Cytokinesis is the final step of cell division that leads to the physical separation of two daughter cells by abscission. It is now well established that several components of endosomal machineries are involved in cytokinesis in human cells. Among these are the components of the Endosomal Sorting Complex Required for Transport, or ESCRT, machinery. Recent studies indicate that ESCRTs and associated proteins play a key role in abscission, the final step of cytokinesis in human cells. However their role in cytokinesis in a multicellular organism is not well known.
The aim of this thesis was to investigate the in vivo roles of the ESCRT-associated protein ALG-2 interacting protein, or ALIX, in cell division and cytokinesis using the fruit fly Drosophila melanogaster as a model organism. It has been shown that ALIX is required for cytokinetic abscission in human cells in culture by recruiting a component of the ESCRT-III complex, CHMP4B, to mediate abscission. ALIX is evolutionarily conserved, and in Drosophila it has the symbol ALiX.
We studied the dynamics of ALiX during cell division in Drosophila cell lines by the use of confocal microscopy. We could detect ALiX at the centrosomes in the early phases of cell division, and at the midbody at cytokinesis. It had been shown previously that ALIX localizes to centrosomes and to the midbody in human cells. However, the dynamics of ALiX moving from the centrosomes to the midbody during cell division had not been demonstrated before. In addition, we studied ALiX depletion in Drosophila cells, and the results showed that this led to increased binucleation of the cells, suggesting that ALiX has a role in cytokinesis.
In our in vivo studies, we found that alix1 mutant female flies show a reduced fertility. A closer analysis of the oogenesis of these flies uncovered an abnormal number of germ cells and intercellular bridges, called ring canals in Drosophila, in the egg chambers, the functional unit of the ovaries. Studies of germ stem cell division in oogenesis suggested that abscission in the first cell division step in the germline was defective in the alix1 mutant flies, leading to the aberrant number of cells in the egg chamber.
From these results in this thesis, we propose a novel in vivo role of ALiX in abscission during cytokinesis of the germ stem cells in the female germline of the fruit fly.