Characterizing the Feedback Loop Between Cells and the Pericellular Region During Cell-Material Interactions

Kelly M. Schultz, Ph.D. 
Larry and Virginia Faith Associate Professor
Purdue University
 

Seminar Abstract: During the wound healing process, human mesenchymal stem cells (hMSCs) are recruited to an injury where they regulate inflammation and initiate healing and tissue regeneration. To aid in healing, synthetic cell-laden hydrogel scaffolds that mimic aspects of native tissue are being designed to deliver additional hMSCs to wounds to enhance or restart the healing process. These scaffolds initially provide well-defined 3D microenvironments to cells, but are also designed to be remodeled by these cells. This creates a feedback loop where cell remodels the pericellular region and respond to dynamically changing cues presented in the environment. In this work, we quantitatively characterize the real-time feedback between the microenvironment cells create and the cues presented to encapsulated cells and how this defines basic cellular processes. To do this, we encapsulate hMSCs in a well-defined synthetic hydrogel that consists of a 4-arm poly(ethylene glycol) (PEG) backbone end-functionalized with norbornene which is chemically cross-linked with an matrix metalloproteinase (MMP)-degradable peptide sequence. We use multiple particle tracking microrheology (MPT) to characterize spatio-temporal cellmediated degradation in the pericellular region. In MPT, fluorescently labeled particles are embedded in the material and their Brownian motion is measured and related to rheological properties using the Generalized Stokes-Einstein Relation. We start by characterizing the strategies that hMSCs use to engineer their microenvironments prior to and during motility in soft materials that mimic adipose tissue. We measure that hMSCs regulate the molecules they secrete to maintain material stiffness directly around them prior to migration. We hypothesize that hMSCs are creating an environment that enables spreading and attachment prior to motility by secreting a combination of MMPs and tissue inhibitors of metalloproteinases (TIMPs). We then inhibit TIMPs and reverse the pericellular degradation profile and increase cell motility. 

Cell-mediated pericellular re-engineering is also inherently length-scale dependent and both structure and rheological properties are important factors in retaining native function of hMSCs. During hMSC-mediated scaffold remodeling single cross-links break on the nanometer scale, cellular extensions pull material and degrade paths through the scaffold to enable motility on the micrometer scale and bulk scaffold degradation occurs on macroscopic scales. We measure length-scale dependent rheology during cell-mediated remodeling of the pericellular region using bi-disperse MPT. Bi-disperse MPT extends MPT to characterize material evolution across length-scales by tracking a bi-disperse population of particles, 0.5 and 2 micron particles. We measure that cells preferentially re-engineer their microenvironment across length-scales using enzymatic secretions (irreversible degradation) and cytoskeletal tension (reversible degradation). From these measurements, we identify that 2 micron particles are stuck in a loose gel network that has been partially degraded and is being reversibly remodeled by the hMSC. At the same time, 0.5 micron particles are measuring irreversible scaffold degradation due to cellsecreted enzymes and are able to diffuse through the larger network structure cells are pulling on. Characterization of spatio-temporal evolution due to cell-mediated degradation could lead to better design of implantable biomaterials for cell delivery to wounded areas.