Reprogramming progressive cells display low CAG promoter activity.

There is wide variability in the propensity of somatic cells to reprogram into pluripotency in response to the Yamanaka factors. How to segregate these variabilities to enrich for cells of specific traits that reprogram efficiently remains challenging. Here we report that the variability in reprogramming propensity is associated with the activity of the MKL1/SRF transcription factor and concurs with small cell size as well as rapid cell cycle. Reprogramming progressive cells can be prospectively identified by their low activity of a widely used synthetic promoter, CAG. CAGlow cells arise and expand during cell cycle acceleration in the early reprogramming culture of both mouse and human fibroblasts. Our work illustrates a molecular scenario underlying the distinct reprogramming propensities and demonstrates a convenient practical approach for their enrichment.

Resolving Cell Cycle Speed in One Snapshot with a Live-Cell Fluorescent Reporter.

Cell proliferation changes concomitantly with fate transitions during reprogramming, differentiation, regeneration, and oncogenesis. Methods to resolve cell cycle length heterogeneity in real time are currently lacking. Here, we describe a genetically encoded fluorescent reporter that captures live-cell cycle speed using a single measurement. This reporter is based on the color-changing fluorescent timer (FT) protein, which emits blue fluorescence when newly synthesized before maturing into a red fluorescent protein. We generated a mouse strain expressing an H2B-FT fusion reporter from a universally active locus and demonstrate that faster cycling cells can be distinguished from slower cycling ones on the basis of the intracellular fluorescence ratio between the FT's blue and red states. Using this reporter, we reveal the native cell cycle speed distributions of fresh hematopoietic cells and demonstrate its utility in analyzing cell proliferation in solid tissues. This system is broadly applicable for dissecting functional heterogeneity associated with cell cycle dynamics in complex tissues.

YAP Non-cell-autonomously Promotes Pluripotency Induction in Mouse Cells.

Yes-associated protein (YAP) is known to promote the stemness of multiple stem cell types, including pluripotent stem cells, while also antagonizing pluripotency during early embryogenesis. How YAP accomplishes these distinct functions remains unclear. Here, we report that, depending on the specific cells in which it is expressed, YAP could exhibit opposing effects on pluripotency induction from mouse somatic cells. Specifically, YAP inhibits pluripotency induction cell-autonomously but promotes it non-cell-autonomously. For its non-cell-autonomous role, YAP alters the expression of many secreted and matricellular proteins, including CYR61. YAP's non-cell-autonomous promoting effect could be recapitulated by recombinant CYR61 and abrogated by CYR61 depletion. Thus, we define a YAP-driven effect on enhancing pluripotency induction largely mediated by CYR61. Our work highlights the importance of considering the distinct contributions from heterologous cell types in deciphering cell fate control mechanisms and calls for careful re-examination of the co-existing bystander cells in complex cultures and tissues.

Publisher Correction: MLL-AF9 initiates transformation from fast-proliferating myeloid progenitors.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

MLL-AF9 initiates transformation from fast-proliferating myeloid progenitors.

Cancer is a hyper-proliferative disease. Whether the proliferative state originates from the cell-of-origin or emerges later remains difficult to resolve. By tracking de novo transformation from normal hematopoietic progenitors expressing an acute myeloid leukemia (AML) oncogene MLL-AF9, we reveal that the cell cycle rate heterogeneity among granulocyte-macrophage progenitors (GMPs) determines their probability of transformation. A fast cell cycle intrinsic to these progenitors provide permissiveness for transformation, with the fastest cycling 3% GMPs acquiring malignancy with near certainty. Molecularly, we propose that MLL-AF9 preserves gene expression of the cellular states in which it is expressed. As such, when expressed in the naturally-existing, rapidly-cycling immature myeloid progenitors, this cell state becomes perpetuated, yielding malignancy. In humans, high CCND1 expression predicts worse prognosis for MLL fusion AMLs. Our work elucidates one of the earliest steps toward malignancy and suggests that modifying the cycling state of the cell-of-origin could be a preventative approach against malignancy.

MKL1-actin pathway restricts chromatin accessibility and prevents mature pluripotency activation.

Actin cytoskeleton is well-known for providing structural/mechanical support, but whether and how it regulates chromatin and cell fate reprogramming is far less clear. Here, we report that MKL1, the key transcriptional co-activator of many actin cytoskeletal genes, regulates genomic accessibility and cell fate reprogramming. The MKL1-actin pathway weakens during somatic cell reprogramming by pluripotency transcription factors. Cells that reprogram efficiently display low endogenous MKL1 and inhibition of actin polymerization promotes mature pluripotency activation. Sustained MKL1 expression at a level seen in typical fibroblasts yields excessive actin cytoskeleton, decreases nuclear volume and reduces global chromatin accessibility, stalling cells on their trajectory toward mature pluripotency. In addition, the MKL1-actin imposed block of pluripotency can be bypassed, at least partially, when the Sun2-containing linker of the nucleoskeleton and cytoskeleton (LINC) complex is inhibited. Thus, we unveil a previously unappreciated aspect of control on chromatin and cell fate reprogramming exerted by the MKL1-actin pathway.