The authors would like to thank Drs. Heping Cheng, Huiliang Zhang and Stephen Kolwicz for their helpful comments and technical support in developing this method. This study was supported by NIH grants and the Scientist Development Grant from American Heart Association to WW.

1. Experiment Preparation
<ol>
	<li>Prepare the isotonic balanced salt solution (50 ml) containing: 140 mM NaCl, 5 mM KCl, 2.5 mM CaCl2, 2 mM MgCl2 and 10 mM HEPES (pH 7.2) for in vivo skeletal muscle imaging.</li>
	<li>Prepare 1 L of Krebs-Henseleit buffer (KHB) containing: 118 mM NaCl, 5.3 mM KCl, 1.2 mM MgSO4, 0.5 mM EDTA and 25 mM NaHCO3.</li>
	<li>Bubble the KHB with gas containing 95% O2/5% CO2 for 10-15 min prior to the addition of 2 mM CaCl2. Add metabolic substrates (e.g.&#160;10 mM glucose and 0.5 mM pyruvate).</li>
	<li>Prepare the surgical instruments by sterilizing the clamp, scissors, and forceps in 70% ethanol and then rinsing in sterilized ddH2O.</li>
	<li>Turn on the heart perfusion system, adjust the temperature on the water bath and circulator, set the speed of peristaltic pump to 6 ml/min.</li>
	<li>Bubble the KHB with 95% O2/5% CO2 gas, and monitor the flow rate and temperature (37 &#176;C, by a fiber optic temperature probe) of the effluent. The system needs about 20 min for gas and temperature equilibration.</li>
</ol>
2. Confocal Imaging of Skeletal Muscles In Vivo
<ol>
	<li>Anesthetize mouse with pentobarbital (80 mg/kg, i.p.). The animal will reach surgical anesthesia (no response to toe pinch) within 10-15 min and remain in this status for 1-1.5 hr, sufficient for the in vivo imaging of skeletal muscles.</li>
	<li>Remove hairs on one of the hindlimbs and sterilize the skin with 70% ethanol.</li>
	<li>Make an incision on the skin along the outer side of the limb to expose the gastrocnemius muscles.</li>
	<li>Pick up the epimysium gently with a sharp forceps and make an incision through it with scissors. Further dissect to remove the epimysium and expose the muscle fibers beneath it.</li>
	<li>For in vivo loading of TMRM into the skeletal muscle, include TMRM (500 nM) in the isotonic balanced salt solution to immerse muscle for 30 min and then wash out by indicator-free solution.</li>
	<li>Put the mouse on its side on the confocal microscope (Zeiss LSM 510) stage and restrain the rear limb in a position that the exposed skeletal muscle is facing against the coverslip that forms the bottom of the chamber. The coverslip is in between the muscle tissue and the inverted objective (40X oil).</li>
	<li>Press the leg down gently to make tight contact between the tissue and the coverslip. Immerse the exposed muscles in isotonic balanced salt solution.</li>
	<li>Record two dimensional (2D, xy) confocal images at a sampling rate of one second per frame. The intensity of each pixel is digitized at 8-bit depth. Usually, a serial scan contains 100 frames.</li>
	<li>Collect sequential images by first exciting at 405 nm and collecting at &#62;505 nm then at 488 nm while collecting at &#62;505 nm for dual wavelength excitation imaging of mt-cpYFP. Use sequential excitation at 405, 488 and 543 nm, and collect emission at 505-545, 505-545, and &#62;560 nm, respectively, for triple wavelength excitation imaging of mt-cpYFP and TMRM.</li>
</ol>
3. Confocal Imaging of Perfused Mouse Heart
<ol>
	<li>Immediately after the in vivo imaging of skeletal muscles, inject the mouse with 200 units of heparin (i.p.). Ten minutes later, euthanize the mouse by thoracotomy and quickly remove the heart with lungs and thymus attached to it.</li>
	<li>Quickly remove the lung in ice-cold buffer. Identify the lobes of the thymus and gently peel back to expose the ascending aorta.</li>
	<li>Remove the thymus and isolate the aorta by carefully removing any surrounding tissue.</li>
	<li>Make a cut at the upper end of ascending aorta before the first branch of aortic arch.</li>
	<li>Hold the wall of the aorta gently with two micro suturing forceps to expose the lumen and carefully place the aorta onto the cannula (PE50 tube). The aorta is held in place with a micro vessel clamp while sutures are quickly tied around the aorta.</li>
	<li>Remove the clamp, carefully check the cannula with forceps to make sure the tip of the cannula is above aortic root. Additional ties are added as necessary to hold the heart in place.</li>
	<li>Turn on the peristaltic pump and perfuse the heart at 1 ml/min. The heart will resume beating upon perfusion.</li>
	<li>Adjust the position of the perfusion system and put the heart on the microscope stage. The stage has an adaptor that allows heating of the chamber that holds the heart.</li>
	<li>Add 1 ml of KHB perfusion solution in the chamber to partially submerge the heart. Monitor temperature of the chamber at 37 &#176;C. Remove effluent from the chamber by using a peristaltic pump.</li>
	<li>Increase the speed of peristaltic pump gradually to provide sufficient flow (approximately 2 ml/min) to the heart.</li>
	<li>After 10 min of stabilization, perfuse the heart with 10 &#956;M blebbistatin and 100-500 nM TMRM. Heart beat will slow down after 10 min.</li>
	<li>Apply gentle pressure on the heart to ensure tight contact of the heart with the coverslip at the bottom of the chamber and to further suppress heartbeat.</li>
	<li>Follow the same procedure for confocal imaging of intact myocardium as described in step 2.8 above. Carefully adjust the focal plane to reveal the clearest image possible.</li>
</ol>
4. Image Processing and Data Analysis
<ol>
	<li>Use tools provided by the &#34;Physiological Analysis&#34; module of the confocal software to analyze single mitochondrial flashes and membrane potential changes. This module is often included in the image acquiring software of the confocal system and allows determination of certain areas in the image as well as output of fluorescence intensity with time labels.</li>
	<li>Open the database and then the serial 2D image file to be analyzed.</li>
	<li>Click the &#34;Region of Interest (ROI) mean&#34; to switch to the &#34;mean of ROIs&#34; mode. Click the &#34;Region of Interest (ROI) mean&#34; to switch to the &#34;mean of ROIs&#34; mode.</li>
	<li>Turn off the display of other channels except the channel of cpYFP 488 nm for selecting the flashes.</li>
	<li>Zoom in the image and manually move slide bar to play the serial 2D images.</li>
	<li>Identify single mitochondrial superoxide flashes by locating the site where fluorescent signal increases transiently. Use the appropriate ROI tool to mark the flashes. The trace showing time-dependent fluorescence change of each ROI will show up beside the image.</li>
	<li>After selecting all the flashes, turn on the display of other channels. Select an ROI on the image outside of the cell for background signal subtraction. Output the average fluorescence of each ROI together with the time labels.</li>
	<li>Record the number of flashes in each of the serial scanning image file together with the scan time and area of the cell. Use Excel to calculate flash frequency as number of flashes per 100 sec/1,000 &#956;m2 cell area.</li>
	<li>Use a custom-developed program written in Interactive Data Language (IDL, ResearchSystems) to calculate the parameters of each flash (amplitude, time to peak and decay time). IDL software is commercially available and the custom-developed program for flash parameter analysis can be obtained from the author upon request.</li>
</ol>

<ol>
	<li>Brookes, P. S., Yoon, Y., Robotham, J. L., Anders, M. W., Calcium Sheu, 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=Calcium,+ATP,+and+ROS:+a+mitochondrial+love-hate+triangle.">Calcium, ATP, and ROS: a mitochondrial love-hate triangle.</a> Am. J. Physiol. Cell Physiol. 287, 817-833 (2004).</li><li>Scheffler, I. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Mitochondria. , (2008).</li><li>Rosca, M. G., Hoppel, C. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondria+in+heart+failure.">Mitochondria in heart failure.</a> Cardiovasc. Res. 88, 40-50 (2010).</li><li>Griffiths, E. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondria+and+heart+disease.">Mitochondria and heart disease.</a> Adv. Exp. Med. Biol. 942, 249-267 (2012).</li><li>Winklhofer, K. F., Haass, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondrial+dysfunction+in+Parkinson's+disease.">Mitochondrial dysfunction in Parkinson's disease.</a> Biochim. Biophys. Acta. 1802, 29-44 (2010).</li><li>Pieczenik, S. R., Neustadt, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondrial+dysfunction+and+molecular+pathways+of+disease.">Mitochondrial dysfunction and molecular pathways of disease.</a> Exp. Mol. Pathol. 83, 84-92 (2007).</li><li>Szentesi, P., Zaremba, R., van Mechelen, W., Stienen, G. J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=ATP+utilization+for+calcium+uptake+and+force+production+in+different+types+of+human+skeletal+muscle+fibres.">ATP utilization for calcium uptake and force production in different types of human skeletal muscle fibres.</a> J. Physiol. 531, 393-403 (2001).</li><li>Lemieux, H., Hoppel, C. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondria+in+the+human+heart.">Mitochondria in the human heart.</a> J. Bioenerg. Biomembr. 41, 99-106 (2009).</li><li>Chance, B., Williams, G. R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Respiratory+enzymes+in+oxidative+phosphorylation.+III.+The+steady+state.">Respiratory enzymes in oxidative phosphorylation. III. 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T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+dynamic+redox+changes+in+mammalian+cells+with+green+fluorescent+protein+indicators.">Imaging dynamic redox changes in mammalian cells with green fluorescent protein indicators.</a> J. Biol. Chem. 279, 22284-22293 (2004).</li><li>Hanson, G. T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Investigating+mitochondrial+redox+potential+with+redox-sensitive+green+fluorescent+protein+indicators.">Investigating mitochondrial redox potential with redox-sensitive green fluorescent protein indicators.</a> J. Biol. Chem. 279, 13044-13053 (2004).</li><li>Belousov, V. 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Chem. 288, 10567-10577 (2013).</li><li>Fang, H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+superoxide+flash+and+metabolism-coupled+mitochondrial+permeability+transition+in+living+animals.">Imaging superoxide flash and metabolism-coupled mitochondrial permeability transition in living animals.</a> Cell Res. 21, 1295-1304 (2011).</li><li>Wei, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Mitochondrial+superoxide+flashes:+metabolic+biomarkers+of+skeletal+muscle+activity+and+disease.">Mitochondrial superoxide flashes: metabolic biomarkers of skeletal muscle activity and disease.</a> FASEB J. 25, 3068-3078 (2011).</li><li>Sheu, S. S., Wang, W., Cheng, H., Dirksen, R. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superoxide+flashes:+illuminating+new+insights+into+cardiac+ischemia/reperfusion+injury.">Superoxide flashes: illuminating new insights into cardiac ischemia/reperfusion injury.</a> Future Cardiol. 4, 551-554 (2008).</li><li>Wei, L., Dirksen, R. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Perspectives+on:+SGP+symposium+on+mitochondrial+physiology+and+medicine:+mitochondrial+superoxide+flashes:+from+discovery+to+new+controversies.">Perspectives on: SGP symposium on mitochondrial physiology and medicine: mitochondrial superoxide flashes: from discovery to new controversies.</a> J. Gen. Physiol. 139, 425-434 (2012).</li><li>Johnson, I., Spence, M. T. 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Cell Cardiol. 52, 940-948 (2012).</li><li>Li, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Superoxide+flashes+reveal+novel+properties+of+mitochondrial+reactive+oxygen+species+excitability+in+cardiomyocytes.">Superoxide flashes reveal novel properties of mitochondrial reactive oxygen species excitability in cardiomyocytes.</a> Biophys. J. 102, 1011-1021 (2012).</li><li>Bell, R. M., Mocanu, M. M., Yellon, D. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Retrograde+heart+perfusion:+The+Langendorff+technique+of+isolated+heart+perfusion.">Retrograde heart perfusion: The Langendorff technique of isolated heart perfusion.</a> J. Mol. Cell. Cardiol. 50, 940-950 (2011).</li><li>Pasdois, P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effect+of+diazoxide+on+flavoprotein+oxidation+and+reactive+oxygen+species+generation+during+ischemia-reperfusion:+a+study+on+Langendorff-perfused+rat+hearts+using+optic+fibers.">Effect of diazoxide on flavoprotein oxidation and reactive oxygen species generation during ischemia-reperfusion: a study on Langendorff-perfused rat hearts using optic fibers.</a> Am. J. Physiol. 294, H2088-H2097 (2008).</li><li>Granville, D. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Reduction+of+ischemia+and+reperfusion-induced+myocardial+damage+by+cytochrome+P450+inhibitors.">Reduction of ischemia and reperfusion-induced myocardial damage by cytochrome P450 inhibitors.</a> Proc. Natl. Acad. Sci. U.S.A. 101, 1321-1326 (2004).</li><li>Davidson, S. M., Yellon, D. M., Murphy, M. P., 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=Slow+calcium+waves+and+redox+changes+precede+mitochondrial+permeability+transition+pore+opening+in+the+intact+heart+during+hypoxia+and+reoxygenation.">Slow calcium waves and redox changes precede mitochondrial permeability transition pore opening in the intact heart during hypoxia and reoxygenation.</a> Cardiovasc. Res. 93, 445-453 (2012).</li></ol>

The authors declare that they have no competing financial interests.

Imaging single mitochondrial events in live animal or perfused organs has significant advantage over traditional methods for mitochondrial function evaluation17,19,21,22,24,25. The technique described here can achieve real-time in situ determination of mitochondrial function in a real physiological condition at the subcellular resolution. This is particularly useful, when combined with other measurements, to systemically study the role of mitochondria in the normal function of a particular organ or ce...

Mitochondria play a central role in cell bioenergetics, free radical signaling, redox homeostasis, ion regulation, and cell fate determination1,2. Mitochondria dysfunction often accompanies and underlies the pathogenesis of diseases3-6. Especially in the muscle systems such as the heart and skeletal muscles, mitochondrial respiration provides the majority of ATP to support timely regulation of intracellular calcium and robust force development7,8. These muscles possess a large number of mitochondria that often occupy up to 20-40% of the total cell volume and are &#34;fixed&#34; in between myofilaments2.
Despite numerous studies, our understanding of the mitochondrial function regulation, specifically in vivo and under physiologically relevant conditions, is limited. One of the reasons is that majority of the methods developed for evaluating mitochondrial function rely on in vitro or ex vivo approaches, such as monitoring the oxygen consumption of isolated mitochondria supplemented with artificial substrates, and the indirect determination of mitochondrial function through morphology (e.g.&#160;electron microscopy), enzyme activity (e.g.&#160;aconitase activity), or intracellular ATP levels9-11.
Recently, small molecule fluorescent indicators with relative mitochondrial enrichment have been applied to provide a glimpse of the mitochondrial signals, including membrane potential, calcium and reactive oxygen species (ROS), in intact cells11-13. Moreover, several green fluorescent protein (GFP) based redox and ROS indicators have been developed to achieve more specific evaluation of the compartmentalized intracellular redox or ROS signals14-16. Among this, we developed a genetically encoded superoxide indicator, the circular permuted yellow fluorescent protein, and targeted it into mitochondria (mt-cpYFP)17. mt-cpYFP can be excited at 405 or 488 nm with both emission peaks at 515 nm. The emission at 488 nm excitation is specifically responsive to superoxide as shown by previous in vitro and in vivo calibrations17,18. The emission at 405 nm excitation is used as internal control (please refer to Figure 1 of Ref 17 for detailed information on the emission and excitation spectra of mt-cpYFP under various conditions). With time-lapse confocal imaging, this indicator detects bursting superoxide production events, named superoxide flashes, in single mitochondria of intact cells. Superoxide flash serves as a composite function of mitochondrial respiration, accompanying transient mitochondrial membrane depolarization and ROS production17-20. Recently, we have generated the pan-tissue mt-cpYFP transgenic mice using the pUC-CAGGS-mt-cpYFP vector17,19 on C57/BL6 background and verified the strong expression of this indicator in the heart, skeletal muscles and other tissues (Figure 2). The transgenic mice will be available for interested academic investigators upon request and MTA approval by the University of Washington.
In this study, we describe in situ imaging of superoxide flashes in Langendorff perfused heart as well as in vivo imaging of flash events in skeletal muscles of anesthetized mt-cpYFP transgenic mice17,19. This technology allows real time monitoring of single mitochondrial ROS production events in a physiologically relevant condition or in vivo 21,22. It is also feasible to use the system to monitor other single mitochondrial parameters such as membrane potential and calcium with appropriate fluorescent indicators. Further, simultaneous or parallel evaluation of mitochondrial function with intracellular events (e.g.&#160;calcium transients) or heart function (e.g.&#160;ejection fraction) can be achieved. Pathological perturbations, such as ischemia and reperfusion, can be applied to the perfused heart to assess the impact of stress on single mitochondrial function in the intact myocardium.

<table border="1">
	<tbody>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>REAGENTS</td>
		</tr>
		<tr>
			<td>Blebbistatin</td>
			<td>Toronto Research Chemicals</td>
			<td>B592500</td>
			<td></td>
		</tr>
		<tr>
			<td>CaCl2</td>
			<td>Acros Organics</td>
			<td>AC34961-5000</td>
			<td></td>
		</tr>
		<tr>
			<td>EDTA</td>
			<td>Fisher Scientific</td>
			<td>BP120-500</td>
			<td></td>
		</tr>
		<tr>
			<td>D-Glucose</td>
			<td>Sigma-Aldrich</td>
			<td>G8270-1</td>
			<td></td>
		</tr>
		<tr>
			<td>HEPES</td>
			<td>Sigma-Aldrich</td>
			<td>H7006-500</td>
			<td></td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>Sigma-Aldrich</td>
			<td>P9541-1</td>
			<td></td>
		</tr>
		<tr>
			<td>MgCl2&#8226;6H2O</td>
			<td>Fisher Scientific</td>
			<td>BP214-500</td>
			<td></td>
		</tr>
		<tr>
			<td>MgSO4&#8226;7H2O</td>
			<td>Sigma-Aldrich</td>
			<td>M1880-1</td>
			<td></td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>Fisher Scientific</td>
			<td>BP358-212</td>
			<td></td>
		</tr>
		<tr>
			<td>NaH2PO4</td>
			<td>Sigma-Aldrich</td>
			<td>S8282-500</td>
			<td></td>
		</tr>
		<tr>
			<td>NaHCO3</td>
			<td>Sigma-Aldrich</td>
			<td>S6014-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Pyruvate</td>
			<td>Sigma-Aldrich</td>
			<td>P2256-25</td>
			<td></td>
		</tr>
		<tr>
			<td>TMRM</td>
			<td>Invitrogen</td>
			<td>T-668</td>
			<td></td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>[header]</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>EQUIPMENT</td>
		</tr>
		<tr>
			<td></td>
			<td></td>
			<td></td>
			<td>Confocal Line Scanning Microscope (LSM 510 Meta, Zeiss), software version 4.2 SP1 including &#34;Physiological Analysis&#34; module.</td>
		</tr>
	</tbody>
</table>

confocal imaging of single mitochondrial superoxide flashes in intact heart or in vivo

Mitochondrion is a critical intracellular organelle responsible for energy production and intracellular signaling in eukaryotic systems. Mitochondrial dysfunction often accompanies and contributes to human disease. Majority of the approaches that have been developed to evaluate mitochondrial function and dysfunction are based on in vitro or ex vivo measurements. Results from these experiments have limited ability in determining mitochondrial function in vivo. Here, we describe a novel approach that utilizes confocal scanning microscopy for the imaging of intact tissues in live aminals, which allows the evaluation of single mitochondrial function in a real-time manner in vivo. First, we generate transgenic mice expressing the mitochondrial targeted superoxide indicator, circularly permuted yellow fluorescent protein (mt-cpYFP). Anesthetized mt-cpYFP mouse is fixed on a custom-made stage adaptor and time-lapse images are taken from the exposed skeletal muscles of the hindlimb. The mouse is subsequently sacrificed and the heart is set up for Langendorff perfusion with physiological solutions at 37 &deg;C. The perfused heart is positioned in a special chamber on the confocal microscope stage and gentle pressure is applied to immobilize the heart and suppress heart beat induced motion artifact. Superoxide flashes are detected by real-time 2D confocal imaging at a frequency of one frame per second. The perfusion solution can be modified to contain different respiration substrates or other fluorescent indicators. The perfusion can also be adjusted to produce disease models such as ischemia and reperfusion. This technique is a unique approach for determining the function of single mitochondrion in intact tissues and in vivo.

According to this protocol, in vivo imaging of single mitochondrial events can be done in skeletal muscles of anesthetized mice followed by in situ imaging in perfused heart (Figure 1). The optimal setting of the imaging conditions will ensure clear images of the intact muscle tissues and with single mitochondrion resolution (Figure 2). TMRM is often used to verify the location of mt-cpYFP and should show a complete overlapping pattern with the mt-cpYFP signal (...

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Confocal Imaging of Single Mitochondrial Superoxide Flashes in Intact Heart or In Vivo

Confocal scanning microscopy is applied for imaging single mitochondrial events in perfused heart or skeletal muscles in live animal. Real-time monitoring of single mitochondrial processes such as superoxide flashes and membrane potential fluctuations enables the evaluation of mitochondrial function in a physiologically relevant context and during pathological perturbations.

Confocal Imaging of Single Mitochondrial Superoxide Flashes in Intact Heart or In Vivo

Mitochondrion is a critical intracellular organelle responsible for energy production and intracellular signaling in eukaryotic systems. Mitochondrial ...

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14.5K Views.  University of Washington. Confocal scanning microscopy is applied for imaging single mitochondrial events in perfused heart or skeletal muscles in live animal. Real-time monitoring of single mitochondrial processes such as superoxide flashes and membrane potential fluctuations enables the evaluation of mitochondrial function in a physiologically relevant context and during pathological perturbations.

Video: Confocal Imaging of Single Mitochondrial Superoxide Flashes in Intact Heart or In Vivo

Single Cell Electroporation in vivo within the Intact Developing Brain

Single-cell electroporation (SCE) is a specialized technique allowing the delivery of DNA or other macromolecules into individual cells within intact tissue, including in vivo preparations. The distinct advantage of this technique is that experimental manipulations may be performed on individual cells while leaving the surrounding tissue unaltered, thereby distinguishing cell-autonomous effects from those resulting from global treatments. When combined with advanced in vivo imaging techniques, SCE of fluorescent markers permits direct visualization of cellular morphology, cell growth, and intracellular events over timescales ranging from seconds to days. While this technique is used in a variety of in vivo and ex vivo preparations, we have optimized this technique for use in Xenopus laevis tadpoles. In this video article, we detail the procedure for SCE of a fluorescent dye or plasmid DNA into neurons within the intact brain of the albino Xenopus tadpole. We also discuss methods to optimize yield, and show examples of live two-photon fluorescence imaging of neurons fluorescently labeled by SCE.

Single-cell electroporation (SCE) is a specialized technique allowing delivery of DNA or other macromolecules into individual cells within intact tissue, including in vivo preparations. Here we detail the procedure for SCE of a fluorescent dye or plasmid DNA into neurons within the intact brain of the Xenopus laevis tadpole.

Single-cell electroporation (SCE) is a specialized technique allowing the delivery of DNA or other macromolecules into individual cells within intact ...

In vivo Imaging of Intact Drosophila Larvae at Sub-cellular Resolution

Recent improvements in optical imaging, genetically encoded fluorophores and genetic tools allowing efficient establishment of desired transgenic animal lines have enabled biological processes to be studied in the context of a living, and in some instances even behaving, organism. In this protocol we will describe how to anesthetize intact Drosophila larvae, using the volatile anesthetic desflurane, to follow the development and plasticity of synaptic populations at sub-cellular resolution1-3. While other useful methods to anesthetize Drosophila melanogaster larvae have been previously described4,5,6,7,8, the protocol presented herein demonstrates significant improvements due to the following combined key features: (1) A very high degree of anesthetization; even the heart beat is arrested allowing for lateral resolution of up to 150 nm1, (2) a high survival rate of &gt; 90% per anesthetization cycle, permitting the recording of more than five time-points over a period of hours to days2 and (3) a high sensitivity enabling us in 2 instances to study the dynamics of proteins expressed at physiological levels. In detail, we were able to visualize the postsynaptic glutamate receptor subunit GluR-IIA expressed via the endogenous promoter1 in stable transgenic lines and the exon trap line FasII-GFP1. (4) In contrast to other methods4,7 the larvae can be imaged not only alive, but also intact (i.e. non-dissected) allowing observation to occur over a number of days1. The accompanying video details the function of individual parts of the in vivo imaging chamber2,3, the correct mounting of the larvae, the anesthetization procedure, how to re-identify specific positions within a larva and the safe removal of the larvae from the imaging chamber.

In vivo Imaging of Intact Drosophila Larvae at Sub-cellular Resolution

This protocol describes a reliable method for anesthetization and imaging of intact Drosophila melanogaster larvae. We have utilized the volatile anesthetic desflurane to allow for repetitive imaging at sub-cellular resolution and re-identification of structures for up to a few days1.

Recent improvements in optical imaging, genetically encoded fluorophores and genetic tools allowing efficient establishment of desired transgenic animal ...

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 ...

Assessment of Calcium Sparks in Intact Skeletal Muscle Fibers

Maintaining homeostatic Ca2+ signaling is a fundamental physiological process in living cells. Ca2+ sparks are the elementary units of Ca2+ signaling in the striated muscle fibers that appear as highly localized Ca2+ release events mediated by ryanodine receptor (RyR) Ca2+ release channels on the sarcoplasmic reticulum (SR) membrane. Proper assessment of muscle Ca2+ sparks could provide information on the intracellular Ca2+&#160;handling properties of healthy and diseased striated muscles. Although Ca2+ sparks events are commonly seen in resting cardiomyocytes, they are rarely observed in resting skeletal muscle fibers; thus there is a need for methods to generate and analyze sparks in skeletal muscle fibers.
Detailed here is an experimental protocol for measuring Ca2+ sparks in isolated flexor digitorm brevis (FDB) muscle fibers using fluorescent Ca2+ indictors and laser scanning confocal microscopy. In this approach, isolated FDB fibers are exposed to transient hypoosmotic stress followed by a return to isotonic physiological solution. Under these conditions, a robust Ca2+ sparks response is detected adjacent to the sarcolemmal membrane in young healthy FDB muscle fibers. Altered Ca2+ sparks response is detected in dystrophic or aged skeletal muscle fibers. This approach has recently demonstrated that membrane-delimited signaling involving cross-talk between inositol (1,4,5)-triphosphate receptor (IP3R) and RyR contributes to Ca2+ spark activation in skeletal muscle. In summary, our studies using osmotic stress induced Ca2+ sparks showed that this intracellular response reflects a muscle signaling mechanism in physiology and aging/disease states, including mouse models of muscle dystrophy (mdx mice) or amyotrophic lateral sclerosis (ALS model).

Described here is a method to directly measure calcium sparks, the elementary units of Ca2+ release from sarcoplasmic reticulum&#160;in intact skeletal muscle fibers. This method utilizes osmotic-stress-mediated triggering of Ca2+ release from ryanodine receptor in isolated muscle fibers. The dynamics and homeostatic capacity of intracellular Ca2+ signaling can be employed to assess muscle function in health and disease.

Maintaining homeostatic Ca2+ signaling is a fundamental physiological process in living cells. Ca2+ sparks are the elementary units of Ca2+ signaling in ...

En Face Detection of Nitric Oxide and Superoxide in Endothelial Layer of Intact Arteries

Endothelium-derived nitric oxide (NO) produced from endothelial NO-synthase (eNOS) is one of the most important vasoprotective molecules in cardiovascular physiology. Dysfunctional eNOS such as uncoupling of eNOS leads to decrease in NO bioavailability and increase in superoxide anion (O2.&#8722;) production, and in turn promotes cardiovascular diseases. Therefore, appropriate measurement of NO and O2.&#8722; levels in the endothelial cells are pivotal for research on cardiovascular diseases and complications. Because of the extremely labile nature of NO and O2.&#8722;, it is difficult to measure NO and O2.&#8722; directly in a blood vessel. Numerous methods have been developed to measure NO and O2.&#8722; production. It is, however, either insensitive, or non-specific, or technically demanding and requires special equipment. Here we describe an adaption of the fluorescence dye method for en face simultaneous detection and visualization of intracellular NO and O2.&#8722; using the cell permeable diaminofluorescein-2 diacetate (DAF-2DA) and dihydroethidium (DHE), respectively, in intact aortas of an obesity mouse model induced by high-fat-diet feeding. We could demonstrate decreased intracellular NO and enhanced O2.&#8722; levels in the freshly isolated intact aortas of obesity mouse as compared to the control lean mouse. We demonstrate that this method is an easy technique for direct detection and visualization of NO and O2.&#8722; in the intact blood vessels and can be widely applied for investigation of endothelial (dys)function under (physio)pathological conditions.

En Face Detection of Nitric Oxide and Superoxide in Endothelial Layer of Intact Arteries

In this article we describe an adapted relatively easy method using the fluorescence dye diaminofluorescein-2 diacetate (DAF-2DA) and dihydroethidium (DHE) for en face simultaneous detection and visualization of intracellular nitric oxide (NO) and superoxide anion (O2.&#8722;) respectively, in freshly isolated intact aortas of an obesity mouse model.

Endothelium-derived nitric oxide (NO) produced from endothelial NO-synthase (eNOS) is one of the most important vasoprotective molecules in cardiovascular ...

High-resolution Respirometry to Assess Mitochondrial Function in Permeabilized and Intact Cells

A high-resolution oxygraph is a device for measuring cellular oxygen consumption in a closed-chamber system with very high resolution and sensitivity in biological samples (intact and permeabilized cells, tissues or isolated mitochondria). The high-resolution oxygraph device is equipped with two chambers and uses polarographic oxygen sensors to measure oxygen concentration and calculate oxygen consumption within each chamber. Oxygen consumption rates are calculated using software and expressed as picomoles per second per number of cells. Each high-resolution oxygraph chamber contains a stopper with injection ports, which makes it ideal for substrate-uncoupler-inhibitor titrations or detergent titration protocols for determining effective and optimum concentrations for plasma membrane permeabilization. The technique can be applied to measure respiration in a wide range of cell types and also provides information on mitochondrial quality and integrity, and maximal mitochondrial respiratory electron transport system capacity.

High-resolution respirometry is used to determine mitochondrial oxygen consumption. This is a straightforward technique to determine mitochondrial respiratory chain complexes&#39; (I-IV) respiratory rates, maximal mitochondrial electron transport system capacity, and mitochondrial outer membrane integrity.

A high-resolution oxygraph is a device for measuring cellular oxygen consumption in a closed-chamber system with very high resolution and sensitivity in ...

Drosophila Preparation and Longitudinal Imaging of Heart Function In Vivo Using Optical Coherence Microscopy (OCM)

Longitudinal study of the heartbeat in small animals contributes to understanding structural and functional changes during heart development. Optical coherence microscopy (OCM) has been demonstrated to be capable of imaging small animal hearts with high spatial resolution and ultrahigh imaging speed. The high image contrast and noninvasive properties make OCM ideal for performing longitudinal studies without requiring tissue dissections or staining. Drosophila has been widely used as a model organism in cardiac developmental studies due to its high number of orthologous human disease genes, its similarity of molecular mechanisms and genetic pathways with vertebrates, its short life cycle, and its low culture cost. Here, the experimental protocols are described for the preparation of Drosophila and optical imaging of the heartbeat with a custom OCM system throughout the life cycle of the specimen. By following the steps provided in this report, transverse M-mode and 3D OCM images can be acquired to conduct longitudinal studies of the Drosophila cardiac morphology and function. The en face and axial sectional OCM images and the heart rate (HR) and cardiac activity period (CAP) histograms, were also shown to analyze the heart structural changes and to quantify the heart dynamics during Drosophila metamorphosis, combined with the videos constructed with M-mode images to trace cardiac activity intuitively. Due to the genetic similarity between Drosophila and vertebrates, longitudinal study of heart morphology and dynamics in fruit flies could help reveal the origins of human heart diseases. The protocol here would provide an effective method to perform a wide range of studies to understand the mechanisms of cardiac diseases in humans.

Drosophila Preparation and Longitudinal Imaging of Heart Function In Vivo Using Optical Coherence Microscopy OCM

Here, the experimental protocols are described for preparing Drosophila at different developmental stages and performing longitudinal optical imaging of Drosophila heartbeats using a custom optical coherence microscopy (OCM) system. The cardiac morphological and dynamical changes can be quantitatively characterized by analyzing the heart structural and functional parameters from OCM images.

Longitudinal study of the heartbeat in small animals contributes to understanding structural and functional changes during heart development. Optical ...

Confocal Imaging of Neuropeptide Y-pHluorin: A Technique to Visualize Insulin Granule Exocytosis in Intact Murine and Human Islets

Insulin secretion plays a central role in glucose homeostasis under normal physiological conditions as well as in disease. Current approaches to study insulin granule exocytosis either use electrophysiology or microscopy coupled to the expression of fluorescent reporters. However most of these techniques have been optimized for clonal cell lines or require dissociating pancreatic islets. In contrast, the method presented here allows for real time visualization of insulin granule exocytosis in intact pancreatic islets. In this protocol, we first describe the viral infection of isolated pancreatic islets with adenovirus that encodes a pH-sensitive green fluorescent protein (GFP), pHluorin, coupled to neuropeptide Y (NPY). Second, we describe the confocal imaging of islets five days after viral infection and how to monitor the insulin granule secretion. Briefly, the infected islets are placed on a coverslip on an imaging chamber and imaged under an upright laser-scanning confocal microscope while being continuously perfused with extracellular solution containing various stimuli. Confocal images spanning 50 &#181;m of the islet are acquired as time-lapse recordings using a fast-resonant scanner. The fusion of insulin granules with the plasma membrane can be followed over time. This procedure also allows for testing a battery of stimuli in a single experiment, is compatible with both mouse and human islets, and can be combined with various dyes for functional imaging (e.g., membrane potential or cytosolic calcium dyes).

We describe a protocol for visualization of insulin exocytosis in intact islets using pHluorin, a pH-sensitive green fluorescent protein. Isolated islets are infected with adenovirus encoding pHluorin coupled to the vesicle cargo neuropeptide Y. This allows for the detection of insulin granule fusion events by confocal microscopy.

Insulin secretion plays a central role in glucose homeostasis under normal physiological conditions as well as in disease. Current approaches to study ...

Single-Cell Resolution Three-Dimensional Imaging of Intact Organoids

Organoid technology, in vitro 3D culturing of miniature tissue, has opened a new experimental window for cellular processes that govern organ development and function as well as disease. Fluorescence microscopy has played a major role in characterizing their cellular composition in detail and demonstrating their similarity to the tissue they originate from. In this article, we present a comprehensive protocol for high-resolution 3D imaging of whole organoids upon immunofluorescent labeling. This method is widely applicable for imaging of organoids differing in origin, size and shape. Thus far we have applied the method to airway, colon, kidney, and liver organoids derived from healthy human tissue, as well as human breast tumor organoids and mouse mammary gland organoids. We use an optical clearing agent, FUnGI, which enables the acquisition of whole 3D organoids with the opportunity for single-cell quantification of markers. This three-day protocol from organoid harvesting to image analysis is optimized for 3D imaging using confocal microscopy.

The entire 3D structure and cellular content of organoids, as well as their phenotypic resemblance to the original tissue can be captured using the single-cell resolution 3D imaging protocol described here. This protocol can be applied to a wide range of organoids varying in origin, size and shape.

Organoid technology, in vitro 3D culturing of miniature tissue, has opened a new experimental window for cellular processes that govern organ development ...

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, ...