| Connective tissue cells have been directly converted into functional neurons, avoiding stem cell intermediates likely to be carcinogenic, an important step towards replacing cells lost in Parkinson's disease. |
A limitation of stem cells is that, as currently used, they can cause cancer and immune problems. Recent research aims to address this concern by understanding stem cell epigenetics.
An interesting alternative is to avoid the use of stem cells altogether. Directly converting adult cells into another cell type, avoiding stem cell intermediates, might avoid the medical safety concerns of stem cells.
This result was achieved last year with mouse cells. Malin Parmar (Lund University, Sweden) and coworkers have now done the same thing with human cells, in early-stage research that will be helpful in the long-term for treating Parkinson's disease.
Converting fibroblasts into functional neurons in one month.
A brief summary of the scientists' research is that they directly converted human fibroblasts (connective tissue) into functional neurons. Stem cell intermediates were avoided.
The scientists obtained their fibroblasts from legally aborted fetuses, 5.5-7 weeks after conception. The head, vertebral column, dorsal root ganglia, and all internal organs were carefully removed to eliminate cells with the obvious potential to develop into neurons (numerous molecular controls excluded the presence of cell contaminants).
Genes known to convert fibroblasts to neurons were delivered by a virus. Genetic delivery was confirmed by biochemical assays.
These cells began to develop the elongated appearance of neurons after three or four days. After twelve days, many of the cells had undergone the transformation, confirmed by the presence of an indicative protein biochemical marker (βIII-tubulin).
Elaborate nerve cell morphologies were almost exclusive of the converted cells after 24 days. Cells not treated with the virus did not become apparent neurons, and thus genetic delivery was the cause of this cellular transformation.
Further genetic modifications were able to convert these neurons into specialized neurons, most notably those which transmit the nerve cell communication molecule dopamine (lost in Parkinson's disease). However, success was very limited at this point in the scientists' research.
Cellular conversion efficiency.
How efficient was the cellular transformation? Not all of the fibroblasts were converted into functional neurons.
The overall conversion efficiency was 16.3%±4.3%. Over 95% of these converted cells possessed a biochemical marker indicative of neurons (the cytoskeletal protein MAP2), and over 90% of them possessed the protein synaptophysin, a protein important to nerve cell communication.
The cells were immature until one month after transformation. However, at this time, 90% of the converted cells were able to transmit action potentials (nerve cell electrical impulses); further indicators of neuron identity were also observed.
How permanent is the cellular transformation?
A possible limitation that might immediately come to mind is related to the genetic conversion mechanism. Doxycycline is required to induce the original genetic reprogramming, and it was not clear whether continual cycling of this molecule was required for the cells to continue behaving as neurons.
The scientists found that doxycycline was required for the first three days. After that, their cells behaved as neurons even without doxycycline, indicating that genetic transformation into neurons is permanent by this point (all three virally-delivered genes are required).
Future directions.
The scientists found that prenatal fibroblasts were not required. Foreskin cells (which are readily available and already in medical use) were also successfully converted into neurons, but at a lower efficiency (4.3%±1.1%).
The avoidance of highly proliferative stem cell intermediates in this research may reduce the possibility of tumor formation, but it also limits the number of neurons that can be produced. The scientists intend to find the optimal cells for a medically-sufficient conversion efficiency, and to more rigorously investigate the safety of their genetically reprogrammed cells.
NOTE: The scientists' research was funded by the European Community's 7th Framework Programme, the Swedish Research Council, Bagadilico, Stem-Therapy, the Crafoord Foundation, the Swedish Parkinson Foundation, the Jeansson Foundation, the Lundbeck Foundation, the M. Lundqvist Foundation, the Knut and Alice Wallenberg Foundation, and the Human Frontiers Science Program.
Ulrich Pfisterer, Agnete Kirkeby, Olof Torper, James Wood, Jenny Nelander, Audrey Dufour, Anders Björklund, Olle Lindvall, Johan Jakobsson, & Malin Parmar (2011). Direct conversion of human fibroblasts to dopaminergic neurons Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1105135108