Targeting microglia for the treatment of Alzheimer's Disease

Epigallocatechin Gallate Modulates Microglia Phenotype to Suppress Pro-inflammatory Signalling Cues and Inhibit Phagocytosis

Microglia are crucial players in the pathogenesis of late-onset Alzheimer's disease (AD), with evidence for both deleterious and beneficial effects. Identifying interventions to modulate microglial responsiveness, promote amyloid β (Aβ) clearance, disrupt plaque formation, or dampen excessive inflammation has therapeutic potential. Bioavailable flavonoids, such as the flavan 3-ols, are of interest due to their antioxidant, metal chelating, signalling, and anti-inflammatory potential. Primary microglia were treated with a series of structurally related flavanol 3-ols to assess effects on phagocytosis, cytokine release, and transcriptional responses by RNA sequencing. Data indicated that the extent of hydroxylation and the presence of the galloyl moiety were strong determinants of flavan 3-ol activity. Epigallocatechin gallate (EGCG) was the most effective flavan-3-ol tested and strongly inhibited phagocytosis of Aβ independent of any metal chelating properties, suggesting a more direct modulation of microglia responsiveness. EGCG was broadly anti-inflammatory, reducing cytokine release and downregulating transcription, particularly of components of the microglia extracellular matrix such as MMP3 and SerpinB2. Collectively, this brings new insight into the actions of flavonoids on microglial responsiveness with potential implications for the therapeutic use of EGCG and structurally related flavanol-3-ols in AD.

Decoding the role of the CCL2/CCR2 axis in Alzheimer's disease and innovating therapeutic approaches: Keeping All options open

Alzheimer's disease (AD), as a neurodegenerative disorder, distresses the elderly in large numbers and is characterized by β-amyloid (Aβ) accumulation, elevated tau protein levels, and chronic inflammation. The brain's immune system is aided by microglia and astrocytes, which produce chemokines and cytokines. Nevertheless, dysregulated expression can cause hyperinflammation and lead to neurodegeneration. CCL2/CCR2 chemokines are implicated in neurodegenerative diseases exacerbating. Inflicting damage on nerves and central nervous system (CNS) cells is the function of this axis, which recruits and migrates immune cells, including monocytes and macrophages. It has been shown that targeting the CCL2/CCR2 axis may be a therapeutic option for inflammatory diseases. Using the current knowledge about the involvement of the CCL2/CCR2 axis in the immunopathogenesis of AD, this comprehensive review synthesizes existing information. It also explores potential therapeutic options, including modulation of the CCL2/CCR2 axis as a possible strategy in AD.

PKCδ serves as a potential biomarker and therapeutic target for microglia-mediated neuroinflammation in Alzheimer's disease

INTRODUCTION

To investigate the role of a novel type of protein kinase C delta (PKCδ) in the neuroinflammation of Alzheimer's disease (AD).

METHODS

We analyzed PKCδ and inflammatory cytokines levels in cerebrospinal fluid (CSF) of AD and normal controls, as well as their correlations. The cellular expression pattern of PKCδ and the effects of PKCδ modulation on microglia-mediated neuroinflammation were evaluated by quantitative real-time polymerase chain reaction (qRT-PCR), western blot, RNA sequencing (RNA-seq), and immunofluorescence staining.

RESULTS

PKCδ levels were increased dramatically in the CSF of AD patients and positively correlated with cytokines. PKCδ is expressed mainly in microglia in the brain. Amyloid beta (Aβ) stimulation increased PKCδ expression and secretion, which led to upregulation of the nuclear factor kappa B (NF-κB) pathway and overproduction of proinflammatory cytokines. Downregulation or inhibition of PKCδ attenuated Aβ-induced microglial responses and improved cognitive function in an AD mouse model.

DISCUSSION

Our study identifies PKCδ as a potential biomarker and therapeutic target for microglia-mediated neuroinflammation in AD.

HIGHLIGHTS

Protein kinase C delta (PKCδ) levels increase in cerebrospinal fluid (CSF) of patients with Alzheimer's disease (AD), and positively correlate with elevated inflammatory cytokines in human subjects. PKCδ is expressed mainly in microglia in vivo, whereas amyloid beta (Aβ) stimulation increases PKCδ expression and secretion, causing upregulation of the nuclear factor kappa B (NF-κB) pathway and production of inflammatory cytokines. Downregulation or inhibition of PKCδ attenuates Aβ-enhanced NF-κB signaling and cytokine production in microglia and improves cognitive function in AD mice. PKCδ serves as a potential biomarker and therapeutic target for microglia-mediated neuroinflammation in AD.

Microglial apolipoprotein E particles contribute to neuronal senescence and synaptotoxicity

Apolipoprotein E (apoE) plays a crucial role in the pathogenesis of Alzheimer's disease (AD). Microglia exhibit a substantial upregulation of apoE in AD-associated circumstances, despite astrocytes being the primary source of apoE expression and secretion in the brain. Although the role of astrocytic apoE in the brain has been extensively investigated, it remains unclear that whether and how apoE particles generated from astrocytes and microglia differ in biological characteristic and function. Here, we demonstrate the differences in size between apoE particles generated from microglia and astrocytes. Microglial apoE particles impair neurite growth and synapses, and promote neuronal senescence, whereas depletion of GPNMB (glycoprotein non-metastatic melanoma protein B) in microglial apoE particles mitigated these deleterious effects. In addition, human APOE4-expressing microglia are more neurotoxic than APOE3-bearing microglia. For the first time, these results offer concrete evidence that apoE particles produced by microglia are involved in neuronal senescence and toxicity.

Current understanding on TREM-2 molecular biology and physiopathological functions

Triggering receptor expressed on myeloid cells 2 (TREM-2), a glycosylated receptor belonging to the immunoglobin superfamily and especially expressed in the myeloid cell lineage, is frequently explained as a reminiscent receptor for both adaptive and innate immunity regulation. TREM-2 is also acknowledged to influence NK cell differentiation via the PI3K and PLCγ signaling pathways, as well as the partial activation or direct inhibition of T cells. Additionally, TREM-2 overexpression is substantially linked to cell-specific functions, such as enhanced phagocytosis, reduced toll-like receptor (TLR)-mediated inflammatory cytokine production, increased transcription of anti-inflammatory cytokines, and reshaped T cell function. Whereas TREM-2-deficient cells exhibit diminished phagocytic function and enhanced proinflammatory cytokines production, proceeding to inflammatory injuries and an immunosuppressive environment for disease progression. Despite the growing literature supporting TREM-2+ cells in various diseases, such as neurodegenerative disorders and cancer, substantial facets of TREM-2-mediated signaling remain inadequately understood relevant to pathophysiology conditions. In this direction, herein, we have summarized the current knowledge on TREM-2 biology and cell-specific TREM-2 expression, particularly in the modulation of pivotal TREM-2-dependent functions under physiopathological conditions. Furthermore, molecular regulation and generic biological relevance of TREM-2 are also discussed, which might provide an alternative approach for preventing or reducing TREM-2-associated deformities. At last, we discussed the TREM-2 function in supporting an immunosuppressive cancer environment and as a potential drug target for cancer immunotherapy. Hence, summarized knowledge of TREM-2 might provide a window to overcome challenges in clinically effective therapies for TREM-2-induced diseases in humans.

Potential of Neuroinflammation-Modulating Strategies in Tuberculous Meningitis: Targeting Microglia

Tuberculous meningitis (TBM) is the most severe complication of tuberculosis (TB) and is associated with high rates of disability and mortality. Mycobacterium tuberculosis (M. tb), the infectious agent of TB, disseminates from the respiratory epithelium, breaks through the blood-brain barrier, and establishes a primary infection in the meninges. Microglia are the core of the immune network in the central nervous system (CNS) and interact with glial cells and neurons to fight against harmful pathogens and maintain homeostasis in the brain through pleiotropic functions. However, M. tb directly infects microglia and resides in them as the primary host for bacillus infections. Largely, microglial activation slows disease progression. The non-productive inflammatory response that initiates the secretion of pro-inflammatory cytokines and chemokines may be neurotoxic and aggravate tissue injuries based on damages caused by M. tb. Host-directed therapy (HDT) is an emerging strategy for modulating host immune responses against diverse diseases. Recent studies have shown that HDT can control neuroinflammation in TBM and act as an adjunct therapy to antibiotic treatment. In this review, we discuss the diverse roles of microglia in TBM and potential host-directed TB therapies that target microglia to treat TBM. We also discuss the limitations of applying each HDT and suggest a course of action for the near future.

Gut instincts: Unveiling the connection between gut microbiota and Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder marked by neuroinflammation and gradual cognitive decline. Recent research has revealed that the gut microbiota (GM) plays an important role in the pathogenesis of AD through the microbiota-gut-brain axis. However, the mechanism by which GM and microbial metabolites alter brain function is not clearly understood. GM dysbiosis increases the permeability of the intestine, alters the blood-brain barrier permeability, and elevates proinflammatory mediators causing neurodegeneration. This review article introduced us to the composition and functions of GM along with its repercussions of dysbiosis in relation to AD. We also discussed the importance of the gut-brain axis and its role in communication. Later we focused on the mechanism behind gut dysbiosis and the progression of AD including neuroinflammation, oxidative stress, and changes in neurotransmitter levels. Furthermore, we highlighted recent developments in AD management, such as microbiota-based therapy, dietary interventions like prebiotics, probiotics, and fecal microbiota transplantation. Finally, we concluded with challenges and future directions in AD research based on GM.

Single-Cell Transcriptional Profiling and Gene Regulatory Network Modeling in Tg2576 Mice Reveal Gender-Dependent Molecular Features Preceding Alzheimer-Like Pathologies

Alzheimer's disease (AD) onset and progression is influenced by a complex interplay of several environmental and genetic factors, one of them gender. Pronounced gender differences have been observed both in the relative risk of developing AD and in clinical disease manifestations. A molecular level understanding of these gender disparities is still missing, but could provide important clues on cellular mechanisms modulating the disease and reveal new targets for gender-oriented disease-modifying precision therapies. We therefore present here a comprehensive single-cell analysis of disease-associated molecular gender differences in transcriptomics data from the neocortex, one of the brain regions most susceptible to AD, in one of the most widely used AD mouse models, the Tg2576 model. Cortical areas are also most commonly used in studies of post-mortem AD brains. To identify disease-linked molecular processes that occur before the onset of detectable neuropathology, we focused our analyses on an age with no detectable plaques and microgliosis. Cell-type specific alterations were investigated at the level of individual genes, pathways, and gene regulatory networks. The number of differentially expressed genes (DEGs) was not large enough to build context-specific gene regulatory networks for each individual cell type, and thus, we focused on the study of cell types with dominant changes and included analyses of changes across the combination of cell types. We observed significant disease-associated gender differences in cellular processes related to synapse organization and reactive oxygen species metabolism, and identified a limited set of transcription factors, including Egr1 and Klf6, as key regulators of many of the disease-associated and gender-dependent gene expression changes in the model. Overall, our analyses revealed significant cell-type specific gene expression changes in individual genes, pathways and sub-networks, including gender-specific and gender-dimorphic changes in both upstream transcription factors and their downstream targets, in the Tg2576 AD model before the onset of overt disease. This opens a window into molecular events that could determine gender-susceptibility to AD, and uncovers tractable target candidates for potential gender-specific precision medicine for AD.

Interactions between CNS and immune cells in tuberculous meningitis

The central nervous system (CNS) harbors its own special immune system composed of microglia in the parenchyma, CNS-associated macrophages (CAMs), dendritic cells, monocytes, and the barrier systems within the brain. Recently, advances in the immune cells in the CNS provided new insights to understand the development of tuberculous meningitis (TBM), which is the predominant form of Mycobacterium tuberculosis (M.tb) infection in the CNS and accompanied with high mortality and disability. The development of the CNS requires the protection of immune cells, including macrophages and microglia, during embryogenesis to ensure the accurate development of the CNS and immune response following pathogenic invasion. In this review, we summarize the current understanding on the CNS immune cells during the initiation and development of the TBM. We also explore the interactions of immune cells with the CNS in TBM. In the future, the combination of modern techniques should be applied to explore the role of immune cells of CNS in TBM.

Monoclonal Antibody Therapy in Alzheimer's Disease

Alzheimer's disease is a neurodegenerative condition marked by the progressive deterioration of cognitive abilities, memory impairment, and the accumulation of abnormal proteins, specifically beta-amyloid plaques and tau tangles, within the brain. Despite extensive research efforts, Alzheimer's disease remains without a cure, presenting a significant global healthcare challenge. Recently, there has been an increased focus on antibody-based treatments as a potentially effective method for dealing with Alzheimer's disease. This paper offers a comprehensive overview of the current status of research on antibody-based molecules as therapies for Alzheimer's disease. We will briefly mention their mechanisms of action, therapeutic efficacy, and safety profiles while addressing the challenges and limitations encountered during their development. We also highlight some crucial considerations in antibody-based treatment development, including patient selection criteria, dosing regimens, or safety concerns. In conclusion, antibody-based therapies present a hopeful outlook for addressing Alzheimer's disease. While challenges remain, the accumulating evidence suggests that these therapies may offer substantial promise in ameliorating or preventing the progression of this debilitating condition, thus potentially enhancing the quality of life for the millions of individuals and families affected by Alzheimer's disease worldwide.

Baicalein promotes the microglia M2 polarization and suppresses apoptosis by targeting HMOX1/PDE4D to alleviate Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder that has quickly becoming one of the most expensive, lethal, and burdening diseases of this century. In the past twenty years, hundreds of drugs have been tested while only several have been authorized by FDA for AD treatment, hence, searching for candidate agent with therapeutic potential for AD is imminent. Controlling polarization direction of microglia is crucial in AD therapy. Recent research suggests that baicalein has potential to reduce neuroinflammation and prevent neurodegenerative diseases by affecting microglia, while the specific molecular mechanism of baicalein in regulating microglia in the treatment of AD is still unclear. In this study, we investigated how baicalein affected microglial polarization in AD and potential biological mechanisms. In cell experiments, it was verified that baicalein significantly shifted the BV-2 microglia phenotype from the pro-inflammatory M1 to the anti-inflammatory M2 phenotype, inhibited the microglial apoptosis and pro-inflammatory factors, promoted the microglial Aβ uptake and anti-inflammatory factors after LPS stimulated. In APP/PS1 mice, it was found that baicalein decreased the Aβ plaque deposition in brain, attenuated NLRP3 inflammasome activation and neuronal apoptosis in APP/PS1 mice. Furthermore, bioinformatics analysis and experiment validated that HMOX1 is a target of baicalein, and we elucidated that baicalein modulated the microglial polarization to inhibit neuroinflammation and neural injury through targeting on the HMOX1/PDE4D axis in AD. In conclusion, our findings indicate the therapeutic effect of baicalein for AD, and baicalein might serve a potential agent for AD treatment.

Long-term radiofrequency electromagnetic fields exposure attenuates cognitive dysfunction in 5×FAD mice by regulating microglial function

We have previously found that long-term effects of exposure to radiofrequency electromagnetic fields in 5×FAD mice with severe late-stage Alzheimer's disease reduced both amyloid-β deposition and glial activation, including microglia. To examine whether this therapeutic effect is due to the regulation of activated microglia, we analyzed microglial gene expression profiles and the existence of microglia in the brain in this study. 5×FAD mice at the age of 1.5 months were assigned to sham- and radiofrequency electromagnetic fields-exposed groups and then animals were exposed to 1950 MHz radiofrequency electromagnetic fields at a specific absorption rate of 5 W/kg for 2 hours/day and 5 days/week for 6 months. We conducted behavioral tests including the object recognition and Y-maze tests and molecular and histopathological analysis of amyloid precursor protein/amyloid-beta metabolism in brain tissue. We confirmed that radiofrequency electromagnetic field exposure for 6 months ameliorated cognitive impairment and amyloid-β deposition. The expression levels of Iba1 (pan-microglial marker) and colony-stimulating factor 1 receptor (CSF1R; regulates microglial proliferation) in the hippocampus in 5×FAD mice treated with radiofrequency electromagnetic fields were significantly reduced compared with those of the sham-exposed group. Subsequently, we analyzed the expression levels of genes related to microgliosis and microglial function in the radiofrequency electromagnetic fields-exposed group compared to those of a CSF1R inhibitor (PLX3397)-treated group. Both radiofrequency electromagnetic fields and PLX3397 suppressed the levels of genes related to microgliosis (Csf1r, CD68, and Ccl6) and pro-inflammatory cytokine interleukin-1β. Notably, the expression levels of genes related to microglial function, including Trem2, Fcgr1a, Ctss, and Spi1, were decreased after long-term radiofrequency electromagnetic field exposure, which was also observed in response to microglial suppression by PLX3397. These results showed that radiofrequency electromagnetic fields ameliorated amyloid-β pathology and cognitive impairment by suppressing amyloid-β deposition-induced microgliosis and their key regulator, CSF1R.

Druggable targets for the immunopathy of Alzheimer's disease

Alzheimer's disease (AD) is one of the leading threats to the health and socioeconomic well-being of humankind. Though research to develop disease modifying therapies for AD has traditionally focussed on the misfolding and aggregation of proteins, this approach has failed to yield a definitively curative agent. Accordingly, the search for additional or alternative approaches is a medicinal chemistry priority. Dysfunction of the brain's neuroimmune-neuroinflammation axis has emerged as a leading contender. Neuroimmunity however is mechanistically complex, rendering the recognition of candidate receptors a challenging task. Herein, a review of the role of neuroimmunity in the biomolecular pathogenesis of AD is presented with the identification of a 'druggable dozen' targets; in turn, each identified target represents one or more discrete receptors centred on a common biochemical mechanism. The druggable dozen is composed of both cellular and molecular messenger targets, with a 'targetable ten' microglial targets as well as two cytokine-based targets. For each target, the underlying molecular basis, with a consideration of strengths and weaknesses, is considered.

Comparative physiological anthropogeny: exploring molecular underpinnings of distinctly human phenotypes

Anthropogeny is a classic term encompassing transdisciplinary investigations of the origins of the human species. Comparative anthropogeny is a systematic comparison of humans and other living nonhuman hominids (so-called "great apes"), aiming to identify distinctly human features in health and disease, with the overall goal of explaining human origins. We begin with a historical perspective, briefly describing how the field progressed from the earliest evolutionary insights to the current emphasis on in-depth molecular and genomic investigations of "human-specific" biology and an increased appreciation for cultural impacts on human biology. While many such genetic differences between humans and other hominids have been revealed over the last two decades, this information remains insufficient to explain the most distinctive phenotypic traits distinguishing humans from other living hominids. Here we undertake a complementary approach of "comparative physiological anthropogeny," along the lines of the preclinical medical curriculum, i.e., beginning with anatomy and considering each physiological system and in each case considering genetic and molecular components that are relevant. What is ultimately needed is a systematic comparative approach at all levels from molecular to physiological to sociocultural, building networks of related information, drawing inferences, and generating testable hypotheses. The concluding section will touch on distinctive considerations in the study of human evolution, including the importance of gene-culture interactions.

The triggering receptor expressed on myeloid cells 2-apolipoprotein E signaling pathway in diseases

ABSTRACT

Triggering receptor expressed on myeloid cells 2 (TREM2) is a membrane receptor on myeloid cells and plays an important role in the body's immune defense. Recently, TREM2 has received extensive attention from researchers, and its activity has been found in Alzheimer's disease, neuroinflammation, and traumatic brain injury. The appearance of TREM2 is usually accompanied by changes in apolipoprotein E (ApoE), and there has been a lot of research into their structure, as well as the interaction mode and signal pathways involved in them. As two molecules with broad and important roles in the human body, understanding their correlation may provide therapeutic targets for certain diseases. In this article, we reviewed several diseases in which TREM2 and ApoE are synergistically involved in the development. We further discussed the positive or negative effects of the TREM2-ApoE pathway on nervous system immunity and inflammation.

APOEε4 and risk of Alzheimer's disease - time to move forward

The inheritance of Apolipoprotein E4 (APOEε4) brings the highest genetic risk of Alzheimer's disease (AD), arguably the highest genetic risk in human pathology. Since the discovery of the association, APOE protein isoforms have been at the center of tens of thousands of studies and reports. While, without a doubt, our knowledge about the normal physiological function of APOE isoforms in the brain has increased tremendously, the questions of how the inheritance of the APOEε4 allele translates into a risk of AD, and the risk is materialized, remain unanswered. Moreover, the knowledge about the risk associated with APOEε4 has not helped design a meaningful preventative or therapeutic strategy. Animal models with targeted replacement of Apoe have been generated and, thanks to the recent NIH/NIA/Alzheimer's disease Association initiative, are now freely available to AD researchers. While helpful in many aspects, none of the available models recapitulates normal physiological transcriptional regulation of the human APOE gene cluster. Changes in epigenetic regulation of APOE alleles in animal models in response to external insults have rarely been if ever, addressed. However, these animal models provide a useful tool to handle questions and investigate protein-protein interactions with proteins expressed by other recently discovered genes and gene variants considered genetic risk factors of AD, like Triggering Receptor expressed on Myeloid cells 2 (TREM2). In this review, we discuss genetic and epigenetic regulatory mechanisms controlling and influencing APOE expression and focus on interactions of APOE and TREM2 in the context of microglia and astrocytes' role in AD-like pathology in animal models.

Menstrual blood-derived endometrial stem cells inhibit neuroinflammation by regulating microglia through the TLR4/MyD88/NLRP3/Casp1 pathway

Neuroinflammation is a common response in various neurological disorders. Mesenchymal stem cell-based treatment has become a promising therapy for neuroinflammation-associated diseases. However, the effects of mesenchymal stem cells are controversial, and the underlying mechanism is incompletely understood. In the present study, menstrual blood-derived endometrial stem cells were intravenously transplanted into a mouse model of neuroinflammation established by peripheral injection of lipopolysaccharide. Microglial cells challenged with lipopolysaccharide were cultured with conditioned medium from endometrial stem cells. The levels of cytokines were detected by enzyme-linked immunosorbent assay. Cell proliferation and death were detected by Cell Counting Kit 8 and flow cytometry, respectively. The expression levels of Toll-like receptor 4 (TLR4), myeloid differentiation primary response gene 88 (MyD88), NLR family pyrin domain containing 3 (NLRP3) and caspase 1 (Casp1) were evaluated by western blotting. The results showed that intravenous transplantation of endometrial stem cells downregulated proinflammatory factors and upregulated anti-inflammatory factors in the brain of mice with neuroinflammation. Conditioned medium suppressed the inflammatory reaction and hyperactivation of microglial cells and protected microglial cells from cell death induced by lipopolysaccharide in vitro. The expression of TLR4, MyD88, NLRP3 and Casp1 in the brain of mice with neuroinflammation and in lipopolysaccharide-stimulated microglial cells was downregulated by endometrial stem cells and conditioned medium, respectively. These data suggested that menstrual blood-derived endometrial stem cells may suppress neuroinflammatory reactions partially by regulating microglia through the TLR4/MyD88/NLRP3/Casp1 signalling pathway. Our findings may be very useful for the development of an alternative stem cell-based therapy for neuroinflammation-associated disorders.

Exposure to Low Levels of Radiofrequency Electromagnetic Fields Emitted from Cell-phones as a Promising Treatment of Alzheimer's Disease: A Scoping Review Study

BACKGROUND

Alzheimer's disease (AD) is one of the most significant public health concerns and tremendous economic challenges. Studies conducted over the past decades show that exposure to radiofrequency electromagnetic fields (RF-EMFs) may relieve AD symptoms.

OBJECTIVE

To determine if exposure to RF-EMFs emitted by cellphones affect the risk of AD.

MATERIAL AND METHODS

In this review, all relevant published articles reporting an association of cell phone use with AD were studied. We systematically searched international datasets to identify relevant studies. Finally, 33 studies were included in the review. Our review discusses the effects of RF-EMFs on the amyloid β (Aβ), oxidative stress, apoptosis, reactive oxygen species (ROS), neuronal death, and astrocyte responses. Moreover, the role of exposure parameters, including the type of exposure, its duration, and specific absorption rate (SAR), are discussed.

RESULTS

Progressive factors of AD such as Aβ, myelin basic protein (MBP), nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, and neurofilament light polypeptide (NFL) were decreased. While tau protein showed no change, factors affecting brain activity such as glial fibrillary acidic protein (GFAP), mitogen-activated protein kinases (MAPKs), cerebral blood flow (CBF), brain temperature, and neuronal activity were increased.

CONCLUSION

Exposure to low levels of RF-EMFs can reduce the risk of AD by increasing MAPK and GFAP and decreasing MBP. Considering the role of apoptosis in AD and the effect of RF-EMF on the progression of the process, this review indicates the positive effect of these exposures.

Gut-brain axis through the lens of gut microbiota and their relationships with Alzheimer's disease pathology: Review and recommendations

Alzheimer's disease (AD) is a neurodegenerative disorder that affects millions of people worldwide. Growing evidence suggests that the gut microbiome (GM) plays a pivotal role in the pathogenesis of AD through the microbiota-gut-brain axis (MGB). Alterations in GM composition and diversity have been observed in both animal models and in human patients with AD. GM dysbiosis has been implicated in increased intestinal permeability, blood-brain barrier (BBB) impairment, neuroinflammation and the development of hallmarks of AD. Further elucidation of the role of GM in AD could pave way for the development of holistic predictive methods for determining AD risk and progression of disease. Furthermore, accumulating evidence suggests that GM modulation could alleviate adverse symptoms of AD or serve as a preventive measure. In addition, increasing evidence shows that Type 2 Diabetes Mellitus (T2DM) is often comorbid with AD, with common GM alterations and inflammatory response, which could chart the development of GM-related treatment interventions for both diseases. We conclude by exploring the therapeutic potential of GM in alleviating symptoms of AD and in reducing risk. Furthermore, we also propose future directions in AD research, namely fecal microbiota transplantation (FMT) and precision medicine.

Licochalcone E, a β-Amyloid Aggregation Inhibitor, Regulates Microglial M1/M2 Polarization via Inhibition of CTL1-Mediated Choline Uptake

Alzheimer's disease (AD) is thought to be a series of neuroinflammatory diseases caused by abnormal deposits of amyloid-β (Aβ) and tau protein in the brain as part of its etiology. We focused on Aβ aggregation and M1 and M2 microglial polarity in microglia to search for novel therapeutic agents. It has been reported that the inhibition of choline uptake via choline transporter-like protein 1 (CTL1) in microglia preferentially induces M2 microglial polarity. However, the role of the choline transport system on the regulation of microglial M1/M2 polarity in AD is not fully understood. Licochalcones (Licos) A-E, flavonoids extracted from licorice, have been reported to have immunological anti-inflammatory effects, and Lico A inhibits Aβ aggregation. In this study, we compared the efficacy of five Licos, from Lico A to E, at inhibiting Aβ1-42 aggregation. Among the five Licos, Lico E was selected to investigate the relationship between the inhibition of choline uptake and microglial M1/M2 polarization using the immortalized mouse microglial cell line SIM-A9. We newly found that Lico E inhibited choline uptake and Aβ1-42 aggregation in SIM-A9 cells in a concentration-dependent manner, suggesting that the inhibitory effect of Lico E on choline uptake is mediated by CTL1. The mRNA expression of tumor necrosis factor (TNF-α), a marker of M1 microglia, was increased by Aβ1-42, and its effect was inhibited by choline deprivation and Lico E in a concentration-dependent manner. In contrast, the mRNA expression of arginase-1 (Arg-1), a marker of M2 microglia, was increased by IL-4, and its effect was enhanced by choline deprivation and Lico E. We found that Lico E has an inhibitory effect on Aβ aggregation and promotes polarity from M1 to M2 microglia via inhibition of the CTL1 function in microglia. Thus, Lico E may become a leading compound for a novel treatment of AD.

Untangling the Role of TREM2 in Conjugation with Microglia in Neuronal Dysfunction: A Hypothesis on a Novel Pathway in the Pathophysiology of Alzheimer's Disease

Alzheimer's disease (AD) is a complex neurodegenerative disorder involving heterogenous pathophysiological characteristics, which has become a challenge to therapeutics. The major pathophysiology of AD comprises amyloid-β (Aβ), tau, oxidative stress, and apoptosis. Recent studies indicate the significance of Triggering receptor expressed on myeloid cells 2 (TREM2) and its mutant variants in AD. TREM2 are the transmembrane receptors of microglial cells that performs a broad range of physiological cell processes. Phagocytosis of Aβ is one of the physiological roles of TREM2, which plays a pivotal role in AD progression. R47H, a mutant variant of TREM2, increases the risk of AD by impairing TREM2-Aβ binding. Inconclusive evidence regarding the TREM2 signaling cascade mechanism of Aβ phagocytosis motivates the current review to propose a new hypothesis. The review systematically assesses the cross talk between TREM2 and other AD pathological domains and the influence of TREM2 on amyloid and tau seeding. Disease associated microglia (DAM), a novel state of microglia with unique transcriptional and functional signatures reported in neurodegenerative conditions, also depend on the TREM2 pathway for its differentiation. DAM is suggested to have a neuroprotective role. We hypothesize that TREM2, along with its signaling adaptors and endogenous proteins, play a key role in ameliorating Aβ clearance. We indicate that TREM2 has the potential to ameliorate the Aβ burden, though with differential clearance ability and may act as a potential therapeutic target.

Clinical significance and potential role of trimethylamine N-oxide in neurological and neuropsychiatric disorders

In the past three decades, research on the gut microbiome and its metabolites, such as trimethylamines (TMA), trimethylamine N-oxide (TMAO), short-chain fatty acids (SCFAs), branched-chain amino acids (BCAAs), bile acids, tryptophan and indole derivatives, has attracted the attention of many scientists and industrialists. Among these metabolites, TMAO is produced from dietary choline, phosphatidylcholine, carnitine,andbetaine. TMAO and other gut metabolites, such as TMA and SCFAs, reach the brain by crossing the blood-brain barrier (BBB) and are involved in brain development, neurogenesis, and behavior. Gut-microbiota composition is influenced by diet, lifestyle, antibiotics, and age. Several studies have confirmed that altered TMAO levels contribute to metabolic, vascular, psychiatric, and neurodegenerative disorders. This review focuses on how altered TMAO levels impact oxidative stress, microglial activation, and the apoptosis of neurons, and may lead to neuroinflammation, which can subsequently result in the development of psychiatric, cognitive, and behavioral disorders.

TREM2 drives microglia response to amyloid-β via SYK-dependent and -independent pathways

Genetic studies have highlighted microglia as pivotal in orchestrating Alzheimer's disease (AD). Microglia that adhere to Aβ plaques acquire a transcriptional signature, "disease-associated microglia" (DAM), which largely emanates from the TREM2-DAP12 receptor complex that transmits intracellular signals through the protein tyrosine kinase SYK. The human TREM2R47H variant associated with high AD risk fails to activate microglia via SYK. We found that SYK-deficient microglia cannot encase Aβ plaques, accelerating brain pathology and behavioral deficits. SYK deficiency impaired the PI3K-AKT-GSK-3β-mTOR pathway, incapacitating anabolic support required for attaining the DAM profile. However, SYK-deficient microglia proliferated and advanced to an Apoe-expressing prodromal stage of DAM; this pathway relied on the adapter DAP10, which also binds TREM2. Thus, microglial responses to Aβ involve non-redundant SYK- and DAP10-pathways. Systemic administration of an antibody against CLEC7A, a receptor that directly activates SYK, rescued microglia activation in mice expressing the TREM2R47H allele, unveiling new options for AD immunotherapy.

Role of CD40(L)-TRAF signaling in inflammation and resolution—a double-edged sword

Cardiovascular diseases (CVD) and cardiovascular risk factors are the leading cause of death in the world today. According to the Global Burden of Disease Study, hypertension together with ischemic heart and cerebrovascular diseases is responsible for approximately 40% of all deaths worldwide. The major pathomechanism underlying almost all CVD is atherosclerosis, an inflammatory disorder of the vascular system. Recent large-scale clinical trials demonstrated that inflammation itself is an independent cardiovascular risk factor. Specific anti-inflammatory therapy could decrease cardiovascular mortality in patients with atherosclerosis (increased markers of inflammation). Inflammation, however, can also be beneficial by conferring so-called resolution, a process that contributes to clearing damaged tissue from cell debris upon cell death and thereby represents an essential step for recovery from, e.g., ischemia/reperfusion damage. Based on these considerations, the present review highlights features of the detrimental inflammatory reactions as well as of the beneficial process of immune cell-triggered resolution. In this context, we discuss the polarization of macrophages to either M1 or M2 phenotype and critically assess the role of the CD40L-CD40-TRAF signaling cascade in atherosclerosis and its potential link to resolution. As CD40L can bind to different cellular receptors, it can initiate a broad range of inflammatory processes that may be detrimental or beneficial. Likewise, the signaling of CD40L downstream of CD40 is mainly determined by activation of TRAF1-6 pathways that again can be detrimental or beneficial. Accordingly, CD40(L)-based therapies may be Janus-faced and require sophisticated fine-tuning in order to promote cardioprotection.

Sodium propionate improves cognitive and memory function in mouse models of Alzheimer's disease

This study was designed to explore whether sodium propionate (SP) alleviates cognitive damage in a mouse model of Alzheimer's disease (AD). We evaluated behavioral and biochemical aspects in an animal model of AD made by intracerebroventricular injection of Aβ_1-42 peptide. Two-month-old ICR mice were treated with SP or normal saline for 21 days (control group, Aβ_1-42 group, Aβ_1-42+SP50 mg/kg group, Aβ_1-42+SP100 mg/kg group, and Aβ_1-42+SP200 mg/kg group). Behavioral tests showed that SP alleviated cognitive and memory impairments in AD mice. Moreover, SP treatment significantly suppressed the level of inducible nitric oxide synthase (iNOS) in the hippocampus. Concomitantly, the overexpression of interleukin-1α (IL-1α), interleukin-1β (IL-1β) and interleukin-6 (IL-6) in the hippocampus induced by Aβ_1-42 was significantly reduced following treatment with SP. In addition, SP was able to increase the levels of synaptophysin (SYN) and postsynaptic dense protein 95 (PSD95). Our study shows that SP could significantly improve Aβ_1-42-induced spatial learning and memory impairment by reducing neuroinflammation via inhibition of proinflammatory cytokines and iNOS activation and restoring synapse plasticity by increasing synaptically associated protein levels, suggesting that SP has a positive effect and potential for AD therapies.

Lipoprotein-Inspired Nanoscavenger for the Three-Pronged Modulation of Microglia-Derived Neuroinflammation in Alzheimer's Disease Therapy

The inflammatory dysfunction of microglia from excess amyloid-β peptide (Aβ) disposal is an overlooked but pathogenic event in Alzheimer's disease (AD). Here, we exploit a native high-density lipoprotein (HDL)-inspired nanoscavenger (pHDL/Cur-siBACE1) that combines the trinity of phosphatidic acid-functionalized HDL (pHDL), curcumin (Cur), and β-site APP cleavage enzyme 1 targeted siRNA (siBACE1) to modulate microglial dysfunction. By mimicking the natural lipoprotein transport route, pHDL can penetrate the blood-brain barrier and sequentially target Aβ plaque, where Aβ catabolism is accelerated without microglial dysfunction. The benefit results are from a three-pronged modulation strategy, including promoted Aβ clearance with an antibody-like Aβ binding affinity, normalized microglial dysfunction by blocking the NF-κB pathway, and reduced Aβ production by gene silence (44%). After treatment, the memory deficit and neuroinflammation of APPswe/PSEN 1dE9 mice are reversed. Collectively, this study highlights the double-edged sword role of microglia and provides a promising tactic for modulating microglial dysfunction in AD treatment.

Microglia in Alzheimer's Disease: An Unprecedented Opportunity as Prospective Drug Target

Alzheimer's disease (AD) is an ever more common neurodegenerative disease among the elderly, characterized by recurrent neuroinflammation and amyloid beta (Aβ) accumulation in the brain parenchyma. Recent genome-wide association studies (GWAS) have shown a distinct role for the innate immune system in AD, with microglia playing a key role. The function of microglial cells is stringently regulated by the neighboring microenvironment in the brain. Upon interruption in diseases, like AD, it demonstrates neurotoxic and neuroprotective action by M1 (neurotoxic) and M2 (neuroprotective) microglial phenotypes, respectively, in the brain. Microglial cells on activation by complement factors, toll-like receptors, and genetic variants result in Aβ' phagocytosis, synaptic pruning, and reactivation of complement pathway. Recent studies have demonstrated the presence of potential therapeutic targets in microglial cells. Immune receptors revealed on microglia as potential drug targets can be paired immunoglobulin-like type 2 receptor (PILR), CD3358, and triggering receptor expressed on myeloid cells 2 (TREM2), as they can have impact on late-onset AD occurrence and progression. Thus, targeting these receptors can accentuate the beneficial effects of microglial cells required to decelerate the progression of AD. This review emphasizes the microglial phenotypes, its function in AD brain, and potential immunological and therapeutic targets to fight this highly progressive neurodegenerative disorder.

An overlooked subset of Cx3cr1wt/wt microglia in the Cx3cr1CreER-Eyfp/wt mouse has a repopulation advantage over Cx3cr1CreER-Eyfp/wt microglia following microglial depletion

Background

Fluorescent reporter labeling and promoter-driven Cre-recombinant technologies have facilitated cellular investigations of physiological and pathological processes, including the widespread use of the Cx3cr1^{CreER-Eyfp/wt} mouse strain for studies of microglia.

Methods

Immunohistochemistry, Flow Cytometry, RNA sequencing and whole-genome sequencing were used to identify the subpopulation of microglia in Cx3cr1^{CreER-Eyfp/wt} mouse brains. Genetically mediated microglia depletion using Cx3cr1^{CreER-Eyfp/wt}Rosa26^DTA/wt mice and CSF1 receptor inhibitor PLX3397 were used to deplete microglia. Primary microglia proliferation and migration assay were used for in vitro studies.

Results

We unexpectedly identified a subpopulation of microglia devoid of genetic modification, exhibiting higher Cx3cr1 and CX3CR1 expression than Cx3cr1^{CreER-Eyfp/wt}Cre⁺Eyfp⁺ microglia in Cx3cr1^{CreER-Eyfp/wt} mouse brains, thus termed Cx3cr1^highCre⁻Eyfp⁻ microglia. This subpopulation constituted less than 1% of all microglia under homeostatic conditions, but after Cre-driven DTA-mediated microglial depletion, Cx3cr1^highCre⁻Eyfp⁻ microglia escaped depletion and proliferated extensively, eventually occupying one-third of the total microglial pool. We further demonstrated that the Cx3cr1^highCre⁻Eyfp⁻ microglia had lost their genetic heterozygosity and become homozygous for wild-type Cx3cr1. Therefore, Cx3cr1^highCre⁻Eyfp⁻ microglia are Cx3cr1^wt/wtCre⁻Eyfp⁻. Finally, we demonstrated that CX3CL1–CX3CR1 signaling regulates microglial repopulation both in vivo and in vitro.

Conclusions

Our results raise a cautionary note regarding the use of Cx3cr1^{CreER-Eyfp/wt} mouse strains, particularly when interpreting the results of fate mapping, and microglial depletion and repopulation studies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02381-6.

Sex Differences in Neurodegeneration: The Role of the Immune System in Humans

Growing evidence supports significant involvement of immune dysfunction in the etiology of neurodegenerative diseases, several of which also display prominent sex differences across prevalence, pathology, and symptomology. In this review, we summarize evidence from human studies of established and recent findings of sex differences in multiple sclerosis, Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis and discuss how sex-specific central nervous system innate immune activity could contribute to downstream sex differences in these diseases. We examine human genomic and transcriptomics studies in each neurodegenerative disease through the lens of sex differences in the neuroimmune system and highlight the importance of stratifying sex in clinical and translational research studies. Finally, we discuss the limitations of the existing studies and outline recommendations for further advancing sex-based analyses to uncover novel disease mechanisms that could ultimately help treat both sexes.

Anti-inflammatory effects of 9-cis-retinoic acid on β-amyloid treated human microglial cells

Objective: Alzheimer’s disease (AD) is a neurodegenerative disease that abolishes cognitive and analytical abilities to perform basic day-to-day tasks. Microglia are involved in AD-associated neuroinflammation in response to amyloid β‐peptide (Aβ). This study focused on observing the immunomodulatory effects of 9-cis-retinoic acid (9-Cis-RA) the active metabolite of vitamin A, on Aβ-treated human microglial HMO6 cells.

Methods: HMO6 cells were treated with Aβ42 in the absence or presence of 9-cis-RA, and the expression of M1-and M2-associated molecules, Toll like receptors (TLRs), and triggering receptor expressed on myeloid cells 2 (TREM2) were examined.

Results: The levels of M1-markers [cluster of differentiation (CD86) and inducible nitric oxide synthase (iNOS)] and -cytokines [tumor necrosis factor (TNF-α), interleukin (IL)-6, and IL-1β], inflammatory receptors (TLR2 and TLR4), and reactive oxygen species increased significantly in Aβ-treated HMO6 cells. In contrast, the levels of M2-markers (CD206 and arginase-1) and -cytokines (IL-10, IL-4, and C-C motif chemokine ligand 17) the anti-inflammatory receptor TLR10 was significantly suppressed. However, 9-cis-RA treatment reversed the Aβ-induced upregulation of expression of M1-associated molecules and upregulated the expression of M2-associated molecules. Moreover, 9-cis-RA treatment augmented Aβ uptake by HMO6 cells, possibly by increasing the cell surface protein levels of TREM2, which is a receptor of Aβ that promotes Aβ phagocytosis by microglia.

Conclusion: Our results suggest that 9-cis-RA is a potential therapeutic agent for AD.

Regulation of the Late Onset alzheimer's Disease Associated HLA-DQA1/DRB1 Expression

(Genome-wide Association Studies) GWAS have identified ∼42 late-onset Alzheimer's disease (LOAD)-associated loci, each of which contains multiple single nucleotide polymorphisms (SNPs) in linkage disequilibrium (LD) and most of these SNPs are in the non-coding region of human genome. However, how these SNPs regulate risk gene expression remains unknown. In this work, by using a set of novel techniques, we identified 6 functional SNPs (fSNPs) rs9271198, rs9271200, rs9281945, rs9271243, and rs9271247 on the LOAD-associated HLA-DRB1/DQA1 locus and 42 proteins specifically binding to five of these 6 fSNPs. As a proof of evidence, we verified the allele-specific binding of GATA2 and GATA3, ELAVL1 and HNRNPA0, ILF2 and ILF3, NFIB and NFIC, as well as CUX1 to these five fSNPs, respectively. Moreover, we demonstrate that all these nine proteins regulate the expression of both HLA-DQA1 and HLA-DRB1 in human microglial cells. The contribution of HLA class II to the susceptibility of LOAD is discussed.

Neuroimmune contributions to Alzheimer's disease: a focus on human data

The past decade has seen the convergence of a series of new insights that arose from genetic and systems analyses of Alzheimer's disease (AD) with a wealth of epidemiological data from a variety of fields; this resulted in renewed interest in immune responses as important, potentially causal components of AD. Here, we focus primarily on a review of human data which has recently yielded a set of robust, reproducible results that exist in a much larger universe of conflicting reports stemming from small studies with important limitations in their study design. Thus, we are at an important crossroads in efforts to first understand at which step of the long, multiphasic course of AD a given immune response may play a causal role and then modulate this response to slow or block the pathophysiology of AD. We have a wealth of new experimental tools, analysis methods, and capacity to sample human participants at large scale longitudinally; these resources, when coupled to a foundation of reproducible results and novel study designs, will enable us to monitor human immune function in the CNS at the level of complexity that is required while simultaneously capturing the state of the peripheral immune system. This integration of peripheral and central perturbations in immune responses results in pathologic responses in the central nervous system parenchyma where specialized cellular microenvironments composed of multiple cell subtypes respond to these immune perturbations as well as to environmental exposures, comorbidities and the impact of the advancing life course. Here, we offer an overview that seeks to illustrate the large number of interconnecting factors that ultimately yield the neuroimmune component of AD.

Current Understanding of the Physiopathology, Diagnosis and Therapeutic Approach to Alzheimer's Disease

Alzheimer's disease (AD) is the most common cause of dementia. It is characterized by cognitive decline and progressive memory loss. The aim of this review was to update the state of knowledge on the pathophysiological mechanisms, diagnostic methods and therapeutic approach to AD. Currently, the amyloid cascade hypothesis remains the leading theory in the pathophysiology of AD. This hypothesis states that amyloid-β (Aβ) deposition triggers a chemical cascade of events leading to the development of AD dementia. The antemortem diagnosis of AD is still based on clinical parameters. Diagnostic procedures in AD include fluid-based biomarkers such as those present in cerebrospinal fluid and plasma or diagnostic imaging methods. Currently, the therapeutic armory available focuses on symptom control and is based on four pillars: pharmacological treatment where acetylcholinesterase inhibitors stand out; pharmacological treatment under investigation which includes drugs focused on the control of Aβ pathology and tau hyperphosphorylation; treatment focusing on risk factors such as diabetes; or nonpharmacological treatments aimed at preventing development of the disease or treating symptoms through occupational therapy or psychological help. AD remains a largely unknown disease. Further research is needed to identify new biomarkers and therapies that can prevent progression of the pathology.

Potentiation of microglial endocannabinoid signaling alleviates neuroinflammation in Alzheimer's disease

Alzheimer's disease (AD) isaprogressive neurodegenerative disorder characterized by chronic inflammation due to the presence of neurotoxic Aβ and tau proteins. Increased microglial activation and inflated immune response are the other factors to be considered in AD pathology. Microglial cells own biochemical machinery that synthesizes and release endocannabinoids. The exploitation of therapeutic actions of endocannabinoid system has newly emerged in the field of Alzheimer's disease. The activation of cannabinoid receptors/ cannabinoid system modulates inflammatory responses. This review assesses the association between the microglial endocannabinoid system and neuroinflammation in AD. The data supporting the anti-inflammatory role of pharmacological agents modulating cannabinoid system are also reviewed.

Untangling huge literature to disinter genetic underpinnings of Alzheimer's Disease: A systematic review and meta-analysis

Drug discovery for Alzheimer's Disease (AD) is channeled towards unravelling key disease specific drug targets/genes to predict promising therapeutic candidates. Though enormous literature on AD genetics is available, there exists dearth in data pertinent to drug targets and crucial pathological pathways intertwined in disease progression. Further, the research findings revealing genetic associations failed to demonstrate consistency across different studies. This scenario prompted us to initiate a systematic review and meta-analysis with an aim of unearthing significant genetic hallmarks of AD. Initially, a Boolean search strategy was developed to retrieve case-control studies from PubMed, Cochrane, ProQuest, Europe PMC, grey literature and HuGE navigator. Subsequently, certain inclusion and exclusion criteria were framed to shortlist the relevant studies. These studies were later critically appraised using New Castle Ottawa Scale and Q-Genie followed by data extraction. Later, meta-analysis was performed only for those Single Nucleotide Polymorphisms (SNPs) which were evaluated in at least two different ethnicities from two different reports. Among, 204,351 studies retrieved, 820 met our eligibility criteria and 117 were processed for systematic review after critical appraisal. Ultimately, meta-analysis was performed for 23 SNPs associated with 15 genes which revealed significant associations of rs3865444 (CD33), rs7561528 (BIN1) and rs1801133 (MTHFR) with AD risk.

Anti-inflammatory Activity of a Polypeptide Fraction From Achyranthes bidentate in Amyloid β Oligomers Induced Model of Alzheimer's Disease

Neuroinflammation plays a crucial role in neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD), and anti-inflammation has been considered as a potential therapeutic strategy. Achyranthes bidentate polypeptide fraction k (ABPPk) was shown to protect neurons from death and suppress microglia and astrocyte activation in PD model mice. However, how ABPPk regulates neuroinflammation to exert a neuroprotective role remains unclear. Toxic Aβ oligomers (AβOs) can trigger inflammatory response and play an important role in the pathogenesis of AD. In the present study, for the first time, we investigated the effects and underlying mechanisms of ABPPk on neuroinflammation in AβOs-induced models of AD. In vitro, ABPPk pretreatment dose-dependently inhibited AβOs-induced pro-inflammatory cytokines mRNA levels in BV2 and primary microglia. ABPPk pretreatment also reduced the neurotoxicity of BV2 microglia-conditioned media on primary hippocampal neurons. Furthermore, ABPPk down-regulated the AβOs-induced phosphorylation of IκBα and NF-κB p65 as well as the expression of NLRP3 in BV2 microglia. In vivo, ABPPk pre-administration significantly improved locomotor activity, alleviated memory deficits, and rescued neuronal degeneration and loss in the hippocampus of AβOs-injected mice. ABPPk inhibited the activation of microglia in hippocampal CA3 region and suppressed the activation of NF-κB as well as the expression of NLRP3, cleaved caspase-1, and ASC in the brain after AβOs injection. ABPPk hindered the release of pro-inflammatory cytokines and promoted the release of anti-inflammatory cytokines in the brain. Notably, the polarization experiment on BV2 microglia demonstrated that ABPPk inhibited M1-phenotype polarization and promoted M2-phenotype polarization by activating the LPS- or AβOs-impaired autophagy in microglia. Taken together, our observations indicate that ABPPk can restore the autophagy of microglia damaged by AβOs, thereby promoting M2-phenotype polarization and inhibiting M1-phenotype polarization, thus playing a role in regulating neuroinflammation and alleviating neurotoxicity.

Microglia and its Genetics in Alzheimer's Disease

Alzheimer's Disease (AD) is the most prevalent form of dementia across the world. While its discovery and pathological manifestations are centered on protein aggregations of amyloid- beta (Aβ) and hyperphosphorylated tau protein, neuroinflammation has emerged in the last decade as a main component of the disease in terms of both pathogenesis and progression. As the main innate immune cell type in the central nervous system (CNS), microglia play a very important role in regulating neuroinflammation, which occurs commonly in neurodegenerative conditions, including AD. Under inflammatory response, microglia undergo morphological changes and status transition from homeostatic to activated forms. Different microglia subtypes displaying distinct genetic profiles have been identified in AD, and these signatures often link to AD risk genes identified from the genome-wide association studies (GWAS), such as APOE and TREM2. Furthermore, many AD risk genes are highly enriched in microglia and specifically influence the functions of microglia in pathogenesis, e.g. releasing inflammatory cytokines and clearing Aβ. Therefore, building up a landscape of these risk genes in microglia, based on current preclinical studies and in the context of their pathogenic or protective effects, would largely help us to understand the complex etiology of AD and provide new insight into the unmet need for effective treatment.

In silico screening of neurokinin receptor antagonists as a therapeutic strategy for neuroinflammation in Alzheimer's disease

Neuroinflammation is one of the detrimental factors leading to neurodegeneration in Alzheimer’s disease (AD) and other neurodegenerative disorders. The activation of microglial neurokinin 1 receptor (NK1R) by substance P (SP) enhances neuroinflammation which is mediated through pro-inflammatory pathways involving NFkB, ERK1/2, and P38 and thus projects the scope and importance of NK1R inhibitors. Emphasizing the inhibitory role of N Acetyl l Tryptophan (l-NAT) on NK1R, this is the first in silico screening of l-NAT mediated NK1R antagonism. In addition, FDA- approved ligands were screened for their potential NK1R antagonism. The l-NAT was docked in XP (Extra Precision) mode while FDA-approved ligands were screened in HTVS (High Throughput Virtual Screening), SP (Standard Precision), and XP mode onto NK1R (PDB:6HLO). The l-NAT and top 3 compounds FDA-approved ligands were subjected to molecular dynamics (MD) studies of 100 ns simulation time. The XP docking of l-NAT, indacaterol, modafinil and alosetron showed good docking scores. Their 100 ns MD showed brief protein–ligand interactions with an acceptable root mean square deviation. The protein–ligand contacts depicted pi-pi stacking, pi-cation, hydrogen bonds, and water bridges with the amino acids necessary for NK1R inhibition. The variable colour band intensities on the protein–ligand contact map indicated their binding strength with amino acids. The molecular mechanics/generalized born surface area (MM-GBSA) scores suggested favourable binding free energy of the complexes. Thus, our study predicted the ability of l-NAT, indacaterol, modafinil, and alosetron as capable NK1R inhibitors that can aid to curb neuroinflammation in conditions of AD which could be further ascertained in subsequent studies.

Integration of Multimodal Data for Deciphering Brain Disorders

The accumulation of vast amounts of multimodal data for the human brain, in both normal and disease conditions, has provided unprecedented opportunities for understanding why and how brain disorders arise. Compared with traditional analyses of single datasets, the integration of multimodal datasets covering different types of data (i.e., genomics, transcriptomics, imaging, etc.) has shed light on the mechanisms underlying brain disorders in greater detail across both the microscopic and macroscopic levels. In this review, we first briefly introduce the popular large datasets for the brain. Then, we discuss in detail how integration of multimodal human brain datasets can reveal the genetic predispositions and the abnormal molecular pathways of brain disorders. Finally, we present an outlook on how future data integration efforts may advance the diagnosis and treatment of brain disorders.

Recent Trends in the Management of Alzheimer's Disease: Current Therapeutic Options and Drug Repurposing Approaches

Alzheimer's disease is one of the most progressive forms of dementia, ultimately leading to death in aged populations. The major hallmarks of Alzheimer's disease include deposition of extracellular amyloid senile plaques and intracellular neurofibrillary tangles in brain neuronal cells. Although there are classical therapeutic options available for the treatment of the diseases, however, they provide only a symptomatic relief and do not modify the molecular pathophysiological course of the disease. Recent research advances in Alzheimer's disease have highlighted the potential role of anti-amyloid, anti-tau, and anti-inflammatory therapies. However, these therapies are still in different phases of pre-clinical/clinical development. In addition, drug repositioning/repurposing is another interesting and promising approach to explore rationalized options for the treatment of Alzheimer's disease. This review discusses the different aspects of the pathophysiological mechanism involved in the progression of Alzheimer's disease along with the limitations of current therapies. Furthermore, this review also highlights emerging investigational drugs along with recent drug repurposing approaches for Alzheimer's disease.

Acute systemic inflammation exacerbates neuroinflammation in Alzheimer's disease: IL-1β drives amplified responses in primed astrocytes and neuronal network dysfunction

Neuroinflammation contributes to Alzheimer's disease (AD) progression. Secondary inflammatory insults trigger delirium and can accelerate cognitive decline. Individual cellular contributors to this vulnerability require elucidation. Using APP/PS1 mice and AD brain, we studied secondary inflammatory insults to investigate hypersensitive responses in microglia, astrocytes, neurons, and human brain tissue. The NLRP3 inflammasome was assembled surrounding amyloid beta, and microglia were primed, facilitating exaggerated interleukin-1β (IL-1β) responses to subsequent LPS stimulation. Astrocytes were primed to produce exaggerated chemokine responses to intrahippocampal IL-1β. Systemic LPS triggered microglial IL-1β, astrocytic chemokines, IL-6, and acute cognitive dysfunction, whereas IL-1β disrupted hippocampal gamma rhythm, all selectively in APP/PS1 mice. Brains from AD patients with infection showed elevated IL-1β and IL-6 levels. Therefore, amyloid leaves the brain vulnerable to secondary inflammation at microglial, astrocytic, neuronal, and cognitive levels, and infection amplifies neuroinflammatory cytokine synthesis in humans. Exacerbation of neuroinflammation to produce deleterious outcomes like delirium and accelerated disease progression merits careful investigation in humans.

Andrographolide ameliorates neuroinflammation in APP/PS1 transgenic mice

Alzheimer's disease is a devastating neurodegenerative disorder, with no disease-modifying treatment available yet. There is increasing evidence that neuroinflammation plays a critical role in the pathogenesis of AD. Andrographolide (Andro), a labdane diterpene extracted from the herb Andrographis paniculata, has been reported to exhibit neuroprotective property in central nervous system diseases. However, its effects on Aβ and Aβ-induced neuroinflammation have not yet been studied. In the present study, we found that Andro administration significantly alleviated cognitive impairments, reduced amyloid-β deposition, inhibited microglial activation, and decreased the secretion of proinflammatory factors in APP/PS1 mice. Furthermore, transcriptome sequencing analysis revealed that Andro could significantly decrease the expression of Itgax, TLR2, CD14, CCL3, CCL4, TLR1, and C3ar1 in APP/PS1 mice, which was further validated by qRT-PCR. Our results suggest that Andro might be a potential therapeutic drug for AD by regulating neuroinflammation.

The Potential of Induced Pluripotent Stem Cells to Treat and Model Alzheimer's Disease

An estimated 6.2 million Americans aged 65 or older are currently living with Alzheimer's disease (AD), a neurodegenerative disease that disrupts an individual's ability to function independently through the degeneration of key regions in the brain, including but not limited to the hippocampus, the prefrontal cortex, and the motor cortex. The cause of this degeneration is not known, but research has found two proteins that undergo posttranslational modifications: tau, a protein concentrated in the axons of neurons, and amyloid precursor protein (APP), a protein concentrated near the synapse. Through mechanisms that have yet to be elucidated, the accumulation of these two proteins in their abnormal aggregate forms leads to the neurodegeneration that is characteristic of AD. Until the invention of induced pluripotent stem cells (iPSCs) in 2006, the bulk of research was carried out using transgenic animal models that offered little promise in their ability to translate well from benchtop to bedside, creating a bottleneck in the development of therapeutics. However, with iPSC, patient-specific cell cultures can be utilized to create models based on human cells. These human cells have the potential to avoid issues in translatability that have plagued animal models by providing researchers with a model that closely resembles and mimics the neurons found in humans. By using human iPSC technology, researchers can create more accurate models of AD ex vivo while also focusing on regenerative medicine using iPSC in vivo. The following review focuses on the current uses of iPSC and how they have the potential to regenerate damaged neuronal tissue, in the hopes that these technologies can assist in getting through the bottleneck of AD therapeutic research.

Neuroinflammation: Integrated Nervous Tissue Response through Intercellular Interactions at the "Whole System" Scale

Different cell populations in the nervous tissue establish numerous, heterotypic interactions and perform specific, frequently intersecting activities devoted to the maintenance of homeostasis. Microglia and astrocytes, respectively the immune and the "housekeeper" cells of nervous tissue, play a key role in neurodegenerative diseases. Alterations of tissue homeostasis trigger neuroinflammation, a collective dynamic response of glial cells. Reactive astrocytes and microglia express various functional phenotypes, ranging from anti-inflammatory to pro-inflammatory. Chronic neuroinflammation is characterized by a gradual shift of astroglial and microglial phenotypes from anti-inflammatory to pro-inflammatory, switching their activities from cytoprotective to cytotoxic. In this scenario, the different cell populations reciprocally modulate their phenotypes through intense, reverberating signaling. Current evidence suggests that heterotypic interactions are links in an intricate network of mutual influences and interdependencies connecting all cell types in the nervous system. In this view, activation, modulation, as well as outcomes of neuroinflammation, should be ascribed to the nervous tissue as a whole. While the need remains of identifying further links in this network, a step back to rethink our view of neuroinflammation in the light of the "whole system" scale, could help us to understand some of its most controversial and puzzling features.

Sialometabolism in Brain Health and Alzheimer's Disease

Sialic acids refer to a unique family of acidic sugars with a 9-carbon backbone that are mostly found as terminal residues in glycan structures of glycoconjugates including both glycoproteins and glycolipids. The highest levels of sialic acids are expressed in the brain where they regulate neuronal sprouting and plasticity, axon myelination and myelin stability, as well as remodeling of mature neuronal connections. Moreover, sialic acids are the sole ligands for microglial Siglecs (sialic acid-binding immunoglobulin-type lectins), and sialic acid-Siglec interactions have been indicated to play a critical role in the regulation of microglial homeostasis in a healthy brain. The recent discovery of CD33, a microglial Siglec, as a novel genetic risk factor for late-onset Alzheimer's disease (AD), highlights the potential role of sialic acids in the development of microglial dysfunction and neuroinflammation in AD. Apart from microglia, sialic acids have been found to be involved in several other major changes associated with AD. Elevated levels of serum sialic acids have been reported in AD patients. Alterations in ganglioside (major sialic acid carrier) metabolism have been demonstrated as an aggravating factor in the formation of amyloid pathology in AD. Polysialic acids are linear homopolymers of sialic acids and have been implicated to be an important regulator of neurogenesis that contributes to neuronal repair and recovery from neurodegeneration such as in AD. In summary, this article reviews current understanding of neural functions of sialic acids and alterations of sialometabolism in aging and AD brains. Furthermore, we discuss the possibility of looking at sialic acids as a promising novel therapeutic target for AD intervention.

Mitochondria-Targeted Therapeutics for Alzheimer's Disease: The Good, the Bad, the Potential

Significance: Alzheimer's disease (AD) is the leading cause of dementia. Thus far, 99.6% of clinical trials, including those targeting energy metabolism, have failed to exert disease-modifying efficacy. Altered mitochondrial function and disruption to the brain bioenergetic system have long-been documented as early events during the pathological progression of AD. Recent Advances: While therapeutic approaches that directly promote mitochondrial bioenergetic machinery or eliminate reactive oxygen species have exhibited limited translatability, emerging strategies targeting nonenergetic aspects of mitochondria provide novel therapeutic targets with the potential to modify AD risk and progression. Growing evidence also reveals a critical link between mitochondrial phenotype and neuroinflammation via metabolic reprogramming of glial cells. Critical Issues: Herein, we summarize major classes of mitochondrion-centered AD therapeutic strategies. In addition, the discrepancy in their efficacy when translated from preclinical models to clinical trials is addressed. Key factors that differentiate the responsiveness to bioenergetic interventions, including sex, apolipoprotein E genotype, and cellular diversity in the brain, are discussed. Future Directions: We propose that the future development of mitochondria-targeted AD therapeutics should consider the interactions between bioenergetics and other disease mechanisms, which may require cell-type-specific targeting to distinguish neurons and non-neuronal cells. Moreover, a successful strategy will likely include stratification by metabolic phenotype, which varies by sex and genetic risk profile and dynamically changes throughout the course of disease. As the network of mitochondrial integration expands across intracellular and systems level biology, assessment of intended, the good, versus unintended consequences, the bad, will be required to reach the potential of mitochondrial therapeutics.

Reduction of Amyloid Burden by Proliferated Homeostatic Microglia in Toxoplasma gondii-Infected Alzheimer's Disease Model Mice

In this study, we confirmed that the number of resident homeostatic microglia increases during chronic Toxoplasma gondii infection. Given that the progression of Alzheimer’s disease (AD) worsens with the accumulation of amyloid β (Aβ) plaques, which are eliminated through microglial phagocytosis, we hypothesized that T. gondii-induced microglial proliferation would reduce AD progression. Therefore, we investigated the association between microglial proliferation and Aβ plaque burden using brain tissues isolated from 5XFAD AD mice (AD group) and T. gondii-infected AD mice (AD + Toxo group). In the AD + Toxo group, amyloid plaque burden significantly decreased compared with the AD group; conversely, homeostatic microglial proliferation, and number of plaque-associated microglia significantly increased. As most plaque-associated microglia shifted to the disease-associated microglia (DAM) phenotype in both AD and AD + Toxo groups and underwent apoptosis after the lysosomal degradation of phagocytosed Aβ plaques, this indicates that a sustained supply of homeostatic microglia is required for alleviating Aβ plaque burden. Thus, chronic T. gondii infection can induce microglial proliferation in the brains of mice with progressed AD; a sustained supply of homeostatic microglia is a promising prospect for AD treatment.

Microglia increase tight-junction permeability in coordination with Müller cells under hypoxic condition in an in vitro model of inner blood-retinal barrier

Microglia and Müller cells (MCs) are believed to be critically involved in hypoxia-induced blood-retinal barrier (BRB) disruption, which is a major pathogenic factor of various retinopathies. However, the underlying mechanism remains poorly defined. The inner BRB (iBRB) is primarily formed of microvascular endothelial cells (ECs) with tight junction (TJ), which are surrounded and supported by retinal glial cells. We developed a novel in vitro iBRB model sheet by sandwiching Transwell membrane with layered mouse brain microvascular ECs (bEnd.3) and mouse retinal MCs (QMMuC-1) on each side of the membrane. Using this model, we tested the hypothesis that under hypoxic condition, activated microglia produce inflammatory cytokines such as interleukin (IL)-1β, which may promote vascular endothelial growth factor (VEGF) production from MCs, leading to TJ disruption. The iBRB model cell sheets were exposed to 1% oxygen for 6 h with or without mouse brain microglia (BV2) or IL-1β. TJ structure and function were examined by zonula occludens (ZO)-1 immunostaining and fluorescein isothiocyanate permeability assay, respectively. Relative gene expression of IL-1β in BV2 under normoxic and hypoxic conditions was examined by real-time reverse transcription-polymerase chain reaction. VEGF protein concentration in QMMuC-1 supernatants was measured by enzyme-linked immunosorbent assay. The bEnd.3 cell sheet incubated with BV2 in hypoxic condition or with IL-1β in normoxic condition showed abnormal localization of ZO-1 and aberrated barrier function. Under normoxic condition, EC-MC iBRB model cell sheet showed lower permeability than bEnd.3 cell sheet. Under hypoxic conditions, the barrier function of EC-MC iBRB model cell sheet was more deteriorated compared to bEnd.3 cell sheet. Under hypoxic condition, incubation of EC-MC iBRB model cell sheet with BV2 cells or IL-1β significantly increased barrier permeability, and hypoxia-treated BV2 cells expressed significantly higher levels of IL-1β mRNA. Incubation of QMMuC-1 with IL-1β increased VEGF production. These results suggest that under hypoxic condition, microglia are activated to release proinflammatory cytokines such as IL-1β that promote VEGF production from MCs, leading to disruption of iBRB function. Modulating microglia and MCs function may be a novel approach to treat hypoxia-induced retinal BRB dysfunction.

Alzheimer's Disease and Diabetes: Role of Diet, Microbiota and Inflammation in Preclinical Models

Alzheimer's disease (AD) is the most common cause of dementia. Epidemiological studies show the association between AD and type 2 diabetes (T2DM), although the mechanisms are not fully understood. Dietary habits and lifestyle, that are risk factors in both diseases, strongly modulate gut microbiota composition. Also, the brain-gut axis plays a relevant role in AD, diabetes and inflammation, through products of bacterial metabolism, like short-chain fatty acids. We provide a comprehensive review of current literature on the relation between dysbiosis, altered inflammatory cytokines profile and microglia in preclinical models of AD, T2DM and models that reproduce both diseases as commonly observed in the clinic. Increased proinflammatory cytokines, such as IL-1β and TNF-α, are widely detected. Microbiome analysis shows alterations in Actinobacteria, Bacteroidetes or Firmicutes phyla, among others. Altered α- and β-diversity is observed in mice depending on genotype, gender and age; therefore, alterations in bacteria taxa highly depend on the models and approaches. We also review the use of pre- and probiotic supplements, that by favoring a healthy microbiome ameliorate AD and T2DM pathologies. Whereas extensive studies have been carried out, further research would be necessary to fully understand the relation between diet, microbiome and inflammation in AD and T2DM.

A reporter cell system for the triggering receptor expressed on myeloid cells 2 reveals differential effects of disease-associated variants on receptor signaling and activation by antibodies against the stalk region

The triggering receptor expressed on myeloid cells 2 (TREM2) is an immune receptor expressed on myeloid-derived cell types. The extracellular immunoglobulin-like domain of TREM2 binds anionic ligands including Apolipoprotein E and Amyloid-β. The transmembrane domain interacts with its adaptor protein DAP12/TYROBP that is responsible for propagation of downstream signaling upon ligand interaction. Several sequence variants of TREM2 have been linked to different neurodegenerative diseases including Alzheimer's disease. Here, we generated HEK 293 Flp-In cell lines stably expressing human TREM2 and DAP12 using a bicistronic construct with a T2A linker sequence allowing initial expression of both proteins in stoichiometric amounts. Cell biological and biochemical analyses revealed transport of TREM2 to the cell surface, and canonical sequential proteolytic processing and shedding of TREM2 (sTREM2). The functionality of this cell system was demonstrated by detection of phosphorylated spleen tyrosine kinase (SYK) upon stimulation of TREM2 with the anionic membrane lipid phosphatidylserine or anti-TREM2 antibodies. Using this cell model, we demonstrated impaired signaling of disease associated TREM2 variants. We also identified a monoclonal antibody against the stalk region of TREM2 with agonistic activity. Activation of TREM2-DAP12 signaling with the monoclonal antibody and the partial loss of function of disease associated variants were recapitulated in induced pluripotent stem cell derived microglia. Thus, this reporter cell model represents a suitable experimental system to investigate signaling of TREM2 variants, and for the identification of ligands and compounds that modulate TREM2-DAP12 signaling. MAIN POINTS: Disease associated variants impair the signaling activity of TREM2 by distinct mechanisms. Targeting the stalk region of TREM2 with bivalent antibodies activates TREM2 signaling.