In vitro Generation of Megakaryocytes and Platelets

Efficient Generation of Megakaryocyte Progenitors and Platelets From HSPCs via JAK2/STAT3 Signaling

The supply of platelets for clinical transfusion is often insufficient to meet growing demand. Platelet regeneration from stem cells offers a potential solution to reduce reliance on donor-based transfusions. However, the current differentiation efficiency is suboptimal. A novel approach is presented that significantly enhances platelet yield from hematopoietic stem and progenitor cells (HSPCs) by increasing the production of megakaryocyte progenitors (MkPs) and mature megakaryocytes (MKs). This method employs the overexpression of HES7 combined with the HDAC inhibitor and GABA agonist (collectively termed the VGM cocktail). The VGM cocktail induces MkP production with an efficiency of up to 90%, validated across HSPCs from various donors. These MkPs exhibit extended proliferative capacity, remaining viable for up to 51 days in prolonged culture, and show enhanced maturation into MKs. This differentiation system effectively replicates in vivo thrombocytopoiesis, as evidenced by polyploidization, long protrusions, and proplatelet formation. Transfusion of VGM-induced MkPs into thrombocytopenic mice results in the release of platelets into circulation. Mechanistic investigation identifies the JAK2/STAT3 signaling pathway as critical in promoting megakaryopoiesis within this system. Therefore, this study demonstrates that the VGM cocktail facilitates enhanced platelet production by promoting MkP generation, offering a promising strategy for in vitro platelet regeneration for clinical applications.

Programming megakaryocytes to produce engineered platelets for delivering non-native proteins

Platelets are anucleate cells naturally filled with secretory granules that store large amounts of protein to be released in response to certain physiological conditions. Cell engineering can endow platelets with the ability to deliver non-native proteins by modifying them as they develop during the cell fate process. This study presents a strategy to efficiently generate mouse platelets from pluripotent stem cells and demonstrates their potential as bioengineered protein delivery platforms. By modifying megakaryocytes, the progenitor cells of platelets, we successfully engineered platelets capable of packaging and delivering non-native proteins. These engineered platelets can offer flexible delivery platforms to release non-native proteins in a controlled manner upon activation when packaged into α-granules or deliver active enzymes to genetically alter recipient cells. Our findings highlight platelets as a promising tool for protein delivery in cell therapy applications.

Biological Aging and Venous Thromboembolism: A Review of Telomeres and Beyond

Although venous thromboembolism (VTE) is the third most common cardiovascular disease, and the risk of VTE increases sharply with advancing age, approximately 40% of VTE cases are currently classified as unprovoked, highlighting the importance of risk factor research. While chronological aging is associated with the risk of VTE, the association with biological aging remains unclear. Biological aging is highly complex, influenced by several dysregulated cellular and biochemical mechanisms. In the last decade, advancements in omics methodologies provided insights into the molecular complexity of biological aging. Techniques such as high-throughput genomics, epigenomics, transcriptomics, proteomics, and metabolomics analyses identified and quantified numerous epigenetic markers, transcripts, proteins, and metabolites. These methods have also revealed the molecular alterations organisms undergo as they age. Despite the progress, there is still a lack of consensus regarding the methods for assessing and validating these biomarkers, and their application lacks standardization. This review gives an overview of biomarkers of biological aging, including telomere length, and their potential role for VTE. Furthermore, we critically examine the advantages and disadvantages of the proposed methods and discuss possible future directions for investigating biological aging in VTE.

Platelet generation in vivo and in vitro

ABSTRACT

Platelets play crucial roles in hemostasis, thrombosis, and immunity, but our understanding of their complex biogenesis (thrombopoiesis) is currently incomplete. Deeper insight into the mechanisms of platelet biogenesis inside and outside the body is fundamental for managing hematological disorders and for the development of novel cell-based therapies. In this article, we address the current understanding of in vivo thrombopoiesis, including mechanisms of platelet generation from megakaryocytes (proplatelet formation, cytoplasmic fragmentation, and membrane budding) and their physiological location. Progress has been made in replicating these processes in vitro for potential therapeutic application, notably in platelet transfusion and bioengineering of platelets for novel targeted therapies. The current platelet-generating systems and their limitations, particularly yield, scalability, and functionality, are discussed. Finally, we highlight the current controversies and challenges in the field that need to be addressed to achieve a full understanding of these processes, in vivo and in vitro.

A modern overview of the process of platelet formation (thrombocytopoiesis) and its dependence on several factors

Structural and functional alterations in platelets are an actual problem that requires more attention. The treatment of these illnesses proves challenging, inefficient and heavily relies on platelet donations. A difficult task confronting science is producing platelets in vitro, which calls for meticulous examination of factors affecting platelet generation. It is known that megakaryocytes produce platelets in vitro and in vivo differently: in the laboratory we can get a smaller number of platelets compared to the human body. This review primarily examines the stages of megakaryocyte maturation and the processes involved in platelet formation. The article reflects the results of both fundamental research on the problem and the new results obtained over the past decade. Currently, most scientists accept the pro-platelets theory of platelet formation. This review aims to explore in detail each stage of pro-platelet formation and the platelet formation process. It explains on the processes of polyploidization, endomitosis, and apoptosis, as well as the functions of structural cell components (microtubules, mitochondria, T- and α-granules) and pro-platelet migration. The microenvironment influence is acknowledged for the osteoblastic and vascular niches that affect thrombocytopoiesis. The additional aspect is the contribution of specific proteins to thrombocytopoiesis such as RhoA, β1-tubulin, cytokines IL-6, IL-8, Toll-like receptors, etc.

Implementing evidence-based restrictive neonatal intensive care unit platelet transfusion guidelines

Platelet transfusions are life-saving treatments for specific populations of neonates. However, recent evidence indicates that liberal prophylactic platelet transfusion practices cause harm to premature neonates. New efforts to better balance benefits and risks are leading to the adoption of more restrictive platelet transfusion guidelines in neonatal intensive care units (NICU). Although restrictive guidelines have the potential to improve outcomes, implementation barriers exist. We postulate that as neonatologists become more familiar with the data on the harm of liberal platelet transfusions, enthusiasm for restrictive guidelines will increase and barriers to implementation will decrease. Thus, we focused this educational review on; (1) the adverse effects of platelet transfusions to neonates, (2) awareness of platelet transfusion "refractoriness" in thrombocytopenic neonates and its association with poor outcomes, and (3) the impetus to find alternatives to transfusing platelets from adult donors to NICU patients.

G2 arrest primes hematopoietic stem cells for megakaryopoiesis

In contrast to most hematopoietic lineages, megakaryocytes (MKs) can derive rapidly and directly from hematopoietic stem cells (HSCs). The underlying mechanism is unclear, however. Here, we show that DNA damage induces MK markers in HSCs and that G2 arrest, an integral part of the DNA damage response, suffices for MK priming followed by irreversible MK differentiation in HSCs, but not in progenitors. We also show that replication stress causes DNA damage in HSCs and is at least in part due to uracil misincorporation in vitro and in vivo. Consistent with this notion, thymidine attenuated DNA damage, improved HSC maintenance, and reduced the generation of CD41+ MK-committed HSCs. Replication stress and concomitant MK differentiation is therefore one of the barriers to HSC maintenance. DNA damage-induced MK priming may allow rapid generation of a lineage essential to immediate organismal survival, while also removing damaged cells from the HSC pool.

Cell cycle-dependent centrosome clustering precedes proplatelet formation

Platelet-producing megakaryocytes (MKs) primarily reside in the bone marrow, where they duplicate their DNA content with each cell cycle resulting in polyploid cells with an intricate demarcation membrane system. While key elements of the cytoskeletal reorganizations during proplatelet formation have been identified, what initiates the release of platelets into vessel sinusoids remains largely elusive. Using a cell cycle indicator, we observed a unique phenomenon, during which amplified centrosomes in MKs underwent clustering following mitosis, closely followed by proplatelet formation, which exclusively occurred in G1 of interphase. Forced cell cycle arrest in G1 increased proplatelet formation not only in vitro but also in vivo following short-term starvation of mice. We identified that inhibition of the centrosomal protein kinesin family member C1 (KIFC1) impaired clustering and subsequent proplatelet formation, while KIFC1-deficient mice exhibited reduced platelet counts. In summary, we identified KIFC1- and cell cycle-mediated centrosome clustering as an important initiator of proplatelet formation from MKs.

Thrombopoietin-independent generation of platelet-like particles from megakaryoblastic cells

The use of megakaryoblastic leukemia MEG-01 cells can help reveal the mechanisms of thrombopoiesis. However, conventional in vitro activation of platelet release from MEG-01 cells requires thrombopoietin, which is costly. Here, we aim to develop a more straightforward and affordable method. Synchronization of the MEG-01 cells was initially performed using serum-free culture, followed by spontaneous cell differentiation in the presence of serum. Different stages of megakaryoblast differentiation were classified based on cell morphology, DNA content, and cell cycle. The MEG-01 cells released platelet-like particles at a level comparable to that of the thrombopoietin-activated MEG-01 cells. The platelet-like particles were distinguishable from PLP-derived extracellular vesicles and could express P-selectin following ADP activation. Importantly, the platelet-like particles induced fibrin clotting in vitro using platelet-poor plasma. Therefore, this thrombopoietin-independent cell synchronization method is an effective and straightforward method for studying megakaryopoiesis and thrombopoiesis.

Baffled-flow culture system enables the mass production of megakaryocytes from human embryonic stem cells by enhancing mitochondrial function

Human embryonic stem cells (hESCs) have become an ideal cell source for the ex vivo generation of megakaryocyte (MK) and platelet products for clinical applications. However, an ongoing challenge is to establish scalable culture systems to maximize the yield of stem cell-derived MKs that release platelets. We defined a specific dynamic 3D manufacturing system in a baffled-flow manner that could remarkably facilitate megakaryopoiesis and increase the yield of platelet-producing MKs from hESCs within a 12-day induction period. Additionally, an increased number of >16N ploidy MKs, proplatelets, and platelets were generated from induced cells harvested on Day 12 using the specific dynamic culture method. The specific dynamic culture method significantly enhanced endothelium-to-haematopoietic transition and early haematopoiesis. More importantly, MK fate was significantly facilitated in a specific dynamic manner during early haematopoiesis. Mechanistically, this dynamic culture significantly enhanced mitochondrial function via the oxidative phosphorylation pathway and caused differentiation skewing of hESCs toward megakaryopoiesis. This study can aid in the automatic and scalable production of MKs from stem cells using baffled-flow bioreactors and assist in the manufacturing of hESC-derived MK and platelet products.

Humanized mouse models for inherited thrombocytopenia studies

Inherited thrombocytopenia (IT) is a group of hereditary disorders characterized by a reduced platelet count as the main clinical manifestation, and often with abnormal platelet function, which can subsequently lead to impaired hemostasis. In the past decades, humanized mouse models (HMMs), that are mice engrafted with human cells or genes, have been widely used in different research areas including immunology, oncology, and virology. With advances of the development of immunodeficient mice, the engraftment, and reconstitution of functional human platelets in HMM permit studies of occurrence and development of platelet disorders including IT and treatment strategies. This article mainly reviews the development of humanized mice models, the construction methods, research status, and problems of using humanized mice for the in vivo study of human thrombopoiesis.

Engineered cord blood megakaryocytes evade killing by allogeneic T-cells for refractory thrombocytopenia

The current global platelet supply is often insufficient to meet all the transfusion needs of patients, in particular for those with alloimmune thrombocytopenia. To address this issue, we have developed a strategy employing a combination of approaches to achieve more efficient production of functional megakaryocytes (MKs) and platelets collected from cord blood (CB)-derived CD34+ hematopoietic cells. This strategy is based on ex-vivo expansion and differentiation of MKs in the presence of bone marrow niche-mimicking mesenchymal stem cells (MSCs), together with two other key components: (1) To enhance MK polyploidization, we used the potent pharmacological Rho-associated coiled-coil kinase (ROCK) inhibitor, KD045, resulting in liberation of increased numbers of functional platelets both in-vitro and in-vivo; (2) To evade HLA class I T-cell-driven killing of these expanded MKs, we employed CRISPR-Cas9-mediated β-2 microglobulin (β2M) gene knockout (KO). We found that coculturing with MSCs and MK-lineage-specific cytokines significantly increased MK expansion. This was further increased by ROCK inhibition, which induced MK polyploidization and platelet production. Additionally, ex-vivo treatment of MKs with KD045 resulted in significantly higher levels of engraftment and donor chimerism in a mouse model of thrombocytopenia. Finally, β2M KO allowed MKs to evade killing by allogeneic T-cells. Overall, our approaches offer a novel, readily translatable roadmap for producing adult donor-independent platelet products for a variety of clinical indications.

Prognostic Value of Platelet to Lymphocyte Ratio in Sepsis: A Systematic Review and Meta-analysis

The goal of this study was to conduct a systematic review of the literature on the relationship between peripheral blood platelet to lymphocyte ratio (PLR) and mortality in sepsis and to integrate the findings in a meta-analysis. An electronic search of three main databases was performed: PubMed, Embase, and Scopus on 19 December 2021. Finally, 16 studies comprising 2403 septic patients, including 1249 survivors and 1154 nonsurvivors, were included in this meta-analysis. We found that PLR levels were significantly higher in nonsurvivors than in survivors (random effect model: SMD = 0.72, 95% CI; 0.35–1.10, p < 0.001). However, significant heterogeneity was observed across the studies (I² = 94.1%, p < 0.01). So, we used random effect model in our meta-analysis. In the subgroup analysis, according to mortality time, patients deceased during one month after sepsis had elevated levels of PLR compared to survivors (SMD = 1.03, 95% CI = 0.15-1.92, p = 0.22). However, in-hospital mortality was not associated with PLR level (SMD = 0.41, 95% CI = −0.18-0.99, p = 0.175). Our findings support PLR to be a promising biomarker that can be readily integrated into clinical settings to aid in the prediction and prevention of sepsis mortality.

Ectopic Expression of FVIII in HPCs and MSCs Derived from hiPSCs with Site-Specific Integration of ITGA2B Promoter-Driven BDDF8 Gene in Hemophilia A

Hemophilia A (HA) is caused by mutations in the coagulation factor VIII (FVIII) gene (F8). Gene therapy is a hopeful cure for HA; however, FVIII inhibitors formation hinders its clinical application. Given that platelets promote coagulation via locally releasing α-granule, FVIII ectopically expressed in platelets has been attempted, with promising results for HA treatment. The B-domain-deleted F8 (BDDF8), driven by a truncated ITGA2B promoter, was targeted at the ribosomal DNA (rDNA) locus of HA patient-specific induced pluripotent stem cells (HA-iPSCs). The F8-modified, human induced pluripotent stem cells (2bF8-iPSCs) were differentiated into induced hematopoietic progenitor cells (iHPCs), induced megakaryocytes (iMKs), and mesenchymal stem cells (iMSCs), and the FVIII expression was detected. The ITGA2B promoter-driven BDDF8 was site-specifically integrated into the rDNA locus of HA-iPSCs. The 2bF8-iPSCs were efficiently differentiated into 2bF8-iHPCs, 2bF8-iMKs, and 2bF8-iMSCs. FVIII was 10.31 ng/10⁶ cells in lysates of 2bF8-iHPCs, compared to 1.56 ng/10⁶ cells in HA-iHPCs, and FVIII was 3.64 ng/10⁶ cells in 2bF8-iMSCs lysates, while 1.31 ng/10⁶ cells in iMSCs with CMV-driven BDDF8. Our results demonstrated a high expression of FVIII in iHPCs and iMSCs derived from hiPSCs with site-specific integration of ITGA2B promoter-driven BDDF8, indicating potential clinical prospects of this platelet-targeted strategy for HA gene therapy.