PKCα negatively regulates in vitro proplatelet formation and in vivo platelet production in mice

Functional enrichment of alternative splicing events with NEASE reveals insights into tissue identity and diseases

Alternative splicing (AS) is an important aspect of gene regulation. Nevertheless, its role in molecular processes and pathobiology is far from understood. A roadblock is that tools for the functional analysis of AS-set events are lacking. To mitigate this, we developed NEASE, a tool integrating pathways with structural annotations of protein-protein interactions to functionally characterize AS events. We show in four application cases how NEASE can identify pathways contributing to tissue identity and cell type development, and how it highlights splicing-related biomarkers. With a unique view on AS, NEASE generates unique and meaningful biological insights complementary to classical pathways analysis.

Rapamycin induces megakaryocytic differentiation through increasing autophagy in Dami cells

: Autophagy is a conserved cellular process that involves the degradation of cytoplasmic components in eukaryotic cells. However, the correlation between autophagy and megakaryocyte development is unclear. This study aims to explore the role of autophagy in megakaryocyte differentiation. To test our hypothesis, we used the Dami cell line in-vitro experiments. Rapamycin and Bafilomycin A1 were used to stimulate Dami cells. CD41 expression and apoptosis were analysed by flow cytometry. Autophagy-related proteins were detected by Western blotting. 12-O-Tetradecanoylphorbol 13-acetate-treated Dami cells can simulate endomitosis of megakaryocytes in vitro. Rapamycin-induced autophagic cell death was verified by LC3-II conversion upregulation. Meanwhile, Bafilomycin A1 blocked endomitosis and autophagy of Dami cells. Our results provide evidence that autophagy is involved in megakaryocyte endomitosis and platelet development. Rapamycin inhibited cell viability and induced multiple cellular events, including apoptosis, autophagic cell death, and megakaryocytic differentiation, in human Dami cells. Upregulated autophagy triggered by rapamycin can promote the differentiation of Dami cells, while endomitosis is accompanied by enhanced autophagy.

Downregulation of hypoxia-inducible factor-1α contributes to impaired megakaryopoiesis in immune thrombocytopenia

Impaired megakaryocyte maturation and exaggerated platelet destruction play a pivotal role in the pathogenesis of immune thrombocytopenia (ITP). Previous studies have shown that HIF-1α promotes the homing and engraftment of haematopoietic stem cells (HSCs), thereby stimulating HSC differentiation. However, whether HIF-1α plays a role in megakaryocytic maturation and platelet destruction in ITP remains elusive. Using enzyme-linked immunosorbent assays (ELISAs), we demonstrated that there were lower HIF-1α levels in the bone marrow (BM) of ITP patients than in that of healthy donors and patients with chemotherapy-related thrombocytopenia. Subjects with lower megakaryocyte (<100/slide) and platelet (<30 × 109/L) counts exhibited significantly decreased BM HIF-1α levels, compared to those with higher megakaryocyte (≥100/slide) and platelet (≥30 × 109/L) counts. To test whether HIF-1α regulates megakaryopoiesis and platelet production, megakaryocytes derived from mouse BM cells were treated with an HIF-1α activator (IOX-2; 50 µM) or inhibitor (PX-478; 50 µM). PX-478 significantly decreased HIF-1α expression, cell size, and the populations of CD41-positive and high-ploidy cells. Importantly, to evaluate the role of HIF-1α as a potential therapeutic target in ITP, mouse BM cells were incubated with plasma from ITP patients in the presence or absence of IOX-2. IOX-2 significantly attenuated the ITP plasma-induced decrease in cell size as well as the proportions of CD41-positive and high-ploidy cells. In addition, IOX-2 increased the number of megakaryocytes from mouse BM cells treated with ITP plasma. Our findings indicate that decreased HIF-1α may contribute to impaired megakaryopoiesis in ITP, and HIF-1α may provide a potential therapy for ITP patients.

Supplementary Material to this article is available online at www.thrombosis-online.com.

PKC-epsilon deficiency alters progenitor cell populations in favor of megakaryopoiesis

BACKGROUND

It has long been postulated that Protein Kinase C (PKC) is an important regulator of megakaryopoiesis. Recent contributions to the literature have outlined the functions of several individual PKC isoforms with regard to megakaryocyte differentiation and platelet production. However, the exact role of PKCε remains elusive.

OBJECTIVE

To delineate the role of PKCε in megakaryopoiesis.

APPROACH AND RESULTS

We used a PKCε knockout mouse model to examine the effect of PKCε deficiency on platelet mass, megakaryocyte mass, and bone marrow progenitor cell distribution. We also investigated platelet recovery in PKCε null mice and TPO-mediated signaling in PKCε null megakaryocytes. PKCε null mice have higher platelet counts due to increased platelet production compared to WT littermate controls (p<0.05, n = 8). Furthermore, PKCε null mice have more bone marrow megakaryocyte progenitor cells than WT littermate control mice. Additionally, thrombopoietin-mediated signaling is perturbed in PKCε null mice as Akt and ERK1/2 phosphorylation are enhanced in PKCε null megakaryocytes stimulated with thrombopoietin. Finally, in response to immune-induced thrombocytopenia, PKCε null mice recovered faster and had higher rebound thrombocytosis than WT littermate control mice.

CONCLUSIONS

Enhanced platelet recovery could be due to an increase in megakaryocyte progenitor cells found in PKCε null mice as well as enhanced thrombopoietin-mediated signaling observed in PKCε deficient megakaryocytes. These data suggest that PKCε is a negative regulator of megakaryopoiesis.

The Phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P3) Binder Rasa3 Regulates Phosphoinositide 3-kinase (PI3K)-dependent Integrin αIIbβ3 Outside-in Signaling

The class I PI3K family of lipid kinases plays an important role in integrin α_IIbβ₃ function, thereby supporting thrombus growth and consolidation. Here, we identify Ras/Rap1GAP Rasa3 (GAP1^IP4BP) as a major phosphatidylinositol 3,4,5-trisphosphate-binding protein in human platelets and a key regulator of integrin α_IIbβ₃ outside-in signaling. We demonstrate that cytosolic Rasa3 translocates to the plasma membrane in a PI3K-dependent manner upon activation of human platelets. Expression of wild-type Rasa3 in integrin α_IIbβ₃-expressing CHO cells blocked Rap1 activity and integrin α_IIbβ₃-mediated spreading on fibrinogen. In contrast, Rap1GAP-deficient (P489V) and Ras/Rap1GAP-deficient (R371Q) Rasa3 had no effect. We furthermore show that two Rasa3 mutants (H794L and G125V), which are expressed in different mouse models of thrombocytopenia, lack both Ras and Rap1GAP activity and do not affect integrin α_IIbβ₃-mediated spreading of CHO cells on fibrinogen. Platelets from thrombocytopenic mice expressing GAP-deficient Rasa3 (H794L) show increased spreading on fibrinogen, which in contrast to wild-type platelets is insensitive to PI3K inhibitors. Together, these results support an important role for Rasa3 in PI3K-dependent integrin α_IIbβ₃-mediated outside-in signaling and cell spreading.

Ral A, via activating the mitotic checkpoint, sensitizes cells lacking a functional Nf1 to apoptosis in the absence of protein kinase C

Nf1 mutations or deletions are suggested to underlie the tumor predisposition of NF1 (neurofibromatosis type 1) and few treatments are available for treating NF1 patients with advanced malignant tumors. Aberrant activation of Ras in Nf1-deficient conditions is responsible for the promotion of tumorigenesis in NF1. PKC is proven to be an important factor in supporting the viability of Nf1-defected cells, but the molecular mechanisms are not fully understood. In this study, we demonstrate that the inhibition of protein kinase C (PKC) by 1-O-Hexadecyl-2-O-methyl-rac-glycerol (HMG, a PKC inhibitor) preferentially sensitizes Nf1-defected cells to apoptosis, via triggering a persistent mitotic arrest. In this process, Ral A is activated. Subsequently, Chk1 is phosphorylated and translocated to the nucleus. Silencing Ral A significantly blocks Chk1 nuclear translocation and releases HMG-treated Nf1-deficient cells from mitotic arrest, resulting in the reduction of the magnitude of apoptosis. Thus, our study reveals that PKC is able to maintain the homeostasis or viability of Nf1-defected cells and may serve as a potential target for developing new therapeutic strategies.

Discovery and refinement of genetic loci associated with cardiometabolic risk using dense imputation maps

Large-scale whole-genome sequence data sets offer novel opportunities to identify genetic variation underlying human traits. Here we apply genotype imputation based on whole-genome sequence data from the UK10K and 1000 Genomes Project into 35,981 study participants of European ancestry, followed by association analysis with 20 quantitative cardiometabolic and hematological traits. We describe 17 new associations, including 6 rare (minor allele frequency (MAF) < 1%) or low-frequency (1% < MAF < 5%) variants with platelet count (PLT), red blood cell indices (MCH and MCV) and HDL cholesterol. Applying fine-mapping analysis to 233 known and new loci associated with the 20 traits, we resolve the associations of 59 loci to credible sets of 20 or fewer variants and describe trait enrichments within regions of predicted regulatory function. These findings improve understanding of the allelic architecture of risk factors for cardiometabolic and hematological diseases and provide additional functional insights with the identification of potentially novel biological targets.

Rapamycin and bafilomycin A1 alter autophagy and megakaryopoiesis

Autophagy is an effective strategy for cell development by recycling cytoplasmic constituents. Genetic deletion of autophagy mediator Atg7 in hematopoietic stem cells (HSCs) can lead to failure of megakaryopoiesis and enhanced autophagy has been implicated in various hematological disorders such as immune thrombocytopenia and myelodysplastic syndrome. Here, we examined the hypothesis that optimal autophagy is essential for megakaryopoiesis and thrombopoiesis by altering autophagy using pharmacological approaches. When autophagy was induced by rapamycin or inhibited by bafilomycin A1 in fetal liver cells, we observed a significant decrease in high ploidy megakaryocytes, a reduction of CD41 and CD61 co-expressing cells, and less proplatelet or platelet formation. Additionally, reduced cell size was shown in megakaryocytes derived from rapamycin, but not bafilomycin A1-treated mouse fetal liver cells. However, when autophagy was altered in mature megakaryocytes, we observed no significant change in proplatelet formation, which was consistent with normal platelet counts, megakaryocyte numbers, and ploidy in Atg7^flox/flox PF4-Cre mice with megakaryocyte- and platelet-specific deletion of autophagy-related gene Atg7. Therefore, our findings suggest that either induction or inhibition of autophagy in the early stage of megakaryopoiesis suppresses megakaryopoiesis and thrombopoiesis.

Synthesis and dephosphorylation of MARCKS in the late stages of megakaryocyte maturation drive proplatelet formation

Platelets are essential for hemostasis, and thrombocytopenia is a major clinical problem. Megakaryocytes (MKs) generate platelets by extending long processes, proplatelets, into sinusoidal blood vessels. However, very little is known about what regulates proplatelet formation. To uncover which proteins were dynamically changing during this process, we compared the proteome and transcriptome of round vs proplatelet-producing MKs by 2D difference gel electrophoresis (DIGE) and polysome profiling, respectively. Our data revealed a significant increase in a poorly-characterized MK protein, myristoylated alanine-rich C-kinase substrate (MARCKS), which was upregulated 3.4- and 5.7-fold in proplatelet-producing MKs in 2D DIGE and polysome profiling analyses, respectively. MARCKS is a protein kinase C (PKC) substrate that binds PIP2. In MKs, it localized to both the plasma and demarcation membranes. MARCKS inhibition by peptide significantly decreased proplatelet formation 53%. To examine the role of MARCKS in the PKC pathway, we treated MKs with polymethacrylate (PMA), which markedly increased MARCKS phosphorylation while significantly inhibiting proplatelet formation 84%, suggesting that MARCKS phosphorylation reduces proplatelet formation. We hypothesized that MARCKS phosphorylation promotes Arp2/3 phosphorylation, which subsequently downregulates proplatelet formation; both MARCKS and Arp2 were dephosphorylated in MKs making proplatelets, and Arp2 inhibition enhanced proplatelet formation. Finally, we used MARCKS knockout (KO) mice to probe the direct role of MARCKS in proplatelet formation; MARCKS KO MKs displayed significantly decreased proplatelet levels. MARCKS expression and signaling in primary MKs is a novel finding. We propose that MARCKS acts as a "molecular switch," binding to and regulating PIP2 signaling to regulate processes like proplatelet extension (microtubule-driven) vs proplatelet branching (Arp2/3 and actin polymerization-driven).

PKCs in thrombus formation

The protein kinase C (PKC) family has been implicated in several physiological processes regulating platelet activation. Each isoform of PKC expressed on platelets, may have a positive and/or negative role depending on the nature and concentration of the agonist. Mice lacking PKCα show much reduced thrombus formation in vivo, while PKCθ(-/-) showed inhibition of aggregation in response to PAR4. On the other hand, PKCδ by associating with Fyn, inhibits platelet aggregation. In addition, PKCβ by interacting with its receptor RACK1 has been implicated in the primary phases of signaling via the αIIbβ3 and finally PKCɛ appears to be involved in platelet function downstream GPVI. The present review discusses the latest observations relevant to the role of individual PKC isoforms in platelet activation and thrombus formation.

Cross talk between serine/threonine and tyrosine kinases regulates ADP-induced thromboxane generation in platelets

ADP-induced thromboxane generation depends on Src family kinases (SFKs) and is enhanced with pan-protein kinase C (PKC) inhibitors, but it is not clear how these two events are linked. The aim of the current study is to investigate the role of Y311 phosphorylated PKCδ in regulating ADP-induced platelet activation. In the current study, we employed various inhibitors and murine platelets from mice deficient in specific molecules to evaluate the role of PKCδ in ADP-induced platelet responses. We show that, upon stimulation of platelets with 2MeSADP, Y311 on PKCδ is phosphorylated in a P2Y1/Gq and Lyn-dependent manner. By using PKCδ and Lyn knockout murine platelets, we also show that tyrosine phosphorylated PKCδ plays a functional role in mediating 2MeSADP-induced thromboxane generation. 2MeSADP-induced PKCδ Y311 phosphorylation and thromboxane generation were potentiated in human platelets pre-treated with either a pan-PKC inhibitor, GF109203X or a PKC α/β inhibitor and in PKC α or β knockout murine platelets compared to controls. Furthermore, we show that PKC α/β inhibition potentiates the activity of SFK, which further hyper-phosphorylates PKCδ and potentiates thromboxane generation. These results show for the first time that tyrosine phosphorylated PKCδ regulates ADP-induced thromboxane generation independent of its catalytic activity and that classical PKC isoforms α/β regulate the tyrosine phosphorylation on PKCδ and subsequent thromboxane generation through tyrosine kinase, Lyn, in platelets.

Induction of mitotic catastrophe by PKC inhibition in Nf1-deficient cells

Mutations of tumor suppressor Nf1 gene deregulate Ras-mediated signaling, which confers the predisposition for developing benign or malignant tumors. Inhibition of protein kinase C (PKC) was shown to be in synergy with aberrant Ras for the induction of apoptosis in various types of cancer cells. However, it has not been investigated whether loss of PKC is lethal for Nf1-deficient cells. In this study, using HMG (3-hydroxy-3-methylgutaryl, a PKC inhibitor), we demonstrate that the inhibition of PKC by HMG treatment triggered a persistently mitotic arrest, resulting in the occurrence of mitotic catastrophe in Nf1-deficient ST8814 cells. However, the introduction of the Nf1 effective domain gene into ST8814 cells abolished this mitotic crisis. In addition, HMG injection significantly attenuated the growth of the xenografted ST8814 tumors. Moreover, Chk1 was phosphorylated, accompanied with the persistent increase of cyclin B1 expression in HMG-treated ST8814 cells. The knockdown of Chk1 by the siRNA prevented the Nf1-deficient cells from undergoing HMG-mediated mitotic arrest as well as mitotic catastrophe. Thus, our data suggested that the suppression of PKC activates the Chk1-mediated mitotic exit checkpoint in Nf1-deficient cells, leading to the induction of apoptosis via mitotic catastrophe. Collectively, the study indicates that targeting PKC may be a potential option for developing new strategies to treat Nf1-deficiency-related diseases.

Protein kinase C δ deficiency enhances megakaryopoiesis and recovery from thrombocytopenia

OBJECTIVE

We previously determined that protein kinase C δ (PKCδ) regulates platelet function. However, the function of PKCδ in megakaryopoiesis is unknown.

APPROACH AND RESULTS

Using PKCδ(-/-) and wild-type littermate mice, we found that deficiency of PKCδ caused an increase in white blood cells and platelet counts, as well as in bone marrow and splenic megakaryocytes (P<0.05). Additionally, the megakaryocyte number and DNA content were enhanced in PKCδ(-/-) mouse bone marrow after culturing with exogenous thrombopoietin compared with wild-type (P<0.05). Importantly, thrombopoietin-induced signaling was also altered with PKCδ deletion because both extracellular signal-regulated kinase and Akt308 phosphorylation were heightened in PKCδ(-/-) megakaryocytes compared with wild-type. Finally, PKCδ(-/-) mice recovered faster and had a heightened rebound thrombocytosis after thrombocytopenic challenge.

CONCLUSIONS

These data suggest that PKCδ is an important megakaryopoietic protein, which regulates signaling induced by thrombopoietin and represents a potential therapeutic target.

Protein kinase Cα (PKCα) regulates bone architecture and osteoblast activity

Bones' strength is achieved and maintained through adaptation to load bearing. The role of the protein kinase PKCα in this process has not been previously reported. However, we observed a phenotype in the long bones of Prkca^−/− female but not male mice, in which bone tissue progressively invades the medullary cavity in the mid-diaphysis. This bone deposition progresses with age and is prevented by disuse but unaffected by ovariectomy. Castration of male Prkca^−/− but not WT mice results in the formation of small amounts of intramedullary bone. Osteoblast differentiation markers and Wnt target gene expression were up-regulated in osteoblast-like cells derived from cortical bone of female Prkca^−/− mice compared with WT. Additionally, although osteoblastic cells derived from WT proliferate following exposure to estradiol or mechanical strain, those from Prkca^−/− mice do not. Female Prkca^−/− mice develop splenomegaly and reduced marrow GBA1 expression reminiscent of Gaucher disease, in which PKC involvement has been suggested previously. From these data, we infer that in female mice, PKCα normally serves to prevent endosteal bone formation stimulated by load bearing. This phenotype appears to be suppressed by testicular hormones in male Prkca^−/− mice. Within osteoblastic cells, PKCα enhances proliferation and suppresses differentiation, and this regulation involves the Wnt pathway. These findings implicate PKCα as a target gene for therapeutic approaches in low bone mass conditions.