Mitotic chromosomes scale to nuclear-cytoplasmic ratio and cell size in Xenopus
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This study combines experiments in developing embryos and embryo extracts to investigate a fundamental relationship in biology - how the size of mitotic chromosomes scales with changes in cell size during development. By combining the unique tools available in the Xenopus system with modern genomic approaches, the authors convincingly demonstrate that mitotic chromosome scaling is mediated by differential loading of maternal chromatin remodeling factors during interphase. Although it remains unclear exactly how these factors impact chromosome size, the findings reported here will be of broad interest to the cell biology community and are likely to spawn new avenues of experimental inquiry aimed at understanding intracellular scaling relationships.
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Abstract
During the rapid and reductive cleavage divisions of early embryogenesis, subcellular structures such as the nucleus and mitotic spindle scale to decreasing cell size. Mitotic chromosomes also decrease in size during development, presumably to scale coordinately with mitotic spindles, but the underlying mechanisms are unclear. Here we combine in vivo and in vitro approaches using eggs and embryos from the frog Xenopus laevis to show that mitotic chromosome scaling is mechanistically distinct from other forms of subcellular scaling. We found that mitotic chromosomes scale continuously with cell, spindle, and nuclear size in vivo. However, unlike for spindles and nuclei, mitotic chromosome size cannot be reset by cytoplasmic factors from earlier developmental stages. In vitro, increasing nuclear-cytoplasmic (N/C) ratio is sufficient to recapitulate mitotic chromosome scaling, but not nuclear or spindle scaling, through differential loading of maternal factors during interphase. An additional pathway involving importin α scales mitotic chromosomes to cell surface area/volume ratio (SA/V) during metaphase. Finally, single-chromosome immunofluorescence and Hi-C data suggest that mitotic chromosomes shrink during embryogenesis through decreased recruitment of condensin I, resulting in major rearrangements of DNA loop architecture to accommodate the same amount of DNA on a shorter chromosome axis. Together, our findings demonstrate how mitotic chromosome size is set by spatially and temporally distinct developmental cues in the early embryo.
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eLife assessment
This study combines experiments in developing embryos and embryo extracts to investigate a fundamental relationship in biology - how the size of mitotic chromosomes scales with changes in cell size during development. By combining the unique tools available in the Xenopus system with modern genomic approaches, the authors convincingly demonstrate that mitotic chromosome scaling is mediated by differential loading of maternal chromatin remodeling factors during interphase. Although it remains unclear exactly how these factors impact chromosome size, the findings reported here will be of broad interest to the cell biology community and are likely to spawn new avenues of experimental inquiry aimed at understanding intracellular scaling relationships.
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Reviewer #1 (Public Review):
Zhou et al. investigated the factors that regulate mitotic chromosome size scaling during the early embryo divisions in Xenopus laevis using imaging of intact whole embryos and of embryo extracts with different sources of nuclei. They find that chromosome volume decreases during embryogenesis, and scales with nuclear and spindle volume throughout a broad range of embryo stages (stages 3 to 9) and cell sizes. They show that extracts from stage 3 or stage 8 embryos demonstrate significant differences in chromosome length, mirroring changes to chromosome volume observed in vivo. Using extracts from eggs or stage 8 embryos, and nuclei from sperm or stage 8 embryos, the authors demonstrate that chromosome length is dictated by the chromosomes and not the maternal mitotic environment, and find that the major …
Reviewer #1 (Public Review):
Zhou et al. investigated the factors that regulate mitotic chromosome size scaling during the early embryo divisions in Xenopus laevis using imaging of intact whole embryos and of embryo extracts with different sources of nuclei. They find that chromosome volume decreases during embryogenesis, and scales with nuclear and spindle volume throughout a broad range of embryo stages (stages 3 to 9) and cell sizes. They show that extracts from stage 3 or stage 8 embryos demonstrate significant differences in chromosome length, mirroring changes to chromosome volume observed in vivo. Using extracts from eggs or stage 8 embryos, and nuclei from sperm or stage 8 embryos, the authors demonstrate that chromosome length is dictated by the chromosomes and not the maternal mitotic environment, and find that the major determining factor is the amount of condensin I loading on mitotic chromosomes, which they correlate to changes in DNA loop size and layering. Interestingly, they find that the prior state of nuclei prior to entry into mitosis dictates mitotic chromosome length. They attribute this phenomenon to the nuclear to-cytoplasmic ratio during the prior interphase and suggest that some factor is titrated on chromatin that sets condensin I loading in mitosis. Notably, they found that chromosome length does not scale with nuclear or spindle size in vitro. In another set of experiments, the authors found that artificially increasing the palmitoylation of importin resulted in decreased chromosome length. However, this scaling effect is not due to condensin I loading differences, but to some unidentified importin cargo that would get released as cell size decreases during development. Overall, the conclusions of this paper are well supported by data, but some aspects of data interpretation and analysis need to be clarified and extended. The approaches used here are quite impressive and creative and provide compelling evidence for factors that regulate chromosome scaling during development in a vertebrate organism.
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Reviewer #2 (Public Review):
In this work from Zhou et al., the authors address mechanisms of mitotic chromosome size scaling during development. Their approach, which employs complementary use of in vivo (Xenopus embryos) and in vitro systems (Xenopus extracts), rendered investigation of this relationship experimentally tractable and allowed the authors to convincingly demonstrate that mitotic chromosome scaling is mediated by differential loading of maternal chromatin remodeling factors during interphase. The authors show that this scaling is dependent on an increasing nucleo-cytoplasmic (N/C) and that condensin I is titrated away from chromosomes as the N/C ratio is increased. Interestingly, the authors found that spindle and nuclei did not scale with changes in N/C ratio, suggesting that although mitotic chromosome scaling …
Reviewer #2 (Public Review):
In this work from Zhou et al., the authors address mechanisms of mitotic chromosome size scaling during development. Their approach, which employs complementary use of in vivo (Xenopus embryos) and in vitro systems (Xenopus extracts), rendered investigation of this relationship experimentally tractable and allowed the authors to convincingly demonstrate that mitotic chromosome scaling is mediated by differential loading of maternal chromatin remodeling factors during interphase. The authors show that this scaling is dependent on an increasing nucleo-cytoplasmic (N/C) and that condensin I is titrated away from chromosomes as the N/C ratio is increased. Interestingly, the authors found that spindle and nuclei did not scale with changes in N/C ratio, suggesting that although mitotic chromosome scaling correlates with spindle and nuclei scaling, it is mechanistically distinct. Complementary Hi-C analyses of chromatin architectures of both larger condensin I-rich chromosomes and smaller condensin I-poor chromosomes support a condensin-based looping model to explain the inverse relationship between chromosome-associated condensin and chromosome length, however, this model seems somewhat contrived due to inherent limitations of the approach. A characterization of an independent importin-α-dependent mitotic chromosome scaling mechanism, though potentially interesting, is too premature to be included and a bit of a non sequitur in terms of the overarching narrative and major findings of the work. Though there is some room for improvement in terms of image analysis and measurements, the work is well-written, comprehensive in scope, and addresses a fundamental biological question. Furthermore, the authors' major conclusions and substantive claims are well-supported by the experimental results.
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Reviewer #3 (Public Review):
In the work by Zhou, et.al., the authors pursue a mechanistic understanding of chromosome size scaling in development, a problem first noted in 1912 by Conklin and largely unstudied until very recently. Using the tools available in the Xenopus genus developmental biology system (cell extracts, related species of differing size, etc.), they nicely show that condensin I levels directly correlates with chromosome size. Further, importin levels decrease leading to axial shortening of chromosomes during development. The combined physical outcome is that Mitotic Chromatin looping changes, resulting in axial compression of chromosomes. This work represents a major step in the molecular understanding of how the genome is regulated through development and changing cell size, which also occurs in many other adult …
Reviewer #3 (Public Review):
In the work by Zhou, et.al., the authors pursue a mechanistic understanding of chromosome size scaling in development, a problem first noted in 1912 by Conklin and largely unstudied until very recently. Using the tools available in the Xenopus genus developmental biology system (cell extracts, related species of differing size, etc.), they nicely show that condensin I levels directly correlates with chromosome size. Further, importin levels decrease leading to axial shortening of chromosomes during development. The combined physical outcome is that Mitotic Chromatin looping changes, resulting in axial compression of chromosomes. This work represents a major step in the molecular understanding of how the genome is regulated through development and changing cell size, which also occurs in many other adult tissues and cancers. Further work to understand other contributing factors and understanding how loop structure changes the polymer dynamics of mitotic chromatin will be exciting in the future.
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