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

Restrictions in time and space--new insights into generation of specific neuronal subtypes in the adult mammalian brain

Key questions in regard to neuronal repair strategies are which cells are best suited to regenerate specific neuronal subtypes and how much of a neuronal circuit needs to persist in order to allow its functional repair. Here we discuss recent findings in the field of adult neurogenesis, which shed new light on these questions. Neural stem cells in the adult brain generate very distinct types of neurons depending on their regional and temporal specification. Moreover, distinct brain regions differ in the mode of neuron addition in adult neurogenesis, suggesting that different brain circuits may be able to cope differently with the incorporation of new neurons. These new insights are then considered in regard to the choice of cells with the appropriate region-specific identity for repair strategies.

A novel function of the proneural factor Ascl1 in progenitor proliferation identified by genome-wide characterization of its targets

Proneural genes such as Ascl1 are known to promote cell cycle exit and neuronal differentiation when expressed in neural progenitor cells. The mechanisms by which proneural genes activate neurogenesis--and, in particular, the genes that they regulate--however, are mostly unknown. We performed a genome-wide characterization of the transcriptional targets of Ascl1 in the embryonic brain and in neural stem cell cultures by location analysis and expression profiling of embryos overexpressing or mutant for Ascl1. The wide range of molecular and cellular functions represented among these targets suggests that Ascl1 directly controls the specification of neural progenitors as well as the later steps of neuronal differentiation and neurite outgrowth. Surprisingly, Ascl1 also regulates the expression of a large number of genes involved in cell cycle progression, including canonical cell cycle regulators and oncogenic transcription factors. Mutational analysis in the embryonic brain and manipulation of Ascl1 activity in neural stem cell cultures revealed that Ascl1 is indeed required for normal proliferation of neural progenitors. This study identified a novel and unexpected activity of the proneural gene Ascl1, and revealed a direct molecular link between the phase of expansion of neural progenitors and the subsequent phases of cell cycle exit and neuronal differentiation.

Oligodendrogenesis in the subventricular zone and the role of epidermal growth factor

Demyelinating diseases are characterized by an extensive loss of oligodendrocytes and myelin sheaths from axolemma. These neurological disorders are a common cause of disability in young adults, but so far, there is no effective treatment against them. It has been suggested that neural stem cells (NSCs) may play an important role in brain repair therapies. NSCs in the adult subventricular zone (SVZ), also known as Type-B cells, are multipotential cells that can self-renew and give rise to neurons and glia. Recent findings have shown that cells derived from SVZ Type-B cells actively respond to epidermal-growth-factor (EGF) stimulation becoming highly migratory and proliferative. Interestingly, a subpopulation of these EGF-activated cells expresses markers of oligodendrocyte precursor cells (OPCs). When EGF administration is removed, SVZ-derived OPCs differentiate into myelinating and pre-myelinating oligodendrocytes in the white matter tracts of corpus callosum, fimbria fornix and striatum. In the presence of a demyelinating lesion, OPCs derived from EGF-stimulated SVZ progenitors contribute to myelin repair. Given their high migratory potential and their ability to differentiate into myelin-forming cells, SVZ NSCs represent an important endogenous source of OPCs for preserving the oligodendrocyte population in the white matter and for the repair of demyelinating injuries.

Refinement of the Region for Split Hand/Foot Malformation 5 on 2q31.1

Background: Deletions that encompass 2q31.1 have been proposed as a microdeletion syndrome with common clinical features, including intellectual disability/developmental delay, microcephaly, cleft palate, growth delay, and hand/foot anomalies. In addition, several genes within this region have been proposed as candidates for split hand-foot malformation 5 (SHFM5). Methods: To delineate the genotype-phenotype correlation between deletions of this region, we identified 14 individuals with deletions at 2q31.1 detected by microarray analysis for physical and developmental disabilities. Results: All subjects for whom detailed clinical records were available had neurological deficits of varying degree. Seven subjects with deletions encompassing the HOXD cluster had hand/foot anomalies of varying severity, including syndactyly, brachydactyly, and ectrodactyly. Of 7 subjects with deletions proximal to the HOXD cluster, 5 of which encompassed DLX1/DLX2, none had clinically significant hand/foot anomalies. In contrast to previous reports, the individuals in our study did not display a characteristic gestalt of dysmorphic facial features. Conclusion: The absence of hand/foot anomalies in any of the individuals with deletions of DLX1/DLX2 but not the HOXD cluster supports the hypothesis that haploinsufficiency of the HOXD cluster, rather than DLX1/DLX2, accounts for the skeletal abnormalities in subjects with 2q31.1 microdeletions.

Phosphorylation state of Olig2 regulates proliferation of neural progenitors

The bHLH transcription factors that regulate early development of the central nervous system can generally be classified as either antineural or proneural. Initial expression of antineural factors prevents cell cycle exit and thereby expands the pool of neural progenitors. Subsequent (and typically transient) expression of proneural factors promotes cell cycle exit, subtype specification, and differentiation. Against this backdrop, the bHLH transcription factor Olig2 in the oligodendrocyte lineage is unorthodox, showing antineural functions in multipotent CNS progenitor cells but also sustained expression and proneural functions in the formation of oligodendrocytes. We show here that the proliferative function of Olig2 is controlled by developmentally regulated phosphorylation of a conserved triple serine motif within the amino-terminal domain. In the phosphorylated state, Olig2 maintains antineural (i.e., promitotic) functions that are reflected in human glioma cells and in a genetically defined murine model of primary glioma.

Annual Research Review: Development of the cerebral cortex: implications for neurodevelopmental disorders

The cerebral cortex has a central role in cognitive and emotional processing. As such, understanding the mechanisms that govern its development and function will be central to understanding the bases of severe neuropsychiatric disorders, particularly those that first appear in childhood. In this review, I highlight recent progress in elucidating genetic, molecular and cellular mechanisms that control cortical development. I discuss basic aspects of cortical developmental anatomy, and mechanisms that regulate cortical size and area formation, with an emphasis on the roles of fibroblast growth factor (Fgf) signaling and specific transcription factors. I then examine how specific types of cortical excitatory projection neurons are generated, and how their axons grow along stereotyped pathways to their targets. Next, I address how cortical inhibitory (GABAergic) neurons are generated, and point out the role of these cells in controlling cortical plasticity and critical periods. The paper concludes with an examination of four possible developmental mechanisms that could contribute to some forms of neurodevelopmental disorders, such as autism.

Proneural transcription factors Dlx2 and Pax6 are altered in adult SVZ neural precursor cells following striatal cell loss

Compensatory replacement of neurons by endogenous subventricular zone (SVZ)-derived neural precursor cells has been demonstrated in the adult brain following striatal cell loss. Such cell replacement is associated with increased SVZ cell proliferation and neuroblast expansion in the rostral migratory stream (RMS). SVZ-derived neural precursor cells co-express multiple transcription factors involved in lineage restriction and cell fate determination. We propose that compensatory neurogenesis in response to striatal cell loss will alter the temporal expression of transcription factors in discrete populations of SVZ-derived neural precursor cells. We therefore examined the expression of Mash1, Dlx2, Pax6 and Olig2 in SVZ-derived neural precursor cell populations across a range of times following quinolinic acid (QA) induced striatal cell death. We have identified a heterogeneous population of SVZ-derived neural precursor cells that respond independently to striatal cell loss. In both the anterior SVZ (aSVZ) and RMS we observed an increase in a sub-population of Dlx2+ transit amplifying precursor (TAP) cells and neuroblasts following QA lesioning when compared to controls. Subsequently, the number of Pax6+ TAPs and neuroblasts in the QA lesioned aSVZ and RMS was also increased. Olig2 expression was not however altered in response to QA-induced cell loss. Our results suggest Dlx2 and Pax6 may play a prominent role in directing neural precursor cell proliferation and neuroblast generation following striatal cell loss. Selective alteration of specific transcription factors in the SVZ and during migration through the RMS in response to cell loss may predetermine the subsequent generation of specific neuronal subclasses for endogenous replacement.

Milestones of directed differentiation of mouse and human embryonic stem cells into telencephalic neurons based on neural development in vivo

Understanding the normal development of individual neural subtypes provides an essential framework for the design of rational approaches to embryonic stem cell differentiation for in vitro studies and cell replacement therapies. Of particular interest and a particular challenge are the cells that build-up the telencephalon. Recent research has unraveled key developmental mechanisms contributing to the generation of specific telencephalic cells. We focus on morphogens and transcription factors known to regulate distinct developmental processes. These include early anterior/posterior patterning, dorsal/ventral patterning, and generation of progenitor domains and neuronal specification into major classes of telencephalic cells: glutamatergic projection neurons, different subtypes of γ-aminobutyric acid-ergic interneurons and projection neurons, as well as cholinergic interneurons and projection neurons. Based on a comparison with in vivo telencephalic neurogenesis, we propose that the specific combinations of transcription factors expressed during development can serve as milestones for the in vitro differentiation of embryonic stem cells toward specific telencephalic neurons.

The Wnt receptor Ryk controls specification of GABAergic neurons versus oligodendrocytes during telencephalon development

GABAergic neurons and oligodendrocytes originate from progenitors within the ventral telencephalon. However, the molecular mechanisms that control neuron-glial cell-fate segregation, especially how extrinsic factors regulate cell-fate changes, are poorly understood. We have discovered that the Wnt receptor Ryk promotes GABAergic neuron production while repressing oligodendrocyte formation in the ventral telencephalon. We demonstrate that Ryk controls the cell-fate switch by negatively regulating expression of the intrinsic oligodendrogenic factor Olig2 while inducing expression of the interneuron fate determinant Dlx2. In addition, we demonstrate that Ryk is required for GABAergic neuron induction and oligodendrogenesis inhibition caused by Wnt3a stimulation. Furthermore, we showed that the cleaved intracellular domain of Ryk is sufficient to regulate the cell-fate switch by regulating the expression of intrinsic cell-fate determinants. These results identify Ryk as a multi-functional receptor that is able to transduce extrinsic cues into progenitor cells, promote GABAergic neuron formation, and inhibit oligodendrogenesis during ventral embryonic brain development.

Developmental genetics of vertebrate glial-cell specification

Oligodendrocytes and astrocytes are macroglial cells of the vertebrate central nervous system. These cells have diverse roles in the maintenance of neurological function. In the embryo, the genetic mechanisms that underlie the specification of macroglial precursors in vivo appear strikingly similar to those that regulate the development of the diverse neuron types. The switch from producing neuronal to glial subtype-specific precursors can be modelled as an interplay between region-restricted components and temporal regulators that determine neurogenic or gliogenic phases of development, contributing to glial diversity. Gaining insight into the developmental genetics of macroglia has great potential to improve our understanding of a variety of neurological disorders in humans.

Sox10 directs neural stem cells toward the oligodendrocyte lineage by decreasing Suppressor of Fused expression

Oligodendrocyte precursor cells (OPCs) are lineage-restricted progenitors generally limited in vivo to producing oligodendrocytes. Mechanisms controlling genesis of OPCs are of interest because of their importance in myelin development and their potential for regenerative therapies in multiple sclerosis and dysmyelinating syndromes. We show here that the SoxE transcription factors (comprising Sox8, 9, and 10) induce multipotent neural precursor cells (NPCs) from the early postnatal subventricular zone (SVZ) to become OPCs in an autonomous manner. We performed a chromatin immunoprecipitation-based bioinformatic screen and identified Suppressor of Fused (Sufu) as a direct target of repression by Sox10. In vitro, overexpression of Sufu blocked OPC production, whereas RNAi-mediated inhibition augmented OPC production. Furthermore, mice heterozygous for Sufu have increased numbers of OPCs in the telencephalon during development. We conclude that Sox10 acts to restrict the potential of NPCs toward the oligodendrocyte lineage in part by regulating the expression of Sufu.

The germinal zones of the basal ganglia but not the septum generate GABAergic interneurons for the cortex

Cortical interneurons originate from subpallial precursors and migrate into the cortex during development. Using genetic lineage tracing in transgenic mice we examine the contribution of two germinal zones, the septum and the lateral ganglionic eminence/caudal ganglionic eminence (LGE/CGE) to interneurons of the cortex. We find that the septal neuroepithelium does not generate interneurons for the neocortex. There is, however, clear migration of cells from the LGE/CGE to the cortex. Comparison of the dynamics of cortical colonization by the two major cohorts of interneurons originating in the medial ganglionic eminence (MGE) and the LGE/CGE has shown differences in the timing of migration and initial route of entry into the cortex. LGE/CGE-derived interneurons enter the cortex later than the MGE-derived ones. They invade the cortex through the subventricular/intermediate zone route and only later disperse within the cortical plate and the marginal zone. During the first postnatal week MGE interneurons move extensively to acquire their laminar position within the cortical plate whereas LGE/CGE-derived cells remain largely within the upper layers of the cortex. The two populations intermingle in the adult cortex but have distinct neurochemical properties and different overall distributions. LGE/CGE-derived interneurons account for one third of the total GABAergic interneuron population in the adult cortex.

ERK1 and ERK2 are required for radial glial maintenance and cortical lamination

ERK1/2 is involved in a variety of cellular processes during development, but the functions of these isoforms in brain development remain to be determined. Here, we generated double knockout (DKO) mice to study the individual and combined roles of ERK1 and ERK2 during cortical development. Mice deficient in Erk2, and more dramatically in the DKOs, displayed proliferation defects in late radial glial progenitors within the ventricular zone, and a severe disruption of lamination in the cerebral cortex. Immunohistochemical analyses revealed that late-generated cortical neurons were misplaced and failed to migrate the upper cortical layers in DKO mice. Moreover, these mice displayed fewer radial glial fibers, which provide architectural guides for radially migrating neurons. These results suggest that extracellular signal-regulated kinase signaling is essential for the expansion of the radial glial population and for the maintenance of radial glial scaffolding. Tangential migration of interneurons and oligodendrocytes from the ganglionic eminences (GE) to the dorsal cortex was more severely impaired in DKO mice than in mice deficient for Erk2 alone, because of reduced progenitor proliferation in the GE of the ventral telencephalon. These data demonstrate functional overlaps between ERK1 and ERK2 and indicate that extracellular signal-regulated kinase signaling plays a crucial role in cortical development.

Genotype-phenotype correlation in eight new patients with a deletion encompassing 2q31.1

Microdeletions of the 2q31.1 region are rare. We present the clinical and molecular findings of eight previously unreported patients with overlapping deletions in 2q31.1. The patients have a variable clinical phenotype and present with developmental delay (7/8), growth retardation (5/8), seizures (2/8) and a craniofacial dysmorphism consisting of microcephaly (4/8), short palpebral fissures (7/8), broad eyebrows with lateral flare (7/8), low-set ears with thickened helices and lobules (5/8), and micrognathia (6/8). Additional congenital anomalies were noted, including limb abnormalities (8/8), heart defects (3/8), genital anomalies (3/8), and craniosynostosis (1/8). Six of these microdeletions, ranging in size from 1.24 to 8.35 Mb, were identified by array CGH, one larger deletion (19.7 Mb) was detected by conventional karyotyping and further characterized by array CGH analysis. The smallest region of overlap in all eight patients spans at most 88 kb and includes only the WIPF1 gene. This gene codes for the WAS/WASL interacting protein family member 1. The patients described here do not present with clinical signs of the Wiskott-Aldrich syndrome and the deletion of this single gene does not allow explaining the phenotype in our patients. It is likely that the deletion of different but overlapping sets of genes from 2q31 is responsible for the clinical variability in these patients. To further dissect the complex phenotype associated with deletions in 2q31, additional patients with overlapping phenotypes should be examined with array CGH. This should help to link particular phenotypes to specific genes, and add to our understanding of the underlying developmental processes.

Three hypotheses for developmental defects that may underlie some forms of autism spectrum disorder

PURPOSE OF REVIEW

Molecular and genetic insights into the etiology of autism spectrum disorders are now available. The field now needs to understand how these perturbations affect development and function of the brain.

RECENT FINDINGS

Herein I review the genetic mechanisms known to predispose to autism spectrum disorders, and attempt to consolidate many of these within cellular/molecular pathways that regulate development of neural systems that underlie cognition and social behaviors. In addition to the clear relationship of many susceptibility genes to activity-dependent neural responses, I propose the existence of three additional mechanisms that may contribute to autism spectrum disorders: evolutionary-driven expansion of cerebrum and cerebellar size; imbalance in the excitatory/inhibitory ratio in local and extended circuits; the hormonal effects of the male genotype.

SUMMARY

Understanding these mechanisms opens the possibility to therapeutic interventions.

Epigenetic control of neural precursor cell fate during development

The temporally and spatially restricted nature of the differentiation capacity of cells in the neural lineage has been studied extensively in recent years. Epigenetic control of developmental genes, which is heritable through cell divisions, has emerged as a key mechanism defining the differentiation potential of cells. Short-term or reversible repression of developmental genes puts them in a 'poised state', ready to be activated in response to differentiation-inducing cues, whereas long-term or permanent repression of developmental genes restricts the cell fates they regulate. Here, we review the molecular mechanisms that underlie the establishment and regulation of differentiation potential along the neural lineage during development.

Directing astroglia from the cerebral cortex into subtype specific functional neurons

Forced expression of single defined transcription factors can selectively and stably convert cultured astroglia into synapse-forming excitatory and inhibitory neurons.

Astroglia from the postnatal cerebral cortex can be reprogrammed in vitro to generate neurons following forced expression of neurogenic transcription factors, thus opening new avenues towards a potential use of endogenous astroglia for brain repair. However, in previous attempts astroglia-derived neurons failed to establish functional synapses, a severe limitation towards functional neurogenesis. It remained therefore also unknown whether neurons derived from reprogrammed astroglia could be directed towards distinct neuronal subtype identities by selective expression of distinct neurogenic fate determinants. Here we show that strong and persistent expression of neurogenic fate determinants driven by silencing-resistant retroviral vectors instructs astroglia from the postnatal cortex in vitro to mature into fully functional, synapse-forming neurons. Importantly, the neurotransmitter fate choice of astroglia-derived neurons can be controlled by selective expression of distinct neurogenic transcription factors: forced expression of the dorsal telencephalic fate determinant neurogenin-2 (Neurog2) directs cortical astroglia to generate synapse-forming glutamatergic neurons; in contrast, the ventral telencephalic fate determinant Dlx2 induces a GABAergic identity, although the overall efficiency of Dlx2-mediated neuronal reprogramming is much lower compared to Neurog2, suggesting that cortical astroglia possess a higher competence to respond to the dorsal telencephalic fate determinant. Interestingly, however, reprogramming of astroglia towards the generation of GABAergic neurons was greatly facilitated when the astroglial cells were first expanded as neurosphere cells prior to transduction with Dlx2. Importantly, this approach of expansion under neurosphere conditions and subsequent reprogramming with distinct neurogenic transcription factors can also be extended to reactive astroglia isolated from the adult injured cerebral cortex, allowing for the selective generation of glutamatergic or GABAergic neurons. These data provide evidence that cortical astroglia can undergo a conversion across cell lineages by forced expression of a single neurogenic transcription factor, stably generating fully differentiated neurons. Moreover, neuronal reprogramming of astroglia is not restricted to postnatal stages but can also be achieved from terminally differentiated astroglia of the adult cerebral cortex following injury-induced reactivation.

The brain consists of two major cell types: neurons, which transmit information, and glial cells, which support and protect neurons. Interestingly, evidence suggests that some glial cells, including astroglia, can be directly converted into neurons by specific proteins, a transformation that may aid in the functional repair of damaged brain tissue. However, in order for the repaired brain areas to function properly, it is important that astroglia be directed into appropriate neuronal subclasses. In this study, we show that non-neurogenic astroglia from the cerebral cortex can be reprogrammed in vitro using just a single transcription factor to yield fully functional excitatory or inhibitory neurons. We achieved this result through forced expression of the same transcription factors that instruct the genesis of these distinct neuronal subtypes during embryonic forebrain development. Moreover we demonstrate that reactive astroglia isolated from the adult cortex after local injury can be reprogrammed into synapse-forming excitatory or inhibitory neurons following a similar strategy. Our findings provide evidence that endogenous glial cells may prove a promising strategy for replacing neurons that have degenerated due to trauma or disease.

Dlx5 and Dlx6 regulate the development of parvalbumin-expressing cortical interneurons

Dlx5 and Dlx6 homeobox genes are expressed in developing and mature cortical interneurons. Simultaneous deletion of Dlx5 and 6 results in exencephaly of the anterior brain; despite this defect, prenatal basal ganglia differentiation appeared largely intact, while tangential migration of Lhx6(+) and Mafb(+) interneurons to the cortex was reduced and disordered. The migration deficits were associated with reduced CXCR4 expression. Transplantation of mutant immature interneurons into a wild-type brain demonstrated that loss of either Dlx5 or Dlx5&6 preferentially reduced the number of mature parvalbumin(+) interneurons; those parvalbumin(+) interneurons that were present had increased dendritic branching. Dlx5/6(+/-) mice, which appear normal histologically, show spontaneous electrographic seizures and reduced power of gamma oscillations. Thus, Dlx5&6 appeared to be required for development and function of somal innervating (parvalbumin(+)) neocortical interneurons. This contrasts with Dlx1, whose function is required for dendrite innervating (calretinin(+), somatostatin(+), and neuropeptide Y(+)) interneurons (Cobos et al., 2005).

The progenitor zone of the ventral medial ganglionic eminence requires Nkx2-1 to generate most of the globus pallidus but few neocortical interneurons

We show that most globus pallidus neurons, but very few neocortical interneurons, are generated from the ventral medial ganglionic eminence and dorsal preoptic area based on fate mapping using an Shh-Cre allele. The Shh-expressing subpallial lineage produces parvalbumin(+) GABAergic neurons, ChAT(+) cholinergic neurons, and oligodendrocytes. Loss of Nkx2-1 function from the Shh-expressing domain eliminated most globus pallidus neurons, whereas most cortical and striatal interneurons continued to be generated, except for striatal cholinergic neurons. Finally, our analysis provided evidence for a novel cellular component (Nkx2-1(-);Npas1(+)) of the globus pallidus.

NG2 cells: Properties, progeny and origin

The NG2 proteoglycan is a type 1-transmembrane protein expressed by a range of cell types within and outside the mammalian nervous system. NG2-expressing (NG2) cells are found in grey and white matter tracts of the developing and adult CNS and have previously been assumed to represent oligodendrocyte precursor cells: new work using transgenic mice has shown that NG2 cells generate oligodendrocytes, protoplasmic astrocytes and in some instances neurons in vivo. NG2 cells express GABAA receptors and the AMPA subtype of glutamate receptors. They make intimate contact to neurons prior to myelinating axons and also form electron-dense synaptic specialisations with axons in the cerebellum, cortex and hippocampus and with non-myelinated axons in the corpus callosum. These synaptic NG2 cells respond to neuronal release of glutamate and GABA. This neuron-glia interaction may thus regulate the differentiation and proliferation of NG2 cells. The C-terminal PDZ-binding motif of the NG2 protein binds several PDZ proteins including Mupp1, Syntenin and the Glutamate Receptor Interacting Protein (GRIP). Since GRIP can bind subunits of the AMPA receptors expressed by NG2 cells, the interaction between GRIP and NG2 may orientate the glial AMPA receptors towards sites of neuronal glutamate release. The origin, heterogeneity and function of NG2 cells as modulators of the neuronal network are important incompletely resolved questions.

Multipotent precursors in the anterior and hippocampal subventricular zone display similar transcription factor signatures but their proliferation and maintenance are differentially regulated

Precursors within the subventricular zone (SVZ) exhibit regional variations in the expression of transcription factors important for the regulation of their proliferation and differentiation. In the anterior SVZ (aSVZ) the homeobox transcription factor distalless (Dlx)2 modulates both processes by promoting neural stem cell (NSC) activation as well as neurogenesis. Activated NSCs and transit-amplifying precursors (TAPs) in the aSVZ both express high levels of epidermal growth factor receptor (EGFR(high)) and form clones in response to exogenous EGF. EGF-responsive cells are also present in the hippocampal subependyma (hSVZ). However, it is not clear whether they represent NSCs or TAPs and whether their proliferation and differentiation are regulated as in the aSVZ. Here we have purified EGFR(high) cells from both the aSVZ and hSVZ at different ages. When isolated from perinatal tissue both populations were enriched in multipotent clonogenic precursors, which generated GABAergic neurons. Although they differed in absolute expression levels, activated NSCs and TAPs in both regions displayed similar signatures of transcription factor expression. However, activated NSCs were less frequent in the hSVZ than in the aSVZ. Furthermore, increasing age had a greater inhibitory effect on NSC proliferation in the hSVZ than in the aSVZ. This suggests that NSC activation is differentially regulated in the two regions. Consistent with this hypothesis, we found that in hippocampal precursors Dlx2 promoted neurogenesis but not NSC activation. Thus, most clonogenic EGFR(high) precursors in the hSVZ represent TAPs and NSC proliferation in the aSVZ and hSVZ is regulated by different mechanisms.

The specific role of histone deacetylase 2 in adult neurogenesis

Gene expression changes during cell differentiation are thought to be coordinated by histone modifications, but still little is known about the role of specific histone deacetylases (HDACs) in cell fate decisions in vivo. Here we demonstrate that the catalytic function of HDAC2 is required in adult, but not embryonic neurogenesis. While brain development and adult stem cell fate were normal upon conditional deletion of HDAC2 or in mice lacking the catalytic activity of HDAC2, neurons derived from both zones of adult neurogenesis die at a specific maturation stage. This phenotype is correlated with an increase in proliferation and the aberrant maintenance of proteins normally expressed only in progenitors, such as Sox2, also into some differentiating neurons, suggesting that HDAC2 is critically required to silence progenitor transcripts during neuronal differentiation of adult generated neurons. This cell-autonomous function of HDAC2 exclusively in adult neurogenesis reveals clear differences in the molecular mechanisms regulating neurogenesis during development and in adulthood.

Molecular control of brain size: regulators of neural stem cell life, death and beyond

The proper development of the brain and other organs depends on multiple parameters, including strictly controlled expansion of specific progenitor pools. The regulation of such expansion events includes enzymatic activities that govern the correct number of specific cells to be generated via an orchestrated control of cell proliferation, cell cycle exit, differentiation, cell death etc. Certain proteins in turn exert direct control of these enzymatic activities and thus progenitor pool expansion and organ size. The members of the Cip/Kip family (p21Cip1/p27Kip1/p57Kip2) are well-known regulators of cell cycle exit that interact with and inhibit the activity of cyclin-CDK complexes, whereas members of the p53/p63/p73 family are traditionally associated with regulation of cell death. It has however become clear that the roles for these proteins are not as clear-cut as initially thought. In this review, we discuss the roles for proteins of the Cip/Kip and p53/p63/p73 families in the regulation of cell cycle control, differentiation, and death of neural stem cells. We suggest that these proteins act as molecular interfaces, or "pilots", to assure the correct assembly of protein complexes with enzymatic activities at the right place at the right time, thereby regulating essential decisions in multiple cellular events.

Long noncoding RNAs in neuronal-glial fate specification and oligodendrocyte lineage maturation

Background

Long non-protein-coding RNAs (ncRNAs) are emerging as important regulators of cellular differentiation and are widely expressed in the brain.

Results

Here we show that many long ncRNAs exhibit dynamic expression patterns during neuronal and oligodendrocyte (OL) lineage specification, neuronal-glial fate transitions, and progressive stages of OL lineage elaboration including myelination. Consideration of the genomic context of these dynamically regulated ncRNAs showed they were part of complex transcriptional loci that encompass key neural developmental protein-coding genes, with which they exhibit concordant expression profiles as indicated by both microarray and in situ hybridization analyses. These included ncRNAs associated with differentiation-specific nuclear subdomains such as Gomafu and Neat1, and ncRNAs associated with developmental enhancers and genes encoding important transcription factors and homeotic proteins. We also observed changes in ncRNA expression profiles in response to treatment with trichostatin A, a histone deacetylase inhibitor that prevents the progression of OL progenitors into post-mitotic OLs by altering lineage-specific gene expression programs.

Conclusion

This is the first report of long ncRNA expression in neuronal and glial cell differentiation and of the modulation of ncRNA expression by modification of chromatin architecture. These observations explicitly link ncRNA dynamics to neural stem cell fate decisions, specification and epigenetic reprogramming and may have important implications for understanding and treating neuropsychiatric diseases.

Arcuate nucleus expression of NKX2.1 and DLX and lineages expressing these transcription factors in neuropeptide Y(+), proopiomelanocortin(+), and tyrosine hydroxylase(+) neurons in neonatal and adult mice

Despite its small size, the arcuate nucleus of the hypothalamus has a critical role in regulating energy homeostasis. We have begun to define genetic approaches to express genes in specific cell types within the developing arcuate nucleus, to allow precise molecular perturbations of these cells. Furthermore, our analysis aims to contribute to defining the transcriptional networks that regulate the development of function of the arcuate neurons. Here, we define the neuronal cells types within the arcuate that express Nkx2.1 and Dlx homeobox genes. In addition, we used mice expressing Cre recombinase from the Dlx5/6 intergenic enhancer (Dlx5/6i) and from the Nkx2.1 locus to follow the fate of embryonic cells expressing these genes within the arcuate nucleus. We demonstrate that NKX2.1(+) cells and their lineages are broadly expressed in arcuate neurons [gamma-aminobutyric acid (GABA)(+), neuropeptide Y (NPY)(+), proopiomelanocortin (POMC)(+), tyrosine hydroxylase (TH)(+)] and glia (tanycytes). On the other hand, DLX(+) cells and their lineages mark only GABA(+) and TH(+) (dopaminergic) neurons, and Dlx1(-/-) mutants have fewer TH(+) neurons. These results have implications for the genetic control of arcuate development and function and for the utility of the Nkx2.1-Cre and Dlx5/6i-Cre mouse lines to alter gene expression in the developing arcuate.

Making neurons from mature glia: a far-fetched dream?

The fact that cells with glial characteristics such as forebrain radial glia during development and astroglial stem cells in the adult neurogenic zones serve as neuronal precursors provokes the question why glia in most other areas of the adult central nervous system are apparently incapable of generating new neurons. Besides being of pivotal biological interest answers to this question may also open new avenues for cell-based therapies of neurodegenerative diseases that involve a permanent loss of neurons which are not replaced naturally. For if one could indeed instruct glia to generate neurons, such a strategy would carry the enormous advantage of making use of a large pool of endogenous, and hence autologous cells, thereby circumventing many of the problems associated with therapeutic strategies based on transplantation. Accordingly, the recent years have seen increasing effort in assessing the plasticity of astroglia and other types of resident non-neuronal cells as a potential source for new neurons in the injured brain or eye. For instance, following injury astroglia in the cerebral cortex and Müller glia in the retina can de-differentiate and acquire stem or precursor cell like properties. Moreover, it has been shown that astroglia can be reprogrammed in vitro by forced expression of neurogenic transcription factors to transgress their lineage restriction and stably acquire a neuronal identity. In this review I will discuss the status quo of these early attempts, the limitations currently encountered and the future challenges before the full potential of this approach can be weighed.

Her6 regulates the neurogenetic gradient and neuronal identity in the thalamus

During vertebrate brain development, the onset of neuronal differentiation is under strict temporal control. In the mammalian thalamus and other brain regions, neurogenesis is regulated also in a spatially progressive manner referred to as a neurogenetic gradient, the underlying mechanism of which is unknown. Here we describe the existence of a neurogenetic gradient in the zebrafish thalamus and show that the progression of neurogenesis is controlled by dynamic expression of the bHLH repressor her6. Members of the Hes/Her family are known to regulate proneural genes, such as Neurogenin and Ascl. Here we find that Her6 determines not only the onset of neurogenesis but also the identity of thalamic neurons, marked by proneural and neurotransmitter gene expression: loss of Her6 leads to premature Neurogenin1-mediated genesis of glutamatergic (excitatory) neurons, whereas maintenance of Her6 leads to Ascl1-mediated production of GABAergic (inhibitory) neurons. Thus, the presence or absence of a single upstream regulator of proneural gene expression, Her6, leads to the establishment of discrete neuronal domains in the thalamus.

The glial nature of embryonic and adult neural stem cells

Glial cells were long considered end products of neural differentiation, specialized supportive cells with an origin very different from that of neurons. New studies have shown that some glial cells--radial glia (RG) in development and specific subpopulations of astrocytes in adult mammals--function as primary progenitors or neural stem cells (NSCs). This is a fundamental departure from classical views separating neuronal and glial lineages early in development. Direct visualization of the behavior of NSCs and lineage-tracing studies reveal how neuronal lineages emerge. In development and in the adult brain, many neurons and glial cells are not the direct progeny of NSCs, but instead originate from transit amplifying, or intermediate, progenitor cells (IPCs). Within NSCs and IPCs, genetic programs unfold for generating the extraordinary diversity of cell types in the central nervous system. The timing in development and location of NSCs, a property tightly linked to their neuroepithelial origin, appear to be the key determinants of the types of neurons generated. Identification of NSCs and IPCs is critical to understand brain development and adult neurogenesis and to develop new strategies for brain repair.

GABAergic differentiation induced by Mash1 is compromised by the bHLH proteins Neurogenin2, NeuroD1, and NeuroD2

During forebrain development, Mash1 directs gamma-aminobutyric acid (GABA)ergic neuron differentiation ventrally in the ganglionic eminences. Repression of Mash1 in the cortex is necessary to prevent the formation of GABAergic interneurons. Negative regulation of Mash1 has been attributed to members of the Neurogenin family; the genetic ablation of Neurogenin2 (Ngn2) leads to the derepression of Mash1 and the formation of ectopic GABAergic neurons in the cortex. We have developed an in vitro system to clarify the importance of NeuroD proteins in the Mash1 regulatory pathway. Using a neurosphere culture system, we show that the downstream effectors of the Ngn2 pathway NeuroD1 and NeuroD2 can abrogate GABAergic differentiation directed by Mash1. The ectopic expression of either of these genes in Mash1-expressing cells derived from the lateral ganglionic eminence, independently downregulate Mash1 expression without affecting expression of distal less homeodomain genes. This results in a complete loss of the GABAergic phenotype. Moreover, we demonstrate that ectopic expression of Mash1 in cortical progenitors is sufficient to phenocopy the loss of Ngn2 and strongly enhances ectopic GABAergic differentiation. Collectively, our results define the compensatory and cross-regulatory mechanisms that exist among basic helix-loop-helix transcription factors during neuronal fate specification.

A critical period in cortical interneuron neurogenesis in down syndrome revealed by human neural progenitor cells

Down syndrome (DS) is a developmental disorder whose mental impairment is due to defective cortical development. Human neural progenitor cells (hNPCs) derived from fetal DS cortex initially produce normal numbers of neurons, but generate fewer neurons with time in culture, similar to the pattern of neurogenesis that occurs in DS in vivo. Microarray analysis of DS hNPCs at this critical time reveals gene changes indicative of defects in interneuron progenitor development. In addition, dysregulated expression of many genes involved in neural progenitor cell biology points to changes in the progenitor population and subsequent reduction in interneuron neurogenesis. Delineation of a critical period in interneuron development in DS provides a foundation for investigation of the basis of reduced neurogenesis in DS and defines a time when these progenitor cells may be amenable to therapeutic treatment.

Experimental autoimmune encephalomyelitis-induced upregulation of tumor necrosis factor-alpha in the dorsal root ganglia

Background: Multiple sclerosis (MS) is a chronic, neurological disease characterized by targeted destruction of central nervous system (CNS) myelin. The autoimmune theory is the most widely accepted explanation of disease pathology. Circulating Th1 cells become activated by exposure to CNS-specific antigens such as myelin basic protein. The activated Th1 cells secrete inflammatory cytokines, which are pivotal for inflammatory responses. We hypothesize that enhanced production of inflammatory cytokines triggers cellular events within the dorsal root ganglia (DRG) and/or spinal cord, facilitating the development of neuropathic pain (NPP) in MS. NPP, the second worst disease-induced symptom suffered by patients with MS, is normally regulated by DRG and/or spinal cord.

Objective: To determine gene and protein expression levels of tumor necrosis factor-alpha (TNFα) within DRG and/or spinal cord in an animal model of MS.

Methods: Experimental autoimmune encephalomyelitis (EAE) was induced in adolescent female Lewis rats. Animals were sacrificed every 3 days post-disease induction. DRG and spinal cords were harvested for protein and gene expression analysis.

Results: We show significant increases in TNFα expression in the DRG and of EAE animals at peak disease stage, as assessed by clinical symptoms.

Conclusion: Antigen-induced production of inflammatory cytokines such as TNFα within the DRG identifies a potential novel mechanism for MS-induced NPP.

Identification of Wnt-responsive cells in the zebrafish hypothalamus

In all vertebrate brains, there is a period of widespread embryonic neurogenesis followed by specific regional neurogenesis that continues into adult stages. The Wnt signaling pathway, which is essential for numerous developmental processes, has also been suggested to be involved in neurogenesis. To help investigate the exact roles of canonical Wnt signaling in neurogenesis, here we examine the identity of Wnt-responsive cells in the zebrafish hypothalamus. This tissue is a useful diencephalic neurogenesis model containing evolutionarily conserved populations of neurons. We first performed in situ hybridization to show the expression patterns of Tcf family members and a canonical Wnt signaling reporter in the 50 hpf embryonic hypothalamus and larval/adult hypothalamus. We then used immunohistochemistry to identify the cell types of Wnt-responsive and Lef1-positive cells in both 50 hpf embryonic and adult hypothalamus. Our results indicate that Wnt-responsive cells in the hypothalamus are likely to be both mitotic progenitors and postmitotic precursors at embryonic stages, but only precursors at the adult stage. These data suggest that canonical Wnt signaling may be functionally required for maintenance of neural progenitor and precursor pools in the embryo, and for ongoing neurogenesis in the adult zebrafish.

p57Kip2 is a repressor of Mash1 activity and neuronal differentiation in neural stem cells

Mammalian central nervous system (CNS) development is a highly organized process involving the precise and coordinated timing of cell-cycle exit, differentiation, survival, and migration. These events require proper expression of pro-neuronal genes but also repression of alternative cell fates and restriction of cell-type-specific gene expression. Here, we show that the cyclin-dependent kinase (CDK) inhibitor p57Kip2 interacted with pro-neuronal basic helix-loop-helix (bHLH) factors such as Mash1, NeuroD, and Nex/Math2. Increased levels of p57Kip2 inhibited Mash1 transcriptional activity independently of CDK interactions and acted as a direct repressor in transcriptional assays. Proliferating telencephalic neural progenitors co-expressed basal levels of Mash1 and p57Kip2, and endogenous p57Kip2 accumulated transiently in the nuclei of neural stem cells (NSCs) during early stages of astrocyte differentiation mediated by ciliary neurotrophic factor (CNTF), independent of cell-cycle exit and at times when Mash1 expression was still prominent. In accordance with these observations, gain- and loss-of-function studies showed that p57Kip2 repressed neuronal differentiation after mitogen withdrawal, but exerted little or no effect on CNTF-mediated astroglial differentiation of NSCs. Our data suggest a novel role for p57Kip2 as a context-dependent repressor of neurogenic transcription factors and telencephalic neuronal differentiation.

The developmental integration of cortical interneurons into a functional network

The central goal of this manuscript is to survey our present knowledge of how cortical interneuron subtypes are generated. To achieve this, we will first define what is meant by subtype diversity. To this end, we begin by considering the mature properties that differentiate between the different populations of cortical interneurons. This requires us to address the difficulties involved in determining which characteristics allow particular interneurons to be assigned to distinct subclasses. Having grappled with this thorny issue, we will then proceed to review the progressive events in development involved in the generation of interneuron diversity. Starting with their origin and specification within the subpallium, we will follow them up through the first postnatal weeks during their integration into a functional network. Finally, we will conclude by calling the readers attention to the devastating consequences that result from developmental failures in the formation of inhibitory circuits within the cortex.

Role of neuroepithelial Sonic hedgehog in hypothalamic patterning

The hypothalamus is a region of the diencephalon with particularly complex patterning. Sonic hedgehog (Shh), encoding a protein with key developmental roles, shows a peculiar and dynamic diencephalic expression pattern. Here, we use transgenic strategies and in vitro experiments to test the hypothesis that Shh expressed in the diencephalic neuroepithelium (neural Shh) coordinates tissue growth and patterning in the hypothalamus. Our results show that neural Shh coordinates anteroposterior and dorsoventral patterning in the hypothalamus and in the diencephalon-telencephalon junction. Neural Shh also coordinates mediolateral hypothalamic patterning, since it is necessary for the lateral hypothalamus to attain proper size and is required for the specification of hypocretin/orexin cells. Finally, neural Shh is necessary to maintain expression of differentiation markers including survival factor Foxb1.

Olig2 transcription factor in the developing and injured forebrain; cell lineage and glial development

Olig2 transcription factor is widely expressed throughout the central nervous system; therefore, it is considered to have multiple functions in the developing, mature and injured brain. In this mini-review, we focus on Olig2 in the forebrain (telencephalon and diencephalon) and discuss the functional significance of Olig2 and the differentiation properties of Olig2-expressing progenitors in the development and injured states. Short- and long-term lineage analysis in the developing forebrain elucidated that not all late Olig2+ cells are direct cohorts of early cells and that Olig2 lineage cells differentiate into neurons or glial cells in a region- and stage-dependent manner. Olig2-deficient mice revealed large elimination of oligodendrocyte precursor cells and a decreased number of astrocyte progenitors in the dorsal cortex, whereas no reduction in the number of GABAergic neurons. In addition to Olig2 function in the developing cortex, Olig2 is also reported to be important for glial scar formation after injury. Thus, Olig2 can be essential for glial differentiation during development and after injury.

Dlx1&2 and Mash1 transcription factors control MGE and CGE patterning and differentiation through parallel and overlapping pathways

Here we define the expression of approximately 100 transcription factors (TFs) in progenitors and neurons of the developing mouse medial and caudal ganglionic eminences, anlage of the basal ganglia and pallial interneurons. We have begun to elucidate the transcriptional hierarchy of these genes with respect to the Dlx homeodomain genes, which are essential for differentiation of most gamma-aminobutyric acidergic projection neurons of the basal ganglia. This analysis identified Dlx-dependent and Dlx-independent pathways. The Dlx-independent pathway depends in part on the function of the Mash1 basic helix-loop-helix (b-HLH) TF. These analyses define core transcriptional components that differentially specify the identity and differentiation of the globus pallidus, basal telencephalon, and pallial interneurons.

Netrin-1-alpha3beta1 integrin interactions regulate the migration of interneurons through the cortical marginal zone

Cortical GABAergic interneurons, most of which originate in the ganglionic eminences, take distinct tangential migratory trajectories into the developing cerebral cortex. However, the ligand-receptor systems that modulate the tangential migration of distinct groups of interneurons into the emerging cerebral wall remain unclear. Here, we show that netrin-1, a diffusible guidance cue expressed along the migratory routes traversed by GABAergic interneurons, interacts with alpha3beta1 integrin to promote interneuronal migration. In vivo analysis of interneuron-specific alpha3beta1 integrin, netrin-1-deficient mice (alpha3(lox/-)Dlx5/6-CIE, netrin-1(-/-)) reveals specific deficits in the patterns of interneuronal migration along the top of the developing cortical plate, resulting in aberrant interneuronal positioning throughout the cerebral cortex and hippocampus of conditional alpha3(lox/-)Dlx5/6-CIE, netrin-1(-/-) mice. These results indicate that specific guidance mechanisms, such as netrin-1-alpha3beta1 integrin interactions, modulate distinct routes of interneuronal migration and the consequent positioning of groups of cortical interneurons in the developing cerebral cortex.

Ultrastructural and antigenic properties of neural stem cells and their progeny in adult rat subventricular zone

Neural stem cells (NSCs) in the subventricular zone (SVZ) continuously generate olfactory bulb interneurons in the adult rodent brain. Based on their ultrastructural and antigenic properties, NSCs, transient amplifying precursor cells, and neuroblasts (B, C, and A cells, respectively) have been distinguished in mouse SVZ. Here, we aimed to identify these cell types in rat SVZ ultrastructurally and at the light microscopy level, and to determine the antigenic properties of each cell type using gold and fluorescence immunolabeling. We found astrocytes with single cilia (NSCs, correspond to B cells) and neuroblasts (A cells). We also observed mitotic cells, ependymal cells, displaced ependymal cells, and mature astrocytes. In contrast, transient amplifying precursor cells (C cells) were not detected. The NSCs and neuroblasts had epidermal growth factor receptor (EGFR) and platelet-derived growth factor receptor alpha (PDGFRalpha) expressed on the ciliary apparatus and were the only cell types incorporating the proliferation marker BrdU. Throughout mitosis, EGFR and PDGFRalpha were associated with the microtubule of the mitotic spindle. Ependymal and displaced ependymal cells also expressed EGFR and PDGFRalpha on their cilia but did not incorporate BrdU. Our findings indicate that the NSCs in adult rat SVZ give rise directly to neuroblasts. During mitosis, the NSCs disassemble the primary cilium and symmetrically distribute EGFR and PDGFRalpha among their progeny.

Origins and control of the differentiation of inhibitory interneurons and glia in the cerebellum

Cerebellar GABAergic interneurons and glia originate from progenitors that delaminate from the ventricular neuroepithelium and proliferate in the prospective white matter. Even though this population of progenitor cells is multipotent as a whole, clonal analysis indicates that different lineages are already separated during postnatal development and little is known about the mechanisms that regulate the specification and differentiation of these cerebellar types at earlier stages. Here, we investigate the role of Ascl1 in the development of inhibitory interneurons and glial cells in the cerebellum. This gene is expressed by maturing oligodendrocytes and GABAergic interneurons and is required for the production of appropriate quantities of these cells, which are severely reduced in Ascl1(-/-) mouse cerebella. Nevertheless, the two lineages are not related and the majority of oligodendrocytes populating the developing cerebellum actually derive from extracerebellar sources. Targeted electroporation of Ascl1-expression vectors to ventricular neuroepithelium progenitors enhances the production of interneurons and completely suppresses astrocytic differentiation, whereas loss of Ascl1 function has opposite effects on both cell types. Our results indicate that Ascl1 directs ventricular neuroepithelium progenitors towards inhibitory interneuron fate and restricts their ability to differentiate along the astroglial lineage.

Dlx1&2 and Mash1 transcription factors control striatal patterning and differentiation through parallel and overlapping pathways

Here we define the expression of approximately 100 transcription factors in progenitors and neurons of the developing basal ganglia. We have begun to elucidate the transcriptional hierarchy of these genes with respect to the Dlx homeodomain genes, which are essential for differentiation of most GABAergic projection neurons of the basal ganglia. This analysis identified Dlx-dependent and Dlx-independent pathways. The Dlx-independent pathway depends in part on the function of the Mash1 b-HLH transcription factor. These analyses define core transcriptional components that differentially specify the identity and differentiation of the striatum, nucleus accumbens, and septum.

Paving the axonal highway: from stem cells to myelin repair

Multiple sclerosis (MS), a demyelinating disorder of the central nervous system (CNS), remains among the most prominent and devastating diseases in contemporary neurology. Despite remarkable advances in anti-inflammatory therapies, the inefficiency or failure of myelin-forming oligodendrocytes to remyelinate axons and preserve axonal integrity remains a major impediment for the repair of MS lesions. To this end, the enhancement of remyelination through endogenous and exogenous repair mechanisms and the prevention of axonal degeneration are critical objectives for myelin repair therapies. Thus, recent advances in uncovering myelinating cell sources and the intrinsic and extrinsic factors that govern neural progenitor differentiation and myelination may pave a way to novel strategies for myelin regeneration. The scope of this review is to discuss the potential sources of stem/progenitor cells for CNS remyelination and the molecular mechanisms underlying oligodendrocyte myelination.

Fate plasticity of adult hippocampal progenitors: biological relevance and therapeutic use

Adult hippocampal stem/progenitor cells (AHPs) continuously give rise to new neurons throughout life, which might be an important determinant of hippocampus-dependent function. Strikingly, the fate potential of AHPs is not restricted to the neuronal lineage because AHPs can be genetically induced to generate oligodendrocytes within their in vivo niche by AHP-specific ectopic expression of the basic-helix-loop-helix (bHLH) transcription factor achaete-scute complex-like 1 (ASCL1). Fate plasticity of AHPs is controlled by cell-autonomous and also niche-dependent mechanisms. Here, we discuss the biological importance and potential therapeutic applications of retained fate plasticity of AHPs in the adult mammalian brain in addition to the future scientific inquiries indicated by this finding.

Generation of Cre-transgenic mice using Dlx1/Dlx2 enhancers and their characterization in GABAergic interneurons

DLX1 and DLX2 transcription factors are necessary for forebrain GABAergic neuron differentiation, migration, and survival. We generated transgenic mice that express Cre-recombinase under the control of two ultra-conserved DNA elements near the Dlx1 and 2 locus termed I12b and URE2. We show that Cre-recombinase is active in a "Dlx-pattern" in the embryonic forebrain of transgenic mice. I12b-Cre is more active than URE2-Cre in the medial ganglionic eminences and its derivatives. Fate-mapping of EGFP+ cells in adult Cre;Z/EG animals demonstrated that GABAergic neurons, but not glia, are labeled. Most NPY+, nNOS+, parvalbumin+, and somatostatin+ cells are marked by I12b-Cre in the cortex and hippocampus, while 25-40% of these interneuron subtypes are labeled by URE2-Cre. Labeling of neurons generated between E12.5 to E15.5 indicated differences in birth-dates of EGFP+ cells that populate the olfactory bulb, hippocampus, and cortex. Finally, we provide the first in vivo evidence that both I12b and URE2 are direct targets of DLX2 and require Dlx1 and Dlx2 expression for proper activity.

Arx is a direct target of Dlx2 and thereby contributes to the tangential migration of GABAergic interneurons

The Arx transcription factor is expressed in the developing ventral telencephalon and subsets of its derivatives. Mutation of human ARX ortholog causes neurological disorders including epilepsy, lissencephaly, and mental retardation. We have isolated the mouse Arx endogenous enhancer modules that control its tightly compartmentalized forebrain expression. Interestingly, they are scattered downstream of its coding region and partially included within the introns of the downstream PolA1 gene. These enhancers are ultraconserved noncoding sequences that are highly conserved throughout the vertebrate phylum. Functional characterization of the Arx GABAergic enhancer element revealed its strict dependence on the activity of Dlx transcription factors. Dlx overexpression induces ectopic expression of endogenous Arx and its isolated enhancer, whereas loss of Dlx expression results in reduced Arx expression, suggesting that Arx is a key mediator of Dlx function. To further elucidate the mechanisms involved, a combination of gain-of-function studies in mutant Arx or Dlx tissues was pursued. This analysis provided evidence that, although Arx is necessary for the Dlx-dependent promotion of interneuron migration, it is not required for the GABAergic cell fate commitment mediated by Dlx factors. Although Arx has additional functions independent of the Dlx pathway, we have established a direct genetic relationship that controls critical steps in the development of telencephalic GABAergic neurons. These findings contribute elucidating the genetic hierarchy that likely underlies the etiology of a variety of human neurodevelopmental disorders.

A time and a place for nkx2-1 in interneuron specification and migration

The homeobox transcription factor, Nkx2-1, plays multiple roles during forebrain development. Using restricted genetic ablation of Nkx2-1, in this issue of Neuron, Butt et al. show that Nkx2-1 in telencephalic progenitors regulates interneuron subtype specification, while Nóbrega-Pereira et al. demonstrate that postmitotic Nkx2-1 regulates migration and sorting of interneurons to the striatum or cortex by controlling the expression of the guidance receptor, Neuropilin-2.

Context dependence of proneural bHLH proteins

A key point in neural development is the commitment of progenitor cells to a specific neural fate. In all animals studied, proneural proteins - transcription factors of the basic helix-loop-helix (bHLH) family - are central to this process. The function of these factors is strongly influenced by the spatial and temporal context in which they are expressed. It is important to understand the molecular mechanisms by which developmental context interacts with and modifies the intrinsic functions and properties of the proneural proteins. Recent insights have been obtained in Drosophila and vertebrates from analysis of how bHLH proteins interact with other transcription factors to regulate target genes.