Determining the Position of the Cell Division Plane

Nature 424:1074 (2003)

Ted Salmon @ UNC

I should have written this review a long time ago since it is highly correlative to what I am doing right now.

How do cells properly position the cell division plane is a big, important but unsolved question. The cleavage plane appears in the equatorial plane which is in the middle of spindle poles. Therefore a bipolar microtubule array has been proposed to be a minimum requirement for furrow positioning, with furrows forming at the site of microtubule plus-end overlap between the spindle poles. In this paper, Julie made cells in mitosis with monopolar spindle by applying monostral which is essential for bipolar spindle formation. Then a Mad2 antibody was microinjected to inactivate the spindle checkpoint to force cytokinesis entry.



As the movie shown, since there was only one pole and no overlapped microtubules, surprisingly cells could still form a cleavage furrow in the side facing the chromosomes but not centrosomes. Further, Julie found there are some stable microtubules passing by kinetochores/centromeres to the cortex. So she concluded this supported the hypothesis that chromosomes provide some factors enhancing microtubules.

The cell Julie used in her paper was Ptk1 which has small chromosome numbers and is easier for live imaging because it is flat in mitosis. Unlike HeLa cells in my hands, the monopolar spindles are polarized in Ptk1 cells, that is, spindles are symmetrically distributed, with pole in one side of the cell and all microtubules growing to the other side of the cell in metaphase. The common argue about this paper is the furrow always appears on the side microinjected. However that is the downside of microinjection; it is much easier to inject to the thickest part of the cell, usually the center. Since centrosomes polarized no longer in the center, it is very difficult to avoid this problem.

This is a descriptive paper which described a surprising and interesting novel finding but rarely addressed the detailed mechanism in it. The immunostaining pictures are bad, especially for INCENP in figure 2. Julie showed and claimed in that picture INCENP accumulated in the equatorial region. It is well known that INCENP was transported by MKLP2 motor proteins on microtubules. It clearly showed that INCENP is positioned beyond the microtubule ends, but no more detail was addressed further. One point Julie didn’t address publicly is microtubules can be stabilized without antiparallel microtubule arrays (spindle-midzone). This could be a very unique finding for microtubule studies.