Laminar fate of cortical GABAergic interneurons is dependent on both birthdate and phenotype

Pioneering work indicates that the final position of neurons in specific layers of the mammalian cerebral cortex is determined primarily by birthdate. Glutamatergic projection neurons are born in the cortical proliferative zones of the dorsal telencephalon, and follow an "inside-out" neurogenesis gradient: later-born cohorts migrate radially past earlier-born neurons to populate more superficial layers. GABAergic interneurons, the major source of cortical inhibition, comprise a heterogeneous population and are produced in proliferative zones of the ventral telencephalon. Mechanisms by which interneuron subclasses find appropriate layer-specific cortical addresses remain largely unexplored. Major cortical interneuron subclasses can be identified based on expression of distinct calcium-binding proteins including parvalbumin, calretinin, or calbindin. We determined whether cortical layer-patterning of interneurons is dependent on phenotype. Parvalbumin-positive interneurons populate cortical layers with an inside-out gradient, and birthdate is isochronous to projection neurons in the same layers. In contrast, another major GABAergic subtype, labeled using calretinin, populates the cerebral cortex using an opposite "outside-in" gradient, heterochronous to neighboring neurons. In addition to birthdate, phenotype is also a determinant of cortical patterning. Discovery of a cortical subpopulation that does not follow the well-established inside-out gradient has important implications for mechanisms of layer formation in the cerebral cortex.

Involvement of the myelin-associated inhibitor Nogo-A in early cortical development and neuronal maturation

Nogo-A is a myelin-associated protein expressed by neurons and myelinating mature oligodendrocytes in the central nervous system. Although most research has focused on the participation of Nogo-A in the prevention of axonal regeneration and plasticity in the adult, little attention has been paid to the putative functions of Nogo-A during embryonic development. Here we examined the general pattern and cell-specific distribution of Nogo-A in the prenatal mouse telencephalon. In addition, we studied the development of the major axon tracts and radial and tangential migration in Nogo-A/B/C knockout mice. The pattern of Nogo-A showed distinct distribution in radial glia and postmitotic neurons, in which it is particularly enriched in developing axons. Similarly, Nogo-A was enriched at the leading process of tangentially migrating interneurons but not detectable in radial migrating neurons. Although a low level of Nogo-A appears to be on the surface of many cortical neurons, most proteins have intracellular localization. In Nogo-deficient background, neurons displayed early polarization and increased branching in vitro, probably reflecting a cell-intrinsic role of Nogo proteins in branching reduction, and early tangential migration was delayed. On the basis of these observations, we propose that Nogo proteins, particularly Nogo-A, are involved in multiple processes during cortical development.

Comparative aspects of p73 and Reelin expression in Cajal-Retzius cells and the cortical hem in lizard, mouse and human

Cajal-Retzius (CR) cells of the mammalian neocortex co-express the extracellular matrix protein Reelin and p73, a transcription factor involved in cell death and survival. Most neocortical CR cells derive from the cortical hem, with minor additional sources. We analyzed the distribution of Reelin and p73 immunoreactive (ir) neurons in the telencephalon of Lacerta galloti from early embryonic stages to hatching. Numerous Reelin-ir cells appeared in the pallial MZ from the preplate stage onward. Conversely, p73-ir cells were rare in the pallial preplate and not observed in the cortical plate. Subpallial p73-ir cells spread from the septum and the telencephalic-diencephalic boundary to the pial surface of the basal forebrain and amygdala, respectively, where they co-expressed Reelin and p73. A small group of Reelin/p73-ir CR cells appeared in a rudimentary cortical hem at the interface of the medial cortex and choroid plexus. Comparison with early embryonic stages of mice and humans showed similar foci of p73-ir cells in the septum and at the telencephalic-diencephalic boundary and revealed an increasing prominence of the cortical hem, in parallel with increasing numbers of neocortical Reelin/p73 positive CR cells, which attain highest differentiation in the human brain. Our data show that Reelin-expression in the pallium is evolutionarily conserved and independent of a cortical hem, and suggest that p73 in the cortical hem may be involved in the evolutionary increase in number and complexity of the mammalian neocortical CR cells.

Genetic patterning of the mammalian telencephalon by morphogenetic molecules and transcription factors

Patterning centers that produce gradients of morphogenetic molecules, including fibroblast growth factor (FGF), bone morphogenetic proteins (BMP), Wnt, Sonic hedgehog (Shh), and retinoic acid (RA), are located in telencephalic anlage during early stages of development. Genetic evidence based on loss-of-function and gain-of-function studies indicate that they are involved in regional specification of the dorsal, ventral, and lateral telencephalon. For patterning of the dorsal telencephalon, FGF8 controls the anteroposterior patterning, while BMP and Wnt molecules regulate the mediolateral patterning. Shh and retinoic acid regulate patterning of the ventral and the lateral telencephalon. The regionalization of telencephalon is accompanied by expression of region-specific codes of transcription factors, which in turn regulate different phases of neuronal development to generate different cell types in each brain region. Therefore, bioactive signals of morphogenetic molecules are translated into transcription factor codes for regional specification, which subsequently leads to neurogenesis of the diversity of cell types in different regions of the telencephalon.

Molecular characterization of adult mouse subventricular zone progenitor cells during the onset of differentiation

Adult mouse subventricular zone (SVZ) neural progenitor cells (NPCs) retain the capacity to generate multiple lineages in vitro and in vivo. Thus far, the mechanisms involved in the regulation of these cells have not been well elucidated. We have carried out RNA profiling of adult SVZ cell cultures undergoing differentiation, to identify pathways that regulate progenitor cell proliferation and to define a set of transcripts that can be used as molecular tools in the drug discovery process. We carried out a stepwise stratification of the results to identify transcripts specifically enriched in NPCs and validated some of these using comparative literature analysis, quantitative polymerase chain reaction and immunological techniques. The results show a set of transcription factors, secreted molecules and plasma membrane markers that are differentially regulated during differentiation. Pathway analysis highlights alterations in insulin growth factor, Wnt and transforming growth factor beta signalling cascades. Further characterization of these components could provide greater insight into the mechanisms involved in the regulation of neurogenesis in the adult brain.

Development of cerebral sulci and gyri in fetuses of cynomolgus monkeys (Macaca fascicularis)

This study aimed to clarify the development of sulci and gyri on the external surface of the cerebrum of cynomolgus monkeys. Sulcus formation began with the appearance of the lateral fissure on embryonic day (ED) 70, followed by delineations of four cerebral lobes by the emergence of the parietooccipital sulcus, central sulcus, and preoccipital notch on EDs 80-90. The following primary sulci were then visible until ED 120: the superior temporal sulcus on ED 90; the intraparietal sulcus, lunate sulcus, inferior occipital sulcus, and arcuate sulcus on ED 100; and the principle sulcus on ED 110; the occipitotemporal sulcus, anterior middle temporal sulcus, and superior postcentral dimple on ED 120. These sulci demarcated the superior temporal gyrus on ED 90, the precentral gyrus, supramarginal gyrus, and angular gyrus on ED 100, and the inferior and middle temporal gyri, postocentral gyrus, superior parietal lobule, superior, middle and inferior frontal gyri, and inferior occipital gyrus on ED 120. Except for the intermediate and lateral orbitofrontal sulci, the sulci that appeared on ED 130 and thereafter were not related to the gyrus demarcations. Intriguingly, the brain markedly gained weight on EDs 100 and 120, corresponding to the embryonic ages when almost all gyri were visible. The results suggest that a rapid growth of the cerebrum involves convolutions of the gyri by a regular sequence of the sulcus formation in cynomolgus monkeys. This study further provides a standard of reference for normal development in the cerebral cortical morphology of cynomolgus monkeys.

Abnormal positioning of diencephalic cell types in neocortical tissue in the dorsal telencephalon of mice lacking functional Gli3

The transcription factor Gli3 (glioma-associated oncogene homolog) is essential for normal development of the mammalian forebrain. One extreme requirement for Gli3 is at the dorsomedial telencephalon, which does not form in Gli3(Xt/Xt) mutant mice lacking functional Gli3. In this study, we analyzed expression of Gli3 in the wild-type telencephalon and observed a (high)dorsal-to-(low)ventral gradient of Gli3 expression and predominance of the cleaved form of the Gli3 protein dorsally. This graded expression correlates with the (severe)dorsal-to-(mild)ventral telencephalic phenotype observed in Gli3(Xt/Xt) mice. We characterized the abnormal joining of the telencephalon to the diencephalon and defined the medial limit of the dorsal telencephalon in Gli3(Xt/Xt) mice early in corticogenesis. Based on this analysis, we concluded that some of the abnormal expression of ventral telencephalic markers previously described as being in the dorsal telencephalon is, in fact, expression in adjacent diencephalic tissue, which expresses many of the same genes that mark the ventral telencephalon. We observed occasional cells with diencephalic character in the Foxg1 (forkhead box)-expressing Gli3(Xt/Xt) telencephalon at embryonic day 10.5, a day after the anatomical subdivision of the forebrain vesicle. Large clusters of such cells appear in the Gli3(Xt/Xt) neocortical region at later ages, when the neocortex becomes highly disorganized, forming rosettes comprising mainly neural progenitors. We propose that Gli3 is indispensable for formation of an intact telencephalic-diencephalic boundary and for preventing the abnormal positioning of diencephalic cells in the dorsal telencephalon.

The amygdaloid complex and the medial and lateral ventricular eminences in staged human embryos

The amygdaloid complex was investigated in 36 serially sectioned staged human embryos, including 20 impregnated with silver. This is the first such account based on graphic reconstructions, 28 of which were prepared. Significant findings in the human include the following. (1) The medial (first) and (then) lateral ventricular eminences arise independently at stages 14 and 15, and unite only at stage 18 to form the floor of the lateral ventricle. (2) The future amygdaloid region is discernible at stage 14 and the amygdaloid primordium at stage 15. (3) The anterior amygdaloid area and the corticomedial and basolateral complexes appear at stage 16. (4) These three major divisions arise initially from the medial ventricular eminence, which is diencephalic. (5) Individual nuclei begin to be detectable at stages 17-21, the central nucleus at stage 23 and the lateral nucleus shortly thereafter. (6) The ontogenetic findings in the human embryonic period accord best with the classification used by Humphrey. (7) The lateral eminence, which is telencephalic, contributes to the cortical nucleus at stage 18. (8) The primordial plexiform layer develops independently of the cortical nucleus. (9) Spatial changes of the nuclei within the amygdaloid complex and of the complex as a whole begin in the embryonic period and continue during the fetal period, during the early part of which the definitive amygdaloid topography in relation to the corpus striatum is attained. (10) The developing amygdaloid nuclei are closely related to the medial forebrain bundle, which has already appeared in stage 15. (11) Fibre connections develop successively between the amygdaloid nuclei and the septal, hippocampal and diencephalic formations, constituting the beginning of the limbic system before the end of the embryonic period. Although the nucleus accumbens also appears relatively early (stage 19), connections between it and the amygdaloid complex are not evident during the embryonic period. (12) Influence of the olfactory bulb and tubercle on initial amygdaloid development, as postulated for rodents, is unlikely in the human. The findings exemplify the necessity of beginning developmental studies with the embryonic period proper.

Central roles of the roof plate in telencephalic development and holoprosencephaly

The roof plate is a well known signaling center in CNS development, but its roles in the developing telencephalon and the common holoprosencephaly (HPE) malformation have been uncertain. Using cellular ablations in mice, we show that roof plate cell loss causes failed midline induction and HPE in the dorsal telencephalon. This morphologic phenotype is accompanied by selective deficits in midline gene expression and a reduced activity gradient for bone morphogenetic proteins (Bmps), the major signals produced by the roof plate. In dissociated cells and mutant explants, exogenous Bmp4 is sufficient to mimic roof plate selectivity in midline gene regulation and to rescue roof plate-dependent midline patterning. Previously unrecognized neuroanatomical defects predicted by the mouse model are then confirmed in human HPE patients. These findings establish selective roles for roof plate-dependent Bmp signaling in dorsal telencephalic patterning and HPE and define novel candidate genes for the human disorder.

Shh maintains Nkx2.1 in the MGE by a Gli3-independent mechanism

Gamma-aminobutyric acidergic (GABAergic) interneurons perform crucial roles in cortical development and function. These roles are executed by diverse subgroups of interneurons, and dysfunction of particular subgroups may contribute to a variety of neuropsychiatric diseases. In rodents, most cortical interneurons originate in the pallidal telencephalon that also gives rise to the GABAergic neurons of the striatum and other ventral structures. In this paper, we examine the evidence for distinct origins of interneuron subgroups and then discuss the role of the homeobox gene thyroid transcription factor1 (Ttf-1 or Nkx2.1) in the specification of interneurons from the medial ganglionic eminence (MGE). Nkx2.1 is induced in the pallidal telencephalon by the action of sonic hedgehog (Shh) that antagonizes formation of the dorsalizing Gli3 repressor (Gli3R) protein. Recent evidence suggests that Shh is also required to maintain Nkx2.1 expression, and thus MGE interneuron specification, during embryonic neurogenesis. Here we provide evidence that, in contrast to the initial induction of Nkx2.1 during telencephalic patterning, the Nkx2.1 maintenance function of Shh does not require blocking the formation of the Gli3R. The plastic nature of Nkx2.1 expression during the age range of interneuron genesis suggests that factors regulating this gene can be critical determinants of the balance of excitation and inhibition in the postnatal cerebral cortex.

Removal of Pax6 partially rescues the loss of ventral structures in Shh null mice

Pax6 and Gli3 are dorsally expressed genes that are known to antagonize sonic hedgehog (Shh) activity. We have previously shown that dorsoventral patterning defects seen in Shh(-/-) mutants are rescued in Shh(-/-);Gli3(-/-) compound mutants. Here we investigate if the loss of Pax6 can also ameliorate defects seen in Shh(-/-) mutants. In support of this notion, we observe that the fusion of the cerebral vesicles seen in Shh(-/-) mutants is partially corrected in E12.5 Shh(-/-);Pax6(-/-) compound mutants. Investigation of pan-ventral markers such as Dlx2 also shows that, unlike Shh(-/-), a broad domain of expression of this gene is observed in Shh(-/-);Pax6(-/-) mice. Interestingly, we observe that while the expression of ER81 in the ventral telencephalon is expanded, the expression of Ebf1 is lost. This suggests that the rescued ventral domain observed in Shh(-/-);Pax6(-/-) mice is the dorsal lateral ganglionic eminence region. With regard to dorsal telencephalic patterning, we also observe rescue of the pallial-subpallial boundary, as well as a partial rescue of the dorsal midline. Together, our findings are consistent with Pax6 function being required for aspects of Gli3-mediated telencephalic patterning.

The Rho-GTPase cdc42 regulates neural progenitor fate at the apical surface

Stem cell persistence into adulthood requires self-renewal from early developmental stages. In the developing mouse brain, only apical progenitors located at the ventricle are self-renewing, whereas basal progenitors gradually deplete. However, nothing is known about the mechanisms regulating the fundamental difference between these progenitors. Here we show that the conditional deletion of the small Rho-GTPase cdc42 at different stages of neurogenesis in mouse telencephalon results in an immediate increase in basal mitoses. Whereas cdc42-deficient progenitors have normal cell cycle length, orientation of cell division and basement membrane contact, the apical location of the Par complex and adherens junctions are gradually lost, leading to an increasing failure of apically directed interkinetic nuclear migration. These cells then undergo mitoses at basal positions and acquire the fate of basal progenitors. Thus, cdc42 has a crucial role at the apical pole of progenitors, thereby regulating the position of mitoses and cell fate.

The origin and specification of cortical interneurons

GABA-containing interneurons are crucial to both the development and function of the cerebral cortex. Unlike cortical projection neurons, which have a relatively conserved set of characteristics, interneurons include multiple phenotypes that vary on morphological, physiological and neurochemical axes. This diversity, and the relatively late, context-dependent maturation of defining features, has challenged efforts to uncover the transcriptional control of cortical interneuron development. Here, we discuss recent data that are beginning to illuminate the origins and specification of distinct subgroups of cortical interneurons.

FGF signalling generates ventral telencephalic cells independently of SHH

Sonic hedgehog (SHH) is required to generate ventral cell types throughout the central nervous system. Its role in directly specifying ventral cells,however, has recently been questioned because loss of the Shh gene has little effect on ventral development if the Gli3 gene is also mutant. Consequently, another ventral determinant must exist. Here, genetic evidence establishes that FGFs are required for ventral telencephalon development. First, simultaneous deletion of Fgfr1 and Fgfr3specifically in the telencephalon results in the loss of differentiated ventromedial cells; and second, in the Fgfr1;Fgfr2 double mutant, ventral precursor cells are lost, mimicking the phenotype obtained previously with a loss of SHH signalling. Yet, in the Fgfr1;Fgfr2 mutant, Shh remains expressed, as does Gli1, the transcription of which depends on SHH activity, suggesting that FGF signalling acts independently of SHH to generate ventral precursors. Moreover, the Fgfr1;Fgfr2 phenotype, unlike the Shhphenotype, is not rescued by loss of Gli3, further indicating that FGFs act downstream of Shh and Gli3 to generate ventral telencephalic cell types.

Segregation of telencephalic and eye-field identities inside the zebrafish forebrain territory is controlled by Rx3

Anteroposterior patterning of the vertebrate forebrain during gastrulation involves graded Wnt signaling, which segregates anterior fields (telencephalon and eye) from the diencephalon. How the telencephalic and retinal primordia are subsequently subdivided remains largely unknown. We demonstrate that at late gastrulation the Paired-like homeodomain transcription factor Rx3 biases cell specification choices towards the retinal fate within a population of bipotential precursors of the anterior forebrain: direct cell tracing demonstrates that retinal precursors acquire a telencephalic fate in embryos homozygous for the rx3-null allele ckh(ne2611), characterized by an enlarged telencephalon and a lack of eyes. Chimera analyses further indicate that this function of Rx3 is cell autonomous. Transfating of the eye field in the absence of Rx3 function correlates with a substantial posterior expansion of expression of the Wnt antagonist Tlc and the winged-helix transcription factor Foxg1. These results suggest that the process segregating the telencephalic and eye fields is isolated from diencephalic patterning, and is mediated by Rx3.

Environmental Signals Elicit Multiple Responses in Dorsal Telencephalic Progenitors by Threshold-Dependent Mechanisms

Environmental signals including epidermal growth factor family members, Shh, fibroblast growth factor, and bone morphogenetic protein (BMP) can affect multiple processes during the development of the central nervous system, raising questions about the mechanisms that determine how these pleiotropic signals are interpreted to elicit appropriate responses at specific times and locations. Here we address the idea that different thresholds of stimulation determine how progenitors in the dorsal telencephalon interpret these signals. One mechanism for achieving different thresholds of signaling is illustrated by the developmental increase in the level of epidermal growth factor receptor (EGFR) expression among a subset of progenitors in the late embryonic telencephalon. Another mechanism is illustrated by the antagonistic interaction of BMP with Shh, which can influence EGFR expression and neuron subtype choice. We focus on the similarities and differences in the control of these responses and address the possibility that the gamma-aminobutyric acidergic neuron specification might be linked to progenitor expression of a higher level of EGFRs.

P-GAP-43 Is Enriched in Horizontal Cell Divisions throughout Rat Cortical Development

Asymmetric cell divisions are correlated to neurogenesis in the mammalian cortex and occur often with a horizontal orientation of cell division. However, the molecular mechanisms of spindle orientation or asymmetric cell divisions are not well understood in the developing mammalian central nervous system. Here we show a new molecular marker for horizontally dividing precursors in the mammalian telencephalon. The antibody 2G12 directed against phosphorylated serine of growth associated protein 43 (GAP-43) labels postmitotic neurons and a subset of cells in mitoses in the developing rat telencephalon. 2G12 immunoreactivity was found at a high frequency in mitotic cells dividing parallel to the ventricular surface throughout neurogenesis (embryonic day 13-17) in the cerebral cortex and ganglionic eminence. Interestingly, we detected the same predominance of 2G12 immunoreactivity in horizontally dividing cells in the subventricular zone, the second proliferative layer that has recently been involved in the generation of neurons. Moreover, 2G12 immunostaining is no longer detectable in mitotic cells of the ventricular zones at E21, the onset of gliogenesis in rat telencephalon. These data imply GAP-43 phosphorylation in the phase of neuronal commitment during M-phase and present to our knowledge the first molecular correlate to horizontally dividing precursors in mammalian neurogenesis.

D2-40 antibody immunoreactivity in developing human brain, brain tumors and cultured neural cells

D2-40 antibody is raised against an oncofetal antigen, the M2A antigen. It has been used as a marker for lymphatic endothelium as well as mesothelioma and cerebellar hemangioblastoma. We demonstrate here that positive D2-40 immunoreactivity was found in the developing cerebrum, particularly in the germinal matrix layer, immature ependyma, choroid plexus and meninges. In the developing cerebellum, positive D2-40 immunoreactivity was found in the external granular layer particularly of the outer portion and the Purkinje cell layer as well as meninges. Some brain tumors such as anaplastic ependymoma, some medulloblastomas, glioblastoma, pineal germinoma, craniopharyngioma, choroid plexus papilloma, choroid plexus carcinoma, and meningioma showed positive immunoreactivity with D2-40. Therefore, D2-40 antibody is considered a useful marker for research on developing brain and diagnosis of brain tumors, differentiation between choroid plexus carcinoma and metastatic carcinoma. In addition, on cultured human neural cells, D2-40 immunoreactivity was found in nestin-positive neural stem/progenitor cells and neuronal lineage cells. As D2-40 antibody recognizes cell surface antigen M2A, it might be a candidate cell surface marker for isolation of human neural stem cells/neuronal lineage cells in the fluorescence-activated cell sorting technique.

Basic fibroblast growth factor endows dorsal telencephalic neural progenitors with the ability to differentiate into oligodendrocytes but not gamma-aminobutyric acidergic neurons

Basic fibroblast growth factor (bFGF) is commonly used to enrich and maintain neural stem cells in vitro. Olig2 is an essential transcription factor for oligodendrocyte lineage specification and is expressed predominantly in ventral neuroepithelial cells in the medial and lateral ganglionic eminence (GE), where oligodendrocyte progenitors originate. Here we report significant induction of Olig2 expression in dorsal neuroepithelium-derived cells cultured in the presence of bFGF, in which Olig2-expressing cells were initially negligible. Among Olig2-expressing cells appearing after a 5-day treatment with bFGF, 99.8% coexpressed nestin. There was no significant difference in proliferation or apoptosis in dorsal and ventral neuroepithelial cultures in the presence of bFGF, suggesting that bFGF induces ectopic expression of Olig2 in dorsal "cortical" neuroepithelial cells. Similarly, expression of Mash1, another ventral neuroepithelial cell marker gene, was also induced in cultured dorsal neuroepithelial cells in the presence of bFGF. Conversely, in this culture, expression of dorsal neuroepithelial cell markers, such as Neurogenin1, Neurogenin2, Pax6, and Emx2, was down-regulated. These results suggested a possible ventralizing activity of bFGF. In fact, bFGF-treated dorsal neuroepithelial cells acquired the potential to generate O4-positive oligodendrocytes with efficacy comparable to that observed with GE-derived cells. In marked contrast, bFGF did not enable dorsal neuroepithelial cells to generate gamma-aminobutyric acid (GABA) neurons, which normally develop only from GE in vivo. Thus, bFGF endows dorsal telencephalic neural progenitors with the ability to differentiate into oligodendrocytes but not GABAergic neurons, suggesting the presence of different mechanisms governing specification of dorsoventral cell identities of neuronal and glial cell lineages.

Notch3 signaling promotes radial glial/progenitor character in the mammalian telencephalon

The Notch signaling pathway is known to influence cell fate in the developing mammalian nervous system. Previous work in the mouse telencephalon has shown that activated Notch1 promotes radial glial and astrocytic character in vivo, and fibroblast growth factor (FGF)-responsive neural progenitor character in vitro. In light of studies suggesting that Notch3 can antagonize Notch1, we tested the effects of activated Notch3 (NICD3) in the mouse telencephalon. Infection of embryonic day 9.5 telencephalic progenitors in vivo with NICD3 promoted radial glial/progenitor character embryonically and astrocyte fate postnatally. In addition, expression of NICD3 in telencephalic progenitors in vitro increased neurosphere frequency in FGF2, but was incompatible with neurosphere growth in epidermal growth factor (EGF). Thus, in the developing telencephalon, Notch1 and Notch3 function similarly, and may activate similar signaling cascades. Consistent with this notion, expression of an activated form of the Notch effector CBF1 (CBF1-VP16), or of the pathway target Hes5 promoted radial glial/progenitor character in vivo. Interestingly, unlike NICD1 and NICD3, CBF1-VP16 and Hes5 did not inhibit neurosphere growth in EGF, suggesting that this effect may be mediated at least in part by CBF1/Hes-independent signaling.

Cell cycle progression is required for nuclear migration of neural progenitor cells

In the developing brain, neural progenitor cells in the ventricular zone (VZ) show a typical migration pattern-interkinetic nuclear migration, in which nuclear position within the VZ is correlated with the cell cycle. However, the mechanisms underlying this regulation remain unclear. To clarify whether the cell cycle progression controls nuclear migration of neural progenitor cells, we determined whether chemically induced cell cycle arrest affected nuclear migration patterns in the VZ. Administration of 5-azacytidine (5AzC) or cyclophosphamide (CP) to pregnant mice induced cell cycle arrest in the fetal neural progenitor cells of the telencephalon: 5AzC induced G2/M-phase arrest, and CP induced S-phase arrest. We used 5-bromo-2'-deoxyuridine (BrdU) labeling to determine the position of the cell in the cell cycle and the nuclei within the VZ at the same time. Cells arrested in G2/M-phase stopped migrating in the inner area of the VZ. Cells arrested in S-phase stopped migrating in the outer area. These results indicate that nuclear position within the VZ was correlated with cell cycle phase, even when the cell cycle was disrupted, and that the nuclei of neural progenitor cells can migrate only when their cell cycle is going. Our results suggest that cell cycle regulators might control the machinery of migration through a common regulatory mechanism.

Dose-dependent functions of Fgf8 in regulating telencephalic patterning centers

Mouse embryos bearing hypomorphic and conditional null Fgf8mutations have small and abnormally patterned telencephalons. We provide evidence that the hypoplasia results from decreased Foxg1 expression,reduced cell proliferation and increased cell death. In addition, alterations in the expression of Bmp4, Wnt8b, Nkx2.1 and Shh are associated with abnormal development of dorsal and ventral structures. Furthermore, nonlinear effects of Fgf8 gene dose on the expression of a subset of genes, including Bmp4 and Msx1, correlate with a holoprosencephaly phenotype and with the nonlinear expression of transcription factors that regulate neocortical patterning. These data suggest that Fgf8 functions to coordinate multiple patterning centers, and that modifications in the relative strength of FGF signaling can have profound effects on the relative size and nature of telencephalic subdivisions.

Clonal origin of the mammalian forebrain from widespread oriented mixing of early regionalized neuroepithelium precursors

The forebrain is formed by remodeling and growth of the anterior neural plate. This morphogenesis occurs in response to inductive signals during gastrulation and neurulation but is poorly understood at the cellular level. Here, we have used the LaacZ method of single cells labeling to visualize, at E12.5, clones originated at early stages of mouse forebrain development. The largest clones show that single progenitors can give rise to neuroepithelial cells dispersed across the forebrain. A significant fraction of the clones, and even relatively small ones, populated both the diencephalon and the telencephalon, indicating that the clonal separation between diencephalic and telencephalic progenitors is transient and/or partial. However, two groups of large clones, populating either the diencephalon or the telencephalon, dispersed within their respective domains, suggesting an early regionalization between some diencephalic and telencephalic progenitors. Widespread oriented mixing within these territories and then clonal expansion into smaller domains probably follow this initial regionalization. These data are consistent with a model of progressive specification of forebrain domains. We propose that the ordered expansion of early regionalized progenitor pools for the diencephalon and telencephalon could establish a potential link between early inductive signals and forebrain morphogenesis.

belladonna/(Ihx2) is required for neural patterning and midline axon guidance in the zebrafish forebrain

Some of the earliest axon pathways to form in the vertebrate forebrain are established as commissural and retinal axons cross the midline of the diencephalon and telencephalon. To better understand axon guidance in the forebrain, we characterized the zebrafish belladonna (bel) mutation, which disrupts commissural and retinal axon guidance in the forebrain. Using a positional cloning strategy, we determined that the bel locus encodes zebrafish Lhx2, a lim-homeodomain transcription factor expressed in the brain, eye and fin buds. We show that bel(Ihx2) function is required for patterning in the ventral forebrain and eye, and that loss of bel function leads to alterations in regulatory gene expression, perturbations in axon guidance factors, and the absence of an optic chiasm and forebrain commissures. Our analysis reveals new roles for Ihx2 in midline axon guidance, forebrain patterning and eye morphogenesis.