Stem Cells and iPSCs

Study Notes

iPSCs (induced pluripotent stem cells) are a type of pluripotent stem cell artificially derived from a non-pluripotent cell, typically an adult somatic cell, by inducing a "forced" expression of certain genes.
First produced in 2006 from mouse cells and in 2007 from human cells.

Group 1: ES cell-specific transcription factors, essential for pluripotency in ES cells and early embryos, including Oct3/4, Sox2, and Nanog.
Group 2: Proto-oncogenes, important for proliferation of ES cells, but not in early embryos, including TCLI, Stat3, c-Myc, and ERas.
Group 3: Less famous genes, specifically expressed in ES cells, but with less defined functions, including ECATI, Esg1, and Fbx15.

Oct3/4: involved in the maintenance of self-renewal of pluripotent cells, repression leads to formation of trophoectoderm, and overexpression leads to formation of various lineages, including primitive endoderm.
Sox2: essential for embryonic development, downregulation leads to differentiation of cells in murine ES cells.
KIf4: represses p53 directly, contributes to activation of Nanog and other ES cell-specific genes, acts as an inhibitor of c-Myc-induced apoptosis, and activates p21CIP, thereby suppressing cell proliferation.
Nanog: necessary for promoting pluripotency in embryonic stem cells, along with Oct-3/4 and Sox2.
LIN28: an mRNA binding protein expressed in embryonic stem cells and embryonic carcinoma cells, associated with differentiation and proliferation.

Typically derived by transfecting certain stem cell-associated genes into non-pluripotent cells, such as adult fibroblasts, using viral vectors, such as retroviruses.
Small numbers of transfected cells become morphologically and biochemically similar to pluripotent stem cells, and are isolated through morphological selection, doubling time, or through a reporter gene and antibiotic selection.

First generated by Shinya Yamanaka's team at Kyoto University, Japan in 2006, using four key pluripotency genes: Oct-3/4, Sox2, c-Myc, and KIf4.
Retroviruses were used to transfect mouse fibroblasts, and cells were isolated by antibiotic selection of Fbx15+ cells.
Limitations: DNA methylation errors compared to original patterns in ESC lines and failed to produce viable chimeras if injected into developing embryos.

Generated in June 2007, by the same group, using the same four endogenous pluripotent factors, but with Nanog instead of Fbx15.
DNA methylation patterns and producing viable chimeras indicated that Nanog is a major determinant of cellular pluripotency.
Limitations: one of the four genes used (c-Myc) is oncogenic, and 20% of the chimeric mice developed tumors.