The APC/C Coactivator Cdh1 Controls Self-Renewal and Differentiation of Human and Murine HSPCs

Introduction: The balance between differentiation and self-renewal in hematopoietic stem and progenitor cells (HSPCs) is crucial for homeostasis and lifelong blood cell production. Differentiation is predominantly initiated in the G1 phase of the cell cycle when the E3 ligase anaphase-promoting complex or cyclosome (APC/C) is highly active. Its coactivator Cdh1 determines substrate specificity and mediates proteasomal degradation. Relevant target proteins are associated with cell fate decisions in G₁/G₀, and there is growing evidence that Cdh1 is an important regulator of differentiation. While this has already been demonstrated in neurons, muscle cells or osteoblasts, little is known about the role of APC/C^Cdh1 in hematopoiesis. Here we report on the function of Cdh1 in human and murine HSPCs in vitro and in vivo.

Methods: Human CD34⁺ cells from the peripheral blood of G-CSF mobilized donors were exposed to different cytokine combinations and gains or losses of surface marker expression during cell division were determined. By using the established culture conditions Cdh1 expression was detected in distinct hematopoietic lineages and developmental states. CD34⁺ cells were transduced with a lentivirus to deplete Cdh1 by stably expressing shRNA and was then used for in vitro differentiation in liquid culture or CFU assay. In a second miR-based RNAi approach murine BM cells were depleted of Cdh1 and used for competitive transplantation assays. Complementary xenotransplantation of human Cdh1-depleted CD34⁺cells was carried out with NSG mice.

Results: The stimulation of freshly thawed CD34⁺ cells with cytokines led to cell cycle entry and proliferation. Self-renewing cells preserved CD34 expression for up to 7 cell divisions with a low proliferation rate. In contrast, during granulopoiesis and erythropoiesis cells divided more frequently with rapid down-regulation of CD34. Cdh1 expression was tightly connected to differentiation status and proliferation properties. In vitro cultured CD34⁺ cellsand those from BM of healthy human donors showed the highest Cdh1 level compared to moderate or low expression in lymphoid and myeloid cells. Cdh1 is highly expressed at the transcriptional and translational level during both self-renewal and also when cells were directed toward erythroid differentiation. Therefore, high Cdh1 expression is characteristic of immature hematopoietic cells and differentiating precursors. The knockdown of Cdh1 (Cdh1-kd) did not affect proliferation or viability as detected by CFSE staining and measuring the cell cycle length via live-cell imaging. However, Cdh1-kd cells showed a significant maintenance of CD34⁺ cells under self-renewal conditions and during erythropoiesis with a lower frequency of glycophorin A⁺ cells. The functional relevance of Cdh1 depletion was verified in CFU assays. Cells with Cdh1-kd formed fewer primary colonies but significantly more secondary colonies, indicating a preference for self-renewal over differentiation. After competitive transplantation Cdh1-depleted murine BM cells showed a significant enhancement in the repopulation of PB, BM and spleen at week 3, while there was no change in cell cycle properties. However, after 8 weeks chimerism in each of the compartments was reduced to that of the control cells. Accordingly, higher LK and LSK frequencies supported the engraftment of Cdh1-depleted cells at week 3, but there was a significant decrease at week 8 compared to control cells, suggestive of stem cell exhaustion. The Cdh1 level also affected cell differentiation in vivo. After 8 weeks the population of B cells (B220⁺) was increased in transplanted Cdh1-kd cells and the frequency of mature granulocytes (CD11b⁺ Gr1^high) was reduced. Consistently, human Cdh1-depleted CD34⁺ cells engrafted to a much higher degree in the murine BM 8 and 12 weeks after xenotransplantation, as shown by a higher frequency of human CD45⁺ cells. Moreover, the increase of human CD19⁺ B cells with Cdh1-kd confirmed the results of the competitive transplantation.

Conclusions: Loss of the APC/C coactivator Cdh1 supports repopulation of murine HSPCs after transplantation with a lymphoid-biased differentiation, and was confirmed in xenotranplantation experiments. In the long-term, Cdh1 loss led to exhaustion of primitive LK and LSK population, highlighting the role of Cdh1 as a critical regulator of HSPC self-renewal and differentiation.