Alzheimer's disease: microglia targets and their modulation to promote amyloid phagocytosis and mitigate neuroinflammation

P2Y6R Inhibition Induces Microglial M2 Polarization by Promoting PINK1/Parkin-Dependent Mitophagy After Spinal Cord Injury

Secondary injury presents a significant hurdle to neural regeneration following spinal cord injury (SCI), primarily driven by inflammation in which microglial cells play a crucial role. Despite the growing interest in mitophagy, studies on its occurrence post-spinal cord injury, particularly within microglial cells, are scarce. While P2Y6R has been implicated in inflammation regulation in various neurological conditions, its specific role in SCI remains uncertain. Our study revealed an upregulation of P2Y6R expression following SCI notably in microglial cells. Treatment with the P2Y6R-specific inhibitor, MRS2578, in mice facilitated M2 polarization of microglial cells and alleviated secondary damage, ultimately enhancing neural regeneration and functional recovery. In an in vitro BV2 inflammation model, our findings indicate that P2Y6R inhibition induced M2 polarization of BV2 cells and reduced neuroinflammation through PINK/Parkin-dependent mitophagy activation. In summary, our results underscore the potential of P2Y6R inhibition in promoting mitophagy-induced M2 polarization of microglial cells, thereby ameliorating secondary injury following spinal cord injury.

The evolving pathophysiology of TBI and the advantages of temporally-guided combination therapies

Several clinical and experimental studies have demonstrated that traumatic brain injury (TBI) activates cascades of biochemical, molecular, structural, and pathological changes in the brain. These changes combine to contribute to the various outcomes observed after TBI. Given the breadth and complexity of changes, combination treatments may be an effective approach for targeting multiple detrimental pathways to yield meaningful improvements. In order to identify targets for therapy development, the temporally evolving pathophysiology of TBI needs to be elucidated in detail at both the cellular and molecular levels, as it has been shown that the mechanisms contributing to cognitive dysfunction change over time. Thus, a combination of individual mechanism-based therapies is likely to be effective when maintained based on the time courses of the cellular and molecular changes being targeted. In this review, we will discuss the temporal changes of some of the key clinical pathologies of human TBI, the underlying cellular and molecular mechanisms, and the results from preclinical and clinical studies aimed at mitigating their consequences. As most of the pathological events that occur after TBI are likely to have subsided in the chronic stage of the disease, combination treatments aimed at attenuating chronic conditions such as cognitive dysfunction may not require the initiation of individual treatments at a specific time. We propose that a combination of acute, subacute, and chronic interventions may be necessary to maximally improve health-related quality of life (HRQoL) for persons who have sustained a TBI.

A Survey on Computational Methods in Drug Discovery for Neurodegenerative Diseases

Currently, the age structure of the world population is changing due to declining birth rates and increasing life expectancy. As a result, physicians worldwide have to treat an increasing number of age-related diseases, of which neurological disorders represent a significant part. In this context, there is an urgent need to discover new therapeutic approaches to counteract the effects of neurodegeneration on human health, and computational science can be of pivotal importance for more effective neurodrug discovery. The knowledge of the molecular structure of the receptors and other biomolecules involved in neurological pathogenesis facilitates the design of new molecules as potential drugs to be used in the fight against diseases of high social relevance such as dementia, Alzheimer's disease (AD) and Parkinson's disease (PD), to cite only a few. However, the absence of comprehensive guidelines regarding the strengths and weaknesses of alternative approaches creates a fragmented and disconnected field, resulting in missed opportunities to enhance performance and achieve successful applications. This review aims to summarize some of the most innovative strategies based on computational methods used for neurodrug development. In particular, recent applications and the state-of-the-art of molecular docking and artificial intelligence for ligand- and target-based approaches in novel drug design were reviewed, highlighting the crucial role of in silico methods in the context of neurodrug discovery for neurodegenerative diseases.

Modification of Neural Circuit Functions by Microglial P2Y6 Receptors in Health and Neurodegeneration

Neural circuits consisting of neurons and glial cells help to establish all functions of the CNS. Microglia, the resident immunocytes of the CNS, are endowed with UDP-sensitive P2Y6 receptors (P2Y6Rs) which regulate phagocytosis/pruning of excessive synapses during individual development and refine synapses in an activity-dependent manner during adulthood. In addition, this type of receptor plays a decisive role in primary (Alzheimer's disease, Parkinson's disease, neuropathic pain) and secondary (epilepsy, ischemic-, mechanical-, or irradiation-induced) neurodegeneration. A whole range of microglial cytokines controlled by P2Y6Rs, such as the interleukins IL-1β, IL-6, IL-8, and tumor necrosis factor-α (TNF-α), leads to neuroinflammation, resulting in neurodegeneration. Hence, small molecular antagonists of P2Y6Rs and genetic knockdown of this receptor provide feasible ways to alleviate inflammation-induced neurological disorders but might also interfere with the regulation of the synaptic circuitry. The present review aims at investigating this dual role of P2Y6Rs in microglia, both in shaping neural circuits by targeted phagocytosis and promoting neurodegenerative illnesses by fostering neuroinflammation through multiple transduction mechanisms.

Overview of pro-inflammatory and pro-survival components in neuroinflammatory signalling and neurodegeneration

Neurodegenerative diseases (NDDs) are identified by the progressive deterioration of neurons and a subsequent decline in cognitive function, creating an enormous burden on the healthcare system globally. Neuroinflammation is an intricate procedure that initiates the immune response in the central nervous system (CNS) and significantly impacts the expansion of NDDs. This study scrutinizes the complicated interaction between neuronal degeneration and neuroinflammation, with an appropriate emphasis on their reciprocal impacts. It also describes how neuroinflammatory reactions in NDDs are controlled by activating certain pro-inflammatory transcription factors, including p38 MAPK, FAF1, Toll-like receptors (TLRs), and STAT3. Alternatively, it evaluates the impact of pro-survival transcription factors, such as the SOCS pathway, YY1, SIRT1, and MEF2, which provide neuroprotective protection against damage triggered by neuroinflammation. Moreover, we study the feasibility of accommodating drug repositioning as a therapeutic approach for treating neuroinflammatory disorders. This suggests the use of existing medications for novel utilization in the treatment of NDDs. Furthermore, the study intends to reveal novel biomarkers of neuroinflammation that contribute fundamental observation for the initial detection and diagnosis of these disorders. This study aims to strengthen therapy interference and augment patient outcomes by combining ongoing data and evaluating novel therapeutic and diagnostic approaches. The goal is to devote the growth of an effective strategy to reducing the impact of neuroinflammation on neuronal protection in NDDs.

Rational fusion design inspired by cell-penetrating peptide: SS31/S-14 G Humanin hybrid peptide with amplified multimodal efficacy and bio-permeability for the treatment of Alzheimer's disease

Alzheimer's disease is a neurodegenerative disease induced by multiple interconnected mechanisms. Peptide drug candidates with multi-modal efficacy generated from fusion strategy are suitable for addressing multi-facet pathology. However, clinical translation of peptide drugs is greatly hampered by their low permeability into brain. Herein, a hybrid peptide HNSS is generated by merging two therapeutic peptides (SS31 and S-14 G Humanin (HNG)), using a different approach from the classical shuttle-therapeutic peptide conjugate design. HNSS demonstrated increased bio-permeability, with a 2-fold improvement in brain distribution over HNG, thanks to its structure mimicking the design of signal peptide-derived cell-penetrating peptides. HNSS efficiently alleviated mitochondrial dysfunction through the combined effects of mitochondrial targeting, ROS scavenging and p-STAT3 activation. Meanwhile, HNSS with increased Aβ affinity greatly inhibited Aβ oligomerization/fibrillation, and interrupted Aβ interaction with neuron/microglia by reducing neuronal mitochondrial Aβ deposition and promoting microglial phagocytosis of Aβ. In 3× Tg-AD transgenic mice, HNSS treatment efficiently inhibited brain neuron loss and improved the cognitive performance. This work validates the rational fusion design-based strategy for bio-permeability improvement and efficacy amplification, providing a paradigm for developing therapeutic peptide candidates against neurodegenerative disease.

Microglial purinergic signaling in Alzheimer's disease

Alzheimer's disease (AD) is a progressive and fatal neurodegenerative disease. The prevalent features of AD pathogenesis are the appearance of β-amyloid (Aβ) plaques and neurofibrillary tangles, which cause microglial activation, synaptic deficiency, and neuronal loss. Microglia accompanies AD pathological processes and is also linked to cognitive deficits. Purinergic signaling has been shown to play a complex and tight interplay with the chemotaxis, phagocytosis, and production of pro-inflammatory factors in microglia, which is an important mechanism for regulating microglia activation. Here, we review recent evidence for interactions between AD, microglia, and purinergic signaling and find that the purinergic P2 receptors pertinently expressed on microglia are the ionotropic receptors P2X4 and P2X7, and the subtypes of P2YRs expressed by microglia are metabotropic receptors P2Y2, P2Y6, P2Y12, and P2Y13. The adenosine P1 receptors expressed in microglia include A1R, A2AR, and A2BR. Among them, the activation of P2X4, P2X7, and adenosine A1, A2A receptors expressed in microglia can aggravate the pathological process of AD, whereas P2Y2, P2Y6, P2Y12, and P2Y13 receptors expressed by microglia can induce neuroprotective effects. However, A1R activation also has a strong neuroprotective effect and has a significant anti-inflammatory effect in chronic neuroinflammation. These receptors regulate a variety of pathophysiological processes in AD, including APP processing, Aβ production, tau phosphorylation, neuroinflammation, synaptic dysfunction, and mitochondrial dysfunction. This review also provides key pharmacological advances in purinergic signaling receptors.

Human Microglia-Like Cells Differentiated from Monocytes with GM-CSF and IL-34 Show Phagocytosis of α-Synuclein Aggregates and C/EBPβ-Dependent Proinflammatory Activation

Microglia, the main resident immune cells in the central nervous system, are implicated in the pathogenesis of various neurological disorders. Much of our knowledge on microglial biology was obtained using rodent microglial cultures. To understand the role of microglia in human disease, reliable in vitro models of human microglia are necessary. Monocyte-derived microglia-like cells (MDMi) are a promising approach. This study aimed to characterize MDMi cells generated from adult human monocytes using granulocyte-macrophage colony-stimulating factor and interleukin-34. To this end, 49 independent cultures of MDMI were prepared, and various methodological and functional studies were performed. We show that with this protocol, adult human monocytes develop into microglia-like cells, a coating is unnecessary, and high cell density seeding is preferable. When compared to monocytes, MDMi upregulate the expression of many, but not all, microglial markers, indicating that, although these cells display a microglia-like phenotype, they cannot be considered bona fide human microglia. At the functional level, MDMi phagocytose α-synuclein aggregates and responds to lipopolysaccharide (LPS) by nuclear translocation of the transcription factor nuclear factor-kappaB (NFkappaB) and the upregulation of proinflammatory genes. Finally, a long-lasting silencing of the transcription factor CCAAT/enhancer protein β (C/EBPβ) was achieved by small interfering RNA, resulting in the subsequent downregulation of proinflammatory genes. This supports the hypothesis that C/EBPβ plays a key role in proinflammatory gene program activation in human microglia. Altogether, this study sheds new light on the properties of MDMi cells and supports these cells as a promising in vitro model for studying adult human microglia-like cells.

Sera from people with HIV and depression induce commensurate metabolic alterations in astrocytes: toward precision diagnoses and therapies

Objectives

People with HIV (PWH) have high rates of depression and neurocognitive impairment (NCI) despite viral suppression on antiretroviral therapy (ART). Mounting evidence suggests that immunometabolic disruptions may contribute to these conditions in some PWH. We hypothesized that metabolic dysfunction in astrocytes is associated with depressive symptoms and cognitive function in PWH.

Methods

Human astrocytes were exposed to sera from PWH (n=40) with varying degrees of depressive symptomatology and cognitive function. MitoTrackerTM Deep Red FM (MT) was used to visualize mitochondrial activity and glial fibrillary acidic protein (GFAP) as an indicator of astrocyte reactivity using the high-throughput fluorescent microscopy and image analyses platform, CellInsight CX5 (CX5). The Seahorse platform was used to assess glycolytic and mitochondrial metabolism.

Results

More severe depression, as indexed by higher Beck's Depression Inventory (BDI-II) scores, was associated with lower MT signal measures. Better cognitive function, as assessed by neuropsychiatric testing t-scores, was associated with increased MT signal measures. GFAP intensity negatively correlated with several cognitive t-scores. Age positively correlated with (higher) MT signal measures and GFAP intensity. Worse depressive symptoms (higher BDI-II scores) were associated with decreased oxygen consumption rate and spare respiratory capacity, concomitant with increased extracellular acidification rate in astrocytes.

Conclusions

These findings show that factors in the sera of PWH alter mitochondrial activity in cultured human astrocytes, suggesting that mechanisms that alter mitochondrial and astrocyte homeostasis can be detected peripherally. Thus, in vitro cultures may provide a model to identify neuropathogenic mechanisms of depression or neurocognitive impairment in PWH and test personalized therapeutics for neurologic and psychiatric disorders.

[On the road to biological blood diagnosis of Alzheimer's disease?]

The growing number of people suffering from Alzheimer's disease (AD) represents a major public health problem. The diagnosis of AD is multidisciplinary and involves the use of amyloid and tau biomarkers measured in cerebrospinal fluid. Recent advances in analytical techniques now allow us to measure these biomarkers in blood. Blood biomarkers offer particularly promising potential for early, minimally invasive detection of AD, as well as for differential diagnosis of dementia and patient follow-up. The aim of this review is to provide an overview of current and candidate blood biomarkers for AD, their informative value, and their potential to be integrated into clinical practice in the near future.

Hemicyanine-based sensor for mitochondrial viscosity imaging in BV2 cells

Mitochondrial viscosity is a critical factor affecting numerous physiological processes, including phagocytosis. Abnormal viscosity in mitochondria is related to some pathological activities and diseases. Evaluating and detecting the changes in mitochondrial viscosity in vivo is crucial. Thus, a mitochondria-targeted red-emitting fluorescent probe (VP) was prepared, and can be used to detect viscosity with high selectivity and sensitivity. The synthesis of probe VP was as simple as two steps and the cost was low. In addition, the fluorescence intensity (log I615) exhibited an excellent relationship with viscosity (log η) in the range of 0.5 - 2.5 (R2 = 0.9985) in water/glycerol mixture. It is noteworthy that the probe VP displayed the highest signal-to-noise ratio (about 50-fold) for viscosity in water and glycerol system. The probe VP can visualize the mitochondrial viscosity change in living cells. More importantly, phagocytic test for BV2 cells further demonstrated that phagocytosis decreased with increased viscosity. Furthermore, VP was successfully used for monitoring the mitophagy process induced by starvation, and mitochondrial viscosity exhibited enhancement during mitophagy. The probe was a potential tool for studying viscosity and phagocytosis.

Non-Coding RNA in Microglia Activation and Neuroinflammation in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by complex pathophysiological features. Amyloid plaques resulting from extracellular amyloid deposition and neurofibrillary tangles formed by intracellular hyperphosphorylated tau accumulation serve as primary neuropathological criteria for AD diagnosis. The activation of microglia has been closely associated with these pathological manifestations. Non-coding RNA (ncRNA), a versatile molecule involved in various cellular functions such as genetic information storage and transport, as well as catalysis of biochemical reactions, plays a crucial role in microglial activation. This review aims to investigate the regulatory role of ncRNAs in protein expression by directly targeting genes, proteins, and interactions. Furthermore, it explores the ability of ncRNAs to modulate inflammatory pathways, influence the expression of inflammatory factors, and regulate microglia activation, all of which contribute to neuroinflammation and AD. However, there are still significant controversies surrounding microglial activation and polarization. The categorization into M1 and M2 phenotypes may oversimplify the intricate and multifaceted regulatory processes in microglial response to neuroinflammation. Limited research has been conducted on the role of ncRNAs in regulating microglial activation and inducing distinct polarization states in the context of neuroinflammation. Moreover, the regulatory mechanisms through which ncRNAs govern microglial function continue to be refined. The current understanding of ncRNA regulatory pathways involved in microglial activation remains incomplete and may be influenced by spatial, temporal, and tissue-specific factors. Therefore, further in-depth investigations are warranted. In conclusion, there are ongoing debates and uncertainties regarding the activation and polarization of microglial cells, particularly concerning the categorization into M1 and M2 phenotypes. The study of ncRNA regulation in microglial activation and polarization, as well as its mechanisms, is still in its early stages and requires further investigation. However, this review offers new insights and opportunities for therapeutic approaches in AD. The development of ncRNA-based drugs may hold promise as a new direction in AD treatment.

The effect of the sulfation patterns of dermatan and chondroitin sulfate from vertebrates and ascidians on their neuritogenic and neuroprotective properties

Neurodegeneration is caused by the progressive loss of the structure and function of neurons, leading to cell death, and it is the main cause of many neurodegenerative diseases. Many molecules, such as glycosaminoglycans (GAGs), have been studied for their potential to prevent or treat these diseases. They are widespread in nature and perform an important role in neuritogenesis and neuroprotection. Here we investigated the neuritogenic and neuroprotective role of Phallusia nigra dermatan sulfate (PnD2,6S) and compared it with two distinct structures of chondroitin sulfate (C6S) and dermatan sulfate (D4S). For this study, a neuro 2A murine neuroblastoma cell line was used, and a chemical lesion was induced by the pesticide rotenone (ROT). We observed that PnD2,6S + ROT had a better neuritogenic effect than either C6S + ROT or D4S + ROT at a lower concentration (0.05 μg/mL). When evaluating the mitochondrial membrane potential, PnD2,6S showed a neuroprotective effect at a concentration of 0.4 μg/mL. These data indicate different mechanisms underlying this neuronal potential, in which the sulfation pattern is important for neuritogenic activity, while for neuroprotection all DS/CS structures had similar effects. This finding leads to a better understanding the chemical structures of PnD2,6S, C6S, and D4S and their therapeutic potential.

Study on the Role of an Erythrocyte Membrane-Coated Nanotheranostic System in Targeted Immune Regulation of Alzheimer's Disease

Alzheimer's disease (AD) is one of the most common neurodegenerative diseases in the elderly population. Despite significant advances in studies of the pathobiology on AD, there is still no effective treatment. Here, an erythrocyte membrane-camouflaged nanodrug delivery system (TR-ZRA) modified with transferrin receptor aptamers that can be targeted across the blood-brain barrier to ameliorate AD immune environment is established. Based on metal-organic framework (Zn-CA), TR-ZRA is loaded with CD22shRNA plasmid to silence the abnormally high expression molecule CD22 in aging microglia. Most importantly, TR-ZRA can enhance the ability of microglia to phagocytose Aβ and alleviate complement activation, which can promote neuronal activity and decrease inflammation level in the AD brain. Moreover, TR-ZRA is also loaded with Aβ aptamers, which allow rapid and low-cost monitoring of Aβ plaques in vitro. After treatment with TR-ZRA, learning, and memory abilities are enhanced in AD mice. In conclusion, the biomimetic delivery nanosystem TR-ZRA in this study provides a promising strategy and novel immune targets for AD therapy.

Parthenolide alleviates microglia-mediated neuroinflammation via MAPK/TRIM31/NLRP3 signaling to ameliorate cognitive disorder

BACKGROUND AND PURPOSE

Neuroinflammation, mainly mediated by microglia, is involved in the evolution of Alzheimer's disease (AD). Parthenolide (PTL) has diverse pharmacological effects such as anti-inflammatory and antioxidative stress. However, whether PTL can modulate microglia-mediated neuroinflammation to improve cognitive impairment in amyloid precursor protein/presenilin 1 (APP/PS1) mice is unclear.

METHODS

LPS/IFN-γ-induced BV2 and HMC3 microglia were used for in vitro experiments; the roles of PTL on anti-inflammatory, anti-oxidative, phagocytic activity, and neuroprotection were assessed by inflammatory cytokines assays, dichlorodihydrofluorescein diacetate, phagocytosis, and cell counting kit-8 assays. Western blot and immunofluorescence（IF） were used to examine related molecular mechanisms. In vivo, IF and western blot were applied in LPS-treated wild-type (WT) mice and APP/PS1 mice models. The Morris water maze test was performed to evaluate the effects of PTL on cognitive disorders.

RESULTS

In vitro, PTL dramatically suppressed proinflammatory cytokines IL-6, IL-1β, and TNF-α release and increased IL-10 levels. Moreover, PTL decreased reactive oxygen species and restored microglial phagocytic activities via the AKT/MAPK/ NF-κB signaling pathway. Importantly, we discovered that PTL obviously enhanced TRIM31 expression and siTRIM31 elevated proinflammatory cytokine levels. Furthermore, we determined that the anti-inflammatory role of PTL was mostly TRIM31/NLRP3 signaling-dependent. In vivo, PTL alleviated microgliosis and astrogliosis in LPS-treated WT and APP/PS1 mice. Additionally, PTL significantly ameliorated memory and learning deficits in cognitive behaviors.

CONCLUSIONS

PTL improved cognitive and behavioral dysfunction, inhibited neuroinflammation, and showed potent anti-neuroinflammatory activity and neuroprotective effects by improving the MAPK/TRIM31/NLRP3 axis. Our study emphasized the therapeutic potential of PTL for improving cognitive disorders during AD progression.

Purinergic signaling in cognitive impairment and neuropsychiatric symptoms of Alzheimer's disease

About 10 million new cases of dementia develop worldwide each year, of which up to 70% are attributable to Alzheimer's disease (AD). In addition to the widely known symptoms of memory loss and cognitive impairment, AD patients frequently develop non-cognitive symptoms, referred to as behavioral and psychological symptoms of dementia (BPSDs). Sleep disorders are often associated with AD, but mood alterations, notably depression and apathy, comprise the most frequent class of BPSDs. BPSDs negatively affect the lives of AD patients and their caregivers, and have a significant impact on public health systems and the economy. Because treatments currently available for AD are not disease-modifying and mainly aim to ameliorate some of the cognitive symptoms, elucidating the mechanisms underlying mood alterations and other BPSDs in AD may reveal novel avenues for progress in AD therapy. Purinergic signaling is implicated in the pathophysiology of several central nervous system (CNS) disorders, such as AD, depression and sleep disorders. Here, we review recent findings indicating that purinergic receptors, mainly the A₁, A_2A, and P2X7 subtypes, are associated with the development/progression of AD. Current evidence suggests that targeting purinergic signaling may represent a promising therapeutic approach in AD and related conditions. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Proinflammatory S100A9 stimulates TLR4/NF-κB signaling pathways causing enhanced phagocytic capacity of microglial cells

Alzheimer's disease (AD) is the main cause of dementia, affecting the increasingly aging population. Growing evidence indicates that neuro-inflammation plays crucial roles, e.g., the association between AD risk genes with innate immune functions. In this study, we demonstrate that moderate concentrations of pro-inflammatory cytokine S100A9 regulate immune response of BV2 microglial cells, i.e., the phagocytic capacity, reflected by elevated number of 1 μm diameter Dsred-stained latex beads in the cytoplasm. In contrast, at high S100A9 concentrations, both the viability and phagocytic capacity of BV2 cells drop substantially. Furthermore, it is uncovered that S100A9 affects phagocytosis of microglia via NF-κB signaling pathways. Application of related target-specific drugs, i.e., IKK and TLR4 inhibitors, effectively suppresses BV2 cells' immune responses. These results suggest that pro-inflammatory S100A9 activates microglial phagocytosis, and possibly contributes to the clearance of amyloidogenic species at the early stage of AD.

Beyond the classical amyloid hypothesis in Alzheimer's disease: Molecular insights into current concepts of pathogenesis, therapeutic targets, and study models

Alzheimer's disease (AD) is one of the progressive neurodegenerative disorders and the most common cause of dementia in the elderly worldwide causing difficulties in the daily life of the patient. AD is characterized by the aberrant accumulation of β-amyloid plaques and tau protein-containing neurofibrillary tangles (NFTs) in the brain giving rise to neuroinflammation, oxidative stress, synaptic failure, and eventual neuronal cell death. The total cost of care in AD treatment and related health care activities is enormous and pharmaceutical drugs approved by Food and Drug Administration have not manifested sufficient efficacy in protection and therapy. In recent years, there are growing studies that contribute a fundamental understanding to AD pathogenesis, AD-associated risk factors, and pharmacological intervention. However, greater molecular process-oriented research in company with suitable experimental models is still of the essence to enhance the prospects for AD therapy and cell lines as a disease model are still the major part of this milestone. In this review, we provide an insight into molecular mechanisms, particularly the recent concept in gut-brain axis, vascular dysfunction and autophagy, and current models used in the study of AD. Here, we emphasized the importance of therapeutic strategy targeting multiple mechanisms together with utilizing appropriate models for the discovery of novel effective AD therapy. This article is categorized under: Neurological Diseases > Molecular and Cellular Physiology.

GRK5 Deficiency in the Hippocampus Leads to Cognitive Impairment via Abnormal Microglial Alterations

GRK5 is a member of the G protein-coupled receptor (GPCR) kinase family and is closely associated with heart and nervous system disease. It has been reported that GRK5 is closely related to cerebral nerve function and neurodegenerative diseases. However, the biological function of GRK5 in the brain and the influence of GRK5 deficiency on cognitive dysfunction associated with neurodegenerative diseases are unknown. Here, we reported that mice with reduced GRK5 in the hippocampus exhibit cognitive impairment and some Alzheimer's disease (AD)-related molecular pathologies, such as significant neuronal damage and loss, enhanced tau protein phosphorylation, and increased levels of Aβ peptides in the hippocampus. Mechanistically, we observed that GRK5 is located in microglia and plays an essential role in maintaining the morphology and function of microglia. GRK5 deficiency elicits microglial morphology changes and proinflammatory-associated gene increases. In addition, transcriptional analysis of hippocampal tissues revealed striking changes in neuroactive ligand‒receptor interactions and TNF signaling in GRK5-deficient mice. In conclusion, our results further confirm the vital role of GRK5 in maintaining normal cognitive function in mice. This finding suggests a possible mechanism by which GRK5 maintains microglial homeostasis, and its loss may induce microglial function deficits and cause some AD-related molecular pathogenesis.

Overview of the blood biomarkers in Alzheimer's disease: Promises and challenges

The increasing number of people with advanced Alzheimer's disease (AD) represents a significant psychological and financial cost to the world population. Accurate detection of the earliest phase of preclinical AD is of major importance for the success of preventive and therapeutic strategies (Cullen et al., 2021). Advances in analytical techniques have been essential for the development of sensitive, specific and reliable diagnostic tests for AD biomarkers in biological fluids (cerebrospinal fluid and blood). Blood biomarkers hold promising potential for early and minimally invasive detection of AD, but also for differential diagnosis of dementia and for monitoring the course of the disease. The aim of this review is to provide an overview of current blood biomarkers of AD, from tau proteins and amyloid peptides to biomarkers of neuronal degeneration and inflammation, reactive and metabolic factors. We thus discuss the informative value of currently candidate blood biomarkers and their potential to be integrated into clinical practice for the management of AD in the near future.

Microglia-Mediated Neurovascular Unit Dysfunction in Alzheimer's Disease

The neurovascular unit (NVU) is involved in the pathological changes in Alzheimer's disease (AD). The NVU is a structural and functional complex that maintains microenvironmental homeostasis and metabolic balance in the central nervous system. As one of the most important components of the NVU, microglia not only induce blood-brain barrier breakdown by promoting neuroinflammation, the infiltration of peripheral white blood cells and oxidative stress but also mediate neurovascular uncoupling by inducing mitochondrial dysfunction in neurons, abnormal contraction of cerebral vessels, and pericyte loss in AD. In addition, microglia-mediated dysfunction of cellular components in the NVU, such as astrocytes and pericytes, can destroy the integrity of the NVU and lead to NVU impairment. Therefore, we review the mechanisms of microglia-mediated NVU dysfunction in AD. Furthermore, existing therapeutic advancements aimed at restoring the function of microglia and the NVU in AD are discussed. Finally, we predict the role of pericytes in microglia-mediated NVU dysfunction in AD is the hotspot in the future.

The Role of Bromodomain and Extraterminal (BET) Proteins in Controlling the Phagocytic Activity of Microglia In Vitro: Relevance to Alzheimer's Disease

The correct phagocytic activity of microglia is a prerequisite for maintaining homeostasis in the brain. In the analysis of mechanisms regulating microglial phagocytosis, we focused on the bromodomain and extraterminal domain (BET) proteins: Brd2, Brd3, and Brd4, the acetylation code readers that control gene expression in cooperation with transcription factors. We used pharmacological (JQ1) and genetic (siRNA) inhibition of BET proteins in murine microglial cell line BV2. Inhibition of BET proteins reduced the phagocytic activity of BV2, as determined by using a fluorescent microspheres-based assay and fluorescently labelled amyloid-beta peptides. Gene silencing experiments demonstrated that all brain-existing BET isoforms control phagocytosis in microglia. From a set of 84 phagocytosis-related genes, we have found the attenuation of the expression of 14: Siglec1, Sirpb1a, Cd36, Clec7a, Itgam, Tlr3, Fcgr1, Cd14, Marco, Pld1, Fcgr2b, Anxa1, Tnf, Nod1, upon BET inhibition. Further analysis of the mRNA level of other phagocytosis-related genes which were involved in the pathomechanism of Alzheimer's disease demonstrated that JQ1 significantly reduced the expression of Cd33, Trem2, and Zyx. Our results indicate the important role of BET proteins in controlling microglial phagocytosis; therefore, targeting BET may be the efficient method of modulating microglial activity.

Role of Zerumbone, a Phytochemical Sesquiterpenoid from Zingiber zerumbet Smith, in Maintaining Macrophage Polarization and Redox Homeostasis

Macrophages and microglia are highly versatile cells that can be polarized into M1 and M2 phenotypes in response to diverse environmental stimuli, thus exhibiting different biological functions. In the central nervous system, activated resident macrophages and microglial cells trigger the production of proinflammatory mediators that contribute to neurodegenerative diseases and psychiatric disorders. Therefore, modulating the activation of macrophages and microglia by optimizing the inflammatory environment is beneficial for disease management. Several naturally occurring compounds have been reported to have anti-inflammatory and neuroprotective properties. Zerumbone is a phytochemical sesquiterpenoid and also a cyclic ketone isolated from Zingiber zerumbet Smith. In this study, we found that zerumbone effectively reduced the expression of lipocalin-2 in macrophages and microglial cell lines. Lipocalin-2, also known as neutrophil gelatinase-associated lipocalin (NGAL), has been characterized as an adipokine/cytokine implicated in inflammation. Moreover, supplement with zerumbone inhibited reactive oxygen species production. Phagocytic activity was decreased following the zerumbone supplement. In addition, the zerumbone supplement remarkably reduced the production of M1-polarization-associated chemokines CXC10 and CCL-2, as well as M1-polarization-associated cytokines interleukin (IL)-6, IL-1β, and tumor necrosis factor-α. Furthermore, the expression of inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 and the production of NO were attenuated in macrophages and microglial cells supplemented with zerumbone. Notably, we discovered that zerumbone effectively promoted the production of the endogenous antioxidants heme oxygenase-1, glutamate-cysteine ligase modifier subunit, glutamate-cysteine ligase catalytic subunit, and NAD(P)H quinone oxidoreductase-1 and remarkably enhanced IL-10, a marker of M2 macrophage polarization. Endogenous antioxidant production and M2 macrophage polarization were increased through activation of the AMPK/Akt and Akt/GSK3 signaling pathways. In summary, this study demonstrated the protective role of zerumbone in maintaining M1 and M2 polarization homeostasis by decreasing inflammatory responses and enhancing the production of endogenous antioxidants in both macrophages and microglia cells. This study suggests that zerumbone can be used as a potential therapeutic drug for the supplement of neuroinflammatory diseases.

The effects and potential of microglial polarization and crosstalk with other cells of the central nervous system in the treatment of Alzheimer's disease

Microglia are resident immune cells in the central nervous system. During the pathogenesis of Alzheimer's disease, stimulatory factors continuously act on the microglia causing abnormal activation and unbalanced phenotypic changes; these events have become a significant and promising area of research. In this review, we summarize the effects of microglial polarization and crosstalk with other cells in the central nervous system in the treatment of Alzheimer's disease. Our literature search found that phenotypic changes occur continuously in Alzheimer's disease and that microglia exhibit extensive crosstalk with astrocytes, oligodendrocytes, neurons, and penetrated peripheral innate immune cells via specific signaling pathways and cytokines. Collectively, unlike previous efforts to modulate microglial phenotypes at a single level, targeting the phenotypes of microglia and the crosstalk with other cells in the central nervous system may be more effective in reducing inflammation in the central nervous system in Alzheimer's disease. This would establish a theoretical basis for reducing neuronal death from central nervous system inflammation and provide an appropriate environment to promote neuronal regeneration in the treatment of Alzheimer's disease.

In Silico Prediction of the Bioactive Profile and Metabolites of Satureja nepeta in Diseases Related to the Excessive Production of Interleukin-6

Inflammatory bowel diseases are caused by an abnormal reaction of the immune system, which becomes hyperactive because the mechanisms responsible for regulating it get out of control. For an effective immune response, many proinflammatory cytokines are secreted, particularly interleukin-6 (IL-6) keystone cytokine inflammation. Many synthetic and natural compounds targeting IL-6 have been studied. The genus Satureja of the Lamiaceae family is generally known for its many virtues, in particular anti-inflammatory properties. However, the mechanism of action is unclear. This study aims to predict the impact of characterized bioactive molecules of Moroccan Satureja nepeta in the potential control of inflammatory response mediated by IL-6 cytokine. A list of 9 previously characterized natural compounds of S. nepeta was compiled, and a list of 6 potential protein targets involved in intestinal inflammation was made. The 2 lists of natural compound-target proteins were analyzed by the STITCH software (http://stitch.embl.de/) to develop protein-compound and protein-protein interaction networks (PPINs). An ontological enrichment (GO) analysis was performed by the Clue GO plugin to evaluate the PPIN generated by STITCH; finally, the molecular docking to predict the mode underlying the anti-inflammatory effects. STITCH results revealed direct and indirect interactions of S. nepeta chemical compounds with a key protein target IL-6. The array results by ClueGO showed that most compounds involved in the regulation of several biological processes related to IL-6 such as inflammation apoptosis, cell differentiation, and metabolic regulation. The targets directly related to IL-6 have been used for molecular docking. Quercetin, catechin, and gallic acid have a strong affinity with the IL-6 receptor (respectively −7.1; −6.1; −5.3). This study strongly suggests that the bioactive compounds of S. nepeta could constitute a new therapeutic alternative in the treatment of diseases related to IL-6. However, to validate the results obtained in this work, it is necessary to explore the mechanism of action of potential bioactive molecules by experimentation.

Vesicles of yeast cell wall-sitagliptin to alleviate neuroinflammation in Alzheimer's disease

A cell-based drug delivery system based on yeast-cell wall loaded with sitagliptin, a drug with an anti-inflammatory effect, was developed to control neuroinflammation associated with Alzheimer's disease. The optimized nanoparticles had a spherical shape with a negative surface charge, and were shown to be less toxic than the carrier and sitagliptin. Moreover, the nanoparticles caused anti-inflammatory effects against tumor necrosis factor-alpha in mice model of neuroinflammation. The pharmacokinetics study showed the brain concentration of drug in the nanoparticles group was much higher than in the control group. To evaluate the effect of P-glycoprotein on brain entry of sitagliptin, the experiment was repeated with verapamil, as a P-glycoprotein inhibitor. Brain concentration of the nanoparticles group remained approximately unchanged, proving the "Trojan Horse" effect of the developed nanocarriers. The results are promising for using yeast-cell wall as a carrier for targeted delivery to immune cells for the management of inflammation.

Spermidine reduces neuroinflammation and soluble amyloid beta in an Alzheimer's disease mouse model

Background

Deposition of amyloid beta (Aβ) and hyperphosphorylated tau along with glial cell-mediated neuroinflammation are prominent pathogenic hallmarks of Alzheimer’s disease (AD). In recent years, impairment of autophagy has been identified as another important feature contributing to AD progression. Therefore, the potential of the autophagy activator spermidine, a small body-endogenous polyamine often used as dietary supplement, was assessed on Aβ pathology and glial cell-mediated neuroinflammation.

Results

Oral treatment of the amyloid prone AD-like APPPS1 mice with spermidine reduced neurotoxic soluble Aβ and decreased AD-associated neuroinflammation. Mechanistically, single nuclei sequencing revealed AD-associated microglia to be the main target of spermidine. This microglia population was characterized by increased AXL levels and expression of genes implicated in cell migration and phagocytosis. A subsequent proteome analysis of isolated microglia confirmed the anti-inflammatory and cytoskeletal effects of spermidine in APPPS1 mice. In primary microglia and astrocytes, spermidine-induced autophagy subsequently affected TLR3- and TLR4-mediated inflammatory processes, phagocytosis of Aβ and motility. Interestingly, spermidine regulated the neuroinflammatory response of microglia beyond transcriptional control by interfering with the assembly of the inflammasome.

Conclusions

Our data highlight that the autophagy activator spermidine holds the potential to enhance Aβ degradation and to counteract glia-mediated neuroinflammation in AD pathology.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02534-7.

Microglia Phenotypes in Aging and Neurodegenerative Diseases

Neuroinflammation is a hallmark of many neurodegenerative diseases (NDs) and plays a fundamental role in mediating the onset and progression of disease. Microglia, which function as first-line immune guardians of the central nervous system (CNS), are the central drivers of neuroinflammation. Numerous human postmortem studies and in vivo imaging analyses have shown chronically activated microglia in patients with various acute and chronic neuropathological diseases. While microglial activation is a common feature of many NDs, the exact role of microglia in various pathological states is complex and often contradictory. However, there is a consensus that microglia play a biphasic role in pathological conditions, with detrimental and protective phenotypes, and the overall response of microglia and the activation of different phenotypes depends on the nature and duration of the inflammatory insult, as well as the stage of disease development. This review provides a comprehensive overview of current research on the various microglia phenotypes and inflammatory responses in health, aging, and NDs, with a special emphasis on the heterogeneous phenotypic response of microglia in acute and chronic diseases such as hemorrhagic stroke (HS), Alzheimer's disease (AD), and Parkinson's disease (PD). The primary focus is translational research in preclinical animal models and bulk/single-cell transcriptome studies in human postmortem samples. Additionally, this review covers key microglial receptors and signaling pathways that are potential therapeutic targets to regulate microglial inflammatory responses during aging and in NDs. Additionally, age-, sex-, and species-specific microglial differences will be briefly reviewed.

Identification of potential therapeutic and diagnostic characteristics of Alzheimer disease by targeting the miR-132-3p/FOXO3a-PPM1F axis in APP/PS1 mice

Alzheimer disease (AD) is a neurodegenerative disorder, which is characterized by progressive impairment of memory and cognition. Early diagnosis and treatment of AD has become a leading topic of research. In this study, we explored the effects of the miR-132-3p/FOXO3a-PPM1F axis on the onset of AD for possible early diagnosis and therapy. We found that miR-132-3p levels in the hippocampus and blood were drastically decreased in APP/PS1 mice from 9 months of age, and bi-directional manipulation of miR-132-3p levels induced magnified effects on learning memory behaviors, and manifestation of AD-related pathological characteristics and inflammatory cytokines in APP/PS1 mice of relevant ages. The hippocampal PPM1F expression levels were significantly elevated in APP/PS1 mice from 3 months of age, which was correlated with miR-132-3p levels at different ages. Overexpression of PPM1F remarkably accelerated the progression of learning memory deficits and associated pathological factors in APP/PS1 mice. Further, we showed that miR-132-3p modulated the expression of PPM1F via FOXO3a in HT22 cells. Finally, using peripheral blood samples of human study participants, we found that the miR-132-3p and PPM1F expression levels in patients with AD were also altered with prominent correlations. In conclusion, miR-132-3p indirectly regulates PPM1F expression by targeting FOXO3a, which could play an extensive role in contributing to the establishment of early diagnosis, treatment, and pathogenesis of AD.

Optimization of measurement of mitochondrial electron transport activity in postmortem human brain samples and measurement of susceptibility to rotenone and 4-hydroxynonenal inhibition

Mitochondrial function is required to meet the energetic and metabolic requirements of the brain. Abnormalities in mitochondrial function, due to genetic or developmental factors, mitochondrial toxins, aging or insufficient mitochondrial quality control contribute to neurological and psychiatric diseases. Studying bioenergetics from postmortem human tissues has been challenging due to the diverse range of human genetics, health conditions, sex, age, and postmortem interval. Furthermore, fresh tissues that were in the past required for assessment of mitochondrial respiratory function were rarely available. Recent studies established protocols to use in bioenergetic analyses from frozen tissues using animal models and cell cultures. In this study we optimized these methods to determine the activities of mitochondrial electron transport in postmortem human brain. Further we demonstrate how these samples can be used to assess the susceptibility to the mitochondrial toxin rotenone and exposure to the reactive lipid species 4-hydroxynonenal. The establishment of such an approach will significantly impact translational studies of human diseases by allowing measurement of mitochondrial function in human tissue repositories.

Targeting whole body metabolism and mitochondrial bioenergetics in the drug development for Alzheimer's disease

Aging is by far the most prominent risk factor for Alzheimer's disease (AD), and both aging and AD are associated with apparent metabolic alterations. As developing effective therapeutic interventions to treat AD is clearly in urgent need, the impact of modulating whole-body and intracellular metabolism in preclinical models and in human patients, on disease pathogenesis, have been explored. There is also an increasing awareness of differential risk and potential targeting strategies related to biological sex, microbiome, and circadian regulation. As a major part of intracellular metabolism, mitochondrial bioenergetics, mitochondrial quality-control mechanisms, and mitochondria-linked inflammatory responses have been considered for AD therapeutic interventions. This review summarizes and highlights these efforts.

N-acetyldopamine dimer inhibits neuroinflammation through the TLR4/NF-κB and NLRP3/Caspase-1 pathways

Neuroinflammation mediated by microglia is an important pathophysiological mechanism in neurodegenerative diseases. However, there is a lack of effective drugs to treat neuroinflammation. N-acetyldopamine dimer (NADD) is a natural compound from the traditional Chinese medicine Isaria cicada. In our previous study, we found that NADD can attenuate DSS-induced ulcerative colitis by suppressing the NF-κB and MAPK pathways. Does NADD inhibit neuroinflammation, and what is the target of NADD? To answer this question, lipopolysaccharide (LPS)-stimulated BV-2 microglia was used as a cell model to investigate the effect of NADD on neuroinflammation. Nitric oxide (NO) detection, reactive oxygen species (ROS) detection and enzyme-linked immunosorbent assay (ELISA) results show that NADD attenuates inflammatory signals and proinflammatory cytokines in LPS-stimulated BV-2 microglia, including NO, ROS, tumor necrosis factor (TNF)-α, interleukin (IL)-1β and interleukin-6 (IL-6). Western blot analysis show that NADD inhibits the protein levels of Toll-like receptor 4 (TLR4), nuclear factor kappa-B (NF-κB), NOD-like receptor thermal protein domain associated protein 3 (NLRP3), ASC and cysteinyl aspartate specific proteinase (Caspase)-1, indicating that NADD may inhibit neuroinflammation through the TLR4/NF-κB and NLRP3/Caspase-1 signaling pathways. In addition, surface plasmon resonance assays and molecular docking demonstrate that NADD binds with TLR4 directly. Our study reveals a new role of NADD in inhibiting the TLR4/NF-κB and NLRP3/Caspase-1 pathways, and shows that TLR4-MD2 is the direct target of NADD, which may provide a potential therapeutic candidate for the treatment of neuroinflammation.

Gardenia jasminoides J.Ellis extract GJ-4 alleviated cognitive deficits of APP/PS1 transgenic mice

BACKGROUND

Accumulating evidence demonstrates that traditional Chinese medicines that act on multiple targets could effectively treat various multi-etiological diseases, including cerebrovascular diseases, Alzheimer's disease (AD), Parkinson's disease (PD) and so on. Previous studies have shown that crocin richments (GJ-4), Gardenia jasminoides J.Ellis extract, provide neuroprotective effects on cognitive impairments in AD mouse models. However, the mechanism how GJ-4 improves cognition remains still unclear.

PURPOSE

The aim of this study was to uncover the protective effects and underlying mechanism of GJ-4 on PrP-hAβPPswe/PS1^ΔE9 (APP/PS1) transgenic mice.

METHODS

APP/PS1 mice were given GJ-4 (10, 20, and 50 mg/kg), donepezil (5 mg/kg) and memantine (5 mg/kg) orally at eight months of age for 12 consecutive weeks. Morris water maze and novel object recognition were conducted to assess the cognitive ability of mice. The release of inflammatory cytokines was determined by RT-PCR assay, and the pathological features of neurons and microglia were assayed by immunohistochemistry and immunofluorescence assay. The expression of Aβ-related proteins and signaling pathways were determined by Western blot.

RESULTS

The behavioral results revealed that GJ-4 ameliorated the cognitive deficits of APP/PS1 mice measured by Morris water maze and novel object recognition tests. Mechanism studies indicated that GJ-4 significantly decreased β-amyloid (Aβ) level through reducing Aβ production and promoting Aβ degradation. It has been reported that Aβ plaques trigger the hyper-phosphorylation of tau protein in APP/PS1 mice. Consistent with previous studies, hyper-phosphorylation of tau was also occurred in APP/PS1 mice in the present study, and GJ-4 inhibited Tau phosphorylation at different sites. Overwhelming evidence indicates that neuroinflammation stimulated by Aβ and hyperphosphorylated tau is involved in the pathological progression of AD. We found that GJ-4 suppressed neuroinflammatory responses in the brain through regulating phosphatidylinositide 3-kinase/AKT (PI3K/AKT) signaling pathway activation, and subsequent expression of inflammatory proteins and release of inflammatory cytokines.

CONCLUSION

Altogether, GJ-4 ameliorated cognition of APP/PS1 transgenic mice through multiple targets, including Aβ, tau and neuroinflammation. This study provides a solid research basis for further development of GJ-4 as a potential candidate for the treatment of AD.

Cellular and molecular influencers of neuroinflammation in Alzheimer's disease: Recent concepts & roles

Alzheimer's disease (AD), an extremely common neurodegenerative disorder of the older generation, is one of the leading causes of death globally. Besides the conventional hallmarks i.e. Amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs), neuroinflammation also serves as a major contributing factor in the pathogenesis of AD. There are mounting evidences to support the fundamental role of cellular (microglia, astrocytes, mast cells, and T-cells) and molecular (cytokines, chemokines, caspases, and complement proteins) influencers of neuroinflammation in producing/promoting neurodegeneration and dementia in AD. Genome-wide association studies (GWAS) have revealed the involvement of various single nucleotide polymorphisms (SNPs) of genes related to neuroinflammation with the risk of developing AD. Modulating the release of the neuroinflammatory molecules and targeting their relevant mechanisms may have beneficial effects on the onset, progress and severity of the disease. Here, we review the distinct role of various mediators and modulators of neuroinflammation that impact the pathogenesis and progression of AD as well as incite further research efforts for the treatment of AD through a neuroinflammatory approach.

Innate immunity stimulation via CpG oligodeoxynucleotides ameliorates Alzheimer's disease pathology in aged squirrel monkeys

Alzheimer's disease is the most common cause of dementia and the only illness among the top 10 causes of death for which there is no disease-modifying therapy. The failure rate of clinical trials is very high, in part due to the premature translation of successful results in transgenic mouse models to patients. Extensive evidence suggests that dysregulation of innate immunity and microglia/macrophages plays a key role in Alzheimer's disease pathogenesis. Activated resident microglia and peripheral macrophages can display protective or detrimental phenotypes depending on the stimulus and environment. Toll-like receptors (TLRs) are a family of innate immune regulators known to play an important role in governing the phenotypic status of microglia. We have shown in multiple transgenic Alzheimer's disease mouse models that harnessing innate immunity via TLR9 agonist CpG oligodeoxynucleotides (ODNs) modulates age-related defects associated with immune cells and safely reduces amyloid plaques, oligomeric amyloid-β, tau pathology, and cerebral amyloid angiopathy (CAA) while promoting cognitive benefits. In the current study we have used a non-human primate model of sporadic Alzheimer's disease pathology that develops extensive CAA-elderly squirrel monkeys. The major complications in current immunotherapeutic trials for Alzheimer's disease are amyloid-related imaging abnormalities, which are linked to the presence and extent of CAA; hence, the prominence of CAA in elderly squirrel monkeys makes them a valuable model for studying the safety of the CpG ODN-based concept of immunomodulation. We demonstrate that long-term use of Class B CpG ODN 2006 induces a favourable degree of innate immunity stimulation without producing excessive or sustained inflammation, resulting in efficient amelioration of both CAA and tau Alzheimer's disease-related pathologies in association with behavioural improvements and in the absence of microhaemorrhages in aged elderly squirrel monkeys. CpG ODN 2006 has been well established in numerous human trials for a variety of diseases. The present evidence together with our earlier, extensive preclinical research, validates the beneficial therapeutic outcomes and safety of this innovative immunomodulatory approach, increasing the likelihood of CpG ODN therapeutic efficacy in future clinical trials.

Epigenetic Modulation of Microglia Function and Phenotypes in Neurodegenerative Diseases

Microglia-mediated neuroinflammation is one of the most remarkable hallmarks of neurodegenerative diseases (NDDs), including AD, PD, and ALS. Accumulating evidence indicates that microglia play both neuroprotective and detrimental roles in the onset and progression of NDDs. Yet, the specific mechanisms of action surrounding microglia are not clear. Modulation of microglia function and phenotypes appears to be a potential strategy to reverse NDDs. Until recently, research into the epigenetic mechanisms of diseases has been gradually developed, making it possible to elucidate the molecular mechanisms underlying the epigenetic regulation of microglia in NDDs. This review highlights the function and phenotypes of microglia, elucidates the relationship between microglia, epigenetic modifications, and NDDs, as well as the possible mechanisms underlying the epigenetic modulation of microglia in NDDs with a focus on potential intervention strategies.

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.

Minocycline attenuation of rat corpus callosum abnormality mediated by low-dose lipopolysaccharide-induced microglia activation

Background

Microglia are resident innate immune cells in the brain, and activation of these myeloid cells results in secretion of a variety of pro-inflammatory molecules, leading to the development of neurodegenerative disorders. Lipopolysaccharide (LPS) is a widely used experimental stimulant in microglia activation. We have previously shown that LPS produced microglia activation and evoked detectable functional abnormalities in rat corpus callosum (CC) in vitro. Here, we further validated the effects of low-dose LPS-induced microglia activation and resultant white matter abnormality in the CC in an animal model and examined its attenuation by an anti-inflammatory agent minocycline.

Methods

Twenty-four SD rats were divided randomly into three groups and intra-peritoneally injected daily with saline, LPS, and LPS + minocycline, respectively. All animals were subject to MRI tests 6 days post-injection. The animals were then sacrificed to harvest the CC tissues for electrophysiology, western blotting, and immunocytochemistry. One-way ANOVA with Tukey’s post-test of all pair of columns was employed statistical analyses.

Results

Systemic administration of LPS produced microglial activation in the CC as illustrated by Iba-1 immunofluorescent staining. We observed that a large number of Iba-1-positive microglial cells were hyper-ramified with hypertrophic somata or even amoeba like in the LPS-treated animals, and such changes were significantly reduced by co-administration of minocycline. Electrophysiological recordings of axonal compound action potential (CAP) in the brain slices contained the CC revealed an impairment on the CC functionality as detected by a reduction in CAP magnitude. Such an impairment was supported by a reduction of fast axonal transportation evidenced by β-amyloid precursor protein accumulation. These alterations were attenuated by minocycline, demonstrating minocycline reduction of microglia-mediated interruption of white matter integrity and function in the CC.

Conclusions

Systemic administration of LPS produced microglia activation in the CC and resultant functional abnormalities that were attenuated by an anti-inflammatory agent minocycline.

The sTREM2 Concentrations in the Blood: A Marker of Neurodegeneration?

Microglia performs a variety of functions during brain development designed to maintain brain homeostasis. Triggering receptor expressed on myeloid cells 2 (TREM2) is expressed in microglial cells modulating phagocytosis, cytokine production, cell proliferation, and cell survival. Interestingly, the levels of soluble TREM2 (the secreted ectodomain of TREM2, sTREM2) were higher in cerebrospinal fluid (CSF) from Alzheimer's disease (AD) patients than subjects without cognitive decline. It is noteworthy that, while CSF sTREM2 levels have been extensively studied, few studies have investigated sTREM2 in blood producing conflicting results. We aimed to investigate the levels of sTREM2 in CSF and blood from a cohort of well-characterized AD comparing the results to those obtained in patients suffering from idiopathic normal pressure hydrocephalus (iNPH), a potentially reversible cognitive impairment. Our findings underlined a significantly lower plasma sTREM2 concentration in AD patients compared to iNPH subjects [39.1 ng/mL (standard deviation (SD), 15.0) and 47.2 ng/mL (SD, 19.5), respectively; p = 0.01], whereas no difference was revealed between the two groups in the CSF sTREM2 levels. The adjusted regression analyses evidenced in AD patients an association between plasma and CSF sTREM2 levels [B = 0.411; 95% confidence interval (CI), 0.137-0.685, p = 0.004], as well as β-amyloid concentrations (B = 0.035; 95% CI, 0.007-0.063, p = 0.01) and an association between CSF sTREM2 and phospho-Tau concentrations (B = 0.248; 95% CI, 0.053-0.443; p = 0.01). No significant relation was found in iNPH patients. In conclusion, these differences in sTREM2 profiles between AD and iNPH reinforce the notion that this receptor has a role in neurodegeneration.

The dialog between neurons and microglia in Alzheimer's disease: The neurotransmitters view

Microglia play a vital role in maintaining brain homeostasis. Their continuous sensing of surrounding micro-environments is crucial for their activity. Cross talk between specific neurons and microglia might occur through specific neurotransmitter receptors on microglia. Impairment with this interaction might result in pathological activity of microglia against potential insults. The reason for this activity in many neurodegenerative diseases such as Alzheimer's disease (AD) is not known. However, several papers report of the effects of different neurotransmitter agonists on microglial cells function that relate to their activity in AD. This review aims to summarize those works and to raise potential fundamental questions for future research.

Bixin Attenuates Experimental Autoimmune Encephalomyelitis by Suppressing TXNIP/NLRP3 Inflammasome Activity and Activating NRF2 Signaling

Multiple sclerosis (MS), an autoimmune and degenerative disease, is characterized by demyelination and chronic neuroinflammation. Bixin is a carotenoid isolated from the seeds of Bixa orellana that exhibits various potent pharmacological activities, including antioxidant, anti-inflammatory, and anti-tumor properties. However, the effects of bixin on MS have not yet been examined. To evaluate the effects and underlying molecular mechanisms of bixin on MS, experimental autoimmune encephalomyelitis (EAE) was established in C57BL/6 mice, which were treated via intragastric administration of bixin solutions. To evaluate the molecular mechanisms of bixin, quantitative reverse-transcription PCR, western blot, immunohistochemistry, flow cytometry, and enzyme-linked immunosorbent assay analyses were performed. We found that bixin significantly improved the symptoms and pathology in EAE mice, reduced the release of inflammatory cytokines TNF-α, IL-6, IL-8, IL-17, and IFN-γ, and increased the expression of the anti-inflammatory cytokine IL-10. And bixin reduced the proportion of Th1 and Th17 cells in the spleen and CNS, and suppressed microglia aggregation, and TXNIP/NLRP3 inflammasome activity by scavenging excessive reactive oxygen species (ROS) in EAE mice. Furthermore, bixin inhibited inflammation and oxidative stress via activating nuclear factor erythroid 2-related factor 2 (NRF2), and its downstream genes in EAE mice, meanwhile, these effects were suppressed upon treatment with an NRF2 inhibitor, ML385. Bixin prevented neuroinflammation and demyelination in EAE mice primarily by scavenging ROS through activation of the NRF2 signaling pathway. Taken together, our results indicate that bixin is a promising therapeutic candidate for treatment of MS.

Repositioning of Immunomodulators: A Ray of Hope for Alzheimer's Disease?

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder characterized by cognitive decline and by the presence of amyloid β plaques and neurofibrillary tangles in the brain. Despite recent advances in understanding its pathophysiological mechanisms, to date, there are no disease-modifying therapeutic options, to slow or halt the evolution of neurodegenerative processes in AD. Current pharmacological treatments only transiently mitigate the severity of symptoms, with modest or null overall improvement. Emerging evidence supports the concept that AD is affected by the impaired ability of the immune system to restrain the brain's pathology. Deep understanding of the relationship between the nervous and the immune system may provide a novel arena to develop effective and safe drugs for AD treatment. Considering the crucial role of inflammatory/immune pathways in AD, here we discuss the current status of the immuno-oncological, immunomodulatory and anti-TNF-α drugs which are being used in preclinical studies or in ongoing clinical trials by means of the drug-repositioning approach.

How Microglia Manages Non-cell Autonomous Vicious Cycling of Aβ Toxicity in the Pathogenesis of AD

Alzheimer’s disease (AD) is a progressive neurodegenerative disease and a common form of dementia that affects cognition and memory mostly in aged people. AD pathology is characterized by the accumulation of β-amyloid (Aβ) senile plaques and the neurofibrillary tangles of phosphorylated tau, resulting in cell damage and neurodegeneration. The extracellular deposition of Aβ is regarded as an important pathological marker and a principal-agent of neurodegeneration. However, the exact mechanism of Aβ-mediated pathogenesis is not fully understood yet. Recently, a growing body of evidence provides novel insights on the major role of microglia and its non-cell-autonomous cycling of Aβ toxicity. Hence, this article provides a comprehensive overview of microglia as a significant player in uncovering the underlying disease mechanisms of AD.

Salvianic Acid A Sodium Promotes the Recovery of Motor Function After Spinal Cord Injury in Rats by Reducing Microglia Inflammation through Regulating MIP2/Vdac1/Ndufa12 Signaling Axis

Objective

To clarify the effects on and the mechanism of salvianic acid A sodium (SAAS) in the recovery of motor function after spinal cord injury.

Methods

In vivo and in vitro experiments were carried out in this research to determine the effects of SAAS on tissue damage, neuron survival, microglia polarization, and inflammation after spinal cord injury (SCI). Differentially expressed genes treated with SAAS were screened by transcriptome sequencing, and the molecular mechanism was investigated simultaneously.

Results

The results revealed that SAAS could promote type M2 polarization of microglia and reduce the proportion of type M1. In this way, it reduced the secretion and expression of inflammatory factors. Compared with Lipopolysaccharides(LPS), 345 genes were upregulated and 407 genes were downregulated in the LPS + SAAS treatment group. In the SAAS group, expression levels of Ndufa12, IL‐6, TNF‐α, and Vdac1 were significantly reduced, while a marked elevation was found in MIP2. In addition, results found in an animal model showed that SAAS could obviously facilitate motor function recovery of mice after spinal cord injury, and it had a good protective effect on spinal cord tissue and neuron cells.

Conclusion

As a result, the present study clarified both the protective effect of SAAS on neurons after spinal cord injury and the anti‐inflammatory effect of microglia, which is expected to serve as a theoretical basis for clinical treatment.

Microglia TREM2: A Potential Role in the Mechanism of Action of Electroacupuncture in an Alzheimer's Disease Animal Model

Alzheimer's disease (AD) is one of the most serious public health concerns facing the world. Its characteristic feature is neuroinflammation due to microglial activation. Electroacupuncture is one of the therapies employed to improve the condition of patients with AD, although its mechanism of action is still to be determined. Triggering receptor expressed on myeloid cells 2 (TREM2) is a microglia-specific receptor that is involved in regulating neuroinflammation in AD. In this study, we applied senescence-accelerated mouse-prone 8 mice as the AD animal model, used the Morris water maze, and applied hematoxylin and eosin staining, immunofluorescence double staining, and Western blotting, to explore the effects and potential mechanisms of action of electroacupuncture. In summary, this study suggested that electroacupuncture treatment could improve the learning and memory abilities (p < 0.05) and protect neurons. These effects result from acupuncture could upregulate TREM2 expression in the hippocampus (p < 0.01), which was essential for the anti-inflammatory effects in the AD animal model. However, further studies are needed to conclusively demonstrate the mechanism of action of electroacupuncture in AD.

Microglia Purinoceptor P2Y6: An Emerging Therapeutic Target in CNS Diseases

The purinergic receptor P2Y6 is expressed in immune cells, including the microglia that are implicated in neurological disorders. Its ligand, UDP, is a signaling molecule that can serve as an “find-me” signal when released in significant quantities by damaged/dying cells. The binding of UDP by P2Y6R leads to the activation of different biochemical pathways, depending on the disease context and the pathological environment. Generally, P2Y6R stimulates phagocytosis. However, whether or not phagocytosis coincides with cell activation or the secretion of pro-inflammatory cytokines needs further investigation. The current review aims to discuss the various functions of P2Y6R in some CNS disorders. We present evidence that P2Y6R may have a detrimental or beneficial role in the nervous system, in the context of neurological pathologies, such as ischemic stroke, Alzheimer’s disease, Parkinson’s disease, radiation-induced brain injury, and neuropathic pain.