Cell coupling compensates for changes in single-cell Her6 dynamics and provides phenotypic robustness

This paper investigates the effect of altering the protein expression dynamics of the bHLH transcription factor Her6 at the single-cell level in the embryonic zebrafish telencephalon. Using a homozygote endogenous Her6:Venus reporter and 4D single-cell tracking, we show that Her6 oscillates in neural telencephalic progenitors and that the fusion of protein destabilisation (PEST) domain alters its expression dynamics, causing most cells to downregulate Her6 prematurely. However, counterintuitively, oscillatory cells increase, with some expressing Her6 at high levels, resulting in increased heterogeneity of Her6 expression in the population. These tissue-level changes appear to be an emergent property of coupling between single-cells, as revealed by experimentally disrupting Notch signalling and by computationally modelling alterations in Her6 protein stability. Despite the profound differences in the single-cell Her6 dynamics, the size of the telencephalon is only transiently altered and differentiation markers do not exhibit significant differences early on; however, a small increase is observed at later developmental stages. Our study suggests that cell coupling provides a compensation strategy, whereby an almost normal phenotype is maintained even though single-cell gene expression dynamics are abnormal, granting phenotypic robustness.

Molecular programs of regional specification and neural stem cell fate progression in macaque telencephalon

During early telencephalic development, intricate processes of regional patterning and neural stem cell (NSC) fate specification take place. However, our understanding of these processes in primates, including both conserved and species-specific features, remains limited. Here, we profiled 761,529 single-cell transcriptomes from multiple regions of the prenatal macaque telencephalon. We deciphered the molecular programs of the early organizing centers and their cross-talk with NSCs, revealing primate-biased galanin-like peptide (GALP) signaling in the anteroventral telencephalon. Regional transcriptomic variations were observed along the frontotemporal axis during early stages of neocortical NSC progression and in neurons and astrocytes. Additionally, we found that genes associated with neuropsychiatric disorders and brain cancer risk might play critical roles in the early telencephalic organizers and during NSC progression.

Tetrabromobisphenol A Disturbs Brain Development in Both Thyroid Hormone-Dependent and -Independent Manners in Xenopus laevis

Although tetrabromobisphenol A (TBBPA) has been well proven to disturb TH signaling in both in vitro and in vivo assays, it is still unclear whether TBBPA can affect brain development due to TH signaling disruption. Here, we employed the T3-induced Xenopus metamorphosis assay (TIXMA) and the spontaneous metamorphosis assay to address this issue. In the TIXMA, 5–500 nmol/L TBBPA affected T3-induced TH-response gene expression and T3-induced brain development (brain morphological changes, cell proliferation, and neurodifferentiation) at premetamorphic stages in a complicated biphasic concentration-response manner. Notably, 500 nmol/L TBBPA treatment alone exerted a stimulatory effect on tadpole growth and brain development at these stages, in parallel with a lack of TH signaling activation, suggesting the involvement of other signaling pathways. As expected, at the metamorphic climax, we observed inhibitory effects of 50–500 nmol/L TBBPA on metamorphic development and brain development, which was in agreement with the antagonistic effects of higher concentrations on T3-induced brain development at premetamorphic stages. Taken together, all results demonstrate that TBBPA can disturb TH signaling and subsequently interfere with TH-dependent brain development in Xenopus; meanwhile, other signaling pathways besides TH signaling could be involved in this process. Our study improves the understanding of the effects of TBBPA on vertebrate brain development.

DCC regulates astroglial development essential for telencephalic morphogenesis and corpus callosum formation

The forebrain hemispheres are predominantly separated during embryogenesis by the interhemispheric fissure (IHF). Radial astroglia remodel the IHF to form a continuous substrate between the hemispheres for midline crossing of the corpus callosum (CC) and hippocampal commissure (HC). Deleted in colorectal carcinoma (DCC) and netrin 1 (NTN1) are molecules that have an evolutionarily conserved function in commissural axon guidance. The CC and HC are absent in Dcc and Ntn1 knockout mice, while other commissures are only partially affected, suggesting an additional aetiology in forebrain commissure formation. Here, we find that these molecules play a critical role in regulating astroglial development and IHF remodelling during CC and HC formation. Human subjects with DCC mutations display disrupted IHF remodelling associated with CC and HC malformations. Thus, axon guidance molecules such as DCC and NTN1 first regulate the formation of a midline substrate for dorsal commissures prior to their role in regulating axonal growth and guidance across it.

Differential roles of ARID1B in excitatory and inhibitory neural progenitors in the developing cortex

Genetic evidence indicates that haploinsufficiency of ARID1B causes intellectual disability (ID) and autism spectrum disorder (ASD), but the neural function of ARID1B is largely unknown. Using both conditional and global Arid1b knockout mouse strains, we examined the role of ARID1B in neural progenitors. We detected an overall decrease in the proliferation of cortical and ventral neural progenitors following homozygous deletion of Arid1b, as well as altered cell cycle regulation and increased cell death. Each of these phenotypes was more pronounced in ventral neural progenitors. Furthermore, we observed decreased nuclear localization of β-catenin in Arid1b-deficient neurons. Conditional homozygous deletion of Arid1b in ventral neural progenitors led to pronounced ID- and ASD-like behaviors in mice, whereas the deletion in cortical neural progenitors resulted in minor cognitive deficits. This study suggests an essential role for ARID1B in forebrain neurogenesis and clarifies its more pronounced role in inhibitory neural progenitors. Our findings also provide insights into the pathogenesis of ID and ASD.

A Chromatin Accessibility Atlas of the Developing Human Telencephalon

To discover regulatory elements driving the specificity of gene expression in different cell types and regions of the developing human brain, we generated an atlas of open chromatin from nine dissected regions of the mid-gestation human telencephalon, as well as microdissected upper and deep layers of the prefrontal cortex. We identified a subset of open chromatin regions (OCRs), termed predicted regulatory elements (pREs), that are likely to function as developmental brain enhancers. pREs showed temporal, regional, and laminar differences in chromatin accessibility and were correlated with gene expression differences across regions and gestational ages. We identified two functional de novo variants in a pRE for autism risk gene SLC6A1, and using CRISPRa, demonstrated that this pRE regulates SCL6A1. Additionally, mouse transgenic experiments validated enhancer activity for pREs proximal to FEZF2 and BCL11A. Thus, this atlas serves as a resource for decoding neurodevelopmental gene regulation in health and disease.

Vascularized human cortical organoids (vOrganoids) model cortical development in vivo

Modeling the processes of neuronal progenitor proliferation and differentiation to produce mature cortical neuron subtypes is essential for the study of human brain development and the search for potential cell therapies. We demonstrated a novel paradigm for the generation of vascularized organoids (vOrganoids) consisting of typical human cortical cell types and a vascular structure for over 200 days as a vascularized and functional brain organoid model. The observation of spontaneous excitatory postsynaptic currents (sEPSCs), spontaneous inhibitory postsynaptic currents (sIPSCs), and bidirectional electrical transmission indicated the presence of chemical and electrical synapses in vOrganoids. More importantly, single-cell RNA-sequencing analysis illustrated that vOrganoids exhibited robust neurogenesis and that cells of vOrganoids differentially expressed genes (DEGs) related to blood vessel morphogenesis. The transplantation of vOrganoids into the mouse S1 cortex resulted in the construction of functional human-mouse blood vessels in the grafts that promoted cell survival in the grafts. This vOrganoid culture method could not only serve as a model to study human cortical development and explore brain disease pathology but also provide potential prospects for new cell therapies for nervous system disorders and injury.

This study establishes a method to generate vascularized cortical organoids. This shows that in addition to reducing hypoxia and cell death, the vascular system promotes neural development in organoids. When transplanting these organoids into host mice, a graft-host vascular system could be reconstructed.

march5 Governs the Convergence and Extension Movement for Organization of the Telencephalon and Diencephalon in Zebrafish Embryos

MARCH5 is a RING finger E3 ligase involved in mitochondrial integrity, cellular protein homeostasis, and the regulation of mitochondrial fusion and fission. To determine the function of MARCH5 during development, we assessed transcript expression in zebrafish embryos. We found that march5 transcripts were of maternal origin and evenly distributed at the 1-cell stage, except for the mid-blastula transition, with expression predominantly in the developing central nervous system at later stages of embryogenesis. Overexpression of march5 impaired convergent extension movement during gastrulation, resulting in reduced patterning along the dorsoventral axis and alterations in the ventral cell types. Overexpression and knockdown of march5 disrupted the organization of the developing telencephalon and diencephalon. Lastly, we found that the transcription of march5 was tightly regulated by the transcriptional regulators CHOP, C/EBPα, Staf, Znf143a, and Znf76. These results demonstrate the essential role of March5 in the development of zebrafish embryos.

Developmental dynamics of the periventricular parietal crossroads of growing cortical pathways in the fetal brain - In vivo fetal MRI with histological correlation

The periventricular crossroads have been described as transient structures of the fetal brain where major systems of developing fibers intersect. The triangular parietal crossroad constitutes one major crossroad region. By combining in vivo and post-mortem fetal MRI with histological and immunohistochemical methods, we aimed to characterize these structures. Data from 529 in vivo and 66 post-mortem MRI examinations of fetal brains between gestational weeks (GW) 18-39 were retrospectively reviewed. In each fetus, the area adjacent to the trigone of the lateral ventricles at the exit of the posterior limb of the internal capsule (PLIC) was assessed with respect to signal intensity, size, and shape on T2-weighted images. In addition, by using in vivo diffusion tensor imaging (DTI), the main fiber pathways that intersect in these areas were identified. In order to explain the in vivo features of the parietal crossroads (signal intensity and developmental profile), we analyzed 23 post-mortem fetal human brains, between 16 and ​40 GW of age, processed by histological and immunohistochemical methods. The parietal crossroads were triangular-shaped areas with the base in the continuity of the PLIC, adjacent to the germinal matrix and the trigone of the lateral ventricles, with the tip pointing toward the subplate. These areas appeared hyperintense to the subplate, and corresponded to a convergence zone of the developing external capsule, the PLIC, and the fronto-occipital association fibers. They were best detected between GW 25-26, and, at term, they became isointense to the adjacent structures. The immunohistochemical results showed a distinct cellular, fibrillar, and extracellular matrix arrangement in the parietal crossroads, depending on the stage of development, which influenced the MRI features. The parietal crossroads are transient, but important structures in white matter maturation and their damage may be indicative of a poor prognosis for a fetus with regard to neurological development. In addition, impairment of this region may explain the complex neurodevelopmental deficits in preterm infants with periventricular hypoxic/ischemic or inflammatory lesions.

Conserved and divergent expression dynamics during early patterning of the telencephalon in mouse and chick embryos

The mammalian and the avian telencephalon are nearly indistinguishable at early embryonic vesicle stages but differ substantially in form and function at their adult stage. We sequenced and analyzed RNA populations present in mouse and chick during the early stages of embryonic telencephalon to understand conserved and lineage-specific developmental differences. We found approximately 3000 genes that orchestrate telencephalon development. Many chromatin-associated epigenetic and transcription regulators show high expression in both species and some show species-specific expression dynamics. Interestingly, previous studies associated them to autism, intellectual disabilities, and mental retardation supporting a causal link between their impaired functions during telencephalon development and brain dysfunction. Strikingly, the conserved up-regulated genes were differentially enriched in ontologies related to development or functions of the adult brain. Moreover, a differential enrichment of distinct repertoires of transcription factor binding motifs in their upstream promoter regions suggest a species-specific regulation of the various gene groups identified. Overall, our results reveal that the ontogenetic divergences between the mouse and chick telencephalon result from subtle differences in the regulation of common patterning signaling cascades and regulatory networks unique to each species at their very early stages of development.

Spatial distribution of the full-length members of the Grg family during embryonic neurogenesis reveals a "Grg-mediated repression map" in the mouse telencephalon

The full-length members of the Groucho/Transducin-like Enhancer of split gene family, namely Grg1-4, encode nuclear corepressors that act either directly, via interaction with transcription factors, or indirectly by modifying histone acetylation or chromatin structure. In this work we describe a detailed expression analysis of Grg1-4 family members during embryonic neurogenesis in the developing murine telencephalon. Grg1-4 presented a unique, complex yet overlapping expression pattern; Grg1 and Grg3 were mainly detected in the proliferative zones of the telencephalon, Grg2 mainly in the subpallium and finally, Grg4 mainly in the subpallial post mitotic neurons. In addition, comparative analysis of the expression of Grg1-4 revealed that, at these stages, distinct telencephalic progenitor domains or structures are characterized by the presence of different combinations of Grg repressors, thus forming a “Grg-mediated repression map”.

CRISPR/Cas9-induced disruption of gene expression in mouse embryonic brain and single neural stem cells in vivo

We have applied the CRISPR/Cas9 system in vivo to disrupt gene expression in neural stem cells in the developing mammalian brain. Two days after in utero electroporation of a single plasmid encoding Cas9 and an appropriate guide RNA (gRNA) into the embryonic neocortex of Tis21::GFP knock-in mice, expression of GFP, which occurs specifically in neural stem cells committed to neurogenesis, was found to be nearly completely (≈ 90%) abolished in the progeny of the targeted cells. Importantly, upon in utero electroporation directly of recombinant Cas9/gRNA complex, near-maximal efficiency of disruption of GFP expression was achieved already after 24 h. Furthermore, by using microinjection of the Cas9 protein/gRNA complex into neural stem cells in organotypic slice culture, we obtained disruption of GFP expression within a single cell cycle. Finally, we used either Cas9 plasmid in utero electroporation or Cas9 protein complex microinjection to disrupt the expression of Eomes/Tbr2, a gene fundamental for neocortical neurogenesis. This resulted in a reduction in basal progenitors and an increase in neuronal differentiation. Thus, the present in vivo application of the CRISPR/Cas9 system in neural stem cells provides a rapid, efficient and enduring disruption of expression of specific genes to dissect their role in mammalian brain development.

Apical versus Basal Neurogenesis Directs Cortical Interneuron Subclass Fate

Fate determination in the mammalian telencephalon, with its diversity of neuronal subtypes and relevance to neuropsychiatric disease, remains a critical area of study in neuroscience. Most studies investigating this topic focus on the diversity of neural progenitors within spatial and temporal domains along the lateral ventricles. Often overlooked is whether the location of neurogenesis within a fate-restricted domain is associated with, or instructive for, distinct neuronal fates. Here, we use in vivo fate mapping and the manipulation of neurogenic location to demonstrate that apical versus basal neurogenesis influences the fate determination of major subgroups of cortical interneurons derived from the subcortical telencephalon. Somatostatin-expressing interneurons arise mainly from apical divisions along the ventricular surface, whereas parvalbumin-expressing interneurons originate predominantly from basal divisions in the subventricular zone. As manipulations that shift neurogenic location alter interneuron subclass fate, these results add an additional dimension to the spatial-temporal determinants of neuronal fate determination.

Identification and Epigenetic Analysis of a Maternally Imprinted Gene Qpct

Most imprinted genes are concerned with embryonic development, especially placental development. Here, we identified a placenta-specific imprinted gene Qpct. Our results show that Qpct is widely expressed during early embryonic development and can be detected in the telecephalon, midbrain, and rhombencephalon at E9.5b-E11.5. Moreover, Qpct is strikingly expressed in the brain, lung and liver in E15.5. Expression signals for Qpct achieved a peak at E15.5 during placental development and were only detected in the labyrinth layer in E15.5 placenta. ChIP assay results suggest that the modification of histone H3K4me3 can result in maternal activating of Qpct.

Targeted deletion of Vglut2 expression in the embryonal telencephalon promotes an anxiolytic phenotype of the adult mouse

BACKGROUND

Anxiety is a natural emotion experienced by all individuals. However, when anxiety becomes excessive, it contributes to the substantial group of anxiety disorders that affect one in three people and thus are among the most common psychiatric disorders. Anxiolysis, the reduction of anxiety, is mediated via several large groups of therapeutical compounds, but the relief is often only temporary, and increased knowledge of the neurobiology underlying anxiety is needed in order to improve future therapies.

AIM

We previously demonstrated that mice lacking forebrain expression of the Vesicular glutamate transporter 2 (Vglut2) from adolescence showed a strong anxiolytic behaviour as adults. In the current study, we wished to analyse if removal of Vglut2 expression already from mid-gestation of the mouse embryo would give rise to similar anxiolysis in the adult mouse.

METHODS

We produced transgenic mice lacking Vglut2 from mid-gestation and analysed their affective behaviour, including anxiety, when they had reached adulthood.

RESULTS

The transgenic mice lacking Vglut2 expression from mid-gestation showed certain signs of anxiolytic behaviour, but this phenotype was not as prominent as when Vglut2 was removed during adolescence.

CONCLUSION

Our results suggest that both embryonal and adolescent forebrain expression of Vglut2 normally contributes to balancing the level of anxiety. As the neurobiological basis for anxiety is similar across species, our results in mice may help improve the current understanding of the neurocircuitry of anxiety, and hence anxiolysis, also in humans.

Exposure to benzidine caused apoptosis and malformation of telencephalon region in zebrafish

Exposure to benzidine has been known to induce human cancers, particularly bladder carcinomas. In this study, the zebrafish model was used to investigate the developmental toxicity of benzidine. Embryos at 6 h postfertilization (hpf) that were exposed to benzidine exhibited embryonic death in a dose- and time-dependent manner. Benzidine induced malformations in zebrafish, such as small brain development, shorter axes, and a slight pericardial edema. High concentrations (50, 100, and 200 µM) of benzidine triggered widespread apoptosis in the brain and dorsal neurons, as evidenced by acridine orange and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assays. Real-time polymerase chain reaction analysis also showed that benzidine treatment affected p53, bax, and noxa expression. Decreases in specific brain markers, such as emx1 in the telencephalon, ngn1 in differentiated neurons, and otx2 in the midbrain, were observed in benzidine-treated embryos at 24 hpf. Conversely, no overt changes to pax2.1 expression in the midbrain-hindbrain boundary were found. Moreover, the use of Tg(HuC:GFP) zebrafish showed that benzidine caused a malformation of the telencephalon region. Our findings show that benzidine exposure triggers widespread apoptosis in the zebrafish brain and dorsal neurons, resulting in the development of an abnormal telencephalon.

Ontogeny of small RNA in the regulation of mammalian brain development

BACKGROUND

MicroRNAs (miRNAs) play a pivotal role in coordinating messenger RNA (mRNA) transcription and stability in almost all known biological processes, including the development of the central nervous system. Despite our broad understanding of their involvement, we still have a very sparse understanding of specifically how miRNA contribute to the strict regional and temporal regulation of brain development. Accordingly, in the current study we have examined the contribution of miRNA in the developing rat telencephalon and mesencephalon from just after neural tube closure till birth using a genome-wide microarray strategy.

RESULTS

We identified temporally distinct expression patterns in both the telencephalon and mesencephalon for both miRNAs and their target genes. We demonstrate direct miRNA targeting of several genes involved with the migration, differentiation and maturation of neurons.

CONCLUSIONS

Our findings suggest that miRNA have significant implications for the development of neural structure and support important mechanisms that if disrupted, may contribute to or drive neurodevelopmental disorders.

Eph/Ephrin signalling maintains eye field segregation from adjacent neural plate territories during forebrain morphogenesis

During forebrain morphogenesis, there is extensive reorganisation of the cells destined to form the eyes, telencephalon and diencephalon. Little is known about the molecular mechanisms that regulate region-specific behaviours and that maintain the coherence of cell populations undergoing specific morphogenetic processes. In this study, we show that the activity of the Eph/Ephrin signalling pathway maintains segregation between the prospective eyes and adjacent regions of the anterior neural plate during the early stages of forebrain morphogenesis in zebrafish. Several Ephrins and Ephs are expressed in complementary domains in the prospective forebrain and combinatorial abrogation of their activity results in incomplete segregation of the eyes and telencephalon and in defective evagination of the optic vesicles. Conversely, expression of exogenous Ephs or Ephrins in regions of the prospective forebrain where they are not usually expressed changes the adhesion properties of the cells, resulting in segregation to the wrong domain without changing their regional fate. The failure of eye morphogenesis in rx3 mutants is accompanied by a loss of complementary expression of Ephs and Ephrins, suggesting that this pathway is activated downstream of the regional fate specification machinery to establish boundaries between domains undergoing different programmes of morphogenesis.

Arl13b-regulated cilia activities are essential for polarized radial glial scaffold formation

The construction of cerebral cortex begins with the formation of radial glia. Once formed, polarized radial glial cells divide either symmetrically or asymmetrically to balance appropriate production of progenitor cells and neurons. Following birth, neurons use the processes of radial glia as scaffolding for oriented migration. Radial glia therefore provide an instructive structural matrix to coordinate the generation and placement of distinct groups of cortical neurons in the developing cerebral cortex. We found that Arl13b, a cilia-enriched small GTPase that is mutated in Joubert syndrome, was critical for the initial formation of the polarized radial progenitor scaffold. Using developmental stage-specific deletion of Arl13b in mouse cortical progenitors, we found that early neuroepithelial deletion of ciliary Arl13b led to a reversal of the apical-basal polarity of radial progenitors and aberrant neuronal placement. Arl13b modulated ciliary signaling necessary for radial glial polarity. Our findings indicate that Arl13b signaling in primary cilia is crucial for the initial formation of a polarized radial glial scaffold and suggest that disruption of this process may contribute to aberrant neurodevelopment and brain abnormalities in Joubert syndrome-related ciliopathies.

GABA progenitors grafted into the adult epileptic brain control seizures and abnormal behavior

Impaired GABA-mediated neurotransmission has been implicated in many neurologic diseases, including epilepsy, intellectual disability and psychiatric disorders. We found that inhibitory neuron transplantation into the hippocampus of adult mice with confirmed epilepsy at the time of grafting markedly reduced the occurrence of electrographic seizures and restored behavioral deficits in spatial learning, hyperactivity and the aggressive response to handling. In the recipient brain, GABA progenitors migrated up to 1,500 μm from the injection site, expressed genes and proteins characteristic for interneurons, differentiated into functional inhibitory neurons and received excitatory synaptic input. In contrast with hippocampus, cell grafts into basolateral amygdala rescued the hyperactivity deficit, but did not alter seizure activity or other abnormal behaviors. Our results highlight a critical role for interneurons in epilepsy and suggest that interneuron cell transplantation is a powerful approach to halting seizures and rescuing accompanying deficits in severely epileptic mice.

Expression of the murine transcription factor SOX3 during embryonic and adult neurogenesis

Previous studies have shown that Sox3 is expressed in nascent neuroprogenitor cells and is functionally required in mammals for development of the dorsal telencephalon and hypothalamus. However, Sox3 expression during embryonic and adult neurogenesis has not been examined in detail. Using a SOX3-specific antibody, we show that murine SOX3 expression is maintained throughout telencephalic neurogenesis and is restricted to progenitor cells with neuroepithelial and radial glial morphologies. We also demonstrate that SOX3 is expressed within the adult neurogenic regions and is coexpressed extensively with the neural stem cell marker SOX2 indicating that it is a lifelong marker of neuroprogenitor cells. In contrast to the telencephalon, Sox3 expression within the developing hypothalamus is upregulated in developing neurons and is maintained in a subset of differentiated hypothalamic cells through to adulthood. Together, these data show that Sox3 regulation is region-specific, consistent with it playing distinct biological roles in the dorsal telencephalon and hypothalamus.

Otx2 is involved in the regional specification of the developing retinal pigment epithelium by preventing the expression of sox2 and fgf8, factors that induce neural retina differentiation

The retinal pigment epithelium (RPE) shares its developmental origin with the neural retina (NR). When RPE development is disrupted, cells in the presumptive RPE region abnormally differentiate into NR-like cells. Therefore, the prevention of NR differentiation in the presumptive RPE area seems to be essential for regionalizing the RPE during eye development. However, its molecular mechanisms are not fully understood. In this study, we conducted a functional inhibition of a transcription factor Otx2, which is required for RPE development, using early chick embryos. The functional inhibition of Otx2 in chick eyes, using a recombinant gene encoding a dominant negative form of Otx2, caused the outer layer of the optic cup (the region forming the RPE, when embryos normally develop) to abnormally form an ectopic NR. In that ectopic NR, the characteristics of the RPE did not appear and NR markers were ectopically expressed. Intriguingly, the repression of Otx2 function also caused the ectopic expression of Fgf8 and Sox2 in the outer layer of the optic cup (the presumptive RPE region of normally developing eyes). These two factors are known to be capable of inducing NR cell differentiation in the presumptive RPE region, and are not expressed in the normally developing RPE region. Here, we suggest that Otx2 prevents the presumptive RPE region from forming the NR by repressing the expression of both Fgf8 and Sox2 which induce the NR cell fate.

Expression at the imprinted dlk1-gtl2 locus is regulated by proneural genes in the developing telencephalon

Imprinting is an epigenetic mechanism that restrains the expression of about 100 genes to one allele depending on its parental origin. Several imprinted genes are implicated in neurodevelopmental brain disorders, such as autism, Angelman, and Prader-Willi syndromes. However, how expression of these imprinted genes is regulated during neural development is poorly understood. Here, using single and double KO animals for the transcription factors Neurogenin2 (Ngn2) and Achaete-scute homolog 1 (Ascl1), we found that the expression of a specific subset of imprinted genes is controlled by these proneural genes. Using in situ hybridization and quantitative PCR, we determined that five imprinted transcripts situated at the Dlk1-Gtl2 locus (Dlk1, Gtl2, Mirg, Rian, Rtl1) are upregulated in the dorsal telencephalon of Ngn2 KO mice. This suggests that Ngn2 influences the expression of the entire Dlk1-Gtl2 locus, independently of the parental origin of the transcripts. Interestingly 14 other imprinted genes situated at other imprinted loci were not affected by the loss of Ngn2. Finally, using Ngn2/Ascl1 double KO mice, we show that the upregulation of genes at the Dlk1-Gtl2 locus in Ngn2 KO animals requires a functional copy of Ascl1. Our data suggest a complex interplay between proneural genes in the developing forebrain that control the level of expression at the imprinted Dlk1-Gtl2 locus (but not of other imprinted genes). This raises the possibility that the transcripts of this selective locus participate in the biological effects of proneural genes in the developing telencephalon.

Role of the cellular prion protein in oligodendrocyte precursor cell proliferation and differentiation in the developing and adult mouse CNS

There are numerous studies describing the signaling mechanisms that mediate oligodendrocyte precursor cell (OPC) proliferation and differentiation, although the contribution of the cellular prion protein (PrP^c) to this process remains unclear. PrP^c is a glycosyl-phosphatidylinositol (GPI)-anchored glycoprotein involved in diverse cellular processes during the development and maturation of the mammalian central nervous system (CNS). Here we describe how PrP^c influences oligodendrocyte proliferation in the developing and adult CNS. OPCs that lack PrP^c proliferate more vigorously at the expense of a delay in differentiation, which correlates with changes in the expression of oligodendrocyte lineage markers. In addition, numerous NG2-positive cells were observed in cortical regions of adult PrP^c knockout mice, although no significant changes in myelination can be seen, probably due to the death of surplus cells.

The avian subpallium: new insights into structural and functional subdivisions occupying the lateral subpallial wall and their embryological origins

The subpallial region of the avian telencephalon contains neural systems whose functions are critical to the survival of individual vertebrates and their species. The subpallial neural structures can be grouped into five major functional systems, namely the dorsal somatomotor basal ganglia; ventral viscerolimbic basal ganglia; subpallial extended amygdala including the central and medial extended amygdala and bed nuclei of the stria terminalis; basal telencephalic cholinergic and non-cholinergic corticopetal systems; and septum. The paper provides an overview of the major developmental, neuroanatomical and functional characteristics of the first four of these neural systems, all of which belong to the lateral telencephalic wall. The review particularly focuses on new findings that have emerged since the identity, extent and terminology for the regions were considered by the Avian Brain Nomenclature Forum. New terminology is introduced as appropriate based on the new findings. The paper also addresses regional similarities and differences between birds and mammals, and notes areas where gaps in knowledge occur for birds.

Fezf2 regulates telencephalic precursor differentiation from mouse embryonic stem cells

The mechanisms by which transcription factors control stepwise lineage restriction during the specification of cortical neurons remain largely unknown. Here, we investigated the role of forebrain embryonic zinc finger like (Fezf2) in this process by generating Fezf2 knockdown and tetracycline-inducible Fezf2 overexpression mouse embryonic stem cell (mESC) lines. The overexpression of Fezf2 at early time points significantly increased the generation of rostral forebrain progenitors (Foxg1(+), Six3(+)) and inhibited the expression of transcription factors which are expressed by the midbrain and caudal diencephalon (En1(+), Irx(+)). This effect was partially achieved by the regulation of Wnt signaling during this critical early time window. The role of Fezf2 in regulating the rostrocaudal patterning was further confirmed by the significant decrease in the expression of Foxg1 and Six3 and the increase in the expression of En1 when Fezf2 was knocked down. In addition, Fezf2 overexpression at later time points had little effect on the expression of Foxg1 and Six3. Instead, Fezf2 promotes the generation of dorsal telencephalic progenitors and deep-layer cortical neurons at later stages. Collectively, our data suggest that Fezf2 controls the specification of telencephalic progenitors from mESCs through differentially regulating the expression of rostrocaudal and dorsoventral patterning genes.

Functional dicer is necessary for appropriate specification of radial glia during early development of mouse telencephalon

Early telencephalic development involves transformation of neuroepithelial stem cells into radial glia, which are themselves neuronal progenitors, around the time when the tissue begins to generate postmitotic neurons. To achieve this transformation, radial precursors express a specific combination of proteins. We investigate the hypothesis that micro RNAs regulate the ability of the early telencephalic progenitors to establish radial glia. We ablate functional Dicer, which is required for the generation of mature micro RNAs, by conditionally mutating the Dicer1 gene in the early embryonic telencephalon and analyse the molecular specification of radial glia as well as their progeny, namely postmitotic neurons and basal progenitors. Conditional mutation of Dicer1 from the telencephalon at around embryonic day 8 does not prevent morphological development of radial glia, but their expression of Nestin, Sox9, and ErbB2 is abnormally low. The population of basal progenitors, which are generated by the radial glia, is disorganised and expanded in Dicer1^-/- dorsal telencephalon. While the proportion of cells expressing markers of postmitotic neurons is unchanged, their laminar organisation in the telencephalic wall is disrupted suggesting a defect in radial glial guided migration. We found that the laminar disruption could not be accounted for by a reduction of the population of Cajal Retzius neurons. Together, our data suggest novel roles for micro RNAs during early development of progenitor cells in the embryonic telencephalon.

ApoER2 and VLDLR in the developing human telencephalon

The Reelin-Dab1 signaling pathway plays a crucial role in regulating the migration and position of cortical neurons during the development of the cerebral cortex. Mutation in Reelin may result in severe developmental disorders such as autosomal recessive lissencephaly. Apolipoprotein E receptor type-2 (ApoER2) and very low-density lipoprotein receptor (VLDLR) are canonical receptors of Reelin, through which extracellular Reelin activates the intracellular adapter, Disabled1(Dab1), and subsequently interacts with other molecules. Although it is widely accepted that ApoER2 and VLDLR are indispensable components of the Reelin signaling pathway, little is known of their expression pattern in the laminated developing human brain. Here, we collected 18 cases of human fetal brains of 6-18 gestational weeks (GW) old and examined the expression of ApoER2 and VLDLR in the their telencephalon using immunocytochemical staining. We found that both receptors were absent in the preplate (PP) and the earliest stage of the cortical plate (CP). In later stages of CP development, ApoER2 was expressed earlier than VLDLR in the migrating neurons. Thus, the Reelin-Dab1 signaling pathway may not be involved in the formation of the preplate and deep layers of the CP. Instead, the pathway may act on neurons that are destined to form the more superficial layers of the CP. In addition, the pathway required ApoER2 only rather than both ApoER2 and VLDLR at the initiation of activity.

Essential regulation of CNS angiogenesis by the orphan G protein-coupled receptor GPR124

The orphan G protein-coupled receptor (GPCR) GPR124/tumor endothelial marker 5 is highly expressed in central nervous system (CNS) endothelium. Here, we show that complete null or endothelial-specific GPR124 deletion resulted in embryonic lethality from CNS-specific angiogenesis arrest in forebrain and neural tube. Conversely, GPR124 overexpression throughout all adult vascular beds produced CNS-specific hyperproliferative vascular malformations. In vivo, GPR124 functioned cell-autonomously in endothelium to regulate sprouting, migration, and developmental expression of the blood-brain barrier marker Glut1, whereas in vitro, GPR124 mediated Cdc42-dependent directional migration to forebrain-derived, vascular endothelial growth factor-independent cues. Our results demonstrate CNS-specific angiogenesis regulation by an endothelial receptor and illuminate functions of the poorly understood adhesion GPCR subfamily. Further, the functional tropism of GPR124 marks this receptor as a therapeutic target for CNS-related vascular pathologies.

Monoamine oxidases regulate telencephalic neural progenitors in late embryonic and early postnatal development

Monoamine neurotransmitters play major roles in regulating a range of brain functions in adults and increasing evidence suggests roles for monoamines in brain development. Here we show that mice lacking the monoamine metabolic enzymes MAO A and MAO B (MAO AB-deficient mice) exhibit diminished proliferation of neural stem cells (NSC) in the developing telencephalon beginning in late gestation [embryonic day (E) 17.5], a deficit that persists in neonatal and adult mice. These mice showed significantly increased monoamine levels and anxiety-like behaviors as adults. Assessments of markers of intermediate progenitor cells (IPC) and mitosis showed that NSC in the subventricular zone (SVZ), but not in the ventricular zone, are reduced in MAO AB-deficient mice. A developmental time course of monoamines in frontal cortical tissues revealed increased serotonin levels as early as E14.5, and a further large increase was found between E17.5 and postnatal day 2. Administration of an inhibitor of serotonin synthesis (parachlorophenylalanine) between E14.5 and E19.5 restored the IPC numbers and SVZ thickness, suggesting the role of serotonin in the suppression of IPC proliferation. Studies of neurosphere cultures prepared from the telencephalon at different embryonic and postnatal ages showed that serotonin stimulates proliferation in wild-type, but not in MAO AB-deficient, NSC. Together, these results suggest that a MAO-dependent long-lasting alteration in the proliferation capacity of NSC occurs late in embryonic development and is mediated by serotonin. Our findings reveal novel roles for MAOs and serotonin in the regulation of IPC proliferation in the developing brain.

Role of Fgf8 signalling in the specification of rostral Cajal-Retzius cells

Cajal-Retzius (CR) cells play a key role in the formation of the cerebral cortex. These pioneer neurons are distributed throughout the cortical marginal zone in distinct graded distributions. Fate mapping and cell lineage tracing studies have recently shown that CR cells arise from restricted domains of the pallial ventricular zone, which are associated with signalling centres involved in the early regionalisation of the telencephalic vesicles. In this study, we identified a subpopulation of CR cells in the rostral telencephalon that expresses Er81, a downstream target of Fgf8 signalling. We investigated the role of the rostral telencephalic patterning centre, which secretes FGF molecules, in the specification of these cells. Using pharmacological inhibitors and genetic inactivation of Fgf8, we showed that production of Fgf8 by the rostral telencephalic signalling centre is required for the specification of the Er81+ CR cell population. Moreover, the analysis of Fgf8 gain-of-function in cultivated mouse embryos and of Emx2 and Gli3 mutant embryos revealed that ectopic Fgf8 signalling promotes the generation of CR cells with a rostral phenotype from the dorsal pallium. These data showed that Fgf8 signalling is both required and sufficient to induce rostral CR cells. Together, our results shed light on the mechanisms specifying rostral CR cells and further emphasise the crucial role of telencephalic signalling centres in the generation of distinct CR cell populations.

Normal ventral telencephalic expression of Pax6 is required for normal development of thalamocortical axons in embryonic mice

BACKGROUND

In addition to its well-known expression in dorsal telencephalic progenitor cells, where it regulates cell proliferation and identity, the transcription factor Pax6 is expressed in some ventral telencephalic cells, including many postmitotic neurons. Its functions in these cells are unknown.

RESULTS

We generated a new floxed allele of Pax6 and tested the consequences of a highly specific ventral telencephalic depletion of Pax6. We used the Six3A1A2-Cre allele that drives production of Cre recombinase in a specific region of Pax6-expression close to the internal capsule, through which thalamic axons navigate to cerebral cortex. Depletion in this region caused many thalamic axons to take aberrant routes, either failing to turn normally into ventral telencephalon to form the internal capsule or exiting the developing internal capsule ventrally. We tested whether these defects might have resulted from abnormalities of two structural features proposed to guide thalamic axons into and through the developing internal capsule. First, we looked for the early pioneer axons that project from the region of the future internal capsule to the thalamus and are thought to guide thalamocortical axons to the internal capsule: we found that they are present in conditional mutants. Second, we examined the development of the corridor of Islet1-expressing cells that guides thalamic axons through ventral telencephalon and found that it was broader and less dense than normal in conditional mutants. We also examined corticofugal axons that are thought to interact with ascending thalamocortical axons, resulting in each set providing guidance to the other, and found that some are misrouted to lateral telencephalon.

CONCLUSION

These findings indicate that ventral telencephalic Pax6 is important for formation of the Islet1-expressing corridor and the thalamic and cortical axons that grow through it. We suggest that Pax6 might affect thalamic axonal growth indirectly via its effect on the corridor.

Bhlhb5 regulates the postmitotic acquisition of area identities in layers II-V of the developing neocortex

While progenitor-restricted factors broadly specify area identities in developing neocortex, the downstream regulatory elements involved in acquisition of those identities in postmitotic neurons are largely unknown. Here, we identify Bhlhb5, a transcription factor expressed in layers II-V, as a postmitotic regulator of area identity. Bhlhb5 is initially expressed in a high caudomedial to low rostrolateral gradient that transforms into a sharp border between sensory and rostral motor cortices. Bhlhb5 null mice exhibit aberrant expression of area-specific genes and structural organization in the somatosensory and caudal motor cortices. In somatosensory cortex, Bhlhb5 null mice display postsynaptic disorganization of vibrissal barrels. In caudal motor cortex, Bhlhb5 null mice exhibit anomalous differentiation of corticospinal motor neurons, accompanied by failure of corticospinal tract formation. Together, these results demonstrate Bhlhb5's function as an area-specific transcription factor that regulates the postmitotic acquisition of area identities and elucidate the genetic hierarchy between progenitors and postmitotic neurons driving neocortical arealization.

The genetics of early telencephalon patterning: some assembly required

The immense range of human behaviours is rooted in the complex neural networks of the cerebrum. The creation of these networks depends on the precise integration of specific neuronal subtypes that are born in different regions of the telencephalon. Here, using the mouse as a model system, we review how these proliferative zones are established. Moreover, we discuss how these regions can be traced back in development to the function of a few key genes, including those that encode fibroblast growth factors (FGFs), sonic hedgehog (SHH), bone morphogenetic proteins (BMPs), forkhead box G1 (FOXG1), paired box 6 (PAX6) and LIM homeobox protein 2 (LHX2), that pattern the early telencephalon.

Topography of thalamic projections requires attractive and repulsive functions of Netrin-1 in the ventral telencephalon

Recent studies have demonstrated that the topography of thalamocortical (TC) axon projections is initiated before they reach the cortex, in the ventral telencephalon (VTel). However, at this point, the molecular mechanisms patterning the topography of TC projections in the VTel remains poorly understood. Here, we show that a long-range, high-rostral to low-caudal gradient of Netrin-1 in the VTel is required in vivo for the topographic sorting of TC axons to distinct cortical domains. We demonstrate that Netrin-1 is a chemoattractant for rostral thalamic axons but functions as a chemorepulsive cue for caudal thalamic axons. In accordance with this model, DCC is expressed in a high-rostromedial to low-caudolateral gradient in the dorsal thalamus (DTh), whereas three Unc5 receptors (Unc5A–C) show graded expression in the reverse orientation. Finally, we show that DCC is required for the attraction of rostromedial thalamic axons to the Netrin-1–rich, anterior part of the VTel, whereas DCC and Unc5A/C receptors are required for the repulsion of caudolateral TC axons from the same Netrin-1–rich region of the VTel. Our results demonstrate that a long-range gradient of Netrin-1 acts as a counteracting force from ephrin-A5 to control the topography of TC projections before they enter the cortex.

The functional properties of each structure in the central nervous system are critically dependent on the precision of neuronal connectivity. The cerebral cortex in particular is a highly organized structure divided into many distinct cortical areas underlying important sensory, motor, and cognitive functions in the brain. Each primary cortical area receives its synaptic inputs from the periphery via the dorsal thalamus. The main relay station for sensory information to the cortex, the thalamus, can be divided into specific nuclei projecting topographically to individual cortical areas. How is the complex topography of thalamic axon projection to individual cortical areas specified during development? Recent evidence demonstrated that thalamic axons are routed to different cortical domains before they enter the cortex, by putative axon guidance cues present in the ventral forebrain. In the present study, we provide evidence that a secreted axon guidance cue, Netrin-1, expressed in a long-range gradient in the ventral forebrain, plays a critical role in the establishment of the topography of thalamic projections by directing different subsets of axons to specific cortical domains. These results provide important insights into the molecular mechanisms responsible for shaping the topographical patterns of thalamocortical axon projections in mammals.

A long-range gradient of Netrin-1 plays a critical role in the specification of the topography of thalamocortical projections in the ventral telencephalon. The function of Netrin-1 requires both its attractive and repulsive functions to guide different subsets of thalamic axons to specific cortical domains.

Genetic regulation of arealization of the neocortex

Arealization of the neocortex is controlled by a regulatory hierarchy beginning with morphogens secreted from patterning centers positioned at the perimeter of the dorsal telencephalon. These morphogens act in part to establish within cortical progenitors the differential expression of transcription factors that specify their area identity, which is inherited by their neuronal progeny, providing the genetic framework for area patterning. The two patterning centers most directly implicated in arealization are the commissural plate, which expresses fibroblast growth factors, and the cortical hem, which expresses bone morphogenetic proteins and vertebrate orthologs of Drosophila wingless, the Wnts. A third, albeit putative, patterning center is the antihem, identified by its expression of multiple signaling molecules. We describe recent findings on roles for these patterning centers in arealization. We also present the most recent evidence on functions of the four transcription factors, Emx2, COUP-TFI, Pax6, and Sp8, thus far implicated in arealization. We also describe screens for candidate target genes of these transcription factors, or other genes potentially involved in arealization. We conclude with an assessment of a forward genetics approach for identifying genes involved in determining area size based in part on quantitative trait locus mapping, and the implications for significant differences between individuals in area size on behavioral performance.

Region-specific requirement for cholesterol modification of sonic hedgehog in patterning the telencephalon and spinal cord

Sonic hedgehog (Shh) secreted from the axial signaling centers of the notochord and prechordal plate functions as a morphogen in dorsoventral patterning of the neural tube. Active Shh is uniquely cholesterol-modified and the hydrophobic nature of cholesterol suggests that it might regulate Shh spreading in the neural tube. Here, we examined the capacity of Shh lacking the cholesterol moiety (ShhN) to pattern different cell types in the telencephalon and spinal cord. In mice expressing ShhN, we detected low-level ShhN in the prechordal plate and notochord, consistent with the notion that ShhN can rapidly spread from its site of synthesis. Surprisingly, we found that low-level ShhN can elicit the generation of a full spectrum of ventral cell types in the spinal cord, whereas ventral neuronal specification and ganglionic eminence development in the Shh(N/-) telencephalon were severely impaired, suggesting that telencephalic patterning is more sensitive to alterations in local Shh concentration and spreading. In agreement, we observed induction of Shh pathway activity and expression of ventral markers at ectopic sites in the dorsal telencephalon indicative of long-range ShhN activity. Our findings indicate an essential role for the cholesterol moiety in restricting Shh dilution and deregulated spread for patterning the telencephalon. We propose that the differential effect of ShhN in patterning the spinal cord versus telencephalon may be attributed to regional differences in the maintenance of Shh expression in the ventral neuroepithelium and differences in dorsal tissue responsiveness to deregulated Shh spreading behavior.

Six3 inactivation causes progressive caudalization and aberrant patterning of the mammalian diencephalon

The homeobox gene Six3 represses Wnt1 transcription. It is also required in the anterior neural plate for the development of the mammalian rostral forebrain. We have now determined that at the 15- to 17-somite stage, the prospective diencephalon is the most-anterior structure in the Six3-null brain, and Wnt1 expression is anteriorly expanded. Consequently, the brain caudalizes, and at the 22- to 24-somite stage, the prospective thalamic territory is the most-anterior structure. At around E11.0, the pretectum replaces this structure. Analysis of Six3;Wnt1 double-null mice revealed that Six3-mediated repression of Wnt1 is necessary for the formation of the rostral diencephalon and that Six3 activity is required for the formation of the telencephalon. These results provide insight into the mechanisms that establish anteroposterior identity in the developing mammalian brain.

Neurotrophin receptor-mediated death of misspecified neurons generated from embryonic stem cells lacking Pax6

Pax6-positive radial glial (RG) cells are the progenitors of most glutamatergic neurons in the cortex, a lineage that can be recapitulated in vitro using embryonic stem (ES) cells. We show here that ES cells lacking Pax6, a transcription factor long known to be essential for cortical development, generate Mash1-positive RG cells that differentiate in GABAergic neurons. These neurons express high levels of the neurotrophin receptor p75NTR causing their rapid death. Pax6 function was also investigated following transplantation of ES cells in the developing chick telencephalon and in mice lacking both Pax6 and p75NTR. Taken together, our results indicate that reliable predictions can be made with cultured ES cells when used to explore the role of genes impacting early aspects of mammalian neurogenesis. They also provide a novel opportunity to compare the molecular constituents of glutamatergic with those of GABA-ergic neurons and to explore the mechanisms of their generation.

Delineation of multiple subpallial progenitor domains by the combinatorial expression of transcriptional codes

The mammalian telencephalon is considered the most complex of all biological structures. It comprises a large number of functionally and morphologically distinct types of neurons that coordinately control most aspects of cognition and behavior. The subpallium, for example, not only gives rise to multiple neuronal types that form the basal ganglia and parts of the amygdala and septum but also is the origin of an astonishing diversity of cortical interneurons. Despite our detailed knowledge on the molecular, morphological, and physiological properties of most of these neuronal populations, the mechanisms underlying their generation are still poorly understood. Here, we comprehensively analyzed the expression patterns of several transcription factors in the ventricular zone of the developing subpallium in the mouse to generate a detailed molecular map of the different progenitor domains present in this region. Our study demonstrates that the ventricular zone of the mouse subpallium contains at least 18 domains that are uniquely defined by the combinatorial expression of several transcription factors. Furthermore, the results of microtransplantation experiments in vivo corroborate that anatomically defined regions of the mouse subpallium, such as the medial ganglionic eminence, can be subdivided into functionally distinct domains.

Cytoskeletal abnormalities in long-term embryonic CNS transplants isolated within peripheral nerve

Cells from the fetal central nervous system (CNS) of rat embryos survive and differentiate when transplanted into the peripheral nervous system (PNS) of adult rats. The experiments described here were aimed at investigating selected molecular and ultrastructural features of dissociated CNS cells from the telencephalon of 12-day-old embryos isolated for long periods of time within PNS segments. Neurons and glia of grafts examined 6-12 months after transplantation into the PNS developed several cytoskeletal abnormalities. In neurons, these changes included Hirano bodies within dendrites and a marked perikaryal immunoreactivity to RT97, a monoclonal antibody that normally recognizes in neuronal processes the phosphorylated 200 kDa protein subunit of neurofilaments. Rosenthal fibres were seen within the glial cells. Similar-looking abnormalities have been described in certain human and animal neurodegenerative diseases and in ageing. Although a relationship between the changes in these long-term neural transplants and such diseases is unknown, these observations provide an opportunity for studying their pathogenesis within laboratory conditions.

Dlx1 and Dlx2 control neuronal versus oligodendroglial cell fate acquisition in the developing forebrain

Progenitors within the ventral telencephalon can generate GABAergic neurons and oligodendrocytes, but regulation of the neuron-glial switch is poorly understood. We investigated the combinatorial expression and function of Dlx1&2, Olig2, and Mash1 transcription factors in the ventral telencephalon. We show that Dlx homeobox transcription factors, required for GABAergic interneuron production, repress oligodendrocyte precursor cell (OPC) formation by acting on a common progenitor to determine neuronal versus oligodendroglial cell fate acquisition. We demonstrate that Dlx1&2 negatively regulate Olig2-dependant OPC formation and that Mash1 promotes OPC formation by restricting the number of Dlx+ progenitors. Progenitors transplanted from Dlx1&2 mutant ventral telencephalon into newborn wild-type mice do not produce neurons but differentiate into myelinating oligodendrocytes that survive into adulthood. Our results identify another role for Dlx genes as modulators of neuron versus oligodendrocyte development in the ventral embryonic forebrain.

Early telencephalic migration topographically converging in the olfactory cortex

Neurons that participate in the olfactory system arise in different areas of the developing mouse telencephalon. The generation of these different cell populations and their tangential migration into the olfactory cortex (OC) was tracked by tracer injection and in toto embryo culture. Cells originating in the dorsal lateral ganglionic eminence (LGE) migrate tangentially along the anteroposterior axis to settle in the piriform cortex (PC). Those originating in the ventral domain of this structure occupy the thickness of the olfactory tubercle (OT), whereas cells from the rostral LGE migrate tangentially into the most anterior telencephalon, at the level of the prospective olfactory bulb (pOB). Neurons from the dorsal telencephalon migrate ventrally, bordering the PC, toward olfactory structures. Two cell populations migrate tangentially from the rostromedial telencephalic wall to the OT and the PC, passing through the ventromedial and dorsolateral face of the telencephalon. Some cells from the germinative area of the rostral telencephalon, at the level of the septoeminential sulcus, migrate rostrally to the pOB or caudally to the OC. Thus, we demonstrate multiple telencephalic origins for the first olfactory neurons and each population following different migratory routes to colonize the OC according to an accurate topographic map.

Multiphoton microscope imaging: The behavior of neural progenitor cells in the rostral migratory stream

Neural progenitor cells (NPCs) in the subventricular zone (SVZ) travel a long distance along the rostral migratory stream (RMS) to give rise to interneurons in the olfactory bulb (OB). Using the multiphoton microscope and time-lapse recording techniques we here report the behavior of NPCs in the RMS under both intact and ischemic conditions in living brain slices. The NPCs were visualized in 3-week-old transgenic mice that carry the reporter gene, green fluorescent protein (GFP), driven by the nestin promoter. Cortical brain ischemia was induced by permanent occlusion of the right common carotid artery and the middle cerebral artery. We observed that the RMS contained two populations of NPCs: nonmigrating cells (bridge cells) and migrating cells. Bridge cells enabled migrating cells to travel and also produced new cells in the RMS. The direction of NPC migration in the RMS was bidirectional in both intact and ischemic conditions. Cortical ischemia impeded NPC travel in the RMS next to the lesion area during the early period of ischemia. Cell-cell contact was a prominent feature affecting NPC translocation and migratory direction. These data suggest that behavior and function of nestin-positive NPCs in the RMS are variable. Cell-cell contacts and microenvironmental changes influence NPC behavior in the RMS. This study may provide insights to help in understanding NPC biology.

Death effector activation in the subventricular zone subsequent to perinatal hypoxia/ischemia

Perinatal hypoxia/ischemia (H/I) is the leading cause of neurological injury resulting from birth complications and pre-maturity. Our studies have demonstrated that this injury depletes the subventricular zone (SVZ) of progenitors. In this study, we sought to reveal which cell death pathways are activated within these progenitors after H/I. We found that calpain activity is detected as early as 4 h of reperfusion and is sustained for 48 h, while caspase 3 activation does not occur until 8 h and peaks at 24 h post-insult. Activated calpains and caspase 3 co-localized within precursors situated in the lateral aspects of the SVZ (which coincides with progenitor cell death), whereas neither enzyme was activated in the medial SVZ (which harbors the neural stem cells that are resilient to this insult). These studies reveal targets for neuroprotective agents to protect precursors from cell death towards the goal of restoring normal brain development after H/I.

REVIEW ARTICLE: Neurotransmitters regulate cell migration in the telencephalon

The complex cytoarchitectonic organization of the adult mammalian telencephalon reflects the elaborate patterns of cell migration that contribute to its generation. The migration by neurons in the CNS can broadly be divided into two categories: radial and tangential. Experimental observations in the telencephalon have shown that glutamatergic projection neurons are born in the progenitor compartment of the dorsal telencephalon and migrate radially to integrate the cortical plate, whereas most gamma-aminobutyric acid (GABA)ergic interneurons are generated in the ganglionic eminences and navigate through multiple tangential paths to settle into distinct telencephalic structures. Despite progress towards the understanding of the genetic determinants that specify the fate of neuronal progenitors, much remains unknown about the mechanisms that direct their migration into specific regions of the telencephalon. Interestingly, besides their function in synaptic transmission, neurotransmitters have been shown to promote several developmental processes that contribute to the establishment and maintenance of the CNS. In this respect, recent studies have highlighted a role for neurotransmitters through activation of their receptors in regulating cell migration in the telencephalon. This review summarizes and discusses the growing body of literature implicating neurotransmitters and their cognate receptors as part of a complex molecular machinery that regulate the migration of immature neurons in the telencephalon during development and in adulthood.

A subpopulation of olfactory bulb GABAergic interneurons is derived from Emx1- and Dlx5/6-expressing progenitors

The subventricular zone (SVZ) of the postnatal brain continuously generates olfactory bulb (OB) interneurons. We show that calretinin+, calbindin+, and dopaminergic (TH+) periglomerular OB interneurons correspond to distinct subtypes of GABAergic cells; all were produced in the postnatal mouse brain, but they matured and were eliminated at different rates. The embryonic lateral ganglionic eminence (LGE) is thought to be the site of origin of postnatal SVZ neural progenitors. Consistently, grafts of the embryonic LGE into the adult brain SVZ generated many OB interneurons, including TH+ and calbindin+ periglomerular interneurons. However, calretinin+ cells were not produced from these LGE grafts. Surprisingly, pallial and septal embryonic progenitors transplanted into the adult brain SVZ also resulted in the generation of OB interneurons, including calretinin+ cells. A subset of Dlx2+ OB interneurons was derived from cells expressing Emx1, a transcription factor largely restricted to the pallium during development. Emx1 lineage-derived cells contributed a substantial portion of GABAergic cells in the OB, including calretinin+ interneurons. This is in contrast to cortex, in which Emx1 lineage-derived cells do not differentiate into GABAergic neurons. Our results suggest that some OB interneurons are derived from progenitors outside the LGE and that precursors expressing what has classically been considered a pallial transcription factor generate GABAergic interneurons.

Directing neurotransmitter identity of neurones derived from expanded adult neural stem cells

In-vitro expanded neural stem cells (NSCs) of the adult subependymal zone (SEZ) may serve as a source for replacing degenerating neurones in disease and trauma. Crucial for the viability of this approach is the ability to selectively generate specific types of neurones from these cells. Here we show that NSCs derived from the adult mouse SEZ and expanded in vitro as neurosphere cells lose their in-vivo specification and generate a mixture of progeny comprising both GABAergic and also, surprisingly, glutamatergic neurones. When forced to express the pro-neural transcription factor neurogenin 2, virtually all progeny of in-vitro expanded adult NSCs acquire a glutamatergic identity, whereas only GABAergic neurones are generated upon expression of the transcription factor Mash1. Respecification of expanded NSCs from the adult SEZ by neurogenin 2 was accompanied by upregulation of the T-box transcription factor Tbr1, suggesting that their progeny had acquired a dorsal telencephalic identity. Thus, in-vitro expanded adult NSCs have the competence to become directed towards distinct functional neurotransmitter phenotypes when the appropriate transcriptional cues are provided.

Neural transplantation in Huntington disease: long-term grafts in two patients

OBJECTIVE

Clinical trials of fetal neural tissue transplantation for Huntington disease (HD) have been conducted with variable clinical results. However, no long-term analysis of graft survival and integration has been published. Here, we report the pathologic findings in two patients with HD who died 74 and 79 months after transplantation.

METHODS

Methods used were pathologic examination, histochemistry, and immunohistochemistry.

RESULTS

Neostriatum from both patients showed typical neuropathologic changes of advanced HD. Surviving grafts were identified in both patients (6/6 sites and 7/8 sites, respectively) as well-demarcated nests within host neostriatum with associated needle tracts. Grafted neurons adopted either dominant calbindin/parvalbumin or calretinin immunoreactivity (IR). Few neurofilament, MAP-2, DARPP-32, tyrosine hydroxylase, or calbindin IR processes traversed the host parenchyma-graft interface despite minimal junctional gliosis. Immunohistochemistry for CD68 showed microgliosis that was more pronounced in host striatum than graft. Scattered CD45 and CD3 IR cells were present within grafts and host parenchyma. No ubiquitin IR neuronal intranuclear inclusions were identified in graft neurons, although these were prevalent in host cells.

CONCLUSIONS

These two autopsies confirm previous findings of neuronal differentiation and survival of transplanted fetal tissue from the ganglionic eminence and also demonstrate viability of neurons from fetal transplants in human neostriatum for more than 6 years. Despite prolonged survival, these grafts had poor integration with host striatum that is likely responsible for lack of clear clinical improvement in these patients.

Conserved pattern of tangential neuronal migration during forebrain development

Origin, timing and direction of neuronal migration during brain development determine the distinct organization of adult structures. Changes in these processes might have driven the evolution of the forebrain in vertebrates. GABAergic neurons originate from the ganglionic eminence in mammals and migrate tangentially to the cortex. We are interested in differences and similarities in tangential migration patterns across corresponding telencephalic territories in mammals and reptiles. Using morphological criteria and expression patterns of Darpp-32, Tbr1, Nkx2.1 and Pax6 genes, we show in slice cultures of turtle embryos that early cohorts of tangentially migrating cells are released from the medial ganglionic eminence between stages 14 and 18. Additional populations migrate tangentially from the dorsal subpallium. Large cohorts of tangentially migrating neurons originate ventral to the dorsal ventricular ridge at stage 14 and from the lateral ganglionic eminence from stage 15. Release of GABAergic cells from these regions was investigated further in explant cultures. Tangential migration in turtle proceeds in a fashion similar to mammals. In chimeric slice culture and in ovo graft experiments, the tangentially migrating cells behaved according to the host environment - turtle cells responded to the available cues in mouse slices and mouse cells assumed characteristic migratory routes in turtle brains, indicating highly conserved embryonic signals between these distant species. Our study contributes to the evaluation of theories on the origin of the dorsal cortex and indicates that tangential migration is universal in mammals and sauropsids.