Immunohistochemical localization of neurocan and L1 in the formation of thalamocortical pathway of developing rats

Histological Analysis of a Mouse Model of the 22q11.2 Microdeletion Syndrome

22q11.2 deletion syndrome (22q11.2DS) is associated with a high risk of developing various psychiatric and developmental disorders, including schizophrenia and early-onset Parkinson's disease. Recently, a mouse model of this disease, Del(3.0Mb)/+, mimicking the 3.0 Mb deletion which is most frequently found in patients with 22q11.2DS, was generated. The behavior of this mouse model was extensively studied and several abnormalities related to the symptoms of 22q11.2DS were found. However, the histological features of their brains have been little addressed. Here we describe the cytoarchitectures of the brains of Del(3.0Mb)/+ mice. First, we investigated the overall histology of the embryonic and adult cerebral cortices, but they were indistinguishable from the wild type. However, the morphologies of individual neurons were slightly but significantly changed from the wild type counterparts in a region-specific manner. The dendritic branches and/or dendritic spine densities of neurons in the medial prefrontal cortex, nucleus accumbens, and primary somatosensory cortex were reduced. We also observed reduced axon innervation of dopaminergic neurons into the prefrontal cortex. Given these affected neurons function together as the dopamine system to control animal behaviors, the impairment we observed may explain a part of the abnormal behaviors of Del(3.0Mb)/+ mice and the psychiatric symptoms of 22q11.2DS.

Comprehensive characterization of migration profiles of murine cerebral cortical neurons during development using FlashTag labeling

In the mammalian cerebral neocortex, different regions have different cytoarchitecture, neuronal birthdates, and functions. In most regions, neuronal migratory profiles are speculated similar based on observations using thymidine analogs. Few reports have investigated regional migratory differences from mitosis at the ventricular surface. In this study, we applied FlashTag technology, in which dyes are injected intraventricularly, to describe migratory profiles. We revealed a mediolateral regional difference in the migratory profiles of neurons that is dependent on developmental stage; for example, neurons labeled at embryonic day 12.5–15.5 reached their destination earlier dorsomedially than dorsolaterally, even where there were underlying ventricular surfaces, reflecting sojourning below the subplate. This difference was hardly recapitulated by thymidine analogs, which visualize neurogenic gradients, suggesting a biological significance different from the neurogenic gradient. These observations advance our understanding of cortical development and the power of FlashTag in studying migration and are thus resources for future neurodevelopmental studies.

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FlashTag visualized mediolateral regional differences of cortical migratory profiles

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Mediolateral differences were observed when neurons were labeled at E12.5–15.5

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Late-born neurons transiently sojourned below the dorsolateral subplate (SP) cells

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The difference was unclear in reeler cortex, where SP cells position superficially

The enigmatic fetal subplate compartment forms an early tangential cortical nexus and provides the framework for construction of cortical connectivity

The most prominent transient compartment of the primate fetal cortex is the deep, cell-sparse, synapse-containing subplate compartment (SPC). The developmental role of the SPC and its extraordinary size in humans remain enigmatic. This paper evaluates evidence on the development and connectivity of the SPC and discusses its role in the pathogenesis of neurodevelopmental disorders. A synthesis of data shows that the subplate becomes a prominent compartment by its expansion from the deep cortical plate (CP), appearing well-delineated on MR scans and forming a tangential nexus across the hemisphere, consisting of an extracellular matrix, randomly distributed postmigratory neurons, multiple branches of thalamic and long corticocortical axons. The SPC generates early spontaneous non-synaptic and synaptic activity and mediates cortical response upon thalamic stimulation. The subplate nexus provides large-scale interareal connectivity possibly underlying fMR resting-state activity, before corticocortical pathways are established. In late fetal phase, when synapses appear within the CP, transient the SPC coexists with permanent circuitry. The histogenetic role of the SPC is to provide interactive milieu and capacity for guidance, sorting, "waiting" and target selection of thalamocortical and corticocortical pathways. The new evolutionary role of the SPC and its remnant white matter neurons is linked to the increasing number of associative pathways in the human neocortex. These roles attributed to the SPC are regulated using a spatiotemporal gene expression during critical periods, when pathogenic factors may disturb vulnerable circuitry of the SPC, causing neurodevelopmental cognitive circuitry disorders.

hPSC Modeling Reveals that Fate Selection of Cortical Deep Projection Neurons Occurs in the Subplate

Cortical deep projection neurons (DPNs) are implicated in neurodevelopmental disorders. Although recent findings emphasize post-mitotic programs in projection neuron fate selection, the establishment of primate DPN identity during layer formation is not well understood. The subplate lies underneath the developing cortex and is a post-mitotic compartment that is transiently and disproportionately enlarged in primates in the second trimester. The evolutionary significance of subplate expansion, the molecular identity of its neurons, and its contribution to primate corticogenesis remain open questions. By modeling subplate formation with human pluripotent stem cells (hPSCs), we show that all classes of cortical DPNs can be specified from subplate neurons (SPNs). Post-mitotic WNT signaling regulates DPN class selection, and DPNs in the caudal fetal cortex appear to exclusively derive from SPNs. Our findings indicate that SPNs have evolved in primates as an important source of DPNs that contribute to cortical lamination prior to their known role in circuit formation.

Reciprocal Connections Between Cortex and Thalamus Contribute to Retinal Axon Targeting to Dorsal Lateral Geniculate Nucleus

The dorsal Lateral Geniculate Nucleus (dLGN) is the primary image-forming target of the retina and shares a reciprocal connection with primary visual cortex (V1). Previous studies showed that corticothalamic input is essential for the development of thalamocortical projections, but less is known about the potential role of this reciprocal connection in the development of retinal projections. Here, we show a deficit of retinal innervation in the dLGN around E18.5 in Tra2β conditional knockout (cKO) "cortexless" mice, an age when apoptosis occurs along the thalamocortical tract and in some dLGN neurons. In vivo electrophysiology experiments in the dLGN further confirmed the loss of functional retinal input. Experiments with N-methyl-d-aspartic acid-induced V1 lesion as well as Fezf2 cKO mice confirmed that the disruption of connections between the dLGN and V1 lead to abnormal retinal projections to the dLGN. Interestingly, retinal projections to the ventral Lateral Geniculate Nucleus (vLGN) and Superior Colliculus (SC) were normal in all 3 mice models. Finally, we show that the cortexless mice had worse performance than control mice in a go-no go task with visual cues. Our results provide evidence that the wiring of visual circuit from the retina to the dLGN and V1 thereafter is coordinated at a surprisingly early stage of circuit development.

β-amyloid increases neurocan expression through regulating Sox9 in astrocytes: A potential relationship between Sox9 and chondroitin sulfate proteoglycans in Alzheimer's disease

OBJECTIVE

This study aimed to investigate whether β-amyloid (Aβ) was able to enhance neurocan expression in a Sox9 dependent manner in astrocytes.

METHODS AND MATERIALS

Astrocytes were incubated with Aβ at different concentrations, the expression of Sox9 and neurocan was detected by Western blot assay. Meanwhile, the viability and proliferation of astrocytes were assessed by MTT assay. Then, the Sox9 expression was silenced, and the expression of Sox9 and neurocan was examined.

RESULTS

After incubation with Aβ, the viability of astrocytes was increased regardless silencing of Sox9 (all P<0.05). The proliferation of astrocytes was also gradually increased with the increase in the time of Aβ incubation (all P<0.05). With the increase in Aβ concentration, the expression of Sox9 and neurocan was also increased (all P<0.05). However, after silencing of Sox9 expression, the neurocan expression was significantly reduced as compared to control group and scra-siRNA group (all P<0.05).

CONCLUSION

Our study shows the viability and proliferation of astrocytes are significantly increased by Aβ in a dose dependent manner. Moreover, Aβ may effectively up-regulate the neurocan expression via regulating Sox9.

Draxin from neocortical neurons controls the guidance of thalamocortical projections into the neocortex

The thalamocortical tract carries sensory information to the neocortex. It has long been recognized that the neocortical pioneer axons of subplate neurons are essential for thalamocortical development. Herein we report that an axon guidance cue, draxin, is expressed in early-born neocortical neurons, including subplate neurons, and is necessary for thalamocortical development. In draxin^−/− mice, thalamocortical axons do not enter the neocortex. This phenotype is sufficiently rescued by the transgenic expression of draxin in neocortical neurons. Genetic interaction data suggest that draxin acts through Deleted in colorectal cancer (DCC) and Neogenin (Neo1), to regulate thalamocortical projections in vivo. Draxin promotes the outgrowth of thalamic axons in vitro and this effect is abolished in thalamic neurons from Dcc and Neo1 double mutants. These results suggest that draxin from neocortical neurons controls thalamocortical projections into the neocortex, and that this effect is mediated through the DCC and Neo1 receptors.

During neural development thalamocortical axons follow corticofugal projections into the neocortex. Here, using a combination of knock down and rescue experiments, the authors show that Draxin expression in neocortical cells promotes thalamic axon projections from the internal capsule.

Astrocyte roles in traumatic brain injury

Astrocytes sense changes in neural activity and extracellular space composition. In response, they exert homeostatic mechanisms critical for maintaining neural circuit function, such as buffering neurotransmitters, modulating extracellular osmolarity and calibrating neurovascular coupling. In addition to upholding normal brain activities, astrocytes respond to diverse forms of brain injury with heterogeneous and progressive changes of gene expression, morphology, proliferative capacity and function that are collectively referred to as reactive astrogliosis. Traumatic brain injury (TBI) sets in motion complex events in which noxious mechanical forces cause tissue damage and disrupt central nervous system (CNS) homeostasis, which in turn trigger diverse multi-cellular responses that evolve over time and can lead either to neural repair or secondary cellular injury. In response to TBI, astrocytes in different cellular microenvironments tune their reactivity to varying degrees of axonal injury, vascular disruption, ischemia and inflammation. Here we review different forms of TBI-induced astrocyte reactivity and the functional consequences of these responses for TBI pathobiology. Evidence regarding astrocyte contribution to post-traumatic tissue repair and synaptic remodeling is examined, and the potential for targeting specific aspects of astrogliosis to ameliorate TBI sequelae is considered.

The role of chondroitinase as an adjuvant to peripheral nerve repair

Chondroitin sulfate proteoglycans (CSPGs) are potent inhibitors of neural regeneration in the peripheral nervous system. Following nerve injury, inhibitory CSPGs accumulate within the endoneurium and Schwann cell basal lamina of the distal nerve stump. The utilization of chondroitinase ABC (chABC) has led to a marked increase in the ability of injured axons to regenerate across gaps through the CSPG-laden extracellular matrix. Experimental models have repeatedly shown chABC to be capable of degrading the CSPGs that hinder neurite outgrowth. In this article, the characterization of CSPGs, their upregulation following peripheral nerve injury, and potential mechanisms behind their growth and inhibition are described. To date, the literature supports that the adjunct use of chABC may be beneficial to peripheral nerve repair in digesting inhibitory CSPGs. chABC has also shown some indication of synergism with other therapies, such as stem cell transplantation. Evidence supporting the use of chondroitinase as a treatment modality in nerve repair, either alone or in combination with other agents, is reviewed within. Finally, several shortcomings of chABC are addressed, notably its thermal stability and physiologic longevity - both hindering its widespread clinical adoption. Future studies are warranted in order to optimize the therapeutic benefits of the chondroitinase enzyme.

Pioneering axons regulate neuronal polarization in the developing cerebral cortex

The polarization of neurons, which mainly includes the differentiation of axons and dendrites, is regulated by cell-autonomous and non-cell-autonomous factors. In the developing central nervous system, neuronal development occurs in a heterogeneous environment that also comprises extracellular matrices, radial glial cells, and neurons. Although many cell-autonomous factors that affect neuronal polarization have been identified, the microenvironmental cues involved in neuronal polarization remain largely unknown. Here, we show that neuronal polarization occurs in a microenvironment in the lower intermediate zone, where the cell adhesion molecule transient axonal glycoprotein-1 (TAG-1) is expressed in cortical efferent axons. The immature neurites of multipolar cells closely contact TAG-1-positive axons and generate axons. Inhibition of TAG-1-mediated cell-to-cell interaction or its downstream kinase Lyn impairs neuronal polarization. These results show that the TAG-1-mediated cell-to-cell interaction between the unpolarized multipolar cells and the pioneering axons regulates the polarization of multipolar cells partly through Lyn kinase and Rac1.

EphB receptor forward signaling regulates area-specific reciprocal thalamic and cortical axon pathfinding

In early brain development, ascending thalamocortical axons (TCAs) navigate through the ventral telencephalon (VTel) to reach their target regions in the young cerebral cortex. Descending, deep-layer cortical axons subsequently target appropriate thalamic and subcortical target regions. However, precisely how and when corticothalamic axons (CTAs) identify their appropriate, reciprocal thalamic targets remains unclear. We show here that EphB1 and EphB2 receptors control proper navigation of a subset of TCA and CTA projections through the VTel. We show in vivo that EphB receptor forward signaling and the ephrinB1 ligand are required during the early navigation of L1-CAM(+) thalamic fibers in the VTel, and that the misguided thalamic fibers in EphB1/2 KO mice appear to interact with cortical subregion-specific axon populations during reciprocal cortical axon guidance. As such, our findings suggest that descending cortical axons identify specific TCA subpopulations in the dorsal VTel to coordinate reciprocal cortical-thalamic connectivity in the early developing brain.

From basics to clinical: a comprehensive review on spinal cord injury

Spinal cord injury (SCI) is a devastating neurological disorder that affects thousands of individuals each year. Over the past decades an enormous progress has been made in our understanding of the molecular and cellular events generated by SCI, providing insights into crucial mechanisms that contribute to tissue damage and regenerative failure of injured neurons. Current treatment options for SCI include the use of high dose methylprednisolone, surgical interventions to stabilize and decompress the spinal cord, and rehabilitative care. Nonetheless, SCI is still a harmful condition for which there is yet no cure. Cellular, molecular, rehabilitative training and combinatorial therapies have shown promising results in animal models. Nevertheless, work remains to be done to ascertain whether any of these therapies can safely improve patient's condition after human SCI. This review provides an extensive overview of SCI research, as well as its clinical component. It starts covering areas from physiology and anatomy of the spinal cord, neuropathology of the SCI, current clinical options, neuronal plasticity after SCI, animal models and techniques to assess recovery, focusing the subsequent discussion on a variety of promising neuroprotective, cell-based and combinatorial therapeutic approaches that have recently moved, or are close, to clinical testing.

The subplate and early cortical circuits

The developing mammalian cerebral cortex contains a distinct class of cells, subplate neurons (SPns), that play an important role during early development. SPns are the first neurons to be generated in the cerebral cortex, they reside in the cortical white matter, and they are the first to mature physiologically. SPns receive thalamic and neuromodulatory inputs and project into the developing cortical plate, mostly to layer 4. Thus SPns form one of the first functional cortical circuits and are required to relay early oscillatory activity into the developing cortical plate. Pathophysiological impairment or removal of SPns profoundly affects functional cortical development. SPn removal in visual cortex prevents the maturation of thalamocortical synapses, the maturation of inhibition in layer 4, the development of orientation selective responses and the formation of ocular dominance columns. SPn removal also alters ocular dominance plasticity during the critical period. Therefore, SPns are a key regulator of cortical development and plasticity. SPns are vulnerable to injury during prenatal stages and might provide a crucial link between brain injury in development and later cognitive malfunction.

Manipulating the glial scar: chondroitinase ABC as a therapy for spinal cord injury

Chondroitin sulphate proteoglycans (CSPGs) are potent inhibitors of growth in the adult CNS. Use of the enzyme chondroitinase ABC (ChABC) as a strategy to reduce CSPG inhibition in experimental models of spinal cord injury has led to observations of a remarkable capacity for repair. Here we review the evidence that treatment with ChABC, either as an individual therapy or in combination with other strategies, can have multiple beneficial effects on promoting repair following spinal cord injury. These include promoting regeneration of injured axons, plasticity of uninjured pathways and neuroprotection of injured projection neurons. More importantly, ChABC therapy has been demonstrated to promote significant recovery of function to spinal injured animals. Thus, there is robust pre-clinical evidence demonstrating beneficial effects of ChABC treatment following spinal cord injury. Furthermore, these effects have been replicated in a number of different injury models, with independent confirmation by different laboratories, providing an important validation of ChABC as a promising therapeutic strategy. We discuss putative mechanisms underlying ChABC-mediated repair as well as potential issues and considerations in translating ChABC treatment into a clinical therapy for spinal cord injury.

L1 and CHL1 Cooperate in Thalamocortical Axon Targeting

Neural cell adhesion molecule close homolog of L1 (CHL1) is a regulator of topographic targeting of thalamic axons to the somatosensory cortex (S1) but little is known about its cooperation with other L1 class molecules. To investigate this, CHL1(-/-)/L1(-/y) double mutant mice were generated and analyzed for thalamocortical axon topography. Double mutants exhibited a striking posterior shift of axons from motor thalamic nuclei to the visual cortex (V1), which was not observed in single mutants. In wild-type (WT) embryos, L1 and CHL1 were coexpressed in the dorsal thalamus (DT) and on fibers along the thalamocortical projection in the ventral telencephalon and cortex. L1 and CHL1 colocalized on growth cones and neurites of cortical and thalamic neurons in culture. Growth cone collapse assays with WT and mutant neurons demonstrated a requirement for L1 and CHL1 in repellent responses to EphrinA5, a guidance factor for thalamic axons. L1 coimmunoprecipitated with the principal EphrinA5 receptors expressed in the DT (EphA3, EphA4, and EphA7), whereas CHL1 associated selectively with EphA7. These results implicate a novel mechanism in which L1 and CHL1 interact with individual EphA receptors and cooperate to guide subpopulations of thalamic axons to distinct neocortical areas essential for thalamocortical connectivity.

Lrp12/Mig13a reveals changing patterns of preplate neuronal polarity during corticogenesis that are absent in reeler mutant mice

During corticogenesis, the earliest generated neurons form the preplate, which evolves into the marginal zone and subplate. Lrp12/Mig13a, a mammalian gene related to the Caenorhabditis elegans neuroblast migration gene mig-13, is expressed in a subpopulation of preplate neurons that undergo ventrally directed tangential migrations in the preplate layer and pioneer axon projections to the anterior commissure. As the preplate separates, Lrp12/Mig13a-positive neurons polarize in the radial plane and form a pseudocolumnar pattern, prior to moving to a deeper position within the emerging subplate layer. These changes in neuronal polarity do not occur in reeler mutant mice, revealing the earliest known defect in reeler cortical patterning and suggesting that the alignment of preplate neurons into a pseudolayer facilitates the movement of later-born radially migrating neurons into the emerging cortical plate.

Gene deletion mutants reveal a role for semaphorin receptors of the plexin-B family in mechanisms underlying corticogenesis

Semaphorins and their receptors, plexins, are emerging as key regulators of various aspects of neural and nonneural development. Semaphorin 4D (Sema4D) and B-type plexins demonstrate distinct expression patterns over critical time windows during the development of the murine neocortex. Here, analysis of mice genetically lacking plexin-B1 or plexin-B2 revealed the significance of Sema4D-plexin-B signaling in cortical development. Deficiency of plexin-B2 resulted in abnormal cortical layering and defective migration and differentiation of several subtypes of cortical neurons, including Cajal-Retzius cells, GABAergic interneurons, and principal cells in vivo. In contrast, a lack of plexin-B1 did not impact on cortical development in vivo. In various ex vivo assays on embryonic forebrain, Sema4D enhanced the radial and tangential migration of developing neurons in a plexin-B2-dependent manner. These results suggest that Sema4D-plexin-B2 interactions regulate mechanisms underlying cell specification, differentiation, and migration during corticogenesis.

Role of dual leucine zipper-bearing kinase (DLK/MUK/ZPK) in axonal growth

In developing cerebral cortices, post-mitotic neurons migrate toward the pial surface, elongating their axons concurrently. It has been reported that targeted-deletion of the dual leucine zipper-bearing kinase (DLK)/mitogen-activated protein kinase upstream protein kinase (MUK)/leucine-zipper protein kinase (ZPK) gene, which encodes a MAP kinase kinase kinase (MAPKKK) for c-Jun N-terminal kinase (JNK), leads to a neuronal migration-defect and hypoplasia of axonal fiber tracts including those of the anterior commissure and corpus callosum. However, there is no evidence that DLK directly regulates axonal development, because another possibility, i.e. that the defective axonal development in the DLK mutant might be caused secondary to migration failure cannot be ruled out. In this study, we first examined the distributions of DLK mRNA and its protein in the developing cerebral cortex, and found that major portion of DLK proteins appear to be transported into axons. Using dissociated cortical neurons and PC12 cells, we provide direct evidence that DLK regulates axonal elongation. Furthermore, knock-down of DLK decreased the phosphorylation of JNK and its substrate, microtubule-associated protein 1B (MAP1B), which is known to be involved in axonal elongation. These results suggest that the DLK/MUK/ZPK-JNK pathway directly regulates axonal growth through phosphorylation of MAP1B.

The transcriptional repressor RP58 is crucial for cell-division patterning and neuronal survival in the developing cortex

The neocortex and the hippocampus comprise several specific layers containing distinct neurons that originate from progenitors at specific development times, under the control of an adequate cell-division patterning mechanism. Although many molecules are known to regulate this cell-division patterning process, its details are not well understood. Here, we show that, in the developing cerebral cortex, the RP58 transcription repressor protein was expressed both in postmitotic glutamatergic projection neurons and in their progenitor cells, but not in GABAergic interneurons. Targeted deletion of the RP58 gene led to dysplasia of the neocortex and of the hippocampus, reduction of the number of mature cortical neurons, and defects of laminar organization, which reflect abnormal neuronal migration within the cortical plate. We demonstrate an impairment of the cell-division patterning during the late embryonic stage and an enhancement of apoptosis of the postmitotic neurons in the RP58-deficient cortex. These results suggest that RP58 controls cell division of progenitor cells and regulates the survival of postmitotic cortical neurons.

Gene expression profiling of preplate neurons destined for the subplate: genes involved in transcription, axon extension, neurotransmitter regulation, steroid hormone signaling, and neuronal survival

During mammalian corticogenesis a series of transient cell layers establish laminar architectonics. The preplate, which forms from the earliest-generated neurons, separates into the marginal zone and subplate layer. To provide a systematic screen for genes involved in subplate development and function, we screened lines of transgenic mice, generated using bacterial artificial chromosome methodology (GENSAT Project), to identify transgenic lines of mice that express the enhanced green fluorescent protein (EGFP) reporter in preplate neurons destined for the subplate. Gene expression profiling of RNA purified from EGFP-positive neurons identified over 200 genes with enriched expression in future subplate neurons. Major classes of subplate-enriched genes included genes involved in transcriptional processes, cortical development, cell and axon motility, protein trafficking and steroid hormone signaling. Additionally, we identified 10 genes related to degenerative diseases of the cerebral and cerebellar cortex. Cre recombinase-based fate mapping of cells expressing Phosphodiesterase 1c (Pde1c) revealed beta-galactosidase positive cells in the ventricular zone, as well as the subplate, suggesting that subplate neurons and cortical projection neurons may be derived from common progenitors. These experiments therefore reveal genetic markers, which identify subplate neurons from the earliest stages of their development, and genes with enriched expression in subplate neurons during early stages of corticogenesis.

Expression of Leukaemia associated transcription factor Af9/Mllt3 in the cerebral cortex of the mouse

Mutations of leukaemia associated AF9/MLLT3 are implicated in neurodevelopmental diseases such as epilepsia and ataxia. This study shows for the first time, that murine Af9 is transcribed in various CNS structures including the subventricular zone (SVZ) of the cerebral cortex, hippocampus, cerebellar cortex, septum and various thalamic structures, the choroid plexus, and the midbrain/hindbrain boundary. Expression of Af9 in the SVZ overlaps with Svet1, Cux2, and partially with Tbr2, confining its activity to the neurogenic compartment of the SVZ. In contrast to Svet1 and Cux2 expression, Af9 transcription is not limited to upper layer neurons but is found in the entire cortical plate. As part of an extensive network of interacting proteins involved in epigenetic DNA modification, we could show overlapping expression of Af9 with Af4/Aff1 and Fmr2/Aff2, two genes that are also related to neurodevelopmental diseases, as well as with the highly homologous Enl.

Satb2 is a postmitotic determinant for upper-layer neuron specification in the neocortex

Pyramidal neurons of the neocortex can be subdivided into two major groups: deep- (DL) and upper-layer (UL) neurons. Here we report that the expression of the AT-rich DNA-binding protein Satb2 defines two subclasses of UL neurons: UL1 (Satb2 positive) and UL2 (Satb2 negative). In the absence of Satb2, UL1 neurons lose their identity and activate DL- and UL2-specific genetic programs. UL1 neurons in Satb2 mutants fail to migrate to superficial layers and do not contribute to the corpus callosum but to the corticospinal tract, which is normally populated by DL axons. Ctip2, a gene required for the formation of the corticospinal tract, is ectopically expressed in all UL1 neurons in the absence of Satb2. Satb2 protein interacts with the Ctip2 genomic region and controls chromatin remodeling at this locus. Satb2 therefore is required for the initiation of the UL1-specific genetic program and for the inactivation of DL- and UL2-specific genes.

Pattern of chondroitin sulfate proteoglycan expression after ablation of the sensorimotor cortex of the neonatal and adult rat brain

The central nervous system has a limited capacity for self-repair after damage. However, the neonatal brain has agreater capacity for recovery than the adult brain. These differences in the regenerative capability depend on local environmental factors and the maturational stage of growing axons. Among molecules which have both growth-promoting and growth-inhibiting activities is the heterogeneous class of chondroitin sulfate proteoglycans (CSPGs). In this paper, we investigated the chondroitin-4 and chondroitin-6 sulfate proteoglycan expression profile after left sensorimotor cortex ablation of the neonatal and adult rat brain. Immunohistochemical analysis revealed that compared to the normal uninjured cortex, lesion provoked up regulation of CSPGs showing a different pattern of expression in the neonatal vs. the adult brain. Punctuate and membrane-bound labeling was predominate after neonatal lesion, where as heavy deposition of staining in the extracellular matrix was observed after adult lesion. Heavy deposition of CSPG immunoreactivity around the lesionsite in adult rats, in contrast to a less CSPG-rich environment in neonatal rats, indicated that enhancement of the recovery process after neonatal injury is due to amore permissive environment.

Spatial learning and expression of neural cell adhesion molecule L1 in rats X-irradiated prenatally

The present study was designed to present evidence to clarify the relationships between learning ability, neuronal cell adhesion molecule L1 expression and hippocampal structural changes in the rat model received X-irradiation at an embryonic stage (E15). Water maze task indicated that all of the irradiated rats failed to learn the task in the whole training procedure. Their latency to the platform and swimming distance were significant differences from those sham-treated controls. Histological studies showed that the hippocampal ectopias induced by X-rays in the CA1 were involved in the spatial learning impairment, in which they hampered normal processes in learning development and transmission of information. Number, size and positions of the ectopias in the dorsal parts of the hippocampus were confirmed to be related to degrees of spatial learning impairment. On the other hand, L1 expression in the hippocampus was examined with Western blot analysis. The results indicated a lower content of L1 in the irradiated rats. A decrease in L1 might be one of reasons to cause disorganization of the septohippocampal pathways. These findings suggest some mechanisms of spatial learning impairment can be attributed to the formation of the hippocampal ectopias and redaction of L1 following prenatal exposure to X-irradiation.

Defects in reciprocal projections between the thalamus and cerebral cortex in the early development of Fezl-deficient mice

Fez-like (Fezl), the forebrain embryonic zinc finger-like protein, is a transcriptional repressor selectively expressed in the deep layers of the developing cortex. We examined the thalamocortical and corticofugal pathways in Fezl-deficient fetal mice by using immunohistochemistry and by axonal labeling with the lipophilic dyes DiI and DiA, with special attention to the spatiotemporal relation between thalamocortical and corticofugal axons. In normal mice, thalamic and cortical axons meet in the internal capsule between embryonic day (E) 13.5 and E14.5 and fasciculate with each other as they extend to their targets, the cortex and thalamus, respectively. In Fezl-deficient mice, most of the thalamic and cortical axons stop in the internal capsule and at the pallial-subpallial boundary at E14.5, respectively. This abnormality is transient, and the thalamic and cortical axons reach their targets at E15.5, although the number of thalamic axons is remarkably reduced in the cortical anlage. Double labeling with DiI and DiA demonstrated close apposition of the thalamic and cortical axons in the subpallium and pallium as well as in the external capsule of this mutant after E15.5. Because the expression of genes that define the pallial-subpallial boundary and guidance molecules of thalamocortical axons did not show remarkable changes in Fezl-deficient mice, abnormal formation of thalamocortical pathway in this mutant may be caused by the defect of axons of cortical efferent neurons that express Fezl.

Entry and Distribution of Microglial Cells in Human Embryonic and Fetal Cerebral Cortex

Microglial cells penetrate into and scatter throughout the human cortical grey and white matter according to a specific spatiotemporal pattern during the first 2 trimesters of gestation. Routes of entry were quantitatively and qualitatively different from those identified in the diencephalon. Starting at 4.5 gestational weeks, amoeboid microglial cells, characterized by different antibodies as Iba1, CD68, CD45, and MHC-II, entered the cerebral wall from the ventricular lumen and the leptomeninges. Migration was mainly radial and tangential toward the immature white matter, subplate layer, and cortical plate, whereas pial cells populated the prospective layer I. The intraparenchymal vascular route of entry was detectable only from 12 gestational weeks. Interestingly, microglial cells accumulated in restricted laminar bands particularly at 19 to 24 gestational weeks among the corona radiata fibers rostrally, extending caudally in the immature white matter to reach the visual radiations. This accumulation of proliferating MIB1-positive microglia (as shown by MIB1-Iba1 double immunolabeling) was located at the site of white matter injury in premature neonates. The spatiotemporal organization of microglia in the immature white and grey matter suggests that these cells may play active roles in developmental processes and in injury to the developing brain.

DSD-1-Proteoglycan/Phosphacan and Receptor Protein Tyrosine Phosphatase-Beta Isoforms during Development and Regeneration of Neural Tissues

Interactions between neurons and glial cells play important roles in regulating key events of development and regeneration of the CNS. Thus, migrating neurons are partly guided by radial glia to their target, and glial scaffolds direct the growth and directional choice of advancing axons, e.g., at the midline. In the adult, reactive astrocytes and myelin components play a pivotal role in the inhibition of regeneration. The past years have shown that astrocytic functions are mediated on the molecular level by extracellular matrix components, which include various glycoproteins and proteoglycans. One important, developmentally regulated chondroitin sulfate proteoglycan is DSD-1-PG/phosphacan, a glial derived proteoglycan which represents a splice variant of the receptor protein tyrosine phosphatase (RPTP)-beta (also known as PTP-zeta). Current evidence suggests that this proteoglycan influences axon growth in development and regeneration, displaying inhibitory or stimulatory effects dependent on the mode of presentation, and the neuronal lineage. These effects seem to be mediated by neuronal receptors of the Ig-CAM superfamily.

Tangential neuronal migration controls axon guidance: a role for neuregulin-1 in thalamocortical axon navigation

Neuronal migration and axon guidance constitute fundamental processes in brain development that are generally studied independently. Although both share common mechanisms of cell biology and biochemistry, little is known about their coordinated integration in the formation of neural circuits. Here we show that the development of the thalamocortical projection, one of the most prominent tracts in the mammalian brain, depends on the early tangential migration of a population of neurons derived from the ventral telencephalon. This tangential migration contributes to the establishment of a permissive corridor that is essential for thalamocortical axon pathfinding. Our results also demonstrate that in this process two different products of the Neuregulin-1 gene, CRD-NRG1 and Ig-NRG1, mediate the guidance of thalamocortical axons. These results show that neuronal tangential migration constitutes a novel mechanism to control the timely arrangement of guidance cues required for axonal tract formation in the mammalian brain.

Identification and Functions of Chondroitin Sulfate in the Milieu of Neural Stem Cells

The behavior of cells is generally considered to be regulated by environmental factors, but the molecules in the milieu of neural stem cells have been little studied. We found by immunohistochemistry that chondroitin sulfate (CS) existed in the surroundings of nestin-positive cells or neural stem/progenitor cells in the rat ventricular zone of the telencephalon at embryonic day 14. Brain-specific chondroitin sulfate proteoglycans (CSPGs), including neurocan, phosphacan/receptor-type protein-tyrosine phosphatase beta, and neuroglycan C, were detected in the ventricular zone. Neurospheres formed by cells from the fetal telencephalon also expressed these CSPGs and NG2 proteoglycan. To examine the structural features and functions of CS polysaccharides in the milieu of neural stem cells, we isolated and purified CS from embryonic day 14 telencephalons. The CS preparation consisted of two fractions differing in size and extent of sulfation: small CS polysaccharides with low sulfation and large CS polysaccharides with high sulfation. Interestingly, both CS polysaccharides and commercial preparations of dermatan sulfate CS-B and an E-type of highly sulfated CS promoted the fibroblast growth factor-2-mediated proliferation of neural stem/progenitor cells. None of these CS preparations promoted the epidermal growth factor-mediated neural stem cell proliferation. These results suggest that these CSPGs are involved in the proliferation of neural stem cells as a group of cell microenvironmental factors.

Prenatal alcohol exposure delays the development of the cortical barrel field in neonatal rats

In-utero alcohol exposure produces sensorimotor developmental abnormalities that often persist into adulthood. The rodent cortical barrel field associated with the representation of the body surface was used as our model system to examine the effect of prenatal alcohol exposure (PAE) on early somatosensory cortical development. In this study, pregnant female rats were intragastrically gavaged daily with high doses of alcohol (6 gm/kg body weight) throughout the first 20 days of pregnancy. Blood alcohol levels were measured in the pregnant dams on gestational days 13 (G13) and G20. The ethanol treated group (EtOH) was compared to the normal control chowfed (CF) group, nutritionally matched pairfed (PF) group, and cross-foster (XF) group. Cortical barrel development was examined in pups across all treatment groups from G25, corresponding to postnatal day 2 (P2), to G32 corresponding to P9. The EtOH and control group pups were weighed, anesthetized, and perfused. Brains were removed and weighed with, and without cerebellum and olfactory bulbs, and neocortex was removed and weighed. Cortices were then flattened, sectioned tangentially, and stained with a metabolic marker, cytochrome oxidase (CO) to reveal the barrel field. Progression of barrel development was distinguished into three categories: (a) absent, (b) cloudy barrel-like pattern, and (c) well-formed barrels with intervening septae. The major findings are: (1) PAE delayed barrel field development by one or more days, (2) the barrel field first appeared as a cloudy pattern that gave way on subsequent days to an adult-like pattern with clearly demarcated intervening septal regions, (3) the barrel field developed differentially in a lateral-to-medial gradient in both alcohol and control groups, (4) PAE delayed birth by one or more days in 53% of the pups, (5) regardless of whether pups were born on G23 (normal expected birth date for non-alcohol controls) or as in the case for the alcohol-delayed pups born as late as G27, the barrel field was never present at birth suggesting the importance of postnatal experience on barrel field development, and (6) PAE did not disrupt the normal barrel field pattern, although both total body and brain weights were compromised. These findings suggest that PAE delays the development of the somatosensory cortex (SI); such delays may interfere with timing and formation of cortical circuits. It is unknown whether other nuclei along the somatosensory pathway undergo similar delays in development or if PAE selectively disrupts cortical circuitry.

A sulfated disaccharide derived from chondroitin sulfate proteoglycan protects against inflammation-associated neurodegeneration

Chondroitin sulfate proteoglycan (CSPG), a matrix protein that occurs naturally in the central nervous system (CNS), is considered to be a major inhibitor of axonal regeneration and is known to participate in activation of the inflammatory response. The degradation of CSPG by a specific enzyme, chondroitinase ABC, promotes repair. We postulated that a disaccharidic degradation product of this glycoprotein (CSPG-DS), generated following such degradation, participates in the modulation of the inflammatory responses and can, therefore, promote recovery in immune-induced neuropathologies of the CNS, such as experimental autoimmune encephalomyelitis (EAE) and experimental autoimmune uveitis (EAU). In these pathologies, the dramatic increase in T cells infiltrating the CNS is far in excess of the numbers needed for regular maintenance. Here, we show that CSPG-DS markedly alleviated the clinical symptoms of EAE and protected against the neuronal loss in EAU. The last effect was associated with a reduction in the numbers of infiltrating T cells and marked microglia activation. This is further supported by our in vitro results indicating that CSPG-DS attenuated T cell motility and decreased secretion of the cytokines interferon-gamma and tumor necrosis factor-alpha. Mechanistically, these effects are associated with an increase in SOCS-3 levels and a decrease in NF-kappaB. Our results point to a potential therapeutic modality, in which a compound derived from an endogenous CNS-resident molecule, known for its destructive role in CNS recovery, might be helpful in overcoming inflammation-induced neurodegenerative conditions.

Aberrant trajectory of thalamocortical axons associated with abnormal localization of neurocan immunoreactivity in the cerebral neocortex of reeler mutant mice

We examined the molecular mechanisms underlying the formation of the thalamocortical pathway in the cerebral neocortex of normal and reeler mutant mice. During normal development of the mouse neocortex, thalamic axons immunoreactive for the neural cell adhesion molecule L1 rarely invaded the cortical plate and ran centered in the subplate which is immunoreactive for neurocan, a brain-specific chondroitin sulfate proteoglycan. On the other hand, in homozygous reeler mutant mice, thalamic axons took an aberrant course to run obliquely through the cortical plate. Injection of bromodeoxyuridine at embryonic day 11 specifically labeled subplate neurons in normal mice, whilst in the reeler neocortex it labeled cells scattered in the cortical plate as well as in the superficial layer (superplate). Neurocan immunoreactivity was associated with the bromodeoxyuridine-positive cells in the superplate, as well as being present in oblique bands within the cortical plate, along which L1-bearing thalamic axons preferentially ran. The present results support our previous hypothesis proposed for normal rats that a heterophilic molecular interaction between L1 and neurocan is involved in determining the thalamocortical pathway within the neocortical anlage [T. Fukuda et al. (1997) Journal of Comparative Neurology, 382, 141-152].

Inhibition of collagen synthesis overrides the age-related failure of regeneration of nigrostriatal dopaminergic axons

To investigate the mechanism of the age-related failure of regeneration of transected axons, nigrostriatal dopaminergic axons were unilaterally transected in the lateral hypothalamus in adult mice and in immature mice aged postnatal days 7, 14, and 21. Ten days after the transection, tyrosine hydroxylase-immunoreactive axons had regenerated from caudally to rostrally across the lesion site in mice transected at postnatal day 7, whereas they stopped and did not extend across the lesion site in mice transected at postnatal day 14 or older. Reactive astrocytes bearing chondroitin sulfate proteoglycans were observed around the lesion in mice transected at all ages. However, a fibrotic scar containing type IV collagen-immunoreactive deposits, which was consistently formed at the lesion site in mice transected at postnatal day 14 or older, was not formed in mice lesioned at postnatal day 7. When 2,2'-dipyridyl, an inhibitor of collagen synthesis, was injected into the lesion site at the time of transection in both postnatal day 14 and adult mice, the deposition of type IV collagen and the formation of a fibrotic scar were completely prevented, and large numbers of tyrosine hydroxylase-immunoreactive axons extended across the lesion and reinnervated the striatum. These results imply that the fibrotic scar formed in the lesion site is a crucial impediment to the regeneration of ascending dopaminergic axons in adult mice and suggest that type IV collagen is required for the development of the fibrotic response to adult brain injury.

CNR/Pcdhalpha family in subplate neurons, and developing cortical connectivity

The cadherin-related neuronal receptor (CNR)/protocadherin (Pcdh) alpha family is one of the diverse protocadherin families identified as a candidate diversified membrane-associated component regulating the formation of neuronal connectivity. However, its expression during neural circuit formation has not been examined in detail. Here, we used a conserved sequence to study the expression of this protein family during the development of neocortical connectivity, by immunohistochemistry and in situ hybridization. The proteins were detected in developing thalamocortical and corticofugal axons, and in subplate neurons, which pioneer these axon tracts. The expression in subplate neurons was confirmed by birth-date labeling with BrdU, and by examination in homozygous reeler mice. This pattern of CNR/Pcdhalpha expression suggests its involvement in the development of neocortical connectivity.

Distribution of L1cam mRNA in the adult mouse brain: In situ hybridization and Northern blot analyses

Previous immunohistochemical analysis revealed a wide distribution of L1cam-positive neural and nonneural structures in adult mouse brain. Although there were numerous punctate immunoreactive nerve terminals, only a few immunoreactive neuronal cell somata were present (Munakata et al. [2003] BMC Neurosci. 4:7). To explore the distribution of L1cam mRNA-containing cells, which are interpreted to be L1cam-producing cells, we performed in situ hybridization histochemistry with an antisense L1cam cRNA probe. L1cam mRNA was distributed widely from the olfactory bulb to the upper cervical cord with an uneven localization pattern in adult brain. All positive cell somata with silver grains after emulsion autoradiography were neuronal, and no grains were detected on nonneural cells in the present study. A high density of signals for neuronal L1cam mRNA was found in the thalamus, mammillary body, and hippocampus. In addition, strong hybridization signals were localized in various nuclei: main and accessory olfactory bulb, compact part of the substantia nigra, pontine gray matter, tegmental reticular nucleus, Edinger-Westphal nucleus, trigeminal motor nucleus, locus coeruleus, mesencephalic trigeminal nucleus, raphe nuclei, facial nucleus, ambiguus nucleus, dorsal motor vagal nucleus, and inferior olivary nucleus. Some long projection neurons such as the pyramidal, mitral, principal neurons of several cranial nuclei, and presumably monoaminergic cells containing noradrenalin, dopamine, and serotonin, expressed high levels of L1cam.

Multiple defects in the formation of rat cortical axonal pathways following prenatal X-ray irradiation

Prenatal X-ray irradiation is known to result in severe defects of neuronal migration and laminar formation in the cerebral cortex. We examined the formation of cortical afferent and efferent pathways in rats that had been exposed to X-ray irradiation (1.0 Gy) at embryonic day 14 (E14), by birthdating with bromodeoxyuridine (BrdU) and axonal labeling with 1-1'-dioctodecyl-3,3,3',3'- tetramethyl-indocarbocyanine perchlorate (DiI), in addition to immunohistochemical staining for various axonal markers including neurofilament, and cell adhesion molecules L1 and TAG-1. The results obtained were as follows. (i) The neuroepithelium formed germinal rosettes and concavities in the cortical anlage from 2 days after irradiation. Neurons generated in the neuroepithelium accumulated to form subcortical heterotopia and obstructed pathway formation in the intermediate zone, resulting in an aberrant trajectory of TAG-1-immunoreactive cortical efferent axons. (ii) In rats exposed to X-ray irradiation at E14, cystic cavities were formed in the cortex-striatum boundary region between E15 and E17, probably because of delayed cell death of neurons generated at E14. These cavities transiently interrupted both cortical afferent (L1-positive) and efferent (TAG-1-positive) axons. (iii) X-ray irradiation at E14 partially destroyed subplate neurons (transient targets of thalamic afferent axons) and disturbed the arrangement of the subplate layer. This resulted in a misrouting of neurofilament- and L1-immunoreactive thalamocortical axons that obliquely traversed the cortical plate to run up to the superficial layer. The present study demonstrates for the first time that X-ray irradiation during initial cortical development causes multiple defects in the formation of cortical afferent and efferent pathways.

Expression of neurocan after transient middle cerebral artery occlusion in adult rat brain

Neurocan is one of the major chondroitin sulfate proteoglycans in the nervous tissues. The expression and proteolytic cleavage of neurocan are developmentally regulated in the normal rat brain, and the full-length neurocan is detected in juvenile brains but not in normal adult brains. Recently, some studies showed that the full-length neurocan was detectable even in the adult brain when it was exposed to mechanical incision or epileptic stimulation. In the present study, we demonstrated by Western blot analysis that the full-length neurocan transiently appeared in the peri-ischemic region of transient middle cerebral artery occlusion (tMCAO) in adult rat with a peak level at 4 days after tMCAO. Immunohistochemical analysis showed that a clear positive signal of neurocan was observed 4 days after tMCAO in the peri-ischemic region of cerebral cortex and caudate, where cells strongly positive in GFAP expression were also distributed. These results indicate that accumulation of the full-length neurocan produced by reactive astrocytes may be one of the processes for tissue repair and reconstruction of neural networks after focal brain ischemia as well.

Distribution of RA175/TSLC1/SynCAM, a member of the immunoglobulin superfamily, in the developing nervous system

RA175 is a new member of the immunoglobulin superfamily with trans interaction activity, and it plays a role as a tumor suppressor in lung carcinoma (TSLC1) and as a cell adhesion molecule promoting the formation of functional synapses (SynCAM). Little is known about the biological function of RA175/TSLC1/SynCAM neural network formation during neurogenesis. We examined the distribution and colocalization of the RA175/TSLC1/SynCAM protein with other members of the immunoglobulin superfamily such as NCAM, L1, and TAG-1 in the mouse developing nervous system. Consistent with the expression of RA175/TSLC1/SynCAM mRNA, the protein was localized in the brain neuroepithelium at embryonic day (E) 9.5, neural crest at E10.5, motor neurons at E10.5, and olfactory epithelium at E16.5. In contrast with its mRNA, the protein was intensely detected on the fasciculated axons in the floor plates, ventral root, and dorsal funiculus in the E10.5-11.5 spinal cord and colocalized with NCAM and L1 on the ventral root and dorsal funiculus and partly colocalized with TAG-1 on the commissural axons and dorsal funiculus. In the E13.5-15.5 brain, RA175/TSLC1/SynCAM colocalized with NCAM and L1 on the developing thalamocortical fibers from the internal capsule (IC) and partly colocalized with TAG-1 on the cortical efferent axons in the intermediate zone (IZ). RA175/TSLC1/SynCAM was localized on the axons of some of the cortical neurons cultured in vitro. Thus, in addition to cell adhesion activity in the neuroepithelium and the synapses, RA175/TSLC1/SynCAM may be involved in neuronal migration, axon growth, pathfinding, and fasciculation on the axons of differentiating neurons.

Cellular and molecular tunnels surrounding the forebrain commissures of human fetuses

Glial cells and extracellular matrix (ECM) molecules surround developing fiber tracts and are implicated in axonal pathfinding. These and other molecules are produced by these strategically located glial cells and have been shown to influence axonal growth across the midline in rodents. We searched for similar cellular and molecular structures surrounding the telencephalic commissures of fetal human brains. Paraffin-embedded brain sections were immunostained for glial fibrillary acidic protein (GFAP) and vimentin (VN) to identify glial cells; for microtubule-associated protein-2 (MAP-2) and neuronal nuclear protein (NeuN) to document neurons; for neurofilament (NF) to identify axons; and for chondroitin sulfate (CS), tenascin (TN), and fibronectin (FN) to show the ECM. As in rodents, three cellular clusters surrounding the corpus callosum were identified by their expression of GFAP and VN (but not MAP-2 or NeuN) from 13 to at least 18 weeks postovulation (wpo): the glial wedge, the glia of the indusium griseum, and the midline sling. CS and TN (but not FN) were expressed pericellularly in these cell groups. The anterior commissure was surrounded by a GFAP+/VN+ glial tunnel from 12 wpo, with TN expression seen between the GFAP+ cell bodies. The fimbria showed GFAP+/VN+ cells at its lateral and medial borders from 12 wpo, with pericellular expression of CS. The fornix showed GFAP+ cells somewhat later (16 wpo). Because these structures are similar to those described for rodents, we concluded that the axon guiding mechanisms postulated for commissural formation in nonhuman mammals may also be operant in the developing human brain.

Expression profiles of EphA3 at both the RNA and protein level in the developing mammalian forebrain

The ephrin/Eph system is well known to regulate various aspects of brain development. In this study, we analyzed the expression profiles of EphA3 at both the RNA and protein level in developing mouse forebrains. Although the EphA3 gene is known to encode two isoforms of the receptors, a full-length transmembrane form, and a short, secretory form, only the full-length isoform was detected in the developing forebrain. We found that, in the early developmental stages, while EphA3 mRNA was expressed in the dorsal thalamus and the cortical intermediate zone (IMZ), the EphA3 protein was detected in the IMZ and the internal capsule, but not in the dorsal thalamus. In the later stages the mRNA was expressed in the most superficial region of the cortical plate, while the protein was expressed in the IMZ. This discrepancy between the mRNA and protein expression patterns might be attributed to the possibility of the protein being transported to the axons to regulate the thalamocortical and corticofugal projection. The results of double-immunostaining for L1 and EphA3 or TAG-1 and EphA3 suggested that EphA3 protein was produced mainly in the thalamocortical axons and only partially in the corticofugal axons. In addition, the EphA3 protein was also detected in various other structures, such as the lateral olfactory tract, anterior commissure, and corpus callosum, suggesting the possibility that EphA3 might regulate the formation of various neuronal networks in the developing brain, including the TC projection and the commissural fibers.

Zinc finger gene fez-like functions in the formation of subplate neurons and thalamocortical axons

fez-like (fezl) is a forebrain-expressed zinc finger gene required for the formation of the hypothalamic dopaminergic and serotonergic (monoaminergic) neurons in zebrafish. To reveal its function in mammals, we analyzed the expression of the mouse orthologue of fezl and generated fezl-deficient mice by homologous recombination. Mouse fezl was expressed specifically in the forebrain from embryonic day 8.5. At mid-gestation, fezl expression was detected in subdomains of the forebrain, including the dorsal telencephalon and ventral diencephalon. Unlike the zebrafish fezl mutant too few, the fezl-deficient mice displayed normal development of hypothalamic monoaminergic neurons, but showed abnormal "hyperactive" behavior. In fezl(-/-) mice, the thalamocortical axons (TCA) were reduced in number and aberrantly projected to the cortex. These mutants had a reduced number of subplate neurons, which are involved in guiding the TCA from the dorsal thalamus, although the subplate neurons were born normally. These results suggest that fezl is required for differentiation or survival of the subplate neurons, and reduction of the subplate neurons in fezl-deficient mice leads to abnormal development of the TCA, providing a possible link between the transcriptional regulation of forebrain development and hyperactive behavior.

Axon mediated interneuron migration

Mammalian forebrain development requires extensive cell migration for cells to reach their appropriate location in the adult brain. Defects in this migration result in human malformations and neurologic deficits. Thus, understanding the mechanisms underlying normal cell migration during development is essential to understanding the pathogenesis of human malformations. Radial glia are known to support radial cell migration, while axons have been proposed as substrate for some non-radially migrating cells. Herein we have directly tested the hypothesis that axons can support non-radial cell migration. One population of cells known to migrate non-radially is the inhibitory interneurons that move from the ganglionic eminence to the cerebral cortex. We first show that early born GABAergic cells colocalize with TAG-1-positive (TAG-1+) axons, while later born cells colocalize with intermediate weight neurofilament-positive, TAG-1-negative (TAG-1-) processes, suggesting temporal differences in substrate specificities. We next developed an in vitro assay that allows us to observe cell migration on axons in culture. Using this assay we find that early born medial ganglionic eminence-derived interneurons migrate preferentially on TAG-1+ axons, while later born cells only migrate on neurofilament-positive/TAG-1- processes. These data provide the first direct evidence that ganglionic eminence cells migrate on axons and that there is an age-dependent substrate preference. Furthermore, the assay developed and characterized herein provides a robust method to further study the molecular substrates and guidance cues of axonophilic cell migration in neural development.

Cell adhesion molecule L1 promotes neurite outgrowth of septal neurons

To establish if the cell adhesion molecule L1 could promote neurite outgrowth of septal neurons, L1-positive substrates were prepared by genetically modifying 3T3 fibroblasts with a retroviral vector encoding human L1 under the control of a negative tetracycline-regulatory system. In several clones of L1-transfected fibroblasts, L1 expression at the cell surface was prominent and efficiently regulated by doxycycline, a tetracycline analogue. In co-culture of septal neurons and fibroblasts, a two-dimensional fractionator probe provided systematic random sampling of the neurites to be measured. Septal neurons, isolated at embryonic Day 17, were found to express L1 in vitro and to extend significantly longer neurites when plated on L1-expressing fibroblasts compared to control fibroblasts. The neurite outgrowth-promoting effect of L1 was inhibited after a doxycycline treatment, which specifically suppressed L1 expression from the modified fibroblasts. The findings that septal neurons at embryonic Day 17 in vitro express L1 and respond to L1-modulation suggest that this molecule is involved in development of the septohippocampal pathway.

Neural cell adhesion molecule L1 is required for fasciculation and routing of thalamocortical fibres and corticothalamic fibres

To examine the role of neural cell adhesion molecule L1 in thalamocortical projections, we analysed L1 deficient (L1-/y) mice. Immunohistochemistry of pleiotrophin/HB-GAM, a marker for thalamocortical axons and axonal tracing experiments showed that thalamocortical axons were abnormally and highly fasciculated when they pass through the developing internal capsule. Within the cortex, however, their course was more diffuse. The corticofugal fibres immunoreactive for TAG-1 were also more strongly fasciculated and their number was decreased in L1-/y mice. Furthermore, no TAG-1-positive corticofugal axons reached the dorsal thalamus. These data suggest that L1 plays an important role in the fasciculation and routing of axons connecting between the thalamus and the cortex.

Altered neurotrophin receptor function in the developing prefrontal cortex leads to adult-onset dopaminergic hyperresponsivity and impaired prepulse inhibition of acoustic startle

BACKGROUND

Survival and differentiation of neurons and the formation and maintenance of synapses in the cerebral cortex may be affected in schizophrenia. Since neurotrophins play an important role in these events, behavioral effects relevant to schizophrenia were investigated in rats that had compromised neurotrophin function during prefrontal cortical development.

METHODS

Neonatal rat pups were injected into the developing prefrontal cortex with a depot preparation of p75 receptor antibody conjugated to saporin. Animals were tested for dopaminergic hyperresponsivity and prepulse inhibition of acoustic startle at 5 or 10 weeks. Neonatal and adult brain sections were examined for morphologic abnormality.

RESULTS

Animals that received neonatal injections of p75 antibody conjugated to saporin showed significantly increased amphetamine-induced locomotion and rearing and impairment of prepulse inhibition of acoustic startle at 10 weeks of age but not at 5 weeks. Examination of adult brain sections revealed apparently normal structure, whereas neonatal brain sections showed apoptotic cells in the developing prefrontal cortex in pups that received p75 antibody conjugated to saporin.

CONCLUSIONS

Compromised p75 neurotrophin receptor function in the developing prefrontal cortex may be associated with the manifestation of adult-onset dopaminergic hyperresponsivity and impaired prepulse inhibition and therefore may be involved in the pathogenesis of schizophrenia.

Developmental regulation of notochord-derived repulsion for dorsal root ganglion axons

During the initial stages of development, the notochord provides repulsive signals for dorsal root ganglion (DRG) axons via semaphorin 3A/neuropilin-1, axonin-1/SC2, and other unknown repulsive molecules. The notochord is known to produce aggrecan, one of the chondroitin sulfate proteoglycans (CSPGs). We report here that adding aggrecan to the culture medium cannot only induce DRG growth cone collapse, but also inhibit DRG axonal growth. Using cocultures composed of tissues derived from chick embryos or neuropilin-1-deficient mice treated with chondroitinase ABC, we show the direct evidence that CSPGs are involved in notochord-derived repulsion for DRG axons. At later developmental stages, CSPGs are involved in perinotochordal sheath-derived axon repulsion, but not in notochord core-derived repulsion. We further demonstrate that TAG-1/axonin-1/SC2 is not involved in mediating repulsive activities by CSPGs, but is required for notochord core-derived axon repulsion. Thus, notochord-derived multiple axon repulsions act in a spatiotemporal-specific manner to shape the initial trajectories of DRG axons.

Receptor-like protein tyrosine phosphatase zeta/RPTP beta is expressed on tangentially aligned neurons in early mouse neocortex

Protein tyrosine phosphatase zeta (PTPzeta)/RPTPbeta is a chondroitin sulfate proteoglycan predominantly expressed in the brain. In this study, we examined immunohistochemical localisation of PTPzeta in the mouse telencephalon from embryonic day 9.5 (E9.5) to E15.5. During E10.5-E12.5, immunoreactivities for PTPzeta are specifically observed on the tangentially aligned neurons at the preplate (PP) of the neocortex, as well as on the neurons at the mantle layer (ML) of the ganglionic eminences (GEs). Likewise, neurons immunoreactive for CR50, a marker for Cajal-Retzius neurons, are aligned from the ML of the ganglionic eminences to the PP of the neocortex and co-express PTPzeta. During E13.5-E15.5, PTPzeta-positive neurons are present at the subplate (SP) as well as at the marginal zone (MZ) of the neocortex. These results indicate that PTPzeta is a useful marker for early-generated neocortical neurons in mice: Cajal-Retzius neurons as well as the subplate neurons.

Kainic acid-induced convulsions cause prolonged changes in the chondroitin sulfate proteoglycans neurocan and phosphacan in the limbic structures

Systemic administration of kainic acid induces repeated convulsive seizures (KA convulsions) that result in neuropathological changes similar to temporal lobe epilepsy and the appearance of spontaneous recurrent seizures (SRS). The appearance of SRS is considered a result of the remodeling of neuronal networks following neuronal degeneration. We investigated the changes in chondroitin sulfate proteoglycans (CSPGs) in the limbic structures after KA convulsions in the rat using monoclonal antibodies 1G2, which recognizes full-length neurocan and the C-terminal half of neurocan, neurocan C, and 6B4, which recognize phosphacan and protein tyrosine phosphatase zeta. After KA convulsions, full-length neurocan appeared by 24 h and reached a peak by 48 to 72 h, whereas phosphacan decreased within 24 h in the hippocampus. In immunohistochemistry, neurocan increased in the limbic structures coincident with the appearance of reactive astrocytes. Phosphacan decreased coincident with pyramidal cell loss in the hippocampus, and the number of phosphacan-positive perineuronal nets around parvalbumin neurons decreased, whereas parvalbumin neurons were relatively conserved. In contrast, phosphacan increased in the entorhinal and piriform cortices in correlation with the severity of neuronal loss. Both neurocan and phosphacan recovered to the control level by 8 weeks after KA convulsions in some rats, but the changes in neurocan and phosphacan described above still persisted in more than half the rats. The results indicate that KA convulsions induce prolonged changes in neurocan and phosphacan similar to those in the developing rat brain and suggest a role of these CSPGs in the remodeling of neuronal networks related to the establishment or enhancement of epileptogenesis.

Neurite elongation on chondroitin sulfate proteoglycans is characterized by axonal fasciculation

In the developing or regenerating nervous system, migrating growth cones are exposed to regulatory molecules that positively and/or negatively affect guidance. Chondroitin sulfate proteoglycans (CSPGs) are complex macromolecules that are typically negative regulators of growth cone migration in vivo and in vitro. However, in certain cases, neurites sometimes traverse regions expressing relatively high levels of CSPGs, seemingly a paradox. In our continuing efforts to characterize CSPG inhibition in vitro, we manipulated the ratio of CSPGs to growth-promoting laminin-1 to produce a substratum that supports outgrowth of a subpopulation of dorsal root ganglia (DRG) neurites, while still being inhibitory to other populations of DRG neurons [Exp. Neurol. 109 (1990), 111; J. Neurobiol. 51 (2002), 285]. This model comprises a useful tool in the analysis of mechanisms of growth cone guidance and is particularly useful to analyze how CSPGs can be inhibitory under some conditions, and growth permissive under others. We grew embryonic (E9-10) chicken DRG neurons on nervous system-isolated, substratum-bound CSPGs at a concentration that supports an intermittent pattern of outgrowth, alternating with regions adsorbed with growth-promoting laminin-1 alone, and analyzed outgrowth behaviors qualitatively and quantitatively. A novel finding of the study was that DRG neurites that elongated onto CSPGs were predominantly fasciculated, but immediately returned to a defasciculated state upon contact with laminin-1. Further, cursory inspection suggests that outgrowth onto CSPGs may be initially accomplished by pioneer axons, along which subsequent axons migrate. The outgrowth patterns characterized in vitro may accurately reflect outgrowth in vivo in locations where inhibitory CSPGs and growth-promoting molecules are coexpressed, e.g., in the developing retina where fasciculated outgrowth may be instrumental in the guidance of retinal ganglion cells from the periphery to the optic fissure.