Auxetic nuclei in embryonic stem cells exiting pluripotency

Embryonic stem cells (ESCs) self-renew in a state of naïve pluripotency in which they are competent to generate all somatic cells1. It has been hypothesized that, before irreversibly committing, ESCs pass through at least one metastable transition state2-4. This transition would represent a gateway for differentiation and reprogramming of somatic cells5,6. We sought a mechanical phenotype of transition by probing the nuclear response to compressive and tensile forces and found that, during transition, nuclei of ESCs are auxetic: they displayed a cross-sectional expansion when stretched and a cross-sectional contraction when compressed, and their stiffness increased under compression. We show that the auxetic phenotype of transition ESC nuclei is driven at least in part by global chromatin decondensation. Through the regulation of molecular turnover in the differentiating nucleus by external forces, auxeticity could be a key element in mechanotransduction. Our findings highlight the importance of nuclear structure in the regulation of differentiation and reprogramming.