title: Lysosomes behave as Ca2+-regulated exocytic vesicles in fibroblasts and epithelial cells.
abstract: Lysosomes are considered to be a terminal degradative compartment of the endocytic pathway, into which transport is mostly unidirectional. However, specialized secretory vesicles regulated by Ca2+, such as neutrophil azurophil granules, mast cell-specific granules, and cytotoxic lymphocyte lytic granules, share characteristics with lysosomes that may reflect a common biogenesis. In addition, the involvement of Ca2+ transients in the invasion mechanism of the parasite Trypanosoma cruzi, which occurs by fusion of lysosomes with the plasma membrane, suggested that lysosome exocytosis might be a generalized process present in most cell types. Here we demonstrate that elevation in the intracellular free Ca2+ concentration of normal rat kidney (NRK) fibroblasts induces fusion of lysosomes with the plasma membrane. This was verified by measuring the release of the lysosomal enzyme beta-hexosaminidase, the appearance on the plasma membrane of the lysosomal glycoprotein lgp120, the release of fluid-phase tracers previously loaded into lysosomes, and the release of the lysosomally processed form of cathepsin D. Exposure to the Ca2+ ionophore ionomycin or addition of Ca2+-containing buffers to streptolysin O-permeabilized cells induced exocytosis of approximately 10% of the total lysosomes of NRK cells. The process was also detected in other cell types such as epithelial cells and myoblasts. Lysosomal exocytosis was found to require micromolar levels of Ca2+ and to be temperature and ATP dependent, similar to Ca2+-regulated secretory mechanisms in specialized cells. These findings highlight a novel role for lysosomes in cellular membrane traffic and suggest that fusion of lysosomes with the plasma membrane may be an ubiquitous form of Ca2+-regulated exocytosis.
authors.affiliation: Department of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06520, USA.
authors.forename: A
authors.initials: A
authors.lastname: Rodríguez
authors.forename: P
authors.initials: P
authors.lastname: Webster
authors.forename: J
authors.initials: J
authors.lastname: Ortego
authors.forename: N W
authors.initials: NW
authors.lastname: Andrews
mesh_terms: D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D015703:Antigens, CD / Q000378:metabolism; D002118:Calcium / Q000494:pharmacology* / Q000502:physiology; D002402:Cathepsin D / Q000378:metabolism; D002462:Cell Membrane / Q000737:chemistry / Q000502:physiology / Q000648:ultrastructure; D002463:Cell Membrane Permeability; D002478:Cells, Cultured / Q000737:chemistry / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D011992:Endosomes / Q000737:chemistry / Q000378:metabolism; D004847:Epithelial Cells; D004848:Epithelium / Q000502:physiology / Q000648:ultrastructure; D005089:Exocytosis / Q000187:drug effects / Q000502:physiology*; D005347:Fibroblasts / Q000166:cytology / Q000187:drug effects / Q000502:physiology; D016650:Fluorescein-5-isothiocyanate; D005456:Fluorescent Dyes; D006801:Humans; D015759:Ionomycin / Q000494:pharmacology; D007476:Ionophores / Q000494:pharmacology; D007668:Kidney / Q000166:cytology; D052118:Lysosomal-Associated Membrane Protein 1; D051907:Lysosomal Membrane Proteins; D008247:Lysosomes / Q000187:drug effects / Q000378:metabolism / Q000502:physiology*; D008562:Membrane Glycoproteins / Q000378:metabolism; D008854:Microscopy, Electron; D051381:Rats; D011990:Receptors, Transferrin / Q000378:metabolism
chemical_list: D015703:Antigens, CD; D005456:Fluorescent Dyes; D007476:Ionophores; D052118:Lysosomal-Associated Membrane Protein 1; D051907:Lysosomal Membrane Proteins; D008562:Membrane Glycoproteins; D011990:Receptors, Transferrin; D015759:Ionomycin; D000255:Adenosine Triphosphate; D002402:Cathepsin D; D016650:Fluorescein-5-isothiocyanate; D002118:Calcium