Red blood cells (RBCs) influence rheology, and release ADP, ATP, and nitric oxide, suggesting a role for RBCs in hemostasis and thrombosis. Here, we provide evidence for a significant contribution of RBCs to thrombus formation. Anemic mice showed enhanced occlusion times upon injury of the carotid artery. A small population of RBCs was located to platelet thrombi and enhanced platelet activation by a direct cell contact via the FasL/FasR (CD95) pathway known to induce apoptosis. Activation of platelets in the presence of RBCs led to platelet FasL exposure that activated FasR on RBCs responsible for externalization of phosphatidylserine (PS) on the RBC membrane. Inhibition or genetic deletion of either FasL or FasR resulted in reduced PS exposure of RBCs and platelets, decreased thrombin generation, and reduced thrombus formation in vitro and protection against arterial thrombosis in vivo. Direct cell contacts between platelets and RBCs via FasL/FasR were shown after ligation of the inferior vena cava (IVC) and in surgical specimens of patients after thrombectomy. In a flow restriction model of the IVC, reduced thrombus formation was observed in FasL-/- mice. Taken together, our data reveal a significant contribution of RBCs to thrombosis by the FasL/FasR pathway.

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Tau aggregation into amyloid fibers based on the cross-beta structure is a hallmark of several Tauopathies, including Alzheimer Disease (AD). Trans-cellular propagation of Tau with pathological conformation has been suggested as a key disease mechanism. This is thought to cause the spreading of Tau pathology in AD by templated conversion of naive Tau in recipient cells into a pathological state, followed by assembly of pathological Tau fibers, similar to the mechanism of nucleated polymerization proposed for prion pathogenesis. In cell cultures, the process is often monitored by a FRET assay where the recipient cell expresses the Tau repeat domain (Tau RD) with a pro-aggregant mutation, fused to GFP-based FRET pairs. Since the size of the reporter GFP (barrel of~3 nm × 4 nm) is~7 times larger than the β-strand distance (0.47 nm), this points to a potential steric clash. Hence, we investigated the influence of the GFP tag on Tau FL or Tau RD aggregation. Using biophysical methods (light scattering, atomic force microscopy (AFM), and scanning-transmission electron microscopy (STEM)), we found that the assembly of Tau RD-GFP was severely inhibited and incompatible with that of Alzheimer filaments. These observations argue against the hypothesis that the propagation of Tau pathology in AD is caused by the prion-like templated aggregation of Tau protein, transmitted via cell-to-cell spreading of Tau. Thus, even though the observed local increase of FRET in recipient cells may be a valid hallmark of a pathological reaction, our data argue that it is caused by a process distinct from assembly of Tau RD filaments.

The results of the present study identified OPHN1 as an important regulator of platelet cytoskeletal reorganization and demonstrate that abnormal regulation of Rho proteins leads to increased platelet adhesion and thrombus formation under low shear conditions in vitro and in vivo, suggesting a prothrombotic phenotype of mice critical for acute thrombotic occlusions.

Rationale: Lymphotoxin β receptor (LTbR) regulates immune cell trafficking and communication in inflammatory diseases. However, the role of LTbR in atherosclerosis is still unclear. Objective: The aim of this study was to elucidate the role of LTbR in atherosclerosis. Methods and Results: After 15 weeks of feeding a Western-type diet, mice double-deficient in apolipoprotein E and LTbR (apoE −/− /LTbR −/− ) exhibited lower aortic plaque burden than did apoE −/− littermates. Macrophage content at the aortic root and in the aorta was reduced, as determined by immunohistochemistry and flow cytometry. In line with a decrease in plaque inflammation, chemokine (C–C motif) ligand 5 ( Ccl5 ) and other chemokines were transcriptionally downregulated in aortic tissue from apoE −/− /LTbR −/− mice. Moreover, bone marrow chimeras demonstrated that LTbR deficiency in hematopoietic cells mediated the atheroprotection. Furthermore, during atheroprogression, apoE −/− mice exhibited increased concentrations of cytokines, for example, Ccl5, whereas apoE −/− /LTbR −/− mice did not. Despite this decreased plaque macrophage content, flow cytometric analysis showed that the numbers of circulating lymphocyte antigen 6C (Ly6C) low monocytes were markedly elevated in apoE −/− /LTbR −/− mice. The influx of these cells into atherosclerotic lesions was significantly reduced, whereas apoptosis and macrophage proliferation in atherosclerotic lesions were unaffected. Gene array analysis pointed to chemokine (C–C motif) receptor 5 as the most regulated pathway in isolated CD115 + cells in apoE −/− /LTbR −/− mice. Furthermore, stimulating monocytes from apoE −/− mice with agonistic anti-LTbR antibody or the natural ligand lymphotoxin-α1β2, increased Ccl5 mRNA expression. Conclusions: These findings suggest that LTbR plays a role in macrophage-driven inflammation in atherosclerotic lesions, probably by augmenting the Ccl5-mediated recruitment of monocytes.

Tau aggregation into amyloid fibers based on the cross-beta structure is a hallmark of several Tauopathies, including Alzheimer Disease (AD). Trans-cellular propagation of Tau with pathological conformation has been suggested as a key disease mechanism. This is thought to cause the spreading of Tau pathology in AD by templated conversion of naive Tau in recipient cells into a pathological state, followed by assembly of pathological Tau fibers, similar to the mechanism of nucleated polymerization proposed for prion pathogenesis. In cell cultures, the process is often monitored by a FRET assay where the recipient cell expresses the Tau repeat domain (TauRD) with a pro-aggregant mutation, fused to GFP-based FRET pairs. Since the size of the reporter GFP (barrel of ~3nm x 4nm) is ~7 times larger than the β-strand distance (0.47nm), this points to a potential steric clash. Hence, we investigated the influence of the GFP tag on Tau or TauRD aggregation. Using biophysical methods (light scattering, atomic force microscopy (AFM), and scanning-transmission electron microscopy (STEM)), we found that the assembly of TauRD-GFP was severely inhibited and incompatible with that of Alzheimer filaments. These observations argue against the hypothesis that the propagation of Tau pathology in AD is caused by the prion-like templated aggregation of Tau protein, transmitted via cell-to-cell spreading of Tau. Thus, even though the observed local increase of FRET in recipient cells may be a valid hallmark of a pathological reaction, our data argue that it is caused a process distinct from assembly of TauRD filaments.

Abbreviations: AD Alzheimer disease AFM atomic force microscopy CNS central nervous system FRET fluorescence resonance energy transfer GFP green fluorescent protein PHF paired helical filament Tau FL full-length Tau protein, largest isoform in human CNS (Uniprot P10636-F, htau40) Tau FLΔK full-length Tau protein with pro-aggregant mutation ΔK280 Tau RD Tau repeat domain (amyloidogenic domain) Tau RDΔK Tau repeat domain with pro-aggregant mutation ΔK280 Sf9 cell line from Spodoptera frugiperda STEM scanning transmission electron microscopy EM electron microscopy MPL mass per length Wt wildtype AbstractTau aggregation into amyloid fibers based on the cross-beta structure is a hallmark of several Tauopathies, including Alzheimer Disease (AD). Trans-cellular propagation of Tau with pathological conformation has been suggested as a key disease mechanism. This is thought to cause the spreading of Tau pathology in AD by templated conversion of naive Tau in recipient cells into a pathological state, followed by assembly of pathological Tau fibers, similar to the mechanism proposed for prion pathogenesis. In cell cultures, the process is usually monitored by a FRET assay where the recipient cell expresses the Tau repeat domain (Tau RD , with pro-aggregant mutation, e.g., ΔK280 or P301L, ~13.5 kDa) fused to GFP-based FRET pairs (YFP or CFP, ~28 kD). Since the diameter of the reporter GFP (~3 nm) is ~6.5 times larger than the β-strand distance (0.47nm), this points to a potential steric clash. Hence, we investigated the influence of GFP tagged (N-or C-terminally) Tau RD and Tau FL (full-length Tau) on their aggregation behavior in vitro. Using biophysical methods (light scattering, atomic force microscopy (AFM), and scanningtransmission electron microscopy (STEM)), we found that the assembly of Tau RDΔK -GFP was severely inhibited, even in the presence of nucleation enhancers (heparin and/or pre-formed PHFs from Tau RDΔK ). Some rare fiber-like particles had a very different subunit packing from proper PHFs, as judged by STEM. The mass per length (MPL) values of Tau RDΔK fibrils are equivalent to 4.45 molecules/nm, close to the expected value for a paired-helical fiber with 2 protofilaments and cross-β structure. By contrast, the elongated particles formed by Tau RDΔK -GFP have MPL values around ~2, less than half of the values expected for PHFs, indicating that the subunit packing is distinct. Thus, both kinetic and structural observations are incompatible with a model whereby external Tau can form a template for PHF assembly of Tau-GFP in recipient cells. As a consequence, the observed local increase of FRET in recipient cells must be caused by other signalling processes.

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