A recent study published in ASCB – Molecular Biology of the Cell by Diane Barber and her BARI team introduces a practical new model for investigating frontotemporal dementia (FTD), a neurodegenerative disorder that primarily affects behavior, language, and movement, often striking people under 65. Because animal models don’t faithfully recapitulate the FTD phenotype, researchers often generate induced pluripotent stem cells (iPSCs) from patients and differentiate them in a laborious method into neurons to study the cellular pathology. What makes work from the Barber group stand out is that they found that the iPSCs themselves already display key FTD cellular pathologies, bypassing the time-consuming and costly process of creating mature neurons. By showing that these undifferentiated iPSCs already display key FTD features, the study opens a more efficient path to study disease hallmarks and test potential treatments.
FTD researchers are in need of new experimental models
FTD symptoms arise from a progressive degeneration of neurons in the frontal and temporal lobes of the brain. A key genetic cause in many cases is a repeat expansion of C9orf72, which disrupts normal cellular processes and contributes to neurodegeneration through several interconnected mechanisms. These mechanisms include impaired endolysosomal function, where lysosomes (organelles responsible for breaking down waste), become less acidic, reducing the activity of lysosomal enzymes and leading to the accumulation of toxic debris in the cell. This dysfunction is often accompanied by TDP-43 proteinopathy, where the nuclear RNA regulation protein, TDP-43 mislocalizes to the cytoplasm and forms harmful aggregates. Together, these pathologies trigger neuroinflammation, oxidative stress, and eventual neuronal death, though the exact sequence and interplay remain incompletely understood.
This lack of understanding of FTD pathobiology is in large part due to limitations of current experimental models. Postmortem human brain samples offer valuable insights into tissue-level pathologies but provide only a static view of end-stage disease, lacking the ability to observe dynamic processes or perform genetic manipulations for mechanistic studies or drug testing. Animal models such as mice, zebrafish, or fruit flies engineered with C9orf72 mutations, allow for systems-level analysis and genetic interventions but often fail to fully replicate human-specific aspects of FTD due to species differences in brain structure, genetics, and disease progression. Cellular models using neurons differentiated from patient-derived iPSCs capture human-relevant features but are labor-intensive, expensive, and time-consuming to produce. Moreover, these cells are post-mitotic, making expansion and maintenance more difficult and limiting their scalability for high-throughput screening.
A familiar, but new model for FTD
The Barber team found that FTD iPSCs exhibit several established disease markers without needing neuronal differentiation. Using a fluorescent lysosome pH biosensor they developed called pHLARE, they measured higher lysosomal pH in FTD iPSCs compared with wildtype iPSCs derived from unaffected individuals, which impairs waste breakdown. This tied to reduced activities of cathepsin (enzymes that are active in acidic environments) also measured via fluorescent probes. Another hallmark was TDP-43 mislocalization: in healthy cells, this protein shuttles between the nucleus and cytoplasm to regulate RNA, but in FTD iPSCs, it accumulated abnormally in the cytoplasm, forming aggregates linked to neuron death.
uclear buildup of TFEB, a transcription factor that boosts lysosome production when stressed, was also elevated in FTD iPSCs, signaling compensatory efforts against dysfunction. Notably, treating these cells with a compound that lowers lysosomal pH without affecting overall cell (cytosolic) pH reduced TDP-43 aggregates. This suggests lysosomal pH as a therapeutic target. These findings align with observations in patient brains and differentiated neurons, but in a more accessible model.
Transcriptomic changes and broader implications
RNA sequencing revealed hundreds of dysregulated genes in FTD iPSCs compared with wildtype iPSCs, pointing to disrupted pathways in calcium signaling (critical for cell communication), cell survival, synaptic function, and neuronal development. Protein-level changes were validated for several novel candidates: reduced CNTFR, which supports neuron survival, reduced ANXA2, an anti-cell-death protein, reduced NANOG, involved in stem cell regulation and potentially neuronal growth, and increased MSN, which affects cell shape and movement. Pharmacological lowering of lysosomal pH partially reversed some of these changes, reinforcing lysosomal dysfunction as playing a central role. Overall, this work positions undifferentiated iPSCs as a robust, cost-effective tool for FTD research, ideal for high-throughput drug screening. By highlighting shared pathologies across cell types, it broadens our view of FTD beyond neurons and could accelerate discoveries for treatments, especially given the lack of cures today.
What’s next for the Barber lab?
The group is now using FTD iPSCs, which can be propagated for longitudinal studies, to identify small molecules that decrease lysosome pH as potential therapeutics. Working with the UCSF Small Molecule Discovery Center (SMDC) they recently used FTD iPSCs expressing the lysosome pH biosensor pHLARE and having a higher lysosome pH compared with wildtype iPSCs to complete a screen of small molecules that lower lysosome pH. After months of assay development, they successfully identified several “hits” they are now testing for reversing FTD cellular pathologies, including decreasing cytosolic TDP-43 proteinopathy and increasing lysosome cathepsin activity, with promise for new therapeutics to treat FTD and possibly other neurodegenerative disorders with a hallmark of increased lysosome pH.