Pas de deux: the coordinated coupling of erythroid differentiation with the cell cycle

Abstract

Purpose of review: Recent work reveals that cell cycle duration and structure are remodeled in lock-step with distinct stages of erythroid differentiation. These cell cycle features have regulatory roles in differentiation, beyond the generic function of increasing cell number.

Recent findings: Developmental progression through the early erythroid progenitor stage (known as colony-forming-erythroid, or 'CFU-e') is characterized by gradual shortening of G1 phase of the cycle. This process culminates in a key transcriptional switch to erythroid terminal differentiation (ETD) that is synchronized with, and dependent on, S phase progression. Further, the CFU-e/ETD switch takes place during an unusually short S phase, part of an exceptionally short cell cycle that is characterized by globally fast replication fork speeds. Cell cycle and S phase speed can alter developmental events during erythroid differentiation, through pathways that are targeted by glucocorticoid and erythropoietin signaling during the erythroid stress response.

Summary: There is close inter-dependence between cell cycle structure and duration, S phase and replication fork speeds, and erythroid differentiation stage. Further, modulation of cell cycle structure and speed cycle impacts developmental progression and cell fate decisions during erythroid differentiation. These pathways may offer novel mechanistic insights and potential therapeutic targets.

Figure 1:. Flow-cytometric identification of the CFU-e/ETD switch

A. The mouse fetal liver (FL) may be divided into 6 subsets by the cell-surface markers CD71 and Ter119. The transition from S0 to S1 marks the switch from the progenitor amplification stage (CFU-e) to erythroid terminal differentiation (ETD). B. Differentiation of Epor^−/− FLs is blocked at the S0/S1 transition, suggesting that it marks the onset of dependence on signaling by EpoR (from [3]).

Figure 2:. Global acceleration in replication fork speed at the transition from S0 to S1

Replication fork speeds in FL S0 and S1 subsets. p57^KIP2 downregulation at the transition from S0 to S1 is in part responsible for the increased speed of S1 replication forks: baseline fork speeds in p57^KIP2 -deficient S0 cells is increased. The violin plots show the distribution of speeds of individual forks measured during a 10 minute pulse with iododeoxyuridine using DNA combing. Mean fork speed for each subset is next to each violin. p57^+/−m; p57^KIP2 heterozygous with maternal null allele (from [17]).

Figure 3:. Progressive increase in S phase and cell cycle shortening in differentiating erythroid progenitor

A. Single-cell transcriptomics shows ramping up of S phase genes with developmental progression along the erythroid pseudotime, which starts with multipotential progenitors (MPP, 0%) and progresses through the erythroid progenitor continuum to ETD (completion of ETD= 100%). S phase gene expression peaks at the CFU-e/ETD transition. Vertical lines denote successive progenitor stages in the following order along the x axis: MPP, EBMP (erythroid/basophil-mast cell/megakaryocytic progenitors), EEP (early erythroid progenitors with BFU- activity), CEP (committed erythroid progenitors with CFU-e activity), ETD (from [6]). B. Functional cell cycle analysis of adult bone marrow, using a brief pulse of BrdU in vivo, shows progressive increase in the fraction of cells in S phase, and a converse decrease in G1 cells (from [6]).

Figure 4:. Regulatory roles of cell cycle remodeling during erythroid differentiation

The erythroid progenitor developmental continuum (cells in blue) is associated with progressive shortening of the cell cycle and G1 phase. The switch (cell in purple) to ETD (cells in red) is dependent on an unusually short S phase. Hypoxic stress or anemia increase blood levels of glucocorticoids and of Epo. Glucocorticoids amplify the early progenitor pool by inducing p57KIP2, which slows the cycle and S phase and thereby preserve cells in the (CFU-e) progenitor state, at the expense of developmental progression to ETD. Conversely, Epo acts early in ETD, increasing the number and speed of early ETD cell cycles. The shorter cycles of early ETD promote global loss of DNA methylation, accelerating induction of erythroid genes.