Jacques Pecreaux et al. (2006) European Worm Meeting "Mechanical Oscillations during Asymmetric Spindle Positioning Require a Threshold Number of Active Cortical Force Generators"

Jonathon Howard, Frank Julicher, Stephan W. Grill, Anthony A. Hyman, Karsten Kruse, Jens-Christian Roper, Jacques Pecreaux

[European Worm Meeting, 2006]

Jacques Pecreaux1, Jens-Christian Rper2, Karsten Kruse3, Frank Julicher3, Anthony A. Hyman1, Stephan W. Grill, Jonathon Howard1 Background. Asymmetric division of the C. elegans zygote is due to the posterior-directed movement of the mitotic spindle during metaphase and anaphase. During this movement along the anterior-posterior axis, the spindle oscillates transversely. A theoretical analysis indicates that oscillations might occur as a result of the concerted action of many cortical force generators that pull on astral microtubules in a tug-of-war situation. This model predicts a threshold of motor activity below which no oscillations occur. Results: We have tested the existence of a threshold by using RNA interference to gradually reduce the levels of GPR-1 and GPR-2 that are involved in the G-protein-mediated regulation of the force generators. We found an abrupt cessation of oscillations as expected if the activity drops below a threshold. Furthermore, we could account for the complex choreography of the mitotic spindle - the precise temporal coordination of the build-up and die-down of the transverse oscillations with the posterior displacement - by a gradual increase in the processivity of the force generators during metaphase and anaphase. Conclusions: The agreement between our results and modeling suggests that the same motor machinery underlies two different spindle motions in the embryo: the equal and opposite motors on each side of the AP axis drive oscillations whereas the imbalanced motors in the two halves of the embryo drive posterior displacement.