Design of Elastin-like Protein and Poly(ethylene glycol) Hydrogels for Neural Tissue Engineering Applications

Biomaterial scaffolds are promising tools in developing treatments for central nervous system (CNS) injury and disease. In vitro biomaterial systems can be designed to help elucidate the complex signaling interplay between cells and their extracellular matrix (ECM). Recombinant engineered proteins are ideal biomaterials to create in vitro models of native ECM because they are derived from native proteins, have a well-defined protein primary structure, bioactivity can be imbedded in the amino acid sequence, and amino acid residue side chains can be modified to adapt the cross-linking mechanism to form hydrogels suitable for a variety of applications. Recombinant engineered elastin-like proteins (ELPs) are derived from tropoelastin and are composed of repeating VPGxG penta-peptide sequences, where x is any amino acid guest residue except for proline. ELPs undergo a unique lower critical solution temperature (LCST) transition above which they aggregate into a protein-rich coacervate phase in aqueous solution. The ELP LCST transition was used to develop poly(ethylene glycol) (PEG) and ELP hydrogels with micro-architecture. Two distinct hydrogels with differing properties were formed by controlling the temperature at which the cross-linking reactions occurred. Bioactivity is incorporated into ELP hydrogels by imbedding small bioactive peptide sequences, such as cell-adhesive RGD, into the protein sequence. ELP sequences with differing bioactive peptides can be combined to form hydrogels with multiple bioactivities, and the small differences in the amino acid content result in hydrogels with almost identical physical properties. This allows for independent tuning of ELP hydrogel physical properties, which is controlled by adjusting protein concentration and crosslinking ratios, from bioactivity, which is controlled by adjusting the mixture of ELP sequences. Hydrogels that were either enzymatically degradable by urokinase plasminogen activator (uPA) or non-degradable had similar stiffness and stress relaxation properties to native brain tissue, making them suitable in vitro models of brain ECM. uPA degradable hydrogels promoted encapsulated oligodendrocyte precursor cell (OPC) maturation when compared to non-degradable hydrogels. This work identifies the usefulness of ELPs in designing in vitro hydrogel models of the CNS ECM, and illustrates the types of interactions between cells and their ECM that can be analyzed through ELP hydrogel systems.