Divergent Impact of Actin Isoforms on Division of Epithelial Cells

We investigated distribution and functions of beta- and gamma-cytoplasmic actins (CYAs) at different stages of non-neoplastic epithelial cell division using laser scanning microscopy (LSM). Here, we demonstrated that beta- and gamma-CYAs are spatially segregated in the early prophase, anaphase, telophase, and cytokinesis. Small interfering RNA (siRNA) experiments revealed that in both beta-CYA- and gamma-CYA-depleted cells, the number of cells was significantly reduced compared with the siRNA controls. Beta-CYA depletion resulted in an enlargement of the cell area in metaphase and high percentage of polynuclear cells compared with the siRNA control, indicating a potential failure of cytokinesis. Gamma-CYA depletion resulted in a reduced percentage of mitotic cells. We also observed the interdependence between the actin isoforms and the microtubule system in mitosis: (i) a decrease in the gamma-CYA led to impaired mitotic spindle organization; (ii) suppression of tubulin polymerization caused impaired beta-CYA reorganization, as incubation with colcemid blocked the transfer of short beta-actin polymers from the basal to the cortical compartment. We conclude that both actin isoforms are essential for proper cell division, but each isoform has its own specific functional role in this process.