Influence of FTDP-17 mutants on circular tau RNAs

At least 53 mutations in the microtubule associated protein tau gene (MAPT) have been identified that cause frontotemporal dementia. 47 of these mutations are localized between exons 7 and 13. They could thus affect the formation of circular RNAs (circRNAs) from the MAPT gene that occurs through backsplicing from exon 12 to either exon 10 or exon 7. We analyzed representative mutants and found that five FTDP-17 mutations increase the formation of 12➔7 circRNA and three different mutations increase the amount of 12➔10 circRNA. CircRNAs are translated after undergoing adenosine to inosine RNA editing, catalyzed by ADAR enzymes. We found that the interferon induced ADAR1-p150 isoform has the strongest effect on circTau RNA translation. ADAR1-p150 activity had a stronger effect on circTau RNA expression and strongly decreased 12➔7 circRNA, but unexpectedly increased 12➔10 circRNA. In both cases, ADAR-activity strongly promoted translation of circTau RNAs. Unexpectedly, we found that the 12➔7 circTau protein interacts with eukaryotic initiation factor 4B (eIF4B), which is reduced by four FTDP-17 mutations located in the second microtubule domain. These are the first studies of the effect of human mutations on circular RNA formation and translation. They show that point mutations influence circRNA expression levels, likely through changes in pre-mRNA structures. The effect of the mutations is surpassed by editing of the circular RNAs, leading to their translation. Thus, circular RNAs and their editing status should be considered when analyzing FTDP-17 mutations.

Effects of altered tau expression on dentate granule cell excitability in mice

Tauopathies, including Alzheimer's disease, are characterized by progressive accumulation of hyperphosphorylated and pathologic tau protein in association with onset of cognitive and behavioral impairment. Tau pathology is also associated with increased susceptibility to seizures and epilepsy, with tau^-/- mice showing seizure resistance in some epilepsy models. To better understand how tau pathology is related to neuronal excitability, we performed whole-cell patch-clamp electrophysiology in dentate gyrus granule cells of tau^-/- and human-tau expressing, htau mice. The htau mouse is unique from other transgenic tau models in that the endogenous murine tau gene has been and replaced with readily phosphorylated human tau. We assessed several measures of neuronal excitability, including evoked action potential frequency and excitatory synaptic responses in dentate granule cells from tau^-/-, htau, and non-transgenic control mice at 1.5, 4, and 9 months of age. Compared to age matched controls, dentate granule cells from both tau^-/- and htau mice had a lower peak frequency of evoked action potentials and greater paired pulse facilitation, suggesting reduced neuronal excitability. Our results suggest that neuronal excitability is more strongly influenced by the absence of functional tau than by the presence of pathologic tau. These results also suggest that tau's effect on neuronal excitability is more complex than previously understood.

Synergistic toxicity in an in vivo model of neurodegeneration through the co-expression of human TDP-43M337V and tauT175D protein

Although it has been suggested that the co-expression of multiple pathological proteins associated with neurodegeneration may act synergistically to induce more widespread neuropathology, experimental evidence of this is sparse. We have previously shown that the expression of Thr¹⁷⁵Asp-tau (tau^T175D) using somatic gene transfer with a stereotaxically-injected recombinant adeno-associated virus (rAAV9) vector induces tau pathology in rat hippocampus. In this study, we have examined whether the co-expression of human tau^T175D with mutant human TDP-43 (TDP-43^M337V) will act synergistically. Transgenic female Sprague-Dawley rats that inducibly express mutant human TDP-43^M337V using the choline acetyltransferase (ChAT) tetracycline response element (TRE) driver with activity modulating tetracycline-controlled transactivator (tTA) were utilized in these studies. Adult rats were injected with GFP-tagged tau protein constructs in a rAAV9 vector through bilateral stereotaxic injection into the hippocampus. Injected tau constructs were: wild-type GFP-tagged 2N4R human tau (tau^WT; n = 8), GFP-tagged tau^T175D 2N4R human tau (tau^T175D, pseudophosphorylated, toxic variant, n = 8), and GFP (control, n = 8). Six months post-injection, mutant TDP-43^M337V expression was induced for 30 days. Behaviour testing identified motor deficits within 3 weeks after TDP-43 expression irrespective of tau expression, though social behaviour and sensorimotor gating remained unchanged. Increased tau pathology was observed in the hippocampus of both tau^WT and tau^T175D expressing rats and tau^T175D pathology was increased in the presence of cholinergic neuronal expression of human TDP-43^M337V. These data indicate that co-expression of pathological TDP-43 and tau protein exacerbate the pathology associated with either individual protein.

Untangling the Tauopathy for Alzheimer's disease and parkinsonism

Tau is a microtubule-associated protein that mainly localizes to the axon to stabilize axonal microtubule structure and neuronal connectivity. Tau pathology is one of the most common proteinopathies that associates with age-dependent neurodegenerative diseases including Alzheimer's disease (AD), and various Parkinsonism. Tau protein undergoes a plethora of intra-molecular modifications and some altered forms promote the production of toxic oligomeric tau and paired helical filaments, and through which further assemble into neurofibrillary tangles, also known as tauopathy. In this review, we will discuss the recent advances of the tauopathy research, primarily focusing on its association with the early axonal manifestation of axonal transport defect, axonal mitochondrial stress, autophagic vesicle accumulation and the proceeding of axon destruction, and the pathogenic Tau spreading across the synapse. Two alternative strategies either by targeting tau protein itself or by improving the age-related physiological decline are currently racing to find the hopeful treatment for tauopathy. Undoubtedly, more studies are needed to combat this devastating condition that has already affected millions of people in our aging population.

Characterization of Detergent Insoluble Proteome in Chronic Traumatic Encephalopathy

Quantitative proteomics of postmortem human brain can identify dysfunctional proteins that contribute to neurodegenerative disorders like Alzheimer disease (AD) and frontotemporal dementia. Similar studies in chronic traumatic encephalopathy (CTE) are limited, therefore we hypothesized that proteomic sequencing of CTE frontal cortex brain homogenates from varying CTE pathologic stages may provide important new insights into this disorder. Quantitative proteomics of control, CTE and AD brains was performed to characterize differentially expressed proteins, and we identified over 4000 proteins in CTE brains, including significant enrichment of the microtubule associated protein tau. We also found enrichment and pathologic aggregation of RNA processing factors as seen previously in AD, supporting the previously recognized overlap between AD and CTE. In addition to these similarities, we identified CTE-specific enrichment of proteins which increase with increasing severity of CTE pathology. NADPH dehydrogenase quinone 1 (NQO1) was one of the proteins which showed significant enrichment in CTE and also correlated with increasing CTE stage. NQO1 demonstrated neuropathologic correlation with hyperphosphorylated tau in glial cells, mainly astrocytes. These results demonstrate that quantitative proteomic analysis of CTE postmortem human brain can identify disease relevant findings and novel cellular pathways involved in CTE pathogenesis.

The Polyphenol Altenusin Inhibits in Vitro Fibrillization of Tau and Reduces Induced Tau Pathology in Primary Neurons

In Alzheimer's disease, the microtubule-associated protein tau forms intracellular neurofibrillary tangles (NFTs). A critical step in the formation of NFTs is the conversion of soluble tau into insoluble filaments. Accordingly, a current therapeutic strategy in clinical trials is aimed at preventing tau aggregation. Here, we assessed altenusin, a bioactive polyphenolic compound, for its potential to inhibit tau aggregation. Altenusin inhibits aggregation of tau protein into paired helical filaments in vitro. This was associated with stabilization of tau dimers and other oligomers into globular structures as revealed by atomic force microscopy. Moreover, altenusin reduced tau phosphorylation in cells expressing pathogenic tau, and prevented neuritic tau pathology induced by incubation of primary neurons with tau fibrils. However, treatment of tau transgenic mice did not improve neuropathology and functional deficits. Taken together, altenusin prevents tau fibrillization in vitro and induced tau pathology in neurons.

ALS/FTLD: experimental models and reality

Amyotrophic lateral sclerosis is characterised by a loss of upper and lower motor neurons and characteristic muscle weakness and wasting, the most common form being sporadic disease with neuronal inclusions containing the tar DNA-binding protein 43 (TDP-43). Frontotemporal lobar degeneration is characterised by atrophy of the frontal and/or temporal lobes, the most common clinical form being the behavioural variant, in which neuronal inclusions containing either TDP-43 or 3-repeat tau are most prevalent. Although the genetic mutations associated with these diseases have allowed various experimental models to be developed, the initial genetic forms identified remain the most common models employed to date. It is now known that these first models faithfully recapitulate only some aspects of these diseases and do not represent the majority of cases or the most common overlapping pathologies. Newer models targeting the main molecular pathologies are still rare and in some instances, lack significant aspects of the molecular pathology. However, these diseases are complex and multigenic, indicating that experimental models may need to be targeted to different disease aspects. This would allow information to be gleaned from a variety of different yet relevant models, each of which has the capacity to capture a certain aspect of the disease, and together will enable a more complete understanding of these complex and multi-layered diseases.

Threonine175, a novel pathological phosphorylation site on tau protein linked to multiple tauopathies

Microtubule associated protein tau (tau) deposition is associated with a spectrum of neurodegenerative diseases collectively termed tauopathies. We have previously shown that amyotrophic lateral sclerosis (ALS) with cognitive impairment (ALSci) is associated with tau phosphorylation at Thr¹⁷⁵ and that this leads to activation of GSK3β which then induces phosphorylation at tau Thr²³¹. This latter step leads to dissociation of tau from microtubules and pathological tau fibril formation. To determine the extent to which this pathway is unique to ALS, we have investigated the expression of pThr¹⁷⁵ tau and pThr²³¹ tau across a range of frontotemporal degenerations. Representative sections from the superior frontal cortex, anterior cingulate cortex (ACC), amygdala, hippocampal formation, basal ganglia, and substantia nigra were selected from neuropathologically confirmed cases of Alzheimer’s disease (AD; n = 3), vascular dementia (n = 2), frontotemporal lobar degeneration (FTLD; n = 4), ALS (n = 5), ALSci (n = 6), Parkinson’s disease (PD; n = 5), corticobasal degeneration (CBD; n = 2), diffuse Lewy body dementia (DLBD; n = 2), mixed DLBD (n = 3), multisystem atrophy (MSA; n = 6) and Pick’s disease (n = 1) and three neuropathologically-normal control groups aged 50–60 (n = 6), 60–70 (n = 6) and 70–80 (n = 8). Sections were examined using a panel of phospho-tau antibodies (pSer^208,210, pThr²¹⁷, pThr¹⁷⁵, pThr²³¹, pSer²⁰² and T22 (oligomeric tau)). Across diseases, phospho-tau load was most prominent in layers II/III of the entorhinal cortex, amygdala and hippocampus. This is in contrast to the preferential deposition of phospho-tau in the ACC and frontal cortex in ALSci. Controls showed pThr¹⁷⁵ tau expression only in the 7^th decade of life and only in the presence of tau pathology and tau oligomers. With the exception of DLBD, we observed pThr¹⁷⁵ co-localizing with pThr²³¹ in the same cell populations as T22 positivity. This suggests that this pathway may be a common mechanism of toxicity across the tauopathies.