Arx homeobox gene is essential for development of mouse olfactory system

Arx revisited: involved in the development of GABAergic interneurons

The aristaless-related homeobox (Arx) transcription factor, located on the X chromosome, has been implicated in a wide range of neurological disorders, including intellectual disability and epilepsy, as well as diabetes and pancreatic developmental disorders. In the mouse brain, Arx is expressed not only in the olfactory bulb (OB) and cerebral cortex progenitor cells but also in these gamma-aminobutyric acid (GABA)-releasing interneurons. In the initial study, constitutive Arx knockout (KO) mice showed aberrant migration and a reduction in GABAergic interneurons in the neonatal OB. However, constitutive Arx KO mice with perinatal lethality preclude further analysis in adolescent or adult mice. To overcome this, Arx-floxed mice have been crossed with Cre driver mice to generate conditional KO mice with selective Arx deletion in distinct interneuron progenitors. These studies have identified Arx as a key transcriptional regulator involved in the generation, fate determination, and migration of cortical interneurons. This review focuses on the critical role of Arx in the development of progenitor cells and the migration of interneurons in the mouse OB and cerebral cortex, and discusses differences in Arx mutant-based abnormality between mouse mutants and human patients.

A protein-coding gene expression atlas from the brain of pregnant and non-pregnant goats

Background: The brain is an extraordinarily complex organ with multiple anatomical structures involved in highly specialized functions related with behavior and physiological homeostasis. Our goal was to build an atlas of protein-coding gene expression in the goat brain by sequencing the transcriptomes of 12 brain regions in seven female Murciano-Granadina goats, from which three of them were 1-month pregnant. Results: Between 14,889 (cerebellar hemisphere) and 15,592 (pineal gland) protein-coding genes were expressed in goat brain regions, and most of them displayed ubiquitous or broad patterns of expression across tissues. Principal component analysis and hierarchical clustering based on the patterns of mRNA expression revealed that samples from certain brain regions tend to group according to their position in the anterior-posterior axis of the neural tube, i.e., hindbrain (pons and medulla oblongata), midbrain (rostral colliculus) and forebrain (frontal neocortex, olfactory bulb, hypothalamus, and hippocampus). Exceptions to this observation were cerebellum and glandular tissues (pineal gland and hypophysis), which showed highly divergent mRNA expression profiles. Differential expression analysis between pregnant and non-pregnant goats revealed moderate changes of mRNA expression in the frontal neocortex, hippocampus, adenohypophysis and pons, and very dramatic changes in the olfactory bulb. Many genes showing differential expression in this organ are related to olfactory function and behavior in humans. Conclusion: With the exception of cerebellum and glandular tissues, there is a relationship between the cellular origin of sampled regions along the anterior-posterior axis of the neural tube and their mRNA expression patterns in the goat adult brain. Gestation induces substantial changes in the mRNA expression of the olfactory bulb, a finding consistent with the key role of this anatomical structure on the development of maternal behavior.

The P-body protein 4E-T represses translation to regulate the balance between cell genesis and establishment of the postnatal NSC pool

Here, we ask how developing precursors maintain the balance between cell genesis for tissue growth and establishment of adult stem cell pools, focusing on postnatal forebrain neural precursor cells (NPCs). We show that these NPCs are transcriptionally primed to differentiate and that the primed mRNAs are associated with the translational repressor 4E-T. 4E-T also broadly associates with other NPC mRNAs encoding transcriptional regulators, and these are preferentially depleted from ribosomes, consistent with repression. By contrast, a second translational regulator, Cpeb4, associates with diverse target mRNAs that are largely ribosome associated. The 4E-T-dependent mRNA association is functionally important because 4E-T knockdown or conditional knockout derepresses proneurogenic mRNA translation and perturbs maintenance versus differentiation of early postnatal NPCs in culture and in vivo. Thus, early postnatal NPCs are primed to differentiate, and 4E-T regulates the balance between cell genesis and stem cell expansion by sequestering and repressing mRNAs encoding transcriptional regulators.

Spatiotemporal dynamics of the development of mouse olfactory system from prenatal to postnatal period

Introduction

The olfactory epithelium (OE) and olfactory bulb (OB) are the major components of the olfactory system and play critical roles in olfactory perception. However, the embryonic development of OE and OB by using the olfactory specific genes has not been comprehensively investigated yet. Most previous studies were limited to a specific embryonic stage, and very little is known, till date, about the development of OE.

Methods

The current study aimed to explore the development of mouse olfactory system by spatiotemporal analysis of the histological features by using the olfactory specific genes of olfactory system from the prenatal to postnatal period.

Results

We found that OE is divided into endo-turbinate, ecto-turbinate, and vomeronasal organs, and that putative OB with putative main and accessory OB is formed in the early developmental stage. The OE and OB became multilayered in the later developmental stages, accompanied by the differentiation of olfactory neurons. Remarkably, we found the development of layers of olfactory cilia and differentiation of OE to progress dramatically after birth, suggesting that the exposure to air may facilitate the final development of OE.

Discussion

Overall, the present study laid the groundwork for a better understanding of the spatial and temporal developmental events of the olfactory system.

Development of the gonadotropin-releasing hormone system

This review summarizes the current understanding of the development of the neuroendocrine gonadotropin-releasing hormone (GnRH) system, including discussion on open questions regarding (1) transcriptional regulation of the Gnrh1 gene; (2) prenatal development of the GnRH1 system in rodents and humans; and (3) paracrine and synaptic communication during migration of the GnRH cells.

Genetic Regulation of Vertebrate Forebrain Development by Homeobox Genes

Forebrain development in vertebrates is regulated by transcription factors encoded by homeobox, bHLH and forkhead gene families throughout the progressive and overlapping stages of neural induction and patterning, regional specification and generation of neurons and glia from central nervous system (CNS) progenitor cells. Moreover, cell fate decisions, differentiation and migration of these committed CNS progenitors are controlled by the gene regulatory networks that are regulated by various homeodomain-containing transcription factors, including but not limited to those of the Pax (paired), Nkx, Otx (orthodenticle), Gsx/Gsh (genetic screened), and Dlx (distal-less) homeobox gene families. This comprehensive review outlines the integral role of key homeobox transcription factors and their target genes on forebrain development, focused primarily on the telencephalon. Furthermore, links of these transcription factors to human diseases, such as neurodevelopmental disorders and brain tumors are provided.

Development of the mammalian main olfactory bulb

The mammalian main olfactory bulb is a crucial processing centre for the sense of smell. The olfactory bulb forms early during development and is functional from birth. However, the olfactory system continues to mature and change throughout life as a target of constitutive adult neurogenesis. Our Review synthesises current knowledge of prenatal, postnatal and adult olfactory bulb development, focusing on the maturation, morphology, functions and interactions of its diverse constituent glutamatergic and GABAergic cell types. We highlight not only the great advances in the understanding of olfactory bulb development made in recent years, but also the gaps in our present knowledge that most urgently require addressing.

Summary: This Review describes the morphological and functional maturation of cells in the mammalian main olfactory bulb, from embryonic development to adult neurogenesis.

The conserved core of the nereid brain: Circular CNS, apical nervous system and lhx6-arx-dlx neurons

When bilaterian animals first emerged, an enhanced perception of the Precambrian environment was key to their stunning success. This occurred through the acquisition of an anterior brain, as found in most extant bilaterians. What were the core circuits of the first brain, and how do they relate to today's diversity? With two landmark resources - the full connectome and a multimodal cellular atlas combining gene expression and ultrastructure - the young worm of the marine annelid Platynereis dumerilii takes center stage in comparative bilaterian neuroanatomy. The new data suggest a composite structure of the ancestral bilaterian brain, with the anterior end of a circular CNS fused to a sensory-neurosecretory apical system, and with lhx6-arx-dlx chemosensory circuits giving rise to associative centers in the descending bilaterian lineages.

Neural processing of the reward value of pleasant odorants

Pleasant odorants are represented in the posterior olfactory bulb (pOB) in mice. How does this hedonic information generate odor-motivated behaviors? Using optogenetics, we report here that stimulating the representation of pleasant odorants in a sensory structure, the pOB, can be rewarding, self-motivating, and is accompanied by ventral tegmental area activation. To explore the underlying neural circuitry downstream of the olfactory bulb (OB), we use 3D high-resolution imaging and optogenetics and determine that the pOB preferentially projects to the olfactory tubercle, whose increased activity is related to odorant attraction. We further show that attractive odorants act as reinforcers in dopamine-dependent place preference learning. Finally, we extend those findings to humans, who exhibit place preference learning and an increase BOLD signal in the olfactory tubercle in response to attractive odorants. Thus, strong and persistent attraction induced by some odorants is due to a direct gateway from the pOB to the reward system.

Expression of Oncofetal Antigen 5T4 in Murine Taste Papillae

Background: Multicellular taste buds located within taste papillae on the tongue mediate taste sensation. In taste papillae, taste bud cells (TBCs), such as taste receptor cells and taste precursor cells, and the surrounding lingual epithelium including epithelial progenitors (also called taste stem/progenitor cells) are maintained by continuous cell turnover throughout life. However, it remains unknown how the cells constituting taste buds proliferate and differentiate to maintain taste bud tissue. Based on in situ hybridization (ISH) screening, we demonstrated that the oncofetal antigen 5T4 (also known as trophoblast glycoprotein: TPBG) gene is expressed in the adult mouse tongue. Results: In immunohistochemistry of coronal tongue sections, 5T4 protein was detected at a low level exclusively in the basal part of the lingual epithelium in developing and adult mice, and at a high level particularly in foliate papillae and circumvallate papillae (CVPs). Furthermore, immunohistochemistry of the basal part of CVPs indicated that the proliferation marker PCNA (proliferating cell nuclear antigen) co-localized with 5T4. 5T4 was strongly expressed in Krt5+ epithelial progenitors and Shh+ taste precursor cells, but weakly in mature taste receptor cells. The number of proliferating cells in the CVP was higher in 5T4-knockout mice than in wild-type (WT) mice, while neither cell differentiation nor the size of taste buds differed between these two groups of mice. Notably, X-ray irradiation enhanced cell proliferation more in 5T4-knockout mice than in WT mice. Conclusion: Our results suggest that 5T4, expressed in epithelial progenitors (taste stem/progenitor cells), and taste precursor cells, may influence the maintenance of taste papillae under both normal and injury conditions.

Mitral cell development in the olfactory bulb of sharks: evidences of a conserved pattern of glutamatergic neurogenesis

In mammals, the development of the olfactory bulb (OB) relies in part on the expression of transcription factors involved in the specifications/differentiation of glutamatergic cells. In a previous study from our group, a high molecular similarity was reported between mammals and cartilaginous fishes regarding the neurogenic mechanisms underlying the development of glutamatergic cells in the telencephalon. However, information about the transcriptional program operating in the development of the glutamatergic system (mainly represented by mitral cells) in the OB is lacking in the catshark Scyliorhinus canicula, a cartilaginous fish. Using immunohistochemistry and in situ hybridization techniques, we have found that, previously to the appearance of the olfactory primordium (OP), proliferating cells expressing Pax6 with molecular hallmarks of progenitor radial glia were located in the ventrolateral pallial ventricular zone. Later in development, when the OP is recognizable, a stream of Pax6-positive cells were observed between the ventricular zone and the OP, where transcription factors involved in mitral cell development in mammals (ScTbr2, ScNeuroD, Tbr1) are expressed. Later in development, these transcription factors became expressed in a layered-like structure where ScVglut1, a marker of mitral cells, is also present. Our data suggest that the transcriptional program related with the specification/differentiation of glutamatergic cells in the telencephalon has been conserved throughout the evolution of vertebrates. These results, in combination with previous studies concerning GABAergic neurogenesis in sharks, have evidenced that the OB of mammals and sharks shares similarities in the timing and molecular programs of development.

Regional differences in mitral cell development in mouse olfactory bulb

Olfactory sensory neurons (OSNs) located in the dorsomedial and ventromedial regions of the olfactory epithelium (OE) are distinguished from one another based on their molecular expression patterns. This difference is reflected in the separation of the glomerular layer of the olfactory bulb (OB) into dorsomedial and ventrolateral regions. However, it is unclear whether a complementary separation is also evident in the projection neurons that innervate the OB glomeruli. In this study, we compared the development of the OB between different regions by focusing on the transcription factor, Tbx21, which is expressed by mitral and tufted cells in the mature OB. Examining the OB at different developmental ages, we found that Tbx21 expression commenced in the anteromedial region called the tongue-shaped area, followed by the dorsomedial and then ventrolateral areas. We also showed that the tongue-shaped area was innervated by the OSNs located in the most dorsomedial part of the ventrolateral OE, the V-zone:DM. Interestingly, the generation of OSNs occurred first in the dorsomedial zone including the V-zone:DM, suggesting a correlation between the time course of OSN generation in the OE and Tbx21 expression in their target region of the OB. In contrast, expression of vGluT1, which is also found in all mitral cells in the mature OB, was first detected in the ventrolateral region during development. Our findings demonstrate that the development of projection neurons occurs in a compartmentalized manner in the OB; tongue-shaped, dorsomedial, and ventrolateral areas, and that not all projection neurons follow the same developmental pathway.

Mitral cells and the glucagon-like peptide 1 receptor: The sweet smell of success?

The olfactory bulb (OB) is often affected at very early stages of neurodegenerative disorders, in the so-called "prodromal" phase. In Parkinson's disease (PD), olfactory disturbances appear years before motor symptoms arise. Additionally, pathological alpha-synuclein aggregates are found in olfactory regions before spreading to other areas of the brain. Being positioned at the frontier between the brain and a potentially hostile environment, could explain the particular vulnerability of the OB. Mitral cells (MCs), the principal projecting neurons of the olfactory system, are involved in the pathogenesis and in the prion-like progression of PD. They are affected by Lewy pathology and are thought to contribute to the axonal transport of misfolded alpha-synuclein to other regions of the brain. Here, we first describe the main markers reported to distinguish MCs from other olfactory neurons. We focus on the glucagon-like peptide 1 receptor (GLP-1R), a membrane protein specifically expressed in MCs. After summarizing OB pathology, we explore the idea of targeting specifically MCs with GLP-1 or its analogues. Exenatide has shown great promise as a neuroprotective and neurorestorative agent and has been used in a clinical trial for clinical PD. Since GLP-1R activation has the ability to mitigate many facets of prodromal PD pathology, we postulate that once a robust biomarker is in place that is capable of identifying individuals in the prodromal phase of PD, homing in on GLP-1R could assist in deferring, or eradicating to a significant degree, the clinical manifestation of this debilitating human disorder.

The transcription factor Tfap2e/AP-2ε plays a pivotal role in maintaining the identity of basal vomeronasal sensory neurons

The identity of individual neuronal cell types is defined and maintained by the expression of specific combinations of transcriptional regulators that control cell type-specific genetic programs. The epithelium of the vomeronasal organ of mice contains two major types of vomeronasal sensory neurons (VSNs): 1) the apical VSNs which express vomeronasal 1 receptors (V1r) and the G-protein subunit Gαi2 and; 2) the basal VSNs which express vomeronasal 2 receptors (V2r) and the G-protein subunit Gαo. Both cell types originate from a common pool of progenitors and eventually acquire apical or basal identity through largely unknown mechanisms. The transcription factor AP-2ε, encoded by the Tfap2e gene, plays a role in controlling the development of GABAergic interneurons in the main and accessory olfactory bulb (AOB), moreover AP-2ε has been previously described to be expressed in the basal VSNs. Here we show that AP-2ε is expressed in post-mitotic VSNs after they commit to the basal differentiation program. Loss of AP-2ε function resulted in reduced number of basal VSNs and in an increased number of neurons expressing markers of the apical lineage. Our work suggests that AP-2ε, which is expressed in late phases of differentiation, is not needed to initiate the apical-basal differentiation dichotomy but for maintaining the basal VSNs' identity. In AP-2ε mutants we observed a large number of cells that entered the basal program can express apical genes, our data suggest that differentiated VSNs of mice retain a notable level of plasticity.

Transcriptional and Epigenetic Control of Mammalian Olfactory Epithelium Development

The postnatal mammalian olfactory epithelium (OE) represents a major aspect of the peripheral olfactory system. It is a pseudostratified tissue that originates from the olfactory placode and is composed of diverse cells, some of which are specialized receptor neurons capable of transducing odorant stimuli to afford the perception of smell (olfaction). The OE is known to offer a tractable miniature model for studying the systematic generation of neurons and glia that typify neural tissue development. During OE development, stem/progenitor cells that will become olfactory sensory neurons and/or non-neuronal cell types display fine spatiotemporal expression of neuronal and non-neuronal genes that ensures their proper proliferation, differentiation, survival, and regeneration. Many factors, including transcription and epigenetic factors, have been identified as key regulators of the expression of such requisite genes to permit normal OE morphogenesis. Typically, specific interactive regulatory networks established between transcription and epigenetic factors/cofactors orchestrate histogenesis in the embryonic and adult OE. Hence, investigation of these regulatory networks critical for OE development promises to disclose strategies that may be employed in manipulating the stepwise transition of olfactory precursor cells to become fully differentiated and functional neuronal and non-neuronal cell types. Such strategies potentially offer formidable means of replacing injured or degenerated neural cells as therapeutics for nervous system perturbations. This review recapitulates the developmental cellular diversity of the olfactory neuroepithelium and discusses findings on how the precise and cooperative molecular control by transcriptional and epigenetic machinery is indispensable for OE ontogeny.

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

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

The terminal nerve plays a prominent role in GnRH-1 neuronal migration independent from proper olfactory and vomeronasal connections to the olfactory bulbs

Gonadotropin-releasing hormone-1 (GnRH-1) neurons (GnRH-1 ns) migrate from the developing olfactory pit into the hypothalamus during embryonic development. Migration of the GnRH-1 neurons is required for mammalian reproduction as these cells control release of gonadotropins from the anterior pituitary gland. Disturbances in GnRH-1 ns migration, GnRH-1 synthesis, secretion or signaling lead to varying degrees of hypogonadotropic hypogonadism (HH), which impairs pubertal onset and fertility. HH associated with congenital olfactory defects is clinically defined as Kallmann Syndrome (KS). The association of olfactory defects with HH in KS suggested a potential direct relationship between defective olfactory axonal routing, lack of olfactory bulbs (OBs) and aberrant GnRH-1 ns migration. However, it has never been experimentally proven that the formation of axonal connections of the olfactory/vomeronasal neurons to their functional targets are necessary for the migration of GnRH-1 ns to the hypothalamus. Loss-of-function of the Arx-1 homeobox gene leads to the lack of proper formation of the OBs with abnormal axonal termination of olfactory sensory neurons ( Yoshihara et al., 2005). Our data prove that correct development of the OBs and axonal connection of the olfactory/vomeronasal sensory neurons to the forebrain are not required for GnRH-1 ns migration, and suggest that the terminal nerve, which forms the GnRH-1 migratory scaffold, follows different guidance cues and differs in gene expression from olfactory/vomeronasal sensory neurons.

Summary: Our work reveals that correct olfactory bulb development is not required for GnRH-1 neuronal migration. This study challenges the idea that GnRH-1 neuronal migration to the hypothalamus relies on correct routing of the olfactory and vomeronasal neurons and supports the existence of the TN in mammals.

A Subtype of Olfactory Bulb Interneurons Is Required for Odor Detection and Discrimination Behaviors

Neural circuits that undergo reorganization by newborn interneurons in the olfactory bulb (OB) are necessary for odor detection and discrimination, olfactory memory, and innate olfactory responses, including predator avoidance and sexual behaviors. The OB possesses many interneurons, including various types of granule cells (GCs); however, the contribution that each type of interneuron makes to olfactory behavioral control remains unknown. Here, we investigated the in vivo functional role of oncofetal trophoblast glycoprotein 5T4, a regulator for dendritic arborization of 5T4-expressing GCs (5T4 GCs), the level of which is reduced in the OB of 5T4 knock-out (KO) mice. Electrophysiological recordings with acute OB slices indicated that external tufted cells (ETCs) can be divided into two types, bursting and nonbursting. Optogenetic stimulation of 5T4 GCs revealed their connection to both bursting and nonbursting ETCs, as well as to mitral cells (MCs). Interestingly, nonbursting ETCs received fewer inhibitory inputs from GCs in 5T4 KO mice than from those in wild-type (WT) mice, whereas bursting ETCs and MCs received similar inputs in both mice. Furthermore, 5T4 GCs received significantly fewer excitatory inputs in 5T4 KO mice. Remarkably, in olfactory behavior tests, 5T4 KO mice had higher odor detection thresholds than the WT, as well as defects in odor discrimination learning. Therefore, the loss of 5T4 attenuates inhibitory inputs from 5T4 GCs to nonbursting ETCs and excitatory inputs to 5T4 GCs, contributing to disturbances in olfactory behavior. Our novel findings suggest that, among the various types of OB interneurons, the 5T4 GC subtype is required for odor detection and discrimination behaviors.

SIGNIFICANCE STATEMENT

Neuronal circuits in the brain include glutamatergic principal neurons and GABAergic interneurons. Although the latter is a minority cell type, they are vital for normal brain function because they regulate the activity of principal neurons. If interneuron function is impaired, brain function may be damaged, leading to behavior disorder. The olfactory bulb (OB) possesses various types of interneurons, including granule cells (GCs); however, the contribution that each type of interneuron makes to the control of olfactory behavior remains unknown. Here, we analyzed electrophysiologically and behaviorally the function of oncofetal trophoblast glycoprotein 5T4, a regulator for dendritic branching in OB GCs. We found that, among the various types of OB interneuron, the 5T4 GC subtype is required for odor detection and odor discrimination behaviors.

Profiles of Periglomerular Cells in the Olfactory Bulb of Prokineticin Type 2 Receptor-deficient Mice

Both prokineticin receptor 2 (pkr2) and prokineticin 2 (pk2) gene-deficient mice have hypoplasia of the main olfactory bulb (MOB). This hypoplasia has been attributed to disruption of the glomerulus that is caused by loss of afferent projection from olfactory sensory neurons (OSN), and to the impaired migration of granule cells, a type of interneuron. In the present study, we examined whether migration of the second type of interneuron, periglomerular cells (PGC), is dependent on the pkr2 expression by observing the localization of distinct subpopulations of PGC: calretinin (CR)-, calbindin (CB)- and tyrosine hydroxylase (TH)-expressing neurons. In the Pkr2^−/− mice, the construction of the layered structure of the MOB was partially preserved, with the exception of the internal plexiform layer (IPL) and the glomerular layer (GL). In the outermost layer of the MOB, abundant CR- and CB-immunopositive neurons were observed in the hypoplastic olfactory bulb. In addition, although markedly decreased, TH-immunopositive neurons were also observed in the outermost cell-dense region in the Pkr2^−/−. The findings suggest that the migration of PGC to the MOB, as well as the migration from the core to the surface region of the MOB, is not driven by the PK2-PKR2 system.

Determination of the connectivity of newborn neurons in mammalian olfactory circuits

The mammalian olfactory bulb is a forebrain structure just one synapse downstream from the olfactory sensory neurons and performs the complex computations of sensory inputs. The formation of this sensory circuit is shaped through activity-dependent and cell-intrinsic mechanisms. Recent studies have revealed that cell-type specific connectivity and the organization of synapses in dendritic compartments are determined through cell-intrinsic programs already preset in progenitor cells. These progenitor programs give rise to subpopulations within a neuron type that have distinct synaptic organizations. The intrinsically determined formation of distinct synaptic organizations requires factors from contacting cells that match the cell-intrinsic programs. While certain genes control wiring within the newly generated neurons, other regulatory genes provide intercellular signals and are only expressed in neurons that will form contacts with the newly generated cells. Here, the olfactory system has provided a useful model circuit to reveal the factors regulating assembly of the highly structured connectivity in mammals.

Olfactory Sensory Neurons Control Dendritic Complexity of Mitral Cells via Notch Signaling

Mitral cells (MCs) of the mammalian olfactory bulb have a single primary dendrite extending into a single glomerulus, where they receive odor information from olfactory sensory neurons (OSNs). Molecular mechanisms for controlling dendritic arbors of MCs, which dynamically change during development, are largely unknown. Here we found that MCs displayed more complex dendritic morphologies in mouse mutants of Maml1, a crucial gene in Notch signaling. Similar phenotypes were observed by conditionally misexpressing a dominant negative form of MAML1 (dnMAML1) in MCs after their migration. Conversely, conditional misexpression of a constitutively active form of Notch reduced their dendritic complexity. Furthermore, the intracellular domain of Notch1 (NICD1) was localized to nuclei of MCs. These findings suggest that Notch signaling at embryonic stages is involved in the dendritic complexity of MCs. After the embryonic misexpression of dnMAML1, many MCs aberrantly extended dendrites to more than one glomerulus at postnatal stages, suggesting that Notch signaling is essential for proper formation of olfactory circuits. Moreover, dendrites in cultured MCs were shortened by Jag1-expressing cells. Finally, blocking the activity of Notch ligands in OSNs led to an increase in dendritic complexity as well as a decrease in NICD1 signals in MCs. These results demonstrate that the dendritic complexity of MCs is controlled by their presynaptic partners, OSNs.

Olfactory circuits are critical for the survival of many animals. Odor information is transmitted from olfactory sensory neurons (OSNs) to relay neurons, the morphology of which is crucial for processing of the information and similar among species. The major relay neurons, mitral cells (MCs) in mammals and projection neurons in flies, have a single primary dendrite at the mature stage. Molecular mechanisms to control the formation of the dendrite are largely unknown. MCs dynamically change their dendrites during development. In this study, we show that the dendritic morphologies of MCs are controlled by Notch signaling, many factors of which are well conserved among species. Moreover, we have found that Notch signaling in MCs is activated by OSNs, and that Notch operates in the relay neurons in the mouse olfactory system, in contrast to the fly system, where Notch functions in OSNs. Therefore, our study has revealed a novel step for shaping the dendritic morphologies of MCs.

Axonal branching in lateral olfactory tract is promoted by Nogo signaling

Mitral cells are major projection neurons of the olfactory bulb (OB) that form an axonal bundle known as the lateral olfactory tract (LOT). After axonal bundle formation, collateral branches sprout from primary axons of the LOT. Recently, we identified LOT usher substance (LOTUS) as an endogenous Nogo receptor-1 (NgR1) antagonist and demonstrated that LOTUS contributes to the formation of the LOT axonal bundle. Immunoblots revealed that the expression level of Nogo-A in the OB developmentally increased during axonal collateral formation. Next, we found that the axonal collateral branches were increased in cultured OB neurons from LOTUS-knockout (KO) mice, whereas they were decreased in cultured OB neurons from NgR1-KO mice. Knockdown of Nogo-A in cultured OB neurons reduced the number of axonal collateral branches, suggesting that endogenous Nogo-A induces axonal branching. Finally, the collateral branches of the LOT were increased in LOTUS-KO mice, whereas those in NgR1-KO mice were decreased. Moreover, the abnormal increase of axonal branching observed in LOTUS-KO mice was rescued in the double mutant of LOTUS- and NgR1-KO mice. These findings suggest that Nogo-A and NgR1 interactions may contribute to axonal branching in LOT development.

Lgl1 Is Required for Olfaction and Development of Olfactory Bulb in Mice

Lethal giant larvae 1 (Lgl1) was initially identified as a tumor suppressor in Drosophila and functioned as a key regulator of epithelial polarity and asymmetric cell division. In this study, we generated Lgl1 conditional knockout mice mediated by Pax2-Cre, which is expressed in olfactory bulb (OB). Next, we examined the effects of Lgl1 loss in the OB. First, we determined the expression patterns of Lgl1 in the neurogenic regions of the embryonic dorsal region of the LGE (dLGE) and postnatal OB. Furthermore, the Lgl1 conditional mutants exhibited abnormal morphological characteristics of the OB. Our behavioral analysis exhibited greatly impaired olfaction in Lgl1 mutant mice. To elucidate the possible mechanisms of impaired olfaction in Lgl1 mutant mice, we investigated the development of the OB. Interestingly, reduced thickness of the MCL and decreased density of mitral cells (MCs) were observed in Lgl1 mutant mice. Additionally, we observed a dramatic loss in SP8⁺ interneurons (e.g. calretinin and GABAergic/non-dopaminergic interneurons) in the GL of the OB. Our results demonstrate that Lgl1 is required for the development of the OB and the deletion of Lgl1 results in impaired olfaction in mice.

Cell migration in the developing rodent olfactory system

The components of the nervous system are assembled in development by the process of cell migration. Although the principles of cell migration are conserved throughout the brain, different subsystems may predominantly utilize specific migratory mechanisms, or may display unusual features during migration. Examining these subsystems offers not only the potential for insights into the development of the system, but may also help in understanding disorders arising from aberrant cell migration. The olfactory system is an ancient sensory circuit that is essential for the survival and reproduction of a species. The organization of this circuit displays many evolutionarily conserved features in vertebrates, including molecular mechanisms and complex migratory pathways. In this review, we describe the elaborate migrations that populate each component of the olfactory system in rodents and compare them with those described in the well-studied neocortex. Understanding how the components of the olfactory system are assembled will not only shed light on the etiology of olfactory and sexual disorders, but will also offer insights into how conserved migratory mechanisms may have shaped the evolution of the brain.

RNA-seq analysis of developing olfactory bulb projection neurons

Transmission of olfactory information to higher brain regions is mediated by olfactory bulb (OB) projection neurons, the mitral and tufted cells. Although mitral/tufted cells are often characterized as the OB counterpart of cortical projection neurons (also known as pyramidal neurons), they possess several unique morphological characteristics and project specifically to the olfactory cortices. Moreover, the molecular networks contributing to the generation of mitral/tufted cells during development are largely unknown. To understand the developmental patterns of gene expression in mitral/tufted cells in the OB, we performed transcriptome analyses targeting purified OB projection neurons at different developmental time points with next-generation RNA sequencing (RNA-seq). Through these analyses, we found 1202 protein-coding genes that are temporally differentially-regulated in developing projection neurons. Among them, 388 genes temporally changed their expression level only in projection neurons. The data provide useful resource to study the molecular mechanisms regulating development of mitral/tufted cells. We further compared the gene expression profiles of developing mitral/tufted cells with those of three cortical projection neuron subtypes, subcerebral projection neurons, corticothalamic projection neurons, and callosal projection neurons, and found that the molecular signature of developing olfactory projection neuron bears resemblance to that of subcerebral neurons. We also identified 3422 events that change the ratio of splicing isoforms in mitral/tufted cells during maturation. Interestingly, several genes expressed a novel isoform not previously reported. These results provide us with a broad perspective of the molecular networks underlying the development of OB projection neurons.

Activating transcription factor 5 is required for mouse olfactory bulb development via interneuron

Activating transcription factor 5 (ATF5) is a stress response transcription factor of the cAMP-responsive element-binding/ATF family. Earlier, we reported that ATF5 expression is up-regulated in response to stress, such as amino acid limitation or arsenite exposure. Although ATF5 is widely expressed in the brain and the olfactory epithelium, the role of ATF5 is not fully understood. Here, the olfactory bulbs (OBs) of ATF5-deficient mice are smaller than those of wild-type mice. Histological analysis reveals the disturbed laminar structure of the OB, showing the thinner olfactory nerve layer, and a reduced number of interneurons. This is mainly due to the reduced number of bromodeoxyuridine-positive proliferating cells in the subventricular zone, where the interneuron progenitors are formed and migrate to the OBs. Moreover, the olfaction-related aggressive behavior of ATF5-deficient mice is reduced compared to wild-type mice. Our data suggest that ATF5 plays a crucial role in mouse OB development via interneuron.

Segregated labeling of olfactory bulb projection neurons based on their birthdates

Mitral and tufted cells are the projection neurons of the olfactory bulb (OB). We previously reported that somata location and innervation patterns were different between early- and late-born mitral cells (Imamura et al., 2011). Here, we introduced a plasmid that drives the expression of a GFP gene into the mouse OB using in utero electroporation, and demonstrated that we can deliver the plasmid vectors into distinct subsets of OB projection neurons by changing the timing of electroporation after fertilisation. The electroporation performed at embryonic day (E)10 preferentially labeled mitral cells in the accessory OB and main OB mitral cells in dorsomedial mitral cell layer (MCL). In contrast, the E12 electroporation introduced the plasmid vectors preferentially into main OB mitral cells in the ventrolateral MCL and tufted cells. Combining these data with BrdU injections, we confirmed that E10 and E12 electroporation preferentially labeled early- and late-born projection neurons, respectively. This work introduces a novel method for segregated labeling of mouse olfactory bulb projection neurons based on their birthdates. With this technique we found that early- and late-born projection neurons extend their secondary dendrites in the deep and superficial external plexiform layer (EPL), respectively. Although a similar segregation has been suggested for mitral vs. tufted cell dendrites, we found mitral cells projecting secondary dendrites into the superficial EPL in E12-electroporated main OB. Our observations indicate that timing of neurogenesis regulates not only somata location and innervation patterns but also the laminar organisation of projection neuron dendrites in the EPL.

ARX/Arx is expressed in germ cells during spermatogenesis in both marsupial and mouse

The X-linked aristaless gene, ARX, is essential for the development of the gonads, forebrain, olfactory bulb, pancreas, and skeletal muscle in mice and humans. Mutations cause neurological diseases, often accompanied by ambiguous genitalia. There are a disproportionately high number of testis and brain genes on the human and mouse X chromosomes. It is still unknown whether the X chromosome accrued these genes during its evolution or whether genes that find themselves on the X chromosome evolve such roles. ARX was originally autosomal in mammals and remains so in marsupials, whereas in eutherian mammals it translocated to the X chromosome. In this study, we examined autosomal ARX in tammars and compared it with the X-linked Arx in mice. We detected ARX mRNA in the neural cells of the forebrain, midbrain and hindbrain, and olfactory bulbs in developing tammars, consistent with the expression in mice. ARX was detected by RT-PCR and mRNA in situ hybridization in the developing tammar wallaby gonads of both sexes, suggestive of a role in sexual development as in mice. We also detected ARX/Arx mRNA in the adult testis in both tammars and mice, suggesting a potential novel role for ARX/Arx in spermiogenesis. ARX transcripts were predominantly observed in round spermatids. Arx mRNA localization distributions in the mouse adult testis suggest that it escaped meiotic sex chromosome inactivation during spermatogenesis. Our findings suggest that ARX in the therian mammal ancestor already played a role in male reproduction before it was recruited to the X chromosome in eutherians.

Parvalbumin-expressing interneurons linearly control olfactory bulb output

In the olfactory bulb, odor representations by principal mitral cells are modulated by local inhibitory circuits. While dendrodendritic synapses between mitral and granule cells are typically thought to be a major source of this modulation, the contributions of other inhibitory neurons remain unclear. Here we demonstrate the functional properties of olfactory bulb parvalbumin-expressing interneurons (PV cells) and identify their important role in odor coding. Using paired recordings, we find that PV cells form reciprocal connections with the majority of nearby mitral cells, in contrast to the sparse connectivity between mitral and granule cells. In vivo calcium imaging in awake mice reveals that PV cells are broadly tuned to odors. Furthermore, selective PV cell inactivation enhances mitral cell responses in a linear fashion while maintaining mitral cell odor preferences. Thus, dense connections between mitral and PV cells underlie an inhibitory circuit poised to modulate the gain of olfactory bulb output.

Aristaless related homeobox gene, Arx, is implicated in mouse fetal Leydig cell differentiation possibly through expressing in the progenitor cells

Development of the testis begins with the expression of the SRY gene in pre-Sertoli cells. Soon after, testis cords containing Sertoli and germ cells are formed and fetal Leydig cells subsequently develop in the interstitial space. Studies using knockout mice have indicated that multiple genes encoding growth factors and transcription factors are implicated in fetal Leydig cell differentiation. Previously, we demonstrated that the Arx gene is implicated in this process. However, how ARX regulates Leydig cell differentiation remained unknown. In this study, we examined Arx KO testes and revealed that fetal Leydig cell numbers largely decrease throughout the fetal life. Since our study shows that fetal Leydig cells rarely proliferate, this decrease in the KO testes is thought to be due to defects of fetal Leydig progenitor cells. In sexually indifferent fetal gonads of wild type, ARX was expressed in the coelomic epithelial cells and cells underneath the epithelium as well as cells at the gonad-mesonephros border, both of which have been described to contain progenitors of fetal Leydig cells. After testis differentiation, ARX was expressed in a large population of the interstitial cells but not in fetal Leydig cells, raising the possibility that ARX-positive cells contain fetal Leydig progenitor cells. When examining marker gene expression, we observed cells as if they were differentiating into fetal Leydig cells from the progenitor cells. Based on these results, we propose that ARX acts as a positive factor for differentiation of fetal Leydig cells through functioning at the progenitor stage.

Nuclear import of aristaless-related homeobox protein via its NLS1 regulates its transcriptional function

Nucleocytoplasmic transport of transcription factors is essential in eukaryotes. We previously reported the presence of two functional NLSs in the homeodomain protein, aristaless-related homeobox (Arx) protein, which is a key transcriptional repressor of LMO1, SHOX2, and PAX4 during development. NLS2, that overlaps the homeodomain, is recognized directly by multiple importin βs, but not by importin αs. In this study, we found that the N-terminal NLS1 of Arx is targeted by multiple importin α proteins, including importin α3 and α5. Both in vivo and in vitro assays demonstrated that nuclear import of Arx via NLS1 is mediated by the importin α/β pathway. Mutagenesis analysis indicated that two basic amino acids, (84)K and (87)R, are essential to the function of NLS1, and that their mutation prevents interactions of Arx with importin αs. Interestingly, inhibition of nuclear import of Arx via NLS1 clearly attenuates its ability of transcriptional repression, suggesting that nuclear import of Arx via NLS1 contributes to its transcriptional function.

Mitral and tufted cells are potential cellular targets of nitration in the olfactory bulb of aged mice

Olfactory sensory function declines with age; though, the underlying molecular changes that occur in the olfactory bulb (OB) are relatively unknown. An important cellular signaling molecule involved in the processing, modulation, and formation of olfactory memories is nitric oxide (NO). However, excess NO can result in the production of peroxynitrite to cause oxidative and nitrosative stress. In this study, we assessed whether changes in the expression of 3-nitrotyrosine (3-NT), a neurochemical marker of peroxynitrite and thus oxidative damage, exists in the OB of young, adult, middle-aged, and aged mice. Our results demonstrate that OB 3-NT levels increase with age in normal C57BL/6 mice. Moreover, in aged mice, 3-NT immunoreactivity was found in some blood vessels and microglia throughout the OB. Notably, large and strongly immunoreactive puncta were found in mitral and tufted cells, and these were identified as lipofuscin granules. Additionally, we found many small-labeled puncta within the glomeruli of the glomerular layer and in the external plexiform layer, and these were localized to mitochondria and discrete segments of mitral and tufted dendritic plasma membranes. These results suggest that mitral and tufted cells are potential cellular targets of nitration, along with microglia and blood vessels, in the OB during aging.

Transitory and activity-dependent expression of neurogranin in olfactory bulb tufted cells during mouse postnatal development

Neurogranin (Ng) is a brain-specific postsynaptic calmodulin-binding protein involved in synaptic activity-dependent plasticity. In the adult olfactory bulb (OB), Ng is expressed by a large population of GABAergic interneurons in the granule cell layer. We show here that, during postnatal development, Ng is also expressed by OB neurons in the superficial external plexiform layer (sEPL) and glomerular layer (GL). These Ng-positive neurons display morphological and neurochemical features of superficial and external tufted cells. Ng expression in these cells is transient during OB development: few elements express Ng at postnatal day (P) 5, increasing in number and reaching a peak at P10, then progressively decreasing. At P30, Ng is rarely detectable in these neurons. Ng expression in developing tufted cells is also modulated at the cellular level: at earlier stages, Ng labeling is distributed throughout the cell body and dendritic arborization in the GL, but, at P20, when the glomerular circuits are fully matured, Ng becomes restricted to the soma and proximal portion of tufted cell apical dendrites. We show that olfactory deprivation at early postnatal stages induces a strong increase in Ng-positive tufted cells from P10 to P20, whereas no changes have been observed following olfactory deprivation in adult mice. These findings demonstrate that Ng expression in sEPL-GL is restricted to developmental stages and indicate its activity-dependent regulation in a time window critical for glomerular circuit development, suggesting a role for Ng in maturation and dendritic remodeling of tufted cells.

Dynamic sensory representations in the olfactory bulb: modulation by wakefulness and experience

How are sensory representations in the brain influenced by the state of an animal? Here we use chronic two-photon calcium imaging to explore how wakefulness and experience shape odor representations in the mouse olfactory bulb. Comparing the awake and anesthetized state, we show that wakefulness greatly enhances the activity of inhibitory granule cells and makes principal mitral cell odor responses more sparse and temporally dynamic. In awake mice, brief repeated odor experience leads to a gradual and long-lasting (months) weakening of mitral cell odor representations. This mitral cell plasticity is odor specific, recovers gradually over months, and can be repeated with different odors. Furthermore, the expression of this experience-dependent plasticity is prevented by anesthesia. Together, our results demonstrate the dynamic nature of mitral cell odor representations in awake animals, which is constantly shaped by recent odor experience.

Dynamic changes in the transcriptional profile of subventricular zone-derived postnatally born neuroblasts

The subventricular zone (SVZ) of the lateral ventricles is a major neurogenic region in the postnatal mammalian brain. Thousands of neuroblasts are generated daily throughout the life of an animal. Newly born neuroblasts migrate via the rostral migratory stream (RMS) into the olfactory bulb where they mature into distinct neuronal subtypes. Neuroblasts exiting the SVZ retain the ability to proliferate, however, proliferation declines in the course of migration to the olfactory bulb. While migrating in the RMS, neuroblasts receive a plethora of stimuli that modify transcription according to the local microenvironment, and eventually modulate neuroblast migration. In the target area, the olfactory bulb, neuroblasts develop into mature neurons. In this review, we discuss dynamic changes of the transcriptome that occur during the "lifetime" of a neuroblast, thereby governing the activation or inhibition of distinct genes/pathways that are responsible for proliferation, migration and differentiation.

Tbr2 deficiency in mitral and tufted cells disrupts excitatory-inhibitory balance of neural circuitry in the mouse olfactory bulb

The olfactory bulb (OB) is the first relay station in the brain where odor information from the olfactory epithelium is integrated, processed through its intrinsic neural circuitry, and conveyed to higher olfactory centers. Compared with profound mechanistic insights into olfactory axon wiring from the nose to the OB, little is known about the molecular mechanisms underlying the formation of functional neural circuitry among various types of neurons inside the OB. T-box transcription factor Tbr2 is expressed in various types of glutamatergic excitatory neurons in the brain including the OB projection neurons, mitral and tufted cells. Here we generated conditional knockout mice in which the Tbr2 gene is inactivated specifically in mitral and tufted cells from late embryonic stages. Tbr2 deficiency caused cell-autonomous changes in molecular expression including a compensatory increase of another T-box member, Tbr1, and a concomitant shift of vesicular glutamate transporter (VGluT) subtypes from VGluT1 to VGluT2. Tbr2-deficient mitral and tufted cells also exhibited anatomical abnormalities in their dendritic morphology and projection patterns. Additionally, several non-cell-autonomous phenotypes were observed in parvalbumin-, calbindin-, and 5T4-positive GABAergic interneurons. Furthermore, the number of dendrodendritic reciprocal synapses between mitral/tufted cells and GABAergic interneurons was significantly reduced. Upon stimulation with odorants, larger numbers of mitral and tufted cells were activated in Tbr2 conditional knockout mice. These results suggest that Tbr2 is required for not only the proper differentiation of mitral and tufted cells, but also for the establishment of functional neuronal circuitry in the OB and maintenance of excitatory-inhibitory balance crucial for odor information processing.

Neurog1 and Neurog2 coordinately regulate development of the olfactory system

Background

Proneural genes encode basic helix–loop–helix transcription factors that specify distinct neuronal identities in different regions of the nervous system. In the embryonic telencephalon, the proneural genes Neurog1 and Neurog2 specify a dorsal regional identity and glutamatergic projection neuron phenotype in the presumptive neocortex, but their roles in cell fate specification in the olfactory bulb, which is also partly derived from dorsal telencephalic progenitors, have yet to be assessed. Given that olfactory bulb development is guided by interactions with the olfactory epithelium in the periphery, where proneural genes are also expressed, we investigated the roles of Neurog1 and Neurog2 in the coordinated development of these two olfactory structures.

Results

Neurog1/2 are co-expressed in olfactory bulb progenitors, while only Neurog1 is widely expressed in progenitors for olfactory sensory neurons in the olfactory epithelium. Strikingly, only a remnant of an olfactory bulb forms in Neurog1^−/−;Neurog2^−/− double mutants, while this structure is smaller but distinguishable in Neurog1^−/− single mutants and morphologically normal in Neurog2^−/− single mutants. At the cellular level, fewer glutamatergic mitral and juxtaglomerular cells differentiate in Neurog1^−/−;Neurog2^−/− double-mutant olfactory bulbs. Instead, ectopic olfactory bulb interneurons are derived from dorsal telencephalic lineages in Neurog1^−/−;Neurog2^−/− double mutants and to a lesser extent in Neurog2^−/− single mutants. Conversely, cell fate specification is normal in Neurog1^−/− olfactory bulbs, but aberrant patterns of cell proliferation and neuronal migration are observed in Neurog1^−/− single and Neurog1^−/−;Neurog2^−/− double mutants, probably contributing to their altered morphologies. Finally, in Neurog1^−/− and Neurog1^−/−;Neurog2^−/− embryos, olfactory sensory neurons in the epithelium, which normally project to the olfactory bulb to guide its morphogenesis, fail to innervate the olfactory bulb.

Conclusions

We have identified a cell autonomous role for Neurog1/2 in specifying the glutamatergic identity of olfactory bulb neurons. Furthermore, Neurog1 (and not Neurog2) is required to guide olfactory sensory neuron innervation of the olfactory bulb, the loss of which results in defects in olfactory bulb proliferation and tissue morphogenesis. We thus conclude that Neurog1/2 together coordinate development of the olfactory system, which depends on tissue interactions between the olfactory bulb and epithelium.

Analysis of transcriptional codes for zebrafish dopaminergic neurons reveals essential functions of Arx and Isl1 in prethalamic dopaminergic neuron development

Distinct groups of dopaminergic neurons develop at defined anatomical sites in the brain to modulate function of a large diversity of local and far-ranging circuits. However, the molecular identity as judged from transcription factor expression has not been determined for all dopaminergic groups. Here, we analyze regional expression of transcription factors in the larval zebrafish brain to determine co-expression with the Tyrosine hydroxylase marker in dopaminergic neurons. We define sets of transcription factors that clearly identify each dopaminergic group. These data confirm postulated relations to dopaminergic groups defined for mammalian systems. We focus our functional analysis on prethalamic dopaminergic neurons, which co-express the transcription factors Arx and Isl1. Morpholino-based knockdown reveals that both Arx and Isl1 are strictly required for prethalamic dopaminergic neuron development and appear to act in parallel. We further show that Arx contributes to patterning in the prethalamic region, while Isl1 is required for differentiation of prethalamic dopaminergic neurons.

5T4 glycoprotein regulates the sensory input-dependent development of a specific subtype of newborn interneurons in the mouse olfactory bulb

Sensory input has been shown to regulate development in a variety of species and in various structures, including the retina, cortex, and olfactory bulb (OB). Within the mammalian OB specifically, the development of dendrites in mitral/tufted cells is well known to be odor-evoked activity dependent. However, little is known about the developmental role of sensory input in the other major OB population of the GABAgenic interneurons, such as granule cells and periglomerular cells. Here, we identified, with DNA microarray and in situ hybridization screenings, a trophoblast glycoprotein gene, 5T4, whose expression in a specific subtype of OB interneurons is dependent on sensory input. 5T4 is a type I membrane protein, whose extracellular domain contains seven leucine-rich repeats (LRR) flanked by characteristic LRR-N-flanking and C-flanking regions, and a cytoplasmic domain. 5T4 overexpression in the newborn OB interneurons facilitated their dendritic arborization even under the sensory input-deprived condition. By contrast, both 5T4 knockdown with RNAi and 5T4 knockout with mice resulted in a significant reduction in the dendritic arborization of 5T4(+) granule cells. Further, we identified the amino acid sequence in the 5T4 cytoplasmic domain that is necessary and sufficient for the sensory input-dependent dendritic shaping of specific neuronal subtypes in the OB. Thus, these results demonstrate that 5T4 glycoprotein contributes in the regulation of activity-dependent dendritic development of interneurons and the formation of functional neural circuitry in the OB.

Further characterization of the juxtaglomerular neurons in the mouse main olfactory bulb by transcription factors, Sp8 and Tbx21

Juxtaglomerular neurons in the mouse main olfactory bulb consist of various types of neurons, especially classified by their chemical properties such as transmitter-related molecules and calcium binding proteins. In addition several transcription factors have been revealed to characterize neuronal subpopulations. In this study we examined the immunoreactivities of two transcription factors, Sp8 and Tbx21, in the juxtaglomerular neuronal subpopulations containing calretinin, calbindin, secretagogin, tyrosine hydroxylase (TH) and nitric oxide synthase (NOS). Both Sp8 and Tbx21 immunoreactivities were so diverse in their staining intensities. Almost all calretinin and secretagogin positive neurons were relatively strongly Sp8 positive, whereas none of calbindin positive neurons were Sp8 positive. TH positive neurons were also usually Sp8 positive, although some were faintly positive. These four types of interneurons were Tbx21 negative. On the other hand large faintly NOS positive external tufted cells were occasionally Tbx21 positive but always Sp8 negative, whereas small NOS positive periglomerular cells without distinctly stained dendrites were usually Sp8 positive and Tbx21 negative. Strangely, most of strongly NOS positive periglomerular cells with distinctly stained dendritic processes were Sp8 negative and Tbx21 negative. Thus Sp8 and Tbx21 immunoreactivities further characterized juxtaglomerular neurons and, especially confirmed the heterogeneity of NOS positive juxtaglomerular neurons.

Distinct DNA binding and transcriptional repression characteristics related to different ARX mutations

Mutations in the Aristaless-related homeobox gene (ARX) are associated with a wide variety of neurologic disorders including lissencephaly, hydrocephaly, West syndrome, Partington syndrome, and X-linked intellectual disability with or without epilepsy. A genotype-phenotype correlation exists for ARX mutations; however, the molecular basis for this association has not been investigated. To begin understanding the molecular basis for ARX mutations, we tested the DNA binding sequence preference and transcriptional repression activity for Arx, deletion mutants and mutants associated with various neurologic disorders. We found DNA binding preferences of Arx are influenced by the amino acid sequences adjacent to the homeodomain. Mutations in the homeodomain show a loss of DNA binding activity, while the T333N and P353R homeodomain mutants still possess DNA binding activities, although less than the wild type. Transcription repression activity, the primary function of ARX, is reduced in all mutants except the L343Q, which has no DNA binding activity and does not functionally repress Arx targets. These data indicate that mutations in the homeodomain result in not only a loss of DNA binding activity but also loss of transcriptional repression activity. Our results provide novel insights into the pathogenesis of ARX-related disorders and possible directions to pursue potential therapeutic interventions.

Bcl11b/Ctip2 controls the differentiation of vomeronasal sensory neurons in mice

The transcription factor Bcl11b/Ctip2 plays critical roles in the development of several systems and organs, including the immune system, CNS, skin, and teeth. Here, we show that Bcl11b/Ctip2 is highly expressed in the developing vomeronasal system in mice and is required for its proper development. Bcl11b/Ctip2 is expressed in postmitotic vomeronasal sensory neurons (VSNs) in the vomeronasal epithelium (VNE) as well as projection neurons and GABAergic interneurons in the accessory olfactory bulb (AOB). In the absence of Bcl11b, these neurons are born in the correct number, but VSNs selectively die by apoptosis. The critical role of Bcl11b in vomeronasal system development is demonstrated by the abnormal phenotypes of Bcl11b-deficient mice: disorganization of layer formation of the AOB, impaired axonal projections of VSNs, a significant reduction in the expression of vomeronasal receptor genes, and defective mature differentiation of VSNs. VSNs can be classified into two major types of neurons, vomeronasal 1 receptor (V1r)/Gα(i2)-positive and vomeronasal 2 receptor (V2r)/Gα(o)-positive VSNs. We found that all Gα(i2)-positive cells coexpressed Gα(o) during embryogenesis. This coexpression is also observed in newly differentiated neurons in the adult VNE. Interestingly, loss of Bcl11b function resulted in an increased number of V1r/Gα(i2)-type VSNs and a decreased number of V2r/Gα(o)-type VSNs, suggesting that Bcl11b regulates the fate choice between these two VSN types. These results indicate that Bcl11b/Ctip2 is an essential regulator of the differentiation and dichotomy of VSNs.

Caenorhabditis elegans aristaless/Arx gene alr-1 restricts variable gene expression

Variable expressivity of mutant phenotypes in genetically identical individuals is a phenomenon widely reported but poorly understood. For example, mutations in the gene encoding the transcription factor ALR-1 in Caenorhabditis elegans result in variable touch receptor neuron (TRN) function. Using single-molecule in situ hybridization, we demonstrate that this phenotypic variability reflects enhanced variability in the expression of the selector gene mec-3, which is needed, together with unc-86, for the differentiation of the TRNs. In a yeast expression system, ALR-1 enhances MEC-3/UNC-86-dependent transcription from the mec-3 promoter, showing that ALR-1 can enhance bulk mec-3 expression. We show that, due to stochastic fluctuations, autoregulation of mec-3 is not sufficient for TRN differentiation; ALR-1 provides a second positive feedback loop that increases mec-3 expression, by restricting variability, and thus ensures TRN differentiation. Our results link fluctuations in gene expression to phenotypic variability, which is seen in many mutant strains, and provide an explicit demonstration of how variable gene expression can be curtailed in developing cells to ensure their differentiation. Because ALR-1 and similar proteins (Drosophila Aristaless and human ARX) are needed for the expression of other transcription factors, we propose that proteins in this family may act to ensure differentiation more generally.

Genetic visualization of the secondary olfactory pathway in Tbx21 transgenic mice

Background

Mitral and tufted cells are the projection neurons in the olfactory bulb, conveying odour information to various regions of the olfactory cortex. In spite of their functional importance, there are few molecular and genetic tools that can be used for selective labelling or manipulation of mitral and tufted cells. Tbx21 was first identified as a T-box family transcription factor regulating the differentiation and function of T lymphocytes. In the brain, Tbx21 is specifically expressed in mitral and tufted cells of the olfactory bulb.

Results

In this study, we performed a promoter/enhancer analysis of mouse Tbx21 gene by comparing nucleotide sequence similarity of Tbx21 genes among several mammalian species and generating transgenic mouse lines with various lengths of 5' upstream region fused to a fluorescent reporter gapVenus. We identified the cis-regulatory enhancer element (~300 nucleotides) at ~ 3.0 kb upstream of the transcription start site of Tbx21 gene, which is both necessary and sufficient for transgene expression in mitral and tufted cells. In contrast, the 2.6-kb 5'-flanking region of mouse Tbx21 gene induced transgene expression with variable patterns in restricted populations of neurons predominantly located along the olfactory pathway. Furthermore, we generated transgenic mice expressing the genetically-encoded fluorescent exocytosis indicator, synaptopHluorin, in mitral and tufted cells for visualization of presynaptic neural activities in the piriform cortex.

Conclusions

The transcriptional enhancer of Tbx21 gene provides a powerful tool for genetic manipulations of mitral and tufted cells in studying the development and function of the secondary olfactory pathways from the bulb to the cortex.

The SRC homology 2 domain protein Shep1 plays an important role in the penetration of olfactory sensory axons into the forebrain

Shep1 is a multidomain signaling protein that forms a complex with Cas, a key scaffolding component of integrin signaling pathways, to promote the migration of non-neuronal cells. However, the physiological function of Shep1 in the nervous system remains unknown. Interestingly, we found that Shep1 and Cas are both concentrated in the axons of developing olfactory sensory neurons (OSNs). These neurons extend their axons from the olfactory epithelium to the olfactory bulb located at the anterior tip of the forebrain. However, in developing Shep1 knock-out mice, we did not detect penetration of OSN axons across the pial basement membrane surrounding the olfactory bulb, suggesting that Shep1 function is important for the establishment of OSN connections with the olfactory bulb. Interestingly, we observed reduced levels of Cas tyrosine phosphorylation in OSN axons of Shep1 knock-out mice, suggesting compromised Cas signaling function. Indeed, when embedded in a three-dimensional gel of basement membrane proteins, explants from Shep1 knock-out olfactory epithelium extend neuronal processes less efficiently than explants from control epithelium. Furthermore, ectopic expression of Shep1 in non-neuronal cells promotes cell migration through a collagen gel. Later in development, loss of Shep1 function also causes a marked reduction in olfactory bulb size and disruption of bulb lamination, which may be primarily attributable to the defective innervation. The greatly reduced OSN connections and hypoplasia of the olfactory bulb, likely resulting in anosmia, are reminiscent of the symptoms of Kallmann syndrome, a human developmental disease that can be caused by mutations in a growing number of genes.