Extracellular chaperones modulate AL light chains fibrillar aggregation and contribute to amyloid structural heterogeneity

In AL amyloidosis, monoclonal immunoglobulin light chains (LCs) aggregate as amyloid fibrils in tissues. In synergy with the intrinsic aggregation propensity of specific LC sequences, microenvironment factors may be involved in tuning the disease pathophysiology, in particular proteolytic LC remodelling and heterotypic interactions in the extracellular milieu. Accounting for extrinsic modulators is critical for understanding the phenotypic variability of AL, usually imputed mainly to the LC diversity. We investigated the effects of apolipoprotein E (allele 3, apoE3) and clusterin (CLU), two amyloid-signature proteins involved in extracellular proteostasis, on the fibrillogenesis kinetics of amyloidogenic LC fragments from patient-derived sequences, as well as on aggregate composition, fibril morphology and thermodynamic stability. We show that apoE3 and CLU act as heterotypic interactors of prefibrillar and fibrillar LCs, significantly modulating LC amyloidogenesis, with complex and non-monotypic effects that range from anti- to pro-amyloidogenic depending on their concentration and on the LCs intrinsic amyloidogenicity. ApoE3 and CLU also influence fibril morphology, possibly by modifying protofilament association, and alter their thermodynamic properties. LC interactors may play a significant and insofar underappreciated role in the AL pathophysiology in vivo, likely contributing to phenotypic variability and structural polymorphisms and, possibly, to fibril resilience to amyloid reabsorption strategies.