Genetic and proteomic analysis identifies BAG3 as an amyloid-responsive regulator of neuronal proteostasis.

The autophagy-lysosome pathway (ALP) and the ubiquitin-proteasome system (UPS) are the primary protein degradative mechanisms maintaining proteostasis in neurons. However, the impact of human genetic variation on these pathways and the role of BAG3 are poorly understood, particularly in the context of Alzheimer's disease, where proteostatic dysfunction is a defining hallmark. We utilized a large panel of iPSCs from deeply phenotyped cohorts to interrogate genetic contributions to baseline autophagic flux and UPS activity in human neurons, and protein turnover was assessed using SILAC-based quantitative proteomics. Across this panel of neurons, we observed substantial inter-individual differences in autophagic flux, which was inversely correlated with UPS activity. This reciprocal relationship extended to tau homeostasis, where higher autophagic flux resulted in reduced accumulation of aggregated, phosphorylated tau. Proteomic analyses revealed that global protein turnover dynamics stratified based on degradation pathway activity and could predict pathway-specific substrate dependencies. Interestingly, Bcl-2-associated athanogene 3 (BAG3), an important member of the chaperone-assisted selective autophagy pathway, emerged as a dynamically regulated autophagy chaperone, responsive to pharmacological inhibition of both the UPS and ALP. BAG3 knockout in neurons decreased autophagic flux and increased levels of high-molecular-weight phosphorylated tau. Notably, familial AD mutations and Aβ exposure induced BAG3 expression in neurons, while elevated BAG3 levels in human brain tissue were associated with higher neuropathological burden and disease progression. Our findings identify BAG3 as a key modulator of proteostasis in human neurons. Its regulation across genetic backgrounds and pathological stimuli suggests a central role in maintaining degradation activities in Alzheimer's disease and related disorders.