Fig. 5: Osmotic pressure controls CBX2 condensation to gate gene repression.
From: Mechano-osmotic signals control chromatin state and fate transitions in pluripotent stem cells
a, Heatmap and Euclidian distance dendrogram of differentially abundant phosphosites quantified by mass spectrometry in cells subjected to compression (Comp) or hypertonic (Hyper) stress. b, Distance-based clustering of phosphosites and GO-term analyses show changes specific or common to the specific stresses. c, Example heatmaps of differentially abundant phosphoproteins from b. d, Representative snapshots of live imaging and quantification of CBX2 condensation dynamics from hiPS cells with an endogenously tagged CBX2 allele. Note the rapid dissolution and re-establishment of condensates upon removal of pluripotency (pluri) factors or exposure to axial compression or hypertonic shock. Arrows mark dissolving condensates; dotted arrows mark newly formed condensates (scale bars, 20 µm; n = 136 (Pluri), 172 (Basal), 99 (Comp); 146 (Hyper) total nuclei tracked over time and pooled across 3 independent experiments; mean ± s.e.m.; two-way-ANOVA/Tukey’s). e, Heatmap and Euclidian distance dendrogram of differential CBX2 occupancy quantified by CUT&Run in cells subjected to removal of pluripotency factors (basal), compression (comp) or compression in basal medium, normalized to pluripotency condition. Asterisks mark transcription factors that control differentiation. f, Reactome analysis of genes in clusters 1 and 4 implicate metal-binding genes with reduced CBX2 occupancy in both basal medium and basal medium compression condition, whereas differentiation genes show reduced CBX2 in compression in basal medium. g, Representative tracks of genes with altered CBX2. h, Model of how intranuclear and cytoskeletal forces influence hiPS cell exit from pluripotency. Under conditions with pluripotency growth factors (GFs), nuclear mechanics are maintained and differentiation is prevented under volumetric stress, restoring pluripotency gene expression. In the absence of pluripotency GFs, osmotic stress leads to nuclear envelope fluctuations and CBX2 condensation, priming chromatin for a cell state transition. This ultimately causes derepression of CBX2 target genes, facilitating exit from pluripotency.
