Environmental enrichment reverses Aβ pathology during pregnancy in a mouse model of Alzheimer's disease

The intricate interplay between microglia and adult neurogenesis in Alzheimer's disease

Microglia, the resident immune cells of the central nervous system, play a crucial role in regulating adult neurogenesis and contribute significantly to the pathogenesis of Alzheimer's disease (AD). Under physiological conditions, microglia support and modulate neurogenesis through the secretion of neurotrophic factors, phagocytosis of apoptotic cells, and synaptic pruning, thereby promoting the proliferation, differentiation, and survival of neural progenitor cells (NPCs). However, in AD, microglial function becomes dysregulated, leading to chronic neuroinflammation and impaired neurogenesis. This review explores the intricate interplay between microglia and adult neurogenesis in health and AD, synthesizing recent findings to provide a comprehensive overview of the current understanding of microglia-mediated regulation of adult neurogenesis. Furthermore, it highlights the potential of microglia-targeted therapies to modulate neurogenesis and offers insights into potential avenues for developing novel therapeutic interventions.

Rodent Models of Alzheimer's Disease: Past Misconceptions and Future Prospects

Alzheimer's disease (AD) is a progressive neurodegenerative disease with no effective treatments, not least due to the lack of authentic animal models. Typically, rodent models recapitulate the effects but not causes of AD, such as cholinergic neuron loss: lesioning of cholinergic neurons mimics the cognitive decline reminiscent of AD but not its neuropathology. Alternative models rely on the overexpression of genes associated with familial AD, such as amyloid precursor protein, or have genetically amplified expression of mutant tau. Yet transgenic rodent models poorly replicate the neuropathogenesis and protein overexpression patterns of sporadic AD. Seeding rodents with amyloid or tau facilitates the formation of these pathologies but cannot account for their initial accumulation. Intracerebral infusion of proinflammatory agents offer an alternative model, but these fail to replicate the cause of AD. A novel model is therefore needed, perhaps similar to those used for Parkinson's disease, namely adult wildtype rodents with neuron-specific (dopaminergic) lesions within the same vulnerable brainstem nuclei, 'the isodendritic core', which are the first to degenerate in AD. Site-selective targeting of these nuclei in adult rodents may recapitulate the initial neurodegenerative processes in AD to faithfully mimic its pathogenesis and progression, ultimately leading to presymptomatic biomarkers and preventative therapies.

Emerging Pro-neurogenic Therapeutic Strategies for Neurodegenerative Diseases: A Review of Pre-clinical and Clinical Research

The neuroscience community has largely accepted the notion that functional neurons can be generated from neural stem cells in the adult brain, especially in two brain regions: the subventricular zone of the lateral ventricles and the subgranular zone in the dentate gyrus of the hippocampus. However, impaired neurogenesis has been observed in some neurodegenerative diseases, particularly in Alzheimer's, Parkinson's, and Huntington's diseases, and also in Lewy Body dementia. Therefore, restoration of neurogenic function in neurodegenerative diseases emerges as a potential therapeutic strategy to counteract, or at least delay, disease progression. Considering this, the present study summarizes the different neuronal niches, provides a collection of the therapeutic potential of different pro-neurogenic strategies in pre-clinical and clinical research, providing details about their possible modes of action, to guide future research and clinical practice.

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.

Effects of lifespan-extending interventions on cognitive healthspan

Ageing is known to be the primary risk factor for most neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease and Huntington's disease. They are currently incurable and worsen over time, which has broad implications in the context of lifespan and healthspan extension. Adding years to life and even to physical health is suboptimal or even insufficient, if cognitive ageing is not adequately improved. In this review, we will examine how interventions that have the potential to extend lifespan in animals affect the brain, and if they would be able to thwart or delay the development of cognitive dysfunction and/or neurodegeneration. These interventions range from lifestyle (caloric restriction, physical exercise and environmental enrichment) through pharmacological (nicotinamide adenine dinucleotide precursors, resveratrol, rapamycin, metformin, spermidine and senolytics) to epigenetic reprogramming. We argue that while many of these interventions have clear potential to improve cognitive health and resilience, large-scale and long-term randomised controlled trials are needed, along with studies utilising washout periods to determine the effects of supplementation cessation, particularly in aged individuals.

Neurogenesis in aging and age-related neurodegenerative diseases

Adult neurogenesis, the process by which neurons are generated in certain areas of the adult brain, declines in an age-dependent manner and is one potential target for extending cognitive healthspan. Aging is a major risk factor for neurodegenerative diseases and, as lifespans are increasing, these health challenges are becoming more prevalent. An age-associated loss in neural stem cell number and/or activity could cause this decline in brain function, so interventions that reverse aging in stem cells might increase the human cognitive healthspan. In this review, we describe the involvement of adult neurogenesis in neurodegenerative diseases and address the molecular mechanistic aspects of neurogenesis that involve some of the key aggregation-prone proteins in the brain (i.e., tau, Aβ, α-synuclein, …). We summarize the research pertaining to interventions that increase neurogenesis and regulate known targets in aging research, such as mTOR and sirtuins. Lastly, we share our outlook on restoring the levels of neurogenesis to physiological levels in elderly individuals and those with neurodegeneration. We suggest that modulating neurogenesis represents a potential target for interventions that could help in the fight against neurodegeneration and cognitive decline.

Seed-induced Aβ deposition alters neuronal function and impairs olfaction in a mouse model of Alzheimer's disease

Alzheimer's disease (AD) is characterized by the accumulation of amyloid-β (Aβ) which ultimately forms plaques. These Aβ deposits can be induced in APP transgenic mouse models by prion-like seeding. It has been widely accepted that anosmia and hyposmia occur during the early stages of AD, even before cognitive deficits are present. In order to determine the impact of seed-induced Aβ deposits on olfaction, we performed intracerebral injections of seed-competent brain homogenate into the olfactory bulb of young pre-depositing APP transgenic mice. Remarkably, we observed a dramatic olfactory impairment in those mice. Furthermore, the number of newborn neurons as well as the activity of cells in the mitral cell layer was decreased. Notably, exposure to an enriched environment reduced Aβ seeding, vivified neurogenesis and most importantly reversed olfactory deficits. Based on our findings, we conclude that altered neuronal function as a result of induced Aβ pathology might contribute to olfactory dysfunction in AD.

MiR-29c-3p May Promote the Progression of Alzheimer's Disease through BACE1

The aim of this study was to explore the specific role of miR-29c-3p in Alzheimer's disease (AD). Animal models of AD were established by injecting streptozotocin (STZ) into mice through the lateral ventricle, while cell models of AD were induced by 10 μM β-amyloid (Aβ). We detected miR-29c-3p and β-site amyloid precursor protein cleaving enzyme 1 (BACE1) contents and measured AD cell proliferation and apoptosis. A low miR-29c-3p level and a high BACE1 level were detected in the brain tissue of AD animal models and AD cell models. Aβ-processed cells had markedly lower proliferation activity, higher apoptosis, increased phosphorylation of tau protein was over phosphorylated, but the overexpression of miR-29c-3p or the silencing of BACE1 significantly enhanced the cell proliferation activity and reduced cell apoptosis by regulating the contents of related proteins. Inhibition of miR-29c-3p or overexpression of BACE1 aggravated Aβ-induced side effects. We used Targetscan7.2 to predict the downstream target genes of miR-29c-3p. Then, we detected that there were target binding sites between miR-29c-3p and BACE1. The rescue experiment identified BACE1 as a functional target for miR-29c-3p. AD leads to decreased miR-29c-3p level and increased BACE1 level. MiR-29c-3p has specific binding sites with the 3′-untranslated region (3′-UTR) of BACE1 and thus negatively regulates the BACE1 level, thereby affecting the progression of AD.

MEHP induces alteration of mitochondrial function and inhibition of steroid biosynthesis in MA-10 mouse tumor Leydig cells

Di-(2-ethylhexyl) phthalate (DEHP) is a plasticizer that is widely used in manufacturing. Previous studies have shown that mono-(2-ethylhexyl) phthalate (MEHP), the active metabolite of DEHP, has inhibitory effects on luteinizing hormone (LH)-stimulated steroid biosynthesis by Leydig cells. The molecular mechanisms underlying its effects, however, remain unclear. In the present study, we examined the effects of MEHP on changes in mitochondrial function in relationship to reduced progesterone formation by MA-10 mouse tumor Leydig cells. Treatment of MA-10 cells with MEHP (0-300 μM for 24 h) resulted in dose-dependent inhibition of LH-stimulated progesterone biosynthesis. Biochemical analysis data revealed that the levels of the mature steroidogenic acute regulatory protein (STAR), a protein that works at the outer mitochondrial membrane to facilitate the translocation of cholesterol for steroid formation, was significantly reduced in response to MEHP exposures. MEHP also caused reductions in MA-10 cell mitochondrial membrane potential (ΔΨm) and mitochondrial respiration as evidenced by decreases in the ability of the mitochondria to consume molecular oxygen. Additionally, significant increases in the generation of mitochondrial superoxide were observed. Taken together, these results indicate that MEHP inhibits steroid formation in MA-10 cells at least in part by its effects on mitochondrial function.

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.

Molecular mechanisms of altered adult hippocampal neurogenesis in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia globally. AD is a progressive neurodegenerative disorder, eventually manifesting as severe cognitive impairment. Adult hippocampal neurogenesis (AHN) occurs throughout adulthood and plays an important role in hippocampus-dependent learning and memory. The stages of AHN, predominantly comprising the proliferation, differentiation, survival, and maturation of newborn neurons, are affected to varying degrees in AD. However, the exact molecular mechanisms remain to be elucidated. Recent evidence suggests that the molecules involved in AD pathology contribute to the compromised AHN in AD. Notably, various interventions may have common signaling pathways that, once identified, could be harnessed to enhance adult neurogenesis. This in turn could putatively rescue cognitive deficits associated with impaired neurogenesis as observed in animal models of AD. In this manuscript, we review the current knowledge concerning AHN under normal physiological and AD pathological conditions and highlight the possible role of specific molecules in AHN alteration in AD. In addition, we summarize in vivo experiments with emphasis on the effect of the activation of certain key signalings on AHN in AD rodent models. We propose that these signaling targets and corresponding interventions should be considered when developing novel therapies for AD.

Different effects of constitutive and induced microbiota modulation on microglia in a mouse model of Alzheimer's disease

It was recently revealed that gut microbiota promote amyloid-beta (Aβ) burden in mouse models of Alzheimer’s disease (AD). However, the underlying mechanisms when using either germ-free (GF) housing conditions or treatments with antibiotics (ABX) remained unknown. In this study, we show that GF and ABX-treated 5x familial AD (5xFAD) mice developed attenuated hippocampal Aβ pathology and associated neuronal loss, and thereby delayed disease-related memory deficits. While Aβ production remained unaffected in both GF and ABX-treated 5xFAD mice, we noticed in GF 5xFAD mice enhanced microglial Aβ uptake at early stages of the disease compared to ABX-treated 5xFAD mice. Furthermore, RNA-sequencing of hippocampal microglia from SPF, GF and ABX-treated 5xFAD mice revealed distinct microbiota-dependent gene expression profiles associated with phagocytosis and altered microglial activation states. Taken together, we observed that constitutive or induced microbiota modulation in 5xFAD mice differentially controls microglial Aβ clearance mechanisms preventing neurodegeneration and cognitive deficits.

Environmental enrichment restores impaired spatial memory and synaptic plasticity in prenatally stress exposed rats: The role of GABAergic neurotransmission

Evidence shows that prenatal stress negatively affects cognitive functions and activity of neuronal circuits in postnatal age. Environmental enrichment counteracts deficits induced by early life stress. We examined if behavioural function and synaptic plasticity are sensitive to prenatal stress and, how much environmental enrichment and GABAergic system impact these phenomena. Animals were exposed to noise stress during the third trimester of foetal life. Groups of the stressed animals remained intact (S-SH) or received enrichment (S-EE) from postnatal day 22 for one month. Also, two groups received either saline (S-SH-S) or bicuculline (S-SH-B). One enriched group received muscimol (S-EE-M). The control groups were intact (C-SH), enriched (C-EE), or received bicuculline (C-SH-B) or saline (C-SH-S). We assessed learning and memory and, hippocampal long-term potentiation (LTP). Serum corticosterone levels were detected as a measure of stress condition. We found that stress reduced spatial performance and suppressed LTP in the S-SH animals. Postnatal enrichment restored both spatial learning and memory and synaptic plasticity in the S-EE rats. GABAergic antagonism strengthens maze performance and LTP induction in the S-SH-B group. However, muscimol prevented the positive effects of enrichment in the S-EE-M animals. Environmental enrichment and GABAergic modulation may improve disrupted spatial performance and synaptic plasticity.

Multiparity, Brain Atrophy, and Cognitive Decline

Background

Multiparity – grand multiparity (i.e., five or more childbirths) in particular – has been reported to have an association with increased risk of Alzheimer’s disease (AD) dementia or related cognitive decline in women. However, the pathological links underlying this relationship are still unknown. This study was conducted to examine the relationships of multiparity with cerebral amyloid-beta (Aβ) deposition, brain atrophy, and white matter hyperintensities (WMHs).

Methods

In this study, total of 237 older women with 148 cognitively normal and 89 mild cognitive impairment from the Korean Brain Aging Study for Early Diagnosis and Prediction of Alzheimer’s Disease (KBASE) were included. Participants underwent clinical and neuropsychological assessments in addition to ¹¹C-labeled Pittsburgh Compound B positron emission tomography, and magnetic resonance imaging. The associations of parity with Aβ deposition, hippocampal volume, cortical volume, WMH volume and mini-mental status examination (MMSE) score were examined.

Results

Participants with grand multiparity showed significantly reduced adjusted hippocampal volume, spatial pattern of atrophy for recognition of AD volume and spatial pattern of atrophy for recognition of brain aging volume even after controlling for potential confounders. Furthermore, MMSE score was also significantly lower in this group. In contrast, grand multiparity did not show any association with global Aβ retention, Aβ positivity rate, or WMH volume, regardless of covariates.

Conclusion

Our findings suggest that grand multiparity contributes to cognitive decline or increased dementia risk in older women by aggravating amyloid-independent hippocampal or cortical atrophy.

Redox regulation of hormone sensitive lipase: Potential role in the mechanism of MEHP-induced stimulation of basal steroid synthesis in MA-10 Leydig cells

Mono-(2-ethylhexyl) phthalate (MEHP), the active metabolite of di-(2-ethylhexyl) phthalate (DEHP), is a plasticizer with endocrine disruptor activity that has been shown to stimulate basal steroid biosynthesis in Leydig cells. The mechanism by which it does so is unknown. Using MA-10 mouse tumor Leydig cells, we assessed the effects of MEHP on reactive oxygen species (ROS) levels, and on the signal transduction pathways that mobilize cholesterol. Exposure to 0-300 μM MEHP stimulated basal progesterone production in a dose-dependent manner. Progesterone stimulation was correlated with increases in the phosphorylation of hormone-sensitive lipase (HSL; aka cholesteryl ester hydrolase), which is involved in the production of free cholesterol, and of steroidogenic acute regulatory (STAR) protein expression. Co-treating MA-10 cells with MEHP and the ROS scavenger N-acetyl cysteine (NAC) blocked the activation of HSL, blunted MEHP-induced STAR, and reduced basal progesterone formation. These observations suggest that ROS generation by MEHP leads to activation of HSL and increase in STAR which, together, result in increased free-cholesterol bioavailability and progesterone formation.

The Role of Glial Cells and Synapse Loss in Mouse Models of Alzheimer's Disease

Synapse loss has detrimental effects on cellular communication, leading to network disruptions within the central nervous system (CNS) such as in Alzheimer’s disease (AD). AD is characterized by a progressive decline of memory function, cognition, neuronal and synapse loss. The two main neuropathological hallmarks are amyloid-β (Aβ) plaques and neurofibrillary tangles. In the brain of AD patients and in mouse models of AD several morphological and functional changes, such as microgliosis and astrogliosis around Aβ plaques, as well as dendritic and synaptic alterations, are associated with these lesions. In this review article, we will summarize the current literature on synapse loss in mouse models of AD and discuss current and prospective treatments for AD.