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For Making Stem Cells, Viruses more than 'Trojan Horses'

Ashley Yeager

Viruses that carry genes to transform adult cells to stem cell also cause an immune response in the cell that could be key to safer cell-reprogramming methods.

Sometimes the vehicle can be just as important as its passengers. Scientists now think that's the case for making stem cells from already differentiated cells, such as skin cells.

This protein, TLR 3 pathway, could make cell reprogramming from adult cells to stem cells more efficient. Source: Wikimedia Commons.

Scientists put four specific genes in either a virus or cell-permeable protein and use the carriers to transfect a differentiated cell with DNA to coax it into a stem cell. Stanford researcher John Cooke and his colleagues observed that the gene expression a virus carrier causes is different than the gene expression of a protein carrier. They also observed that the virus carrier activates inflammatory genes, such as toll-like receptor 3 (TLR 3).

The results of the study, which appear in the October 26 issue of Cell (1), show that a virus carrying reprogramming genes is more than a Trojan horse moving the genetic material into a cell, said Nazish Sayed, a postdoctoral fellow at Stanford and co-lead author of the new paper. The findings also suggest that a cell's innate immunity plays a role in cell reprogramming.

Understanding that role, and how to manipulate it, could change the way that scientists work with genetic and epigenetic pathways to transform differentiated cells into stem cells or directly to other cell types, said Sayed.

Transforming a differentiated cell, such as a skin cell, back into an undifferentiated form won scientists Shinya Yamanaka and John Gurdon the 2012 Nobel Prize in Physiology or Medicine. The technique is promising for disease treatments, but is still risky because inserting the genes and other DNA from the virus could make the cells cancerous.

Sayed, Cooke, and their colleagues, along with other researchers, have been trying to find alternatives to a virus carrier for the four cell-reprogramming genes. But, the cell-permeable proteins are "exceedingly inefficient," at about a 0.001% success rate, compared to reprogramming with viruses, which has a success rate of 0.1 to 1%, the authors wrote in the paper.

Comparing the gene expression of a virus and a protein carrier, the scientists discovered that the virus did more than ferry the reprogramming genes into the cell. The infection also causes the cell to loosen its chromatin, making the DNA more accessible to revert the cell to an undifferentiated, or pluripotent, state. Based on the results, Cooke's group then paired a protein carrier with virus particles encoding a gene not involved in reprogramming.

The pairing "substantially enhanced" the expression of the targeted reprogramming gene, Sox2, so much so that it matched the pattern of gene expression observed with the virus carrier. The team also noticed that a virus – but not a protein – carrier activated inflammatory genes, including the TLR 3 pathway. The scientist then used a small molecule to activate the TLR 3 pathway while also smuggling in reprogramming genes with a protein carrier. Together, the two boosted the efficiency of a cell's induction of the genes, suggesting that inflammation pathways are important for reprogramming cells, said Sayed.

The discovery could make proteins a better vehicle for carrying reprogramming genes to cells, which could make stem cells safe to use in human therapies. The information could also help scientists manipulate direct cell reprogramming, where a skin cell turns straight into a blood cell. But scientists still need to do more work on the make and model of the vehicles to ensure the reprogrammed cells are safe for use.


1. Lee, J., N. Sayed, A. Hunter, K.F. Au, W.H. Wong, E.S. Mocarski, et al. 2012. Activation of Innate Immunity Is Required for Efficient Nuclear Reprogramming. Cell. 151: 547–558.

Keywords:  stem cells