Medin drives Aβ40 to adopt Aβ42-like fibril polymorphs in vitro

Medin, a vascular amyloid derived from MFG-E8, is the most prevalent form of localized human amyloid and co-localizes with A{beta} in Alzheimers disease and, in particular, cerebral amyloid angiopathy (CAA). While it was shown that medin can promote A{beta} aggregation, it remains unclear whether this amyloid-amyloid interaction affects the structure of the resulting fibrils. Here, we investigate how medin modulates A{beta}40 fibril assembly in vitro using cryo-electron microscopy, aggregation kinetics, and immunogold electron microscopy. We show that medin accelerates A{beta}40 aggregation, co-assembles into hybrid fibrils, and modulates fibril morphology. Cryo-EM analysis reveals two fibril populations: one corresponding to a previously described in vitro A{beta}40 morphology, and a second, previously unobserved polymorph with A{beta}42-like features, including a structured N-terminus and a compact hydrophobic C-terminal core. The presence of a peripheral, unresolved cryo-EM density near the fibril surface suggests that the new polymorph is stabilised through heterotypic interactions, yet the atomic details remain unresolved, likely due to substantial structural heterogeneity. Rather than representing a limitation, this highlights how not all determinants critical for fibril assembly are necessarily ordered or resolvable in the final fibril structure, reflecting the inherent dynamic and heterogeneous nature of amyloid interactions. Our findings provide structural evidence that heterotypic co-aggregation can redirect A{beta}40 into distinct conformational states and suggest that dynamic or transient interactions contribute to fibril polymorphism beyond what can be fully captured in static structural models.