Localization and role of the molecular scaffold testin during mitosis and in actin-based processes

The cytoskeleton is an important system in health and disease. Composed by three major classes of filaments, it ensures the maintaining of the cellular shape, it is involved in cell motility and cell division. The complex regulation of this system is mediated by a various number of proteins which often display multiple and differential functions. Testin is one of these cytoskeletal proteins described in important processes involving the actin cytoskeleton such as cell migration and cell spreading.
This thesis is dedicated to the study of testin during the cell division process which is mainly regulated by the actin and microtubule cytoskeleton. Our results provided evidence that, during mitosis, testin is able to dynamically localize to mitotic structures like centrosome, midzone and midbody. Further investigations of this localization, particularly in cytokinesis, permit to conclude that testin is a midbody bulge protein capable to concentrate to this intercellular bridge through its LIM1 and LIM2 domains. Testin is involved in the regulation of the actin cytoskeleton through being part of a protein complex with amongst other zyxin and talin, also able to concentrate to the midbody. Therefore, using RNAi experiments, we showed that the localization of testin to the midbody is zyxin and talin independent.
In order to understand the potential function of testin during the cell division process, we initiate the identification of the potential partners of testin during mitosis. Based on the known interactors of testin, themselves described to localize to the midbody, we performed pull down as well as knock-down experiments. We also started the screening of potential mitotic partners of testin using the BioID approach.
Given these observations about the localization of testin during mitosis the question arose whether testin plays a role in this process of cell division. Indeed, modulating the expression of testin results in different phenotypes. Overexpression of testin leads in faulty spindle formation or in a lack of abscission thus ultimately to an increase in multinucleated cells. Conversely, down regulation of testin leads to a perturbation of the nuclear shape and the microtubule structure of the midbody.
Taken together, these data suggest that, in addition to its important role in actin-based processes in interphase, testin, a novel midbody bulge protein is involved in mitosis where it appears required for the integrity of a microtubule structure.