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.”

BMP-2 Release By Synthetic Coacervates Improves Bone Making Ability of Muscle Stem Cells

Johnny Huard and his co-workers from the McGowan Institute for Regenerative Medicine at the University of Pittsburgh have isolated a slowly-adherent stem cell population from skeletal muscle called muscle-derived stem cells or MDSCs (see Deasy et al Blood Cells Mol Dis 2001 27: 924-933). These stem cells can form bone and cartilage tissue in culture when induced properly, but more importantly when MDSCs are engineered to express the growth factor Bone Morphogen Protein-2 (BMP-2), they make better bone and do a better job of healing bone lesions than other engineered muscle-derived cells (Gates et al., J Am Acad Orthop Surg 2008 16: 68-76).

In most experiments, MDSCs are infected with genetically engineered viruses to deliver the BMP-2 genes, but the use of viruses is not preferred if such a technique is to come to the clinic. Viruses elicit and immune response and can also introduce mutations into stem cells. Therefore a new way to introduce BMP-2 into stem cells is preferable.

To that end, Huard and his colleagues devised an ingenious technique to feed BMP-2 to implanted MDSCs without using viruses. They utilized a particle composed of heparin (a component of blood vessels) and a synthetic molecule called poly(ethylene arginylaspartate diglyceride), which is mercifully abbreviated PEAD. The PEAD-heparin delivery system formed a so-called “coacervate,” which is a tiny spherical droplet that is held together by internal forces and composed of organic molecules. These PEAD-heparin coacervates could be loaded with BMP-2 protein and they released slowly and steadily to provide the proper stimulus to the MDSCs to form bone.

When tested in culture dishes, the BMP-2-loaded coacervates more than tripled the amount of bone made by the MDSCs, but when they were implanted in living rodents the presence of the BMP-2-loaded coacervates quadrupled the amount of bone made by the MDSCs.

This technique provides a way to continuously deliver BMP-2 to MDSCs without using viral vectors to infect them. These carriers do inhibit the growth or function of the MDSCs and activate their production of bone.

This paper used a “heterotropic bone formation assay” which is to say that cells were injected into the middle of muscle and they formed ectopic bone. The real test is to see if these cells can repair actual bone lesions with this system.