Synaptotagmin 2 Couples Mucin Granule Exocytosis to Ca2+ Signaling from Endoplasmic Reticulum

Michael J Tuvim,Andrea Rossi Mospan,Kimberlie A Burns,Michael Chua,Peter J Mohler,Ernestina Melicoff,Roberto Adachi,Zoulikha Ammar-Aouchiche,C.William Davis,Burton F Dickey

doi:10.1074/jbc.m807849200

Abstract

Synaptotagmin 2 (Syt2) functions as a low affinity, fast exocytic Ca(2+) sensor in neurons, where it is activated by Ca(2+) influx through voltage-gated channels. Targeted insertion of lacZ into the mouse syt2 locus reveals expression in mucin-secreting goblet cells of the airways. In these cells, rapid Ca(2+) entry from the extracellular medium does not contribute significantly to stimulated secretion (Davis, C. W., and Dickey, B. F. (2008) Annu. Rev. Physiol. 70, 487-512). Nonetheless, Syt2(-/-) mice show a severe defect in acute agonist-stimulated airway mucin secretion, and Syt2(+/-) mice show a partial defect. In contrast to Munc13-2(-/-) mice (Zhu, Y., Ehre, C., Abdullah, L. H., Sheehan, J. K., Roy, M., Evans, C. M., Dickey, B. F., and Davis, C. W. (2008) J. Physiol. (Lond.) 586, 1977-1992), Syt2(-/-) mice show no spontaneous mucin accumulation, consistent with the inhibitory action of Syt2 at resting cytoplasmic Ca(2+) in neurons. In human airway goblet cells, inositol trisphosphate receptors are found in rough endoplasmic reticulum that closely invests apical mucin granules, consistent with the known dependence of exocytic Ca(2+) signaling on intracellular stores in these cells. Hence, Syt2 can serve as an exocytic sensor for diverse Ca(2+) signaling systems, and its levels are limiting for stimulated secretory function in airway goblet cells.

Highlights

Grants HL072984, HL094848, CA105352, CA016672, and HL063756
Synaptotagmin 2 (Syt2) Expression in Airway Epithelium—We found Syt2 in the lungs of WT but not null mice by immunoblot analysis (Fig. 1A), raising the possibility that Syt2 serves as an exocytic Ca2ϩ sensor in secretory epithelial cells
Syt2 null mice had been generated by knocking IRES-lacZ with a nuclear localization signal into the syt2 locus under the transcriptional control of the native Syt2 regulatory elements [14]

Summary

Introduction

Syt2 Expression in Airway Epithelium—We found Syt2 in the lungs of WT but not null mice by immunoblot analysis (Fig. 1A), raising the possibility that Syt2 serves as an exocytic Ca2ϩ sensor in secretory epithelial cells. Proximity of ER to Mucin Secretory Granules—ATP-stimulated elevations in cytoplasmic Ca2ϩ in airway goblet cells are due primarily to IP3-mediated release from intracellular stores [3, 7, 9].

Results

Conclusion