title: Cascade pathway of filopodia formation downstream of SCAR.
abstract: The protrusion of two distinct actin-containing organelles, lamellipodia and filopodia, is thought to be regulated by two parallel pathways: from Rac1 through Scar/WAVEs to lamellipodia, and from Cdc42 through N-WASP to filopodia. We tested this hypothesis in Drosophila, which contains a single gene for each WASP subfamilies, SCAR and WASp. We performed targeted depletion of SCAR or WASp by dsRNA-mediated interference in two Drosophila cultured cell lines expressing lamellipodial and filopodial protrusion. Knockdown was verified by laser capture microdissection and RT-PCR, as well as western blotting. Morphometrical, kinetic and electron microscopy analyses of the SCAR-depleted phenotype in both cell types revealed strong inhibition of lamellipodial formation and cell spreading, as expected. More importantly, filopodia formation was also strongly inhibited, which is not consistent with the parallel pathway hypothesis. By contrast, depletion of WASp did not produce any significant phenotype, except for a slight inhibition of spreading, showing that both lamellipodia and filopodia in Drosophila cells are regulated predominantly by SCAR. We propose a new, cascade pathway model of filopodia regulation in which SCAR signals to lamellipodia and then filopodia arise from lamellipodia in response to additional signal(s).
authors.affiliation: Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA. a-biyasheva@northwestern.edu
authors.forename: Assel
authors.initials: A
authors.lastname: Biyasheva
authors.forename: Tatyana
authors.initials: T
authors.lastname: Svitkina
authors.forename: Patricia
authors.initials: P
authors.lastname: Kunda
authors.forename: Buzz
authors.initials: B
authors.lastname: Baum
authors.forename: Gary
authors.initials: G
authors.lastname: Borisy
mesh_terms: D000818:Animals; D002460:Cell Line; D004330:Drosophila; D029721:Drosophila Proteins / Q000235:genetics / Q000378:metabolism*; D019204:GTP-Binding Proteins / Q000378:metabolism; D008840:Microfilament Proteins / Q000235:genetics / Q000378:metabolism*; D011554:Pseudopodia / Q000502:physiology* / Q000648:ultrastructure; D034622:RNA Interference; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology*; D051316:Wiskott-Aldrich Syndrome Protein; D020744:rac GTP-Binding Proteins / Q000378:metabolism
chemical_list: C090145:Cdc42 protein, Drosophila; D029721:Drosophila Proteins; D008840:Microfilament Proteins; C090146:Rac1 protein, Drosophila; C449671:SCAR protein, Drosophila; C477859:WASp protein, Drosophila; D051316:Wiskott-Aldrich Syndrome Protein; D019204:GTP-Binding Proteins; D020744:rac GTP-Binding Proteins