Using an ingenious CRISPR-based screening technique, scientists have found a protein that tags tau for degradation and is more strongly expressed in tau-resilient neurons [1].
Some neurons are more equal than others
The accumulation of tau protein fibrils in neurons is a hallmark of Alzheimer’s and several other diseases [2]. Scientists have long noticed that even in the brains of people who died of Alzheimer’s, some neurons are markedly healthier than others, suggesting that neurons differ in how they handle tau and that these differences may explain selective vulnerability in tauopathies [3].
In a new study published in the journal Cell, scientists from the University of California San Francisco built a human-neuron CRISPR interference (CRISPRi) screening platform and asked, genome-wide, which genes push tau toward or away from oligomer accumulation. Tau oligomers, which consist of chains of several tau molecules, are considered a crucial step in the formation of tau fibrils.
Full-genome screening
The team compared isogenic iPSC-derived neurons with or without a familial tauopathy mutation (V337M) in the gene MAPT. Using the oligomer-selective antibody T22, they found elevated tau oligomer levels in the mutated neurons. The signal dropped with MAPT knockdown, showing that the assay depends on tau expression and can report genetically driven changes.
The researchers used a genome-wide CRISPR “turn-down” screen in human neurons to find genes that control tau oligomers – without having to test 20,000 genes one by one. Essentially, they washed the cells in a cocktail of viral vectors, each one carrying a CRISPR-based construct to silence a particular gene. The concentration was such that the vast majority of cells received only one vector or none at all, creating a variety of cells with one different gene turned down.
The researchers then stained the neurons with T22, an antibody that recognizes tau oligomers, and used flow cytometry to sort cells into low-oligomer and high-oligomer bins. They then sequenced the CRISPR guide RNAs present in each bin to see which genes were knocked down.
This produced a ranked list of candidate genes, which the authors stress-tested in follow-up screens. One gene in particular, CUL5 – part of the ubiquitin-proteasome machinery that tags proteins for degradation – was a top hit across these different screens, making it a natural focus for a deeper mechanistic analysis.
CUL-ling tau
Because CUL5 is a key component of an E3 ubiquitin ligase, which is part of the cellular machinery that tags specific proteins for proteasomal destruction, the authors suspected that altering CUL5 would change how efficiently neurons can clear tau. To test this, they used individual CRISPRi guides to dial down CUL5 and RNF7, its core partner, and then directly measured tau.
The team found that tau levels rose when this ligase machinery was impaired. They then showed that the effect was post-translational, meaning that, with CUL5 knocked down, tau was becoming more stable, not just more expressed. Blocking the proteasome eliminated the CUL5-linked difference, tying the pathway to proteasomal clearance. Finally, they found the specific region of tau (around residues 80-130) that the ligase complex uses to target it for disposal.
In multiple human single-cell datasets, higher expression of CUL5 and key complex members was linked to neuronal resilience in Alzheimer’s disease and other tauopathies, suggesting that stronger CUL5-based ubiquitin-proteasome capacity may help certain neuron populations better withstand tau stress.
“CUL5 is uniquely suited to getting rid of tau,” said Martin Kampmann, Ph.D., professor of Biochemistry and Biophysics at UCSF. “Maybe a future therapy could enhance the body’s natural mechanism for avoiding neurodegeneration. It’s the first time we’ve been able to screen human neurons for genes that determine their resilience to tau. We hope that CUL5 can be the first of many new targets for drug discovery against dementias.”
Mitochondrial function flagged, too
A separate signal from the same CRISPRi screens also demanded attention: beyond the CUL5 “tau clearance” pathway, the strongest pathway-level hits pointed to mitochondrial oxidative phosphorylation/ETC (electron transport chain) genes as major modifiers of tau-oligomer burden.
The authors pivoted to ask what mitochondrial dysfunction does to tau. Using drugs that inhibit the ETC (notably rotenone and antimycin A), they caused tau levels to rise. Moreover, neurons started producing a form of tau that resembles what Alzheimer’s biomarker tests are designed to detect.
They traced this effect to oxidative stress (ROS) rather than generic energy failure: ROS increased alongside fragment formation, adding hydrogen peroxide to generate ROS could reproduce the effect, and antioxidants blunted it. While the role of mitochondrial dysfunction in dementias is known, this study provides more details that may be relevant for future therapies.
Literature
[1] Samelson, A. J., Ariqat, N., McKetney, J., Rohanitazangi, G., Parra Bravo, C., Bose, R. S., Travaglini, K. J., Lam, V. L., Goodness, D., Ta, T., Dixon, G., Marzette, E., Jin, J., Tian, R., Tse, E., Abskharon, R., Pan, H. S., Carroll, E. C., Lawrence, R. E., … Kampmann, M. (2025). CRISPR screens in iPSC-derived neurons reveal principles of tau proteostasis. Cell.
[2] Serrano-Pozo, A., Frosch, M. P., Masliah, E., & Hyman, B. T. (2011). Neuropathological alterations in Alzheimer disease. Cold Spring Harbor perspectives in medicine, 1(1), a006189.
[3] Roussarie, J. P., Yao, V., Rodriguez-Rodriguez, P., Oughtred, R., Rust, J., Plautz, Z., … & Greengard, P. (2020). Selective neuronal vulnerability in Alzheimer’s disease: a network-based analysis. Neuron, 107(5), 821-835.
View the article at lifespan.io
