Intracellular pH mapping with SNARF-1 and confocal microscopy. II: pH gradients within single cultured cells

Vacuolar H+-ATPase in the nuclear membranes regulates nucleo-cytosolic proton gradients

The regulation of the luminal pH of each organelle is crucial for its function and must be controlled tightly. Nevertheless, it has been assumed that the nuclear pH is regulated by the cytoplasmic proton transporters via the diffusion of H⁺ across the nuclear pores because of their large diameter. However, it has been demonstrated that ion gradients exist between cytosol and nucleus, suggesting that the permeability of ions across the nuclear pores is restricted. Vacuolar H⁺-ATPase (V-H⁺-ATPase) is responsible for the creation and maintenance of trans-membrane electrochemical gradient. We hypothesize that V-H⁺-ATPase located in the nuclear membranes functions as the primary mechanism to regulate nuclear pH and generate H⁺ gradients across the nuclear envelope. We studied the subcellular heterogeneity of H⁺ concentration in the nucleus and cytosol using ratio imaging microscopy and SNARF-1, a pH indicator, in prostate cells. Our results indicate that there are proton gradients across the nuclear membranes that are generated by V-H⁺-ATPase located in the outer and inner nuclear membranes. We demonstrated that these gradients are mostly dissipated by inhibiting V-H⁺-ATPase. Immunoblots and V-H⁺-ATPase activity corroborated the existence of V-H⁺-ATPase in the nuclear membranes. This study demonstrates that V-H⁺-ATPase is functionally expressed in nuclear membranes and is responsible for nuclear H⁺ gradients that may promote not only the coupled transport of substrates, but also most electrochemically driven events across the nuclear membranes. This study represents a paradigm shift that the nucleus can regulate its own pH microenvironment, providing new insights into nuclear ion homeostasis and signaling.

Retinal ganglion cells: Energetics, compartmentation, axonal transport, cytoskeletons and vulnerability

Retinal ganglion cells (RGCs) are specialized projection neurons that relay an immense amount of visual information from the retina to the brain. RGC signal inputs are collected by dendrites and output is distributed from the cell body via very thin (0.5-1 μm) and long (∼50 mm) axons. The RGC cell body is larger than other retinal neurons, but is still only a very small fraction (one ten thousandths) of the length and total surface area of the axon. The total distance traversed by RGCs extends from the retina, starting from synapses with bipolar and amacrine cells, to the brain, to synapses with neurons in the lateral geniculate nucleus. This review will focus on the energy demands of RGCs and the relevant tissues that surround them. RGC survival and function unexceptionally depends upon free energy, predominantly adenosine triphosphate (ATP). RGC energy metabolism is vastly different when compared to that of the photoreceptors. Each subcellular component of the RGC is remarkably different in terms of structure, function and extracellular environment. The energy demands and distribution of each component are also distinct as evidenced by the uneven distribution of mitochondria and ATP within the RGC - signifying the presence of intracellular energy gradients. In this review we will describe RGCs as having four subcellular components, (1) Dendrites, (2) Cell body, (3) Non-myelinated axon, including intraocular and optic nerve head portions, and (4) Myelinated axon, including the intra-orbital and intracranial portions. We will also describe how RGCs integrate information from each subcellular component in order achieve intracellular homeostatic stability as well as respond to perturbations in the extracellular environment. The possible cellular mechanisms such as axonal transport and axonal cytoskeleton proteins that are involved in maintaining RGC energy homeostasis during normal and disease conditions will also be discussed in depth. The emphasis of this review will be on energetic mechanisms within RGC components that have the most relevance to clinical ophthalmology.

The influence of photosynthesis on host intracellular pH in scleractinian corals

The regulation of intracellular pH is a fundamental aspect of cell physiology that has received little attention in reef building corals and symbiotic cnidarians. Here, we investigated the hypothesis that dynamic changes in the pHi of coral host cells are controlled by the photosynthetic activity of the coral’s dinoflagellate symbionts. Using live cell imaging and the pH sensitive dye SNARF-1, we tracked pH in symbiont-containing and symbiont-free cells isolated from the reef coral Stylophora pistillata. We characterized the response of coral pHi in the presence of a photosynthetic inhibitor, the dynamics of coral pHi during light exposure and how pHi values vary on exposure to a range of irradiance levels lying within the coral's photosynthesis-irradiance (PI) response curve. Our results demonstrate that increases in coral pHi are dependent on photosynthetic activity of intracellular symbionts and that pHi recovers under darkness to pHi values that match symbiont-free cells. Furthermore, we show that the timing of the pHi response is governed by irradiance level and that pHi increases to irradiance-specific steady state values. Minimum steady state values of pHi 7.05±0.05 were obtained under darkness and maximum values of 7.46±0.07 under saturating irradiance. As changes in pHi affect organism homeostasis there is a need for continued research into acid/base regulation in symbiotic corals. More generally, these results represent the first characterization of photosynthesis-driven pHi changes in animal cells.

The intranuclear environment

Many of the chapters in this volume are concerned with processes or structures inside the nucleus, and it is relevant to consider the properties of their environment, or rather of the multiple different and specific environments that must exist in local regions of the highly heterogeneous intranuclear space. Relatively little is known about the fundamental physical properties of these environments, and theoretical treatments of phenomena in such concentrated mixtures of charged macromolecules are complex and as yet poorly developed. Some of the phenomena that occur at the molecular level are unexpected and counterintuitive for biologists, although well known to colloid and polymer scientists; for example, the existence of short-range attractive forces between macromolecules or structures with like charges. As a background for the chapters that follow, we consider here some of the particular features of intranuclear environments, how they may influence processes and structures in the nucleus, and their implications for working with nuclei.

Direct pH measurements by using subcellular targeting of 5(and 6-) carboxyseminaphthorhodafluor in mammalian cells

As a means of reliably measuring intracellular pH, we have precisely targeted 5(and 6-) carboxyseminaphthorhodafluor to cellular subcompartments. This was accomplished by combining the well-established pH-sensitive dye with a protein-based reporter system. When expressed in cells, the reporter protein is designed to covalently bind ligands composed of a functional group and a reactive linker. In order to make a pH-sensitive ligand, we chemically coupled the pH sensor to a reactive linker. Several ligands of differing linker lengths were made and tested for their pH responsiveness in vitro. The most responsive of these ligands was then evaluated for its efficacy in live cell labeling and its use as an intracellular pH sensor for ratiometric confocal microscopy. Here we show that we could target this pH sensor within mammalian cells exclusively to either the nucleus or cytoplasm. Exhibiting the versatility of this reporter technology, we were also able to specifically limit pH sensor labeling to within the trafficking pathway of integrins and directly measure pH of this environment. Results correspond well with previously published reports. Both the simplicity and flexibility of the technology used in this study make possible the development of diverse targeted microenvironmental sensors or other moieties of interest.

Imaging intracellular pH in a reef coral and symbiotic anemone

The challenges corals and symbiotic cnidarians face from global environmental change brings new urgency to understanding fundamental elements of their physiology. Intracellular pH (pHi) influences almost all aspects of cellular physiology but has never been described in anthozoans or symbiotic cnidarians, despite its pivotal role in carbon concentration for photosynthesis and calcification. Using confocal microscopy and the pH sensitive probe carboxy SNARF-1, we mapped pHi in short-term light and dark-incubated cells of the reef coral Stylophora pistillata and the symbiotic anemone Anemonia viridis. In all cells isolated from both species, pHi was markedly lower than the surrounding seawater pH of 8.1. In cells that contained symbiotic algae, mean values of pHi were significantly higher in light treated cells than dark treated cells (7.41 +/- 0.22 versus 7.13 +/- 0.24 for S. pistillata; and 7.29 +/- 0.15 versus 7.01 +/- 0.27 for A. viridis). In contrast, there was no significant difference in pHi in light and dark treated cells without algal symbionts. Close inspection of the interface between host cytoplasm and algal symbionts revealed a distinct area of lower pH adjacent to the symbionts in both light and dark treated cells, possibly associated with the symbiosome membrane complex. These findings are significant developments for the elucidation of models of inorganic carbon transport for photosynthesis and calcification and also provide a cell imaging procedure for future investigations into how pHi and other fundamental intracellular parameters in corals respond to changes in the external environment such as reductions in seawater pH.

Development of a novel GFP-based ratiometric excitation and emission pH indicator for intracellular studies

We report on the development of the F64L/S65T/T203Y/L231H GFP mutant (E2GFP) as an effective ratiometric pH indicator for intracellular studies. E2GFP shows two distinct spectral forms that are convertible upon pH changes both in excitation and in emission with pK close to 7.0. The excitation of the protein at 488 and 458 nm represents the best choice in terms of signal dynamic range and ratiometric deviation from the thermodynamic pK. This makes E2GFP ideally suited for imaging setups equipped with the most widespread light sources and filter settings. We used E2GFP to determine the average intracellular pH (pH(i)) and spatial pH(i) maps in two different cell lines, CHO and U-2 OS, under physiological conditions. In CHO, we monitored the evolution of the pH(i) during mitosis. We also showed the possibility to target specific subcellular compartments such as nucleoli (by fusing E2GFP with the transactivator protein of HIV, (Tat) and nuclear promyelocytic leukemia bodies (by coexpression of promyelocytic leukemia protein).

Application of fluorescence resonance energy transfer resolved by fluorescence lifetime imaging microscopy for the detection of enterovirus 71 infection in cells

Timely and effective virus infection detection is critical for the clinical management and prevention of the disease spread in communities during an outbreak. A range of methods have been developed for this purpose, of which classical serological and viral nucleic acids detection are the most popular. We describe an alternative, imaging-based approach that utilizes fluorescence resonance energy transfer (FRET) resolved by fluorescence lifetime imaging microscopy (FLIM) and demonstrate it on the example of enterovirus 71 (EV71) infection detection. A plasmid construct is developed with the sequence for GFP2 and DsRed2 fluorescent proteins, linked by a 12-amino-acid-long cleavage recognition site for the 2A protease (2A(pro)), encoded by the EV71 genome and specific for the members of Picornaviridae family. In the construct expressed in HeLa cells, the linker binds the fluorophores within the Forster distance and creates a condition for FRET to occur, thus resulting in shortening of the GFP2 fluorescence lifetime. On cells infection with EV71, viral 2A(pro) released to the cytoplasm cleaves the recognition site, causing disruption of FRET through separation of the fluorophores. Thus, increased GFP2 lifetime to the native values, manifested by the time-correlated single-photon counting, serves as an efficient and specific indicator of the EV71 virus infection.

Intracellular pH homeostasis during cell-cycle progression and growth state transition in Schizosaccharomyces pombe

Accurate measurement of intracellular pH in unperturbed cells is fraught with difficulty. Nevertheless, using a variety of methods, intracellular pH oscillations have been reported to play a regulatory role in the control of the cell cycle in several eukaryotic systems. Here, we examine pH homeostasis in Schizosaccharomyces pombe using a non-perturbing ratiometric pH sensitive GFP reporter. This method allows for accurate intracellular pH measurements in living, entirely undisturbed, logarithmically growing cells. In addition, the use of a flow cell allows internal pH to be monitored in real time during nutritional, or growth state transition. We can find no evidence for cell-cycle-related changes in intracellular pH. By contrast, all data are consistent with a very tight homeostatic regulation of intracellular pH near 7.3 at all points in the cell cycle. Interestingly, pH set point changes are associated with growth state. Spores, as well as vegetative cells starved of either nitrogen, or a carbon source, show a marked reduction in their internal pH compared with logarithmically growing vegetative cells. However, in both cases, homeostatic regulation is maintained.

Regulation of cytosol-nucleus pH gradients by K+/H+ exchange mechanism in the nuclear envelope of neonatal rat astrocytes

In order to study the subcellular heterogeneity of intracellular H+ concentration in reactive astrocytes, the pH in the nucleus and cytosol of cultured astrocytes was measured using a confocal laser scanning microscope (CLSM) and pH indicator dye, 5'(and 6')-carboxyseminaphthofluorescein (carboxy SNAFL-1). The change in intracellular pH was indexed by the fluorescence ratio (F535/F610) at an excitation wavelength of 514.5 nm. The in vitro fluorescence ratio increased as pH decreased. This ratio in the nucleus was significantly lower than that in the cytosol of astrocytes when perfused by HEPES-buffered Hanks' balanced salt solution (HHBSS) at pH 7.4. Acid stimulations of cells (pH 5.0) raised the fluorescence ratio in both nucleus and cytosol. However, the increase in the fluorescence ratio of the nucleus was less than that of cytosol. Treatment with a K+/H+ ionophore, nigericin (20 microM), reversibly nullified this cytosol-nucleus pH gradient. These findings suggest that a buffering mechanism(s) for maintaining of intracellular pH exists between the nucleus and cytosol, and a K+/H+ exchanger may act on the nuclear envelope to eventuate intranuclear pH maintenance in the living cells.

Intracellular pH of the preimplantation mouse embryo: effects of extracellular pH and weak acids

Although intracellular pH (pHi), is a regulator of numerous biological processes, it has received relatively little attention with regard to the physiology of the mammalian preimplantation embryo. Interestingly, there is some controversy as to whether the early embryo can recover from an acid load. The significance of this is that two constituents of mouse embryo culture media are pyruvate and lactate. These carboxylic acids are utilised by the early mouse embryo for energy production. However, as weak acids, pyruvate and lactate may induce perturbations in the pHi and thus alter the physiology of the embryo. The aims of this study were therefore to measure the pHi of the mouse preimplantation embryo and to determine the effect of lactate on pHi at different developmental stages. The pHi was measured using the ratio-metric fluorophore carboxy-seminaphthorhodafluor-1-acetoxymethylester (SNARF-1) in conjunction with confocal microscopy. The pHi increased significantly with development from the zygote to the morula stage. Furthermore, at concentrations greater than 5 mM, lactate caused the pHi of the zygote to become significantly more acidic. It was demonstrated that facilitative transport in association with a smaller passive component was responsible for the movement of lactate into the zygote. Metabolic studies revealed that, through their acidifying effect, weak acids caused a reduction in glycolytic activity in the early embryo. In contrast, the pHi of the compacted embryo remained unchanged by the presence of lactate in the external media. Furthermore, incubation with weak acids did not affect the rate of glycolysis in the morula. These data suggest that, by the generation of a transporting epithelium at compaction, the embryo develops the ability to regulate pHi against an acid load.

Measurement of intracellular pH and pCa with a confocal microscope

In the late 1980s, the field of biological confocal microscopy exploded. So did traffic on the Internet. Considering the ongoing interest in the role of intracellular pH and pCa in all aspects of cell physiology, it is not surprising that the most frequently asked question on the Internet's confocal forum has been: 'How do I measure pH/pCa with a confocal microscope?' This article was inspired by these Internet discussions and attempts to answer this question by presenting the rationale for using (or not using) a confocal approach to measure intracellular ion concentration, assessing the feasibility of performing this task with currently prevailing hardware, assembling the currently available 'know-how' and telling 'how'.

Nuclear pH gradient in mammalian cells revealed by laser microspectrofluorimetry

Intracellular pH has been measured by laser microspectrofluorimetry, using the pH-sensitive dyes SNARF-1, SNARF-calcein and SNARF-1-dextran. By this technique it was possible to accurately determine pH in volumes as small as 0.5 × 0.5 × 1 microns 3. The probes were loaded into the cells either by diffusion of their acetoxymethylester derivatives (SNARF-1-AM, SNARF-calcein-AM) or by microinjection (SNARF-1-dextran). When the five types of cells were studied in RPMI medium, the nuclear pH was consistently found to be 0.3 to 0.5 units above that of the cytosol. Although the presence of pores in the nuclear membrane has been taken as evidence that free diffusion of ions and small molecules can occur in and out the nucleus, we conclude that the nuclear membrane of these cells presents a permeability barrier to H+. The pH gradient was not observed in cells suspended in PBS.

Intracellular pH of Candida albicans blastospores as measured by laser microspectrofluorimetry and 31P-NMR

The intracellular pH (pHi) of Candida albicans blastospores harvested from 8 h or 48 h cultures was determined under identical experimental conditions by two different techniques: 31P-NMR and laser microspectrofluorimetry. Time dependence of pHi was monitored by 31P-NMR on the whole cell population. Microspectrofluorimetry, after loading of the cells with SNARF-1, enabled the determination of pHi in isolated cells and its distribution among the cell population. By this method, the vacuolar pH could not be distinguished from the cytoplasmic pH in C. albicans blastospores, but alkalization of pHi was observed at the beginning of germ tubes. The absolute values of pHi determined by 31P-NMR were slightly different from those obtained by laser microspectrofluorimetry. However, the pH distributions in the cell population were converging. For blastospores in exponential phase a gaussian distribution of pHi was observed with both methods, the cells maintained a steady pHi value when the external pH was varied from 5.5 to 8.5. For cells in stationary phase two pools were identified: the combination of the two techniques demonstrated the presence of two different subpopulations. One of these population (with lower pH) was able to commute to the other one with time as shown by 31P-NMR kinetics. This information is reported here for the first time in C. albicans.

Mechanisms of calcium release and propagation in cardiac cells. Do studies with confocal microscopy add to our understanding?

Laser-scanning confocal microscopy (LSCM) has a number of recognised advantages over other techniques of light microscopy for the study of cell and tissue structure. These include increased image spatial resolution, and even more importantly, removal of out-of-focus information from 2-dimensional images of 3-dimensional structures. Moreover, these features have also recently proved to be of immense benefit when coupled with ion-sensitive fluorescent probes, in the study of second messenger systems in relation to cell function. This review summarises the contribution that recent studies with LSCM have made to our understanding of the important patho-physiological state, spontaneous Ca(2+)-release (SCR) in isolated cardiac myocytes, and the relationship of this phenomenon to the induction of abnormal cell automaticity or cardiac arrhythmia. In some components of SCR and propagation, our existing knowledge has only been confirmed by recent results, while in others facets of this complex process, our understanding is being greatly enhanced by LSCM.