WASP family proteins and formins compete in pseudopod- and bleb-based migration.

Andrew J Davidson,Peter A Thomason,Clelia Amato,Robert H Insall

doi:10.1083/jcb.201705160

Andrew J Davidson, Peter A Thomason + Show 2 more

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https://doi.org/10.1083/jcb.201705160

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Abstract

Actin pseudopods induced by SCAR/WAVE drive normal migration and chemotaxis in eukaryotic cells. Cells can also migrate using blebs, in which the edge is driven forward by hydrostatic pressure instead of actin. In Dictyostelium discoideum, loss of SCAR is compensated by WASP moving to the leading edge to generate morphologically normal pseudopods. Here we use an inducible double knockout to show that cells lacking both SCAR and WASP are unable to grow, make pseudopods or, unexpectedly, migrate using blebs. Remarkably, amounts and dynamics of actin polymerization are normal. Pseudopods are replaced in double SCAR/WASP mutants by aberrant filopods, induced by the formin dDia2. Further disruption of the gene for dDia2 restores cells' ability to initiate blebs and thus migrate, though pseudopods are still lost. Triple knockout cells still contain near-normal F-actin levels. This work shows that SCAR, WASP, and dDia2 compete for actin. Loss of SCAR and WASP causes excessive dDia2 activity, maintaining F-actin levels but blocking pseudopod and bleb formation and migration.

Highlights

The actin cytoskeleton is involved in almost all aspects of cell behavior, but most clearly cell migration, endocytosis, adhesion, and cell division (Insall and Machesky, 2009)
Contractility driven by actin and the motor protein myosin II is essential for the initial separation of the plasma membrane from the underlying actin cortex (Diz-Muñoz et al, 2010), but the regulators that catalyze the actin polymerization observed as blebs are filled are unknown
We recently found that WASP is able to substitute for SCAR and appears to be responsible for the residual pseudopods extended by Dictyostelium scar knockout cells (Veltman et al, 2012); this was unexpected as the two are typically thought to be regulated by different upstream pathways, but has since been confirmed in Caenorhabditis elegans (Zhu et al, 2016)

Summary

Introduction

The actin cytoskeleton is involved in almost all aspects of cell behavior, but most clearly cell migration, endocytosis, adhesion, and cell division (Insall and Machesky, 2009). How different combinations of these proteins are brought together at the right time and place within the cell to promote the formation of a particular actin-based structure is still poorly understood. The Arp2/3 complex generates thick meshes of branched F-actin (Pollard, 2007) that support the formation of broad cellular protrusions known as pseudopods or lamellipods. Formins form unbranched actin filaments (Pollard, 2007), which are used in several different cellular processes. Cells can extend their edges using blebs, which form when actin fills an area of plasma membrane that has detached and bulged outward from the cell (Paluch and Raz, 2013). Contractility driven by actin and the motor protein myosin II (collectively actomyosin) is essential for the initial separation of the plasma membrane from the underlying actin cortex (Diz-Muñoz et al, 2010), but the regulators that catalyze the actin polymerization observed as blebs are filled are unknown

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