Synaptogenesis in the developing mouse visual cortex

Alteration of synaptic protein composition during developmental synapse maturation

The postsynaptic density (PSD) is a collection of specialized proteins assembled beneath the postsynaptic membrane of dendritic spines. The PSD proteome comprises ~1000 proteins, including neurotransmitter receptors, scaffolding proteins and signalling enzymes. Many of these proteins have essential roles in synaptic function and plasticity. During brain development, changes are observed in synapse density and in the stability and shape of spines, reflecting the underlying molecular maturation of synapses. Synaptic protein composition changes in terms of protein abundance and the assembly of protein complexes, supercomplexes and the physical organization of the PSD. Here, we summarize the developmental alterations of postsynaptic protein composition during synapse maturation. We describe major PSD proteins involved in postsynaptic signalling that regulates synaptic plasticity and discuss the effect of altered expression of these proteins during development. We consider the abnormality of synaptic profiles and synaptic protein composition in the brain in neurodevelopmental disorders such as autism spectrum disorders. We also explain differences in synapse development between rodents and primates in terms of synaptic profiles and protein composition. Finally, we introduce recent findings related to synaptic diversity and nanoarchitecture and discuss their impact on future research. Synaptic protein composition can be considered a major determinant and marker of synapse maturation in normality and disease.

Dysregulation of astrocyte-secreted pleiotrophin contributes to neuronal structural and functional deficits in Down Syndrome

Neuronal dendrite patterning and synapse formation are tightly regulated during development to promote proper connectivity. Astrocyte-secreted proteins act as guidance and pro-synaptogenic factors during development, but little is known about how astrocytes may contribute to neurodevelopmental disorders. Here we identify down-regulation of the astrocyte-secreted molecule pleiotrophin as a major contributor to neuronal morphological alterations in the Ts65Dn mouse model of Down Syndrome. We find overlapping deficits in neuronal dendrites, spines and intracortical synapses in Ts65Dn mutant and pleiotrophin knockout mice. By targeting pleiotrophin overexpression to astrocytes in adult Ts65Dn mutant mice in vivo , we show that pleiotrophin can rescue dendrite morphology and spine density and increase excitatory synapse number. We further demonstrate functional improvements in behavior. Our findings identify pleiotrophin as a molecule that can be used in Down Syndrome to promote proper circuit connectivity, importantly at later stages of development after typical periods of circuit refinement have completed.

δ-Catenin controls astrocyte morphogenesis via layer-specific astrocyte-neuron cadherin interactions

Astrocytes control the formation of specific synaptic circuits via cell adhesion and secreted molecules. Astrocyte synaptogenic functions are dependent on the establishment of their complex morphology. However, it is unknown if distinct neuronal cues differentially regulate astrocyte morphogenesis. δ-Catenin was previously thought to be a neuron-specific protein that regulates dendrite morphology. We found δ-catenin is also highly expressed by astrocytes and required both in astrocytes and neurons for astrocyte morphogenesis. δ-Catenin is hypothesized to mediate transcellular interactions through the cadherin family of cell adhesion proteins. We used structural modeling and biochemical analyses to reveal that δ-catenin interacts with the N-cadherin juxtamembrane domain to promote N-cadherin surface expression. An autism-linked δ-catenin point mutation impaired N-cadherin cell surface expression and reduced astrocyte complexity. In the developing mouse cortex, only lower-layer cortical neurons express N-cadherin. Remarkably, when we silenced astrocytic N-cadherin throughout the cortex, only lower-layer astrocyte morphology was disrupted. These findings show that δ-catenin controls astrocyte-neuron cadherin interactions that regulate layer-specific astrocyte morphogenesis.

Uncovering a mammalian neural-specific poly(A) binding protein with unique properties

The mRNA 3' poly(A) tail plays a critical role in regulating both mRNA translation and turnover. It is bound by the cytoplasmic poly(A) binding protein (PABPC), an evolutionarily conserved protein that can interact with translation factors and mRNA decay machineries to regulate gene expression. Mammalian PABPC1, the prototypical PABPC, is expressed in most tissues and interacts with eukaryotic translation initiation factor 4G (eIF4G) to stimulate translation in specific contexts. In this study, we uncovered a new mammalian PABPC, which we named neural PABP (neuPABP), as it is predominantly expressed in the brain. neuPABP maintains a unique architecture as compared with other PABPCs, containing only two RNA recognition motifs (RRMs) and maintaining a unique N-terminal domain of unknown function. neuPABP expression is activated in neurons as they mature during synaptogenesis, where neuPABP localizes to the soma and postsynaptic densities. neuPABP interacts with the noncoding RNA BC1, as well as mRNAs coding for ribosomal and mitochondrial proteins. However, in contrast to PABPC1, neuPABP does not associate with actively translating mRNAs in the brain. In keeping with this, we show that neuPABP has evolved such that it does not bind eIF4G and as a result fails to support protein synthesis in vitro. Taken together, these results indicate that mammals have expanded their PABPC repertoire in the brain and propose that neuPABP may support the translational repression of select mRNAs.

Pyramidal neurons form active, transient, multilayered circuits perturbed by autism-associated mutations at the inception of neocortex

Cortical circuits are composed predominantly of pyramidal-to-pyramidal neuron connections, yet their assembly during embryonic development is not well understood. We show that mouse embryonic Rbp4-Cre cortical neurons, transcriptomically closest to layer 5 pyramidal neurons, display two phases of circuit assembly in vivo. At E14.5, they form a multi-layered circuit motif, composed of only embryonic near-projecting-type neurons. By E17.5, this transitions to a second motif involving all three embryonic types, analogous to the three adult layer 5 types. In vivo patch clamp recordings and two-photon calcium imaging of embryonic Rbp4-Cre neurons reveal active somas and neurites, tetrodotoxin-sensitive voltage-gated conductances, and functional glutamatergic synapses, from E14.5 onwards. Embryonic Rbp4-Cre neurons strongly express autism-associated genes and perturbing these genes interferes with the switch between the two motifs. Hence, pyramidal neurons form active, transient, multi-layered pyramidal-to-pyramidal circuits at the inception of neocortex, and studying these circuits could yield insights into the etiology of autism.

Synaptic Development in Diverse Olfactory Neuron Classes Uses Distinct Temporal and Activity-Related Programs

Developing neurons must meet core molecular, cellular, and temporal requirements to ensure the correct formation of synapses, resulting in functional circuits. However, because of the vast diversity in neuronal class and function, it is unclear whether or not all neurons use the same organizational mechanisms to form synaptic connections and achieve functional and morphologic maturation. Moreover, it remains unknown whether neurons united in a common goal and comprising the same sensory circuit develop on similar timescales and use identical molecular approaches to ensure the formation of the correct number of synapses. To begin to answer these questions, we took advantage of the Drosophila antennal lobe (AL), a model olfactory circuit with remarkable genetic access and synapse-level resolution. Using tissue-specific genetic labeling of active zones, we performed a quantitative analysis of synapse formation in multiple classes of neurons of both sexes throughout development and adulthood. We found that olfactory receptor neurons (ORNs), projection neurons (PNs), and local interneurons (LNs) each have unique time courses of synaptic development, addition, and refinement, demonstrating that each class follows a distinct developmental program. This raised the possibility that these classes may also have distinct molecular requirements for synapse formation. We genetically altered neuronal activity in each neuronal subtype and observed differing effects on synapse number based on the neuronal class examined. Silencing neuronal activity in ORNs, PNs, and LNs impaired synaptic development but only in ORNs did enhancing neuronal activity influence synapse formation. ORNs and LNs demonstrated similar impairment of synaptic development with enhanced activity of a master kinase, GSK-3β, suggesting that neuronal activity and GSK-3β kinase activity function in a common pathway. ORNs also, however, demonstrated impaired synaptic development with GSK-3β loss-of-function, suggesting additional activity-independent roles in development. Ultimately, our results suggest that the requirements for synaptic development are not uniform across all neuronal classes with considerable diversity existing in both their developmental time frames and molecular requirements. These findings provide novel insights into the mechanisms of synaptic development and lay the foundation for future work determining their underlying etiologies.SIGNIFICANCE STATEMENT Distinct olfactory neuron classes in Drosophila develop a mature synaptic complement over unique timelines and using distinct activity-dependent and molecular programs, despite having the same generalized goal of olfactory sensation.

Repeated sevoflurane exposures inhibit neurogenesis by inducing the upregulation of glutamate transporter 1 in astrocytes

Sevoflurane is a widely used general anaesthetic in paediatric patients. Although repeated sevoflurane exposure is known to cause neurodevelopmental disorders in children, the mechanism of this neurotoxicity remains largely unknown. Herein, we investigated the role of glutamate transporter 1 (GLT1) in sevoflurane-induced decreased neurogenesis. Neonatal rat pups (postnatal Day 7, PN7) were exposed to 3% sevoflurane for 2 h for three consecutive days. Neuron loss and decreased neurogenesis have been observed in the neonatal rat brain, along with decreased number of astrocytes. Apoptotic astrocytes were observed after repeated sevoflurane exposure in vitro, resulting in decreased levels of brain-derived neurotrophic factor (BDNF). Calcium overload was observed in astrocytes after repeated sevoflurane exposure, in addition to upregulation of GLT1. Inhibition of GLT1 activity ameliorates repeated sevoflurane exposure-induced cognitive deficits in adult rats. Mechanically, the upregulation of GLT1 was caused by the activation of mRNA translation. RNA-sequencing analysis further confirmed that translation-related genes were activated by repeated sevoflurane exposure. These results indicate that cognitive deficits caused by repeated sevoflurane exposure during PN7-9 are triggered decreased neurogenesis. The proposed underlying mechanism involves upregulation of apoptosis in astrocytes induced by GLT1; therefore, we propose GLT1 as a potential pharmacological target for brain injury in paediatric practice.

Flying under the radar: CDH2 (N-cadherin), an important hub molecule in neurodevelopmental and neurodegenerative diseases

CDH2 belongs to the classic cadherin family of Ca²⁺-dependent cell adhesion molecules with a meticulously described dual role in cell adhesion and β-catenin signaling. During CNS development, CDH2 is involved in a wide range of processes including maintenance of neuroepithelial integrity, neural tube closure (neurulation), confinement of radial glia progenitor cells (RGPCs) to the ventricular zone and maintaining their proliferation-differentiation balance, postmitotic neural precursor migration, axon guidance, synaptic development and maintenance. In the past few years, direct and indirect evidence linked CDH2 to various neurological diseases, and in this review, we summarize recent developments regarding CDH2 function and its involvement in pathological alterations of the CNS.

Acutely increased aquaporin-4 exhibits more potent protective effects in the cortex against single and repeated isoflurane-induced neurotoxicity in the developing rat brain

Damage to hippocampus, cerebellum, and cortex associated with cognitive functions due to anesthetic-induced toxicity early in life may cause cognitive decline later. Aquaporin 4 (AQP4), a key protein in waste clearance pathway of brain, is involved in synaptic plasticity and neurocognition. We investigated the effects of single and repeated isoflurane (Iso) anesthesia on AQP4 levels and brain damage. Postnatal-day (P)7 Wistar albino rats were randomly assigned to Iso or Control (C) groups. For single-exposure, pups were exposed to 1.5% Iso in 30% oxygenated-air for 3-h at P7 (Iso₁). For repeated-exposure, pups were exposed to Iso for 3 days, 3-h each day, at 1-day intervals (P7 + 9+11) starting at P7 (Iso₃). C₁ and C₃ groups received only 30% oxygenated-air. Based on HE-staining and immunoblotting (Bax/Bcl-2, cleaved-caspase3 and PARP1) analyses, Iso exposures caused a higher degree of apoptosis in hippocampus. Anesthesia increased 4HNE, oxidative stress marker; the highest ROS accumulation was determined in cerebellum. Increased inflammation (TNF-α, NF-κB) was detected. Multiple Iso-exposures caused more significant damage than single exposure. Moreover, 4HNE and TNF-α contributed synergistically to Iso-induced neurotoxicity. After anesthesia, higher expression of AQP4 was detected in cortex than hippocampus and cerebellum. There was an inverse correlation between increased AQP4 levels and apoptosis/ROS/inflammation. Correlation analysis indicated that AQP4 had a more substantial protective profile against oxidative stress than apoptosis. Remarkably, acutely increased AQP4 against Iso exhibited a more potent neuroprotective effect in cortex, especially frontal cortex. These findings promote further research to understand better the mechanisms underlying anesthesia-induced toxicity in the developing brain.

Imaging the fetal nonhuman primate brain with SV2A positron emission tomography (PET)

PURPOSE

Exploring synaptic density changes during brain growth is crucial to understanding brain development. Previous studies in nonhuman primates report a rapid increase in synapse number between the late gestational period and the early neonatal period, such that synaptic density approaches adult levels by birth. Prenatal synaptic development may have an enduring impact on postnatal brain development, but precisely how synaptic density changes in utero are unknown because current methods to quantify synaptic density are invasive and require post-mortem brain tissue.

METHODS

We used synaptic vesicle glycoprotein 2A (SV2A) positron emission tomography (PET) radioligands [¹¹C]UCB-J and [¹⁸F]Syn-VesT-1 to conduct the first assessment of synaptic density in the developing fetal brain in gravid rhesus monkeys. Eight pregnant monkeys were scanned twice during the third trimester at two imaging sites. Fetal post-mortem samples were collected near term in a subset of subjects to quantify SV2A density by Western blot.

RESULTS

Image-derived fetal brain SV2A measures increased during the third trimester. SV2A concentrations were greater in subcortical regions than in cortical regions at both gestational ages. Near term, SV2A density was higher in primary motor and visual areas than respective associative regions. Post-mortem quantification of SV2A density was significantly correlated with regional SV2A PET measures.

CONCLUSION

While further study is needed to determine the exact relationship of SV2A and synaptic density, the imaging paradigm developed in the current study allows for the effective in vivo study of SV2A development in the fetal brain.

The role of astrocyte structural plasticity in regulating neural circuit function and behavior

Brain circuits undergo substantial structural changes during development, driven by the formation, stabilization, and elimination of synapses. Synaptic connections continue to undergo experience‐dependent structural rearrangements throughout life, which are postulated to underlie learning and memory. Astrocytes, a major glial cell type in the brain, are physically in contact with synaptic circuits through their structural ensheathment of synapses. Astrocytes strongly contribute to the remodeling of synaptic structures in healthy and diseased central nervous systems by regulating synaptic connectivity and behaviors. However, whether structural plasticity of astrocytes is involved in their critical functions at the synapse is unknown. This review will discuss the emerging evidence linking astrocytic structural plasticity to synaptic circuit remodeling and regulation of behaviors. Moreover, we will survey possible molecular and cellular mechanisms regulating the structural plasticity of astrocytes and their non‐cell‐autonomous effects on neuronal plasticity. Finally, we will discuss how astrocyte morphological changes in different physiological states and disease conditions contribute to neuronal circuit function and dysfunction.

Vision-dependent specification of cell types and function in the developing cortex

The role of postnatal experience in sculpting cortical circuitry, while long appreciated, is poorly understood at the level of cell types. We explore this in the mouse primary visual cortex (V1) using single-nucleus RNA sequencing, visual deprivation, genetics, and functional imaging. We find that vision selectively drives the specification of glutamatergic cell types in upper layers (L) (L2/3/4), while deeper-layer glutamatergic, GABAergic, and non-neuronal cell types are established prior to eye opening. L2/3 cell types form an experience-dependent spatial continuum defined by the graded expression of ∼200 genes, including regulators of cell adhesion and synapse formation. One of these genes, Igsf9b, a vision-dependent gene encoding an inhibitory synaptic cell adhesion molecule, is required for the normal development of binocular responses in L2/3. In summary, vision preferentially regulates the development of upper-layer glutamatergic cell types through the regulation of cell-type-specific gene expression programs.

Effects and Mechanisms of Bone Marrow Mesenchymal Stem Cell Transplantation for Treatment of Ischemic Stroke in Hypertensive Rats

Background and Objectives

Stroke is the most common cause of human death and functional disability, resulting in more than 5 million deaths worldwide each year. Bone marrow mesenchymal stem cells (BMSCs) are a kind of stem cell that are able to self-renew and differentiate into many types of tissues. Therefore, BMSCs have the potential to replace damaged neurons and promote the reconstruction of nerve conduction pathways and connective tissue. However, it remains unknown whether transplanted BMSCs promote angiogenesis or improve the tissue microenvironment directly or indirectly through paracrine interactions. This study aimed to determine the therapeutic effect of BMSCs on ischemic stroke with hypertension in a rodent model and to explore the possible mechanisms underlying any benefits.

Methods and Results

Middle cerebral artery occlusion was used to establish the experimental stroke model. The area of cerebral infarction, expression of vascular endothelial growth factor (VEGF) and glial cell line-derived neurotrophic factor (GDNF), and increment of astrocyte were measured by TTC staining, western blot, real-time quantitative polymerase chain reaction (RT-qPCR) and immunocytochemistry. The results showed a smaller area of cerebral infarction and improved neurological function scores in animals treated with BMSCs compared to controls. The results of RT-qPCR and western blot assays showed higher expression of VEGF and GDNF in BMSC-treated animals compared with controls. Our study also showed that one round of BMSCs transplantation significantly promoted the proliferation of subventricular zone and cortical cells, especially astrocytes, on the ischemic side following cerebral ischemia.

Conclusions

Above findings support that BMSCs have therapeutic effects for ischemic stroke complicated with hypertension, which may occur via up-regulated expression of VEGF and GDNF and reduction of neuronal apoptosis, thereby promoting the recovery of nerve function.

Activity-dependent modulation of synapse-regulating genes in astrocytes

Astrocytes regulate the formation and function of neuronal synapses via multiple signals; however, what controls regional and temporal expression of these signals during development is unknown. We determined the expression profile of astrocyte synapse-regulating genes in the developing mouse visual cortex, identifying astrocyte signals that show differential temporal and layer-enriched expression. These patterns are not intrinsic to astrocytes, but regulated by visually evoked neuronal activity, as they are absent in mice lacking glutamate release from thalamocortical terminals. Consequently, synapses remain immature. Expression of synapse-regulating genes and synaptic development is also altered when astrocyte signaling is blunted by diminishing calcium release from astrocyte stores. Single-nucleus RNA sequencing identified groups of astrocytic genes regulated by neuronal and astrocyte activity, and a cassette of genes that show layer-specific enrichment. Thus, the development of cortical circuits requires coordinated signaling between astrocytes and neurons, highlighting astrocytes as a target to manipulate in neurodevelopmental disorders.

Modeling the role of gap junctions between excitatory neurons in the developing visual cortex

Recent experiments in the developing mammalian visual cortex have revealed that gap junctions couple excitatory cells and potentially influence the formation of chemical synapses. In particular, cells that were coupled by a gap junction during development tend to share an orientation preference and are preferentially coupled by a chemical synapse in the adult cortex, a property that is diminished when gap junctions are blocked. In this work, we construct a simplified model of the developing mouse visual cortex including spike-timing-dependent plasticity of both the feedforward synaptic inputs and recurrent cortical synapses. We use this model to show that synchrony among gap-junction-coupled cells underlies their preference to form strong recurrent synapses and develop similar orientation preference; this effect decreases with an increase in coupling density. Additionally, we demonstrate that gap-junction coupling works, together with the relative timing of synaptic development of the feedforward and recurrent synapses, to determine the resulting cortical map of orientation preference.

Gap junctions, or sites of direct electrical connections between neurons, have a significant presence in the cortex, both during development and in adulthood. Their primary function during either of these periods, however, is still poorly understood. In the adult cortex, gap junctions between local, inhibitory neurons have been shown to promote synchronous firing, a network characteristic thought to be important for learning, attention, and memory. During development, gap junctions between excitatory, pyramidal cells, have been conjectured to play a role in synaptic plasticity and the formation of cortical circuits. In the visual cortex, where neurons exhibit tuned responses to properties of visual input such as orientation and direction, recent experiments show that excitatory cells are coupled by gap junctions during the first postnatal week and are replaced by chemical synapses during the second week. In this work, we explore the possible contribution of gap-junction coupling during development to the formation of chemical synapses between the visual cortex from the thalamus and between cortical cells within the visual cortex. Specifically, using a mathematical model of the visual cortex during development, we identify the response properties of gap-junction-coupled cells and their influence on the formation of the cortical map of orientation preference.

Intranasal levosimendan prevents cognitive dysfunction and apoptotic response induced by repeated isoflurane exposure in newborn rats

Anesthetic-induced toxicity in early life may lead to risk of cognitive decline at later ages. Notably, multiple exposures to isoflurane (ISO) cause acute apoptotic cell death in the developing brain and long-term cognitive dysfunction. This study is the first to investigate whether levosimendan (LVS), known for its protective myocardial properties, can prevent anesthesia-induced apoptotic response in brain cells and learning and memory impairment. Postnatal day (P)7 Wistar albino pups were randomly assigned to groups consisting of an equal number of males and females in this laboratory investigation. We treated rats with LVS (0.8 mg/kg/day) intranasally 30 min before each ISO exposure (1.5%, 3 h) at P7+9+11. We selected DMSO as the drug vehicle. Also, the control group at P7+9+11 received 50% O2 for 3 h instead of ISO. Neuroprotective activity of LVS against ISO-induced cognitive dysfunction was evaluated by Morris water maze. Expression of apoptotic-related proteins was detected in the whole brain using western blot. LVS pretreatment significantly prevented anesthesia-induced deficit in spatial learning (at P28-32) and memory (at P33, P60, and P90). No sex-dependent difference occurred on any day of the training and probe trial. Intranasal LVS was also found to significantly prevent the ISO-induced apoptosis by reducing Bax and cleaved caspase-3, and by increasing Bcl-2 and Bcl-xL. Our findings support pretreatment with intranasal LVS application as a simple strategy in daily clinical practice in pediatric anesthesia to protect infants and children from the risk of general anesthesia-induced cell death and cognitive declines.

Region-specific sex modulation of central oxytocin receptor by gut microbiota: An ontogenic study

Oxytocin (OT) is a developmentally important neuropeptide recognized to play a dominant role in social functioning and stress-related behaviors, in a sex-dependent manner. Nonetheless, the underlining factors driving OT and OT receptor (OTR) early brain development remain unclear. Recent evidence highlight the critical influence of gut microbiota and its bidirectional interaction with the brain on neurodevelopment via the gut microbiota-brain axis. Therefore, we aimed to determine the impact of gut microbiota on the OTR system of the rat brain at different developmental stages in a pilot study. Quantitative OTR [¹²⁵ I]-OVTA autoradiographic binding was carried out in the forebrain of male and female conventional (CON) and germ-free (GF) rats at postnatal days (PND) 8, 22, and 116-150. OTR binding was also assessed in the eyes of PND 1 and PND 4 GF female rats. Significant "microbiota × sex × region" interaction and age-dependent effects on OTR binding were demonstrated. Microbiota status influenced OTR levels in males but not females with higher levels of OTR observed in GF versus CON rats in the cingulate, prelimbic, and lateral/medial/ventral orbital cortex, and septum across all age groups, while sex differences were observed in GF, but not in CON rats. Interestingly, OTRs present in the eyes of CON rats were abolished in GF rats. This is the first study to uncover a sex-specific role of gut microbiota on the central OTR system, which may have implications in understanding the developmental neuroadaptations critical for behavioral regulation and the etiology of certain neurodevelopmental disorders.

Impact of maternal immune activation on dendritic spine development

Dendritic spines are small dendritic protrusions that harbor most excitatory synapses in the brain. The proper generation and maturation of dendritic spines are crucial for the regulation of synaptic transmission and formation of neuronal circuits. Abnormalities in dendritic spine density and morphology are common pathologies in autism and schizophrenia. According to epidemiological studies, one risk factor for these neurodevelopmental disorders is maternal infection during pregnancy. This review discusses spine alterations in animal models of maternal immune activation in the context of neurodevelopmental disorders. We describe potential mechanisms that might be responsible for prenatal infection-induced changes in the dendritic spine phenotype and behavior in offspring.

The small GTPase Arf6 is dysregulated in a mouse model for fragile X syndrome

Fragile X syndrome (FXS), the most common inherited cause of intellectual disability, results from silencing of the fragile X mental retardation gene 1 (FMR1). The analyses of FXS patients' brain autopsies revealed an increased density of immature dendritic spines in cortical areas. We hypothesize that the small GTPase Arf6, an actin regulator critical for the development of glutamatergic synapses and dendritic spines, is implicated in FXS. Here, we determined the fraction of active, GTP-bound Arf6 in cortical neuron cultures and synaptoneurosomes from Fmr1 knockout mice, measured actin polymerization in neurons expressing Arf6 mutants with variant GTP- or GDP-binding properties, and recorded hippocampal long-term depression induced by metabotropic glutamate receptors (mGluR-LTD) in acute brain slices. We detected a persistently elevated Arf6 activity, a loss of Arf6 sensitivity to synaptic stimulation and an increased Arf6-dependent dendritic actin polymerization in mature Fmr1 knockout neurons. Similar imbalances in Arf6-GTP levels and actin filament assembly were caused in wild-type neurons by RNAi-mediated depletion of the postsynaptic Arf6 guanylate exchange factors IQSEC1 (BRAG2) or IQSEC2 (BRAG1). Targeted deletion of Iqsec1 in hippocampal neurons of 3-week-old mice interfered with mGluR-LTD in wild-type, but not in Fmr1 knockout mice. Collectively, these data suggest an aberrant Arf6 regulation in Fmr1 knockout neurons with consequences for the actin cytoskeleton, spine morphology, and synaptic plasticity. Moreover, FXS and syndromes caused by genetic variants in IQSEC1 and IQSEC2 share intellectual disabilities and developmental delay as main symptoms. Therefore, dysregulation of Arf6 may contribute to the cognitive impairment in FXS.

How Do Electric Fields Coordinate Neuronal Migration and Maturation in the Developing Cortex?

During development the vast majority of cells that will later compose the mature cerebral cortex undergo extensive migration to reach their final position. In addition to intrinsically distinct migratory behaviors, cells encounter and respond to vastly different microenvironments. These range from axonal tracts to cell-dense matrices, electrically active regions and extracellular matrix components, which may all change overtime. Furthermore, migrating neurons themselves not only adapt to their microenvironment but also modify the local niche through cell-cell contacts, secreted factors and ions. In the radial dimension, the developing cortex is roughly divided into dense progenitor and cortical plate territories, and a less crowded intermediate zone. The cortical plate is bordered by the subplate and the marginal zone, which are populated by neurons with high electrical activity and characterized by sophisticated neuritic ramifications. Neuronal migration is influenced by these boundaries resulting in dramatic changes in migratory behaviors as well as morphology and electrical activity. Modifications in the levels of any of these parameters can lead to alterations and even arrest of migration. Recent work indicates that morphology and electrical activity of migrating neuron are interconnected and the aim of this review is to explore the extent of this connection. We will discuss on one hand how the response of migrating neurons is altered upon modification of their intrinsic electrical properties and whether, on the other hand, the electrical properties of the cellular environment can modify the morphology and electrical activity of migrating cortical neurons.

Presynaptic PRRT2 Deficiency Causes Cerebellar Dysfunction and Paroxysmal Kinesigenic Dyskinesia

PRRT2 loss-of-function mutations have been associated with familial paroxysmal kinesigenic dyskinesia (PKD), infantile convulsions and choreoathetosis, and benign familial infantile seizures. Dystonia is the foremost involuntary movement disorder manifest by patients with PKD. Using a lacZ reporter and quantitative reverse-transcriptase PCR, we mapped the temporal and spatial distribution of Prrt2 in mouse brain and showed the highest levels of expression in cerebellar cortex. Further investigation into PRRT2 localization within the cerebellar cortex revealed that Prrt2 transcripts reside in granule cells but not Purkinje cells or interneurons within cerebellar cortex, and PRRT2 is presynaptically localized in the molecular layer. Analysis of synapses in the cerebellar molecular layer via electron microscopy showed that Prrt2^-/- mice have increased numbers of docked vesicles but decreased vesicle numbers overall. In addition to impaired performance on several motor tasks, approximately 5% of Prrt2^-/- mice exhibited overt PKD with clear face validity manifest as dystonia. In Prrt2 mutants, we found reduced parallel fiber facilitation at parallel fiber-Purkinje cell synapses, reduced Purkinje cell excitability, and normal cerebellar nuclear excitability, establishing a potential mechanism by which altered cerebellar activity promotes disinhibition of the cerebellar nuclei, driving motor abnormalities in PKD. Overall, our findings replicate, refine, and expand upon previous work with PRRT2 mouse models, contribute to understanding of paroxysmal disorders of the nervous system, and provide mechanistic insight into the role of cerebellar cortical dysfunction in dystonia.

Identification of cofilin 1 as a candidate protein associated to mouse visual cortex plasticity

In order to characterize the mechanisms controlling plasticity in the mouse visual cortex, we used, for the first time on brain samples, an unconventional proteomic approach to separate acid-extracted proteins by bi-dimensional electrophoresis (AUT/SDS or AUT/AU gels). The analysis was performed on high plasticity critical period young vs. low plasticity adult, and on fluoxetine-induced high plasticity vs. low plasticity untreated adult mice. Mass spectrometry allowed for the identification of 11 proteins that are differentially expressed between critical period and adult mice, and 5 between fluoxetine-treated and control adult mice. We then focused on cofilin 1, as the intensity level of the corresponding spot on 2D gels was higher in both high plasticity conditions. Western blot showed that cofilin 1 expression is dynamically regulated during postnatal life, reaching a peak at the critical period, and decreasing at adult stage, and that it increases in fluoxetine-treated vs. untreated adult mice. In summary, by using a 2D gel electrophoresis differential approach on basic proteins followed by mass spectrometry and immunoblot analysis, we identified cofilin 1 as a potential candidate controlling plasticity levels of the mouse visual cortex.

Lipophilic tracer Dil and fluorescence labeling of acridine orange used for Leishmania major tracing in the fibroblast cells

Background

This study aims to evaluate the use of fluorescent dye Dil and super vital dye acridine orange (AO) in vitro tracking of labeled L. major in the fibroblast cells.

Methods

Dil crystal and AO were used to stain L. major in a co-culture of the fibroblasts with the parasite. AO staining solution was added to 1 × 10⁶ parasites. After 10 min, the stained parasites were centrifuged and washed seven times with phosphate buffered saline (PBS). The stained promastigote was incubated with fibroblasts for 6-8 h. The presence of stained parasites with AO in the fibroblast was assessed using a fluorescence microscope. 1 × 10⁶/mL promastigote of L. major was gently suspended and mixed by Dil staining solution with an ultimate concentration of 0.002 μg/mL and it was kept for 20 min at the room temperature. Subsequently, after washing it in PBS for seven times, it was centrifuged at 3000 rpm for 10 min. The supernatant was removed and the precipitate containing stained promastigote was suspended in fresh DMEM F12 with fibroblasts at 37 °C for 6 h. The presence of stained parasites with Dil in fibroblast was assessed using a fluorescence microscope. Fibroblast characterization was undertaken by a real-time polymerase chain reaction (PCR).

Results

Acridine orange staining assisted in detection of the live parasite in the fibroblast cells. Free promastigote looked green before entering into the fibroblasts after 12 h culture. The parasite entered the cytoplasm of fibroblasts at the beginning of the exposure and gradually entered the nucleus of the fibroblast. The fibroblast nucleus was entirely stained green by AO. The L. major appeared green under the fluorescent microscope. Dil staining revealed that the internalized parasites with red/orange color were localized within the cytoplasm after 6-hours and the nucleus of the fibroblasts after 72-hours following culture. Human fibroblasts were positive at the expression of CD10, CD26, matrix metalloproteinase-1 (MMP-1) and matrix metalloproteinase-3 (MMP-3) and negative for CD106 and integrin alpha 11.

Conclusion

The fluorescent dye Dil staining is a safe, easy to use, inexpensive and fast method for labeling of the Leishmania parasite in the fibroblast cells. Acridine orange staining could be useful for tracing the parasites in the fibroblasts too. In this study, both Dil and AO were compared and considered as suitable vital dyes for identifying labeled Leishmania in the fibroblast in vitro, but Dil was superior to AO with its feature does not transfer from the labeled to unlabeled cells.

Isoflurane exposure in infant rats acutely increases aquaporin 4 and does not cause neurocognitive impairment

Isoflurane is commonly used in pediatric population, but its mechanism of action in cognition is unclear. Aquaporin 4 (AQP4) regulates water content in blood, brain, and cerebrospinal fluid. Various studies have provided evidence for the role of AQP4 in synaptic plasticity and neurocognition. In this study, we aimed to determine whether a prolonged exposure to isoflurane in infant rats is associated with cognition and what effect this exposure has on AQP4 expression. Ten-day-old [postnatal day (P) 10] Wistar albino rats were randomly allocated to isoflurane group (n = 32; 1.5% isoflurane in 50% oxygen for 6 hours) or control group (n = 32; only 50% oxygen for 6 hours). Acute (P11) and long-term (P33) effects of 6-hour anesthetic isoflurane exposure on AQP4 expression were analyzed in whole brains of P11 and P33 rats by RT-qPCR and Western blot. Spatial learning and memory were assessed on P28 to P33 days by Morris Water Maze (MWM) test. The analysis revealed that isoflurane increased acutely both mRNA (~4.5 fold) and protein (~90%) levels of AQP4 in P11 rats compared with control group. The increasing levels of AQP4 in P11 were not observed in P33 rats. Also, no statistically significant change between isoflurane and control groups was observed in the latency to find the platform during MWM training and probe trial. Our results indicate that a single exposure to isoflurane anesthesia does not influence cognition in infant rats. In this case, acutely increased AQP4 after isoflurane anesthesia may have a protective role in neurocognition.

Impaired development of neocortical circuits contributes to the neurological alterations in DYRK1A haploinsufficiency syndrome

Autism spectrum disorders are early onset neurodevelopmental disorders characterized by deficits in social communication and restricted repetitive behaviors, yet they are quite heterogeneous in terms of their genetic basis and phenotypic manifestations. Recently, de novo pathogenic mutations in DYRK1A, a chromosome 21 gene associated to neuropathological traits of Down syndrome, have been identified in patients presenting a recognizable syndrome included in the autism spectrum. These mutations produce DYRK1A kinases with partial or complete absence of the catalytic domain, or they represent missense mutations located within this domain. Here, we undertook an extensive biochemical characterization of the DYRK1A missense mutations reported to date and show that most of them, but not all, result in enzymatically dead DYRK1A proteins. We also show that haploinsufficient Dyrk1a+/- mutant mice mirror the neurological traits associated with the human pathology, such as defective social interactions, stereotypic behaviors and epileptic activity. These mutant mice present altered proportions of excitatory and inhibitory neocortical neurons and synapses. Moreover, we provide evidence that alterations in the production of cortical excitatory neurons are contributing to these defects. Indeed, by the end of the neurogenic period, the expression of developmental regulated genes involved in neuron differentiation and/or activity is altered. Therefore, our data indicate that altered neocortical neurogenesis could critically affect the formation of cortical circuits, thereby contributing to the neuropathological changes in DYRK1A haploinsufficiency syndrome.

Slow Dynamics in Microcolumnar Gap Junction Network of Developing Neocortical Pyramidal Neurons

Gap junctions mediate electrical coupling between neurons and modulate their firing activity. In mouse neocortical layer 5, the major types of pyramidal neurons organize into cell type-specific microcolumns that exhibit modular neuronal activity. During cortical development, microcolumn neurons are electrically coupled in a cell type-specific manner at the stage of synaptogenesis, forming a dense network of gap junctions. However, modulation of neuronal activity by the gap junction network has not been examined. Here, we show that the electrical coupling induces amplification and slow synchronization of action potentials. This slow synchronization is mediated by electrical transmission that is an order of magnitude slower than that of gap junction-coupled neurons of other types. Theoretical and structural analyses suggested that apical dendrites are a major site of electrical coupling, providing slow electrical transmission. These results suggest that the gap junction network organizes neuronal activity of developing cortical circuit modules with unique slow dynamics.

A Complex Code of Extrinsic Influences on Cortical Progenitor Cells of Higher Mammals

Development of the cerebral cortex depends critically on the regulation of progenitor cell proliferation and fate. Cortical progenitor cells are remarkably diverse with regard to their morphology as well as laminar and areal position. Extrinsic factors, such as thalamic axons, have been proposed to play key roles in progenitor cell regulation, but the diversity, extent and timing of interactions between extrinsic elements and each class of cortical progenitor cell in higher mammals remain undefined. Here we use the ferret to demonstrate the existence of a complex set of extrinsic elements that may interact, alone or in combination, with subpopulations of progenitor cells, defining a code of extrinsic influences. This code and its complexity vary significantly between developmental stages, layer of residence and morphology of progenitor cells. By analyzing the spatial-temporal overlap of progenitor cell subtypes with neuronal and axonal populations, we show that multiple sets of migrating neurons and axon tracts overlap extensively with subdivisions of the Subventricular Zones, in an exquisite lamina-specific pattern. Our findings provide a framework for understanding the feedback influence of both intra- and extra-cortical elements onto progenitor cells to modulate their dynamics and fate decisions in gyrencephalic brains.

Astrocytes, neurons, synapses: a tripartite view on cortical circuit development

In the mammalian cerebral cortex neurons are arranged in specific layers and form connections both within the cortex and with other brain regions, thus forming a complex mesh of specialized synaptic connections comprising distinct circuits. The correct establishment of these connections during development is crucial for the proper function of the brain. Astrocytes, a major type of glial cell, are important regulators of synapse formation and function during development. While neurogenesis precedes astrogenesis in the cortex, neuronal synapses only begin to form after astrocytes have been generated, concurrent with neuronal branching and process elaboration. Here we provide a combined overview of the developmental processes of synapse and circuit formation in the rodent cortex, emphasizing the timeline of both neuronal and astrocytic development and maturation. We further discuss the role of astrocytes at the synapse, focusing on astrocyte-synapse contact and the role of synapse-related proteins in promoting formation of distinct cortical circuits.

Astrocytic neuroligins control astrocyte morphogenesis and synaptogenesis

Astrocytes are highly complex glial cells with numerous fine cellular processes which infiltrate the neuropil to interact with synapses. The mechanisms controlling the establishment of astrocytes’ remarkable morphology and how impairing astrocytic infiltration of the neuropil alters synaptic connectivity are largely unknown. Here we find that cortical astrocyte morphogenesis depends on direct contact with neuronal processes and occurs in tune with the growth and activity of synaptic circuits. Neuroligin (NL) family cell adhesion proteins, NL1, NL2, and NL3, which are expressed by cortical astrocytes, control astrocyte morphogenesis through interactions with neuronal neurexins. Furthermore, in the absence of astrocytic NL2, cortical excitatory synapse formation and function is diminished, whereas inhibitory synaptic function is enhanced. Our findings highlight a novel mechanism of action for NLs and link astrocyte morphogenesis to synaptogenesis. Because NL mutations are implicated in various neurological disorders, these findings also offer an astrocyte-based mechanism of neural pathology.

Prepubertal Development of GABAergic Transmission to Gonadotropin-Releasing Hormone (GnRH) Neurons and Postsynaptic Response Are Altered by Prenatal Androgenization

Gonadotropin-releasing hormone (GnRH) neurons regulate reproduction through pulsatile GnRH release. Women with polycystic ovary syndrome (PCOS) have persistently elevated luteinizing hormone release frequency, reflecting GnRH release; this exacerbates hyperandrogenemia and disrupted reproductive cycles that are characteristic of this disorder. Clinical evidence suggests that neuroendocrine features of PCOS may manifest peripubertally. Adult mice prenatally exposed to androgens (PNA) mimic several reproductive features of PCOS. GnRH neurons from these mice have increased firing activity and receive increased GABAergic transmission, which is excitatory. When changes emerge during development is unknown. To study the typical postnatal development of GABAergic transmission and the effects of PNA treatment and sex, whole-cell voltage-clamp recordings were made of GABAergic postsynaptic currents (PSCs) in GnRH neurons in brain slices from prepubertal through adult control and PNA female and male mice. GABAergic transmission was present by 1 week of age in females and males and increased in frequency, reaching adult levels at 3 and 4 weeks, respectively. GABAergic PSC frequency was elevated in 3-week-old PNA versus control females. PSC frequency in both controls and PNA mice was activity independent, suggesting that PNA induces changes in synapse organization. PNA also alters the functional response of GnRH neurons to GABA. GABA induced firing in fewer neurons from 3-week-old PNA than control females; membrane potential depolarization induced by GABA was also reduced in cells from PNA mice at this age. PNA thus induces changes during development in the presynaptic organization of the GABAergic network afferent to GnRH neurons as well as the postsynaptic GnRH neuron response, both of which may contribute to adult reproductive dysfunction.SIGNIFICANCE STATEMENT The central neuronal network that regulates reproduction is overactive in polycystic ovary syndrome (PCOS), a leading cause of infertility. Recent evidence of neuroendocrine dysfunction in midpubertal girls suggests that the pathophysiological mechanisms underlying PCOS may arise before pubertal maturation. Prenatal exposure to androgens (PNA) in mice mimics several neuroendocrine features of PCOS. GABAergic transmission to gonadotropin-releasing hormone (GnRH) neurons is important for reproduction and is increased in adult PNA mice. The typical development of this network and when changes with PNA and sex arise relative to puberty are unknown. These studies provide evidence that PNA alters prepubertal development of the GABAergic network afferent to GnRH neurons, including both the presynaptic organization and postsynaptic response. These changes may contribute to reproductive dysfunction in adults.

Synaptic aging disrupts synaptic morphology and function in cerebellar Purkinje cells

Synapses are key structures in neural networks, and are involved in learning and memory in the central nervous system. Investigating synaptogenesis and synaptic aging is important in understanding neural development and neural degeneration in diseases such as Alzheimer disease and Parkinson's disease. Our previous study found that synaptogenesis and synaptic maturation were harmonized with brain development and maturation. However, synaptic damage and loss in the aging cerebellum are not well understood. This study was designed to investigate the occurrence of synaptic aging in the cerebellum by observing the ultrastructural changes of dendritic spines and synapses in cerebellar Purkinje cells of aging mice. Immunocytochemistry, DiI diolistic assays, and transmission electron microscopy were used to visualize the morphological characteristics of synaptic buttons, dendritic spines and synapses of Purkinje cells in mice at various ages. With synaptic aging in the cerebellum, dendritic spines and synaptic buttons were lost, and the synaptic ultrastructure was altered, including a reduction in the number of synaptic vesicles and mitochondria in presynaptic termini and smaller thin specialized zones in pre- and post-synaptic membranes. These findings confirm that synaptic morphology and function is disrupted in aging synapses, which may be an important pathological cause of neurodegenerative diseases.

Maternal IL-17A in autism

Although autism spectrum disorder (ASD) has a strong genetic basis, its etiology is complex, with several genetic factors likely to be involved as well as environmental factors. Immune dysregulation has gained significant attention as a causal mechanism in ASD pathogenesis. ASD has been associated with immune abnormalities in the brain and periphery, including inflammatory disorders and autoimmunity in not only the affected individuals but also their mothers. Prenatal exposure to maternal immune activation (MIA) has been implicated as an environmental risk factor for ASD. In support of this notion, animal models have shown that MIA results in offspring with behavioral, neurological, and immunological abnormalities similar to those observed in ASD. This raises the question of how MIA exposure can lead to ASD in susceptible individuals. Recent evidence points to a potential inflammation pathway linking MIA-associated ASD with the activity of T helper 17 (Th17) lymphocytes and their effector cytokine interleukin-17A (IL-17A). IL-17A has been implicated from human studies and elevated IL-17A levels in the blood have been found to correlate with phenotypic severity in a subset of ASD individuals. In MIA model mice, elevated IL-17A levels also have been observed. Additionally, antibody blockade to inhibit IL-17A signaling was found to prevent ASD-like behaviors in offspring exposed to MIA. Therefore, IL-17A dysregulation may play a causal role in the development of ASD. The source of increased IL-17A in the MIA mouse model was attributed to maternal Th17 cells because genetic removal of the transcription factor RORγt to selectively inhibit Th17 differentiation in pregnant mice was able to prevent ASD-like behaviors in the offspring. Similar to ASD individuals, the MIA-exposed offspring also displayed cortical dysplasia which could be prevented by inhibition of IL-17A signaling in pregnant mice. This finding reveals one possible cellular mechanism through which ASD-related cognitive and behavioral deficits may emerge following maternal inflammation. IL-17A can exert strong effects on cell survival and differentiation and the activity of signal transduction cascades, which can have important consequences during cortical development on neural function. This review examines IL-17A signaling pathways in the context of both immunity and neural function that may contribute to the development of ASD associated with MIA.

Dynamics of the mouse brain cortical synaptic proteome during postnatal brain development

Development of the brain involves the formation and maturation of numerous synapses. This process requires prominent changes of the synaptic proteome and potentially involves thousands of different proteins at every synapse. To date the proteome analysis of synapse development has been studied sparsely. Here, we analyzed the cortical synaptic membrane proteome of juvenile postnatal days 9 (P9), P15, P21, P27, adolescent (P35) and different adult ages P70, P140 and P280 of C57Bl6/J mice. Using a quantitative proteomics workflow we quantified 1560 proteins of which 696 showed statistically significant differences over time. Synaptic proteins generally showed increased levels during maturation, whereas proteins involved in protein synthesis generally decreased in abundance. In several cases, proteins from a single functional molecular entity, e.g., subunits of the NMDA receptor, showed differences in their temporal regulation, which may reflect specific synaptic development features of connectivity, strength and plasticity. SNARE proteins, Snap 29/47 and Stx 7/8/12, showed higher expression in immature animals. Finally, we evaluated the function of Cxadr that showed high expression levels at P9 and a fast decline in expression during neuronal development. Knock down of the expression of Cxadr in cultured primary mouse neurons revealed a significant decrease in synapse density.

Mutations in mitochondrial enzyme GPT2 cause metabolic dysfunction and neurological disease with developmental and progressive features

Mutations that cause neurological phenotypes are highly informative with regard to mechanisms governing human brain function and disease. We report autosomal recessive mutations in the enzyme glutamate pyruvate transaminase 2 (GPT2) in large kindreds initially ascertained for intellectual and developmental disability (IDD). GPT2 [also known as alanine transaminase 2 (ALT2)] is one of two related transaminases that catalyze the reversible addition of an amino group from glutamate to pyruvate, yielding alanine and α-ketoglutarate. In addition to IDD, all affected individuals show postnatal microcephaly and ∼80% of those followed over time show progressive motor symptoms, a spastic paraplegia. Homozygous nonsense p.Arg404* and missense p.Pro272Leu mutations are shown biochemically to be loss of function. The GPT2 gene demonstrates increasing expression in brain in the early postnatal period, and GPT2 protein localizes to mitochondria. Akin to the human phenotype, Gpt2-null mice exhibit reduced brain growth. Through metabolomics and direct isotope tracing experiments, we find a number of metabolic abnormalities associated with loss of Gpt2. These include defects in amino acid metabolism such as low alanine levels and elevated essential amino acids. Also, we find defects in anaplerosis, the metabolic process involved in replenishing TCA cycle intermediates. Finally, mutant brains demonstrate misregulated metabolites in pathways implicated in neuroprotective mechanisms previously associated with neurodegenerative disorders. Overall, our data reveal an important role for the GPT2 enzyme in mitochondrial metabolism with relevance to developmental as well as potentially to neurodegenerative mechanisms.

Development of Synaptic Boutons in Layer 4 of the Barrel Field of the Rat Somatosensory Cortex: A Quantitative Analysis

Understanding the structural and functional mechanisms underlying the development of individual brain microcircuits is critical for elucidating their computational properties. As synapses are the key structures defining a given microcircuit, it is imperative to investigate their development and precise structural features. Here, synapses in cortical layer 4 were analyzed throughout the first postnatal month using high-end electron microscopy to generate realistic quantitative 3D models. Besides their overall geometry, the size of active zones and the pools of synaptic vesicles were analyzed. At postnatal day 2 only a few shaft synapses were found, but spine synapses steadily increased with ongoing corticogenesis. From postnatal day 2 to 30 synaptic boutons significantly decreased in size whereas that of active zones remained nearly unchanged despite a reshaping. During the first 2 weeks of postnatal development, a rearrangement of synaptic vesicles from a loose distribution toward a densely packed organization close to the presynaptic density was observed, accompanied by the formation of, first a putative readily releasable pool and later a recycling and reserve pool. The quantitative 3D reconstructions of synapses will enable the comparison of structural and functional aspects of signal transduction thus leading to a better understanding of networks in the developing neocortex.

Carbon monoxide pollution and neurodevelopment: A public health concern

Although an association between air pollution and adverse systemic health effects has been known for years, the effect of pollutants on neurodevelopment has been underappreciated. Recent evidence suggests a possible link between air pollution and neurocognitive impairment and behavioral disorders in children, however, the exact nature of this relationship remains poorly understood. Infants and children are uniquely vulnerable due to the potential for exposure in both the fetal and postnatal environments during critical periods in development. Carbon monoxide (CO), a common component of indoor and outdoor air pollution, can cross the placenta to gain access to the fetal circulation and the developing brain. Thus, CO is of particular interest as a known neurotoxin and a potential public health threat. Here we review overt CO toxicity and the policies regulating CO exposure, detail the evidence suggesting a potential link between CO-associated ambient air pollution, tobacco smoke, and learning and behavioral abnormalities in children, describe the effects of subclinical CO exposure on the brain during development, and provide mechanistic insight into a potential connection between CO exposure and neurodevelopmental outcome. CO can disrupt a number of critical processes in the developing brain, providing a better understanding of how this specific neurotoxin may impair neurodevelopment. However, further investigation is needed to better define the effects of perinatal CO exposure on the immature brain. Current policies regarding CO standards were established based on evidence of cardiovascular risk in adults with pre-existing comorbidities. Thus, recent and emerging data highlighted in this review regarding CO exposure in the fetus and developing child may be important to consider when the standards and guidelines are evaluated and revised in the future.

Neural cell adhesion molecule NrCAM regulates Semaphorin 3F-induced dendritic spine remodeling

Neuron-glial related cell adhesion molecule (NrCAM) is a regulator of axon growth and repellent guidance, and has been implicated in autism spectrum disorders. Here a novel postsynaptic role for NrCAM in Semaphorin3F (Sema3F)-induced dendritic spine remodeling was identified in pyramidal neurons of the primary visual cortex (V1). NrCAM localized to dendritic spines of star pyramidal cells in postnatal V1, where it was coexpressed with Sema3F. NrCAM deletion in mice resulted in elevated spine densities on apical dendrites of star pyramidal cells at both postnatal and adult stages, and electron microscopy revealed increased numbers of asymmetric synapses in layer 4 of V1. Whole-cell recordings in cortical slices from NrCAM-null mice revealed increased frequency of mEPSCs in star pyramidal neurons. Recombinant Sema3F-Fc protein induced spine retraction on apical dendrites of wild-type, but not NrCAM-null cortical neurons in culture, while re-expression of NrCAM rescued the spine retraction response. NrCAM formed a complex in brain with Sema3F receptor subunits Neuropilin-2 (Npn-2) and PlexinA3 (PlexA3) through an Npn-2-binding sequence (TARNER) in the extracellular Ig1 domain. A trans heterozygous genetic interaction test demonstrated that Sema3F and NrCAM pathways interacted in vivo to regulate spine density in star pyramidal neurons. These findings reveal NrCAM as a novel postnatal regulator of dendritic spine density in cortical pyramidal neurons, and an integral component of the Sema3F receptor complex. The results implicate NrCAM as a contributor to excitatory/inhibitory balance in neocortical circuits.

Astrocytes refine cortical connectivity at dendritic spines

During cortical synaptic development, thalamic axons must establish synaptic connections despite the presence of the more abundant intracortical projections. How thalamocortical synapses are formed and maintained in this competitive environment is unknown. Here, we show that astrocyte-secreted protein hevin is required for normal thalamocortical synaptic connectivity in the mouse cortex. Absence of hevin results in a profound, long-lasting reduction in thalamocortical synapses accompanied by a transient increase in intracortical excitatory connections. Three-dimensional reconstructions of cortical neurons from serial section electron microscopy (ssEM) revealed that, during early postnatal development, dendritic spines often receive multiple excitatory inputs. Immuno-EM and confocal analyses revealed that majority of the spines with multiple excitatory contacts (SMECs) receive simultaneous thalamic and cortical inputs. Proportion of SMECs diminishes as the brain develops, but SMECs remain abundant in Hevin-null mice. These findings reveal that, through secretion of hevin, astrocytes control an important developmental synaptic refinement process at dendritic spines.

Subcellular localization of intercellular adhesion molecule-5 (telencephalin) in the visual cortex is not developmentally regulated in the absence of matrix metalloproteinase-9

The telencephalon-associated intercellular adhesion molecule-5 (telencephalin; ICAM-5) regulates dendritic morphology in the developing brain. In vitro studies have shown that ICAM-5 is found predominantly within dendrites and immature dendritic protrusions, with reduced expression in mushroom spines, suggesting that ICAM-5 downregulation is critical for the maturation of synaptic structures. However, developmental expression of ICAM-5 has not been explored in depth at the ultrastructural level in intact brain tissue. To investigate the ultrastructural localization of ICAM-5 with transmission electron microscopy, we performed immunoperoxidase histochemistry for ICAM-5 in mouse visual cortex at postnatal day (P)14, a period of intense synaptogenesis, and at P28, when synapses mature. We observed the expected ICAM-5 expression in dendritic protrusions and shafts at both P14 and P28. ICAM-5 expression in these dendritic protrusions decreased in prevalence with developmental age to become localized predominantly to dendritic shafts by P28. To understand better the relationship between ICAM-5 and the endopeptidase metalloproteinase-9 (MMP-9), which mediates ICAM-5 cleavage following glutamate activation during postnatal development, we also explored ICAM-5 expression in MMP-9 null animals. This analysis revealed a similar expression of ICAM-5 in dendritic elements at P14 and P28; however, an increased prevalence of ICAM-5 was noted in dendritic protrusions at P28 in the MMP-9 null animals, indicating that, in the absence of MMP-9, there is no developmental shift in ICAM-5 subcellular localization. Our ultrastructural observations shed light on possible functions mediated by ICAM-5 and their regulation by extracellular proteases.

Fluorescent labeling of dendritic spines in cell cultures with the carbocyanine dye "DiI"

Analyzing cell morphology is a key component to understand neuronal function. Several staining techniques have been developed to facilitate the morphological analysis of neurons, including the use of fluorescent markers, such as DiI (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate). DiI is a carbocyanine membrane dye that exhibits enhanced fluorescence upon insertion of its lipophilic hydrocarbon chains into the lipid membrane of cells. The high photostability and prominent fluorescence of the dye serves as an effective means of illuminating cellular architecture in individual neurons, including detailed dendritic arborizations and spines in cell culture and tissue sections. Here, we specifically optimized a simple and reliable method to fluorescently label and visualize dissociated hippocampal neurons using DiI and high-resolution confocal microscopic imaging. With high efficacy, this method accurately labels neuronal and synaptic morphology to permit quantitative analysis of dendritic spines. Accurate imaging techniques of these fine neuronal specializations are vital to the study of their morphology and can help delineate structure-function relationships in the central nervous system.

A period of structural plasticity at the axon initial segment in developing visual cortex

Cortical networks are shaped by sensory experience and are most susceptible to modifications during critical periods characterized by enhanced plasticity at the structural and functional level. A system particularly well-studied in this context is the mammalian visual system. Plasticity has been documented for the somatodendritic compartment of neurons in detail. A neuronal microdomain not yet studied in this context is the axon initial segment (AIS) located at the proximal axon segment. It is a specific electrogenic axonal domain and the site of action potential (AP) generation. Recent studies showed that structure and function of the AIS can be dynamically regulated. Here we hypothesize that the AIS shows a dynamic regulation during maturation of the visual cortex. We therefore analyzed AIS length development from embryonic day (E) 12.5 to adulthood in mice. A tri-phasic time course of AIS length remodeling during development was observed. AIS first appeared at E14.5 and increased in length throughout the postnatal period to a peak between postnatal day (P) 10 to P15 (eyes open P13–14). Then, AIS length was reduced significantly around the beginning of the critical period for ocular dominance plasticity (CP, P21). Shortest AIS were observed at the peak of the CP (P28), followed by a moderate elongation toward the end of the CP (P35). To test if the dynamic maturation of the AIS is influenced by eye opening (onset of activity), animals were deprived of visual input before and during the CP. Deprivation for 1 week prior to eye opening did not affect AIS length development. However, deprivation from P0 to 28 and P14 to 28 resulted in AIS length distribution similar to the peak at P15. In other words, deprivation from birth prevents the transient shortening of the AIS and maintains an immature AIS length. These results are the first to suggest a dynamic maturation of the AIS in cortical neurons and point to novel mechanisms in the development of neuronal excitability.

Oxytocin receptor ligand binding in embryonic tissue and postnatal brain development of the C57BL/6J mouse

Oxytocin (OXT) has drawn increasing attention as a developmentally relevant neuropeptide given its role in the brain regulation of social behavior. It has been suggested that OXT plays an important role in the infant brain during caregiver attachment in nurturing familial contexts, but there is incomplete experimental evidence. Mouse models of OXT system genes have been particularly informative for the role of the OXT system in social behavior, however, the developing brain areas that could respond to ligand activation of the OXT receptor (OXTR) have yet to be identified in this species. Here we report new data revealing dynamic ligand-binding distribution of OXTR in the developing mouse brain. Using male and female C57BL/6J mice at postnatal days (P) 0, 7, 14, 21, 35, and 60 we quantified OXTR ligand binding in several brain areas which changed across development. Further, we describe OXTR ligand binding in select tissues of the near-term whole embryo at E18.5. Together, these data aid in the interpretation of findings in mouse models of the OXT system and generate new testable hypotheses for developmental roles for OXT in mammalian systems. We discuss our findings in the context of developmental disorders (including autism), attachment biology, and infant physiological regulation.

Summary: Quantitative mapping of selective OXTR ligand binding during postnatal development in the mouse reveals an unexpected, transient expression in layers II/III throughout the mouse neocortex. OXTR are also identified in several tissues in the whole late embryo, including the adrenal glands, brown adipose tissue, and the oronasal cavity.

Restoration of Intracortical and Thalamocortical Circuits after Transplantation of Bone Marrow Mesenchymal Stem Cells into the Ischemic Brain of Mice

Transplantation of bone marrow mesenchymal stem cells (BMSCs) provides a promising regenerative medicine for stroke. Whether BMSC therapy could repair ischemia-damaged neuronal circuits and recover electrophysiological activity has largely been unknown. To address this issue, BMSCs were implanted into the ischemic barrel cortex of adult mice 1 and 7 days after focal barrel cortex stroke. Two days after the first transplantation (3 days after stroke), the infarct volume determined by TTC staining was significantly smaller in BMSC-treated compared to vehicle-treated stroke mice. The behavioral corner test showed better long-term recovery of sensorimotor function in BMSC-treated mice. Six weeks poststroke, thalamocortical slices were prepared and neuronal circuit activity in the peri-infarct region of the barrel cortex was determined by extracellular recordings of evoked field potentials. In BMSC-transplanted brain slices, the ischemia-disrupted intracortical activity from layer 4 to layer 2/3 was noticeably recovered, and the thalamocortical circuit connection was also partially restored. In contrast, much less and slower recovery was seen in control animals of barrel cortex stroke. Immunohistochemical staining disclosed that the density of neurons, axons, and blood vessels in the peri-infarct region was significantly higher in BMSC-treated mice, accompanied with enhanced local blood flow recovery. Western blotting showed that BMSC treatment increased the expression of stromal cell-derived factor-1 (SDF-1), vascular endothelial growth factor (VEGF), and brain-derived neurotrophic factor (BDNF) in the peri-infarct region. Moreover, the expression of the axonal growth associated protein-43 (GAP-43) was markedly increased, whereas the axonal growth inhibiting proteins ROCK II and NG2 were suppressed in the BMSC-treated brains. BMSC transplantation also promoted directional migration and survival of doublecortin (DCX)-positive neuroblasts in the peri-infarct region. The present investigation thus provides novel evidence that BMSC transplantation has the potential to repair the ischemia-damaged neural networks and restore lost neuronal connections. The recovered circuit activity likely contributes to the improved sensorimotor function after focal ischemic stroke and BMSC transplantation.

Dendritic translocation establishes the winner in cerebellar climbing fiber synapse elimination

In many regions of the developing mammalian nervous system, functional synaptic circuitry is formed by competitive elimination of early formed redundant synapses. However, how winning synapses emerge through competition remains unclear in the brain largely because of the technical difficulty of directly observing this dynamic cellular process in vivo. Here, we developed a method of two-photon multicolor vital imaging to observe competitive elimination of supernumerary climbing fibers (CFs) in the cerebellum of live mouse pups. At birth, each Purkinje cell (PC) in the cerebellar cortex is innervated by multiple CFs; an activity-dependent regression of supernumerary CFs ultimately yields a single innervation for most PCs by postnatal day 21. As supernumerary CFs are pruned, the terminal field of CFs translocates from the soma to the dendrites of PCs. In vivo time-lapse imaging of CF elimination revealed that (1) CF terminals were highly motile on the soma, but their motility was significantly reduced on dendrites; (2) only one CF could translocate to the dendrites whereas their competitors were restricted to perisomatic regions; and (3) the CF that began dendritic translocation became the winner. Moreover, selective photo-ablation of the winning CF (that undergoes dendritic translocation) reversed the fate of its losing competitor. These results indicate that dendritic translocation is a key cellular event that determines the winner during CF elimination. We propose that CF terminals are selectively stabilized on dendrites, providing irreversible competitive vigor to the first CF to form dendritic synapses.

Reversal of disease-related pathologies in the fragile X mouse model by selective activation of GABAB receptors with arbaclofen

Fragile X syndrome (FXS), the most common inherited cause of intellectual disability and autism, results from the transcriptional silencing of FMR1 and loss of the mRNA translational repressor protein fragile X mental retardation protein (FMRP). Patients with FXS exhibit changes in neuronal dendritic spine morphology, a pathology associated with altered synaptic function. Studies in the mouse model of fragile X have shown that loss of FMRP causes excessive synaptic protein synthesis, which results in synaptic dysfunction and altered spine morphology. We tested whether the pharmacologic activation of the γ-aminobutyric acid type B (GABA(B)) receptor could correct or reverse these phenotypes in Fmr1-knockout mice. Basal protein synthesis, which is elevated in the hippocampus of Fmr1-knockout mice, was corrected by the in vitro application of the selective GABA(B) receptor agonist STX209 (arbaclofen, R-baclofen). STX209 also reduced to wild-type values the elevated AMPA receptor internalization in Fmr1-knockout cultured neurons, a known functional consequence of increased protein synthesis. Acute administration of STX209 in vivo, at doses that modify behavior, decreased mRNA translation in the cortex of Fmr1-knockout mice. Finally, the chronic administration of STX209 in juvenile mice corrected the increased spine density in Fmr1-knockout mice without affecting spine density in wild-type mice. Thus, activation of the GABA(B) receptor with STX209 corrected synaptic abnormalities considered central to fragile X pathophysiology, a finding that suggests that STX209 may be a potentially effective therapy to treat the core symptoms of FXS.

Comparing development of synaptic proteins in rat visual, somatosensory, and frontal cortex

Two theories have influenced our understanding of cortical development: the integrated network theory, where synaptic development is coordinated across areas; and the cascade theory, where the cortex develops in a wave-like manner from sensory to non-sensory areas. These different views on cortical development raise challenges for current studies aimed at comparing detailed maturation of the connectome among cortical areas. We have taken a different approach to compare synaptic development in rat visual, somatosensory, and frontal cortex by measuring expression of pre-synaptic (synapsin and synaptophysin) proteins that regulate vesicle cycling, and post-synaptic density (PSD-95 and Gephyrin) proteins that anchor excitatory or inhibitory (E-I) receptors. We also compared development of the balances between the pairs of pre- or post-synaptic proteins, and the overall pre- to post-synaptic balance, to address functional maturation and emergence of the E-I balance. We found that development of the individual proteins and the post-synaptic index overlapped among the three cortical areas, but the pre-synaptic index matured later in frontal cortex. Finally, we applied a neuroinformatics approach using principal component analysis and found that three components captured development of the synaptic proteins. The first component accounted for 64% of the variance in protein expression and reflected total protein expression, which overlapped among the three cortical areas. The second component was gephyrin and the E-I balance, it emerged as sequential waves starting in somatosensory, then frontal, and finally visual cortex. The third component was the balance between pre- and post-synaptic proteins, and this followed a different developmental trajectory in somatosensory cortex. Together, these results give the most support to an integrated network of synaptic development, but also highlight more complex patterns of development that vary in timing and end point among the cortical areas.

Overexpression of ALS-associated p.M337V human TDP-43 in mice worsens disease features compared to wild-type human TDP-43 mice

Mutations in TAR DNA-binding protein 43 (TDP-43) are associated with familial forms of amyotrophic lateral sclerosis (ALS), while wild-type TDP-43 is a pathological hallmark of patients with sporadic ALS and frontotemporal lobar degeneration (FTLD). Various in vitro and in vivo studies have also demonstrated toxicity of both mutant and wild-type TDP-43 to neuronal cells. To study the potential additional toxicity incurred by mutant TDP-43 in vivo, we generated mutant human TDP-43 (p.M337V) transgenic mouse lines driven by the Thy-1.2 promoter (Mt-TAR) and compared them in the same experimental setting to the disease phenotype observed in wild-type TDP-43 transgenic lines (Wt-TAR) expressing comparable TDP-43 levels. Overexpression of mutant TDP-43 leads to a worsened dose-dependent disease phenotype in terms of motor dysfunction, neurodegeneration, gliosis, and development of ubiquitin and phosphorylated TDP-43 pathology. Furthermore, we show that cellular aggregate formation or accumulation of TDP-43 C-terminal fragments (CTFs) are not primarily responsible for development of the observed disease phenotype in both mutant and wild-type TDP-43 mice.

Ablation of TNAP function compromises myelination and synaptogenesis in the mouse brain

Mutations in the tissue-nonspecific alkaline phosphatase (TNAP) gene can result in skeletal and dental hypomineralization and severe neurological symptoms. TNAP is expressed in the synaptic cleft and the node of Ranvier in normal adults. Using TNAP knockout (KO) mice (Akp2(-/-)), we studied synaptogenesis and myelination with light- and electron microscopy during the early postnatal days. Ablation of TNAP function resulted in a significant decrease of the white matter of the spinal cord accompanied by ultrastructural evidence of cellular degradation around the paranodal regions and a decreased ratio and diameter of the myelinated axons. In the cerebral cortex, myelinated axons, while present in wild-type, were absent in the Akp2( -/- ) mice and these animals also displayed a significantly increased proportion of immature cortical synapses. The results suggest that TNAP deficiency could contribute to neurological symptoms related to myelin abnormalities and synaptic dysfunction, among which epilepsy, consistently present in the Akp2(-/-) mice and observed in severe cases of hypophosphatasia.