Axonal swelling as a neuron-specific compartment to sequester and expel misfolded proteins
Neurons are long-lived, highly specialized cells with extended neurites, requiring precise control of misfolded proteins over time and space. Yet, where misfolded proteins are directed and how quality-control pathways adapt during aging are still unresolved. Here, we identify a neuron-specific quality-control compartment that emerges in neurites as aggresome function declines with age, which we call SolAS (Soluble Misfolded Proteins-induced Axonal Swellings). These structures not only sequester misfolded proteins but also facilitate their clearance via microvesicles and exophers. During aging, neurite SolAS and soma aggresomes function hierarchically to maintain proteostasis, with aggresomes acting as the primary sequestration sites in young neurons and SolAS taking over this role as their function declines with age. This transition is driven by a shift from a ubiquitin-dominant to a SUMO-dominant balance. Moreover, solid pathogenic amyloids, such as GA50, can be converted into soluble forms and sequestered into SolAS via SUMO fusion, thereby reducing neurotoxicity. Our findings identify a previously unrecognized neuronal quality-control pathway critical for proteostasis during aging.