The actin-binding protein Canoe/AF-6 forms a complex with Robo and is required for Slit-Robo signaling during axon pathfinding at the CNS midline

SRGP-1/srGAP and AFD-1/afadin stabilize HMP-1/⍺-catenin at rosettes to seal internalization sites following gastrulation in C. elegans

A hallmark of gastrulation is the establishment of germ layers by internalization of cells initially on the exterior. In C. elegans the end of gastrulation is marked by the closure of the ventral cleft, a structure formed as cells internalize during gastrulation, and the subsequent rearrangement of adjacent neuroblasts that remain on the surface. We found that a nonsense allele of srgp-1/srGAP leads to 10-15% cleft closure failure. Deletion of the SRGP-1/srGAP C-terminal domain led to a comparable rate of cleft closure failure, whereas deletion of the N-terminal F-BAR region resulted in milder defects. Loss of the SRGP-1/srGAP C-terminus or F-BAR domain results in defects in rosette formation and defective clustering of HMP-1/⍺-catenin in surface cells during cleft closure. A mutant form of HMP-1/⍺-catenin with an open M domain can suppress cleft closure defects in srgp-1 mutant backgrounds, suggesting that this mutation acts as a gain-of-function allele. Since SRGP-1 binding to HMP-1/⍺-catenin is not favored in this case, we sought another HMP-1 interactor that might be recruited when HMP-1/⍺-catenin is constitutively open. A good candidate is AFD-1/afadin, which genetically interacts with cadherin-based adhesion later during embryonic elongation. AFD-1/afadin is prominently expressed at the vertex of neuroblast rosettes in wildtype, and depletion of AFD-1/afadin increases cleft closure defects in srgp-1/srGAP and hmp-1R551/554A/⍺-catenin backgrounds. We propose that SRGP-1/srGAP promotes nascent junction formation in rosettes; as junctions mature and sustain higher levels of tension, the M domain of HMP-1/⍺-catenin opens, allowing maturing junctions to transition from recruitment of SRGP-1/srGAP to AFD-1/afadin. Our work identifies new roles for ⍺-catenin interactors during a process crucial to metazoan development.

Drosophila Primary Neuronal Cultures as a Useful Cellular Model to Study and Image Axonal Transport

The use of primary neuronal cultures generated from Drosophila tissue provides a powerful model for studies of transport mechanisms. Cultured fly neurons provide similarly detailed subcellular resolution and applicability of pharmacology or fluorescent dyes as mammalian primary neurons. As an experimental advantage for the mechanistic dissection of transport, fly primary neurons can be combined with the fast and highly efficient combinatorial genetics of Drosophila, and genetic tools for the manipulation of virtually every fly gene are readily available. This strategy can be performed in parallel to in vivo transport studies to address relevance of any findings. Here we will describe the generation of primary neuronal cultures from Drosophila embryos and larvae, the use of external fluorescent dyes and genetic tools to label cargo, and the key strategies for live imaging and subsequent analysis.

Afadin Signaling at the Spinal Neuroepithelium Regulates Central Canal Formation and Gait Selection

Afadin, a scaffold protein controlling the activity of the nectin family of cell adhesion molecules, regulates important morphogenetic processes during development. In the central nervous system, afadin has critical roles in neuronal migration, axonal elongation, and synapse formation. Here we examine the role of afadin in development of spinal motor circuits. Afadin elimination in motor neuron progenitors results in striking locomotor behavior: left-right limb alternation is substituted by synchronous activation, characteristic of bound gait. We find that afadin function at the neuroepithelium is required for structural organization of the spinal midline and central canal morphogenesis. Perturbation of afadin results in formation of two central canals, aberrant contralateral wiring of different classes of spinal premotor interneurons, and loss of left-right limb alternation, highlighting important developmental principles controlling the assembly of spinal motor circuits.

System-Level Analysis of Alzheimer's Disease Prioritizes Candidate Genes for Neurodegeneration

Alzheimer’s disease (AD) is a debilitating neurodegenerative disorder. Since the advent of the genome-wide association study (GWAS) we have come to understand much about the genes involved in AD heritability and pathophysiology. Large case-control meta-GWAS studies have increased our ability to prioritize weaker effect alleles, while the recent development of network-based functional prediction has provided a mechanism by which we can use machine learning to reprioritize GWAS hits in the functional context of relevant brain tissues like the hippocampus and amygdala. In parallel with these developments, groups like the Alzheimer’s Disease Neuroimaging Initiative (ADNI) have compiled rich compendia of AD patient data including genotype and biomarker information, including derived volume measures for relevant structures like the hippocampus and the amygdala. In this study we wanted to identify genes involved in AD-related atrophy of these two structures, which are often critically impaired over the course of the disease. To do this we developed a combined score prioritization method which uses the cumulative distribution function of a gene’s functional and positional score, to prioritize top genes that not only segregate with disease status, but also with hippocampal and amygdalar atrophy. Our method identified a mix of genes that had previously been identified in AD GWAS including APOE, TOMM40, and NECTIN2(PVRL2) and several others that have not been identified in AD genetic studies, but play integral roles in AD-effected functional pathways including IQSEC1, PFN1, and PAK2. Our findings support the viability of our novel combined score as a method for prioritizing region- and even cell-specific AD risk genes.

Robo2 regulates synaptic oxytocin content by affecting actin dynamics

The regulation of neuropeptide level at the site of release is essential for proper neurophysiological functions. We focused on a prominent neuropeptide, oxytocin (OXT) in the zebrafish as an in vivo model to visualize and quantify OXT content at the resolution of a single synapse. We found that OXT-loaded synapses were enriched with polymerized actin. Perturbation of actin filaments by either cytochalasin-D or conditional Cofilin expression resulted in decreased synaptic OXT levels. Genetic loss of robo2 or slit3 displayed decreased synaptic OXT content and robo2 mutants displayed reduced mobility of the actin probe Lifeact-EGFP in OXT synapses. Using a novel transgenic reporter allowing real-time monitoring of OXT-loaded vesicles, we show that robo2 mutants display slower rate of vesicles accumulation. OXT-specific expression of dominant-negative Cdc42, which is a key regulator of actin dynamics and a downstream effector of Robo2, led to a dose-dependent increase in OXT content in WT, and a dampened effect in robo2 mutants. Our results link Slit3-Robo2-Cdc42, which controls local actin dynamics, with the maintenance of synaptic neuropeptide levels.

In Vivo Functional Analysis of Drosophila Robo1 Fibronectin Type-III Repeats

The repellant ligand Slit and its Roundabout (Robo) family receptors regulate midline crossing of axons during development of the embryonic central nervous system (CNS). Slit proteins are produced at the midline and signal through Robo receptors to repel axons from the midline. Disruption of Slit-Robo signaling causes ectopic midline-crossing phenotypes in the CNS of a broad range of animals, including insects and vertebrates. While previous studies have investigated the roles of Drosophila melanogaster Robo1's five Immunoglobulin-like (Ig) domains, little is known about the importance of the three evolutionarily conserved Fibronectin (Fn) type-III repeats. We have individually deleted each of Drosophila Robo1's three Fn repeats, and then tested these Robo1 variants in vitro to determine their ability to bind Slit in cultured Drosophila cells and in vivo to investigate the requirement for each domain in regulating Robo1's embryonic expression pattern, axonal localization, midline repulsive function, and sensitivity to Commissureless (Comm) downregulation. We demonstrate that the Fn repeats are not required for Robo1 to bind Slit or for proper expression of Robo1 in Drosophila embryonic neurons. When expressed in a robo1 mutant background, these variants are able to restore midline repulsion to an extent equivalent to full-length Robo1. We identify a novel requirement for Fn3 in the exclusion of Robo1 from commissures and downregulation of Robo1 by Comm. Our results indicate that each of the Drosophila Robo1 Fn repeats are individually dispensable for the protein's role in midline repulsion, despite the evolutionarily conserved "5 + 3" protein structure.

Control of nerve cord formation by Engrailed and Gooseberry-Neuro: A multi-step, coordinated process

One way to better understand the molecular mechanisms involved in the construction of a nervous system is to identify the downstream effectors of major regulatory proteins. We previously showed that Engrailed (EN) and Gooseberry-Neuro (GsbN) transcription factors act in partnership to drive the formation of posterior commissures in the central nervous system of Drosophila. In this report, we identified genes regulated by both EN and GsbN through chromatin immunoprecipitation ("ChIP on chip") and transcriptome experiments, combined to a genetic screen relied to the gene dose titration method. The genomic-scale approaches allowed us to define 175 potential targets of EN-GsbN regulation. We chose a subset of these genes to examine ventral nerve cord (VNC) defects and found that half of the mutated targets show clear VNC phenotypes when doubly heterozygous with en or gsbn mutations, or when homozygous. This strategy revealed new groups of genes never described for their implication in the construction of the nerve cord. Their identification suggests that, to construct the nerve cord, EN-GsbN may act at three levels, in: (i) sequential control of the attractive-repulsive signaling that ensures contralateral projection of the commissural axons, (ii) temporal control of the translation of some mRNAs, (iii) regulation of the capability of glial cells to act as commissural guideposts for developing axons. These results illustrate how an early, coordinated transcriptional control may orchestrate the various mechanisms involved in the formation of stereotyped neuronal networks. They also validate the overall strategy to identify genes that play crucial role in axonal pathfinding.

Midline axon guidance in the Drosophila embryonic central nervous system

Studies in the fruit fly Drosophila melanogaster have provided many fundamental insights into the genetic regulation of neural development, including the identification and characterization of evolutionarily conserved axon guidance pathways and their roles in important guidance decisions. Due to its highly organized and fast-developing embryonic nervous system, relatively small number of neurons, and molecular and genetic tools for identifying, labeling, and manipulating individual neurons or small neuronal subsets, studies of axon guidance in the Drosophila embryonic CNS have allowed researchers to dissect these genetic mechanisms with a high degree of precision. In this review, we discuss the major axon guidance pathways that regulate midline crossing of axons and the formation and guidance of longitudinal axon tracts, two processes that contribute to the development of the precise three-dimensional structure of the insect nerve cord. We focus particularly on recent insights into the roles and regulation of canonical midline axon guidance pathways, and on additional factors and pathways that have recently been shown to contribute to axon guidance decisions at and near the midline.

Robo Ig4 Is a Dimerization Domain

Robo receptors play pivotal roles in axonal guidance as well as in neurogenesis, angiogenesis, cell migration, and cancer progression and invasiveness. They are considered to be attractive drug targets for the treatment of cancer, ocular neovascular disorders, chronic kidney diseases, and more. Despite their great importance, the mechanisms by which Robo receptors switch from their "off" to "on" states remain obscure. One possibility involves a monomer-to-dimer or dimer-to-monomer transition that facilitates the recruitment and activation of enzymatic effectors to instigate intracellular signaling. However, it is not known which domains mediate Robo dimerization, or the structural properties of the dimeric interactions. Here, we identify the extracellular Ig4 (D4) as a Robo dimerization domain. We have determined the crystal structure of the tandem Ig4-5 domains (D4-5) of human Robo2 and found that a hydrophobic surface on D4 mediates close homotypic contacts with a reciprocal D4. Analytical ultracentrifugation measurements of intact and mutated D4-5 shows that dimerization through the D4 interface is specific and has a dimerization dissociation constant of 16.9μM in solution. Direct fluorescence resonance energy transfer dimerization measurements in HEK293 cells corroborate the dimerization of transmembrane hRobo2 through D4, and a functional COS-7 cell collapse assay links D4-mediated dimerization with Robo intracellular signaling. The high level of conservation in the D4 dimerization interface throughout all Robo orthologs and paralogs implies that D4-mediated dimerization is a central hallmark in Robo activation and signaling.

Compound Heterozygous Variants in ROBO1 Cause a Neurodevelopmental Disorder With Absence of Transverse Pontine Fibers and Thinning of the Anterior Commissure and Corpus Callosum

BACKGROUND

Axonal guidance disorders are characterized by white matter tracts with an anomalous course, failure to cross the midline, or presence of anomalous white matter tracts. Diffusion tensor imaging (DTI) is a suitable noninvasive, in vivo neuroimaging tool to study axonal guidance disorders. We describe a novel disorder in a boy with compound heterozygous variants in the ROBO1 gene.

PATIENT DESCRIPTION

The child was referred at age 13 months because of developmental delay. At age nine years, he had severe intellectual disability and hyperactivity. He was nonverbal and wheelchair dependent because of spastic diplegia and ataxia. Brain magnetic resonance imaging with DTI revealed marked pontine hypoplasia, thinning of the anterior commissure and corpus callosum, and absence of the transverse pontine fibers. In addition, at the level of the pons the corticospinal tracts and medial lemnisci were not clearly separated from each other. Whole exome sequencing revealed compound heterozygous variants in the ROBO1 gene.

CONCLUSION

This child's neuroimaging phenotype (absence of the transverse pontine fibers and thinning of the anterior commissure and corpus callosum as shown by DTI) is suggestive of an axonal guidance disorder and supports a pathogenic role of the compound heterozygous variants in the ROBO1 gene.

Disorders of Microtubule Function in Neurons: Imaging Correlates

BACKGROUND AND PURPOSE

A number of recent studies have described malformations of cortical development with mutations of components of microtubules and microtubule-associated proteins. Despite examinations of a large number of MRIs, good phenotype-genotype correlations have been elusive. Additionally, most of these studies focused exclusively on cerebral cortical findings. The purpose of this study was to characterize imaging findings associated with disorders of microtubule function.

MATERIALS AND METHODS

MRIs from 18 patients with confirmed tubulin mutations (8 TUBA1A, 5 TUBB2B, and 5 TUBB3) and 15 patients with known mutations of the genes encoding microtubule-associated proteins (5 LIS1, 4 DCX, and 6 DYNC1H1) were carefully visually analyzed and compared. Specific note was made of the cortical gyral pattern, basal ganglia, and white matter to assess internal capsular size, cortical thickness, ventricular and cisternal size, and the size and contours of the brain stem, cerebellar hemispheres and vermis, and the corpus callosum of patients with tubulin and microtubule-associated protein gene mutations. Results were determined by unanimous consensus of the authors.

RESULTS

All patients had abnormal findings on MR imaging. A large number of patients with tubulin gene mutations were found to have multiple cortical and subcortical abnormalities, including microcephaly, ventriculomegaly, abnormal gyral and sulcal patterns (termed "dysgyria"), a small or absent corpus callosum, and a small pons. All patients with microtubule-associated protein mutations also had abnormal cerebral cortices (predominantly pachygyria and agyria), but fewer subcortical abnormalities were noted.

CONCLUSIONS

Comparison of MRIs from patients with known mutations of tubulin genes and microtubule-associated proteins allows the establishment of some early correlations of phenotype with genotype and may assist in identification and diagnosis of these rare disorders.

Building Blocks of Functioning Brain: Cytoskeletal Dynamics in Neuronal Development

Neural connectivity requires proper polarization of neurons, guidance to appropriate target locations, and establishment of synaptic connections. From when neurons are born to when they finally reach their synaptic partners, neurons undergo constant rearrangment of the cytoskeleton to achieve appropriate shape and polarity. Of particular importance to neuronal guidance to target locations is the growth cone at the tip of the axon. Growth-cone steering is also dictated by the underlying cytoskeleton. All these changes require spatiotemporal control of the cytoskeletal machinery. This review summarizes the proteins that are involved in modulating the actin and microtubule cytoskeleton during the various stages of neuronal development.

Slit-Dependent Endocytic Trafficking of the Robo Receptor Is Required for Son of Sevenless Recruitment and Midline Axon Repulsion

Understanding how axon guidance receptors are activated by their extracellular ligands to regulate growth cone motility is critical to learning how proper wiring is established during development. Roundabout (Robo) is one such guidance receptor that mediates repulsion from its ligand Slit in both invertebrates and vertebrates. Here we show that endocytic trafficking of the Robo receptor in response to Slit-binding is necessary for its repulsive signaling output. Dose-dependent genetic interactions and in vitro Robo activation assays support a role for Clathrin-dependent endocytosis, and entry into both the early and late endosomes as positive regulators of Slit-Robo signaling. We identify two conserved motifs in Robo's cytoplasmic domain that are required for its Clathrin-dependent endocytosis and activation in vitro; gain of function and genetic rescue experiments provide strong evidence that these trafficking events are required for Robo repulsive guidance activity in vivo. Our data support a model in which Robo's ligand-dependent internalization from the cell surface to the late endosome is essential for receptor activation and proper repulsive guidance at the midline by allowing recruitment of the downstream effector Son of Sevenless in a spatially constrained endocytic trafficking compartment.

miR-124 controls Drosophila behavior and is required for neural development

MicroRNA-124 (miR-124) is an evolutionarily conserved, small, noncoding RNA molecule that participates in the central nervous system (CNS) developmental control of gene expression. In the current study, we found that Drosophila embryos lacking the mir-124 gene did not exhibit detectable defects in axon growth or CNS development. On the other hand, adult mutants showed severe problems in locomotion, flight, and female fertility. Furthermore, the deficits that we observed in the adult stage could be marginally rescued with elav-GAL4 driven expression of miR-124, making elav-GAL4 an excellently simulated driver to induce entopic over-expression of miR-124. Further developmental assessment in the third instar larval neuromuscular junction (NMJ) and dendritic arborization (DA) neurons was performed with miR-124 knock outs, flies over-expressing miR-124, and rescue models. Typically, the absence and over-abundance of a molecular signal exerts opposite effects on development or phenotype. However, we determined that both miR-124 knock-outs and over-expressing flies displayed reduced NMJ 6/7 bouton number and branch length. Similarly, reduced ddaE branching numbers were observed between the two mutant lines. As to ddaF, we found that branching number was not influenced by mir-124 knock out, but was statistically reduced by miR-124 over-expression. While we were not able to determine any causal relationship between behavioral defects and dysplasia of NMJs or DA neurons, there were some accompanying relationships among behavioral phenotypes, NMJ abnormalities, and ddaE/ddaF dendritic branching which were all controlled by miR-124.

Roundabout receptors

Roundabout receptors (Robo) and their Slit ligands were discovered in the 1990s and found to be key players in axon guidance. Slit was initially described s an extracellular matrix protein that was expressed by midline glia in Drosophila. A few years later, it was shown that, in vertebrates and invertebrates, Slits acted as chemorepellents for axons crossing the midline. Robo proteins were originally discovered in Drosophila in a mutant screen for genes involved in the regulation of midline crossing. This ligand-receptor pair has since been implicated in a variety of other neuronal and non-neuronal processes ranging from cell migration to angiogenesis, tumourigenesis and even organogenesis of tissues such as kidneys, lungs and breasts.

Formin-mediated actin polymerization cooperates with Mushroom body defect (Mud)-Dynein during Frizzled-Dishevelled spindle orientation

To position the mitotic spindle, cytoskeletal components must be coordinated to generate cortical forces on astral microtubules. Although the dynein motor is common to many spindle orientation systems, 'accessory pathways' are often also required. In this work, we identified an accessory spindle orientation pathway in Drosophila that functions with Dynein during planar cell polarity, downstream of the Frizzled (Fz) effector Dishevelled (Dsh). Dsh contains a PDZ ligand and a Dynein-recruiting DEP domain that are both required for spindle orientation. The Dsh PDZ ligand recruits Canoe/Afadin and ultimately leads to Rho GTPase signaling mediated through RhoGEF2. The formin Diaphanous (Dia) functions as the Rho effector in this pathway, inducing F-actin enrichment at sites of cortical Dsh. Chimeric protein experiments show that the Dia-actin accessory pathway can be replaced by an independent kinesin (Khc73) accessory pathway for Dsh-mediated spindle orientation. Our results define two 'modular' spindle orientation pathways and show an essential role for actin regulation in Dsh-mediated spindle orientation.

Expression, derivatization, crystallization and experimental phasing of an extracellular segment of the human Robo1 receptor

Robo receptors participate in the orchestration of several developmental responses, most notably axonal guidance in the central nervous system. Robo1 contains five tandem Ig-like and three fibronectin type-III (FnIII) domains in its ectodomain, followed by a single-pass transmembrane segment and an intracellular region. A human Robo1 construct that includes the two extracellular membrane-proximal fibronectin (Fn) domains and the juxtamembrane linker was overexpressed in Escherichia coli and purified. Crystals were obtained using the vapour-diffusion method at 293 K and X-ray diffraction data were collected. Molecular-replacement attempts using related Fn domains as search models did not result in a solution. After introducing two additional methionine residues using PCR site-directed mutagenesis, selenomethionine-derivative crystals were produced. These crystals belonged to the primitive orthorhombic space group P212121, with unit-cell parameters a = 27.24, b = 77.64, c = 91.91 Å. Assuming the presence of a monomer in the asymmetric unit gave a crystal volume per protein weight (VM) of 1.97 Å(3) Da(-1) and a solvent content of 37.6%. Anisotropic diffraction data and a fragmented single-wavelength anomalous dispersion electron-density map, to which homology-modelled domains were docked, were obtained.

Role of Slit and Robo proteins in the development of dopaminergic neurons

Dopamine plays a number of important roles in the nervous system and the dopaminergic system is affected in several brain disorders. It is therefore of great interest to study the axonal guidance systems that specifically participate in the correct establishment of dopaminergic projections during development and possibly during regenerative processes. In recent years, several reports have shown that Slits and their Robo receptors control the growth of longitudinal (both ascending and descending) mesodiencephalic dopaminergic axons to their appropriate target areas. In vitro studies have shown that Slit1, 2 and 3 are potent repellents of dopamine neurite extension. In vivo studies using both mice and zebrafish mutants for Slits and Robos have shown that Slits and Robos control the lateral and dorsoventral positioning of dopaminergic longitudinal projections during early development. In the present review, we aimed to compile the existing knowledge from both in vitro and in vivo studies on the role of Slit and Robo proteins in the development of dopaminergic neurons as a basis for future studies.

Using fly genetics to dissect the cytoskeletal machinery of neurons during axonal growth and maintenance

The extension of long slender axons is a key process of neuronal circuit formation, both during brain development and regeneration. For this, growth cones at the tips of axons are guided towards their correct target cells by signals. Growth cone behaviour downstream of these signals is implemented by their actin and microtubule cytoskeleton. In the first part of this Commentary, we discuss the fundamental roles of the cytoskeleton during axon growth. We present the various classes of actin- and microtubule-binding proteins that regulate the cytoskeleton, and highlight the important gaps in our understanding of how these proteins functionally integrate into the complex machinery that implements growth cone behaviour. Deciphering such machinery requires multidisciplinary approaches, including genetics and the use of simple model organisms. In the second part of this Commentary, we discuss how the application of combinatorial genetics in the versatile genetic model organism Drosophila melanogaster has started to contribute to the understanding of actin and microtubule regulation during axon growth. Using the example of dystonin-linked neuron degeneration, we explain how knowledge acquired by studying axonal growth in flies can also deliver new understanding in other aspects of neuron biology, such as axon maintenance in higher animals and humans.