Embryonic stem cells become mechanoresponsive upon exit from ground state of pluripotency.

Stem cell fate decisions are driven by a broad array of signals, both chemical and mechanical. Although much progress has been made in our understanding of the impact of chemical signals on cell fate choice, much less is known about the role and influence of mechanical signalling, particularly in embryonic stem (ES) cells. Many studies use substrates with different stiffness to study mechanical signalling, but changing substrate stiffness can induce secondary effects which are difficult to disentangle from the direct effects of forces/mechanical signals. To probe the direct impact of mechanical stress on cells, we developed an adaptable cell substrate stretcher to exert specific, reproducible forces on cells. Using this device to test the response of ES cells to tensile strain, we found that cells experienced a transient influx of calcium followed by an upregulation of the so-called immediate and early genes. On longer time scales, however, ES cells in ground state conditions were largely insensitive to mechanical stress. Nonetheless, as ES cells exited the ground state, their susceptibility to mechanical signals increased, resulting in broad transcriptional changes. Our findings suggest that exit from ground state of pluripotency is unaffected by mechanical signals, but that these signals could become important during the next stage of lineage specification. A better understanding of this process could improve our understanding of cell fate choice in early development and improve protocols for differentiation guided by mechanical cues.

Active GSK3ß and an intact ß-catenin TCF complex are essential for the differentiation of human myogenic progenitor cells.

Wnt-ß-catenin signalling is essential for skeletal muscle myogenesis during development, but its role in adult human skeletal muscle remains unknown. Here we have used human primary CD56Pos satellite cell-derived myogenic progenitors obtained from healthy individuals to study the role of Wnt-ß-catenin signalling in myogenic differentiation. We show that dephosphorylated ß-catenin (active-ß-catenin), the central effector of the canonical Wnt cascade, is strongly upregulated at the onset of differentiation and undergoes nuclear translocation as differentiation progresses. To establish the role of Wnt signalling in regulating the differentiation process we manipulated key nodes of this pathway through a series of ß-catenin gain-of-function (GSK3 inhibition and ß-catenin overexpression) or loss-of-function experiments (dominant negative TCF4). Our data showed that manipulation of these critical pathway components led to varying degrees of disruption to the normal differentiation phenotype indicating the importance of Wnt signalling in regulating this process. We reveal an independent necessity for active-ß-catenin in the fusion and differentiation of human myogenic progenitors and that dominant negative inhibition of TCF4 prevents differentiation completely. Together these data add new mechanistic insights into both Wnt signalling and adult human myogenic progenitor differentiation.