Reduction of cortical pulling at mitotic entry facilitates aster centration

Abstract

Although it has been studied for more than a century, the question of how one cell divides into two equal parts is still not fully resolved. Zygotes have provided much of the mechanistic insight into how the mitotic apparatus finds the center of the cell since the centrally-located mitotic apparatus is created from a large sperm aster that forms at the cortex and thus far from the zygote center. Here we show that in ascidians, the sperm aster extends throughout the cytoplasm during interphase yet remains located near the cortex and does not migrate towards the zygote center. It is only at mitotic entry, when the sperm aster has duplicated and the mitotic apparatus is being assembled, that most of the migration and centration occurs. This temporal pattern of centration behavior is mirrored by primate zygotes (including human). The current mechanisms of aster centration include cytoplasmic pulling that scale with microtubule (MT) length, MT pushing against the proximal cortex or MT-based cortical pulling. However, it is not yet known whether and how these 3 mechanisms are coordinated to prevent aster migration during interphase and trigger migration at mitotic entry. By monitoring quantitatively all three mechanisms (cytoplasmic pulling, pushing and cortical pulling) we have discovered that cortical pulling is switched off as the zygote enters mitosis while both cytoplasmic pulling and proximal cortical pushing remain active. Physical simulations could recapitulate both the static and migratory aspects of sperm aster and mitotic apparatus behavior. We therefore surmise that the reduction in cortical pulling at mitotic entry represents a switch that allows proximal cortical pushing forces and cytoplasmic pulling forces to center the nascent mitotic apparatus. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=156 SRC="FIGDIR/small/533625v1_ufig1.gif" ALT="Figure 1"> View larger version (35K): org.highwire.dtl.DTLVardef@135189dorg.highwire.dtl.DTLVardef@e2ef1dorg.highwire.dtl.DTLVardef@c070f7org.highwire.dtl.DTLVardef@38832f_HPS_FORMAT_FIGEXP M_FIG C_FIG HighlightsO_LISperm aster/mitotic apparatus centration occurs at entry into first mitosis C_LIO_LIMT-based cortical pulling is active during interphase and switched off at mitotic entry C_LIO_LILoss of cortical pulling at mitosis entry facilitates centration of the aster C_LIO_LIMT-based cytoplasmic pulling is active during both interphase and mitosis C_LIO_LIAgent-based simulations advocate the need for cytoplasmic pulling, a switch in cortical pulling and a minor role of pushing for aster centration at mitotic entry. C_LI