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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - PolarNet (Principles of Polarity â€“ Integrating genetic, biophysical and computational approaches to understand cell and tissue polarity)

Teaser

Summary

The ability to generate an axis of polarity is a fundamental property of animal cells that is a prerequisite for many cellular functions. Migrating cells for example establish a front-rear axis, and distribute specific protein components to the front and rear compartments. In development, cell polarity is critical for the generation of different cell types through asymmetric cell divisions. Finally, many cell types need to establish functionally distinct domains to perform their functions. The importance of cell polarity is underscored by the fact that cell polarity is essential for animal development and is perturbed in disease states such as cancer. Understanding of cell polarity requires knowledge of the molecular players involved and a quantitative description of their biochemical interactions, as well as the mechanical processes underlying polarity establishment and maintenance.

The main objectives of PolarNet were to improve our understanding of molecular mechanisms underlying cell polarity, to provide rigorous training to ESRs, and to increase awareness of polarity research and disseminate PolarNetâ€™s scientific accomplishments. PolarNet brought together experts in molecular, cell and developmental biology, biochemistry, biophysics, microscopy, and mathematical modelling. PolarNet provided individual research projects to 15 ESRs, organized in three thematic work packages, and organized twice-yearly network wide training events.
Results are summarized here and in more detail in deliverables D1-15.

Work performed

Scientific accomplishments
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Work package 1 addressed the fundamental principles of polarity establishment. Research in this work package used the C. elegans one-cell embryo and yeast as model systems, because polarity in these systems is established by minimal well-defined mechanisms, for which theoretical frameworks to build on had already been established. Two ESRs used quantitative imaging of polarity proteins in the C. elegans one-cell embryo, combined with mathematical modeling, to better understand how PAR proteins and actomyosin dynamics organize cell polarity. The results improved our understanding of how PAR proteins respond to polarizing cues and give rise to cell scale polarity patterns, and identified a novel role for phase transitions in regulating F-actin and the actin regulator N-WASP.
Two further ESRs studied the role and contribution of localized enrichment of phospholipids and G protein activity on polarized growth in yeast and cell polarity in the C. elegans embryo. This work demonstrated that Cdc42 distribution in C. albicans filaments controls formation of dynamic clusters of secretory vesicles that ultimately establish a new growth site and generated important data on Arf1 regulators in C. elegans.

Work package 2 addressed cell polarity in more complex and dynamic systems. Two ESRs studied cell polarity during migration of astrocytes, an important model for understanding the relationship between cell polarity and cell migration. The studies performed here have yielded new insights into the role of the lipid phosphatase PTEN in collective cell migration, and into how lipid modifications of Cdc42 control localization of this key polarity regulator in migrating astrocytes.
A third ESR investigated the role of the Crumbs and PAR complexes in epithelial tissues of the nematode C. elegans, including the epidermis. The studies performed here identified a novel role for PAR-6 in regulating microtubule formation in the epidermal epithelium, as well as candidate proteins functioning with the apical Crumbs complex in this tissue.
The final project of work package 2 was more translational in nature and aimed to improve the delivery of drugs across the blood-brain barrier (BBB). Work within this project provided a better understanding of receptor-mediated transcytosis and made progress in use of nanobodies as carrier protein to transport therapeutic drugs across the BBB by making use of the transcytosis machinery.

Work package 3 addressed how cell polarity is coordinated with the development of multicellular tissues.
This work package emphasized biophysical characterizations and modelling of cell polarity. Two ESRs performed biophysical characterizations and modelling of the interplay between mechanical properties of tissues and cell polarity in Drosophila epithelia. This led to a better understanding of the role of tri-cellular junctions in determining the cleavage plane in response to cellular force, and a new 3D-computational model of wound healing that shed light on a novel role of polarized basolateral forces in wound closure.
Two further ESRs examined basolateral microtubule attachments and control of microtubule spindle positioning by Wnt signaling in 3D cultures of epithelial cells. This work demonstrated that microtubules can function as load-bearing structures that can support cell elongation by withstanding compression, and yielded new insights into the role of Wnt signaling in regulating distinct aspects of liver biology.
Two projects within this work package utilized multicellular polarized tissues of C. elegans larval stages, to study the contribution of core polarity proteins to tissue organization, and the maintenance of tissue integrity as a tumour suppressor function in the C. elegans seam epithelium.
The final project of this work package was again more applied in nature. The ESR working on this project developed technologies to scale up the culturi

Final results

The projects performed in PolarNet have pushed the boundaries of what can be achieved. Examples includes the development of methods for the inducible and tissue-specific degradation of proteins or knockout of genes, and advanced culturing of organoids. We have also generated novel computational models that incorporate quantitative measurements for the development of the one-cell C. elegans embryo, and wound healing in epithelial tissues. The scientific results have broadened our understanding of the core mechanisms of cell polarity establishment, and how cell polarity is controlled and coordinated with developmental processes. While the long term societal impact is difficult to predict, a greater understanding of the mechanisms driving cell and tissue polarity has the potential impact our ability to combat diseases in which polarity is disrupted, including retinal dystrophies, cystic kidney disease, and cancer.