The (not necessarily) convoluted role of basal radial glia in cortical neurogenesis

Recent advances in cell labeling and imaging techniques have dramatically expanded our knowledge of the neural precursor cells responsible for corticogenesis. In particular, radial glial cells are now known to generate several classes of restricted progenitors and neurons. While radial glial cells in the ventricular zone have received the most attention, it has become increasingly clear that a distinct subclass of radial glial cells situated in the subventricular zone (SVZ) and intermediate zone also play an important role in corticogenesis. These delaminated radial glial cells, which lack an apical process attached to the ventricular surface but maintain a basal process, were discovered over 3 decades ago. Recently, they have been further characterized as cortical progenitors and renamed outer, intermediate, or basal radial glia (bRG). Some of these studies indicated that bRG abundance in the outer SVZ (oSVZ) is correlated with enhanced gyrencephaly, particularly in primates and especially human, and therefore suggested that bRG may be responsible for the emergence and evolution of cerebral convolutions. In this issue of Cerebral Cortex, 2 papers provide new information about bRG in common marmosets, a near-lissencephalic primate, and in agouti, a near-gyrencephalic rodent (Garcia-Moreno et al. 2011; Kelava et al. 2011). They demonstrate that bRG are abundant and proliferate in inner as well as oSVZ, in both species. Together, these findings indicate that bRG and the oSVZ might not be correlated with gyrification or phylogeny. Rather, differential regulation of bRG and other progenitor types may enhance the adaptability and diversity of cortical morphogenesis.

Expression and actions of transcription factors in adult hippocampal neurogenesis

Neurogenesis in the adult brain, a process once thought to be essentially absent, has now been demonstrated to occur throughout adult mammalian life within several brain regions. Adult neurogenesis normally occurs only within the subventricular zone (SVZ) bordering the lateral ventricles and the subgranular zone (SGZ) of the hippocampal dentate gyrus (DG). Neurogenic progenitors within these regions produce distinct neuron types, with progenitors in the SGZ giving rise to glutamatergic neurons that populate the DG granule cell layer and those within the SVZ producing neurons destined for the olfactory bulb. In this review, we highlight recent research on transcription factor expression and function during adult hippocampal neurogenesis. In this regard, recent evidence indicates that adult neurogenesis replicates important aspects of progenitor cell development in the embryonic brain. Specifically, work from our laboratory and others indicates that transcription factor cascades active in progenitor cells during neurogenesis in the embryonic cerebral cortex are also activated in adult hippocampal progenitor cells, where they play an important role in determining neuronal fate and regulating progenitor cell proliferation and maintenance. These findings suggest that conserved transcription factor cascades regulate genetic programs that delineate progenitor cell lineages and control progenitor cell proliferation and differentiation.

Cell-type-specific consequences of Reelin deficiency in the mouse neocortex, hippocampus, and amygdala

The disrupted cortical lamination phenotype in reeler mice and subsequent identification of the Reelin signaling pathway have strongly informed models of cortical development. We describe here a marker-based phenotyping approach to reexamine the cytoarchitectural consequences of Reelin deficiency, using high-throughput histology and newly identified panels of highly specific molecular markers. The resulting cell-type-level cytoarchitectural analysis revealed novel features of abnormal patterning in the male reeler mouse not obvious with less specific markers or histology. The reeler cortex has been described as a rough laminar inversion, but the data presented here are not compatible with this model. The reeler cortex is disrupted in a more complex fashion, with some regions showing a mirror-image laminar phenotype. Major rostrocaudal and cell-type-specific differences in the laminar phenotype between cortical areas are detailed. These and similar findings in hippocampus and amygdala have implications for mechanisms of normal brain development and abnormalities in neurodevelopmental disorders.

Fetal akinesia deformation sequence with delayed skeletal muscle maturation and polymicrogyria: evidence for a hypoxic/ischemic pathogenesis

Multiple congenital contractures, also known as fetal akinesia deformation sequence (FADS) and related terms, result from decreased fetal movement. The underlying etiologies are diverse and include central nervous system (CNS) dysgeneses and primary myopathies. Persistent central nuclei or the presence of myotubes is often regarded as evidence of a primary myopathic etiology; however, these findings are also associated with impaired fetal innervation. We report 7 fetuses, estimated gestational age 20 to 23 weeks, with persistent myotubular morphology, a change that could be (mis)interpreted as a primary myopathy. In 4 of the patients, CNS histology showed hypoxic/ischemic injury, polymicrogyria, mineralized neurons, and microinfarcts with or without loss of anterior horn neurons. FADS cases with polymicrogyria have frequently been interpreted as a consequence of a primary brain malformation. Only a few descriptions of FADS associate polymicrogyria with CNS hypoxic/ischemic injury, however, and do not describe skeletal muscle maturation delay. We hypothesize that this combination of neural and muscular pathology is an under-recognized pattern in FADS, which results from diffuse hypoxic/ischemic injury involving the brain and spinal cord during early to middle gestation.

Autism susceptibility candidate 2 (Auts2) encodes a nuclear protein expressed in developing brain regions implicated in autism neuropathology

Autism susceptibility candidate 2 (Auts2) is a gene associated with autism and mental retardation, whose function is unknown. Expression of Auts2 mRNA and protein were studied in the developing mouse brain by in situ hybridization, immunohistochemistry, and western blotting. Auts2 mRNA was highly expressed in the developing cerebral cortex and cerebellum, regions often affected by neuropathological changes in autism, and a few other brain regions. On embryonic day (E) 12, Auts2 mRNA was expressed in the cortical preplate, where it colocalized with Tbr1, a transcription factor specific for postmitotic projection neurons. From E16 to postnatal day 21, Auts2 was expressed most abundantly in frontal cortex, hippocampus and cerebellum, including Purkinje cells and deep nuclei. High levels of Auts2 were also detected in developing dorsal thalamus, olfactory bulb, inferior colliculus and substantia nigra. Auts2 protein showed similar regional expression patterns as the mRNA. At the cellular level, Auts2 protein was localized in the nuclei of neurons and some neuronal progenitors.

Adult generation of glutamatergic olfactory bulb interneurons

The adult mouse subependymal zone (SEZ) harbours neural stem cells that are thought to generate exclusively GABAergic interneurons of the olfactory bulb. Here we describe the adult generation of glutamatergic juxtaglomerular neurons, with dendritic arborizations that project into adjacent glomeruli identifying them as short-axon cells. Fate mapping revealed that these originate from Neurogenin2- and Tbr2-expressing progenitors located in the dorsal region of the SEZ. Progenitors of these glutamatergic interneurons recapitulate the sequential expression of transcription factors that hallmark glutamatergic neurogenesis in the developing cerebral cortex and adult hippocampus. Indeed, the molecular specification of these SEZ progenitors allows for their recruitment into the cerebral cortex upon lesion. Taken together, our data show that SEZ progenitors not only produce a novel population of adult-born glutamatergic juxtaglomerular neurons, but may also provide a new source of progenitors for endogenous repair.

Fibroblast growth factor signaling in development of the cerebral cortex

Despite substantial and exciting recent progress in our understanding of developmental processes in the cerebral cortex, there is still much to be learned about the molecular and cellular mechanisms that account for formation of the cortical structures, and in turn, how the regulation of these mechanisms is linked to cortical functions and behaviors in animals and humans. Fibroblast growth factors (FGFs) are a classic family of growth factors that are important in neural development and whose structures and signaling have been well-studied molecularly and biochemically. Recent advances have revealed their diverse but specific functions in patterning and neurogenesis during cortical development, as evidenced by multiple experimental approaches using in vivo models. Importantly, changes in FGF signaling during development have been shown to influence structure and function of the cerebral cortex as well as animal behavior, and have been implicated in disorders of nervous system function and intellectual development in humans. For example, disturbance of FGF pathways during development has been implicated in the pathogenesis of autism spectrum disorders. Experimental models with altered cortical structure due to perturbations of FGF signaling present a unique opportunity whereby molecular and cellular mechanisms that underlie cortical function and animal behavior can be directly studied and linked to each other.

A patient with Huntington's disease and long-surviving fetal neural transplants that developed mass lesions

Transplantation of human fetal neural tissue into adult neostriatum is an experimental therapy for Huntington's disease (HD). Here we describe a patient with HD who received ten intrastriatal human fetal neural transplants and, at one site, an autologous sural nerve co-graft. Although initially clinically stable, she developed worsening asymmetric upper motor neuron symptoms in addition to progression of HD, and ultimately died 121 months post transplantation. Eight neural transplants, up to 2.9 cm, and three ependymal cysts, up to 2.0 cm, were identified. The autologous sural nerve co-graft was found adjacent to the largest mass lesion, which, along with the ependymal cyst, exhibited pronounced mass effect on the internal capsules bilaterally. Grafts were composed of neurons and glia embedded in disorganized neuropil; robust Y chromosome labeling was present in a subset of grafts and cysts. The graft-host border was discrete, and there was no evidence of graft rejection or HD pathologic changes within donor neurons. This report, for the first time, highlights the potential for graft overgrowth in a patient receiving fetal neural transplantation.

Fgf receptor 3 activation promotes selective growth and expansion of occipitotemporal cortex

BACKGROUND

Fibroblast growth factors (Fgfs) are important regulators of cerebral cortex development. Fgf2, Fgf8 and Fgf17 promote growth and specification of rostromedial (frontoparietal) cortical areas. Recently, the function of Fgf15 in antagonizing Fgf8 in the rostral signaling center was also reported. However, regulation of caudal area formation by Fgf signaling remains unknown.

RESULTS

In mutant mice with constitutive activation of Fgf receptor 3 (Fgfr3) in the forebrain, surface area of the caudolateral cortex was markedly expanded at early postnatal stage, while rostromedial surface area remained normal. Cortical thickness was also increased in caudal regions. The expression domain and levels of Fgf8, as well as overall patterning, were unchanged. In contrast, the changes in caudolateral surface area were associated with accelerated cell cycle in early stages of neurogenesis without an alteration of cell cycle exit. Moreover, a marked overproduction of intermediate neuronal progenitors was observed in later stages, indicating prolongation of neurogenesis.

CONCLUSION

Activation of Fgfr3 selectively promotes growth of caudolateral (occipitotemporal) cortex. These observations support the 'radial unit' and 'radial amplification' hypotheses and may explain premature sulcation of the occipitotemporal cortex in thanatophoric dysplasia, a human FGFR3 disorder. Together with previous work, this study suggests that formation of rostral and caudal areas are differentially regulated by Fgf signaling in the cerebral cortex.

Intermediate neuronal progenitors (basal progenitors) produce pyramidal-projection neurons for all layers of cerebral cortex

The developing cerebral cortex contains apical and basal types of neurogenic progenitor cells. Here, we investigated the cellular properties and neurogenic output of basal progenitors, also called intermediate neuronal progenitors (INPs). We found that basal mitoses expressing transcription factor Tbr2 (an INP marker) were present throughout corticogenesis, from embryonic day 10.5 through birth. Postnatally, Tbr2(+) progenitors were present in the dentate gyrus, subventricular zone (SVZ), and posterior periventricle (pPV). Two morphological subtypes of INPs were distinguished in the embryonic cortex, "short radial" in the ventricular zone (VZ) and multipolar in the SVZ, probably corresponding to molecularly defined INP subtypes. Unexpectedly, many short radial INPs appeared to contact the apical (ventricular) surface and some divided there. Time-lapse video microscopy suggested that apical INP divisions produced daughter INPs. Analysis of neurogenic divisions (Tis21-green fluorescent protein [GFP](+)) indicated that INPs may produce the majority of projection neurons for preplate, deep, and superficial layers. Conversely, proliferative INP divisions (Tis21-GFP(-)) increased from early to middle corticogenesis, concomitant with SVZ growth. Our findings support the hypothesis that regulated amplification of INPs may be an important factor controlling the balance of neurogenesis among different cortical layers.

Organotypic slice culture of embryonic brain tissue

INTRODUCTIONThis protocol describes how to dissect, assemble, and cultivate mouse embryonic (E) brain tissue from age E11.5 to E18.5 (days) for organotypic slice culture. These preparations can be used for a variety of assays and studies including coculture of different brain regions, cell migration assays, axon guidance assays, and DNA electroporation experiments. During electroporation, an electric current is applied to the surface of a specific target area of the brain slice in order to open holes in the plasma membrane and introduce a plasmid of coding DNA. The floating slice-on-membrane construct helps to preserve the structural integrity of the brain slices, while maintaining easy experimental access and optimal viability. Experiments can be monitored in living slices (e.g., with confocal imaging), and further studies can be completed using slices that have been fixed and cryosectioned at the end of the experiment. Any region of embryonic brain or spinal tissue can be used in this protocol.

A stream of cells migrating from the caudal telencephalon reveals a link between the amygdala and neocortex

The amygdaloid complex consists of diverse nuclei that belong to distinct functional systems, yet many issues about its development are poorly understood. Here, we identify a stream of migrating cells that form specific amygdaloid nuclei in mice. In utero electroporation showed that this caudal amygdaloid stream (CAS) originated in a unique domain at the caudal telencephalic pole that is contiguous with the dorsal pallium, which was previously thought to generate only neocortical cells. The CAS and the neocortex share mechanisms for specification (transcription factors Tbr1, Lhx2 and Emx1/2) and migration (reelin and Cdk5). Reelin, a critical cue for migration in the neocortex, and Cdk5, which is specifically required for migration along radial glia in the neocortex, were both selectively required for the normal migration of the CAS, but not for that of other amygdaloid nuclei. This is first evidence of a dorsal pallial contribution to the amygdala, demonstrating a developmental and mechanistic link between the amygdala and the neocortex.

Layer-specific markers as probes for neuron type identity in human neocortex and malformations of cortical development

Malformations of cortical development (MCDs) are heterogeneous disorders caused by abnormalities of cell proliferation, apoptosis, cell migration, cortical organization, and axon pathfinding. In severe MCDs, the cerebral cortex can appear completely disorganized, or may be replaced by aberrant laminar patterns, as in "4-layered" types of lissencephaly and polymicrogyria. Little is known about the abnormal layers in MCDs and whether they bear any relation to normal cortical layers or how MCDs affect specific neuron types. Normally, each layer contains a defined mixture of different types of pyramidal and nonpyramidal neurons. The neuron types are distinguished by molecular expression as well as morphologic, neurochemical, and electrophysiologic criteria. Patterns of layer-specific mRNA and protein expression reflect the segregation of different neuron types into different layers (e.g. corticospinal projection neurons in layer V). Numerous layer-specific markers have been described in rodent cortex, and increasing numbers are being documented in human and monkey cortex. Applied to MCDs, layer-specific markers have the potential to reveal new insights on pathogenesis, treatment possibilities, and genotype-phenotype correlations. However, much work remains before layer-specific markers become practical tools in diagnostic neuropathology. Additional markers, more extensive documentation of normal expression, and better antibodies compatible with paraffin-embedded tissues will be necessary.

Unipolar brush cells of the cerebellum are produced in the rhombic lip and migrate through developing white matter

Unipolar brush cells (UBCs) are glutamatergic interneurons in the cerebellar cortex and dorsal cochlear nucleus. We studied the development of UBCs, using transcription factor Tbr2/Eomes as a marker for UBCs and their progenitors in embryonic and postnatal mouse cerebellum. Tbr2+ UBCs appeared to migrate out of the upper rhombic lip via two cellular streams: a dorsal pathway into developing cerebellar white matter, where the migrating cells dispersed widely before entering the internal granular layer, and a rostral pathway along the cerebellar ventricular zone toward the brainstem. Ablation of the rhombic lip in organotypic slice cultures substantially reduced the production of Tbr2+ UBCs. In coculture experiments, Tbr2+ UBCs migrated from rhombic lip explants directly into the developing white matter of adjacent cerebellar slices. The origin of Tbr2+ UBCs was confirmed by colocalization with beta-galactosidase expressed from the Math1 locus, a molecular marker of rhombic lip lineages. Moreover, the production of Tbr2+ UBCs was Math1 dependent, as Tbr2+ UBCs were severely reduced in Math1-null cerebellum. In reeler mutant mice, Tbr2+ UBCs accumulated near the rhombic lip, consistent with impaired migration through developing white matter. Our results suggest that UBCs arise from the rhombic lip and migrate via novel pathways to their final destinations in the cerebellum and dorsal cochlear nucleus. Our findings support a model of cerebellar neurogenesis, in which glutamatergic and GABAergic neurons are produced from separate progenitor pools located mainly in the rhombic lip and the cerebellar ventricular zone, respectively.

Pax6 controls cerebral cortical cell number by regulating exit from the cell cycle and specifies cortical cell identity by a cell autonomous mechanism

Many cerebral cortical neurons and glia are produced by apical progenitors dividing at the ventricular surface of the embryonic dorsal telencephalon. Other neurons are produced by basal progenitor cells, which are derived from apical progenitors, dividing away from the ventricular surface. The transcription factor Pax6 is expressed in apical progenitors and is downregulated in basal progenitors, which upregulate the transcription factor Tbr2. Here we show that Pax6^−/− cells are under-represented in the cortex of Pax6^+/+↔Pax6^−/− chimeras early in corticogenesis, indicating that Pax6 is required for the production of normal numbers of cortical cells. We provide evidence that this underproduction is attributable to an early depletion of the progenitor pool caused by greater than normal proportions of newly divided cells exiting the cell cycle. We show that most progenitor cells dividing away from the ventricular surface in Pax6^−/− embryos fail to express the transcription factor Tbr2 and that Pax6 is required cell autonomously for Tbr2 expression in the developing cortex of Pax6^+/+↔Pax6^−/− chimeras. Transcription factors normally expressed ventrally in the telencephalic ganglionic eminences (Mash1, Dlx2 and Gsh2) are upregulated cell autonomously in mutant cells in the developing cortex of Pax6^+/+↔Pax6^−/− chimeras; Nkx2.1, which is expressed only in the medial ganglionic eminence, is not. These data indicate that early functions of Pax6 in developing cortical cells are to repress expression of transcription factors normally found in the lateral ganglionic eminence, to prevent precocious differentiation and depletion of the progenitor pool, and to induce normal development of cortical basal progenitor cells.

Development of the deep cerebellar nuclei: transcription factors and cell migration from the rhombic lip

The deep cerebellar nuclei (DCN) are the main output centers of the cerebellum, but little is known about their development. Using transcription factors as cell type-specific markers, we found that DCN neurons in mice are produced in the rhombic lip and migrate rostrally in a subpial stream to the nuclear transitory zone (NTZ). The rhombic lip-derived cells express transcription factors Pax6, Tbr2, and Tbr1 sequentially as they enter the NTZ. A subset of rhombic lip-derived cells also express reelin, a key regulator of Purkinje cell migrations. In organotypic slice cultures, the rhombic lip was necessary and sufficient to produce cells that migrate in the subpial stream, enter the NTZ, and express Pax6, Tbr2, Tbr1, and reelin. In later stages of development, the subpial stream is replaced by the external granular layer, and the NTZ organizes into distinct DCN nuclei. Tbr1 expression persists to adulthood in a subset of medial DCN projection neurons. In reeler mutant mice, which have a severe cerebellar malformation, rhombic lip-derived cells migrated to the NTZ, despite reelin deficiency. Studies in Tbr1 mutant mice suggested that Tbr1 plays a role in DCN morphogenesis but is not required for reelin expression, glutamatergic differentiation, or the initial formation of efferent axon pathways. Our findings reveal underlying similarities in the transcriptional programs for glutamatergic neuron production in the DCN and the cerebral cortex, and they support a model of cerebellar neurogenesis in which glutamatergic and GABAergic neurons are produced from separate progenitor compartments.

Transcription factors in glutamatergic neurogenesis: conserved programs in neocortex, cerebellum, and adult hippocampus

Glutamatergic, pyramidal-projection neurons are produced in the embryonic cerebral cortex by a series of genetically programmed fate choices, implemented in large part by developmental transcription factors. Our work has focused on Pax6, Tbr2/Eomes, NeuroD, and Tbr1, which are expressed sequentially during the neurogenesis of pyramidal-projection neurons. Recently, we have found that the same transcription factors are expressed, in the same order, during glutamatergic neurogenesis in the adult dentate gyrus, and (with modifications) in the developing cerebellum. While the precise functional significance of this transcription factor expression sequence is unknown, its common appearance in embryonic and adult neurogenesis, and in different brain regions, suggests it is part of a conserved genetic program that specifies general properties of glutamatergic neurons in these regions. Subtypes of glutamatergic neurons (e.g., layer-specific fates in the cortex) are further determined by combinations of transcription factors, superimposed on general sequential programs. These new perspectives on neurogenesis add to the conceptual framework for strategies to engineer neural stem cells for the repair of specific brain circuits.

From Radial Glia to Pyramidal-Projection Neuron: Transcription Factor Cascades in Cerebral Cortex Development

Pyramidal-projection neurons are glutamatergic neurons that develop from progenitors in the ventricular and subventricular zones of the embryonic cortex. Recently, much has been learned about the cortical progenitor cells and the cellular and molecular mechanisms by which they produce projection neurons. We now know that radial glia are the progenitors of most or all projection neurons and that they generate neurons by two distinct mitotic sequences: direct neurogenesis to produce a single daughter neuron or indirect neurogenesis to produce two to four neurons via intermediate progenitor cells. The underlying genetic programs for proliferation and differentiation are controlled and implemented by specific transcription factors, whose interactions largely determine the cortical surface area, thickness, and neuronal subtype composition. In turn, transcription factor expression is modulated by extrinsic signals from patterning centers and adjacent cells and by intrinsic signals distributed asymmetrically within progenitors and daughter cells. Together, the new findings provide a coherent framework for understanding cortical neurogenesis.

Essential role of Shp2-binding sites on FRS2alpha for corticogenesis and for FGF2-dependent proliferation of neural progenitor cells

Mammalian corticogenesis occurs through a complex process that includes neurogenesis, in which neural progenitor cells proliferate, differentiate, and migrate. It has been reported recently that neurogenesis occurs in the subventricular zone (SVZ), a region previously thought to be the primary site of gliogenesis. It has been recognized that in the SVZ, intermediate progenitor cells, derived from radial glial cells that are multipotent neural stem cells, produce only neurons. However, the molecular mechanisms underlying the regulation of neural stem cells and intermediate progenitor cells as well as their contribution to overall corticogenesis remain unknown. The docking protein FRS2alpha is a major mediator of signaling by means of FGFs and neurotrophins. FRS2alpha mediates many of its pleiotropic cellular responses by recruiting the adaptor protein Grb2 and the protein tyrosine phosphatase Shp2 upon ligand stimulation. Here, we report that targeted disruption of Shp2-binding sites in FRS2alpha leads to severe impairment in cerebral cortex development in mutant mice. The defect in corticogenesis appears to be due at least in part to abnormalities in intermediate progenitor cells. Genetic evidence is provided that FRS2alpha plays critical roles in the maintenance of intermediate progenitor cells and in neurogenesis in the cerebral cortex. Moreover, FGF2-responsive neurospheres, which are cell aggregates derived from neural stem/progenitor cells (NSPCs), from FRS2alpha mutant mice were smaller than those of WT mice. However, mutant NSPCs were able to self-renew, demonstrating that Shp2-binding sites on FRS2alpha play an important role in NSPC proliferation but are dispensable for NSPC self-renewing capacity after FGF2 stimulation.