BAG3 modulates clathrin-mediated endocytosis and tau uptake in astrocytes

Astrocytes maintain neuronal homeostasis by removing extracellular disease-relevant proteins, such as tau, to prevent their uptake by neurons. In Alzheimer's disease (AD), this astrocytic function is impaired, contributing to pathological tau accumulation. Many AD-associated risk genes are linked to endocytosis pathways, suggesting their role in AD pathogenesis. While astrocytes can internalize, degrade, and release tau, the mechanisms governing these processes remain unclear. Bcl2-associated athanogene 3 (BAG3), a multifunctional protein regulating vacuolar processes, interacts with components of clathrin-mediated endocytosis (CME), including clathrin heavy chain, dynamin, and AP-2 complex members. However, BAG3's role in astrocytic CME and tau processing is not fully understood. We demonstrate for the first time that BAG3 depletion in astrocytes reduces clathrin-AP-2 interactions, inhibits CME-dependent epidermal growth factor receptor internalization, and decreases tau uptake. Live-cell imaging reveals impaired CME dynamics with BAG3 depletion, marked by prolonged clathrin particle lifetimes. BAG3 depletion also increases Lamp1+ puncta and co-localization of tau with Lamp1-positive structures, indicating vacuolar disturbances beyond CME. These findings suggest BAG3 facilitates CME, tau uptake, and trafficking in astrocytes, playing a critical role in vacuolar processes and tau proteostasis. Alterations in astrocytic BAG3 may contribute to AD pathogenesis and other proteinopathies.