Munc18b is an essential gene in mice whose expression is limiting for secretion by airway epithelial and mast cells

Abstract

Airway mucin secretion and MC (mast cell) degranulation must be tightly controlled for homoeostasis of the lungs and immune system respectively. We found the exocytic protein Munc18b to be highly expressed in mouse airway epithelial cells and MCs, and localized to the apical pole of airway secretory cells. To address its functions, we created a mouse with a severely hypomorphic Munc18b allele such that protein expression in heterozygotes was reduced by ~50%. Homozygous mutant mice were not viable, but heterozygotes showed a ~50% reduction in stimulated release of mucin from epithelial cells and granule contents from MCs. The defect in MCs affected only regulated secretion and not constitutive or transporter-mediated secretion. The severity of passive cutaneous anaphylaxis was also reduced by ~50%, showing that reduction of Munc18b expression results in an attenuation of physiological responses dependent on MC degranulation. The Munc18b promoter is controlled by INR (initiator), Sp1 (specificity protein 1), Ets, CRE (cAMP-response element), GRE (glucocorticoid-response element), GATA and E-box elements in airway epithelial cells; however, protein levels did not change during mucous metaplasia induced by allergic inflammation. Taken together, the results of the present study identify Munc18b as an essential gene that is a limiting component of the exocytic machinery of epithelial cells and MCs.

Chaperoning of closed syntaxin-3 through Lys46 and Glu59 in domain 1 of Munc18 proteins is indispensable for mast cell exocytosis

“…In the immune tissue of these patients, there is a markedly reduced degranulation of lytic granules from cytotoxic T-lymphocytes and natural killer cells (Côte et al, 2009). In mice, Munc18-2 heterozygous knockdown results in significant impairment of mast cell degranulation (Kim et al, 2012). However, despite its importance, understanding the functions and its structural determinants of Munc18-2, which are necessary for immune cell exocytosis, has just begun recently .…”

Genetic Diseases Predisposing to HLH

“…43 We observed male infertility in an older non-transplanted patient with MUNC18-2 deficiency; this observation may be relevant in the context of recent findings in a mouse model. 112 Platelet secretory defects, sometimes associated with abnormal bleeding, 113 can be observed in MUNC18-2 deficiency. In one patient with biallelic STXBP2 mutations, 43 Hodgkin disease was diagnosed at the age of 9 years.…”

Regulated Mucin Secretion from Airway Epithelial Cells

“…The Syntaxin that mediates mucin granule exocytosis remains unknown. Candidates are Stx 2, 3, and 11, all of which have been shown to functionally pair with Munc18b in other cell types since Munc18b has been definitively implicated in airway mucin exocytosis (56) (see below). Efforts are underway in the Dickey laboratory using genetically modified mice to test the roles of these Syntaxins in mucin secretion.…”

“…The scaffolding function of SM proteins in exocytosis in different cell types is mediated by three Munc18 proteins (54, 56). Munc18a (Stxbp1) and Munc18b (Stxbp2) appear to be paralogs functioning in axonal/apical secretion, whereas Munc18c (Stxbp3) is a ubiquitous isoform functioning in dendritic/basolateral secretion.…”