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Establishment of cortical polarity in the one-cell C. elegans embryo

Final Report Summary - GOEHRING-POLARITY (Establishment of cortical polarity in the one-cell C. elegans embryo)

This project sought to uncover the molecular mechanisms underlying the establishment of cell polarity in the c. elegans embryo by the conserved PAR proteins. This so-called PAR polarity pathway is essential for animal development and underlies numerous examples of cellular asymmetry. It is perhaps most strikingly apparent in asymmetric segregation of cell fate determinants during self-renewing stem-cell like cell divisions. Through quantitative measurements, we sought to define the key kinetic behaviours of PAR proteins in live embryos and, in so doing, provide the basis for developing and distinguishing between theoretical models for cell polarisation.

In this context, we developed several novel fluorescence microscopy techniques to address the mobility of the PAR proteins within embryos. These included an analytical solution for extracting lateral diffusion and membrane-cytoplasm exchange kinetics from fluorescence recovery after photobleaching (FRAP) data (refer to Goehring et al., 2010). Using these techniques, we discovered that PAR proteins diffused laterally on the membrane and they appeared to diffuse freely across the anterior-posterior boundary. Importantly, this indicated that there was no diffusion barrier or transport process maintaining PAR proteins within the correct halves of the cell.

Furthermore, through establishing a method to introduce pharmacologic agents at defined times during development we showed that the actin cortex did not play a key role in defining PAR polarity domains. Relevant results were published by Redemann et al., 2010, as part of a study on the actin cortex in spindle positioning. Our data rather suggested a self-organising model for cell polarity in which the interactions between PAR proteins gave rise to spatial asymmetries in membrane association and dissociation. These findings were recently published as a featured cover article in the Journal of Cell Biology (Goehring et al., 2011).

Based on these observations, we were developing, by the time of the project completion, a physical model for cell polarity inspired by a well-known class of reaction-diffusion processes. We found that the ability of diffusible PAR proteins to displace one another from the membrane combined with the limiting amounts of PAR proteins in cells could result in stable pattern formation. It should be noted that using this model we could largely reproduce the distribution of PAR proteins measured in embryos. Moreover, we recently confirmed several key predictions of this model and were working up a manuscript to describe these findings.

Our work provided new insight into the PAR proteins, a fundamental and conserved polarity network that is essential in a wide range of biological processes including stem-cell like divisions, the form of differentiated cells such a epithelial cells and neurons and the establishment of tissue architecture. Defects in polarity have even been associated with metastatic spread of epithelial cancer cells. In addition to the advancement in understanding this central biological process, the tools developed here, particularly the FRAP analysis, should be useful to researchers addressing the kinetics of membrane associated proteins throughout biology.

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