Physiology of Airway Mucus Secretion.pdf

Conference Proceedings

Physiology of Airway Mucus Secretionand Pathophysiology of Hypersecretion

Duncan F Rogers PhD FIBiol

IntroductionAirway MucusRespiratory Tract MucinsMucin Genes and Gene ProductsMechanisms of Goblet Cell ExocytosisAirway Mucus Hypersecretory Phenotype in COPDAirway Mucus Hypersecretory Phenotype in AsthmaMechanisms of Airway Goblet Cell HyperplasiaSummary

Mucus secretion is the first-line defense against the barrage of irritants that inhalation of approx-imately 500 L of air an hour brings into the lungs. The inhaled soot, dust, microbes, and gases canall damage the airway epithelium. Consequently, mucus secretion is extremely rapid, occurring intens of milliseconds. In addition, mucus is held in cytoplasmic granules in a highly condensed statein which high concentrations of Ca2� nullify the repulsive forces of the highly polyanionic mucinmolecules. Upon initiation of secretion and dilution of the Ca2�, the repulsion forces of the mucinmolecules cause many-hundred-fold swelling of the secreted mucus, to cover and protect the epi-thelium. Secretion is a highly regulated process, with coordination by several molecules, includingsoluble N-ethyl-maleimide-sensitive factor attachment protein receptor (SNARE) proteins, myris-toylated alanine-rich C kinase substrate (MARCKS), and Munc proteins, to dock the mucin gran-ules to the secretory cell membrane prior to exocytosis. Because mucus secretion appears to be sucha fundamental airway homeostatic process, virtually all regulatory and inflammatory mediatorsand interventions that have been investigated increase secretion acutely. When given longer-term,many of these same mediators also increase mucin gene expression and mucin synthesis, and inducegoblet cell hyperplasia. These responses induce (in contrast to the protective effects of acute secre-tion) long-term, chronic hypersecretion of airway mucus, which contributes to respiratory disease.In this case the homeostatic, protective function of airway mucus secretion is lost, and, instead,mucus hypersecretion contributes to pathophysiology of a number of severe respiratory conditions,including asthma, chronic obstructive pulmonary disease, and cystic fibrosis. Key words: mucin,mucus, asthma, chronic obstructive pulmonary disease, cystic fibrosis. [Respir Care 2007;52(9):1134–1146. © 2007 Daedalus Enterprises]

Duncan F Rogers PhD FIBiol is affiliated with the National Heart &Lung Institute, Imperial College London, London, United Kingdom.

Dr Rogers presented a version of this paper at the 39th RESPIRATORY

CARE Journal Conference, “Airway Clearance: Physiology, Pharmacol-ogy, Techniques, and Practice,” held April 21–23, 2007, in Cancun,Mexico.

Dr Rogers has a financial relationship with Syntaxin Ltd, Porton Down,Salisbury, United Kingdom.

Correspondence: Duncan F Rogers PhD FIBiol, Airway Disease, Na-tional Heart & Lung Institute, Imperial College London, DovehouseStreet, London, United Kingdom, SW3 6LY. E-mail: [email protected].

1134 RESPIRATORY CARE • SEPTEMBER 2007 VOL 52 NO 9

Introduction

Inhalation of approximately 12,000 L of air a day bom-bards the airway epithelium with up to 25 million particlesan hour.1 Cigarette smoking more than doubles thatamount.2,3 As a result, the airway epithelium has devel-oped ways to combat this onslaught of soot, dust, mi-crobes, and allergens. The first-line defense against in-haled insult impinging on and damaging the epithelium isthe production of mucus. This mucus is a viscoelastic gelthat forms a thin film on the surface of the airways (Fig. 1).It is an important homeostatic defense mechanism with avariety of functions (Table 1) that have evolved to reduceepithelial damage by inhaled irritants. Under normal cir-cumstances, airway mucus protects the epithelial lining byentrapping foreign debris, bacteria, and viruses, and clear-ing them from the airway by ciliary movement, a processtermed mucociliary clearance4 (Fig. 2). In contrast, in clin-ical conditions associated with airway mucus hypersecre-tion, such as asthma,5 chronic obstructive pulmonary dis-ease (COPD),6,7 and cystic fibrosis (CF),8 the mucus shiftsfrom a protective role to one that contributes to respiratorydisease (see Fig. 2). Excessive production of airway mu-cus, termed mucus hypersecretion, and changes in the bio-physical properties of the mucus can impair mucociliaryclearance, with associated accumulation of mucus in thelungs (Fig. 3), leading to difficulty in breathing, morbidity,and, in severe cases, mortality. The latter aspects are cov-ered in the present article, after a general introduction toairway mucus and the physiology of airway mucus secre-tion.

Airway Mucus

Airway mucus is a complex dilute aqueous solution oflipids, glycoconjugates, and proteins. It contains electro-

lytes, enzymes and anti-enzymes, oxidants and antioxi-dants, exogenous bacterial products, endogenous antibac-terial secretions, cell-derived mediators and proteins,plasma-derived mediators and proteins (see Fig. 2), andcell debris such as deoxyribonucleic acid (DNA). Airwaymucus is believed to form a liquid bi-layer; an upper gellayer floats above a lower, more watery sol, or periciliaryliquid, layer9 (see Fig. 2). It is likely that a thin layer ofsurfactant lies between the sol and gel phases10 (see Fig. 2).The function(s) of the sol layer is debated, but is presumedto include “lubrication” of the beating cilia. The surfactantlayer might facilitate spreading of mucus over the epithe-lial surface. The gel layer traps particles and is moved onthe tips of the beating cilia. The inhaled particles are trappedin the sticky gel layer and are removed from the airways—aprocess termed mucociliary clearance. When the mucusreaches the throat, it is either swallowed and delivered tothe gastrointestinal tract for degradation, or, if excessive,as in respiratory disease, it is coughed out as “sputum.”4

Respiratory tract mucus requires the correct combina-tion of viscosity and elasticity (viscoelasticity) for optimalefficiency of ciliary interaction.6,11 Viscosity is a liquid-like characteristic and is the resistance to flow and thecapacity to absorb energy when moving. Elasticity is asolid-like property and is the capacity to store energy thatmoves or deforms the fluid. Viscoelasticity is conferred onthe mucus primarily by high-molecular-weight mucous gly-coproteins, termed mucins.

Respiratory Tract Mucins

In health, mucins comprise up to 2% by weight of theairway mucus.12 In the airways, mucins are produced bygoblet cells in the epithelium13 (see Figs. 2 and 4) andsero-mucous glands in the submucosa14 (Fig. 5).

Fig. 1. Scanning electron micrograph of human bronchus, show-ing mucus (M), appearing as “rafts” and strands, resting on cilia(C). (Courtesy of Peter K Jeffery, Department of Gene Therapy,Royal Brompton Hospital, Imperial College London, United King-dom.)

Table 1. Functions of Airway Mucus

Physical barrier to inhaled airborne particles, irritants, microbes, and toaspirated foods and liquids

Entrapment of organisms, particles, and irritantsFormation of the vehicle on which irritants are transported by

mucociliary activity for clearance from the airwaysProvision of a waterproof layer over the epithelium to limit

dehydrationHumidification of inspired airInsulationpH buffering capacityLubricationNeutralization of toxic gasesSelective macromolecular sieveSource of antibacterial and other protective enzymes, and provision of

extracellular surface for their activitySource of immunoglobulins, and provision of extracellular surface for

their activity

PHYSIOLOGY OF AIRWAY MUCUS SECRETION AND PATHOPHYSIOLOGY OF HYPERSECRETION

RESPIRATORY CARE • SEPTEMBER 2007 VOL 52 NO 9 1135

Mucins are long, thread-like, complex glycoconjugates(see Fig. 4). A mucin consists of a linear peptide backbone(termed apomucin), which is encoded by specific mucin(MUC) genes (see below), to which hundreds of carbohy-drate side-chains are O-linked, but also with additionalN-linked glycans. The glycosylation pattern is complex

and extremely diverse,15 and is associated with comple-mentary motifs on bacterial cell walls, which facilitatesbroad-spectrum bacterial attachment and subsequent clear-ance.16,17 Within the main protein core are variable num-bers of tandemly repeated serine-rich and/or threonine-rich regions, which are unique in size and sequence foreach mucin,4 and represent sites for mucin glycosylation.These complex glycoproteins are polydisperse, linear poly-mers that can be fragmented, by reduction, to create mono-mers (see Fig. 4) termed “reduced subunits.”18–21 Thereare at least 2 structurally and functionally distinct classesof mucin, namely, the membrane-associated mucins (Ta-ble 2) and the secreted mucins (either gel-forming or non-gel-forming) (Tables 3 and 4). Membrane-tethered mu-cins, which have a hydrophobic domain that anchors themucin in the plasma membrane, contribute to the forma-tion of the epithelial surface.4 Secretory mucins are storedintracellularly in secretory granules and are released at theapical surface of the cell in response to a stimulus (seebelow). It would appear that mucus production is such afundamental homeostatic process that virtually all the in-terventions that have been investigated trigger airway mu-cin secretion (Table 5). In addition, many of these samemediators when administered longer-term not only inducemucin secretion but also up-regulate MUC gene expres-sion, with concomitant increases in mucin synthesis andassociated goblet cell hyperplasia (see Table 5).

Mucin Genes and Gene Products

Twenty human MUC genes have so far been identified(see Tables 2–4). Of these, only nine, namely, MUC1,MUC2, MUC4, MUC5AC, MUC5B, MUC7, MUC8,MUC11, and MUC13, are expressed in the human respi-ratory tract.4 Of these, only MUC2, MUC5AC, and MUC5B(the classic gel-forming mucins, see Fig. 4), are found inairway secretions. MUC5AC and MUC5B glycoproteins,localized adjacent to each other on chromosome 11p15.5,are considered the major gel-forming mucins in both nor-mal respiratory tract secretions and airway secretions frompatients with respiratory diseases.22–27 Interestingly,MUC5B appears to be unique in that it is not polymorphic.Small amounts of MUC2 may, however, be found in se-cretions from “irritated” airways (see below).

In general, the MUC gene products are poorly charac-terized biochemically and biophysically.12 The predictedsequences of the MUC1, 3A, 3B, 4, 11–12, 13, 15–18, and20 gene products suggest they are membrane-bound, withan extracellular mucin domain and a hydrophobic mem-brane-spanning domain (see Table 2). In contrast, MUC2,5AC, 5B, 6–9, and 19 gene products are secreted mucins(see Tables 3 and 4). The technology for studying thecontribution to physiology and pathophysiology of the in-dividual MUC gene products lags well behind that of in-

Fig. 2. Airway mucus secretion and hypersecretion. Upper Panel:In healthy airways, mucus forms a bi-layer over the epithelium,with surfactant (dotted line) separating the gel and sol layers. Mu-cins secreted by goblet cells and submucosal glands confer vis-coelasticity on the mucus, which facilitates mucociliary clearanceof inhaled particles and irritants. Mucus hydration is regulated bysalt (and, hence, water) flux across the epithelium. The glands alsosecrete water. Plasma proteins exuded from the tracheobronchialmicrovasculature bathe the submucosa and contribute to the for-mation of mucus. The above processes are under the control ofnerves and regulatory mediators. Lower Panel: Airway inflamma-tion (in asthma, chronic obstructive pulmonary disease [COPD],and possibly cystic fibrosis [CF]) induces changes associated witha mucus hypersecretory phenotype, including increased plasmaexudation (more predominant in asthma than COPD or CF), gobletcell hyperplasia, via differentiation from basal cells, and associ-ated increased mucus synthesis and secretion, and submucosalgland hypertrophy (with associated increased mucus production),leading to increased luminal mucus (and airway obstruction).



vestigation of gene expression.4 MUC1, 2, and 8 genes areexpressed in both the epithelium and submucosal glands,whereas MUC4, 5AC and 13 are expressed primarily inthe epithelium. In contrast, MUC5B and MUC7 genes areexpressed primarily in the glands. Use of currently avail-able antibodies confirms that the MUC5AC gene productis a goblet cell mucin, whereas MUC5B predominates in

the glands, albeit that some MUC5AC (and possibly MUC7)is also usually present.12 Interestingly, MUC4 mucin lo-calizes to the ciliated cells.

The mucin content of secretions from patients with hy-persecretory respiratory diseases may differ from normal.For example, MUC5AC mucin, initially isolated as a tra-cheobronchial mucin,28 is found in airway secretions pooled

Fig. 3. Mucus obstruction of the airways in asthma and chronic obstructive pulmonary disease (COPD). A: Mucus plugging in asthma.Complete occlusion by mucus (M) plugs of an intrapulmonary bronchus (arrow), cut in longitudinal section, in a patient who died of an acutesevere asthma attack. B: Mucus (M) partially obstructing an extrapulmonary bronchus (transverse section: arrow) of an elderly, male,long-term cigarette smoker. C: Bronchoconstriction and luminal mucus in fatal asthma. Intrapulmonary airway (transverse section) of apatient who died of an acute severe asthma attack, showing airway epithelium (arrow) thrown into folds by smooth-muscle contraction, andocclusion by mucus (M) of remaining luminal space. This relatively small amount of mucus would not be expected to significantly reduceairflow in a relaxed, nonconstricted airway. D: Mucus (M) blocking an intrapulmonary airway (transverse section) of an elderly, male,long-term cigarette smoker (a different patient than that in panel B). Note the lack of airway constriction (in contrast to panel B) and thecellular infiltrate in the mucus. The arrow points to the epithelium.



Fig. 4. Airway mucin and mucin secretion. A: Goblet cell (GC) and ciliated cell (CC) in human bronchus. M � mucin-containing granules.C � cilia. L � lumen. B: Visualization of mucin exocytosis by a guinea pig tracheal goblet cell. Fusion of an intracellular mucin granule (arrow)with the apical membrane of the cell (arrow head) leads to the formation of a bi-membrane-spanning pore that rapidly opens out to forman “omega” (�) profile (shown), which allows release of stored mucin (M). In this image the mucin is retained in the granule, due to the useof tannic acid fixation. C: Predominant airway mucin gene products: modular motifs in amino acid backbones. MUC2, MUC5AC, andMUC5B are cysteine-rich secretory mucins. See Reference 4 for details of modular motifs. D: Mucin subunit. Each subunit (approximately500 nm in length) comprises an amino acid backbone with highly glycosylated (linear) domains and folded regions, stabilized via disulphidebonds, with little or no glycosylation. Glycosylation is via O-linkages and is highly diverse. E: Mature mucin molecule. In secretions, themucin subunits are joined end-to-end by disulphide bonds (S-S) to form long, thread-like mature mucin molecules.



from healthy individuals,24,22 and increased levels arepresent in the airways of patients with asthma.29 The ex-pression of many genes, such as MUC5AC, in airwayepithelial cells is regulated by various neurohumoral fac-tors and inflammatory mediators (see Table 5). MUC5Bmucins are a major component of tenacious mucus plugsfrom the lungs of a patient who died in status asthmati-cus,25,30 and in sputum from patients with chronic bron-chitis.26

From the above it appears that, in healthy individuals,MUC5B is mainly expressed in the airway submucosalglands, which are restricted to the more proximal, car-tilaginous airways. In contrast, MUC5AC expression isgenerally restricted to goblet cells in the upper and lowerrespiratory tracts.31,32 Thus, the composition of normalmucus can be altered, depending on the relative contri-bution to the secretions of these different cellular sourc-es.33 In respiratory diseases associated with airway mu-cus hypersecretion, such as asthma, COPD, and CF,further changes in the composition of the mucus, and in

the mucus secretory phenotype in general, are observed(see below).

Mechanisms of Goblet Cell Exocytosis

Exocytosis is an evolutionarily conserved and ubiqui-tous process whereby hormones, mediators, and other mol-ecules are released from cells. For exocytosis of mostvesicles to occur, a soluble N-ethyl-maleimide-sensitivefactor attachment protein receptor (SNARE) complex hasto be formed, which links the vesicle (v-SNARE) and thetarget cell membrane (t-SNARE) together, to facilitate re-lease of vesicle content from the cell.34

There are 2 general mechanisms by which exocytosisoccurs, and these apply also to mucin exocytosis:

• Constitutive (basal) secretion: this secretion is unreg-ulated and of a low level.

• Stimulated secretion: regulated exocytosis of granulesin response to extracellular stimuli.

Fig. 5. Airway submucosal glands. A: Isolated dog tracheal gland, showing complex structure of secretory acini that feed secretions intoa collecting duct, which are then wafted out via the ciliated duct. Mucus is seen being secreted from the top of the gland. (Courtesy of SanaeShimura, First Department of Internal Medicine, Tohoku University School of Medicine, Sendai, Japan). B: Computer-generated image ofhuman bronchial gland. Serial histological sections were translated into a false-colored reconstruction of the gland. The distal serous aciniproduce relatively watery secretions that contain antibacterial enzymes (lysozyme, lactoferrin). It has been proposed that they wash overthe more viscous mucin secretions produced by the more proximal mucous acini and flush them into the collecting duct. (Courtesy ofWilliam F Whimster [deceased], Department of Histopathology, King’s College School of Medicine, London, United Kingdom.)



Mucin granules are present in the cytoplasm of airwaymucin-secreting cells (see Fig. 4). Mucins are first synthe-sized on the rough endoplasmic reticulum, then oligomerizedand sent to the Golgi for glycosylation and subsequent pack-aging and budding into mature mucin granules. The granulesare stored in the cytoplasm, in preparation for release.

In airway goblet cells, exocytosis of mucin involves themovement of the mucin granule to the apical surface of the

goblet cell. This movement is dependent upon many fac-tors, including chaperoning to the plasma membrane viamyristoylated alanine-rich C-kinase substrate (MARCKS)protein. Upon stimulation, MARCKS is phosphorylatedby protein kinase C, released from the plasma membrane,and de-phosphorylated by protein phosphatase 2A, acti-vated by protein kinase G. This allows MARCKS to beunbound and ready for actin/myosin binding to form aninteraction with the secretory vesicle. Targeting to the se-cretory vesicle is mediated by its chaperone protein,Hsp70.35 Binding allows MARCKS to chaperone the mu-cin-containing vesicle to the apical membrane of the gob-

Table 2. Human MUC Genes That Produce Membrane-AssociatedMucins

Gene Tissue Distribution

MUC 1 Lung, cornea, salivary glands, esophagus, stomach,pancreas, large intestine, breast, prostate, ovary,kidney, uterus, cervix, dendritic cells

MUC 3A Thymus, small intestine, colon, kidneyMUC 3B Small intestine, colonMUC 4 Lung, cornea, salivary glands, esophagus, small

intestine, kidney, endocervixMUC 11 Lung, middle ear, thymus, small intestine, pancreas,

colon, liver, kidney, uterus, prostateMUC 12 Middle ear, pancreas, colon, uterus, prostateMUC 13 Lung, conjunctiva, stomach, small intestine, colon,

kidneyMUC 15 Conjunctiva, tonsils, thymus, lymph node, breast,

small intestine, colon, liver, spleen, prostate, testis,ovary, leukocytes, bone marrow

MUC 16 Conjunctiva, ovaryMUC 17 Intestinal cells, conjunctival epitheliumMUC 18 ProstateMUC 20 Lung, liver, kidney, colon, placenta, prostate

Table 3. Human MUC Genes That Produce Secreted, Cysteine-Rich(Gel-Forming) Mucins


MUC 2 Lung, conjunctiva, middle ear, stomach, smallintestine, colon, nasopharynx, prostate

MUC 5AC Lung, conjunctiva, middle ear, stomach, gall bladder,nasopharynx

MUC 5B Lung, middle ear, sublingual gland, laryngealsubmucosal glands, esophageal glands, stomach,duodenum, gall bladder, nasopharynx

MUC 6 Stomach, duodenum, gall bladder, pancreas, kidneyMUC 19 Lung, salivary gland, kidney, liver, colon, placenta,

prostate

Table 4. Human MUC Genes That Produce Secreted, Cysteine-PoorMucins


MUC 7 Lung, lachrymal glands, salivary glands, noseMUC 8 OviductMUC 9 Submandibular glands

Table 5. Inducers of Airway Mucus Secretion, Goblet CellHyperplasia, and Mucin Gene Expression/Mucin Synthesis

Stimulant Secretion* Hyperplasia MUC Gene

CytokinesIL-1� � NR NRIL-6 � NR YesIL-9 NR NR YesIL-13 (IL-4) � Yes YesTNF-� �� Yes† Yes1

GasesIrritant gases (eg, cigarette

smoke)�� Yes Yes

Nitric oxide NE/� NR NRReactive oxygen species 0/� NR NR

Inflammatory mediatorsBradykinin � NR NRCysteinyl leukotrienes �� NR NREndothelin 0/� NR NRHistamine � NR NRPlatelet activating factor � Yes1 Yes†Prostaglandins 0/� NR NRProteinases �� Yes NRPurine nucleotides �� NR NR

Neural pathwaysCholinergic nerves �� NR NRCholinoceptor agonists �� Yes NRNicotine �� Yes NRTachykininergic nerves � NR NRSubstance P �� NR NRNeurokinin A � NR NR

MiscellaneousEpidermal growth factor

(� TNF-�)NR Yes Yes

Sensitization followed bychallenge

� Yes Yes

*�� highly potent, �� marked effect, � � lesser effect, 0 � minimal effect, NE �

no effect, NR � effect not reported.† Effect only observed with platelet activating factor and tumor necrosis factor alpha (TNF-�)in combination.IL � interleukin



let cell. As a result of MARCKS phosphorylation, theactin/myosin contracts, which allows the vesicle to fusewith the plasmalemma, and release mucin out of the cell.Munc-18 is also required for syntaxin binding to the plas-malemma. Docked granules have to mature to fusion-com-petence before they can undergo exocytosis. Munc-13–4participates in this “priming” of airway goblet cell gran-ules.36 Once at the plasma membrane, the mucin-contain-ing granule forms a SNARE complex, irreversibly tether-ing the granule. Correct and complete formation of thisMARCKS-guided SNARE complex has to occur beforemucin exocytosis can take place, which forms an openconformation with the profile of an omega symbol (�),that links the mucin granule and apical cell membranes37

(see Fig. 4).Once initiated, goblet cell mucin exocytosis obeys first-

order kinetics: it is extremely rapid, taking only tens ofmilliseconds, during which time the released mucin ex-pands many hundred-fold38 (Fig. 6). Rapid expansion oc-curs because the mucin is highly condensed within thegranules, with the mucin threads bound together by highintragranular concentrations of Ca2�, which acts as a shield-ing cation. Mucin granules are polyanioic, so without thecalcium present within the mucin matrix, such close pack-aging could not occur. Upon exocytosis, the Ca2� is pro-gressively diluted, allowing electrostatic expulsion to oc-cur, a process accelerated by water uptake, resulting inexpansion of the mucin into the airways. This is a normal,homeostatic process. However, excessive and prolongedexocytosis results in airway mucus hypersecretion, as seenin respiratory diseases such as COPD and asthma.

Airway Mucus Hypersecretory Phenotype in COPD

COPD comprises 3 overlapping conditions, namely,chronic bronchitis (airway mucus hypersecretion), chronicbronchiolitis (small airways disease), and emphysema (airspace enlargement due to alveolar destruction).39 The fol-lowing discussion considers the “bronchitic” componentof COPD. The airways of patients with COPD containexcessive amounts of mucus40 (see Fig. 3), which is mark-edly increased above that in control subjects.41,42 The ex-cessive luminal mucus is associated with increased amountsof mucus-secreting tissue. Goblet cell hyperplasia is a car-dinal feature of chronic bronchitis,40 with increased num-bers of goblet cells in the airways of cigarette smokers,either with chronic bronchitis and chronic airflow limita-tion,43 or with or without productive cough.44 Submucosalgland hypertrophy also characterizes chronic bronchi-tis,40,41,45,46 and the amount of gland correlates with theamount of luminal mucus.41

Airway Mucus Hypersecretory Phenotype in Asthma

Asthma is a chronic inflammatory condition of the air-ways, characterized by variable airflow limitation that is atleast partially reversible, either spontaneously or with treat-ment.47,48 It has specific clinical and pathophysiologicalfeatures,49 including mucus obstruction of the airways.50

There is more mucus in the central and peripheral airwaysin patients with chronic or severe asthma than in controlsubjects.51 The increased luminal mucus reflects an in-crease in the amount of airway secretory tissue, due toboth goblet cell hyperplasia29,51 and submucosal gland hy-pertrophy,52 although the latter is not characteristic of allpatients with asthma.51

Airway mucus obstruction in asthma is particularly ev-ident in a proportion of patients who die in status asth-

Fig. 6. Kinetics of airway mucin secretion. A: Highly polyanionicmucin molecules repel each other to form a tangled network thatspreads over the epithelial surface. For packaging into intracellularmucin granules, high concentrations of intragranular Ca2� act as ashielding cation to nullify anionic repulsion and allow condensa-tion of the mucin in the granules. For mucin secretion, after poreformation and expansion into an omega profile (see Fig. 4), waterenters the granule and progressively dilutes out the Ca2�, withconsequent loss of capacity to nullify anionic repulsion, whichallows the mucin to undergo a Donnan shift and expand out of thecell, in the manner of a “jack-in-the-box.” B: Kinetics of mucinexpansion. Condensed intragranular mucin undergoes size ex-pansion of many hundred-fold upon secretion, reaching a plateauexpansion in tens of milliseconds. C: Mucin secretion from a guineapig secretory epithelial cell. The arrowheads point to mucin, exo-cytosed in sequence from different individual granules, around thecell. Note the relative size of the mucin “globules” to the cell,indicating large, post-secretory size expansion. The whole se-quence for the 4 exocytotic events is 40 s.



maticus, where many airways are occluded by mucusplugs52–54 (see Fig. 3). The plugs are highly viscous andcontain large amounts of plasma proteins (such as serumalbumin), as well as DNA, cells, proteoglycans,55 and mu-cins.30,52,55 The plasma proteins are a result of increasedplasma exudation56,57 (Fig. 7), which is a pathophysiolog-ical feature of asthma.58 Importantly, incomplete mucusplugs are found in the airways of asthmatic subjects whohave died from causes other than asthma,59 which indi-cates that plug formation is a chronic, progressive process.The increased viscosity of the airway mucus in asthmacould be due to an intrinsic abnormality in the secretedmucins30 or to interactions between mucins and plasma,whereby plasma synergistically increases the viscosity ofmucus60,61 (Fig. 8). The mechanisms underlying the latterincreased viscosity are unclear, but may be due to plasma-induced rupturing of hydrogen bonds between adjacentmucin molecules, which promotes greater inter-tanglingbetween mucin and albumin molecules, or to plasma lim-iting the normal hydration and swelling of secreted mu-cin.62 In addition to its thickening effect on mucin, luminalplasma would directly contribute to the increased amountof airway mucus, and may itself induce mucin secretion,63

leading to further increases in luminal mucus with highviscosity (see Fig. 8).

Mechanisms of Airway Goblet Cell Hyperplasia

Airway goblet cell hyperplasia is a prominent patho-physiological feature of COPD, asthma, and CF (see above),and is an often-used end point in animal models of respi-ratory disease.64 The cellular composition of the airwayepithelium can alter both by cell division and by differen-tiation of one cell into another.65 There are at least 8 celltypes in the airway epithelium of the conducting airways.In terms of goblet cell hyperplasia, differentiation is themajor pathway for production of new goblet cells, and celldivision is the major carcinoma pathway. The basal serousand Clara cells are considered the primary progenitor cells,because they have the capacity to undergo division, fol-lowed by differentiation into “mature” ciliated or gobletcells. In specific experimental conditions (eg, exposure tocigarette smoke), goblet cell division contributes in part tothe hyperplasia. However, differentiation of nongranulatedairway epithelial cells is a major route for production ofnew goblet cells.65–67 In experimental animals, productionof goblet cells is usually at the “expense” of the progenitorcells, most notably serous and Clara cells, which decreasein number as goblet cell numbers increase. Serous-likecells and Clara cells are found in macroscopically normalbronchioles in human lung.68 Whether there is a reductionin number in respiratory disease has not been reported, butmerits investigation. Reduction in the relative proportionof serous and Clara cells has pathophysiological impor-tance because they produce a number of anti-inflamma-

Fig. 7. Airway plasma exudation in asthma. Patients with seasonalallergic asthma underwent a bronchoalveolar lavage (BAL) beforeand then immediately after local challenge of the airways withallergen. BAL fluid concentrations of low- and high-molecular-weight proteins were expressed as a ratio of total BAL protein, andthen expressed as a ratio of serum proteins (again as a ratio oftotal serum protein). Compared with pre-challenge BAL, allergenchallenge caused a greater increase in BAL low-molecular-weightproteins (plasma markers) (double-headed arrow) compared withhigh-molecular-weight markers of secretion (arrows), which indi-cates increased plasma exudation in response to allergen chal-lenge. �1AT � alpha-1 antitrypsin. Alb � albumin. Trans � trans-ferrin. Cer � ceruloplasmin. Fib � fibrinogen. �2M � alpha-2macroglobulin. (Redrawn using data in Reference 57.)

Fig. 8. Effect of mucin secretion and plasma exudation on airwaymucus. Mucus secretion and plasma exudation both increase thevolume of luminal liquid. In addition, plasma can induce mucinsecretion, which further increases the volume of liquid. Plasmaand mucin interact such that plasma proteins (eg, albumin) syn-ergistically increase the viscosity of mucin. Thus, mucin secretioncoupled with plasma exudation potentially results in a greater vol-ume of more viscous liquid than either mucin secretion or plasmaexudation alone.



tory, immunomodulatory, and antibacterial molecules vitalto host defense.69,70 For example, serous cells producelysozyme, lactoferrin, secretory immunoglobin A, perox-idase, and at least 2 protease inhibitors. Clara cells pro-duce Clara cell 10-kDa protein (also known as uteroglobu-lin), Clara cell 55-kDa protein, Clara cell tryptase,�-galactoside-binding lectin, possibly a specific phospho-lipase, and surfactant proteins A, B, and D. Thus, in re-spiratory diseases associated with airway mucus hyperse-cretion it seems that not only is there goblet cell hyperplasia,with associated mucus hypersecretion, but also a reductionin serous and Clara cells, with concomitant impaired po-tential for host defense.

Some of the mechanisms for development of airwaygoblet cell hyperplasia and the associated mucus hyperse-cretory phenotype in asthma and COPD are becoming clear-er.50,71 Many regulatory and inflammatory mediators andenzymes increase mucus secretion and induce MUC geneexpression, mucin synthesis, and goblet cell hyperplasia inexperimental systems (see Table 5). These mediators are

intermediates in a cascade of pathophysiological eventsleading from initiating factors (such as allergen exposurein asthma) to a chronic inflammatory/repair response, whichin turn leads to mucus hypersecretion and associated air-way obstruction and clinical symptoms (Fig. 9). A smallnumber of key molecules may be involved in translatingthe actions of the different inflammatory mediators intoairway mucus hypersecretion, namely, epidermal growthfactor and its receptor tyrosine signaling pathway,72 themitogen activated kinase and extracellular signal-regulatedkinase (MEK/ERK) pathway,73 calcium-activated chloridechannels,74,75 and the retinoic acid receptor (RAR)-� sig-naling pathway.76 A wide variety of small molecule an-tagonists and inhibitors of these pathways are currently inpharmacotherapeutic development.77

Summary

Secretion of airway mucus is a vital homeostatic mech-anism that protects the respiratory tract from a barrage of

Fig. 9. Pathophysiology of airway mucus hypersecretion. Initiating factors, including allergen exposure (in asthma), cigarette smoking (inchronic obstructive pulmonary disease [COPD]), defects in the cystic fibrosis (CF) transmembrane-conductance regulator (CFTR), andbacterial infection (COPD and CF), set up a cycle of inflammation, and are also associated with nerve activation. The inflammation of asthma(predominantly Th2 lymphocytes [T] and eosinophils [E]) differs from that in COPD or CF (predominantly macrophages [M] and neutrophils[N]). Secretagogues produced during inflammation and nerve activation induce a number of signaling pathways associated with increasedmucin secretion, mucin gene expression, and mucin synthesis, which, in turn, are associated with secretory cell hyperplasia, airway mucushypersecretion and respiratory problems. EGF-R � epidermal growth factor receptor. MAP � mitogen-activated protein (kinases). hCLCA1 �human calcium-activated chloride channel. RAR � retinoic acid receptor.



inhaled insult. The mucus has to be of the correct viscosityand elasticity for optimal interaction with the cilia andeffective mucociliary clearance of particles from the lungs.Presumably because of the potential damage that inhaledirritants can do to the airway epithelium, the process ofsecretion is extremely rapid. In addition, because of themarked condensation of intragranular mucins, deconden-sation and subsequent secretion releases vast amounts ofmucin onto the airway surface. However, over and abovethe rapid secretion in response to temporary inhaled insult,long-term, chronic increased secretion of airway mucuscontributes to respiratory disease. In this case, the homeo-static, protective function of airway mucus secretion is lostand, instead, mucus hypersecretion contributes to disease.Airway obstruction by mucus is a common feature of anumber of severe respiratory conditions, including asthma,COPD, and CF. To a certain extent, each disease has aparticular hypersecretory phenotype, although a number ofpathophysiological features are shared (eg, submucosalgland hypertrophy and goblet cell hyperplasia). Goblet cellhyperplasia, and the associated mucus hypersecretion, areparticularly important in small airways. Mucus in theseairways cannot be cleared by cough and tends to accumu-late and cause obstruction. Goblet cell hyperplasia is at theexpense of serous cells and Clara cells. The loss of thevarious anti-inflammatory, immunomodulatory, and anti-bacterial molecules normally secreted by these cells fur-ther compromises host defense.

In summary, mucus secretion is homeostatic, and mucushypersecretion is not. Where the division lies betweensecretion and hypersecretion is not clear, but needs to bedelineated for more precise and clinically useful diagnosisof airway mucus hypersecretory diseases.

REFERENCES

1. Seaton A, MacNee W, Donaldson K, Godden D. Particulate airpollution and acute health effects. Lancet 1995;345(8943):176–178.

2. Lippmann M, Yeates DB, Albert RE. Deposition, retention, andclearance of inhaled particles. Br J Ind Med 1980;37(4):337–362.

3. Hollander W, . Aerosols of smoke, respiratory physiology and dep-osition. Arch Toxicol Suppl 1986;9:74–87.

4. Rose MC, Voynow JA. Respiratory tract mucin genes and mucinglycoproteins in health and disease. Physiol Rev 2006;86(1):245–278.

5. Del Donno M, Bittesnich D, Chetta A, Olivieri D, Lopez-VidrieroMT. The effect of inflammation on mucociliary clearance in asthma:an overview. Chest 2000;118(4):1142–1149.

6. Houtmeyers E, Gosselink R, Gayan-Ramirez G, Decramer M. Reg-ulation of mucociliary clearance in health and disease. Eur Respir J1999;13(5):1177–1188.

7. Maestrelli P, Saetta M, Mapp CE, Fabbri LM. Remodeling in re-sponse to infection and injury: airway inflammation and hypersecre-tion of mucus in smoking subjects with chronic obstructive pulmo-nary disease. Am J Respir Crit Care Med 2001;164(10 Pt 2):S76–S80.

8. Robinson M, Bye PT. Mucociliary clearance in cystic fibrosis. Pe-diatr Pulmonol 2002;33(4):293–306.

9. Knowles MR, Boucher RC. Mucus clearance as a primary innatedefense mechanism for mammalian airways. J Clin Invest 2002;109(5):571–577.

10. Morgenroth K, Bolz J. Morphological features of the interactionbetween mucus and surfactant on the bronchial mucosa. Respiration1985;47(3):225–231.

11. Sleigh MA, Blake JR, Liron N. The propulsion of mucus by cilia.Am Rev Respir Dis 1988;137(3):726–741.

12. Davies JR, Herrmann A, Russell W, Svitacheva N, Wickstrom C,Carlstedt I. Respiratory tract mucins: structure and expression pat-terns. Novartis Found Symp 2002;248:76–88.

13. Rogers DF. Airway goblet cell hyperplasia in asthma: hypersecretoryand anti-inflammatory? Clin Exp Allergy 2002;32(8):1124–1127.

14. Finkbeiner WE. Physiology and pathology of tracheobronchial glands.Respir Physiol 1999;118(2–3):77–83.

15. Hanisch FG. O-glycosylation of the mucin type. Biol Chem 2001;382(2):143–149.

16. Dell A, Morris HR. Glycoprotein structure determination by massspectrometry. Science 2001;291(5512):2351–2356.

17. Moniaux N, Escande F, Porchet N, Aubert JP, Batra SK. Structuralorganization and classification of the human mucin genes. FrontBiosci 2001;6:D1192–D1206.

18. Thornton DJ, Davies JR, Kraayenbrink M, Richardson PS, SheehanJK, Carlstedt I. Mucus glycoproteins from ‘normal’ human tracheo-bronchial secretion. Biochem J 1990;265(1):179–186.

19. Thornton DJ, Sheehan JK, Carlstedt I. Heterogeneity of mucus gly-coproteins from cystic fibrotic sputum: are there different families ofmucins? Biochem J 1991;276(Pt 3):677–682.

20. Sheehan JK, Thornton DJ, Somerville M, Carlstedt I. Mucin struc-ture: the structure and heterogeneity of respiratory mucus glycopro-teins. Am Rev Respir Dis 1991;144(3 Pt 2):S4–S9.

21. Thornton DJ, Devine PL, Hanski C, Howard M, Sheehan JK. Iden-tification of two major populations of mucins in respiratory secre-tions. Am J Respir Crit Care Med 1994;150(3):823–832.

22. Hovenberg HW, Davies JR, Herrmann A, Linden CJ, Carlstedt I.MUC5AC, but not MUC2, is a prominent mucin in respiratory se-cretions. Glycoconj J 1996;13(5):839–847.

23. Hovenberg HW, Davies JR, Carlstedt I. Different mucins are pro-duced by the surface epithelium and the submucosa in human tra-chea: identification of MUC5AC as a major mucin from the gobletcells. Biochem J 1996;318(Pt 1):319–324.

24. Thornton DJ, Carlstedt I, Howard M, Devine PL, Price MR, SheehanJK. Respiratory mucins: identification of core proteins and glyco-forms. Biochem J 1996;316(Pt 3):967–975.

25. Thornton DJ, Howard M, Khan N, Sheehan JK. Identification of twoglycoforms of the MUC5B mucin in human respiratory mucus. Ev-idence for a cysteine-rich sequence repeated within the molecule.J Biol Chem 1997;272(14):9561–9566.

26. Wickstrom C, Davies JR, Eriksen GV, Veerman EC, Carlstedt I.MUC5B is a major gel-forming, oligomeric mucin from human sal-ivary gland, respiratory tract and endocervix: identification of gly-coforms and C-terminal cleavage. Biochem J 1998;334(Pt 3):685–693.

27. Sheehan JK, Howard M, Richardson PS, Longwill T, Thornton DJ.Physical characterization of a low-charge glycoform of the MUC5Bmucin comprising the gel-phase of an asthmatic respiratory mucousplug. Biochem J 1999;338(Pt 2):507–513.

28. Guyonnet DuperatV, Audie JP, Debailleul V, Laine A, Buisine MP,Galiegue-Zouitina S, et al. Characterization of the human mucingene MUC5AC: a consensus cysteine-rich domain for 11p15 mucingenes? Biochem J 1995;305(Pt 1):211–219.

29. Ordonez CL, Khashayar R, Wong HH, Ferrando R, Wu R, HydeDM, et al. Mild and moderate asthma is associated with airway



goblet cell hyperplasia and abnormalities in mucin gene expression.Am J Respir Crit Care Med 2001;163(2):517–523.

30. Sheehan JK, Richardson PS, Fung DC, Howard M, Thornton DJ.Analysis of respiratory mucus glycoproteins in asthma: a detailedstudy from a patient who died in status asthmaticus. Am J RespirCell Mol Biol 1995;13(6):748–756.

31. Audie JP, Janin A, Porchet N, Copin MC, Gosselin B, Aubert JP.Expression of human mucin genes in respiratory, digestive, and re-productive tracts ascertained by in situ hybridization. J HistochemCytochem 1993;41(10):1479–1485.

32. Reid CJ, Gould S, Harris A. Developmental expression of mucingenes in the human respiratory tract. Am J Respir Cell Mol Biol1997;17(5):592–598.

33. Kirkham S, Sheehan JK, Knight D, Richardson PS, Thornton DJ. Het-erogeneity of airways mucus: variations in the amounts and glyco-forms of the major oligomeric mucins MUC5AC and MUC5B. Bio-chem J 2002;361(Pt 3):537–546.

34. Foster KA. A new wrinkle on pain relief: re-engineering clostridialneurotoxins for analgesics. Drug Discov Today 2005;10(8):563–569.

35. Li Y, Martin LD, Spizz G, Adler KB. MARCKS protein is a keymolecule regulating mucin secretion by human airway epithelial cellsin vitro. J Biol Chem 2001;276(44):40982–40990.

36. Koch H, Hofmann K, Brose N. Definition of Munc13-homology-domains and characterization of a novel ubiquitously expressedMunc13 isoform. Biochem J 2000;349(Pt 1):247–253.

37. Newman TM, Robichaud A, Rogers DF. Microanatomy of secretorygranule release from guinea pig tracheal goblet cells. Am J RespirCell Mol Biol 1996;15(4):529–539.

38. Rogers DF. The airway goblet cell. Int J Biochem Cell Biol 2003;35(1):1–6.

39. Buist AS. Executive summary: Global strategy for the diagnosis,management, and prevention of chronic obstructive pulmonary dis-ease. Global Initiative for Chronic Obstructive Lung Disease (GOLD)website. http://goldcopd.org/GuidelineItem.asp?intId�996. Pub-lished September 2003. Updated December 2006.

40. Reid L. Pathology of chronic bronchitis. Proc R Soc Med 1956;49(10):771–773.

41. Aikawa T, Shimura S, Sasaki H, Takishima T, Yaegashi H, Taka-hashi T. Morphometric analysis of intraluminal mucus in airways inchronic obstructive pulmonary disease. Am Rev Respir Dis 1989;140(2):477–482.

42. Steiger D, Fahy J, Boushey H, Finkbeiner WE, Basbaum C. Use ofmucin antibodies and cDNA probes to quantify hypersecretion invivo in human airways. Am J Respir Cell Mol Biol 1994;10(5):538–545.

43. Saetta M, Turato G, Baraldo S, Zanin A, Braccioni F, Mapp CE, etal. Goblet cell hyperplasia and epithelial inflammation in peripheralairways of smokers with both symptoms of chronic bronchitis andchronic airflow limitation. Am J Respir Crit Care Med 2000;161(3 Pt1):1016–1021.

44. Mullen JB, Wright JL, Wiggs BR, Pare PD, Hogg JC. Structure ofcentral airways in current smokers and ex-smokers with and withoutmucus hypersecretion: relationship to lung function. Thorax 1987;42(11):843–848.

45. Reid L. Measurement of the bronchial mucous gland layer: a diag-nostic yardstick in chronic bronchitis. Thorax 1960;15:132–141.

46. Restrepo G, Heard BE. The size of the bronchial glands in chronicbronchitis. J Pathol Bacteriol 1963;85:305–310.

47. American Thoracic Society. Standards for the diagnosis and care ofpatients with chronic obstructive pulmonary disease (COPD) andasthma. Am Rev Respir Dis 1987;136:225–244.

48. British Thoracic Society. The British guidelines on asthma manage-ment. Thorax 1997;52(Suppl 1):S1–S21.

49. Eapen SS, Busse WW. Asthma. Clin Allergy Immunol 2002;16:325–353.

50. Rogers DF. Airway mucus hypersecretion in asthma: an undervaluedpathology? Curr Opin Pharmacol 2004;4(3):241–250.

51. Aikawa T, Shimura S, Sasaki H, Ebina M, Takishima T. Markedgoblet cell hyperplasia with mucus accumulation in the airways ofpatients who died of severe acute asthma attack. Chest 1992;101(4):916–921.

52. Dunnill MS. The pathology of asthma with special reference tochanges in the bronchial mucosa. J Clin Pathol 1960;13:27–33.

53. Houston JC, De Navasquez S, Trounce JR. A clinical and pathologicalstudy of fatal cases of status asthmaticus. Thorax 1953;8(3):207–213.

54. Saetta M, Di Stefano A, Rosina C, Thiene G, Fabbri LM. Quantita-tive structural analysis of peripheral airways and arteries in suddenfatal asthma. Am Rev Respir Dis 1991;143(1):138–143.

55. Bhaskar KR, O’Sullivan DD, Coles SJ, Kozakewich H, Vawter GP,Reid LM. Characterization of airway mucus from a fatal case ofstatus asthmaticus. Pediatr Pulmonol 1988;5(3):176–182.

56. Metzger WJ, Zavala D, Richerson HB, Moseley P, Iwamota P, MonickM, et al. Local allergen challenge and bronchoalveolar lavage ofallergic asthmatic lungs. Description of the model and local airwayinflammation. Am Rev Respir Dis 1987;135(2):433–440.

57. Fick RB Jr, Metzger WJ, Richerson HB, Zavala DC, Moseley PL,Schoderbek WE, et al. Increased bronchovascular permeability afterallergen exposure in sensitive asthmatics. J Appl Physiol 1987;63(3):1147–1155.

58. Rogers DF, Evans TW. Plasma exudation and oedema in asthma. BrMed Bull 1992;48(1):120–134.

59. Dunnill MS. The morphology of the airways in bronchial asthma. In:Stein M, editor. New directions in asthma: proceedings of the Inter-national Conference on Asthma, 1974, Asilomar, California. ParkRidge IL: American College of Chest Physicians, 1975; pp. 213–222.

60. List SJ, Findlay BP, Forstner GG, Forstner JF. Enhancement of theviscosity of mucin by serum albumin. Biochem J 1978;175(2):565–571.

61. Marriott C, Beeson MF, Brown DT. Biopolymer induced changes inmucus viscoelasticity. Adv Exp Med Biol 1982;144:89–92.

62. Aitken ML, Verdugo P. Donnan mechanism of mucin release andconditioning in goblet cells: the role of polyions. Symp Soc Exp Biol1989;43:73–80.

63. Williams IP, Rich B, Richardson PS. Action of serum on the outputof secretory glycoproteins from human bronchi in vitro. Thorax 1983;38(9):682–685.

64. Rogers DF. In vivo preclinical test models for studying airway mu-cus secretion. Pulm Pharmacol Ther 1997;10(3):121–128.

65. Ayers MM, Jeffery PK. Proliferation and differentiation in mamma-lian airway epithelium. Eur Respir J 1988;1(1):58–80.

66. Rogers DF. Airway goblet cells: responsive and adaptable front-linedefenders. Eur Respir J 1994;7(9):1690–1706.

67. Nadel JA, Burgel PR. The role of epidermal growth factor in mucusproduction. Curr Opin Pharmacol 2001;1(3):254–258.

68. Rogers AV, Dewar A, Corrin B, Jeffery PK. Identification of serous-like cells in the surface epithelium of human bronchioles. Eur RespirJ 1993;6(4):498–504.

69. Basbaum CB, Jany B, Finkbeiner WE. The serous cell. Annu RevPhysiol 1990;52:97–113.

70. Singh G, Katyal SL. Clara cell proteins. Ann N Y Acad Sci 2000;923:43–58.

71. Rogers DF. The role of airway secretions in COPD: pathophysiol-ogy, epidemiology and pharmacotherapeutic options. COPD 2005;2(3):341–353.

72. Burgel PR, Nadel JA. Roles of epidermal growth factor receptoractivation in epithelial cell repair and mucin production in airwayepithelium. Thorax 2004;59(11):992–996.



73. Hewson CA, Edbrooke MR, Johnston SL. PMA induces the MUC5ACrespiratory mucin in human bronchial epithelial cells, via PKC, EGF/TGF-alpha, Ras/Raf, MEK, ERK and Sp1-dependent mechanisms. JMol Biol 2004;344(3):683–695.

74. Toda M, Tulic MK, Levitt RC, Hamid Q. A calcium-activated chlo-ride channel (HCLCA1) is strongly related to IL-9 expression andmucus production in bronchial epithelium of patients with asthma. JAllergy Clin Immunol 2002;109(2):246–250.

75. Zhou Y, Shapiro M, Dong Q, Louahed J, Weiss C, Wan S, et al. A calcium-activated chloride channel blocker inhibits goblet cell metaplasia and mucusoverproduction. Novartis Found Symp 2002;248:150–165.

76. Donnelly LE, Rogers DF. Antiproteases and retinoids for treatmentof chronic obstructive pulmonary disease. Expert Opin Ther Patents2003;13(9):1345–1372.

77. Rogers DF, Barnes PJ. Treatment of airway mucus hypersecretion.Ann Med 2006;38(2):116–125.

Discussion

MacIntyre: That was terrific. Forsomebody who doesn’t deal with thisvery often, that was very clear, thankyou. I have 2 somewhat separate ques-tions. Number one, and this is proba-bly oversimplified, but you describedsecretions—a normal secretion and ahypersecretion. You say normal is nor-mal and hypersecretion is abnormal.Is there not a state in the middle wherea little hypersecretion is appropriate?It’s sort of like the sepsis cascade. Youknow, we think of these inflammatorymediators as all being bad.

Well, as a matter of fact, we evolvedinto creatures where this increased in-flammatory response in sepsis is prob-ably protective, and it’s only bad whenit starts spinning out of control. Sothere probably is a hypersecretion statethat is good, and protects us againstviral infections and other infections,and stuff like that. I guess the ques-tion is that it’s not normal versus ab-normal; it’s like normal at rest, nor-mal hypersecretion, and then abnormalhypersecretion. Does that make sense?

Rogers: I couldn’t agree more withthat. That’s the basis of the last ques-tion on my final slide. The airway ep-ithelium has got to have mucus on it:mucus is clearly protective. It does allthese wonderful things. Not least thatit provides a barrier. We’re inhalingabout 12,000 liters of air a day. Incentral London, where I work, it’sbringing millions upon millions of par-ticulate matter into the airways on anhourly basis, and we’ve got to haveairway mucus. It’s got viscoelasticityfor mucociliary clearance, as well asthe other protective mechanisms that

it provides. And when, for example,asthmatics inhale pollen or other irri-tant, acute production of mucus is pre-sumably protection against the inhaledpollen. But then the secretion subsides;so there is only transitory mucus hy-persecretion, after which mucus secre-tion returns to “baseline.” So you’vegot normal mucus secretion, andyou’ve got protective, transitory mu-cus hypersecretion.

MacIntyre: Which is good.

Rogers: Which is good; and abso-lutely what you want. But at somestage, if your airways are being re-peatedly challenged by inhaled partic-ulates, which in turn are setting up achronic inflammatory response lead-ing to airway remodeling into a mu-cus hypersecretory phenotype (eg,goblet cell hyperplasia and submuco-sal gland hypertrophy), you must reacha cutoff point where protective, tran-sitory mucus hypersecretion shiftsover into a chronic mucus hypersecre-tory phenotype, whereby the perpet-ual presence of excess mucus in theairways is pathophysiological. I don’tknow where that cutoff point will be,because the chronic mucus hyperse-cretion will still be trying to be pro-tective, but will merge into being non-protective: but, where is that mergepoint?

MacIntyre: Can I ask you anotherquestion? I am struck by the fact thatemphysema doesn’t have any mucus.As you go down the human tracheo-bronchial tree, where do you start los-ing mucus glands? Emphysematouspatients are dyspneic, but they cer-tainly don’t cough mucus.

Rogers: This is the thing. You canvisualize COPD as 3 interlinkedOlympic rings set in a triangle.1 Oneis chronic bronchitis, or mucus hy-persecretion; one is small airwaysdisease, which is fibrosis, and theresultant stenosis, of the bronchioli;and the third is alveolar destruction,or emphysema. Clearly, in any onepatient, you will not necessarilyknow the relative contribution ofthose 3 components to the patho-physiology and clinical symptom-atology of the patient, unless theyare hawking up great quantities ofsputum. In the paper by Aikawa etal, the patients just had emphysema;they weren’t producing sputum.2

They had alveolar destruction, withbig holes in their lungs. So, theydidn’t seem to have the bronchiticcomponent to their COPD, for what-ever reason; but it might be genetic.

1. Rogers DF. Mucoactive agents for airwaymucus hypersecretory diseases. Respir Care2007;52(9):1176-1193.

2. Aikawa T, Shimura S, Sasaki H, TakishimaT, Yaegashi H, Takahashi T. Morphometricanalysis of intraluminal mucus in airways inchronic obstructive pulmonary disease. AmRev Respir Dis 1989;140(2):477-482.

MacIntyre: But anatomically,where do you lose mucus glands?

Rubin: You go down until you losecartilage, so you’ve got mucus glandsuntil you’ve got cartilage.

MacIntyre: And how far down isthat? 20. . .?

Rubin: Not that far down. I’m notsure how far, but I don’t think it’s thatfar.



Rogers: Terminal bronchioles don’thave cartilage and don’t have glands,so in general you have glands whereyou have cartilage.

Rubin: The corollary to that is thatsome of the animal models that areused don’t have submucosal glands–typically, mice, rodents. So one of thereasons we think that there may be noair lung disease in the CF mouse modelis the complete lack of submucosalglands, except for a single little sub-mucosal gland right below the vocalcords. So there are very significant spe-cies differences, which makes it hardto extrapolate some of these things.Very nicely done, Duncan.

Also, to Neil: There are accumulat-ing data now that there are conditionsthat are associated with decreased mu-cus secretion that may lead to morechronic infection inflammation.1,2

And primary mucus hyposecretionmay actually be pathophysiologic insome of the diseases that we know of.But for asthma, in the last couple ofyears there’s been an interest in thisentity called “secretory hyperrespon-siveness” where patients with asthmaare given methacholine as a challenge,and some of them clearly get a bron-choconstrictive element and then bron-chodilate. Others still get a drop, butdon’t respond to bronchodilators.

If you look at this, these are thepatients who appear to respond moreto cholinergic agent by producingthese buckets of mucus, presumablysomething may be related to these pa-tients, whether it’s the presence of neu-trophil elastase, which can induce thisphenotype, that may lead the reallybad asthmatics to end up drowning intheir secretions, because you’ve shownthat. Have you any more informationon what would make somebody havea hypersecretory response to these ir-ritants, as opposed to a bronchospas-tic response?

1. Henke MO, Renner A, Huber RM, SeedsMC, Rubin BK. MUC5AC and MUC5B mu-cins are decreased in cystic fibrosis airway

secretions. Am J Respir Cell Mol Biol 2004;31:86-91.

2. Henke MO, Gerrit J, Germann M, Linde-mann H, Rubin BK. MUC5AC and MUC5Bmucins increase in cystic fibrosis airway se-cretions during a pulmonary exacerbation.Am J Respir Crit Care Med 2007;175:816-821.

Rogers: Yes, that’s a very interest-ing question. It’s something that fas-cinates me, and I’ve spoken about itwith some of my clinical colleagues.There are clearly patients who have amore bronchospastic response withless secretion. In contrast, other pa-tients produce a lot of mucus com-pared with the smooth muscle con-traction. I’m not sure why that is,because the nerves go to the 2 differ-ent structures, to the airway submu-cosal glands and the smooth muscle.I’m not sure why there should be adifference.

Some patients certainly have “bron-chorrhea,” which is a Japanese term,and one we don’t usually use in En-gland or the USA. Bronchorrhea maybe associated with excessive water se-cretion. Submucosal glands secretewater and, in experimental studies,cholinergic stimulation of glands willinduce mucus secretion and also wa-ter secretion; this is via interaction withmuscarinic M3 receptors. It could bethat these patients have a polymor-phism in the receptor whereby theyproduce more water in the secretionthan mucus. Bronchorrhea secretionsare excessive and certainly more wa-tery than a typical mucus secretion,indicating a preferential stimulation ofwater than of mucus. Why that wouldbe, I don’t know.

One possibility is that there’s a dis-proportionate change in the glands,whereby you get an increase in serouscells, which produce a more waterysecretion, compared with mucouscells, which produce a more viscoussecretion. Human glands compriseboth serous and mucous acini. InCOPD, there are many patients whodemonstrate a disproportionate in-crease in mucus cells compared to se-

rous cells. So there is the possibilityof the reverse situation: a dispropor-tionate increase in serous cells. Itwould be very interesting to make ahistological study of the airway glandsof patients with bronchorrhea.

Rubin: I know that Ruben Restrepowill be talking about adrenergic agents,but some of us have been interestedthat as asthmatics have come in withthis hypersecretion, they may beginwith very watery secretions, very muchthe way your nose runs during the al-lergy season. But during severeasthma, they develop these massivelyviscous secretions, and the concernmight be that this may be a result offlogging in the airways with the � ago-nists. You showed a number of yearsago that �-adrenergic stimulation pro-duces a very viscous secretion. And Iwonder if some of our asthmatics whoare up in the critical care unit are do-ing so because they’ve been given suchmassive doses of � agonists.

Rogers: That’s a possibility. One ofthe things about � agonists is whatyou’ve alluded to already with yourreference to the mouse, is that animalsand human beings respond differentlyto drugs, and � agonists are one ex-ample. It’s very easy in research ani-mals to show that � agonists stimulatemucus secretion. � agonists will do it,�-adrenergic agonists will do it, andyou can show various interactions.However, in human airways, it’s muchmore difficult to demonstrate a secre-tory response to � agonists. It maydepend on the distribution of recep-tors on the cells.

For example, cholinergic stimula-tion was once considered to inducesubmucosal gland secretion from bothmucous and serous acini. In contrast,�-receptor stimulation would stimu-late the mucus cells, whilst �-adren-ergic stimulation causes the serouscells to secrete. But it depends on thedistribution of the relevant receptors,and that will presumably have a ge-netic component. So, theoretically,



there is a scenario where there may bepreferential stimulation of mucusrather than serous secretion. But I don’tthink that’s been systematically lookedat.

Homnick: I’m interested in this con-cept of “tethered mucus” in asthma. Ithink it’s tethered to the goblet cells,is that correct?

Rogers: Yes, yes. I’m going to beshowing a slide of that in my nexttalk. But we can discuss it now.

Homnick: I’d like to know, what isthe mechanism? Is it incomplete exo-cytosis, or what is it?

Rogers: Well, that’s a very interest-ing question. In asthma there’s some-thing strange about either the mucusitself, or the secretory process that re-fuses to release the mucus when it’sbeing extruded from the goblet cells.1

There is continuity of mucus betweenthe lumen and the cells. This incom-plete release is not found in the air-ways of patients with COPD. Sothere’s something about asthma thatleads to this phenotype. And the ex-planation by the authors was that inasthma the airway inflammatory cellprofile comprises Th2 lymphocytesand eosinophils, whereas in COPD,macrophages and neutrophils predom-inate. The macrophages and neutro-phils produce proteases, includingelastase and matrix metalloproteases.And the hypothesis is that these pro-teases cleave the mucus, once it hasbeen secreted, away from the gobletcells.

In contrast, lymphocytes and eosin-ophils do not produce appropriate pro-teases to cleave away the secreted mu-cus from the goblet cells—hence thetethering. But you would have to lookat the kinetics of the system and athow the COPD protease enzymes ac-tually work. For example, are theylikely to cleave mucin molecules? Al-ternatively, it could be something morefundamental, like a difference in the

exocytotic mechanism betweenasthma and COPD. But it hasn’t beenexplored.

1. Rogers DF. Mucoactive agents for airwaymucus hypersecretory diseases. Respir Care;2007;52(9):1176-1193.

Restrepo: What is the importanceof the regional distribution of the se-rous cells or the mucus secretion? Thereason why I ask is because, whenyou look at the studies for sympatho-mimetics and anticholinergics, they al-ways talk about these regions of in-terest. And they will divide these areasof radioaerosol penetration and clear-ings into peripheral, transitional, andcentral regions. It looked to me, basedon this computerized picture, that theserous cells and the mucus cells don’thave any homogenous distribution.

Rogers: That distribution is theo-retically very important. The serouscells are at the periphery of the gland;moving in from there are the mucuscells that are producing mucus; thenthere is the collecting duct and cili-ated duct. Based on this distributionof the different secretory acini, the the-ory is that on the outside there are theserous cells, which produce a morewatery secretion (containing antibac-terial enzymes). Inside of them are themucus cells, which produce a moreviscous secretion (ie, mucin). And thetheory is that the more watery secre-tion washes the more viscous mucussecretion into the collecting duct andout of the top of the gland. In addi-tion, the glands have got smooth mus-cle bundles around them, which makesthe glands contract, for example, inresponse to cholinergic stimulation.That contraction, combined with themore watery secretion flooding overthe more viscous secretion, would helpforce the secretions out.

Restrepo: I guess the clinical im-portance of this distribution is becausethe sympathomimetics have the ten-dency to distribute toward the periph-ery, which actually correlates very well

with the pathological changes of pa-tients with COPD or chronic obstruc-tive airway disease.

Rubin: Ruben, I think Dr Rogerswas talking about the periphery in thecenter of an individual gland, distrib-uted throughout the airways . . .

Restrepo: Oh, I’m sorry . . .

Rubin: . . . You’re discussing smallairways having greater � agonist re-ceptors and proximal to the tracheahaving cholinergic receptors, I think,so when we’re talking periphery, we’reusing the term differently.

Restrepo: I was actually talkingabout the lung distribution. When youhave this computerized version of thedistribution of the. . .

Rogers: That was an individualgland. Professor Bill Whimster cut nu-merous histological sections througha human airway, and found that he’dalso fortuitously cut sections througha submucosal gland. He “recon-structed” the gland using a computer-based image analysis system.1

1. Rogers DF. Physiology of airway mucus se-cretion and pathophysiology of hypersecre-tion. Respir Care 2007;52(9):1134-1146.

Restrepo: Well, if that is the case,what is the importance of the lung dis-tribution of these cells? Are they ho-mogenously distributed throughout thelung, or is there any difference on theregional distribution?

Rogers: That’s an interesting ques-tion, but it’s not really been addressedbecause of the magnitude of the task:sampling a whole lung to determinethe regional distribution of the glandswould be prohibitively demanding.However, many years ago, Restrepoand Heard1,2 did a lot of work withsubmucosal glands, but not extensiveinvestigation of regional distributionalong the airways.



1. Restrepo GL, Heard BE. The size of thebronchial glands in chronic bronchitis.J Pathol Bacteriol 1963;85:305-310.

2. Restrepo GL, Heard BE. Mucous gland en-largement in chronic bronchitis: extent ofenlargement in the tracheo-bronchial tree.Thorax 1963;18:334-339.

Schechter: I’d like to ask about theproposition that some patients are mu-cus hypersecretors. That’s an interest-ing concept, and you even showed aslide indicating that there were differ-ences in mortality, depending uponhow these patients were categorized.How does one categorize a patient asa mucus hypersecretor? What criteriado you use for that classification? Arethere specific criteria, or is it just agestalt classification?

Rogers: It was what we might call agestalt evaluation. There was a ques-tionnaire, and one question asked, “Doyou produce a lot of sputum?” Theyeither did or they didn’t. So that was

the definition of mucus hypersecre-tion in the metric. The data from theCopenhagen Heart Study where thepopulation of Copenhagen was sam-pled. Originally, it was primarily aheart study, and acquired as much in-formation as they could from every-body in Copenhagen, mostly by ques-tionnaire. Professor Jorgen Vestbo andcolleagues looked at the data from theperspective of respiratory epidemiol-ogists interested in the respiratory pa-rameters, one of which was chronicmucus hypersecretion, and how it re-lated to parameters such as mortality,infection, hospitalization.1

1. Vestbo J. Epidemiological studies in mucushypersecretion. Novartis Found Symp 2002;248:3-12.

Schechter: So that creates a prob-lem with interpretation. Even if therewas some objective measure of se-cretion that would allow you to cat-egorize patients as hypersecretors,

the problem would still be that theunderlying disease process ratherthat the individual patient character-istics may be the cause of the mucushypersecretion. So the cause of in-creased mortality might be the dis-ease that is causing it rather than thepresence or absence of the mucusper se.

Rogers: I agree. I’m not an epide-miologist, nor would I want to be; theyhave a very difficult job—dealing withpopulations is just too complex.

Schechter: Well, this is the point Iam trying to make. We can have in-teresting discussions that focus at themolecular and at the cellular level, butthe jump from the cellular level to thepatient outcome level is one that wemust make with caution.

Rogers: Absolutely!



Physiology of Airway Mucus Secretion.pdf

Documents

Transcript of Physiology of Airway Mucus Secretion.pdf