Regulation of cell fate asymmetry in Caulobacter crescentus by a complex of two component signaling proteins

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Biology, 2011.Cataloged from PDF version of thesis.Includes bibliographical references.Cellular asymmetry is critical to the generation of complexity in both metazoans and many microbes. However, several molecular mechanisms responsible for translating asymmetry into differential cell fates remain unknown. Caulobacter crescentus provides an excellent model to study this process because every division is asymmetric. One daughter cell, the stalked cell, is sessile and commits immediately to S phase. The other daughter, the swarmer cell, is motile and locked in G1. Cellular differentiation requires asymmetric distribution or activation of regulatory factors. In Caulobacter, the master cell cycle regulator CtrA is selectively activated in swarmer cells, deactivated in stalked cells, and reactivated in predivisional cells. CtrA controls DNA replication, polar morphogenesis and cell division, and its cell-type and cell cycle-specific regulation is essential to the life cycle of Caulobacter. In swarmer cells, activated CtrA binds to the origin of replication and holds cells in G1. In stalked cells, CtrA deactivation allows for the initiation of DNA replication. Finally, in predivisional cells, CtrA is reactivated and acts as a transcription factor for >100 genes including those involved in polar morphogenesis and cell division. CtrA regulation is determined by the polarly localized histidine kinase CckA, but how CckA is differentially regulated in each cell type and why activity depends on localization are unknown. This thesis demonstrates that the unorthodox kinase DivL promotes CckA activity and that the phosphorylated regulator DivK inhibits CckA by binding to DivL. Differential cellular fates are achieved by regulating the phosphorylation state of DivK. In swarmer cells, DivK is dephosphorylated, thereby activating CckA and arresting the cells in G1. In stalked cells, phosphorylated DivK inactivates CckA, thus allowing for DNA replication initiation. Paradoxically, in predivisional cells, while phosphorylated DivK levels remain high, CckA is reactivated to initiate cellular division and morphogenesis. CckA activation in this cell type relies on polar localization with a DivK phosphatase. Localization thus creates a protected zone for CckA within the cell, without the use of membrane-enclosed compartments. These results reveal the mechanisms by which CckA is regulated in a cell-type-dependent manner. More generally, these findings reveal how cells exploit subcellular localization to orchestrate sophisticated regulation.by Christos G. Tsokos.Ph.D

Dynamics of two phosphorelays controlling cell cycle progression in 1 Caulobacter crescentus

In Caulobacter crescentus, progression through the cell cycle is governed by the periodic activation and inactivation of the master regulator CtrA. Two phosphorelays, each initiating with the histidine kinase CckA, promote CtrA activation by driving its phosphorylation and by inactivating its proteolysis. Here, we examined whether the CckA phosphorelays also influence the downregulation of CtrA. We demonstrate that CckA is bifunctional, capable of acting as either a kinase or phosphatase to drive the activation or inactivation, respectively, of CtrA. By identifying mutations that uncouple these two activities, we show that CckA's phosphatase activity is important for downregulating CtrA prior to DNA replication initiation in vivo but that other phosphatases may exist. Our results demonstrate that cell cycle transitions in Caulobacter require and are likely driven by the toggling of CckA between its kinase and phosphatase states. More generally, our results emphasize how the bifunctional nature of histidine kinases can help switch cells between mutually exclusive states

Modeling outcomes of soccer matches

We compare various extensions of the Bradley–Terry model and a hierarchical Poisson log-linear model in terms of their performance in predicting the outcome of soccer matches (win, draw, or loss). The parameters of the Bradley–Terry extensions are estimated by maximizing the log-likelihood, or an appropriately penalized version of it, while the posterior densities of the parameters of the hierarchical Poisson log-linear model are approximated using integrated nested Laplace approximations. The prediction performance of the various modeling approaches is assessed using a novel, context-specific framework for temporal validation that is found to deliver accurate estimates of the test error. The direct modeling of outcomes via the various Bradley–Terry extensions and the modeling of match scores using the hierarchical Poisson log-linear model demonstrate similar behavior in terms of predictive performance

Finite Fermion Density Effects on the Electroweak String

We consider an Electroweak string in the background of a uniform distribution of cold fermionic matter. As a consequence of the fermion number non-conservation in the Weinberg-Salam model, the string produces a long-range magnetic field.Comment: 10 pages, IC/94/22

Disseminated Microsporidiosis in an Immunosuppressed Patient

We report a case of disseminated microsporidiosis in a patient with multiple myeloma who had received an allogeneic stem cell transplant requiring substantial immunosuppression. The causative organism was identified as Tubulinosema acridophagus, confirming this genus of microsporidia as a novel human pathogen

Fermion Back-Reaction and the Sphaleron

Using a simple model, a new sphaleron solution which incorporates finite fermionic density effects is obtained. The main result is that the height of the potential barrier (sphaleron energy) decreases as the fermion density increases. This suggests that the rate of sphaleron-induced transitions increases when the fermionic density increases. However the rate increase is not expected to change significantly the predictions from the standard sphaleron-induced baryogenesis scenarios.Comment: 11 pages, Revtex (2 figures available upon request), to appear in Phys. Rev. D (Rapid Communication

Spleen Tyrosine Kinase (Syk) Regulates Systemic Lupus Erythematosus (SLE) T Cell Signaling

Engagement of the CD3/T cell receptor complex in systemic lupus erythematosus (SLE) T cells involves Syk rather than the zeta-associated protein. Because Syk is being considered as a therapeutic target we asked whether Syk is central to the multiple aberrantly modulated molecules in SLE T cells. Using a gene expression array, we demonstrate that forced expression of Syk in normal T cells reproduces most of the aberrantly expressed molecules whereas silencing of Syk in SLE T cells normalizes the expression of most abnormally expressed molecules. Protein along with gene expression modulation for select molecules was confirmed. Specifically, levels of cytokine IL-21, cell surface receptor CD44, and intracellular molecules PP2A and OAS2 increased following Syk overexpression in normal T cells and decreased after Syk silencing in SLE T cells. Our results demonstrate that levels of Syk affect the expression of a number of enzymes, cytokines and receptors that play a key role in the development of disease pathogenesis in SLE and provide support for therapeutic targeting in SLE patients