Dynamic microenvironments created by human mesenchymal stem cells*

During wound healing, human mesenchymal stem cells (hMSCs) orchestrate the healing process by regulating inflammation and coordinating tissue regeneration. Cell-laden hydrogels are designed to deliver additional hMSCs to wounds to enhance or restart healing. These scaffolds are designed to mimic native tissues, including physical and chemical cues. We characterize the feedback between microenvironmental cues presented to cells and the microenvironments cells engineer in response. We encapsulate hMSCs in a well-defined hydrogel that recapitulates aspects of the native extracellular matrix. Our hydrogel consists of a 4-arm poly(ethylene glycol) end-functionalized with norbornene which is cross-linked with a matrix metalloproteinase (MMP) degradable peptide that is cleaved by hMSC secretions. We use multiple particle tracking microrheology (MPT) and bi-disperse MPT to characterize spatio-temporal cell-mediated degradation. In MPT, particles are embedded in the material and their Brownian motion is measured and related to rheological properties. Bi-disperse MPT embeds two different particle sizes into the material to simultaneously measure lengthscale-dependent rheology. Using MPT, we determine hMSCs create a microenvironment where the cross-link density decreases as distance from the cell increases, which enables spreading and attachment prior to motility. The cell simultaneous secretes scaffold degrading MMPs and tissue inhibitors of metalloproteinases (TIMPs), which inhibit MMP activity and scaffold degradation. We reverse this degradation profile by inhibiting TIMPs, which increases hMSC motility. Using bi-disperse MPT, we simultaneously measure cell-mediated degradation and reversible remodeling. This work highlights the ability for a cell to selectively remodel their microenvironment during motility. These measurements can inform design of implantable biomaterials that instruct cellular processes for cell delivery to wounds.