The Planar Cell Polarity (PCP) system polarizes cells in some epithelial sheets along an axis orthogonal to their apical-basal axis, and is necessary for numerous physiological functions. Studies in the fruit fly, Drosophila, have led to the concept of a modular system controlling PCP. The PCP genes can be grouped together into functional modules, each representing a genetically and biochemically related unit. However, conflicting models describing the relationships between the principal (global, core and effector) PCP modules have been proposed, suggesting either a series or parallel relationship upstream of the various tissue specific effector modules. Notably, the connectivity between the PCP modules, and between the PCP modules and their targets is controversial. Targets of PCP signaling may be discrete systems that act within single cells to build polarized structures, or may be multicellular units that themselves constitute modules in which signals within and between cells contribute to patterning. The components of the PCP signaling system, and the effector systems with which they interact, function together to produce emergent patterns. Manipulation of individual PCP signaling molecules in specified groups of cells not only perturbs the polarization of the targeted cells at a subcellular level, but also perturbs patterns of polarity at the multicellular level, often affecting nearby cells in characteristic ways. These kinds of experiments should, in principle, allow us to infer the architecture of the governing control systems, but the relationships between molecular interactions and tissue-level pattern are sufficiently complex that they defy intuitive understanding. Mathematical modeling has been an important tool to address this problem. Here, we propose to combine novel, hybrid models, amenable to analysis, with biological experimentation to better understand the PCP signaling network architecture and whether a single or multiple architectures function in different contexts. Specifically, we propose to first probe the global and core modules and network architecture in two tissues, wing and abdomen, in which the output is hair polarization, but in which mutually exclusive model architectures have been proposed. Next, we will probe the corresponding modules and network architecture in a third system, the bristles, in which the effector is a more complex multicellular system that may be more divergent in how it responds to PCP input. We believe that this work will result in an enhanced understanding of PCP, which will be important in understanding the many vertebrate developmental defects and diseases to which PCP contributes. The work will also produce broadly applicable mathematical tools as well as mathematical model components that can be integrated with existing developmental models. PUBLIC HEALTH RELEVANCE: Defects in planar cell polarity (PCP) result in a range of developmental anomalies and diseases including open neural tube defects, polycystic kidneys, deafness, and a specific class of heart defects. PCP is also believed to underlie the pathogenesis of idiopathic pulmonary hypertension and the directed migration that occurs during invasion and metastasis of malignant cells. This project will result in an enhanced understanding of the network architecture controlling PCP, facilitating the development of diagnostic and therapeutic approaches for PCP related disorders, and will also produce broadly applicable mathematical tools as well as mathematical model components that can be integrated with existing developmental models. The goal of the project is to combine novel, hybrid models, amenable to analysis, with biological experimentation to better understand the PCP signaling network architecture and whether a single or multiple architectures function in different contexts.