Embryonic stem cells have great potential in medicine, but the current methods used to grow them prevent their therapeutic use. A dual-action compound has been discovered that may help solve this problem.
Last year, the International Stem Cell Forum stated that a reliable method for growing stem cells that does not depend on animal-derived products is a requirement for future work1. Reporting in Proceedings of the National Academy of Sciences, Chen et al.2 describe their progress towards this goal. Using high-throughput screening, they have discovered a chemical that allows mouse embryonic stem (ES) cells to perpetuate themselves. This compound is a valuable tool for studying ES cell self-renewal, and may bring us closer to developing these cells for therapeutic purposes.
Great hopes have been raised that ES cells will one day be used to replace damaged cells, and to provide therapies beyond the reach of conventional drugs. However, a serious obstacle to their use is our lack of insight into the mechanisms that regulate a stem cell's behaviour — more specifically, whether it undergoes self-renewal or differentiates to become a more specialized type of cell. Furthermore, most human stem-cell lines, including all human ES cell lines approved for study with US federal funds, were grown using animal products, so the potential for cross-species contamination is too high for these lines to be developed for therapeutic purposes3. A synthetic compound that promotes ES cell self-renewal could help to address both of these issues.
Intriguingly, Chen and colleagues' compound — named SC1 — hits two targets in a protein network. Both of these targets are necessary to promote ES cell self-renewal. The authors discovered the compound by high-throughput screening of a chemical library4 they had designed to target kinase enzymes; kinases pass on signals inside the cell. The authors tested their chemicals in mouse ES cells that were engineered to produce green fluorescent protein (GFP) only when they were perpetuating themselves. For the screen, the cells were grown under conditions that should cause them to stop self-renewing and to differentiate, a process that causes the levels of GFP to drop and the fluorescence to disappear. Chen et al. looked for compounds that had the ability to maintain fluorescence under these conditions.
Surprisingly, although the library was designed to target kinase enzymes, only one of the two targets of SC1 is a kinase. This illustrates an interesting notion in the design of chemical libraries: certain chemical groups bind well to a variety of cellular proteins, even though the structures of those proteins may be very different. In addition, SC1 highlights the value of using small molecules in screens that probe signalling pathways as an alternative to genetic methods, such as RNA interference or complementary DNA expression. As the authors explain2, the “advantage (and complexity) in the use of small molecules...is that compounds can modulate more than one target to achieve a desired biological effect”. Moreover, using current technology for protein identification, one can identify and dissect these multiple relevant targets, as the authors did here.
Chen et al. found that the two proteins targeted by SC1 are in the Ras signalling network (Fig. 1). Ras proteins are principal players in a complex network that regulates many cellular functions, including growth and mobility. These proteins stimulate growth when activated, that is, when they are bound to the molecule guanosine triphosphate (GTP). One of the targets of SC1 — Ras-GAP — is a protein that modulates Ras activity by stimulating the Ras protein to convert its GTP into guanosine diphosphate, turning off Ras signalling. So, inhibition of Ras-GAP by SC1 increases Ras signalling; Ras then activates its signalling partners farther along the network — including the kinase enzyme PI3K, which has been implicated in stem-cell self-renewal5. Think of the old aphorism “the enemy of my enemy is my friend”; SC1 inhibits the inhibitor of Ras, thereby activating Ras and PI3K signalling.
However, activation of the entire Ras network is unlikely to result in stem-cell self-renewal, as two aspects of Ras signalling oppose each other in regulating perpetuation. The PI3K arm of the Ras network promotes self-renewal, whereas another arm — containing the kinases ERK1 and ERK2 — inhibits self-renewal in mouse ES cells. By inhibiting ERK1 and ERK2 signalling, SC1 funnels the activated Ras signal towards the PI3K arm. This is a sophisticated effect for a small molecule, and it illustrates the power of this screening approach to identify unexpected ways of intervening in biological systems.
The authors have yet to test SC1 in human embryonic or adult stem cells, and it is possible that differences between signalling pathways in mice and humans may prevent SC1 from being active in human cell lines. For example, inhibition of ERK1 and ERK2 signalling in mouse ES cells promotes self-renewal, but the opposite is true for human ES cells. However, even if SC1 is inactive in human cell lines, the discovery of a compound that can promote self-renewal in mouse cells strongly suggests that there may be a parallel pathway to exploit in their human counterparts. This would simplify the method of growing human ES cells, provide information about the mechanisms controlling self-renewal, and offer a contaminant-free method of growing these cells for therapeutic purposes.
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