Lower rhombic lip-derived cells undergo transmedian tangential migration followed by radial migration in the chick embryo brainstem

Disruption of the anterior commissure in Olig2 deficient mice

In the present study, we examined neural circuit formation in the forebrain of the Olig2 knockout (Olig2-KO) mouse model and found disruption of the anterior commissure at the late foetal stage. Axon bundles of the anterior commissure encountered the wall of the third ventricle and ceased axonal extension. L1-CAM immunohistochemistry showed that Olig2-KO mice lose decussation formation in the basal forebrain. DiI tracing revealed that the thin bundles of the anterior commissure axons crossed the midline but ceased further extension into the deep part of the contralateral side. Furthermore, some fractions of DiI-labelled axons were oriented dorsolaterally, which was not observed in the control mouse forebrain. The rostral part of the third ventricle was much wider in the Olig2-KO mice than in wild-type mice, which likely resulted in the delay of midline fusion and subsequent delay and malformation of the anterior commissure. We analysed gene expression alterations in the Olig2-KO mice using a public database and found multiple genes, which are related to axon guidance and epithelial-mesenchymal transition, showing subtle expression changes. These results suggest that Olig2 is essential for anterior commissure formation, likely by regulating multiple biological processes.

The fornix acts as a permissive corridor for septal neuron migration beyond the diencephalic-telencephalic boundary

Neuronal migration is essential for constructing functional neural networks. Two posterior septal (PS) nuclei, the triangular septal nucleus and bed nuclei of the anterior commissure, are involved in fear and anxiety. During development, glutamatergic PS neurons undergo long-distance rostrodorsal migration from the thalamic eminence (TE) of the diencephalon, then settle in the caudalmost telencephalon. However, the developmental behavior of PS neurons and the guidance structures facilitating their migration remain unknown. We previously demonstrated the migration of PS neurons along the fornix, a major efferent pathway from the hippocampal formation. Here, we show that the postcommissural fornix is essential for PS neuron migration which is largely confined to its axonal tract, which grows in the opposite direction as PS neuron migration. Fornical axons reach the TE prior to initiation of PS neuron rostrodorsal migration. Ectopic expression of Semaphorin 3 A in the dorsomedial cortex resulted in defective fornix formation. Furthermore, loss of the postcommissural fornix stalled PS neuron migration resulting in abnormal accumulation near their origin. This suggests that PS neurons utilize the postcommissural fornix as a permissive corridor during migration beyond the diencephalic-telencephalic boundary. This axonal support is essential for the functional organization of the heterogeneous septal nuclear complex.

Birthdate-dependent heterogeneity of oculomotor neurons is involved in transmedian migration in the developing mouse midbrain

During the formation of the oculomotor nucleus (nIII), a subset of cells undergoes transmedian migration, crossing the midline to join the contralateral nucleus. A recent study reported that the onset of transmedian migration of nIII neurons is regulated by Slit/Robo signaling. However, developmental programs that differentiate migratory subpopulations of the nIII remain elusive. Here, we identified cellular and molecular characteristics of nIII neurons that are correlated with their migratory behaviors. Birthdate analysis revealed that contralaterally migrating neurons in the caudal part of the nIII are generated at later stages than uncrossed neurons in the rostral part of the nIII. Furthermore, we found that Slit2 is expressed in the ventral midline of the midbrain and contralaterally migrating neurons. On the other hand, Robo2, a receptor of Sli2, is differentially expressed in subpopulations of rostral and caudal parts of the nIII: uncrossed neurons expressed Robo2 in the developing nIII. These results suggest that spatio-temporal regulation of developmental timings and the molecular signatures of oculomotor neurons are crucial for transmedian migration, which underlies appropriate positioning and stereotyped circuit formation of the nIII in the developing mouse midbrain.

Development of the prethalamus is crucial for thalamocortical projection formation and is regulated by Olig2

Thalamocortical axons (TCAs) pass through the prethalamus in the first step of their neural circuit formation. Although it has been supposed that the prethalamus is an intermediate target for thalamocortical projection formation, much less is known about the molecular mechanisms of this targeting. Here, we demonstrated the functional implications of the prethalamus in the formation of this neural circuit. We show that Olig2 transcription factor, which is expressed in the ventricular zone (VZ) of prosomere 3, regulates prethalamus formation, and loss of Olig2 results in reduced prethalamus size in early development, which is accompanied by expansion of the thalamic eminence (TE). Extension of TCAs is disorganized in the Olig2-KO dorsal thalamus, and initial elongation of TCAs is retarded in the Olig2-KO forebrain. Microarray analysis demonstrated upregulation of several axon guidance molecules, including Epha3 and Epha5, in the Olig2-KO basal forebrain. In situ hybridization showed that the prethalamus in the wild type excluded the expression of Epha3 and Epha5, whereas loss of Olig2 resulted in reduction of this Ephas-negative area and the corresponding expansion of the Ephas-positive TE. Dissociated cultures of thalamic progenitor cells demonstrated that substrate-bound EphA3 suppresses neurite extension from dorsal thalamic neurons. These results indicate that Olig2 is involved in correct formation of the prethalamus, which leads to exclusion of the EphA3-expressing region and is crucial for proper TCA formation. Our observation is the first report showing the molecular mechanisms underlying how the prethalamus acts on initial thalamocortical projection formation.

Calcium release-dependent actin flow in the leading process mediates axophilic migration

Proper assembly of neural circuits requires newly born neurons to migrate from their place of origin to their final location. Little is known about the mechanisms of axophilic neuronal migration, whereby neurons travel along axon pathways to navigate to their destinations. Gonadotropin-releasing hormone (GnRH)-expressing neurons migrate along olfactory axons from the nose into the forebrain during development, and were used as a model of axophilic migration. After migrating, GnRH neurons are located in the hypothalamus and are essential for puberty and maintenance of reproductive function. To gain a better understanding of the mechanisms underlying axophilic migration, we investigated in mice the regulation of movement from calcium signals to cytoskeletal dynamics. Live imaging revealed robust calcium activity during axophilic migration, and calcium release through IP3 receptors was found to stimulate migration. This occurred through a signaling pathway involving the calcium sensor calcium/calmodulin protein kinase kinase, AMP-activated kinase, and RhoA/ROCK. By imaging GnRH neurons expressing actin-GFP or Lifeact-RFP, calcium release was found to stimulate leading process actin flow away from the cell body. In contrast, actin contractions at the cell rear were unaffected by this calcium signaling pathway. These findings are the first to test the regulation of cytoskeletal dynamics in axophilic migration, and reveal mechanisms of movement that have broad implications for the migration of other CNS populations.

The long adventurous journey of rhombic lip cells in jawed vertebrates: a comparative developmental analysis

This review summarizes vertebrate rhombic lip and early cerebellar development covering classic approaches up to modern developmental genetics which identifies the relevant differential gene expression domains and their progeny. Most of this information is derived from amniotes. However, progress in anamniotes, particularly in the zebrafish, has recently been made. The current picture suggests that rhombic lip and cerebellar development in jawed vertebrates (gnathostomes) share many characteristics. Regarding cerebellar development, these include a ptf1a expressing ventral cerebellar proliferation (VCP) giving rise to Purkinje cells and other inhibitory cerebellar cell types, and an atoh1 expressing upper rhombic lip giving rise to an external granular layer (EGL, i.e., excitatory granule cells) and an early ventral migration into the anterior rhombencephalon (cholinergic nuclei). As for the lower rhombic lip (LRL), gnathostome commonalities likely include the formation of precerebellar nuclei (mossy fiber origins) and partially primary auditory nuclei (likely convergently evolved) from the atoh1 expressing dorsal zone. The fate of the ptf1a expressing ventral LRL zone which gives rise to (excitatory cells of) the inferior olive (climbing fiber origin) and (inhibitory cells of ) cochlear nuclei in amniotes, has not been determined in anamniotes. Special for the zebrafish in comparison to amniotes is the predominant origin of anamniote excitatory deep cerebellar nuclei homologs (i.e., eurydendroid cells) from ptf1a expressing VCP cells, the sequential activity of various atoh1 paralogs and the incomplete coverage of the subpial cerebellar plate with proliferative EGL cells. Nevertheless, the conclusion that a rhombic lip and its major derivatives evolved with gnathostome vertebrates only and are thus not an ancestral craniate character complex is supported by the absence of a cerebellum (and likely absence of its afferent and efferent nuclei) in jawless fishes

Genetically-defined lineage tracing of Nkx2.2-expressing cells in chick spinal cord

In the spinal cord, generation of oligodendrocytes (OLs) is totally dependent on the presence of Olig2, a basic helix-loop-helix transcription factor. However, it also requires Nkx2.2 for its generation, whose expression follows the expression of Olig2. Although it is believed that oligodendrocytes originate from the pMN domain, Nkx2.2 is present in the p3 domain located ventral to the pMN domain. According to recent reports, it is possible that oligodendrocytes are directly derived from the p3 domain in addition to the pMN domain in the chick spinal cord. We examined this hypothesis in this paper. To analyze OL development in the spinal cord, chick embryos are widely used for genetic modification by electroporation or for transplantation experiments, because it is relatively easy to manipulate them compared with mouse embryos. However, genetic modification by electroporation is not appropriate for glial development analyses because glia proliferate vigorously before maturation. In order to overcome these problems, we established a novel method to permanently introduce exogenous gene into a specific cell type. We introduced the CAT1 gene, a murine retroviral receptor, by electroporation followed by injection of murine retrovirus. By using this method, we successfully transduced murine retrovirus into the chick neural tube. We analyzed cell lineage from the p3 domain by restricting CAT1 expression by Nkx2.2-enhancer and found that most of the labeled cells became OLs when the cells were labeled at cE4. Moreover, the labeled OLs were found throughout the white matter in the spinal cord including the most dorsal spinal cord. Thus p3 domain directly generates spinal cord OLs in the chick spinal cord.

Netrin-1 acts as a repulsive guidance cue for sensory axonal projections toward the spinal cord

During early development, the ventral spinal cord expresses chemorepulsive signals that act on dorsal root ganglion (DRG) axons to help orient them toward the dorsolateral part of the spinal cord. However, the molecular nature of this chemorepulsion is mostly unknown. We report here that netrin-1 acts as an early ventral spinal cord-derived chemorepellent for DRG axons. In the developing mouse spinal cord, netrin-1 is expressed in the floor plate of the spinal cord, and the netrin receptor Unc5c is expressed in DRG neurons. We show that human embryonic kidney cell aggregates secreting netrin-1 repel DRG axons and that netrin-1-deficient ventral spinal cord explants lose their repulsive influence on DRG axons. In embryonic day 10 netrin-1 mutant mice, we find that DRG axons exhibit transient misorientation. Furthermore, by means of gain-of-function analyses, we show that ectopic netrin-1 in the dorsal and intermediate spinal cord prevents DRG axons from being directed toward the dorsal spinal cord. Together, these findings suggest that netrin-1 contributes to the formation of the initial trajectories of developing DRG axons as a repulsive guidance cue.

Regional- and temporal-dependent changes in the differentiation of Olig2 progenitors in the forebrain, and the impact on astrocyte development in the dorsal pallium

Olig2 is a basic helix-loop-helix transcription factor essential for oligodendrocyte and motoneuron development in the spinal cord. Olig2-positive (Olig2+) cells in the ventricular zone of the ventral telencephalon have been shown to differentiate into GABAergic and cholinergic neurons. However, the fate of Olig2 lineage cells in the postnatal forebrain has not been fully described and Olig2 may regulate the development of both astrocytes and oligodendrocytes. Here, we examined the fate of embryonic Olig2+ progenitors using a tamoxifen-inducible Cre/loxP system. Using long-term lineage tracing, Olig2+ cells in the early fetal stage primarily differentiated into GABAergic neurons in the adult telencephalon, while those in later stages gave rise to macroglial cells, both astrocytes and oligodendrocytes. Olig2+ progenitors in the diencephalon developed into oligodendrocytes, as observed in the spinal cord, and a fraction developed into glutamatergic neurons. Olig2 lineage oligodendrocytes tended to form clusters, probably due to local proliferation at the site of terminal differentiation. In spite of the abundance of Olig2 lineage GABAergic neurons in the normal neocortex, GABAergic neurons seemed to develop at normal density in the Olig2 deficient mouse. Thus, Olig2 is dispensable for GABAergic neuron specification. In contrast, at the late fetal stage in the Olig2 deficient mouse, astrocyte development was retarded in the dorsal neocortex, but not in the basal forebrain. Olig2 functions, therefore, in gliogenesis in the dorsal pallium. Short-term lineage tracing experiments revealed that the majority of late Olig2+ cells were not direct descendants of early Olig2+ progenitors in the fetal forebrain. These observations indicate that embryonic Olig2+ progenitor cells change their differentiative properties during development, and also that Olig2 plays a role in astrocyte development in a region-specific manner.

Development and migration of GABAergic neurons in the mouse myelencephalon

GABAergic neurons are the major inhibitory interneurons that are widely distributed in the central nervous system. It is well established that they originate from a focal region in the embryonic forebrain during development, and then migrate to other regions such as the neocortex. However, the migration of GABAergic neurons remains obscure in other axial levels of the brain. We examined the early development of myelencephalic GABAergic neurons using glutamate decarboxylase 67 / green fluorescent protein (GAD67-GFP) knocking mice. Observation of fixed tissues in coronal sections and flat whole-mount preparations indicated that, while GFP-positive cells are restricted to the subpial region in the ventral aspect of the myelencephalon at an early stage, they spread dorsally and eventually occupy the entire region of the myelencephalon as development proceeds. We developed a flat-mount in vitro preparation in which these patterns of development could be recapitulated. Transplantation of dorsal myelencephalic tissue of a wildtype embryo to a corresponding region of GAD67-GFP mouse embryos clearly demonstrated invasion of dorsally oriented GABAergic neurons from host to donor tissue. These results indicate that ventral-to-dorsal tangential migration of GABAergic neurons takes place in the myelencephalon. Our results extend the observations in the forebrain that inhibitory and excitatory neurons in a specific brain compartment take distinct migratory paths.

Migration and nucleogenesis of mouse precerebellar neurons visualized by in utero electroporation of a green fluorescent protein gene

Neural migration is a critical step for accurate CNS development, but the molecular mechanisms that regulate migration, settlement and nucleogenesis remain largely unknown. The precerebellar neurons (PCNs), generated in the lower rhombic lip (LRL), migrate towards their destinations: some neurons form the pontine gray nucleus (PGN) and reticulotegmental nucleus (RTN) in the ipsilateral pons, while others form the lateral reticular and external cuneate nuclei in the contralateral medulla after crossing the midline. Here, by introducing an EGFP gene into a unilateral LRL of mouse embryos by in utero electroporation, we specifically labeled and tracked the PCNs in vivo. We found that a substantial number of the labeled neurons crossed the midline and formed PGN/RTN on the contralateral side. In addition, we found that a subpopulation of the interpolar subnucleus of the spinal trigeminal nucleus, which projects the axons to the cerebellum, was one of the PCNs derived from the LRL. Furthermore, because the electroporated mice were born and grew up healthy, we could visualize the PCNs and their mossy fibers in the adult brain. Therefore, the EGFP labeling of PCNs can be applied to studying the physiology of the mossy fiber system as well as PCN development in embryos.

Dorsally derived netrin 1 provides an inhibitory cue and elaborates the`waiting period' for primary sensory axons in the developing spinal cord

Dorsal root ganglion (DRG) neurons extend axons to specific targets in the gray matter of the spinal cord. During development, DRG axons grow into the dorsolateral margin of the spinal cord and projection into the dorsal mantle layer occurs after a `waiting period' of a few days. Netrin 1 is a long-range diffusible factor expressed in the ventral midline of the developing neural tube, and has chemoattractive and chemorepulsive effects on growing axons. Netrin 1 is also expressed in the dorsal spinal cord. However, the roles of dorsally derived netrin 1 remain totally unknown. Here, we show that dorsal netrin 1 controls the correct guidance of primary sensory axons. During the waiting period, netrin 1 is transiently expressed or upregulated in the dorsal spinal cord, and the absence of netrin 1 results in the aberrant projection of sensory axons, including both cutaneous and proprioceptive afferents, into the dorsal mantle layer. Netrin 1 derived from the dorsal spinal cord, but not the floor plate, is involved in the correct projection of DRG axons. Furthermore,netrin 1 suppresses axon outgrowth from DRG in vitro. Unc5crcm mutant shows abnormal invasion of DRG axons as observed in netrin 1 mutants. These results are the first direct evidence that netrin 1 in the dorsal spinal cord acts as an inhibitory cue for primary sensory axons and is a crucial signal for the formation of sensory afferent neural networks.

Short-term lineage analysis of dorsally derived Olig3 cells in the developing spinal cord

We examined the migration and differentiation of cells expressing Olig3, a basic helix-loop-helix transcriptional factor, in the developing spinal cord. Distribution of Olig3 lineage cells was demonstrated with in situ hybridization and X-gal staining in an Olig3-lacZ knock-in mouse. Olig3-positive cells first appeared in the dorsal spinal cord, except for the roof plate. Some of the dorsal Olig3 lineage cells co-expressed Islet1/2, Math1, or Brn3a, markers for dorsal interneuron. LacZ-positive cells were observed in the ventral-most part of the E10.5 spinal cord, suggesting that some dorsal Olig3 lineage cells migrate into the ventral-most part by E10.5. Ventral-ward migration of dorsal cells and contribution to commissural interneurons were substantiated by electroporation of EGFP expression plasmid in the dorsal spinal cord of chick embryo. Dorsal midline cells were also LacZ-positive during development. These findings suggest that dorsal Olig3 cells contribute to dorsal midline cells and commissural interneurons at intermediate and ventral levels.