Actin dynamics in microgravity

Organisms on earth develop in the presence of gravity. A good opportunity to study the effects of gravity on organisms is to expose organisms or cells to conditions of altered gravity, such as microgravity in space. Microgravity has been described to affect numerous processes that take place in organisms. Effects of exposure to microgravity conditions were also described on a cellular level, such as changes in signal transduction cascades, modification of gene expression, and changes in cell morphology. On the basis of results from previous research, it has been suggested that many of these changes observed in cells result from a changing actin cytoskeleton. This thesis further explores the effects on the actin cytoskeleton in cells that are exposed to microgravity conditions. Microgravity was achieved during rocket missions in space and was simulated on-ground by two methods that have been described to simulate microgravity, i.e. Random Positioning Machine (RPM) rotation and magnetic levitation. In cells, a dynamic network of filaments is formed by several types of proteins. This results in the formation of a kind of skeleton that is referred to as the cytoskeleton of cells. Actin is a major component of this cytoskeleton, in addition to tubulin and intermediate filaments. Actin is present in cells as polymerized actin (F-actin) and non-polymerized actin (G-actin). Actin filaments are continuously formed and degraded, a process that is tightly regulated in cells. This results in the local formation of several subcellular actin structures that play specific roles in cells. The aim of this study was to investigate the behavior of actin in cells upon exposure to conditions of microgravity. To this end, several experimental models were selected, i.e. cell cycle progression, membrane ruffling, and cellular movement. To be able to compare results obtained in microgravity with the situation at 1g, these models were first extensively studied at 1g on-ground. In the first part of this thesis, the models are described. Subsequently, experiments are described that were conducted both in simulated microgravity and in conditions of real microgravity