Ran-GTP assembles a specialized spindle structure for accurate chromosome segregation in medaka early embryos

Abstract

Despite drastic cellular changes during cleavage divisions, a mitotic spindle is assembled in each blastomere to accurately segregate duplicated chromosomes. Recent studies indicate that early embryonic divisions are highly error-prone in bovines and humans. However, processes and mechanisms of embryonic spindle assembly remain little understood in vertebrates. Here, we established live functional assay systems in medaka fish (Oryzias latipes) embryos by combining CRISPR knock-in with an auxin-inducible degron technology. In contrast to mammals, mitoses during cleavage divisions are very rapid (<12 min), but segregation errors are rarely observed. Importantly, we found that the Ran-GTP gradient assembles a specialized, dense microtubule network at the spindle midplane during metaphase, which is essential for faithful chromosome segregation in early embryos. In contrast, Ran-GTP becomes dispensable for chromosome segregation in later stages, where spindles are morphologically remodeled into short, somatic-like spindles lacking the dense microtubule network. We propose that the specialized Ran-based spindle structure ensures high fidelity of chromosome segregation in large, vertebrate early embryos.

Cell State-Specific Cytoplasmic Material Properties Control Spindle Architecture and Scaling

“…The current understanding is that in very large cells, spindle scaling is mainly achieved via microtubule nucleation to create sufficient amounts of polymer mass, while in small cells regulating microtubule dynamics can be sufficient to scale spindle size 9 . Indeed, the dominance of nucleation pathways has been shown to shift in the course of mitosis 84 or even gradually over the course of development 6,10 . This cooperation of pathways is thought to provide robustness and mitotic fidelity 85 , but as we show here, can also result in the construction of spindles with differing architectures, providing the necessary plasticity to adapt spindles to evolving cellular environments.…”

“…In large cells, the size of the spindle becomes decoupled from cell size and reaches an upper limit. Such key observations of spindle scaling mostly stem from fast reductive divisions in early mouse, frog, fish, and worm embryos [3][4][5][6][7][8][9][10] . However, later in development, cellular differentiation poses a similar challenge: differentiating cells change size, morphology, and function and therefore must reorganise and adapt their internal architecture.…”