Microglia as possible therapeutic targets for autism spectrum disorders

A comparison between children and adolescents with autism spectrum disorders and healthy controls in biomedical factors, trace elements, and microbiota biomarkers: a meta-analysis

Introduction

Autism spectrum disorder (ASD) is a multifaceted developmental condition that commonly appears during early childhood. The etiology of ASD remains multifactorial and not yet fully understood. The identification of biomarkers may provide insights into the underlying mechanisms and pathophysiology of the disorder. The present study aimed to explore the causes of ASD by investigating the key biomedical markers, trace elements, and microbiota factors between children with autism spectrum disorder (ASD) and control subjects.

Methods

Medline, PubMed, ProQuest, EMBASE, Cochrane Library, PsycINFO, Web of Science, and EMBSCO databases have been searched for publications from 2012 to 2023 with no language restrictions using the population, intervention, control, and outcome (PICO) approach. Keywords including "autism spectrum disorder," "oxytocin," "GABA," "Serotonin," "CRP," "IL-6," "Fe," "Zn," "Cu," and "gut microbiota" were used for the search. The Joanna Briggs Institute (JBI) critical appraisal checklist was used to assess the article quality, and a random model was used to assess the mean difference and standardized difference between ASD and the control group in all biomedical markers, trace elements, and microbiota factors.

Results

From 76,217 records, 43 studies met the inclusion and exclusion criteria and were included in this meta-analysis. The pooled analyses showed that children with ASD had significantly lower levels of oxytocin (mean differences, MD = -45.691, 95% confidence interval, CI: -61.667, -29.717), iron (MD = -3.203, 95% CI: -4.891, -1.514), and zinc (MD = -6.707, 95% CI: -12.691, -0.722), lower relative abundance of Bifidobacterium (MD = -1.321, 95% CI: -2.403, -0.238) and Parabacteroides (MD = -0.081, 95% CI: -0.148, -0.013), higher levels of c-reactive protein, CRP (MD = 0.401, 95% CI: 0.036, 0.772), and GABA (MD = 0.115, 95% CI: 0.045, 0.186), and higher relative abundance of Bacteroides (MD = 1.386, 95% CI: 0.717, 2.055) and Clostridium (MD = 0.281, 95% CI: 0.035, 0.526) when compared with controls. The results of the overall analyses were stable after performing the sensitivity analyses. Additionally, no substantial publication bias was observed among the studies.

Interpretation

Children with ASD have significantly higher levels of CRP and GABA, lower levels of oxytocin, iron, and zinc, lower relative abundance of Bifidobacterium and Parabacteroides, and higher relative abundance of Faecalibacterium, Bacteroides, and Clostridium when compared with controls. These results suggest that these indicators may be a potential biomarker panel for the diagnosis or determining therapeutic targets of ASD. Furthermore, large, sample-based, and randomized controlled trials are needed to confirm these results.

Glial-mediated dysregulation of neurodevelopment in Fragile X Syndrome

Astrocytes are highly involved in a multitude of developmental processes that are known to be dysregulated in Fragile X Syndrome. Here, we examine these processes individually and review the roles astrocytes play in contributing to the pathology of this syndrome. As a growing area of interest in the field, new and exciting insight is continually emerging. Understanding these glial-mediated roles is imperative for elucidating the underlying molecular mechanisms at play, not only in Fragile X Syndrome, but also other ASD-related disorders. Understanding these roles will be central to the future development of effective, clinically-relevant treatments of these disorders.

Minocycline improves autism-related behaviors by modulating microglia polarization in a mouse model of autism

Autism spectrum disorder (ASD) is a heterogeneous neurodevelopmental disorder with few pharmacological treatments. Minocycline, a tetracycline derivative that inhibits microglial activation, has been well-identified with anti-inflammatory properties and neuroprotective effects. A growing body of research suggests that ASD is associated with neuroinflammation, abnormal neurotransmitter levels, and neurogenesis. Thus, we hypothesized that minocycline could improve autism-related behaviors by inhibiting microglia activation and altering neuroinflammation. To verify our hypothesis, we used a mouse model of autism, BTBR T + Itpr3tf/J (BTBR). As expected, minocycline administration rescued the sociability and repetitive, stereotyped behaviors of BTBR mice while having no effect in C57BL/6J mice. We also found that minocycline improved neurogenesis and inhibited microglia activation in the hippocampus of BTBR mice. In addition, minocycline treatment inhibited Erk1/2 phosphorylation in the hippocampus of BTBR mice. Our findings show that minocycline administration alleviates ASD-like behaviors in BTBR mice and improves neurogenesis, suggesting that minocycline supplementation might be a potential strategy for improving ASD symptoms.

Identification of target hub genes and construction of a novel miRNA regulatory network in autism spectrum disorder by integrated analysis

The incidence of autism spectrum disorder (ASD) is increasing year by year in children. The aim of the study was to find possible biomarkers for ASD diagnosis as well as examine MicroRNA (miRNA) signatures and crucial pathways. We conducted a two-stage study to explore potential target genes and functional miRNAs. Peripheral blood samples of children with ASD were enrolled and performed RNA sequencing analysis. The overlapped candidate genes were further screened in combination with differentially expressed genes (DEGs) of GSE77103 datasets. STRING established a protein-protein interaction network comprising DEGs. The hub genes were filtered out using the CytoHubba. Then, we set up a miRNA-mRNA regulatory network. Correlational analyses between hub genes and immune cells associated with ASD were carried out using the CIBERSORT software to assess the diversity of immune cell types in ASD. RNA-sequencing analysis was used to confirm the differential expression of 3 hub genes. Briefly, after blood samples were sequenced interrogating 867 differential genes in our internal screening dataset. After screening GEO databases, 551 DEGs obtained from GSE77103. Fourteen common genes were overlapped through DEGs of GEO datasets and internal screening dataset. Among protein-protein interaction network, 10 hub genes with high degree algorithm were screened out and 3 hub genes of them - ADIPOR1, LGALS3, and GZMB - that were thought to be most associated with the emergence of ASD. Then, we developed a network of miRNA-mRNA regulatory interactions by screening miRNAs (such as hsa-miR-20b-5p, hsa-miR-17-5p, and hsa-miR-216b-5p) that were closely associated to 3 hub genes. Additionally, we discovered 18 different immune cell types associated with ASD using the CIBERSORT algorithm, and we discovered that mononuclear macrophages differed considerably between the 2 groups. Overall, 3 hub genes (ADIPOR1, LGALS3, and GZMB) and 15 candidates miRNAs-target 3 genes regulatory pathways representing potentially novel biomarkers of ASD diseases were revealed. These findings could enhance our knowledge of ASD and offer possible therapeutic targets of ASD patients in the future.

Roles of the Notch signaling pathway and microglia in autism

The Notch signaling pathway is mainly involved in the regulation of neural stem cell proliferation, survival and differentiation during the development of the central nervous system. As a neurodevelopmental disorder, autism is associated with an abnormal increase in the number of microglia in several brain regions. These findings suggest that the pathogenesis of autism may be related to the Notch signaling pathway and microglia. In this review, we discuss how Notch pathway activity leads to behavioral abnormalities such as learning and memory impairment by influencing neuronal biological activities. An increase in microglial protein synthesis and abnormal autophagy can affect synaptic development and lead to behavioral abnormalities, and all of these changes can lead to autism. Furthermore, the Notch signaling pathway regulates the activation and differentiation of microglia and promotes inflammatory responses, leading to the occurrence of autism. When excessive reactive oxygen species (ROS) secreted by microglia cannot be cleared by autophagy in a timely manner, Notch signaling pathway activity is affected, possibly further increasing susceptibility to autism. This review reveals the mechanism underlying the role of the Notch signaling pathway, microglia and their interaction in the pathogenesis of autism and provides a theoretical reference for targeted clinical therapies for autism.

Effects of different doses of lithium on the central nervous system in the rat valproic acid model of autism

Epidemiological studies have shown that low doses of lithium in the environment can have beneficial effects on mental health. Autism spectrum disorder, a neurodevelopmental disorder in which patients exhibit abnormal behaviors, pharmacological interventions usually relied on a range of psychotropic medications. However, such medications often produce severe side effects or are ineffective in symptoms. Finding alternative ways to improve abnormal behaviors in individuals with autism are warranted, in which case lithium may be a relatively safe and effective medication. Lithium salt therapy is used to treat a variety of neuropsychiatric disorders and has neuroprotective effects. In this study, we investigated the effects of different doses of lithium on neurobehavioural disorders using the rat model of autism established by valproic acid (VPA) injection. Lithium was observed to have an ameliorative effect on the social cognitive, social memory and anxiety levels in the rat model of autism. Immunofluorescence staining showed that subchronic LiCl administration (1.0 mmol/kg) significantly reduced the number of Iba-1 positive cells in the CA1 region of the hippocampus in VPA group and brought it close to the levels of control group. Significantly lower levels of the pro-inflammatory marker IL-6 were observed in the hippocampus and serum after lithium treatment. In addition, the lithium treatment increased the levels of H3K9 acetylation in the hippocampus of VPA-exposed rats. The results showed a defensive effect of environment-related lithium exposure doses on neurobehavioural deficits in the rat valproic acid model of autism, suggesting that it may be a potential drug for the treatment of autism.

SCGN deficiency is a risk factor for autism spectrum disorder

Autism spectrum disorder (ASD) affects 1-2% of all children and poses a great social and economic challenge for the globe. As a highly heterogeneous neurodevelopmental disorder, the development of its treatment is extremely challenging. Multiple pathways have been linked to the pathogenesis of ASD, including signaling involved in synaptic function, oxytocinergic activities, immune homeostasis, chromatin modifications, and mitochondrial functions. Here, we identify secretagogin (SCGN), a regulator of synaptic transmission, as a new risk gene for ASD. Two heterozygous loss-of-function mutations in SCGN are presented in ASD probands. Deletion of Scgn in zebrafish or mice leads to autism-like behaviors and impairs brain development. Mechanistically, Scgn deficiency disrupts the oxytocin signaling and abnormally activates inflammation in both animal models. Both ASD probands carrying Scgn mutations also show reduced oxytocin levels. Importantly, we demonstrate that the administration of oxytocin and anti-inflammatory drugs can attenuate ASD-associated defects caused by SCGN deficiency. Altogether, we identify a convergence between a potential autism genetic risk factor SCGN, and the pathological deregulation in oxytocinergic signaling and immune responses, providing potential treatment for ASD patients suffering from SCGN deficiency. Our study also indicates that it is critical to identify and stratify ASD patient populations based on their disease mechanisms, which could greatly enhance therapeutic success.

Microglia and astrocytes underlie neuroinflammation and synaptic susceptibility in autism spectrum disorder

Autism spectrum disorder (ASD) is a common neurodevelopmental disorder with onset in childhood. The mechanisms underlying ASD are unclear. In recent years, the role of microglia and astrocytes in ASD has received increasing attention. Microglia prune the synapses or respond to injury by sequestrating the injury site and expressing inflammatory cytokines. Astrocytes maintain homeostasis in the brain microenvironment through the uptake of ions and neurotransmitters. However, the molecular link between ASD and microglia and, or astrocytes remains unknown. Previous research has shown the significant role of microglia and astrocytes in ASD, with reports of increased numbers of reactive microglia and astrocytes in postmortem tissues and animal models of ASD. Therefore, an enhanced understanding of the roles of microglia and astrocytes in ASD is essential for developing effective therapies. This review aimed to summarize the functions of microglia and astrocytes and their contributions to ASD.

Modelling the Interplay Between Neuron-Glia Cell Dysfunction and Glial Therapy in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a complicated, interpersonally defined, static condition of the underdeveloped brain. Although the aetiology of autism remains unclear, disturbance of neuronglia interactions has lately been proposed as a significant event in the pathophysiology of ASD. In recent years, the contribution of glial cells to autism has been overlooked. In addition to neurons, glial cells play an essential role in mental activities, and a new strategy that emphasises neuron-glia interactions should be applied. Disturbance of neuron-glia connections has lately been proposed as a significant event in the pathophysiology of ASD because aberrant neuronal network formation and dysfunctional neurotransmission are fundamental to the pathology of the condition. In ASD, neuron and glial cell number changes cause brain circuits to malfunction and impact behaviour. A study revealed that reactive glial cells result in the loss of synaptic functioning and induce autism under inflammatory conditions. Recent discoveries also suggest that dysfunction or changes in the ability of microglia to carry out physiological and defensive functions (such as failure in synaptic elimination or aberrant microglial activation) may be crucial for developing brain diseases, especially autism. The cerebellum, white matter, and cortical regions of autistic patients showed significant microglial activation. Reactive glial cells result in the loss of synaptic functioning and induce autism under inflammatory conditions. Replacement of defective glial cells (Cell-replacement treatment), glial progenitor cell-based therapy, and medication therapy (inhibition of microglia activation) are all utilised to treat glial dysfunction. This review discusses the role of glial cells in ASD and the various potential approaches to treating glial cell dysfunction.

Genomic Strategies for Understanding the Pathophysiology of Autism Spectrum Disorder

Recent breakthroughs in sequencing technology and technological developments have made it easier to analyze the entire human genome than ever before. In addition to disease-specific genetic mutations and chromosomal aberrations, epigenetic alterations in individuals can also be analyzed using genomics. Autism spectrum disorder (ASD) is a neurodevelopmental disorder (NDD) caused by genetic and/or environmental factors. More than a thousand genes associated with ASD have been identified which are known to be involved in brain development. However, it is difficult to decode the roles of ASD-associated genes without in vitro and in vivo validations, particularly in the process of brain development. In this review, we discuss genomic strategies for understanding the pathological mechanisms underlying ASD. For this purpose, we discuss ASD-associated genes and their functions, as well as analytical strategies and their strengths and weaknesses in cellular and animal models from a basic research perspective.

Synapse Maturation and Developmental Impairment in the Medial Nucleus of the Trapezoid Body

Sound localization requires rapid interpretation of signal speed, intensity, and frequency. Precise neurotransmission of auditory signals relies on specialized auditory brainstem synapses including the calyx of Held, the large encapsulating input to principal neurons in the medial nucleus of the trapezoid body (MNTB). During development, synapses in the MNTB are established, eliminated, and strengthened, thereby forming an excitatory/inhibitory (E/I) synapse profile. However, in neurodevelopmental disorders such as autism spectrum disorder (ASD), E/I neurotransmission is altered, and auditory phenotypes emerge anatomically, molecularly, and functionally. Here we review factors required for normal synapse development in this auditory brainstem pathway and discuss how it is affected by mutations in ASD-linked genes.

Alpha-Synuclein, cyclooxygenase-2 and prostaglandins-EP2 receptors as neuroinflammatory biomarkers of autism spectrum disorders: Use of combined ROC curves to increase their diagnostic values

BACKGROUND

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by impairments in social interaction and restricted and repetitive behaviors. Neuroinflammation and abnormal lipid mediators have been identified in multiple investigations as an acknowledged etiological mechanism of ASD that can be targeted for therapeutic intervention.

METHODS

In this study, multiple regression and combined receiver operating characteristic (ROC) curve analyses were used to determine the relationship between the neuroinflammatory marker α-synuclein and lipid mediator markers related to inflammation induction, such as cyclooxygenase-2 and prostaglandin-EP2 receptors, in the etiology of ASD. Additionally, the study aimed to determine the linear combination that maximizes the partial area under ROC curves for a set of markers. Forty children with ASD and 40 age- and sex-matched controls were enrolled in the study. Using ELISA, the levels of α-synuclein, cyclo-oxygenase-2, and prostaglandin-EP2 receptors were measured in the plasma of both groups. Statistical analyses using ROC curves and multiple and logistic regression models were performed.

RESULTS

A remarkable increase in the area under the curve was observed using combined ROC curve analyses. Moreover, higher specificity and sensitivity of the combined markers were reported.

CONCLUSIONS

The present study indicates that measurement of the predictive value of selected biomarkers related to neuroinflammation and lipid metabolism in children with ASD using a ROC curve analysis should lead to a better understanding of the etiological mechanism of ASD and its link with metabolism. This information may facilitate early diagnosis and intervention.

Biological implications of genetic variations in autism spectrum disorders from genomics studies

Autism spectrum disorder (ASD) is a highly heterogeneous neurodevelopmental condition characterized by atypical social interaction and communication together with repetitive behaviors and restricted interests. The prevalence of ASD has been increased these years. Compelling evidence has shown that genetic factors contribute largely to the development of ASD. However, knowledge about its genetic etiology and pathogenesis is limited. Broad applications of genomics studies have revealed the importance of gene mutations at protein-coding regions as well as the interrupted non-coding regions in the development of ASD. In this review, we summarize the current evidence for the known molecular genetic basis and possible pathological mechanisms as well as the risk genes and loci of ASD. Functional studies for the underlying mechanisms are also implicated. The understanding of the genetics and genomics of ASD is important for the genetic diagnosis and intervention for this condition.

Microglia regulate synaptic development and plasticity

Synapses are fundamental structures of neural circuits that transmit information between neurons. Thus, the process of neural circuit formation via proper synaptic connections shapes the basis of brain functions and animal behavior. Synapses continuously undergo repeated formation and elimination throughout the lifetime of an organism, reflecting the dynamics of neural circuit function. The structural transformation of synapses has been described mainly in relation to neural activity-dependent strengthening and weakening of synaptic functions, that is, functional plasticity of synapses. An increasing number of studies have unveiled the roles of microglia, brain-resident immune cells that survey the brain parenchyma with highly motile processes, in synapse formation and elimination as well as in regulating synaptic function. Over the past 15 years, the molecular mechanisms underlying microglia-dependent regulation of synaptic plasticity have been thoroughly studied, and researchers have reported that the disruption of microglia-dependent regulation causes synaptic dysfunction that leads to brain diseases. In this review, we will broadly introduce studies that report the roles of microglia in synaptic plasticity and the possible underlying molecular mechanisms.

Synaptic elimination by microglia and disturbed higher brain functions

Microglial cells in normal mature brains have long been considered to be cells that are resting until pathological events take place, activating the microglial cells. However, it is currently well known that the microglia that have resting ramified morphology in normal mature brains move actively in the brain parenchyma and phagocytose synapses, thus forming and maintaining neural circuits. This review summarizes recent findings on the roles of microglia in mature brains, with special reference to phagocytosis of synapses and higher brain functions. Phagocytic elimination of synapses by microglia may affect the balance between excitatory and inhibitory synaptic transmission, termed the E/I balance. When impaired synaptic elimination by microglia leads to disturbed E/I balance, various problems may follow in brain functions: in memory and cognitive functions, sleep, movement, social behaviors, and thinking. In addition to the roles of microglia in normal developing and mature brains, impaired microglial phagocytosis functions also correlate with disturbances to these higher brain functions that are caused by neurological, mental, and developmental disorders; Parkinson's and Alzheimer's diseases, autism spectrum disorder, attention deficit/hyperactivity disorder, and schizophrenia.

Microglia in animal models of autism spectrum disorders

Various genetic and environmental factors have been suggested to cause autism spectrum disorders (ASDs). A variety of animal models of ASDs have been developed and used to investigate the mechanisms underlying the pathogenesis of ASDs. These animal models have contributed to clarifying that abnormalities in neuronal morphology and neurotransmission are responsible for the onset of ASDs. In recent years, researchers have started to focus not only on neurons but also on glial cells, particularly microglia. This is because microglial malfunction is strongly associated with structural and functional abnormalities of neurons, as well as the inflammation that is commonly observed both in the brains of patients with ASDs and in animal models of ASDs. In this chapter, we first introduce a list of commonly available animal models of ASDs and describe the validity of each model from the viewpoint of behaviors and neuroanatomy. We next detail the malfunction of microglia that has been reported in animal models of ASDs and discuss the roles of microglia in ASD pathogenesis. We will further propose possible therapeutic strategies to tackle ASDs by controlling microglial functions.