Engineering the cellular mechanical microenvironment to regulate stem cell chondrogenesis: Insights from a microgel model

Biophysical cues (especially mechanical cues) embedded in cellular microenvironments show a critical impact on stem cell fate. Despite the capability of traditional hydrogels to mimic the feature of extracellular matrix (ECM) and tune their physicochemical properties via diverse approaches, their relatively large size not only induces biased results, but also hinders high-throughput screening and analysis. In this paper, a microgel model is proposed to recapitulate the role of 3D mechanical microenvironment on stem cell behaviors especially chondrogenesis in vitro. The small diameter of microgels brings the high surface area to volume ratio and then the enlarged diffusion area and shortened diffusion distance of soluble molecules, leading to uniform distribution of nutrients and negligible biochemical gradient inside microgels. To construct ECM-like microenvironment with tunable mechanical strength, three gelatin/hyaluronic acid hybrid microgels with low, medium and high crosslinking densities, i.e., Gel-HA(L), Gel-HA(M) and Gel-HA(H), are fabricated in microfluidic devices by Michael addition reaction between thiolated gelatin (Gel-SH) and ethylsulfated hyaluronic acid (HA-VS) with different substitution degrees of vinyl sulfone groups. Our results show that mouse bone marrow mesenchymal stem cell (BMSC) proliferation, distribution and chondrogenesis are all closely dependent on mechanical microenvironments in microgels. Noteworthily, BMSCs show a clear trend of differentiating into hyaline cartilage in Gel-HA(L) and fibrocartilage in Gel-HA(M) and Gel-HA(H). Whole transcriptome RNA sequencing reveals that mechanical microenvironment of microgels affects BMSC differentiation via TGF-β/Smad signaling pathway, Hippo signaling pathway and Integrin/YAP/TAZ signaling pathway. We believe this microgel model provides a new way to further explore the interaction between cells and 3D microenvironment. In recent years, hydrogels have been frequently used to construct 3D microenvironment for cells. However, their relatively large size not only brings biased experimental results, but also limits high-throughput screening and analysis. Herein we propose a gelatin/hyaluronic acid microgel model to explore the effects of 3D cellular mechanical microenvironment (biophysical cues) on BMSC behaviors especially chondrogenesis, which can minimize the interference of biochemical gradients. Our results reveal that BMSC differentiation into either hyaline cartilage or fibrocartilage can be regulated via tailoring the mechanical properties of microgels. Whole transcriptome RNA sequencing proves that "TGF-β/Smad signaling pathway", "Hippo signaling pathway" and "Integrins/YAP/ TAZ signaling pathway" are activated or inhibited in this process.