Improved fundamental understanding of how cells interpret microenvironmental signals is integral to designing better biomaterial therapies. YAP/TAZ are key mediators of mechanosensitive signaling; however, it is not clear how they are regulated by the complex interplay of microenvironmental factors (e.g., stiffness and degradability) and culture dimensionality. Using covalently crosslinked norbornene-functionalized hyaluronic acid (HA) hydrogels with controlled stiffness (via crosslink density) and degradability (via susceptibility of crosslinks to proteolysis), we found that human mesenchymal stem cells (MSCs) displayed increased spreading and YAP/TAZ nuclear localization when cultured atop stiffer hydrogels; however, the opposite trend was observed when MSCs were encapsulated within degradable hydrogels. When stiffnessmatched hydrogels of reduced degradability were used, YAP/TAZ nuclear translocation was greater in cells that were able to spread, which was confirmed through pharmacological inhibition of YAP/TAZ and actin polymerization. Together, these data illustrate that YAP/TAZ signaling is responsive to hydrogel stiffness and degradability, but the outcome is dependent on the dimensionality of cell-biomaterial interactions.

The relationship between ECM mechanics and cell behavior is dynamic, as cells remodel and respond to changes in their local environment. Most in vitro substrates are static and supraphysiologically stiff; thus, platforms with dynamic and reversible mechanical changes are needed. Here, we developed hyaluronic acid-based substrates capable of sequential photodegradation and photoinitiated crosslinking reactions to “soften” and then “stiffen” the hydrogels over a physiologically-relevant range of moduli. Reversible mechanical signaling to adhered cells was demonstrated with human mesenchymal stem cells. In situ hydrogel softening (from ~14 to 3.5 kPa) led to a decrease in cell area and nuclear localization of YAP/TAZ, and subsequent stiffening (from ~3.5 to 28 kPa) increased cell area and nuclear localization of YAP/TAZ. Each photoreaction was cytocompatible and tunable, rendering this platform amenable to studies of dynamic mechanics on cell behavior across many cell types and contexts.

Articular cartilage is a load-bearing tissue that lines the surface of bones in diarthrodial joints. Unfortunately, this avascular tissue has a limited capacity for intrinsic repair. Treatment options for articular cartilage defects include microfracture and arthroplasty; however, these strategies fail to generate tissue that adequately restores damaged cartilage. Limitations of current treatments for cartilage defects have prompted the field of cartilage tissue engineering, which seeks to integrate engineering and biological principles to promote the growth of new cartilage to replace damaged tissue. To-date, a wide range of scaffolds and cell sources have emerged towards cartilage tissue engineering, with a focus on recapitulating microenvironments present during development or in adult tissue to induce the formation of cartilaginous constructs with biochemical and mechanical properties of native tissue. Hydrogels have emerged as a promising scaffold due to the wide range of properties that are possible and the ability to entrap cells within the material. Towards improving cartilage repair, hydrogel design has advanced in recent years to improve their utility. Some of these advances include the development of improved network crosslinking (e.g., double-networks), new techniques to process hydrogels (e.g., 3D printing), and the better incorporation of biological signals (e.g., controlled release). This review summarizes these innovative approaches to engineer hydrogels towards cartilage repair, with an eye towards eventual clinical translation.

3D microenvironmental parameters control cell behavior, but can be challenging to investigate over a wide range of conditions. Here, a combinatorial hydrogel platform is developed that uses light-mediated thiol-norbornene chemistry to encapsulate cells within hydrogels with biochemical gradients made by spatially varied light exposure. Specifically, mesenchymal stem cells are photoencapsulated in norbornene-modified hyaluronic acid hydrogels functionalized with gradients (0–5 mM) of peptides that mimic cell-cell or cell-matrix interactions, either as single or orthogonal gradients. Chondrogenesis varied spatially in these hydrogels based on the local biochemical formulation, as indicated by Sox9 and aggrecan expression levels. From 100 combinations investigated, discrete hydrogels are formulated and early gene expression and long-term cartilage-specific matrix production are assayed and found to be consistent with screening predictions. This platform is a scalable, high-throughput technique that enables the screening of the effects of multiple biochemical signals on 3D cell behavior.

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