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

From active affordance to active inference: vertical integration of cognition in the cerebral cortex through dual subcortical control systems

In previous papers, we proposed that the dorsal attention system's top-down control is regulated by the dorsal division of the limbic system, providing a feedforward or impulsive form of control generating expectancies during active inference. In contrast, we proposed that the ventral attention system is regulated by the ventral limbic division, regulating feedback constraints and error-correction for active inference within the neocortical hierarchy. Here, we propose that these forms of cognitive control reflect vertical integration of subcortical arousal control systems that evolved for specific forms of behavior control. The feedforward impetus to action is regulated by phasic arousal, mediated by lemnothalamic projections from the reticular activating system of the lower brainstem, and then elaborated by the hippocampus and dorsal limbic division. In contrast, feedback constraint-based on environmental requirements-is regulated by the tonic activation furnished by collothalamic projections from the midbrain arousal control centers, and then sustained and elaborated by the amygdala, basal ganglia, and ventral limbic division. In an evolutionary-developmental analysis, understanding these differing forms of active affordance-for arousal and motor control within the subcortical vertebrate neuraxis-may help explain the evolution of active inference regulating the cognition of expectancy and error-correction within the mammalian 6-layered neocortex.

Analysis of the Expression Pattern of Cajal-Retzius Cell Markers in the Xenopus laevis Forebrain

Cajal-Retzius cells are essential for cortical development in mammals, and their involvement in the evolution of this structure has been widely postulated, but very little is known about their progenitor domains in non-mammalian vertebrates. Using in situhybridization and immunofluorescence techniques we analyzed the expression of some of the main Cajal-Retzius cell markers such as Dbx1, Ebf3, ER81, Lhx1, Lhx5, p73, Reelin, Wnt3a, Zic1, and Zic2 in the forebrain of the anuran Xenopus laevis, because amphibians are the only class of anamniote tetrapods and show a tetrapartite evaginated pallium, but no layered or nuclear organization. Our results suggested that the Cajal-Retzius cell progenitor domains were comparable to those previously described in amniotes. Thus, at dorsomedial telencephalic portions a region comparable to the cortical hem was defined in Xenopus based on the expression of Wnt3a, p73, Reelin, Zic1, and Zic2. In the septum, two different domains were observed: a periventricular dorsal septum, at the limit between the pallium and the subpallium, expressing Reelin, Zic1, and Zic2, and a related septal domain, expressing Ebf3, Zic1, and Zic2. In the lateral telencephalon, the ventral pallium next to the pallio-subpallial boundary, the lack of Dbx1 and the unique expression of Reelin during development defined this territory as the most divergent with respect to mammals. Finally, we also analyzed the expression of these markers at the prethalamic eminence region, suggested as Cajal-Retzius progenitor domain in amniotes, observing there Zic1, Zic2, ER81, and Lhx1 expression. Our data show that in anurans there are different subtypes and progenitor domains of Cajal-Retzius cells, which probably contribute to the cortical regional specification and territory-specific properties. This supports the notion that the basic organization of pallial derivatives in vertebrates follows a comparable fundamental arrangement, even in those that do not have a sophisticated stratified cortical structure like the mammalian cerebral cortex.

Wiring of higher-order cortical areas: Spatiotemporal development of cortical hierarchy

A hierarchical development of cortical areas was suggested over a century ago, but the diversity and complexity of cortical hierarchy properties have so far prevented a formal demonstration. The aim of this review is to clarify the similarities and differences in the developmental processes underlying cortical development of primary and higher-order areas. We start by recapitulating the historical and recent advances underlying the biological principle of cortical hierarchy in adults. We then revisit the arguments for a hierarchical maturation of cortical areas, and further integrate the principles of cortical areas specification during embryonic and postnatal development. We highlight how the dramatic expansion in cortical size might have contributed to the increased number of association areas sustaining cognitive complexification in evolution. Finally, we summarize the recent observations of an alteration of cortical hierarchy in neuropsychiatric disorders and discuss their potential developmental origins.

Mouse induced pluripotent stem cells-derived Alzheimer's disease cerebral organoid culture and neural differentiation disorders

Alzheimer's disease (AD) is a progressive neurodegenerative disease, characterized by cognitive impairment. However, the pathogenesis of AD are very complicated, and the theories of Aβ and neurofibrillary tangles cannot explain all pathological alterations and clinical symptoms. Here, we used three-dimensional (3D) neural organoids culture derived from mouse induced pluripotent stem cells (iPSCs) to investigate the pathological mechanisms of AD. In this study, AD cerebral organoids were generated by overexpressing familial AD mutations (APP and PS1 genes) in mouse induced pluripotent stem cells, so that the early pathogenesis of AD could be investigated well with protein and cellular phenotype analyses. The results showed that AD cerebral organoids appeared some AD pathological alterations, and high levels of Aβ and p-Tau were induced as well. Furthermore, the number of GFAP-positive astrocytes and glutamatergic excitatory neurons increased significantly, but the number of GABAergic interneurons decreased. In conclusion, we suggest that cerebral organoids are a suitable AD model for scientific study, and that will provide us a novel insight into the understanding of the pathogenesis of AD.

Dbx1-Derived Pyramidal Neurons Are Generated Locally in the Developing Murine Neocortex

The neocortex (NCx) generates at the dorsal region of the pallium in the forebrain. Several adjacent structures also contribute with neurons to NCx. Ventral pallium (VP) is considered to generate several populations of neurons that arrive through tangential migration to the NCx. Amongst them are the Cajal-Retzius cells and some transient pyramidal neurons. However, the specific site and timing of generation, trajectory of migration and actual contribution to the pyramidal population remains elusive. Here, we investigate the spatio-temporal origin of neuronal populations from VP in an in vivo model, using a transposase mediated in utero electroporation method in embryonic mouse. From E11 to E14 cells born at the lateral corner of the neocortical neuroepithelium including the VP migrated ventro-laterally to settle all areas of the ventral telencephalon. Specifically, neurons migrated into amygdala (Ag), olfactory cortices, and claustrum (Cl). However, we found no evidence for any neurons migrating tangentially toward the NCx, regardless the antero-posterior level and developmental time of the electroporation. Our results challenge the described ventral-pallial origin of the transient pyramidal neuron population. In order to find the exact origin of cortical neurons that were previously Dbx1-fate mapped we used the promoter region of the murine Dbx1 locus to selectively target Dbx1-expressing progenitors and label their lineage. We found these progenitors in low numbers in all pallial areas, and not only in the ventral pallial ventricular zone. Our findings on the local cortical origin of the Dbx1-derived pyramidal neurons reconcile the observation of Dbx1-derived neurons in the cortex without evidence of dorsal tangential migration from VP and provide a new framework for the origin of the transient Dbx1-derived pyramidal neuron population. We conclude that these neurons are born locally within the dorsal pallial neuroepithelium.

Enhanced Abventricular Proliferation Compensates Cell Death in the Embryonic Cerebral Cortex

Loss of neurons in the neocortex is generally thought to result in a final reduction of cerebral volume. Yet, little is known on how the developing cerebral cortex copes with death of early-born neurons. Here, we tackled this issue by taking advantage of a transgenic mouse model in which, from early embryonic stages to mid-corticogenesis, abundant apoptosis is induced in the postmitotic compartment. Unexpectedly, the thickness of the mutant cortical plate at E18.5 was normal, due to an overproduction of upper layer neurons at E14.5. We developed and simulated a mathematical model to investigate theoretically the recovering capacity of the system and found that a minor increase in the probability of proliferative divisions of intermediate progenitors (IPs) is a powerful compensation lever. We confirmed experimentally that mutant mice showed an enhanced number of abventricular progenitors including basal radial glia-like cells and IPs. The latter displayed increased proliferation rate, sustained Pax6 expression and shorter cell cycle duration. Altogether, these results demonstrate the remarkable plasticity of neocortical progenitors to adapt to major embryonic insults via the modulation of abventricular divisions thereby ensuring the production of an appropriate number of neurons.

The caudo-ventral pallium is a novel pallial domain expressing Gdf10 and generating Ebf3-positive neurons of the medial amygdala

In rodents, the medial nucleus of the amygdala receives direct inputs from the accessory olfactory bulbs and is mainly implicated in pheromone-mediated reproductive and defensive behaviors. The principal neurons of the medial amygdala are GABAergic neurons generated principally in the caudo-ventral medial ganglionic eminence and preoptic area. Beside GABAergic neurons, the medial amygdala also contains glutamatergic Otp-expressing neurons cells generated in the lateral hypothalamic neuroepithelium and a non-well characterized Pax6-positive population. In the present work, we describe a novel glutamatergic Ebf3-expressing neuronal subpopulation distributed within the periphery of the postero-ventral medial amygdala. These neurons are generated in a pallial domain characterized by high expression of Gdf10. This territory is topologically the most caudal tier of the ventral pallium and accordingly, we named it Caudo-Ventral Pallium (CVP). In the absence of Pax6, the CVP is disrupted and Ebf3-expressing neurons fail to be generated. Overall, this work proposes a novel model of the neuronal composition of the medial amygdala and unravels for the first time a new novel pallial subpopulation originating from the CVP and expressing the transcription factor Ebf3.

Cortical developmental death: selected to survive or fated to die

The mature cerebral cortex only contains a fraction of the cells that are generated during embryonic development. Indeed some neuronal populations are produced in excess and later subjected to partial elimination whereas others are almost completely removed during the first two postnatal weeks in mice. Although the identity of cells that disappear, the time course and mechanisms of their death are becoming reasonably well established, the meaning of producing supernumerary cells still remains elusive. In this review, we focus on recent data that shed a new light on the mechanisms involved in adjusting cell numbers and discuss the significance of refinement versus complete elimination of cell populations in the developing cortex.

Rostro-Caudal and Caudo-Rostral Migrations in the Telencephalon: Going Forward or Backward?

The generation and differentiation of an appropriate number of neurons, as well as its distribution in different parts of the brain, is crucial for the proper establishment, maintenance and plasticity of neural circuitries. Newborn neurons travel along the brain in a process known as neuronal migration, to finalize their correct position in the nervous system. Defects in neuronal migration produce abnormalities in the brain that can generate neurodevelopmental pathologies, such as autism, schizophrenia and intellectual disability. In this review, we present an overview of the developmental origin of the different telencephalic subdivisions and a description of migratory pathways taken by distinct neural populations traveling long distances before reaching their target position in the brain. In addition, we discuss some of the molecules implicated in the guidance of these migratory paths and transcription factors that contribute to the correct migration and integration of these neurons.

Pioneer neurog1 expressing cells ingress into the otic epithelium and instruct neuronal specification

Neural patterning involves regionalised cell specification. Recent studies indicate that cell dynamics play instrumental roles in neural pattern refinement and progression, but the impact of cell behaviour and morphogenesis on neural specification is not understood. Here we combine 4D analysis of cell behaviours with dynamic quantification of proneural expression to uncover the construction of the zebrafish otic neurogenic domain. We identify pioneer cells expressing neurog1 outside the otic epithelium that migrate and ingress into the epithelialising placode to become the first otic neuronal progenitors. Subsequently, neighbouring cells express neurog1 inside the placode, and apical symmetric divisions amplify the specified pool. Interestingly, pioneer cells delaminate shortly after ingression. Ablation experiments reveal that pioneer cells promote neurog1 expression in other otic cells. Finally, ingression relies on the epithelialisation timing controlled by FGF activity. We propose a novel view for otic neurogenesis integrating cell dynamics whereby ingression of pioneer cells instructs neuronal specification.

DOI:http://dx.doi.org/10.7554/eLife.25543.001

The inner ear is responsible for our senses of hearing and balance, and is made up of a series of fluid-filled cavities. Sounds, and movements of the head, cause the fluid within these cavities to move. This activates neurons that line the cavities, causing them to increase their firing rates and pass on information about the sounds or head movements to the brain. Damage to these neurons can result in deafness or vertigo. But where do the neurons themselves come from?

It is generally assumed that all inner ear neurons develop inside an area of the embryo called the inner ear epithelium. Cells in this region are thought to switch on a gene called neurog1, triggering a series of changes that turn them into inner ear neurons. However, using advanced microscopy techniques in zebrafish embryos, Hoijman, Fargas et al. now show that this is not the whole story.

While zebrafish do not have external ears, they do possess fluid-filled structures for balance and hearing that are similar to those of other vertebrates. Zebrafish embryos are also transparent, which means that activation of genes can be visualized directly. By imaging zebrafish embryos in real time, Hoijman, Fargas et al. show that the first cells to switch on neurog1 do so outside the inner ear epithelium. These pioneer cells then migrate into the inner ear epithelium and switch on neurog1 in their new neighbors. A substance called fibroblast growth factor tells the inner ear epithelium to let the pioneers enter, and thereby controls the final number of inner ear neurons.

The work of Hoijman, Fargas et al. reveals how coordinated activation of genes and movement of cells gives rise to inner ear neurons. This should provide insights into the mechanisms that generate other types of sensory tissue. In the long term, the advances made in this study may lead to new strategies for repairing damaged sensory nerves.

DOI:http://dx.doi.org/10.7554/eLife.25543.002

Developmental Pattern Formation in Phases

Cells in developing organs undergo a series of changes in their transcriptional state until a complete repertoire of cell types is specified. These changes in cell identity, together with the control of tissue growth, determine the pattern of gene expression in the tissue. Recent studies explore the dynamics of pattern formation during development and provide new insights into the control mechanisms. Changes in morphogen signalling and transcriptional networks control the specification of cell types. This is often followed by a distinct second phase, where pattern is elaborated by tissue growth. Here, we discuss the transitions between distinct phases in pattern formation. We consider the implications of the underlying mechanisms for understanding how reproducible patterns form during development.

Radial derivatives of the mouse ventral pallium traced with Dbx1-LacZ reporters

The progeny of Dbx1-expressing progenitors was studied in the developing mouse pallium, using two transgenic mouse lines: (1) Dbx1(nlslacZ) mice, in which the gene of the β-galactosidase reporter (LacZ) is inserted directly under the control of the Dbx1 promoter, allowing short-term lineage tracing of Dbx1-derived cells; and (2) Dbx1(CRE) mice crossed with a Cre-dependent reporter strain (ROSA26(loxP-stop-loxP-LacZ)), in which the Dbx1-derived cells result permanently labeled (Bielle et al., 2005). We thus examined in detail the derivatives of the postulated longitudinal ventral pallium (VPall) sector, which has been defined among other features by its selective ventricular zone expression of Dbx1 (the recent ascription by Puelles, 2014 of the whole olfactory cortex primordium to the VPall was tested). Earlier notions about a gradiental caudorostral reduction of Dbx1 signal were corroborated, so that virtually no signal was found at the olfactory bulb and the anterior olfactory area. The piriform cortex was increasingly labeled caudalwards. The only endopiriform grisea labeled were the ventral endopiriform nucleus and the bed nucleus of the external capsule. Anterior and basolateral parts of the whole pallial amygdala also were densely marked, in contrast to the negative posterior parts of these pallial amygdalar nuclei (leaving apart medial amygdalar parts ascribed to subpallial or extratelencephalic sources of Dbx1-derived GABAergic and non-GABAergic neurons). Alternative tentative interpretations are discussed to explain the partial labeling obtained of both olfactory and amygdaloid structures. This includes the hypothesis of an as yet undefined part of the pallium, potentially responsible for the posterior amygdala, or the hypothesis that the VPall may not be wholly characterized by Dbx1 expression (this gene not being necessary for VPall molecular distinctness and histogenetic potency), which would leave a dorsal Dbx1-negative VPall subdomain of variable size that might contribute partially to olfactory and posterior amygdalar structures.

Molecular Pathways Underlying Projection Neuron Production and Migration during Cerebral Cortical Development

Glutamatergic neurons of the mammalian cerebral cortex originate from radial glia (RG) progenitors in the ventricular zone (VZ). During corticogenesis, neuroblasts migrate toward the pial surface using two different migration modes. One is multipolar (MP) migration with random directional movement, and the other is locomotion, which is a unidirectional movement guided by the RG fiber. After reaching their final destination, the neurons finalize their migration by terminal translocation, which is followed by maturation via dendrite extension to initiate synaptogenesis and thereby complete neural circuit formation. This switching of migration modes during cortical development is unique in mammals, which suggests that the RG-guided locomotion mode may contribute to the evolution of the mammalian neocortical 6-layer structure. Many factors have been reported to be involved in the regulation of this radial neuronal migration process. In general, the radial migration can be largely divided into four steps; (1) maintenance and departure from the VZ of neural progenitor cells, (2) MP migration and transition to bipolar cells, (3) RG-guided locomotion, and (4) terminal translocation and dendrite maturation. Among these, many different gene mutations or knockdown effects have resulted in failure of the MP to bipolar transition (step 2), suggesting that it is a critical step, particularly in radial migration. Moreover, this transition occurs at the subplate layer. In this review, we summarize recent advances in our understanding of the molecular mechanisms underlying each of these steps. Finally, we discuss the evolutionary aspects of neuronal migration in corticogenesis.

Tangential migration of glutamatergic neurons and cortical patterning during development: Lessons from Cajal-Retzius cells

Tangential migration is a mode of cell movement, which in the developing cerebral cortex, is defined by displacement parallel to the ventricular surface and orthogonal to the radial glial fibers. This mode of long-range migration is a strategy by which distinct neuronal classes generated from spatially and molecularly distinct origins can integrate to form appropriate neural circuits within the cortical plate. While it was previously believed that only GABAergic cortical interneurons migrate tangentially from their origins in the subpallial ganglionic eminences to integrate in the cortical plate, it is now known that transient populations of glutamatergic neurons also adopt this mode of migration. These include Cajal-Retzius cells (CRs), subplate neurons (SPs), and cortical plate transient neurons (CPTs), which have crucial roles in orchestrating the radial and tangential development of the embryonic cerebral cortex in a noncell-autonomous manner. While CRs have been extensively studied, it is only in the last decade that the molecular mechanisms governing their tangential migration have begun to be elucidated. To date, the mechanisms of SPs and CPTs tangential migration remain unknown. We therefore review the known signaling pathways, which regulate parameters of CRs migration including their motility, contact-redistribution and adhesion to the pial surface, and discuss this in the context of how CR migration may regulate their signaling activity in a spatial and temporal manner. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 847-881, 2016.

Switching modes in corticogenesis: mechanisms of neuronal subtype transitions and integration in the cerebral cortex

Information processing in the cerebral cortex requires the activation of diverse neurons across layers and columns, which are established through the coordinated production of distinct neuronal subtypes and their placement along the three-dimensional axis. Over recent years, our knowledge of the regulatory mechanisms of the specification and integration of neuronal subtypes in the cerebral cortex has progressed rapidly. In this review, we address how the unique cytoarchitecture of the neocortex is established from a limited number of progenitors featuring neuronal identity transitions during development. We further illuminate the molecular mechanisms of the subtype-specific integration of these neurons into the cerebral cortex along the radial and tangential axis, and we discuss these key features to exemplify how neocortical circuit formation accomplishes economical connectivity while maintaining plasticity and evolvability to adapt to environmental changes.

A hypothesis for the evolution of the upper layers of the neocortex through co-option of the olfactory cortex developmental program

The neocortex is unique to mammals and its evolutionary origin is still highly debated. The neocortex is generated by the dorsal pallium ventricular zone, a germinative domain that in reptiles give rise to the dorsal cortex. Whether this latter allocortical structure contains homologs of all neocortical cell types it is unclear. Recently we described a population of DCX+/Tbr1+ cells that is specifically associated with the layer II of higher order areas of both the neocortex and of the more evolutionary conserved piriform cortex. In a reptile similar cells are present in the layer II of the olfactory cortex and the DVR but not in the dorsal cortex. These data are consistent with the proposal that the reptilian dorsal cortex is homologous only to the deep layers of the neocortex while the upper layers are a mammalian innovation. Based on our observations we extended these ideas by hypothesizing that this innovation was obtained by co-opting a lateral and/or ventral pallium developmental program. Interestingly, an analysis in the Allen brain atlas revealed a striking similarity in gene expression between neocortical layers II/III and piriform cortex. We thus propose a model in which the early neocortical column originated by the superposition of the lateral olfactory and dorsal cortex. This model is consistent with the fossil record and may account not only for the topological position of the neocortex, but also for its basic cytoarchitectural and hodological features. This idea is also consistent with previous hypotheses that the peri-allocortex represents the more ancient neocortical part. The great advances in deciphering the molecular logic of the amniote pallium developmental programs will hopefully enable to directly test our hypotheses in the next future.

Quiescent neuronal progenitors are activated in the juvenile guinea pig lateral striatum and give rise to transient neurons

In the adult brain, active stem cells are a subset of astrocytes residing in the subventricular zone (SVZ) and the dentate gyrus (DG) of the hippocampus. Whether quiescent neuronal progenitors occur in other brain regions is unclear. Here, we describe a novel neurogenic system in the external capsule and lateral striatum (EC-LS) of the juvenile guinea pig that is quiescent at birth but becomes active around weaning. Activation of neurogenesis in this region was accompanied by the emergence of a neurogenic-like niche in the ventral EC characterized by chains of neuroblasts, intermediate-like progenitors and glial cells expressing markers of immature astrocytes. Like neurogenic astrocytes of the SVZ and DG, these latter cells showed a slow rate of proliferation and retained BrdU labeling for up to 65 days, suggesting that they are the primary progenitors of the EC-LS neurogenic system. Injections of GFP-tagged lentiviral vectors into the SVZ and the EC-LS of newborn animals confirmed that new LS neuroblasts originate from the activation of local progenitors and further supported their astroglial nature. Newborn EC-LS neurons existed transiently and did not contribute to neuronal addition or replacement. Nevertheless, they expressed Sp8 and showed strong tropism for white matter tracts, wherein they acquired complex morphologies. For these reasons, we propose that EC-LS neuroblasts represent a novel striatal cell type, possibly related to those populations of transient interneurons that regulate the development of fiber tracts during embryonic life.

Neuronal subtype specification in establishing mammalian neocortical circuits

The functional integrity of the neocortical circuit relies on the precise production of diverse neuron populations and their assembly during development. In recent years, extensive progress has been made in the understanding of the mechanisms that control differentiation of each neuronal type within the neocortex. In this review, we address how the elaborate neocortical cytoarchitecture is established from a simple neuroepithelium based on recent studies examining the spatiotemporal mechanisms of neuronal subtype specification. We further discuss the critical events that underlie the conversion of the stem amniotes cerebrum to a mammalian-type neocortex, and extend these key findings in the light of mammalian evolution to understand how the neocortex in humans evolved from common ancestral mammals.

The cell biology of neurogenesis: toward an understanding of the development and evolution of the neocortex

Neural stem and progenitor cells have a central role in the development and evolution of the mammalian neocortex. In this review, we first provide a set of criteria to classify the various types of cortical stem and progenitor cells. We then discuss the issue of cell polarity, as well as specific subcellular features of these cells that are relevant for their modes of division and daughter cell fate. In addition, cortical stem and progenitor cell behavior is placed into a tissue context, with consideration of extracellular signals and cell-cell interactions. Finally, the differences across species regarding cortical stem and progenitor cells are dissected to gain insight into key developmental and evolutionary mechanisms underlying neocortex expansion.

Neurogenesis during development of the vertebrate central nervous system

During vertebrate development, a wide variety of cell types and tissues emerge from a single fertilized oocyte. One of these tissues, the central nervous system, contains many types of neurons and glial cells that were born during the period of embryonic and post-natal neuro- and gliogenesis. As to neurogenesis, neural progenitors initially divide symmetrically to expand their pool and switch to asymmetric neurogenic divisions at the onset of neurogenesis. This process involves various mechanisms involving intrinsic as well as extrinsic factors. Here, we discuss the recent advances and insights into regulation of neurogenesis in the developing vertebrate central nervous system. Topics include mechanisms of (a)symmetric cell division, transcriptional and epigenetic regulation, and signaling pathways, using mostly examples from the developing mammalian neocortex.

Neural progenitors, patterning and ecology in neocortical origins

The anatomical organization of the mammalian neocortex stands out among vertebrates for its laminar and columnar arrangement, featuring vertically oriented, excitatory pyramidal neurons. The evolutionary origin of this structure is discussed here in relation to the brain organization of other amniotes, i.e., the sauropsids (reptiles and birds). Specifically, we address the developmental modifications that had to take place to generate the neocortex, and to what extent these modifications were shared by other amniote lineages or can be considered unique to mammals. In this article, we propose a hypothesis that combines the control of proliferation in neural progenitor pools with the specification of regional morphogenetic gradients, yielding different anatomical results by virtue of the differential modulation of these processes in each lineage. Thus, there is a highly conserved genetic and developmental battery that becomes modulated in different directions according to specific selective pressures. In the case of early mammals, ecological conditions like nocturnal habits and reproductive strategies are considered to have played a key role in the selection of the particular brain patterning mechanisms that led to the origin of the neocortex.

The homeobox gene Gsx2 controls the timing of oligodendroglial fate specification in mouse lateral ganglionic eminence progenitors

The homeobox gene Gsx2 has previously been shown to be required for the specification of distinct neuronal subtypes derived from lateral ganglionic eminence (LGE) progenitors at specific embryonic time points. However, its role in the subsequent generation of oligodendrocytes from these progenitors remains unclear. We have utilized conditional gain-of-function and loss-of-function approaches in order to elucidate the role of Gsx2 in the switch between neurogenesis and oligodendrogenesis within the embryonic ventral telencephalon. In the absence of Gsx2 expression, an increase in oligodendrocyte precursor cells (OPCs) with a concomitant decrease in neurogenesis is observed in the subventricular zone of the LGE at mid-stages of embryogenesis (i.e. E12.5-15.5), which subsequently leads to an increased number of Gsx2-derived OPCs within the adjacent mantle regions of the cortex before birth at E18.5. Moreover, using Olig2(cre) to conditionally inactivate Gsx2 throughout the ventral telencephalon with the exception of the dorsal (d)LGE, we found that the increase in cortical OPCs in Gsx2 germline mutants are derived from dLGE progenitors. We also show that Ascl1 is required for the expansion of these dLGE-derived OPCs in the cortex of Gsx2 mutants. Complementing these results, gain-of-function experiments in which Gsx2 was expressed throughout most of the late-stage embryonic telencephalon (i.e. E15.5-18.5) result in a significant decrease in the number of cortical OPCs. These results support the notion that high levels of Gsx2 suppress OPC specification in dLGE progenitors and that its downregulation is required for the transition from neurogenesis to oligodendrogenesis.

Neocortical arealization: evolution, mechanisms, and open questions

The mammalian neocortex is a structure with no equals in the vertebrates and is the seat of the highest cerebral functions, such as thoughts and consciousness. It is radially organized into six layers and tangentially subdivided into functional areas deputed to the elaboration of sensory information, association between different stimuli, and selection and triggering of voluntary movements. The process subdividing the neocortical field into several functional areas is called "arealization". Each area has its own cytoarchitecture, connectivity, and peculiar functions. In the last century, several neuroscientists have investigated areal structure and the mechanisms that have led during evolution to the rising of the neocortex and its organization. The extreme conservation in the positioning and wiring of neocortical areas among different mammalian families suggests a conserved genetic program orchestrating neocortical patterning. However, the impressive plasticity of the neocortex, which is able to rewire and reorganize areal structures and connectivity after impairments of sensory pathways, argues for a more complex scenario. Indeed, even if genetics and molecular biology helped in identifying several genes involved in the arealization process, the logic underlying the neocortical bauplan is still beyond our comprehension. In this review, we will introduce the present knowledge and hypotheses on the ontogenesis and evolution of neocortical areas. Then, we will focus our attention on some open issues, which are still unresolved, and discuss some recent studies that might open new directions to be explored in the next few years.

Reptiles: a new model for brain evo-devo research

Vertebrate brains exhibit vast amounts of anatomical diversity. In particular, the elaborate and complex nervous system of amniotes is correlated with the size of their behavioral repertoire. However, the evolutionary mechanisms underlying species-specific brain morphogenesis remain elusive. In this review we introduce reptiles as a new model organism for understanding brain evolution. These animal groups inherited ancestral traits of brain architectures. We will describe several unique aspects of the reptilian nervous system with a special focus on the telencephalon, and discuss the genetic mechanisms underlying reptile-specific brain morphology. The establishment of experimental evo-devo approaches to studying reptiles will help to shed light on the origin of the amniote brains.

A wide diversity of cortical GABAergic interneurons derives from the embryonic preoptic area

GABA-containing (GABAergic) interneurons comprise a very heterogeneous group of cells that are crucial for cortical function. Different classes of interneurons specialize in targeting specific subcellular domains of excitatory pyramidal cells or other interneurons, which provides cortical circuits with an enormous capability for information processing. As in other regions of the CNS, cortical interneuron diversity is thought to emerge from the genetic specification of different groups of progenitor cells within the subpallium. Most cortical interneurons originate from two main regions, the medial and the caudal ganglionic eminences (MGE and CGE, respectively). In addition, it has been shown that progenitors in the embryonic preoptic area (POA) also produce a small population of cortical GABAergic interneurons. Here, we show that the contribution of the POA to the complement of cortical GABAergic interneurons is larger than previously believed. Using genetic fate mapping and in utero transplantation experiments, we demonstrate that Dbx1-expressing progenitor cells in the POA give rise to a small but highly diverse cohort of cortical interneurons, with some neurochemical and electrophysiological characteristics that were previously attributed to MGE- or CGE-derived interneurons. There are, however, some features that seem to distinguish POA-derived interneurons from MGE- or CGE-derived cells, such as their preferential laminar location. These results indicate that the mechanisms controlling the specification of different classes of cortical interneurons might be more complex than previously expected. Together with earlier findings, our results also suggest that the POA generates nearly 10% of the GABAergic interneurons in the cerebral cortex of the mouse.

The homeobox gene Gsx2 regulates the self-renewal and differentiation of neural stem cells and the cell fate of postnatal progenitors

The Genetic screened homeobox 2 (Gsx2) transcription factor is required for the development of olfactory bulb (OB) and striatal neurons, and for the regional specification of the embryonic telencephalon. Although Gsx2 is expressed abundantly by progenitor cells in the ventral telencephalon, its precise function in the generation of neurons from neural stem cells (NSCs) is not clear. Similarly, the role of Gsx2 in regulating the self-renewal and multipotentiality of NSCs has been little explored. Using retroviral vectors to express Gsx2, we have studied the effect of Gsx2 on the growth of NSCs isolated from the OB and ganglionic eminences (GE), as well as its influence on the proliferation and cell fate of progenitors in the postnatal mouse OB. Expression of Gsx2 reduces proliferation and the self-renewal capacity of NSCs, without significantly affecting cell death. Furthermore, Gsx2 overexpression decreases the differentiation of NSCs into neurons and glia, and it maintains the cells that do not differentiate as cycling progenitors. These effects were stronger in GESCs than in OBSCs, indicating that the actions of Gsx2 are cell-dependent. In vivo, Gsx2 produces a decrease in the number of Pax6+ cells and doublecortin+ neuroblasts, and an increase in Olig2+ cells. In summary, our findings show that Gsx2 inhibits the ability of NSCs to proliferate and self-renew, as well as the capacity of NSC-derived progenitors to differentiate, suggesting that this transcription factor regulates the quiescent and undifferentiated state of NSCs and progenitors. Furthermore, our data indicate that Gsx2 negatively regulates neurogenesis from postnatal progenitor cells.