Forebrain microglia from wild-type but not adult 5xFAD mice prevent amyloid-beta plaque formation in organotypic hippocampal slice cultures

The role of microglia in amyloid-beta (A beta) deposition is controversial. In the present study, an organotypic hippocampal slice culture (OHSC) system with an in vivo-like microglial-neuronal environment was used to investigate the potential contribution of microglia to A beta plaque formation. We found that microglia ingested A beta, thereby preventing plaque formation in OHSCs. Conversely, A beta deposits formed rapidly in microglia-free wild-type slices. The capacity to prevent A beta plaque formation was absent in forebrain microglia from young adult but not juvenile 5xFamilial Alzheimer's disease (FAD) mice. Since no loss of A beta clearance capacity was observed in both wild-type and cerebellar microglia from 5xFAD animals, the high A beta(1-42) burden in the forebrain of 5xFAD animals likely underlies the exhaustion of microglial A beta clearance capacity. These data may therefore explain why A beta plaque formation has never been described in wild-type mice, and point to a beneficial role of microglia in AD pathology. We also describe a new method to study A beta plaque formation in a cell culture setting

T cell mediated cerebral hemorrhages and microhemorrhages during passive Aβ immunization in APPPS1 transgenic mice

<p>Abstract</p> <p>Background</p> <p>Immunization against amyloid-β (Aβ), the peptide that accumulates in the form of senile plaques and in the cerebrovasculature in Alzheimer's disease (AD), causes a dramatic immune response that prevents plaque formation and clears accumulated Aβ in transgenic mice. In a clinical trial of Aβ immunization, some patients developed meningoencephalitis and hemorrhages. Neuropathological investigations of patients who died after the trial showed clearance of amyloid pathology, but also a powerful immune response involving activated T cells probably underlying the negative effects of the immunization.</p> <p>Results</p> <p>To define the impact of T cells on this inflammatory response we used passive immunization and adoptive transfer to separate the effect of IgG and T cell mediated effects on microhemorrhage in APPPS1 transgenic mice. Neither anti Aβ IgG nor adoptively transferred T cells, alone, led to increased cerebrovascular damage. However, the combination of adoptively transferred T cells and passive immunization led to massive cerebrovascular bleeding that ranged from multiple microhemorrhages in the parenchyma to large hematomas.</p> <p>Conclusions</p> <p>Our results indicate that vaccination can lead to Aβ and T cell induced cerebral micro-hemorrhages and acute hematomas, which are greatly exacerbated by T cell mediated activity.</p

A reporter of local dendritic translocation shows plaque- related loss of neural system function in APP-transgenic mice

Although neuronal communication is thought to be summated within local dendritic segments, no technique is currently available to monitor activity in vivo at this level of resolution. To overcome this challenge, we developed an optical reporter of neuronal activity utilizing the coding sequence of Venus, flanked by short stretches of the 5' and 3' untranslated regions from CAMKIIα. This reporter takes advantage of the fact that CAMKIIα mRNA is transported to the dendrite and locally translated in an activity dependent manner (Aakalu et al., 2001). Using adeno associated virus (AAV), we used this reporter to study neuronal activity in adult mice. Exposure of the mice to an enriched environment led to enhancement of Venus expression in dendritic segments of somatosensory cortex, demonstrating in vivo that dendritic mRNA translocation and local transcription occur in response to physiologically relevant stimuli. We then utilized this system to examine the impact of Alzheimer related local amyloid-β deposits on neural system function, to test the hypothesis that plaques are toxic. In APPswe/PS1d9 (APP/PS1) mice, neurons close to plaques, and dendritic segments close to plaques, both showed diminished fluorescent intensity and therefore neuronal activity. In contrast to wildtype mice, fluorescent intensity in neurons near plaques in transgenic mice did not increase after environmental enrichment. These data indicate that neuronal activity in dendritic segments and neurons in the vicinity of a plaque is decreased compared to wildtype mice, supporting the idea that plaques are a focal lesion leading to impaired neural system function

Dendritic spine abnormalities in amyloid precursor protein transgenic mice demonstrated by gene transfer and intravital multiphoton microscopy

Accumulation of amyloid-beta (Aβ) into senile plaques in Alzheimer’s disease (AD) is a hallmark neuropathological feature of the disorder, which likely contributes to alterations in neuronal structure and function. Recent work has revealed changes in neurite architecture associated with plaques and functional changes in cortical signaling in amyloid precursor protein (APP) expressing mouse models of AD. Here we developed a method using gene transfer techniques to introduce GFP into neurons allowing the investigation of neuronal processes in the vicinity of plaques. Multiphoton imaging of GFP-labeled neurons in living Tg2576 APP mice revealed disrupted neurite trajectories and reductions in dendritic spine density compared to age-matched control mice. A profound deficit in spine density (∼50%) extends approximately 20 μm from plaque edges. Importantly, a robust decrement (∼25%) also occurs on dendrites not associated with plaques, suggesting widespread loss of postsynaptic apparatus. Plaques and dendrites remained stable over the course of weeks of imaging. Post-mortem analysis of axonal immunostaining and co-localization of synaptophysin and postsynaptic density 95 (PSD-95) protein staining around plaques indicate a parallel loss of pre- and postsynaptic partners. These results show considerable changes in dendrites and dendritic spines in APP transgenic mice, demonstrating a dramatic synaptotoxic effect of dense core plaques. Decreased spine density will likely contribute to altered neural system function and behavioral impairments observed in Tg2576 mice

Monitoring protein aggregation and toxicity in Alzheimer's disease mouse models using in vivo imaging

Aggregation of amyloid beta peptide into senile plaques and hyperphosphorylated tau protein into neurofibrillary tangles in the brain are the pathological hallmarks of Alzheimer’s disease. Despite over a century of research into these lesions, the exact relationship between pathology and neurotoxicity has yet to be fully elucidated. In order to study the formation of plaques and tangles and their effects on the brain, we have applied multiphoton in vivo imaging of transgenic mouse models of Alzheimer’s disease. This technique allows longitudinal imaging of pathological aggregation of proteins and the subsequent changes in surrounding neuropil neurodegeneration and recovery after therapeutic interventions

Rapid appearance and local toxicity of amyloid-beta plaques in a mouse model of Alzheimer's disease

Senile plaques accumulate over the course of decades in the brains of patients with Alzheimer’s disease. A fundamental tenet of the amyloid hypothesis of Alzheimer’s disease is that the deposition of amyloid-β precedes and induces the neuronal abnormalities that underlie dementia(1). This idea has been challenged, however, by the suggestion that alterations in axonal trafficking and morphological abnormalities precede and lead to senile plaques(2). The role of microglia in accelerating or retarding these processes has been uncertain. To investigate the temporal relation between plaque formation and the changes in local neuritic architecture, we used longitudinal in vivo multiphoton microscopy to sequentially image young APPswe/PS1d9xYFP (B6C3-YFP) transgenic mice(3). Here we show that plaques form extraordinarily quickly, over 24 h. Within 1–2 days of a new plaque’s appearance, microglia are activated and recruited to the site. Progressive neuritic changes ensue, leading to increasingly dysmorphic neurites over the next days to weeks. These data establish plaques as a critical mediator of neuritic pathology

Passive immunotherapy rapidly increases structural plasticity in a mouse model of Alzheimer disease

Senile plaque-associated changes in neuronal connectivity such as altered neurite trajectory, dystrophic swellings, and synapse and dendritic spine loss are thought to contribute to cognitive dysfunction in Alzheimer’s disease and mouse models. Immunotherapy to remove amyloid beta is a promising therapy that causes recovery of neurite trajectory and dystrophic neurites over a period of days. The acute effects of immunotherapy on neurite morphology at a time point when soluble amyloid has been cleared but dense plaques are not yet affected are unknown. To examine whether removal of soluble amyloid β (Aβ) has a therapeutic effect on dendritic spines, we explored spine dynamics within one hour of applying a neutralizing anti Aβ antibody. This acute treatment caused a small but significant increase in dendritic spine formation in PDAPP brain far from plaques, without affecting spine plasticity near plaques or average dendritic spine density. These data support the hypothesis that removing toxic soluble forms of amyloid-beta rapidly increases structural plasticity possibly allowing functional recovery of neural circuits

Impaired spine stability underlies plaque-related spine loss in an Alzheimer's disease mouse model

Dendritic spines, the site of most excitatory synapses in the brain, are lost in Alzheimer’s disease and in related mouse models, undoubtedly contributing to cognitive dysfunction. We hypothesized that spine loss results from plaque-associated alterations of spine stability, causing an imbalance in spine forma-tion and elimination. To investigate effects of plaques on spine stability in vivo, we observed cortical neu-rons using multiphoton microscopy in a mouse model of amyloid pathology before and after extensive plaque deposition. We also observed age-matched non-transgenic mice to study normal effects of aging on spine plasticity. We found that spine density and struc-tural plasticity are maintained during normal aging. Tg2576 mice had normal spine density and plasticity before plaques appeared, but after amyloid pathology is established, severe disruptions were observed. In con-trol animals, spine formation and elimination were equivalent over 1 hour of observation (5 % of observed spines), resulting in stable spine density. However, in aged Tg2576 mice spine elimination increased, specifi-cally in the immediate vicinity of plaques. Spine forma-tion was unchanged, resulting in spine loss. These data show a small population of rapidly changing spines in adult and even elderly mouse cortex; further, in the vicinity of amyloid plaques, spine stability is markedly impaired leading to loss of synaptic structural in

Substantia nigra pars reticulata neurons code initiation of a serial pattern: implications for natural action sequences and sequential disorders

Sequences of movements are initiated abnormally in neurological disorders involving basal ganglia dysfunction, such as Parkinson's disease or Tourette's syndrome. The substantia nigra pars reticulata (SNpr) is one of the two primary output structures of the basal ganglia. However, little is known about how substantia nigra mediates the initiation of normal movement sequences. We studied its role in coding initiation of a sequentially stereotyped but natural movement sequence by recording neuronal activity in SNpr during behavioural performance of ‘syntactic grooming chains’. These are rule-governed sequences of up to 25 grooming movements emitted in four predictable (syntactic) phases, which occur spontaneously during grooming behaviour by rats and other rodents. Our results show that neuronal activation in central SNpr codes the onset of this entire rule-governed sequential pattern of grooming actions, not elemental grooming movements. We conclude that the context of sequential pattern may be more important than the elemental motor parameters in determining SNpr neuronal activation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/71795/1/j.1460-9568.2002.02210.x.pd

Restricted diffusion in the brain of transgenic mice with cerebral amyloidosis.

A prominent hallmark of Alzheimer's disease pathology is cerebral amyloidosis. However, it is not clear how extracellular amyloid-beta peptide (A beta) deposition and amyloid formation compromise brain function and lead to dementia. It has been argued that extracellular amyloid deposition is neurotoxic and/or that soluble A beta oligomers impair synaptic function. Amyloid deposits, by contrast, may affect diffusion properties of the brain interstitium with implications for the transport of endogenous signalling molecules during synaptic and/or extrasynaptic transmission. We have used diffusion-weighted magnetic resonance imaging to study diffusion properties in brains of young (6-month-old) and aged (25-month-old) APP23 transgenic mice and control littermates. Our results demonstrate that fibrillar amyloid deposits and associated gliosis in brains of aged APP23 transgenic mice are accompanied by a reduction in the apparent diffusion coefficient. This decrease was most pronounced in neocortical areas with a high percentage of congophilic amyloid and was not significant in the caudate putamen, an area with only modest and diffuse amyloid deposition. These findings suggest that extracellular deposition of fibrillar amyloid and/or associated glial proliferation and hypertrophy cause restrictions to interstitial fluid diffusion. Reduced diffusivity within the interstitial space may alter volume transmission and therefore contribute to the cognitive impairment in Alzheimer's disease