Transcription factors and effectors that regulate neuronal morphology

Fruit Ripening Retardant Daminozide Induces Cognitive Impairment, Cell Specific Neurotoxicity and Genotoxicity in Drosophila melanogaster

BACKGROUND

We explored neurotoxic and genotoxic effects of Daminozide, a fruit ripening retardant, on the brain of Drosophila melanogaster, based on our previous finding of DNA fragmentation in larval brain cell in the flies experimentally exposed to this chemicals.

METHODS

Adult flies were subjected to two distinct concentrations of daminozide (200mg/L and 400mg/L) mixed in culture medium, followed by an examination of specific behaviors such as courtship conditioning and aversive phototaxis, which serve as indicators of cognitive functions. We investigated brain histology and histochemistry to assess the overall toxicity of daminozide, focusing on neuron type-specific effects. Additionally, we conducted studies on gene expression specific to neuronal function. Statistical comparisons were then made between the exposed and control flies across all tested attributes.

RESULTS

The outcome of behavioral assays suggested deleterious effects of Daminozide on learning, short term and long term memory function. Histological examination of brain sections revealed cellular degeneration, within Kenyon cell neuropiles in Daminozide-exposed flies. Neurone specific Immuno-histochemistry study revealed significant reduction of dopaminergic and glutaminergic neurones with discernible reduction in cellular counts, alteration in cell and nuclear morphology among daminozide exposed flies. Gene expression analyses demonstrated upregulation of rutabaga (rut), hb9 and down regulation of PKa- C1, CrebB, Ace and nAchRbeta-1 in exposed flies which suggest dysregulation of gene functions involved in motor neuron activity, learning, and memory.

CONCLUSION

Taken together, our findings suggests that Daminozide induces multifaceted harmful impacts on the neural terrain of Drosophila melanogaster, posing a threat to its cognitive abilities.

Stochastic growth and selective stabilization generate stereotyped dendritic arbors

Stereotyped dendritic arbors are shaped by dynamic and stochastic growth during neuronal development. It remains unclear how guidance receptors and ligands coordinate branch dynamic growth, retraction, and stabilization to specify dendritic arbors. We previously showed that extracellular ligand SAX-7/LICAM dictates the shape of the PVD sensory neuron via binding to the dendritic guidance receptor DMA-1, a single transmembrane adhesion molecule. Here, we perform structure-function analyses of DMA-1 and unexpectedly find that robust, stochastic dendritic growth does not require ligand-binding. Instead, ligand-binding inhibits growth, prevents retraction, and specifies arbor shape. Furthermore, we demonstrate that dendritic growth requires a pool of ligand-free DMA-1, which is maintained by receptor endocytosis and reinsertion to the plasma membrane via recycling endosomes. Mutants defective of DMA-1 endocytosis show severely truncated dendritic arbors. We present a model in which ligand-free guidance receptor mediates intrinsic, stochastic dendritic growth, while extracellular ligands instruct dendrite shape by inhibiting growth.

Strain in the Midbrain: Impact of Traumatic Brain Injury on the Central Serotonin System

Traumatic brain injury (TBI) is a pervasive public health crisis that severely impacts the quality of life of affected individuals. Like peripheral forms of trauma, TBI results from extraordinarily heterogeneous environmental forces being imparted on the cranial space, resulting in heterogeneous disease pathologies. This has made therapies for TBI notoriously difficult to develop, and currently, there are no FDA-approved pharmacotherapies specifically for the acute or chronic treatment of TBI. TBI is associated with changes in cognition and can precipitate the onset of debilitating psychiatric disorders like major depressive disorder (MDD), generalized anxiety disorder (GAD), and post-traumatic stress disorder (PTSD). Complicating these effects of TBI, FDA-approved pharmacotherapies utilized to treat these disorders often fail to reach the desired level of efficacy in the context of neurotrauma. Although a complicated association, decades of work have linked central serotonin (5-HT) neurotransmission as being involved in the etiology of a myriad of neuropsychiatric disorders, including MDD and GAD. 5-HT is a biogenic monoamine neurotransmitter that is highly conserved across scales of biology. Though the majority of 5-HT is isolated to peripheral sites such as the gastrointestinal (GI) tract, 5-HT neurotransmission within the CNS exerts exquisite control over diverse biological functions, including sleep, appetite and respiration, while simultaneously establishing normal mood, perception, and attention. Although several key studies have begun to elucidate how various forms of neurotrauma impact central 5-HT neurotransmission, a full determination of precisely how TBI disrupts the highly regulated dynamics of 5-HT neuron function and/or 5-HT neurotransmission has yet to be conceptually or experimentally resolved. The purpose of the current review is, therefore, to integrate the disparate bodies of 5-HT and TBI research and synthesize insight into how new combinatorial research regarding 5-HT neurotransmission and TBI may offer an informed perspective into the nature of TBI-induced neuropsychiatric complications.

Homeodomain proteins hierarchically specify neuronal diversity and synaptic connectivity

How our brain generates diverse neuron types that assemble into precise neural circuits remains unclear. Using Drosophila lamina neuron types (L1-L5), we show that the primary homeodomain transcription factor (HDTF) brain-specific homeobox (Bsh) is initiated in progenitors and maintained in L4/L5 neurons to adulthood. Bsh activates secondary HDTFs Ap (L4) and Pdm3 (L5) and specifies L4/L5 neuronal fates while repressing the HDTF Zfh1 to prevent ectopic L1/L3 fates (control: L1-L5; Bsh-knockdown: L1-L3), thereby generating lamina neuronal diversity for normal visual sensitivity. Subsequently, in L4 neurons, Bsh and Ap function in a feed-forward loop to activate the synapse recognition molecule DIP-β, thereby bridging neuronal fate decision to synaptic connectivity. Expression of a Bsh:Dam, specifically in L4, reveals Bsh binding to the DIP-β locus and additional candidate L4 functional identity genes. We propose that HDTFs function hierarchically to coordinate neuronal molecular identity, circuit formation, and function. Hierarchical HDTFs may represent a conserved mechanism for linking neuronal diversity to circuit assembly and function.

De novo missense variants in ZBTB47 are associated with developmental delays, hypotonia, seizures, gait abnormalities, and variable movement abnormalities

The collection of known genetic etiologies of neurodevelopmental disorders continues to increase, including several syndromes associated with defects in zinc finger protein transcription factors (ZNFs) that vary in clinical severity from mild learning disabilities and developmental delay to refractory seizures and severe autism spectrum disorder. Here we describe a new neurodevelopmental disorder associated with variants in ZBTB47 (also known as ZNF651), which encodes zinc finger and BTB domain-containing protein 47. Exome sequencing (ES) was performed for five unrelated patients with neurodevelopmental disorders. All five patients are heterozygous for a de novo missense variant in ZBTB47, with p.(Glu680Gly) (c.2039A>G) detected in one patient and p.(Glu477Lys) (c.1429G>A) identified in the other four patients. Both variants impact conserved amino acid residues. Bioinformatic analysis of each variant is consistent with pathogenicity. We present five unrelated patients with de novo missense variants in ZBTB47 and a phenotype characterized by developmental delay with intellectual disability, seizures, hypotonia, gait abnormalities, and variable movement abnormalities. We propose that these variants in ZBTB47 are the basis of a new neurodevelopmental disorder.

MBL-1 and EEL-1 affect the splicing and protein levels of MEC-3 to control dendrite complexity

Transcription factors (TFs) play critical roles in specifying many aspects of neuronal cell fate including dendritic morphology. How TFs are accurately regulated during neuronal morphogenesis is not fully understood. Here, we show that LIM homeodomain protein MEC-3, the key TF for C. elegans PVD dendrite morphogenesis, is regulated by both alternative splicing and an E3 ubiquitin ligase. The mec-3 gene generates several transcripts by alternative splicing. We find that mbl-1, the orthologue of the muscular dystrophy disease gene muscleblind-like (MBNL), is required for PVD dendrite arbor formation. Our data suggest mbl-1 regulates the alternative splicing of mec-3 to produce its long isoform. Deleting the long isoform of mec-3(deExon2) causes reduction of dendrite complexity. Through a genetic modifier screen, we find that mutation in the E3 ubiquitin ligase EEL-1 suppresses mbl-1 phenotype. eel-1 mutants also suppress mec-3(deExon2) mutant but not the mec-3 null phenotype. Loss of EEL-1 alone leads to excessive dendrite branches. Together, these results indicate that MEC-3 is fine-tuned by alternative splicing and the ubiquitin system to produce the optimal level of dendrite branches.

The Zinc-BED Transcription Factor Bedwarfed Promotes Proportional Dendritic Growth and Branching through Transcriptional and Translational Regulation in Drosophila

Dendrites are the primary points of sensory or synaptic input to a neuron and play an essential role in synaptic integration and neural function. Despite the functional importance of dendrites, relatively less is known about the underlying mechanisms regulating cell type-specific dendritic patterning. Herein, we have dissected the functional roles of a previously uncharacterized gene, CG3995, in cell type-specific dendritic development in Drosophila melanogaster. CG3995, which we have named bedwarfed (bdwf), encodes a zinc-finger BED-type protein that is required for proportional growth and branching of dendritic arbors. It also exhibits nucleocytoplasmic expression and functions in both transcriptional and translational cellular pathways. At the transcriptional level, we demonstrate a reciprocal regulatory relationship between Bdwf and the homeodomain transcription factor (TF) Cut. We show that Cut positively regulates Bdwf expression and that Bdwf acts as a downstream effector of Cut-mediated dendritic development, whereas overexpression of Bdwf negatively regulates Cut expression in multidendritic sensory neurons. Proteomic analyses revealed that Bdwf interacts with ribosomal proteins and disruption of these proteins resulted in phenotypically similar dendritic hypotrophy defects as observed in bdwf mutant neurons. We further demonstrate that Bdwf and its ribosomal protein interactors are required for normal microtubule and F-actin cytoskeletal architecture. Finally, our findings reveal that Bdwf is required to promote protein translation and ribosome trafficking along the dendritic arbor. These findings shed light on the complex, combinatorial, and multi-functional roles of transcription factors (TFs) in directing the diversification of cell type-specific dendritic development.

The Zinc-BED transcription factor Bedwarfed promotes proportional dendritic growth and branching through transcriptional and translational regulation in Drosophila

Dendrites are the primary points of sensory or synaptic inputs to a neuron and play an essential role in synaptic integration and neural function. Despite the functional importance of dendrites, relatively less is known about the underlying mechanisms regulating cell-type specific dendritic patterning. Herein, we have dissected functional roles of a previously uncharacterized gene, CG3995 , in cell-type specific dendritic development in Drosophila melanogaster . CG3995 , which we have named bedwarfed ( bdwf ), encodes a zinc-finger BED-type protein which is required for proportional growth and branching of dendritic arbors, exhibits nucleocytoplasmic expression, and functions in both transcriptional and translational cellular pathways. At the transcriptional level, we demonstrate a reciprocal regulatory relationship between Bdwf and the homeodomain transcription factor (TF) Cut. We show that Cut positively regulates Bdwf expression and that Bdwf acts as a downstream effector of Cut-mediated dendritic development, whereas overexpression of Bdwf negatively regulates Cut expression in multidendritic sensory neurons. Proteomic analyses revealed that Bdwf interacts with ribosomal proteins and disruption of these proteins produced phenotypically similar dendritic hypotrophy defects as observed in bdwf mutant neurons. We further demonstrate that Bdwf and its ribosomal protein interactors are required for normal microtubule and F-actin cytoskeletal architecture. Finally, our findings reveal that Bdwf is required to promote protein translation and ribosome trafficking along the dendritic arbor. Taken together, these results provide new insights into the complex, combinatorial and multi-functional roles of transcription factors (TFs) in directing diversification of cell-type specific dendritic development.

JNK signaling in pioneer neurons organizes ventral nerve cord architecture in Drosophila embryos

Morphogenesis of the Central Nervous System (CNS) is a complex process that obeys precise architectural rules. Yet, the mechanisms dictating these rules remain unknown. Analyzing morphogenesis of the Drosophila embryo Ventral Nerve Cord (VNC), we observe that a tight control of JNK signaling is essential for attaining the final VNC architecture. JNK signaling in a specific subset of pioneer neurons autonomously regulates the expression of Fasciclin 2 (Fas 2) and Neurexin IV (Nrx IV) adhesion molecules, probably via the transcription factor zfh1. Interfering at any step in this cascade affects fasciculation along pioneer axons, leading to secondary cumulative scaffolding defects during the structural organization of the axonal network. The global disorder of architectural landmarks ultimately influences nervous system condensation. In summary, our data point to JNK signaling in a subset of pioneer neurons as a key element underpinning VNC architecture, revealing critical milestones on the mechanism of control of its structural organization.

Progress and challenges in directing the differentiation of human iPSCs into spinal motor neurons

Motor neuron (MN) diseases, including amyotrophic lateral sclerosis, progressive bulbar palsy, primary lateral sclerosis and spinal muscular atrophy, cause progressive paralysis and, in many cases, death. A better understanding of the molecular mechanisms of pathogenesis is urgently needed to identify more effective therapies. However, studying MNs has been extremely difficult because they are inaccessible in the spinal cord. Induced pluripotent stem cells (iPSCs) can generate a theoretically limitless number of MNs from a specific patient, making them powerful tools for studying MN diseases. However, to reach their potential, iPSCs need to be directed to efficiently differentiate into functional MNs. Here, we review the reported differentiation protocols for spinal MNs, including induction with small molecules, expression of lineage-specific transcription factors, 2-dimensional and 3-dimensional cultures, as well as the implementation of microfluidics devices and co-cultures with other cell types, including skeletal muscle. We will summarize the advantages and disadvantages of each strategy. In addition, we will provide insights into how to address some of the remaining challenges, including reproducibly obtaining mature and aged MNs.

PP2A phosphatase regulates cell-type specific cytoskeletal organization to drive dendrite diversity

Uncovering molecular mechanisms regulating dendritic diversification is essential to understanding the formation and modulation of functional neural circuitry. Transcription factors play critical roles in promoting dendritic diversity and here, we identify PP2A phosphatase function as a downstream effector of Cut-mediated transcriptional regulation of dendrite development. Mutant analyses of the PP2A catalytic subunit (mts) or the scaffolding subunit (PP2A-29B) reveal cell-type specific regulatory effects with the PP2A complex required to promote dendritic growth and branching in Drosophila Class IV (CIV) multidendritic (md) neurons, whereas in Class I (CI) md neurons, PP2A functions in restricting dendritic arborization. Cytoskeletal analyses reveal requirements for Mts in regulating microtubule stability/polarity and F-actin organization/dynamics. In CIV neurons, mts knockdown leads to reductions in dendritic localization of organelles including mitochondria and satellite Golgi outposts, while CI neurons show increased Golgi outpost trafficking along the dendritic arbor. Further, mts mutant neurons exhibit defects in neuronal polarity/compartmentalization. Finally, genetic interaction analyses suggest β-tubulin subunit 85D is a common PP2A target in CI and CIV neurons, while FoxO is a putative target in CI neurons.

The regulatory landscape of neurite development in Caenorhabditis elegans

Neuronal communication requires precise connectivity of neurite projections (axons and dendrites). Developing neurites express cell-surface receptors that interpret extracellular cues to enable correct guidance toward, and connection with, target cells. Spatiotemporal regulation of neurite guidance molecule expression by transcription factors (TFs) is critical for nervous system development and function. Here, we review how neurite development is regulated by TFs in the Caenorhabditis elegans nervous system. By collecting publicly available transcriptome and ChIP-sequencing data, we reveal gene expression dynamics during neurite development, providing insight into transcriptional mechanisms governing construction of the nervous system architecture.

Cracking the combinatorial code of neuronal wiring

How transcription factors orchestrate the combinatorial expression of cell-surface proteins that, in turn, specify the wiring of the nervous system is an open question. In this issue of Neuron, Xie et al. reveal a new, unexpected layer of complexity.

Transcription factor Acj6 controls dendrite targeting via a combinatorial cell-surface code

Transcription factors specify the fate and connectivity of developing neurons. We investigate how a lineage-specific transcription factor, Acj6, controls the precise dendrite targeting of Drosophila olfactory projection neurons (PNs) by regulating the expression of cell-surface proteins. Quantitative cell-surface proteomic profiling of wild-type and acj6 mutant PNs in intact developing brains, and a proteome-informed genetic screen identified PN surface proteins that execute Acj6-regulated wiring decisions. These include canonical cell adhesion molecules and proteins previously not associated with wiring, such as Piezo, whose mechanosensitive ion channel activity is dispensable for its function in PN dendrite targeting. Comprehensive genetic analyses revealed that Acj6 employs unique sets of cell-surface proteins in different PN types for dendrite targeting. Combined expression of Acj6 wiring executors rescued acj6 mutant phenotypes with higher efficacy and breadth than expression of individual executors. Thus, Acj6 controls wiring specificity of different neuron types by specifying distinct combinatorial expression of cell-surface executors.

Visceral mesoderm signaling regulates assembly position and function of the Drosophila testis niche

Tissue homeostasis often requires a properly placed niche to support stem cells. Morphogenetic processes that position a niche are just being described. For the Drosophila testis, we recently showed that pro-niche cells, specified at disparate positions during early gonadogenesis, must assemble into one collective at the anterior of the gonad. We now find that Slit and FGF signals emanating from adjacent visceral mesoderm regulate assembly. In response to signaling, niche cells express islet, which we find is also required for niche assembly. Without signaling, niche cells specified furthest from the anterior are unable to migrate, remaining dispersed. The function of such niches is severely disrupted, with niche cells evading cell cycle quiescence, compromised in their ability to signal the incipient stem cell pool, and failing to orient stem cell divisions properly. Our work identifies both extrinsic signaling and intrinsic responses required for proper assembly and placement of the testis niche.

The Role of Even-Skipped in Drosophila Larval Somatosensory Circuit Assembly

Proper somatosensory circuit assembly is critical for processing somatosensory stimuli and for responding accordingly. In comparison to other sensory circuits (e.g., olfactory and visual), somatosensory circuits have unique anatomy and function. However, understanding of somatosensory circuit development lags far behind that of other sensory systems. For example, there are few identified transcription factors required for integration of interneurons into functional somatosensory circuits. Here, as a model, we examine one type of somatosensory interneuron, Even-skipped (Eve) expressing laterally placed interneurons (ELs) of the Drosophila larval nerve cord. Eve is a highly conserved, homeodomain transcription factor known to play a role in cell fate specification and neuronal axon guidance. Because marker genes are often functionally important in the cell types they define, we deleted eve expression specifically from EL interneurons. On the cell biological level, using single neuron labeling, we find eve plays several previously undescribed roles in refinement of neuron morphogenesis. Eve suppresses aberrant neurite branching, promotes axon elongation, and regulates dorsal-ventral dendrite position. On the circuit level, using optogenetics, calcium imaging, and behavioral analysis, we find eve expression is required in EL interneurons for the normal encoding of somatosensory stimuli and for normal mapping of outputs to behavior. We conclude that the eve gene product coordinately regulates multiple aspects of EL interneuron morphogenesis and is critically required to properly integrate EL interneurons into somatosensory circuits. Our data shed light on the genetic regulation of somatosensory circuit assembly.

Pleiotropic predisposition to Alzheimer's disease and educational attainment: insights from the summary statistics analysis

Epidemiological studies report beneficial associations of higher educational attainment (EDU) with Alzheimer's disease (AD). Prior genome-wide association studies (GWAS) also reported variants associated with AD and EDU separately. The analysis of pleiotropic associations with these phenotypes may shed light on EDU-related protection against AD. We performed pleiotropic meta-analyses using Fisher's method and omnibus test applied to summary statistics for single nucleotide polymorphisms (SNPs) associated with AD and EDU in large-scale univariate GWAS at suggestive-effect (5 × 10-8 < p < 0.1) and genome-wide (p ≤ 5 × 10-8) significance levels. We report 53 SNPs that attained p ≤ 5 × 10-8 at least in one of the pleiotropic meta-analyses and were reported in the univariate GWAS at 5 × 10-8 < p < 0.1. Of them, there were 46 pleiotropic SNPs according to Fisher's method. Additionally, Fisher's method identified 25 of 206 SNPs with pleiotropic effects, which attained p ≤ 5 × 10-8 in the univariate GWAS. We showed that a large fraction of the pleiotropic associations was affected by a counterintuitive phenomenon of antagonistic genetic heterogeneity, which explains the increase, rather than decrease, of the significance of the pleiotropic associations in the omnibus test. Functional enrichment analysis showed that apart from cancers, gene set harboring the non-pleiotropic SNPs was characterized by late-onset AD and neurodevelopmental disorders. The pleiotropic gene set was characterized by a broad spectrum of progressive neurological and neuromuscular diseases and immune-mediated conditions, including progressive motor neuropathy, multiple sclerosis, Parkinson's disease, and severe AD. Our results suggest that disentangling genes harboring variants with and without pleiotropic associations with AD and EDU is promising for dissecting heterogeneity in biological mechanisms of AD.

Transcriptional regulation of neuronal identity

Neuronal diversity is an intrinsic feature of the nervous system. Transcription factors (TFs) are key regulators in the establishment of different neuronal identities; how are the actions of different TFs coordinated to orchestrate this diversity? Are there common features shared among the different neuron types of an organism or even among different animal groups? In this review, we provide a brief overview on common traits emerging on the transcriptional regulation of neuron type diversification with a special focus on the comparison between mouse and Caenorhabditis elegans model systems. In the first part, we describe general concepts on neuronal identity and transcriptional regulation of gene expression. In the second part of the review, TFs are classified in different categories according to their key roles at specific steps along the protracted process of neuronal specification and differentiation. The same TF categories can be identified both in mammals and nematodes. Importantly, TFs are very pleiotropic: Depending on the neuron type or the time in development, the same TF can fulfil functions belonging to different categories. Finally, we describe the key role of transcriptional repression at all steps controlling neuronal diversity and propose that acquisition of neuronal identities could be considered a metastable process.

Satb2 Regulates EphA7 to Control Soma Spacing and Self-Avoidance of Cortical Pyramidal Neurons

Soma spacing and dendritic arborization during brain development are key events for the establishment of proper neural circuitry and function. Transcription factor Satb2 is a molecular node in regulating the development of the cerebral cortex, as shown by the facts that Satb2 is required for the regionalization of retrosplenial cortex, the determination of callosal neuron fate, and the regulation of soma spacing and dendritic self-avoidance of cortical pyramidal neurons. In this study, we explored downstream effectors that mediate the Satb2-implicated soma spacing and dendritic self-avoidance. First, RNA-seq analysis of the cortex revealed differentially expressed genes between control and Satb2 CKO mice. Among them, EphA7 transcription was dramatically increased in layers II/III of Satb2 CKO cortex. Overexpression of EphA7 in the late-born cortical neurons of wild-type mice via in utero electroporation resulted in soma clumping and impaired self-avoidance of affected pyramidal neurons, which resembles the phenotypes caused by knockdown of Satb2 expression. Importantly, the phenotypes by Satb2 knockdown was rescued by reducing EphA7 expression in the cortex. Finally, ChIP and luciferase reporter assays indicated a direct suppression of EphA7 expression by Satb2. These findings provide new insights into the complexity of transcriptional regulation of the morphogenesis of cerebral cortex.

Role of Satb1 and Satb2 Transcription Factors in the Glutamate Receptors Expression and Ca2+ Signaling in the Cortical Neurons In Vitro

Transcription factors Satb1 and Satb2 are involved in the processes of cortex development and maturation of neurons. Alterations in the expression of their target genes can lead to neurodegenerative processes. Molecular and cellular mechanisms of regulation of neurotransmission by these transcription factors remain poorly understood. In this study, we have shown that transcription factors Satb1 and Satb2 participate in the regulation of genes encoding the NMDA-, AMPA-, and KA- receptor subunits and the inhibitory GABA(A) receptor. Deletion of gene for either Satb1 or Satb2 homologous factors induces the expression of genes encoding the NMDA receptor subunits, thereby leading to higher amplitudes of Ca²⁺-signals in neurons derived from the Satb1-deficient (Satb1^fl/+ * Nex^Cre/+) and Satb1-null mice (Satb1^fl/fl * Nex^Cre/+) in response to the selective agonist reducing the EC50 for the NMDA receptor. Simultaneously, there is an increase in the expression of the Gria2 gene, encoding the AMPA receptor subunit, thus decreasing the Ca²⁺-signals of neurons in response to the treatment with a selective agonist (5-Fluorowillardiine (FW)). The Satb1 deletion increases the sensitivity of the KA receptor to the agonist (domoic acid), in the cortical neurons of the Satb1-deficient mice but decreases it in the Satb1-null mice. At the same time, the Satb2 deletion decreases Ca²⁺-signals and the sensitivity of the KA receptor to the agonist in neurons from the Satb1-null and the Satb1-deficient mice. The Satb1 deletion affects the development of the inhibitory system of neurotransmission resulting in the suppression of the neuron maturation process and switching the GABAergic responses from excitatory to inhibitory, while the Satb2 deletion has a similar effect only in the Satb1-null mice. We show that the Satb1 and Satb2 transcription factors are involved in the regulation of the transmission of excitatory signals and inhibition of the neuronal network in the cortical cell culture.

Role of Exendin-4 in Brain Insulin Resistance, Mitochondrial Function, and Neurite Outgrowth in Neurons under Palmitic Acid-Induced Oxidative Stress

Glucagon like peptide 1 (GLP-1) is an incretin hormone produced by the gut and brain, and is currently being used as a therapeutic drug for type 2 diabetes and obesity, suggesting that it regulates abnormal appetite patterns, and ameliorates impaired glucose metabolism. Many researchers have demonstrated that GLP-1 agonists and GLP-1 receptor agonists exert neuroprotective effects against brain damage. Palmitic acid (PA) is a saturated fatty acid, and increases the risk of neuroinflammation, lipotoxicity, impaired glucose metabolism, and cognitive decline. In this study, we investigated whether or not Exentin-4 (Ex-4; GLP-1 agonist) inhibits higher production of reactive oxygen species (ROS) in an SH-SY5Y neuronal cell line under PA-induced apoptosis conditions. Moreover, pre-treatment with Ex-4 in SH-SY5Y neuronal cells prevents neural apoptosis and mitochondrial dysfunction through several cellular signal pathways. In addition, insulin sensitivity in neurons is improved by Ex-4 treatment under PA-induced insulin resistance. Additionally, our imaging data showed that neuronal morphology is improved by EX-4 treatment, in spite of PA-induced neuronal damage. Furthermore, we identified that Ex-4 inhibits neuronal damage and enhanced neural complexity, such as neurite length, secondary branches, and number of neurites from soma in PA-treated SH-SY5Y. We observed that Ex-4 significantly increases neural complexity, dendritic spine morphogenesis, and development in PA treated primary cortical neurons. Hence, we suggest that GLP-1 administration may be a crucial therapeutic solution for improving neuropathology in the obese brain.

Distinct Relations of Microtubules and Actin Filaments with Dendritic Architecture

Microtubules (MTs) and F-actin (F-act) have long been recognized as key regulators of dendritic morphology. Nevertheless, precisely ascertaining their distinct influences on dendritic trees have been hampered until now by the lack of direct, arbor-wide cytoskeletal quantification. We pair live confocal imaging of fluorescently labeled dendritic arborization (da) neurons in Drosophila larvae with complete multi-signal neural tracing to separately measure MTs and F-act. We demonstrate that dendritic arbor length is highly interrelated with local MT quantity, whereas local F-act enrichment is associated with dendritic branching. Computational simulation of arbor structure solely constrained by experimentally observed subcellular distributions of these cytoskeletal components generated synthetic morphological and molecular patterns statistically equivalent to those of real da neurons, corroborating the efficacy of local MT and F-act in describing dendritic architecture. The analysis and modeling outcomes hold true for the simplest (class I), most complex (class IV), and genetically altered (Formin3 overexpression) da neuron types.

Experience-Dependent Development of Dendritic Arbors in Mouse Visual Cortex

The dendritic arbor of neurons constrains the pool of available synaptic partners and influences the electrical integration of synaptic currents. Despite these critical functions, our knowledge of the dendritic structure of cortical neurons during early postnatal development and how these dendritic structures are modified by visual experience is incomplete. Here, we present a large-scale dataset of 849 3D reconstructions of the basal arbor of pyramidal neurons collected across early postnatal development in visual cortex of mice of either sex. We found that the basal arbor grew substantially between postnatal day 7 (P7) and P30, undergoing a 45% increase in total length. However, the gross number of primary neurites and dendritic segments was largely determined by P7. Growth from P7 to P30 occurred primarily through extension of dendritic segments. Surprisingly, comparisons of dark-reared and typically reared mice revealed that a net gain of only 15% arbor length could be attributed to visual experience; most growth was independent of experience. To examine molecular contributions, we characterized the role of the activity-regulated small GTPase Rem2 in both arbor development and the maintenance of established basal arbors. We showed that Rem2 is an experience-dependent negative regulator of dendritic segment number during the visual critical period. Acute deletion of Rem2 reduced directionality of dendritic arbors. The data presented here establish a highly detailed, quantitative analysis of basal arbor development that we believe has high utility both in understanding circuit development as well as providing a framework for computationalists wishing to generate anatomically accurate neuronal models.SIGNIFICANCE STATEMENT Dendrites are the sites of the synaptic connections among neurons. Despite their importance for neural circuit function, only a little is known about the postnatal development of dendritic arbors of cortical pyramidal neurons and the influence of experience. Here we show that the number of primary basal dendritic arbors is already established before eye opening, and that these arbors primarily grow through lengthening of dendritic segments and not through addition of dendritic segments. Surprisingly, visual experience has a modest net impact on overall arbor length (15%). Experiments in KO animals revealed that the gene Rem2 is positive regulator of dendritic length and a negative regulator of dendritic segments.

DiNeR: a Differential graphical model for analysis of co-regulation Network Rewiring

BACKGROUND

During transcription, numerous transcription factors (TFs) bind to targets in a highly coordinated manner to control the gene expression. Alterations in groups of TF-binding profiles (i.e. "co-binding changes") can affect the co-regulating associations between TFs (i.e. "rewiring the co-regulator network"). This, in turn, can potentially drive downstream expression changes, phenotypic variation, and even disease. However, quantification of co-regulatory network rewiring has not been comprehensively studied.

RESULTS

To address this, we propose DiNeR, a computational method to directly construct a differential TF co-regulation network from paired disease-to-normal ChIP-seq data. Specifically, DiNeR uses a graphical model to capture the gained and lost edges in the co-regulation network. Then, it adopts a stability-based, sparsity-tuning criterion -- by sub-sampling the complete binding profiles to remove spurious edges -- to report only significant co-regulation alterations. Finally, DiNeR highlights hubs in the resultant differential network as key TFs associated with disease. We assembled genome-wide binding profiles of 104 TFs in the K562 and GM12878 cell lines, which loosely model the transition between normal and cancerous states in chronic myeloid leukemia (CML). In total, we identified 351 significantly altered TF co-regulation pairs. In particular, we found that the co-binding of the tumor suppressor BRCA1 and RNA polymerase II, a well-known transcriptional pair in healthy cells, was disrupted in tumors. Thus, DiNeR successfully extracted hub regulators and discovered well-known risk genes.

CONCLUSIONS

Our method DiNeR makes it possible to quantify changes in co-regulatory networks and identify alterations to TF co-binding patterns, highlighting key disease regulators. Our method DiNeR makes it possible to quantify changes in co-regulatory networks and identify alterations to TF co-binding patterns, highlighting key disease regulators.

Lhx2/9 and Etv1 Transcription Factors have Complementary roles in Regulating the Expression of Guidance Genes slit1 and sema3a

During neural network development, growing axons read a map of guidance cues expressed in the surrounding tissue that lead the axons toward their targets. In particular, Xenopus retinal ganglion axons use the cues Slit1 and Semaphorin 3a (Sema3a) at a key guidance decision point in the mid-diencephalon in order to continue on to their midbrain target, the optic tectum. The mechanisms that control the expression of these cues, however, are poorly understood. Extrinsic Fibroblast Growth Factor (Fgf) signals are known to help coordinate the development of the brain by regulating gene expression. Here, we propose Lhx2/9 and Etv1 as potential downstream effectors of Fgf signalling to regulate slit1 and sema3a expression in the Xenopus forebrain. We find that lhx2/9 and etv1 mRNAs are expressed complementary to and within slit1/sema3a expression domains, respectively. Our data indicate that Lhx2 functions as an indirect repressor in that lhx2 overexpression within the forebrain downregulates the mRNA expression of both guidance genes, and in vitro lhx2/9 overexpression decreases the activity of slit1 and sema3a promoters. The Lhx2-VP16 constitutive activator fusion reduces sema3a promoter function, and the Lhx2-En constitutive repressor fusion increases slit1 induction. In contrast, etv1 gain of function transactivates both guidance genes in vitro and in the forebrain. Based on these data, together with our previous work, we hypothesize that Fgf signalling promotes both slit1 and sema3a expression in the forebrain through Etv1, while using Lhx2/9 to limit the extent of expression, thereby establishing the proper boundaries of guidance cue expression.

C2H2-Type Zinc Finger Proteins in Brain Development, Neurodevelopmental, and Other Neuropsychiatric Disorders: Systematic Literature-Based Analysis

Neurodevelopmental disorders (NDDs) are multifaceted pathologic conditions manifested with intellectual disability, autistic features, psychiatric problems, motor dysfunction, and/or genetic/chromosomal abnormalities. They are associated with skewed neurogenesis and brain development, in part through dysfunction of the neural stem cells (NSCs) where abnormal transcriptional regulation on key genes play significant roles. Recent accumulated evidence highlights C₂H₂-type zinc finger proteins (C₂H₂-ZNFs), the largest transcription factor family in humans, as important targets for the pathologic processes associated with NDDs. In this review, we identified their significant accumulation (74 C₂H₂-ZNFs: ~10% of all human member proteins) in brain physiology and pathology. Specifically, we discuss their physiologic contribution to brain development, particularly focusing on their actions in NSCs. We then explain their pathologic implications in various forms of NDDs, such as morphological brain abnormalities, intellectual disabilities, and psychiatric disorders. We found an important tendency that poly-ZNFs and KRAB-ZNFs tend to be involved in the diseases that compromise gross brain structure and human-specific higher-order functions, respectively. This may be consistent with their characteristic appearance in the course of species evolution and corresponding contribution to these brain activities.

Lmx1b is required at multiple stages to build expansive serotonergic axon architectures

Formation of long-range axons occurs over multiple stages of morphological maturation. However, the intrinsic transcriptional mechanisms that temporally control different stages of axon projection development are unknown. Here, we addressed this question by studying the formation of mouse serotonin (5-HT) axons, the exemplar of long-range profusely arborized axon architectures. We report that LIM homeodomain factor 1b (Lmx1b)-deficient 5-HT neurons fail to generate axonal projections to the forebrain and spinal cord. Stage-specific targeting demonstrates that Lmx1b is required at successive stages to control 5-HT axon primary outgrowth, selective routing, and terminal arborization. We show a Lmx1b→Pet1 regulatory cascade is temporally required for 5-HT arborization and upregulation of the 5-HT axon arborization gene, Protocadherin-alphac2, during postnatal development of forebrain 5-HT axons. Our findings identify a temporal regulatory mechanism in which a single continuously expressed transcription factor functions at successive stages to orchestrate the progressive development of long-range axon architectures enabling expansive neuromodulation.

Transcriptomic correlates of electrophysiological and morphological diversity within and across excitatory and inhibitory neuron classes

In order to further our understanding of how gene expression contributes to key functional properties of neurons, we combined publicly accessible gene expression, electrophysiology, and morphology measurements to identify cross-cell type correlations between these data modalities. Building on our previous work using a similar approach, we distinguished between correlations which were "class-driven," meaning those that could be explained by differences between excitatory and inhibitory cell classes, and those that reflected graded phenotypic differences within classes. Taking cell class identity into account increased the degree to which our results replicated in an independent dataset as well as their correspondence with known modes of ion channel function based on the literature. We also found a smaller set of genes whose relationships to electrophysiological or morphological properties appear to be specific to either excitatory or inhibitory cell types. Next, using data from PatchSeq experiments, allowing simultaneous single-cell characterization of gene expression and electrophysiology, we found that some of the gene-property correlations observed across cell types were further predictive of within-cell type heterogeneity. In summary, we have identified a number of relationships between gene expression, electrophysiology, and morphology that provide testable hypotheses for future studies.

Stereotyped terminal axon branching of leg motor neurons mediated by IgSF proteins DIP-α and Dpr10

For animals to perform coordinated movements requires the precise organization of neural circuits controlling motor function. Motor neurons (MNs), key components of these circuits, project their axons from the central nervous system and form precise terminal branching patterns at specific muscles. Focusing on the Drosophila leg neuromuscular system, we show that the stereotyped terminal branching of a subset of MNs is mediated by interacting transmembrane Ig superfamily proteins DIP-α and Dpr10, present in MNs and target muscles, respectively. The DIP-α/Dpr10 interaction is needed only after MN axons reach the vicinity of their muscle targets. Live imaging suggests that precise terminal branching patterns are gradually established by DIP-α/Dpr10-dependent interactions between fine axon filopodia and developing muscles. Further, different leg MNs depend on the DIP-α and Dpr10 interaction to varying degrees that correlate with the morphological complexity of the MNs and their muscle targets.

MTOC Organization and Competition During Neuron Differentiation

Neurons are polarized cells with long branched axons and dendrites. Microtubule generation and organization machineries are crucial to grow and pattern these complex cellular extensions. Microtubule organizing centers (MTOCs) concentrate the molecular machinery for templating microtubules, stabilizing the nascent polymer, and organizing the resultant microtubules into higher-order structures. MTOC formation and function are well described at the centrosome, in the spindle, and at interphase Golgi; we review these studies and then describe recent results about how the machineries acting at these classic MTOCs are repurposed in the postmitotic neuron for axon and dendrite differentiation. We further discuss a constant tug-of-war interplay between different MTOC activities in the cell and how this process can be used as a substrate for transcription factor-mediated diversification of neuron types.

Recent advances in branching mechanisms underlying neuronal morphogenesis

Proper neuronal wiring is central to all bodily functions, sensory perception, cognition, memory, and learning. Establishment of a functional neuronal circuit is a highly regulated and dynamic process involving axonal and dendritic branching and navigation toward appropriate targets and connection partners. This intricate circuitry includes axo-dendritic synapse formation, synaptic connections formed with effector cells, and extensive dendritic arborization that function to receive and transmit mechanical and chemical sensory inputs. Such complexity is primarily achieved by extensive axonal and dendritic branch formation and pruning. Fundamental to neuronal branching are cytoskeletal dynamics and plasma membrane expansion, both of which are regulated via numerous extracellular and intracellular signaling mechanisms and molecules. This review focuses on recent advances in understanding the biology of neuronal branching.

Basal autophagy is required for promoting dendritic terminal branching in Drosophila sensory neurons

Dendrites function as the primary sites for synaptic input and integration with impairments in dendritic arborization being associated with dysfunctional neuronal circuitry. Post-mitotic neurons require high levels of basal autophagy to clear cytotoxic materials and autophagic dysfunction under native or cellular stress conditions has been linked to neuronal cell death as well as axo-dendritic degeneration. However, relatively little is known regarding the developmental role of basal autophagy in directing aspects of dendritic arborization or the mechanisms by which the autophagic machinery may be transcriptionally regulated to promote dendritic diversification. We demonstrate that autophagy-related (Atg) genes are positively regulated by the homeodomain transcription factor Cut, and that basal autophagy functions as a downstream effector pathway for Cut-mediated dendritic terminal branching in Drosophila multidendritic (md) sensory neurons. Further, loss of function analyses implicate Atg genes in promoting cell type-specific dendritic arborization and terminal branching, while gain of function studies suggest that excessive autophagy leads to dramatic reductions in dendritic complexity. We demonstrate that the Atg1 initiator kinase interacts with the dual leucine zipper kinase (DLK) pathway by negatively regulating the E3 ubiquitin ligase Highwire and positively regulating the MAPKKK Wallenda. Finally, autophagic induction partially rescues dendritic atrophy defects observed in a model of polyglutamine toxicity. Collectively, these studies implicate transcriptional control of basal autophagy in directing dendritic terminal branching and demonstrate the importance of homeostatic control of autophagic levels for dendritic arbor complexity under native or cellular stress conditions.

Myocyte Enhancer Factor 2c Regulates Dendritic Complexity and Connectivity of Cerebellar Purkinje Cells

Mef2c haploinsufficiency is implicated in behavioral deficits related to autism, schizophrenia, and intellectual disability. Although perturbations in the cerebellum, notably Purkinje cells, have been linked to these neurological disorders, the underlying mechanisms remain poorly understood. In this study, we investigated the roles of Mef2c in cerebellar Purkinje cells during the first three weeks of postnatal development. Our analysis revealed that in comparison to other members of the Mef2 family, Mef2c expression is limited to postnatal Purkinje cells. Because the role of Mef2c has not been assessed in GABAergic neurons, we set out to determine the functional significance of Mef2c by knocking down the expression of Mef2c selectively in Purkinje cells. We found that the loss of Mef2c expression during the first and second postnatal week results in an increase in dendritic arborization without impact on the general growth and migration of Purkinje cells. The influence of Mef2c on dendritic arborization persists throughout the first three weeks, but is most prominent during the first postnatal week suggesting a critical period of Mef2c activity. Additionally, the loss of Mef2c expression results in an increase in the number of spines accompanied by an increase in Gad67 and vGluT1 puncta and decrease in vGluT2 puncta. Thus, our results reveal the specific expression and functional relevance of Mef2c in developing Purkinje cells and offer insight to how disruption of the expression of Mef2c in a GABAergic neuronal subtype may lead to pathogenesis of cerebellar-associated disorders.

Phenotypic Convergence: Distinct Transcription Factors Regulate Common Terminal Features

Transcription factors regulate the molecular, morphological, and physiological characteristics of neurons and generate their impressive cell-type diversity. To gain insight into the general principles that govern how transcription factors regulate cell-type diversity, we used large-scale single-cell RNA sequencing to characterize the extensive cellular diversity in the Drosophila optic lobes. We sequenced 55,000 single cells and assigned them to 52 clusters. We validated and annotated many clusters using RNA sequencing of FACS-sorted single-cell types and cluster-specific genes. To identify transcription factors responsible for inducing specific terminal differentiation features, we generated a "random forest" model, and we showed that the transcription factors Apterous and Traffic-jam are required in many but not all cholinergic and glutamatergic neurons, respectively. In fact, the same terminal characters often can be regulated by different transcription factors in different cell types, arguing for extensive phenotypic convergence. Our data provide a deep understanding of the developmental and functional specification of a complex brain structure.

Dendritic Cytoskeletal Architecture Is Modulated by Combinatorial Transcriptional Regulation in Drosophila melanogaster

Transcription factors (TFs) have emerged as essential cell autonomous mediators of subtype specific dendritogenesis; however, the downstream effectors of these TFs remain largely unknown, as are the cellular events that TFs control to direct morphological change. As dendritic morphology is largely dictated by the organization of the actin and microtubule (MT) cytoskeletons, elucidating TF-mediated cytoskeletal regulatory programs is key to understanding molecular control of diverse dendritic morphologies. Previous studies in Drosophila melanogaster have demonstrated that the conserved TFs Cut and Knot exert combinatorial control over aspects of dendritic cytoskeleton development, promoting actin and MT-based arbor morphology, respectively. To investigate transcriptional targets of Cut and/or Knot regulation, we conducted systematic neurogenomic studies, coupled with in vivo genetic screens utilizing multi-fluor cytoskeletal and membrane marker reporters. These analyses identified a host of putative Cut and/or Knot effector molecules, and a subset of these putative TF targets converge on modulating dendritic cytoskeletal architecture, which are grouped into three major phenotypic categories, based upon neuromorphometric analyses: complexity enhancer, complexity shifter, and complexity suppressor. Complexity enhancer genes normally function to promote higher order dendritic growth and branching with variable effects on MT stabilization and F-actin organization, whereas complexity shifter and complexity suppressor genes normally function in regulating proximal-distal branching distribution or in restricting higher order branching complexity, respectively, with spatially restricted impacts on the dendritic cytoskeleton. Collectively, we implicate novel genes and cellular programs by which TFs distinctly and combinatorially govern dendritogenesis via cytoskeletal modulation.

Islet Coordinately Regulates Motor Axon Guidance and Dendrite Targeting through the Frazzled/DCC Receptor

Motor neuron axon targeting in the periphery is correlated with the positions of motor neuron inputs in the CNS, but how these processes are coordinated to form a myotopic map remains poorly understood. We show that the LIM homeodomain factor Islet (Isl) controls targeting of both axons and dendrites in Drosophila motor neurons through regulation of the Frazzled (Fra)/DCC receptor. Isl is required for fra expression in ventrally projecting motor neurons, and isl and fra mutants have similar axon guidance defects. Single-cell labeling indicates that isl and fra are also required for dendrite targeting in a subset of motor neurons. Finally, overexpression of Fra rescues axon and dendrite targeting defects in isl mutants. These results indicate that Fra acts downstream of Isl in both the periphery and the CNS, demonstrating how a single regulatory relationship is used in multiple cellular compartments to coordinate neural circuit wiring.

Satb1 Regulates Contactin 5 to Pattern Dendrites of a Mammalian Retinal Ganglion Cell

The size and shape of dendritic arbors are prime determinants of neuronal connectivity and function. We asked how ON-OFF direction-selective ganglion cells (ooDSGCs) in mouse retina acquire their bistratified dendrites, in which responses to light onset and light offset are segregated to distinct strata. We found that the transcriptional regulator Satb1 is selectively expressed by ooDSGCs. In Satb1 mutant mice, ooDSGC dendrites lack ON arbors, and the cells selectively lose ON responses. Satb1 regulates expression of a homophilic adhesion molecule, Contactin 5 (Cntn5). Both Cntn5 and its co-receptor Caspr4 are expressed not only by ooDSGCs, but also by interneurons that form a scaffold on which ooDSGC ON dendrites fasciculate. Removing Cntn5 from either ooDSGCs or interneurons partially phenocopies Satb1 mutants, demonstrating that Satb1-dependent Cntn5 expression in ooDSGCs leads to branch-specific homophilic interactions with interneurons. Thus, Satb1 directs formation of a morphologically and functionally specialized compartment within a complex dendritic arbor.

Sox11 Balances Dendritic Morphogenesis with Neuronal Migration in the Developing Cerebral Cortex

The coordinated mechanisms balancing promotion and suppression of dendritic morphogenesis are crucial for the development of the cerebral cortex. Although previous studies have revealed important transcription factors that promote dendritic morphogenesis during development, those that suppress dendritic morphogenesis are still largely unknown. Here we found that the expression levels of the transcription factor Sox11 decreased dramatically during dendritic morphogenesis. Our loss- and gain-of-function studies using postnatal electroporation and in utero electroporation indicate that Sox11 is necessary and sufficient for inhibiting dendritic morphogenesis of excitatory neurons in the mouse cerebral cortex during development. Interestingly, we found that precocious suppression of Sox11 expression caused precocious branching of neurites and a neuronal migration defect. We also found that the end of radial migration induced the reduction of Sox11 expression. These findings indicate that suppression of dendritic morphogenesis by Sox11 during radial migration is crucial for the formation of the cerebral cortex.

SIGNIFICANCE STATEMENT

Because dendritic morphology has profound impacts on neuronal information processing, the mechanisms underlying dendritic morphogenesis during development are of great interest. Our loss- and gain-of-function studies indicate that Sox11 is necessary and sufficient for inhibiting dendritic morphogenesis of excitatory neurons in the mouse cerebral cortex during development. Interestingly, we found that precocious suppression of Sox11 expression caused a neuronal migration defect. These findings indicate that suppression of dendritic morphogenesis by Sox11 during radial migration is crucial for the formation of the cerebral cortex.

Experience-Dependent and Differential Regulation of Local and Long-Range Excitatory Neocortical Circuits by Postsynaptic Mef2c

Development of proper cortical circuits requires an interaction of sensory experience and genetic programs. Little is known of how experience and specific transcription factors interact to determine the development of specific neocortical circuits. Here, we demonstrate that the activity-dependent transcription factor, Myocyte enhancer factor-2C (Mef2c), differentially regulates development of local versus long-range excitatory synaptic inputs onto layer 2/3 neurons in the somatosensory neocortex in vivo. Postnatal, postsynaptic deletion of Mef2c in a sparse population of L2/3 neurons suppressed development of excitatory synaptic connections from all local input pathways tested. In the same cell population, Mef2c deletion promoted the strength of excitatory inputs originating from contralateral neocortex. Both the synapse promoting and synapse suppressing effects of Mef2c deletion required normal whisking experience. These results reveal a role of Mef2c in experience-dependent development of specific sensory neocortical circuits.

The Drosophila Hox gene Ultrabithorax acts in both muscles and motoneurons to orchestrate formation of specific neuromuscular connections

Hox genes are known to specify motoneuron pools in the developing vertebrate spinal cord and to control motoneuronal targeting in several species. However, the mechanisms controlling axial diversification of muscle innervation patterns are still largely unknown. We present data showing that the Drosophila Hox gene Ultrabithorax (Ubx) acts in the late embryo to establish target specificity of ventrally projecting RP motoneurons. In abdominal segments A2 to A7, RP motoneurons innervate the ventrolateral muscles VL1-4, with VL1 and VL2 being innervated in a Wnt4-dependent manner. In Ubx mutants, these motoneurons fail to make correct contacts with muscle VL1, a phenotype partially resembling that of the Wnt4 mutant. We show that Ubx regulates expression of Wnt4 in muscle VL2 and that it interacts with the Wnt4 response pathway in the respective motoneurons. Ubx thus orchestrates the interaction between two cell types, muscles and motoneurons, to regulate establishment of the ventrolateral neuromuscular network.

Summary: Ultrabithorax controls correct innervation of ventrolateral muscles by coordinating Wnt4 ligand expression in muscles with the signalling pathway response in motoneurons.

In vivo matching of postsynaptic excitability with spontaneous synaptic inputs during formation of the rat calyx of Held synapse

KEY POINTS

Neurons in the medial nucleus of the trapezoid body of anaesthetized rats of postnatal day (P)2-6 showed burst firing with a preferred interval of about 100 ms, which was stable, and a second preferred interval of 5-30 ms, which shortened during development. In 3 out of 132 cases, evidence for the presence of two large inputs was found. In vivo whole-cell recordings revealed that the excitability of the principal neuron and the size of its largest synaptic inputs were developmentally matched. At P2-4, action potentials were triggered by barrages of small synaptic events that summated to plateau potentials, while at later stages firing depended on a single, large and often prespike-associated input, which is probably the nascent calyx of Held. Simulations with a Hodgkin-Huxley-like model, which was based on fits of the intrinsic postsynaptic properties, suggested an essential role for the low-threshold potassium conductance in this transition.

ABSTRACT

In the adult, principal neurons of the medial nucleus of the trapezoid body (MNTB) are typically contacted by a single, giant terminal called the calyx of Held, whereas during early development a principal neuron receives inputs from many axons. How these changes in innervation impact the postsynaptic activity has not yet been studied in vivo. We therefore recorded spontaneous inputs and intrinsic properties of principal neurons in anaesthetized rat pups during the developmental period in which the calyx forms. A characteristic bursting pattern could already be observed at postnatal day (P)2, before formation of the calyx. At this age, action potentials (APs) were triggered by barrages of summating EPSPs causing plateau depolarizations. In contrast, at P5, a single EPSP reliably triggered APs, resulting in a close match between pre- and postsynaptic firing. Postsynaptic excitability and the size of the largest synaptic events were developmentally matched. The developmental changes in intrinsic properties were estimated by fitting in vivo current injections to a Hodgkin-Huxley-type model of the principal neuron. Our simulations indicated that the developmental increases in I_h , low-threshold K⁺ channels and leak currents contributed to the reduction in postsynaptic excitability, but that low-threshold K⁺ channels specifically functioned as a dampening influence in the near-threshold range, thus precluding small inputs from triggering APs. Together, these coincident changes help to propagate bursting activity along the auditory brainstem, and are essential steps towards establishing the relay function of the calyx of Held synapse.

FoxO regulates microtubule dynamics and polarity to promote dendrite branching in Drosophila sensory neurons

The size and shape of dendrite arbors are defining features of neurons and critical determinants of neuronal function. The molecular mechanisms establishing arborization patterns during development are not well understood, though properly regulated microtubule (MT) dynamics and polarity are essential. We previously found that FoxO regulates axonal MTs, raising the question of whether it also regulates dendritic MTs and morphology. Here we demonstrate that FoxO promotes dendrite branching in all classes of Drosophila dendritic arborization (da) neurons. FoxO is required both for initiating growth of new branches and for maintaining existing branches. To elucidate FoxO function, we characterized MT organization in both foxO null and overexpressing neurons. We find that FoxO directs MT organization and dynamics in dendrites. Moreover, it is both necessary and sufficient for anterograde MT polymerization, which is known to promote dendrite branching. Lastly, FoxO promotes proper larval nociception, indicating a functional consequence of impaired da neuron morphology in foxO mutants. Together, our results indicate that FoxO regulates dendrite structure and function and suggest that FoxO-mediated pathways control MT dynamics and polarity.

Regulation of dendritic development by semaphorin 3A through novel intracellular remote signaling

Numerous cell adhesion molecules, extracellular matrix proteins and axon guidance molecules participate in neuronal network formation through local effects at axo-dendritic, axo-axonic or dendro-dendritic contact sites. In contrast, neurotrophins and their receptors play crucial roles in neural wiring by sending retrograde signals to remote cell bodies. Semaphorin 3A (Sema3A), a prototype of secreted type 3 semaphorins, is implicated in axon repulsion, dendritic branching and synapse formation via binding protein neuropilin-1 (NRP1) and the signal transducing protein PlexinAs (PlexAs) complex. This review focuses on Sema3A retrograde signaling that regulates dendritic localization of AMPA-type glutamate receptor GluA2 and dendritic patterning. This signaling is elicited by activation of NRP1 in growth cones and is propagated to cell bodies by dynein-dependent retrograde axonal transport of PlexAs. It also requires interaction between PlexAs and a high-affinity receptor for nerve growth factor, toropomyosin receptor kinase A. We propose a control mechanism by which retrograde Sema3A signaling regulates the glutamate receptor localization through trafficking of cis-interacting PlexAs with GluA2 along dendrites; this remote signaling may be an alternative mechanism to local adhesive contacts for neural network formation.

Temporal Dysynchrony in brain connectivity gene expression following hypoxia

Background

Despite the fundamental biological importance and clinical relevance of characterizing the effects of chronic hypoxia exposure on central nervous system (CNS) development, the changes in gene expression from hypoxia are unknown. It is not known if there are unifying principles, properties, or logic in the response of the developing CNS to hypoxic exposure. Here, we use the small vertebrate zebrafish (Danio rerio) to study the effects of hypoxia on connectivity gene expression across development. We perform transcriptional profiling at high temporal resolution to systematically determine and then experimentally validate the response of CNS connectivity genes to hypoxia exposure.

Results

We characterized mRNA changes during development, comparing the effects of chronic hypoxia exposure at different time-points. We focused on changes in expression levels of a subset of 1270 genes selected for their roles in development of CNS connectivity, including axon pathfinding and synapse formation. We found that the majority of CNS connectivity genes were unaffected by hypoxia. However, for a small subset of genes hypoxia significantly affected their gene expression profiles. In particular, hypoxia appeared to affect both the timing and levels of expression, including altering expression of interacting gene pairs in a fashion that would potentially disrupt normal function.

Conclusions

Overall, our study identifies the response of CNS connectivity genes to hypoxia exposure during development. While for most genes hypoxia did not significantly affect expression, for a subset of genes hypoxia changed both levels and timing of expression. Importantly, we identified that some genes with interacting proteins, for example receptor/ligand pairs, had dissimilar responses to hypoxia that would be expected to interfere with their function. The observed dysynchrony of gene expression could impair the development of normal CNS connectivity maps.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2638-x) contains supplementary material, which is available to authorized users.

Microtubule nucleation and organization in dendrites

Dendrite branching is an essential process for building complex nervous systems. It determines the number, distribution and integration of inputs into a neuron, and is regulated to create the diverse dendrite arbor branching patterns characteristic of different neuron types. The microtubule cytoskeleton is critical to provide structure and exert force during dendrite branching. It also supports the functional requirements of dendrites, reflected by differential microtubule architectural organization between neuron types, illustrated here for sensory neurons. Both anterograde and retrograde microtubule polymerization occur within growing dendrites, and recent studies indicate that branching is enhanced by anterograde microtubule polymerization events in nascent branches. The polarities of microtubule polymerization events are regulated by the position and orientation of microtubule nucleation events in the dendrite arbor. Golgi outposts are a primary microtubule nucleation center in dendrites and share common nucleation machinery with the centrosome. In addition, pre-existing dendrite microtubules may act as nucleation sites. We discuss how balancing the activities of distinct nucleation machineries within the growing dendrite can alter microtubule polymerization polarity and dendrite branching, and how regulating this balance can generate neuron type-specific morphologies.

Reciprocal autoregulation by NFI occupancy and ETV1 promotes the developmental expression of dendrite-synapse genes in cerebellar granule neurons

Temporal control of dendritogenesis is poorly understood. Mutual feedback between NFIA temporal occupancy and ETV1 drives the timing of gene expression associated with dendrite formation in maturing neurons. A sequential timing model is proposed in which ETV1 autoregulation precedes activation of downstream NFIA/ETV1 coregulated genes.

Nuclear Factor One (NFI) transcription factors regulate temporal gene expression required for dendritogenesis and synaptogenesis via delayed occupancy of target promoters in developing cerebellar granule neurons (CGNs). Mechanisms that promote NFI temporal occupancy have not been previously defined. We show here that the transcription factor ETV1 directly binds to and is required for expression and NFI occupancy of a cohort of NFI-dependent genes in CGNs maturing in vivo. Expression of ETV1 is low in early postnatal cerebellum and increases with maturation, mirroring NFI temporal occupancy of coregulated target genes. Precocious expression of ETV1 in mouse CGNs accelerated onset of expression and NFI temporal occupancy of late target genes and enhanced Map2(+) neurite outgrowth. ETV1 also activated expression and NFI occupancy of the Etv1 gene itself, and this autoregulatory loop preceded ETV1 binding and activation of other coregulated target genes in vivo. These findings suggest a potential model in which ETV1 activates NFI temporal binding to a subset of late-expressed genes in a stepwise manner by initial positive feedback regulation of the Etv1 gene itself followed by activation of downstream coregulated targets as ETV1 expression increases. Sequential transcription factor autoregulation and subsequent binding to downstream promoters may provide an intrinsic developmental timer for dendrite/synapse gene expression.

Hox Proteins Coordinate Motor Neuron Differentiation and Connectivity Programs through Ret/Gfrα Genes

The accuracy of neural circuit assembly relies on the precise spatial and temporal control of synaptic specificity determinants during development. Hox transcription factors govern key aspects of motor neuron (MN) differentiation; however, the terminal effectors of their actions are largely unknown. We show that Hox/Hox cofactor interactions coordinate MN subtype diversification and connectivity through Ret/Gfrα receptor genes. Hox and Meis proteins determine the levels of Ret in MNs and define the intrasegmental profiles of Gfrα1 and Gfrα3 expression. Loss of Ret or Gfrα3 leads to MN specification and innervation defects similar to those observed in Hox mutants, while expression of Ret and Gfrα1 can bypass the requirement for Hox genes during MN pool differentiation. These studies indicate that Hox proteins contribute to neuronal fate and muscle connectivity through controlling the levels and pattern of cell surface receptor expression, consequently gating the ability of MNs to respond to limb-derived instructive cues.