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Dicing with dogma: de-branching the lamellipodium.

Small JV - Trends Cell Biol. (2010)

Bottom Line: The primary event in the movement of a migrating eukaryotic cell is the extension of cytoplasmic sheets termed lamellipodia composed of networks of actin filaments.Lamellipodia networks are thought to arise through the branching of new filaments from the sides of old filaments, producing a dendritic array.These findings signal a reconsideration of the structural basis of protrusion and about the roles of the different actin nucleating and elongating complexes involved in the process.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Biotechnology, Austrian Academy of Sciences, Dr Bohr-Gasse 3, Vienna, Austria. vic.small@imba.oeaw.ac.at

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Proposed tethered nucleation/elongation model of actin network formation in lamellipodia and transitions to filopodia. (a) Based on the results from electron tomography [20], actin filaments in lamellipodia form un-branched networks that include cross-linked filament pairs that can serve as potential precursors of filopodia. (b) Enlargement of the area boxed in yellow in A. In this hypothetical scheme, actin filaments are nucleated via the interaction of the Arp2/3 complex with the activated WAVE complex on the membrane. The Arp2/3 complex remains at the pointed end of the filament and treadmills with actin. The filament plus end elongates while being tethered to WAVE or to proximal Ena/VASP oligomers after transfer from WAVE. Filaments elongating on VASP can associate in pairs that are stabilized by a short actin cross-linker, like fascin. Long cross-linkers, such as filamin, stabilize the actin network. (c) Enlargement of the area boxed in white in A. Filopodia form via the clustering and bundling of actin filament pairs. At the filopodia tip, Ena/VASP proteins and formins cooperate in filament elongation. The inventory of proteins at lamellipodia and filopodia tips is far from complete; aside from missing adaptors, formins may also be involved in the generation of filament pairs in lamellipodia [20] and one of the WAVE isoforms, WAVE 2, is found at filopodia tips [58].
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fig0005: Proposed tethered nucleation/elongation model of actin network formation in lamellipodia and transitions to filopodia. (a) Based on the results from electron tomography [20], actin filaments in lamellipodia form un-branched networks that include cross-linked filament pairs that can serve as potential precursors of filopodia. (b) Enlargement of the area boxed in yellow in A. In this hypothetical scheme, actin filaments are nucleated via the interaction of the Arp2/3 complex with the activated WAVE complex on the membrane. The Arp2/3 complex remains at the pointed end of the filament and treadmills with actin. The filament plus end elongates while being tethered to WAVE or to proximal Ena/VASP oligomers after transfer from WAVE. Filaments elongating on VASP can associate in pairs that are stabilized by a short actin cross-linker, like fascin. Long cross-linkers, such as filamin, stabilize the actin network. (c) Enlargement of the area boxed in white in A. Filopodia form via the clustering and bundling of actin filament pairs. At the filopodia tip, Ena/VASP proteins and formins cooperate in filament elongation. The inventory of proteins at lamellipodia and filopodia tips is far from complete; aside from missing adaptors, formins may also be involved in the generation of filament pairs in lamellipodia [20] and one of the WAVE isoforms, WAVE 2, is found at filopodia tips [58].

Mentions: Advances in electron microscope technology have recently opened the way to electron tomography, also in combination with rapid freezing to avoid chemical fixation [33–35]. Early results of 3D imaging by this technique, with Dictyostelium amoeba, showed that actin filaments could be resolved in frozen cells [36], but the motile activity of the regions imaged was unknown. Electron tomography can be applied to those parts of cells thin enough to allow penetration of the electron beam. Lamellipodia, which vary from 0.1-0.3 μm in thickness, fall within this range, and tomograms of lamellipodia in cells frozen live and imaged in vitreous ice have been acquired recently [20]. Tracking of actin filaments through the tomograms showed that the lamellipodium is composed mainly of overlapping actin filaments. The same result was obtained for cytoskeletons dried in negative stain, which retain a three dimensional organization, albeit with a collapse to around 50% of the thickness of frozen preparations. These recent findings, including data from four cell types [20], indicate that the Arp2/3 complex nucleates un-branched arrays of actin in lamellipodia (Figure 1).


Dicing with dogma: de-branching the lamellipodium.

Small JV - Trends Cell Biol. (2010)

Proposed tethered nucleation/elongation model of actin network formation in lamellipodia and transitions to filopodia. (a) Based on the results from electron tomography [20], actin filaments in lamellipodia form un-branched networks that include cross-linked filament pairs that can serve as potential precursors of filopodia. (b) Enlargement of the area boxed in yellow in A. In this hypothetical scheme, actin filaments are nucleated via the interaction of the Arp2/3 complex with the activated WAVE complex on the membrane. The Arp2/3 complex remains at the pointed end of the filament and treadmills with actin. The filament plus end elongates while being tethered to WAVE or to proximal Ena/VASP oligomers after transfer from WAVE. Filaments elongating on VASP can associate in pairs that are stabilized by a short actin cross-linker, like fascin. Long cross-linkers, such as filamin, stabilize the actin network. (c) Enlargement of the area boxed in white in A. Filopodia form via the clustering and bundling of actin filament pairs. At the filopodia tip, Ena/VASP proteins and formins cooperate in filament elongation. The inventory of proteins at lamellipodia and filopodia tips is far from complete; aside from missing adaptors, formins may also be involved in the generation of filament pairs in lamellipodia [20] and one of the WAVE isoforms, WAVE 2, is found at filopodia tips [58].
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fig0005: Proposed tethered nucleation/elongation model of actin network formation in lamellipodia and transitions to filopodia. (a) Based on the results from electron tomography [20], actin filaments in lamellipodia form un-branched networks that include cross-linked filament pairs that can serve as potential precursors of filopodia. (b) Enlargement of the area boxed in yellow in A. In this hypothetical scheme, actin filaments are nucleated via the interaction of the Arp2/3 complex with the activated WAVE complex on the membrane. The Arp2/3 complex remains at the pointed end of the filament and treadmills with actin. The filament plus end elongates while being tethered to WAVE or to proximal Ena/VASP oligomers after transfer from WAVE. Filaments elongating on VASP can associate in pairs that are stabilized by a short actin cross-linker, like fascin. Long cross-linkers, such as filamin, stabilize the actin network. (c) Enlargement of the area boxed in white in A. Filopodia form via the clustering and bundling of actin filament pairs. At the filopodia tip, Ena/VASP proteins and formins cooperate in filament elongation. The inventory of proteins at lamellipodia and filopodia tips is far from complete; aside from missing adaptors, formins may also be involved in the generation of filament pairs in lamellipodia [20] and one of the WAVE isoforms, WAVE 2, is found at filopodia tips [58].
Mentions: Advances in electron microscope technology have recently opened the way to electron tomography, also in combination with rapid freezing to avoid chemical fixation [33–35]. Early results of 3D imaging by this technique, with Dictyostelium amoeba, showed that actin filaments could be resolved in frozen cells [36], but the motile activity of the regions imaged was unknown. Electron tomography can be applied to those parts of cells thin enough to allow penetration of the electron beam. Lamellipodia, which vary from 0.1-0.3 μm in thickness, fall within this range, and tomograms of lamellipodia in cells frozen live and imaged in vitreous ice have been acquired recently [20]. Tracking of actin filaments through the tomograms showed that the lamellipodium is composed mainly of overlapping actin filaments. The same result was obtained for cytoskeletons dried in negative stain, which retain a three dimensional organization, albeit with a collapse to around 50% of the thickness of frozen preparations. These recent findings, including data from four cell types [20], indicate that the Arp2/3 complex nucleates un-branched arrays of actin in lamellipodia (Figure 1).

Bottom Line: The primary event in the movement of a migrating eukaryotic cell is the extension of cytoplasmic sheets termed lamellipodia composed of networks of actin filaments.Lamellipodia networks are thought to arise through the branching of new filaments from the sides of old filaments, producing a dendritic array.These findings signal a reconsideration of the structural basis of protrusion and about the roles of the different actin nucleating and elongating complexes involved in the process.

View Article: PubMed Central - PubMed

Affiliation: Institute of Molecular Biotechnology, Austrian Academy of Sciences, Dr Bohr-Gasse 3, Vienna, Austria. vic.small@imba.oeaw.ac.at

Show MeSH
Related in: MedlinePlus