Alzheimer's disease and non-demented high pathology control nonagenarians: comparing and contrasting the biochemistry of cognitively successful aging

The amyloid cascade hypothesis provides an economical mechanistic explanation for Alzheimer's disease (AD) dementia and correlated neuropathology. However, some nonagenarian individuals (high pathology controls, HPC) remain cognitively intact while enduring high amyloid plaque loads for decades. If amyloid accumulation is the prime instigator of neurotoxicity and dementia, specific protective mechanisms must enable these HPC to evade cognitive decline. We evaluated the neuropathological and biochemical differences existing between non-demented (ND)-HPC and an age-matched cohort with AD dementia. The ND-HPC selected for our study were clinically assessed as ND and possessed high amyloid plaque burdens. ELISA and Western blot analyses were used to quantify a group of proteins related to APP/Aβ/tau metabolism and other neurotrophic and inflammation-related molecules that have been found to be altered in neurodegenerative disorders and are pivotal to brain homeostasis and mental health. The molecules assumed to be critical in AD dementia, such as soluble or insoluble Aβ40, Aβ42 and tau were quantified by ELISA. Interestingly, only Aβ42 demonstrated a significant increase in ND-HPC when compared to the AD group. The vascular amyloid load which was not used in the selection of cases, was on the average almost 2-fold greater in AD than the ND-HPC, suggesting that a higher degree of microvascular dysfunction and perfusion compromise was present in the demented cohort. Neurofibrillary tangles were less frequent in the frontal cortices of ND-HPC. Biochemical findings included elevated vascular endothelial growth factor, apolipoprotein E and the neuroprotective factor S100B in ND-HPC, while anti-angiogenic pigment epithelium derived factor levels were lower. The lack of clear Aβ-related pathological/biochemical demarcation between AD and ND-HPC suggests that in addition to amyloid plaques other factors, such as neurofibrillary tangle density and vascular integrity, must play important roles in cognitive failure.     

The emerging roles of protein homeostasis‐governing pathways in Alzheimer's disease

Pathways governing protein homeostasis are involved in maintaining the structural, quantitative, and functional stability of intracellular proteins and involve the ubiquitin–proteasome system, autophagy, endoplasmic reticulum, and mTOR pathway. Due to the broad physiological implications of protein homeostasis pathways, dysregulation of proteostasis is often involved in the development of multiple pathological conditions, including Alzheimer's disease (AD). Similar to other neurodegenerative diseases that feature pathogenic accumulation of misfolded proteins, Alzheimer's disease is characterized by two pathological hallmarks, amyloid‐β (Aβ) plaques and tau aggregates. Knockout or transgenic overexpression of various proteostatic components in mice results in AD‐like phenotypes. While both Aβ plaques and tau aggregates could in turn enhance the dysfunction of these proteostatic pathways, eventually leading to apoptotic or necrotic neuronal death and pathogenesis of Alzheimer's disease. Therefore, targeting the components of proteostasis pathways may be a promising therapeutic strategy against Alzheimer's disease.     

Wnt signalling pathway and tau phosphorylation: A comprehensive study on known connections

The Wnt pathway is the most important cascade in the nervous system; evidence has indicated that deregulation of the Wnt pathway induced pathogenic hallmarks of neurodegenerative diseases. Glycogen synthase kinase-3β (GSK-3β) as the main member of the Wnt pathway increases tau inclusions, the main marker in the neurodegenerative diseases. Phosphorylated tau is observed in the pre-tangle of the neurons in the early stage of neurodegenerative diseases. The researchers always try to improve pharmacological approaches of new therapeutic strategies to the treatment of neurodegenerative diseases that are required to represent a significant entry point by understanding the theoretical interactions of the molecular pathways. In this review, we have discussed the recent knowledge about the canonical and non-canonical Wnt signalling pathway, GSK-3β, Wnt/β-catenin antagonists, tau phosphorylation, and their important roles in the neurodegenerative diseases.     

Tau Proteins and Tauopathies in Alzheimer’s Disease

Alzheimer’s disease (AD) is characterized by progressive memory loss and cognitive function deficits. There are two major pathological hallmarks that contribute to the pathogenesis of AD which are the presence of extracellular amyloid plaques composed of amyloid-β (Aβ) and intracellular neurofibrillary tangles composed of hyperphosphorylated tau. Despite extensive research that has been done on Aβ in the last two decades, therapies targeting Aβ were not very fruitful at treating AD as the efficacy of Aβ therapies observed in animal models is not reflected in human clinical trials. Hence, tau-directed therapies have received tremendous attention as the potential treatments for AD. Tauopathies are closely correlated with dementia and immunotherapy has been effective at reducing tau pathology and improving cognitive deficits in animal models. Thus, in this review article, we discussed the pathological mechanism of tau proteins, the key factors contributing to tauopathies, and therapeutic approaches for tauopathies in AD based on the recent progress in tau-based research.     

A standard model of Alzheimer’s disease?

ABSTRACT

The recent Research Framework proposed by the US National Institute on Aging and the Alzheimer’s Association (NIA-AA) recommends that Alzheimer’s disease be defined by its specific biology rather than by non-specific neurodegenerative and syndromal features. By affirming markers of abnormal Aβ and tau proteins as the essential pathobiological signature of Alzheimer’s disease, the Framework tacitly reinforces the amyloid (Aβ) cascade as the leading theory of Alzheimer pathogenesis. In light of recent evidence that the cascade is driven by the misfolding and templated aggregation of Aβ and tau, we believe that an empirically grounded Standard Model of Alzheimer’s pathogenesis is within reach. A Standard Model can clarify and consolidate existing information, contextualize risk factors and the complex disease phenotype, identify testable hypotheses for future research, and pave the most direct path to effective prevention and treatment.     

A Non-transgenic Mouse Model (icv-STZ Mouse) of Alzheimer’s Disease: Similarities to and Differences from the Transgenic Model (3xTg-AD Mouse)

Alzheimer’s disease (AD) can be divided into sporadic AD (SAD) and familial AD (FAD). Most AD cases are sporadic and result from multiple etiologic factors, including environmental, genetic, and metabolic factors, whereas FAD is caused by mutations in the presenilins or amyloid-β (Aβ) precursor protein (APP) genes. A commonly used animal model for AD is the 3xTg-AD transgenic mouse model, which harbors mutated presenilin 1, APP, and tau genes and thus represents a model of FAD. There is an unmet need in the field to characterize animal models representing different AD mechanisms, so that potential drugs for SAD can be evaluated preclinically in these animal models. A mouse model generated by intracerebroventricular (icv) administration of streptozocin (STZ), the icv-STZ mouse, shows many aspects of SAD. In this study, we compared the non-cognitive and cognitive behaviors as well as biochemical and immunohistochemical alterations between the icv-STZ mouse and the 3xTg-AD mouse. We found that both mouse models showed increased exploratory activity as well as impaired learning and spatial memory. Both models also demonstrated neuroinflammation, altered synaptic proteins and insulin/IGF-1 (insulin-like growth factor-1) signaling, and increased hyperphosphorylated tau in the brain. The most prominent brain abnormality in the icv-STZ mouse was neuroinflammation, and in the 3xTg-AD mouse it was elevation of hyperphosphorylated tau. These observations demonstrate the behavioral and neuropathological similarities and differences between the icv-STZ mouse and the 3xTg-AD mouse models and will help guide future studies using these two mouse models for the development of AD drugs.     

The significance of tau and alpha-synuclein inclusions in neurodegenerative diseases

Intracellular filamentous inclusions made of either the microtubule-associated protein tau or the protein alpha-synuclein define the majority of cases of neurodegenerative disease. Mutations in the tau gene in familial forms of frontotemporal dementia and in the alpha-synuclein gene in familial cases of Parkinson's disease have provided causal links between the dysfunction of these proteins and neurodegeneration. Over the past year, several novel tau gene mutations have been identified and more has been learned about possible mechanisms by which tau gene mutations lead to frontotemporal dementia. Experimental animal models have provided a link between tau filament formation and nerve cell degeneration. Along similar lines, animal models have been produced that result in the formation of alpha-synuclein filaments and the degeneration of dopaminergic nerve cells. Building on previous work, synthetic alpha-synuclein filaments have been shown to exhibit the characteristics of amyloid.     

Tau Mediated Neurodegeneration: An Insight into Alzheimer’s Disease Pathology

Extracellular accumulations of Aβ, hyperphosphorylation of tau and intracellular neurofibrillary tangle formation have been the hallmarks of Alzheimer’s Disease (AD). Although tau and its phosphorylation play a pivotal role in the normal physiology yet its hyperphosphorylation has been a pathological manifestation in neurodegenerative disorders like AD. In this review physiology of tau, its phosphorylation, hyperphosphorylation with the intervention of various kinases, aggregation and formation of paired helical filaments has been discussed. A brief account of various animal models employed to study the pathological manifestation of tau in AD and therapeutic strategies streamlined to counter the tau induced pathology has been given. The reasons for the failure to have suitable animal model to study AD pathology and recent success in achieving this has been included. The role of caspase cascade in tau cleavage has been emphasized. The summary of current studies on tau and the need for future studies has been accentuated.     

Autophagy,a guardian against neurodegeneration

Autophagy is an intracellular degradation process responsible for the clearance of most long-lived proteins and organelles. Cytoplasmic components are enclosed by double-membrane autophagosomes, which subsequently fuse with lysosomes for degradation. Autophagy dysfunction may contribute to the pathology of various neurodegenerative disorders, which manifest abnormal protein accumulation. As autophagy induction enhances the clearance of aggregate-prone intracytoplasmic proteins that cause neurodegeneration (like mutant huntingtin, tau and ataxin 3) and confers cytoprotective roles in cell and animal models, upregulating autophagy may be a tractable therapeutic strategy for diseases caused by such proteins. Here, we will review the molecular machinery of autophagy and its role in neurodegenerative diseases. Drugs and associated signalling pathways that may be targeted for pharmacological induction of autophagy will also be discussed.     

Tau story: from frontotemporal dementia to other tauopathies

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, the recent discovery of tau gene mutations in fronto-temporal dementia with parkinsonism linked to chromosome 17 has reinforced the direct role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies. Conversely, recent data in myotonic dystrophy has demonstrated that indirect effect (CTG repeat expansion) leading to variations in tau alternative splicing also produce neurofibrillary degeneration.     

Proteomics analysis of the neurodegeneration in the brain of tau transgenic mice

Protein tau, a major microtubule-binding protein in the brain, comprises six isoforms generated through alternative mRNA splicing. A dysfunctional form of mutant and normal tau is associated or implicated in the pathogenesis of several neurodegenerative disorders. The neuropathological hallmark of these tau-opathies are intraneuronal depositions of fibrillary aggregates of which neurofibrillary tangles are most common. Several distinct transgene mouse models confirmed that tau protein can cause neurodegeneration directly. This study was aimed at identifying proteins that might play a role in the cellular disturbances caused by overexpression of the longest isoform of human tau in the brain of transgenic mice. We found 34 proteins which differed in integrated intensity by a factor of at least 1.5. These proteins could be sorted into several categories. Some of the phenotypic characteristics found in the htau transgenic mice could be related to proteins found in this study. Several proteins are linked to processes involving apoptosis and neuronal death and have been discussed in papers describing neurodegenerative disorders.     

Microtubule-stabilizing agents as potential therapeutics for neurodegenerative disease

Microtubules (MTs), cytoskeletal elements found in all mammalian cells, play a significant role in cell structure and in cell division. They are especially critical in the proper functioning of post-mitotic central nervous system neurons, where MTs serve as the structures on which key cellular constituents are trafficked in axonal projections. MTs are stabilized in axons by the MT-associated protein tau, and in several neurodegenerative diseases, including Alzheimer’s disease, frontotemporal lobar degeneration, and Parkinson’s disease, tau function appears to be compromised due to the protein dissociating from MTs and depositing into insoluble inclusions referred to as neurofibrillary tangles. This loss of tau function is believed to result in alterations of MT structure and function, resulting in aberrant axonal transport that likely contributes to the neurodegenerative process. There is also evidence of axonal transport deficiencies in other neurodegenerative diseases, including amyotrophic lateral sclerosis and Huntington’s disease, which may result, at least in part, from MT alterations. Accordingly, a possible therapeutic strategy for such neurodegenerative conditions is to treat with MT-stabilizing agents, such as those that have been used in the treatment of cancer. Here, we review evidence of axonal transport and MT deficiencies in a number of neurodegenerative diseases, and summarize the various classes of known MT-stabilizing agents. Finally, we highlight the growing evidence that small molecule MT-stabilizing agents provide benefit in animal models of neurodegenerative disease and discuss the desired features of such molecules for the treatment of these central nervous system disorders.     

Murine models of Alzheimer's disease and their use in developing immunotherapies

Alzheimer's disease (AD) is one of the categories of neurodegenerative diseases characterized by a conformational change of a normal protein into a pathological conformer with a high β-sheet content that renders it resistant to degradation and neurotoxic. In AD, the normal soluble amyloid β (sAβ) peptide is converted into oligomeric/fibrillar Aβ. The oligomeric forms of Aβ are thought to be the most toxic, while fibrillar Aβ becomes deposited as amyloid plaques and congophilic angiopathy, which both serve as neuropathological markers of the disease. An additional important feature of AD is the accumulation of abnormally phosphorylated tau as soluble toxic oligomers and as neurofibrillary tangles. Many therapeutic interventions are under investigation to prevent and treat AD. The testing of these diverse approaches to ameliorate AD pathology has been made possible by the existence of numerous transgenic mouse models which each mirror specific aspects of AD pathology. None of the current murine models is a perfect match of the human disease. Perhaps the most exciting of the therapeutic approaches being developed is immunomodulation targeting the aggregating proteins, Aβ and tau. This type of AD therapy is currently being assessed in many transgenic mouse models, and promising findings have led to clinical trials. However, there is a discrepancy between results in murine models and ongoing clinical trials, which highlight the limitations of these models and also of our understanding of the underlying etiology and pathogenesis of AD. Because of these uncertainties, Tg models for AD are continuously being refined with the aim to better understand the disease and to enhance the predictive validity of potential treatments such as immunotherapies.     

Neuroprotective activity and evaluation of Hsp90 inhibitors in an immortalized neuronal cell line

Alzheimer’s disease (AD) neuropathology is characterized by loss of synapses and neurons, neuritic plaques consisting of β-amyloid (Aβ) peptides, and neurofibrillary tangles consisting of intracellular aggregates of hyperphosphorylated tau protein in susceptible brain regions. Aβ oligomers trigger a cascade of pathogenic events including tau hyperphosphorylation and aggregation, inflammatory reactions, and excitotoxicity that contribute to the progression of AD. The molecular chaperone Hsp90 facilitates the folding of newly synthesized and denatured proteins and is believed to play a role in neurodegenerative disorders in which the defining pathology results in misfolded proteins and the accumulation of protein aggregates. Some agents that inhibit Hsp90 protect neurons against Aβ toxicity and tau aggregation, and assays for rapidly screening potential Hsp90 inhibitors are of interest. We used the release of the soluble cytosolic enzyme lactate dehydrogenase (LDH) as an indicator of the loss of cell membrane integrity and cytotoxicity resulting from exposure to Aβ peptides to evaluate the neuroprotective properties of novel novobiocin analogues and established Hsp90 inhibitors. Compounds were assessed for potency in protecting proliferating and differentiated SH-SY5Y neuronal cells against Aβ-induced cell death; the potential toxicity of each agent alone was also determined. The data indicated that several of the compounds decreased Aβ toxicity even at low nanomolar concentrations and, unexpectedly, were more potent in protecting the undifferentiated cells against Aβ. The novobiocin analogues alone were not toxic even up to 10 μM concentrations whereas GDA and the parent compound, novobiocin, were toxic at 1 and 10 μM, respectively. The results suggest that novobiocin analogues may provide novel leads for the development of neuroprotective drugs.     

Proteins that mediate protein aggregation and cytotoxicity distinguish Alzheimer's hippocampus from normal controls

Neurodegenerative diseases are distinguished by characteristic protein aggregates initiated by disease‐specific ‘seed’ proteins; however, roles of other co‐aggregated proteins remain largely unexplored. Compact hippocampal aggregates were purified from Alzheimer's and control‐subject pools using magnetic‐bead immunoaffinity pulldowns. Their components were fractionated by electrophoretic mobility and analyzed by high‐resolution proteomics. Although total detergent‐insoluble aggregates from Alzheimer's and controls had similar protein content, within the fractions isolated by tau or Aβ_1–42 pulldown, the protein constituents of Alzheimer‐derived aggregates were more abundant, diverse, and post‐translationally modified than those from controls. Tau‐ and Aβ‐containing aggregates were distinguished by multiple components, and yet shared >90% of their protein constituents, implying similar accretion mechanisms. Alzheimer‐specific protein enrichment in tau‐containing aggregates was corroborated for individuals by three analyses. Five proteins inferred to co‐aggregate with tau were confirmed by precise in situ methods, including proximity ligation amplification that requires co‐localization within 40 nm. Nematode orthologs of 21 proteins, which showed Alzheimer‐specific enrichment in tau‐containing aggregates, were assessed for aggregation‐promoting roles in C. elegans by RNA‐interference ‘knockdown’. Fifteen knockdowns (71%) rescued paralysis of worms expressing muscle Aβ, and 12 (57%) rescued chemotaxis disrupted by neuronal Aβ expression. Proteins identified in compact human aggregates, bound by antibody to total tau, were thus shown to play causal roles in aggregation based on nematode models triggered by Aβ_1–42. These observations imply shared mechanisms driving both types of aggregation, and/or aggregate‐mediated cross‐talk between tau and Aβ. Knowledge of protein components that promote protein accrual in diverse aggregate types implicates common mechanisms and identifies novel targets for drug intervention.     

Tau-like Proteins Associated with Centrosomes in Cultured Cells

The subcellular association of tau-like proteins with centrosomes in cultured cell lines and its effects in nucleating microtubule assembly were analyzed using biochemical and immunocytochemical approaches. Tau proteins, major components of microtubules, appear to be tightly associated with actin filaments in a variety of cell lines, while in pathological conditions of neurons, they are part of paired helical filaments found in Alzheimer's disease. Different studies suggest that, in addition to tau interactions with the components of the cytoskeletal network, tau polypeptides appear to be associated with highly structured cellular elements, in both interphase and mitotic cells. An in-depth analysis of tau subcellular distribution us- ing different polyclonal and monoclonal antibodies showed colocalization of tau-like components with centrosomes in interphase cells of the human Huh-7 hepatoma, in SW-13 adenocarcinoma, and in normal human fibroblasts. Tau associated with centrosomes in mitotic Huh-7 cells was also identified. However, antibodies against the tau binding repeats did not stain centrosomes. A set of different tau isoforms was also identified by Western blot analysis on isolated centrosomal preparations from Huh-7 cells, obtained by differential centrifugation through sucrose gradients. Microtubule nucleationin vitroover isolated centrosomes was inhibited by both the polyclonal antibody against native tau and an antibody to the N-terminal tau sequence, as revealed by immunofluorescence analysis and assembly kinetics experiments. The antibody TRS1.2 against the fragment containing the first binding repeat on tau did not affect nucleation. These studies allowed us to characterize tau association with the isolated centrosomal preparation and its involvement in microtubule assembly nucleated over centrosomes, thus suggesting possible structural and functional roles for these interactions.     

Role of autophagy in the clearance of mutant huntingtin: a step towards therapy?

Macroautophagy (henceforth referred to simply as autophagy) is a bulk degradation process involved in the clearance of long-lived proteins, protein complexes and organelles. A portion of the cytosol, with its contents to be degraded, is enclosed by double-membrane structures called autophagosomes/autophagic vacuoles, which ultimately fuse with lysosomes where their contents are degraded. In this review, we will describe how induction of autophagy is protective against toxic intracytosolic aggregate-prone proteins that cause a range of neurodegenerative diseases. Autophagy is a key clearance pathway involved in the removal of such proteins, including mutant huntingtin (that causes Huntington’s disease), mutant ataxin-3 (that causes spinocerebellar ataxia type 3), forms of tau that cause tauopathies, and forms of alpha-synuclein that cause familial Parkinson’s disease. Induction of autophagy enhances the clearance of both soluble and aggregated forms of such proteins, and protects against toxicity of a range of these mutations in cell and animal models. Interestingly, the aggregates formed by mutant huntingtin sequester and inactivate the mammalian target of rapamycin (mTOR), a key negative regulator of autophagy. This results in induction of autophagy in cells with these aggregates.     

Critical roles of protein disulfide isomerases in balancing proteostasis in the nervous system

Protein disulfide isomerases (PDIs) constitute a family of oxidoreductases promoting redox protein folding and quality control in the endoplasmic reticulum. PDIs catalyze disulfide bond formation, isomerization, and reduction, operating in concert with molecular chaperones to fold secretory cargoes in addition to directing misfolded proteins to be refolded or degraded. Importantly, PDIs are emerging as key components of the proteostasis network, integrating protein folding status with central surveillance mechanisms to balance proteome stability according to cellular needs. Recent advances in the field driven by the generation of new mouse models, human genetic studies, and omics methodologies, in addition to interventions using small molecules and gene therapy, have revealed the significance of PDIs to the physiology of the nervous system. PDIs are also implicated in diverse pathologies, ranging from neurodevelopmental conditions to neurodegenerative diseases and traumatic injuries. Here, we review the principles of redox protein folding in the ER with a focus on current evidence linking genetic mutations and biochemical alterations to PDIs in the etiology of neurological conditions.     

Improved Behavioral Response as a Valid Biomarker for Drug Screening Program in Transgenic Rodent Models of Tauopathies

Neurodegenerative tauopathies are defined as a group of dementia and movement disorders characterized by prominent filamentous tau inclusions and degeneration located within certain brain regions. Their common sign is a presence of proteinaceous aggregates composed of hyperphosphorylated and truncated tau proteins. The molecular mechanisms of the disease still remain unresolved, therefore transgenic organisms displaying tau-related neurodegenerative cascade have been created to allow decoding of individual pathways involved in human pathological conditions. Moreover, use of transgenic model organisms enables the application of potential therapeutic approaches. The expression of mutated or misfolded tau as a transgene in vivo leads to significant alteration of neurobehavioral features of experimental animal, therefore detailed classification of behavioral phenotype become one of the first crucial analyses, while it functionally correlates with central nervous system impairment. Currently, two major types of behavioral impairment have been described in transgenic rodent models of tauopathies, (1) progressive motor impairment associated with muscular weakness and premature death and (2) age-related impairment of cognitive functions attended with unaffected motor status. Up to the present, only transgenic models displaying motor impairment were successfully applied into the drug trials targeting misfolded tau protein, despite their behavioral inconsistence with clinical profile of progressive human tauopathy. The aim of this study was, therefore, to summarize the pros and cons of used transgenic rodent models mimicking human tauopathies in connection with development of therapeutic strategies.     

Interaction of NF-κB and Wnt/β-catenin Signaling Pathways in Alzheimer’s Disease and Potential Active Drug Treatments

The most important neuropathological features of Alzheimer's disease (AD) are extracellular amyloid-β protein (Aβ) deposition, tau protein hyperphosphorylation and activation of neurometabolic reaction in the brain accompanied by neuronal and synaptic damage, and impaired learning and memory function. According to the amyloid cascade hypothesis, increased Aβ deposits in the brain to form the core of the senile plaques that initiate cascade reactions, affecting the synapses and stimulating activation of microglia, resulting in neuroinflammation. A growing number of studies has shown that NF-κB and Wnt/β-catenin pathways play important roles in neurodegenerative diseases, especially AD. In this review, we briefly introduce the connection between neuroinflammation-mediated synaptic dysfunction in AD and elaborated on the mechanism of these two signaling pathways in AD-related pathological changes, as well as their interaction. Based on our interest in natural compounds, we also briefly introduce and conduct preliminary screening of potential therapeutics for AD.