Cytoplasmic Mislocalization of RNA Polymerase II Subunit RPB1 in Alzheimer Disease Is Linked to Pathologic Tau

Abnormal protein accumulation and mislocalization is a general hallmark of Alzheimer disease. Recent data suggest nucleocytoplasmic transport may be compromised by tau in Alzheimer disease. In this context, we have examined the RNA polymerase II subunit RPB1, which is the catalytic subunit that plays a critical role in transcription. Using immunofluorescence staining in control and Alzheimer disease hippocampal tissue, we show that 2 phosphoisoforms of RPB1 mislocalize from the nucleus to the cytoplasm of neurons in Alzheimer disease. The number of neurons with this cytoplasmic mislocalization is correlated with the burden of pathologic tau (AT8-immunopositive neurons). In order to test whether there is a causal relationship between pathologic tau and cytoplasmic RPB1 accumulation, we used the rTg4510 mouse model, which expresses a regulatable pathologic human tau species harboring the P301L mutation. Using immunofluorescence staining on brain tissue from young (2.5-month-old) and aged (8.5- to 10-month-old) rTg4510 mice, we found a tau- and age-dependent increase in cytoplasmic mislocalization of Rpb1. In summary, this study provides evidence that tau induces mislocalization of RPB1 in Alzheimer disease, and since RPB1 is essential for transcription, this raises the possibility that RPB1 mislocalization could lead to fundamental alterations in neuronal health.

Cell Population Effects in a Mouse Tauopathy Model Identified by Single Cell Sequencing

Neurodegenerative disorders are complex multifactorial diseases that have poorly understood selective vulnerabilities among discrete cell populations. We performed single cell RNA sequencing of whole hippocampi from the rTg4510 mouse tauopathy model, which expresses a P301L MAPT mutation at two time points—before and after the onset of pathology. One population of neurons showed a robust size reduction in both the young and the old transgenic animals. Differential expression of genes expressed in this group of neurons suggested an enrichment in granule cell neurons. We identified genes that characterize this population of neurons using Pareto optimization of the specificity and precision of gene pairs for the population of interest. The resulting optimal marker genes were overwhelmingly associated with neuronal projections and their expression was enriched in the dentate gyrus suggesting that the rTg4510 mouse is a good model for Pick’s disease. This observation suggested that the tau mutation affects the population of neurons associated with neuronal projections even before overt tau inclusions can be observed. Out of the optimal pairs of genes identified as markers of the population of neurons of interest, we selected Purkinje cell protein 4 (Pcp4+) and Syntaxin binding protein 6 (Stxbp6+) for experimental validation. Single-molecule RNA fluorescence in situ hybridization confirmed preferential expression of these markers and localized them to the dentate gyrus.

Factors other than hTau overexpression that contribute to tauopathy-like phenotype in rTg4510 mice

The tauopathy-like phenotype observed in the rTg4510 mouse line, in which human tauP301L expression specifically within the forebrain can be temporally controlled, has largely been attributed to high overexpression of mutant human tau in the forebrain region. Unexpectedly, we found that in a different mouse line with a targeted-insertion of the same transgene driven by the same tetracycline-TransActivator (tTA) allele, but with even higher overexpression of tauP301L than rTg4510, atrophy and tau histopathology are delayed, and a different behavioral profile is observed. This suggests that it is not overexpression of mutant human tau alone that contributes to the phenotype in rTg4510 mice. Furthermore we show that the tauopathy-like phenotype seen in rTg4510 requires a ~70-copy tau-transgene insertion in a 244kb deletion in Fgf14, a ~7-copy tTA-transgene insertion in a 508kb deletion that disrupts another five genes, in addition to high transgene overexpression. We propose that these additional effects need to be accounted for in any studies using rTg4510, and that Tg-INDEL mutations and their impacts on phenotype should be defined for all transgenic models used in biomedical research.

A farnesyltransferase inhibitor activates lysosomes and reduces tau pathology in mice with tauopathy

Tau inclusions are a shared feature of many neurodegenerative diseases, among them frontotemporal dementia caused by tau mutations. Treatment approaches for these conditions include targeting posttranslational modifications of tau proteins, maintaining a steady-state amount of tau, and preventing its tendency to aggregate. We discovered a new regulatory pathway for tau degradation that operates through the farnesylated protein, Rhes, a GTPase in the Ras family. Here, we show that treatment with the farnesyltransferase inhibitor lonafarnib reduced Rhes and decreased brain atrophy, tau inclusions, tau sumoylation, and tau ubiquitination in the rTg4510 mouse model of tauopathy. In addition, lonafarnib treatment attenuated behavioral abnormalities in rTg4510 mice and reduced microgliosis in mouse brain. Direct reduction of Rhes in the rTg4510 mouse by siRNA reproduced the results observed with lonafarnib treatment. The mechanism of lonafarnib action mediated by Rhes to reduce tau pathology was shown to operate through activation of lysosomes. We finally showed in mouse brain and in human induced pluripotent stem cell–derived neurons a normal developmental increase in Rhes that was initially suppressed by tau mutations. The known safety of lonafarnib revealed in human clinical trials for cancer suggests that this drug could be repurposed for treating tauopathies.