title: Actin coating and compression of fused secretory vesicles are essential for surfactant secretion--a role for Rho, formins and myosin II.
abstract: Secretion of vesicular contents by exocytosis is a fundamental cellular process. Increasing evidence suggests that post-fusion events play an important role in determining the composition and quantity of the secretory output. In particular, regulation of fusion pore dilation and closure is considered a key regulator of the post-fusion phase. However, depending on the nature of the cargo, additional mechanisms might be essential to facilitate effective release. We have recently described that in alveolar type II (ATII) cells, lamellar bodies (LBs), which are secretory vesicles that store lung surfactant, are coated with actin following fusion with the plasma membrane. Surfactant, a lipoprotein complex, does not readily diffuse out of fused LBs following opening and dilation of the fusion pore. Using fluorescence microscopy, atomic force microscopy and biochemical assays, we present evidence that actin coating and subsequent contraction of the actin coat is essential to facilitate surfactant secretion. Latrunculin B prevents actin coating of fused LBs and inhibits surfactant secretion almost completely. Simultaneous imaging of the vesicle membrane and the actin coat revealed that contraction of the actin coat compresses the vesicle following fusion. This leads to active extrusion of vesicle contents. Initial actin coating of fused vesicles is dependent on activation of Rho and formin-dependent actin nucleation. Actin coat contraction is facilitated by myosin II. In summary, our data suggest that fusion pore opening and dilation itself is not sufficient for release of bulky vesicle cargos and that active extrusion mechanisms are required.
authors.affiliation: Department of General Physiology, University of Ulm, Ulm, Germany.
authors.forename: Pika
authors.initials: P
authors.lastname: Miklavc
authors.forename: Elena
authors.initials: E
authors.lastname: Hecht
authors.forename: Nina
authors.initials: N
authors.lastname: Hobi
authors.forename: Oliver H
authors.initials: OH
authors.lastname: Wittekindt
authors.forename: Paul
authors.initials: P
authors.lastname: Dietl
authors.forename: Christine
authors.initials: C
authors.lastname: Kranz
authors.forename: Manfred
authors.initials: M
authors.lastname: Frick
mesh_terms: D000199:Actins / Q000378:metabolism*; D000818:Animals; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000494:pharmacology; D047630:Depsipeptides / Q000494:pharmacology; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism; D005326:Fetal Proteins / Q000378:metabolism*; D036681:Fluorescence Recovery After Photobleaching; D000080865:Formins; D008561:Membrane Fusion / Q000187:drug effects; D008840:Microfilament Proteins / Q000378:metabolism*; D018625:Microscopy, Atomic Force; D024462:Myosin Type II / Q000378:metabolism*; D009687:Nuclear Proteins / Q000378:metabolism*; D011663:Pulmonary Surfactants / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D022142:Secretory Vesicles / Q000187:drug effects / Q000378:metabolism* / Q000648:ultrastructure; D053778:Thiazolidines / Q000494:pharmacology; D020742:rhoA GTP-Binding Protein / Q000378:metabolism*
chemical_list: D000199:Actins; D019086:Bridged Bicyclo Compounds, Heterocyclic; D047630:Depsipeptides; D005326:Fetal Proteins; D000080865:Formins; D008840:Microfilament Proteins; D009687:Nuclear Proteins; D011663:Pulmonary Surfactants; D053778:Thiazolidines; C057531:jasplakinolide; D024462:Myosin Type II; D020742:rhoA GTP-Binding Protein; C037068:latrunculin B