Gestodene, a novel positive allosteric modulator of PAR1, enhances PAR1-mediated human platelet aggregation

Background: Protease-activated receptor 1 (PAR1) is expressed in human platelets and can be activated by low concentrations of thrombin. Vorapaxar, a selective antagonist of PAR1, inhibits thrombin-induced calcium mobilization in human platelet, which is associated with an increased risk of bleeding. Conversely, the administration of a positive allosteric modulator (PAM) of PAR1 may pose a substantial risk of thrombosis due to inducing excessive platelet activation. In this study, we discovered a novel PAM of PAR1 and investigated the effect of enhanced PAR1 activation by PAM of PAR1 on platelet activation. Methods: To find PAMs of PAR1, a cell-based screen was performed in HT29 cells, and finally, gestodene, an oral contraceptive drug (OC), was identified as a novel PAM of PAR1. The mechanism of action of gestodene and its effects on platelet activation were investigated in human megakaryocytic leukemia cell line MEG-01 cells and human platelet. Results: Gestodene enhanced both thrombin- and PAR1-activating peptide (AP)-induced intracellular calcium levels in a dose-dependent manner without altering PAR2 and PAR4 activity. Gestodene significantly increased PAR1-AP-induced internalization of PAR1 and phosphorylation of ERK1/2, and the enhancing effects were significantly blocked by vorapaxar. Furthermore, gestodene potently increased PAR1-AP induced morphological changes in MEG-01 cells. Remarkably, in human blood, gestodene exerted a robust augmentation of PAR1-AP-induced platelet aggregation, and vorapaxar effectively attenuated the gestodene-induced enhancement of platelet aggregation mediated by PAR1. Conclusion: Gestodene is a selective PAM of PAR1 and suggest one possible mechanism for the increased risk of venous thromboembolism associated with OCs containing gestodene.

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.

Dengue and Zika virus differential infection of human megakaryoblast MEG-01 reveals unique cellular markers

Platelet count is widely used for the diagnosis and follow-up of patients with dengue. Despite its close viral structural and symptomatic homology, ZIKV infection does not typically induce significant thrombocytopenia. To determine the effect of DENV-2 and ZIKV infection on human platelet precursors we utilized MEG-01 cell line to evaluate the viral infection, viability, innate gene expression and release of platelet-like particles (PLPs). DENV-2 induced a higher proportion of cell death at 48-72 h post-infection than ZIKV. The median range of intracellular NS1+/E+ cells was 11.2% (3.3%-25%) and 5% (3%-8.1%) for DENV-2 and ZIKV, respectively (p = 0.03). MEG-01 cells infected with DENV-2 quickly expressed higher levels of IFN-β, indolamine 2,3-dioxygenase and CXCL10 mRNA compared to ZIKV infected cells and DENV-2 but not ZIKV infection reduced the number PLPs from stimulated MEG-01 cells. The results shed light into mechanisms including thrombocytopenia present in patients with DENV but absent in ZIKV infections.

Dengue Virus and Platelets: From the Biology to the Clinic

Dengue is one of the most important vector-borne viral illnesses found in tropical and subtropical regions. Colombia has one of the highest rates of dengue cases in the Americas. Severe dengue virus (DENV) infection presents with capillary leakage, hemorrhage, and organ compromise, eventually leading to death. Over the years, there have been many efforts to develop a vaccine that guarantees protective immunity, but they have been partially successful, as such immunity would need to guarantee protection against four distinct viral serotypes. Absolute platelet count is a laboratory parameter used to monitor the clinical progression of DENV, as infection is often accompanied by thrombocytopenia. Although this finding is well described with respect to the natural history of the disease, there are various hypotheses as to the cause of this rapid decrease, and several in vivo and ex vivo models have been used to explain the effect of DENV infection on platelets and their precursors. DENV infects and activates platelets, facilitating their elimination through recognition by phagocytic cells and peripheral margination. However, infection also affects the precursors in the bone marrow by modulating megakaryopoiesis. The objective of this article is to explore various proposed mechanisms of DENV-induced thrombocytopenia to better understand the pathophysiology and clinical presentations of this highly relevant viral infection.

PAR4-Mediated PI3K/Akt and RhoA/ROCK Signaling Pathways Are Essential for Thrombin-Induced Morphological Changes in MEG-01 Cells

Thrombin stimulates platelets via a dual receptor system of protease-activated receptors (PARs): PAR1 and PAR4. PAR1 activation induces a rapid and transient signal associated with the initiation of platelet aggregation, whereas PAR4 activation results in a prolonged signal, required for later phases, that regulates the stable formation of thrombus. In this study, we observed differential signaling pathways for thrombin-induced PAR1 and PAR4 activation in a human megakaryoblastic leukemia cell line, MEG-01. Interestingly, thrombin induced both calcium signaling and morphological changes in MEG-01 cells via the activation of PAR1 and PAR4, and these intracellular events were very similar to those observed in platelets shown in previous studies. We developed a novel image-based assay to quantitatively measure the morphological changes in living cells, and observed the underlying mechanism for PAR1- and PAR4-mediated morphological changes in MEG-01 cells. Selective inhibition of PAR1 and PAR4 by vorapaxar and BMS-986120, respectively, showed that thrombin-induced morphological changes were primarily mediated by PAR4 activation. Treatment of a set of kinase inhibitors and 2-aminoethoxydiphenyl borate (2-APB) revealed that thrombin-mediated morphological changes were primarily regulated by calcium-independent pathways and PAR4 activation-induced PI3K/Akt and RhoA/ROCK signaling pathways in MEG-01 cells. These results indicate the importance of PAR4-mediated signaling pathways in thrombin-induced morphological changes in MEG-01 cells and provide a useful in vitro cellular model for platelet research.

High-Throughput Production of Platelet-Like Particles

The in vitro production of platelets could provide a life-saving intervention for patients that would otherwise require donor-derived platelets. Producing large numbers of platelets in vitro from their progenitor cells, megakaryocytes, remains remarkably difficult and inefficient. Here, a human megakaryoblast leukemia cell line (MEG-01) was used to assess the maturation of megakaryocytes and to develop a new methodology for producing high numbers of platelet-like particles from mature MEG-01 cells in vitro.

Dengue virus infection impedes megakaryopoiesis in MEG-01 cells where the virus envelope protein interacts with the transcription factor TAL-1

Dengue virus (DENV) infection causes dengue fever in humans, which can lead to thrombocytopenia showing a marked reduction in platelet counts, and dengue hemorrhagic fever. The virus may cause thrombocytopenia either by destroying the platelets or by interfering with their generation via the process of megakaryopoiesis. MEG-01 is the human megakaryoblastic leukemia cell line that can be differentiated in vitro by phorbol-12-myristate-13-acetate (PMA) treatment to produce platelet-like-particles (PLPs). We have studied DENV infection of MEG-01 cells to understand its effect on megakaryopoiesis and the generation of PLPs. We observed that DENV could infect only naive MEG-01 cells, and differentiated cells were refractory to virus infection/replication. However, DENV-infected MEG-01 cells, when induced for differentiation with PMA, supported an enhanced viral replication. Following the virus infection, the MEG-01 cells showed a marked reduction in the surface expression of platelet markers (CD41, CD42a, and CD61), a decreased polyploidy, and significantly reduced PLP counts. DENV infection caused an enhanced Notch signaling in MEG-01 cells where the virus envelope protein was shown to interact with TAL-1, a host protein important for megakaryopoiesis. These observations provide new insight into the role of DENV in modulating the megakaryopoiesis and platelet production process.

Pro-apoptotic properties and mitochondrial functionality in platelet-like-particles generated from low Aspirin-incubated Meg-01 cells

Long-term therapy with low Aspirin (ASA) dose is basis to prevent thrombotic acute events. However, the anti-platelet mechanisms of ASA remain not completely known. The aim was to analyze if in vitro exposure of human megakaryocytes to low ASA concentration may alter the apoptotic features of the newly formed platelets. Cultured Meg-01 cells, a human megakaryoblastic cell line, were stimulated to form platelets with 10 nmol/L phorbol 12-myristate-13-acetate (PMA) in the presence and absence of ASA (0.33 mmol/L). Results revealed that platelet-like particles (PLPs) derived from ASA-exposed Meg-01 cells, showed higher content of pro-apoptotic proteins Bax and Bak than PLPs from non-ASA incubated Meg-01 cells. It was accompanied of reduced cytochrome C oxidase activity and higher mitochondrial content of PTEN-induced putative kinase-1 in PLPs from ASA-incubated Meg-01 cells. However, only after calcium ionophore A23187 stimulation, caspase-3 activity, the cytosolic cytochrome C content, and reduction of mitochondrial membrane potential were higher in PLPs from ASA-incubated megakaryocytes than in those from Meg-01 without ASA. Nitric oxide synthase 3 content was higher in PLPs from ASA-exposed Meg-01 cells than in PLPs from non-ASA incubated Meg-01 cells. The L-arginine antagonist, NG-Nitro-L-arginine Methyl Ester, reduced caspase-3 activity in A23187-stimulated PLPs generated from ASA-incubated Meg-01 cells. As conclusions exposure of megakaryocyte to ASA promotes that the newly generated PLPs have, under stimulating condition, higher sensitivity to go into apoptosis than those PLPs generated from Meg-01 cells without ASA. It could be associated with differences in mitochondrial functionality and NO formation.

N-Methyl-D-Aspartate Receptor Hypofunction in Meg-01 Cells Reveals a Role for Intracellular Calcium Homeostasis in Balancing Megakaryocytic-Erythroid Differentiation

The release of calcium ions (Ca ²⁺ ) from the endoplasmic reticulum (ER) and related store-operated calcium entry (SOCE) regulate maturation of normal megakaryocytes. The N -methyl-D-aspartate (NMDA) receptor (NMDAR) provides an additional mechanism for Ca ²⁺ influx in megakaryocytic cells, but its role remains unclear. We created a model of NMDAR hypofunction in Meg-01 cells using CRISPR-Cas9 mediated knockout of the GRIN1 gene, which encodes an obligate, GluN1 subunit of the NMDAR. We found that compared with unmodified Meg-01 cells, Meg-01- GRIN1^−/− cells underwent atypical differentiation biased toward erythropoiesis, associated with increased basal ER stress and cell death. Resting cytoplasmic Ca ²⁺ levels were higher in Meg-01- GRIN1^−/− cells, but ER Ca ²⁺ release and SOCE were lower after activation. Lysosome-related organelles accumulated including immature dense granules that may have contributed an alternative source of intracellular Ca ²⁺ . Microarray analysis revealed that Meg-01- GRIN1^−/− cells had deregulated expression of transcripts involved in Ca ²⁺ metabolism, together with a shift in the pattern of hematopoietic transcription factors toward erythropoiesis. In keeping with the observed pro-cell death phenotype induced by GRIN1 deletion, memantine (NMDAR inhibitor) increased cytotoxic effects of cytarabine in unmodified Meg-01 cells. In conclusion, NMDARs comprise an integral component of the Ca ²⁺ regulatory network in Meg-01 cells that help balance ER stress and megakaryocytic-erythroid differentiation. We also provide the first evidence that megakaryocytic NMDARs regulate biogenesis of lysosome-related organelles, including dense granules. Our results argue that intracellular Ca ²⁺ homeostasis may be more important for normal megakaryocytic and erythroid differentiation than currently recognized; thus, modulation may offer therapeutic opportunities.

Anagrelide Modulates Proplatelet Formation Resulting in Decreased Number and Increased Size of Platelets

We retrospectively evaluated 48 essential thrombocythemia (ET) patients who were treated in our institute (male/female, 14/34, median age, 61.5 years). In 14 patients treated with anagrelide (ANA), the degree of platelet count reduction (median, -56.6%) was strongly correlated with increase of mean platelet volume (MPV) (median, +11.7%) (R = 0.777). This correlation was not observed in ET patients treated with hydroxycarbamide alone (R = 0.245). The change in size of platelets strongly suggested that ANA affected the final process of platelet production. Thus, we hypothesized that ANA modifies the process by which platelets are released from proplatelets. To verify the association in an in vitro setting, we compared MEG-01 cells treated with PMA ± ANA. The number of platelet-like particles (PLPs) was decreased (P < 0.05) and the size of PLPs estimated by using flow cytometry was significantly increased when MEG-01 cells were treated with PMA + ANA (P < 0.05 vs PMA alone), recapitulating the clinical findings. The cytoplasmic protrusions extending from MEG-01 cells were shorter and thicker and the number of proplatelets was decreased when MEG-01 cells were treated with PMA + ANA (P < 0.01 vs PMA alone). Western blotting analysis showed that ANA treatment resulted in increased phosphorylation of MLC2 and reduced phosphorylation of focal adhesion kinase (FAK). The morphological change of proplatelets were reversed by blebbistatin, a specific inhibitor of myosin II. These findings indicated that ANA modulates the FAK-RhoA-ROCK-MLC2-myosine IIA pathway and suppresses proplatelet maturation, leading to a decrease in platelet count and increase in MPV.

Regulation of differentiation of MEG01 to megakaryocytes and platelet-like particles by Valproic acid through Notch3 mediated actin polymerization

Valproic acid (VPA) is one of the HDAC inhibitors used for the treatment of neurological disorders and hematological malignancies. Its role in self-renewal and proliferation of hematopoietic stem cells (HSCs) is well studied, but little is known about its involvement in regulating megakaryopoiesis and thrombopoiesis. In this study, we evaluated the role of VPA in megakaryopoiesis by using MEG-01, a megakaryoblast cell line. Our results show that VPA treatment differentiates MEG-01 cells to megakaryocytes (MK) and platelet-like particles. It was confirmed by augmented expression of MK and PLT-specific markers, higher ploidy, and PLT functionality. We assessed the molecular events underlying megakaryopoiesis. In the present study, we found an upregulation of Notch3 and its downstream target PDGFR-β upon VPA treatment. The direct role of Notch3 in megakaryopoiesis has not yet been studied. PDGFR-β is known to control actin organization during vascular smooth muscle cell differentiation. The actin cytoskeleton plays important role during proplatelet and PLT formation. We found an upregulation of Rac/Cdc42 GTPase and its downstream effectors that are the key players during actin polymerization events. We speculate that VPA induces PLT formation through Notch-3 signaling that in turn modulates actin polymerization that is one of the crucial steps necessary for thrombopoiesis. These studies were also confirmed with knockdown of Notch3 in MEG01 by using ShRNA approach as well as with apheresis-derived CD34⁺ cells. Altogether, these findings provide an evidence for a novel role of Notch3 in regulating platelet formation.

Platelet WDR1 suppresses platelet activity and is associated with cardiovascular disease

Platelet activity plays a major role in hemostasis with increased platelet activity likely contributing to the pathogenesis of atherothrombosis. We sought to identify associations between platelet activity variability and platelet-related genes in healthy controls. Transcriptional profiling of platelets revealed that WD-40 repeat domain 1 (WDR1), an enhancer of actin-depolymerizing factor activity, is downregulated in platelet messenger RNA (mRNA) from subjects with a hyperreactive platelet phenotype. We used the human megakaryoblastic cell line MEG-01 as an in vitro model for human megakaryocytes and platelets. Stimulation of MEG-01 with thrombin reduced levels of WDR1 transcripts and protein. WDR1 knockdown (KD) in MEG-01 cells increased adhesion and spreading in both the basal and activated states, increased F-actin content, and increased the basal intracellular calcium concentration. Platelet-like particles (PLPs) produced by WDR1 KD cells were fewer in number but larger than PLPs produced from unmodified MEG-01 cells, and had significantly increased adhesion in the basal state and upon thrombin activation. In contrast, WDR1 overexpression reversed the WDR1 KD phenotype of megakaryocytes and PLPs. To translate the clinical significance of these findings, WDR1 expression was measured in platelet RNA from subjects with established cardiovascular disease (n = 27) and age- and sex-matched controls (n = 10). The WDR1 mRNA and protein level was significantly lower in subjects with cardiovascular disease. These data suggest that WDR1 plays an important role in suppressing platelet activity, where it alters the actin cytoskeleton dynamics, and downregulation of WDR1 may contribute to the platelet-mediated pathogenesis of cardiovascular disease.

Platelets contain tissue factor pathway inhibitor-2 derived from megakaryocytes and inhibits fibrinolysis

Tissue factor pathway inhibitor-2 (TFPI-2) is a homologue of TFPI-1 and contains three Kunitz-type domains and a basic C terminus region. The N-terminal domain of TFPI-2 is the only inhibitory domain, and it inhibits plasma kallikrein, factor XIa, and plasmin. However, plasma TFPI-2 levels are negligible (≤20 pM) in the context of influencing clotting or fibrinolysis. Here, we report that platelets contain significant amounts of TFPI-2 derived from megakaryocytes. We employed RT-PCR, Western blotting, immunohistochemistry, and confocal microscopy to determine that platelets, MEG-01 megakaryoblastic cells, and bone marrow megakaryocytes contain TFPI-2. ELISA data reveal that TFPI-2 binds factor V (FV) and partially B-domain-deleted FV (FV-1033) with K(d) ~9 nM and binds FVa with K(d) ~100 nM. Steady state analysis of surface plasmon resonance data reveal that TFPI-2 and TFPI-1 bind FV-1033 with K(d) ~36-48 nM and bind FVa with K(d) ~252-456 nM. Further, TFPI-1 (but not TFPI-1161) competes with TFPI-2 in binding to FV. These data indicate that the C-terminal basic region of TFPI-2 is similar to that of TFPI-1 and plays a role in binding to the FV B-domain acidic region. Using pull-down assays and Western blots, we show that TFPI-2 is associated with platelet FV/FVa. TFPI-2 (~7 nM) in plasma of women at the onset of labor is also, in part, associated with FV. Importantly, TFPI-2 in platelets and in plasma of pregnant women inhibits FXIa and tissue-type plasminogen activator-induced clot fibrinolysis. In conclusion, TFPI-2 in platelets from normal or pregnant subjects and in plasma from pregnant women binds FV/Va and regulates intrinsic coagulation and fibrinolysis.

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.

Platelets and platelet-like particles mediate intercellular RNA transfer

The role of platelets in hemostasis and thrombosis is clearly established; however, the mechanisms by which platelets mediate inflammatory and immune pathways are less well understood. Platelets interact and modulate the function of blood and vascular cells by releasing bioactive molecules. Although the platelet is anucleate, it contains transcripts that may mirror disease. Platelet mRNA is only associated with low-level protein translation; however, platelets have a unique membrane structure allowing for the passage of small molecules, leading to the possibility that its cytoplasmic RNA may be passed to nucleated cells. To examine this question, platelet-like particles with labeled RNA were cocultured with vascular cells. Coculture of platelet-like particles with activated THP-1, monocytic, and endothelial cells led to visual and functional RNA transfer. Posttransfer microarray gene expression analysis of THP-1 cells showed an increase in HBG1/HBG2 and HBA1/HBA2 expression that was directly related to the transfer. Infusion of wild-type platelets into a TLR2-deficient mouse model established in vivo confirmation of select platelet RNA transfer to leukocytes. By specifically transferring green fluorescent protein, we also observed external RNA was functional in the recipient cells. The observation that platelets possess the capacity to transfer cytosolic RNA suggests a new function for platelets in the regulation of vascular homeostasis.

Identification of human thrombin-activatable fibrinolysis inhibitor in vascular and inflammatory cells

TAFI (thrombin-activatable fibrinolysis inhibitor) is a carboxypeptidase zymogen originally identified in plasma. The TAFI pathway helps to regulate the balance between the coagulation and fibrinolytic cascades. Activated TAFI (TAFIa) can also inactivate certain pro-inflammatory mediators, suggesting that the TAFI pathway may also regulate communication between coagulation and inflammation. Expression in the liver is considered to be the source of plasma TAFI. TAFI has also been identified in platelets and CPB2 (the gene encoding TAFI) mRNA has been detected in megakaryocytic cell lines as well as in endothelial cells. We have undertaken a quantitative analysis of CPB2 mRNA and TAFI protein in extrahepatic cell types relevant to vascular disease. Using RT-PCR and quantitative RT-PCR, we detected CPB2 mRNA in the human megakaryoblastic cell lines MEG-01 and Dami, the human monocytoid cell line THP-1 as well as THP-1 cells differentiated into a macrophage-like phenotype, and in primary human umbilical vein and coronary artery endothelial cells. CPB2 mRNA abundance in MEG-01, Dami, and THP-1 cells was modulated by the state of differentiation of these cells. Using a recently developed TAFIa assay, we detected TAFI protein in the lysates of the human hepatocellular carcinoma cell line HepG2 as well as in MEG-01 and Dami cells and in the conditioned medium of HepG2 cells, differentiated Dami cells, and THP-1 macrophages. We have obtained clear evidence for extrahepatic expression of TAFI, which has clear implications for the physiological and pathophysiological functions of the TAFI pathway.

SNARE-dependent glutamate release in megakaryocytes

Objective

The identification of signaling pathways involved in megakaryocytopoiesis is essential for development of novel therapeutics to treat hematological disorders. Following our previous findings that megakaryocytes express functional channel-forming N-methyl-D-aspartate-type glutamate receptors, here we aimed to determine the glutamate release capacity in undifferentiated and differentiated megakaryocytes and the role of soluble N-ethyl maleimide-sensitive factor attachment protein receptor (SNARE) proteins that are known to be associated with vesicular exocytosis.

Materials and Methods

Using the megakaryocytic cell line MEG-01, primary megakaryocytes, and tissue sections of bone marrow, reverse transcription polymerase chain reaction, Western blot analysis, and immunolocalization were employed to detect factors required for vesicular glutamate release. Vesicle recycling was monitored by acridine orange and FM1-43 staining and glutamate release activity was assessed by an enzyme-linked fluorimetric assay. Genetically modified MEG-01 cells, with deletion or overexpression of SNARE and vesicular proteins, were also examined for glutamate release activity.

Results

We demonstrated that megakaryocytes express numerous proteins required for vesicular glutamate release, including core SNARE proteins, vesicle-associated membrane protein, soluble N-ethyl maleimide-sensitive factor attachment protein−23, and syntaxin, as well as specific glutamate-loading vesicle proteins, VGLUT1 and VGLUT2. Moreover, active vesicle recycling and differentiation-dependent glutamate release were observed in megakaryocytes. Vesicle-associated membrane protein−deficient MEG-01 cells, which are impaired in vesicle recycling, showed a 30% decrease in released glutamate, whereas overexpression of VGLUT1 exhibited up to a 2.2-fold increase in glutamate release.

Conclusion

These data show that glutamate release from megakaryocytes occurs in a SNARE-dependent, exocytotic manner and is increased during differentiation, suggesting that manipulation of glutamate signaling could influence megakaryocytopoiesis and, therefore, offer a suitable target for the treatment of thrombosis and other hematological disorders.

Human serotonin transporter expression during megakaryocytic differentiation of MEG-01 cells

The serotonin (5-HT) transporter (SERT) has been found altered in platelets of patients with genetically complex disorders, including mood-anxiety, pain and eating disorders. In this study, we used cell cultures of platelet precursors as models of investigation on mechanisms of SERT regulation: SERT expression was appraised during megakaryocytic differentiation of human megakaryoblastic MEG-01 cells. Cells were cultured for 8 days with 10(-7)M 4-beta-12-tetradecanoylphorbol-13-acetate (beta-TPA) in the presence of 10% fetal bovine serum (FBS) and SERT was assessed by real time PCR, immunofluorescence microscopy, Western blot and [(3)H]5-HT re-uptake. Results revealed that SERT is present in control-untreated MEG-01 cells. beta-TPA-differentiating MEG-01 cells showed a redistribution of SERT fluorescence, diffuse to cell bodies and blebs along with a 3-fold SERT mRNA increase and a moderate raise in SERT protein (1.5/1.4-fold) by immunoblot and re-uptake assays. In summary, we have shown herein that control megakaryoblasts express the SERT protein. SERT is modulated by differentiation events, implying that SERT density in platelets is under the control of megakaryocytopoiesis stages. Differentiation of MEG-01 cells can provide considerable insight into interactions between SERT genetics, transmitter-hormonal/homeostatic mechanisms and signaling pathways.