title: Two distinct modes of exocytotic fusion pore expansion in large astrocytic vesicles.
abstract: Formation of the fusion pore is a central question for regulated exocytosis by which secretory cells release neurotransmitters or hormones. Here, by dynamically monitoring exocytosis of large vesicles (2-7 μM) in astrocytes with two-photon microscopy imaging, we found that the exocytotic fusion pore was generated from the SNARE-dependent fusion at a ring shape of the docked plasma-vesicular membrane and the movement of a fusion-produced membrane fragment. We observed two modes of fragment movements, 1) a shift fragment that shifted to expand the fusion pore and 2) a fall-in fragment that fell into the collapsed vesicle to expand the fusion pore. Shift and fall-in modes are associated with full and partial collapses of large vesicles, respectively. The astrocytic marker, sulforhodamine 101, stained the fusion-produced membrane fragment more brightly than FM 1-43. Sulforhodamine 101 imaging showed that double fusion pores could simultaneously occur in a single vesicle (16% of large vesicles) to accelerate discharge of vesicular contents. Electron microscopy of large astrocytic vesicles showed shift and fall-in membrane fragments. Two modes of fusion pore formation demonstrate a novel mechanism underlying fusion pore expansion and provide a new explanation for full and partial collapses of large secretory vesicles.
authors.affiliation: Department of Cell Biology and Anatomy, New York Medical College, Valhalla, New York 10595.
authors.forename: Hong
authors.initials: H
authors.lastname: Peng
authors.affiliation: Department of Cell Biology and Anatomy, New York Medical College, Valhalla, New York 10595.
authors.forename: Ning
authors.initials: N
authors.lastname: Kang
authors.affiliation: East Hospital, Tongji University School of Medicine, Shanghai 200120, China. Electronic address: xunymc2000@yahoo.com.
authors.forename: Jun
authors.initials: J
authors.lastname: Xu
authors.affiliation: Department of Cell Biology and Anatomy, New York Medical College, Valhalla, New York 10595.
authors.forename: Patric K
authors.initials: PK
authors.lastname: Stanton
authors.affiliation: Department of Cell Biology and Anatomy, New York Medical College, Valhalla, New York 10595. Electronic address: jian_kang@NYMC.edu.
authors.forename: Jian
authors.initials: J
authors.lastname: Kang
mesh_terms: D000818:Animals; D001253:Astrocytes / Q000166:cytology / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism*; D005089:Exocytosis / Q000502:physiology*; D008561:Membrane Fusion / Q000502:physiology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D050600:SNARE Proteins / Q000378:metabolism*; D022142:Secretory Vesicles / Q000378:metabolism*
chemical_list: D050600:SNARE Proteins
keywords: Astrocytes; Exocytosis; Fluorescence; Membrane Fusion; Pore Expansion; Vesicles