Tara L. Spires

Dendritic Spine Abnormalities in Amyloid Precursor Protein Transgenic Mice Demonstrated by Gene Transfer and Intravital Multiphoton Microscopy

Accumulation of amyloid-β (Aβ) into senile plaques in Alzheimers 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 green fluorescent protein (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 with age-matched control mice. A profound deficit in spine density (∼50%) extends ∼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. Postmortem analysis of axonal immunostaining and colocalization of synaptophysin and postsynaptic density 95 protein staining around plaques indicate a parallel loss of presynaptic and postsynaptic partners. These results show considerable changes in dendrites and dendritic spines in APP transgenic mice, demonstrating a dramatic synaptotoxic effect of dense-cored plaques. Decreased spine density will likely contribute to altered neural system function and behavioral impairments observed in Tg2576 mice.

Environmental Enrichment Rescues Protein Deficits in a Mouse Model of Huntington's Disease, Indicating a Possible Disease Mechanism

Huntingtons disease (HD) is a devastating neurodegenerative disorder caused by a CAG repeat expansion encoding an extended polyglutamine tract in the huntingtin protein. Transgenic mice expressing a human huntingtin transgene containing an expanded CAG repeat (R6/1 model) develop a neurodegenerative disorder closely resembling human HD. Previous work demonstrated that environmental enrichment delays the onset of motor symptoms in this mouse model. We confirmed that at 5 months of age, enrichment ameliorates motor symptoms (assessed using the rotarod test) and prevents loss of body weight induced by the HD transgene. We further examined molecular consequences of enrichment by determining changes in protein levels in the neostriatum, hippocampus, and anterior cortex using quantitative Western blot analysis. Non-enriched HD mice have severe reductions in BDNF in the hippocampus and striatum at 5 months, which are entirely rescued by enrichment. BDNF levels are unaltered by HD in the anterior cortex, suggesting that enrichment might prevent HD-induced impairment of anterograde transport of this neurotrophin to the striatum. NGF is unaffected by HD. Non-enriched HD mice also exhibit deficits in dopamine and cAMP-regulated phosphoprotein (32 kDa) in striatum and anterior cortex. Environmental enrichment rescues the cortical but not the striatal deficit at 5 months. These results suggest that environmental enrichment benefits animals at early stages of the disease by rescuing protein deficits, possibly through rescuing transcription or protein transport problems.

Abnormal bundling and accumulation of F-actin mediates tau-induced neuronal degeneration in vivo

Hyperphosphorylated forms of the microtubule-associated protein (MAP) tau accumulate in Alzheimers disease and related tauopathies and are thought to have an important role in neurodegeneration. However, the mechanisms through which phosphorylated tau induces neurodegeneration have remained elusive. Here, we show that tau-induced neurodegeneration is associated with accumulation of filamentous actin (F-actin) and the formation of actin-rich rods in Drosophila and mouse models of tauopathy. Importantly, modulating F-actin levels genetically leads to dramatic modification of tau-induced neurodegeneration. The ability of tau to interact with F-actin in vivo and in vitro provides a molecular mechanism for the observed phenotypes. Finally, we show that the Alzheimers disease-linked human β-amyloid protein (Aβ) synergistically enhances the ability of wild-type tau to promote alterations in the actin cytoskeleton and neurodegeneration. These findings raise the possibility that a direct interaction between tau and actin may be a critical mediator of tau-induced neurotoxicity in Alzheimers disease and related disorders.

Transgenic Models of Alzheimer's Disease: Learning from Animals

SummaryAs the scope of the problem of Alzheimer’s disease (AD) grows due to an aging population, research into the devastating condition has taken on added urgency. Rare inherited forms of AD provide insight into the molecular pathways leading to degeneration and have made possible the development of transgenic animal models. Several of these models are based on the overexpression of amyloid precursor protein (APP), presenilins, or tau to cause production and accumulation of amyloid-β into plaques or hyperphosphorylated tau into neurofibrillary tangles. Producing these characteristic neuropathological lesions in animals causes progressive neurodegeneration and in some cases similar behavioral disruptions to those seen in AD patients. Knockout models of proteins involved in AD have also been generated to explore the native functions of these genes and examine whether pathogenesis is due to loss of function or toxic gain of function in these systems. Although none of the transgenic lines models the human condition exactly, the ability to study similar pathological processes in living animals have provided numerous insights into disease mechanisms and opportunities to test therapeutic agents. This chapter reviews animal models of AD and their contributions to developing therapeutic approaches for AD.

Dendritic spine pathology and deficits in experience-dependent dendritic plasticity in R6/1 Huntington's disease transgenic mice

Huntingtons disease (HD) is a fatal neurodegenerative disease caused by a CAG repeat expansion coding for an expanded polyglutamine tract in the huntingtin protein. Dendritic abnormalities occur in human HD patients and in several transgenic mouse models of the disease. In this study, we examine, for the first time, dendrite and spine pathology in the R6/1 mouse model of HD, which mimics neurodegeneration seen in human HD. Enriching the environment of HD transgenic mice delays the onset of symptoms, so we also examine the effects of enrichment on dendrite pathology. Golgi‐impregnated tissue from symptomatic R6/1 HD mice reveals a decrease in dendritic spine density and dendritic spine length in striatal medium spiny neurons and cortical pyramidal neurons. HD also causes a specific reduction in the proportion of bifurcated dendritic spines on basal dendrites of cortical pyramidal neurons. No differences in soma size, recurving distal dendrites, or dendritic branching were observed. Although home‐cage environmental enrichment from 1 to 8 months of age increases spine density in wild‐type mice, it has no effect on the spine pathology in HD mice. These results show that dendritic spine pathology in R6/1 HD mice resembles degenerative changes seen in human HD and in other transgenic mouse models of the disease. We thus provide further evidence that the HD mutation disrupts the connectivity in both neostriatum and cerebral cortex, which will contribute to motor and cognitive disease symptoms. Furthermore, we demonstrate that Huntingtons disease pathology interferes with the normal plastic response of dendritic spines to environmental enrichment.

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 Huntingtons, Alzheimers, and Parkinsons 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 Huntingtons disease has shown that enriching the environment of transgenic animals delays the onset and slows the progression of Huntingtons 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.

Neuropathology of Alzheimer's Disease

Publisher Summary This chapter emphasizes that neurofibrillary tangles correlate tightly with anatomical regions affected clinically and correlate well numerically with both neuronal loss and severity of cognitive changes. The initial involvement by neurofibrillary tangles involves the entorhinal cortex and subsequently CA1/ subiculum of hippocampus, other limbic and high order association cortices, and ascending neurotransmitter specific systems. This parallels in a general way the initial symptoms of short-term memory impairments followed by more generalized dementia with prominent language impairments and so forth. The chapter also focuses on the neurofibrillary tangles that are most prominent in a laminar distribution matching well with putative cortical-cortical projection neurons. There is a loss of synapses, dendritic spines, and marked abnormalities of dendrite and axonal morphology that leads to disconnection phenomena and at least loss of synchrony in major cortical projections. The chapter concludes that recent evidence suggests that dystrophic neurites around plaques, which are often axonal in origin, and even morphologically changed axons, have functional impairments in axon trafficking, although the details of this are still uncertain. To this effect on neural processes, plaques clearly attract and activate astrocytic or microglial cells in their immediate vicinity, leading to a wide cascade of potentially damaging inflammatory phenomena.

Environmental Enrichment Reduces Neuronal Intranuclear Inclusion Load But Has No Effect on Messenger RNA Expression in a Mouse Model of Huntington Disease

Huntington disease (HD) is a fatal neurodegenerative disease with no effective treatment. In the R6/1 mouse model of HD, environmental enrichment delays the neurologic phenotype onset and prevents cerebral volume loss by unknown molecular mechanisms. We examined the effects of environmental enrichment on well-characterized neuropathological parameters in a mouse model of HD. We found a trend toward preservation of downregulated neurotransmitter receptors in striatum of environmentally enriched mice and assessed possible enrichment-related modifications in gene expression using microarrays. We observed similar gene expression changes in R6/1 and R6/2 transgenic mice but found no specific changes in enrichment-related microarray expression profiles in either transgenic or wild-type mice. Furthermore, specific corrections in transprotein-induced transcriptional dysregulation in R6/1 mice were not detected by microarray profiling. However, gene-specific analyses suggested that long-term environmental enrichment may beneficially modulate gene expression dysregulation. Finally, environmental enrichment significantly decreased neuronal intranuclear inclusion load, despite unaffected transgene expression levels. Thus, the therapeutic effects of environmental enrichment likely contribute to decreasing aggregated polyglutamine protein levels without exerting strong effects on gene expression.

Molecular mechanisms mediating pathological plasticity in Huntington's disease and Alzheimer's disease.

Neurodegenerative diseases such as Huntingtons disease and Alzheimers disease, although very different in etiology, share common degenerative processes. These include neuronal dysfunction, decreased neural connectivity, and disruption of cellular plasticity. Understanding the molecular mechanisms underlying the neural plasticity deficits in these devastating conditions may lead the way toward new therapeutic targets, both disease‐specific and more generalized, which can ameliorate degenerative cognitive deficits. Furthermore, investigations of ‘pathological plasticity’ in these diseases lend insight into normal brain function. This review will present evidence for altered plasticity in Huntingtons and Alzheimers diseases, relate these findings to symptomatology, and review possible causes and commonalities.

An automatic method for spine detection and spine tracking in in vivo images

The variation in dendritic branch morphology and spine density offers the scientists information about the function of the treatment to neuron disease. In particular we study the passive immunotherapy treatment on dendritic spine loss of Alzheimers disease. This paper presents an automated approach for the detection of spines and tracking of spine evolution at different time points. Most automated processing methods are developed for in vitro images. Here we investigate the possibility of automated detection and tracking of the spines on lower contrast in vivo confocal microscopy images. We propose a curvilinear structure detector to determine the medial axis of the dendritic backbone and the spines connected to the backbone. In addition, we present a maximum likelihood based technique optimized through dynamic programming to find the graph homomorphism between two image graph structures at different time points to track the growth or loss of spines. Our results show that on eight data samples, we can achieve accuracies of 91.2% for detecting spines and 78.3% for tracking spine correspondences at different time points.