We thank Dr. Gilles Dupuis for critical reading of the manuscript. 
Funding
This work was supported by the Canadian Institutes for Health Research, grant # MOP-126173. C.M.D is a member of the Fonds de la Recherche en Sant&#233; (FRSQ)-funded Centre de Recherche Clinique &#201;tienne LeBel.

1. Preparation of Solutions for Cellular pH Calibration
<ol>
	<li>Add the following compounds to five separate 50 ml conical tubes to prepare 5 solutions to be used for calibration:
	<ul>
		<li>NaCl (1 ml of 1 M NaCl) (1 M NaCl = 0.58 g in 10 ml H20)</li>
		<li>KCl (6.75 ml of 1 M KCl) (1 M KCl = 0.75 g in 10 ml H20)</li>
		<li>Glucose (1 ml of 1 M D-Glucose) ( 1M D-Glucose = 1.80 g in 10 ml H20)</li>
		<li>MgS04 (0.05 ml of 1 M MgSO4) (1 M MgSO4= 1.20 g in 10 ml H20)</li>
		<li>20 mM HEPES (1 ml of 1M HEPES) (1 M HEPES = 2.38 g in 10 ml H20)</li>
		<li>CaCl2 (0.05 ml of 1M CaCl2) (1 M CaCl2= 1.47 g in 10 ml H20) 
		Prepare all solutions in double distilled H2O (ddH2O).</li>
	</ul>
	</li>
	<li>Complete each solution with 35 ml of ddH2O and mix with a magnetic stirrer until solution is achieved.</li>
	<li>Adjust the pH of each solution with 1 N KOH or 1 N HCl to obtain pH values of 5.5, 6.0, 6.5, 7.0 and 7.5. Then, adjust the volume with deionized water to a final volume of 50 ml.</li>
	<li>Measure 5 mg of nigericin and add it to 670 &#956;l of absolute EtOH to prepare a 10 mM nigericin stock solution. 
	Caution: nigericin is highly toxic and must be handled with great care (gloves, safety glasses and mask). Mix the solution by inversion until the nigericin is completely dissolved.</li>
	<li>Add 0.05 ml of 10 mM nigericin (final concentration 10 &#956;M) to each calibration solution. Nigericin is an ionophore that increases cell membrane permeability and causes equilibration of intracellular and extracellular pH in the presence of a depolarizing concentration of extracellular K+.</li>
	<li>Calibration solutions can be stored up to 1 month at 4 &#176;C</li>
</ol>
2. Cell Preparation
<ol>
	<li>Under a biological safety hood, seed cells in 35 mm x 10 mm culture dishes in 2 ml of growth medium supplemented with 10% fetal bovine serum and antibiotics. Use the amount of cells needed to ensure approximately 50% confluence after 24 hr. 
	NOTE: In our experiments, we use the HT-1080 cell line and plate 1 x 105 cells per culture dish.</li>
	<li>Determine the number of culture dishes needed for the experiment and plate 5 additional culture dishes to be used for calibration.</li>
	<li>Place culture dishes in a 37 &#176;C/5% CO2 incubator for at least 24 hr.</li>
</ol>
3. Preparation of pH-sensing Probes
<ol>
	<li>Prepare a 1M stock solution of the fluorescent pH probe 8-hydroxypyrene-1,3,6 trisulfonic acid, trisodium salt (HPTS/pyranine) in ddH20. This solution must be stored at RT and protected from light.</li>
	<li>Prepare a stock solution of 1 mM 5-(and-6)-carboxyl seminaphthorhodafluor acetoxymethyl ester acetate (C-SNARF-1 AM; called thereafter SNARF-1) by dissolving 1 mg of SNARF-1 in 1.76 ml of DMSO. Thoroughly mix by repeated pipetting. SNARF-1 solution must be stored at -20 &#176;C and protected from light.</li>
</ol>
4. Cell Labeling
<ol>
	<li>Sixteen hours before live-cell imaging, add 2 &#956;l of 1M HPTS stock solution (final concentration 1 mM) to each culture dish containing 2 ml of complete medium.</li>
	<li>Return the culture dishes to the&#160;incubator. The HPTS probe is cell-impermeant, therefore to label organelles the probe must be taken up by pinocytosis. This condition requires that cell incubations with HPTS are performed in serum-supplemented culture medium.</li>
	<li>Following incubation with HPTS, wash the cells twice with sterile PBS and add 2 ml of serum-free medium for at least 2 hr. Incubation with SNARF-1 needs to be performed in serum-free medium because serum may prevent the uptake of the probe.</li>
	<li>Add 10 &#956;l of a 1 mM SNARF-1 stock solution to obtain a final concentration of 5 &#956;M and allow cells to incubate for 20 min at 37&#176; C, under dim light. C-SNARF-1 concentration and time of incubation may vary according to cell type.</li>
	<li>At the end of incubation, wash cells twice in sterile PBS and add 2 ml of serum-free medium.</li>
</ol>
5. Addition of pH Modulators
<ol>
	<li>To assess whether the method can detect pH changes in cells, different reagents that modulate intracellular pH can be used:
	<ul>
		<li>Bafilomycin A1, a specific inhibitor of V-ATPase5</li>
		<li>Na+/H+ exchanger isoform 1 (NHE-1)6</li>
		<li>NH4Cl, a weak base that increases intracellular pH7</li>
	</ul>
	<ol>
		<li>Prepare stock solutions of the reagents mentioned above:
		<ul>
			<li>Bafilomycin A1: add 10 &#956;g of bafilomycin A1 to 160 &#956;l of DMSO to obtain a stock solution of 100 &#956;M</li>
			<li>EIPA: add 25 mg of EIPA to 1.67 ml of methanol to obtain a final concentration of 50 mM</li>
			<li>NH4Cl: dissolve 0.53 g of ammonium chloride in 10 ml of ddH2O to obtain a final concentration of 1 mM.</li>
		</ul>
		 
		Note: All stock solutions must be kept at 4 &#176;C except for bafilomycin A1 which must be stored at -20 &#176;C and protected from light.</li>
		<li>In separate culture dishes, add one of the reagents as described below:
		<ul>
			<li>2 &#956;l of 100 &#956;M bafilomycin A1 (final concentration 100 nM)</li>
			<li>1 &#956;l of 50 mM EIPA (final concentration 25 &#956;M)</li>
			<li>40 &#956;l of 1M NH4Cl (final concentration 20 mM)</li>
		</ul>
		</li>
		<li>Incubate the cells for 30 min at 37 &#176;C.</li>
	</ol>
	</li>
	<li>Proceed to image acquisition.</li>
</ol>
6. Specifications of the Live-cell Imaging Microscope and Acquisition Settings
<ol>
	<li>Perform live-cell imaging using a 40X objective mounted on a spectral inverted scanning confocal microscope equipped with a motorized XY stage, a chamber that accommodates glass bottom culture dishes in an environmentally controlled (temperature, humidity, CO2) stage incubator.</li>
	<li>The confocal instrument (Olympus FV1000) used here is equipped with:
	<ul>
		<li>A diode laser 405 nm</li>
		<li>A 40 mW argon laser (458 nm, 488 nm, 515 nm)</li>
		<li>A helium neon laser (543 nm)</li>
		<li>Fluorite 10X objective, NA0.4</li>
		<li>A LUCPLFLN 40X objective, NA0.60</li>
		<li>A PLAPONSC PLAN APO 60X oil objective, NA1.4</li>
		<li>A dichroic mirror DM 405/458/488/543</li>
		<li>A SDM 560 and a SDM 640 dichroic filters</li>
		<li>A BA655-755 barrier filter</li>
		<li>A beam splitter BS20/80</li>
		<li>Data analysis software for image acquisition and analysis,</li>
	</ul>
	</li>
	<li>Once the stage incubator has warmed up to 37 &#176;C and the microscope is set up, place a culture dish previously incubated with HPTS and SNARF-1 into the confocal stage imaging chamber.</li>
	<li>After locating an area of interest, adjust the software acquisition settings Figure 1.
	<ul>
		<li>Set up two virtual channel sets.</li>
		<li>Initiate the first phase that corresponds to HPTS excitation at 405 nm and emission collection at 505 - 525 nm and SNARF-1 excitation at 543 nm and emission collection at 570 - 600 nm.</li>
		<li>Initiate the second phase that corresponds to HPTS excitation at 458 nm and emission collection at 505 - 525 nm and SNARF-1 excitation at 543 nm and emission collection at 640 - 650 nm.</li>
		<li>The confocal aperture is set to 175 &#956;m by default and optimal aperture needs to be manually set at 300 &#956;m.</li>
		<li>Take Image by stacking serial horizontal optical sections (800 x 800 pixels) of 0.4 &#956;m thickness.</li>
	</ul>
	</li>
</ol>
NOTE: A 40X objective is useful to observe several cells simultaneously and works well for organelle viewing. However, spatial resolution and signal intensity are better achieved using a 60X oil objective. In addition, there is no excitation or emission cross-talk between the two dyes.
7. Calibration of pH-sensing Probes
<ol>
	<li>Before calibration, remove medium from the culture dish previously incubated with HPTS and SNARF-1, wash twice with sterile PBS and then add 2 ml of one of the five calibration solutions.</li>
	<li>Place the culture dish into the confocal stage chamber.</li>
	<li>Proceed to time-lapse image acquisition according to the following settings:
	<ol>
		<li>Set the number of images and interval duration times for the experiment. 
		NOTE: We typically use a 1 min interval between images for up to 30 min.</li>
		<li>Program the shutter to close between the acquisition of each image.</li>
	</ol>
	</li>
	<li>Record the fluorescence intensity per min in a spreadsheet software and determine the time when fluorescence intensity is steady. Intracellular pH is usually stable after 15 - 20 min.</li>
	<li>Repeat steps 7.1 to 7.4 using a new culture dish for each remaining calibration solution.</li>
</ol>
8. Data Acquisition
<ol>
	<li>Once calibration has been done, acquire sample data from 4-5 randomly chosen fields containing 15-20 cells per field. Two images are obtained for each field: one for HPTS labeling (vesicles) and the second for SNARF-1 labeling (cytosol).</li>
	<li>Background fluorescence (extracellular area) is digitally subtracted from fluorescent images of both probes. Labeled vesicles and cytosol from their respective stored images are selected using a binary mask. For each pH-sensing probe, fluorescence intensities are measured on a numerical scale (0-4,095 gray level scale). A mean intensity for all pixels in the organelle or cytoplasm is calculated and values are used to calculate fluorescence ratios.</li>
</ol>
9. Data Analysis
<ol>
	<li>Various software packages are commercially available for data analysis. i.e.&#160;Fluoview Software.</li>
	<li>In a spreadsheet, record values and calculate fluorescence ratios as follows: In the case of HPTS, R = (F458 nm/F405 nm) and, in the case of SNARF-1, R = (F644 nm/F584 nm).</li>
	<li>Plot the data using scientific graphing software such as GraphPad Prism. The plot should portray fluorescence ratios (Y-axis) as a function of pH (X-axis). Curve fitting is achieved using non-linear regression.</li>
	<li>Collect fluorescence intensities of the samples as described in step 8.2 and calculate fluorescence ratios. Use calibration curves to transform ratios into pH values.</li>
</ol>

<ol>
	<li>Demaurex, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=pH+Homeostasis+of+cellular+organelles.">pH Homeostasis of cellular organelles.</a> News Physiol Sci. 17, 1-5 (2002).</li><li>Webb, B. A., Chimenti, M., Jacobson, M. P., Barber, D. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dysregulated+pH:+a+perfect+storm+for+cancer+progression.">Dysregulated pH: a perfect storm for cancer progression.</a> Nat Rev Cancer. 11, 671-677 (2011).</li><li>Hinton, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Function+of+a+subunit+isoforms+of+the+V-ATPase+in+pH+homeostasis+and+in+vitro+invasion+of+MDA-MB231+human+breast+cancer+cells.">Function of a subunit isoforms of the V-ATPase in pH homeostasis and in vitro invasion of MDA-MB231 human breast cancer cells.</a> J Biol Chem. 284, 16400-16408 (2009).</li><li>Overly, C. C., Lee, K. D., Berthiaume, E., Hollenbeck, P. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+measurement+of+intraorganelle+pH+in+the+endosomal-lysosomal+pathway+in+neurons+by+using+ratiometric+imaging+with+pyranine.">Quantitative measurement of intraorganelle pH in the endosomal-lysosomal pathway in neurons by using ratiometric imaging with pyranine.</a> Proc Natl Acad Sci U S A. 92, 3156-3160 (1995).</li><li>Drose, S., Altendorf, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bafilomycins+and+concanamycins+as+inhibitors+of+V-ATPases+and+P-ATPases.">Bafilomycins and concanamycins as inhibitors of V-ATPases and P-ATPases.</a> J Exp Biol. 200, 1-8 (1997).</li><li>Pedersen, S. F., King, S. A., Nygaard, E. B., Rigor, R. R., Cala, P. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=NHE1+inhibition+by+amiloride-+and+benzoylguanidine-type+compounds.+Inhibitor+binding+loci+deduced+from+chimeras+of+NHE1+homologues+with+endogenous+differences+in+inhibitor+sensitivity.">NHE1 inhibition by amiloride- and benzoylguanidine-type compounds. Inhibitor binding loci deduced from chimeras of NHE1 homologues with endogenous differences in inhibitor sensitivity.</a> J Biol Chem. 282, 19716-19727 (2007).</li><li>Alfonso, A., Cabado, A. G., Vieytes, M. R., Botana, L. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calcium-pH+crosstalks+in+rat+mast+cells:+cytosolic+alkalinization,+but+not+intracellular+calcium+release,+is+a+sufficient+signal+for+degranulation.">Calcium-pH crosstalks in rat mast cells: cytosolic alkalinization, but not intracellular calcium release, is a sufficient signal for degranulation.</a> Br J Pharmacol. 130, 1809-1816 (2000).</li><li>Han, J., Burgess, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fluorescent+indicators+for+intracellular+pH.">Fluorescent indicators for intracellular pH.</a> Chem Rev. 110, 2709-2728 (2010).</li><li>Giuliano, K. A., Gillies, R. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Determination+of+intracellular+pH+of+BALB/c-3T3+cells+using+the+fluorescence+of+pyranine.">Determination of intracellular pH of BALB/c-3T3 cells using the fluorescence of pyranine.</a> Anal Biochem. 167, 362-371 (1987).</li><li>Willoughby, D., Thomas, R., Schwiening, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+effects+of+intracellular+pH+changes+on+resting+cytosolic+calcium+in+voltage-clamped+snail+neurones.">The effects of intracellular pH changes on resting cytosolic calcium in voltage-clamped snail neurones.</a> J Physiol. 530, 405-416 (2001).</li></ol>

No conflict of interest declared.

Quantitative pH measurement in live-cell imaging of different cellular compartments is needed to accurately measure pH variations in cell responses to external challenges. However, one of the major hurdles that remains is to easily and specifically target cellular compartments. In this respect, a few studies have highlighted the use of HPTS as an intracellular pH indicator introduced into cells by electroporation or microinjection8-10. These techniques suffer from serious drawbacks, namely electroporation caus...

<table border="0" cellpadding="0" cellspacing="0" width="872">
	<tbody>
		<tr height="20">
			<td colspan="4" height="20" style="height:20px;width:872px;">Solutions for calibration</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Sodium chloride</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">L-11621</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Potassium chloride</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">M-12445</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">D-Glucose (dextrose)</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">D16-1</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Magnesium sulfate</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">M65-500</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">HEPES</td>
			<td style="width:136px;">Sigma-Aldrich</td>
			<td style="width:119px;">H3375</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Calcium chloride dihydrate</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">M-1612</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Deionized water</td>
			<td style="width:136px;">N/A</td>
			<td style="width:119px;">N/A</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Nigericin</td>
			<td style="width:136px;">Calbiochem</td>
			<td align="right" style="width:119px;">481990</td>
			<td>Toxic by inhalation or in contact with skin</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;"></td>
			<td style="width:136px;"></td>
			<td style="width:119px;"></td>
			<td style="width:315px;">Keep protected from the light at 4&#176;C</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;"></td>
			<td style="width:136px;"></td>
			<td style="width:119px;"></td>
			<td style="width:315px;"></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Culture dishes &#8709; 35mm</td>
			<td style="width:136px;">BD Biosciences</td>
			<td align="right" style="width:119px;">354456</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;"></td>
			<td style="width:136px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td colspan="4" height="20" style="height:20px;width:872px;">pH-sensing probes</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;width:303px;">8-Hydroxypyrene-1,3,6-trisulfonic acid (HPTS)</td>
			<td style="width:136px;">Life technologies</td>
			<td style="width:119px;">H-348</td>
			<td>Keep away from the light at room temperature</td>
		</tr>
		<tr height="40">
			<td height="40" style="height:40px;width:303px;">5-(and-6)-Carboxy SNARF&#174;-1, Acetoxymethyl Ester, Acetate</td>
			<td style="width:136px;">Life technologies</td>
			<td style="width:119px;">C-1271</td>
			<td>Keep away from the light and humidity at -20&#176;C</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;"></td>
			<td style="width:136px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Dimethyl Sufoxide (DMSO)</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">BP231-1&#160;</td>
			<td>Harmful</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;"></td>
			<td style="width:136px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td colspan="4" height="20" style="height:20px;width:872px;">Phosphate buffered saline (PBS) 100X</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Potassium chloride</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">M-12445</td>
			<td>20 g</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Sodium phosphate monobasic</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">S369-1</td>
			<td>115 g</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Potassium phosphate monobasic</td>
			<td style="width:136px;">J.T Baker</td>
			<td style="width:119px;">3246-01</td>
			<td>20 g</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Deionized water</td>
			<td style="width:136px;">N/A</td>
			<td style="width:119px;">N/A</td>
			<td>Bring to 1 L</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Autoclave and adjust pH to 7.4</td>
			<td style="width:136px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;"></td>
			<td style="width:136px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td colspan="4" height="20" style="height:20px;width:872px;">Phosphate buffered saline (PBS) 1X</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">PBS 100X</td>
			<td style="width:136px;">N/A</td>
			<td style="width:119px;">N/A</td>
			<td>50 mL</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Sodium chloride</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">L-11621</td>
			<td>40,5 g</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Deoinized water</td>
			<td style="width:136px;">N/A</td>
			<td style="width:119px;">N/A</td>
			<td>Bring to 5 L</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;"></td>
			<td style="width:136px;"></td>
			<td style="width:119px;"></td>
			<td></td>
		</tr>
		<tr height="20">
			<td colspan="4" height="20" style="height:20px;width:872px;">pH modulators</td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">&#160;Bafilomycin A1</td>
			<td style="width:136px;">Sigma-Aldrich</td>
			<td style="width:119px;">B-1793</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;">5-(N-Ethyl-N-isopropyl)amiloride (EIPA)</td>
			<td style="width:136px;">Sigma-Aldrich</td>
			<td style="width:119px;">A3085</td>
			<td></td>
		</tr>
		<tr height="20">
			<td height="20" style="height:20px;width:303px;">Ammonium chloride</td>
			<td style="width:136px;">Fischer</td>
			<td style="width:119px;">A649-500</td>
			<td></td>
		</tr>
	</tbody>
</table>

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.

For the simultaneous measurement of pH in both intracellular and endosomal/lysosomal cell compartments, the ratiometric fluorescent pH-sensing probes SNARF-1 and HPTS were used. SNARF-1 is restricted to the cytosolic compartment whereas HPTS allows ratiometric pH measurement of the endosomal/lysosomal compartment. Cell loading with HPTS for 16 hr allows its selective localization in endosomes/lysosomes4. A typical example of fluorescence recorded from HT-1080 cells loaded with the pH-sensing probes is shown in...

Watch this Scientific Journal Video about Simultaneous pH Measurement in Endocytic and Cytosolic Compartments in Living Cells using Confocal Microscopy at JoVE.com

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

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 ...

simultaneous-ph-measurement-endocytic-cytosolic-compartments-living

Research

JoVE Journal

Biology

14.9K Views.  Université de Sherbrooke. 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. 

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

Single Liposome Measurements for the Study of Proton-Pumping Membrane Enzymes Using Electrochemistry and Fluorescent Microscopy

Proton-pumping enzymes of electron transfer chains couple redox reactions to proton translocation across the membrane, creating a proton-motive force used for ATP production. The amphiphilic nature of membrane proteins requires particular attention to their handling, and reconstitution into the natural lipid environment is indispensable when studying membrane transport processes like proton translocation. Here, we detail a method that has been used for the investigation of the proton-pumping mechanism of membrane redox enzymes, taking cytochrome bo3 from Escherichia coli as an example. A combination of electrochemistry and fluorescence microscopy is used to control the redox state of the quinone pool and monitor pH changes in the lumen. Due to the spatial resolution of fluorescent microscopy, hundreds of liposomes can be measured simultaneously while the enzyme content can be scaled down to a single enzyme or transporter per liposome. The respective single enzyme analysis can reveal patterns in the enzyme functional dynamics that might be otherwise hidden by the behavior of the whole population. We include a description of a script for automated image analysis.

Here, we present a protocol to study the molecular mechanism of proton translocation across lipid membranes of single liposomes, using cytochrome bo3 as an example. Combining electrochemistry and fluorescence microscopy, pH changes in the lumen of single vesicles, containing single or multiple enzyme, can be detected and analyzed individually.

Proton-pumping enzymes of electron transfer chains couple redox reactions to proton translocation across the membrane, creating a proton-motive force used ...

Biochemistry

In vitro Monitoring of Extracellular pH in Real-Time

Early accumulation of neutrophils (PMN) is a hallmark of acute intestinal inflammation. This acute inflammation is either resolved or progresses to chronic inflammation. Without efficient PMN clearance at sites of infiltration, PMN can accumulate and contribute to chronic inflammatory conditions, including the intestinal diseases ulcerative colitis (UC) and Crohn's Disease (CD). The pH in the distal colon in individuals with active UC can range between a pH of 5 and 6, whereas healthy individuals maintain colonic pH in the range of 6.8-7.4. Extracellular pH has been shown to influence both intestinal epithelial cells and the infiltrating immune cells. More specifically, extracellular acidosis significantly impacts PMN. At pH below 6.5, there are increases in the production of H2O2, inhibition of apoptosis, and increases in the functional lifespan of PMN. Given the significant presence of PMN and extracellular acidification at sites of inflammation, we developed a novel model that allows for the monitoring of extracellular pH during PMN transepithelial migration in real time. Here, we describe this model and how it can be utilized to measure both the apical and basal pH during PMN trafficking. This model can be utilized to monitor extracellular pH under a wide range of conditions; including, hypoxia, PMN transepithelial migration, and for extended periods of time.

In vitro Monitoring of Extracellular pH in Real-Time

This article represents a useful in vitro assay to measure changes in extracellular pH during neutrophil (PMN) transepithelial migration (TEM)

Early accumulation of neutrophils (PMN) is a hallmark of acute intestinal inflammation. This acute inflammation is either resolved or progresses to ...

Measuring Phagosome pH by Ratiometric Fluorescence Microscopy

Phagocytosis is a fundamental process through which innate immune cells engulf bacteria, apoptotic cells or other foreign particles in order to kill or neutralize the ingested material, or to present it as antigens and initiate adaptive immune responses. The pH of phagosomes is a critical parameter regulating fission or fusion with endomembranes and activation of proteolytic enzymes, events that allow the phagocytic vacuole to mature into a degradative organelle. In addition, translocation of H+ is required for the production of high levels of reactive oxygen species (ROS), which are essential for efficient killing and signaling to other host tissues. Many intracellular pathogens subvert phagocytic killing by limiting phagosomal acidification, highlighting the importance of pH in phagosome biology. Here we describe a ratiometric method for measuring phagosomal pH in neutrophils using fluorescein isothiocyanate (FITC)-labeled zymosan as phagocytic targets, and live-cell imaging. The assay is based on the fluorescence properties of FITC, which is quenched by acidic pH when excited at 490 nm but not when excited at 440 nm, allowing quantification of a pH-dependent ratio, rather than absolute fluorescence, of a single dye. A detailed protocol for performing in situ dye calibration and conversion of ratio to real pH values is also provided. Single-dye ratiometric methods are generally considered superior to single wavelength or dual-dye pseudo-ratiometric protocols, as they are less sensitive to perturbations such as bleaching, focus changes, laser variations, and uneven labeling, which distort the measured signal. This method can be easily modified to measure pH in other phagocytic cell types, and zymosan can be replaced by any other amine-containing particle, from inert beads to living microorganisms. Finally, this method can be adapted to make use of other fluorescent probes sensitive to different pH ranges or other phagosomal activities, making it a generalized protocol for the functional imaging of phagosomes.

Phagosomal pH influences phagosome maturation, oxidant production, phagosomal killing as well as antigen presentation. Here we describe a ratiometric method for measuring time-course and endpoint pH changes in individual phagosomes in living phagocytes using fluorescence microscopy.

Phagocytosis is a fundamental process through which innate immune cells engulf bacteria, apoptotic cells or other foreign particles in order to kill or ...

Immunology and Infection

Imaging the Neutrophil Phagosome and Cytoplasm Using a Ratiometric pH Indicator

Neutrophils are crucial to host innate defense and, consequently, constitute an important area of medical research. The phagosome, the intracellular compartment where the killing and digestion of engulfed particles take place, is the main arena for neutrophil pathogen killing that requires tight regulation. Phagosomal pH is one aspect that is carefully controlled, in turn regulating antimicrobial protease activity. Many fluorescent pH-sensitive dyes have been used to visualize the phagosomal environment. S-1 has several advantages over other pH-sensitive dyes, including its dual emission spectra, its resistance to photo-bleaching, and its high pKa. Using this method, we have demonstrated that the neutrophil phagosome is unusually alkaline in comparison to other phagocytes. By using different biochemical conjugations of the dye, the phagosome can be delineated from the cytoplasm so that changes in the size and shape of the phagosome can be assessed. This allows for further monitoring of ionic movement.

This manuscript describes a simple method to measure the phagosomal pH and area as well as the cytoplasmic pH of human and mouse neutrophils using the ratiometric indicator seminaphthorhodafluor (SNARF)-1, or S-1. This is achieved using live-cell confocal fluorescence microscopy and image analysis.

Neutrophils are crucial to host innate defense and, consequently, constitute an important area of medical research. The phagosome, the intracellular ...

Monitoring Extracellular pH in Cross-Kingdom Biofilms using Confocal Microscopy

Cross-kingdom biofilms consisting of both fungal and bacterial cells are involved in a variety of oral diseases, such as endodontic infections, periodontitis, mucosal infections and, most notably, early childhood caries. In all of these conditions, the pH in the biofilm matrix impacts microbe-host interactions and thus the disease progression. The present protocol describes a confocal microscopy-based method to monitor pH dynamics inside cross-kingdom biofilms comprising Candida albicans and Streptococcus mutans. The pH-dependent dual-emission spectrum and the staining properties of the ratiometric probe C-SNARF-4 are exploited to determine drops in pH in extracellular areas of the biofilms. Use of pH ratiometry with the probe requires a meticulous choice of imaging parameters, a thorough calibration of the dye, and careful, threshold-based post-processing of the image data. When used correctly, the technique allows for the rapid assessment of extracellular pH in different areas of a biofilm and thus the monitoring of both horizontal and vertical pH gradients over time. While the use of confocal microscopy limits Z-profiling to thin biofilms of 75 &#181;m or less, the use of pH ratiometry is ideally suited for the noninvasive study of an important virulence factor in cross-kingdom biofilms.

The protocol describes the cultivation of cross-kingdom biofilms consisting of Candida albicans and Streptococcus mutans and presents a confocal microscopy-based method for the monitoring of extracellular pH inside these biofilms.

Cross-kingdom biofilms consisting of both fungal and bacterial cells are involved in a variety of oral diseases, such as endodontic infections, ...

Measurement of Vacuolar and Cytosolic pH In Vivo in Yeast Cell Suspensions

Vacuolar and cytosolic pH are highly regulated in yeast cells and occupy a central role in overall pH homeostasis. We describe protocols for ratiometric measurement of pH in vivo using pH-sensitive fluorophores localized to the vacuole or cytosol. Vacuolar pH is measured using BCECF, which localizes to the vacuole in yeast when introduced into cells in its acetoxymethyl ester form. Cytosolic pH is measured with a pH-sensitive GFP expressed under control of a yeast promoter, yeast pHluorin. Methods for measurement of fluorescence ratios in yeast cell suspensions in a fluorimeter are described. Through these protocols, single time point measurements of pH under different conditions or in different yeast mutants have been compared and changes in pH over time have been monitored. These methods have also been adapted to a fluorescence plate reader format for high-throughput experiments. Advantages of ratiometric pH measurements over other approaches currently in use, potential experimental problems and solutions, and prospects for future use of these techniques are also described.

Measurement of Vacuolar and Cytosolic pH In Vivo in Yeast Cell Suspensions

Vacuolar and cytosolic pH can be measured in live yeast (S. cerevisiae) cells using ratiometric fluorescent dyes localized to specific cellular compartments. We describe procedures for measuring vacuolar pH with BCECF-AM, which localizes to the vacuole in yeast, and cytosolic pH with a cytosolic ratiometric pH-sensitive GFP (yeast pHluorin).

Vacuolar and  cytosolic pH are highly regulated in yeast cells and occupy a central role in  overall pH homeostasis. We describe  protocols for ...

Measuring the pH, Redox Chemistries, and Degradative Capacity of Macropinosomes using Dual-Fluorophore Ratiometric Microscopy

In recent years, the field of macropinocytosis has grown rapidly. Macropinocytosis has emerged as a central mechanism by which innate immune cells maintain organismal homeostasis and immunity. Simultaneously, and in contrast to its homeostatic role, it can also drive various pathologies, including cancer and viral infections. Unlike other modes of endocytosis, the tools developed for studying the maturation of macropinosomes remain underdeveloped. Here the protocol describes newly developed tools for studying the redox environment within the lumen of early and maturing macropinosomes. Methodologies for using ratiometric fluorescence microscopy in assessing the pH, production of reactive oxygen species, and the degradative capacity within the lumen of individual macropinosomes in live cells are described. Single organelle measurements offer the advantage of revealing spatiotemporal heterogeneity, which is often lost with population-based approaches. Emphasis is placed on the basic principles of dual fluorophore ratiometric microscopy, including probe selection, instrumentation, calibration, and single-cell versus population-based methods.

We describe protocols for measuring pH, oxidative events, and protein digestion in individual macropinosomes in live cells. An emphasis is placed on dual-fluorophore ratiometric microscopy and the advantages it offers over population-based techniques.

In recent years, the field of macropinocytosis has grown rapidly. Macropinocytosis has emerged as a central mechanism by which innate immune cells ...

Applications of pHluorin for Quantitative, Kinetic and High-throughput Analysis of Endocytosis in Budding Yeast

Green fluorescent protein (GFP) and its variants are widely used tools for studying protein localization and dynamics of events such as cytoskeletal remodeling and vesicular trafficking in living cells. Quantitative methodologies using chimeric GFP fusions have been developed for many applications; however, GFP is somewhat resistant to proteolysis, thus its fluorescence persists in the lysosome/vacuole, which can impede quantification of cargo trafficking in the endocytic pathway. An alternative method for quantifying endocytosis and post-endocytic trafficking events makes use of superecliptic pHluorin, a pH-sensitive variant of GFP that is quenched in acidic environments. Chimeric fusion of pHluorin to the cytoplasmic tail of transmembrane cargo proteins results in a dampening of fluorescence upon incorporation of the cargo into multivesicular bodies (MVBs) and delivery to the lysosome/vacuole lumen. Thus, quenching of vacuolar fluorescence facilitates quantification of endocytosis and early events in the endocytic pathway. This paper describes methods using pHluorin-tagged cargos for quantification of endocytosis via fluorescence microscopy, as well as population-based assays using flow cytometry.

Accurate quantification of vesicular trafficking events often provides key insights into roles for specific proteins and the effects of mutations. This paper presents methods for using superecliptic pHluorin, a pH-sensitive GFP variant, as a tool for quantification of endocytic events in living cells using quantitative fluorescence microscopy and flow cytometry.

Green fluorescent protein (GFP) and its variants are widely used tools for studying protein localization and dynamics of events such as cytoskeletal ...

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.

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+ ...

Utilizing pHluorin-tagged Receptors to Monitor Subcellular Localization and Trafficking

Understanding membrane protein trafficking, assembly, and expression requires an approach that differentiates between those residing in intracellular organelles and those localized on the plasma membrane. Traditional fluorescence-based measurements lack the capability to distinguish membrane proteins residing in different organelles. Cutting edge methodologies transcend traditional methods by coupling pH-sensitive fluorophores with total internal reflection fluorescence microscopy (TIRFM). TIRF illumination excites the sample up to approximately 150 nm from the glass-sample interface, thus decreasing background, increasing the signal to noise ratio, and enhancing resolution. The excitation volume in TIRFM encompasses the plasma membrane and nearby organelles such as the peripheral ER. Superecliptic pHluorin (SEP) is a pH sensitive version of GFP. Genetically encoding SEP into the extracellular domain of a membrane protein of interest positions the fluorophore on the luminal side of the ER and in the extracellular region of the cell. SEP is fluorescent when the pH is greater than 6, but remains in an off state at lower pH values. Therefore, receptors tagged with SEP fluoresce when residing in the endoplasmic reticulum (ER) or upon insertion in the plasma membrane (PM) but not when confined to a trafficking vesicle or other organelles such as the Golgi. The extracellular pH can be adjusted to dictate the fluorescence of receptors on the plasma membrane. The difference in fluorescence between TIRF images at neutral and acidic extracellular pH for the same cell corresponds to a relative number of receptors on the plasma membrane. This allows a simultaneous measurement of intracellular and plasma membrane resident receptors. Single vesicle insertion events can also be measured when the extracellular pH is neutral, corresponding to a low pH trafficking vesicle fusing with the plasma membrane and transitioning into a fluorescent state. This versatile technique can be exploited to study localization, expression, and trafficking of membrane proteins.

Labeling the extracellular domain of a membrane protein with a pH sensitive fluorophore, superecliptic pHluorin (SEP), allows subcellular localization, expression, and trafficking to be determined. Imaging SEP-labeled proteins with total internal reflection fluorescence microscopy (TIRFM) enables the quantification of protein levels in the peripheral ER and plasma membrane.

Understanding membrane protein trafficking, assembly, and expression requires an approach that differentiates between those residing in intracellular ...