Evidence for activity-dependent cortical wiring: formation of interhemispheric connections in neonatal mouse visual cortex requires projection neuron activity

Callosal responses in a retrosplenial column

The axons forming the corpus callosum sustain the interhemispheric communication across homotopic cortical areas. We have studied how neurons throughout the columnar extension of the retrosplenial cortex integrate the contralateral input from callosal projecting neurons in cortical slices. Our results show that pyramidal neurons in layers 2/3 and the large, thick-tufted pyramidal neurons in layer 5B showed larger excitatory callosal responses than layer 5A and layer 5B thin-tufted pyramidal neurons, while layer 6 remained silent to this input. Feed-forward inhibitory currents generated by fast spiking, parvalbumin expressing  interneurons recruited by callosal axons mimicked the response size distribution of excitatory responses across pyramidal subtypes, being larger in those of superficial layers and in the layer 5B thick-tufted pyramidal cells. Overall, the combination of the excitatory and inhibitory currents evoked by callosal input had a strong and opposed effect in different layers of the cortex; while layer 2/3 pyramidal neurons were powerfully inhibited, the thick-tufted but not thin-tufted pyramidal neurons in layer 5 were strongly recruited. We believe that these results will help to understand the functional role of callosal connections in physiology and disease.

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

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

SIGNIFICANCE STATEMENT

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

Proteome dynamics during postnatal mouse corpus callosum development

Formation of cortical connections requires the precise coordination of numerous discrete phases. This is particularly significant with regard to the corpus callosum, whose development undergoes several dynamic stages including the crossing of axon projections, elimination of exuberant projections, and myelination of established tracts. To comprehensively characterize the molecular events in this dynamic process, we set to determine the distinct temporal expression of proteins regulating the formation of the corpus callosum and their respective developmental functions. Mass spectrometry-based proteomic profiling was performed on early postnatal mouse corpus callosi, for which limited evidence has been obtained previously, using stable isotope of labeled amino acids in mammals (SILAM). The analyzed corpus callosi had distinct proteomic profiles depending on age, indicating rapid progression of specific molecular events during this period. The proteomic profiles were then segregated into five separate clusters, each with distinct trajectories relevant to their intended developmental functions. Our analysis both confirms many previously-identified proteins in aspects of corpus callosum development, and identifies new candidates in understudied areas of development including callosal axon refinement. We present a valuable resource for identifying new proteins integral to corpus callosum development that will provide new insights into the development and diseases afflicting this structure.

The anatomy, organisation and development of contralateral callosal projections of the mouse somatosensory cortex

Background:

Alterations in the development of neuronal connectivity can result in dramatic outcomes for brain function. In the cerebral cortex, most sensorimotor and higher-order functions require coordination between precise regions of both hemispheres through the axons that form the corpus callosum. However, little is known about how callosal axons locate and innervate their contralateral targets.

Methods:

Here, we use a combination of in utero electroporation, retrograde tracing, sensory deprivation and high-resolution axonal quantification to investigate the development, organisation and activity dependence of callosal axons arising from the primary somatosensory cortex of mice.

Results:

We show that distinct contralateral projections arise from different neuronal populations and form homotopic and heterotopic circuits. Callosal axons innervate the contralateral hemisphere following a dorsomedial to ventrolateral and region-specific order. Furthermore, we identify two periods of region- and layer-specific developmental exuberance that correspond to initial callosal axon innervation and subsequent arborisation. Early sensory deprivation affects only the latter of these events.

Conclusion:

Taken together, these results reveal the main developmental events of contralateral callosal targeting and may aid future understanding of the formation and pathologies of brain connectivity.

Binocular matching of thalamocortical and intracortical circuits in the mouse visual cortex

Visual cortical neurons are tuned to similar orientations through the two eyes. The binocularly-matched orientation preference is established during a critical period in early life, but the underlying circuit mechanisms remain unknown. Here, we optogenetically isolated the thalamocortical and intracortical excitatory inputs to individual layer 4 neurons and studied their binocular matching. In adult mice, the thalamic and cortical inputs representing the same eyes are similarly tuned and both are matched binocularly. In mice before the critical period, the thalamic input is already slightly matched, but the weak matching is not manifested due to random connections in the cortex, especially those serving the ipsilateral eye. Binocular matching is thus mediated by orientation-specific changes in intracortical connections and further improvement of thalamic matching. Together, our results suggest that the feed-forward thalamic input may play a key role in initiating and guiding the functional refinement of cortical circuits in critical period development.

DOI:http://dx.doi.org/10.7554/eLife.22032.001

A balancing Akt: How to fine-tune neuronal migration speed

In the developing mammalian neocortex, newborn neurons produced deep in the brain from neural stem/progenitor cells set out for a long journey to reach their final destination at the brain surface. This process called radial neuronal migration is prerequisite for the formation of appropriate layers and networks in the cortex, and its dysregulation has been implicated in cortical malformation and neurological diseases. Considering a fine correlation between temporal order of cortical neuronal cell types and their spatial distribution, migration speed needs to be tightly controlled to achieve correct neocortical layering, although the underlying molecular mechanisms remain not fully understood. Recently, we discovered that the kinase Akt and its activator PDK1 regulate the migration speed of mouse neocortical neurons through the cortical plate. We further found that the PDK1-Akt pathway controls coordinated movement of the nucleus and the centrosome during migration. Our data also suggested that control of neuronal migration by the PDK1-Akt pathway is mediated at the level of microtubules, possibly through regulation of the cytoplasmic dynein/dynactin complex. Our findings thus identified a signaling pathway controlling neuronal migration speed as well as a novel link between Akt signaling and cytoplasmic dynein/dynactin complex.

Reorganization of Visual Callosal Connections Following Alterations of Retinal Input and Brain Damage

Vision is a very important sensory modality in humans. Visual disorders are numerous and arising from diverse and complex causes. Deficits in visual function are highly disabling from a social point of view and in addition cause a considerable economic burden. For all these reasons there is an intense effort by the scientific community to gather knowledge on visual deficit mechanisms and to find possible new strategies for recovery and treatment. In this review, we focus on an important and sometimes neglected player of the visual function, the corpus callosum (CC). The CC is the major white matter structure in the brain and is involved in information processing between the two hemispheres. In particular, visual callosal connections interconnect homologous areas of visual cortices, binding together the two halves of the visual field. This interhemispheric communication plays a significant role in visual cortical output. Here, we will first review the essential literature on the physiology of the callosal connections in normal vision. The available data support the view that the callosum contributes to both excitation and inhibition to the target hemisphere, with a dynamic adaptation to the strength of the incoming visual input. Next, we will focus on data showing how callosal connections may sense visual alterations and respond to the classical paradigm for the study of visual plasticity, i.e., monocular deprivation (MD). This is a prototypical example of a model for the study of callosal plasticity in pathological conditions (e.g., strabismus and amblyopia) characterized by unbalanced input from the two eyes. We will also discuss the findings of callosal alterations in blind subjects. Noteworthy, we will discuss data showing that inter-hemispheric transfer mediates recovery of visual responsiveness following cortical damage. Finally, we will provide an overview of how callosal projections dysfunction could contribute to pathologies such as neglect and occipital epilepsy. A particular focus will be on reviewing noninvasive brain stimulation techniques and optogenetic approaches that allow to selectively manipulate callosal function and to probe its involvement in cortical processing and plasticity. Overall, the data indicate that experience can potently impact on transcallosal connectivity, and that the callosum itself is crucial for plasticity and recovery in various disorders of the visual pathway.

Supernova: A Versatile Vector System for Single-Cell Labeling and Gene Function Studies in vivo

Here we describe “Supernova” series of vector systems that enable single-cell labeling and labeled cell-specific gene manipulation, when introduced by in utero electroporation (IUE) or adeno-associated virus (AAV)-mediated gene delivery. In Supernova, sparse labeling relies on low TRE leakage. In a small population of cells with over-threshold leakage, initial tTA-independent weak expression is enhanced by tTA/TRE-positive feedback along with a site-specific recombination system (e.g., Cre/loxP, Flpe/FRT). Sparse and bright labeling by Supernova with little background enables the visualization of the morphological details of individual neurons in densely packed brain areas such as the cortex and hippocampus, both during development and in adulthood. Sparseness levels are adjustable. Labeled cell-specific gene knockout was accomplished by introducing Cre/loxP-based Supernova vectors into floxed mice. Furthermore, by combining with RNAi, TALEN, and CRISPR/Cas9 technologies, IUE-based Supernova achieved labeled cell-specific gene knockdown and editing/knockout without requiring genetically altered mice. Thus, Supernova system is highly extensible and widely applicable for single-cell analyses in complex organs, such as the mammalian brain.

Aberrant Subcellular Dynamics of Sigma-1 Receptor Mutants Underlying Neuromuscular Diseases

The sigma-1 receptor (σ-1R) is an endoplasmic reticulum resident chaperone protein involved in a plethora of cellular functions, and whose disruption has been implicated in a wide range of diseases. Genetic analysis has revealed two σ-1R mutants involved in neuromuscular disorders. A point mutation (E102Q) in the ligand-binding domain results in the juvenile form of amyotrophic lateral sclerosis (ALS16), and a 20 amino-acid deletion (Δ31-50) in the putative cytosolic domain leads to a form of distal hereditary motor neuropathy. We investigated the localization and functional properties of these mutants in cell lines using confocal imaging and electrophysiology. The σ-1R mutants exhibited a significant increase in mobility, aberrant localization, and enhanced block of the inwardly rectifying K(+) channel Kir2.1, compared with the wild-type σ-1R. Thus, these σ-1R mutants have different functional properties that could contribute to their disease phenotypes.

Glycine release from astrocytes via functional reversal of GlyT1

It was previously reported that functional glycine receptors were expressed in neonatal prefrontal cortex; however, the glycine-releasing cells were unknown. We hypothesized that astrocytes might be a major glycine source, and examined the glycine release properties of astrocytes. We also hypothesized that dopamine (DA) might be a trigger for the astrocytic glycine release, as numerous DA terminals localize in the cortex. We combined two different methods to confirm the glycine release from astrocytes. Firstly, we analyzed the supernatant of astrocytes by amino acid analyzer after DA stimulation, and detect significant glycine peak. Furthermore, we utilized a patch-clamp biosensor method to confirm the glycine release from astrocytes by using GlyRα1 and Glyβ-expressing HEK293T cells, and detected significant glycine-evoked current upon DA stimulation. Thus, we clearly demonstrated that DA induces glycine release from astrocytes. Surprisingly, DA caused a functional reversal of astrocytic glycine transporter 1, an astrocytic type of glycine transporter, causing astrocytes to release glycine. Hence, astrocytes transduce pre-synaptic DA signals to glycine signals through a reversal of astrocytic glycine transporter 1 to regulate neuronal excitability. Cover Image for this issue: doi: 10.1111/jnc.13785.

Quantitative Analysis of Axonal Branch Dynamics in the Developing Nervous System

Branching is an important mechanism by which axons navigate to their targets during neural development. For instance, in the developing zebrafish retinotectal system, selective branching plays a critical role during both initial pathfinding and subsequent arborisation once the target zone has been reached. Here we show how quantitative methods can help extract new information from time-lapse imaging about the nature of the underlying branch dynamics. First, we introduce Dynamic Time Warping to this domain as a method for automatically matching branches between frames, replacing the effort required for manual matching. Second, we model branch dynamics as a birth-death process, i.e. a special case of a continuous-time Markov process. This reveals that the birth rate for branches from zebrafish retinotectal axons, as they navigate across the tectum, increased over time. We observed no significant change in the death rate for branches over this time period. However, blocking neuronal activity with TTX slightly increased the death rate, without a detectable change in the birth rate. Third, we show how the extraction of these rates allows computational simulations of branch dynamics whose statistics closely match the data. Together these results reveal new aspects of the biology of retinotectal pathfinding, and introduce computational techniques which are applicable to the study of axon branching more generally.

The complex morphologies of neurons present challenges for analysis. Large data sets can be gathered, but extracting meaningful data from the hundreds of branches from one axon over a few hundred time points can be difficult. One problem in particular is matching a single unique branch through several images, when the branches can extend, retract, or be removed entirely. In addition, if the imaging is done in vivo, the environment itself can grow and shift. Here we introduce Dynamic Time Warping (DTW) analysis to follow the complex structures of neurons through time. DTW identifies individual branches and therefore allows the determination of branch lifetimes. Using this approach we find that for retinal ganglion cell axons, the branch birth rate increases over time as axons navigate to their targets, and that blocking neural activity slightly increases the branch death rate without impacting the birth rate. From the estimated birth and death rate parameters we create simulations based on a continuous-time Markov chain process. These tools expand the techniques available to study the development of neuronal structures and provide more information from large time-lapse imaging datasets.

Neuronal activity is not required for the initial formation and maturation of visual selectivity

Neuronal activity is important for the functional refinement of neuronal circuits in the early visual system. At the level of the cerebral cortex, however, it is still unknown whether the formation of fundamental functions such as orientation selectivity depends on neuronal activity, as it has been difficult to suppress activity throughout development. Using genetic silencing of cortical activity starting before the formation of orientation selectivity, we found that the orientation selectivity of neurons in the mouse visual cortex formed and matured normally despite a strong suppression of both spontaneous and visually evoked activity throughout development. After the orientation selectivity formed, the distribution of the preferred orientations of neurons was reorganized. We found that this process required spontaneous activity, but not visually evoked activity. Thus, the initial formation and maturation of orientation selectivity is largely independent of neuronal activity, and the initial selectivity is subsequently modified depending on neuronal activity.

Rapid Changes in Cortical and Subcortical Brain Regions after Early Bilateral Enucleation in the Mouse

Functional sensory and motor areas in the developing mammalian neocortex are formed through a complex interaction of cortically intrinsic mechanisms, such as gene expression, and cortically extrinsic mechanisms such as those mediated by thalamic input from the senses. Both intrinsic and extrinsic mechanisms are believed to be involved in cortical patterning and the establishment of areal boundaries in early development; however, the nature of the interaction between intrinsic and extrinsic processes is not well understood. In a previous study, we used a perinatal bilateral enucleation mouse model to test some aspects of this interaction by reweighting sensory input to the developing cortex. Visual deprivation at birth resulted in a shift of intraneocortical connections (INCs) that aligned with ectopic ephrin A5 expression in the same location ten days later at postnatal day (P) 10. A prevailing question remained: Does visual deprivation first induce a change in gene expression, followed by a shift in INCs, or vice versa? In the present study, we address this question by investigating the neuroanatomy and patterns of gene expression in post-natal day (P) 1 and 4 mice following bilateral enucleation at birth. Our results demonstrate a rapid reduction in dorsal lateral geniculate nucleus (dLGN) size and ephrin A5 gene expression 24-hours post-enucleation, with more profound effects apparent at P4. The reduced nuclear size and diminished gene expression mirrors subtle changes in ephrin A5 expression evident in P1 and P4 enucleated neocortex, 11 and 8 days prior to natural eye opening, respectively. Somatosensory and visual INCs were indistinguishable between P1 and P4 mice bilaterally enucleated at birth, indicating that perinatal bilateral enucleation initiates a rapid change in gene expression (within one day) followed by an alteration of sensory INCs later on (second postnatal week). With these results, we gain a deeper understanding of how gene expression and sensory input together regulate cortical arealization and plasticity during early development.

GABA-ergic interneurons involved in transcallosal inhibition of the visual cortices in vivo in mice

In the current study we investigated the role of the corpus callosum, particularly the gamma-aminobutyric acid-ergic (GABAergic) projection neurons involved in interhemispheric inhibition (IHI). In order to explore IHI in primary visual cortices, we adopted a protocol whereby we performed a direct current lesion of the unilateral primary visual cortex with or without posterior callosotomy, and used two-photon Ca(2+)in vivo imaging on the opposite unaffected region to detect neural activities in mice. Following this procedure, the numbers of vesicular GABAergic transporters (VGATs) and GABAergic interneurons in the unaffected primary cortex were determined using immunofluorescence staining. Results indicated that following unilateral visual cortical lesioning without callosotomy, the neuronal Ca(2+) activities in the opposite side were significantly increased. However, the neuronal activities of the unaffected visual cortex in animals with unilateral cortical lesion with callosotomy were not significantly different. Additionally, there was no significant difference in the numbers of GABAergic interneurons in the unaffected region between each group, while the number of VGATs in the unaffected region was significantly decreased following unilateral visual cortical lesion without callosotomy, which was unchanged once with callosotomy. Finally, callosotomy alone without cortical lesioning produced no change in neuronal activities, the number of GABAergic interneurons or VGATs. Our results demonstrate that IHI between the homologous primary visual cortices occurs via the corpus callosum, and further indicate the important involvement of long-range GABAergic interneurons in transcallosal inhibition.

Axon Dynamics during Neocortical Laminar Innervation

The cerebral cortex is a densely interconnected structure with neural circuits that form between cortical laminae and also between distinct cortical areas. However, the precise cell biological and developmental mechanisms that underlie the formation of these neural circuits remain unknown. Here, we visualize laminar innervation of the developing mouse cerebral cortex by layer II/III pyramidal neurons in real time, describing cytoskeletal dynamics during this process. We find that layer II/III pyramidal neurons achieve local laminar-specific innervation through the stabilization of collateral axon branches in target laminae. We also find that loss of neural activity does not abolish local laminar-specific innervation and that cells within the local environment are the likely source of cues that direct layer-specific cortical innervation.

Synapse-dependent and independent mechanisms of thalamocortical axon branching are regulated by neuronal activity

Axon branching and synapse formation are critical processes for establishing precise circuit connectivity. These processes are tightly regulated by neural activity, but the relationship between them remains largely unclear. We use organotypic coculture preparations to examine the role of synapse formation in the activity-dependent axon branching of thalamocortical (TC) projections. To visualize TC axons and their presynaptic sites, two plasmids encoding DsRed and EGFP-tagged synaptophysin (SYP-EGFP) were cotransfected into a small number of thalamic neurons. Time-lapse imaging of individual TC axons showed that most branches emerged from SYP-EGFP puncta, indicating that synapse formation precedes emergences of axonal branches. We also investigated the effects of neuronal activity on axon branching and synapse formation by manipulating spontaneous firing activity of thalamic cells. An inward rectifying potassium channel, Kir2.1, and a bacterial voltage-gated sodium channel, NaChBac, were used to suppress and promote firing activity, respectively. We found suppressing neural activity reduced both axon branching and synapse formation. In contrast, increasing neural activity promoted only axonal branch formation. Time-lapse imaging of NaChBac-expressing cells further revealed that new branches frequently appeared from the locations other than SYP-EGFP puncta, indicating that enhancing activity promotes axonal branch formation due to an increase of branch emergence at nonsynaptic sites. These results suggest that presynaptic locations are hotspots for branch emergence, and that frequent firing activity can shift branch emergence to a synapse-independent process.

DCC functions as an accelerator of thalamocortical axonal growth downstream of spontaneous thalamic activity

Controlling the axon growth rate is fundamental when establishing brain connections. Using the thalamocortical system as a model, we previously showed that spontaneous calcium activity influences the growth rate of thalamocortical axons by regulating the transcription of Robo1 through an NF-κB-binding site in its promoter. Robo1 acts as a brake on the growth of thalamocortical axons in vivo. Here, we have identified the Netrin-1 receptor DCC as an accelerator for thalamic axon growth. Dcc transcription is regulated by spontaneous calcium activity in thalamocortical neurons and activating DCC signaling restores normal axon growth in electrically silenced neurons. Moreover, we identified an AP-1-binding site in the Dcc promoter that is crucial for the activity-dependent regulation of this gene. In summary, we have identified the Dcc gene as a novel downstream target of spontaneous calcium activity involved in axon growth. Together with our previous data, we demonstrate a mechanism to control axon growth that relies on the activity-dependent regulation of two functionally opposed receptors, Robo1 and DCC. These two proteins establish a tight and efficient means to regulate activity-guided axon growth in order to correctly establish neuronal connections during development.

Contralateral targeting of the corpus callosum in normal and pathological brain function

The corpus callosum connects the two cortical hemispheres of the mammalian brain and is susceptible to structural defects during development, which often result in significant neuropsychological dysfunction. To date, such individuals have been studied primarily with regards to the integrity of the callosal tract at the midline. However, the mechanisms regulating the contralateral targeting of the corpus callosum, after midline crossing has occurred, are less well understood. Recent evidence suggests that defects in contralateral targeting can occur in isolation from midline-tract malformations, and may have significant functional implications. We propose that contralateral targeting is a crucially important and relatively under-investigated event in callosal development, and that defects in this process may constitute an undiagnosed phenotype in several neurological disorders.

Imaging the awake visual cortex with a genetically encoded voltage indicator

Genetically encoded voltage indicators (GEVIs) promise to reveal the membrane potential of genetically targeted neuronal populations through noninvasive, chronic imaging of large portions of cortical space. Here we test a promising GEVI in mouse cortex during wakefulness, a challenging condition due to large hemodynamic activity, and we introduce a straightforward projection method to separate a signal dominated by membrane voltage from a signal dominated by hemodynamic activity. We expressed VSFP-Butterfly 1.2 plasmid in layer 2/3 pyramidal cells of visual cortex through electroporation in utero. We then used wide-field imaging with two cameras to measure both fluorophores of the indicator in response to visual stimuli. By taking weighted sums and differences of the two measurements, we obtained clear separation of hemodynamic and voltage signals. The hemodynamic signal showed strong heartbeat oscillations, superimposed on slow dynamics similar to blood oxygen level-dependent (BOLD) or "intrinsic" signals. The voltage signal had fast dynamics similar to neural responses measured electrically, and showed an orderly retinotopic mapping. We compared this voltage signal with calcium signals imaged in transgenic mice that express a calcium indicator (GCaMP3) throughout cortex. The voltage signal from VSFP had similar signal-to-noise ratios as the calcium signal, it was more immune to vascular artifacts, and it integrated over larger regions of visual space, which was consistent with its reporting mostly subthreshold activity rather than the spiking activity revealed by calcium signals. These results demonstrate that GEVIs provide a powerful tool to study the dynamics of neural populations at mesoscopic spatial scales in the awake cortex.

Up-regulation of HP1γ expression during neuronal maturation promotes axonal and dendritic development in mouse embryonic neocortex

Immature neurons undergo morphological and physiological changes including axonal and dendritic development to establish neuronal networks. As the transcriptional status changes at a large number of genes during neuronal maturation, global changes in chromatin modifiers may take place in this process. We now show that the amount of heterochromatin protein 1γ (HP1γ) increases during neuronal maturation in the mouse neocortex. Knockdown of HP1γ suppressed axonal and dendritic development in mouse embryonic neocortical neurons in culture, and either knockdown or knockout of HP1γ impaired the projection of callosal axons of superficial layer neurons to the contralateral hemisphere in the developing neocortex. Conversely, forced expression of HP1γ facilitated axonal and dendritic development, suggesting that the increase of HP1γ is a rate limiting step in neuronal maturation. These results together show an important role for HP1γ in promoting axonal and dendritic development in maturing neurons.

DSCAM and DSCAML1 regulate the radial migration and callosal projection in developing cerebral cortex

Down syndrome cell adhesion molecule (Dscam) is essential for self-avoidance and tiling of dendritic development in sensory neurons in Drosophila. Recent studies also show that DSCAM together with its closely related protein DSCAML1 functions in dendritic self-avoidance of a certain types of interneuron in mammalian retina. However, the functions of these DSCAMs in developing mammalian cerebral cortex are not well understood. Here we reduced the expression of DSCAM or DSCAML1 in mouse cortical neurons by RNA interference both in vitro and in vivo. We found that knockdown of DSCAM or DSCAML1 increases the complexity of proximal dendritic branching, and impedes the axon growth in cultured neurons. In vivo knockdown experiments showed that both DSCAM and DSCAML1 contribute to normal radial migration and callosal projection during the postnatal development. Our results indicate an important role of DSCAM and DSCAML1 in the development of cortical neural network.

Cortico-cortical interactions influence binocularity of the primary visual cortex of adult mice

Electrophysiological studies have revealed that a large proportion of the mouse primary visual cortex (V1) receives input also from the ipsilateral eye. This is surprising as most optic nerve fibers cross at the optic chiasm in mice. Inactivating V1 of one hemisphere has recently demonstrated a strong contribution of one hemisphere's activity on binocularity of single units and visually evoked potentials of V1 in the other hemisphere of young rats and of single units in young adult mice. Here we used intrinsic signal optical imaging to quantitatively study the influence of cortico-cortical connections on the magnitude of neuronal activation in the entire binocular zone of adult mouse V1. We simultaneously measured V1-activity of both hemispheres in adult C57BL/6J mice before and after blocking sensory-driven activity in one hemisphere with muscimol. In V1 contralateral to the inactivation, ipsilateral eye evoked activity was reduced by on average 18% while contralateral eye evoked activity did not change. Our results clearly show that cortico-cortical interactions exert a global amplification of ipsilateral eye evoked activity in adult mouse V1.

Balanced interhemispheric cortical activity is required for correct targeting of the corpus callosum

Bilateral integration of sensory and associative brain processing is achieved by precise connections between homologous regions in the two hemispheres via the corpus callosum. These connections form postnatally, and unilateral deprivation of sensory or spontaneous cortical activity during a critical period severely disrupts callosal wiring. However, little is known about how this early activity affects precise circuit formation. Here, using in utero electroporation of reporter genes, optogenetic constructs, and direct disruption of activity in callosal neurons combined with whisker ablations, we show that balanced interhemispheric activity, and not simply intact cortical activity in either hemisphere, is required for functional callosal targeting. Moreover, bilateral ablation of whiskers in symmetric or asymmetric configurations shows that spatially symmetric interhemispheric activity is required for appropriate callosal targeting. Our findings reveal a principle governing axon targeting, where spatially balanced activity between regions is required to establish their appropriate connectivity.

Normotopic cortex is the major contributor to epilepsy in experimental double cortex

OBJECTIVE

Subcortical band heterotopia (SBH) is a cortical malformation formed when neocortical neurons prematurely stop their migration in the white matter, forming a heterotopic band below the normotopic cortex, and is generally associated with intractable epilepsy. Although it is clear that the band heterotopia and the overlying cortex both contribute to creating an abnormal circuit prone to generate epileptic discharges, it is less understood which part of this circuitry is the most critical. Here, we sought to identify the origin of epileptiform activity in a targeted genetic model of SBH in rats.

METHODS

Rats with SBH (Dcx-KD rats) were generated by knocking down the Dcx gene using shRNA vectors transfected into neocortical progenitors of rat embryos. Origin, spatial extent, and laminar profile of bicuculline-induced interictal-like activity on neocortical slices were analyzed by using extracellular recordings from 60-channel microelectrode arrays. Susceptibility to pentylenetetrazole-induced seizures was assessed by electrocorticography in head-restrained nonanesthetized rats.

RESULTS

We show that the band heterotopia does not constitute a primary origin for interictal-like epileptiform activity in vitro and is dispensable for generating induced seizures in vivo. Furthermore, we report that most interictal-like discharges originating in the overlying cortex secondarily propagate to the band heterotopia. Importantly, we found that in vivo suppression of neuronal excitability in SBH does not alter the higher propensity of Dcx-KD rats to display seizures.

INTERPRETATION

These results suggest a major role of the normotopic cortex over the band heterotopia in generating interictal epileptiform activity and seizures in brains with SBH.

Non-cell-autonomous mechanism of activity-dependent neurotransmitter switching

Activity-dependent neurotransmitter switching engages genetic programs regulating transmitter synthesis, but the mechanism by which activity is transduced is unknown. We suppressed activity in single neurons in the embryonic spinal cord to determine whether glutamate-gamma-aminobutyric acid (GABA) switching is cell autonomous. Transmitter respecification did not occur, suggesting that it is homeostatically regulated by the level of activity in surrounding neurons. Graded increase in the number of silenced neurons in cultures led to graded decrease in the number of neurons expressing GABA, supporting non-cell-autonomous transmitter switching. We found that brain-derived neurotrophic factor (BDNF) is expressed in the spinal cord during the period of transmitter respecification and that spike activity causes release of BDNF. Activation of TrkB receptors triggers a signaling cascade involving JNK-mediated activation of cJun that regulates tlx3, a glutamate/GABA selector gene, accounting for calcium-spike BDNF-dependent transmitter switching. Our findings identify a molecular mechanism for activity-dependent respecification of neurotransmitter phenotype in developing spinal neurons.

Evolution and development of interhemispheric connections in the vertebrate forebrain

Axonal connections between the left and right sides of the brain are crucial for bilateral integration of lateralized sensory, motor, and associative functions. Throughout vertebrate species, forebrain commissures share a conserved developmental plan, a similar position relative to each other within the brain and similar patterns of connectivity. However, major events in the evolution of the vertebrate brain, such as the expansion of the telencephalon in tetrapods and the origin of the six-layered isocortex in mammals, resulted in the emergence and diversification of new commissural routes. These new interhemispheric connections include the pallial commissure, which appeared in the ancestors of tetrapods and connects the left and right sides of the medial pallium (hippocampus in mammals), and the corpus callosum, which is exclusive to eutherian (placental) mammals and connects both isocortical hemispheres. A comparative analysis of commissural systems in vertebrates reveals that the emergence of new commissural routes may have involved co-option of developmental mechanisms and anatomical substrates of preexistent commissural pathways. One of the embryonic regions of interest for studying these processes is the commissural plate, a portion of the early telencephalic midline that provides molecular specification and a cellular scaffold for the development of commissural axons. Further investigations into these embryonic processes in carefully selected species will provide insights not only into the mechanisms driving commissural evolution, but also regarding more general biological problems such as the role of developmental plasticity in evolutionary change.

Transcallosal Projections Require Glycoprotein M6-Dependent Neurite Growth and Guidance

The function of mature neurons critically relies on the developmental outgrowth and projection of their cellular processes. It has long been postulated that the neuronal glycoproteins M6a and M6b are involved in axon growth because these four-transmembrane domain-proteins of the proteolipid protein family are highly enriched on growth cones, but in vivo evidence has been lacking. Here, we report that the function of M6 proteins is required for normal axonal extension and guidance in vivo. In mice lacking both M6a and M6b, a severe hypoplasia of axon tracts was manifested. Most strikingly, the corpus callosum was reduced in thickness despite normal densities of cortical projection neurons. In single neuron tracing, many axons appeared shorter and disorganized in the double-mutant cortex, and some of them were even misdirected laterally toward the subcortex. Probst bundles were not observed. Upon culturing, double-mutant cortical and cerebellar neurons displayed impaired neurite outgrowth, indicating a cell-intrinsic function of M6 proteins. A rescue experiment showed that the intracellular loop of M6a is essential for the support of neurite extension. We propose that M6 proteins are required for proper extension and guidance of callosal axons that follow one of the most complex trajectories in the mammalian nervous system.

Septins promote dendrite and axon development by negatively regulating microtubule stability via HDAC6-mediated deacetylation

Neurite growth requires two guanine nucleotide-binding protein polymers of tubulins and septins. However, whether and how those cytoskeletal systems are coordinated was unknown. Here we show that the acute knockdown or knockout of the pivotal septin subunit SEPT7 from cerebrocortical neurons impairs their interhemispheric and cerebrospinal axon projections and dendritogenesis in perinatal mice, when the microtubules are severely hyperacetylated. The resulting hyperstabilization and growth retardation of microtubules are demonstrated in vitro. The phenotypic similarity between SEPT7 depletion and the pharmacological inhibition of α-tubulin deacetylase HDAC6 reveals that HDAC6 requires SEPT7 not for its enzymatic activity, but to associate with acetylated α-tubulin. These and other findings indicate that septins provide a physical scaffold for HDAC6 to achieve efficient microtubule deacetylation, thereby negatively regulating microtubule stability to an optimal level for neuritogenesis. Our findings shed light on the mechanisms underlying the HDAC6-mediated coupling of the two ubiquitous cytoskeletal systems during neural development.

Septins are a family of heteropolymerizing GTP/GDP-binding proteins and are implicated in neuritogenesis in nematodes. Ageta-Ishihara et al. show that septins also facilitate this process in the developing mouse brain as scaffolds that coordinate HDAC6-mediated deacetylation of microtubules.

In vivo visuotopic brain mapping with manganese-enhanced MRI and resting-state functional connectivity MRI

The rodents are an increasingly important model for understanding the mechanisms of development, plasticity, functional specialization and disease in the visual system. However, limited tools have been available for assessing the structural and functional connectivity of the visual brain network globally, in vivo and longitudinally. There are also ongoing debates on whether functional brain connectivity directly reflects structural brain connectivity. In this study, we explored the feasibility of manganese-enhanced MRI (MEMRI) via 3 different routes of Mn(2+) administration for visuotopic brain mapping and understanding of physiological transport in normal and visually deprived adult rats. In addition, resting-state functional connectivity MRI (RSfcMRI) was performed to evaluate the intrinsic functional network and structural-functional relationships in the corresponding anatomical visual brain connections traced by MEMRI. Upon intravitreal, subcortical, and intracortical Mn(2+) injection, different topographic and layer-specific Mn enhancement patterns could be revealed in the visual cortex and subcortical visual nuclei along retinal, callosal, cortico-subcortical, transsynaptic and intracortical horizontal connections. Loss of visual input upon monocular enucleation to adult rats appeared to reduce interhemispheric polysynaptic Mn(2+) transfer but not intra- or inter-hemispheric monosynaptic Mn(2+) transport after Mn(2+) injection into visual cortex. In normal adults, both structural and functional connectivity by MEMRI and RSfcMRI was stronger interhemispherically between bilateral primary/secondary visual cortex (V1/V2) transition zones (TZ) than between V1/V2 TZ and other cortical nuclei. Intrahemispherically, structural and functional connectivity was stronger between visual cortex and subcortical visual nuclei than between visual cortex and other subcortical nuclei. The current results demonstrated the sensitivity of MEMRI and RSfcMRI for assessing the neuroarchitecture, neurophysiology and structural-functional relationships of the visual brains in vivo. These may possess great potentials for effective monitoring and understanding of the basic anatomical and functional connections in the visual system during development, plasticity, disease, pharmacological interventions and genetic modifications in future studies.

A switch from inter-ocular to inter-hemispheric suppression following monocular deprivation in the rat visual cortex

Binocularity is a key property of primary visual cortex (V1) neurons that is widely used to study synaptic integration in the brain and plastic mechanisms following an altered visual experience. However, it is not clear how the inputs from the two eyes converge onto binocular neurons, and how their interaction is modified by an unbalanced visual drive. Here, using visual evoked potentials recorded in the juvenile rat V1, we report evidence for a suppressive mechanism by which contralateral eye activity inhibits responses from the ipsilateral eye. Accordingly, we found a lack of additivity of the responses evoked independently by the two eyes in the V1, and acute silencing of the contralateral eye resulted in the enhancement of ipsilateral eye responses in cortical neurons. We reverted the relative cortical strength of the two eyes by suturing the contralateral eye shut [monocular deprivation (MD)]. After 7 days of MD, there was a loss of interocular suppression mediated by the contralateral, deprived eye, and weak inputs from the closed eye were functionally inhibited by interhemispheric callosal pathways. We conclude that interocular suppressive mechanisms play a crucial role in shaping normal binocularity in visual cortical neurons, and a switch from interocular to interhemispheric suppression represents a key step in the ocular dominance changes induced by MD. These data have important implications for a deeper understanding of the key mechanisms that underlie activity-dependent rearrangements of cortical circuits following alteration of sensory experience.

Branch management: mechanisms of axon branching in the developing vertebrate CNS

The remarkable ability of a single axon to extend multiple branches and form terminal arbors enables vertebrate neurons to integrate information from divergent regions of the nervous system. Axons select appropriate pathways during development, but it is the branches that extend interstitially from the axon shaft and arborize at specific targets that are responsible for virtually all of the synaptic connectivity in the vertebrate CNS. How do axons form branches at specific target regions? Recent studies have identified molecular cues that activate intracellular signalling pathways in axons and mediate dynamic reorganization of the cytoskeleton to promote the formation of axon branches.

Distal connectivity causes summation and division across mouse visual cortex

Neurons in different locations across the cortex are connected through polysynaptic networks involving both excitation and inhibition. To probe the functional effect of such networks, we used optogenetic stimulation to trigger antidromic spikes in a local region of primary visual cortex (V1). This local activity had two effects at distal V1 locations: summation and division. The balance between the two depended on visual contrast, and a normalization model precisely captured these effects.

Sensory input is required for callosal axon targeting in the somatosensory cortex

Background

Sensory input is generally thought to be necessary for refining and consolidating neuronal connections during brain development. We here report that cortical callosal axons in somatosensory cortex require sensory input for their target selection in contralateral cortex.

Results

Eliminating sensory input to either hemisphere by unilateral transection of infraorbital nerve (ION) prevents target selection of callosal axons in contralateral cortex. Strikingly, blocking sensory input bilaterally, by simultaneously transecting both IONs, results in rescued callosal projection. In contrast, non-simultaneous bilateral ION transection has the same effect as unilateral transection. Similar results are obtained by lesion of whisker hair follicles. c-Fos-positive neurons in brain slices treated with KCl is decreased more in contralateral cortex with unilateral removal of sensory input, but decreased similarly in both cortices in mice with simultaneous bilateral removal of sensory input. Frequency of sEPSC of cortical neurons is also reduced in contralateral cortex with the unilateral removal of sensory input, but equally reduced on both sides with the bilateral removal of sensory input, suggesting that unbalanced bilateral sensory input might lead to mismatched neuronal activity between the two cortices and contribute to the formation of callosal projection.

Conclusion

Our data demonstrate a critical role of balanced bilateral somatosensory input in the formation of callosal connections, and thus reveal a new role of sensory input in wiring brain circuits.

Cell biology in neuroscience: Cellular and molecular mechanisms underlying axon formation, growth, and branching

Proper brain wiring during development is pivotal for adult brain function. Neurons display a high degree of polarization both morphologically and functionally, and this polarization requires the segregation of mRNA, proteins, and lipids into the axonal or somatodendritic domains. Recent discoveries have provided insight into many aspects of the cell biology of axonal development including axon specification during neuronal polarization, axon growth, and terminal axon branching during synaptogenesis.

Chronic depression of hypothalamic paraventricular neuronal activity produces sustained hypotension in hypertensive rats

Changes in the sympathetic nervous system are responsible for the initiation, development and maintenance of hypertension. An important central sympathoexcitatory region is the paraventricular nucleus (PVN) of the hypothalamus, which may become more active in hypertensive conditions, as shown in acute studies previously. Our objective was to depress PVN neuronal activity chronically by the overexpression of an inwardly rectifying potassium channel (hKir2.1), while evaluating the consequences on blood pressure (BP) and its reflex regulation. In spontaneously hypertensive rats (SHRs) and Wistar rats (WKY) lentiviral vectors (LVV-hKir2.1; LV-TREtight-Kir-cIRES-GFP5 4 × 10(9) IU and LV-Syn-Eff-G4BS-Syn-Tetoff 6.2 × 10(9) IU in a ratio 1:4) were stereotaxically microinjected bilaterally into the PVN. Sham-treated SHRs and WKY received bilateral PVN microinjections of LVV-eGFP (LV-Syn-Eff-G4BS-Syn-Tetoff 6.2 × 10(9) IU and LV-TREtight-GFP 5.7 × 10(9) IU in a ratio 1:4). Blood pressure was monitored continuously by radio-telemetry and evaluated over 75 days. Baroreflex gain was evaluated using phenylephrine (25 μg ml(-1), i.v.), whereas lobeline (25 μg ml(-1), i.v.) was used to stimulate peripheral chemoreceptors. In SHRs but not normotensive WKY rats, LVV-hKir2.1 expression in the PVN produced time-dependent and significant decreases in systolic (from 158 ± 3 to 132 ± 6 mmHg; P < 0.05) and diastolic BP (from 135 ± 4 to 113 ± 5 mmHg; P < 0.05). The systolic BP low-frequency band was reduced (from 0.79 ± 0.13 to 0.42 ± 0.09 mmHg(2); P < 0.05), suggesting reduced sympathetic vasomotor tone. Baroreflex gain was increased and peripheral chemoreflex depressed after PVN microinjection of LVV-hKir2.1. We conclude that the PVN plays a major role in long-term control of BP and sympathetic nervous system activity in SHRs. This is associated with reductions in both peripheral chemosensitivity and respiratory-induced sympathetic modulation and an improvement in baroreflex sensitivity. Our results support the PVN as a powerful site to control BP in neurogenic hypertension.

A theory of the transition to critical period plasticity: inhibition selectively suppresses spontaneous activity

What causes critical periods (CPs) to open? For the best-studied case, ocular dominance plasticity in primary visual cortex in response to monocular deprivation (MD), the maturation of inhibition is necessary and sufficient. How does inhibition open the CP? We present a theory: the transition from pre-CP to CP plasticity arises because inhibition preferentially suppresses responses to spontaneous relative to visually driven input activity, switching learning cues from internal to external sources. This differs from previous proposals in (1) arguing that the CP can open without changes in plasticity mechanisms when activity patterns become more sensitive to sensory experience through circuit development, and (2) explaining not simply a transition from no plasticity to plasticity, but a change in outcome of MD-induced plasticity from pre-CP to CP. More broadly, hierarchical organization of sensory-motor pathways may develop through a cascade of CPs induced as circuit maturation progresses from "lower" to "higher" cortical areas.

Terminal axon branching is regulated by the LKB1-NUAK1 kinase pathway via presynaptic mitochondrial capture

The molecular mechanisms underlying the axon arborization of mammalian neurons are poorly understood but are critical for the establishment of functional neural circuits. We identified a pathway defined by two kinases, LKB1 and NUAK1, required for cortical axon branching in vivo. Conditional deletion of LKB1 after axon specification or knockdown of NUAK1 drastically reduced axon branching in vivo, whereas their overexpression was sufficient to increase axon branching. The LKB1-NUAK1 pathway controls mitochondria immobilization in axons. Using manipulation of Syntaphilin, a protein necessary and sufficient to arrest mitochondrial transport specifically in the axon, we demonstrate that the LKB1-NUAK1 kinase pathway regulates axon branching by promoting mitochondria immobilization. Finally, we show that LKB1 and NUAK1 are necessary and sufficient to immobilize mitochondria specifically at nascent presynaptic sites. Our results unravel a link between presynaptic mitochondrial capture and axon branching.

Retinal input regulates the timing of corticogeniculate innervation

Neurons in layer VI of visual cortex represent one of the largest sources of nonretinal input to the dorsal lateral geniculate nucleus (dLGN) and play a major role in modulating the gain of thalamic signal transmission. However, little is known about how and when these descending projections arrive and make functional connections with dLGN cells. Here we used a transgenic mouse to visualize corticogeniculate projections to examine the timing of cortical innervation in dLGN. Corticogeniculate innervation occurred at postnatal ages and was delayed compared with the arrival of retinal afferents. Cortical fibers began to enter dLGN at postnatal day 3 (P3) to P4, a time when retinogeniculate innervation is complete. However, cortical projections did not fully innervate dLGN until eye opening (P12), well after the time when retinal inputs from the two eyes segregate to form nonoverlapping eye-specific domains. In vitro thalamic slice recordings revealed that newly arriving cortical axons form functional connections with dLGN cells. However, adult-like responses that exhibited paired pulse facilitation did not fully emerge until 2 weeks of age. Finally, surgical or genetic elimination of retinal input greatly accelerated the rate of corticogeniculate innervation, with axons invading between P2 and P3 and fully innervating dLGN by P8 to P10. However, recordings in genetically deafferented mice showed that corticogeniculate synapses continued to mature at the same rate as controls. These studies suggest that retinal and cortical innervation of dLGN is highly coordinated and that input from retina plays an important role in regulating the rate of corticogeniculate innervation.

The cell-autonomous role of excitatory synaptic transmission in the regulation of neuronal structure and function

The cell-autonomous role of synaptic transmission in the regulation of neuronal structural and electrical properties is unclear. We have now employed a genetic approach to eliminate glutamatergic synaptic transmission onto individual CA1 pyramidal neurons in a mosaic fashion in vivo. Surprisingly, while electrical properties are profoundly affected in these neurons, as well as inhibitory synaptic transmission, we found little perturbation of neuronal morphology, demonstrating a functional segregation of excitatory synaptic transmission from neuronal morphological development.

Visual callosal topography in the absence of retinal input

Using probabilistic diffusion tractography, we examined the retinotopic organization of splenial callosal connections within early blind, anophthalmic, and control subjects. Early blind subjects experienced prenatal retinal "waves" of spontaneous activity similar to those of sighted subjects, and only lack postnatal visual experience. In anophthalmia, the eye is either absent or arrested at an early prenatal stage, depriving these subjects of both pre- and postnatal visual input. Therefore, comparing these two groups provides a way of separating the influence of pre- and postnatal retinal input on the organization of visual connections across hemispheres. We found that retinotopic mapping within the splenium was not measurably disrupted in early blind or anophthalmic subjects compared to visually normal controls. No significant differences in splenial volume were observed across groups. No significant differences in diffusivity were found between early blind subjects and sighted controls, though some differences in diffusivity were noted between anophthalmic subjects and controls. These results suggest that neither prenatal retinal activity nor postnatal visual experience plays a role in the large-scale topographic organization of visual callosal connections within the splenium.

Axon guidance mechanisms for establishment of callosal connections

Numerous studies have investigated the formation of interhemispheric connections which are involved in high-ordered functions of the cerebral cortex in eutherian animals, including humans. The development of callosal axons, which transfer and integrate information between the right/left hemispheres and represent the most prominent commissural system, must be strictly regulated. From the beginning of their growth, until reaching their targets in the contralateral cortex, the callosal axons are guided mainly by two environmental cues: (1) the midline structures and (2) neighboring? axons. Recent studies have shown the importance of axona guidance by such cues and the underlying molecular mechanisms. In this paper, we review these guidance mechanisms during the development of the callosal neurons. Midline populations express and secrete guidance molecules, and "pioneer" axons as well as interactions between the medial and lateral axons are also involved in the axon pathfinding of the callosal neurons. Finally, we describe callosal dysgenesis in humans and mice, that results from a disruption of these navigational mechanisms.

Long-term channelrhodopsin-2 (ChR2) expression can induce abnormal axonal morphology and targeting in cerebral cortex

Long-term expression of optogenetic proteins including channelrhodopsin-2 (ChR2) is widely used to study neural circuit function, but whether ChR2 expression itself perturbs circuits is not known. We expressed a common construct, CAG::ChR2 (H134R)-EYFP-WPRE, in L2/3 pyramidal cells in rat somatosensory cortex via in utero DNA electroporation (IUE). L2/3 pyramidal cells expressed ChR2-EYFP, but histology revealed abnormal morphology and targeting of ChR2-EYFP expressing axons, beginning at postnatal day (P) 33 and increasing with age. Axonal abnormalities included cylinders that enveloped pyramidal cell proximal apical dendrites, and spherical, calyx-like structures that surrounded neuronal cell bodies, including in L4. These are abnormal subcellular and laminar targets for L2/3 pyramidal cell synapses. Abnormalities did not occur in cells expressing GFP instead of ChR2, or in intermixed ChR2-negative axons. Long-term viral-mediated expression (80 d) did not cause axonal abnormalities when the CAG promoter was used, but produced some abnormalities with the stronger αCaMKII promoter (albeit much less than with in utero electroporation). Thus, under some circumstances high-level, long-term expression of ChR2-EYFP can perturb the structural organization of cortical circuits.