Abstract
The cellular cytoskeleton provides the cell with a mechanical rigidity that allows mechanical interaction between cells and the extracellular environment. The actin structure plays a key role in mechanical events such as motility or the establishment of cell polarity. From the earliest stages of development, as represented by the ex vivo expansion of naïve embryonic stem cells (ESCs), the critical mechanical role of the actin structure is becoming recognized as a vital cue for correct segregation and lineage control of cells and as a regulatory structure that controls several transcription factors. Naïve ESCs have a characteristic morphology, and the ultrastructure that underlies this condition remains to be further investigated. Here, we investigate the 3D actin cytoskeleton of naïve mouse ESCs using super-resolution optical reconstruction microscopy (STORM). We investigate the morphological, cytoskeletal, and mechanical changes in cells cultured in 2i or Serum/LIF media reflecting, respectively, a homogeneous preimplantation cell state and a state that is closer to embarking on differentiation. STORM imaging showed that the peripheral actin structure undergoes a dramatic change between the two culturing conditions. We also detected micro-rheological differences in the cell periphery between the cells cultured in these two media correlating well with the observed nano-architecture of the ESCs in the two different culture conditions. These results pave the way for linking physical properties and cytoskeletal architecture to cell morphology during early development.
Highlights
The biomechanical properties of cells allow them to sense and react in response to their physical environment, for example, by sensing the rigidity of the extracellular matrix
Actin was observed to mainly localize in the cell cortex both in cells cultured in Serum/LIF and in 2i, with important differences: (i) cells cultured in Serum/LIF have a higher number of filopodia and actin protrusions along the surface than those cultured in 2i, (ii) colonies of embryonic stem cells (ESCs) cultured in 2i grow in rounded relatively high dome-like colonies, as has previously been observed [46], whereas cells cultured in Serum/LIF media grow in flat and more spreading colonies of cells, as is apparent from the x-z projection (Figure 2A,B)
We found that ESCs grown in Serum/LIF media exhibit significantly larger spreading on the surface than cells grown in 2i, corresponding well with the more spread-out and flatter geometry of colonies grown in Serum/LIF than in 2i
Summary
The biomechanical properties of cells allow them to sense and react in response to their physical environment, for example, by sensing the rigidity of the extracellular matrix. The mechanical properties, both of the cell and its environment, play an important role in cell organization, migration, and differentiation. The physical properties of cells change drastically in terms of viscoelasticity and morphology, with the cell generally changing stiffness, as it progresses through development [1,2]. As the exact mechanisms and effects of both mechanical properties and mechano-sensing of cells during differentiation and development are not yet well-understood, it is important to map and understand the changes in physical properties of cells during differentiation and to correlate this with mapping of the differential expression of transcription factors
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