Observations in THY-Tau22 mice that resemble behavioral and psychological signs and symptoms of dementia

Apathy-like behaviour in tau mouse models of Alzheimer's disease and frontotemporal dementia

Apathy is the most common behavioural and psychological symptom in Alzheimer's disease (AD) and other neurodegenerative diseases including frontotemporal dementia (FTD) and Parkinson's disease (PD). In patients, apathy can include symptoms of loss of motivation, initiative, and interest, listlessness, and indifference, flattening of emotions, absence of drive and passion. Researchers have later refined this to a reduction in goal direct behaviours. In animals, specific symptoms of apathy-like behaviour have been modelled including goal directed or nest-building behaviour which are seen as indicative of proxies for motivation and daily activities. In the present study a nest-building protocol was established using four different inbred mouse strains (CD1, BALB/c, C57Bl/6J, C3H) before assessing AD and FTD tau transgenic mice of Line 1 (L1) and Line 66 (L66) in this paradigm. Female mice aged 5 - 6 months were assessed in the home cage over a period of 7 days with nest-building behaviour scored by three independent experimenters at intervals of 1-, 2- and 7-days post nestlet introduction. Inbred mouse strains displayed different levels of nesting behaviour. BALB/c mice were more proficient than CD1 and C3H mice, while all strains displayed similar nest-building behaviour by day 7. In the tau mouse models, L66 presented with impaired nesting compared to wild-type on days 1 and 2 (not day 7), whereas L1 performed like wild-type on all days. Anhedonia measured in a sucrose preference test was only observed in L66. Anhedonia and low nesting scores in L66 mice are indicative of apathy-like phenotypes. Differences evident between the L1 and L66 tau transgenic mouse models are likely due to the different human tau species expressed in these mice.

Longitudinal assessment of aggression and circadian rhythms in the APPswe mouse model of Alzheimer`s disease

Agitation, which comprises verbal or physical aggression and hyperactivity, is one of the most frequent neuropsychiatric symptoms observed in patients with Alzheimer's disease (AD). It often co-occurs with dysregulated circadian rhythms. Current medications are associated with serious adverse effects, and novel therapeutics are therefore needed. Rodent models can be instrumental to provide a first signal for potential efficacy of novel drug candidates. Longitudinal data assessing the face validity of such models for AD-related agitation are largely missing. We employed telemeterized APPswe mice, a frequently used AD transgenic mouse line overexpressing the human beta-amyloid precursor protein (APP) with the Swedish KM670/671NL mutation, to study the occurrence and progression of changes in reactive aggressive behavior as well as the circadian profile of locomotor activity and body temperature. Analysis was conducted between 5 and 11 months of age, at regular 2-months intervals. The aggressivity of all mice was highest at 5 months and waned with increasing age. APPswe mice were more aggressive than WT at 5 and 7 months of age. The locomotor activity and body temperature of WT mice declined with increasing age, while that of APPswe mice remained rather constant. This genotype difference was solely evident during the active, dark phase. APPswe mice did not display a phase shift of their circadian rhythms. We conclude that the APPswe mouse line can recapitulate some of the behavioral disturbances observed in AD, including an agitation-relevant phenotype characterized by active phase hyperactivity and aggressivity. It does not recapitulate the nighttime disturbances (also characterized by hyperactivity) and the shift of circadian rhythms observed in AD patients. Therefore, the APPswe strain could be used at specific ages to model a subset of agitation-relevant behavioral problems and to test the modulatory effects of drugs.

THY-Tau22 mouse model accumulates more tauopathy at late stage of the disease in response to microglia deactivation through TREM2 deficiency

The role played by microglia has taken the center of the stage in the etiology of Alzheimer's disease (AD). Several genome-wide association studies carried out on large cohorts of patients have indeed revealed a large number of genetic susceptibility factors corresponding to genes involved in neuroinflammation and expressed specifically by microglia in the brain. Among these genes TREM2, a cell surface receptor expressed by microglia, arouses strong interest because its R47H variant confers a risk of developing AD comparable to the ε4 allele of the APOE gene. Since this discovery, a growing number of studies have therefore examined the role played by TREM2 in the evolution of amyloid plaques and neurofibrillary tangles, the two brain lesions characteristic of AD. Many studies report conflicting results, reflecting the complex nature of microglial activation in AD. Here, we investigated the impact of TREM2 deficiency in the THY-Tau22 transgenic line, a well-characterized model of tauopathy. Our study reports an increase in the severity of tauopathy lesions in mice deficient in TREM2 occurring at an advanced stage of the pathology. This exacerbation of pathology was associated with a reduction in microglial activation indicated by typical morphological features and altered expression of specific markers. However, it was not accompanied by any further changes in memory performance. Our longitudinal study confirms that a defect in microglial TREM2 signaling leads to an increase in neuronal tauopathy occurring only at late stages of the disease.

Chronic Sodium Selenate Treatment Restores Deficits in Cognition and Synaptic Plasticity in a Murine Model of Tauopathy

A major goal in diseases is identifying a potential therapeutic agent that is cost-effective and can remedy some, if not all, disease symptoms. In Alzheimer’s disease (AD), aggregation of hyperphosphorylated tau protein is one of the neuropathological hallmarks, and Tau pathology correlates better with cognitive impairments in AD patients than amyloid-β load, supporting a key role of tau-related mechanisms. Selenium is a non-metallic trace element that is incorporated in the brain into selenoproteins. Chronic treatment with sodium selenate, a non-toxic selenium compound, was recently reported to rescue behavioral phenotypes in tau mouse models. Here, we focused on the effects of chronic selenate application on synaptic transmission and synaptic plasticity in THY-Tau22 mice, a transgenic animal model of tauopathies. Three months with a supplement of sodium selenate in the drinking water (12 μg/ml) restored not only impaired neurocognitive functions but also rescued long-term depression (LTD), a major form of synaptic plasticity. Furthermore, selenate reduced the inactive demethylated catalytic subunit of protein phosphatase 2A (PP2A) in THY-Tau22 without affecting total PP2A.Our study provides evidence that chronic dietary selenate rescues functional synaptic deficits of tauopathy and identifies activation of PP2A as the putative mechanism.

Behavioural and psychological symptoms of dementia in mouse models of Alzheimer's disease-related pathology

Transgenic mouse models have been used extensively to model the cognitive impairments arising from Alzheimer's disease (AD)-related pathology. However, less is known about the relationship between AD-related pathology and the behavioural and psychological symptoms of dementia (BPSD) commonly presented by patients. This review discusses the BPSD-like behaviours recapitulated by several mouse models of AD-related pathology, including the APP/PS1, Tg2576, 3xTg-AD, 5xFAD, and APP23 models. Current evidence suggests that social withdrawal and depressive-like behaviours increase with progressive neuropathology, and increased aggression and sleep-wake disturbances are present even at early stages; however, there is no clear evidence to support increased anxiety-like behaviours, agitation (hyperactivity), or general apathy. Overall, transgenic mouse models of AD-related pathology recapitulate some of the BPSD-like behaviours associated with AD, but these behaviours vary by model. This reflects the patient population, where AD patients typically exhibit one or more BPSD, but rarely all symptoms at once. As a result, we suggest that transgenic mouse models are an important tool to investigate the pathology underlying BPSD in human AD patients.

Novel Behavioural Characteristics of Male Human P301S Mutant Tau Transgenic Mice - A Model for Tauopathy

Alzheimer's disease (AD) is a neurodegenerative disease characterised by progressive cognitive decline and the accumulation of two hallmark proteins, amyloid-beta (Aβ) and tau. Traditionally, transgenic mouse models for AD have generally focused on Aβ pathology, however, in recent years a number of tauopathy transgenic mouse models have been developed, including the TAU58/2 mouse model. These mice develop tau pathology and neurofibrillary tangles from 2 months of age and show motor impairments and alterations in the behavioural response to elevated plus maze (EPM) testing. The cognitive and social phenotype of this model has not yet been assessed comprehensively. Furthermore, the behavioural changes seen in the EPM have previously been linked to both anxiety and disinhibitory phenotypes. Thus, this study assessed 4-month-old TAU58/2 males comprehensively for disinhibitory and social behaviours, social recognition memory, and sensorimotor gating. TAU58/2 males demonstrated reduced exploration and anxiety-like behaviours but no changes to disinhibitory behaviours, reduced sociability in the social preference test and impaired acoustic startle and prepulse inhibition. Aggressive and socio-positive behaviours were not affected except a reduction in the occurrence of nosing and anogenital sniffing. Our study identified new phenotypic characteristics of young adult male TAU58/2 transgenic mice and clarified the nature of changes detected in the behavioural response of these mice to EPM testing. Social withdrawal and inappropriate social behaviours are common symptoms in both AD and FTD patients and impaired sensorimotor gating is seen in moderate-late stage AD, emphasising the relevance of the TAU58/2 model to these diseases.

Cachexia and advanced dementia

Cachexia is a complex metabolic process that is associated with several end-stage organ diseases. It is known to be also associated with advanced dementia, although the pathophysiologic mechanisms are still largely unknown. The present narrative review is aimed at presenting recent insights concerning the pathophysiology of weight loss and wasting syndrome in dementia, the putative mechanisms involved in the dysregulation of energy balance, and the interplay among the chronic clinical conditions of sarcopenia, malnutrition, and frailty in the elderly. We discuss the clinical implications of these new insights, with particular attention to the challenging question of nutritional needs in advanced dementia and the utility of tube feeding in order to optimize the management of end-stage dementia.

Negative allosteric modulation of alpha 5-containing GABAA receptors engenders antidepressant-like effects and selectively prevents age-associated hyperactivity in tau-depositing mice

RATIONALE

Associated with frank neuropathology, patients with Alzheimer's disease suffer from a host of neuropsychiatric symptoms that include depression, apathy, agitation, and aggression. Negative allosteric modulators (NAMs) of α5-containing GABA_A receptors have been suggested to be a novel target for antidepressant action. We hypothesized that pharmacological modulation of this target would engender increased motivation in stressful environments.

METHODS

We utilized electrophysiological recordings from Xenopus oocytes and behavioral measures in mice to address this hypothesis.

RESULTS

In the forced-swim assay in mice that detects antidepressant drugs, the α5β3γ2 GABA_Α receptor NAM, RY-080 produced a marked antidepressant phenotype. Another compound, PWZ-029, was characterized as an α5β3γ2 receptor NAM of lower intrinsic efficacy in electrophysiological studies in Xenopus oocytes. In contrast to RY-080, PWZ-029 was only moderately active in the forced-swim assay and the α5β3γ2 receptor antagonist, Xli-093, was not active at all. The effects of RY-080 were prevented by the non-selective benzodiazepine receptor antagonist flumazenil as well as by the selective ligands, PWZ-029 and Xli-093. These findings demonstrate that this effect of RY-080 is driven by negative allosteric modulation of α5βγ2 GABA_A receptors. RY-080 was not active in the tail-suspension test. We also demonstrated a reduction in the age-dependent hyperactivity exhibited by transgenic mice that accumulate pathological tau (rTg4510 mice) by RY-080. The decrease in hyperactivity by RY-080 was selective for the hyperactivity of the rTg4510 mice since the locomotion of control strains of mice were not significantly affected by RY-080.

CONCLUSIONS

α5βγ2 GABA_A receptor NAMs might function as a pharmacological treatment for mood, amotivational syndromes, and psychomotor agitation in patients with Alzheimer's and other neurodegenerative disorders.

Monoaminergic neuropathology in Alzheimer's disease

None of the proposed mechanisms of Alzheimer's disease (AD) fully explains the distribution patterns of the neuropathological changes at the cellular and regional levels, and their clinical correlates. One aspect of this problem lies in the complex genetic, epigenetic, and environmental landscape of AD: early-onset AD is often familial with autosomal dominant inheritance, while the vast majority of AD cases are late-onset, with the ε4 variant of the gene encoding apolipoprotein E (APOE) known to confer a 5-20 fold increased risk with partial penetrance. Mechanisms by which genetic variants and environmental factors influence the development of AD pathological changes, especially neurofibrillary degeneration, are not yet known. Here we review current knowledge of the involvement of the monoaminergic systems in AD. The changes in the serotonergic, noradrenergic, dopaminergic, histaminergic, and melatonergic systems in AD are briefly described. We also summarize the possibilities for monoamine-based treatment in AD. Besides neuropathologic AD criteria that include the noradrenergic locus coeruleus (LC), special emphasis is given to the serotonergic dorsal raphe nucleus (DRN). Both of these brainstem nuclei are among the first to be affected by tau protein abnormalities in the course of sporadic AD, causing behavioral and cognitive symptoms of variable severity. The possibility that most of the tangle-bearing neurons of the LC and DRN may release amyloid β as well as soluble monomeric or oligomeric tau protein trans-synaptically by their diffuse projections to the cerebral cortex emphasizes their selective vulnerability and warrants further investigations of the monoaminergic systems in AD.

Metabolic changes over the course of aging in a mouse model of tau deposition

Weight loss and food intake disturbances that often precede cognitive decline and diagnosis have been extensively reported in Alzheimer's disease patients. Previously, we observed that transgenic mice overexpressing tau seemed to eat more food yet weigh less than nontransgenic littermates. Thus, the present longitudinal study measured the time course of changes in metabolic state over the lifespan of the tau depositing Tg4510 mouse model of tau deposition. Although body weight was comparable to nontransgenic littermates at 2 months of age, Tg4510 mice weighed less at older ages. This was accompanied by the accumulation of tau pathology and by dramatically increased activity in all phases of the 24-hour cycle. Resting metabolic rate was also increased at 7 months of age. At 12 months near the end of the Tg4510 lifespan, there was a wasting phase, with a considerable decrease of resting metabolic rate, although hyperactivity was maintained. These diverse changes in metabolism in a mouse model of tau deposition are discussed in the context of known changes in energy metabolism in Alzheimer's disease.

Mutant Tau knock-in mice display frontotemporal dementia relevant behaviour and histopathology

Models of Tau pathology related to frontotemporal dementia (FTD) are essential to determine underlying neurodegenerative pathologies and resulting tauopathy relevant behavioural changes. However, existing models are often limited in their translational value due to Tau overexpression, and the frequent occurrence of motor deficits which prevent comprehensive behavioural assessments. In order to address these limitations, a forebrain-specific (CaMKIIα promoter), human mutated Tau (hTauP301L+R406W) knock-in mouse was generated out of the previously characterised PLB1Triple mouse, and named PLB2Tau. After confirmation of an additional hTau species (~60kDa) in forebrain samples, we identified age-dependent progressive Tau phosphorylation which coincided with the emergence of FTD relevant behavioural traits. In line with the non-cognitive symptomatology of FTD, PLB2Tau mice demonstrated early emerging (~6months) phenotypes of heightened anxiety in the elevated plus maze, depressive/apathetic behaviour in a sucrose preference test and generally reduced exploratory activity in the absence of motor impairments. Investigations of cognitive performance indicated prominent dysfunctions in semantic memory, as assessed by social transmission of food preference, and in behavioural flexibility during spatial reversal learning in a home cage corner-learning task. Spatial learning was only mildly affected and task-specific, with impairments at 12months of age in the corner learning but not in the water maze task. Electroencephalographic (EEG) investigations indicated a vigilance-stage specific loss of alpha power during wakefulness at both parietal and prefrontal recording sites, and site-specific EEG changes during non-rapid eye movement sleep (prefrontal) and rapid eye movement sleep (parietal). Further investigation of hippocampal electrophysiology conducted in slice preparations indicated a modest reduction in efficacy of synaptic transmission in the absence of altered synaptic plasticity. Together, our data demonstrate that the transgenic PLB2Tau mouse model presents with a striking behavioural and physiological face validity relevant for FTD, driven by the low level expression of mutant FTD hTau.

[The testicular microtubule-associated protein Tau: Where, when during spermatogenesis?]

The Tau protein (Tubulin Associated Unit) is a phosphoprotein of the microtubule-associated protein family (MAPs). Its role is the regulation of the microtubule polymerization. The Tau protein is naturally present in brain, heart, muscle, lung, kidney, pancreas and liver. An expression of Tau protein and RNA messengers was also highlighted in the testis that is an organ rich in microtubules. The role of microtubules is essential in the stabilization of the cellular shape and in cell divisions. In the testis, Tau protein could be involved in the division process of the spermatogenesis by acting on the microtubular dynamics in the arrangement of the spermatozoon polarity. This review synthesizes the current knowledge, the localization and the main functions of the Tau protein focused on the testis. The localization and the potential roles of the Tau protein during the spermatogenesis are discussed by emphasizing the link with the microtubular structures of seminiferous tubules.

Cholesterol 24-hydroxylase defect is implicated in memory impairments associated with Alzheimer-like Tau pathology

Alzheimer's disease (AD) is characterized by both amyloid and Tau pathologies. The amyloid component and altered cholesterol metabolism are closely linked, but the relationship between Tau pathology and cholesterol is currently unclear. Brain cholesterol is synthesized in situ and cannot cross the blood-brain barrier: to be exported from the central nervous system into the blood circuit, excess cholesterol must be converted to 24S-hydroxycholesterol by the cholesterol 24-hydroxylase encoded by the CYP46A1 gene. In AD patients, the concentration of 24S-hydroxycholesterol in the plasma and the cerebrospinal fluid are lower than in healthy controls. The THY-Tau22 mouse is a model of AD-like Tau pathology without amyloid pathology. We used this model to investigate the potential association between Tau pathology and CYP46A1 modulation. The amounts of CYP46A1 and 24S-hydroxycholesterol in the hippocampus were lower in THY-Tau22 than control mice. We used an adeno-associated virus (AAV) gene transfer strategy to increase CYP46A1 expression in order to investigate the consequences on THY-Tau22 mouse phenotype. Injection of the AAV-CYP46A1 vector into the hippocampus of THY-Tau22 mice led to CYP46A1 and 24S-hydroxycholesterol content normalization. The cognitive deficits, impaired long-term depression and spine defects that characterize the THY-Tau22 model were completely rescued, whereas Tau hyperphosphorylation and associated gliosis were unaffected. These results argue for a causal link between CYP46A1 protein content and memory impairments that result from Tau pathology. Therefore, CYP46A1 may be a relevant therapeutic target for Tauopathies and especially for AD.

Telencephalic neurocircuitry and synaptic plasticity in rodent spatial learning and memory

Spatial learning and memory in rodents represent close equivalents of human episodic declarative memory, which is especially sensitive to cerebral aging, neurodegeneration, and various neuropsychiatric disorders. Many tests and protocols are available for use in laboratory rodents, but Morris water maze and radial-arm maze remain the most widely used as well as the most valid and reliable spatial tests. Telencephalic neurocircuitry that plays functional roles in spatial learning and memory includes hippocampus, dorsal striatum and medial prefrontal cortex. Prefrontal-hippocampal circuitry comprises the major associative system in the rodent brain, and is critical for navigation in physical space, whereas interconnections between prefrontal cortex and dorsal striatum are probably more important for motivational or goal-directed aspects of spatial learning. Two major forms of synaptic plasticity, namely long-term potentiation, a lasting increase in synaptic strength between simultaneously activated neurons, and long-term depression, a decrease in synaptic strength, have been found to occur in hippocampus, dorsal striatum and medial prefrontal cortex. These and other phenomena of synaptic plasticity are probably crucial for the involvement of telencephalic neurocircuitry in spatial learning and memory. They also seem to play a role in the pathophysiology of two brain pathologies with episodic declarative memory impairments as core symptoms, namely Alzheimer's disease and schizophrenia. Further research emphasis on rodent telencephalic neurocircuitry could be relevant to more valid and reliable preclinical research on these most devastating brain disorders. This article is part of a Special Issue entitled SI: Brain and Memory.

Aging, but not tau pathology, impacts olfactory performances and somatostatin systems in THY-Tau22 mice

Somatostatin (SOM) cortical levels decline in Alzheimer's disease (AD) in correlation with cognitive impairment severity, the latter being closely related to the presence of neurofibrillary tangles. Impaired olfaction is another hallmark of AD tightly related to tau pathology in the olfactory pathways. Recent studies showed that SOM modulates olfactory processing, suggesting that alterations in SOM levels participate to olfactory deficits in AD. Herein, we first observed that human olfactory peduncle and cortex are enriched in SOM cells and fibers, in aged postmortem brains. Then, the possible link between SOM alterations and olfactory deficits was evaluated by exploring the impact of age and tau hyperphosphorylation on olfactory SOM networks and behavioral performances in THY-Tau22 mice, a tauopathy transgenic model. Distinct molecular repertoires of SOM peptide and receptors were associated to sensory or cortical olfactory processing structures. Aging mainly affected SOM neurotransmission in piriform and entorhinal cortex in wild-type mice, although olfactory performances decreased. However, no further olfactory impairment was evidenced in THY-Tau22 mice until 12 months although tau pathology early affected olfactory cortical structures. Thus, tau hyperphosphorylation per se has a limited impact on olfactory performances in THY-Tau22 mice.

Rescue of impaired late-phase long-term depression in a tau transgenic mouse model

Cognitive decline, the hallmark of Alzheimer's disease, and accompanying neuropsychiatric symptoms share dysfunctions of synaptic processes as a common cellular pathomechanism. Long-term potentiation has proven to be a sensitive tool for the "diagnosis" of such synaptic dysfunctions. Much less, however, is known about how long-term depression (LTD), an alternative mechanism for the storage of memory, is affected by Alzheimer's disease progression. Here, we demonstrate that impaired late LTD (>3 hours) in THY-Tau22 mice can be rescued by either inhibition of glycogen synthase kinase-3 (GSK3β) activity or by application of the protein-phosphatase 2A agonist selenate. In line with these findings, we observed increased phosphorylation of GSK3β at Y216 and reduced total phosphatase activity in biochemical assays of hippocampal tissue of THY-Tau22 mice. Interestingly, LTD induction and pharmacologic inhibition of GSK3β appeared to downregulate GSK3ß activity via a marked upregulation of phosphorylation at the inhibitory Ser9 residue. Our results point to alterations in phosphorylation and/or dephosphorylation homeostasis as key mechanisms underlying the deficits in LTD and hippocampus-dependent learning found in THY-Tau22 mice.