Accelerating Bone Regeneration with Combination Gene Therapy and Novel Scaffolds

A truly remarkable paper in the journal Advanced Healthcare Materials by Fergal J. O’Brien and his co-workers from the Tissue Engineering Research Group at the Royal College of Surgeons in Dublin, Ireland has examined a unique way to greatly speed up bone regeneration.

Mesenchymal stem cells from bone marrow (other locations as well) can differentiate into bone-making cells (osteoblasts) that will make architecturally normal bone under particular conditions. The use of mesenchymal stem cells and a variety of manufactured biomaterial matrices and administered growth factors enhance bone formation by mesenchymal stem cells (M. Noelle Knight and Kurt D. Hankenson, Adv Wound Care 2013; 2(6): 306–316; also see Marx RE, Harrell DB. Int J Oral Maxillofac Implants 2014 29(2)e201-9; and Kaigler D, et al., Cell Transplant 2013;22(5):767-77).

Protein growth factors tend to have rather short half-lives when applied to growth scaffolds. A better way to apply growth factors is to use the genes for these growth factors and apply them to “gene activated scaffolds.” Gene-activated scaffolds consist of biomaterial scaffolds modified to act as depots for gene delivery while simultaneously offering structural support and a matrix for new tissue deposition. A gene-activated scaffold can therefore induce the body’s own cells to steadily produce specific proteins providing a much more efficient alternative.

In this paper by O’Brien and his groups, the genes for two growth factors, VEGF and BMP2, were applied to a gene-activated scaffold that consisted of collagen-nanohydroxyapatite. VEGF drives the formation of new blood vessels, and this fresh vascularization, coupled with increase bone deposition, which is induced by BMP2, accelerated bone repair.

Mind you, the assays in the paper were conducted in cell culture systems. However, O’Brien and his colleagues implanted these gene-activated scaffolds with their mesenchymal stem cells into rats that had large gaps in their skulls. In this animal model system for bone repair, stem cell-mediated bone production, in addition to increased blood vessel formation accelerated bone repair in these animals. Tissue examinations of the newly-formed bone showed that bone made from gene-activated scaffolds with mesenchymal stem cells embedded in them made thicker, more vascularized bone than the other types of strategies.

This is not a clinical trial, but this preclinical trial shows that vascularization and bone repair by host cells is enhanced by the use of nanohydroxyapatite vectors to deliver a combination of genes, thus markedly enhancing bone healing.

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.

Training Stem Cells to Differentiate Properly

Pluripotent stem cells have the ability to differentiate into a whole host of adult cell types. Unfortunately this ability to differentiate into any adult cell type also comes with it the tendency to form tumors. Controlling stem cell differentiation requires that you give a little “push” in the right direction. What is the nature of that push? It varies from stem cell to stem cell and it also depends on what type of cell you want you stem cells to make. Therefore, pluripotent stem cell differentiation is sometimes a matter of art as much as a matter of science.

A research group at Stanford University School of Medicine have designed an experimental protocol that uses the signals in the body to direct the differentiation of stem cells to a desired end.

Stanford University professor Michael Longaker, who is also the director of the Institute for Stem Cell Biology and Regenerative Medicine at Stanford University, explained it this way: “Before we can use these cells, we have to differentiate or ‘coach,’ them down a specific developmental pathway.” Longaker continued: “But there’s always a question as to exactly how to do that, and how many developmental doors we have to close before we can use the cells. In this study, we found that, with appropriate environmental cues, we could let the body do the work.”

Allowing the patient’s body to direct differentiation of pluripotent stem cells could potentially speed approval of stem cell-based treatments by the US Food and Drug Administration (FDA). If Longeker’s protocol pans out, it could eliminate long period of extended laboratory manipulation in order to force stem cells to differentiate into the desired cell type.

“Once we identify the key proteins and signals coaching the tissue within the body, we can try to mimic then when we use the stem cells,” said Longaker. “Just as the shape of water is determined by its container, cells respond to external cues. For example, in the future, if you want to replace a failing liver, you could put cells in a scaffold or microenvironment that strongly promotes liver cell differentiation and place the cell-seeded scaffold into the liver to let them differentiate in the optimal macroenvironment.”

Longaker does not work on liver, but bone. As a pedatric plastic and reconstructive surgeon who specializes in craniofacial malformations, finding ways to coax pluripotent stem cells to make bone is his research Holy Grail. “Imagin being able to treat children and adults who require craniofacial skeletal reconstruction, not with surgery, but with stem cells,” opined Longaker.

In this experiment, Longaker and his colleagues removed a four-millimeter circle of bone taken from the skulls of anesthetized mice and implanted a tiny, artificial scaffold coated with a bone-promoting protein called BMP-2 (bone morphogen protein-2) that was seeded with one million human pluripotent stem cells.

According to Longaker, these implants formed bone and repaired the defect in the skulls of the mice even the original stem cells were not differentiated when added to the wound. These human stem cells made human bone that was then replaced by mouse bone as time progressed. This shows that the repair was physiologically normal.

This bone growth was stimulated by the presence of BMP-2 and the microenvironment that induced the stem cells to differentiate into bone-making cells that made normal bone.

In this experiment, Longaker and his group used human embryonic stem cells and induced pluripotent stem cells and both stem types seemed to work equally well at repairing the skull defect.

Teratomas (tumors made by pluripotent stem cells) were observed but only rarely (two of the 42 animals that received the stem cell implants developed tumors). Interestingly, the few teratomas that formed developed in two laboratory animals that received embryonic stem cell implants and not induced pluripotent stem cell implants. This is surprising, since most stem cells researchers consider induced pluripotent stem cells to be more tumorigenic than embryonic stem cells. Standard tests of these stem cells (implantation under the kidneys of immunodeficient mice) showed that they did produce teratomas under these conditions.

Longaker commented: “We still have work to do to completely eliminate teratoma formation, but we are highly encouraged.” Longaker also thinks that by combining this technique with other strategies, he and his group might be able to completely prevent teratoma formation. For example, including other cell types that can act as shepherds for the stem cells as they differentiate into the desired cell type can also increase differentiation into a desired cell type.

Longaker said, “I want to see how broadly applicable this technique may be.” He was referring to tissues that do not heal well. For example, cartilage heals very poorly if at all. Longaker wonders if you could “add some cells that can form replacement tissue in this macroenvironment while you’re already looking at the joint.”