Extended Exposure to Stiff Microenvironments Leads to Persistent Chromatin Remodeling in Human Mesenchymal Stem Cells

Abstract

Bone marrow derived human mesenchymal stem cells (hMSCs) are a promising cell source for regenerative therapies; however, ex vivo expansion is often required to achieve clinically useful cells numbers. Recent results reveal that when MSCs are cultured in stiff microenvironments, their regenerative capacity can be altered in a manner that is dependent on time (e.g., a mechanical dosing analogous to a chemical one). It is hypothesized that epigenomic modifications are involved in storing these mechanical cues, regulating gene expression, and ultimately leading to a mechanical memory. Using hydrogels containing an allyl sulfide cross‐linker and a radical‐mediated addition‐fragmentation chain transfer process, in situ softened hMSC‐laden hydrogels at different time points are achieved and the effects of short‐term and long‐term mechanical dosing on epigenetic modifications in hMSCs are quantified. Results show that histone acetylation and chromatin organization adapt rapidly after softening and can be reversible or irreversible depending on time of exposure to stiff microenvironments. Furthermore, epigenetic modulators are differentially expressed depending on the culture history. Collectively, these experiments suggest that epigenetic remodeling can be persistent and might be a memory keeper.

Mechanical memory in cells emerges from mechanotransduction with transcriptional feedback and epigenetic plasticity

“…4E), which also validates the measured mechanical memory response in stem cells (Fig. 4A) (22,23,33). In this case, there is ample time for accumulation of stiff-memory factors beyond the threshold value, to ~0.9 (Fig.…”

“…Increasing contractility has been shown to regulate levels of HDACs, whereas available spreading area has been implicated in modulating HAT (54). Recently, it has been shown that stiff-primed stem cells continue to express an epigenetic modification factor HAT1 and suppress histone deacetylation enzymes (HDACs) even after they move to a soft matrix (33). Thus, prior stiff-primed epigenetic state persists during the memory phase, which in turn could disable new chromatin remodeling and opening of TF binding sites required for soft adaptation.…”

“…During priming, cells produce memory-regulating factors and reduce their epigenetic plasticity. In previous studies, it has been shown that YAP and microRNA-21 expressions regulate short-and longterm memory responses, respectively (22)(23)(24), and the epigenetic modifiers HAT1 and HDACs may serve as memory-keepers (33), all of which could serve as memory-regulating factors. It is also likely that different cell types vary in their ability to maintain memory factors (α) due to varying rate constants for production ( 4 ), due to translation, and degradation ( ' ), due to autophagy and protein half-life.…”

“…Since the epigenetic landscape fundamentally regulates transcription factor (TF) binding and gene expression (30)(31)(32), the connection between stiffness-sensitive cytoskeletal signaling and epigenetic programming is a critical missing piece in the theoretical framework of mechanotransduction. Indeed, when stem cells are stiff-primed and moved to a soft matrix, an epigenetic modification factor HAT1 is persistently expressed and histone deacetylation enzymes (HDACs) remain low (33), both of which enhance chromatin resistance. Based on this work and related experimental findings of primingdependent cellular response (22)(23)(24)33), we propose that mechanical priming of cells requires the production of memory-regulating factors, such as epigenetic modifiers (e.g., HAT1, HDACs) and nuclear structure regulators (e.g., Lamin A/C).…”

“…Indeed, when stem cells are stiff-primed and moved to a soft matrix, an epigenetic modification factor HAT1 is persistently expressed and histone deacetylation enzymes (HDACs) remain low (33), both of which enhance chromatin resistance. Based on this work and related experimental findings of primingdependent cellular response (22)(23)(24)33), we propose that mechanical priming of cells requires the production of memory-regulating factors, such as epigenetic modifiers (e.g., HAT1, HDACs) and nuclear structure regulators (e.g., Lamin A/C). These memory factors reduce epigenetic plasticity, defined as the ability of cells to alter their chromatin configuration and expose TF binding sites.…”