The ability to clone an animal, by transferring the nucleus of a differentiated cell into a fertilised, enucleated oocyte, showed that, in fact, the profile of gene expression could be reversed, wiped clean and restored to a pristine, undifferentiated state. Groundbreaking experiments over the last few years, led by Shinya Yamanaka and colleagues, have established an even simpler method to take a differentiated somatic cell and convert it back to an undifferentiated state, a so-called induced pluripotent stem cell (iPS cell). This is achieved by forcing the expression of a set of transcription factors that define the stem cell fate – in essence, driving the biochemical profile to a stem cell pattern. Auto- and cross-regulatory interactions between these transcription factors and others in the cell ensure a stable transformation of cell identity. So, one can take a differentiated cell and reverse the direction of development, driving it back to an undifferentiated state. This is obviously a hugely powerful technique as the iPS cells can then be differentiated in vitro into many other cell types, for research or clinical purposes.
What was not clear, however, was how general this strategy might be. Was there something special about the stem cell fate or could the same approach be used to convert one differentiated cell-type directly into another, without having to go via a stem cell intermediate? Wernig and colleagues have now shown that this can be done.
They forced fibroblasts, derived from either embryonic or adult mice, to express a combination of three transcription factors which characterise a general neuronal fate, by transfecting them with lentiviruses carrying these genes. These genes, Ascl, Brn2 and Mytl1, were defined from an initial set of nineteen candidates based on genes known to be important in neuronal differentiation during development. Transfection with these genes very efficiently, over a period of several days, transformed the fibroblasts into neurons. They express neuronal proteins, show a neuronal morphology, elaborate axons and dendrites, have characteristic electrical membrane properties, fire action potentials and even synapse with each other. By all possible criteria, these are neurons. In particular, 99% of them seem to be glutamatergic and many express markers characteristic of cortical neurons.
One can expect a flurry of follow-on studies using different combinations of transcription factors to drive cells into a variety of more specific neuronal subtypes – likely to include dopaminergic neurons, motor neurons, photoreceptors and many others – as well as non-neuronal cell types.
There is every reason to expect the same approach will work with human cells, as was the case with the iPS cell technology. This will offer a rapid and very efficient method to generate a large supply of neurons from individuals with their own genotype. These are most obviously of possible use in cell replacement therapies for neurodegenerative disease, though the prospect of this in clinical practice remains some years off.
The value for clinical research is also immense, and likely more immediate. The ability to grow neurons with the genotypes of patients with psychiatric or neurological disorders, for example, offers a kind of virtual biopsy – access to neurons of the patient previously unobtainable. This will allow analysis of the possible genetic defects in each patient and their cellular consequences, likely to be invaluable in elucidating pathogenic mechanisms at a molecular level.
The future is now.
Vierbuchen, T., Ostermeier, A., Pang, Z., Kokubu, Y., Südhof, T., & Wernig, M. (2010). Direct conversion of fibroblasts to functional neurons by defined factors Nature DOI: 10.1038/nature08797