Investigation of structure/composition, physiological effects, and dynamics of Hirano bodies
Davis, Richard Croft
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Hirano bodies are paracrystalline F-actin aggregations reported in various tissues. Their function and physiological effect is not understood as analysis has been limited to post mortem tissue. We have recently demonstrated that we can induce Hirano body formation in Dictyostelium discoideum as well as mammalian cells including primary neurons, HeLa, and mouse broblasts cells by expressing the carboxyl-terminal (CT) region (a.a. 124-295) of the 34 kDa actin bundling protein. Numerous cell lines were investigated and veried to have formed Hirano bodies using immunouorescence and transmission electron microscopy. Initial expression of CT induces a small fraction of cells to die via p53 regulated apoptosis. However, cells constitutively forming Hirano bodies are healthy and appear to have an advantage when exposed to different types of acute stresses. Cells with Hirano bodies, such as L broblasts, survive acute oxidative CTstress better than cells without Hirano bodies. Furthermore, the percentage of cells with Hirano bodies increases following the stress suggesting a possible relationship. Cells experiencing DNA damage induced by the topoisomerase II inhibitor, etoposide, have enhanced viability if they contain Hirano bodies. These experiments suggest that Hirano bodies confer an adaptive advantage to cells containing the F-actin inclusions. As very little is known about the dynamics of Hirano bodies, investigations evaluating the dynamics of these inclusions in cultured mammalian cells were done. Cells constitutively expressing CT-GFP reveal a dynamic process that leads to the aggregation. This dynamic Àux requires an intact microtubule network. Evaluation of the small particle movement provided insight into the rates of movement that were consistent with active transport. Transmission electron micrographs indicate that Hirano bodies becomes membrane enclosed, consistent with degradation by autophagy, and that the structural order of Hirano bodies is altered in cells treated with inhibitors of ubiquitin-dependent protein degradation.