The cell cycle and Myc intersect with mechanisms that regulate pluripotency and reprogramming

Abstract

Pluripotent stem cells have long-term proliferative capacity and an unusual mode of cell-cycle regulation and can divide independently of extrinsic mitogenic signals. The last few years has seen evidence emerge that links cell-cycle regulation to the maintenance and establishment of pluripotency. Myc transcription factors appear to be central to this regulation. This review addresses these links and discusses how cell-cycle controls and Myc impact on the maintenance and establishment of pluripotency.

Figure 1. Cell cycle structure of human and murine pluripotent cells and fibroblasts

This figure represents typical cell cycle profiles from a range of pluripotent cell types (mESC, hESC, miPSC, hiPSC and mEpiSC) and primary fibroblasts. Pluripotent cells spend a high proportion of time in S-phase cells and a low proportion of time in G1 (Savatier et al., 1996; Stead et al., 2002; Fluckiger et al., 2006; Ohtsuka and Dalton, 2008; Dalton, 2009; Neganova et al., 2009; Zhang et al., 2009). MEFs and human IMR90 fibroblasts show a lower % of cells in S-phase and an increased proportion in G1, relative to pluripotent cells. hPRCs display a cell cycle structure intermediate between differentiated fibroblasts and pluripotent cells. The positions of G1 cells (2n DNA content), S-phase cells and G2/M cells (4n DNA content) are indicated. The Y-axis represents the relative number of cells and the X-axis represents the DNA content of cells, a read-out for cell cycle position.

Figure 2. The relationships between pluripotency, cell cycle and cell size control

The figure represents a summary of the pathways discussed throughout this review, now thought to be important for cell cycle and cell size control in pluripotent cells. The core transcriptional network inherent to pluripotent stem cells impacts on this network at several points. Evidence links Myc to all major points in the regulatory network. For example, Myc regulates the cell cycle directly through regulation of Cdk activity and indirectly through miRNAs. We speculate that accelerated progress through G1 can account for the small size of pluripoent cells. Interestingly, cells with inactive Rb-family members progress through G1 rapidly and have reduced cell volume.

Figure 3. How Myc and cell cycle regulatory controls intersect with different stages of reprogramming

Cells subject to reprogramming cues undergo a three step process as they transition towards iPSCs. The initial step involves Myc which causes global changes in genes expression, including repression of differentiation genes, without induction of pluripotency genes (see Sridharan et al., 2009; Zhao and Daley et al., 2008; Mahereli et al., 2008; Judson et al., 2009; Silva et al., 2008; Mikkelesen et al., 2008). Other reprogramming factors, possibly in combination with Myc (Huangfu et al., 2008), then drive the formation of partially reprogrammed cells (PRCs) and then fully reprogrammed iPSCs. Roles for Myc have also been defined in the maintenance of iPSCs and other pluripotent cell types, some of which are directly related to cell cycle control (Cartwright et al., 2005; Wang et al., 2008; Judson et al., 2009; Hanna et al., 2009). Possible points at which Myc could be involved, but for which there are no solid data, are indicated by broken arrows. For example, it is likely that for reprogramming cells need to be constantly kept cycling by Myc.