Rooted in mythology, legends, and folklore, the prospect of re-growing body parts has fascinated humanity since ancient times. One mythical creature from ancient Greece, the nine-headed Hydra, was able to grow two heads in place of one.
And Prometheus, whose liver regenerated overnight, often prompts questions on whether early societies were aware of tissue repair and regeneration.
The field of stem cell biology opened the attractive possibility to use cells that can be obtained from adult organisms to generate any cell type. However, translating basic research advancements into clinical benefits presents challenges at multiple levels.
“In any kind of cell replacement therapy, I believe the integration of the cells made in the laboratory into a patient is going to be essential, and one of the most challenging problems to be solved is to find out how to get laboratory-made cells to integrate into a patient,” says Sir John B. Gurdon, Ph.D., emeritus professor at the University of Cambridge, distinguished group leader in the Wellcome Trust/CRUK Gurdon Institute, and co-recipient of the 2012 Nobel Prize in Physiology or Medicine.
Research efforts in Dr. Gurdon’s group are focusing on understanding cellular and molecular mechanisms that allow cells to reach and maintain the differentiated state, and on unveiling the intricacies of embryonic gene reactivation. While regenerative medicine has the potential to impact virtually every clinical area, its promises are particularly important for diseases that have therapeutic options that are currently limited, ineffective, or nonexistent.
“Clearly, any advances in the field of neurodegeneration would be of maximum importance, but any other field that looks really promising, like the restoration of vision, would greatly improve the lives of very many people,” says Dr. Gurdon.
Advances in stem cell biology not only brought therapeutic promises through innovative approaches, but have also created the need to revisit long-standing concepts.
A New Paradigm
“For years, we have defined bone marrow stem cells in terms of their ability to differentiate along a specific lineage, but we missed the possibility that the bone marrow could be a resource to generate adult tissues,” says Richard A. Lerner, M.D., professor of immunochemistry at The Scripps Research Institute.
Investigators in Dr. Lerner’s lab recently reported the possibility to transdifferentiate CD34+ human myeloid bone marrow stem cells into neural progenitor cells using antibodies, in an approach different from those employed to date. To identify a large number of agonist antibodies that each can bind to a different region of the G-CSF receptor (G-CSFR) molecule, Dr. Lerner and colleagues designed a two-step selection strategy.
First, antibodies derived from a phage combinatorial library, which had been converted to a plasma membrane-binding format, are selected based on their ability to bind a membrane-bound version of G-CSFR. Second, based both on binding and function, selected agonists are enriched for those possessing unusual or rare functions.
This strategy favors the selection of antibodies that are generally not identified in solution and provides the opportunity to explore a large number of agonists, each of them binding to a different region of the receptor. Additional strengths of this approach are the presentation of the receptor in its natural environment, where it assumes physiologically relevant conformations, and its ability to test for direct binding.
Among the antibodies they generated, Dr. Lerner and his colleagues identified one that, unlike the natural ligand G-CSF, was also able to activate CD34+ stem cells and initiate neurogenesis.
“What we found is a special antibody to a known receptor whose activation generates more white blood cells but, in this case, the antibody stimulated differentiation into brain cells,” Dr. Lerner explains.
This revealed a new paradigm, an agonist antibody binding, in a population of identical cells, the same receptor as the ligand, but inducing a distinct cell fate, by biasing signaling to specific downstream pathways. While antibodies that can generate other cell types, such as red blood cells, platelets, or dendritic cells, have previously been described, in each of those examples the respective differentiated cells represented a known potential of that specific cell lineage. “In this case, what we observed is not a known potential, and the broad question that emerges is whether we have missed the overall capacity of the bone marrow,” he says.
One of the key questions is making choices in terms of the many available sources of stem cells and the different approaches that exist to grow and select for them,” says Edward L. Field, COO at Cytomedix.
Historically, the identification of stem cells has relied on surface markers characteristic of each cell type. “What makes Cytomedix unique is that we select stem cell progenitors based on an intracellular marker, the enzyme aldehyde dehydrogenase,” Field says.
A fluorescent aldehyde dehydrogenase substrate allows the prospective isolation of stem and progenitor cells in which this enzyme is upregulated. “We are using this population of ALDH-bright stem cells that we believe are very potent in tissue repair and regeneration,” he explains.
Another distinguishing feature of the Cytomedix technology is that the ALDH-bright stem cells are a heterogeneous population that include hematopoietic, mesenchymal, neural, and endothelial stem cells. The ability of each of these cell types to participate in tissue repair and regeneration in very specific ways promises advantages over other approaches that rely on homogeneous stem cell populations. “Investigators are increasingly finding that it is the complementary activity of these stem cell types, that provides most benefits,” Field says.
The company recently became part of the first Phase II randomized trial exploring the benefits of autologous stem cell therapy in patients with intermittent claudication, an NIH-funded initiative managed by the Cardiovascular Cell Therapy Research Network.