MUC2 exocytosis in intestinal goblet cells

Abstract

MUC2 in the major secreted gel‐forming mucin produced by goblet cells (GC) that form the protective mucus blanket covering the epithelium as the first line of innate host defense in the gut. The protein complex and the cellular mechanisms by which MUC2 granulae are released by compound exocytosis from GC under basal or in response to mucus secretagogues are not known. In this study we identified the putative exocytosis components expressed in human GC, LS 174T. The exocytosis protein complex formation between SNAP23, Syntaxin 3 and Munc18b was studied in response to the PKC agonist, phorbol 12‐myristate 13‐acetate (PMA) using immunoprecipitation and mass spectrometry. In contrast to MUC2, the exocytosis proteins were not regulated at the transcriptional level, as mRNA and protein expression was unaltered after PMA. However, PMA treatment phosphorylated Syntaxin 3 in active SNARE complex formation. Surprisingly, confocal imaging revealed that Syntaxin 3 and Munc18b are not only localized to the apical cap of the GC theca but also to areas around the theca where the SNARE components co‐localized with actin filaments. These findings were corroborated using differential centrifugation where Syntaxin 3 was identified on both the granulae and the plasma membrane. Our studies reveal complex exocytosis protein machinery that regulates both basal and stimulated MUC2 granulae release from goblet cells. Supported by CIHR.