Stem Cells
mias6. These iPS cells were differentiated to func- tional cardiomyocytes which were found to reca- pitulate the longer action potentials observed in the patients. Small molecules were screened against these cells to see which could correct the underly- ing electrophysiological defect. iPS cells have been generated from patients with many other diseases, eg Huntingdon’s, ALS, SCID, juvenile diabetes and spinal muscular atrophy (SMA)9.
For stem cells to be utilised to their full poten- tial, two major challenges have to be overcome. The first is to be able to expand stem cell numbers in vitro, while maintaining a homogeneous culture of undifferentiated cells. The second is to be able to routinely direct stem cell differentiation in vitro to generate fully functional, specific cell types of choice. Production of stem cells and their differen- tiated progeny at large scale, in a robust and cost- effective manner, as required for cell therapy and drug discovery applications, is even more challeng- ing. There are many different types of stem cells, of diverse origins and differentiation potential (Figure 1) and each requires unique culture conditions for growth and differentiation.
Figure 1
Stem cell sources and their differentiation potential:
different types of stem cells exist which differ in their
longevity in culture and in the variety of mature cell types
they can generate. Pluripotent stem cells – either embryonic or induced – are the most potent stem cells and are
capable of infinite self-renewal in vitro and can generate all somatic cell types. Embryonic stem cells are isolated from the inner cell mass of
blastocysts, whereas induced pluripotent stem cells are generated by reprogramming somatic cells. Adult, or tissue specific, stem cells are more restricted in their
differentiation potential,
typically only being able to generate cells of the tissue
from which they were isolated. From: McNeish, J et al. High-
throughput screening in embryonic stem cell-derived neurons identifies potentiators of alpha-amino-3- hydroxyl-5-methyl-4-
isoxazolepropionate-type glutamate receptors. J Biol Chem, 2010. 285(22): p. 17209-17
concept for this application7. Mouse embryonic stem (mES) cells were differentiated into neuronal cells that express AMPA receptors and are phar- macologically responsive to standard AMPA potentiation agents. A library of 2.4 x 106 com- pounds was screened against these cells and novel chemical hits for AMPA potentiation were identi- fied, followed by validation of leads in secondary assays using human embryonic stem (hES) cell- derived neurons. There is increasing evidence that pharmaceutical companies are realising the poten- tial of stem cells for drug discovery applications. For example, Roche has invested $20 million in a deal with Harvard University to use cell lines and protocols to screen for drugs to treat cardiovascu- lar and other diseases and are already using iPS- derived cardiomyocytes8 (supplied by Cellular Dynamics International) in their drug discovery and toxicity processes.
Another application of stem cells is the develop- ment of disease models, either by generation of dis- ease-specific somatic cells or in vivo animal mod- els. iPS cells hold particular promise for this appli- cation since they can be generated from patients with a variety of diseases. iPS cells can then be dif- ferentiated to specific lineages to generate disease and patient-specific somatic cells. An example of this is the generation of iPS cells from patients with a K+ channel mutation found in congenital long QT syndrome associated with cardiac arrhyth-
72 Stem cell types
Pluripotent stem cells are the most potent of all stem cells, being able to self-renew indefinitely in vitro and differentiate into all somatic cell types in vivo and many in vitro (Figure 1). For example, of particular interest to the pharmaceutical industry, human pluripotent stem cells have been differenti- ated in vitro to haemopoietic, cardiac, multiple neuronal (eg dopaminergic, GABAergic, motor neuron), hepatic and pancreatic cells. There are two types of pluripotent stem cells. Embryonic stem (ES) cells10, are derived from the inner cell mass of pre-implantation embryos. Induced pluripotent stem (iPS) cells are generated by repro- gramming adult somatic cells to a pluripotent state through expression of a combination of genes or reprogramming factors1. iPS cells share many of the characteristics of ES cells, although there is speculation as to the true similarity of the cells, particularly in relation to the epigenetic state of their DNA11. In addition, it has been discovered that reprogramming of somatic cells can induce genomic alterations such as copy number varia- tions and point mutations12,13.
Adult stem cells, or tissue specific stem cells, have more restricted differentiation potential than pluripotent stem cells, typically limited to generation of cell types of the tissue from which they were isolated (Figure 1), eg neural stem cells under normal circumstances are only capable of
Drug Discovery World Winter 2011/12
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