Elucidating the protective role of CLU in modulating astrocyte reactivity and Alzheimer's disease pathologies in human brain and cellular models

Both common and rare variants in the gene Clusterin (CLU) are implicated in the pathogenesis of Alzheimer’s disease (AD), and CLU levels are often elevated in the brain of AD patients independent of the presence of CLU variants. Despite decades of research, the implications of CLU upregulation in AD remains uncertain, with no clear determination on whether it offers neuroprotection or exacerbates pathology. Through analysis of RNA and protein profiles from over 800 human brains in the ROSMAP cohorts, we observed significant upregulation of CLU in AD-affected brains. Intriguingly, the risk allele at the CLU locus is associated with reduced CLU levels, specifically in astrocytes. We hypothesize that heightened CLU expression in AD brains may serve as a protective mechanism against pathology, while diminished CLU protein levels confer increased disease risk.

To interrogate both cell autonomous and non-autonomous consequences of CLU in human biology, we utilized CRISPR/Cas9 editing to induce CLU loss-of-function mutations in human astrocytes from cognitively unimpaired participants in the ROSMAP cohort. Employing unbiased proteomic profiling, we identified cell-type-specific mechanisms influenced by CLU loss. Notably, our investigation revealed that CLU depletion triggers an inflammatory response in astrocytes independently of other AD-relevant stressors. Our data suggest that the elevation of C3 and other inflammatory response genes is mediated by NfkB signaling in human astrocytes. To explore the non-cell autonomous effects of astrocytic CLU dysfunction, we used our recently developed tri-culture system comprising astrocytes, microglia, and neurons, allowing for a comprehensive examination of intercellular interactions. This approach unveiled that astrocyte CLU communicates with other brain cells to modulate ABETA and phosphorylated tau levels in neurons, as well as microglial TREM2 processing.

Taken together, our study underscores the pivotal role of astrocytic CLU in modulating NfkB signaling, amyloid and tau pathogenesis, and TREM2 shedding. Elevating CLU levels in the brain may provide protection against AD by mitigating inflammation, reducing neuronal pathology, and regulating TREM2-dependent processes, thereby offering potential therapeutic avenues.