Place Spindle Here

Abstract

Nonpolarized cultured cells that adhere to the substrate on which they are growing orient their spindles parallel to the substratum, thereby allowing both daughter cells to maintain contact with the substrate. Interactions between the astral microtubules and various microtubule-organizing proteins, including the minus end-directed motor complex and the dynein-dynactin complex, are involved in connecting the spindle to the actin cytoskeleton and the cell cortex. Now Toyoshima et al . find that, in HeLa cells, integrin-mediated stimulation of phosphoinositide 3-kinase (PI3K) leading to the production of phosphatidylinositol 3,4,5-trisphosphate [PI(3,4,5)P 3 ] at the midsection of the cell cortex in metaphase cells was required for proper spindle orientation. PI(3,4,5)P 3 concentration was monitored with a green fluorescent protein fused to the pleckstrin homology domain of Akt (Akt-PH-GFP), and the midsection cortical localization, as well as proper spindle orientation, was lost if PI3K activity was inhibited pharmacologically or if β1 integrin was knocked down with RNAi. In contrast, polarized MDCK cells did not require PI3K signaling to achieve proper spindle orientation and instead appeared to rely on an interaction of the spindle with the adherens junction (see Gachet and Tournier). The dynein-dynactin complex associates with the astral microtubules, and inhibition of PI3K prevented the localization of the dynein-dynactin complex to the midsection of the cell. Furthermore, in cells in which PI3K was inhibited, the chromosomes rotated in the z axis, and this rotation was eliminated in cells in which dynein activity was inhibited using RNAi techniques. An open question is how integrin signaling, which occurs at the bottom of the cell at the sites of cell attachment to the substrate, leads to the accumulation of PI(3,4,5)P 3 at the midsection of the cell cortex. F. Toyoshima, S. Matsumura, H. Morimoto, M. Mitsushima, E. Nishida, PtdIns(3,4,5)P3 regulates spindle orientation in adherent cells. Dev. Cell 13 , 796-811 (2007). [PubMed] Y. Gachet, S. Tournier, Lost your orientation? Find your way with PtdIns(3,4,5)P3! Dev. Cell 13 , 753-754 (2007). [PubMed]