The role of Munc18b in mucin secretion in airway secretory cells

Abstract

Tight control of airway mucin secretion is required for lung homeostasis, yet the composition and function of the airway exocytic machinery are poorly understood. Munc18 family proteins are critical regulators of exocytosis in neurons and yeast. We found Munc18b protein strongly expressed in mouse lungs and airway epithelial cell lines by immunoblot and Munc18b transcripts in airway secretory cells by in situ hybridization. Endogenous Munc18b localized immunohistochemically to the apical pole of airway epithelial secretory cells. We engineered a mouse with a severely hypomorphic Munc18b allele such that protein expression in heterozygotes was reduced by ~50% and no homozygotes were obtained. Heterozygous mutant mice showed a ~50% reduction in acute ATP-stimulated release of intracellular mucin. Munc18 proteins function by regulating the activity of Syntaxins, the central components of the exocytic machinery. The exocytic Syntaxins 1, 2, 3 and 4 are all expressed in airway epithelial cell lines, but Munc18b interacted preferentially with Syntaxins 2 and 3 by co-immunoprecipitation. These results identify Munc18b as an essential gene that is a limiting component of the airway mucin exocytic machinery, and demonstrate selective interactions of Munc18b with Syntaxins 2 and 3 in airway secretory cells.