A converging-methods approach to fragile X syndrome

The effect of single-cell knockout of Fragile X Messenger Ribonucleoprotein on synaptic structural plasticity

Fragile X Syndrome (FXS) is the best-known form of inherited intellectual disability caused by the loss-of-function mutation in a single gene. The FMR1 gene mutation abolishes the expression of Fragile X Messenger Ribonucleoprotein (FMRP), which regulates the expression of many synaptic proteins. Cortical pyramidal neurons in postmortem FXS patient brains show abnormally high density and immature morphology of dendritic spines; this phenotype is replicated in the Fmr1 knockout (KO) mouse. While FMRP is well-positioned in the dendrite to regulate synaptic plasticity, intriguing in vitro and in vivo data show that wild type neurons embedded in a network of Fmr1 KO neurons or glia exhibit spine abnormalities just as neurons in Fmr1 global KO mice. This raises the question: does FMRP regulate synaptic morphology and dynamics in a cell-autonomous manner, or do the synaptic phenotypes arise from abnormal pre-synaptic inputs? To address this question, we combined viral and mouse genetic approaches to delete FMRP from a very sparse subset of cortical layer 5 pyramidal neurons (L5 PyrNs) either during early postnatal development or in adulthood. We then followed the structural dynamics of dendritic spines on these Fmr1 KO neurons by in vivo two-photon microscopy. We found that, while L5 PyrNs in adult Fmr1 global KO mice have abnormally high density of thin spines, single-cell Fmr1 KO in adulthood does not affect spine density, morphology, or dynamics. On the contrary, neurons with neonatal FMRP deletion have normal spine density but elevated spine formation at 1 month of age, replicating the phenotype in Fmr1 global KO mice. Interestingly, these neurons exhibit elevated thin spine density, but normal total spine density, by adulthood. Together, our data reveal cell-autonomous FMRP regulation of cortical synaptic dynamics during adolescence, but spine defects in adulthood also implicate non-cell-autonomous factors.

Restoration of FMRP expression in adult V1 neurons rescues visual deficits in a mouse model of fragile X syndrome

Many people affected by fragile X syndrome (FXS) and autism spectrum disorders have sensory processing deficits, such as hypersensitivity to auditory, tactile, and visual stimuli. Like FXS in humans, loss of Fmr1 in rodents also cause sensory, behavioral, and cognitive deficits. However, the neural mechanisms underlying sensory impairment, especially vision impairment, remain unclear. It remains elusive whether the visual processing deficits originate from corrupted inputs, impaired perception in the primary sensory cortex, or altered integration in the higher cortex, and there is no effective treatment. In this study, we used a genetic knockout mouse model (Fmr1^KO), in vivo imaging, and behavioral measurements to show that the loss of Fmr1 impaired signal processing in the primary visual cortex (V1). Specifically, Fmr1^KO mice showed enhanced responses to low-intensity stimuli but normal responses to high-intensity stimuli. This abnormality was accompanied by enhancements in local network connectivity in V1 microcircuits and increased dendritic complexity of V1 neurons. These effects were ameliorated by the acute application of GABA_A receptor activators, which enhanced the activity of inhibitory neurons, or by reintroducing Fmr1 gene expression in knockout V1 neurons in both juvenile and young-adult mice. Overall, V1 plays an important role in the visual abnormalities of Fmr1^KO mice and it could be possible to rescue the sensory disturbances in developed FXS and autism patients.

The effects of early-life immune activation on microglia-mediated neuronal remodeling and the associated ontogeny of hippocampal-dependent learning in juvenile rats

Many neurodevelopmental disorders and associated learning deficits have been linked to early-life immune activation or ongoing immune dysregulation (Laskaris et al., 2016; O'Connor et al., 2014; Frick et al., 2013). Neuroscientists have begun to understand how the maturation of neural circuits allows for the emergence of cognitive and learning behaviors; yet we know very little about how these developing neural circuits are perturbed by certain events, including risk-factors such as early-life immune activation and immune dysregulation. To answer these questions, we examined the impact of early-life immune activation on the emergence of hippocampal-dependent learning in juvenile male and female rats using a well-characterized hippocampal-dependent learning task and we investigated the corresponding, dynamic multicellular interactions in the hippocampus that may contribute to these learning deficits. We found that even low levels of immune activation can result in hippocampal-depedent learning deficits days later, but only when this activation occurs during a sensitive period of development. The initial immune response and associated cytokine production in the hippocampus resolved within 24 h, several days prior to the observed learning deficit, but notably the initial immune response was followed by altered microglial-neuronal communication and synapse remodeling that changed the structure of hippocampal neurons during this period of juvenile brain development. We conclude that immune activation or dysregulation during a sensitive period of hippocampal development can precipitate the emergence of learning deficits via a multi-cellular process that may be initiated by, but not the direct result of the initial cytokine response. SIGNIFICANCE STATEMENT: Many neurodevelopmental disorders have been linked to early-life immune activation or immune dysregulation; however, very little is known about how dynamic changes in neuroimmune cells mediate the transition from normal brain function to the early stages of cognitive disorders, or how changes in immune signaling are subsequently integrated into developing neuronal networks. The current experiments examined the consequences of immune activation on the cellular and molecular changes that accompany the emergence of learning deficits during a sensitive period of hippocampal development. These findings have the potential to significantly advance our understanding of how early-life immune activation or dysregulation can result in the emergence of cognitive and learning deficits that are the largest source of years lived with disability in humans.

Multifaceted Changes in Synaptic Composition and Astrocytic Involvement in a Mouse Model of Fragile X Syndrome

Fragile X Syndrome (FXS), a common inheritable form of intellectual disability, is known to alter neocortical circuits. However, its impact on the diverse synapse types comprising these circuits, or on the involvement of astrocytes, is not well known. We used immunofluorescent array tomography to quantify different synaptic populations and their association with astrocytes in layers 1 through 4 of the adult somatosensory cortex of a FXS mouse model, the FMR1 knockout mouse. The collected multi-channel data contained approximately 1.6 million synapses which were analyzed using a probabilistic synapse detector. Our study reveals complex, synapse-type and layer specific changes in the neocortical circuitry of FMR1 knockout mice. We report an increase of small glutamatergic VGluT1 synapses in layer 4 accompanied by a decrease in large VGluT1 synapses in layers 1 and 4. VGluT2 synapses show a rather consistent decrease in density in layers 1 and 2/3. In all layers, we observe the loss of large inhibitory synapses. Lastly, astrocytic association of excitatory synapses decreases. The ability to dissect the circuit deficits by synapse type and astrocytic involvement will be crucial for understanding how these changes affect circuit function, and ultimately defining targets for therapeutic intervention.

Caspase-2 promotes AMPA receptor internalization and cognitive flexibility via mTORC2-AKT-GSK3β signaling

Caspase-2 is the most evolutionarily conserved member in the caspase family of proteases and is constitutively expressed in most cell types including neurons; however, its physiological function remains largely unknown. Here we report that caspase-2 plays a critical role in synaptic plasticity and cognitive flexibility. We found that caspase-2 deficiency led to deficits in dendritic spine pruning, internalization of AMPA receptors and long-term depression. Our results indicate that caspase-2 degrades Rictor, a key mTOR complex 2 (mTORC2) component, to inhibit Akt activation, which leads to enhancement of the GSK3β activity and thereby long-term depression. Furthermore, we found that mice lacking caspase-2 displayed elevated levels of anxiety, impairment in reversal water maze learning, and little memory loss over time. These results not only uncover a caspase-2-mTORC2-Akt-GSK3β signaling pathway, but also suggest that caspase-2 is important for memory erasing and normal behaviors by regulating synaptic number and transmission.

Sensory Processing Phenotypes in Fragile X Syndrome

Fragile X syndrome (FXS) is a neurodevelopmental disorder that causes intellectual disability. It is a leading known genetic cause of autism. In addition to cognitive, social, and communication deficits, humans with FXS demonstrate abnormal sensory processing including sensory hypersensitivity. Sensory hypersensitivity commonly manifests as auditory, tactile, or visual defensiveness or avoidance. Clinical, behavioral, and electrophysiological studies consistently show auditory hypersensitivity, impaired habituation to repeated sounds, and reduced auditory attention in humans with FXS. Children with FXS also exhibit significant visuospatial impairments. Studies in infants and toddlers with FXS have documented impairments in processing texture-defined motion stimuli, temporal flicker, perceiving ordinal numerical sequence, and the ability to maintain the identity of dynamic object information during occlusion. Consistent with the observations in humans with FXS, fragile X mental retardation 1 ( Fmr1) gene knockout (KO) rodent models of FXS also show seizures, abnormal visual-evoked responses, auditory hypersensitivity, and abnormal processing at multiple levels of the auditory system, including altered acoustic startle responses. Among other sensory symptoms, individuals with FXS exhibit tactile defensiveness. Fmr1 KO mice also show impaired encoding of tactile stimulation frequency and larger size of receptive fields in the somatosensory cortex. Since sensory deficits are relatively more tractable from circuit mechanisms and developmental perspectives than more complex social behaviors, the focus of this review is on clinical, functional, and structural studies that outline the auditory, visual, and somatosensory processing deficits in FXS. The similarities in sensory phenotypes between humans with FXS and animal models suggest a likely conservation of basic sensory processing circuits across species and may provide a translational platform to not just develop biomarkers but also to understand underlying mechanisms. We argue that preclinical studies in animal models of FXS can facilitate the ongoing search for new therapeutic approaches in FXS by understanding mechanisms of basic sensory processing circuits and behaviors that are conserved across species.

The role of reduced expression of fragile X mental retardation protein in neurons and increased expression in astrocytes in idiopathic and syndromic autism (duplications 15q11.2-q13)

Fragile X syndrome (FXS), caused by lack of fragile X mental retardation protein (FMRP), is associated with a high prevalence of autism. The deficit of FMRP reported in idiopathic autism suggests a mechanistic overlap between FXS and autism. The overall goal of this study is to detect neuropathological commonalities of FMRP deficits in the brains of people with idiopathic autism and with syndromic autism caused by dup15q11.2-q13 (dup15). This study tests the hypothesis based on our preliminary data that both idiopathic and syndromic autism are associated with brain region-specific deficits of neuronal FMRP and structural changes of the affected neurons. This immunocytochemical study revealed neuronal FMRP deficits and shrinkage of deficient neurons in the cerebral cortex, subcortical structures, and cerebellum in subjects with idiopathic and dup(15)/autism. Neuronal FMRP deficit coexists with surprising infiltration of the brains of autistic children and adults with FMRP-positive astrocytes known to be typical only for the fetal and short postnatal periods. In the examined autistic subjects, these astrocytes selectively infiltrate the border between white and gray matter in the cerebral and cerebellar cortex, the molecular layer of the cortex, part of the amygdala and thalamus, central cerebellar white matter, and dentate nucleus. Astrocyte pathology results in an additional local loss of FMRP in neurons and their shrinkage. Neuronal deficit of FMRP and shrinkage of affected neurons in structures free of FMRP-positive astrocytes and regions infiltrated with FMRP-expressing astrocytes appear to reflect mechanistic, neuropathological, and functional commonalities of FMRP abnormalities in FXS and autism spectrum disorder. Autism Res 2018, 11: 1316-1331. © 2018 International Society for Autism Research, Wiley Periodicals, Inc. LAY SUMMARY: Immunocytochemistry reveals a deficit of fragile X mental retardation protein (FMRP) in neurons of cortical and subcortical brain structures but increased FMRP expression in astrocytes infiltrating gray and white matter. The detected shrinkage of FMRP-deficient neurons may provide a mechanistic explanation of reported neuronal structural and functional changes in autism. This study contributes to growing evidence of mechanistic commonalities between fragile X syndrome and autism spectrum disorder.

The Etiology of Neuronal Development in Craniosynostosis: A Working Hypothesis

Craniosynostosis is one of the most common craniofacial conditions treated by neurologic and plastic surgeons. In addition to disfigurement, children with craniosynostosis experience significant cognitive dysfunction later in life. Surgery is performed in infancy to correct skull deformity; however, the field is at a crossroads regarding the best approach for correction. Since the cause of brain dysfunction in these patients has remained uncertain, the role and type of surgery might have in attenuating the later-observed cognitive deficits through impact on the brain has been unclear. Recently, however, advances in imaging such as event-related potentials, diffusion tensor imaging, and functional MRI, in conjunction with more robust clinical studies, are providing important insight into the potential etiologies of brain dysfunction in syndromic and nonsyndromic craniosynostosis patients. This review aims to outline the cause(s) of such brain dysfunction including the role extrinsic vault constriction might have on brain development and the current evidence for an intrinsic modular developmental error in brain development. Illuminating the cause of brain dysfunction will identify the role of surgery can play in improving observed functional deficits and thus direct optimal primary and adjuvant treatment.

Amygdala - and serum - neurotrophic factor levels depend on rearing condition in male rats

Early life experiences could determine brain and behavioral development. Neurotrophic factors are likely to mediate the effects of the experience on brain structures and function. Brain-derived neurotrophic factor (BDNF) plays a central role in psychiatric disorders. To investigate the effects of early rearing condition on the amygdala - and serum - BDNF levels, we reared male Wistar rats from weaning (postnatal days 21) to adulthood (postnatal days 119) in three different rearing conditions: (1) enriched, (2) standard and (3) isolated. We found that long-term post-weaning environmental enrichment leads to lower amygdala - and serum - BDNF levels as well as lower brain weights. Grouped rearing in standard laboratory cages enhanced body weight. Thus, early rearing condition might play a crucial role in adult healthiness by predetermining individual BDNF profiles.

Fragile X Mental Retardation-1 Knockout Zebrafish Shows Precocious Development in Social Behavior

Fragile X syndrome (FXS) is a generally hereditary form of human mental retardation that is caused by triplet repeat expansion (CGG) mutation in fragile X mental retardation 1 (fmr1) gene promoter and that results in the absence of the fragile X mental retardation protein (FMRP) expression. The common symptoms of FXS patients include learning disabilities, anxiety, autistic behaviors, as well as other behavioral abnormalities. Our previous results demonstrated the behavioral abnormalities in fmr1 knockout (KO) zebrafish such as fear memory impairment and autism-like behavior. Here, we studied the functional role of fmr1 gene on the development of social behavior by behavioral experiments, including shoaling behavior, shoaling preference, light/dark test, and novel tank task. Our results demonstrated that precocious development of shoaling behavior is found in fmr1 KO zebrafish without affecting the shoaling preference on conspecific zebrafish. The shoaling behavior appeared after 14 days postfertilization (dpf), and the level of shoaling elevated in fmr1 KO zebrafish. Furthermore, the fmr1 KO zebrafish at 28 dpf expressed higher anxiety level in novel tank task. These results suggest that the change of shoaling behavior in fmr1 KO zebrafish may result from hyperactivity and an increase of anxiety.

Is Synaesthesia an X-Linked Dominant Trait with Lethality in Males?

In previous research the inheritance patterns of synaesthesia (eg experiencing colours from graphemes) has been studied and it was concluded that synaesthesia is most likely to be the outcome of a single gene passed on the X chromosome in a dominant fashion. In addition, it has been reported that the female – male ratio of synaesthetes is as high as 6: 1 and the families of synaesthetes contain more female than male members. This raises the possibility that the gene may be associated with lethality in males. In this study we replicate and extend previous research by investigating the female – male ratio and inheritance patterns in a large sample of synaesthetic families ( N = 85). We were able to verify the authenticity of grapheme – colour associates in at least one proband from each family using internal consistency. As before, our results show a female – male bias and are broadly consistent with an X-linked dominant mode of inheritance. However, there was no evidence of male lethality (eg synaesthetes are just as likely to give birth to sons as to daughters). Moreover, our female – male ratio of synaesthetes within families was 2: 1 —considerably lower than previous estimates. We speculate that men may be more reluctant to disclose synaesthesia than women (indeed, our female – male ratio based on self-referral was 3.7: 1). Finally, we discuss how the genotype may give rise to the phenotype in terms of changes in synaptogenesis or plasticity extending into childhood, to be subsequently shaped by the environment.

Control of spine maturation and pruning through proBDNF synthesized and released in dendrites

Excess synapses formed during early postnatal development are pruned over an extended period, while the remaining synapses mature. Synapse pruning is critical for activity-dependent refinement of neuronal connections and its dysregulation has been found in neurodevelopmental disorders such as autism spectrum disorders; however, the mechanism underlying synapse pruning remains largely unknown. As dendritic spines are the postsynaptic sites for the vast majority of excitatory synapses, spine maturation and pruning are indicators for maturation and elimination of these synapses. Our previous studies have found that dendritically localized mRNA for brain-derived neurotrophic factor (BDNF) regulates spine maturation and pruning. Here we investigated the mechanism by which dendritic Bdnf mRNA, but not somatically restricted Bdnf mRNA, promotes spine maturation and pruning. We found that neuronal activity stimulates both translation of dendritic Bdnf mRNA and secretion of its translation product mainly as proBDNF. The secreted proBDNF promotes spine maturation and pruning, and its effect on spine pruning is in part mediated by the p75(NTR) receptor via RhoA activation. Furthermore, some proBDNF is extracellularly converted to mature BDNF and then promotes maturation of stimulated spines by activating Rac1 through the TrkB receptor. In contrast, translation of somatic Bdnf mRNA and the release of its translation product mainly as mature BDNF are independent of action potentials. These results not only reveal a biochemical pathway regulating synapse pruning, but also suggest that BDNF synthesized in the soma and dendrites is released through distinct secretory pathways.

Distinct roles for somatically and dendritically synthesized brain-derived neurotrophic factor in morphogenesis of dendritic spines

Dendritic spines undergo the processes of formation, maturation, and pruning during development. Molecular mechanisms controlling spine maturation and pruning remain largely unknown. The gene for brain-derived neurotrophic factor (BDNF) produces two pools of mRNA, with either a short or long 3' untranslated region (3' UTR). Our previous results show that short 3' UTR Bdnf mRNA is restricted to cell bodies, whereas long 3' UTR Bdnf mRNA is also trafficked to dendrites for local translation. Mutant mice lacking long 3' UTR Bdnf mRNA display normal spines at 3 weeks of age, but thinner and denser spines in adults compared to wild-type littermates. These observations suggest that BDNF translated from long 3' UTR Bdnf mRNA, likely in dendrites, is required for spine maturation and pruning. In this study, using rat hippocampal neuronal cultures, we found that knocking down long 3' UTR Bdnf mRNA blocked spine head enlargement and spine elimination, whereas overexpressing long 3' UTR Bdnf mRNA had the opposite effect. The effect of long 3' UTR Bdnf mRNA on spine head enlargement and spine elimination was diminished by a human single-nucleotide polymorphism (SNP, rs712442) in its 3' UTR that inhibited dendritic localization of Bdnf mRNA. Furthermore, we found that overexpression of either Bdnf mRNA increased spine density at earlier time points. Spine morphological alterations were associated with corresponding changes in density, size, and function of synapses. These results indicate that somatically synthesized BDNF promotes spine formation, whereas dendritically synthesized BDNF is a key regulator of spine head growth and spine pruning.

Reflections of experience-expectant development in repair of the adult damaged brain

Behavioral experience has long been known to influence functional outcome after brain injury, but only recently has its pervasive role in the reorganization of the adult brain after damage become appreciated. We briefly review findings from animal models on the role of experience in shaping neuronal events after stroke-like injury. Experience-dependent neural plasticity can be enhanced or impaired by brain damage, depending upon injury parameters and timing. The neuronal growth response to some experiences is heightened due to interactions with denervation-induced plasticity. This includes compensatory behavioral strategies developed in response to functional impairments. Early behavioral experiences can constrain later experience-dependent plasticity, leading to suboptimal functional outcome. Time dependencies and facets of neural growth patterns are reminiscent of experience-expectant processes that shape brain development. As with sensitive periods in brain development, this process may establish behavioral patterns early after brain injury which are relatively resistant to later change.

Developmental characteristics of dendritic spines in the dentate gyrus of Fmr1 knockout mice

Fragile X Syndrome (FXS) is the most common form of inherited mental retardation. The neuroanatomical phenotype of adult FXS patients, as well as adult Fmr1 knockout (KO) mice, includes elevated dendritic spine density and a spine morphology profile in neocortex that resembles younger individuals. Developmental studies in mouse neocortex have revealed a dynamic phenotype that varies with age, especially during the period of synaptic pruning. Here we investigated the hippocampal dentate gyrus to determine if the FXS spine phenotype is similarly tied to periods of maturation and pruning in this brain region. We used high-voltage electron microscopy to characterize Golgi-stained spines along granule cell dendrites in Fmr1 KO and wildtype (WT) mouse dentate gyrus at postnatal days 15, 21, 30, and 60. In contrast to neocortex, dendritic spine density was higher in Fmr1 KO mice across development. Interestingly, neither genotype showed specific phases of synaptogenesis or pruning, potentially explaining the phenotypic differences from neocortex. Similarly, although the KO mice showed a more immature morphological phenotype overall than WT (higher proportion of thin headed spines, lower proportion of mushroom and stubby spines), both genotypes showed gradual development, rather than impairments during specific phases of maturation. Finally, spine length showed a complex developmental pattern that differs from other brain regions examined, suggesting dynamic regulation by FMRP and other brain region-specific proteins. These findings shed new light on FMRP's role in development and highlight the need for new techniques to further understand the mechanisms by which FMRP affects synaptic maturation.

New insights in the biology of BDNF synthesis and release: implications in CNS function

BDNF has pleiotropic effects on neuronal development and synaptic plasticity that underlie circuit formation and cognitive function. Recent breakthroughs reveal that neuronal activity regulates BDNF cell biology, including Bdnf transcription, dendritic targeting and trafficking of BDNF mRNA and protein, and secretion and extracellular conversion of proBDNF to mature BDNF. Defects in these mechanisms contribute differentially to cognitive dysfunction and anxiety-like behaviors. Here we review recent studies, presented at a symposium at Neuroscience 2009, that describe regulatory mechanisms that permit rapid and dynamic refinement of BDNF actions in neurons.

Phonological accuracy and intelligibility in connected speech of boys with fragile X syndrome or Down syndrome

PURPOSE

To compare the phonological accuracy and speech intelligibility of boys with fragile X syndrome with autism spectrum disorder (FXS-ASD), fragile X syndrome only (FXS-O), Down syndrome (DS), and typically developing (TD) boys.

METHOD

Participants were 32 boys with FXS-O (3-14 years), 31 with FXS-ASD (5-15 years), 34 with DS (4-16 years), and 45 TD boys of similar nonverbal mental age. We used connected speech samples to compute measures of phonological accuracy, phonological process occurrence, and intelligibility.

RESULTS

The boys with FXS, regardless of autism status, did not differ from TD boys on phonological accuracy and phonological process occurrence but produced fewer intelligible words than did TD boys. The boys with DS scored lower on measures of phonological accuracy and occurrence of phonological processes than all other groups and used fewer intelligible words than did TD boys. The boys with FXS and the boys with DS did not differ on measures of intelligibility.

CONCLUSIONS

Boys with FXS, regardless of autism status, exhibited phonological characteristics similar to those of younger TD children but were less intelligible in connected speech. Boys with DS showed greater delays in all phonological measures than the boys with FXS and the TD boys.

Neurodevelopment of children with single suture craniosynostosis: a review

INTRODUCTION

Rates of neurocognitive risk range from 35-50% of school-aged children with isolated single suture craniosynostosis (SSC). It has been hypothesized that early surgical intervention to release suture fusion reduces risk for increased intracranial pressure (ICP) and the corresponding risk to neurodevelopment. However, studies assessing children with SSC have been inconsistent in finding an association between neurocognitive development, age of surgery, and ICP.

REVIEW

SSC produces notable distortion of the cranial vault and underlying brain mass. Although a linear relationship between skull distortion, ICP, and neurocognitive deficits has generally been assumed, recent studies have postulated an interactive process between the skull and developing brain that results in neuroanatomical changes that are not limited to areas directly beneath the fused suture. The specific neuropsychological deficits identified in children with SSC including problems with attention and planning, processing speed, visual spatial skills, language, reading, and spelling may be related to the anatomic differences that persist after correction of suture fusion.

CONCLUSIONS

Available literature on neurocognitive development of children with SSC is suggestive of mild but persistent neuropsychological deficits, which become more significant as cognitive demands increase at school age. Anatomical studies of children without SSC are beginning to identify particular groups of brain structures that if disrupted or malformed, may be associated with specific cognitive deficits. Controlled research investigating the relationship between persistent anatomical changes and neurocognitive functioning of school-aged children with SSC is needed.

Neuroanatomical, molecular genetic, and behavioral correlates of fragile X syndrome

Fragile X syndrome (FXS) is a leading cause of inherited mental retardation. In the vast majority of cases, this X-linked disorder is due to a CGG expansion in the 5' untranslated region of the fmr-1 gene and the resulting decreased expression of its associated protein, FMRP. FXS is characterized by a number of cognitive, behavioral, anatomical, and biological abnormalities. FXS provides a unique opportunity to study the consequence of mutation in a single gene on the development and proper functioning of the CNS. The current focus on the role of FMRP in neuronal maturation makes it timely to assemble the extant information on how reduced expression of the fmr-1 gene leads to neuronal dysmorphology. The purpose of this review is to summarize recent genetic, neuroanatomical, and behavioral studies of fragile X syndrome and to offer potential mechanisms to account for the pleiotropic phenotype of this disorder.

Temporal lobe anatomy and psychiatric symptoms in velocardiofacial syndrome (22q11.2 deletion syndrome)

OBJECTIVE

To investigate the association between mesial temporal lobe morphology, ratios of prefrontal cortex to amygdala and hippocampus volumes, and psychiatric symptomatology in children and adolescents with velocardiofacial syndrome (VCFS).

METHOD

Scores on behavioral rating scales and volumetric measures of the amygdala, hippocampus, and prefrontal cortex based on high-resolution magnetic resonance imaging were compared among 47 children with VCFS, 15 of their siblings, and 18 community controls.

RESULTS

After covarying for whole brain volume, children with VCFS exhibited 11% greater volume of the left amygdala (p =.002) and 8% greater volume of the right amygdala (p =.01). Children with VCFS exhibited smaller volumes of the hippocampus, but not disproportionately to reductions in whole brain volume. Children with VCFS exhibited smaller volumetric ratios of prefrontal and orbitofrontal cortex to amygdala, but not prefrontal cortex to hippocampus. For children with VCFS, but not for the comparison sample, larger volumes of the amygdala and smaller ratios of prefrontal cortex to amygdala were associated with higher scores on the Internalizing, Externalizing, Anxiety, and Aggression scales of the Child Behavior Checklist and on the parent version of the Young Mania Rating Scale.

CONCLUSIONS

These findings suggest that the prefrontal cortex-amygdala circuit that underlies emotional processing is disrupted in children with VCFS and may be an important neurobiological substrate of psychiatric disorder in these children.

Hippocampal pyramidal cells in adult Fmr1 knockout mice exhibit an immature-appearing profile of dendritic spines

Fragile X syndrome (FXS) is a common form of mental retardation caused by the absence of functional fragile X mental retardation protein (FMRP). FXS is associated with elevated density and length of dendritic spines, as well as an immature-appearing distribution profile of spine morphologies in the neocortex. Mice that lack FMRP (Fmr1 knockout mice) exhibit a similar phenotype in the neocortex, suggesting that FMRP is important for dendritic spine maturation and pruning. Examination of Golgi-stained pyramidal cells in hippocampal subfield CA1 of adult Fmr1 knockout mice reveals longer spines than controls and a morphology profile that, while essentially opposite of that described in the Fmr1 knockout neocortex, appears similarly immature. This finding strongly suggests that FMRP is required for the processes of spine maturation and pruning in multiple brain regions and that the specific pathology depends on the cellular context.

A comparison of phonological skills of boys with fragile X syndrome and Down syndrome

In this study, the authors compared the phonological accuracy and patterns of sound change of boys with fragile X syndrome, boys with Down syndrome, and typically developing mental-age-matched boys. Participants were 50 boys with fragile X syndrome, ages 3 to 14 years; 32 boys with Down syndrome, ages 4 to 13 years; and 33 typically developing boys, ages 2 to 6 years, who were matched for nonverbal mental age to both the boys with fragile X syndrome and the boys with Down syndrome. All participants were administered a standardized articulation test, and their sound accuracy, phonological process, and proportion of whole-word proximity scores were analyzed. Although boys with fragile X syndrome were delayed in their speech development, they did not differ from the typically developing, mental-age-matched boys in the percentage of correct early-, middle-, and late-developing consonants; phonological processes; or whole-word proximity scores. Furthermore, boys with fragile X syndrome had fewer errors on early-, middle-, and late-developing consonants; fewer syllable structure processes; and higher whole-word proximity scores than did boys with Down syndrome. Boys with Down syndrome also were delayed in their speech development, yet their phonological inventories, occurrences of phonological processes, and proportion of whole-word proximity scores indicated greater delays in their phonological development than the younger, typically developing boys. These results suggest that males with fragile X syndrome display phonological characteristics in isolated words similar to younger, typically developing children, whereas males with Down syndrome show greater delays as well as some developmental differences compared with both the males with fragile X syndrome and typically developing males.

What's the difference? Contrasting modular and neural network approaches to understanding developmental variability

Understanding why development differs across individuals is an important challenge for developmental theory. This paper evaluates two approaches to developmental variability observed in domains such as language processing and across populations such as typically developing children, children with developmental disorders, and typical adults. Modular accounts attribute developmental variability to delay, damage, or dysfunction in discrete underlying structures. Neural network approaches attribute developmental variability to emergent effects of graded variations in an interactive, developing system. The authors conclude that neural network approaches offer more formal and parsimonious accounts of the nature and sources of developmental variability.

Sociability and preference for social novelty in five inbred strains: an approach to assess autistic-like behavior in mice

Deficits in social interaction are important early markers for autism and related neurodevelopmental disorders with strong genetic components. Standardized behavioral assays that measure the preference of mice for initiating social interactions with novel conspecifics would be of great value for mutant mouse models of autism. We developed a new procedure to assess sociability and the preference for social novelty in mice. To quantitate sociability, each mouse was scored on measures of exploration in a central habituated area, a side chamber containing an unfamiliar conspecific (stranger 1) in a wire cage, or an empty side chamber. In a secondary test, preference for social novelty was quantitated by presenting the test mouse with a choice between the first, now-familiar, conspecific (stranger 1) in one side chamber, and a second unfamiliar mouse (stranger 2) in the other side chamber. Parameters scored included time spent in each chamber and number of entries into the chambers. Five inbred strains of mice were tested, C57BL/6J, DBA/2J, FVB/NJ, A/J and B6129PF2/J hybrids. Four strains showed significant levels of sociability (spend- ing more time in the chamber containing stranger 1 than in the empty chamber) and a preference for social novelty (spending more time in the chamber containing stranger 2 than in the chamber containing the now-familiar stranger 1). These social preferences were observed in both male and female mice, and in juveniles and adults. The exception was A/J, a strain that demonstrated a preference for the central chamber. Results are discussed in terms of potential applications of the new methods, and the proper controls for the interpretation of social behavior data, including assays for health, relevant sensory abilities and motor functions. This new standardized procedure to quantitate sociability and preference for social novelty in mice provides a method to assess tendencies for social avoidance in mouse models of autism.

Premorbid intellectual functioning, education, and brain size in traumatic brain injury: an investigation of the cognitive reserve hypothesis

Cognitive reserve theories have been postulated in an attempt to explain individual differences in functional outcome following cerebral insult or disease. These theories suggest that higher education and psychometric intelligence may preserve functional capacity regardless of injury or disease severity. This study investigated cognitive reserve in 25 participants with traumatic brain injury (TBI) using high-resolution magnetic resonance imaging (MRI) analyses. We examined the relationships between total intracranial volume (TICV), ventricle-tobrain ratio (VBR), education level, and standardized testing obtained prior to injury with post-injury cognitive outcome. Participants with lower post-injury IQ scores had significantly lower TICV values, irrespective of injury severity, and experienced significantly greater change in IQ from pre- to post-injury. TICV and education correctly predicted participants' post-injury IQ category ( Y 90 or < 90). However, premorbid standardized testing (PST) scores did not predict cognitive outcome. The results of this study suggest that larger premorbid brain volume and higher education level may decrease vulnerability to cognitive deficits following TBI, consistent with the notion of a cognitive reserve.

Effects of X-monosomy and X-linked imprinting on superior temporal gyrus morphology in Turner syndrome

BACKGROUND

Turner syndrome (TS) results from complete or partial monosomy X. The cognitive phenotype of TS involves preservation of verbal skills with visuospatial functioning deficits. The superior temporal gyrus (STG), which is involved in language capacities, has not been investigated in TS.

METHODS

The STG was measured in 30 female subjects (mean age = 14.73 +/- 6.41; range = 7.56-33.30) with TS and 30 age-matched control subjects (mean age = 14.63 +/- 5.90; range = 6.35-32.65) using volumetric magnetic resonance imaging analyses.

RESULTS

-Right STG, including both gray and white matter volumes, was significantly larger in TS compared with control subjects. Overall left STG volume was not significantly different between groups, although left white matter volume was increased in the TS subjects. The TS subgroup with a maternally derived X chromosome (Xm) demonstrated more aberrant STG volumes compared with subjects with a paternally (Xp) derived X and control subjects. The difference in STG volumes between Xm and control subjects involved both white and gray matter. The Xm subjects differed from Xp subjects only in terms of gray matter.

CONCLUSIONS

These findings suggest that X-monosomy and X-linked imprinting negatively affect STG development, possibly by disrupting neural pruning mechanisms.

Experience effects on brain development: possible contributions to psychopathology

Researchers and clinicians are increasingly recognizing that psychological and psychiatric disorders are often developmentally progressive, and that diagnosis often represents a point along that progression that is defined largely by our abilities to detect symptoms. As a result, strategies that guide our searches for the root causes and etiologies of these disorders are beginning to change. This review describes interactions between genetics and experience that influence the development of psychopathologies. Following a discussion of normal brain development that highlights how specific cellular processes may be targeted by genetic or environmental factors, we focus on four disorders whose origins range from genetic (fragile X syndrome) to environmental (fetal alcohol syndrome) or a mixture of both factors (depression and schizophrenia). C.H. Waddington's canalization model (slightly modified) is used as a tool to conceptualize the interactive influences of genetics and experience in the development of these psychopathologies. Although this model was originally proposed to describe the 'canalizing' role of genetics in promoting normative development, it serves here to help visualize, for example, the effects of adverse (stressful) experience in the kindling model of depression, and the multiple etiologies that may underlie the development of schizophrenia. Waddington's model is also useful in understanding the canalizing influence of experience-based therapeutic approaches, which also likely bring about 'organic' changes in the brain. Finally, in light of increased evidence for the role of experience in the development and treatment of psychopathologies, we suggest that future strategies for identifying the underlying causes of these disorders be based less on the mechanisms of action of effective pharmacological treatments, and more on increased knowledge of the brain's cellular mechanisms of plastic change.

Convergence of psychological and biological development

The special issue provides an opportunity to assess the degree of integration that has been achieved between studies of psychological and biological development. This commentary uses the material in the special issue to address integration of the two fields and to suggest some future developments.