Aging impact on amyloid precursor protein neuronal trafficking

Neurons live a lifetime. Neuronal aging may increase the risk of Alzheimer's disease. How does neuronal membrane trafficking maintain synapse function during aging? In the normal aged brain, intraneuronal beta-amyloid (Aβ) accumulates without Alzheimer's disease mutations or risk variants. However, do changes with neuronal aging potentiate Aβ accumulation? We reviewed the membrane trafficking of the amyloid precursor protein in neurons and highlighted its importance in Aβ production. Importantly, we reviewed the evidence supporting the impact of aging on neuronal membrane trafficking, APP processing, and consequently Aβ production. Dissecting the molecular regulators of APP trafficking during neuronal aging is required to identify strategies to delay synaptic decline and protect from Alzheimer's disease.     

Accelerated microglial pathology is associated with Aβ plaques in mouse models of Alzheimer's disease

Microglia integrate within the neural tissue with a distinct ramified morphology through which they scan the surrounding neuronal network. Here, we used a digital tool for the quantitative morphometric characterization of fine cortical microglial structures in mice, and the changes they undergo with aging and in Alzheimer's‐like disease. We show that, compared with microglia in young mice, microglia in old mice are less ramified and possess fewer branches and fine processes along with a slightly increased proinflammatory cytokine expression. A similar microglial pathology appeared 6–12 months earlier in mouse models of Alzheimer's disease (AD), along with a significant increase in brain parenchyma lacking coverage by microglial processes. We further demonstrate that microglia near amyloid plaques acquire unique activated phenotypes with impaired process complexity. We thus show that along with a chronic proinflammatory reaction in the brain, aging causes a significant reduction in the capacity of microglia to scan their environment. This type of pathology is markedly accelerated in mouse models of AD, resulting in a severe microglial process deficiency, and possibly contributing to enhanced cognitive decline.     

n-3 Polyunsaturated fatty acids in animal models with neuroinflammation

Neuroinflammation is present in the majority of acute and chronic neurological disorders. Excess or prolonged inflammation in the brain is thought to exacerbate neuronal damage and loss. Identifying modulators of neuroinflammation is an active area of study since it may lead to novel therapies. Omega-3 polyunsaturated fatty acids (n-3 PUFA) are anti-inflammatory in many non-neural tissues; their role in neuroinflammation is less studied. This review summarizes the relationship between n-3 PUFA and brain inflammation in animal models of brain injury and aging. Evidence by and large shows protective effects of n-3 PUFA in models of sickness behavior, stroke, aging, depression, Parkinson's disease, diabetes, and cytokine- and irradiation-induced cognitive impairments. However, rigorous studies that test the direct effects of n-3 PUFA in neuroinflammation in vivo are lacking. Future research in this area is necessary to determine if, and if so which, n-3 PUFA directly target brain inflammatory pathways. n-3 PUFA bioactive metabolites may provide novel therapeutic targets for neurological disorders with a neuroinflammatory component.     

Cerebral microinfarct is emergency consequence of Alzheimer's disease: a new insight into development of neurodegenerative diseases

Imbalance of Aβ and tau protein production and clearance are the key factors among many causes of Alzheimer''s disease that leading to neurons degeneration and cognitive disorders. As a novel approach, glymphatic system quickly clear metabolic waste (especially Aβ and tau) from cerebral environment, and dysfunction of glymphatic system may relate to occurrence of Alzheimer''s disease. Microinfarct is a common histopathologic situation occurring in aging brain and leads to dramatic increase the generation of metabolic by-product after neuronal injury, hindering the operation of glymphatic system and suppress cerebral spinal fluid (CSF) and cerebral interstitial fluid (interstitial fluid, ISF) exchange. Microinfarcts destruct the integrity of microvascular and microstructural tissue, result in Aβ deposition and tau phosphorylation that form neurofibrillary tangles and associated with the cause of Alzheimer''s disease. Currently, it has been found that glymphatic system is involved in the pathological process of Alzheimer''s disease. Improving the function of glymphatic system after cerebral microinfarcts could be developed as a new approach for Alzheimer''s disease prevention and treatment. In this review, we will provide in-depth discussion on functional changes of glymphatic system after cerebral microinfarcts, further reveal pathogenesis of Alzheimer''s disease and provide a potentially more effective method for treatment of Alzheimer''s disease.     

Gray and White Matter Changes in Subjective Cognitive Impairment,Amnestic Mild Cognitive Impairment and Alzheimer's Disease: A Voxel-Based Analysis Study

Subjective cognitive impairment may be a very early at-risk period of the continuum of dementia. However, it is difficult to discriminate at-risk states from normal aging. Thus, detection of the early pathological changes in the subjective cognitive impairment period is needed. To elucidate these changes, we employed diffusion tensor imaging and volumetry analysis, and compared subjective cognitive impairment with normal, mild cognitive impairment and Alzheimer''s disease. The subjects in this study were 39 Alzheimer''s disease, 43 mild cognitive impairment, 28 subjective cognitive impairment and 41 normal controls. There were no statistically significant differences between the normal control and subjective cognitive impairment groups in all measures. Alzheimer''s disease and mild cognitive impairment had the same extent of brain atrophy and diffusion changes. These results are consistent with the hypothetical model of the dynamic biomarkers of Alzheimer''s disease.     

P2Y receptors in Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia, affecting more than 10% of people over the age of 65. Age is the greatest risk factor for AD, although a combination of genetic, lifestyle and environmental factors also contribute to disease development. Common features of AD are the formation of plaques composed of beta‐amyloid peptides (Aβ) and neuronal death in brain regions involved in learning and memory. Although Aβ is neurotoxic, the primary mechanisms by which Aβ affects AD development remain uncertain and controversial. Mouse models overexpressing amyloid precursor protein and Aβ have revealed that Aβ has potent effects on neuroinflammation and cerebral blood flow that contribute to AD progression. Therefore, it is important to consider how endogenous signalling in the brain responds to Aβ and contributes to AD pathology. In recent years, Aβ has been shown to affect ATP release from brain and blood cells and alter the expression of G protein‐coupled P2Y receptors that respond to ATP and other nucleotides. Accumulating evidence reveals a prominent role for P2Y receptors in AD pathology, including Aβ production and elimination, neuroinflammation, neuronal function and cerebral blood flow.     

The modulation of neuroinflammation by inducible nitric oxide synthase

The accumulation and propagation of misfolded proteins in the brain is a pathological hallmark shared by many neurodegenerative diseases, such as the depositions of β-amyloid and hyperphosphorylated tau proteins in Alzheimer''s disease. Initial evidence shows the role of nitric oxide synthases in the development of neurodegenerative diseases. A recent, in an exciting paper (Bourgognon et al. in Proc Natl Acad Sci USA 118, 1–11, 2021. 10.1073/pnas.2009579118) it was shown that the inducible nitric oxide synthase plays an important role in promoting oxidative and nitrergic stress leading to neuroinflammation and consequently neuronal function impairments and decline in synaptic strength in mouse prion disease. In this context, we reviewed the possible mechanisms of nitric oxide synthase in the generation of neurodegenerative diseases.     

The decline and resurgence of vascular dementia.

Arteriosclerotic narrowing of cerebral arteries was once viewed as the key to mental decline. As Alzheimer''s disease gained recognition and the concept of multi-infarct dementia achieved acceptance, vascular dementia came to be regarded as uncommon. The changing nature of cerebral vascular disease, the aging of the population and the widespread use of brain imaging techniques have brought new prominence to vascular dementia, chiefly in the form of an epidemic of "Binswanger''s disease". Growing evidence suggests that not only grey matter lesions but also white matter lesions contribute to dementia, that vascular factors commonly coexist and interact with Alzheimer changes and that Alzheimer''s disease has a vascular and potentially treatable component. Vascular dementia needs to be redefined, reappraised and reinvestigated.     

Luteinizing hormone: Evidence for direct action in the CNS

This article is part of a Special Issue “SBN 2014”.Hormonal dysfunction due to aging, especially during menopause, plays a substantial role in cognitive decline as well as the progression and development of neurodegenerative diseases. The hypothalamic–pituitary–gonadal (HPG) axis has long been implicated in changes in behavior and neuronal morphology. Most notably, estrogens have proven beneficial in the healthy brain through a host of different mechanisms. Recently, luteinizing hormone (LH) has emerged as a candidate for further investigation for its role in the CNS. The basis of this is that both LH and the LH receptor are expressed in the brain, and serum levels of LH correlate with cognitive deficits and Alzheimer's disease (AD) incidence. The study of LH in cognition and AD primarily focuses on evaluating the effects of downregulation of this peptide. This literature has shown that decreasing peripheral LH, through a variety of pharmacological interventions, reduces cognitive deficits in ovariectomy and AD models. However, few studies have researched the direct actions of LH on neurons and glial cells. Here we summarize the role of luteinizing hormone in modulating cognition, and we propose a mechanism that underlies a role for brain LH in this process.     

Functional effects of gut microbiota-derived metabolites in Alzheimer's disease

The precise causation of Alzheimer's disease (AD) is unknown, and the factors that contribute to its etiology are highly complicated. Numerous research has been conducted to investigate the potential impact of various factors to the risk of AD development or prevention against it. A growing body of evidence suggests to the importance of the gut microbiota-brain axis in the modulation of AD, which is characterized by altered gut microbiota composition. These changes can alter the production of microbial-derived metabolites, which may play a detrimental role in disease progression by being involved in cognitive decline, neurodegeneration, neuroinflammation, and accumulation of Aβ and tau. The focus of this review is on the relationship between the key metabolic products of the gut microbiota and AD pathogenesis in the brain. Understanding the action of microbial metabolites can open up new avenues for the development of AD treatment targets.     

Retracted: Aging is associated with altered inflammatory,arachidonic acid cascade,and synaptic markers,influenced by epigenetic modifications,in the human frontal cortex

Aging is a risk factor for Alzheimer's disease (AD) and is associated with cognitive decline. However, underlying molecular mechanisms of brain aging are not clear. Recent studies suggest epigenetic influences on gene expression in AD, as DNA methylation levels influence protein and mRNA expression in postmortem AD brain. We hypothesized that some of these changes occur with normal aging. To test this hypothesis, we measured markers of the arachidonic acid (AA) cascade, neuroinflammation, pro‐ and anti‐apoptosis factors, and gene specific epigenetic modifications in postmortem frontal cortex from nine middle‐aged [41 ± 1 (SEM) years] and 10 aged subjects (70 ± 3 years). The aged compared with middle‐aged brain showed elevated levels of neuroinflammatory and AA cascade markers, altered pro and anti‐apoptosis factors and loss of synaptophysin. Some of these changes correlated with promoter hypermethylation of brain derived neurotrophic factor (BDNF), cyclic AMP responsive element binding protein (CREB), and synaptophysin and hypomethylation of BCL‐2 associated X protein (BAX). These molecular alterations in aging are different from or more subtle than changes associated with AD pathology. The degree to which they are related to changes in cognition or behavior during normal aging remains to be evaluated.     

Cholesterol in brain disease: sometimes determinant and frequently implicated

Cholesterol is essential for neuronal physiology, both during development and in the adult life: as a major component of cell membranes and precursor of steroid hormones, it contributes to the regulation of ion permeability, cell shape, cell–cell interaction, and transmembrane signaling. Consistently, hereditary diseases with mutations in cholesterol‐related genes result in impaired brain function during early life. In addition, defects in brain cholesterol metabolism may contribute to neurological syndromes, such as Alzheimer's disease (AD), Huntington's disease (HD), and Parkinson's disease (PD), and even to the cognitive deficits typical of the old age. In these cases, brain cholesterol defects may be secondary to disease‐causing elements and contribute to the functional deficits by altering synaptic functions. In the first part of this review, we will describe hereditary and non‐hereditary causes of cholesterol dyshomeostasis and the relationship to brain diseases. In the second part, we will focus on the mechanisms by which perturbation of cholesterol metabolism can affect synaptic function.     

Alzheimer's Therapeutics Targeting Amyloid Beta 1–42 Oligomers I: Abeta 42 Oligomer Binding to Specific Neuronal Receptors Is Displaced by Drug Candidates That Improve Cognitive Deficits

Synaptic dysfunction and loss caused by age-dependent accumulation of synaptotoxic beta amyloid (Abeta) 1–42 oligomers is proposed to underlie cognitive decline in Alzheimer''s disease (AD). Alterations in membrane trafficking induced by Abeta oligomers mediates reduction in neuronal surface receptor expression that is the basis for inhibition of electrophysiological measures of synaptic plasticity and thus learning and memory. We have utilized phenotypic screens in mature, in vitro cultures of rat brain cells to identify small molecules which block or prevent the binding and effects of Abeta oligomers. Synthetic Abeta oligomers bind saturably to a single site on neuronal synapses and induce deficits in membrane trafficking in neuronal cultures with an EC₅₀ that corresponds to its binding affinity. The therapeutic lead compounds we have found are pharmacological antagonists of Abeta oligomers, reducing the binding of Abeta oligomers to neurons in vitro, preventing spine loss in neurons and preventing and treating oligomer-induced deficits in membrane trafficking. These molecules are highly brain penetrant and prevent and restore cognitive deficits in mouse models of Alzheimer''s disease. Counter-screening these compounds against a broad panel of potential CNS targets revealed they are highly potent and specific ligands of the sigma-2/PGRMC1 receptor. Brain concentrations of the compounds corresponding to greater than 80% receptor occupancy at the sigma-2/PGRMC1 receptor restore cognitive function in transgenic hAPP Swe/Ldn mice. These studies demonstrate that synthetic and human-derived Abeta oligomers act as pharmacologically-behaved ligands at neuronal receptors - i.e. they exhibit saturable binding to a target, they exert a functional effect related to their binding and their displacement by small molecule antagonists blocks their functional effect. The first-in-class small molecule receptor antagonists described here restore memory to normal in multiple AD models and sustain improvement long-term, representing a novel mechanism of action for disease-modifying Alzheimer''s therapeutics.     

Brain cholesterol in normal and pathological aging

Correct lipid homeostasis at the plasma membrane is essential for cell survival and performance. These are critically challenged in the aging brain. Changes in the levels of cholesterol, a major membrane component especially enriched in neurons, accompany the brain aging process. They also occur in neurodegenerative diseases. Understanding the causes and consequences of these changes is a crucial step when trying to delay the cognitive decline, which comes with age, or to design strategies to fight neurodegenerative disorders such as Alzheimer's disease. We here review work that has contributed to this understanding.     

Ensheathing glia promote increased lifespan and healthy brain aging

Glia have an emergent role in brain aging and disease. In the Drosophila melanogaster brain, ensheathing glia function as phagocytic cells and respond to acute neuronal damage, analogous to mammalian microglia. We previously reported changes in glia composition over the life of ants and fruit flies, including a decline in the relative proportion of ensheathing glia with time. How these changes influence brain health and life expectancy is unknown. Here, we show that ensheathing glia but not astrocytes decrease in number during Drosophila melanogaster brain aging. The remaining ensheathing glia display dysregulated expression of genes involved in lipid metabolism and apoptosis, which may lead to lipid droplet accumulation, cellular dysfunction, and death. Inhibition of apoptosis rescued the decline of ensheathing glia with age, improved the neuromotor performance of aged flies, and extended lifespan. Furthermore, an expanded ensheathing glia population prevented amyloid-beta accumulation in a fly model of Alzheimer's disease and delayed the premature death of the diseased animals. These findings suggest that ensheathing glia play a vital role in regulating brain health and animal longevity.     

Early Decline in Glucose Transport and Metabolism Precedes Shift to Ketogenic System in Female Aging and Alzheimer's Mouse Brain: Implication for Bioenergetic Intervention

We previously demonstrated that mitochondrial bioenergetic deficits in the female brain accompanied reproductive senescence and was accompanied by a shift from an aerobic glycolytic to a ketogenic phenotype. Herein, we investigated the relationship between systems of fuel supply, transport and mitochondrial metabolic enzyme expression/activity during aging (3–15 months) in the hippocampus of nontransgenic (nonTg) background and 3xTgAD female mice. Results indicate that during female brain aging, both nonTg and 3xTgAD brains undergo significant decline in glucose transport, as detected by FDG-microPET, between 6–9 months of age just prior to the transition into reproductive senescence. The deficit in brain metabolism was sustained thereafter. Decline in glucose transport coincided with significant decline in neuronal glucose transporter expression and hexokinase activity with a concomitant rise in phosphorylated/inactivated pyruvate dehydrogenase. Lactate utilization declined in parallel to the decline in glucose transport suggesting lactate did not serve as an alternative fuel. An adaptive response in the nonTg hippocampus was a shift to transport and utilization of ketone bodies as an alternative fuel. In the 3xTgAD brain, utilization of ketone bodies as an alternative fuel was evident at the earliest age investigated and declined thereafter. The 3xTgAD adaptive response was to substantially increase monocarboxylate transporters in neurons while decreasing their expression at the BBB and in astrocytes. Collectively, these data indicate that the earliest change in the metabolic system of the aging female brain is the decline in neuronal glucose transport and metabolism followed by decline in mitochondrial function. The adaptive shift to the ketogenic system as an alternative fuel coincided with decline in mitochondrial function. Translationally, these data provide insights into the earliest events in bioenergetic aging of the female brain and provide potential targets for preventing shifts to less efficient bioenergetic fuels and transition to the ketogenic phenotype of the Alzheimer''s brain.     

Cyclooxygenase‐1 inhibition reduces amyloid pathology and improves memory deficits in a mouse model of Alzheimer's disease

Several epidemiological and preclinical studies suggest that non‐steroidal anti‐inflammatory drugs (NSAIDs), which inhibit cyclooxygenase (COX), reduce the risk of Alzheimer's disease (AD) and can lower β‐amyloid (Aβ) production and inhibit neuroinflammation. However, follow‐up clinical trials, mostly using selective cyclooxygenase (COX)‐2 inhibitors, failed to show any beneficial effect in AD patients with mild to severe cognitive deficits. Recent data indicated that COX‐1, classically viewed as the homeostatic isoform, is localized in microglia and is actively involved in brain injury induced by pro‐inflammatory stimuli including Aβ, lipopolysaccharide, and interleukins. We hypothesized that neuroinflammation is critical for disease progression and selective COX‐1 inhibition, rather than COX‐2 inhibition, can reduce neuroinflammation and AD pathology. Here, we show that treatment of 20‐month‐old triple transgenic AD (3 × Tg‐AD) mice with the COX‐1 selective inhibitor SC‐560 improved spatial learning and memory, and reduced amyloid deposits and tau hyperphosphorylation. SC‐560 also reduced glial activation and brain expression of inflammatory markers in 3 × Tg‐AD mice, and switched the activated microglia phenotype promoting their phagocytic ability. The present findings are the first to demonstrate that selective COX‐1 inhibition reduces neuroinflammation, neuropathology, and improves cognitive function in 3 × Tg‐AD mice. Thus, selective COX‐1 inhibition should be further investigated as a potential therapeutic approach for AD.     

PKR knockout in the 5xFAD model of Alzheimer's disease reveals beneficial effects on spatial memory and brain lesions

Brain lesions in Alzheimer's disease (AD) include amyloid plaques made of Aβ peptides and neurofibrillary tangles composed of hyperphosphorylated tau protein with synaptic and neuronal loss and neuroinflammation. Aβ oligomers can trigger tau phosphorylation and neuronal alterations through activation of neuronal kinases leading to progressive cognitive decline. PKR is a ubiquitous pro‐apoptotic serine/threonine kinase, and levels of activated PKR are increased in AD brains and AD CSF. In addition, PKR regulates negatively memory formation in mice. To assess the role of PKR in an AD in vivo model, we crossed 5xFAD transgenic mice with PKR knockout (PKRKO) mice and we explored the contribution of PKR on cognition and brain lesions in the 5xFAD mouse model of AD as well as in neuron–microglia co‐cultures exposed to the innate immunity activator lipopolysaccharide (LPS). Nine‐month‐old double‐mutant mice revealed significantly improved memory consolidation with the new object location test, starmaze test, and elevated plus maze test as compared to 5xFAD mice. Brain amyloid accumulation and BACE1 levels were statistically decreased in double‐mutant mice. Apoptosis, neurodegeneration markers, and synaptic alterations were significantly reduced in double‐mutant mice as well as neuroinflammation markers such as microglial load and brain cytokine levels. Using cocultures, we found that PKR in neurons was essential for LPS microglia‐induced neuronal death. Our results demonstrate the clear involvement of PKR in abnormal spatial memory and brain lesions in the 5xFAD model and underline its interest as a target for neuroprotection in AD.