Retarded axonal transport of R406W mutant tau in transgenic mice with a neurodegenerative tauopathy

Limitations and Applications of Rodent Models in Tauopathy and Synucleinopathy Research

Rodent models that accurately recapitulate key aspects of human disease have long been fundamental to the successful development of clinical interventions. This is greatly underscored in the neurodegenerative disease field, where preclinical testing of anti-prion therapeutics against rodent-adapted prions resulted in the development of small molecules effective against rodent-adapted prions but not against human prions. These findings provided critical lessons for ongoing efforts to develop treatments for patients with neurodegenerative diseases caused by misfolding and accumulation of the proteins tau and α-synuclein, or tauopathies and synucleinopathies, respectively. To avoid the potential pitfalls previously identified in the prion field, this review focuses on rodent models currently available to study tau and α-synuclein disease pathogenesis, emphasizing the strengths and limitations of each with the particular goal of better supporting preclinical research.

A small-molecule microtubule-stabilizing agent safely reduces Aβ plaque and tau pathology in transgenic mouse models of Alzheimer's disease

INTRODUCTION

Intraneuronal inclusions composed of tau protein are found in Alzheimer's disease (AD) and other tauopathies. Tau normally binds microtubules (MTs), and its disengagement from MTs and misfolding in AD is thought to result in MT abnormalities. We previously identified triazolopyrimidine-containing MT-stabilizing compounds that provided benefit in AD mouse models and herein describe the characterization and efficacy testing of an optimized candidate, CNDR-51997.

METHODS

CNDR-51997 underwent pharmacokinetic, pharmacodynamic, safety pharmacology, and mouse tolerability testing. In addition, the compound was examined for efficacy in 5XFAD amyloid beta (Aβ) plaque mice and PS19 tauopathy mice.

RESULTS

CNDR-51997 significantly reduced Aβ plaques in 5XFAD mice and tau pathology in PS19 mice, with the latter also showing attenuated axonal dystrophy and gliosis. CNDR-51997 was well tolerated at doses that exceeded efficacy doses, with a good safety pharmacology profile.

DISCUSSION

CNDR-51997 may be a candidate for advancement as a potential therapeutic agent for AD and/or other tauopathies. Highlights There is evidence of microtubule alterations (MT) in Alzheimer's disease (AD) brain and in mouse models of AD pathology. Intermittent dosing with an optimized, brain-penetrant MT-stabilizing small-molecule, CNDR-51997, reduced both Aβ plaque and tau inclusion pathology in established mouse models of AD. CNDR-51997 attenuated axonal dystrophy and gliosis in a tauopathy mouse model, with a strong trend toward reduced hippocampal neuron loss. CNDR-51997 is well tolerated in mice at doses that are meaningfully greater than required for efficacy in AD mouse models, and the compound has a good safety pharmacology profile.

KIBRA repairs synaptic plasticity and promotes resilience to tauopathy-related memory loss

Synaptic plasticity is obstructed by pathogenic tau in the brain, representing a key mechanism that underlies memory loss in Alzheimer's disease (AD) and related tauopathies. Here, we found that reduced levels of the memory-associated protein KIdney/BRAin (KIBRA) in the brain and increased KIBRA protein levels in cerebrospinal fluid are associated with cognitive impairment and pathological tau levels in disease. We next defined a mechanism for plasticity repair in vulnerable neurons using the C-terminus of the KIBRA protein (CT-KIBRA). We showed that CT-KIBRA restored plasticity and memory in transgenic mice expressing pathogenic human tau; however, CT-KIBRA did not alter tau levels or prevent tau-induced synapse loss. Instead, we found that CT-KIBRA stabilized the protein kinase Mζ (PKMζ) to maintain synaptic plasticity and memory despite tau-mediated pathogenesis. Thus, our results distinguished KIBRA both as a biomarker of synapse dysfunction and as the foundation for a synapse repair mechanism to reverse cognitive impairment in tauopathy.

Methods for Biochemical Isolation of Insoluble Tau in Rodent Models of Tauopathies

The intracellular accumulation of microtubule-associated protein tau is a characteristic feature of tauopathies, a group of neurodegenerative diseases including Alzheimer's disease. Formation of insoluble tau aggregates is initiated by the abnormal hyperphosphorylation and oligomerization of tau. Over the past decades, multiple transgenic rodent models mimicking tauopathies have been develop, showcasing this neuropathological hallmark. The biochemical analysis of insoluble tau in these models has served as a valuable tool to understand the progression of tau-related pathology. In this chapter, we provide a comprehensive review of the two primary methods for isolating insoluble tau, namely, sarkosyl and formic acid extraction (and their variants), which are employed for biochemical analysis in transgenic mouse models of tauopathy. We also analyze the strengths and limitations of these methods.

KIBRA repairs synaptic plasticity and promotes resilience to tauopathy-related memory loss

Synaptic plasticity is obstructed by pathogenic tau in the brain, representing a key mechanism that underlies memory loss in Alzheimer's disease (AD) and related tauopathies. Here, we define a mechanism for plasticity repair in vulnerable neurons using the C-terminus of the KIdney/BRAin (KIBRA) protein (CT-KIBRA). We show that CT-KIBRA restores plasticity and memory in transgenic mice expressing pathogenic human tau; however, CT-KIBRA did not alter tau levels or prevent tau-induced synapse loss. Instead, we find that CT-KIBRA binds to and stabilizes protein kinase Mζ (PKMζ) to maintain synaptic plasticity and memory despite tau-mediated pathogenesis. In humans we find that reduced KIBRA in brain and increased KIBRA in cerebrospinal fluid are associated with cognitive impairment and pathological tau levels in disease. Thus, our results distinguish KIBRA both as a novel biomarker of synapse dysfunction in AD and as the foundation for a synapse repair mechanism to reverse cognitive impairment in tauopathy.

Can accelerated ageing models inform us on age-related tauopathies?

Ageing is the greatest risk factor of late-onset neurodegenerative diseases. In the realm of sporadic tauopathies, modelling the process of biological ageing in experimental animals forms the foundation of searching for the molecular origin of pathogenic tau and developing potential therapeutic interventions. Although prior research into transgenic tau models offers valuable lessons for studying how tau mutations and overexpression can drive tau pathologies, the underlying mechanisms by which ageing leads to abnormal tau accumulation remains poorly understood. Mutations associated with human progeroid syndromes have been proposed to be able to mimic an aged environment in animal models. Here, we summarise recent attempts in modelling ageing in relation to tauopathies using animal models that carry mutations associated with human progeroid syndromes, or genetic elements unrelated to human progeroid syndromes, or have exceptional natural lifespans, or a remarkable resistance to ageing-related disorders.

Axonal transport impairment and its relationship with diffusion tensor imaging metrics of a murine model of p301L tau induced tauopathy

Diffusion Tensor Imaging (DTI) and Manganese Enhanced MRI (MEMRI) are noninvasive tools to characterize neural fiber microstructure and axonal transport. A combination of both may provide novel insights into the progress of neurodegeneration. To investigate the relationship of DTI and MEMRI in white matter of tauopathy, twelve optic nerves of 11-month-old p301L tau mice were imaged and finished with postmortem immunohistochemistry. MEMRI was used to quantify Mn2+ accumulation rates in the optic nerve (ON, termed ONAR) and the Superior Colliculus (SC, termed SCAR), the primary terminal site of ON in mice. We found that both ONAR and SCAR revealed a significant linear correlation with mean diffusion (mD) and radial diffusion (rD) but not with other DTI quantities. Immunohistochemistry findings showed that ONAR, mD, and rD are significantly correlated with the myelin content (Myelin Basic Protein, p < 0.05) but not with the axonal density (SMI-31), tubulin density, or tau aggregates (AT8 staining). In summary, slower axonal transport appeared to have less myelinated axons and thinner remaining axons, associated with reduced rD and mD of in vivo DTI. A combination of in vivo MEMRI and DTI can provide critical information to delineate the progress of white matter deficits in neurodegenerative diseases.

Heparan Sulfate Proteoglycans (HSPGs) Serve as the Mediator Between Monomeric Tau and Its Subsequent Intracellular ERK1/2 Pathway Activation

The conversion of soluble tau protein to insoluble, hyperphosphorylated neurofibrillary tangles (NFTs) is a major hallmark leading to neuronal death observed in neurodegenerative tauopathies. Unlike NFTs, the involvement of monomeric tau in the progression of tau pathology has been less investigated. Using live-cell confocal microscopy and flow cytometry, we demonstrate that soluble 0N4R monomers were rapidly endocytosed by SH-SY5Y and C6 glioma cells via actin-dependent macropinocytosis. Further, cellular endocytosis of monomeric tau has been demonstrated to be HSPG-dependent, as shown in C6 glial cells with genetic knockouts of xylosyltransferase-1-a key enzyme in HSPG synthesis-with a reduced level of tau uptake. Tau internalization subsequently triggers ERK1/2 activation and therefore, the upregulation of IL-6 and IL-1β. The role of ERK1/2 in regulating the levels of pro-inflammatory gene transcripts was confirmed by inhibiting the MEK-ERK1/2 signaling pathway, which led to the attenuated IL-6 and IL-1β expressions but not that of TNF-α. Moreover, as a key regulator of tau internalization, LRP1 (low-density lipoprotein receptor-related protein 1) levels were downregulated in response to monomeric tau added to C6 cells, while it was upregulated in HSPG-deficient cells, suggesting that the involvement of LRP1 in tau uptake depends on the presence of HSPGs on the cell surface. The subsequent LRP1 knockdown experiment we performed shows that LRP1 deficiency leads to an attenuated propensity for tau uptake and further elevated IL-6 gene expression. Collectively, our data suggest that tau has multiple extracellular binding partners that mediate its internalization through distinct mechanisms. Additionally, this study demonstrates the important role of both HSPGs and LRP1 in regulating cellular immune responses to tau protein monomers, providing a novel target for alleviating the neuroinflammatory environment before the formation of neurofibrillary tangles.

Quantitative transportomics identifies Kif5a as a major regulator of neurodegeneration

Many neurons in the adult central nervous system, including retinal ganglion cells (RGCs), degenerate and die after injury. Early axon protein and organelle trafficking failure is a key component in many neurodegenerative disorders yet changes to axoplasmic transport in disease models have not been quantified. We analyzed early changes in the protein ‘transportome’ from RGC somas to their axons after optic nerve injury and identified transport failure of an anterograde motor protein Kif5a early in RGC degeneration. We demonstrated that manipulating Kif5a expression affects anterograde mitochondrial trafficking in RGCs and characterized axon transport in Kif5a knockout mice to identify proteins whose axon localization was Kif5a-dependent. Finally, we found that knockout of Kif5a in RGCs resulted in progressive RGC degeneration in the absence of injury. Together with expression data localizing Kif5a to human RGCs, these data identify Kif5a transport failure as a cause of RGC neurodegeneration and point to a mechanism for future therapeutics.

Regulation Of Microtubule: Current Concepts And Relevance To Neurodegenerative Diseases

Neurodevelopmental disorders (NDDs) are abnormalities linked to neuronal structure and irregularities associated with the proliferation of cells, transportation, and differentiation. NDD also involves synaptic circuitry and neural network alterations known as synaptopathies. Microtubules (MTs) and MTs-associated proteins help to maintain neuronal health as well as their development. The microtubular dynamic structure plays a crucial role in the division of cells and forms mitotic spindles, thus take part in initiating stages of differentiation and polarization for various types of cells. The MTs also take part in the cellular death but MT-based cellular degenerations are not yet well excavated. In the last few years, studies have provided the protagonist activity of MTs in neuronal degeneration. In this review, we largely engrossed our discussion on the change of MT cytoskeleton structure, describing their organization, dynamics, transportation, and their failure causing NDDs. At end of this review, we are targeting the therapeutic neuroprotective strategies on clinical priority and also try to discuss the clues for the development of new MT-based therapy as a new pharmacological intervention. This will be a new potential site to block not only neurodegeneration but also promotes the regeneration of neurons.

Axonal TAU Sorting Requires the C-terminus of TAU but is Independent of ANKG and TRIM46 Enrichment at the AIS

Somatodendritic missorting of the axonal protein TAU is a hallmark of Alzheimer's disease and related tauopathies. Rodent primary neurons and iPSC-derived neurons are used for studying mechanisms of neuronal polarity, including TAU trafficking. However, these models are expensive, time-consuming, and/or require the killing of animals. In this study, we tested four differentiation procedures to generate mature neuron cultures from human SH-SY5Y neuroblastoma cells and assessed the TAU sorting capacity. We show that SH-SY5Y-derived neurons, differentiated with sequential RA/BDNF treatment, are suitable for investigating axonal TAU sorting. These human neurons show pronounced neuronal polarity, axodendritic outgrowth, expression of the neuronal maturation markers TAU and MAP2, and, importantly, efficient axonal sorting of endogenous and transfected human wild-type TAU, similar to mouse primary neurons. We demonstrate that the N-terminal half of TAU is not sufficient for axonal targeting, as a C-terminus-lacking construct (N-term-TAU^HA) is not axonally enriched in both neuronal cell models. Importantly, SH-SY5Y-derived neurons do not show the formation of a classical axon initial segment (AIS), indicated by the lack of ankyrin G (ANKG) and tripartite motif-containing protein 46 (TRIM46) at the proximal axon, which suggests that successful axonal TAU sorting is independent of classical AIS formation. Taken together, our results provide evidence that (i) SH-SY5Y-derived neurons are a valuable human neuronal cell model for studying TAU sorting readily accessible at low cost and without animal need, and that (ii) efficient axonal TAU targeting is independent of ANKG or TRIM46 enrichment at the proximal axon in these neurons.

Tau Isoform-Driven CBD Pathology Transmission in Oligodendrocytes in Humanized Tau Mice

The aggregation of abnormally phosphorylated tau protein in neurons and glia is a neuropathological hallmark of several neurodegenerative disorders, collectively known as tauopathies. They are further subclassified based on the preferential pathological aggregation of three carboxyl-terminal repeat domains (3R) and/or 4R tau. Corticobasal degeneration (CBD) is a rare neurodegenerative disorder classified as a 4R tauopathy. In the present study, we extend analysis of CBD-tau cell-type specific pathology transmission with 3R and 4R tau isoform distinguishable changes. We use a humanized tau (hTau) mouse line, which overexpress all six human tau isoforms in a murine tau knockout background and perform intrastriatal inoculation of control and CBD-tau enriched human brain homogenate. We show that CBD-tau causes hyperphosphorylation of tau at Ser202 predominantly in oligodendrocytes. Next, we demonstrate the spread of tau pathology from striatum to the overlaying corpus callosum and further to the contralateral side. Finally, we demonstrate that the almost exclusive oligodendrocyte-based transmission of hyperphosphorylated tau is reflected in the endogenous 4R tau isoform expression and corresponds to subclassification of CBD as a 4R tauopathy. Additionally, we identify functional changes in oligodendrocytes reflected by myelin basic protein abnormalities upon CBD-tau inoculation. These changes are not observed in murine tau knockout mice lacking both human and murine tau. Our study presents not only in vivo tau isoform–driven region- and cell-specific tau pathology, but also underlines that tau pathology seeding and transmission might be oligodendrocyte-based. These results, which need to be extended to more cases, give new insights into why tauopathies might vary greatly in both histopathological and neuroanatomical patterns.

Mechanisms of Pathogenic Tau and Aβ Protein Spreading in Alzheimer's Disease

Alzheimer’s disease (AD) is pathologically defined by extracellular accumulation of amyloid-β (Aβ) peptides generated by the cleavage of amyloid precursor protein (APP), strings of hyperphosphorylated Tau proteins accumulating inside neurons known as neurofibrillary tangles (NFTs) and neuronal loss. The association between the two hallmarks and cognitive decline has been known since the beginning of the 20th century when the first description of the disease was carried out by Alois Alzheimer. Today, more than 40 million people worldwide are affected by AD that represents the most common cause of dementia and there is still no effective treatment available to cure the disease. In general, the aggregation of Aβ is considered an essential trigger in AD pathogenesis that gives rise to NFTs, neuronal dysfunction and dementia. During the process leading to AD, tau and Aβ first misfold and form aggregates in one brain region, from where they spread to interconnected areas of the brain thereby inducing its gradual morphological and functional deterioration. In this mini-review article, we present an overview of the current literature on the spreading mechanisms of Aβ and tau pathology in AD since a more profound understanding is necessary to design therapeutic approaches aimed at preventing or halting disease progression.

Resveratrol targeting tau proteins, amyloid-beta aggregations, and their adverse effects: An updated review

Resveratrol (Res) is a non-flavonoid compound with pharmacological actions such as antioxidant, antiinflammatory, hepatoprotective, antidiabetes, and antitumor. This plant-derived chemical has a long history usage in treatment of diseases. The excellent therapeutic impacts of Res and its capability in penetration into blood-brain barrier have made it an appropriate candidate in the treatment of neurological disorders (NDs). Tau protein aggregations and amyloid-beta (Aβ) deposits are responsible for the induction of NDs. A variety of studies have elucidated the role of these aggregations in NDs and the underlying molecular pathways in their development. In the present review, based on the recently published articles, we describe that how Res administration could inhibit amyloidogenic pathway and stimulate processes such as autophagy to degrade Aβ aggregations. Besides, we demonstrate that Res supplementation is beneficial in dephosphorylation of tau proteins and suppressing their aggregations. Then, we discuss molecular pathways and relate them to the treatment of NDs.

Mechanisms of secretion and spreading of pathological tau protein

Accumulation of misfolded and aggregated forms of tau protein in the brain is a neuropathological hallmark of tauopathies, such as Alzheimer's disease and frontotemporal lobar degeneration. Tau aggregates have the ability to transfer from one cell to another and to induce templated misfolding and aggregation of healthy tau molecules in previously healthy cells, thereby propagating tau pathology across different brain areas in a prion-like manner. The molecular mechanisms involved in cell-to-cell transfer of tau aggregates are diverse, not mutually exclusive and only partially understood. Intracellular accumulation of misfolded tau induces several mechanisms that aim to reduce the cellular burden of aggregated proteins and also promote secretion of tau aggregates. However, tau may also be released from cells physiologically unrelated to protein aggregation. Tau secretion involves multiple vesicular and non-vesicle-mediated pathways, including secretion directly through the plasma membrane. Consequently, extracellular tau can be found in various forms, both as a free protein and in vesicles, such as exosomes and ectosomes. Once in the extracellular space, tau aggregates can be internalized by neighboring cells, both neurons and glial cells, via endocytic, pinocytic and phagocytic mechanisms. Importantly, accumulating evidence suggests that prion-like propagation of misfolding protein pathology could provide a general mechanism for disease progression in tauopathies and other related neurodegenerative diseases. Here, we review the recent literature on cellular mechanisms involved in cell-to-cell transfer of tau, with a particular focus in tau secretion.

Pathogenic Effects of Impaired Retrieval between the Endoplasmic Reticulum and Golgi Complex

Cellular activities, such as growth and secretion, are dependent on correct protein folding and intracellular protein transport. Injury, like ischemia, malnutrition, and invasion of toxic substances, affect the folding environment in the endoplasmic reticulum (ER). The ER senses this information, following which cells adapt their response to varied situations through the unfolded protein response. Activation of the KDEL receptor, resulting from the secretion from the ER of chaperones containing the KDEL sequence, plays an important role in this adaptation. The KDEL receptor was initially shown to be necessary for the retention of KDEL sequence-containing proteins in the ER. However, it has become clear that the activated KDEL receptor also regulates bidirectional transport between the ER and the Golgi complex, as well as from the Golgi to the secretory pathway. In addition, it has been suggested that the signal for KDEL receptor activation may also affect several other cellular activities. In this review, we discuss KDEL receptor-mediated bidirectional transport and signaling and describe disease models and human diseases related to KDEL receptor dysfunction.

Pathological Progression Induced by the Frontotemporal Dementia-Associated R406W Tau Mutation in Patient-Derived iPSCs

Mutations in the microtubule-associated protein tau (MAPT) gene are known to cause familial frontotemporal dementia (FTD). The R406W tau mutation is a unique missense mutation whose patients have been reported to exhibit Alzheimer's disease (AD)-like phenotypes rather than the more typical FTD phenotypes. In this study, we established patient-derived induced pluripotent stem cell (iPSC) models to investigate the disease pathology induced by the R406W mutation. We generated iPSCs from patients and established isogenic lines using CRISPR/Cas9. The iPSCs were induced into cerebral organoids, which were dissociated into cortical neurons with high purity. In this neuronal culture, the mutant tau protein exhibited reduced phosphorylation levels and was increasingly fragmented by calpain. Furthermore, the mutant tau protein was mislocalized and the axons of the patient-derived neurons displayed morphological and functional abnormalities, which were rescued by microtubule stabilization. The findings of our study provide mechanistic insight into tau pathology and a potential for therapeutic intervention.

Regulation of Tau Homeostasis and Toxicity by Acetylation

Multiple neurodegenerative conditions including Alzheimer's disease and frontotemporal dementia are characterized by the accumulation of tau in the brain, associated with synapse loss and cognitive decline. Currently, the molecular events that lead to tau aggregation, and the pathological effects of the tau protein, are incompletely understood. Recent work has highlighted aberrant acetylation of tau as a key to understanding the pathophysiological roles of this protein. Specific acetylation sites regulate the formation of tau aggregates, synaptic signaling and long-term potentiation. Unraveling the details of this emerging story may offer novel insights into potential therapeutic approaches for devastating neurodegenerative diseases.

Integrative system biology analyses of CRISPR-edited iPSC-derived neurons and human brains reveal deficiencies of presynaptic signaling in FTLD and PSP

Mutations in the microtubule-associated protein tau (MAPT) gene cause autosomal dominant frontotemporal lobar degeneration with tau inclusions (FTLD-tau). MAPT p.R406W carriers present clinically with progressive memory loss and neuropathologically with neuronal and glial tauopathy. However, the pathogenic events triggered by the expression of the mutant tau protein remain poorly understood. To identify the genes and pathways that are dysregulated in FTLD-tau, we performed transcriptomic analyses in induced pluripotent stem cell (iPSC)-derived neurons carrying MAPT p.R406W and CRISPR/Cas9-corrected isogenic controls. We found that the expression of the MAPT p.R406W mutation was sufficient to create a significantly different transcriptomic profile compared with that of the isogeneic controls and to cause the differential expression of 328 genes. Sixty-one of these genes were also differentially expressed in the same direction between MAPT p.R406W carriers and pathology-free human control brains. We found that genes differentially expressed in the stem cell models and human brains were enriched for pathways involving gamma-aminobutyric acid (GABA) receptors and pre-synaptic function. The expression of GABA receptor genes, including GABRB2 and GABRG2, were consistently reduced in iPSC-derived neurons and brains from MAPT p.R406W carriers. Interestingly, we found that GABA receptor genes, including GABRB2 and GABRG2, are significantly lower in symptomatic mouse models of tauopathy, as well as in brains with progressive supranuclear palsy. Genome wide association analyses reveal that common variants within GABRB2 are associated with increased risk for frontotemporal dementia (P < 1 × 10-3). Thus, our systems biology approach, which leverages molecular data from stem cells, animal models, and human brain tissue can reveal novel disease mechanisms. Here, we demonstrate that MAPT p.R406W is sufficient to induce changes in GABA-mediated signaling and synaptic function, which may contribute to the pathogenesis of FTLD-tau and other primary tauopathies.

Decreased Protein Quality Control Promotes the Cognitive Dysfunction Associated With Aging and Environmental Insults

Background: Most neurodegenerative diseases are sporadic and develop with age. Degenerative neural tissues often contain intra- and extracellular protein aggregates, suggesting that the proteostasis network that combats protein misfolding could be dysfunctional in the setting of neurodegenerative disease. Binding immunoglobulin protein (BiP) is an endoplasmic reticulum (ER) chaperone that is crucial for protein folding and modulating the adaptive response in early secretory pathways. The interaction between BiP and unfolded proteins is mediated by the substrate-binding domain and nucleotide-binding domain with ATPase activity. The interaction facilitates protein folding and maturation. BiP has a recovery motif at the carboxyl terminus. The aim of this study is to examine cognitive function in model mice with an impaired proteostasis network by expressing a mutant form of BiP lacking the recovery motif. We also investigated if impairments of cognitive function were exacerbated by exposure to environmental insults, such as inhaled anesthetics.

Methods: We examined cognitive function by performing radial maze testing with mutant BiP mice and assessed the additional impact of general anesthesia in the context of proteostasis dysfunction. Testing over 8 days was performed 10 weeks, 6 months, and 1 year after birth.

Results: Age-related cognitive decline occurred in both forms of mice. The mutant BiP and anesthetic exposure promoted cognitive dysfunction prior to the senile period. After senescence, when mice were tested at 6 months of age and at 1 year old, there were no significant differences between the two genotypes in terms of the radial maze testing; furthermore, there was no significant difference when tested with and without anesthetic exposure.

Conclusion: Our data suggest that aging was the predominant factor underlying the impairment of cognitive function in this study. Impairment of the proteostasis network may promote age-related neurodegeneration, and this is exacerbated by external insults.

Modifying Rap1-signalling by targeting Pde6δ is neuroprotective in models of Alzheimer's disease

Background

Neuronal Ca²⁺ dyshomeostasis and hyperactivity play a central role in Alzheimer’s disease pathology and progression. Amyloid-beta together with non-genetic risk-factors of Alzheimer’s disease contributes to increased Ca²⁺ influx and aberrant neuronal activity, which accelerates neurodegeneration in a feed-forward fashion. As such, identifying new targets and drugs to modulate excessive Ca²⁺ signalling and neuronal hyperactivity, without overly suppressing them, has promising therapeutic potential.

Methods

Here we show, using biochemical, electrophysiological, imaging, and behavioural tools, that pharmacological modulation of Rap1 signalling by inhibiting its interaction with Pde6δ normalises disease associated Ca²⁺ aberrations and neuronal activity, conferring neuroprotection in models of Alzheimer’s disease.

Results

The newly identified inhibitors of the Rap1-Pde6δ interaction counteract AD phenotypes, by reconfiguring Rap1 signalling underlying synaptic efficacy, Ca²⁺ influx, and neuronal repolarisation, without adverse effects in-cellulo or in-vivo. Thus, modulation of Rap1 by Pde6δ accommodates key mechanisms underlying neuronal activity, and therefore represents a promising new drug target for early or late intervention in neurodegenerative disorders.

Conclusion

Targeting the Pde6δ-Rap1 interaction has promising therapeutic potential for disorders characterised by neuronal hyperactivity, such as Alzheimer’s disease.

Electronic supplementary material

The online version of this article (10.1186/s13024-018-0283-3) contains supplementary material, which is available to authorized users.

The Role of Axon Transport in Neuroprotection and Regeneration

Retinal ganglion cells and other central nervous system neurons fail to regenerate after injury. Understanding the obstacles to survival and regeneration, and overcoming them, is key to preserving and restoring function. While comparisons in the cellular changes seen in these non-regenerative cells with those that do have intrinsic regenerative ability has yielded many candidate genes for regenerative therapies, complete visual recovery has not yet been achieved. Insights gained from neurodegenerative diseases, like glaucoma, underscore the importance of axonal transport of organelles, mRNA, and effector proteins in injury and disease. Targeting molecular motor networks, and their cargoes, may be necessary for realizing complete axonal regeneration and vision restoration.

Untangling Tau and Iron: Exploring the Interaction Between Iron and Tau in Neurodegeneration

There is an emerging link between the accumulation of iron in the brain and abnormal tau pathology in a number of neurodegenerative disorders, such as Alzheimer’s disease (AD). Studies have demonstrated that iron can regulate tau phosphorylation by inducing the activity of multiple kinases that promote tau hyperphosphorylation and potentially also by impacting protein phosphatase 2A activity. Iron is also reported to induce the aggregation of hyperphosphorylated tau, possibly through a direct interaction via a putative iron binding motif in the tau protein, facilitating the formation of neurofibrillary tangles (NFTs). Furthermore, in human studies high levels of iron have been reported to co-localize with tau in NFT-bearing neurons. These data, together with our own work showing that tau has a role in mediating cellular iron efflux, provide evidence supporting a critical tau:iron interaction that may impact both the symptomatic presentation and the progression of disease. Importantly, this may also have relevance for therapeutic directions, and indeed, the use of iron chelators such as deferiprone and deferoxamine have been reported to alleviate the phenotypes, reduce phosphorylated tau levels and stabilize iron regulation in various animal models. As these compounds are also moving towards clinical translation, then it is imperative that we understand the intersection between iron and tau in neurodegeneration. In this article, we provide an overview of the key pathological and biochemical interactions between tau and iron. We also review the role of iron and tau in disease pathology and the potential of metal-based therapies for tauopathies.

Tau-mediated synaptic and neuronal dysfunction in neurodegenerative disease

The accumulation of pathological tau in the brain is associated with neuronal deterioration and cognitive impairments in tauopathies including Alzheimer's disease. Tau, while primarily localized in the axons of healthy neurons, accumulates in the soma and dendrites of neurons under pathogenic conditions. Tau is found in both presynaptic and postsynaptic compartments of neurons in Alzheimer's disease. New research supports that soluble forms of tau trigger pathophysiology in the brain by altering properties of synaptic and neuronal function at the early stages of disease progression, before neurons die. Here we review the current understanding of how tau-mediated synaptic and neuronal dysfunction contributes to cognitive decline. Delineating the mechanisms by which pathogenic tau alters synapses, dendrites and axons will help lay the foundation for new strategies that can restore neuronal function in tauopathy.

Early intervention of tau pathology prevents behavioral changes in the rTg4510 mouse model of tauopathy

Although tau pathology, behavioral deficits, and neuronal loss are observed in patients with tauopathies, the relationship between these endpoints has not been clearly established. Here we found that rTg4510 mice, which overexpress human mutant tau in the forebrain, develop progressive age-dependent increases in locomotor activity (LMA), which correlates with neurofibrillary tangle (NFT) pathology, hyperphosphorylated tau levels, and brain atrophy. To further clarify the relationship between these endpoints, we treated the rTg4510 mice with either doxycycline to reduce mutant tau expression or an O-GlcNAcase inhibitor Thiamet G, which has been shown to ameliorate tau pathology in animal models. We found that both doxycycline and Thiamet G treatments starting at 2 months of age prevented the progression of hyperactivity, slowed brain atrophy, and reduced brain hyperphosphorylated tau. In contrast, initiating doxycycline treatment at 4 months reduced neither brain hyperphosphorylated tau nor hyperactivity, further confirming the relationship between these measures. Collectively, our results demonstrate a unique behavioral phenotype in the rTg4510 mouse model of tauopathy that strongly correlates with disease progression, and that early interventions which reduce tau pathology ameliorate the progression of the locomotor dysfunction. These findings suggest that better understanding the relationship between locomotor deficits and tau pathology in the rTg4510 model may improve our understanding of the mechanisms underlying behavioral disturbances in patients with tauopathies.

Interaction between a MAPT variant causing frontotemporal dementia and mutant APP affects axonal transport

In Alzheimer's disease, many indicators point to a central role for poor axonal transport, but the potential for stimulating axonal transport to alleviate the disease remains largely untested. Previously, we reported enhanced anterograde axonal transport of mitochondria in 8- to 11-month-old MAPT^P301L knockin mice, a genetic model of frontotemporal dementia with parkinsonism-17T. In this study, we further characterized the axonal transport of mitochondria in younger MAPT^P301L mice crossed with the familial Alzheimer's disease model, TgCRND8, aiming to test whether boosting axonal transport in young TgCRND8 mice can alleviate axonal swelling. We successfully replicated the enhancement of anterograde axonal transport in young MAPT^P301L/P301L knockin animals. Surprisingly, we found that in the presence of the amyloid precursor protein mutations, MAPT^P301L/P3101L impaired anterograde axonal transport. The numbers of plaque-associated axonal swellings or amyloid plaques in TgCRND8 brains were unaltered. These findings suggest that amyloid-β promotes an action of mutant tau that impairs axonal transport. As amyloid-β levels increase with age even without amyloid precursor protein mutation, we suggest that this rise could contribute to age-related decline in frontotemporal dementia.

In Vivo Evaluation of Neuronal Transport in Murine Models of Neurodegeneration Using Manganese-Enhanced MRI

Manganese-enhanced MRI (MRI) is a technique that allows for a noninvasive in vivo estimation of neuronal transport. It relies on the physicochemical properties of manganese, which is both a calcium analogue being transported along neurons by active transport, and a paramagnetic compound that can be detected on conventional T1-weighted images. Here, we report a multi-session MEMRI protocol that helps establish time-dependent curves relating to neuronal transport along the olfactory tract over several days. The characterization of these curves via unbiased fitting enables us to infer objectively a set of three parameters (the rate of manganese transport from the maximum slope, the peak intensity, and the time to peak intensity). These parameters, measured previously in wild type mice during normal aging, have served as a baseline to demonstrate their significant sensitivity to pathogenic processes associated with Tau pathology. Importantly, the evaluation of these three parameters and their use as indicators can be extended to monitor any normal and pathogenic processes where neuronal transport is altered. This approach can be applied to characterize and quantify the effect of any neurological disease conditions on neuronal transport in animal models, together with the efficacy of potential therapies.

Functional analyses of major cancer-related signaling pathways in Alzheimer's disease etiology

Alzheimer's disease (AD) is an aging-related neurodegenerative disease and accounts for majority of human dementia. The hyper-phosphorylated tau-mediated intracellular neurofibrillary tangle and amyloid β-mediated extracellular senile plaque are characterized as major pathological lesions of AD. Different from the dysregulated growth control and ample genetic mutations associated with human cancers, AD displays damage and death of brain neurons in the absence of genomic alterations. Although various biological processes predominately governing tumorigenesis such as inflammation, metabolic alteration, oxidative stress and insulin resistance have been associated with AD genesis, the mechanistic connection of these biological processes and signaling pathways including mTOR, MAPK, SIRT, HIF, and the FOXO pathway controlling aging and the pathological lesions of AD are not well recapitulated. Hence, we performed a thorough review by summarizing the physiological roles of these key cancer-related signaling pathways in AD pathogenesis, comprising of the crosstalk of these pathways with neurofibrillary tangle and senile plaque formation to impact AD phenotypes. Importantly, the pharmaceutical investigations of anti-aging and AD relevant medications have also been highlighted. In summary, in this review, we discuss the potential role that cancer-related signaling pathways may play in governing the pathogenesis of AD, as well as their potential as future targeted strategies to delay or prevent aging-related diseases and combating AD.

Neurobiology of axonal transport defects in motor neuron diseases: Opportunities for translational research?

Intracellular trafficking of cargoes is an essential process to maintain the structure and function of all mammalian cell types, but especially of neurons because of their extreme axon/dendrite polarisation. Axonal transport mediates the movement of cargoes such as proteins, mRNA, lipids, membrane-bound vesicles and organelles that are mostly synthesised in the cell body and in doing so is responsible for their correct spatiotemporal distribution in the axon, for example at specialised sites such as nodes of Ranvier and synaptic terminals. In addition, axonal transport maintains the essential long-distance communication between the cell body and synaptic terminals that allows neurons to react to their surroundings via trafficking of for example signalling endosomes. Axonal transport defects are a common observation in a variety of neurodegenerative diseases, and mutations in components of the axonal transport machinery have unequivocally shown that impaired axonal transport can cause neurodegeneration (reviewed in El-Kadi et al., 2007, De Vos et al., 2008; Millecamps and Julien, 2013). Here we review our current understanding of axonal transport defects and the role they play in motor neuron diseases (MNDs) with a specific focus on the most common form of MND, amyotrophic lateral sclerosis (ALS).

Evaluation of the dynactin 1 gene in Leonbergers and Labrador Retrievers with laryngeal paralysis

OBJECTIVE To sequence exons and splice consensus sites of the dynactin subunit 1 (DCTN1) gene in Leonbergers and Labrador Retrievers with clinical laryngeal paralysis. ANIMALS 5 unrelated Leonbergers with laryngeal paralysis, 2 clinically normal Leonbergers, 7 unrelated Labrador Retrievers with laryngeal paralysis, and 2 clinically normal Labrador Retrievers. PROCEDURES Primers were designed for the entire coding regions of the DCTN1 gene, a noncoding exon at the 5´ end of the gene, and a 900-bp single-nucleotide polymorphism (SNP)-rich region located 17 kb upstream of the DCTN1 gene by use of the CanFam3 assembly of the canine genome sequence. Sequences were generated and compared between clinically normal and affected dogs. The SNPs flanking the DCTN1 gene as well as a previously identified nonsynonymous SNP in exon 32 were genotyped in affected and clinically normal Leonbergers and Labrador Retrievers. RESULTS None of the affected dogs were homozygous for any mutation affecting coding regions or splicing consensus sequences. Of the 16 dogs tested for the missense SNP in exon 32, all were homozygous for the reference allele, except for 2 affected and 1 clinically normal Labrador Retriever and 1 clinically normal Leonberger. The DCTN1 gene sequences (5 dogs) and haplotypes of polymorphic markers surrounding the DCTN1 gene (all dogs) were not consistent with the hypothesis that laryngeal paralysis was associated with inheritance of the same DCTN1 disease-causing allele within all Labrador Retrievers or Leonbergers evaluated. CONCLUSIONS AND CLINICAL RELEVANCE Mutations in the DCTN1 gene did not appear to cause laryngeal paralysis in Leonbergers or Labrador Retrievers.

Calcium dysregulation contributes to neurodegeneration in FTLD patient iPSC-derived neurons

Mutations in the gene MAPT encoding tau, a microtubules-associated protein, cause a subtype of familial neurodegenerative disorder, known as frontotemporal lobar degeneration tauopathy (FTLD-Tau), which presents with dementia and is characterized by atrophy in the frontal and temporal lobes of the brain. Although induced pluripotent stem cell (iPSC) technology has facilitated the investigation of phenotypes of FTLD-Tau patient neuronal cells in vitro, it remains unclear how FTLD-Tau patient neurons degenerate. Here, we established neuronal models of FTLD-Tau by Neurogenin2-induced direct neuronal differentiation from FTLD-Tau patient iPSCs. We found that FTLD-Tau neurons, either with an intronic MAPT mutation or with an exonic mutation, developed accumulation and extracellular release of misfolded tau followed by neuronal death, which we confirmed by correction of the intronic mutation with CRISPR/Cas9. FTLD-Tau neurons showed dysregulation of the augmentation of Ca²⁺ transients evoked by electrical stimulation. Chemogenetic or pharmacological control of neuronal activity-relevant Ca²⁺ influx by the introduction of designer receptors exclusively activated by designer drugs (DREADDs) or by the treatment with glutamate receptor blockers attenuated misfolded tau accumulation and neuronal death. These data suggest that neuronal activity may regulate neurodegeneration in tauopathy. This FTLD-Tau model provides mechanistic insights into tauopathy pathogenesis and potential avenues for treatments.

Intraneuronal aggregation of the β-CTF fragment of APP (C99) induces Aβ-independent lysosomal-autophagic pathology

Endosomal-autophagic-lysosomal (EAL) dysfunction is an early and prominent neuropathological feature of Alzheimers’s disease, yet the exact molecular mechanisms contributing to this pathology remain undefined. By combined biochemical, immunohistochemical and ultrastructural approaches, we demonstrate a link between EAL pathology and the intraneuronal accumulation of the β-secretase-derived βAPP fragment (C99) in two in vivo models, 3xTgAD mice and adeno-associated viral-mediated C99-infected mice. We present a pathological loop in which the accumulation of C99 is both the effect and causality of impaired lysosomal-autophagic function. The deleterious effect of C99 was found to be linked to its aggregation within EAL-vesicle membranes leading to disrupted lysosomal proteolysis and autophagic impairment. This effect was Aβ independent and was even exacerbated when γ-secretase was pharmacologically inhibited. No effect was observed in inhibitor-treated wild-type animals suggesting that lysosomal dysfunction was indeed directly linked to C99 accumulation. In some brain areas, strong C99 expression also led to inflammatory responses and synaptic dysfunction. Taken together, this work demonstrates a toxic effect of C99 which could underlie some of the early-stage anatomical hallmarks of Alzheimer’s disease pathology. Our work also proposes molecular mechanisms likely explaining some of the unfavorable side-effects associated with γ-secretase inhibitor-directed therapies.

Tau physiology and pathomechanisms in frontotemporal lobar degeneration

Frontotemporal lobar degeneration (FTLD) has been associated with toxic intracellular aggregates of hyperphosphorylated tau (FTLD-tau). Moreover, genetic studies identified mutations in the MAPT gene encoding tau in familial cases of the disease. In this review, we cover a range of aspects of tau function, both in the healthy and diseased brain, discussing several in vitro and in vivo models. Tau structure and function in the healthy brain is presented, accentuating its distinct compartmentalization in neurons and its role in microtubule stabilization and axonal transport. Furthermore, tau-driven pathology is discussed, introducing current concepts and the underlying experimental evidence. Different aspects of pathological tau phosphorylation, the protein's genomic and domain organization as well as its spreading in disease, together with MAPT-associated mutations and their respective models are presented. Dysfunction related to other post-transcriptional modifications and their effect on normal neuronal functions such as cell cycle, epigenetics and synapse dynamics are also discussed, providing a mechanistic explanation for the observations made in FTLD-tau cases, with the possibility for therapeutic intervention. In this review, we cover aspects of tau function, both in the healthy and diseased brain, referring to different in vitro and in vivo models. In healthy neurons, tau is compartmentalized, with higher concentrations found in the distal part of the axon. Cargo molecules are sensitive to this gradient. A disturbed tau distribution, as found in frontotemporal lobar degeneration (FTLD-tau), has severe consequences for cellular physiology: tau accumulates in the neuronal soma and dendrites, leading among others to microtubule depolymerization and impaired axonal transport. Tau forms insoluble aggregates that sequester additional molecules stalling cellular physiology. Neuronal communication is gradually lost as toxic tau accumulates in dendritic spines with subsequent degeneration of synapses and synaptic loss. Thus, by providing a mechanistic explanation for the observations made in FTLD-tau cases, arises a possibility for therapeutic interventions. This article is part of the Frontotemporal Dementia special issue.

Expression of A152T human tau causes age-dependent neuronal dysfunction and loss in transgenic mice

A152T-variant human tau (hTau-A152T) increases risk for tauopathies, including Alzheimer's disease. Comparing mice with regulatable expression of hTau-A152T or wild-type hTau (hTau-WT), we find age-dependent neuronal loss, cognitive impairments, and spontaneous nonconvulsive epileptiform activity primarily in hTau-A152T mice. However, overexpression of either hTau species enhances neuronal responses to electrical stimulation of synaptic inputs and to an epileptogenic chemical. hTau-A152T mice have higher hTau protein/mRNA ratios in brain, suggesting that A152T increases production or decreases clearance of hTau protein. Despite their functional abnormalities, aging hTau-A152T mice show no evidence for accumulation of insoluble tau aggregates, suggesting that their dysfunctions are caused by soluble tau. In human amyloid precursor protein (hAPP) transgenic mice, co-expression of hTau-A152T enhances risk of early death and epileptic activity, suggesting copathogenic interactions between hTau-A152T and amyloid-β peptides or other hAPP metabolites. Thus, the A152T substitution may augment risk for neurodegenerative diseases by increasing hTau protein levels, promoting network hyperexcitability, and synergizing with the adverse effects of other pathogenic factors.

Mutant Tau knock-in mice display frontotemporal dementia relevant behaviour and histopathology

Models of Tau pathology related to frontotemporal dementia (FTD) are essential to determine underlying neurodegenerative pathologies and resulting tauopathy relevant behavioural changes. However, existing models are often limited in their translational value due to Tau overexpression, and the frequent occurrence of motor deficits which prevent comprehensive behavioural assessments. In order to address these limitations, a forebrain-specific (CaMKIIα promoter), human mutated Tau (hTauP301L+R406W) knock-in mouse was generated out of the previously characterised PLB1Triple mouse, and named PLB2Tau. After confirmation of an additional hTau species (~60kDa) in forebrain samples, we identified age-dependent progressive Tau phosphorylation which coincided with the emergence of FTD relevant behavioural traits. In line with the non-cognitive symptomatology of FTD, PLB2Tau mice demonstrated early emerging (~6months) phenotypes of heightened anxiety in the elevated plus maze, depressive/apathetic behaviour in a sucrose preference test and generally reduced exploratory activity in the absence of motor impairments. Investigations of cognitive performance indicated prominent dysfunctions in semantic memory, as assessed by social transmission of food preference, and in behavioural flexibility during spatial reversal learning in a home cage corner-learning task. Spatial learning was only mildly affected and task-specific, with impairments at 12months of age in the corner learning but not in the water maze task. Electroencephalographic (EEG) investigations indicated a vigilance-stage specific loss of alpha power during wakefulness at both parietal and prefrontal recording sites, and site-specific EEG changes during non-rapid eye movement sleep (prefrontal) and rapid eye movement sleep (parietal). Further investigation of hippocampal electrophysiology conducted in slice preparations indicated a modest reduction in efficacy of synaptic transmission in the absence of altered synaptic plasticity. Together, our data demonstrate that the transgenic PLB2Tau mouse model presents with a striking behavioural and physiological face validity relevant for FTD, driven by the low level expression of mutant FTD hTau.

Cellular and molecular modifier pathways in tauopathies: the big picture from screening invertebrate models

Abnormal tau accumulations were observed and documented in post-mortem brains of patients affected by Alzheimer's disease (AD) long before the identification of mutations in the Microtubule-associated protein tau (MAPT) gene, encoding the tau protein, in a different neurodegenerative disease called Frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). The discovery of mutations in the MAPT gene associated with FTDP-17 highlighted that dysfunctions in tau alone are sufficient to cause neurodegeneration. Invertebrate models have been diligently utilized in investigating tauopathies, contributing to the understanding of cellular and molecular pathways involved in disease etiology. An important discovery came with the demonstration that over-expression of human tau in Drosophila leads to premature mortality and neuronal dysfunction including neurodegeneration, recapitulating some key neuropathological features of the human disease. The simplicity of handling invertebrate models combined with the availability of a diverse range of experimental resources make these models, in particular Drosophila a powerful invertebrate screening tool. Consequently, several large-scale screens have been performed using Drosophila, to identify modifiers of tau toxicity. The screens have revealed not only common cellular and molecular pathways, but in some instances the same modifier has been independently identified in two or more screens suggesting a possible role for these modifiers in regulating tau toxicity. The purpose of this review is to discuss the genetic modifier screens on tauopathies performed in Drosophila and C. elegans models, and to highlight the common cellular and molecular pathways that have emerged from these studies. Here, we summarize results of tau toxicity screens providing mechanistic insights into pathological alterations in tauopathies. Key pathways or modifiers that have been identified are associated with a broad range of processes including, but not limited to, phosphorylation, cytoskeleton organization, axonal transport, regulation of cellular proteostasis, transcription, RNA metabolism, cell cycle regulation, and apoptosis. We discuss the utility and application of invertebrate models in elucidating the cellular and molecular functions of novel and uncharacterized disease modifiers identified in large-scale screens as well as for investigating the function of genes identified as risk factors in genome-wide association studies from human patients in the post-genomic era. In this review, we combined and summarized several large-scale modifier screens performed in invertebrate models to identify modifiers of tau toxicity. A summary of the screens show that diverse cellular processes are implicated in the modification of tau toxicity. Kinases and phosphatases are the most predominant class of modifiers followed by components required for cellular proteostasis and axonal transport and cytoskeleton elements.

Behind the curtain of tauopathy: a show of multiple players orchestrating tau toxicity

tau, a microtubule-associated protein, directly binds with microtubules to dynamically regulate the organization of cellular cytoskeletons, and is especially abundant in neurons of the central nervous system. Under disease conditions such as Pick's disease, progressive supranuclear palsy, frontotemporal dementia, parkinsonism linked to chromosome 17 and Alzheimer's disease, tau proteins can self-assemble to paired helical filaments progressing to neurofibrillary tangles. In these diseases, collectively referred to as "tauopathies", alterations of diverse tau modifications including phosphorylation, metal ion binding, glycosylation, as well as structural changes of tau proteins have all been observed, indicating the complexity and variability of factors in the regulation of tau toxicity. Here, we review our current knowledge and hypotheses from relevant studies on tau toxicity, emphasizing the roles of phosphorylations, metal ions, folding and clearance control underlining tau etiology and their regulations. A summary of clinical efforts and associated findings of drug candidates under development is also presented. It is hoped that a more comprehensive understanding of tau regulation will provide us with a better blueprint of tau networking in neuronal cells and offer hints for the design of more efficient strategies to tackle tau-related diseases in the future.

Pathological interface between oligomeric alpha-synuclein and tau in synucleinopathies

BACKGROUND

Aberrant accumulation of α-synuclein constitutes inclusion bodies that are considered a characteristic feature of a group of neurological disorders described as synucleinopathies. Often, multiple disease-causing proteins overlap within a given disease pathology. An emerging body of research focuses on the oligomeric populations of various pathogenic proteins, considering them as the culprits causing neuronal damage and degeneration. To this end, the use of conformation-specific antibodies has proven to be an effective tool. Previous work from our laboratory and others has shown that oligomeric entities of α-synuclein and tau accumulate in their respective diseases, but their interrelationship at this higher order has yet to be shown in synucleinopathies.

METHODS

Here, we used two novel conformation-specific antibodies, F8H7 and Syn33, which recognize α-synuclein oligomers and were developed in our laboratory. We investigated brain tissue from five of each Parkinson's disease and dementia with Lewy bodies patients by performing biophysical and biochemical assays using these antibodies, in addition to the previously characterized anti-tau oligomer antibody T22.

RESULTS

We demonstrate that in addition to the deposition of oligomeric α-synuclein, tau oligomers accumulate in these diseased brains compared with control brains. Moreover, we observed that oligomers of tau and α-synuclein exist in the same aggregates, forming hybrid oligomers in these patients' brains.

CONCLUSIONS

In addition to the deposition of tau oligomers, our results also provide compelling evidence of co-occurrence of α-synuclein and tau into their most toxic forms, i.e., oligomers suggesting that these species interact and influence each other's aggregation via an interface in synucleinopathies.

Tau reduction prevents Aβ-induced axonal transport deficits by blocking activation of GSK3β

Tau ablation, knockdown, and reconstitution studies in primary mouse neurons show that tau enables amyloid β oligomers to inhibit axonal transport through activation of GSK3β and through functions of tau that do not depend on its microtubule binding activity.

Axonal transport deficits in Alzheimer’s disease (AD) are attributed to amyloid β (Aβ) peptides and pathological forms of the microtubule-associated protein tau. Genetic ablation of tau prevents neuronal overexcitation and axonal transport deficits caused by recombinant Aβ oligomers. Relevance of these findings to naturally secreted Aβ and mechanisms underlying tau’s enabling effect are unknown. Here we demonstrate deficits in anterograde axonal transport of mitochondria in primary neurons from transgenic mice expressing familial AD-linked forms of human amyloid precursor protein. We show that these deficits depend on Aβ_1–42 production and are prevented by tau reduction. The copathogenic effect of tau did not depend on its microtubule binding, interactions with Fyn, or potential role in neuronal development. Inhibition of neuronal activity, N-methyl-d-aspartate receptor function, or glycogen synthase kinase 3β (GSK3β) activity or expression also abolished Aβ-induced transport deficits. Tau ablation prevented Aβ-induced GSK3β activation. Thus, tau allows Aβ oligomers to inhibit axonal transport through activation of GSK3β, possibly by facilitating aberrant neuronal activity.

Cellular factors modulating the mechanism of tau protein aggregation

Pathological accumulation of the microtubule-associated protein tau, in the form of neurofibrillary tangles, is a major hallmark of Alzheimer's disease, the most prevalent neurodegenerative condition worldwide. In addition to Alzheimer's disease, a number of neurodegenerative diseases, called tauopathies, are characterized by the accumulation of aggregated tau in a variety of brain regions. While tau normally plays an important role in stabilizing the microtubule network of the cytoskeleton, its dissociation from microtubules and eventual aggregation into pathological deposits is an area of intense focus for therapeutic development. Here we discuss the known cellular factors that affect tau aggregation, from post-translational modifications to molecular chaperones.

Transport and diffusion of Tau protein in neurons

In highly polarized and elongated cells such as neurons, Tau protein must enter and move down the axon to fulfill its biological task of stabilizing axonal microtubules. Therefore, cellular systems for distributing Tau molecules are needed. This review discusses different mechanisms that have been proposed to contribute to the dispersion of Tau molecules in neurons. They include (1) directed transport along microtubules as cargo of tubulin complexes and/or motor proteins, (2) diffusion, either through the cytosolic space or along microtubules, and (3) mRNA-based mechanisms such as transport of Tau mRNA into axons and local translation. Diffusion along the microtubule lattice or through the cytosol appear to be the major mechanisms for axonal distribution of Tau protein in the short-to-intermediate range over distances of up to a millimetre. The high diffusion coefficients ensure that Tau can distribute evenly throughout the axonal volume as well as along microtubules. Motor protein-dependent transport of Tau dominates over longer distances and time scales. At low near-physiological levels, Tau is co-transported along with short microtubules from cell bodies into axons by cytoplasmic dynein and kinesin family members at rates of slow axonal transport.

Global axonal transport rates are unaltered in htau mice in vivo

Microtubule-based axonal transport is believed to become globally disrupted in Alzheimer's disease in part due to alterations of tau expression or phosphorylation. We previously showed that axonal transport rates along retinal ganglion axons are unaffected by deletion of normal mouse tau or by overexpression of wild-type human tau. Here, we report that htau mice expressing 3-fold higher levels of human tau in the absence of mouse tau also display normal fast and slow transport kinetics despite the presence of abnormally hyperphosphorylated tau in some neurons. In addition, markers of slow transport (neurofilament light subunit) and fast transport (snap25) exhibit normal distributions along optic axons of these mice. These studies demonstrate that human tau overexpression, even when associated with a limited degree of tau pathology, does not necessarily impair general axonal transport function in vivo.