Late recruitment of synapsin to nascent synapses is regulated by Cdk5

A zebrafish gephyrinb mutant distinguishes synaptic and enzymatic functions of Gephyrin

Gephyrin is thought to play a critical role in clustering glycine receptors at synapses within the central nervous system (CNS). The main in vivo evidence for this comes from Gephyrin (Gphn)-null mice, where glycine receptors are depleted from synaptic regions. However, these mice die at birth, possibly due to impaired molybdenum cofactor (MoCo) synthesis, an essential role Gephyrin assumes throughout an animal. This complicates the interpretation of synaptic phenotypes in Gphn-null mice and raises the question whether the synaptic and enzymatic functions of Gephyrin can be investigated separately. Here, we generated a gephyrinb zebrafish mutant, vo84, that almost entirely lacks Gephyrin staining in the spinal cord. gephyrinbvo84 mutants exhibit normal gross morphology at both larval and adult stages. In contrast to Gphn-null mice, gephyrinbvo84 mutants exhibit normal motor activity and MoCo-dependent enzyme activity. Instead, gephyrinbvo84 mutants display impaired rheotaxis and increased mortality in late development. To investigate what may mediate these defects in gephyrinbvo84 mutants, we examined the cell density of neurons and myelin in the spinal cord and found no obvious changes. Surprisingly, in gephyrinbvo84 mutants, glycine receptors are still present in the synaptic regions. However, their abundance is reduced, potentially contributing to the observed defects. These findings challenge the notion that Gephyrin is absolutely required to cluster glycine receptors at synapses and reveals a new role of Gephyrin in regulating glycine receptor abundance and rheotaxis. They also establish a powerful new model for studying the mechanisms underlying synaptic, rather than enzymatic, functions of Gephyrin.

Synaptic input and Ca2+ activity in zebrafish oligodendrocyte precursor cells contribute to myelin sheath formation

In the nervous system, only one type of neuron-glial synapse is known to exist: that between neurons and oligodendrocyte precursor cells (OPCs), yet their composition, assembly, downstream signaling and in vivo functions remain largely unclear. Here, we address these questions using in vivo microscopy in zebrafish spinal cord and identify postsynaptic molecules PSD-95 and gephyrin in OPCs. The puncta containing these molecules in OPCs increase during early development and decrease upon OPC differentiation. These puncta are highly dynamic and frequently assemble at 'hotspots'. Gephyrin hotspots and synapse-associated Ca2+ activity in OPCs predict where a subset of myelin sheaths forms in differentiated oligodendrocytes. Further analyses reveal that spontaneous synaptic release is integral to OPC Ca2+ activity, while evoked synaptic release contributes only in early development. Finally, disruption of the synaptic genes dlg4a/dlg4b, gphnb and nlgn3b impairs OPC differentiation and myelination. Together, we propose that neuron-OPC synapses are dynamically assembled and can predetermine myelination patterns through Ca2+ signaling.

A maturational shift in the frontal cortex synaptic transcriptional landscape underlies schizophrenia-relevant behavioural traits: A congenital rat model

Disruption of brain development early in life may underlie the neurobiology behind schizophrenia. We have reported more immature synaptic spines in the frontal cortex (FC) of adult Roman High-Avoidance (RHA-I) rats, a behavioural model displaying schizophrenia-like traits. Here, we performed a whole transcriptome analysis in the FC of 4 months old male RHA-I (n=8) and its counterpart, the Roman Low-Avoidance (RLA-I) (n=8). We identified 203 significant genes with overrepresentation of genes involved in synaptic function. Next, we performed a gene set enrichment analysis (GSEA) for genes co-expressed during neurodevelopment. Gene networks were obtained by weighted gene co-expression network analysis (WGCNA) of a transcriptomic dataset containing human FC during lifespan (n=269). Out of thirty-one functional gene networks, six were significantly enriched in the RHA-I. These were differentially regulated during infancy and enriched in biological ontologies related to myelination, synaptic function, and immune response. We validated differential gene expression in a new cohort of adolescent (<=2 months old) and young-adult (>=3 months old) RHA-I and RLA-I rats. The results confirmed overexpression of Gsn, Nt5cd1, Ppp1r1b, and Slc9a3r1 in young-adult RHA-I, while Cables1, a regulator of Cdk5 phosphorylation in actin regulation and involved in synaptic plasticity and maturation, was significantly downregulated in adolescent RHA-I. This age-related expression change was also observed for presynaptic components Snap25 and Snap29. Our results show a different maturational expression profile of synaptic components in the RHA-I strain, supporting a shift in FC maturation underlying schizophrenia-like behavioural traits and adding construct validity to this strain as a neurodevelopmental model.

Neurobeachin controls the asymmetric subcellular distribution of electrical synapse proteins

The subcellular positioning of synapses and their specialized molecular compositions form the fundamental basis of neural circuits. Like chemical synapses, electrical synapses are constructed from an assortment of adhesion, scaffolding, and regulatory molecules, yet little is known about how these molecules localize to specific neuronal compartments. Here, we investigate the relationship between the autism- and epilepsy-associated gene Neurobeachin, the neuronal gap junction channel-forming Connexins, and the electrical synapse scaffold ZO1. Using the zebrafish Mauthner circuit, we find Neurobeachin localizes to the electrical synapse independently of ZO1 and Connexins. By contrast, we show Neurobeachin is required postsynaptically for the robust localization of ZO1 and Connexins. We demonstrate that Neurobeachin binds ZO1 but not Connexins. Finally, we find Neurobeachin is required to restrict electrical postsynaptic proteins to dendrites, but not electrical presynaptic proteins to axons. Together, the results reveal an expanded understanding of electrical synapse molecular complexity and the hierarchical interactions required to build neuronal gap junctions. Further, these findings provide novel insight into the mechanisms by which neurons compartmentalize the localization of electrical synapse proteins and provide a cell biological mechanism for the subcellular specificity of electrical synapse formation and function.

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

A neuronal blueprint for directional mechanosensation in larval zebrafish

Animals have a remarkable ability to use local cues to orient in space in the absence of a panoramic fixed reference frame. Here we use the mechanosensory lateral line in larval zebrafish to understand rheotaxis, an innate oriented swimming evoked by water currents. We generated a comprehensive light-microscopy cell-resolution projectome of lateralis afferent neurons (LANs) and used clustering techniques for morphological classification. We find surprising structural constancy among LANs. Laser-mediated microlesions indicate that precise topographic mapping of lateral-line receptors is not essential for rheotaxis. Recording neuronal-activity during controlled mechanical stimulation of neuromasts reveals unequal representation of water-flow direction in the hindbrain. We explored potential circuit architectures constrained by anatomical and functional data to suggest a parsimonious model under which the integration of lateralized signals transmitted by direction-selective LANs underlies the encoding of water-flow direction in the brain. These data provide a new framework to understand how animals use local mechanical cues to orient in space.

Late onset of Synaptotagmin 2a expression at synapses relevant to social behavior

As they form, synapses go through various stages of maturation and refinement. These steps are linked to significant changes in synaptic function, potentially resulting in emergence and maturation of behavioral outputs. Synaptotagmins are calcium-sensing proteins of the synaptic vesicle exocytosis machinery, and changes in Synaptotagmin proteins at synapses have significant effects on vesicle release and synaptic function. Here, we examined the distribution of the synaptic vesicle protein Synaptotagmin 2a (Syt2a) during development of the zebrafish nervous system. Syt2a is widely distributed throughout the midbrain and hindbrain early during larval development but very weakly expressed in the forebrain. Later in development, Syt2a expression levels in the forebrain increase, particularly in regions associated with social behavior, and most intriguingly, around the time social behavior becomes apparent. We provide evidence that Syt2a localizes to synapses onto neurons implicated in social behavior in the ventral forebrain and show that Syt2a is colocalized with tyrosine hydroxylase, a biosynthetic enzyme in the dopamine pathway. Our results suggest a developmentally important role for Syt2a in maturing synapses in the forebrain, coinciding with the emergence of social behavior.

Integrated Proteogenomic Characterization across Major Histological Types of Pediatric Brain Cancer

We report a comprehensive proteogenomics analysis, including whole-genome sequencing, RNA sequencing, and proteomics and phosphoproteomics profiling, of 218 tumors across 7 histological types of childhood brain cancer: low-grade glioma (n = 93), ependymoma (32), high-grade glioma (25), medulloblastoma (22), ganglioglioma (18), craniopharyngioma (16), and atypical teratoid rhabdoid tumor (12). Proteomics data identify common biological themes that span histological boundaries, suggesting that treatments used for one histological type may be applied effectively to other tumors sharing similar proteomics features. Immune landscape characterization reveals diverse tumor microenvironments across and within diagnoses. Proteomics data further reveal functional effects of somatic mutations and copy number variations (CNVs) not evident in transcriptomics data. Kinase-substrate association and co-expression network analysis identify important biological mechanisms of tumorigenesis. This is the first large-scale proteogenomics analysis across traditional histological boundaries to uncover foundational pediatric brain tumor biology and inform rational treatment selection.

Rapid Assembly of Presynaptic Materials behind the Growth Cone in Dopaminergic Neurons Is Mediated by Precise Regulation of Axonal Transport

The proper assembly of neural circuits depends on the process of synaptogenesis, or the formation of synapses between partner neurons. Using the dopaminergic PDE neurons in C. elegans, we developed an in vivo system to study the earliest steps of the formation of en passant presynaptic specializations behind an extending growth cone. We find that presynaptic materials coalesce into puncta in as little as a few minutes and that both synaptic vesicle (SV) and active zone (AZ) proteins arrive nearly simultaneously at the nascent sites of synapse formation. We show that precise regulation of UNC-104/Kinesin-3 determines the distribution of SV proteins along the axon. The localization of AZ proteins to en passant puncta, however, is largely independent of the major axonal kinesins: UNC-104/Kinesin-3 and UNC-116/ Kinesin-1. Moreover, AZ proteins play a crucial role in recruiting and tethering SV precursors (SVPs).

Lipton et al. explore the initial steps of synapse formation in vivo. They find that clustering of major presynaptic material occurs extremely rapidly (<5 min). Both synaptic vesicle precursors and active zone proteins accumulate simultaneously at developing puncta. Precise regulation of the Kinesin-3 activation state strongly influences the positioning of vesicles along the axon during development.

Regeneration of Spinal Cord Connectivity Through Stem Cell Transplantation and Biomaterial Scaffolds

Significant progress has been made in the treatment of spinal cord injury (SCI). Advances in post-trauma management and intensive rehabilitation have significantly improved the prognosis of SCI and converted what was once an “ailment not to be treated” into a survivable injury, but the cold hard fact is that we still do not have a validated method to improve the paralysis of SCI. The irreversible functional impairment of the injured spinal cord is caused by the disruption of neuronal transduction across the injury lesion, which is brought about by demyelination, axonal degeneration, and loss of synapses. Furthermore, refractory substrates generated in the injured spinal cord inhibit spontaneous recovery. The discovery of the regenerative capability of central nervous system neurons in the proper environment and the verification of neural stem cells in the spinal cord once incited hope that a cure for SCI was on the horizon. That hope was gradually replaced with mounting frustration when neuroprotective drugs, cell transplantation, and strategies to enhance remyelination, axonal regeneration, and neuronal plasticity demonstrated significant improvement in animal models of SCI but did not translate into a cure in human patients. However, recent advances in SCI research have greatly increased our understanding of the fundamental processes underlying SCI and fostered increasing optimism that these multiple treatment strategies are finally coming together to bring about a new era in which we will be able to propose encouraging therapies that will lead to appreciable improvements in SCI patients. In this review, we outline the pathophysiology of SCI that makes the spinal cord refractory to regeneration and discuss the research that has been done with cell replacement and biomaterial implantation strategies, both by itself and as a combined treatment. We will focus on the capacity of these strategies to facilitate the regeneration of neural connectivity necessary to achieve meaningful functional recovery after SCI.

4.1Ba is necessary for glutamatergic synapse formation in the sensorimotor circuit of developing zebrafish

During the process of synapse formation, thousands of proteins assemble at prospective sites of cell-cell communication. Although many of these proteins have been identified, the roles they play in generating functional connections during development remain unknown. 4.1 scaffolding proteins have been implicated in synapse formation and maturation in vitro, but in vivo studies for some family members have suggested these proteins are not important for this role. We examined the role of family member 4.1B because it has been implicated in glutamatergic synaptogenesis, but has not been described in vivo. We identified two 4.1B genes in zebrafish, 4.1Ba and 4.1Bb, by sequence comparisons and synteny analysis. In situ hybridization shows these genes are differentially expressed, with 4.1Ba expressed primarily in the nervous system and 4.1Bb expressed in the nervous system and muscle, but not the spinal cord. We focused our studies on 4.1Ba in the spinal cord. 4.1Ba knockdown reduced the number of glutamatergic synapses at caudal primary motor neurons and caused an increase in the duration of touch-evoked coiling. These results suggest 4.1Ba is important for the formation of functional glutamatergic synapses in the developing zebrafish spinal cord.

Principles Governing Locomotion in Vertebrates: Lessons From Zebrafish

Locomotor behaviors are critical for survival and enable animals to navigate their environment, find food and evade predators. The circuits in the brain and spinal cord that initiate and maintain such different modes of locomotion in vertebrates have been studied in numerous species for over a century. In recent decades, the zebrafish has emerged as one of the main model systems for the study of locomotion, owing to its experimental amenability, and work in zebrafish has revealed numerous new insights into locomotor circuit function. Here, we review the literature that has led to our current understanding of the neural circuits controlling swimming and escape in zebrafish. We highlight recent studies that have enriched our comprehension of key topics, such as the interactions between premotor excitatory interneurons (INs) and motoneurons (MNs), supraspinal and spinal circuits that coordinate escape maneuvers, and developmental changes in overall circuit composition. We also discuss roles for neuromodulators and sensory inputs in modifying the relative strengths of constituent circuit components to provide flexibility in zebrafish behavior, allowing the animal to accommodate changes in the environment. We aim to provide a coherent framework for understanding the circuitry in the brain and spinal cord of zebrafish that allows the animal to flexibly transition between different speeds, and modes, of locomotion.

Opportunities and challenges for using the zebrafish to study neuronal connectivity as an endpoint of developmental neurotoxicity

Chemical exposures have been implicated as environmental risk factors that interact with genetic susceptibilities to influence individual risk for complex neurodevelopmental disorders, including autism spectrum disorder, schizophrenia, attention deficit hyperactivity disorder and intellectual disabilities. Altered patterns of neuronal connectivity represent a convergent mechanism of pathogenesis for these and other neurodevelopmental disorders, and growing evidence suggests that chemicals can interfere with specific signaling pathways that regulate the development of neuronal connections. There is, therefore, a growing interest in developing screening platforms to identify chemicals that alter neuronal connectivity. Cell-cell, cell-matrix interactions and systemic influences are known to be important in defining neuronal connectivity in the developing brain, thus, a systems-based model offers significant advantages over cell-based models for screening chemicals for effects on neuronal connectivity. The embryonic zebrafish represents a vertebrate model amenable to higher throughput chemical screening that has proven useful in characterizing conserved mechanisms of neurodevelopment. Moreover, the zebrafish is readily amenable to gene editing to integrate genetic susceptibilities. Although use of the zebrafish model in toxicity testing has increased in recent years, the diverse tools available for imaging structural differences in the developing zebrafish brain have not been widely applied to studies of the influence of gene by environment interactions on neuronal connectivity in the developing zebrafish brain. Here, we discuss tools available for imaging of neuronal connectivity in the developing zebrafish, review what has been published in this regard, and suggest a path forward for applying this information to developmental neurotoxicity testing.

Mechanosensory neurons control the timing of spinal microcircuit selection during locomotion

Despite numerous physiological studies about reflexes in the spinal cord, the contribution of mechanosensory feedback to active locomotion and the nature of underlying spinal circuits remains elusive. Here we investigate how mechanosensory feedback shapes active locomotion in a genetic model organism exhibiting simple locomotion-the zebrafish larva. We show that mechanosensory feedback enhances the recruitment of motor pools during active locomotion. Furthermore, we demonstrate that inputs from mechanosensory neurons increase locomotor speed by prolonging fast swimming at the expense of slow swimming during stereotyped acoustic escape responses. This effect could be mediated by distinct mechanosensory neurons. In the spinal cord, we show that connections compatible with monosynaptic inputs from mechanosensory Rohon-Beard neurons onto ipsilateral V2a interneurons selectively recruited at high speed can contribute to the observed enhancement of speed. Altogether, our study reveals the basic principles and a circuit diagram enabling speed modulation by mechanosensory feedback in the vertebrate spinal cord.

Optogenetic stimulation of neuronal repair

Environmental insult, disease or trauma can affect the physical integrity of neuronal circuits, and the inability of many neurons to regenerate injured axons invariably leads to irreversible neural dysfunction. The conserved second messenger cyclic adenosine monophosphate (cAMP) can promote axonal re-growth. Widely used pharmacological or genetic approaches to increase intracellular levels of cAMP are often inadequate for precise neural-circuit reconstruction because their activity cannot be easily timed to specific target cells. These shortcomings have prevented the controlled repair of pre-defined neurons at selected time points in whole specimens. Thus, technologies to guide neuronal repair in time and space would enable studies of neural-circuit recovery with unprecedented resolution. Towards this aim, we have implemented a proof-of-principle optogenetic method to promote the selective regeneration of refractory axons in a living vertebrate.

Puzzling out presynaptic differentiation

Proper brain function in the nervous system relies on the accurate establishment of synaptic contacts during development. Countless synapses populate the adult brain in an orderly fashion. In each synapse, a presynaptic terminal loaded with neurotransmitters-containing synaptic vesicles is perfectly aligned to an array of receptors in the postsynaptic membrane. Presynaptic differentiation, which encompasses the events underlying assembly of new presynaptic units, has seen notable advances in recent years. It is now consensual that as a growing axon encounters the receptive dendrites of its partner, presynaptic assembly will be triggered and specified by multiple postsynaptically-derived factors including soluble molecules and cell adhesion complexes. Presynaptic material that reaches these distant sites by axonal transport in the form of pre-assembled packets will be retained and clustered, ultimately giving rise to a presynaptic bouton. This review focuses on the cellular and molecular aspects of presynaptic differentiation in the central nervous system, with a particular emphasis on the identity of the instructive factors and the intracellular processes used by neuronal cells to assemble functional presynaptic terminals. We provide a detailed description of the mechanisms leading to the formation of new presynaptic terminals. In brief, soma-derived packets of pre-assembled material are trafficked to distant axonal sites. Synaptogenic factors from dendritic or glial provenance activate downstream intra-axonal mediators to trigger clustering of passing material and their correct organization into a new presynaptic bouton. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Huntingtin-associated protein-1 is a synapsin I-binding protein regulating synaptic vesicle exocytosis and synapsin I trafficking

Huntingtin-associated protein-1 (HAP1) is involved in intracellular trafficking, vesicle transport, and membrane receptor endocytosis. However, despite such diverse functions, the role of HAP1 in the synaptic vesicle (SV) cycle in nerve terminals remains unclear. Here, we report that HAP1 functions in SV exocytosis, controls total SV turnover and the speed of vesicle fusion in nerve terminals and regulates glutamate release in cortical brain slices. We found that HAP1 interacts with synapsin I, an abundant neuronal phosphoprotein that associates with SVs during neurotransmitter release and regulates synaptic plasticity and neuronal development. The interaction between HAP1 with synapsin I was confirmed by reciprocal co-immunoprecipitation of the endogenous proteins. Furthermore, HAP1 co-localizes with synapsin I in cortical neurons as discrete puncta. Interestingly, we find that synapsin I localization is specifically altered in Hap1(-/-) cortical neurons without an effect on the localization of other SV proteins. This effect on synapsin I localization was not because of changes in the levels of synapsin I or its phosphorylation status in Hap1(-/-) brains. Furthermore, fluorescence recovery after photobleaching in transfected neurons expressing enhanced green fluorescent protein-synapsin Ia demonstrates that loss of HAP1 protein inhibits synapsin I transport. Thus, we demonstrate that HAP1 regulates SV exocytosis and may do so through binding to synapsin I. The Proposed mechanism of synapsin I transport mediated by HAP1 in neurons. HAP1 interacts with synapsin I, regulating the trafficking of synapsin I containing vesicles and/or transport packets, possibly through its engagement of microtubule motors. The absence of HAP1 reduces synapsin I transport and neuronal exocytosis. These findings provide insights into the processes of neuronal trafficking and synaptic signaling.

Conventional kinesin: Biochemical heterogeneity and functional implications in health and disease

Intracellular trafficking events powered by microtubule-based molecular motors facilitate the targeted delivery of selected molecular components to specific neuronal subdomains. Within this context, we provide a brief review of mechanisms underlying the execution of axonal transport (AT) by conventional kinesin, the most abundant kinesin-related motor protein in the mature nervous system. We emphasize the biochemical heterogeneity of this multi-subunit motor protein, further discussing its significance in light of recent discoveries revealing its regulation by various protein kinases. In addition, we raise issues relevant to the mode of conventional kinesin attachment to cargoes and examine recent evidence linking alterations in conventional kinesin phosphorylation to the pathogenesis of adult-onset neurodegenerative diseases.

Individual Neuronal Subtypes Exhibit Diversity in CNS Myelination Mediated by Synaptic Vesicle Release

Regulation of myelination by oligodendrocytes in the CNS has important consequences for higher-order nervous system function (e.g., [1, 2, 3, 4]), and there is growing consensus that neuronal activity regulates CNS myelination (e.g., [5, 6, 7, 8, 9]) through local axon-oligodendrocyte synaptic-vesicle-release-mediated signaling [10, 11, 12]. Recent analyses have indicated that myelination along axons of distinct neuronal subtypes can differ [13, 14], but it is not known whether regulation of myelination by activity is common to all neuronal subtypes or only some. This limits insight into how specific neurons regulate their own conduction. Here, we use a novel fluorescent fusion protein reporter to study myelination along the axons of distinct neuronal subtypes over time in zebrafish. We find that the axons of reticulospinal and commissural primary ascending (CoPA) neurons are among the first myelinated in the zebrafish CNS. To investigate how activity regulates myelination by different neuronal subtypes, we express tetanus toxin (TeNT) in individual reticulospinal or CoPA neurons to prevent synaptic vesicle release. We find that the axons of individual tetanus toxin expressing reticulospinal neurons have fewer myelin sheaths than controls and that their myelin sheaths are 50% shorter than controls. In stark contrast, myelination along tetanus-toxin-expressing CoPA neuron axons is entirely normal. These results indicate that while some neuronal subtypes modulate myelination by synaptic vesicle release to a striking degree in vivo, others do not. These data have implications for our understanding of how different neurons regulate myelination and thus their own function within specific neuronal circuits.

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Live imaging of myelin sheath dynamics along single axons over time in zebrafish

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Vesicular release from reticulospinal axons required for normal myelination

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Vesicular release from CoPA axons not required for myelination

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Evidence of neuronal subtype diversity of activity-regulated myelination

PSD-Zip70 Deficiency Causes Prefrontal Hypofunction Associated with Glutamatergic Synapse Maturation Defects by Dysregulation of Rap2 Activity

Dysregulation of synapse formation and plasticity is closely related to the pathophysiology of psychiatric and neurodevelopmental disorders. The prefrontal cortex (PFC) is particularly important for executive functions such as working memory, cognition, and emotional control, which are impaired in the disorders. PSD-Zip70 (Lzts1/FEZ1) is a postsynaptic density (PSD) protein predominantly expressed in the frontal cortex, olfactory bulb, striatum, and hippocampus. Here we found that PSD-Zip70 knock-out (PSD-Zip70KO) mice exhibit working memory and cognitive defects, and enhanced anxiety-like behaviors. These abnormal behaviors are caused by impaired glutamatergic synapse transmission accompanied by tiny-headed immature dendritic spines in the PFC, due to aberrant Rap2 activation, which has roles in synapse formation and plasticity. PSD-Zip70 modulates the Rap2 activity by interacting with SPAR (spine-associated RapGAP) and PDZ-GEF1 (RapGEF) in the postsynapse. Furthermore, suppression of the aberrant Rap2 activation in the PFC rescued the behavioral defects in PSD-Zip70KO mice. Our data demonstrate a critical role for PSD-Zip70 in Rap2-dependent spine synapse development in the PFC and underscore the importance of this regulation in PFC-dependent behaviors.

SIGNIFICANCE STATEMENT

PSD-Zip70 deficiency causes behavioral defects in working memory and cognition, and enhanced anxiety due to prefrontal hypofunction. This study revealed that PSD-Zip70 plays essential roles in glutamatergic synapse maturation via modulation of the Rap2 activity in the PFC. PSD-Zip70 interacts with both SPAR (spine-associated RapGAP) and PDZ-GEF1 (RapGEF) and modulates the Rap2 activity in postsynaptic sites. Our results provide a novel Rap2-specific regulatory mechanism in synaptic maturation involving PSD-Zip70.

Transgenic FingRs for Live Mapping of Synaptic Dynamics in Genetically-Defined Neurons

Tools for genetically-determined visualization of synaptic circuits and interactions are necessary to build connectomics of the vertebrate brain and to screen synaptic properties in neurological disease models. Here we develop a transgenic FingR (fibronectin intrabodies generated by mRNA display) technology for monitoring synapses in live zebrafish. We demonstrate FingR labeling of defined excitatory and inhibitory synapses, and show FingR applicability for dissecting synapse dynamics in normal and disease states. Using our system we show that chronic hypoxia, associated with neurological defects in preterm birth, affects dopaminergic neuron synapse number depending on the developmental timing of hypoxia.

The Role of Cdk5 in Alzheimer's Disease

Alzheimer's disease (AD) is known as the most fatal chronic neurodegenerative disease in adults along with progressive loss of memory and other cognitive function disorders. Cyclin-dependent kinase 5 (Cdk5), a unique member of the cyclin-dependent kinases (Cdks), is reported to intimately associate with the process of the pathogenesis of AD. Cdk5 is of vital importance in the development of CNS and neuron movements such as neuronal migration and differentiation, synaptic functions, and memory consolidation. However, when neurons suffer from pathological stimuli, Cdk5 activity becomes hyperactive and causes aberrant hyperphosphorylation of various substrates of Cdk5 like amyloid precursor protein (APP), tau and neurofilament, resulting in neurodegenerative diseases like AD. Deregulation of Cdk5 contributes to an array of pathological events in AD, ranging from formation of senile plaques and neurofibrillary tangles, synaptic damage, mitochondrial dysfunction to cell cycle reactivation as well as neuronal cell apoptosis. More importantly, an inhibition of Cdk5 activity with inhibitors such as RNA inference (RNAi) could protect from memory decline and neuronal cell loss through suppressing β-amyloid (Aβ)-induced neurotoxicity and tauopathies. This review will briefly describe the above-mentioned possible roles of Cdk5 in the physiological and pathological mechanisms of AD, further discussing recent advances and challenges in Cdk5 as a therapeutic target.

Building a Terminal: Mechanisms of Presynaptic Development in the CNS

To create a presynaptic terminal, molecular signaling events must be orchestrated across a number of subcellular compartments. In the soma, presynaptic proteins need to be synthesized, packaged together, and attached to microtubule motors for shipment through the axon. Within the axon, transport of presynaptic packages is regulated to ensure that developing synapses receive an adequate supply of components. At individual axonal sites, extracellular interactions must be translated into intracellular signals that can incorporate mobile transport vesicles into the nascent presynaptic terminal. Even once the initial recruitment process is complete, the components and subsequent functionality of presynaptic terminals need to constantly be remodeled. Perhaps most remarkably, all of these processes need to be coordinated in space and time. In this review, we discuss how these dynamic cellular processes occur in neurons of the central nervous system in order to generate presynaptic terminals in the brain.

SUMOylation of synapsin Ia maintains synaptic vesicle availability and is reduced in an autism mutation

Synapsins are key components of the presynaptic neurotransmitter release machinery. Their main role is to cluster synaptic vesicles (SVs) to each other and anchor them to the actin cytoskeleton to establish the reserve vesicle pool, and then release them in response to appropriate membrane depolarization. Here we demonstrate that SUMOylation of synapsin Ia (SynIa) at K687 is necessary for SynIa function. Replacement of endogenous SynIa with a non-SUMOylatable mutant decreases the size of the releasable vesicle pool and impairs stimulated SV exocytosis. SUMOylation enhances SynIa association with SVs to promote the efficient reclustering of SynIa following neuronal stimulation and maintain its presynaptic localization. The A548T mutation in SynIa is strongly associated with autism and epilepsy and we show that it leads to defective SynIa SUMOylation. These results identify SUMOylation as a fundamental regulator of SynIa function and reveal a novel link between reduced SUMOylation of SynIa and neurological disorders.

Synapse assembly and neurodevelopmental disorders

In this review we examine the current understanding of how genetic deficits associated with neurodevelopmental disorders may impact synapse assembly. We then go on to discuss how the critical periods for these genetic deficits will shape the nature of future clinical interventions.

Dynamics of nascent and active zone ultrastructure as synapses enlarge during long-term potentiation in mature hippocampus

Nascent zones and active zones are adjacent synaptic regions that share a postsynaptic density, but nascent zones lack the presynaptic vesicles found at active zones. Here dendritic spine synapses were reconstructed through serial section electron microscopy (3DEM) and EM tomography to investigate nascent zone dynamics during long-term potentiation (LTP) in mature rat hippocampus. LTP was induced with theta-burst stimulation, and comparisons were made with control stimulation in the same hippocampal slices at 5 minutes, 30 minutes, and 2 hours post-induction and to perfusion-fixed hippocampus in vivo. Nascent zones were present at the edges of ∼35% of synapses in perfusion-fixed hippocampus and as many as ∼50% of synapses in some hippocampal slice conditions. By 5 minutes, small dense-core vesicles known to transport active zone proteins moved into more presynaptic boutons. By 30 minutes, nascent zone area decreased, without significant change in synapse area, suggesting that presynaptic vesicles were recruited to preexisting nascent zones. By 2 hours, both nascent and active zones were enlarged. Immunogold labeling revealed glutamate receptors in nascent zones; however, average distances from nascent zones to docked presynaptic vesicles ranged from 170 ± 5 nm in perfusion-fixed hippocampus to 251 ± 4 nm at enlarged synapses by 2 hours during LTP. Prior stochastic modeling suggests that decrease in glutamate concentration reduces the probability of glutamate receptor activation from 0.4 at the center of release to 0.1 just 200 nm away. Thus, conversion of nascent zones to functional active zones likely requires the recruitment of presynaptic vesicles during LTP.

Sensory gating of an embryonic zebrafish interneuron during spontaneous motor behaviors

In all but the simplest monosynaptic reflex arcs, sensory stimuli are encoded by sensory neurons that transmit a signal via sensory interneurons to downstream partners in order to elicit a response. In the embryonic zebrafish (Danio rerio), cutaneous Rohon-Beard (RB) sensory neurons fire in response to mechanical stimuli and excite downstream glutamatergic commissural primary ascending (CoPA) interneurons to produce a flexion response contralateral to the site of stimulus. In the absence of sensory stimuli, zebrafish spinal locomotor circuits are spontaneously active during development due to pacemaker activity resulting in repetitive coiling of the trunk. Self-generated movement must therefore be distinguishable from external stimuli in order to ensure the appropriate activation of touch reflexes. Here, we recorded from CoPAs during spontaneous and evoked fictive motor behaviors in order to examine how responses to self-movement are gated in sensory interneurons. During spontaneous coiling, CoPAs received glycinergic inputs coincident with contralateral flexions that shunted firing for the duration of the coiling event. Shunting inactivation of CoPAs was caused by a slowly deactivating chloride conductance that resulted in lowered membrane resistance and increased action potential threshold. During spontaneous burst swimming, which develops later, CoPAs received glycinergic inputs that arrived in phase with excitation to ipsilateral motoneurons and provided persistent shunting. During a touch stimulus, short latency glutamatergic inputs produced cationic currents through AMPA receptors that drove a single, large amplitude action potential in the CoPA before shunting inhibition began, providing a brief window for the activation of downstream neurons. We compared the properties of CoPAs to those of other spinal neurons and propose that glycinergic signaling onto CoPAs acts as a corollary discharge signal for reflex inhibition during movement.

A hybrid electrical/chemical circuit in the spinal cord generates a transient embryonic motor behavior

Spontaneous network activity is a highly stereotyped early feature of developing circuits throughout the nervous system, including in the spinal cord. Spinal locomotor circuits produce a series of behaviors during development before locomotion that reflect the continual integration of spinal neurons into a functional network, but how the circuitry is reconfigured is not understood. The first behavior of the zebrafish embryo (spontaneous coiling) is mediated by an electrical circuit that subsequently generates mature locomotion (swimming) as chemical neurotransmission develops. We describe here a new spontaneous behavior, double coiling, that consists of two alternating contractions of the tail in rapid succession. Double coiling was glutamate-dependent and required descending hindbrain excitation, similar to but preceding swimming, making it a discrete intermediary developmental behavior. At the cellular level, motoneurons had a distinctive glutamate-dependent activity pattern that correlated with double coiling. Two glutamatergic interneurons, CoPAs and CiDs, had different activity profiles during this novel behavior. CoPA neurons failed to show changes in activity patterns during the period in which double coiling appears, whereas CiD neurons developed a glutamate-dependent activity pattern that correlated with double coiling and they innervated motoneurons at that time. Additionally, double coils were modified after pharmacological reduction of glycinergic neurotransmission such that embryos produced three or more rapidly alternating coils. We propose that double coiling behavior represents an important transition of the motor network from an electrically coupled spinal cord circuit that produces simple periodic coils to a spinal network driven by descending chemical neurotransmission, which generates more complex behaviors.

Phosphorylation of synapsin I by cyclin-dependent kinase-5 sets the ratio between the resting and recycling pools of synaptic vesicles at hippocampal synapses

Cyclin-dependent kinase-5 (Cdk5) was reported to downscale neurotransmission by sequestering synaptic vesicles (SVs) in the release-reluctant resting pool, but the molecular targets mediating this activity remain unknown. Synapsin I (SynI), a major SV phosphoprotein involved in the regulation of SV trafficking and neurotransmitter release, is one of the presynaptic substrates of Cdk5, which phosphorylates it in its C-terminal region at Ser(549) (site 6) and Ser(551) (site 7). Here we demonstrate that Cdk5 phosphorylation of SynI fine tunes the recruitment of SVs to the active recycling pool and contributes to the Cdk5-mediated homeostatic responses. Phosphorylation of SynI by Cdk5 is physiologically regulated and enhances its binding to F-actin. The effects of Cdk5 inhibition on the size and depletion kinetics of the recycling pool, as well as on SV distribution within the nerve terminal, are virtually abolished in mouse SynI knock-out (KO) neurons or in KO neurons expressing the dephosphomimetic SynI mutants at sites 6,7 or site 7 only. The observation that the single site-7 mutant phenocopies the effects of the deletion of SynI identifies this site as the central switch in mediating the synaptic effects of Cdk5 and demonstrates that SynI is necessary and sufficient for achieving the effects of the kinase on SV trafficking. The phosphorylation state of SynI by Cdk5 at site 7 is regulated during chronic modification of neuronal activity and is an essential downstream effector for the Cdk5-mediated homeostatic scaling.

Complexes of Usher proteins preassemble at the endoplasmic reticulum and are required for trafficking and ER homeostasis

Usher syndrome (USH), the leading cause of hereditary combined hearing and vision loss, is characterized by sensorineural deafness and progressive retinal degeneration. Mutations in several different genes produce USH, but the proximal cause of sensory cell death remains mysterious. We adapted a proximity ligation assay to analyze associations among three of the USH proteins, Cdh23, Harmonin and Myo7aa, and the microtubule-based transporter Ift88 in zebrafish inner ear mechanosensory hair cells. We found that the proteins are in close enough proximity to form complexes and that these complexes preassemble at the endoplasmic reticulum (ER). Defects in any one of the three USH proteins disrupt formation and trafficking of the complex and result in diminished levels of the other proteins, generalized trafficking defects and ER stress that triggers apoptosis. ER stress, thus, contributes to sensory hair cell loss and provides a new target to explore for protective therapies for USH.

Overexpression of the calpain-specific inhibitor calpastatin reduces human alpha-Synuclein processing, aggregation and synaptic impairment in [A30P]αSyn transgenic mice

Lewy bodies, a pathological hallmark of Parkinson's disease (PD), contain aggregated alpha-synuclein (αSyn), which is found in several modified forms and can be discovered phosphorylated, ubiquitinated and truncated. Aggregation-prone truncated species of αSyn caused by aberrant cleavage of this fibrillogenic protein are hypothesized to participate in its sequestration into inclusions subsequently leading to synaptic dysfunction and neuronal death. Here, we investigated the role of calpain cleavage of αSyn in vivo by generating two opposing mouse models. We crossed into human [A30P]αSyn transgenic (i) mice deficient for calpastatin, a calpain-specific inhibitor, thus enhancing calpain activity (SynCAST(-)) and (ii) mice overexpressing human calpastatin leading to reduced calpain activity (SynCAST(+)). As anticipated, a reduced calpain activity led to a decreased number of αSyn-positive aggregates, whereas loss of calpastatin led to increased truncation of αSyn in SynCAST(-). Furthermore, overexpression of calpastatin decreased astrogliosis and the calpain-dependent degradation of synaptic proteins, potentially ameliorating the observed neuropathology in [A30P]αSyn and SynCAST(+) mice. Overall, our data further support a crucial role of calpains, particularly of calpain 1, in the pathogenesis of PD and in disease-associated aggregation of αSyn, indicating a therapeutic potential of calpain inhibition in PD.

Molecular mechanisms driving homeostatic plasticity of neurotransmitter release

Homeostatic plasticity is a process by which neurons adapt to the overall network activity to keep their firing rates in a reasonable range. At the cellular level this kind of plasticity comprises modulation of cellular excitability and tuning of synaptic strength. In this review we concentrate on presynaptic homeostatic plasticity controlling the efficacy of neurotransmitter release from presynaptic boutons. While morphological and electrophysiological approaches were successful to describe homeostatic plasticity-induced changes in the presynaptic architecture and function, cellular and molecular mechanisms underlying those modifications remained largely unknown for a long time. We summarize the latest progress made in the understanding of homeostasis-induced regulation of different steps of the synaptic vesicle cycle and the molecular machineries involved in this process. We particularly focus on the role of presynaptic scaffolding proteins, which functionally and spatially organize synaptic vesicle clusters, neurotransmitter release sites and the associated endocytic machinery. These proteins turned out to be major presynaptic substrates for remodeling during homeostatic plasticity. Finally, we discuss cellular processes and signaling pathways acting during homeostatic molecular remodeling and their potential involvement in the maladaptive plasticity occurring in multiple neuropathologic conditions such as neurodegeneration, epilepsy and neuropsychiatric disorders.