Inhibition of ectopic microtubule assembly by the kinesin-13 KLP-7MCAK prevents chromosome segregation and cytokinesis defects in oocytes

Abstract

In most species, oocytes lack centrosomes. Accurate meiotic spindle assembly and chromosome segregation -essential to prevent miscarriage or developmental defects -thus occur through atypical mechanisms that are not well characterized. Using quantitative in vitro and in vivo functional assays in the C. elegans oocyte, we provide novel evidence that the kinesin-13 KLP-7 promotes destabilization of the whole cellular microtubule network. By counteracting ectopic microtubule assembly and disorganization of the microtubule network, this function is strictly required for spindle organization, chromosome segregation and cytokinesis in meiotic cells. Strikingly, when centrosome activity was experimentally reduced, the absence of KLP-7 or the mammalian kinesin-13 protein MCAK (KIF2C) also resulted in ectopic microtubule asters during mitosis in C. elegans zygotes or HeLa cells, respectively. Our results highlight the general function of kinesin-13 microtubule depolymerases in preventing ectopic, spontaneous microtubule assembly when centrosome activity is defective or absent, which would otherwise lead to spindle microtubule disorganization and aneuploidy.

Interplay between microtubule bundling and sorting factors ensures acentriolar spindle stability during C. elegans oocyte meiosis

“…(D) Movie stills of oocytes expressing mCherry::histone and SPD-1::GFP in both control and klp-15/16(RNAi) . In control spindles, SPD-1 localizes between chromosomes as they begin to segregate and continues to accumulate as anaphase progresses, consistent with previous studies [ 19 , 20 ]. Following klp-15/16(RNAi) , SPD-1 begins to load all over the microtubules, and as SPD-1 accumulates, long bundles begin to form that then orient along the same axis and the chromosomes segregate (4/4 movies analyzed).…”

“…Given this finding, we more carefully assessed the dynamics of SPD-1 loading onto the spindle during anaphase. Live imaging of control oocytes expressing SPD-1::GFP and mCherry::histone revealed that SPD-1 begins to load onto the spindle between segregating chromosomes shortly after spindle rotation and then continues to accumulate as anaphase progresses ( Fig 4D ; S7 Movie ), consistent with previous studies [ 19 , 20 ]. Similar to control oocytes, following depletion of KLP-15/16, SPD-1 loads onto microtubules in early anaphase, at the microtubule ball stage ( Fig 4D ; S7 Movie ).…”

“…(B) Movie stills from control and klp-15/16(RNAi) oocytes expressing GFP::tubulin and mCherry::histone. In the control, after NEBD, diffuse tubulin can be seen inside the nucleus as was recently reported [ 20 ]. Then microtubules are nucleated and bundled to form a cage-like structure before reorganizing into a multipolar spindle and achieving bipolarity (in 5/5 control movies examined, spindles achieved bipolarity prior to anaphase onset).…”

“…elegans mitosis [ 14 , 16 , 18 , 34 ] and have been shown to concentrate at the midzone of the anaphase spindle in C . elegans oocytes [ 8 , 19 , 20 ]. Therefore, we assessed the localization of SPD-1 and ZEN-4 at high resolution on C .…”

“…Another component important for anaphase spindle organization during C . elegans mitosis is the microtubule bundling protein SPD-1 (PRC1), which is required for proper central spindle structure [ 16 – 18 ], and localizes to the midzone in mitosis [ 16 ] and meiosis [ 19 , 20 ]. However, depletion of SPD-1 from C .…”