Direct Reprogramming

From Cell Stem Cell
By Stuart P. Atkinson

Direct conversion of one somatic cell to another somatic cell type, completely bypassing the pluripotent stage through the forced expression of lineage specific transcription factors has emerged as a large “splinter group” of research, taking many lessons from induced pluripotent stem cell (iPSC) technology. The direct generation of induced neuronal cells (iN) from human fibroblasts has been previously demonstrated in several papers (Ambasuhan et al, Caiazzo et al, Pang et al, Pfisterer et al, Qiang et al, Son et al and Yoo et al.) however fibroblasts represent a heterogeneous mixture of cells, potentially including cells of the neural crest, and so the reprogrammed cell of origin remains undefined. Therefore, researchers from the group of Marius Wernig at the Stanford University School of Medicine, USA, decided to attempt to identify a specific somatic cell type from one germ lineage and reprogram these cells across the germ layer barrier into iN cells. The study, published as a short article in Cell Stem Cell demonstrates the direct conversion of mouse hepatocytes to iN cells and analyses the reprogramming process, demonstrating the faithful silencing of the hepatocyte expression program and the expression of the neuronal expression program (Marro et al).

From PNAS
By Carla Mellough

It is just 5 years since Takahashi and Yamanaka demonstrated that both embryonic and adult mouse somatic cells could be reprogrammed back to a pluripotent state, creating induced pluripotent stem cells (iPSCs). In the short time since this achievement the equivalent has been demonstrated across multiple human somatic cell types, which has paved the way for a new era of disease modelling and a focus towards the generation of autologous transplantable tissues by differentiating patient-specific iPSCs into mature cell types for cell replacement. The availability of disease- and indeed patient-specific iPSC disease models have enabled rapid advances in other scientific fields, the most notable of which may be genetics and gene therapy. Further to this, not only does the list of reprogrammable somatic cells expand with time, more recent studies have demonstrated that the iPSC stage can be completely bypassed by the direct conversion of somatic cells into other mature somatic cell types. One of these studies, published in January this year, reported that the expression of three neuronal transcription factors (Ascl1, Brn2, and Myt1l) in mouse fibroblasts could efficiently convert these cells into induced neurons (iN). Four months later, a study published in PNAS by Pfisterer et al. demonstrated that, using the same strategy, the same direct lineage conversion is also achievable in human.

From Nature
By Stuart P. Atkinson

Of the most widely used transcription factors in reprogramming experiments, MYC is often considered the “problem child”. Although MYC boosts the efficiency of induced pluripotent stem cell (iPSC) generation, it also increases the tumorigenic nature of the resultant cells thereby presenting clear cautionary implications for clinical use (Nakagawa et al). It is for this reason that studies have attempted to describe protocols for iPSC generation which remove MYC from the reprogramming cocktail or attempt to replace it with alternative reprogramming factors or small-molecules in order to generate “safer” iPSCs. Maekawa et al from the lab of Shinya Yamanaka now show that GLIS1, a GLI-related Kruppel-like zinc finger protein that functions both as an activator and repressor of transcription (Kim et al) and which is enriched in oocytes and 1-cell embryos, can effectively replace MYC in the reprogramming of mouse and human fibroblasts when used alongside OCT4, SOX2 and KLF4 (OSK), resulting in enhanced generation of fully pluripotent iPSCs. This study is published in Nature.