Presynaptic failure in Alzheimer's disease

Synaptic loss is the best correlate of cognitive deficits in Alzheimer's disease (AD). Extensive experimental evidence also indicates alterations of synaptic properties at the early stages of disease progression, before synapse loss and neuronal degeneration. A majority of studies in mouse models of AD have focused on post-synaptic mechanisms, including impairment of long-term plasticity, spine structure and glutamate receptor-mediated transmission. Here we review the literature indicating that the synaptic pathology in AD includes a strong presynaptic component. We describe the evidence indicating presynaptic physiological functions of the major molecular players in AD. These include the amyloid precursor protein (APP) and the two presenilin (PS) paralogs PS1 or PS2, genetically linked to the early-onset form of AD, in addition to tau which accumulates in a pathological form in the AD brain. Three main mechanisms participating in presynaptic functions are highlighted. APP fragments bind to presynaptic receptors (e.g. nAChRs and GABA_B receptors), presenilins control Ca²⁺ homeostasis and Ca²⁺-sensors, and tau regulates the localization of presynaptic molecules and synaptic vesicles. We then discuss how impairment of these presynaptic physiological functions can explain or forecast the hallmarks of synaptic impairment and associated dysfunction of neuronal circuits in AD. Beyond the physiological roles of the AD-related proteins, studies in AD brains also support preferential presynaptic alteration. This review features presynaptic failure as a strong component of pathological mechanisms in AD.

Mechanisms of neurodegeneration - Insights from familial Alzheimer's disease

The rising prevalence of Alzheimer's disease (AD), together with the lack of effective treatments, portray it as one of the major health challenges of our times. Untangling AD implies advancing the knowledge of the biology that gets disrupted during the disease while deciphering the molecular and cellular mechanisms leading to AD-related neurodegeneration. In fact, a solid mechanistic understanding of the disease processes stands as an essential prerequisite for the development of safe and effective treatments. Genetics has provided invaluable clues to the genesis of the disease by revealing deterministic genes - Presenilins (PSENs) and the Amyloid Precursor Protein (APP) - that, when affected, lead in an autosomal dominant manner to early-onset, familial AD (FAD). PSEN is the catalytic subunit of the membrane-embedded γ-secretase complexes, which act as proteolytic switches regulating key cell signalling cascades. Importantly, these intramembrane proteases are responsible for the production of Amyloid β (Aβ) peptides from APP. The convergence of pathogenic mutations on one functional pathway, the amyloidogenic cleavage of APP, strongly supports the significance of this process in AD pathogenesis. Here, we review and discuss the state-of-the-art knowledge of the molecular mechanisms underlying FAD, their implications for the sporadic form of the disease and for the development of safe AD therapeutics.

The cellular expression and proteolytic processing of the amyloid precursor protein is independent of TDP-43

Alzheimer's disease (AD) is a neurodegenerative condition, of which one of the cardinal pathological hallmarks is the extracellular accumulation of amyloid β (Aβ) peptides. These peptides are generated via proteolysis of the amyloid precursor protein (APP), in a manner dependent on the β-secretase, BACE1 and the multicomponent γ-secretase complex. Recent data also suggest a contributory role in AD of transactive response DNA binding protein 43 (TDP-43). There is little insight into a possible mechanism linking TDP-43 and APP processing. To this end, we used cultured human neuronal cells to investigate the ability of TDP-43 to interact with APP and modulate its proteolytic processing. Immunocytochemistry showed TDP-43 to be spatially segregated from both the extranuclear APP holoprotein and its nuclear C-terminal fragment. The latter (APP intracellular domain) was shown to predominantly localise to nucleoli, from which TDP-43 was excluded. Furthermore, neither overexpression of each of the APP isoforms nor siRNA-mediated knockdown of APP had any effect on TDP-43 expression. Doxycycline-stimulated overexpression of TDP-43 was explored in an inducible cell line. Overexpression of TDP-43 had no effect on expression of the APP holoprotein, nor any of the key proteins involved in its proteolysis. Furthermore, increased TDP-43 expression had no effect on BACE1 enzymatic activity or immunoreactivity of Aβ1-40, Aβ1-42 or the Aβ1-40:Aβ1-42 ratio. Also, siRNA-mediated knockdown of TDP-43 had no effect on BACE1 immunoreactivity. Taken together, these data indicate that TDP-43 function and/or dysfunction in AD is likely independent from dysregulation of APP expression and proteolytic processing and Aβ generation.

Recent Studies on Design and Development of Drugs Against Alzheimer's Disease (AD) Based on Inhibition of BACE-1 and Other AD-causative Agents

BACKGROUND

Alzheimer's disease (AD) is one of the neurodegenerative diseases and has been hypothesized to be a protein misfolding disease. In the generation of AD, β-secretase, γ-secretase, and tau protein play an important role. A literature search reflects ever increasing interest in the design and development of anti-AD drugs targeting β-secretase, γ-secretase, and tau protein.

OBJECTIVE

The objective is to explore the structural aspects and role of β-secretase, γ-secretase, and tau protein in AD and the efforts made to exploit them for the design of effective anti-AD drugs.

METHODS

The manuscript covers the recent studies on design and development of anti-AD drugs exploiting amyloid and cholinergic hypotheses.

RESULTS

Based on amyloid and cholinergic hypotheses, effective anti-AD drugs have been searched out in which non-peptidic BACE1 inhibitors have been most prominent.

CONCLUSION

Further exploitation of the structural aspects and the inhibition mechanism for β-secretase, γ-secretase, and tau protein and the use of cholinergic hypothesis may lead still more potent anti-AD drugs.

Determining the Protein Stability of Alzheimer's Disease Protein, Amyloid Precursor Protein

Determining protein thermal stability is integral in biomedical research. Here, with the use of two thermal stability assays, we show the melting temperature of amyloid precursor protein, an Alzheimer's disease related protein. The average melting temperature for amyloid precursor protein of 55.9 °C was derived from differential scanning fluorometry (55.1 ± 0.3 °C) and cellular thermal melt (56.7 ± 0.7 °C). These experimental methods have significant application for Alzheimer's disease research including their use for amyloid precursor protein stability profiling and for the identification of additional binding partners to further elucidate novel protein functions.

Interaction of Sp1 and APP promoter elucidates a mechanism for Pb2+ caused neurodegeneration

A ubiquitously expressed transcription factor, specificity protein 1 (Sp1), interacts with the amyloid precursor protein (APP) promoter and likely mediates APP expression. Promoter-interaction strengths variably regulate the level of APP expression. Here, we examined the interactions of finger 3 of Sp1 (Sp1-f3) with a DNA fragment containing the APP promoter in different ionic solutions using atomic force microscope (AFM) spectroscopy. Sp1-f3 molecules immobilized on an Si substrate were bound to the APP promoter, which was linked to the AFM tips via covalent bonds. The interactions were strongly influenced by Pb2+, considering that substituting Zn2+ with Pb2+ increased the binding affinity of Sp1 for the APP promoter. The results revealed that the enhanced interaction force facilitated APP expression and that APP overexpression could confer a high-risk for disease incidence. An increased interaction force between Sp1-f3 and the APP promoter in Pb2+ solutions was consistent with a lower binding free energy, as determined by computer-assisted analysis. The impact of Pb2+ on cell morphology and related mechanical properties were also detected by AFM. The overexpression of APP caused by the enhanced interaction force triggered actin reorganization and further resulted in an increased Young's modulus and viscosity. The correlation with single-force measurements revealed that altered cellular activities could result from alternation of Sp1-APP promoter interaction. Our AFM findings offer a new approach in understanding Pb2+ associated neurodegeneration.

Prominent amyloid plaque pathology and cerebral amyloid angiopathy in APP V717I (London) carrier - phenotypic variability in autosomal dominant Alzheimer's disease

The discovery of mutations associated with familial forms of Alzheimer's disease (AD), has brought imperative insights into basic mechanisms of disease pathogenesis and progression and has allowed researchers to create animal models that assist in the elucidation of the molecular pathways and development of therapeutic interventions. Position 717 in the amyloid precursor protein (APP) is a hotspot for mutations associated with autosomal dominant AD (ADAD) and the valine to isoleucine amino acid substitution (V717I) at this position was among the first ADAD mutations identified, spearheading the formulation of the amyloid cascade hypothesis of AD pathogenesis. While this mutation is well described in multiple kindreds and has served as the basis for the generation of widely used animal models of disease, neuropathologic data on patients carrying this mutation are scarce. Here we present the detailed clinical and neuropathologic characterization of an APP V717I carrier, which reveals important novel insights into the phenotypic variability of ADAD cases. While age at onset, clinical presentation and widespread parenchymal beta-amyloid (Aβ) deposition are in line with previous reports, our case also shows widespread and severe cerebral amyloid angiopathy (CAA). This patient also presented with TDP-43 pathology in the hippocampus and amygdala, consistent with limbic predominant age-related TDP-43 proteinopathy (LATE). The APOE ε2/ε3 genotype may have been a major driver of the prominent vascular pathology seen in our case. These findings highlight the importance of neuropathologic examinations of genetically determined AD cases and demonstrate striking phenotypic variability in ADAD cases.

The amyloid precursor protein affects glyceraldehyde 3-phosphate dehydrogenase levels, organelle localisation and thermal stability

Glyceraldehyde 3-phosphate dehydrogenase's (GAPDH) proapoptotic response to cellular oxidative stress has suspected implication for Alzheimer's disease (AD). Interestingly, the overexpression of the amyloid precursor protein (APP) can initiate oxidative stress responses within mammalian cell lines. Here, APP695 and APP770 overexpression significantly increased the level of GAPDH, while no effect was observed when the APP homologues APLP1 or APLP2 were used. Heterologous expression of APP695 was shown to increase the level of GAPDH within the cytoplasm by over 100% and within the mitochondria by approximately 50%. Moreover, a shift in organelle distribution from cytoplasm > nucleus > mitochondria in control cell lines to cytoplasm > mitochondria > nucleus in the APP695 overexpressing cell line was also observed. Further, the overexpression of APP695 increased GAPDH aggregation temperature by 3.09 ± 0.46 °C, indicative of greater thermal stability. These results demonstrate a clear correlation between APP overexpression and GAPDH levels, organelle distribution and thermal stability.

Angiopoietin-1 accelerates Alzheimer's disease via FOXA2/PEN2/APP pathway in APP/PS1 mice

Angiopoietin-1 (Ang-1), a regulatory angiogenesis protein and it has been found to be involved in the occurrence and progression of Alzheimer's disease. However, it was still to be addressed the distinctly role and the molecular mechanisms of Ang-1 affects Alzheimer's disease. Our data suggest that Ang-1 aggravated the accumulation of Aβ₄₂ and cognitive decline in APP/PS1 mice. The upregulation of APPβ is essential for Aβ₄₂ production in N2a cells overexpressing the mutational human APP gene (N2a/APP695 cells), while downregulation of PEN2 could reduce APP expression. Silencing of FOXA2 lead to inhibition of APP expression, as well as decrease of Aβ₄₂ contents. In conclusion, Ang-1 has an accelerative effect on Alzheimer's disease by increasing the secretion of Aβ₄₂ via FOXA2/PEN2/APP pathway.

Tau-tubulin kinase 1 and amyloid-β peptide induce phosphorylation of collapsin response mediator protein-2 and enhance neurite degeneration in Alzheimer disease mouse models

The accumulation of phosphorylated tau protein (pTau) in the entorhinal cortex (EC) is the earliest tau pathology in Alzheimer’s disease (AD). Tau tubulin kinase-1 (TTBK1) is a neuron-specific tau kinase and expressed in the EC and hippocampal regions in both human and mouse brains. Here we report that collapsin response mediator protein-2 (CRMP2), a critical mediator of growth cone collapse, is a new downstream target of TTBK1 and is accumulated in the EC region of early stage AD brains. TTBK1 transgenic mice show severe axonal degeneration in the perforant path, which is exacerbated by crossing with Tg2576 mice expressing Swedish familial AD mutant of amyloid precursor protein (APP). TTBK1 mice show accumulation of phosphorylated CRMP2 (pCRMP2), in the EC at 10 months of age, whereas age-matched APP/TTBK1 bigenic mice show pCRMP2 accumulation in both the EC and hippocampal regions. Amyloid-β peptide (Aβ) and TTBK1 suppress the kinetics of microtubule polymerization and TTBK1 reduces the neurite length of primary cultured neurons in Rho kinase-dependent manner in vitro. Silencing of TTBK1 or expression of dominant-negative Rho kinase demonstrates that Aβ induces CRMP2 phosphorylation at threonine 514 in a TTBK1-dependent manner, and TTBK1 enhances Aβ-induced CRMP2 phosphorylation in Rho kinase-dependent manner in vitro. Furthermore, TTBK1 expression induces pCRMP2 complex formation with pTau in vitro, which is enhanced upon Aβ stimulation in vitro. Finally, pCRMP2 forms a complex with pTau in the EC tissue of TTBK1 mice in vivo, which is exacerbated in both the EC and hippocampal tissues in APP/TTBK1 mice. These results suggest that TTBK1 and Aβ induce phosphorylation of CRMP2, which may be causative for the neurite degeneration and somal accumulation of pTau in the EC neurons, indicating critical involvement of TTBK1 and pCRMP2 in the early AD pathology.

Proximity ligation assay reveals both pre- and postsynaptic localization of the APP-processing enzymes ADAM10 and BACE1 in rat and human adult brain

BACKGROUND

Synaptic degeneration and accumulation of amyloid β-peptides (Aβ) are hallmarks of the Alzheimer diseased brain. Aβ is synaptotoxic and produced by sequential cleavage of the amyloid precursor protein (APP) by the β-secretase BACE1 and by γ-secretase. If APP is instead cleaved by the α-secretase ADAM10, Aβ will not be generated. Although BACE1 is considered to be a presynaptic protein and ADAM10 has been reported to mainly localize to the postsynaptic density, we have previously shown that both ADAM10 and BACE1 are highly enriched in synaptic vesicles of rat brain and mouse primary hippocampal neurons.

RESULTS

Here, using brightfield proximity ligation assay, we expanded our previous result in primary neurons and investigated the in situ synaptic localization of ADAM10 and BACE1 in rat and human adult brain using both pre- and postsynaptic markers. We found that ADAM10 and BACE1 were in close proximity with both the presynaptic marker synaptophysin and the postsynaptic marker PSD-95. The substrate APP was also detected both pre- and postsynaptically. Subcellular fractionation confirmed that ADAM10 and BACE1 are enriched to a similar degree in synaptic vesicles and as well as in the postsynaptic density.

CONCLUSIONS

We show that the α-secretase ADAM10 and the β-secretase BACE1 are located in both the pre- and postsynaptic compartments in intact brain sections. These findings increase our understanding of the regulation of APP processing, thereby facilitating development of more specific treatment strategies.

Intracerebral seeding of amyloid-β and tau pathology in mice: Factors underlying prion-like spreading and comparisons with α-synuclein

Alzheimer's disease (AD) is characterized neuropathologically by progressive neurodegeneration and by the presence of amyloid plaques and neurofibrillary tangles. These plaques and tangles are composed, respectively, of amyloid-beta (Aβ) and tau proteins. While long recognized as hallmarks of AD, it remains unclear what causes the formation of these insoluble deposits. One theory holds that prion-like templated misfolding of Aβ and tau induces these proteins to form pathological aggregates, and propagation of this misfolding causes the stereotyped progression of pathology commonly seen in AD. Supporting this theory, numerous studies have been conducted in which aggregated Aβ, tau, or α-synuclein is injected intracerebrally into pathology-free host animals, resulting in robust formation of pathology. Here, we review this literature, focusing on in vivo intracerebral seeding of Aβ and tau in mice. We compare the results of these experiments to what is known about the seeding and spread of α-synuclein pathology, and we discuss how this research informs our understanding of the factors underlying the onset, progression, and outcomes of proteinaceous pathologies.

Interactions between amyloid precursor protein-like (APPL) and MAGUK scaffolding proteins contribute to appetitive long-term memory in Drosophila melanogaster

Amyloid precursor protein (APP), the precursor of amyloid beta peptide, plays a central role in Alzheimer's disease (AD), a pathology characterized by memory decline and synaptic loss upon aging. Understanding the physiological role of APP is fundamental in deciphering the progression of AD, and several studies suggest a synaptic function via protein-protein interactions. Nevertheless, it remains unclear whether and how these interactions contribute to memory. In Drosophila, we previously showed that APP-like (APPL), the fly APP homolog, is required for aversive associative memory in the olfactory memory center, the mushroom body (MB). In the present study, we show that APPL is required for appetitive long-term memory (LTM), another form of associative memory, in a specific neuronal subpopulation of the MB, the α'/β' Kenyon cells. Using a biochemical approach, we identify the synaptic MAGUK (membrane-associated guanylate kinase) proteins X11, CASK, Dlgh2 and Dlgh4 as interactants of the APP intracellular domain (AICD). Next, we show that the Drosophila homologs CASK and Dlg are also required for appetitive LTM in the α'/β' neurons. Finally, using a double RNAi approach, we demonstrate that genetic interactions between APPL and CASK, as well as between APPL and Dlg, are critical for appetitive LTM. In summary, our results suggest that APPL contributes to associative long-term memory through its interactions with the main synaptic scaffolding proteins CASK and Dlg. This function should be conserved across species.

The effect of age and obesity on platelet amyloid precursor protein processing and plasma markers of oxidative stress and inflammation

INTRODUCTION

Advancing age is a major risk factor for a range of diseases such as, cardiovascular disease, diabetes, cancer and neurodegenerative diseases. In addition, over a third of the population are overweight and obesity is becoming more prevalent in younger people. Ageing and obesity are both linked to a chronic proinflammatory state and elevated oxidative stress, which have both been implicated in cardiovascular and neurodegenerative diseases. Platelets contain all the necessary machinery to process the Amyloid precursor protein AβPP, a pathway thought to be perturbed in Alzheimer's Disease (AD). The ratio of AβPP isoforms present in platelets, and the amount of alpha secretase ADAM10, that works to process AβPP, appear to be associated with cognitive decline and a diagnosis of Alzheimer's disease. The aim of this study was to assess changes in AβPP ratio, ADAM10 and markers of inflammation and oxidative stress with ageing and obesity.

MATERIALS AND METHODS

Ninety participants were recruited to this study to provide one blood sample. Platelet-rich plasma and platelet lysates were collected and the expression of AβPPr, proADAM10 and mADAM10 was assessed by Western blotting. In addition, markers of inflammation (IL-6) and oxidative stress (8-Isoprostane) were assessed in plasma.

RESULTS

Participants were placed into one of four groups based on their age and body mass index (Young/Lean, Young/obese, Old/Lean and Old/Obese). IL-6 was able to significantly distinguish obese from lean participants (AUC of 0.80, SE = 0.05, P < 0.001). Plasma isoprostanes were able to distinguish between both young/old (AUC of 0.73, SE = 0.05, P < 0.01), and obese/non-obese participants (AUC of 0.66, SE = 0.01, P < 0.01). Plasma protein carbonyls could distinguish young and old participants (AUC of 0.69, SE = 0.07 P < 0.02). Old Lean participants had significantly lower mADAM10 expression than both Young Lean and Young Obese participants (p < 0.05). Old obese participants had significantly lower proADAM10 expression compared to both Young Lean and Young Obese (p < 0.05). Both mADAM10 and proADAM10 were significantly decreased with advancing age (p < 0.05).

CONCLUSIONS

The findings presented in this study provide evidence that blood-based biomarkers related to the pathology of AD are altered with age and obesity in otherwise healthy adults. Ageing was more strongly associated with elevated markers of oxidative stress whereas obesity was associated with elevated inflammatory IL-6.

Contribution of GABAergic interneurons to amyloid-β plaque pathology in an APP knock-in mouse model

The amyloid-β (Aβ) peptide, the primary constituent of amyloid plaques found in Alzheimer’s disease (AD) brains, is derived from sequential proteolytic processing of the Amyloid Precursor Protein (APP). However, the contribution of different cell types to Aβ deposition has not yet been examined in an in vivo, non-overexpression system. Here, we show that endogenous APP is highly expressed in a heterogeneous subset of GABAergic interneurons throughout various laminae of the hippocampus, suggesting that these cells may have a profound contribution to AD plaque pathology. We then characterized the laminar distribution of amyloid burden in the hippocampus of an APP knock-in mouse model of AD. To examine the contribution of GABAergic interneurons to plaque pathology, we blocked Aβ production specifically in these cells using a cell type-specific knock-out of BACE1. We found that during early stages of plaque deposition, interneurons contribute to approximately 30% of the total plaque load in the hippocampus. The greatest contribution to plaque load (75%) occurs in the stratum pyramidale of CA1, where plaques in human AD cases are most prevalent and where pyramidal cell bodies and synaptic boutons from perisomatic-targeting interneurons are located. These findings reveal a crucial role of GABAergic interneurons in the pathology of AD. Our study also highlights the necessity of using APP knock-in models to correctly evaluate the cellular contribution to amyloid burden since APP overexpressing transgenic models drive expression in cell types according to the promoter and integration site and not according to physiologically relevant expression mechanisms.

Mechanisms of action of amyloid-beta and its precursor protein in neuronal cell death

Extracellular senile plaques and intracellular neurofibrillary tangles are the neuropathological findings of the Alzheimer's disease (AD). Based on the amyloid cascade hypothesis, the main component of senile plaques, the amyloid-beta (Aβ) peptide, and its derivative called amyloid precursor protein (APP) both have been found to place their central roles in AD development for years. However, the recent therapeutics have yet to reverse or halt this disease. Previous evidence demonstrates that the accumulation of Aβ peptides and APP can exert neurotoxicity and ultimately neuronal cell death. Hence, we discuss the mechanisms of excessive production of Aβ peptides and APP serving as pathophysiologic stimuli for the initiation of various cell signalling pathways including apoptosis, necrosis, necroptosis and autophagy which lead to neuronal cell death. Conversely, the activation of such pathways could also result in the abnormal generation of APP and Aβ peptides. An elucidation of actions of APP and its metabolite, Aβ, could be vital in suggesting novel therapeutic opportunities.

Icaritin, an inhibitor of beta-site amyloid cleaving enzyme-1, inhibits secretion of amyloid precursor protein in APP-PS1-HEK293 cells by impeding the amyloidogenic pathway

Background

Icaritin (ICT) is a prenylflavonoid derivative from Epimedium brevicornum Maxim. ICT has been shown to have neuroprotective effects. We investigate how ICT affects secretion of amyloid precursor protein (APP).

Methods

We exposed APP-PS1-HEK293 cells to ICT to investigate its effect on beta-site amyloid cleaving enzyme (BACE)1. Cell viability was evaluated by MTT and lactate dehydrogenase (LDH) assays. The half-maximal inhibitory concentration (IC₅₀) of ICT for BACE1 was measured using fluorescence resonance energy transfer. Effects of ICT on the mRNA expression of APP were assessed by quantitative polymerase chain reaction, and protein expression was measured by western blotting and immunofluorescence.

Results

Icaritin inhibited BACE1 activity and IC₅₀ was 5.70 ± 1.09 μM. Compared with the control group, at ICT concentrations of 5 μM and 10 μM, the viability increased and LDH leakage decreased in APP-PS1-293 cells. Also, mRNA expression of A disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) increased, while that of BACE1 and presenilin-1 (PS1) decreased, upon ICT treatment. Western blotting and immunofluorescence confirmed that protein expression of ADAM10, BACE1 and PS1 showed the same trend. Expression of the APP fragments sAPPβ and C-terminal fragment β decreased, while that of sAPPα increased, upon ICT treatment. Expression of amyloid β peptides in APP-PS1-HEK293 cells was lower in ICT-treated groups compared with that in the control group.

Conclusions

Icaritin, as a BACE1 inhibitor, inhibits APP secretion in APP-PS1-HEK293 cells by impeding the amyloidogenic pathway.

Loss of amyloid precursor protein exacerbates early inflammation in Niemann-Pick disease type C

BACKGROUND

Niemann-Pick disease type C (NPC) is a progressive neurodegenerative condition that results in early fatality. NPC is inherited in an autosomal recessive pattern from mutations in NPC1 or NPC2 genes. The etiology of NPC is poorly defined. In that regard, neuroinflammation occurs early in the disease and we have recently unveiled an atypical pattern of interferon signaling in pre-symptomatic Npc1^-/- mice, with microglial activation, anti-viral response, activation of antigen-presenting cells, and activation and chemotaxis of T lymphocytes as the key affected pathologic pathways. Furthermore, IP-10/CXCL10, a potent IFN-γ-responsive cytokine, was identified as the potential mediator of these early inflammatory abnormalities. Here, we asked whether this aberrant signaling may be exacerbated by the loss of amyloid precursor protein (APP) function, a loss known to shorten lifespan and accelerate neurodegeneration in Npc1^-/- mice.

METHODS

We carried out genome-wide comparative transcriptome analyses of pre-symptomatic Npc1^+/+/App^+/+, Npc1^-/-/App^+/+, Npc1^+/+/App^-/-, and Npc1^-/-/App^-/- mouse cerebella to identify biological pathways in the NPC brain further affected by the loss of APP. Gene Set Enrichment Analysis and Ingenuity Pathway Analysis were utilized for molecular mapping and functional upstream pathway analyses of highly differentially expressed genes. We simultaneously measured the expression of 32 inflammatory cytokines and chemokines in the cerebella from these mice, including those identified in our genome-wide analyses. Finally, we used immunohistochemistry to measure T cell infiltration in the cerebellum.

RESULTS

Expression of IFN-γ- and IFN-α-responsive genes in pre-symptomatic Npc1^-/-/App^-/- cerebella is upregulated compared with Npc1^-/-/App^+/+ mice, compounding the dysregulation of microglial activation, anti-viral response, activation of antigen-presenting cells, and T-lymphocyte activation and chemotaxis pathways present in the NPC brain. Multiplex protein analysis further showed elevated expression of IP-10/CXCL10, a potent downstream effector of IFN-γ, as well as RANTES/CCL5, eotaxin/CCL11 and IL-10, prior to symptomatic onset in Npc1^-/-/App^-/- cerebella, compared with Npc1^-/-/App^+/+mice. In the terminal disease stage, loss of APP caused pleiotropic differential expression of the vast majority of cytokines evaluated. Finally, we present evidence of T cell infiltration in Npc1^-/-/App^-/- cerebella.

CONCLUSIONS

Loss of APP exacerbates the pathogenic neuroinflammation that occurs prior to symptomatic onset in the NPC brain. These findings shed new light on the function of APP as a cytoprotective modulator in the CNS, offering potential evidence-based therapies against NPC.

Kinesin light chain-1 serine-460 phosphorylation is altered in Alzheimer's disease and regulates axonal transport and processing of the amyloid precursor protein

Damage to axonal transport is an early pathogenic event in Alzheimer’s disease. The amyloid precursor protein (APP) is a key axonal transport cargo since disruption to APP transport promotes amyloidogenic processing of APP. Moreover, altered APP processing itself disrupts axonal transport. The mechanisms that regulate axonal transport of APP are therefore directly relevant to Alzheimer’s disease pathogenesis. APP is transported anterogradely through axons on kinesin-1 motors and one route for this transport involves calsyntenin-1, a type-1 membrane spanning protein that acts as a direct ligand for kinesin-1 light chains (KLCs). Thus, loss of calsyntenin-1 disrupts APP axonal transport and promotes amyloidogenic processing of APP. Phosphorylation of KLC1 on serine-460 has been shown to reduce anterograde axonal transport of calsyntenin-1 by inhibiting the KLC1-calsyntenin-1 interaction. Here we demonstrate that in Alzheimer’s disease frontal cortex, KLC1 levels are reduced and the relative levels of KLC1 serine-460 phosphorylation are increased; these changes occur relatively early in the disease process. We also show that a KLC1 serine-460 phosphomimetic mutant inhibits axonal transport of APP in both mammalian neurons in culture and in Drosophila neurons in vivo. Finally, we demonstrate that expression of the KLC1 serine-460 phosphomimetic mutant promotes amyloidogenic processing of APP. Together, these results suggest that increased KLC1 serine-460 phosphorylation contributes to Alzheimer’s disease.

Deregulated Local Protein Synthesis in the Brain Synaptosomes of a Mouse Model for Alzheimer's Disease

While protein synthesis in neurons is largely attributed to cell body and dendrites, the capability of synaptic regions to synthesize new proteins independently of the cell body has been widely demonstrated as an advantageous mechanism subserving synaptic plasticity. Thus, the contribution that local protein synthesis at synapses makes to physiology and pathology of brain plasticity may be more prevalent than initially thought. In this study, we tested if local protein synthesis at synapses is deregulated in the brains of TgCRND8 mice, an animal model for Alzheimer's disease (AD) overexpressing mutant human amyloid precursor protein (APP). To this end, we used synaptosomes as a model system to study the functionality of the synaptic regions in mouse brains. Our results showed that, while TgCRND8 mice exhibit early signs of brain inflammation and deficits in learning, the electrophoretic profile of newly synthesized proteins in their synaptosomes was subtly different from that of the control mice. Interestingly, APP itself was, in part, locally synthesized in the synaptosomes, underscoring the potential importance of local translation at synapses. More importantly, after the contextual fear conditioning, de novo synthesis of some individual proteins was significantly enhanced in the synaptosomes of control animals, but the TgCRND8 mice failed to display such synaptic modulation by training. Taken together, our results demonstrate that synaptic synthesis of proteins is impaired in the brain of a mouse model for AD, and raise the possibility that this deregulation may contribute to the early progression of the pathology.

Is brain iron trafficking part of the physiology of the amyloid precursor protein?

The amyloid precursor protein is so named, because a proteolytic fragment of it was found associated with a neuropathic disorder now known as Alzheimer's disease. This fragment, Aβ, along with tau makes up the plaques and tangles that are the hallmark of AD. Iron (and other first-row transition metals) is found associated with these proteinaceous deposits. Much research has focused on the relationship of the plaques and iron to the etiology of the disease. This commentary asks another question, one only more recently addressed namely, what is the physiologic function of the amyloid precursor protein (APP) and of its secretase-generated soluble species? Overall, the data make clear that APP and its products have neurotrophic functions and some data indicate one of these may be to modulate the trafficking of iron in the brain.

Long-term cognitive impairment without diffuse axonal injury following repetitive mild traumatic brain injury in rats

Repetitive mild traumatic brain injuries (TBI) impair cognitive abilities and increase risk of neurodegenerative disorders in humans. We developed two repetitive mild TBI models in rats with different time intervals between successive weight-drop injuries. Rats were subjected to repetitive Sham (no injury), single mild (mTBI), repetitive mild (rmTBI - 5 hits, 24 h apart), rapid repetitive mild (rapTBI - 5 hits, 5 min apart) or a single severe (sTBI) TBI. Cognitive performance was assessed 2 and 8 weeks after TBI in the novel object recognition test (NOR), and 6-7 weeks after TBI in the water maze (MWM). Acute immunohistochemical markers were evaluated 24 h after TBI, and blood biomarkers were measured with ELISA 8 weeks after TBI. In the NOR, both rmTBI and rapTBI showed poor performance at 2 weeks post-injury. At 8 weeks post-injury, the rmTBI group still performed worse than the Sham and mTBI groups, while the rapTBI group recovered. In the MWM, the rapTBI group performed worse than the Sham and mTBI groups. Acute APP and RMO-14 immunohistochemistry showed axonal injury at the pontomedullary junction in the sTBI, but not in other groups. ELISA showed increased serum GFAP levels 8 weeks after sTBI, while no differences were found between the injury groups in the levels of phosphorylated-tau and S100β. Results suggest that the rmTBI protocol is the most suitable model for testing cognitive impairment after mild repetitive head injuries and that the prolonged cognitive impairment after repetitive mild TBI originates from different structural and molecular mechanisms compared to similar impairments after single sTBI.

Alzheimer's Disease and Its Potential Alternative Therapeutics

Alzheimer’s Disease (AD) is a chronic neurodegenerative disease that affects over 5 million individuals in the United States alone. Currently, there are only two kinds of pharmacological interventions available for symptomatic relief of AD; Acetyl Cholinesterase Inhibitors (AChEI) and N-methyl-D-aspartic Acid (NMDA) receptor antagonists and these drugs do not slow down or stop the progression of the disease. Several molecular targets have been implicated in the pathophysiology of AD, such as the tau (τ) protein, Amyloid-beta (Aβ), the Amyloid Precursor Protein (APP) and more and several responses have also been observed in the advancement of the disease, such as reduced neurogenesis, neuroinflammation, oxidative stress and iron overload. In this review, we discuss general features of AD and several small molecules across different experimental AD drug classes that have been studied for their effects in the context of the molecular targets and responses associated with the AD progression. These drugs include: Paroxetine, Desferrioxamine (DFO), N-acetylcysteine (NAC), Posiphen/-(−)Phenserine, JTR-009, Carvedilol, LY450139, Intravenous immunoglobulin G 10%, Indomethacin and Lithium Carbonate (Li₂CO₃).

Novel amyloid precursor protein mutation, Val669Leu ("Seoul APP"), in a Korean patient with early-onset Alzheimer's disease

In this study, a novel mutation in APP gene, Val669Leu ("Seoul APP"), was reported in a Korean female patient with Alzheimer's disease. She developed cognitive decline at 56 years of age, and her memory declined rapidly over one-year period from her 1st visit to the hospital. Her Mini-Mental State Examination scores dropped from 25/30 to 13/30. Two years later, she developed parkinsonian features, myoclonic jerk, and generalized seizure. As the disease progressed, aggravated diffuse brain atrophy and small-vessel ischemic lesion was also observed, and she became mute and vegetative in 4 years from the symptom onset. Magnetic resonance imaging showed mild medial temporal lobe and hippocampal atrophy, and 18F-fluoro-deoxyglucose positron emission tomography showed bilateral temporoparietal hypometabolism. Plasma amyloid oligomer analysis revealed highly elevated Aβ oligomers levels in the proband patient. Family history revealed positive without biochemical confirmation because family members testified similar type of cognitive decline from the proband's mother and one of her aunt/uncle. Her half-siblings did not present any signs of memory impairment. Sanger sequencing of the proband patient revealed a novel mutation in APP gene, Val669Leu, but mutation was not found in her unaffected half-sisters. A designed algorithm by Guerreiro et al. on early-onset Alzheimer's disease-associated mutations suggested the mutation as possibly pathogenic mutation. On the other hand, PolyPhen2 and SIFT tools suggested as otherwise. Since the mutation was located nearby the β-secretase cleavage site of APP, right next to the Swedish APP (Lys,Met670/671Asn,Leu) mutation, it was named as "Seoul APP" mutation. 3D modeling revealed that this mutation could result in significant changes in loop orientation of APP and also its intramolecular interactions. Hence, a novel APP Val669Leu mutation could alter the binding interactions between APP and β-secretase, which may influence the Aβ40 and Aβ42 generations.

A Novel Apolipoprotein E Antagonist Functionally Blocks Apolipoprotein E Interaction With N-terminal Amyloid Precursor Protein, Reduces β-Amyloid-Associated Pathology, and Improves Cognition

BACKGROUND

The ɛ4 isoform of apolipoprotein E (apoE4) is a major genetic risk factor for the development of sporadic Alzheimer's disease (AD), and its modification has been an intense focus for treatment of AD during recent years.

METHODS

We investigated the binding of apoE, a peptide corresponding to its low-density lipoprotein receptor binding domain (amino acids 133-152; ApoEp), and modified ApoEp to amyloid precursor protein (APP) and their effects on amyloid-β (Aβ) production in cultured cells. Having discovered a peptide (6KApoEp) that blocks the interaction of apoE with N-terminal APP, we investigated the effects of this peptide and ApoEp on AD-like pathology and behavioral impairment in 3XTg-AD and 5XFAD transgenic mice.

RESULTS

ApoE and ApoEp, but not truncated apoE lacking the low-density lipoprotein receptor binding domain, physically interacted with N-terminal APP and thereby mediated Aβ production. Interestingly, the addition of 6 lysine residues to the N-terminus of ApoEp (6KApoEp) directly inhibited apoE binding to N-terminal APP and markedly limited apoE- and ApoEp-mediated Aβ generation, presumably through decreasing APP cellular membrane trafficking and p44/42 mitogen-activated protein kinase phosphorylation. Moreover, while promoting apoE interaction with APP by ApoEp exacerbated Aβ and tau brain pathologies in 3XTg-AD mice, disrupting this interaction by 6KApoEp ameliorated cerebral Aβ and tau pathologies, neuronal apoptosis, synaptic loss, and hippocampal-dependent learning and memory impairment in 5XFAD mice without altering cholesterol, low-density lipoprotein receptor, and apoE expression levels.

CONCLUSIONS

These data suggest that disrupting apoE interaction with N-terminal APP may be a novel disease-modifying therapeutic strategy for AD.