The use of small molecules in somatic-cell reprogramming

Abstract

Pioneering work over the past years has highlighted the remarkable ability of manipulating cell states through exogenous, mostly transcription factor-induced reprogramming. The use of small molecules and reprogramming by transcription factors share a common history starting with the early AZA and MyoD experiments in fibroblast cells. Recent work shows that a combination of small molecules can replace all of the reprogramming factors and many previous studies have demonstrated their use in enhancing efficiencies or replacing individual factors. Here we provide a brief introduction to reprogramming followed by a detailed review of the major classes of small molecules that have been used to date and what future opportunities can be expected from these.

Keywords: chemical biology; epigenetics; induced pluripotent stem cells; reprogramming; small molecules.

Figure 1

Dynamics of selected molecular events during reprogramming. The reprogramming of somatic cells occurs in two general phases, the first being transgene dependent wherein removal of exogenous expression of OSK(M) results in the reversion to a differentiated state. During the second phase, removal of exogenous transgene expression no longer prevents the final transition to pluripotency. (A) A set of morphological changes are notable during the transgene-dependent phase, as early reprogramming cells divide rapidly, becoming round and beginning to form clusters. Widespread apoptosis is seen following this expansion, and eventually the cells form compact colonies of fully reprogrammed iPSCs [79]. (B) Many of the early changes in transcription result from a c-Myc driven effect. This is followed by the mesenchymal to epithelial transition seen at an intermediate phase in reprogramming, ultimately leading to the silencing of transgenes and the permanent re-activation of the core pluripotency network. (C) The initial binding locations of the OSKM factors are defined by the chromatin landscape of the somatic cell. Early in reprogramming, widespread changes in histone modifications are seen, followed by a general loss of repressive histone and DNA modifications. In fully reprogrammed cells, bivalent domains are re-established at loci important for development as seen in ESCs. Chromatin remodeling continues during the transgene-independent phase with X-chromosome reactivation and telomere elongation. Abbreviations: ESCs, embryonic stem cells; iPSCs, induced pluripotent stem cells; OSKM, Oct4, Sox2, Klf4, and c-Myc.

Figure 2

Simplified schematic of key signaling pathways (Wnt/Gsk3β, TGFβ, and MEK) that have been targeted by small molecule inhibitors to enhance the reprogramming of somatic cells to a pluripotent state. Three small molecules and their targets within the pathways are shown.