Refine
Has Fulltext
- yes (2)
Is part of the Bibliography
- yes (2)
Document Type
- Doctoral Thesis (2)
Language
- English (2)
Keywords
- Computersimulation (1)
- Fluoreszenzmikroskopie (1)
- Light sheet microscopy (1)
- Megakaryozyt (1)
- Schlaganfall (1)
- ischemia reperfusion injury (1)
- ischemic stroke (1)
- liver (1)
- megakaryopoiesis (1)
- multi-photon microscopy (1)
Institute
In mammals, anucleate platelets circulate in the blood flow and are primarily responsible for maintaining functional hemostasis. Platelets are generated in the bone marrow (BM) by megakaryocytes (MKs), which mainly reside directly next to the BM sinusoids to release proplatelets into the blood. MKs originate from hematopoietic stem cells and are thought to migrate from the endosteal to the vascular niche during their maturation, a process, which is, despite being intensively investigated, still not fully understood.
Long-term intravital two photon microscopy (2PM) of MKs and vasculature in murine bone marrow was performed and mean squared displacement analysis of cell migration was performed. The MKs exhibited no migration, but wobbling-like movement on time scales of 3 h. Directed cell migration always results in non-random spatial distribution. Thus, a computational modelling algorithm simulating random MK distribution using real 3D light-sheet fluorescence microscopy data sets was developed. Direct comparison of real and simulated random MK distributions showed, that MKs exhibit a strong bias to vessel-contact. However, this bias is not caused by cell migration, as non-vessel-associated MKs were randomly distributed in the intervascular space. Furthermore, simulation studies revealed that MKs strongly impair migration of other cells in the bone marrow by acting as large-sized obstacles. MKs are thought to migrate from the regions close to the endosteum towards the vasculature during their maturation process. MK distribution as a function of their localization relative to the endosteal regions of the bones was investigated by light sheet fluorescence microscopy (LSFM). The results show no bone-region dependent distribution of MKs. Taken together, the newly established methods and obtained results refute the model of MK migration during their maturation.
Ischemia reperfusion (I/R) injury is a frequent complication of cerebral ischemic stroke, where brain tissue damage occurs despite successful recanalization. Platelets, endothelial cells and immune cells have been demonstrated to affect the progression of I/R injury in experimental mouse models 24 h after recanalization. However, the underlying Pathomechanisms, especially in the first hours after recanalization, are poorly understood.
Here, LSFM, 2PM and complemental advanced image analysis workflows were established for investigation of platelets, the vasculature and neutrophils in ischemic brains. Quantitative analysis of thrombus formation in the ipsilateral and contralateral hemispheres at different time points revealed that platelet aggregate formation is minimal during the first 8 h after recanalization and occurs in both hemispheres. Considering that maximal tissue damage already is present at this time point, it can be concluded that infarct progression and neurological damage do not result from platelet aggregated formation. Furthermore, LSFM allowed to confirm neutrophil infiltration into the infarcted hemisphere and, here, the levels of endothelial cell marker PECAM1 were strongly reduced. However, further investigations must be carried out to clearly identify the role of neutrophils and the endothelial cells in I/R injury.
Platelets are the second most abundant blood cells and their main function is maintenance of vascular integrity. In addition, platelets are increasingly recognized as cells with immune functions, as they participate in the recruitment of immune cells and modulate the progression and severity of an immune response. So-called lipid mediators, which are – besides other cells – released by activated platelets, influence the immune response. LTB4 is one of these potent lipid mediators and is able to activate neutrophils and induce their infiltration into injured tissue.
In order to investigate the involvement of platelets in inflammatory processes, a murine model of hepatic ischemia reperfusion injury as well as confocal intravital microscopy of the liver were established. Both methods were used to analyze the influence of platelets on the inflammation that follows sterile liver inflammation. We found platelet function to be unaltered after three hours of reperfusion and platelet aggregation to be irrelevant for the outcome of hepatic ischemia reperfusion injury. However, a strong impact of the GPIb-vWF axis could be observed, as antibody mediated blockade of GPIb as well as vWF-deficiency significantly reduced liver damage markers and decreased neutrophil infiltration. GPIb-IL-4R mice were used to exclude the possibility that the protective effects of the anti-GPIbα antibody treatment (p0p/B) results from something else than blocking GPIbα. Furthermore, the slope of neutrophil infiltration was decreased in p0p/B-treated mice, leading to overall decreased neutrophil numbers in the liver after three hours of reperfusion. Blockade of the integrin αIIbβ3, however, showed no reduction in neutrophil infiltration into the post-ischemic liver, in line with unaltered liver damage.
To study the role of leukotriene B4, conditional and constitutive knockout mice for the LTA4 hydrolase, which catalyzes the last step in LTB4 synthesis, were generated. Lta4h deficiency did not affect general platelet functionality in hemostasis and thrombosis. Interestingly,
Lta4h-/- mice were not protected from cellular damage following hepatic ischemia, despite lower neutrophil numbers in the post-ischemic liver.
Intravital microscopy of the pancreas was established and revealed increased CD4+ T cell numbers in GPVI-deficient animals compared to WT controls in line with the pre-diabetic phenotype of Gp6-/- mice that was revealed in Grzegorz Sumara’s group. Furthermore, platelet ‘behavior’ in pancreatic islets was observed following glucose injection. We found a high number of platelets adherent to islet sinusoids under basal conditions and no rolling/decelerating of platelets following glucose injection. This was accompanied by temporary sinusoidal constriction and stop of the blood flow. This phenomenon was not observed in control settings (injection of PBS, insulin or L-glucose).
In a side project, which was carried out jointly with Tobias Heib, a side by side comparison of the classical syringe-based flushing and the centrifugation-based spinning method to isolate murine bone marrow was conducted. Flow cytometry revealed no differences in the distribution of hematopoietic stem cells and immune cells and functional analysis with primary and cultured megakaryocytes (MKs) showed comparable results in all conducted assays. Thus, our data demonstrated that the faster and more efficient spinning method can be used for the isolation of bone marrow cells.