Controlling Transplanted Stem Cells from the Inside Out

Scientists have worked very hard to understand how to control stem cell differentiation.  However, despite how well you direct stem cell behavior in culture, once those stem cells have been transplanted, they will often do as they wish.  Sometimes, transplanted stem cells surprise people.

Several publications describe stem cells that, once transplanted undergo “heterotropic differentiation.” Heterotropic differentiation refers to tissues that form in the wrong place. For example, one lab found that transplantation of mesenchymal stem cells into mouse hearts after a heart attack produced bone (don’t believe me – see Martin Breitbach and others, “Potential risks of bone marrow cell transplantation into infarcted hearts.” Blood 2007 110:1362-1369).  Bone in the heart – that can’t be good. Therefore, new ways to control the differentiation of cells once they have been transplanted are a desirable goal for stem cell research.

From this motivation comes a weird but wonderful paper from Jeffrey Karp and James Ankrum of Brigham and Women’s Hospital and MIT, respectively, that loads stem cells with microparticles that give the transplanted stem cell continuous cues that tell them how to behave over the course of days or weeks as the particles degrade.

“Regardless of where the cell in the body, it’s going to be receiving its cues from the inside,” said Karp. “This is a completely different strategy than the current method of placing cells onto drug-doped microcarriers or scaffolds, which is limiting because the cells need to remain in close proximity to those materials in order to function. Also these types of materials are too large to be infused into the bloodstream.”

Controlling cells in culture is relatively easy. If cells take up the right molecules, they will change their behavior. This level of control, however, is lost after the cell is transplanted. Sometimes implanted cells readily respond to the environment within the body,. but other times, their behavior is erratic and unpredictable. Karp’s strategy, which her called “particle engineering,” corrects this problem by turning cells into pre-programmable units. The internalized particles stably remain inside the transplanted cell and instruct it precisely how to act. It can direct cells to release anti-inflammatory factors, or regenerate lost tissue and heal lesions or wounds.

“Once those particles are internalized into the cells, which can take on the order of 6-24 hours, we can deliver the transplant immediately or even cryopreserve the cells,” said Karp. “When the cells are thawed at the patient’s bedside, they can be administrated and the agents will start to be released inside the cells to control differentiation, immune modulation or matrix production, for example.”

It could take more than a decade for this type of cell therapy to be a common medical practice, but to speed up the pace of this research, Karp published the study to encourage others in the scientific community to apply the technique to their various fields. Karp’s paper also illustrates the range of different cell types that can be controlled by particle engineering, including stem cells, cells of the immune system, and pancreatic cells.

“With this versatile platform, which leveraged Harvard and MIT experts in drug delivery, cell engineering, and biology, we’ve demonstrated the ability to track cells in the body, control stem cell differentiation, and even change the way cells interact with immune cells, said Ankrum, who is a former graduate student in Karp’s laboratory. “We’re excited to see what applications other researchers will imagine using this platform.”

One-Step iPS cells

Three years ago, Kyoto University scientist Shinya Yamanaka made induced pluripotent stem cells (iPSCs) by inserting four genes into skin cells from laboratory mice. To get these genes into cells he used genetically engineered retroviruses, which insert copies of genes directly into the chromosomes of the cells they infect.  In order to convert the skin cells into iPSCs, he had to infect the cells with four different recombinant viruses, and subject the cells to four different insertion events.  Because retroviruses randomly insert into the gene, there is the possibility that they could insert their DNA right in the middle of an active gene.  Thus Yamanaka’s procedure left people worried about making mutations in tumor suppressor genes or other genes necessary for life.  If iPSCs were going to be used for clinical purposed, then how could such a procedure be used in the clinic if it was so potentially dangerous?

Now two labs have shown that you can do the same experiment by inserting one piece of DNA to do the same job. Rudolf Jaenisch’s lab at MIT and Konrad Hochedlinger at Harvard University have combined the four mouse reprogramming genes onto a single piece of DNA, known as a cassette, and inserted it at a single locus in the mouse genome. The mice with the insert were bred, and their somatic cells were transformed into iPS cells following the addition of the antibiotic doxycycline, which triggers the cassette to express the four reprogramming genes. This innovation saves time and money and brings iPSCs one step closer to clinical trials.

One group of experiments that scientists hope to do with these newly made mouse strains is test the ability of iPSCs to form particular cell types versus traditionally made embryonic stem cells (ESCs).  In a study published earlier this year in Cell Stem Cell, hundreds of genes are differentially expressed between iPSCs and ESCs (Chin, M. H. et al. (2009) Cell Stem Cell 5, 111-123).  Another study in Nature showed that iPS cells are not as efficient as embryonic stem cells at differentiating into all cell types (Zhao, X.-Y. et al. (2009) Nature 461, 86-90).