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

Alzheimer's Disease and Age-Related Changes in the Cu Isotopic Composition of Blood Plasma and Brain Tissues of the APPNL-G-F Murine Model Revealed by Multi-Collector ICP-Mass Spectrometry

Alzheimer's' disease (AD) is characterized by the formation of β-amyloid (Aβ) plaques and neurofibrillary tangles of tau protein in the brain. Aβ plaques are formed by the cleavage of the β-amyloid precursor protein (APP). In addition to protein aggregations, the metabolism of the essential mineral element Cu is also altered during the pathogenesis of AD. The concentration and the natural isotopic composition of Cu were investigated in blood plasma and multiple brain regions (brain stem, cerebellum, cortex, and hippocampus) of young (3-4 weeks) and aged (27-30 weeks) APP^NL-G-F knock-in mice and wild-type controls to assess potential alterations associated with ageing and AD. Tandem inductively coupled plasma-mass spectrometry (ICP-MS/MS) was used for elemental analysis and multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS) for high-precision isotopic analysis. The blood plasma Cu concentration was significantly altered in response to both age- and AD-related effects, whereas the blood plasma Cu isotope ratio was only affected by the development of AD. Changes in the Cu isotopic signature of the cerebellum were significantly correlated with the changes observed in blood plasma. The brain stem showed a significant increase in Cu concentration for both young and aged AD transgenic mice compared with healthy controls, whereas the Cu isotopic signature became lighter as a result of age-related changes. In this work, ICP-MS/MS and MC-ICP-MS provided relevant and complementary information on the potential role of Cu in ageing and AD.

Alzheimer's Disease as a Membrane Disorder: Spatial Cross-Talk Among Beta-Amyloid Peptides, Nicotinic Acetylcholine Receptors and Lipid Rafts

Biological membranes show lateral and transverse asymmetric lipid distribution. Cholesterol (Chol) localizes in both hemilayers, but in the external one it is mostly condensed in lipid-ordered microdomains (raft domains), together with saturated phosphatidyl lipids and sphingolipids (including sphingomyelin and glycosphingolipids). Membrane asymmetries induce special membrane biophysical properties and behave as signals for several physiological and/or pathological processes. Alzheimer’s disease (AD) is associated with a perturbation in different membrane properties. Amyloid-β (Aβ) plaques and neurofibrillary tangles of tau protein together with neuroinflammation and neurodegeneration are the most characteristic cellular changes observed in this disease. The extracellular presence of Aβ peptides forming senile plaques, together with soluble oligomeric species of Aβ, are considered the major cause of the synaptic dysfunction of AD. The association between Aβ peptide and membrane lipids has been extensively studied. It has been postulated that Chol content and Chol distribution condition Aβ production and posterior accumulation in membranes and, hence, cell dysfunction. Several lines of evidence suggest that Aβ partitions in the cell membrane accumulate mostly in raft domains, the site where the cleavage of the precursor AβPP by β- and γ- secretase is also thought to occur. The main consequence of the pathogenesis of AD is the disruption of the cholinergic pathways in the cerebral cortex and in the basal forebrain. In parallel, the nicotinic acetylcholine receptor has been extensively linked to membrane properties. Since its transmembrane domain exhibits extensive contacts with the surrounding lipids, the acetylcholine receptor function is conditioned by its lipid microenvironment. The nicotinic acetylcholine receptor is present in high-density clusters in the cell membrane where it localizes mainly in lipid-ordered domains. Perturbations of sphingomyelin or cholesterol composition alter acetylcholine receptor location. Therefore, Aβ processing, Aβ partitioning, and acetylcholine receptor location and function can be manipulated by changes in membrane lipid biophysics. Understanding these mechanisms should provide insights into new therapeutic strategies for prevention and/or treatment of AD. Here, we discuss the implications of lipid-protein interactions at the cell membrane level in AD.

Improvements of symptoms of Alzheimer`s disease by inhibition of the angiotensin system

With ageing of the global society, the frequency of ageing-related neurodegenerative diseases such as Alzheimer`s disease (AD) is on the rise worldwide. Currently, there is no cure for AD, and the four drugs approved for AD only have very small effects on AD symptoms. Consequently, there are enormous efforts worldwide to identify new targets for treatment of AD. Approaches that interfere with classical neuropathologic features of AD, such as extracellular senile plaques formed of aggregated amyloid-beta (Abeta), and intracellular neurofibrillary tangles of hyperphosphorylated tau have not been successful so far. In search for a treatment approach of AD, we found that inhibition of the angiotensin-converting enzyme (ACE) by a centrally acting ACE inhibitor retards symptoms of neurodegeneration, Abeta plaque formation and tau hyperphosphorylation in experimental models of AD. Our approach is currently being investigated in a clinical setting. Initial evidence with AD patients shows that a brain-penetrating ACE inhibitor counteracts the process of neurodegeneration and dementia. Moreover, centrally acting ACE inhibitors given in addition to the standard therapy, cholinesterase inhibition, can improve cognitive function of AD patients for several months. This is one of the most promising results for AD treatment since more than a decade.

A novel vector for transgenesis in the rat CNS

The larger brain of the rat enables a much greater repertoire of complex behaviors than mice, likely making rats preferential for investigating neurodegeneration. Because molecular tools for specific expression of transgenes in the rat brain are sparse, we chose Prnp encoding the prion protein (PrP) to develop a novel vector to drive transgene expression in the rat brain. We compared the rat Prnp sequence with mouse and Syrian hamster Prnp sequences, identifying conserved genetic elements and hypothesizing that these elements would be able to drive neuronal transgene expression. We investigated this by generating a vector termed RaPrnp that encompasses portions of the rat Prnp gene. Importantly, we replaced the rat Prnp open reading frame (ORF) with a cloning site for rapid and seamless In-Fusion cloning. To validate the in vivo neuronal specificity of the RaPrnp vector in rats, we generated stable RaPrnp-LacZ/enhanced green fluorescent protein (EGFP) transgenic (Tg) rat lines, which led to robust LacZ activity and high EGFP fluorescence in the central nervous system of embryos and adult animals. Next, we restored the rat Prnp ORF and generated multiple Tg(RaPrnp-PrP) lines, demonstrating that overexpression of Prnp accelerates the onset of scrapie. While the incubation time in wild-type (WT) rats was 175 ± 3 days post inoculation (dpi), one line, Tg2919, overexpressed RaPrPC at 4.4-fold and exhibited a reduced incubation time of 149 ± 2 dpi. The second line, Tg2922, overexpressed RaPrPC at 9.7-fold compared with WT animals and had an incubation time of 112 ± 0 dpi. Tg2922 rats inoculated with rat RML showed extensive vacuolation of the brainstem in contrast to WT and Tg2919 animals in which vacuolation was most prominent in the hippocampus and striatum as well as the motor and sensory cortices. It is possible that construction of Tg rats with modified phenotypes will prove more advantageous than mice for neurodegeneration studies.

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 Mouse Lemur, a Genetic Model Organism for Primate Biology, Behavior, and Health

Systematic genetic studies of a handful of diverse organisms over the past 50 years have transformed our understanding of biology. However, many aspects of primate biology, behavior, and disease are absent or poorly modeled in any of the current genetic model organisms including mice. We surveyed the animal kingdom to find other animals with advantages similar to mice that might better exemplify primate biology, and identified mouse lemurs (Microcebus spp.) as the outstanding candidate. Mouse lemurs are prosimian primates, roughly half the genetic distance between mice and humans. They are the smallest, fastest developing, and among the most prolific and abundant primates in the world, distributed throughout the island of Madagascar, many in separate breeding populations due to habitat destruction. Their physiology, behavior, and phylogeny have been studied for decades in laboratory colonies in Europe and in field studies in Malagasy rainforests, and a high quality reference genome sequence has recently been completed. To initiate a classical genetic approach, we developed a deep phenotyping protocol and have screened hundreds of laboratory and wild mouse lemurs for interesting phenotypes and begun mapping the underlying mutations, in collaboration with leading mouse lemur biologists. We also seek to establish a mouse lemur gene "knockout" library by sequencing the genomes of thousands of mouse lemurs to identify null alleles in most genes from the large pool of natural genetic variants. As part of this effort, we have begun a citizen science project in which students across Madagascar explore the remarkable biology around their schools, including longitudinal studies of the local mouse lemurs. We hope this work spawns a new model organism and cultivates a deep genetic understanding of primate biology and health. We also hope it establishes a new and ethical method of genetics that bridges biological, behavioral, medical, and conservation disciplines, while providing an example of how hands-on science education can help transform developing countries.

Telencephalic neurocircuitry and synaptic plasticity in rodent spatial learning and memory

Spatial learning and memory in rodents represent close equivalents of human episodic declarative memory, which is especially sensitive to cerebral aging, neurodegeneration, and various neuropsychiatric disorders. Many tests and protocols are available for use in laboratory rodents, but Morris water maze and radial-arm maze remain the most widely used as well as the most valid and reliable spatial tests. Telencephalic neurocircuitry that plays functional roles in spatial learning and memory includes hippocampus, dorsal striatum and medial prefrontal cortex. Prefrontal-hippocampal circuitry comprises the major associative system in the rodent brain, and is critical for navigation in physical space, whereas interconnections between prefrontal cortex and dorsal striatum are probably more important for motivational or goal-directed aspects of spatial learning. Two major forms of synaptic plasticity, namely long-term potentiation, a lasting increase in synaptic strength between simultaneously activated neurons, and long-term depression, a decrease in synaptic strength, have been found to occur in hippocampus, dorsal striatum and medial prefrontal cortex. These and other phenomena of synaptic plasticity are probably crucial for the involvement of telencephalic neurocircuitry in spatial learning and memory. They also seem to play a role in the pathophysiology of two brain pathologies with episodic declarative memory impairments as core symptoms, namely Alzheimer's disease and schizophrenia. Further research emphasis on rodent telencephalic neurocircuitry could be relevant to more valid and reliable preclinical research on these most devastating brain disorders. This article is part of a Special Issue entitled SI: Brain and Memory.

Development of isotope labeling liquid chromatography mass spectrometry for mouse urine metabolomics: quantitative metabolomic study of transgenic mice related to Alzheimer's disease

Because of a limited volume of urine that can be collected from a mouse, it is very difficult to apply the common strategy of using multiple analytical techniques to analyze the metabolites to increase the metabolome coverage for mouse urine metabolomics. We report an enabling method based on differential isotope labeling liquid chromatography mass spectrometry (LC-MS) for relative quantification of over 950 putative metabolites using 20 μL of urine as the starting material. The workflow involves aliquoting 10 μL of an individual urine sample for ¹²C-dansylation labeling that target amines and phenols. Another 10 μL of aliquot was taken from each sample to generate a pooled sample that was subjected to ¹³C-dansylation labeling. The ¹²C-labeled individual sample was mixed with an equal volume of the ¹³C-labeled pooled sample. The mixture was then analyzed by LC-MS to generate information on metabolite concentration differences among different individual samples. The interday repeatability for the LC-MS runs was assessed, and the median relative standard deviation over 4 days was 5.0%. This workflow was then applied to a metabolomic biomarker discovery study using urine samples obtained from the TgCRND8 mouse model of early onset familial Alzheimer's disease (FAD) throughout the course of their pathological deposition of beta amyloid (Aβ). It was showed that there was a distinct metabolomic separation between the AD prone mice and the wild type (control) group. As early as 15-17 weeks of age (presymptomatic), metabolomic differences were observed between the two groups, and after the age of 25 weeks the metabolomic alterations became more pronounced. The metabolomic changes at different ages corroborated well with the phenotype changes in this transgenic mice model. Several useful candidate biomarkers including methionine, desaminotyrosine, taurine, N1-acetylspermidine, and 5-hydroxyindoleacetic acid were identified. Some of them were found in previous metabolomics studies in human cerebrospinal fluid or blood samples. This work illustrates the utility of this isotope labeling LC-MS method for biomarker discovery using mouse urine metabolomics.

Neuroinflammation in Alzheimer's disease: from pathogenesis to a therapeutic target

The top-down, reductionist approach of the past three decades has resulted in remarkable progress in identifying genes and proteins involved in Alzheimer's disease (AD), including β-amyloid (Aβ) peptides and tau protein. Recently, a number of genes of the innate immune pathway have been identified as AD risk factors and several microglial proteins have been shown to be chronically activated in AD brains. Together, these observations suggest a crucial role for neuroinflammation in AD pathogenesis and emerging evidence suggests that neuroinflammation is both a cause and a consequence of AD. Epidemiological studies show that long-term users of anti-inflammatory drugs are protected from AD but anti-inflammatory treatment in mild AD patients has not been successful. These observations suggest that anti-inflammatory treatment is likely to be successful if initiated prior to the onset of neurological symptoms. Finally, after the remarkable success of the reductionist approach, a complimentary bottom-up systems approach is necessary to gain a better understanding of the highly complex, multifactorial nature of AD pathogenesis.

Role of p73 in Alzheimer disease: lack of association in mouse models or in human cohorts

Background

P73 belongs to the p53 family of cell survival regulators with the corresponding locus Trp73 producing the N-terminally distinct isoforms, TAp73 and DeltaNp73. Recently, two studies have implicated the murine Trp73 in the modulation in phospho-tau accumulation in aged wild type mice and in young mice modeling Alzheimer’s disease (AD) suggesting that Trp73, particularly the DeltaNp73 isoform, links the accumulation of amyloid peptides to the creation of neurofibrillary tangles (NFTs). Here, we reevaluated tau pathologies in the same TgCRND8 mouse model as the previous studies.

Results

Despite the use of the same animal models, our in vivo studies failed to demonstrate biochemical or histological evidence for misprocessing of tau in young compound Trp73^+/- + TgCRND8 mice or in aged Trp73^+/- mice analyzed at the ages reported previously, or older. Secondly, we analyzed an additional mouse model where the DeltaNp73 was specifically deleted and confirmed a lack of impact of the DeltaNp73 allele, either in heterozygous or homozygous form, upon tau pathology in aged mice. Lastly, we also examined human TP73 for single nucleotide polymorphisms (SNPs) and/or copy number variants in a meta-analysis of 10 AD genome-wide association datasets. No SNPs reached significance after correction for multiple testing and no duplications/deletions in TP73 were found in 549 cases of AD and 544 non-demented controls.

Conclusion

Our results fail to support P73 as a contributor to AD pathogenesis.

Aβ inhibition of ionic conductance in mouse basal forebrain neurons is dependent upon the cellular prion protein PrPC

Current therapies for Alzheimer's disease (AD) address a loss of cholinergic neurons, while accumulation of neurotoxic amyloid β (Aβ) peptide assemblies is thought central to molecular pathogenesis. Overlaps may exist between prionopathies and AD wherein Aβ oligomers bind to the cellular prion protein PrP(C) and inhibit synaptic plasticity in the hippocampus (Laurén et al., 2009). Here we applied oligomeric Aβ to neurons with different PrP (Prnp) gene dosage. Whole-cell recordings were obtained from dissociated neurons of the diagonal band of Broca (DBB), a cholinergic basal forebrain nucleus. In wild-type (wt) mice, Aβ₁₋₄₂ evoked a concentration-dependent reduction of whole-cell outward currents in a voltage range between -30 and +30 mV; reduction occurred through a combined modulation of a suite of potassium conductances including the delayed rectifier (I(K)), the transient outward (I(A)), and the iberiotoxin-sensitive (calcium-activated potassium, I(C)) currents. Inhibition was not seen with Aβ₄₂₋₁ peptide, while Aβ₁₋₄₂-induced responses were reduced by application of anti-PrP antibody, attenuated in cells from Prnp⁰/⁺ hemizygotes, and absent in Prnp⁰/⁰ homozygotes. Similarly, amyloidogenic amylin peptide depressed DBB whole-cell currents in DBB cells from wt mice, but not Prnp⁰/⁰ homozygotes. While prior studies give broad support for a neuroprotective function for PrP(C), our data define a latent pro-pathogenic role in the presence of amyloid assemblies.

Age-related changes of neuron numbers in the frontal cortex of a transgenic mouse model of Alzheimer's disease

Alzheimer’s disease (AD) is a neurodegenerative disorder, characterized by amyloid plaque accumulation, intracellular tangles and neuronal loss in selective brain regions. The frontal cortex, important for executive functioning, is one of the regions that are affected. Here, we investigated the neurodegenerative effects of mutant human amyloid precursor protein (APP) and presenilin 1 (PS1) on frontal cortex neurons in APP/PS1KI mice, a transgenic mouse model of AD, expressing two mutations in the human APP, as well as two human PS1 mutations knocked-in into the mouse PS1 gene in a homozygous (ho) manner. Although the hippocampus is significantly affected in these mice, very little is known about the effects of these mutations on selective neuronal populations and plaque load in the frontal cortex. In this study, cytoarchitectural changes were characterized using high precision design-based stereology to evaluate plaque load, total neuron numbers, as well as total numbers of parvalbumin- (PV) and calretinin- (CR) immunoreactive (ir) neurons in the frontal cortex of 2- and 10-month-old APP/PS1KI mice. The frontal cortex was divided into two subfields: layers II–IV and layers V–VI, the latter of which showed substantially more extracellular amyloid-beta aggregates. We found a 34% neuron loss in layers V–VI in the frontal cortex of 10-month-old APP/PS1KI mice compared to 2-month-old, while there was no change in PV- and CR-ir neurons in these mice. In addition, the plaque load in layers V–VI of 10-month-old APP/PS1KI mice was only 11% and did not fully account for the extent of neuronal loss. Interestingly, an increase was found in the total number of PV-ir neurons in all frontal cortical layers of single transgenic APP mice and in layers II–IV of single transgenic PS1ho mice between 2 and 10 months of age. In conclusion, the APP/PS1KI mice provide novel insights into the regional selective vulnerability in the frontal cortex during AD that, together with previous findings in the hippocampus, are remarkably similar to the human situation.

Investigating regeneration and functional integration of CNS neurons: lessons from zebrafish genetics and other fish species

Zebrafish possess a robust, innate CNS regenerative ability. Combined with their genetic tractability and vertebrate CNS architecture, this ability makes zebrafish an attractive model to gain requisite knowledge for clinical CNS regeneration. In treatment of neurological disorders, one can envisage replacing lost neurons through stem cell therapy or through activation of latent stem cells in the CNS. Here we review the evidence that radial glia are a major source of CNS stem cells in zebrafish and thus activation of radial glia is an attractive therapeutic target. We discuss the regenerative potential and the molecular mechanisms thereof, in the zebrafish spinal cord, retina, optic nerve and higher brain centres. We evaluate various cell ablation paradigms developed to induce regeneration, with particular emphasis on the need for (high throughput) indicators that neuronal regeneration has restored sensory or motor function. We also examine the potential confound that regeneration imposes as the community develops zebrafish models of neurodegeneration. We conclude that zebrafish combine several characters that make them a potent resource for testing hypotheses and discovering therapeutic targets in functional CNS regeneration. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

The functional neurophysiology of the amyloid precursor protein (APP) processing pathway

Amyloid beta (Abeta) peptides derived from proteolytic cleavage of amyloid precursor protein (APP) are thought to be a pivotal toxic species in the pathogenesis of Alzheimer's disease (AD). Furthermore, evidence has been accumulating that components of APP processing pathway are involved in non-pathological normal function of the CNS. In this review we aim to cover the extensive body of research aimed at understanding how components of this pathway contribute to neurophysiological function of the CNS in health and disease. We briefly outline changes to clinical neurophysiology seen in AD patients before discussing functional changes in mouse models of AD which range from changes to basal synaptic transmission and synaptic plasticity through to abnormal synchronous network activity. We then describe the various neurophysiological actions that are produced by application of exogenous Abeta in various forms, and finally discuss a number or other neurophysiological aspects of the APP pathway, including functional activities of components of secretase complexes other than Abeta production.

Examination of potential mechanisms of amyloid-induced defects in neuronal transport

Microtubule-based neuronal transport pathways are impaired during the progression of Alzheimer's disease and other neurodegenerative conditions. However, mechanisms leading to defects in transport remain to be determined. We quantified morphological changes in neuronal cells following treatment with fibrils and unaggregated peptides of beta-amyloid (Abeta). Abeta fibrils induce axonal and dendritic swellings indicative of impaired transport. In contrast, Abeta peptides induce a necrotic phenotype in both neurons and non-neuronal cells. We tested several popular hypotheses by which aggregated Abeta could disrupt transport. Using fluorescent polystyrene beads, we developed experimental models of physical blockage and localized release of reactive oxygen species (ROS) that reliably induce swellings. Like the beads, Abeta fibrils localize in close proximity to swellings; however, fibril internalization is not required for disrupting transport. ROS and membrane permeability are also unlikely to be responsible for fibril-mediated toxicity. Collectively, our results indicate that multiple initiating factors converge upon pathways of defective transport.

Reassessing the amyloid cascade hypothesis of Alzheimer's disease

Since its inception, the amyloid cascade hypothesis has dominated the field of Alzheimer's disease (AD) research and has provided the intellectual framework for therapeutic intervention. Although the details of the hypothesis continue to evolve, its core principle has remained essentially unaltered. It posits that the amyloid-beta peptides, derived from amyloid precursor protein (APP), are the root cause of AD. Substantial genetic and biochemical data support this view, and yet a number of findings also run contrary to its tenets. The presence of familial AD mutations in APP and presenilins, demonstration of Abeta toxicity, and studies in mouse models of AD all support the hypothesis, whereas the presence of Abeta plaques in normal individuals, the uncertain nature of the pathogenic Abeta species, and repeated disappointments with Abeta-centered therapeutic trials are inconsistent with the hypothesis. The current state of knowledge does not prove nor disprove the amyloid hypothesis, but rather points to the need for its reassessment. A view that Abeta is one of the factors, as opposed to the factor, that causes AD is more consistent with the present knowledge, and is more likely to promote comprehensive and effective therapeutic strategies.

The effects of NOS2 gene deletion on mice expressing mutated human AbetaPP

Nitric oxide synthase 2 (NOS2) and its gene product, inducible NOS (iNOS) play an important role in neuroinflammation by generating nitric oxide (NO), a critical signaling and redox factor in the brain. Although NO is associated with tissue damage, it can also promote cell survival. We hypothesize that during long-term exposure to amyloid-beta (Abeta) in Alzheimer's disease (AD), NO levels fall in the brain to a threshold at which the protective effects of NO cannot be sustained, promoting Abeta mediated damage. Two new mouse models of AD have been developed that utilize this concept of NO's action. These mice express human amyloid-beta protein precursor (AbetaPP) mutations that generate Abeta peptides on a mouse NOS2 knockout background. The APP/NOS2(-/-) bigenic mice progress from Abeta production and amyloid deposition to hyperphosphorylated normal mouse tau at AD-associated epitopes, aggregation and redistribution of tau to somatodendritic regions of neurons and significant neuronal loss including loss of interneurons. This AD-like pathology is accompanied by robust behavioral changes. As APP/NOS2(-/-) bigenic mice more fully model the human AD disease pathology, they may serve as a tool to better understand disease progression in AD and the role of NO in altering chronic neurological disease processes.

Dense-core and diffuse Abeta plaques in TgCRND8 mice studied with synchrotron FTIR microspectroscopy

Plaques composed of the Abeta peptide are the main pathological feature of Alzheimer's disease. Dense-core plaques are fibrillar deposits of Abeta, showing all the classical properties of amyloid including beta-sheet secondary structure, while diffuse plaques are amorphous deposits. We studied both plaque types, using synchrotron infrared (IR) microspectroscopy, a technique that allows the chemical composition and average protein secondary structure to be investigated in situ. We examined plaques in hippocampal, cortical and caudal tissue from 5- to 21-month-old TgCRND8 mice, a transgenic model expressing doubly mutant amyloid precursor protein, and displaying impaired hippocampal function and robust pathology from an early age. Spectral analysis confirmed that the congophilic plaque cores were composed of protein in a beta-sheet conformation. The amide I maximum of plaque cores was at 1623 cm(-1), and unlike for in vitro Abeta fibrils, the high-frequency (1680-1690 cm(-1)) component attributed to antiparallel beta-sheet was not observed. A significant elevation in phospholipids was found around dense-core plaques in TgCRND8 mice ranging in age from 5 to 21 months. In contrast, diffuse plaques were not associated with IR detectable changes in protein secondary structure or relative concentrations of any other tissue components.

Proteinase-activated receptor-2 exerts protective and pathogenic cell type-specific effects in Alzheimer's disease

The proteinase-activated receptors (PARs) are a novel family of G protein-coupled receptors, and their effects in neurodegenerative diseases remain uncertain. Alzheimer's disease (AD) is a neurodegenerative disorder defined by misfolded protein accumulation with concurrent neuroinflammation and neuronal death. We report suppression of proteinase-activated receptor-2 (PAR2) expression in neurons of brains from AD patients, whereas PAR2 expression was increased in proximate glial cells, together with up-regulation of proinflammatory cytokines and chemokines and reduced IL-4 expression (p < 0.05). Glial PAR2 activation increased expression of formyl peptide receptor-2 (p < 0.01), a cognate receptor for a fibrillar 42-aa form of beta-amyloid (Abeta(1-42)), enhanced microglia-mediated proinflammatory responses, and suppressed astrocytic IL-4 expression, resulting in neuronal death (p < 0.05). Conversely, neuronal PAR2 activation protected human neurons against the toxic effects of Abeta(1-42) (p < 0.05), a key component of AD neuropathogenesis. Amyloid precursor protein-transgenic mice, displayed glial fibrillary acidic protein and IL-4 induction (p < 0.05) in the absence of proinflammatory gene up-regulation and neuronal injury, whereas PAR2 was up-regulated at this early stage of disease progression. PAR2-deficient mice, after hippocampal Abeta(1-42) implantation, exhibited enhanced IL-4 induction and less neuroinflammation (p < 0.05), together with improved neurobehavioral outcomes (p < 0.05). Thus, PAR2 exerted protective properties in neurons, but its activation in glia was pathogenic with secretion of neurotoxic factors and suppression of astrocytic anti-inflammatory mechanisms contributing to Abeta(1-42)-mediated neurodegeneration.

Altered levels and distribution of IGF-II/M6P receptor and lysosomal enzymes in mutant APP and APP + PS1 transgenic mouse brains

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor participates in the trafficking of lysosomal enzymes from the trans-Golgi network or the cell surface to lysosomes. In Alzheimer's disease (AD) brains, marked up-regulation of the lysosomal system in vulnerable neuronal populations has been correlated with altered metabolic functions. To establish whether IGF-II/M6P receptors and lysosomal enzymes are altered in the brain of transgenic mice harboring different familial AD mutations, we measured the levels and distribution of the receptor and lysosomal enzymes cathepsins B and D in select brain regions of transgenic mice overexpressing either mutant presenilin 1 (PS1; PS1(M146L+L286V)), amyloid precursor protein (APP; APP(KM670/671NL+V717F)) or APP+PS1 (APP(KM670/671NL+V717F)+PS1(M146L+L286V)) transgenes. Our results revealed that levels and expression of the IGF-II/M6P receptor and lysosomal enzymes are increased in the hippocampus and frontal cortex of APP and APP+PS1, but not in PS1, transgenic mouse brains compared with wild-type controls. The changes were more prominent in APP+PS1 than in APP single transgenic mice. Additionally, all beta-amyloid-containing neuritic plaques in the hippocampal and cortical regions of APP and APP+PS1 transgenic mice were immunopositive for both lysosomal enzymes, whereas only a subset of the plaques displayed IGF-II/M6P receptor immunoreactivity. These results suggest that up-regulation of the IGF-II/M6P receptor and lysosomal enzymes in neurons located in vulnerable regions reflects an altered functioning of the endosomal-lysosomal system which may be associated with the increased intracellular and/or extracellular A beta deposits observed in APP and APP+PS1 transgenic mouse brains.

Modeling familial British dementia in transgenic mice

The chromosome 13 linked amyloidopathies familial British dementia (FBD) and familial Danish dementia (FDD) are caused by mutations in the C-terminus of the BRI2 gene. In both diseases, novel peptides are deposited in amyloid plaques in the brain. Several laboratories have attempted to model these diseases in BRI2 transgenic mice with limited success. While high expression levels of BRI protein were achieved in transgenic lines, no ABri-amyloidosis was observed in aged mice. This review discusses the strategies chosen and problems experienced with the development of FBD/FDD models and suggests novel approaches to model the diseases in murine models.

Mendelian disorders deserve more attention

The study of inherited monogenic diseases has contributed greatly to our mechanistic understanding of pathogenic mutations and gene regulation, and to the development of effective diagnostic tools. But interest has gradually shifted away from monogenic diseases, which collectively affect only a small fraction of the world's population, towards multifactorial, common diseases. The quest for the genetic variability associated with common traits should not be done at the expense of Mendelian disorders, because the latter could still contribute greatly to understanding the aetiology of complex traits.

Endomorphin‐2, an endogenous tetrapeptide, protects against Aβ1‐42 in vitro and in vivo

The underlying cause of Alzheimer's disease (AD) is thought to be the beta-amyloid aggregates formed mainly by Abeta1-42 peptide. Protective pentapeptides [e.g., Leu-Pro-Phe-Phe-Asp (LPFFD)] have been shown to prevent neuronal toxicity of Abeta1-42 by arresting and reversing fibril formation. Here we report that an endogenous tetrapeptide, endomorphin-2 (End-2, amino acid sequence: YPFF), defends against Abeta1-42 induced neuromodulatory effects at the cellular level. Although End-2 does not interfere with the kinetics of Abeta fibrillogenesis according to transmission electron microscopic studies and quasielastic light scattering measurements, it binds to Abeta1-42 during aggregation, as revealed by tritium-labeled End-2 binding assay and circular dichroism measurements. The tetrapeptide attenuates the inhibitory effect on cellular redox activity of Abeta1-42 in a dose-dependent manner, as measured by 3-(4,5-dimethylthiazolyl-2)-2,-5-diphenyltetrazolium bromide (MTT) assay. In vitro and in vivo electrophysiological experiments show that End-2 also protects against the field excitatory postsynaptic potential attenuating and the NMDA-evoked response-enhancing effect of Abeta1-42. Studies using [D-Ala (2), N-Me-Phe (4), Gly (5)-ol]-enkephalin (DAMGO), a mu-opioid receptor agonist, show that the protective effects of the tetrapeptide are not mu-receptor modulated. The endogenous tetrapeptide End-2 may serve as a lead compound for the drug development in the treatment of AD.

Microcebus murinus: a useful primate model for human cerebral aging and Alzheimer's disease?

Age-associated dementia, in particular Alzheimer's disease (AD), will be a major concern of the 21st century. Research into normal brain aging and AD will therefore become increasingly important. As for other areas of medicine, the availability of good animal models will be a limiting factor for progress. Given the complexity of the human brain, the identification of appropriate primate models will be essential to further knowledge of the disease. In this review, we describe the features of brain aging and age-associated neurodegeneration in a small lemurian primate, the Microcebus murinus, also referred to as the mouse lemur. The mouse lemur has a relatively short life expectancy, and animals over 5 years of age are considered to be elderly. Among elderly mouse lemurs, the majority show normal brain aging, whereas approximately 20% develop neurodegeneration. This Microcebus age-associated neurodegeneration is characterized by a massive brain atrophy, abundant amyloid plaques, a cytoskeletal Tau pathology and a loss of cholinergic neurons. While elderly mouse lemurs with normal brain aging maintain memory function and social interaction, animals with age-associated neurodegeneration lose their cognitive and social capacities and demonstrate certain similarities with age-associated human AD. We conclude that M. murinus is an interesting primate model for the study of normal brain aging and the biochemical dysfunctions occurring in age-associated neurodegeneration. Mouse lemurs might also become an increasingly important model for the development of novel treatments in this domain.

Mechanisms of memory loss in Abeta and tau mouse models

Although memory loss is the central symptom of Alzheimer's disease, the pathophysiological mechanisms leading to dementia are poorly understood. It is difficult to answer this issue with studies in humans and impossible in cultured cells. Therefore animal models are needed to elucidate the molecular mechanisms leading to dementia. The chief neuropathological changes during Alzheimer's disease, namely neurofibrillary tangles and amyloid plaques, have helped us to determine which molecules to focus upon in the animal models, specifically Abeta (amyloid beta) and tau. This paper presents my perspective on what we have learnt about mechanisms of memory loss from Abeta and tau mouse models of Alzheimer's disease.

Nature, nurture and neurology: gene-environment interactions in neurodegenerative disease. FEBS Anniversary Prize Lecture delivered on 27 June 2004 at the 29th FEBS Congress in Warsaw

Neurodegenerative disorders, such as Huntington's, Alzheimer's, and Parkinson's diseases, affect millions of people worldwide and currently there are few effective treatments and no cures for these diseases. Transgenic mice expressing human transgenes for huntingtin, amyloid precursor protein, and other genes associated with familial forms of neurodegenerative disease in humans provide remarkable tools for studying neurodegeneration because they mimic many of the pathological and behavioural features of the human conditions. One of the recurring themes revealed by these various transgenic models is that different diseases may share similar molecular and cellular mechanisms of pathogenesis. Cellular mechanisms known to be disrupted at early stages in multiple neurodegenerative disorders include gene expression, protein interactions (manifesting as pathological protein aggregation and disrupted signaling), synaptic function and plasticity. Recent work in mouse models of Huntington's disease has shown that enriching the environment of transgenic animals delays the onset and slows the progression of Huntington's disease-associated motor and cognitive symptoms. Environmental enrichment is known to induce various molecular and cellular changes in specific brain regions of wild-type animals, including altered gene expression profiles, enhanced neurogenesis and synaptic plasticity. The promising effects of environmental stimulation, demonstrated recently in models of neurodegenerative disease, suggest that therapy based on the principles of environmental enrichment might benefit disease sufferers and provide insight into possible mechanisms of neurodegeneration and subsequent identification of novel therapeutic targets. Here, we review the studies of environmental enrichment relevant to some major neurodegenerative diseases and discuss their research and clinical implications.

Neurodegenerative memory disorders: a potential role of environmental toxins

The hypothesis that neurotoxins may play a role in neurodegenerative disorders remains an elusive one, given that epidemiologic studies often provide conflicting results. Although these conflicting results may result from methodological differences within and between studies, the complexity of chemical disruption of the central nervous system cannot be ignored in attempts to evaluate this hypothesis in different neurodegenerative disorders. Spencer provides a detailed review of the complex processes involved in defining the neurotoxic potential of naturally occurring and synthetic agents. Even concepts such as exposure and dose, as often reported in studies attempting to evaluate the risk imparted by a potential compound, can be deceptive. For example, although dose reflects "that amount of chemical transferred to the exposed subject", factors such as time and concentration in the organism, the ability to access the central nervous system, and how a compound reaches the central nervous system (routes of administration) or secondarily affects other organ systems leading to central nervous system disruption are clearly important to the concept of neurotoxic risk in neurodegenerative disorders. These factors would appear to explain the observed disagreements between studies using animal or neuronal models of neurotoxicity and population-based studies in humans. The importance of these factors and how a potential neurotoxin is investigated are clearly seen in the data on AD and aluminum. In contrast, the impact of MTPT on the central nervous system is more direct and compelling. Added complexity in the study of neurotoxins in human neurodegeneration is derived from data showing that agents may have additive, potentiating, synergistic, or antagonistic effects. Therefore, data from studies evaluating EMF risks could be readily confounded by the presence or absence of heavy metals (eg, arc welding). Other factors that may conceal neurotoxic causes for a given disorder focus on additional features such as genetic predispositions, physiologic changes that occur in aging, and even nutritional status that can support or hinder the affect of a given agent on the central nervous system. Finally, many studies that investigate exposure risk do not readily incorporate the five criteria proposed by Schaumburg for establishing causation. For example, if we apply Schaumburg's first criterion, epidemiologic studies often determines the presence of an agent through history, yet they cannot readily confirm exposure based on environmental or clinical chemical analyses to fulfill this criterion for causation. Additional limitations in research design along with the populations and methods that are sued to study neurotoxins in human neurodegenerative disorders often fail to meet other criteria such as linking the severity and onset with duration and exposure level. Therefore, although studies of agents such as MTPT provide compelling models of neurotoxins and neurodegeneration in humans, disorders such as ALS, PD, and particularly AD will require additional effort if research is to determine the contribution (presence or absence) of neurotoxins to these neurologic disorders.

Phospholipid transfer protein (PLTP) deficiency reduces brain vitamin E content and increases anxiety in mice

Vitamin E supplementation constitutes a promising strategy in the prevention of neurodegenerative diseases. Here, we show that a phospholipid transfer protein (PLTP) is widely expressed in the brain where it appears to function as a transfer factor for alpha-tocopherol, the main isomer of vitamin E. PLTP deficiency results in significant depletion of brain alpha-tocopherol in both homozygous (-30.1%, P<0.0002) and heterozygous (-18.0%, P<0.05) PLTP knocked-out mice. Alpha-tocopherol depletion in PLTP-deficient homozygotes is associated with the elevation of lipofuscin (+25% and +450% increases in cortex and substantia nigra, respectively), cholesterol oxides (+54.5%, P<0.05), and cellular peroxides (+32.3%, P<0.01) in the brain. Complete PLTP deficiency in homozygotes is accompanied by increased anxiety as shown by fewer entries (8.3% vs. 44.4% in controls, P<0.01) and less time spent (1.7% vs. 41.3% in controls, P<0.05) in the open arms of an elevated plus-maze, in the absence of locomotor deterioration. Thus, the vitamin E transfer activity of PLTP appears to be a key process in preventing oxidative damage in the brain, and PLTP-deficient mice could be a new model of the contribution of oxidative brain injury in the etiology of neurodegenerative diseases.

Creation of non-human primate neurogenetic disease models by gene targeting and nuclear transfer

Genetically modified rhesus macaques are necessary because mouse models are not suitable for a number of important neurogenetic disorders; for example, Kallmann's syndrome, Lesch-Nyhan's disease and Ataxia-Telangiectasia. Mouse models may not be suitable because there may be no mouse ortholog of the human gene of interest, as is the case for Kallmann's syndrome, or because mutant mice do not exhibit the same phenotype observed in humans, as is the the case for Lesch-Nyhan's disease and Ataxia-Telangiectasia. Non-human primate models of neurogenetic diseases are expected to more closely resemble human diseases than existing mouse models. Genetically modified rhesus macaques can be created by modifying the genome of a somatic cell and then transferring the nucleus from this cell to an enucleated oocyte. Random integration of a transgene is sufficient to create models of gain-of-function genetic diseases. Stable expression of green fluorescent protein has been achieved in rhesus macaque fibroblasts. However, gene targeting is necessary to create models of loss-of-function genetic diseases. Several technical challenges must be overcome before null mutant non-human primates can be produced. In our experience, fetal fibroblasts frequently become senescent before selection procedures can be completed. We have overcome this problem by transfecting somatic cells with human telomerase reverse transcriptase. This enzyme extends the telomeres, and lifespan, of somatic cells. Long and accurate polymerase chain reaction can be used to obtain sufficient regions of homology of isogenic rhesus genomic DNA for targeting constructs. This should improve gene targeting efficiency. Gene targeting experiments are currently underway. Null mutant rhesus macaques will likely result in breakthrough advances in the understanding of neurogenetic disease and prove invaluable for preclinical trials of new therapies.