A cell death-independent role for MLKL in TGF-β receptor recycling in Marfan syndrome

Marfan syndrome (MFS) causes thoracic aortic aneurysm through mutations in the fibrillin-1 gene and subsequent dysregulated TGF-β 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-β receptor recycling and aortic wall integrity without causing cell death. Phosphorylation-dead MLKL ^S345A/S347A ( Mlkl ^AA ) mice developed aortic root dilation with disorganised collagen architecture and a secretory-pathway proteomic signature. Crossing them with Fbn1 ^{C1039G /+} 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-β-driven matrix remodelling and impaired RAB11-dependent TGFβR1 recycling. These findings suggest that phosphorylation-dependent, non-lytic functions of MLKL may contribute to the maintenance of thoracic aortic integrity in MFS.