The late neuropathological effects of traumatic brain injury have yet to be fully elucidated, particularly with respect to community-based cohorts. To contribute to this critical gap in knowledge, we designed a multimodal neuropathological study, integrating traditional and quantitative approaches to detect pathologic changes in 532 consecutive brain autopsies from participants in the Adult Changes in Thought (ACT) study. Diagnostic evaluation including assessment for chronic traumatic encephalopathy (CTE) and quantitative immunoassay-based methods were deployed to examine levels of pathological (hyperphosphorylated) tau (pTau) and amyloid (A) β in brains from ACT participants with ( n = 107) and without ( n = 425) history of remote TBI with loss of consciousness (w/LOC). Further neuropathological assessments included immunohistochemistry for α-synuclein and phospho-TDP-43 pathology and astro- (GFAP) and micro- (Iba1) gliosis, mass spectrometry analysis of free radical injury, and gene expression evaluation (RNA sequencing) in a smaller sub-cohort of matched samples (49 cases with TBI and 49 non-exposed matched controls). Out of 532 cases, only 3 (0.6%–none with TBI w/LOC history) showed evidence of the neuropathologic signature of chronic traumatic encephalopathy (CTE). Across the entire cohort, the levels of pTau and Aβ showed expected differences for brain region (higher levels in temporal cortex), neuropathological diagnosis (higher in participants with Alzheimer's disease), and APOE genotype (higher in participants with one or more APOE ε4 allele). However, no differences in PHF-tau or Aβ 1−42 were identified by Histelide with respect to the history of TBI w/LOC. In a subset of TBI cases with more carefully matched control samples and more extensive analysis, those with TBI w/LOC history had higher levels of hippocampal pTau but no significant differences in Aβ, α-synuclein, pTDP-43, GFAP, Iba1, or free radical injury. RNA-sequencing also did not reveal significant gene expression associated with any measure of TBI exposure. Combined, these findings suggest long term neuropathological changes associated with TBI w/LOC may be subtle, involve non-traditional pathways of neurotoxicity and neurodegeneration, and/or differ from those in autopsy cohorts specifically selected for neurotrauma exposure.

In the central nervous system, immunologic surveillance and response are carried out, in large part, by microglia. These resident macrophages derive from myeloid precursors in the embryonic yolk sac, migrating to the brain and eventually populating local tissue prior to blood-brain barrier formation. Preserved for the duration of lifespan, microglia serve the host as more than just a central arm of innate immunity, also contributing significantly to the development and maintenance of neurons and neural networks, as well as neuroregeneration. The critical nature of these varied functions makes the characterization of key roles played by microglia in neurodegenerative disorders, especially Alzheimer’s disease, of paramount importance. While genetic models and rudimentary pharmacologic approaches for microglial manipulation have greatly improved our understanding of central nervous system health and disease, significant advances in the selective and near complete in vitro and in vivo depletion of microglia for neuroscience application continue to push the boundaries of research. Here we discuss the research efficacy and utility of various microglial depletion strategies, including the highly effective CSF1R inhibitor models, noteworthy insights into the relationship between microglia and neurodegeneration, and the potential for therapeutic repurposing of microglial depletion and repopulation.

Alzheimer's disease (AD) is a devastating neurodegenerative disorder characterized by the deposition of amyloid-beta (Aβ) plaques and widespread neuroinflammation. While the cause of AD remains unknown, multiple factors likely contribute to the disease, including heart disease, diabetes, previous head injury, as well as a number of genetic determinants. Inheritance of the apolipoprotein (APOE) ε4 allele represents the strongest genetic risk factor for development of AD, driving pathogenesis and increasing overall disease severity. APOE has long been recognized as a key regulator of cholesterol homeostasis, although a greater appreciation now exists for its role in various innate immune system processes. Indeed, APOE modulates inflammatory environments in brain in large part by altering gene expression profiles in glia, important mediators of immunity in the CNS. While the association between APOE and AD was first observed nearly three decades ago, the mechanism by which APOE ε4 influences the etiology and pathophysiology of AD is not well characterized. Overwhelming data supports the hypothesis that APOE ε4 dysregulates central amyloid metabolism by an undetermined molecular mechanism, thus laying the foundation for disease. A host of amyloid-degrading enzymes (ADEs) regulate Aβ accumulation in brain, and therefore represent valuable therapeutic targets. Neprilysin (NEP), a metalloendopeptidase expressed by activated microglia and astrocytes, is a broad-spectrum ADE able to degrade a variety of Aβ species. Here we describe in vivo and in vitro experiments designed to investigate the potential for APOE genotype to differentially regulate glial NEP in brain under neuroinflammatory conditions. Our results provide a novel mechanism by which APOE genotype-dependent differential expression of NEP by glia during neuroinflammation may contribute to AD pathogenesis.

GENERAL COMMENTARY article Front. Immunol., 15 September 2020Sec. Inflammation Volume 11 - 2020 | https://doi.org/10.3389/fimmu.2020.01939

Multiple Sclerosis (MS) is an autoimmune disease that demyelinates the central nervous system (CNS) and leads to a loss of neuron conductivity. T helper (Th) cells promote inflammation in the CNS by responding to myelin antigens and activating microglia cells and astrocytes, the resident immune cells of the brain. Proinflammatory cytokines synthesized and secreted by Th cells have profound effects on the brain and represent a possible initiator and key mediator of MS pathogenesis. While the precise molecular and cellular mechanisms underlying CNS sensitivity to Th cells and their secreted products remain poorly understood, genetic risk factors offer important insight to the puzzle. APOE encodes apolipoprotein E and is uniquely polymorphic in humans. Inheritance of the ɛ4 allele is associated with increased disease severity and progression, as well as poorer outcomes in many neurodegenerative diseases, including MS. In addition, APOE genotype determines microglial and astrocyte responses to a wide range of inflammatory stimuli. Here we present data supporting APOE genotype‐dependent differential glial response to MS‐relevant stimuli.

Alzheimer's disease (AD) is a devastating neurodegenerative disorder set to become one of the greatest public health challenges in modern history. Attempts at disease intervention have thus far proven unsuccessful, and so the need for identification of novel therapeutic targets has never been greater. The association between exaggerated brain inflammation and AD strongly suggests that a more complete understanding of the mechanisms underlying dysfunctional immune activity may allow for novel target identification. The APOE gene is uniquely polymorphic in humans and differentially modulates innate immune activity in brain, with the ɛ4 allele representing the greatest known genetic risk factor for development of AD. However, while genetic association helps to identify risk, it does little to inform us about the molecular mechanisms that underlie disease. Amyloid (A) β, a pleiotropic neurotoxic pepetide, aggregates to form insoluble senile plaques and represents the pathologic hallmark of AD. Indeed, significant evidence supports dysfunctional clearance of Aβ in the etiology and/or pathophysiology of AD. Furthermore, Aβ is a potent inflammagen, promoting neuroinflammation through direct activation of both microglia and astrocytes. Here we present data supporting APOE genotype‐dependent differential expression of key glial targets associated with AD pathophysiology.

As more people live longer, age-related neurodegenerative diseases are an increasingly important societal health issue. Treatments targeting specific pathologies such as amyloid beta in Alzheimer’s disease (AD) have not led to effective treatments, and there is increasing evidence of a disconnect between traditional pathology and cognitive abilities with advancing age, indicative of individual variation in resilience to pathology. Here, we generated a comprehensive neuropathological, molecular, and transcriptomic characterization of hippocampus and two regions cortex in 107 aged donors (median = 90) from the Adult Changes in Thought (ACT) study as a freely-available resource (http://aging.brain-map.org/). We confirm established associations between AD pathology and dementia, albeit with increased, presumably aging-related variability, and identify sets of co-expressed genes correlated with pathological tau and inflammation markers. Finally, we demonstrate a relationship between dementia and RNA quality, and find common gene signatures, highlighting the importance of properly controlling for RNA quality when studying dementia.

Multiple sclerosis (MS) is an autoimmune demyelinating disease characterized by chronic inflammatory foci within the central nervous system (CNS). Resident immunocompetent cells of the CNS, namely microglia and astrocytes, respond to a variety of sterile and non‐sterile inflammagens by initiating neuroinflammatory cascades. Although these inflammatory processes are necessary for pathogen destruction, clearance, and tissue repair, disproportionate or protracted responses are thought to contribute to the development and progression of many brain disorders, including MS. While the molecular determinants influencing sensitivity and responsiveness of the affected brain to these inflammatory insults remain poorly understood, genetic studies have identified many potential disease risk alleles. Although the precise mechanisms responsible for this influence remain unclear, APOE represents a key modulator of microglial and astrocyte function. Therefore, we decided to examine how APOE genotype modulates glial function in response to general inflammagens. Our results further elucidate the CNS inflammatory mechanisms underlying MS, allowing for identification of potential therapeutic agents to mitigate disease progression, severity, and improve clinical outcomes.

Human cerebrospinal fluid (CSF) contains diverse lipid particles, including lipoproteins that are distinct from their plasma counterparts and contain apolipoprotein (apo) E isoforms, apoJ, and apoAI, and extracellular vesicles, which can be detected by annexin V binding. The aim of this study was to develop a method to quantify CSF particles and evaluate their relationship to aging and neurodegenerative diseases. We used a flow cytometric assay to detect annexin V-, apoE-, apoAI-, apoJ-, and amyloid (A) β42-positive particles in CSF from 131 research volunteers who were neurologically normal or had mild cognitive impairment (MCI), Alzheimer disease (AD) dementia, or Parkinson disease. APOE ε4/ε4 participants had CSF apoE-positive particles that were more frequently larger but at an 88% lower level versus those in APOE ε3/ε3 or APOE ε3/ε4 patients; this finding was reproduced in conditioned medium from mouse primary glial cell cultures with targeted replacement of apoE. Cerebrospinal fluid apoE-positive and β-amyloid (Aβ42)-positive particle concentrations were persistently reduced one-third to one-half in middle and older age subjects; apoAI-positive particle concentration progressively increased approximately 2-fold with age. Both apoAI-positive and annexin V-positive CSF particle levels were reduced one-third to one-half in CSF of MCI and/or AD dementia patients versus age-matched controls. Our approach provides new methods to investigate CNS lipid biology in relation to neurodegeneration and perhaps develop new biomarkers for diagnosis or treatment monitoring.

Endophilin-B1, also known as Bax-interacting factor 1 (Bif-1, and encoded by SH3GLB1), is a multifunctional protein involved in apoptosis, autophagy and mitochondrial function. We recently described a unique neuroprotective role for neuron-specific alternatively spliced isoforms of endophilin-B1. To examine whether endophilin-B1-mediated neuroprotection could be a novel therapeutic target for Alzheimer's disease we used a double mutant amyloid precursor protein and presenilin 1 (APPswe/PSEN1dE9) mouse model of Alzheimer's disease and observed that expression of neuron-specific endophilin-B1 isoforms declined with disease progression. To determine if this reduction in endophilin-B1 has a functional role in Alzheimer's disease pathogenesis, we crossed endophilin-B1(-/-) mice with APPswe/PSEN1dE9 mice. Deletion of endophilin-B1 accelerated disease onset and progression in 6-month-old APPswe/PSEN1dE9/endophilin-B1(-/-) mice, which showed more plaques, astrogliosis, synaptic degeneration, cognitive impairment and mortality than APPswe/PSEN1dE9 mice. In mouse primary cortical neuron cultures, overexpression of neuron-specific endophilin-B1 isoforms protected against amyloid-β-induced apoptosis and mitochondrial dysfunction. Additionally, protein and mRNA levels of neuron-specific endophilin-B1 isoforms were also selectively decreased in the cerebral cortex and in the synaptic compartment of patients with Alzheimer's disease. Flow sorting of synaptosomes from patients with Alzheimer's disease demonstrated a negative correlation between amyloid-β and endophilin-B1 levels. The importance of endophilin-B1 in neuronal function was further underscored by the development of synaptic degeneration and cognitive and motor impairment in endophilin-B1(-/-) mice by 12 months. Our findings suggest that endophilin-B1 is a key mediator of a feed-forward mechanism of Alzheimer's disease pathogenesis where amyloid-β reduces neuron-specific endophilin-B1, which in turn enhances amyloid-β accumulation and neuronal vulnerability to stress.

Parkinson's disease and Alzheimer's disease (AD) are recognized to coexist on a spectrum of neurodegeneration, and it has been proposed that molecular interactions among pathogenic proteins are a basis for the overlap between these two diseases. We instead hypothesized that degeneration of the nigrostriatal dopaminergic system enhances the clinical penetrance of early‐stage AD. To determine the effect of striatal dopamine (DA) on the pathological effects in an experimental model of AD, APP SWE /PS1ΔE9 mice received striatal injections of the neurotoxin 6‐hydroxydopamine (6OHDA). Animals were tested in a Barnes maze protocol and in a water T‐maze protocol at different ages to determine the onset of cognitive impairment. APP SWE /PS1ΔE9 mice that received 6OHDA injections showed significant impairment in Barnes maze performance at an earlier age than controls. Additionally, at 12 months of age, APP swe /PS1ΔE9 + 6OHDA mice demonstrated worse behavioral flexibility than other groups in a task‐switch phase of the water T‐maze. To determine the neuroprotective effects of dopaminergic neurotransmission against amyloid‐β 42 (Aβ 42 ) toxicity, neuronal branch order and dendrite length were quantified in primary medium spiny neuron (MSN) cultures pretreated with increasing doses of the D 1 and D 2 receptor agonists before being exposed to oligomerized Aβ 42 . Although there were no differences in Aβ peptide levels or plaque burden among the groups, in murine MSN culture dopaminergic agonists prevented a toxic response to Aβ 42. Depletion of DA in the striatum exacerbated the cognitive impairment seen in a mouse model of early‐stage AD; this may be due to a protective effect of dopaminergic innervation against Aβ striatal neurotoxicity. © 2015 Wiley Periodicals, Inc.

Frontotemporal dementia (FTD) is a neurodegenerative disease with devastating changes in behavioral performance and social function. Mutations in the progranulin gene (GRN) are one of the most common causes of inherited FTD due to reduced progranulin expression or activity, including in brain where it is expressed primarily by neurons and microglia. Thus, efforts aimed at enhancing progranulin levels might be a promising therapeutic strategy. Bone marrow (BM)-derived cells are able to engraft in the brain and adopt a microglial phenotype under myeloablative irradiation conditioning. This ability makes BM-derived cells a potential cellular vehicle for transferring therapeutic molecules to the central nervous system. Here, we utilized BM cells from Grn+/+ (wild type or wt) mice labeled with green fluorescence protein for delivery of progranulin to progranulin-deficient (Grn−/−) mice. Our results showed that wt bone marrow transplantation (BMT) partially reconstituted progranulin in the periphery and in cerebral cortex of Grn−/− mice. We demonstrated a pro-inflammatory effect in vivo and in ex vivo preparations of cerebral cortex of Grn−/− mice that was partially to fully reversed 5 months after BMT. Our findings suggest that BMT can be administered as a stem cell-based approach to prevent or to treat neurodegenerative diseases.

Cannabinoids affect immune responses in ways that may be beneficial for autoimmune diseases. We sought to determine whether chronic Cannabis use differentially modulates a select number of immune parameters in healthy controls and individuals with multiple sclerosis (MS cases). Subjects were enrolled and consented to a single blood draw, matched for age and BMI. We measured monocyte migration isolated from each subject, as well as plasma levels of endocannabinoids and cytokines. Cases met definition of MS by international diagnostic criteria. Monocyte cell migration measured in control subjects and individuals with MS was similarly inhibited by a set ratio of phytocannabinoids. The plasma levels of CCL2 and IL17 were reduced in non-naïve cannabis users irrespective of the cohorts. We detected a significant increase in the endocannabinoid arachidonoylethanolamine (AEA) in serum from individuals with MS compared to control subjects, and no significant difference in levels of other endocannabinoids and signaling lipids irrespective of Cannabis use. Chronic Cannabis use may affect the immune response to similar extent in individuals with MS and control subjects through the ability of phytocannabinoids to reduce both monocyte migration and cytokine levels in serum. From a panel of signaling lipids, only the levels of AEA are increased in individuals with MS, irrespective of Cannabis use or not. Our results suggest that both MS cases and controls respond similarly to chronic Cannabis use with respect to the immune parameters measured in this study.

An important pathologic hallmark of Alzheimer's disease (AD) is neuroinflammation, a process characterized in AD by disproportionate activation of cells (microglia and astrocytes, primarily) of the non-specific innate immune system within the CNS. While inflammation itself is not intrinsically detrimental, a delicate balance of pro- and anti-inflammatory signals must be maintained to ensure that long-term exaggerated responses do not damage the brain over time. Non-steroidal anti-inflammatory drugs (NSAIDs) represent a broad class of powerful therapeutics that temper inflammation by inhibiting cyclooxygenase-mediated signaling pathways including prostaglandins, which are the principal mediators of CNS neuroinflammation. While historically used to treat discrete or systemic inflammatory conditions, epidemiologic evidence suggests that protracted NSAID use may delay AD onset, as well as decrease disease severity and rate of progression. Unfortunately, clinical trials with NSAIDs have thus far yielded disappointing results, including premature discontinuation of a large-scale prevention trial due to unexpected cardiovascular side effects. Here we review the literature and make the argument that more targeted exploitation of downstream prostaglandin signaling pathways may offer significant therapeutic benefits for AD while minimizing adverse side effects. Directed strategies such as these may ultimately help to delay the deleterious consequences of brain aging and might someday lead to new therapies for AD and other chronic neurodegenerative diseases.

Cerebrospinal fluid (CSF) lipoprotein particles and cell derived microparticles have been shown to be associated with pathological conditions. Acknowledgment of their roles both as markers and pathogenic effectors in neurodegenerative disease has increased the interest of their measurement in clinical practice. However, assessment of their clinical use is impeded by technological issues. Here, we have described a quantitative flow cytometric assay by which annexin V + microparticles,apolipoprotein (apo)-E, -AI, -J containing lipoprotein particles, and Aβ 42 containing particles can be measured for concentration and size distribution in human CSF. We applied this techniqueto CSF from 126 research volunteers, and analyzed our results with respect to age, APOE genotype, and Alzheimer's disease (AD) or Parkinson's disease (PD) status. APOE4/4 subjects with mild cognitive impairment (MCI) or AD dementia had apoE + particles that were larger but less concentrated on average than in APOE3/3 or APOE3/4. In vitro studies using mice with targeted replacement of mouse apoE gene with human apoE3 (E3 +/+) or apoE4 (E4 +/+) demonstrated that E4 +/+ mice had significantly lower apoE + particle levels than E3 +/+ mice in astrocyte conditioned medium, and E4 +/+ mice has significantly greater % of larger apoE + particles than E3 +/+ mice in microglia conditioned medium. Our data suggest that the effects of APOE genotype on the size and concentration of apoE containing particles in human CSF can be reproduced, at least in part, by primary cultures of mouse glial cells with no influence by processes of disease. We also observed that Aβ 42 was co-labeled with both apoE + and apoJ + particles in human CSF. Interestingly, the concentration of Aβ 42 containing particles was not significantly associated with the concentration of CSF Aβ 42 as determined by luminex. Both apoA-I + and annexin V + particle levels were significantly lower in human CSF of AD dementia than in older controls. Exploratory analysis for correlations among CSF particles demonstrated annexin V + and apoA-I + particle levels were positively correlated with each other, suggesting a potential biological interaction. Our results suggest annexin V and perhaps apoA-I particles in CSF might be diagnostically helpful addition to existing biomarkers for AD patients.