This work was supported by NIH R01 GM50322 to P.M. Kane. The authors thank Dr. Rajini Rao, Johns Hopkins University for providing the yeast pHluorin plasmids and for advice on ratiometric pH measurements, and Dr. Gloria A. Martinez Munoz for working out these protocols for our lab.

1. Measurement of Vacuolar pH In Vivo Using BCECF-AM
<ol>
	<li>Grow a 50 ml liquid culture of the yeast strain to be measured in the desired medium overnight. The goal is to have cells in mid-log phase (OD600 (optical density at 600 nm) measurement of approximately 0.8 for the suspension).</li>
	<li>Pellet the yeast cells by centrifugation. Resuspend the pellet in 0.6 ml of the growth medium and transfer to a microcentrifuge tube that has been weighed previously. Pellet the cells again in a microcentrifuge at 2,000 x g for 60 sec. Remove the supernatant as completely as possible and then weigh the cell pellet. Resuspend the pellet to a final density of 0.5 g/ml (w/v); a culture of this volume will give a cell pellet of approximately 200 mg, and 200 &mu;l of buffer would be added to give a final volume of 400 &mu;l.</li>
	<li>Add BCECF-AM to the cell suspension at a final concentration of 50 mM from a 12 mM stock prepared in DMSO. Mix well and then incubate the cells at 30 &deg;C for 30 min. on a rocking platform or roller drum.</li>
	<li>While the cells are incubating, prepare calibration buffers. The calibration buffer contains 50 mM MES (2-(N-morpholino)ethanesulfonic acid), 50 mM HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid), 50 mM KCl, 50 mM NaCl, 0.2 M ammonium acetate, 10 mM sodium azide, and 10 mM 2-deoxyglucose, and is adjusted to the pH values appropriate for your calibration range with NaOH or HCl. (Caution: Sodium azide is highly toxic and should be handled with care.) For measurement of pH in wild-type cells, we would prepare several calibration mixtures at pH 5.5 to 6.5, but for mutants expected to have a more alkaline vacuolar pH (vma mutants), we would prepare additional, higher pH buffers.</li>
	<li>Aliquot 2 ml of calibration buffer for each pH into 15 ml conical tubes, and add monensin (15 mM stock) and nigericin (2 mM stock) to give final concentrations of 110 &mu;M monensin and 15 &mu;M nigericin, respectively. Mix well on a vortex mixer. (Caution: Both monensin and nigericin are toxic and should be handled with care.)</li>
	<li>When the BCECF-AM incubation time is completed, pellet the cell suspension by centrifugation for 30 sec at 2,000 x g in a microcentrifuge. Resuspend the cells is 1 ml of growth medium lacking glucose and centrifuge as above. Repeat this wash step once, and then resuspend the pellet in 200 &mu;l of growth medium without glucose. Place on ice.</li>
	<li>Add 20 &mu;l of the cell suspension to each pH calibration tube prepared above. Incubate at 30 &deg;C for 30-60 min. on a roller drum.</li>
	<li>Set the fluorimeter to alternately measure at excitation wavelengths 450 nm and 490 nM, both with an emission wavelength of 535 nm. Set the sample chamber temperature to 30 &deg;C. Perform all measurements with continuous stirring of the mixture in the cuvette.</li>
	<li>Add 1.96 ml of 1 mM MES (adjusted to pH 5 or 7 depending on the pH of the cells' growth medium and the experimental design). Add 20 &mu;l of cell suspension into the cuvette. Initiate fluorescence measurements. We collect both continuous kinetic and time-point data in different experiments. For continuous kinetic data, we take measurements every 6 sec for 5 min, then add glucose to a final glucose concentration of 50 mM, and continue measurement for 5-10 min. For single time-point measurements, we generally take measurements at 1 and 5 min. after addition of cells to the cuvette, add glucose as in the kinetic measurements, and then take another measurement 5 min. after glucose addition. These additions can be varied, and additional components (inhibitors, etc) can also be added.</li>
	<li>After the experimental measurements are complete, remove the calibration tubes from the 30 &deg;C incubation and transfer the entire 2 ml volume for each to the fluorimeter cuvette. Measure fluorescence at the same settings (see 1.8) every 5 sec over a total of 30 sec for each sample.</li>
	<li>Export fluorescence data to Microsoft Excel. (For our fluorimeter, this requires export of data as text in tab-delimited form and import into Excel.) Obtain a calibration curve by calculating the ratio of fluorescence at 490 nm to 450 nm for each calibration mixture. The fluorescence ratio is then plotted vs. pH to get a calibration curve.</li>
	<li>Convert the experimental data to fluorescence ratio and calculate pH using the standard curve. Vacuolar pH can then plotted against time (kinetics of pH changes) or compared under various conditions (Figure 1) or in different mutant strains.</li>
</ol>
2. Measurement of Cytosolic pH In Vivo Using Yeast pHluorin
<ol>
	<li>Transform the desired yeast strain with the yeast pHluorin plasmid by standard protocols, and select for transformants on supplemented minimal medium lacking uracil (SC-uracil).</li>
	<li>Grow a 50 ml liquid culture of transformed cells in SC-uracil to mid-log phase (OD600 = 0.8 or lower).</li>
	<li>Prepare calibration standards as described for vacuolar pH, but buffer to a pH range appropriate for cytosolic pH measurements, generally pH 6-8. Aliquot 2 ml of calibration buffer adjusted to the desired pH into several 15 ml conical tubes. Add monensin and nigericin as described above (1.4) and mix well.</li>
	<li>Harvest the cells by centrifugation as described above, and resuspend the pellet in 600 &mu;l of the growth medium. Transfer to a weighed microcentrifuge tube, and pellet cells by centrifugation at 5,000 rpm for 30 sec. Resuspend the pellet in 1 ml of growth medium without glucose (SC-uracil, -glucose), pellet cells again and repeat. After the final centrifugation, remove the supernatant as thoroughly as possible and weigh the cell pellet. Resuspend cells to a final density of 0.5 g/ml in SC-uracil with no glucose.</li>
	<li>Add 20 &mu;l of cell suspension to each tube of calibration buffer and mix well on a vortex mixer. Incubate at 30 &deg;C on a spinning drum rotor for 60 min.</li>
	<li>Set up the fluorimeter for excitation at wavelengths 405 and 485 nm and an emission wavelength of 508 nm. Proceed with single time point or kinetic measurements and addition of glucose as described above for BCECF measurement.</li>
	<li>Measure fluorescence of calibration samples, and construct a calibration curve as for BCECF (see 1.10-1.11). A plot of fluorescence ratio vs. pH is linear over the range of most cytosolic pH measurements (pH 6.0-8.0) 9, so experimental fluorescence ratio measurements are easily converted to pH.</li>
</ol>

<ol>
	<li>Casey, J. R., Grinstein, S., Orlowski, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sensors+and+regulators+of+intracellular+pH.">Sensors and regulators of intracellular pH.</a> Nat. Rev. Mol. Cell Biol. 11, 50-61 (2010).</li><li>Orij, R., Brul, S., Smits, G. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intracellular+pH+is+a+tightly+controlled+signal+in+yeast.">Intracellular pH is a tightly controlled signal in yeast.</a> Biochim. Biophys. Acta. 1810, 933-944 (2011).</li><li>Diakov, T. T., Kane, 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=Regulation+of+vacuolar+proton-translocating+ATPase+activity+and+assembly+by+extracellular+pH.">Regulation of vacuolar proton-translocating ATPase activity and assembly by extracellular pH.</a> J. Biol. Chem. 285, 23771-23778 (2010).</li><li>Martinez-Munoz, G. A., Kane, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Vacuolar+and+plasma+membrane+proton+pumps+collaborate+to+achieve+cytosolic+pH+homeostasis+in+yeast.">Vacuolar and plasma membrane proton pumps collaborate to achieve cytosolic pH homeostasis in yeast.</a> J. Biol. Chem. 283, 20309-20319 (2008).</li><li>Tarsio, M., Zheng, H., Smardon, A. M., Martinez-Munoz, G. A., Kane, 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=Consequences+of+loss+of+Vph1+protein-containing+vacuolar+ATPases+(V-ATPases)+for+overall+cellular+pH+homeostasis.">Consequences of loss of Vph1 protein-containing vacuolar ATPases (V-ATPases) for overall cellular pH homeostasis.</a> J. Biol. Chem. 286, 28089-28096 (2011).</li><li>Plant, P. J., Manolson, M. F., Grinstein, S., 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=Alternative+mechanisms+of+vacuolar+acidification+in+H(+)-ATPase-deficient+yeast.">Alternative mechanisms of vacuolar acidification in H(+)-ATPase-deficient yeast.</a> J. Biol. Chem. 274, 37270-37279 (1999).</li><li>Ali, R., Brett, C. L., Mukherjee, S., Rao, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inhibition+of+sodium/proton+exchange+by+a+Rab-GTPase-activating+protein+regulates+endosomal+traffic+in+yeast.">Inhibition of sodium/proton exchange by a Rab-GTPase-activating protein regulates endosomal traffic in yeast.</a> J. Biol. Chem. 279, 4498-4506 (2004).</li><li>Miesenbock, G., De Angelis, D. A., Rothman, J. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Visualizing+secretion+and+synaptic+transmission+with+pH-sensitive+green+fluorescent+proteins.">Visualizing secretion and synaptic transmission with pH-sensitive green fluorescent proteins.</a> Nature. 394, 192-195 (1998).</li><li>Brett, C. L., Tukaye, D. N., Mukherjee, S., Rao, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+yeast+endosomal+Na+K+/H++exchanger+Nhx1+regulates+cellular+pH+to+control+vesicle+trafficking.">The yeast endosomal Na+K+/H+ exchanger Nhx1 regulates cellular pH to control vesicle trafficking.</a> Mol. Biol. Cell. 16, 1396-1405 (2005).</li><li>Owen, C. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Comparison+of+spectrum-shifting+intracellular+pH+probes+5'(and+6')-carboxy-10-dimethylamino-3-hydroxyspiro[7H-benzo[c]xanthene-7,+1'(3'H)-isobenzofuran]-3'-one+and+2',7'-biscarboxyethyl-5(and+6)-carboxyfluorescein.">Comparison of spectrum-shifting intracellular pH probes 5'(and 6')-carboxy-10-dimethylamino-3-hydroxyspiro[7H-benzo[c]xanthene-7, 1'(3'H)-isobenzofuran]-3'-one and 2',7'-biscarboxyethyl-5(and 6)-carboxyfluorescein.</a> Anal. Biochem. 204, 65-71 (1992).</li><li>Dechant, R., 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=Cytosolic+pH+is+a+second+messenger+for+glucose+and+regulates+the+PKA+pathway+through+V-ATPase.">Cytosolic pH is a second messenger for glucose and regulates the PKA pathway through V-ATPase.</a> Embo J. 29, 2515-2526 (2010).</li><li>Gustavsson, M., Barmark, G., Larsson, J., Muren, E., Ronne, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Functional+genomics+of+monensin+sensitivity+in+yeast:+implications+for+post-Golgi+traffic+and+vacuolar+H+-ATPase+function.">Functional genomics of monensin sensitivity in yeast: implications for post-Golgi traffic and vacuolar H+-ATPase function.</a> Mol. Genet. Genomics. 280, 233-248 (2008).</li><li>Kovac, L., Bohmerova, E., Butko, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ionophores+and+intact+cells.+I.+Valinomycin+and+nigericin+act+preferentially+on+mitochondria+and+not+on+the+plasma+membrane+of+Saccharomyces+cerevisiae.">Ionophores and intact cells. I. Valinomycin and nigericin act preferentially on mitochondria and not on the plasma membrane of Saccharomyces cerevisiae.</a> Biochim. Biophys. Acta. 721, 341-348 (1982).</li><li>Braun, N. A., Morgan, B., Dick, T. P., Schwappach, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+yeast+CLC+protein+counteracts+vesicular+acidification+during+iron+starvation.">The yeast CLC protein counteracts vesicular acidification during iron starvation.</a> J. Cell Sci. 123, 2342-2350 (2010).</li><li>Orij, R., Postmus, J., Beek, T. e. r., Brul, A., S, G. J., Smits, <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vivo+measurement+of+cytosolic+and+mitochondrial+pH+using+a+pH-sensitive+GFP+derivative+in+Saccharomyces+cerevisiae+reveals+a+relation+between+intracellular+pH+and+growth.">In vivo measurement of cytosolic and mitochondrial pH using a pH-sensitive GFP derivative in Saccharomyces cerevisiae reveals a relation between intracellular pH and growth.</a> Microbiology. 155, 268-278 (2009).</li><li>Zhang, Y. Q., 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=Requirement+for+ergosterol+in+V-ATPase+function+underlies+antifungal+activity+of+azole+drugs.">Requirement for ergosterol in V-ATPase function underlies antifungal activity of azole drugs.</a> PLoS Pathog. 6, e1000939 (2010).</li><li>Brett, C. 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=Genome-wide+analysis+reveals+the+vacuolar+pH-stat+of+Saccharomyces+cerevisiae.">Genome-wide analysis reveals the vacuolar pH-stat of Saccharomyces cerevisiae.</a> PLoS One. 6, e17619 (2011).</li><li>Roberts, C. J., Raymond, C. K., Yamashiro, C. T., Stevens, T. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Methods+for+studying+the+yeast+vacuole.">Methods for studying the yeast vacuole.</a> Methods Enzymol. 194, 644-661 (1991).</li><li>Chan, C. Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inhibitors+of+V-ATPase+proton+transport+reveal+uncoupling+functions+of+tether+linking+cytosolic+and+membrane+domains+of+V0+subunit+a+(Vph1p).">Inhibitors of V-ATPase proton transport reveal uncoupling functions of tether linking cytosolic and membrane domains of V0 subunit a (Vph1p).</a> J. Biol. Chem. 287, 10236-10250 (2012).</li><li>Johnson, R. M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Identification+of+inhibitors+of+vacuolar+proton-translocating+ATPase+pumps+in+yeast+by+high-throughput+screening+flow+cytometry.">Identification of inhibitors of vacuolar proton-translocating ATPase pumps in yeast by high-throughput screening flow cytometry.</a> Anal. Biochem. 398, 203-211 (2010).</li></ol>

The authors have no conflicts of interest to disclose.

We have utilized these protocols to address a number of aspects of pH homeostasis. For example, we have compared cytosolic and pH responses of wild-type and V-ATPase-deficient mutant cells 4,5. We have also examined the effects of altered growth conditions, particularly extracellular pH, on vacuolar pH response to glucose 3. Importantly, the responses we observe are both consistent with other methods of quantitative pH measurement and with biochemical data describing altered activities of the ...

pH homeostasis is a dynamic and highly regulated process in all organisms 1,2. Biochemical processes are tightly regulated by pH, and intracellular environments are tuned to narrow pH ranges to allow optimal activity of the resident enzymes. However, intracellular pH homeostasis can be challenged by rapid changes in environmental pH, metabolic shifts, and certain signaling pathways. In addition, intracellular pH can itself serve as an important signal. Finally, many organelles maintain lumenal pH values that are distinct from the surrounding cytosol and essential for organelle-specific functions.
The yeast Saccharomyces cerevisiae shares a number of pH homeostasis mechanisms with higher eukaryotes 2. In the acidic organelles of the endocytic/lysosomal pathway, pH is primarily controlled by the highly conserved vacuolar proton-translocating ATPase (V-ATPase), acting in tandem with many exchangers dependent on the pH gradient. All eukaryotic cells also have proton export mechanisms. In fungi and plants, a second, distinct proton pump at the plasma membrane, Pma1, exports metabolic protons and is believed to be the major determinant of cytosolic pH and plasma membrane potential. The genetic flexibility of S. cerevisiae and its commercial importance, have made it a very interesting and important model for studying pH homeostasis 2.
In addition to being the primary drivers of organelle acidification, V-ATPases are highly regulated enzymes and our lab is interested in understanding mechanisms of V-ATPase regulation. Towards this goal, we have been using in vivo pH measurements of vacuolar and cytosolic pH: 1) to monitor responses to changing extracellular conditions, such as glucose deprivation and readdition, 2) to examine the effects of mutations that compromise V-ATPase activity, and 3) to explore the coordination of organelle and plasma membrane proton pumps 3-5. These experiments only became possible through the development of robust ratiometric pH indicators amenable to use in yeast cells. Plant et al. first showed that BCECF (2'7'-Bis-(2-carboxyethyl)-5-(and 6)-carboxyfluorescein), which has been used widely to measure cytosolic pH in mammalian cells, accumulates in the yeast vacuole instead of the cytosol 6. This difference in BCECF localization has been attributed to the many hydrolytic enzymes in the vacuole, which are likely responsible for cleavage of the acetoxy methyl ester from BCECF-AM (acetoxymethyl ester of BCECF) and vacuolar retention 6. Ali et al. 7 further developed vacuolar pH measurement using BCECF and adapted these measurements to a fluorescence plate reader format. Brett et al. introduced yeast pHluorin as a means of measuring cytosolic pH in yeast by expressing a plasmid-borne ratiometric pH-sensitive GFP 8 under control of a yeast-specific promoter 9.
The excitation spectra of both BCECF and yeast pHluorin are sensitive to pH, so they are used as ratiometric pH indicators in which the ratio of fluorescence at two excitation wavelengths, measured at a single emission wavelength, provides a measure of pH 8,10. These yeast vacuolar and cytosolic pH sensors have been used for both single-cell and population-based measurements. Single-cell measurements 6,11 are performed by fluorescence microscopy and image analysis. Vacuolar or cytosolic fluorescence at the two wavelengths is measured for each cell. The population-based measurements are performed in either a microplate reader with appropriate fluorescence capabilities or in a fluorimeter. We have generally done our measurements in a fluorimeter, because it provides easy access for addition of components such as glucose during continuous kinetic measurements. Our current lab protocols for measurement of vacuolar and cytosolic pH are listed below; both are also easily adapted to microplate assays.

<table><tbody><tr><td>Spectrofluorometer</td><td>Horiba Jobin Yvon</td><td>Model Fluoromax-4</td><td>Temperature control and stirring capability are desirable.</td></tr><tr><td>BCECF-AM</td><td>Invitrogen/Molecular Probes</td><td>B1150</td><td>Prepare a 12 mM stock in dry DMSO, store as aliquots at -20 &amp;deg;C </td></tr><tr><td>monensin</td><td>Sigma</td><td>M5273</td><td>Toxic. </td></tr><tr><td>nigericin</td><td>Sigma</td><td>N7143</td><td>Toxic. </td></tr><tr><td>MES</td><td>Sigma</td><td>M8250</td><td></td></tr></tbody></table>

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.

Figure 1 presents vacuolar pH data obtained on wild-type yeast cells grown in rich medium (yeast extract-peptone-dextran; YEPD) buffered to pH 5 with 50 mM MES. We often grow the cells in buffered medium because the pH of the medium can change quite dramatically during overnight growth, particularly for minimal medium, and we have found that the pH of the growth medium can affect vacuolar pH responses 3. However, it is also acceptable for many experiments to grow the cells in unbuffered...

Watch this Scientific Journal Video about Measurement of Vacuolar and Cytosolic pH In Vivo in Yeast Cell Suspensions at JoVE.com

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).

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 ...

measurement-vacuolar-cytosolic-ph-vivo-yeast-cell

Research

JoVE Journal

Biology

19.8K Views.  SUNY Upstate Medical University. 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).

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

Radiolabeling and Quantification of Cellular Levels of Phosphoinositides by High Performance Liquid Chromatography-coupled Flow Scintillation

Phosphoinositides (PtdInsPs) are essential signaling lipids responsible for recruiting specific effectors and conferring organelles with molecular identity and function. Each of the seven PtdInsPs varies in their distribution and abundance, which are tightly regulated by specific kinases and phosphatases. The abundance of PtdInsPs can change abruptly in response to various signaling events or disturbance of the regulatory machinery. To understand how these events lead to changes in the amount of PtdInsPs and their resulting impact, it is important to quantify PtdInsP levels before and after a signaling event or between control and abnormal conditions. However, due to their low abundance and similarity, quantifying the relative amounts of each PtdInsP can be challenging. This article describes a method for quantifying PtdInsP levels by metabolically labeling cells with 3H-myo-inositol, which is incorporated into PtdInsPs. Phospholipids are then precipitated and deacylated. The resulting soluble 3H-glycero-inositides are further extracted, separated by high-performance liquid chromatography (HPLC), and detected by flow scintillation. The labeling and processing of yeast samples is described in detail, as well as the instrumental setup for the HPLC and flow scintillator. Despite losing structural information regarding acyl chain content, this method is sensitive and can be optimized to concurrently quantify all seven PtdInsPs in cells.

Phosphoinositides are signaling lipids whose relative abundance rapidly changes in response to various stimuli. This article describes a method to measure the abundance of phosphoinositides by metabolically labeling cells with 3H-myo-inositol, followed by extraction and deacylation. Extracted glycero-inositides are then separated by high-performance liquid chromatography and quantified by flow scintillation.

Phosphoinositides (PtdInsPs) are essential signaling lipids responsible for recruiting specific effectors and conferring organelles with molecular ...

Chemistry

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 ...

Biochemistry

Quantification of Cytosolic vs. Vacuolar Salmonella in Primary Macrophages by Differential Permeabilization

Intracellular bacterial pathogens can replicate in the cytosol or in specialized pathogen-containing vacuoles (PCVs). To reach the cytosol, bacteria like Shigella flexneri and Francisella novicida need to induce the rupture of the phagosome. In contrast, Salmonella typhimurium replicates in a vacuolar compartment, known as Salmonella-containing vacuole (SCV). However certain mutants of Salmonella fail to maintain SCV integrity and are thus released into the cytosol. The percentage of cytosolic vs. vacuolar bacteria on the level of single bacteria can be measured by differential permeabilization, also known as phagosome-protection assay. The approach makes use of the property of detergent digitonin to selectively bind cholesterol. Since the plasma membrane contains more cholesterol than other cellular membranes, digitonin can be used to selectively permeabilize the plasma membrane while leaving intracellular membranes intact. In brief, following infection with the pathogen expressing a fluorescent marker protein (e.g. mCherry among others), the plasma membrane of host cells is permeabilized with a short incubation in digitonin containing buffer. Cells are then washed and incubated with a primary antibody (coupled to a fluorophore of choice) directed against the bacterium of choice (e.g. anti-Salmonella-FITC) and washed again. If unmarked bacteria are used, an additional step can be done, in which all membranes are permeabilized and all bacteria stained with a corresponding antibody. Following the staining, the percentage of vacuolar and cytosolic bacteria can be quantified by FACS or microscopy by counting single or double-positive events. Here we provide experimental details for use of this technique with the bacterium Salmonella typhimurium. The advantage of this assay is that, in contrast to other assay, it provides a quantification on the level of single bacteria, and if analyzed by microscopy provides the exact number of cytosolic and vacuolar bacteria in a given cell.

Quantification of Cytosolic vs. Vacuolar Salmonella in Primary Macrophages by Differential Permeabilization

We describe the quantification of cytosolic and vacuolar Salmonella typhimurium in bone-marrow derived macrophages using differential digitonin permeabilization.

Intracellular bacterial pathogens can replicate in the cytosol or in specialized pathogen-containing vacuoles (PCVs). To reach the cytosol, bacteria like ...

Immunology and Infection

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 ...

FRET Analysis of IDR Structural Sensitivity to Study Hyperosmotic Stress in Yeast

Estimation of Structural Sensitivity of Intrinsically Disordered Regions in Response to Hyperosmotic Stress in Living Cells Using FRET

Intrinsically disordered regions (IDRs) are protein domains that participate in crucial cellular processes. During stress conditions, the physicochemical properties of the cellular environment change, directly impacting the conformational ensemble of IDRs. IDRs are inherently sensitive to environmental perturbations. Studying how the physicochemical properties of the cell regulate the conformational ensemble of IDRs is essential for understanding the environmental control of their function. Here, we describe a step-by-step method for measuring the structural sensitivity of IDRs in living Saccharomyces cerevisiae cells in response to hyperosmotic stress conditions. We present the use of ensemble fluorescence resonance energy transfer (FRET) to estimate how the global dimensions of IDRs change during a progressive increase of hyperosmotic stress imposed on cells with any osmolyte. In addition, we provide a script for processing fluorescence measurements and comparing structural sensitivity for different IDRs. By following this method, researchers can obtain valuable insights into the conformational changes that IDRs undergo in the complex intracellular milieu upon changing environments.

Author Spotlight: Unlocking the World of Intrinsically Disordered Regions with Cellular Sensing and Responses

Intrinsically disordered regions (IDRs) are flexible protein domains that modify their conformation in response to environmental changes. Ensemble fluorescence resonance energy transfer (FRET) can estimate protein dimensions under different conditions. We present a FRET approach to assess IDR structural sensitivity in living Saccharomyces cerevisiae cells under hyperosmotic stress.

Intrinsically disordered regions (IDRs) are protein domains that participate in crucial cellular processes. During stress conditions, the physicochemical ...

A Fluorescence Microscopy Assay for Monitoring Mitophagy in the Yeast Saccharomyces cerevisiae

Autophagy is important for turnover of cellular components under a range of different conditions. It serves an essential homeostatic function as well as a quality control mechanism that can target and selectively degrade cellular material including organelles1-4. For example, damaged or redundant mitochondria (Fig. 1), not disposed of by autophagy, can represent a threat to cellular homeostasis and cell survival. In the yeast, Saccharomyces cerevisiae, nutrient deprivation (e.g., nitrogen starvation) or damage can promote selective turnover of mitochondria by autophagy in a process termed mitophagy 5-9.
We describe a simple fluorescence microscopy approach to assess autophagy. For clarity we restrict our description here to show how the approach can be used to monitor mitophagy in yeast cells. The assay makes use of a fluorescent reporter, Rosella, which is a dual-emission biosensor comprising a relatively pH-stable red fluorescent protein linked to a pH-sensitive green fluorescent protein. The operation of this reporter relies on differences in pH between the vacuole (pH ~ 5.0-5.5) and mitochondria (pH ~ 8.2) in living cells. Under growing conditions, wild type cells exhibit both red and green fluorescence distributed in a manner characteristic of the mitochondria. Fluorescence emission is not associated with the vacuole. When subjected to nitrogen starvation, a condition which induces mitophagy, in addition to red and green fluorescence labeling the mitochondria, cells exhibit the accumulation of red, but not green fluorescence, in the acidic vacuolar lumen representing the delivery of mitochondria to the vacuole. Scoring cells with red, but not green fluorescent vacuoles can be used as a measure of mitophagic activity in cells5,10-12.

A Fluorescence Microscopy Assay for Monitoring Mitophagy in the Yeast Saccharomyces cerevisiae

A robust approach to monitor the delivery of organelles to the acidic lumen of the yeast vacuole for degradation and recycling is described. The method relies on the specific labeling of target organelles with a genetically encoded dual-emission fluorescence pH-biosensor, and visualization of individual cells using fluorescence microscopy.

Autophagy is important for turnover of cellular components under a range of different conditions. It serves an essential homeostatic function as well as a ...

Measuring Yeast Mitochondrial Respiration While Maintaining Cellular Context

Mitochondrial Respiration Quantification in Yeast Whole Cells

Metabolism is mainly coordinated by cellular energy availability and environmental conditions. Assays for knowing how cells adapt energetic metabolism to different nutritional and environmental conditions give valuable information to elucidate molecular mechanisms. Oxidative phosphorylation is the primary source of ATP in most cells, and mitochondrial respiration activity is a key component of oxidative phosphorylation, maintaining mitochondrial membrane potential for ATP synthesis. Mitochondrial respiration is often studied in isolated mitochondria that are missing the cellular context. Here, we present a method for quantifying mitochondrial respiration in yeast-intact cells. This method applies to any yeast species, although it has been generally used for Saccharomyces cerevisiae cells. First, the yeast growth in specific conditions is tested. Then, cells are washed and resuspended in deionized water with a 1:1 ratio (w/v). Cells are then placed in an oximeter chamber with constant stirring, and a Clark electrode is used to quantify oxygen consumption. Some molecules are sequentially placed into the chamber and selected according to this effect on the electron transport chain or ATP synthesis. ATPase inhibitor oligomycin is first added to measure respiration coupled to ATP synthesis. Afterward, an uncoupler is used to measure the maximal respiratory capacity. Finally, a mix of electron transport chain inhibitors is added to discard oxygen consumption unrelated to mitochondrial respiration. Data are analyzed using a linear regression to obtain the slope, representing the oxygen consumption rate. The advantage of this method is that it is specific for yeast mitochondrial respiration, maintaining the cellular context. It is essential to highlight that inhibitors used in mitochondrial respiration quantification could vary between yeast species.

Mitochondrial respiration in yeast whole cells is a valuable indicator of cell bioenergetics. Here, we present a protocol to quantify this phenotype applicable to different yeast species.

Metabolism is mainly coordinated by cellular energy availability and environmental conditions. Assays for knowing how cells adapt energetic metabolism to ...

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

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

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

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

Development of pHluorin2-Based Sensor for Glycosomal pH in Trypanosoma brucei

Measuring Dynamic Glycosomal pH Changes in Living Trypanosoma brucei

Glucose metabolism is critical for the African trypanosome, Trypanosoma brucei, as an essential metabolic process and regulator of parasite development. Little is known about the cellular responses generated when environmental glucose levels change. In both bloodstream and procyclic form (insect stage) parasites, glycosomes house most of glycolysis. These organelles are rapidly acidified in response to glucose deprivation, which likely results in the allosteric regulation of glycolytic enzymes such as hexokinase. In previous work, localizing the chemical probe used to make pH measurements was challenging, limiting its utility in other applications.
This paper describes the development and use of parasites that express glycosomally localized pHluorin2, a heritable protein pH biosensor. pHluorin2 is a ratiometric pHluorin variant that displays a pH (acid)-dependent decrease in excitation at 395 nm while simultaneously yielding an increase in excitation at 475 nm. Transgenic parasites were generated by cloning the pHluorin2 open reading frame into the trypanosome expression vector pLEW100v5, enabling inducible protein expression in either lifecycle stage. Immunofluorescence was used to confirm the glycosomal localization of the pHluorin2 biosensor, comparing the localization of the biosensor to the glycosomal resident protein aldolase. The sensor responsiveness was calibrated at differing pH levels by incubating cells in a series of buffers that ranged in pH from 4 to 8, an approach we have previously used to calibrate a fluorescein-based pH sensor. We then measured pHluorin2 fluorescence at 405 nm and 488 nm using flow cytometry to determine glycosomal pH. We validated the performance of the live transgenic pHluorin2-expressing parasites, monitoring pH over time in response to glucose deprivation, a known trigger of glycosomal acidification in PF parasites. This tool has a range of potential applications, including potentially being used in high-throughput drug screening. Beyond glycosomal pH, the sensor could be adapted to other organelles or used in other trypanosomatids to understand pH dynamics&#160;in the live cell setting.

Author Spotlight: Advancements in Glycosomal pH Monitoring in Trypanosoma brucei Using pHluorin2 Biosensor

We describe a method to study how pH responds to environmental cues in the glycosomes of the bloodstream form of African trypanosomes. This approach involves a pH-sensitive heritable protein sensor in combination with flow cytometry to measure pH dynamics, both as a time-course assay and in a high-throughput screen format.

Glucose metabolism is critical for the African trypanosome, Trypanosoma brucei, as an essential metabolic process and regulator of parasite development. ...

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 ...

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

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Radiolabeling and Quantification of Cellular Levels of Phosphoinositides by High Performance Liquid Chromatography-coupled Flow Scintillation

In vitro Monitoring of Extracellular pH in Real-Time

Quantification of Cytosolic vs. Vacuolar Salmonella in Primary Macrophages by Differential Permeabilization

Measuring Phagosome pH by Ratiometric Fluorescence Microscopy

Estimation of Structural Sensitivity of Intrinsically Disordered Regions in Response to Hyperosmotic Stress in Living Cells Using FRET

A Fluorescence Microscopy Assay for Monitoring Mitophagy in the Yeast Saccharomyces cerevisiae

Mitochondrial Respiration Quantification in Yeast Whole Cells

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

Measuring Dynamic Glycosomal pH Changes in Living Trypanosoma brucei

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