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Annexin A2-dependent actin bundling promotes secretory granule docking to the plasma membrane and exocytosis.

Gabel M, Delavoie F, Demais V, Royer C, Bailly Y, Vitale N, Bader MF, Chasserot-Golaz S - J. Cell Biol. (2015)

Bottom Line: Annexin A2, a calcium-, actin-, and lipid-binding protein involved in exocytosis, mediates the formation of lipid microdomains required for the structural and spatial organization of fusion sites at the plasma membrane.When an annexin A2 mutant with impaired actin filament-bundling activity was expressed, the formation of plasma membrane lipid microdomains and the number of exocytotic events were decreased and the fusion kinetics were slower, whereas the pharmacological activation of the intrinsic actin-bundling activity of endogenous annexin A2 had the opposite effects.Thus, annexin A2-induced actin bundling is apparently essential for generating active exocytotic sites.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institut des Neurosciences Cellulaires et Intégratives, UPR3212 Centre National de la Recherche Scientifique, Université de Strasbourg, F-67084 Strasbourg, France.

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Spatial organization of actin filaments connecting secretory granules to the plasma membrane. (A) TEM images at zero tilt of a docked secretory granule and a series of secretory granules at different stages of fusion (left) and the corresponding clipping plane of the side view of the isosurface representation of their tomogram (right). Actin cytoskeleton favoring exocytosis (asterisks) of secretory granules (SG) is clearly linked to the plasma membrane (PM) through specific associated-membrane structures (V). Bars, 200 nm. (B) Slices through a tomogram of docked and fusing secretory granules. Red broken circles define an area of 90–250 nm around the granules, wherein anchor points for cortical actin at the plasma membrane were systematically localized. Bars, 100 nm.
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fig3: Spatial organization of actin filaments connecting secretory granules to the plasma membrane. (A) TEM images at zero tilt of a docked secretory granule and a series of secretory granules at different stages of fusion (left) and the corresponding clipping plane of the side view of the isosurface representation of their tomogram (right). Actin cytoskeleton favoring exocytosis (asterisks) of secretory granules (SG) is clearly linked to the plasma membrane (PM) through specific associated-membrane structures (V). Bars, 200 nm. (B) Slices through a tomogram of docked and fusing secretory granules. Red broken circles define an area of 90–250 nm around the granules, wherein anchor points for cortical actin at the plasma membrane were systematically localized. Bars, 100 nm.

Mentions: The side-view of tilt transmission electron microscopy (TEM) series reconstructions of docked granules at different stages of fusion clearly showed that actin bundles linked secretory granules to the plasma membrane (PM) through specific membrane-associated structures (Fig. 3 A, V). These structures could correspond to the fine strands cross-linking granules to the plasma membrane previously revealed by quick-freeze, deep-etch electron microscopy (Nakata et al., 1990). These actin filaments were also observed when granules collapsed gradually to fuse with the plasma membrane. As recently described by Chiang et al. (2014), the size of the granules undergoing exocytosis seemed to decrease. It is interesting to note the positive curvature of the plasma membrane under docked granules that could be caused by forces exerted by the formation of actin bundles. Fig. 3 B shows that actin filaments connected secretory granules to the plasma membrane at anchor points localized in an area of 90–250 nm around the granule. This area corresponds to the 0.1-µm zone from the edge of granules where GM1 and AnxA2 beads were concentrated (Fig. 1 D; also see Fig. 4 B) and may constitute the dynamic platform in which protein and lipid diffusion is restricted (Saka et al., 2014).


Annexin A2-dependent actin bundling promotes secretory granule docking to the plasma membrane and exocytosis.

Gabel M, Delavoie F, Demais V, Royer C, Bailly Y, Vitale N, Bader MF, Chasserot-Golaz S - J. Cell Biol. (2015)

Spatial organization of actin filaments connecting secretory granules to the plasma membrane. (A) TEM images at zero tilt of a docked secretory granule and a series of secretory granules at different stages of fusion (left) and the corresponding clipping plane of the side view of the isosurface representation of their tomogram (right). Actin cytoskeleton favoring exocytosis (asterisks) of secretory granules (SG) is clearly linked to the plasma membrane (PM) through specific associated-membrane structures (V). Bars, 200 nm. (B) Slices through a tomogram of docked and fusing secretory granules. Red broken circles define an area of 90–250 nm around the granules, wherein anchor points for cortical actin at the plasma membrane were systematically localized. Bars, 100 nm.
© Copyright Policy - openaccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4555831&req=5

fig3: Spatial organization of actin filaments connecting secretory granules to the plasma membrane. (A) TEM images at zero tilt of a docked secretory granule and a series of secretory granules at different stages of fusion (left) and the corresponding clipping plane of the side view of the isosurface representation of their tomogram (right). Actin cytoskeleton favoring exocytosis (asterisks) of secretory granules (SG) is clearly linked to the plasma membrane (PM) through specific associated-membrane structures (V). Bars, 200 nm. (B) Slices through a tomogram of docked and fusing secretory granules. Red broken circles define an area of 90–250 nm around the granules, wherein anchor points for cortical actin at the plasma membrane were systematically localized. Bars, 100 nm.
Mentions: The side-view of tilt transmission electron microscopy (TEM) series reconstructions of docked granules at different stages of fusion clearly showed that actin bundles linked secretory granules to the plasma membrane (PM) through specific membrane-associated structures (Fig. 3 A, V). These structures could correspond to the fine strands cross-linking granules to the plasma membrane previously revealed by quick-freeze, deep-etch electron microscopy (Nakata et al., 1990). These actin filaments were also observed when granules collapsed gradually to fuse with the plasma membrane. As recently described by Chiang et al. (2014), the size of the granules undergoing exocytosis seemed to decrease. It is interesting to note the positive curvature of the plasma membrane under docked granules that could be caused by forces exerted by the formation of actin bundles. Fig. 3 B shows that actin filaments connected secretory granules to the plasma membrane at anchor points localized in an area of 90–250 nm around the granule. This area corresponds to the 0.1-µm zone from the edge of granules where GM1 and AnxA2 beads were concentrated (Fig. 1 D; also see Fig. 4 B) and may constitute the dynamic platform in which protein and lipid diffusion is restricted (Saka et al., 2014).

Bottom Line: Annexin A2, a calcium-, actin-, and lipid-binding protein involved in exocytosis, mediates the formation of lipid microdomains required for the structural and spatial organization of fusion sites at the plasma membrane.When an annexin A2 mutant with impaired actin filament-bundling activity was expressed, the formation of plasma membrane lipid microdomains and the number of exocytotic events were decreased and the fusion kinetics were slower, whereas the pharmacological activation of the intrinsic actin-bundling activity of endogenous annexin A2 had the opposite effects.Thus, annexin A2-induced actin bundling is apparently essential for generating active exocytotic sites.

View Article: PubMed Central - HTML - PubMed

Affiliation: Institut des Neurosciences Cellulaires et Intégratives, UPR3212 Centre National de la Recherche Scientifique, Université de Strasbourg, F-67084 Strasbourg, France.

Show MeSH