A promising treatment for multiple neurological disorders including stroke, Huntington's, and Parkinson's is the transplantation of stem cells into the diseased site to promote regeneration of the neural tissue. However, viability of transplanted cells is often low (15-35%) and unpredictable. Cell viability has been directly correlated with functional outcome of the treatment, motivating the development of more reliable cell transplantation procedures. To protect transplanted cells from shear stress during injection and from the hostile, inflammatory environment of the diseased brain tissue, many research groups are exploring physical hydrogels as a protective, growth-permissive matrix to enhance cell viability. However, physical hydrogels require an environmental trigger to induce gelation. These environmental triggers include sudden changes in temperature, pH, or salt concentration - all of which are detrimental to encapsulated cells and proteins and complicate their use in a clinical environment. To address this need, we have designed a two-component, protein-based hydrogel system that can self-assemble under constant physiological conditions.
Both components of the hydrogel system are created using recombinant protein technology, which allows synthesis of exact monomer sequences within monodisperse polymers. The first hydrogel component is a block copolymer made of several repeats of a peptide sequence encoding the WW domain-fold, a short triple-stranded, anti-parallel, beta-sheet. The WW domains are interspersed with a random-coil, hydrophilic spacer to enhance polymer flexibility and solubility. The second hydrogel component is made of several repeats of a polyproline rich peptide sequence interspersed with a random-coil, hydrophilic spacer. Upon mixing the two hydrogel components together, the WW-domains in component 1 and the polyproline rich peptides in component 2 bind together with 1:1 stoichiometry. This binding has an apparent association constant of 2.2×105 M, as measured by isothermal titration calorimetry. This peptide-binding event serves as the physical crosslinks to form a polymeric network composed of the two components. Because gelation is initiated by simply mixing the two components together at physiological pH, temperature, and ionic strength, this system is highly biocompatible and easy to use. Furthermore, the precision of protein engineering allows both components to be easily modified. For example, increasing the length of the hydrophilic spacers will increase the resulting network pore size. Additionally, bioactive peptide sequences, such as the RGD cell-binding domain, have been introduced into the hydrophilic spacers to modify cell-scaffold interactions. Our long-term objective is to design a self-assembling hydrogel system for cell delivery that will both improve cell viability and mimic many of the essential cues in the developmental niche to encourage cell differentiation and outgrowth.