Selective phosphorylation of adult tau isoforms in mature hippocampal neurons exposed to fibrillar A beta

Lung endothelium, tau, and amyloids in health and disease

Lung endothelia in the arteries, capillaries, and veins are heterogeneous in structure and function. Lung capillaries in particular represent a unique vascular niche, with a thin yet highly restrictive alveolar-capillary barrier that optimizes gas exchange. Capillary endothelium surveys the blood while simultaneously interpreting cues initiated within the alveolus and communicated via immediately adjacent type I and type II epithelial cells, fibroblasts, and pericytes. This cell-cell communication is necessary to coordinate the immune response to lower respiratory tract infection. Recent discoveries identify an important role for the microtubule-associated protein tau that is expressed in lung capillary endothelia in the host-pathogen interaction. This endothelial tau stabilizes microtubules necessary for barrier integrity, yet infection drives production of cytotoxic tau variants that are released into the airways and circulation, where they contribute to end-organ dysfunction. Similarly, beta-amyloid is produced during infection. Beta-amyloid has antimicrobial activity, but during infection it can acquire cytotoxic activity that is deleterious to the host. The production and function of these cytotoxic tau and amyloid variants are the subject of this review. Lung-derived cytotoxic tau and amyloid variants are a recently discovered mechanism of end-organ dysfunction, including neurocognitive dysfunction, during and in the aftermath of infection.

Polyherbal and Multimodal Treatments: Kaempferol- and Quercetin-Rich Herbs Alleviate Symptoms of Alzheimer's Disease

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder impairing cognition and memory in the elderly. This disorder has a complex etiology, including senile plaque and neurofibrillary tangle formation, neuroinflammation, oxidative stress, and damaged neuroplasticity. Current treatment options are limited, so alternative treatments such as herbal medicine could suppress symptoms while slowing cognitive decline. We followed PRISMA guidelines to identify potential herbal treatments, their associated medicinal phytochemicals, and the potential mechanisms of these treatments. Common herbs, including Ginkgo biloba, Camellia sinensis, Glycyrrhiza uralensis, Cyperus rotundus, and Buplerum falcatum, produced promising pre-clinical results. These herbs are rich in kaempferol and quercetin, flavonoids with a polyphenolic structure that facilitate multiple mechanisms of action. These mechanisms include the inhibition of Aβ plaque formation, a reduction in tau hyperphosphorylation, the suppression of oxidative stress, and the modulation of BDNF and PI3K/AKT pathways. Using pre-clinical findings from quercetin research and the comparatively limited data on kaempferol, we proposed that kaempferol ameliorates the neuroinflammatory state, maintains proper cellular function, and restores pro-neuroplastic signaling. In this review, we discuss the anti-AD mechanisms of quercetin and kaempferol and their limitations, and we suggest a potential alternative treatment for AD. Our findings lead us to conclude that a polyherbal kaempferol- and quercetin-rich cocktail could treat AD-related brain damage.

The hidden players: Shedding light on the significance of post-translational modifications and miRNAs in Alzheimer's disease development

Alzheimer's disease (AD) is the most prevalent, expensive, lethal, and burdening neurodegenerative disease of this century. The initial stages of this disease are characterized by a reduced ability to encode and store new memories. Subsequent cognitive and behavioral deterioration occurs during the later stages. Abnormal cleavage of amyloid precursor protein (APP) resulting in amyloid-beta (Aβ) accumulation along with hyperphosphorylation of tau protein are the two characteristic hallmarks of AD. Recently, several post-translational modifications (PTMs) have been identified on both Aβ as well as tau proteins. However, a complete understanding of how different PTMs influence the structure and function of proteins in both healthy and diseased conditions is still lacking. It has been speculated that these PTMs might play vital roles in the progression of AD. In addition, several short non-coding microRNA (miRNA) sequences have been found to be deregulated in the peripheral blood of Alzheimer patients. The miRNAs are single-stranded RNAs that control gene expression by causing mRNA degradation, deadenylation, or translational repression and have been implicated in the regulation of several neuronal and glial activities. The lack of comprehensive understanding regarding disease mechanisms, biomarkers, and therapeutic targets greatly hampers the development of effective strategies for early diagnosis and the identification of viable therapeutic targets. Moreover, existing treatment options for managing the disease have proven to be ineffective and provide only temporary relief. Therefore, understanding the role of miRNAs and PTMs in AD can provide valuable insights into disease mechanisms, aid in the identification of biomarkers, facilitate the discovery of novel therapeutic targets, and inspire innovative treatments for this challenging condition.

Discovery of 2-(Anilino)pyrimidine-4-carboxamides as Highly Potent, Selective, and Orally Active Glycogen Synthase Kinase-3 (GSK-3) Inhibitors

Glycogen synthase kinase-3 (GSK-3) is a serine/threonine kinase that serves as an important regulator of a broad range of cellular functions. It has been linked to Alzheimer's disease as well as various other diseases, including mood disorders, type 2 diabetes, and cancer. There is considerable evidence indicating that GSK-3β in the central nervous system plays a role in the production of abnormal, hyperphosphorylated, microtubule-associated tau protein found in neurofibrillary tangles associated with Alzheimer's disease. A series of analogues containing a pyrimidine-based hinge-binding heterocycle was synthesized and evaluated, leading to the identification of highly potent GSK-3 inhibitors with excellent kinase selectivity. Further evaluation of 34 and 40 in vivo demonstrated that these compounds are orally bioavailable, brain-penetrant GSK-3 inhibitors that lowered levels of phosphorylated tau in a triple-transgenic mouse Alzheimer's disease model.

Homocysteine, Cognitive Functions, and Degenerative Dementias: State of the Art

There is strong evidence that homocysteine is a risk factor not only for cerebrovascular diseases but also for degenerative dementias. A recent consensus statement renewed the importance and the role of high levels of homocysteine in cognitive decline in several forms of degenerative dementia, such as Alzheimer's disease. Although the molecular mechanisms by which homocysteine causes cell dysfunction are known, both the impact of homocysteine on specific cognitive functions and the relationship between homocysteine level and non-Alzheimer dementias have been poorly investigated. Most of the studies addressing the impact of hyperhomocysteinemia on dementias have not examined the profile of performance across different cognitive domains, and have only relied on screening tests, which provide a very general and coarse-grained picture of the cognitive status of the patients. Yet, trying to understand whether hyperhomocysteinemia is associated with the impairment of specific cognitive functions would be crucial, as it would be, in parallel, learning whether some brain circuits are particularly susceptible to the damage caused by hyperhomocysteinemia. These steps would allow one to (i) understand the actual role of homocysteine in the pathogenesis of cognitive decline and (ii) improve the diagnostic accuracy, differential diagnosis and prognostic implications. This review is aimed at exploring and revising the state of the art of these two strictly related domains. Suggestions for future research are provided.

Populations of Tau Conformers Drive Prion-like Strain Effects in Alzheimer's Disease and Related Dementias

Recent findings of diverse populations of prion-like conformers of misfolded tau protein expand the prion concept to Alzheimer's disease (AD) and monogenic frontotemporal lobar degeneration (FTLD)-MAPT P301L, and suggest that distinct strains of misfolded proteins drive the phenotypes and progression rates in many neurodegenerative diseases. Notable progress in the previous decades has generated many lines of proof arguing that yeast, fungal, and mammalian prions determine heritable as well as infectious traits. The extraordinary phenotypic diversity of human prion diseases arises from structurally distinct prion strains that target, at different progression speeds, variable brain structures and cells. Although human prion research presents beneficial lessons and methods to study the mechanism of strain diversity of protein-only pathogens, the fundamental molecular mechanism by which tau conformers are formed and replicate in diverse tauopathies is still poorly understood. In this review, we summarize up to date advances in identification of diverse tau conformers through biophysical and cellular experimental paradigms, and the impact of heterogeneity of pathological tau strains on personalized structure- and strain-specific therapeutic approaches in major tauopathies.

MEK1/2 inhibition rescues neurodegeneration by TFEB-mediated activation of autophagic lysosomal function in a model of Alzheimer's Disease

Alzheimer's Disease (AD) is a progressive neurodegenerative disorder, which is characterized by cognitive deficit due to synaptic loss and neuronal death. Extracellular amyloid β plaques are one of the pathological hallmarks of AD. The autophagic lysosomal pathway is the essential mechanism to maintain cellular homeostasis by driving clearance of protein aggregates and is dysfunctional in AD. Here, we showed that inhibiting MEK/ERK signaling using a clinically available MEK1/2 inhibitor, trametinib (GSK1120212, SNR1611), induces the protection of neurons through autophagic lysosomal activation mediated by transcription factor EB (TFEB) in a model of AD. Orally administered trametinib recovered impaired neural structures, cognitive functions, and hippocampal long-term potentiation (LTP) in 5XFAD mice. Trametinib also reduced Aβ deposition via induction of autophagic lysosomal activation. RNA-sequencing analysis revealed upregulation of autophagic lysosomal genes by trametinib administration. In addition, trametinib inhibited TFEB phosphorylation at Ser142 and promoted its nuclear translocation, which in turn induced autophagic lysosomal related genes, indicating that trametinib activates the autophagic lysosomal process through TFEB activation. From these observations, we concluded that MEK inhibition provides neuronal protection from the Aβ burden by increasing autophagic lysosomal activity. Thus, MEK inhibition may be an effective therapeutic strategy for AD.

The formation of small aggregates contributes to the neurotoxic effects of tau45-230

Intracellular deposits of hyperphosphorylated tau are commonly detected in tauopathies. Furthermore, these aggregates seem to play an important role in the pathobiology of these diseases. In the present study, we determined whether the recently identified neurotoxic tau_45-230 fragment also formed aggregates in neurodegenerative disorders. The presence of such aggregates was examined in brain samples obtained from Alzheimer's disease (AD) subjects by means of Western blot analysis performed under non-denaturing conditions. Our results showed that a mixture of tau_45-230 oligomers of different sizes was easily detectable in brain samples obtained from AD subjects. Our data also suggested that tau_45-230 oligomers could be internalized by cultured hippocampal neurons, mainly through a clathrin-mediated mechanism, triggering their degeneration. In addition, in vitro aggregation studies showed that tau_45-230 modulated full-length tau aggregation thereby inducing the formation of smaller, and potentially more toxic, aggregates of this microtubule-associated protein. Together, these data identified alternative mechanisms underlying the toxic effects of tau_45-230.

Probiotics Fermentation Technology, a Novel Kefir Product, Ameliorates Cognitive Impairment in Streptozotocin-Induced Sporadic Alzheimer's Disease in Mice

Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive impairment. Gut microbiota dysfunction (dysbiosis) is implicated in the pathology of AD and is associated with several detrimental consequences, including neurotransmitter depletion, oxidative stress, inflammation, apoptosis, and insulin resistance, which all contribute to the onset of AD. The objective of this study was to assess the effectiveness of Probiotics Fermentation Technology (PFT), a kefir product, in alleviating AD symptoms via regulation of the gut microbiota using a streptozotocin- (STZ-) induced AD mouse model and to compare its activity with simvastatin, which has been proven to effectively treat AD. Mice received one intracerebroventricular injection of STZ (3 mg/kg). PFT (100, 300, 600 mg/kg) and simvastatin (20 mg/kg) were administered orally for 3 weeks. PFT supplementation mitigated STZ-induced neuronal degeneration in the cortex and hippocampus, restored hippocampal acetylcholine levels, and improved cognition in a dose-dependent manner. These effects were accompanied by reductions in oxidative damage, proinflammatory cytokine expression, apoptosis, and tau hyperphosphorylation. Moreover, PFT hindered amyloid plaque accumulation via the enhancement of insulin-degrading enzyme. These beneficial effects were comparable to those produced by simvastatin. The results suggest that PFT can alleviate AD symptoms by regulating the gut microbiota and by inhibiting AD-related pathological events.

Tau in the gut, does it really matter?

The enteric nervous system plays a critical role in the regulation of gastrointestinal tract functions and is often referred to as the 'second brain' because it shares many features with the central nervous system. These similarities include among others a large panel of neurotransmitters, a large population of glial cells and a susceptibility to neurodegeneration. This close homology between the central and enteric nervous systems suggests that a disease process affecting the central nervous system could also involve its enteric counterpart. This was already documented in Parkinson's disease, the most common synucleinopathy, in which alpha-synuclein deposits are reported in the enteric nervous system in the vast majority of patients. Tau is another key protein involved in neurodegenerative disorders of the brain. Whether changes in tau also occur in the enteric nervous system during gut or brain disorders has just begun to be explored. The scope of the present article is therefore to review existing studies on the expression and phosphorylation pattern of tau in the enteric nervous system under physiological and pathological conditions and to discuss the possible occurrence of 'enteric tauopathies'.

Homocysteine in Neurology: A Possible Contributing Factor to Small Vessel Disease

Homocysteine (Hcy) is a sulfur-containing amino acid generated during methionine metabolism, accumulation of which may be caused by genetic defects or the deficit of vitamin B12 and folate. A serum level greater than 15 micro-mols/L is defined as hyperhomocysteinemia (HHcy). Hcy has many roles, the most important being the active participation in the transmethylation reactions, fundamental for the brain. Many studies focused on the role of homocysteine accumulation in vascular or degenerative neurological diseases, but the results are still undefined. More is known in cardiovascular disease. HHcy is a determinant for the development and progression of inflammation, atherosclerotic plaque formation, endothelium, arteriolar damage, smooth muscle cell proliferation, and altered-oxidative stress response. Conversely, few studies focused on the relationship between HHcy and small vessel disease (SVD), despite the evidence that mice with HHcy showed a significant end-feet disruption of astrocytes with a diffuse SVD. A severe reduction of vascular aquaporin-4-water channels, lower levels of high-functioning potassium channels, and higher metalloproteinases are also observed. HHcy modulates the N-homocysteinylation process, promoting a pro-coagulative state and damage of the cellular protein integrity. This altered process could be directly involved in the altered endothelium activation, typical of SVD and protein quality, inhibiting the ubiquitin-proteasome system control. HHcy also promotes a constant enhancement of microglia activation, inducing the sustained pro-inflammatory status observed in SVD. This review article addresses the possible role of HHcy in small-vessel disease and understands its pathogenic impact.

Modulation of the Astrocyte-Neuron Lactate Shuttle System contributes to Neuroprotective action of Fibroblast Growth Factor 21

A viewpoint considering Alzheimer's disease (AD) as “type 3 diabetes” emphasizes the pivotal role of dysfunctional brain energy metabolism in AD. The hormone fibroblast growth factor 21 (FGF21) is a crucial regulator in energy metabolism; however, our understanding of the therapeutic potential and mechanisms underlying the effect of FGF21 on neurodegeneration of AD is far from complete.

Methods: To further elucidate the effect of FGF21 on AD-related neurodegeneration, we used APP/PS1 transgenic mice to assess the effects of FGF21 on memory dysfunction, amyloid plaque pathology and pathological tau hyperphosphorylation. We also established an in vitro system to mimic astrocyte-neuron communication and an in vivo model of acute injury. Based on the in vivo and in vitro models, we analyzed the neuroprotective actions of FGF21 and pathways related to astrocyte-neuron communication and further focused on the astrocyte-neuron lactate shuttle system.

Results: Here, we report that FGF21 can ameliorate Alzheimer-like neurodegeneration in APP/PS1 transgenic mice. We detected defects in the astrocyte-neuron lactate shuttle system in the in vivo and in vitro models of AD and identified FGF21 as a neuroprotective molecule that can rescue these deficits. Administration of FGF21 can alleviate memory dysfunction, amyloid plaque pathology and pathological tau hyperphosphorylation, and the function of FGF21 in neurodegeneration is mediated in part by monocarboxylate transporters (MCTs). In vivo evidence also suggests that FGF21 acts centrally in mice to exert its effects on neurodegeneration and energy metabolism via its regulation of MCTs.

Conclusions: These results suggest that FGF21 alters metabolic parameters to mediate its neuroprotective functions. Modulation of the astrocyte-neuron lactate shuttle system can be one of the most efficient strategies for FGF21 in Alzheimer-like degeneration and contributes to improvements in brain metabolic defects and amyloid β-induced cytotoxicity. Our findings provide insights into the mechanisms underlying the effects of FGF21 on neurodegeneration and brain energy metabolism and suggest that FGF21 may have therapeutic value in the treatment of AD and other neurodegenerative diseases.

Characterising the spatial and temporal brain metal profile in a mouse model of tauopathy

A dysregulation in the homeostasis of metals such as copper, iron and zinc is speculated to be involved in the pathogenesis of tauopathies, which includes Alzheimer's disease (AD). In particular, there is a growing body of evidence to support a role for iron in facilitating the hyperphosphorylation and aggregation of the tau protein into neurofibrillary tangles (NFTs) - a primary neuropathological hallmark of tauopathies. Therefore, the aim of this study was to characterize the spatial and temporal brain metallomic profile in a mouse model of tauopathy (rTg(tauP301L)4510), so as to provide some insight into the potential interaction between tau pathology and iron. Using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), our results revealed an age-dependent increase in brain iron levels in both WT and rTg(tauP301L)4510 mice. In addition, size exclusion chromatography-ICP-MS (SEC-ICP-MS) revealed significant age-related changes in iron bound to metalloproteins such as ferritin. The outcomes from this study may provide valuable insight into the inter-relationship between iron and tau in ageing and neurodegeneration.

Moderate Overexpression of Tau in Drosophila Exacerbates Amyloid-β-Induced Neuronal Phenotypes and Correlates with Tau Oligomerization

Alzheimer's disease (AD) is neuropathologically defined by two key hallmarks: extracellular senile plaques composed primarily of amyloid-β (Aβ) peptide and intraneuronal neurofibrillary tangles, containing abnormally hyperphosphorylated tau protein. The tau protein is encoded by the MAPT gene. Recently, the H1 and H2 haplotypes of the MAPT gene were associated with AD risk. The minor MAPT H2 haplotype has been linked with a decreased risk of developing late-onset AD (LOAD). MAPT haplotypes show different levels of MAPT/Tau expression with H1 being ∼1.5-fold more expressed than H2, suggesting that MAPT expression level could be related to LOAD risk. In this study, we investigated whether this moderate difference in MAPT/Tau expression could influence Aβ-induced toxicity in vivo. We show that modest overexpression of tau protein in Drosophila exacerbates neuronal phenotypes in AβPP/BACE1 flies. The exacerbation of neuronal defects correlates with the accumulation of insoluble dTau oligomers, suggesting that the moderate difference in level of tau expression observed between H1 and H2 haplotypes could influence Aβ toxicity through the production of oligomeric tau insoluble species.

B Vitamins and Fatty Acids: What Do They Share with Small Vessel Disease-Related Dementia?

Many studies have been written on vitamin supplementation, fatty acid, and dementia, but results are still under debate, and no definite conclusion has yet been drawn. Nevertheless, a significant amount of lab evidence confirms that vitamins of the B group are tightly related to gene control for endothelium protection, act as antioxidants, play a co-enzymatic role in the most critical biochemical reactions inside the brain, and cooperate with many other elements, such as choline, for the synthesis of polyunsaturated phosphatidylcholine, through S-adenosyl-methionine (SAM) methyl donation. B-vitamins have anti-inflammatory properties and act in protective roles against neurodegenerative mechanisms, for example, through modulation of the glutamate currents and a reduction of the calcium currents. In addition, they also have extraordinary antioxidant properties. However, laboratory data are far from clinical practice. Many studies have tried to apply these results in everyday clinical activity, but results have been discouraging and far from a possible resolution of the associated mysteries, like those represented by Alzheimer's disease (AD) or small vessel disease dementia. Above all, two significant problems emerge from the research: No consensus exists on general diagnostic criteria-MCI or AD? Which diagnostic criteria should be applied for small vessel disease-related dementia? In addition, no general schema exists for determining a possible correct time of implementation to have effective results. Here we present an up-to-date review of the literature on such topics, shedding some light on the possible interaction of vitamins and phosphatidylcholine, and their role in brain metabolism and catabolism. Further studies should take into account all of these questions, with well-designed and world-homogeneous trials.

β-Amyloid Induces Pathology-Related Patterns of Tau Hyperphosphorylation at Synaptic Terminals

A synergy between β-amyloid (Aβ) and tau appears to occur in Alzheimer disease (AD), but the mechanisms of interaction, and potential locations, are little understood. This study investigates the possibility of such interactions within the cortical synaptic compartments of APP/PS1 mice. We used label-free quantitative mass spectrometry to study the phosphoproteome of synaptosomes, covering 2400 phosphopeptides and providing an unbiased survey of phosphorylation changes associated with amyloid pathology. Hyperphosphorylation was detected on 36 synaptic proteins, many of which are associated with the cytoskeleton. Importantly, tau is one of the most hyperphosphorylated proteins at the synapse, upregulated at both proline-directed kinase (PDK) sites (S199/S202, S396/S404) and nonPDK sites (S400). These PDK sites correspond to well-known pathological tau epitopes in AD patients, recognized by AT8 and PHF-1 antibodies, respectively. Hyperphosphorylation at S199/S202, a rarely examined combination, was further validated in patient-derived human synaptosomes by immunoblotting. Global surveys of upregulated phosphosites revealed 2 potential kinase motifs, which resemble those of cyclin-dependent kinase 5 (CDK5, a PDK) and casein kinase II (CK2, a nonPDK). Our data demonstrate that, within synaptic compartments, amyloid pathology is associated with tau hyperphosphorylation at disease-relevant epitopes. This provides a plausible mechanism by which Aβ promotes the spreading of tauopathy.

Altered Cytoskeletal Composition and Delayed Neurite Elongation in tau45-230-Expressing Hippocampal Neurons

Calpain-mediated tau cleavage into the neurotoxic tau_45-230 fragment plays an important role in Alzheimer's disease (AD). This tau fragment accumulates mainly in the cytoplasm of degenerating neurons. However, subcellular localization studies indicated that a pool of tau_45-230 associates with the cytoskeleton in hippocampal neurons. In the present study, we assessed whether such localization could underlie tau_45-230 neurotoxic effects. Quantitative Western blot analysis showed decreased levels of full-length tau bound to microtubules in tau_45-230-expressing hippocampal neurons when compared to controls. In addition, the presence of this tau fragment induced a transient increase in tyrosinated tubulin, a marker of unstable microtubules, followed by a significant decrease in the levels of this tubulin isoform. The data obtained also showed a significant reduction in actin filaments in tau_45-230-expressing neurons. These changes in microtubules and actin filaments correlated with delayed neurite elongation and axonal differentiation in the presence of this tau fragment. Together, these results suggest that tau_45-230 could exert its toxic effects, at least in part, by modifying the composition of the neuronal cytoskeleton and impairing neurite elongation in neurons undergoing degeneration.

The Controversial Role of Homocysteine in Neurology: From Labs to Clinical Practice

Homocysteine (Hcy) is a sulfur-containing amino acid that is generated during methionine metabolism. Physiologic Hcy levels are determined primarily by dietary intake and vitamin status. Elevated plasma levels of Hcy can be caused by deficiency of either vitamin B12 or folate. Hyperhomocysteinemia (HHcy) can be responsible of different systemic and neurological disease. Actually, HHcy has been considered as a risk factor for systemic atherosclerosis and cardiovascular disease (CVD) and HHcy has been reported in many neurologic disorders including cognitive impairment and stroke, independent of long-recognized factors such as hyperlipidemia, hypertension, diabetes mellitus, and smoking. HHcy is typically defined as levels >15 micromol/L. Treatment of hyperhomocysteinemia with folic acid and B vitamins seems to be effective in the prevention of the development of atherosclerosis, CVD, and strokes. However, data from literature show controversial results regarding the significance of homocysteine as a risk factor for CVD and stroke and whether patients should be routinely screened for homocysteine. HHcy-induced oxidative stress, endothelial dysfunction, inflammation, smooth muscle cell proliferation, and endoplasmic reticulum (ER) stress have been considered to play an important role in the pathogenesis of several diseases including atherosclerosis and stroke. The aim of our research is to review the possible role of HHcy in neurodegenerative disease and stroke and to understand its pathogenesis.

MicroRNA-132 provides neuroprotection for tauopathies via multiple signaling pathways

MicroRNAs (miRNA) regulate fundamental biological processes, including neuronal plasticity, stress response, and survival. Here, we describe a neuroprotective function of miR-132, the miRNA most significantly downregulated in neurons in Alzheimer’s disease. We demonstrate that miR-132 protects primary mouse and human wild-type neurons and more vulnerable Tau-mutant neurons against amyloid β-peptide (Aβ) and glutamate excitotoxicity. It lowers the levels of total, phosphorylated, acetylated, and cleaved forms of Tau implicated in tauopathies, promotes neurite elongation and branching, and reduces neuronal death. Similarly, miR-132 attenuates PHF-Tau pathology and neurodegeneration, and enhances long-term potentiation in the P301S Tau transgenic mice. The neuroprotective effects are mediated by direct regulation of the Tau modifiers acetyltransferase EP300, kinase GSK3β, RNA-binding protein Rbfox1, and proteases Calpain 2 and Caspases 3/7. These data suggest miR-132 as a master regulator of neuronal health and indicate that miR-132 supplementation could be of therapeutic benefit for the treatment of Tau-associated neurodegenerative disorders.

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.

Tau45-230 association with the cytoskeleton and membrane-bound organelles: Functional implications in neurodegeneration

The dysregulation of posttranslational modifications of the microtubule-associated protein (MAP) tau plays a key role in Alzheimer's disease (AD) and related disorders. Thus, we have previously shown that beta amyloid (Aβ)-induced neurotoxicity was mediated, at least in part, by tau cleavage into the tau_45-230 fragment. However, the mechanisms underlying the toxicity of tau_45-230 remain unknown. To get insights into such mechanisms, we first determined the subcellular localization of this tau fragment in hippocampal neurons. Tau_45-230 was easily detectable in cell bodies and processes extended by these neurons. In addition, cell extraction experiments performed using Triton X-100 and saponin showed that a pool of tau_45-230 was associated with the cytoskeleton and the cytoskeleton plus membrane-bound organelles, respectively, in cultured hippocampal neurons. Furthermore, they suggested that these associations were independent of the presence of full-length tau. We also assessed whether this tau fragment could alter axonal transport. Our results indicated that tau_45-230 significantly reduced the number of organelles transported along hippocampal axons. This altered axonal transport did not correlate with changes in the total number of organelles present in these cells or in motor protein levels. Together these results suggested that tau_45-230 could exert its toxic effects by partially blocking axonal transport along microtubules thus contributing to the early pathology of AD.

Physiological and Pathophysiological Implications of Synaptic Tau

Tauopathies encompass a broad range of neurodegenerative diseases featuring extensive neuronal death and cognitive decline. However, research over the past 30 years has failed to significantly advance our understanding of how tau causes dementia, limiting the design of rational therapeutics. It has become evident that we need to expand our understanding of tau in physiology, in order to delineate how tau may contribute to pathology. This review discusses recent evidence that has uncovered a novel aspect of tau function, based on its previously uncharacterized localization to the synapse. Here, multiple streams of evidence support a critical role for synaptic tau in the regulation of synapse physiology. In particular, long-term depression, a form of synaptic weakening, is dependent on the presence of tau in hippocampal neurons. The regulation of tau by specific phosphorylation events downstream of GSK-3β activation appears to be integral to this signaling role. We also describe how the regulation of synapse physiology by tau and its phosphorylation may inform our understanding of tauopathies and comorbid diseases. This work should provide a platform for future tau biology research in addition to therapeutic design.

Stabilization of Microtubule-Unbound Tau via Tau Phosphorylation at Ser262/356 by Par-1/MARK Contributes to Augmentation of AD-Related Phosphorylation and Aβ42-Induced Tau Toxicity

Abnormal accumulation of the microtubule-interacting protein tau is associated with neurodegenerative diseases including Alzheimer’s disease (AD). β-amyloid (Aβ) lies upstream of abnormal tau behavior, including detachment from microtubules, phosphorylation at several disease-specific sites, and self-aggregation into toxic tau species in AD brains. To prevent the cascade of events leading to neurodegeneration in AD, it is essential to elucidate the mechanisms underlying the initial events of tau mismetabolism. Currently, however, these mechanisms remain unclear. In this study, using transgenic Drosophila co-expressing human tau and Aβ, we found that tau phosphorylation at AD-related Ser262/356 stabilized microtubule-unbound tau in the early phase of tau mismetabolism, leading to neurodegeneration. Aβ increased the level of tau detached from microtubules, independent of the phosphorylation status at GSK3-targeted SP/TP sites. Such mislocalized tau proteins, especially the less phosphorylated species, were stabilized by phosphorylation at Ser262/356 via PAR-1/MARK. Levels of Ser262 phosphorylation were increased by Aβ42, and blocking this stabilization of tau suppressed Aβ42-mediated augmentation of tau toxicity and an increase in the levels of tau phosphorylation at the SP/TP site Thr231, suggesting that this process may be involved in AD pathogenesis. In contrast to PAR-1/MARK, blocking tau phosphorylation at SP/TP sites by knockdown of Sgg/GSK3 did not reduce tau levels, suppress tau mislocalization to the cytosol, or diminish Aβ-mediated augmentation of tau toxicity. These results suggest that stabilization of microtubule-unbound tau by phosphorylation at Ser262/356 via the PAR-1/MARK may act in the initial steps of tau mismetabolism in AD pathogenesis, and that such tau species may represent a potential therapeutic target for AD.

Alzheimer’s disease (AD) is the most common cause of dementia resulting from progressive neuron loss. Two proteins, β-amyloid (Aβ) and tau, accumulate in AD brains and are involved in AD pathogenesis. In healthy neurons, tau binds to microtubules to regulate its stability; in AD brains, however, tau is detached from microtubules and phosphorylated at multiple sites. Such abnormal tau behavior, which is likely to be triggered by Aβ, results in generation of pathological tau species that mediate neuron loss. However, the detailed mechanisms underlying this event remain incompletely understood. Using transgenic flies expressing human tau and Aβ as a model system, we found that tau phosphorylation at specific AD-related sites stabilized microtubule-unbound tau in the early phase of tau mismetabolism to generate toxic tau species. Moreover, this process is critical for Aβ to promote subsequent tau phosphorylation and neurodegeneration. Our results reveal a critical step in the initiation of tau mismetabolism, and this process may represent a potential therapeutic target for AD.

Characterization of Neuronal Tau Protein as a Target of Extracellular Signal-regulated Kinase

Tau neuronal protein has a central role in neurodegeneration and is implicated in Alzheimer disease development. Abnormal phosphorylation of Tau impairs its interaction with other proteins and is associated with its dysregulation in pathological conditions. Molecular mechanisms leading to hyperphosphorylation of Tau in pathological conditions are unknown. Here, we characterize phosphorylation of Tau by extracellular-regulated kinase (ERK2), a mitogen-activated kinase (MAPK) that responds to extracellular signals. Analysis ofin vitrophosphorylated Tau by activated recombinant ERK2 with nuclear magnetic resonance spectroscopy (NMR) reveals phosphorylation of 15 Ser/Thr sites.In vitrophosphorylation of Tau using rat brain extract and subsequent NMR analysis identifies the same sites. Phosphorylation with rat brain extract is known to transform Tau into an Alzheimer disease-like state. Our results indicate that phosphorylation of Tau by ERK2 alone is sufficient to produce the same characteristics. We further investigate the mechanism of ERK2 phosphorylation of Tau. Kinases are known to recognize their protein substrates not only by their specificity for a targeted Ser or Thr phosphorylation site but also by binding to linear-peptide motifs called docking sites. We identify two main ERK2 docking sites in Tau sequence using NMR. Our results suggest that ERK2 dysregulation in Alzheimer disease could lead to abnormal phosphorylation of Tau resulting in the pathology of the disease.

Three dimensions of the amyloid hypothesis: time, space and 'wingmen'

The amyloid hypothesis, which has been the predominant framework for research in Alzheimer's disease (AD), has been the source of considerable controversy. The amyloid hypothesis postulates that amyloid-β peptide (Aβ) is the causative agent in AD. It is strongly supported by data from rare autosomal dominant forms of AD. However, the evidence that Aβ causes or contributes to age-associated sporadic AD is more complex and less clear, prompting criticism of the hypothesis. We provide an overview of the major arguments for and against the amyloid hypothesis. We conclude that Aβ likely is the key initiator of a complex pathogenic cascade that causes AD. However, we argue that Aβ acts primarily as a trigger of other downstream processes, particularly tau aggregation, which mediate neurodegeneration. Aβ appears to be necessary, but not sufficient, to cause AD. Its major pathogenic effects may occur very early in the disease process.

Growth factor treatment to overcome Alzheimer's dysfunctional signaling

The number of people suffering from Alzheimer's disease (AD) will increase as the world population ages, creating a huge socio-economic burden. The three pathophysiological hallmarks of AD are the cholinergic system dysfunction, the β-amyloid peptide deposition and the Tau protein hyperphosphorylation. Current treatments have only transient effects and each tends to concentrate on a single pathophysiological aspect of AD. This review first provides an overall view of AD in terms of its pathophysiological symptoms and signaling dysfunction. We then examine the therapeutic potential of growth factors (GFs) by showing how they can overcome the dysfunctional cell signaling that occurs in AD. Finally, we discuss new alternatives to GFs that help overcome the problem of brain uptake, such as small peptides, with evidence from some of our unpublished data on human neuronal cell line.

Selective acetyl- and butyrylcholinesterase inhibitors reduce amyloid-β ex vivo activation of peripheral chemo-cytokines from Alzheimer's disease subjects: exploring the cholinergic anti-inflammatory pathway

Increasing evidence suggests that elevated production and/or reduced clearance of amyloid-β peptide (Aβ) drives the early pathogenesis of Alzheimer's disease (AD). Aβ soluble oligomers trigger a neurotoxic cascade that leads to neuronal dysfunction, neurodegeneration and, ultimately, clinical dementia. Inflammation, both within brain and systemically, together with a deficiency in the neurotransmitter acetylcholine (ACh) that underpinned the development of anticholinesterases for AD symptomatic treatment, are invariable hallmarks of the disease. The inter-relation between Aβ, inflammation and cholinergic signaling is complex, with each feeding back onto the others to drive disease progression. To elucidate these interactions plasma samples and peripheral blood mononuclear cells (PBMCs) were evaluated from healthy controls (HC) and AD patients. Plasma levels of acetylcholinesterase (AChE), butyrylcholinesterase (BuChE) and Aβ were significantly elevated in AD vs. HC subjects, and ACh showed a trend towards reduced levels. Aβ challenge of PBMCs induced a greater release of inflammatory cytokines interleukin-1β (IL-1β), monocyte chemotactic protein-1 (MCP-1) and tumor necrosis factor-alpha (TNF-α) from AD vs. HC subjects, with IL-10 being similarly affected. THP-1 monocytic cells, a cell culture counterpart of PBMCs and brain microglial cells, responded similarly to Aβ as well as to phytohaemagglutinin (PHA) challenge, to allow preliminary analysis of the cellular and molecular pathways underpinning Aβ-induced changes in cytokine expression. As amyloid-β precursor protein expression, and hence Aβ, has been reported regulated by particular cytokines and anticholinesterases, the latter were evaluated on Aβ- and PHA-induced chemocytokine expression. Co-incubation with selective AChE/BuChE inhibitors, (-)-phenserine (AChE) and (-)-cymserine analogues (BuChE), mitigated the rise in cytokine levels and suggest that augmentation of the cholinergic anti-inflammatory pathway may prove valuable in AD.

The Ubiquitin-Proteasome System and Molecular Chaperone Deregulation in Alzheimer's Disease

One of the shared hallmarks of neurodegenerative diseases is the accumulation of misfolded proteins. Therefore, it is suspected that normal proteostasis is crucial for neuronal survival in the brain and that the malfunction of this mechanism may be the underlying cause of neurodegenerative diseases. The accumulation of amyloid plaques (APs) composed of amyloid-beta peptide (Aβ) aggregates and neurofibrillary tangles (NFTs) composed of misfolded Tau proteins are the defining pathological markers of Alzheimer's disease (AD). The accumulation of these proteins indicates a faulty protein quality control in the AD brain. An impaired ubiquitin-proteasome system (UPS) could lead to negative consequences for protein regulation, including loss of function. Another pivotal mechanism for the prevention of misfolded protein accumulation is the utilization of molecular chaperones. Molecular chaperones, such as heat shock proteins (HSPs) and FK506-binding proteins (FKBPs), are highly involved in protein regulation to ensure proper folding and normal function. In this review, we elaborate on the molecular basis of AD pathophysiology using recent data, with a particular focus on the role of the UPS and molecular chaperones as the defensive mechanism against misfolded proteins that have prion-like properties. In addition, we propose a rational therapy approach based on this mechanism.

Recapitulating amyloid β and tau pathology in human neural cell culture models: clinical implications

The "amyloid β hypothesis" of Alzheimer's disease (AD) has been the reigning hypothesis explaining pathogenic mechanisms of AD over the last two decades. However, this hypothesis has not been fully validated in animal models, and several major unresolved issues remain. We recently developed a human neural cell culture model of AD based on a three-dimensional (3D) cell culture system. This unique, cellular model recapitulates key events of the AD pathogenic cascade, including β-amyloid plaques and neurofibrillary tangles. Our 3D human neural cell culture model system provides a premise for a new generation of cellular AD models that can serve as a novel platform for studying pathogenic mechanisms and for high-throughput drug screening in a human brain-like environment.

Models of β-amyloid induced Tau-pathology: the long and "folded" road to understand the mechanism

The amyloid cascade hypothesis has been the prevailing hypothesis in Alzheimer’s Disease research, although the final and most wanted proof i.e. fully successful anti-amyloid clinical trials in patients, is still lacking. This may require a better in depth understanding of the cascade. Particularly, the exact toxic forms of Aβ and Tau, the molecular link between them and their respective contributions to the disease process need to be identified in detail. Although the lack of final proof has raised substantial criticism on the hypothesis per se, accumulating experimental evidence in in vitro models, in vivo models and from biomarkers analysis in patients supports the amyloid cascade and particularly Aβ-induced Tau-pathology, which is the focus of this review. We here discuss available models that recapitulate Aβ-induced Tau-pathology and review some potential underlying mechanisms. The availability and diversity of these models that mimic the amyloid cascade partially or more complete, provide tools to study remaining questions, which are crucial for development of therapeutic strategies for Alzheimer’s Disease.

Interactions between amyloid-β and Tau fragments promote aberrant aggregates: implications for amyloid toxicity

We have investigated at the oligomeric level interactions between Aβ(25–35) and Tau(273–284), two important fragments of the amyloid-β and Tau proteins, implicated in Alzheimer’s disease. We are able to directly observe the coaggregation of these two peptides by probing the conformations of early heteroligomers and the macroscopic morphologies of the aggregates. Ion-mobility experiment and theoretical modeling indicate that the interactions of the two fragments affect the self-assembly processes of both peptides. Tau(273–284) shows a high affinity to form heteroligomers with existing Aβ(25–35) monomer and oligomers in solution. The configurations and characteristics of the heteroligomers are determined by whether the population of Aβ(25–35) or Tau(273–284) is dominant. As a result, two types of aggregates are observed in the mixture with distinct morphologies and dimensions from those of pure Aβ(25–35) fibrils. The incorporation of some Tau into β-rich Aβ(25–35) oligomers reduces the aggregation propensity of Aβ(25–35) but does not fully abolish fibril formation. On the other hand, by forming complexes with Aβ(25–35), Tau monomers and dimers can advance to larger oligomers and form granular aggregates. These heteroligomers may contribute to toxicity through loss of normal function of Tau or inherent toxicity of the aggregates themselves.

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

Tau is a microtubule-associated-protein that is sorted into neuronal axons in physiological conditions. In Alzheimer disease (AD) and other tauopathies, Tau sorting mechanisms fail and Tau becomes missorted into the somatodendritic compartment. In AD, aberrant amyloid-β (Aβ) production might trigger Tau missorting. The physiological axonal sorting of Tau depends on the developmental stage of the neuron, the phosphorylation state of Tau and the microtubule cytoskeleton. Disease-associated missorting of Tau is connected to increased phosphorylation and aggregation of Tau, and impaired microtubule interactions. Disease-causing mechanisms involve impaired transport, aberrant kinase activation, non-physiological interactions of Tau, and prion-like spreading. In this review we focus on the physiological and pathological (mis)sorting of Tau, the underlying mechanisms, and effects in disease.

Neuronal degeneration, synaptic defects, and behavioral abnormalities in tau₄₅₋₂₃₀ transgenic mice

The complement of mechanisms underlying tau pathology in neurodegenerative disorders has yet to be elucidated. Among these mechanisms, abnormal tau phosphorylation has received the most attention because neurofibrillary tangles present in Alzheimer's disease (AD) and related disorders known as tauopathies are composed of hyperphosphorylated forms of this microtubule-associated protein. More recently, we showed that calpain-mediated cleavage leading to the generation of the 17kDa tau₄₅₋₂₃₀ fragment is a conserved mechanism in these diseases. To obtain insights into the role of this fragment in neurodegeneration, we generated transgenic mice that express tau₄₅₋₂₃₀ and characterized their phenotype. Our results showed a significant increase in cell death in the hippocampal pyramidal cell layer of transgenic tau₄₅₋₂₃₀ mice when compared to wild-type controls. In addition, significant synapse loss was detected as early as six months after birth in transgenic hippocampal neurons. These synaptic changes were accompanied by alterations in the expression of the N-methyl-d-aspartate glutamate (NMDA) receptor subunits. Furthermore, functional abnormalities were detected in the transgenic mice using Morris Water Maze and fear conditioning tests. These results suggest that the accumulation of tau₄₅₋₂₃₀ is responsible, at least in part, for neuronal degeneration and some behavioral changes in AD and other tauopathies. Collectively, these data provide the first direct evidence of the toxic effects of a tau fragment biologically produced in the context of these diseases in vertebrate neurons that develop in situ.

Severe diffuse axon injury in chronic alcoholic rat medulla oblongata following a concussion blow

AIMS

We investigated the axonal morphological changes and expression of both tau protein and β-APP following concussion to the medulla oblongata, in a rat model of chronic alcoholism.

METHODS

Fifty-nine male Sprague-Dawley rats were randomly divided into EtOH, EtOH-TBI and control groups (water group, water-TBI group). To establish chronic alcoholic rats, rats were intragastrically given edible spirituous liquor twice daily. Rats also received a blow on the occipital tuberosity with an iron pendulum. Morphological changes and expression of tau and β-APP proteins in the medulla oblongata were examined.

RESULTS

(a) Nerve fibre thickening and twisting were observed in alcoholic rats, with nerve fibre changes becoming more significant following a concussion blow, which leads to some nerve fibres fracturing. (b) Transmission electron microscopy revealed that the nerve fibre myelin became loosened and displayed lamellar separation, which became more significant following concussion. (c) The integral optical density (IOD) sum value of β-APP of the EtOH-TBI group was lower than that in the EtOH group (P < 0.05); the Tau IOD sum value of the EtOH-TBI group was higher than that in the EtOH group (P < 0.05).

CONCLUSION

(a) Chronic alcoholism caused nerve fibre and neuronal morphology damage in the rat medulla oblongata, with structural damage becoming more significant following concussion. (b) Concussion changed the expression of β-APP and tau protein in chronic alcoholic rat medulla oblongata, suggesting that chronic alcoholism can lead to severe axonal injury following a concussion blow. (c) The effect of chronic alcoholism may be synergistic the concussion blow to promote animal injury and death.

The Aβ₁₋₄₂ peptide regulates microtubule stability independently of tau

Interference with microtubule stability by beta-amyloid peptide (Aβ) has been shown to disrupt dendritic function and axonal trafficking, both early events in Alzheimer's disease. However, it is unclear whether Aβ regulation of microtubule dynamics can occur independently of its action on tau. RhoA has been implicated in neurotoxicity by Aβ but the mechanism by which this activation generates cytoskeletal changes is also unclear. We found that oligomeric Aβ1-42 induced the formation of stable detyrosinated microtubules in NIH3T3 cells and this function resulted from the activation of a RhoA-dependent microtubule stabilization pathway regulated by integrin signaling and the formin mDia1. Induction of microtubule stability by Aβ was also initiated by dimerization of APP and required caspase activity, two previously characterized regulators of neurotoxicity downstream of Aβ. Finally, we found that this function was conserved in primary neurons and abolished by Rho inactivation, reinforcing a link between induction of stable detyrosinated microtubules and neuropathogenesis by Aβ. Our study reveals a novel activity of Aβ on the microtubule cytoskeleton that is independent of tau and associated with pathways linked to microtubule stabilization and Aβ-mediated neurotoxicity.

Design, synthesis, and biological evaluation of 1-phenylpyrazolo[3,4-e]pyrrolo[3,4-g]indolizine-4,6(1H,5H)-diones as new glycogen synthase kinase-3β inhibitors

Compound 5 was selected from our in-house library as a suitable starting point for the rational design of new GSK-3β inhibitors. MC/FEP calculations of 5 led to the identification of a structural class of new GSK-3β inhibitors. Compound 18 inhibited GSK-3β with an IC50 of 0.24 μM and inhibited tau phosphorylation in a cell-based assay. It proved to be a selective inhibitor of GSK-3 against a panel of 17 kinases and showed >10-fold selectivity against CDK2. Calculated physicochemical properties and Volsurf predictions suggested that compound 18 has the potential to diffuse passively across the blood-brain barrier.

Proteasome inhibition induces stress kinase dependent transport deficits--implications for Alzheimer's disease

Alzheimer's disease (AD) is characterized by accumulation of two misfolded and aggregated proteins, β-amyloid and hyperphosphorylated tau. Both cellular systems responsible for clearance of misfolded and aggregated proteins, the lysosomal and the proteasomal, have been shown to be malfunctioning in the aged brain and more so in patients with neurodegenerative diseases, including AD. This malfunction could be contributing to β-amyloid and tau accumulation, eventually aggregating in plaques and tangles. We have investigated the impact of decreased proteasome activity on tau phosphorylation as well as on microtubule stability and transport. To do this, we used our recently developed neuronal model where human SH-SY5Y cells obtain neuronal morphology and function through differentiation. We found that exposure to low doses of the proteasome inhibitor MG-115 caused tau phosphorylation, microtubule destabilization and disturbed neuritic transport. Furthermore, reduced proteasome activity activated several proteins implicated in tau phosphorylation and AD pathology, including c-Jun N-terminal kinase, c-Jun and extracellular signal-regulated protein kinase (ERK) 1/2. Restoration of the microtubule transport was achieved by inhibiting ERK 1/2 activation, and simultaneous inhibition of both ERK 1/2 and c-Jun reversed the proteasome inhibition-induced tau phosphorylation. Taken together, this study suggests that a decrease in proteasome activity can, through activation of c-Jun and ERK 1/2, result in several events related to neurodegenerative diseases. Restoration of proteasome activity or modulation of ERK 1/2 and c-Jun function can open new treatment possibilities against neurodegenerative diseases such as AD.

Neurofibrillary tangle-like tau pathology induced by synthetic tau fibrils in primary neurons over-expressing mutant tau

Increasing evidence demonstrates the transmissibility of fibrillar species of tau protein, but this has never been directly tested in neurons, the cell type most affected by formation of tau inclusions in neurodegenerative tauopathies. Here we show that synthetic tau fibrils made from recombinant protein not only time-dependently recruit normal tau into neurofibrillary tangle-like insoluble aggregates in primary hippocampal neurons over-expressing human tau, but also induce neuritic tau pathology in non-transgenic neurons. This study provides highly compelling support for the protein-only hypothesis of pathological tau transmission in primary neurons and describes a useful neuronal model for studying the pathogenesis of tauopathies.

Focal expression of adeno-associated viral-mutant tau induces widespread impairment in an APP mouse model

Adeno-associated virus serotype 6 (AAV6) viral vectors encoding mutant and normal tau were used to produce focal tau pathology. Two mutant forms of tau were used; the P301S tau mutation is associated with neurofibrillary tangle formation in humans, and the 3PO mutation leads to rapid tau aggregation and is associated with pathogenic phosphorylation and cytotoxicity in vitro. We show that adeno-associated viral injection into entorhinal cortex of normal and tau knockout animals leads to local overexpression of tau, and the presence of human tau in axons projecting to and emanating from the entorhinal cortex. Starting at 2 months and increasing by 6 months post-injection neurons expressing mutant tau developed hyperphosphorylated tau pathology, in addition to dystrophic neurites. There was neuronal loss in tau-expressing regions, which was similar in normal and in TASTPM mice injected with mutant tau. There was neuroinflammation around plaques, and in regions expressing mutant tau. We saw no evidence that mutant tau had affected amyloid-beta pathology or vice versa. Morris water maze behavioral tests demonstrated mild memory impairment attributable to amyloid-beta pathology at 2 and 4 months, with severe impairment at 6 months in animals receiving adeno-associated viral-3PO. Therefore, TASTPM mice injected with mutant tau displayed many of the main features characteristic of human Alzheimer's disease patients and might be used as a model to test new drugs to ameliorate clinical features of Alzheimer's disease.

Origins of Alzheimer's disease: reconciling cerebrospinal fluid biomarker and neuropathology data regarding the temporal sequence of amyloid-beta and tau involvement

PURPOSE OF REVIEW

This review aims to address the temporal sequencing of involvement of amyloid-beta (Aβ) and tau in the pathogenesis of Alzheimer's disease and reconcile apparently conflicting neuropathologic and biomarker data.

RECENT FINDINGS

Although neuropathologic studies show that limbic system tau disease occurs ubiquitously in middle-aged individuals before the appearance of amyloid plaques, biomarker studies in living individuals suggest that Aβ disease is the initiating event in Alzheimer's disease and precedes cerebrospinal fluid tau changes. Evidence from neuropathologic, biomarker, genetic and cellular/mouse studies shows that tau accumulation in limbic regions occurs slowly with age and does not induce widespread neurodegeneration, but that Aβ interacts with tau in some way to accelerate neurofibrillary disease and induce neurodegeneration.

SUMMARY

Aβ aggregation is the key initial trigger of Alzheimer's disease pathologic changes and interacts with tau to exacerbate age-related tauopathy and induce neurodegeneration.

Loss of prohibitin membrane scaffolds impairs mitochondrial architecture and leads to tau hyperphosphorylation and neurodegeneration

Fusion and fission of mitochondria maintain the functional integrity of mitochondria and protect against neurodegeneration, but how mitochondrial dysfunctions trigger neuronal loss remains ill-defined. Prohibitins form large ring complexes in the inner membrane that are composed of PHB1 and PHB2 subunits and are thought to function as membrane scaffolds. In Caenorhabditis elegans, prohibitin genes affect aging by moderating fat metabolism and energy production. Knockdown experiments in mammalian cells link the function of prohibitins to membrane fusion, as they were found to stabilize the dynamin-like GTPase OPA1 (optic atrophy 1), which mediates mitochondrial inner membrane fusion and cristae morphogenesis. Mutations in OPA1 are associated with dominant optic atrophy characterized by the progressive loss of retinal ganglion cells, highlighting the importance of OPA1 function in neurons. Here, we show that neuron-specific inactivation of Phb2 in the mouse forebrain causes extensive neurodegeneration associated with behavioral impairments and cognitive deficiencies. We observe early onset tau hyperphosphorylation and filament formation in the hippocampus, demonstrating a direct link between mitochondrial defects and tau pathology. Loss of PHB2 impairs the stability of OPA1, affects mitochondrial ultrastructure, and induces the perinuclear clustering of mitochondria in hippocampal neurons. A destabilization of the mitochondrial genome and respiratory deficiencies manifest in aged neurons only, while the appearance of mitochondrial morphology defects correlates with tau hyperphosphorylation in the absence of PHB2. These results establish an essential role of prohibitin complexes for neuronal survival in vivo and demonstrate that OPA1 stability, mitochondrial fusion, and the maintenance of the mitochondrial genome in neurons depend on these scaffolding proteins. Moreover, our findings establish prohibitin-deficient mice as a novel genetic model for tau pathologies caused by a dysfunction of mitochondria and raise the possibility that tau pathologies are associated with other neurodegenerative disorders caused by deficiencies in mitochondrial dynamics.

Mitochondria are the major site of cellular ATP production and are essential for the survival of neurons. High ATP levels are required to sustain neuronal activities and axonal transport of macromolecules and organelles. The functional integrity of mitochondria depends on fusion and fission of their membranes, which maintain a dynamic mitochondrial network in cells. Interference with these processes causes neurodegenerative disorders that are characterized by axonal degeneration of distinct neurons. However, how an impaired fusion affects mitochondrial activities and neuronal survival remains poorly understood. Here, we have addressed this question by analyzing forebrain-specific knockout mice lacking prohibitins. Prohibitin complexes form membrane scaffolds in the inner membrane, which we now show are required for mitochondrial fusion, ultrastructure, and genome stability in neurons. Loss of prohibitins triggers extensive neurodegeneration associated with behavioral and cognitive deficiencies. Surprisingly, we observe hyperphosphorylation and filament formation of the microtubule-associated protein tau, reminiscent of a large group of neurodegenerative disorders termed tauopathies. Our findings, therefore, not only provide new insight into how defects in mitochondrial fusion affect neuronal survival, but also point to an intimate relationship of deficiencies in mitochondrial dynamics and tau pathologies.

Calpain dysregulation in Alzheimer's disease

Alzheimer's disease (AD) is characterized by the presence of senile plaques and neurofibrillary tangles in the neocortex and hippocampus of AD patients. In addition, a marked decrease in synaptic contacts has been detected in these affected brain areas. Due to its prevalence in the aging population, this disease has been the focus of numerous studies. The data obtained from those studies suggest that the mechanisms leading to the formation of the hallmark lesions of AD might be linked. One of such mechanisms seems to be the dysregulation of calcium homeostasis that results in the abnormal activation of calpains. Calpains are a family of Ca(2+)-dependent cysteine proteases that play a key role in multiple cell functions including cell development, differentiation and proliferation, axonal guidance, growth cone motility, and cell death, among others. In this paper, we briefly reviewed data on the structure of these proteases and their regulation under normal conditions. We also summarized data underscoring the participation of calpains in the neurodegenerative mechanisms associated with AD.

Alzheimer's disease: biological aspects, therapeutic perspectives and diagnostic tools

Alzheimer's disease (AD) is the most common form of dementia among older people. Dementia is an irreversible brain disorder that seriously affects a person's ability to carry out daily activities. It is characterized by loss of cognitive functioning and behavioral abilities, to such an extent that it interferes with the daily life and activities of the affected patients. Although it is still unknown how the disease process begins, it seems that brain damage starts a decade or more before problems become evident. Scientific data seem to indicate that changes in the generation or the degradation of the amyloid-b peptide (Aβ) lead to the formation of aggregated structures that are the triggering molecular events in the pathogenic cascade of AD. This review summarizes some characteristic features of Aβ misfolding and aggregation and how cell damage and death mechanisms are induced by these supramolecular and toxic structures. Further, some interventions for the early diagnosis of AD are described and in the last part the potential therapeutic strategies adoptable to slow down, or better block, the progression of the pathology are reported.