(408f) Determining How Human Mesenchymal Stem Cells Change Their Degradation Strategy in Response to Microenvironmental Stiffness

During the wound healing process injured tissues secrete different chemical cues to recruit cells to regulate inflammation, initiate healing and regenerate tissue. Important cells in this process are human mesenchymal stem cells (hMSCs), which migrate from their native tissue niche to the wounded site to begin the healing process. To migrate, hMSCs degrade the native extracellular matrix (ECM) which is composed of various fibrous proteins. hMSCs secrete matrix metalloproteinases (MMPs) to degrade the ECM and create micron-sized channels for facile migration. The native ECM is complex and cells experience different physical and chemical cues unique to each tissue which can manipulate basic cellular functions. To understand how hMSCs reengineer their surrounding microenvironment, we characterize cell-mediated rheological changes in the pericellular region as a function of the physical microenvironmental stiffness in well-defined synthetic hydrogels. These hydrogels are engineered to mimic both physical and chemical aspects of the native ECM and enable 3D cell encapsulation. Our hydrogels scaffold consists of a 4-arm star poly(ethylene glycol) (PEG) end-functionalized with norbornene which is chemically cross-linked with an MMP degradable peptide sequence through radical mediated step-growth photopolymerization. This peptide sequence is cleaved by cell secreted MMPs. The elastic modulus of our swollen hydrogels varies from 81 Pa to 2400 Pa which mimics the stiffness of soft tissues in the human body such as neural, lung, breast and endothelial tissue. Multiple particle tracking (MPT) microrheology is used to characterize spatio-temporal rheological changes in the pericellular region. In MPT, Brownian motion of fluorescent probe particles embedded in the hydrogel is measured. MPT measurements around an encapsulated hMSCs in low-crosslink density hydrogels (G’ = 81 – 269 Pa) measure a reverse reaction-diffusion degradation profile in the pericellular region. This degradation profile is where material directly around the cells remain in a gel state and degradation is greatest far from the cell center. Increasing the cross-link density (G’ = 640 Pa) changes the degradation profile to a reaction-diffusion degradation profile. In this profile, the greatest degradation is directly around the cell and decreases as distance from the cell increases. We hypothesize that in stiffer material hMSCs secrete more MMPs to degrade the cross-linkers and enable migration. We measure no degradation around hMSCs encapsulated in high cross-link density gels (G’ = 1780 – 2400 Pa). We hypothesize that the increase in MMP activity for the highest stiffnesses is not enough to degrade the cross-linkers and create micron-sized channels which are necessary for migration. Furthermore, cellular speed decreased significantly when hydrogel stiffness is increased. These measurements help us to better understand how changes in the degradation mechanism in response to different physical microenvironments will change the cellular migration in 3D and can lead to design of biomaterials that mimic physical aspects of tissues in the body for cell delivery into wounded areas.