Carbohydrate Polymers and Human Embryonic Stem Cell Pluripotency
Our experience in activating signaling with multivalent carbohydrate derivatives led us to become interested in using surfaces to deliver signals that direct cellular decisions. Like polymers, surfaces can display multiple copies of binding epitopes and therefore cluster proteins that activate signaling. This mode of directing and controlling cellular responses is largely unexplored. To test the feasibility of the strategy, we developed an array strategy to pattern surfaces, such that materials that alter cell adhesion and/or cell behavior can be identified (Fig. 5). The surfaces that we have begun to study display peptides, which can bind to cell surface receptors or cell surface carbohydrates.
Fig. 5. Strategy for synthesis of arrays for identifying surfaces that support cell attachment or elicit cellular responses. Alkane thiols functionalized with peptides or small molecules are immobilized on a gold surface to generate array elements. The immobilized alkane thiols assemble to form a well-ordered array.
We used our arrays to identify surfaces that would support the growth of human embryonic stem (hES) cells. The ability to grow human cells in culture is the basis for remarkable advances in fields ranging from cell biology to medicine. Human ES cells are remarkable because they can be cultured indefinitely and can differentiate into every cell type in the body. The ability to propagate hES cells provides new opportunities to test drug candidates on human cells and to generate cells for therapies. Still, to exploit the full potential of hES cells for regenerative medicine, developmental biology, and drug discovery, chemically defined culture conditions are needed. Reproducible cellular responses depend on having a fully defined environment. In addition, contamination of animal cells and/or animal-derived components typically used for hES cell culture remains a safety concern. We have addressed the limitations of standard methods. Using our surface array strategy, we identified synthetic surfaces that can support the long-term culture of hES cells. A surprising result of this screen is the simplicity of the identified surfaces. They do not present complex mixtures of proteins, but rather display a single glycosaminoglycan-binding peptide. This finding was especially exciting to us because glycosaminoglycans are polysaccharides present on the surface of all mammalian cells. While others have examined surfaces that can interact with cell-surface proteins, our data indicate that carbohydrate polymers are important in pluripotency. The surfaces we devised highlight the critical role of cell surface polysaccharides in regulating cell fate decisions. They also offer a chemically defined environment for propagating hES cells. The simplicity and efficiency of our results led the Director of NIGMS to highlight them to Congress last year as an example of how basic science can yield economic and scientific benefits.