Thomas Moreau

Large-scale production of megakaryocytes from human pluripotent stem cells by chemically defined forward programming

The production of megakaryocytes (MKs)—the precursors of blood platelets—from human pluripotent stem cells (hPSCs) offers exciting clinical opportunities for transfusion medicine. Here we describe an original approach for the large-scale generation of MKs in chemically defined conditions using a forward programming strategy relying on the concurrent exogenous expression of three transcription factors: GATA1, FLI1 and TAL1. The forward programmed MKs proliferate and differentiate in culture for several months with MK purity over 90% reaching up to 2 × 105 mature MKs per input hPSC. Functional platelets are generated throughout the culture allowing the prospective collection of several transfusion units from as few as 1 million starting hPSCs. The high cell purity and yield achieved by MK forward programming, combined with efficient cryopreservation and good manufacturing practice (GMP)-compatible culture, make this approach eminently suitable to both in vitro production of platelets for transfusion and basic research in MK and platelet biology.

Brain-derived neurotrophic factor in megakaryocytes

The biosynthesis of endogenous brain-derived neurotrophic factor (BDNF) has thus far been examined in neurons where it is expressed at very low levels, in an activity-dependent fashion. In humans, BDNF has long been known to accumulate in circulating platelets, at levels far higher than in the brain. During the process of blood coagulation, BDNF is released from platelets, which has led to its extensive use as a readily accessible biomarker, under the assumption that serum levels may somehow reflect brain levels. To identify the cellular origin of BDNF in platelets, we established primary cultures of megakaryocytes, the progenitors of platelets, and we found that human and rat megakaryocytes express the BDNF gene. Surprisingly, the pattern of mRNA transcripts is similar to neurons. In the presence of thapsigargin and external calcium, the levels of the mRNA species leading to efficient BDNF translation rapidly increase. Under these conditions, pro-BDNF, the obligatory precursor of biologically active BDNF, becomes readily detectable. Megakaryocytes store BDNF in α-granules, with more than 80% of them also containing platelet factor 4. By contrast, BDNF is undetectable in mouse megakaryocytes, in line with the absence of BDNF in mouse serum. These findings suggest that alterations of BDNF levels in human serum as reported in studies dealing with depression or physical exercise may primarily reflect changes occurring in megakaryocytes and platelets, including the ability of the latter to retain and release BDNF.

Differentiation of Human Pluripotent Stem Cells to Megakaryocytes by Transcription Factor-Driven Forward Programming.

The differentiation of megakaryocytes from human pluripotent stem cells in vitro offers intriguing new perspectives for research and transfusion medicine. However, applications have been hampered by the low efficiency of cytokine driven differentiation protocols leading to poor megakaryocyte purity and yield. Here we describe a novel forward programming approach relying on the combined ectopic expression of the three transcription factors GATA1, FLI1, and TAL1 in human pluripotent stem cells for large scale production of mature megakaryocytes using chemically defined culture and minimum cytokines.

Structurally graduated collagen scaffolds applied to the ex vivo generation of platelets from human pluripotent stem cell-derived megakaryocytes: Enhancing production and purity

Platelet transfusions are a key treatment option for a range of life threatening conditions including cancer, chemotherapy and surgery. Efficient ex vivo systems to generate donor independent platelets in clinically relevant numbers could provide a useful substitute. Large quantities of megakaryocytes (MKs) can be produced from human pluripotent stem cells, but in 2D culture the ratio of platelets harvested from MK cells has been limited and restricts production rate. The development of biomaterial cell supports that replicate vital hematopoietic micro-environment cues are one strategy that may increase in vitro platelet production rates from iPS derived Megakaryocyte cells. In this paper, we present the results obtained generating, simulating and using a novel structurally-graded collagen scaffold within a flow bioreactor system seeded with programmed stem cells. Theoretical analysis of porosity using micro-computed tomography analysis and synthetic micro-particle filtration provided a predictive tool to tailor cell distribution throughout the material. When used with MK programmed stem cells the graded scaffolds influenced cell location while maintaining the ability to continuously release metabolically active CD41 + CD42 + functional platelets. This scaffold design and novel fabrication technique offers a significant advance in understanding the influence of scaffold architectures on cell seeding, retention and platelet production.

Corrigendum: Large-scale production of megakaryocytes from human pluripotent stem cells by chemically defined forward programming

This corrects the article DOI: 10.1038/ncomms11208.

Transcriptional Regulation of Platelet Formation: Harnessing the Complexity for Efficient Platelet Production In Vitro

It is now common knowledge that specific repertoires of transcription factors (TFs) determine a cell’s protein content and thereby its phenotype. The expression of a given TF is not necessarily cell specific, and many TFs play a pivotal role in several different cell types. For example, TAL1, FLI1, RUNX1, ERG and GATA2 are important regulators of stem cells, but also play a vital role in megakaryopoiesis. Although the megakaryocyte (MK) and its closest relative, the red blood cell, share key TFs like GATA1 and NFE2, the bifurcation between the two lineages has been associated with pairs of TFs that act as a toggle switch (such as FLI1 and KLF1). This chapter will summarise the current knowledge of key transcriptional regulators of MK differentiation and how some of these TFs, despite being expressed in several cell types, can impose MK cell identity. Since the discovery of TPO in 1994, our knowledge of MK biology and differentiation has increased exponentially, but we still lack a deep understanding of what triggers the transition from MK growth and maturation to proplatelet formation. We describe how some well-known TFs control the expression of proteins that play a pivotal role in the dramatic cytoplasmic and cytoskeletal events that accompany proplatelet formation. Finally, we show how TFs can be harnessed in a powerful way to produce MKs and, potentially, platelets in vitro for future clinical applications.