Expression of a foreign protein in human megakaryocytes and platelets by retrovirally mediated gene transfer

Super-resolution imaging and quantification of megakaryocytes and platelets

Recent advances in super-resolution (sub-diffraction limited) microscopy have yielded remarkable insights into the nanoscale architecture and behavior of cells. In addition to the capacity to provide sub 100 nm resolution, these technologies offer unique quantitative opportunities with particular relevance to platelet and megakaryocyte biology. In this review, we provide a short introduction to modern super-resolution microscopy, its applications in the field of platelet and megakaryocyte biology, and emerging quantitative approaches which will allow for unprecedented insights into the biology of these unique cell types.

Methods for genetic modification of megakaryocytes and platelets

During recent decades there have been major advances in the fields of thrombosis and haemostasis, in part through development of powerful molecular and genetic technologies. Nevertheless, genetic modification of megakaryocytes and generation of mutant platelets in vitro remains a highly specialized area of research. Developments are hampered by the low frequency of megakaryocytes and their progenitors, a poor efficiency of transfection and a lack of understanding with regard to the mechanism by which megakaryocytes release platelets. Current methods used in the generation of genetically modified megakaryocytes and platelets include mutant mouse models, cell line studies and use of viruses to transform primary megakaryocytes or haematopoietic precursor cells. This review summarizes the advantages, limitations and technical challenges of such methods, with a particular focus on recent successes and advances in this rapidly progressing field including the potential for use in gene therapy for treatment of patients with platelet disorders.

Efficient gene expression in megakaryocytic cell line using nucleofection

To clarify the mechanism of platelet production from megakaryocytes, expression of target proteins by gene transfection was examined using various gene delivery techniques. Transfection into hematopoietic cells, including megakaryocytes, by conventional gene delivery techniques such as electroporation and lipofection are known to be difficult. In this study, in addition to electroporation and lipofection, we tested other gene-transfer methods (nucleofection, transfection using inactivated virus envelope, and transferrin-linked cationic polymer) with the green fluorescent protein (GFP) gene into the human megakaryocytic cell line MEG-01. We found that nucleofection, which uses a combination of special electrical parameters and specific solutions, was the best, judging from the expression ratio of GFP-positive cells (approximately 70% of cells) and low toxicity. The efficiency of GFP expression was not related to the amount of pDNA delivered into the MEG-01 cells. To verify the utility of nucleofection, the thrombopoietin (TPO) receptor c-mpl was transfected into MEG-01 cells. Transfected cells showed a higher responsiveness to TPO than mock-transfected MEG-01 cells. We propose that nucleofection is a useful method for transfecting target genes to megakaryocytic cells when addressing the mechanism of platelet production.

Retroviral Infection and Selection of Culture-Derived Platelets Allows Study of the Effect of Transgenes on Platelet Physiology Ex Vivo and on Thrombus Formation In Vivo

Background— We recently reported the development of culture-derived (CD) platelets with the aim to express any protein of interest in these platelets. 1 We now report a specific protocol of retroviral infection into the progenitor cells and subsequent selection, which allows to generate large amounts of highly homogenous transgene-expressing CD platelets and to study transgene function rapidly and reliably at large-scale ex vivo and in vivo settings.

Methods and Results— After retroviral infection and selection, the activation-dependent expression profile of surface markers, aggregation, and granule release were investigated. The function of transgene-expressing CD platelets, the precursor cells of which had been retrovirally infected, compared well to noninfected CD platelets or freshly isolated platelets. Hence, the retroviral infection protocol did not alter platelet physiology. In contrast, adenoviral infection of precursors to CD platelets resulted in marked functional alterations that obviated their use in analytic experiments. Additionally, sufficient amounts of selected CD platelets were generated to warrant intravenous injections into living mice. This approach permitted study of their adhesive profile at endothelial lesions and their effect on thrombus formation in vivo by intravital videofluorescence microscopy.

Conclusion— The novel selection method allowed us to produce recombinant transgene-expressing platelets in sufficient amounts to study genetically modified platelets in vitro and in vivo.

Generation of functional culture-derived platelets from CD34+ progenitor cells to study transgenes in the platelet environment

The possibility of evaluating the function of transgenes in platelets requires the generation of platelets from nucleated progenitor cells in vitro. In this article, we provide effective culture conditions for generating functional culture-derived (CD) human and mouse platelets from CD34(+) progenitor cells that allow expression of any foreign protein of interest. We have evolved an effective cytokine cocktail (thrombopoietin, stem cell factor, interleukin [IL]-1beta, IL-6) that induces a high yield of CD platelets and optimal shedding from cultivated megakaryocytes generated from CD34(+) progenitor cells. CD platelets showed similar functional and morphological characteristics compared with isolated blood platelets, including surface expression of platelet antigens (CD41, CD42, CD62P), aggregation, release of granule constituents (P-selectin, platelet factor 4, serotonin). Moreover, transmission electron microscopy revealed the presence of typical alpha- and dense granules and dense tubular system in CD platelets. Additionally, we showed that stable transgene expression in CD platelets can be performed through infection of CD34(+) progenitor cells using adenoviral vectors. Thus, we describe a methodology that enables studying functional consequences of transgenes of interest in the natural environment of platelets that may impose substantial impact on potential future platelet research and therapeutic target evaluation. The full text of this article is available online at http://circres.ahajournals.org.

In vivo induction of human immunodeficiency virus type 1 entry into nucleus-free cells by CD4 gene transfer to hematopoietic stem cells: a hypothetical possible strategy for therapeutic intervention

As a useful alternative to employing soluble CD4 to inhibit binding of human immunodeficiency virus type 1 (HIV-1) to target cells, the introduction of CD4-bearing erythrocyte has been proposed by two study groups (see Refs. (5,6)). Prominently, Nicolau and colleagues demonstrated that the electroinserted CD4 molecules in the membranes of erythrocytes are capable of mediating HIV-1 entry. The implications of the studies are that inactivation of the integration-dependent retrovirus by the facilitation of entry into the nucleus-free cells, referred to as 'fake host trap' or 'host cell decoy', may be a possible therapeutic approach. Here we expand this concept to include genetic modification of autologous hematopoietic stem cells and review the relevant theoretical basis. Effective application of molecular technologies to induce partial replacement of hematopoiesis may be critical for this strategy.

Primary megakaryocytes reveal a role for transcription factor NF-E2 in integrin alpha IIb beta 3 signaling

Platelet integrin alphaIIbbeta3 responds to intracellular signals by binding fibrinogen and triggering cytoskeletal reorganization, but the mechanisms of alphaIIbbeta3 signaling remain poorly understood. To better understand this process, we established conditions to study alphaIIbbeta3 signaling in primary murine megakaryocytes. Unlike platelets, these platelet precursors are amenable to genetic manipulation. Cytokine-stimulated bone marrow cultures produced three arbitrary populations of alphaIIbbeta3-expressing cells with increasing size and DNA ploidy: small progenitors, intermediate-size young megakaryocytes, and large mature megakaryocytes. A majority of the large megakaryocytes bound fibrinogen in response to agonists, while almost none of the smaller cells did. Fibrinogen binding to large megakaryocytes was inhibited by Sindbis virus-mediated expression of isolated beta3 integrin cytoplasmic tails. Strikingly, large megakaryocytes from mice deficient in the transcription factor NF-E2 failed to bind fibrinogen in response to agonists, despite normal surface expression of alphaIIbbeta3. Furthermore, while megakaryocytes from wild-type mice spread on immobilized fibrinogen and exhibited filopodia, lamellipodia and Rho-dependent focal adhesions and stress fibers, NF-E2-deficient megakaryocytes adhered poorly. These studies establish that agonist-induced activation of alphaIIbbeta3 is controlled by NF-E2-regulated signaling pathways that mature late in megakaryocyte development and converge at the beta3 cytoplasmic tail. Megakaryocytes provide a physiologically relevant and tractable system for analysis of bidirectional alphaIIbbeta3 signaling.