Is there sexual dimorphism of hyperserotonemia in autism spectrum disorder?

A Systematic Review on Autism and Hyperserotonemia: State-of-the-Art, Limitations, and Future Directions

Hyperserotonemia is one of the most studied endophenotypes in autism spectrum disorder (ASD), but there are still no unequivocal results about its causes or biological and behavioral outcomes. This systematic review summarizes the studies investigating the relationship between blood serotonin (5-HT) levels and ASD, comparing diagnostic tools, analytical methods, and clinical outcomes. A literature search on peripheral 5-HT levels and ASD was conducted. In total, 1104 publications were screened, of which 113 entered the present systematic review. Of these, 59 articles reported hyperserotonemia in subjects with ASD, and 26 presented correlations between 5-HT levels and ASD-core clinical outcomes. The 5-HT levels are increased in about half, and correlations between hyperserotonemia and clinical outcomes are detected in a quarter of the studies. The present research highlights a large amount of heterogeneity in this field, ranging from the characterization of ASD and control groups to diagnostic and clinical assessments, from blood sampling procedures to analytical methods, allowing us to delineate critical topics for future studies.

Contribution of the serotonergic system to developmental brain abnormalities in autism spectrum disorder

This review highlights a key role of the serotonergic system in brain development and in distortions of normal brain development in early stages of fetal life resulting in cascades of abnormalities, including defects of neurogenesis, neuronal migration, neuronal growth, differentiation, and arborization, as well as defective neuronal circuit formation in the cortex, subcortical structures, brainstem, and cerebellum of autistic subjects. In autism, defects in regulation of neuronal growth are the most frequent and ubiquitous developmental changes associated with impaired neuron differentiation, smaller size, distorted shape, loss of spatial orientation, and distortion of cortex organization. Common developmental defects of the brain in autism include multiregional focal dysplastic changes contributing to local neuronal circuit distortion, epileptogenic activity, and epilepsy. There is a discrepancy between more than 500 reports demonstrating the contribution of the serotonergic system to autism's behavioral anomalies, highlighted by lack of studies of autistic subjects' brainstem raphe nuclei, the center of brain serotonergic innervation, and of the contribution of the serotonergic system to the diagnostic features of autism spectrum disorder (ASD). Discovery of severe fetal brainstem auditory system neuronal deficits and other anomalies leading to a spectrum of hearing deficits contributing to a cascade of behavioral alterations, including deficits of social and verbal communication in individuals with autism, is another argument to intensify postmortem studies of the type and topography of, and the severity of developmental defects in raphe nuclei and their contribution to abnormal brain development and to the broad spectrum of functional deficits and comorbid conditions in ASD.

Candidate biomarkers in psychiatric disorders: state of the field

The field of psychiatry is hampered by a lack of robust, reliable and valid biomarkers that can aid in objectively diagnosing patients and providing individualized treatment recommendations. Here we review and critically evaluate the evidence for the most promising biomarkers in the psychiatric neuroscience literature for autism spectrum disorder, schizophrenia, anxiety disorders and post-traumatic stress disorder, major depression and bipolar disorder, and substance use disorders. Candidate biomarkers reviewed include various neuroimaging, genetic, molecular and peripheral assays, for the purposes of determining susceptibility or presence of illness, and predicting treatment response or safety. This review highlights a critical gap in the biomarker validation process. An enormous societal investment over the past 50 years has identified numerous candidate biomarkers. However, to date, the overwhelming majority of these measures have not been proven sufficiently reliable, valid and useful to be adopted clinically. It is time to consider whether strategic investments might break this impasse, focusing on a limited number of promising candidates to advance through a process of definitive testing for a specific indication. Some promising candidates for definitive testing include the N170 signal, an event-related brain potential measured using electroencephalography, for subgroup identification within autism spectrum disorder; striatal resting-state functional magnetic resonance imaging (fMRI) measures, such as the striatal connectivity index (SCI) and the functional striatal abnormalities (FSA) index, for prediction of treatment response in schizophrenia; error-related negativity (ERN), an electrophysiological index, for prediction of first onset of generalized anxiety disorder, and resting-state and structural brain connectomic measures for prediction of treatment response in social anxiety disorder. Alternate forms of classification may be useful for conceptualizing and testing potential biomarkers. Collaborative efforts allowing the inclusion of biosystems beyond genetics and neuroimaging are needed, and online remote acquisition of selected measures in a naturalistic setting using mobile health tools may significantly advance the field. Setting specific benchmarks for well-defined target application, along with development of appropriate funding and partnership mechanisms, would also be crucial. Finally, it should never be forgotten that, for a biomarker to be actionable, it will need to be clinically predictive at the individual level and viable in clinical settings.

Sex Differences in Tryptophan Metabolism: A Systematic Review Focused on Neuropsychiatric Disorders

Tryptophan (Tryp) is an essential amino acid and the precursor of several neuroactive compounds within the central nervous system (CNS). Tryp metabolism, the common denominator linking serotonin (5-HT) dysfunctions and neuroinflammation, is involved in several neuropsychiatric conditions, including neurological, neurodevelopmental, neurodegenerative, and psychiatric diseases. Interestingly, most of those conditions occur and progress in a sex-specific manner. Here, we explore the most relevant observations about the influence of biological sex on Tryp metabolism and its possible relation to neuropsychiatric diseases. Consistent evidence suggests that women have a higher susceptibility than men to suffer serotoninergic alterations due to changes in the levels of its precursor Tryp. Indeed, female sex bias in neuropsychiatric diseases is involved in a reduced availability of this amino acid pool and 5-HT synthesis. These changes in Tryp metabolism could lead to sexual dimorphism on the prevalence and severity of some neuropsychiatric disorders. This review identifies gaps in the current state of the art, thus suggesting future research directions. Specifically, there is a need for further research on the impact of diet and sex steroids, both involved in this molecular mechanism as they have been poorly addressed for this topic.

Intestinal Predictors of Whole Blood Serotonin Levels in Children With or Without Autism

Hyperserotonemia, or elevated levels of whole blood serotonin (WB5-HT), was the first biomarker linked to autism spectrum disorder (ASD). Despite numerous studies investigating the etiology of hyperserotonemia, results have been inconsistent. Recent findings suggest a relationship between the immune system and hyperserotonemia. The current study investigated whether intestinal 5-HT levels, 5-HT gene expression, or intestinal cell types predict WB5-HT. Participants included thirty-one males aged 3-18 who were classified into one of three groups: ASD and functional GI issues, typically developing with GI issues, and typically developing without GI issues. Samples from a lower endoscopy were analyzed to examine the pathways in predicting WB-5HT. Results demonstrated an association between T-Lymphocytes and WB5-HT.

Rare Opportunities for Insights Into Serotonergic Contributions to Brain and Bowel Disorders: Studies of the SERT Ala56 Mouse

Altered structure, expression, and regulation of the presynaptic serotonin (5-HT) transporter (SERT) have been associated with multiple neurobehavioral disorders, including mood disorders, obsessive-compulsive disorder (OCD), and autism spectrum disorder (ASD). Opportunities to investigate mechanistic links supporting these associations were spurred with the identification of multiple, rare human SERT coding variants in a study that established a male-specific linkage of ASD to a linkage marker on chromosome 17 which encompassed the location of the SERT gene (SLC6A4). We have explored the most common of these variants, SERT Ala56, in vitro and in vivo. Results support a tonic elevation of 5-HT transport activity in transfected cells and human lymphoblasts by the variant in vitro that leads to an increased 5-HT clearance rate in vivo when studied in the SERT Ala56 mouse model, along with altered sensitivity to SERT regulatory signaling pathways. Importantly, hyperserotonemia, or an elevated whole blood 5-HT, level, was found in SERT Ala56 mice, reproducing a well-replicated trait observed in a significant fraction of ASD subjects. Additionally, we found multiple biochemical, physiological, and behavioral alterations in the SERT Ala56 mice that can be analogized to those observed in ASD and its medical comorbidities. The similarity of the functional impact of the SERT Ala56 variant to the consequences of p38α MAPK activation, ascribed to the induction of a biased conformation of the transporter toward an outward-facing conformation, has resulted in successful efforts to restore normal behavioral and bowel function via pharmacological and genetic p38α MAPK targeting. Moreover, the ability of the inflammatory cytokine IL-1β to enhance SERT activity via a p38α MAPK-dependent pathway suggests that the SERT Ala56 conformation mimics that of a chronic inflammatory state, supporting findings in ASD of elevated inflammatory cytokine levels. In this report, we review studies of the SERT Ala56 variant, discussing opportunities for continued insight into how chronically altered synaptic 5-HT homeostasis can drive reversible, functional perturbations in 5-HT sensitive pathways in the brain and periphery, and how targeting the SERT regulome, particularly through activating pathways such as those involving IL-1β/p38α MAPK, may be of benefit for neurobehavioral disorders, including ASD.

Quantitative and Qualitative Sex Modulations in the Brain Anatomy of Autism

BACKGROUND

Sex-based neurobiological heterogeneity in autism is poorly understood. Research is disproportionately biased to males, leading to an unwarranted presumption that autism neurobiology is the same across sexes. Previous neuroimaging studies using amalgamated multicenter datasets to increase autistic female samples are characterized by large statistical noise.

METHODS

We used a better-powered dataset of 1183 scans of 839 individuals-299 (467 scans) autistic males, 74 (102 scans) autistic females, 240 (334 scans) control males, and 226 (280 scans) control females-to test two whole-brain models of overall/global sex modulations on autism neuroanatomy, by summary measures computed across the brain: the local magnitude model, in which the same brain regions/circuitries are involved across sexes but effect sizes are larger in females, indicating quantitative sex modulation; and spatial dissimilarity model, in which the neuroanatomy differs spatially between sexes, indicating qualitative sex modulation. The male and female autism groups were matched on age, IQ, and autism symptoms. Autism brain features were defined by comparisons with same-sex control individuals.

RESULTS

Across five metrics (cortical thickness, surface area, volume, mean absolute curvature, and subcortical volume), we found no evidence supporting the local magnitude model. We found indicators supporting the spatial dissimilarity model on cortical mean absolute curvature and subcortical volume, but not on other metrics.

CONCLUSIONS

The overall/global autism neuroanatomy in females and males does not simply differ quantitatively in the same brain regions/circuitries. They may differ qualitatively in spatial involvement in cortical curvature and subcortical volume. The neuroanatomy of autism may be partly sex specific. Sex stratification to inform autism preclinical/clinical research is needed to identify sex-informed neurodevelopmental targets.

A Novel and Reliable Rat Model of Autism

Background: Autism spectrum disorders (ASD) is a complex neurodevelopmental disorder that lacks an ideal animal model to recapitulate the disease state of ASD. Previous studies have reported that transplanting gut microbiota of ASD patients into pregnant mice is sufficient to promote the changes of autism-like behavior in offspring. This study aims to explore whether fecal microbiota transplantation (FMT) can be used as a new method to establish the ASD animal model. Methods: We transplanted the fecal sample extract of ASD children into pregnant rats (rFMT) repeatedly to establish an ASD rat model (oFMT) and compare it with the classical valproic acid (VPA) model (oVPA). Results: First, we reveal that oFMT shows hypoevolutism and typical behavioral characteristics of ASD, consistent with the previous study. Second, the gut microbiota of oFMT mainly consists of Firmicutes and Bacteroidetes, recapitulating the abnormal gut microbiota of ASD. In oFMT, the abundance of Lactobacillus and Collinsella increased (Lactobacillus: oFMT 60.16%, oVPA 64.13%, oCON 40.11%; Collinsella: oFMT 3.73%, oVPA 1.39%, oCON 1.28%), compared with oVPA, gut microbiota also showed high consistency. Third, the expression of 5-hydroxytryptamine (5-HT) in oFMT serum increased, γ-aminobutyric acid (GABA) and norepinephrine (NE) in oFMT serum decreased. Fourth, the gut microbiota of oFMT also has some ASD characteristic gut microbiota not found in oVPA. Fifth, pregnant rat with VPA showed significant immune activation, while those with FMT showed relatively minor immune activation. Limitations: Although the mechanism of establishing FMT autism rat model (oFMT) has not clearly defined, the data show that the model has high structural validity, and FMT model is likely to be a new and reliable potential animal model of ASD, and will have potential value in studying gut microbiota of ASD. Conclusions: The FMT autism rat model has high structural validity, and the FMT model is likely to be a new and reliable potential animal model of ASD.

Blood platelet research in autism spectrum disorders: In search of biomarkers

Autism spectrum disorder (ASD) is a clinically heterogeneous neurodevelopmental disorder that is caused by gene-environment interactions. To improve its diagnosis and treatment, numerous efforts have been undertaken to identify reliable biomarkers for autism. None of them have delivered the holy grail that represents a reproducible, quantifiable, and sensitive biomarker. Though blood platelets are mainly known to prevent bleeding, they also play pivotal roles in cancer, inflammation, and neurological disorders. Platelets could serve as a peripheral biomarker or cellular model for autism as they share common biological and molecular characteristics with neurons. In particular, platelet-dense granules contain neurotransmitters such as serotonin and gamma-aminobutyric acid. Molecular players controlling granule formation and secretion are similarly regulated in platelets and neurons. The major platelet integrin receptor αIIbβ3 has recently been linked to ASD as a regulator of serotonin transport. Though many studies revealed associations between platelet markers and ASD, there is an important knowledge gap in linking these markers with autism and explaining the altered platelet phenotypes detected in autism patients. The present review enumerates studies of different biomarkers detected in ASD using platelets and highlights the future needs to bring this research to the next level and advance our understanding of this complex disorder.

Whole Blood Serotonin Levels and Platelet 5-HT2A Binding in Autism Spectrum Disorder

Elevated whole blood serotonin (WB5-HT) is a well-replicated biomarker in autism spectrum disorder (ASD). Decreased platelet serotonin receptor 5-HT_2A binding has been reported in ASD. WB5-HT levels and platelet 5-HT_2A specific binding were obtained from 110 individuals with ASD and 18 controls. Individuals with ASD had significantly higher WB5-HT levels than controls. There was no difference in the platelet 5-HT_2A specific binding between groups. Multiple regression analyses revealed that platelet 5-HT_2A binding significantly predicted WB5-HT in the control sample but not in the ASD sample. These results indicate that the relationship between WB5-HT and platelet 5-HT_2A binding differs depending on ASD diagnosis, suggesting differences in platelet 5-HT system regulation in ASD.

Effects of Diagnostic Severity upon Sex Differences in Behavioural Profiles of Young Males and Females with Autism Spectrum Disorder

To determine if diagnostic severity of Autistic Spectrum Disorder (ASD) affected sex differences in the detailed ASD-related behavioural profiles of young males and female, two samples of males and females with a diagnosis of ASD, aged between 6 and 17 years, were compared across the 65 items of the Social Responsiveness Scale (2nd ed.). Results are reported for a sample of males and females matched on age and IQ (n = 51 pairs) and a smaller sample matched on age, IQ and ADOS-2 severity (n = 32 pairs). ASD-related behaviours from the SRS-2 that were significantly and meaningfully different across sexes were identified for both samples. ADOS-2 diagnostic severity was associated with different sets of sex-based differences in SRS-2 item scores.

Astrogliosis Associated With Behavioral Abnormality in a Non-anaphylactic Mouse Model of Cow's Milk Allergy

Etiology of neuropsychiatric disorders is complex, involving multiple factors that can affect the type and severity of symptoms. Although precise causes are far from being identified, allergy or other forms of hypersensitivity to dietary ingredients have been implicated in triggering or worsening of behavioral and emotional symptoms, especially in patients suffering from depression, anxiety, attention-deficit hyperactivity, and/or autism. Among such ingredients, cow's milk, along with wheat gluten, is commonly suspected. However, the contributory role of cow's milk in these disorders has not been elucidated due to insufficient pathophysiological evidence. In the present study, we therefore investigated neuroinflammatory changes that are associated with behavioral abnormality using a non-anaphylactic mouse model of cow's milk allergy (CMA). Male and female C57BL/6J mice were subjected to a 5-week oral sensitization procedure without or with a major milk allergen, beta-lactoglobulin (BLG). All mice were then later challenged with BLG, and their anxiety- and depression-associated behaviors were subsequently assessed during the 6th and 7th weeks. We found that BLG-sensitized male mice exhibited significantly increased anxiety- and depression-like behavior, although they did not display anaphylactic reactions when challenged with BLG. Female behavior was not noticeably affected by BLG sensitization. Upon examination of the small intestines, reduced immunoreactivity to occludin was detected in the ileal mucosa of BLG-sensitized mice although the transcriptional expression of this tight-junction protein was not significantly altered when measured by quantitative RT-PCR. On the other hand, the expression of tumor necrosis factor alpha (TNFα) in the ileal mucosa was significantly elevated in BLG-sensitized mice, suggesting the sensitization had resulted in intestinal inflammation. Inflammatory responses were also detected in the brain of BLG-sensitized mice, determined by the hypertrophic morphology of GFAP-immunoreactive astrocytes. These reactive astrocytes were particularly evident near the blood vessels in the midbrain region, resembling the perivascular barrier previously reported by others in experimental autoimmune encephalitis (EAE) mouse models. Interestingly, increased levels of COX-2 and TNFα were also found in this region. Taken together, our results demonstrated that BLG sensitization elicits inflammatory responses in the intestine and brain without overt anaphylactic signs of milk allergy, signifying food allergy as a potential pathogenic factor of neuropsychiatric disorders.

Extreme enhancement or depletion of serotonin transporter function and serotonin availability in autism spectrum disorder

A variety of human and animal studies support the hypothesis that serotonin (5-hydroxytryptamine or 5-HT) system dysfunction is a contributing factor to the development of autism in some patients. However, many questions remain about how developmental manipulation of various components that influence 5-HT signaling (5-HT synthesis, transport, metabolism) persistently impair social behaviors. This review will summarize key aspects of central 5-HT function important for normal brain development, and review evidence implicating perinatal disruptions in 5-HT signaling in the pathophysiology of autism spectrum disorder. We discuss the importance, and relative dearth, of studies that explore the possible correlation to autism in the interactions between important intrinsic and extrinsic factors that may disrupt 5-HT homeostasis during development. In particular, we focus on exposure to 5-HT transport altering mechanisms such as selective serotonin-reuptake inhibitors or genetic polymorphisms in primary or auxiliary transporters of 5-HT, and how they relate to neurological stores of serotonin and its precursors. A deeper understanding of the many mechanisms by which 5-HT signaling can be disrupted, alone and in concert, may contribute to an improved understanding of the etiologies and heterogeneous nature of this disorder. We postulate that extreme bidirectional perturbations of these factors during development likely compound or synergize to facilitate enduring neurochemical changes resulting in insufficient or excessive 5-HT signaling, that could underlie the persistent behavioral characteristics of autism spectrum disorder.

Maternal Serotonin Levels Are Associated With Cognitive Ability and Core Symptoms in Autism Spectrum Disorder

OBJECTIVE

The serotonin (5-hydroxytryptamine [HT]) system has long been implicated in autism spectrum disorder (ASD). Whole-blood 5-HT level (WB5-HT) is a stable, heritable biomarker that is elevated in more than 25% of children with ASD. Recent findings indicate that the maternal 5-HT system may influence embryonic neurodevelopment, but maternal WB5-HT has not been examined in relation to ASD phenotypes.

METHOD

WB5-HT levels were obtained from 181 individuals (3-27 years of age) diagnosed with ASD, 99 of their fathers, and 119 of their mothers. Standardized assessments were used to evaluate cognitive, behavioral, and language phenotypes.

RESULTS

Exploratory regression analyses found relationships between maternal WB5-HT and nonverbal IQ (NVIQ), Autism Diagnostic Interview-Revised (ADI-R) Nonverbal Communication Algorithm scores, and overall adaptive function on the Vineland Adaptive Behavior Scales-II. Latent class analysis identified a three-class structure in the assessment data, describing children with low, intermediate, and high severity across measures of behavior, cognition, and adaptive function. Mean maternal WB5-HT differed across classes, with the lowest maternal WB5-HT levels seen in the highest-severity group (Welch F_2,46.048 = 17.394, p < .001). Paternal and proband WB5-HT did not differ between classes.

CONCLUSION

Maternal WB5-HT is associated with neurodevelopmental outcomes in offspring with ASD. Prospective, longitudinal studies will be needed to better understand the relationship between the function of the maternal serotonin system during pregnancy and brain development. Further studies in animal models may be able to reveal the mechanisms underlying these findings.

Sex Differences in Autism Spectrum Disorder: a Review

PURPOSE OF REVIEW

Neurodevelopmental disorders disproportionately affect males. The mechanisms underlying male vulnerability or female protection are not known and remain understudied. Determining the processes involved is crucial to understanding the etiology and advancing treatment of neurodevelopmental disorders. Here, we review current findings and theories that contribute to male preponderance of neurodevelopmental disorders, with a focus on autism.

RECENT FINDINGS

Recent work on the biological basis of the male preponderance of autism and other neurodevelopmental disorders includes discussion of a higher genetic burden in females and sex-specific gene mutations or epigenetic changes that differentially confer risk to males or protection to females. Other mechanisms discussed are sex chromosome and sex hormone involvement. Specifically, fetal testosterone is involved in many aspects of development and may interact with neurotransmitter, neuropeptide, or immune pathways to contribute to male vulnerability. Finally, the possibilities of female underdiagnosis and a multi-hit hypothesis are discussed. This review highlights current theories of male bias in developmental disorders. Topics include environmental, genetic, and epigenetic mechanisms; theories of sex chromosomes, hormones, neuroendocrine, and immune function; underdiagnosis of females; and a multi-hit hypothesis.

Evidence the U.S. autism epidemic initiated by acetaminophen (Tylenol) is aggravated by oral antibiotic amoxicillin/clavulanate (Augmentin) and now exponentially by herbicide glyphosate (Roundup)

Because certain hereditary diseases show autistic behavior, and autism often runs in families, researchers seek genes underlying the pathophysiology of autism, thus core behaviors. Other researchers argue environmental factors are decisive, citing compelling evidence of an autism epidemic in the United States beginning about 1980. Recognition that environmental factors influence gene expression led to synthesis of these views - an 'epigenetic epidemic' provoked by pervasive environmental agents altering expression of vulnerable genes, inducing characteristic autistic biochemistries in many mothers and infants. Two toxins most implicated in the U.S. autism epidemic are analgesic/antipyretic acetaminophen (Tylenol) and oral antibiotic amoxicillin/clavulanate (Augmentin). Recently herbicide glyphosate (Roundup) was exponentially implicated. What do these toxins have in common? Acetaminophen depletes sulfate and glutathione required to detoxify it. Oral antibiotics kill and glyphosate inhibits intestinal bacteria that synthesize methionine (precursor of sulfate and glutathione, and required to methylate DNA), bacteria that synthesize tryptophan (sole precursor of neuroinhibitor serotonin), and bacteria that restrain ammonia-generating anaerobes. Sulfate plus glutathione normally sulfate fetal adrenal androgen dehydroepiandrosterone to DHEAS - major precursor of placental/postnatal estrogens. Glyphosate (and heavy metals) also inhibit aromatase that turns androgens to estrogens. Placental/postnatal estrogens dehydrate/mature brain myelin sheaths, mature corpus callosum and left hemisphere preferentially, dilate brain blood vessels, and elevate brain serotonin and oxytocin. Stress-induced weak androgens and estrogen depletion coherently explain white matter asymmetry and dysconnection in autism, extreme male brain, low brain blood flow, hyperexcitability, social anxiety, and insufficient maternal oxytocin at birth to limit fetal brain chloride/water and mature GABA.