damage and a signaling molecule. This protocol describes how to measure mitochondrial H2O2 using mitochondria-targeted HyPer7 (mtHyPer7), a genetically encoded ratiometric biosensor, in living yeast. It details how to optimize imaging conditions and perform quantitative cellular and subcellular analysis using freely available software.

Mitochondrial dysfunction, or functional alteration, is found in many diseases and conditions, including neurodegenerative and musculoskeletal disorders, ...