Structural Transitions in Higher Order Chromatin Assembly and Nuclear Plasticity during Stem Cell Differentiation

Abstract

Undifferentiated cells integrate physico-chemical cues from the local microenvironment to elicit lineage specific gene expression programs. However the underlying mechanisms of this physical plasticity and how it impinges on gene expression is still unclear. In this study, using high resolution live-cell fluorescence polarization imaging, we analyze the spatio-temporal aspects of nuclear organization and chromatin structure in mouse embryonic stem (ES) cells and contrast them to a primary embryonic fibroblasts (PMEF). Higher-order chromatin compaction states exhibit unique features in ES cells, marked by homogeneous chromatin compaction but heterogeneity at the population level, but PMEFs evidence an inverse correlation. In addition, the nuclear lamina and actin cytoskeleton is highly flexible in ES cells but are frozen in PMEFs. This transition in nuclear architecture resembles that of fluid-like to solid-like transitions. Further the temporal evolution of nuclear plasticity is studied by differentiating ES cells on gelatin coated dishes. Taken together these results suggest that ES cells exhibit a broad epigenetic free energy landscape transitioning into a frozen configuration in higher-order chromatin assembly as lineage specific gene expression programs emerge.