Drug Discovery
Figure 2
Potential uses for iPS cells for target identification, screening, ADME/toxicity testing and therapy
Several growth factors have been identified that promote growth of human ES cells in culture, most notably basic fibroblast growth factor (bFGF). However, the use of bFGF for human ES cell cul- ture, particularly at the high levels used by some for ‘feeder-free’ culture, must be considered carefully, as bFGF may drive ES cells towards differentiation. Xeno-free media that do not rely upon high lev- els of bFGF have been developed that contain only humanised or synthetic components and are engi- neered to support the undifferentiated growth and expansion of human ES cells. Use of human feed- ers derived under xeno-free conditions can main- tain the xeno-free nature of the culture system and allow for the use of low levels of bFGF. Use of progenitor cell lines can eliminate the need to culture challenging human ES cells. Progenitor cells are similar to embryonic stem cells in their capacity to differentiate into various cell types. However, progenitor cells can only differen- tiate into a limited number of cell types. Progenitor cells can be far easier to handle in culture than ES cells. For example, neural progenitor cells derived from a human ES cell line are easily propagated and require less handling than human ES cells. In addition, they do not require feeder layers and can differentiate into many neural lineages under the appropriate conditions1.
The need to control differentiation of embryonic stem cells in vitro presents another set of challenges. Identifying the right cocktail of media condi- tions, supplements and growth factors that suc- cessfully drive stem cells toward a desired lineage on a reproducible basis is a time-consuming, itera- tive exercise. A carefully choreographed series of
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signals must be recreated to guide cells down the chosen pathway. Fortunately, this labour-intensive work has already been done for a number of cell types. Kits and media containing an optimised set of factors necessary to differentiate stem cells to a chosen lineage are commercially available for gen- erating neurons, oligodendrocytes, mesenchymal cells and osteocytes.
Cellular reprogramming
The discovery of a process whereby fully differen- tiated, adult somatic cells can be reprogrammed into induced pluripotent stem cells (iPS cells) has provided researchers entirely new approaches for target discovery, screening, metabolic profiling and toxicity evaluation2.
iPS cells, which are similar to embryonic stem cells in their ability to differentiate into a wide variety of cell types, are now routinely generated from adult cells. Fibroblasts derived from a simple skin biopsy are a common starting point. For example, fibroblasts from a patient with Alzheimer’s disease or amyotrophic lateral sclerosis (ALS) can be reprogrammed to yield iPS cells. The iPS cells can then be induced to differentiate into neurons and other cell types that might be affected in the disease.
Initial efforts to generate iPS cells required simultaneous co-infection of cells with four sepa- rate retroviral expression vectors. Each vector carried one transcription factor, which resulted in a high number of genomic integrations which may activate or inactivate critical host genes. Alternative approaches to iPS generation have included use of plasmids and non-integrating
Drug Discovery World Spring 2011
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