Activated Microglia in Cortical White Matter Across Cognitive Aging Trajectories

An overview on microglial origin, distribution, and phenotype in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease that is responsible for about one-third of dementia cases worldwide. It is believed that AD is initiated with the deposition of Ab plaques in the brain. Genetic studies have shown that a high number of AD risk genes are expressed by microglia, the resident macrophages of brain. Common mode of action by microglia cells is neuroinflammation and phagocytosis. Moreover, it has been discovered that inflammatory marker levels are increased in AD patients. Recent studies advocate that neuroinflammation plays a major role in AD progression. Microglia have different activation profiles depending on the region of brain and stimuli. In different activation, profile microglia can generate either pro-inflammatory or anti-inflammatory responses. Microglia defend brain cells from pathogens and respond to injuries; also, microglia can lead to neuronal death along the way. In this review, we will bring the different roles played by microglia and microglia-related genes in the progression of AD.

Immune proteins C1q and CD47 may contribute to aberrant microglia-mediated synapse loss in the aging monkey brain that is associated with cognitive impairment

Cognitive impairment in learning, memory, and executive function occurs in normal aging even in the absence of Alzheimer's disease (AD). While neurons do not degenerate in humans or monkeys free of AD, there are structural changes including synapse loss and dendritic atrophy, especially in the dorsolateral prefrontal cortex (dlPFC), and these correlate with cognitive age-related impairment. Developmental studies revealed activity-dependent neuronal properties that lead to synapse remodeling by microglia. Microglia-mediated phagocytosis that may eliminate synapses is regulated by immune "eat me" and "don't eat me" signaling proteins in an activity-dependent manner, so that less active synapses are eliminated. Whether this process contributes to age-related synapse loss remains unknown. The present study used a rhesus monkey model of normal aging to investigate the balance between the "eat me" signal, complement component C1q, and the "don't eat me" signal, transmembrane glycoprotein CD47, relative to age-related synapse loss in dlPFC Area 46. Results showed an age-related elevation of C1q and reduction of CD47 at PSD95+ synapses that is associated with cognitive impairment. Additionally, reduced neuronal CD47 RNA expression was found, indicating that aged neurons were less able to produce the protective signal CD47. Interestingly, microglia do not show the hypertrophic morphology indicative of phagocytic activity. These findings suggest that in the aging brain, changes in the balance of immunologic proteins give microglia instructions favoring synapse elimination of less active synapses, but this may occur by a process other than classic phagocytosis such as trogocytosis.

Ferroptosis of Microglia in Aging Human White Matter Injury

OBJECTIVE

Because the role of white matter (WM) degenerating microglia (DM) in remyelination failure is unclear, we sought to define the core features of this novel population of aging human microglia.

METHODS

We analyzed postmortem human brain tissue to define a population of DM in aging WM lesions. We used immunofluorescence staining and gene expression analysis to investigate molecular mechanisms related to the degeneration of DM.

RESULTS

We found that DM, which accumulated myelin debris were selectively enriched in the iron-binding protein light chain ferritin, and accumulated PLIN2-labeled lipid droplets. DM displayed lipid peroxidation injury and enhanced expression for TOM20, a mitochondrial translocase, and a sensor of oxidative stress. DM also displayed enhanced expression of the DNA fragmentation marker phospho-histone H2A.X. We identified a unique set of ferroptosis-related genes involving iron-mediated lipid dysmetabolism and oxidative stress that were preferentially expressed in WM injury relative to gray matter neurodegeneration.

INTERPRETATION

Ferroptosis appears to be a major mechanism of WM injury in Alzheimer's disease and vascular dementia. WM DM are a novel therapeutic target to potentially reduce the impact of WM injury and myelin loss on the progression of cognitive impairment. ANN NEUROL 2023;94:1048-1066.

The multifactorial nature of healthy brain ageing: Brain changes, functional decline and protective factors

As the global population faces a progressive shift towards a higher median age, understanding the mechanisms underlying healthy brain ageing has become of paramount importance for the preservation of cognitive abilities. The first part of the present review aims to provide a comprehensive look at the anatomical changes the healthy brain endures with advanced age, while also summarizing up to date findings on modifiable risk factors to support a healthy ageing process. Subsequently, we describe the typical cognitive profile displayed by healthy older adults, conceptualizing the well-established age-related decline as an impairment of four main cognitive factors and relating them to their neural substrate previously described; different cognitive trajectories displayed by typical Alzheimer's Disease patients and successful agers with a high cognitive reserve are discussed. Finally, potential effective interventions and protective strategies to promote cognitive reserve and defer cognitive decline are reviewed and proposed.

Microglial TREM2 at the Intersection of Brain Aging and Alzheimer's Disease

As resident immune cells of the brain, microglia serve pivotal roles in regulating neuronal function under both physiological and pathological conditions, including aging and the most prevalent neurodegenerative disease, Alzheimer's disease (AD). Instructed by neurons, microglia regulate synaptic function and guard brain homeostasis throughout life. Dysregulation of microglial function, however, can lead to dire consequences, including aggravated cognitive decline during aging and exacerbated neuropathology in diseases. The triggering receptor expressed on myeloid cells 2 (TREM2) is a key regulator of microglial function. Loss-of-function variants of TREM2 are associated with an increased risk of AD. TREM2 orchestrates the switch of microglial transcriptome programming that modulates microglial chemotaxis, phagocytosis, and inflammatory responses, as well as microglial regulation of synaptic function in health and disease. Intriguingly, the outcome of microglial/TREM2 function is influenced by age and the context of neuropathology. This review summarizes the rapidly growing research on TREM2 under physiological conditions and in AD, particularly highlighting the impact of TREM2 on neuronal function.

The association of astrogliosis and microglial activation with aging and Alzheimer's disease pathology in the chimpanzee brain

Aging and neurodegenerative disorders, such as Alzheimer's disease (AD), trigger an immune response known as glial activation in the brain. Recent evidence indicates species differences in inflammatory responses to AD pathology, highlighting the need for additional comparative studies to further understand human-specific neuropathologies. In the present study, we report on the occurrence of astrogliosis, microglial activation, and their relationship with age and AD-like pathology in a cohort of male and female chimpanzees (Pan troglodytes). Chimpanzees with severe astrogliosis exhibited widespread upregulation of hypertrophic astrocytes immunoreactive for glial fibrillary acidic protein (GFAP) throughout all layers of the dorsolateral prefrontal cortex and a loss of the interlaminar palisade. In addition, extreme astrogliosis was associated with increased astrocyte density in the absence of significant microglial activation and AD lesions. A shift from decreased resting to increased phagocytotic microglia occurred with aging, although proliferation was absent and no changes in astrogliosis was observed. Vascular amyloid correlated with decreased astrocyte and microglia densities, while tau lesions were associated with morphological changes in microglia and greater total glia density and glia: neuron ratio. These results further our understanding of inflammatory processes within the chimpanzee brain and provide comparative data to improve our understanding of human aging and neuropathological processes.

Integrity of Neuronal Size in the Entorhinal Cortex Is a Biological Substrate of Exceptional Cognitive Aging

Average aging is associated with a gradual decline of memory capacity. SuperAgers are humans ≥80 years of age who show exceptional episodic memory at least as good as individuals 20-30 years their junior. This study investigated whether neuronal integrity in the entorhinal cortex (ERC), an area critical for memory and selectively vulnerable to neurofibrillary degeneration, differentiated SuperAgers from cognitively healthy younger individuals, cognitively average peers ("Normal Elderly"), and individuals with amnestic mild cognitive impairment. Postmortem sections of the ERC were stained with cresyl violet to visualize neurons and immunostained with mouse monoclonal antibody PHF-1 to visualize neurofibrillary tangles. The cross-sectional area (i.e., size) of layer II and layer III/V ERC neurons were quantified. Two-thirds of total participants were female. Unbiased stereology was used to quantitate tangles in a subgroup of SuperAgers and Normal Elderly. Linear mixed-effect models were used to determine differences across groups. Quantitative measurements found that the soma size of layer II ERC neurons in postmortem brain specimens were significantly larger in SuperAgers compared with all groups (p < 0.05)-including younger individuals 20-30 years their junior (p < 0.005). SuperAgers had significantly fewer stereologically quantified Alzheimer's disease-related neurofibrillary tangles in layer II ERC than Normal Elderly (p < 0.05). This difference in tangle burden in layer II between SuperAgers and Normal Elderly suggests that tangle-bearing neurons may be prone to shrinkage during aging. The finding that SuperAgers show ERC layer II neurons that are substantially larger even compared with individuals 20-30 years younger is remarkable, suggesting that layer II ERC integrity is a biological substrate of exceptional memory in old age.SIGNIFICANCE STATEMENT Average aging is associated with a gradual decline of memory. Previous research shows that an area critical for memory, the entorhinal cortex (ERC), is susceptible to the early formation of Alzheimer's disease neuropathology, even during average (or typical) trajectories of aging. The Northwestern University SuperAging Research Program studies unique individuals known as SuperAgers, individuals ≥80 years old who show exceptional memory that is at least as good as individuals 20-30 years their junior. In this study, we show that SuperAgers harbor larger, healthier neurons in the ERC compared with their cognitively average same-aged peers, those with amnestic mild cognitive impairment, and - remarkably - even compared with individuals 20-30 years younger. We conclude that larger ERC neurons are a biological signature of the SuperAging trajectory.

Transcriptomic architecture of nuclei in the marmoset CNS

To understand the cellular composition and region-specific specialization of white matter - a disease-relevant, glia-rich tissue highly expanded in primates relative to rodents - we profiled transcriptomes of ~500,000 nuclei from 19 tissue types of the central nervous system of healthy common marmoset and mapped 87 subclusters spatially onto a 3D MRI atlas. We performed cross-species comparison, explored regulatory pathways, modeled regional intercellular communication, and surveyed cellular determinants of neurological disorders. Here, we analyze this resource and find strong spatial segregation of microglia, oligodendrocyte progenitor cells, and astrocytes. White matter glia are diverse, enriched with genes involved in stimulus-response and biomolecule modification, and predicted to interact with other resident cells more extensively than their gray matter counterparts. Conversely, gray matter glia preserve the expression of neural tube patterning genes into adulthood and share six transcription factors that restrict transcriptome complexity. A companion Callithrix jacchus Primate Cell Atlas (CjPCA) is available through https://cjpca.ninds.nih.gov .

miR-214-3p Deficiency Enhances Caspase-1-Dependent Pyroptosis of Microglia in White Matter Injury

White matter injury (WMI) is the most frequent impairment of neurodevelopment in preterm infants. Here, we report that the caspase-1 inflammasome is abundantly activated in the microglia of WMI mice and results in increased pyroptosis of microglia. Pharmacology inhibition of caspase-1 cleavage alleviated the pathogenesis of WMI mice. The expression of microRNA miR-214-3p was largely reduced in the microglia of WMI mice compared to controls. Compromised expression of miR-214-3p on microglia gives rise to the inflammasome activation and microglial pyroptosis. Treatment with miR-214-3p agomir is sufficient to relieve the white matter lesion and demyelination in WMI mice. miR-214-3p is able to bind to the 3 $\prime$ region of the NLRP-3 inflammasome compartment NEK7, preventing the transcription of NEK7 mRNA. As a result, in WMI mice, the lack of miR-214-3p leads to the accumulation of NEK7 which supports NLRP 3 inflammasome activation, microglial pyroptosis, and white matter pathogenesis.

Structural integrity of the anterior mid-cingulate cortex contributes to resilience to delirium in SuperAging

Despite its devastating clinical and societal impact, approaches to treat delirium in older adults remain elusive, making it important to identify factors that may confer resilience to this syndrome. Here, we investigated a cohort of 93 cognitively normal older patients undergoing elective surgery recruited as part of the Successful Aging after Elective Surgery study. Each participant was classified either as a SuperAger (n = 19) or typically aging older adult (n = 74) based on neuropsychological criteria, where the former was defined as those older adults whose memory function rivals that of young adults. We compared these subgroups to examine the role of preoperative memory function in the incidence and severity of postoperative delirium. We additionally investigated the association between indices of postoperative delirium symptoms and cortical thickness in functional networks implicated in SuperAging based on structural magnetic resonance imaging data that were collected preoperatively. We found that SuperAging confers the real-world benefit of resilience to delirium, as shown by lower (i.e. zero) incidence of postoperative delirium and decreased severity scores compared with typical older adults. Furthermore, greater baseline cortical thickness of the anterior mid-cingulate cortex—a key node of the brain’s salience network that is also consistently implicated in SuperAging—predicted lower postoperative delirium severity scores in all patients. Taken together, these findings suggest that baseline memory function in older adults may be a useful predictor of postoperative delirium risk and severity and that superior memory function may contribute to resilience to delirium. In particular, the integrity of the anterior mid-cingulate cortex may be a potential biomarker of resilience to delirium, pointing to this region as a potential target for preventive or therapeutic interventions designed to mitigate the risk or consequences of developing this prevalent clinical syndrome.

Potential Mechanisms Underlying Resistance to Dementia in Non-Demented Individuals with Alzheimer's Disease Neuropathology

Alzheimer’s disease (AD) is the most common form of dementia and typically characterized by the accumulation of amyloid-β plaques and tau tangles. Intriguingly, there also exists a group of elderly which do not develop dementia during their life, despite the AD neuropathology, the so-called non-demented individuals with AD neuropathology (NDAN). In this review, we provide extensive background on AD pathology and normal aging and discuss potential mechanisms that enable these NDAN individuals to remain cognitively intact. Studies presented in this review show that NDAN subjects are generally higher educated and have a larger cognitive reserve. Furthermore, enhanced neural hypertrophy could compensate for hippocampal and cingulate neural atrophy in NDAN individuals. On a cellular level, these individuals show increased levels of neural stem cells and ‘von Economo neurons’. Furthermore, in NDAN brains, binding of Aβ oligomers to synapses is prevented, resulting in decreased glial activation and reduced neuroinflammation. Overall, the evidence stated here strengthens the idea that some individuals are more resistant to AD pathology, or at least show an elongation of the asymptomatic state of the disease compared to others. Insights into the mechanisms underlying this resistance could provide new insight in understanding normal aging and AD itself. Further research should focus on factors and mechanisms that govern the NDAN cognitive resilience in order to find clues on novel biomarkers, targets, and better treatments of AD.

White matter-associated microglia: New players in brain aging and neurodegenerative diseases

There has been growing interest in brain aging and rejuvenation. It is well known that brain aging is one of the leading causes of neurodegenerative diseases, such as Alzheimer's disease, but brain aging alone can cause cognitive decline. Microglia are thought to act as 'conductors' of white matter aging by modulating diverse glial cells and phagocytosing white matter-derived myelin debris. A recent study identified a specific subpopulation of microglia in the white matter of aged mice, termed white matter-associated microglia (WAM). Additionally, senescent microglia show impaired phagocytic function and altered lipid metabolism, which cause accumulation of lipid metabolites and eventually lead to myelin sheath degeneration. These results suggest that senescent WAM could be pivotal players in axonal loss during brain aging. The aim of this review is to assess the current state of knowledge on brain aging, with an emphasis on the roles of the white matter and microglia, and suggest potential approaches for rejuvenating the aged brain.

Diabetes Mellitus: A Path to Amnesia, Personality, and Behavior Change

Simple Summary

Diabetes Mellitus (DM) is a metabolic disorder resulting from a disturbance of insulin secretion, action, or both. Hyperglycemia and overproduction of superoxide induce the development and progression of chronic complications of DM. The impact of DM and its complication on the central nervous system (CNS) such as dementia and Alzheimer’s Disease (AD) still remain obscure. In dementia, there is a gradual decline in cognitive function. The incidence of dementia increases with age, and patient become socially, physically, and mentally more vulnerable and dependent. The symptoms often emerge decades after the onset of pathophysiology, thus impairing early therapeutic intervention. Most diabetic subjects who develop dementia are above the age of 65, but diabetes may also cause an increased risk of developing dementia before 65 years. Vascular dementia is the second most common form of dementia after AD. Type 2 DM (T2DM) increases the incidence of vascular dementia (since its covers the vascular system) and AD. The functional and structural integrity of the CNS is altered in T2DM due to increased synthesis of Aβ. Additionally, hyperphosphorylation of Tau protein also results from dysregulation of various signaling cascades in T2DM, thereby causing neuronal damage and AD. There is the prospect for development of a therapy that may help prevent or halt the progress of dementia resulting from T2DM.

Abstract

Type 2 diabetes mellitus is increasingly being associated with cognition dysfunction. Dementia, including vascular dementia and Alzheimer’s Disease, is being recognized as comorbidities of this metabolic disorder. The progressive hallmarks of this cognitive dysfunction include mild impairment of cognition and cognitive decline. Dementia and mild impairment of cognition appear primarily in older patients. Studies on risk factors, neuropathology, and brain imaging have provided important suggestions for mechanisms that lie behind the development of dementia. It is a significant challenge to understand the disease processes related to diabetes that affect the brain and lead to dementia development. The connection between diabetes mellitus and dysfunction of cognition has been observed in many human and animal studies that have noted that mechanisms related to diabetes mellitus are possibly responsible for aggravating cognitive dysfunction. This article attempts to narrate the possible association between Type 2 diabetes and dementia, reviewing studies that have noted this association in vascular dementia and Alzheimer’s Disease and helping to explain the potential mechanisms behind the disease process. A Google search for “Diabetes Mellitus and Dementia” was carried out. Search was also done for “Diabetes Mellitus”, “Vascular Dementia”, and “Alzheimer’s Disease”. The literature search was done using Google Scholar, Pubmed, Embase, ScienceDirect, and MEDLINE. Keeping in mind the increasing rate of Diabetes Mellitus, it is important to establish the Type 2 diabetes’ effect on the brain and diseases of neurodegeneration. This narrative review aims to build awareness regarding the different types of dementia and their relationship with diabetes.

Characteristics of Neural Network Changes in Normal Aging and Early Dementia

To understand the mechanisms underlying preserved and impaired cognitive function in healthy aging and dementia, respectively, the spatial relationships of brain networks and mechanisms of their resilience should be understood. The hub regions of the brain, such as the multisensory integration and default mode networks, are critical for within- and between-network communication, remain well-preserved during aging, and play an essential role in compensatory processes. On the other hand, these brain hubs are the preferred sites for lesions in neurodegenerative dementias, such as Alzheimer's disease. Disrupted primary information processing networks, such as the auditory, visual, and sensorimotor networks, may lead to overactivity of the multisensory integration networks and accumulation of pathological proteins that cause dementia. At the cellular level, the brain hub regions contain many synapses and require a large amount of energy. These regions are rich in ATP-related gene expression and had high glucose metabolism as demonstrated on positron emission tomography (PET). Importantly, the number and function of mitochondria, which are the center of ATP production, decline by about 8% every 10 years. Dementia patients often have dysfunction of the ubiquitin-proteasome and autophagy-lysosome systems, which require large amounts of ATP. If there is low energy supply but the demand is high, the risk of disease can be high. Imbalance between energy supply and demand may cause accumulation of pathological proteins and play an important role in the development of dementia. This energy imbalance may explain why brain hub regions are vulnerable to damage in different dementias. Here, we review (1) the characteristics of gray matter network, white matter network, and resting state functional network changes related to resilience in healthy aging, (2) the mode of resting state functional network disruption in neurodegenerative dementia, and (3) the cellular mechanisms associated with the disruption.

Comparative neuropathology in aging primates: A perspective

While humans exhibit a significant degree of neuropathological changes associated with deficits in cognitive and memory functions during aging, non-human primates (NHP) present with more variable expressions of pathological alterations among individuals and species. As such, NHP with long life expectancy in captivity offer an opportunity to study brain senescence in the absence of the typical cellular pathology caused by age-related neurodegenerative illnesses commonly seen in humans. Age-related changes at neuronal population, single cell, and synaptic levels have been well documented in macaques and marmosets, while age-related and Alzheimer's disease-like neuropathology has been characterized in additional species including lemurs as well as great apes. We present a comparative overview of existing neuropathologic observations across the primate order, including classic age-related changes such as cell loss, amyloid deposition, amyloid angiopathy, and tau accumulation. We also review existing cellular and ultrastructural data on neuronal changes, such as dendritic attrition and spine alterations, synaptic loss and pathology, and axonal and myelin pathology, and discuss their repercussions on cellular and systems function and cognition.

Statin therapy associated with decreased neuronal injury measured by serum S100β levels in patients with acute ischemic stroke

Background

Acute ischemic strokes (AIS) are a common cause of morbidity, mortality, and disability. The serum biomarker S100β correlates with poor neurological outcomes in the setting of AIS. This study describes the impact of statin treatment on S100β levels following AIS.

Methods

This was a prospective case-control study of AIS patients compared to healthy controls. Patients were stratified into three groups: (1) AIS patients on statin therapy, (2) AIS patients not on statin therapy, and (3) healthy controls. Demographics, clinical parameters, stroke risk scores (SRS), and S100β levels were recorded for all patients.

Results

Blood pressure, lipids, and SRS scores were higher in stroke versus control patients (all P < 0.05), and lower in Group I versus II (all P < 0.05). S100β levels were higher in stroke versus nonstroke patients (P = 0.001), and lower in Group I versus II (P = 0.001). Furthermore, patients on atorvastatin showed greater S100β reductions than those on rosuvastatin therapy (P = 0.01).

Conclusion

In acute stroke patients, statins therapy correlated with reductions in the neuronal injury biomarker S100β, with greater reductions observed for atorvastatin than rosuvastatin therapy.

Neurovascular Alterations in Vascular Dementia: Emphasis on Risk Factors

Vascular dementia (VaD) constitutes the second most prevalent cause of dementia in the world after Alzheimer’s disease (AD). VaD regroups heterogeneous neurological conditions in which the decline of cognitive functions, including executive functions, is associated with structural and functional alterations in the cerebral vasculature. Among these cerebrovascular disorders, major stroke, and cerebral small vessel disease (cSVD) constitute the major risk factors for VaD. These conditions alter neurovascular functions leading to blood-brain barrier (BBB) deregulation, neurovascular coupling dysfunction, and inflammation. Accumulation of neurovascular impairments over time underlies the cognitive function decline associated with VaD. Furthermore, several vascular risk factors, such as hypertension, obesity, and diabetes have been shown to exacerbate neurovascular impairments and thus increase VaD prevalence. Importantly, air pollution constitutes an underestimated risk factor that triggers vascular dysfunction via inflammation and oxidative stress. The review summarizes the current knowledge related to the pathological mechanisms linking neurovascular impairments associated with stroke, cSVD, and vascular risk factors with a particular emphasis on air pollution, to VaD etiology and progression. Furthermore, the review discusses the major challenges to fully elucidate the pathobiology of VaD, as well as research directions to outline new therapeutic interventions.

Cognitive and brain cytokine profile of non-demented individuals with cerebral amyloid-beta deposition

BACKGROUND

Brain inflammation has been increasingly associated with early amyloid accumulation in Alzheimer's disease models; however, evidence of its occurrence in humans remains scarce. To elucidate whether amyloid deposition is associated with neuroinflammation and cognitive deficits, we studied brain inflammatory cytokine expression and cognitive decline in non-demented elderly individuals with and without cerebral amyloid-beta deposition.

METHODS

Global cognition, episodic, working, and semantic memory, perceptual speed, visuospatial ability, and longitudinal decline (5.7 ± 3.6 years) in each cognitive domain were compared between elderly individuals (66-79 years) with and without cerebral amyloid-beta deposition. The expression of 20 inflammatory cytokines was analyzed in frozen temporal, parietal, and frontal cortices and compared between older individuals with and without amyloid-beta deposition in each brain region. Correlation analyses were performed to analyze associations between amyloid-beta load, cytokine expression, and cognitive decline.

RESULTS

Individuals with cortical amyloid-beta deposition displayed deficits and a faster rate of cognitive decline in perceptual speed as compared with those individuals without amyloid-beta. This decline was positively associated with cortical amyloid-beta levels. Elderly individuals with amyloid-beta deposition had higher levels of IL-1β, IL-6, and eotaxin-3 in the temporal cortex accompanied by an increase in MCP-1 and IL-1β in the parietal cortex and a trend towards higher levels of IL-1β and MCP-1 in the frontal cortex as compared with age-matched amyloid-free individuals. Brain IL-1β levels displayed a positive association with cortical amyloid burden in each brain region. Finally, differential cytokine expression in each cortical region was associated with cognitive decline.

CONCLUSIONS

Elderly individuals with amyloid-beta neuropathology but no symptomatic manifestation of dementia, exhibit cognitive decline and increased brain cytokine expression. Such observations suggest that increased cytokine expression might be an early event in the Alzheimer's continuum.

Microglia in Aging and Alzheimer's Disease: A Comparative Species Review

Microglia are the primary immune cells of the central nervous system that help nourish and support neurons, clear debris, and respond to foreign stimuli. Greatly impacted by their environment, microglia go through rapid changes in cell shape, gene expression, and functional behavior during states of infection, trauma, and neurodegeneration. Aging also has a profound effect on microglia, leading to chronic inflammation and an increase in the brain's susceptibility to neurodegenerative processes that occur in Alzheimer's disease. Despite the scientific community's growing knowledge in the field of neuroinflammation, the overall success rate of drug treatment for age-related and neurodegenerative diseases remains incredibly low. Potential reasons for the lack of translation from animal models to the clinic include the use of a single species model, an assumption of similarity in humans, and ignoring contradictory data or information from other species. To aid in the selection of validated and predictive animal models and to bridge the translational gap, this review evaluates similarities and differences among species in microglial activation and density, morphology and phenotype, cytokine expression, phagocytosis, and production of oxidative species in aging and Alzheimer's disease.

Paucity of Entorhinal Cortex Pathology of the Alzheimer's Type in SuperAgers with Superior Memory Performance

Advancing age is typically associated with declining memory capacity and increased risk of Alzheimer's disease (AD). Markers of AD such as amyloid plaques (AP) and neurofibrillary tangles (NFTs) are commonly found in the brains of cognitively average elderly but in more limited distribution than in those at the mild cognitive impairment and dementia stages of AD. Cognitive SuperAgers are individuals over age 80 who show superior memory capacity, at a level consistent with individuals 20-30 years their junior. Using a stereological approach, the current study quantitated the presence of AD markers in the memory-associated entorhinal cortex (ERC) of seven SuperAgers compared with six age-matched cognitively average normal control individuals. Amyloid plaques and NFTs were visualized using Thioflavin-S histofluorescence, 6E10, and PHF-1 immunohistochemistry. Unbiased stereological analysis revealed significantly more NFTs in ERC in cognitively average normal controls compared with SuperAgers (P < 0.05) by a difference of ~3-fold. There were no significant differences in plaque density. To highlight relative magnitude, cases with typical amnestic dementia of AD showed nearly 100 times more entorhinal NFTs than SuperAgers. The results suggest that resistance to age-related neurofibrillary degeneration in the ERC may be one factor contributing to preserved memory in SuperAgers.

Hearing loss and dementia: radiologic and biomolecular basis of their shared characteristics. A systematic review

Dementia and hearing loss share radiologic and biologic findings that might explain their coexistence, especially in the elderly population. Brain atrophy has been observed in both conditions, as well as the presence of areas of gliosis. The brain atrophy is usually focal; it is located in the temporal lobe in patients with hearing loss, while it involves different part of brain in patients with dementia. Radiological studies have shown white matter hyperintensities (WMHs) in both conditions. WMHs have been correlated with the inability to correctly understand words in elderly persons with normal auditory thresholds and, the identification of these lesion in brain magnetic resonance imaging studies has been linked with an increased risk of developing cognitive loss. In addition to WMHs, some anatomopathological studies identified the presence of brain gliosis in the elderly's brain. The cause-effect link between hearing loss and dementia is still unknown, despite they might share some common findings. The aim of this systematic review is to analyze radiologic and biomolecular findings that these two conditions might share, identify a common pathological basis, and discuss the effects of hearing aids on prevention and treatment of cognitive decline in elderly patients with hearing loss.

Distribution of microglial phenotypes as a function of age and Alzheimer's disease neuropathology in the brains of people with Down syndrome

INTRODUCTION

Microglial cells play an important role in the development of Alzheimer's disease (AD). People with Down syndrome (DS) inevitably develop AD neuropathology (DSAD) by 40 years of age. We characterized the distribution of different microglial phenotypes in the brains of people with DS and DSAD.

METHODS

Autopsy tissue from the posterior cingulate cortex (PCC) from people with DS, DSAD, and neurotypical controls was immunostained with the microglial marker Iba1 to assess five microglia morphological types.

RESULTS

Individuals with DS have more hypertrophic microglial cells in their white matter. In the gray matter, individuals with DSAD had significantly fewer ramified microglia and more dystrophic microglia than controls and the younger individuals with DS. The DSAD group also exhibited more rod-shaped and amoeboid cells than the AD group.

DISCUSSION

Individuals with DS and DSAD show a microglial phenotype that distinguishes them from non-DS controls.

Sex-Dependent Pathology in the HPA Axis at a Sub-acute Period After Experimental Traumatic Brain Injury

Over 2.8 million traumatic brain injuries (TBIs) are reported in the United States annually, of which, over 75% are mild TBIs with diffuse axonal injury (DAI) as the primary pathology. TBI instigates a stress response that stimulates the hypothalamic-pituitary-adrenal (HPA) axis concurrently with DAI in brain regions responsible for feedback regulation. While the incidence of affective symptoms is high in both men and women, presentation is more prevalent and severe in women. Few studies have longitudinally evaluated the etiology underlying late-onset affective symptoms after mild TBI and even fewer have included females in the experimental design. In the experimental TBI model employed in this study, evidence of chronic HPA dysregulation has been reported at 2 months post-injury in male rats, with peak neuropathology in other regions of the brain at 7 days post-injury (DPI). We predicted that mechanisms leading to dysregulation of the HPA axis in male and female rats would be most evident at 7 DPI, the sub-acute time point. Young adult age-matched male and naturally cycling female Sprague Dawley rats were subjected to midline fluid percussion injury (mFPI) or sham surgery. Corticotropin releasing hormone, gliosis, and glucocorticoid receptor (GR) levels were evaluated in the hypothalamus and hippocampus, along with baseline plasma adrenocorticotropic hormone (ACTH) and adrenal gland weights. Microglial response in the paraventricular nucleus of the hypothalamus indicated mild neuroinflammation in males compared to sex-matched shams, but not females. Evidence of microglia activation in the dentate gyrus of the hippocampus was robust in both sexes compared with uninjured shams and there was evidence of a significant interaction between sex and injury regarding microglial cell count. GFAP intensity and astrocyte numbers increased as a function of injury, indicative of astrocytosis. GR protein levels were elevated 30% in the hippocampus of females in comparison to sex-matched shams. These data indicate sex-differences in sub-acute pathophysiology following DAI that precede late-onset HPA axis dysregulation. Further understanding of the etiology leading up to late-onset HPA axis dysregulation following DAI could identify targets to stabilize feedback, attenuate symptoms, and improve efficacy of rehabilitation and overall recovery.

Royal Jelly as an Intelligent Anti-Aging Agent-A Focus on Cognitive Aging and Alzheimer's Disease: A Review

The astronomical increase of the world's aged population is associated with the increased prevalence of neurodegenerative diseases, heightened disability, and extremely high costs of care. Alzheimer's Disease (AD) is a widespread, age-related, multifactorial neurodegenerative disease that has enormous social and financial drawbacks worldwide. The unsatisfactory outcomes of available AD pharmacotherapy necessitate the search for alternative natural resources that can target various the underlying mechanisms of AD pathology and reduce disease occurrence and/or progression. Royal jelly (RJ) is the main food of bee queens; it contributes to their fertility, long lifespan, and memory performance. It represents a potent nutraceutical with various pharmacological properties, and has been used in a number of preclinical studies to target AD and age-related cognitive deterioration. To understand the mechanisms through which RJ affects cognitive performance both in natural aging and AD, we reviewed the literature, elaborating on the metabolic, molecular, and cellular mechanisms that mediate its anti-AD effects. Preclinical findings revealed that RJ acts as a multidomain cognitive enhancer that can restore cognitive performance in aged and AD models. It promotes brain cell survival and function by targeting multiple adversities in the neuronal microenvironment such as inflammation, oxidative stress, mitochondrial alterations, impaired proteostasis, amyloid-β toxicity, Ca excitotoxicity, and bioenergetic challenges. Human trials using RJ in AD are limited in quantity and quality. Here, the limitations of RJ-based treatment strategies are discussed, and directions for future studies examining the effect of RJ in cognitively impaired subjects are noted.

Receptors for pro-resolving mediators are increased in Alzheimer's disease brain

Neuroinflammation is a key element of AD pathology and conceivably a result of a disturbed resolution. Resolution of inflammation is an active process which is strictly orchestrated following the acute inflammatory response after removal of the inflammatory stimuli. Acute inflammation is actively terminated by specialized pro‐resolving mediators (SPMs) thereby promoting healing and return to homeostasis. Failed resolution may contribute to persistent neuroinflammation and aggravate AD pathology. BLT1 (leukotriene B₄ receptor) and ChemR23 (chemerin receptor 23) are receptors for the SPM resolvin (Rv) E1 and are important clinical targets for ending inflammation. In AD, the levels of SPMs are decreased, and pro‐inflammatory mediators are increased. In the current study, the distribution of BLT1 and ChemR23 receptors in control brains and in AD as well as correlations with AD pathology was examined for the first time. BLT1 and ChemR23 were analyzed in different regions of post‐mortem human brain from cases with AD, early‐onset AD and mild cognitive impairment (MCI) and healthy controls, using western blotting and immunohistochemistry. BLT1 and ChemR23 were detected in neurons and glial cells in all examined regions of the human brain, with markedly higher levels in AD than in controls. The receptor levels correlated with the density of staining for the inflammation markers HLA‐DR and YKL‐40 for microglia and astrocytes, respectively, and elevated staining coincided with high Braak stages in AD. The relative staining densities of these receptors were higher in the basal forebrain, cingulate gyrus and hippocampal regions compared to the cerebellum and frontal cortex (BA46).

In conclusion, alterations in the expression of the resolution receptor BLT1 in AD have not been reported previously and the changes in both BLT1 and ChemR23 suggest a disturbed resolution pathway in several regions of the AD brain that may play a role in disease pathology.