Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Universiẗat, Diss., 2000--München.

Universiẗat, Diss., 2003--Kassel.

Thesis (Ph. D.)--University of Texas at Austin, 2006. / Vita. Includes bibliographical references.

Universiẗat, Diss., 2004--München.

Univ., Diss., 2006--Kassel

Universiẗat, Diss., 2006--Frankfurt (Main).

No description available.

Dictyostelium discoideum is a simple model organism that can be used to study endocytic pathways such as phagocytosis and macropinocytosis because of its homology to cells of the mammalian innate immune system, namely macrophages and neutrophils. Consequently, Dictyostelium can also be used to study the process of exocytosis. In our laboratory, we generated Dictyostelium cells lacking superoxide dismutase SodC. Our data suggest that cells that lack SodC are defective in macropinocytosis and exocytosis when compared to wild type cells. In this study I describe a regulatory mechanism of macropinocytosis by SodC via regulation of RasG, which in turn controls PI3K activation and thus macropinocytosis. Our results show that proper metabolism of superoxide is critical for efficient particle uptake, for the proper trafficking of internalized particles, and a timely exocytosis of fluid uptake in Dictyostelium cells.

Migrating cells have two basic ways of extending their leading edge: by dendritic actin polymerization beneath the membrane, or by fluid pressure, which produces blebs. Most cells are believed to move using actin-driven projections, but in more physiological conditions, blebbing motility is also apparent. It has been shown that certain cells even can switch between these two modes of motility, although it is not known how this switch is triggered. Besides, it is unclear whether blebbing can be regulated by chemotactic stimuli, and generally, how blebbing is controlled in the cell. In this study I employed a popular model organism – Dictyostelium discoideum – to investigate the role of blebbing in chemotaxis. Here I confirm that in standard conditions Dictyostelium cells move by a combination of F-actin-driven protrusions and blebs. Blebbing is characterized by the rapid projection of hemispherical patches of plasma membrane at 2-4 times the speed of an actin-driven projection, and leaves transient scars of F-actin marking the original cortex in the base of blebs. I demonstrate that Dictyostelium cells can adjust their mode of movement according to the conditions: in a resistive environment they switch almost entirely to “bleb mode”. I show that in chemotaxing cells, blebs are mainly restricted to the leading edge, and they often lead the way when a cell is forced to re-orientate. Bleb location appears to be controlled directly by chemotactic gradients. To investigate how chemoattractant induces blebbing, I have screened signal transduction mutants for altered blebbing. I have found that blebbing is unaffected in many chemotactic mutants, but unexpectedly depends on PI3-kinases and two downstream PIP3-binding proteins of unknown function – PhdA and CRAC. I conclude that Dictyostelium cells move using a hybrid motor in which hydrostatic pressure-driven bleb formation is as important as F-actin-driven membrane extension, and that cells can change the balance between modes as required. I propose that blebbing motility of Dictyostelium cells is a direct response to mechanical resistance of environment. More generally, bleb-driven motility may be a ‘”high-force” mode of movement that is suited to penetrating tissues. Blebs are chemotactic and their induction may involve branches of the chemotactic signal transduction pathway distinct from F-actin regulation.