Summary Exposure to a plethora of environmental challenges commonly triggers pathological type 2 cell-mediated inflammation. Here we report the pathological role of the Wnt antagonist Dickkopf-1 (Dkk-1) upon allergen challenge or non-healing parasitic infection. The increased circulating amounts of Dkk-1 polarized T cells to T helper 2 (Th2) cells, stimulating a marked simultaneous induction of the transcription factors c-Maf and Gata-3, mediated by the kinases p38 MAPK and SGK-1, resulting in Th2 cell cytokine production. Circulating Dkk-1 was primarily from platelets, and the increase of Dkk-1 resulted in formation of leukocyte-platelet aggregates (LPA) that facilitated leukocytes infiltration to the affected tissue. Functional inhibition of Dkk-1 impaired Th2 cell cytokine production and leukocyte infiltration, protecting mice from house dust mite (HDM)-induced asthma or Leishmania major infection. These results highlight that Dkk-1 from thrombocytes is an important regulator of leukocyte infiltration and polarization of immune responses in pathological type 2 cell-mediated inflammation.

Discovery of the cellular and molecular mechanisms of induced pluripotency has been hampered by its low efficiency and slow kinetics. Here, we report an experimental system with multi-color time-lapse microscopy that permits direct observation of pluripotency induction at single cell resolution, with temporal intervals as short as five minutes. Using granulocyte-monocyte progenitors as source cells, we visualized nascent pluripotent cells emerge from a hematopoietic state. We engineered a suite of image processing and analysis software to annotate the behaviors of the reprogramming cells, which revealed the highly dynamic cell-cell interactions associated with early reprogramming. We observed frequent cell migration, which can lead to sister colonies, satellite colonies and colonies of mixed genetic makeup. In addition, we discovered a previously unknown morphologically distinct 2-cell intermediate of reprogramming, which occurs prior to other reprogramming landmarks. By directly visualizing the reprogramming process with E-cadherin inhibition, we demonstrate the requirement of E-cadherin for proper cellular interactions from an early stage of reprogramming, including the 2-cell intermediate. The detailed cell-cell interactions revealed by this imaging platform shed light on previously unappreciated early reprogramming dynamics. This experimental system could serve as a powerful tool to dissect the complex mechanisms of early reprogramming by focusing on the relevant but rare cells with superb temporal and spatial resolution.

• RhoA-induced actin polymerization promotes nuclear accumulation of MKL1 and transcriptional activation.• Thrombopoietin activates nuclear accumulation of MKL1 and transcriptional activation in primary megakarocytes.How components of the cytoskeleton regulate complex cellular responses is fundamental to understanding cellular function. Megakaryoblast leukemia 1 (MKL1), an activator of serum response factor (SRF) transcriptional activity, promotes muscle, neuron, and megakaryocyte differentiation. In muscle cells, where MKL1 subcellular localization is one mechanism by which cells control SRF activity, MKL1 translocation from the cytoplasm to the nucleus in response to actin polymerization is critical for its function as a transcriptional regulator. MKL1 localization is cell-type specific; it is predominantly cytoplasmic in unstimulated fibroblasts and some muscle cell types and is constitutively nuclear in neuronal cells. In the present study, we report that in megakaryocytes, subcellular localization and regulation of MKL1 is dependent on RhoA activity and actin organization. Induction of megakaryocytic differentiation of human erythroleukemia cells by 12-O-tetradecanoylphorbol-13-acetate and primary megakaryocytes by thrombopoietin promotes MKL1 nuclear localization. This MKL1 localization is blocked by drugs inhibiting RhoA activity or actin polymerization. We also show that nuclear-localized MKL1 activates the transcription of SRF target genes. This report broadens our knowledge of the molecular mechanisms regulating megakaryocyte differentiation. (Blood. 2013;121(7):1094-1101) IntroductionAlthough megakaryoblastic leukemia 1 (MKL1, also known as MRTF-A, MAL, or BSAC) plays a role in normal megakaryocytopoiesis, 1-3 much of what is known about this transcriptional coactivator of serum response factor (SRF) has been defined in fibroblasts and muscle cells. MKL1 promotes musclespecific gene expression, maintains mammary myoepithelial cell differentiation, and contributes to myocardial infarction-induced fibrosis and myofibroblast activation. [4][5][6][7] Other members of the MKL1 family include MKL2 and Myocardin. All 3 genes have been implicated in muscle cell differentiation, but have different patterns of cellular and developmental expression, which likely explains some of the differences in their knockout (KO) phenotypes. Although Mkl2-and Myocardin-KO mice are embryonic lethal with severe cardiac abnormalities, Mkl1-KO mice are viable with a less severe phenotype. Female Mkl1-KO mice have premature mammary gland involution that prevents lactation. 6,8 In addition, Mkl1-KO mice have impaired megakaryocytopoiesis defined by increased numbers of megakaryocytes in the BM, decreased ploidy of BM megakaryocytes, and low peripheral blood platelet counts. 1,3 In fibroblast cell lines, MKL1 activity is regulated posttranslationally by its subcellular localization, which is dependent on the actin cytoskeleton. 9-11 When MKL1 is bound to monomeric (G)-actin via its N-terminal RPEL domains, it is predominantly local...

Genome-wide association studies have identified a genetic variant at 3p14.3 (SNP rs1354034) that strongly associates with platelet number and mean platelet volume in humans. While originally proposed to be intronic, analysis of mRNA expression in primary human hematopoietic subpopulations reveals that this SNP is located directly upstream of the predominantly expressed ARHGEF3 isoform in megakaryocytes (MK). We found that ARHGEF3, which encodes a Rho guanine exchange factor, is dramatically upregulated during both human and murine MK maturation. We show that the SNP (rs1354034) is located in a DNase I hypersensitive region in human MKs and is an expression quantitative locus (eQTL) associated with ARHGEF3 expression level in human platelets, suggesting that it may be the causal SNP that accounts for the variations observed in human platelet traits and ARHGEF3 expression. In vitro human platelet activation assays revealed that rs1354034 is highly correlated with human platelet activation by ADP. In order to test whether ARHGEF3 plays a role in MK development and/or platelet function, we developed an Arhgef3 KO/LacZ reporter mouse model. Reflecting changes in gene expression, LacZ expression increases during MK maturation in these mice. Although Arhgef3 KO mice have significantly larger platelets, loss of Arhgef3 does not affect baseline MK or platelets nor does it affect platelet function or platelet recovery in response to antibody-mediated platelet depletion compared to littermate controls. In summary, our data suggest that modulation of ARHGEF3 gene expression in humans with a promoter-localized SNP plays a role in human MKs and human platelet function—a finding resulting from the biological follow-up of human genetic studies. Arhgef3 KO mice partially recapitulate the human phenotype.

3440 Background: How components of the cytoskeleton regulate complex cellular responses is fundamental to understanding cellular function. Megakaryocyte Leukemia 1 (MKL1), an activator of serum response factor (SRF) transcriptional activity, plays critical roles in muscle, neuron, and megakaryocyte differentiation. Regulation of MKL1 subcellular localization is one mechanism by which a cell can control SRF activity with MKL1 localization to the nucleus being critical for its function as a transcriptional activator. MKL1 subcellular localization is cell-type specific; MKL1 is predominantly cytoplasmic in unstimulated fibroblasts and some muscle cell types until it is sequestered in the nucleus following actin polymerization. In contrast, MKL1 is constitutively localized to the nucleus in neuronal cells. Objective: We tested the hypothesis that MKL1 subcellular localization is tightly regulated in megakaryocytic cells during induction of maturation. Methods and Results: Using a human erythroleukemia (HEL) cell line, we systematically dissected the events that occur after 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced megakaryocytic differentiation to assess the relationships between RhoA activation, actin polymerization, MKL1 subcellular localization, and upregulation of SRF-target genes essential for megakaryocyte differentiation. In response to treatment with TPA, the percentage of HEL cells with predominantly nuclear localization went from <5% to over 50% in 2 hours. We found that TPA triggered RhoA activation and subsequent actin polymerization, each of which was necessary for MKL1 nuclear accumulation. In the absence of TPA, activation of RhoA and actin polymerization by calpeptin and Jasplakinolide, respectively, resulted in a statistically significant (p<.001) increase in MKL1 nuclear localization. Conversely, in cells exposed to TPA, Rho inhibition and actin depolymerization by a cell permeable C3 Transferase and Latrunculin B, respectively, caused a decrease in MKL1 nuclear localization (p<.01). Interference with MKL1 transcriptional activity using either dominant negative MKL1 (lacking the transcriptional activation domain) or the MKL1 chemical inhibitor CCG-1423 was sufficient to prevent TPA-induced expression of the SRF target genes MYL9, MYH9, and MMP9, all of which are necessary for proper megakaryocyte differentiation and maturation. Finally, we used timelapse microscopy to analyze the subcellular movement of a GFP tagged MKL1 in primary megakaryocytes. As predicted, Rho activation and actin polymerization were sufficient to drive MKL1 to the nucleus. Importantly, exposure of primary megakaryocytes to the physiological agonist thrombopoietin (TPO) stimulated MKL1 nuclear localization within minutes. Conclusions: Subcellular localization and regulation of MKL1 in megakaryocytes is dependent on RhoA activity and actin organization, similar to its regulation in fibroblasts and smooth muscle cells. Induction of megakaryocytic differentiation of HEL cells with TPA and primary megakaryocytes with TPO promotes MKL1 nuclear localization and downstream gene activation. This report broadens our knowledge of the mechanisms of action by which TPO promotes megakaryocyte differentiation. Disclosures: No relevant conflicts of interest to declare.

Cadherins play a major role in mediating cell–cell adhesion, which shares many parallels with platelet–platelet interactions during aggregate formation and clot stabilization. Platelets express epithelial (E)-cadherin, but its contribution to platelet function and/or platelet production is currently unknown. To assess the role of E-cadherin in platelet production and function in vitro and in vivo, we utilized a megakaryocyte-specific E-cadherin knockout mouse model. Loss of E-cadherin in megakaryocytes does not affect megakaryocyte maturation, platelet number or size. However, platelet dysfunction in the absence of E-cadherin is revealed when conditional knockout mice are challenged with acute antibody-mediated platelet depletion. Unlike wild-type mice that recover fully, knockout mice die within 72 hours post-antibody administration, likely from haemorrhage. Furthermore, conditional knockout mice have prolonged tail bleeding times, unstable clot formation, reduced clot retraction and reduced fibrin deposition in in vivo injury models. Murine platelet aggregation in vitro in response to thrombin and thrombin receptor activating peptide is compromised in E-cadherin null platelets, while aggregation in response to adenosine diphosphate (ADP) is not significantly different. Consistent with this, in vitro aggregation of primary human platelets in response to thrombin is decreased by an inhibitory E-cadherin antibody. Integrin activation and granule secretion in response to ADP and thrombin are not affected in E-cadherin null platelets, but Akt and glycogen synthase kinase 3β (GSK3β) activation are attenuated, suggesting a that E-cadherin contributes to aggregation, clot stabilization and retraction that is mediated by phosphoinositide 3-kinase/Akt/GSK3β signalling. In summary, E-cadherin plays a salient role in platelet aggregation and clot stability.

Summary Leukemia-Associated RhoGEF (LARG) is highly expressed in platelets, which are essential for maintaining normal hemostasis. We studied the function of LARG in murine and human megakaryocytes and platelets with Larg knockout, shRNA-mediated knockdown and small molecule-mediated inhibition. We found that LARG is important for human, but not murine, megakaryocyte maturation. Larg KO mice exhibit macrothrombocytopenia, internal bleeding in the ovaries and prolonged bleeding times. KO platelets have impaired aggregation, α-granule release and integrin α2bβ3 activation in response to thrombin and thromboxane, but not to ADP. The same agonist-specific reductions in platelet aggregation occur in human platelets treated with a LARG inhibitor. Larg KO platelets have reduced RhoA activation and myosin light chain phosphorylation, suggesting that Larg plays an agonist-specific role in platelet signal transduction. Using 2 different in vivo assays, Larg KO mice are protected from in vivo thrombus formation. Together, these results establish that LARG regulates human megakaryocyte maturation, and is critical for platelet function in both humans and mice.

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