Developmental Biology Smurfed Up PCP

Wei Wong

Science Signaling, AAAS, Washington, DC 20005, USA

Planar cell polarity (PCP) orients cells within polarized epithelia, such as the stereocilia of hair cells in the cochlea. Animals with mutations in the PCP pathway also exhibit defects in convergent extension (CE), a process in which cells simultaneously migrate and intercalate to elongate the embryo along the anteroposterior axis. Narimatsu et al. found that the ubiquitin ligases Smurf1 (Smad ubiquitination regulatory factor 1) and Smurf2, which have been previously shown to regulate cell polarity downstream of Par6, have additional roles in PCP. Some mouse embryos lacking both Smurf1 and Smurf2 (Smurf DKO) had open neural tubes and displayed CE defects reminiscent of mutants in the PCP pathway. Mice lacking three of the four Smurf alleles had less severe neural tube and CE defects than Smurf DKO mice; however, the stereocilia in the cochlea of Smurf1^–/–;Smurf2^+/– and Smurf1^+/–;Smurf2^+/– mice, unlike those in wild-type mice, were unaligned. The Wnt receptor Frizzled and its intracellular effector Dishevelled (Dvl) are crucial for establishing PCP. Biochemical assays showed that Smurf1 and Smurf2 interacted with phosphorylated, but not unphosphorylated, Dvl2 but that Par6 interacted with both wild-type and a phosphorylation-deficient Dvl2 mutant, suggesting that activated, phosphorylated Dvl2 recruits Smurf1 or Smurf2 to Par6. Accordingly, Wnt5A treatment (which would be expected to activate Dvl2) was required to coprecipitate Par6 with Smurf2 from human mammary carcinoma cells. Prickle 1 (Pk1), which displays a subcellular distribution opposite to that of Dvl and antagonizes Dvl-mediated signaling, associated with both wild-type Par6 and a mutant unable to bind Dvl2 (Par6250-295). In human embryonic kidney cells, proteosomal degradation of Pk1 increased when wild-type, but not catalytically inactive, Smurf2 was cotransfected, whereas it decreased with the cotransfection of Par6250-295, suggesting that Dvl2 binding to Par6 is a prerequisite for Smurf-mediated degradation of Pk1. This degradation appeared to be involved in determining the subcellular localization of Pk1, because the asymmetric distribution of Pk1 in sensory hair and neuroepithelial cells in wild-type mice was lost in Smurf DKO mice. Moreover, mouse embryos expressing the Par6250-295 mutant had CE defects and diffuse Pk1 localization similar to that seen in the Smurf DKO mice. In his commentary, Mlodzik notes that ubiquitination and proteosomal degradation may help to explain the antagonistic nature between signaling modules in the PCP pathway.

M. Narimatsu, R. Bose, M. Pye, L. Zhang, B. Miller, P. Ching, R. Sakuma, V. Luga, L. Roncari, L. Attisano, J. L. Wrana, Regulation of planar cell polarity by Smurf ubiquitin ligases. Cell 137, 295–307 (2009). [PubMed]

M. Mlodzik, Ubiquitin connects with planar cell polarity. Cell 137, 209–211 (2009). [PubMed]