New Nerve Cells Made from Skin
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| Action Points For the first time, human skin cells have been directly transformed into functional nerve cells, opening a way for cell replacement therapies for neurodegenerative conditions such as Alzheimer's disease, researchers reported. Transduction using a lentivirus vector that carried the transcription-regulating genes Ascl1, Brn2, and Zic1 resulted in conversion of 62% of human fibroblasts into neuronal cells, according to Asa Abeliovich, MD, PhD, of Columbia University in New York City, and colleagues.
Addition of the gene Myt1 to the cell culture increased the conversion rate to 85%, and led to the cellular expression of neuronal markers such as MAP2 and Tau-1. "A major goal in regenerative medicine is the facile generation of human neurons for cell replacement therapeutics or disease modeling," Abeliovich and colleagues wrote in the Aug. 4 online issue of Cell. The battle over the ethics of stem cell use in research led to a search for alternative sources of pluripotent cells -- those that can differentiate into any type of cell. One such technique involves genetic programming of skin cells into an intermediate pluripotent state, from which the cells can subsequently be converted to neuronal cells. However, the process of transforming fibroblasts into this intermediate pluripotent state is inefficient, with only 1% of cells being converted, and the intermediate state itself is associated with DNA alterations and tumorigenesis. So Abeliovich's group sought to skip the intermediate step and transform the cells directly. Once they determined that the fibroblasts had been converted to a neuronal phenotype through expression of genetic markers, they performed electrophysiologic testing and found that the transformed cells demonstrated the sodium, calcium, and potassium channel activity typical of neuronal cells. The neuronal cells also exhibited resting membrane potentials of −67 mV to −32 mV, which is also characteristic. The researchers next transplanted labeled human-derived neuronal cells into the brains of embryonic mice, and demonstrated the integration of the neural cells into the murine neural circuitry by immunostaining using an antibody to a human neural cell adhesion molecule. Finally, "as proof of principle" for the use of these transformed neuronal cells in human neurodegenerative disease, they cultured fibroblast cells from patients with familial Alzheimer's disease carrying mutations in the presenilin (PSEN)-1 or -2 genes. These two genes encode for proteins involved in the γ-secretase enzyme system through which amyloid precursor protein is cleaved to Aβ. The neuropathology of Alzheimer's disease is characterized by the presence of plaques consisting of fragments of this amyloid precursor protein, with an increased ratio of Aβ42 fragments to Aβ40 fragments, the researchers explained. This "amyloid hypothesis" of Alzheimer's disease "proposes that modified cleavage of [amyloid precursor protein] by β-secretase and γ-secretase leads to the generation of a pathogenic Aβ42 fragment," they wrote. In these cell cultures from familial Alzheimer's patients, the researchers observed a marked increase in the Aβ42/Aβ40 ratio compared with cultures from unaffected individuals (P<0.001). Combining all the cell lines from Alzheimer's patients found a significant and amplified increase in the Aβ42/Aβ40 ratio (P<1 ×10−9). In comparison, the Aβ42/Aβ40 ratio in cell lines from unaffected individuals was not increased (P>0.05). These in vitro findings were "consistent with patient brain pathology," in Alzheimer's patients, they observed. The results of these experiments in which fibroblasts can be directly transformed into neuronal cells should provide researchers with an opportunity for in vitro exploration of other abnormalities associated with Alzheimer's disease, such as in synaptic function. And future murine experiments can focus on actually testing the therapeutic effects of integration of the induced human neuronal cells, Abeliovich and colleagues predicted. They noted that a limitation of their work is that thus far they have only analyzed two of the PSEN mutations, and that there are many others that could yield further information. Funding was provided by New York Stem Cell Science. The authors declared no disclosures. | ||
| Primary source: Cell Source reference: Qiang L, et al "Directed conversion of Alzheimer's disease patient skin fibroblasts into functional neurons" Cell 2011; DOI:10.1016/j.cell.2011.07.007. |
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