C. eleganschromosomes connect to centrosomes by anchoring into the spindle network

Abstract

The mitotic spindle ensures the faithful segregation of chromosomes. Here we combine the first large-scale serial electron tomography of whole mitotic spindles in early C. elegans embryos with live-cell imaging to reconstruct all microtubules in 3D and identify their plus-and minus-ends. We classify them as kinetochore (KMTs), spindle (SMTs) or astral microtubules (AMTs) according to their positions, and quantify distinct properties of each class. While our light microscopy and mutant studies show that microtubules are nucleated from the centrosomes, we find only a few KMTs directly connected to the centrosomes. Indeed, by quantitatively analysing several models of microtubule growth, we conclude that minus-ends of KMTs have selectively detached and depolymerized from the centrosome. In toto, our results show that the connection between centrosomes and chromosomes is mediated by an anchoring into the entire spindle network and that any direct connections through KMTs are few and likely very transient.

Mechanically Distinct Microtubule Arrays Determine the Length and Force Response of the Meiotic Spindle

“…of total filament number) (Ohi et al, 2007), their length must be coupled to the spindle's pole-to-pole distance in order to prevent chromosomes from detaching. A recent electron tomography study revealed that kinetochore microtubules of C. elegans spindles are not continuous, but rather, their minus ends are terminated midway and embedded within a network of short microtubules assembled from the pole (Redemann et al, 2017). In addition to the previously reported force-based mechanisms regulating filament depolymerization at the fiber ends (Dumont and Mitchison, 2009a;Skibbens and Salmon, 1997), we predict that the linkage between these parallel filaments is mechanically compliant and can adapt to force.…”

Mechanisms of chromosome biorientation and bipolar spindle assembly analyzed by computational modeling

“…Computational modeling has been used previously to study the mitotic spindle [3,4,67]. Recent work on spindle and MT organization includes studies of spindle elongation and force balance [59,68], the formation and maintenance of antiparallel MT overlaps [69,70], MT bundling and sliding [15], spindle movements and positioning [71,72], spindle length and shape [15,51,52,73,74], MT organization [75], and spindle assembly from a bipolar initial condition [32,76]. Models of kinetochore-MT attachment and biorientation have examined capture of lost kinetochores [63,77], chromosome reorientation after MT attachment [31], attachment error correction [33,39,78,79], and chromosome movement on the spindle [52,61,[80][81][82].…”

Four Syntheses of 4‐Amino‐3,5‐dinitropyrazole

“…The nitration of pyrazole into 4-nitropyrazole (2) was nearly quantitative (95%). The reduction of the latter in aqueous, basic sodium dithionite solution [11] to afford 4-aminopyrazole (7), protected as a carbamate (8), had a yield of 60%. The reduction and protection is a one-pot procedure.…”

“…This compound was readily nitrated in mixed acids at ambient temperature to give corresponding 4-nitro derivative (3), which could be oxidized [18] to 4-nitropyrazole-3,5-dicarboxylic acid (10) in 60% yield. The latter compound was reduced with sodium dithionite to 4-aminopyrazole-3,5-dicarboxylic acid (11), as described by Redemann [11].…”