Intracellular pH mapping with SNARF-1 and confocal microscopy. I: A quantitative technique for living tissue and isolated cells

Lactic Acidosis in Patients with Solid Cancer

Significance: Cancer-associated tissue-specific lactic acidosis stimulates and mediates tumor invasion and metastasis and is druggable. Rarely, malignancy causes systemic lactic acidosis, the role of which is poorly understood. Recent Advances: The understanding of the role of lactate has shifted dramatically since its discovery. Long recognized as only a waste product, lactate has become known as an alternative metabolism substrate and a secreted nutrient that is exchanged between the tumor and the microenvironment. Tissue-specific lactic acidosis is targeted to improve the host body's anticancer defense and serves as a tool that allows the targeting of anticancer compounds. Systemic lactic acidosis is associated with poor survival. In patients with solid cancer, systemic lactic acidosis is associated with an extremely poor prognosis, as revealed by the analysis of 57 published cases in this study. Although it is considered a pathology worth treating, targeting systemic lactic acidosis in patients with solid cancer is usually inefficient. Critical Issues: Research gaps include simple questions, such as the unknown nuclear pH of the cancer cells and its effects on chemotherapy outcomes, pH sensitivity of glycosylation in cancer cells, in vivo mechanisms of response to acidosis in the absence of lactate, and overinterpretation of in vitro results that were obtained by using cells that were not preadapted to acidic environments. Future Directions: Numerous metabolism-targeting anticancer compounds induce lactatemia, lactic acidosis, or other types of acidosis. Their potential to induce acidic environments is largely overlooked, although the acidosis might contribute to a substantial portion of the observed clinical effects. Antioxid. Redox Signal. 37, 1130-1152.

Designing bioresponsive nanomaterials for intracellular self-assembly

Supramolecular assemblies are essential components of living organisms. Cellular scaffolds, such as the cytoskeleton or the cell membrane, are formed via secondary interactions between proteins or lipids and direct biological processes such as metabolism, proliferation and transport. Inspired by nature’s evolution of function through structure formation, a range of synthetic nanomaterials has been developed in the past decade, with the goal of creating non-natural supramolecular assemblies inside living mammalian cells. Given the intricacy of biological pathways and the compartmentalization of the cell, different strategies can be employed to control the assembly formation within the highly crowded, dynamic cellular environment. In this Review, we highlight emerging molecular design concepts aimed at creating precursors that respond to endogenous stimuli to build nanostructures within the cell. We describe the underlying reaction mechanisms that can provide spatial and temporal control over the subcellular formation of synthetic nanostructures. Showcasing recent advances in the development of bioresponsive nanomaterials for intracellular self-assembly, we also discuss their impact on cellular function and the challenges associated with establishing structure–bioactivity relationships, as well as their relevance for the discovery of novel drugs and imaging agents, to address the shortfall of current solutions to pressing health issues.

Creating artificial nanostructures inside living cells requires the careful design of molecules that can transform into active monomers within a complex cellular environment. This Review explores the recent development of bioresponsive precursors for the controlled formation of intracellular supramolecular assemblies.

Imidazole-Based pH-Sensitive Convertible Liposomes for Anticancer Drug Delivery

In efforts to enhance the activity of liposomal drugs against solid tumors, three novel lipids that carry imidazole-based headgroups of incremental basicity were prepared and incorporated into the membrane of PEGylated liposomes containing doxorubicin (DOX) to render pH-sensitive convertible liposomes (ICL). The imidazole lipids were designed to protonate and cluster with negatively charged phosphatidylethanolamine-polyethylene glycol when pH drops from 7.4 to 6.0, thereby triggering ICL in acidic tumor interstitium. Upon the drop of pH, ICL gained more positive surface charges, displayed lipid phase separation in TEM and DSC, and aggregated with cell membrane-mimetic model liposomes. The drop of pH also enhanced DOX release from ICL consisting of one of the imidazole lipids, sn-2-((2,3-dihexadecyloxypropyl)thio)-5-methyl-1H-imidazole. ICL demonstrated superior activities against monolayer cells and several 3D MCS than the analogous PEGylated, pH-insensitive liposomes containing DOX, which serves as a control and clinical benchmark. The presence of cholesterol in ICL enhanced their colloidal stability but diminished their pH-sensitivity. ICL with the most basic imidazole lipid showed the highest activity in monolayer Hela cells; ICL with the imidazole lipid of medium basicity showed the highest anticancer activity in 3D MCS. ICL that balances the needs of tissue penetration, cell-binding, and drug release would yield optimal activity against solid tumors.

Microenvironment pH-Induced Selective Cell Death for Potential Cancer Therapy Using Nanofibrous Self-Assembly of a Peptide Amphiphile

Self-assembly of synthetic molecules has been drawing broad attention as a novel emerging approach in drug discovery. Here, we report selective cell death induced by a novel peptide amphiphile that self-assembles to form entangled nanofibers (hydrogel) based on intracellular pH (pH_i). We found that a palmitoylated hexapeptide (C₁₆-VVAEEE) formed a hydrogel below pH 7. The formation of the nanofibrous self-assembly was responsive to a small pH change around pH 7. The cytotoxicity of C₁₆-VVAEEE was correlated with pH_i of cells. Microscope observation demonstrated the self-assembly of C₁₆-VVAEEE inside HEK293 cells. In vivo experiments revealed that the transcutaneous administration of C₁₆-VVAEEE showed remarkable anti-tumor activity. This study proposes that distinct microenvironment inside living cells can be used as a trigger for the intracellular self-assembly of a peptide amphiphile, which provide a new clue to drug discovery.

Overcoming Chemoresistance: Altering pH of Cellular Compartments by Chloroquine and Hydroxychloroquine

Resistance to the anti-cancer effects of chemotherapeutic agents (chemoresistance) is a major issue for people living with cancer and their providers. A diverse set of cellular and inter-organellar signaling changes have been implicated in chemoresistance, but it is still unclear what processes lead to chemoresistance and effective strategies to overcome chemoresistance are lacking. The anti-malaria drugs, chloroquine (CQ) and its derivative hydroxychloroquine (HCQ) are being used for the treatment of various cancers and CQ and HCQ are used in combination with chemotherapeutic drugs to enhance their anti-cancer effects. The widely accepted anti-cancer effect of CQ and HCQ is their ability to inhibit autophagic flux. As diprotic weak bases, CQ and HCQ preferentially accumulate in acidic organelles and neutralize their luminal pH. In addition, CQ and HCQ acidify the cytosolic and extracellular environments; processes implicated in tumorigenesis and cancer. Thus, the anti-cancer effects of CQ and HCQ extend beyond autophagy inhibition. The present review summarizes effects of CQ, HCQ and proton pump inhibitors on pH of various cellular compartments and discuss potential mechanisms underlying their pH-dependent anti-cancer effects. The mechanisms considered here include their ability to de-acidify lysosomes and inhibit autophagosome lysosome fusion, to de-acidify Golgi apparatus and secretory vesicles thus affecting secretion, and to acidify cytoplasm thus disturbing aerobic metabolism. Further, we review the ability of these agents to prevent chemotherapeutic drugs from accumulating in acidic organelles and altering their cytosolic concentrations.

Novel application ofMacrolampissp2 firefly luciferase for intracellular pH-biosensing in mammalian cells

Bioluminescence is widely used in biosensors.

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.

Fast loading ester fluorescent Ca2+ and pH indicators into pollen of Pyrus pyrifolia

Loading of Ca(2+)-sensitive fluorescent probes into plant cells is an essential step to measure activities of free Ca(2+) ions in cytoplasm with a fluorescent imaging technique. Fluo-3 is one of the most suitable Ca(2+) indicators for CLSM. We loaded pollen with fluo-3/AM at three different temperatures. Fluo-3/AM was successfully loaded into pollen at both low (4°C) and high (37°C) temperatures. However, high loading temperature was best suited for pollen, because germination rate of pollen and growth of pollen tubes were relatively little impaired and loading time was shortened. Moreover, Ca(2+) distribution increased in the three apertures of pollen after hydration and showed a Ca(2+) gradient, similar to the tip of growing pollen tubes. The same protocol can be used with the AM-forms of other fluorescent dyes for effective labeling. When loading BCECF-AM into pollen at high temperature, the pollen did not show a pH gradient after hydration. Ca(2+) activities and fluxes had the same periodicity as pollen germination, but pH did not show the same phase and mostly lagged behind. However, the clear zone was alkaline when pollen tube growth was slowed or stopped and turned acidic when growth recovered. It is likely that apical pH(i) regulated pollen tube growth.

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.

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).

Carboxy SNARF-4F as a Fluorescent pH Probe for Ensemble and Fluorescence Correlation Spectroscopies

The optical spectroscopic properties of 1,4(and 5)-benzenedicarboxylic acid, 2-[10-(dimethylamino)-4-fluoro-3-oxo-3H-benzo[c]xanthen-7-yl] (carboxy SNARF-4F), a commercial promising fluorescent pH probe, are investigated in buffered aqueous solutions in the 5.6-8.2 pH range by steady-state and time-resolved spectroscopy. A multiexponential global analysis of the picosecond time-resolved data is performed. The nonprotonated A species decays monoexponentially (0.73 ns), while the protonated species AH decays following a biexponential law with time constants of 0.40 and 1.87 ns. A kinetic scheme is proposed to explain the observations, which involves AH in equilibrium with a species denoted Y. The nature of Y is discussed in terms of a possible structural change in the molecule producing the lactone form, although the formation of a hydrogen-bonded complex to the solvent cannot be ruled out. Finally, the ability of the dye to probe pH at the single-molecule level is explored using fluorescence correlation spectroscopy.

Dexamethasone inhibits endotoxin-induced changes in calcium and contractility in rat isolated papillary muscle

This study investigates whether endotoxin-induced contractile dysfunction is associated with a defect in the modulation of calcium homeostasis and the potential mechanisms involved. Treatment of rats in vivo with endotoxin significantly decreased the magnitude of contractile transients in electrically stimulated left ventricular papillary muscle isolated after an equilibration period of 6 hours. Although no significant difference was found in the peak intracellular calcium concentration ([Ca2+]i) between the endotoxin-treated and control groups, resting [Ca2+]i) was significantly elevated in the endotoxin-treated group, producing a smaller Ca2+ transient (basal-peak difference) in this group. Pretreatment of rats with dexamethasone prevented the endotoxin-induced decrease in peak tension and inhibited the elevation in resting [Ca2+]i, with a resultant maintenance of Ca2+ transient magnitude. Similar observations were made during stimulation of the muscles by the beta-adrenoceptor agonist, isoprenaline. These results show that endotoxin-induced reduction of cardiac contractile performance is mediated, at least in part, by elevating resting [Ca2+]i, and a glucocorticoid protected from these negative effects. While endotoxin reduces the magnitude of the Ca2+ transient it does not alter peak [Ca2+]i availability. Further investigation is required to determine whether endotoxin decreases contractile performance by reducing the sensitivity of cardiac myofilaments to calcium.

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.

Real time in situ confocal imaging of calcium wave in the perfused whole heart of the rat

To understand the calcium handling in whole heart having automaticity of the sinus node, we have developed a system of in situ imaging the intracellular calcium ion concentration in the perfused whole heart of the rat. The system consists of a stage-fixed upright microscope equipped with a real-time confocal laser scanning device of a multipinhole type with a water-immersion objective lens for observation. This in situ imaging system rendered observations and analyses of the rapidly changing images of intracellular calcium dynamics possible in the whole rat heart loaded with fluo-3. The scanning was conducted at a video rate of 30 frames per second, and the confocal effects included both X and Y planes. Calcium waves were frequently interrupted by calcium transients from either external electro-stimulation pulses or spontaneous sinus rhythm. Our findings suggest that abnormal calcium waves in minute areas cannot disturb the excitation-contraction coupling in the whole heart if the myocardial cells have orderly end-on-end intercellular electric paths.

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'.

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.

Quantitative recording of vitality patterns in living multicellular spheroids by confocal microscopy

Fluorescent dyes were used in conjunction with confocal microscopy to record the vitality status of cells in multicellular glioma spheroids. Multicellular spheroids are in vitro models for micrometastases or intravascular microregions of large tumors. With progressing growth three distinct concentric annular shells develop. A rim of proliferating cells in the periphery is followed towards the center by layers of quiescent cells and at a defined spheroid diameter cell death occurs in the central core. Fluorescein diacetate (FDA) and Calcein/AM were used as vital stains and Lucifer Yellow/VS (LYVS) was used as a marker for dead cells. For loading multicellular spheroids with the esterase substrate dyes we used a two step cold incubation technique to avoid dye accumulation in the most peripheral cell layers. Homogenously stained tissue allowed to describe the fluorescence attenuation in depth as a monoexponential decay. An attenuation coefficient C was calculated from calibration experiments to be 12.5 x 10(-3) in vital stained tissue and 17.9 x 10(-3) in lethal stained tissue. Using the respective attenuation coefficient the raw data were corrected for light absorption and scattering in depth. In radial recordings of the vitality status of multicellular glioma spheroids using CLSM-technique we showed that spheroids up to a diameter of 250 microns were homogenously stained with Calcein/AM and FDA. Spheroids larger than 250 microns consist of vital stained cells and unstained cells. They do not show dead cell staining until they reach a diameter of about 400 microns. The thickness of the rim of vital stained cells decreased with increasing diameter of the spheroids to 64 +/- 7 microns in spheroids of a diameter of 550 +/- 25 microns. Thereafter the thickness of the Calcein/AM or FDA stained rim augmented again, reaching 93 +/- 9 microns in spheroids of 700 microns in diameter. The first signs of dead cell staining in the central core occurred at a diameter of 400 +/- 25 microns. The radius of the core increased in an exponential way. The cell layer which was stained neither by vital nor by lethal dyes showed a thickness of 150 microns in spheroids of 550 +/- 25 microns in diameter. Our staining technique and the radial recording of mean field fluorescence signals in living multicellular spheroids will be a valuable tool for experimental cancer research providing a non invasive quantification of cell vitality in living multicellular spheroids.

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.