42.7: Methods of Nuclear Reprogramming
Nuclear reprogramming is a process of transforming one cell type into an unrelated cell type by epigenetic changes that alter the cell’s original gene expression pattern. Such epigenetic changes force cells to express a different set of genes, which play a significant role in inducing transformation into other cell types. Nuclear reprogramming offers applications in reproductive cloning for livestock propagation and regenerative medicine — developing patient-specific cells for injury repair.
The prerequisite for successful nuclear reprogramming involves the efficient uncoiling of the genomic DNA inside the nucleus that allows access to regulatory proteins. This is achieved through chromatin decondensation and histone modification that includes acetylation, methylation, and phosphorylation. In somatic cell nuclear transfer (SCNT), the histone modification pattern in the chromatin of a transplanted nucleus changes to that of the oocyte. These changes are enforced by transcription factors present in the oocyte cytoplasm. For example, oocyte-specific B4 or H1foo histone linkers replace the H1 histone and promote changes in gene expression patterns.
In 2006, Shinya Yamanaka discovered the Oct-4, Sox-2, Klf-4, and c-Myc transcription factors, referred to as Yamanaka factors, that are highly expressed in embryonic stem (ES) cells. The transcription factor transduction method reprograms the nucleus by overexpressing these transcription factors, thus inducing embryonic gene expression in somatic cells. These transformed cells are called induced pluripotent stem cells or iPSCs. The Oct-4 (octamer-binding transcription factor 4) is a protein encoded by the POU5F1 human gene. It plays a vital role in deciding the fate of the inner mass and embryonic stem cells and helps them to maintain pluripotency during embryonic development. The sex-determining region Y-box 2, referred to as Sox-2, plays an essential role in maintaining the self-renewal potential in ES cells. Klf-4 or Kruppel-like factor 4 belongs to the KLF family of zinc finger transcription factors and is majorly involved in the proliferation and differentiation of stem cells. c-Myc is the transcription factor belonging to the Myc family of protooncogenes, and it has an important role in cellular proliferation and metabolism.