Deepak Srivastava, director of the UCSF-affiliated Gladstone Institute of Cardiovascular Disease, stands with George Daley, a leading stem cell scientist from Harvard Medical School who presented a seminar talk at UCSF Mission Bay.
Research focused on turning adult cells into potent stem cells capable of morphing into many different tissues -- nerves, heart or liver -- has been viewed by some as a way to circumvent a need for embryonic stem cells.
However, there has been recent evidence of growing pains for this embryonic stem cell alternative, and a bit of bad press.
The cells in question hit the limelight as soon as they were created. In 2006, Shinya Yamanaka, MD, found a way to coax mature skin cells from mice into becoming potent stem cells. A year later he accomplished the same with human cells. The technique is viewed as a unique way to study the behavior of cells in inborn and chronic diseases. It also is seen as the key to developing personalized cell therapies, derived from the cells of each individual patient.
Although the initial derivation of these so-called induced pluripotent stem (iPS) cells from adult cells by Yamanaka – now a UCSF faculty member and recipient of the 2009 Lasker Award for stem cell research -- occurred just a few years ago, advances and variations in techniques used to generate iPS cells have rapidly proliferated. Hundreds of labs have leaped into the burgeoning field.
To generate an iPS cell, researchers manipulate a specialized adult cell, transforming it back to an unspecialized state. The ideal stem cell is like a blank slate. It is capable of becoming any other tissue, and is immortal.
Just in the past few weeks, however, the potential of iPS cells has been questioned and even called “hyped,” in light of research reports showing that iPS cells differ from embryonic stem cells and suffer from the comparison.
Poor Growth and Survival
According to stem cell scientist Robert Lanza, MD, nerve cells and blood cells grown from iPS cells grow and survive poorly -- and age quickly -- in comparison to embryonic stem cells. Lanza said iPS cells cannot now be considered a practical choice for stem cell therapy. Lanza and colleagues published their findings in February in the journal Stem Cells.
“Before clinical application, it will be necessary to determine the cause and extent of such abnormalities, and whether they also occur in stem cells generated using different reprogramming methods,” Lanza stated in a press release issued by Advanced Cell Technology, the company where he serves as chief scientific officer.
Similarly, another research report published in the February 16 issue of the Proceedings of the National Academy of Sciences by a team led by University of Wisconsin researcher Su-Chun Zhang, MD, PhD, also found that a variety of iPS cell lines were less efficient and reliable in generating nerve cells.
But emphasizing iPS shortcomings and talking of hype seems a bit like attacking a straw man. UCSF researchers, for instance -- who are keen to exploit the potential of both iPS and embryonic stem cells – are not making claims that iPS cells are identical to embryonic stem cells.
In fact, Arnold Kriegstein, MD, PhD, director of the Eli and Edythe Broad Center for Regeneration Medicine and Stem Cell Research at UCSF, has referred to embryonic stem cells as the “gold standard” to which iPS cells should be compared through continuing research.
Researchers are well aware of ongoing challenges. They remain hopeful about the promise of iPS cells, which are thought to have greater potential for giving rise to cells that can be used to model specific diseases.
According to stem cell researcher Deepak Srivastava, MD, director of the UCSF-affiliated Gladstone Institute of Cardiovascular Disease, “iPS cells will face many of the same hurdles as embryonic stem cells for therapeutic use, although most ethical issues and concerns about immune rejection will be obviated. … However, for disease-modeling, the technology can be used now; the utility will be dependent upon the reproducibility of the disease phenotype that can be generated in cells derived from iPS cells.”
George Daley, MD, PhD, a leading stem cell researcher from Harvard Medical School, said, “One of the major questions that really can’t be addressed by using most embryonic stem cells are questions that relate to disease modeling. An embryonic stem cell is a generic cell. It allows you to do research on generic questions, but it’s the disease oriented research and ultimately the disease-oriented therapy that requires customized cells.”
Highlighting Differences Among Stem Cells
Daley spoke about his own recent research comparing iPS cell lines to each other and to embryonic stem cell lines when he visited the UCSF Mission Bay campus in January. Daley, introduced by Yamanaka, presented a seminar talk at the J. David Gladstone Institutes.
During the seminar Daley said that iPS cells and embryonic stem cells appear to be “functionally the same in a generic way.” But he also presented data from his own lab group’s experiments showing that there actually are significant differences. In other words, many iPS cell lines are not really blank slates like embryonic stem cells.
Daley found that stem cells from iPS cell lines were better able to generate specialized cells more similar to the cells from which they were derived. Stem cells made by reprogramming blood cells were better able to form types of blood cells than types of nerve cells, whereas iPS cells made by reprogramming nerve cells were better at regenerating nerve tissue. The iPS cells became more potent in generating different cell types when treated with certain chemicals, Daley said.
To induce an adult cell to become a stem cell, many genes that are active must be silenced, and many genes that are silent must become active. Yamanaka originally found that this reprogramming could be set in motion by using a virus to insert just four genes into the cell and by forcing these genes to become active, or “expressed.”
Daley, by one overall measure of gene activation - the presence of messenger RNA that encodes proteins - found that patterns of gene activation were similar among stem cell lines. However, many individual genes were activated or silenced differently between the different cell lines.
During development it’s clear that factors beyond DNA affect which genes are turned on and off. Daley identified some of these “epigenetic” differences between stem cell lines.
Daley analyzed production of a type of RNA that does not encode proteins. Some of these RNAs continued to be produced in the iPS cell lines in a manner similar to the adult cell types from which the iPS cells were derived, such as the connective-tissue cells called fibroblasts. These iPS cells “represent a sort of residual effect of the fibroblast gene expression program, and in some sense an epigenetic memory,” he said.
Others RNAs seemed to become especially abundant in the different iPS cell lines due to the way the cells are derived from adult cells. “This set seems to be dramatically over-expressed in iPS cells,” Daley said. “Their over-expression appears to be selected for in the context of reprogramming.”
Daley’s group and other research teams also have found differences between iPS cells and embryonic stem cells in another epigenetic marker, called DNA methylation. Methyl groups are chemical tags that often are appended to or removed from particular DNA sequences depending on cell type and developmental state. Methyl groups often serve to prevent the activation of specific genes. In general, DNA in iPS cells is more methylated, Daley said.
In Search of Consistency
“In a practical sense, when isolating iPS cells from various [cell] populations of the adult -- depending on the derivation method, the tissue of origin and donor age -- you are going to see consistent differences,” Daley said.
“We need to come up with strategies to attempt to get iPS cells back to a more uniform, embryonic-stem-cell-like state.
“As the field is emerging and lots of laboratories are jumping in to work on iPS cells, we have to adhere to strict criteria for documentation, so we know we are working with faithfully reprogrammed cells.”
Despite the variability in iPS cell lines, they still hold great promise, said UCSF stem cell researcher Robert Blelloch, MD, PhD, who attended Daley’s talk. He noted that researchers already have succeeded in growing mice from iPS cells.
“I still believe in them,” Blelloch said. “There are going to be setbacks here and there, but overall the field is advancing at a remarkable pace.”
Photo by Chris Goodfellow
Still No Truce in the Stem-Cell Wars
Newsweek, February 10, 2010
Stem Cell Alternatives Show Early Aging Abnormalities
USA Today, February 12, 2010
Induced stem cells don't meet “gold standard”
The Cap Times (Madison, WI), February 15, 2010