AMPK α2 subunit is involved in platelet signaling, clot retraction, and thrombus stability

Combining RNA-seq, molecular docking and experimental verification to explore the mechanism of BAM15 as a potential drug for atherosclerosis

BAM15 is a novel mitochondrial uncoupling agent derived from a synthetic source, that has been wildly explored for its ability to enhance mitochondrial respiration and metabolic flexibility. In this study, we investigated the underlying mechanisms of BAM15 on atherosclerosis (AS) through experimental validation, RNA-seq and molecular docking. The results showed that oral administration of BAM15 suppressed atherosclerosis in western diet (WD)-fed ApoE(-/-) mice and significantly improved the hyperlipidemia. And the increased serum ALT, AST and liver TC, TG, ALT, AST in ApoE(-/-) mice were reduced by BAM15 treatment. In in vitro experiments BAM15 inhibited RAW264.7 macrophages invasive ability and reduced palmitic acid-induced lipid accumulation. RNA-seq results confirmed the differential genes after BAM15 treatment and 140 common targets were identified by intersecting with AS-related targets. A protein-protein interaction (PPI) network analysis high-lighted IL1A, SRC and CSF3 as key targets of BAM15 against AS, which is further verified by molecular docking and western blot. Molecular dynamics analysis results confirmed that BAM15 exhibits strong affinity with the IL-1α, SRC and CSF3 proteins. This study indicates that BAM15 inhibits atherosclerosis through a multi-molecular mechanism, and we propose it as a novel anti-atherosclerotic drug.

Role of Liver Kinase 1B in Platelet Activation and Host Defense During Klebsiella pneumoniae-Induced Pneumosepsis

Pneumonia is the most common cause of sepsis, with Klebsiella pneumoniae frequently implicated as a causative pathogen. Platelets play a crucial role in host defense during sepsis, and their activation is essential for effective immune responses, which is at least in part induced through activation of the collagen receptor glycoprotein (GP)VI. Platelets require energy for their activation, and Liver kinase B1 (LKB1) is a key regulator of energy metabolism. We sought to determine the role of LKB1 in platelet function and host response during K. pneumoniae-induced pneumosepsis. Platelet-specific-Lkb1-deficient mice were generated and compared to control littermates. Platelet counts were unaffected by Lkb1 deficiency in naïve mice. However, Lkb1-deficient platelets exhibited significant hyperreactivity to GPVI stimulation, an effect not observed after stimulation of the thrombin receptor protease-activated receptor 4. During K. pneumoniae infection, platelets of both Lkb1-deficient and control mice became equally hyporesponsive to GPVI stimulation, without differences between genotypes. Platelet-specific Lkb1 deficiency did not alter bacterial outgrowth or dissemination, inflammatory responses, or lung pathology. These findings suggest that while Lkb1 plays a role in regulating platelet activation in response to GPVI stimulation, it does not significantly impact platelet activation or the host response during pneumonia-induced sepsis.

Activation of AMPK in platelets promotes the production of offspring

Platelets are terminally differentiated anucleated cells, but they still have cell-like functions and can even produce progeny platelets. However, the mechanism of platelet sprouting has not been elucidated so far. Here, we show that when platelet-rich plasma(PRP) was cultured at 37°C, platelets showed a spore phenomenon. The number of platelets increased when given a specific shear force. It is found that AMP-related signaling pathways, such as PKA and AMPK are activated in platelets in the spore state. Meanwhile, the mRNA expression levels of genes, such as CNN3, CAPZB, DBNL, KRT19, and ESPN related to PLS1 skeleton proteins also changed. Moreover, when we use the AMPK activator AICAR(AI) to treat washed platelets, cultured platelets can still appear spore phenomenon. We further demonstrate that washed platelets treated with Forskolin, an activator of PKA, not only platelet sprouting after culture but also the AMPK is activated. Taken together, these data demonstrate that AMPK plays a key role in the process of platelet budding and proliferation, suggesting a novel strategy to solve the problem of clinical platelet shortage.

The mechanism of oleic acid inhibiting platelet activation stimulated by collagen

BACKGROUND

Abnormal platelet activation is a key factor in the occurrence and development of thrombotic diseases. However, the physiological mechanisms that underlie platelet homeostasis remain unclear. Oleic acid, one of the most abundant lipids in the human diet, has potential antithrombotic effects. This study aimed to investigate the effects of oleic acid on platelet activation and thrombosis.

METHODS

Platelet aggregation, ATP release, and fibrinogen spread were evaluated to determine the role of oleic acid in platelet activation. A ferric chloride-induced carotid injury model was used to establish the effect of oleic acid on thrombus formation in vivo. Western blotting analysis and transfection experiments were performed to determine the mechanisms involved in this process.

RESULTS

Oleic acid inhibited platelet aggregation, granule release, and calcium mobilization. Furthermore, it inhibited the spread of platelets on fibrinogen. We also found that oleic acid delayed arterial thrombosis in mice, as demonstrated in a murine model of ferric chloride-induced carotid artery thrombosis. The molecular mechanism of its inhibition of platelet activity may be through the Syk-PLCγ2 and CaMKKβ/AMPKα/VASP pathways. In addition, we demonstrated that the phosphorylation of AMPK at Ser496 was an important mechanism of platelet activation.

CONCLUSIONS

Our study showed that oleic acid inhibits platelet activation and reduces thrombogenesis by inhibiting the phosphorylation of multiple signaling molecules, offering new insights into the research and development of antiplatelet drugs. Video Abstract.

Non-canonical Sonic Hedgehog signaling amplifies platelet reactivity and thrombogenicity

Sonic Hedgehog signaling amplifies platelet activation.

Targeting Shh signaling attenuates hemostasis and thrombosis.

Sonic Hedgehog (Shh) is a morphogen in vertebrate embryos that is also associated with organ homeostasis in adults. We report here that human platelets, though enucleate, synthesize Shh from preexisting mRNAs upon agonist stimulation, and mobilize it for surface expression and release on extracellular vesicles, thus alluding to its putative role in platelet activation. Shh, in turn, induced a wave of noncanonical signaling in platelets leading to activation of small GTPase Ras homolog family member A and phosphorylation of myosin light chain in activated protein kinase-dependent manner. Remarkably, agonist-induced thrombogenic responses in platelets, which include platelet aggregation, granule secretion, and spreading on immobilized fibrinogen, were significantly attenuated by inhibition of Hedgehog signaling, thus, implicating inputs from Shh in potentiation of agonist-mediated platelet activation. In consistence, inhibition of the Shh pathway significantly impaired arterial thrombosis in mice. Taken together, the above observations strongly support a feed-forward loop of platelet stimulation triggered locally by Shh, similar to ADP and thromboxane A2, that contributes significantly to the stability of occlusive arterial thrombus and that can be investigated as a potential therapeutic target in thrombotic disorders.

Donor genetic variants as risk factors for thrombosis after liver transplantation: A genome-wide association study

Thrombosis after liver transplantation substantially impairs graft- and patient survival. Inevitably, heritable disorders of coagulation originating in the donor liver are transmitted by transplantation. We hypothesized that genetic variants in donor thrombophilia genes are associated with increased risk of posttransplant thrombosis. We genotyped 775 donors for adult recipients and 310 donors for pediatric recipients transplanted between 1993 and 2018. We determined the association between known donor thrombophilia gene variants and recipient posttransplant thrombosis. In addition, we performed a genome-wide association study (GWAS) and meta-analyzed 1085 liver transplantations. In our donor cohort, known thrombosis risk loci were not associated with posttransplant thrombosis, suggesting that it is unnecessary to exclude liver donors based on thrombosis-susceptible polymorphisms. By performing a meta-GWAS from children and adults, we identified 280 variants in 55 loci at suggestive genetic significance threshold. Downstream prioritization strategies identified biologically plausible candidate genes, among which were AK4 (rs11208611-T, p = 4.22 × 10^-05 ) which encodes a protein that regulates cellular ATP levels and concurrent activation of AMPK and mTOR, and RGS5 (rs10917696-C, p = 2.62 × 10^-05 ) which is involved in vascular development. We provide evidence that common genetic variants in the donor, but not previously known thrombophilia-related variants, are associated with increased risk of thrombosis after liver transplantation.

Platelet autophagic machinery involved in thrombosis through a novel linkage of AMPK-MTOR to sphingolipid metabolism

Basal macroautophagy/autophagy has recently been found in anucleate platelets. Platelet autophagy is involved in platelet activation and thrombus formation. However, the mechanism underlying autophagy in anucleate platelets require further clarification. Our data revealed that LC3-II formation and SQSTM1/p62 degradation were noted in H₂O₂-activated human platelets, which could be blocked by 3-methyladenine and bafilomycin A₁, indicating that platelet activation may cause platelet autophagy. AMPK phosphorylation and MTOR dephosphorylation were also detected, and block of AMPK activity by the AMPK inhibitor dorsomorphin reversed SQSTM1 degradation and LC3-II formation. Moreover, autophagosome formation was observed through transmission electron microscopy and deconvolution microscopy. These findings suggest that platelet autophagy was induced partly through the AMPK-MTOR pathway. In addition, increased LC3-II expression occurred only in H₂O₂-treated Atg5^f/f platelets, but not in H₂O₂-treated atg5^-/- platelets, suggesting that platelet autophagy occurs during platelet activation. atg5^-/- platelets also exhibited a lower aggregation in response to agonists, and platelet-specific atg5^-/- mice exhibited delayed thrombus formation in mesenteric microvessles and decreased mortality rate due to pulmonary thrombosis. Notably, metabolic analysis revealed that sphingolipid metabolism is involved in platelet activation, as evidenced by observed several altered metabolites, which could be reversed by dorsomorphin. Therefore, platelet autophagy and platelet activation are positively correlated, partly through the interconnected network of sphingolipid metabolism. In conclusion, this study for the first time demonstrated that AMPK-MTOR signaling could regulate platelet autophagy. A novel linkage between AMPK-MTOR and sphingolipid metabolism in anucleate platelet autophagy was also identified: platelet autophagy and platelet activation are positively correlated.Abbreviations: 3-MA: 3-methyladenine; A.C.D.: citric acid/sod. citrate/glucose; ADP: adenosine diphosphate; AKT: AKT serine/threonine kinase; AMPK: AMP-activated protein kinase; ANOVA: analysis of variance; ATG: autophagy-related; B4GALT/LacCS: beta-1,4-galactosyltransferase; Baf-A1: bafilomycin A₁; BECN1: beclin 1; BHT: butylate hydrooxytoluene; BSA: bovine serum albumin; DAG: diacylglycerol; ECL: enhanced chemiluminescence; EDTA: ethylenediamine tetraacetic acid; ELISA: enzyme-linked immunosorbent assay; GALC/GCDase: galactosylceramidase; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GBA/GluSDase: glucosylceramidase beta; GPI: glycosylphosphatidylinositol; H₂O₂: hydrogen peroxide; HMDB: human metabolome database; HRP: horseradish peroxidase; IF: immunofluorescence; IgG: immunoglobulin G; KEGG: Kyoto Encyclopedia of Genes and Genomes; LAMP1: lysosomal associated membrane protein 1; LC-MS/MS: liquid chromatography-tandem mass spectrometry; mAb: monoclonal antibody; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MPV: mean platelet volume; MTOR: mechanistic target of rapamycin kinase; ox-LDL: oxidized low-density lipoprotein; pAb: polyclonal antibody; PC: phosphatidylcholine; PCR: polymerase chain reaction; PI3K: phosphoinositide 3-kinase; PLS-DA: partial least-squares discriminant analysis; PRP: platelet-rich plasma; Q-TOF: quadrupole-time of flight; RBC: red blood cell; ROS: reactive oxygen species; RPS6KB/p70S6K: ribosomal protein S6 kinase B; SDS: sodium dodecyl sulfate; S.E.M.: standard error of the mean; SEM: scanning electron microscopy; SGMS: sphingomyelin synthase; SM: sphingomyelin; SMPD/SMase: sphingomyelin phosphodiesterase; SQSTM1/p62: sequestosome 1; TEM: transmission electron microscopy; UGT8/CGT: UDP glycosyltransferase 8; UGCG/GCS: UDP-glucose ceramide glucosyltransferase; ULK1: unc-51 like autophagy activating kinase 1; UPLC: ultra-performance liquid chromatography; PIK3C3/VPS34: phosphatidylinositol 3-kinase catalytic subunit type 3; PtdIns3P: phosphatidylinositol-3-phosphate; WBC: white blood cell; WT: wild type.

AMP-activated protein kinase: A remarkable contributor to preserve a healthy heart against ROS injury

Heart failure is one of the leading causes of death and disability worldwide. Left ventricle remodeling, fibrosis, and ischemia/reperfusion injury all contribute to the deterioration of cardiac function and predispose to the onset of heart failure. Adenosine monophosphate-activated protein kinase (AMPK) is the universally recognized energy sensor which responds to low ATP levels and restores cellular metabolism. AMPK activation controls numerous cellular processes and, in the heart, it plays a pivotal role in preventing onset and progression of disease. Excessive reactive oxygen species (ROS) generation, known as oxidative stress, can activate AMPK, conferring an additional role of AMPK as a redox-sensor. In this review, we discuss recent insights into the crosstalk between ROS and AMPK. We describe the molecular mechanisms by which ROS activate AMPK and how AMPK signaling can further prevent heart failure progression. Ultimately, we review the potential therapeutic approaches to target AMPK for the treatment of cardiovascular disease and prevention of heart failure.

Cyclin Y is expressed in Platelets and Modulates Integrin Outside-in Signaling

Diabetes is associated with platelet hyper-reactivity and enhanced risk of thrombosis development. Here we compared protein expression in platelets from healthy donors and diabetic patients to identify differentially expressed proteins and their possible function in platelet activation. Mass spectrometry analyses identified cyclin Y (CCNY) in platelets and its reduced expression in platelets from diabetic patients, a phenomenon that could be attributed to the increased activity of calpains. To determine the role of CCNY in platelets, mice globally lacking the protein were studied. CCNY^-/- mice demonstrated lower numbers of circulating platelets but platelet responsiveness to thrombin and a thromboxane A₂ analogue were comparable with that of wild-type mice, as was agonist-induced α and dense granule secretion. CCNY-deficient platelets demonstrated enhanced adhesion to fibronectin and collagen as well as an attenuated spreading and clot retraction, indicating an alteration in "outside in" integrin signalling. This phenotype was accompanied by a significant reduction in the agonist-induced tyrosine phosphorylation of β3 integrin. Taken together we have shown that CCNY is present in anucleated platelets where it is involved in the regulation of integrin-mediated outside in signalling associated with thrombin stimulation.

AMPK inhibition protects against arterial thrombosis while sparing hemostasis through differential modulation of platelet responses

AMP-activated protein kinase (AMPK) is a metabolic master switch that has critical role in wide range of pathologies including cardiovascular disorders. As AMPK-α2 knockout mice exhibit impaired thrombus stability, we asked whether pharmacological inhibition of AMPK with a specific small-molecule inhibitor, compound C, could protect against arterial thrombosis without affecting hemostasis. Mice pre-administered with compound C exhibited decreased mesenteric arteriolar thrombosis but normal tail bleeding time compared to vehicle-treated animals. Compound C potently restricted platelet aggregation, clot retraction and integrin activation induced by thrombin and collagen. It impaired platelet spreading on both immobilized fibrinogen and collagen matrices; it, however, had no significant effect on thrombin-induced phosphatidylserine exposure that is characteristic of procoagulant platelets. In parallel, compound C brought about significant drop in thrombin-induced phosphorylation of myosin light chain (MLC) and MLC phosphatase (MYPT1) as well as abrogated rise in level of RhoA-GTP in thrombin-stimulated platelets. Thus, effects of compound C on agonist-induced platelet responses could be at least in part attributed to modulation of cytoskeletal changes mediated by RhoA-MYPT1-MLC signaling. An ideal antithrombotic drug would spare hemostatic responses that maintain vascular integrity while preferentially protecting against thrombosis. The present study suggests that AMPK could be one such potential therapeutic target.

Curdione inhibits thrombin-induced platelet aggregation via regulating the AMP-activated protein kinase-vinculin/talin-integrin αIIbβ3 sign pathway

BACKGROUND

Curdione, a sesquiterpene compound isolated from the essential oil of Curcuma aromatica Salisb. inhibits platelet aggregation, suggesting its significant anticoagulant and antithrombotic effects. However, the mechanisms have not been fully elucidated.

HYPOTHESIS

We hypothesized that curdione inhibits thrombin-induced platelet aggregation via regulating the AMP-activated protein kinase-vinculin/talin-integrin αIIbβ3 signaling pathway.

STUDY DESIGN

We performed in vitro assays to evaluate the effect of curdione on thrombin-induced expression levels of the AMPK signaling molecule and integrin αIIbβ3 signaling pathway components.

METHODS

Platelet proteins were extracted from washed human platelets, and the effects of curdione on thrombin-induced platelet aggregation were evaluated. The expression levels of the AMPK signaling molecule and integrin αIIbβ3 signaling pathway-related proteins were examined using western blot and RT-PCR. The binding of vinculin and talin were studied using immunoprecipitation, double immunofluorescence staining and microscale thermophoresis.

RESULTS

Platelet aggregation analysis showed that 0.02 U/ml thrombin significantly induces platelet aggregation. Western blot and RT-PCR analysis revealed that AMPK inhibits the vinculin/talin-mediated integrin αIIbβ3 signaling pathway, and curdione downregulates the thrombin-induced expression of phosphorylated AMPK (P-AMPK) and P-integrin at both the protein and mRNA levels and downregulates vinculin and talin at the protein level. Furthermore, microscale thermophoresis experiments showed that curdione inhibits the binding of vinculin and talin. The results from the immunoprecipitation and double immunofluorescence staining were consistent with the results of the microscale thermophoresis experiments.

CONCLUSION

Curdione inhibits thrombin-induced platelet aggregation via regulating the AMP-activated protein kinase-vinculin/talin-integrin αIIbβ3 signaling pathway, which suggests its therapeutic potential in ethnomedicinal applications as an anti-platelet and anti-thrombotic compound to prevent thrombotic diseases.

Glutamate induces synthesis of thrombogenic peptides and extracellular vesicle release from human platelets

Platelets are highly sensitive blood cells, which play central role in hemostasis and thrombosis. Platelet dense granules carry considerable amount of neurotransmitter glutamate that is exocytosed upon cell activation. As platelets also express glutamate receptors on their surface, it is pertinent to ask whether exposure to glutamate would affect their signalling within a growing thrombus. In this study we demonstrate that, glutamate per se induced synthesis of thrombogenic peptides, plasminogen activator inhibitor-1 and hypoxia-inducible factor-2α, from pre-existing mRNAs in enucleate platelets, stimulated cytosolic calcium entry, upregulated RhoA-ROCK-myosin light chain/myosin light chain phosphatase axis, and elicited extensive shedding of extracellular vesicles from platelets. Glutamate, too, incited platelet spreading and adhesion on to immobilized matrix under arterial shear, raised mitochondrial transmembrane potential associated with generation of reactive oxygen species and induced activation of AMP-activated protein kinase in platelets. Taken together, glutamate switches human platelets to pro-activation phenotype mediated mostly through AMPA receptors and thus targeting glutamate receptors may be a promising anti-platelet strategy.

Aerobic glycolysis fuels platelet activation: small-molecule modulators of platelet metabolism as anti-thrombotic agents

Platelets are critical to arterial thrombosis, which underlies myocardial infarction and stroke. Activated platelets, regardless of the nature of their stimulus, initiate energy-intensive processes that sustain thrombus, while adapting to potential adversities of hypoxia and nutrient deprivation within the densely packed thrombotic milieu. We report here that stimulated platelets switch their energy metabolism to aerobic glycolysis by modulating enzymes at key checkpoints in glucose metabolism. We found that aerobic glycolysis, in turn, accelerates flux through the pentose phosphate pathway and supports platelet activation. Hence, reversing metabolic adaptations of platelets could be an effective alternative to conventional anti-platelet approaches, which are crippled by remarkable redundancy in platelet agonists and ensuing signaling pathways. In support of this hypothesis, small-molecule modulators of pyruvate dehydrogenase, pyruvate kinase M2 and glucose-6-phosphate dehydrogenase, all of which impede aerobic glycolysis and/or the pentose phosphate pathway, restrained the agonist-induced platelet responses ex vivo These drugs, which include the anti-neoplastic candidate, dichloroacetate, and the Food and Drug Administration-approved dehydroepiandrosterone, profoundly impaired thrombosis in mice, thereby exhibiting potential as anti-thrombotic agents.

An Antithrombotic Strategy by Targeting Phospholipase D in Human Platelets

Phospholipase D (PLD) is involved in many biological processes. PLD1 plays a crucial role in regulating the platelet activity of mice; however, the role of PLD in the platelet activation of humans remains unclear. Therefore, we investigated whether PLD is involved in the platelet activation of humans. Our data revealed that inhibition of PLD1 or PLD2 using pharmacological inhibitors effectively inhibits platelet aggregation in humans. However, previous studies have showed that PLD1 or PLD2 deletion did not affect mouse platelet aggregation in vitro, whereas only PLD1 deletion inhibited thrombus formation in vivo. Intriguingly, our data also showed that the pharmacological inhibition of PLD1 or PLD2 does not affect mouse platelet aggregation in vitro, whereas the inhibition of only PLD1 delayed thrombus formation in vivo. These findings indicate that PLD may play differential roles in humans and mice. In humans, PLD inhibition attenuates platelet activation, adhesion, spreading, and clot retraction. For the first time, we demonstrated that PLD1 and PLD2 are essential for platelet activation in humans, and PLD plays different roles in platelet function in humans and mice. Our findings also indicate that targeting PLD may provide a safe and alternative therapeutic approach for preventing thromboembolic disorders.

High-dose Factor XIII administration induces effective hemostasis for trauma-associated coagulopathy (TAC) both in vitro and in rat hemorrhagic shock in vivo models

BACKGROUND

Trauma-associated coagulopathy (TAC) is an early and primary complication in severe trauma patients. Factor XIII (FXIII) is reported to stabilize a clot in the late phase of the coagulation cascade. The goal of this study was to investigate whether the administration of FXIII improves the condition of TAC both in vitro and in vivo.

METHODS

We evaluated the effects of different doses, including a very high dose of FXIII (3.6-32.4 IU/mL) on tissue-plasminogen activator-induced hyperfibrinolysis and the combined condition of dilutional coagulopathy and tissue-plasminogen activator-induced hyperfibrinolysis in vitro. The coagulation status was analyzed by rotational thromboelastometry (ROTEM) and Sonoclot. Then, we evaluated the effect of high-dose FXIII (300 IU/kg) for severe coagulopathy in vivo using a rat liver trauma model in which coagulopathy similar to TAC was observed. Survival time and the amount of intra-abdominal bleeding of rats were measured, and a coagulation test was also performed. Histologic evaluations of rats' lung and kidney after FXIII administration were completed.

RESULTS

High-dose FXIII significantly improved clot strength as well as increased resistance to hyperfibrinolysis in vitro which was confirmed by ROTEM. Platelet function on Sonoclot was significantly increased by FXIII in a dose-dependent manner. Factor XIII significantly decreased the total amount of bleeding and prolonged the survival time compared to control (control vs FXIII: 108.9 ± 11.4 vs 32.6 ± 5.5 mL/kg; p < 0.001; 26.0 ± 8.8 vs 120 minutes, p < 0.001) in a rat model. Rotational thromboelastometry parameters and platelet function on Sonoclot were significantly improved in the FXIII (+) group compared to control. No adverse effects of FXIII were detected histologically.

CONCLUSION

Factor XIII not only generated stable clot resistance to hyperfibrinolysis but also enhanced platelet function by facilitating clot retraction. High-dose FXIII administration therapy has significant clinical impact for severe trauma accompanied with TAC.

STUDY TYPE

Human in vitro and rat in vivo experimental study.

AMPK-ACC signaling modulates platelet phospholipids and potentiates thrombus formation

AMP-activated protein kinase (AMPK) α1 is activated in platelets on thrombin or collagen stimulation, and as a consequence, phosphorylates and inhibits acetyl-CoA carboxylase (ACC). Because ACC is crucial for the synthesis of fatty acids, which are essential for platelet activation, we hypothesized that this enzyme plays a central regulatory role in platelet function. To investigate this, we used a double knock-in (DKI) mouse model in which the AMPK phosphorylation sites Ser79 on ACC1 and Ser212 on ACC2 were mutated to prevent AMPK signaling to ACC. Suppression of ACC phosphorylation promoted injury-induced arterial thrombosis in vivo and enhanced thrombus growth ex vivo on collagen-coated surfaces under flow. After collagen stimulation, loss of AMPK-ACC signaling was associated with amplified thromboxane generation and dense granule secretion. ACC DKI platelets had increased arachidonic acid-containing phosphatidylethanolamine plasmalogen lipids. In conclusion, AMPK-ACC signaling is coupled to the control of thrombosis by specifically modulating thromboxane and granule release in response to collagen. It appears to achieve this by increasing platelet phospholipid content required for the generation of arachidonic acid, a key mediator of platelet activation.

Biphasic myosin II light chain activation during clot retraction

Clot retraction is an essential step during primary haemostasis, thereby promoting thrombus stability and wound healing. Integrin αIIbβ3 plays a critical role in clot retraction, by inducing acto-myosin interactions that allow platelet cytoskeleton reorganisation. However, the signalling pathways that lead to clot retraction are still misunderstood. In this study, we report the first data on the kinetics of myosin II light chain (MLC) phosphorylation during clot retraction. We found an early phosphorylation peak followed by a second peak. By using specific inhibitors of kinases and small G proteins, we showed that MLC kinase (MLCK), RhoA/ROCK, and Rac-1 were involved in clot retraction and in the early MLC phosphorylation peak. Only Rac-1 and actin polymerisation, controlled by outside-in signalling, were crucial to the second MLC phosphorylation peak.

Activation of CaMKKβ/AMPKα pathway by 2-AG in human platelets

The objective of this study was to determine whether AMPK is activated by 2-arachidonoylglycerol (2-AG) and participates to the cytoskeleton control in human platelets. We found that 2-AG stimulates the AMPKα activation through a Ca²⁺ /Calmodulin-dependent pathway as the specific inhibition of the CaMKKβ by STO-609 inhibits the AMPKα phosphorylation/activation. Moreover, the CaMKKβ/AMPKα pathway activated by 2-AG is involved in the phosphorylation of cofilin, vasodilator stimulated phosphoprotein (VASP), and myosin light chain (MLCs). These proteins participate to actin cytoskeletal remodelling during aggregation. We found that the phosphorylation/activation inhibition of these proteins is associated with a significant reduction in actin polymerization, aggregation, ATP, and α-granule secretion. Finally, AMPKα activation, Cofilin, VASP, and MLCs phosphorylation are significantly reduced by SR141716, the specific inhibitor of type 1 cannabinoid (CB1) receptor, suggesting that the CB1 receptor is involved in the 2-AG effect. In conclusion, we have shown that the CaMKKβ/AMPKα pathway is activated by 2-AG in human platelets and controls the phosphorylation of key proteins involved in actin polymerization and aggregation.

Functional differences between AMPK α1 and α2 subunits in osteogenesis, osteoblast-associated induction of osteoclastogenesis, and adipogenesis

The endocrine role of the skeleton-which is impaired in human diseases including osteoporosis, obesity and diabetes-has been highlighted previously. In these diseases, the role of AMPK, a sensor and regulator of energy metabolism, is of biological and clinical importance. Since AMPK's main catalytic subunit α has two isoforms, it is unclear whether functional differences between them exist in the skeletal system. The current study overexpressed AMPKα1 and α2 in MC3T3-E1 cells, primary osteoblasts and mouse BMSCs by lentiviral transduction. Cells overexpressing AMPKα2 showed higher osteogenesis potential than AMPKα1, wherein androgen receptor (AR) and osteoactivin played important roles. RANKL and M-CSF were secreted at lower levels from cells overexpressing α2 than α1, resulting in decreased osteoblast-associated osteoclastogenesis. Adipogenesis was inhibited to a greater degree in 3T3-L1 cells overexpressing α2 than α1, which was modulated by AR. An abnormal downregulation of AMPKα2 was observed in human BMSCs exhibiting the fibrous dysplasia (FD) phenotype. Overexpression of AMPKα2 in these cells rescued the defect in osteogenesis, suggesting that AMPKα2 plays a role in FD pathogenesis. These findings highlight functional differences between AMPKα1 and α2, and provide a basis for investigating the molecular mechanisms of diseases associated with impaired functioning of the skeletal system.

Metformin reduces hyper-reactivity of platelets from patients with polycystic ovary syndrome by improving mitochondrial integrity

Polycystic ovary syndrome (PCOS) is associated with decreased fertility, insulin resistance and an increased risk of developing cardiovascular disease. Treating PCOS patients with metformin improves fertility and decreases cardiovascular complications. Given that platelet activation contributes to both infertility and cardiovascular disease development, we assessed platelet reactivity in PCOS patients and the consequences of metformin treatment. Compared to washed platelets from healthy donors, platelets from PCOS patients demonstrated enhanced reactivity and impaired activation of the AMP-activated kinase (AMPK). PCOS platelets also demonstrated enhanced expression of mitochondrial proteins such as the cytochrome c reductase, ATP synthase and the voltage-dependent anion channel-1. However, mitochondrial function was impaired as demonstrated by a decreased respiration rate. In parallel, the phosphorylation of dynamin-related protein-1 (Drp-1) on Ser616 was increased while that on Ser637 decreased. The latter changes were accompanied by decreased mitochondrial size. In insulin-resistant PCOS patients (HOMA-IR> 2) metformin treatment (1.7 g per day for 4 weeks to 6 months) improved insulin sensitivity, restored mitochondrial integrity and function and normalised platelet aggregation. Treatment was without effect in PCOS patients with HOMA-IR< 2. Moreover, treatment of megakaryocytes with metformin enhanced mitochondrial content and in the same cells metformin enhanced the phosphorylation of the Drp-1 on Ser637 via an AMPKα1-dependent mechanism. In conclusion, the improvement of mitochondrial integrity and platelet reactivity may contribute to the beneficial effects of metformin on cardiovascular disease.

The Ca(2+) /calmodulin-dependent kinase kinase β-AMP-activated protein kinase-α1 pathway regulates phosphorylation of cytoskeletal targets in thrombin-stimulated human platelets

BACKGROUND

Platelet activation requires sweeping morphologic changes, supported by contraction and remodeling of the platelet actin cytoskeleton. In various other cell types, AMP-activated protein kinase (AMPK) controls the phosphorylation state of cytoskeletal targets.

OBJECTIVE

To determine whether AMPK is activated during platelet aggregation and contributes to the control of cytoskeletal targets.

RESULTS

We found that AMPK-α1 was mainly activated by thrombin, and not by other platelet agonists, in purified human platelets. Thrombin activated AMPK-α1 ex vivo via a Ca(2+) /calmodulin-dependent kinase kinase β (CaMKKβ)-dependent pathway. Pharmacologic inhibition of CaMKKβ blocked thrombin-induced platelet aggregation and counteracted thrombin-induced phosphorylation of several cytoskeletal proteins, namely, regulatory myosin light chains (MLCs), cofilin, and vasodilator-stimulated phosphoprotein (VASP), three key elements involved in actin cytoskeletal contraction and polymerization. Platelets isolated from mice lacking AMPK-α1 showed reduced aggregation in response to thrombin, and this was associated with defects in MLC, cofilin and VASP phosphorylation and actin polymerization. More importantly, we show, for the first time, that the AMPK pathway is activated in platelets of patients undergoing major cardiac surgery, in a heparin-sensitive manner.

CONCLUSION

AMPK-α1 is activated by thrombin in human platelets. It controls the phosphorylation of key cytoskeletal targets and actin cytoskeletal remodeling during platelet aggregation.

Gene expression profiles and phosphorylation patterns of AMP-activated protein kinase subunits in various mesenchymal cell types

BACKGROUND

Recent studies on bone have shown an endocrine role of the skeleton, which could be impaired in various human diseases, including osteoporosis, obesity, and diabetes-associated bone diseases. As a sensor and regulator of energy metabolism, AMP-activated protein kinase (AMPK) may also play an important role in the regulation of bone metabolism. The current study aimed to establish the expression profiles and phosphorylation patterns of AMPK subunits in several mesenchymal cell types.

METHODS

Reverse transcription-polymerase chain reaction (PCR) for relative quantification, real-time PCR for absolute quantification, and Western blotting were used to investigate the gene expression profiles and phosphorylation patterns of AMPK subunits in several mesenchymal cell types, including primary human mesenchymal stem cells (hMSCs) and hFOB, Saos-2, C3H/10T1/2, MC3T3-E1, 3T3-L1, and C2C12 cells.

RESULTS

AMPKα1 and AMPKβ1 mRNAs were abundantly expressed in all cell types. AMPKγ1 mRNA was abundantly expressed in C3H/10T1/2, MC3T3-E1, 3T3-L1, and C2C12 but not detected in human-derived cell types. AMPKγ2 mRNA was mildly expressed in all cell types. AMPKα1 protein was highly expressed in all cell types and AMPKα2 protein was highly expressed only in hFOB and Saos-2 cells. AMPKβ1 protein was abundantly expressed in all cell types except for Saos-2, in which AMPKβ2 protein overwhelmed AMPKβ1 expression. AMPKγ1 and AMPKγ2 proteins were expressed in C3H/10T1/2, MC3T3-E1, 3T3-L1, and C2C12 cells and only AMPKγ2 protein was expressed in hMSCs, hFOB and Saos-2 cells. AMPKα was phosphorylated at Thr172 and Ser485 and AMPKβ1 was phosphorylated at Ser108 and Ser182 in all cell types with a specific pattern in each cell type.

CONCLUSION

The combination of AMPK α, β, and γ subunits and phosphorylation of AMPKα (Thr172 and Ser485) and AMPKβ1 (Ser108 and Ser182) showed a specific pattern in each cell type.

Loss of ATE1-mediated arginylation leads to impaired platelet myosin phosphorylation, clot retraction, and in vivo thrombosis formation

Protein arginylation by arginyl-transfer RNA protein transferase (ATE1) is emerging as a regulator protein function that is reminiscent of phosphorylation. For example, arginylation of β-actin has been found to regulate lamellipodial formation at the leading edge in fibroblasts. This finding suggests that similar functions of β-actin in other cell types may also require arginylation. Here, we have tested the hypothesis that ATE1 regulates the cytoskeletal dynamics essential for in vivo platelet adhesion and thrombus formation. To test this hypothesis, we generated conditional knockout mice specifically lacking ATE1 in their platelets and in their megakaryocytes and analyzed the role of arginylation during platelet activation. Surprisingly, rather than finding an impairment of the actin cytoskeleton structure and its rearrangement during platelet activation, we observed that the platelet-specific ATE1 knockout led to enhanced clot retraction and in vivo thrombus formation. This effect might be regulated by myosin II contractility since it was accompanied by enhanced phosphorylation of the myosin regulatory light chain on Ser19, which is an event that activates myosin in vivo. Furthermore, ATE1 and myosin co-immunoprecipitate from platelet lysates. This finding suggests that these proteins directly interact within platelets. These results provide the first evidence that arginylation is involved in phosphorylation-dependent protein regulation, and that arginylation affects myosin function in platelets during clot retraction.

Methylglyoxal induces platelet hyperaggregation and reduces thrombus stability by activating PKC and inhibiting PI3K/Akt pathway

Diabetes is characterized by a dysregulation of glucose homeostasis and platelets from patients with diabetes are known to be hyper-reactive and contribute to the accelerated development of vascular diseases. Since many of the deleterious effects of glucose have been attributed to its metabolite methylgyloxal (MG) rather than to hyperglycemia itself, the aim of the present study was to characterize the effects of MG on platelet function. Washed human platelets were pre-incubated for 15 min with MG and platelet aggregation, adhesion on matrix-coated slides and signaling (Western blot) were assessed ex vivo. In vivo, the effect of MG on thrombus formation was determined using the FeCl₃-induced carotid artery injury model. MG potentiated thrombin-induced platelet aggregation and dense granule release, but inhibited platelet spreading on fibronectin and collagen. In vivo, MG accelerated thrombus formation but decreased thrombus stability. At the molecular level, MG increased intracellular Ca²⁺ and activated classical PKCs at the same time as inhibiting PI3K/Akt and the β3-integrin outside-in signaling. In conclusion, these findings indicate that the enhanced MG concentration measured in diabetic patients can directly contribute to the platelet dysfunction associated with diabetes characterized by hyperaggregability and reduced thrombus stability.

Antiplatelet effect of AMP-activated protein kinase activator and its potentiation by the phosphodiesterase inhibitor dipyridamole

AMP-activated protein kinase (AMPK) activates endothelial nitric oxide synthase (eNOS) via phosphorylation at the activating site. The eNOS-nitric oxide (NO)/soluble guanylate cyclase (sGC)-cGMP/cGMP-dependent protein kinase (PKG) signaling axis is a major antiaggregatory mechanism residing in platelets. Based on the hypothesis that direct activation of AMPK might be a potential strategy to inhibit platelet aggregation, the antiplatelet effect of AMPK activators was investigated. Treatment of isolated platelets with the AMPK activator, 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside (AICAR) resulted in AMPK activation and a decrease in aggregation, which was abolished by pretreatment with the AMPK inhibitors compound C (CC) and ara-A. Such an AMPK-dependent antiaggregatory effect was also observed with other AMPK activators such as A-769662 and PT1. AICAR induced eNOS activation was followed by NO synthesis, cGMP production, and subsequent phosphorylation of vasodilator-stimulated phosphoprotein (VASP), a PKG substrate. All these events were blocked by CC or ara-A pretreatment, and each event was inhibited by the eNOS inhibitor L-NAME, the sGC inhibitor ODQ, and the PKG inhibitor Rp-8-pCPT-cGMPS. Simultaneous treatment of dipyridamole, a phosphodiesterase (PDE) inhibitor, with AICAR potentiated the antiaggregatory effect by enhancing the cGMP elevation. Administration of AICAR increased platelet cGMP and prolonged FeCl3-induced arterial occlusion time in rats, which further increased in combination with dipyridamole. In conclusion, AMPK activators inhibited platelet aggregation by stimulating the eNOS-NO/sGC-cGMP/PKG signaling pathway. The antiplatelet effect of AMPK activators could be potentiated in combination with a PDE inhibitor through the common mechanism of elevating cGMP. Thus, AMPK may serve as a potential target for antiplatelet therapy.

Regulation of STIM1/Orai1-dependent Ca2+ signalling in platelets

Platelet secretion and aggregation as well as thrombus formation of blood platelets critically depend on increase of cytosolic Ca2+ concentration ([Ca2+]i) mainly resulting from intracellular Ca2+ release followed by store operated Ca2+ entry (SOCE) through Ca2+ release activated channels (CRAC). SOCE is in part accomplished by the pore forming unit Orai and its regulator stromal interaction molecule (STIM). Orai1 and STIM1 transcription is stimulated by NF-κB (nuclear factor kappa B). Serum- and glucocorticoid-inducible kinase 1 (SGK1) up-regulates NF-κB-activity in megakaryocytes and thus Orai1-expression and SOCE in platelets. SGK1 is thus a powerful regulator of platelet Ca2+-signalling and thrombus formation and presumably participates in the regulation of platelet activation by a variety of hormones as well as clinical conditions (e.g. type 2 diabetes or metabolic syndrome) associated with platelet hyperaggregability and increased risk of thromboocclusive events. SOCE in platelets is further regulated by scaffolding protein Homer and chaperone protein cyclophilin A (CyPA). Additional potential regulators of Orai1/STIM1 and thus SOCE in platelets include AMP activated kinase (AMPK), protein kinase A (PKA), reactive oxygen species, lipid rafts, pH and mitochondrial Ca2+ buffering. Future studies are required defining the significance of those mechanisms for platelet Orai1 abundance and function, for SOCE into platelets and for platelet function in cardiovascular diseases.

The effects of arterial flow on platelet activation, thrombus growth, and stabilization

Injury of an arterial vessel wall acutely triggers a multifaceted process of thrombus formation, which is dictated by the high-shear flow conditions in the artery. In this overview, we describe how the classical concept of arterial thrombus formation and vascular occlusion, driven by platelet activation and fibrin formation, can be extended and fine-tuned. This has become possible because of recent insight into the mechanisms of: (i) platelet-vessel wall and platelet-platelet communication, (ii) autocrine platelet activation, and (iii) platelet-coagulation interactions, in relation to blood flow dynamics. We list over 40 studies with genetically modified mice showing a role of platelet and plasma proteins in the control of thrombus stability after vascular injury. These include multiple platelet adhesive receptors and other junctional molecules, components of the ADP receptor signalling cascade to integrin activation, proteins controlling platelet shape, and autocrine activation processes, as well as multiple plasma proteins binding to platelets and proteins of the intrinsic coagulation cascade. Regulatory roles herein of the endothelium and other blood cells are recapitulated as well. Patient studies support the contribution of platelet- and coagulation activation in the regulation of thrombus stability. Analysis of the factors determining flow-dependent thrombus stabilization and embolus formation in mice will help to understand the regulation of this process in human arterial disease.

AMPK activation by oncogenesis is required to maintain cancer cell proliferation in astrocytic tumors

5'-AMP-activated protein kinase (AMPK) is an energy sensor that controls cell metabolism, and it has been related to apoptosis and cell-cycle arrest. Although its role in metabolic homeostasis is well documented, its function in cancer is much less clear. In this study, we examined the role of AMPK in a mouse model of astrocytoma driven by oncogenic H-Ras(V12) and/or with PTEN deletion based on the common constitutive activation of the Raf/MEK/ERK and PI3K/AKT cascades in human astrocytomas. We also evaluated the activity and role of AMPK in human glioblastoma cells and xenografts. AMPK was constitutively activated in astrocytes expressing oncogenic H-Ras(V12) in parallel with high cell division rates. Genetic deletion of AMPK or attenuation of its activity in these cells was sufficient to reduce cell proliferation. The levels of pAMK were always related to the levels of phosphorylated retinoblastoma (Rb) at Ser804, which may indicate an AMPK-mediated phosphorylation of Rb. We confirmed this AMPK-Rb relationship in human glioblastoma cell lines and xenografts. In clinical specimens of human glioblastoma, elevated levels of activated AMPK appeared especially in areas of high proliferation surrounding the blood vessels. Together, our findings indicate that the initiation and progression of astrocytic tumors relies upon AMPK-dependent control of the cell cycle, thereby identifying AMPK as a candidate therapeutic target in this setting.

Platelet-based coagulation: different populations, different functions

Platelets in a thrombus interact with (anti)coagulation factors and support blood coagulation. In the concept of cell-based control of coagulation, three different roles of platelets can be distinguished: control of thrombin generation, support of fibrin formation, and regulation of fibrin clot retraction. Here, we postulate that different populations of platelets with distinct surface properties are involved in these coagulant functions. Platelets with elevated Ca(2+) and exposed phosphatidylserine control thrombin and fibrin generation, while platelets with activated α(IIb) β(3) regulate clot retraction. We review how coagulation factor binding depends on the platelet activation state. Furthermore, we discuss the ligands, platelet receptors and downstream intracellular signaling pathways implicated in these coagulant functions. These insights lead to an adapted model of platelet-based coagulation.

Hydrogen sulfide-releasing aspirin derivative ACS14 exerts strong antithrombotic effects in vitro and in vivo

OBJECTIVE

Hydrogen sulfide (H(2)S)-releasing NSAIDs exert potent anti-inflammatory effects beyond classical cyclooxygenase inhibition. Here, we compared the platelet inhibitory effects of the H(2)S-releasing aspirin derivative ACS14 with its mother compound aspirin to analyze additional effects on platelets.

METHODS AND RESULTS

In platelets of mice fed with ACS14 for 6 days (50 mg/kg per day), not only arachidonic acid-induced platelet aggregation but also ADP-dependent aggregation was decreased, an effect that was not observed with an equimolar dose of aspirin (23 mg/kg per day). ACS14 led to a significantly longer arterial occlusion time after light-dye-induced endothelial injury as well as decreased thrombus formation after ferric chloride-induced injury in the carotid artery. Bleeding time was not prolonged compared with animals treated with equimolar doses of aspirin. In vitro, in human whole blood, ACS14 (25-500 µmol/L) inhibited arachidonic acid-induced platelet aggregation, but compared with aspirin additionally reduced thrombin receptor-activating peptide-, ADP-, and collagen-dependent aggregation. In washed human platelets, ACS14 (500 µmol/L) attenuated αIIbβ3 integrin activation and fibrinogen binding and increased intracellular cAMP levels and cAMP-dependent vasodilator-stimulated phosphoprotein (VASP) phosphorylation.

CONCLUSIONS

The H(2)S-releasing aspirin derivative ACS14 exerts strong antiaggregatory effects by impairing the activation of the fibrinogen receptor by mechanisms involving increased intracellular cyclic nucleotides. These additional antithrombotic properties result in a more efficient inhibition of thrombus formation in vivo as achieved with aspirin alone.

AMPK in myocardial infarction and diabetes: the yin/yang effect

There has been considerable progress in our understanding of cardiac cell metabolism in health and disease, yet important gaps remain in basic knowledge and its translation to clinical care. AMP-activated protein kinase (AMPK) functions either to conserve ATP or to promote alternative methods of ATP generation. Since the discovery of AMPK more than three decades ago and demonstration of its expression in the heart, interest has grown exponentially in this major fuel gauge as a modulator of the cellular response to ischemia. Such pathway may potentially explain the strong association between metabolic syndrome and ischemic heart disease. Still missing from our most recent cardiology textbooks, this article aims to summarize our understanding so far of the role of AMPK in coordinating the cellular response to ischemic stress and reperfusion injury in the heart. We aim to provide a focused update on the pharmacological agents activating AMPK for treatment of diabetes that show potential cardioprotective effects. Our hope is to stimulate future researchers to the potential benefits of harnessing the AMPK signaling pathway, or better one of its novel downstream targets for the treatment of myocardial ischemia.

Plant food delphinidin-3-glucoside significantly inhibits platelet activation and thrombosis: novel protective roles against cardiovascular diseases

Delphinidin-3-glucoside (Dp-3-g) is one of the predominant bioactive compounds of anthocyanins in many plant foods. Although several anthocyanin compounds have been reported to be protective against cardiovascular diseases (CVDs), the direct effect of anthocyanins on platelets, the key players in atherothrombosis, has not been studied. The roles of Dp-3-g in platelet function are completely unknown. The present study investigated the effects of Dp-3-g on platelet activation and several thrombosis models in vitro and in vivo. We found that Dp-3-g significantly inhibited human and murine platelet aggregation in both platelet-rich plasma and purified platelets. It also markedly reduced thrombus growth in human and murine blood in perfusion chambers at both low and high shear rates. Using intravital microscopy, we observed that Dp-3-g decreased platelet deposition, destabilized thrombi, and prolonged the time required for vessel occlusion. Dp-3-g also significantly inhibited thrombus growth in a carotid artery thrombosis model. To elucidate the mechanisms, we examined platelet activation markers via flow cytometry and found that Dp-3-g significantly inhibited the expression of P-selectin, CD63, CD40L, which reflect platelet α- and δ-granule release, and cytosol protein secretion, respectively. We further demonstrated that Dp-3-g downregulated the expression of active integrin αIIbβ3 on platelets, and attenuated fibrinogen binding to platelets following agonist treatment, without interfering with the direct interaction between fibrinogen and integrin αIIbβ3. We found that Dp-3-g reduced phosphorylation of adenosine monophosphate-activated protein kinase, which may contribute to the observed inhibitory effects on platelet activation. Thus, Dp-3-g significantly inhibits platelet activation and attenuates thrombus growth at both arterial and venous shear stresses, which likely contributes to its protective roles against thrombosis and CVDs.

Regulation of NAD(P)H oxidases by AMPK in cardiovascular systems

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are ubiquitously produced in cardiovascular systems. Under physiological conditions, ROS/RNS function as signaling molecules that are essential in maintaining cardiovascular function. Aberrant concentrations of ROS/RNS have been demonstrated in cardiovascular diseases owing to increased production or decreased scavenging, which have been considered common pathways for the initiation and progression of cardiovascular diseases such as atherosclerosis, hypertension, (re)stenosis, and congestive heart failure. NAD(P)H oxidases are primary sources of ROS and can be induced or activated by all known cardiovascular risk factors. Stresses, hormones, vasoactive agents, and cytokines via different signaling cascades control the expression and activity of these enzymes and of their regulatory subunits. But the molecular mechanisms by which NAD(P)H oxidase is regulated in cardiovascular systems remain poorly characterized. Investigations by us and others suggest that adenosine monophosphate-activated protein kinase (AMPK), as an energy sensor and modulator, is highly sensitive to ROS/RNS. We have also obtained convincing evidence that AMPK is a physiological suppressor of NAD(P)H oxidase in multiple cardiovascular cell systems. In this review, we summarize our current understanding of how AMPK functions as a physiological repressor of NAD(P)H oxidase.

Factor XIII is a key molecule at the intersection of coagulation and fibrinolysis as well as inflammation and infection control

Factor XIII (FXIII) is a transglutaminase consisting of two catalytic A subunits (FXIII-A) and two non-catalytic B subunits (FXIII-B) in plasma. FXIII-B protects FXIII-A from its clearance. FXIII-A is also present as a homodimer inside megakaryocytes/platelets and monocytes/macrophages. Although possible functions of intracellular FXIII-A have been proposed, these remain to be established. Intra- and extra-cellular FXIIIs support platelet adhesion and spreading as well as clot retraction, suggesting that FXIII is important for the stabilization of platelet-fibrin clots. Intra- and extra-cellular FXIIIs also support immobilization and killing of bacteria as well as phagocytosis by macrophages. Thus, FXIII may function in innate immunity. Congenital FXIII deficiency due to defective F13-A genes manifests as a life-long bleeding tendency, abnormal wound healing, and recurrent miscarriage. Although congenital FXIII-B deficiency used to be thought rare, reports of such cases have increased recently. As the bleeding tendency is often mild, patients with FXIII-B deficiency may be overlooked by physicians. Patients with acquired FXIII deficiency, in particular those with autoimmune hemorrhaphilia due to anti-FXIII antibodies, are on the increase, at least in Japan. It is important to diagnose such cases as early as possible, and to treat them with immunosuppression in combination with FXIII replacement therapy as their bleeding symptoms can be life-threatening.

Clot retraction

The study of clot retraction in vitro has been adopted as a simple and reproducible approach to assess platelet function. Plasma clots should retract away from the sides of a glass tube within a few hours allowing the rapid characterization of outside-in signaling through platelet integrin α(IIb)β(3). In this chapter, we describe the role of platelets in fibrin clot retraction and provide a detailed description of the methods used to assess this process.