体細胞から iPS 細胞への再プログラミング

The four transcription factors are Oct4, Sox2, Klf4, and c-Myc, collectively known as Yamanaka factors. These genes are introduced into somatic cells using viral vectors and, when expressed, alter the cell's gene expression pattern to activate cell growth, change metabolism, and remodel the cytoskeleton. Together, they regulate over 300 genes required for pluripotency.

c-Myc activates genes that promote cell proliferation and reorganizes chromatin structure to allow Oct4, Sox2, and Klf4 to bind and regulate genes required for pluripotency. This chromatin reorganization is essential for enabling the other transcription factors to access and control the genes necessary for transforming somatic cells into pluripotent cells.

Klf4 forms a complex with Oct4 and Sox2 to activate Nanog, a transcription factor required for self-renewal. Additionally, Klf4 represses genes involved in cell senescence, further maintaining the pluripotent state. This dual function makes Klf4 critical for establishing and sustaining pluripotency in reprogrammed cells.

During initiation, genes specific to the somatic cell are downregulated while genes involved in proliferation are upregulated, and telomerase is reactivated. Cells like fibroblasts undergo mesenchymal to epithelial transition, acquiring apical-basal polarity and expressing epithelial markers such as cadherin and tight junctions.

Reprogramming cells shift from oxidative phosphorylation to preferentially using glycolysis for ATP generation. This metabolic change occurs because reprogramming factors transform elongated mitochondria into spherical ones with very few cristae, fundamentally altering the cell's energy production machinery and supporting pluripotency establishment.

Less than 1% of cells become pluripotent after reprogramming. Efficiency can be increased by altering chromatin structure, repressing proteins like p53 that promote cell senescence, and suppressing signaling pathways or enzymes that act as barriers to reprogramming. These modifications help overcome natural resistance to pluripotency induction.

The reprogramming process alters the expression of approximately 1,500 genes across multiple phases. Oct4, Sox2, and Nanog, the core pluripotency factors, regulate over 300 of these genes, including those responsible for metabolic changes and cytoskeletal reorganization that transform somatic cells into induced pluripotent stem cells.