The nucleolus is the busiest manufacturing hub in the cell. It forms around clusters of ribosomal DNA repeats, where the enzyme RNA polymerase I transcribes them into precursor ribosomal RNA at a ferocious pace, producing the raw material for ribosome assembly. Because this transcriptional output consumes enormous cellular resources, the nucleolus doubles as a sensitive sensor of cellular well-being. When nucleolar function is disturbed, the structure disintegrates and stabilises the tumour suppressor p53, which can halt the cell cycle or trigger programmed cell death. Previous work had hinted that tau, conventionally viewed as a cytoskeletal protein, also shuttles to the nucleus and may interact with nucleolar components, but the consequences of tau pathology for nucleolar homeostasis remained poorly charted territory.

To map that territory, the team used two complementary cellular systems: human neuroblastoma-derived SH-SY5Y cells engineered to express mutant tau, and induced pluripotent stem cell neurons generated from patients carrying disease-associated MAPT mutations. The researchers focused on three mutations known to cause familial forms of frontotemporal dementia and related tauopathies: P301S, one of the most widely studied mutations in tau transgenic models; S305N, which alters a critical phosphorylation-sensitive region; and the splice-site mutation IVS 10+16, which disrupts the normal ratio of tau isoforms by promoting inclusion of exon 10. Across both models, the team found that tau localised to the nucleolus, and that the disease-associated mutations were consistently associated with greater accumulation of tau within this structure compared with cells expressing the normal protein.

Using high-content imaging, an automated microscopy approach that quantifies features across thousands of cells, the researchers documented structural consequences of this mutant tau accumulation. Nuclei and nucleoli in cells expressing mutant tau were measurably enlarged, and key molecular markers drawn from all three canonical nucleolar sub-compartments, the fibrillar centre, the dense fibrillar component and the granular component, were upregulated. In a healthy cell, expansion of these compartments and elevation of their marker proteins generally signal increased biosynthetic demand rather than shutdown, and the pattern pointed the investigators toward a provocative conclusion: mutant tau appeared to be revving up the nucleolus rather than simply poisoning it.

Functional assays reinforced that interpretation. Quantitative PCR measurements indicated increased transcription from the ribosomal DNA repeats, and staining with a nucleolar RNA-selective dye revealed enhanced ribosomal RNA processing, both consistent with an elevated nucleolar biosynthetic output in mutant tau-expressing cells. In other words, the cellular protein-factory was running hotter than it should. But hyperactivity of this kind carries a cost. Alongside the signs of increased rDNA transcription, the team detected a suite of markers associated with nucleolar stress and cell death: stabilisation of p53, activation of the executioner caspases 3 and 7, and positive results in TUNEL assays, which flag the DNA fragmentation characteristic of apoptosis. Mutant tau-expressing cells were, in effect, working themselves to death.

The most therapeutically intriguing finding came when the researchers pharmacologically throttled back the nucleolar engine. By inhibiting RNA polymerase I, the enzyme responsible for transcribing ribosomal DNA, in the inducible SH-SY5Y model, they were able to attenuate caspase 3/7 activity. This result supports a functional link between the elevated rDNA transcriptional activity and the downstream apoptotic signalling, rather than the two phenomena merely co-occurring. If hyperactive ribosome biogenesis is genuinely contributing to cell death in mutant tau neurons, then RNA polymerase I emerges as a plausible point of intervention, a hypothesis that aligns with growing interest in nucleolar stress as a convergent mechanism across neurodegenerative diseases.

The study reframes tau in an unfamiliar light. For most of the modern era of dementia research, tau’s pathogenic identity has been tied to its misfolding and aggregation into paired helical filaments and neurofibrillary tangles, and to the toxic gain-of-function properties attributed to aggregated species. The new findings add a nuclear dimension to this picture, consistent with a body of recent evidence indicating that tau performs important roles within the nucleus and nucleolus, including reported interactions with DNA and effects on genome stability. If disease mutations perturb these nuclear functions early in the course of illness, nucleolar dysfunction could represent an upstream event that precedes, and potentially contributes to, the aggregation-centred pathology that dominates the late disease stage.

There are important caveats to keep in view. The experiments were conducted in cellular models, differentiated SH-SY5Y cells and iPSC-derived neurons, rather than in patient brain tissue or animal models, so the extent to which nucleolar hyperactivation operates in the living human brain with a MAPT mutation remains an open question. The authors also describe their work as identifying altered nucleolar homeostasis as a feature associated with mutant tau expression, a deliberately careful formulation that stops short of claiming causation for the broader disease process. The association between the mutations and increased nucleolar tau accumulation was consistent across the three variants examined, but the precise molecular mechanism by which mutant tau drives rDNA transcriptional hyperactivation, whether through direct effects on chromatin, on RNA polymerase I machinery, or on nucleolar stress pathways, has yet to be resolved.

Even with those limitations, the study opens several concrete lines of investigation. Nucleolar stress markers could serve as early biomarkers in cells derived from MAPT mutation carriers, potentially detectable before overt neurodegeneration. The finding that RNA polymerase I inhibition reduces caspase activation suggests that existing or in-development polymerase I inhibitors, some of which have been explored in oncology precisely because cancer cells depend on runaway ribosome biogenesis, might be repurposed or adapted for neurodegenerative contexts, though any such translation would need to reckon with the essential role of ribosome production in healthy neurons. The work also raises questions about whether other tauopathies, including sporadic Alzheimer’s disease, involve similar nucleolar perturbations even in the absence of MAPT mutations.

For a field that has poured enormous resources into anti-aggregation therapies with limited clinical success, the message that tau pathology may exert some of its damage through an entirely different cellular route is likely to resonate widely. The study, funded in part by the Rainwater Charitable Foundation, the Alzheimer’s Association, the Wellcome Trust and Alzheimer’s Research UK, was published open access, and its authors declare no competing interests. Whether targeting the nucleolus will ultimately yield new treatments for MAPT-linked frontotemporal dementia is far from settled, but the research makes a compelling case that the quiet factory at the heart of the nucleus deserves a far louder voice in conversations about how tau mutations kill neurons.

Subject of Research: Effects of MAPT tau mutations on nucleolar function and ribosomal DNA transcription in cellular models of frontotemporal dementia

Article Title: MAPT mutations are associated with rDNA transcriptional hyperactivation and nucleolar stress in cellular models

Article References: Muhammad, Z., Gu, Y., Kwairanga, S. H., Bailey, L. J., Khan, A., Nasser, M., Aljarrah, D., Barri, M., Arber, C., Wray, S., Serpell, L. C., Karch, C. M., & Maina, M. B. (2026). MAPT mutations are associated with rDNA transcriptional hyperactivation and nucleolar stress in cellular models. Translational Psychiatry. https://doi.org/10.1038/s41398-026-04484-7

Image Credits: AI Generated

DOI: 10.1038/s41398-026-04484-7

Keywords: tau, MAPT, nucleolus, rDNA transcription, nucleolar stress, frontotemporal dementia, Alzheimer's disease, RNA polymerase I, p53, apoptosis, iPSC neurons, tauopathies