The Robo3 receptor, a key player in the development, evolution, and function of commissural systems

Glu592 of the axon guidance receptor ROBO3 mediates a pH-dependent interaction with NELL2 ligand

There are only a few studies on the function of neuronal axon guidance molecules during low brain pH conditions. We previously reported that roundabout (ROBO) 2, a receptor for the axon guidance molecule SLIT, can bind to the neural epidermal growth factor-like-like (NELL) ligands in acidic conditions by conformational change of its ectodomain. Here, we show that the ROBO3 receptor also exhibits a pH-dependent increase in binding to the NELL2 ligand. We found that the Glu592 residue of ROBO3 at the binding interface between NELL2 and ROBO3 is a pH sensor and that the formation of a new hydrogen bonding network, due to protonation of the Glu592, leads to increased binding in acidic conditions. These results suggest that NELL2-ROBO3 signaling could be regulated by extracellular pH.

Thyroid hormone remodels cortex to coordinate body-wide metabolism and exploration

Animals adapt to environmental conditions by modifying the function of their internal organs, including the brain. To be adaptive, alterations in behavior must be coordinated with the functional state of organs throughout the body. Here, we find that thyroid hormone-a regulator of metabolism in many peripheral organs-directly activates cell-type-specific transcriptional programs in the frontal cortex of adult male mice. These programs are enriched for axon-guidance genes in glutamatergic projection neurons, synaptic regulatory genes in both astrocytes and neurons, and pro-myelination factors in oligodendrocytes, suggesting widespread plasticity of cortical circuits. Indeed, whole-cell electrophysiology revealed that thyroid hormone alters excitatory and inhibitory synaptic transmission, an effect that requires thyroid hormone-induced gene regulatory programs in presynaptic neurons. Furthermore, thyroid hormone action in the frontal cortex regulates innate exploratory behaviors and causally promotes exploratory decision-making. Thus, thyroid hormone acts directly on the cerebral cortex in males to coordinate exploratory behaviors with whole-body metabolic state.

Accelerated evolution in the human lineage led to gain and loss of transcriptional enhancers in the RBFOX1 locus

A long-standing goal of evolutionary biology is to decode how changes in gene regulatory networks contribute to human-specific traits. Human accelerated regions (HARs) are prime candidates for driving gene regulatory modifications in human development. The RBFOX1 locus is densely populated with HARs, providing a set of potential regulatory elements that could have changed its expression in the human lineage. Here, we examined the role of RBFOX1-HARs using transgenic zebrafish reporter assays and identified 15 transcriptional enhancers that are active in the developing nervous system, 9 of which displayed differential activity between the human and chimpanzee sequences. The engineered loss of two selected RBFOX1-HARs in knockout mouse models modified Rbfox1 expression at specific developmental stages and tissues in the brain, influencing the expression and splicing of a high number of Rbfox1 target genes. Our results provided insight into the spatial and temporal changes in gene expression driven by RBFOX1-HARs.

A global gene regulatory program and its region-specific regulator partition neurons into commissural and ipsilateral projection types

Understanding the genetic programs that drive neuronal diversification into classes and subclasses is key to understand nervous system development. All neurons can be classified into two types: commissural and ipsilateral, based on whether their axons cross the midline or not. However, the gene regulatory program underlying this binary division is poorly understood. We identified a pair of basic helix-loop-helix transcription factors, Nhlh1 and Nhlh2, as a global transcriptional mechanism that controls the laterality of all floor plate-crossing commissural axons in mice. Mechanistically, Nhlh1/2 play an essential role in the expression of Robo3, the key guidance molecule for commissural axon projections. This genetic program appears to be evolutionarily conserved in chick. We further discovered that Isl1, primarily expressed in ipsilateral neurons within neural tubes, negatively regulates the Robo3 induction by Nhlh1/2. Our findings elucidate a gene regulatory strategy where a conserved global mechanism intersects with neuron class-specific regulators to control the partitioning of neurons based on axon laterality.

Evolution and development of extraocular motor neurons, nerves and muscles in vertebrates

The purpose of this review is to analyze the origin of ocular motor neurons, define the pattern of innervation of nerve fibers that project to the extraocular eye muscles (EOMs), describe congenital disorders that alter the development of ocular motor neurons, and provide an overview of vestibular pathway inputs to ocular motor nuclei. Six eye muscles are innervated by axons of three ocular motor neurons, the oculomotor (CNIII), trochlear (CNIV), and abducens (CNVI) neurons. Ocular motor neurons (CNIII) originate in the midbrain and innervate the ipsilateral orbit, except for the superior rectus and the levator palpebrae, which are contralaterally innervated. Trochlear motor neurons (CNIV) originate at the midbrain-hindbrain junction and innervate the contralateral superior oblique muscle. Abducens motor neurons (CNVI) originate variously in the hindbrain of rhombomeres r4-6 that innervate the posterior (or lateral) rectus muscle and innervate the retractor bulbi. Genes allow a distinction between special somatic (CNIII, IV) and somatic (CNVI) ocular motor neurons. Development of ocular motor neurons and their axonal projections to the EOMs may be derailed by various genetic causes, resulting in the congenital cranial dysinnervation disorders. The ocular motor neurons innervate EOMs while the vestibular nuclei connect with the midbrain-brainstem motor neurons.

Molecular mechanisms of corpus callosum development: a four-step journey

The Corpus Callosum (CC) is a bundle of axons connecting the cerebral hemispheres. It is the most recent structure to have appeared during evolution of placental mammals. Its development is controlled by a very complex interplay of many molecules. In humans it contains almost 80% of all commissural axons in the brain. The formation of the CC can be divided into four main stages, each controlled by numerous intracellular and extracellular molecular factors. First, a newborn neuron has to specify an axon, leave proliferative compartments, the Ventricular Zone (VZ) and Subventricular Zone (SVZ), migrate through the Intermediate Zone (IZ), and then settle at the Cortical Plate (CP). During the second stage, callosal axons navigate toward the midline within a compact bundle. Next stage is the midline crossing into contralateral hemisphere. The last step is targeting a defined area and synapse formation. This review provides an insight into these four phases of callosal axons development, as well as a description of the main molecular players involved.

Thyroid hormone rewires cortical circuits to coordinate body-wide metabolism and exploratory drive

Animals adapt to varying environmental conditions by modifying the function of their internal organs, including the brain. To be adaptive, alterations in behavior must be coordinated with the functional state of organs throughout the body. Here we find that thyroid hormone- a prominent regulator of metabolism in many peripheral organs- activates cell-type specific transcriptional programs in anterior regions of cortex of adult mice via direct activation of thyroid hormone receptors. These programs are enriched for axon-guidance genes in glutamatergic projection neurons, synaptic regulators across both astrocytes and neurons, and pro-myelination factors in oligodendrocytes, suggesting widespread remodeling of cortical circuits. Indeed, whole-cell electrophysiology recordings revealed that thyroid hormone induces local transcriptional programs that rewire cortical neural circuits via pre-synaptic mechanisms, resulting in increased excitatory drive with a concomitant sensitization of recruited inhibition. We find that thyroid hormone bidirectionally regulates innate exploratory behaviors and that the transcriptionally mediated circuit changes in anterior cortex causally promote exploratory decision-making. Thus, thyroid hormone acts directly on adult cerebral cortex to coordinate exploratory behaviors with whole-body metabolic state.

A Genetic Variant of the ROBO3 Gene is Associated With Adolescent Idiopathic Scoliosis in the Chinese Population

STUDY DESIGN

A case-control association study.

OBJECTIVES

This study aimed to reveal whether mutations within roundabout receptor 3 ( ROBO3 ) gene were related to adolescent idiopathic scoliosis (AIS) in Chinese Han population and to investigate the functional role of ROBO3 in the pathogenesis and progression of AIS.

SUMMARY OF BACKGROUND DATA

ROBO3 is essential for the regulation of hindbrain axonal cell migration and midline crossing. Studies have demonstrated that ROBO3 homozygous mutations are associated with horizontal gaze palsy with progressive scoliosis. However, whether and how ROBO3 contributed to the development of scoliosis remains unclear.

MATERIALS AND METHODS

Whole exome sequencing was performed in 135 AIS patients and 267 healthy controls to evaluate the differences of single nucleotide polymorphism variants within ROBO3 . Then the identified variant of ROBO3 was genotyped in another cohort included 1140 AIS patients and 1580 controls. Moreover, paraspinal muscles were collected from 39 AIS patients and 45 lumbar disk herniation patients for the measurement of ROBO3 mRNA expression. The χ 2 test, Fisher exact test or the Student t test were used to compare intergroup data. Pearson correlation was used to determine the association between ROBO3 expression and clinical phenotypes.

RESULTS

A significant association was identified between the gene variant (rs74787566) of ROBO3 and the development of AIS through exome sequencing. The genotyping cohort demonstrated a higher frequency of allele A in AIS patients compared to controls (7.89% vs . 4.30%, P <0.001, odds ratio=1.87). In addition, the expression of ROBO3 in paraspinal muscles was inversely correlated with the Cobb angle ( P =0.043, r2 =0.1059).

CONCLUSION

A significant association was identified between the gene variant (rs74787566) of ROBO3 and the development of AIS. The reduced expression of ROBO3 could result in the progression of curve magnitude in patients with AIS. Further studies are needed to verify the functional role of ROBO3 in the development of AIS.

LEVEL OF EVIDENCE

Level III.

KLF4 inhibits early neural differentiation of ESCs by coordinating specific 3D chromatin structure

Neural differentiation of embryonic stem cells (ESCs) requires precisely orchestrated gene regulation, a process governed in part by changes in 3D chromatin structure. How these changes regulate gene expression in this context remains unclear. In this study, we observed enrichment of the transcription factor KLF4 at some poised or closed enhancers at TSS-linked regions of genes associated with neural differentiation. Combination analysis of ChIP, HiChIP and RNA-seq data indicated that KLF4 loss in ESCs induced changes in 3D chromatin structure, including increased chromatin interaction loops between neural differentiation-associated genes and active enhancers, leading to upregulated expression of neural differentiation-associated genes and therefore early neural differentiation. This study suggests KLF4 inhibits early neural differentiation by regulation of 3D chromatin structure, which is a new mechanism of early neural differentiation.

ROBO3s: a novel ROBO3 short isoform promoting breast cancer aggressiveness

Basal-like breast cancer (BLBC) is a highly aggressive breast cancer subtype frequently associated with poor prognosis. Due to the scarcity of targeted treatment options, conventional cytotoxic chemotherapies frequently remain the standard of care. Unfortunately, their efficacy is limited as BLBC malignancies rapidly develop resistant phenotypes. Using transcriptomic and proteomic approaches in human and murine BLBC cells, we aimed to elucidate the molecular mechanisms underlying the acquisition of aggressive and chemotherapy-resistant phenotypes in these mammary tumors. Specifically, we identified and characterized a novel short isoform of Roundabout Guidance Receptor 3 (ROBO3s), upregulated in BLBC in response to chemotherapy and encoding for a protein variant lacking the transmembrane domain. We established an important role for the ROBO3s isoform, mediating cancer stem cell properties by stimulating the Hippo-YAP signaling pathway, and thus driving resistance of BLBC cells to cytotoxic drugs. By uncovering the conservation of ROBO3s expression across multiple cancer types, as well as its association with reduced BLBC-patient survival, we emphasize its potential as a prognostic marker and identify a novel attractive target for anti-cancer drug development.

Uncoupling axon guidance and neuronal migration in Robo3-deficient inferior olivary neurons

Inferior olivary (IO) neurons are born in the dorsal hindbrain and migrate tangentially toward the ventral midline. During their dorsoventral migration, IO neurons extend long leading processes that cross the midline, transform into axons, and project into the contralateral cerebellum. In absence of the axon guidance receptor Robo3, IO axons fail to cross the midline and project to the ipsilateral cerebellum. Remarkably, the IO cell bodies still reach the midline where they form a nucleus of abnormal cytoarchitecture. The mechanisms underlying the migration of Robo3-deficient IO neurons are unknown. Here, we used three-dimensional imaging and transgenic mice to label subsets of IO neurons and study their migratory behavior in Robo3 knockout. We show that IO migration is delayed in absence of Robo3. Strikingly, Robo3-deficient IO neurons progress toward the midline in a direction opposite to their axons. This occurs through a change of polarity and the generation of a second leading process at the rear of the cell. These results suggest that Robo3 receptor controls the establishment of neuronal polarity and the coupling of axonogenesis and cell body migration in IO neurons.

N6-methyladenosine and Neurological Diseases

N6-methyladenosine (m6A) is a dynamic reversible methylation modification of the adenosine N6 position and is the most common chemical epigenetic modification among mRNA post-transcriptional modifications, including methylation, demethylation, and recognition. Post-transcriptional modification involves multiple protein molecules, including METTL3, METTL14, WTAP, KIAA1429, ALKBH5, YTHDF1/2/3, and YTHDC1/2. m6A-related proteins are expressed in almost all cells. However, the abnormal expression of m6A-related proteins may occur in the nervous system, thereby affecting neuritogenesis, brain volume, learning and memory, memory formation and consolidation, etc., and is implicated in the development of diseases, such as Parkinson's disease, Alzheimer's disease, multiple sclerosis, depression, epilepsy, and brain tumors. This review focuses on the functions of m6A in the development of central nervous system diseases, thus contributing to a deeper understanding of disease pathogenesis and providing potential clinical therapeutic targets for neurological diseases.

The Fine Art of Writing a Message: RNA Metabolism in the Shaping and Remodeling of the Nervous System

Neuronal morphogenesis, integration into circuits, and remodeling of synaptic connections occur in temporally and spatially defined steps. Accordingly, the expression of proteins and specific protein isoforms that contribute to these processes must be controlled quantitatively in time and space. A wide variety of post-transcriptional regulatory mechanisms, which act on pre-mRNA and mRNA molecules contribute to this control. They are thereby critically involved in physiological and pathophysiological nervous system development, function, and maintenance. Here, we review recent findings on how mRNA metabolism contributes to neuronal development, from neural stem cell maintenance to synapse specification, with a particular focus on axon growth, guidance, branching, and synapse formation. We emphasize the role of RNA-binding proteins, and highlight their emerging roles in the poorly understood molecular processes of RNA editing, alternative polyadenylation, and temporal control of splicing, while also discussing alternative splicing, RNA localization, and local translation. We illustrate with the example of the evolutionary conserved Musashi protein family how individual RNA-binding proteins are, on the one hand, acting in different processes of RNA metabolism, and, on the other hand, impacting multiple steps in neuronal development and circuit formation. Finally, we provide links to diseases that have been associated with the malfunction of RNA-binding proteins and disrupted post-transcriptional regulation.

Compound Heterozygous ROBO3 Mutation in Two Siblings Presenting with Horizontal Gaze Palsy without Scoliosis: Case-Based Review

Horizontal gaze palsy with progressive scoliosis (HGPPS) is a rare, autosomal recessively inherited disorder characterized by a congenital absence of conjugated horizontal eye movements with progressive scoliosis developing in childhood and adolescence. HGPPS is caused by mutations of the ROBO3 gene that disrupts the midline crossing of the descending corticospinal and ascending lemniscal sensory tracts in the medulla. We present two siblings, 5-year-old and 2-year-old boys with HGPPS, from non-consanguineous parents. The older brother was brought for the evaluation of moderate psychomotor retardation. He had bilateral horizontal gaze palsy with preserved vertical gaze and convergence. Scoliosis was absent. Cranial MRI showed brainstem abnormalities, and diffusion tensor imaging showed absent decussation of cortico-spinal tracts in the medulla. Clinical diagnosis of HGPPS was confirmed by sequencing of ROBO3 gene, IVS4–1G > A (c.767–1G > A) and c.328_329delinsCCC (p.Asp110Profs*57) compound heterozygous variations were found, and segregated in parents. The younger boy was first reported at 16 months of age and had the same clinical and neuroradiological findings, unlike mild psychomotor retardation. ROBO3 gene analysis showed the same variants in his brother. Our cases show the importance of evaluating eye movements in children with neurodevelopmental abnormalities and looking for brainstem abnormalities in children with bilateral horizontal gaze palsy.

Tissue clearing and 3D imaging in developmental biology

Tissue clearing increases the transparency of late developmental stages and enables deep imaging in fixed organisms. Successful implementation of these methodologies requires a good grasp of sample processing, imaging and the possibilities offered by image analysis. In this Primer, we highlight how tissue clearing can revolutionize the histological analysis of developmental processes and we advise on how to implement effective clearing protocols, imaging strategies and analysis methods for developmental biology.

The developmental hourglass model is applicable to the spinal cord based on single-cell transcriptomes and non-conserved cis-regulatory elements

The developmental hourglass model predicts that embryonic morphology is most conserved at the mid-embryonic stage and diverges at the early and late stages. To date, this model has been verified by examining the anatomical features or gene expression profiles at the whole embryonic level. Here, by data mining approach utilizing multiple genomic and transcriptomic datasets from different species in combination, and by experimental validation, we demonstrate that the hourglass model is also applicable to a reduced element, the spinal cord. In the middle of spinal cord development, dorsoventrally arrayed neuronal progenitor domains are established, which are conserved among vertebrates. By comparing the publicly available single-cell transcriptome datasets of mice and zebrafish, we found that ventral subpopulations of post-mitotic spinal neurons display divergent molecular profiles. We also detected the non-conservation of cis-regulatory elements located around the progenitor fate determinants, indicating that the cis-regulatory elements contributing to the progenitor specification are evolvable. These results demonstrate that, despite the conservation of the progenitor domains, the processes before and after the progenitor domain specification diverged. This study will be helpful to understand the molecular basis of the developmental hourglass model.

Robo2 Receptor Gates the Anatomical Divergence of Neurons Derived From a Common Precursor Origin

Sensory information relayed to the brain is dependent on complex, yet precise spatial organization of neurons. This anatomical complexity is generated during development from a surprisingly small number of neural stem cell domains. This raises the question of how neurons derived from a common precursor domain respond uniquely to their environment to elaborate correct spatial organization and connectivity. We addressed this question by exploiting genetically labeled mouse embryonic dorsal interneuron 1 (dI1) neurons that are derived from a common precursor domain and give rise to spinal projection neurons with distinct organization of cell bodies with axons projecting either commissurally (dI1c) or ipsilaterally (dI1i). In this study, we examined how the guidance receptor, Robo2, which is a canonical Robo receptor, influenced dI1 guidance during embryonic development. Robo2 was enriched in embryonic dI1i neurons, and loss of Robo2 resulted in misguidance of dI1i axons, whereas dI1c axons remained unperturbed within the mantle zone and ventral commissure. Further, Robo2 profoundly influenced dI1 cell body migration, a feature that was partly dependent on Slit2 signaling. These data suggest that dI1 neurons are dependent on Robo2 for their organization. This work integrated with the field support of a model whereby canonical Robo2 vs. non-canonical Robo3 receptor expression facilitates projection neurons derived from a common precursor domain to read out the tissue environment uniquely giving rise to correct anatomical organization.

Axonal Growth Abnormalities Underlying Ocular Cranial Nerve Disorders

Abnormalities in cranial motor nerve development cause paralytic strabismus syndromes, collectively referred to as congenital cranial dysinnervation disorders, in which patients cannot fully move their eyes. These disorders can arise through one of two mechanisms: (a) defective motor neuron specification, usually by loss of a transcription factor necessary for brainstem patterning, or (b) axon growth and guidance abnormalities of the oculomotor, trochlear, and abducens nerves. This review focuses on our current understanding of axon guidance mechanisms in the cranial motor nerves and how disease-causing mutations disrupt axon targeting. Abnormalities of axon growth and guidance are often limited to a single nerve or subdivision, even when the causative gene is ubiquitously expressed. Additionally, when one nerve is absent, its normal target muscles attract other motor neurons. Study of these disorders highlights the complexities of axon guidance and how each population of neurons uses a unique but overlapping set of axon guidance pathways. Expected final online publication date for the Annual Review of Vision Science, Volume 7 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Characterizing the diverse cells that associate with the developing commissures of the zebrafish forebrain

During embryonic development of bilaterally symmetrical organisms, neurons send axons across the midline at specific points to connect the two halves of the nervous system with a commissure. Little is known about the cells at the midline that facilitate this tightly regulated process. We exploit the conserved process of vertebrate embryonic development in the zebrafish model system to elucidate the identity of cells at the midline that may facilitate postoptic (POC) and anterior commissure (AC) development. We have discovered that three different gfap+ astroglial cell morphologies persist in contact with pathfinding axons throughout commissure formation. Similarly, olig2+ progenitor cells occupy delineated portions of the postoptic and anterior commissures where they act as multipotent, neural progenitors. Moreover, we conclude that both gfap+ and olig2+ progenitor cells give rise to neuronal populations in both the telencephalon and diencephalon; however, these varied cell populations showed significant developmental timing differences between the telencephalon and diencephalon. Lastly, we also showed that fli1a+ mesenchymal cells migrate along the presumptive commissure regions before and during midline axon crossing. Furthermore, following commissure maturation, specific blood vessels formed at the midline of the POC and immediately ventral and parallel to the AC. This comprehensive account of the cellular populations that correlate with the timing and position of commissural axon pathfinding has supported the conceptual modeling and identification of the early forebrain architecture that may be necessary for proper commissure development.

Neural EGFL like 2 expressed in myoepithelial cells and suppressed breast cancer cell migration

Breast tissue has a branching structure that contains double-layered cells, consisting primarily of luminal epithelial cells inside and myoepithelial cells outside. Ductal carcinoma in situ (DCIS) still has myoepithelial cells surrounding the cancer cells. However, myoepithelial cells disappear in invasive ductal carcinoma. In this study, we detected expression of neural EGFL like (NELL) 2 and one of its receptors, roundabout guidance receptor (ROBO) 3, in myoepithelial and luminal epithelial cells (respectively) in normal breast tissue. NELL2 also was expressed in myoepithelial cells surrounding the non-cancerous intraductal proliferative lesions and DCIS. However, the expression level and proportion of NELL2-positive cells in DCIS were lower than those in normal and non-cancerous intraductal proliferative lesions. ROBO3 expression was decreased in invasive ductal carcinoma compared to that in normal and non-cancerous intraductal proliferative lesions. An evaluation of NELL2's function in breast cancer cell lines demonstrated that full-length NELL2 suppressed cell adhesion and migration in vitro. In contrast, the N-terminal domain of NELL2 increased cell adhesion in the early phase and migration in vitro in some breast cancer cells. These results suggested that full-length NELL2 protein, when expressed in myoepithelial cells, might serve as an inhibitor of breast cancer cell migration.

A spinal organ of proprioception for integrated motor action feedback

Proprioception is essential for behavior and provides a sense of our body movements in physical space. Proprioceptor organs are thought to be only in the periphery. Whether the central nervous system can intrinsically sense its own movement remains unclear. Here we identify a segmental organ of proprioception in the adult zebrafish spinal cord, which is embedded by intraspinal mechanosensory neurons expressing Piezo2 channels. These cells are late-born, inhibitory, commissural neurons with unique molecular and physiological profiles reflecting a dual sensory and motor function. The central proprioceptive organ locally detects lateral body movements during locomotion and provides direct inhibitory feedback onto rhythm-generating interneurons responsible for the central motor program. This dynamically aligns central pattern generation with movement outcome for efficient locomotion. Our results demonstrate that a central proprioceptive organ monitors self-movement using hybrid neurons that merge sensory and motor entities into a unified network.

Family-effects in the epigenomic response of red blood cells to a challenge test in the European sea bass (Dicentrarchus labrax, L.)

Abstract

Background

In fish, minimally invasive blood sampling is widely used to monitor physiological stress with blood plasma biomarkers. As fish blood cells are nucleated, they might be a source a potential new markers derived from ‘omics technologies. We modified the epiGBS (epiGenotyping By Sequencing) technique to explore changes in genome-wide cytosine methylation in the red blood cells (RBCs) of challenged European sea bass (Dicentrarchus labrax), a species widely studied in both natural and farmed environments.

Results

We retrieved 501,108,033 sequencing reads after trimming, with a mean mapping efficiency of 73.0% (unique best hits). Minor changes in RBC methylome appeared to manifest after the challenge test and a family-effect was detected. Only fifty-seven differentially methylated cytosines (DMCs) close to 51 distinct genes distributed on 17 of 24 linkage groups (LGs) were detected between RBCs of pre- and post-challenge individuals. Thirty-seven of these genes were previously reported as differentially expressed in the brain of zebrafish, most of them involved in stress coping differences. While further investigation remains necessary, few DMC-related genes associated to the Brain Derived Neurotrophic Factor, a protein that favors stress adaptation and fear memory, appear relevant to integrate a centrally produced stress response in RBCs.

Conclusion

Our modified epiGBS protocol was powerful to analyze patterns of cytosine methylation in RBCs of D. labrax and to evaluate the impact of a challenge using minimally invasive blood samples. This study is the first approximation to identify epigenetic biomarkers of exposure to stress in fish.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07420-9.

New insights into the molecular mechanisms of axon guidance receptor regulation and signaling

As the nervous system develops, newly differentiated neurons need to extend their axons toward their synaptic targets to form functional neural circuits. During this highly dynamic process of axon pathfinding, guidance receptors expressed at the tips of motile axons interact with soluble guidance cues or membrane tethered molecules present in the environment to be either attracted toward or repelled away from the source of these cues. As competing cues are often present at the same location and during the same developmental period, guidance receptors need to be both spatially and temporally regulated in order for the navigating axons to make appropriate guidance decisions. This regulation is exerted by a diverse array of molecular mechanisms that have come into focus over the past several decades and these mechanisms ensure that the correct complement of surface receptors is present on the growth cone, a fan-shaped expansion at the tip of the axon. This dynamic, highly motile structure is defined by a lamellipodial network lining the periphery of the growth cone interspersed with finger-like filopodial projections that serve to explore the surrounding environment. Once axon guidance receptors are deployed at the right place and time at the growth cone surface, they respond to their respective ligands by initiating a complex set of signaling events that serve to rearrange the growth cone membrane and the actin and microtubule cytoskeleton to affect axon growth and guidance. In this review, we highlight recent advances that shed light on the rich complexity of mechanisms that regulate axon guidance receptor distribution, activation and downstream signaling.

SlitC-PlexinA1 mediates iterative inhibition for orderly passage of spinal commissural axons through the floor plate

Spinal commissural axon navigation across the midline in the floor plate requires repulsive forces from local Slit repellents. The long-held view is that Slits push growth cones forward and prevent them from turning back once they became sensitized to these cues after midline crossing. We analyzed with fluorescent reporters Slits distribution and FP glia morphology. We observed clusters of Slit-N and Slit-C fragments decorating a complex architecture of glial basal process ramifications. We found that PC2 proprotein convertase activity contributes to this pattern of ligands. Next, we studied Slit-C acting via PlexinA1 receptor shared with another FP repellent, the Semaphorin3B, through generation of a mouse model baring PlexinA1Y1815F mutation abrogating SlitC but not Sema3B responsiveness, manipulations in the chicken embryo, and ex vivo live imaging. This revealed a guidance mechanism by which SlitC constantly limits growth cone exploration, imposing ordered and forward-directed progression through aligned corridors formed by FP basal ramifications.

Uncovering memory-related gene expression in contextual fear conditioning using ribosome profiling

Contextual fear conditioning (CFC) in rodents is the most widely used behavioural paradigm in neuroscience research to elucidate the neurobiological mechanisms underlying learning and memory. It is based on the pairing of an aversive unconditioned stimulus (US; e.g. mild footshock) with a neutral conditioned stimulus (CS; e.g. context of the test chamber) in order to acquire associative long-term memory (LTM), which persists for days and even months. Using genome-wide analysis, several studies have generated lists of genes modulated in response to CFC in an attempt to identify the “memory genes”, which orchestrate memory formation. Yet, most studies use naïve animals as a baseline for assessing gene-expression changes, while only few studies have examined the effect of the US alone, without pairing to context, using genome-wide analysis of gene-expression. Herein, using the ribosome profiling methodology, we show that in male mice an immediate shock, which does not lead to LTM formation, elicits pervasive translational and transcriptional changes in the expression of Immediate Early Genes (IEGs) in dorsal hippocampus (such as Fos and Arc), a fact which has been disregarded by the majority of CFC studies. By removing the effect of the immediate shock, we identify and validate a new set of genes, which are translationally and transcriptionally responsive to the association of context-to-footshock in CFC, and thus constitute salient “memory genes”.

Molecular mechanisms regulating axon responsiveness at the midline

During embryonic development in bilaterally symmetric organisms, correct midline crossing is important for the proper formation of functional neural circuits. The aberrant development of neural circuits can result in multiple neurodevelopmental disorders, including horizontal gaze palsy, congenital mirror movement disorder, and autism spectrum disorder. Thus, understanding the molecular mechanisms that regulate proper axon guidance at the midline can provide insights into the pathology of neurological disorders. The signaling mechanisms that regulate midline crossing have been extensively studied in the Drosophila ventral nerve cord and the mouse embryonic spinal cord. In this review, we discuss these axon guidance mechanisms, highlighting the most recent advances in the understanding of how commissural axons switch their responsiveness from attractants to repellents during midline crossing.

Mutation in ROBO3 Gene in Patients with Horizontal Gaze Palsy with Progressive Scoliosis Syndrome: A Systematic Review

Horizontal gaze palsy with progressive scoliosis (HGPPS) is a rare, inherited disorder characterized by a congenital absence of conjugate horizontal eye movements with progressive scoliosis developing in childhood and adolescence. Mutations in the Roundabout (ROBO3) gene located on chromosome 11q23-25 are responsible for the development of horizontal gaze palsy and progressive scoliosis. However, some studies redefined the locus responsible for this pathology to a 9-cM region. This study carried out a systematic review in which 25 documents were analyzed, following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) standards. The search was made in the following electronic databases from January 1995 to October 2019: PubMed, Scopus, Web of Science, PEDRO, SPORT Discus, and CINAHL. HGPPS requires a multidisciplinary diagnostic approach, in which magnetic resonance imaging might be the first technique to suggest the diagnosis, which should be verified by an analysis of the ROBO3 gene. This is important to allow for adequate ocular follow up, apply supportive therapies to prevent the rapid progression of scoliosis, and lead to appropriate genetic counseling.

Roles of axon guidance molecules in neuronal wiring in the developing spinal cord

The spinal cord receives, relays and processes sensory information from the periphery and integrates this information with descending inputs from supraspinal centres to elicit precise and appropriate behavioural responses and orchestrate body movements. Understanding how the spinal cord circuits that achieve this integration are wired during development is the focus of much research interest. Several families of proteins have well-established roles in guiding developing spinal cord axons, and recent findings have identified new axon guidance molecules. Nevertheless, an integrated view of spinal cord network development is lacking, and many current models have neglected the cellular and functional diversity of spinal cord circuits. Recent advances challenge the existing spinal cord axon guidance dogmas and have provided a more complex, but more faithful, picture of the ontogenesis of vertebrate spinal cord circuits.

Clinical and Genetic Heterogeneity in Six Tunisian Families With Horizontal Gaze Palsy With Progressive Scoliosis: A Retrospective Study of 13 Cases

Background: Horizontal Gaze Palsy with Progressive Scoliosis (HGPPS) is a rare autosomal recessive congenital disorder characterized by the absence of conjugate horizontal eye movements, and progressive debilitating scoliosis during childhood and adolescence. HGPPS is associated with mutations of the ROBO3 gene. In this study, the objective is to identify pathogenic variants in a cohort of Tunisian patients with HGPPS and to further define ROBO3 genotype-phenotype correlations. Methods: Thirteen Tunisian patients from six unrelated consanguineous families all manifesting HGPPS were genetically investigated. We searched for the causative variants for HGPPS using classical Sanger and whole exome sequencing. Results: Four distinct homozygous mutations were identified in ROBO3 gene. Two of these were newly identified homozygous and non-synonymous mutations, causing effectively damage to the protein by in silico analysis. The other two mutations were previously reported in Tunisian patients with HGPPS. Mutations were validated by Sanger sequencing in parents and affected individuals. Conclusion: To the best of our knowledge, this is the largest ever reported cohort on families with HGPPS in whom ROBO3 mutations were identified. These molecular findings have expanded our knowledge of the ROBO3 mutational spectrum. The relevance of our current study is two-fold; first to assist proper management of the scoliosis and second to protect families at risk.

Construction and reconstruction of brain circuits: normal and pathological axon guidance

Perception of our environment entirely depends on the close interaction between the central and peripheral nervous system. In order to communicate each other, both systems must develop in parallel and in coordination. During development, axonal projections from the CNS as well as the PNS must extend over large distances to reach their appropriate target cells. To do so, they read and follow a series of axon guidance molecules. Interestingly, while these molecules play critical roles in guiding developing axons, they have also been shown to be critical in other major neurodevelopmental processes, such as the migration of cortical progenitors. Currently, a major hurdle for brain repair after injury or neurodegeneration is the absence of axonal regeneration in the mammalian CNS. By contrasts, PNS axons can regenerate. Many hypotheses have been put forward to explain this paradox but recent studies suggest that hacking neurodevelopmental mechanisms may be the key to promote CNS regeneration. Here we provide a seminar report written by trainees attending the second Flagship school held in Alpbach, Austria in September 2018 organized by the International Society for Neurochemistry (ISN) together with the Journal of Neurochemistry (JCN). This advanced school has brought together leaders in the fields of neurodevelopment and regeneration in order to discuss major keystones and future challenges in these respective fields.

The m6A reader YTHDF1 regulates axon guidance through translational control of Robo3.1 expression

N 6-Methyladenosine (m6A) is a dynamic mRNA modification which regulates protein expression in various posttranscriptional levels. Functional studies of m6A in nervous system have focused on its writers and erasers so far, whether and how m6A readers mediate m6A functions through recognizing and binding their target mRNA remains poorly understood. Here, we find that the expression of axon guidance receptor Robo3.1 which plays important roles in midline crossing of spinal commissural axons is regulated precisely at translational level. The m6A reader YTHDF1 binds to and positively regulates translation of m6A-modified Robo3.1 mRNA. Either mutation of m6A sites in Robo3.1 mRNA or YTHDF1 knockdown or knockout leads to dramatic reduction of Robo3.1 protein without affecting Robo3.1 mRNA level. Specific ablation of Ythdf1 in spinal commissural neurons results in pre-crossing axon guidance defects. Our findings identify a mechanism that YTHDF1-mediated translation of m6A-modified Robo3.1 mRNA controls pre-crossing axon guidance in spinal cord.

GnRH-1 Neural Migration From the Nose to the Brain Is Independent From Slit2, Robo3 and NELL2 Signaling

Gonadotropin releasing hormone-1 (GnRH-1) neurons play a pivotal role in controlling pubertal onset and fertility once they reach their hypothalamic location. During embryonic development, GnRH-1 neurons migrate from the nasal area to the hypothalamus where they modulate gonadotropin release from the pituitary gland. Defective migration of the GnRH-1 neurons to the brain, lack of GnRH-1 secretion or signaling cause hypogonadotropic hypogonadism (HH), a pathology characterized by delayed or absence of puberty. Binding of the guidance cue Slit2 to the receptor roundabout 3 (Robo3) has been proposed to modulate GnRH-1 cell motility and basal forebrain (bFB) access during migration. However, evidence suggests that Neural EGFL Like 2 (NELL2), not Slit2, binds to Robo3. To resolve this discrepancy, we analyzed GnRH-1 neuronal migration in NELL2, Robo3, and Slit2 knock-out mouse lines. Our data do not confirm a negative effect for monogenic Robo3 and Slit2 mutations on GnRH-1 neuronal migration from the nasal area to the brain. Moreover, we found no changes in GnRH-1 neuronal migration in the brain after NELL2 loss-of-function. However, we found that Slit2 loss-of-function alters the patterning of GnRH-1 cells in the brain, suggesting that Slit2 loss-of-function affects GnRH-1 cell positioning in the brain in a Robo3 independent fashion. Our results challenge previous theories on GnRH-1 neuronal migration mechanisms and provide a new impetus to identify and understand the complex genetic mechanisms causing disorders like Kallmann syndrome (KS) and HH.

Keratan sulfate (KS)-proteoglycans and neuronal regulation in health and disease: the importance of KS-glycodynamics and interactive capability with neuroregulatory ligands

Compared to the other classes of glycosaminoglycans (GAGs), that is, chondroitin/dermatan sulfate, heparin/heparan sulfate and hyaluronan, keratan sulfate (KS), have the least known of its interactive properties. In the human body, the cornea and the brain are the two most abundant tissue sources of KS. Embryonic KS is synthesized as a linear poly-N-acetyllactosamine chain of d-galactose-GlcNAc repeat disaccharides which become progressively sulfated with development, sulfation of GlcNAc is more predominant than galactose. KS contains multi-sulfated high-charge density, monosulfated and non-sulfated poly-N-acetyllactosamine regions and thus is a heterogeneous molecule in terms of chain length and charge distribution. A recent proteomics study on corneal KS demonstrated its interactivity with members of the Slit-Robbo and Ephrin-Ephrin receptor families and proteins which regulate Rho GTPase signaling and actin polymerization/depolymerization in neural development and differentiation. KS decorates a number of peripheral nervous system/CNS proteoglycan (PG) core proteins. The astrocyte KS-PG abakan defines functional margins of the brain and is up-regulated following trauma. The chondroitin sulfate/KS PG aggrecan forms perineuronal nets which are dynamic neuroprotective structures with anti-oxidant properties and roles in neural differentiation, development and synaptic plasticity. Brain phosphacan a chondroitin sulfate, KS, HNK-1 PG have roles in neural development and repair. The intracellular microtubule and synaptic vesicle KS-PGs MAP1B and SV2 have roles in metabolite transport, storage, and export of neurotransmitters and cytoskeletal assembly. MAP1B has binding sites for tubulin and actin through which it promotes cytoskeletal development in growth cones and is highly expressed during neurite extension. The interactive capability of KS with neuroregulatory ligands indicate varied roles for KS-PGs in development and regenerative neural processes.

A conserved role for Syntaxin-1 in pre- and post-commissural midline axonal guidance in fly, chick, and mouse

Axonal growth and guidance rely on correct growth cone responses to guidance cues. Unlike the signaling cascades that link axonal growth to cytoskeletal dynamics, little is known about the crosstalk mechanisms between guidance and membrane dynamics and turnover. Recent studies indicate that whereas axonal attraction requires exocytosis, chemorepulsion relies on endocytosis. Indeed, our own studies have shown that Netrin-1/Deleted in Colorectal Cancer (DCC) signaling triggers exocytosis through the SNARE Syntaxin-1 (STX1). However, limited in vivo evidence is available about the role of SNARE proteins in axonal guidance. To address this issue, here we systematically deleted SNARE genes in three species. We show that loss-of-function of STX1 results in pre- and post-commissural axonal guidance defects in the midline of fly, chick, and mouse embryos. Inactivation of VAMP2, Ti-VAMP, and SNAP25 led to additional abnormalities in axonal guidance. We also confirmed that STX1 loss-of-function results in reduced sensitivity of commissural axons to Slit-2 and Netrin-1. Finally, genetic interaction studies in Drosophila show that STX1 interacts with both the Netrin-1/DCC and Robo/Slit pathways. Our data provide evidence of an evolutionarily conserved role of STX1 and SNARE proteins in midline axonal guidance in vivo, by regulating both pre- and post-commissural guidance mechanisms.

Syntaxin-1 is a core factor in tethering synaptic vesicles and mediating their fusion to the cell membrane at the synapse. Thus, Syntaxin-1 mediates neurotransmission in the adult nervous system. Here we show that this protein is also involved in axonal guidance in the CNS of vertebrates and invertebrates during the development of the nervous system: our systematic analysis of the phenotypes in the nervous system midline of fly, chick, and mouse embryos mutant for Syntaxin-1 unveils an evolutionarily conserved role for this protein in midline axonal guidance. Further, we also dissect the contribution of other proteins regulating neuronal exocytosis in axonal development. We propose that the coupling of the guidance molecule machinery to proteins that regulate exocytosis is a general mechanism linking chemotropism to axonal growth.

Understanding axon guidance: are we nearly there yet?

During nervous system development, neurons extend axons to reach their targets and form functional circuits. The faulty assembly or disintegration of such circuits results in disorders of the nervous system. Thus, understanding the molecular mechanisms that guide axons and lead to neural circuit formation is of interest not only to developmental neuroscientists but also for a better comprehension of neural disorders. Recent studies have demonstrated how crosstalk between different families of guidance receptors can regulate axonal navigation at choice points, and how changes in growth cone behaviour at intermediate targets require changes in the surface expression of receptors. These changes can be achieved by a variety of mechanisms, including transcription, translation, protein-protein interactions, and the specific trafficking of proteins and mRNAs. Here, I review these axon guidance mechanisms, highlighting the most recent advances in the field that challenge the textbook model of axon guidance.

Commissural neurons transgress the CNS/PNS boundary in absence of ventricular zone-derived netrin 1

During the development of the central nervous system (CNS), only motor axons project into peripheral nerves. Little is known about the cellular and molecular mechanisms that control the development of a boundary at the CNS surface and prevent CNS neuron emigration from the neural tube. It has previously been shown that a subset of spinal cord commissural axons abnormally invades sensory nerves in Ntn1 hypomorphic embryos and Dcc knockouts. However, whether netrin 1 also plays a similar role in the brain is unknown. In the hindbrain, precerebellar neurons migrate tangentially under the pial surface, and their ventral migration is guided by netrin 1. Here, we show that pontine neurons and inferior olivary neurons, two types of precerebellar neurons, are not confined to the CNS in Ntn1 and Dcc mutant mice, but that they invade the trigeminal, auditory and vagus nerves. Using a Ntn1 conditional knockout, we show that netrin 1, which is released at the pial surface by ventricular zone progenitors is responsible for the CNS confinement of precerebellar neurons. We propose, that netrin 1 distribution sculpts the CNS boundary by keeping CNS neurons in netrin 1-rich domains.

Behavioral Consequences of a Bifacial Map in the Mouse Somatosensory Cortex

The whisker system is an important sensory organ with extensive neural representations in the brain of the mouse. Patterned neural modules (barrelettes) in the ipsilateral principal sensory nucleus of the trigeminal nerve (PrV) correspond to the whiskers. Axons of the PrV barrelette neurons cross the midline and confer the whisker-related patterning to the contralateral ventroposteromedial nucleus of the thalamus, and subsequently to the cortex. In this way, specific neural modules called barreloids and barrels in the contralateral thalamus and cortex represent each whisker. Partial midline crossing of the PrV axons, in a conditional Robo3 mutant (Robo3R3-5cKO) mouse line, leads to the formation of bilateral whisker maps in the ventroposteromedial, as well as the barrel cortex. We used voltage-sensitive dye optical imaging and somatosensory and motor behavioral tests to characterize the consequences of bifacial maps in the thalamocortical system. Voltage-sensitive dye optical imaging verified functional, bilateral whisker representation in the barrel cortex and activation of distinct cortical loci following ipsilateral and contralateral stimulation of the specific whiskers. The mutant animals were comparable with the control animals in sensorimotor tests. However, they showed noticeable deficits in all of the whisker-dependent or -related tests, including Y-maze exploration, horizontal surface approach, bridge crossing, gap crossing, texture discrimination, floating in water, and whisking laterality. Our results indicate that bifacial maps along the thalamocortical system do not offer a functional advantage. Instead, they lead to impairments, possibly due to the smaller size of the whisker-related modules and interference between the ipsilateral and contralateral whisker representations in the same thalamus and cortex.SIGNIFICANCE STATEMENT The whisker sensory system plays a quintessentially important role in exploratory behavior of mice and other nocturnal rodents. Here, we studied a novel mutant mouse line, in which the projections from the brainstem to the thalamus are disrupted. This led to formation of bilateral whisker maps in both the thalamus and the cortex. The two whisker maps crowd in a space normally devoted to the contralateral map alone and in a nonoverlapping fashion. Stimulation of the whiskers on either side activates the corresponding region of the map. Mice with bilateral whisker maps perform well in general sensorimotor tasks but show poor performance in specific tests that require whisker-dependent tactile discrimination. These observations indicate that contralateral, instead of bilateral, representation of the sensory space plays a critical role in acuity and fine discrimination during somesthesis.