Astrocytes in Alzheimer's Disease: Pathological Significance and Molecular Pathways

Challenges and Future Perspectives in Modeling Neurodegenerative Diseases Using Organ-on-a-Chip Technology

Neurodegenerative diseases (NDDs) affect more than 50 million people worldwide, posing a significant global health challenge as well as a high socioeconomic burden. With aging constituting one of the main risk factors for some NDDs such as Alzheimer's disease (AD) and Parkinson's disease (PD), this societal toll is expected to rise considering the predicted increase in the aging population as well as the limited progress in the development of effective therapeutics. To address the high failure rates in clinical trials, legislative changes permitting the use of alternatives to traditional pre-clinical in vivo models are implemented. In this regard, microphysiological systems (MPS) such as organ-on-a-chip (OoC) platforms constitute a promising tool, due to their ability to mimic complex and human-specific tissue niches in vitro. This review summarizes the current progress in modeling NDDs using OoC technology and discusses five critical aspects still insufficiently addressed in OoC models to date. Taking these aspects into consideration in the future MPS will advance the modeling of NDDs in vitro and increase their translational value in the clinical setting.

Pongamol Alleviates Neuroinflammation and Promotes Autophagy in Alzheimer's Disease by Regulating the Akt/mTOR Signaling Pathway

Alzheimer's disease (AD), one of the neurodegenerative disorders, is highly correlated with the abnormal hyperphosphorylation of Tau and aggregation of β-amyloid (Aβ). Oxidative stress, neuroinflammation, and abnormal autophagy are key drivers of AD and how they contribute to neuropathology remains largely unknown. The flavonoid compound pongamol is reported to possess a variety of pharmacological activities, such as antioxidant, antibacterial, and anti-inflammatory. This study investigated the neuroprotective effect and its mechanisms of pongamol in lipopolysaccharide (LPS)-induced BV2 cells, d-galactose/sodium nitrite/aluminum chloride (d-gal/NaNO2/AlCl3)-induced AD mice, and Caenorhabditis elegans models. Our research revealed that pongamol reduced the release of inflammatory factors IL-1β, TNF-α, COX-2, and iNOS in LPS-induced BV2 cells. Pongamol also protected neurons and significantly restored memory function, inhibited Tau phosphorylation, downregulated Aβ aggregation, and increased oxidoreductase activity in the hippocampus of AD mice. In addition, pongamol reversed the nuclear transfer of NF-κB and increased the levels of Beclin 1 and LC3 II/LC3 I. Most importantly, the anti-inflammatory and promoter autophagy effects of pongamol may be related to the regulation of the Akt/mTOR signaling pathway. In summary, these results showed that pongamol has a potential neuroprotective effect, which greatly enriched the research on the pharmacological activity of pongamol for improving AD.

ER-stress response in retinal Müller glia occurs significantly earlier than amyloid pathology in the Alzheimer's mouse brain and retina

Alzheimer's Disease (AD) pathogenesis is thought to begin up to 20 years before cognitive symptoms appear, suggesting the need for more sensitive diagnostic biomarkers of AD. In this report, we demonstrated pathological changes in retinal Müller glia significantly earlier than amyloid pathology in AD mouse models. By utilizing the knock-in NLGF mouse model, we surprisingly discovered an increase in reticulon 3 (RTN3) protein levels in the NLGF retina as early as postnatal day 30 (P30). Despite RTN3 being a canonically neuronal protein, this increase was noted in the retinal Müller glia, confirmed by immunohistochemical characterization. Further unbiased transcriptomic assays of the P30 NLGF retina revealed that retinal Müller glia were the most sensitive responding cells in this mouse retina, compared with other cell types including photoreceptor cells and ganglion neurons. Pathway analyses of differentially expressed genes in glia cells showed activation of ER stress response via the upregulation of unfolded protein response (UPR) proteins such as ATF4 and CHOP. Early elevation of RTN3 in response to challenges by toxic Aβ likely facilitated UPR. Altogether, these findings suggest that Müller glia act as a sentinel for AD pathology in the retina and should aid for both intervention and diagnosis.

Amyloid precursor protein induces reactive astrogliosis

AIM

Astrocytes respond to stressors by acquiring a reactive state characterized by changes in their morphology and function. Molecules underlying reactive astrogliosis, however, remain largely unknown. Given that several studies observed increase in the Amyloid Precursor Protein (APP) in reactive astrocytes, we here test whether APP plays a role in reactive astrogliosis.

METHODS

We investigated whether APP instigates reactive astroglios by examining in vitro and in vivo the morphology and function of naive and APP-deficient astrocytes in response to APP and well-established stressors.

RESULTS

Overexpression of APP in cultured astrocytes led to remodeling of the intermediate filament network, enhancement of cytokine production, and activation of cellular programs centered around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion abrogated remodeling of the intermediate filament network and blunted expression of IFN-stimulated gene products in response to lipopolysaccharide. Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein observed canonically in astrocytes in response to TBI.

CONCLUSIONS

The APP thus represents a candidate molecular inducer and regulator of reactive astrogliosis. This finding has implications for understanding pathophysiology of neurodegenerative and other diseases of the nervous system characterized by reactive astrogliosis and opens potential new therapeutic avenues targeting APP and its pathways to modulate reactive astrogliosis.

Tryptophan Metabolism in Alzheimer's Disease with the Involvement of Microglia and Astrocyte Crosstalk and Gut-Brain Axis

Alzheimer's disease (AD) is an age-dependent neurodegenerative disease characterized by extracellular Amyloid Aβ peptide (Aβ) deposition and intracellular Tau protein aggregation. Glia, especially microglia and astrocytes are core participants during the progression of AD and these cells are the mediators of Aβ clearance and degradation. The microbiota-gut-brain axis (MGBA) is a complex interactive network between the gut and brain involved in neurodegeneration. MGBA affects the function of glia in the central nervous system (CNS), and microbial metabolites regulate the communication between astrocytes and microglia; however, whether such communication is part of AD pathophysiology remains unknown. One of the potential links in bilateral gut-brain communication is tryptophan (Trp) metabolism. The microbiota-originated Trp and its metabolites enter the CNS to control microglial activation, and the activated microglia subsequently affect astrocyte functions. The present review highlights the role of MGBA in AD pathology, especially the roles of Trp per se and its metabolism as a part of the gut microbiota and brain communications. We (i) discuss the roles of Trp derivatives in microglia-astrocyte crosstalk from a bioinformatics perspective, (ii) describe the role of glia polarization in the microglia-astrocyte crosstalk and AD pathology, and (iii) summarize the potential of Trp metabolism as a therapeutic target. Finally, we review the role of Trp in AD from the perspective of the gut-brain axis and microglia, as well as astrocyte crosstalk, to inspire the discovery of novel AD therapeutics.

Exploring the properties and potential of the neural extracellular matrix for next-generation regenerative therapies

The extracellular matrix (ECM) is a dynamic and complex network of proteins and molecules that surrounds cells and tissues in the nervous system and orchestrates a myriad of biological functions. This review carefully examines the diverse interactions between cells and the ECM, as well as the transformative chemical and physical changes that the ECM undergoes during neural development, aging, and disease. These transformations play a pivotal role in shaping tissue morphogenesis and neural activity, thereby influencing the functionality of the central nervous system (CNS). In our comprehensive review, we describe the diverse behaviors of the CNS ECM in different physiological and pathological scenarios and explore the unique properties that make ECM-based strategies attractive for CNS repair and regeneration. Addressing the challenges of scalability, variability, and integration with host tissues, we review how advanced natural, synthetic, and combinatorial matrix approaches enhance biocompatibility, mechanical properties, and functional recovery. Overall, this review highlights the potential of decellularized ECM as a powerful tool for CNS modeling and regenerative purposes and sets the stage for future research in this exciting field. This article is categorized under: Implantable Materials and Surgical Technologies > Nanotechnology in Tissue Repair and Replacement Therapeutic Approaches and Drug Discovery > Nanomedicine for Neurological Disease Implantable Materials and Surgical Technologies > Nanomaterials and Implants.

Death-associated protein kinase 1 as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly and represents a major clinical challenge in the ageing society. Neuropathological hallmarks of AD include neurofibrillary tangles composed of hyperphosphorylated tau, senile plaques derived from the deposition of amyloid-β (Aβ) peptides, brain atrophy induced by neuronal loss, and synaptic dysfunctions. Death-associated protein kinase 1 (DAPK1) is ubiquitously expressed in the central nervous system. Dysregulation of DAPK1 has been shown to contribute to various neurological diseases including AD, ischemic stroke and Parkinson's disease (PD). We have established an upstream effect of DAPK1 on Aβ and tau pathologies and neuronal apoptosis through kinase-mediated protein phosphorylation, supporting a causal role of DAPK1 in the pathophysiology of AD. In this review, we summarize current knowledge about how DAPK1 is involved in various AD pathological changes including tau hyperphosphorylation, Aβ deposition, neuronal cell death and synaptic degeneration. The underlying molecular mechanisms of DAPK1 dysregulation in AD are discussed. We also review the recent progress regarding the development of novel DAPK1 modulators and their potential applications in AD intervention. These findings substantiate DAPK1 as a novel therapeutic target for the development of multifunctional disease-modifying treatments for AD and other neurological disorders.

Astrocyte S100β expression and selective differentiation to GFAP and GS in the entorhinal cortex during ageing in the 3xTg-Alzheimer's disease mouse model

The study of astrocytes and its role in the development and evolution of neurodegenerative diseases, including Alzheimer's disease (AD) is essential to fully understand their aetiology. The aim if this study is to deepen into the concept of the heterogeneity of astrocyte subpopulations in the EC and in particular the identification of differentially functioning astrocyte subpopulations that respond differently to AD progression. S100β protein belongs to group of small calcium regulators of cell membrane channels and pumps that are expressed by astrocytes and is hypothesised to play and have a relevant role in AD development. We analysed the selective differentiation of S100β-positive astrocytes into Glutamine synthetase (GS) and Glial fibrillary acidic protein (GFAP)-positive sub-groups in the entorhinal cortex (EC) of AD triple transgenic animal model (3xTg-AD). EC is the brain region earliest affected in humans AD but also in this closest animal model regarding their pathology and time course. We observed no changes in the number of S100β-positive astrocytes between 1 and 18 months of age in the EC of 3xTg-AD mice. However, we identified relevant morphological changes in S100β/GFAP positive astrocytes showing a significant reduction in their surface and volume whilst an increase in number and percentage. Furthermore, the percentage of S100β/GS positive astrocyte population was also increased in 18 months old 3xTg-AD mice compared to the non-Tg mice. Our findings reveal the presence of differentially controlled astrocyte populations that respond differently to AD progression in the EC of 3xTg-AD mice. These results highpoints the major astrocytic role together with its early and marked affection in AD and arguing in favour of its importance in neurogenerative diseases and potential target for new therapeutic approaches.

Amyloid precursor protein induces reactive astrogliosis

We present in vitro and in vivo evidence demonstrating that Amyloid Precursor Protein (APP) acts as an essential instigator of reactive astrogliosis. Cell-specific overexpression of APP in cultured astrocytes led to remodelling of the intermediate filament network, enhancement of cytokine production and activation of cellular programs centred around the interferon (IFN) pathway, all signs of reactive astrogliosis. Conversely, APP deletion in cultured astrocytes abrogated remodelling of the intermediate filament network and blunted expression of IFN stimulated gene (ISG) products in response to lipopolysaccharide (LPS). Following traumatic brain injury (TBI), mouse reactive astrocytes also exhibited an association between APP and IFN, while APP deletion curbed the increase in glial fibrillary acidic protein (GFAP) observed canonically in astrocytes in response to TBI. Thus, APP represents a molecular inducer and regulator of reactive astrogliosis.

Effects of amyloid β (Aβ)42 and Gasdermin D on the progression of Alzheimer's disease in vitro and in vivo through the regulation of astrocyte pyroptosis

PURPOSE

The study aimed to investigate whether astrocyte pyroptosis, and the subsequent neuroinflammatory response that exerts amyloid β (Aβ) neurotoxic effects, has an effect on endothelial cells, along with the underlying mechanisms.

METHODS

In vivo, 5 μL of disease venom was injected into the lateral ventricle of APP/PS1 mice for treatment. Pyroptosis was induced by treating astrocytes with Aβ42 in vitro. Small interfering RNA (siRNA) was used to silence caspase-1 and Gasdermin D (GSDMD) mRNA expression. Cell viability was determined using a CCK-8 detection kit. Scanning electron microscopy (SEM), Annexin V/propidium iodide (PI) double staining, RT-qPCR, immunofluorescence, western blotting, and enzyme-linked immunosorbent assay (ELISA) were used to detect cell pyroptosis. The degree of pathological damage to the brain and aortic tissue was assessed by hematoxylin-eosin staining and immunohistochemistry.

RESULTS

Aβ42 induced astrocyte pyroptosis dependent on the GSDMD/Gasdermin E (GSDME)/Caspase 11/NLRP3 pathway, releasing large amounts of inflammatory factors, such as TNF-α, IL-1α, IL-1β, and IL-18. Astrocyte pyroptosis caused endothelial cell dysfunction and release of large amounts of vasoconstrictors (ET and vWF). Knockdown of GSDMD reduced astrocyte pyroptosis in the cerebral cortex and hippocampal tissue, decreased the release of inflammatory factors IL-1 β and IL-18, reduced Aβ deposition and tau protein, increased the release of peripheral vasodilator substances (eNOS), and decreased the release of vasoconstrictor substances (ET, vWF), thereby reducing brain tissue damage and vascular injury in APP/PS1 mice.

CONCLUSION

Aβ42 induced astrocyte pyroptosis, while GSDMD knockout inhibited astrocyte pyroptosis, reduced the release of inflammatory factors, and alleviated brain tissue damage and vascular damage in APP/PS1 mice. Therefore, GSDMD is a novel therapeutic target for Alzheimer's disease.

PURPOSE

The study aimed to investigate whether astrocyte pyroptosis, and the subsequent neuroinflammatory response that exerts amyloid β (Aβ) neurotoxic effects, has an effect on endothelial cells, along with the underlying mechanisms.

METHODS

In vivo, 5 μL of disease venom was injected into the lateral ventricle of APP/PS1 mice for treatment. Pyroptosis was induced by treating astrocytes with Aβ42 in vitro. Small interfering RNA (siRNA) was used to silence caspase-1 and Gasdermin D (GSDMD) mRNA expression. Cell viability was determined using a CCK-8 detection kit. Scanning electron microscopy (SEM), Annexin V/propidium iodide (PI) double staining, RT-qPCR, immunofluorescence, western blotting, and enzyme-linked immunosorbent assay (ELISA) were used to detect cell pyroptosis. The degree of pathological damage to the brain and aortic tissue was assessed by hematoxylin-eosin staining and immunohistochemistry.

RESULTS

Aβ42 induced astrocyte pyroptosis dependent on the GSDMD/Gasdermin E (GSDME)/Caspase 11/NLRP3 pathway, releasing large amounts of inflammatory factors, such as TNF-α, IL-1α, IL-1β, and IL-18. Astrocyte pyroptosis caused endothelial cell dysfunction and release of large amounts of vasoconstrictors (ET and vWF). Knockdown of GSDMD reduced astrocyte pyroptosis in the cerebral cortex and hippocampal tissue, decreased the release of inflammatory factors IL-1 β and IL-18, reduced Aβ deposition and tau protein, increased the release of peripheral vasodilator substances (eNOS), and decreased the release of vasoconstrictor substances (ET, vWF), thereby reducing brain tissue damage and vascular injury in APP/PS1 mice.

CONCLUSION

Aβ42 induced astrocyte pyroptosis, while GSDMD knockout inhibited astrocyte pyroptosis, reduced the release of inflammatory factors, and alleviated brain tissue damage and vascular damage in APP/PS1 mice. Therefore, GSDMD is a novel therapeutic target for Alzheimer's disease.

An Overview of Chemistry, Kinetics, Toxicity and Therapeutic Potential of Boldine in Neurological Disorders

Boldine is an alkaloid obtained from the medicinal herb Peumus boldus (Mol.) (Chilean boldo tree; boldo) and belongs to the family Monimiaceae. It exhibits a wide range of pharmacological effects such as antioxidant, anticancer, hepatoprotective, neuroprotective, and anti-diabetic properties. There is a dearth of information regarding its pharmacokinetics and toxicity in addition to its potential pharmacological activity. Boldine belongs to the aporphine alkaloid class and possesses lipophilic properties which enable its efficient absorption and distribution throughout the body, including the central nervous system. It exhibits potent free radical scavenging activity, thereby reducing oxidative stress and preventing neuronal damage. Through a variety of neuroprotective mechanisms, including suppression of AChE and BuChE activity, blocking of connexin-43 hemichannels, pannexin 1 channel, reduction of NF-κβ mediated interleukin release, and glutamate excitotoxicity which successfully reduces neuronal damage. These results point to its probable application in reducing neuroinflammation and oxidative stress in epilepsy, Alzheimer's disease (AD), and Parkinson's disease (PD). Moreover, its effects on serotonergic, dopaminergic, opioid, and cholinergic receptors were further investigated in order to determine its applicability for neurobehavioral dysfunctions. The article investigates the pharmacokinetics of boldine and reveals that it has a low oral bioavailability and a short half-life, requiring regular dosage to maintain therapeutic levels. The review studies boldine's potential therapeutic uses and mode of action while summarizing its neuroprotective benefits.  Given the favorable results for boldine as a potential neurotherapeutic drug in laboratory animals, more research is required. However, in order to optimise its therapeutic potential, it must be more bioavailable with fewer detrimental side effects.

Astrocytes in human central nervous system diseases: a frontier for new therapies

Astroglia are a broad class of neural parenchymal cells primarily dedicated to homoeostasis and defence of the central nervous system (CNS). Astroglia contribute to the pathophysiology of all neurological and neuropsychiatric disorders in ways that can be either beneficial or detrimental to disorder outcome. Pathophysiological changes in astroglia can be primary or secondary and can result in gain or loss of functions. Astroglia respond to external, non-cell autonomous signals associated with any form of CNS pathology by undergoing complex and variable changes in their structure, molecular expression, and function. In addition, internally driven, cell autonomous changes of astroglial innate properties can lead to CNS pathologies. Astroglial pathophysiology is complex, with different pathophysiological cell states and cell phenotypes that are context-specific and vary with disorder, disorder-stage, comorbidities, age, and sex. Here, we classify astroglial pathophysiology into (i) reactive astrogliosis, (ii) astroglial atrophy with loss of function, (iii) astroglial degeneration and death, and (iv) astrocytopathies characterised by aberrant forms that drive disease. We review astroglial pathophysiology across the spectrum of human CNS diseases and disorders, including neurotrauma, stroke, neuroinfection, autoimmune attack and epilepsy, as well as neurodevelopmental, neurodegenerative, metabolic and neuropsychiatric disorders. Characterising cellular and molecular mechanisms of astroglial pathophysiology represents a new frontier to identify novel therapeutic strategies.

Primary Hypothyroidism and Alzheimer's Disease: A Tale of Two

Hypothyroidism (HPT) HPT could be a risk factor for the development and progression of Alzheimer's disease (AD). In addition, progressive neurodegeneration in AD may affect the metabolism of thyroid hormones (THs) in the brain causing local brain HPT. Hence, the present review aimed to clarify the potential association between HPT and AD. HPT promotes the progression of AD by inducing the production of amyloid beta (Aβ) and tau protein phosphorylation with the development of synaptic plasticity and memory dysfunction. Besides, the metabolism of THs is dysregulated in AD due to the accumulation of Aβ and tau protein phosphorylation leading to local brain HPT. Additionally, HPT can affect AD neuropathology through various mechanistic pathways including dysregulation of transthyretin, oxidative stress, ER stress, autophagy dysfunction mitochondrial dysfunction, and inhibition of brain-derived neurotrophic factor. Taken together there is a potential link between HPT and AD, as HPT adversely impacts AD neuropathology and the reverse is also true.

Altered Brain Energy Metabolism Related to Astrocytes in Alzheimer's Disease

OBJECTIVE

Increasing evidence suggests that reactive astrocytes are associated with Alzheimer's disease (AD). However, its underlying pathogenesis remains unknown. Given the role of astrocytes in energy metabolism, reactive astrocytes may contribute to altered brain energy metabolism. Astrocytes are primarily considered glycolytic cells, suggesting a preference for lactate production. This study aimed to examine alterations in astrocytic activities and their association with brain lactate levels in AD.

METHODS

The study included 30 AD and 30 cognitively unimpaired participants. For AD participants, amyloid and tau depositions were confirmed by positron emission tomography using [11 C]PiB and [18 F]florzolotau, respectively. Myo-inositol, an astroglial marker, and lactate in the posterior cingulate cortex were quantified by magnetic resonance spectroscopy. These magnetic resonance spectroscopy metabolites were compared with plasma biomarkers, including glial fibrillary acidic protein as another astrocytic marker, and amyloid and tau positron emission tomography.

RESULTS

Myo-inositol and lactate levels were higher in AD patients than in cognitively unimpaired participants (p < 0.05). Myo-inositol levels correlated with lactate levels (r = 0.272, p = 0.047). Myo-inositol and lactate levels were positively associated with the Clinical Dementia Rating sum-of-boxes scores (p < 0.05). Significant correlations were noted between myo-inositol levels and plasma glial fibrillary acidic protein, tau phosphorylated at threonine 181 levels, and amyloid and tau positron emission tomography accumulation in the posterior cingulate cortex (p < 0.05).

INTERPRETATION

We found high myo-inositol levels accompanied by increased lactate levels in the posterior cingulate cortex in AD patients, indicating a link between reactive astrocytes and altered brain energy metabolism. Myo-inositol and plasma glial fibrillary acidic protein may reflect similar astrocytic changes as biomarkers of AD. ANN NEUROL 2023.

Astrocyte metabolism and signaling pathways in the CNS

Astrocytes comprise half of the cells in the central nervous system and play a critical role in maintaining metabolic homeostasis. Metabolic dysfunction in astrocytes has been indicated as the primary cause of neurological diseases, such as depression, Alzheimer's disease, and epilepsy. Although the metabolic functionalities of astrocytes are well known, their relationship to neurological disorders is poorly understood. The ways in which astrocytes regulate the metabolism of glucose, amino acids, and lipids have all been implicated in neurological diseases. Metabolism in astrocytes has also exhibited a significant influence on neuron functionality and the brain's neuro-network. In this review, we focused on metabolic processes present in astrocytes, most notably the glucose metabolic pathway, the fatty acid metabolic pathway, and the amino-acid metabolic pathway. For glucose metabolism, we focused on the glycolysis pathway, pentose-phosphate pathway, and oxidative phosphorylation pathway. In fatty acid metabolism, we followed fatty acid oxidation, ketone body metabolism, and sphingolipid metabolism. For amino acid metabolism, we summarized neurotransmitter metabolism and the serine and kynurenine metabolic pathways. This review will provide an overview of functional changes in astrocyte metabolism and provide an overall perspective of current treatment and therapy for neurological disorders.

Effect of resveratrol and combination of resveratrol and donepezil on the expression of microglial cells and astrocytes in Wistar albino rats of colchicine-induced Alzheimer's disease

Aim

The goal was to evaluate the effect of resveratrol (RS) and combination therapy of RS and donepezil (DPZ), on the numerical expression of microglial cells and astrocytes, in the frontal cortex, regions of the hippocampus in colchicine-induced Alzheimer's disease (AD) model.

Methods

The study involved male albino Wistar rats of three months, age and consisted of 6 groups, with six animals each. The immunohistochemical staining with mouse monoclonal anti-human CD 68 and mouse monoclonal anti-GFAP was performed to assess the number of microglial cells and astrocytes, respectively.

Results

AD group showed an increase in the number of microglia, and the numbers declined in the treatment groups, RS 10, RS 20, RS10/10 and DPZ + RS (p < 0.001). Astrocyte count was increased in the treatment groups in contrast to the AD group (p < 0.05). The DPZ + RS combination group revealed substantial elevation in the number of astrocytes and decreased microglial number among all the groups (p < 0.001).

Conclusion

RS administration has diminished the microglial number and elevated the number of astrocytes. The elevated reactive astrocytes have decreased the microglial population. However, the limitation of our study is utilizing the colchicine for the induction of neurodegeneration. Using the transgenic models of AD may give a better insight into the pathogenesis and effect of RS. Another limitation of this study is the administration of RS and DPZ through different routes. The prospects of this research include studying the probiotic nature of RS and the effect of RS in other neurodegenerative disorders.

Cooperation between neurovascular dysfunction and Aβ in Alzheimer's disease

The amyloid-β (Aβ) hypothesis was once believed to represent the pathogenic process of Alzheimer's disease (AD). However, with the failure of clinical drug development and the increasing understanding of the disease, the Aβ hypothesis has been challenged. Numerous recent investigations have demonstrated that the vascular system plays a significant role in the course of AD, with vascular damage occurring prior to the deposition of Aβ and neurofibrillary tangles (NFTs). The question of how Aβ relates to neurovascular function and which is the trigger for AD has recently come into sharp focus. In this review, we outline the various vascular dysfunctions associated with AD, including changes in vascular hemodynamics, vascular cell function, vascular coverage, and blood-brain barrier (BBB) permeability. We reviewed the most recent findings about the complicated Aβ-neurovascular unit (NVU) interaction and highlighted its vital importance to understanding disease pathophysiology. Vascular defects may lead to Aβ deposition, neurotoxicity, glial cell activation, and metabolic dysfunction; In contrast, Aβ and oxidative stress can aggravate vascular damage, forming a vicious cycle loop.

Disease-associated astrocytes and microglia markers are upregulated in mice fed high fat diet

High-fat diet (HFD) is associated with Alzheimer's disease (AD) and type 2 diabetes risk, which share features such as insulin resistance and amylin deposition. We examined gene expression associated with astrocytes and microglia since dysfunction of these cell types is implicated in AD pathogenesis. We hypothesize gene expression changes in disease-associated astrocytes (DAA), disease-associated microglia and human Alzheimer's microglia exist in diabetic and obese individuals before AD development. By analyzing bulk RNA-sequencing (RNA-seq) data generated from brains of mice fed HFD and humans with AD, 11 overlapping AD-associated differentially expressed genes were identified, including Kcnj2, C4b and Ddr1, which are upregulated in response to both HFD and AD. Analysis of single cell RNA-seq (scRNA-seq) data indicated C4b is astrocyte specific. Spatial transcriptomics (ST) revealed C4b colocalizes with Gfad, a known astrocyte marker, and the colocalization of C4b expressing cells with Gad2 expressing cells, i.e., GABAergic neurons, in mouse brain. There also exists a positive correlation between C4b and Gad2 expression in ST indicating a potential interaction between DAA and GABAergic neurons. These findings provide novel links between the pathogenesis of obesity, diabetes and AD and identify C4b as a potential early marker for AD in obese or diabetic individuals.

Striatal astrocytic A2A-D2 receptor-receptor interactions and their role in neuropsychiatric disorders

It is now generally accepted that astrocytes are active players in synaptic transmission, so that a neurocentric perspective of the integrative signal communication in the central nervous system is shifting towards a neuro-astrocentric perspective. Astrocytes respond to synaptic activity, release chemical signals (gliotransmitters) and express neurotransmitter receptors (G protein-coupled and ionotropic receptors), thus behaving as co-actors with neurons in signal communication in the central nervous system. The ability of G protein-coupled receptors to physically interact through heteromerization, forming heteromers and receptor mosaics with new distinct signal recognition and transduction pathways, has been intensively studied at neuronal plasma membrane, and has changed the view of the integrative signal communication in the central nervous system. One of the best-known examples of receptor-receptor interaction through heteromerization, with relevant consequences for both the physiological and the pharmacological points of view, is given by adenosine A2A and dopamine D2 receptors on the plasma membrane of striatal neurons. Here we review evidence that native A2A and D2 receptors can interact through heteromerization at the plasma membrane of astrocytes as well. Astrocytic A2A-D2 heteromers were found able to control the release of glutamate from the striatal astrocyte processes. A2A-D2 heteromers on striatal astrocytes and astrocyte processes are discussed as far as their potential relevance in the control of glutamatergic transmission in striatum is concerned, including potential roles in glutamatergic transmission dysregulation in pathological conditions including schizophrenia or the Parkinson's disease.

Neuron-astrocyte omnidirectional signaling in neurological health and disease

Astrocytes are an abundantly distributed population of glial cells in the central nervous system (CNS) that perform myriad functions in the normal and injured/diseased brain. Astrocytes exhibit heterogeneous phenotypes in response to various insults, a process known as astrocyte reactivity. The accuracy and precision of brain signaling are primarily based on interactions involving neurons, astrocytes, oligodendrocytes, microglia, pericytes, and dendritic cells within the CNS. Astrocytes have emerged as a critical entity within the brain because of their unique role in recycling neurotransmitters, actively modulating the ionic environment, regulating cholesterol and sphingolipid metabolism, and influencing cellular crosstalk in diverse neural injury conditions and neurodegenerative disorders. However, little is known about how an astrocyte functions in synapse formation, axon specification, neuroplasticity, neural homeostasis, neural network activity following dynamic surveillance, and CNS structure in neurological diseases. Interestingly, the tripartite synapse hypothesis came to light to fill some knowledge gaps that constitute an interaction of a subpopulation of astrocytes, neurons, and synapses. This review highlights astrocytes' role in health and neurological/neurodegenerative diseases arising from the omnidirectional signaling between astrocytes and neurons at the tripartite synapse. The review also recapitulates the disruption of the tripartite synapse with a focus on perturbations of the homeostatic astrocytic function as a key driver to modulate the molecular and physiological processes toward neurodegenerative diseases.

Integrated multimodal cell atlas of Alzheimer's disease

Alzheimer's disease (AD) is the most common cause of dementia in older adults. Neuropathological and imaging studies have demonstrated a progressive and stereotyped accumulation of protein aggregates, but the underlying molecular and cellular mechanisms driving AD progression and vulnerable cell populations affected by disease remain coarsely understood. The current study harnesses single cell and spatial genomics tools and knowledge from the BRAIN Initiative Cell Census Network to understand the impact of disease progression on middle temporal gyrus cell types. We used image-based quantitative neuropathology to place 84 donors spanning the spectrum of AD pathology along a continuous disease pseudoprogression score and multiomic technologies to profile single nuclei from each donor, mapping their transcriptomes, epigenomes, and spatial coordinates to a common cell type reference with unprecedented resolution. Temporal analysis of cell-type proportions indicated an early reduction of Somatostatin-expressing neuronal subtypes and a late decrease of supragranular intratelencephalic-projecting excitatory and Parvalbumin-expressing neurons, with increases in disease-associated microglial and astrocytic states. We found complex gene expression differences, ranging from global to cell type-specific effects. These effects showed different temporal patterns indicating diverse cellular perturbations as a function of disease progression. A subset of donors showed a particularly severe cellular and molecular phenotype, which correlated with steeper cognitive decline. We have created a freely available public resource to explore these data and to accelerate progress in AD research at SEA-AD.org.

The effects of voluntary running exercise on the astrocytes of the medial prefrontal cortex in APP/PS1 mice

Pathological changes in the medial prefrontal cortex (mPFC) and astrocytes are closely associated with Alzheimer's disease (AD). Voluntary running has been found to effectively delay AD. However, the effects of voluntary running on mPFC astrocytes in AD are unclear. A total of 40 10-month-old male amyloid precursor protein/presenilin 1 (APP/PS1) mice and 40 wild-type (WT) mice were randomly divided into control and running groups, and the running groups underwent voluntary running for 3 months. Mouse cognition was assessed by the novel object recognition (NOR), Morris water maze (MWM), and Y maze tests. The effects of voluntary running on mPFC astrocytes were investigated using immunohistochemistry, immunofluorescence, western blotting, and stereology. APP/PS1 mice performed significantly worse than WT mice in the NOR, MWM, and Y maze tests, and voluntary running improved the performance of APP/PS1 mice in these tests. The total number of mPFC astrocytes was increased, cell bodies were enlarged, and protrusion number and length were increased in AD mice compared with WT mice, but there was no difference in component 3 (C3) levels in the mPFC (total mPFC level); however, C3 and S100B levels in astrocytes were increased in AD mice. Voluntary running reduced the total number of astrocytes and S100B levels in astrocytes and increased the density of PSD95⁺ puncta in direct contact with astrocyte protrusions in the APP/PS1 mouse mPFC. Three months of voluntary running inhibited astrocyte hyperplasia and S100B expression in astrocytes, increased the density of synapses in contact with astrocytes, and improved cognitive function in APP/PS1 mice.

The Adult Neurogenesis Theory of Alzheimer's Disease

Alzheimer's disease starts in neural stem cells (NSCs) in the niches of adult neurogenesis. All primary factors responsible for pathological tau hyperphosphorylation are inherent to adult neurogenesis and migration. However, when amyloid pathology is present, it strongly amplifies tau pathogenesis. Indeed, the progressive accumulation of extracellular amyloid-β deposits in the brain triggers a state of chronic inflammation by microglia. Microglial activation has a significant pro-neurogenic effect that fosters the process of adult neurogenesis and supports neuronal migration. Unfortunately, this "reactive" pro-neurogenic activity ultimately perturbs homeostatic equilibrium in the niches of adult neurogenesis by amplifying tau pathogenesis in AD. This scenario involves NSCs in the subgranular zone of the hippocampal dentate gyrus in late-onset AD (LOAD) and NSCs in the ventricular-subventricular zone along the lateral ventricles in early-onset AD (EOAD), including familial AD (FAD). Neuroblasts carrying the initial seed of tau pathology travel throughout the brain via neuronal migration driven by complex signals and convey the disease from the niches of adult neurogenesis to near (LOAD) or distant (EOAD) brain regions. In these locations, or in close proximity, a focus of degeneration begins to develop. Then, tau pathology spreads from the initial foci to large neuronal networks along neural connections through neuron-to-neuron transmission.

A comparative study of acetyl-l-carnitine and caloric restriction impact on hippocampal autophagy, apoptosis, neurogenesis, and astroglial function in AlCl3-induced Alzheimer's in rats

Alzheimer's disease (AD) is a worldwide chronic progressive neurodegenerative disease. We aimed to investigate and compare the neuroprotective impact of acetyl-l-carnitine and caloric restriction (CR) on AlCl₃-induced AD to explore the pathogenesis and therapeutic strategies of AD. Sixty-seven adult male Wistar rats were allocated into Control, AlCl₃, AlCl₃-acetyl-l-carnitine, and AlCl₃-CR groups. Each of AlCl₃ and acetyl-l-carnitine were given by gavage in a daily dose of 100 mg/kg and CR was conducted by giving 70% of the daily average caloric intake of the control group. Rats were subjected to behavioral assessment using open field test, Y maze, novel object recognition test and passive avoidance test, biochemical assay of serum phosphorylated tau (pTau), hippocampal homogenate phosphorylated adenosine monophosphate-activated protein kinase, Beclin-1, Bcl-2-associated X protein, and B cell lymphoma 2 (Bcl2) as well as hippocampal Ki-67 and glial fibrillary acidic protein immunohistochemistry. AlCl₃-induced cognitive and behavioral deficits coincident with impaired autophagy and enhanced apoptosis associated with defective neurogenesis and defective astrocyte activation. Acetyl-l-carnitine and CR partially protect against AlCl₃-induced behavioral, cognitive, biochemical, and histological changes, with more ameliorative effect of acetyl-l-carnitine on hippocampal apoptotic markers, and more obvious behavioral and histological improvement with CR.

Astrocytes in the pathophysiology of neuroinfection

Key homeostasis providing cells in the central nervous system (CNS) are astrocytes, which belong to the class of cells known as atroglia, a highly heterogeneous type of neuroglia and a prominent element of the brain defence. Diseases evolve due to altered homeostatic state, associated with pathology-induced astroglia remodelling represented by reactive astrocytes, astroglial atrophy and astrodegeneration. These features are hallmarks of most infectious insults, mediated by bacteria, protozoa and viruses; they are also prominent in the systemic infection. The COVID-19 pandemic revived the focus into neurotropic viruses such as SARS-CoV2 (Coronaviridae) but also the Flaviviridae viruses including tick-borne encephalitis (TBEV) and Zika virus (ZIKV) causing the epidemic in South America prior to COVID-19. Astrocytes provide a key response to neurotropic infections in the CNS. Astrocytes form a parenchymal part of the blood-brain barrier, the site of virus entry into the CNS. Astrocytes exhibit aerobic glycolysis, a form of metabolism characteristic of highly morphologically plastic cells, like cancer cells, hence a suitable milieu for multiplication of infectious agent, including viral particles. However, why the protection afforded by astrocytes fails in some circumstances is an open question to be studied in the future.

A Purine Derivative Containing an Organoselenium Group Protects Against Memory Impairment, Sensitivity to Nociception, Oxidative Damage, and Neuroinflammation in a Mouse Model of Alzheimer's Disease

In the present study, the effect of 6-((4-fluorophenyl) selanyl)-9H-purine (FSP) was tested against memory impairment and sensitivity to nociception induced by intracerebroventricular injection of amyloid-beta peptide (Aβ) (25-35 fragment), 3 nmol/3 μl/per site in mice. Memory impairment was determined by the object recognition task (ORT) and nociception by the Von-Frey test (VFT). Aβ caused neuroinflammation with upregulation of glial fibrillary acidic protein (GFAP) (in hippocampus), nuclear factor-κB (NF-κB), and the proinflammatory cytokines interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α) in cerebral cortex and hippocampus. Additionally, Aβ increased oxidant levels and lipid peroxidation in cerebral cortex and hippocampus, but decreased heme oxygenase-1 (HO-1) and peroxiredoxin-1 (Prdx1) expression in the hippocampus. Anti-neuroinflammatory effects of FSP were demonstrated by a decrease in the expression of GFAP and NF-κB in the hippocampus, as well as a decrease in proinflammatory cytokines in both the hippocampus and cerebral cortex FSP protected against oxidative stress by decreasing oxidant levels and lipid peroxidation and by increasing HO-1 and Prdx1 expressions in the hippocampus of mice. Moreover, FSP prevented the activation of nuclear factor erythroid 2-related factor 2 (Nrf-2) in the hippocampus of mice induced by Aβ. In conclusion, treatment with FSP attenuated memory impairment, nociception sensitivity by decreasing oxidative stress, and neuroinflammation in a mouse model of Alzheimer's disease.

Fundamental Neurochemistry Review: Incorporating a greater diversity of cell types, including microglia, in brain organoid cultures improves clinical translation

Brain organoids have the potential to improve clinical translation, with the added benefit of reducing any extraneous use of experimental animals. As brain organoids are three-dimensional in vitro constructs that emulate the human brain, they bridge in vitro and in vivo studies more appropriately than monocultures. Although many factors contribute to the failure of extrapolating monoculture-based information to animal-based experiments and clinical trials, for the purpose of this review, we will focus on glia (non-neuronal brain cells), whose functions and transcriptome are particularly abnormal in monocultures. As discussed herein, glia require signals from-and contact with-other cell types to exist in their homeostatic state, which likely contributes to some of the differences between data derived from monocultures and data derived from brain organoids and even two-dimensional co-cultures. Furthermore, we highlight transcriptomic differences between humans and mice in regard to aging and Alzheimer's disease, emphasizing need for a model using the human genome-again, a benefit of brain organoids-to complement data derived from animals. We also identify an urgency for guidelines to improve the reporting and transparency of research using organoids. The lack of reporting standards creates challenges for the comparison and discussion of data from different articles. Importantly, brain organoids mark the first human model enabling the study of brain cytoarchitecture and development.

The application of alcian blue to identify astrocyte-associated amyloid plaques by using fluorescence and confocal microscopy

BACKGROUND

Astrocytes play an essential role in the normal functioning of the nervous system and are active contributors to the pathogenesis of neurodegenerative diseases such as Alzheimer's disease (AD). Therefore, to comprehend the astrocytes and amyloid plaques relationship there is a need for imaging techniques providing simultaneous visualization of astrocytes using fluorescence and amyloid plaques revealed by transmitted light microscopy.

NEW METHOD

The possibility of simultaneous detection of astrocytes by immunocytochemistry (fluorescent) and amyloid plaques by cytochemical Alcian Blue (transparent) using confocal microscopy in 8-month-old 5хFAD mice samples shown.

RESULTS

The described method supposes performing astrocytes fluorescent labelling by GFAP or S100beta and amyloid plaques staining by Alcian Blue.

COMPARISON WITH EXISTING METHODS

Proposed approach circumvents some limitations of fluorescence microscopy, such as weak fluorescence, low contrast, fluorophore broad excitation/emission profile and chemical instability.

CONCLUSIONS

The proposed technique provides high-quality resulting images of GFAP/s100beta- labelled astrocytes and Alcian Blue-stained amyloid plaques. These images are appliable for prospective qualitative and quantitative three-dimensional analysis due to the z-axis scanning. Moreover, it demonstrated the formation of stable Alcian Blue staining.

Electron microscopy analysis of astrocyte-synapse interactions shows altered dynamics in an Alzheimer's disease mouse model

Introduction

Astrocyte-synapse bi-directional communication is required for neuronal development and synaptic plasticity. Astrocytes structurally interact with synapses using their distal processes also known as leaflets or perisynaptic astrocytic processes (PAPs). We recently showed that these PAPs are retracted from hippocampal synapses, and involved in the consolidation of fear memory. However, whether astrocytic synaptic coverage is affected when memory is impaired is unknown.

Methods

Here, we describe in detail an electron microscopy method that makes use of a large number of 2D images to investigate structural astrocyte-synapse interaction in paraformaldehyde fixed brain tissue of mice.

Results and discussion

We show that fear memory-induced synaptic activation reduces the interaction between the PAPs and the presynapse, but not the postsynapse, accompanied by retraction of the PAP tip from the synaptic cleft. Interestingly, this retraction is absent in the APP/PS1 mouse model of Alzheimer's disease, supporting the concept that alterations in astrocyte-synapse coverage contribute to memory processing.

The Dual Role of Astrocyte-Derived Exosomes and Their Contents in the Process of Alzheimer's Disease

Millions of patients worldwide are affected by Alzheimer's disease (AD), and the number of patients with AD is increasing. However, current treatment can only improve symptoms but cannot cure the disease. Astrocytes, glial cells in the central nervous system, play important roles in support, nutrition, protection, and information transmission in the nervous system. Pathological changes in astrocytes are closely associated with the development and progression of AD. As carriers for material and information exchange between astrocytes and other neural cells, astrocyte-derived exosomes (ADEs) have been widely studied in recent years, and ADE secretion has been shown to be increased in patients with AD and animal models of AD. ADEs contain a variety of substances, including nucleic acids, proteins, and lipids. The contents of ADEs can effectively control oxidative stress and detoxification during the early development of AD, thereby playing positive and negative roles in the occurrence and development of AD. In this review, we elaborate on the functions of ADEs and their components in AD and discuss their applications in AD research and clinical practice.

Sex and APOE Genotype Alter the Basal and Induced Inflammatory States of Primary Astrocytes from Humanized Targeted Replacement Mice

Apolipoprotein E4 (APOE4) genotype and sex are significant risk factors for Alzheimer's disease (AD), with females demonstrating increased risk modulated by APOE genotype. APOE is predominantly expressed in astrocytes, however, there is a lack of comprehensive assessments of sex differences in astrocytes stratified by APOE genotype. Here, we examined the response of mixed-sex and sex-specific neonatal APOE3 and APOE4 primary mouse astrocytes (PMA) to a cytokine mix of IL1b, TNFa, and IFNg. Pro-inflammatory and anti-inflammatory cytokine profiles were assessed by qRT-PCR and Meso Scale Discovery multiplex assay. Mixed-sex APOE4 PMA were found to have higher basal messenger RNA expression of several pro-inflammatory cytokines including Il6, Tnfa, Il1b, Mcp1, Mip1a, and Nos2 compared to APOE3 PMA, which was accompanied by increased levels of these secreted cytokines. In sex-specific cultures, basal expression of Il1b, Il6, and Nos2 was 1.5 to 2.5 fold higher in APOE4 female PMA compared to APOE4 males, with both being higher than APOE3 PMA. Similar results were found for secreted levels of these cytokines. Together, these findings indicate that APOE4 genotype and female sex, contribute to a greater inflammatory response in primary astrocytes and these data may provide a framework for investigating the mechanisms contributing to genotype and sex differences in AD-related neuroinflammation.

The role of transforming growth factor β1 /Smad pathway in Alzheimer's disease inflammation pathology

Alzheimer's disease (AD), a progressive disorder, has become a global health problem and is now the main cause of dementia. The aetiology of AD is complex and remains elusive making effective AD treatment difficult. Current drugs for AD only improve symptoms but do not interfere with pathogenic mechanisms. Three main hypotheses have been brought forward regarding AD aetiology, one of them being the 'inflammation hypothesis'. A number of studies have demonstrated that inflammation plays a critical role in AD. Self-limiting neuroinflammation is considered beneficial to AD, whereas chronic inflammation aggravates brain injury and neuronal death. Transforming growth factor β 1(TGF-β1) is an anti-inflammatory cytokine with neuroprotective properties. Smad proteins are downstream molecules of TGF-β signalling. They are cytoplasmic transcription factors that can regulate targeted gene expression. In AD, impairments of TGF-β1/Smad pathways have been observed. Moreover, microglia, astrocytes, inflammasomes, and insulin resistance also have been implicated in AD pathogenesis. Elucidating the molecular mechanisms underlying AD pathogenesis is a fundamental step toward designing new treatment options. In this review, we detail the changes in TGF-β1/Smad pathways in AD and hope this will facilitate further research on AD treatment.

Molecular mechanisms of Alzheimer's disease: From therapeutic targets to promising drugs

Alzheimer's disease (AD) is a neurodegenerative disease characterized by cognitive impairment so widespread that it interferes with a person's ability to complete daily activities. AD is becoming increasingly common, and it is estimated that the number of patients will reach 152 million by 2050. Current treatment options for AD are symptomatic and have modest benefits. Therefore, considering the human, social, and economic burden of the disease, the development of drugs with the potential to alter disease progression has become a global priority. In this review, the molecular mechanisms involved in the pathology of AD were evaluated as therapeutic targets. The main aim of the review is to focus on new knowledge about mitochondrial dysfunction, oxidative stress, and neuronal transmission in AD, as well as a range of cellular signaling mechanisms and associated treatments. Important molecular interactions leading to AD were described in amyloid cascade and in tau protein function, oxidative stress, mitochondrial dysfunction, cholinergic and glutamatergic neurotransmission, cAMP-regulatory element-binding protein (CREB), the silent mating type information regulation 2 homolog 1 (SIRT-1), neuroinflammation (glial cells), and synaptic alterations. This review summarizes recent experimental and clinical research in AD pathology and analyzes the potential of therapeutic applications based on molecular disease mechanisms.

Evolution of neuroglia

The evolution of the nervous system progressed through cellular diversification and specialization of functions. Conceptually, the nervous system is composed of electrically excitable neuronal networks connected by chemical synapses and nonexcitable glial cells that provide for homeostasis and defense. The evolution of neuroglia began with the emergence of the centralized nervous system and proceeded through a continuous increase in their complexity. In the primate brain, especially in the brain of humans, the astrocyte lineage is exceedingly complex, with the emergence of new types of astroglial cells possibly involved in interlayer communication and integration.

The role of ADAM10 in astrocytes: Implications for Alzheimer’s disease

Much of the early research into AD relies on a neuron-centric view of the brain, however, evidence of multiple altered cellular interactions between glial cells and the vasculature early in AD has been demonstrated. As such, alterations in astrocyte function are widely recognized a contributing factor in the pathogenesis of AD. The processes by which astrocytes may be involved in AD make them an interesting target for therapeutic intervention, but in order for this to be most effective, there is a need for the specific mechanisms involving astrocyte dysfunction to be investigated. “α disintegrin and metalloproteinase” 10 (ADAM10) is capable of proteolytic cleavage of the amyloid precursor protein which prevents amyloid-β generation. As such ADAM10 has been identified as an interesting enzyme in AD pathology. ADAM10 is also known to play a role in a significant number of cellular processes, most notable in notch signaling and in inflammatory processes. There is a growing research base for the involvement of ADAM10 in regulating astrocytic function, primarily from an immune perspective. This review aims to bring together available evidence for ADAM10 activity in astrocytes, and how this relates to AD pathology.

2021 Editor's Choice Articles in the Section "Cells of the Nervous System"

Referenced below are the top 10 cited papers in 2021 from the Section "Cells of the Nervous System", published in Cells (ISSN: 2073-4409) [...].

Emerging Role of miR-21-5p in Neuron-Glia Dysregulation and Exosome Transfer Using Multiple Models of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder associated with neuron-glia dysfunction and dysregulated miRNAs. We previously reported upregulated miR-124/miR-21 in AD neurons and their exosomes. However, their glial distribution, phenotypic alterations and exosomal spread are scarcely documented. Here, we show glial cell activation and miR-21 overexpression in mouse organotypic hippocampal slices transplanted with SH-SY5Y cells expressing the human APP695 Swedish mutation. The upregulation of miR-21 only in the CSF from a small series of mild cognitive impairment (MCI) AD patients, but not in non-AD MCI individuals, supports its discriminatory potential. Microglia, neurons, and astrocytes differentiated from the same induced pluripotent stem cells from PSEN1ΔE9 AD patients all showed miR-21 elevation. In AD neurons, miR-124/miR-21 overexpression was recapitulated in their exosomes. In AD microglia, the upregulation of iNOS and miR-21/miR-146a supports their activation. AD astrocytes manifested a restrained inflammatory profile, with high miR-21 but low miR-155 and depleted exosomal miRNAs. Their immunostimulation with C1q + IL-1α + TNF-α induced morphological alterations and increased S100B, inflammatory transcripts, sAPPβ, cytokine release and exosomal miR-21. PPARα, a target of miR-21, was found to be repressed in all models, except in neurons, likely due to concomitant miR-125b elevation. The data from these AD models highlight miR-21 as a promising biomarker and a disease-modifying target to be further explored.

Astrocyte calcium dysfunction causes early network hyperactivity in Alzheimer's disease

Dysfunctions of network activity and functional connectivity (FC) represent early events in Alzheimer’s disease (AD), but the underlying mechanisms remain unclear. Astrocytes regulate local neuronal activity in the healthy brain, but their involvement in early network hyperactivity in AD is unknown. We show increased FC in the human cingulate cortex several years before amyloid deposition. We find the same early cingulate FC disruption and neuronal hyperactivity in App^NL-F mice. Crucially, these network disruptions are accompanied by decreased astrocyte calcium signaling. Recovery of astrocytic calcium activity normalizes neuronal hyperactivity and FC, as well as seizure susceptibility and day/night behavioral disruptions. In conclusion, we show that astrocytes mediate initial features of AD and drive clinically relevant phenotypes.

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The cingulate cortex of humans and mice shows early functional deficits in AD

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Astrocyte calcium signaling is decreased before the presence of amyloid plaques

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Recovery of astrocyte calcium signals mitigates neuronal hyperactivity

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Recovery of astrocytes normalizes cingulate connectivity and behavior disruptions

Sulforaphane Upregulates Cultured Mouse Astrocytic Aquaporin-4 Expression through p38 MAPK Pathway

Protein misfolding and/or aggregation are common pathological features associated with a number of neurodegenerative diseases, including Alzheimer's disease (AD) and Parkinson disease (PD). Abnormal protein aggregation may be caused by misfolding of the protein and/or dysfunction of the protein clearance system. Recent studies have demonstrated that the specific water channel protein, aquaporin-4 (AQP4), plays a role in the pathogenesis of neurodegenerative diseases involving protein clearance system. In this study, we aimed to investigate the role of sulforaphane (SFN) in the upregulation of AQP4 expression, along with its underlying mechanism using cultured mouse astrocytes as a model system. At low concentrations, SFN was found to increase cell proliferation and result in the activation of astrocytes. However, high SFN concentrations were found to suppress cell proliferation of astrocytes. In addition, our study found that a 1 μM concentration of SFN resulted in the upregulation of AQP4 expression and p38 MAPK phosphorylation in cultured mouse astrocytes. Moreover, we demonstrated that the upregulation of AQP4 expression was significantly attenuated when cells were pretreated with SB203580, a p38 MAPK inhibitor. In conclusion, our findings from this study revealed that SFN exerts hormesis effect on cultured mouse astrocytes and can upregulate astrocytic AQP4 expression by targeting the p38 MAPK pathway.

Induction of ICAM1 in Brain Vessels is Implicated in an Early AD Pathogenesis by Modulating Neprilysin

Intercellular adhesion molecule 1 (ICAM1) is a vessel adhesion protein induced during brain vascular inflammation, which could be closely linked with the development of Alzheimer's disease (AD). This study investigated the effect of ICAM1 on amyloid-degrading enzymes (ADEs) in endothelial cells and their potential involvement in inflammation and AD progression. TNF-α treatment increased ICAM1 in human brain microvascular endothelial cells (HBMVECs) but decreased the neprilysin (NEP) protein level. Knock-down of ICAM1 using siRNA enhanced NEP, which increased the degradation of amyloid-β. In the brains of 4-month-old AD transgenic mice (APPswe/PSEN1dE9), there were significantly higher levels of ICAM1 expression and amyloid deposits but lower levels of NEP and insulin-degrading enzymes (IDE), demonstrating an inverse correlation of ICAM1 with NEP and IDE expression. Further studies demonstrated significantly increased GFAP protein levels in the brain, specifically localized near blood vessels, of both TNF-α-injected and 4-month-old AD transgenic mice. Taken together, the induction of ICAM1 in endothelial cells suppresses NEP expression, accelerating the accumulation of amyloid-β in blood vessels. It also enhances leukocyte adhesion to blood vessels stimulating the migration of leukocytes into the brain, subsequently triggering brain inflammation.

Targeting gut microbiota to alleviate neuroinflammation in Alzheimer's disease

The gut microbiota came into focus within the last years regarding being associated with or even underlying neuropsychiatric diseases. The existence of the gut-brain-axis makes it highly plausible that bacterial metabolites or toxins that escape the intestinal environment or approach the vagal connections towards the brain, exert devastating effects on the central nervous system. In Alzheimer's disease (AD), growing evidence for dysbiotic changes in the gut microbiota is obtained, even though the question for cause or consequence remains open. Nevertheless, using modulation of microbiota to address inflammatory processes seems an attractive therapeutic approach as certain microbial products such as short chain fatty acids have been proven to exert beneficial cognitive effects. In this review, we summarize, contemporary knowledge on neuroinflammation and inflammatory processes within the brain and even more detailed in the gut in AD, try to conclude whom to target regarding human microbial commensals and report on current interventional trials.

Innate Immune Cell Death in Neuroinflammation and Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disorder molecularly characterized by the formation of amyloid β (Aβ) plaques and type 2 microtubule-associated protein (Tau) abnormalities. Multiple studies have shown that many of the brain’s immunological cells, specifically microglia and astrocytes, are involved in AD pathogenesis. Cells of the innate immune system play an essential role in eliminating pathogens but also regulate brain homeostasis and AD. When activated, innate immune cells can cause programmed cell death through multiple pathways, including pyroptosis, apoptosis, necroptosis, and PANoptosis. The cell death often results in the release of proinflammatory cytokines that propagate the innate immune response and can eliminate Aβ plaques and aggregated Tau proteins. However, chronic neuroinflammation, which can result from cell death, has been linked to neurodegenerative diseases and can worsen AD. Therefore, the innate immune response must be tightly balanced to appropriately clear these AD-related structural abnormalities without inducing chronic neuroinflammation. In this review, we discuss neuroinflammation, innate immune responses, inflammatory cell death pathways, and cytokine secretion as they relate to AD. Therapeutic strategies targeting these innate immune cell death mechanisms will be critical to consider for future preventive or palliative treatments for AD.

The Sedentary Lifestyle and Masticatory Dysfunction: Time to Review the Contribution to Age-Associated Cognitive Decline and Astrocyte Morphotypes in the Dentate Gyrus

As aging and cognitive decline progresses, the impact of a sedentary lifestyle on the appearance of environment-dependent cellular morphologies in the brain becomes more apparent. Sedentary living is also associated with poor oral health, which is known to correlate with the rate of cognitive decline. Here, we will review the evidence for the interplay between mastication and environmental enrichment and assess the impact of each on the structure of the brain. In previous studies, we explored the relationship between behavior and the morphological features of dentate gyrus glial fibrillary acidic protein (GFAP)-positive astrocytes during aging in contrasting environments and in the context of induced masticatory dysfunction. Hierarchical cluster and discriminant analysis of GFAP-positive astrocytes from the dentate gyrus molecular layer revealed that the proportion of AST1 (astrocyte arbors with greater complexity phenotype) and AST2 (lower complexity) are differentially affected by environment, aging and masticatory dysfunction, but the relationship is not straightforward. Here we re-evaluated our previous reconstructions by comparing dorsal and ventral astrocyte morphologies in the dentate gyrus, and we found that morphological complexity was the variable that contributed most to cluster formation across the experimental groups. In general, reducing masticatory activity increases astrocyte morphological complexity, and the effect is most marked in the ventral dentate gyrus, whereas the effect of environment was more marked in the dorsal dentate gyrus. All morphotypes retained their basic structural organization in intact tissue, suggesting that they are subtypes with a non-proliferative astrocyte profile. In summary, the increased complexity of astrocytes in situations where neuronal loss and behavioral deficits are present is counterintuitive, but highlights the need to better understand the role of the astrocyte in these conditions.

Microglia in Alzheimer’s Disease: A Favorable Cellular Target to Ameliorate Alzheimer’s Pathogenesis

Microglial cells serve as molecular sensors of the brain that play a role in physiological and pathological conditions. Under normal physiology, microglia are primarily responsible for regulating central nervous system homeostasis through the phagocytic clearance of redundant protein aggregates, apoptotic cells, damaged neurons, and synapses. Furthermore, microglial cells can promote and mitigate amyloid β phagocytosis and tau phosphorylation. Dysregulation of the microglial programming alters cellular morphology, molecular signaling, and secretory inflammatory molecules that contribute to various neurodegenerative disorders especially Alzheimer’s disease (AD). Furthermore, microglia are considered primary sources of inflammatory molecules and can induce or regulate a broad spectrum of cellular responses. Interestingly, in AD, microglia play a double-edged role in disease progression; for instance, the detrimental microglial effects increase in AD while microglial beneficiary mechanisms are jeopardized. Depending on the disease stages, microglial cells are expressed differently, which may open new avenues for AD therapy. However, the disease-related role of microglial cells and their receptors in the AD brain remain unclear. Therefore, this review represents the role of microglial cells and their involvement in AD pathogenesis.

TGF-β as a Key Modulator of Astrocyte Reactivity: Disease Relevance and Therapeutic Implications

Astrocytes are essential for normal brain development and functioning. They respond to brain injury and disease through a process referred to as reactive astrogliosis, where the reactivity is highly heterogenous and context-dependent. Reactive astrocytes are active contributors to brain pathology and can exert beneficial, detrimental, or mixed effects following brain insults. Transforming growth factor-β (TGF-β) has been identified as one of the key factors regulating astrocyte reactivity. The genetic and pharmacological manipulation of the TGF-β signaling pathway in animal models of central nervous system (CNS) injury and disease alters pathological and functional outcomes. This review aims to provide recent understanding regarding astrocyte reactivity and TGF-β signaling in brain injury, aging, and neurodegeneration. Further, it explores how TGF-β signaling modulates astrocyte reactivity and function in the context of CNS disease and injury.

Controlling the spontaneous firing behavior of a neuron with astrocyte

Mounting evidence in recent years suggests that astrocytes, a sub-type of glial cells, not only serve metabolic and structural support for neurons and synapses but also play critical roles in the regulation of proper functioning of the nervous system. In this work, we investigate the effect of astrocytes on the spontaneous firing activity of a neuron through a combined model that includes a neuron-astrocyte pair. First, we show that an astrocyte may provide a kind of multistability in neuron dynamics by inducing different firing modes such as random and bursty spiking. Then, we identify the underlying mechanism of this behavior and search for the astrocytic factors that may have regulatory roles in different firing regimes. More specifically, we explore how an astrocyte can participate in the occurrence and control of spontaneous irregular spiking activity of a neuron in random spiking mode. Additionally, we systematically investigate the bursty firing regime dynamics of the neuron under the variation of biophysical facts related to the intracellular environment of the astrocyte. It is found that an astrocyte coupled to a neuron can provide a control mechanism for both spontaneous firing irregularity and burst firing statistics, i.e., burst regularity and size.

Coupled Neural–Glial Dynamics and the Role of Astrocytes in Alzheimer’s Disease

Neurodegenerative diseases such as Alzheimer’s (AD) are associated with the propagation and aggregation of toxic proteins. In the case of AD, it was Alzheimer himself who showed the importance of both amyloid beta (Aβ) plaques and tau protein neurofibrillary tangles (NFTs) in what he called the “disease of forgetfulness”. The amyloid beta forms extracellular aggregates and plaques, whereas tau proteins are intracellular proteins that stabilize axons by cross-linking microtubules that can form largely messy tangles. On the other hand, astrocytes and microglial cells constantly clear these plaques and NFTs from the brain. Astrocytes transport nutrients from the blood to neurons. Activated astrocytes produce monocyte chemoattractant protein-1 (MCP-1), which attracts anti-inflammatory macrophages and clears Aβ. At the same time, the microglia cells are poorly phagocytic for Aβ compared to proinflammatory and anti-inflammatory macrophages. In addition to such distinctive neuropathological features of AD as amyloid beta and tau proteins, neuroinflammation has to be brought into the picture as well. Taking advantage of a coupled mathematical modelling framework, we formulate a network model, accounting for the coupling between neurons and astroglia and integrating all three main neuropathological features with the brain connectome data. We provide details on the coupled dynamics involving cytokines, astrocytes, and microglia. Further, we apply the tumour necrosis factor alpha (TNF-α) inhibitor and anti-Aβ drug and analyze their influence on the brain cells, suggesting conditions under which the drug can prevent cell damage. The important role of astrocytes and TNF-α inhibitors in AD pathophysiology is emphasized, along with potentially promising pathways for developing new AD therapies.

Dihydromyricetin improves social isolation-induced cognitive impairments and astrocytic changes in mice

Social isolation induces stress, anxiety, and mild cognitive impairment that could progress towards irreversible brain damage. A probable player in the mechanism of social isolation-induced anxiety is astrocytes, specialized glial cells that support proper brain function. Using a social isolation mouse model, we observed worsened cognitive and memory abilities with reductions of Object Recognition Index (ORI) in novel object recognition test and Recognition Index (RI) in novel context recognition test. Social isolation also increased astrocyte density, reduced astrocyte size with shorter branches, and reduced morphological complexity in the hippocampus. Dihydromyricetin, a flavonoid that we previously demonstrated to have anxiolytic properties, improved memory/cognition and restored astrocyte plasticity in these mice. Our study indicates astrocytic involvement in social isolation-induced cognitive impairment as well as anxiety and suggest dihydromyricetin as an early-stage intervention against anxiety, cognitive impairment, and potential permanent brain damage.

Astroglial cannabinoid signaling and behavior

In neuroscience, the explosion of innovative and advanced technical accomplishments is fundamental to understanding brain functioning. For example, the possibility to distinguish glial and neuronal activities at the synaptic level and/or the appearance of new genetic tools to specifically monitor and manipulate astroglial functions revealed that astrocytes are involved in several facets of behavioral control. In this sense, the discovery of functional presence of type-1 cannabinoid receptors in astrocytes has led to identify important behavioral responses mediated by this specific pool of cannabinoid receptors. Thus, astroglial type-1 cannabinoid receptors are in the perfect place to play a role in a complex scenario in which astrocytes sense neuronal activity, release gliotransmitters and modulate the activity of other neurons, ultimately controlling behavioral responses. In this review, we will describe the known behavioral implications of astroglial cannabinoid signaling and highlight exciting unexplored research avenues on how astroglial cannabinoid signaling could affect behavior.

Novel Therapeutic Target for Prevention of Neurodegenerative Diseases: Modulation of Neuroinflammation with Sig-1R Ligands

Neurodegenerative diseases (NDDs) are characterized by progressive deterioration of the structure and function of cells and their networks in the nervous system. There are currently no drugs or other treatments that can stop the progression of NDDs. NDDs have many similarities and common pathways, e.g., formation of misfolded amyloid proteins, intra- and extracellular amyloid deposits, and chronic inflammation. Initially, the inflammation process has a cytoprotective function; however, an elevated and prolonged immune response has damaging effects and causes cell death. Neuroinflammation has been a target of drug development for treating and curing NDDs. Treatment of different NDDs with non-steroid anti-inflammatory drugs (NSAIDs) has failed or has given inconsistent results. The use of NSAIDs in diagnosed Alzheimer's disease is currently not recommended. Sigma-1 receptor (Sig-1R) is a novel target for NDD drug development. Sig-1R plays a key role in cellular stress signaling, and it regulates endoplasmic reticulum stress and unfolded protein response. Activation of Sig-1R provides neuroprotection in cell cultures and animal studies. Clinical trials demonstrated that several Sig-1R agonists (pridopidine, ANAVEX3-71, fluvoxamine, dextrometorphan) and their combinations have a neuroprotective effect and slow down the progression of distinct NDDs.