Progress in the reprogramming of somatic cells

Abstract

Pluripotent stem cells can differentiate into nearly all types of cells in the body. This unique potential provides significant promise for cell-based therapies to restore tissues or organs destroyed by injuries, degenerative diseases, aging, or cancer. The discovery of induced pluripotent stem cell (iPSC) technology offers a possible strategy to generate patient-specific pluripotent stem cells. However, because of concerns about the specificity, efficiency, kinetics, and safety of iPSC reprogramming, improvements or fundamental changes in this process are required before their effective clinical use. A chemical approach is regarded as a promising strategy to improve and change the iPSC process. Dozens of small molecules have been identified that can functionally replace reprogramming factors and significantly improve iPSC reprogramming. In addition to the prospect of deriving patient-specific tissues and organs from iPSCs, another attractive strategy for regenerative medicine is transdifferentiation-the direct conversion of one somatic cell type to another. Recent studies revealed a new paradigm of transdifferentiation: using transcription factors used in iPSC generation to induce transdifferentiation or called iPSC transcription factor-based transdifferentiation. This type of transdifferentiation not only reveals and uses the developmentally plastic intermediates generated during iPSC reprogramming but also produces a wide range of cells, including expandable tissue-specific precursor cells. Here, we review recent progress of small molecule approaches in the generation of iPSCs. In addition, we summarize the new concept of iPSC transcription factor-based transdifferentiation and discuss its application in generating various lineage-specific cells, especially cardiovascular cells.

Figure 1. Mechanisms of reprogramming to pluripotency induced by exogenous transcription factors and hypothetical mechanisms of small molecule-mediated reprogramming to pluripotency

A. Somatic cell maintains its identity by transcription factor (TF) mediated activation of lineage specific genes, and TF or heterochromatin mediated silencing of pluripoteny genes. B. In the paradigm of TF-induced reprogramming to pluripotency, exogenous reprogramming TF complexes (e.g., pluripotency TF complexes coming from oocyte in SCNT, pluripotent cell in cell fusion, or ectopically expressed iPSC TFs), which can recognize and bind to specific sequences across the whole genome, interact with other TFs binding to nearby or distal sites and recruit other transcription cofactors (e.g., activators, repressors, and epigenetic enzyme complexes) to co-occupy and epigenetically modify the genome in a sequence-specific manner. As a result, the gene expression profile and epigenetic pattern of somatic cells gradually becomes iPSC specific. C. Unlike TFs, small molecules do not have the exquisite ability of molecular recognition possessed by TFs, and cannot specifically interact with both DNA and other transcription cofactors. Consequently, small molecules would hypothetically target endogenous components (e.g., TFs in purple and epigenetic enzyme complexes in light blue) in somatic cells to indirectly initiate iPSC reprogramming. D. Pluripotent stem cell generated by either a TF or small-molecule approach maintains its pluripotency by TF-mediated activation of pluripoteny genes and repression of lineage specific genes.

Figure 2. A simplified and conceptual paradigm of iPSC TF–based transdifferentiation

Temporally restricted overexpression of iPSC TFs in fibroblasts leads to the rapid generation of epigenetically “activated” cells, which can be further reprogrammed to iPSC by both extended expression of iPSC TFs and culture of iPSC medium, and parallelly can be coaxed by other signals and small molecule inhibitor of pluripotency to “relax” back into various differentiated state(s), ultimately giving rise to somatic cells entirely distinct from the starting population.