Optimizing Peptide Crosslinks for Cell-Responsive Hydrogels

Abstract

Cells dynamically modify their local extracellular matrix by expressing proteases that degrade matrix proteins. This enables cells to spread and migrate within tissues, and this process is often mimicked in hydrogels through the incorporation of peptide crosslinks that can be degraded by cell-secreted proteases. However, the cleavage of hydrogel crosslinks will also reduce the local matrix mechanical properties, and most crosslinking peptides, such as the widely used GPQGIWGQ "PanMMP" sequence, lead to bulk degradation of the hydrogel. A subset of proteases are localized to the cell membrane and are only active in the pericellular region in the immediate vicinity of the cell surface. These membrane-type proteases have important physiological roles and enable cells to migrate within tissues. In this work we developed an approach to identify and optimize peptide sequences that are specifically degraded by membrane-type proteases. We utilized a proteomic screen to identify peptide targets, and coupled this with a functional assay that both quantifies peptide degradation by individual cell types and can elucidate whether the peptides are primarily cleaved by soluble proteases or membrane-type proteases. We then used a split-and-pool synthesis approach to generate more than 300 variants of the target peptide to improve the degradation behavior. We identified an optimized peptide sequence, KLVADLMASAE, which is primarily degraded by membrane-type proteases, but enables both endothelial cells and stem cells grown in KLVADLMASAE-crosslinked hydrogels to spread and have viabilities similar to the gels crosslinked by the PanMMP peptide. Notably, the biological performance of the KLVADLMASAE peptide-cross linked gels was significantly improved from the initial peptide target found in the proteomic screen. This work introduces a functional approach to identifying and refining protease-substrate peptides as a way to enhance the properties of hydrogel matrices.