Progressive Supranuclear Palsy (PSP) Cell Models for Research

Progressive supranuclear palsy (PSP) is a rare, rapidly progressive neurodegenerative disorder classified as an atypical parkinsonian syndrome. Global prevalence estimates range from 5 to 7 per 100,000 individuals, with an annual incidence of approximately 5.3 per 100,000 person-years (WHO, 2023). The disease typically manifests after age 60 and is more common in men. Median survival is 5 to 8 years from symptom onset, with a 5-year survival rate of approximately 50% and a 10-year survival rate below 20% (NCI SEER, 2022). Key risk factors include advanced age, male sex, and genetic variants in the MAPT gene. No disease-modifying therapies exist, and current treatments are purely symptomatic, underscoring the urgent need for mechanistic research and novel therapeutic targets.

PSP is an ideal model for studying tauopathies because it is a primary 4-repeat (4R) tauopathy with distinct neuropathological features, including globose neurofibrillary tangles, tufted astrocytes, and coiled bodies. Unlike Alzheimer's disease, PSP tau pathology is largely restricted to 4R tau isoforms, offering a clearer window into tau isoform-specific toxicity. Public datasets such as the PSP RNA-seq and whole-genome sequencing data in GEO and dbGaP provide rich resources for target discovery. Open questions include the role of MAPT haplotypes, the contribution of non-neuronal cells, and the mechanisms of selective vulnerability in the basal ganglia and brainstem. These factors make PSP an attractive disease area for developing gene-edited cell models to dissect disease mechanisms and validate therapeutic targets.

Although PSP is not a neoplastic disease, its molecular pathogenesis involves several key pathways that can be modeled in cell systems:

1. Tau aggregation and propagation:

• • Hyperphosphorylation of tau at sites such as Ser202/Thr205 and Thr231 promotes detachment from microtubules.
• • 4R tau isoforms aggregate into oligomers and fibrils, forming neurofibrillary tangles.
• • Prion-like propagation of tau seeds across connected brain regions.

2. Microtubule destabilization and axonal transport defects:

• • Tau hyperphosphorylation reduces microtubule binding, impairing axonal transport.
• • Disruption of mitochondrial transport leads to energy failure and synaptic dysfunction.

3. Neuroinflammation and glial dysfunction:

• • Activated microglia and astrocytes release pro-inflammatory cytokines (IL-1β, TNF-α).
• • Astrocytic tau inclusions (tufted astrocytes) contribute to blood-brain barrier dysfunction.

4. Mitochondrial dysfunction and oxidative stress:

• • Complex I inhibition and reactive oxygen species accumulation exacerbate tau pathology.
• • Impaired mitophagy leads to accumulation of damaged mitochondria.

The genetic architecture of PSP is dominated by common variants in MAPT, but rare variants in other genes also contribute. The table below summarizes key alterations.

Data sources: ClinVar, NCBI Gene, and large-scale exome sequencing studies (e.g., PSP Genetics Consortium).

Several signaling networks are dysregulated in PSP and can be interrogated using gene-edited cell models:

• • Tau phosphorylation kinases:
• • GSK-3β: hyperactive in PSP, phosphorylates tau at multiple sites.
• • CDK5: activated by p25, contributes to tau hyperphosphorylation.
• • ERK1/2: mediates stress-induced tau phosphorylation.
• • Autophagy-lysosomal pathway:
• • mTORC1: hyperactivated, inhibiting autophagy.
• • TFEB: reduced nuclear translocation, impairing lysosomal biogenesis.
• • Beclin-1: decreased expression, leading to defective autophagosome formation.
• • Neuroinflammation signaling:
• • NF-κB: activated in microglia, driving cytokine release.
• • NLRP3 inflammasome: promotes IL-1β maturation.
• • TREM2: loss-of-function variants impair microglial phagocytosis of tau.
• • Oxidative stress response:
• • Nrf2: reduced nuclear translocation, decreasing antioxidant gene expression.
• • SOD1: impaired activity leads to superoxide accumulation.

Several cell lines and organoid models are used to study PSP mechanisms. The table below lists commonly used models.

Organoid advantages: 3D architecture, cell-type diversity, and recapitulation of tau pathology and propagation. They enable studies of tau spreading and glial-neuronal interactions.

Animal models are essential for understanding PSP pathogenesis and testing therapeutics. Key models include:

• • Transgenic mice:
• • P301S tau mice: express human 4R tau with P301S mutation, develop tau tangles and motor deficits.
• • rTg4510 mice: inducible expression of P301L tau, allowing temporal control of tau pathology.
• • hTau mice: express all six human tau isoforms, but lack 4R tau dominance.
• • Knock-in mice:
• • MAPT H1 haplotype knock-in: models genetic risk associated with PSP.
• • Tau knockout mice: used to study loss-of-function effects.
• • Viral vector-based models:
• • AAV-mediated expression of mutant tau in basal ganglia or brainstem.
• • Tau seed injection models: propagate tau pathology from injection site.
• • Non-mammalian models:
• • Zebrafish and Drosophila expressing human tau: used for high-throughput screening.

These models have limitations, including species differences in tau isoforms and incomplete recapitulation of PSP pathology.

CRISPR-Cas9 gene editing enables the creation of isogenic cell lines that differ only at specific genetic loci, providing powerful tools for PSP research. Examples include:

• • MAPT knockout lines: generated in SH-SY5Y or iPSCs to study loss of tau function and identify tau-dependent pathways.
• • MAPT knock-in lines: introducing PSP-associated mutations (e.g., G303V) or the H1 haplotype to model genetic risk.
• • 4R tau isoform-specific reporter lines: using CRISPR to tag endogenous MAPT with fluorescent proteins to monitor isoform expression and aggregation.
• • TBK1 or TRIM11 knockout lines: to investigate autophagy and protein clearance defects.

Commercially available, sequence-verified gene-edited cell models accelerate research by providing reproducible, validated tools. These models are engineered using CRISPR and other gene-editing technologies, and are available from commercial sources without naming specific companies. They enable researchers to focus on mechanistic studies and drug discovery rather than time-consuming model generation.

Gene-edited cell models are indispensable for functional genomics in PSP. Knockout lines can validate candidate genes identified from genome-wide association studies (GWAS) or exome sequencing. For example, knocking out TBK1 in neuroblastoma cells can reveal its role in autophagy and tau clearance. Knock-in lines expressing mutant MAPT can be used to study isoform-specific toxicity and identify modifiers of tau aggregation. These models enable high-throughput screens to discover genes that modulate tau pathology.

Isogenic pairs of cell lines (e.g., MAPT wild-type vs. knockout) are ideal for drug screening. Researchers can compare the efficacy of tau-lowering compounds or kinase inhibitors in the presence and absence of tau. Resistance modeling can be performed by chronically exposing cells to a drug and identifying secondary mutations or gene expression changes that confer resistance. Gene-edited lines with specific mutations can also be used to test targeted therapies, such as tau aggregation inhibitors or autophagy enhancers.

CRISPR-based synthetic lethality screens can identify genes that are essential only in the context of PSP-associated mutations. For example, a genome-wide knockout screen in MAPT mutant cells can reveal vulnerabilities that could serve as biomarkers or therapeutic targets. Additionally, reporter lines can be used to monitor tau aggregation and identify small molecules or genetic modifiers that reduce aggregation, leading to novel biomarkers for early diagnosis and disease progression.