During embryonic development, precursor germ cells contain aggregates of protein and RNA known as germ granules. These germ granules are important in the specifi- cation of a functioning germ line, i.e. functioning sex cells within mature organisms. In the single cell fertilized embryo of the nematode worm C.elegans, germ granules (referred to as P granules) localize to the posterior side of the cell. After cell division occurs, they are found only in the posterior daughter cell. The localization behav- ior of P granules has been a topic of much interest, and considered an important aspect of symmetry breaking during development. We learn the fundamental prop- erties of P granule localization, and determine possible parameters and features of this biological system by developing theory in close collaboration with experimental evidence.
In this study, experimental evidence is presented which shows that P granules are liquid droplets, and that their localization occurs through preferential nucleation and growth behavior on one side of the cell and simultaneous preferential dissolution on the opposite side. It is also shown that this behavior is linked to the concentration gradient of the protein Mex-5 along the anterio-posterior axis of the cell, which is necessary to induce the preferential growth of P granules.
From this experimental data, a theoretical model for the preferential growth of P granules is developed, where the localization of P granules occurs by phase separa- tion. That is, P granules separate from the bulk cytoplasm by a process described by a first order liquid-liquid phase transitition, where a liquid droplet granule phase nucleates and then grows out of the bulk liquid cytoplasmic phase. In this model, a spatial gradient is imposed on the saturation point, the boundary point between the single phase state consisting only of the cytoplasm, and a metastable state which includes both a P granule and cytoplasm phase. This gradient mimics the properties of the Mex-5 gradient and is sufficient in explaining P granule localization.
Using numerical simulations, the theoretical model is studied. It is found suffi- cient to both successfully describe P granule localizaion, and to describe interesting behavior in a system with assymetric growth due to a spatial gradient. From a purely theoretical standpoint, we observe cyclical non-equilibrium steady states, where material is cycled back and forth along the gradient. From the biological side, experimental properties of the system, such as the diffusion coefficient of P granules and P granule growth rates are determined through both simulation and image analysis of data. In addition, the possiblility of different types of growth behavior at later cell stages, and a method of long range intracellular signalling are suggested from the theoretical model.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa:de:qucosa:26745 |
| Date | 07 January 2013 |
| Creators | Gharakhani, Jöbin |
| Contributors | Jülicher, Frank, Sommer, Jens-Uwe, 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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