Investigating mechanisms controlling tissue-level coordination of Planar Cell Polarity - PROJECT SUMMARY Cells are collectively oriented along tissue axes to execute coordinated behaviors during embryonic development. The highly conserved Planar Cell Polarity (PCP) pathway allows cells to communicate directional information with each other to direct essential processes such as neural tube closure, axis elongation, and heart morphogenesis. The core PCP pathway includes three transmembrane proteins (Celsr1, Fzd6, and Vangl2), which form complexes on opposite sides of the cell. Through interactions with intracellular binding partners, these proteins are able to self-organize such that Vangl2/Celsr1 complexes enrich on one side while Fzd6/Celsr1 complexes enrich on the other side of the cell. Extracellularly, Vangl2/Celsr1 complexes form an asymmetric junction with Fzd6/Celsr1 complexes to directly link PCP asymmetry between neighboring cells. A defining characteristic of PCP organization is its alignment with a tissue axis which allows cells to uniformly orient PCP asymmetry across great distances. This stereotypic pattern cannot be achieved by self-organization alone as cells could spontaneously organize in any orientation with respect to the tissue axis. For this reason, a well-accepted hypothesis emerged that a ‘directional cue’ acts in a gradient across a tissue to bias the distribution of PCP proteins along the same axis. Currently, the identity of potential ‘directional cues’ remain elusive, and how the core PCP pathway interacts with the cue to generate tissue-level asymmetry is unknown. Forward genetic screens are a powerful tool to identify genes involved in a particular process based on the phenotype. In fact, core PCP genes were identified in Drosophila based on disrupted bristle orientations in the wing and thorax of PCP mutants. However, the same approach is less successful in identifying genes that encode ‘directional cues’ as they are also thought to be essential for the development of the tissue itself. Fortunately, breeders have performed their own forward genetic screen in guinea pigs and mice, selecting for naturally occurring genetic variants with altered hair follicle orientation. Similar to Drosophila bristles, mammalian hair follicles are oriented by the PCP pathway. The altered fur orientation in these natural variants shows that planar polarity alignment is decoupled from the body axis in a region-specific and uniform manner, suggesting the causative mutations alter ‘directional cues.’ Remarkably, the viability of the animals and the overall architecture of the skin itself remain unaltered. By turning to these natural variants, I hypothesize that I will be able to identify ‘directional cues’ that link PCP asymmetry to a tissue axis. By computationally mapping the genetic variants, I will reveal genes required to link PCP alignment to a tissue axis. Through phenotypic characterization in vivo and in vitro, I will define how the altered gene impacts PCP asymmetry at individual junctions and its alignment across the entire tissue. This research will uncover mechanisms coordinating cell behaviors across tissues, lending insight into a fundamental process that is required for proper development.
