Selective vulnerability of dentate granule cells prior to amyloid deposition in PDAPP mice: digital morphometric analyses

Procognitive and neurotrophic benefits of α5-GABA-A receptor positive allosteric modulation in a β-amyloid deposition mouse model of Alzheimer's disease pathology

Reduced somatostatin (SST) and SST-expressing GABAergic neurons are well-replicated findings in Alzheimer's disease (AD) and are associated with cognitive deficits. SST cells inhibit pyramidal cell dendrites through α5-GABA-A receptors (α5-GABAA-R). α5-GABAAR positive allosteric modulation (α5-PAM) has procognitive and neurotrophic effects in stress and aging models. We tested whether α5-PAM (GL-II-73) could prevent cognitive deficits and neuronal spine loss in early stages, and reverse them in late stages of β-amyloid deposition in the 5xFAD model (N = 48/study; 50 % female). Acute administration of GL-II-73 prevented spatial working memory deficits in 5xFAD mice at 2 months of age, while chronic administration reversed the deficits at 5 months of age. Chronic GL-II-73 treatment prevented 5xFAD-induced loss of spine density, spine count and dendritic length at both time points, despite β-amyloid accumulation. These results demonstrate procognitive and neurotrophic effects of GL-II-73 in early and late stages of Alzheimer-related β-amyloid deposition. This suggests α5-PAM as a novel β-amyloid-independent symptomatic therapeutic approach.

Gulf War toxicant-induced reductions in dendritic arbors and spine densities of dentate granule cells are improved by treatment with a Nrf2 activator

Gulf War Illness (GWI) is a chronic multi-symptom disorder affecting approximately 30 % of Veterans deployed to the Persian Gulf from 1990 to 91. GWI encompasses a wide spectrum of symptoms which frequently include neurological problems such as learning and memory impairments, mood disorders, and an increased incidence of neurodegenerative disorders. Combined exposure to both reversible and irreversible acetylcholinesterase (AChE) inhibitors has been identified as a likely risk factor for GWI. It is possible that the exposures affected connectivity in the brain, and it was also unknown whether this could benefit from treatment. We assessed chronic changes in dendritic architecture in granule cells of the dentate gyrus following exposure to pyridostigmine bromide (PB, 0.7 mg/kg), chlorpyrifos (CPF, 12.5 mg/kg), and N,N-diethyl-m-toluamide (DEET, 7.5 mg/kg) in male C57Bl/6J mice. We also evaluated the therapeutic effects of dietary administration for eight weeks of 1 % tert-butylhydroquinone (tBHQ), a Nrf2 activator, on long-term neuronal morphology. We found that Gulf War toxicant exposure resulted in reduced dendritic length and branching as well as overall spine density in dentate granule cells at 14 weeks post-exposure and that these effects were ameliorated by treatment with tBHQ. These findings indicate that Gulf War toxicant exposure results in chronic changes to dentate granule cell morphology and that modulation of neuroprotective transcription factors such as Nrf2 may improve long-term neuronal health in the hippocampus.

Symptomatic and neurotrophic effects of GABAA receptor positive allosteric modulation in a mouse model of chronic stress

Chronic stress is a risk factor for Major Depressive Disorder (MDD), and in rodents, it recapitulates human behavioral, cellular and molecular changes. In MDD and after chronic stress, neuronal dysfunctions and deficits in GABAergic signaling are observed and responsible for symptom severity. GABA signals predominantly through GABAA receptors (GABAA-R) composed of various subunit types that relate to downstream outcomes. Activity at α2-GABAA-Rs contributes to anxiolytic properties, α5-GABAA-Rs to cognitive functions, and α1-GABAA-Rs to sedation. Therefore, a therapy aiming at increasing α2- and α5-GABAA-Rs activity, but devoid of α1-GABAA-R activity, has potential to address several symptomologies of depression while avoiding side-effects. This study investigated the activity profiles and behavioral efficacy of two enantiomers of each other (GL-II-73 and GL-I-54), separately and as a racemic mixture (GL-RM), and potential disease-modifying effects on neuronal morphology. Results confirm GL-I-54 and GL-II-73 exert positive allosteric modulation at the α2-, α3-, α5-GABAA-Rs and α5-containing GABAA-Rs, respectively, and separately reduces immobility in the forced swim test and improves stress-induced spatial working memory deficits. Using unpredictable chronic mild stress (UCMS), we show that acute and chronic administration of GL-RM provide pro-cognitive effects, with mild efficacy on mood symptoms, although at lower doses avoiding sedation. Morphology studies showed reversal of spine density loss caused by UCMS after chronic GL-RM treatment at apical and basal dendrites of the PFC and CA1. Together, these results support using a racemic mixture with combined α2-, α3-, α5-GABAA-R profile to reverse chronic stress-induced mood symptoms, cognitive deficits, and with anti-stress neurotrophic effects.

Magnetic Resonance Imaging in Animal Models of Alzheimer's Disease Amyloidosis

Amyloid-beta (Aβ) plays an important role in the pathogenesis of Alzheimer's disease. Aberrant Aβ accumulation induces neuroinflammation, cerebrovascular alterations, and synaptic deficits, leading to cognitive impairment. Animal models recapitulating the Aβ pathology, such as transgenic, knock-in mouse and rat models, have facilitated the understanding of disease mechanisms and the development of therapeutics targeting Aβ. There is a rapid advance in high-field MRI in small animals. Versatile high-field magnetic resonance imaging (MRI) sequences, such as diffusion tensor imaging, arterial spin labeling, resting-state functional MRI, anatomical MRI, and MR spectroscopy, as well as contrast agents, have been developed for preclinical imaging in animal models. These tools have enabled high-resolution in vivo structural, functional, and molecular readouts with a whole-brain field of view. MRI has been used to visualize non-invasively the Aβ deposits, synaptic deficits, regional brain atrophy, impairment in white matter integrity, functional connectivity, and cerebrovascular and glymphatic system in animal models of Alzheimer's disease amyloidosis. Many of the readouts are translational toward clinical MRI applications in patients with Alzheimer's disease. In this review, we summarize the recent advances in MRI for visualizing the pathophysiology in amyloidosis animal models. We discuss the outstanding challenges in brain imaging using MRI in small animals and propose future outlook in visualizing Aβ-related alterations in the brains of animal models.

Neuroinflammation in Alzheimer's disease and beneficial action of luteolin

Alzheimer's disease (AD), already the world's most common form of dementia, is projected to continue increasing in prevalence over the next several decades. The current lack of understanding of the pathogenesis of AD has hampered the development of effective treatments. Historically, AD research has been predicated on the amyloid cascade hypothesis (ACH), which attributes disease progression to the build-up of amyloid protein. However, multiple clinical studies of drugs interfering with ACH have failed to show any benefit demonstrating that AD etiology is more complex than previously thought. Here we review the current literature on the emerging key role of neuroinflammation, especially activation of microglia, in AD pathogenesis. Moreover, we provide compelling evidence that certain flavonoids, especially luteolin formulated in olive pomace oil together with hydroxytyrosol, offers a reasonable prophylactic treatment approach due to its many beneficial actions.

Reversal of Age-Related Neuronal Atrophy by α5-GABAA Receptor Positive Allosteric Modulation

Aging is associated with reduced brain volume, altered neural activity, and neuronal atrophy in cortical-like structures, comprising the frontal cortex and hippocampus, together contributing to cognitive impairments. Therapeutic efforts aimed at reversing these deficits have focused on excitatory or neurotrophic mechanisms, although recent findings show that reduced dendritic inhibition mediated by α5-subunit containing GABA-A receptors (α5-GABAA-Rs) occurs during aging and contributes to cognitive impairment. Here, we aimed to confirm the beneficial effect on working memory of augmenting α5-GABAA-R activity in old mice and tested its potential at reversing age-related neuronal atrophy. We show that GL-II-73, a novel ligand with positive allosteric modulatory activity at α5-GABAA-R (α5-PAM), increases dendritic branching complexity and spine numbers of cortical neurons in vitro. Using old mice, we confirm that α5-PAM reverses age-related working memory deficits and show that chronic treatment (3 months) significantly reverses age-related dendritic shrinkage and spine loss in frontal cortex and hippocampus. A subsequent 1-week treatment cessation (separate cohort) resulted in loss of efficacy on working memory but maintained morphological neurotrophic effects. Together, the results demonstrate the beneficial effect on working memory and neurotrophic efficacy of augmenting α5-GABAA-R function in old mice, suggesting symptomatic and disease-modifying potential in age-related brain disorders.

Dendritic arbor complexity and spine density changes after repetitive mild traumatic brain injury and neuroprotective treatments

Traumatic brain injury has been described as the signature affliction of recent military conflicts and repetitive TBIs, particularly associated with military and athletic activities, typically result in more severe clinical effects. The majority of TBIs are mild, but they can result in long term cognitive deficits for which there is no effective treatment. One of the most significant deficits observed in TBI patients is memory loss, which suggests that TBI can induce pathological changes within the hippocampus. tert-butylhydroquinone (tBHQ) and pioglitazone activate the Nrf2 and PPAR-γ transcription factors, respectively, and both have been shown to be neuroprotective in model systems. We examined the morphological changes within the hippocampus following repetitive mild TBI and simultaneous treatment with both factors. We utilized a closed head injury mouse model with five injuries over 5 weeks. Our results showed marked morphological changes among the dendrites and dendritic spines of the neurons of the dentate gyrus of the hippocampus. We observed decreases in overall dendritic length, as well as in the quantity and density of dendritic spines. Our treatment partially ameliorated these effects, suggesting that the Nrf2 and PPAR-γ transcription factors may be important targets for future drug development in the treatment of TBI in humans.

Recalibrating the Relevance of Adult Neurogenesis

Conflicting reports about whether adult hippocampal neurogenesis occurs in humans raise questions about its significance for human health and the relevance of animal models. Drawing upon published data, I review species' neurogenesis rates across the lifespan and propose that accelerated neurodevelopmental timing is consistent with lower rates of neurogenesis in adult primates and humans. Nonetheless, protracted neurogenesis may produce populations of neurons that retain plastic properties for long intervals, and have distinct functions depending on when in the lifespan they were born. With some conceptual recalibration we may therefore be able to reconcile seemingly disparate findings and continue to ask how adult neurogenesis, as studied in animals, is relevant for human health.

Novel targets in Alzheimer's disease: A special focus on microglia

Several years after the intriguing novelty in the β-amyloid (Aβ) cascade hypothesis, where the Aβ oligomers emerged as the most detrimental species in the neuropathogenic process of Alzheimer's disease (AD) in place of fibrillar plaques, more recently innate immune system have come on stage as the other prominent factor. Neuroinflammation apparently contributes to AD eziopathogenesis, in large part through overactivation of microglia cells. Genetic and experimental studies strongly support the contribution of the immune system to increasing the risk of AD and participating in its progression. Besides the central immune response mediated by resident microglial cells, peripheral immune challenges may have profound negative effects on brain physiology as well, such as those originating from the gut microbiota. Despite the initial immune response to defend the organism, perpetuation seemingly turns into a chronic detrimental phenomenon that contributes to neuronal dysfunction and exacerbation of the disease. Several new immune-druggable targets are now under investigation, but much still remains to be defined about their precise role and whether and how their physiological activity changes in the injurious context of AD. From a therapeutic perspective, we can undoubtedly consider that AD is no longer solely an Aβ pathology, but rather a multifaceted disorder calling for multi-target therapies. New therapies fighting AD must still counteract Aβ but must also restore appropriate immune defences by tempering maladaptive factors and enabling beneficial responses.

Abnormal dendritic calcium activity and synaptic depotentiation occur early in a mouse model of Alzheimer's disease

Background

Alzheimer’s disease (AD) is characterized by amyloid deposition, tangle formation as well as synapse loss. Synaptic abnormalities occur early in the pathogenesis of AD. Identifying early synaptic abnormalities and their underlying mechanisms is likely important for the prevention and treatment of AD.

Methods

We performed in vivo two-photon calcium imaging to examine the activities of somas, dendrites and dendritic spines of layer 2/3 pyramidal neurons in the primary motor cortex in the APPswe/PS1dE9 mouse model of AD and age-matched wild type control mice. We also performed calcium imaging to determine the effect of Aβ oligomers on dendritic calcium activity. In addition, structural and functional two-photon imaging were used to examine the link between abnormal dendritic calcium activity and changes in dendritic spine size in the AD mouse model.

Results

We found that somatic calcium activities of layer 2/3 neurons were significantly lower in the primary motor cortex of 3-month-old APPswe/PS1dE9 mice than in wild type mice during quiet resting, but not during running on a treadmill. Notably, a significantly larger fraction of apical dendrites of layer 2/3 pyramidal neurons showed calcium transients with abnormally long duration and high peak amplitudes during treadmill running in AD mice. Administration of Aβ oligomers into the brain of wild type mice also induced abnormal dendritic calcium transients during running. Furthermore, we found that the activity and size of dendritic spines were significantly reduced on dendritic branches with abnormally prolonged dendritic calcium transients in AD mice.

Conclusion

Our findings show that abnormal dendritic calcium transients and synaptic depotentiation occur before amyloid plaque formation in the motor cortex of the APPswe/PS1dE9 mouse model of AD. Dendritic calcium transients with abnormally long durations and high amplitudes could be induced by soluble Aβ oligomers and contribute to synaptic deficits in the early pathogenesis of AD.

Electronic supplementary material

The online version of this article (10.1186/s13024-017-0228-2) contains supplementary material, which is available to authorized users.

Alzheimer's disease as oligomeropathy

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder and is characterized by pathological aggregates of amyloid β-protein (Aβ) and tau protein. On the basis of genetic evidence, biochemical data, and animal models, Aβ has been suggested to be responsible for the pathogenesis of AD (the amyloid hypothesis). Aβ molecules tend to aggregate to form oligomers, protofibrils, and mature fibrils. Although mature fibrils in the final stage have been thought to be the cause of AD pathogenesis, recent studies using synthetic Aβ peptides, a cell culture model, Aβ precursor protein transgenic mice models, and human samples, such as cerebrospinal fluids and postmortem brains of AD patients, suggest that pre-fibrillar forms (oligomers of Aβ) are more deleterious than are extracellular fibril forms. Based on this recent evidence showing that oligomers have a central role in the pathogenesis of AD, the term "oligomeropathy" could be used to define AD and other protein-misfolding diseases. In this review, I discuss recent developments in the "oligomer hypothesis" including our research findings regarding the pathogenesis of AD.

Plaque formation and the intraneuronal accumulation of β-amyloid in Alzheimer's disease

Amyloid plaques and neurofibrillary tangles (NFTs) in the brain are the neuropathological hallmarks of Alzheimer's disease (AD). Amyloid plaques are composed of β-amyloid peptides (Aβ), while NFTs contain hyperphosphorylated tau proteins. Patients with familial AD who have mutations in the amyloid precursor protein (APP) gene have either increased production of Aβ or generate more aggregation-prone forms of Aβ. The findings of familial AD mutations in the APP gene suggest that Aβ plays a central role in the pathophysiology of AD. Aβ42, composed of 42 amino acid residues, aggregates readily and is considered to form amyloid plaque. However, the processes of plaque formation are still not well known. It is generally thought that Aβ is secreted into the extracellular space and aggregates to form amyloid plaques. Aβ as extracellular aggregates and amyloid plaques are thought to be toxic to the surrounding neurons. The intraneuronal accumulation of Aβ has more recently been demonstrated and is reported to be involved in synaptic dysfunction, cognitive impairment, and the formation of amyloid plaques in AD. We herein provide an overview of the process of the intraneuronal accumulation of Aβ and plaque formation, and discuss its implications for the pathology, early diagnosis, and therapy of AD.

Microtubule Dynamics in Neuronal Development, Plasticity, and Neurodegeneration

Neurons are the basic information-processing units of the nervous system. In fulfilling their task, they establish a structural polarity with an axon that can be over a meter long and dendrites with a complex arbor, which can harbor ten-thousands of spines. Microtubules and their associated proteins play important roles during the development of neuronal morphology, the plasticity of neurons, and neurodegenerative processes. They are dynamic structures, which can quickly adapt to changes in the environment and establish a structural scaffold with high local variations in composition and stability. This review presents a comprehensive overview about the role of microtubules and their dynamic behavior during the formation and maturation of processes and spines in the healthy brain, during aging and under neurodegenerative conditions. The review ends with a discussion of microtubule-targeted therapies as a perspective for the supportive treatment of neurodegenerative disorders.

The keystone of Alzheimer pathogenesis might be sought in Aβ physiology

For several years Amyloid-beta peptide (Aβ) has been considered the main pathogenetic factor of Alzheimer's disease (AD). According to the so called Amyloid Cascade Hypothesis the increase of Aβ triggers a series of events leading to synaptic dysfunction and memory loss as well as to the structural brain damage in the later stage of the disease. However, several evidences suggest that this hypothesis is not sufficient to explain AD pathogenesis, especially considering that most of the clinical trials aimed to decrease Aβ levels have been unsuccessful. Moreover, Aβ is physiologically produced in the healthy brain during neuronal activity and it is needed for synaptic plasticity and memory. Here we propose a model interpreting AD pathogenesis as an alteration of the negative feedback loop between Aβ and its physiological receptors, focusing on alpha7 nicotinic acetylcholine receptors (α7-nAchRs). According to this vision, when Aβ cannot exert its physiological function a negative feedback mechanism would induce a compensatory increase of its production leading to an abnormal accumulation that reduces α7-nAchR function, leading to synaptic dysfunction and memory loss. In this perspective, the indiscriminate Aβ removal might worsen neuronal homeostasis, causing a further impoverishment of learning and memory. Even if further studies are needed to better understand and validate these mechanisms, we believe that to deepen the role of Aβ in physiological conditions might represent the keystone to elucidate important aspects of AD pathogenesis.

Gene expression parallels synaptic excitability and plasticity changes in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by abnormal accumulation of β-amyloid and tau and synapse dysfunction in memory-related neural circuits. Pathological and functional changes in the medial temporal lobe, a region essential for explicit memory encoding, contribute to cognitive decline in AD. Surprisingly, functional imaging studies show increased activity of the hippocampus and associated cortical regions during memory tasks in presymptomatic and early AD stages, whereas brain activity declines as the disease progresses. These findings suggest an emerging scenario where early pathogenic events might increase neuronal excitability leading to enhanced brain activity before clinical manifestations of the disease, a stage that is followed by decreased brain activity as neurodegeneration progresses. The mechanisms linking pathology with synaptic excitability and plasticity changes leading to memory loss in AD remain largely unclear. Recent studies suggest that increased brain activity parallels enhanced expression of genes involved in synaptic transmission and plasticity in preclinical stages, whereas expression of synaptic and activity-dependent genes are reduced by the onset of pathological and cognitive symptoms. Here, we review recent evidences indicating a relationship between transcriptional deregulation of synaptic genes and neuronal activity and memory loss in AD and mouse models. These findings provide the basis for potential clinical applications of memory-related transcriptional programs and their regulatory mechanisms as novel biomarkers and therapeutic targets to restore brain function in AD and other cognitive disorders.

Early hyperactivity in lateral entorhinal cortex is associated with elevated levels of AβPP metabolites in the Tg2576 mouse model of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder which is the most common cause of dementia in the elderly today. One of the earliest symptoms of AD is olfactory dysfunction. The present study investigated the effects of amyloid β precursor protein (AβPP) metabolites, including amyloid-β (Aβ) and AβPP C-terminal fragments (CTF), on olfactory processing in the lateral entorhinal cortex (LEC) using the Tg2576 mouse model of human AβPP over-expression. The entorhinal cortex is an early target of AD related neuropathology, and the LEC plays an important role in fine odor discrimination and memory. Cohorts of transgenic and age-matched wild-type (WT) mice at 3, 6, and 16months of age (MO) were anesthetized and acute, single-unit electrophysiology was performed in the LEC. Results showed that Tg2576 exhibited early LEC hyperactivity at 3 and 6MO compared to WT mice in both local field potential and single-unit spontaneous activity. However, LEC single-unit odor responses and odor receptive fields showed no detectable difference compared to WT at any age. Finally, the very early emergence of olfactory system hyper-excitability corresponded not to detectable Aβ deposition in the olfactory system, but rather to high levels of intracellular AβPP-CTF and soluble Aβ in the anterior piriform cortex (aPCX), a major afferent input to the LEC, by 3MO. The present results add to the growing evidence of AβPP-related hyper-excitability, and further implicate both soluble Aβ and non-Aβ AβPP metabolites in its early emergence.

Increasing the Content of High-Content Screening: An Overview

Target-based high-throughput screening (HTS) has recently been critiqued for its relatively poor yield compared to phenotypic screening approaches. One type of phenotypic screening, image-based high-content screening (HCS), has been seen as particularly promising.

In this article, we assess whether HCS is as high content as it can be. We analyze HCS publications and find that although the number of HCS experiments published each year continues to grow steadily, the information content lags behind. We find that a majority of high-content screens published so far (60−80%) made use of only one or two image-based features measured from each sample and disregarded the distribution of those features among each cell population. We discuss several potential explanations, focusing on the hypothesis that data analysis traditions are to blame. This includes practical problems related to managing large and multidimensional HCS data sets as well as the adoption of assay quality statistics from HTS to HCS. Both may have led to the simplification or systematic rejection of assays carrying complex and valuable phenotypic information.

We predict that advanced data analysis methods that enable full multiparametric data to be harvested for entire cell populations will enable HCS to finally reach its potential.

Why looking at the whole hippocampus is not enough-a critical role for anteroposterior axis, subfield and activation analyses to enhance predictive value of hippocampal changes for Alzheimer's disease diagnosis

The hippocampus is one of the earliest affected brain regions in Alzheimer's disease (AD) and its dysfunction is believed to underlie the core feature of the disease-memory impairment. Given that hippocampal volume is one of the best AD biomarkers, our review focuses on distinct subfields within the hippocampus, pinpointing regions that might enhance the predictive value of current diagnostic methods. Our review presents how changes in hippocampal volume, shape, symmetry and activation are reflected by cognitive impairment and how they are linked with neurogenesis alterations. Moreover, we revisit the functional differentiation along the anteroposterior longitudinal axis of the hippocampus and discuss its relevance for AD diagnosis. Finally, we indicate that apart from hippocampal subfield volumetry, the characteristic pattern of hippocampal hyperactivation associated with seizures and neurogenesis changes is another promising candidate for an early AD biomarker that could become also a target for early interventions.

Accumulation of intraneuronal β-amyloid 42 peptides is associated with early changes in microtubule-associated protein 2 in neurites and synapses

Pathologic aggregation of β-amyloid (Aβ) peptide and the axonal microtubule-associated protein tau protein are hallmarks of Alzheimer's disease (AD). Evidence supports that Aβ peptide accumulation precedes microtubule-related pathology, although the link between Aβ and tau remains unclear. We previously provided evidence for early co-localization of Aβ42 peptides and hyperphosphorylated tau within postsynaptic terminals of CA1 dendrites in the hippocampus of AD transgenic mice. Here, we explore the relation between Aβ peptide accumulation and the dendritic, microtubule-associated protein 2 (MAP2) in the well-characterized amyloid precursor protein Swedish mutant transgenic mouse (Tg2576). We provide evidence that localized intraneuronal accumulation of Aβ42 peptides is spatially associated with reductions of MAP2 in dendrites and postsynaptic compartments of Tg2576 mice at early ages. Our data support that reduction in MAP2 begins at sites of Aβ42 monomer and low molecular weight oligomer (M/LMW) peptide accumulation. Cumulative evidence suggests that accumulation of M/LMW Aβ42 peptides occurs early, before high molecular weight oligomerization and plaque formation. Since synaptic alteration is the best pathologic correlate of cognitive dysfunction in AD, the spatial association of M/LMW Aβ peptide accumulation with pathology of MAP2 within neuronal processes and synaptic compartments early in the disease process reinforces the importance of intraneuronal Aβ accumulation in AD pathogenesis.

Transgenic mouse models of Alzheimer disease: developing a better model as a tool for therapeutic interventions

Alzheimer disease (AD) is the leading cause of dementia among elderly. Currently, no effective treatment is available for AD. Analysis of transgenic mouse models of AD has facilitated our understanding of disease mechanisms and provided valuable tools for evaluating potential therapeutic strategies. In this review, we will discuss the strengths and weaknesses of current mouse models of AD and the contribution towards understanding the pathological mechanisms and developing effective therapies.

Synaptic changes in Alzheimer's disease and its models

Alzheimer's disease (AD) is a highly prevalent neurodegenerative disorder characterized by a progressive loss of cognition and the presence of two hallmark lesions, senile plaques (SP) and neurofibrillary tangles (NFT), which result from the accumulation and deposition of the β-amyloid peptide (Aβ) and the aggregation of hyperphosphorylated tau protein, respectively. Initially, it was thought that Aβ fibrils, which make up SP, were the root cause of the massive neurodegeneration usual found in AD brains. Over time, the longstanding emphasis on fibrillar Aβ deposits and neuronal death slowly gave way to a new paradigm involving soluble oligomeric forms of Aβ, which play a prominent role in triggering the cognitive deficits by specifically targeting synapses and disrupting synaptic signaling pathways. While this paradigm is widely accepted today in the AD field, the molecular details have not been fully elucidated. In this review, we address some of the important evidence, which has led to the Aβ oligomer-centric hypothesis as well as some of the key findings concerning the effects of Aβ oligomers on synapses at a morphological and functional level. Understanding how Aβ oligomers target synapses provides an important framework for ongoing AD research, which can lead to the development of successful therapeutic strategies designed to alter or perhaps reverse the course of the disease.

APP transgenic mice for modelling behavioural and psychological symptoms of dementia (BPSD)

The discovery of gene mutations responsible for autosomal dominant Alzheimer's disease has enabled researchers to reproduce in transgenic mice several hallmarks of this disorder, notably Aβ accumulation, though in most cases without neurofibrillary tangles. Mice expressing mutated and wild-type APP as well as C-terminal fragments of APP exhibit variations in exploratory activity reminiscent of behavioural and psychological symptoms of Alzheimer dementia (BPSD). In particular, open-field, spontaneous alternation, and elevated plus-maze tasks as well as aggression are modified in several APP transgenic mice relative to non-transgenic controls. However, depending on the precise murine models, changes in open-field and elevated plus-maze exploration occur in either direction, either increased or decreased relative to controls. It remains to be determined which neurotransmitter changes are responsible for this variability, in particular with respect to GABA, 5HT, and dopamine.

β-Amyloid 42/40 ratio and kalirin expression in Alzheimer disease with psychosis

Psychosis in Alzheimer disease differentiates a subgroup with more rapid decline, is heritable, and aggregates within families, suggesting a distinct neurobiology. Evidence indicates that greater impairments of cerebral cortical synapses, particularly in dorsolateral prefrontal cortex, may contribute to the pathogenesis of psychosis in Alzheimer disease (AD) phenotype. Soluble β-amyloid induces loss of dendritic spine synapses through impairment of long-term potentiation. In contrast, the Rho guanine nucleotide exchange factor (GEF) kalirin is an essential mediator of spine maintenance and growth in cerebral cortex. We therefore hypothesized that psychosis in AD would be associated with increased soluble β-amyloid and reduced expression of kalirin in the cortex. We tested this hypothesis in postmortem cortical gray matter extracts from 52 AD subjects with and without psychosis. In subjects with psychosis, the β-amyloid(1-42)/β-amyloid(1-40) ratio was increased, due primarily to reduced soluble β-amyloid(1-40), and kalirin-7, -9, and -12 were reduced. These findings suggest that increased cortical β-amyloid(1-42)/β-amyloid(1-40) ratio and decreased kalirin expression may both contribute to the pathogenesis of psychosis in AD.

Dispersible amyloid β-protein oligomers, protofibrils, and fibrils represent diffusible but not soluble aggregates: their role in neurodegeneration in amyloid precursor protein (APP) transgenic mice

Soluble amyloid β-protein (Aβ) aggregates have been identified in the Alzheimer's disease (AD) brain. Dispersed Aβ aggregates in the brain parenchyma are different from soluble, membrane-associated and plaque-associated solid aggregates. They are in mixture with the extra- or intracellular fluid but can be separated from soluble proteins by ultracentrifugation. To clarify the role of dispersible Aβ aggregates for neurodegeneration we analyzed 2 different amyloid precursor protein (APP)-transgenic mouse models. APP23 mice overexpress human mutant APP with the Swedish mutation. APP51/16 mice express high levels of human wild type APP. Both mice develop Aβ-plaques. Dendritic degeneration, neuron loss, and loss of asymmetric synapses were seen in APP23 but not in APP51/16 mice. The soluble and dispersible fractions not separated from one another were received as supernatant after centrifugation of native forebrain homogenates at 14,000 × g. Subsequent ultracentrifugation separated the soluble, i.e., the supernatant, from the dispersible fraction, i.e., the resuspended pellet. The major biochemical difference between APP23 and APP51/16 mice was that APP23 mice exhibited higher levels of dispersible Aβ oligomers, protofibrils and fibrils precipitated with oligomer (A11) and protofibril/fibril (B10AP) specific antibodies than APP51/16 mice. These differences, rather than soluble Aβ and Aβ plaque pathology were associated with dendritic degeneration, neuron, and synapse loss in APP23 mice in comparison with APP51/16 mice. Immunoprecipitation of dispersible Aβ oligomers, protofibrils, and fibrils revealed that they were associated with APP C-terminal fragments (APP-CTFs). These results indicate that dispersible Aβ oligomers, protofibrils, and fibrils represent an important pool of Aβ aggregates in the brain that critically interact with membrane-associated APP C-terminal fragments. The concentration of dispersible Aβ aggregates, thereby, presumably determines its toxicity.

Localized hippocampus measures are associated with Alzheimer pathology and cognition independent of total hippocampal volume

Hippocampal injury in the Alzheimer's disease (AD) pathological process is region-specific and magnetic resonance imaging (MRI)-based measures of localized hippocampus (HP) atrophy are known to detect region-specific changes associated with clinical AD, but it is unclear whether these measures provide information that is independent of that already provided by measures of total HP volume. Therefore, this study assessed the strength of association between localized HP atrophy measures and AD-related measures including cerebrospinal fluid (CSF) amyloid beta and tau concentrations, and cognitive performance, in statistical models that also included total HP volume as a covariate. A computational technique termed localized components analysis (LoCA) was used to identify 7 independent patterns of HP atrophy among 390 semiautomatically delineated HP from baseline magnetic resonance imaging of participants in the Alzheimer's Disease Neuroimaging Initiative (ADNI). Among cognitively normal participants, multiple measures of localized HP atrophy were significantly associated with CSF amyloid concentration, while total HP volume was not. In addition, among all participants, localized HP atrophy measures and total HP volume were both independently and additively associated with CSF tau concentration, performance on numerous neuropsychological tests, and discrimination between normal, mild cognitive impairment (MCI), and AD clinical diagnostic groups. Together, these results suggest that regional measures of hippocampal atrophy provided by localized components analysis may be more sensitive than total HP volume to the effects of AD pathology burden among cognitively normal individuals and may provide information about HP regions whose deficits may have especially profound cognitive consequences throughout the AD clinical course.

Truncation of tau at E391 promotes early pathologic changes in transgenic mice

Proteolytic cleavage of tau at glutamic acid 391 (E391) is linked to the pathogenesis of Alzheimer disease (AD). This C-terminal-truncated tau species exists in neurofibrillary tangles and abnormal neurites in the brains of AD patients and may potentiate tau polymerization. We generated a mouse model that expresses human tau truncated at E391 to begin to elucidate the role of this C-terminal-truncated tau species in the development of tau pathology. Our results show that truncated but otherwise wild-type human tau is sufficient to drive pretangle pathologic changes in tau, including accumulation of insoluble tau, somatodendritic redistribution, formation of pathologic conformations, and dual phosphorylation of tau at sites associated with AD pathology. In addition, these mice exhibit atypical neuritic tau immunoreactivity, including abnormal neuritic processes and dystrophic neurites. These results suggest that changes in tau proteolysis can initiate tauopathy.

Functional consequences of the lack of amyloid precursor protein in the mouse dentate gyrus in vivo

The amyloid precursor protein (APP) plays a crucial role in the pathogenesis of Alzheimer's disease. Here, we studied whether the lack of APP affects the synaptic properties in the dentate gyrus by measuring granule cell field potentials evoked by perforant path stimulation in anesthetized 9-11-month-old APP-deficient mice in vivo. We found decreased paired-pulse facilitation, indicating altered presynaptic short-term plasticity in the APP-deficient dentate gyrus. In contrast, excitatory synaptic strength and granule cell firing were unchanged in APP knockout mice. Likewise, long-term potentiation (LTP) induced by a theta-burst stimulation protocol was not impaired in the absence of APP. These findings suggest that the deletion of APP may affect presynaptic plasticity of synaptic transmission at the perforant path-granule cell synapse but leaves synaptic efficacy intact and LTP preserved, possibly due to functional redundancy within the APP gene family.

Alzheimer's disease

Over the last three decades, advances in biochemical pathology and human genetics have illuminated one of the most enigmatic subjects in biomedicine--neurodegeneration. Eponymic diseases of the nervous system such as Alzheimer's, Parkinson's, and Huntington's diseases that were long characterized by mechanistic ignorance have yielded striking progress in our understanding of their molecular underpinnings. A central theme in these and related disorders is the concept that certain normally soluble neuronal proteins can misfold and aggregate into oligomers and amyloid fibrils which can confer profound cytotoxicity. Perhaps the foremost example, both in terms of its societal impact and how far knowledge has moved toward the clinic, is that of Alzheimer's disease (AD). Here, we will review the classical protein lesions of the disorder that have provided a road map to etiology and pathogenesis. We will discuss how elucidating the genotype-to-phenotype relationships of familial forms of Alzheimer's disease has highlighted the importance of the misfolding and altered proteostasis of two otherwise soluble proteins, amyloid β-protein and tau, suggesting mechanism-based therapeutic targets that have led to clinical trials.

Short-term environmental enrichment rescues adult neurogenesis and memory deficits in APP(Sw,Ind) transgenic mice

Epidemiological studies indicate that intellectual activity prevents or delays the onset of Alzheimer's disease (AD). Similarly, cognitive stimulation using environmental enrichment (EE), which increases adult neurogenesis and functional integration of newborn neurons into neural circuits of the hippocampus, protects against memory decline in transgenic mouse models of AD, but the mechanisms involved are poorly understood. To study the therapeutic benefits of cognitive stimulation in AD we examined the effects of EE in hippocampal neurogenesis and memory in a transgenic mouse model of AD expressing the human mutant β-amyloid (Aβ) precursor protein (APP_Sw,Ind). By using molecular markers of new generated neurons (bromodeoxiuridine, NeuN and doublecortin), we found reduced neurogenesis and decreased dendritic length and projections of doublecortin-expressing cells of the dentate gyrus in young APP_Sw,Ind transgenic mice. Moreover, we detected a lower number of mature neurons (NeuN positive) in the granular cell layer and a reduced volume of the dentate gyrus that could be due to a sustained decrease in the incorporation of new generated neurons. We found that short-term EE for 7 weeks efficiently ameliorates early hippocampal-dependent spatial learning and memory deficits in APP_Sw,Ind transgenic mice. The cognitive benefits of enrichment in APP_Sw,Ind transgenic mice were associated with increased number, dendritic length and projections to the CA3 region of the most mature adult newborn neurons. By contrast, Aβ levels and the total number of neurons in the dentate gyrus were unchanged by EE in APP_Sw,Ind mice. These results suggest that promoting the survival and maturation of adult generated newborn neurons in the hippocampus may contribute to cognitive benefits in AD mouse models.

Low-n oligomers as therapeutic targets of Alzheimer's disease

The pathogenesis of Alzheimer's disease involves the progressive accumulation of amyloid β-protein (Aβ). Recent studies using synthetic Aβ peptides, a cell culture model, Aβ precursor protein transgenic mice models suggest that pre-fibrillar forms of Aβ are more deleterious than extracellular fibril forms. Recent findings obtained using synthetic Aβ peptides and human samples indicated that low-n oligomers (from dimers to octamers) may be proximate toxins for neuron and synapse. Here, we review the recent studies on the soluble oligomers, especially low-n oligomers in Alzheimer's disease.

Amyloid-β protein modulates the perivascular clearance of neuronal apolipoprotein E in mouse models of Alzheimer's disease

The deposition of amyloid-β protein (Aβ) in the brain is a hallmark of Alzheimer's disease (AD). Apolipoprotein E (apoE) is involved in the clearance of Aβ from brain and the APOE ε4 allele is a major risk factor for sporadic AD. We have recently shown that apoE is drained into the perivascular space (PVS), where it co-localizes with Aβ. To further clarify the role of apoE in perivascular clearance of Aβ, we studied apoE-transgenic mice over-expressing human apoE4 either in astrocytes (GE4) or in neurons (TE4). These animals were crossbred with amyloid precursor protein (APP)-transgenic mice and with APP-presenilin-1 (APP-PS1) double transgenic mice. Using an antibody that specifically detects human apoE (h-apoE), we observed that astroglial expression of h-apoE in GE4 mice leads to its perivascular drainage, whereas neuronal expression in TE4 mice does not, indicating that neuron-derived apoE is usually not the subject of perivascular drainage. However, h-apoE was observed not only in the PVS of APP-GE4 and APP-PS1-GE4 mice, but also in that of APP-TE4 and APP-PS1-TE4 mice. In all these mouse lines, we found co-localization of neuron-derived h-apoE and Aβ in the PVS. Aβ and h-apoE were also found in the cytoplasm of perivascular astrocytes indicating that astrocytes take up the neuron-derived apoE bound to Aβ, presumably prior to its clearance into the PVS. The uptake of apoE-Aβ complexes into glial cells was further investigated in glioblastoma cells. It was mediated by α(2)macroglobulin receptor/low density lipoprotein receptor-related protein (LRP-1) and inhibited by adding receptor-associated protein (RAP). It results in endosomal Aβ accumulation within these cells. These results suggest that neuronal apoE-Aβ complexes, but not neuronal apoE alone, are substrates for LRP-1-mediated astroglial uptake, transcytosis, and subsequent perivascular drainage. Thus, the production of Aβ and its interaction with apoE lead to the pathological perivascular drainage of neuronal apoE and provide insight into the pathological interactions of Aβ with neuronal apoE metabolism.

Influence of aging and neurodegeneration on dendritic spine morphology

In neuronal circuits, excitatory synaptic transmission predominantly occurs at postsynaptic protrusions called dendritic spines. Spines are highly plastic structures capable of formation, enlargement, shrinkage, and elimination over time. Individual spine morphology is widely variable, and evidence suggests these differences in morphology are relevant to spine function. Recent reports provide evidence that spine structural plasticity underlies functional synaptic changes, including those seen in animal models of learning and memory plasticity. Conversely, impairments in cognitive functions, such as those commonly seen in aging, have recently been linked to and correlated with alterations in spine density and morphology. In addition, dendritic spine density and morphology also appear to be altered in various transgenic animal models of neurodegenerative diseases. Ultimately, an understanding of the synaptic basis of age- and disease-related cognitive impairments may lead to the development of drug treatments that can restore or protect synaptic profiles in neural circuits that mediate cognition.

Insights into Alzheimer disease pathogenesis from studies in transgenic animal models

Alzheimer disease is the most common cause of dementia among the elderly, accounting for ~60-70% of all cases of dementia. The neuropathological hallmarks of Alzheimer disease are senile plaques (mainly containing p-amyloid peptide derived from amyloid precursor protein) and neurofibrillary tangles (containing hyperphosphorylated Tau protein), along with neuronal loss. At present there is no effective treatment for Alzheimer disease. Given the prevalence and poor prognosis of the disease, the development of animal models has been a research priority to understand pathogenic mechanisms and to test therapeutic strategies. Most cases of Alzheimer disease occur sporadically in people over 65 years old, and are not genetically inherited. Roughly 5% of patients with Alzheimer disease have familial Alzheimer disease--that is, related to a genetic predisposition, including mutations in the amyloid precursor protein, presenilin 1, and presenilin 2 genes. The discovery of genes for familial Alzheimer disease has allowed transgenic models to be generated through the overexpression of the amyloid precursor protein and/or presenilins harboring one or several mutations found in familial Alzheimer disease. Although none of these models fully replicates the human disease, they have provided valuable insights into disease mechanisms as well as opportunities to test therapeutic approaches. This review describes the main transgenic mouse models of Alzheimer disease which have been adopted in Alzheimer disease research, and discusses the insights into Alzheimer disease pathogenesis from studies in such models. In summary, the Alzheimer disease mouse models have been the key to understanding the roles of soluble b-amyloid oligomers in disease pathogenesis, as well as of the relationship between p-amyloid and Tau pathologies.

APP transgenic mice: their use and limitations

Alzheimer's disease is the most widespread form of dementia. Its histopathological hallmarks include vascular and extracellular β-amyloid (Aβ) deposition and intraneuronal neurofibrillary tangles (NFTs). Gradual decline of cognitive functions linked to progressive synaptic loss makes patients unable to store new information in the earlier stages of the pathology, later becoming completely dependent because they are unable to do even elementary daily life actions. Although more than a hundred years have passed since Alois Alzheimer described the first case of AD, and despite many years of intense research, there are still many crucial points to be discovered in the neuropathological pathway. The development of transgenic mouse models engineered with overexpression of the amyloid precursor protein carrying familial AD mutations has been extremely useful. Transgenic mice present the hallmarks of the pathology, and histological and behavioural examination supports the amyloid hypothesis. As in human AD, extracellular Aβ deposits surrounded by activated astrocytes and microglia are typical features, together with synaptic and cognitive defects. Although animal models have been widely used, they are still being continuously developed in order to recapitulate some missing aspects of the disease. For instance, AD therapeutic agents tested in transgenic mice gave encouraging results which, however, were very disappointing in clinical trials. Neuronal cell death and NFTs typical of AD are much harder to replicate in these mice, which thus offer a fundamental but still imperfect tool for understanding and solving dementia pathology.

Advanced microscopy techniques for quantitative analysis in neuromorphology and neuropathology research: current status and requirements for the future

Visualizing neuromorphology and in particular neuropathology has been the focus of many researchers in the quest to solve the numerous questions that are still remaining related to several neurological and neuropsychiatric diseases. Over the last years, intense research into microscopy techniques has resulted in the development of various new types of microscopes, software imaging systems, and analysis programs. This review briefly discusses some key techniques, such as confocal stereology and automated neuron tracing and reconstruction, and their applications in neuroscience research. Special emphasis is placed on needs for further developments, such as the demand for higher-throughput analyses in quantitative neuromorphology. These developments will advance basic neuroscience research as well as pharmaceutical and biotechnology research and development.

Brain slices as models for neurodegenerative disease and screening platforms to identify novel therapeutics

Recent improvements in brain slice technology have made this biological preparation increasingly useful for examining pathophysiology of brain diseases in a tissue context. Brain slices maintain many aspects of in vivo biology, including functional local synaptic circuitry with preserved brain architecture, while allowing good experimental access and precise control of the extracellular environment, making them ideal platforms for dissection of molecular pathways underlying neuronal dysfunction. Importantly, these ex vivo systems permit direct treatment with pharmacological agents modulating these responses and thus provide surrogate therapeutic screening systems without recourse to whole animal studies. Virus or particle mediated transgenic expression can also be accomplished relatively easily to study the function of novel genes in a normal or injured brain tissue context.In this review we will discuss acute brain injury models in organotypic hippocampal and co-culture systems and the effects of pharmacological modulation on neurodegeneration. The review will also cover the evidence of developmental plasticity in these ex vivo models, demonstrating emergence of injury-stimulated neuronal progenitor cells, and neurite sprouting and axonal regeneration following pathway lesioning. Neuro-and axo-genesis are emerging as significant factors contributing to brain repair following many acute and chronic neurodegenerative disorders. Therefore brain slice models may provide a critical contextual experimental system to explore regenerative mechanisms in vitro.

Transgenic mouse models of Alzheimer's disease

Alzheimer's disease is the most common cause of senile dementia in the United States and Europe. At present, there is no effective treatment. Given the disease's prevalence and poor prognosis, the development of animal models has been a high research priority. Transgenic modeling has been pursued on the basis of the amyloid hypothesis and has taken advantage of mutations in the amyloid precursor protein and the presenilins that cause familial forms of Alzheimer's disease. Modeling has been most aggressively pursued in mice, for which the techniques of genetic modification are well developed. Transgenic mouse models now exist that mimic a range of Alzheimer's disease-related pathologies. Although none of the models fully replicates the human disease, the models have contributed significant insights into the pathophysiology of beta-amyloid toxicity, particularly with respect to the effects of different beta-amyloid species and the possible pathogenic role of beta-amyloid oligomers. They have also been widely used in the preclinical testing of potential therapeutic modalities and have played a pivotal role in the development of immunotherapies for Alzheimer's disease that are currently in clinical trials. These models will, without a doubt, continue to play central roles in preclinical testing and be used as tools for developing insights into the biological basis of Alzheimer's disease.

Transgenic mouse models of Alzheimer's disease

Alzheimer's disease is the most common cause of senile dementia in the United States and Europe. At present, there is no effective treatment. Given the disease's prevalence and poor prognosis, the development of animal models has been a high research priority. Transgenic modeling has been pursued on the basis of the amyloid hypothesis and has taken advantage of mutations in the amyloid precursor protein and the presenilins that cause familial forms of Alzheimer's disease. Modeling has been most aggressively pursued in mice, for which the techniques of genetic modification are well developed. Transgenic mouse models now exist that mimic a range of Alzheimer's disease-related pathologies. Although none of the models fully replicates the human disease, the models have contributed significant insights into the pathophysiology of beta-amyloid toxicity, particularly with respect to the effects of different beta-amyloid species and the possible pathogenic role of beta-amyloid oligomers. They have also been widely used in the preclinical testing of potential therapeutic modalities and have played a pivotal role in the development of immunotherapies for Alzheimer's disease that are currently in clinical trials. These models will, without a doubt, continue to play central roles in preclinical testing and be used as tools for developing insights into the biological basis of Alzheimer's disease.

Adenosine A2A receptor blockade prevents synaptotoxicity and memory dysfunction caused by beta-amyloid peptides via p38 mitogen-activated protein kinase pathway

Alzheimer's disease (AD) is characterized by memory impairment, neurochemically by accumulation of beta-amyloid peptide (namely Abeta(1-42)) and morphologically by an initial loss of nerve terminals. Caffeine consumption prevents memory dysfunction in different models, which is mimicked by antagonists of adenosine A(2A) receptors (A(2A)Rs), which are located in synapses. Thus, we now tested whether A(2A)R blockade prevents the early Abeta(1-42)-induced synaptotoxicity and memory dysfunction and what are the underlying signaling pathways. The intracerebral administration of soluble Abeta(1-42) (2 nmol) in rats or mice caused, 2 weeks later, memory impairment (decreased performance in the Y-maze and object recognition tests) and a loss of nerve terminal markers (synaptophysin, SNAP-25) without overt neuronal loss, astrogliosis, or microgliosis. These were prevented by pharmacological blockade [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH58261); 0.05 mg . kg(-1) . d(-1), i.p.; for 15 d] in rats, and genetic inactivation of A(2A)Rs in mice. Moreover, these were synaptic events since purified nerve terminals acutely exposed to Abeta(1-42) (500 nm) displayed mitochondrial dysfunction, which was prevented by A(2A)R blockade. SCH58261 (50 nm) also prevented the initial synaptotoxicity (loss of MAP-2, synaptophysin, and SNAP-25 immunoreactivity) and subsequent loss of viability of cultured hippocampal neurons exposed to Abeta(1-42) (500 nm). This A(2A)R-mediated control of neurotoxicity involved the control of Abeta(1-42)-induced p38 phosphorylation and was independent from cAMP/PKA (protein kinase A) pathway. Together, these results show that A(2A)Rs play a crucial role in the development of Abeta-induced synaptotoxicity leading to memory dysfunction through a p38 MAPK (mitogen-activated protein kinase)-dependent pathway and provide a molecular basis for the benefits of caffeine consumption in AD.

Reversal of long-term dendritic spine alterations in Alzheimer disease models

Synapse loss is strongly correlated with cognitive impairment in Alzheimer's disease (AD). We have previously reported the loss of dendritic spines and the presence of dystrophic neurites in both the hippocampi of transgenic mice overexpressing amyloid precursor protein (APP) and in the human brain affected with AD. In the studies reported here we have asked whether the acute alterations in dendritic spines induced by Abeta, as well as the chronic loss of spine density seen in hAPP transgenic mice, are reversible by treatments that restore the cAMP/PKA/CREB signaling pathway or proteasome function to control levels. The results show that both rolipram and TAT-HA-Uch-L1 restore spine density to near control conditions, even in elderly mice. The results suggest that changes in dendritic structure and function that occur after Abeta elevation are reversible even after long periods of time, and that one could envision therapeutic approaches to AD based on this restoration that could work independently of therapies aimed at lowering Abeta levels in the brain.

Biochemical and immunohistochemical analysis of an Alzheimer's disease mouse model reveals the presence of multiple cerebral Abeta assembly forms throughout life

The amyloid beta-protein (Abeta) is believed to play a causal role in Alzheimer's disease, however, the mechanism by which Abeta mediates its effect and the assembly form(s) of Abeta responsible remain unclear. Several APP transgenic mice have been shown to accumulate Abeta and to develop cognitive deficits. We have studied one such model, the J20 mouse. Using an immunoprecipitation/Western blotting technique we find an age-dependent increase in Abeta monomer and SDS-stable dimer. But prior to the earliest detection of Abeta dimers, immunohistochemical analysis revealed an increase in oligomer immunoreactivity that was coincident with reduced hippocampal MAP2 and synaptophysin staining. Moreover, biochemical fractionation and ELISA analysis revealed evidence of TBS and triton-insoluble sedimentable Abeta aggregates at the earliest ages studied. These data demonstrate the presence of multiple assembly forms of Abeta throughout the life of J20 mice and highlight the difficulty in attributing synaptotoxicity to a single Abeta species.

Ballistic delivery of dyes for structural and functional studies of the nervous system

This protocol describes detailed procedures for rapid labeling of cells in a variety of preparations by means of particle-mediated ballistic (i.e., Gene Gun) delivery of fluorescent dyes. The method has been used for rapid labeling of cells with either lipid- or water-soluble dyes, in a variety of preparations at different ages. Tissue preparations include fixed mouse brain slices (described here), cell cultures, and tissue explants. This ballistic labeling technique is useful for studying neuronal connectivity, function, and pathology in the nervous system of living as well as fixed specimens.

Parenchymal and vascular Abeta-deposition and its effects on the degeneration of neurons and cognition in Alzheimer's disease

The deposition of the amyloid beta-protein (Abeta) is one of the pathological hallmarks of Alzheimer's disease (AD). Abeta-deposits show the morphology of senile plaques and cerebral amyloid angiopathy (CAA). Senile plaques and vascular Abeta-deposits occur first in neocorti-cal areas. Then, they expand hierarchically into further brain regions. The distribution of Abeta plaques throughout the entire brain, thereby correlates with the clinical status of the patients. Imaging techniques for Abeta make use of the hierarchical distribution of Abeta to distinguish AD patients from non-AD patients. However, pathology seen in AD patients represents a late stage of a pathological process starting 10-30 years earlier in cognitively normal individuals. In addition to the fibrillar amyloid of senile plaques, oligomeric and monomeric Abeta is found in the brain. Recent studies revealed that oligomeric Abeta is presumably the most toxic Abeta-aggregate, which interacts with glutamatergic synapses. In doing so, dendrites are presumed to be the primary target for Abeta-toxicity. In addition, vascular Abeta-deposits can lead to capillary occlusion and blood flow disturbances presumably contributing to the alteration of neurons in addition to the direct neurotoxic effects of Abeta. All these findings point to an important role of Abeta and its aggregates in the neurodegenerative process of AD. Since there is already significant neuron loss in AD patients, treatment strategies aimed at reducing the amyloid load will presumably not cure the symptoms of dementia but they may stop disease progression. Therefore, it seems to be necessary to protect the brain from Abeta-toxicity already in stages of the disease with minor neuron loss before the onset of cognitive symptoms.

Dendritic spine loss and synaptic alterations in Alzheimer's disease

Dendritic spines are tiny protrusions along dendrites, which constitute major postsynaptic sites for excitatory synaptic transmission. These spines are highly motile and can undergo remodeling even in the adult nervous system. Spine remodeling and the formation of new synapses are activity-dependent processes that provide a basis for memory formation. A loss or alteration of these structures has been described in patients with neurodegenerative disorders such as Alzheimer's disease (AD), and in mouse models for these disorders. Such alteration is thought to be responsible for cognitive deficits long before or even in the absence of neuronal loss, but the underlying mechanisms are poorly understood. This review will describe recent findings and discoveries on the loss or alteration of dendritic spines induced by the amyloid beta (Abeta) peptide in the context of AD.

Tracing of temporo-entorhinal connections in the human brain: cognitively impaired argyrophilic grain disease cases show dendritic alterations but no axonal disconnection of temporo-entorhinal association neurons

Argyrophilic grain disease (AGD), a neurodegenerative disorder, is often associated with mild to moderate Alzheimer’s disease (AD)-related pathology. The development of dementia in AGD is associated with the extent of coexisting AD-related pathology. Therefore, the question arises whether the degenerative changes in the neuronal network of demented AGD-patients represent a distinct pattern or show similar changes of disconnection as considered for AD. We were able to apply DiI-tracing in two human autopsy cases with mild AD-related pathology (controls), in one AD-patient, in one non-demented patient with advanced AD-related pathology, and in three cognitively impaired AGD-patients. DiI-crystals were injected into the entorhinal cortex. Pyramidal neurons of layers III and V of the adjacent temporal neocortex (area 35) were retrogradely marked with the tracer and analyzed. The AD case did not exhibit any retrogradely labeled neurons in the temporal neocortex. In the non-demented case with advanced AD-related pathology, the number of traced neurons was reduced as compared to that in the two controls and in the three AGD cases. In contrast, all three cognitively impaired AGD cases exhibited labeled pyramidal neurons in area 35 in an almost similar number as in the controls. However, alterations in the dendritic tree were observed in the AGD cases. These results show the existence of temporo-entorhinal connections in the adult human brain similar to those reported in animal models. Furthermore, the present study based on seven cases is the first attempt to study changes in the neuronal network in a human tauopathy with targeted axonal tracing techniques. Our findings in three cognitively impaired AGD cases suggest that AGD-related dementia constitutes a distinct disorder with a characteristic pattern of degeneration in the neuronal network.