title: Secretory proteins without a transport signal are retained in secretory granules during maturation in rat parotid acinar cells.
abstract: OBJECTIVE
  The acinar cells of the parotid gland are filled with numerous secretory granules (SGs), which accumulate the digestion enzyme amylase. SGs mature accompanied with membrane remodelling such as fusion and budding of small vesicles. However, little is understood about the mechanism of the condensation of SG contents during maturation. In this study, we examined whether secretory proteins need a specific signal to be retained in SGs.
  
  
  METHODS
  To induce internalization of the luminal membrane after exocytosis, we injected the β-adrenergic agonist isoproterenol into rats. Acinar cells were then incubated with Lucifer Yellow (LY) dye as a tracer for 3h for uptake into immature secretory granules (ISGs). To observe whether LY was retained in SGs after maturation, we continued incubating the cultured acinar cells for 2 days.
  
  
  RESULTS
  The localization of LY into ISGs was confirmed by the following four methods: (1) co-localization of the fluorescence of LY and amylase by confocal laser microscopy, (2) detection of the fluorescence from purified ISGs, (3) secretion of the fluorescence together with amylase upon stimulation, and (4) observation of the intracellular localization of LY by electron microscopy. Moreover, we observed co-localization of some of the SGs with the fluorescence of LY after cell culture.
  
  
  CONCLUSIONS
  Although the fusion and budding of small vesicles may contribute to the process of granule maturation, LY remained in the SGs even after maturation. These results suggest that secretory proteins that have no transport signal are not excluded from SGs, and they are retained in SGs during granule maturation in exocrine parotid glands.
authors.affiliation: Department of Physiology and Research Institute of Oral Science, Nihon University School of Dentistry at Matsudo, 2-870-1 Sakaecho-Nishi, Matsudo, Chiba 271-8587, Japan. Electronic address: katsumata.osamu@nihon-u.ac.jp.
authors.forename: Osamu
authors.initials: O
authors.lastname: Katsumata-Kato
authors.affiliation: Department of Physiology and Research Institute of Oral Science, Nihon University School of Dentistry at Matsudo, 2-870-1 Sakaecho-Nishi, Matsudo, Chiba 271-8587, Japan.
authors.forename: Megumi
authors.initials: M
authors.lastname: Yokoyama
authors.affiliation: Department of Physiology and Research Institute of Oral Science, Nihon University School of Dentistry at Matsudo, 2-870-1 Sakaecho-Nishi, Matsudo, Chiba 271-8587, Japan.
authors.forename: Miwako
authors.initials: M
authors.lastname: Matsuki-Fukushima
authors.affiliation: Laboratory of Veterinary Biochemistry, Nihon University College of Bioresource Sciences, 1866 Kameino, Fujisawa, Kanagawa 252-0880, Japan.
authors.forename: Takanori
authors.initials: T
authors.lastname: Narita
authors.affiliation: Laboratory of Veterinary Biochemistry, Nihon University College of Bioresource Sciences, 1866 Kameino, Fujisawa, Kanagawa 252-0880, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Sugiya
authors.affiliation: Department of Physiology and Research Institute of Oral Science, Nihon University School of Dentistry at Matsudo, 2-870-1 Sakaecho-Nishi, Matsudo, Chiba 271-8587, Japan.
authors.forename: Junko
authors.initials: J
authors.lastname: Fujita-Yoshigaki
mesh_terms: D061354:Acinar Cells / Q000378:metabolism*; D000681:Amylases / Q000378:metabolism; D000818:Animals; D002478:Cells, Cultured; D003594:Cytoplasmic Granules / Q000378:metabolism*; D004705:Endocytosis; D007545:Isoproterenol / Q000494:pharmacology*; D018613:Microscopy, Confocal; D008854:Microscopy, Electron; D010306:Parotid Gland / Q000378:metabolism*; D051381:Rats; D022142:Secretory Vesicles / Q000378:metabolism*; D015398:Signal Transduction
chemical_list: D000681:Amylases; D007545:Isoproterenol
keywords: Endocytosis; Granule maturation; Parotid gland; Salivary gland; Secretory granules; Secretory vesicles