An in vitro reconstitution system for the assessment of chromatin protein fluidity during Xenopus development

Chromatin fluidity, which is one of the indicators of higher-order structures in chromatin, is associated with cell differentiation. However, little is known about the relationships between chromatin fluidity and cell differentiation status in embryonic development. We established an in vitro reconstitution system that uses isolated nuclei and cytoplasmic extracts of Xenopus embryos and a fluorescence recovery after photobleaching assay to measure the fluidities of heterochromatin protein 1 (HP1) and histone H1 during development. The HP1 and H1 fluidities of nuclei isolated from the tailbuds of early tadpole stage (stage 32) embryos in the cytoplasmic extracts of eggs and of late blastula stage (stage 9) embryos were higher than those in the cytoplasmic extracts of mid-neurula stage (stage 15) embryos. The HP1 fluidities of nuclei isolated from animal cap cells of early gastrula stage (stage 10) embryos and from the neural plates of neural stage (stage 20) embryos were higher than those isolated from the tailbuds of stage 32 embryos in egg extracts, whereas the HP1 fluidities of these nuclei were the same in the cytoplasmic extracts of stage 15 embryos. These results suggest that chromatin fluidity is dependent upon both cytoplasmic and nuclear factors and decreases during development.