Severe bleeding symptoms are one of the main complications in patients with VAD, ECLS or ECMO. Our working group identified for the first time Acquired von Willebrand syndrome (AvWS) as the main cause of these bleeding symptoms and published these results for large cohorts with VAD, ECLS or ECMO. In addition, we identified thrombocytopathy (platelet secretion defect) in these patients. AvWS and thrombocytopathy seem to be due to the increased shear stress caused by these devices leading to hemolysis and activation of primary and secondary hemostasis. Identifying these acquired disorders leads to improved diagnostic and therapeutical algorithms and better clinical outcome. In addition, these results may help to develop devices which will lead to less shear stress.

Von Willebrand factor (vWF) multimeric analysis (ECLS: loss of high molecular weight multimers) and ratio of vWF:collagen binding activity/vWF:antigen pre, during and post ECMO (Heilmann et al., Intensive care medicine. 2012; 38(1):62-8; Kalbhenn et al., J Heart Lung Transplant. 2018; 37(8):985-91.

Septins (SEPTs) are a family of filamentous GTPases, which have been described for the first time in the context of septum formation in yeast cells. So far, 13 mammalian septins have been published to form heteropolymeric complexes with other septins, actin filaments, microtubules, cellular membranes, and/or diverse proteins. Thus, they function as multimodular scaffolds that can recruit components of several signaling pathways and exert diverse functions in a cell type-specific manner. There is more and more evidence that links dysregulated septins to diseases, including neurological diseases or cancer. Originally, our group cloned four septins (SEPT4, SEPT5, SEPT8, SEPT11) and investigated septin expression and septin interaction with other proteins. We have identified the first patient with a deletion in the SEPT5 gene and severe bleeding symptoms due to a platelet secretion defect. Unrecognized platelet dysfunction can lead to severe perioperative bleeding complications, making research regarding the precise mechanisms of septins in platelets of great importance. We identified a platelet secretion defect for a pathogenic variant in the SEPT9 and SEPT6 gene using “next generation sequencing” (NGS) in patients. In a knockout mouse model, we demonstrated that Sept8 plays an essential role in platelet function. Loss of Sept8 in mice led to a reduction in platelet activation (integrin αIIbβ3), α-granule secretion, aggregation, spreading, pro-coagulant activity, and thrombin generation. Sept4-deficient platelets do not show the marked platelet dysfunction described for Sept8, but they do exhibit an enhanced procoagulant or apoptotic state after activation compared with controls. These different platelet phenotypes in the Sept4- and Sept8-knockout mouse models demonstrate that the analysis of each of the individual septins is essential. For platelets, there is currently no functional data for 10 of the 13 septins available. Currently, we investigate the role of septins (Sept2, 6, 7, and 9) in platelet function using knockout mouse models and patients with septin defects (including superresolution- and electron microscopy, proteome analyses and flow cytometry). We perform NGS-screening of patients with thrombocytopathies/-penias for different septin variants, in particular those with cytoskeletal defects or platelet secretory disorders. Analysis of genetically modified mice and patients with septin defects will provide fundamental insights into the relevance of septins in platelets, particularly for cytoskeletal reorganization, granule degranulation, and spreading of filo/lamellipodia. Since the secretion processes in platelets and endothelial cells appear to be similar, we investigate the role of septins in these cell types in our mouse models as well. The knowledge gained in this work will lead to a better understanding of the functional mechanisms of septins in platelets and endothelial cells.

Immunohistochemical staining of SEPT2, β-tubulin, and F-actin in activated human platelets.