Synaptic circuit abnormalities of motor-frontal layer 2/3 pyramidal neurons in an RNA interference model of methyl-CpG-binding protein 2 deficiency

Neocortical arealization: evolution, mechanisms, and open questions

The mammalian neocortex is a structure with no equals in the vertebrates and is the seat of the highest cerebral functions, such as thoughts and consciousness. It is radially organized into six layers and tangentially subdivided into functional areas deputed to the elaboration of sensory information, association between different stimuli, and selection and triggering of voluntary movements. The process subdividing the neocortical field into several functional areas is called "arealization". Each area has its own cytoarchitecture, connectivity, and peculiar functions. In the last century, several neuroscientists have investigated areal structure and the mechanisms that have led during evolution to the rising of the neocortex and its organization. The extreme conservation in the positioning and wiring of neocortical areas among different mammalian families suggests a conserved genetic program orchestrating neocortical patterning. However, the impressive plasticity of the neocortex, which is able to rewire and reorganize areal structures and connectivity after impairments of sensory pathways, argues for a more complex scenario. Indeed, even if genetics and molecular biology helped in identifying several genes involved in the arealization process, the logic underlying the neocortical bauplan is still beyond our comprehension. In this review, we will introduce the present knowledge and hypotheses on the ontogenesis and evolution of neocortical areas. Then, we will focus our attention on some open issues, which are still unresolved, and discuss some recent studies that might open new directions to be explored in the next few years.

NMDA receptor regulation prevents regression of visual cortical function in the absence of Mecp2

Brain function is shaped by postnatal experience and vulnerable to disruption of Methyl-CpG-binding protein, Mecp2, in multiple neurodevelopmental disorders. How Mecp2 contributes to the experience-dependent refinement of specific cortical circuits and their impairment remains unknown. We analyzed vision in gene-targeted mice and observed an initial normal development in the absence of Mecp2. Visual acuity then rapidly regressed after postnatal day P35-40 and cortical circuits largely fell silent by P55-60. Enhanced inhibitory gating and an excess of parvalbumin-positive, perisomatic input preceded the loss of vision. Both cortical function and inhibitory hyperconnectivity were strikingly rescued independent of Mecp2 by early sensory deprivation or genetic deletion of the excitatory NMDA receptor subunit, NR2A. Thus, vision is a sensitive biomarker of progressive cortical dysfunction and may guide novel, circuit-based therapies for Mecp2 deficiency.

Brain activity mapping in Mecp2 mutant mice reveals functional deficits in forebrain circuits, including key nodes in the default mode network, that are reversed with ketamine treatment

Excitatory-inhibitory imbalance has been identified within specific brain microcircuits in models of Rett syndrome (RTT) and other autism spectrum disorders (ASDs). However, macrocircuit dysfunction across the RTT brain as a whole has not been defined. To approach this issue, we mapped expression of the activity-dependent, immediate-early gene product Fos in the brains of wild-type (Wt) and methyl-CpG-binding protein 2 (Mecp2)-null (Null) mice, a model of RTT, before and after the appearance of overt symptoms (3 and 6 weeks of age, respectively). At 6 weeks, Null mice exhibit significantly less Fos labeling than Wt in limbic cortices and subcortical structures, including key nodes in the default mode network. In contrast, Null mice exhibit significantly more Fos labeling than Wt in the hindbrain, most notably in cardiorespiratory regions of the nucleus tractus solitarius (nTS). Using nTS as a model, whole-cell recordings demonstrated that increased Fos expression in Nulls at 6 weeks of age is associated with synaptic hyperexcitability, including increased frequency of spontaneous and miniature EPSCs and increased amplitude of evoked EPSCs in Nulls. No such effect of genotype on Fos or synaptic function was seen at 3 weeks. In the mutant forebrain, reduced Fos expression, as well as abnormal sensorimotor function, were reversed by the NMDA receptor antagonist ketamine. In light of recent findings that the default mode network is hypoactive in autism, our data raise the possibility that hypofunction within this meta-circuit is a shared feature of RTT and other ASDs and is reversible.

Reduced seizure threshold and altered network oscillatory properties in a mouse model of Rett syndrome

Rett syndrome (RTT) is a disorder with a pronounced neurological phenotype and is caused mainly by mutations in the X-linked gene MECP2. A common feature of RTT is an abnormal electroencephalography and a propensity for seizures. In the current study we aimed to assess brain network excitability and seizure propensity in a mouse model of RTT. Mice in which Mecp2 expression was silenced (Mecp2(stop/y)) showed a higher seizure score (mean=6 ± 0.8 compared to 4±0.2 in wild-type [WT]) and more rapid seizure onset (median onset=10 min in Mecp2(stop/y) and 32 min in WT) when challenged with the convulsant drug kainic acid (25mg/kg). Hippocampal slices from Mecp2(stop/y) brain displayed no spontaneous field potential activities under control conditions but showed higher power gamma frequency field potential oscillations compared to WT in response to kainic acid (400 nM) in vitro. Brain slices challenged with the GABA(A)-receptor antagonist bicuculline (0.1-10 μM) and the potassium channel blocker 4-aminopyridine (1-50 μM) also revealed differences between genotypes with hippocampal circuits from Mecp2(stop/y) mouse slices showing enhanced epileptiform burst duration and frequency. In contrast to these network level findings, single cell analysis of pyramidal cells by whole-cell patch clamp recording revealed no detectable differences in synaptic or biophysical properties between methyl-CpG-binding protein 2 (MeCP2)-containing and MeCP2-deficient neurons. These data support the proposal that loss of MeCP2 alters network level excitability in the brain to promote epileptogenesis.

A critical and cell-autonomous role for MeCP2 in synaptic scaling up

Rett syndrome (Rett) is the leading genetic cause of mental retardation in females. Most cases of Rett are caused by loss-of-function mutations in the gene coding for the transcriptional regulator methyl-CpG binding protein 2 (MeCP2), but despite much effort, it remains unclear how a loss of MeCP2 function generates the neurological deficits of Rett. Here we show that MeCP2 plays an essential and cell-autonomous role in homeostatic synaptic scaling up in response to reduced firing or reduced sensory drive in rat visual cortical pyramidal neurons. We found that acute RNAi knockdown of MeCP2 blocked synaptic scaling within targeted neocortical pyramidal neurons. Furthermore, MeCP2 knockdown decreased excitatory synapse number without affecting basal mEPSC amplitude or AMPAR accumulation at spared synapses, demonstrating that MeCP2 acts cell-autonomously to maintain both excitatory synapse number and synaptic scaling in individual neocortical neurons. Finally, we used a mouse model of Rett to show that MeCP2 loss prevents homeostatic synaptic scaling up in response to visual deprivation in vivo, demonstrating for the first time that MeCP2 loss disrupts homeostatic plasticity within the intact developing neocortex. Our results establish MeCP2 as a critical mediator of synaptic scaling and raise the possibility that some of the neurological defects of Rett arise from a disruption of homeostatic plasticity.

Laminar circuit organization and response modulation in mouse visual cortex

The mouse has become an increasingly important animal model for visual system studies, but few studies have investigated local functional circuit organization of mouse visual cortex. Here we used our newly developed mapping technique combining laser scanning photostimulation (LSPS) with fast voltage-sensitive dye (VSD) imaging to examine the spatial organization and temporal dynamics of laminar circuit responses in living slice preparations of mouse primary visual cortex (V1). During experiments, LSPS using caged glutamate provided spatially restricted neuronal activation in a specific cortical layer, and evoked responses from the stimulated layer to its functionally connected regions were detected by VSD imaging. In this study, we first provided a detailed analysis of spatiotemporal activation patterns at specific V1 laminar locations and measured local circuit connectivity. Then we examined the role of cortical inhibition in the propagation of evoked cortical responses by comparing circuit activity patterns in control and in the presence of GABAa receptor antagonists. We found that GABAergic inhibition was critical in restricting layer-specific excitatory activity spread and maintaining topographical projections. In addition, we investigated how AMPA and NMDA receptors influenced cortical responses and found that blocking AMPA receptors abolished interlaminar functional projections, and the NMDA receptor activity was important in controlling visual cortical circuit excitability and modulating activity propagation. The NMDA receptor antagonist reduced neuronal population activity in time-dependent and laminar-specific manners. Finally, we used the quantitative information derived from the mapping experiments and presented computational modeling analysis of V1 circuit organization. Taken together, the present study has provided important new information about mouse V1 circuit organization and response modulation.

The neural circuit basis of Rett syndrome

Rett syndrome is an Autism Spectrum Disorder caused by mutations in the gene encoding methyl-CpG binding protein (MeCP2). Following a period of normal development, patients lose learned communication and motor skills, and develop a number of symptoms including motor disturbances, cognitive impairments and often seizures. In this review, we discuss the role of MeCP2 in regulating synaptic function and how synaptic dysfunctions lead to neuronal network impairments and alterations in sensory information processing. We propose that Rett syndrome is a disorder of neural circuits as a result of non-linear accumulated dysfunction of synapses at the level of individual cell populations across multiple neurotransmitter systems and brain regions.

Circuit mapping by ultraviolet uncaging of glutamate

In laser photostimulation, small clusters of neurons in brain slices are induced to fire action potentials by focal glutamate uncaging, and synaptic connectivity between photoexcited presynaptic neurons and individual postsynaptic neurons is assessed by intracellular recording of synaptic events. With a scanner, this process can be repeated sequentially across a patterned array of stimulus locations, generating maps of neurons' local sources of synaptic inputs. Laser scanning photostimulation (LSPS) based on patterned glutamate uncaging offers an efficient, quantitative, optical-electrophysiological way to map synaptic circuits in brain slices. The efficacy of glutamate-based photostimulation for circuit mapping (in contrast to electrical stimulation) derives from the ability to stimulate neurons with high precision and speed, and without stimulating axons of passage. This protocol describes the components, assembly, and operation of a laser scanning microscope for ultraviolet (UV) uncaging, along with experimental methods for circuit mapping in brain slices. It presents a general approach and a set of guidelines for quantitative circuit mapping using "standard" LSPS methods based on single-photon glutamate uncaging using a UV laser, a pair of scanning mirror galvanometers, a patch-clamp setup, and open-source data acquisition software.

High-performance and site-directed in utero electroporation by a triple-electrode probe

In utero electroporation is a powerful tool to transfect and manipulate neural-precursor cells of the rodent parietal cortex and their progeny in vivo. Although this technique can potentially target numerous brain areas, reliability of transfection in some brain regions is low or physical access is limited. Here, we present a new in utero-electroporation configuration based on the use of three electrodes, the relative position and polarities of which can be adjusted. The technique allows easy access and exceedingly reliable monolateral or bilateral transfection at brain locations that could only be sporadically targeted before. By improvement in the efficiency of the electrical field distribution, demonstrated here by a mathematical simulation, the multi-electrode configuration also extends the developmental timeframe for reliable in utero electroporation, allowing for the first time specific transfection of Purkinje cells in the rat cerebellum.

Laminarly orthogonal excitation of fast-spiking and low-threshold-spiking interneurons in mouse motor cortex

In motor cortex, long-range output to subcortical motor circuits depends on excitatory and inhibitory inputs converging on projection neurons in layers 5A/B. How interneurons interconnect with these projection neurons, and whether these microcircuits are interneuron and/or projection specific, is unclear. We found that fast-spiking interneurons received strong intralaminar (horizontal) excitation from pyramidal neurons in layers 5A/B including corticostriatal and corticospinal neurons, implicating them in mediating disynaptic recurrent, feedforward, and feedback inhibition within and across the two projection classes. Low-threshold-spiking (LTS) interneurons were instead strongly excited by descending interlaminar (vertical) input from layer 2/3 pyramidal neurons, implicating them in mediating disynaptic feedforward inhibition to both projection classes. Furthermore, in a novel pattern, lower layer 2/3 preferentially excited interneurons in one layer (5A/LTS) and excitatory neurons in another (5B/corticospinal). Thus, these inhibitory microcircuits in mouse motor cortex follow an orderly arrangement that is laminarly orthogonalized by interneuron-specific, projection-nonspecific connectivity.

Modeling of autism genetic variations in mice: focusing on synaptic and microcircuit dysfunctions

Autism spectrum disorders (ASD) are heterogeneous neurodevelopmental disorders that are characterized by deficits in social interaction, verbal and nonverbal communication, and restrictive interests and repetitive behaviors. While human genetic studies have revealed marked heritability in ASD, it has been challenging to translate this genetic risk into a biological mechanism that influences brain development relevant to the disorder phenotypes. This is partly due to the complex genetic architecture of ASD, which involves de novo gene mutations, genomic abnormalities, and common genetic variants. Rather than trying to reconstitute the clinical disorder, using genetic model animals to examine specific features of core ASD pathophysiology offers unique opportunities for refining our understanding of neurodevelopmental mechanisms in ASD. A variety of ASD-relevant phenotypes can now be investigated in rodents, including stereotyped and repetitive behaviors, and deficits in social interaction and communication. In this review, we focus on several prevailing mouse models and discuss how studies have advanced our understanding of synaptic mechanisms that may underlie ASD pathophysiology. Although synaptic perturbations are not the only alterations relevant for ASD, we reason that understanding the synaptic underpinnings of ASD using mouse models may provide mechanistic insights into its etiology and lead to novel therapeutic and interventional strategies.

PTEN regulation of local and long-range connections in mouse auditory cortex

Autism spectrum disorders (ASDs) are highly heritable developmental disorders caused by a heterogeneous collection of genetic lesions. Here we use a mouse model to study the effect on cortical connectivity of disrupting the ASD candidate gene PTEN (phosphatase and tensin homolog deleted on chromosome 10). Through Cre-mediated recombination, we conditionally knocked out PTEN expression in a subset of auditory cortical neurons. Analysis of long-range connectivity using channelrhodopsin-2 revealed that the strength of synaptic inputs from both the contralateral auditory cortex and from the thalamus onto PTEN-cko neurons was enhanced compared with nearby neurons with normal PTEN expression. Laser-scanning photostimulation showed that local inputs onto PTEN-cko neurons in the auditory cortex were similarly enhanced. The hyperconnectivity caused by PTEN-cko could be blocked by rapamycin, a specific inhibitor of the PTEN downstream molecule mammalian target of rapamycin complex 1. Together, our results suggest that local and long-range hyperconnectivity may constitute a physiological basis for the effects of mutations in PTEN and possibly other ASD candidate genes.

Rett syndrome: genes, synapses, circuits, and therapeutics

Development of the nervous system proceeds through a set of complex checkpoints which arise from a combination of sequential gene expression and early neural activity sculpted by the environment. Genetic and environmental insults lead to neurodevelopmental disorders which encompass a large group of diseases that result from anatomical and physiological abnormalities during maturation and development of brain circuits. Rett syndrome (RTT) is a neurological disorder of genetic origin, caused by mutations in the X-linked gene methyl-CpG binding protein 2 (MeCP2). It features a range of neuropsychiatric abnormalities including motor dysfunctions and mild to severe cognitive impairment. Here, we discuss key questions and recent studies describing animal models, cell-type specific functions of methyl-CpG binding protein 2 (MeCP2), defects in neural circuit plasticity, and attempts to evaluate possible therapeutic strategies for RTT. We also discuss how genes, proteins, and overlapping signaling pathways affect the molecular etiology of apparently unrelated neuropsychiatric disorders, an understanding of which can offer novel therapeutic strategies for a range of autism spectrum disorders (ASDs).

Synaptic microcircuit dysfunction in genetic models of neurodevelopmental disorders: focus on Mecp2 and Met

Recent findings in the genetics of neurodevelopmental syndromes have ushered in an exciting era of discovery in which substrates of neurologic dysfunction are being identified at the synaptic and microcircuit levels in mouse models of these disorders. We review recent progress in this area, focusing on two examples of mouse models of autism spectrum disorders (ASDs): Mecp2 models of Rett syndrome, and a Met-knockout model of non-syndromic forms of autism. In both cases, a dominant theme is changes in synaptic strength, associated with hyper-connectivity or hypo-connectivity in specific microcircuits. Alterations in intrinsic neuronal excitability are also found, but do not appear to be as common. The microcircuit-specific nature of synaptic changes observed in these ASD models indicates that it will be necessary to define mechanisms of circuit dysfunction on a case-by-case basis, not only in neocortex but also in brainstem and other sub-cortical areas. Thus, functional microcircuit analysis is emerging as an important line of investigation, highly complementary to neurogenetic and molecular strategies, and holds promise for generating models of the underlying pathophysiology and for guiding development of novel therapeutic strategies.

Normal mitral cell dendritic development in the setting of Mecp2 mutation

Rett syndrome (RTT) is an autism spectrum disorder caused by mutation in the gene encoding methyl CpG binding protein 2 (MECP2). Evidence to date suggests that these disorders display defects in synaptic organization and plasticity. A hallmark of the pathology in RTT has been identified as decreased dendritic arborization, which has been interpreted to represent abnormal dendritic formation and pruning during development. Our previous studies revealed that olfactory axons display defective pathfinding and targeting in the setting of Mecp2 mutation. In the present work, we use Mecp2 mutant mouse models and the olfactory system to investigate dendritic development. Here, we demonstrate that mitral cell dendritic development proceeds normally in mutant mice, resulting in typical dendritic morphology at early postnatal ages. We also failed to detect abnormalities in dendritic inputs at symptomatic stages when glomeruli from mutant mice appear smaller in area than the wild type (WT) (6 weeks postnatally). Collectively, these findings suggest that the initial defects in glomeruli impairment seen with Mecp2 mutation do not result from abnormal dendritic development. Our results using the olfactory system indicate that dendritic abnormalities are not an early feature in the abnormalities incurred by Mecp2 mutation.

Rett syndrome mutation MeCP2 T158A disrupts DNA binding, protein stability and ERP responses

Mutations in the MECP2 gene cause the autism spectrum disorder Rett Syndrome (RTT). One of the most common mutations associated with RTT occurs at MeCP2 Threonine 158 converting it to Methionine (T158M) or Alanine (T158A). To understand the role of T158 mutation in the pathogenesis of RTT, we generated knockin mice recapitulating MeCP2 T158A mutation. Here we show a causal role for T158A mutation in the development of RTT-like phenotypes including developmental regression, motor dysfunction, and learning and memory deficits. These phenotypes resemble those in Mecp2-null mice and manifest through a reduction in MeCP2 binding to methylated DNA and a decrease in MeCP2 protein stability. Importantly, the age-dependent development of event-related neuronal responses are disrupted by MeCP2 mutation, suggesting that impaired neuronal circuitry underlies the pathogenesis of RTT and that assessment of event-related potentials may serve as a biomarker for RTT and treatment evaluation.

Corticospinal-specific HCN expression in mouse motor cortex: I(h)-dependent synaptic integration as a candidate microcircuit mechanism involved in motor control

Motor cortex is a key brain center involved in motor control in rodents and other mammals, but specific intracortical mechanisms at the microcircuit level are largely unknown. Neuronal expression of hyperpolarization-activated current (I(h)) is cell class specific throughout the nervous system, but in neocortex, where pyramidal neurons are classified in various ways, a systematic pattern of expression has not been identified. We tested whether I(h) is differentially expressed among projection classes of pyramidal neurons in mouse motor cortex. I(h) expression was high in corticospinal neurons and low in corticostriatal and corticocortical neurons, a pattern mirrored by mRNA levels for HCN1 and Trip8b subunits. Optical mapping experiments showed that I(h) attenuated glutamatergic responses evoked across the apical and basal dendritic arbors of corticospinal but not corticostriatal neurons. Due to I(h), corticospinal neurons resonated, with a broad peak at ∼4 Hz, and were selectively modulated by α-adrenergic stimulation. I(h) reduced the summation of short trains of artificial excitatory postsynaptic potentials (EPSPs) injected at the soma, and similar effects were observed for short trains of actual EPSPs evoked from layer 2/3 neurons. I(h) narrowed the coincidence detection window for EPSPs arriving from separate layer 2/3 inputs, indicating that the dampening effect of I(h) extended to spatially disperse inputs. To test the role of corticospinal I(h) in transforming EPSPs into action potentials, we transfected layer 2/3 pyramidal neurons with channelrhodopsin-2 and used rapid photostimulation across multiple sites to synaptically drive spiking activity in postsynaptic neurons. Blocking I(h) increased layer 2/3-driven spiking in corticospinal but not corticostriatal neurons. Our results imply that I(h)-dependent synaptic integration in corticospinal neurons constitutes an intracortical control mechanism, regulating the efficacy with which local activity in motor cortex is transferred to downstream circuits in the spinal cord. We speculate that modulation of I(h) in corticospinal neurons could provide a microcircuit-level mechanism involved in translating action planning into action execution.

Mechanisms of specificity in neuronal activity-regulated gene transcription

The brain is a highly adaptable organ that is capable of converting sensory information into changes in neuronal function. This plasticity allows behavior to be accommodated to the environment, providing an important evolutionary advantage. Neurons convert environmental stimuli into long-lasting changes in their physiology in part through the synaptic activity-regulated transcription of new gene products. Since the neurotransmitter-dependent regulation of Fos transcription was first discovered nearly 25 years ago, a wealth of studies have enriched our understanding of the molecular pathways that mediate activity-regulated changes in gene transcription. These findings show that a broad range of signaling pathways and transcriptional regulators can be engaged by neuronal activity to sculpt complex programs of stimulus-regulated gene transcription. However, the shear scope of the transcriptional pathways engaged by neuronal activity raises the question of how specificity in the nature of the transcriptional response is achieved in order to encode physiologically relevant responses to divergent stimuli. Here we summarize the general paradigms by which neuronal activity regulates transcription while focusing on the molecular mechanisms that confer differential stimulus-, cell-type-, and developmental-specificity upon activity-regulated programs of neuronal gene transcription. In addition, we preview some of the new technologies that will advance our future understanding of the mechanisms and consequences of activity-regulated gene transcription in the brain.

Experience-dependent retinogeniculate synapse remodeling is abnormal in MeCP2-deficient mice

Mutations in MECP2 underlie the neurodevelopmental disorder Rett syndrome (RTT). One hallmark of RTT is relatively normal development followed by a later onset of symptoms. Growing evidence suggests an etiology of disrupted synaptic function, yet it is unclear how these abnormalities explain the clinical presentation of RTT. Here we investigate synapse maturation in Mecp2-deficient mice at a circuit with distinct developmental phases: the retinogeniculate synapse. We find that synapse development in mutants is comparable to that of wild-type littermates between postnatal days 9 and 21, indicating that initial phases of synapse formation, elimination, and strengthening are not significantly affected by MeCP2 absence. However, during the subsequent experience-dependent phase of synapse remodeling, the circuit becomes abnormal in mutants as retinal innervation of relay neurons increases and retinal inputs fail to strengthen further. Moreover, synaptic plasticity in response to visual deprivation is disrupted in mutants. These results suggest a crucial role for Mecp2 in experience-dependent refinement of synaptic circuits.

Neuronal activity-regulated gene transcription in synapse development and cognitive function

Activity-dependent plasticity of vertebrate neurons allows the brain to respond to its environment. During brain development, both spontaneous and sensory-driven neural activity are essential for instructively guiding the process of synapse development. These effects of neuronal activity are transduced in part through the concerted regulation of a set of activity-dependent transcription factors that coordinate a program of gene expression required for the formation and maturation of synapses. Here we review the cellular signaling networks that regulate the activity of transcription factors during brain development and discuss the functional roles of specific activity-regulated transcription factors in specific stages of synapse formation, refinement, and maturation. Interestingly, a number of neurodevelopmental disorders have been linked to abnormalities in activity-regulated transcriptional pathways, indicating that these signaling networks are critical for cognitive development and function.

Selective impact of MeCP2 and associated histone deacetylases on the dynamics of evoked excitatory neurotransmission

An imbalance between the strengths of excitatory and inhibitory synaptic inputs has been proposed as the cellular basis of autism and related neurodevelopmental disorders. Previous studies examining spontaneous levels of excitatory and inhibitory neurotransmission in the forebrain regions of methyl-CpG-binding protein 2 (Mecp2) mutant mice, models of the autism spectrum disorder Rett syndrome, have identified a decrease in excitatory drive, in some cases coupled with an increase in inhibitory synaptic strength, as a major source of this imbalance. Here, we reevaluated this question by examining the short-term dynamics of evoked neurotransmission between hippocampal neurons cultured from MeCP2 knockout mice and found a marked increase in evoked excitatory neurotransmission that is consistent with an increase in presynaptic release probability. This increase in evoked excitatory drive was not matched with alterations in evoked inhibitory neurotransmission. Moreover, we observed similar excitatory drive specific changes after the loss of key histone deacetylases (histone deacetylase 1 and 2) that form a complex with MeCP2 and mediate transcriptional regulation. These findings suggest a distinct role for MeCP2 and its cofactors in the regulation of evoked excitatory neurotransmission compared with their essential role in basal synaptic activity.

Circuit-specific intracortical hyperconnectivity in mice with deletion of the autism-associated Met receptor tyrosine kinase

Local hyperconnectivity in the neocortex is a hypothesized pathophysiological state in autism spectrum disorder (ASD). MET, a receptor tyrosine kinase that regulates dendrite and spine morphogenesis, has been established as a risk gene for ASD. Here, we analyzed the synaptic circuit organization of identified pyramidal neurons in the anterior frontal cortex of mice with a dorsal pallium-derived, conditional knock-out (cKO) of Met. Synaptic mapping by glutamate uncaging identified layer 2/3 as the main source of local excitatory input to layer 5 projection neurons in controls. In both cKO and heterozygotes, this pathway was stronger by a factor of approximately 2. This increase was both sublayer and projection-class specific, restricted to corticostriatal neurons in upper layer 5B and not neighboring corticopontine neurons. Paired recordings in cKO slices demonstrated increased unitary connectivity. We propose that excitatory hyperconnectivity in specific neocortical microcircuits constitutes a physiological basis for Met-mediated ASD risk.

Short-hairpin RNA silencing of endogenous fibroblast growth factor 2 in rat hippocampus increases anxiety behavior

BACKGROUND

The fibroblast growth factor system has been implicated in the pathophysiology of mood disorders in humans and in affective behavior in animal models. However, the studies have been either correlative or involved exogenous administration of fibroblast growth factor 2 (FGF2). None of them have directly linked endogenous FGF2 to changes in emotional responses. Therefore, we began a series of studies to knockdown FGF2 by RNA interference to examine the role of brain FGF2 in emotional responsiveness.

METHODS

We assessed the efficacy of short-hairpin RNA (shRNA) sequences targeted to FGF2 in COS7 cells transfected with a plasmid vector containing the full-length FGF2 sequence. We then sought to assess the effects of knocking down FGF2 gene expression in vivo on behavior. We microinjected a lentiviral vector containing either a shRNA targeting FGF2 or a nonsilencing sequence bilaterally into the dentate gyrus of the rat.

RESULTS

In a reporter assay system, three different shRNA sequences resulted in significant FGF2 knockdown in vitro. Five weeks following a single microinjection of one of those sequences in vivo, we observed a significant decrease in FGF2 gene expression by messenger RNA in situ hybridization in the hippocampus. The FGF2 knockdown increased the time spent in the closed arms of the elevated-plus maze, a test of anxiety behavior.

CONCLUSIONS

The FGF2 knockdown in the hippocampus resulted in an anxiogenic effect. Together with our findings of an inverse correlation between anxiety and FGF2 expression levels, these results implicate FGF2 in the genesis and expression of anxiety disorders.

Epigenetics in the mature mammalian brain: effects on behavior and synaptic transmission

The role of epigenetic mechanisms in control of gene expression during mammalian development is well established. Associations between specific DNA or histone modifications and numerous neurodevelopmental and neurodegenerative disorders implies significant consequences of epigenetic dysregulation in both the developing and mature brain, the latter of which is the general focus of this review. Accumulating evidence suggests that epigenetic changes are involved in normal cognitive processes in addition to neurological and psychiatric disorders. Recent investigations into the regulation of epigenetic modifications in the adult brain have revealed novel and surprisingly dynamic mechanisms for controlling learning and memory-related behaviors as well as long-term synaptic plasticity. DNA methylation and histone acetylation have also been implicated in the modulation of basal synaptic transmission and the balance between excitation and inhibition in various brain regions. Studies have begun to uncover some of the alterations in gene expression that appear to mediate many of these effects, but an understanding of the precise mechanisms involved is still lacking. Nevertheless, the fundamental importance of epigenetic processes in influencing neuronal activity is becoming increasingly evident.

Experimental models of Rett syndrome based on Mecp2 dysfunction

Rett syndrome (RTT) is a neurodevelopmental disorder predominantly occurring in females with an incidence of 1:10,000 births and caused by sporadic mutations in the MECP2 gene, which encodes methyl-CpG-binding protein-2, an epigenetic transcription factor that binds methylated DNA. The clinical hallmarks include a period of apparently normal early development followed by a plateau and then subsequent frank regression. Impaired visual and aural contact often lead to an initial diagnosis of autism. The characterization of experimental models based on the loss-of-function of the mouse Mecp2 gene revealed that subtle changes in the morphology and function of brain cells and synapses have profound consequences on network activities that underlie critical brain functions. Furthermore, these experimental models have been used for successful reversals of RTT-like symptoms by genetic, pharmacological and environmental manipulations, raising hope for novel therapeutic strategies to improve the quality of life of RTT individuals.

Genome-wide analysis reveals methyl-CpG-binding protein 2-dependent regulation of microRNAs in a mouse model of Rett syndrome

MicroRNAs (miRNAs) are a class of small, noncoding RNAs that function as posttranscriptional regulators of gene expression. Many miRNAs are expressed in the developing brain and regulate multiple aspects of neural development, including neurogenesis, dendritogenesis, and synapse formation. Rett syndrome (RTT) is a progressive neurodevelopmental disorder caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MECP2). Although Mecp2 is known to act as a global transcriptional regulator, miRNAs that are directly regulated by Mecp2 in the brain are not known. Using massively parallel sequencing methods, we have identified miRNAs whose expression is altered in cerebella of Mecp2-null mice before and after the onset of severe neurological symptoms. In vivo genome-wide analyses indicate that promoter regions of a significant fraction of dysregulated miRNA transcripts, including a large polycistronic cluster of brain-specific miRNAs, are DNA-methylated and are bound directly by Mecp2. Functional analysis demonstrates that the 3' UTR of messenger RNA encoding Brain-derived neurotrophic factor (Bdnf) can be targeted by multiple miRNAs aberrantly up-regulated in the absence of Mecp2. Taken together, these results suggest that dysregulation of miRNAs may contribute to RTT pathoetiology and also may provide a valuable resource for further investigations of the role of miRNAs in RTT.

Ephus: multipurpose data acquisition software for neuroscience experiments

Physiological measurements in neuroscience experiments often involve complex stimulus paradigms and multiple data channels. Ephus (http://www.ephus.org) is an open-source software package designed for general-purpose data acquisition and instrument control. Ephus operates as a collection of modular programs, including an ephys program for standard whole-cell recording with single or multiple electrodes in typical electrophysiological experiments, and a mapper program for synaptic circuit mapping experiments involving laser scanning photostimulation based on glutamate uncaging or channelrhodopsin-2 excitation. Custom user functions allow user-extensibility at multiple levels, including on-line analysis and closed-loop experiments, where experimental parameters can be changed based on recently acquired data, such as during in vivo behavioral experiments. Ephus is compatible with a variety of data acquisition and imaging hardware. This paper describes the main features and modules of Ephus and their use in representative experimental applications.

Genetic controls balancing excitatory and inhibitory synaptogenesis in neurodevelopmental disorder models

Proper brain function requires stringent balance of excitatory and inhibitory synapse formation during neural circuit assembly. Mutation of genes that normally sculpt and maintain this balance results in severe dysfunction, causing neurodevelopmental disorders including autism, epilepsy and Rett syndrome. Such mutations may result in defective architectural structuring of synaptic connections, molecular assembly of synapses and/or functional synaptogenesis. The affected genes often encode synaptic components directly, but also include regulators that secondarily mediate the synthesis or assembly of synaptic proteins. The prime example is Fragile X syndrome (FXS), the leading heritable cause of both intellectual disability and autism spectrum disorders. FXS results from loss of mRNA-binding FMRP, which regulates synaptic transcript trafficking, stability and translation in activity-dependent synaptogenesis and plasticity mechanisms. Genetic models of FXS exhibit striking excitatory and inhibitory synapse imbalance, associated with impaired cognitive and social interaction behaviors. Downstream of translation control, a number of specific synaptic proteins regulate excitatory versus inhibitory synaptogenesis, independently or combinatorially, and loss of these proteins is also linked to disrupted neurodevelopment. The current effort is to define the cascade of events linking transcription, translation and the role of specific synaptic proteins in the maintenance of excitatory versus inhibitory synapses during neural circuit formation. This focus includes mechanisms that fine-tune excitation and inhibition during the refinement of functional synaptic circuits, and later modulate this balance throughout life. The use of powerful new genetic models has begun to shed light on the mechanistic bases of excitation/inhibition imbalance for a range of neurodevelopmental disease states.

Sublayer-specific microcircuits of corticospinal and corticostriatal neurons in motor cortex

The mammalian motor system is organized around distinct subcortical subsystems, suggesting that the intracortical circuits immediately upstream of spinal cord and basal ganglia might be functionally differentiated as well. We found that the main excitatory pathway in mouse motor cortex, layer 2/3-->5, is fractionated into distinct pathways targeting corticospinal and corticostriatal neurons, which are involved in motor control. However, connections were selective for neurons in certain sublayers: corticospinal neurons in upper layer 5B and corticostriatal neurons in lower 5A. A simple structural combinatorial principle accounts for this highly specific functional circuit architecture: potential connectivity is established by neuronal sublayer positioning and actual connectivity in this framework is determined by long-range axonal projection targets. Thus, intracortical circuits of these pyramidal neurons are specified not only by their long-range axonal targets or their layer or sublayer positions, but by both, in specific combinations.

Synaptic circuit abnormalities of motor-frontal layer 2/3 pyramidal neurons in a mutant mouse model of Rett syndrome

Motor and cognitive functions are severely impaired in Rett syndrome (RTT). Here, we examined local synaptic circuits of layer 2/3 (L2/3) pyramidal neurons in motor-frontal cortex of male hemizygous MeCP2-null mice at 3 to 4weeks of age. We mapped local excitatory input to L2/3 neurons using glutamate uncaging and laser scanning photostimulation, and compared synaptic input maps recorded from MeCP2-null and wild type (WT) mice. Local excitatory input was significantly reduced in the mutants. The strongest phenotype was observed for lateral (horizontal, intralaminar) inputs, that is, L2/3-->2/3 inputs, which showed a large reduction in MeCP2(-/y) animals. Neither the amount of local inhibitory input to these L2/3 pyramidal neurons nor their intrinsic electrophysiological properties differed by genotype. Our findings provide further evidence that excitatory networks are selectively reduced in RTT. We discuss our findings in the context of recently published parallel studies using selective MeCP2 knockdown in individual L2/3 neurons.

Rett syndrome microglia damage dendrites and synapses by the elevated release of glutamate

MECP2, an X-linked gene encoding the epigenetic factor methyl-CpG-binding protein-2, is mutated in Rett syndrome (RTT) and aberrantly expressed in autism. Most children affected by RTT are heterozygous Mecp2(-/+) females whose brain function is impaired postnatally due to MeCP2 deficiency. Recent studies suggest a role of glia in causing neuronal dysfunction via a non-cell-autonomous effect in RTT. Here we report a potent neurotoxic activity in the conditioned medium (CM) obtained from Mecp2-null microglia. Hippocampal neurons treated with CM from Mecp2-null microglia showed an abnormal stunted and beaded dendritic morphology, and signs of microtubule disruption and damage of postsynaptic glutamatergic components within 24 h. We identified that the toxic factor in the CM is glutamate, because (1) Mecp2-null microglia released a fivefold higher level of glutamate, (2) blockage of microglial glutamate synthesis by a glutaminase inhibitor abolished the neurotoxic activity, (3) blockage of microglial glutamate release by gap junction hemichannel blockers abolished the neurotoxic activity, and (4) glutamate receptor antagonists blocked the neurotoxicity of the Mecp2-null microglia CM. We further identified that increased levels of glutaminase and connexin 32 in Mecp2-null microglia are responsible for increased glutamate production and release, respectively. In contrast, the CM from highly pure Mecp2-null astrocyte cultures showed no toxic effect. Our results suggest that microglia may influence the onset and progression of RTT and that microglia glutamate synthesis or release could be a therapeutic target for RTT.

Diminished prevalence but preserved synaptic distribution of N-methyl-D-aspartate receptor subunits in the methyl CpG binding protein 2(MeCP2)-null mouse brain

In this study, we examined the prevalence and distribution of NMDA receptor subunits within crude synaptic membranes derived from the brains of mice lacking methyl CpG binding protein 2 (MeCP2). Our results show that the distribution of NMDA receptor subunits within the detergent soluble, detergent resistant, and postsynaptic density microdomains is preserved at MeCP2-null synapses. However, analysis of the NMDA receptor subunit expression revealed a decrease in the prevalence of the GluN1 and GluN2A subunits in MeCP2-null tissue. Collectively, these results indicate that synaptic membrane microdomains at synapses of the MeCP2-null brain develop normally, and that NMDA receptor subunits are properly targeted and distributed within them. The under-representation of the GluN1 and GluN2A subunits suggests that MeCP2-null synapses contain fewer mature NMDA receptor complexes, and raises the possibility that impaired NMDA receptor ontogeny could contribute to Rett syndrome pathophysiology.

Intracortical cartography in an agranular area

A well-defined granular layer 4 is a defining cytoarchitectonic feature associated with sensory areas of mammalian cerebral cortex, and one with hodological significance: the local axons ascending from cells in thalamorecipient layer 4 and connecting to layer 2/3 pyramidal neurons form a major feedforward excitatory interlaminar projection. Conversely, agranular cortical areas, lacking a distinct layer 4, pose a hodological conundrum: without a laminar basis for the canonical layer 4→2/3 pathway, what is the basic circuit organization? This review highlights current challenges and prospects for local-circuit electroanatomy and electrophysiology in agranular cortex, focusing on the mouse. Different lines of evidence, drawn primarily from studies of motor areas in frontal cortex in rodents, support the view that synaptic circuits in agranular cortex are organized around prominent descending excitatory layer 2/3→5 pathways targeting multiple classes of projection neurons.