Two-dimensional (2D) models fail to mimic a three-dimensional (3D) environment in studying mechanosensing of stem cells. Here, we present a 3D culture model to investigate how 3D matrix stiffness influences YAP activation and osteogenic differentiation of human periodontal ligament stem cells (hPDLSCs) and evaluate the osteogenic potential of hydrogels in vivo. In this study, a 3D culture model with an adjustable matrix stiffness was established. In the in vitro study, first the osteogenic differentiation of hPDLSCs by the expression of OCN, ALP, COL-1, and RUNX-2 was assessed using qRT-PCR, accompanied by ALP staining, and then YAP expression was evaluated by immunofluorescence. In the in vivo study, hPDLSCs, together with gelatin methacrylate (GelMA) hydrogels of different stiffnesses, were implanted into a rat alveolar bone defect model. As matrix stiffness increased, hPDLSCs showed reduced spreading and significantly decreased expression of OCN, ALP, COL-1, RUNX-2, and YAP activation. Specifically, COL-1 expression in the low-stiffness group was 4.3-fold higher than that in the high-stiffness group, and the YAP nuclear/cytoplasmic ratio under low stiffness was 5.5-fold greater than that under high stiffness at day 7. In vivo, the soft-matrix-cell-laden group exhibited more new bone (86.04%) and collagen formation (74.43%) in the defect area than other groups at week 6. Reduced matrix stiffness likely promotes hPDLSC proliferation, spreading, and osteogenic differentiation through YAP activation, and low-stiffness GelMA hydrogels loaded with hPDLSCs significantly enhance alveolar bone regeneration in vivo.
Biomechanical stimuli derived from the extracellular matrix (ECM) extremely tune stem cell fate through three-dimensional (3D) and spatiotemporal changes in vivo. The matrix stiffness as a crucial factor during bone tissue development is dynamic stiffening. However, most in vitro models to study the osteogenesis of mesenchymal stem cells (MSCs) are static or stiffening in a 2D environment. Here, we create a dynamic and controllable stiffening 3D biomimetic model to regulate the osteoblastic differentiation of MSCs with a dual-functional gelatin macromer that can generate a double-network hydrogel by sequential enzymatic and light-triggered crosslinking reactions. Our findings showed that these dynamic hydrogels allowed cells to spread and volume expansion prior to secondary crosslinking and to sense high stiffness after stiffening. The MSCs in the dynamic hydrogels, especially the hydrogel stiffened at the late period, presented the significantly elevated osteogenic ECM secretion, gene expression, and YAP/TAZ nuclear localization. In vivo evaluation of animal experiments further indicated that the enhancement of dynamic stiffening on osteogenesis of MSCs substantially promoted bone remodeling. Consequently, this work reveals that the 3D dynamic stiffening microenvironment as a critical biophysical cue not only mediates the stem cell fate in vitro, but also augments bone restoration in vivo. This article is protected by copyright. All rights reserved
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