TheDrosophilaplanar polarity genemultiple wing hairsdirectly regulates the actin cytoskeleton

Abstract

The evolutionarily conserved frizzled/starry night (fz/stan) pathway regulates planar cell polarity (PCP) in vertebrates and invertebrates. This pathway has been extensively studied in the Drosophila wing, where it is manifested by an array of distally pointing cuticular hairs. Using in vivo imaging we found that, early in hair growth, cells have multiple actin bundles and hairs that subsequently fuse into a single growing hair. The downstream PCP gene multiple wing hairs (mwh) plays a key role in this process and acts to antagonize the actin cytoskeleton. In mwh mutants hair initiation is not limited to a small region at the distal edge of pupal wing cells as in wild type, resulting in multiple hairs with aberrant polarity. Extra actin bundles/hairs are formed and do not completely fuse, in contrast to wild type. As development proceeded additional hairs continued to form, further increasing hair number. We identified a fragment of Mwh with in vivo rescue activity and that bound and bundled F-actin filaments and inhibited actin polymerization in in vitro actin assays. The loss of these activities can explain the mwh mutant phenotype. Our data suggest a model whereby, prior to hair initiation, proximally localized Mwh inhibits actin polymerization resulting in polarized activation of the cytoskeleton and hair formation on the distal side of wing cells. During hair growth Mwh is found in growing hairs, where we suggest it functions to promote the fusion of actin bundles and inhibit the formation of additional actin bundles that could lead to extra hairs.

From Planar Cell Polarity to Ciliogenesis and Back: The Curious Tale of the PPE and CPLANE proteins

“…The first sign of hair initiation is the accumulation of F-actin in a small apical region around the distal most vertex of these hexagonally shaped cells. Multiple thin extensions form and these coalesce to form the growing hair [33] (Fig. 1B).…”

“…Mutations in PCP and PPE genes lead to an increase in the region where F-actin accumulation is seen, and it is no longer seen only in the vicinity of the distal most region [10, 33, 34]. These phenotypes argue the PCP pathway functions to restrict activation of the actin cytoskeleton to a small region of the cell, and one PCP/PPE effector that functions as a cytoskeletal antagonist has been identified (Fig.…”

Tissue morphodynamics: Translating planar polarity cues into polarized cell behaviors

“…In mwh mutants, three or more ectopic hairs protrude from each wing cell due to excessive F-actin accumulation over the entire apical surface as well as a failure of actin pre-hairs to fully coalesce [9, 10, 16, 17]. The Mwh protein is distributed in a gradient within each wing cell with maximal enrichment on the proximal side.…”

“…Overexpression of Mwh delays pre-hair initiation and blocks the formation of F-actin structures in cultured S2 cells [10, 16]. Recent biochemical experiments found that Mwh can antagonize actin directly, as purified Mwh-GBD-FH3 fragments bind to actin in vitro and inhibit actin elongation into long filaments [17]. Mwh formin-like domains also have actin bundling activities in vitro, which explains why actin pre-hairs fail to fuse in mwh mutants [17].…”

“…Recent biochemical experiments found that Mwh can antagonize actin directly, as purified Mwh-GBD-FH3 fragments bind to actin in vitro and inhibit actin elongation into long filaments [17]. Mwh formin-like domains also have actin bundling activities in vitro, which explains why actin pre-hairs fail to fuse in mwh mutants [17]. Thus, there is a functional hierarchy controlling wing hair polarization where the core PCP module acts upstream to localize the PCP effector genes, which in turn localize and perhaps activate Mwh to restrict actin-polymerizing activities to the distal side of each cell.…”