Identification of Phospholipase C Gamma 1 As a Key Regulator of Erythropoiesis

Abstract 4744

Erythropoiesis is a complex multistage process in which the development of red blood cells occurs through expansion and differentiation of hematopoietic stem cells (HSCs) into more committed progenitors. Regulation of survival, expansion and differentiation of erythroid progenitors is dependent on a well-coordinated cohort of transcription factors and an intricate network of finely tuned regulatory signalling pathways. In vivo and in vitro studies have highlighted erythropoietin receptor (EpoR) signaling through JAK2 tyrosine kinase as a crucial regulator of erythropoiesis. This leads to the subsequent activation of downstream effectors such as STAT5, MAPK, and PI-3K/Akt pathways. However, detailed knowledge about signalling pathways involved in EPO/EpoR induced differentiation of erythroid progenitors remain elusive.

Phosphatidylinositol-specific phospholipase C gamma1 (PLCg1) is known to act as key mediator of calcium-signalling that can substitute for PI-3K/AKT signalling in oncogenic models. Moreover, its loss is associated with lack of erythropoiesis in a straight knockout mouse model. As it is tempting to speculate on the role of Plcg1/Ca-signalling downstream of EpoR/JAK in regulation of erythroid development we aimed to investigate its influence on differentiation and proliferation of hematopoietic cells in vitro and in vivo.

Using different cellular models (Ba/F3, 32D) stably transfected with EpoR and wildtype JAK2 we could provide evidence that PLCg1 is a downstream target of EpoR/JAK2 signalling. Knockdown of PLCg1 led to a decreased proliferation of PLCg1-deficient cells compared to control cells whereas survival of these cells was not affected. In contrast, other downstream targets of EpoR signalling were not affected by PLCg1 knockdown. In order to assess specifically its role in erythroid development, we used the murine pro-erythroblast cell line I-11 as well as primary fetal liver cells (FLC). The I-11 cell line was isolated from p53-deficient fetal livers and is able to differentiate upon dexamethasone-/stem cell factor-withdrawal combined with erythropoietin stimulation; primary FLC were harvested at E13.5. PLCg1 knockdown by using RNA-interference technology led to a significant delay in erythroid differentiation and accumulation of immature erythroid progenitors (e.g. pro-erythroblasts) as assessed by cytology and flow cytometry technology. In addition, we tested the colony-forming potential of PLCg1-deficient I-11 and fetal liver cells compared to controls. Colony formation was significantly impaired in both - I-11 and primary FLC - when compared to control cells (shRNA-scr). We performed gene-expression analysis by Q-RT-PCR on sorted hematopoietic stem and progenitor cells and found a higher expression in MEP compared to GMP or CMP. To clarify, whether the effects of Plcg1 knockdown are restricted to erythroid development at the stage of MEP or erythroid progenitors, we aimed to investigate adult hematopoietic stem cells in erythroid development. We infected lineage-depleted/erythroid-enriched (Gr1-, B220-, CD3/4/8, CD19-/ IL7Ra- negative) bone marrow cells with either PLCg1 or control shRNA. Using flow cytometry analysis to examine differentiation we could observe a reduction of megakaryocyte/erythroid progenitor cells (MEP) in PLCg1 knockdown cells compared to control cells while development of other lineages (e.g. GMP) remained unaffected. Currently, competitive repopulation assays investigating the repopulation and differentiation capacity of hematopoietic stem cells after Plcg1 knockdown (or scr controls) are under way to explore the role of Plcg1 signalling in hematopoietic and erythroid development in vivo.

Taken together, our findings presume PLCg1 to be a key regulator in erythroid development and understanding of its relevance in development and maintenance of normal hematopoiesis will be a crucial prerequisite for targeting this important pathway in myeloproliferative disease.