Abundance of Aβ₅-x like immunoreactivity in transgenic 5XFAD, APP/PS1KI and 3xTG mice, sporadic and familial Alzheimer's disease

Amyloid Precursor Protein: A Regulatory Hub in Alzheimer's Disease

Decades of research have demonstrated an incontrovertible role of amyloid-β (Aβ) in the etiology of Alzheimer's disease (AD). However, the overemphasis on the pathological impacts of Aβ may obscure the role of its metabolic precursor, amyloid precursor protein (APP), as a significant hub in the occurrence and progression of AD. The complicated enzymatic processing, ubiquitous receptor-like properties, and abundant expression of APP in the brain, as well as its close links with systemic metabolism, mitochondrial function and neuroinflammation, imply that APP plays multifaceted roles in AD. In this review, we briefly describe the evolutionarily conserved biological characteristics of APP, including its structure, functions and enzymatic processing. We also discuss the possible involvement of APP and its enzymatic metabolites in AD, both detrimental and beneficial. Finally, we describe pharmacological agents or genetic approaches with the capability to reduce APP expression or inhibit its cellular internalization, which can ameliorate multiple aspects of AD pathologies and halt disease progression. These approaches provide a basis for further drug development to combat this terrible disease.

Amyloid β, Lipid Metabolism, Basal Cholinergic System, and Therapeutics in Alzheimer’s Disease

The presence of insoluble aggregates of amyloid β (Aβ) in the form of neuritic plaques (NPs) is one of the main features that define Alzheimer’s disease. Studies have suggested that the accumulation of these peptides in the brain significantly contributes to extensive neuronal loss. Furthermore, the content and distribution of cholesterol in the membrane have been shown to have an important effect on the production and subsequent accumulation of Aβ peptides in the plasma membrane, contributing to dysfunction and neuronal death. The monomeric forms of these membrane-bound peptides undergo several conformational changes, ranging from oligomeric forms to beta-sheet structures, each presenting different levels of toxicity. Aβ peptides can be internalized by particular receptors and trigger changes from Tau phosphorylation to alterations in cognitive function, through dysfunction of the cholinergic system. The goal of this review is to summarize the current knowledge on the role of lipids in Alzheimer’s disease and their relationship with the basal cholinergic system, as well as potential disease-modifying therapies.

Cu, Fe, and Zn isotope ratios in murine Alzheimer's disease models suggest specific signatures of amyloidogenesis and tauopathy

Alzheimer’s disease (AD) is characterized by accumulation of tau and amyloid-beta in the brain, and recent evidence suggests a correlation between associated protein aggregates and trace elements, such as copper, iron, and zinc. In AD, a distorted brain redox homeostasis and complexation by amyloid-beta and hyperphosphorylated tau may alter the isotopic composition of essential mineral elements. Therefore, high-precision isotopic analysis may reveal changes in the homeostasis of these elements. We used inductively coupled plasma–mass spectrometry (ICP-MS)-based techniques to determine the total Cu, Fe, and Zn contents in the brain, as well as their isotopic compositions in both mouse brain and serum. Results for male transgenic tau (Line 66, L66) and amyloid/presenilin (5xFAD) mice were compared with those for the corresponding age- and sex-matched wild-type control mice (WT). Our data show that L66 brains showed significantly higher Fe levels than did those from the corresponding WT. Significantly less Cu, but more Zn was found in 5xFAD brains. We observed significantly lighter isotopic compositions of Fe (enrichment in the lighter isotopes) in the brain and serum of L66 mice compared with WT. For 5xFAD mice, Zn exhibited a trend toward a lighter isotopic composition in the brain and a heavier isotopic composition in serum compared with WT. Neither mouse model yielded differences in the isotopic composition of Cu. Our findings indicate significant pathology-specific alterations of Fe and Zn brain homeostasis in mouse models of AD. The associated changes in isotopic composition may serve as a marker for proteinopathies underlying AD and other types of dementia.

Treatment with scFv-h3D6 Prevented Neuronal Loss and Improved Spatial Memory in Young 3xTg-AD Mice by Reducing the Intracellular Amyloid-β Burden

The intracellular deposition of amyloid-β (Aβ) peptides has been described in the brains of both Alzheimer's disease (AD) patients and animal models. A correlation between the intracellular amyloid burden and neurodegeneration has recently been reported in a triple-transgenic AD (3xTg-AD) murine model. In the present study, we assessed the effect of scFv-h3D6, an anti-Aβ single-chain variable fragment (scFv) derived from the antibody bapineuzumab, on amyloid pathology in 5-month-old 3xTg-AD female mice, focusing on intracellular Aβ clearance, neuronal survival, and functional abilities. We also examined neuroinflammation and the histology of peripheral organ samples to detect any adverse effects. A single intraperitoneal injection of scFv-h3D6 dramatically reduced intracellular Aβ burden in the deep layers of the cerebral cortex, pyramidal cells layer of the hippocampus, and basolateral amygdalar nucleus. The treatment prevented neuronal loss in the hippocampus and amygdala, while neither astrogliosis nor microgliosis was induced. Instead, an increase in the size of the white pulp after the treatment indicated that the spleen could be involved in the clearance mechanism. Although the treatment did not ameliorate behavioral and psychological symptoms of dementia-like symptoms, the results of cognitive testing pointed to a noticeable improvement in spatial memory. These findings indicated that the mechanism underlying the therapeutic effect of scFv-h3D6 was the clearance of intracellular Aβ, with subsequent prevention of neuronal loss and amelioration of cognitive disabilities. The treatment was safe in terms of neuroinflammation and kidney and liver function, whereas some effects on the spleen were observed.

Rodent Models of Amyloid-Beta Feature of Alzheimer's Disease: Development and Potential Treatment Implications

Alzheimer's disease (AD) is the most common neurodegenerative disorder worldwide and causes severe financial and social burdens. Despite much research on the pathogenesis of AD, the neuropathological mechanisms remain obscure and current treatments have proven ineffective. In the past decades, transgenic rodent models have been used to try to unravel this disease, which is crucial for early diagnosis and the assessment of disease-modifying compounds. In this review, we focus on transgenic rodent models used to study amyloid-beta pathology in AD. We also discuss their possible use as promising tools for AD research. There is still no effective treatment for AD and the development of potent therapeutics are urgently needed. Many molecular pathways are susceptible to AD, ranging from neuroinflammation, immune response, and neuroplasticity to neurotrophic factors. Studying these pathways may shed light on AD pathophysiology as well as provide potential targets for the development of more effective treatments. This review discusses the advantages and limitations of these models and their potential therapeutic implications for AD.

Both Amyloid-β Peptide and Tau Protein Are Affected by an Anti-Amyloid-β Antibody Fragment in Elderly 3xTg-AD Mice

Alzheimer's disease (AD) is the most common dementia worldwide. According to the amyloid hypothesis, the early accumulation of the Aβ-peptide triggers tau phosphorylation, synaptic dysfunction, and eventually neuronal death leading to cognitive impairment, as well as behavioral and psychological symptoms of dementia. ScFv-h3D6 is a single-chain variable fragment that has already shown its ability to diminish the amyloid burden in 5-month-old 3xTg-AD mice. However, tau pathology is not evident at this early stage of the disease in this mouse model. In this study, the effects of scFv-h3D6 on Aβ and tau pathologies have been assessed in 22-month-old 3xTg-AD mice. Briefly, 3xTg-AD female mice were treated for 2 weeks with scFv-h3D6 and compared with 3xTg-AD and non-transgenic (NTg) mice treated with PBS. The treatment with scFv-h3D6 was unequivocally effective in reducing the area of Aβ staining. Furthermore, a tendency for a reduction in tau levels was also observed after treatment that points to the interplay between Aβ and tau pathologies. The pro-inflammatory state observed in the 3xTg-AD mice did not progress after scFv-h3D6 treatment. In addition, the treatment did not alter the levels of apolipoprotein E or apolipoprotein J. Thus, a 2-week treatment with scFv-h3D6 was able to reduce AD-like pathology in elderly 3xTg-AD female mice.

5XFAD mice show early-onset gap encoding deficits in the auditory cortex

Early detection will be crucial for effective treatment or prevention of Alzheimer's disease. The identification and validation of early, noninvasive biomarkers is therefore key to avoiding the most devastating aspects of Alzheimer's disease. Measures of central auditory processing such as gap detection have recently emerged as potential biomarkers in both human patients and the 5XFAD mouse model of Alzheimer's disease. Full validation of gap detection deficits as a biomarker will require detailed understanding of the underlying neuropathology, including which brain structures are involved and how the operation of neural circuits is affected. Here we show that 5XFAD mice exhibit gap detection deficits as early as 2 months of age, well before development of Alzheimer's disease-associated pathology. We then examined responses of neurons in the auditory cortex to gaps in white noise. Both gap responses and baseline firing rates were robustly and progressively degraded in 5XFAD mice compared to littermate controls. These impairments were first evident at 2-4 months of age in males, and 4-6 months in females. This demonstrates early-onset impairments to the central auditory system, which could be due to damage in the auditory cortex, upstream subcortical structures, or both.

Recommendations for measuring whisker movements and locomotion in mice with sensory, motor and cognitive deficits

BACKGROUND

Previous studies have measured whisker movements and locomotion to characterise mouse models of neurodegenerative disease. However, these studies have always been completed in isolation, and do not involve standardized procedures for comparisons across multiple mouse models and background strains.

NEW METHOD

We present a standard method for conducting whisker movement and locomotion studies, by carrying out qualitative scoring and quantitative measurement of whisker movements from high-speed video footage of mouse models of Amyotrophic Lateral Sclerosis, Huntington's disease, Parkinson's disease, Alzheimer's disease, Cerebellar Ataxia, Somatosensory Cortex Development and Ischemic stroke.

RESULTS

Sex, background strain, source breeder and genotype all affected whisker movements. All mouse models, apart from Parkinson's disease, revealed differences in whisker movements during locomotion. R6/2 CAG250 Huntington's disease mice had the strongest behavioural phenotype. Robo3R^3-5-CKO and RIM-DKOSert mouse models have abnormal somatosensory cortex development and revealed significant changes in whisker movements during object exploration.

COMPARISON WITH EXISTING METHOD(S)

Our results have good agreement with past studies, which indicates the robustness and reliability of measuring whisking. We recommend that differences in whisker movements of mice with motor deficits can be captured in open field arenas, but that mice with impairments to sensory or cognitive functioning should also be filmed investigating objects. Scoring clips qualitatively before tracking will help to structure later analyses.

CONCLUSIONS

Studying whisker movements provides a quantitative measure of sensing, motor control and exploration. However, the effect of background strain, sex and age on whisker movements needs to be better understood.

Vulnerability and resilience to Alzheimer's disease: early life conditions modulate neuropathology and determine cognitive reserve

BACKGROUND

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with a high prevalence among the elderly and a huge personal and societal impact. Recent epidemiological studies have indicated that the incidence and age of onset of sporadic AD can be modified by lifestyle factors such as education, exercise, and (early) stress exposure. Early life adversity is known to promote cognitive decline at a later age and to accelerate aging, which are both primary risk factors for AD. In rodent models, exposure to 'negative' or 'positive' early life experiences was recently found to modulate various measures of AD neuropathology, such as amyloid-beta levels and cognition at later ages. Although there is emerging interest in understanding whether experiences during early postnatal life also modulate AD risk in humans, the mechanisms and possible substrates underlying these long-lasting effects remain elusive.

METHODS

We review literature and discuss the role of early life experiences in determining later age and AD-related processes from a brain and cognitive 'reserve' perspective. We focus on rodent studies and the identification of possible early determinants of later AD vulnerability or resilience in relation to early life adversity/enrichment.

RESULTS

Potential substrates and mediators of early life experiences that may influence the development of AD pathology and cognitive decline are: programming of the hypothalamic-pituitary-adrenal axis, priming of the neuroinflammatory response, dendritic and synaptic complexity and function, overall brain plasticity, and proteins such as early growth response protein 1 (EGR1), activity regulated cytoskeleton-associated protein (Arc), and repressor element-1 silencing transcription factor (REST).

CONCLUSIONS

We conclude from these rodent studies that the early postnatal period is an important and sensitive phase that influences the vulnerability to develop AD pathology. Yet translational studies are required to investigate whether early life experiences also modify AD development in human studies, and whether similar molecular mediators can be identified in the sensitivity to develop AD in humans.

Intracerebroventricular streptozotocin-induced Alzheimer's disease-like sleep disorders in rats: Role of the GABAergic system in the parabrachial complex

AIM

Sleep disorders are common in Alzheimer's disease (AD) and assumed to directly influence cognitive function and disease progression. This study evaluated sleep characteristics in a rat model of AD that was induced by intracerebroventricular streptozotocin (STZ) administration and assessed the possible underlying mechanisms.

METHODS

Cognition ability was assessed in the Morris water maze in rats. Sleep parameters were analyzed by electroencephalographic and electromyographic recordings. Neuronal activity in brain areas that regulate sleep-wake states was evaluated by double-staining immunohistochemistry. High-performance liquid chromatography with electrochemical detection was used to detect neurotransmitter levels.

RESULTS

Fourteen days after the STZ injection, the rats exhibited sleep disorders that were similar to those in AD patients, reflected by a significant increase in wakefulness and decreases in nonrapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. The c-Fos expression analysis indicated that neuronal activity and the number of neurons in the dorsal raphe nucleus and locus coeruleus decreased in STZ-injected rats. In the ventrolateral preoptic nucleus (VLPO), the activity of γ-aminobutyric acid (GABA) neurons was suppressed. In the arousal-driving parabrachial nucleus (PBN), GABAergic activity was suppressed, whereas glutamatergic activity was promoted. The neurotransmitter analysis revealed a reduction in GABA in the VLPO and PBN and elevation of glutamate in the PBN. A direct injection of the GABA_A receptor antagonist bicuculline in the PBN in normal rats induced a similar pattern of sleep disorder as in STZ-injected rats. A microinjection of GABA in the PBN improved sleep disorders that were induced by STZ.

CONCLUSION

These results suggest that the reduction in GABAergic inhibition in the PBN and VLPO may be involved in sleep disorders that are induced by STZ. Our novel findings encourage further studies that investigate mechanisms of sleep regulation in sporadic AD.

Synergistic Effect on Neurodegeneration by N-Truncated Aβ4-42 and Pyroglutamate Aβ3-42 in a Mouse Model of Alzheimer's Disease

The N-terminally truncated pyroglutamate Aβ_3-42 (Aβ_pE3-42) and Aβ_4-42 peptides are known to be highly abundant in the brain of Alzheimer's disease (AD) patients. Both peptides show enhanced aggregation and neurotoxicity in comparison to full-length Aβ, suggesting that these amyloid peptides may play an important role in the pathogenesis of AD. The aim of the present work was to study the direct effect of the combination of Aβ_pE3-42 and Aβ_4-42 on ongoing AD-related neuron loss, pathology, and neurological deficits in transgenic mice. Bigenic mice were generated by crossing the established TBA42 and Tg4-42 mouse models expressing the N-truncated Aβ peptides Aβ_pE3-42 and Aβ_4-42, respectively. After generation of the bigenic mice, detailed phenotypical characterization was performed using either immunostainings to evaluate amyloid pathology or quantification of neuron numbers using design-based stereology. The elevated plus maze was used to study anxiety levels. In order to evaluate sensori-motor deficits, the inverted grid, the balance beam and the string suspension tasks were applied. We could demonstrate that co-expression of Aβ_pE3-42 and Aβ_4-42 accelerates neuron loss in the CA1 pyramidal layer of young bigenic mice as seen by reduced neuron numbers in comparison to single transgenic homozygous mice expressing either Aβ_pE3-42 or Aβ_4-42. This observation coincides with the robust intraneuronal Aβ accumulation observed in the bigenic mice. In addition, loss of anxiety and motor deficits were enhanced in an age-dependent manner. The sensori-motor deficits correlate with the abundant spinal cord pathology, as demonstrated by robust intracellular Aβ accumulation within motor neurons and extracellular Aβ deposition. Our observations demonstrate that a combination of Aβ_pE3-42 and Aβ_4-42 has a stronger effect on ongoing AD pathology than the peptides alone. Therefore, Aβ_pE3-42 and Aβ_4-42 might represent excellent potential therapeutic targets and diagnostic markers for AD.

N-truncated Aβ4-x peptides in sporadic Alzheimer's disease cases and transgenic Alzheimer mouse models

Background

The deposition of neurotoxic amyloid-β (Aβ) peptides in plaques in the brain parenchyma and in cerebral blood vessels is considered to be a key event in Alzheimer’s disease (AD) pathogenesis. Although the presence and impact of full-length Aβ peptides such as Aβ_1–40 and Aβ_1–42 have been analyzed extensively, the deposition of N-terminally truncated Aβ peptide species has received much less attention, largely because of the lack of specific antibodies.

Methods

This paper describes the generation and characterization of novel antibodies selective for Aβ_4–x peptides and provides immunohistochemical evidence of Aβ_4–x in the human brain and its distribution in the APP/PS1KI and 5XFAD transgenic mouse models.

Results

The Aβ_4–x staining pattern was restricted mainly to amyloid plaque cores and cerebral amyloid angiopathy in AD and Down syndrome cases and in both AD mouse models. In contrast, diffuse amyloid deposits were largely negative for Aβ_4–x immunoreactivity. No overt intraneuronal staining was observed.

Conclusions

The findings of this study are consistent with previous reports demonstrating a high aggregation propensity of Aβ_4–x peptides and suggest an important role of these N-truncated Aβ species in the process of amyloidogenesis and plaque core formation.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-017-0309-z) contains supplementary material, which is available to authorized users.

Spatial Memory Impairment is Associated with Intraneural Amyloid-β Immunoreactivity and Dysfunctional Arc Expression in the Hippocampal-CA3 Region of a Transgenic Mouse Model of Alzheimer's Disease

Dysfunction of synaptic communication in cortical and hippocampal networks has been suggested as one of the neuropathological hallmarks of the early stages of Alzheimer's disease (AD). Also, several lines of evidence have linked disrupted levels of activity-regulated cytoskeletal associated protein (Arc), an immediate early gene product that plays a central role in synaptic plasticity, with AD "synaptopathy". The mapping of Arc expression patterns in brain networks has been extensively used as a marker of memory-relevant neuronal activity history. Here we evaluated basal and behavior-induced Arc expression in hippocampal networks of the 3xTg-AD mouse model of AD. The basal percentage of Arc-expressing cells in 10-month-old 3xTg-AD mice was higher than wild type in CA3 (4.88% versus 1.77% , respectively) but similar in CA1 (1.75% versus 2.75% ). Noteworthy, this difference was not observed at 3 months of age. Furthermore, although a Morris water maze test probe induced a steep (∼4-fold) increment in the percentage of Arc+ cells in the CA3 region of the 10-month-old wild-type group, no such increment was observed in age-matched 3xTg-AD, whereas the amount of Arc+ cells in CA1 increased in both groups. Further, we detected that CA3 neurons with amyloid-β were much more likely to express Arc protein under basal conditions. We propose that in 3xTg-AD mice, intraneuronal amyloid-β expression in CA3 could increase unspecific neuronal activation and subsequent Arc protein expression, which might impair further memory-stabilizing processes.

Implications of GABAergic Neurotransmission in Alzheimer's Disease

Alzheimer's disease (AD) is characterized pathologically by the deposition of β-amyloid peptides (Aβ) and the accumulation of neurofibrillary tangles (NFTs) composed of hyper-phosphorylated tau. Regardless of the pathological hallmarks, synaptic dysfunction is widely accepted as a causal event in AD. Of the two major types of synapses in the central nervous system (CNS): glutamatergic and GABAergic, which provide excitatory and inhibitory outputs respectively, abundant data implicate an impaired glutamatergic system during disease progression. However, emerging evidence supports the notion that disrupted default neuronal network underlies impaired memory, and that alterations of GABAergic circuits, either plays a primary role or as a compensatory response to excitotoxicity, may also contribute to AD by disrupting the overall network function. The goal of this review is to provide an overview of the involvement of Aβ, tau and apolipoprotein E4 (apoE4), the major genetic risk factor in late-onset AD (LOAD), in GABAergic neurotransmission and the potential of modulating the GABAergic function as AD therapy.

Intraneuronal Aβ accumulation induces hippocampal neuron hyperexcitability through A-type K(+) current inhibition mediated by activation of caspases and GSK-3

Amyloid β-protein (Aβ) pathologies have been linked to dysfunction of excitability in neurons of the hippocampal circuit, but the molecular mechanisms underlying this process are still poorly understood. Here, we applied whole-cell patch-clamp electrophysiology to primary hippocampal neurons and show that intracellular Aβ42 delivery leads to increased spike discharge and action potential broadening through downregulation of A-type K(+) currents. Pharmacologic studies showed that caspases and glycogen synthase kinase 3 (GSK-3) activation are required for these Aβ42-induced effects. Extracellular perfusion and subsequent internalization of Aβ42 increase spike discharge and promote GSK-3-dependent phosphorylation of the Kv4.2 α-subunit, a molecular determinant of A-type K(+) currents, at Ser-616. In acute hippocampal slices derived from an adult triple-transgenic Alzheimer's mouse model, characterized by endogenous intracellular accumulation of Aβ42, CA1 pyramidal neurons exhibit hyperexcitability accompanied by increased phosphorylation of Kv4.2 at Ser-616. Collectively, these data suggest that intraneuronal Aβ42 accumulation leads to an intracellular cascade culminating into caspases activation and GSK-3-dependent phosphorylation of Kv4.2 channels. These findings provide new insights into the toxic mechanisms triggered by intracellular Aβ42 and offer potentially new therapeutic targets for Alzheimer's disease treatment.

Focusing the amyloid cascade hypothesis on N-truncated Abeta peptides as drug targets against Alzheimer's disease

Although N-truncated Aβ variants are known to be the main constituent of amyloid plaques in the brains of patients with Alzheimer’s disease, their potential as targets for pharmacological intervention has only recently been investigated. In the last few years, the Alzheimer field has experienced a paradigm shift with the ever increasing understanding that targeting amyloid plaques has not led to a successful immunotherapy. On the other hand, there can be no doubt that the amyloid cascade hypothesis is central to the etiology of Alzheimer’s disease, raising the question as to why it is apparently failing to translate into the clinic. In this review, we aim to refocus the amyloid hypothesis integrating N-truncated Aβ peptides based on mounting evidence that they may represent better targets than full-length Aβ. In addition to Aβ peptides starting with an Asp at position 1, a variety of different N-truncated Aβ peptides have been identified starting with amino residue Ala-2, pyroglutamylated Glu-3, Phe-4, Arg-5, His-6, Asp-7, Ser-8, Gly-9, Tyr-10 and pyroglutamylated Glu-11. Certain forms of N-truncated species are better correlates for early pathological changes found pre-symptomatically more often than others. There is also evidence that, together with full-length Aβ, they might be physiologically detectable and are naturally secreted by neurons. Others are known to form soluble aggregates, which have neurotoxic properties in transgenic mouse models. It has been clearly demonstrated by several groups that some N-truncated Aβs dominate full-length Aβ in the brains of Alzheimer’s patients. We try to address which of the N-truncated variants may be promising therapeutic targets and which enzymes might be involved in the generation of these peptides