In vitro megakaryocyte culture from human bone marrow aspirates as a research and diagnostic tool

Megakaryocytes (MKs) are relatively rare in bone marrow, comprising <0.05% of the nucleated cells, which makes direct isolation from human bone marrow impractical. As such, in vitro expansion of primary MKs from patient samples offers exciting fundamental and clinical opportunities. As most of the developed ex vivo methods require a substantial volume of biomaterial, they are not widely performed on young patients. Here we propose a simple, robust, and adapted method of primary human MK culture from 1 mL of bone marrow aspirate. Our technique uses a small volume of bone marrow per culture, uses straightforward isolation methods, and generates approximately 6 × 10⁵ mature MKs per culture. The relative high cell purity and yield achieved by this technique, combined with efficient use of low volumes of bone marrow, make this approach suitable for diagnostic and basic research of human megakaryopoiesis.

PAK1 Regulates MEC-17 Acetyltransferase Activity and Microtubule Acetylation during Proplatelet Extension

Mature megakaryocytes extend long processes called proplatelets from which platelets are released in the blood stream. The Rho GTPases Cdc42 and Rac as well as their downstream target, p21-activated kinase 2 (PAK2), have been demonstrated to be important for platelet formation. Here we address the role, during platelet formation, of PAK1, another target of the Rho GTPases. PAK1 decorates the bundled microtubules (MTs) of megakaryocyte proplatelets. Using a validated cell model which recapitulates proplatelet formation, elongation and platelet release, we show that lack of PAK1 activity increases the number of proplatelets but restrains their elongation. Moreover, in the absence of PAK1 activity, cells have hyperacetylated MTs and lose their MT network integrity. Using inhibitors of the tubulin deacetylase HDAC6, we demonstrate that abnormally high levels of MT acetylation are not sufficient to increase the number of proplatelets but cause loss of MT integrity. Taken together with our previous demonstration that MT acetylation is required for proplatelet formation, our data reveal that MT acetylation levels need to be tightly regulated during proplatelet formation. We identify PAK1 as a direct regulator of the MT acetylation levels during this process as we found that PAK1 phosphorylates the MT acetyltransferase MEC-17 and inhibits its activity.

Cyanidin-3-O-β-glucoside, a Natural Polyphenol, Exerts Proapoptotic Effects on Activated Platelets and Enhances Megakaryocytic Proplatelet Formation

This study investigated whether the anthocyanin cyanidin-3-O-β-glucoside (Cy-3-g) could affect platelet apoptosis and proplatelet formation in vitro. Thrombin-stimulated or resting human platelets and Meg-01 megakaryocytes were incubated with Cy-3-g (0, 0.5, 5, or 50 μM). We found that the percentage of the platelet mitochondrial membrane potential treated with 5 and 50 μM Cy-3-g was significantly higher than control (15.50% ± 3.24% and 29.77% ± 4.06% versus 2.76% ± 1.33%, respectively; P < 0.05). Treatment with 5 and 50 μM Cy-3-g significantly increased phosphatidylserine exposure compared with control (40.56% ± 10.53% and 76.62% ± 8.28% versus 15.43% ± 3.93%, respectively; P < 0.05). Moreover, Cy-3-g significantly increased the expression of Bax, Bak, and cytochrome c while markedly decreasing Bcl-xL and Bcl-2 expression as well as stimulating caspase-3, caspase-9, caspase-8, Bid, and gelsolin cleavage in thrombin-activated platelets in a dose-dependent manner ( P < 0.05). However, no significant differences were observed in the apoptosis of resting platelets when treated with Cy-3-g ( P > 0.05). Furthermore, Cy-3-g significantly ( P < 0.05) enhanced cell viability (50 μM versus control, 1.34 ± 0.01 versus 0.35 ± 0.02), the number of colony-forming unit-megakaryocytes (50 μM versus control, 38 ± 3 versus 8 ± 3), CD41 expression (50 μM versus control, 96.80% ± 2.55% versus 25.57% ± 2.86%), DNA ploidy (16N) (50 μM versus control, 19.73% ± 2.34% versus 4.42% ± 1.96%), and proplatelet formation (50 μM versus control, 27.5% ± 3.77% versus 7.67% ± 2.25%) in Meg-01 cells. In conclusion, Cy-3-g promotes activated platelet apoptosis and enhances megakaryocyte proliferation, differentiation, and proplatelet formation in vitro.

In vitro culture of murine megakaryocytes from fetal liver-derived hematopoietic stem cells

Megakaryocytes (MKs) are specialized precursor cells committed to producing and proliferating platelets. In a cytoskeletal-driven process, mature MKs generate platelets by releasing thin cytoplasmic extensions, named proplatelets, into the sinusoids. Due to knowledge gaps in this process and mounting clinical demand for non-donor-based platelet sources, investigators are successfully developing artificial culture systems to recreate the environment of platelet biogenesis. Nevertheless, drawbacks in current methods entail elaborate procedures for stem cell enrichment, extensive growth periods, low MK yield, and poor proplatelet production. We propose a simple, robust method of primary MK culture that utilizes fetal livers from pregnant mice. Our technique reduces expansion time to 4 days, and generates ~15,000-20,000 MKs per liver. Approximately, 20-50% of these MKs produce structurally dense, high-quality proplatelets. In this review, we outline our method of MK culture and isolation.

Endocytosed factor V is trafficked to CD42b+ proplatelet extensions during differentiation of human umbilical cord blood-derived megakaryocytes

Plasma- and platelet-derived factor Va are essential for thrombin generation catalyzed by the prothrombinase complex; however, several observations demonstrate that the platelet-derived cofactor, which is formed following megakaryocyte endocytosis and modification of the plasma procofactor, factor V, is more hemostatically relevant. Factor V endocytosis, as a function of megakaryocyte differentiation and proplatelet formation, was assessed by flow cytometry and microscopy in CD34⁺ hematopoietic progenitor cells isolated from human umbilical cord blood and cultured for 12 days in the presence of cytokines to induce ex vivo differentiation into megakaryocytes. Expression of an early marker of megakaryocyte differentiation, CD41, endocytosis of factor V, and the percentage of CD41⁺ cells that endocytosed factor V increased from days 6 to 12 of differentiation. In contrast, statistically significant decreases in expression of the stem cell marker, CD34, and in the percentage of CD34⁺ cells that endocytosed factor V were observed. A statistically significant increase in the expression of CD42b, a late marker of megakaryocyte differentiation, was also observed over time, such that by Day 12, all CD42b⁺ cells endocytosed factor V and expressed CD41. This endocytosed factor V was trafficked to proplatelet extensions and was localized in a punctate pattern in the cytoplasm consistent with its storage in α-granules. In conclusion, loss of CD34 and expression of CD42b define cells capable of factor V endocytosis and trafficking to proplatelet extensions during differentiation of megakaryocytes ex vivo from progenitor cells isolated from umbilical cord blood.

Platelet dense granules begin to selectively accumulate mepacrine during proplatelet formation

Platelet dense granules (DGs) are storage organelles for calcium ions, small organic molecules such as ADP and serotonin, and larger polyphosphates that are secreted upon platelet stimulation to enhance platelet activation, adhesion, and stabilization at sites of vascular damage. DGs are thought to fully mature within megakaryocytes (MKs) prior to platelet formation. Here we challenge this notion by exploiting vital fluorescent dyes to distinguish mildly acidic DGs from highly acidic compartments by microscopy in platelets and MKs. In isolated primary mouse platelets, compartments labeled by mepacrine - a fluorescent weak base that accumulates in DGs - are readily distinguishable from highly acidic compartments, likely lysosomes, that are labeled by the acidic pH indicator, LysoTracker, and from endolysosomes and alpha granules labeled by internalized and partially digested DQ™ BSA. By contrast, in murine fetal liver- and human CD34⁺ cell-derived MKs and the megakaryocytoid cell lines, MEG-01 and differentiated G1ME2, labeling by mepacrine overlapped nearly completely with labeling by LysoTracker and partially with labeling by DQ™ BSA. Mepacrine labeling in G1ME2-derived MKs was fully sensitive to proton ATPase inhibitors, but was only partially sensitive in platelets. These data indicate that mepacrine in MKs accumulates as a weak base in endolysosomes but is likely pumped into or retained in separate DGs in platelets. Fluorescent puncta that labeled uniquely for mepacrine were first evident in G1ME2-derived proplatelets, suggesting that DGs undergo a maturation step that initiates in the final stages of MK differentiation.

Interpreting the developmental dance of the megakaryocyte: a review of the cellular and molecular processes mediating platelet formation

Platelets are essential for haemostasis, and thrombocytopenia (platelet counts <150 × 10(9) /l) is a major clinical problem encountered across a number of conditions, including immune thrombocytopenic purpura, myelodysplastic syndromes, chemotherapy, aplastic anaemia, human immunodeficiency virus infection, complications during pregnancy and delivery, and surgery. Circulating blood platelets are specialized cells that function to prevent bleeding and minimize blood vessel injury. Platelets circulate in their quiescent form, and upon stimulation, activate to release their granule contents and spread on the affected tissue to create a physical barrier that prevents blood loss. The current model of platelet formation states that large progenitor cells in the bone marrow, called megakaryocytes, release platelets by extending long, branching processes, designated proplatelets, into sinusoidal blood vessels. This review will focus on different factors that impact megakaryocyte development, proplatelet formation and platelet release. It will highlight recent studies on thrombopoeitin-dependent megakaryocyte maturation, endomitosis and granule formation, cytoskeletal contributions to proplatelet formation, the role of apoptosis, and terminal platelet formation and release.

Thrombocytogenesis by megakaryocyte; Interpretation by protoplatelet hypothesis

Serial transmission electron microscopy of human megakaryocytes (MKs) revealed their polyploidization and gradual maturation through consecutive transition in characteristics of various organelles and others. At the beginning of differentiation, MK with ploidy 32N, e.g., has 16 centrosomes in the cell center surrounded by 32N nucleus. Each bundle of microtubules (MTs) emanated from the respective centrosome supports and organizes 16 equally volumed cytoplasmic compartments which together compose one single 32N MK. During the differentiation, single centriole separated from the centriole pair, i.e., centrosome, migrates to the most periphery of the cell through MT bundle, corresponding to a half of the interphase array originated from one centrosome, supporting one "putative cytoplasmic compartment" (PCC). Platelet demarcation membrane (DM) is constructed on the boundary surface between neighbouring PCCs. Matured PCC, composing of a tandem array of platelet territories covered by a sheet of DM is designated as protoplatelet. Eventually, the rupture of MK results in release of platelets from protoplatelets. (Communicated by Tadamitsu Kishimoto, M.J.A.).