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Intravital Microscopy of Leukocyte-endothelial and Platelet-leukocyte Interactions in Mesenterial Veins in Mice

Intravital microscopy is a method that can be used to investigate different processes in different regions and vessels in living animals. In this ...

Heart Center, University of Freiburg

University of Freiburg

Increased residual platelet reactivity remains a burden for coronary artery disease (CAD) patients who received a coronary stent and do not respond sufficiently to treatment with acetylsalicylic acid and clopidogrel. We hypothesized that serotonin antagonism reduces high on-treatment platelet reactivity. Whole blood impedance aggregometry was performed with arachidonic acid (AA, 0.5 mM) and adenosine diphosphate (ADP, 6.5 µM) in addition to different concentrations of serotonin (1-100 µM) in whole blood from 42 CAD patients after coronary stent placement and 10 healthy subjects. Serotonin increased aggregation dose-dependently in CAD patients who responded to clopidogrel treatment: After activation with ADP, aggregation increased from 33.7 ± 1.3% to 40.9 ± 2.0% in the presence of 50 µM serotonin (p<0.05) and to 48.2 ± 2.0% with 100 µM serotonin (p<0.001). The platelet serotonin receptor antagonist ketanserin decreased ADP-induced aggregation significantly in clopidogrel low-responders (from 59.9 ± 3.1% to 37.4 ± 3.5, p<0.01), but not in clopidogrel responders. These results were confirmed with light transmission aggregometry in platelet-rich plasma in a subset of patients. Serotonin hence increased residual platelet reactivity in patients who respond to clopidogrel after coronary stent placement. In clopidogrel low-responders, serotonin receptor antagonism improved platelet inhibition, almost reaching responder levels. This may justify further investigation of triple antiplatelet therapy with anti-serotonergic agents.

Serotonin antagonism improves platelet inhibition in clopidogrel low-responders after coronary stent placement: an in vitro pilot study.

The majority of peripheral serotonin is stored in platelets, which secrete it on activation. Serotonin releases Weibel-Palade bodies (WPBs) and we asked whether absence of platelet serotonin affects neutrophil recruitment in inflammatory responses. Tryptophan hydroxylase (Tph)1–deficient mice, lacking non-neuronal serotonin, showed mild leukocytosis compared with wild-type (WT), primarily driven by an elevated neutrophil count. Despite this, 50% fewer leukocytes rolled on unstimulated mesenteric venous endothelium of Tph1(-/-) mice. The velocity of rolling leukocytes was higher in Tph1(-/-) mice, indicating fewer selectin-mediated interactions with endothelium. Stimulation of endothelium with histamine, a secretagogue of WPBs, or injection of serotonin normalized the rolling in Tph1(-/-) mice. Diminished rolling in Tph1(-/-) mice resulted in reduced firm adhesion of leukocytes after lipopolysaccharide treatment. Blocking platelet serotonin uptake with fluoxetine in WT mice reduced serum serotonin by > 80% and similarly reduced leukocyte rolling and adhesion. Four hours after inflammatory stimulation, neutrophil extravasation into lung, peritoneum, and skin wounds was reduced in Tph1(-/-) mice, whereas in vitro neutrophil chemotaxis was independent of serotonin. Survival of lipopolysaccharide-induced endotoxic shock was improved in Tph1(-/-) mice. In conclusion, platelet serotonin promotes the recruitment of neutrophils in acute inflammation, supporting an important role for platelet serotonin in innate immunity.

Platelet serotonin promotes the recruitment of neutrophils to sites of acute inflammation in mice.

We tested the feasibility of local thrombolytic therapy via a novel hollow flexible and perforated wire in a mouse model of deep vein thrombosis. Inferior vena cava (IVC) thrombosis was induced by vessel wall exposure to ferric chloride after laparotomy in anesthetized C57Bl/6 mice. Thrombus formation was visualized by intravital microscopy of rhodamine-labeled platelets and leukocytes. A nitinol hypotube coronary wire with perforated tip was inserted via a 0.8 × 40 mm canula into the IVC lumen distal to the site of ferric chloride exposure. Either tissue plasminogen activator (tPA, alteplase) or saline (control) was administered via the platinum wire distal to the thrombus, avoiding mechanical fragmentation. Thrombus size was assessed by immunohistochemistry (platelet CD41 staining). Intravital microscopy of the IVC demonstrated platelet-containing thrombus growth starting 1 min after ferric chloride exposure. Alteplase administration resulted in significant thrombus size reduction within 10-20 min observed by intravital microscopy and confirmed by histological assessment of IVC cross-sections. Saline-treated mice (n = 4) demonstrated near total IVC occlusion with thrombotic material (84 ± 8% of cross-sectional area in serial sections), whereas alteplase-treated mice showed a dose-dependent decrease of thrombotic area [56 ± 5% with 1.5, 39 ± 4 % with 15 and 21 ± 6% with 150 mg/kg, respectively (n = 4)]. We demonstrate that a flexible hollow and perforated wire enables the successful application of thrombolytic therapy to IVC thrombi in mice without vessel wall perforation. Flexible wire-based thrombolytic therapy appears to be a safe and reliable method for thrombus dissolution even in fragile small veins and may become a promising strategy for targeted therapy of small vessel thrombosis.

A novel hollow and perforated flexible wire allows the safe and effective local application of thrombolytic therapy in a mouse model of deep vein thrombosis.

Activated platelets release serotonin at sites of inflammation where it acts as inflammatory mediator and enhances recruitment of neutrophils. Chronic treatment with selective serotonin reuptake inhibitors (SSRI) depletes the serotonin storage pool in platelets, leading to reduced leukocyte recruitment in murine experiments. Here, we examined the direct and acute effects of SSRI on leukocyte recruitment in murine peritonitis.

Acute fluoxetine treatment induces slow rolling of leukocytes on endothelium in mice.

Inflammation and myocardial necrosis play important roles in ischemia/reperfusion injury after coronary artery occlusion and recanalization. The detection of inflammatory activity and the extent of myocardial necrosis itself are of great clinical and prognostic interest. We developed a dual, noninvasive imaging approach using molecular magnetic resonance imaging in an in vivo mouse model of myocardial ischemia and reperfusion.

Dual-contrast molecular imaging allows noninvasive characterization of myocardial ischemia/reperfusion injury after coronary vessel occlusion in mice by magnetic resonance imaging.

Maximilian Mauler

Department of Cardiology and Angiology I

Heart Center, University of Freiburg

Intravital Microscopy of Leukocyte-endothelial and Platelet-leukocyte Interactions in Mesenterial Veins in Mice

Maximilian Mauler

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Cardiology and Angiology I

Heart Center, University of Freiburg

Intravital Microscopy of Leukocyte-endothelial and Platelet-leukocyte Interactions in Mesenterial Veins in Mice

Department of Cardiology and Angiology I