Multiple motors cooperate to establish and maintain acentrosomal spindle bipolarity inC. elegansoocyte meiosis

Abstract

While centrosomes organize spindle poles during mitosis, oocyte meiosis can occur in their absence. Spindles in human oocytes frequently fail to maintain bipolarity and consequently undergo chromosome segregation errors, making it important to understand mechanisms that promote acentrosomal spindle stability. To this end, we have optimized the auxin-inducible degron system in C. elegans to remove factors from pre-formed oocyte spindles within minutes and assess effects on spindle structure. This approach revealed that dynein is required to maintain the integrity of acentrosomal poles; removal of dynein from bipolar spindles caused pole splaying, and when coupled with a monopolar spindle induced by depletion of kinesin-12 motor KLP-18, dynein depletion led to a complete dissolution of the monopole. Surprisingly, we went on to discover that following monopole disruption, individual chromosomes were able to reorganize local microtubules and re-establish a miniature bipolar spindle that mediated chromosome segregation. This revealed the existence of redundant microtubule sorting forces that are undetectable when KLP-18 and dynein are active. We found that the kinesin-5 family motor BMK-1 provides this force, uncovering the first evidence that kinesin-5 contributes to C. elegans meiotic spindle organization. Altogether, our studies have revealed how multiple motors are working synchronously to establish and maintain bipolarity in the absence of centrosomes.

Mechanism of spindle pole organization and instability in human oocytes

“…Dynein and KIFC1 appear to have similar roles in pole focusing in Xenopus egg extracts, Drosophila S2 cells, and nonmammalian oocytes ( 22 – 25 , 29 , 109 – 114 ). By contrast, perturbation of NUMA-dynein and of KIFC1 result in distinct phenotypes in mammalian oocytes that lack aMTOCs, suggesting that they have nonredundant roles at the spindle poles.…”

“…How spindle poles are organized in the absence of centrosomes has been studied mostly in nonmammalian oocytes and in mouse oocytes. In nonmammalian oocytes, spindle poles are focused by motor and nonmotor proteins that cross-link microtubules (22)(23)(24)(25)(26)(27)(28)(29). In mouse oocytes, canonical centrosomes are functionally replaced by acentriolar MTOCs (aMTOCs) (30).…”