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

Transcriptional profiling of the chick retina identifies down-regulation of VIP and UTS2B genes during early lens-induced myopia

Gene expression of the chick retina was examined during the early development of lens-induced myopia (LIM) using whole transcriptome sequencing. Monocular treatment of the right eyes with −10 diopter (D)...

Cilia, ciliopathies and hedgehog-related forebrain developmental disorders

Development of the forebrain critically depends on the Sonic Hedgehog (Shh) signaling pathway, as illustrated in humans by the frequent perturbation of this pathway in holoprosencephaly, a condition defined as a defect in the formation of midline structures of the forebrain and face. The Shh pathway requires functional primary cilia, microtubule-based organelles present on virtually every cell and acting as cellular antennae to receive and transduce diverse chemical, mechanical or light signals. The dysfunction of cilia in humans leads to inherited diseases called ciliopathies, which often affect many organs and show diverse manifestations including forebrain malformations for the most severe forms. The purpose of this review is to provide the reader with a framework to understand the developmental origin of the forebrain defects observed in severe ciliopathies with respect to perturbations of the Shh pathway. We propose that many of these defects can be interpreted as an imbalance in the ratio of activator to repressor forms of the Gli transcription factors, which are effectors of the Shh pathway. We also discuss the complexity of ciliopathies and their relationships with forebrain disorders such as holoprosencephaly or malformations of cortical development, and emphasize the need for a closer examination of forebrain defects in ciliopathies, not only through the lens of animal models but also taking advantage of the increasing potential of the research on human tissues and organoids.

A transient role of the ciliary gene Inpp5e in controlling direct versus indirect neurogenesis in cortical development

During the development of the cerebral cortex, neurons are generated directly from radial glial cells or indirectly via basal progenitors. The balance between these division modes determines the number and types of neurons formed in the cortex thereby affecting cortical functioning. Here, we investigate the role of primary cilia in controlling the decision between forming neurons directly or indirectly. We show that a mutation in the ciliary gene Inpp5e leads to a transient increase in direct neurogenesis and subsequently to an overproduction of layer V neurons in newborn mice. Loss of Inpp5e also affects ciliary structure coinciding with reduced Gli3 repressor levels. Genetically restoring Gli3 repressor rescues the decreased indirect neurogenesis in Inpp5e mutants. Overall, our analyses reveal how primary cilia determine neuronal subtype composition of the cortex by controlling direct versus indirect neurogenesis. These findings have implications for understanding cortical malformations in ciliopathies with INPP5E mutations.

The Neocortical Progenitor Specification Program Is Established through Combined Modulation of SHH and FGF Signaling

Neuronal progenitors in the developing forebrain undergo dynamic competence states to ensure timely generation of specific excitatory and inhibitory neuronal subtypes from distinct neurogenic niches of the dorsal and ventral forebrain, respectively. Here we show evidence of progenitor plasticity when Sonic hedgehog (SHH) signaling is left unmodulated in the embryonic neocortex of the mammalian dorsal forebrain. We found that, at early stages of corticogenesis, loss of Suppressor of Fused (Sufu), a potent inhibitor of SHH signaling, in neocortical progenitors, altered the transcriptomic landscape of male mouse embryos. Ectopic activation of SHH signaling occurred, via degradation of Gli3R, resulting in significant upregulation of fibroblast growth factor 15 (FGF15) gene expression in all E12.5 Sufu-cKO neocortex regardless of sex. Consequently, activation of FGF signaling, and its downstream effector the MAPK signaling, facilitated expression of genes characteristic of ventral forebrain progenitors. Our studies identify the importance of modulating extrinsic niche signals such as SHH and FGF15, to maintain the competency and specification program of neocortical progenitors throughout corticogenesis.SIGNIFICANCE STATEMENT Low levels of FGF15 control progenitor proliferation and differentiation during neocortical development, but little is known on how FGF15 expression is maintained. Our studies identified SHH signaling as a critical activator of FGF15 expression during corticogenesis. We found that Sufu, via Gli3R, ensured low levels of FGF15 was expressed to prevent abnormal specification of neocortical progenitors. These studies advance our knowledge on the molecular mechanisms guiding the generation of specific neocortical neuronal lineages, their implications in neurodevelopmental diseases, and may guide future studies on how progenitor cells may be used for brain repair.

Gli3 Regulates Vomeronasal Neurogenesis, Olfactory Ensheathing Cell Formation, and GnRH-1 Neuronal Migration

During mammalian development, gonadotropin-releasing-hormone-1 neurons (GnRH-1ns) migrate from the developing vomeronasal organ (VNO) into the brain asserting control of pubertal onset and fertility. Recent data suggest that correct development of the olfactory ensheathing cells (OEC) is imperative for normal GnRH-1 neuronal migration. However, the full ensemble of molecular pathways that regulate OEC development remains to be fully deciphered. Loss-of-function of the transcription factor Gli3 is known to disrupt olfactory development, however, if Gli3 plays a role in GnRH-1 neuronal development is unclear. By analyzing Gli3 extra-toe mutants (Gli3Xt/Xt), we found that Gli3 loss-of-function compromises the onset of achaete-scute family bHLH transcription factor 1 (Ascl-1)+ vomeronasal progenitors and the formation of OEC in the nasal mucosa. Surprisingly, GnRH-1 neurogenesis was intact in Gli3Xt/Xt mice but they displayed significant defects in GnRH-1 neuronal migration. In contrast, Ascl-1null mutants showed reduced neurogenesis for both vomeronasal and GnRH-1ns but less severe defects in OEC development. These observations suggest that Gli3 is critical for OEC development in the nasal mucosa and subsequent GnRH-1 neuronal migration. However, the nonoverlapping phenotypes between Ascl-1 and Gli3 mutants indicate that Ascl-1, while crucial for GnRH-1 neurogenesis, is not required for normal OEC development. Because Kallmann syndrome (KS) is characterized by abnormal GnRH-1ns migration, we examined whole-exome sequencing data from KS subjects. We identified and validated a GLI3 loss-of-function variant in a KS individual. These findings provide new insights into GnRH-1 and OECs development and demonstrate that human GLI3 mutations contribute to KS etiology.SIGNIFICANCE STATEMENT The transcription factor Gli3 is necessary for correct development of the olfactory system. However, if Gli3 plays a role in controlling GnRH-1 neuronal development has not been addressed. We found that Gli3 loss-of-function compromises the onset of Ascl-1+ vomeronasal progenitors, formation of olfactory ensheathing cells in the nasal mucosa, and impairs GnRH-1 neuronal migration to the brain. By analyzing Ascl-1null mutants we dissociated the neurogenic defects observed in Gli3 mutants from lack of olfactory ensheathing cells in the nasal mucosa, moreover, we discovered that Ascl-1 is necessary for GnRH-1 ontogeny. Analyzing human whole-exome sequencing data, we identified a GLI3 loss-of-function variant in a KS individual. Our data suggest that GLI3 is a candidate gene contributing to KS etiology.

Diencephalic progenitors contribute to the posterior septum through rostral migration along the hippocampal axonal pathway

Septal nuclei are telencephalic structures associated with a variety of brain functions as part of the limbic system. The two posterior septal nuclei, the triangular septal nucleus (TS) and the bed nuclei of the anterior commissure (BAC), are involved in fear and anxiety through their projections to the medial habenular nucleus. However, the development of both the TS and BAC remains unclear. Here, we found a novel caudal origin and putative migratory stream of mouse posterior septal neurons arising from the thalamic eminence (TE), a transient developmental structure at the rostral end of the rodent diencephalon. TE-derived cells, which have glutamatergic identity, migrated rostrally and entered the telencephalic territory by passing beneath the third ventricle. Subsequently, they turned dorsally toward the posterior septum. We also observed that TS and BAC neurons in the postnatal septum were labeled with GFP by in utero electroporation into the TE, suggesting a shared origin. Furthermore, TE-derived septal neurons migrated along the fornix, an efferent pathway from the hippocampus. These results demonstrate that posterior septal neurons have a distinct extratelencephalic origin from other septal nuclei. This heterogeneous origin may contribute to neuronal diversity of the septal nuclear complex.

Direct Interactions Between Gli3, Wnt8b, and Fgfs Underlie Patterning of the Dorsal Telencephalon

A key step in the development of the cerebral cortex is a patterning process, which subdivides the telencephalon into several molecularly distinct domains and is critical for cortical arealization. This process is dependent on a complex network of interactions between signaling molecules of the Fgf and Wnt gene families and the Gli3 transcription factor gene, but a better knowledge of the molecular basis of the interplay between these factors is required to gain a deeper understanding of the genetic circuitry underlying telencephalic patterning. Using DNA-binding and reporter gene assays, we here investigate the possibility that Gli3 and these signaling molecules interact by directly regulating each other's expression. We show that Fgf signaling is required for Wnt8b enhancer activity in the cortical hem, whereas Wnt/β-catenin signaling represses Fgf17 forebrain enhancer activity. In contrast, Fgf and Wnt/β-catenin signaling cooperate to regulate Gli3 expression. Taken together, these findings indicate that mutual interactions between Gli3, Wnt8b, and Fgf17 are crucial elements of the balance between these factors thereby conferring robustness to the patterning process. Hence, our study provides a framework for understanding the genetic circuitry underlying telencephalic patterning and how defects in this process can affect the formation of cortical areas.

Lateral Thalamic Eminence: A Novel Origin for mGluR1/Lot Cells

A unique population of cells, called "lot cells," circumscribes the path of the lateral olfactory tract (LOT) in the rodent brain and acts to restrict its position at the lateral margin of the telencephalon. Lot cells were believed to originate in the dorsal pallium (DP). We show that Lhx2 null mice that lack a DP show a significant increase in the number of mGluR1/lot cells in the piriform cortex, indicating a non-DP origin of these cells. Since lot cells present common developmental features with Cajal-Retzius (CR) cells, we analyzed Wnt3a- and Dbx1-reporter mouse lines and found that mGluR1/lot cells are not generated in the cortical hem, ventral pallium, or septum, the best characterized sources of CR cells. Finally, we identified a novel origin for the lot cells by combining in utero electroporation assays and histochemical characterization. We show that mGluR1/lot cells are specifically generated in the lateral thalamic eminence and that they express mitral cell markers, although a minority of them express ΔNp73 instead. We conclude that most mGluR1/lot cells are prospective mitral cells migrating to the accessory olfactory bulb (OB), whereas mGluR1+, ΔNp73+ cells are CR cells that migrate through the LOT to the piriform cortex and the OB.

Transcriptome variations among human embryonic stem cell lines are associated with their differentiation propensity

Human embryonic stem cells (hESCs) have the potential to form any cell type in the body, making them attractive cell sources in drug screening, regenerative medicine, disease and developmental processes modeling. However, not all hESC lines have the equal potency to generate desired cell types in vitro. Significant variations have been observed for the differentiation efficiency of various human ESC lines. The precise underpinning molecular mechanisms are still unclear. In this work, we compared transcriptome variations of four hESC lines H7, HUES1, HUES8 and HUES9. We found that hESC lines have different gene expression profiles, and these differentially expressed genes (DEGs) are significantly enriched in developmental processes, such as ectodermal, mesodermal and endodermal development. The enrichment difference between hESC lines was consistent with its lineage bias. Among these DEGs, some pluripotency factors and genes involved in signaling transduction showed great variations as well. The pleiotropic functions of these genes in controlling hESC identity and early lineage specification, implicated that different hESC lines may utilize distinct balance mechanisms to maintain pluripotent state. When the balance is broken in a certain environment, gene expression variation between them could impact on their different lineage specification behavior.

Primary Cilia in Brain Development and Diseases

The primary cilium, a sensory appendage that is present in most mammalian cells, plays critical roles in signaling pathways and cell cycle progression. Mutations that affect the structure or function of primary cilia result in ciliopathies, a group of developmental and degenerative diseases that affect almost all organs and tissues. Our understanding of the constituents, development, and function of primary cilia has advanced considerably in recent years, revealing pathogenic mechanisms that potentially underlie ciliopathies. In the brain, the primary cilia are crucial for early patterning, neurogenesis, neuronal maturation and survival, and tumorigenesis, mostly through regulating cell cycle progression, Hedgehog signaling, and WNT signaling. We review these advances in our knowledge of primary cilia, focusing on brain development, and discuss the mechanisms that may underlie brain abnormalities in ciliopathies.

The Transcription Factor Foxg1 Promotes Optic Fissure Closure in the Mouse by Suppressing Wnt8b in the Nasal Optic Stalk

During vertebrate eye morphogenesis, a transient fissure forms at its inferior part, known as the optic fissure. This will gradually close, giving rise to a healthy, spherical optic cup. Failure of the optic fissure to close gives rise to an ocular disorder known as coloboma. During this developmental process, Foxg1 is expressed in the optic neuroepithelium, with highest levels of expression in the nasal optic stalk. Foxg1-/- mutant mice have microphthalmic eyes with a large ventral coloboma. We found Wnt8b expression upregulated in the Foxg1-/- optic stalk and hypothesized that, similar to what is observed in telencephalic development, Foxg1 directs development of the optic neuroepithelium through transcriptional suppression of Wnt8b To test this, we generated Foxg1-/-;Wnt8b-/- double mutants of either sex and found that the morphology of the optic cup and stalk and the closure of the optic fissure were substantially rescued in these embryos. This rescue correlates with restored Pax2 expression in the anterior tip of the optic fissure. In addition, although we do not find evidence implicating altered proliferation in the rescue, we observe a significant increase in apoptotic cell density in Foxg1-/-;Wnt8b-/- double mutants compared with the Foxg1-/- single mutant. Upregulation of Wnt/β-catenin target molecules in the optic cup and stalk may underlie the molecular and morphological defects in the Foxg1-/- mutant. Our results show that proper optic fissure closure relies on Wnt8b suppression by Foxg1 in the nasal optic stalk to maintain balanced apoptosis and Pax2 expression in the nasal and temporal edges of the fissure.SIGNIFICANCE STATEMENT Coloboma is an ocular disorder that may result in a loss of visual acuity and accounts for ∼10% of childhood blindness. It results from errors in the sealing of the optic fissure (OF), a transient structure at the bottom of the eye. Here, we investigate the colobomatous phenotype of the Foxg1-/- mutant mouse. We identify upregulated expression of Wnt8b in the optic stalk of Foxg1-/- mutants before OF closure initiates. Foxg1-/-;Wnt8b-/- double mutants show a substantial rescue of the Foxg1-/- coloboma phenotype, which correlates with a rescue in molecular and cellular defects of Foxg1-/- mutants. Our results unravel a new role of Foxg1 in promoting OF closure providing additional knowledge about the molecules and cellular mechanisms underlying coloboma formation.

The Many Hats of Sonic Hedgehog Signaling in Nervous System Development and Disease

Sonic hedgehog (Shh) signaling occurs concurrently with the many processes that constitute nervous system development. Although Shh is mostly known for its proliferative and morphogenic action through its effects on neural stem cells and progenitors, it also contributes to neuronal differentiation, axonal pathfinding and synapse formation and function. To participate in these diverse events, Shh signaling manifests differently depending on the maturational state of the responsive cell, on the other signaling pathways regulating neural cell function and the environmental cues that surround target cells. Shh signaling is particularly dynamic in the nervous system, ranging from canonical transcription-dependent, to non-canonical and localized to axonal growth cones. Here, we review the variety of Shh functions in the developing nervous system and their consequences for neurodevelopmental diseases and neural regeneration, with particular emphasis on the signaling mechanisms underlying Shh action.

The molecular and cellular signatures of the mouse eminentia thalami support its role as a signalling centre in the developing forebrain

The mammalian eminentia thalami (EmT) (or thalamic eminence) is an embryonic forebrain structure of unknown function. Here, we examined the molecular and cellular properties of the mouse EmT. We first studied mRNA expression of signalling molecules and found that the EmT is a structure, rich in expression of secreted factors, with Wnts being the most abundantly detected. We then examined whether EmT tissue could induce cell fate changes when grafted ectopically. For this, we transplanted EmT tissue from a tau-GFP mouse to the ventral telencephalon of a wild type host, a telencephalic region where Wnt signalling is not normally active but which we showed in culture experiments is competent to respond to Wnts. We observed that the EmT was able to induce in adjacent ventral telencephalic cells ectopic expression of Lef1, a transcriptional activator and a target gene of the Wnt/β-catenin pathway. These Lef1-positive;GFP-negative cells expressed the telencephalic marker Foxg1 but not Ascl1, which is normally expressed by ventral telencephalic cells. These results suggest that the EmT has the capacity to activate Wnt/β-catenin signalling in the ventral telencephalon and to suppress ventral telencephalic gene expression. Altogether, our data support a role of the EmT as a signalling centre in the developing mouse forebrain.

Suppressor of Fused Is Critical for Maintenance of Neuronal Progenitor Identity during Corticogenesis

Proper lineage progression and diversification of neural progenitor cells (NPCs) ensures the generation of projection neuron (PN) subtypes in the mammalian neocortex. Here, we show that Suppressor of Fused (Sufu) controls PN specification by maintaining the identity of NPCs in the embryonic neocortex. Deletion of Sufu in NPCs of the E10.5 mouse neocortex led to improper specification of progenitors and a reduction in intermediate progenitors (IPs) during corticogenesis. We found that Sufu deletion resulted in unstable Gli2 and Gli3 activity, leading to the ectopic activation of Sonic hedgehog (Shh) signaling. The role of Sufu in maintaining progenitor identity is critical at early stages of corticogenesis, since deletion of Sufu at E13.5 did not cause similar abnormalities. Our studies revealed that Sufu critically modulates Shh signaling at early stages of neurogenesis for proper specification and maintenance of cortical NPCs to ensure the appropriate generation of cortical PN lineages.

The role of primary cilia in corpus callosum formation is mediated by production of the Gli3 repressor

Agenesis of the corpus callosum (AgCC) is a frequent brain disorder found in over 80 human congenital syndromes including ciliopathies. Here, we report a severe AgCC in Ftm/Rpgrip1l knockout mouse, which provides a valuable model for Meckel-Grüber syndrome. Rpgrip1l encodes a protein of the ciliary transition zone, which is essential for ciliogenesis in several cell types in mouse including neuroepithelial cells in the developing forebrain. We show that AgCC in Rpgrip1l(-/-) mouse is associated with a disturbed location of guidepost cells in the dorsomedial telencephalon. This mislocalization results from early patterning defects and abnormal cortico-septal boundary (CSB) formation in the medial telencephalon. We demonstrate that all these defects primarily result from altered GLI3 processing. Indeed, AgCC, together with patterning defects and mispositioning of guidepost cells, is rescued by overexpressing in Rpgrip1l(-/-) embryos, the short repressor form of the GLI3 transcription factor (GLI3R), provided by the Gli3(Δ699) allele. Furthermore, Gli3(Δ699) also rescues AgCC in Rfx3(-/-) embryos deficient for the ciliogenic RFX3 transcription factor that regulates the expression of several ciliary genes. These data demonstrate that GLI3 processing is a major outcome of primary cilia function in dorsal telencephalon morphogenesis. Rescuing CC formation in two independent ciliary mutants by GLI3(Δ699) highlights the crucial role of primary cilia in maintaining the proper level of GLI3R required for morphogenesis of the CC.

Gli3 repressor controls cell fates and cell adhesion for proper establishment of neurogenic niche

Neural stem cells (NSCs) in the subventricular zone (SVZ) rely on environmental signals provided by the neurogenic niche for their proper function. However, little is known about the initial steps of niche establishment, as embryonic radial glia transition to postnatal NSCs. Here, we identify Gli3 repressor (Gli3R), a component of the Sonic hedgehog (Shh) pathway, as a critical factor controlling both cell-type specification and structural organization of the developing SVZ. We demonstrate that Gli3R expressed in radial glia temporally regulates gp130/STAT3 signaling at the transcriptional level to suppress glial characteristics in differentiating ependymal cells. In addition, Gli3R maintains the proper level of Numb in ependymal cells to allow localization of cell adhesion molecules such as vascular cell adhesion molecule (VCAM) and E-cadherin. Thus, our findings reveal a role for Gli3R as a mediator of niche establishment and provide insights into the conditions required for proper SVZ neurogenic niche formation.

Expansion of the piriform cortex contributes to corticothalamic pathfinding defects in Gli3 conditional mutants

The corticothalamic and thalamocortical tracts play essential roles in the communication between the cortex and thalamus. During development, axons forming these tracts have to follow a complex path to reach their target areas. While much attention has been paid to the mechanisms regulating their passage through the ventral telencephalon, very little is known about how the developing cortex contributes to corticothalamic/thalamocortical tract formation. Gli3 encodes a zinc finger transcription factor widely expressed in telencephalic progenitors which has important roles in corticothalamic and thalamocortical pathfinding. Here, we conditionally inactivated Gli3 in dorsal telencephalic progenitors to determine its role in corticothalamic tract formation. In Emx1Cre;Gli3(fl/fl) mutants, only a few corticothalamic axons enter the striatum in a restricted dorsal domain. This restricted entry correlates with a medial expansion of the piriform cortex. Transplantation experiments showed that the expanded piriform cortex repels corticofugal axons. Moreover, expression of Sema5B, a chemorepellent for corticofugal axons produced by the piriform cortex, is similarly expanded. Finally, time course analysis revealed an expansion of the ventral pallial progenitor domain which gives rise to the piriform cortex. Hence, control of lateral cortical development by Gli3 at the progenitor level is crucial for corticothalamic pathfinding.

Foxg1 is required to limit the formation of ciliary margin tissue and Wnt/β-catenin signalling in the developing nasal retina of the mouse

The ciliary margin (CM) develops in the peripheral retina and gives rise to the iris and the ciliary body. The Wnt/β-catenin signalling pathway has been implicated in ciliary margin development. Here, we tested the hypothesis that in the developing mouse retina Foxg1 is responsible for suppressing the Wnt/β-catenin pathway and restricting CM development. We showed that there is excess CM tissue in Foxg1^−/− null embryos and this expansion is more pronounced in the nasal retina where Foxg1 normally shows its highest expression levels. Results on expression of a reporter allele for Wnt/β-catenin signalling and of Lef1, a target of Wnt/β-catenin signalling, displayed significant upregulation of this pathway in Foxg1^−/− nulls at embryonic days 12.5 and 14.5. Interestingly, this upregulation was observed specifically in the nasal retina, where normally very few Wnt-responsive cells are observed. These results indicate a suppressive role of Foxg1 on this signalling pathway. Our results reveal a new role of Foxg1 in limiting CM development in the nasal peripheral retina and add a new molecular player in the developmental network involved in CM specification.

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Foxg1 is expressed in a nasal-high to temporal-low gradient in developing retina.

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Ciliary margin expansion is observed nasally in the Foxg1^−/− mutant retina.

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Wnt/β-catenin signalling is upregulated in the Foxg1^−/− peripheral retina nasally.

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A new role of Foxg1 in controlling ciliary margin development is proposed.

Gli3 is required in Emx1+ progenitors for the development of the corpus callosum

The corpus callosum (CC) is the largest commissure in the forebrain and mediates the transfer of sensory, motor and cognitive information between the cerebral hemispheres. During CC development, a number of strategically located glial and neuronal guidepost structures serve to guide callosal axons across the midline at the corticoseptal boundary (CSB). Correct positioning of these guideposts requires the Gli3 gene, mutations of which result in callosal defects in humans and mice. However, as Gli3 is widely expressed during critical stages of forebrain development, the precise temporal and spatial requirements for Gli3 function in callosal development remain unclear. Here, we used a conditional mouse mutant approach to inactivate Gli3 in specific regions of the developing telencephalon in order to delineate the domain(s) in which Gli3 is required for normal development of the corpus callosum. Inactivation of Gli3 in the septum or in the medial ganglionic eminence had no effect on CC formation, however Gli3 inactivation in the developing cerebral cortex led to the formation of a severely hypoplastic CC at E18.5 due to a severe disorganization of midline guideposts. Glial wedge cells translocate prematurely and Slit1/2 are ectopically expressed in the septum. These changes coincide with altered Fgf and Wnt/β-catenin signalling during CSB formation. Collectively, these data demonstrate a crucial role for Gli3 in cortical progenitors to control CC formation and indicate how defects in CSB formation affect the positioning of callosal guidepost cells.

Ofd1 controls dorso-ventral patterning and axoneme elongation during embryonic brain development

Oral-facial-digital type I syndrome (OFDI) is a human X-linked dominant-male-lethal developmental disorder caused by mutations in the OFD1 gene. Similar to other inherited disorders associated to ciliary dysfunction OFD type I patients display neurological abnormalities. We characterized the neuronal phenotype that results from Ofd1 inactivation in early phases of mouse embryonic development and at post-natal stages. We determined that Ofd1 plays a crucial role in forebrain development, and in particular, in the control of dorso-ventral patterning and early corticogenesis. We observed abnormal activation of Sonic hedgehog (Shh), a major pathway modulating brain development. Ultrastructural studies demonstrated that early Ofd1 inactivation results in the absence of ciliary axonemes despite the presence of mature basal bodies that are correctly orientated and docked. Ofd1 inducible-mediated inactivation at birth does not affect ciliogenesis in the cortex, suggesting a developmental stage-dependent role for a basal body protein in ciliogenesis. Moreover, we showed defects in cytoskeletal organization and apical-basal polarity in Ofd1 mutant embryos, most likely due to lack of ciliary axonemes. Thus, the present study identifies Ofd1 as a developmental disease gene that is critical for forebrain development and ciliogenesis in embryonic life, and indicates that Ofd1 functions after docking and before elaboration of the axoneme in vivo.

Primary cilia and forebrain development

With a microtubule-based axoneme supporting its plasma membrane-ensheathed projection from the basal body of almost all cell types in the human body, and present in only one copy per cell, the primary cilium can be considered an organelle sui generis. Although it was first observed and recorded in histological studies from the late 19th century, the tiny structure was essentially forgotten for many decades. In the past ten years, however, scientists have turned their eyes once again upon primary cilia and realized that they are very important for the development of almost all organs in the mammalian body, especially those dependent upon the signaling from members Hedgehog family, such as Indian and Sonic hedgehog. In this review, we outline the roles that primary cilia play in forebrain development, not just in the crucial transduction of Sonic hedgehog signaling, but also new results showing that cilia are important for cell cycle progression in proliferating neural precursors. We will focus upon cerebral cortex development but will also discuss the importance of cilia for the embryonic hippocampus, olfactory bulb, and diencephalon.

Gli3 controls corpus callosum formation by positioning midline guideposts during telencephalic patterning

The corpus callosum (CC) represents the major forebrain commissure connecting the 2 cerebral hemispheres. Midline crossing of callosal axons is controlled by several glial and neuronal guideposts specifically located along the callosal path, but it remains unknown how these cells acquire their position. Here, we show that the Gli3 hypomorphic mouse mutant Polydactyly Nagoya (Pdn) displays agenesis of the CC and mislocation of the glial and neuronal guidepost cells. Using transplantation experiments, we demonstrate that agenesis of the CC is primarily caused by midline defects. These defects originate during telencephalic patterning and involve an up-regulation of Slit2 expression and altered Fgf and Wnt/β-catenin signaling. Mutations in sprouty1/2 which mimic the changes in these signaling pathways cause a disorganization of midline guideposts and CC agenesis. Moreover, a partial recovery of midline abnormalities in Pdn/Pdn;Slit2(-/-) embryos mutants confirms the functional importance of correct Slit2 expression levels for callosal development. Hence, Gli3 controlled restriction of Fgf and Wnt/β-catenin signaling and of Slit2 expression is crucial for positioning midline guideposts and callosal development.

The doublesex homolog Dmrt5 is required for the development of the caudomedial cerebral cortex in mammals

Regional patterning of the cerebral cortex is initiated by morphogens secreted by patterning centers that establish graded expression of transcription factors within cortical progenitors. Here, we show that Dmrt5 is expressed in cortical progenitors in a high-caudomedial to low-rostrolateral gradient. In its absence, the cortex is strongly reduced and exhibits severe abnormalities, including agenesis of the hippocampus and choroid plexus and defects in commissural and thalamocortical tracts. Loss of Dmrt5 results in decreased Wnt and Bmp in one of the major telencephalic patterning centers, the dorsomedial telencephalon, and in a reduction of Cajal-Retzius cells. Expression of the dorsal midline signaling center-dependent transcription factors is downregulated, including Emx2, which promotes caudomedial fates, while the rostral determinant Pax6, which is inhibited by midline signals, is upregulated. Consistently, Dmrt5(-/-) brains exhibit patterning defects with a dramatic reduction of the caudomedial cortex. Dmrt5 is increased upon the activation of Wnt signaling and downregulated in Gli3(xt/xt) mutants. We conclude that Dmrt5 is a novel Wnt-dependent transcription factor required for early cortical development and that it may regulate initial cortical patterning by promoting dorsal midline signaling center formation and thereby helping to establish the graded expression of the other transcription regulators of cortical identity.

Gli3 controls subplate formation and growth of cortical axons

The formation of a functional cortical circuitry requires the coordinated growth of cortical axons to their target areas. While the mechanisms guiding cortical axons to their targets have extensively been studied, very little is known about the processes which promote their growth in vivo. Gli3 encodes a zinc finger transcription factor which is expressed in cortical progenitor cells and has crucial roles in cortical development. Here, we characterize the Gli3 compound mutant Gli3(Xt/Pdn), which largely lacks Neurofilament(+) fibers in the rostral and intermediate neocortex. DiI labeling and Golli-τGFP immunofluorescence indicate that Gli3(Xt/Pdn) cortical neurons form short and stunted axons. Using transplantation experiments we demonstrate that this axon growth defect is primarily caused by a nonpermissive cortical environment. Furthermore, in Emx1Cre;Gli3(Pdn/fl) conditional mutants, which mimic the reduction of Gli3 expression in the dorsal telencephalon of Gli3(Xt/Pdn) embryos, the growth of cortical axons is not impaired, suggesting that Gli3 controls this process early in telencephalic development. In contrast to cortical plate neurons, Gli3(Xt/Pdn) embryos largely lack subplate (SP) neurons which normally pioneer cortical projections. Collectively, these findings show that Gli3 specifies a cortical environment permissive to the growth of cortical axons at the progenitor level by controlling the formation of SP neurons.

Primary cilia and Gli3 activity regulate cerebral cortical size

During neural development patterning, neurogenesis, and overall growth are highly regulated and coordinated between different brain regions. Here, we show that primary cilia and the regulation of Gli activity are necessary for the normal expansion of the cerebral cortex. We show that loss of Kif3a, an important functional component of primary cilia, leads to the degeneration of primary cilia, marked overgrowth of the cortex, and altered cell cycle kinetics within cortical progenitors. The G1 phase of the cell cycle is shortened through a mechanism likely involving reduced Gli3 activity and a resulting increase in expression of cyclin D1 and Fgf15. The defects in Gli3 activity alone are sufficient to accelerate cell cycle kinetics and cause the molecular changes seen in brains that lack cilia. Finally, we show that levels of full-length and repressor Gli3 proteins are tightly regulated during normal development and correlate with changes in expression of two known Shh-target genes, CyclinD1 and Fgf15, and with the normal lengthening of the cell cycle during corticogenesis. These data suggest that Gli3 activity is regulated through the primary cilium to control cell cycle length in the cortex and thus determine cortical size.

Mouse thalamic differentiation: gli-dependent pattern and gli-independent prepattern

Sonic hedgehog (Shh) signaling is essential for thalamic development. The Gli transcription factors act downstream of Shh – while Gli2 is the major activator (GliA), Gli3 acts primarily as a repressor (GliR). The thalamus is remarkable among dorsal structures because of its proximity to the mid-diencephalic organizer, a unique dorsal Shh source. This lends complexity to the interactions between Shh, Gli2, and Gli3, suggesting the presence of a dorsal Gli activator which elsewhere is found only ventrally, and making the dissection of thalamic Gli functions particularly interesting. A current model based on mutant phenotypes in telencephalon and midbrain postulates a degree of reciprocal antagonism of Shh and Gli3 in dorsal brain regions. To approach the role of Gli factors in thalamic specification we first analyzed mice deficient in Gli2 or Gli3. In Gli2 mutants, the thalamus is small and poorly differentiated with the exception of the medial and intralaminar nuclei which, in contrast, are specifically and severely affected by Gli3 inactivation. Gbx2 expression is very reduced in the Gli3 mutant. Most thalamic nuclei are present in both mutants, although incompletely differentiated, as reflected by the loss of specific markers. The ventral posterior group, revealed by novel specific marker Hes1, is present in both mutants and extends axons to the telencephalon. To test the Gli3/Shh interaction we generated a novel mutant deficient in Gli3 and neuroepithelial Shh. The thalamus of the n-Shh/Gli3 double mutants is very large and very poorly differentiated except for a broad domain of Gbx2, Lhx2, and Calb2 expression. In utero electroporation experiments on wild type embryos suggest that a stage-specific factor acting early is responsible for this prepattern. We show that, in the thalamus, GliA acts downstream of Shh to specify pattern and size of the thalamic nuclei to the exception of the medial and intralaminar groups. Gli3A can partially substitute for Gli2A in the Gli2 mutant. GliR is essential for specification and growth of the medial and intralaminar nuclei, contributes to the specification of other thalamic nuclei and reduces thalamic size. GliA (from neuroepithelial Shh signaling) and GliR do not show reciprocal antagonism in the thalamus, and their joint abolition does not rescue the wild type phenotype.

Transcriptional analysis of Gli3 mutants identifies Wnt target genes in the developing hippocampus

Early development of the hippocampus, which is essential for spatial memory and learning, is controlled by secreted signaling molecules of the Wnt gene family and by Wnt/β-catenin signaling. Despite its importance, little is known, however, about Wnt-regulated genes during hippocampal development. Here, we used the Gli3 mutant mouse extra-toes (Xt(J)), in which Wnt gene expression in the forebrain is severely affected, as a tool in a microarray analyses to identify potential Wnt target genes. This approach revealed 53 candidate genes with restricted or graded expression patterns in the dorsomedial telencephalon. We identified conserved Tcf/Lef-binding sites in telencephalon-specific enhancers of several of these genes, including Dmrt3, Gli3, Nfia, and Wnt8b. Binding of Lef1 to these sites was confirmed using electrophoretic mobility shift assays. Mutations in these Tcf/Lef-binding sites disrupted or reduced enhancer activity in vivo. Moreover, ectopic activation of Wnt/β-catenin signaling in an ex vivo explant system led to increased telencephalic expression of these genes. Finally, conditional inactivation of Gli3 results in defective hippocampal growth. Collectively, these data strongly suggest that we have identified a set of direct Wnt target genes in the developing hippocampus and provide inside into the genetic hierarchy underlying Wnt-regulated hippocampal development.

Wnt/β-catenin signalling is active in a highly dynamic pattern during development of the mouse cerebellum

The adult cerebellum is composed of several distinct cell types with well defined developmental origins. However, the molecular mechanisms that govern the generation of these cell types are only partially resolved. Wnt/β-catenin signalling has a wide variety of roles in generation of the central nervous system, though the specific activity of this pathway during cerebellum development is not well understood. Here, we present data that delineate the spatio-temporal specific pattern of Wnt/β-catenin signaling during mouse cerebellum development between E12.5 and P21. Using the BAT-gal Wnt/β-catenin reporter mouse, we found that Wnt/β-catenin activity is present transiently at the embryonic rhombic lip but not at later stages during the expansion of cell populations that arise from there. At late embryonic and early postnatal stages, Wnt/β-catenin activity shifts to the cerebellar ventricular zone and to cells arising from this germinal centre. Subsequently, the expression pattern becomes progressively restricted to Bergmann glial cells, which show expression of the reporter at P21. These results indicate a variety of potential functions for Wnt/β-catenin activity during cerebellum development.

Wnt/β-catenin signaling is disrupted in the extra-toes (Gli3(Xt/Xt) ) mutant from early stages of forebrain development, concomitant with anterior neural plate patterning defects

The zinc finger transcription factor Gli3 is essential for normal development of the forebrain. Mutant mice with no functional Gli3 (extra-toes, Gli3(Xt/Xt) mutants) display a massive reduction in the size of the telencephalic lobes and absence of dorsomedial telencephalic structures, including the cortical hem, which normally expresses a number of Wnt molecules essential for patterning the hippocampus. Dorsomedial telencephalic Wnt activity, transduced through the Wnt/β-catenin signaling pathway, is also required for hippocampal specification and dorsoventral telencephalic patterning. Wnts whose normal expression is restricted to the cortical hem are completely absent in Gli3(Xt/Xt) embryos, but some expression of those Wnts with a broader expression domain persists, raising the possibility that Wnt/β-catenin signaling may still be active in this mutant. We examined whether the Wnt expression that persists in the Gli3(Xt/Xt) mutant neocortex activates Wnt/β-catenin signaling, using the BAT-gal transgenic reporter. We found Wnt/β-catenin signaling consistently decreased in the forebrains of Gli3(Xt/Xt) mutants, even prior to the formation of the cortical hem. This is accompanied by a severe reduction in expression of Wnt7b and Wnt8b at the lateral edges of the anterior neural plate that will give rise to the pallium. In addition, we found a significant increase in the expression of rostroventral markers of the anterior neural plate that will give rise to the basal forebrain. Our data reveal that Gli3 is required at the neural plate stage to regulate Wnt expression and Wnt/β-catenin signaling in the presumptive forebrain and confirm its previously proposed role in patterning the anterior neural plate.

Primary cilia and organogenesis: is Hedgehog the only sculptor?

The primary cilium is a small microtubule-based organelle projecting from the plasma membrane of practically all cells in the mammalian body. In the past 8 years, a flurry of papers has indicated a crucial role of this long-neglected organelle in the development of a wide variety of organs, including derivatives of all three germ layers. A common theme of these studies is the critical dependency of signal transduction of the Hedgehog pathway upon functionally intact cilia to regulate organogenesis. Another common theme is the role that the cilium plays, not necessarily in the determination of the embryonic anlagen of these organs, although this too occurs but rather in the proliferation and morphogenesis of the previously determined organ. We outline the various organ systems that are dependent upon primary cilia for their proper development and we discuss the cilia-dependent roles that Sonic and Indian Hedgehog play in these processes. In addition and most importantly for the field, we discuss the controversial involvement of another major developmental pathway, Wnt signaling, in cilia-dependent organogenesis.

Gli3 is required for maintenance and fate specification of cortical progenitors

Gli3, one of three vertebrate Gli transcription factors in Hedgehog (Hh) pathway, is processed into a repressor form (Gli3R) in the absence of Hh signal and acts as the major negative transducer of the pathway. Although the role of Gli3 in embryonic patterning has been extensively studied, its role in cortical neurogenesis, especially in the regulation of neural progenitors in proliferation and cell fate specification, is largely unknown. To bypass the patterning defects caused by loss of Gli3, we conditionally deleted Gli3 after patterning was complete in mouse. Our results from birthdating and in utero electroporation experiments demonstrate that the Gli3, specifically Gli3R, is critical for specifying the fate of cortical neurons that are generated following a stereotypical temporal order. Moreover, Gli3 is required for maintaining the cortical progenitors in active cell cycle, suggesting that cells may acquire differentiated status as they turn off Gli3 expression during neurogenesis.

The Gli3 hypomorphic mutation Pdn causes selective impairment in the growth, patterning, and axon guidance capability of the lateral ganglionic eminence

Previous studies have defined a requirement for Sonic hedgehog (Shh) signaling in patterning the ventral telencephalon, a major source of the neuronal diversity found in the mature telencephalon. The zinc finger transcription factor Gli3 is a critical component of the Shh signaling pathway and its loss causes major defects in telencephalic development. Gli3 is expressed in a graded manner along the dorsoventral axis of the telencephalon but it is unknown whether Gli3 expression levels are important for dorsoventral telencephalic patterning. To address this, we used the Gli3 hypomorphic mouse mutant Polydactyly Nagoya (Pdn). We show that in Pdn/Pdn embryos, the telencephalic expression of Gli3 remains graded, but Gli3 mRNA and protein levels are reduced, resulting in an upregulation of Shh expression and signaling. These changes mainly affect the development of the lateral ganglionic eminence (LGE), with some disorganization of the medial ganglionic eminence mantle zone. The pallial/subpallial boundary is shifted dorsally and the production of postmitotic neurons is reduced. Moreover, LGE pioneer neurons that guide corticofugal axons into the LGE do not form properly, delaying the entry of corticofugal axons into the ventral telencephalon. Pdn/Pdn mutants also show severe pathfinding defects of thalamocortical axons in the ventral telencephalon. Transplantation experiments demonstrate that the intrinsic ability of the Pdn ventral telencephalon to guide thalamocortical axons is compromised. We conclude that correct Gli3 levels are particularly important for the LGE's growth, patterning, and development of axon guidance capabilities.

Sonic hedgehog functions through dynamic changes in temporal competence in the developing forebrain

Morphogens act during development to provide graded spatial information that controls patterning and cell lineage specification in the nervous system. The role of morphogen signaling in instructing the expression of downstream effector transcription factors has been well established. However, a key requirement for morphogen signaling is the existence of functional intracellular machinery able to mediate the appropriate response in target cells. Here we suggest that dynamic changes in the temporal responses to Shh in the developing ventral telencephalon occur through alterations in progenitor competence. We suggest these developmental changes in competence are mediated by a transcriptional mechanism that intrinsically integrates information from the distinct signaling pathways that act to pattern the telencephalic neuroepithelium.

Loss of Wnt8b has no overt effect on hippocampus development but leads to altered Wnt gene expression levels in dorsomedial telencephalon

Wnt signalling proteins regulate many aspects of animal development. We have investigated the function of mouse Wnt8b during forebrain development. Wnt8b is expressed in a highly restricted pattern including the prospective hippocampus and hypothalamus. Mutant mice lacking Wnt8b are viable and healthy. The size and morphology of the hippocampus appeared normal in mutant embryos and adults, and we found no evidence of hypothalamic defects in mutants. Wnt8b is also expressed in the neurogenic region of the adult dentate gyrus, however, cell proliferation was unchanged in Wnt8b(-/-) mutants. Mutant embryos did, however, display altered levels of expression of other Wnt genes normally expressed in forebrain. The spatial expression patterns of other Wnt genes and the overall level of canonical Wnt activity were indistinguishable from wild-types. Thus, loss of Wnt8b does not give rise to an overt morphological phenotype, but does affect expression levels of other Wnts in developing forebrain.

The transcription factor Foxg1 regulates the competence of telencephalic cells to adopt subpallial fates in mice

Foxg1 is required for development of the ventral telencephalon in the embryonic mammalian forebrain. Although one existing hypothesis suggests that failed ventral telencephalic development in the absence of Foxg1 is due to reduced production of the morphogens sonic hedgehog (Shh) and fibroblast growth factor 8 (Fgf8), the possibility that telencephalic cells lacking Foxg1 are intrinsically incompetent to generate the ventral telencephalon has remained untested. We examined the ability of Foxg1(-/-) telencephalic cells to respond to Shh and Fgf8 by examining the expression of genes whose activation requires Shh or Fgf8 in vivo and by testing their responses to Shh and Fgf8 in culture. We found that many elements of the Shh and Fgf8 signalling pathways continue to function in the absence of Foxg1 but, nevertheless, we were unable to elicit normal responses of key ventral telencephalic marker genes in Foxg1(-/-) telencephalic tissue following a range of in vivo and in vitro manipulations. We explored the development of Foxg1(-/-) cells in Foxg1(-/-) Foxg1(+/+) chimeric embryos that contained ventral telencephalon created by normally patterned wild-type cells. We found that Foxg1(-/-) cells contributed to the chimeric ventral telencephalon, but that they retained abnormal specification, expressing dorsal rather than ventral telencephalic markers. These findings indicate that, in addition to regulating the production of ventralising signals, Foxg1 acts cell-autonomously in the telencephalon to ensure that cells develop the competence to adopt ventral identities.

Role of primary cilia in brain development and cancer

The primary cilium, a hair-like extension from a cell's surface, acts as a sensory organelle to receive signals that regulate cellular behavior and physiology. Like most mammalian cells, neural progenitors and neurons have primary cilia. Recent studies show that this tiny projection plays important roles in brain development and diseases. Ciliary mutant mice show defects in brain patterning, progenitor proliferation, and specification of adult neural stem cells. Primary cilia also have dual opposing functions in the development of brain tumors. Ciliary defects are associated with genetic syndromes that frequently have neurological symptoms. Understanding the multifaceted roles that primary cilia have in brain development will provide important insights into the mechanism of brain development and diseases.

Patterning of ventral telencephalon requires positive function of Gli transcription factors

The ability of neuroepithelial cells to generate a diverse array of neurons is influenced by locally secreted signals. In the spinal cord, Sonic Hedgehog (Shh) is known to induce distinct cell fates in a concentration-dependent manner by regulating the activities of the three Gli transcription factors in neural precursors. However, whether Gli-mediated Shh signaling is also required to induce different cell types in the ventral telencephalon has been controversial. In particular, loss of Shh has little effect on dorsoventral patterning of the telencephalon when Gli3 is also removed. Furthermore, no ventral telencephalic phenotypes have been found in individual Gli mutants. To address this issue, we first characterized Shh-responding ventral telencephalic progenitors between E9.5 and E12.5 and found that they produce neurons migrating to different layers of the cortex. We also discovered a loss of Nkx2.1 and Nkx6.2 expression in two subgroups of progenitors in embryos lacking major Gli activators. Finally, we analyzed the telencephalic phenotypes of embryos lacking all Gli genes and found that the ventral telencephalon was highly disorganized with intermingling of distinct neuronal cell types. Together, these studies unravel a role for Gli transcription factors in mediating Shh signaling to control specification, differentiation and positioning of ventral telencephalic neurons.

Regulation of mouse embryonic stem cell neural differentiation by retinoic acid

Pluripotent mouse embryonic stem cells (ESCs) derived from the early blastocyst can differentiate in vitro into a variety of somatic cell types including lineages from all three embryonic germ layers. Protocols for ES cell neural differentiation typically involve induction by retinoic acid (RA), or by exposure to growth factors or medium conditioned by other cell types. A serum-free differentiation (SFD) medium completely lacking exogenous retinoids was devised that allows for efficient conversion of aggregated mouse ESCs into neural precursors and immature neurons. Neural cells produced in this medium express neuronal ion channels, establish polarity, and form functional excitatory and inhibitory synapses. Brief exposure to RA during the period of cell aggregation speeds neuronal maturation and suppresses cell proliferation. Differentiation without RA yields neurons and neural progenitors with apparent telencephalic identity, whereas cells differentiated with exposure to RA express markers of hindbrain and spinal cord. Transcriptional profiling indicates a substantial representation of transit amplifying neuroblasts in SFD cultures not exposed to RA.

Analysis of early ventral telencephalic defects in mice lacking functional Gli3 protein

The transcription factor Gli3 is expressed throughout developing telencephalon. Previous studies have focused on Gli3's role in dorsal telencephalon, which is greatly reduced in size in Gli3(Xt/Xt) mutants. We examined the effects of loss of Gli3 on early development of ventral telencephalon. Ventral telencephalon was defined in both wildtypes and Gli3(Xt/Xt) mutants on the basis of its expression of Olig2, Nkx2.1, Mash1, and Foxg1 and its lack of expression of Pax6. We found that at embryonic day (E)10.5 the volume of the ventral telencephalon is about 50% greater in Gli3(Xt/Xt) mutants than in wildtypes. By E12.5, however, the volume of the ventral telencephalon is about 20% lower in Gli3(Xt/Xt) mutants than in wildtypes. We observed a significant increase in the number of both apoptotic cells and newly differentiated neurons in the E10.5 Gli3(Xt/Xt) ventral telencephalon, suggesting that increased cell death and withdrawal of cells from the cell cycle might account for the failure of the Gli3(Xt/Xt) ventral telencephalon to grow normally by E12.5. We found no changes in the lengths of the cell cycles of proliferating ventral telencephalic cells at E10.5. We used marker analysis and optical projection tomography to assess the Gli3(Xt/Xt) forebrain in three dimensions and found that the Gli3(Xt/Xt) diencephalon is shifted relatively rostrally. We conclude that in the absence of Gli3 an abnormally large portion of the newly formed telencephalon is specified to a ventral fate but this then suffers impaired growth, due to defects of cell differentiation and death, contributing to severe distortion of the forebrain.

A crucial role for primary cilia in cortical morphogenesis

Primary cilia are important sites of signal transduction involved in a wide range of developmental and postnatal functions. Proteolytic processing of the transcription factor Gli3, for example, occurs in primary cilia, and defects in intraflagellar transport (IFT), which is crucial for the maintenance of primary cilia, can lead to severe developmental defects and diseases. Here we report an essential role of primary cilia in forebrain development. Uncovered by N-ethyl-N-nitrosourea-mutagenesis, cobblestone is a hypomorphic allele of the IFT gene Ift88, in which Ift88 mRNA and protein levels are reduced by 70-80%. cobblestone mutants are distinguished by subpial heterotopias in the forebrain. Mutants show both severe defects in the formation of dorsomedial telencephalic structures, such as the choroid plexus, cortical hem and hippocampus, and also a relaxation of both dorsal-ventral and rostral-caudal compartmental boundaries. These defects phenocopy many of the abnormalities seen in the Gli3 mutant forebrain, and we show that Gli3 proteolytic processing is reduced, leading to an accumulation of the full-length activator isoform. In addition, we observe an upregulation of canonical Wnt signaling in the neocortex and in the caudal forebrain. Interestingly, the ultrastructure and morphology of ventricular cilia in the cobblestone mutants remains intact. Together, these results indicate a critical role for ciliary function in the developing forebrain.

Gli3 is required autonomously for dorsal telencephalic cells to adopt appropriate fates during embryonic forebrain development

The Gli3 zinc finger transcription factor is expressed in developing forebrain, with the highest levels of expression in dorsal telencephalon. In Gli3(-/-) embryos the dorsal telencephalon is abnormally small and fails to develop dorsomedial telencephalic structures, including hippocampus and cortical hem, while the ventral telencephalon appears to expand. A hurdle to understanding the underlying mechanisms is that abnormalities of developing Gli3(-/-) telencephalic cells in Gli3(-/-) mutants result from a combination of their own cell autonomous defects and defects in the Gli3(-/-) cells that surround them. Here we used chimeras to identify some of the defects of Gli3(-/-) telencephalic cells that are likely to be autonomous by studying how Gli3(-/-) cells develop when surrounded by a majority of wild-type cells. We found that Gli3(-/-) cells are present in all components of the Gli3(-/-)<-->Gli3(+/+) chimeric forebrain, including dorsomedial structures, in proportions that either equal or exceed proportions found elsewhere in the embryo. Gli3(-/-) cells segregate from Gli3(+/+) cells to form many abnormal structures particularly in dorsal telencephalon. Gli3(-/-) cells in some locations are misspecified: in those parts of the dorsal telencephalon near to its boundaries with the diencephalon and the ventral telencephalon, mutant cells express sets of transcription factors expressed by wild-type cells on the other side of the boundary. Elsewhere in the dorsal telencephalon, in the diencephalon and in the ventral telencephalon, mutant cells express sets of transcription factors similar to those expressed by their immediately surrounding wild-type cells. We propose that an important cell autonomous action of Gli3 is to regulate the competence of dorsal telencephalic cells, preventing cells near to its boundaries expressing regulatory factors normally restricted to adjacent tissues.

Molecular specification and patterning of progenitor cells in the lateral and medial ganglionic eminences

We characterized intrinsic and extrinsic specification of progenitors in the lateral and medial ganglionic eminences (LGE and MGE). We identified seven genes whose expression is enriched or restricted in either the LGE [biregional cell adhesion molecule-related/downregulated by oncogenes binding protein (Boc), Frizzled homolog 8 (Fzd8), Ankrd43 (ankyrin repeat domain-containing protein 43), and Ikzf1 (Ikaros family zinc finger 1)] or MGE [Map3k12 binding inhibitory protein 1 (Mbip); zinc-finger, SWIM domain containing 5 (Zswim5); and Adamts5 [a disintegrin-like and metallopeptidase (reprolysin type) with thrombospondin type 1 motif, 5]]. Boc, Fzd8, Mbip, and Zswim5 are apparently expressed in LGE or MGE progenitors, whereas the remaining three are seen in the postmitotic mantle zone. Relative expression levels are altered and regional distinctions are lost for each gene in LGE or MGE cells propagated as neurospheres, indicating that these newly identified molecular characteristics of LGE or MGE progenitors depend on forebrain signals not available in the neurosphere assay. Analyses of Pax6(Sey/Sey), Shh(-/-), and Gli3(XtJ/XtJ) mutants suggests that LGE and MGE progenitor identity does not rely exclusively on previously established forebrain-intrinsic patterning mechanisms. Among a limited number of additional potential patterning mechanisms, we found that extrinsic signals from the frontonasal mesenchyme are essential for Shh- and Fgf8-dependent regulation of LGE and MGE genes. Thus, extrinsic and intrinsic forebrain patterning mechanisms cooperate to establish LGE and MGE progenitor identity, and presumably their capacities to generate distinct classes of neuronal progeny.

Newly identified patterns of Pax2 expression in the developing mouse forebrain

BACKGROUND

The availability of specific markers expressed in different regions of the developing nervous system provides a useful tool for the study of mouse mutants. One such marker, the transcription factor Pax2, is expressed at the midbrain-hindbrain boundary and in the cerebellum, spinal cord, retina, optic stalk, and optic chiasm. We recently described a group of diencephalic cells that express Pax2 as early as embryonic day (E) 10.5, and become part of the eminentia thalami by E11.5. The discovery of this previously undescribed cell population prompted us to examine Pax2 protein expression in the developing mouse forebrain in more detail.

RESULTS

We determined the expression pattern of Pax2 in the forebrain of wild type mouse embryos between E10.5 and postnatal day (P) 15. Pax2 expression was detected in the septum of the basal forebrain, hypothalamus, eminentia thalami and in the subfornical organ. To evaluate Pax2 as a marker for septal cells, we examined Pax2 expression in Pax6Sey/Sey mutants, which have an enlarged septum. We found that Pax2 clearly marks a population of septal cells equivalent to that seen in wild types, indicating its utility as a marker of septal identity. These cells did not express the GABAergic marker calbindin nor the cholinergic marker choline acetyltransferase and were not detectable after P15.

CONCLUSION

Pax2 is expressed in populations of cells within the developing septum, hypothalamus, and eminentia thalami. It seems especially useful as a marker of the telencephalic septum, because of its early, strong and characteristic expression in this structure. Further, its expression is maintained in the enlarged septum of Pax6Sey/Sey mutants.

Lamination of the cerebral cortex is disturbed in Gli3 mutant mice

The layered organization of the cerebral cortex develops in an inside-out pattern, a process which is controlled by the secreted protein reelin. Here we report on cortical lamination in the Gli3 hypomorphic mouse mutant Xt(J)/Pdn which lacks the cortical hem, a major source of reelin(+) Cajal Retzius cells in the cerebral cortex. Unlike other previously described mouse mutants with hem defects, cortical lamination is disturbed in Xt(J)/Pdn animals. Surprisingly, these layering defects occur in the presence of reelin(+) cells which are probably derived from an expanded Dbx1(+) progenitor pool in the mutant. However, while these reelin(+) neurons and also Calretinin(+) cells are initially evenly distributed over the cortical surface they form clusters later during development suggesting a novel role for Gli3 in maintaining the proper arrangement of these cells in the marginal zone. Moreover, the radial glial network is disturbed in the regions of these clusters. In addition, the differentiation of subplate cells is affected which serve as a framework for developing a properly laminated cortex.

Role of annexin 1 gene expression in mouse craniofacial bone development

BACKGROUND

Annexin 1 is a 37-kDa protein that has complex intra- and extracellular effects. To discover whether the absence of this protein alters bone development, we monitored this event in the annexin-A1 null mice in comparison with littermate wild-type controls.

METHODS

Radiographic and densitometry methods were used for the assessment of bone in annexin-A1 null mice at a gross level. We used whole-skeleton staining, histological analysis, and Western blotting techniques to monitor changes at the tissue and cellular levels.

RESULTS

There were no gross differences in the appendicular skeleton between the genotypes, but an anomalous development of the skull was observed in the annexin-A1 null mice. This was characterized in the newborn annexin-A1 null animals by a delayed intramembranous ossification of the skull, incomplete fusion of the interfrontal suture and palatine bone, and the presence of an abnormal suture structure. The annexin-A1 gene was shown to be active in osteocytes during this phase and COX-2 was abundantly expressed in cartilage and bone taken from annexin-A1 null mice.

CONCLUSIONS

Expression of the annexin-A1 gene is important for the normal development of the skull in mice, possibly through the regulation of osteoblast differentiation and a secondary effect on the expression of components of the cPLA2-COX-2 system.