The type of Aβ-related neuronal degeneration differs between amyloid precursor protein (APP23) and amyloid β-peptide (APP48) transgenic mice

What is the gold standard model for Alzheimer's disease drug discovery and development?

Introduction: Alzheimer's disease models (ADMs) are currently used for drug development (DD). More than 20,000 molecules were screened for AD treatment over decades, with only one drug (Aducanumab)FDA-approved over the past 18 years. A revision of pathogenic concepts and ADMs are needed.Areas covered: The authors discuss herein preclinical models including: (i) in vitro models (cell lines, primary neuron cell cultures, iPSC-derived brain cells), (ii) ex vivo models, and (iii) in vivo models (artificial, transgenic, non-transgenic and induced).Expert opinion: The following types of ADMs have been reported: Mouse models (45.08%), Rat models (15.04%), Non-human Primate models (0.76%), Rabbit models (0.46%), Cat models (0.53%), Pig models (0.30%), Guinea pig models (0.15%), Octodon degu models (0.02%), Dog models (0.54%), Drosophila melanogaster models (1.79%), Zebrafish models (0.50%), Caenorhabditis elegans (1.21%), Cell culture models (3.31%), Cholinergic models (8.26%), Neurotoxic models (6.79%), Neuroinflammation models (6.92%), Neurovascular models (7.88%), and Microbiome models (0.45%).No single ADM faithfully reproduces all the pathogenic events in the human AD phenotype spectrum. ADMs should be different for (i) pathogenic studies vs basic DD, and (ii) preventive interventions vs symptomatic treatments. There cannot be an ideal ADM for DD, because AD is a spectrum of syndromes. DD can integrate pathogenic, mechanistic, metabolic, transporter and pleiotropic genes in a multisystem model.

Neuron Loss in Alzheimer's Disease: Translation in Transgenic Mouse Models

Transgenic mouse models represent an essential tool for the exploration of Alzheimer’s disease (AD) pathological mechanisms and the development of novel treatments, which at present provide only symptomatic and transient effects. While a variety of mouse models successfully reflects the main neuropathological hallmarks of AD, such as extracellular amyloid-β (Aβ) deposits, intracellular accumulation of Tau protein, the development of micro- and astrogliosis, as well as behavioral deficits, substantial neuron loss, as a key feature of the disease, seems to be more difficult to achieve. In this review, we summarize information on classic and more recent transgenic mouse models for AD, focusing in particular on loss of pyramidal, inter-, and cholinergic neurons. Although the cause of neuron loss in AD is still a matter of scientific debate, it seems to be linked to intraneuronal Aβ accumulation in several transgenic mouse models, especially in pyramidal neurons.

Effect of AmyTrap, an amyloid-β binding drug, on Aβ induced mitochondrial dysfunction and tau phosphorylation in cultured neuroblastoma cells

Alzheimer's Disease (AD) is the most common cause of dementia, affecting 25 million people worldwide. Accumulation of Amyloid-β (Aβ) in the mitochondria has been shown to adversely affect key enzymes including pyruvate dehydrogenase (PDH), succinate dehydrogenase (SDH), oxoglutarate dehydrogenase (OGDH). Accumulation of Aβ is also believed to increase Tau expression and pathology. Tau, in its toxic state, results in synaptic damage causing memory and cognitive dysfunction. We are developing a drug to treat AD namely AmyTrap. The active pharmacological ingredient is a retro inverso, Aβ-binding peptide which sequesters Aβ. We wanted to examine the effect of AmyTrap peptide on Aβ-induced mitochondrial dysfunction and Tau phosphorylation. Therefore, we treated SH-SY5Y neuroblastoma cells with wild-type Aβ, a mutant AβY10A, AmyTrap peptide (RI-peptide), or Aβ and RI-peptide for 72 h. The mutant AβY10A is known to impact the self-aggregating property of Aβ as this Tyr10 is essential for self-aggregation. As expected, AβY10A reversed PDH, OGDH and SDH dysfunction to near normal levels. Further, AβY10A successfully reversed Tau phosphorylation, suggesting that Tyr10 is also associated with Aβ-induced cytotoxicity. RI-peptide was able to significantly reverse SDH dysfunction with limited effect on PDH and Tau phosphorylation. The findings are suggestive that the Tyr10 on Aβ plays a critical role in the self-aggregation. Further studies are warranted to expand these findings.

Microglial Trem2 induces synaptic impairment at early stage and prevents amyloidosis at late stage in APP/PS1 mice

Triggering receptor expressed in myeloid cells (TREM)2 is a genetic high-risk factor for sporadic Alzheimer's disease (AD) and is considered a potential target for AD diagnosis and therapy, although its role in the different stages of AD remains controversial. We generated an embryonic deletion of Trem2 (whole body deletion) and induced hippocampal- and cortical-specific knockdown of microglial Trem2 at different stages of the AD process in amyloid precursor protein/Psen1 mice by adeno-associated virus (AAV) infection. AAV infection induced microglial Trem2 overexpression in the hippocampus of wild-type (WT) and thymus cell antigen 1-enhanced green fluorescent protein mice. Mice were subjected to ethological and pathologic tests. Whole body genetic deletion of Trem2 exerted different electrophysiological outcomes between different AD pathologic stages, which results from a complex integration of synaptic loss and amyloid aggregation. Interestingly, knockdown of Trem2 at the early-middle stage of AD (2-6 mo) prevents synaptic loss through directly inhibiting microglial phagocytosis, whereas knockdown of Trem2 at the middle-late stage of AD (6-10 mo) accelerates synaptic dysfunction because of more severe amyloid deposition caused by the depression of microglial phagocytosis. Additionally, hippocampal overexpression of Trem2 in WT mice results in significant synaptic impairment. Here, with transgenic technology and electrophysiological assay, we revealed that TREM2 up-regulation promotes microglial phagocytosis equally against synapse and amyloid plaques and eventually results in different outcomes. During the early-middle pathologic stage, TREM2 enhancing microglial phagocytosis mainly causes synaptic loss. However, TREM2 up-regulating microglial phagocytosis gradually supports a positive role when amyloid deposition occupies the leading position at the middle-late pathologic stage. In this study, we highlighted that TREM2 triggers synaptic loss during AD pathology development.-Sheng, L., Chen, M., Cai, K., Song, Y., Yu, D., Zhang, H., Xu, G. Microglial Trem2 induces synaptic impairment at early stage and prevents amyloidosis at late stage in APP/PS1 mice.

Reduced plaque size and inflammation in the APP23 mouse model for Alzheimer's disease after chronic application of polymeric nanoparticles for CNS targeted zinc delivery

A local dyshomeostasis of zinc ions in the vicinity of amyloid aggregates has been proposed in Alzheimer's disease (AD) due to the sequestration of zinc in senile plaques. While an increase in zinc levels may promote the aggregation of amyloid beta (Aβ), increased brain zinc might also be beneficial rescuing some pathological alterations caused by local zinc deficiency. For example, increased Aβ degradation by metalloproteinases, and a reduction in inflammation can be hypothesized. In addition, zinc may allow a stabilization of the number of synapses in AD brains. Thus, to evaluate whether altering zinc-levels within the brain is a promising new target for the prevention and treatment of AD, we employed novel zinc loaded nanoparticles able to deliver zinc into the brain across the blood-brain barrier. We performed in vivo studies using wild type (WT) and APP23 mice to assess plaque load, inflammatory status and synapse loss. Furthermore, we performed behavioral analyses. After chronically injecting these nanoparticles for 14 days, our results show a significant reduction in plaque size and effects on the pro-inflammatory cytokines IL-6 and IL-18. On behavioral level we could not detect negative effects of increased brain zinc levels in APP23 mice and treatment with g7-NP-Zn normalized the observed hyperlocomotion of APP23 mice. Therefore, we conclude that a targeted increase in brain zinc levels may have beneficial effects in AD.

Interactions of pathological proteins in neurodegenerative diseases

Neurodegenerative diseases such as Alzheimer's disease (AD), frontotemporal lobar degeneration (FTD), Lewy body disease (LBD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS) have in common that protein aggregates represent pathological hallmark lesions. Amyloid β-protein, τ-protein, α-synuclein, and TDP-43 are the most frequently aggregated proteins in these disorders. Although they are assumed to form disease-characteristic aggregates, such as amyloid plaques and neurofibrillary tangles in AD or Lewy bodies in LBD/PD, they are not restricted to these clinical presentations. They also occur in non-diseased individuals and can co-exist in the same brain without or with a clinical picture of a distinct dementing or movement disorder. In this review, we discuss the co-existence of these pathologies and potential additive effects in the human brain as well as related functional findings on cross-seeding and molecular interactions between these aggregates/proteins. We conclude that there is evidence for interactions at the molecular level as well as for additive effects on brain damage by multiple pathologies occurring in different functionally important neurons. Based upon this information, we hypothesize a cascade of events that may explain general mechanisms in the development of neurodegenerative disorders: (1) distinct lesions are a prerequisite for the development of a distinct disease (e.g., primary age-related tauopathy for AD), (2) disease-specific pathogenic events further trigger the development of a specific disease (e.g., Aβ aggregation in AD that exaggerate further Aβ and AD-related τ pathology), (3) the symptomatic disease manifests, and (4) neurodegenerative co-pathologies may be either purely coincidental or (more likely) have influence on the disease development and/or its clinical presentation.

Selective deletion of apolipoprotein E in astrocytes ameliorates the spatial learning and memory deficits in Alzheimer's disease (APP/PS1) mice by inhibiting TGF-β/Smad2/STAT3 signaling

Astrocytes and apolipoprotein E (apoE) play critical roles in cognitive function, not only under physiological conditions but also in some pathological situations, particularly in the pathological progression of Alzheimer's disease (AD). The regulatory mechanisms underlying the effect of apoE, derived from astrocytes, on cognitive deficits during AD pathology development are unclear. In this study, we generated amyloid precursor protein/apoE knockout (APP/apoE^KO) and APP/glial fibrillary acidic protein (GFAP)-apoE^KO mice (the AD mice model used in this study was based on the APP-familial Alzheimer disease overexpression) to investigate the role of apoE, derived from astrocytes, in AD pathology and cognitive function. To explore the mechanism, we investigated the amyloidogenic process related transforming growth factor β/mothers against decapentaplegic homolog 2/signal transducer and activator of transcription 3 (TGF-β/Smad2/STAT3) signaling pathway and further confirmed by administering TGF-β-overexpression adeno-associated virus (specific to astrocytes) to APP/GFAP-apoE^KO mice and TGF-β-inhibition adeno-associated virus (specific to astrocytes) to APP/WT mice. Whole body deletion of apoE significantly ameliorated the spatial learning and memory impairment, reduced amyloid β-protein production and inhibited astrogliosis in APP/apoE^KO mice, as well as specific deletion apoE in astrocytes in APP/GFAP-apoE^KO mice. Moreover, amyloid β-protein accumulation was increased due to promotion of amyloidogenesis of APP, and astrogliosis was upregulated by activation of TGF-β/Smad2/STAT3 signaling. Furthermore, the overexpression of TGF-β in astrocytes in APP/GFAP-apoE^KO mice abrogated the effects of apoE knockout. In contrast, repression of TGF-β in astrocytes of APP/WT mice exerted a therapeutic effect similar to apoE knockout. These data suggested that apoE derived from astrocytes contributes to the risk of AD through TGF-β/Smad2/STAT3 signaling activation. These findings enhance our understanding of the role of apoE, derived from astrocytes, in AD and suggest it to be a potential biomarker and therapeutic target for AD.

Metabolic Characterization of Intact Cells Reveals Intracellular Amyloid Beta but Not Its Precursor Protein to Reduce Mitochondrial Respiration

One hallmark of Alzheimer´s disease are senile plaques consisting of amyloid beta (Aβ), which derives from the processing of the amyloid precursor protein (APP). Mitochondrial dysfunction has been linked to the pathogenesis of Alzheimer´s disease and both Aβ and APP have been reported to affect mitochondrial function in isolated systems. However, in intact cells, considering a physiological localization of APP and Aβ, it is pending what triggers the mitochondrial defect. Thus, the aim of this study was to dissect the impact of APP versus Aβ in inducing mitochondrial alterations with respect to their subcellular localization. We performed an overexpression of APP or beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), increasing APP and Aβ levels or Aβ alone, respectively. Conducting a comprehensive metabolic characterization we demonstrate that only APP overexpression reduced mitochondrial respiration, despite lower extracellular Aβ levels compared to BACE overexpression. Surprisingly, this could be rescued by a gamma secretase inhibitor, oppositionally indicating an Aβ-mediated mitochondrial toxicity. Analyzing Aβ localization revealed that intracellular levels of Aβ and an increased spatial association of APP/Aβ with mitochondria are associated with reduced mitochondrial respiration. Thus, our data provide marked evidence for a prominent role of intracellular Aβ accumulation in Alzheimer´s disease associated mitochondrial dysfunction. Thereby it highlights the importance of the localization of APP processing and intracellular transport as a decisive factor for mitochondrial function, linking two prominent hallmarks of neurodegenerative diseases.

Impact of amyloid β aggregate maturation on antibody treatment in APP23 mice

Introduction

The deposition of the amyloid β protein (Aβ) in the brain is a hallmark of Alzheimer's disease (AD). Removal of Aβ by Aβ-antibody treatment has been developed as a potential treatment strategy against AD. First clinical trials showed neither a stop nor a reduction of disease progression. Recently, we have shown that the formation of soluble and insoluble Aβ aggregates in the human brain follows a hierarchical sequence of three biochemical maturation stages (B-Aβ stages). To test the impact of the B-Aβ stage on Aβ immunotherapy, we treated transgenic mice expressing human amyloid precursor protein (APP) carrying the Swedish mutation (KM670/671NL; APP23) with the Aβ-antibody β1 or phosphate-buffered saline (PBS) beginning 1) at 3 months, before the onset of dendrite degeneration and plaque deposition, and 2) at 7 months, after the start of Aβ plaque deposition and dendrite degeneration.

Results

At 5 months of age, first Aβ aggregates in APP23 brain consisted of non-modified Aβ (representing B-Aβ stage 1) whereas mature Aβ-aggregates containing N-terminal truncated, pyroglutamate-modified Aβ_N3pE and phosphorylated Aβ (representing B-Aβ stage 3) were found at 11 months of age in both β1- and PBS-treated animals. Protective effects on commissural neurons with highly ramified dendritic trees were observed only in 3-month-old β1-treated animals sacrificed at 5 months. When treatment started at 7 months of age, no differences in the numbers of healthy commissural neurons were observed between β1- and PBS-treated APP23 mice sacrificed with 11 months.

Conclusions

Aβ antibody treatment was capable of protecting neurons from dendritic degeneration as long as Aβ aggregation was absent or represented B-Aβ stage 1 but had no protective or curative effect in later stages with mature Aβ aggregates (B-Aβ stage 3). These data indicate that the maturation stage of Aβ aggregates has impact on potential treatment effects in APP23 mice.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0217-z) contains supplementary material, which is available to authorized users.

Analyzing dendritic spine pathology in Alzheimer's disease: problems and opportunities

Synaptic failure is an immediate cause of cognitive decline and memory dysfunction in Alzheimer’s disease. Dendritic spines are specialized structures on neuronal processes, on which excitatory synaptic contacts take place and the loss of dendritic spines directly correlates with the loss of synaptic function. Dendritic spines are readily accessible for both in vitro and in vivo experiments and have, therefore, been studied in great detail in Alzheimer’s disease mouse models. To date, a large number of different mechanisms have been proposed to cause dendritic spine dysfunction and loss in Alzheimer’s disease. For instance, amyloid beta fibrils, diffusible oligomers or the intracellular accumulation of amyloid beta have been found to alter the function and structure of dendritic spines by distinct mechanisms. Furthermore, tau hyperphosphorylation and microglia activation, which are thought to be consequences of amyloidosis in Alzheimer’s disease, may also contribute to spine loss. Lastly, genetic and therapeutic interventions employed to model the disease and elucidate its pathogenetic mechanisms in experimental animals may cause alterations of dendritic spines on their own. However, to date none of these mechanisms have been translated into successful therapeutic approaches for the human disease. Here, we critically review the most intensely studied mechanisms of spine loss in Alzheimer’s disease as well as the possible pitfalls inherent in the animal models of such a complex neurodegenerative disorder.

Neuropathology and biochemistry of Aβ and its aggregates in Alzheimer's disease

Alzheimer's disease (AD) is characterized by β-amyloid plaques and intraneuronal τ aggregation usually associated with cerebral amyloid angiopathy (CAA). Both β-amyloid plaques and CAA deposits contain fibrillar aggregates of the amyloid β-peptide (Aβ). Aβ plaques and CAA develop first in neocortical areas of preclinical AD patients and, then, expand in a characteristic sequence into further brain regions with end-stage pathology in symptomatic AD patients. Aβ aggregates are not restricted to amyloid plaques and CAA. Soluble and several types of insoluble non-plaque- and non-CAA-associated Aβ aggregates have been described. Amyloid fibrils are products of a complex self-assembly process that involves different types of transient intermediates. Amongst these intermediate species are protofibrils and oligomers. Different variants of Aβ peptides may result from alternative processing or from mutations that lead to rare forms of familial AD. These variants can exhibit different self-assembly and aggregation properties. In addition, several post-translational modifications of Aβ have been described that result, for example, in the production of N-terminal truncated Aβ with pyroglutamate modification at position 3 (AβN3pE) or of Aβ phosphorylated at serine 8 (pSer8Aβ). Both AβN3pE and pSer8Aβ show enhanced aggregation into oligomers and fibrils. However, the earliest detectable soluble and insoluble Aβ aggregates in the human brain exhibit non-modified Aβ, whereas AβN3pE and pSer8Aβ are detected in later stages. This finding indicates the existence of different biochemical stages of Aβ aggregate maturation with pSer8Aβ being related mainly to cases with symptomatic AD. The conversion from preclinical to symptomatic AD could thereby be related to combined effects of increased Aβ concentration, maturation of aggregates and spread of deposits into additional brain regions. Thus, the inhibition of Aβ aggregation and maturation before entering the symptomatic stage of the disease as indicated by the accumulation of pSer8Aβ may represent an attractive treatment strategy for preventing disease progression.

Deciphering the molecular profile of plaques, memory decline and neuron loss in two mouse models for Alzheimer's disease by deep sequencing

One of the central research questions on the etiology of Alzheimer’s disease (AD) is the elucidation of the molecular signatures triggered by the amyloid cascade of pathological events. Next-generation sequencing allows the identification of genes involved in disease processes in an unbiased manner. We have combined this technique with the analysis of two AD mouse models: (1) The 5XFAD model develops early plaque formation, intraneuronal Aβ aggregation, neuron loss, and behavioral deficits. (2) The Tg4–42 model expresses N-truncated Aβ_4–42 and develops neuron loss and behavioral deficits albeit without plaque formation. Our results show that learning and memory deficits in the Morris water maze and fear conditioning tasks in Tg4–42 mice at 12 months of age are similar to the deficits in 5XFAD animals. This suggested that comparative gene expression analysis between the models would allow the dissection of plaque-related and -unrelated disease relevant factors. Using deep sequencing differentially expressed genes (DEGs) were identified and subsequently verified by quantitative PCR. Nineteen DEGs were identified in pre-symptomatic young 5XFAD mice, and none in young Tg4–42 mice. In the aged cohort, 131 DEGs were found in 5XFAD and 56 DEGs in Tg4–42 mice. Many of the DEGs specific to the 5XFAD model belong to neuroinflammatory processes typically associated with plaques. Interestingly, 36 DEGs were identified in both mouse models indicating common disease pathways associated with behavioral deficits and neuron loss.