Late-life environmental enrichment preserves short-term memory and may attenuate microglia in male APP/PS1 mice

Environmental enrichment improves cognitive function, learning, memory and anxiety-related behaviours in rodent models of dementia: Implications for future study

Environmental enrichment (EE) is a process of brain stimulation by modifying the surroundings, for example, by changing the sensory, social, or physical conditions. Rodents have been used in such experimental strategies through exposure to diverse physical, social, and exploration conditions. The present study conducted an extensive analysis of the existing literature surrounding the impact of EE on dementia rodent models. The review emphasised the two principal aspects that are very closely related to dementia: cognitive function (learning and memory) as well as psychological factors (anxiety-related behaviours such as phobias and unrealistic worries). Also highlighted were the mechanisms involved in the rodent models of dementia showing EE effects. Two search engines, PubMed and Science Direct, were used for data collection using the following keywords: environmental enrichment, dementia, rodent model, cognitive performance, and anxiety-related behaviour. Fifty-five articles were chosen depending on the criteria for inclusion and exclusion. The rodent models with dementia demonstrated improved learning and memory in the form of hampered inflammatory responses, enhanced neuronal plasticity, and sustained neuronal activity. EE housing also prevented memory impairment through the prevention of amyloid beta (Aβ) seeding formation, an early stage of Aβ plaque formation. The rodents subjected to EE were observed to present increased exploratory activity and exert less anxiety-related behaviour, compared to those in standard housing. However, some studies have proposed that EE intervention through exercise would be too mild to counteract the anxiety-related behaviour and risk assessment behaviour deficits in the Alzheimer's disease rodent model. Future studies should be conducted on old-aged rodents and the duration of EE exposure that would elicit the greatest benefits since the existing studies have been conducted on a range of ages and EE durations. In summary, EE had a considerable effect on dementia rodent models, with the most evident being improved cognitive function.

Life Experience Matters: Enrichment and Stress Can Influence the Likelihood of Developing Alzheimer's Disease via Gut Microbiome

Alzheimer's disease (AD) is a chronic neurodegenerative disease, characterized by the presence of β-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs) formed from abnormally phosphorylated tau proteins (ptau). To date, there is no cure for AD. Earlier therapeutic efforts have focused on the clinical stages of AD. Despite paramount efforts and costs, pharmaceutical interventions including antibody therapies targeting Aβ have largely failed. This highlights the need to alternate treatment strategies and a shift of focus to early pre-clinical stages. Approximately 25-40% of AD cases can be attributed to environmental factors including chronic stress. Gut dysbiosis has been associated with stress and the pathogenesis of AD and can increase both Aβ and NFTs in animal models of the disease. Both stress and enrichment have been shown to alter AD progression and gut health. Targeting stress-induced gut dysbiosis through probiotic supplementation could provide a promising intervention to delay disease progression. In this review, we discuss the effects of stress, enrichment, and gut dysbiosis in AD models and the promising evidence from probiotic intervention studies.

Mid-Adulthood Cognitive Training Improves Performance in a Spatial Task but Does Not Ameliorate Hippocampal Pathology in a Mouse Model of Alzheimer's Disease

BACKGROUND

Prior experience in early life has been shown to improve performance in aging and mice with Alzheimer's disease (AD) pathology. However, whether cognitive training at a later life stage would benefit subsequent cognition and reduce pathology in AD mice needs to be better understood.

OBJECTIVE

This study aimed to verify if behavioral training in mid-adulthood would improve subsequent cognition and reduce AD pathology and astrogliosis.

METHODS

Mixed-sex APP/PS1 and wildtype littermate mice received a battery of behavioral training, composed of spontaneous alternation in the Y-maze, novel object recognition and location tasks, and spatial training in the water maze, or handling only at 7 months of age. The impact of AD genotype and prior training on subsequent learning and memory of aforementioned tasks were assessed at 9 months.

RESULTS

APP/PS1 mice made more errors than wildtype littermates in the radial-arm water maze (RAWM) task. Prior training prevented this impairment in APP/PS1 mice. Prior training also contributed to better efficiency in finding the escape platform in both APP/PS1 mice and wildtype littermates. Short-term and long-term memory of this RAWM task, of a reversal task, and of a transfer task were comparable among APP/PS1 and wildtype mice, with or without prior training. Amyloid pathology and astrogliosis in the hippocampus were also comparable between the APP/PS1 groups.

CONCLUSION

These data suggest that cognitive training in mid-adulthood improves subsequent accuracy in AD mice and efficiency in all mice in the spatial task. Cognitive training in mid-adulthood provides no clear benefit on memory or on amyloid pathology in midlife.

The Molecular Effects of Environmental Enrichment on Alzheimer's Disease

Environmental enrichment (EE) is an environmental paradigm encompassing sensory, cognitive, and physical stimulation at a heightened level. Previous studies have reported the beneficial effects of EE in the brain, particularly in the hippocampus. EE improves cognitive function as well as ameliorates depressive and anxiety-like behaviors, making it a potentially effective neuroprotective strategy against neurodegenerative diseases such as Alzheimer's disease (AD). Here, we summarize the current evidence for EE as a neuroprotective strategy as well as the potential molecular pathways that can explain the effects of EE from a biochemical perspective using animal models. The effectiveness of EE in enhancing brain activity against neurodegeneration is explored with a view to differences present in early and late life EE exposure, with its potential application in human being discussed. We discuss EE as one of the non pharmacological approaches in preventing or delaying the onset of AD for future research.

Multisensory Stimulation Improves Cognition and Behavior in Adult Male Rats Born to LT4-treated Thyroidectomized Dams

Gestational hypothyroidism can impair development, cognition, and mood. Here, we tested whether multisensory stimulation (MS) improves the phenotype of rats born to surgically thyroidectomized (Tx) dams suboptimally treated with LT4. 8-week-old female Tx Wistar rats were kept on daily LT4 (0.7 µg/100 g body weight) dosed by gavage (serum TSH and T4 levels indicated moderate hypothyroidism) and 3 weeks later placed for breeding. MS of the litter started at age 60 days and lasted for 8 weeks. It consisted of twice per week of physical, cognitive, sensorial, and food stimuli. The offspring were assessed before and after MS for standardized tests of locomotor activity, cognition, and mood. Gestational hypothyroidism resulted in reduced litter size and increased offspring mortality. The pups exhibited delayed physical development, impairment of short- and long-term memory, and anxiety- and depressive-like behaviors. Nonetheless, ambulatory activity, social memory, and social preference were not affected by gestational hypothyroidism. MS restored short-term memory and anxiety while improving depressive like-behaviors. MS did not improve long-term memory. MS also did not modify the performance of control litter born to intact dams. We conclude that cognition and mood impairments caused by moderate gestational hypothyroidism were reversed or minimized in rats through MS. Further studies should define the molecular mechanisms involved.

The relationship between cognitive engagement and better memory in midlife

INTRODUCTION

Engagement in cognitively stimulating work and activities may slow cognitive decline and dementia. We examined the individual and combined associations of four cognitive engagement indices (educational attainment, occupational complexity, social engagement, and cognitively stimulating leisure activities) with objective and subjective cognition.

METHODS

Middle-aged adults (n = 1864) enrolled in the Healthy Brain Project completed the Cogstate Brief Battery, the Cognitive Function Instrument, and self-report questionnaires of cognitive engagement.

RESULTS

Educational attainment and leisure activity engagement were individually associated with memory performance. Participants were classified based on whether they rated highly in zero to four cognitive engagement indices. Compared to participants with no indices, participants with two or more indices performed moderately better on memory.

DISCUSSION

Results suggest that greater variety of cognitive engagement across different areas of life is related to better memory in midlife. Possible explanation for this relationship may be increased opportunity for enhancing cognitive reserve, but further investigations are required.

Reduced ccl11/eotaxin mediates the beneficial effects of environmental stimulation on the aged hippocampus

A deterioration in cognitive performance accompanies brain aging, even in the absence of neurodegenerative pathologies. However, the rate of cognitive decline can be slowed down by enhanced cognitive and sensorimotor stimulation protocols, such as environmental enrichment (EE). Understanding how EE exerts its beneficial effects on the aged brain pathophysiology can help in identifying new therapeutic targets. In this regard, the inflammatory chemokine ccl11/eotaxin-1 is a marker of aging with a strong relevance for neurodegenerative processes. Here, we demonstrate that EE in both elderly humans and aged mice decreases circulating levels of ccl11. Interfering, in mice, with the ccl11 decrease induced by EE ablated the beneficial effects on long-term memory retention, hippocampal neurogenesis, activation of local microglia and of ribosomal protein S6. On the other hand, treatment of standard-reared aged mice with an anti-ccl11 antibody resulted in EE-like improvements in spatial memory, hippocampal neurogenesis, and microglial activation. Taken together, our findings point to a decrease in circulating ccl11 concentration as a key mediator of the enhanced hippocampal function resulting from exposure to EE.

An enriched environment delays the progression from mild cognitive impairment to Alzheimer's disease in senescence-accelerated mouse prone 8 mice

A previous study demonstrated that middle-aged (5-6 months of age) senescence-accelerated mouse prone 8 (SAMP8) mice can be used as animal models of mild cognitive impairment (MCI). An enriched environment (EE) can mitigate cognitive decline and decrease the pathological changes associated with various neurodegenerative diseases. In the present study, the learning-memory abilities of SAMP8 mice during the MCI phase (5 months of age) was evaluated and neuropathological changes in the hippocampus were examined after the mice were exposed to an EE for 60 days. In the Morris water maze test, EE-exposed mice demonstrated significantly decreased escape latency and increased time spent in the target quadrant and number of platform crossings compared with control mice. Terminal deoxynucleotidyl transferase dUTP nick end labeling and Nissl staining showed that EE-exposed mice had reduced neuronal apoptosis and increased number of surviving neurons compared with control mice. Golgi staining, transmission electron microscopy, and immunohistochemical staining demonstrated that EE-exposed mice exhibited increased dendritic spine densities among secondary and tertiary apical dendrites; increases in synaptic numerical density, synaptic surface density, and expression of synaptophysin; and reduced deposition of amyloid-β (Aβ) and expression of amyloid-precursor protein (APP) in the hippocampal CA1 region compared with control mice. These results demonstrate that EE exposure effectively decreases neuronal loss and regulates neuronal synaptic plasticity by reducing the expression of APP and the deposition of Aβ in the hippocampal CA1 region, thereby mitigating cognitive decline in SAMP8 mice during the MCI phase and delaying the progression from MCI to Alzheimer's disease.

Peony seed oil ameliorates neuroinflammation-mediated cognitive deficits by suppressing microglial activation through inhibition of NF-κB pathway in presenilin 1/2 conditional double knockout mice

Chronic neuroinflammation has been shown to exert adverse influences on the pathology of Alzheimer's disease (AD), associated with the release of abundant proinflammatory mediators by excessively activated microglia, causing synaptic dysfunction, neuronal degeneration, and memory deficits. Thus, the prevention of microglial activation-associated neuroinflammation is important target for deterring neurodegenerative disorders. Peony seed oil (PSO) is a new food resource, rich in α-linolenic acid, the precursor of long chain omega-3 polyunsaturated fatty acids, including docosahexaenoic acid and eicosapentaenoic acid, which exhibit anti-inflammatory properties by altering cell membrane phospholipid fatty acid compositions, disrupting lipid rafts, and inhibiting the activation of the proinflammatory transcription factor NF-κB. However, few studies have examined the anti-neuroinflammatory effects of PSO in AD, and the relevant molecular mechanisms remain unclear. Presenilin1/2 conditional double knockout (PS cDKO) mice display obvious AD-like phenotypes, such as neuroinflammatory responses, synaptic dysfunction, and cognitive deficits. Here, we assessed the potential neuroprotective effects of PSO against neuroinflammation-mediated cognitive deficits in PS cDKO using behavioral tests and molecular biologic analyses. Our study demonstrated that PSO suppressed microglial activation and neuroinflammation through the down-regulation of proinflammatory mediators, such as inducible NOS, COX-2, IL-1β, and TNF-α, in the prefrontal cortex and hippocampus of PS cDKO mice. Further, PSO significantly lessened memory impairment by reversing hyperphosphorylated tau and synaptic proteins deficits in PS cDKO mice. Importantly, PSO's therapeutic effects on cognitive deficits were due to inhibiting neuroinflammatory responses mediated by NF-κB signaling pathway. Taken together, PSO may represent an effective dietary supplementation to restrain the neurodegenerative processes of AD.

Lifestyle-dependent microglial plasticity: training the brain guardians

Lifestyle is one of the most powerful instruments shaping mankind; the lifestyle includes many aspects of interactions with the environment, from nourishment and education to physical activity and quality of sleep. All these factors taken in complex affect neuroplasticity and define brain performance and cognitive longevity. In particular, physical exercise, exposure to enriched environment and dieting act through complex modifications of microglial cells, which change their phenotype and modulate their functional activity thus translating lifestyle events into remodelling of brain homoeostasis and reshaping neural networks ultimately enhancing neuroprotection and cognitive longevity.

How environmental enrichment balances out neuroinflammation in chronic pain and comorbid depression and anxiety disorders

Depression and anxiety commonly occur in chronic pain states and the coexistence of these diseases worsens outcomes for both disorders and may reduce treatment adherence and response. Despite the advances in the knowledge of chronic pain mechanisms, pharmacological treatment is still unsatisfactory. Research based on exposure to environmental enrichment is currently under investigation and seems to offer a promising low-cost strategy with no side effects. In this review, we discuss the role of inflammation as a major biological substrate and aetiological factor of chronic pain and depression/anxiety and report a collection of preclinical evidence of the effects and mechanisms of environmental enrichment. As microglia participates in the development of both conditions, we also discuss microglia as a potential target underlying the beneficial actions of environmental enrichment in chronic pain and comorbid depression/anxiety. We also discuss how alternative interventions under clinical guidelines, such as environmental enrichment, may improve treatment compliance and patient outcomes.

Environmental enrichment combined with fasudil treatment inhibits neuronal death in the hippocampal CA1 region and ameliorates memory deficits

Currently, no specific treatment exists to promote recovery from cognitive impairment after a stroke. Dysfunction of the actin cytoskeleton correlates well with poststroke cognitive declines, and its reorganization requires proper regulation of Rho-associated kinase (ROCK) proteins. Fasudil downregulates ROCK activation and protects neurons against cytoskeleton collapse in the acute phase after stroke. An enriched environment can reduce poststroke cognitive impairment. However, the efficacy of environmental enrichment combined with fasudil treatment remains poorly understood. A photothrombotic stroke model was established in 6-week-old male C57BL/6 mice. Twenty-four hours after modeling, these animals were intraperitoneally administered fasudil (10 mg/kg) once daily for 14 successive days and/or provided with environmental enrichment for 21 successive days. After exposure to environmental enrichment combined with fasudil treatment, the number of neurons in the hippocampal CA1 region increased significantly, the expression and proportion of p-cofilin in the hippocampus decreased, and the distribution of F-actin in the hippocampal CA1 region increased significantly. Furthermore, the performance of mouse stroke models in the tail suspension test and step-through passive avoidance test improved significantly. These findings suggest that environmental enrichment combined with fasudil treatment can ameliorate memory dysfunction through inhibition of the hippocampal ROCK/cofilin pathway, alteration of the dynamic distribution of F-actin, and inhibition of neuronal death in the hippocampal CA1 region. The efficacy of environmental enrichment combined with fasudil treatment was superior to that of fasudil treatment alone. This study was approved by the Animal Ethics Committee of Fudan University of China (approval No. 2019-Huashan Hospital JS-139) on February 20, 2019.

Regional differences in beta amyloid plaque deposition and variable response to midlife environmental enrichment in the cortex of APP/PS1 mice

Environmentally enriched housing conditions can increase performance on cognitive tasks in APP/PS1 mice; however, the potential effects of environmental enrichment (EE) on disease modification in terms of pathological change are inconclusive. We hypothesized that previous contrasting findings may be attributable to regional differences in susceptibility to amyloid beta (Aβ) plaque deposition in cortical regions that are functionally associated with EE. We characterized fibrillar plaque deposition in 6, 12, and 18-22 months old APP/PS1 mice in the prefrontal (PFC), somatosensory (SS2), and primary motor cortex (M1). We found a significant increase in plaque load between 6 and 12 months in all regions. In animals over 12 months, only the PFC region continued to significantly accumulate plaques. Additionally, 12 months old animals subjected to 6 months of EE showed improved spatial navigation and had significantly fewer plaques in M1 and SS2, but not in the PFC. These findings suggest that the PFC region is selectively susceptible to Aβ deposition and less responsive to the attenuating effects of EE. In contrast, M1 and SS2 regions plateau with respect to Aβ deposition by 12 months of age and are susceptible to amyloid pathology modification by midlife EE.

Lifestyle modifications with anti-neuroinflammatory benefits in the aging population

Aging-associated microglial priming results in the potential for an exaggerated neuroinflammatory response to a subsequent inflammatory challenge in regions of the brain known to support learning and memory. This excessive neuroinflammation in the aging brain is known to occur following a variety of peripheral insults, including infection and surgery, where it has been associated with precipitous declines in cognition and memory. As the average lifespan increases worldwide, identifying interventions to prevent and treat aging-associated excessive neuroinflammation and ensuing cognitive impairments is of critical importance. Lifestyle has emerged as a potential non-pharmacological target in this endeavor. Here, we review important and recent preclinical and clinical literature demonstrating the anti-inflammatory effects of lifestyle modifications such as exercise, diet, and environmental enrichment in the context of aging and memory. Importantly, we focus on research indicating that these lifestyle modifications do not need to be lifelong, suggesting that such interventions may be efficacious in the prevention and treatment of aging- and neuroinflammation-associated cognitive impairment, even when initiated in older age.

Microglia Demonstrate Local Mixed Inflammation and a Defined Morphological Shift in an APP/PS1 Mouse Model

Background: Microglia are traditionally described as the immune cells of the brain and have an inflammatory role in Alzheimer’s disease (AD). Microglial morphological and phenotypic shifts in AD have not been fully characterized; however, microglia are often described as either pro- or anti-inflammatory. Objective: To determine microglial if microglial morphology and phenotype changes with disease status. Methods: This study observed morphology through Iba1 immunohistochemistry on tissue sections encompassing the primary motor cortex and somatosensory barrel fields. Immunohistochemistry for pro-inflammatory markers: CD14 and CD40; and anti-inflammatory markers: CD16 and TREM2, was performed at 3, 6, and 12 months of age which correlated with pre-plaque, onset, and significant plaque load in APP/PS1 brains (n = 6) and compared to age-matched littermate controls (n = 6). Results: Microglia demonstrated a defined morphological shift with time. Deramified morphologies increased in the APP/PS1, at both 6 months (p < 0.0001) and 12 months (p < 0.0001). At 12 months, there were significantly lower numbers of ramified microglia (p < 0.001). Results indicated that microglia have a heterogenic marker immunoreactivity as CD16, TREM2, and CD40 were associated with an activated morphology at the same time points. All inflammatory markers were significantly upregulated at 12 months in the APP/PS1 mice (TREM2 (F (2,30)  = 10.75, p = 0.0003), CD40 (F (2,30)  = 15.86, p < 0.0001), CD14 (F (2,30)  = 6.84, p = 0.0036), and CD16 (F (2,30)  = 3.026, p = 0.0635)). Conclusion: Our data indicate that pro- and anti-inflammatory factors of microglia occur in APP/PS1 mice.

Pathological Links between Traumatic Brain Injury and Dementia: Australian Pre-Clinical Research

Traumatic brain injury (TBI) can cause persistent cognitive changes and ongoing neurodegeneration in the brain. Accumulating epidemiological and pathological evidence implicates TBI in the development of Alzheimer's disease, the most common cause of dementia. Further, the TBI-induced form of dementia, called chronic traumatic encephalopathy, shares many pathological hallmarks present in multiple different diseases which cause dementia. The inflammatory and neuritic responses to TBI and dementia overlap, indicating that they may share common pathological mechanisms and that TBI may ultimately cause a pathological cascade culminating in the development of dementia. This review explores Australian pre-clinical research investigating the pathological links between TBI and dementia.