Age-Dependent Degradation of Locomotion Encoding in Huntington's Disease R6/2 Model Mice.

BACKGROUND: Huntington's disease (HD) is an inherited fatal neurodegenerative disease, leading to neocortical and striatal atrophy. The commonly studied R6/2 HD transgenic mouse model displays progressive motor and cognitive deficits in parallel to major pathological changes in corticostriatal circuitry. OBJECTIVE: To study how disease progression influences striatal encoding of movement. METHODS: We chronically recorded neuronal activity in the dorsal striatum of R6/2 transgenic (Tg) mice and their age-matched nontransgenic littermate controls (WTs) during novel environment exposure, a paradigm which engages locomotion to explore the novel environment. RESULTS: Exploratory locomotion degraded with age in Tg mice as compared to WTs. We encountered fewer putative medium spiny neurons (MSNs)-striatal projection neurons, and more inhibitory interneurons-putative fast spiking interneurons (FSIs) in Tg mice as compared to WTs. MSNs from Tg mice fired less spikes in bursts without changing their firing rate, while FSIs from these mice had a lower firing rate and more of them were task-responsive as compared to WTs. Additionally, MSNs from Tg mice displayed a reduced ability to encode locomotion across age groups, likely associated with their low prevalence in Tg mice, whereas the encoding of locomotion by FSIs from Tg mice was substantially reduced solely in old Tg mice as compared to WTs. CONCLUSION: Our findings reveal an age-dependent decay in striatal information processing in transgenic mice. We propose that the ability of FSIs to compensate for the loss of MSNs by processes of recruitment and enhanced task-responsiveness diminishes with disease progression, possibly manifested in the displayed age-dependent degradation of exploratory locomotion.

Learning to classify neural activity from a mouse model of Alzheimer's disease amyloidosis versus controls.

The mechanisms underlying Alzheimer's disease (AD) onset and progression are not yet elucidated. The extent to which alterations in the activity of individual neurons of an AD model are significant, and the phase at which they can be captured, point to the intensity of the pathology and imply the stage at which it can be detected. Using a machine-learning algorithm, we present a successful cell-by-cell classification of intracellularly recorded neurons from the B6C3 APPswe/PS1dE9 AD model, versus wildtypes controls, at both a late stage and at an early stage, when the plaque pathology and behavioral deficits are absent or rare. These results suggest that the deficits present in neuronal networks of both old and young transgenic animals are large enough to be apparent at the level of individual neurons, and that the pathology could be detected in nearly any given sample, even before pathologic signs.

Abnormal Population Responses in the Somatosensory Cortex of Alzheimer's Disease Model Mice.

Alzheimer's disease (AD) is the most common form of dementia. One of the neuropathological hallmarks of AD is the accumulation of amyloid-ß plaques. Overexpression of human amyloid precursor protein in transgenic mice induces hippocampal and neocortical amyloid-ß accumulation and plaque deposition that increases with age. The impact of these effects on neuronal population responses and network activity in sensory cortex is not well understood. We used Voltage Sensitive Dye Imaging, to investigate at high spatial and temporal resolution, the sensory evoked population responses in the barrel cortex of aged transgenic (Tg) mice and of age-matched non-transgenic littermate controls (Ctrl) mice. We found that a whisker deflection evoked abnormal sensory responses in the barrel cortex of Tg mice. The response amplitude and the spatial spread of the cortical responses were significantly larger in Tg than in Ctrl mice. At the network level, spontaneous activity was less synchronized over cortical space than in Ctrl mice, however synchronization during evoked responses induced by whisker deflection did not differ between the two groups. Thus, the presence of elevated Aß and plaques may alter population responses and disrupts neural synchronization in large-scale networks, leading to abnormalities in sensory processing.

Amyloid-ß disrupts ongoing spontaneous activity in sensory cortex.

UNLABELLED: The effect of Alzheimer's disease pathology on activity of individual neocortical neurons in the intact neural network remains obscure. Ongoing spontaneous activity, which constitutes most of neocortical activity, is the background template on which further evoked-activity is superimposed. We compared in vivo intracellular recordings and local field potentials (LFP) of ongoing activity in the barrel cortex of APP/PS1 transgenic mice and age-matched littermate CONTROLS, following significant amyloid-ß (Aß) accumulation and aggregation. We found that membrane potential dynamics of neurons in Aß-burdened cortex significantly differed from those of nontransgenic CONTROLS: durations of the depolarized state were considerably shorter, and transitions to that state frequently failed. The spiking properties of APP/PS1 neurons showed alterations from those of CONTROLS: both firing patterns and spike shape were changed in the APP/PS1 group. At the population level, LFP recordings indicated reduced coherence within neuronal assemblies of APP/PS1 mice. In addition to the physiological effects, we show that morphology of neurites within the barrel cortex of the APP/PS1 model is altered compared to CONTROLS. These results are consistent with a process where the effect of Aß on spontaneous activity of individual neurons amplifies into a network effect, reducing network integrity and leading to a wide cortical dysfunction.

Neuronal response impedance mechanism implementing cooperative networks with low firing rates and µs precision.

Realizations of low firing rates in neural networks usually require globally balanced distributions among excitatory and inhibitory links, while feasibility of temporal coding is limited by neuronal millisecond precision. We show that cooperation, governing global network features, emerges through nodal properties, as opposed to link distributions. Using in vitro and in vivo experiments we demonstrate microsecond precision of neuronal response timings under low stimulation frequencies, whereas moderate frequencies result in a chaotic neuronal phase characterized by degraded precision. Above a critical stimulation frequency, which varies among neurons, response failures were found to emerge stochastically such that the neuron functions as a low pass filter, saturating the average inter-spike-interval. This intrinsic neuronal response impedance mechanism leads to cooperation on a network level, such that firing rates are suppressed toward the lowest neuronal critical frequency simultaneously with neuronal microsecond precision. Our findings open up opportunities of controlling global features of network dynamics through few nodes with extreme properties.

Pathological tau disrupts ongoing network activity.

Pathological tau leads to dementia and neurodegeneration in tauopathies, including Alzheimer's disease. It has been shown to disrupt cellular and synaptic functions, yet its effects on the function of the intact neocortical network remain unknown. Using in vivo intracellular and extracellular recordings, we measured ongoing activity of neocortical pyramidal cells during various arousal states in the rTg4510 mouse model of tauopathy, prior to significant cell death, when only a fraction of the neurons show pathological tau. In transgenic mice, membrane potential oscillations are slower during slow-wave sleep and under anesthesia. Intracellular recordings revealed that these changes are due to longer Down states and state transitions of membrane potentials. Firing rates of transgenic neurons are reduced, and firing patterns within Up states are altered, with longer latencies and inter-spike intervals. By changing the activity patterns of a subpopulation of affected neurons, pathological tau reduces the activity of the neocortical network.

Amyloid-ß alters ongoing neuronal activity and excitability in the frontal cortex.

The effects of amyloid-ß on the activity and excitability of individual neurons in the early and advanced stages of the pathological progression of Alzheimer's disease remain unknown. We used in vivo intracellular recordings to measure the ongoing and evoked activity of pyramidal neurons in the frontal cortex of APPswe/PS1dE9 transgenic mice and age-matched nontransgenic littermate controls. Evoked excitability was altered in both transgenic groups: neurons in young transgenic mice displayed hypoexcitability, whereas those in older transgenic mice displayed hyperexcitability, suggesting changes in intrinsic electrical properties of the neurons. However, the ongoing activity of neurons in both young and old transgenic groups showed signs of hyperexcitability in the depolarized state of the membrane potential. The membrane potential of neurons in old transgenic mice had an increased tendency to fail to transition to the depolarized state, and the depolarized states had shorter durations on average than did controls. This suggests a combination of both intrinsic electrical and synaptic dysfunctions as mechanisms for activity changes at later stages of the neuropathological progression.

Age-dependent effects of microglial inhibition in vivo on Alzheimer's disease neuropathology using bioactive-conjugated iron oxide nanoparticles.

BACKGROUND: Tau dysfunction is believed to be the primary cause of neurodegenerative disorders referred to as tauopathies, including Alzheimer's disease, Pick's disease, frontotemporal dementia and Parkinsonism. The role of microglial cells in the pathogenesis of tauopathies is still unclear. The activation of microglial cells has been correlated with neuroprotective effects through the release of neurotrophic factors and through clearance of cell debris and phagocytosis of cells with intracellular inclusions. In contrast, microglial activation has also been linked with chronic neuroinflammation contributing to the development of neurodegenerative diseases such as tauopathies. Microglial activation has been recently reported to precede tangle formation and the attenuation of tau pathology occurs after immunosuppression of transgenic mice. METHODS: Here we report the specific inhibition of microglial cells in rTg4510 tau-mutant mice by using fibrin γ377-395 peptide conjugated to iron oxide (γ-Fe2O3) nanoparticles of 21 ± 3.5 nm diameter. RESULTS: Stabilization of the peptide by its covalent conjugation to the γ-Fe2O3 nanoparticles significantly decreased the number of the microglial cells compared to the same concentration of the free peptide. The specific microglial inhibition induces different effects on tau pathology in an age dependent manner. The reduction of activation of microglial cells at an early age increases the number of neurons with hyperphosphorylated tau in transgenic mice. In contrast, reduction of activation of microglial cells reduced the severity of the tau pathology in older mice. The number of neurons with hyperphosphorylated tau and the number of neurons with tangles are reduced than those in animals not receiving the fibrin γ377-395 peptide-nanoparticle conjugate. CONCLUSIONS: These results demonstrate a differential effect of microglial activity on tau pathology using the fibrin γ377-395 peptide-nanoparticle conjugate, depending on age and/or stage of the neuropathological accumulation and aggregation.

Mitigation of X-ray damage in macromolecular crystallography by submicrometre line focusing.

Reported here are measurements of the penetration depth and spatial distribution of photoelectron (PE) damage excited by 18.6 keV X-ray photons in a lysozyme crystal with a vertical submicrometre line-focus beam of 0.7 µm full-width half-maximum (FWHM). The experimental results determined that the penetration depth of PEs is 5 ± 0.5 µm with a monotonically decreasing spatial distribution shape, resulting in mitigation of diffraction signal damage. This does not agree with previous theoretical predication that the mitigation of damage requires a peak of damage outside the focus. A new improved calculation provides some qualitative agreement with the experimental results, but significant errors still remain. The mitigation of radiation damage by line focusing was measured experimentally by comparing the damage in the X-ray-irradiated regions of the submicrometre focus with the large-beam case under conditions of equal exposure and equal volumes of the protein crystal, and a mitigation factor of 4.4 ± 0.4 was determined. The mitigation of radiation damage is caused by spatial separation of the dominant PE radiation-damage component from the crystal region of the line-focus beam that contributes the diffraction signal. The diffraction signal is generated by coherent scattering of incident X-rays (which introduces no damage), while the overwhelming proportion of damage is caused by PE emission as X-ray photons are absorbed.

Parallel processing of environmental recognition and locomotion in the mouse striatum.

Information processing in behaving animals has been the target of many studies in the striatum; however, its dynamics and complexity remain to a large extent unknown. Here, we chronically recorded neuronal populations in dorsal striatum as mice were exposed to a novel environment, a paradigm which enables the dissociation of locomotion and environmental recognition. The findings indicate that non-overlapping populations of striatal projection neurons-the medium spiny neurons-reliably encode locomotion and environmental identity, whereas two subpopulations of short-spike interneurons encode distinct information: the fast spiking interneurons preferentially encode locomotion whereas the second type of interneurons preferentially encodes environmental identity. The three neuronal subgroups used cell-type specific coding schemes. This study provides evidence for the existence of parallel processing circuits within the sensorimotor region of the striatum.

Interaction between amyloid-ß pathology and cortical functional columnar organization.

Amyloid-ß plaques are one of the major neuropathological features in Alzheimer's disease (AD). Plaques are found in the extracellular space of telencephalic structures, and have been shown to disrupt neuronal connectivity. Since the disruption of connectivity may underlie a number of the symptoms of AD, understanding the distribution of plaques in the neuropil in relation to the connectivity pattern of the neuronal network is crucial. We measured the distribution and clustering patterns of plaques in the vibrissae-receptive primary sensory cortex (barrel cortex), in which the cortical columnar structure is anatomically demarcated by boundaries in Layer IV. We found that the plaques are not distributed randomly with respect to the barrel structures in Layer IV; rather, they are more concentrated in the septal areas than in the barrels. This difference was not preserved in the supragranular extensions of the functional columns. When comparing the degree of clustering of plaques between primary sensory cortices, we found that the degree of plaques clustering is significantly higher in somatosensory cortex than in visual cortex, and these differences are preserved in Layers II/III. The degree of areal discontinuity is therefore correlated with the patterns of neuropathological deposits. The discontinuous anatomical structure of this area allows us to make predictions about the functional effects of plaques on specific patterns of computational disruption in the AD brain.

Functional Changes in Neocortical Activity in Huntington's Disease Model Mice: An in vivo Intracellular Study.

Studies of animal models of Huntington's disease (HD) have revealed that neocortical and neostriatal neurons of these animals in vitro exhibit a number of morphological and physiological changes, including increased input resistance and changes in neocortical synaptic inputs. We measured the functional effects of polyglutamate accumulation in neocortical neurons in R6/2 mice (8-14 weeks of age) and their age-matched non-transgenic littermates using in vivo intracellular recordings. All neurons showed spontaneous membrane potential fluctuations. The current/voltage and the firing properties of the HD neocortical neurons were significantly altered, especially in the physiologically relevant current range around and below threshold. As a result, membrane potential transitions from the Down state to Up state were evoked with smaller currents in HD neocortical neurons than in controls. The excitation-to-frequency curves of the HD mice were significantly steeper than those of controls, indicating a smaller input-output dynamic range for these neurons. Increased likelihood of Down to Up state transitions could cause pathological recruitment of corticostriatal assemblies by increasing correlated neuronal activity. We measured coherence of the in vivo intracellular recordings with simultaneously recorded electrocorticograms. We found that the peak of the coherence at <5 Hz was significantly smaller in the HD animals, indicating that the amount of coherence in the state transitions of single neurons is less correlated with global activity than non-transgenic controls. We propose that decreased correlation of neocortical inputs may be a major physiological cause underlying the errors in sensorimotor pattern generation in HD.

Spatial dependence and mitigation of radiation damage by a line-focus mini-beam.

Recently, strategies to reduce primary radiation damage have been proposed which depend on focusing X-rays to dimensions smaller than the penetration depth of excited photoelectrons. For a line focus as used here the penetration depth is the maximum distance from the irradiated region along the X-ray polarization direction that the photoelectrons penetrate. Reported here are measurements of the penetration depth and distribution of photoelectron damage excited by 18.6 keV photons in a lysozyme crystal. The experimental results showed that the penetration depth of ~17.35 keV photoelectrons is 1.5 ± 0.2 µm, which is well below previous theoretical estimates of 2.8 µm. Such a small penetration depth raises challenging technical issues in mitigating damage by line-focus mini-beams. The optimum requirements to reduce damage in large crystals by a factor of 2.0-2.5 are Gaussian line-focus mini-beams with a root-mean-square width of 0.2 µm and a distance between lines of 2.0 µm. The use of higher energy X-rays (> 26 keV) would help to alleviate some of these requirements by more than doubling the penetration depth. It was found that the X-ray dose has a significant contribution from the crystal's solvent, which initially contained 9.0%(w/v) NaCl. The 15.8 keV photoelectrons of the Cl atoms and their accompanying 2.8 keV local dose from the decay of the resulting excited atoms more than doubles the dose deposited in the X-ray-irradiated region because of the much greater cross-section and higher energy of the excited atom, degrading the mitigation of radiation damage from 2.5 to 2.0. Eliminating heavier atoms from the solvent and data collection far from heavy-atom absorption edges will significantly improve the mitigation of damage by line-focus mini-beams.

A single dose of passive immunotherapy has extended benefits on synapses and neurites in an Alzheimer's disease mouse model.

Alzheimer's disease (AD) is a neurodegenerative disorder that impairs memory and cognition. One of the major neuropathological hallmarks is the accumulation of the extracellular senile plaques that are mainly composed of amyloid beta (Abeta) protein. Plaques are associated with synapse loss, dystrophic neurites and altered neurite trajectories. A reversal of such morphological changes has been observed days after single dose anti-Abeta immunotherapy. In this study we investigated the extended effects of a single dose of passive anti-Abeta immunotherapy on morphological changes associated with senile plaques. We found that although plaque burden was not reduced 30 days after immunotherapy, there were fewer dystrophic neurites around each plaque, a recovery of synapse density, and normalization of neurite curvature near plaques. Taken together these results suggest that a single dose of immunotherapy is sufficient to cause lasting benefits to the morphology of cortical neurons, implying substantial plasticity of neural circuits despite the continued presence of plaques.

Reducing radiation damage in macromolecular crystals at synchrotron sources.

A new strategy is presented to reduce primary X-ray damage in macromolecular crystallography. The strategy is based on separating the diffracting and damaged regions as much as feasible. The source of the radiation damage to macromolecular crystals is from two primary mechanisms: the direct excitations of electrons by absorption, and inelastic scattering of the X-rays. The first produces photoelectrons with their accompanying Auger electrons from relaxation of the core hole and the second creates Compton electrons. The properties of these two mechanisms and calculations of primary X-ray damage quantify how to modify the spatial distribution of X-rays to reduce the deleterious effects of radiation damage. By focusing the incident X-rays into vertical stripes, it is estimated that the survival (the time during which quality diffraction data can be obtained with a given X-ray flux) of large crystals can be increased by at least a factor of 1.6, while for very small platelet crystals the survival can be increased by up to a factor of 14.

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 formation and elimination. To investigate effects of plaques on spine stability in vivo, we observed cortical neurons using multiphoton microscopy in a mouse model of amyloid pathology before and after extensive plaque deposition. We also observed age-matched nontransgenic mice to study normal effects of aging on spine plasticity. We found that spine density and structural 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 control animals, spine formation and elimination were equivalent over 1 hour of observation ( approximately 5% of observed spines), resulting in stable spine density. However, in aged Tg2576 mice spine elimination increased, specifically in the immediate vicinity of plaques. Spine formation 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 integrity.

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

Accumulation of amyloid-beta (Abeta) 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 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 (approximately 50%) extends approximately 20 mum from plaque edges. Importantly, a robust decrement (approximately 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.

Birdbrains could teach basal ganglia research a new song.

Recent advances in anatomical, physiological and histochemical characterization of avian basal ganglia neurons and circuitry have revealed remarkable similarities to mammalian basal ganglia. A modern revision of the avian anatomical nomenclature has now provided a common language for studying the function of the cortical-basal-ganglia-cortical loop, enabling neuroscientists to take advantage of the specialization of basal ganglia areas in various avian species. For instance, songbirds, which learn their vocal motor behavior using sensory feedback, have specialized a portion of their cortical-basal ganglia circuitry for song learning and production. This discrete circuit dedicated to a specific sensorimotor task could be especially tractable for elucidating the interwoven sensory, motor and reward signals carried by basal ganglia, and the function of these signals in task learning and execution.