Designing Cell-Responsive Biomaterials Through Protein Engineering

Currently, surgeons use two primary types of materials inside the body to replace damaged body parts: either common industrial materials or harvested natural materials. Common industrial materials include stainless steel and titanium alloys in artificial joints and synthetic polymers in vascular bypasses. While these materials often perform well at replacing mechanical functions in the body, they lack much of the micro- and nano-scale organization necessary for proper biochemical function within the body.

In order to replace these biochemical functions, harvested natural materials such as heart valves and other organ transplants are generally the gold standard. However, these natural materials have a number of limitations: they are expensive and time-intensive to harvest and purify, they risk the transmission of diseases, they lack the strict quality control reproducibility found in the synthesis of industrial materials, and they are nearly impossible to customize for specific applications.

In response, Professor Sarah Heilshorn and her team at Heilshorn Biomaterials Group has been exploring the possibility of designing and synthesizing materials made of engineered proteins.¹ Proteins are one of the main material components of our bodies, and they have evolved over time to form materials with a dazzling array of biomechanical and biochemical properties. A protein is made of several amino acids strung together into a long chain.

By varying the sequence of the amino acids in the chain, proteins with different material properties are created. For naturally evolved proteins, the instructions that contain the sequence of amino acids for each protein chain are encoded in our genetic material, DNA, found in the nucleus of cells. To mimic this process, materials scientists can encode new amino acid sequences into bits of engineered DNA that are chemically synthesized. This genetic message is inserted into a host organism, such as non-infectious bacteria. The natural synthetic machinery in the host organism then translates the engineered DNA message and synthesizes a protein chain with the exact amino acid sequence specified. The engineered protein product can then be purified and processed into a material.

Professor Sarah Heilshorn and her team are using this technique, recombinant protein engineering, to design new protein materials for medical applications. These protein materials combine many of the advantages of common industrial materials, such as reproducibility and easy customization, with the advantages of harvested natural materials, such as cell compatibility and biodegradability. These new materials work by imitating the natural proteins found inside our bodies.

Just like natural proteins, these engineered proteins contain biomechanical and biochemical "instructions" that can guide cells to behave in certain ways. Using these new materials, we are working to determine the correct set of instructions that will cause cells to adhere, migrate, proliferate, and differentiate into a specific tissue type, Figure 1.

Figure 1. Digitally enhanced microscope image of neural stem cells (shown in green and blue) grown within a protein-engineered biomaterial. Several of the cells have started to differentiate into neurons (shown in green) and to extend long neuritic sprouts. Original micrograph acquired by Cheryl Wong Po Foo; Digital Artwork performed by Chelsea Castillo.

My laboratory is currently focused on materials to treat diseases and injuries to the central nervous system, although this material design strategy can be applied to any tissue type.² Because cells simultaneously respond to both biomechanical and biochemical signals, it is critical to design materials that enable independent modulation of each variable.

As one example, we have designed protein materials that combine elastic-like mechanical properties (they are stretchy and resilient) with cell-adhesive biochemical properties.³ These materials can be further designed to respond to changes in the local cell environment by reacting with cell-secreted enzymes.⁴ These material-enzyme reactions can be used to trigger the emergence of three-dimensional patterns within the material or to trigger the delivery of multiple drugs with distinct temporal and spatial release profiles. By imitating the materials naturally found in our bodies, these protein-engineered materials will enable the development of new medical therapies.

Acknowledgments

The work described in this article was supported by the Hellman Faculty Scholar Award, the deLarios Family Scholar Fund, the National Academies Keck Futures Initiative, the National Science Foundation (EFRI-CBE-0735551 and DMR-0846363), and the National Institutes of Health (1DP2-OD-006477-01 and R01-DK-085720-01).

References

1. Wong Po Foo C, Heilshorn SC. Protein Engineered Biomaterials, Chapter 8, Protein Engineering and Design, editors Cochran J and Park SJ, CRC Press, Boca Raton, FL, 2009.
2. Wong Po Foo C, Lee JS, Mulyasasmita W, Parisi-Amon A, Heilshorn SC. Two-component protein-engineered physical hydrogels for cell encapsulation. Proceedings of the National Academy of Sciences USA 2009, 106 (52):22067-22072.
3. Straley K, Heilshorn SC. Independent tuning of multiple biomaterial properties using protein engineering. Soft Matter 2009; 5:114-124.
4. Straley K, Heilshorn SC. Dynamic, three-dimensional pattern formation within enzyme-responsive hydrogels. Advanced Materials 2009, 21:4148-4152.