Aquaporin-5 Dependent Fluid Secretion in Airway Submucosal Glands

Abstract

Submucosal glands in mammalian airways secrete a mixture of water, ions, and macromolecules onto the airway surface.The composition of the airway surface liquid (ASL), 1 the thin layer of fluid coating the airway epithelium, is determined in large part by the composition of fluids secreted by submucosal glands (1-3). Submucosal gland function is thought to play an important role in antimicrobial defense and in the regulation of ASL volume and composition (3-5). Abnormally viscous gland secretions in cystic fibrosis have been proposed to promote bacterial adhesion and inhibit bacterial clearance by impeding ciliary function (6 -8).Submucosal glands consist of serous tubules where active salt secretion into the gland lumen creates a small osmotic gradient driving water transport across a presumably highly water permeable epithelium. Salt secretion by serous epithelial cells is believed to involve Cl Ϫ transport by the cystic fibrosis transmembrane conductance regulator protein (CFTR), the protein which is defective in cystic fibrosis (9, 10). The glands also contain mucous cells and tubules where viscous glycoproteins are secreted. The fluid then passes through a collecting duct containing ciliated epithelial cells and onto the airway surface. Based on evidence that aquaporins facilitate nearisosmolar fluid transport in kidney proximal tubule (11) and salivary gland (12), we postulated that aquaporins and high water permeability are required for near-isosmolar fluid secretion by the epithelium of airway submucosal glands.The purpose of this study was to determine whether aquaporin water channels facilitate fluid secretion from airway submucosal glands. Aquaporin water channels AQP4 and AQP5 were immunolocalized to the basolateral and luminal membranes, respectively, of submucosal gland serous epithelial cells in mouse nasopharynx and upper airways. Methodology was developed to quantify the volume and composition of fluid secreted from submucosal glands in living mice. Comparative measurements done on wild type mice and transgenic knockout mice lacking AQP4 and AQP5 indicated an important role for AQP5 in submucosal gland fluid secretion. AQP5 is thus a rate-limiting barrier in gland fluid secretion, and so regulation of its activity may alter gland fluid secretion in cystic fibrosis and other airway diseases. EXPERIMENTAL PROCEDURESTransgenic Mice-Knockout mice deficient in AQP4 and AQP5 in a CD1 genetic background were generated by targeted gene disruption (13,14). Measurements were done in litter-matched mice (age 8 -10 weeks, body weight 20 -25 grams) produced by intercrossing of heterozygous mice. The investigators were blinded to genotype information in all measurements. Protocols were approved by the U.C.S.F. Committee on Animal Research.Measurement of Total Submucosal Gland Fluid Secretion RateMice were anesthetized with pentobarbital (50 mg/kg). After exposure

Transient receptor potential vanilloid 4 regulates aquaporin-5 abundance under hypotonic conditions

“…Our investigation suggests distinct mechanisms for this regulation, with TRPV4 activation mediating hypotonic reduction, and EGFR activation mediating hypertonic induction. These findings, coupled with observations indicating that the presence of AQP5 is fundamental to ASL secretion (54) and cell volume regulation (6), suggest important potential roles for this pathway in airway surface regulation, and may extend to regulation of water homeostasis at other sites where these proteins are present, including salivary and sweat glands.…”

Antenatal Dexamethasone Treatment Leads to Changes in Gene Expression in a Murine Late Placenta

“…Placenta expresses aquaporins 1, 3, 8 and 9 and aquaporin 5 is normally expressed in the lung and salivary gland and plays a critical role in regulation of cell volume in response to changes in osmolarity [49]. Aquaporin 5 expression has been shown to be enhanced by steroids in other tissues, thus this may be the reason for its increased expression in the DEX-exposed placentas and may explain the hydropic appearance of these placentas.…”

Role of proneuregulin 1 cleavage and human epidermal growth factor receptor activation in hypertonic aquaporin induction

“…AQP5 plays an important role in the generation of the airway surface liquid, as evidenced by reduced surface layer secretions in AQP5-null mice (56). Tight regulation of the constitution and volume of the airway surface layer is necessary for the maintenance of normal lung defense.…”