Superresolution Fluorescence Microscopy Reveals the Nanoscale Architecture of Actin Cytoskeleton and Integrin-Based Adhesions in Mouse Embryonic Stem Cells

Efficient differentiation of embryonic stem cells (ESCs) into specific cell types is a major focus of regenerative medicine. In addition to soluble biochemical cues, the specific lineage commitment of ESCs also depends on mechanical cues, such as substrate rigidity. However, the underlying mechanisms that couple stem cell mechano-environment sensing to differentiation and maintenance remain obscure. While in many specialized cells, integrin-based adhesion complexes are organized into multi-layer nanostructures that link the extracellular matrix to the actin cytoskeleton and constitute major sites of mechanotransduction, few studies have investigated the structural basis of integrin adhesions and actin cytoskeleton in ESCs. Here we characterize the actin cortex and integrin adhesions in mouse ESC (mESCs) using superresolution fluorescence microscopy. We found that in mESCs, sparse and isotropic actin network comprises the cell cortex, with filopodia and actin bundles confined to the cell periphery. Actin filaments in the cortical meshwork appears to converge into several nodal structures which contain cortactin and a-actinin, while, surprisingly, actin polymerization activities appear to be evenly distributed throughout the cortex. We further characterized the integrin adhesions of ESCs, observing myosin II contractility-dependent localization of hallmark focal adhesions proteins such as focal adhesion kinase, paxillin, talin, vinculin, zyxin, and a-actinin to adhesions formed on fibronectin, laminin, and gelatin. Furthermore, at the nanoscale level these proteins are stratified into a multi-layer architecture, consisting of the integrin-signalling layer, force transduction layer, and actin regulatory layer. Thus, despite the small size, low density, and sparse peripheral distributions, the mESC integrin adhesions are mechanosensitive, compositionally mature, and organized into a conserved nanoscale architecture. Our results suggest that mESCs may adopt an alternate cellular organization paradigm whereby a sparse isotropic cortex is anchored by mechanosensitive integrin adhesions. Their integrated functions may account for the high pliability of ESCs that supports rapid sensing and adaptation to their microenvironment.