An olfactory sensory map develops in the absence of normal projection neurons or GABAergic interneurons

IGF-I promotes neuronal migration and positioning in the olfactory bulb and the exit of neuroblasts from the subventricular zone

While insulin-like growth factor-I (IGF-I) supports neuronal and glial differentiation in the CNS, it is largely unknown whether IGF-I also influences neuronal migration and positioning. We show here that the pattern of olfactory bulb (OB) layering is altered in Igf-I (-/-) mice. In these animals, Tbr1(+)-glutamatergic neurons are misplaced in the mitral cell layer (ML) and the external plexiform layer (EPL). In addition, there are fewer interneurons in the glomerular layer and the EPL of the Igf-I (-/-) mice, and fewer newborn neurons are incorporated into the OB from the forebrain subventricular zone (SVZ). Indeed, neuroblasts accumulate in the postnatal/adult SVZ of Igf-I (-/-) mice. Significantly, the positioning of Tbr1(+)-cells in a primitive ML is stimulated by IGF-I in cultured embryonic OB slices, an effect that is partially repressed by the phosphoinositide 3-kinase (PI3K) inhibitor. In OB cell cultures, IGF-I increases the phosphorylation of disabled1 (P-Dab1), an adaptor protein that is a target of Src family kinases (SFK) in the reelin signalling pathway, whereas reduced P-Dab1 levels were found in Igf-I (-/-) mice. Neuroblast migration from the rostral migratory stream (RMS) explants of postnatal Igf-I (-/-) was similar to that from Igf-I (+/+) explants. However, cell migration was significantly enhanced by IGF-I added to the explants, an effect that was repressed by PI3K and SFK inhibitors. These findings suggest that IGF-I promotes neuronal positioning in the OB and support a role for IGF-I in stimulating neuroblast exit from the SVZ into the RMS, thereby promoting the incorporation of newly formed neurons into the OB.

How the olfactory bulb got its glomeruli: a just so story?

The nearly 2,000 glomeruli that cover the surface of the olfactory bulb are so distinctive that they were noted specifically in the earliest of Cajal's catalogues. They have variously been considered a functional unit, an organizational unit and a crucial component of the olfactory coding circuit. Despite their central position in olfactory processing, the development of the glomeruli has only recently begun to be investigated with new and powerful genetic tools. Some unexpected findings have been made that may lead to a new understanding of the processes involved in wiring sensory regions of the brain. It may no longer be sufficient to simply invoke genes, spikes and their interplay in the construction of brain circuits. The story of 'how the olfactory bulb got its glomeruli' may be more complex, and more revealing, than has been supposed.

The developmental integration of cortical interneurons into a functional network

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

Disorganized olfactory bulb lamination in mice deficient for transcription factor AP-2epsilon

Within the olfactory bulb, neurons and their axonal connections are organized into distinct layers corresponding to different functionalities. Here we demonstrate that transcription factor AP-2epsilon is required for olfactory bulb development, specifically the establishment of appropriate neuronal lamination. During normal mouse embryogenesis, AP-2epsilon transcripts are one of the earliest markers of olfactory bulb formation, and expression eventually becomes refined to the projection neurons, the mitral and tufted cells. To assess the function of AP-2epsilon in olfaction, we generated a null allele (the "AK" allele) by inserting a Cre recombinase transgene into the endogenous AP-2epsilon genomic locus. AP-2epsilon-null mice exhibited defective olfactory bulb architecture. The mitral cell layer was disorganized, typified by misoriented and aberrantly positioned projection neurons, and the adjacent internal plexiform layer was absent. Despite the significant disruption of olfactory bulb organization, AP-2epsilon null mice were viable and could discriminate a variety of odors. AP-2epsilon-null mice thus provide compelling evidence for the robust nature of the mouse olfactory system, and serve as a model system to probe both the regulation of neuronal lamination and the functional circuitry of the olfactory bulb. We also show that Cre recombinase expression directed by the AP-2epsilon locus can specifically target floxed genes within the olfactory bulb, extending the utility of this AK allele.

Olfactory sensory axon growth and branching is influenced by sonic hedgehog

Olfactory sensory neuron (OSN) axons extend from the olfactory epithelium to the olfactory bulb without branching until they reach their target region, the glomerulus. In this report, we present evidence to support the involvement of sonic hedgehog in promoting rat olfactory sensory axons to branch and to enter into the glomerulus. Sonic hedgehog (Shh) protein is detected in the glomeruli of the olfactory bulb, whereas its transcript is expressed in the mitral and tufted cells, suggesting that Shh in the glomeruli is produced by mitral and tufted cells. In primary OSN cultures, Shh-N peptide promotes olfactory axon branching. When Shh function is neutralized in vivo by its antibody, growth of newly generated OSN axons into the glomeruli is vastly reduced.

Mapping of class I and class II odorant receptors to glomerular domains by two distinct types of olfactory sensory neurons in the mouse

The repertoire of approximately 1200 odorant receptors (ORs) is mapped onto the array of approximately 1800 glomeruli in the mouse olfactory bulb (OB). The spatial organization of this array is influenced by the ORs. Here we show that glomerular mapping to broad domains in the dorsal OB is determined by two types of olfactory sensory neurons (OSNs), which reside in the dorsal olfactory epithelium. The OSN types express either class I or class II OR genes. Axons from the two OSN types segregate already within the olfactory nerve and form distinct domains of glomeruli in the OB. These class-specific anatomical domains correlate with known functional odorant response domains. However, axonal segregation and domain formation are not determined by the class of the expressed OR protein. Thus, the two OSN types are determinants of axonal wiring, operate at a higher level than ORs, and contribute to the functional organization of the glomerular array.

Ascl1 is a required downstream effector of Gsx gene function in the embryonic mouse telencephalon

Background

The homeobox gene Gsx2 (formerly Gsh2) is known to regulate patterning in the lateral ganglionic eminence (LGE) of the embryonic telencephalon. In its absence, the closely related gene Gsx1 (previously known as Gsh1) can partially compensate in the patterning and differentiation of ventral telencephalic structures, such as the striatum. However, the cellular and molecular mechanisms underlying this compensation remain unclear.

Results

We show here that in the Gsx2 mutants Gsx1 is expressed in only a subset of the ventral telencephalic progenitors that normally express Gsx2. Based on the similarities in the expression of Gsx1 and Ascl1 (Mash1) within the Gsx2 mutant LGE, we examined whether Ascl1 plays an integral part in the Gsx1-based recovery. Ascl1 mutants show only modest alterations in striatal development; however, in Gsx2;Ascl1 double mutants, striatal development is severely affected, similar to that seen in the Gsx1;Gsx2 double mutants. This is despite the fact that Gsx1 is expressed, and even expands, in the Gsx2;Ascl1 mutant LGE, comparable to that seen in the Gsx2 mutant. Finally, Notch signaling has recently been suggested to be required for normal striatal development. In spite of the fact that Notch signaling is severely disrupted in Ascl1 mutants, it actually appears to be improved in the Gsx2;Ascl1 double mutants.

Conclusion

These results, therefore, reveal a non-proneural requirement of Ascl1 that together with Gsx1 compensates for the loss of Gsx2 in a subset of LGE progenitors.

Mutations of CASK cause an X-linked brain malformation phenotype with microcephaly and hypoplasia of the brainstem and cerebellum

CASK is a multi-domain scaffolding protein that interacts with the transcription factor TBR1 and regulates expression of genes involved in cortical development such as RELN. Here we describe a previously unreported X-linked brain malformation syndrome caused by mutations of CASK. All five affected individuals with CASK mutations had congenital or postnatal microcephaly, disproportionate brainstem and cerebellar hypoplasia, and severe mental retardation.

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

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

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

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

The T-box transcription factor Eomes/Tbr2 regulates neurogenesis in the cortical subventricular zone

The embryonic subventricular zone (SVZ) is a critical site for generating cortical projection neurons; however, molecular mechanisms regulating neurogenesis specifically in the SVZ are largely unknown. The transcription factor Eomes/Tbr2 is transiently expressed in cortical SVZ progenitor cells. Here we demonstrate that conditional inactivation of Tbr2 during early brain development causes microcephaly and severe behavioral deficits. In Tbr2 mutants the number of SVZ progenitor cells is reduced and the differentiation of upper cortical layer neurons is disturbed. Neurogenesis in the adult dentate gyrus but not the subependymal zone is abolished. These studies establish Tbr2 as a key regulator of neurogenesis in the SVZ.

Characterization of a distinct subpopulation of striatal projection neurons expressing the Dlx genes in the basal ganglia through the activity of the I56ii enhancer

Regulation of region-specific neuronal differentiation and migration in the embryonic forebrain is a complex mechanism that involves a variety of transcription factors such as the Dlx genes. At least four cis-acting regulatory elements (CREs) are responsible for the Dlx transcriptional regulation in the subcortical telencephalon and the rostral diencephalon. These include I12b and URE2 in the Dlx1/2 bigene cluster, and, I56i and I56ii in the Dlx5/6 cluster. We previously reported that URE2, I12b, and I56i, mark different progenitor cell populations in the ganglionic eminences as well as different subtypes of adult cortical interneurons. Here, we carried out a detailed spatial and temporal analysis of the I56ii CRE activity in the developing telencephalon between E10.5 and E15.5, and compared its activity with the other three Dlx CREs using lacZ reporter genes in transgenic mice. We show that I56ii marks distinct group(s) of neurons located in the superficial mantle of the LGE and MGE between E11.5 and E13.5. The I56ii-positive cells are Dlx- and GABA-immunoreactive. However, unlike the other CREs, I56ii does not label interneuron progenitors in the basal ganglia, nor tangentially migrating cells to the cortex at E13.5. Instead, I56ii-positive cells mark a subpopulation(s) of post-mitotic projection neurons that tangentially migrate from the LGE to the deep mantle of the MGE and reside between the subventricular zone and the globus pallidus during midgestation. The majority of these neurons express the striatal markers Meis2 and Islet1. Moreover, both Meis2 and Islet1 activate transcription of a reporter gene containing the I56ii sequence in co-transfection assays, indicating that these transcriptional factors may be potential upstream modulators of the Dlx genes in vivo.

Bergmann glia and the recognition molecule CHL1 organize GABAergic axons and direct innervation of Purkinje cell dendrites

The geometric and subcellular organization of axon arbors distributes and regulates electrical signaling in neurons and networks, but the underlying mechanisms have remained elusive. In rodent cerebellar cortex, stellate interneurons elaborate characteristic axon arbors that selectively innervate Purkinje cell dendrites and likely regulate dendritic integration. We used GFP BAC transgenic reporter mice to examine the cellular processes and molecular mechanisms underlying the development of stellate cell axons and their innervation pattern. We show that stellate axons are organized and guided towards Purkinje cell dendrites by an intermediate scaffold of Bergmann glial (BG) fibers. The L1 family immunoglobulin protein Close Homologue of L1 (CHL1) is localized to apical BG fibers and stellate cells during the development of stellate axon arbors. In the absence of CHL1, stellate axons deviate from BG fibers and show aberrant branching and orientation. Furthermore, synapse formation between aberrant stellate axons and Purkinje dendrites is reduced and cannot be maintained, leading to progressive atrophy of axon terminals. These results establish BG fibers as a guiding scaffold and CHL1 a molecular signal in the organization of stellate axon arbors and in directing their dendritic innervation.

Large principal neurons in vertebrate neural circuits often consist of distinct anatomical and physiological compartments, which allow distributed and compartmentalized signaling and greatly increase the computational power of single neurons. Superimposed upon this intrinsic compartmental architecture is the subcellular organization of synaptic inputs, which exert local control over the biophysical properties and differentially regulate the input, integration, and output of principal neurons. In the cerebellar cortex, Purkinje neurons are innervated by GABA inhibitory synapses from the stellate and basket cells at dendrites and soma-axon initial (AIS) segments, respectively. Previous studies have shown that an L1 family immunoglobulin cell adhesion molecule (neurofascin186) is distributed as a subcellular gradient and directs basket cell axons to innervate Purkinje cell AIS. Here, we examine the mechanisms underlying the innervation of Purkinje cell dendrites by stellate axons. We found that stellate axons are organized into characteristic trajectories and guided towards Purkinje dendrites by an intermediate scaffold of astroglia—the Bergmann glial (BG) fibers. Another member of L1 family, Close Homologue of L1 (CHL1), is localized to BG fibers and stellate cells, and contributes to the organization of stellate axons along BG fibers and to the innervation of Purkinje cell dendrites.

Subcellular synapse organization regulates the input, integration, and output of target neurons. An astroglial scaffold and an L1 family cell adhesion molecule contribute to dendritic innervation by GABA inhibitory synapses.

Sall3 is required for the terminal maturation of olfactory glomerular interneurons

Sall3 is a zinc finger containing putative transcription factor and a member of the Sall gene family. Members of the Sall gene family are highly expressed during development. Sall3-deficient mice die in the perinatal period because of dehydration and display alterations in palate formation and cranial nerve formation (Parrish et al. [2004] Mol Cell Biol 24:7102-7112). We examined the role of Sall3 in the development of the olfactory system. We determined that Sall3 is expressed by cells in the olfactory epithelium and olfactory bulb. Sall3 deficiency specifically alters formation of the glomerular layer. The glomerular layer was hypocellular, because of a decrease in the number of interneurons. The lateral ganglionic eminence and rostral migratory stream developed normally in Sall3-deficient animals, which suggests that Sall3 is not required for the initial specification of olfactory bulb interneurons. Fewer GAD65/67-, Pax6-, calretinin-, and calbindin-positive cells were detected in the glomerular layer, accompanied by an increase in cells positive for these markers in the granule cell layer. In addition, a complete absence of tyrosine hydroxylase expression was observed in the olfactory bulb in the absence of Sall3. However, expression of Nurr1, a marker of dopaminergic precursors, was maintained, indicating that dopaminergic precursors were present. Our data suggest that Sall3 is required for the terminal maturation of neurons destined for the glomerular layer.

The distinct temporal origins of olfactory bulb interneuron subtypes

Olfactory bulb (OB) interneurons are a heterogeneous population produced beginning in embryogenesis and continuing through adulthood. Understanding how this diversity arises will provide insight into how OB microcircuitry is established as well as adult neurogenesis. Particular spatial domains have been shown to contribute specific interneuron subtypes. However, the temporal profile by which OB interneuron subtypes are produced is unknown. Using inducible genetic fate mapping of Dlx1/2 precursors, we analyzed the production of seven OB interneuron subtypes and found that the generation of each subpopulation has a unique temporal signature. Within the glomerular layer, the production of tyrosine hydroxylase-positive interneurons is maximal during early embryogenesis and decreases thereafter. In contrast, the generation of calbindin interneurons is maximal during late embryogenesis and declines postnatally, whereas calretinin (CR) cell production is low during embryogenesis and increases postnatally. Parvalbumin interneurons within the external plexiform layer are produced only perinatally, whereas the generation of 5T4-positive granule cells in the mitral cell layer does not change significantly over time. CR-positive granule cells are not produced at early embryonic time points, but constitute a large percentage of the granule cells born after birth. Blanes cells in contrast are produced in greatest number during embryogenesis. Together we provide the first comprehensive analysis of the temporal generation of OB interneuron subtypes and demonstrate that the timing by which these populations are produced is tightly orchestrated.

Role of IGF signaling in olfactory sensory map formation and axon guidance

Olfactory neurons project their axons to spatially invariant glomeruli in the olfactory bulb, forming an ordered pattern of innervation comprising the olfactory sensory map. A mirror symmetry exists within this map, such that neurons expressing a given receptor typically project to one glomerulus on the medial face and one glomerulus on the lateral face of the bulb. The mechanisms underlying an olfactory neuron's choice to project medially versus laterally remain largely unknown, however. Here we demonstrate that insulin-like growth factor (IGF) signaling is required for sensory innervation of the lateral olfactory bulb. Mutations that eliminate IGF signaling cause axons destined for targets in the lateral bulb to shift to ectopic sites on the ventral-medial surface. Using primary cultures of olfactory and cerebellar neurons, we further show that IGF is a chemoattractant for axon growth cones. Together these observations reveal a role of IGF signaling in sensory map formation and axon guidance.

Olfactory axon guidance: the modified rules

The olfactory system represents a complex model for the investigation of factors that influence the guidance of sensory axon populations to specific targets in the CNS. In the mouse, the projections of approximately 1,000 neuronal subsets, each defined by expression of a distinct odorant receptor (OR), converge at unique glomerular loci in the olfactory bulb (OB). Unlike the case in other sensory systems, proper guidance is achieved without benefit of any known cues in the target itself that are capable of attracting or repelling specific axons. It has long been argued that OR proteins are the critical molecules orchestrating guidance. However, recent studies suggest that axon identity may be dependent on the graded expression of a variety of unique olfactory axon guidance cues. This review focuses attention on these non-OR factors and their roles in olfactory axon guidance.

Expression patterns of the three Teashirt-related genes define specific boundaries in the developing and postnatal mouse forebrain

We compare the expression patterns of the three mouse Teashirt (mTsh) genes during development of the forebrain and at a postnatal stage. During development, mTsh genes are expressed in domains that are restricted both dorsoventrally and rostrocaudally, with major changes in expression level coinciding with compartment boundaries. Striking complementarities in the distribution of mTsh transcripts were observed in the developing diencephalon, telencephalon, and olfactory bulb (OB). A mTsh1-positive cell population is part of the DLX-positive population localized in the dorsalmost portion of the lateral ganglionic eminence (dLGE). Comparison of the mTsh1 expression domain with the domains of Er81 and Islet1, which mark two distinct progenitor populations in the subventricular zone of the LGE, suggests that mTsh1 marks OB interneuron progenitors. Furthermore, the distinct expression patterns of mTsh1 and mTsh2 in the ventral LGE and the dLGE highlight the differential contributions of these structures to the striatum and the amydaloid complex. For Sey/Sey mutants, we show that Pax6 function is critical for the correct specification of the mTsh1+ population in the dLGE during embryogenesis. At postnatal stages in the OB, mTsh1 is expressed in granule and periglomerular cells, which originate from the subpallium during development. Furthermore, mTsh1+ cells line the walls of the anterior lateral ventricle, a region that gives rise to the interneurons that migrate in the rostral migratory streams and populate the OB postnatally. Our results suggest a role for mTsh genes in the establishment of regional identity and specification of cell types in the developing and adult forebrain.

Deletion of voltage-gated channel affects glomerular refinement and odorant receptor expression in the mouse olfactory system

Olfactory sensory neurons (OSNs) expressing a specific odorant receptor (OR) gene send axonal projections to specific glomeruli, creating a stereotypic olfactory sensory map. Odorant receptor sequence, G-protein cAMP signaling, and axon guidance molecules have been shown to direct axons of OSNs toward central targets in the olfactory bulb (OB). Although the OR sequence may act as one determinant, our objective was to elucidate the extent by which voltage-dependent activity of postsynaptic projection neurons in the OB centrally influences peripheral development and target destination of OSNs. We bred OR-tagged transgenic mice to homozygosity with mice that had a gene-targeted deletion of the Shaker potassium ion channel (Kv1.3) to elucidate how activity modulates synaptic connections that formulate the sensory map. Here we report that the Kv1.3 ion channel, which is predominantly expressed in mitral cells and whose gene-targeted deletion causes a "super-smeller" phenotype, alters synaptic refinement of axonal projections from OSNs expressing P2, M72, and MOR28 ORs. Absence of Kv1.3 voltage-gated activity caused the formation of small, heterogeneous, and supernumerary glomeruli that failed to undergo neural pruning over development. These changes were accompanied by a significant decrease in the number of P2-, M72-, and MOR28-expressing OSNs, which contained an overexpression of OR protein and G-protein G(olf) in the cilia of the olfactory epithelium. These findings suggest that voltage-gated activity of projection neurons is essential to refine primary olfactory projections and that it regulates proper expression of the transduction machinery at the periphery.

Homeobox genes in vertebrate forebrain development and disease

Homeobox genes are an evolutionarily conserved class of transcription factors that are key regulators of developmental processes such as regional specification, patterning, migration and differentiation. In both mouse and humans, the developing forebrain is marked by distinct boundaries of homeobox gene expression at different developmental time points. These genes regulate the patterning of the forebrain along the dorsal/ventral and rostral/caudal axes and are also essential for the differentiation of specific neuronal subtypes. Inhibitory interneurons that arise from the ganglionic eminences and migrate tangentially to the neocortex and hippocampus are dramatically affected by mutations in several homeobox genes. In this review, we discuss the identification, expression patterns, loss- and/or gain-of-function models, and confirmed transcriptional targets for a set of homeobox genes required for the correct development of the forebrain in the mouse. In humans, mutations of homeobox genes expressed in the forebrain have been shown to result in mental retardation, epilepsy or movement disorders. The number of homeobox genes currently linked to human nervous system disease is surprisingly low, perhaps reflecting the essential functions of these genes throughout embryogenesis or the degree of functional redundancy during central nervous system development.

How neuronal migration contributes to the morphogenesis of the CNS: insights from the zebrafish

We used transgenic zebrafish expressing GFP or YFP in subpopulations of neurons to study the migration, homing process and axon extension of groups of CNS neurons in different regions of the zebrafish brain. We found that extensive migration takes place at all levels of the CNS and gives rise to nuclei or cell populations with specific identities. Here, we describe 4 previously unknown or only partially characterized migratory events taking place in the zebrafish telencephalon and rhombic lip, using 3 different transgenic lines, and identify the phenotypes of the cells undertaking these migrations. The migration of a subgroup of mitral cell precursors from the dorsocaudal telencephalon to the olfactory bulb, visualized in the tg(tbr1:YFP) transgenic line, is coupled with morphogenetic transformation of the dorsal telencephalon. The tg(1.4dlx5a-6a:GFP) transgenic line provides a means to analyze the migration of GABAergic interneurons from the ventral to the dorsal telencephalon, thus extending the occurrence of this migration to another vertebrate. The tg(Xeom:GFP) transgenic line provides the first demonstration of the dorsoventral migration of glutamatergic septal neurons, present in mammals and now described in fish, thus reconciling the contrasting evidence of dorsal patterning genes (tbr1, eomes) expressed in a ventral cell population. Furthermore, migration studies in the tg(1.4dlx5a-6a:GFP) and tg(Xeom:GFP) lines help determine the origin of 2 important cell populations in the fish cerebellum: projection neurons and Purkinje cells. These examples reinforce the concept that migratory events contribute to the distribution of cell types with diverse identities through the CNS and that zebrafish transgenic lines represent excellent tools to study these events.

Dlx2 homeobox gene transcriptional regulation of Trkb neurotrophin receptor expression during mouse retinal development

Dlx homeobox genes are first expressed in embryonic retina at E11.5. The Dlx1/Dlx2 null retina has a reduced ganglion cell layer (GCL), with loss of late-born differentiated retinal ganglion cells (RGCs) due to increased apoptosis. TrkB signaling is proposed to regulate the dynamics of RGC apoptosis throughout development. DLX2 expression markedly precedes the onset of TrkB expression in the GCL; TrkB co-expression with Dlx2 and RGC markers is well-established by E13.5. In the Dlx1/Dlx2 null retina, TrkB expression is significantly reduced by E16.5. We demonstrated that DLX2 binds to a specific region of the TrkB promoter in retinal neuroepithelium during embryogenesis. In vitro confirmation and the functional consequences of DLX2 binding to this TrkB regulatory region support TrkB as a Dlx2 transcriptional target. Furthermore, ectopic Dlx2 expression in retinal explants activates TrkB expression and Dlx2 knockdown in primary retinal cultures results in reduced TrkB expression. RGC differentiation and survival require the coordinated expression of transcription factors. This study establishes a direct transcriptional relationship between a homeodomain protein involved in RGC differentiation and a neurotrophin receptor implicated in RGC survival. Signaling mediated by TrkB may contribute to survival of late-born RGCs whose terminal differentiation is regulated by Dlx gene function.

Olfactory bulb hypoplasia in Prokr2 null mice stems from defective neuronal progenitor migration and differentiation

New neurons are added on a daily basis to the olfactory bulb (OB) of a mammal, and this phenomenon exists throughout its lifetime. These new cells are born in the subventricular zone and migrate to the OB via the rostral migratory stream (RMS). To examine the role of the prokineticin receptor 2 (Prokr2) in neurogenesis, we created a Prokr2 null mouse, and report a decrease in the volume of its OB and also a decrease in the number of bromodeoxyuridine (BrdU)-positive cells. There is disrupted architecture of the OB, with the glomerular layer containing terminal dUTP nick-end labeling (TUNEL) -positive nuclei and also a decrease in tyrosine hydroxylase-positive neurons in this layer. In addition, there are increased numbers of doublecortin-positive neuroblasts in the RMS and increased PSA-NCAM (polysialylated form of the neural cell adhesion molecule) -positive neuronal progenitors around the olfactory ventricle, indicating their detachment from homotypic chains is compromised. Finally, in support of this, Prokr2-deficient cells expanded in vitro as neurospheres are incapable of migrating towards a source of recombinant human prokineticin 2 (PROK2). Together, these findings suggest an important role for Prokr2 in OB neurogenesis.

Sall1 regulates mitral cell development and olfactory nerve extension in the developing olfactory bulb

Sall1 is a zinc finger containing transcription factor that is highly expressed during mammalian embryogenesis. In humans, the developmental disorder Townes Brocks Syndrome is associated with mutations in the SALL1 gene. Sall1-deficient animals die at birth due to kidney deficits; however, its function in the nervous system has not been characterized. We examined the role of Sall1 in the developing olfactory system. We demonstrate that Sall1 is expressed by cells in the olfactory epithelium and olfactory bulb (OB). Sall1-deficient OBs are reduced in size and exhibit alterations in neurogenesis and mitral cell production. In addition, the olfactory nerve failed to extend past the ventral-medial region of the OB in Sall1-deficient animals. We observed intrinsic patterns of neurogenesis during olfactory development in control animals. In Sall1-mutant animals, these patterns of neurogenesis were disrupted. These findings suggest a role for Sall1 in regulating neuronal differentiation and maturation in developing neural structures.

Early telencephalic migration topographically converging in the olfactory cortex

Neurons that participate in the olfactory system arise in different areas of the developing mouse telencephalon. The generation of these different cell populations and their tangential migration into the olfactory cortex (OC) was tracked by tracer injection and in toto embryo culture. Cells originating in the dorsal lateral ganglionic eminence (LGE) migrate tangentially along the anteroposterior axis to settle in the piriform cortex (PC). Those originating in the ventral domain of this structure occupy the thickness of the olfactory tubercle (OT), whereas cells from the rostral LGE migrate tangentially into the most anterior telencephalon, at the level of the prospective olfactory bulb (pOB). Neurons from the dorsal telencephalon migrate ventrally, bordering the PC, toward olfactory structures. Two cell populations migrate tangentially from the rostromedial telencephalic wall to the OT and the PC, passing through the ventromedial and dorsolateral face of the telencephalon. Some cells from the germinative area of the rostral telencephalon, at the level of the septoeminential sulcus, migrate rostrally to the pOB or caudally to the OC. Thus, we demonstrate multiple telencephalic origins for the first olfactory neurons and each population following different migratory routes to colonize the OC according to an accurate topographic map.

A subpopulation of olfactory bulb GABAergic interneurons is derived from Emx1- and Dlx5/6-expressing progenitors

The subventricular zone (SVZ) of the postnatal brain continuously generates olfactory bulb (OB) interneurons. We show that calretinin+, calbindin+, and dopaminergic (TH+) periglomerular OB interneurons correspond to distinct subtypes of GABAergic cells; all were produced in the postnatal mouse brain, but they matured and were eliminated at different rates. The embryonic lateral ganglionic eminence (LGE) is thought to be the site of origin of postnatal SVZ neural progenitors. Consistently, grafts of the embryonic LGE into the adult brain SVZ generated many OB interneurons, including TH+ and calbindin+ periglomerular interneurons. However, calretinin+ cells were not produced from these LGE grafts. Surprisingly, pallial and septal embryonic progenitors transplanted into the adult brain SVZ also resulted in the generation of OB interneurons, including calretinin+ cells. A subset of Dlx2+ OB interneurons was derived from cells expressing Emx1, a transcription factor largely restricted to the pallium during development. Emx1 lineage-derived cells contributed a substantial portion of GABAergic cells in the OB, including calretinin+ interneurons. This is in contrast to cortex, in which Emx1 lineage-derived cells do not differentiate into GABAergic neurons. Our results suggest that some OB interneurons are derived from progenitors outside the LGE and that precursors expressing what has classically been considered a pallial transcription factor generate GABAergic interneurons.

The shape of the olfactory bulb influences axon targeting

Each primary olfactory neuron in the mouse expresses a single type of odorant receptor. All neurons expressing the same odorant receptor gene typically project to two topographically fixed glomeruli, one each on the medial and lateral surfaces of the olfactory bulb. While topographic gradients of guidance receptors and their ligands help to establish the retinotectal projection, similar orthogonal distributions of cues have not yet been detected within the olfactory system. While odorant receptors are crucial for the final targeting of axons to glomeruli, it is unclear whether the olfactory bulb itself provides instructive cues for the establishment of the topographic map. To begin to understand the role of the olfactory bulb in the formation of the olfactory nerve pathway, we developed a model whereby the gross shape of the bulb in the P2-IRES-tau-LacZ line of mice was radically altered during postnatal development. We have shown here that the topography of axons expressing the P2 odorant receptor is dependent on the shape of the olfactory bulb. When the dorsoventral axis of the olfactory bulb was compressed during the early postnatal period, newly developing P2 axons projected to multiple inappropriate glomeruli surrounding their normal target site. These results suggest that the distribution of local guidance cues within the olfactory bulb is influenced by the shape of the olfactory bulb and that these cues contribute to the topographic positioning of glomeruli.

Absence of adenylyl cyclase 3 perturbs peripheral olfactory projections in mice

A remarkable feature of peripheral olfactory projections in mammals is the convergence of axons from olfactory sensory neurons (OSNs) expressing the same odorant receptor (OR) into the same glomeruli. There is mounting evidence that the ORs play critical roles in glomerular formation. However, it remains unclear how the OR exerts its function of sorting axons into homogeneity. We and others have shown previously that activation of the G-protein/cAMP signaling cascade underlies glomerular formation. Here, we further investigated whether establishment of the mature glomerular array requires adenylyl cyclase 3 (AC3), a key component of the OR-mediated cAMP-dependent signaling cascade. We found robust AC3 expression in both OSN cilia and axons during the period of active glomerular formation in neonatal mice. Examination of OR-tagged mice in an AC3 knock-out background revealed that the absence of AC3 drastically and differentially perturbed the formation of several representative glomeruli. Furthermore, heterogeneous glomeruli innervated by axons of multiple OSN populations persisted in such mice well into adulthood. In addition, reproducible aberrations in axonal projections in AC3-/- mice appeared to correlate with the activation of specific OR loci, regardless of the expressed receptor sequence, suggesting that OR expression is but one factor in determining OSN axonal projections. Together, our results indicate that cAMP signaling is critical for axonal sorting and the establishment of axonal identity.

Response of olfactory axons to loss of synaptic targets in the adult mouse

Glomerular convergence has been proposed to rely on interactions between like olfactory axons, however topographic targeting is influenced by guidance molecules encountered in the olfactory bulb. Disruption of these cues during development misdirects sensory axons, however little is known about the role of bulb-derived signals in later life, as new axons arise during turnover of the olfactory sensory neuron (OSN) population. To evaluate the contribution of bulb neurons in maintaining topographic projections in adults, we ablated them with N-methyl-d-aspartate (NMDA) in P2-IRES-tauLacZ mice and examined how sensory axons responded to loss of their postsynaptic partners. NMDA lesion eliminated bulb neurons without damage to sensory axons or olfactory ensheathing glia. P2 axons contained within glomeruli at the time of lesion maintained convergence at these locations; there was no evidence of compensatory growth into the remnant tissue. Delayed apoptosis of OSNs in the target-deprived epithelium led to declines in P2 neuron number as well as the gradual atrophy, and in some cases complete loss, of P2 glomeruli in lesioned bulbs by 3 weeks. Increased cell proliferation in the epithelium partially restored the OSN population, and by 8 weeks, new P2 axons distributed within diverse locations in the bulb remnant and within the anterior olfactory nucleus. Prior studies have suggested that initial development of olfactory topography does not rely on synapse formation with target neurons, however the present data demonstrate that continued maintenance of the sensory map requires the presence of sufficient numbers and/or types of available bulbar synaptic targets.

Spatio-temporal specification of olfactory bulb interneurons

Olfactory bulb (OB) interneurons are continuously generated throughout development and in adulthood, and are derived from different progenitor zones. Once integrated in the OB circuits, interneurons play essential roles in olfactory information processing by modulating the activity of major output neurons. These functions are performed by multiple classes of neurons that differ in their spatial distribution, morphology, neurochemical and synaptic properties. This diversity, and the continuous neurogenesis make the understanding of the specification mechanisms in the OB a challenging task. New studies suggest that both intrinsic and extrinsic cues are involved in fate determination of OB interneurons. In both development and adulthood the expression of specific transcription factors not only defines different progenitor regions but also precise interneuronal phenotypes. Here we discuss recent findings on the molecular mechanisms regulating production and diversity of OB interneurons with respect to the spatial and temporal parameters.

Novel subdomains of the mouse olfactory bulb defined by molecular heterogeneity in the nascent external plexiform and glomerular layers

Background

In the mouse olfactory system, the role of the olfactory bulb in guiding olfactory sensory neuron (OSN) axons to their targets is poorly understood. What cell types within the bulb are necessary for targeting is unknown. What genes are important for this process is also unknown. Although projection neurons are not required, other cell-types within the external plexiform and glomerular layers also form synapses with OSNs. We hypothesized that these cells are important for targeting, and express spatially differentially expressed guidance cues that act to guide OSN axons within the bulb.

Results

We used laser microdissection and microarray analysis to find genes that are differentially expressed along the dorsal-ventral, medial-lateral, and anterior-posterior axes of the bulb. The expression patterns of these genes divide the bulb into previously unrecognized subdomains. Interestingly, some genes are expressed in both the medial and lateral bulb, showing for the first time the existence of symmetric expression along this axis. We use a regeneration paradigm to show that several of these genes are altered in expression in response to deafferentation, consistent with the interpretation that they are expressed in cells that interact with OSNs.

Conclusion

We demonstrate that the nascent external plexiform and glomerular layers of the bulb can be divided into multiple domains based on the expression of these genes, several of which are known to function in axon guidance, synaptogenesis, and angiogenesis. These genes represent candidate guidance cues that may act to guide OSN axons within the bulb during targeting.

Dlx-dependent and -independent regulation of olfactory bulb interneuron differentiation

Olfactory bulb interneuron development is a complex multistep process that involves cell specification in the ventral telencephalon, tangential migration into the olfactory bulb, and local neuronal maturation. Although several transcription factors have been implicated in this process, how or when they act remains to be elucidated. Here we explore the mechanisms that result in olfactory bulb interneuron defects in Dlx1&2-/- (distal-less homeobox 1 and 2) and Mash1-/- (mammalian achaete-schute homolog 1) mutants. We provide evidence that Dlx1&2 and Mash1 regulate parallel molecular pathways that are required for the generation of these cells, thereby providing new insights into the mechanisms underlying olfactory bulb development. The analysis also defined distinct anatomical zones related to olfactory bulb development. Finally we show that Dlx1&2 are required for promoting tangential migration to the olfactory bulb, potentially via regulating the expression of ErbB4 (v-erb-a erythroblastic leukemia viral oncogene homolog 4), Robo2 (roundabout homolog 2), Slit1 (slit homolog 1), and PK2 (prokineticin 2), which have all been shown to play essential roles in this migration.

Dual role for LIM-homeodomain gene Lhx2 in the formation of the lateral olfactory tract

The development of the olfactory system in vertebrates is a multistep process, in which several regulatory molecules are required at different stages. The development of the olfactory sensory epithelium and its projection to the olfactory bulb are both known to require the LIM-homeodomain transcription factor Lhx2. We examined whether Lhx2 plays a role in the development of the OB itself, as well as its projection to the olfactory cortex. Although there is no morphological OB protuberance in the Lhx2 mutant, mitral cells are normally specified and cluster in a displaced olfactory bulb-like structure (OBLS). The OBLS is not able to pioneer the lateral olfactory tract (LOT) projection in vivo or when provided control (host) telencephalic territory in an in vitro assay. Strikingly, the mutant OBLS is capable of projecting along the LOT if provided with an existing normal LOT in the host explant. This is the first report of a role for a transcription factor expressed in the OB that selectively affects the axon guidance but not the specification of mitral cells. Furthermore, the Lhx2 mutant lateral telencephalon does not support growth of an LOT projection from control OB explants. The defect correlates with the disruption of a cellular mechanism that is thought to be critical for LOT pathfinding: a specialized cell population, the "lot cells," is mislocalized in the Lhx2 mutant. In addition, the expression of Sema6A is aberrantly upregulated. Together, these findings reveal a dual role for Lhx2, in the OB as well as in the lateral telencephalon, for establishing the LOT projection.

Generation of GABAergic and dopaminergic interneurons from endogenous embryonic olfactory bulb precursor cells

During the embryonic period, many olfactory bulb (OB) interneurons arise in the lateral ganglionic eminence (LGE) from precursor cells expressing Dlx2, Gsh2 and Er81 transcription factors. Whether GABAergic and dopaminergic interneurons are also generated within the embryonic OB has not been studied thoroughly. In contrast to abundant Dlx2 and Gsh2 expression in ganglionic eminences (GE), Dlx2 and Gsh2 proteins are not expressed in the E12.5-13.5 mouse OB, whereas the telencephalic pallial domain marker Pax6 is abundant. We found GABAergic and dopaminergic neurons originating from dividing precursor cells in E13.5 OB and in short-term dissociated cultures prepared from the rostral half of E13.5 OB. In OB cultures, 22% of neurons were GAD+, of which 53% were Dlx2+, whereas none expressed Gsh2. By contrast, 70% of GAD+ cells in GE cultures were Dlx2+ and 16% expressed Gsh2. In E13.5 OB slices transplanted with EGFP-labeled E13.5 OB precursor cells, 31.7% of EGFP+ cells differentiated to GABAergic neurons. OB and LGE precursors transplanted into early postnatal OB migrated and differentiated in distinct patterns. Transplanted OB precursors gave rise to interneurons with dendritic spines in close proximity to synaptophysin-positive boutons. Interneurons were also abundant in differentiating OB neural stem cell cultures; the neurons responded to the neurotrophin Bdnf and expressed presynaptic proteins. In vivo, the Bdnf receptor TrkB colocalized with synaptic proteins at the glomeruli. These findings suggest that, in addition to receiving interneurons from the LGE, the embryonic OB contains molecularly distinct local precursor cells that generate mature GABAergic and dopaminergic neurons.

Patterning the developing and regenerating olfactory system

The olfactory system is a remarkable model for investigating the factors that influence the guidance of sensory axon populations to specific targets in the CNS. Since the initial discovery of the vast odorant receptor (ORs) gene family in rodents and the subsequent finding that these molecules directly influence targeting, several additional olfactory axon guidance cues have been identified. Two of these, ephrins and semaphorins, have well-established functions in patterning axon connections in other systems. In addition, lactosamine-containing glycans are also required for proper targeting and maintenance of olfactory axons, and may also function in other sensory regions. It is now apparent that these and likely other additional molecules are required along with ORs to orchestrate the complex pattern of convergence and divergence that is unique to the olfactory system.

Dlx homeobox genes promote cortical interneuron migration from the basal forebrain by direct repression of the semaphorin receptor neuropilin-2

Dlx homeobox genes play an important role in vertebrate forebrain development. Dlx1/Dlx2 null mice die at birth with an abnormal cortical phenotype, including impaired differentiation and migration of GABAergic interneurons to the neocortex. However, the molecular basis for these defects downstream of loss of Dlx1/Dlx2 function is unknown. Neuropilin-2 (NRP-2) is a receptor for Class III semaphorins, which inhibit neuronal migration. Herein, we show that Neuropilin-2 is a specific DLX1 and DLX2 transcriptional target by applying chromatin immunoprecipitation to embryonic forebrain tissues. Both homeobox proteins repress Nrp-2 expression in vitro, confirming the functional significance of DLX binding. Furthermore, the homeodomain of DLX1 and DLX2 is necessary for DNA binding and this binding is essential for Dlx repression of Nrp-2 expression. Of importance, there is up-regulated and aberrant expression of NRP-2 in the forebrains of Dlx1/Dlx2 null mice. This is the first report that DLX1 or DLX2 can function as transcriptional repressors. Our data show that DLX proteins specifically mediate the repression of Neuropilin-2 in the developing forebrain. As well, our results support the hypothesis that down-regulation of Neuropilin-2 expression may facilitate tangential interneuron migration from the basal forebrain.

Diversity of the vomeronasal system in mammals: The singularities of the sheep model

The enormous morphological diversity and heterogeneity of the vomeronasal system (VNS) in mammals--as well as its complete absence in some cases--complicates the extrapolation of data from one species to another, making any physiological and functional conclusions valid for the whole Mammalian Class difficult and risky to draw. Some highly-evolved macrosmatic mammals, like sheep, have been previously used in interesting behavioral studies concerning the main and accessory olfactory systems. However, in this species, certain crucial morphological peculiarities have not until now been considered. Following histological, histochemical and immunohistochemical procedures, we have studied the vomeronasal organ (VNO) and the accessory olfactory bulb (AOB) of adult sheep. We have determined: (1) that all structures which classically define the VNO in mammals are present and well developed, providing the morphological basis for functional activity. (2) that, conversely, there is only a scant population of scattered mitral/tufted cells. One morphological consequence of both details is that the strata of the AOB in adult sheep are not as sharply defined as in other species; moreover, the small number of the mitral/tufted cells in the AOB may imply that the VNS of adult sheep is not capable of functioning in the way a well-developed VNS does in other species. (3) the zone to zone projection from the apical and basal sensory epithelium of the VNO to the anterior and posterior part of the AOB, respectively, typical in rodents, lagomorphs and marsupials, is not present in adult sheep.

Axonal wiring in the mouse olfactory system

The main olfactory epithelium of the mouse is a mosaic of 2000 populations of olfactory sensory neurons (OSNs). Each population expresses one allele of one of the 1000 intact odorant receptor (OR) genes. An OSN projects a single unbranched axon to a single glomerulus, from an array of 1600-1800 glomeruli in the main olfactory bulb. Within a glomerulus the OSN axon synapses with the dendrites of second-order neurons and interneurons. Axons of OSNs that express the same OR project to the same glomeruli-typically one glomerulus per half-bulb and thus four glomeruli per mouse. These glomeruli are located at characteristic positions within the glomerular layer of the bulb. ORs determine both the odorant response profile of the OSN and the projection of its axon to a specific glomerulus. I focus on genetic approaches to the axonal wiring problem, particularly on how ORs may function in axonal wiring.

Mice without transcription factor KLF7 provide new insight into olfactory bulb development

Recent genetic studies have excluded that peripheral innervation plays a substantial role in the initial outgrowth of the olfactory bulb. Mice without Kruppel-like factor 7 activity die at birth and display hypoplastic olfactory bulbs which lack peripheral innervation. Here, we report that incomplete penetrance of the mutation is responsible for partial bulb innervation in a small fraction of Klf7 null mice. Analysis of the partially innervated bulbs of mutant embryos, newborns and adult mice revealed an obligatory correlation with local restoration of laminar architecture, neuronal cell differentiation and neuronal activity. The degree of normal OB maturation in Klf7-/- OBs was proportional to the degree of peripheral innervation. These findings therefore indicate that peripheral innervation contributes to bulb maturation late in development by promoting cell morphogenesis and differentiation.

Time frame of mitral cell development in the mice olfactory bulb

Along with tufted cells, mitral cells are the principal projection neurons in the olfactory bulb (OB). During the development of the OB, mitral cells migrate from the ventricular zone to the intermediate zone, where they begin to send axons along the lateral olfactory tract (LOT) to the cortical olfactory zones. Subsequently, they lose their tangential orientation, enabling them to make contact with the axons of the olfactory sensory neurons (OSN) that innervate the whole OB. Here, we investigated the distinct morphological features displayed by developing mitral cells and analyzed the relationship between the changes undertaken by these neurons and the arrival of the OSN axons. Immunostaining for specific markers of developing axons and dendrites, coupled with the use of fluorescent tracers, revealed the morphological changes, the continuous reorientation, and the final refinement that these cells undergo. We found that some of these changes are dependent on the arrival of the OSN axons. Indeed, we identified three main chronological events: 1) newly generated neurons become established in the intermediate zone and project to the LOT; 2) the cells reorient and spread their dendrites at the same time as OSN axons penetrate the OB (this is a sensitive period between embryonic day (E)15-16, in which the arrival of afferents establishes a spatial and temporal gradient that facilitates protoglomerulus and glomerulus formation); and 3) final refinement of the radially orientated cells to adopt a mature morphology. These results suggest that both afferent inputs and intrinsic factors participate to produce the well-defined sensory system.

Necdin promotes GABAergic neuron differentiation in cooperation with Dlx homeodomain proteins

Necdin, a member of the MAGE (melanoma antigen) protein family, is expressed predominantly in terminally differentiated neurons. The necdin gene NDN is maternally imprinted and expressed only from the paternal allele, the deficiency of which is implicated in the pathogenesis of the neurodevelopmental disorder Prader-Willi syndrome. Necdin binds to its homologous MAGE protein MAGE-D1 (also known as NRAGE or Dlxin-1), which interacts with Msx (msh homeobox) and Dlx (distal-less homeobox) family homeodomain transcription factors. Members of the Dlx homeobox gene family are involved in the differentiation and specification of forebrain GABAergic neurons. Here we demonstrate that necdin associates with Dlx homeodomain proteins via MAGE-D1 to promote the differentiation of GABAergic neurons in mouse embryonic forebrain. Immunohistochemical analysis revealed that necdin was coexpressed with Dlx2, Dlx5, or MAGE-D1 in a subpopulation of embryonic forebrain cells. Necdin bound to Dlx2 and Dlx5 via MAGE-D1 and enhanced Dlx2-dependent activation of the Wnt1 (wingless-type MMTV integration site family) promoter. Necdin significantly increased the populations of cells expressing the GABAergic neuron markers calbindin D-28k and glutamic acid decarboxylase when overexpressed by electroporation in cultured forebrain slices. In this assay, Dlx5N, a truncated Dlx5 mutant that competes with Dlx2 to bind MAGE-D1, diminished the effect of necdin on GABAergic neuron differentiation. Furthermore, mutant mice lacking the paternal necdin allele showed a significant reduction in the differentiation of forebrain GABAergic neurons in vivo and in vitro. These results suggest that paternally expressed necdin facilitates the differentiation and specification of GABAergic neurons in cooperation with Dlx homeodomain proteins.

Position and time specify the migration of a pioneering population of olfactory bulb interneurons

We defined the cellular mechanisms for genesis, migration, and differentiation of the initial population of olfactory bulb (OB) interneurons. This cohort of early generated cells, many of which become postmitotic on embryonic day (E) 14.5, differentiates into a wide range of mature OB interneurons by postnatal day (P) 21, and a substantial number remains in the OB at P60. Their precursors autonomously acquire a distinct identity defined by their position in the lateral ganglionic eminence (LGE). The progeny migrate selectively to the OB rudiment in a pathway that presages the rostral migratory stream. After arriving in the OB rudiment, these early generated cells acquire cellular and molecular hallmarks of OB interneurons. Other precursors--including those from the medial ganglionic eminence (MGE) and OB--fail to generate neuroblasts with similar migratory capacity when transplanted to the LGE. The positional identity and migratory specificity of the LGE precursors is rigidly established between E12.5 and E14.5. Thus, the pioneering population of OB interneurons is generated from spatially and temporally determined LGE precursors whose progeny uniquely recognize a distinct migratory trajectory.

The zinc finger transcription factor Sp8 regulates the generation and diversity of olfactory bulb interneurons

The molecular mechanisms that regulate the production and diversity of olfactory bulb interneurons remain poorly understood. With the exception of the GABAergic/dopaminergic subtype in the glomerular layer, no information exists concerning the generation of the other subtypes. Here we show that the recently identified zinc finger transcription factor Sp8 is expressed in neurogenic regions, which give rise to olfactory bulb interneurons at embryonic and postnatal time points and remains expressed in the calretinin-expressing and GABAergic/nondopaminergic interneurons of the glomerular layer. Conditional inactivation of Sp8 in the embryonic ventral telencephalon reveals a requirement for the normal generation of these interneuron subtypes. Sp8 conditional mutants exhibit an increase in cell death within the lateral ganglionic eminence and rostral migratory stream. Moreover, mutant neuroblasts/interneurons are misspecified and display abnormal migration patterns in the olfactory bulb, indicating that Sp8 contributes to olfactory bulb interneuron diversity by regulating the survival, migration, and molecular specification of neuroblasts/interneurons.

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.