Differential effects of apolipoprotein E isoforms on phosphorylation at specific sites on tau by glycogen synthase kinase-3 beta identified by nano-electrospray mass spectrometry

Molecular insights into the potential effects of selective estrogen receptor β agonists in Alzheimer's and Parkinson's diseases

Alzheimer's disease (AD) and Parkinson's disease (PD) are the most common neurodegenerative disorders. Pathologically, AD and PD are characterized by the accumulation of misfolded proteins. Hence, they are also called as proteinopathy diseases. Gender is considered as one of the risk factors in both diseases. Estrogens are widely accepted to be neuroprotective in several neurodegenerative disorders. Estrogens can be produced in the central nervous system, where they are called as neurosteroids. Estrogens mediate their neuroprotective action mainly through their actions on estrogen receptor alpha (ERα) and estrogen receptor beta (ERβ). However, ERα is mainly involved in the growth and development of the primary and secondary sexual organs in females. Hence, the activation of ERα is associated with undesired side effects such as gynecomastia and increase in the risk of breast cancer, thromboembolism, and feminization. Therefore, selective activation of ERβ is often considered to be safer. In this review, we explore the role of ERβ in regulating the expression and functions of AD- and PD-associated genes. Additionally, we discuss the association of these genes with the amyloid-beta peptide (Aβ) and α-synuclein mediated toxicity. Ultimately, we established a correlation between the importance of ERβ activation and the process underlying ERβ's neuroprotective mechanisms in AD and PD.

Alzheimer's Disease: Models and Molecular Mechanisms Informing Disease and Treatments

Alzheimer's Disease (AD) is a complex neurodegenerative disease resulting in progressive loss of memory, language and motor abilities caused by cortical and hippocampal degeneration. This review captures the landscape of understanding of AD pathology, diagnostics, and current therapies. Two major mechanisms direct AD pathology: (1) accumulation of amyloid β (Aβ) plaque and (2) tau-derived neurofibrillary tangles (NFT). The most common variants in the Aβ pathway in APP, PSEN1, and PSEN2 are largely responsible for early-onset AD (EOAD), while MAPT, APOE, TREM2 and ABCA7 have a modifying effect on late-onset AD (LOAD). More recent studies implicate chaperone proteins and Aβ degrading proteins in AD. Several tests, such as cognitive function, brain imaging, and cerebral spinal fluid (CSF) and blood tests, are used for AD diagnosis. Additionally, several biomarkers seem to have a unique AD specific combination of expression and could potentially be used in improved, less invasive diagnostics. In addition to genetic perturbations, environmental influences, such as altered gut microbiome signatures, affect AD. Effective AD treatments have been challenging to develop. Currently, there are several FDA approved drugs (cholinesterase inhibitors, Aß-targeting antibodies and an NMDA antagonist) that could mitigate AD rate of decline and symptoms of distress.

Unraveling Molecular and Genetic Insights into Neurodegenerative Diseases: Advances in Understanding Alzheimer's, Parkinson's, and Huntington's Diseases and Amyotrophic Lateral Sclerosis

Neurodegenerative diseases are, according to recent studies, one of the main causes of disability and death worldwide. Interest in molecular genetics has started to experience exponential growth thanks to numerous advancements in technology, shifts in the understanding of the disease as a phenomenon, and the change in the perspective regarding gene editing and the advantages of this action. The aim of this paper is to analyze the newest approaches in genetics and molecular sciences regarding four of the most important neurodegenerative disorders: Alzheimer's disease, Parkinson's disease, Huntington's disease, and amyotrophic lateral sclerosis. We intend through this review to focus on the newest treatment, diagnosis, and predictions regarding this large group of diseases, in order to obtain a more accurate analysis and to identify the emerging signs that could lead to a better outcome in order to increase both the quality and the life span of the patient. Moreover, this review could provide evidence of future possible novel therapies that target the specific genes and that could be useful to be taken into consideration when the classical approaches fail to shed light.

ApoE in Alzheimer’s disease: pathophysiology and therapeutic strategies

Alzheimer’s disease (AD) is the most common cause of dementia worldwide, and its prevalence is rapidly increasing due to extended lifespans. Among the increasing number of genetic risk factors identified, the apolipoprotein E (APOE) gene remains the strongest and most prevalent, impacting more than half of all AD cases. While the ε4 allele of the APOE gene significantly increases AD risk, the ε2 allele is protective relative to the common ε3 allele. These gene alleles encode three apoE protein isoforms that differ at two amino acid positions. The primary physiological function of apoE is to mediate lipid transport in the brain and periphery; however, additional functions of apoE in diverse biological functions have been recognized. Pathogenically, apoE seeds amyloid-β (Aβ) plaques in the brain with apoE4 driving earlier and more abundant amyloids. ApoE isoforms also have differential effects on multiple Aβ-related or Aβ-independent pathways. The complexity of apoE biology and pathobiology presents challenges to designing effective apoE-targeted therapeutic strategies. This review examines the key pathobiological pathways of apoE and related targeting strategies with a specific focus on the latest technological advances and tools.

The Role of Post-Translational Modifications on the Structure and Function of Tau Protein

Involving addition of chemical groups or protein units to specific residues of the target protein, post-translational modifications (PTMs) alter the charge, hydrophobicity, and conformation of a protein, which in tune influences protein function, protein - protein interaction, and protein aggregation. While the occurrence of PTMs is dynamic and subject to regulations, conformational disorder of the target protein facilitates PTMs. The microtubule-associated protein tau is a typical intrinsically disordered protein that undergoes a variety of PTMs including phosphorylation, acetylation, ubiquitination, methylation, and oxidation. Accumulated evidence shows that these PTMs play a critical role in regulating tau-microtubule interaction, tau localization, tau degradation and aggregation, and reinforces the correlation between tau PTMs and pathogenesis of neurodegenerative disease. Here, we review tau PTMs with an emphasis on their influence on tau structure. With available biophysical characterization results, we describe how PTMs induce conformational changes in tau monomer and regulate tau aggregation. Compared to functional analysis of tau PTMs, biophysical characterization of tau PTMs is lagging. While it is challenging, characterizing the specific effects of PTMs on tau conformation and interaction is indispensable to unravel the tau PTM code.

The Interplay of Tau Protein and β-Amyloid: While Tauopathy Spreads More Profoundly Than Amyloidopathy, Both Processes Are Almost Equally Pathogenic

Alzheimer's disease (AD) is a neurodegenerative disorder, in which amyloid precursor protein (APP) misprocessing and tau protein hyperphosphorylation are well-established pathogenic cascades. Despite extensive considerations, the central mediator of neuronal cell death upon AD remains under debate. Therefore, we examined the direct interplay between tauopathy and amyloidopathy processes. We employed primary culture neurons and examined pathogenic P-tau and Aβ oligomers upon hypoxia treatment by immunofluorescence and immunoblotting. We observed both tauopathy and amyloidopathy processes upon the hypoxia condition. We also applied Aβ1-42 or P-tau onto primary cultured neurons. We overexpressed P-tau in SH-SY5Y cells and found Aβ accumulation. Furthermore, adult male rats received Aβ1-42 or pathogenic P-tau in the dorsal hippocampus and were examined for 8 weeks. Learning and memory performance, as well as anxiety behaviors, were assessed by Morris water maze and elevated plus-maze tests. Both Aβ1-42 and pathogenic P-tau significantly induced learning and memory deficits and enhanced anxiety behavior after treatment 2 weeks. Aβ administration induced robust tauopathy distribution in the cortex, striatum, and corpus callosum as well as CA1. On the other hand, P-tau treatment developed Aβ oligomers in the cortex and CA1 only. Our findings indicate that Aβ1-42 and pathogenic P-tau may induce each other and cause almost identical neurotoxicity in a time-dependent manner, while tauopathy seems to be more distributable than amyloidopathy.

History and progress of hypotheses and clinical trials for Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disease characterized by progressive memory loss along with neuropsychiatric symptoms and a decline in activities of daily life. Its main pathological features are cerebral atrophy, amyloid plaques, and neurofibrillary tangles in the brains of patients. There are various descriptive hypotheses regarding the causes of AD, including the cholinergic hypothesis, amyloid hypothesis, tau propagation hypothesis, mitochondrial cascade hypothesis, calcium homeostasis hypothesis, neurovascular hypothesis, inflammatory hypothesis, metal ion hypothesis, and lymphatic system hypothesis. However, the ultimate etiology of AD remains obscure. In this review, we discuss the main hypotheses of AD and related clinical trials. Wealthy puzzles and lessons have made it possible to develop explanatory theories and identify potential strategies for therapeutic interventions for AD. The combination of hypometabolism and autophagy deficiency is likely to be a causative factor for AD. We further propose that fluoxetine, a selective serotonin reuptake inhibitor, has the potential to treat AD.

Genetic discoveries in AD using CSF amyloid and tau

The use of cerebrospinal fluid levels of Aβ42 and pTau181 as endophenotypes for genetic studies of Alzheimer's disease (AD) has led to successful identification of both rare and common AD risk variants. In addition, this approach has provided meaningful hypotheses for the biological mechanisms by which known AD risk variants modulate the disease process. In this article we discuss these successes and outline challenges to effective and continued applications of this approach. We contrast the statistical power of this approach with traditional case-control designs and discuss solutions to address challenges in quality control and data analysis for these phenotypes. Finally, we discuss the potential for the use of this approach with larger samples as well as the incorporation of next generation sequencing and for future work with other endophenotypes for AD.

Cognitive deficits and disruption of neurogenesis in a mouse model of apolipoprotein E4 domain interaction

Apolipoprotein E4 (apoE4) allele is the major genetic risk factor for sporadic Alzheimer disease (AD) due to the higher prevalence and earlier onset of AD in apoE4 carriers. Accumulating data suggest that the interaction between the N- and the C-terminal domains in the protein may be the main pathologic feature of apoE4. To test this hypothesis, we used Arg-61 mice, a model of apoE4 domain interaction, by introducing the domain interaction feature of human apoE4 into native mouse apoE. We carried out hippocampus-dependent learning and memory tests and related cellular and molecular assays on 12- and 3-month-old Arg-61 and age-matched background C57BL/6J mice. Learning and memory task performance were impaired in Arg-61 mice at both old and young ages compared with C57BL/6J mice. Surprisingly, young Arg-61 mice had more mitotic doublecortin-positive cells in the subgranular zone; mRNA levels of brain-derived neurotrophic factor (BDNF) and TrkB were also higher in 3-month-old Arg-61 hippocampus compared with C57BL/6J mice. These early-age neurotrophic and neurogenic (proliferative) effects in the Arg-61 mouse may be an inadequate compensatory but eventually detrimental attempt by the system to "repair" itself. This is supported by the higher cleaved caspase-3 levels in the young animals that not only persisted, but increased in old age, and the lower levels of doublecortin at old age in the hippocampus of Arg-61 mice. These results are consistent with human apoE4-dependent cognitive and neuro-pathologic changes, supporting the principal role of domain interaction in the pathologic effect of apoE4. Domain interaction is, therefore, a viable therapeutic/prophylactic target for cognitive impairment and AD in apoE4 subjects.

GWAS of cerebrospinal fluid tau levels identifies risk variants for Alzheimer's disease

Cerebrospinal fluid (CSF) tau, tau phosphorylated at threonine 181 (ptau), and Aβ₄₂ are established biomarkers for Alzheimer's disease (AD) and have been used as quantitative traits for genetic analyses. We performed the largest genome-wide association study for cerebrospinal fluid (CSF) tau/ptau levels published to date (n = 1,269), identifying three genome-wide significant loci for CSF tau and ptau: rs9877502 (p = 4.89 × 10⁻⁹ for tau) located at 3q28 between GEMC1 and OSTN, rs514716 (p = 1.07 × 10⁻⁸ and p = 3.22 × 10⁻⁹ for tau and ptau, respectively), located at 9p24.2 within GLIS3 and rs6922617 (p = 3.58 × 10⁻⁸ for CSF ptau) at 6p21.1 within the TREM gene cluster, a region recently reported to harbor rare variants that increase AD risk. In independent data sets, rs9877502 showed a strong association with risk for AD, tangle pathology, and global cognitive decline (p = 2.67 × 10⁻⁴, 0.039, 4.86 × 10⁻⁵, respectively) illustrating how this endophenotype-based approach can be used to identify new AD risk loci.

Linking Abeta and tau in late-onset Alzheimer's disease: a dual pathway hypothesis

Alzheimer's disease is characterized by abnormal elevation of Abeta peptide and abnormal hyperphosphorylation of the tau protein. The "amyloid hypothesis," which is based on molecular defects observed in autosomal-dominant early-onset Alzheimer's disease (EOAD), suggests a serial model of causality, whereby elevation of Abeta drives other disease features including tau hyperphosphorylation. Here, we review recent evidence from drug trials, genetic studies, and experimental work in animal models that suggests that an alternative model might exist in late-onset AD (LOAD), the complex and more common form of the disease. Specifically, we hypothesize a "dual pathway" model of causality, whereby Abeta and tau can be linked by separate mechanisms driven by a common upstream driver. This model may account for the results of recent drug trials and, if confirmed, may guide future drug development.

Phosphorylation of tau regulates its axonal transport by controlling its binding to kinesin

Defective axonal transport has been proposed as an underlying mechanism that may give rise to neurodegeneration. We investigated the effect of phosphorylation on the axonal transport of tau, a neuronal protein that stabilizes microtubules and is hyperphosphorylated and mislocalized in Alzheimer's disease. We report here that specific inhibition of glycogen synthase kinase-3 (GSK-3) reduces tau phosphorylation and significantly decreases the overall rate of axonal transport of tau in rat cortical neurons. Tau mutants, with serine/threonine targets of GSK-3 mutated to glutamate to mimic a permanent state of phosphorylation, were transported at a significantly increased rate compared to wild-type tau. Conversely, tau mutants, in which alanine replaced serine/threonine to mimic permanent dephosphorylation, were transported at a decreased rate compared to wild-type tau. We also found that tau interacts with the light chain of kinesin-1 and that this is dependent on the phosphorylation state of tau. Tau phosphorylation by GSK-3 increased binding, and dephosphorylated tau exhibited a reduced association with kinesin-1. We conclude that GSK-3 phosphorylation of tau modulates its axonal transport by regulating binding to kinesin-1. Hyperphosphorylated tau in Alzheimer's disease appearing first in distal portions of axons may result from aberrant axonal transport of phosphorylated tau reported here.

The CDC2 I-G-T haplotype associated with the APOE ɛ4 allele increases the risk of sporadic Alzheimer's disease in Sicily

The cell division cycle 2 (CDC2) gene is a candidate susceptibility gene for Alzheimer's disease (AD). We investigated the CDC2 genotype, and allele and haplotype frequencies in AD patients and matched controls, distinguishing between apolipoprotein E (APOE) epsilon4 allele carriers and non-carriers. APOE epsilon4 is an established predictor of AD risk. APOE and CDC2 genotypes were examined in 109 sporadic AD patients and in 110 healthy age- and sex-matched controls from Sicily. The epsilon4 allele of APOE was predictive of AD risk in our study group (odds ratio: 5.37, 95% CI 2.77-10.41; P<0.0001). Genotype and allele frequencies of the three tested CDC2 polymorphisms (Ex6+7I/D, Ex7-15 G>A, Ex7-14 T>A) were not significantly different between AD patients and controls. However, a significant different distribution of a specific CDC2 haplotype (I-G-T) was found between AD patients and controls when analyzing APOE epsilon4-positive subjects (P=0.0288). Moreover, the combined presence of the I-G-T haplotype and the epsilon4 allele almost doubled the risk of AD (odds ratio: 10.09, 95% CI 3.88-26.25; P<0.0001) compared to carriers of epsilon4 alone. This study suggests that the I-G-T haplotype of the CDC2 gene increases the risk of AD in APOE epsilon4 carriers.

Apolipoprotein E decreases tau kinases and phospho-tau levels in primary neurons

Apolipoprotein E (apoE) receptors act as signaling molecules in neurons, altering phosphorylation of numerous proteins after extracellular ligand binding and affecting neurite outgrowth, synapse formation, and neuronal migration. Since apoE is important in the pathogenesis of Alzheimer's disease (AD), we tested whether apoE treatment of neurons affected molecules important to phosphorylation of tau, such as GSK 3β, P35, and CDK5, and the phosphorylation of tau itself. Treatment of primary neurons with 2 uM apoE (or an apoE-derived peptide) decreased levels of phospho-GSK 3β, P35 and CDK5, and decreased levels of phosphorylated forms of tau. A lower concentration of apoE (100 nM) had no effect on these molecules. The alteration of tau phosphorylation by apoE was blocked by an inhibitor of the low-density lipoprotein receptor family, demonstrating the effects were due to receptor interactions. These results demonstrate that apoE affects several downstream signaling cascades in neurons: decreased tau kinases phosphorylation and inhibition of tau phosphorylation at Thr171 and Ser202/Thr205 epitopes. We conclude that apoE can alter levels of tau kinases and phospho-tau epitopes, potentially affecting tau neuropathological changes seen in AD brains.

ApoE allelic variability influences pupil response to cholinergic challenge and cognitive impairment

Exaggerated pupil response to dilute tropicamide has been suggested as an early biological marker for Alzheimer's disease. The current study links apolipoprotein E (ApoE) allelic variability to the magnitude of pupil response in a sample of community-dwelling elderly without a diagnosis of Alzheimer's disease or dementia. Possession of an epsilon 4 allele influences both the likelihood of exhibiting an exaggerated pupil response above a predetermined cut-off (13% above baseline diameter) and the absolute overall magnitude of the response. Allelic variability was also shown to correlate with cognitive impairments in memory and attention. The data in this study further elucidate the nature of the biological bond between an exaggerated pupil response and the pathology of Alzheimer's disease. ApoE allelic variability is probably linked to pupil response through its influence on tau hyperphosphorylation. The early Alzheimer's pathology seen in the Edinger-Westphal area of cranial nerve III, a major centre for pupil control, is primarily tau-based with significant cell loss in this nucleus leading to central denervation hypersensitivity even in elderly who are clinically silent but who have early pathology.

Tau phosphorylation: physiological and pathological consequences

The microtubule-associated protein tau, abundant in neurons, has gained notoriety due to the fact that it is deposited in cells as fibrillar lesions in numerous neurodegenerative diseases, and most notably Alzheimer's disease. Regulation of microtubule dynamics is the most well-recognized function of tau, but it is becoming increasingly evident that tau plays additional roles in the cell. The functions of tau are regulated by site-specific phosphorylation events, which if dysregulated, as they are in the disease state, result in tau dysfunction and mislocalization, which is potentially followed by tau polymerization, neuronal dysfunction and death. Given the increasing evidence that a disruption in the normal phosphorylation state of tau plays a key role in the pathogenic events that occur in Alzheimer's disease and other neurodegenerative conditions, it is of crucial importance that the protein kinases and phosphatases that regulate tau phosphorylation in vivo as well as the signaling cascades that regulate them be identified. This review focuses on recent literature pertaining to the regulation of tau phosphorylation and function in cell culture and animal model systems, and the role that a dysregulation of tau phosphorylation may play in the neuronal dysfunction and death that occur in neurodegenerative diseases that have tau pathology.

Proteomics in Alzheimer's disease: insights into potential mechanisms of neurodegeneration

Proteomics involves the identification of unknown proteins following their separation, often using two-dimensional electrophoresis, digestion of particular proteins of interest by trypsin, determination of the molecular weight of the resulting peptides, and database searching to make the identification of the proteins. Application of proteomics to Alzheimer's disease (AD), the major dementing disorder of the elderly, has just begun. Differences in protein expression and post-translational modification (mostly oxidative modification) of proteins from AD brain and peripheral tissue, as well as in brain from rodent models of AD, have yielded insights into potential molecular mechanisms of neurodegeneration in this dementing disorder. This review surveys the proteomics studies relevant to AD, from which new understandings of the pathology, biochemistry, and physiology of AD are beginning to emerge.

Apolipoprotein E (apoE) uptake and distribution in mammalian cell lines is dependent upon source of apoE and can be monitored in living cells

As part of investigations of the cellular uptake of apolipoprotein E (apoE) relevant to Alzheimer's disease we have found that different preparations of apoE are handled differently by cells expressing the LDL-receptor. Comparing recombinant, cellular and native apoE, complexed with different preparations of lipid we find that only cellular and native apoE enter a vesicular compartment. Some, but not all of these apoE containing vesicles are lysosomes. In order to further examine the intracellular fate of apoE we demonstrate that apoE-Enhanced green fluorescent protein chimeric protein can be taken up from medium by recipient cells and tracked within these cells for extended periods.

Truncated apoE forms tangle-like structures in a neuronal cell line

Apolipoprotein E is the predominant brain lipoprotein and polymorphic variation in the APOE gene the major genetic susceptibly factor for late onset Alzheimer's disease (AD). Recently it was reported that carboxyl-truncated ApoE fragments induce tangle-like structures in neurons. We confirm the finding: in mouse neuroblastoma cells truncated apoE fragments lacking the carboxyterminus induce structures that have the appearance of neurofibrillary tangles. However these tangles are not induced in non-neuronal cells even in the presence of co-expressed neurofilaments or tau. Further understanding of the basis of this cell specificity might add to understanding of the cell specificity of tangles in AD.

Apolipoprotein E: a major piece in the Alzheimer's disease puzzle

Alzheimer's disease (AD) is a complex neurodegenerative disorder with multiple etiologies. The presence of the E4 isoform of apolipoprotein E (apoE) has been shown to increase the risk and to decrease the age of onset for AD and is the major susceptibility factor known for the disease. ApoE4 has been shown to intensify all the biochemical disturbances characteristic of AD, including beta amyloid (Abeta) deposition, tangle formation, neuronal cell death, oxidative stress, synaptic plasticity and dysfunctions of lipid homeostasis and cholinergic signalling. In contrast, other apoE isoforms are protective. Here we review and discuss these major hypotheses of the apoE4-AD association.

Effects of FTDP-17 mutations on the in vitro phosphorylation of tau by glycogen synthase kinase 3beta identified by mass spectrometry demonstrate certain mutations exert long-range conformational changes

In vitro phosphorylation of recombinant wild-type 2N4R tau and FTDP-17 exonic mutant forms P301L, V337M and R406W by glycogen synthase kinase 3beta (GSK3beta) was examined by two dimensional phosphopeptide mapping analysis on thin layer cellulose plates. Comparison of these peptide maps with those generated from wild-type 1N4R tau isoform from which the phosphopeptide constituents and sites of phosphorylation had been determined previously, enabled us to monitor directly changes in phosphorylation of the individual tau proteins. No differences were found in the phosphorylation of wild-type, P301L or V337M tau by GSK3beta but the R406W mutant showed at least two clear differences from the other three tau proteins. The peptides, identified by mass spectrometry corresponding to phosphorylation at both threonine 231 and serine 235 (spot 3), serines 396, 400 and 404 (spot 6a) and serines 195 and 199 (spot 6b) were absent from the R406W peptide map. The findings imply that the R406W mutation in tau exerts long-range conformational effects on the structure of tau.