Microglia Express Insulin-Like Growth Factor-1 in the Hippocampus of Aged APPswe/PS1ΔE9 Transgenic Mice

Serotonin-2B receptor (5-HT2BR) expression and binding in the brain of APPswe/PS1dE9 transgenic mice and in Alzheimer's disease brain tissue

Despite well-documented dysregulation in central serotonergic signaling in Alzheimer's disease (AD), knowledge about the potential involvement of the serotonin-2B receptor (5-HT2BR) subtype remains sparse. Here, we assessed the levels of 5-HT2BRs in brain tissue from APPswe/PS1dE9 transgenic (TG) mice, AD patients, and adult microglial cells. 5-HT2BR mRNA was measured by RT-qPCR in ageing TG and wild-type (WT) mice, in samples from the middle frontal gyrus of female, AD and control subjects, and in microglia from the cerebral cortex of WT mice. The density of 5-HT2BRs was measured by autoradiography using [3H]RS 127445. Both mouse and human brains had low levels of 5-HT2BR mRNA. In whole-brain mouse samples, mRNA expression was significantly lower in TG mice compared to WT at > 18 months of age. In the Aβ-plaque-burdened neocortex and hippocampus of old TG mice, however, levels of 5-HT2BR mRNA were two-fold higher over control, with similar elevations observed in the Aβ-plaque-burdened frontal cortex of human AD patients. 5-HT2BR mRNA expression varied widely in adult microglia and was higher compared to other cortical cell subtypes. In mice, specific [3H]RS-127445 binding in the cortex was first detected after 3 months of age. The density of 5-HT2BRs was low and overall reduced in TG, compared to WT mice. Binding was detectable but too low to be reliably quantified in the human cortex. Our results document Aβ-associated increases in 5-HT2BR mRNA expression and suggest reduced receptor binding in the context of AD. Studies investigating the functional involvement of microglial 5-HT2BRs in AD are considered relevant.

Peptide discovery across the spectrum of neuroinflammation; microglia and astrocyte phenotypical targeting, mediation, and mechanistic understanding

Uncontrolled and chronic inflammatory states in the Central Nervous System (CNS) are the hallmark of neurodegenerative pathology and every injury or stroke-related insult. The key mediators of these neuroinflammatory states are glial cells known as microglia, the resident immune cell at the core of the inflammatory event, and astroglia, which encapsulate inflammatory insults in proteoglycan-rich scar tissue. Since the majority of neuroinflammation is exclusively based on the responses of said glia, their phenotypes have been identified to be on an inflammatory spectrum encompassing developmental, homeostatic, and reparative behaviors as opposed to their ability to affect devastating cell death cascades and scar tissue formation. Recently, research groups have focused on peptide discovery to identify these phenotypes, find novel mechanisms, and mediate or re-engineer their actions. Peptides retain the diverse function of proteins but significantly reduce the activity dependence on delicate 3D structures. Several peptides targeting unique phenotypes of microglia and astroglia have been identified, along with several capable of mediating deleterious behaviors or promoting beneficial outcomes in the context of neuroinflammation. A comprehensive review of the peptides unique to microglia and astroglia will be provided along with their primary discovery methodologies, including top-down approaches using known biomolecules and naïve strategies using peptide and phage libraries.

Microglia signaling in health and disease - Implications in sex-specific brain development and plasticity

Microglia, the intrinsic neuroimmune cells residing in the central nervous system (CNS), exert a pivotal influence on brain development, homeostasis, and functionality, encompassing critical roles during both aging and pathological states. Recent advancements in comprehending brain plasticity and functions have spotlighted conspicuous variances between male and female brains, notably in neurogenesis, neuronal myelination, axon fasciculation, and synaptogenesis. Nevertheless, the precise impact of microglia on sex-specific brain cell plasticity, sculpting diverse neural network architectures and circuits, remains largely unexplored. This article seeks to unravel the present understanding of microglial involvement in brain development, plasticity, and function, with a specific emphasis on microglial signaling in brain sex polymorphism. Commencing with an overview of microglia in the CNS and their associated signaling cascades, we subsequently probe recent revelations regarding molecular signaling by microglia in sex-dependent brain developmental plasticity, functions, and diseases. Notably, C-X3-C motif chemokine receptor 1 (CX3CR1), triggering receptors expressed on myeloid cells 2 (TREM2), calcium (Ca2+), and apolipoprotein E (APOE) emerge as molecular candidates significantly contributing to sex-dependent brain development and plasticity. In conclusion, we address burgeoning inquiries surrounding microglia's pivotal role in the functional diversity of developing and aging brains, contemplating their potential implications for gender-tailored therapeutic strategies in neurodegenerative diseases.

The intricate interplay between microglia and adult neurogenesis in Alzheimer's disease

Microglia, the resident immune cells of the central nervous system, play a crucial role in regulating adult neurogenesis and contribute significantly to the pathogenesis of Alzheimer's disease (AD). Under physiological conditions, microglia support and modulate neurogenesis through the secretion of neurotrophic factors, phagocytosis of apoptotic cells, and synaptic pruning, thereby promoting the proliferation, differentiation, and survival of neural progenitor cells (NPCs). However, in AD, microglial function becomes dysregulated, leading to chronic neuroinflammation and impaired neurogenesis. This review explores the intricate interplay between microglia and adult neurogenesis in health and AD, synthesizing recent findings to provide a comprehensive overview of the current understanding of microglia-mediated regulation of adult neurogenesis. Furthermore, it highlights the potential of microglia-targeted therapies to modulate neurogenesis and offers insights into potential avenues for developing novel therapeutic interventions.

Spatial dynamics of mammalian brain development and neuroinflammation by multimodal tri-omics mapping

The ability to spatially map multiple layers of the omics information over different time points allows for exploring the mechanisms driving brain development, differentiation, arealization, and alterations in disease. Herein we developed and applied spatial tri-omic sequencing technologies, DBiT ARP-seq (spatial ATAC-RNA-Protein-seq) and DBiT CTRP-seq (spatial CUT&Tag-RNA-Protein-seq) together with multiplexed immunofluorescence imaging (CODEX) to map spatial dynamic remodeling in brain development and neuroinflammation. A spatiotemporal tri-omic atlas of the mouse brain was obtained at different stages from postnatal day P0 to P21, and compared to the regions of interest in the human developing brains. Specifically, in the cortical area, we discovered temporal persistence and spatial spreading of chromatin accessibility for the layer-defining transcription factors. In corpus callosum, we observed dynamic chromatin priming of myelin genes across the subregions. Together, it suggests a role for layer specific projection neurons to coordinate axonogenesis and myelination. We further mapped the brain of a lysolecithin (LPC) neuroinflammation mouse model and observed common molecular programs in development and neuroinflammation. Microglia, exhibiting both conserved and distinct programs for inflammation and resolution, are transiently activated not only at the core of the LPC lesion, but also at distal locations presumably through neuronal circuitry. Thus, this work unveiled common and differential mechanisms in brain development and neuroinflammation, resulting in a valuable data resource to investigate brain development, function and disease.

Spatial dynamics of mammalian brain development and neuroinflammation by multimodal tri-omics mapping

The ability to spatially map multiple layers of the omics information over different time points allows for exploring the mechanisms driving brain development, differentiation, arealization, and alterations in disease. Herein we developed and applied spatial tri-omic sequencing technologies, DBiT ARP-seq (spatial ATAC-RNA-Protein-seq) and DBiT CTRP-seq (spatial CUT&Tag-RNA-Protein-seq) together with multiplexed immunofluorescence imaging (CODEX) to map spatial dynamic remodeling in brain development and neuroinflammation. A spatiotemporal tri-omic atlas of the mouse brain was obtained at different stages from postnatal day P0 to P21, and compared to the regions of interest in the human developing brains. Specifically, in the cortical area, we discovered temporal persistence and spatial spreading of chromatin accessibility for the layer-defining transcription factors. In corpus callosum, we observed dynamic chromatin priming of myelin genes across the subregions. Together, it suggests a role for layer specific projection neurons to coordinate axonogenesis and myelination. We further mapped the brain of a lysolecithin (LPC) neuroinflammation mouse model and observed common molecular programs in development and neuroinflammation. Microglia, exhibiting both conserved and distinct programs for inflammation and resolution, are transiently activated not only at the core of the LPC lesion, but also at distal locations presumably through neuronal circuitry. Thus, this work unveiled common and differential mechanisms in brain development and neuroinflammation, resulting in a valuable data resource to investigate brain development, function and disease.

Palmitic Acid Modulation of the Insulin-Like Growth Factor System in Hypothalamic Astrocytes and Neurons

INTRODUCTION

Insulin-like growth factor (IGF)1 and IGF2 have neuroprotective effects, but less is known regarding how other members of the IGF system, including IGF binding proteins (IGFBPs) and the regulatory proteinase pappalysin-1 (PAPP-A) and its endogenous inhibitor stanniocalcin-2 (STC2) participate in this process. Here, we analyzed whether these members of the IGF system are modified in neurons and astrocytes in response to palmitic acid (PA), a fatty acid that induces cell stress when increased centrally.

METHODS

Primary hypothalamic astrocyte cultures from male and female PND2 rats and the pro-opiomelanocortin (POMC) neuronal cell line, mHypoA-POMC/GFP-2, were treated with PA, IGF1 or both. To analyze the role of STC2 in astrocytes, siRNA assays were employed.

RESULTS

In astrocytes of both sexes, PA rapidly increased cell stress factors followed by increased Pappa and Stc2 mRNA levels and then a decrease in Igf1, Igf2, and Igfbp2 expression and cell number. Exogenous IGF1 did not revert these effects. In mHypoA-POMC/GFP-2 neurons, PA reduced cell number and Pomc and Igf1 mRNA levels, and increased Igfbp2 and Stc2, again with no effect of exogenous IGF1. PA increased STC2 expression, but no effects of decreasing its levels by interference assays or exogenous STC2 treatment in astrocytes were found.

CONCLUSIONS

The response of the IGF system to PA was cell and sex specific, but no protective effects of the IGFs were found. However, the modifications in hypothalamic PAPP-A and STC2 indicate that further studies are required to determine their role in the response to fatty acids and possibly in metabolic control.

Caffeic acid and diabetic neuropathy: Investigating protective effects and insulin-like growth factor 1 (IGF-1)-related antioxidative and anti-inflammatory mechanisms in mice

Diabetic neuropathy (DN) represents a common and debilitating complication of diabetes, affecting a significant proportion of patients. Despite available treatments focusing on symptom management, there remains an unmet need for therapies that address the underlying pathophysiology. In pursuit of novel interventions, this study evaluated the therapeutic effects of caffeic acid-a natural phenolic compound prevalent in various foods-on diabetic neuropathy using a mouse model, particularly examining its interaction with the Insulin-like Growth Factor 1 (IGF-1) signaling pathway. Caffeic acid was administered orally at two dosages (5 mg/kg and 10 mg/kg), and a comprehensive set of outcomes including fasting blood glucose levels, body weight, sensory behavior, spinal cord oxidative stress markers, inflammatory cytokines, and components of the IGF-1 signaling cascade were assessed. Additionally, to determine the specific contribution of IGF-1 signaling to the observed benefits, IGF1R inhibitor Picropodophyllin (PPP) was co-administered with caffeic acid. Our results demonstrated that caffeic acid, at both dosages, effectively reduced hyperglycemia and alleviated sensory behavioral deficits in diabetic mice. This was accompanied by a marked decrease in oxidative stress markers and an increase in antioxidant enzyme activities within the spinal cord. Significantly lowered microglial activation and inflammatory cytokine expression highlighted the potent antioxidative and anti-inflammatory effects of caffeic acid. Moreover, increases in both serum and spinal levels of IGF-1, along with elevated phosphorylated IGF1R, implicated the IGF-1 signaling pathway as a mediator of caffeic acid's neuroprotective actions. The partial reversal of caffeic acid's benefits by PPP substantiated the pivotal engagement of IGF-1 signaling in mediating its effects. Our findings delineate the capability of caffeic acid to mitigate DN symptoms, particularly through reducing spinal oxidative stress and inflammation, and pinpoint the integral role of IGF-1 signaling in these protective mechanisms. The insights gleaned from this study not only position caffeic acid as a promising dietary adjunct for managing diabetic neuropathy but also highlight the therapeutic potential of targeting spinal IGF-1 signaling as part of a strategic treatment approach.

CNS Resident Innate Immune Cells: Guardians of CNS Homeostasis

Although the CNS has been considered for a long time an immune-privileged organ, it is now well known that both the parenchyma and non-parenchymal tissue (meninges, perivascular space, and choroid plexus) are richly populated in resident immune cells. The advent of more powerful tools for multiplex immunophenotyping, such as single-cell RNA sequencing technique and upscale multiparametric flow and mass spectrometry, helped in discriminating between resident and infiltrating cells and, above all, the different spectrum of phenotypes distinguishing border-associated macrophages. Here, we focus our attention on resident innate immune players and their primary role in both CNS homeostasis and pathological neuroinflammation and neurodegeneration, two key interconnected aspects of the immunopathology of multiple sclerosis.

Role of the Insulin-like Growth Factor System in Neurodegenerative Disease

The insulin-like growth factor (IGF) system has paracrine and endocrine roles in the central nervous system. There is evidence that IGF signalling pathways have roles in the pathophysiology of neurodegenerative disease. This review focusses on Alzheimer's disease and Parkinson's disease, the two most common neurodegenerative disorders that are increasing in prevalence globally in relation to the aging population and the increasing prevalence of obesity and type 2 diabetes. Rodent models used in the study of the molecular pathways involved in neurodegeneration are described. However, currently, no animal model fully replicates these diseases. Mice with triple mutations in APP, PSEN and MAPT show promise as models for the testing of novel Alzheimer's therapies. While a causal relationship is not proven, the fact that age, obesity and T2D are risk factors in both strengthens the case for the involvement of the IGF system in these disorders. The IGF system is an attractive target for new approaches to management; however, there are gaps in our understanding that first need to be addressed. These include a focus beyond IGF-I on other members of the IGF system, including IGF-II, IGF-binding proteins and the type 2 IGF receptor.

Oral administration of ellagic acid mitigates perioperative neurocognitive disorders, hippocampal oxidative stress, and neuroinflammation in aged mice by restoring IGF-1 signaling

This study investigates the potential of ellagic acid (EA), a phytochemical with antioxidant and anti-inflammatory properties, in managing perioperative neurocognitive disorders (PND). PND, which represents a spectrum of cognitive impairments often faced by elderly patients, is principally linked to surgical and anesthesia procedures, and heavily impacted by oxidative stress in the hippocampus and microglia-induced neuroinflammation. Employing an aged mice model subjected to abdominal surgery, we delve into EA's ability to counteract postoperative oxidative stress and cerebral inflammation by engaging the Insulin-like growth factor-1 (IGF-1) pathway. Our findings revealed that administering EA orally notably alleviated post-surgical cognitive decline in older mice, a fact that was manifested in improved performance during maze tests. This enhancement in the behavioral performance of the EA-treated mice corresponded with the rejuvenation of IGF-1 signaling, a decrease in oxidative stress markers in the hippocampus (like MDA and carbonylated protein), and an increase in the activity of antioxidant enzymes such as SOD and CAT. Alongside these, we observed a decrease in microglia-driven neuroinflammation in the hippocampus, thus underscoring the antioxidant and anti-inflammatory roles of EA. Interestingly, when EA was given in conjunction with an IGF1R inhibitor, these benefits were annulled, accentuating the pivotal role that the IGF-1 pathway plays in the neuroprotective potential of EA. Hence, EA could serve as a potent candidate for safeguarding against PND in older patients by curbing oxidative stress and neuroinflammation through the activation of the IGF-1 pathway.

Diet-induced glial insulin resistance impairs the clearance of neuronal debris in Drosophila brain

Obesity significantly increases the risk of developing neurodegenerative disorders, yet the precise mechanisms underlying this connection remain unclear. Defects in glial phagocytic function are a key feature of neurodegenerative disorders, as delayed clearance of neuronal debris can result in inflammation, neuronal death, and poor nervous system recovery. Mounting evidence indicates that glial function can affect feeding behavior, weight, and systemic metabolism, suggesting that diet may play a role in regulating glial function. While it is appreciated that glial cells are insulin sensitive, whether obesogenic diets can induce glial insulin resistance and thereby impair glial phagocytic function remains unknown. Here, using a Drosophila model, we show that a chronic obesogenic diet induces glial insulin resistance and impairs the clearance of neuronal debris. Specifically, obesogenic diet exposure down-regulates the basal and injury-induced expression of the glia-associated phagocytic receptor, Draper. Constitutive activation of systemic insulin release from Drosophila insulin-producing cells (IPCs) mimics the effect of diet-induced obesity on glial Draper expression. In contrast, genetically attenuating systemic insulin release from the IPCs rescues diet-induced glial insulin resistance and Draper expression. Significantly, we show that genetically stimulating phosphoinositide 3-kinase (Pi3k), a downstream effector of insulin receptor (IR) signaling, rescues high-sugar diet (HSD)-induced glial defects. Hence, we establish that obesogenic diets impair glial phagocytic function and delays the clearance of neuronal debris.

The metabolic effects of resumption of a high fat diet after weight loss are sex dependent in mice

Dietary restriction is a frequent strategy for weight loss, but adherence is difficult and returning to poor dietary habits can result in more weight gain than that previously lost. How weight loss due to unrestricted intake of a healthy diet affects the response to resumption of poor dietary habits is less studied. Moreover, whether this response differs between the sexes and if the insulin-like growth factor (IGF) system, sex dependent and involved in metabolic control, participates is unknown. Mice received rodent chow (6% Kcal from fat) or a high-fat diet (HFD, 62% Kcal from fat) for 4 months, chow for 3 months plus 1 month of HFD, or HFD for 2 months, chow for 1 month then HFD for 1 month. Males and females gained weight on HFD and lost weight when returned to chow at different rates (p < 0.001), but weight gain after resumption of HFD intake was not affected by previous weight loss in either sex. Glucose metabolism was more affected by HFD, as well as the re-exposure to HFD after weight loss, in males. This was associated with increases in hypothalamic mRNA levels of IGF2 (p < 0.01) and IGF binding protein (IGFBP) 2 (p < 0.05), factors involved in glucose metabolism, again only in males. Likewise, IGF2 increased IGFBP2 mRNA levels only in hypothalamic astrocytes from males (p < 0.05). In conclusion, the metabolic responses to dietary changes were less severe and more delayed in females and the IGF system might be involved in some of the sex specific observations.

Local autocrine plasticity signaling in single dendritic spines by insulin-like growth factors

The insulin superfamily of peptides is essential for homeostasis as well as neuronal plasticity, learning, and memory. Here, we show that insulin-like growth factors 1 and 2 (IGF1 and IGF2) are differentially expressed in hippocampal neurons and released in an activity-dependent manner. Using a new fluorescence resonance energy transfer sensor for IGF1 receptor (IGF1R) with two-photon fluorescence lifetime imaging, we find that the release of IGF1 triggers rapid local autocrine IGF1R activation on the same spine and more than several micrometers along the stimulated dendrite, regulating the plasticity of the activated spine in CA1 pyramidal neurons. In CA3 neurons, IGF2, instead of IGF1, is responsible for IGF1R autocrine activation and synaptic plasticity. Thus, our study demonstrates the cell type-specific roles of IGF1 and IGF2 in hippocampal plasticity and a plasticity mechanism mediated by the synthesis and autocrine signaling of IGF peptides in pyramidal neurons.

Study on the mechanism of Coptis chinensis Franch. And its main active components in treating Alzheimer's disease based on SCFAs using Orbitrap Fusion Lumos Tribrid MS

ETHNOPHARMACOLOGICAL RELEVANCE

Coptis chinensis Franch. (CCF), as an extensively used traditional Chinese medicine, has therapeutic effects on Alzheimer's disease (AD), but its mechanism of action has not yet been elucidated.

AIM OF THE STUDY

This study aims to reveal the mechanism of action of CCF via the gut-brain axis, and provide a new strategy for the clinical treatment of AD.

MATERIALS AND METHODS

APPswe/PS1ΔE9 mice were used as AD models, and were given CCF extract by intragastric administration. Barnes maze was used to test the therapeutic effect of CCF on the treatment of AD. To reveal the mechanism of action of CCF in the treatment of AD, Vanquish Flex UHPLC-orbitrap fusion lumos mass was chosen to detect endogenous differential metabolite; MetaboAnalyst 5.0 was applied to derive relevant metabolic pathways; similarly, to explore the effects of CCF on the gut-brain axis, Vanquish Flex UPLC-Orbitrap fusion lumos mass was utilized to detect the changes in the content of SCFAs in AD mice after CCF administration; the prototype components and metabolites in CCF were identified by UPLC/ESI/qTOF-MS, then their effects on Bifidobacterium breve were explored.

RESULTS

CCF shortened the latency time of AD mice, improved the target quadrant ratio of AD mice, and made the maze roadmap simpler of AD mice; CCF regulated fifteen potential metabolites of AD mice, interestingly, ILA (indole-3-lactic acid) in SCFAs (short-chain fatty acids) was also included; CCF acted on histidine and phenylalanine metabolic pathways of AD mice; CCF increased the contents of acetic acid and ILA in AD mice; magnoflorine, jatrorrhizine, coptisine, groenlandicine, thalifendine, palmatine, berberine, epiberberine, hydroxylated jatrorrhizine, and 3-methoxydemethyleneberberine in CCF were detected in fecal samples of AD mice; magnoflorine, palmatrubine, 13-methylberberine, berberine, coptisine, and palmatine promoted the growth of Bifidobacterium breve.

CONCLUSIONS

we have demonstrated that CCF acts on the gut-brain axis by regulating SCFAs to treat AD.

Insulin and Disorders of Behavioural Flexibility

Behavioural inflexibility is a symptom of neuropsychiatric and neurodegenerative disorders such as Obsessive-Compulsive Disorder, Autism Spectrum Disorder and Alzheimer's Disease, encompassing the maintenance of a behaviour even when no longer appropriate. Recent evidence suggests that insulin signalling has roles apart from its regulation of peripheral metabolism and mediates behaviourally-relevant central nervous system (CNS) functions including behavioural flexibility. Indeed, insulin resistance is reported to generate anxious, perseverative phenotypes in animal models, with the Type 2 diabetes medication metformin proving to be beneficial for disorders including Alzheimer's Disease. Structural and functional neuroimaging studies of Type 2 diabetes patients have highlighted aberrant connectivity in regions governing salience detection, attention, inhibition and memory. As currently available therapeutic strategies feature high rates of resistance, there is an urgent need to better understand the complex aetiology of behaviour and develop improved therapeutics. In this review, we explore the circuitry underlying behavioural flexibility, changes in Type 2 diabetes, the role of insulin in CNS outcomes and mechanisms of insulin involvement across disorders of behavioural inflexibility.

Diet-Induced Glial Insulin Resistance Impairs The Clearance Of Neuronal Debris

Obesity significantly increases the risk of developing neurodegenerative disorders, yet the precise mechanisms underlying this connection remain unclear. Defects in glial phagocytic function are a key feature of neurodegenerative disorders, as delayed clearance of neuronal debris can result in inflammation, neuronal death, and poor nervous system recovery. Mounting evidence indicates that glial function can affect feeding behavior, weight, and systemic metabolism, suggesting that diet may play a role in regulating glial function. While it is appreciated that glial cells are insulin sensitive, whether obesogenic diets can induce glial insulin resistance and thereby impair glial phagocytic function remains unknown. Here, using a Drosophila model, we show that a chronic obesogenic diet induces glial insulin resistance and impairs the clearance of neuronal debris. Specifically, obesogenic diet exposure downregulates the basal and injury-induced expression of the glia-associated phagocytic receptor, Draper. Constitutive activation of systemic insulin release from Drosophila Insulin-producing cells (IPCs) mimics the effect of diet-induced obesity on glial draper expression. In contrast, genetically attenuating systemic insulin release from the IPCs rescues diet-induced glial insulin resistance and draper expression. Significantly, we show that genetically stimulating Phosphoinositide 3-kinase (PI3K), a downstream effector of Insulin receptor signaling, rescues HSD-induced glial defects. Hence, we establish that obesogenic diets impair glial phagocytic function and delays the clearance of neuronal debris.

The link between neuroinflammation and the neurovascular unit in synucleinopathies

The neurovascular unit (NVU) is composed of vascular cells, glial cells, and neurons. As a fundamental functional module in the central nervous system, the NVU maintains homeostasis in the microenvironment and the integrity of the blood-brain barrier. Disruption of the NVU and interactions among its components are involved in the pathophysiology of synucleinopathies, which are characterized by the pathological accumulation of α-synuclein. Neuroinflammation contributes to the pathophysiology of synucleinopathies, including Parkinson's disease, multiple system atrophy, and dementia with Lewy bodies. This review aims to summarize the neuroinflammatory response of glial cells and vascular cells in the NVU. We also review neuroinflammation in the context of the cross-talk between glial cells and vascular cells, between glial cells and pericytes, and between microglia and astroglia. Last, we discuss how α-synuclein affects neuroinflammation and how neuroinflammation influences the aggregation and spread of α-synuclein and analyze different properties of α-synuclein in synucleinopathies.

An Early Enriched Experience Drives an Activated Microglial Profile at Site of Corrective Neuroplasticity in Ten-m3 Knock-Out Mice

Environmental enrichment (EE) is beneficial for brain development and function, but our understanding of its capacity to drive circuit repair, the underlying mechanisms, and how this might vary with age remains limited. Ten-m3 knock-out (KO) mice exhibit a dramatic and stereotyped mistargeting of ipsilateral retinal inputs to the thalamus, resulting in visual deficits. We have recently shown a previously unexpected capacity for EE during early postnatal life (from birth for six weeks) to drive the partial elimination of miswired axonal projections, along with a recovery of visually mediated behavior, but the timeline of this repair was unclear. Here, we reveal that with just 3.5 weeks of EE from birth, Ten-m3 KOs exhibit a partial behavioral rescue, accompanied by pruning of the most profoundly miswired retinogeniculate terminals. Analysis suggests that the pruning is underway at this time point, providing an ideal opportunity to probe potential mechanisms. With the shorter EE-period, we found a localized increase in microglial density and activation profile within the identified geniculate region where corrective pruning was observed. No comparable response to EE was found in age-matched wild-type (WT) mice. These findings identify microglia as a potential mechanistic link through which EE drives the elimination of miswired neural circuits during early postnatal development. Activity driven, atypical recruitment of microglia to prune aberrant connectivity and restore function may have important therapeutic implications for neurodevelopmental disorders such as autistic spectrum disorder.

Astrocytic IGF-1 and IGF-1R Orchestrate Mitophagy in Traumatic Brain Injury via Exosomal miR-let-7e

Defective brain hormonal signaling and autophagy have been associated with neurodegeneration after brain insults, characterized by neuronal loss and cognitive dysfunction. However, few studies have linked them in the context of brain injury. Insulin-like growth factor-1 (IGF-1) is an important hormone that contributes to growth, cell proliferation, and autophagy and is also expressed in the brain. Here, we assessed the clinical data from TBI patients and performed both in vitro and in vivo experiments with proteomic and gene-chip analysis to assess the functions of IGF-1 in mitophagy following TBI. We show that reduced plasma IGF-1 is correlated with cognition in TBI patients. Overexpression of astrocytic IGF-1 improves cognitive dysfunction and mitophagy in TBI mice. Mechanically, proteomics data show that the IGF-1-related NF-κB pathway transcriptionally regulates decapping mRNA2 (Dcp2) and miR-let-7, together with IGF-1R to orchestrate mitophagy in TBI. Finally, we demonstrate that brain injury induces impaired mitophagy at the chronic stage and that IGF-1 treatment could facilitate the mitophagy markers via exosomal miR-let-7e. By showing that IGF-1 is an important mediator of the beneficial effect of the neural-endocrine network in TBI models, our findings place IGF-1/IGF-1R as a potential target capable of noncoding RNAs and opposing mitophagy failure and cognitive impairment in TBI.

Olfactory sensory experience regulates gliomagenesis via neuronal IGF1

Animals constantly receive various sensory stimuli, such as odours, sounds, light and touch, from the surrounding environment. These sensory inputs are essential for animals to search for food and avoid predators, but they also affect their physiological status, and may cause diseases such as cancer. Malignant gliomas-the most lethal form of brain tumour¹-are known to intimately communicate with neurons at the cellular level^2,3. However, it remains unclear whether external sensory stimuli can directly affect the development of malignant glioma under normal living conditions. Here we show that olfaction can directly regulate gliomagenesis. In an autochthonous mouse model that recapitulates adult gliomagenesis^4-6 originating in oligodendrocyte precursor cells (OPCs), gliomas preferentially emerge in the olfactory bulb-the first relay of brain olfactory circuitry. Manipulating the activity of olfactory receptor neurons (ORNs) affects the development of glioma. Mechanistically, olfaction excites mitral and tufted (M/T) cells, which receive sensory information from ORNs and release insulin-like growth factor 1 (IGF1) in an activity-dependent manner. Specific knockout of Igf1 in M/T cells suppresses gliomagenesis. In addition, knocking out the IGF1 receptor in pre-cancerous mutant OPCs abolishes the ORN-activity-dependent mitogenic effects. Our findings establish a link between sensory experience and gliomagenesis through their corresponding sensory neuronal circuits.

Efficacy of Chronic Paroxetine Treatment in Mitigating Amyloid Pathology and Microgliosis in APPSWE/PS1ΔE9 Transgenic Mice

BACKGROUND

Modulation of serotonergic signaling by treatment with selective serotonin reuptake inhibitors (SSRIs) has been suggested to mitigate amyloid-β (Aβ) pathology in Alzheimer's disease, in addition to exerting an anti-depressant action.

OBJECTIVE

To investigate the efficacy of chronic treatment with the SSRI paroxetine, in mitigating Aβ pathology and Aβ plaque-induced microgliosis in the hippocampus of 18-month-old APPswe/PS1 ΔE9 mice.

METHODS

Plaque-bearing APPswe/PS1 ΔE9 and wildtype mice were treated with paroxetine per os at a dose of 5 mg/kg/day, from 9 to 18 months of age. The per os treatment was monitored by recording of the body weights and serum paroxetine concentrations, and by assessment of the serotonin transporter occupancy by [3H]DASB-binding in wildtype mice. Additionally, 5,7-dihydroxytryptamine was administered to 9-month-old APPswe/PS1 ΔE9 mice, to examine the effect of serotonin depletion on Aβ pathology. Aβ pathology was evaluated by Aβ plaque load estimation and the Aβ 42/Aβ 40 ratio by ELISA.

RESULTS

Paroxetine treatment led to >  80% serotonin transporter occupancy. The treatment increased the body weight of wildtype mice, but not of APPswe/PS1 ΔE9 mice. The treatment had no effect on the Aβ plaque load (p = 0.39), the number and size of plaques, or the Aβ plaque-induced increases in microglial numbers in the dentate gyrus. Three months of serotonin depletion did not significantly impact the Aβ plaque load or Aβ 42/Aβ 40 ratio in APPswe/PS1 ΔE9 mice at 12 months.

CONCLUSION

Our results show that chronic treatment with the SSRI paroxetine does not mitigate Aβ pathology and Aβ plaque-induced microgliosis in the hippocampus of APPswe/PS1 ΔE9 mice.

The Combined Effects of Amyloidosis and Serotonin Deficiency by Tryptophan Hydroxylase-2 Knockout Impacts Viability of the APP/PS1 Mouse Model of Alzheimer’s Disease

Background: A decline of brain serotonin (5-HT) is held responsible for the changes in mood that can be observed in Alzheimer’s disease (AD). However, 5-HT’ergic signaling is also suggested to reduce the production of pathogenic amyloid-4β (Aβ). Objective: To investigate the effect of targeted inactivation of tryptophan hydroxylase-2 (Tph2), which is essential for neuronal 5-HT synthesis, on amyloidosis in amyloid precursor protein (APP)swe/presenilin 1 (PS1) ΔE9 transgenic mice. Methods: Triple-transgenic (3xTg) APP/PS1 mice with partial (+/-) or complete Tph2 knockout (–/–) were allowed to survive until 6 months old with APP/PS1, Tph2–/–, and wildtype mice. Survival and weight were recorded. Levels of Aβ 42/40/38, soluble APPα (sAβPPα) and sAβPPβ, and cytokines were analyzed by mesoscale, neurotransmitters by mass spectrometry, and gene expression by quantitative PCR. Tph2, microglia, and Aβ were visualized histologically. Results: Tph2 inactivation in APP/PS1 mice significantly reduced viability, without impacting soluble and insoluble Aβ 42 and Aβ 40 in neocortex and hippocampus, and with only mild changes of soluble Aβ 42/Aβ 40. However, sAβPPα and sAβPPβ in hippocampus and Aβ 38 and Aβ 40 in cerebrospinal fluid were reduced. 3xTg–/–mice were devoid of Tph2 immunopositive fibers and 5-HT. Cytokines were unaffected by genotype, as were neocortical TNF, HTR2a and HTR2b mRNA levels in Tph2–/– mice. Microglia clustered around Aβ plaques regardless of genotype. Conclusion: The results suggest that Tph2 inactivation influences AβPP processing, at least in the hippocampus, although levels of Aβ are unchanged. The reduced viability of 3xTg–/–mice could indicate that 5-HT protects against the seizures that can impact the viability of APP/PS1 mice.

Infection and Immunometabolism in the Central Nervous System: A Possible Mechanistic Link Between Metabolic Imbalance and Dementia

Metabolic syndromes are frequently associated with dementia, suggesting that the dysregulation of energy metabolism can increase the risk of neurodegeneration and cognitive impairment. In addition, growing evidence suggests the link between infections and brain disorders, including Alzheimer's disease. The immune system and energy metabolism are in an intricate relationship. Infection triggers immune responses, which are accompanied by imbalance in cellular and organismal energy metabolism, while metabolic disorders can lead to immune dysregulation and higher infection susceptibility. In the brain, the activities of brain-resident immune cells, including microglia, are associated with their metabolic signatures, which may be affected by central nervous system (CNS) infection. Conversely, metabolic dysregulation can compromise innate immunity in the brain, leading to enhanced CNS infection susceptibility. Thus, infection and metabolic imbalance can be intertwined to each other in the etiology of brain disorders, including dementia. Insulin and leptin play pivotal roles in the regulation of immunometabolism in the CNS and periphery, and dysfunction of these signaling pathways are associated with cognitive impairment. Meanwhile, infectious complications are often comorbid with diabetes and obesity, which are characterized by insulin resistance and leptin signaling deficiency. Examples include human immunodeficiency virus (HIV) infection and periodontal disease caused by an oral pathogen Porphyromonas gingivalis. This review explores potential interactions between infectious agents and insulin and leptin signaling pathways, and discuss possible mechanisms underlying the relationship between infection, metabolic dysregulation, and brain disorders, particularly focusing on the roles of insulin and leptin.

Inflammation and Insulin Resistance as Risk Factors and Potential Therapeutic Targets for Alzheimer's Disease

Overnutrition and modern diets containing high proportions of saturated fat are among the major factors contributing to a low-grade state of inflammation, hyperglycemia and dyslipidemia. In the last decades, the global rise of type 2 diabetes and obesity prevalence has elicited a great interest in understanding how changes in metabolic function lead to an increased risk for premature brain aging and the development of neurodegenerative disorders such as Alzheimer's disease (AD). Cognitive impairment and decreased neurogenic capacity could be a consequence of metabolic disturbances. In these scenarios, the interplay between inflammation and insulin resistance could represent a potential therapeutic target to prevent or ameliorate neurodegeneration and cognitive impairment. The present review aims to provide an update on the impact of metabolic stress pathways on AD with a focus on inflammation and insulin resistance as risk factors and therapeutic targets.

Alzheimer's Disease Pathogenesis: Role of Autophagy and Mitophagy Focusing in Microglia

Alzheimer's disease (AD) is a debilitating neurological disorder, and currently, there is no cure for it. Several pathologic alterations have been described in the brain of AD patients, but the ultimate causative mechanisms of AD are still elusive. The classic hallmarks of AD, including amyloid plaques (Aβ) and tau tangles (tau), are the most studied features of AD. Unfortunately, all the efforts targeting these pathologies have failed to show the desired efficacy in AD patients so far. Neuroinflammation and impaired autophagy are two other main known pathologies in AD. It has been reported that these pathologies exist in AD brain long before the emergence of any clinical manifestation of AD. Microglia are the main inflammatory cells in the brain and are considered by many researchers as the next hope for finding a viable therapeutic target in AD. Interestingly, it appears that the autophagy and mitophagy are also changed in these cells in AD. Inside the cells, autophagy and inflammation interact in a bidirectional manner. In the current review, we briefly discussed an overview on autophagy and mitophagy in AD and then provided a comprehensive discussion on the role of these pathways in microglia and their involvement in AD pathogenesis.

Tumor Necrosis Factor (TNF) Is Required for Spatial Learning and Memory in Male Mice under Physiological, but Not Immune-Challenged Conditions

Increasing evidence demonstrates that inflammatory cytokines-such as tumor necrosis factor (TNF)-are produced at low levels in the brain under physiological conditions and may be crucial for synaptic plasticity, neurogenesis, learning and memory. Here, we examined the effects of developmental TNF deletion on spatial learning and memory using 11-13-month-old TNF knockout (KO) and C57BL6/J wild-type (WT) mice. The animals were tested in the Barnes maze (BM) arena under baseline conditions and 48 h following an injection of the endotoxin lipopolysaccharide (LPS), which was administered at a dose of 0.5 mg/kg. Vehicle-treated KO mice were impaired compared to WT mice during the acquisition and memory-probing phases of the BM test. No behavioral differences were observed between WT and TNF-KO mice after LPS treatment. Moreover, there were no differences in the hippocampal content of glutamate and noradrenaline between groups. The effects of TNF deletion on spatial learning and memory were observed in male, but not female mice, which were not different compared to WT mice under baseline conditions. These results indicate that TNF is required for spatial learning and memory in male mice under physiological, non-inflammatory conditions, however not following the administration of LPS. Inflammatory signalling can thereby modulate spatial cognition in male subjects, highlighting the importance of sex- and probably age-stratified analysis when examining the role of TNF in the brain.

Modulating Microglial Cells for Promoting Brain Recovery and Repair

Representing the brain’s innate immune cells that interact vividly with blood-derived immune cells and brain parenchymal cells, microglia set the stage for successful brain remodeling and repair in the aftermath of brain damage. With the development of pharmacological colony-stimulating factor-1 receptor inhibitors, which allow inhibiting or depleting microglial cells, and of transgenic mice, allowing the inducible depletion of microglial cells, experimental tools have become available for studying roles of microglia in neurodegenerative and neurorestorative processes. These models open fundamental insights into roles of microglia in controlling synaptic plasticity in the healthy and the injured brain. Acting as a switch from injury to repair, microglial cells might open opportunities for promoting neurological recovery in human patients upon brain injury.

Role of Microglia in Modulating Adult Neurogenesis in Health and Neurodegeneration

Microglia are the resident immune cells of the brain, constituting the powerhouse of brain innate immunity. They originate from hematopoietic precursors that infiltrate the developing brain during different stages of embryogenesis, acquiring a phenotype characterized by the presence of dense ramifications. Microglial cells play key roles in maintaining brain homeostasis and regulating brain immune responses. They continuously scan and sense the brain environment to detect any occurring changes. Upon detection of a signal related to physiological or pathological processes, the cells are activated and transform to an amoeboid-like phenotype, mounting adequate responses that range from phagocytosis to secretion of inflammatory and trophic factors. The overwhelming evidence suggests that microglia are crucially implicated in influencing neuronal proliferation and differentiation, as well as synaptic connections, and thereby cognitive and behavioral functions. Here, we review the role of microglia in adult neurogenesis under physiological conditions, and how this role is affected in neurodegenerative diseases.

Shifting equilibriums in Alzheimer's disease: the complex roles of microglia in neuroinflammation, neuronal survival and neurogenesis

Alzheimer's disease is the leading cause of dementia. Its increased prevalence in developed countries, due to the sharp rise in ageing populations, presents one of the costliest challenges to modern medicine. In order to find disease-modifying therapies to confront this challenge, a more complete understanding of the pathogenesis of Alzheimer's disease is necessary. Recent studies have revealed increasing evidence for the roles played by microglia, the resident innate immune system cells of the brain. Reflecting the well-established roles of microglia in reacting to pathogens and inflammatory stimuli, there is now a growing literature describing both protective and detrimental effects for individual cytokines and chemokines produced by microglia in Alzheimer's disease. A smaller but increasing number of studies have also addressed the divergent roles played by microglial neurotrophic and neurogenic factors, and how their perturbation may play a key role in the pathogenesis of Alzheimer's disease. Here we review recent findings on the roles played by microglia in neuroinflammation, neuronal survival and neurogenesis in Alzheimer's disease. In each case, landmark studies have provided evidence for the divergent ways in which microglia can either promote neuronal function and survival, or perturb neuronal function, leading to cell death. In many cases, the secreted molecules of microglia can lead to divergent effects depending on the magnitude and context of microglial activation. This suggests that microglial functions must be maintained in a fine equilibrium, in order to support healthy neuronal function, and that the cellular microenvironment in the Alzheimer's disease brain disrupts this fine balance, leading to neurodegeneration. Thus, an understanding of microglial homeostasis, both in health and across the trajectory of the disease state, will improve our understanding of the pathogenic mechanisms underlying Alzheimer's disease, and will hopefully lead to the development of microglial-based therapeutic strategies to restore equilibrium in the Alzheimer's disease brain.

Protective Microglial Subset in Development, Aging, and Disease: Lessons From Transcriptomic Studies

Microglial heterogeneity has been the topic of much discussion in the scientific community. Elucidation of their plasticity and adaptability to disease states triggered early efforts to characterize microglial subsets. Over time, their phenotypes, and later on their homeostatic signature, were revealed, through the use of increasingly advanced transcriptomic techniques. Recently, an increasing number of these "microglial signatures" have been reported in various homeostatic and disease contexts. Remarkably, many of these states show similar overlapping microglial gene expression patterns, both in homeostasis and in disease or injury. In this review, we integrate information from these studies, and we propose a unique subset, for which we introduce a core signature, based on our own research and reports from the literature. We describe that this subset is found in development and in normal aging as well as in diverse diseases. We discuss the functions of this subset as well as how it is induced.