Transgenic mice expressing APP-C100 in the brain

Spatial differences in gene expression across the dorsal raphe nucleus in a model of early Alzheimer's disease

BACKGROUND

Persons with Alzheimer's disease (AD) present with changes in mood, sleep, and arousal that may precede the clinical manifestation of cognitive decline. These early symptoms can be driven by changes in the serotonergic (5-HT) nuclei of the brainstem, particularly the dorsal raphe nucleus (DRN). It is unclear why all 5-HT neurons do not simultaneously develop AD pathology that progresses at the same rate.

OBJECTIVE

We sought to identify any underlying genetic components associated with susceptibility or resistance of 5-HT neurons to AD pathology.

METHODS

The Visium Spatial Gene Expression platform was used to identify transcriptomic changes across the DRN in a preclinical model of early AD, human tau-overexpressing mice (htau mice). We further used RNAscope and immunohistochemical assessment to validate findings of primary interest.

RESULTS

We find that the DRN of htau mice differentially expresses AD-related genes, including those related to kinase binding, ion channel activity, ligand-receptor interactions, and regulation of serine/threonine kinases. We further find that computational sub-clustering of the DRN is consistent with previous circuitry-driven characterizations, allowing for spatial bounding of distinct subregions within the DRN. Of these, we find the dorsolateral DRN is preferentially impacted by 5-HT neuron loss and development of tau pathology, which coincides with increased expression of the long noncoding RNA Map2k3os.

CONCLUSIONS

Map2k3os may serve regulatory roles relevant for tau phosphorylation and warrants further investigation to characterize its interactions. Overall, this report demonstrates the power of large-scale spatial transcriptomics technologies, while underscoring the need for convergent-data validation to overcome their limitations.

Towards a Unitary Hypothesis of Alzheimer's Disease Pathogenesis

The "amyloid cascade" hypothesis of Alzheimer's disease (AD) pathogenesis invokes the accumulation in the brain of plaques (containing the amyloid-β protein precursor [AβPP] cleavage product amyloid-β [Aβ]) and tangles (containing hyperphosphorylated tau) as drivers of pathogenesis. However, the poor track record of clinical trials based on this hypothesis suggests that the accumulation of these peptides is not the only cause of AD. Here, an alternative hypothesis is proposed in which the AβPP cleavage product C99, not Aβ, is the main culprit, via its role as a regulator of cholesterol metabolism. C99, which is a cholesterol sensor, promotes the formation of mitochondria-associated endoplasmic reticulum (ER) membranes (MAM), a cholesterol-rich lipid raft-like subdomain of the ER that communicates, both physically and biochemically, with mitochondria. We propose that in early-onset AD (EOAD), MAM-localized C99 is elevated above normal levels, resulting in increased transport of cholesterol from the plasma membrane to membranes of intracellular organelles, such as ER/endosomes, thereby upregulating MAM function and driving pathology. By the same token, late-onset AD (LOAD) is triggered by any genetic variant that increases the accumulation of intracellular cholesterol that, in turn, boosts the levels of C99 and again upregulates MAM function. Thus, the functional cause of AD is upregulated MAM function that, in turn, causes the hallmark disease phenotypes, including the plaques and tangles. Accordingly, the MAM hypothesis invokes two key interrelated elements, C99 and cholesterol, that converge at the MAM to drive AD pathogenesis. From this perspective, AD is, at bottom, a lipid disorder.

Exosome Production Is Key to Neuronal Endosomal Pathway Integrity in Neurodegenerative Diseases

Dysfunction of the endosomal–lysosomal system is a prominent pathogenic factor in Alzheimer’s disease (AD) and other neurodevelopmental and neurodegenerative disorders. We and others have extensively characterized the neuronal endosomal pathway pathology that results from either triplication of the amyloid-β precursor protein (APP) gene in Down syndrome (DS) or from expression of the apolipoprotein E ε4 allele (APOE4), the greatest genetic risk factor for late-onset AD. More recently brain exosomes, extracellular vesicles that are generated within and released from endosomal compartments, have been shown to be altered in DS and by APOE4 expression. In this review, we discuss the emerging data arguing for an interdependence between exosome production and endosomal pathway integrity in the brain. In vitro and in vivo studies indicate that altered trafficking through the endosomal pathway or compromised cargo turnover within lysosomes can affect the production, secretion, and content of exosomes. Conversely, exosome biogenesis can affect the endosomal–lysosomal system. Indeed, we propose that efficient exosome release helps to modulate flux through the neuronal endosomal pathway by decompressing potential “traffic jams.” Exosome secretion may have the added benefit of unburdening the neuron’s lysosomal system by delivering endosomal–lysosomal material into the extracellular space, where other cell types may contribute to the degradation of neuronal debris. Thus, maintaining robust neuronal exosome production may prevent or mitigate endosomal and lysosomal abnormalities linked to aging and neurodegenerative diseases. While the current evidence suggests that the exosomal system in the brain can be modulated both by membrane lipid composition and the expression of key proteins that contribute to the formation and secretion of exosomes, how exosomal pathway-regulatory elements sense and respond to perturbations in the endosomal pathway is not well understood. Based upon findings from the extensively studied DS and APOE4 models, we propose that enhanced neuronal exosome secretion can be a protective response, reducing pathological disruption of the endosomal–lysosomal system in disease-vulnerable neurons. Developing therapeutic approaches that help to maintain or enhance neuronal exosome biogenesis and release may be beneficial in a range of disorders of the central nervous system.

Emerging Alternative Proteinases in APP Metabolism and Alzheimer's Disease Pathogenesis: A Focus on MT1-MMP and MT5-MMP

Processing of amyloid beta precursor protein (APP) into amyloid-beta peptide (Aβ) by β-secretase and γ-secretase complex is at the heart of the pathogenesis of Alzheimer’s disease (AD). Targeting this proteolytic pathway effectively reduces/prevents pathology and cognitive decline in preclinical experimental models of the disease, but therapeutic strategies based on secretase activity modifying drugs have so far failed in clinical trials. Although this may raise some doubts on the relevance of β- and γ-secretases as targets, new APP-cleaving enzymes, including meprin-β, legumain (δ-secretase), rhomboid-like protein-4 (RHBDL4), caspases and membrane-type matrix metalloproteinases (MT-MMPs/η-secretases) have confirmed that APP processing remains a solid mechanism in AD pathophysiology. This review will discuss recent findings on the roles of all these proteinases in the nervous system, and in particular on the roles of MT-MMPs, which are at the crossroads of pathological events involving not only amyloidogenesis, but also inflammation and synaptic dysfunctions. Assessing the potential of these emerging proteinases in the Alzheimer’s field opens up new research prospects to improve our knowledge of fundamental mechanisms of the disease and help us establish new therapeutic strategies.

Inactivation of γ-secretases leads to accumulation of substrates and non-Alzheimer neurodegeneration

γ-Secretases are a family of intramembrane cleaving aspartyl proteases and important drug targets in Alzheimer's disease. Here, we generated mice deficient for all γ-secretases in the pyramidal neurons of the postnatal forebrain by deleting the three anterior pharynx defective 1 (Aph1) subunits (Aph1abc cKO Cre⁺). The mice show progressive cortical atrophy, neuronal loss, and gliosis. Interestingly, this is associated with more than 10-fold accumulation of membrane-bound fragments of App, Aplp1, Nrg1, and Dcc, while other known substrates of γ-secretase such as Aplp2, Lrp1, and Sdc3 accumulate to lesser extents. Despite numerous reports linking neurodegeneration to accumulation of membrane-bound App fragments, deletion of App expression in the combined Aph1 knockout does not rescue this phenotype. Importantly, knockout of only Aph1a- or Aph1bc-secretases causes limited and differential accumulation of substrates. This was not associated with neurodegeneration. Further development of selective Aph1-γ-secretase inhibitors should be considered for treatment of Alzheimer's disease.

Dysregulation of Rab5-mediated endocytic pathways in Alzheimer's disease

Increasing evidence has pointed to that dysregulation of the endo-lysosomal system is an early cellular phenotype of pathogenesis for Alzheimer's disease (AD). Rab5, a small GTPase, plays a critical role in mediating these processes. Abnormal overactivation of Rab5 has been observed in post-mortem brain samples of Alzheimer's patients as well as brain samples of mouse models of AD. Recent genome-wide association studies of AD have identified RIN3 (Ras and Rab interactor 3) as a novel risk factor for the disease. RIN3 that functions as a guanine nucleotide exchange factor for Rab5 may serve as an important activator for Rab5 in AD pathogenesis. In this review, we present recent research highlights on the possible roles of dysregulation of Rab5-mediated endocytic pathways in contributing to early pathogenesis of AD.

Increased localization of APP-C99 in mitochondria-associated ER membranes causes mitochondrial dysfunction in Alzheimer disease

In the amyloidogenic pathway associated with Alzheimer disease (AD), the amyloid precursor protein (APP) is cleaved by β‐secretase to generate a 99‐aa C‐terminal fragment (C99) that is then cleaved by γ‐secretase to generate the β‐amyloid (Aβ) found in senile plaques. In previous reports, we and others have shown that γ‐secretase activity is enriched in mitochondria‐associated endoplasmic reticulum (ER) membranes (MAM) and that ER–mitochondrial connectivity and MAM function are upregulated in AD. We now show that C99, in addition to its localization in endosomes, can also be found in MAM, where it is normally processed rapidly by γ‐secretase. In cell models of AD, however, the concentration of unprocessed C99 increases in MAM regions, resulting in elevated sphingolipid turnover and an altered lipid composition of both MAM and mitochondrial membranes. In turn, this change in mitochondrial membrane composition interferes with the proper assembly and activity of mitochondrial respiratory supercomplexes, thereby likely contributing to the bioenergetic defects characteristic of AD.

Exosomes in the Diseased Brain: First Insights from In vivo Studies

Extracellular vesicles (EVs) are nanoscale size vesicles secreted by cells and are important mediators of intercellular communication and genetic exchange. Exosomes, EVs generated in endosomal multivesicular bodies, have been the focus of numerous publications as they have emerged as clinically valuable markers of disease states. Exosomes have been mostly studied from conditioned culture media and body fluids, with the difficulty of isolating exosomes from tissues having delayed their study in vivo. The implementation of a method designed to isolate exosomes from tissues, however, has yielded the first insights into characteristics of exosomes in the brain. It has been observed that brain exosomes from murine models of neurodegenerative diseases and human postmortem brains tend to mirror the protein content of the cells of origin, and interestingly, they are enriched with toxic proteins. Whether this enrichment with neurotoxic proteins is beneficial by relieving neurons of accumulated toxic material or detrimental to the brain by propagating pathogenicity throughout the brain remains to be answered. Here is summarized the first group of studies describing exosomes isolated from brain, results that demonstrate that exosomes in vivo reflect complex multicellular pathogenic processes in neurodegenerative disorders and the brain's response to injury and damage.

Differential contribution of APP metabolites to early cognitive deficits in a TgCRND8 mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative pathology commonly characterized by a progressive and irreversible deterioration of cognitive functions, especially memory. Although the etiology of AD remains unknown, a consensus has emerged on the amyloid hypothesis, which posits that increased production of soluble amyloid β (Aβ) peptide induces neuronal network dysfunctions and cognitive deficits. However, the relative failures of Aβ-centric therapeutics suggest that the amyloid hypothesis is incomplete and/or that the treatments were given too late in the course of AD, when neuronal damages were already too extensive. Hence, it is striking to see that very few studies have extensively characterized, from anatomy to behavior, the alterations associated with pre-amyloid stages in mouse models of AD amyloid pathology. To fulfill this gap, we examined memory capacities as well as hippocampal network anatomy and dynamics in young adult pre-plaque TgCRND8 mice when hippocampal Aβ levels are still low. We showed that TgCRND8 mice present alterations in hippocampal inhibitory networks and γ oscillations at this stage. Further, these mice exhibited deficits only in a subset of hippocampal-dependent memory tasks, which are all affected at later stages. Last, using a pharmacological approach, we showed that some of these early memory deficits were Aβ-independent. Our results could partly explain the limited efficacy of Aβ-directed treatments and favor multitherapy approaches for early symptomatic treatment for AD.

APP and APLP2 interact with the synaptic release machinery and facilitate transmitter release at hippocampal synapses

The amyloid precursor protein (APP), whose mutations cause familial Alzheimer's disease, interacts with the synaptic release machinery, suggesting a role in neurotransmission. Here we mapped this interaction to the NH2-terminal region of the APP intracellular domain. A peptide encompassing this binding domain -named JCasp- is naturally produced by a γ-secretase/caspase double-cut of APP. JCasp interferes with the APP-presynaptic proteins interaction and, if linked to a cell-penetrating peptide, reduces glutamate release in acute hippocampal slices from wild-type but not APP deficient mice, indicating that JCasp inhibits APP function.The APP-like protein-2 (APLP2) also binds the synaptic release machinery. Deletion of APP and APLP2 produces synaptic deficits similar to those caused by JCasp. Our data support the notion that APP and APLP2 facilitate transmitter release, likely through the interaction with the neurotransmitter release machinery. Given the link of APP to Alzheimer's disease, alterations of this synaptic role of APP could contribute to dementia.

The multifaceted nature of amyloid precursor protein and its proteolytic fragments: friends and foes

The amyloid precursor protein (APP) has occupied a central position in Alzheimer's disease (AD) pathophysiology, in large part due to the seminal role of amyloid-β peptide (Aβ), a proteolytic fragment derived from APP. Although the contribution of Aβ to AD pathogenesis is accepted by many in the research community, recent studies have unveiled a more complicated picture of APP's involvement in neurodegeneration in that other APP-derived fragments have been shown to exert pathological influences on neuronal function. However, not all APP-derived peptides are neurotoxic, and some even harbor neuroprotective effects. In this review, we will explore this complex picture by first discussing the pleiotropic effects of the major APP-derived peptides cleaved by multiple proteases, including soluble APP peptides (sAPPα, sAPPβ), various C- and N-terminal fragments, p3, and APP intracellular domain fragments. In addition, we will highlight two interesting sequences within APP that likely contribute to this duality in APP function. First, it has been found that caspase-mediated cleavage of APP in the cytosolic region may release a cytotoxic peptide, C31, which plays a role in synapse loss and neuronal death. Second, recent studies have implicated the -YENPTY- motif in the cytoplasmic region as a domain that modulates several APP activities through phosphorylation and dephosphorylation of the first tyrosine residue. Thus, this review summarizes the current understanding of various APP proteolytic products and the interplay among them to gain deeper insights into the possible mechanisms underlying neurodegeneration and AD pathophysiology.

Bidirectional links between Alzheimer's disease and Niemann-Pick type C disease

Alzheimer's disease (AD) and Niemann-Pick type C (NPC) disease are progressive neurodegenerative diseases with very different epidemiology and etiology. AD is a common cause of dementia with a complex polyfactorial etiology, including both genetic and environmental risk factors, while NPC is a very rare autosomal recessive disease. However, the diseases share some disease-related molecular pathways, including abnormal cholesterol metabolism, and involvement of amyloid-β (Aβ) and tau pathology. Here we review recent studies on these pathological traits, focusing on studies of Aβ and tau pathology in NPC, and the importance of the NPC1 gene in AD. Further studies of similarities and differences between AD and NPC may be useful to increase the understanding of both these devastating neurological diseases.

The dark sides of amyloid in Alzheimer's disease pathogenesis

Although widely explored, the pathogenesis of Alzheimer's disease (AD) has yet to be cleared. Over the past twenty years the so call amyloid cascade hypothesis represented the main research paradigm in AD pathogenesis. In spite of its large consensus, the proposed role of β-amyloid (Aβ) remain to be elucidated. Many evidences are starting to cast doubt on Aβ as the primary causative factor in AD. For instance, Aβ is deposited in the brain following many different kinds of injury. Also, concentration of Aβ needed to induce toxicity in vitro are never reached in vivo. In this review we propose an amyloid-independent interpretation of several AD pathogenic features, such as synaptic plasticity, endo-lysosomal trafficking, cell cycle regulation and neuronal survival.

Biology and pathophysiology of the amyloid precursor protein

The amyloid precursor protein (APP) plays a central role in the pathophysiology of Alzheimer's disease in large part due to the sequential proteolytic cleavages that result in the generation of β-amyloid peptides (Aβ). Not surprisingly, the biological properties of APP have also been the subject of great interest and intense investigations. Since our 2006 review, the body of literature on APP continues to expand, thereby offering further insights into the biochemical, cellular and functional properties of this interesting molecule. Sophisticated mouse models have been created to allow in vivo examination of cell type-specific functions of APP together with the many functional domains. This review provides an overview and update on our current understanding of the pathobiology of APP.

Amyloid-independent mechanisms in Alzheimer's disease pathogenesis

Despite the progress of the past two decades, the cause of Alzheimer's disease (AD) and effective treatments against it remain elusive. The hypothesis that amyloid-β (Aβ) peptides are the primary causative agents of AD retains significant support among researchers. Nonetheless, a growing body of evidence shows that Aβ peptides are unlikely to be the sole factor in AD etiology. Evidence that Aβ/amyloid-independent factors, including the actions of AD-related genes, also contribute significantly to AD pathogenesis was presented in a symposium at the 2010 Annual Meeting of the Society for Neuroscience. Here we summarize the studies showing how amyloid-independent mechanisms cause defective endo-lysosomal trafficking, altered intracellular signaling cascades, or impaired neurotransmitter release and contribute to synaptic dysfunction and/or neurodegeneration, leading to dementia in AD. A view of AD pathogenesis that encompasses both the amyloid-dependent and -independent mechanisms will help fill the gaps in our knowledge and reconcile the findings that cannot be explained solely by the amyloid hypothesis.

Troubleshooting methods for APP processing in vitro

INTRODUCTION

The amyloid hypothesis states that Abeta is the main trigger for Alzheimer's disease. This report is focused in the study of the processing of the Amyloid Precursor Protein (APP) as a procedure to investigate the molecular mechanisms that may result in changes in the levels of Abeta.

METHODS

Here we analyse different methodologies for Abeta determination, soluble APP, APP-Carboxy terminus fragments (CTFs) and enzymes for synthesis (secretases) and degradation of Abeta. In addition the advantages and disadvantages of different methodologies are discussed.

DISCUSSION

The potential value of these procedures is described in the context of the function of APP and the different fragments derived from its cleavage.

Behavioural phenotype of APPC100.V717F transgenic mice over-expressing a mutant Abeta-bearing fragment is associated with reduced NMDA receptor density

The aim of this study was to characterize APPC100.V717F transgenic (TgC100.V717F) mice which over-express a mutant C100 fragment of the amyloid precursor protein. The mice were compared to TgC100 wild type mice (TgC100.WT) and non-transgenic controls at 4-9 and 16-22 months of age. TgC100.V717F mice showed behavioural hyperactivity, particularly at a younger age, as shown by increased numbers of elevated plus maze arm entries and Y-maze arm entries, enhanced baseline locomotor activity in the open field, and enhanced amphetamine-induced hyperlocomotion. This hyperactivity was less pronounced in TgC100.WT which only displayed significant differences to non-transgenic controls at a younger age for the number of Y-maze arm entries and baseline locomotor activity in the open field. In addition, TgC100.V717F mice, but not TgC100.WT, demonstrated cognitive deficits, as shown by reduced spontaneous alternation in the Y-maze and markedly reduced retention in a passive avoidance test. At an older age, TgC100.V717F mice showed enhanced startle and increased immobility time in the forced swim test. In the TgC100.V717F mice, but not TgC100.WT, the behavioural changes were paralleled by a significant reduction in the expression of hippocampal NMDA receptor subunits types 1 and 2A. Concomitantly, we detected axonal disruption and apoptosis in the hippocampus of TgC100.V717F mice. In conclusion, these data demonstrate that the mutant C100 fragment is an effector of biochemical and both cognitive and non-cognitive behaviours. These transgenic mice may be a model for the psychotic features associated with early Alzheimer's disease.

Histopathological and molecular heterogeneity among individuals with dementia associated with Presenilin mutations

Background

Mutations in the presenilin (PSEN) genes are associated with early-onset familial Alzheimer's disease (FAD). Biochemical characterizations and comparisons have revealed that many PSEN mutations alter γ-secretase activity to promote accumulation of toxic Aβ42 peptides. In this study, we compared the histopathologic and biochemical profiles of ten FAD cases expressing independent PSEN mutations and determined the degradation patterns of amyloid-β precursor protein (AβPP), Notch, N-cadherin and Erb-B4 by γ-secretase. In addition, the levels of Aβ40/42 peptides were quantified by ELISA.

Results

We observed a wide variation in type, number and distribution of amyloid deposits and neurofibrillary tangles. Four of the ten cases examined exhibited a substantial enrichment in the relative proportions of Aβ40 over Aβ42. The AβPP N-terminal and C-terminal fragments and Tau species, assessed by Western blots and scanning densitometry, also demonstrated a wide variation. The Notch-1 intracellular domain was negligible by Western blotting in seven PSEN cases. There was significant N-cadherin and Erb-B4 peptide heterogeneity among the different PSEN mutations.

Conclusion

These observations imply that missense mutations in PSEN genes can alter a range of key γ-secretase activities to produce an array of subtly different biochemical, neuropathological and clinical manifestations. Beyond the broad common features of dementia, plaques and tangles, the various PSEN mutations resulted in a wide heterogeneity and complexity and differed from sporadic AD.

Detailed immunohistochemical characterization of temporal and spatial progression of Alzheimer's disease-related pathologies in male triple-transgenic mice

Background

Several transgenic animal models genetically predisposed to develop Alzheimer's disease (AD)-like pathology have been engineered to facilitate the study of disease pathophysiology and the vetting of potential disease-modifying therapeutics. The triple transgenic mouse model of AD (3xTg-AD) harbors three AD-related genetic loci: human PS1^M146V, human APP^swe, and human tau^P301L. These mice develop both amyloid plaques and neurofibrillary tangle-like pathology in a progressive and age-dependent manner, while these pathological hallmarks are predominantly restricted to the hippocampus, amygdala, and the cerebral cortex the main foci of AD neuropathology in humans. This model represents, at present, one of the most advanced preclinical tools available and is being employed ever increasingly in the study of mechanisms underlying AD, yet a detailed regional and temporal assessment of the subtleties of disease-related pathologies has not been reported.

Methods and results

In this study, we immunohistochemically documented the evolution of AD-related transgene expression, amyloid deposition, tau phosphorylation, astrogliosis, and microglial activation throughout the hippocampus, entorhinal cortex, primary motor cortex, and amygdala over a 26-month period in male 3xTg-AD mice. Intracellular amyloid-beta accumulation is detectable the earliest of AD-related pathologies, followed temporally by phospho-tau, extracellular amyloid-beta, and finally paired helical filament pathology. Pathology appears to be most severe in medial and caudal hippocampus. While astrocytic staining remains relatively constant at all ages and regions assessed, microglial activation appears to progressively increase temporally, especially within the hippocampal formation.

Conclusion

These data fulfill an unmet need in the ever-widening community of investigators studying 3xTg-AD mice and provide a foundation upon which to design future experiments that seek to examine stage-specific disease mechanisms and/or novel therapeutic interventions for AD.

Astroglial activation of extracellular-regulated kinase in early stages of Alzheimer disease

Characterization of the earliest neuropathologic features of Alzheimer disease (AD) indicates that synaptic degeneration accompanied by tau hyperphosphorylation and amyloid deposition might be an important feature. The mechanisms involved are unclear; however, dysregulation of signaling cascades such as the extracellular signal-regulated kinase (ERK) pathway might play a role. In this context, the main objective of this study was to determine whether ERK hyperactivation occurs in early stages of AD. We compared the patterns of total and phosphorylated ERK (pERK) expression in the midfrontal cortex of patients clinically and neuropathologically characterized with early, intermediate, or advanced AD. Immunocytochemical and Western blot analysis showed that in early AD, there was extensive activation of ERK in astroglial cells in the white matter accompanied by intense astrogliosis. In contrast, in patients with more advanced AD, pERK immunoreactivity was associated with neuronal cell bodies and dystrophic neurites around plaques. Levels of astroglial pERK immunoreactivity in the white matter were strongly correlated with scores of cognitive performance (Blessed, Mini-Mental Status Examination, and Clinical Dementia Rating) and with the severity of AD neuropathology (Braak stage). These findings suggest that astroglial ERK activation may be an important early response to the onset of AD pathology. Identification of cell signaling events unique to early AD may provide therapeutic targets for the prevention or delay of dementia.

Progressive neuronal loss and behavioral impairments of transgenic C57BL/6 inbred mice expressing the carboxy terminus of amyloid precursor protein

The beta-secretase cleaved Abeta-bearing carboxy-terminal fragments (betaCTFs) of amyloid precursor protein (APP) in neural cells have been suggested to be cytotoxic. However, the functional significance of betaCTFs in vivo remains elusive. We created a transgenic mouse line Tg-betaCTF99/B6 expressing the human betaCTF99 in the brain of inbred C57BL/6 strain. Tg-betaCTF99/B6 mouse brain at 12-16 months showed severely down-regulated calbindin, phospho-CREB, and Bcl-xL expression and up-regulated phospho-JNK, Bcl-2, and Bax expression. Neuronal cell density in the Tg-betaCTF99/B6 cerebral cortex at 16-18 months was lower than that of the non-transgenic control, but not at 5 months. At 11-14 months, Tg-betaCTF99/B6 mice displayed cognitive impairments and increased anxiety, which were not observed at 5 months. These results suggest that increased betaCTF99 expression is highly detrimental to the aging brain and that it produces a progressive and age-dependent AD-like pathogenesis.

Recent advances in transgenic model development for Alzheimer's disease

The lack of a small animal model that represents major features of Alzheimer's disease has long been considered a major handicap for research and drug development. Transgenic technology has been used to introduce potential pathological start points as well as established genetic causes of the disease to trigger pathogenesis in a small animal model. This review describes various approaches, discusses the available transgenic mouse models and compares their similarities and differences, and their applicability for the testing of drugs aiming at a causal treatment of the disease.

Signal transduction through tyrosine-phosphorylated carboxy-terminal fragments of APP via an enhanced interaction with Shc/Grb2 adaptor proteins in reactive astrocytes of Alzheimer's disease brain

The processing of the amyloid precursor protein (APP) through the formation of C-terminal fragments (CTFs) and the production of beta-amyloid, are events likely to influence the development and the progression of Alzheimer's disease (AD). APP is a transmembrane protein similar to a cell-surface receptor with the intraluminal NPTY motif in the cytosolic C terminus. Although APP holoprotein can be bound to intracellular proteins like Fe65, X11, and mDab, the ultimate function and the mechanisms through which this putative receptor transfers its message are unclear. Here it is shown that in human brain, a subset of tyrosine-phosphorylated CTFs represent docking sites for the adaptor protein ShcA. ShcA immunoreactivity is greatly enhanced in Alzheimer's patients; it is mainly localized to glial cells and occurs at reactive astrocytes surrounding cerebral vessels and amyloid plaques. Grb2 also is involved in complexes with ShcA and tyrosine-phosphorylated CTFs, and in AD brain the interaction between Grb2-ShcA and CTFs is enhanced. Also, a higher amount of phospho-ERK1,2 is present in AD brain in comparison with control cases, likely as a result of the ShcA activation. In vitro experiments show that the ShcA-CTFs interaction is strictly confined to glial cells when treated with thrombin, which is a well-known ShcA and ERK1,2 activator, mitogen, and regulator of APP cleavage. In untreated cells ShcA does not interact with either APP or CTFs, although they are normally produced. Altogether these data suggest that CTFs are implicated in cell signaling via Shc transduction machinery, likely influencing MAPK activity and glial reaction in AD patients.

Mouse models of Alzheimer's disease: a quest for plaques and tangles

Many genetically altered mice have been designed to help understand the role of specific gene mutations in the pathogenesis of Alzheimer's disease (AD) based on the realization that specific mutations in the genes for amyloid precursor protein--the presenilins and tau--are associated with early-onset familial AD or, in the case of tau mutations, other neurodegenerative diseases with neurofibrillary tangles. However, attempts to reproduce the neuropathology of AD in the mouse have been frustrating. Transgenic designs emphasizing amyloid precursor protein produced mice that develop amyloid plaques, but neurodegeneration and neurofibrillary tangles failed to form. Strategies emphasizing tau resulted in increased phosphorylation of tau and tangle formation, although amyloid plaques were absent. Nevertheless, crossing transgenic animals expressing mutated tau and amyloid precursor protein has produced a mouse that closely recapitulates the neuropathology of AD. A review of the various murine models, their role in understanding the pathogenesis of AD and their use in testing therapeutic regimens, is provided.

Amino-terminal modification and tyrosine phosphorylation of [corrected] carboxy-terminal fragments of the amyloid precursor protein in Alzheimer's disease and Down's syndrome brain

The carboxy-terminal fragments (CTFs) of the amyloid precursor protein (APP) are considered beta-amyloid (Abeta) precursors as well as molecular species possibly amyloidogenic and neurotoxic by [corrected] in vitro or in animal models. The CTF's role in the pathogenesis of Alzheimer's disease (AD) is however relatively unexplored in human brain. In this study, we analyzed brain extracted CTFs in subjects with AD, non-AD control, and Down's syndrome (DS) cases. Our data indicate that: (i) In fetal DS subjects CTFs levels are increased in comparison to age-matched control, suggesting that the enhanced CTFs formation is important for the early occurrence of plaques deposition in DS. No significant difference in CTFs level [corrected] between AD and age-matched control cases. (ii) CTFs modified at their N-terminus are the direct precursors of similarly N-terminally modified Abeta peptides, which constitute the most abundant species in AD and DS plaques. This observation suggests that N-truncated Abeta peptides are formed directly at beta-secretase level and not through a progressive proteolysis of full-length Abeta1-40/42. (iii) Among the differently cleaved CTFs, only the 22- and 12.5-kDa CTF polypeptides are tyrosine phosphorylated in both AD and control brain while the full-length APP and the CTFs migrating below the 12.5-kDa marker are not phosphorylated, suggesting that APP and CTFs may be involved in different pathways depending on their length and sequences. This study provides evidence that CTFs constitute in human brain a molecular species directly involved in AD pathogenesis and in the development of the AD-like pathology in DS subjects.

ASP1 (BACE2) cleaves the amyloid precursor protein at the beta-secretase site

Sequential proteolytic processing of the Amyloid Precursor Protein (APP) by beta- and gamma-secretases generates the 4-kDa amyloid (A beta) peptide, a key component of the amyloid plaques seen in Alzheimer's disease (AD). We and others have recently reported the identification and characterisation of an aspartic proteinase, Asp2 (BACE), as beta-secretase. Here we describe the characterization of a second highly related aspartic proteinase, Asp1 as a second beta-secretase candidate. Asp1 is expressed in brain as detected at the mRNA level and at the protein level. Transient expression of Asp1 in APP-expressing cells results in an increase in the level of beta-secretase-derived soluble APP and the corresponding carboxy-terminal fragment. Paradoxically there is a decrease in the level of soluble A beta secreted from the cells. Asp1 colocalizes with APP in the Golgi/endoplasmic reticulum compartments of cultured cells. Asp1, when expressed as an Fc fusion protein (Asp1-Fc), has the N-terminal sequence ALEP..., indicating that it has lost the prodomain. Asp1-Fc exhibits beta-secretase activity by cleaving both wild-type and Swedish variant (KM/NL) APP peptides at the beta-secretase site.

Alzheimer's disease: transgenic mouse models and drug assessment

Alzheimer's disease (AD), characterized by neuritic plaques and neurofibrillary tangles of the brain, is experienced by more and more elderly people in a form of senile dementia. Four genes are closely linked with AD and are located on chromosomes 21, 19, 14 and 1. Transgenic technology enables the development of animal models for research into this human disease. Recently reported transgenic AD mouse models, which express AD-related mutant human genes, develop some significant aspects of AD-like pathology. The specific role of these mice in representing different targets, the consequent pathology of AD and the availability of this increasingly popular tool for investigating new therapeutic strategies for AD are reviewed.

Enhanced BK-induced calcium responsiveness in PC12 cells expressing the C100 fragment of the amyloid precursor protein

Several lines of evidence have implicated the amyloid precursor protein (APP) and its metabolic products as key players in Alzheimer's disease (AD) pathophysiology. The approximately 100 amino acid C-terminal fragment (C100) of APP has been shown to accumulate intracellularly in neurons expressing familial AD (FAD) mutants of APP and to cause neurodegeneration when expressed in transfected neuronal cells. Transgenic animals expressing this fragment in the brain also exhibit some neuropathological and behavioral AD-like deficits. Here, we present evidence that PC12 cells expressing the C100 fragment either via stable transfections or herpes simplex virus-mediated infections show alterations in calcium handling that are similar to those previously shown in fibroblasts from AD patients. This alteration in calcium homeostasis may contribute to the deleterious effects of C100 in PC12 cells. Our data also lend support for a pathophysiological role for C100 since it induces an alteration thought to play an important role in AD pathology.

Transgenic animals in Alzheimer's disease research

Alzheimer's disease (AD) is a neurodegenerative disorder of the brain accounting for about 50-70% of the typical late onset cases of dementia. The pathological and diagnostic hallmarks of the disease are principally the presence of extracellular deposits called neuritic amyloid plaques and the intracellular aggregation of neurofibrillary tangles. In addition selective neuronal cell loss accompanied by cerebrovascular amyloidosis is detectable. In the case of familial AD, defects in at least three different genes (APP, PS1, PS2) leading to indistinguishable pathology are now well defined. There is as yet no real treatment for AD. Therefore the availability of an easily manipulable animal model is crucial for the development of new drugs, which could slow down or, even better, stop the progression of the disease. The development and originality of such experimental models that could greatly facilitate the investigation of the aetiology and pathogenesis of AD are described and discussed in this review. They are based mainly on the attempt to reproduce the neurofibrillary tangles or the amyloid deposits and plaque formation.

Intracellular accumulation of detergent-soluble amyloidogenic A beta fragment of Alzheimer's disease precursor protein in the hippocampus of aged transgenic mice

To study amyloid beta-protein (A beta) production and aggregation in vivo, we created two transgenic (Tg) mouse lines expressing the C-terminal 100 amino acids of human amyloid precursor protein (APP): Tg C100.V717F and Tg C100.WT. Western blot analysis showed that human APP-C100 and A beta were produced in brain and some peripheral tissues and A beta was produced in serum. Using antibodies specific for the A beta C terminus we found that Tg C100.V717F produced a 1.6-fold increase in A beta42/A beta40 compared with Tg C100.WT. Approximately 30% of total brain A beta (approximately 122 ng/g of wet tissue) was water-soluble. The remaining 70% of A beta partitioned into the particulate fraction and was completely sodium dodecyl sulfate-soluble. In contrast, human Alzheimer's disease brain has predominantly sodium dodecyl sulfate-insoluble A beta. Immunohistochemistry with an A beta(5-8) antibody showed that A beta or A beta-containing fragments accumulated intracellularly in the hippocampus of aged Tg C100.V717F mice. The soluble A beta levels in Tg brain are similar to those in normal human brain, and this may explain the lack of microscopic amyloid deposits in the Tg mice. However, this mouse model provides a system to study the intracellular processing and accumulation of A beta or A beta-containing fragments and to screen for compounds directed at the gamma-secretase activity.

Death of PC12 cells and hippocampal neurons induced by adenoviral-mediated FAD human amyloid precursor protein gene expression

We used adenoviral-mediated gene transfer of human amyloid precursor proteins (h-APPs) to evaluate the role of various h-APPs in causing neuronal cell death. We were able to infect PC12 cells with very high efficiency because approximately 90% of the cells were cytochemically positive for beta-galactosidase activity when an adenoviral vector containing LacZ cDNA was used to infect cells. Cells infected with adenovirus containing h-APP cDNA showed high-level transcription and expression of h-APP as measured by reverse transcriptase-polymerase chain reaction and Western immunoblot analyses, respectively. Intracellular and extracellular levels of h-APP were elevated approximately 17-and 24-fold in cultures infected with recombinant adenovirus containing wild-type mutant and 13- and 17-fold with V642F mutant. No elevation in h-APP was seen in cultures infected with antisense h-APP or null adenovirus. H-APP levels were maximal 3 days after infection. Overexpression of V642F mutant h-APP in PC12 cells and hippocampal neurons resulted in about a twofold increase in death compared with overexpression of wild-type h-APP. These results demonstrate the usefulness of recombinant adenoviral mediated gene transfer in cell culture studies and suggest that overexpression of a familial Alzheimer's disease mutant APP may be toxic to neuronal cells.

Age-related toxicity to lactate, glutamate, and beta-amyloid in cultured adult neurons

The age-related susceptibility of the brain to neurodegenerative disease may be inherent in the susceptibility of individual neurons to various stressors. Neurons were isolated from embryonic, young- and old-aged rat hippocampus, cultured in serum-free medium and exposed to lactic acid, glutamate or beta-amyloid. Yields of isolated adult cells were 1 million cells/hippocampus, 12,000 cells/mg tissue, independent of age. For lactic acidosis, there was a non-significant 10% increment in killing of neuron-like cells from old rats compared to young. For glutamate, there was a 5-10% increment in killing of neuron-like cells from old rats compared to young rats and embryonic neurons. For cells exposed to the toxic fragment of beta-amyloid, A beta (25-35), toxicity was age, dose and time-dependent. Maximum toxicity in cells treated for 1 day with 25 microM A beta (25-35) was 16%, 24%, and 33% for embryonic, young and old cells. Similar results were found for A beta (1-40) (LD50 = 2 microM). These results suggest that aging imparts to individual cells an increased susceptibility to toxic substances relevant to neurodegenerative diseases.

Relative increase in Alzheimer's disease of soluble forms of cerebral Abeta amyloid protein precursor containing the Kunitz protease inhibitory domain

Although a number of studies have examined amyloid precursor protein (APP) mRNA levels in Alzheimer's disease (AD), no clear consensus has emerged as to whether the levels of transcripts for isoforms containing a Kunitz protease inhibitory (KPI)-encoded region are increased or decreased in AD. Here we compare AD and control brain for the relative amounts of APP protein containing KPI to APP protein lacking this domain. APP protein was purified from the soluble subcellular fraction and Triton X-100 membrane pellet extract of one hemisphere of AD (n = 10), normal (n = 7), and neurological control (n = 5) brains. The amount of KPI-containing APP in the purified protein samples was determined using two independent assay methods. The first assay exploited the inhibitory action of KPI-containing APP on trypsin. The second assay employed reflectance analysis of Western blots. The proportion of KPI-containing forms of APP in the soluble subcellular fraction of AD brains is significantly elevated (p < 0.01) compared with controls. Species containing a KPI domain comprise 32-41 and 76-77% of purified soluble APP from control and AD brains, respectively. For purified membrane-associated APP, 72-77 and 65-82% of control and AD samples, respectively, contain a KPI domain. Since KPI-containing species of APP may be more amyloidogenic (Ho, L., Fukuchi, K., and Yonkin, S. G. (1996) J. Biol. Chem. 271, 30929-30934), our findings support an imbalance of isoforms as one possible mechanism for amyloid deposition in sporadic AD.

Murine models of brain aging and age-related neurodegenerative diseases

In the past, structural changes in the brain with aging have been studied using a variety of animal models, with rats and nonhuman primates being the most popular. With the rapid evolution of mouse genetics, murine models have gained increased attention in the neurobiology of aging. The genetic contribution of age-related traits as well as specific mechanistic hypotheses underlying brain aging and age-related neurodegenerative diseases can now be assessed by using genetically-selected and genetically-manipulated mice. Against this background of increased demand for aging research in mouse models, relatively few studies have examined structural alterations with aging in the normal mouse brain, and the data available are almost exclusively restricted to the C57BL/6 strain. Moreover, many older studies have used quantitative techniques which today can be questioned regarding their accuracy. Here we review the state of knowledge about structural changes with aging in outbred, inbred, genetically-selected, and genetically-engineered murine models. Moreover, we suggest several new opportunities that are emerging to study brain aging and age-related neurodegenerative diseases using genetically-defined mouse models. By reviewing the literature, it has become clear to us that in light of the rapid progress in genetically-engineered and selected mouse models for brain aging and age-related neurodegenerative diseases, there is a great and urgent need to study and define morphological changes in the aging brain of normal inbred mice and to analyze the structural changes in genetically-engineered mice more carefully and completely than accomplished to date. Such investigations will broaden knowledge in the neurobiology of aging, particularly regarding the genetics of aging, and possibly identify the most useful murine models.