Human embryonic stem cells (hESCs) may take a back seat to human somatic progenitor cells for regenerative medicine applications. Scientists are becoming more adept at reprogramming committed stem cells back to pluripotence and then nudging them toward development as the desired specialized cell type. In practice, such cells could be expanded in cell culture and then used to repair or replace damaged tissues in the body.
The use of somatic progenitor cells could potentially avoid issues that arise with hESCs, including the need for immunosuppressive drugs as well as the risk of neoplasia and phenotypic or genomic instability. Scientists recently showed that somatic progenitor cells can differentiate in animals to produce functional improvement without unwanted effects.
Stem cells are pluripotent, undifferentiated cells capable of self-renewal through numerous cycles of cell division or proliferation. Stem cells differentiate into progenitor cells, which are more developmentally committed to a cell line than stem cells but relatively undifferentiated compared to cells that have matured into specialized tissue cells. A significant difference between stem cells and progenitor cells is that stem cells can replicate indefinitely, whereas progenitor cells can only divide a limited number of times.
While the field has been moving forward at warp speed, scientists counsel caution despite the tremendous potential this technology has to circumvent many of the issues around hESCs. Some scientists noted that the acquisition of immortality is a key, rate-limiting step toward the establishment of a pluripotent state in somatic cells. This likely underscores the similarities between induced pluripotency and tumorigenicity.
Scientists also point out that reprogramming somatic cells through expression of pluripotency factors and oncogenes has occurred at a relatively low frequency. Low recovery of iPSCs and the tendency to induce malignant transformation compromise, they say, the clinical utility of the approach.
Induced Conditional Self-Renewing Progenitor Cells
A new approach may allow development and expansion of iSPCs without the risk of runaway replication. Evan Snyder, M.D., Ph.D., of Sanford-Burnham Research Institute in La Jolla, and colleagues at several other institutions may have developed a method to conditionally control the capacity for sustained self-renewal. Details are reported in the January issue of PNAS.
Dr. Snyder told GEN that his technique may solve the practical problem of getting enough of these tissue-specific progenitor cells to treat diseases. The approach may also reduce time, effort, the number of genes needed, and the risks of neoplasia and developing unwanted cell types, he added.
Dr. Snyder’s team, with the addition of a single gene, the v-myc gene, successfully instructed neural progenitor cells to self-renew in a laboratory dish. Once they had a sufficient number of cells, the researchers moved the cells into a rodent stroke model, where the cells stopped proliferating, started differentiating, and improved brain function.
According to Dr. Snyder, the team started by questioning the premise of whether an end-committed somatic cell from a so-called irrelevant tissue is necessarily the best initiator cell for regenerative therapy in a particular organ. They asked whether reprogramming all the way back to pluripotence may involve more reverse steps than necessary.
The scientists concluded that it would make more sense to start with cells firmly within the neural lineage, endow them with enough self renewal to allow expansion in cell culture while delaying terminal differentiation, then reverse the steps once they had enough cells to implant for disease treatment.
“While it’s cool to take a skin cell and de-differentiate it all the way back, we said why don’t we start with a cell we don’t have to push all the way back, just get it to a point where it’s self-renewing, but not pluripotent, and sustainable in the target organ,” Dr. Snyder explained.
To test this approach they began with telecephalon cells from the brains of human fetal cadavers that were between 11 to 14 weeks of gestational age. From these they developed cell lines transfected with an inducible oncogene v-myc under the control of doxycycline, a form of tetracycline.
When grown in tissue culture without tetracycline, the cells did not express myc, began to develop a nerve cell like morphology, and only a few expressed the neural stem cell/progenitor cell marker, nestin. When tetracycline was added the cells began to express myc as well as proliferate, and nearly all of them expressed nestin.
“Take your favorite oncogene like myc in a regulated format—in this case, a tetracycline-inducible form—to endow the cells with self renewal, induce its expression in culture, then turn the gene off,” Dr. Snyder explained. He and his team dubbed the cells induced conditional self-renewing progenitor cells, or ICSPCs.
When transplanted into a rodent model of intracerebral hemorrhagic stroke, the rats showed improvement in other measures of brain recovery and function, including behavioral performance. The cells did not grow uncontrollably.
Additionally, absent the inducer doxycylcine, v-myc RNA and protein were undetectable, meaning the expression of the oncogene was completely shut off. The scientists also did not use immunosuppressive drugs to enhance the survival of the implanted cells because they wished to determine how well the cells would persist, differentiate, and impact function without the side effects and risks of such drugs.
Dr. Snyder emphasized that they do not claim functional improvement in adult rats with stroke as a result of the cell replacement but instead that there is likely a chaperone effect in which the progenitor cells provided trophic and neuroprotective support to injured cells.
Nonetheless, this strategy of inducing conditional self renewal in lineage-bounded progenitor cells—in this case, neuronal progenitor cells—may provide a safer, more broadly applicable, and more practical method for studying the conditions under which progenitor cells can be produced and expanded for research, and potential therapeutic use. It remains to be seen whether the strategy can be extended to progenitor cells from other tissues.