Measurement of Protein Synthesis Rate in Rat by [11C]Leucine PET Imaging: Application to the TgF344-AD Model of Alzheimer's Disease.

Long-term memory requires stable protein synthesis and is altered in Alzheimer's disease (AD). This study aimed to implement a method to measure the cerebral protein synthesis rate (PSR) with [11C]leucine PET in vivo in rats and evaluate potential PSR alterations longitudinally (6, 12 and 18 months old) in the TgF344-AD rat model of AD. Wistar, wild-type (WT) and TgF344-AD rats (TG) were scanned for 60 min with [11C]leucine. Arterial blood activity was monitored online and with discrete whole blood and plasma samples by γ-counting in Wistar rats, WT (n = 4) and TG (n = 5). Unlabelled amino acids were measured in plasma. The sensitivity of [11C]leucine PET to measure alterations in PSR was assessed in Wistar rats by injection of PSR inhibitor anisomycin before PET acquisition. Anisomycin administration significantly reduced the net uptake rate constant (Kcplx) of [11C]leucine and PSR, proving the suitability of the method. For the longitudinal study, averaged population-based input functions were used to calculate PSR. We found a significant genotype effect on PSR (decrease in TG vs WT) only in the globus pallidus. This study suggests that [11C]leucine PET is sensitive enough to measure brain PSR in rat but that cross-sectional design with individual input function should be preferred.

Long-term exercise pre-training attenuates Alzheimer's disease-related pathology in a transgenic rat model of Alzheimer's disease.

Alzheimer's disease (AD) is the most common form of dementia. Despite enormous efforts around the world, there remains no effective cure for AD. This study was performed to investigate the effects of long-term exercise pretreatment on the typical pathology of AD in a novel transgenic AD rat model. Male 2-month-old animals were divided into the following groups: wild-type (WT) rats, AD rats, and AD rats with treadmill exercise pretreatment (AD-Exe). After exercise pretreatment, the Barnes maze task, passive avoidance task, and cued fear conditioning test were performed to test learning and memory function. The elevated plus maze, open field test, sucrose preference test, and forced swim test were conducted to measure anxious-depressive-like behavior. Immunofluorescence staining, Golgi staining, transmission electron microscopy, Western blot analysis, F-Jade C staining, TUNEL staining, and related assay kits were conducted to measure Aß plaques, tau hyperphosphorylation, neuronal damage, neuronal degeneration, dendritic spine density, synapses, synaptic vesicles, mitochondrial morphology, mitochondrial dynamic, oxidative stress, and neuroinflammation. Behavioral tests revealed that long-term exercise pretreatment significantly alleviated learning and memory dysfunction and anxious-depressive-like behaviors in AD animals. In addition, exercise pretreatment attenuated amyloid-ß deposition and tau hyperphosphorylation and preserved spine density, synapses, and presynaptic vesicles. Exercise also inhibited neuronal damage, neuronal apoptosis, and neuronal degeneration. Additional studies revealed the imbalance of mitochondrial dynamics was significantly inhibited by exercise pretreatment accompanied by a remarkable suppression of oxidative stress and neuroinflammation. Our findings suggest that long-term exercise pretreatment alleviated behavioral deficits and typical pathologies of the AD rat model, supporting long-term exercise pretreatment as a potential approach to delay the progression of AD.

Spatiotemporal immunolocalisation of REST in the brain of healthy ageing and Alzheimer's disease rats.

In the brain, REST (Repressor Element-1 Silencing Transcription factor) is a key regulator of neuron cell-specific gene expression. Nuclear translocation of neuronal REST has been shown to be neuroprotective in a healthy ageing context. In contrast, inability to upregulate nuclear REST is thought to leave ageing neurons vulnerable to neurodegenerative stimuli, such as Alzheimer's disease (AD) pathology. Hippocampal and cortical neurons are known to be particularly susceptible to AD-associated neurodegeneration. However, REST expression has not been extensively characterised in the healthy ageing brain. Here, we examined the spatiotemporal immunolocalisation of REST in the brains of healthy ageing wild-type Fischer-344 and transgenic Alzheimer's disease rats (TgF344-AD). Nuclear expression of REST increased from 6 months to 18 months of age in the hippocampus, frontal cortex and subiculum of wild-type rats, but not in TgF344-AD rats. No changes in REST were measured in more posterior cortical regions or in the thalamus. Interestingly, levels of the presynaptic marker synaptophysin, a known gene target of REST, were lower in CA1 hippocampal neurons of 18-month TgF344-AD rats compared to 18-month wild-types, suggesting that elevated nuclear REST may protect against synapse loss in the CA1 of 18-month wild-type rats. High REST expression in ageing wild-type rats did not, however, protect against axonal loss nor against astroglial reactivity in the hippocampus. Taken together, our data confirm that changes in nuclear REST expression are context-, age- and brain region-specific. Moreover, key brain structures involved in learning and memory display elevated REST expression in healthy ageing wild-type rats but not TgF344-AD rats.

Infection Augments Expression of Mechanosensing Piezo1 Channels in Amyloid Plaque-Reactive Astrocytes.

A defining pathophysiological hallmark of Alzheimer's disease (AD) is the amyloid plaque; an extracellular deposit of aggregated fibrillar Aß1-42 peptides. Amyloid plaques are hard, brittle structures scattered throughout the hippocampus and cerebral cortex and are thought to cause hyperphosphorylation of tau, neurofibrillary tangles, and progressive neurodegeneration. Reactive astrocytes and microglia envelop the exterior of amyloid plaques and infiltrate their inner core. Glia are highly mechanosensitive cells and can almost certainly sense the mismatch between the normally soft mechanical environment of the brain and very stiff amyloid plaques via mechanosensing ion channels. Piezo1, a non-selective cation channel, can translate extracellular mechanical forces to intracellular molecular signaling cascades through a process known as mechanotransduction. Here, we utilized an aging transgenic rat model of AD (TgF344-AD) to study expression of mechanosensing Piezo1 ion channels in amyloid plaque-reactive astrocytes. We found that Piezo1 is upregulated with age in the hippocampus and cortex of 18-month old wild-type rats. However, more striking increases in Piezo1 were measured in the hippocampus of TgF344-AD rats compared to age-matched wild-type controls. Interestingly, repeated urinary tract infections with Escherichia coli bacteria, a common comorbidity in elderly people with dementia, caused further elevations in Piezo1 channel expression in the hippocampus and cortex of TgF344-AD rats. Taken together, we report that aging and peripheral infection augment amyloid plaque-induced upregulation of mechanoresponsive ion channels, such as Piezo1, in astrocytes. Further research is required to investigate the role of astrocytic Piezo1 in the Alzheimer's brain, whether modulating channel opening will protect or exacerbate the disease state, and most importantly, if Piezo1 could prove to be a novel drug target for age-related dementia.

Neurophysiological signals as predictive translational biomarkers for Alzheimer's disease treatment: effects of donepezil on neuronal network oscillations in TgF344-AD rats.

BACKGROUND: Translational research in Alzheimer's disease (AD) pathology provides evidence that accumulation of amyloid-ß and hyperphosphorylated tau, neuropathological hallmarks of AD, is associated with complex disturbances in synaptic and neuronal function leading to oscillatory abnormalities in the neuronal networks that support memory and cognition. Accordingly, our recent study on transgenic TgF344-AD rats modeling AD showed an age-dependent reduction of stimulation-induced oscillations in the hippocampus, and disrupted long-range connectivity together with enhanced neuronal excitability in the cortex, reflected in greatly increased expression of high-voltage spindles, an epileptic absence seizure-like activity. To better understand the translational value of observed oscillatory abnormalities in these rats, we examine here the effects of donepezil, an acetylcholine esterase inhibitor clinically approved for AD treatment. METHODS: Brainstem nucleus pontis oralis stimulation-induced hippocampal oscillations were recorded under urethane anesthesia in adult (6-month-old) and aged (12-month-old) TgF344-AD and wild-type rats. Spontaneous cortical activity was monitored in a cohort of freely behaving aged rats implanted with frontal and occipital cortical electroencephalography (EEG) electrodes. RESULTS: Subcutaneous administration of donepezil significantly augmented stimulation-induced hippocampal theta oscillation in aged wild-type rats and both adult and aged TgF344-AD rats, which have been previously shown to have diminished response to nucleus pontis oralis stimulation. Moreover, in adult TgF344-AD rats, donepezil also significantly increased theta phase-gamma amplitude coupling in the hippocampus during stimulation. However, neither of these effects were significantly changed in adult wild-type rats. Under freely behaving conditions, donepezil treatment had the opposite effect on cortical oscillatory connectivity in TgF344-AD and wild-type rats, and it reduced the occurrence of high-voltage spindle activity in TgF344-AD rats. CONCLUSIONS: Together, these results imply that pharmacologically enhancing cholinergic tone with donepezil could partially reverse oscillatory abnormalities in TgF344-AD rats, which is in line with its clinical effectiveness in AD patients. Therefore, our study suggests good translational opportunities for these neurophysiological signals recorded in TgF344-AD rats, and their application could be considered in drug discovery efforts for developing therapies with disease-modifying potential.