Tyrosine phosphatase STEP61 in human dementia and in animal models with amyloid and tau pathology.

Synaptic degeneration is a precursor of synaptic and neuronal loss in neurodegenerative diseases such as Alzheimer's disease (AD) and frontotemporal dementia with tau pathology (FTD-tau), a group of primary tauopathies. A critical role in this degenerative process is assumed by enzymes such as the kinase Fyn and its counterpart, the phosphatase striatal-enriched tyrosine phosphatase 61 (STEP61). Whereas the role of Fyn has been widely explored, less is known about STEP61 that localises to the postsynaptic density (PSD) of glutamatergic neurons. In dementias, synaptic loss is associated with an increased burden of pathological aggregates. Tau pathology is a hallmark of both AD (together with amyloid-ß deposition) and FTD-tau. Here, we examined STEP61 and its activity in human and animal brain tissue and observed a correlation between STEP61 and disease progression. In early-stage human AD, an initial increase in the level and activity of STEP61 was observed, which decreased with the loss of the synaptic marker PSD-95; in FTD-tau, there was a reduction in STEP61 and PSD-95 which correlated with clinical diagnosis. In APP23 mice with an amyloid-ß pathology, the level and activity of STEP61 were increased in the synaptic fraction compared to wild-type littermates. Similarly, in the K3 mouse model of FTD-tau, which we assessed at two ages compared to wild-type, expression and activity of STEP61 were increased with ageing. Together, these findings suggest that STEP contributes differently to the pathogenic process in AD and FTD-tau, and that its activation may be an early response to a degenerative process.

CRISPRi screening reveals regulators of tau pathology shared between exosomal and vesicle-free tau.

The aggregation of the microtubule-associated protein tau is a defining feature of Alzheimer's disease and other tauopathies. Tau pathology is believed to be driven by free tau aggregates and tau carried within exosome-like extracellular vesicles, both of which propagate trans-synaptically and induce tau pathology in recipient neurons by a corrupting process of seeding. Here, we performed a genome-wide CRISPRi screen in tau biosensor cells and identified cellular regulators shared by both mechanisms of tau seeding. We identified ANKLE2, BANF1, NUSAP1, EIF1AD, and VPS18 as the top validated regulators that restrict tau aggregation initiated by both exosomal and vesicle-free tau seeds. None of our validated hits affected the uptake of either form of tau seeds, supporting the notion that they operate through a cell-autonomous mechanism downstream of the seed uptake. Lastly, validation studies with human brain tissue also revealed that several of the identified protein hits are down-regulated in the brains of Alzheimer's patients, suggesting that their decreased activity may be required for the emergence or progression of tau pathology in the human brain.

A Practical Approach to Differentiate the Frontotemporal Tauopathy Subtypes.

This study proposes a practical approach, using the minimum number of brain regions and stains, to consolidate previously published neuropathological criteria into one operationalized schema to differentiate subtypes of frontotemporal lobar degeneration with tau-immunopositive inclusions (FTLD-tau). This approach uses the superior frontal and precentral cortices and hippocampus stained for phosphorylated-tau, p62 and modified Bielschowsky silver, and the midbrain stained only for modified Bielschowsky silver. Accuracy of interrater reliability was determined by 10 raters in 24 FTLD-tau cases (Pick disease = 4, corticobasal degeneration = 9, progressive supranuclear palsy = 5, globular glial tauopathy = 6) including 4 with a mutation in MAPT collected with consent by Sydney Brain Bank. All brain regions and stains assessed proved informative for accurate pathological subtyping, and many neuropathological features were identified as common across the FTLD-tau subtypes. By identifying subtype-specific neuropathological features in the sections selected, 10 independent observers assigned the cases to a FTLD-tau subtype with almost perfect agreement between raters, emphasizing the requirement for the assessment of subtype-specific features for the accurate subtyping of FTLD-tau. This study consolidates current consensus diagnostic criteria for classifying FTLD-tau subtypes with an efficient, simple and accurate approach that can be implemented in future clinicopathological studies.

Decreased synthesis of ribosomal proteins in tauopathy revealed by non-canonical amino acid labelling.

Tau is a scaffolding protein that serves multiple cellular functions that are perturbed in neurodegenerative diseases, including Alzheimer's disease (AD) and frontotemporal dementia (FTD). We have recently shown that amyloid-ß, the second hallmark of AD, induces de novo protein synthesis of tau. Importantly, this activation was found to be tau-dependent, raising the question of whether FTD-tau by itself affects protein synthesis. We therefore applied non-canonical amino acid labelling to visualise and identify newly synthesised proteins in the K369I tau transgenic K3 mouse model of FTD. This revealed massively decreased protein synthesis in neurons containing pathologically phosphorylated tau, a finding confirmed in P301L mutant tau transgenic rTg4510 mice. Using quantitative SWATH-MS proteomics, we identified changes in 247 proteins of the de novo proteome of K3 mice. These included decreased synthesis of the ribosomal proteins RPL23, RPLP0, RPL19 and RPS16, a finding that was validated in both K3 and rTg4510 mice. Together, our findings present a potential pathomechanism by which pathological tau interferes with cellular functions through the dysregulation of ribosomal protein synthesis.

Increased Tau Phosphorylation in Motor Neurons From Clinically Pure Sporadic Amyotrophic Lateral Sclerosis Patients.

Amyotrophic lateral sclerosis (ALS) is characterized by the progressive degeneration of motor neurons. There is a pathological and genetic link between ALS and frontotemporal lobar degeneration (FTLD). Although FTLD is characterized by abnormal phosphorylated tau deposition, it is unknown whether tau is phosphorylated in ALS motor neurons. Therefore, this study assessed tau epitopes that are commonly phosphorylated in FTLD, including serine 396 (pS396), 214 (pS214), and 404 (pS404) in motor neurons from clinically pure sporadic ALS cases compared with controls. In ALS lower motor neurons, tau pS396 was observed in the nucleus or the nucleus and cytoplasm. In ALS upper motor neurons, tau pS396 was observed in the nucleus, cytoplasm, or both the nucleus and cytoplasm. Tau pS214 and pS404 was observed only in the cytoplasm of upper and lower motor neurons in ALS. The number of motor neurons (per mm2) positive for tau pS396 and pS214, but not pS404, was significantly increased in ALS. Furthermore, there was a significant loss of phosphorylated tau-negative motor neurons in ALS compared with controls. Together, our data identified a complex relationship between motor neurons positive for tau phosphorylated at specific residues and disease duration, suggesting that tau phosphorylation plays a role in ALS.