Stabilization of axon branch dynamics by synaptic maturation

Hebbian instruction of axonal connectivity by endogenous correlated spontaneous activity

Spontaneous activity refines neural connectivity prior to the onset of sensory experience, but it remains unclear how such activity instructs axonal connectivity with subcellular precision. We simultaneously measured spontaneous retinal waves and the activity of individual retinocollicular axons and tracked morphological changes in axonal arbors across hours in vivo in neonatal mice. We demonstrate that the correlation of an axon branch's activity with neighboring axons or postsynaptic neurons predicts whether the branch will be added, stabilized, or eliminated. Desynchronizing individual axons from their local networks, changing the pattern of correlated activity, or blocking N-methyl-d-aspartate receptors all significantly altered single-axon morphology. These observations provide the first direct evidence in vivo that endogenous patterns of correlated neuronal activity instruct fine-scale refinement of axonal processes.

Mutation of the ALS-/FTD-Associated RNA-Binding Protein FUS Affects Axonal Development

Aberrant condensation and localization of the RNA-binding protein (RBP) fused in sarcoma (FUS) occur in variants of amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Changes in RBP function are commonly associated with changes in axonal cytoskeletal organization and branching in neurodevelopmental disorders. Here, we asked whether branching defects also occur in vivo in a model of FUS-associated disease. We use two reported Xenopus models of ALS/FTD (of either sex), the ALS-associated mutant FUS(P525L) and a mimic of hypomethylated FUS, FUS(16R). Both mutants strongly reduced axonal complexity in vivo. We also observed an axon looping defect for FUS(P525L) in the target area, which presumably arises due to errors in stop cue signaling. To assess whether the loss of axon complexity also had a cue-independent component, we assessed axonal cytoskeletal integrity in vitro. Using a novel combination of fluorescence and atomic force microscopy, we found that mutant FUS reduced actin density in the growth cone, altering its mechanical properties. Therefore, FUS mutants may induce defects during early axonal development.

Physical presence of chemical synapses is necessary for turning behavior of anterograde synaptic vesicles at the branch point of PLM neurons in C. elegans

Neurons exhibit complex branched axonal morphologies in both vertebrate and invertebrate systems, and show heterogeneity in the distribution of synaptic cargo across multiple synapses. It is possible that differences in transport across multiple branches contribute to the heterogeneity in cargo distribution across multiple synapses. However, the regulation of transport at axonal branch points is not well understood. We demonstrate that branch-specific transport of synaptic vesicles is dependent on the existence of a connection between the branch and synapses. The loss of this connection causes an immediate decrease in branch-specific transport of synaptic vesicles in the PLM neuron of C. elegans .

The regulation of enteric neuron connectivity by semaphorin 5A is affected by the autism-associated S956G missense mutation

The neural network of the enteric nervous system (ENS) underlies gastrointestinal functions. However, the molecular mechanisms involved in enteric neuronal connectivity are poorly characterized. Here, we studied the role of semaphorin 5A (Sema5A), previously characterized in the central nervous system, on ENS neuronal connectivity. Sema5A is linked to autism spectrum disorder (ASD), a neurodevelopmental disorder frequently associated with gastrointestinal comorbidities, and potentially associated with ENS impairments. This study investigated in rat enteric neuron cultures and gut explants the role of Sema5A on enteric neuron connectivity and the impact of ASD-associated mutations on Sema5A activity. Our findings demonstrated that Sema5A promoted axonal complexity and reduced functional connectivity in enteric neurons. Strikingly, the ASD-associated mutation S956G in Sema5A strongly affected these activities. This study identifies a critical role of Sema5A in the ENS as a regulator of neuronal connectivity that might be compromised in ASD.

The AMPK-related kinase NUAK1 controls cortical axons branching by locally modulating mitochondrial metabolic functions

The cellular mechanisms underlying axonal morphogenesis are essential to the formation of functional neuronal networks. We previously identified the autism-linked kinase NUAK1 as a central regulator of axon branching through the control of mitochondria trafficking. However, (1) the relationship between mitochondrial position, function and axon branching and (2) the downstream effectors whereby NUAK1 regulates axon branching remain unknown. Here, we report that mitochondria recruitment to synaptic boutons supports collateral branches stabilization rather than formation in mouse cortical neurons. NUAK1 deficiency significantly impairs mitochondrial metabolism and axonal ATP concentration, and upregulation of mitochondrial function is sufficient to rescue axonal branching in NUAK1 null neurons in vitro and in vivo. Finally, we found that NUAK1 regulates axon branching through the mitochondria-targeted microprotein BRAWNIN. Our results demonstrate that NUAK1 exerts a dual function during axon branching through its ability to control mitochondrial distribution and metabolic activity.

Myelination-independent functions of oligodendrocyte precursor cells in health and disease

Oligodendrocyte precursor cells (OPCs) are a population of tissue-resident glial cells found throughout the CNS, constituting approximately 5% of all CNS cells and persisting from development to adulthood and aging. The canonical role of OPCs is to give rise to myelinating oligodendrocytes. However, additional functions of OPCs beyond this traditional role as precursors have been suggested for a long time. In this Perspective, we provide an overview of the multiple myelination-independent functions that have been described for OPCs in the context of neuron development, angiogenesis, inflammatory response, axon regeneration and their recently discovered roles in neural circuit remodeling.

Activity-Dependent Synapse Refinement: From Mechanisms to Molecules

The refinement of immature neuronal networks into efficient mature ones is critical to nervous system development and function. This process of synapse refinement is driven by the neuronal activity-dependent competition of converging synaptic inputs, resulting in the elimination of weak inputs and the stabilization of strong ones. Neuronal activity, whether in the form of spontaneous activity or experience-evoked activity, is known to drive synapse refinement in numerous brain regions. More recent studies are now revealing the manner and mechanisms by which neuronal activity is detected and converted into molecular signals that appropriately regulate the elimination of weaker synapses and stabilization of stronger ones. Here, we highlight how spontaneous activity and evoked activity instruct neuronal activity-dependent competition during synapse refinement. We then focus on how neuronal activity is transformed into the molecular cues that determine and execute synapse refinement. A comprehensive understanding of the mechanisms underlying synapse refinement can lead to novel therapeutic strategies in neuropsychiatric diseases characterized by aberrant synaptic function.

The synaptic basis of activity-dependent eye-specific competition

Binocular vision requires proper developmental wiring of eye-specific inputs to the brain. In the thalamus, axons from the two eyes initially overlap in the dorsal lateral geniculate nucleus and undergo activity-dependent competition to segregate into target domains. Here, we combine eye-specific tract tracing with volumetric super-resolution imaging to measure the nanoscale molecular reorganization of developing retinogeniculate eye-specific synapses in the mouse brain. We show there are eye-specific differences in presynaptic vesicle pool size and vesicle association with the active zone at the earliest stages of retinogeniculate refinement but find no evidence of eye-specific differences in subsynaptic domain number, size, or transsynaptic alignment across development. Genetic disruption of spontaneous retinal activity decreases retinogeniculate synapse density, delays the emergence eye-specific differences in vesicle organization, and disrupts subsynaptic domain maturation. These results suggest that activity-dependent eye-specific presynaptic maturation underlies synaptic competition in the mammalian visual system.

EGFR-dependent suppression of synaptic autophagy is required for neuronal circuit development

The development of neuronal connectivity requires stabilization of dynamic axonal branches at sites of synapse formation. Models that explain how axonal branching is coupled to synaptogenesis postulate molecular regulators acting in a spatiotemporally restricted fashion to ensure branching toward future synaptic partners while also stabilizing the emerging synaptic contacts between such partners. We investigated this question using neuronal circuit development in the Drosophila brain as a model system. We report that epidermal growth factor receptor (EGFR) activity is required in presynaptic axonal branches during two distinct temporal intervals to regulate circuit wiring in the developing Drosophila visual system. EGFR is required early to regulate primary axonal branching. EGFR activity is then independently required at a later stage to prevent degradation of the synaptic active zone protein Bruchpilot (Brp). Inactivation of EGFR results in a local increase of autophagy in presynaptic branches and the translocation of active zone proteins into autophagic vesicles. The protection of synaptic material during this later interval of wiring ensures the stabilization of terminal branches, circuit connectivity, and appropriate visual behavior. Phenotypes of EGFR inactivation can be rescued by increasing Brp levels or downregulating autophagy. In summary, we identify a temporally restricted molecular mechanism required for coupling axonal branching and synaptic stabilization that contributes to the emergence of neuronal wiring specificity.

NMDA Receptor Antagonist MK801 Reduces Dendritic Spine Density and Stability in Zebrafish Pyramidal Neurons

NMDA-type glutamate receptors play a critical role in activity-dependent neurite growth. We employed cell type-specific genetic labeling in zebrafish to examine the effects of NMDA receptor antagonism on the morphological development of tectal pyramidal neurons (PyrNs). Our data demonstrate that the NMDA receptor antagonist MK801 reduces PyrN spine density and stability without significantly altering dendritic growth and branching. However, the axons that synapse onto PyrN dendritic spines do exhibit reduced arbor growth and branching in response to MK801 treatment. Axons that synapse with PyrNs, but not on spines, are unaffected by MK801 treatment. These findings may reflect different roles for NMDARs during the development of spiny and aspiny dendrites.

Neuromodulatory control of inhibitory network arborization in the developing postnatal neocortex

Interregional neuronal communication is pivotal to instructing and adjusting cortical circuit assembly. Subcortical neuromodulatory systems project long-range axons to the cortex and affect cortical processing. However, their roles and signaling mechanisms in cortical wiring remain poorly understood. Here, we explored whether and how the cholinergic system regulates inhibitory axonal ramification of neocortical chandelier cells (ChCs), which control spike generation by innervating axon initial segments of pyramidal neurons. We found that acetylcholine (ACh) signaling through nicotinic ACh receptors (nAChRs) and downstream T-type voltage-dependent calcium (Ca²⁺) channels cell-autonomously controls axonal arborization in developing ChCs through regulating filopodia initiation. This signaling axis shapes the basal Ca²⁺ level range in varicosities where filopodia originate. Furthermore, the normal development of ChC axonal arbors requires proper levels of activity in subcortical cholinergic neurons. Thus, the cholinergic system regulates inhibitory network arborization in the developing neocortex and may tune cortical circuit properties depending on early-life experiences.

Live imaging of retinotectal mapping reveals topographic map dynamics and a previously undescribed role for Contactin 2 in map sharpening

Organization of neuronal connections into topographic maps is essential for processing information. Yet, our understanding of topographic mapping has remained limited by our inability to observe maps forming and refining directly in vivo. Here, we used Cre-mediated recombination of a new colorswitch reporter in zebrafish to generate the first transgenic model allowing the dynamic analysis of retinotectal mapping in vivo. We found that the antero-posterior retinotopic map forms early but remains dynamic, with nasal and temporal retinal axons expanding their projection domains over time. Nasal projections initially arborize in the anterior tectum but progressively refine their projection domain to the posterior tectum, leading to the sharpening of the retinotopic map along the antero-posterior axis. Finally, using a CRISPR-mediated mutagenesis approach, we demonstrate that the refinement of nasal retinal projections requires the adhesion molecule Contactin 2. Altogether, our study provides the first analysis of a topographic map maturing in real time in a live animal and opens new strategies for dissecting the molecular mechanisms underlying precise topographic mapping in vertebrates.

Cytoskeleton and Membrane Organization at Axon Branches

Axon branching is a critical process ensuring a high degree of interconnectivity for neural network formation. As branching occurs at sites distant from the soma, it is necessary that axons have a local system to dynamically control and regulate axonal growth. This machinery depends on the orchestration of cellular functions such as cytoskeleton, subcellular transport, energy production, protein- and membrane synthesis that are adapted for branch formation. Compared to the axon shaft, branching sites show a distinct and dynamic arrangement of cytoskeleton components, endoplasmic reticulum and mitochondria. This review discusses the regulation of axon branching in the context of cytoskeleton and membrane remodeling.

Imaging Structural and Functional Dynamics in Xenopus Neurons

In vivo time-lapse imaging has been a fruitful approach to identify structural and functional changes in the Xenopus nervous system in tadpoles and adult frogs. Structural imaging studies have identified fundamental aspects of brain connectivity, development, plasticity, and disease and have been instrumental in elucidating mechanisms regulating these events in vivo. Similarly, assessment of nervous system function using dynamic changes in calcium signals as a proxy for neuronal activity has demonstrated principles of neuron and circuit function and principles of information organization and transfer within the brain of living animals. Because of its many advantages as an experimental system, use of Xenopus has often been at the forefront of developing these imaging methods for in vivo applications. Protocols for in vivo structural and functional imaging-including cellular labeling strategies, image collection, and image analysis-will expand the use of Xenopus to understand brain development, function, and plasticity.

The RNA-binding protein Musashi controls axon compartment-specific synaptic connectivity through ptp69D mRNA poly(A)-tailing

Synaptic targeting with subcellular specificity is essential for neural circuit assembly. Developing neurons use mechanisms to curb promiscuous synaptic connections and to direct synapse formation to defined subcellular compartments. How this selectivity is achieved molecularly remains enigmatic. Here, we discover a link between mRNA poly(A)-tailing and axon collateral branch-specific synaptic connectivity within the CNS. We reveal that the RNA-binding protein Musashi binds to the mRNA encoding the receptor protein tyrosine phosphatase Ptp69D, thereby increasing poly(A) tail length and Ptp69D protein levels. This regulation specifically promotes synaptic connectivity in one axon collateral characterized by a high degree of arborization and strong synaptogenic potential. In a different compartment of the same axon, Musashi prevents ectopic synaptogenesis, revealing antagonistic, compartment-specific functions. Moreover, Musashi-dependent Ptp69D regulation controls synaptic connectivity in the olfactory circuit. Thus, Musashi differentially shapes synaptic connectivity at the level of individual subcellular compartments and within different developmental and neuron type-specific contexts.

Application of Recombinant Rabies Virus to Xenopus Tadpole Brain

The Xenopus laevis experimental system has provided significant insight into the development and plasticity of neural circuits. Xenopus neuroscience research would be enhanced by additional tools to study neural circuit structure and function. Rabies viruses are powerful tools to label and manipulate neural circuits and have been widely used to study mesoscale connectomics. Whether rabies virus can be used to transduce neurons and express transgenes in Xenopus has not been systematically investigated. Glycoprotein-deleted rabies virus transduces neurons at the axon terminal and retrogradely labels their cell bodies. We show that glycoprotein-deleted rabies virus infects local and projection neurons in the Xenopus tadpole when directly injected into brain tissue. Pseudotyping glycoprotein-deleted rabies with EnvA restricts infection to cells with exogenous expression of the EnvA receptor, TVA. EnvA pseudotyped virus specifically infects tadpole neurons with promoter-driven expression of TVA, demonstrating its utility to label targeted neuronal populations. Neuronal cell types are defined by a combination of features including anatomical location, expression of genetic markers, axon projection sites, morphology, and physiological properties. We show that driving TVA expression in one hemisphere and injecting EnvA pseudotyped virus into the contralateral hemisphere, retrogradely labels neurons defined by cell body location and axon projection site. Using this approach, rabies can be used to identify cell types in Xenopus brain and simultaneously to express transgenes which enable monitoring or manipulation of neuronal activity. This makes rabies a valuable tool to study the structure and function of neural circuits in Xenopus.Significance StatementStudies in Xenopus have contributed a great deal to our understanding of brain circuit development and plasticity, regeneration, and hormonal regulation of behavior and metamorphosis. Here, we show that recombinant rabies virus transduces neurons in the Xenopus tadpole, enlarging the toolbox that can be applied to studying Xenopus brain. Rabies can be used for retrograde labeling and expression of a broad range of transgenes including fluorescent proteins for anatomical tracing and studying neuronal morphology, voltage or calcium indicators to visualize neuronal activity, and photo- or chemosensitive channels to control neuronal activity. The versatility of these tools enables diverse experiments to analyze and manipulate Xenopus brain structure and function, including mesoscale connectivity.

Precisely controlled visual stimulation to study experience-dependent neural plasticity in Xenopus tadpoles

Studies on visual experience-dependent plasticity can benefit tremendously from experimental protocols in which sensory stimulation is precisely controlled for extended periods over which neuronal, circuit, and behavioral plasticity occurs. Small vertebrates, such as Xenopus tadpoles and zebrafish, are excellent systems for studying brain plasticity. Here, we present a detailed protocol to perform controlled visual stimulation for extended time periods. These methods have been used to study structural plasticity induced by temporally controlled visual stimulation in Xenopus tadpoles.

For further details on the use and execution of this protocol, please refer to Hiramoto and Cline (2014, 2020).

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A detailed protocol to immobilize live tadpoles over 10–12 h

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Strategies to evoke reproducible visual responses in the retinal ganglion cells

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Design of devices for visual stimulation

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Minimally invasive, efficient electroporation of the retinal ganglion cells

Microglial trogocytosis and the complement system regulate axonal pruning in vivo

Partial phagocytosis-called trogocytosis-of axons by microglia has been documented in ex vivo preparations but has not been directly observed in vivo. The mechanisms that modulate microglial trogocytosis of axons and its function in neural circuit development remain poorly understood. Here, we directly observe axon trogocytosis by microglia in vivo in the developing Xenopus laevis retinotectal circuit. We show that microglia regulate pruning of retinal ganglion cell axons and are important for proper behavioral response to dark and bright looming stimuli. Using bioinformatics, we identify amphibian regulator of complement activation 3, a homolog of human CD46, as a neuronally expressed synapse-associated complement inhibitory molecule that inhibits trogocytosis and axonal pruning. Using a membrane-bound complement C3 fusion protein, we demonstrate that enhancing complement activity enhances axonal pruning. Our results support the model that microglia remodel axons via trogocytosis and that neurons can control this process through expression of complement inhibitory proteins.

Single-Cell Visualization Deep in Brain Structures by Gene Transfer

A projection neuron targets multiple regions beyond the functional brain area. In order to map neuronal connectivity in a massive neural network, a means for visualizing the entire morphology of a single neuron is needed. Progress has facilitated single-neuron analysis in the cerebral cortex, but individual neurons in deep brain structures remain difficult to visualize. To this end, we developed an in vivo single-cell electroporation method for juvenile and adult brains that can be performed under a standard stereomicroscope. This technique involves rapid gene transfection and allows the visualization of dendritic and axonal morphologies of individual neurons located deep in brain structures. The transfection efficiency was enhanced by directly injecting the expression vector encoding green fluorescent protein instead of monitoring cell attachment to the electrode tip. We obtained similar transfection efficiencies in both young adult (≥P40) and juvenile mice (P21-30). By tracing the axons of thalamocortical neurons, we identified a specific subtype of neuron distinguished by its projection pattern. Additionally, transfected mOrange-tagged vesicle-associated membrane protein 2-a presynaptic protein-was strongly localized in terminal boutons of thalamocortical neurons. Thus, our in vivo single-cell gene transfer system offers rapid single-neuron analysis deep in brain. Our approach combines observation of neuronal morphology with functional analysis of genes of interest, which can be useful for monitoring changes in neuronal activity corresponding to specific behaviors in living animals.

The GTPase Arl8B Plays a Principle Role in the Positioning of Interstitial Axon Branches by Spatially Controlling Autophagosome and Lysosome Location

Interstitial axon branching is an essential step during the establishment of neuronal connectivity. However, the exact mechanisms on how the number and position of branches are determined are still not fully understood. Here, we investigated the role of Arl8B, an adaptor molecule between lysosomes and kinesins. In chick retinal ganglion cells (RGCs), downregulation of Arl8B reduces axon branch density and shifts their location more proximally, while Arl8B overexpression leads to increased density and more distal positions of branches. These alterations correlate with changes in the location and density of lysosomes and autophagosomes along the axon shaft. Diminishing autophagy directly by knock-down of atg7, a key autophagy gene, reduces branch density, while induction of autophagy by rapamycin increases axon branching, indicating that autophagy plays a prominent role in axon branch formation. In vivo, local inactivation of autophagy in the retina using a mouse conditional knock-out approach disturbs retino-collicular map formation which is dependent on the formation of interstitial axon branches. These data suggest that Arl8B plays a principal role in the positioning of axon branches by spatially controlling autophagy, thus directly controlling formation of neural connectivity in the brain.SIGNIFICANCE STATEMENT The formation of interstitial axonal branches plays a prominent role in numerous places of the developing brain during neural circuit establishment. We show here that the GTPase Arl8B controls density and location of interstitial axon branches, and at the same time controls also density and location of the autophagy machinery. Upregulation or downregulation of autophagy in vitro promotes or inhibits axon branching. Local disruption of autophagy in vivo disturbs retino-collicular mapping. Our data suggest that Arl8B controls axon branching by controlling locally autophagy. This work is one of the first reports showing a role of autophagy during early neural circuit development and suggests that autophagy in general plays a much more prominent role during brain development than previously anticipated.

Local Translation in Growth Cones and Presynapses, Two Axonal Compartments for Local Neuronal Functions

During neural development, growth cones, very motile compartments of tips of axons, lead axonal extension to the correct targets. Subsequently, presynapses, another axonal compartment with vigorous trafficking of synaptic vesicles, emerge to form functional synapses with postsynapses. In response to extracellular stimuli, the immediate supply of proteins by local translation within these two axonal compartments far from cell bodies confers high motility of growth cones and active vesicle trafficking in presynapses. Although local translation in growth cones and presynapses occurs at a very low level compared with cell bodies and even dendrites, recent progress in omics and visualization techniques with subcellular fractionation of these compartments has revealed the actual situation of local translation within these two axonal compartments. Here, the increasing evidence for local protein synthesis in growth cones and presynapses for axonal and synaptic functions has been reviewed. Furthermore, the mechanisms regulating local translation in these two compartments and pathophysiological conditions caused by dysregulated local translation are highlighted.

Maturation of newborn neurons predicts social memory persistence in mice

Memory transience is essential to gain cognitive flexibility. Recently, hippocampal neurogenesis is emerging as one of the mechanisms involved in the balance between persistence and forgetting. Social recognition memory (SRM) has its duration prolonged by neurogenesis. However, it is still to be determined whether boosting neurogenesis in distinct phases of SRM may favor forgetting over persistence. In the present study, we used enriched environment (EE) and memantine (MEM) to increase neurogenesis. SRM was ubiquitously prolonged by both, while EE after the memory acquisition did not favor forgetting. Interestingly, the proportion of newborn neurons with mature morphology in the dorsal hippocampus was higher in animals where persistence prevailed. Finally, one of the main factors for dendritic growth is the formation of cytoskeleton. We found that Latrunculin A, an inhibitor of actin polymerization, blunted the promnesic effect of EE. Altogether, our results indicate that the mechanisms triggered by EE to improve SRM are not limited to increasing the number of newborn neurons.

Morpholino Studies in Xenopus Brain Development

Antisense morpholino oligonucleotides (MOs) have become a valuable method to knockdown protein levels, to block with mRNA splicing and to interfere with miRNA function. MOs are widely used to alter gene expression in development of Xenopus and Zebrafish, where they are typically injected into the fertilized egg or blastomeres. Here we present methods to use electroporation to target delivery of MOs to the central nervous system of Xenopus laevis or Xenopus tropicalis tadpoles. Briefly, MO electroporation is accomplished by injecting MO solution into the brain ventricle and driving the MOs into cells of the brain with current passing between 2 platinum plate electrodes, positioned on either side of the target brain area. The method is relatively straightforward and uses standard equipment found in many neuroscience labs. A major advantage of electroporation is that it allows spatial and temporal control of MO delivery and therefore knockdown. Co-electroporation of MOs with cell type-specific fluorescent protein expression plasmids allows morphological analysis of cellular phenotypes. Furthermore, co-electroporation of MOs with rescuing plasmids allows assessment of specificity of the knockdown and phenotypic outcome. By combining MO-mediated manipulations with sophisticated assays of neuronal function, such as electrophysiological recording, behavioral assays, or in vivo time-lapse imaging of neuronal development, the functions of specific proteins and miRNAs within the developing nervous system can be elucidated. These methods can be adapted to apply antisense morpholinos to study protein and RNA function in a variety of complex tissues.

Post-crossing segment of dI1 commissural axons forms collateral branches to motor neurons in the developing spinal cord

The dI1 commissural axons in the developing spinal cord, upon crossing the midline through the floor plate, make a sharp turn to grow rostrally. These post-crossing axons initially just extend adjacent to the floor plate without entering nearby motor columns. However, it remains poorly characterized how these post-crossing dI1 axons behave subsequently to this process. In the present study, to address this issue, we examined in detail the behavior of post-crossing dI1 axons in mice, using the Atoh1 enhancer-based conditional expression system that enables selective and sparse labeling of individual dI1 axons, together with Hb9 and ChAT immunohistochemistry for precise identification of spinal motor neurons (MNs). We found unexpectedly that the post-crossing segment of dI1 axons later gave off collateral branches that extended laterally to invade motor columns. Interestingly, these collateral branches emerged at around the time when their primary growth cones initiated invasion into motor columns. In addition, although the length of the laterally growing collateral branches increased with age, the majority of them remained within motor columns. Strikingly, these collateral branches further gave rise to multiple secondary branches in the region of MNs that innervate muscles close to the body axis. Moreover, these axonal branches formed presynaptic terminals on MNs. These observations demonstrate that dI1 commissural neurons develop axonal projection to spinal MNs via collateral branches arising later from the post-crossing segment of these axons. Our findings thus reveal a previously unrecognized projection of dI1 commissural axons that may contribute directly to generating proper motor output.

Mitochondria at the neuronal presynapse in health and disease

Synapses enable neurons to communicate with each other and are therefore a prerequisite for normal brain function. Presynaptically, this communication requires energy and generates large fluctuations in calcium concentrations. Mitochondria are optimized for supplying energy and buffering calcium, and they are actively recruited to presynapses. However, not all presynapses contain mitochondria; thus, how might synapses with and without mitochondria differ? Mitochondria are also increasingly recognized to serve additional functions at the presynapse. Here, we discuss the importance of presynaptic mitochondria in maintaining neuronal homeostasis and how dysfunctional presynaptic mitochondria might contribute to the development of disease.

Branch-restricted localization of phosphatase Prl-1 specifies axonal synaptogenesis domains

Central nervous system (CNS) circuit development requires subcellular control of synapse formation and patterning of synapse abundance. We identified the Drosophila membrane-anchored phosphatase of regenerating liver (Prl-1) as an axon-intrinsic factor that promotes synapse formation in a spatially restricted fashion. The loss of Prl-1 in mechanosensory neurons reduced the number of CNS presynapses localized on a single axon collateral and organized as a terminal arbor. Flies lacking all Prl-1 protein had locomotor defects. The overexpression of Prl-1 induced ectopic synapses. In mechanosensory neurons, Prl-1 modulates the insulin receptor (InR) signaling pathway within a single contralateral axon compartment, thereby affecting the number of synapses. The axon branch–specific localization and function of Prl-1 depend on untranslated regions of the prl-1 messenger RNA (mRNA). Therefore, compartmentalized restriction of Prl-1 serves as a specificity factor for the subcellular control of axonal synaptogenesis.

Enhanced visual experience rehabilitates the injured brain in Xenopus tadpoles in an NMDAR-dependent manner

Traumatic brain injuries introduce functional and structural circuit deficits that must be repaired for an organism to regain function. We developed an injury model in which Xenopus laevis tadpoles are given a penetrating stab wound that damages the optic tectal circuit and impairs visuomotor behavior. In tadpoles, as in other systems, injury induces neurogenesis. The newly generated neurons are thought to integrate into the existing circuit; however, whether they integrate via the same mechanisms that govern normal neuronal maturation during development is not understood. Development of the functional visuomotor circuit in Xenopus is driven by sensory activity. We hypothesized that enhanced visual experience would improve recovery from injury by facilitating integration of newly generated neurons into the tectal circuit. We labeled newly generated neurons in the injured tectum by green fluorescent protein expression and examined their circuit integration using electrophysiology and in vivo imaging. Providing animals with brief bouts of enhanced visual experience starting 24 h after injury increased synaptogenesis and circuit integration of new neurons and facilitated behavioral recovery. To investigate mechanisms of neuronal integration and behavioral recovery after injury, we interfered with N-methyl-d-aspartate (NMDA) receptor function. Ifenprodil, which blocks GluN2B-containing NMDA receptors, impaired dendritic arbor elaboration. GluN2B blockade inhibited functional integration of neurons generated in response to injury and prevented behavioral recovery. Furthermore, tectal GluN2B knockdown blocked the beneficial effects of enhanced visual experience on functional plasticity and behavioral recovery. We conclude that visual experience-mediated rehabilitation of the injured tectal circuit occurs by GluN2B-containing NMDA receptor-dependent integration of newly generated neurons. NEW & NOTEWORTHY Recovery from brain injury is difficult in most systems. The study of regenerative animal models that are capable of injury repair can provide insight into cellular and circuit mechanisms underlying repair. Using Xenopus tadpoles, we show enhanced sensory experience rehabilitates the injured visual circuit and that this experience-dependent recovery depends on N-methyl-d-aspartate receptor function. Understanding the mechanisms of rehabilitation in this system may facilitate recovery in brain regions and systems where repair is currently impossible.

MFF-dependent mitochondrial fission regulates presynaptic release and axon branching by limiting axonal mitochondria size

Neurons display extreme degrees of polarization, including compartment-specific organelle morphology. In cortical, long-range projecting, pyramidal neurons (PNs), dendritic mitochondria are long and tubular whereas axonal mitochondria display uniformly short length. Here we explored the functional significance of maintaining small mitochondria for axonal development in vitro and in vivo. We report that the Drp1 'receptor' Mitochondrial fission factor (MFF) is required for determining the size of mitochondria entering the axon and then for maintenance of their size along the distal portions of the axon without affecting their trafficking properties, presynaptic capture, membrane potential or ability to generate ATP. Strikingly, this increase in presynaptic mitochondrial size upon MFF downregulation augments their capacity for Ca²⁺ ([Ca²⁺]_m) uptake during neurotransmission, leading to reduced presynaptic [Ca²⁺]_c accumulation, decreased presynaptic release and terminal axon branching. Our results uncover a novel mechanism controlling neurotransmitter release and axon branching through fission-dependent regulation of presynaptic mitochondrial size.

Presenilin Regulates Retinotectal Synapse Formation through EphB2 Receptor Processing

As the catalytic component of γ-secretase, presenilin (PS) has long been studied in the context of Alzheimer's disease through cleaving the amyloid precursor protein. PS/γ-secretase, however, also cleaves a multitude of single-pass transmembrane proteins that are important during development, including Notch, the netrin receptor DCC, cadherins, drebrin-A, and the EphB2 receptor. Because transgenic PS-KO mice do not survive to birth, studies of this molecule during later embryonic or early postnatal stages of development have been carried out using cell cultures or conditional knock-out mice, respectively. As a result, the function of PS in synapse formation had not been well-addressed. Here, we study the role of PS in the developing Xenopus tadpole retinotectal circuit, an in-vivo model that allows for protein expression to be manipulated specifically during the peak of synapse formation between retinal ganglion cells and tectal neurons. We found that inhibiting PS in the postsynaptic tectal neurons impaired tadpole visual avoidance behavior. Whole cell recordings indicated weaker retinotectal synaptic transmission which was characterized by significant reductions in both NMDA receptor (NMDAR)- and AMPA receptor (AMPAR)-mediated currents. We also found that expression of the C-tail fragment of the EphB2 receptor, which is normally cleaved by PS/γ-secretase and which has been shown to upregulate NMDARs at the synapse, rescued the reduced NMDAR-mediated responses. Our data determine that normal PS function is important for proper formation and strengthening of retinotectal synapses through cleaving the EphB2 receptor.

A transgenic zebrafish model for in vivo long-term imaging of retinotectal synaptogenesis

The retinotectal synapse in larval zebrafish, combined with live time-lapse imaging, provides an advantageous model for study of the development and remodelling of central synapses in vivo. In previous studies, these synapses were labelled by transient expression of fluorescence-tagged synaptic proteins, which resulted in the dramatic variation of labelling patterns in each larva. Here, using GAL4-Upstream Activating Sequence (GAL4-UAS) methodology, we generated stable transgenic lines, which express EGFP-tagged synaptophysin (a presynaptic protein) in retinal ganglion cells (RGCs), to reliably label the pre-synaptic site of retinotectal synapses. This tool avoids the variable labelling of RGCs that occurs in transient transgenic larvae. We obtained several stable transgenic lines that differ consistently in the number of labelled RGCs. Using stable lines that consistently had a single labelled RGC, we could trace synaptogenic dynamics on an individual RGC axonal arbor across different developmental stages. In the stable lines that consistently had multiple labelled RGCs, we could simultaneously monitor both pre- and post-synaptic compartments by combining transient labelling of post-synaptic sites on individual tectal neurons. These tools allowed us to investigate molecular events underlying synaptogenesis and found that the microRNA-132 (miR-132) is required for developmental synaptogenesis. Thus, these transgenic zebrafish stable lines provide appropriate tools for studying central synaptogenesis and underlying molecular mechanisms in intact vertebrate brain.

The EGF-LIKE domain of thrombospondin-4 is a key determinant in the development of pain states due to increased excitatory synaptogenesis

Up-regulation of thrombospondin-4 (TSP4) or voltage-gated calcium channel subunit α₂δ₁ (Ca_vα₂δ₁) proteins in the spinal cord contributes to neuropathic pain development through an unidentified mechanism. We have previously shown that TSP4 interacts with Ca_vα₂δ₁ to promote excitatory synaptogenesis and the development of chronic pain states. However, the TSP4 determinants responsible for these changes are not known. Here, we tested the hypothesis that the Ca_vα₂δ₁-binding domains of TSP4 are synaptogenic and pronociceptive. We mapped the major Ca_vα₂δ₁-binding domains of TSP4 within the coiled-coil and epidermal growth factor (EGF)-like domains in vitro Intrathecal injection of TSP4 fragment proteins containing the EGF-like domain (EGF-LIKE) into naïve rodents was sufficient for inducing behavioral hypersensitivity similar to that produced by an equal molar dose of full-length TSP4. Gabapentin, a drug that binds to Ca_vα₂δ₁, blocked EGF-LIKE-induced behavioral hypersensitivity in a dose-dependent manner, supporting the notion that EGF-LIKE interacts with Ca_vα₂δ₁ and thereby mediates behavioral hypersensitivity. This notion was further supported by our findings that a peptide within EGF-LIKE (EGFD355-369) could block TSP4- or Ca_vα₂δ₁-induced behavioral hypersensitivity after intrathecal injections. Furthermore, only TSP4 proteins that contained EGF-LIKE could promote excitatory synaptogenesis between sensory and spinal cord neurons, which could be blocked by peptide EGFD355-369. Together, these findings indicate that EGF-LIKE is the molecular determinant that mediates aberrant excitatory synaptogenesis and chronic pain development. Blocking interactions between EGF-LIKE and Ca_vα₂δ₁ could be an alternative approach in designing target-specific pain medications.

The interplay between electrical and chemical synaptogenesis

Neurons communicate with each other via electrical or chemical synaptic connections. The pattern and strength of connections between neurons are critical for generating appropriate output. What mechanisms govern the formation of electrical and/or chemical synapses between two neurons? Recent studies indicate that common molecular players could regulate the formation of both of these classes of synapses. In addition, electrical and chemical synapses can mutually coregulate each other’s formation. Electrical activity, generated spontaneously by the nervous system or initiated from sensory experience, plays an important role in this process, leading to the selection of appropriate connections and the elimination of inappropriate ones. In this review, we discuss recent studies that shed light on the formation and developmental interactions of chemical and electrical synapses.

The Microtubule Regulator NEK7 Coordinates the Wiring of Cortical Parvalbumin Interneurons

Functional networks in the mammalian cerebral cortex rely on the interaction between glutamatergic pyramidal cells and GABAergic interneurons. Both neuronal populations exhibit an extraordinary divergence in morphology and targeting areas, which ultimately dictate their precise function in cortical circuits. How these prominent morphological differences arise during development is not well understood. Here, we conducted a high-throughput screen for genes differentially expressed by pyramidal cells and interneurons during cortical wiring. We found that NEK7, a kinase involved in microtubule polymerization, is mostly expressed in parvalbumin (PV+) interneurons at the time when they establish their connectivity. Functional experiments revealed that NEK7-deficient PV+ interneurons show altered microtubule dynamics, axon growth cone steering and reduced axon length, arbor complexity, and total number of synaptic contacts formed with pyramidal cells. Altogether, our results reveal a molecular mechanism by which the microtubule-associated kinase NEK7 regulates the wiring of PV+ interneurons.

Synapse maintenance and restoration in the retina by NGL2

Synaptic cell adhesion molecules (CAMs) promote synapse formation in the developing nervous system. To what extent they maintain and can restore connections in the mature nervous system is unknown. Furthermore, how synaptic CAMs affect the growth of synapse-bearing neurites is unclear. Here, we use adeno-associated viruses (AAVs) to delete, re-, and overexpress the synaptic CAM NGL2 in individual retinal horizontal cells. When we removed NGL2 from horizontal cells, their axons overgrew and formed fewer synapses, irrespective of whether Ngl2 was deleted during development or in mature circuits. When we re-expressed NGL2 in knockout mice, horizontal cell axon territories and synapse numbers were restored, even if AAVs were injected after phenotypes had developed. Finally, overexpression of NGL2 in wild-type horizontal cells elevated synapse numbers above normal levels. Thus, NGL2 promotes the formation, maintenance, and restoration of synapses in the developing and mature retina, and restricts axon growth throughout life.

Neurexin and Neuroligin-based adhesion complexes drive axonal arborisation growth independent of synaptic activity

Building arborisations of the right size and shape is fundamental for neural network function. Live imaging in vertebrate brains strongly suggests that nascent synapses are critical for branch growth during development. The molecular mechanisms underlying this are largely unknown. Here we present a novel system in Drosophila for studying the development of complex arborisations live, in vivo during metamorphosis. In growing arborisations we see branch dynamics and localisations of presynaptic proteins very similar to the ‘synaptotropic growth’ described in fish/frogs. These accumulations of presynaptic proteins do not appear to be presynaptic release sites and are not paired with neurotransmitter receptors. Knockdowns of either evoked or spontaneous neurotransmission do not impact arbor growth. Instead, we find that axonal branch growth is regulated by dynamic, focal localisations of Neurexin and Neuroligin. These adhesion complexes provide stability for filopodia by a ‘stick-and-grow’ based mechanism wholly independent of synaptic activity.

The dorsal striatum and the dynamics of the consensus connectomes in the frontal lobe of the human brain

In the applications of the graph theory, it is unusual that one considers numerous, pairwise different graphs on the very same set of vertices. In the case of human braingraphs or connectomes, however, this is the standard situation: the nodes correspond to anatomically identified cerebral regions, and two vertices are connected by an edge if a diffusion MRI-based workflow identifies a fiber of axons, running between the two regions, corresponding to the two vertices. Therefore, if we examine the braingraphs of n subjects, then we have n graphs on the very same, anatomically identified vertex set. It is a natural idea to describe the k-frequently appearing edges in these graphs: the edges that are present between the same two vertices in at least k out of the n graphs. Based on the NIH-funded large Human Connectome Project's public data release, we have reported the construction of the Budapest Reference Connectome Server http://www.connectome.pitgroup.org that generates and visualizes these k-frequently appearing edges. We call the graphs of the k-frequently appearing edges "k-consensus connectomes" since an edge could be included only if it is present in at least k graphs out of n. Considering the whole human brain, we have reported a surprising property of these consensus connectomes earlier. In the present work we are focusing on the frontal lobe of the brain, and we report here a similarly surprising dynamical property of the consensus connectomes when k is gradually changed from k = n to k = 1: the connections between the nodes of the frontal lobe are seemingly emanating from those nodes that were connected to sub-cortical structures of the dorsal striatum: the caudate nucleus, and the putamen. We hypothesize that this dynamic behavior copies the axonal fiber development of the frontal lobe. An animation of the phenomenon is presented at https://youtu.be/wBciB2eW6_8.

Direct intertectal inputs are an integral component of the bilateral sensorimotor circuit for behavior in Xenopus tadpoles

The circuit controlling visually guided behavior in nonmammalian vertebrates, such as Xenopus tadpoles, includes retinal projections to the contralateral optic tectum, where visual information is processed, and tectal motor outputs projecting ipsilaterally to hindbrain and spinal cord. Tadpoles have an intertectal commissure whose function is unknown, but it might transfer information between the tectal lobes. Differences in visual experience between the two eyes have profound effects on the development and function of visual circuits in animals with binocular vision, but the effects on animals with fully crossed retinal projections are not clear. We tested the effect of monocular visual experience on the visuomotor circuit in Xenopus tadpoles. We show that cutting the intertectal commissure or providing visual experience to one eye (monocular visual experience) is sufficient to disrupt tectally mediated visual avoidance behavior. Monocular visual experience induces asymmetry in tectal circuit activity across the midline. Repeated exposure to monocular visual experience drives maturation of the stimulated retinotectal synapses, seen as increased AMPA-to-NMDA ratios, induces synaptic plasticity in intertectal synaptic connections, and induces bilaterally asymmetric changes in the tectal excitation-to-inhibition ratio (E/I). We show that unilateral expression of peptides that interfere with AMPA or GABA_A receptor trafficking alters E/I in the transfected tectum and is sufficient to degrade visuomotor behavior. Our study demonstrates that monocular visual experience in animals with fully crossed visual systems produces asymmetric circuit function across the midline and degrades visuomotor behavior. The data further suggest that intertectal inputs are an integral component of a bilateral visuomotor circuit critical for behavior. NEW & NOTEWORTHY The developing optic tectum of Xenopus tadpoles represents a unique circuit in which laterally positioned eyes provide sensory input to a circuit that is transiently monocular, but which will be binocular in the animal's adulthood. We challenge the idea that the two lobes of tadpole optic tectum function independently by testing the requirement of interhemispheric communication and demonstrate that unbalanced sensory input can induce structural and functional plasticity in the tectum sufficient to disrupt function.

RNA Docking and Local Translation Regulate Site-Specific Axon Remodeling In Vivo

Nascent proteins can be positioned rapidly at precise subcellular locations by local protein synthesis (LPS) to facilitate localized growth responses. Axon arbor architecture, a major determinant of synaptic connectivity, is shaped by localized growth responses, but it is unknown whether LPS influences these responses in vivo. Using high-resolution live imaging, we examined the spatiotemporal dynamics of RNA and LPS in retinal axons during arborization in vivo. Endogenous RNA tracking reveals that RNA granules dock at sites of branch emergence and invade stabilized branches. Live translation reporter analysis reveals that de novo β-actin hotspots colocalize with docked RNA granules at the bases and tips of new branches. Inhibition of axonal β-actin mRNA translation disrupts arbor dynamics primarily by reducing new branch emergence and leads to impoverished terminal arbors. The results demonstrate a requirement for LPS in building arbor complexity and suggest a key role for pre-synaptic LPS in assembling neural circuits.

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Tracking endogenous RNA shows that RNA docking predicts axon branch emergence in vivo

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Axon arbor complexity in vivo depends on local protein synthesis

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Axonal β-actin synthesis regulates branching by increased branch initiation

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Live imaging reveals de novo synthesis of β-actin hotspots during branch formation

The Gliotransmitter d-Serine Promotes Synapse Maturation and Axonal Stabilization In Vivo

The NMDAR is thought to play a key role in the refinement of connectivity in developing neural circuits. Pharmacological blockade or genetic loss-of-function manipulations that prevent NMDAR function during development result in the disorganization of topographic axonal projections. However, because NMDARs contribute to overall glutamatergic neurotransmission, such loss-of-function experiments fail to adequately distinguish between the roles played by NMDARs and neural activity in general. The gliotransmitter d-serine is a coagonist of the NMDAR that is required for NMDAR channel opening, but which cannot mediate neurotransmission on its own. Here we demonstrate that acute administration of d-serine has no immediate effect on glutamate release or AMPA-mediated neurotransmission. We show that endogenous d-serine is normally present below saturating levels in the developing visual system of the Xenopus tadpole. Using an amperometric enzymatic biosensor, we demonstrate that glutamatergic activation elevates ambient endogenous d-serine levels in the optic tectum. Chronically elevating levels of d-serine promoted synaptic maturation and resulted in the hyperstabilization of developing axon branches in the tadpole visual system. Conversely, treatment with an enzyme that degrades endogenous d-serine resulted in impaired synaptic maturation. Despite the reduction in axon arbor complexity seen in d-serine-treated animals, tectal neuron visual receptive fields were expanded, suggesting a failure to prune divergent retinal inputs. Together, these findings positively implicate NMDAR-mediated neurotransmission in developmental synapse maturation and the stabilization of axonal inputs and reveal a potential role for d-serine as an endogenous modulator of circuit refinement.SIGNIFICANCE STATEMENT Activation of NMDARs is critical for the activity-dependent development and maintenance of highly organized topographic maps. d-Serine, a coagonist of the NMDAR, plays a significant role in modulating NMDAR-mediated synaptic transmission and plasticity in many brain areas. However, it remains unknown whether d-serine participates in the establishment of precise neuronal connections during development. Using an in vivo model, we show that glutamate receptor activation can evoke endogenous d-serine release, which promotes glutamatergic synapse maturation and stabilizes axonal structural and functional inputs. These results reveal a pivotal modulatory role for d-serine in neurodevelopment.

Neocortical Chandelier Cells Developmentally Shape Axonal Arbors through Reorganization but Establish Subcellular Synapse Specificity without Refinement

Diverse types of cortical interneurons (INs) mediate various kinds of inhibitory control mechanisms to balance and shape network activity. Distinct IN subtypes develop uniquely organized axonal arbors that innervate different subcellular compartments of excitatory principal neurons (PNs), which critically contribute to determining their output properties. However, it remains poorly understood how they establish this peculiar axonal organization and synaptic connectivity during development. Here, taking advantage of genetic labeling of IN progenitors, we examined developmental processes of axonal arbors and synaptic connections formed by murine chandelier cells (ChCs), which innervate axon initial segments (AISs) of PNs and thus powerfully regulate their spike generation. Our quantitative analysis by light microscopy revealed that ChCs overgrow and subsequently refine axonal branches as well as varicosities. Interestingly, we found that although a significant number of axonal varicosities are formed off AISs in addition to on AISs, presynaptic markers are predominantly colocalized with those on AISs throughout development. Immunoelectron microscopic (IEM) analysis also demonstrated that only varicosities apposed to AISs contain presynaptic profiles. These results suggest that subcellular synapse specificity of ChCs is genetically predetermined while axonal geometry is shaped through remodeling. Molecular cues localized at AISs may regulate target recognition and synapse formation by ChCs.

Visualizing Morphogenesis with the Processing Programming Language

We used Processing, a visual artists' programming language developed at MIT Media Lab, to simulate cellular mechanisms of morphogenesis - the generation of form and shape in embryonic tissues. Based on observations of in vivo time-lapse image sequences, we created animations of neural cell motility responsible for elongating the spinal cord, and of optic axon branching dynamics that establish primary visual connectivity. These visual models underscore the significance of the computational decomposition of cellular dynamics underlying morphogenesis.

Rules for Shaping Neural Connections in the Developing Brain

It is well established that spontaneous activity in the developing mammalian brain plays a fundamental role in setting up the precise connectivity found in mature sensory circuits. Experiments that produce abnormal activity or that systematically alter neural firing patterns during periods of circuit development strongly suggest that the specific patterns and the degree of correlation in firing may contribute in an instructive manner to circuit refinement. In fish and amphibians, unlike amniotic vertebrates, sensory input directly drives patterned activity during the period of initial projection outgrowth and innervation. Experiments combining sensory stimulation with live imaging, which can be performed non-invasively in these simple vertebrate models, have provided important insights into the mechanisms by which neurons read out and respond to activity patterns. This article reviews the classic and recent literature on spontaneous and evoked activity-dependent circuit refinement in sensory systems and formalizes a set of mechanistic rules for the transformation of patterned activity into accurate neuronal connectivity in the developing brain.

An NMDA receptor-dependent mechanism for subcellular segregation of sensory inputs in the tadpole optic tectum

In the vertebrate CNS, afferent sensory inputs are targeted to specific depths or layers of their target neuropil. This patterning exists ab initio, from the very beginning, and therefore has been considered an activity-independent process. However, here we report that, during circuit development, the subcellular segregation of the visual and mechanosensory inputs to specific regions of tectal neuron dendrites in the tadpole optic tectum requires NMDA receptor activity. Blocking NMDARs during the formation of these sensory circuits, or removing the visual set of inputs, leads to less defined segregation, and suggests a correlation-based mechanism in which correlated inputs wire to common regions of dendrites. This can account for how two sets of inputs form synapses onto different regions of the same dendrite. Blocking NMDA receptors during later stages of circuit development did not disrupt segregation, indicating a critical period for activity-dependent shaping of patterns of innervation.

DOI:http://dx.doi.org/10.7554/eLife.20502.001

Experience-dependent plasticity of excitatory and inhibitory intertectal inputs in Xenopus tadpoles

Communication between optic tecta/superior colliculi is thought to be required for sensorimotor behaviors by comparing inputs across the midline, however the development of and the role of visual experience in the function and plasticity of intertectal connections are unclear. We combined neuronal tracing, in vivo time-lapse imaging, and electrophysiology to characterize the structural and functional development of intertectal axons and synapses in Xenopus tadpole optic tectum. We find that intertectal connections are established early during optic tectal circuit development. We determined the neurotransmitter identity of intertectal neurons using both rabies virus-mediated tracing combined with post-hoc immunohistochemistry, and electrophysiology. Excitatory and inhibitory intertectal neuronal somata are similarly distributed throughout the tectum. Excitatory and inhibitory intertectal axons are structurally similar and elaborate broadly in the contralateral tectum. We demonstrate that intertectal and retinotectal axons converge onto tectal neurons by recording postsynaptic currents after stimulating intertectal and retinotectal inputs. Cutting the intertectal commissure removes synaptic responses to contralateral tectal stimulation. In vivo time-lapse imaging demonstrated that visual experience drives plasticity in intertectal bouton size and dynamics. Finally, visual experience coordinately drives the maturation of excitatory and inhibitory intertectal inputs by increasing AMPA- and GABA-receptor mediated currents, comparable to experience-dependent maturation of retinotectal inputs. These data indicate that visual experience regulates plasticity of excitatory and inhibitory intertectal inputs, maintaining the excitatory: inhibitory ratio of intertectal input. These studies place intertectal inputs as key players in tectal circuit development and suggest that they may play a role in sensory information processing critical to sensorimotor behaviors.

Endocannabinoid signaling enhances visual responses through modulation of intracellular chloride levels in retinal ganglion cells

Type 1 cannabinoid receptors (CB1Rs) are widely expressed in the vertebrate retina, but the role of endocannabinoids in vision is not fully understood. Here, we identified a novel mechanism underlying a CB1R-mediated increase in retinal ganglion cell (RGC) intrinsic excitability acting through AMPK-dependent inhibition of NKCC1 activity. Clomeleon imaging and patch clamp recordings revealed that inhibition of NKCC1 downstream of CB1R activation reduces intracellular Cl⁻ levels in RGCs, hyperpolarizing the resting membrane potential. We confirmed that such hyperpolarization enhances RGC action potential firing in response to subsequent depolarization, consistent with the increased intrinsic excitability of RGCs observed with CB1R activation. Using a dot avoidance assay in freely swimming Xenopus tadpoles, we demonstrate that CB1R activation markedly improves visual contrast sensitivity under low-light conditions. These results highlight a role for endocannabinoids in vision and present a novel mechanism for cannabinoid modulation of neuronal activity through Cl⁻ regulation.

DOI:http://dx.doi.org/10.7554/eLife.15932.001

Early development and function of the Xenopus tadpole retinotectal circuit

The retinotectal circuit is the major component of the amphibian visual system. It is comprised of the retinal ganglion cells (RGCs) in the eye, which project their axons to the optic tectum and form synapses onto postsynaptic tectal neurons. The retinotectal circuit is relatively simple, and develops quickly: Xenopus tadpoles begin displaying retinotectal-dependent visual avoidance behaviors by approximately 7-8 days post-fertilization, early larval stage. In this review we first provide a summary of the dynamic development of the retinotectal circuit, including the microcircuitry formed by local tectal-tectal connections within the tectum. Second, we discuss the basic visual avoidance behavior generated specifically by this circuit, and how this behavior is being used as an assay to test visual system function.

Opposing Effects of Neuronal Activity on Structural Plasticity

The connectivity of the brain is continuously adjusted to new environmental influences by several activity-dependent adaptive processes. The most investigated adaptive mechanism is activity-dependent functional or synaptic plasticity regulating the transmission efficacy of existing synapses. Another important but less prominently discussed adaptive process is structural plasticity, which changes the connectivity by the formation and deletion of synapses. In this review, we show, based on experimental evidence, that structural plasticity can be classified similar to synaptic plasticity into two categories: (i) Hebbian structural plasticity, which leads to an increase (decrease) of the number of synapses during phases of high (low) neuronal activity and (ii) homeostatic structural plasticity, which balances these changes by removing and adding synapses. Furthermore, based on experimental and theoretical insights, we argue that each type of structural plasticity fulfills a different function. While Hebbian structural changes enhance memory lifetime, storage capacity, and memory robustness, homeostatic structural plasticity self-organizes the connectivity of the neural network to assure stability. However, the link between functional synaptic and structural plasticity as well as the detailed interactions between Hebbian and homeostatic structural plasticity are more complex. This implies even richer dynamics requiring further experimental and theoretical investigations.

Brain white matter structure and COMT gene are linked to second-language learning in adults

Adult human brains retain the capacity to undergo tissue reorganization during second-language learning. Brain-imaging studies show a relationship between neuroanatomical properties and learning for adults exposed to a second language. However, the role of genetic factors in this relationship has not been investigated. The goal of the current study was twofold: (i) to characterize the relationship between brain white matter fiber-tract properties and second-language immersion using diffusion tensor imaging, and (ii) to determine whether polymorphisms in the catechol-O-methyltransferase (COMT) gene affect the relationship. We recruited incoming Chinese students enrolled in the University of Washington and scanned their brains one time. We measured the diffusion properties of the white matter fiber tracts and correlated them with the number of days each student had been in the immersion program at the time of the brain scan. We found that higher numbers of days in the English immersion program correlated with higher fractional anisotropy and lower radial diffusivity in the right superior longitudinal fasciculus. We show that fractional anisotropy declined once the subjects finished the immersion program. The relationship between brain white matter fiber-tract properties and immersion varied in subjects with different COMT genotypes. Subjects with the Methionine (Met)/Valine (Val) and Val/Val genotypes showed higher fractional anisotropy and lower radial diffusivity during immersion, which reversed immediately after immersion ended, whereas those with the Met/Met genotype did not show these relationships. Statistical modeling revealed that subjects' grades in the language immersion program were best predicted by fractional anisotropy and COMT genotype.

The PHR proteins: intracellular signaling hubs in neuronal development and axon degeneration

During development, a coordinated and integrated series of events must be accomplished in order to generate functional neural circuits. Axons must navigate toward target cells, build synaptic connections, and terminate outgrowth. The PHR proteins (consisting of mammalian Phr1/MYCBP2, Drosophila Highwire and C. elegans RPM-1) function in each of these events in development. Here, we review PHR function across species, as well as the myriad of signaling pathways PHR proteins regulate. These findings collectively suggest that the PHR proteins are intracellular signaling hubs, a concept we explore in depth. Consistent with prominent developmental functions, genetic links have begun to emerge between PHR signaling networks and neurodevelopmental disorders, such as autism, schizophrenia and intellectual disability. Finally, we discuss the recent and important finding that PHR proteins regulate axon degeneration, which has further heightened interest in this fascinating group of molecules.

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.