Characterisation of murine and human septins and their interaction partners

• Applicant: Professorin Dr. Barbara Zieger
• Subject Area: Pediatric and Adolescent Medicine
• Term: since 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 347212107

Septins (septs) are GTPases, which have been described for the first time in the context of septum formation in yeast cells. So far, in humans 13 septins (SEPT1-12 and 14) have been described 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. 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. In the last funding period, 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 precisely 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, which, interestingly, also appears to be calcium-dependent. 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. In this funding period, we aim to 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 will 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 plan to 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.

• DFG Programme: Research Grants