Neuropathological characterization of mutant amyloid precursor protein yeast artificial chromosome transgenic mice

SPLICER: A Highly Efficient Base Editing Toolbox That Enables In Vivo Therapeutic Exon Skipping

Exon skipping technologies enable exclusion of targeted exons from mature mRNA transcripts, which has broad applications in molecular biology, medicine, and biotechnology. Existing exon skipping techniques include antisense oligonucleotides, targetable nucleases, and base editors, which, while effective for specific applications at some target exons, remain hindered by shortcomings, including transient effects for oligonucleotides, genotoxicity for nucleases and inconsistent exon skipping for base editors. To overcome these limitations, we created SPLICER, a toolbox of next-generation base editors consisting of near-PAMless Cas9 nickase variants fused to adenosine or cytosine deaminases for the simultaneous editing of splice acceptor (SA) and splice donor (SD) sequences. Synchronized SA and SD editing with SPLICER improves exon skipping, reduces aberrant outcomes, including cryptic splicing and intron retention, and enables skipping of exons refractory to single splice-site editing. To demonstrate the therapeutic potential of SPLICER, we targeted APP exon 17, which encodes the amino acid residues that are cleaved to form the Aβ plaques in Alzheimer's disease. SPLICER reduced the formation of Aβ42 peptides in vitro and enabled efficient exon skipping in a mouse model of Alzheimer's disease. Overall, SPLICER is a widely applicable and efficient toolbox for exon skipping with broad therapeutic applications.

Critical thinking of Alzheimer's transgenic mouse model: current research and future perspective

Transgenic models are useful tools for studying the pathogenesis of and drug development for Alzheimer's Disease (AD). AD models are constructed usually using overexpression or knock-in of multiple pathogenic gene mutations from familial AD. Each transgenic model has its unique behavioral and pathological features. This review summarizes the research progress of transgenic mouse models, and their progress in the unique mechanism of amyloid-β oligomers, including the first transgenic mouse model built in China based on a single gene mutation (PSEN1 V97L) found in Chinese familial AD. We further summarized the preclinical findings of drugs using the models, and their future application in exploring the upstream mechanisms and multitarget drug development in AD.

Cuprizone drives divergent neuropathological changes in different mouse models of Alzheimer's disease

Myelin degradation is a normal feature of brain aging that accelerates in Alzheimer's disease (AD). To date, however, the underlying biological basis of this correlation remains elusive. The amyloid cascade hypothesis predicts that demyelination is caused by increased levels of the β-amyloid (Aβ) peptide. Here we report on work supporting the alternative hypothesis that early demyelination is upstream of amyloid. We challenged two different mouse models of AD (R1.40 and APP/PS1) using cuprizone-induced demyelination and tracked the responses with both neuroimaging and neuropathology. In oppose to amyloid cascade hypothesis, R1.40 mice, carrying only a single human mutant APP (Swedish; APP SWE ) transgene, showed a more abnormal changes of magnetization transfer ratio and diffusivity than in APP/PS1 mice, which carry both APP SWE and a second PSEN1 transgene (delta exon 9; PSEN1 dE9 ). Although cuprizone targets oligodendrocytes (OL), magnetic resonance spectroscopy and targeted RNA-seq data in R1.40 mice suggested a possible metabolic alternation in axons. In support of alternative hypotheses, cuprizone induced significant intraneuronal amyloid deposition in young APP/PS1, but not in R1.40 mice, and it suggested the presence of PSEN deficiencies, may accelerate Aβ deposition upon demyelination. In APP/PS1, mature OL is highly vulnerable to cuprizone with significant DNA double strand breaks (53BP1 + ) formation. Despite these major changes in myelin, OLs, and Aβ immunoreactivity, no cognitive impairment or hippocampal pathology was detected in APP/PS1 mice after cuprizone treatment. Together, our data supports the hypothesis that myelin loss can be the cause, but not the consequence, of AD pathology.

SIGNIFICANCE STATEMENT

The causal relationship between early myelin loss and the progression of Alzheimer's disease remains unclear. Using two different AD mouse models, R1.40 and APP/PS1, our study supports the hypothesis that myelin abnormalities are upstream of amyloid production and deposition. We find that acute demyelination initiates intraneuronal amyloid deposition in the frontal cortex. Further, the loss of oligodendrocytes, coupled with the accelerated intraneuronal amyloid deposition, interferes with myelin tract diffusivity at a stage before any hippocampus pathology or cognitive impairments occur. We propose that myelin loss could be the cause, not the consequence, of amyloid pathology during the early stages of Alzheimer's disease.

The Amyloid Precursor Protein Modulates the Position and Length of the Axon Initial Segment

The amyloid precursor protein (APP) is linked to the genetics and pathogenesis of Alzheimer's disease (AD). It is the parent protein of the β-amyloid (Aβ) peptide, the main constituent of the amyloid plaques found in an AD brain. The pathways from APP to Aβ are intensively studied, yet the normal functions of APP itself have generated less interest. We report here that glutamate stimulation of neuronal activity leads to a rapid increase in App gene expression. In mouse and human neurons, elevated APP protein changes the structure of the axon initial segment (AIS) where action potentials are initiated. The AIS is shortened in length and shifts away from the cell body. The GCaMP8f Ca2+ reporter confirms the predicted decrease in neuronal activity. NMDA antagonists or knockdown of App block the glutamate effects. The actions of APP on the AIS are cell-autonomous; exogenous Aβ, either fibrillar or oligomeric, has no effect. In culture, APPSwe (a familial AD mutation) induces larger AIS changes than wild type APP. Ankyrin G and βIV-spectrin, scaffolding proteins of the AIS, both physically associate with APP, more so in AD brains. Finally, in humans with sporadic AD or in the R1.40 AD mouse model, both females and males, neurons have elevated levels of APP protein that invade the AIS. In vivo as in vitro, this increased APP is associated with a significant shortening of the AIS. The findings outline a new role for the APP and encourage a reconsideration of its relationship to AD.SIGNIFICANCE STATEMENT While the amyloid precursor protein (APP) has long been associated with Alzheimer's disease (AD), the normal functions of the full-length Type I membrane protein have been largely unexplored. We report here that the levels of APP protein increase with neuronal activity. In vivo and in vitro, modest amounts of excess APP alter the properties of the axon initial segment. The β-amyloid peptide derived from APP is without effect. Consistent with the observed changes in the axon initial segment which would be expected to decrease action potential firing, we show that APP expression depresses neuronal activity. In mouse AD models and human sporadic AD, APP physically associates with the scaffolding proteins of the axon initial segment, suggesting a relationship with AD dementia.

Exogenous Aβ seeds induce Aβ depositions in the blood vessels rather than the brain parenchyma, independently of Aβ strain-specific information

Little is known about the effects of parenchymal or vascular amyloid β peptide (Aβ) deposition in the brain. We hypothesized that Aβ strain-specific information defines whether Aβ deposits on the brain parenchyma or blood vessels. We investigated 12 autopsied patients with different severities of Aβ plaques and cerebral amyloid angiopathy (CAA), and performed a seeding study using an Alzheimer’s disease (AD) mouse model in which brain homogenates derived from the autopsied patients were injected intracerebrally. Based on the predominant pathological features, we classified the autopsied patients into four groups: AD, CAA, AD + CAA, and less Aβ. One year after the injection, the pathological and biochemical features of Aβ in the autopsied human brains were not preserved in the human brain extract-injected mice. The CAA counts in the mice injected with all four types of human brain extracts were significantly higher than those in mice injected with PBS. Interestingly, parenchymal and vascular Aβ depositions were observed in the mice that were injected with the human brain homogenate from the less Aβ group. The Aβ and CAA seeding activities, which had significant positive correlations with the Aβ oligomer ratio in the human brain extracts, were significantly higher in the human brain homogenate from the less Aβ group than in the other three groups. These results indicate that exogenous Aβ seeds from different Aβ pathologies induced Aβ deposition in the blood vessels rather than the brain parenchyma without being influenced by Aβ strain-specific information, which might be why CAA is a predominant feature of Aβ pathology in iatrogenic transmission cases. Furthermore, our results suggest that iatrogenic transmission of Aβ pathology might occur due to contamination of brain tissues from patients with little Aβ pathology, and the development of inactivation methods for Aβ seeding activity to prevent iatrogenic transmission is urgently required.

Higher levels of kallikrein-8 in female brain may increase the risk for Alzheimer's disease

Women seem to have a higher vulnerability to Alzheimer's disease (AD), but the underlying mechanisms of this sex dichotomy are not well understood. Here, we first determined the influence of sex on various aspects of Alzheimer's pathology in transgenic CRND8 mice. We demonstrate that beta-amyloid (Aβ) plaque burden starts to be more severe around P180 (moderate disease stage) in female transgenics when compared to males and that aging aggravates this sex-specific difference. Furthermore, we show that female transgenics suffer from higher levels of neurovascular dysfunction around P180, resulting in impaired Aβ peptide clearance across the blood-brain-barrier at P360. Female transgenics show also higher levels of diffuse microgliosis and inflammation, but the density of microglial cells surrounding Aβ plaques is less in females. In line with this finding, testosterone compared to estradiol was able to improve microglial viability and Aβ clearance in vitro. The spatial memory of transgenics was in general poorer than in wildtypes and at P360 worse in females irrespective of their genotype. This difference was accompanied by a slightly diminished dendritic complexity in females. While all the above-named sex-differences emerged after the onset of Aβ pathology, kallikrein-8 (KLK8) protease levels were, as an exception, higher in female than in male brains very early when virtually no plaques were detectable. In a second step, we quantified cerebral KLK8 levels in AD patients and healthy controls, and could ascertain, similar to mice, higher KLK8 levels not only in AD-affected but also in healthy brains of women. Accordingly, we could demonstrate that estradiol but not testosterone induces KLK8 synthesis in neuronal and microglial cells. In conclusion, multiple features of AD are more pronounced in females. Here, we show for the first time that this sex-specific difference may be meditated by estrogen-induced KLK8 overproduction long before AD pathology emerges.

Quantitative Comparison of Dense-Core Amyloid Plaque Accumulation in Amyloid-β Protein Precursor Transgenic Mice

There exist several dozen lines of transgenic mice that express human amyloid-β protein precursor (AβPP) with Alzheimer's disease (AD)-linked mutations. AβPP transgenic mouse lines differ in the types and amounts of Aβ that they generate and in their spatiotemporal patterns of expression of Aβ assemblies, providing a toolkit to study Aβ amyloidosis and the influence of Aβ aggregation on brain function. More complete quantitative descriptions of the types of Aβ assemblies present in transgenic mice and in humans during disease progression should add to our understanding of how Aβ toxicity in mice relates to the pathogenesis of AD. Here, we provide a direct quantitative comparison of amyloid plaque burdens and plaque sizes in four lines of AβPP transgenic mice. We measured the fraction of cortex and hippocampus occupied by dense-core plaques, visualized by staining with Thioflavin S, in mice from young adulthood through advanced age. We found that the plaque burdens among the transgenic lines varied by an order of magnitude: at 15 months of age, the oldest age studied, the median cortical plaque burden in 5XFAD mice was already ∼4.5 times that of 21-month-old Tg2576 mice and ∼15 times that of 21-24-month-old rTg9191 mice. Plaque-size distributions changed across the lifespan in a line- and region-dependent manner. We also compared the dense-core plaque burdens in the mice to those measured in a set of pathologically-confirmed AD cases from the Nun Study. Cortical plaque burdens in Tg2576, APPSwePS1ΔE9, and 5XFAD mice eventually far exceeded those measured in the human cohort.

The CNS in inbred transgenic models of 4-repeat Tauopathy develops consistent tau seeding capacity yet focal and diverse patterns of protein deposition

BACKGROUND

MAPT mutations cause neurodegenerative diseases such as frontotemporal dementia but, strikingly, patients with the same mutation may have different clinical phenotypes.

METHODS

Given heterogeneities observed in a transgenic (Tg) mouse line expressing low levels of human (2 N, 4R) P301L Tau, we backcrossed founder stocks of mice to C57BL/6Tac, 129/SvEvTac and FVB/NJ inbred backgrounds to discern the role of genetic versus environmental effects on disease-related phenotypes.

RESULTS

Three inbred derivatives of a TgTau^P301L founder line had similar quality and steady-state quantity of Tau production, accumulation of abnormally phosphorylated 64-68 kDa Tau species from 90 days of age onwards and neuronal loss in aged Tg mice. Variegation was not seen in the pattern of transgene expression and seeding properties in a fluorescence-based cellular assay indicated a single "strain" of misfolded Tau. However, in other regards, the aged Tg mice were heterogeneous; there was incomplete penetrance for Tau deposition despite maintained transgene expression in aged animals and, for animals with Tau deposits, distinctions were noted even within each subline. Three classes of rostral deposition in the cortex, hippocampus and striatum accounted for 75% of pathology-positive mice yet the mean ages of mice scored as class I, II or III were not significantly different and, hence, did not fit with a predictable progression from one class to another defined by chronological age. Two other patterns of Tau deposition designated as classes IV and V, occurred in caudal structures. Other pathology-positive Tg mice of similar age not falling within classes I-V presented with focal accumulations in additional caudal neuroanatomical areas including the locus coeruleus. Electron microscopy revealed that brains of Classes I, II and IV animals all exhibit straight filaments, but with coiled filaments and occasional twisted filaments apparent in Class I. Most strikingly, Class I, II and IV animals presented with distinct western blot signatures after trypsin digestion of sarkosyl-insoluble Tau.

CONCLUSIONS

Qualitative variations in the neuroanatomy of Tau deposition in genetically constrained slow models of primary Tauopathy establish that non-synchronous, focal events contribute to the pathogenic process. Phenotypic diversity in these models suggests a potential parallel to the phenotypic variation seen in P301L patients.

The role of neuropathological markers in the interpretation of neuropsychiatric disorders: Focus on fetal and perinatal programming

The study of neuropathological markers in patients affected by mental/psychiatric disorders is relevant for the comprehension of the pathogenesis and the correlation with the clinical symptomatology. The neuropathology of Alzheimer's disease (AD) recognizes intraneuronal and extracellular neurofibrillary formation responsible for neuronal degeneration. Immunohistochemical studies discovered many interesting results for a better interpretation of the AD pathogenesis, while the "metal hypothesis" supports that metal ions might differentially influence the formation of amyloid aggregates. The most relevant pathological findings reported in schizophrenia originate from computer assisted tomography (CT), Magnetic Resonance Imaging (MRI) studies and Diffusion Tensor Imaging (DTI), suggesting the brain abnormalities involved in the pathophysiology of schizophrenia. The theory of fetal programming illustrates the epigenetic factors that may act during the intrauterine life on brain development, with relevant consequences on the susceptibility to develop AD or schizophrenia later in life. The neuropathological interpretation of AD and schizophrenia shows that the presence of severe neuropathological changes is not always associated with severe cognitive impairment. A better dialogue between psychiatrics and pathologists might help to halt insurgence and progression of neurodegenerative diseases.

Sex-related dimorphism in dentate gyrus atrophy and behavioral phenotypes in an inducible tTa:APPsi transgenic model of Alzheimer's disease

Sex differences are a well-known phenomenon in Alzheimer's disease (AD), with women having a higher risk for AD than men. Many AD mouse models display a similar sex-dependent pattern, with females showing earlier cognitive deficits and more severe neuropathology than males. However, whether those differences are relevant to human disease is unclear. Here we show that in AD mouse models that overexpress amyloid precursor protein (APP) under control of the prion protein promoter (PrP), female transgenic mice have higher APP expression than males, complicating interpretations of the role of sex-related factors in such models. By contrast, in a tTa:APPsi model, in which APP expression is driven by the tetracycline transactivator (tTa) from the CaMKIIα promoter, there are no sex-related differences in expression or processing of APP. In addition, the levels of Aβ dimers and tetramers, as well as Aβ peptide accumulation, are similar between sexes. Behavioral testing demonstrated that both male and female tTa:APPsi mice develop age-dependent deficits in spatial recognition memory and conditional freezing to context. These cognitive deficits were accompanied by habituation-associated hyperlocomotion and startle hyper-reactivity. Significant sex-related dimorphisms were observed, due to females showing earlier onsets of the deficits in conditioned freezing and hyperlocomotion. In addition, tTa:APPsi males but not females demonstrated a lack of novelty-induced activation. Both males and females showed atrophy of the dentate gyrus (DG) of the dorsal hippocampus, associated with widening of the pyramidal layer of the CA1 area in both sexes. Ventral DG was preserved. Sex-related differences were limited to the DG, with females showing more advanced degeneration than males. Collectively, our data show that the tTa:APPsi model is characterized by a lack of sex-related differences in APP expression, making this model useful in deciphering the mechanisms of sex differences in AD pathogenesis. Sex-related dimorphisms observed in this model under conditions of equal APP expression between sexes suggest a higher sensitivity of females to the effects of APP and/or Aβ production.

Multiple factors contribute to the peripheral induction of cerebral β-amyloidosis

Deposition of aggregated amyloid-β (Aβ) peptide in brain is an early event and hallmark pathology of Alzheimer's disease and cerebral Aβ angiopathy. Experimental evidence supports the concept that Aβ multimers can act as seeds and structurally corrupt other Aβ peptides by a self-propagating mechanism. Here we compare the induction of cerebral β-amyloidosis by intraperitoneal applications of Aβ-containing brain extracts in three Aβ-precursor protein (APP) transgenic mouse lines that differ in levels of transgene expression in brain and periphery (APP23 mice, APP23 mice lacking murine APP, and R1.40 mice). Results revealed that beta-amyloidosis induction, which could be blocked with an anti-Aβ antibody, was dependent on the amount of inoculated brain extract and on the level of APP/Aβ expression in the brain but not in the periphery. The induced Aβ deposits in brain occurred in a characteristic pattern consistent with the entry of Aβ seeds at multiple brain locations. Intraperitoneally injected Aβ could be detected in blood monocytes and some peripheral tissues (liver, spleen) up to 30 d after the injection but escaped histological and biochemical detection thereafter. These results suggest that intraperitoneally inoculated Aβ seeds are transported from the periphery to the brain in which corruptive templating of host Aβ occurs at multiple sites, most efficiently in regions with high availability of soluble Aβ.

Mitochondria and vascular lesions as a central target for the development of Alzheimer's disease and Alzheimer disease-like pathology in transgenic mice

Accumulating evidence strongly suggests that the AD brain is characterized by impairments in energy metabolism, and vascular hypoperfusion, whereby oxidative stress appears to be an especially important contributor to neuronal death and development of AD pathology. We hypothesized that mitochondria play a key role in the generation of reactive oxygen species, resulting in oxidative damage to neuronal cell bodies, as well as other cellular compartments in the AD brain. All of these changes have been found to accompany AD pathology. In this review we have outlined recent evidence from the literature and our own original studies concerning the role of mitochondrial abnormalities and vascular damage in the pathogenesis of AD and AD-like pathology in transgenic mice (as a model for human AD). We examined ultrastructural features of vascular lesions and mitochondria from vascular wall cells in human AD brain biopsies, in human short post-mortem brain tissues and in yeast artificial chromosome (YAC) and C57B6/SJL transgenic positive (Tg+) mice overexpressing amyloid beta precursor protein (A beta PP). In situ hybridization using mitochondrial DNA (mtDNA) probes for human wild type, 5kb deleted and mouse mtDNA was performed along with immunocytochemistry using antibodies against amyloid beta precursor protein (A beta PP), 8-hydroxy-2'-guanosine (8OHG) and cytochrome C oxidase (COX) were studied at the electron microscopic levels. There was a higher degree of amyloid deposition in the vascular walls of the human AD, YAC and C57B6/SJL Tg(+) mice compared to aged-matched controls. In addition, vessels with more severe lesions showed immunopositive staining for APP and possessed large, lipid-laden vacuoles in the cytoplasm of endothelial cells (EC). Significantly more mitochondrial abnormalities were seen in human AD, YAC and C57B6/SJL Tg(+) mouse microvessels where lesions occurred. In situ hybridization using wild and chimera (5 kB) mtDNA probes revealed positive signals in damaged mitochondria from the vascular endothelium and in perivascular cells of lesioned microvessels close to regions of large amyloid deposition. These features were absent in undamaged regions of human AD tissues, YAC and C57B6/SJL Tg(+) mouse tissues and in aged-matched control subjects. In addition, vessels with atherosclerotic lesions revealed endothelium and perivascular cells possessing clusters of wild and deleted mtDNA positive probes. These mtDNA deletions were accompanied by increased amounts of immunoreactive APP, 8OHG and COX in the same cellular compartment. Our observations first time demonstrate that vascular wall cells, especially their mitochondria, appear to be a central target for oxidative stress induced damage.

Short-term treatment with tolfenamic acid improves cognitive functions in Alzheimer's disease mice

Tolfenamic acid lowers the levels of the amyloid precursor protein (APP) and amyloid beta (Aβ) when administered to C57BL/6 mice by lowering their transcriptional regulator specificity protein 1 (SP1). To determine whether changes upstream in the amyloidogenic pathway that forms Aβ plaques would improve cognitive outcomes, we administered tolfenamic acid for 34 days to hemizygous R1.40 transgenic mice. After the characterization of cognitive deficits in these mice, assessment of spatial learning and memory functions revealed that treatment with tolfenamic acid attenuated long-term memory and working memory deficits, determined using Morris water maze and the Y-maze. These improvements occurred within a shorter period of exposure than that seen with clinically approved drugs. Cognitive enhancement was accompanied by reduction in the levels of the SP1 protein (but not messenger RNA [mRNA]), followed by lowering both the mRNA and the protein levels of APP and subsequent Aβ levels. These findings provide evidence that tolfenamic acid can disrupt the pathologic processes associated with Alzheimer's disease (AD) and are relevant to its scheduled biomarker study in AD patients.

Creation and characterization of BAC-transgenic mice with physiological overexpression of epitope-tagged RCAN1 (DSCR1)

The chromosome 21 gene RCAN1, encoding a modulator of the calcineurin (CaN) phosphatase, is a candidate gene for contributing to cognitive disability in people with Down syndrome (DS; trisomy 21). To develop a physiologically relevant model for studying the biochemistry of RCAN1 and its contribution to DS, we generated bacterial artificial chromosome-transgenic (BAC-Tg) mouse lines containing the human RCAN1 gene with a C-terminal HA-FLAG epitope tag incorporated by recombineering. The BAC-Tg was expressed at levels only moderately higher than the native Rcan1 gene: approximately 1.5-fold in RCAN1 (BAC-Tg1) and twofold in RCAN1 (BAC-Tg2). Affinity purification of the RCAN1 protein complex from brains of these mice revealed a core complex of RCAN1 with CaN, glycogen synthase kinase 3-beta (Gsk3b), and calmodulin, with substoichiometric components, including LOC73419. The BAC-Tg mice are fully viable, but long-term synaptic potentiation is impaired in proportion to BAC-Tg dosage in hippocampal brain slices from these mice. RCAN1 can act as a tumor suppressor in some systems, but we found that the RCAN1 BAC-Tg did not reduce mammary cancer growth when present at a low copy number in Tp53;WAP-Cre mice. This work establishes a useful mouse model for investigating the biochemistry and dose-dependent functions of the RCAN1 protein in vivo.

The presence of Aβ seeds, and not age per se, is critical to the initiation of Aβ deposition in the brain

The deposition of the β-amyloid (Aβ) peptide in senile plaques and cerebral Aβ-amyloid angiopathy can be seeded in β-amyloid precursor protein (APP)-transgenic mice by the intracerebral infusion of brain extracts containing aggregated Aβ. Previous studies of seeded β-amyloid induction have used relatively short incubation periods to dissociate seeded β-amyloid induction from endogenous β-amyloid deposition of the host, thus precluding the analysis of the impact of age and extended incubation periods on the instigation and spread of Aβ lesions in brain. In the present study using R1.40 APP-transgenic mice (which do not develop endogenous Aβ deposition up to 15 months of age) we show that: (1) seeding at 9 months of age does not induce more Aβ deposition than seeding at 3 months of age, provided that the incubation period (6 months) is the same; and (2) very long-term (12 months) incubation after a focal application of the seed results in the emergence of Aβ deposits throughout the forebrain. These findings indicate that the presence of Aβ seeds, and not the age of the host per se, is critical to the initiation of Aβ aggregation in the brain, and that Aβ deposition, actuated in one brain area, eventually spreads throughout the brain.

A comparative study of five mouse models of Alzheimer's disease: cell cycle events reveal new insights into neurons at risk for death

Ectopic cell cycle events (CCEs) in postmitotic neurons link the neurodegenerative process in human Alzheimer's disease (AD) with the brain phenotype of transgenic mouse models with known familial AD genes. Most reports on the mouse models use the appearance of brain amyloid pathology as a key outcome measure. In the current paper, we focus on the induction of neurodegeneration using CCEs as markers for impending neuronal loss. We compare 5 mouse models of familial AD for the appearance of CCEs in subcortical regions—deep cerebellar nuclei, amygdala, locus coeruleus, hippocampus, and dorsal raphe. We find that the models differ in their CCE involvement as well as in the appearance of phosphorylated tau and amyloid deposition, suggesting that each model represents a different disease phenotype. Comparison with the pattern of neuron death in human AD suggests that each may represent a distinctly different disease model when used in preclinical trials.

Peripheral reduction of β-amyloid is sufficient to reduce brain β-amyloid: implications for Alzheimer's disease

Three loci that modify β-amyloid (Aβ) accumulation and deposition in the brains of a mouse model of Alzheimer's disease have been previously described. One encompasses the Psen2 gene encoding presenilin 2, a component of the γ-secretase activity responsible for generating Aβ by proteolysis. We show that the activity of mouse Psen2, as measured by levels of mRNA accumulation, unexpectedly is heritable in the liver but not the brain, suggesting liver as the origin of brain Aβ deposits. Administration of STI571, a cancer therapeutic that does not cross the blood-brain barrier, reduced accumulation of Aβ in both the blood and the brain, confirming brain Aβ's peripheral origin and suggesting that STI571 and related compounds might have therapeutic/prophylactic value in human Alzheimer's disease. The genes Cib1 and Zfhx1b reside within the other modifier loci and also exhibit heritable expression in the liver, suggesting that they too contribute to Aβ accumulation.

CX3CR1 deficiency alters microglial activation and reduces beta-amyloid deposition in two Alzheimer's disease mouse models

Microglia, the primary immune effector cells in the brain, continually monitor the tissue parenchyma for pathological alterations and become activated in Alzheimer's disease. Loss of signaling between neurons and microglia via deletion of the microglial receptor, CX3CR1, worsens phenotypes in various models of neurodegenerative diseases. In contrast, CX3CR1 deficiency ameliorates pathology in murine stroke models. To examine the role of CX3CR1 in Alzheimer's disease-related β-amyloid pathology, we generated APPPS1 and R1.40 transgenic mouse models of Alzheimer's disease deficient for CX3CR1. Surprisingly, CX3CR1 deficiency resulted in a gene dose-dependent reduction in β-amyloid deposition in both the APPPS1 and R1.40 mouse models of AD. Immunohistochemical analysis revealed reduced staining for CD68, a marker of microglial activation. Furthermore, quantitative immunohistochemical analysis revealed reduced numbers of microglia surrounding β-amyloid deposits in the CX3CR1-deficient APPPS1 animals. The reduced β-amyloid pathology correlated with reduced levels of TNFα and CCL2 mRNAs, but elevated IL1β mRNA levels, suggesting an altered neuroinflammatory milieu. Finally, to account for these seemingly disparate results, both in vitro and in vivo studies provided evidence that CX3CL1/CX3CR1 signaling alters the phagocytic capacity of microglia, including the uptake of Aβ fibrils. Taken together, these results demonstrate that loss of neuron-microglial fractalkine signaling leads to reduced β-amyloid deposition in mouse models of AD that is potentially mediated by altered activation and phagocytic capability of CX3CR1-deficient microglia.

Ibuprofen attenuates oxidative damage through NOX2 inhibition in Alzheimer's disease

Considerable evidence points to important roles for inflammation in Alzheimer's disease (AD) pathophysiology. Epidemiological studies have suggested that long-term nonsteroidal anti-inflammatory drug (NSAID) therapy reduces the risk for Alzheimer's disease; however, the mechanism remains unknown. We report that a 9-month treatment of aged R1.40 mice resulted in 90% decrease in plaque burden and a similar reduction in microglial activation. Ibuprofen treatment reduced levels of lipid peroxidation, tyrosine nitration, and protein oxidation, demonstrating a dramatic effect on oxidative damage in vivo. Fibrillar β-amyloid (Aβ) stimulation has previously been demonstrated to induce the assembly and activation of the microglial nicotinamide adenine dinucleotide phosphate (NADPH) oxidase leading to superoxide production through a tyrosine kinase-based signaling cascade. Ibuprofen treatment of microglia or monocytes with racemic or S-ibuprofen inhibited Aβ-stimulated Vav tyrosine phosphorylation, NADPH oxidase assembly, and superoxide production. Interestingly, Aβ-stimulated Vav phosphorylation was not inhibited by COX inhibitors. These findings suggest that ibuprofen acts independently of cyclooxygenase COX inhibition to disrupt signaling cascades leading to microglial NADPH oxidase (NOX2) activation, preventing oxidative damage and enhancing plaque clearance in the brain.

Recent advances in the manipulation of murine gene expression and its utility for the study of human neurological disease

Transgenic mouse models have vastly contributed to our knowledge of the genetic and molecular pathways underlying the pathogenesis of neurological disorders that affect millions of people worldwide. Not only have they allowed the generation of disease models mimicking the human pathological state but they have also permitted the exploration of the pathological role of specific genes through the generation of knock-out and knock-in models. Classical constitutive transgenic mice have several limitations however, due to behavioral adaptation process occurring and conditional mouse models are time-consuming and often lack extensive spatial or temporal control of gene manipulation. These limitations could be overcome by means of innovative methods that are now available such as RNAi, viral vectors and large cloning DNA vectors. These tools have been extensively used for the generation of mouse models and are characterized by the superior control of transgene expression that has been proven invaluable in the assessment of novel treatments for neurological diseases and to further investigate the molecular processes underlying the etiopathology of neurological disorders. Furthermore, in association with classical transgenic mouse models, they have allowed the validation of innovative therapeutic strategies for the treatment of human neurological disorders. This review describes how these tools have overcome the limitations of classical transgenic mouse models and how they have been of value for the study of human neurological diseases.

NSAIDs prevent, but do not reverse, neuronal cell cycle reentry in a mouse model of Alzheimer disease

Ectopic cell cycle events (CCEs) mark vulnerable neuronal populations in human Alzheimer disease (AD) and are observed early in disease progression. In transgenic mouse models of AD, CCEs are found before the onset of beta-amyloid peptide (Abeta) deposition to form senile plaques, a hallmark of AD. Here, we have demonstrated that alterations in brain microglia occur coincidently with the appearance of CCEs in the R1.40 transgenic mouse model of AD. Furthermore, promotion of inflammation with LPS at young ages in R1.40 mice induced the early appearance of neuronal CCEs, whereas treatment with 2 different nonsteroidal antiinflammatory drugs (NSAIDs) blocked neuronal CCEs and alterations in brain microglia without altering amyloid precursor protein (APP) processing and steady-state Abeta levels. In addition, NSAID treatment of older R1.40 animals prevented new neuronal CCEs, although it failed to reverse existing ones. Retrospective human epidemiological studies have identified long-term use of NSAIDs as protective against AD. Prospective clinical trials, however, have failed to demonstrate a similar benefit. Our use of CCEs as an outcome measure offers fresh insight into this discrepancy and provides important information for future clinical trials, as it suggests that NSAID use in human AD may need to be initiated as early as possible to prevent disease progression.

Versatile somatic gene transfer for modeling neurodegenerative diseases

A growing variety of technical approaches allow control over the expression of selected genes in living organisms. The ability to deliver functional exogenous genes involved in neurodegenerative diseases has opened pathological processes to experimental analysis and targeted therapeutic development in rodent and primate preclinical models. Biological adaptability, economic animal use, and reduced model development costs complement improved control over spatial and temporal gene expression compared with conventional transgenic models. A review of viral vector studies, typically adeno-associated virus or lentivirus, for expression of three proteins that are central to major neurodegenerative diseases, will illustrate how this approach has powered new advances and opportunities in CNS disease research.

Nuclear localization of Cdk5 is a key determinant in the postmitotic state of neurons

Cyclin-dependent kinase 5 (Cdk5) is a nontraditional Cdk that is primarily active in postmitotic neurons. Its best known substrates are cytoskeletal proteins. Less appreciated is its role in the maintenance of a postmitotic state. We show here that in cycling cells (NIH 3T3), the localization of Cdk5 changes from predominantly nuclear to cytoplasmic as cells reenter a cell cycle after serum starvation. Similarly, when beta-amyloid peptide is used to stimulate cultured primary neurons to reenter a cell cycle, they too show a loss of nuclear Cdk5. Blocking nuclear export pharmacologically abolishes cell cycle reentry in wild-type but not Cdk5(-/-) neurons, suggesting a Cdk5-specific effect. Cdk5 overexpression targeted to the nucleus of Cdk5(-/-) neurons effectively blocks the cell cycle, but cytoplasmic targeting is ineffective. Further, in both human Alzheimer's disease as well as in the R1.40 mouse Alzheimer's model and the E2f1(-/-) mouse, neurons expressing cell cycle markers consistently show reduced nuclear Cdk5. Thus, both in vivo and in vitro, neurons that reenter a cell cycle lose nuclear Cdk5. We propose that the nuclear Cdk5 plays an active role in allowing neurons to remain postmitotic as they mature and that loss of nuclear Cdk5 leads to cell cycle entry.

Alzheimer disease models and human neuropathology: similarities and differences

Animal models aim to replicate the symptoms, the lesions or the cause(s) of Alzheimer disease. Numerous mouse transgenic lines have now succeeded in partially reproducing its lesions: the extracellular deposits of Abeta peptide and the intracellular accumulation of tau protein. Mutated human APP transgenes result in the deposition of Abeta peptide, similar but not identical to the Abeta peptide of human senile plaque. Amyloid angiopathy is common. Besides the deposition of Abeta, axon dystrophy and alteration of dendrites have been observed. All of the mutations cause an increase in Abeta 42 levels, except for the Arctic mutation, which alters the Abeta sequence itself. Overexpressing wild-type APP alone (as in the murine models of human trisomy 21) causes no Abeta deposition in most mouse lines. Doubly (APP x mutated PS1) transgenic mice develop the lesions earlier. Transgenic mice in which BACE1 has been knocked out or overexpressed have been produced, as well as lines with altered expression of neprilysin, the main degrading enzyme of Abeta. The APP transgenic mice have raised new questions concerning the mechanisms of neuronal loss, the accumulation of Abeta in the cell body of the neurons, inflammation and gliosis, and the dendritic alterations. They have allowed some insight to be gained into the kinetics of the changes. The connection between the symptoms, the lesions and the increase in Abeta oligomers has been found to be difficult to unravel. Neurofibrillary tangles are only found in mouse lines that overexpress mutated tau or human tau on a murine tau -/- background. A triply transgenic model (mutated APP, PS1 and tau) recapitulates the alterations seen in AD but its physiological relevance may be discussed. A number of modulators of Abeta or of tau accumulation have been tested. A transgenic model may be analyzed at three levels at least (symptoms, lesions, cause of the disease), and a reading key is proposed to summarize this analysis.

Ectopic cell cycle events link human Alzheimer's disease and amyloid precursor protein transgenic mouse models

Nerve cells that re-enter a cell cycle will die rather than divide, a fact that likely underlies the neurodegeneration in Alzheimer's disease (AD). Several mouse models of familial AD have been created, and although many display amyloid plaques in their brains, none captures the extensive pattern of nerve cell death found in the human disease. Using both immunocytochemistry and fluorescent in situ hybridization, we show that neurons in three different mouse models reproduce the ectopic cell cycling found in human AD. The temporal and spatial appearance of the cell cycle events in the mouse closely mimics the human disease progression. The cell-cycle events are evident 6 months before the first amyloid deposits and significantly precede the appearance of the first CD45+ microglia. These data suggest that the ectopic initiation of cell-cycle processes in neurons is an early sign of neuronal distress in both human and mouse AD. The close phenotypic correspondence indicates a previously unsuspected level of fidelity of the mouse model to the human disease. Finally, the relative timing suggests that neither the activated microglia nor the amyloid plaques themselves are necessary to initiate the pathogenic events in AD.

Spatial and temporal control of age-related APP processing in genomic-based beta-secretase transgenic mice

Genetic mutations associated with Alzheimer's disease (AD) in the Amyloid Precursor Protein (APP) gene specifically alter the production of the APP processing product, amyloid-beta (Abeta) peptide, generated by beta- and gamma-secretases. The accumulation and deposition of Abeta is hypothesized to cause AD pathogenesis, leading to the debilitating neurological deficits observed in AD patients. However, it is unclear how processing of APP to generate Abeta corresponds with the age-dependent pattern of brain-regional neurodegeneration common in AD. We have previously shown that overexpression of BACE1, the primary beta-secretase gene, in mice expressing an AD mutant form of APP leads to significantly elevated regional Abeta levels, which coincide with the regional pattern of Abeta deposition. In the current study, we have used our genomic-based beta-secretase transgenic mice to determine how BACE1 regulates the spatial and temporal pattern of Abeta production throughout post-natal development. Specifically, we observed unique differences in the brain-regional expression pattern between neonatal and adult BACE1 transgenic mice. These alterations in the BACE1 expression profile directly corresponds with age-related differences in regional Abeta production and deposition. These studies indicate that modulation of BACE1 expression leads to dramatic alterations in APP processing and AD-like neuropathology. Furthermore, our studies provide further evidence that BACE1 plays a major role in the regulation of the APP processing pathway, influencing the age-dependent onset of AD pathogenesis.

Cycloxygenase-2 activity promotes cognitive deficits but not increased amyloid burden in a model of Alzheimer’s disease in a sex-dimorphic pattern

Administration of non-steroidal anti-inflammatory agents reduces the risk of developing Alzheimer's disease in normal aging populations, an effect that may occur from inhibition of the cyclooxygenases, the rate-limiting enzymes in the formation of prostaglandins. In this study, we investigated whether increased activity of cyclooxygenase-2 (COX-2), the inducible isoform of cyclooxygenase, potentiates disease progression in a transgenic mouse model of Alzheimer's disease. To study the functional effects of COX-2 activity, male and female bigenic mice (amyloid precursor protein with Swedish mutation [APPswe]-presenilin-1 protein with deletion of exon 9 [PS1dE9] and trigenic COX-2/APPswe-PS1dE9) were behaviorally tested +/-administration of the selective COX-2 inhibitor celecoxib. Behavioral testing included a three-trial Y maze that measures spatial working and recognition memories and an open field task that tested levels of hyperactivity. Overexpression of COX-2 in APPswe-PS1dE9 mice resulted in specific deficits in spatial working memory in female but not male mice. These sex-specific deficits were abolished by pharmacological inhibition of COX-2 activity. Importantly, COX-2-associated deficits were dependent on co-expression of all three transgenes since COX-2 single transgenic and APPswe-PS1dE9 bigenic mice showed normal memory. Quantification of amyloid plaque load and total Abeta 40 and 42 peptides did not reveal significant differences in trigenic versus bigenic mice treated with either vehicle or celecoxib. Taken together, these data indicate an interaction between the effects of COX-2 and Abeta peptides on cognition that occurs in a sex-specific manner in the absence of significant changes in amyloid burden. These findings suggest that pathological activation of COX-2 may potentiate the toxicity of Abeta peptides, particularly in females, without significantly affecting Abeta accumulation.

Mapping cellular transcriptosomes in autopsied Alzheimer's disease subjects and relevant animal models

Alzheimer's disease (AD) is a late-onset and progressive neurodegenerative disorder characterized clinically by memory loss, impairment of other cognitive functions, and changes in behavior and personality. The overall aim of this review is to summarize recent advances in studies of AD progression and the use of animal models in gene expression studies of AD progression. Genetic causes of AD are known only for early-onset AD patients. For a majority of late-onset AD patients, causal factors are still unknown. Currently, there are no early detectable biomarkers for late-onset AD, and there is a lack of understanding of AD pathophysiology, particularly at the early stages of disease progression, before pathology develops. Human histopathological and biochemical studies provide valuable information regarding the last stages of AD pathogenesis. However, to understand early cellular changes in AD progression before symptoms develop, animal models are still our only alternative. Several research groups have created genetically engineered animal models, particularly models of the mouse, rat, fly, and worm, which have allowed us to better, understand the initiating events of AD progression. Recently, state-of-the-art methods have helped elucidate gene expression changes in affected and unaffected tissues from postmortem AD brains and from animal models developed for AD studies. These methods allow the investigation of mRNA-based transcriptosomal profiles of brain specimens from AD humans and transgenic animals. The major finding from these studies is that AD progression and pathogenesis involve multiple cellular pathways, which suggests that AD is a complex and heterogeneous disease.

Altered amyloid-beta metabolism and deposition in genomic-based beta-secretase transgenic mice

Amyloid-beta (Abeta) the primary component of the senile plaques found in Alzheimer's disease (AD) is generated by the rate-limiting cleavage of amyloid precursor protein (APP) by beta-secretase followed by gamma-secretase cleavage. Identification of the primary beta-secretase gene, BACE1, provides a unique opportunity to examine the role this unique aspartyl protease plays in altering Abeta metabolism and deposition that occurs in AD. The current experiments seek to examine how modulating beta-secretase expression and activity alters APP processing and Abeta metabolism in vivo. Genomic-based BACE1 transgenic mice were generated that overexpress human BACE1 mRNA and protein. The highest expressing BACE1 transgenic line was mated to transgenic mice containing human APP transgenes. Our biochemical and histochemical studies demonstrate that mice overexpressing both BACE1 and APP show specific alterations in APP processing and age-dependent Abeta deposition. We observed elevated levels of Abeta isoforms as well as significant increases of Abeta deposits in these double transgenic animals. In particular, the double transgenics exhibited a unique cortical deposition profile, which is consistent with a significant increase of BACE1 expression in the cortex relative to other brain regions. Elevated BACE1 expression coupled with increased deposition provides functional evidence for beta-secretase as a primary effector in regional amyloid deposition in the AD brain. Our studies demonstrate, for the first time, that modulation of BACE1 activity may play a significant role in AD pathogenesis in vivo.

Mouse models of Alzheimer's disease: the long and filamentous road

Alzheimer's disease (AD) is characterized by memory impairment leading to dementia, deposition of amyloid plaques and neurofibrillary tangles (NFTs), and neuronal loss. The major component of plaques is the amyloid beta peptide, A beta, whereas NFTs contain hyperphosphorylated forms of the microtubule-associated protein tau (tau). Familial AD (FAD) mutations either elevate A beta synthesis by favoring 'secretase' of the Alzheimer beta-amyloid precursor protein (APP) or enhance the fibrillogenic properties of this peptide. Mutations in the tau gene cause a different disease denoted FTPD-17, but suggest that the aberrant forms of tau seen in AD are unlikely to be benign. These findings imply a complex pathogenic cascade in AD and important goals of transgenic modeling are to capture and stratify this pathogenic process. Several laboratories have created APP transgenic (Tg) mice that exhibit AD-like amyloid pathology and A beta burdens. These Tg lines also exhibit deficits in spatial reference and/or working memory, with immunization against A beta attenuating both AD-associated phenotypes. Tangle-like pathologies are observed in mice expressing FTPD-17 mutant forms of tau, but florid tau pathologies based upon the wild type (wt) tau isoforms present in AD have proven more elusive. Creation of animal models with robust amyloid and tau pathologies, yet free of irrelevant confounding pathologies, remains a major objective in this field.

Alterations in beta-amyloid production and deposition in brain regions of two transgenic models

Mutations in the amyloid precursor protein (APP) gene are associated with altered production and deposition of amyloid beta (Abeta) peptide in the Alzheimer's disease (AD) brain. The pathways that regulate APP processing, Abeta production and Abeta deposition in different tissues and brain regions remain unclear. To address this, we examined levels of various APP processing products as well as Abeta deposition in a genomic-based (R1.40) and a cDNA-based (Tg2576) transgenic mouse model of AD. In tissues, only brain generated detectable levels of the penultimate precursor to Abeta, APP C-terminal fragment-beta. In brain regions, holoAPP levels remained constant, but ratios of APP C-terminal fragments and levels of Abeta differed significantly. Surprisingly, cortex had the lowest steady-state levels of Abeta compared to other brain regions. Comparison of Abeta deposition in Tg2576 and R1.40 animals revealed that R1.40 exhibited more abundant deposition in cortex while Tg2576 exhibited extensive deposition in the hippocampus. Our results suggest that AD transgenic models are not equal; their unique characteristics must be considered when studying AD pathogenesis and therapies.

Role of vascular hypoperfusion-induced oxidative stress and mitochondria failure in the pathogenesis of Azheimer disease

Chronic vascular hypoperfusion induces oxidative stress and brain energy failure, and leads to neuronal death, which manifests as cognitive impairment and the development of brain pathology as in Alzheimer disease (AD). It is becoming more widely accepted that AD is characterized by impairments in energy metabolism. We hypothesize that hypoperfusion-induced mitochondrial failure plays a central role in the generation of reactive oxygen species, resulting in oxidative damage to brain cellular compartments, especially in the vascular endothelium and neuronal cell bodies in AD. All of these changes have been found to occur before pathology and coexist during the progression of AD. In this review we have summarized recent evidence and our own knowledge regarding the relationship between the hypoperfusion-induced vascular damage that initiates oxidative stress and mitochondrial abnormalities that appear to be a key target for the development of AD pathology. Future investigations into both the mechanisms behind amyloid beta (Abeta) deposition and the possible accelerating effects of environmental factors, such as chronic hypoxia/reperfusion may open the door for effective pharmacological treatments of AD. We hypothesize that an imbalance between endothelium derived vasoconstrictors and vasodilators, along with an antioxidant system deficiency and mitochondria lesions are prominent in AD. Future studies examining the importance of mitochondrial pathophysiology in different brain cellular compartments may provide insight not only into neurodegenerative and/or cerebrovascular disease pathobiology but may also provide targets for treating these conditions.