Lost after translation: missorting of Tau protein and consequences for Alzheimer disease

Phospholipids and calmodulin modulate the inhibition of PMCA activity by tau

The disruption of Ca²⁺ signaling in neurons, together with a failure to keep optimal intracellular Ca²⁺ concentrations, have been proposed as significant factors for neuronal dysfunction in the Ca²⁺ hypothesis of Alzheimer's disease (AD). Tau is a protein that plays an essential role in axonal transport and can form abnormal structures such as neurofibrillary tangles that constitute one of the hallmarks of AD. We have recently shown that plasma membrane Ca²⁺-ATPase (PMCA), a key enzyme in the maintenance of optimal cytosolic Ca²⁺ levels in cells, is inhibited by tau in membrane vesicles. In the present study we show that tau inhibits synaptosomal PMCA purified from pig cerebrum, and reconstituted in phosphatidylserine-containing lipid bilayers, with a K_i value of 1.5±0.2nM tau. Noteworthy, the inhibitory effect of tau is dependent on the charge of the phospholipid used for PMCA reconstitution. In addition, nanomolar concentrations of calmodulin, the major endogenous activator of PMCA, protects against inhibition of the Ca²⁺-ATPase activity by tau. Our results in a cellular model such as SH-SY5Y human neuroblastoma cells yielded an inhibition of PMCA by nanomolar tau concentrations and protection by calmodulin against this inhibition similar to those obtained with purified synaptosomal PMCA. Functional studies were also performed with native and truncated versions of human cerebral PMCA4b, an isoform that has been showed to be functionally regulated by amyloid peptides, whose aggregates constitutes another hallmark of AD. Kinetic assays point out that tau binds to the C-terminal tail of PMCA, at a site distinct but close to the calmodulin binding domain. In conclusion, PMCA can be seen as a molecular target for tau-induced cytosolic calcium dysregulation in synaptic terminals. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

Microtubule Destabilization Paves the Way to Parkinson's Disease

Microtubules are dynamic structures normally associated to the cell division, during which they form the mitotic spindle, as well as to the initial phases of specification and polarization of various cell types, including neurons. Although microtubules could have a role in the death of many cells and tissues, the microtubule-based degenerative mechanisms have been poorly investigated; nevertheless, during the last two decades, many clues have been accumulated suggesting the importance of the microtubule system during neurodegeneration. Thus, the aim of this review is to analyse how the changes of the microtubule cytoskeleton, in terms of organization and dynamics, as well as the failure of the microtubule-dependent neuronal processes, as axonal transport, may play a pivotal role in the chain of events leading to Parkinson's disease. Last but not least, since disease-modifying or neuroprotective strategies are a clinical priority in Parkinson's disease, we will also present the hints about the concrete possibility of a microtubule-targeted therapy, which would have the potentiality to block the running degenerative events and to prompt the regeneration of the lost tissues.

A Mitocentric View of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease with an increasing morbidity, mortality, and economic cost. Plaques formed by amyloid beta peptide (Aβ) and neurofibrillary tangles formed by microtubule-associated protein tau are two main characters of AD. Though previous studies have focused on Aβ and tau and got some progressions on their toxicity mechanisms, no significantly effective treatments targeting the Aβ and tau have been found. However, it is worth noting that mounting evidences showed that mitochondrial dysfunction is an early event during the process of AD pathologic changes. What is more, these studies also showed an obvious association between mitochondrial dysfunction and Aβ/tau toxicity. Furthermore, both genetic and environmental factors may increase the oxidative stress and the mitochondria are also the sensitive target of ROS, which may form a vicious feedback between mitochondrial dysfunction and oxidative stress, eventually resulting in deficient energy, synaptic failure, and cell death. This article reviews the previous related studies from different aspects and concludes the critical roles of mitochondrial dysfunction in AD, suggesting a different route to AD therapy, which may guide the research and treatment direction.

Regional tau deposition measured by [18F]THK5317 positron emission tomography is associated to cognition via glucose metabolism in Alzheimer's disease

Background

The recent development of tau-specific positron emission tomography (PET) tracers has allowed in vivo quantification of regional tau deposition and offers the opportunity to monitor the progression of tau pathology along with cognitive impairment. In this study, we investigated the relationships of cerebral tau deposition ([¹⁸F]THK5317-PET) and metabolism ([¹⁸F]FDG-PET) with concomitant cognitive function in patients with probable Alzheimer’s disease (AD).

Methods

Nine patients diagnosed with AD dementia and 11 with prodromal AD (mild cognitive impairment, amyloid-positive on [¹¹C]PiB-PET) were included in this study. All patients underwent PET scans using each tracer, as well as episodic memory and global cognition assessment. Linear models were used to investigate the association of regional [¹⁸F]THK5317 retention and [¹⁸F]FDG uptake with cognition. The possible mediating effect of local metabolism on the relationship between tau deposition and cognitive performance was investigated using mediation analyses.

Results

Significant negative associations were found between [¹⁸F]THK5317 regional retention, mainly in temporal regions, and both episodic memory and global cognition. Significant positive associations were found between [¹⁸F]FDG regional uptake and cognition. The association of [¹⁸F]FDG with global cognition was regionally more extensive than that of [¹⁸F]THK5317, while the opposite was observed with episodic memory, suggesting that [¹⁸F]THK5317 retention might be more sensitive than [¹⁸F]FDG regional uptake to early cognitive impairment. Finally, [¹⁸F]FDG uptake had a mediating effect on the relationship between [¹⁸F]THK5317 retention in temporal regions and global cognition.

Conclusions

These findings suggest a mediating role for local glucose metabolism in the observed association between in vivo tau deposition and concomitant cognitive impairment in AD.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-016-0204-z) contains supplementary material, which is available to authorized users.

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.

Alzheimer disease: modeling an Aβ-centered biological network

In genetically complex diseases, the search for missing heritability is focusing on rare variants with large effect. Thanks to next generation sequencing technologies, genome-wide characterization of these variants is now feasible in every individual. However, a lesson from current studies is that collapsing rare variants at the gene level is often insufficient to obtain a statistically significant signal in case-control studies, and that network-based analyses are an attractive complement to classical approaches. In Alzheimer disease (AD), according to the prevalent amyloid cascade hypothesis, the pathology is driven by the amyloid beta (Aβ) peptide. In past years, based on experimental studies, several hundreds of proteins have been shown to interfere with Aβ production, clearance, aggregation or toxicity. Thanks to a manual curation of the literature, we identified 335 genes/proteins involved in this biological network and classified them according to their cellular function. The complete list of genes, or its subcomponents, will be of interest in ongoing AD genetic studies.

Acetylated tau destabilizes the cytoskeleton in the axon initial segment and is mislocalized to the somatodendritic compartment

BACKGROUND

Neurons are highly polarized cells in which asymmetric axonal-dendritic distribution of proteins is crucial for neuronal function. Loss of polarized distribution of the axonal protein tau is an early sign of Alzheimer's disease (AD) and other neurodegenerative disorders. The cytoskeletal network in the axon initial segment (AIS) forms a barrier between the axon and the somatodentritic compartment, contributing to axonal retention of tau. Although perturbation of the AIS cytoskeleton has been implicated in neurological disorders, the molecular triggers and functional consequence of AIS perturbation are incompletely understood.

RESULTS

Here we report that tau acetylation and consequent destabilization of the AIS cytoskeleton promote the somatodendritic mislocalization of tau. AIS cytoskeletal proteins, including ankyrin G and βIV-spectrin, were downregulated in AD brains and negatively correlated with an increase in tau acetylated at K274 and K281. AIS proteins were also diminished in transgenic mice expressing tauK274/281Q, a tau mutant that mimics K274 and K281 acetylation. In primary neuronal cultures, the tauK274/281Q mutant caused hyperdynamic microtubules (MTs) in the AIS, shown by live-imaging of MT mobility and fluorescence recovery after photobleaching. Using photoconvertible tau constructs, we found that axonal tauK274/281Q was missorted into the somatodendritic compartment. Stabilizing MTs with epothilone D to restore the cytoskeletal barrier in the AIS prevented tau mislocalization in primary neuronal cultures.

CONCLUSIONS

Together, these findings demonstrate that tau acetylation contributes to the pathogenesis of neurodegenerative disease by compromising the cytoskeletal sorting machinery in the AIS.

Interaction of tau with the RNA-Binding Protein TIA1 Regulates tau Pathophysiology and Toxicity

Dendritic mislocalization of microtubule associated protein tau is a hallmark of tauopathies, but the role of dendritic tau is unknown. We now report that tau interacts with the RNA-binding protein (RBP) TIA1 in brain tissue, and we present the brain-protein interactome network for TIA1. Analysis of the TIA1 interactome in brain tissue from wild-type (WT) and tau knockout mice demonstrates that tau is required for normal interactions of TIA1 with proteins linked to RNA metabolism, including ribosomal proteins and RBPs. Expression studies show that tau regulates the distribution of TIA1, and tau accelerates stress granule (SG) formation. Conversely, TIA1 knockdown or knockout inhibits tau misfolding and associated toxicity in cultured hippocampal neurons, while overexpressing TIA1 induces tau misfolding and stimulates neurodegeneration. Pharmacological interventions that prevent SG formation also inhibit tau pathophysiology. These studies suggest that the pathophysiology of tauopathy requires an intimate interaction with RNA-binding proteins.

Acetylated Tau Obstructs KIBRA-Mediated Signaling in Synaptic Plasticity and Promotes Tauopathy-Related Memory Loss

Tau toxicity has been implicated in the emergence of synaptic dysfunction in Alzheimer's disease (AD), but the mechanism by which tau alters synapse physiology and leads to cognitive decline is unclear. Here we report abnormal acetylation of K274 and K281 on tau, identified in AD brains, promotes memory loss and disrupts synaptic plasticity by reducing postsynaptic KIdney/BRAin (KIBRA) protein, a memory-associated protein. Transgenic mice expressing human tau with lysine-to-glutamine mutations to mimic K274 and K281 acetylation (tauKQ) exhibit AD-related memory deficits and impaired hippocampal long-term potentiation (LTP). TauKQ reduces synaptic KIBRA levels and disrupts activity-induced postsynaptic actin remodeling and AMPA receptor insertion. The LTP deficit was rescued by promoting actin polymerization or by KIBRA expression. In AD patients with dementia, we found enhanced tau acetylation is linked to loss of KIBRA. These findings suggest a novel mechanism by which pathogenic tau causes synaptic dysfunction and cognitive decline in AD pathogenesis.

Experimental Milestones in the Discovery of Molecular Chaperones as Polypeptide Unfolding Enzymes

Molecular chaperones control the cellular folding, assembly, unfolding, disassembly, translocation, activation, inactivation, disaggregation, and degradation of proteins. In 1989, groundbreaking experiments demonstrated that a purified chaperone can bind and prevent the aggregation of artificially unfolded polypeptides and use ATP to dissociate and convert them into native proteins. A decade later, other chaperones were shown to use ATP hydrolysis to unfold and solubilize stable protein aggregates, leading to their native refolding. Presently, the main conserved chaperone families Hsp70, Hsp104, Hsp90, Hsp60, and small heat-shock proteins (sHsps) apparently act as unfolding nanomachines capable of converting functional alternatively folded or toxic misfolded polypeptides into harmless protease-degradable or biologically active native proteins. Being unfoldases, the chaperones can proofread three-dimensional protein structures and thus control protein quality in the cell. Understanding the mechanisms of the cellular unfoldases is central to the design of new therapies against aging, degenerative protein conformational diseases, and specific cancers.

Structure-function integrity of the adult hippocampus depends on the transcription factor Bcl11b/Ctip2

The dentate gyrus is one of the only two brain regions where adult neurogenesis occurs. Throughout life, cells of the neuronal stem cell niche undergo proliferation, differentiation and integration into the hippocampal neural circuitry. Ongoing adult neurogenesis is a prerequisite for the maintenance of adult hippocampal functionality. Bcl11b, a zinc finger transcription factor, is expressed by postmitotic granule cells in the developing as well as adult dentate gyrus. We previously showed a critical role of Bcl11b for hippocampal development. Whether Bcl11b is also required for adult hippocampal functions has not been investigated. Using a tetracycline‐dependent inducible mouse model under the control of the forebrain‐specific CaMKIIα promoter, we show here that the adult expression of Bcl11b is essential for survival, differentiation and functional integration of adult‐born granule cell neurons. In addition, Bcl11b is required for survival of pre‐existing mature neurons. Consequently, loss of Bcl11b expression selectively in the adult hippocampus results in impaired spatial working memory. Together, our data uncover for the first time a specific role of Bcl11b in adult hippocampal neurogenesis and function.

Tau pathology-mediated presynaptic dysfunction

Brain tauopathies are characterized by abnormal processing of tau protein. While somatodendritic tau mislocalization has attracted considerable attention in tauopathies, the role of tau pathology in axonal transport, connectivity and related dysfunctions remains obscure. We have previously shown using the squid giant synapse that presynaptic microinjection of recombinant human tau protein (htau42) results in failure of synaptic transmission. Here, we evaluated molecular mechanisms mediating this effect. Thus, the initial event, observed after htau42 presynaptic injection, was an increase in transmitter release. This event was mediated by calcium release from intracellular stores and was followed by a reduction in evoked transmitter release. The effect of htau42 on synaptic transmission was recapitulated by a peptide comprising the phosphatase-activating domain of tau, suggesting activation of phosphotransferases. Accordingly, findings indicated that htau42-mediated toxicity involves the activities of both GSK3 and Cdk5 kinases.

Structural studies on the mechanism of protein aggregation in age related neurodegenerative diseases

The progression of many neurodegenerative diseases is assumed to be caused by misfolding of specific characteristic diseases related proteins, resulting in aggregation and fibril formation of these proteins. Protein misfolding associated age related diseases, although different in disease manifestations, share striking similarities. In all cases, one disease protein aggregates and loses its function or additionally shows a toxic gain of function. However, the clear link between these individual amyloid-like protein aggregates and cellular toxicity is often still uncertain. The similar features of protein misfolding and aggregation in this group of proteins, all involved in age related neurodegenerative diseases, results in high interest in characterization of their structural properties. We review here recent findings on structural properties of some age related disease proteins, in the context of their biological importance in disease.

Co-immunoprecipitation with Tau Isoform-specific Antibodies Reveals Distinct Protein Interactions and Highlights a Putative Role for 2N Tau in Disease

Alternative splicing generates multiple isoforms of the microtubule-associated protein Tau, but little is known about their specific function. In the adult mouse brain, three Tau isoforms are expressed that contain either 0, 1, or 2 N-terminal inserts (0N, 1N, and 2N). We generated Tau isoform-specific antibodies and performed co-immunoprecipitations followed by tandem mass tag multiplexed quantitative mass spectrometry. We identified novel Tau-interacting proteins of which one-half comprised membrane-bound proteins, localized to the plasma membrane, mitochondria, and other organelles. Tau was also found to interact with proteins involved in presynaptic signal transduction. MetaCore analysis revealed one major Tau interaction cluster that contained 33 Tau pulldown proteins. To explore the pathways in which these proteins are involved, we conducted an ingenuity pathway analysis that revealed two significant overlapping pathways, “cell-to-cell signaling and interaction” and “neurological disease.” The functional enrichment tool DAVID showed that in particular the 2N Tau-interacting proteins were specifically associated with neurological disease. Finally, for a subset of Tau interactions (apolipoprotein A1 (apoA1), apoE, mitochondrial creatine kinase U-type, β-synuclein, synaptogyrin-3, synaptophysin, syntaxin 1B, synaptotagmin, and synapsin 1), we performed reverse co-immunoprecipitations, confirming the preferential interaction of specific isoforms. For example, apoA1 displayed a 5-fold preference for the interaction with 2N, whereas β-synuclein showed preference for 0N. Remarkably, a reverse immunoprecipitation with apoA1 detected only the 2N isoform. This highlights distinct protein interactions of the different Tau isoforms, suggesting that they execute different functions in brain tissue.

Bicaudal D Family of Motor Adaptors: Linking Dynein Motility to Cargo Binding

Transport of different intracellular cargoes along cytoskeleton filaments is essential for the morphogenesis and function of a broad variety of eukaryotic cells. Intracellular transport is mediated by cytoskeletal motors including myosin, kinesin, and dynein, which are typically linked to various cargoes by adaptor proteins. Recent studies suggest that adaptor proteins can also act as essential transport cofactors, which control motor activity and coordination. Characterization of the evolutionary conserved Bicaudal D (BICD) family of dynein adaptor proteins has provided important insights into the fundamental mechanisms governing cargo trafficking. This review highlights the advances in the current understanding of how BICD adaptors regulate microtubule-based transport and how they contribute to developmental processes and human disease.

Metals and Neuronal Metal Binding Proteins Implicated in Alzheimer's Disease

Alzheimer's disease (AD) is the most prevalent age-related dementia affecting millions of people worldwide. Its main pathological hallmark feature is the formation of insoluble protein deposits of amyloid-β and hyperphosphorylated tau protein into extracellular plaques and intracellular neurofibrillary tangles, respectively. Many of the mechanistic details of this process remain unknown, but a well-established consequence of protein aggregation is synapse dysfunction and neuronal loss in the AD brain. Different pathways including mitochondrial dysfunction, oxidative stress, inflammation, and metal metabolism have been suggested to be implicated in this process. In particular, a body of evidence suggests that neuronal metal ions such as copper, zinc, and iron play important roles in brain function in health and disease states and altered homeostasis and distribution as a common feature across different neurodegenerative diseases and aging. In this focused review, we overview neuronal proteins that are involved in AD and whose metal binding properties may underlie important biochemical and regulatory processes occurring in the brain during the AD pathophysiological process.

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.

Tau missorting and spastin-induced microtubule disruption in neurodegeneration: Alzheimer Disease and Hereditary Spastic Paraplegia

In Alzheimer Disease (AD), the mechanistic connection of the two major pathological hallmarks, namely deposition of Amyloid-beta (Aβ) in the form of extracellular plaques, and the pathological changes of the intracellular protein Tau (such as phosphorylation, missorting, aggregation), is not well understood. Genetic evidence from AD and Down Syndrome (Trisomy 21), and animal models thereof, suggests that aberrant production of Aβ is upstream of Tau aggregation, but also points to Tau as a critical effector in the pathological process. Yet, the cascade of events leading from increased levels of Aβ to Tau-dependent toxicity remains a matter of debate.

Using primary neurons exposed to oligomeric forms of Aβ, we have found that Tau becomes mislocalized (missorted) into the somatodendritic compartment. Missorting of Tau correlates with loss of microtubules and downstream consequences such as loss of mature spines, loss of synaptic activity, and mislocalization of mitochondria.

In this cascade, missorting of Tau induces mislocalization of TTLL6 (Tubulin-Tyrosine-Ligase-Like 6) into the dendrites. TTLL6 induces polyglutamylation of microtubules, which acts as a trigger for spastin mediated severing of dendritic microtubules. Loss of microtubules makes cells unable to maintain transport of mitochondria, which in turn results in synaptic dysfunction and loss of mature spines. These pathological changes are absent in TauKO derived primary neurons. Thus, Tau mediated mislocalization of TTLL6 and spastin activation reveals a pathological gain of function for Tau and spastin in this cellular model system of AD.

In contrast, in hereditary spastic paraplegia (HSP) caused by mutations of the gene encoding spastin (spg4 alias SPAST), spastin function in terms of microtubule severing is decreased at least for the gene product of the mutated allele, resulting in overstable microtubules in disease model systems. Whether total spastin severing activity or microtubule stability in human disease is also affected is not yet clear. No human disease has been associated so far with the long-chain polyglutamylation enzyme TTLL6, or the other TTLLs (1,5,11) possibly involved.

Here we review the findings supporting a role for Tau, spastin and TTLL6 in AD and other tauopathies, HSP and neurodegeneration, and summarize possible therapeutic approaches for AD and HSP.

Obesity and sex interact in the regulation of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder, for which a number of genetic, environmental, and lifestyle risk factors have been identified. A significant modifiable risk factor is obesity in mid-life. Interestingly, both obesity and AD exhibit sex differences and are regulated by sex steroid hormones. Accumulating evidence suggests interactions between obesity and sex in regulation of AD risk, although the pathways underlying this relationship are unclear. Inflammation and the E4 allele of apolipoprotein E have been identified as independent risk factors for AD and both interact with obesity and sex steroid hormones. We review the individual and cooperative effects of obesity and sex on development of AD and examine the potential contributions of apolipoprotein E, inflammation, and their interactions to this relationship.

Mislocalization of neuronal tau in the absence of tangle pathology in phosphomutant tau knockin mice

Hyperphosphorylation and fibrillar aggregation of the microtubule-associated protein tau are key features of Alzheimer's disease and other tauopathies. To investigate the involvement of tau phosphorylation in the pathological process, we generated a pair of complementary phosphomutant tau knockin mouse lines. One exclusively expresses phosphomimetic tau with 18 glutamate substitutions at serine and/or threonine residues in the proline-rich and first microtubule-binding domains to model hyperphosphorylation, whereas its phosphodefective counterpart has matched alanine substitutions. Consistent with expected effects of genuine phosphorylation, association of the phosphomimetic tau with microtubules and neuronal membranes is severely disrupted in vivo, whereas the phosphodefective mutations have more limited or no effect. Surprisingly, however, age-related mislocalization of tau is evident in both lines, although redistribution appears more widespread and more pronounced in the phosphomimetic tau knockin. Despite these changes, we found no biochemical or immunohistological evidence of pathological tau aggregation in mice of either line up to at least 2 years of age. These findings raise important questions about the role of tau phosphorylation in driving pathology in human tauopathies.

Polymeric alkylpyridinium salts permit intracellular delivery of human Tau in rat hippocampal neurons: requirement of Tau phosphorylation for functional deficits

Patients suffering from tauopathies including frontotemporal dementia (FTD) and Alzheimer's disease (AD) present with intra-neuronal aggregation of microtubule-associated protein Tau. During the disease process, Tau undergoes excessive phosphorylation, dissociates from microtubules and aggregates into insoluble neurofibrillary tangles (NFTs), accumulating in the soma. While many aspects of the disease pathology have been replicated in transgenic mouse models, a region-specific non-transgenic expression model is missing. Complementing existing models, we here report a novel region-specific approach to modelling Tau pathology. Local co-administration of the pore-former polymeric 1,3-alkylpyridinium salts (Poly-APS) extracted from marine sponges, and synthetic full-length 4R recombinant human Tau (hTau) was performed in vitro and in vivo. At low doses, Poly-APS was non-toxic and cultured cells exposed to Poly-APS (0.5 µg/ml) and hTau (1 µg/ml; ~22 µM) had normal input resistance, resting-state membrane potentials and Ca(2+) transients induced either by glutamate or KCl, as did cells exposed to a low concentration of the phosphatase inhibitor Okadaic acid (OA; 1 nM, 24 h). Combined hTau loading and phosphatase inhibition resulted in a collapse of the membrane potential, suppressed excitation and diminished glutamate and KCl-stimulated Ca(2+) transients. Stereotaxic infusions of Poly-APS (0.005 µg/ml) and hTau (1 µg/ml) bilaterally into the dorsal hippocampus at multiple sites resulted in hTau loading of neurons in rats. A separate cohort received an additional 7-day minipump infusion of OA (1.2 nM) intrahippocampally. When tested 2 weeks after surgery, rats treated with Poly-APS+hTau+OA presented with subtle learning deficits, but were also impaired in cognitive flexibility and recall. Hippocampal plasticity recorded from slices ex vivo was diminished in Poly-APS+hTau+OA subjects, but not in other treatment groups. Histological sections confirmed the intracellular accumulation of hTau in CA1 pyramidal cells and along their processes; phosphorylated Tau was present only within somata. This study demonstrates that cognitive, physiological and pathological symptoms reminiscent of tauopathies can be induced following non-mutant hTau delivery into CA1 in rats, but functional consequences hinge on increased Tau phosphorylation. Collectively, these data validate a novel model of locally infused recombinant hTau protein as an inducer of Tau pathology in the hippocampus of normal rats; future studies will provide insights into the pathological spread and maturation of Tau pathology.

Frontotemporal lobar degeneration: old knowledge and new insight into the pathogenetic mechanisms of tau mutations

Frontotemporal lobar degeneration (FTLD) is a group of heterogeneous neurodegenerative diseases which includes tauopathies. In the central nervous system (CNS) tau is the major microtubule-associated protein (MAP) of neurons, promoting assembly and stabilization of microtubules (MTs) required for morphogenesis and axonal transport. Primary tauopathies are characterized by deposition of abnormal fibrils of tau in neuronal and glial cells, leading to neuronal death, brain atrophy and eventually dementia. In genetic tauopathies mutations of tau gene impair the ability of tau to bind to MTs, alter the normal ratio among tau isoforms and favor fibril formation. Recently, additional functions have been ascribed to tau and different pathogenetic mechanisms are then emerging. In fact, a role of tau in DNA protection and genome stability has been reported and chromosome aberrations have been found associated with tau mutations. Furthermore, newly structurally and functionally characterized mutations have suggested novel pathological features, such as a tendency to form oligomeric rather than fibrillar aggregates. Tau mutations affecting axonal transport and plasma membrane interaction have also been described. In this article, we will review the pathogenetic mechanisms underlying tau mutations, focusing in particular on the less common aspects, so far poorly investigated.

The axonal cytoskeleton: from organization to function

The axon is the single long fiber that extends from the neuron and transmits electrical signals away from the cell body. The neuronal cytoskeleton, composed of microtubules (MTs), actin filaments and neurofilaments, is not only required for axon formation and axonal transport but also provides the structural basis for several specialized axonal structures, such as the axon initial segment (AIS) and presynaptic boutons. Emerging evidence suggest that the unique cytoskeleton organization in the axon is essential for its structure and integrity. In addition, the increasing number of neurodevelopmental and neurodegenerative diseases linked to defect in actin- and microtubule-dependent processes emphasizes the importance of a properly regulated cytoskeleton for normal axonal functioning. Here, we provide an overview of the current understanding of actin and microtubule organization within the axon and discuss models for the functional role of the cytoskeleton at specialized axonal structures.

Protein aggregation and neurodegeneration in prototypical neurodegenerative diseases: Examples of amyloidopathies, tauopathies and synucleinopathies

Alzheimer's and Parkinson's diseases are the most prevalent neurodegenerative diseases that generate important health-related direct and indirect socio-economic costs. They are characterized by severe neuronal losses in several disease-specific brain regions associated with deposits of aggregated proteins. In Alzheimer's disease, β-amyloid peptide-containing plaques and intraneuronal neurofibrillary tangles composed of hyperphosphorylated microtubule-associated protein tau are the two main neuropathological lesions, while Parkinson's disease is defined by the presence of Lewy Bodies that are intraneuronal proteinaceous cytoplasmic inclusions. α-Synuclein has been identified as a major protein component of Lewy Bodies and heavily implicated in the pathogenesis of Parkinson's disease. In the past few years, evidence has emerged to explain how these aggregate-prone proteins can undergo spontaneous self-aggregation, propagate from cell to cell, and mediate neurotoxicity. Current research now indicates that oligomeric forms are probably the toxic species. This article discusses recent progress in the understanding of the pathogenesis of these diseases, with a focus on the underlying mechanisms of protein aggregation, and emphasizes the pathophysiological molecular mechanisms leading to cellular toxicity. Finally, we present the putative direct link between β-amyloid peptide and tau in causing toxicity in Alzheimer's disease as well as α-synuclein in Parkinson's disease, along with some of the most promising therapeutic strategies currently in development for those incurable neurodegenerative disorders.

Menopause, obesity and inflammation: interactive risk factors for Alzheimer's disease

Alzheimer’s disease (AD) is a multifactorial neurodegenerative disorder, the development of which is regulated by several environmental and genetic risk factors. Two factors theorized to contribute to the initiation and/or progression of AD pathogenesis are age-related increases in inflammation and obesity. These factors may be particularly problematic in women. The onset of menopause in mid-life elevates the vulnerability of women to AD, an increased risk that is likely associated with the depletion of estrogens. Menopause is also linked with an abundance of additional changes, including increased central adiposity and inflammation. Here, we review the current literature to explore the interactions between obesity, inflammation, menopause and AD.

Preventive methylene blue treatment preserves cognition in mice expressing full-length pro-aggregant human Tau

Introduction

Neurofibrillary tangles (NFT) composed of Tau are hallmarks of neurodegeneration in Alzheimer disease. Transgenic mice expressing full-length pro-aggregant human Tau (2N4R Tau-ΔK280, termed Tau^ΔK) or its repeat domain (TauRD-ΔK280, TauRD^ΔK) develop a progressive Tau pathology with missorting, phosphorylation, aggregation of Tau, loss of synapses and functional deficits. Whereas TauRD^ΔK assembles into NFT concomitant with neuronal death, Tau^ΔK accumulates into Tau pretangles without overt neuronal loss. Both forms cause a comparable cognitive decline (with onset at 10mo and 12mo, respectively), which is rescued upon switch-off of transgene expression. Since methylene blue (MB) is able to inhibit Tau aggregation in vitro, we investigated whether MB can prevent or rescue Tau-induced cognitive impairments in our mouse models. Both types of mice received MB orally using different preventive and therapeutic treatment protocols, initiated either before or after disease onset. The cognitive status of the mice was assessed by behavior tasks (open field, Morris water maze) to determine the most successful conditions for therapeutic intervention.

Results

Preventive and therapeutic MB application failed to avert or recover learning and memory deficits of TauRD^ΔK mice. Similarly, therapeutic MB treatment initiated after onset of cognitive impairments was ineffective in Tau^ΔK mice. In contrast, preventive MB application starting before onset of functional deficits preserved cognition of Tau^ΔK mice. Beside improved learning and memory, MB-treated Tau^ΔK mice showed a strong decrease of insoluble Tau, a reduction of conformationally changed (MC1) and phosphorylated Tau species (AT180, PHF1) as well as an upregulation of protein degradation systems (autophagy and proteasome). This argues for additional pleiotropic effects of MB beyond its properties as Tau aggregation inhibitor.

Conclusions

Our data support the use of Tau aggregation inhibitors as potential drugs for the treatment of AD and other tauopathies and highlights the need for preventive treatment before onset of cognitive impairments.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0204-4) contains supplementary material, which is available to authorized users.

Human pluripotent stem cells as tools for neurodegenerative and neurodevelopmental disease modeling and drug discovery

INTRODUCTION

Although intensive efforts have been made, effective treatments for neurodegenerative and neurodevelopmental diseases have not been yet discovered. Possible reasons for this include the lack of appropriate disease models of human neurons and a limited understanding of the etiological and neurobiological mechanisms. Recent advances in pluripotent stem cell (PSC) research have now opened the path to the generation of induced pluripotent stem cells (iPSCs) starting from somatic cells, thus offering an unlimited source of patient-specific disease-relevant neuronal cells.

AREAS COVERED

In this review, the authors focus on the use of human PSC-derived cells in modeling neurological disorders and discovering of new drugs and provide their expert perspectives on the field.

EXPERT OPINION

The advent of human iPSC-based disease models has fuelled renewed enthusiasm and enormous expectations for insights of disease mechanisms and identification of more disease-relevant and novel molecular targets. Human PSCs offer a unique tool that is being profitably exploited for high-throughput screening (HTS) platforms. This process can lead to the identification and optimization of molecules/drugs and thus move forward new pharmacological therapies for a wide range of neurodegenerative and neurodevelopmental conditions. It is predicted that improvements in the production of mature neuronal subtypes, from patient-specific human-induced pluripotent stem cells and their adaptation to culture, to HTS platforms will allow the increased exploitation of human pluripotent stem cells in drug discovery programs.

Alzheimer's disease-related amyloid-β induces synaptotoxicity in human iPS cell-derived neurons

Human induced pluripotent stem cell (iPSC)-derived neurons have been proposed to be a highly valuable cellular model for studying the pathomechanisms of Alzheimer's disease (AD). Studies employing patient-specific human iPSCs as models of familial and sporadic forms of AD described elevated levels of AD-related amyloid-β (Aβ). However, none of the present AD iPSC studies could recapitulate the synaptotoxic actions of Aβ, which are crucial early events in a cascade that eventually leads to vast brain degeneration. Here we established highly reproducible, human iPSC-derived cortical cultures as a cellular model to study the synaptotoxic effects of Aβ. We developed a highly efficient immunopurification procedure yielding immature neurons that express markers of deep layer cortical pyramidal neurons and GABAergic interneurons. Upon long-term cultivation, purified cells differentiated into mature neurons exhibiting the generation of action potentials and excitatory glutamatergic and inhibitory GABAergic synapses. Most interestingly, these iPSC-derived human neurons were strongly susceptible to the synaptotoxic actions of Aβ. Application of Aβ for 8 days led to a reduction in the overall FM4–64 and vGlut1 staining of vesicles in neurites, indicating a loss of vesicle clusters. A selective analysis of presynaptic vesicle clusters on dendrites did not reveal a significant change, thus suggesting that Aβ impaired axonal vesicle clusters. In addition, electrophysiological patch-clamp recordings of AMPA receptor-mediated miniature EPSCs revealed an Aβ-induced reduction in amplitudes, indicating an impairment of postsynaptic AMPA receptors. A loss of postsynaptic AMPA receptor clusters was confirmed by immunocytochemical stainings for GluA1. Incubation with Aβ for 8 days did not result in a significant loss of neurites or cell death. In summary, we describe a highly reproducible cellular AD model based on human iPSC-derived cortical neurons that enables the mechanistic analysis of Aβ-induced synaptic pathomechanisms and the development of novel therapeutic approaches.

Conformational analysis of misfolded protein aggregation by FRET and live-cell imaging techniques

Cellular homeostasis is maintained by several types of protein machinery, including molecular chaperones and proteolysis systems. Dysregulation of the proteome disrupts homeostasis in cells, tissues, and the organism as a whole, and has been hypothesized to cause neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS) and Huntington's disease (HD). A hallmark of neurodegenerative disorders is formation of ubiquitin-positive inclusion bodies in neurons, suggesting that the aggregation process of misfolded proteins changes during disease progression. Hence, high-throughput determination of soluble oligomers during the aggregation process, as well as the conformation of sequestered proteins in inclusion bodies, is essential for elucidation of physiological regulation mechanism and drug discovery in this field. To elucidate the interaction, accumulation, and conformation of aggregation-prone proteins, in situ spectroscopic imaging techniques, such as Förster/fluorescence resonance energy transfer (FRET), fluorescence correlation spectroscopy (FCS), and bimolecular fluorescence complementation (BiFC) have been employed. Here, we summarize recent reports in which these techniques were applied to the analysis of aggregation-prone proteins (in particular their dimerization, interactions, and conformational changes), and describe several fluorescent indicators used for real-time observation of physiological states related to proteostasis.

Three-dimensional collagen type I matrix up-regulates nuclear isoforms of the microtubule associated protein tau implicated in resistance to paclitaxel therapy in ovarian carcinoma

Epithelial ovarian carcinoma is the deadliest gynecologic malignancy. One reason underlying treatment failure is resistance to paclitaxel. Expression of the microtubule associated protein tau has recently been proposed as a predictor of response to paclitaxel in ovarian carcinoma patients. Expression of tau was probed using immunohistochemistry in 312 specimens of primary, and 40 specimens of metastatic, ovarian carcinoma. Serous epithelial ovarian carcinoma cell line models were used to determine the expression of tau by Western blot and immunofluorescence staining. Subcellular fractionation and Western blot were employed to examine nuclear and cytoplasmic localization of tau. Gene silencing and clonogenic assays were used to evaluate paclitaxel response. Tau was expressed in 44% of all tested cases. Among the primary serous epithelial ovarian carcinoma cases, 46% were tau-positive. Among the metastatic serous epithelial ovarian carcinomas, 63% were tau-positive. Cell culture experiments demonstrated that tau was expressed in multiple isoforms. Three-dimensional collagen I matrix culture conditions resulted in up-regulation of tau protein. Silencing of tau with specific siRNAs in a combination with three-dimensional culture conditions led to a significant decrease of the clonogenic ability of cells treated with paclitaxel. The data suggest that reduction of tau expression may sensitize ovarian carcinoma to the paclitaxel treatment.

Amyloid β oligomers in Alzheimer's disease pathogenesis, treatment, and diagnosis

Protein aggregation is common to dozens of diseases including prionoses, diabetes, Parkinson's and Alzheimer's. Over the past 15 years, there has been a paradigm shift in understanding the structural basis for these proteinopathies. Precedent for this shift has come from investigation of soluble Aβ oligomers (AβOs), toxins now widely regarded as instigating neuron damage leading to Alzheimer's dementia. Toxic AβOs accumulate in AD brain and constitute long-lived alternatives to the disease-defining Aβ fibrils deposited in amyloid plaques. Key experiments using fibril-free AβO solutions demonstrated that while Aβ is essential for memory loss, the fibrillar Aβ in amyloid deposits is not the agent. The AD-like cellular pathologies induced by AβOs suggest their impact provides a unifying mechanism for AD pathogenesis, explaining why early stage disease is specific for memory and accounting for major facets of AD neuropathology. Alternative ideas for triggering mechanisms are being actively investigated. Some research favors insertion of AβOs into membrane, while other evidence supports ligand-like accumulation at particular synapses. Over a dozen candidate toxin receptors have been proposed. AβO binding triggers a redistribution of critical synaptic proteins and induces hyperactivity in metabotropic and ionotropic glutamate receptors. This leads to Ca(2+) overload and instigates major facets of AD neuropathology, including tau hyperphosphorylation, insulin resistance, oxidative stress, and synapse loss. Because different species of AβOs have been identified, a remaining question is which oligomer is the major pathogenic culprit. The possibility has been raised that more than one species plays a role. Despite some key unknowns, the clinical relevance of AβOs has been established, and new studies are beginning to point to co-morbidities such as diabetes and hypercholesterolemia as etiological factors. Because pathogenic AβOs appear early in the disease, they offer appealing targets for therapeutics and diagnostics. Promising therapeutic strategies include use of CNS insulin signaling enhancers to protect against the presence of toxins and elimination of the toxins through use of highly specific AβO antibodies. An AD-dependent accumulation of AβOs in CSF suggests their potential use as biomarkers and new AβO probes are opening the door to brain imaging. Overall, current evidence indicates that Aβ oligomers provide a substantive molecular basis for the cause, treatment and diagnosis of Alzheimer's disease.

Pseudophosphorylation of Tau at distinct epitopes or the presence of the P301L mutation targets the microtubule-associated protein Tau to dendritic spines

Alzheimer's disease is characterized by the accumulation of amyloid-β (Aβ) and Tau in the brain. In mature neurons, Tau is concentrated in the axon and found at lower levels in the dendrite where it is required for targeting Fyn to the spines. Here Fyn mediates Aβ toxicity, which is vastly abrogated when Tau is either deleted or a truncated form of Tau (Tau(1-255)) is co-expressed. Interestingly, MAP2, a microtubule-binding protein with mainly dendritic localization that shares Fyn-binding motifs with Tau, does not mediate Aβ's synaptic toxicity in the absence of Tau. Here we show in hippocampal neurons that endogenous Tau enters the entire spine, albeit at low levels, whereas MAP2 only enters its neck or is restricted to the dendritic shaft. Based on an extensive mutagenesis study, we also reveal that the spine localization of Tau is facilitated by deletion of the microtubule-binding repeat domain. When distinct phosphorylation sites (AT180-T231/S235, 12E8-S262/S356, PHF1-S396/S404) were pseudophosphorylated (with glutamic acid, using alanine replacements as controls), Tau targeting to spines was markedly increased, whereas the pseudophosphorylation of the late phospho-epitope S422 had no effect. In determining the role physiological Fyn has in the spine localization of Tau, we found that neither were endogenous Tau levels reduced in Fyn knockout compared with wild-type synaptosomal brain fractions nor was the spine localization of over-expressed pseudophosphorylated or P301L Tau. This demonstrates that although Fyn targeting to the spine is Tau dependent, elevated levels of phosphorylated Tau or P301L Tau can enter the spine in a Fyn-independent manner.

Temporally distinct phosphorylations differentiate Tau-dependent learning deficits and premature mortality in Drosophila

Abnormally phosphorylated Tau protein, the major component of neurofibrillary tangles, is critical in the pathogenesis of Alzheimer's disease and related Tauopathies. We used Drosophila to examine the role of key disease-associated phosphorylation sites on Tau-mediated neurotoxicity. We present evidence that the late-appearing phosphorylation on Ser(238) rather than hyperphosphorylation per se is essential for Tau toxicity underlying premature mortality in adult flies. This site is also occupied at the time of neurodegeneration onset in a mouse Tauopathy model and in damaged brain areas of confirmed Tauopathy patients, suggesting a similar critical role on Tau toxicity in humans. In contrast, occupation of Ser(262) is necessary for Tau-dependent learning deficits in adult Drosophila. Significantly, occupation of Ser(262) precedes and is required for Ser(238) phosphorylation, and these temporally distinct phosphorylations likely reflect conformational changes. Because sequential occupation of Ser(262) and Ser(238) is required for the progression from Tau-mediated learning deficits to premature mortality in Drosophila, they may also play similar roles in the escalating symptom severity in Tauopathy patients, congruent with their presence in damaged regions of their brains.

Olfactory Deprivation Hastens Alzheimer-Like Pathologies in a Human Tau-Overexpressed Mouse Model via Activation of cdk5

Olfactory dysfunction is a recognized risk factor for the pathogenesis of Alzheimer's disease (AD), while the mechanisms are still not clear. Here, we applied bilateral olfactory bulbectomy (OBX), an olfactory deprivation surgery to cause permanent anosmia, in human tau-overexpressed mice (htau mice) to investigate changes of AD-like pathologies including aggregation of abnormally phosphorylated tau and cholinergic neuron loss. We found that tau phosphorylation in hippocampus was increased at Thr-205, Ser-214, Thr-231, and Ser-396 after OBX. OBX also increased the level of sarkosyl-insoluble Tau at those epitopes and accelerated accumulation of somatodendritic tau. Moreover, OBX resulted in the elevation of calpain activity accompanied by an increased expression of the cyclin-dependent kinase 5 (cdk5) neuronal activators, p35 and p25, in hippocampus. Furthermore, OBX induces the loss of the cholinergic neurons in medial septal. Administration of cdk5 pharmacological inhibitor roscovitine into lateral ventricles suppressed tau hyperphosphorylation and mislocalization and restored the cholinergic neuron loss. These findings suggest that olfactory deprivation by OBX hastens tau pathology and cholinergic system impairment in htau mice possibly via activation of cdk5.