Proton leak and CFTR in regulation of Golgi pH in respiratory epithelial cells

Progress in pH-Sensitive sensors: essential tools for organelle pH detection, spotlighting mitochondrion and diverse applications

pH-sensitive fluorescent proteins have revolutionized the field of cellular imaging and physiology, offering insight into the dynamic pH changes that underlie fundamental cellular processes. This comprehensive review explores the diverse applications and recent advances in the use of pH-sensitive fluorescent proteins. These remarkable tools enable researchers to visualize and monitor pH variations within subcellular compartments, especially mitochondria, shedding light on organelle-specific pH regulation. They play pivotal roles in visualizing exocytosis and endocytosis events in synaptic transmission, monitoring cell death and apoptosis, and understanding drug effects and disease progression. Recent advancements have led to improved photostability, pH specificity, and subcellular targeting, enhancing their utility. Techniques for multiplexed imaging, three-dimensional visualization, and super-resolution microscopy are expanding the horizon of pH-sensitive protein applications. The future holds promise for their integration into optogenetics and drug discovery. With their ever-evolving capabilities, pH-sensitive fluorescent proteins remain indispensable tools for unravelling cellular dynamics and driving breakthroughs in biological research. This review serves as a comprehensive resource for researchers seeking to harness the potential of pH-sensitive fluorescent proteins.

Phagosomal chloride dynamics in the alveolar macrophage

Acidification in intracellular organelles is tightly linked to the influx of Cl- counteracting proton translocation by the electrogenic V-ATPase. We quantified the dynamics of Cl- transfer accompanying cargo incorporation into single phagosomes in alveolar macrophages (AMs). Phagosomal Cl- concentration and acidification magnitude were followed in real time with maximal acidification achieved at levels of approximately 200 mM. Live cell confocal microscopy verified that phagosomal Cl- influx utilized predominantly the Cl- channel CFTR. Relative levels of elemental chlorine (Cl) in hard X-ray fluorescence microprobe (XFM) analysis within single phagosomes validated the increase in Cl- content. XFM revealed the complex interplay between elemental K content inside the phagosome and changes in Cl- during phagosomal particle uptake. Cl- -dependent changes in phagosomal membrane potential were obtained using second harmonic generation (SHG) microscopy. These studies provide a mechanistic insight for screening studies in drug development targeting pulmonary inflammatory disease.

The Distribution and Role of the CFTR Protein in the Intracellular Compartments

Cystic fibrosis is a hereditary disease that mainly affects secretory organs in humans. It is caused by mutations in the gene encoding CFTR with the most common phenylalanine deletion at position 508. CFTR is an anion channel mainly conducting Cl⁻ across the apical membranes of many different epithelial cells, the impairment of which causes dysregulation of epithelial fluid secretion and thickening of the mucus. This, in turn, leads to the dysfunction of organs such as the lungs, pancreas, kidney and liver. The CFTR protein is mainly localized in the plasma membrane; however, there is a growing body of evidence that it is also present in the intracellular organelles such as the endosomes, lysosomes, phagosomes and mitochondria. Dysfunction of the CFTR protein affects not only the ion transport across the epithelial tissues, but also has an impact on the proper functioning of the intracellular compartments. The review aims to provide a summary of the present state of knowledge regarding CFTR localization and function in intracellular compartments, the physiological role of this localization and the consequences of protein dysfunction at cellular, epithelial and organ levels. An in-depth understanding of intracellular processes involved in CFTR impairment may reveal novel opportunities in pharmacological agents of cystic fibrosis.

An integrated modular framework for modeling the effect of ammonium on the sialylation process of monoclonal antibodies produced by CHO cells

BACKGROUND

Monoclonal antibodies (mABs) have emerged as one of the most important therapeutic recombinant proteins in the pharmaceutical industry. Their immunogenicity and therapeutic efficacy are influenced by post-translational modifications, specifically the glycosylation process. Bioprocess conditions can influence the intracellular process of glycosylation. Among all the process conditions that have been recognized to affect the mAB glycoforms, the detailed mechanism underlying how ammonium could perturb glycosylation remains to be fully understood. It was shown that ammonium induces heterogeneity in protein glycosylation by altering the sialic acid content of glycoproteins. Hence, understanding this mechanism would aid pharmaceutical manufacturers to ensure consistent protein glycosylation.

METHODS

Three different mechanisms have been proposed to explain how ammonium influences the sialylation process. In the first, the inhibition of CMP-sialic acid transporter, which transports CMP-sialic acid (sialylation substrate) into the Golgi, by an increase in UDP-GlcNAc content that is brought about by the augmented incorporation of ammonium into glucosamine formation. In the second, ammonia diffuses into the Golgi and raises its pH, thereby decreasing the sialyltransferase enzyme activity. In the third, the reduction of sialyltransferase enzyme expression level in the presence of ammonium. We employed these mechanisms in a novel integrated modular platform to link dynamic alteration in mAB sialylation process with extracellular ammonium concentration to elucidate how ammonium alters the sialic acid content of glycoproteins.

RESULTS

Our results show that the sialylation reaction rate is insensitive to the first mechanism. At low ammonium concentration, the second mechanism is the controlling mechanism in mAB sialylation and by increasing the ammonium level (< 8 mM) the third mechanism becomes the controlling mechanism. At higher ammonium concentrations (> 8 mM) the second mechanism becomes predominant again.

CONCLUSION

The presented model in this study provides a connection between extracellular ammonium and the monoclonal antibody sialylation process. This computational tool could help scientists to develop and formulate cell culture media. The model illustrated here can assist the researchers to select culture media that ensure consistent mAB sialylation.

The Golgi as a "Proton Sink" in Cancer

Cancer cells exhibit increased glycolytic flux and adenosine triphosphate (ATP) hydrolysis. These processes increase the acidic burden on the cells through the production of lactate and protons. Nonetheless, cancer cells can maintain an alkaline intracellular pH (pHi) relative to untransformed cells, which sets the stage for optimal functioning of glycolytic enzymes, evasion of cell death, and increased proliferation and motility. Upregulation of plasma membrane transporters allows for H+ and lactate efflux; however, recent evidence suggests that the acidification of organelles can contribute to maintenance of an alkaline cytosol in cancer cells by siphoning off protons, thereby supporting tumor growth. The Golgi is such an acidic organelle, with resting pH ranging from 6.0 to 6.7. Here, we posit that the Golgi represents a "proton sink" in cancer and delineate the proton channels involved in Golgi acidification and the ion channels that influence this process. Furthermore, we discuss ion channel regulators that can affect Golgi pH and Golgi-dependent processes that may contribute to pHi homeostasis in cancer.

Azithromycin and ciprofloxacin have a chloroquine-like effect on respiratory epithelial cells

There is interest in the use of chloroquine/hydroxychloroquine (CQ/HCQ) and azithromycin (AZT) in COVID-19 therapy. Employing cystic fibrosis respiratory epithelial cells, here we show that drugs AZT and ciprofloxacin (CPX) act as acidotropic lipophilic weak bases and confer in vitro effects on intracellular organelles similar to the effects of CQ. These seemingly disparate FDA-approved antimicrobials display a common property of modulating pH of endosomes and trans-Golgi network. We believe this may in part help understand the potentially beneficial effects of CQ/HCQ and AZT in COVID-19, and that the present considerations of HCQ and AZT for clinical trials should be extended to CPX.

Cellular and Subcellular Phosphate Transport Machinery in Plants

Phosphorus (P) is an essential element required for incorporation into several biomolecules and for various biological functions; it is, therefore, vital for optimal growth and development of plants. The extensive research on identifying the processes underlying the uptake, transport, and homeostasis of phosphate (Pi) in various plant organs yielded valuable information. The transport of Pi occurs from the soil into root epidermal cells, followed by loading into the root xylem vessels for distribution into other plant organs. Under conditions of Pi deficiency, Pi is also translocated from the shoot to the root via the phloem. Vacuoles act as a storage pool for extra Pi, enabling its delivery to the cytosol, a process which plays an important role in the homeostatic control of cytoplasmic Pi levels. In mitochondria and chloroplasts, Pi homeostasis regulates ATP synthase activity to maintain optimal ATP levels. Additionally, the endoplasmic reticulum functions to direct Pi transporters and Pi toward various locations. The intracellular membrane potential and pH in the subcellular organelles could also play an important role in the kinetics of Pi transport. The presented review provides an overview of Pi transport mechanisms in subcellular organelles, and also discusses how they affect Pi balancing at cellular, tissue, and whole-plant levels.

Intracellular transport and compartmentation of phosphate in plants

Phosphate (Pi) is an essential macronutrient with structural and metabolic roles within every compartment of the plant cell. Intracellular Pi transporters direct Pi to each organelle and also control its exchange between subcellular compartments thereby providing the means to coordinate compartmented metabolic processes, including glycolysis, photosynthesis, and respiration. In this review we summarize recent advances in the identification and functional analysis of Pi transporters that localize to vacuoles, chloroplasts, non-photosynthetic plastids, mitochondria, and the Golgi apparatus. Electrical potentials across intracellular membranes and the pH of subcellular environments will also be highlighted as key factors influencing the energetics of Pi transport, and therefore pose limits for Pi compartmentation.

Molecular evolution of versatile derivatives from a GFP-like protein in the marine copepod Chiridius poppei

Fluorescent proteins are now indispensable tools in molecular research. They have also been adapted for a wide variety of uses in cases involving creative applications, including textiles, aquarium fish, and ornamental plants. Our colleagues have previously cloned a yellow GFP-like protein derived from the marine copepod Chiridius poppei (YGFP), and moreover, succeeded in generating transgenic flowers with clearly visible fluorescence, without the need for high-sensitivity imaging equipment. However, due to the low Stokes shift of YGFP (10 nm), it is difficult to separate emitted light of a labeled object from the light used for excitation; hence, limitations for various applications remain. In this study, which was aimed at developing YGFP mutants with increased Stokes shifts, we conducted stepwise molecular evolution experiments on YGFP by screening random mutations at three key amino acids, based on their fluorescent characteristics and structural stabilities, followed by optimization of their fluorescence output by DNA shuffling of the entire coding sequence. We successfully identified an eYGFPuv that had an excitation maximum in UV wavelengths and a 24-fold increase in fluorescence intensity compared to the previously reported YGFP mutant (H52D). In addition, eYGFPuv exhibited almost 9-fold higher fluorescence intensity compared to the commercially available GFPuv when expressed in human colon carcinoma HCT116 cells and without any differences in cytotoxicity. Thus, this novel mutant with the desirable characteristics of bright fluorescence, long Stokes shift, and low cytotoxity, may be particularly well suited to a variety of molecular and biological applications.

Monitoring of vacuolar-type H+ ATPase-mediated proton influx into synaptic vesicles

During synaptic vesicle (SV) recycling, the vacuolar-type H(+) ATPase creates a proton electrochemical gradient (ΔμH(+)) that drives neurotransmitter loading into SVs. Given the low estimates of free luminal protons, it has been envisioned that the influx of a limited number of protons suffices to establish ΔμH(+). Consistent with this, the time constant of SV re-acidification was reported to be <5 s, much faster than glutamate loading (τ of ∼ 15 s) and thus unlikely to be rate limiting for neurotransmitter loading. However, such estimates have relied on pHluorin-based probes that lack sensitivity in the lower luminal pH range. Here, we reexamined re-acidification kinetics using the mOrange2-based probe that should report the SV pH more accurately. In recordings from cultured mouse hippocampal neurons, we found that re-acidification took substantially longer (τ of ∼ 15 s) than estimated previously. In addition, we found that the SV lumen exhibited a large buffering capacity (∼ 57 mm/pH), corresponding to an accumulation of ∼ 1200 protons during re-acidification. Together, our results uncover hitherto unrecognized robust proton influx and storage in SVs that can restrict the rate of neurotransmitter refilling.

Polar bears exhibit genome-wide signatures of bioenergetic adaptation to life in the arctic environment

Polar bears (Ursus maritimus) face extremely cold temperatures and periods of fasting, which might result in more severe energetic challenges than those experienced by their sister species, the brown bear (U. arctos). We have examined the mitochondrial and nuclear genomes of polar and brown bears to investigate whether polar bears demonstrate lineage-specific signals of molecular adaptation in genes associated with cellular respiration/energy production. We observed increased evolutionary rates in the mitochondrial cytochrome c oxidase I gene in polar but not brown bears. An amino acid substitution occurred near the interaction site with a nuclear-encoded subunit of the cytochrome c oxidase complex and was predicted to lead to a functional change, although the significance of this remains unclear. The nuclear genomes of brown and polar bears demonstrate different adaptations related to cellular respiration. Analyses of the genomes of brown bears exhibited substitutions that may alter the function of proteins that regulate glucose uptake, which could be beneficial when feeding on carbohydrate-dominated diets during hyperphagia, followed by fasting during hibernation. In polar bears, genes demonstrating signatures of functional divergence and those potentially under positive selection were enriched in functions related to production of nitric oxide (NO), which can regulate energy production in several different ways. This suggests that polar bears may be able to fine-tune intracellular levels of NO as an adaptive response to control trade-offs between energy production in the form of adenosine triphosphate versus generation of heat (thermogenesis).

Illumination of the spatial order of intracellular pH by genetically encoded pH-sensitive sensors

Fluorescent proteins have been extensively used for engineering genetically encoded sensors that can monitor levels of ions, enzyme activities, redox potential, and metabolites. Certain fluorescent proteins possess specific pH-dependent spectroscopic features, and thus can be used as indicators of intracellular pH. Moreover, concatenated pH-sensitive proteins with target proteins pin the pH sensors to a definite location within the cell, compartment, or tissue. This study provides an overview of the continually expanding family of pH-sensitive fluorescent proteins that have become essential tools for studies of pH homeostasis and cell physiology. We describe and discuss the design of intensity-based and ratiometric pH sensors, their spectral properties and pH-dependency, as well as their performance. Finally, we illustrate some examples of the applications of pH sensors targeted at different subcellular compartments.

Functional vacuolar ATPase (V-ATPase) proton pumps traffic to the enterocyte brush border membrane and require CFTR

Vacuolar ATPases (V-ATPases) are highly conserved proton pumps that regulate organelle pH. Epithelial luminal pH is also regulated by cAMP-dependent traffic of specific subunits of the V-ATPase complex from endosomes into the apical membrane. In the intestine, cAMP-dependent traffic of cystic fibrosis transmembrane conductance regulator (CFTR) channels and the sodium hydrogen exchanger (NHE3) in the brush border regulate luminal pH. V-ATPase was found to colocalize with CFTR in intestinal CFTR high expresser (CHE) cells recently. Moreover, apical traffic of V-ATPase and CFTR in rat Brunner's glands was shown to be dependent on cAMP/PKA. These observations support a functional relationship between V-ATPase and CFTR in the intestine. The current study examined V-ATPase and CFTR distribution in intestines from wild-type, CFTR(-/-) mice and polarized intestinal CaCo-2BBe cells following cAMP stimulation and inhibition of CFTR/V-ATPase function. Coimmunoprecipitation studies examined V-ATPase interaction with CFTR. The pH-sensitive dye BCECF determined proton efflux and its dependence on V-ATPase/CFTR in intestinal cells. cAMP increased V-ATPase/CFTR colocalization in the apical domain of intestinal cells and redistributed the V-ATPase Voa1 and Voa2 trafficking subunits from the basolateral membrane to the brush border membrane. Voa1 and Voa2 subunits were localized to endosomes beneath the terminal web in untreated CFTR(-/-) intestine but redistributed to the subapical cytoplasm following cAMP treatment. Inhibition of CFTR or V-ATPase significantly decreased pHi in cells, confirming their functional interdependence. These data establish that V-ATPase traffics into the brush border membrane to regulate proton efflux and this activity is dependent on CFTR in the intestine.

Golgi pH, its regulation and roles in human disease

Most organelles within the exocytic and endocytic pathways typically acidify their interiors, a phenomenon that is known to be crucial for their optimal functioning in eukaryotic cells. This review highlights recent advances in our understanding of how Golgi acidity is maintained and regulated, and how its misregulation contributes to organelle dysfunction and disease. Both its biosynthetic products (glycans) and protein-sorting events are highly sensitive to changes in Golgi luminal pH and are affected in certain human disease states such as cancers and cutis laxa. Other potential disease states that are caused by, or are associated with, Golgi pH misregulation will also be discussed.

CFTR, mucins, and mucus obstruction in cystic fibrosis

Mucus pathology in cystic fibrosis (CF) has been known for as long as the disease has been recognized and is sometimes called mucoviscidosis. The disease is marked by mucus hyperproduction and plugging in many organs, which are usually most fatal in the airways of CF patients, once the problem of meconium ileus at birth is resolved. After the CF gene, CFTR, was cloned and its protein product identified as a cAMP-regulated Cl(-) channel, causal mechanisms underlying the strong mucus phenotype of the disease became obscure. Here we focus on mucin genes and polymeric mucin glycoproteins, examining their regulation and potential relationships to a dysfunctional cystic fibrosis transmembrane conductance regulator (CFTR). Detailed examination of CFTR expression in organs and different cell types indicates that changes in CFTR expression do not always correlate with the severity of CF disease or mucus accumulation. Thus, the mucus hyperproduction that typifies CF does not appear to be a direct cause of a defective CFTR but, rather, to be a downstream consequence. In organs like the lung, up-regulation of mucin gene expression by inflammation results from chronic infection; however, in other instances and organs, the inflammation may have a non-infectious origin. The mucus plugging phenotype of the β-subunit of the epithelial Na(+) channel (βENaC)-overexpressing mouse is proving to be an archetypal example of this kind of inflammation, with a dehydrated airway surface/concentrated mucus gel apparently providing the inflammatory stimulus. Data indicate that the luminal HCO(3)(-) deficiency recently described for CF epithelia may also provide such a stimulus, perhaps by causing a mal-maturation of mucins as they are released onto luminal surfaces. In any event, the path between CFTR dysfunction and mucus hyperproduction has proven tortuous, and its unraveling continues to offer its own twists and turns, along with fascinating glimpses into biology.

Expression of cystic fibrosis transmembrane conductance regulator in paracervical ganglia

The cystic fibrosis transmembrane conductance regulator (CFTR) is an important protein that acts as a chloride channel and regulates many physiological functions, including salt transport and fluid flow. Mutations in the gene encoding the CFTR protein cause cystic fibrosis. CFTR is expressed in the epithelial cells of the lungs, pancreas, intestines, and other organs. In the peripheral and central nervous system, CFTR expression has been detected in the neurons of rat brains, ganglion cells of rat hearts, human hypothalamus, human spinal cord, and human spinal and sympathetic ganglia. However, CFTR has not been identified in other parts of the nervous system. In this study, we used immunohistochemistry, in situ hybridization, and laser-assisted microdissection (LMD) followed by reverse transcriptase (RT) PCR to identify CFTR proteins and messenger RNA in human and rat paracervical ganglion cells. CFTR and its gene expression were both detected in paracervical ganglion cells, a finding that might link this important protein to the neuronal regulation of female urogenital function. These findings could provide new insights into the symptoms related to the reproductive system frequently observed in female cystic fibrosis patients.

Chloride channels of intracellular membranes

Proteins implicated as intracellular chloride channels include the intracellular ClC proteins, the bestrophins, the cystic fibrosis transmembrane conductance regulator, the CLICs, and the recently described Golgi pH regulator. This paper examines current hypotheses regarding roles of intracellular chloride channels and reviews the evidence supporting a role in intracellular chloride transport for each of these proteins.

Expression and distribution of cystic fibrosis transmembrane conductance regulator in neurons of the human brain

The importance of the molecule cystic fibrosis transmembrane conductance regulator (CFTR) is reflected in the many physiological functions it regulates. It is known to be present in epithelial cells of the lungs, pancreas, sweat glands, gut, and other tissues, and gene mutations of CFTR cause cystic fibrosis (CF). We studied the expression and distribution of CFTR in the human brain with reverse transcriptase polymerase chain reaction, in situ hybridization, and immunohistochemistry. This study demonstrates widespread and abundant expression of CFTR in neurons of the human brain. Techniques of double labeling and evaluation of consecutive tissue sections localized CFTR protein and mRNA signals to the cytoplasm of neurons in all regions of the brain studied, but not to glial cells. The presence of CFTR in central neurons not only provides a possible explanation for the neural symptoms observed in CF patients, but also may lead to a better understanding of the functions of CFTR in the human brain.

Red fluorescent protein pH biosensor to detect concentrative nucleoside transport

Human concentrative nucleoside transporter, hCNT3, mediates Na+/nucleoside and H+/nucleoside co-transport. We describe a new approach to monitor H+/uridine co-transport in cultured mammalian cells, using a pH-sensitive monomeric red fluorescent protein variant, mNectarine, whose development and characterization are also reported here. A chimeric protein, mNectarine fused to the N terminus of hCNT3 (mNect.hCNT3), enabled measurement of pH at the intracellular surface of hCNT3. mNectarine fluorescence was monitored in HEK293 cells expressing mNect.hCNT3 or mNect.hCNT3-F563C, an inactive hCNT3 mutant. Free cytosolic mNect, mNect.hCNT3, and the traditional pH-sensitive dye, BCECF, reported cytosolic pH similarly in pH-clamped HEK293 cells. Cells were incubated at the permissive pH for H(+)-coupled nucleoside transport, pH 5.5, under both Na(+)-free and Na(+)-containing conditions. In mNect.hCNT3-expressing cells (but not under negative control conditions) the rate of acidification increased in media containing 0.5 mm uridine, providing the first direct evidence for H(+)-coupled uridine transport. At pH 5.5, there was no significant difference in uridine transport rates (coupled H+ flux) in the presence or absence of Na+ (1.09 +/- 0.11 or 1.18 +/- 0.32 mm min(-1), respectively). This suggests that in acidic Na(+)-containing conditions, 1 Na+ and 1 H+ are transported per uridine molecule, while in acidic Na(+)-free conditions, 1 H+ alone is transported/uridine. In acid environments, including renal proximal tubule, H+/nucleoside co-transport may drive nucleoside accumulation by hCNT3. Fusion of mNect to hCNT3 provided a simple, self-referencing, and effective way to monitor nucleoside transport, suggesting an approach that may have applications in assays of transport activity of other H(+)-coupled transport proteins.

Cystic fibrosis transmembrane conductance regulator expression in human spinal and sympathetic ganglia

Cystic fibrosis transmembrane conductance regulator (CFTR) is a chloride channel protein, and mutations of its gene cause cystic fibrosis. CFTR is known to be expressed in epithelial cells of the respiratory, digestive and reproductive tracts. It is also present in rat neurons and heart ganglion cells. In humans, it is expressed in the hypothalamus, but has not been identified in other parts of the human nervous system. In this study, immunohistochemistry, double-staining immunofluorescence, in situ hybridization, nested reverse transcription-PCR and relative quantification of real-time PCR analysis were performed on spinal and sympathetic ganglia from seven human autopsies with no known nervous system disease. CFTR protein was expressed in most ganglion cells with no obvious difference in the amounts of mRNA transcript in ganglia of different sites. We conclude that CFTR protein and its mRNA were extensively expressed at relatively constant levels in human spinal and sympathetic ganglion cells, and may be important in physiological and pathological conditions. Moreover, CFTR in ganglia may be associated with pathophysiological changes seen in cystic fibrosis.

Defective organellar acidification as a cause of cystic fibrosis lung disease: reexamination of a recurring hypothesis

The cellular mechanisms by which loss-of-function mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel produce cystic fibrosis (CF) lung disease remain uncertain. Defective organellar function has been proposed as an important determinant in the pathogenesis of CF lung disease. According to one hypothesis, reduced CFTR chloride conductance in organelles in CF impairs their acidification by preventing chloride entry into the organelle lumen, which is needed to balance the positive charge produced by proton entry. According to a different hypothesis, CFTR mutation hyperacidifies organelles by an indirect mechanism involving unregulated sodium efflux through epithelial sodium channels. There are reports of defective Golgi, endosomal and lysosomal acidification in CF epithelial cells, defective phagolysosomal acidification in CF alveolar macrophages, and organellar hyperacidification in CF respiratory epithelial cells. The common theme relating too high or low organellar pH to cellular dysfunction and CF pathogenesis is impaired functioning of organellar enzymes, such as those involved in ceramide metabolism and protein processing in epithelial cells and antimicrobial activity in alveolar macrophages. We review here the evidence for defective organellar acidification in CF. Significant technical and conceptual concerns are discussed regarding the validity of data showing too high/low organellar pH in CF cells, and rigorous measurements of organellar pH in CF cells are reviewed that fail to support defective organellar acidification in CF. Indeed, there is an expanding body of evidence supporting the involvement of non-CFTR chloride channels in organellar acidification. We conclude that biologically significant involvement of CFTR in organellar acidification is unlikely.

Revisiting the role of cystic fibrosis transmembrane conductance regulator and counterion permeability in the pH regulation of endocytic organelles

Organellar acidification by the electrogenic vacuolar proton-ATPase is coupled to anion uptake and cation efflux to preserve electroneutrality. The defective organellar pH regulation, caused by impaired counterion conductance of the mutant cystic fibrosis transmembrane conductance regulator (CFTR), remains highly controversial in epithelia and macrophages. Restricting the pH-sensitive probe to CFTR-containing vesicles, the counterion and proton permeability, and the luminal pH of endosomes were measured in various cells, including genetically matched CF and non-CF human respiratory epithelia, as well as cftr(+/+) and cftr(-/-) mouse alveolar macrophages. Passive proton and relative counterion permeabilities, determinants of endosomal, lysosomal, and phagosomal pH-regulation, were probed with FITC-conjugated transferrin, dextran, and Pseudomonas aeruginosa, respectively. Although CFTR function could be documented in recycling endosomes and immature phagosomes, neither channel activation nor inhibition influenced the pH in any of these organelles. CFTR heterologous overexpression also failed to alter endocytic organellar pH. We propose that the relatively large CFTR-independent counterion and small passive proton permeability ensure efficient shunting of the proton-ATPase-generated membrane potential. These results have implications in the regulation of organelle acidification in general and demonstrate that perturbations of the endolysosomal organelles pH homeostasis cannot be linked to the etiology of the CF lung disease.

Unimpaired lysosomal acidification in respiratory epithelial cells in cystic fibrosis

The mechanisms remain uncertain by which mutations in CFTR cause lung disease in cystic fibrosis (CF). Teichgräber et al. recently reported increased ceramide in CF lungs, which was proposed to result from defective lysosomal acidification in airway epithelial cells and consequent impairment of pH-dependent ceramide-metabolizing enzymes (Teichgräber, V., Ulrich, M., Endlich, N., Reithmüller, J., Wilker, B., Conceição Ce Olivereira-Munding, C., van Heeckeren, A. M., Barr, M. L., von Kürthy, G., Schmid, K. W., Weller, M., Tümmler, B., Lang, F., Grassme, H., Döring, G., and Gulbins, E. (2008) Nat. Med. 14, 382-391). Here, we measured lysosomal pH in several CFTR-expressing and -deficient cell lines, freshly isolated airway epithelial cells from non-CF and CF mice and humans, and well-differentiated primary cultures of human non-CF and CF airway epithelial cells. Lysosomal pH was measured by ratio imaging using a fluorescent pH indicator consisting of 40-kDa dextran conjugated to Oregon Green 488 and tetramethylrhodamine. In all cell types, lysosomal pH was approximately 4.5, unaffected by the thiazolidinone CFTR inhibitor CFTR(inh)-172, and increased to approximately 6.5 following bafilomycin inhibition of the vacuolar proton pump. Lysosomal pH did not differ significantly in airway epithelial cells from non-CF versus CF humans or mice. Our results provide direct evidence against the conclusions of Teichgräber et al. that lysosomal acidification is CFTR-dependent, impaired in CF, or responsible for ceramide accumulation. As such, alternative mechanisms are needed to explain increased ceramide in CF airways. Non-CFTR mechanisms, such as ClC-type chloride channels, are likely involved in maintaining electroneutrality during organellar acidification.

Organelle redox of CF and CFTR-corrected airway epithelia

In cystic fibrosis reduced CFTR function may alter redox properties of airway epithelial cells. Redox-sensitive GFP (roGFP1) and imaging microscopy were used to measure the redox potentials of the cytosol, endoplasmic reticulum (ER), mitochondria, and cell surface of cystic fibrosis nasal epithelial cells and CFTR-corrected cells. We also measured glutathione and cysteine thiol redox states in cell lysates and apical fluids to provide coverage over a range of redox potentials and environments that might be affected by CFTR. As measured with roGFP1, redox potentials at the cell surface (approx -207+/-8 mV) and in the ER (approx -217+/-1 mV) and rates of regulation of the apical fluid and ER lumen after DTT treatment were similar for CF and CFTR-corrected cells. CF and CFTR-corrected cells had similar redox potentials in mitochondria (-344+/-9 mV) and cytosol (-322+/-7 mV). Oxidation of carboxydichlorodihydrofluorescein diacetate and of apical Amplex red occurred at equal rates in CF and CFTR-corrected cells. Glutathione and cysteine redox couples in cell lysates and apical fluid were equal in CF and CFTR-corrected cells. These quantitative estimates of organelle redox potentials combined with apical and cell measurements using small-molecule couples confirmed there were no differences in the redox properties of CF and CFTR-corrected cells.

Pharmacological modulation of cGMP levels by phosphodiesterase 5 inhibitors as a therapeutic strategy for treatment of respiratory pathology in cystic fibrosis

The CFTR gene encodes a chloride channel with pleiotropic effects on cell physiology and metabolism. Here, we show that increasing cGMP levels to inhibit epithelial Na(+) channel in cystic fibrosis (CF) respiratory epithelial cells corrects several aspects of the downstream pathology in CF. Cell culture models, using a range of CF cell lines and primary cells, showed that complementary pharmacological approaches to increasing intracellular cGMP, by elevating guanyl cyclase activity though reduced nitric oxide, addition of cell-permeable cGMP analogs, or inhibition of phosphodiesterase 5 corrected multiple aspects of the CF pathological cascade. These included correction of defective protein glycosylation, bacterial adherence, and proinflammatory responses. Furthermore, pharmacological inhibition of phosphodiesterase 5 in tissues ex vivo or in animal models improved transepithelial currents across nasal mucosae from transgenic F508del Cftr(tm1Eur) mice and reduced neutrophil infiltration on bacterial aerosol challenge in Pseudomonas aeruginosa-susceptible DBA/2 mice. Our findings define phosphodiesterase 5 as a specific target for correcting a number of previously disconnected defects in the CF respiratory tract, now linked through this study. Our study suggests that phosphodiesterase 5 inhibition provides an opportunity for simultaneous and concerted correction of seemingly disparate complications in CF.

CFTR regulates phagosome acidification in macrophages and alters bactericidal activity

Acidification of phagosomes has been proposed to have a key role in the microbicidal function of phagocytes. Here, we show that in alveolar macrophages the cystic fibrosis transmembrane conductance regulator Cl- channel (CFTR) participates in phagosomal pH control and has bacterial killing capacity. Alveolar macrophages from Cftr-/- mice retained the ability to phagocytose and generate an oxidative burst, but exhibited defective killing of internalized bacteria. Lysosomes from CFTR-null macrophages failed to acidify, although they retained normal fusogenic capacity with nascent phagosomes. We hypothesize that CFTR contributes to lysosomal acidification and that in its absence phagolysosomes acidify poorly, thus providing an environment conducive to bacterial replication.

Innate immune response in CF airway epithelia: hyperinflammatory?

The lack of functional cystic fibrosis (CF) transmembrane conductance regulator (CFTR) in the apical membranes of CF airway epithelial cells abolishes cAMP-stimulated anion transport, and bacteria, eventually including Pseudomonas aeruginosa, bind to and accumulate in the mucus. Flagellin released from P. aeruginosa triggers airway epithelial Toll-like receptor 5 and subsequent NF-kappaB signaling and production and release of proinflammatory cytokines that recruit neutrophils to the infected region. This response has been termed hyperinflammatory because so many neutrophils accumulate; a response that damages CF lung tissue. We first review the contradictory data both for and against the idea that epithelial cells exhibit larger-than-normal proinflammatory signaling in CF compared with non-CF cells and then review proposals that might explain how reduced CFTR function could activate such proinflammatory signaling. It is concluded that apparent exaggerated innate immune response of CF airway epithelial cells may have resulted not from direct effects of CFTR on cellular signaling or inflammatory mediator production but from indirect effects resulting from the absence of CFTRs apical membrane channel function. Thus, loss of Cl-, HCO3-, and glutathione secretion may lead to reduced volume and increased acidification and oxidation of the airway surface liquid. These changes concentrate proinflammatory mediators, reduce mucociliary clearance of bacteria and subsequently activate cellular signaling. Loss of apical CFTR will also hyperpolarize basolateral membrane potentials, potentially leading to increases in cytosolic [Ca2+], intracellular Ca2+, and NF-kappaB signaling. This hyperinflammatory effect of CF on intracellular Ca2+ and NF-kappaB signaling would be most prominently expressed during exposure to both P. aeruginosa and also endocrine, paracrine, or nervous agonists that activate Ca2+ signaling in the airway epithelia.

Elevated Golgi pH in breast and colorectal cancer cells correlates with the expression of oncofetal carbohydrate T-antigen

Altered glycosylation has turned out to be a universal feature of cancer cells, and in many cases, to correlate with altered expression or localization of relevant glycosyltransferases. However, no such correlation exists between observed enzymatic changes and the expression of the oncofetal Thomsen-Friedenreich (T)-antigen, a core 1 (Gal-beta1 --> 3-GalNAc-ser/thr) carbohydrate structure. Here we report that T-antigen expression, instead, correlates with elevated Golgi pH in cancer cells. Firstly, using a Golgi-targeted green fluorescent protein (GT-EGFP) as a probe, we show that the medial/trans-Golgi pH (pHG) in a high proportion of breast (MCF-7) and colorectal (HT-29, SW-48) cancer cells is significantly more alkaline (pHG > or = 6.75) than that of control cells (pHG 5.9-6.5). The pH gradient between the cytoplasm and the Golgi lumen is also markedly reduced in MCF-7 cells, suggesting a Golgi acidification defect. Secondly, we show that T-antigen expression is highly sensitive to changes in Golgi pH, as only a 0.2 pH unit increase was sufficient to increase T-antigen expression in control cells. Thirdly, we found that T-antigen expressing MCF-7 cells have 0.3 pH units more alkaline Golgi pH than non-expressing MCF-7 cells. Fourthly, in all cell types examined, we observed significant correlation between the number of T-antigen expressing cells and cells with a markedly elevated Golgi pH (pHG > or = 6.75). Consistent with these observations in cultured cells, cells in solid tumors also heterogenously expressed the T-antigen. Thus, elevated Golgi pH appears to be directly linked to T-antigen expression in cancer cells, but it may also act as a more general factor for altered glycosylation in cancer by affecting the distribution of Golgi-localized glycosyltransferases.

Endosomal hyperacidification in cystic fibrosis is due to defective nitric oxide-cylic GMP signalling cascade

Endosomal hyperacidification in cystic fibrosis (CF) respiratory epithelial cells is secondary to a loss of sodium transport control owing to a defective form of the CF transmembrane conductance regulator CFTR. Here, we show that endosomal hyperacidification can be corrected by activating the signalling cascade controlling sodium channels through cyclic GMP. Nitric oxide (NO) donors corrected the endosomal hyperacidification in CF cells. Stimulation of CF cells with guanylate cyclase agonists corrected the pH in endosomes. Exposure of CF cells to an inhibitor of cGMP-specific phosphodiesterase PDE5, Sildenafil, normalized the endosomal pH. Treatment with Sildenafil reduced secretion by CF cells of the proinflammatory chemokine interleukin 8 following stimulation with Pseudomonas aeruginosa products. Thus, the endosomal hyperacidification and excessive proinflammatory response in CF are in part due to deficiencies in NO- and cGMP-regulated processes and can be pharmacologically reversed using PDE5 inhibitors.

Chloride/proton antiporter activity of mammalian CLC proteins ClC-4 and ClC-5

ClC-4 and ClC-5 are members of the CLC gene family, with ClC-5 mutated in Dent's disease, a nephropathy associated with low-molecular-mass proteinuria and eventual renal failure. ClC-5 has been proposed to be an electrically shunting Cl- channel in early endosomes, facilitating intraluminal acidification. Motivated by the discovery that certain bacterial CLC proteins are secondary active Cl-/H+ antiporters, we hypothesized that mammalian CLC proteins might not be classical Cl- ion channels but might exhibit Cl(-)-coupled proton transport activity. Here we report that ClC-4 and ClC-5 carry a substantial amount of protons across the plasma membrane when activated by positive voltages, as revealed by measurements of pH close to the cell surface. Both proteins are able to extrude protons against their electrochemical gradient, demonstrating secondary active transport. H+, but not Cl-, transport was abolished when a pore glutamate was mutated to alanine (E211A). ClC-0, ClC-2 and ClC-Ka proteins showed no significant proton transport. The muscle channel ClC-1 exhibited a small H+ transport that might be physiologically relevant. For ClC-5, we estimated that Cl- and H+ transport contribute about equally to the total charge movement, raising the possibility that the coupled Cl-/H+ transport of ClC-4 and ClC-5 is of significant magnitude in vivo.

Renal vacuolar H+-ATPase

Vacuolar H(+)-ATPases are ubiquitous multisubunit complexes mediating the ATP-dependent transport of protons. In addition to their role in acidifying the lumen of various intracellular organelles, vacuolar H(+)-ATPases fulfill special tasks in the kidney. Vacuolar H(+)-ATPases are expressed in the plasma membrane in the kidney almost along the entire length of the nephron with apical and/or basolateral localization patterns. In the proximal tubule, a high number of vacuolar H(+)-ATPases are also found in endosomes, which are acidified by the pump. In addition, vacuolar H(+)-ATPases contribute to proximal tubular bicarbonate reabsorption. The importance in final urinary acidification along the collecting system is highlighted by monogenic defects in two subunits (ATP6V0A4, ATP6V1B1) of the vacuolar H(+)-ATPase in patients with distal renal tubular acidosis. The activity of vacuolar H(+)-ATPases is tightly regulated by a variety of factors such as the acid-base or electrolyte status. This regulation is at least in part mediated by various hormones and protein-protein interactions between regulatory proteins and multiple subunits of the pump.

The pH of the secretory pathway: measurement, determinants, and regulation

The luminal pH of the secretory pathway plays a critical role in the posttranslational modification and sorting of proteins and lipids. The pH of each one of the organelles that constitute the pathway is unique, becoming more acidic as the biosynthetic cargo approaches its destination. The methods used for measurement of pH in the secretory pathway, its determinants, and its regulation are the subjects of this review.

Mucins and their O-Glycans from human bronchial epithelial cell cultures

A longstanding question in obstructive airway disease is whether observed changes in mucin composition and/or posttranslational glycosylation are due to genetic or to environmental factors. We tested whether the mucins secreted by second-passage primary human bronchial epithelial cell cultures derived from noncystic fibrosis (CF) or CF patients have intrinsically different specific mucin compositions, and whether these mucins are glycosylated differently. Both CF and non-CF cultures produced MUC5B, predominantly, as judged by quantitative agarose gel Western blots with mucin-specific antibodies: MUC5B was present at approximately 10-fold higher levels than MUC5AC, consistent with our previous mRNA studies (Bernacki SH, Nelson AL, Abdullah L, Sheehan JK, Harris A, William DC, and Randell SH. Am J Respir Cell Mol Biol 20: 595-604, 1999). O-linked oligosaccharides released from purified non-CF and CF mucins and studied by HPLC mass spectrometry had highly variable glycan structures, and there were no observable differences between the two groups. Hence, there were no differences in either the specific mucins or their O-glycans that correlated with the CF phenotype under the noninfected/noninflammatory conditions of cell culture. We conclude that the differences observed in the mucins sampled directly from patients are most likely due to environmental factors relating to infection and/or inflammation.

Acidification and protein traffic

Acidification of some organelles, including the Golgi complex, lysosomes, secretory granules, and synaptic vesicles, is important for many of their biochemical functions. In addition, acidic pH in some compartments is also required for the efficient sorting and trafficking of proteins and lipids along the biosynthetic and endocytic pathways. Despite considerable study, however, our understanding of how pH modulates membrane traffic remains limited. In large part, this is due to the diversity of methods to perturb and monitor pH, as well as to the difficulties in isolating individual transport steps within the complex pathways of membrane traffic. This review summarizes old and recent evidence for the role of acidification at various steps of biosynthetic and endocytic transport in mammalian cells. We describe the mechanisms by which organelle pH is regulated and maintained, as well as how organelle pH is monitored and quantitated. General principles that emerge from these studies as well as future directions of interest are discussed.

pH of TGN and recycling endosomes of H+/K+-ATPase-transfected HEK-293 cells: implications for pH regulation in the secretory pathway

The influences of the gastric H+/K+ pump on organelle pH during trafficking to and from the plasma membrane were investigated using HEK-293 cells stably expressing the alpha- and beta-subunits of human H+/K+-ATPase (H+/K+-alpha,beta cells). The pH values of trans-Golgi network (pHTGN) and recycling endosomes (pHRE) were measured by transfecting H+/K+-alpha,beta cells with the pH-sensitive GFP pHluorin fused to targeting sequences of either TGN38 or synaptobrevin, respectively. Immunofluorescence showed that H+/K+-ATPase was present in the plasma membrane, TGN, and RE. The pHTGN was similar in both H+/K+-alpha,beta cells (pHTGN 6.36) and vector-transfected ("mock") cells (pHTGN 6.34); pHRE was also similar in H+/K+-alpha,beta (pHRE 6.40) and mock cells (pHRE 6.37). SCH28080 (inhibits H+/K+-ATPase) caused TGN to alkalinize by 0.12 pH units; subsequent addition of bafilomycin (inhibits H+ v-ATPase) caused TGN to alkalinize from pH 6.4 up to a new steady-state pHTGN of 7.0-7.5, close to pHcytosol. Similar results were observed in RE. Thus H+/K+-ATPases that trafficked to the plasma membrane were active but had small effects to acidify the TGN and RE compared with H+ v-ATPase. Mathematical modeling predicted a large number of H+ v-ATPases (8000) active in the TGN to balance a large, passive H+ leak (with PH approximately 10-3 cm/s) via unidentified pathways out of the TGN. We propose that in the presence of this effective, though inefficient, buffer system in the Golgi and TGN, H+/K+-ATPases (estimated to be approximately 4000 active in the TGN) and other transporters have little effect on luminal pH as they traffic to the plasma membrane.

Voltage-gated proton channels and other proton transfer pathways

Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H(+) channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H(+) channels are expressed in many cells. During the respiratory burst in phagocytes, H(+) current compensates for electron extrusion by NADPH oxidase. Most evidence indicates that the H(+) channel is not part of the NADPH oxidase complex, but rather is a distinct and as yet unidentified molecule.

Determinants of [Cl-] in recycling and late endosomes and Golgi complex measured using fluorescent ligands

Chloride concentration ([Cl-]) was measured in defined organellar compartments using fluorescently labeled transferrin, alpha2-macroglobulin, and cholera toxin B-subunit conjugated with Cl--sensitive and -insensitive dyes. In pulse-chase experiments, [Cl-] in Tf-labeled early/recycling endosomes in J774 cells was 20 mM just after internalization, increasing to 41 mM over approximately 10 min in parallel to a drop in pH from 6.91 to 6.05. The low [Cl-] just after internalization (compared with 137 mM solution [Cl-]) was prevented by reducing the interior-negative Donnan potential. [Cl-] in alpha2-macroglobulin-labeled endosomes, which enter a late compartment, increased from 28 to 58 mM at 1-45 min after internalization, whereas pH decreased from 6.85 to 5.20. Cl- accumulation was prevented by bafilomycin but restored by valinomycin. A Cl- channel inhibitor slowed endosomal acidification and Cl- accumulation by approximately 2.5-fold. [Cl-] was 49 mM and pH was 6.42 in cholera toxin B subunit-labeled Golgi complex in Vero cells; Golgi compartment Cl- accumulation and acidification were reversed by bafilomycin. Our experiments provide evidence that Cl- is the principal counter ion accompanying endosomal and Golgi compartment acidification, and that an interior-negative Donnan potential is responsible for low endosomal [Cl-] early after internalization. We propose that reduced [Cl-] and volume in early endosomes permits endosomal acidification and [Cl-] accumulation without lysis.

Organelle acidification and disease

A subset of cellular compartments maintain acidic interior environments that are critical for the specific functions of each organelle and for cell growth and survival in general. The pH of each organelle reflects the balance between proton pumping, counterion conductance, and proton leak. Alterations in steady-state organelle pH due to defects in either proton pumping activity or counterion conductance have been suggested to contribute to the pathology of several diseases; however, definitive evidence remains elusive. This review describes recent evidence for the misregulation of organelle pH in the progression of cancer, Dent's disease, and cystic fibrosis.

Hyperacidification in cystic fibrosis: links with lung disease and new prospects for treatment

A new link between the genetic defect and lung pathology in cystic fibrosis (CF) has been established by the recent discovery of an abnormally acidic pH in the organelles of CF respiratory epithelial cells, along with an increased acidity of the CF airway surface liquid. The defect in cystic fibrosis transmembrane resistance regulator (CFTR) results in hyperacidification of the trans-Golgi network, an organelle responsible for glycosylation, and protein- and membrane-sorting in mammalian cells. Hyperacidification and altered surface glycoconjugates might contribute to mucus thickening, bacterial adhesion and colonization, inflammation, and irreversible tissue damage. The increased acidity of the intracellular organelles and of the lung lining in CF could be linked, and both represent potential therapeutic targets.