Global axonal transport rates are unaltered in htau mice in vivo

Pathological and physiological functional cross-talks of α-synuclein and tau in the central nervous system

α-Synuclein and tau are abundant multifunctional brain proteins that are mainly expressed in the presynaptic and axonal compartments of neurons, respectively. Previous works have revealed that intracellular deposition of α-synuclein and/or tau causes many neurodegenerative disorders, including Alzheimer's disease and Parkinson's disease. Despite intense investigation, the normal physiological functions and roles of α-synuclein and tau are still unclear, owing to the fact that mice with knockout of either of these proteins do not present apparent phenotypes. Interestingly, the co-occurrence of α-synuclein and tau aggregates was found in post-mortem brains with synucleinopathies and tauopathies, some of which share similarities in clinical manifestations. Furthermore, the direct interaction of α-synuclein with tau is considered to promote the fibrillization of each of the proteins in vitro and in vivo. On the other hand, our recent findings have revealed that α-synuclein and tau are cooperatively involved in brain development in a stage-dependent manner. These findings indicate strong cross-talk between the two proteins in physiology and pathology. In this review, we provide a summary of the recent findings on the functional roles of α-synuclein and tau in the physiological conditions and pathogenesis of neurodegenerative diseases. A deep understanding of the interplay between α-synuclein and tau in physiological and pathological conditions might provide novel targets for clinical diagnosis and therapeutic strategies to treat neurodegenerative diseases.

Tau Interacting Proteins: Gaining Insight into the Roles of Tau in Health and Disease

Tau is most intensely studied in relation to its executive role in Tauopathies, a family of neurodegenerative disorders characterized by the accumulation of Tau aggregates [15, 21, 38, 75, 89, 111, 121, 135, 175, 176, 192]. Tau aggregation in the different Tauopathies differs in the affected cell type, the structure of aggregates and Tau isoform composition. However, in all Tauopathies, accumulation of pathological Tau in well-characterized and well-defined brain regions, correlates strongly with symptoms associated with the dysfunction of this brain region. Hence, symptoms of neurodegenerative Tauopathies can range from motoric to cognitive and behavioral symptoms, even extending to deterioration of vital functions when the disease progresses, or combinations of different symptoms governed by the affected brain regions. The most common Tauopathies are corticobasal degeneration (CBD), Pick's disease, progressive supranuclear palsy (PSP) and frontotemporal dementias with parkinsonism linked to chromosome 17 (FTDP-17). However a growing number of diseases are characterized by Tau aggregation amounting to a large family of more than 20 disorders [176]. Most Tauopathies are sporadic, and are hence linked to a combination of environmental and genetic risk factors. However, mutations in MAPT have been identified which are autosomal dominantly linked to Tauopathies, including FTDP, PSP and CBD [94, 163, 185] (Alzforum, https://www.alzforum.org/mutations/mapt ). More than 80 mutations have been identified in MAPT, both in intronic and exonic regions of the human MAPT. These mutations can be classified as missense mutations or splicing mutations. Most missense mutations cluster in or near the microtubule binding site of Tau, while most splicing mutations affect the splicing of exon 10 (encoding the R2 domain), and hence affect the 3R/4R ratio. While Alzheimer's disease (AD), is the most prevalent Tauopathy, no mutations in MAPT associated with AD have been identified. Brains of AD patients are pathologically characterized by the combined presence of amyloid plaques and neurofibrillary tangles [171]. Familial forms of AD, termed early onset familial AD (EOFAD) with clinical mutations in APP or PS1/2, have an early onset, and are invariably characterized by the combined presence of amyloid and Tau pathology [24, 80, 170]. These EOFAD cases, identify a causal link between APP/PS1 misprocessing and the development of Tau pathology and neurodegeneration [80, 170]. Furthermore, combined genetic, pathological, biomarker and in vivo modelling data, indicate that amyloid pathology precedes Tau pathology, and support a role for Aβ as initiator and Tau as executor in the pathogenetic process of AD [80, 96, 97]. Hence, AD is often considered as a secondary Tauopathy (similar as for Down syndrome patients), in contrast to the primary Tauopathies described above. Tau aggregates in Tauopathies vary with respect to the ratio of different Tau isoforms (3R/4R), to the cell types displaying Tau aggregation and the structure of the aggregates. However, in all Tauopathies a strong correlation between progressive development of pathological Tau accumulation and the loss of the respective brain functions is observed.

Human Tau Expression Does Not Induce Mouse Retina Neurodegeneration, Suggesting Differential Toxicity of Tau in Brain vs. Retinal Neurons

The implication of the microtubule-associated protein (MAP) Tau in the ocular manifestations of Alzheimer’s disease (AD) is elusive due to the lack of relevant animal model. However, signs of AD have been reported in the brain of transgenic mice expressing human Tau (hTau). To assess whether hTau is sufficient to induce AD pathogenesis in the retina as well, in the present study, we compared the retinal structure and function of KO mice deprived of Tau (mTKO) with those of transgenic mice expressing hTau. Our results revealed that hTau is particularly abundant in the inner nuclear layer (INL) cells of the retina. By electroretinogram (ERG) recording, light-induced retinal cell activation was not altered in hTau compared with mTKO littermates. Surprisingly, the ERG response mediated by cone photoreceptor stimulation was even stronger in hTau than in mTKO retinae. Immunofluorescent analysis of retinal sections allowed us to observe thicker inner retina in hTau than in mTKO eyes. By Western Blotting (WB), the upregulation of mTOR that was found in hTau mice may underlie retinal structure and function increases. Taken together, our results not only indicate that hTau expression is not toxic for retinal cells but they also suggest that it may play a positive role in visual physiology. The use of hTau may be envisaged to improve visual recovery in ocular diseases affecting the retinal function such as glaucoma or diabetic retinopathy.

Differential Hyperphosphorylation of Tau-S199, -T231 and -S396 in Organotypic Brain Slices of Alzheimer Mice. A Model to Study Early Tau Hyperphosphorylation Using Okadaic Acid

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder of the brain, characterized by extracellular aggregation of beta-amyloid (Aβ) and hyperphosphorylation of tau causing intraneuronal neurofibrillary tangles (NFTs). There is urgent need to study the interactions between Aβ and tau, especially to solve the question of the pathological cascade. In the present study, we aim to develop a model of organotypic brain slices in which both plaque and tau pathology can be examined. Organotypic brain slices (150 μm thick, coronal, at the hippocampal level) from adult (9 month) wildtype (WT, C57BL/6N) and transgenic AD mice (TG, APP_SweDI) were cultured for 2 weeks. To induce tau hyperphosphorylation 100 nM okadaic acid (OA), 10 μM wortmannin (WM) or both were added to the slices. Hyperphosphorylation of tau was tested at tau-S199, tau-T231 and tau-S396 using Western blot. Our data show that in TG mice with plaques a 50 kDa fragment of tau-S396 was hyperphosphorylated and that OA induced hyperphosphorylation of tau-S199. In WT mice (without plaques) OA caused hyperphosphorylation of a 50 kDa and a 38 kDa tau-T231 form and a 25 kDa sdftau-S396 fragment. The N-methyl-D-aspartate (NMDA) antagonist MK801 (1 μM) did not block these effects. Immunohistochemistry showed diffuse increased tau-S396 and tau-T231-like immunoreactivities at the hippocampal level but no formation of NFTs. Confocal microscopy indicated, that pTau-T231 was preferentially located in cytoplasma surrounding nuclei whereas pTau-S396 was found mainly in nerve fibers and strongly associated with plaques. In conclusion we provide a novel in vitro model to study both plaque and tau hyperphosphorylation but not NFTs, which could be useful to study pathological processes in AD and to screen for drugs.

A role for tau in learning, memory and synaptic plasticity

Tau plays a pivotal role in the pathogenesis of neurodegenerative disorders: mutations in the gene encoding for tau (MAPT) are linked to Fronto-temporal Dementia (FTD) and hyper-phosphorylated aggregates of tau forming neurofibrillary tangles (NFTs) that constitute a pathological hallmark of Alzheimer disease (AD) and FTD. Accordingly, tau is a favored therapeutic target for the treatment of these diseases. Given the criticality of tau to dementia's pathogenesis and therapy, it is important to understand the physiological function of tau in the central nervous system. Analysis of Mapt knock out (Mapt^-/-) mice has yielded inconsistent results. Some studies have shown that tau deletion does not alter memory while others have described synaptic plasticity and memory alterations in Mapt^-/- mice. To help clarifying these contrasting results, we analyzed a distinct Mapt^-/- model on a B6129PF3/J genetic background. We found that tau deletion leads to aging-dependent short-term memory deficits, hyperactivity and synaptic plasticity defects. In contrast, Mapt^+/- mice only showed a mild short memory deficit in the novel object recognition task. Thus, while tau is important for normal neuronal functions underlying learning and memory, partial reduction of tau expression may have fractional deleterious effects.

TDP-43 suppresses tau expression via promoting its mRNA instability

In the brains of individuals with Alzheimer's disease (AD) and chronic traumatic encephalopathy, tau pathology is accompanied usually by intracellular aggregation of transactive response DNA-binding protein 43 (TDP-43). However, the role of TDP-43 in tau pathogenesis is not understood. Here, we investigated the role of TDP-43 in tau expression in vitro and in vivo. We found that TDP-43 suppressed tau expression by promoting its mRNA instability through the UG repeats of its 3΄-untranslated region (3΄-UTR). The C-terminal region of TDP-43 was required for this function. Neurodegenerative diseases-causing TDP-43 mutations affected tau mRNA instability differentially, in that some promoted and others did not significantly affect tau mRNA instability. The expression levels of tau and TDP-43 were inverse in the frontal cortex and the cerebellum. Accompanied with cytoplasmic accumulation of TDP-43, tau expression was elevated in TDP-43_M337V transgenic mouse brains. The level of TDP-43, which is decreased in AD brains, was found to correlate negatively with the tau level in human brain. Our findings indicate that TDP-43 suppresses tau expression by promoting the instability of its mRNA. Down-regulation of TDP-43 may be involved in the tau pathology in AD and related neurodegenerative disorders.

Tau Isoforms Imbalance Impairs the Axonal Transport of the Amyloid Precursor Protein in Human Neurons

Tau, as a microtubule (MT)-associated protein, participates in key neuronal functions such as the regulation of MT dynamics, axonal transport, and neurite outgrowth. Alternative splicing of exon 10 in the tau primary transcript gives rise to protein isoforms with three (3R) or four (4R) MT binding repeats. Although tau isoforms are balanced in the normal adult human brain, imbalances in 3R:4R ratio have been tightly associated with the pathogenesis of several neurodegenerative disorders, yet the underlying molecular mechanisms remain elusive. Several studies exploiting tau overexpression and/or mutations suggested that perturbations in tau metabolism impair axonal transport. Nevertheless, no physiological model has yet demonstrated the consequences of altering the endogenous relative content of tau isoforms over axonal transport regulation. Here, we addressed this issue using a trans-splicing strategy that allows modulating tau exon 10 inclusion/exclusion in differentiated human-derived neurons. Upon changes in 3R:4R tau relative content, neurons showed no morphological changes, but live imaging studies revealed that the dynamics of the amyloid precursor protein (APP) were significantly impaired. Single trajectory analyses of the moving vesicles showed that predominance of 3R tau favored the anterograde movement of APP vesicles, increasing anterograde run lengths and reducing retrograde runs and segmental velocities. Conversely, the imbalance toward the 4R isoform promoted a retrograde bias by a significant reduction of anterograde velocities. These findings suggest that changes in 3R:4R tau ratio has an impact on the regulation of axonal transport and specifically in APP dynamics, which might link tau isoform imbalances with APP abnormal metabolism in neurodegenerative processes.

SIGNIFICANCE STATEMENT

The tau protein has a relevant role in the transport of cargos throughout neurons. Dysfunction in tau metabolism underlies several neurological disorders leading to dementia. In the adult human brain, two tau isoforms are found in equal amounts, whereas changes in such equilibrium have been associated with neurodegenerative diseases. We investigated the role of tau in human neurons in culture and found that perturbations in the endogenous balance of tau isoforms were sufficient to impair the transport of the Alzheimer's disease-related amyloid precursor protein (APP), although neuronal morphology was normal. Our results provide evidence of a direct relationship between tau isoform imbalance and defects in axonal transport, which induce an abnormal APP metabolism with important implications in neurodegeneration.

Imaging organelle transport in primary hippocampal neurons treated with amyloid-β oligomers

We describe a strategy for fluorescent imaging of organelle transport in primary hippocampal neurons treated with amyloid-β (Aβ) peptides that cause Alzheimer's disease (AD). This method enables careful, rigorous analyses of axonal transport defects, which are implicated in AD and other neurodegenerative diseases. Moreover, we present and emphasize guidelines for investigating Aβ-induced mechanisms of axonal transport disruption in the absence of nonspecific, irreversible cellular toxicity. This approach should be accessible to most laboratories equipped with cell culture facilities and a standard fluorescent microscope and may be adapted to other cell types.

Intracellular amyloid β oligomers impair organelle transport and induce dendritic spine loss in primary neurons

Introduction

Synaptic dysfunction and intracellular transport defects are early events in Alzheimer’s disease (AD). Extracellular amyloid β (Aβ) oligomers cause spine alterations and impede the transport of proteins and organelles such as brain-derived neurotrophic factor (BDNF) and mitochondria that are required for synaptic function. Meanwhile, intraneuronal accumulation of Aβ precedes its extracellular deposition and is also associated with synaptic dysfunction in AD. However, the links between intracellular Aβ, spine alteration, and mechanisms that support synaptic maintenance such as organelle trafficking are poorly understood.

Results

We compared the effects of wild-type and Osaka (E693Δ)-mutant amyloid precursor proteins: the former secretes Aβ into extracellular space and the latter accumulates Aβ oligomers within cells. First we investigated the effects of intracellular Aβ oligomers on dendritic spines in primary neurons and their tau-dependency using tau knockout neurons. We found that intracellular Aβ oligomers caused a reduction in mushroom, or mature spines, independently of tau. We also found that intracellular Aβ oligomers significantly impaired the intracellular transport of BDNF, mitochondria, and recycling endosomes: cargoes essential for synaptic maintenance. A reduction in BDNF transport by intracellular Aβ oligomers was also observed in tau knockout neurons.

Conclusions

Our findings indicate that intracellular Aβ oligomers likely contribute to early synaptic pathology in AD and argue against the consensus that Aβ-induced spine loss and transport defects require tau.

Motor and cognitive deficits in aged tau knockout mice in two background strains

Background

We recently reported that Parkinsonian and dementia phenotypes emerge between 7-12 months of age in tau^-/- mice on a Bl6/129sv mixed background. These observations were partially replicated by another group using pure Bl6 background tau^-/- mice, but notably they did not observe a cognitive phenotype. A third group using Bl6 background tau^-/- mice found cognitive impairment at 20-months of age.

Results

To reconcile the observations, here we considered the genetic, dietary and environmental variables in both studies, and performed an extended set of behavioral studies on 12-month old tau^+/+, tau^+/-, and tau^-/- mice comparing Bl6/129sv to Bl6 backgrounds. We found that tau^-/- in both backgrounds exhibited reduced tyrosine hydroxylase-positive nigral neuron and impaired motor function in all assays used, which was ameliorated by oral treatment with L-DOPA, and not confounded by changes in body weight. Tau^-/- in the C57BL6/SV129 background exhibited deficits in the Y-maze cognition task, but the mice on the Bl6 background did not.

Conclusions

These results validate our previous report on the neurodegenerative phenotypes of aged tau^-/- mice, and show that genetic background may impact the extent of cognitive impairment in these mice. Therefore excessive lowering of tau should be avoided in therapeutic strategies for AD.

Tau acts as a mediator for Alzheimer's disease-related synaptic deficits

The two histopathological hallmarks of Alzheimer's disease (AD) are amyloid plaques containing multiple forms of amyloid beta (Aβ) and neurofibrillary tangles containing phosphorylated tau proteins. As mild cognitive impairment frequently occurs long before the clinical diagnosis of AD, the scientific community has been increasingly interested in the roles of Aβ and tau in earlier cellular changes that lead to functional deficits. Therefore, great progress has recently been made in understanding how Aβ or tau causes synaptic dysfunction. However, the interaction between the Aβ and tau-initiated intracellular cascades that lead to synaptic dysfunction remains elusive. The cornerstone of the two-decade-old hypothetical amyloid cascade model is that amyloid pathologies precede tau pathologies. Although the premise of Aβ-tau pathway remains valid, the model keeps evolving as new signaling events are discovered that lead to functional deficits and neurodegeneration. Recent progress has been made in understanding Aβ-PrP(C) -Fyn-mediated neurotoxicity and synaptic deficits. Although still elusive, many novel upstream and downstream signaling molecules have been found to modulate tau mislocalization and tau hyperphosphorylation. Here we will discuss the mechanistic interactions between Aβ-PrP(C) -mediated neurotoxicity and tau-mediated synaptic deficits in an updated amyloid cascade model with calcium and tau as the central mediators.