V363I and V363A mutated tau affect aggregation and neuronal dysfunction differently in C. elegans

Bridging integrator 1 fragment accelerates tau aggregation and propagation by enhancing clathrin-mediated endocytosis in mice

The bridging integrator 1 (BIN1) gene is an important risk locus for late-onset Alzheimer's disease (AD). BIN1 protein has been reported to mediate tau pathology, but the underlying molecular mechanisms remain elusive. Here, we show that neuronal BIN1 is cleaved by the cysteine protease legumain at residues N277 and N288. The legumain-generated BIN1 (1-277) fragment is detected in brain tissues from AD patients and tau P301S transgenic mice. This fragment interacts with tau and accelerates its aggregation. Furthermore, the BIN1 (1-277) fragment promotes the propagation of tau aggregates by enhancing clathrin-mediated endocytosis (CME). Overexpression of the BIN1 (1-277) fragment in tau P301S mice facilitates the propagation of tau pathology, inducing cognitive deficits, while overexpression of mutant BIN1 that blocks its cleavage by legumain halts tau propagation. Furthermore, blocking the cleavage of endogenous BIN1 using the CRISPR/Cas9 gene-editing tool ameliorates tau pathology and behavioral deficits. Our results demonstrate that the legumain-mediated cleavage of BIN1 plays a key role in the progression of tau pathology. Inhibition of legumain-mediated BIN1 cleavage may be a promising therapeutic strategy for treating AD.

Tau mutation S356T in the three repeat isoform leads to microtubule dysfunction and promotes prion-like seeded aggregation

Tauopathies are a group of neurodegenerative diseases, which include frontotemporal dementia (FTD) and Alzheimer's disease (AD), broadly defined by the development of tau brain aggregates. Both missense and splicing tau mutations can directly cause early onset FTD. Tau protein is a microtubule-associated protein that stabilizes and regulates microtubules, but this function can be disrupted in disease states. One contributing factor is the balance of different tau isoforms, which can be categorized into either three repeat (3R) or four repeat (4R) isoforms based on the number of microtubule-binding repeats that are expressed. Imbalance of 3R and 4R isoforms in either direction can cause FTD and neurodegeneration. There is also increasing evidence that 3R tauopathies such as Pick's disease form tau aggregates predominantly comprised of 3R isoforms and these can present differently from 4R and mixed 3R/4R tauopathies. In this study, multiple mutations in 3R tau were assessed for MT binding properties and prion-like aggregation propensity. Different missense tau mutations showed varying effects on MT binding depending on molecular location and properties. Of the mutations that were surveyed, S356T tau is uniquely capable of prion-like seeded aggregation and forms extensive Thioflavin positive aggregates. This unique prion-like tau strain will be useful to model 3R tau aggregation and will contribute to the understanding of diverse presentations of different tauopathies.

A Combined Cell-Worm Approach to Search for Compounds Counteracting the Toxicity of Tau Oligomers In Vivo

A clear relationship between the tau assemblies and toxicity has still to be established. To correlate the tau conformation with its proteotoxic effect in vivo, we developed an innovative cell-worm-based approach. HEK293 cells expressing tau P301L under a tetracycline-inducible system (HEK T-Rex) were employed to produce different tau assemblies whose proteotoxic potential was evaluated using C. elegans. Lysates from cells induced for five days significantly reduced the worm's locomotor activity. This toxic effect was not related to the total amount of tau produced by cells or to its phosphorylation state but was related to the formation of multimeric tau assemblies, particularly tetrameric ones. We investigated the applicability of this approach for testing compounds acting against oligomeric tau toxicity, using doxycycline (Doxy) as a prototype drug. Doxy affected tau solubility and promoted the disassembly of already formed toxic aggregates in lysates of cells induced for five days. These effects translated into a dose-dependent protective action in C. elegans. These findings confirm the validity of the combined HEK T-Rex cells and the C. elegans-based approach as a platform for pharmacological screening.

Generation and characterization of a tractable C. elegans model of tauopathy

Alzheimer's disease(AD) is an age-associated neurodegenerative disease that results in deterioration of memory and cognitive function. As a currently untreatable disorder, AD has emerged as one of the defining biomedical challenges of our time. Thus, new approaches that can examine the cellular and molecular mechanisms underlying age-related AD pathology are sorely needed. One of the hallmarks of Alzheimer's disease is the hyperphosphorylation of the tau protein. Caenorhabditis elegans have been previously used to study the genetic pathways impacted by tau proteotoxic stress; however, currently, available C. elegans tau models express the human protein solely in neurons, which are unresponsive to global RNA interference (RNAi). This limits powerful RNAi screening methods from being utilized effectively in these disease models. Our goal was to develop a C. elegans tau model that has pronounced tau-induced disease phenotypes in cells that can be modified by feeding RNAi methods. Towards this end, we generated a novel C. elegans transgenic line with codon-optimized human 0N4R V337M tau expressed in the body wall muscle under the myo-3 promoter. Immunoblotting experiments revealed that the expressed tau is phosphorylated on epitopes canonically associated with human AD pathology. The tau line has significantly reduced health metrics, including egg laying, growth rate, paralysis, thrashing frequency, crawling speed, and lifespan. These defects are suppressed by RNAi directed against the tau mRNA. Taken together, our results suggest that this alternative tau genetic model could be a useful tool for uncovering the mechanisms that influence the hyperphosphorylation and toxicity of human tau via RNAi screening and other approaches.

Non-Rodent Genetic Animal Models for Studying Tauopathy: Review of Drosophila, Zebrafish, and C. elegans Models

Tauopathy refers to a group of progressive neurodegenerative diseases, including frontotemporal lobar degeneration and Alzheimer's disease, which correlate with the malfunction of microtubule-associated protein Tau (MAPT) due to abnormal hyperphosphorylation, leading to the formation of intracellular aggregates in the brain. Despite extensive efforts to understand tauopathy and develop an efficient therapy, our knowledge is still far from complete. To find a solution for this group of devastating diseases, several animal models that mimic diverse disease phenotypes of tauopathy have been developed. Rodents are the dominating tauopathy models because of their similarity to humans and established disease lines, as well as experimental approaches. However, powerful genetic animal models using Drosophila, zebrafish, and C. elegans have also been developed for modeling tauopathy and have contributed to understanding the pathophysiology of tauopathy. The success of these models stems from the short lifespans, versatile genetic tools, real-time in-vivo imaging, low maintenance costs, and the capability for high-throughput screening. In this review, we summarize the main findings on mechanisms of tauopathy and discuss the current tauopathy models of these non-rodent genetic animals, highlighting their key advantages and limitations in tauopathy research.

C. elegans detects toxicity of traumatic brain injury generated tau

Traumatic brain injury (TBI) is associated with widespread tau pathology in about 30% of patients surviving late after injury. We previously found that TBI in mice induces the formation of an abnormal form of tau (tau^TBI) which progressively spreads from the site of injury to remote brain regions. Intracerebral inoculation of TBI brain homogenates into naïve mice induced progressive tau pathology, synaptic loss and late cognitive decline, suggesting a pivotal role of tau^TBI in post-TBI neurodegeneration. However, the possibility that tau^TBI was a marker of TBI-associated neurodegeneration rather than a toxic driver of functional decline could not be excluded.

Here we employed the nematode C. elegans as a biosensor to test the pathogenic role of TBI generated tau. The motility of this nematode depends on efficient neuromuscular transmission and is exceptionally sensitive to the toxicity of amyloidogenic proteins, providing a tractable model for our tests. We found that worms exposed to brain homogenates from chronic but not acute TBI mice, or from mice in which tau^TBI had been transmitted by intracerebral inoculation, had impaired motility and neuromuscular synaptic transmission. Results were similar when worms were given brain homogenates from transgenic mice overexpressing tau P301L, a tauopathy mouse model, suggesting that TBI-induced and mutant tau have similar toxic properties. P301L brain homogenate toxicity was similar in wild-type and ptl-1 knock-out worms, indicating that the nematode tau homolog protein PTL-1 was not required to mediate the toxic effect. Harsh protease digestion to eliminate the protein component of the homogenates, pre-incubation with anti-tau antibodies or tau depletion by immunoprecipitation, abolished the toxicity. Homogenates of chronic TBI brains from tau knock-out mice were not toxic to C. elegans, whereas oligomeric recombinant tau was sufficient to impair their motility.

This study indicates that tau^TBI impairs motor activity and synaptic transmission in C. elegans and supports a pathogenic role of tau^TBI in the long-term consequences of TBI. It also sets the groundwork for the development of a C. elegans-based platform for screening anti-tau compounds.

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Traumatic brain injury (TBI) in mice induces a progressive tau pathology.

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Brain-injured tissue from chronic but not acute TBI mice impairs C. elegans motility.

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TBI tissue immunodepleted of tau or from tau knock-out mice has no toxic effect.

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Brain-injured tissue from TBI mice impairs neuromuscular transmission in worms.

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C. elegans is a tractable model for investigating tau toxicity generated by TBI.

Caenorhabditis elegans Models to Investigate the Mechanisms Underlying Tau Toxicity in Tauopathies

The understanding of the genetic, biochemical, and structural determinants underlying tau aggregation is pivotal in the elucidation of the pathogenic process driving tauopathies and the design of effective therapies. Relevant information on the molecular basis of human neurodegeneration in vivo can be obtained using the nematode Caenorhabditis elegans (C. elegans). To this end, two main approaches can be applied: the overexpression of genes/proteins leading to neuronal dysfunction and death, and studies in which proteins prone to misfolding are exogenously administered to induce a neurotoxic phenotype. Thanks to the easy generation of transgenic strains expressing human disease genes, C. elegans allows the identification of genes and/or proteins specifically associated with pathology and the specific disruptions of cellular processes involved in disease. Several transgenic strains expressing human wild-type or mutated tau have been developed and offer significant information concerning whether transgene expression regulates protein production and aggregation in soluble or insoluble form, onset of the disease, and the degenerative process. C. elegans is able to specifically react to the toxic assemblies of tau, thus developing a neurodegenerative phenotype that, even when exogenously administered, opens up the use of this assay to investigate in vivo the relationship between the tau sequence, its folding, and its proteotoxicity. These approaches can be employed to screen drugs and small molecules that can interact with the biogenesis and dynamics of formation of tau aggregates and to analyze their interactions with other cellular proteins.

Impaired tau-microtubule interactions are prevalent among pathogenic tau variants arising from missense mutations

tau is a microtubule (MT)-associated protein that promotes tubulin assembly and stabilizes MTs by binding longitudinally along the MT surface. tau can aberrantly aggregate into pathological inclusions that define Alzheimer's disease, frontotemporal dementias, and other tauopathies. A spectrum of missense mutations in the tau-encoding gene microtubule-associated protein tau (MAPT) can cause frontotemporal dementias. tau aggregation is postulated to spread by a prion-like mechanism. Using a cell-based inclusion seeding assay, we recently reported that only a few tau variants are intrinsically prone to this type of aggregation. Here, we extended these studies to additional tau mutants and investigated their MT binding properties in mammalian cell-based assays. A limited number of tau variants exhibited modest aggregation propensity in vivo, but most tau mutants did not aggregate. Reduced MT binding appeared to be the most common dysfunction for the majority of tau variants due to missense mutations, implying that MT-targeting therapies could potentially be effective in the management of tauopathies.