How to Stay in Shape – The Role of the Cell-Spanning Actomyosin Ring in Colonies of Human Induced Pluripotent Stem Cells = Aufrechterhaltung der Koloniestruktur – Die Rolle des zellübergreifenden Aktomyosinrings in humanen induzierten pluripotenten Stammzellen

Over the last years, the mechanical influence on pluripotency regulation has become increasingly evident. Human induced pluripotent stem cells (hiPSCs) are a valuable and accessible tool to study the relationship between cytomechanics and pluripotency regulation in vitro. When cultivated under standard conditions, hiPSC colonies display two distinct actin structures: the previously published actin fence and the here described cell-spanning actomyosin ring. Given the importance of cell-cell interaction during pluripotency dynamics in vivo, this work aimed to elucidate the role of the cell-spanning actomyosin ring in hiPSC
colonies.
HiPSCs self-organize a continuous cell-spanning actomyosin ring, associated with non-muscle myosin IIB (NMIIB), at the apical side of the colony. This cell-spanning ring divides the colony into inner and outer cell populations, where the inner population displays a prominent actomyosin network and enrichment in the apical markers podocalyxin and ezrin. Three key findings underline the significance of the cell-spanning actomyosin ring in hiPSC colony architecture: (1) its establishment is independent of substrate and hiPSCs cell line, (2) it forms on soft substrates comparable to in vivo physical conditions, and (3) it re-establishes after perturbation. ... mehrTaken together, these findings suggest that the cell-spanning actomyosin ring is essential for colony morphology. Furthermore, the results demonstrate that the spatial distribution of NMIIB is strongly associated with the distribution of ezrin throughout the colony.
Additionally, hiPSCs cultivated in mechanical barriers showed alterations in actin cytoskeletal organisation, with two distinct morphologies either displaying a fully formed cell-spanning ring or not. The formation of the cell-spanning ring was primarily observed in barriers with a lower stiffness of 25 MPa whereas a higher stiffness of 1 GPa led to fragmented cell-spanning rings
in the majority of colonies. Finally, interfering with contractile tension resulted in early differentiation towards mesoderm and endoderm lineages in individual cells under self-renewal culture conditions. Based on these observations, three roles of the contractile tension generated by the cell-spanning ring were identified: (1) regulation of apicobasal polarity, (2) maintenance of colony integrity and (3) sustaining pluripotency.
In summary, the results indicate that the cell-spanning actomyosin ring is a major regulator of hiPSC colony morphology, enabling epithelial-like character and pluripotency maintenance.
The structural resemblance to actomyosin cables described in embryonic epithelial tissue of other species suggest a potential role for the cell-spanning actomyosin ring in early human embryonic development. The obtained knowledge of biomechanical regulation can be used to improve stem cell-based models, gaining insights into human embryogenesis.