Nemade, H.; Guthoff, H.; Zimmermann, O.; Riemen, S.; Stiller, H.; Hedbawny, M.; Agbowo, B.; Zahnow, D.; Holtkamp, F.; Arkenberg, P.; Mehrkens, D.; Jahn, P.; Nettersheim, F. S.; Schelemei, P.; Picard, F. R.; Lagunas, K. M.; Jayachandran, J.; Sengle, G.; Ros, U.; Winkels, H.; Riffelmacher, T.; Baldus, S.; Garcia-Saez, A. J.; Adam, M.; Mollenhauer, M.

Marfan syndrome (MFS) causes thoracic aortic aneurysm through mutations in the fibrillin-1 gene and subsequent dysregulated TGF-{beta} signalling. Mixed lineage kinase domain-like protein (MLKL) executes necroptosis after RIPK3-dependent phosphorylation, but also functions in endosomal trafficking. Here we show that MLKL phosphorylation sustains TGF-{beta} receptor recycling and aortic wall integrity without causing cell death. Phosphorylation-dead MLKLS345A/S347A (MlklAA) mice developed aortic root dilation with disorganised collagen architecture and a secretory-pathway proteomic signature. Crossing them with Fbn1C1039G/+ MFS mice aggravated aortic dilation, disrupted collagen fibril ultrastructure and prolyl/lysyl hydroxylation, and caused lethal rupture under angiotensin II. Complete MLKL deficiency reproduced this phenotype. Phosphorylated MLKL was increased in MFS aortas, yet cleaved caspase-3, TUNEL positivity, smooth muscle cell content and leukocyte infiltration were unchanged. In smooth muscle cells, MLKL depletion dysregulated collagen-modifying enzymes, altered TGF-{beta}-driven matrix remodelling and impaired RAB11-dependent TGF{beta}R1 recycling. These findings suggest that phosphorylation-dependent, non-lytic functions of MLKL may contribute to the maintenance of thoracic aortic integrity in MFS.