Cell polarity is required for asymmetric division, a mechanism to generate cell diversity by distributing fate determinants unequally to daughter cells. The establishment of polarity requires the evolutionarily conserved partitioning-defective (PAR) proteins as well as the actin cytoskeleton. In Caenorhabditis elegans one-cell embryos, the PAR proteins are segregated into an anterior (PAR-3, PAR-6) and a posterior (PAR-1, PAR-2) corticaldomain. The formation of PAR polarity correlates with anterior-posterior differences in the contractile activity of the cortex, known as "contractile polarity". It is thought that regulation of contractile polarity controls the establishment of PAR polarity, but detailed evidence to support this idea is lacking. To investigate how modulation of the actomyosin cytoskeleton affects polarity establishment, the acto-myosin cytoskeleton was perturbed by RNA-mediated interference (RNAi) of two Rho GTPases, CDC-42 and RHO-1. To examine how Rho GTPases are implemented in actin remodeling, it is important to analyze how their activity is controlled and how different activities affect polarity formation. The role of two putative Rho GTPase regulators, the Rho GTPase exchange factor (GEF) ECT-2 and the Rho GTPase activating protein (GAP) K09H11.3 were analyzed with respect to polarity formation. The formation of polarity was analyzed by using GFP-labeled proteins, and several different tracking methods were used to investigate the establishment of contractile and PAR polarity in more detail.This study demonstrates that both RHO-1 and CDC-42 are involved in polarity establishment in C. elegans embryos. But importantly, both act by different mechanisms. RHO-1 organizes the acto-myosin cytoskeleton into a contractile network, and therefore is essential for the formation of contractile polarity. The organization of the acto-myosin cytoskeleton is critical to ensure proper PAR protein distribution. Furthermore, a balance of RHO-1 activity by the GEF ECT-2 and the GAP K09H11.3 appears to be important for cortical contractility, for PAR protein domain size and for mutual exclusion of the PAR proteins. Although CDC-42 was shown to be a universal regulator of the actin cytoskeleton, CDC-42 acts downstream of contractile polarity. CDC-42 is required for linking PAR-6 to the cortex. In the absence of RHO-1 and ECT-2, PAR-6 and CDC-42 are not localized to the anterior cortex. This suggests that RHO-1, by organizing the actomyosin cytoskeleton into a contractile network, regulates the segregation of CDC-42 to the anterior cortex, and concomitantly PAR-6 localization. This study shows that the distribution of PAR is related to cortical activity and supports the model that the actin cytoskeleton plays an important role in polarity establishment.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:24640 |
| Date | 29 November 2005 |
| Creators | Schonegg, Stephanie |
| Contributors | Hyman, Anthony, Knoblich, Juergen, Heisenberg, Carl-Philipp |
| Publisher | Technische Universität Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis, info:eu-repo/semantics/doctoralThesis, doc-type:Text |
| Rights | info:eu-repo/semantics/openAccess |
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