Platelets play a fundamental role in hemostasis and thrombosis. They are also involved in pathologic conditions resulting from blocked blood vessels, including myocardial infarction and ischemic stroke. Platelet adhesion, activation, and aggregation at sites of vascular injury are regulated by a diverse repertoire of tyrosine kinase–linked and G protein–coupled receptors. Src family kinases (SFKs) play a central role in initiating and propagating signaling from several platelet surface receptors; however, the underlying mechanism of how SFK activity is regulated in platelets remains unclear. CD148 is the only receptor-like protein tyrosine phosphatase identified in platelets to date. In the present study, we show that mutant mice lacking CD148 exhibited a bleeding tendency and defective arterial thrombosis. Basal SFK activity was found to be markedly reduced in CD148-deficient platelets, resulting in a global hyporesponsiveness to agonists that signal through SFKs, including collagen and fibrinogen. G protein–coupled receptor responses to thrombin and other agonists were also marginally reduced. These results highlight CD148 as a global regulator of platelet activation and a novel antithrombotic drug target.

During thrombopoiesis, maturing megakaryocytes (MKs) migrate within the complex bone marrow stromal microenvironment from the proliferative osteoblastic niche to the capillary-rich vascular niche where proplatelet formation and platelet release occurs. This physiologic process involves proliferation, differentiation, migration, and maturation of MKs before platelet production occurs. In this study, we report a role for the glycoprotein PECAM-1 in thrombopoiesis. We show that following induced thrombocytopenia, recovery of the peripheral platelet count is impaired in PECAM-1-deficient mice. Whereas MK maturation, proplatelet formation, and platelet production under in vitro conditions were unaffected, we identified a migration defect in PECAM-1-deficient MKs in response to a gradient of stromal cell-derived factor 1 (SDF1), a major chemokine regulating MK migration within the bone marrow. This defect could be explained by defective PECAM-1 ؊/؊ MK polarization of the SDF1 receptor CXCR4 and an increase in adhesion to immobilized bone marrow matrix proteins that can be ex- IntroductionMegakaryocytopoiesis involves proliferation and differentiation of megakaryocyte (MK) progenitors to a large, terminally differentiated cell with a multi-lobulated, polyploid nucleus. Nuclear maturation, a process known as endoreplication, proceeds in concert with cytoplasmic maturation and expression of platelet surface markers including the glycoprotein receptors IIb/IIIa, GPIb, GPIX, and GPVI. As the MK matures and differentiates, it migrates to sinusoidal bone marrow endothelial cells where it forms transendothelial projections called proplatelets that release 1000 to 5000 platelets per MK into the intravascular space. [1][2][3][4][5] Thrombopoietin (TPO) participates in the humoral regulation of thrombopoiesis. TPO is produced constitutively in the liver and by bone marrow stromal cells and its levels are regulated by binding to the receptor c-Mpl expressed on platelets. 6 This has the net effect of reducing the concentration of TPO in the circulation and thereby inhibiting differentiation of progenitor cells along the MK lineage. Thus, megakaryocytopoiesis and platelet count are directly regulated by circulating TPO. 2 A key step in thrombopoiesis is migration of maturing MKs from the proliferative osteoblastic niche within the bone marrow microenvironment, where hematopoietic stem cells reside, to the capillary-rich vascular niche, where proplatelets are formed. 6 This process is regulated by a variety of chemokines and cytokines, as well as by adhesive interactions with interstitial cells and extracellular matrix proteins. For example, the chemokine SDF1 directs movement of MK progenitors through its receptor CXCR4 from the proliferative "osteoblastic niche" to the "vascular niche," where platelets are formed. 6 SDF1 therefore acts in combination with TPO to promote differentiation of MK progenitor cells to mature MKs. 7,8 In addition, SDF1 promotes the interaction and transmigration of mature MKs through bone marrow endothel...

Migration of megakaryocytes (MKs) from the proliferative osteoblastic niche to the capillary-rich vascular niche is essential for proplatelet formation and platelet release. In this study, we explore the role of surface glycoprotein receptors and signaling proteins in regulating MK migration and platelet recovery after immuneinduced thrombocytopenia. We show that spreading and migration of mouse primary bone marrow-derived MKs on a fibronectin matrix are abolished by the Src family kinases inhibitor PP1, the Syk kinase inhibitor R406 and the integrin ␣IIb␤3 antagonist lotrafiban. We also demonstrate that these responses are inhibited in primary phospholipase C ␥2 (PLC␥2)-deficient MKs. Conversely, MK spreading and migration were unaltered in the absence of the collagen receptor, the glycoprotein VI-FcR␥-chain complex. We previously reported a correlation between a defect in MK migration and platelet recovery in the absence of platelet endothelial cell adhesion molecule-1 and the tyrosine phosphatase CD148. This correlation also holds for mice deficient in PLC␥2. This study identifies a model in which integrin signaling via Src family kinases and Syk kinase to PLC␥2 is required for MK spreading, migration, and platelet formation. (Blood. 2010;116(5): 793-800)

PECAM-1 is a member of the superfamily of immunoglobulins (Ig) and is expressed on platelets at moderate level. PECAM-1 has been reported to have contrasting effects on platelet activation by the collagen receptor GPVI and the integrin, alphaIIbbeta3, even though both receptors signal through Src-kinase regulation of PLCgamma2. The present study compares the role of PECAM-1 on platelet activation by these two receptors and by the lectin receptor, CLEC-2, which also signals via PLCgamma2. Studies using PECAM-1 knockout-mice and cross-linking of PECAM-1 using specific antibodies demonstrated a minor inhibitory role on platelet responses to the above three receptors and also under some conditions to the G-protein agonist thrombin. The degree of inhibition was considerably less than that produced by PGI2, which elevates cAMP. There was no significant difference in thrombus formation on collagen in PECAM-1-/- platelets relative to litter-matched controls. The very weak inhibitory effect of PECAM-1 on platelet activation relative to that of PGI2 indicate that the Ig-receptor is not a major regulator of platelet activation. PECAM-1 has been reported to have contrasting effects on platelet activation. The present study demonstrates a very mild or negligible effect on platelet activation in response to stimulation by a variety of agonists, thereby questioning the physiological role of the immunoglobulin receptor as a major regulator of platelet activation.

Background Catheter ablation for complex left-atrial arrhythmia is increasing worldwide with many centres admitting patients overnight. Same-day procedures using conscious sedation carry significant benefits to patients/healthcare providers but data are limited. We evaluated the safety and cost-effectiveness of same-day complex left-atrial arrhythmia ablation. Method Multi-centre retrospective cohort study of all consecutive complex elective left-atrial ablation procedures performed between January 2011 and December 2019. Data were collected on planned same-day discharge versus overnight stay, baseline parameters, procedure details/success, ablation technology, post-operative complications, unplanned overnight admissions/outcomes at 4-months and mortality up to April 2020. A cost analysis of potential savings was also performed. Results A total of 967 consecutive patients underwent complex left-ablation using radiofrequency (point-by-point ablation aided by 3D-mapping or PVAC catheter ablation with fluoroscopic screening) or cryoballoon-ablation (mean age: 60.9 ± 11.6 years, range 23-83 yrs., 572 [59%] females). The majority of patients had isolation of pulmonary veins alone ( n = 846, 87%) and most using conscious-sedation alone ( n = 921, 95%). Of the total cohort, 414 (43%) had planned same-day procedure with 35 (8%) admitted overnight due to major ( n = 5) or minor ( n = 30) complications. Overall acute procedural success-rate was 96% ( n = 932). Complications in planned overnight-stay/same-day cohorts were low. At 4-month follow-up there were 62 (6.4%) readmissions (femoral haematomas, palpitation, other reasons); there were 3 deaths at mean follow-up of 42.0 ± 27.6 months, none related to the procedure. Overnight stay costs £350; the same-day ablation policy over this period would have saved £310,450. Conclusions Same-day complex left-atrial catheter ablation using conscious sedation is safe and cost-effective with significant benefits for patients and healthcare providers. This is especially important in the current financial climate and Covid-19 pandemic.