Wnt/PCP proteins regulate stereotyped axon branch extension inDrosophila

Abstract

SUMMARYBranching morphology is a hallmark feature of axons and dendrites and is essential for neuronal connectivity. To understand how this develops, I analyzed the stereotyped pattern of Drosophila mushroom body (MB) neurons, which have single axons branches that extend dorsally and medially. I found that components of the Wnt/Planar Cell Polarity (PCP) pathway control MB axon branching. frizzled mutant animals showed a predominant loss of dorsal branch extension, whereas strabismus (also known as Van Gogh) mutants preferentially lost medial branches. Further results suggest that Frizzled and Strabismus act independently. Nonetheless, branching fates are determined by complex Wnt/PCP interactions, including interactions with Dishevelled and Prickle that function in a context-dependent manner. Branching decisions are MB-autonomous but non-cell-autonomous as mutant and non-mutant neurons regulate these decisions collectively. I found that Wnt/PCP components do not need to be asymmetrically localized to distinct branches to execute branching functions. However, Prickle axonal localization depends on Frizzled and Strabismus. KEY WORDS: Axon Branching, Planar cell polarity (PCP), DrosophilaWnt/PCP proteins regulate stereotyped axon branch extension in Drosophila Julian Ng* DEVELOPMENT 166As branch formation might involve cell-polarizing mechanisms, I decided to study components of the Wnt/Planar Cell Polarity (PCP) pathway. The Wnt/PCP pathway represents a major class of regulators that control planar cell polarity (also termed tissue polarity or planar polarity). This has been extensively studied in Drosophila patterning of the adult cuticle in the wing, abdomen and notum, in the Drosophila eye and in sensory hair cells of the inner ear in mice (Klein and Mlodzik, 2005;Jones and Chen, 2007;Wang and Nathans, 2007;Strutt, 2009;Vladar et al., 2009). The Wnt/PCP pathway acts to orient cells along an axis so that all cells exhibit a grouped polarity. These genes can act both cellautonomously (intracellularly) and non-autonomously (intercellularly between neighboring cells).Despite extensive studies, how they function is unclear and several distinct models are proposed, based principally on Drosophila results (Klein and Mlodzik, 2005;Lawrence et al., 2007;Strutt and Strutt, 2009;Vladar et al., 2009). One key finding is that a seven-pass transmembrane protein Frizzled (Fz) is essential (Gubb and Garcia-Bellido, 1982;Vinson and Adler, 1987;Vinson et al., 1989). How Fz regulates tissue polarity is much debated. One model suggests 'core' components of the Wnt/PCP pathway act to localize Fz asymmetrically within cells (Strutt, 2001;Strutt, 2003;Klein and Mlodzik, 2005;Strutt and Strutt, 2009;Vladar et al., 2009). Acting as an anti-Fz factor, the four-pass transmembrane protein Strabismus (Stbm, also known as Van Gogh or Vang) is thought to act intracellularly to restrict Fz localization to the pole opposite of Stbm (Jenny et al., 2003;Amonlirdviman et al., 2005;Klein and Mlodzik, 2005;Vladar et al., 2009). Interestingly, in g...

Unique and Overlapping Functions of Formins Frl and DAAM During Ommatidial Rotation and Neuronal Development in Drosophila

“…Interestingly, while we found no obvious redundancy between frl and DAAM during PCP signaling in the eye, they cooperate during R-cell morphogenesis or maintenance and during axon growth in the mushroom body, a process also requiring a subset of the PCP genes (Shimizu et al 2011;Ng 2012). Overall, our data suggest that Frl may contribute to ommatidial rotation in the eye and, importantly, has contextdependent, redundant, and nonredundant functions with DAAM during neural development in Drosophila.…”

Genetic evidence that Celsr3 and Celsr2 , together with Fzd3 , regulate forebrain wiring in a Vangl -independent manner

“…During epidermal and eye development in flies, Fmi, Fz, Dsh, Van Gogh, and Pk interact closely (51)(52)(53). Mutations in Van Gogh and Fz also affect branching of axons of mushroom body neurons, albeit with independent effects (54). Similarly, there is ample evidence that Celsr1, Fzd6, and Vangl1,2 act together to regulate skin development (30,32,37) and that Celsr1, Fzd6, Dvl, and Vangl2 collaborate in the regulation of neural tube closure (31,55,56), and in the planar organization of cilia in the mouse ependyma (24,57), inner ear (31,58), and Xenopus skin (59).…”

Partial interchangeability of Fz3 and Fz6 in tissue polarity signaling for epithelial orientation and axon growth and guidance

“…The conserved nature of epithelial PCP signaling can be seen in the morphological and molecular similarities between PCP in mammalian epithelia and in the Drosophila cuticle and wing, both of which require Frizzled, Stan/Fmi/Celsr and Vang/Vangl genes, and both of which feature asymmetric PCP protein complexes (Devenport and Fuchs, 2008;Goodrich and Strutt, 2011;Wang et al, 2006b). In Drosophila and C. elegans, Stan/Fmi/Celsr and Frizzled genes have been implicated in axon guidance, branching and target selection, and Drosophila Stan/Fmi has been implicated in self-avoidance in sensory dendrite tiling (Huarcaya Najarro and Ackley, 2013;Lee et al, 2003;Matsubara et al, 2011;Ng, 2012;Senti et al, 2003;Steinel and Whitington, 2009). In view of the subtly different biological activities of Fz3 and Fz6 observed here, it would be interesting to investigate whether specific classes of mutations in Drosophila Stan/Fmi might differentially affect epidermal polarity versus axonal/dendritic pathfinding/target selection/tiling.…”