Imaging of Mitochondrial pH Using SNARF-1

Laser scanning confocal microscopy provides the ability to image submicron sections in living cells and tissues. In conjunction with pH-indicating fluorescent probes, confocal microscopy can be used to visualize the distribution of pH inside living cells. Here we describe a confocal microscopic technique to image intracellular pH in living cells using carboxyseminaphthorhodafluor-1 (SNARF-1), a ratiometric pH-indicating fluorescent probe. SNARF-1 is ester-loaded into the cytosol and mitochondria of adult cardiac myocytes or other cell type. Using 568-nm excitation, emitted fluorescence longer and shorter than 595-nm is imaged and then ratioed after background subtraction. Ratio values for each pixel are converted to values of pH using a standard curve (lookup table). Images of the intracellular distribution of pH show cytosolic and nuclear areas to have a pH of ~7.1, but in regions corresponding to mitochondria, pH is 8.0, giving a mitochondrial ΔpH of 0.9. During hypoxia, mitochondrial pH decreases to cytosolic values, signifying the collapse of ΔpH. These results illustrate the ability of laser scanning confocal microscopy to image the intracellular distribution of pH in living cells and to determine mitochondrial ΔpH.

Confocal ratio imaging of cytoplasmic pH during germ tube growth and appressorium induction by Magnaporthe grisea

The role of cytosolic pH (pH_c ) in growing germ tubes of the filamentous fungus Magnaporthe grisea was analysed by confocal ratio imaging of the pH-sensitive fluorescent dye 5(6)-carboxyseminaphthorhodafluor-1 (SNARF-1). The cytosol of these cells was successfully loaded with the acetoxymethyl ester of the dye and the pH_c was visualized and quantified during conidium germination, germ tube growth and appressorium induction by simultaneous dual-emission confocal ratio imaging. Calibrations of the free acid in vitro and calibrations in vivo produced results indicating a similar dynamic response in the pH range 6.0-8.0 for both methods. The pH_c in growing germ tubes was consistently pH 7.2±0.1 during all developmental stages analysed. Only slight changes in pH_c (<0.1 pH unit) were found in response to alkaline external pH (pH 8.0). No changes in pH_c occurred in response to an acidic extracellular pH (pH 6.0) or to the presence of nutrients. There was no observation of either pronounced gradients or changes in pH_c in growing germ tubes accompanying conidium germination, germ tube growth or early appressorium formation.

Studying Proton Gradients Across the Nuclear Envelope

The existence of nuclear pore complexes in the nuclear envelope has led to the assumption that ions move freely from the cytosol into the nucleus, and that the molecular mechanisms at the plasma membrane that regulate cytosolic pH also regulate nuclear pH. Furthermore, studies to measure pH in the nucleus have produced contradictory results, since it has been found that the nuclear pH is either similar to the cytosol or more alkaline than the cytosol. However, most studies of nuclear pH have lacked the rigor needed to understand pH regulation in the nucleus. A major problem has been the lack of in situ titrations in the nucleus and cytosol, since the intracellular environment is different in the cytosol and nucleus and the behavior of fluorescent pH probes is different in these environments. Here we present a method that uses the fluorescence of SNARF-1 that labels both cytosol and nucleus. Using ratio imaging microscopy, regions of interest corresponding to the nucleus and cytosol to perform steady-state pH measurements followed by in situ titrations, to correctly assign pH in those cellular domains.