NLRP3 inflammasome activation drives tau pathology

Proteomic Approaches to Study Cysteine Oxidation: Applications in Neurodegenerative Diseases

Oxidative stress appears to be a key feature of many neurodegenerative diseases either as a cause or consequence of disease. A range of molecules are subject to oxidation, but in particular, proteins are an important target and measure of oxidative stress. Proteins are subject to a range of oxidative modifications at reactive cysteine residues, and depending on the level of oxidative stress, these modifications may be reversible or irreversible. A range of experimental approaches has been developed to characterize cysteine oxidation of proteins. In particular, mass spectrometry-based proteomic methods have emerged as a powerful means to identify and quantify cysteine oxidation sites on a proteome scale; however, their application to study neurodegenerative diseases is limited to date. Here we provide a guide to these approaches and highlight the under-exploited utility of these methods to measure oxidative stress in neurodegenerative diseases for biomarker discovery, target engagement and to understand disease mechanisms.

CSF1R inhibition rescues tau pathology and neurodegeneration in an A/T/N model with combined AD pathologies, while preserving plaque associated microglia

Alzheimer's disease (AD) is characterized by a sequential progression of amyloid plaques (A), neurofibrillary tangles (T) and neurodegeneration (N), constituting ATN pathology. While microglia are considered key contributors to AD pathogenesis, their contribution in the combined presence of ATN pathologies remains incompletely understood. As sensors of the brain microenvironment, microglial phenotypes and contributions are importantly defined by the pathologies in the brain, indicating the need for their analysis in preclinical models that recapitulate combined ATN pathologies, besides their role in A and T models only. Here, we report a new tau-seed model in which amyloid pathology facilitates bilateral tau propagation associated with brain atrophy, thereby recapitulating robust ATN pathology. Single-cell RNA sequencing revealed that ATN pathology exacerbated microglial activation towards disease-associated microglia states, with a significant upregulation of Apoe as compared to amyloid-only models (A). Importantly, Colony-Stimulating Factor 1 Receptor inhibition preferentially eliminated non-plaque-associated versus plaque associated microglia. The preferential depletion of non-plaque-associated microglia significantly attenuated tau pathology and neuronal atrophy, indicating their detrimental role during ATN progression. Together, our data reveal the intricacies of microglial activation and their contributions to pathology in a model that recapitulates the combined ATN pathologies of AD. Our data may provide a basis for microglia-targeting therapies selectively targeting detrimental microglial populations, while conserving protective populations.

A comprehensive review on clinical and mechanistic pathophysiological aspects of COVID-19 Malady: How far have we come?

Since its outbreak in 2019, the coronavirus disease (COVID-19) has become a pandemic, affecting more than 52 million people and causing more than 1 million mortalities globally till date. Current research reveals a wide array of disease manifestations and behaviors encompassing multiple organ systems in body and immense systemic inflammation, which have been summarized in this review. Data from a number of scientific reviews, research articles, case series, observational studies, and case reports were retrieved by utilizing online search engines such as Cochrane, PubMed, and Scopus from December 2019 to November 2020. The data for prevalence of signs and symptoms, underlying disease mechanisms and comorbidities were analyzed using SPSS version 25. This review will discuss a wide range of COVID-19 clinical presentations recorded till date, and the current understanding of both the underlying general as well as system specific pathophysiologic, and pathogenetic pathways. These include direct viral penetration into host cells through ACE2 receptors, induction of inflammosomes and immune response through viral proteins, and the initiation of system-wide inflammation and cytokine production. Moreover, peripheral organ damage and underlying comorbid diseases which can lead to short term and long term, reversible and irreversible damage to the body have also been studied. We concluded that underlying comorbidities and their pathological effects on the body contributed immensely and determine the resultant disease severity and mortality of the patients. Presently there is no drug approved for treatment of COVID-19, however multiple vaccines are now in use and research for more is underway.

Microglia in Alzheimer's Disease

Alzheimer's Disease (AD) is the most frequent neurodegenerative disorder. Although proteinaceous aggregates of extracellular Amyloid-β (Aβ) and intracellular hyperphosphorylated microtubule- associated tau have long been identified as characteristic neuropathological hallmarks of AD, a disease- modifying therapy against these targets has not been successful. An emerging concept is that microglia, the innate immune cells of the brain, are major players in AD pathogenesis. Microglia are longlived tissue-resident professional phagocytes that survey and rapidly respond to changes in their microenvironment. Subpopulations of microglia cluster around Aβ plaques and adopt a transcriptomic signature specifically linked to neurodegeneration. A plethora of molecules and pathways associated with microglia function and dysfunction has been identified as important players in mediating neurodegeneration. However, whether microglia exert either beneficial or detrimental effects in AD pathology may depend on the disease stage. In this review, we summarize the current knowledge about the stage-dependent role of microglia in AD, including recent insights from genetic and gene expression profiling studies as well as novel imaging techniques focusing on microglia in human AD pathology and AD mouse models.

A tandem activation of NLRP3 inflammasome induced by copper oxide nanoparticles and dissolved copper ion in J774A.1 macrophage

For the first time, we reported that CuONPs exposure induced interleukin (IL)-1β-mediated inflammation via NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome in J774A.1 macrophage. Mechanistically, CuONPs activated NLRP3 inflammasome is a two-fold process. Firstly, CuONPs challenge caused lysosomal damage, along with the release of cathepsin B, which directly mediated the activation of NLRP3 inflammasomes. Interestingly, after the deposition in lysosomes, CuONPs may release copper ion due to the acidic environment of lysosomes. Consequently, the released copper ions significantly induced cellular oxidative stress and further mediated the activation of NLRP3 inflammasomes. Moreover, CuONPs exposure could prime J774A.1 macrophage to express pro-IL-1β through myeloid differentiation factor 88 (MyD88)-dependent Toll-like receptor 4 (TLR4) signal pathway subsequently activating nuclear transcription factor kappa B (NF-κB).

Distinct responses of human peripheral blood cells to different misfolded protein oligomers

Increasing evidence indicates that peripheral immune cells play a prominent role in neurodegeneration connected to protein misfolding, which are associated with formation of aberrant aggregates, including soluble protein misfolded oligomers. The precise links, however, between the physicochemical features of diverse oligomers and their effects on the immune system, particularly on adaptive immunity, remain currently unexplored, due partly to the transient and heterogeneous nature of the oligomers themselves. To overcome these limitations, we took advantage of two stable and well‐characterized types of model oligomers (A and B), formed by HypF‐N bacterial protein, type B oligomers displaying lower solvent‐exposed hydrophobicity. Exposure to oligomers of human peripheral blood mononuclear cells (PBMCs) revealed differential effects, with type B, but not type A, oligomers leading to a reduction in CD4⁺ cells. Type A oligomers promoted enhanced differentiation towards CD4⁺CD25^HighFoxP3⁺ Tregs and displayed a higher suppressive effect on lymphocyte proliferation than Tregs treated with oligomers B or untreated cells. Moreover, our results reveal Th1 and Th17 lymphocyte differentiation mediated by type A oligomers and a differential balance of TGF‐β, IL‐6, IL‐23, IFN‐γ and IL‐10 mediators. These results indicate that type B oligomers recapitulate some of the biological responses associated with Parkinson's disease in peripheral immunocompetent cells, while type A oligomers resemble responses associated with Alzheimer's disease. We anticipate that further studies characterizing the differential effects of protein misfolded oligomers on the peripheral immune system may lead to the development of blood‐based diagnostics, which could report on the type and properties of oligomers present in patients.

Nanoparticles: A Hope for the Treatment of Inflammation in CNS

Neuroinflammation, an inflammatory response within the central nervous system (CNS), is a main hallmark of common neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS), among others. The over-activated microglia release pro-inflammatory cytokines, which induces neuronal death and accelerates neurodegeneration. Therefore, inhibition of microglia over-activation and microglia-mediated neuroinflammation has been a promising strategy for the treatment of neurodegenerative diseases. Many drugs have shown promising therapeutic effects on microglia and inflammation. However, the blood–brain barrier (BBB)—a natural barrier preventing brain tissue from contact with harmful plasma components—seriously hinders drug delivery to the microglial cells in CNS. As an emerging useful therapeutic tool in CNS-related diseases, nanoparticles (NPs) have been widely applied in biomedical fields for use in diagnosis, biosensing and drug delivery. Recently, many NPs have been reported to be useful vehicles for anti-inflammatory drugs across the BBB to inhibit the over-activation of microglia and neuroinflammation. Therefore, NPs with good biodegradability and biocompatibility have the potential to be developed as an effective and minimally invasive carrier to help other drugs cross the BBB or as a therapeutic agent for the treatment of neuroinflammation-mediated neurodegenerative diseases. In this review, we summarized various nanoparticles applied in CNS, and their mechanisms and effects in the modulation of inflammation responses in neurodegenerative diseases, providing insights and suggestions for the use of NPs in the treatment of neuroinflammation-related neurodegenerative diseases.

Innate Immune Responses and Pulmonary Diseases

Innate immunity is the first defense line of the host against various infectious pathogens, environmental insults, and other stimuli causing cell damages. Upon stimulation, pattern recognition receptors (PRRs) act as sensors to activate innate immune responses, containing NF-κB signaling, IFN response, and inflammasome activation. Toll-like receptors (TLRs), retinoic acid-inducible gene I-like receptors (RLRs), NOD-like receptors (NLRs), and other nucleic acid sensors are involved in innate immune responses. The activation of innate immune responses can facilitate the host to eliminate pathogens and maintain tissue homeostasis. However, the activity of innate immune responses needs to be tightly controlled to ensure the optimal intensity and duration of activation under various contexts. Uncontrolled innate immune responses can lead to various disorders associated with aberrant inflammatory response, including pulmonary diseases such as COPD, asthma, and COVID-19. In this chapter, we will have a broad overview of how innate immune responses function and the regulation and activation of innate immune response at molecular levels as well as their contribution to various pulmonary diseases. A better understanding of such association between innate immune responses and pulmonary diseases may provide potential therapeutic strategies.

Inflammasome NLRP3 Potentially Links Obesity-Associated Low-Grade Systemic Inflammation and Insulin Resistance with Alzheimer's Disease

Alzheimer’s disease (AD) is the most common form of neurodegenerative dementia. Metabolic disorders including obesity and type 2 diabetes mellitus (T2DM) may stimulate amyloid β (Aβ) aggregate formation. AD, obesity, and T2DM share similar features such as chronic inflammation, increased oxidative stress, insulin resistance, and impaired energy metabolism. Adiposity is associated with the pro-inflammatory phenotype. Adiposity-related inflammatory factors lead to the formation of inflammasome complexes, which are responsible for the activation, maturation, and release of the pro-inflammatory cytokines including interleukin-1β (IL-1β) and interleukin-18 (IL-18). Activation of the inflammasome complex, particularly NLRP3, has a crucial role in obesity-induced inflammation, insulin resistance, and T2DM. The abnormal activation of the NLRP3 signaling pathway influences neuroinflammatory processes. NLRP3/IL-1β signaling could underlie the association between adiposity and cognitive impairment in humans. The review includes a broadened approach to the role of obesity-related diseases (obesity, low-grade chronic inflammation, type 2 diabetes, insulin resistance, and enhanced NLRP3 activity) in AD. Moreover, we also discuss the mechanisms by which the NLRP3 activation potentially links inflammation, peripheral and central insulin resistance, and metabolic changes with AD.

Alzheimer and Purinergic Signaling: Just a Matter of Inflammation?

Alzheimer's disease (AD) is a widespread neurodegenerative pathology responsible for about 70% of all cases of dementia. Adenosine is an endogenous nucleoside that affects neurodegeneration by activating four membrane G protein-coupled receptor subtypes, namely P1 receptors. One of them, the A_2A subtype, is particularly expressed in the brain at the striatal and hippocampal levels and appears as the most promising target to counteract neurological damage and adenosine-dependent neuroinflammation. Extracellular nucleotides (ATP, ADP, UTP, UDP, etc.) are also released from the cell or are synthesized extracellularly. They activate P2X and P2Y membrane receptors, eliciting a variety of physiological but also pathological responses. Among the latter, the chronic inflammation underlying AD is mainly caused by the P2X7 receptor subtype. In this review we offer an overview of the scientific evidence linking P1 and P2 mediated purinergic signaling to AD development. We will also discuss potential strategies to exploit this knowledge for drug development.

IL-6 and IL-1β upregulation and tau protein phosphorylation in response to chronic alcohol exposure in the mouse hippocampus

Alcoholism and alcohol abuse can lead to memory loss and cognitive dysfunction. The neuroinflammatory response plays an important role in the neurotoxic mechanism of chronic alcohol exposure. Additionally, the phosphorylation status of the tau protein is closely related to neurotoxicity and synaptic function. As inflammatory cytokines have been shown to regulate tau phosphorylation, in the present study, the aim was to determine whether cognitive impairment caused by chronic alcohol exposure is associated with neuroinflammation and tau hyperphosphorylation in the hippocampus. We established a chronic alcohol exposure model of C57BL/6J mice. The Y maze was used to assess the spatial recognition ability of mice, and ELISA was used to detect the levels of inflammatory cytokines IL-1β and IL-6 in the serum. Immunohistochemical and western blot assays were used to assess the expression levels of IL-1β and IL-6, as well as tau protein and its phosphorylation status in the hippocampus. We also analyzed the mRNA and protein expression of the synapse-associated proteins PSD95 and synaptophysin in the hippocampus. Our results showed that chronic alcohol exposure impaired the spatial recognition ability of mice upregulated the expression of IL-1β and IL-6 in the serum and hippocampus and increased the phosphorylation of tau protein in the hippocampus. In addition, chronic alcohol exposure downregulated PSD95 and synaptophysin protein levels. The present results indicate that hippocampal IL-1β, IL-6, and phosphorylated tau proteins may be involved in the neurotoxic mechanism of chronic alcohol exposure by mediating synaptic dysfunction.

THY-Tau22 mouse model accumulates more tauopathy at late stage of the disease in response to microglia deactivation through TREM2 deficiency

The role played by microglia has taken the center of the stage in the etiology of Alzheimer's disease (AD). Several genome-wide association studies carried out on large cohorts of patients have indeed revealed a large number of genetic susceptibility factors corresponding to genes involved in neuroinflammation and expressed specifically by microglia in the brain. Among these genes TREM2, a cell surface receptor expressed by microglia, arouses strong interest because its R47H variant confers a risk of developing AD comparable to the ε4 allele of the APOE gene. Since this discovery, a growing number of studies have therefore examined the role played by TREM2 in the evolution of amyloid plaques and neurofibrillary tangles, the two brain lesions characteristic of AD. Many studies report conflicting results, reflecting the complex nature of microglial activation in AD. Here, we investigated the impact of TREM2 deficiency in the THY-Tau22 transgenic line, a well-characterized model of tauopathy. Our study reports an increase in the severity of tauopathy lesions in mice deficient in TREM2 occurring at an advanced stage of the pathology. This exacerbation of pathology was associated with a reduction in microglial activation indicated by typical morphological features and altered expression of specific markers. However, it was not accompanied by any further changes in memory performance. Our longitudinal study confirms that a defect in microglial TREM2 signaling leads to an increase in neuronal tauopathy occurring only at late stages of the disease.

The Role of the Effects of Autophagy on NLRP3 Inflammasome in Inflammatory Nervous System Diseases

Autophagy is a stable self-sustaining process in eukaryotic cells. In this process, pathogens, abnormal proteins, and organelles are encapsulated by a bilayer membrane to form autophagosomes, which are then transferred to lysosomes for degradation. Autophagy is involved in many physiological and pathological processes. Nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome, containing NLRP3, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) and pro-caspase-1, can activate caspase-1 to induce pyroptosis and lead to the maturation and secretion of interleukin-1 β (IL-1 β) and IL-18. NLRP3 inflammasome is related to many diseases. In recent years, autophagy has been reported to play a vital role by regulating the NLRP3 inflammasome in inflammatory nervous system diseases. However, the related mechanisms are not completely clarified. In this review, we sum up recent research about the role of the effects of autophagy on NLRP3 inflammasome in Alzheimer’s disease, chronic cerebral hypoperfusion, Parkinson’s disease, depression, cerebral ischemia/reperfusion injury, early brain injury after subarachnoid hemorrhage, and experimental autoimmune encephalomyelitis and analyzed the related mechanism to provide theoretical reference for the future research of inflammatory neurological diseases.

Dexmedetomidine Mitigated NLRP3-Mediated Neuroinflammation via the Ubiquitin-Autophagy Pathway to Improve Perioperative Neurocognitive Disorder in Mice

Background: Surgery and anesthesia-induced perioperative neurocognitive disorder (PND) are closely related to NOD-like receptors (NLR) family, pyrin domain containing 3 (NLRP3) inflammasome microglia inflammatory response. Inhibiting the occurrence of neuroinflammation is an important treatment method to improve postoperative delirium. Fewer NLRP3-targeting molecules are currently available in the clinic to reduce the incidence of postoperative delirium. Dexmedetomidine (DEX), an α2 adrenergic receptor agonist has been shown to have antioxidant and anti-inflammatory activities. The present study showed that DEX reduced the production of cleaved caspase1 (CASP1) and destroyed the NLRP3–PYD And CARD Domain Containing (PYCARD)–CASP1 complex assembly, thereby reducing the secretion of IL-1β interleukin beta (IL-1β). DEX promoted the autophagy process of microglia and reduced NLRP3 expression. More interestingly, it promoted the ubiquitination and degradation of NLRP3. Thus, this study demonstrated that DEX reduced NLRP3-mediated inflammation through the activation of the ubiquitin-autophagy pathway. This study provided a new mechanism for treating PND using DEX.

Methods: C57BL/6 mice were pre-administered DEX 3 days in advance, and an abdominal exploration model was used to establish a perioperative neurocognitive disorder model. The anti-inflammatory effect of DEX was explored in vivo by detecting NLRP3-CASP1/IL-1β protein expression and behavioral testing. Primary microglia were stimulated with lipopolysaccharide (LPS) and adenosine triphosphate (ATP) in vitro, the expression of CASP1 and IL-1β was detected in the supernatant of cells, and the expression of autophagy-related proteins microtubule-associated protein 1 light chain 3 beta (MAP1LC3B) and sequestosome 1 (SQSTM1) was examined in the cytoplasm. Meanwhile, Co-immunoprecipitation (Co-IP) was used to detect NLRP3 protein ubiquitination so as to clarify the new mechanism underlying the anti-inflammatory effect of DEX.

Results: Pre-administration of DEX reduced the protein expression of NLRP3, CASP1, and IL-1β in the hippocampus of mice induced by surgery and also improved the impairment of learning and memory ability. At the same time, DEX also effectively relieved the decrease in spine density of the hippocampal brain induced by surgery. DEX decreased the cleaved CASP1 expression, blocked the assembly of NLRP3–PYCARD–CASP1 complex, and also reduced the secretion of mature IL-1β in vitro. Mechanically, it accelerated the degradation of NLRP3 inflammasome via the autophagy–ubiquitin pathway and reduced the green fluorescent protein/red fluorescent protein MAP1LC3B ratio, which was comparable to the effect when using the autophagy activator rapamycin (Rapa). Furthermore, it increased the ubiquitination of NLRP3 after LPS plus ATP stimulated microglia.

Conclusion: DEX attenuated the hippocampal brain inflammation by promoting NLRP3 inflammasome degradation via the autophagy–ubiquitin pathway, thus improving cognitive impairment in mice.

Using lipid profiling to better characterize metabolic differences in apolipoprotein E (APOE) genotype among community-dwelling older Black men

Apolipoprotein E (APOE) allelic variation is associated with differences in overall circulating lipids and risks of major health outcomes. Lipid profiling provides the opportunity for a more detailed description of lipids that differ by APOE, to potentially inform therapeutic targets for mitigating higher morbidity and mortality associated with certain APOE genotypes. Here, we sought to identify lipids, lipid-like molecules, and important mediators of fatty acid metabolism that differ by APOE among 278 Black men ages 70-81. Using liquid chromatography-mass spectrometry methods, 222 plasma metabolites classified as lipids, lipid-like molecules, or essential in fatty acid metabolism were detected. We applied principal factor analyses to calculate a factor score for each main lipid category. APOE was categorized as ε4 carriers (n = 83; ε3ε4 or ε4ε4), ε2 carriers (n = 58; ε2ε3 or ε2ε2), or ε3 homozygotes (n = 137; ε3ε3). Using analysis of variance, the monoacylglycerol factor, cholesterol ester factor, the factor for triacylglycerols that consist mostly of polyunsaturated fatty acids, sphingosine, and free carnitine significantly differed by APOE (p < 0.05, false discovery rate < 0.30). The monoacylglycerol factor, cholesterol ester factor, and sphingosine were lower, whereas the factor for triacylglycerols that consisted mostly of polyunsaturated fatty acids was higher among ε2 carriers than remaining participants. Free carnitine was lower among ε4 carriers than ε3 homozygotes. Lower monoacylglycerols and cholesteryl esters and higher triacylglycerols that consist mostly of polyunsaturated fatty acids may be protective metabolic characteristics of APOE ε2 carriers, whereas lower carnitine may reflect altered mitochondrial functioning among ε4 carriers in this cohort of older Black men.

The role of the inflammasomes in the pathogenesis of uveitis

Uveitis is a diverse group of sight-threatening intraocular inflammatory diseases usually causing eye redness, pain, blurred vision, and sometimes blindness. Although the exact pathogenesis of uveitis is not yet clear, accumulating evidences have shown that an imbalanced regulation of immune responses caused by a combination of genetic and environmental factors are implicated in the pathogenesis of this disease. As critical regulators of inflammation, inflammasomes have been assumed to play a role in the pathogenesis of uveitis. Recent studies have reported the association between a number of genetic variants in inflammasome related genes (such as NLRP3, NLRP1, NLRC4 and AIM2) with increased risk to uveitis. Mounting evidence have shown an aberrant activation of the NLRP3 inflammasome in both uveitis patients and murine models of uveitis. Some studies explored the intervention of uveitis via modulating inflammasome activity in the eye. This review aims at summarizing the main findings of these studies, proposing the possible mechanism whereby inflammasomes affect the susceptibility to develop uveitis, and giving a perspective for future studies, which may further improve our understanding about the role of inflammasomes and related cytokines in the pathogenesis of uveitis, and may hopefully lead to new therapeutics by targeting inflammasomes.

Small molecule approaches to treat autoimmune and inflammatory diseases (Part II): Nucleic acid sensing antagonists and inhibitors

Nucleic acid sensing pathways play an important role in the innate immune system, protecting hosts against infections. However, a large body of evidence supports a close association between aberrant activation of those pathways and autoimmune and inflammatory diseases. Part II of the digest series on small molecule approaches to autoimmune and inflammatory diseases concentrates on recent advances with respect to small molecule antagonists or inhibitors of the nucleic acid sensing pathways, including endosomal TLRs, NLRP3 inflammasome and cGAS-STING.

Neuroinflammation in Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disease associated with human aging. Ten percent of individuals over 65 years have AD and its prevalence continues to rise with increasing age. There are currently no effective disease modifying treatments for AD, resulting in increasingly large socioeconomic and personal costs. Increasing age is associated with an increase in low-grade chronic inflammation (inflammaging) that may contribute to the neurodegenerative process in AD. Although the exact mechanisms remain unclear, aberrant elevation of reactive oxygen and nitrogen species (RONS) levels from several endogenous and exogenous processes in the brain may not only affect cell signaling, but also trigger cellular senescence, inflammation, and pyroptosis. Moreover, a compromised immune privilege of the brain that allows the infiltration of peripheral immune cells and infectious agents may play a role. Additionally, meta-inflammation as well as gut microbiota dysbiosis may drive the neuroinflammatory process. Considering that inflammatory/immune pathways are dysregulated in parallel with cognitive dysfunction in AD, elucidating the relationship between the central nervous system and the immune system may facilitate the development of a safe and effective therapy for AD. We discuss some current ideas on processes in inflammaging that appear to drive the neurodegenerative process in AD and summarize details on a few immunomodulatory strategies being developed to selectively target the detrimental aspects of neuroinflammation without affecting defense mechanisms against pathogens and tissue damage.

Short-Chain Fatty Acids and Lipopolysaccharide as Mediators Between Gut Dysbiosis and Amyloid Pathology in Alzheimer's Disease

BACKGROUND

Metagenomic data support an association between certain bacterial strains and Alzheimer's disease (AD), but their functional dynamics remain elusive.

OBJECTIVE

To investigate the association between amyloid pathology, bacterial products such as lipopolysaccharide (LPS) and short chain fatty acids (SCFAs: acetate, valerate, butyrate), inflammatory mediators, and markers of endothelial dysfunction in AD.

METHODS

Eighty-nine older persons with cognitive performance from normal to dementia underwent florbetapir amyloid PET and blood collection. Brain amyloidosis was measured with standardized uptake value ratio versus cerebellum. Blood levels of LPS were measured by ELISA, SCFAs by mass spectrometry, cytokines by using real-time PCR, and biomarkers of endothelial dysfunction by flow cytometry. We investigated the association between the variables listed above with Spearman's rank test.

RESULTS

Amyloid SUVR uptake was positively associated with blood LPS (rho≥0.32, p≤0.006), acetate and valerate (rho≥0.45, p < 0.001), pro-inflammatory cytokines (rho≥0.25, p≤0.012), and biomarkers of endothelial dysfunction (rho≥0.25, p≤0.042). In contrast, it was negatively correlated with butyrate (rho≤-0.42, p≤0.020) and the anti-inflammatory cytokine IL10 (rho≤-0.26, p≤0.009). Endothelial dysfunction was positively associated with pro-inflammatory cytokines, acetate and valerate (rho≥0.25, p≤0.045) and negatively with butyrate and IL10 levels (rho≤-0.25, p≤0.038).

CONCLUSION

We report a novel association between gut microbiota-related products and systemic inflammation with brain amyloidosis via endothelial dysfunction, suggesting that SCFAs and LPS represent candidate pathophysiologic links between the gut microbiota and AD pathology.

Necroptosis, pyroptosis and apoptosis: an intricate game of cell death

Cell death is a fundamental physiological process in all living organisms. Its roles extend from embryonic development, organ maintenance, and aging to the coordination of immune responses and autoimmunity. In recent years, our understanding of the mechanisms orchestrating cellular death and its consequences on immunity and homeostasis has increased substantially. Different modalities of what has become known as 'programmed cell death' have been described, and some key players in these processes have been identified. We have learned more about the intricacies that fine tune the activity of common players and ultimately shape the different types of cell death. These studies have highlighted the complex mechanisms tipping the balance between different cell fates. Here, we summarize the latest discoveries in the three most well understood modalities of cell death, namely, apoptosis, necroptosis, and pyroptosis, highlighting common and unique pathways and their effect on the surrounding cells and the organism as a whole.

Cytokine signaling convergence regulates the microglial state transition in Alzheimer's disease

Genetic analyses have revealed the pivotal contribution of microglial dysfunctions to the pathogenesis of Alzheimer's disease (AD). Along AD progression, the accumulation of danger-associated molecular patterns (DAMPs) including beta-amyloid and hyperphosphorylated tau continuously stimulates microglia, which results in their chronic activation. Chronically activated microglia secrete excessive pro-inflammatory cytokines, which further regulate microglial responses towards DAMPs. This has spurred longstanding interest in targeting cytokine-induced microglial responses for AD therapeutic development. However, the cytokine-induced microglial state transition is not comprehensively understood. Cytokines are assumed to induce microglial state transition from a resting state to an activated state. However, recent evidence indicate that this microglial state transition involves multiple sequential functional states. Moreover, the mechanisms by which different functional states within the cytokine-induced microglial state transition regulate AD pathology remain unclear. In this review, we summarize how different cytokine signaling pathways, including those of IL-33 (interleukin-33), NLRP3 inflammasome-IL-1β, IL-10, and IL-12/IL-23, regulate microglial functions in AD. Furthermore, we discuss how the modulation of these cytokine signaling pathways can result in beneficial outcomes in AD. Finally, we describe a stepwise functional state transition of microglia induced by cytokine signaling that can provide insights into the molecular basis of the beneficial effects of cytokine modulation in AD and potentially aid therapeutic development.

Re-emphasizing early Alzheimer's disease pathology starting in select entorhinal neurons, with a special focus on mitophagy

The entorhinal-hippocampal system contains distinct networks subserving declarative memory. This system is selectively vulnerable to changes of ageing and pathological processes. The entorhinal cortex (EC) is a pivotal component of this memory system since it serves as the interface between the neocortex and the hippocampus. EC is heavily affected by the proteinopathies of Alzheimer's disease (AD). These appear in a stereotypical spatiotemporal manner and include increased levels of intracellular amyloid-beta Aβ (iAβ), parenchymal deposition of Aβ plaques, and neurofibrillary tangles (NFTs) containing abnormally processed Tau. Increased levels of iAβ and the formation of NFTs are seen very early on in a population of neurons belonging to EC layer II (EC LII), and recent evidence leads us to believe that this population is made up of highly energy-demanding reelin-positive (RE+) projection neurons. Mitochondria are fundamental to the energy supply, metabolism, and plasticity of neurons. Evidence from AD postmortem brain tissues supports the notion that mitochondrial dysfunction is one of the initial pathological events in AD, and this is likely to take place in the vulnerable RE + EC LII neurons. Here we review and discuss these notions, anchored to the anatomy of AD, and formulate a hypothesis attempting to explain the vulnerability of RE + EC LII neurons to the formation of NFTs. We attempt to link impaired mitochondrial clearance to iAβ and signaling involving both apolipoprotein 4 and reelin, and argue for their relevance to the formation of NFTs specifically in RE + EC LII neurons during the prodromal stages of AD. We believe future studies on these interactions holds promise to advance our understanding of AD etiology and provide new ideas for drug development.

Identification of Conserved Proteomic Networks in Neurodegenerative Dementia

Data-driven analyses are increasingly valued in modern medicine. We integrate quantitative proteomics and transcriptomics from over 1,000 post-mortem brains from six cohorts representing Alzheimer’s disease (AD), asymptomatic AD, progressive supranuclear palsy (PSP), and control patients from the Accelerating Medicines Partnership – Alzheimer’s Disease consortium. We define robust co-expression trajectories related to disease progression, including early neuronal, microglial, astrocyte, and immune response modules, and later mRNA splicing and mitochondrial modules. The majority of, but not all, modules are conserved at the transcriptomic level, including module C3, which is only observed in proteome networks and enriched in mitogen-activated protein kinase (MAPK) signaling. Genetic risk enriches in modules changing early in disease and indicates that AD and PSP have distinct causal biological drivers at the pathway level, despite aspects of similar pathology, including synaptic loss and glial inflammatory changes. The conserved, high-confidence proteomic changes enriched in genetic risk represent targets for drug discovery.

Swarup et al. use a multi-omic, multi-cohort approach to identify robust early and late proteomic changes in AD and other neurodegenerative dementias and find that genetic risk is differentially enriched across disorders. Shared co-expression modules showing consistent molecular alterations at multi-omic levels are ripe for future investigation as drug targets.

Microglial Function and Regulation during Development, Homeostasis and Alzheimer's Disease

Microglia are the resident immune cells of the brain, deriving from yolk sac progenitors that populate the brain parenchyma during development. During development and homeostasis, microglia play critical roles in synaptogenesis and synaptic plasticity, in addition to their primary role as immune sentinels. In aging and neurodegenerative diseases generally, and Alzheimer's disease (AD) specifically, microglial function is altered in ways that significantly diverge from their homeostatic state, inducing a more detrimental inflammatory environment. In this review, we discuss the receptors, signaling, regulation and gene expression patterns of microglia that mediate their phenotype and function contributing to the inflammatory milieu of the AD brain, as well as strategies that target microglia to ameliorate the onset, progression and symptoms of AD.

Precision Nutrition for Alzheimer's Prevention in ApoE4 Carriers

The ApoE4 allele is the most well-studied genetic risk factor for Alzheimer’s disease, a condition that is increasing in prevalence and remains without a cure. Precision nutrition targeting metabolic pathways altered by ApoE4 provides a tool for the potential prevention of disease. However, no long-term human studies have been conducted to determine effective nutritional protocols for the prevention of Alzheimer’s disease in ApoE4 carriers. This may be because relatively little is yet known about the precise mechanisms by which the genetic variant confers an increased risk of dementia. Fortunately, recent research is beginning to shine a spotlight on these mechanisms. These new data open up the opportunity for speculation as to how carriers might ameliorate risk through lifestyle and nutrition. Herein, we review recent discoveries about how ApoE4 differentially impacts microglia and inflammatory pathways, astrocytes and lipid metabolism, pericytes and blood–brain barrier integrity, and insulin resistance and glucose metabolism. We use these data as a basis to speculate a precision nutrition approach for ApoE4 carriers, including a low-glycemic index diet with a ketogenic option, specific Mediterranean-style food choices, and a panel of seven nutritional supplements. Where possible, we integrate basic scientific mechanisms with human observational studies to create a more complete and convincing rationale for this precision nutrition approach. Until recent research discoveries can be translated into long-term human studies, a mechanism-informed practical clinical approach may be useful for clinicians and patients with ApoE4 to adopt a lifestyle and nutrition plan geared towards Alzheimer’s risk reduction.

SGK1 inhibition in glia ameliorates pathologies and symptoms in Parkinson disease animal models

Astrocytes and microglia are brain-resident glia that can establish harmful inflammatory environments in disease contexts and thereby contribute to the progression of neuronal loss in neurodegenerative disorders. Correcting the diseased properties of glia is therefore an appealing strategy for treating brain diseases. Previous studies have shown that serum/ glucocorticoid related kinase 1 (SGK1) is upregulated in the brains of patients with various neurodegenerative disorders, suggesting its involvement in the pathogenesis of those diseases. In this study, we show that inhibiting glial SGK1 corrects the pro-inflammatory properties of glia by suppressing the intracellular NFκB-, NLRP3-inflammasome-, and CGAS-STING-mediated inflammatory pathways. Furthermore, SGK1 inhibition potentiated glial activity to scavenge glutamate toxicity and prevented glial cell senescence and mitochondrial damage, which have recently been reported as critical pathologic features of and therapeutic targets in Parkinson disease (PD) and Alzheimer disease (AD). Along with those anti-inflammatory/neurotrophic functions, silencing and pharmacological inhibition of SGK1 protected midbrain dopamine neurons from degeneration and cured pathologic synuclein alpha (SNCA) aggregation and PD-associated behavioral deficits in multiple in vitro and in vivo PD models. Collectively, these findings suggest that SGK1 inhibition could be a useful strategy for treating PD and other neurodegenerative disorders that share the common pathology of glia-mediated neuroinflammation.

The Potential Role of Herpes Simplex Virus Type 1 and Neuroinflammation in the Pathogenesis of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease affecting ~50 million people worldwide. To date, there is no cure and current therapies have not been effective in delaying disease progression. Therefore, there is an urgent need for better understanding of the pathogenesis of AD and to rethink possible therapies. Herpes simplex virus type 1 (HSV1) has recently received growing attention for its potential role in sporadic AD. The virus is a ubiquitous human pathogen that infects mucosal epithelia and invades the peripheral nervous system (PNS) of its host to establish a reactivable, latent infection. Upon reactivation, HSV1 spreads back to the epithelium and initiates a new infection, causing epithelial lesions. Occasionally, the virus spreads from the PNS to the brain after reactivation. In this review, we discuss current work on the pathogenesis of AD and summarize research results that support a potential role for HSV1 in the infectious hypothesis of AD. We also highlight recent findings on the neuroinflammatory response, which has been proposed to be the main driving force of AD, starting early in the course of the disease. Relevant rodent models to study neuroinflammation in AD and novel therapeutic approaches are also discussed. Throughout this review, we focus on several aspects of HSV1 pathogenesis, including its primary role as an invader of the PNS, that should be considered in the etiology of AD. We also point out some of the contradictory data and remaining knowledge gaps that require further research to finally fully understand the cause of AD in humans.

Astrocyte senescence and SASP in neurodegeneration: tau joins the loop

Tau accumulation is a core component of Alzheimer's disease and other neurodegenerative tauopathies. While tau's impact on neurons is a major area of research, the effect of extracellular tau on astrocytes is largely unknown. This article summarizes our recent studies showing that astrocyte senescence plays a critical role in neurodegenerative diseases and integrates extracellular tau into the regulatory loop of senescent astrocyte-mediated neurotoxicity. Human astrocytes in vitro undergoing senescence were shown to acquire the inflammatory senescence-associated secretory phenotype (SASP) and toxicity to neurons, which may recapitulate aging- and disease-associated neurodegeneration. Here, we show that human astrocytes exposed to extracellular tau in vitro also undergo cellular senescence and acquire a neurotoxic SASP (e.g. IL-6 secretion), with oxidative stress response (indicated by upregulated NRF2 target genes) and a possible activation of inflammasome (indicated by upregulated ASC and IL-1β). These findings suggest that senescent astrocytes induced by various conditions and insults, including tau exposure, may represent a therapeutic target to inhibit or delay the progression of neurodegenerative diseases. We also discuss the pathological activity of extracellular tau in microglia and astrocytes, the disease relevance and diversity of tau forms, therapeutics targeting senescence in neurodegeneration, and the roles of p53 and its isoforms in astrocyte-mediated neurotoxicity and neuroprotection.

Exploiting formyl peptide receptor 2 to promote microglial resolution: a new approach to Alzheimer's disease treatment

Alzheimer's disease and dementia are among the most significant current healthcare challenges given the rapidly growing elderly population, and the almost total lack of effective therapeutic interventions. Alzheimer's disease pathology has long been considered in terms of accumulation of amyloid beta and hyperphosphorylated tau, but the importance of neuroinflammation in driving disease has taken greater precedence over the last 15-20 years. Inflammatory activation of the primary brain immune cells, the microglia, has been implicated in Alzheimer's pathogenesis through genetic, preclinical, imaging and postmortem human studies, and strategies to regulate microglial activity may hold great promise for disease modification. Neuroinflammation is necessary for defence of the brain against pathogen invasion or damage but is normally self-limiting due to the engagement of endogenous pro-resolving circuitry that terminates inflammatory activity, a process that appears to fail in Alzheimer's disease. Here, we discuss the potential for a major regulator and promoter of resolution, the receptor FPR2, to restrain pro-inflammatory microglial activity, and propose that it may serve as a valuable target for therapeutic investigation in Alzheimer's disease.

Reactive microglia enhance the transmission of exosomal alpha-synuclein via toll-like receptor 2

Increasing evidence suggests that microglial activation is strongly linked to the initiation and progression of Parkinson's disease (PD). Cell-to-cell propagation of α-synuclein (α-syn) pathology is a highlighted feature of PD, and the focus of such research has been primarily on neurons. However, recent studies as well as the data contained herein suggest that microglia, the primary phagocytes in the brain, play a direct role in the spread of α-syn pathology. Recent data revealed that plasma exosomes derived from PD patients (PD-EXO) carry pathological α-syn and target microglia preferentially. Hence, PD-EXO is likely a key tool for investigating the role of microglia in α-syn transmission. We showed that intrastriatal injection of PD-EXO resulted in the propagation of exosomal α-syn from microglia to neurons following microglia activation. Toll-like receptor 2 (TLR2) in microglia was activated by exosomal α-syn and acted as a crucial mediator of PD-EXO-induced microglial activation. Additionally, partial microglia depletion resulted in a significant decrease of exogenous α-syn in the substantia nigra (SN). Furthermore, exosomal α-syn internalization was initiated by binding to TLR2 of microglia. Excessive α-syn phagocytosis may induce the inflammatory responses of microglia and provide the seed for microglia-to-neuron transmission. Consistently, TLR2 silencing in microglia mitigated α-syn pathology in vivo. Overall, the present data support the idea that the interaction of exosomal α-syn and microglial TLR2 contribute to excessive α-syn phagocytosis and microglial activation, which lead to the further propagation and spread of α-syn pathology, thereby highlighting the pivotal roles of reactive microglia in α-syn transmission.

Alterations in the Gut-Microbial-Inflammasome-Brain Axis in a Mouse Model of Alzheimer's Disease

Alzheimer’s disease (AD), a progressive neurodegenerative disorder characterized by memory loss and cognitive decline, is a major cause of death and disability among the older population. Despite decades of scientific research, the underlying etiological triggers are unknown. Recent studies suggested that gut microbiota can influence AD progression; however, potential mechanisms linking the gut microbiota with AD pathogenesis remain obscure. In the present study, we provided a potential mechanistic link between dysbiotic gut microbiota and neuroinflammation associated with AD progression. Using a mouse model of AD, we discovered that unfavorable gut microbiota are correlated with abnormally elevated expression of gut NLRP3 and lead to peripheral inflammasome activation, which in turn exacerbates AD-associated neuroinflammation. To this end, we observe significantly altered gut microbiota compositions in young and old 5xFAD mice compared to age-matched non-transgenic mice. Moreover, 5xFAD mice demonstrated compromised gut barrier function as evident from the loss of tight junction and adherens junction proteins compared to non-transgenic mice. Concurrently, we observed increased expression of NLRP3 inflammasome and IL-1β production in the 5xFAD gut. Consistent with our hypothesis, increased gut–microbial–inflammasome activation is positively correlated with enhanced astrogliosis and microglial activation, along with higher expression of NLRP3 inflammasome and IL-1β production in the brains of 5xFAD mice. These data indicate that the elevated expression of gut–microbial–inflammasome components may be an important trigger for subsequent downstream activation of inflammatory and potentially cytotoxic mediators, and gastrointestinal NLRP3 may promote NLRP3 inflammasome-mediated neuroinflammation. Thus, modulation of the gut microbiota may be a potential strategy for the treatment of AD-related neurological disorders in genetically susceptible hosts.

Preoperative Microbiomes and Intestinal Barrier Function Can Differentiate Prodromal Alzheimer's Disease From Normal Neurocognition in Elderly Patients Scheduled to Undergo Orthopedic Surgery

Objective

Emerging evidence links perturbations in the microbiome to neurodegeneration in amnestic mild cognitive impairment (aMCI) and Alzheimer’s disease (AD) and to surgical stress. In this study, we attempted to identify preoperative differences intestinal microbiota (IM) and barrier function between pAD [prodromal AD: Subjective cognitive decline (SCD) and aMCI] patients and normal neurocognition (NC) patients. Additionally, the potential associations between IM and barrier function, inflammation, and the clinical characteristics of pAD were evaluated.

Design

Eighty elderly patients scheduled to undergo orthopedic surgery were consecutively enrolled and grouped as NC, SCD, and aMCI following neuropsychological assessment. IM was determined by 16S rRNA MiSeq sequencing, and PICRUSt was used to predict functional shifts in IM. Furthermore, we investigated the association between IM and plasma claudin-1, occludin, LPS, systemic inflammatory cytokines, neuropsychological assessment, and clinical characteristics.

Results

There was a lower Chao1 index in the SCD group (P = 0.004) and differences in beta diversity among the three groups (PCA: P = 0.026, PCoA: P= 0.004). The relative abundance of Bacteroidetes was higher in the SCD group (P = 0.016, P = 0.008), and Firmicutes were more enriched in the aMCI group than in the SCD group (P= 0.026). At the family level, the total abundance of Gram-negative bacteria was higher in the SCD group than in the aMCI group (P = 0.047), and the Christensenellaceae family was detected at lower levels in the SCD and aMCI groups than in the NC group (P= 0.039). At the genus level, the eleven short-chain fatty acid (SCFA)-producing bacteria exhibited differences among the three groups. PICRUSt analysis showed that the pathways involved in SCFA catabolism, biosynthesis, and adherent junctions were reduced in SCD patients, and lipid synthesis proteins were reduced in pAD patients. Meanwhile, elevated plasma LPS and CRP were observed in SCD patients, and higher plasma occludin in aMCI patients. The IM was correlated with plasma claudin-1, LPS, inflammatory factors, neuropsychological assessment, and clinical characteristics.

Conclusion

The intestines of SCD and aMCI patients preoperatively exhibited IM dysbiosis and barrier dysfunction, and elevated plasma LPS and CRP were observed in SCD patients.

Implications for microglial sex differences in tau-related neurodegenerative diseases

Tauopathies are a group of neurodegenerative diseases that involve pathological changes to the tau protein. Neuroinflammation is a commonly reported feature of tauopathies that has been demonstrated to exacerbate tau pathology and, hence, neurodegeneration. Microglia can mediate the inflammatory response in order to maintain brain homeostasis. In the aged brain, microglia are reported to undergo morphological and functional changes, adopting a pro-inflammatory profile and loss of homeostatic functions. Dystrophic and dysfunctional microglia are associated with tau pathology in the healthy and diseased brain which is proposed to contribute to disease development and progression. Microglia have also been recently demonstrated to possess sexually dimorphic roles in the developing, adult and aged brain. The sex differences in microglial functionality suggest that microglia may contribute to tauopathies which may differ between sexes. This review highlights the detrimental loop between age-related microglial changes and tau pathology with implications for microglial sexual dichotomy.

Plaque associated microglia hyper-secrete extracellular vesicles and accelerate tau propagation in a humanized APP mouse model

Background

Recent studies suggest that microglia contribute to tau pathology progression in Alzheimer’s disease. Amyloid plaque accumulation transforms microglia, the primary innate immune cells in the brain, into neurodegenerative microglia (MGnD), which exhibit enhanced phagocytosis of plaques, apoptotic neurons and dystrophic neurites containing aggregated and phosphorylated tau (p-tau). It remains unclear how microglia promote disease progression while actively phagocytosing pathological proteins, therefore ameliorating pathology.

Methods

Adeno-associated virus expressing P301L tau mutant (AAV-P301L-tau) was stereotaxically injected into the medial entorhinal cortex (MEC) in C57BL/6 (WT) and humanized APP mutant knock-in homozygote (App^NL-G-F) mice at 5 months of age. Mice were fed either chow containing a colony stimulating factor-1 receptor inhibitor (PLX5622) or control chow from 4 to 6 months of age to test the effect of microglia depletion. Animals were tested at 6 months of age for immunofluorescence, biochemistry, and FACS of microglia. In order to monitor microglial extracellular vesicle secretion in vivo, a novel lentiviral EV reporter system was engineered to express mEmerald-CD9 (mE-CD9) specifically in microglia, which was injected into the same region of MEC.

Results

Expressing P301L tau mutant in the MEC induced tau propagation to the granule cell layer of the hippocampal dentate gyrus, which was significantly exacerbated in App^NL-G-F mice compared to WT control mice. Administration of PLX5622 depleted nearly all microglia in mouse brains and dramatically reduced propagation of p-tau in WT and to a greater extent in App^NL-G-F mice, although it increased plaque burden and plaque-associated p-tau⁺ dystrophic neurites. Plaque-associated MGnD microglia strongly expressed an EV marker, tumor susceptibility gene 101, indicative of heightened synthesis of EVs. Intracortical injection of mE-CD9 lentivirus successfully induced microglia-specific expression of mE-CD9⁺ EV particles, which were significantly enhanced in Mac2⁺ MGnD microglia compared to Mac2⁻ homeostatic microglia. Finally, consecutive intracortical injection of mE-CD9 lentivirus and AAV-P301L-tau into App^NL-G-F mice revealed encapsulation of p-tau in microglia-specific mE-CD9⁺ EVs as determined by super-resolution microscopy and immuno-electron microscopy.

Discussion

Our findings suggest that MGnD microglia hyper-secrete p-tau⁺ EVs while compacting Aβ plaques and clearing NP tau, which we propose as a novel mechanistic link between amyloid plaque deposition and exacerbation of tau propagation in App^NL-G-F mice.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00440-9.

Fenamates as Potential Therapeutics for Neurodegenerative Disorders

Neurodegenerative disorders are desperately lacking treatment options. It is imperative that drug repurposing be considered in the fight against neurodegenerative diseases. Fenamates have been studied for efficacy in treating several neurodegenerative diseases. The purpose of this review is to comprehensively present the past and current research on fenamates in the context of neurodegenerative diseases with a special emphasis on tolfenamic acid and Alzheimer's disease. Furthermore, this review discusses the major molecular pathways modulated by fenamates.

Techniques to Study Inflammasome Activation and Inhibition by Small Molecules

Inflammasomes are immune cytosolic oligomers involved in the initiation and progression of multiple pathologies and diseases. The tight regulation of these immune sensors is necessary to control an optimal inflammatory response and recover organism homeostasis. Prolonged activation of inflammasomes result in the development of chronic inflammatory diseases, and the use of small drug-like inhibitory molecules are emerging as promising anti-inflammatory therapies. Different aspects have to be taken in consideration when designing inflammasome inhibitors. This review summarizes the different techniques that can be used to study the mechanism of action of potential inflammasome inhibitory molecules.

Astrocytic Propagation of Tau in the Context of Alzheimer's Disease

More than 6 million Americans are currently living with Alzheimer's disease (AD), and the incidence is growing rapidly with our aging population. Numerous therapeutics have failed to make it to the clinic, potentially due to a focus on presumptive pathogenic proteins instead of cell-type-specific signaling mechanisms. The tau propagation hypothesis that inter-neuronal tau transfer drives AD pathology has recently garnered attention, as accumulation of pathological tau in the brain has high clinical significance in correlating with progression of cognitive AD symptoms. However, studies on tau pathology in AD are classically neuron-centric and have greatly overlooked cell-type specific effects of tau internalization, degradation, and propagation. While the contribution of microglia to tau processing and propagation is beginning to be recognized and understood, astrocytes, glial cells in the brain important for maintaining neuronal metabolic, synaptic, trophic, and immune function which can produce, internalize, degrade, and propagate tau are understudied in their ability to affect AD progression through tau pathology. Here, we showcase evidence for whether tau uptake by astrocytes may be beneficial or detrimental to neuronal health and how astrocytes and their immunometabolic functions may be key targets for future successful AD therapies.

Tau antibody isotype induces differential effects following passive immunisation of tau transgenic mice

One of the main pathological hallmarks of Alzheimer's disease (AD) is the intraneuronal accumulation of hyperphosphorylated tau. Passive immunotherapy is a promising strategy for the treatment of AD and there are currently a number of tau-specific monoclonal antibodies in clinical trials. A proposed mechanism of action is to engage and clear extracellular, pathogenic forms of tau. This process has been shown in vitro to be facilitated by microglial phagocytosis through interactions between the antibody-tau complex and microglial Fc-receptors. As this interaction is mediated by the conformation of the antibody's Fc domain, this suggests that the antibody isotype may affect the microglial phagocytosis and clearance of tau, and hence, the overall efficacy of tau antibodies. We therefore aimed to directly compare the efficacy of the tau-specific antibody, RN2N, cloned into a murine IgG1/κ framework, which has low affinity Fc-receptor binding, to that cloned into a murine IgG2a/κ framework, which has high affinity Fc-receptor binding. Our results demonstrate, for RN2N, that although enhanced microglial activation via the IgG2a/κ isotype increased extracellular tau phagocytosis in vitro, the IgG1/κ isoform demonstrated enhanced ability to reduce tau pathology and microgliosis following passive immunisation of the P301L tau transgenic pR5 mouse model.

Neurodegenerative Disease and the NLRP3 Inflammasome

The prevalence of neurodegenerative disease has increased significantly in recent years, and with a rapidly aging global population, this trend is expected to continue. These diseases are characterised by a progressive neuronal loss in the brain or peripheral nervous system, and generally involve protein aggregation, as well as metabolic abnormalities and immune dysregulation. Although the vast majority of neurodegeneration is idiopathic, there are many known genetic and environmental triggers. In the past decade, research exploring low-grade systemic inflammation and its impact on the development and progression of neurodegenerative disease has increased. A particular research focus has been whether systemic inflammation arises only as a secondary effect of disease or is also a cause of pathology. The inflammasomes, and more specifically the NLRP3 inflammasome, a crucial component of the innate immune system, is usually activated in response to infection or tissue damage. Dysregulation of the NLRP3 inflammasome has been implicated in the progression of several neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and prion diseases. This review aims to summarise current literature on the role of the NLRP3 inflammasome in the pathogenesis of neurodegenerative diseases, and recent work investigating NLRP3 inflammasome inhibition as a potential future therapy.

Tau: Enabler of diverse brain disorders and target of rapidly evolving therapeutic strategies

Several lines of evidence implicate the protein tau in the pathogenesis of multiple brain disorders, including Alzheimer's disease, other neurodegenerative conditions, autism, and epilepsy. Tau is abundant in neurons and interacts with microtubules, but its main functions in the brain remain to be defined. These functions may involve the regulation of signaling pathways relevant to diverse biological processes. Informative disease models have revealed a plethora of abnormal tau species and mechanisms that might contribute to neuronal dysfunction and loss, but the relative importance of their respective contributions is uncertain. This knowledge gap poses major obstacles to the development of truly impactful therapeutic strategies. The current expansion and intensification of efforts to translate mechanistic insights into tau-related therapeutics should address this issue and could deliver better treatments for a host of devastating conditions.

Gamma subunit of complement component 8 is a neuroinflammation inhibitor

The complement system is part of the innate immune system that comprises several small proteins activated by sequential cleavages. The majority of these complement components, such as components 3a (C3a) and C5a, are chemotactic and pro-inflammatory. However, in this study, we revealed an inhibitory role of complement component 8 gamma (C8G) in neuroinflammation. In patients with Alzheimer's disease, who exhibit strong neuroinflammation, we found higher C8G levels in brain tissue, CSF, and plasma. Our novel findings also showed that the expression level of C8G increases in the inflamed mouse brain, and that C8G is mainly localized to brain astrocytes. Experiments using recombinant C8G protein and shRNA-mediated knockdown showed that C8G inhibits glial hyperactivation, neuroinflammation, and cognitive decline in acute and chronic animal models of Alzheimer's disease. Additionally, we identified sphingosine-1-phosphate receptor 2 (S1PR2) as a novel interaction protein of C8G and demonstrated that astrocyte-derived C8G interacts with S1PR2 to antagonize the pro-inflammatory action of S1P in microglia. Taken together, our results reveal the previously unrecognized role of C8G as a neuroinflammation inhibitor. Our findings pave the way towards therapeutic containment of neuroinflammation in Alzheimer's disease and related neurological diseases.

Abundance of P-Glycoprotein and Other Drug Transporters at the Human Blood-Brain Barrier in Alzheimer's Disease: A Quantitative Targeted Proteomic Study

The human blood-brain barrier (BBB) transporter P-gp can efflux amyloid-β (Aβ) out of the central nervous system (CNS). Aβ is thought to be the causative agent for Alzheimer's disease (AD). Using positron emission tomography imaging, we have shown that BBB P-gp activity is reduced in AD, as quantified by the in vivo brain distribution of the P-gp probe [11 C]-verapamil. Therefore, the aim of this study was to determine whether this reduced BBB P-gp activity in AD was due to decreased P-gp abundance at the BBB. Using targeted proteomics, we quantified the abundance of P-gp and other drug transporters in gray matter brain microvessels isolated from 43 subjects with AD and 38 age-matched controls (AMCs) from regions affected by AD (hippocampus and the parietal lobe of the brain cortex) and not affected by AD (cerebellum). First, P-gp abundance was decreased in the BBB of the hippocampus vs. the cerebellum in both subjects with AD and AMCs, and therefore was not AD-related. In addition, gray matter BBB abundance of P-gp (and of other transporters) in the hippocampus and the parietal lobe was not different between AD and AMC. The gray matter BBB abundance of all drug transporters decreased with age, likely due to age-dependent decrease in the density of brain microvessels. Collectively, the observed reduced in vivo cerebral BBB P-gp activity in AD cannot be explained by reduced abundance of P-gp at the BBB. Nevertheless, the drug transporter abundance at the human gray matter BBB data provided here can be used to predict brain distribution of drugs targeted to treat CNS diseases, including AD.

Neuroinflammation and microglial activation in Alzheimer disease: where do we go from here?

Alzheimer disease (AD) is the most common form of neurodegenerative disease, estimated to contribute 60-70% of all cases of dementia worldwide. According to the prevailing amyloid cascade hypothesis, amyloid-β (Aβ) deposition in the brain is the initiating event in AD, although evidence is accumulating that this hypothesis is insufficient to explain many aspects of AD pathogenesis. The discovery of increased levels of inflammatory markers in patients with AD and the identification of AD risk genes associated with innate immune functions suggest that neuroinflammation has a prominent role in the pathogenesis of AD. In this Review, we discuss the interrelationships between neuroinflammation and amyloid and tau pathologies as well as the effect of neuroinflammation on the disease trajectory in AD. We specifically focus on microglia as major players in neuroinflammation and discuss the spatial and temporal variations in microglial phenotypes that are observed under different conditions. We also consider how these cells could be modulated as a therapeutic strategy for AD.

Chalcone and its analogs: Therapeutic and diagnostic applications in Alzheimer's disease

Chalcone [(E)-1,3-diphenyl-2-propene-1-one], a small molecule with α, β unsaturated carbonyl group is a precursor or component of many natural flavonoids and isoflavonoids. It is one of the privileged structures in medicinal chemistry. It possesses a wide range of biological activities encouraging many medicinal chemists to study this scaffold for its usefulness to oncology, infectious diseases, virology and neurodegenerative diseases including Alzheimer's disease (AD). Small molecular size, convenient and cost-effective synthesis, and flexibility for modifications to modulate lipophilicity suitable for blood brain barrier (BBB) permeability make chalcones a preferred candidate for their therapeutic and diagnostic potential in AD. This review summarizes and highlights the importance of chalcone and its analogs as single target small therapeutic agents, multi-target directed ligands (MTDLs) as well as molecular imaging agents for AD. The information summarized here will guide many medicinal chemist and researchers involved in drug discovery to consider chalcone as a potential scaffold for the development of anti-AD agents including theranostics.

Microbiome or Infections: Amyloid-Containing Biofilms as a Trigger for Complex Human Diseases

The human microbiota is the community of microorganisms that live upon or within their human host. The microbiota consists of various microorganisms including bacteria, fungi, viruses, and archaea; the gut microbiota is comprised mostly of bacteria. Many bacterial species within the gut microbiome grow as biofilms, which are multicellular communities embedded in an extracellular matrix. Studies have shown that the relative abundances of bacterial species, and therefore biofilms and bacterial byproducts, change during progression of a variety of human diseases including gastrointestinal, autoimmune, neurodegenerative, and cancer. Studies have shown the location and proximity of the biofilms within the gastrointestinal tract might impact disease outcome. Gram-negative enteric bacteria secrete the amyloid curli, which makes up as much as 85% of the extracellular matrix of enteric biofilms. Curli mediates cell-cell attachment and attachment to various surfaces including extracellular matrix components such as fibronectin and laminin. Structurally, curli is strikingly similar to pathological and immunomodulatory human amyloids such as amyloid-β, which has been implicated in Alzheimer's disease, α-synuclein, which is involved in Parkinson's disease, and serum amyloid A, which is secreted during the acute phase of inflammation. The immune system recognizes both bacterial amyloid curli and human amyloids utilizing the same receptors, so curli also induces inflammation. Moreover, recent work indicates that curli can participate in the self-assembly process of pathological human amyloids. Curli is found within biofilms of commensal enteric bacteria as well as invasive pathogens; therefore, evidence suggests that curli contributes to complex human diseases. In this review, we summarize the recent findings on how bacterial biofilms containing curli participate in the pathological and immunological processes in gastrointestinal diseases, systemic autoimmune diseases, and neurodegenerative diseases.

Neuroinflammation in Prion Disease

Neuroinflammation, typically manifest as microglial activation and astrogliosis accompanied by transcriptomic alterations, represents a common hallmark of various neurodegenerative conditions including prion diseases. Microglia play an overall neuroprotective role in prion disease, whereas reactive astrocytes with aberrant phenotypes propagate prions and contribute to prion-induced neurodegeneration. The existence of heterogeneous subpopulations and dual functions of microglia and astrocytes in prion disease make them potential targets for therapeutic intervention. A variety of neuroinflammation-related molecules are involved in prion pathogenesis. Therapeutics targeting neuroinflammation represents a novel approach to combat prion disease. Deciphering neuroinflammation in prion disease will deepen our understanding of pathogenesis of other neurodegenerative disorders.

A Fundamental Role for Oxidants and Intracellular Calcium Signals in Alzheimer's Pathogenesis-And How a Comprehensive Antioxidant Strategy May Aid Prevention of This Disorder

Oxidative stress and increased cytoplasmic calcium are key mediators of the detrimental effects on neuronal function and survival in Alzheimer's disease (AD). Pathways whereby these perturbations arise, and then prevent dendritic spine formation, promote tau hyperphosphorylation, further amplify amyloid β generation, and induce neuronal apoptosis, are described. A comprehensive program of nutraceutical supplementation, comprised of the NADPH oxidase inhibitor phycocyanobilin, phase two inducers, the mitochondrial antioxidant astaxanthin, and the glutathione precursor N-acetylcysteine, may have important potential for antagonizing the toxic effects of amyloid β on neurons and thereby aiding prevention of AD. Moreover, nutraceutical antioxidant strategies may oppose the adverse impact of amyloid β oligomers on astrocyte clearance of glutamate, and on the ability of brain capillaries to export amyloid β monomers/oligomers from the brain. Antioxidants, docosahexaenoic acid (DHA), and vitamin D, have potential for suppressing microglial production of interleukin-1β, which potentiates the neurotoxicity of amyloid β. Epidemiology suggests that a health-promoting lifestyle, incorporating a prudent diet, regular vigorous exercise, and other feasible measures, can cut the high risk for AD among the elderly by up to 60%. Conceivably, complementing such lifestyle measures with long-term adherence to the sort of nutraceutical regimen outlined here may drive down risk for AD even further.