Migration of sympathetic preganglionic neurons in the spinal cord is regulated by Reelin-dependent Dab1 tyrosine phosphorylation and CrkL

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.

Ethanol Exposure Transiently Elevates but Persistently Inhibits Tyrosine Kinase Activity and Impairs the Growth of the Nascent Apical Dendrite

Dendritogenesis can be impaired by exposure to alcohol, and aspects of this impairment share phenotypic similarities to dendritic defects observed after blockade of the Reelin-Dab1 tyrosine kinase signaling pathway. In this study, we find that 10 min of alcohol exposure (400 mg/dL ethanol) by itself causes an unexpected increase in tyrosine phosphorylation of many proteins including Src and Dab1 that are essential downstream effectors of Reelin signaling. This increase in phosphotyrosine is dose-dependent and blockable by selective inhibitors of Src Family Kinases (SFKs). However, the response is transient, and phosphotyrosine levels return to baseline after 30 min of continuous ethanol exposure, both in vitro and in vivo. During this latter period, Src is inactivated and Reelin application cannot stimulate Dab1 phosphorylation. This suggests that ethanol initially activates but then silences the Reelin-Dab1 signaling pathway by brief activation and then sustained inactivation of SFKs. Time-lapse analyses of dendritic growth dynamics show an overall decrease in growth and branching compared to controls after ethanol-exposure that is similar to that observed with Reelin-deficiency. However, unlike Reelin-signaling disruptions, the dendritic filopodial speeds are decreased after ethanol exposure, and this decrease is associated with sustained dephosphorylation and activation of cofilin, an F-actin severing protein. These findings suggest that persistent Src inactivation coupled to cofilin activation may contribute to the dendritic disruptions observed with fetal alcohol exposure.

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.

MicroRNA and messenger RNA profiling reveals new biomarkers and mechanisms for RDX induced neurotoxicity

Background

RDX is a well-known pollutant to induce neurotoxicity. MicroRNAs (miRNA) and messenger RNA (mRNA) profiles are useful tools for toxicogenomics studies. It is worthy to integrate MiRNA and mRNA expression data to understand RDX-induced neurotoxicity.

Results

Rats were treated with or without RDX for 48 h. Both miRNA and mRNA profiles were conducted using brain tissues. Nine miRNAs were significantly regulated by RDX. Of these, 6 and 3 miRNAs were up- and down-regulated respectively. The putative target genes of RDX-regulated miRNAs were highly nervous system function genes and pathways enriched. Fifteen differentially genes altered by RDX from mRNA profiles were the putative targets of regulated miRNAs. The induction of miR-71, miR-27ab, miR-98, and miR-135a expression by RDX, could reduce the expression of the genes POLE4, C5ORF13, SULF1 and ROCK2, and eventually induce neurotoxicity. Over-expression of miR-27ab, or reduction of the expression of unknown miRNAs by RDX, could up-regulate HMGCR expression and contribute to neurotoxicity. RDX regulated immune and inflammation response miRNAs and genes could contribute to RDX- induced neurotoxicity and other toxicities as well as animal defending reaction response to RDX exposure.

Conclusions

Our results demonstrate that integrating miRNA and mRNA profiles is valuable to indentify novel biomarkers and molecular mechanisms for RDX-induced neurological disorder and neurotoxicity.

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.

Generating spinal motor neuron diversity: a long quest for neuronal identity

Understanding how thousands of different neuronal types are generated in the CNS constitutes a major challenge for developmental neurobiologists and is a prerequisite before considering cell or gene therapies of nervous lesions or pathologies. During embryonic development, spinal motor neurons (MNs) segregate into distinct subpopulations that display specific characteristics and properties including molecular identity, migration pattern, allocation to specific motor columns, and innervation of defined target. Because of the facility to correlate these different characteristics, the diversification of spinal MNs has become the model of choice for studying the molecular and cellular mechanisms underlying the generation of multiple neuronal populations in the developing CNS. Therefore, how spinal motor neuron subpopulations are produced during development has been extensively studied during the last two decades. In this review article, we will provide a comprehensive overview of the genetic and molecular mechanisms that contribute to the diversification of spinal MNs.

The involvement of Reelin in neurodevelopmental disorders

Reelin is a glycoprotein that serves important roles both during development (regulation of neuronal migration and brain lamination) and in adulthood (maintenance of synaptic function). A number of neuropsychiatric disorders including autism, schizophrenia, bipolar disorder, major depression, Alzheimer's disease and lissencephaly share a common feature of abnormal Reelin expression in the brain. Altered Reelin expression has been hypothesized to impair neuronal connectivity and synaptic plasticity, leading ultimately to the cognitive deficits present in these disorders. The mechanisms for abnormal Reelin expression in some of these disorders are currently unknown although possible explanations include early developmental insults, mutations, hypermethylation of the promoter for the Reelin gene (RELN), miRNA silencing of Reelin mRNA, FMRP underexpression and Reelin processing abnormalities. Increasing Reelin expression through pharmacological therapies may help ameliorate symptoms resulting from Reelin deficits. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

Migration of sympathetic preganglionic neurons in the spinal cord of a C3G-deficient mouse suggests that C3G acts in the reelin signaling pathway

Proper positioning of sympathetic preganglionic neurons(SPNs) in the spinal cord is regulated by reelin signaling. SPNs in reeler (which lacks reelin), and in mice deficient in components of the reelin signaling pathway (reelin receptors VldlR and ApoER2, the cytoplasmic adaptor protein Dab1, Src and Fyn of the Src-family of non-receptor protein tyrosine kinases, and CrkL) are located adjacent to the central canal instead of in the intermediolateral column (IML) of the spinal cord. Events downstream of CrkL in control of SPN migration are unclear. The present study asks whether Rap guanine nucleotide exchange factor (GEF) 1 (C3G/Rap-gef1), a Ras family GEF that binds CrkL, could act downstream in the reelin signaling pathway in control of SPN migration. SPN migration was examined in a hypopmorphic C3G mutant mouse (C3G(gt)(/gt)) by using retrograde Dil labeling and NOS immunostaining. The results showed that SPN in the C3G(gt)/(gt) mutant migrate abnormally toward the central canal as in reeler. However, unlike reeler, levels of reelin in the C3G(gt)/(gt) spinal cord are normal, and Dab1 immunostaining is undetectable. These results provide genetic evidence that C3G regulates SPN migration, and suggest that C3G acts downstream in the reelin signaling pathway in control of SPN migration.

Motor neuron position and topographic order imposed by β- and γ-catenin activities

Neurons typically settle at positions that match the location of their synaptic targets, creating topographic maps. In the spinal cord, the organization of motor neurons into discrete clusters is linked to the location of their muscle targets, establishing a topographic map of punctate design. To define the significance of motor pool organization for neuromuscular map formation, we assessed the role of cadherin-catenin signaling in motor neuron positioning and limb muscle innervation. We find that joint inactivation of β- and γ-catenin scrambles motor neuron settling position in the spinal cord but fails to erode the predictive link between motor neuron transcriptional identity and muscle target. Inactivation of N-cadherin perturbs pool positioning in similar ways, albeit with reduced penetrance. These findings reveal that cadherin-catenin signaling directs motor pool patterning and imposes topographic order on an underlying identity-based neural map.

Reelin inhibits migration of sympathetic preganglionic neurons in the spinal cord of the chick

The present study examined the effects of Reelin in the migration of sympathetic preganglionic neurons (SPN) in the spinal cord of the chick. SPN in the chick first migrate from the neuroepithelium to the ventrolateral spinal cord. They then undergo a secondary migration to cluster adjacent to the central canal, forming the column of Terni (CT). During secondary migration, abundant Reelin is found in large areas of the ventral spinal cord; the only areas devoid of Reelin are areas occupied by SPN or somatic motor neurons and the pathway along which SPN migrate. Ectopic expression of Reelin in the pathway of SPN through electroporation of full-length Reelin DNA stopped SPN migration toward their destination. The spatiotemporal pattern of Reelin expression, along with the inhibition of SPN migration by exogenous Reelin, suggests that Reelin functions as a barrier to SPN migration during normal development of the spinal cord.

Song exposure regulates known and novel microRNAs in the zebra finch auditory forebrain

BACKGROUND

In an important model for neuroscience, songbirds learn to discriminate songs they hear during tape-recorded playbacks, as demonstrated by song-specific habituation of both behavioral and neurogenomic responses in the auditory forebrain. We hypothesized that microRNAs (miRNAs or miRs) may participate in the changing pattern of gene expression induced by song exposure. To test this, we used massively parallel Illumina sequencing to analyse small RNAs from auditory forebrain of adult zebra finches exposed to tape-recorded birdsong or silence.

RESULTS

In the auditory forebrain, we identified 121 known miRNAs conserved in other vertebrates. We also identified 34 novel miRNAs that do not align to human or chicken genomes. Five conserved miRNAs showed significant and consistent changes in copy number after song exposure across three biological replications of the song-silence comparison, with two increasing (tgu-miR-25, tgu-miR-192) and three decreasing (tgu-miR-92, tgu-miR-124, tgu-miR-129-5p). We also detected a locus on the Z sex chromosome that produces three different novel miRNAs, with supporting evidence from Northern blot and TaqMan qPCR assays for differential expression in males and females and in response to song playbacks. One of these, tgu-miR-2954-3p, is predicted (by TargetScan) to regulate eight song-responsive mRNAs that all have functions in cellular proliferation and neuronal differentiation.

CONCLUSIONS

The experience of hearing another bird singing alters the profile of miRNAs in the auditory forebrain of zebra finches. The response involves both known conserved miRNAs and novel miRNAs described so far only in the zebra finch, including a novel sex-linked, song-responsive miRNA. These results indicate that miRNAs are likely to contribute to the unique behavioural biology of learned song communication in songbirds.

Role for Reelin-induced cofilin phosphorylation in the assembly of sympathetic preganglionic neurons in the murine intermediolateral column

Sympathetic preganglionic neurons (SPNs) are located in the intermediolateral column (IMLC) of the spinal cord. This specific localization results from primary and secondary migratory processes during spinal cord development. Thus, following neurogenesis in the neuroepithelium, SPNs migrate first in a ventrolateral direction and then, in a secondary step, dorsolaterally to reach the IMLC. These migratory processes are controlled, at least in part, by the glycoprotein Reelin, which is known to be important for the development of laminated brain structures. In reeler mutants deficient in Reelin, SPNs initially migrate ventrolaterally as normal. However, most of them then migrate medially to become eventually located near the central canal. Here, we provide evidence that in wild-type animals this aberrant medial migration towards the central canal is prevented by Reelin-induced cytoskeletal stabilization, brought about by phosphorylation of cofilin. Cofilin plays an important role in actin depolymerization, a process required for the changes in cell shape during migration. Phosphorylation of cofilin renders it unable to depolymerize F-actin, thereby stabilizing the cytoskeleton. Using immunostaining for phosphorylated cofilin (p-cofilin), we demonstrate that SPNs in wild-type animals, but not in reeler mutants and other mutants of the Reelin signalling cascade, are immunoreactive for p-cofilin. These findings suggest that Reelin near the central canal induces cofilin phosphorylation in SPNs, thereby preventing them from aberrant migration towards the central canal. The results extend our previous studies on cortical neurons in which Reelin in the marginal zone was found to stabilize the leading processes of migrating neurons and terminate the migration process.

Crk and CrkL adaptor proteins: networks for physiological and pathological signaling

The Crk adaptor proteins (Crk and CrkL) constitute an integral part of a network of essential signal transduction pathways in humans and other organisms that act as major convergence points in tyrosine kinase signaling. Crk proteins integrate signals from a wide variety of sources, including growth factors, extracellular matrix molecules, bacterial pathogens, and apoptotic cells. Mounting evidence indicates that dysregulation of Crk proteins is associated with human diseases, including cancer and susceptibility to pathogen infections. Recent structural work has identified new and unusual insights into the regulation of Crk proteins, providing a rationale for how Crk can sense diverse signals and produce a myriad of biological responses.

Crk and Crk-like play essential overlapping roles downstream of disabled-1 in the Reelin pathway

Reelin controls neuronal positioning in the developing brain by binding to the two lipoprotein receptors, very-low-density lipoprotein receptor and apolipoprotein E receptor 2, to stimulate phosphorylation of Disabled-1 (Dab1) by the Fyn and Src tyrosine kinases. Crk and Crk-like (CrkL) have been proposed to interact with tyrosine phosphorylated Dab1 to mediate downstream events in the Reelin pathway. However, these adaptor proteins are widely expressed, and they fulfill essential functions during embryonic development. To address their specific roles in Reelin-mediated neuronal migration, we generated mutant mice, by Cre-loxP recombination, lacking Crk and CrkL in most neurons. These animals displayed the major anatomic features of reeler including, cerebellar hypofoliation, failure of Purkinje cell migration, absence of preplate splitting, impaired dendritic development, and disruption of layer formation in the hippocampus and cerebral cortex. However, proximal signaling involving tyrosine phosphorylation and turnover of Dab1 occurred normally in the mutant mouse brain and in primary cortical neurons treated with Reelin. In contrast, two downstream signaling events, Reelin-induced phosphorylation of C3G and Akt, were not observed in the absence of Crk and CrkL in mouse embryonic cortical neurons. These findings place C3G and Akt phosphorylation downstream of Crk and CrkL, which play essential overlapping functions in the Reelin signaling pathway.

Identification of positionally distinct astrocyte subtypes whose identities are specified by a homeodomain code

Astrocytes constitute the most abundant cell type in the central nervous system (CNS) and play diverse functional roles, but the ontogenetic origins of this phenotypic diversity are poorly understood. We have investigated whether positional identity, a fundamental organizing principle governing the generation of neuronal subtype diversity, is also relevant to astrocyte diversification. We identified three positionally distinct subtypes of white-matter astrocytes (WMA) in the spinal cord, which can be distinguished by the combinatorial expression of Reelin and Slit1. These astrocyte subtypes derive from progenitor domains expressing the homeodomain transcription factors Pax6 and Nkx6.1, respectively. Loss- and gain-of-function experiments indicate that the positional identity of these astrocyte subtypes is controlled by Pax6 and Nkx6.1 in a combinatorial manner. Thus, positional identity is an organizing principle underlying astrocyte, as well as neuronal, subtype diversification and is controlled by a homeodomain transcriptional code whose elements are reutilized following the specification of neuronal identity earlier in development.

Reduction of Crk and CrkL expression blocks reelin-induced dendritogenesis

The reelin signaling pathway regulates nervous system function after birth, in addition to its role in regulating neuronal positioning during embryogenesis. The receptor-dependent, reelin-induced tyrosine phosphorylation of the Dab1 docking protein is an established prerequisite for biological responses to this ligand. Here we show that the inactivation of a conditional Dab1 allele reduces process complexity in correctly positioned neurons in the CA1 region of the mouse hippocampus after birth. Reelin stimulation of cultured hippocampal neurons enhances dendritogenesis by approximately twofold and in a manner dependent on Src family kinases. This enhancement is blocked by reducing expression of Crk family proteins, adaptor molecules that interact with Dab1 in a tyrosine phosphorylation-dependent manner. Retrovirally expressed inhibitory RNAs used to reduce Crk and CrkL expression did not block BDNF-enhanced dendritogenesis or influence axonogenesis. Together, this demonstrates that the Crk family proteins are important downstream components of the reelin signaling pathway in the regulation of postnatal hippocampal dendritogenesis.

Ablation of Csk in neural crest lineages causes corneal anomaly by deregulating collagen fibril organization and cell motility

Src family kinases (SFKs) have been implicated in the regulation of cell motility. To verify their in vivo roles during development, we generated mutant mice in which Csk, a negative regulator of SFKs, was inactivated in neural crest lineages using the Protein zero promoter in a Cre-loxP system. Inactivation of Csk caused deformities in various tissues of neural crest origins, including facial dysplasia and corneal opacity. In the cornea, the stromal collagen fibril was disorganized and there was an overproduction of collagen 1a1 and several metalloproteases. The corneal endothelium failed to overlie the central region of the eye and the peripheral endothelium displayed a disorganized cytoskeleton. Corneal mesenchymal cells cultured from mutant mice showed attenuated cell motility. In these cells, p130 Crk-associated substrate (Cas) was hyperphosphorylated and markedly downregulated. The expression of a dominant negative Cas (Cas Delta SD) could suppress the cell motility defects. Fluorescence resonance energy transfer analysis revealed that activation of Rac1 and Cdc42 was depolarized in Csk-inactivated cells, which was restored by the expression of either Csk or Cas Delta SD. These results demonstrate that the SFKs/Csk circuit plays crucial roles in corneal development by controlling stromal organization and by ensuring cell motility via the Cas-Rac/Cdc42 pathways.