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Differentiation of induced pluripotent stem cells (iPSCs) into hematopoietic lineages offers great therapeutic potential. During embryogenesis, hemogenic endothelium (HE) gives rise to hematopoietic stem and progenitor cells through the endothelial-to-hematopoietic transition (EHT). Understanding this process using iPSCs is key to generating functional hematopoietic stem cells (HSCs), a currently unmet challenge. In this study, we examined the role of the transcriptional factor GFI1B and its co-factor LSD1/KDM1A in EHT. To this end, we employed patient-derived iPSC lines with a dominant negative dysfunctional GFI1BQ287* and irreversible pharmacological LSD1/KDM1A inhibition in healthy iPSC lines. The formation of HE remained unaffected; however, hematopoietic output was severely reduced in both conditions. Single-cell RNA sequencing (scRNAseq) performed on the CD144+/CD31+ population derived from healthy iPSCs revealed similar expression dynamics of genes associated with in vivo EHT. Interestingly, LSD1/KDM1A inhibition in healthy lines before EHT resulted in a complete absence of hematopoietic output. However, uncommitted HE cells did not display GFI1B expression, suggesting a timed transcriptional program. To test this hypothesis, we ectopically expressed GFI1B in uncommitted HE cells, leading to downregulation of endothelial genes and upregulation of hematopoietic genes, including GATA2, KIT, RUNX1, and SPI1. Thus, we demonstrate that LSD1/KDM1A and GFI1B can function at distinct temporal points in different cellular subsets during EHT. Although GFI1B is not detected in uncommitted HE cells, its ectopic expression allows for partial hematopoietic specification. These data indicate that precisely timed expression of specific transcriptional regulators during EHT is crucial to the eventual outcome of EHT.

Abstract Background The therapeutic use of [131I]meta-iodobenzylguanidine ([131I]MIBG) is often accompanied by hematological toxicity, primarily consisting of severe and persistent thrombocytopenia. We hypothesize that this is caused by selective uptake of MIBG via the serotonin transporter (SERT) located on platelets and megakaryocytes. In this study, we have investigated whether in vitro cultured human megakaryocytes are capable of selective plasma membrane transport of MIBG and whether pharmacological intervention with selective serotonin reuptake inhibitors (SSRIs) may prevent this radiotoxic MIBG uptake. Methods Peripheral blood CD34+ cells were differentiated to human megakaryocytic cells using a standardized culture protocol. Prior to [3H]serotonin and [125I]MIBG uptake experiments, the differentiation status of megakaryocyte cultures was assessed by flow cytometry. Real-time quantitative polymerase chain reaction (RT-qPCR) was used to assess SERT and NET (norepinephrine transporter) mRNA expression. On day 10 of differentiation, [3H]serotonin and [125I]MIBG uptake assays were conducted. Part of the samples were co-incubated with the SSRI citalopram to assess SERT-specific uptake. HEK293 cells transfected with SERT, NET, and empty vector served as controls. Results In vitro cultured human megakaryocytes are capable of selective plasma membrane transport of MIBG. After 10 days of differentiation, megakaryocytic cell culture batches from three different hematopoietic stem and progenitor cell donors showed on average 9.2 ± 2.4 nmol of MIBG uptake per milligram protein per hour after incubation with 10–7 M MIBG (range: 6.6 ± 1.0 to 11.2 ± 1.0 nmol/mg/h). Co-incubation with the SSRI citalopram led to a significant reduction (30.1%—41.5%) in MIBG uptake, implying SERT-specific uptake of MIBG. A strong correlation between the number of mature megakaryocytes and SERT-specific MIBG uptake was observed. Conclusion Our study demonstrates that human megakaryocytes cultured in vitro are capable of MIBG uptake. Moreover, the SSRI citalopram selectively inhibits MIBG uptake via the serotonin transporter. The concomitant administration of citalopram to neuroblastoma patients during [131I]MIBG therapy might be a promising strategy to prevent the onset of thrombocytopenia.

Hematopoietic differentiation of human induced pluripotent stem cells (iPSCs) provide opportunities not only for fundamental research and disease modelling/drug testing but also for large-scale production of blood effector cells for future clinical application. Although there are multiple ways to differentiate human iPSCs towards hematopoietic lineages, there is a need to develop reproducible and robust protocols. Here we introduce an efficient way to produce three major blood cell types using a standardized differentiation protocol that starts with a single hematopoietic initiation step. This system is feeder-free, avoids EB-formation, starts with a hematopoietic initiation step based on a novel single cell-derived iPSC colony differentiation and produces multi-potential progenitors within 8–10 days. Followed by lineage-specific growth factor supplementation these cells can be matured into well characterized erythroid, megakaryocytic and myeloid cells with high-purity, without transcription factor overexpression or any kind of pre-purification step. This standardized differentiation system provides a simple platform to produce specific blood cells in a reproducible manner for hematopoietic development studies, disease modelling, drug testing and the potential for future therapeutic applications. Keywords: iPSC, Differentiation, Erythroid, Hematopoietic, Megakaryocytic, Myeloid

Peripheral blood mononuclear cells were isolated from an individual harboring a heterozygous c.859C → T p.Q287* mutation in GFI1B, causing an autosomal dominant bleeding disorder, platelet type, 17 (BDPLT17). PBMCs were differentiated to erythroblasts and reprogrammed by lentiviral delivery of a self-silencing hOKSM polycistronic vector. Pluripotency of iPSC line was confirmed by expression of associated markers and by in vitro spontaneous differentiation towards the 3 germ layers. Normal karyotype confirmed the genomic integrity of iPSCs and the presence of disease causing mutation was shown by Sanger sequencing. The generated iPSCs can be used to study BDPLT17 pathophysiology and basic functions of GFI1B.

Mobilized peripheral blood (MPB) CD34+ cells were differentiated to CD34+/CD41+ megakaryoblasts. Cells were sorted to obtain a pure megakaryoblast population which was reprogrammed with a hOKSM self-silencing polycistronic lentiviral vector. Resulting iPSC showed normal karyotype and expression of pluripotency associated markers and in vitro spontaneous differentiation towards the 3 germ layers confirmed pluripotency of iPSC lines. Besides normal iPSC applications, these lines can be used as a control line for other megakaryoid origin iPSC and could be applied for epigenetic based research.

Peripheral blood mononuclear cells were isolated and cultured to a pure pro-EBL population and reprogrammed using episomal plasmids. The pluripotency of transgene-free induced pluripotent stem cell (iPSC) line was verified by the expression of pluripotency-associated markers and by in vitro spontaneous differentiation towards the 3 germ layers. The iPSC line showed normal karyotype. Peripheral blood is a non-invasive easy accessible cell source and combined with EBL outgrowth in vitro, a routine process obtaining sufficient amount of homogenous cells can be obtained within a week. Using episomal delivery, pro-EBLs can be reprogrammed in a transgene-free, cost effective system.

Dominant-negative mutations in the transcription factor Growth Factor Independence-1B (GFI1B), such as GFI1BQ287*, cause a bleeding disorder characterized by a plethora of megakaryocyte and platelet abnormalities. The deregulated molecular mechanisms and pathways are unknown. Here we show that both normal and Q287* mutant GFI1B interacted most strongly with the lysine specific demethylase-1 – REST corepressor - histone deacetylase (LSD1-RCOR-HDAC) complex in megakaryoblasts. Sequestration of this complex by GFI1BQ287* and chemical separation of GFI1B from LSD1 induced abnormalities in normal megakaryocytes comparable to those seen in patients. Megakaryocytes derived from GFI1BQ287*-induced pluripotent stem cells also phenocopied abnormalities seen in patients. Proteome studies on normal and mutant-induced pluripotent stem cell-derived megakaryocytes identified a multitude of deregulated pathways downstream of GFI1BQ287* including cell division and interferon signaling. Proteome studies on platelets from GFI1BQ287* patients showed reduced expression of proteins implicated in platelet function, and elevated expression of proteins normally downregulated during megakaryocyte differentiation. Thus, GFI1B and LSD1 regulate a broad developmental program during megakaryopoiesis, and GFI1BQ287* deregulates this program through LSD1-RCOR-HDAC sequestering.