Genetics and function of neocortical GABAergic interneurons in neurodevelopmental disorders

The neurobiology of autism spectrum disorders

Data is progressively and robustly accumulating regarding the biological basis of autism. Autism spectrum disorders (ASD) are currently considered a group of neurodevelopmental disorders with onset very early in life and a complex, heterogeneous, multifactorial aetiology. A comprehensive search of the last five years of the Medline database was conducted in order to summarize recent evidence on the neurobiological bases of autism. The main findings on genetic influence, neuropathology, neurostructure and brain networks are summarized. In addition, findings from peripheral samples of subjects with autism and animal models, which show immune, oxidative, mitochondrial dysregulations, are reported. Then, other biomarkers from very different systems associated with autism are reported. Finally, an attempt is made to try and integrate the available evidence, which points to a oligogenetic, multifactorial aetiology that converges in an aberrant micro-organization of the cortex, with abnormal functioning of the synapses and abnormalities in very general physiological pathways (such as inflammatory, immune and redox systems).

Evidence for interaction between markers in GABA(A) receptor subunit genes in an Argentinean autism spectrum disorder population

Autism spectrum disorders (ASD) can be conceptualized as a genetic dysfunction that disrupts development and function of brain circuits mediating social cognition and language. At least some forms of ASD may be associated with high level of excitation in neural circuits, and gamma-aminobutyric acid (GABA) has been implicated in its etiology. Single-nucleotide polymorphisms (SNP) located within the GABA receptor (GABAR) subunit genes GABRA1, GABRG2, GABRB3, and GABRD were screened. A hundred and thirty-six Argentinean ASD patients and 150 controls were studied, and the contribution of the SNPs in the etiology of ASD was evaluated independently and/or through gene-gene interaction using multifactor dimensionality reduction (MDR) method. From the 18 SNP studied, 11 were not present in our Argentinean population (patients and controls) and 1 SNP had minor allele frequency < 0.1%. For the remaining six SNPs, none provided statistical significant association with ASD when considering allelic or genotypic frequencies. Non-significant association with ASD was found for the haplotype analysis. MDR identified evidence for synergy between markers in GABRB3 (chromosome 15) and GABRD (chromosome 1), suggesting potential gene-gene interaction across chromosomes associated with increased risk for autism (testing balanced accuracy: 0.6081 and cross-validation consistency: 10/10, P < 0.001). Considering our Argentinean ASD sample, it can be inferred that GABRB3 would be involved in the etiology of autism through interaction with GABRD. These results support the hypothesis that GABAR subunit genes are involved in autism, most likely via complex gene-gene interactions.

Doublecortin knockout mice show normal hippocampal-dependent memory despite CA3 lamination defects

Mutations in the human X-linked doublecortin gene (DCX) cause major neocortical disorganization associated with severe intellectual disability and intractable epilepsy. Although Dcx knockout (KO) mice exhibit normal isocortical development and architecture, they show lamination defects of the hippocampal pyramidal cell layer largely restricted to the CA3 region. Dcx-KO mice also exhibit interneuron abnormalities. As well as the interest of testing their general neurocognitive profile, Dcx-KO mice also provide a relatively unique model to assess the effects of a disorganized CA3 region on learning and memory. Based on its prominent anatomical and physiological features, the CA3 region is believed to contribute to rapid encoding of novel information, formation and storage of arbitrary associations, novelty detection, and short-term memory. We report here that Dcx-KO adult males exhibit remarkably preserved hippocampal- and CA3-dependant cognitive processes using a large battery of classical hippocampus related tests such as the Barnes maze, contextual fear conditioning, paired associate learning and object recognition. In addition, we show that hippocampal adult neurogenesis, in terms of proliferation, survival and differentiation of granule cells, is also remarkably preserved in Dcx-KO mice. In contrast, following social deprivation, Dcx-KO mice exhibit impaired social interaction and reduced aggressive behaviors. In addition, Dcx-KO mice show reduced behavioral lateralization. The Dcx-KO model thus reinforces the association of neuropsychiatric behavioral impairments with mouse models of intellectual disability.

Growth hormone-releasing hormone effects on brain γ-aminobutyric acid levels in mild cognitive impairment and healthy aging

IMPORTANCE

Growth hormone-releasing hormone (GHRH) has been previously shown to have cognition-enhancing effects. The role of neurotransmitter changes, measured by proton magnetic resonance spectroscopy, may inform the mechanisms for this response.

OBJECTIVE

To examine the neurochemical effects of GHRH in a subset of participants from the parent trial.

DESIGN

Randomized, double-blind, placebo-controlled substudy of a larger trial.

SETTING

Clinical research unit at the University of Washington School of Medicine.

PARTICIPANTS

Thirty adults (17 with mild cognitive impairment [MCI]), ranging in age from 55 to 87 years, were enrolled and successfully completed the study.

INTERVENTIONS

Participants self-administered daily subcutaneous injections of tesamorelin (Theratechnologies Inc), a stabilized analogue of human GHRH (1 mg/d), or placebo 30 minutes before bedtime for 20 weeks. At baseline and weeks 10 and 20, participants underwent brain magnetic resonance imaging and spectroscopy protocols and cognitive testing and provided blood samples after fasting. Participants also underwent glucose tolerance tests before and after intervention.

MAIN OUTCOMES AND MEASURES

Brain levels of glutamate, inhibitory transmitters γ-aminobutyric acid (GABA) and N-acetylaspartylglutamate (NAAG), and myo-inositol (MI), an osmolyte linked to Alzheimer disease in humans, were measured in three 2 × 2 × 2-cm3 left-sided brain regions (dorsolateral frontal, posterior cingulate, and posterior parietal). Glutamate, GABA, and MI levels were expressed as ratios to creatine plus phosphocreatine, and NAAG was expressed as a ratio to N-acetylaspartate.

RESULTS

After 20 weeks of GHRH administration, GABA levels were increased in all brain regions (P < .04), NAAG levels were increased (P = .03) in the dorsolateral frontal cortex, and MI levels were decreased in the posterior cingulate (P = .002). These effects were similar in adults with MCI and older adults with normal cognitive function. No changes in the brain levels of glutamate were observed. In the posterior cingulate, treatment-related changes in serum insulin-like growth factor 1 were positively correlated with changes in GABA (r = 0.47; P = .001) and tended to be negatively correlated with MI (r = -0.34; P = .06). Consistent with the results of the parent trial, a favorable treatment effect on cognition was observed in substudy participants (P = .03). No significant associations were observed between treatment-related changes in neurochemical and cognitive outcomes. Glucose homeostasis in the periphery was not reliably affected by GHRH administration and did not account for treatment neurochemical effects.

CONCLUSIONS

Twenty weeks of GHRH administration increased GABA levels in all 3 brain regions, increased NAAG levels in the frontal cortex, and decreased MI levels in the posterior cingulate. To our knowledge, this is the first evidence that 20 weeks of somatotropic supplementation modulates inhibitory neurotransmitter and brain metabolite levels in a clinical trial, and it provides preliminary support for one possible mechanism to explain favorable GHRH effects on cognition in adults with MCI and in healthy older adults. TRIAL REGISTRATION clinicaltrials.gov Identifier: NCT00257712.

MPP3 is required for maintenance of the apical junctional complex, neuronal migration, and stratification in the developing cortex

During mammalian cortical development, division of progenitor cells occurs at the apical ventricular zone. Apical complex proteins and adherens junctions regulate the different modes of division. Here, we have identified the membrane-associated guanylate kinase protein membrane palmitoylated protein 3 (MPP3) as an essential protein for the maintenance of these complexes. MPP3 localizes at the apical membrane in which it shows partial colocalization with adherens junction proteins and apical proteins. We generated Mpp3 conditional knock-out mice and specifically ablated Mpp3 expression in cortical progenitor cells. Conditional deletion of Mpp3 during cortical development resulted in a gradual loss of the apical complex proteins and disrupted adherens junctions. Although there is cellular disorganization in the ventricular zone, gross morphology of the cortex was unaffected during loss of MPP3. However, in the ventricular zone, removal of MPP3 resulted in randomization of spindle orientation and ectopically localized mitotic cells. Loss of MPP3 in the developing cortex resulted in delayed migration of progenitor cells, whereas the rate of cell division and exit from the cell cycle was not affected. This resulted in defects in cortical stratification and ectopically localized layer II-IV pyramidal neurons and interneurons. These data show that MPP3 is required for maintenance of the apical protein complex and adherens junctions and for stratification and proper migration of neurons during the development of the cortex.

Cortical interneurons from human pluripotent stem cells: prospects for neurological and psychiatric disease

Cortical interneurons represent 20% of the cells in the cortex. These cells are local inhibitory neurons whose function is to modulate the firing activities of the excitatory projection neurons. Cortical interneuron dysfunction is believed to lead to runaway excitation underlying (or implicated in) seizure-based diseases, such as epilepsy, autism, and schizophrenia. The complex development of this cell type and the intricacies involved in defining the relative subtypes are being increasingly well defined. This has led to exciting experimental cell therapy in model organisms, whereby fetal-derived interneuron precursors can reverse seizure severity and reduce mortality in adult epileptic rodents. These proof-of-principle studies raise hope for potential interneuron-based transplantation therapies for treating epilepsy. On the other hand, cortical neurons generated from patient iPSCs serve as a valuable tool to explore genetic influences of interneuron development and function. This is a fundamental step in enhancing our understanding of the molecular basis of neuropsychiatric illnesses and the development of targeted treatments. Protocols are currently being developed for inducing cortical interneuron subtypes from mouse and human pluripotent stem cells. This review sets out to summarize the progress made in cortical interneuron development, fetal tissue transplantation and the recent advance in stem cell differentiation toward interneurons.

A sip of GABA for the cerebral cortex

Cortical and striatal interneurons are both generated within the ventral telencephalon, but their migratory journey takes them to very different destinations. Two articles in this issue (van den Berge et al., 2013; McKinsey et al., 2013) add an important molecular component to our understanding of how, during development, interneurons reach the cerebral cortex.

Pharmacotherapy of autism spectrum disorders

Although no pharmacological or behavioral therapy has currently proven effective for treating all core symptoms of autism, many dysfunctional behaviors may be treated pharmacologically. Drug treatments should always be part of a comprehensive management plan that includes behavioral and educational interventions, and should be focused on specific targets. Several classes of psychotropic medications have been used to decrease the wide range of "maladaptive" or "interfering" behaviors and associated medical problems that can interfere with relationships and physical health and hinder the implementation of various non-pharmacological interventions. Atypical neuroleptics have been shown to be useful in the treatment of behavioral symptoms in autism. Attention deficit and hyperactivity disorder medications may be effective for counteracting the additional features of hyperactivity and short attention span. Antiepileptic drugs and selective serotonin reuptake inhibitors have shown promising results, but there are no specific indications for them as of yet. With respect to potential drug targets, some clinical features are caused by a dysfunction in neurochemical signaling systems, and thus may improve with selective pharmacological interventions acting on specific abnormal neurobiological pathways. Recent animal studies can be useful models for understanding the common pathogenic pathways leading to autism spectrum disorders (ASDs), and have the potential to offer new biologically focused treatment options.

Activity-regulated somatostatin expression reduces dendritic spine density and lowers excitatory synaptic transmission via postsynaptic somatostatin receptor 4

Neuronal activity regulates multiple aspects of the morphological and functional development of neural circuits. One mechanism by which it achieves this is through regulation of gene expression. In a screen for activity-induced genes, we identified somatostatin (SST), a neuropeptide secreted by the SST subtype of interneurons. Using real time quantitative PCR and ELISA, we showed that persistent elevation of neuronal activity increased both the gene expression and protein secretion of SST over a relatively prolonged time course of 48 h. Using primary hippocampal neuronal cultures, we found that SST treatment for 1 day significantly reduced the density of dendritic spines, the morphological bases of excitatory synapses. Furthermore, the density of pre- and postsynaptic markers of excitatory synapses was significantly lowered following SST treatment, whereas that of inhibitory synapses was not affected. Consistently, SST treatment reduced the frequency of miniature excitatory postsynaptic currents, without affecting inhibition. Finally, lowering the endogenous level of SST receptor subtype 4 in individual hippocampal pyramidal neurons significantly blocked the effect of SST in reducing spine density and excitatory synaptic transmission in a cell autonomous fashion, suggesting that the effect of SST in regulating excitatory synaptic transmission is mainly mediated by SST receptor subtype 4. Together, our results demonstrated that activity-dependent release of SST reduced the density of dendritic spines and the number of excitatory synapses through postsynaptic activation of SST receptor subtype 4 in pyramidal neurons. To our knowledge, this is the first demonstration of the long term effect of SST on neuronal morphology.

Using mouse models of autism spectrum disorders to study the neurotoxicology of gene-environment interactions

To better study the role of genetics in autism, mouse models have been developed which mimic the genetics of specific autism spectrum and related disorders. These models have facilitated research on the role genetic susceptibility factors in the pathogenesis of autism in the absence of environmental factors. Inbred mouse strains have been similarly studied to assess the role of environmental agents on neurodevelopment, typically without the complications of genetic heterogeneity of the human population. What has not been as actively pursued, however, is the methodical study of the interaction between these factors (e.g., gene and environmental interactions in neurodevelopment). This review suggests that a genetic predisposition paired with exposure to environmental toxicants plays an important role in the etiology of neurodevelopmental disorders including autism, and may contribute to the largely unexplained rise in the number of children diagnosed with autism worldwide. Specifically, descriptions of the major mouse models of autism and toxic mechanisms of prevalent environmental chemicals are provided followed by a discussion of current and future research strategies to evaluate the role of gene and environment interactions in neurodevelopmental disorders.

Autistic-like behaviour in Scn1a+/- mice and rescue by enhanced GABA-mediated neurotransmission

Haploinsufficiency of the SCN1A gene encoding voltage-gated sodium channel Na_V1.1 causes Dravet Syndrome (DS), a childhood neuropsychiatric disorder including recurrent intractable seizures, cognitive deficit, and autism-spectrum behaviors. The neural mechanisms responsible for cognitive deficit and autism-spectrum behaviors in DS are poorly understood. Here we show that mice with Scn1a haploinsufficiency display hyperactivity, stereotyped behaviors, social interaction deficits, and impaired context-dependent spatial memory. Olfactory sensitivity is retained, but novel food odors and social odors are aversive to Scn1a^+/− mice. GABAergic neurotransmission is specifically impaired by this mutation, and selective deletion of Na_V1.1 channels in forebrain interneurons is sufficient to cause these behavioral and cognitive impairments. Remarkably, treatment with low-dose clonazepam, a positive allosteric modulator of GABA_A receptors, completely rescued the abnormal social behaviors and deficits in fear memory in DS mice, demonstrating that they are caused by impaired GABAergic neurotransmission and not by neuronal damage from recurrent seizures. These results demonstrate a critical role for Na_V1.1 channels in neuropsychiatric functions and provide a potential therapeutic strategy for cognitive deficit and autism-spectrum behaviors in DS.

GABA system dysfunction in autism and related disorders: from synapse to symptoms

Autism spectrum disorders (ASDs) are neurodevelopmental syndromes characterised by repetitive behaviours and restricted interests, impairments in social behaviour and relations, and in language and communication. These symptoms are also observed in a number of developmental disorders of known origin, including Fragile X Syndrome, Rett Syndrome, and Foetal Anticonvulsant Syndrome. While these conditions have diverse etiologies, and poorly understood pathologies, emerging evidence suggests that they may all be linked to dysfunction in particular aspects of GABAergic inhibitory signalling in the brain. We review evidence from genetics, molecular neurobiology and systems neuroscience relating to the role of GABA in these conditions. We conclude by discussing how these deficits may relate to the specific symptoms observed.

GABAergic neuron specification in the spinal cord, the cerebellum, and the cochlear nucleus

In the nervous system, there are a wide variety of neuronal cell types that have morphologically, physiologically, and histochemically different characteristics. These various types of neurons can be classified into two groups: excitatory and inhibitory neurons. The elaborate balance of the activities of the two types is very important to elicit higher brain function, because its imbalance may cause neurological disorders, such as epilepsy and hyperalgesia. In the central nervous system, inhibitory neurons are mainly represented by GABAergic ones with some exceptions such as glycinergic. Although the machinery to specify GABAergic neurons was first studied in the telencephalon, identification of key molecules, such as pancreatic transcription factor 1a (Ptf1a), as well as recently developed genetic lineage-tracing methods led to the better understanding of GABAergic specification in other brain regions, such as the spinal cord, the cerebellum, and the cochlear nucleus.

GABA through the ages: regulation of cortical function and plasticity by inhibitory interneurons

Inhibitory interneurons comprise only about 20% of cortical neurons and thus constitute a clear minority compared to the vast number of excitatory projection neurons. They are, however, an influential minority with important roles in cortical maturation, function, and plasticity. In this paper, we will highlight the functional importance of cortical inhibition throughout brain development, starting with the embryonal formation of the cortex, proceeding by the regulation of sensory cortical plasticity in adulthood, and finishing with the GABA involvement in sensory information processing in old age.

The LIM homeodomain protein Lhx6 regulates maturation of interneurons and network excitability in the mammalian cortex

Deletion of LIM homeodomain transcription factor-encoding Lhx6 gene in mice results in defective tangential migration of cortical interneurons and failure of differentiation of the somatostatin (Sst)- and parvalbumin (Pva)-expressing subtypes. Here, we characterize a novel hypomorphic allele of Lhx6 and demonstrate that reduced activity of this locus leads to widespread differentiation defects in Sst(+) interneurons, but relatively minor and localized changes in Pva(+) interneurons. The reduction in the number of Sst-expressing cells was not associated with a loss of interneurons, because the migration and number of Lhx6-expressing interneurons and expression of characteristic molecular markers, such as calretinin or Neuropeptide Y, were not affected in Lhx6 hypomorphic mice. Consistent with a selective deficit in the differentiation of Sst(+) interneurons in the CA1 subfield of the hippocampus, we observed reduced expression of metabotropic Glutamate Receptor 1 in the stratum oriens and characteristic changes in dendritic inhibition, but normal inhibitory input onto the somatic compartment of CA1 pyramidal cells. Moreover, Lhx6 hypomorphs show behavioral, histological, and electroencephalographic signs of recurrent seizure activity, starting from early adulthood. These results demonstrate that Lhx6 plays an important role in the maturation of cortical interneurons and the formation of inhibitory circuits in the mammalian cortex.

Cortical GABAergic interneurons in cross-modal plasticity following early blindness

Early loss of a given sensory input in mammals causes anatomical and functional modifications in the brain via a process called cross-modal plasticity. In the past four decades, several animal models have illuminated our understanding of the biological substrates involved in cross-modal plasticity. Progressively, studies are now starting to emphasise on cell-specific mechanisms that may be responsible for this intermodal sensory plasticity. Inhibitory interneurons expressing γ-aminobutyric acid (GABA) play an important role in maintaining the appropriate dynamic range of cortical excitation, in critical periods of developmental plasticity, in receptive field refinement, and in treatment of sensory information reaching the cerebral cortex. The diverse interneuron population is very sensitive to sensory experience during development. GABAergic neurons are therefore well suited to act as a gate for mediating cross-modal plasticity. This paper attempts to highlight the links between early sensory deprivation, cortical GABAergic interneuron alterations, and cross-modal plasticity, discuss its implications, and further provide insights for future research in the field.

Maternal immune activation by LPS selectively alters specific gene expression profiles of interneuron migration and oxidative stress in the fetus without triggering a fetal immune response

Maternal immune activation (MIA) is a risk factor for the development of schizophrenia and autism. Infections during pregnancy activate the mother's immune system and alter the fetal environment, with consequential effects on CNS function and behavior in the offspring, but the cellular and molecular links between infection-induced altered fetal development and risk for neuropsychiatric disorders are unknown. We investigated the immunological, molecular, and behavioral effects of MIA in the offspring of pregnant Sprague-Dawley rats given an intraperitoneal (0.25 mg/kg) injection of lipopolysaccharide (LPS) on gestational day 15. LPS significantly elevated pro-inflammatory cytokine levels in maternal serum, amniotic fluid, and fetal brain at 4 h, and levels decreased but remained elevated at 24 h. Offspring born to LPS-treated dams exhibited reduced social preference and exploration behaviors as juveniles and young adults. Whole genome microarray analysis of the fetal brain at 4 h post maternal LPS was performed to elucidate the possible molecular mechanisms by which MIA affects the fetal brain. We observed dysregulation of 3285 genes in restricted functional categories, with increased mRNA expression of cellular stress and cell death genes and reduced expression of developmentally-regulated and brain-specific genes, specifically those that regulate neuronal migration of GABAergic interneurons, including the Distal-less (Dlx) family of transcription factors required for tangential migration from progenitor pools within the ganglionic eminences into the cerebral cortex. Our results provide a novel mechanism by which MIA induces the widespread down-regulation of critical neurodevelopmental genes, including those previously associated with autism.

Neural activity and neurotransmission regulate the maturation of the innervation field of cortical GABAergic interneurons in an age-dependent manner

Neural activity guides the patterning of neuron synaptic territory in the developing nervous system. Evidence supporting this hypothesis comes from numerous studies on projection neurons in neuromuscular and visual systems. It is unknown whether the innervation field of GABAergic interneurons, which forms local dense innervations, follows similar rules. Cortical basket cells innervate hundreds of pyramidal cell somata and proximal dendrites. Thanks to this connectivity pattern, they can tightly control neural excitability and synchronization. Here we show that reducing excitation, and thus neurotransmitter release, in mouse cortical single basket cells in slice cultures decreases the number of innervated cells without changing the pattern of perisomatic innervation, both at the peak and after the proliferation phase of perisomatic synapse formation. Conversely, suppressing neurotransmitter release in single basket cells can have completely opposite effects depending on the developmental stage. Our results reveal a remarkably specific and age-dependent role of neural activity and neurotransmission levels in the establishment of the synaptic territory of cortical GABAergic cells.

GABAergic circuit dysfunctions in neurodevelopmental disorders

GABAergic interneurons control neuronal excitability, integration, and plasticity. Further, they regulate the generation of temporal synchrony and oscillatory behavior among networks of pyramidal neurons. Such oscillations within and across neural systems are believed to serve various complex functions, such as perception, movement initiation, and memory. Alterations in the development of GABAergic circuits have been implicated in various brain diseases with neurodevelopmental origin. Here, we highlight recent studies suggesting a role for alterations of GABA transmission in the pathophysiology of two neurodevelopmental diseases, schizophrenia, and autism. We further discuss how manipulations of GABA signaling may be used for novel therapeutic interventions.