Reduced size of corpus callosum in autism

Cognitive and brain morphological deviations in middle-to-old aged autistic adults: A systematic review and meta-analysis

Cognitive challenges and brain structure variations are common in autism spectrum disorder (ASD) but are rarely explored in middle-to-old aged autistic adults. Cognitive deficits that overlap between young autistic individuals and elderlies with dementia raise an important question: does compromised cognitive ability and brain structure during early development drive autistic adults to be more vulnerable to pathological aging conditions, or does it protect them from further decline? To answer this question, we have synthesized current theoretical models of aging in ASD and conducted a systematic literature review (Jan 1, 1980 - Feb 29, 2024) and meta-analysis to summarize empirical studies on cognitive and brain deviations in middle-to-old aged autistic adults. We explored findings that support different aging theories in ASD and addressed study limitations and future directions. This review sheds light on the poorly understood consequences of aging question raised by the autism community to pave the way for future studies to identify sensitive and reliable measures that best predict the onset, progression, and prognosis of pathological aging in ASD.

Abnormal Development of the Corpus Callosum in Autism Spectrum Disorder: An MRI Study

BACKGROUND

Altered size in the corpus callosum (CC) has been reported in individuals with autism spectrum disorder (ASD), but few studies have investigated younger children. Moreover, knowledge about the age-related changes in CC size in individuals with ASD is limited.

OBJECTIVES

Our objective was to investigate the age-related size of the CC and compare them with age-matched healthy controls between the ages of 2 and 18 years.

METHODS

Structural-weighted images were acquired in 97 male patients diagnosed with ASD; published data were used for the control group. The CC was segmented into 7 distinct subregions (rostrum, genu, rostral body, anterior midbody, posterior midbody, isthmus, and splenium) as per Witelson's technique using ITK-SNAP software. We calculated both the total length and volume of the CC as well as the length and height of its 7 subregions. The length of the CC measures was studied as both continuous and categorical forms. For the continuous form, Pearson's correlation was used, while categorical forms were based on age ranges reflecting brain expansion during early postnatal years. Differences in CC measures between adjacent age groups in individuals with ASD were assessed using a Student t-test. Mean and standard deviation scores were compared between ASD and control groups using the Welch t-test.

RESULTS

Age showed a moderate positive association with the total length of the CC (r = 0.43; Padj = 0.003) among individuals with ASD. Among the subregions, a positive association was observed only in the anterior midbody of the CC (r = 0.41; Padj = 0.01). No association was found between the age and the height of individual subregions or with the total volume of the CC. In comparison with healthy controls, individuals with ASD exhibited shorter lengths and heights of the genu and splenium of the CC across wide age ranges.

CONCLUSION

Overall, our results highlight a distinct abnormal developmental trajectory of CC in ASD, particularly in the genu and splenium structures, potentially reflecting underlying pathophysiological mechanisms that warrant further investigation.

Developmental prediction modeling based on diffusion tensor imaging uncovering age-dependent heterogeneity in early childhood autistic brain

OBJECTIVE

There has been increasing evidence for atypical white matter (WM) microstructure in autistic people, but findings have been divergent. The development of autistic people in early childhood is clouded by the concurrently rapid brain growth, which might lead to the inconsistent findings of atypical WM microstructure in autism. Here, we aimed to reveal the developmental nature of autistic children and delineate atypical WM microstructure throughout early childhood while taking developmental considerations into account.

METHOD

In this study, diffusion tensor imaging was acquired from two independent cohorts, containing 91 autistic children and 100 typically developing children (TDC), aged 4-7 years. Developmental prediction modeling using support vector regression based on TDC participants was conducted to estimate the WM atypical development index of autistic children. Then, subgroups of autistic children were identified by using the k-means clustering method and were compared to each other on the basis of demographic information, WM atypical development index, and autistic trait by using two-sample t-test. Relationship of the WM atypical development index with age was estimated by using partial correlation. Furthermore, we performed threshold-free cluster enhancement-based two-sample t-test for the group comparison in WM microstructures of each subgroup of autistic children with the rematched subsets of TDC.

RESULTS

We clustered autistic children into two subgroups according to WM atypical development index. The two subgroups exhibited distinct developmental stages and age-dependent diversity. WM atypical development index was found negatively associated with age. Moreover, an inverse pattern of atypical WM microstructures and different clinical manifestations in the two stages, with subgroup 1 showing overgrowth with low level of autistic traits and subgroup 2 exhibiting delayed maturation with high level of autistic traits, were revealed.

CONCLUSION

This study illustrated age-dependent heterogeneity in early childhood autistic children and delineated developmental stage-specific difference that ranged from an overgrowth pattern to a delayed pattern. Trial registration This study has been registered at ClinicalTrials.gov (Identifier: NCT02807766) on June 21, 2016 ( https://clinicaltrials.gov/ct2/show/NCT02807766 ).

Serotonergic circuit dysregulation underlying autism-related phenotypes in BTBR mouse model of autism

The inbred mouse strain, BTBR T⁺Itpr3^tf/J (BTBR), possesses neuronal and circuit abnormalities that underlie atypical behavioral profiles resembling the major symptoms of human autism spectrum disorder (ASD). Forebrain serotonin (5-HT) transmission has been implicated in ASD-related behavioral alterations. In this study, we assessed 5-HT signals and the functional responsiveness in BTBR mice compared to standard C57BL/6J (B6) control mice to elucidate how 5-HT alterations contribute to behavioral abnormalities in BTBR mice. A lower number of 5-HT neurons in the median raphe, but not in the dorsal raphe, was observed in male and female BTBR mice. Acute systemic injection of buspirone, a 5-HT1A receptor agonist, induced c-Fos in several brain regions in both B6 and BTBR mice; however, blunted c-Fos induction in BTBR mice was documented in the cingulate cortex, basolateral amygdala (BLA), and ventral hippocampus (Hipp). Decreased c-Fos responses in these regions are associated with a lack of buspirone effects on anxiety-like behavior in BTBR mice. Analysis of mRNA expression following acute buspirone injection indicated that 5HTR1a gene downregulation (or upregulation) occurred in the BLA and Hipp of B6 mice, respectively, but not BTBR mice. The mRNA expression of factors associated with neurogenesis or the pro-inflammatory state was not consistently altered by acute buspirone injection. Therefore, 5-HT responsivity via 5-HT1A receptors in the BLA and Hipp are linked to anxiety-like behavior, in which circuits are disrupted in BTBR mice. Other distinct 5-HT circuits from the BLA and Hipp that regulate social behavior are restricted but preserved in BTBR mice.

Tubers Affecting the Fusiform Face Area Are Associated with Autism Diagnosis

OBJECTIVE

Tuberous sclerosis complex (TSC) is associated with focal brain "tubers" and a high incidence of autism spectrum disorder (ASD). The location of brain tubers associated with autism may provide insight into the neuroanatomical substrate of ASD symptoms.

METHODS

We delineated tuber locations for 115 TSC participants with ASD (n = 31) and without ASD (n = 84) from the Tuberous Sclerosis Complex Autism Center of Excellence Research Network. We tested for associations between ASD diagnosis and tuber burden within the whole brain, specific lobes, and at 8 regions of interest derived from the ASD neuroimaging literature, including the anterior cingulate, orbitofrontal and posterior parietal cortices, inferior frontal and fusiform gyri, superior temporal sulcus, amygdala, and supplemental motor area. Next, we performed an unbiased data-driven voxelwise lesion symptom mapping (VLSM) analysis. Finally, we calculated the risk of ASD associated with positive findings from the above analyses.

RESULTS

There were no significant ASD-related differences in tuber burden across the whole brain, within specific lobes, or within a priori regions derived from the ASD literature. However, using VLSM analysis, we found that tubers involving the right fusiform face area (FFA) were associated with a 3.7-fold increased risk of developing ASD.

INTERPRETATION

Although TSC is a rare cause of ASD, there is a strong association between tuber involvement of the right FFA and ASD diagnosis. This highlights a potentially causative mechanism for developing autism in TSC that may guide research into ASD symptoms more generally. ANN NEUROL 2023;93:577-590.

Restoring morphology of light sheet microscopy data based on magnetic resonance histology

The combination of cellular-resolution whole brain light sheet microscopy (LSM) images with an annotated atlas enables quantitation of cellular features in specific brain regions. However, most existing methods register LSM data with existing canonical atlases, e.g., The Allen Brain Atlas (ABA), which have been generated from tissue that has been distorted by removal from the skull, fixation and physical handling. This limits the accuracy of the regional morphologic measurement. Here, we present a method to combine LSM data with magnetic resonance histology (MRH) of the same specimen to restore the morphology of the LSM images to the in-skull geometry. Our registration pipeline which maps 3D LSM big data (terabyte per dataset) to MRH of the same mouse brain provides registration with low displacement error in ∼10 h with limited manual input. The registration pipeline is optimized using multiple stages of transformation at multiple resolution scales. A three-step procedure including pointset initialization, automated ANTs registration with multiple optimized transformation stages, and finalized application of the transforms on high-resolution LSM data has been integrated into a simple, structured, and robust workflow. Excellent agreement has been seen between registered LSM data and reference MRH data both locally and globally. This workflow has been applied to a collection of datasets with varied combinations of MRH contrasts from diffusion tensor images and LSM with varied immunohistochemistry, providing a routine method for streamlined registration of LSM images to MRH. Lastly, the method maps a reduced set of the common coordinate framework (CCFv3) labels from the Allen Brain Atlas onto the geometrically corrected full resolution LSM data. The pipeline maintains the individual brain morphology and allows more accurate regional annotations and measurements of volumes and cell density.

Evaluation of brain structure abnormalities in children with autism spectrum disorder (ASD) using structural magnetic resonance imaging

Background

Autism spectrum disorder (ASD) is a group of developmental disorders of the nervous system. Since the core cause of many of the symptoms of autism spectrum disorder is due to changes in the structure of the brain, the importance of examining the structural abnormalities of the brain in these disorder becomes apparent. The aim of this study is evaluation of brain structure abnormalities in children with autism spectrum disorder (ASD) using structural magnetic resonance imaging (sMRI). sMRI images of 26 autistic and 26 Healthy control subjects in the range of 5–10 years are selected from the ABIDE database. For a better assessment of structural abnormalities, the surface and volume features are extracted together from this images. Then, the extracted features from both groups were compared with the sample t test and the features with significant differences between the two groups were identified.

Results

The results of volume-based features indicate an increase in total brain volume and white matter and a change in white and gray matter volume in brain regions of Hammers atlas in the autism group. In addition, the results of surface-based features indicate an increase in mean and standard deviation of cerebral cortex thickness and changes in cerebral cortex thickness, sulcus depth, surface complexity and gyrification index in the brain regions of the Desikan–Killany cortical atlas.

Conclusions

Identifying structurally abnormal areas of the brain and examining their relationship to the clinical features of Autism Spectrum Disorder can pave the way for the correct and early detection of this disorder using structural magnetic resonance imaging. It is also possible to design treatment for autistic people based on the abnormal areas of the brain, and to see the effectiveness of the treatment using imaging.

Altered transcallosal fiber count and volume in high-functioning adults with autism spectrum disorder

An altered pattern of information processing has been hypothesized in autism spectrum disorder (ASD), characterized by enhanced local network connectivity and reduced long-distance communication. Previous findings of impaired white matter integrity in the genu and the body of the corpus callosum already indicated reduced long-distance connectivity in patients with ASD. However, it remained unclear how this reduced white matter integrity affects the structural connectivity of the corresponding brain areas. To this end, we analyzed magnetic resonance images (MRI) from 30 participants with high-functioning ASD and 30 typically developed individuals using a global tracking approach to estimate the fiber count and volume of the transcallosal fiber tracts of the five corpus callosum subsections. A reduced fiber count and fiber volume in the anterior subsection of the corpus callosum was detected, supporting the hypothesis of reduced long-distance connectivity in ASD.

Insights from an autism imaging biomarker challenge: Promises and threats to biomarker discovery

MRI has been extensively used to identify anatomical and functional differences in Autism Spectrum Disorder (ASD). Yet, many of these findings have proven difficult to replicate because studies rely on small cohorts and are built on many complex, undisclosed, analytic choices. We conducted an international challenge to predict ASD diagnosis from MRI data, where we provided preprocessed anatomical and functional MRI data from > 2,000 individuals. Evaluation of the predictions was rigorously blinded. 146 challengers submitted prediction algorithms, which were evaluated at the end of the challenge using unseen data and an additional acquisition site. On the best algorithms, we studied the importance of MRI modalities, brain regions, and sample size. We found evidence that MRI could predict ASD diagnosis: the 10 best algorithms reliably predicted diagnosis with AUC∼0.80 - far superior to what can be currently obtained using genotyping data in cohorts 20-times larger. We observed that functional MRI was more important for prediction than anatomical MRI, and that increasing sample size steadily increased prediction accuracy, providing an efficient strategy to improve biomarkers. We also observed that despite a strong incentive to generalise to unseen data, model development on a given dataset faces the risk of overfitting: performing well in cross-validation on the data at hand, but not generalising. Finally, we were able to predict ASD diagnosis on an external sample added after the end of the challenge (EU-AIMS), although with a lower prediction accuracy (AUC=0.72). This indicates that despite being based on a large multisite cohort, our challenge still produced biomarkers fragile in the face of dataset shifts.

Involvement of Calcium-Dependent Pathway and β Subunit-Interaction in Neuronal Migration and Callosal Projection Deficits Caused by the Cav1.2 I1166T Mutation in Developing Mouse Neocortex

Introduction: Gain-of-function mutations in the L-type Ca²⁺ channel Cav1.2 cause Timothy syndrome (TS), a multisystem disorder associated with neurologic symptoms, including autism spectrum disorder (ASD), seizures, and intellectual disability. Cav1.2 plays key roles in neural development, and its mutation can affect brain development and connectivity through Ca²⁺-dependent and -independent mechanisms. Recently, a gain-of-function mutation, I1166T, in Cav1.2 was identified in patients with TS-like disorder. Its channel properties have been analyzed in vitro but in vivo effects of this mutation on brain development remain unexplored.

Methods:In utero electroporation was performed on ICR mice at embryonic day 15 to express GFP, wild-type, and mutant Cav1.2 channels into cortical layer 2/3 excitatory neurons in the primary somatosensory area. The brain was fixed at postnatal days 14–16, sliced, and scanned using confocal microscopy. Neuronal migration of electroporated neurons was examined in the cortex of the electroporated hemisphere, and callosal projection was examined in the white matter and contralateral hemisphere.

Results: Expression of the I1166T mutant in layer 2/3 neurons caused migration deficits in approximately 20% of electroporated neurons and almost completely diminished axonal arborization in the contralateral hemisphere. Axonal projection in the white matter was not affected. We introduced second mutations onto Cav1.2 I1166T; L745P mutation blocks Ca²⁺ influx through Cav1.2 channels and inhibits the Ca²⁺-dependent pathway, and the W440A mutation blocks the interaction of the Cav1.2 α1 subunit to the β subunit. Both second mutations recovered migration and projection.

Conclusion: This study demonstrated that the Cav1.2 I1166T mutation could affect two critical steps during cerebrocortical development, migration and axonal projection, in the mouse brain. This is mediated through Ca²⁺-dependent pathway downstream of Cav1.2 and β subunit-interaction.

Hyperexcitability and brain morphological differences in mice lacking the cystine/glutamate antiporter, system xc. J Neurosci Res 2021;99:3339-3353. [PMID: 34747522 DOI: 10.1002/jnr.24971]

System x_c ^- (Sx_c ^- ) is a heteromeric antiporter (L-cystine/L-glutamate exchanger) expressed predominately on astrocytes in the central nervous system. Its activity contributes importantly to the maintenance of the ambient extracellular glutamate levels, as well as, to cellular redox homeostasis. Since alterations in glutamate levels and redox modifications could cause structural changes, we analyzed gross regional morphology of thionin-stained brain sections and cellular and subcellular morphology of Golgi-Cox stained layer V pyramidal neurons in the primary motor cortex (PM1) of mice naturally null for SLC7A11 (SLC7A11^sut/sut )-the gene that encodes the substrate specific light chain (xCT) for Sx_c ^- . Intriguingly, in comparison to age- and sex-matched wild-type (SLC7A11^+/+ ) littermate controls, we found morphologic changes-including increased dendritic complexity and mushroom spine area in males and reduced corpus callosum and soma size in females-that have previously been described, in each case, as morphological correlates of excitability. Consistent with this, we found that both male and female SLC7A11^sut/sut mice had lower convulsive seizure thresholds and greater seizure severity than their sex-matched wild-type (SLC7A11^+/+ ) littermates after acute challenge with two pharmacologically distinct chemoconvulsants: the Glu receptor agonist, kainic acid (KA), or the GABA_A receptor antagonist, pentylenetetrazole (PTZ). These results suggest that the loss of Sx_c ^- signaling in males and females perturbs excitatory/inhibitory (E/I) balance in vivo, potentially through its regulation of cellular and subcellular morphology.

Corpus callosum size and homotopic connectivity in Autism spectrum disorder

By examining how morphology of the corpus callosum (CC) in autism spectrum disorder (ASD) may affect functional communication across hemispheres, we hope to provide new insights into the structure-function relationship in the brain. We used a sample of 94 participants from the Autism Brain Imaging Data Exchange (ABIDE) database (55 typically-developing (TD) and 39 with ASD). The CC was segmented into five sub-regions (anterior, mid-anterior, central, mid-posterior, posterior) using FreeSurfer software, which were further examined for group differences. The total volume and specific sub-region volumes of the CC, and interhemispheric (homotopic) functional connectivity were calculated, along with the relationship between volume and connectivity. These measures were correlated with social ability assessed by the Social Responsiveness Scale (SRS). The central sub-region of CC was significantly smaller in ASD, although there was no group difference in total CC volume. ASD participants also showed stronger homotopic connectivity in the superior frontal gyrus. SRS scores were negatively correlated with the CC central sub-region volumes in ASD. The findings of this study add to the body of research showing morphological differences in the CC in ASD as well as connectivity differences. The absence of a significant relationship between structure and homotopic functional connectivity aligns with previous findings.

PM2.5 as a potential risk factor for autism spectrum disorder: Its possible link to neuroinflammation, oxidative stress and changes in gene expression

Autism spectrum disorder (ASD) is a neurodevelopmental disorder characterized by behavioral deficits including impairments in social communication, social interaction, and repetitive behaviors. Because the etiology of ASD is still largely unknown, there is no cure for ASD thus far. Although it has been established that genetic components play a vital role in ASD development, the influence of epigenetic regulation induced by environmental factors could also contribute to ASD susceptibility. Accumulated evidence has suggested that exposure to atmospheric particulate matter (PM) in polluted air could affect neurodevelopment, thus possibly leading to ASD. Particles with a size of 2.5 μm (PM_2.5) or less have been shown to have negative effects on human health, and could be linked to ASD symptoms in children. This review summarizes evidence from clinical and animal studies to demonstrate the possible linkage between PM_2.5 exposure and the incidence of ASD in children. An attempt was made to explore the possible mechanisms of this linkage, including changes of gene expression, oxidative stress and neuroinflammation induced by PM_2.5 exposure.

Neuronal and glial cell number is altered in a cortical layer-specific manner in autism

LAY ABSTRACT

The cerebral cortex affected with autism spectrum disorder presents changes in the number of neurons and glia cells, possibly leading to a dysregulation of brain circuits and affecting behavior. However, little is known about cell number alteration in specific layers of the cortex in autism spectrum disorder. We found an increase in the number of neurons and a decrease in the number of astrocytes in specific layers of the prefrontal cortex in postmortem human brains from autism spectrum disorder cases. We hypothesize that this may be due to a failure in neural stem cells to shift differentiation from neurons to glial cells during prenatal brain development. These data provide key anatomical findings that contribute to the bases of autism spectrum disorder pathogenesis.

Neurobiological substrates underlying corpus callosum hypoconnectivity and brain metabolic patterns in the valproic acid rat model of autism spectrum disorder

Atypical connectivity between brain regions and altered structure of the corpus callosum (CC) in imaging studies supports the long-distance hypoconnectivity hypothesis proposed for autism spectrum disorder (ASD). The aim of this study was to unveil the CC ultrastructural and cellular changes employing the valproic acid (VPA) rat model of ASD. Male Wistar rats were exposed to VPA (450 mg/kg i.p.) or saline (control) during gestation (embryonic day 10.5), and maturation, exploration, and social behavior were subsequently tested. Myelin content, ultrastructure, and oligodendroglial lineage were studied in the CC at post-natal days 15 (infant) and 36 (juvenile). As a functional outcome, brain metabolic activity was determined by positron emission tomography. Concomitantly with behavioral deficits in juvenile VPA rats, the CC showed reduced myelin basic protein, conserved total number of axons, reduced percentage of myelinated axons, and aberrant and less compact arrangements of myelin sheath ultrastructure. Mature oligodendrocytes decreased and oligodendrocyte precursors increased in the absence of astrogliosis or microgliosis. In medial prefrontal and somatosensory cortices of juvenile VPA rats, myelin ultrastructure and oligodendroglial lineage were preserved. VPA animals exhibited global brain hypometabolism and local hypermetabolism in brain regions relevant for ASD. In turn, the CC of infant VPA rats showed reduced myelin content but preserved oligodendroglial lineage. Our findings indicate that CC hypomyelination is established during infancy and prior to oligodendroglial pattern alterations, which suggests that axon-oligodendroglia communication could be compromised in VPA animals. Thus, CC hypomyelination may underlie white matter alterations and contribute to atypical patterns of connectivity and metabolism found in ASD.

Reduced Inter-hemispheric Resting State Functional Connectivity and Its Association With Social Deficits in Autism

Autism spectrum disorder (ASD) is an early onset developmental disorder which persists throughout life and is increasing in prevalence over the last few decades. Given its early onset and variable cognitive and emotional functional impairments, it is generally challenging to assess ASD individuals using task-based behavioral and functional MRI paradigms. Consequently, resting state functional MRI (rs-fMRI) has become a key approach for examining ASD-associated neural alterations and revealed functional alterations in large-scale brain networks relative to typically developing (TD) individuals, particularly those involved in social-cognitive and affective processes. Recent progress suggests that alterations in inter-hemispheric resting state functional connectivity (rsFC) between regions in the 2 brain hemispheres, particularly homotopic ones, may be of great importance. Here we have reviewed neuroimaging studies examining inter-hemispheric rsFC abnormities in ASD and its associations with symptom severity. As an index of inter-hemispheric functional connectivity, we have additionally reviewed previous studies on corpus callosum (CC) volumetric and fiber changes in ASD. There are converging findings on reduced inter-hemispheric (including homotopic) rsFC in large-scale brain networks particularly in posterior hubs of the default mode network, reduced volumes in the anterior and posterior CC, and on decreased FA and increased MD or RD across CC subregions. Associations between the strength of inter-hemispheric rsFC and social impairments in ASD together with their classification performance in distinguishing ASD subjects from TD controls across ages suggest that the strength of inter-hemispheric rsFC may be a more promising biomarker for assisting in ASD diagnosis than abnormalities in either brain wide rsFC or brain structure.

Atypical Neocortical Development in the Cited2 Conditional Knockout Leads to Behavioral Deficits Associated with Neurodevelopmental Disorders

The mammalian neocortex develops from a single layer of neuroepithelial cells to form a six-layer heterogeneous mosaic of differentiated neurons and glial cells. This process requires a complex choreography of temporally and spatially restricted transcription factors and epigenetic regulators. Even subtle disruptions in this regulation can alter the way the neocortex forms and functions, leading to a neurodevelopmental disorder. One epigenetic regulator that is essential for the precise development of the neocortex is CITED2 (CBP/p300 Interacting Transactivator with ED-rich termini). Cited2 is highly expressed by intermediate progenitor cells in the subventricular zone during the generation of the superficial layers of the neocortex. A forebrain-specific conditional knockout of Cited2 (cKO) exhibits reduced proliferation of intermediate progenitor cells embryonically, leading to reduced thickness of the superficial layers and reduced corpus callosum (CC) volume postnatally. Further, the Cited2 cKO display disruptions in balanced neocortical arealization, with a specific reduction in the somatosensory neocortical length, and dysregulation of precise, area-specific neuronal connectivity. Here, we explore the behavioral consequences resulting from this aberrant neocortical development. We demonstrate that Cited2 cKO mice display decreased maternal separation-induced ultrasonic vocalizations (USVs) as neonates, and an increase in rearing behavior and lack of habituation following repeated acoustic startle as adults. They do not display alterations in anxiety-like behavior, overall locomotor activity, or social interactions. Together with the morphological, molecular, and connectivity disruptions, these results identify the Cited2 cKO neocortex as an ideal system to study mechanisms underlying neurodevelopmental and neuroanatomical disruptions with relevance to human neurodevelopmental disorders.

Dysfunctional d-aspartate metabolism in BTBR mouse model of idiopathic autism

BACKGROUND

Autism spectrum disorders (ASD) comprise a heterogeneous group of neurodevelopmental conditions characterized by impairment in social interaction, deviance in communication, and repetitive behaviors. Dysfunctional ionotropic NMDA and AMPA receptors, and metabotropic glutamate receptor 5 activity at excitatory synapses has been recently linked to multiple forms of ASD. Despite emerging evidence showing that d-aspartate and d-serine are important neuromodulators of glutamatergic transmission, no systematic investigation on the occurrence of these D-amino acids in preclinical ASD models has been carried out.

METHODS

Through HPLC and qPCR analyses we investigated d-aspartate and d-serine metabolism in the brain and serum of four ASD mouse models. These include BTBR mice, an idiopathic model of ASD, and Cntnap2^-/-, Shank3^-/-, and 16p11.2^+/- mice, three established genetic mouse lines recapitulating high confidence ASD-associated mutations.

RESULTS

Biochemical and gene expression mapping in Cntnap2^-/-, Shank3^-/-, and 16p11.2^+/- failed to find gross cerebral and serum alterations in d-aspartate and d-serine metabolism. Conversely, we found a striking and stereoselective increased d-aspartate content in the prefrontal cortex, hippocampus and serum of inbred BTBR mice. Consistent with biochemical assessments, in the same brain areas we also found a robust reduction in mRNA levels of d-aspartate oxidase, encoding the enzyme responsible for d-aspartate catabolism.

CONCLUSIONS

Our results demonstrated the presence of disrupted d-aspartate metabolism in a widely used animal model of idiopathic ASD.

GENERAL SIGNIFICANCE

Overall, this work calls for a deeper investigation of D-amino acids in the etiopathology of ASD and related developmental disorders.

Mapping the Brain-Wide Network Effects by Optogenetic Activation of the Corpus Callosum

Optogenetically driven manipulation of circuit-specific activity enables causality studies, but its global brain-wide effect is rarely reported. Here, we applied simultaneous functional magnetic resonance imaging (fMRI) and calcium recording with optogenetic activation of the corpus callosum (CC) connecting barrel cortices (BC). Robust positive BOLD was detected in the ipsilateral BC due to antidromic activity, spreading to the ipsilateral motor cortex (MC), and posterior thalamus (PO). In the orthodromic target, positive BOLD was reliably evoked by 2 Hz light pulses, whereas 40 Hz light pulses led to reduced calcium, indicative of CC-mediated inhibition. This presumed optogenetic CC-mediated inhibition was further elucidated by pairing light pulses with whisker stimulation at varied interstimulus intervals. Whisker-induced positive BOLD and calcium signals were reduced at intervals of 50/100 ms. The calcium-amplitude-modulation-based correlation with whole-brain fMRI signal revealed that the inhibitory effects spread to contralateral BC, ipsilateral MC, and PO. This work raises the need for fMRI to elucidate the brain-wide network activation in response to optogenetic stimulation.

Vitamin D Supplementation Rescues Aberrant NF-κB Pathway Activation and Partially Ameliorates Rett Syndrome Phenotypes in Mecp2 Mutant Mice

Rett syndrome (RTT) is a severe, progressive X-linked neurodevelopmental disorder caused by mutations in the transcriptional regulator MECP2. We previously identified aberrant NF-κB pathway upregulation in brains of Mecp2-null mice and demonstrated that genetically attenuating NF-κB rescues some characteristic neuronal RTT phenotypes. These results raised the intriguing question of whether NF-κB pathway inhibitors might provide a therapeutic avenue in RTT. Here, we investigate whether the known NF-κB pathway inhibitor vitamin D ameliorates neuronal phenotypes in Mecp2-mutant mice. Vitamin D deficiency is prevalent among RTT patients, and we find that Mecp2-null mice similarly have significantly reduced 25(OH)D serum levels compared with wild-type littermates. We identify that vitamin D rescues aberrant NF-κB pathway activation and reduced neurite outgrowth of Mecp2 knock-down cortical neurons in vitro. Further, dietary supplementation with vitamin D in early symptomatic male Mecp2 hemizygous null and female Mecp2 heterozygous mice ameliorates reduced neocortical dendritic morphology and soma size phenotypes and modestly improves reduced lifespan of Mecp2-nulls. These results elucidate fundamental neurobiology of RTT and provide foundation that NF-κB pathway inhibition might be a therapeutic target for RTT.

GABAARα2 is Decreased in the Axon Initial Segment of Pyramidal Cells in Specific Areas of the Prefrontal Cortex in Autism

Some forms of Autism Spectrum Disorder, a neurodevelopmental syndrome characterized by impaired communication and social skills as well as repetitive behaviors, are purportedly associated with dysregulation of the excitation/inhibition balance in the cerebral cortex. Through human postmortem tissue analysis, we previously found a significant decrease in the number of a gamma-aminobutyric acid (GABA)ergic interneuron subtype, the chandelier (Ch) cell, in the prefrontal cortex of subjects with autism. Ch cells exclusively target the axon initial segment (AIS) of excitatory pyramidal (Pyr) neurons, and a single Ch cell forms synapses on hundreds of Pyr cells, indicating a possible role in maintaining electrical balance. Thus, we herein investigated this crucial link between Ch and Pyr cells in the anatomy of autism neuropathology by examining GABA receptor protein expression in the Pyr cell AIS in subjects with autism. We collected tissue from the prefrontal cortex (Brodmann Areas (BA) 9, 46, and 47) of 20 subjects with autism and 20 age- and sex-matched control subjects. Immunohistochemical staining with antibodies against the GABA_A receptor subunit α2 (GABA_ARα2) - the subunit most prevalent in the Pyr cell AIS - revealed a significantly decreased GABA_ARα2 protein in the Pyr cell AIS in supragranular layers of prefrontal cortical areas BA9 and BA47 in autism. Downregulated GABA_ARα2 protein in the Pyr cell AIS may result from decreased GABA synthesis in the prefrontal cortex of subjects with autism, and thereby contribute to an excitation/inhibition imbalance. Our findings support the potential for GABA receptor agonists asa therapeutic tool for autism.

Diffusion tensor imaging reveals microstructural alterations in corpus callosum and associated transcallosal fiber tracts in adult macaques with neonatal hippocampal lesions

To investigate the effects of neonatal hippocampal lesions on the microstructural integrity of the corpus callosum (CC) in adulthood, macaque monkeys (n = 5) with neonatal bilateral neurotoxic hippocampal lesion (Neo-Hibo) and sham-operated controls (Neo-C, n = 5) were scanned using magnetic resonance diffusion tensor imaging (DTI) technique at 8-10 years old. CC was segmented into seven regionsgrouped into anterior CC (rostrum, genu, rostral body and anterior midbody) and posterior CC (posterior midbody, isthmus and splenium) for data analysis. Associated transcallosal fiber tracts were delineated using probabilistic tractography and evaluated with tract-based spatial statistics (TBSS). Neo-Hibo lesions resulted in significant increased diffusivity indices (mean, axial and radial diffusivity) in CC posterior segments. Also, significant decreased fractional anisotropy (FA) and increased diffusivity indices were seen in the associated transcallosal fiber tracts proximal to motor, posterior parietal and retrosplenial cortices. In Neo-Hibo animals, increased mean diffusivity (MD) in posterior midbody negatively correlated with reduction of CC surface areaand the magnitude of their memory impairments was significantly correlated with FA in transcallosal fiber tracts across splenium. Although no microstructural changes were observed in CC anterior segments, changes in FA values and diffusivity indices were observed in the white matter fibers of the ventromedial prefrontal cortex. Thus, Neo-H lesions resulted in enduring degradation in transcallosal fibers proximal to parietal and retrosplenial cortices, and hemispheric connections through posterior CC. The findings may provide complementary information for understanding the neural substrate of behavioral and cognitive deficits observed in patients with early insult to the hippocampus.

PlexinA1 is crucial for the midline crossing of callosal axons during corpus callosum development in BALB/cAJ mice

The corpus callosum (CC) is the biggest commissure that links cerebral hemispheres. Guidepost structures develop in the cortical midline during CC development and express axon guidance molecules that instruct neurons regarding the proper direction of axonal elongation toward and across the cortical midline. Neuropilin-1 (Npn1), a high affinity receptor for class 3 semaphorins (Sema3s) localized on cingulate pioneering axons, plays a crucial role in axon guidance to the midline through interactions with Sema3s. However, it remains unclear which type of Plexin is a component of Sema3 holoreceptors with Npn1 during the guidance of cingulate pioneering axons. To address the role of PlexinA1 in CC development, we examined with immunohistochemistry the localization of PlexinA1, Npn1, and Sema3s using embryonic brains from wild-type (WT) and PlexinA1-deficient (PlexinA1 knock-out (KO)) mice with a BALB/cAJ background. The immunohistochemistry confirmed the expression of PlexinA1 in callosal axons derived from the cingulate and neocortex of the WT mice on embryonic day 17.5 (E17.5) but not in the PlexinA1 KO mice. To examine the role of PlexinA1 in the navigation of callosal axons, the extension of callosal axons toward and across the midline was traced in brains of WT and PlexinA1 KO mice at E17.5. As a result, callosal axons in the PlexinA1 KO brains had a significantly lower incidence of midline crossing at E17.5 compared with the WT brains. To further examine the role of PlexinA1 in CC development, the CC phenotype was examined in PlexinA1 KO mice at postnatal day 0.5 (P0.5). Most of the PlexinA1 KO mice at P0.5 showed agenesis of the CC. These results indicate the crucial involvement of PlexinA1 in the midline crossing of callosal axons during CC development in BALB/cAJ mice.

Assessment of Apoptosis Pathway in Peripheral Blood of Autistic Patients

Autism spectrum disorder (ASD) includes a number of severe neurodevelopmental disorders known by defects in social interaction, impaired verbal and non-verbal interactions, and stereotypic activities and limited interests. Dysregulation of apoptotic pathways have been demonstrated in brain tissues of affected individuals. In the present study, we evaluated expression levels of apoptosis-related genes and miRNAs in peripheral blood of ASD patients compared with healthy subjects. Transcript levels of BCL2, CASP8, and hsa-29c-3p were significantly lower in total ASD patients compared with total normal children (P values = 0.003, 0.002, and 0.01 respectively). When sex of study participants was considered in the analysis, the difference in transcript levels of these genes was significant only in male subjects. Peripheral expression of BCL2 and hsa-29c-3p had 100% sensitivity 92% specificity in ASD diagnosis. The diagnostic power of combination of transcript levels of these genes was estimated to be 78% based on the calculated AUC value. The present study provides evidences for dysregulation of apoptotic pathways in peripheral blood of ASD patients and suggests certain apoptosis-related genes as biomarkers in this regard.

Gender differences in the rat corpus callosum: An ultrastructure study

Sexual dimorphism exists at all levels of the nervous system, from genetic, anatomical and system levels. The sexual dimorphism in the axonal content of the corpus callosum (CC) has always been controversial; hence, the aim of this study was to analyse the differences in total, myelinated and unmyelinated axons density of various regions of the CC between male and female rats. To assess that, six pairs of adult male and female rats were perfused and the CC was removed and sectioned. Four sections from different subregions of the corpus callosum that represent the genu, anterior body, posterior body, and splenium, were stained, and electron microscopic images were captured using stereological guidelines. Later, the axons density for each subregion was calculated and compared between males and females. The findings of the present study indicated region-specific differences in the myelinated, unmyelinated or the ratio of myelinated/total axons in the CC between male and female rats.

Affective and interpersonal psychopathic traits associated with reduced corpus callosum volume among male inmates - RETRACTED

BACKGROUND

Psychopathy is a personality disorder associated with severe emotional and interpersonal consequences and persistent antisocial behavior. Neurobiological models of psychopathy emphasize impairments in emotional processing, attention, and integration of information across large-scale neural networks in the brain. One of the largest integrative hubs in the brain is the corpus callosum (CC) - a large white matter structure that connects the two cerebral hemispheres.

METHOD

The current study examines CC volume, measured via Freesurfer parcellation, in a large sample (n = 495) of incarcerated men who were assessed for psychopathic traits using the Hare Psychopathy Checklist-Revised (PCL-R).

RESULTS

Psychopathy was associated with reduced volume across all five sub-regions of the CC. These relationships were primarily driven by the affective/interpersonal elements of psychopathy (PCL-R Factor 1), as no significant associations were found between the CC and the lifestyle/antisocial traits of psychopathy. The observed effects were not attributable to differences in substance use severity, age, IQ, or total brain volume.

CONCLUSIONS

These findings align with suggestions that core psychopathic traits may be fostered by reduced integrative capacity across large-scale networks in the brain.

A Review of Functional and Structural Neurobiology of the Action Observation Network in Autism Spectrum Disorder and Developmental Coordination Disorder

Recent research has reported motor impairment similarities between children with developmental coordination disorder (DCD) and a subgroup of individuals with autism spectrum disorder (ASD). However, there is a debate as to whether DCD is a co-occurring diagnosis in individuals with ASD and motor impairments (ASDd), or if motor impairments in ASD are distinct from DCD. However, the etiology of motor impairments is not well understood in either disorder. Clarifying comorbidities in ASD is important to determine different etiopathological phenotyping clusters in ASD and to understand the variety of genetic and environmental factors that contribute to the disorder. Furthermore, this distinction has important therapeutic relevance. Here we explore the current neuroimaging findings in ASD and DCD and discusses possible neural mechanisms that underlie similarities and differences between the disorders.

Brain Morphometry and Longitudinal Relaxation Time of Spontaneously Hypertensive Rats (SHRs) in Early and Intermediate Stages of Hypertension Investigated by 3D VFA-SPGR MRI

Cerebral small vessel disease(s) (SVD) results from pathological changes of the small blood vessels in the brain and is common in older people. The diagnostic features by which SVD manifests in brain includes white matter hyperintensities, lacunes, dilated perivascular spaces, microbleeds, and atrophy. In the present study, we use in vivo magnetic resonance imaging (MRI) to characterize brain morphometry and longitudinal relaxation time (T1) of spontaneously hypertensive rats (SHRs) to study the contribution of chronic hypertension to SVD relevant pathology. Male SHR and Wistar-Kyoto (WKY) rats underwent 3D variable flip angle spoiled gradient echo brain MRI at 9.4 T at early (seven weeks old) and established (19 weeks old) stages of hypertension. The derived proton density weighted and T1 images were utilized for morphometry and to characterize T1 properties in gray matter (GM), white matter (WM) and cerebrospinal fluid (CSF). Custom tissue probability maps were constructed for accurate computerized whole brain tissue segmentations and voxel-wise analyses. Characteristic morphological differences between the two strains included enlarged ventricles, smaller corpus callosum (CC) volumes and general 'thinning' of CC in SHR compared to WKY rats at both age groups. While we did not observe parenchymal T1 differences, the T1 of CSF was elevated in SHR compared to controls. Collectively these findings indicate that SHRs develop WM atrophy which is a clinically robust MRI biomarker associated with WM degeneration.

Correlation of nerve fibers in corpus callosum and number of neurons in cerebral cortex: an innovative mathematical model

Purpose/aim: It is estimated that 10⁹ bits/s information are processed in the human brain. The transmission of this huge amount of information requires all connections in the brain to be highly accurate and have order. The current study attempted to present a new aspect of order and proportion in the ultra-structure of the human brain and to calculate the degree of neural interdependence between the two hemispheres.

MATERIALS AND METHODS

In this model, intensity of interdependence of the brain to hemispheres is estimated to be equal to the mathematical proportion of number of neurons in cerebral cortex divided by 2 (number of hemispheres), divided by number of nerve fibers in the human corpus callosum.

RESULTS

The calculated number is equal to 30-50 and it indicates that for every 30-50 neurons between the two hemispheres, there is a neural interconnecting bridge.

CONCLUSIONS

This connection indicates that the brain's function output follows a mathematical relation.

Developmental Origins of Stress and Psychiatric Disorders

Over the last few decades, evidence has emerged that the pathogenesis of psychiatric disorders such as schizophrenia can involve perturbations of the hypothalamic-pituitary-adrenal (HPA) axis and other neuroendocrine systems. Variations in the manifestation of these effects could be related to differences in clinical symptoms between affected individuals and to differences in treatment response. Such effects can also arise from the complex interaction between genes and environmental factors. Here, we review the effects of maternal stress on abnormalities in HPA axis regulation and the development of psychiatric disorders such as schizophrenia. Studies in this area may prove critical for increasing our understanding of the multidimensional nature of mental disorders and could lead to the development of improved diagnostics and novel therapeutic approaches for treating individuals who suffer from these conditions.

Toddlers later diagnosed with autism exhibit multiple structural abnormalities in temporal corpus callosum fibers

Interhemispheric functional connectivity abnormalities are often reported in autism and it is thus not surprising that structural defects of the corpus callosum (CC) are consistently found using both traditional MRI and DTI techniques. Past DTI studies however, have subdivided the CC into 2 or 3 segments without regard for where fibers may project to within the cortex, thus placing limitations on our ability to understand the nature, timing and neurobehavioral impact of early CC abnormalities in autism. Leveraging a unique cohort of 97 toddlers (68 autism; 29 typical) we utilized a novel technique that identified seven CC tracts according to their cortical projections. Results revealed that younger (<2.5 years old), but not older toddlers with autism exhibited abnormally low mean, radial, and axial diffusivity values in the CC tracts connecting the occipital lobes and the temporal lobes. Fractional anisotropy and the cross sectional area of the temporal CC tract were significantly larger in young toddlers with autism. These findings indicate that water diffusion is more restricted and unidirectional in the temporal CC tract of young toddlers who develop autism. Such results may be explained by a potential overabundance of small caliber axons generated by excessive prenatal neural proliferation as proposed by previous genetic, animal model, and postmortem studies of autism. Furthermore, early diffusion measures in the temporal CC tract of the young toddlers were correlated with outcome measures of autism severity at later ages. These findings regarding the potential nature, timing, and location of early CC abnormalities in autism add to accumulating evidence, which suggests that altered inter-hemispheric connectivity, particularly across the temporal lobes, is a hallmark of the disorder.

Cortical compression rapidly trimmed transcallosal projections and altered axonal anterograde transport machinery

Trauma and tumor compressing the brain distort underlying cortical neurons. Compressed cortical neurons remodel their dendrites instantly. The effects on axons however remain unclear. Using a rat epidural bead implantation model, we studied the effects of unilateral somatosensory cortical compression on its transcallosal projection and the reversibility of the changes following decompression. Compression reduced the density, branching profuseness and boutons of the projection axons in the contralateral homotopic cortex 1week and 1month post-compression. Projection fiber density was higher 1-month than 1-week post-compression, suggesting adaptive temporal changes. Compression reduced contralateral cortical synaptophysin, vesicular glutamate transporter 1 (VGLUT1) and postsynaptic density protein-95 (PSD95) expressions in a week and the first two marker proteins further by 1month. βIII-tubulin and kinesin light chain (KLC) expressions in the corpus callosum (CC) where transcallosal axons traveled were also decreased. Kinesin heavy chain (KHC) level in CC was temporarily increased 1week after compression. Decompression increased transcallosal axon density and branching profuseness to higher than sham while bouton density returned to sham levels. This was accompanied by restoration of synaptophysin, VGLUT1 and PSD95 expressions in the contralateral cortex of the 1-week, but not the 1-month, compression rats. Decompression restored βIII-tubulin, but not KLC and KHC expressions in CC. However, KLC and KHC expressions in the cell bodies of the layer II/III pyramidal neurons partially recovered. Our results show cerebral compression compromised cortical axonal outputs and reduced transcallosal projection. Some of these changes did not recover in long-term decompression.

Altered white-matter integrity in unaffected siblings of probands with autism spectrum disorders

Despite the evidence of altered white-matter tract property in individuals with autism spectrum disorder (ASD), little is known about their unaffected siblings. This study aimed to investigate white-matter integrity in unaffected siblings of ASD probands. Thirty-nine unaffected siblings (mean age 15.6 ± 6.0 years; 27 males, 69.2%) and 39 typically developing controls (TDC) (14.2 ± 5.6 years; 26 males, 66.7%) were assessed with diffusion spectrum images and neuropsychological tests. Using the tract-based automatic analysis and the threshold-free cluster weighted (TFCW) scores, we searched for the segments among 76 tracts with the largest difference over the entire brain compared to TDC. Tract integrity was quantified by calculating the mean generalized fractional anisotropy (mGFA) values of the segments with the largest difference in TFCW scores. Unaffected siblings showed reduced mGFA in the bilateral frontal aslant tracts, the right superior longitudinal fasciculus 2 (SLF2), the frontostriatal tracts from the right dorsolateral and left ventrolateral prefrontal cortices, the thalamic radiations of the left ventral and the right dorsal thalamus, the callosal fibers of the splenium, and the increased mGFA of the callosal fibers of the precuneus and the left inferior longitudinal fasciculus. Among these, reduced right SLF2 mGFA was associated with social awareness deficits; impaired frontostriatal tract was associated with internalizing problems, while right frontal aslant tract integrity was associated with visual memory deficits. In conclusion, unaffected siblings showed the aberrant integrity of several white-matter tracts, which were correlated with clinical symptoms and neurocognitive dysfunction. The altered tract integrity could be further examined in the probands with ASD. Hum Brain Mapp 38:6053-6067, 2017. © 2017 Wiley Periodicals, Inc.

The Emergence of Network Inefficiencies in Infants With Autism Spectrum Disorder

BACKGROUND

Autism spectrum disorder (ASD) is a developmental disorder defined by behavioral features that emerge during the first years of life. Research indicates that abnormalities in brain connectivity are associated with these behavioral features. However, the inclusion of individuals past the age of onset of the defining behaviors complicates interpretation of the observed abnormalities: they may be cascade effects of earlier neuropathology and behavioral abnormalities. Our recent study of network efficiency in a cohort of 24-month-olds at high and low familial risk for ASD reduced this confound; we reported reduced network efficiencies in toddlers classified with ASD. The current study maps the emergence of these inefficiencies in the first year of life.

METHODS

This study uses data from 260 infants at 6 and 12 months of age, including 116 infants with longitudinal data. As in our earlier study, we use diffusion data to obtain measures of the length and strength of connections between brain regions to compute network efficiency. We assess group differences in efficiency within linear mixed-effects models determined by the Akaike information criterion.

RESULTS

Inefficiencies in high-risk infants later classified with ASD were detected from 6 months onward in regions involved in low-level sensory processing. In addition, within the high-risk infants, these inefficiencies predicted 24-month symptom severity.

CONCLUSIONS

These results suggest that infants with ASD, even before 6 months of age, have deficits in connectivity related to low-level processing, which contribute to a developmental cascade affecting brain organization and eventually higher-level cognitive processes and social behavior.

Cited2 Regulates Neocortical Layer II/III Generation and Somatosensory Callosal Projection Neuron Development and Connectivity

The neocortex contains hundreds to thousands of distinct subtypes of precisely connected neurons, allowing it to perform remarkably complex tasks of high-level cognition. Callosal projection neurons (CPN) connect the cerebral hemispheres via the corpus callosum, integrating cortical information and playing key roles in associative cognition. CPN are a strikingly diverse set of neuronal subpopulations, and development of this diversity requires precise control by a complex, interactive set of molecular effectors. We have found that the transcriptional coregulator Cited2 regulates and refines two stages of CPN development. Cited2 is expressed broadly by progenitors in the embryonic day 15.5 subventricular zone, during the peak of superficial layer CPN birth, with a progressive postmitotic refinement in expression, becoming restricted to CPN of the somatosensory cortex postnatally. We generated progenitor-stage and postmitotic forebrain-specific Cited2 conditional knock-out mice, using the Emx1-Cre and NEX-Cre mouse lines, respectively. We demonstrate that Cited2 functions in progenitors, but is not necessary postmitotically, to regulate both (1) broad generation of layer II/III CPN and (2) acquisition of precise area-specific molecular identity and axonal/dendritic connectivity of somatosensory CPN. This novel CPN subtype-specific and area-specific control from progenitor action of Cited2 adds yet another layer of complexity to the multistage developmental regulation of neocortical development.

SIGNIFICANCE STATEMENT

This study identifies Cited2 as a novel subtype-specific and area-specific control over development of distinct subpopulations within the broad population of callosal projection neurons (CPN), whose axons connect the two cerebral hemispheres via the corpus callosum (CC). Currently, how the remarkable diversity of CPN subtypes is specified, and how they differentiate to form highly precise and specific circuits, are largely unknown. We found that Cited2 functions within subventricular zone progenitors to both broadly regulate generation of superficial layer CPN throughout the neocortex, and to refine precise area-specific development and connectivity of somatosensory CPN. Gaining insight into molecular development and heterogeneity of CPN will advance understanding of both diverse functions of CPN and of the remarkable range of neurodevelopmental deficits correlated with CPN/CC development.

Partial Agenesis and Hypoplasia of the Corpus Callosum in Idiopathic Autism

To test the hypothesis that developmental anomalies of the corpus callosum (CC), contribute to the pathogenesis of autism, we characterized the type, topography, and severity of CC pathology corresponding to reduced CC areas that are detected by magnetic resonance imaging in the brains of 11 individuals with autism and 11 controls. In the brains of 3 autistic subjects, partial CC agenesis resulted in complete or partial lack of interhemispheric axonal connections in CC segments III-V. In these cases, a combination of focal agenesis and uniform axonal deficit caused reduction of CC areas by 37%, of axon numbers by 62%, and of the numerical density of axons by 39%. In the CC of 8 autistic subjects without agenesis, there was an 18% deficit of the midsagittal CC area, 48.4% deficit of axon numbers, and 37% reduction of the numerical density of axons. The significantly thinner CC, reduced CC area, and uniform axonal deficit in all autistic subjects were classified as CC hypoplasia. Thus, the byproduct of partial CC agenesis and hypoplasia is reduction of axonal connections between cortical areas known to be involved in behavioral alterations observed in people with autism.

Gestational Exposure to Air Pollution Alters Cortical Volume, Microglial Morphology, and Microglia-Neuron Interactions in a Sex-Specific Manner

Microglia are the resident immune cells of the brain, important for normal neural development in addition to host defense in response to inflammatory stimuli. Air pollution is one of the most pervasive and harmful environmental toxicants in the modern world, and several large scale epidemiological studies have recently linked prenatal air pollution exposure with an increased risk of neurodevelopmental disorders such as autism spectrum disorder (ASD). Diesel exhaust particles (DEP) are a primary toxic component of air pollution, and markedly activate microglia in vitro and in vivo in adult rodents. We have demonstrated that prenatal exposure to DEP in mice, i.e., to the pregnant dams throughout gestation, results in a persistent vulnerability to behavioral deficits in adult offspring, especially in males, which is intriguing given the greater incidence of ASD in males to females (∼4:1). Moreover, there is a striking upregulation of toll-like receptor (TLR) 4 gene expression within the brains of the same mice, and this expression is primarily in microglia. Here we explored the impact of gestational exposure to DEP or vehicle on microglial morphology in the developing brains of male and female mice. DEP exposure increased inflammatory cytokine protein and altered the morphology of microglia, consistent with activation or a delay in maturation, only within the embryonic brains of male mice; and these effects were dependent on TLR4. DEP exposure also increased cortical volume at embryonic day (E)18, which switched to decreased volume by post-natal day (P)30 in males, suggesting an impact on the developing neural stem cell niche. Consistent with this hypothesis, we found increased microglial-neuronal interactions in male offspring that received DEP compared to all other groups. Taken together, these data suggest a mechanism by which prenatal exposure to environmental toxins may affect microglial development and long-term function, and thereby contribute to the risk of neurodevelopmental disorders.

Is functional brain connectivity atypical in autism? A systematic review of EEG and MEG studies

BACKGROUND

Although it is well recognized that autism is associated with altered patterns of over- and under-connectivity, specifics are still a matter of debate. Little has been done so far to synthesize available literature using whole-brain electroencephalography (EEG) and magnetoencephalography (MEG) recordings.

OBJECTIVES

1) To systematically review the literature on EEG/MEG functional and effective connectivity in autism spectrum disorder (ASD), 2) to synthesize and critically appraise findings related with the hypothesis that ASD is characterized by long-range underconnectivity and local overconnectivity, and 3) to provide, based on the literature, an analysis of tentative factors that are likely to mediate association between ASD and atypical connectivity (e.g., development, topography, lateralization).

METHODS

Literature reviews were done using PubMed and PsychInfo databases. Abstracts were screened, and only relevant articles were analyzed based on the objectives of this paper. Special attention was paid to the methodological characteristics that could have created variability in outcomes reported between studies.

RESULTS

Our synthesis provides relatively strong support for long-range underconnectivity in ASD, whereas the status of local connectivity remains unclear. This observation was also mirrored by a similar relationship with lower frequencies being often associated with underconnectivity and higher frequencies being associated with both under- and over-connectivity. Putting together these observations, we propose that ASD is characterized by a general trend toward an under-expression of lower-band wide-spread integrative processes compensated by more focal, higher-frequency, locally specialized, and segregated processes. Further investigation is, however, needed to corroborate the conclusion and its generalizability across different tasks. Of note, abnormal lateralization in ASD, specifically an elevated left-over-right EEG and MEG functional connectivity ratio, has been also reported consistently across studies.

CONCLUSIONS

The large variability in study samples and methodology makes a systematic quantitative analysis (i.e. meta-analysis) of this body of research impossible. Nevertheless, a general trend supporting the hypothesis of long-range functional underconnectivity can be observed. Further research is necessary to more confidently determine the status of the hypothesis of short-range overconnectivity. Frequency-band specific patterns and their relationships with known symptoms of autism also need to be further clarified.

Annual Research Review: Understudied populations within the autism spectrum - current trends and future directions in neuroimaging research

BACKGROUND

Autism spectrum disorders (ASDs) are a heterogeneous group of neurodevelopmental conditions that vary in both etiology and phenotypic expression. Expressions of ASD characterized by a more severe phenotype, including autism with intellectual disability (ASD + ID), autism with a history of developmental regression (ASD + R), and minimally verbal autism (ASD + MV) are understudied generally, and especially in the domain of neuroimaging. However, neuroimaging methods are a potentially powerful tool for understanding the etiology of these ASD subtypes.

SCOPE AND METHODOLOGY

This review evaluates existing neuroimaging research on ASD + MV, ASD + ID, and ASD + R, identified by a search of the literature using the PubMed database, and discusses methodological, theoretical, and practical considerations for future research involving neuroimaging assessment of these populations.

FINDINGS

There is a paucity of neuroimaging research on ASD + ID, ASD + MV, and ASD + R, and what findings do exist are often contradictory, or so sparse as to be ungeneralizable. We suggest that while greater sample sizes and more studies are necessary, more important would be a paradigm shift toward multimodal (e.g. imaging genetics) approaches that allow for the characterization of heterogeneity within etiologically diverse samples.

Multi-scale radiomic analysis of sub-cortical regions in MRI related to autism, gender and age

We propose using multi-scale image textures to investigate links between neuroanatomical regions and clinical variables in MRI. Texture features are derived at multiple scales of resolution based on the Laplacian-of-Gaussian (LoG) filter. Three quantifier functions (Average, Standard Deviation and Entropy) are used to summarize texture statistics within standard, automatically segmented neuroanatomical regions. Significance tests are performed to identify regional texture differences between ASD vs. TDC and male vs. female groups, as well as correlations with age (corrected p < 0.05). The open-access brain imaging data exchange (ABIDE) brain MRI dataset is used to evaluate texture features derived from 31 brain regions from 1112 subjects including 573 typically developing control (TDC, 99 females, 474 males) and 539 Autism spectrum disorder (ASD, 65 female and 474 male) subjects. Statistically significant texture differences between ASD vs. TDC groups are identified asymmetrically in the right hippocampus, left choroid-plexus and corpus callosum (CC), and symmetrically in the cerebellar white matter. Sex-related texture differences in TDC subjects are found in primarily in the left amygdala, left cerebellar white matter, and brain stem. Correlations between age and texture in TDC subjects are found in the thalamus-proper, caudate and pallidum, most exhibiting bilateral symmetry.

Heterotopic Transcallosal Projections Are Present throughout the Mouse Cortex

Transcallosal projection neurons are a population of pyramidal excitatory neurons located in layers II/III and to a lesser extent layer V of the cortex. Their axons form the corpus callosum thereby providing an inter-hemispheric connection in the brain. While transcallosal projection neurons have been described in some detail before, it is so far unclear whether they are uniformly organized throughout the cortex or whether different functional regions of the cortex contain distinct adaptations of their transcallosal connectivity. To address this question, we have therefore conducted a systematic analysis of transcallosal projection neurons and their axons across six distinct stereotactic coordinates in the mouse cortex that cover different areas of the motor and somatosensory cortices. Using anterograde and retrograde tracing techniques, we found that in agreement with previous studies, most of the transcallosal projections show a precise homotopic organization. The somata of these neurons are predominantly located in layer II/III and layer V but notably smaller numbers of these cells are also found in layer IV and layer VI. In addition, regional differences in the distribution of their somata and the precision of their projections exist indicating that while transcallosal neurons show a uniform organization throughout the mouse cortex, there is a sizeable fraction of these connections that are heterotopic. Our study thus provides a comprehensive characterization of transcallosal connectivity in different cortical areas that can serve as the basis for further investigations of the establishment of inter-hemispheric projections in development and their alterations in disease.

Perinatal exposure to the selective serotonin reuptake inhibitor citalopram alters spatial learning and memory, anxiety, depression, and startle in Sprague-Dawley rats

Selective serotonin reuptake inhibitors (SSRIs) block the serotonin (5-HT) reuptake transporter (SERT) and increase synaptic 5-HT. 5-HT is also important in brain development; hence when SSRIs are taken during pregnancy there exists the potential for these drugs to affect CNS ontogeny. Prenatal SSRI exposure has been associated with an increased prevalence of autism spectrum disorder (ASD), and peripheral 5-HT is elevated in some ASD patients. Perinatal SSRI exposure in rodents has been associated with increased depression and anxiety-like behavior, decreased sociability, and impaired learning in the offspring, behaviors often seen in ASD. The present study investigated whether perinatal exposure to citalopram causes persistent neurobehavioral effects. Gravid Sprague-Dawley rats were assigned to two groups and subcutaneously injected twice per day with citalopram (10mg/kg; Cit) or saline (Sal) 6h apart on embryonic day (E)6-21, and then drug was given directly to the pups after delivery from postnatal day (P)1-20. Starting on P60, one male/female from each litter was tested in the Cincinnati water maze (CWM) and open-field before and after MK-801. A second pair from each litter was tested in the Morris water maze (MWM) and open-field before and after (+)-amphetamine. A third pair was tested as follows: elevated zero-maze, open-field, marble burying, prepulse inhibition of acoustic startle, social preference, and forced swim. Cit-exposed rats were impaired in the MWM during acquisition and probe, but not during reversal, shift, or cued trials. Cit-exposed rats also showed increased marble burying, decreased time in the center of the open-field, decreased latency to immobility in forced swim, and increased acoustic startle across prepulse intensities with no effects on CWM. The results are consistent with citalopram inducing several ASD-like effects. The findings add to concerns about use of SSRIs during pregnancy. Further research on different classes of antidepressants, dose-effect relationships, timing of exposure periods, and mechanisms for these effects are needed. It is also important to balance the effects described here against the effects of the disorders for which the drugs are given.

Brain Morphometry and Psychobehavioural Measures in Autistic Low-Functioning Subjects

In the last two decades neurological research has significantly increased knowledge on the neuroanatomic bases of autism. Several autopsy and quantitative magnetic resonance imaging (MRI) studies have reported central nervous system (CNS) abnormalities which may underlie the social, language and cognitive dysfunction typical of the autistic disorder.

Despite the wealth of evidence that the “autistic brain” is different from normal in a number of structures, the relationship between the severity of the developmental impairment in autism and the degree of the brain abnormality remains unknown.

The aim of the present study is to correlate the areas of some brain regions, as calculated on the basis of MRI morphometry, with the Childhood Autism Rating Scale (CARS) and with the Psychoeducational Profile Revised (PEP-R) scores in a group of 22 autistic mentally retarded male subjects.

Comparative Gene Expression Analysis of Two Mouse Models of Autism: Transcriptome Profiling of the BTBR and En2 (-/-) Hippocampus

Autism spectrum disorders (ASD) are characterized by a high degree of genetic heterogeneity. Genomic studies identified common pathological processes underlying the heterogeneous clinical manifestations of ASD, and transcriptome analyses revealed that gene networks involved in synapse development, neuronal activity, and immune function are deregulated in ASD. Mouse models provide unique tools to investigate the neurobiological basis of ASD; however, a comprehensive approach to identify transcriptional abnormalities in different ASD models has never been performed. Here we used two well-recognized ASD mouse models, BTBR T⁺Itpr3^tf/J (BTBR) and Engrailed-2 knockout (En2^−/−), to identify conserved ASD-related molecular signatures. En2^−/− mice bear a mutation within the EN2 transcription factor homeobox, while BTBR is an inbred strain with unknown genetic defects. Hippocampal RNA samples from BTBR, En2^−/− and respective control (C57Bl/6J and En2^+/+) adult mice were assessed for differential gene expression using microarrays. A total of 153 genes were similarly deregulated in the BTBR and En2^−/− hippocampus. Mouse phenotype and gene ontology enrichment analyses were performed on BTBR and En2^−/− hippocampal differentially expressed genes (DEGs). Pathways represented in both BTBR and En2^−/− hippocampal DEGs included abnormal behavioral response and chemokine/MAP kinase signaling. Genes involved in abnormal function of the immune system and abnormal synaptic transmission/seizures were significantly represented among BTBR and En2^−/− DEGs, respectively. Interestingly, both BTBR and En2^−/− hippocampal DEGs showed a significant enrichment of ASD and schizophrenia (SCZ)-associated genes. Specific gene sets were enriched in the two models: microglial genes were significantly enriched among BTBR DEGs, whereas GABAergic/glutamatergic postsynaptic genes, FMRP-interacting genes and epilepsy-related genes were significantly enriched among En2^−/− DEGs. Weighted correlation network analysis (WGCNA) performed on BTBR and En2^−/− hippocampal transcriptomes together identified six modules significantly enriched in ASD-related genes. Each of these modules showed a specific enrichment profile in neuronal and glial genes, as well as in genes associated to ASD comorbidities such as epilepsy and SCZ. Our data reveal significant transcriptional similarities and differences between the BTBR and En2^−/− hippocampus, indicating that transcriptome analysis of ASD mouse models may contribute to identify novel molecular targets for pharmacological studies.

White and gray matter fiber pathways in autism spectrum disorder revealed by ex vivo diffusion MR tractography

INTRODUCTION

The goal of this project was to study the white and gray matter brain pathways of young children with autism spectrum disorder (ASD) and investigate how ASD brains differ from those of typically developing children of the same age.

METHODS

High angular resolution resolution diffusion imaging tractography and diffusion tensor imaging tractography were used to analyze the brains of two 3-year-old children with ASD and two age-matched controls.

RESULTS

In the ASD brains, the callosal and corticopontine pathways were thinner overall and terminal areas in the cortical gray matter were significantly smaller. The ASD brains had more short-range u-fibers in the frontal lobe compared to the control brains. Gray matter pathways were found disorganized with less coherency in the ASD brain, specifically the lateral aspects of the middle part of the brain including motor areas, and both medial and lateral surfaces of the anterior frontal brain regions.

CONCLUSION

These findings show our tractography technique is useful for identifying differences in brain pathways between the ASD and control groups. Given that scanning the brain of 3-year-old children with or even without ASD is challenging, postmortem scanning may offer valuable insights into the connectivity in the brain of young children with ASD.

Investigating the Microstructural Correlation of White Matter in Autism Spectrum Disorder

White matter microstructure forms a complex and dynamical system that is critical for efficient and synchronized brain function. Neuroimaging findings in children with autism spectrum disorder (ASD) suggest this condition is associated with altered white matter microstructure, which may lead to atypical macroscale brain connectivity. In this study, we used diffusion tensor imaging measures to examine the extent that white matter tracts are interrelated within ASD and typical development. We assessed the strength of inter-regional white matter correlations between typically developing and ASD diagnosed individuals. Using hierarchical clustering analysis, clustering patterns of the pairwise white matter correlations were constructed and revealed to be different between the two groups. Additionally, we explored the use of graph theory analysis to examine the characteristics of the patterns formed by inter-regional white matter correlations and compared these properties between ASD and typical development. We demonstrate that the ASD sample has significantly less coherence in white matter microstructure across the brain compared to that in the typical development sample. The ASD group also presented altered topological characteristics, which may implicate less efficient brain networking in ASD. These findings highlight the potential of graph theory based network characteristics to describe the underlying networks as measured by diffusion magnetic resonance imaging and furthermore indicates that ASD may be associated with altered brain network characteristics. Our findings are consistent with those of a growing number of studies and hypotheses that have suggested disrupted brain connectivity in ASD.