Metalloproteinase regulation improves in vitro generation of efficacious platelets from mouse embryonic stem cells

Mass production of iPSC-derived platelets toward the clinical application

The ex vivo production of platelets from induced pluripotent cells (iPSCs) may offer a safer and sustainable alternative for transfusions and drug delivery systems (DDS). However, the mass production of the clinically required number of iPSC-derived platelets (iPSC-PLTs) is challenging. Here, we introduce recent technologies for mass production and the first-in-human clinical trial using ex vivo iPSC-PLTs. To this end, we established immortalized megakaryocyte progenitor cell lines (imMKCLs) as an expandable master cell bank (MCB) through the overexpression of c-MYC, BMI1 and BCL-XL, which modulated megakaryopoiesis and thrombopoiesis. We also optimized a culture cocktail for maturation of the imMKCLs by mixing an aryl hydrocarbon receptor (AhR) antagonist, SR1/GNF-316; a Rho-associated protein kinase (ROCK) inhibitor, Y-27632/Y-39983; and a small-molecule compound replacing recombinant thrombopoietin (TPO), TA-316. Finally, we discovered the importance of turbulence on the manufacturing of intact iPSC-PLTs, allowing us to develop a turbulence-based bioreactor, VerMES. Combination of the MCB and VerMES enabled us to produce more than 100 billion iPSC-PLTs, leading to the first-in-human clinical trial. Despite these advancements, many challenges remain before expanding the clinical implementation of this strategy.

Early Endosomal GTPase Rab5 (Ras-Related Protein in Brain 5) Regulates GPIbβ (Glycoprotein Ib Subunit β) Trafficking and Platelet Production In Vitro

OBJECTIVE

Maturation of megakaryocytes culminates with extensive membrane rearrangements necessary for proplatelet formation. Mechanisms required for proplatelet extension and origin of membranes are still poorly understood. GTPase Rab5 (Ras-related protein in brain 5) regulates endocytic uptake and homotypic fusion of early endosomes and regulates phosphatidylinositol 3-monophosphate production important for binding of effector proteins during early-to-late endosomal/lysosomal maturation. Approach and Results: To investigate the role of Rab5 in megakaryocytes, we expressed GFP (green fluorescent protein)-coupled Rab5 wild type and its point mutants Q79L (active) and N133L (inactive) in primary murine fetal liver-derived megakaryocytes. Active Rab5 Q79L induced the formation of enlarged early endosomes, while inactive Rab5 N133L caused endosomal fragmentation. Consistently, an increased amount of transferrin internalization in Rab5 Q79L was impaired in Rab5 N133L expressing megakaryocytes, when compared with GFP or Rab5 wild type. Moreover, trafficking of GPIbβ (glycoprotein Ib subunit beta), a subunit of major megakaryocytes receptor and membrane marker, was found to be mediated by Rab5 activity. While GPIbβ was mostly present along the plasma membrane, and within cytoplasmic vesicles in Rab5 wild type megakaryocytes, it accumulated in the majority of Rab5 Q79L enlarged endosomes. Conversely, Rab5 N133L caused mostly GPIbβ plasma membrane retention. Furthermore, Rab5 Q79L expression increased incorporation of the membrane dye (PKH26), indicating higher membrane content. Finally, while Rab5 Q79L increased proplatelet production, inactive Rab5 N133L strongly inhibited it and was coupled with a decrease in late endosomes/lysosomes. Localization of GPIbβ in enlarged endosomes was phosphatidylinositol 3-monophosphate dependent.

CONCLUSIONS

Taken together, our results demonstrate that Rab5-dependent endocytosis plays an important role in megakaryocytes receptor trafficking, membrane formation, and thrombopoiesis.

Human induced pluripotent stem cell-derived platelets loaded with lapatinib effectively target HER2+ breast cancer metastasis to the brain

Prognosis of patients with HER2+ breast-to-brain-metastasis (BBM) is dismal even after current standard-of-care treatments, including surgical resection, whole-brain radiation, and systemic chemotherapy. Radiation and systemic chemotherapies can also induce cytotoxicity, leading to significant side effects. Studies indicate that donor-derived platelets can serve as immune-compatible drug carriers that interact with and deliver drugs to cancer cells with fewer side effects, making them a promising therapeutic option with enhanced antitumor activity. Moreover, human induced pluripotent stem cells (hiPSCs) provide a potentially renewable source of clinical-grade transfusable platelets that can be drug-loaded to complement the supply of donor-derived platelets. Here, we describe methods for ex vivo generation of megakaryocytes (MKs) and functional platelets from hiPSCs (hiPSC-platelets) in a scalable fashion. We then loaded hiPSC-platelets with lapatinib and infused them into BBM tumor-bearing NOD/SCID mouse models. Such treatment significantly increased intracellular lapatinib accumulation in BBMs in vivo, potentially via tumor cell-induced activation/aggregation. Lapatinib-loaded hiPSC-platelets exhibited normal morphology and function and released lapatinib pH-dependently. Importantly, lapatinib delivery to BBM cells via hiPSC-platelets inhibited tumor growth and prolonged survival of tumor-bearing mice. Overall, use of lapatinib-loaded hiPSC-platelets effectively reduced adverse effects of free lapatinib and enhanced its therapeutic efficacy, suggesting that they represent a novel means to deliver chemotherapeutic drugs as treatment for BBM.

Oncogenic Gata1 causes stage-specific megakaryocyte differentiation delay

The megakaryocyte/erythroid transient myeloproliferative disorder (TMD) in newborns with Down syndrome (DS) occurs when Nterminal truncating mutations of the hemopoietic transcription factor GATA1, that produce GATA1short protein (GATA1s), are acquired early in development. Prior work has shown that murine GATA1s, by itself, causes a transient yolk sac myeloproliferative disorder. However, it is unclear where in the hemopoietic cellular hierarchy GATA1s exerts its effects to produce this myeloproliferative state. Here, through a detailed examination of hemopoiesis from murine GATA1s embryonic stem cells (ESC) and GATA1s embryos we define defects in erythroid and megakaryocytic differentiation that occur late in hemopoiesis. GATA1s causes an arrest late in erythroid differentiation in vivo, and even more profoundly in ESC-derived cultures, with a marked reduction of Ter-119 cells and reduced erythroid gene expression. In megakaryopoiesis, GATA1s causes a differentiation delay at a specific stage, with accumulation of immature, kit-expressing CD41hi megakaryocytic cells. In this specific megakaryocytic compartment, there are increased numbers of GATA1s cells in S-phase of the cell cycle and a reduced number of apoptotic cells compared to GATA1 cells in the same cell compartment. There is also a delay in maturation of these immature GATA1s megakaryocytic lineage cells compared to GATA1 cells at the same stage of differentiation. Finally, even when GATA1s megakaryocytic cells mature, they mature aberrantly with altered megakaryocyte-specific gene expression and activity of the mature megakaryocyte enzyme, acetylcholinesterase. These studies pinpoint the hemopoietic compartment where GATA1s megakaryocyte myeloproliferation occurs, defining where molecular studies should now be focused to understand the oncogenic action of GATA1s.

Generation and manipulation of human iPSC-derived platelets

The discovery of iPSCs has led to the ex vivo production of differentiated cells for regenerative medicine. In the case of transfusion products, the derivation of platelets from iPSCs is expected to complement our current blood-donor supplied transfusion system through donor-independent production with complete pathogen-free assurance. This derivation can also overcome alloimmune platelet transfusion refractoriness by resulting in autologous, HLA-homologous or HLA-deficient products. Several developments were necessary to produce a massive number of platelets required for a single transfusion. First, expandable megakaryocytes were established from iPSCs through transgene expression. Second, a turbulent-type bioreactor with improved platelet yield and quality was developed. Third, novel drugs that enabled efficient feeder cell-free conditions were developed. Fourth, the platelet-containing suspension was purified and resuspended in an appropriate buffer. Finally, the platelet product needed to be assured for competency and safety including non-tumorigenicity through in vitro and in vivo preclinical tests. Based on these advancements, a clinical trial has started. The generation of human iPSC-derived platelets could evolve transfusion medicine to the next stage and assure a ubiquitous, safe supply of platelet products. Further, considering the feasibility of gene manipulations in iPSCs, other platelet products may bring forth novel therapeutic measures.

Forming megakaryocytes from murine-induced pluripotent stem cells by the inducible overexpression of supporting factors

Background

Platelets are small anucleate cells that circulate in the blood in a resting state but can be activated by external cues. In case of need, platelets from blood donors can be transfused. As an alternative source, platelets can be produced from induced pluripotent stem cells (iPSCs); however, recovered numbers are low.

Objectives

To optimize megakaryocyte (MK) and platelet output from murine iPSCs, we investigated overexpression of the transcription factors GATA‐binding factor 1 (GATA1); nuclear factor, erythroid 2; and pre–B‐cell leukemia transcription factor 1 (Pbx1) and a hyperactive variant of the small guanosine triphosphatase RhoA (RhoAhc).

Methods

To avoid off‐target effects, we generated iPSCs carrying the reverse tetracycline‐responsive transactivator M2 (rtTA‐M2) in the Rosa26 locus and expressed the factors from Tet‐inducible gammaretroviral vectors. Differentiation of iPSCs was initiated by embryoid body (EB) formation. After EB dissociation, early hematopoietic progenitors were enriched and cocultivated on OP9 feeder cells with thrombopoietin and stem cell factor to induce megakaryocyte (MK) differentiation.

Results

Overexpression of GATA1 and Pbx1 increased MK output 2‐ to 2.5‐fold and allowed prolonged collection of MK. Cytologic and ultrastructural analyses identified typical MK with enlarged cells, multilobulated nuclei, granule structures, and an internal membrane system. However, GATA1 and Pbx1 expression did not improve MK maturation or platelet release, although in vitro–generated platelets were functional in spreading on fibrinogen or collagen‐related peptide.

Conclusion

We demonstrate that the use of rtTA‐M2 transgenic iPSCs transduced with Tet‐inducible retroviral vectors allowed for gene expression at later time points during differentiation. With this strategy we could identify factors that increased in vitro MK production.

Induction of differentiation of human stem cells ex vivo: Toward large-scale platelet production

Platelet transfusion is one of the most reliable strategies to cure patients suffering from thrombocytopenia or platelet dysfunction. With the increasing demand for transfusion, however, there is an undersupply of donors to provide the platelet source. Thus, scientists have sought to design methods for deriving clinical-scale platelets ex vivo. Although there has been considerable success ex vivo in the generation of transformative platelets produced by human stem cells (SCs), the platelet yields achieved using these strategies have not been adequate for clinical application. In this review, we provide an overview of the developmental process of megakaryocytes and the production of platelets in vivo and ex vivo, recapitulate the key advances in the production of SC-derived platelets using several SC sources, and discuss some strategies that apply three-dimensional bioreactor devices and biochemical factors synergistically to improve the generation of large-scale platelets for use in future biomedical and clinical settings.

A comparison of haematopoietic stem cells from umbilical cord blood and peripheral blood for platelet production in a microfluidic device

BACKGROUND AND OBJECTIVES

Several sources of haematopoietic stem cells have been used for static culture of megakaryocytes to produce platelets in vitro. This study compares and characterizes platelets produced in shear flow using precursor cells from either umbilical (UCB) or adult peripheral blood (PB).

MATERIALS AND METHODS

The efficiency of platelet production of the cultured cells was studied after perfusion in custom-built von Willebrand factor-coated microfluidic flow chambers. Platelet receptor expression and morphology were investigated by flow cytometry and microscopy, respectively.

RESULTS

Proliferation of stem cells isolated out of UCB was significantly higher (P < 0·0001) compared to PB. Differentiation of these cells towards megakaryocytes was significantly lower from PB compared to UCB where the fraction of CD42b/CD41 double positive events was 44 ± 9% versus 76 ± 11%, respectively (P < 0·0001). However, in vitro platelet production under hydrodynamic conditions was more efficient with 7·4 platelet-like particles per input cell from PB compared to 4·2 from UCB (P = 0·02). The percentage of events positive for CD42b, CD41 and CD61 was comparable between both stem cell sources. The mean number of receptors per platelet from UCB and PB was similar to that on blood bank platelets with on average 28 000 CD42b, 57 000 CD61 and 5500 CD49b receptors. Microscopy revealed platelets appearing similar to blood bank platelets in morphology, size and actin cytoskeleton, alongside smaller fragments and source megakaryocytes.

CONCLUSION

This characterization study suggests that platelets produced in vitro under flow either from UCB or from PB share receptor expression and morphology with donor platelets stored in the blood bank.

Platelet production from induced pluripotent stem cells

Ex vivo production of human platelets has been pursued as an alternative measure to resolve limitations in the supply and safety of current platelet transfusion products. To this end, induced pluripotent stem cells (iPSCs) are considered an ideal global source, as they are not only pluripotent and self-renewing, but are also available from basically any person, have relatively few ethical issues, and are easy to manipulate. From human iPSCs, megakaryocyte (MK) lines with robust proliferation capacity have been established by the introduction of specified sets of genes. These expandable MKs are also cryopreservable and thus would be suitable as master cells for good manufacturing practice (GMP)-grade production of platelets, assuring availability on demand and safety against blood-borne infections. Meanwhile, developments in bioreactors that physically mimic the in vivo environment and discovery of substances that promote thrombopoiesis have yielded competent platelets with improved efficiency. The derivation of platelets from iPSCs could further resolve transfusion-related alloimmune complications through the manufacturing of autologous products and human leukocyte antigen (HLA)-compatible platelets from stocked homologous HLA-type iPSC libraries or by manipulation of HLAs and human platelet antigens (HPAs). Considering these key advances in the field, HLA-deleted platelets could become a universal product that is manufactured at industrial level to safely fulfill almost all demands. In this review, we provide an overview of the ex vivo production of iPSC-derived platelets toward clinical applications, a production that would revolutionize the blood transfusion system and lead the field of iPSC-based regenerative medicine.

Recent developments in ex vivo platelet production

The platelet is a component of blood that functions to initiate blood clotting. Abnormal platelet count and function can lead to a life-threatening condition caused by excessive bleeding. At present, platelet supply for transfusion can be obtained only from platelet donation. However, platelets cannot be stored for longer than 7 days, meaning that routine isolation is required to maintain platelet supply for transfusion. To mitigate for potential platelet shortages, several strategies have been proposed to generate platelets ex vivo. By employing both of natural and artificial approaches, several researchers have successfully generated biomaterials with characteristics similar to human-derived platelets. Their reports indicated that the biomaterials could mimic the aggregation of human-isolated platelets, further suggesting the possibility to substitute or complement human-isolated platelets. The current review summarizes the progress in ex vivo platelet production and gives a prospect for the possible approaches to achieving a feasible platelet factory, based on the Good Manufacturing Practice standards.

MicroRNAs 10a and 10b Regulate the Expression of Human Platelet Glycoprotein Ibα for Normal Megakaryopoiesis

MicroRNAs are a class of small non-coding RNAs that bind to the three prime untranslated region (3′-UTR) of target mRNAs. They cause a cleavage or an inhibition of the translation of target mRNAs, thus regulating gene expression. Here, we employed three prediction tools to search for potential miRNA target sites in the 3′-UTR of the human platelet glycoprotein (GP) 1BA gene. A luciferase reporter assay shows that miR-10a and -10b sites are functional. When miR-10a or -10b mimics were transfected into the GP Ibβ/GP IX-expressing cells, along with a DNA construct harboring both the coding and 3′-UTR sequences of the human GP1BA gene, we found that they inhibit the transient expression of GP Ibα on the cell surface. When the miR-10a or -10b mimics were introduced into murine progenitor cells, upon megakaryocyte differentiation, we found that GP Ibα mRNA expression was markedly reduced, suggesting that a miRNA-induced mRNA degradation is at work. Thus, our study identifies GP Ibα as a novel target of miR-10a and -10b, suggesting that a drastic reduction in the levels of miR-10a and -10b in the late stage of megakaryopoiesis is required to allow the expression of human GP Ibα and the formation of the GP Ib-IX-V complex.

Selective Inhibition of ADAM17 Efficiently Mediates Glycoprotein Ibα Retention During Ex Vivo Generation of Human Induced Pluripotent Stem Cell-Derived Platelets

Donor‐independent platelet concentrates for transfusion can be produced in vitro from induced pluripotent stem cells (iPSCs). However, culture at 37°C induces ectodomain shedding on platelets of glycoprotein Ibα (GPIbα), the von Willebrand factor receptor critical for adhesive function and platelet lifetime in vivo, through temperature‐dependent activation of a disintegrin and metalloproteinase 17 (ADAM17). The shedding can be suppressed by using inhibitors of panmetalloproteinases and possibly of the upstream regulator p38 mitogen‐activated protein kinase (p38 MAPK), but residues of these inhibitors in the final platelet products may be accompanied by harmful risks that prevent clinical application. Here, we optimized the culture conditions for generating human iPSC‐derived GPIbα⁺ platelets, focusing on culture temperature and additives, by comparing a new and safe selective ADAM17 inhibitor, KP‐457, with previous inhibitors. Because cultivation at 24°C (at which conventional platelet concentrates are stored) markedly diminished the yield of platelets with high expression of platelet receptors, 37°C was requisite for normal platelet production from iPSCs. KP‐457 blocked GPIbα shedding from iPSC platelets at a lower half‐maximal inhibitory concentration than panmetalloproteinase inhibitor GM‐6001, whereas p38 MAPK inhibitors did not. iPSC platelets generated in the presence of KP‐457 exhibited improved GPIbα‐dependent aggregation not inferior to human fresh platelets. A thrombus formation model using immunodeficient mice after platelet transfusion revealed that iPSC platelets generated with KP‐457 exerted better hemostatic function in vivo. Our findings suggest that KP‐457, unlike GM‐6001 or p38 MAPK inhibitors, effectively enhances the production of functional human iPSC‐derived platelets at 37°C, which is an important step toward their clinical application. Stem Cells Translational Medicine2017;6:720–730

Metalloproteolytic receptor shedding…platelets "acting their age"

Whilst significant effort has been focused on development of tools and approaches to clinically modulate activation processes that consume platelets, the platelet receptors that initiate activation processes remain untargeted. The modulation of receptor levels is also linked to underlying platelet aging processes which influence normal platelet lifespan and also the functionality and survival of stored platelets that are used in transfusion. In this review, we will focus on platelet adhesion receptors initiating thrombus formation, and discuss how regulation of levels of these receptors impact platelet function and platelet survival.

Structural basis for the specific inhibition of glycoprotein Ibα shedding by an inhibitory antibody

Ectodomain shedding of glycoprotein (GP) Ibα is thought to mediate the clearance of activated, aged or damaged platelets. A monoclonal antibody, 5G6, has been developed recently to specifically bind to the GPIbα shedding cleavage site and to inhibit its shedding. However, the molecular mechanism underlying antigen recognition and inhibitory specificity is not clear. To elucidate the structural basis for 5G6 binding to GPIbα, we determined the crystal structure of 5G6 Fab fragment in complex with its epitope peptide KL10 (GPIbα residues 461–470, KLRGVLQGHL), to 2.4-Å resolution. Key residues in both 5G6 and KL10 were mutated to validate their effects in antibody binding by using isothermal titration calorimetry. The 5G6 Fab-KL10 peptide complex structure confirmed the direct association of 5G6 with its target GPIbα residues and elucidated the molecular basis underlying its binding specificity and high affinity. The similar binding properties of 5G6 Fab fragment to GPIbα on human platelets as those to KL10 suggests that such an interaction may not be affected by the plasma membrane or nearby GPIbβ. This structural information may facilitate further antibody optimization and humanization.

Understanding platelet generation from megakaryocytes: implications for in vitro-derived platelets

Platelets are anucleate cytoplasmic discs derived from megakaryocytes that circulate in the blood and have major roles in hemostasis, thrombosis, inflammation, and vascular biology. Platelet transfusions are required to prevent the potentially life-threatening complications of severe thrombocytopenia seen in a variety of medical settings including cancer therapy, trauma, and sepsis. Platelets used in the clinic are currently donor-derived which is associated with concerns over sufficient availability, quality, and complications due to immunologic and/or infectious issues. To overcome our dependence on donor-derived platelets for transfusion, efforts have been made to generate in vitro-based platelets. Work in this area has advanced our understanding of the complex processes that megakaryocytes must undergo to generate platelets both in vivo and in vitro. This knowledge has also defined the challenges that must be overcome to bring in vitro-based platelet manufacturing to a clinical reality. This review will focus on our understanding of committed megakaryocytes and platelet release in vivo and in vitro, and how this knowledge can guide the development of in vitro-derived platelets for clinical application.

Manipulating megakaryocytes to manufacture platelets ex vivo

Historically, platelet transfusion has proven a reliable way to treat patients suffering from thrombocytopenia or similar ailments. An undersupply of donors, however, has demanded alternative platelet sources. Scientists have therefore sought to recapitulate the biological events that convert hematopoietic stem cells into platelets in the laboratory. Such platelets have shown good function and potential for treatment. Yet the number manufactured ex vivo falls well short of clinical application. Part of the reason is the remarkable gaps in our understanding of the molecular mechanisms driving platelet formation. Using several stem cell sources, scientists have progressively clarified the chemical signaling and physical microenvironment that optimize ex vivo platelets and reconstituted them in synthetic environments. Key advances in cell reprogramming and the ability to propagate self-renewal have extended the lifetime of megakaryocytes to increase the pool of platelet progenitors.

Road blocks in making platelets for transfusion

The production of laboratory-generated human platelets is necessary to meet present and future transfusion needs. This manuscript will identify and define the major roadblocks that must be overcome to make human platelet production possible for clinical use, and propose solutions necessary to accelerate development of laboratory-generated human platelets to market.

Unipotent Megakaryopoietic Pathway Bridging Hematopoietic Stem Cells and Mature Megakaryocytes

Recent identification of platelet/megakaryocyte-biased hematopoietic stem/repopulating cells requires revision of the intermediate pathway for megakaryopoiesis. Here, we show a unipotent megakaryopoietic pathway bypassing the bipotent megakaryocyte/erythroid progenitors (biEMPs). Cells purified from mouse bone marrow by CD42b (GPIbα) marking were demonstrated to be unipotent megakaryocytic progenitors (MKPs) by culture and transplantation. A subpopulation of freshly isolated CD41(+) cells in the lineage Sca1(+) cKit(+) (LSK) fraction (subCD41(+) LSK) differentiated only into MKP and mature megakaryocytes in culture. Although CD41(+) LSK cells as a whole were capable of differentiating into all myeloid and lymphoid cells in vivo, they produced unipotent MKP, mature megakaryocytes, and platelets in vitro and in vivo much more efficiently than Flt3(+) CD41(-) LSK cells, especially at the early phase after transplantation. In single cell polymerase chain reaction and thrombopoietin (TPO) signaling analyses, the MKP and a fraction of CD41(+) LSK, but not the biEMP, showed the similarities in mRNA expression profile and visible TPO-mediated phosphorylation. On increased demand of platelet production after 5-FU treatment, a part of CD41(+) LSK population expressed CD42b on the surface, and 90% of them showed unipotent megakaryopoietic capacity in single cell culture and predominantly produced platelets in vivo at the early phase after transplantation. These results suggest that the CD41(+) CD42b(+) LSK are straightforward progenies of megakaryocytes/platelet-biased stem/repopulating cells, but not progenies of biEMP. Consequently, we show a unipotent/highly biased megakaryopoietic pathway interconnecting stem/repopulating cells and mature megakaryocytes, the one that may play physiologic roles especially in emergency megakaryopoiesis.

Comparative analysis of human ex vivo-generated platelets vs megakaryocyte-generated platelets in mice: a cautionary tale

Thrombopoiesis is the process by which megakaryocytes release platelets that circulate as uniform small, disc-shaped anucleate cytoplasmic fragments with critical roles in hemostasis and related biology. The exact mechanism of thrombopoiesis and the maturation pathways of platelets released into the circulation remain incompletely understood. We showed that ex vivo-generated murine megakaryocytes infused into mice release platelets within the pulmonary vasculature. Here we now show that infused human megakaryocytes also release platelets within the lungs of recipient mice. In addition, we observed a population of platelet-like particles (PLPs) in the infusate, which include platelets released during ex vivo growth conditions. By comparing these 2 platelet populations to human donor platelets, we found marked differences: platelets derived from infused megakaryocytes closely resembled infused donor platelets in morphology, size, and function. On the other hand, the PLP was a mixture of nonplatelet cellular fragments and nonuniform-sized, preactivated platelets mostly lacking surface CD42b that were rapidly cleared by macrophages. These data raise a cautionary note for the clinical use of human platelets released under standard ex vivo conditions. In contrast, human platelets released by intrapulmonary-entrapped megakaryocytes appear more physiologic in nature and nearly comparable to donor platelets for clinical application.

Scalable generation of universal platelets from human induced pluripotent stem cells

Human induced pluripotent stem cells (iPSCs) provide a potentially replenishable source for the production of transfusable platelets. Here, we describe a method to generate megakaryocytes (MKs) and functional platelets from iPSCs in a scalable manner under serum/feeder-free conditions. The method also permits the cryopreservation of MK progenitors, enabling a rapid “surge” capacity when large numbers of platelets are needed. Ultrastructural/morphological analyses show no major differences between iPSC platelets and human blood platelets. iPSC platelets form aggregates, lamellipodia, and filopodia after activation and circulate in macrophage-depleted animals and incorporate into developing mouse thrombi in a manner identical to human platelets. By knocking out the β2-microglobulin gene, we have generated platelets that are negative for the major histocompatibility antigens. The scalable generation of HLA-ABC-negative platelets from a renewable cell source represents an important step toward generating universal platelets for transfusion as well as a potential strategy for the management of platelet refractoriness.

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Large-scale production of platelets from human iPSCs under defined conditions

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iPSC platelets are functional both in vivo and in vitro

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Knockout of β2-microglobulin gene in iPSCs generates universal platelets

SCL/TAL1-mediated transcriptional network enhances megakaryocytic specification of human embryonic stem cells

Human embryonic stem cells (hESCs) are a unique in vitro model for studying human developmental biology and represent a potential source for cell replacement strategies. Platelets can be generated from cord blood progenitors and hESCs; however, the molecular mechanisms and determinants controlling the in vitro megakaryocytic specification of hESCs remain elusive. We have recently shown that stem cell leukemia (SCL) overexpression accelerates the emergence of hemato-endothelial progenitors from hESCs and promotes their subsequent differentiation into blood cells with higher clonogenic potential. Given that SCL participates in megakaryocytic commitment, we hypothesized that it may potentiate megakaryopoiesis from hESCs. We show that ectopic SCL expression enhances the emergence of megakaryocytic precursors, mature megakaryocytes (MKs), and platelets in vitro. SCL-overexpressing MKs and platelets respond to different activating stimuli similar to their control counterparts. Gene expression profiling of megakaryocytic precursors shows that SCL overexpression renders a megakaryopoietic molecular signature. Connectivity Map analysis reveals that trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA), both histone deacetylase (HDAC) inhibitors, functionally mimic SCL-induced effects. Finally, we confirm that both TSA and SAHA treatment promote the emergence of CD34(+) progenitors, whereas valproic acid, another HDAC inhibitor, potentiates MK and platelet production. We demonstrate that SCL and HDAC inhibitors are megakaryopoiesis regulators in hESCs.

Calpain 2 activation of P-TEFb drives megakaryocyte morphogenesis and is disrupted by leukemogenic GATA1 mutation

Megakaryocyte morphogenesis employs a "hypertrophy-like" developmental program that is dependent on P-TEFb kinase activation and cytoskeletal remodeling. P-TEFb activation classically occurs by a feedback-regulated process of signal-induced, reversible release of active Cdk9-cyclin T modules from large, inactive 7SK small nuclear ribonucleoprotein particle (snRNP) complexes. Here, we have identified an alternative pathway of irreversible P-TEFb activation in megakaryopoiesis that is mediated by dissolution of the 7SK snRNP complex. In this pathway, calpain 2 cleavage of the core 7SK snRNP component MePCE promoted P-TEFb release and consequent upregulation of a cohort of cytoskeleton remodeling factors, including α-actinin-1. In a subset of human megakaryocytic leukemias, the transcription factor GATA1 undergoes truncating mutation (GATA1s). Here, we linked the GATA1s mutation to defects in megakaryocytic upregulation of calpain 2 and of P-TEFb-dependent cytoskeletal remodeling factors. Restoring calpain 2 expression in GATA1s mutant megakaryocytes rescued normal development, implicating this morphogenetic pathway as a target in human leukemogenesis.

Expandable megakaryocyte cell lines enable clinically applicable generation of platelets from human induced pluripotent stem cells

The donor-dependent supply of platelets is frequently insufficient to meet transfusion needs. To address this issue, we developed a clinically applicable strategy for the derivation of functional platelets from human pluripotent stem cells (PSCs). This approach involves the establishment of stable immortalized megakaryocyte progenitor cell lines (imMKCLs) from PSC-derived hematopoietic progenitors through the overexpression of BMI1 and BCL-XL to respectively suppress senescence and apoptosis and the constrained overexpression of c-MYC to promote proliferation. The resulting imMKCLs can be expanded in culture over extended periods (4-5 months), even after cryopreservation. Halting the overexpression of c-MYC, BMI1, and BCL-XL in growing imMKCLs led to the production of CD42b(+) platelets with functionality comparable to that of native platelets on the basis of a range of assays in vitro and in vivo. The combination of robust expansion capacity and efficient platelet production means that appropriately selected imMKCL clones represent a potentially inexhaustible source of hPSC-derived platelets for clinical application.

Congenital amegakaryocytic thrombocytopenia iPS cells exhibit defective MPL-mediated signaling

Congenital amegakaryocytic thrombocytopenia (CAMT) is caused by the loss of thrombopoietin receptor-mediated (MPL-mediated) signaling, which causes severe pancytopenia leading to bone marrow failure with onset of thrombocytopenia and anemia prior to leukopenia. Because Mpl(-/-) mice do not exhibit the human disease phenotype, we used an in vitro disease tracing system with induced pluripotent stem cells (iPSCs) derived from a CAMT patient (CAMT iPSCs) and normal iPSCs to investigate the role of MPL signaling in hematopoiesis. We found that MPL signaling is essential for maintenance of the CD34+ multipotent hematopoietic progenitor (MPP) population and development of the CD41+GPA+ megakaryocyte-erythrocyte progenitor (MEP) population, and its role in the fate decision leading differentiation toward megakaryopoiesis or erythropoiesis differs considerably between normal and CAMT cells. Surprisingly, complimentary transduction of MPL into normal or CAMT iPSCs using a retroviral vector showed that MPL overexpression promoted erythropoiesis in normal CD34+ hematopoietic progenitor cells (HPCs), but impaired erythropoiesis and increased aberrant megakaryocyte production in CAMT iPSC-derived CD34+ HPCs, reflecting a difference in the expression of the transcription factor FLI1. These results demonstrate that impaired transcriptional regulation of the MPL signaling that normally governs megakaryopoiesis and erythropoiesis underlies CAMT.

OP9 bone marrow stroma cells differentiate into megakaryocytes and platelets

Platelets are essential for hemostatic plug formation and thrombosis. The mechanisms of megakaryocyte (MK) differentiation and subsequent platelet production from stem cells remain only partially understood. The manufacture of megakaryocytes (MKs) and platelets from cell sources including hematopoietic stem cells and pluripotent stem cells have been highlighted for studying the platelet production mechanisms as well as for the development of new strategies for platelet transfusion. The mouse bone marrow stroma cell line OP9 has been widely used as feeder cells for the differentiation of stem cells into MK lineages. OP9 cells are reported to be pre-adipocytes. We previously reported that 3T3-L1 pre-adipocytes differentiated into MKs and platelets. In the present study, we examined whether OP9 cells differentiate into MKs and platelets using MK lineage induction (MKLI) medium previously established to generate MKs and platelets from hematopoietic stem cells, embryonic stem cells, and pre-adipocytes. OP9 cells cultured in MKLI medium had megakaryocytic features, i.e., positivity for surface markers CD41 and CD42b, polyploidy, and distinct morphology. The OP9-derived platelets had functional characteristics, providing the first evidence for the differentiation of OP9 cells into MKs and platelets. We then analyzed gene expressions of critical factors that regulate megakaryopoiesis and thrombopoiesis. The gene expressions of p45NF-E2, FOG, Fli1, GATA2, RUNX1, thrombopoietin, and c-mpl were observed during the MK differentiation. Among the observed transcription factors of MK lineages, p45NF-E2 expression was increased during differentiation. We further studied MK and platelet generation using p45NF-E2-overexpressing OP9 cells. OP9 cells transfected with p45NF-E2 had enhanced production of MKs and platelets. Our findings revealed that OP9 cells differentiated into MKs and platelets in vitro. OP9 cells have critical factors for megakaryopoiesis and thrombopoiesis, which might be involved in a mechanism of this differentiation. p45NF-E2 might also play important roles in the differentiation of OP9 cells into MK lineages cells.

Challenges and promises for the development of donor-independent platelet transfusions

Platelet transfusions are often a life-saving intervention, and the use of platelet transfusions has been increasing. Donor-derived platelet availability can be challenging. Compounding this concern are additional limitations of donor-derived platelets, including variability in product unit quality and quantity, limited shelf life and the risks of product bacterial contamination, other transfusion-transmitted infections, and immunologic reactions. Because of these issues, there has been an effort to develop strategies to generate platelets from exogenously generated precursor cells. If successful, such platelets have the potential to be a safer, more consistent platelet product, while reducing the necessity for human donations. Moreover, ex vivo-generated autologous platelets or precursors may be beneficial for patients who are refractory to allogeneic platelets. For patients with inherited platelet disorders, ex vivo-generated platelets offer the promise of a treatment via the generation of autologous gene-corrected platelets. Theoretically, ex vivo-generated platelets also offer targeted delivery of ectopic proteins to sites of vascular injury. This review summarizes the current, state-of-the-art methodologies in delivering a clinically relevant ex vivo-derived platelet product, and it discusses significant challenges that must be overcome for this approach to become a clinical reality.

Three-stage ex vivo expansion of high-ploidy megakaryocytic cells: toward large-scale platelet production

Hematopoietic stem and progenitor cells (HSPCs) have been cultured using a wide variety of cytokines to promote differentiation into megakaryocytic cells (Mks), the precursors to platelets. Greater Mk DNA content, or ploidy, has been correlated with increased platelet release. Gradients of pH, pO2, and signaling factors regulate megakaryopoiesis in the bone marrow niche. In this study, we demonstrate that a 3-phase culture process with increasing pH and pO2 and different cytokine cocktails greatly increases megakaryocyte production. CD34(+) HSPCs were first cultured at 5% O2 and pH 7.2 with a cytokine cocktail previously shown to promote Mk progenitor production. At day 5, cells were shifted to 20% O2 and pH 7.4 and maintained in 1 of 17 cytokine cocktails identified using a 2(4) factorial design of experiments method to evaluate the effects of interleukin (IL)-3, IL-6, IL-9, and high- or low-dose stem cell factor (SCF), in conjunction with thrombopoietin (Tpo) and IL-11, on expansion of mature Mks from progenitors. The combination of Tpo, high-dose SCF, IL-3, IL-9, and IL-11 best promoted Mk expansion. IL-3 greatly increased total cell fold expansion, but this was partially offset by lower Mk purity. IL-9 promoted CD41 and CD42b expression. High-dose (100 ng/mL) SCF increased Mk production and ploidy. Different commercial media and IL-3 sources substantially impacted differentiation, and X-VIVO 10 serum-free media best supported mature Mk expansion. Shifting from pH 7.4 to pH 7.6 at day 7 increased Mk production by 30%. Treatment with nicotinamide at day 7 or day 8 more than doubled the fraction of high-ploidy (>4N) Mks. Ultimately, the 3-phase culture system gave rise to 44.5±8.1 Mks and 8.5±3.1 high-ploidy Mks per input HSPC. Further optimization was required to improve platelet production. Using Iscove's modified Dulbecco's medium (IMDM)+20% BSA, insulin and transferin (BIT) 9500 Serum Substitute greatly improved the frequency and quality of Mk proplatelet extensions without affecting Mk expansion, commitment, or polyploidization in the 3-phase process. Mks cultured in IMDM+20% BIT 9500 gave rise to platelets with functional activity similar to that of fresh platelets from normal donors, as evidenced by basal tubulin distribution and the expression of surface markers and spreading in response to platelet agonists.

Ex vivo differentiation of cord blood stem cells into megakaryocytes and platelets

Megakaryocytes (MK) are hematopoietic cells present in the bone marrow that are responsible for the production and release of platelets in the circulation. Given their very low frequency (<1%), human MK often need to be derived in culture to study their development or to generate sufficient material for biological studies. This chapter describes a simplified 14-day culture protocol that efficiently leads to the production of MK and platelets from cord blood enriched progenitor cells. A serum-free medium is suggested for the growth of the CB cells together with an optimized cytokine cocktail developed specifically for MK differentiation, expansion, and maturation. Methodologies for flow cytometry analysis, MK and platelets estimation, and MK progenitor assay are also presented.

Pluripotent stem cells reveal the developmental biology of human megakaryocytes and provide a source of platelets for clinical application

Human pluripotent stem cells [PSCs; including human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs)] can infinitely proliferate in vitro and are easily accessible for gene manipulation. Megakaryocytes (MKs) and platelets can be created from human ESCs and iPSCs in vitro and represent a potential source of blood cells for transfusion and a promising tool for studying the human thrombopoiesis. Moreover, disease-specific iPSCs are a powerful tool for elucidating the pathogenesis of hematological diseases and for drug screening. In that context, we and other groups have developed in vitro MK and platelet differentiation systems from human pluripotent stem cells (PSCs). Combining this co-culture system with a drug-inducible gene expression system enabled us to clarify the novel role played by c-MYC during human thrombopoiesis. In the next decade, technical advances (e.g., high-throughput genomic sequencing) will likely enable the identification of numerous gene mutations associated with abnormal thrombopoiesis. Combined with such technology, an in vitro system for differentiating human PSCs into MKs and platelets could provide a novel platform for studying human gene function associated with thrombopoiesis.

Human endometrial stromal stem cells differentiate into megakaryocytes with the ability to produce functional platelets

Human endometrium is a high dynamic tissue that contains endometrial stromal stem cells (hESSCs). The hESSCs have been differentiated into a number of cell lineages. However, differentiation of hESSCs into megakaryocytes (MKs) has not yet been investigated. The aim of this study was to investigate the feasibility of MK generation from hESSCs and subsequent production of functional platelets (PLTs). In our study, hESSCs were cultured from endometrial stromal cells as confirmed by positive stromal cell specific markers (CD90 and CD29) and negative hematopoietic stem cell markers (CD45 and CD34) expression. Then, hESSCs were differentiated in a medium supplemented with thrombopoietin (TPO) for 18 days. The MK differentiation was analyzed by flow cytometry and confocal microscopy. The differentiation medium was collected for PLT production analysis by flow cytometry, transmission electron microscopy and functional measurements. Our results show: 1) MKs were successfully generated from hESSCs as identified by expression of specific markers (CD41a: 1±0.09% and 39±3.0%; CD42b: 1.2±0.06% and 28±2.0%, control vs. differentiation) accompanied with reduction of pluripotent transcription factors (Oct4 and Sox2) expression; 2) The level of PLTs in the differentiation medium was 16±1 number/µl as determined by size (2–4 µm) and CD41a expression (CD41a: 1±0.4% and 90±2.0%, control vs. differentiation); 3) Generated PLTs were functional as evidenced by the up-regulation of CD62p expression and fibrinogen binding following thrombin stimulation; 4) Released PLTs showed similar ultra-structure characteristics (alpha granules, vacuoles and dense tubular system) as PLTs from peripheral blood determined by electron microscopic analysis. Data demonstrate the feasibility of generating MKs from hESSCs, and that the generated MKs release functional PLTs. Therefore, hESSCs could be a potential new stem cell source for in vitro MK/PLT production.

In vitro generation of megakaryocytes and platelets from human embryonic stem cells and induced pluripotent stem cells

Human embryonic stem cells (hESCs) represent a potential source of blood cells for transfusion therapies and a promising tool for studying the ontogeny of hematopoiesis. Moreover, human-induced pluripotent stem cells (hiPSCs), recently established by defined reprogramming factors expressed in somatic cells, represent a further source for the generation of hematopoietic cells. When undifferentiated hESCs or hiPSCs are cultured on either mesenchymal C3H10T1/2 cells or OP-9 stromal cells, they can be differentiated into a hematopoietic niche that concentrates hematopoietic progenitors, which we named "embryonic stem cell-derived sacs" (ES-sacs). We have optimized the in vitro culture condition for obtaining mature megakaryocytes derived from the hematopoietic progenitors within ES-sacs, which are then able to release platelets. These in vitro-generated platelets display integrin activation capability, indicating normal hemostatic function. This novel protocol thus provides a means of generating platelets from hESCs as well as hiPSCs, for the study of normal human thrombopoiesis and also thrombopoiesis in disease conditions using patient-specific hiPSCs.

Megakaryocyte and platelet production from human cord blood stem cells

The cloning of thrombopoietin together with advances in the culture of hematopoietic stem cells have paved the way for the study of megakaryopoiesis, ongoing clinical trials and, in the future, for the potential therapeutic use of ex vivo produced blood substitutes, such as platelets. This chapter describes a 14-day culture protocol for the production of human megakaryocytes (MKs) and platelets, and assays that can be used to characterize the functional properties of the platelets produced ex vivo. CD34(+) cells isolated from cord blood cells are grown in a serum-free medium supplemented with newly developed cytokine cocktails optimized for MK differentiation, expansion, and maturation. Detailed methodologies for flow cytometry analysis of MKs and platelets, for the purification of platelets and functional assays, are presented together with supporting figures. The chapter also provides a brief review on megakaryocytic differentiation and ex vivo MK cultures.

GPIbα regulates platelet size by controlling the subcellular localization of filamin

Interaction between the cytoplasmic domain of GPIbα with its cytoskeletal binding partner, filamin, is a major determinant of platelet size, and deficiency of either protein results in macrothrombocytopenia. To clarify the mechanism by which GPIbα-filamin interactions regulate platelet production, we manipulated the expression levels of filamin and GPIb in cultured embryonic stem cells (ESCs) that were subsequently differentiated into platelets. Knocking down filamins A and B resulted in the production of ESC-derived proplatelets with abnormally large swellings and proplatelet shafts that generated giant platelets in culture. Large platelets could also be generated by overexpressing GPIbα in ESCs, or by overexpressing in vivo a transgene encoding a chimeric protein containing the cytoplasmic domain of GPIbα. To identify the mechanism by which the GPIb:filamin ratio regulates platelet size, we manipulated filamin and GPIbα levels in HEK293T cells and examined the effects of overexpressing either protein on their ability to traffic to the cell periphery. Accumulation of either protein within the endoplasmic reticulum resulted in trapping of the other. Taken together, these data demonstrate that coordinated expression of GPIbα and filamin is required for efficient trafficking of either protein to the cell surface, and for production of normal-sized platelets.

Platelets generated from human embryonic stem cells are functional in vitro and in the microcirculation of living mice

Platelets play an essential role in hemostasis and atherothrombosis. Owing to their short storage time, there is constant demand for this life-saving blood component. In this study, we report that it is feasible to generate functional megakaryocytes and platelets from human embryonic stem cells (hESCs) on a large scale. Differential-interference contrast and electron microscopy analyses showed that ultrastructural and morphological features of hESC-derived platelets were indistinguishable from those of normal blood platelets. In functional assays, hESC-derived platelets responded to thrombin stimulation, formed microaggregates, and facilitated clot formation/retraction in vitro. Live cell microscopy demonstrated that hESC-platelets formed lamellipodia and filopodia in response to thrombin activation, and tethered to each other as observed in normal blood. Using real-time intravital imaging with high-speed video microscopy, we have also shown that hESC-derived platelets contribute to developing thrombi at sites of laser-induced vascular injury in mice, providing the first evidence for in vivo functionality of hESC-derived platelets. These results represent an important step toward generating an unlimited supply of platelets for transfusion. Since platelets contain no genetic material, they are ideal candidates for early clinical translation involving human pluripotent stem cells.

Transient activation of c-MYC expression is critical for efficient platelet generation from human induced pluripotent stem cells

Human (h) induced pluripotent stem cells (iPSCs) are a potentially abundant source of blood cells, but how best to select iPSC clones suitable for this purpose from among the many clones that can be simultaneously established from an identical source is not clear. Using an in vitro culture system yielding a hematopoietic niche that concentrates hematopoietic progenitors, we show that the pattern of c-MYC reactivation after reprogramming influences platelet generation from hiPSCs. During differentiation, reduction of c-MYC expression after initial reactivation of c-MYC expression in selected hiPSC clones was associated with more efficient in vitro generation of CD41a(+)CD42b(+) platelets. This effect was recapitulated in virus integration-free hiPSCs using a doxycycline-controlled c-MYC expression vector. In vivo imaging revealed that these CD42b(+) platelets were present in thrombi after laser-induced vessel wall injury. In contrast, sustained and excessive c-MYC expression in megakaryocytes was accompanied by increased p14 (ARF) and p16 (INK4A) expression, decreased GATA1 expression, and impaired production of functional platelets. These findings suggest that the pattern of c-MYC expression, particularly its later decline, is key to producing functional platelets from selected iPSC clones.

Regulation of stem cell pluripotency and differentiation by G protein coupled receptors

Stem cell-based therapeutics have the potential to effectively treat many terminal and debilitating human diseases, but the mechanisms by which their growth and differentiation are regulated are incompletely defined. Recent data from multiple systems suggest major roles for G protein coupled receptor (GPCR) pathways in regulating stem cell function in vivo and in vitro. The goal of this review is to illustrate common ground between the growing field of stem cell therapeutics and the long-established field of G protein coupled receptor signaling. Herein, we briefly introduce basic stem cell biology and discuss how several conserved pathways regulate pluripotency and differentiation in mouse and human stem cells. We further discuss general mechanisms by which GPCR signaling may impact these pluripotency and differentiation pathways, and summarize specific examples of receptors from each of the major GPCR subfamilies that have been shown to regulate stem cell function. Finally, we discuss possible therapeutic implications of GPCR regulation of stem cell function.

Infusion of mature megakaryocytes into mice yields functional platelets

Thrombopoiesis, the process by which circulating platelets arise from megakaryocytes, remains incompletely understood. Prior studies suggest that megakaryocytes shed platelets in the pulmonary vasculature. To better understand thrombopoiesis and to develop a potential platelet transfusion strategy that is not dependent upon donors, of which there remains a shortage, we examined whether megakaryocytes infused into mice shed platelets. Infused megakaryocytes led to clinically relevant increases in platelet numbers. The released platelets were normal in size, displayed appropriate surface markers, and had a near-normal circulating half-life. The functionality of the donor-derived platelets was also demonstrated in vivo. The infused megakaryocytes mostly localized to the pulmonary vasculature, where they appeared to shed platelets. These data suggest that it may be unnecessary to generate platelets from ex vivo grown megakaryocytes to achieve clinically relevant increases in platelet numbers.

Are there more tricks in the bag for treating thrombocytopenia?

Thrombocytopenia, an abnormally low number of circulating platelets, results from inadequate platelet production, splenic platelet sequestration, or accelerated platelet clearance. Platelet transfusions are now the cornerstone for treating thrombocytopenia. With an ever-expanding demand for platelets, and with many patients having an inadequate response to platelet transfusions, new strategies are needed to treat thrombocytopenia. In this issue of the JCI, Fuentes et al. present provocative data regarding the use of direct megakaryocyte infusions as a novel approach to manage this vexing clinical problem.

Glycoprotein Ibα receptor instability is associated with loss of quality in platelets produced in culture

The development of culture processes for hematopoietic progenitors could lead to the development of a complementary source of platelets for therapeutic purposes. However, functional characterization of culture-derived platelets remains limited, which raises some uncertainties about the quality of platelets produced in vitro. The aim of this study was to define the proportion of functional platelets produced in cord blood CD34+ cell cultures. Toward this, the morphological and functional properties of culture-derived platelet-like particles (PLPs) were critically compared to that of blood platelets. Flow cytometry combined with transmission electron microscopy analyses revealed that PLPs formed a more heterogeneous population of platelets at a different stage of maturation than blood platelets. The majority of PLPs harbored the fibrinogen receptor αIIbβ3, but a significant proportion failed to maintain glycoprotein (GP)Ibα surface expression, a component of the vWF receptor essential for platelet functions. Importantly, GPIbα extracellular expression correlated closely with platelet function, as the GPIIb+ GPIbα+ PLP subfraction responded normally to agonist stimulation as evidenced by α-granule release, adhesion, spreading, and aggregation. In contrast, the GPIIb+ GPIbα⁻ subfraction was unresponsive in most functional assays and appeared to be metabolically inactive. The present study confirms that functional platelets can be generated in cord blood CD34+ cell cultures, though these are highly susceptible to ectodomain shedding of receptors associated with loss of function. Optimization of culture conditions to prevent these deleterious effects and to homogenize PLPs is necessary to improve the quality and yields of culture-derived platelets before they can be recognized as a suitable complementary source for therapeutic purposes.

In vitro megakaryocyte production and platelet biogenesis: state of the art

The exciting and extraordinary capabilities of stem cells to proliferate and differentiate into numerous cell types not only offers promises for changing how diseases are treated but may also impact how transfusion medicine may be practiced in the future. The possibility of growing platelets in the laboratory to some day supplement and/or replace standard platelet products has clear advantages for blood centers and patients. Because of the high utilization of platelets by patients undergoing chemotherapy or receiving stem cell transplants, platelet transfusions have steadily increased over the past decades. This trend is likely to continue as the number of adult and pediatric patients receiving stem cell transplants is also continuously rising. As a result of increased demand, coupled with the short shelf-life of platelet concentrates, providing platelets to patients can stretch the resources of most blood centers and drive donor recruitment efforts, and on occasion, platelet shortages can compromise the care of thrombocytopenic patients.

Novel approach for formation of platelet-like particles from mouse embryonic stem cells without using feeder cells

Megakaryocytes (MKs) and platelet-like particles (PLPs) have generally been obtained by culturing embryonic stem (ES) cells over feeder cells. However, using feeder cells need many labor-consuming processes and the MK and PLP fractions obtained are often contaminated by such cells and their fragments. Here we describe our new culture system for differentiating mouse ES cells to MKs and PLPs without using feeder cells. ES cells are differentiated to cells with MK-like morphology and properties, including proplatelet formation, high ploidy (>8N), and CD41 expression. The culture medium contained PLPs expressing platelet glycoproteins, CD41 and GPIb. Integrin alpha(IIb)beta(3) of PLPs can be activated by thrombin. Addition of the metalloproteinase inhibitor TAPI-2 to the culture increased the surface expression of GPIbalpha and augmented the adhesion of PLPs to immobilized von Willebrand factor through decreasing the shedding of GPIbalpha. Thus our mouse ES cells culture system is a suitable and efficient method for obtaining MKs and functional PLPs that obviates the need for feeder cells.