Cystic Fibrosis Transmembrane Conductance ... - ATS Journals

Rapid Communication Cystic Fibrosis Transmembrane Conductance Regulator Is Expressed in Mucin Granules from Calu-3 and Primary Human Airway Epithelial Cells Pierre LeSimple1, Julie Goepp1, Melissa L. Palmer2, Scott C. Fahrenkrug2, Scott M. O’Grady2, Pasquale Ferraro3, Renaud Robert1, and John W. Hanrahan1,4 1 Department of Physiology, McGill University, Montreal, Quebec, Canada; 2Departments of Physiology and Animal Science, University of Minnesota, St. Paul, Minnesota; 3Division of Thoracic Surgery, Centre Hospitalier de l’Universite´ de Montre´al, Montreal, Quebec, Canada; and 4 Research Institute of the McGill University Health Centre, Montreal, Quebec, Canada

Cystic fibrosis (CF) is caused by mutations in the tightly regulated anion channel cystic fibrosis transmembrane conductance regulator (CFTR), yet much of the pathology in this disease results from mucus obstruction of the small airways and other organs. Mucus stasis has been attributed to the abnormal luminal environment of CF airways, which results from dehydration of the mucus gel or low bicarbonate concentration. We show here that CFTR and MUC5AC are present in single mucin–containing granules isolated from a human airway epithelial cell line and from highly differentiated airway primary cell cultures. CFTR was not detected in MUC5AC granules from CFTR knockdown cells or CF primary cells. The results suggest a direct link between CFTR and the mucus defect. Keywords: cystic fibrosis; CFTR; mucus

Abnormal mucus accumulation is among the most prominent clinical features of cystic fibrosis (CF), as indicated by the other name given to this disease, mucoviscidosis (1). Mucus plugging of the small airways causes cycles of infection and inflammation, which lead to a gradual decline in pulmonary function and most of the morbidity and mortality in CF. CF is an autosomal recessive disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a plasma membrane anion channel that mediates cyclic adenosine monophosphate–stimulated chloride and bicarbonate secretion. How the dysfunction of this anion channel leads to mucus pathology remains unknown. CFTR is thought to influence mucus properties by altering the airway lumen microenvironment through reduced secretion and fluid hyperabsorption, which dehydrate the mucus gel (2). An alternative hypothesis is that a deficiency in CFTR reduces the availability of luminal bicarbonate for buffering Ca21 and H1, which is required for normal unpacking of mucin chains during exocytosis (3). Although these mechanisms are consistent with the well-established role of CFTR in transepithelial fluid and bicarbonate secretion, mucus stasis also occurs in exocrine ducts (Received in original form October 18, 2012 and in final form May 17, 2013) This work was supported by Cystic Fibrosis Canada (BREATHE Program) and the Canada Foundation for Innovation. Correspondence and requests for reprints should be addressed to John Hanrahan, Ph.D., Department of Physiology, McGill University, 3655 Promenade Sir William Osler, Montre´al, PQ, H3G 1Y6 Canada. E-mail: [email protected] This article has an online supplement, which is accessible from this issue’s table of contents at www.atsjournals.org Am J Respir Cell Mol Biol Vol 49, Iss. 4, pp 511–516, Oct 2013 Copyright ª 2013 by the American Thoracic Society Originally Published in Press as DOI: 10.1165/rcmb.2012-0419RC on June 6, 2013 Internet address: www.atsjournals.org

CLINICAL RELEVANCE Abnormal mucus is a hallmark of cystic fibrosis. However, the dependence of mucus properties on cystic fibrosis transmembrane conductance regulator (CFTR), the anion channel that is mutated in this disease, is not well understood. Here we show CFTR and MUC5AC coimmunostaining in granules isolated from air–liquid interface cultures of the airway epithelial cell line Calu-3 and from primary airway epithelial cells. Conductance of the granules as measured using acridine orange fluorescence was anion dependent and sensitive to the CFTR inhibitor CFTRInh-172. Although CFTR expression in the granule membrane is low, its absence may contribute to the mucus abnormality in cystic fibrosis.

where there is no fluid absorption or evaporation, and therefore an essential role of mucus dehydration in mucus obstruction is not established (4). Bicarbonate is a strong calcium chelator (3); however, it seems unlikely that the reduction in HCO32 concentration from 12.3 to 3.2 mM (5) in CF secretions appearing on the airway surface could explain defective mucin unpacking, and the HCO32 concentration has not been measured within the submucosal glands where most mucus is produced. CFTR could influence the maturation and packaging of mucins as well as its release if it was expressed in airway mucous cells. However, the localization of CFTR in airway epithelia has been controversial. CFTR immunostaining is observed in submucosal gland duct cells and in ciliated surface epithelial cells where it regulates ciliary beat frequency (6). CFTR in ciliated cells may mediate chloride secretion because its expression specifically in ciliated cells restores forskolin-stimulated Cl2 transport across cftr2/2 mouse trachea (7). CFTR has also been identified in goblet (8) and submucosal gland serous cells (9). However, those results have been questioned based on studies using different antibodies (10). Nevertheless, conventional immunostaining seems problematic in airway epithelia; CFTR was also not detected in alveolar epithelial cells (10), where there is now compelling evidence for CFTR function (11–13) and where immunostaining has been reported by some groups (11, 12). We show here using different immunostaining and imaging methods that isolated mucin granules from normal airway epithelial cells contain functional CFTR and the secreted mucin MUC5AC, whereas only MUC5AC is detected in granules from CFTR-knockdown (KD) cells and highly differentiated primary cells from patients with CF. The results have profound implications for the mechanism by which CFTR mutations lead to abnormal mucus properties in CF.

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MATERIALS AND METHODS Cell Culture CFTR knock down Calu-3 cells stably expressing shRNA against CFTR (Calu-3-KD, . 95% knock-down) and control Calu-3 cells expressing mutated shRNA were cultured as described previously (13). CF and non-CF tissues were obtained during lung transplantation using a protocol and informed consent form approved by the Institutional Review Board of the Research Ethics Office of McGill University. Primary cell culture procedures were adapted from Randell and colleagues (17, 18). First-passage cells were used for all primary cultures.

Electron Microscopy Cultures were rinsed with Hanks’ balanced salt solution, fixed, embedded in epon, sectioned, and observed as described in the online supplement.

Histology and Immunostaining of Sections Cultures were fixed in 10% buffered formalin, dehydrated to 100% ethanol, cleared using Slide-Brite (Thermo Fisher Sci., Burlington, ON, Canada), and embedded in paraffin. Calu-3 goblet cells and mucus were demonstrated by alcian blue–periodic acid Schiff staining and immunostained after rehydration in successive xylene and ethanol baths as described in the online supplement.

Immunoblotting Granule preparations from 3.5 million unpolarized Calu-3 cells were mixed in Laemmli buffer and separated by migration on 10% acrylamide gels. Proteins were transferred to nitrocellulose membrane, and the presence of CFTR was revealed using the 23C5 monoclonal antibody (1/50) and a peroxidase-conjugated goat antimouse antibody (1/5,000).

Isolation and Immunostaining of Single Mucin Granules Cells were cultured at the air–liquid interface for at least 10 days postconfluence and scraped into cold PBS, pelleted, resuspended in lysis buffer, disrupted using a Dounce homogenizer, and cleared of nuclei by centrifugation. Granules were isolated on a Percoll gradient, pelleted by centrifugation, and immunostained as described in the online supplement.

Image Acquisition and Processing Confocal images were acquired with a laser scanning microscope (LSM 510 META; Carl Zeiss, Toronto, ON, Canada) and a 363/1.4 numerical aperture oil objective. Structured light illumination (superresolution) imaging was performed as a service by Applied Precision, Issaquah, Washington using a DeltaVision OMX. All images were processed using ImageJ (NIH, Bethesda, MD). Volume and surface reconstitutions were performed using the VolumeJ plugin (14).

Anion Conductance Assay Cl2 conductance of the granule membrane was assayed indirectly by monitoring luminal pH. Granules were resuspended in Cl2–free solution (mM: 2.1 MOPS buffer, 10 CsOH monohydrate, 1 MgSO4) and loaded with acridine orange (A.O.) (1 mM). Protein kinase A (100 U/ml), 4-(chlorophenylthio)adenosine-39,59-cyclic monophosphate (100 mM), and ATP (1 mM) were added to activate CFTR and induce proton pump–mediated acidification of the granules. A.O. fluorescence emission at 528 nm was followed over time using a Cary Eclipse fluorescence spectrophotometer (Varian, Palo Alto, CA) with excitation at 492 nm, and external Cl2 concentration was increased by the addition of 5 M cholineCl2. A saturating level of CFTRInh-172 (nominally 50 mM; solubility z 20 mM) (15) was added as indicated to inhibit CFTR activity.

RESULTS CFTR and MUC5AC Are Expressed in Polarized Calu-3 Cells

Uniform apical CFTR immunostaining was observed in polarized monolayers of control Calu-3 cells (Figure 1A, green,

Figure 1. Coexpression of cystic fibrosis transmembrane conductance regulator (CFTR) and MUC5AC in polarized Calu-3 monolayers. (A) Control cells. (B) CFTR-knockdown (KD) cells immunostained for CFTR (green) and MUC5AC (red). Nuclei are stained blue using diamidino-2phenylindole. Api, apical compartment; Baso, basolateral compartment; Filter, porous support. Images are representative of at least five cultures for each condition. (C and D) Bright field images of granules isolated from polarized monolayers of control cells (C) and CFTR-KD cells (D) suspended in PBS. (E–G) Single granule from control cells immunostained for MUC5AC (E), CFTR (mAb 24–1) (F), and merged images (G). (H–J ) Granule from CFTR-KD cells immunostained for MUC5AC (H), CFTR (mAb 24–1) (I), and merged images ( J ). (K–M) Single granule from control Calu-3 cells immunostained for MUC5AC (K), CFTR (rabbit polyclonal G449) (L), and merged images (M). (N–P) Granule isolated from CFTR-KD cells and immunostained for MUC5AC (N), CFTR (mAb 24–1) (O), and merged images (P). Images are representative of at least three independent immunolabeling experiments for each condition.

solid arrow). Some cells were also immunopositive for MUC5AC (Figure 1B, red staining, open arrow), consistent with a previous report that approximately 30% of Calu-3 cells in polarized, air–liquid interface cultures contain mucin granules (16). By contrast, CFTR immunostaining was not observed in CFTR-KD monolayers (Figure 1B, no green staining), although MUC5AC was detected in about one third of the cells (Figure 1B, red staining, open arrow). The results suggest that CFTR and MUC5AC are coexpressed in Calu-3 cells, although it was not possible to determine if the CFTR was in the apical membrane or subapical vesicles, or to exclude the possibility that different cells containing MUC5AC and CFTR were superimposed in the plane of the section.

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CFTR and MUC5AC Expression in Single Granules Isolated from Polarized Calu-3 Cells

As an alternative approach, granules from polarized cultures were isolated and immunostained using anti-MUC5AC antibody to confirm their identity as mucin granules. Permeabilization of the granules in methanol at 2208 C for 10 minutes provided antibodies access to the luminal contents. Granules from control (Figure 1C) and CFTR-KD Calu-3 cells (Figure 1D) were similar in size and shape and were clearly identified as mucin granules using anti-MUC5AC antibody (Figures 1E, 1H, 1K, and 1N). In control experiments, no MUC5AC immunostaining of the same granule preparations was observed when anti-MUC5AC primary antibody was omitted (see Figures E1A and E1E in the online supplement). CFTR was also clearly detected in control granules that had been incubated with anti-CFTR antibodies 24–1 (Figure 1F) or G449 (Figure 1L), though not in granules from CFTR-KD cultures (Figures 1I and IO), which were nevertheless stained by anti-MUC5AC antibody (Figures 1H and 1N). CFTR immunofluorescence was abolished when the anti-CFTR primary antibody was omitted (Figure E1B) and was brighter with G449 than 24–1 antibody; therefore, the latter was used in subsequent experiments. The yellow merged signals (Figures 1G and 1M) suggest CFTR colocalization with MUC5AC in control granules. Only red MUC5AC immunostaining was observed in granules from CFTR-KD cells, and the MUC5AC appeared evenly distributed throughout the granule lumen, as expected. CFTR immunostaining also appeared homogenous, although optical sections through a spherical granule might be expected to yield a ring of fluorescence for a membrane protein such as CFTR. To better understand the homogeneous appearance of CFTR, subdiffraction limit images of single granules were obtained using three-dimensional structured illumination microscopy. In this method, a structured light pattern is projected onto the sample containing fluorophores to generate interference patterns known as moiré fringes. The illumination pattern is then modulated, and the resulting images are collected and reconstructed to provide lateral resolution of 130 nm and axial resolution of approximately 280 nm (17). This imaging method also detected CFTR immunostaining (Figure 2B) in MUC5AC-positive granules (Figure 2A), confirming the confocal results. More importantly, reconstitution of the Z-axis revealed that the granules were flattened rather than spherical (Figures 2D–2F), probably because they were centrifuged onto the glass slide for imaging. Because the upper and lower surfaces of the granule would be within the illumination volume, this may explain the homogeneous CFTR immunostaining. Figure 3 shows low magnification images for ensembles of granules (open arrows) to minimize data selection. MUC5AC was present in granules from control and CFTR-KD cells (Figures 3A and 3D), whereas CFTR was only detected in those from control cells (Figure 3B); granules isolated from CFTRKD cells were devoid of CFTR, as expected (Figure 3E). Figure 3C also shows a control mucin granule immunostained for both MUC5AC and CFTR (inset: volume reconstruction magnified 315). These results show that CFTR was present in all control Calu-3 mucin granules in the field of view and none of those from CFTR-KD cells. Granule preparations from four different cultures were imaged and submitted to automated object analysis. In total, 1,087 MUC5AC-positive granules were counted, of which 71 6 9% also showed detectable CFTR staining. Thus, CFTR immunostaining in MUC5AC-positive granules appears specific. These granules were collected for ensemble imaging by slow centrifugation. Most CFTR appears at the granule periphery in the volume reconstruction

Figure 2. Superresolution images of a mucin granule. The granule was deposited on a slide by centrifugation at 1,000 RPM, immunostained as in Figure 1, and imaged using three-dimensional structured illumination (17). (A–C) Top view showing immunofluorescence for MUC5AC (A), CFTR (B), and merged images (C). (D–F) Side view of same granule as in A, B, and C reconstructed from subdiffraction limit images. Note the flattened profile after centrifugation at 1,0003 RPM. Representative of two independent immunolabeling experiments.

(Figure 3C, inset, green staining), which is consistent with optical sectioning through spherical granules after slow centrifugation, and membrane localized CFTR. CFTR Is Functional in the Granule Preparations

Like many organelles, mucus granules rely on the vacuolar proton ATPase for acidification and correct mucin organization (18). We took advantage of the proton pump to monitor Cl2 conductance in the granule membrane. Rapid acidification of granules requires a counter ion because the rheogenic proton pump generates a voltage that opposes further proton transport (19). When Cl2 channels are present in the granule membrane, adding Cl2 to the bath shunts the voltage, allowing proton transport and acidification of the luminal contents. We used the propensity of A.O. to segregate into acidic compartments where it is strongly fluorescent to assess granule acidification (20). Adding cholineCl2 to the granule preparation caused A.O. fluorescence to increase in a concentration-dependent manner (Figure 4A, closed circles). This increase was abrogated if granules were preincubated for 10 minutes with a saturating level of CFTRinh-172 (nominal concentration, 50 mM) or when the proton ATPase activity was inhibited by omitting ATP from the solution. A slight decrease in A.O. fluorescence was seen with the first addition of Cl2 even in the absence of granules, presumably due to quenching of the background fluorescence emitted by free A.O. in the bath solution. The experiment in Figure 4A was repeated three times using granules prepared from three independent cultures. For each

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Figure 3. Ensembles of mucin granules viewed at low magnification. Arrows indicate individual granules. (A–C) Granules from control cells centrifuged at 6003 RPM and immunostained for MUC5AC (A), CFTR (B), and the merged images (C). Inset in C shows a surface reconstruction (original magnification: 315) of the granule in the region of interest. (D–F) Multiple granules isolated from CFTR-KD cells. CFTR immunostaining (E) was not detected in any of the MUC5AC-positive granules identified in D. Representative of three independent immunolabeling experiments for each condition.

experiment, the fluorescence with 25 and 500 mM Cl2 was averaged over the relevant time periods (Figure 4B). When buffer was supplemented with ATP, subsequent addition of 25 or 500 mM cholineCl2 increased the fluorescence by 0.54 6 0.38 and 2.63 6 1.93 arbitrary units, respectively (Figure 4B, black bars). By contrast, adding the same Cl2 concentrations to granules that had been maintained in CFTRinh172 or adding the Cl2 fore ATP addition caused small reductions in A.O. fluorescence (20.35 6 0.38 and 20.48 6 0.48 arbitrary units, respectively). The fluorescence measured with each Cl2 concentration was different (P , 0.05). These results indicate the presence of H1–ATPase– driven acidification of the granule preparation, which is dependent on extravesicular Cl2 and sensitive to CFTRInh172. Immunoblot analysis was performed using the same granule preparations, and compared with granules isolated from CFTRKD cultures. CFTR was detected in both preparations but was 21.7 6 4.0-fold higher in granules from control cells compared with CFTR-KD cultures (Figure 4A, insets). A caveat of these immunoblotting and functional assays is that the granule preparation is unlikely to be 100% pure, and some of the CFTR signal may come from other vesicles that have a density similar to that of mucin granules. This uncertainty was our motivation for immunostaining individual granules as our first method of choice. Nevertheless, the results provide further support for the immunostaining results in which MUC5AC and CFTR were detected in the same granules.

Figure 4. CFTR-mediated conductance in granules isolated from Calu3 cultures and suspended initially in Cl2–free solution containing acridine orange (A.O.) (1 mM). (A) Representative traces of the fluorescence emission at 528 nm obtained from a single experiment. In the absence of granules (blue triangles), A.O. fluorescence was reduced slightly upon addition of 25 mM Cl2, consistent with quenching by anions. In the presence of granules, A.O. fluorescence was increased by Cl2 addition in a concentration-dependent manner (black circles), indicating granule acidification and uptake of A.O. (a weak base). This increase was abolished when the CFTR inhibitor CFTRInh172 was present (nominally 50 mM, preincubated for 10 min; red crosses) or when ATP was omitted to inhibit H1–ATPase and CFTR (red diamonds). (B) Effect of CFTRInh172 on A.O. fluorescence when granules were also exposed to ATP and PKA. Based on the experiment shown in A, which was repeated three times. For each experiment, fluorescence was averaged during the entire exposure to 25 or 500 mM Cl2 present. *P , 0.05. (C) Immunoblot confirming the presence of CFTR in granules isolated from 3.5 million control or CFTR-KD Calu-3 cells (insets, n ¼ 3). AU, arbitrary units.

CFTR and MUC5AC Are Colocalized in Single Granules from Highly Differentiated Non-CF Airway Epithelial Primary Cultures but Not from CF Cells

Calu-3 cells have a mixed serous-mucous phenotype. Therefore, we were interested to learn if CFTR is present in goblet cells from more highly differentiated primary cultures. Air–liquid interface cultures of primary airway epithelial cells were comprised of pseudostratified columnar cells with cilia typical of native airway epithelium (Figure 5A, open arrowhead). Alcian blue–periodic acid Schiff staining revealed the presence of goblet cells (Figure 5B, closed arrowhead). In some images these were connected to the overlying mucus gel by fine mucus strands. Immunostaining for b-tubulin, a marker of the base of cilia, was restricted to ciliated cells (Figure 5C, red), which were easily distinguished from MUC5AC-positive goblet cells

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(Figure 5C, green). Electron microscopy confirmed the development of ciliated cells (C), secretory cells with electron-dense granules (S), and goblet cells containing electron-clear mucin granules (G) (Figure 5D). Similar results were obtained using cultures from nasal turbinates. Together, the results indicate a high level of differentiation of these primary epithelial cells at the air–liquid interface. Volume reconstructions of single mucin granules isolated from these primary cultures were immunopositive for MUC5AC (Figure 5E) and CFTR (Figure 5F). Overlays revealed that most CFTR was at the periphery of individual granules, whereas MUC5AC was homogeneously distributed (Figure 5G). This demonstrates that CFTR is expressed in mucin granules that also contain the secreted mucin MUC5AC, both in Calu-3 and in highly differentiated primary cultures of human airway epithelial cells. Most CF is caused by deletion of a phenylalanine residue at position 508 of CFTR (F508 del). This mutation is present on at least one chromosome in approximately 90% of patients with CF and causes misfolding and retention of the protein in the endoplasmic reticulum. We examined if CFTR expression in single mucin granules is affected by this mutation by comparing granules isolated from non-CF and CF primary cultures and immunostaining them using anti-MUC5AC (Figures 5I and 5L) and anti-CFTR antibodies (Figures 5J and 5M). Both proteins were detected in granules from non-CF cultures (Figures 5I–5K), whereas only MUC5AC was immunostained in granules from CF cultures (Figures 5L–5M).

DISCUSSION

Figure 5. CFTR expression in mucin granules from highly differentiated airway epithelial primary cultures. (A) Representative air–liquid interface culture of bronchial epithelial cells used to isolate mucin granules, stained with hematoxylin-eosin. (B) Cell glycoproteins stained using alcian blue– periodic acid Schiff. (C) Primary culture immunostained for b-tubulin (green in this image identifies ciliated cells) and MUC5AC (red in this image identifies mucus-secreting cells). (D) Transmission electron micrograph showing mature cilia (open arrowhead) and mucus granules (solid arrowhead). B, basal cell; C, ciliated cell; G, goblet cell; S, secretory cell. (E–H) Representative granule (volume reconstruction) from highly differentiated nasal turbinate primary culture, immunostained for MUC5AC (E) and CFTR (F). (G) Merged images (scale bar: 2 mm applies to E–H). (H) Transmitted light image of the field in E, F, and G. Representative of three independent experiments performed at different times using P1 cells from one non-CF lung donor, which were cultured from frozen P0 stocks. (I–N) MUC5AC and CFTR in granules isolated from highly differentiated non-CF and CF bronchial epithelial cells. (I–K) Single control granule immunostained for MUC5AC (I), CFTR (J), and merged images (K) (volume reconstruction). (L–N) Granule from CF primary culture immunostained for (L) MUC5AC (L) and CFTR (M). Representative of three independent experiments performed at different times using first passage (P1) cells from one patient with severe CF (transplant recipient), which were cultured at the air–liquid interface from frozen P0 stocks.

The present results indicate that CFTR, an anion channel involved in salt and fluid secretion, is present in mucin granules isolated from normal airway epithelial cells but not in granules from CFTR-KD cells or primary cells from patients with CF. This implies a more direct link between defects in CFTR and abnormal mucus than previously suggested. CFTR channel density in mucus granules is undoubtedly low compared with the apical surface of neighboring ciliated cells, and this may explain why it has not been detected above background in some earlier studies (reviewed in Ref. 21). CFTR was not detected by immunostaining in goblet cell–like Calu-3 cells (16) or in goblet cells in native tissue (10). However, immunostaining has sometimes failed to detect low levels of CFTR in cell types where there is compelling functional evidence for CFTR expression (e.g., alveolar epithelial cells) (11–13) and where positive immunostaining results have also been obtained in some studies (11, 12). CFTR was reported in early studies of airway goblet cells (8), in Clara cells (which secrete significant quantities of mucin and are progenitors of goblet cells [22]), and in mucus cells of mouse gallbladder epithelium (23, 24). Likewise, a small number of putative CFTR peptides were detected in a recent proteomic study of granule-associated proteins (25), but they yielded a cross-correlation score for peptide identification that was below the threshold used to distinguish results from background , 2.0 (K. Raiford, personal communication). Taken together with the present findings, those results suggest that CFTR is expressed in mucin granules but at relatively low levels compared with the apical membrane of adjacent cells, where large numbers of CFTR channels are needed for transepithelial anion and fluid secretion. We can only speculate as to the function of CFTR in the mucus granule membrane. It may allow bicarbonate-dependent expansion of the mucin network, as proposed previously (4), but may enable some bicarbonate to enter the granule from the cytoplasm through CFTR rather than exclusively from the extracellular milieu. Partial dependence on CFTR and intracellular bicarbonate

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might explain why mucus release from CF mouse tissues is not restored when the luminal side is perfused with physiological bicarbonate concentrations (26). Alternatively, CFTR in the granular membrane may be required for normal mucin biosynthesis or organization within the secretory pathway. Mucin maturation requires the formation of dimers in the endoplasmic reticulum and higher-order multimers in the Golgi apparatus. Multimerization occurs through the formation of disulfide bonds between amino-terminal D domains (27), and this process is pH dependent may continue in compartments beyond the Golgi (18). The present results encourage further studies to determine if CFTR and intracellular bicarbonate affect the pH in those compartments or if they influence mucin release or microrheology. Author disclosures are available with the text of this article at www.atsjournals.org. Acknowledgments: The authors thank Dr. Angus Nairn, Yale University, New Haven, CT, for G449 antibody; Dr. Claire Brown, McGill Life Sciences Complex Imaging Facility, for helpful suggestions; and Applied Precision, Issaquah, WA, for superresolution imaging of our samples.

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