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. Although 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 colocalization 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.