The role of Arp2/3 complex in extending the “leading edge” of the cell membrane may be significant for sheet-like protrusions, like lamellipodia, but not for linear protusions such as microvilli, report Dartmouth researchers Susan Nicholson-Dykstra and Henry Higgs in Cell Motility and Cytoskeleton.
Actin, a component of the cytoskeleton, plays a major role in extending the leading edge of the plasma membrane during cell spreading, motility and formation of cell surface structures. Sheet-like protrusions of the cell membrane include two networks, the lamellipodium and the lamellum. The lamellipodium is thinner with highly dynamic, short-lived actin filaments. On the other hand, linear protrusions comprised of tightly-bundled, but unbranched filament networks, include filopodia and microvilli.
The Arp2/3 complex is a set of proteins that help in assembling the branched actin networks. Alone, the complex is inactive, but with other proteins such as WASp/Scar, it nucleates a new filament at the side of a pre-existing one. The exact role of Arp2/3 in actin assembly is not well-understood and studies have presented conflicting evidence on its importance.
Nicholson-Dykstra and Higgs demonstrated the effects of Arp2/3 on actin-based cell spreading and cell surface structures using a knockdown model that suppresses production of the particular protein. They studied Arp2/3 depletion in two kinds of cells, fibroblasts (Swiss 3T3) and Jurkat T-lymphocytes.
Using immunofluorescence and GFP expression to study cell morphology and leading edge protrusions, Nicholson-Dykstra and Higgs showed that Arp2/3 depletion results in a significant decrease in sheet-like protrusions.
In fibroblasts, Arp2/3 depletion inhibited initial cell spreading during replating assays, although over an extended period of time, spreading was not affected greatly. Jurkat T-cells, when grown in suspension, normally have actin-based ruffle structures on their cell surfaces. Arp2/3 depleted Jurkat cells showed defects in sheet-like protrusions overall and a significant decrease in ruffles. Instead, actin-rich structures that resembled filopodia showed an increase in both suspended and spreading Jurkat cells. Overall, however, cell morphology and the cytoskeleton did not show any obvious defects.
On the other hand, Arp2/3 depletion did not appear to affect linear protrusions on the cell surface such as filopodia and microvilli. In lymphocytes, both transient and constitutive linear structures were unaffected, as were linear structures on the periphery of fibroblasts and Jurkat cells.
The study shows that Arp2/3 complex plays an important role in actin assembly for sheet-like lamellipodial structures that are involved in cell spreading. However, it is not necessary in the formation of linear, microvillar structures. Further studies are required for better understanding of the cytoskeletal elements involved in cell motility and surface morphology.