Differential expression of Pax6 and Ngn2 between pair-generated cortical neurons

Role of FOXC1 in regulating APSCs self-renewal via STI-1/PrPC signaling

Forkhead box protein C1 (FOXC1) is known to regulate developmental processes in the skull and brain. Methods: The unique multipotent arachnoid-pia stem cells (APSCs) isolated from human and mouse arachnoid-pia membranes of meninges were grown as 3D spheres and displayed a capacity for self-renewal. Additionally, APSCs also expressed the surface antigens as mesenchymal stem cells. By applying the FOXC1 knockout mice and mouse brain explants, signaling cascade of FOXC1-STI-1-PrP^C was investigated to demonstrate the molecular regulatory pathway for APSCs self-renewal. Moreover, APSCs implantation in stroke model was also verified whether neurogenic property of APSCs could repair the ischemic insult of the stroke brain. Results: Activated FOXC1 regulated the proliferation of APSCs in a cell cycle-dependent manner, whereas FOXC1-mediated APSCs self-renewal was abolished in FOXC1 knockout mice (FOXC1^-/- mice). Moreover, upregulation of STI-1 regulated by FOXC1 enhanced cell survival and self-renewal of APSCs through autocrine signaling of cellular prion protein (PrP^C). Mouse brain explants STI-1 rescues the cortical phenotype in vitro and induces neurogenesis in the FOXC1 ^-/- mouse brain. Furthermore, administration of APSCs in ischemic brain restored the neuroglial microenvironment and improved neurological dysfunction. Conclusion: We identified a novel role for FOXC1 in the direct regulation of the STI-1-PrP^C signaling pathway to promote cell proliferation and self-renewal of APSCs.

Estradiol and the Development of the Cerebral Cortex: An Unexpected Role?

The cerebral cortex undergoes rapid folding in an "inside-outside" manner during embryonic development resulting in the establishment of six discrete cortical layers. This unique cytoarchitecture occurs via the coordinated processes of neurogenesis and cell migration. In addition, these processes are fine-tuned by a number of extracellular cues, which exert their effects by regulating intracellular signaling pathways. Interestingly, multiple brain regions have been shown to develop in a sexually dimorphic manner. In many cases, estrogens have been demonstrated to play an integral role in mediating these sexual dimorphisms in both males and females. Indeed, 17β-estradiol, the main biologically active estrogen, plays a critical organizational role during early brain development and has been shown to be pivotal in the sexually dimorphic development and regulation of the neural circuitry underlying sex-typical and socio-aggressive behaviors in males and females. However, whether and how estrogens, and 17β-estradiol in particular, regulate the development of the cerebral cortex is less well understood. In this review, we outline the evidence that estrogens are not only present but are engaged and regulate molecular machinery required for the fine-tuning of processes central to the cortex. We discuss how estrogens are thought to regulate the function of key molecular players and signaling pathways involved in corticogenesis, and where possible, highlight if these processes are sexually dimorphic. Collectively, we hope this review highlights the need to consider how estrogens may influence the development of brain regions directly involved in the sex-typical and socio-aggressive behaviors as well as development of sexually dimorphic regions such as the cerebral cortex.

Coupling progenitor and neuronal diversity in the developing neocortex

The adult neocortex is composed of several types of glutamatergic neurons, which are sequentially born from progenitors during development. The extent and nature of progenitor diversity, and how it relates to neuronal diversity, is still poorly understood. In this review, we discuss key features of neocortical progenitors across several species, including their morphological properties, cell cycling behaviour and molecular signatures, and how these features relate to the competence of these cells to generate distinct types of progenies.

The 16p11.2 deletion mouse model of autism exhibits altered cortical progenitor proliferation and brain cytoarchitecture linked to the ERK MAPK pathway

Autism spectrum disorders are complex, highly heritable neurodevelopmental disorders affecting ∼1 in 100 children. Copy number variations of human chromosomal region 16p11.2 are genetically linked to 1% of autism-related disorders. This interval contains the MAPK3 gene, which encodes the MAP kinase, ERK1. Mutations in upstream elements regulating the ERK pathway are genetically linked to autism and other disorders of cognition including the neuro-cardio-facial cutaneous syndromes and copy number variations. We report that a murine model of human 16p11.2 deletion exhibits a reduction in brain size and perturbations in cortical cytoarchitecture. We observed enhanced progenitor proliferation and premature cell cycle exit, which are a consequence of altered levels of downstream ERK effectors cyclin D1 and p27(Kip1) during mid-neurogenesis. The increased progenitor proliferation and cell cycle withdrawal resulted in premature depletion of progenitor pools, altering the number and frequency of neurons ultimately populating cortical lamina. Specifically, we found a reduced number of upper layer pyramidal neurons and an increase in layer VI corticothalamic projection neurons, reflecting the altered cortical progenitor proliferation dynamics in these mice. Importantly, we observed a paradoxical increase in ERK signaling in mid-neurogenesis in the 16p11.2del mice, which is coincident with the development of aberrant cortical cytoarchitecture. The 16p11.2del mice exhibit anxiety-like behaviors and impaired memory. Our findings provide evidence of ERK dysregulation, developmental abnormalities in neurogenesis, and behavioral impairment associated with the 16p11.2 chromosomal deletion.

Single-cell analysis reveals transcriptional heterogeneity of neural progenitors in human cortex

The human cerebral cortex depends for its normal development and size on a precisely controlled balance between self-renewal and differentiation of diverse neural progenitor cells. Specialized progenitors that are common in humans but virtually absent in rodents, called outer radial glia (ORG), have been suggested to be crucial to the evolutionary expansion of the human cortex. We combined progenitor subtype-specific sorting with transcriptome-wide RNA sequencing to identify genes enriched in human ORG, which included targets of the transcription factor neurogenin and previously uncharacterized, evolutionarily dynamic long noncoding RNAs. Activating the neurogenin pathway in ferret progenitors promoted delamination and outward migration. Finally, single-cell transcriptional profiling in human, ferret and mouse revealed more cells coexpressing proneural neurogenin targets in human than in other species, suggesting greater neuronal lineage commitment and differentiation of self-renewing progenitors. Thus, we find that the abundance of human ORG is paralleled by increased transcriptional heterogeneity of cortical progenitors.

Neurogenin2-d4Venus and Gadd45g-d4Venus transgenic mice: visualizing mitotic and migratory behaviors of cells committed to the neuronal lineage in the developing mammalian brain

To achieve highly sensitive and comprehensive assessment of the morphology and dynamics of cells committed to the neuronal lineage in mammalian brain primordia, we generated two transgenic mouse lines expressing a destabilized (d4) Venus controlled by regulatory elements of the Neurogenin2 (Neurog2) or Gadd45g gene. In mid-embryonic neocortical walls, expression of Neurog2-d4Venus mostly overlapped with that of Neurog2 protein, with a slightly (1 h) delayed onset. Although Neurog2-d4Venus and Gadd45g-d4Venus mice exhibited very similar labeling patterns in the ventricular zone (VZ), in Gadd45g-d4Venus mice cells could be visualized in more basal areas containing fully differentiated neurons, where Neurog2-d4Venus fluorescence was absent. Time-lapse monitoring revealed that most d4Venus⁺ cells in the VZ had processes extending to the apical surface; many of these cells eventually retracted their apical process and migrated basally to the subventricular zone, where neurons, as well as the intermediate neurogenic progenitors that undergo terminal neuron-producing division, could be live-monitored by d4Venus fluorescence. Some d4Venus⁺ VZ cells instead underwent nuclear migration to the apical surface, where they divided to generate two d4Venus⁺ daughter cells, suggesting that the symmetric terminal division that gives rise to neuron pairs at the apical surface can be reliably live-monitored. Similar lineage-committed cells were observed in other developing neural regions including retina, spinal cord, and cerebellum, as well as in regions of the peripheral nervous system such as dorsal root ganglia. These mouse lines will be useful for elucidating the cellular and molecular mechanisms underlying development of the mammalian nervous system.

POU-III transcription factors (Brn1, Brn2, and Oct6) influence neurogenesis, molecular identity, and migratory destination of upper-layer cells of the cerebral cortex

The upper layers (II-IV) are the most prominent distinguishing feature of mammalian neocortex compared with avian or reptilian dorsal cortex, and are vastly expanded in primates. Although the time-dependent embryonic generation of upper-layer cells is genetically instructed within their parental progenitors, mechanisms governing cell-intrinsic fate transitions remain obscure. POU-homeodomain transcription factors Pou3f3 and Pou3f2 (Brn1 and Brn2) are known to label postmitotic upper-layer cells, and are redundantly required for their production. We find that the onset of Pou3f3/2 expression actually occurs in ventricular zone (VZ) progenitors, and that Pou3f3/2 subsequently label neural progeny switching from deep-layer Ctip2(+) identity to Satb2(+) upper-layer fate as they migrate to proper superficial positions. By using an Engrailed dominant-negative repressor, we show that sustained neurogenesis after the deep- to upper-layer transition requires the proneual action of Pou3fs in VZ progenitors. Conversely, single-gene overexpression of any Pou3f in early neural progenitors is sufficient to specify the precocious birth of Satb2(+) daughter neurons that extend axons to the contralateral hemisphere, as well as exhibit robust pia-directed migration that is characteristic of upper-layer cells. Finally, we demonstrate that Pou3fs influence multiple stages of neurogenesis by suppressing Notch effector Hes5, and promoting the expression of proneural transcription factors Tbr2 and Tbr1.

Sall1 regulates cortical neurogenesis and laminar fate specification in mice: implications for neural abnormalities in Townes-Brocks syndrome

Progenitor cells in the cerebral cortex undergo dynamic cellular and molecular changes during development. Sall1 is a putative transcription factor that is highly expressed in progenitor cells during development. In humans, the autosomal dominant developmental disorder Townes-Brocks syndrome (TBS) is associated with mutations of the SALL1 gene. TBS is characterized by renal, anal, limb and auditory abnormalities. Although neural deficits have not been recognized as a diagnostic characteristic of the disease, ∼10% of patients exhibit neural or behavioral abnormalities. We demonstrate that, in addition to being expressed in peripheral organs, Sall1 is robustly expressed in progenitor cells of the central nervous system in mice. Both classical- and conditional-knockout mouse studies indicate that the cerebral cortex is particularly sensitive to loss of Sall1. In the absence of Sall1, both the surface area and depth of the cerebral cortex were decreased at embryonic day 18.5 (E18.5). These deficiencies are associated with changes in progenitor cell properties during development. In early cortical progenitor cells, Sall1 promotes proliferative over neurogenic division, whereas, at later developmental stages, Sall1 regulates the production and differentiation of intermediate progenitor cells. Furthermore, Sall1 influences the temporal specification of cortical laminae. These findings present novel insights into the function of Sall1 in the developing mouse cortex and provide avenues for future research into potential neural deficits in individuals with TBS.

Basal progenitor cells in the embryonic mouse thalamus - their molecular characterization and the role of neurogenins and Pax6

Background

The size and cell number of each brain region are influenced by the organization and behavior of neural progenitor cells during embryonic development. Recent studies on developing neocortex have revealed the presence of neural progenitor cells that divide away from the ventricular surface and undergo symmetric divisions to generate either two neurons or two progenitor cells. These 'basal' progenitor cells form the subventricular zone and are responsible for generating the majority of neocortical neurons. However, not much has been studied on similar types of progenitor cells in other brain regions.

Results

We have identified and characterized basal progenitor cells in the embryonic mouse thalamus. The progenitor domain that generates all of the cortex-projecting thalamic nuclei contained a remarkably high proportion of basally dividing cells. Fewer basal progenitor cells were found in other progenitor domains that generate non-cortex projecting nuclei. By using intracellular domain of Notch1 (NICD) as a marker for radial glial cells, we found that basally dividing cells extended outside the lateral limit of radial glial cells, indicating that, similar to the neocortex and ventral telencephalon, the thalamus has a distinct subventricular zone. Neocortical and thalamic basal progenitor cells shared expression of some molecular markers, including Insm1, Neurog1, Neurog2 and NeuroD1. Additionally, basal progenitor cells in each region also expressed exclusive markers, such as Tbr2 in the neocortex and Olig2 and Olig3 in the thalamus. In Neurog1/Neurog2 double mutant mice, the number of basally dividing progenitor cells in the thalamus was significantly reduced, which demonstrates the roles of neurogenins in the generation and/or maintenance of basal progenitor cells. In Pax6 mutant mice, the part of the thalamus that showed reduced Neurog1/2 expression also had reduced basal mitosis.

Conclusions

Our current study establishes the existence of a unique and significant population of basal progenitor cells in the thalamus and their dependence on neurogenins and Pax6. These progenitor cells may have important roles in enhancing the generation of neurons within the thalamus and may also be critical for generating neuronal diversity in this complex brain region.

Generating neuronal diversity in the Drosophila central nervous system

Generating diverse neurons in the central nervous system involves three major steps. First, heterogeneous neural progenitors are specified by positional cues at early embryonic stages. Second, neural progenitors sequentially produce neurons or intermediate precursors that acquire different temporal identities based on their birth-order. Third, sister neurons produced during asymmetrical terminal mitoses are given distinct fates. Determining the molecular mechanisms underlying each of these three steps of cellular diversification will unravel brain development and evolution. Drosophila has a relatively simple and tractable CNS, and previous studies on Drosophila CNS development have greatly advanced our understanding of neuron fate specification. Here we review those studies and discuss how the lessons we have learned from fly teach us the process of neuronal diversification in general.

Tangentially migrating transient glutamatergic neurons control neurogenesis and maintenance of cerebral cortical progenitor pools

The relative contribution of intrinsic and extrinsic cues in the regulation of cortical neurogenesis remains a crucial challenge in developmental neurobiology. We previously reported that a transient population of glutamatergic neurons, the cortical plate (CP) transient neurons, migrates from the ventral pallium (VP) over long distances and participate in neocortical development. Here, we show that the genetic ablation of this population leads to a reduction in the number of cortical neurons especially fated to superficial layers. These defects result from precocious neurogenesis followed by a depletion of the progenitor pools. Notably, these changes progress from caudolateral to rostrodorsal pallial territories between E12.5 and E14.5 along the expected trajectory of the ablated cells. Conversely, we describe enhanced proliferation resulting in an increase in the number of cortical neurons in the Gsx2 mutants which present an expansion of the VP and a higher number of CP transient neurons migrating into the pallium. Our findings indicate that these neurons act to maintain the proliferative state of neocortical progenitors and delay differentiation during their migration from extraneocortical regions and, thus, participate in the extrinsic control of cortical neuronal numbers.

Retinoic acid from the meninges regulates cortical neuron generation

Extrinsic signals controlling generation of neocortical neurons during embryonic life have been difficult to identify. In this study we demonstrate that the dorsal forebrain meninges communicate with the adjacent radial glial endfeet and influence cortical development. We took advantage of Foxc1 mutant mice with defects in forebrain meningeal formation. Foxc1 dosage and loss of meninges correlated with a dramatic reduction in both neuron and intermediate progenitor production and elongation of the neuroepithelium. Several types of experiments demonstrate that retinoic acid (RA) is the key component of this secreted activity. In addition, Rdh10- and Raldh2-expressing cells in the dorsal meninges were either reduced or absent in the Foxc1 mutants, and Rdh10 mutants had a cortical phenotype similar to the Foxc1 null mutants. Lastly, in utero RA treatment rescued the cortical phenotype in Foxc1 mutants. These results establish RA as a potent, meningeal-derived cue required for successful corticogenesis.

Patterns of p57Kip2 expression in embryonic rat brain suggest roles in progenitor cell cycle exit and neuronal differentiation

In developing central nervous system, a variety of mechanisms couple cell cycle exit to differentiation during neurogenesis. The cyclin-dependent kinase (CDK) inhibitor p57Kip2 controls the transition from proliferation to differentiation in many tissues, but roles in developing brain remain uncertain. To characterize possible functions, we defined p57Kip2 protein expression in embryonic (E) day 12.5 to 20.5 rat brains using immunohistochemistry combined with markers of proliferation and differentiation. The p57Kip2 was localized primarily in cell nuclei and positive cells formed two distinct patterns including wide dispersion and laminar aggregation that were brain region-specific. From E12.5 to E16.5, p57Kip2 expression was detected mainly in ventricular zone (VZ) and/or mantle zone of hippocampus, septum, basal ganglia, thalamus, hypothalamus, midbrain, and spinal cord. After E18.5, p57Kip2 was detected in select regions undergoing differentiation. The p57Kip2 expression was also compared with regional transcription factors, including Ngn2, Nkx2.1, and Pax6. Time course studies performed in diencephalon showed that p57Kip2 immunoreactivity colocalized with BrdU at 8 hr in nuclei exhibiting the wide dispersion pattern, whereas colocalization in the laminar pattern occurred only later. Moreover, p57Kip2 frequently colocalized with neuronal marker, beta-III tubulin. Finally, we characterized relationships of p57Kip2 to CDK inhibitor p27Kip1: in proliferative regions, p57Kip2 expression preceded p27Kip1 as cells underwent differentiation, though the proteins colocalized in substantial numbers of cells, suggesting potentially related yet distinct functions of Cip/Kip family members during neurogenesis. Our observations that p57Kip2 exhibits nuclear expression as precursors exit the cell cycle and begin expressing neuronal characteristics suggests that the CDK inhibitor contributes to regulating the transition from proliferation to differentiation during brain development.

Molecule-level imaging of Pax6 mRNA distribution in mouse embryonic neocortex by molecular interaction force microscopy

Detection of the cellular and tissue distributions of RNA species is critical in our understanding of the regulatory mechanisms underlying cellular and tissue differentiation. Here, we show that an atomic force microscope tip modified with 27-acid dendron, a cone shaped molecule with 27 monomeric units forming its base, can be successfully used to map the spatial distribution of mouse Pax6 mRNA on sectioned tissues of the mouse embryonic neocortex. Scanning of the sectioned tissue with a 30-mer DNA probe attached to the apex of the dendron resulted in detection of the target mRNA on the tissue section, permitting mapping of the mRNA distribution at nanometer resolution. The unprecedented sensitivity and resolution of this process should be applicable to identification of molecular level distribution of various RNAs in a cell.

Periventricular notch activation and asymmetric Ngn2 and Tbr2 expression in pair-generated neocortical daughter cells

To understand the cellular and molecular mechanisms regulating cytogenesis within the neocortical ventricular zone, we examined at high resolution the spatiotemporal expression patterns of Ngn2 and Tbr2. Individually DiI-labeled daughter cells were tracked from their birth in slice cultures and immunostained for Ngn2 and Tbr2. Both proteins were initially absent from daughter cells during the first 2 h. Ngn2 expression was then initiated asymmetrically in one of the daughter cells, with a bias towards the apical cell, followed by a similar pattern of expression for Tbr2, which we found to be a direct target of Ngn2. How this asymmetric Ngn2 expression is achieved is unclear, but gamma-secretase inhibition at the birth of daughter cells resulted in premature Ngn2 expression, suggesting that Notch signaling in nascent daughter cells suppresses a Ngn2-Tbr2 cascade. Many of the nascent cells exhibited pin-like morphology with a short ventricular process, suggesting periventricular presentation of Notch ligands to these cells.

V2a and V2b neurons are generated by the final divisions of pair-producing progenitors in the zebrafish spinal cord

The p2 progenitor domain in the ventral spinal cord gives rise to two interneuron subtypes: V2a and V2b. Delta-Notch-mediated cell-cell interactions between postmitotic immature neurons have been implicated in the segregation of neuron subtypes. However, lineage relationships between V2a and V2b neurons have not been reported. We address this issue using Tg[vsx1:GFP] zebrafish, a model system in which high GFP expression is initiated near the final stage of p2 progenitors. Cell fates were followed in progeny using time-lapse microscopy. Results indicate that the vast majority, if not all, of GFP-labeled p2 progenitors divide once to produce V2a/V2b neuron pairs, indicating that V2a and V2b neurons are generated by the asymmetric division of pair-producing progenitor cells. Together with evidence that Notch signaling is involved in the cell fate specification process, our results strongly suggest that Delta-Notch interactions between sister cells play a crucial role in the final outcome of these asymmetric divisions. This mechanism for determining cell fate is similar to asymmetric divisions that occur during Drosophila neurogenesis, where ganglion mother cells divide once to produce distinct neurons. However, unlike in Drosophila, the divisional axes of p2 progenitors in zebrafish were not fixed. We report that the terminal division of pair-producing progenitor cells in vertebrate neurogenesis can reproducibly produce two distinct neurons through a mechanism that may not depend on the orientation of the division axis.

Is Pax6 critical for neurogenesis in the human fetal brain?

Transcription factor Pax6 plays an important role in fate determination of neural progenitor cells in animal models, yet, its distribution and role in the human developing brain have not been reported. Here we demonstrated that Pax6 was strongly expressed in dorsal and ventral proliferative zones, mainly in proliferating radial glia (RG) cells, some neuronal and intermediate progenitors, and sporadic deep cortical plate neurons. In contrast to reports in rodents, Pax6 in the human fetal brain occasionally colocalized with ventral transcription factor Olig2 in progenitor cells. Transfection with short interfering RNA abolished Pax6 expression in the cell cultures of human fetal RG, and significantly decreased the number of neurons generated from Pax6 knock-down cells. Hence, Pax6 has a critical role in neurogenic regulation of RG cells in the human forebrain, similar to reports in rodents. What is different in human forebrain is that Pax6 seems to regulate not only the genesis of cortical pyramidal neurons, but also a subpopulation of interneurons from both dorsal and ventral sources. Thus, regional distribution, colocalization with Olig2, and the role of Pax6 in neurogenesis of both projection and interneurons, suggest that developmental regulation by transcription factors may differ in primates and nonprimate mammals.

Par-complex proteins promote proliferative progenitor divisions in the developing mouse cerebral cortex

The size of brain regions depends on the balance between proliferation and differentiation. During development of the mouse cerebral cortex, ventricular zone (VZ) progenitors, neuroepithelial and radial glial cells, enlarge the progenitor pool by proliferative divisions, while basal progenitors located in the subventricular zone (SVZ) mostly divide in a differentiative mode generating two neurons. These differences correlate to the existence of an apico-basal polarity in VZ, but not SVZ, progenitors. Only VZ progenitors possess an apical membrane domain at which proteins of the Par complex are strongly enriched. We describe a prominent decrease in the amount of Par-complex proteins at the apical surface during cortical development and examine the role of these proteins by gain- and loss-of-function experiments. Par3 (Pard3) loss-of-function led to premature cell cycle exit, reflected in reduced clone size in vitro and the restriction of the progeny to the lower cortical layers in vivo. By contrast, Par3 or Par6 (Pard6α)overexpression promoted the generation of Pax6+ self-renewing progenitors in vitro and in vivo and increased the clonal progeny of single progenitors in vitro. Time-lapse video microscopy revealed that a change in the mode of cell division, rather than an alteration of the cell cycle length, causes the Par-complex-mediated increase in progenitors. Taken together, our data demonstrate a key role for the apically located Par-complex proteins in promoting self-renewing progenitor cell divisions at the expense of neurogenic differentiation in the developing cerebral cortex.

Notch signaling regulates neural precursor allocation and binary neuronal fate decisions in zebrafish

Notch signaling plays a well-described role in regulating the formation of neurons from proliferative neural precursors in vertebrates but whether, as in flies, it also specifies sibling cells for different neuronal fates is not known. Ventral spinal cord precursors called pMN cells produce mostly motoneurons and oligodendrocytes, but recent lineage-marking experiments reveal that they also make astrocytes, ependymal cells and interneurons. Our own clonal analysis of pMN cells in zebrafish showed that some produce a primary motoneuron and KA' interneuron at their final division. We investigated the possibility that Notch signaling regulates a motoneuron-interneuron fate decision using a combination of mutant, transgenic and pharmacological manipulations of Notch activity. We show that continuous absence of Notch activity produces excess primary motoneurons and a deficit of KA' interneurons, whereas transient inactivation preceding neurogenesis results in an excess of both cell types. By contrast, activation of Notch signaling at the neural plate stage produces excess KA' interneurons and a deficit of primary motoneurons. Furthermore, individual pMN cells produce similar kinds of neurons at their final division in mib mutant embryos, which lack Notch signaling. These data provide evidence that, among some postmitotic daughters of pMN cells, Notch promotes KA' interneuron identity and inhibits primary motoneuron fate, raising the possibility that Notch signaling diversifies vertebrate neuron type by mediating similar binary fate decisions.

Asymmetric cell division during brain morphogenesis

The division patterns of neural progenitor cells in developing vertebrate brains have traditionally been classified into three types: (i) "symmetric" divisions producing two progenitor cells (P/P division), (ii) "symmetric" divisions producing two neurons (N/N division), and (iii) "asymmetric" divisions producing one progenitor cell and one neuron (P/N division). Many studies examining the mechanism(s) regulating P/N divisions have focused on mitotic cleavage orientation and the possible uneven distribution of cell-fate determining molecules such as Numb. Although these two factors may intrinsically determine daughter cell fate arising from M-phase progenitor cells, no unified explanations have yet to be put forth incorporating all available data. In this review, I will discuss recent advances in techniques allowing the more detailed monitoring of daughter cell behavior in a heterogeneously pseudostratified neuroepithelium that demonstrate previously unrecognized asymmetries in P/P divisions. Careful observations of daughter cell behavior suggest that, immediately after their birth at the apical surface of the neuroepithelium, generated cells may not yet be fate committed but rather integrate extrinsic and intrinsic signals during GI phase before continuing down a developmental pathway.

Induction of oligodendrocyte progenitors in dorsal forebrain by intraventricular microinjection of FGF-2

During embryonic development, oligodendrocyte progenitors (OLPs) originate from the ventral forebrain under the regulation of Sonic hedgehog (Shh). Shh controls the expression of transcription factor Olig2, which is strongly implicated in OLP generation. Studies of mice deficient in Shh expression suggest, however, that an alternative pathway for OLP generation may exist. The generation of OLPs in dorsal forebrain has been suggested since treatment of dorsal-neural progenitor cells in culture with fibroblast growth factor (FGF-2) results in OLP induction. To ask if dorsal induction of OLPs in embryonic forebrain can occur in vivo and if FGF-2 could initiate an alternative pathway of regulation, we used in utero microinjection of FGF-2 into the lateral ventricles of mouse fetal forebrain. A single injection of FGF-2 at E13.5 resulted in the expression of the OLP markers Olig2 and PDGFRalpha mRNA in dorsal forebrain ventricular and intermediate zones. However, FGF-2 did not induce dorsal expression of Shh, Patched1 or Nkx2.1, and co-injection of FGF-2 and a Shh inhibitor did not attenuate the induction of Olig2 and PDGFRalpha, suggesting that Shh signaling was not involved in this FGF-2-mediated dorsal induction. These results demonstrate that the dorsal embryonic forebrain in vivo has the potential to generate OLPs in the presence of normal positional cues and that this can be driven by FGF-2 independent of Shh signaling.

p27kip1 independently promotes neuronal differentiation and migration in the cerebral cortex

The generation of neurons by progenitor cells involves the tight coordination of multiple cellular activities, including cell cycle exit, initiation of neuronal differentiation, and cell migration. The mechanisms that integrate these different events into a coherent developmental program are not well understood. Here we show that the cyclin-dependent kinase inhibitor p27(Kip1) plays an important role in neurogenesis in the mouse cerebral cortex by promoting the differentiation and radial migration of cortical projection neurons. Importantly, these two functions of p27(Kip1) involve distinct activities, which are independent of its role in cell cycle regulation. p27(Kip1) promotes neuronal differentiation by stabilizing Neurogenin2 protein, an activity carried by the N-terminal half of the protein. p27(Kip1) promotes neuronal migration by blocking RhoA signaling, an activity that resides in its C-terminal half. Thus, p27(Kip1) plays a key role in cortical development, acting as a modular protein that independently regulates and couples multiple cellular pathways contributing to neurogenesis.

Cellular and molecular control of neurogenesis in the mammalian telencephalon

The mammalian telencephalon exhibits an amazing diversity of neuronal types. The generation of this diversity relies on multiple developmental strategies, including the regional patterning of progenitors, their temporal specification, and the generation of intermediate progenitor populations. Progress has recently been made in characterizing some of the mechanisms involved. In particular, intermediate progenitors have been shown to play important roles in the generation of neurons in the cerebral cortex, and the properties and lineage relationships between radial glial cells and these intermediate progenitors have recently been examined by elegant time-lapse microscopic studies. Multiple pathways control the progression of neural lineages from multipotent stem cells to intermediate progenitors, postmitotic precursors and finally mature neurons. The regulation of two essential steps, neuronal commitment and specification of subtype identities, is increasingly well understood. These two steps are clearly distinct but co-ordinately regulated by common transcription factors such as neurogenins and Pax6. As our knowledge of the mechanisms of subtype specification of telencephalic neurons progresses, it will become possible to direct stem cells into generating particular telencephalic neuronal populations, opening the way to efficient neuronal replacement therapies.

Symmetric versus asymmetric cell division during neurogenesis in the developing vertebrate central nervous system

The type and number of cell divisions of neuronal progenitors determine the number of neurons generated during the development of the vertebrate central nervous system. Over the past several years, there has been substantial progress in characterizing the various kinds of neuronal progenitors and the types of symmetric and asymmetric divisions they undergo. The understanding of the cell-biological basis of symmetric versus asymmetric progenitor cell division has been consolidated, and the molecular machinery controlling these divisions is beginning to be unravelled. Other recent advances include comparative studies of brain development in rodents and primates, as well as the identification of gene mutations in humans that affect the balance between the various types of cell division of neuronal progenitors.