The neurobiology of autism

Common circuit defect of excitatory-inhibitory balance in mouse models of autism

One unifying explanation for the complexity of Autism Spectrum Disorders (ASD) may lie in the disruption of excitatory/inhibitory (E/I) circuit balance during critical periods of development. We examined whether Parvalbumin (PV)-positive inhibitory neurons, which normally drive experience-dependent circuit refinement (Hensch Nat Rev Neurosci 6:877–888, 1), are disrupted across heterogeneous ASD mouse models. We performed a meta-analysis of PV expression in previously published ASD mouse models and analyzed two additional models, reflecting an embryonic chemical insult (prenatal valproate, VPA) or single-gene mutation identified in human patients (Neuroligin-3, NL-3 R451C). PV-cells were reduced in the neocortex across multiple ASD mouse models. In striking contrast to controls, both VPA and NL-3 mouse models exhibited an asymmetric PV-cell reduction across hemispheres in parietal and occipital cortices (but not the underlying area CA1). ASD mouse models may share a PV-circuit disruption, providing new insight into circuit development and potential prevention by treatment of autism.

Recent advances in the pathogenesis of syndromic autisms

Background. Current advances in genetic technology continue to expand the list of medical conditions associated with autism. Clinicians have to identify specific autistic-related syndromes, and to provide tailored counseling. The aim of this study is to elucidate recent advances in autism research that offer important clues into pathogenetic mechanisms of syndromic autism and relevant implications for clinical practice. Data Sources. The PubMed database was searched with the keywords “autism” and “chromosomal abnormalities,” “metabolic diseases,” “susceptibility loci.” Results. Defined mutations, genetic syndromes, and metabolic diseases account for up to 20% of autistic patients. Metabolic and mitochondrial defects may have toxic effects on the brain cells, causing neuronal loss and altered modulation of neurotransmission systems. Alterations of the neocortical excitatory/inhibitory balance and perturbations of interneurons' development represent the most probable pathogenetic mechanisms underlying the autistic phenotype in Fragile X-Syndrome and Tuberous Sclerosis Complex. Chromosomal abnormalities and potential candidate genes are strongly implicated in the disruption of neural connections, brain growth, and synaptic/dendritic morphology. Conclusion. Metabolic testing may be appropriate if specific symptoms are present. High-resolution chromosome analysis may be recommended if a specific diagnosis is suspected because of obvious dysmorphisms. Identifying cryptic chromosomal abnormalities by whole genome microarray analysis can increase the understanding of the neurobiological pathways to autism.

Genetics of autism spectrum disorders

Autism spectrum disorders (ASDs) are a clinically complex group of childhood disorders that have firm evidence of an underlying genetic etiology. Many techniques have been used to characterize the genetic bases of ASDs. Linkage studies have identified several replicated susceptibility loci, including 2q24-2q31, 7q, and 17q11-17q21. Association studies and mutation analysis of candidate genes have implicated the synaptic genes NRXN1, NLGN3, NLGN4, SHANK3, and CNTNAP2 in ASDs. Traditional cytogenetic approaches highlight the high frequency of large chromosomal abnormalities (3%-7% of patients), including the most frequently observed maternal 15q11-13 duplications (1%-3% of patients). Newly developed techniques include high-resolution DNA microarray technologies, which have discovered formerly undetectable submicroscopic copy number variants, and genomewide association studies, which allow simultaneous detection of multiple genes associated with ASDs. Although great progress has been made in autism genetics, the molecular bases of most ASDs remains enigmatic.

Food allergy and autism spectrum disorders: is there a link?

Gastrointestinal (GI) symptoms are common comorbidities in children with autism spectrum disorders (ASDs). Parents often attribute these GI symptoms to food allergy (FA), although an evaluation for IgE-mediated FA is often unrevealing. Our previous studies indicated a high prevalence of non-IgE-mediated FA in young children with ASDs. Therefore, non-IgE-mediated FA may account for some but not all GI symptoms observed in children with ASDs. This raises the question of what treatment measures are applicable to ASD children with GI symptoms. A wide variety of dietary supplements and dietary intervention measures for ASD children have been promoted by medical professionals practicing complementary and alternative medicine despite the lack of rigorous scientific validation in most instances. This review summarizes possible (or proposed) etiologies of GI symptoms in ASD children and discusses risks and possible benefits of intervention measures promoted by complementary and alternative practitioners, with emphasis on FA.

Polymorphisms in the 3' UTR of the serotonin transporter are associated with cognitive flexibility in rhesus macaques

The serotonin system is an important neurophysiological mediator of many behavioral phenotypes. Genetic variation within this system is thought to contribute not only to the natural range of behavioral differences, but also to neuropsychiatric pathologies. Cognitive flexibility, the ability to change patterns of response as reward context shifts, is an important trait that underlies many complex social interactions. Environmental manipulations of the serotonin system have been shown to alter performance on tests measuring cognitive flexibility. Variation at the serotonin transporter promoter region (5HTTLPR) has recently been shown to associate with the performance of rhesus monkeys on an object discrimination reversal learning task [Izquierdo et al., 2007]. Here, we demonstrate that functional genetic variation at the serotonin transporter 3' untranslated region, independent of 5HTTLPR, also associates with performance in an object discrimination reversal learning task in rhesus macaques. The polymorphisms comprising the T:G:T haplotype (T1970, G1991, and T2327) were associated with fewer errors on a reversal learning test and greater levels of cognitive flexibility. We have previously demonstrated that the T:G:T haplotype renders lower levels of gene expression in vitro, paralleling the functionality of human 3' UTR haplotypes, as well as the short allele of 5HTTLPR found in both macaques and humans. The 3' UTR haplotypes are independent and in linkage equilibrium with the 5HTTLPR locus. Together, these data lead to the intriguing possibility that differences observed in human cognitive flexibility, whether naturally or in pathological states, may be associated with genetic variation in the serotonin transporter 3' untranslated region also.

Intention understanding in autism

When we observe a motor act (e.g. grasping a cup) done by another individual, we extract, according to how the motor act is performed and its context, two types of information: the goal (grasping) and the intention underlying it (e.g. grasping for drinking). Here we examined whether children with autistic spectrum disorder (ASD) are able to understand these two aspects of motor acts. Two experiments were carried out. In the first, one group of high-functioning children with ASD and one of typically developing (TD) children were presented with pictures showing hand-object interactions and asked what the individual was doing and why. In half of the "why" trials the observed grip was congruent with the function of the object ("why-use" trials), in the other half it corresponded to the grip typically used to move that object ("why-place" trials). The results showed that children with ASD have no difficulties in reporting the goals of individual motor acts. In contrast they made several errors in the why task with all errors occurring in the "why-place" trials. In the second experiment the same two groups of children saw pictures showing a hand-grip congruent with the object use, but within a context suggesting either the use of the object or its placement into a container. Here children with ASD performed as TD children, correctly indicating the agent's intention. In conclusion, our data show that understanding others' intentions can occur in two ways: by relying on motor information derived from the hand-object interaction, and by using functional information derived from the object's standard use. Children with ASD have no deficit in the second type of understanding, while they have difficulties in understanding others' intentions when they have to rely exclusively on motor cues.

Autism genetics: strategies, challenges, and opportunities

Although genes have long been appreciated to play a critical role in determining the risk for pervasive developmental disorders, the specific transcripts contributing to autism spectrum disorders (ASD) have been quite difficult to characterize. However, recent findings are now providing the first insights into the molecular mechanisms underlying these syndromes and have begun to shed light on the allelic architecture of ASD. In this article, we address what is known about the relative contributions of various types of genetic variation to ASD, consider the obstacles facing gene discovery in this complex disorder, and evaluate the common methodologies employed to address these issues, including linkage, molecular and array-based cytogenetics, and association strategies. We review the current literature, highlighting recent findings implicating both rare mutations and common genetic polymorphisms in the etiology of autism. Finally, we describe key advances in genomic technologies that are transforming all areas of human genetics and consider both the opportunities and challenges for autism research posed by these rapid changes.

Oxidative and excitatory mechanisms of developmental neurotoxicity: transcriptional profiles for chlorpyrifos, diazinon, dieldrin, and divalent nickel in PC12 cells

BACKGROUND

Oxidative stress and excitotoxicity underlie the developmental neurotoxicity of numerous chemicals.

OBJECTIVES

We compared the effects of organophosphates (chlorpyrifos and diazinon), an organo-chlorine (dieldrin), and a metal [divalent nickel (Ni2+)] to determine how these mechanisms contribute to similar or dissimilar neurotoxic outcomes.

METHODS

We used PC12 cells as a model of developing neurons and evaluated transcriptional profiles for genes for oxidative stress responses and glutamate receptors.

RESULTS

Chlorpyrifos had a greater effect on oxidative-stress-related genes in differentiating cells compared with the undifferentiated state. Chlorpyrifos and diazinon showed significant concordance in their effects on glutathione-related genes, but they were negatively correlated for effects on catalase and superoxide dismutase isoforms and had no concordance for effects on ionotropic glutamate receptors. Surprisingly, the correlations were stronger between diazinon and dieldrin than between the two organophosphates. The effects of Ni2+ were the least similar for genes related to oxidative stress but had significant concordance with dieldrin for effects on glutamate receptors.

CONCLUSIONS

Our results point to underlying mechanisms by which different organophosphates produce disparate neurotoxic outcomes despite their shared property as cholinesterase inhibitors. Further, apparently unrelated neurotoxicants may produce similar outcomes because of convergence on oxidative stress and excitotoxicity. The combined use of cell cultures and microarrays points to specific end points that can distinguish similarities and disparities in the effects of diverse developmental neurotoxicants.

FARP1 promotes the dendritic growth of spinal motor neuron subtypes through transmembrane Semaphorin6A and PlexinA4 signaling

The dendritic morphology of neurons dictates their abilities to process and transmit information; however, the signaling pathways that regulate dendritic growth and complexity are poorly understood. Here, we show that retinoids induce the expression of the FERM Rho-GEF protein FARP1 in the developing spinal cord. FARP1 is expressed in subsets of motor neurons and is enriched in dendrites of lateral motor column (LMC) neurons that innervate the limb. FARP1 is necessary and sufficient to promote LMC dendritic growth but does not affect dendrite number or axonal morphology. We show that FARP1 serves as a specific effector of transmembrane Semaphorin6A and PlexinA4 signals to regulate LMC dendritic growth, and that its Rho-GEF domain is necessary for this function. These findings reveal that retinoid and Sema6A/PlexA4 signaling pathways intersect through FARP1 to control dendritic growth, and uncover the existence of subtype-specific signaling networks that control dendritic developmental programs in spinal motor neurons.

Developmental exposure to polychlorinated biphenyls interferes with experience-dependent dendritic plasticity and ryanodine receptor expression in weanling rats

BACKGROUND

Neurodevelopmental disorders are associated with altered patterns of neuronal connectivity. A critical determinant of neuronal connectivity is the dendritic morphology of individual neurons, which is shaped by experience. The identification of environmental exposures that interfere with dendritic growth and plasticity may, therefore, provide insight into environmental risk factors for neurodevelopmental disorders.

OBJECTIVE

We tested the hypothesis that polychlorinated biphenyls (PCBs) alter dendritic growth and/or plasticity by promoting the activity of ryanodine receptors (RyRs).

METHODS AND RESULTS

The Morris water maze was used to induce experience-dependent neural plasticity in weanling rats exposed to either vehicle or Aroclor 1254 (A1254) in the maternal diet throughout gestation and lactation. Developmental A1254 exposure promoted dendritic growth in cerebellar Purkinje cells and neocortical pyramidal neurons among untrained animals but attenuated or reversed experience-dependent dendritic growth among maze-trained littermates. These structural changes coincided with subtle deficits in spatial learning and memory, increased [3H]-ryanodine binding sites and RyR expression in the cerebellum of untrained animals, and inhibition of training-induced RyR upregulation. A congener with potent RyR activity, PCB95, but not a congener with negligible RyR activity, PCB66, promoted dendritic growth in primary cortical neuron cultures and this effect was blocked by pharmacologic antagonism of RyR activity.

CONCLUSIONS

Developmental exposure to PCBs interferes with normal patterns of dendritic growth and plasticity, and these effects may be linked to changes in RyR expression and function. These findings identify PCBs as candidate environmental risk factors for neurodevelopmental disorders, especially in children with heritable deficits in calcium signaling.

Increased IgG4 levels in children with autism disorder

Accumulating evidence indicates that immune dysfunction is associated with autism disorders in a significant subset of children. Previous reports have shown abnormal immunoglobulin (Ig) levels, including an increased presence of autoreactive antibodies in the circulation of individuals with autism. As IgG is the predominant antibody isotype in circulation, we expected that an altered immune response could result in an abnormal IgG subclass profile in children with autism. We examined circulating plasma levels of IgG1, IgG2, IgG3, and IgG4 in 241 children from the CHARGE (Childhood Autism Risks from Genetics and the Environment) study, a large epidemiologic case-control investigation, including 114 children who meet full criteria for autism disorder (AU), 96 typically developing control children (TD) from a randomly selected sample of the general population, and 31 children with developmental delays (DD). We report significantly increased levels of the IgG4 subclass in children with AU compared with TD control children (p=0.016) and compared with DD controls (p=0.041). These results may suggest an underlying immunological abnormality in AU subjects resulting in elevated IgG4 production. Further investigation is necessary to elucidate the relationship between immunological findings and behavioral impairments in autism.

Nuclear and mitochondrial genome defects in autisms

In this review we will evaluate evidence that altered gene dosage and structure impacts neurodevelopment and neural connectivity through deleterious effects on synaptic structure and function, and evidence that the latter are key contributors to the risk for autism. We will review information on alterations of structure of mitochondrial DNA and abnormal mitochondrial function in autism and indications that interactions of the nuclear and mitochondrial genomes may play a role in autism pathogenesis. In a final section we will present data derived using Affymetrix SNP 6.0 microarray analysis of DNA of a number of subjects and parents recruited to our autism spectrum disorders project. We include data on two sets of monozygotic twins. Collectively these data provide additional evidence of nuclear and mitochondrial genome imbalance in autism and evidence of specific candidate genes in autism. We present data on dosage changes in genes that map on the X chromosomes and the Y chromosome. Precise analyses of Y located genes are often difficult because of the high degree of homology of X- and Y-related genes. However, continued efforts to analyze the latter are important, given the consistent evidence for a 4:1 ratio of males to females affected by autism. It is also important to consider whether environmental factors play a role in generating the nuclear and mitochondrial genomic instability we have observed. The study of autism will benefit from a move to analysis of pathways and multigene clusters for identification of subtypes that share a specific genetic etiology.

Spatiotemporal dynamics of the expression of estrogen receptors in the postnatal mouse brain

This study reports on the spatiotemporal dynamics of the expression of estrogen receptors (ERs) in the mouse central nervous system (CNS) during the early postnatal and the peripubertal period. At postnatal day 7 (P7), neurons with strong nuclear immunostaining for both ERalpha and ERbeta1 were widely distributed throughout the brain. Sucrose density gradient sedimentation followed by western blotting supported the histochemical evidence for high levels of both ERs at P7. Over the following 2 days, there was a rapid downregulation of ERs. At P9, ERalpha expression was visible only in the hypothalamic area. Decline in ERbeta1 expression was slower than that of ERalpha, and ERalpha-negative, ERbeta1-positive cells were observed in the dentate gyrus and walls of third ventricle. Between P14 and P35, ERs were undetectable except for the hypothalamic area. As before P7, the ovary does not produce estrogen but does produce 5alpha-androstane-3beta, 17beta-diol (3betaAdiol), an estrogenic metabolite of dihydrotestosterone, we examined the effects of high levels of 3betaAdiol in the postnatal period. We used CYP7B1 knockout mice which cannot hydroxylate and inactivate 3betaAdiol. The brains of these mice are abnormally large with reduced apoptosis. In the early postnatal period, there was 1-week delay in the timing of the reduction in ER expression in the brain. These data reveal that the time when ERs might be activated in the brain is limited to the first 8 postnatal days. In addition, the importance of aromatase has to be reconsidered as the alternative estrogen, 3betaAdiol, is important in neuronal function in the postnatal brain.

Childhood developmental disorders: an academic and clinical convergence point for psychiatry, neurology, psychology and pediatrics

BACKGROUND

Significant advances in understanding brain development and behavior have not been accompanied by revisions of traditional academic structure. Disciplinary isolation and a lack of meaningful interdisciplinary opportunities are persistent barriers in academic medicine. To enhance clinical practice, research, and training for the next generation, academic centers will need to take bold steps that challenge traditional departmental boundaries. Such change is not only desirable but, in fact, necessary to bring about a truly innovative and more effective approach to treating disorders of the developing brain.

METHODS

I focus on developmental disorders as a convergence point for transcending traditional academic boundaries. First, the current taxonomy of developmental disorders is described with emphasis on how current diagnostic systems inadvertently hinder research progress. Second, I describe the clinical features of autism, a phenomenologically defined condition, and Rett and fragile X syndromes, neurogenetic diseases that are risk factors for autism. Finally, I describe how the fields of psychiatry, psychology, neurology, and pediatrics now have an unprecedented opportunity to promote an interdisciplinary approach to training, research, and clinical practice and, thus, advance a deeper understanding of developmental disorders.

RESULTS

Research focused on autism is increasingly demonstrating the heterogeneity of individuals diagnosed by DSM criteria. This heterogeneity hinders the ability of investigators to replicate research results as well as progress towards more effective, etiology-specific interventions. In contrast, fragile X and Rett syndromes are 'real' diseases for which advances in research are rapidly accelerating towards more disease-specific human treatment trials.

CONCLUSIONS

A major paradigm shift is required to improve our ability to diagnose and treat individuals with developmental disorders. This paradigm shift must take place at all levels - training, research and clinical activity. As clinicians and scientists who are currently constrained by disciplinary-specific history and training, we must move towards redefining ourselves as clinical neuroscientists with shared interests and expertise that permit a more cohesive and effective approach to improving the lives of patients.

Neuroligins and neurexins link synaptic function to cognitive disease

The brain processes information by transmitting signals at synapses, which connect neurons into vast networks of communicating cells. In these networks, synapses not only transmit signals but also transform and refine them. Neurexins and neuroligins are synaptic cell-adhesion molecules that connect presynaptic and postsynaptic neurons at synapses, mediate signalling across the synapse, and shape the properties of neural networks by specifying synaptic functions. In humans, alterations in genes encoding neurexins or neuroligins have recently been implicated in autism and other cognitive diseases, linking synaptic cell adhesion to cognition and its disorders.

Sex-specific programming of offspring emotionality after stress early in pregnancy

Prenatal stress is associated with an increased vulnerability to neurodevelopmental disorders, including autism and schizophrenia. To determine the critical time window when fetal antecedents may induce a disease predisposition, we examined behavioral responses in offspring exposed to stress during early, mid, and late gestation. We found that male offspring exposed to stress early in gestation displayed maladaptive behavioral stress responsivity, anhedonia, and an increased sensitivity to selective serotonin reuptake inhibitor treatment. Long-term alterations in central corticotropin-releasing factor (CRF) and glucocorticoid receptor (GR) expression, as well as increased hypothalamic-pituitary-adrenal (HPA) axis responsivity, were present in these mice and likely contributed to an elevated stress sensitivity. Changes in CRF and GR gene methylation correlated with altered gene expression, providing important evidence of epigenetic programming during early prenatal stress. In addition, we found the core mechanism underlying male vulnerability may involve sex-specific placenta responsivity, where stress early in pregnancy significantly increased expression of PPARalpha (peroxisome proliferator-activated receptor alpha), IGFBP-1 (insulin-like growth factor binding protein 1), HIF3alpha (hypoxia-inducible factor 3a), and GLUT4 (glucose transporter 4) in male placentas but not females. Examination of placental epigenetic machinery revealed basal sex differences, providing further evidence that sex-specific programming begins very early in pregnancy, and may contribute to the timing and vulnerability of the developing fetus to maternal perturbations. Overall, these results indicate that stress experience early in pregnancy may contribute to male neurodevelopmental disorders through impacts on placental function and fetal development.

“Complementary and Alternative Medicine Treatments for Children with Autism Spectrum Disorders”

Complementary and alternative medical (CAM) treatments are commonly used for children with autism spectrum disorders. This review discusses the evidence supporting the most frequently used treatments, including categories of mind-body medicine, energy medicine, and biologically based, manipulative, and body-based practices, with the latter two treatments the most commonly selected by families. Clinical providers need to understand the evidence for efficacy (or lack thereof) and potential side effects. Some CAM practices have evidence to reject their use, such as secretin, whereas others have emerging evidence to support their use, such as melatonin. Most treatments have not been adequately studied and do not have evidence to support their use.

Genetic disorders associated with macrocephaly

Macrocephaly is associated with many genetic disorders and is a frequent cause of referral to the clinical geneticist. In this review we classify the commonly encountered macrocephaly disorders into useful categories and summarize recent genetic advances. Conditions where macrocephaly is a predominant aspect of the clinical presentation are discussed and a diagnostic approach to the common macrocephaly disorders is provided. Some emphasis is placed on familial macrocephaly (sometimes referred to as benign external hydrocephalus) and on the macrocephaly associated with autism spectrum disorders. The more recent conditions associated with the leukodystrophies and the organic acidurias are reviewed, but the well known conditions involving storage disorders and bone dysplasias are mentioned but not discussed. The genetic macrocephaly conditions cover a broad spectrum of gene disorders and their related proteins have diverse biological functions. As of yet it is not clear what precise biological pathways lead to generalized brain overgrowth.

Heterogeneous dysregulation of microRNAs across the autism spectrum

microRNAs (miRNAs) are approximately 21 nt transcripts capable of regulating the expression of many mRNAs and are abundant in the brain. miRNAs have a role in several complex diseases including cancer as well as some neurological diseases such as Tourette's syndrome and Fragile x syndrome. As a genetically complex disease, dysregulation of miRNA expression might be a feature of autism spectrum disorders (ASDs). Using multiplex quantitative polymerase chain reaction (PCR), we compared the expression of 466 human miRNAs from postmortem cerebellar cortex tissue of individuals with ASD (n = 13) and a control set of non-autistic cerebellar samples (n = 13). While most miRNAs levels showed little variation across all samples suggesting that autism does not induce global dysfunction of miRNA expression, some miRNAs among the autistic samples were expressed at significantly different levels compared to the mean control value. Twenty-eight miRNAs were expressed at significantly different levels compared to the non-autism control set in at least one of the autism samples. To validate the finding, we reversed the analysis and compared each non-autism control to a single mean value for each miRNA across all autism cases. In this analysis, the number of dysregulated miRNAs fell from 28 to 9 miRNAs. Among the predicted targets of dysregulated miRNAs are genes that are known genetic causes of autism such Neurexin and SHANK3. This study finds that altered miRNA expression levels are observed in postmortem cerebellar cortex from autism patients, a finding which suggests that dysregulation of miRNAs may contribute to autism spectrum phenotype.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

Redefining the serotonergic system by genetic lineage

Central serotonin-producing neurons are heterogeneous-differing in location, morphology, neurotoxin sensitivity and associated clinical disorders-but the underpinnings of this heterogeneity are largely unknown, as are the markers that distinguish physiological subtypes of serotonergic neurons. Here we redefined serotonergic subtypes on the basis of genetic programs that are differentially enacted in progenitor cells. We uncovered a molecular framework for the serotonergic system that, having genetic lineages as its basis, is likely to have physiological relevance and will permit access to genetically defined subtypes for manipulation.