Self-Renewal and Differentiation in Hematopoietic Stem and Progenitor Cells Is Controlled By the APC/C Coactivator Cdh1

Introduction: Hematopoietic stem and progenitor cells (HSPCs) represent the lifelong source of all blood cells and continuously renew the hematopoietic system by differentiation into mature blood cells. The process of differentiation is predominantly initiated in G1 phase of the cell cycle when stem cells leave their quiescent state. During G1 the anaphase-promoting complex or cyclosome (APC/C) associated with the coactivator Cdh1 is highly active and marks proteins for proteasomal degradation to regulate proliferation. In addition, Cdh1 has been shown to control terminal differentiation in neurons, muscle cells or osteoblasts. Here we show that Cdh1 is also a critical regulator of human HSPC differentiation and self-renewal.

Methods: Human CD34⁺ cells were collected from peripheral blood (PB) of G-CSF mobilized donors and cultured in the presence of different cytokine combinations. To analyze cell division and self-renewal versus differentiation, CFSE staining was used in combination with flow cytometric detection of CD34 expression. The knockdown and overexpression of Cdh1 was achieved by lentiviral delivery of suitable vectors into target cells. After cell sorting transduced (GFP⁺) CD34⁺ cells were used for in vitro differentiation in liquid culture or CFU assay. For in vivo experiments purified cells were transplanted into NSG mice.

Results: G-CSF mobilized CD34⁺ cells showed effective differentiation into granulocytes (SCF, G-CSF), erythrocytes (SCF, EPO) or extended self-renewal (SCF, TPO, Flt3-L) when stimulated in vitro. The differentiation was characterized by a fast downregulation of Cdh1 on protein level, while Cdh1 remained expressed under self-renewal conditions. A detailed analysis of different subsets, both in vitro and in vivo, showed high Cdh1 level in CD34⁺ cells and low expression in myeloid cells. Analysis of proliferation revealed lowest division rates during self-renewal, accompanied by higher frequency of CD34⁺ cells. The fastest proliferation was found after induction of erythropoiesis. These experiments also showed a more rapid decrease of HSPCs' colony-forming ability and of CD34⁺ cells during granulopoiesis after 2-3 cell divisions in contrast to a moderate decline under self-renewal conditions. The depletion of Cdh1 (Cdh1-kd) had no effect on total cell numbers or proliferation detected by CFSE during differentiation and self-renewal, but showed an increase in S phase cells. These results were confirmed at the single cell level by measuring the cell cycle length of individual cells. Independent of cell cycle regulation, Cdh1-kd cells showed a significant maintenance of CD34⁺ cells under self-renewal conditions and during erythropoiesis with lower frequency of Glycophorin A⁺ cells. In CFU assays, the Cdh1-kd resulted in less primary colony formation, notably CFU-GM and BFU-E, but significantly more secondary colonies compared to control cells. These results suggest that the majority of cells reside in a more undifferentiated state due to Cdh1-kd. The overexpression of Cdh1 showed reversed results with less S phase cells and tendency to increased differentiation in liquid culture and CFU assays. To further validate our results in vivo, we have established a NSG xenotransplant mouse model. Human CD34⁺ cells depleted of Cdh1 engrafted to a much higher degree in the murine BM 8 and 12 weeks after injection as shown by higher frequencies of human CD45⁺ cells. Moreover, we also found an increased frequency of human CD19⁺ B cells after transplantation of CD34⁺ Cdh1-kd cells. These results suggest an enhanced in vivo repopulation capacity of human CD34⁺ HSCs in NSG mice when Cdh1 is depleted. Preliminary data in murine hematopoiesis support our hypothesis showing enhanced PB chimerism upon Cdh1-kd. Looking for a mediator of these effects, we found the Cdh1 target protein TRRAP, a cofactor of many HAT complexes, increased upon Cdh1-kd under self-renewal conditions. We use currently RT-qPCR to determine, if this is caused by a transcriptional or post-translational mechanism.

Conclusions: Loss of the APC/C coactivator Cdh1 supports self-renewal of CD34⁺ cells, represses erythropoiesis in vitro and facilitates engraftment capacity and B cell development of human HSPCs in vivo.

This work was supported by Josè Carreras Leukemia Foundation grant DCJLS R10/14 (to ME+RW)