Weissman Laboratory Define Roadmap for Pluripotent Human Stem Cell Differentiation into Mesodermal Fates: Cells Rapidly Generate Bone, Heart Muscle

How do we get stem cells to differentiate into the cell types we want? Implanting undifferentiated stem cells into a living organism can sometimes result in cells that differentiate into unwanted cell types. Such a phenomenon is called heterotropic differentiation and it is a genuine concern of regenerative medicine. What is a clinical researcher to do? Answer: make a road map of the events that drive cells to differentiate into specific cell types and their respective precursors.

Researchers in the laboratory of Irving Weissman at Stanford University Researchers at the Stanford University School of Medicine have mapped out the bifurcating lineage choices that lead from pluripotency to 12 human mesodermal lineages, including bone, muscle, and heart. The experiments also defined the sets of biological and chemical signals necessary to quickly and efficiently direct pluripotent stem cells to differentiate into pure populations of any of 12 cell types. This is certainly a remarkable paper in many aspects, since Weissman and his group defined the extrinsic signals that control each binary lineage decision that occur during stem cell differentiation. This knowledge enables any lab to successfully block differentiation toward unwanted cell fates and rapidly steer pluripotent stem cells toward largely pure human mesodermal lineages at most of these differentiation branchpoints.

The ability to make pure populations of these cells within days rather than the weeks or months is one of the Holy Grails of regenerative medicine. Such abilities can, potentially, allow researchers and clinicians to make new beating heart cells to repair damage after a heart attack, or cartilage for osteoarthritic knees or hips, or bone to reinvigorate broken bones or malfunctioning joints, or heal from accidental or surgical trauma.

The Weissman study also highlights those key, but short-lived, patterns of gene expression that occur during human early embryonic segmentation. By mapping stepwise chromatin and single-cell gene expression changes during the somite segmentation stage of mesodermal development, the Weissman group discovered a previously unobservable human embryonic event transiently marked by expression of the HOPX gene. It turns out that these decisions made during human development rely on processes that are evolutionarily conserved among many animals. These insights may also lead to a better understanding of how congenital defects occur.

“Regenerative medicine relies on the ability to turn pluripotent human stem cells into specialized tissue stem cells that can engraft and function in patients,” said Irving Weissman of Stanford. “It took us years to be able to isolate blood-forming and brain-forming stem cells. Here we used our knowledge of the developmental biology of many other animal models to provide the positive and negative signaling factors to guide the developmental choices of these tissue and organ stem cells. Within five to nine days we can generate virtually all the pure cell populations that we need.”

All in all, this roadmap enables scientists to navigate mesodermal development to produce transplantable, human tissue progenitors, and uncover developmental processes.

This paper was published in the journal Cell: Irving L. Weissman et al., “Mapping the Pairwise Choices Leading from Pluripotency to Human Bone, Heart, and Other Mesoderm Cell Types,” Cell, July 2016 DOI: 10.1016/j.cell.2016.06.011.

Differentiation of Pluripotent Stem Cells for Skeletal Regeneration

Bone injuries and bone diseases sometimes require bone grafts for proper treatment. In order to find bone for implantation, orthopedic surgeons often take bone from other locations in the body, use bone from cadavers or synthetic compounds that promote the formation of new bone. Bone grafting is a complex surgical procedure and even though it can replace missing bone, it poses a significant health risk to the patient, and sometimes completely fails to foster proper healing.

Bone has the ability to regenerate, but it requires very small fracture space or some sort of scaffold in order to make new bone. Bone grafts can provide that scaffold. A bone graft can be “autologous,” which simply means that the bone is harvested from the patient’s own body (often from the iliac crest), or the graft can be an allograft, which consists of cadaveric bone usually obtained from a bone bank. Finally, synthetic bone grafts are made from hydroxyapatite or some other naturally occurring, biocompatible substance such as Bioglass, tricalcium phosphate, or calcium sulfate.

Making natural bone from stem cells is one of the goals of regenerative medicine, and work from Irving I. Weissman at Stanford University has shown that this hope is certainly feasible.

Weissman and his colleagues evaluated the ability of embryonic stem cells and induced pluripotent stem cells to form bone in a culture environment known to induce bone formation in most circumstances. This culture system (known as an osteogenic microniche) consisted of a scaffold made of poly – L-lactate coated with hydroxyapatite and stuffed with a growth factor called bone morphogen protein-2 (BMP-2). BMP-2 is a known inducer of bone formation and this scaffold is placed inside the bone of a laboratory animal that has suffered a fracture.

After implanting pluripotent stem cells into these osteogenic microniches, they were very pleasantly surprised to find that both embryonic stem cells and induced pluripotent stem cells embedded themselves into the scaffold and differentiated into bone making cells (osteoblasts). They also made new bone and did so without forming any tumors.

These results suggest that local signals from the implanted scaffold and the genera environment within the bone directed the cells to survive and differentiate into osteoblasts. Thus pluripotent stem cells may have the clinical capacity to regenerate bone, which would, potentially preclude the need for risky bone grafting procedures.

See Levi, B. et al., In vivo directed differentiation of pluripotent stem cells for skeletal regeneration. PNAS November 20, 2012, doi:10.1073/pnas.1218052109.