Silencing of a targeted protein in in vivo platelets using a lentiviral vector delivering short hairpin RNA sequence

ASC regulates platelet activation and contributes to thrombus formation independent of NLRP3 inflammasome

BACKGROUND

Platelets are critical mediators of vascular homeostasis and thrombosis, and also contribute to the development of inflammation. NLRP3 inflammasome is a cytosolic multi-protein complex that consists of NLRP3, ASC and caspase-1, and regulates IL-1β-mediated inflammation.

METHOD AND RESULTS

Using two mouse models of thrombosis (i.e., occlusion of the middle cerebral artery and inferior vena cava), we found that thrombus formation was significantly enhanced in ASC-deficient (ASC^-/-) mice, compared to that in wild-type (WT) and IL-1β^-/- mice. ASC deficiency had no effects on blood coagulation parameters (i.e., prothrombin time [PT] and activated partial thromboplastin time [APTT]). Platelets from WT mice express ASC, but neither NLRP3 nor caspase-1. ASC deficiency significantly enhanced the expression of P-selectin and GPIIb/IIIa in response to a GPVI agonist (collagen-related peptide [CRP]), but not to thrombin, in platelets. CRP induced ASC speck formation in WT platelets. ASC deficiency also enhanced cytosolic Ca²⁺ elevation and phosphorylation of ERK1/2 and Akt in platelets.

CONCLUSION

Our results demonstrate that ASC negatively regulates GPVI signaling in platelets and enhances thrombus formation, independent of NLRP3 inflammasome and IL-1β, and provide novel insights into the link between inflammation and thrombosis.

Artificial MiRNA Knockdown of Platelet Glycoprotein lbα: A Tool for Platelet Gene Silencing

In recent years, candidate genes and proteins implicated in platelet function have been identified by various genomic approaches. To elucidate their exact role, we aimed to develop a method to apply miRNA interference in platelet progenitor cells by using GPIbα as a proof-of-concept target protein. After in silico and in vitro screening of siRNAs targeting GPIbα (siGPIBAs), we developed artificial miRNAs (miGPIBAs), which were tested in CHO cells stably expressing GPIb-IX complex and megakaryoblastic DAMI cells. Introduction of siGPIBAs in CHO GPIb-IX cells resulted in 44 to 75% and up to 80% knockdown of GPIbα expression using single or combined siRNAs, respectively. Conversion of siGPIBAs to miGPIBAs resulted in reduced silencing efficiency, which could however be circumvented by tandem integration of two hairpins targeting different regions of GPIBA mRNA where 72% GPIbα knockdown was achieved. CHO GPIb-IX cells transfected with the miGPIBA construct displayed a significant decrease in their ability to aggregate characterized by lower aggregate numbers and size compared to control CHO GPIb-IX cells. More importantly, we successfully silenced GPIbα in differentiating megakaryoblastic DAMI cells that exhibited morphological changes associated with actin organization. In conclusion, we here report the successful use of miRNA technology to silence a platelet protein in megakaryoblastic cells and demonstrate its usefulness in functional assays. Hence, we believe that artificial miRNAs are suitable tools to unravel the role of a protein of interest in stem cells, megakaryocytes and platelets, thereby expanding their application to novel fields of basic and translational research.

New approaches to gene and cell therapy for hemophilia

Hemophilia is considered suitable for gene therapy because it is caused by a single gene abnormality, and therapeutic coagulation factor levels may vary across a broad range. Recent success of hemophilia B gene therapy with an adeno-associated virus (AAV) vector in a clinical trial showed the real prospect that, through gene therapy, a cure for hemophilia may become a reality. However, AAV-mediated gene therapy is not applicable to patients with hemophilia A at present, and neutralizing antibodies against AAV reduce the efficacy of AAV-mediated strategies. Because patients that benefit from AAV treatment (hemophilia B without neutralizing antibodies) are estimated to represent only 15% of total patients with hemophilia, the development of basic technologies for hemophilia A and those that result in higher therapeutic effects are critical. In this review, we present an outline of gene therapy methods for hemophilia, including the transition of technical developments thus far and our novel techniques.

Paxillin is an intrinsic negative regulator of platelet activation in mice

Background

Paxillin is a LIM domain protein localized at integrin-mediated focal adhesions. Although paxillin is thought to modulate the functions of integrins, little is known about the contribution of paxillin to signaling pathways in platelets. Here, we studied the role of paxillin in platelet activation in vitro and in vivo.

Methods and results

We generated paxillin knockdown (Pxn-KD) platelets in mice by transplanting bone marrow cells transduced with a lentiviral vector carrying a short hairpin RNA sequence, and confirmed that paxillin expression was significantly reduced in platelets derived from the transduced cells. Pxn-KD platelets showed a slight increased in size and augmented integrin αIIbβ3 activation following stimulation of multiple receptors including glycoprotein VI and G protein-coupled receptors. Thromboxane A₂ biosynthesis and the release of α-granules and dense granules in response to agonist stimulation were also enhanced in Pxn-KD platelets. However, Pxn-KD did not increase tyrosine phosphorylation or intracellular calcium mobilization. Intravital imaging confirmed that Pxn-KD enhanced thrombus formation in vivo.

Conclusions

Our findings suggest that paxillin negatively regulates several common platelet signaling pathways, resulting in the activation of integrin αIIbβ3 and release reactions.

shRNA-induced interferon-stimulated gene analysis

RNA interference (RNAi) is a cellular mechanism to inhibit the expression of gene products in a highly specific manner. In recent years, RNAi has become the cornerstone of gene function studies, shortening the otherwise long process of target identification and validation. In addition, small interfering RNA (siRNA) and short-hairpin RNA (shRNA) therapies are being developed for the treatment of a variety of human diseases. Despite its huge potential for gene silencing, a hurdle to safe and effective RNAi is the activation of innate immune responses. Induction of innate immunity is dose- and sequence-dependent, and is also influenced by target tissue and delivery vehicle. Research on the molecular mechanisms mediating this response is helping to improve the design of the RNAi molecules. Nevertheless, appropriate testing for the presence of this undesired effect is needed prior to making conclusions on the outcome of the silencing treatment.

Model systems of genetically modified platelets

Although platelets are the smallest cells in the blood, they are implied in various processes ranging from immunology and oncology to thrombosis and hemostasis. Many large-scale screening programs, genome-wide association, and "omics" studies have generated lists of genes and loci that are probably involved in the formation or physiology of platelets under normal and pathologic conditions. This creates an increasing demand for new and improved model systems that allow functional assessment of the corresponding gene products in vivo. Such animal models not only render invaluable insight in the platelet biology, but in addition, provide improved test systems for the validation of newly developed anti-thrombotics. This review summarizes the most important models to generate transgenic platelets and to study their influence on platelet physiology in vivo. Here we focus on the zebrafish morpholino oligonucleotide technology, the (platelet-specific) knockout mouse, and the transplantation of genetically modified human or murine platelet progenitor cells in myelo-conditioned mice. The various strengths and pitfalls of these animal models are illustrated by recent examples from the platelet field. Finally, we highlight the latest developments in genetic engineering techniques and their possible application in platelet research.

Vinculin is indispensable for repopulation by hematopoietic stem cells, independent of integrin function

Vinculin is a highly conserved actin-binding protein that is localized in integrin-mediated focal adhesion complexes. Although critical roles have been proposed for integrins in hematopoietic stem cell (HSC) function, little is known about the involvement of intracellular focal adhesion proteins in HSC functions. This study showed that the ability of c-Kit(+)Sca1(+)Lin(-) HSCs to support reconstitution of hematopoiesis after competitive transplantation was severely impaired by lentiviral transduction with short hairpin RNA sequences for vinculin. The potential of these HSCs to differentiate into granulocytic and monocytic lineages, to migrate toward stromal cell-derived factor 1α, and to home to the bone marrow in vivo were not inhibited by the loss of vinculin. However, the capacities to form long term culture-initiating cells and cobblestone-like areas were abolished in vinculin-silenced c-Kit(+)Sca1(+)Lin(-) HSCs. In contrast, adhesion to the extracellular matrix was inhibited by silencing of talin-1, but not of vinculin. Whole body in vivo luminescence analyses to detect transduced HSCs confirmed the role of vinculin in long term HSC reconstitution. Our results suggest that vinculin is an indispensable factor determining HSC repopulation capacity, independent of integrin functions.

Platelets as delivery systems for disease treatments

Platelets are small, anucleate, discoid shaped blood cells that play a fundamental role in hemostasis. Platelets contain a large number of biologically active molecules within cytoplasmic granules that are critical to normal platelet function. Because platelets circulate in blood through out the body, release biological molecules and mediators on demand and participate in hemostasis as well as many other pathophysiologic processes, targeting expression of proteins of interest to platelets and utilizing platelets as delivery systems for disease treatment would be a logical approach. This paper reviews the genetic therapy for inherited bleeding disorders utilizing platelets as delivery system, with a particular focus on platelet-derived FVIII for hemophilia A treatment.

Vinculin activates inside-out signaling of integrin αIIbβ3 in Chinese hamster ovary cells

Although vinculin is used frequently as a marker for integrin-mediated focal adhesion complexes, how it regulates the activation of integrin is mostly unknown. In this study, we examined whether vinculin would activate integrin in Chinese hamster ovary (CHO) cells expressing human integrin αIIbβ3. Silencing of vinculin by lentiviral transduction with a short hairpin RNA sequence affected the binding of PAC-1 (an antibody recognizing activated human αIIbβ3) to a constitutively active form of αIIbβ3 (α6Bβ3) expressed on CHO cells, while its inhibitory effects were much weaker than those of talin-1. Overexpression of an active form of vinculin without intramolecular interactions, but not the full length one, induced PAC-1 binding to native αIIbβ3 expressed on CHO cells in a manner dependent on talin-1. On the other hand, silencing of talin-1, but not vinculin, failed to induce cell spreading of α6Bβ3-CHO cells on fibrinogen, even in the presence of PT 25-2, a monoclonal antibody that activates αIIbβ3. Thus, an active form of vinculin could induce αIIbβ3 inside-out signaling through the actions of talin-1, while vinculin was dispensable for outside-in signaling.

MAL/SRF complex is involved in platelet formation and megakaryocyte migration by regulating MYL9 (MLC2) and MMP9

Megakaryoblastic leukemia 1 (MAL) is a transcriptional coactivator of serum response factor (SRF). In acute megakaryoblastic leukemia, the MAL gene is translocated and fused with the gene encoding one twenty-two (OTT). Herein, we show that MAL expression increases during the late differentiation steps of neonate and adult human megakaryopoiesis and localized into the nucleus after Rho GTPase activation by adhesion on collagen I or convulxin. MAL knockdown in megakaryocyte progenitors reduced the percentage of cells forming filopodia, lamellipodia, and stress fibers after adhesion on the same substrates, and reduced proplatelet formation. MAL repression led to dysmorphic megakaryocytes with disorganized demarcation membranes and alpha granules heterogeneously scattered in the cytoplasm. Gene expression profiling revealed a marked decrease in metalloproteinase 9 (MMP-9) and MYL9 expression after MAL inhibition. Luciferase assays in HEK293T cells and chromatin immunoprecipitation in primary megakaryocytes showed that the MAL/SRF complex directly regulates MYL9 and MMP9 in vitro. Megakaryocyte migration in response to stromal cell-derived factor 1, through Matrigel was considerably decreased after MAL knockdown, implicating MMP9 in migration. Finally, the use of a shRNA to decrease MYL9 expression showed that MYL9 was involved in proplatelet formation. MAL/SRF complex is thus involved in platelet formation and megakaryocyte migration by regulating MYL9 and MMP9.

The evolution of platelet-directed pharmacotherapy

The evolution of platelet directed pharmacotherapy in the prevention and treatment of patients with thrombotic disorders is based soundly on a rapidly expanding knowledge of platelet biology. Traditionally viewed, throughout most of its relatively brief history in medicine, as an anucleate, passive contributor to hemostasis, a more contemporary perspective acknowledges platelets as complex, multidimensional cells that participate actively in coagulation, vascular repair, angiogenesis and thrombosis within the micro and the macro-circulatory systems. Herein, we consider platelet-directed pharmacotherapy from these fundamental, biology-based exemplars--megakaryocytes, signal transduction and the platelet--coagulation protease interface. We also highlight the emerging biopharmacology platform of oligonucleotide platelet adhesion antagonists and their complementary antidotes.

Potentiation and priming of platelet activation: a potential target for antiplatelet therapy

Ischemic cardiovascular events represent the leading cause of mortality and morbidity worldwide, and platelet aggregation and thrombus formation are the main effectors of acute arterial ischemic events. Although antiplatelet therapy is the cornerstone of antithrombotic treatment of ischemic cardiovascular disorders, available antiplatelet agents have less than satisfactory efficacy; thus, the identification of novel potential target candidates for antiplatelet therapy is highly warranted. Recent evidence suggests that several molecules that amplify the aggregation response of platelets to activating stimuli, which are either released by platelets (potentiating molecules) or present in the milieu before platelets get activated (primers), play a major role in pathologic thrombus formation without being significantly involved in primary haemostasis. These molecules appear to be a particularly appealing novel potential pharmacologic target for antiplatelet therapy. Here, we review the present knowledge on some molecules acting as potentiators or primers of platelet activation and discuss their possible pharmacologic modulation for antithrombotic purposes.