The structural role of radial glial endfeet in confining spinal motor neuron somata is controlled by the Reelin and Notch pathways

SASH1 contributes to glial cell migration in the early development of the central nervous system

SAM and SH3 domain-containing 1 (SASH1), a member of the SLy protein family, is a tumor suppressor gene that has been studied for its association with various cancers. SASH1 is highly expressed in the mammalian central nervous system, particularly in glial cells, and is expressed in the central nervous system during zebrafish embryo development. However, SASH1's role in brain development has rarely been investigated. In this study, Morpholino oligonucleotides (MO) were used to down-regulate sash1a expression in zebrafish to observe morphological changes in the brain. Three transgenic zebrafish lines, Tg(gfap:eGFP), Tg(hb9:eGFP), and Tg(coro1a:eGFP) were selected to observe changes in glial cells, neurons, and immune cells after sash1a knockdown. Our results showed that the number of microglia residing in the developmental brain was reduced, whereas the axonal growth of caudal primary motor neurons was unaffected by sash1a downregulation. And more significantly, the gfap + glia presented abnormal arrangements and disordered orientations in sash1a morphants. The similar phenotype was verified in the mutation induced by the injection of cas9 mRNA and sash1a sgRNA. We further performed behavioral experiments in zebrafish larvae that had been injected with sash1a MO at one-cell stage, and found them exhibiting abnormal behavior trajectories. Moreover, injecting the human SASH1 mRNA rescued these phenomena in sash1a MO zebrafish. In summary, our study revealed that the downregulation of SASH1 leads to malformations in the embryonic brain and disorganization of glial cell marshalling, suggesting that SASH1 plays an important role in the migration of glial cells during embryonic brain development.

Selective Inactivation of Reelin in Inhibitory Interneurons Leads to Subtle Changes in the Dentate Gyrus But Leaves Cortical Layering and Behavior Unaffected

Reelin is an extracellular matrix protein, known for its dual role in neuronal migration during brain development and in synaptic plasticity at adult stages. During the perinatal phase, Reelin expression switches from Cajal-Retzius (CR) cells, its main source before birth, to inhibitory interneurons (IN), the main source of Reelin in the adult forebrain. IN-derived Reelin has been associated with schizophrenia and temporal lobe epilepsy; however, the functional role of Reelin from INs is presently unclear. In this study, we used conditional knockout mice, which lack Reelin expression specifically in inhibitory INs, leading to a substantial reduction in total Reelin expression in the neocortex and dentate gyrus. Our results show that IN-specific Reelin knockout mice exhibit normal neuronal layering and normal behavior, including spatial reference memory. Although INs are the major source of Reelin within the adult stem cell niche, Reelin from INs does not contribute substantially to normal adult neurogenesis. While a closer look at the dentate gyrus revealed some unexpected alterations at the cellular level, including an increase in the number of Reelin expressing CR cells, overall our data suggest that Reelin derived from INs is less critical for cortex development and function than Reelin expressed by CR cells.

RNA Localization and Local Translation in Glia in Neurological and Neurodegenerative Diseases: Lessons from Neurons

Cell polarity is crucial for almost every cell in our body to establish distinct structural and functional domains. Polarized cells have an asymmetrical morphology and therefore their proteins need to be asymmetrically distributed to support their function. Subcellular protein distribution is typically achieved by localization peptides within the protein sequence. However, protein delivery to distinct cellular compartments can rely, not only on the transport of the protein itself but also on the transport of the mRNA that is then translated at target sites. This phenomenon is known as local protein synthesis. Local protein synthesis relies on the transport of mRNAs to subcellular domains and their translation to proteins at target sites by the also localized translation machinery. Neurons and glia specially depend upon the accurate subcellular distribution of their proteome to fulfil their polarized functions. In this sense, local protein synthesis has revealed itself as a crucial mechanism that regulates proper protein homeostasis in subcellular compartments. Thus, deregulation of mRNA transport and/or of localized translation can lead to neurological and neurodegenerative diseases. Local translation has been more extensively studied in neurons than in glia. In this review article, we will summarize the state-of-the art research on local protein synthesis in neuronal function and dysfunction, and we will discuss the possibility that local translation in glia and deregulation thereof contributes to neurological and neurodegenerative diseases.

Spinal cord precursors utilize neural crest cell mechanisms to generate hybrid peripheral myelinating glia

During development, oligodendrocytes and Schwann cells myelinate central and peripheral nervous system axons, respectively, while motor exit point (MEP) glia are neural tube-derived, peripheral glia that myelinate axonal territory between these populations at MEP transition zones. From which specific neural tube precursors MEP glia are specified, and how they exit the neural tube to migrate onto peripheral motor axons, remain largely unknown. Here, using zebrafish, we found that MEP glia arise from lateral floor plate precursors and require foxd3 to delaminate and exit the spinal cord. Additionally, we show that similar to Schwann cells, MEP glial development depends on axonally derived neuregulin1. Finally, our data demonstrate that overexpressing axonal cues is sufficient to generate additional MEP glia in the spinal cord. Overall, these studies provide new insight into how a novel population of hybrid, peripheral myelinating glia are generated from neural tube precursors and migrate into the periphery.

TAG-1 Multifunctionality Coordinates Neuronal Migration, Axon Guidance, and Fasciculation

Neuronal migration, axon fasciculation, and axon guidance need to be closely coordinated for neural circuit assembly. Spinal motor neurons (MNs) face unique challenges during development because their cell bodies reside within the central nervous system (CNS) and their axons project to various targets in the body periphery. The molecular mechanisms that contain MN somata within the spinal cord while allowing their axons to exit the CNS and navigate to their final destinations remain incompletely understood. We find that the MN cell surface protein TAG-1 anchors MN cell bodies in the spinal cord to prevent their emigration, mediates motor axon fasciculation during CNS exit, and guides motor axons past dorsal root ganglia. TAG-1 executes these varied functions in MN development independently of one another. Our results identify TAG-1 as a key multifunctional regulator of MN wiring that coordinates neuronal migration, axon fasciculation, and axon guidance.

Suter et al. demonstrate that the motor neuron cell surface molecule TAG-1 confines motor neurons to the central nervous system, promotes motor axon fasciculation, and steers motor axons past inappropriate targets. This study highlights how a single cell adhesion molecule coordinates multiple steps in neuronal wiring through partially divergent mechanisms.

Cell migration and axon guidance at the border between central and peripheral nervous system

The central and peripheral nervous system (CNS and PNS, respectively) are composed of distinct neuronal and glial cell types with specialized functional properties. However, a small number of select cells traverse the CNS-PNS boundary and connect these two major subdivisions of the nervous system. This pattern of segregation and selective connectivity is established during embryonic development, when neurons and glia migrate to their destinations and axons project to their targets. Here, we provide an overview of the cellular and molecular mechanisms that control cell migration and axon guidance at the vertebrate CNS-PNS border. We highlight recent advances on how cell bodies and axons are instructed to either cross or respect this boundary, and present open questions concerning the development and plasticity of the CNS-PNS interface.

Reelin Immunoreactivity in the Adult Spinal Cord: A Comparative Study in Rodents, Carnivores, and Non-human Primates

Reelin is a large extracellular matrix (ECM) glycoprotein secreted by several neuronal populations in a specific manner in both the developing and the adult central nervous system. The extent of Reelin protein distribution and its functional role in the adult neocortex is well documented in different mammal models. However, its role in the adult spinal cord has not been well characterized and its distribution in the rodent spinal cord is fragmentary and has not been investigated in carnivores or primates as of yet. To gain insight into which neuronal populations and specific circuits may be influenced by Reelin in the adult spinal cord, we have conducted light and confocal microscopy study analysis of Reelin-immunoreactive cell types in the adult spinal cord. Here, we describe and compare Reelin immunoreactive cell type and distribution in the spinal cord of adult non-human primate (macaque monkeys, Macaca mulatta), carnivore (ferret, Mustela putorius) and rodent (rat, Rattus norvegicus). Our results show that in all three species studied, Reelin-immunoreactive neurons are present in the intermediate gray matter, ventricular zone and superficial dorsal horn and intermedio-lateral nucleus, while positive cells in the Clarke nucleus are only found in rats and primates. In addition, Reelin intermediolateral neurons colocalize with choline acetyltransferase (ChAT) only in macaque whilst motor neurons also colocalize Reelin and ChAT in macaque, ferret and rat spinal cord. The different expression patterns might reflect a differential role for Reelin in the pathways involved in the coordination of locomotor activity in the fore- and hind limbs.

Dynamics, nanomechanics and signal transduction in reelin repeats

Reelin is a large glycoprotein controlling brain development and cell adhesion. It regulates the positioning of neurons, as well as neurotransmission and memory formation. Perturbations in reelin signaling are linked to psychiatric disorders. Reelin participates in signal transduction by binding to the lipoprotein receptors VLDLR and ApoER2 through its central region. This part is rich in repeating BNR-EGF-BNR modules. We used standard molecular dynamics, steered molecular dynamics, and perturbation response scanning computational methods to characterize unique dynamical properties of reelin modules involved in signaling. Each module has specific sensors and effectors arranged in a similar topology. In the modules studied, disulfide bridges play a protective role, probably making both selective binding and protease activity of reelin possible. Results of single reelin molecule stretching by atomic force microscopy provide the first data on the mechanical stability of individual reelin domains. The forces required for partial unfolding of the modules studied are below 60 pN.

Slit/Robo signals prevent spinal motor neuron emigration by organizing the spinal cord basement membrane

The developing spinal cord builds a boundary between the CNS and the periphery, in the form of a basement membrane. The spinal cord basement membrane is a barrier that retains CNS neuron cell bodies, while being selectively permeable to specific axon types. Spinal motor neuron cell bodies are located in the ventral neural tube next to the floor plate and project their axons out through the basement membrane to peripheral targets. However, little is known about how spinal motor neuron cell bodies are retained inside the ventral neural tube, while their axons can exit. In previous work, we found that disruption of Slit/Robo signals caused motor neuron emigration outside the spinal cord. In the current study, we investigate how Slit/Robo signals are necessary to keep spinal motor neurons within the neural tube. Our findings show that when Slit/Robo signals were removed from motor neurons, they migrated outside the spinal cord. Furthermore, this emigration was associated with abnormal basement membrane protein expression in the ventral spinal cord. Using Robo2 and Slit2 conditional mutants, we found that motor neuron-derived Slit/Robo signals were required to set up a normal basement membrane in the spinal cord. Together, our results suggest that motor neurons produce Slit signals that are required for the basement membrane assembly to retain motor neuron cell bodies within the spinal cord.

Common cues wire the spinal cord: Axon guidance molecules in spinal neuron migration

Topographic arrangement of neuronal cell bodies and axonal tracts are crucial for proper wiring of the nervous system. This involves often-coordinated neuronal migration and axon guidance during development. Most neurons migrate from their birthplace to specific topographic coordinates as they adopt the final cell fates and extend axons. The axons follow temporospatial specific guidance cues to reach the appropriate targets. When neuronal or axonal migration or their coordination is disrupted, severe consequences including neurodevelopmental disorders and neurological diseases, can arise. Neuronal and axonal migration shares some molecular mechanisms, as genes originally identified as axon guidance molecules have been increasingly shown to direct both navigation processes. This review focuses on axon guidance pathways that are shown to also direct neuronal migration in the vertebrate spinal cord.

Netrin-1 Confines Rhombic Lip-Derived Neurons to the CNS

During brainstem development, newborn neurons originating from the rhombic lip embark on exceptionally long migrations to generate nuclei important for audition, movement, and respiration. Along the way, this highly motile population passes several cranial nerves yet remains confined to the CNS. We found that Ntn1 accumulates beneath the pial surface separating the CNS from the PNS, with gaps at nerve entry sites. In mice null for Ntn1 or its receptor DCC, hindbrain neurons enter cranial nerves and migrate into the periphery. CNS neurons also escape when Ntn1 is selectively lost from the sub-pial region (SPR), and conversely, expression of Ntn1 throughout the mutant hindbrain can prevent their departure. These findings identify a permissive role for Ntn1 in maintaining the CNS-PNS boundary. We propose that Ntn1 confines rhombic lip-derived neurons by providing a preferred substrate for tangentially migrating neurons in the SPR, preventing their entry into nerve roots.

Gfap-positive radial glial cells are an essential progenitor population for later-born neurons and glia in the zebrafish spinal cord

Radial glial cells are presumptive neural stem cells (NSCs) in the developing nervous system. The direct requirement of radial glia for the generation of a diverse array of neuronal and glial subtypes, however, has not been tested. We employed two novel transgenic zebrafish lines and endogenous markers of NSCs and radial glia to show for the first time that radial glia are essential for neurogenesis during development. By using the gfap promoter to drive expression of nuclear localized mCherry we discerned two distinct radial glial-derived cell types: a major nestin+/Sox2+ subtype with strong gfap promoter activity and a minor Sox2+ subtype lacking this activity. Fate mapping studies in this line indicate that gfap+ radial glia generate later-born CoSA interneurons, secondary motorneurons, and oligodendroglia. In another transgenic line using the gfap promoter-driven expression of the nitroreductase enzyme, we induced cell autonomous ablation of gfap+ radial glia and observed a reduction in their specific derived lineages, but not Blbp+ and Sox2+/gfap-negative NSCs, which were retained and expanded at later larval stages. Moreover, we provide evidence supporting classical roles of radial glial in axon patterning, blood-brain barrier formation, and locomotion. Our results suggest that gfap+ radial glia represent the major NSC during late neurogenesis for specific lineages, and possess diverse roles to sustain the structure and function of the spinal cord. These new tools will both corroborate the predicted roles of astroglia and reveal novel roles related to development, physiology, and regeneration in the vertebrate nervous system. GLIA 2016;64:1170-1189.

Reelin Signaling in the Migration of Ventral Brain Stem and Spinal Cord Neurons

The extracellular matrix protein Reelin is an important orchestrator of neuronal migration during the development of the central nervous system. While its role and mechanism of action have been extensively studied and reviewed in the formation of dorsal laminar brain structures like the cerebral cortex, hippocampus, and cerebellum, its functions during the neuronal migration events that result in the nuclear organization of the ventral central nervous system are less well understood. In an attempt to delineate an underlying pattern of Reelin action in the formation of neuronal cell clusters, this review highlights the role of Reelin signaling in the migration of neuronal populations that originate in the ventral brain stem and the spinal cord.

Analysis of Expression Pattern and Genetic Deletion of Netrin5 in the Developing Mouse

Boundary cap cells (BCC) are a transient, neural-crest-derived population found at the motor exit point (MEP) and dorsal root entry zone (DREZ) of the embryonic spinal cord. These cells contribute to the central/peripheral nervous system (CNS/PNS) boundary, and in their absence neurons and glia from the CNS migrate into the PNS. We found Netrin5 (Ntn5), a previously unstudied member of the netrin gene family, to be robustly expressed in BCC. We generated Ntn5 knockout mice and examined neurodevelopmental and BCC-related phenotypes. No abnormalities in cranial nerve guidance, dorsal root organization, or sensory projections were found. However, Ntn5 mutant embryos did have ectopic motor neurons (MNs) that migrated out of the ventral horn and into the motor roots. Previous studies have implicated semaphorin6A (Sema6A) in BCC signaling to plexinA2 (PlxnA2)/neuropilin2 (Nrp2) in MNs in restricting MN cell bodies to the ventral horn, particularly in the caudal spinal cord. In Ntn5 mutants, ectopic MNs are likely to be a different population, as more ectopias were found rostrally. Furthermore, ectopic MNs in Ntn5 mutants were not immunoreactive for NRP2. The netrin receptor deleted in colorectal cancer (DCC) is a potential receptor for NTN5 in MNs, as similar ectopic neurons were found in Dcc mutant mice, but not in mice deficient for other netrin receptors. Thus, Ntn5 is a novel netrin family member that is expressed in BCC, functioning to prevent MN migration out of the CNS.

Fstl1 is involved in the regulation of radial glial scaffold development

Background

Radial glial cells (RGCs), the instructive scaffolds for neuronal migration, are well characterized by their unique morphology and polarization; these cells extend elongated basal processes to the pial basement membrane (BM) and parallel to one another. However, little is known about the mechanisms that underlie the developmental regulation and maintenance of this unique morphology.

Results

Here, by crossing Fstl1^fl/fl mice with an EIIa-Cre line, we identified a new role for the secreted glycoprotein Follistatin like-1 (FSTL1). The ablation of Fstl1 in both of its cortical expression domains, the ventricular zone (VZ) and the pia mater, resulted in RGC morphologic disruption; basal processes were not parallel to each other, and endfeet exhibited greater density and branching. However, Fstl1 deletion in only the VZ in the Emx1^IREScre; Fstl1^fl/fl line did not affect RGC morphology, indicating that FSTL1 derived from the pia mater might be more important for RGC morphology. In addition, upper-layer projection neurons, not deeper-layer projection neurons, failed to reach their appropriate positions. We also found that BMP, AKT/PKB, Cdc42, GSK3β, integrin and reelin signals, which have previously been reported to regulate RGC development, were unchanged, indicating that Fstl1 may function through a unique mechanism.

Conclusions

In the present study, we identified a new role for FSTL1 in the development of radial glial scaffolds and the neuronal migration of upper-layer projection neurons. Our findings will improve understanding of the regulation of RGC development and neuronal migration.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-015-0144-8) contains supplementary material, which is available to authorized users.

Chemokine Signaling Controls Integrity of Radial Glial Scaffold in Developing Spinal Cord and Consequential Proper Position of Boundary Cap Cells

Radial glial cells are the neural progenitors of the developing CNS and have long radial processes that guide radially migrating neurons. The integrity of the radial glial scaffold, in particular proper adhesion between the endfeet of radial processes and the pial basement membrane (BM), is important for the cellular organization of the CNS, as indicated by evidence emerging from the developing cortex. However, the mechanisms underlying the maintenance of radial glial scaffold integrity during development, when the neuroepithelium rapidly expands, are still poorly understood. Here, we addressed this issue in the developing mouse spinal cord. We show that CXCR4, a receptor of chemokine CXCL12, is expressed in spinal cord radial glia. Conditional knock-out of Cxcr4 in radial glia caused disrupted radial glial scaffold with gaps at the pial endfeet layer and consequentially led to an invasion of boundary cap (BC) cells into the spinal cord. Because BC cells are PNS cells normally positioned at the incoming and outgoing axonal roots, their invasion into the spinal cord suggests a compromised CNS/PNS boundary in the absence of CXCL12/CXCR4 signaling. Both disrupted radial glial scaffold and invasion of BC cells into the CNS were also present in mice deficient in CXCR7, a second receptor of CXCL12. We further show that CXCL12 signaling promotes the radial glia adhesion to BM components and activates integrin β1 avidity. Our study unravels a novel molecular mechanism that deploys CXCL12/CXCR4/CXCR7 for the maintenance of radial glial scaffold integrity, which in turn safeguards the CNS/PNS boundary during spinal cord development.

Slit and Semaphorin signaling governed by Islet transcription factors positions motor neuron somata within the neural tube

Motor neurons send out axons to peripheral muscles while their cell bodies remain in the ventral spinal cord. The unique configuration of motor neurons spanning the border between the CNS and PNS has been explained by structural barriers such as boundary cap (BC) cells, basal lamina and radial glia. However, mechanisms in motor neurons that retain their position have not been addressed yet. Here we demonstrate that the Islet1 (Isl1) and Islet2 (Isl2) transcription factors, which are essential for acquisition of motor neuron identity, also contribute to restrict motor neurons within the neural tube. In mice that lack both Isl1 and Isl2, large numbers of motor neurons exited the neural tube, even prior to the appearance of BC cells at the ventral exit points. Transcriptional profiling of motor neurons derived from Isl1 null embryonic stem cells revealed that transcripts of major genes involved in repulsive mechanisms were misregulated. Particularly, expression of Neuropilin1 (Npr1) and Slit2 mRNA was diminished in Islet mutant mice, and these could be target genes of the Islet proteins. Consistent with this mechanism, Robo and Slit mutations in mice and knockdown of Npr1 and Slit2 in chick embryos caused motor neurons to migrate to the periphery. Together, our study suggests that Islet genes engage Robo-Slit and Neuropilin-Semaphorin signaling in motor neurons to retain motor somata within the CNS.

Asymmetric inheritance of Cyclin D2 maintains proliferative neural stem/progenitor cells: a critical event in brain development and evolution

Asymmetric cell division and cell cycle regulation are fundamental mechanisms of mammalian brain development and evolution. Cyclin D2, a positive regulator of G1 progression, shows a unique localization within radial glial (RG) cells (i.e., the neural progenitor in the developing neocortex). Cyclin D2 accumulates at the very basal tip of the RG cell (i.e., the basal endfoot) via a unique cis-regulatory sequence found in the 3' untranslated region (3'UTR) of its mRNA. During RG division, Cyclin D2 protein is asymmetrically distributed to two daughter cells following mitosis. The daughter cell that inherits Cyclin D2 mRNA maintains its self-renewal capability, while its sibling undergoes differentiation. A similar localization pattern of Cyclin D2 protein has been observed in the human fetal cortical primordium, suggesting a common mechanism of maintenance of neural progenitors that may be evolutionarily conserved across higher mammals such as primates. Here, we discuss our findings and the Cyclin D2 function in mammalian brain development and evolution.

Repurposing Reelin: the new role of radial glia, Reelin and Notch in motor neuron migration

The role of Reelin during cerebral cortical neuron migration has long been studied, but the Reelin signaling pathway and its possible interactions are just beginning to be unraveled. Reelin is not only important in cerebral cortical migration, but has recently been shown to interact with the Notch signaling pathway and to be critical for radial glial cell number and morphology. Lee and Song (2013) show a new Notch- and Reelin-dependent role for radial glia in the mouse spinal cord: to act as a fine filter that allows somatic motor neuron axons but not cell bodies to traverse out of the CNS. Here, the types of neuronal migration are discussed, focusing on motor neurons and cues for proper localization. The interaction of Reelin signaling with the Notch pathway is reviewed, which dictates the proper formation of radial glia in the spinal cord in order to prevent ectopic motor neuron migration (Lee and Song, 2013). Future studies may reveal novel interactions and further insights as to how Reelin functions throughout the developing nervous system.