The Forces that Center the Mitotic Spindle

Abstract

Precise positioning of the mitotic spindle is important for specifying the plane of cell division and the subsequent partitioning of the cell's contents to the daughter cells. Studies on different organisms and cell types have suggested diverse centering mechanisms: astral microtubules grow out from the spindle and push against the cortex, cortical dynein motors pull on astral microtubules, and dynein-dependent organelle transport on astral microtubules leads to a reactive force on the spindle. The different mechanisms lead to different predictions for the precision of centering, how mutations effect the precision, and the magnitude of the forces associated with spindle centering. We used image processing to accurately track the position and orientation of the mitotic spindle during the first cell division in the C. elegans embryo. The high precision of centering, < 1% of cell diameter transverse to the anterior-posterior axis, increased after RNAi against gpr-1/2, genes encoding activators of the cortical force generators; this suggests that centering is not mediated by gpr-1/2-dependent cortical pulling forces. To measure the forces associated with spindle positioning, we built a magnetic tweezers apparatus so that forces could be exerted on the spindle via beads incorporated into the embryo: forces of approximately 20 pN were required to displace the spindle through 1 μm. These mechanical experiments constrain molecular models of the centering process.