Time frame of mitral cell development in the mice olfactory bulb

Deficiency of CAMSAP2 impairs olfaction and the morphogenesis of mitral cells

In developing olfactory bulb (OB), mitral cells (MCs) remodel their dendrites to establish the precise olfactory circuit, and these circuits are critical for individuals to sense odors and elicit behaviors for survival. However, how microtubules (MTs) participate in the process of dendritic remodeling remains elusive. Here, we reveal that calmodulin-regulated spectrin-associated proteins (CAMSAPs), a family of proteins that bind to the minus-end of the noncentrosomal MTs, play a crucial part in the development of MC dendrites. We observed that Camsap2 knockout (KO) males are infertile while the reproductive tract is normal. Further study showed that the infertility was due to the severe defects of mating behavior in male mice. Besides, mice with loss-of-function displayed defects in the sense of smell. Furthermore, we found that the deficiency of CAMSAP2 impairs the classical morphology of MCs, and the CAMSAP2-dependent dendritic remodeling process is responsible for this defect. Thus, our findings demonstrate that CAMSAP2 plays a vital role in regulating the development of MCs.

Effects of prenatal alcohol exposure on the olfactory system development

Fetal Alcohol Spectrum Disorders (FASD), resulting from maternal alcohol consumption during pregnancy, are a prominent non-genetic cause of physical disabilities and brain damage in children. Alongside common symptoms like distinct facial features and neurocognitive deficits, sensory anomalies, including olfactory dysfunction, are frequently noted in FASD-afflicted children. However, the precise mechanisms underpinning the olfactory abnormalities induced by prenatal alcohol exposure (PAE) remain elusive. Utilizing rodents as a model organism with varying timing, duration, dosage, and administration routes of alcohol exposure, prior studies have documented impairments in olfactory system development caused by PAE. Many reported a reduction in the olfactory bulb (OB) volume accompanied by reduced OB neuron counts, suggesting the OB is a brain region vulnerable to PAE. In contrast, no significant olfactory system defects were observed in some studies, though subtle alterations might exist. These findings suggest that the timing, duration, and extent of fetal alcohol exposure can yield diverse effects on olfactory system development. To enhance comprehension of PAE-induced olfactory dysfunctions, this review summarizes key findings from previous research on the olfactory systems of offspring prenatally exposed to alcohol.

Olfactory bulb anomalies in KBG syndrome mouse model and patients

ANKRD11 (ankyrin repeat domain 11) is a chromatin regulator and the only gene associated with KBG syndrome, a rare neurodevelopmental disorder. We have previously shown that Ankrd11 regulates murine embryonic cortical neurogenesis. Here, we show a novel olfactory bulb phenotype in a KBG syndrome mouse model and two diagnosed patients. Conditional knockout of Ankrd11 in murine embryonic neural stem cells leads to aberrant postnatal olfactory bulb development and reduced size due to reduction of the olfactory bulb granule cell layer. We further show that the rostral migratory stream has incomplete migration of neuroblasts, reduced cell proliferation as well as aberrant differentiation of neurons. This leads to reduced neuroblasts and neurons in the olfactory bulb granule cell layer. In vitro, Ankrd11-deficient neural stem cells from the postnatal subventricular zone display reduced migration, proliferation, and neurogenesis. Finally, we describe two clinically and molecularly confirmed KBG syndrome patients with anosmia and olfactory bulb and groove hypo-dysgenesis/agenesis. Our report provides evidence that Ankrd11 is a novel regulator of olfactory bulb development and neuroblast migration. Moreover, our study highlights a novel clinical sign of KBG syndrome linked to ANKRD11 perturbations in mice and humans.

Suppression of BMP signaling restores mitral cell development impaired by FGF signaling deficits in mouse olfactory bulb

Fibroblast growth factors (FGFs) and bone morphogenic proteins (BMPs) play various important roles in the development of the central nervous system. However, the roles of FGF and BMP signaling in the development of the olfactory bulb (OB) are largely unknown. In this study, we first showed the expression of FGF receptors (FGFRs) and BMP receptors (BMPRs) in OB RGCs, radial glial cells (RGCs) in the developing OB, which generate the OB projection neurons, mitral and tufted cells. When the FGF signaling was inhibited by a dominant-negative form of FGFR1 (dnFGFR1), OB RGCs accelerated their state transition to mitral cell precursors without affecting their transcription cascade and fate. However, the mitral cell precursors could not radially migrate to form the mitral cell layer (MCL). In addition, FGF signaling inhibition reduced the expression of a BMP antagonist, Noggin, in the developing OB. When BMP signaling was suppressed by the ectopic expression of Noggin or a dominant-negative form of BMPR1a (dnBMPR1a) in the developing OB, the defect in MCL formation caused by the dnFGFR1 was rescued. However, the dnBMPR1a did not rescue the accelerated state transition of OB RGCs. These results demonstrate that FGF signaling is important for OB RGCs to maintain their self-renewal state and MCL formation. Moreover, the suppression of BMP signaling is required for mitral cells to form the MCL. This study sheds new light on the roles of FGFs and BMPs in OB development.

Activity-dependent local protection and lateral inhibition control synaptic competition in developing mitral cells in mice

In developing brains, activity-dependent remodeling facilitates the formation of precise neuronal connectivity. Synaptic competition is known to facilitate synapse elimination; however, it has remained unknown how different synapses compete with one another within a post-synaptic cell. Here, we investigate how a mitral cell in the mouse olfactory bulb prunes all but one primary dendrite during the developmental remodeling process. We find that spontaneous activity generated within the olfactory bulb is essential. We show that strong glutamatergic inputs to one dendrite trigger branch-specific changes in RhoA activity to facilitate the pruning of the remaining dendrites: NMDAR-dependent local signals suppress RhoA to protect it from pruning; however, the subsequent neuronal depolarization induces neuron-wide activation of RhoA to prune non-protected dendrites. NMDAR-RhoA signals are also essential for the synaptic competition in the mouse barrel cortex. Our results demonstrate a general principle whereby activity-dependent lateral inhibition across synapses establishes a discrete receptive field of a neuron.

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.

Study of the glial cytoarchitecture of the developing olfactory bulb of a shark using immunochemical markers of radial glia

During development of the olfactory bulb (OB), glial cells play key roles in axonal guiding/targeting, glomerular formation and synaptic plasticity. Studies in mammals have shown that radial glial cells and peripheral olfactory glia (olfactory ensheathing cells, OECs) are involved in the development of the OB. Most studies about the OB glia were carried out in mammals, but data are lacking in most non-mammalian vertebrates. In the present work, we studied the development of the OB glial system in the cartilaginous fish Scyliorhinus canicula (catshark) using antibodies against glial markers, such as glial fibrillary acidic protein (GFAP), brain lipid-binding protein (BLBP), and glutamine synthase (GS). These glial markers were expressed in cells with radial morphology lining the OB ventricle of embryos and this expression continues in ependymal cells (tanycytes) in early juveniles. Astrocyte-like cells were also observed in the granular layer and surrounding glomeruli. Numerous GS-positive cells were present in the primary olfactory pathway of embryos. In the developmental stages analysed, the olfactory nerve layer and the glomerular layer were the regions with higher GFAP, BLBP and GS immuno-reactivity. In addition, numerous BLBP-expressing cells (a marker of mammalian OECs) showing proliferative activity were present in the olfactory nerve layer. Our findings suggest that glial cells of peripheral and central origin coexist in the OB of catshark embryos and early juveniles. These results open the path for future studies about the differential roles of glial cells in the catshark OB during embryonic development and in adulthood.

Relationship between Blood Vessels and Migration of Neuroblasts in the Olfactory Neurogenic Region of the Rodent Brain

Neural precursors originating in the subventricular zone (SVZ), the largest neurogenic region of the adult brain, migrate several millimeters along a restricted migratory pathway, the rostral migratory stream (RMS), toward the olfactory bulb (OB), where they differentiate into interneurons and integrate into the local neuronal circuits. Migration of SVZ-derived neuroblasts in the adult brain differs in many aspects from that in the embryonic period. Unlike in that period, postnatally-generated neuroblasts in the SVZ are able to divide during migration along the RMS, as well as they migrate independently of radial glia. The homophilic mode of migration, i.e., using each other to move, is typical for neuroblast movement in the RMS. In addition, it has recently been demonstrated that specifically-arranged blood vessels navigate SVZ-derived neuroblasts to the OB and provide signals which promote migration. Here we review the development of vasculature in the presumptive neurogenic region of the rodent brain during the embryonic period as well as the development of the vascular scaffold guiding neuroblast migration in the postnatal period, and the significance of blood vessel reorganization during the early postnatal period for proper migration of RMS neuroblasts in adulthood.

Expression of Maf family proteins in glutamatergic neurons of the mouse olfactory bulb

The fate of neurons in the developing brain is largely determined by the combination of transcription factors they express. In particular, stem cells must follow different transcriptional cascades during differentiation in order to generate neurons with different neurotransmitter properties, such as glutamatergic and GABAergic neurons. In the mouse cerebral cortex, it has been shown that large Maf family proteins, MafA, MafB and c-Maf, regulate the development of specific types of GABAergic interneurons but are not expressed in glutamatergic neurons. In this study, we examined the expression of large Maf family proteins in the developing mouse olfactory bulb (OB) by immunohistochemistry and found that the cell populations expressing MafA and MafB are almost identical, and most of them express Tbr2. As Tbr2 is expressed in glutamatergic neurons in the OB, we further examined the expression of glutamatergic and GABAergic neuronal markers in MafA and MafB positive cells. The results showed that in the OB, MafA and MafB are expressed exclusively in glutamatergic neurons, but not in GABAergic neurons. We also found that few cells express c-Maf in the OB. These results indicate that, unlike the cerebral cortex, MafA and/or MafB may regulate the development of glutamatergic neurons in the developing OB. This study advances our knowledge about the development of glutamatergic neurons in the olfactory bulb, and also might suggest that mechanisms for the generation of projection neurons and interneurons differ between the cortex and the olfactory bulb, even though they both develop from the telencephalon.

BMPR-2 gates activity-dependent stabilization of primary dendrites during mitral cell remodeling

Developing neurons initially form excessive neurites and then remodel them based on molecular cues and neuronal activity. Developing mitral cells in the olfactory bulb initially extend multiple primary dendrites. They then stabilize single primary dendrites while eliminating others. However, the mechanisms underlying selective dendrite remodeling remain elusive. Using CRISPR-Cas9-based knockout screening combined with in utero electroporation, we identify BMPR-2 as a key regulator for selective dendrite stabilization. Bmpr2 knockout and its rescue experiments show that BMPR-2 inhibits LIMK without ligands and thereby permits dendrite destabilization. In contrast, the overexpression of antagonists and agonists indicates that ligand-bound BMPR-2 stabilizes dendrites, most likely by releasing LIMK. Using genetic and FRET imaging experiments, we demonstrate that free LIMK is activated by NMDARs via Rac1, facilitating dendrite stabilization through F-actin formation. Thus, the selective stabilization of primary dendrites is ensured by concomitant inputs of BMP ligands and neuronal activity.

Visual Experience Influences Dendritic Orientation but Is Not Required for Asymmetric Wiring of the Retinal Direction Selective Circuit

Changes in dendritic morphology in response to activity have long been thought to be a critical component of how neural circuits develop to properly encode sensory information. Ventral-preferring direction-selective ganglion cells (vDSGCs) have asymmetric dendrites oriented along their preferred direction, and this has been hypothesized to play a critical role in their tuning. Here we report the surprising result that visual experience is critical for the alignment of vDSGC dendrites to their preferred direction. Interestingly, vDSGCs in dark-reared mice lose their inhibition-independent dendritic contribution to direction-selective tuning while maintaining asymmetric inhibitory input. These data indicate that different mechanisms of a cell's computational abilities can be constructed over development through divergent mechanisms.

Cilia, ciliopathies and hedgehog-related forebrain developmental disorders

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

Adeno-Associated Virus-Mediated Single-Cell Labeling of Mitral Cells in the Mouse Olfactory Bulb: Insights into the Developmental Dynamics of Dendrite Remodeling

Neurons typically remodel axons/dendrites for functional refinement of neural circuits in the developing brain. Mitral cells in the mammalian olfactory system remodel their dendritic arbors in the perinatal development, but the underlying molecular and cellular mechanisms remain elusive in part due to a lack of convenient methods to label mitral cells with single-cell resolution. Here we report a novel method for single-cell labeling of mouse mitral cells using adeno-associated virus (AAV)-mediated gene delivery. We first demonstrated that AAV injection into the olfactory ventricle of embryonic day 14.5 (E14.5) mice preferentially labels mitral cells in the olfactory bulb (OB). Birthdate labeling indicated that AAV can transduce mitral cells independently of their birthdates. Furthermore, in combination with the Cre-mediated gene expression system, AAV injection allows visualization of mitral cells at single-cell resolution. Using this AAV-mediated single-cell labeling method, we investigated dendrite development of mitral cells and found that ~50% of mitral cells exhibited mature apical dendrites with a single thick and tufted branch before birth, suggesting that a certain population of mitral cells completes dendrite remodeling during embryonic stages. We also found an atypical subtype of mitral cells that have multiple dendritic shafts innervating the same glomeruli. Our data thus demonstrate that the AAV-mediated labeling method that we reported here provides an efficient way to visualize mitral cells with single-cell resolution and could be utilized to study dynamic aspects as well as functions of mitral cells in the olfactory circuits.

Topographically Distinct Projection Patterns of Early-Generated and Late-Generated Projection Neurons in the Mouse Olfactory Bulb

In the mouse brain, olfactory information is transmitted to the olfactory cortex via olfactory bulb (OB) projection neurons known as mitral and tufted cells. Although mitral and tufted cells share many cellular characteristics, these cell types are distinct in their somata location and in their axonal and dendritic projection patterns. Moreover, mitral cells consist of heterogeneous subpopulations.

In the mouse brain, olfactory information is transmitted to the olfactory cortex via olfactory bulb (OB) projection neurons known as mitral and tufted cells. Although mitral and tufted cells share many cellular characteristics, these cell types are distinct in their somata location and in their axonal and dendritic projection patterns. Moreover, mitral cells consist of heterogeneous subpopulations. We have previously shown that mitral cells generated at different embryonic days differentially localize within the mitral cell layer (MCL) and extend their lateral dendrites to different sublayers of the external plexiform layer (EPL). Here, we examined the axonal projection patterns from the subpopulations of OB projection neurons that are determined by the timing of neurogenesis (neuronal birthdate) to understand the developmental origin of the diversity in olfactory pathways. We separately labeled early-generated and late-generated OB projection neurons using in utero electroporation performed at embryonic day (E)11 and E12, respectively, and quantitatively analyzed their axonal projection patterns in the whole mouse brain using high-resolution 3D imaging. In this study, we demonstrate that the axonal projection of late-generated OB projection neurons is restricted to the anterior portion of the olfactory cortex while those of the early-generated OB projection neurons innervate the entire olfactory cortex. Our results suggest that the late-generated mitral cells do not extend their axons to the posterior regions of the olfactory cortex. Therefore, the mitral cells having different birthdates differ, not only in cell body location and dendritic projections within the OB, but also in their axonal projection pattern to the olfactory cortex.

Bursting mitral cells time the oscillatory coupling between olfactory bulb and entorhinal networks in neonatal mice

KEY POINTS

During early postnatal development, mitral cells show either irregular bursting or non-bursting firing patterns Bursting mitral cells preferentially fire during theta bursts in the neonatal olfactory bulb, being locked to the theta phase Bursting mitral cells preferentially fire during theta bursts in the neonatal lateral entorhinal cortex and are temporally related to both respiration rhythm- and theta phase Bursting mitral cells act as a cellular substrate of the olfactory drive that promotes the oscillatory entrainment of entorhinal networks ABSTRACT: Shortly after birth, the olfactory system provides not only the main source of environmental inputs to blind, deaf, non-whisking and motorically-limited rodents, but also the drive boosting the functional entrainment of limbic circuits. However, the cellular substrate of this early communication remains largely unknown. Here, we combine in vivo and in vitro patch-clamp and extracellular recordings to reveal the contribution of mitral cell (MC) firing to early patterns of network activity in both the neonatal olfactory bulb (OB) and the lateral entorhinal cortex (LEC), the gatekeeper of limbic circuits. We show that MCs predominantly fire either in an irregular bursting or non-bursting pattern during discontinuous theta events in the OB. However, the temporal spike-theta phase coupling is stronger for bursting than non-bursting MCs. In line with the direct OB-to-LEC projections, both bursting and non-bursting discharge augments during co-ordinated patterns of entorhinal activity, albeit with higher magnitude for bursting MCs. For these neurons, temporal coupling to the discontinuous theta events in the LEC is stronger. Thus, bursting MCs might drive the entrainment of the OB-LEC network during neonatal development.

Forces to Drive Neuronal Migration Steps

To establish and maintain proper brain architecture and elaborate neural networks, neurons undergo massive migration. As a unique feature of their migration, neurons move in a saltatory manner by repeating two distinct steps: extension of the leading process and translocation of the cell body. Neurons must therefore generate forces to extend the leading process as well as to translocate the cell body. In addition, neurons need to switch these forces alternately in order to orchestrate their saltatory movement. Recent studies with mechanobiological analyses, including traction force microscopy, cell detachment analyses, live-cell imaging, and loss-of-function analyses, have begun to reveal the forces required for these steps and the molecular mechanics underlying them. Spatiotemporally organized forces produced between cells and their extracellular environment, as well as forces produced within cells, play pivotal roles to drive these neuronal migration steps. Traction force produced by the leading process growth cone extends the leading processes. On the other hand, mechanical tension of the leading process, together with reduction in the adhesion force at the rear and the forces to drive nucleokinesis, translocates the cell body. Traction forces are generated by mechanical coupling between actin filament retrograde flow and the extracellular environment through clutch and adhesion molecules. Forces generated by actomyosin and dynein contribute to the nucleokinesis. In addition to the forces generated in cell-intrinsic manners, external forces provided by neighboring migratory cells coordinate cell movement during collective migration. Here, we review our current understanding of the forces that drive neuronal migration steps and describe the molecular machineries that generate these forces for neuronal migration.

Subpopulations of Projection Neurons in the Olfactory Bulb

Generation of neuronal diversity is a biological strategy widely used in the brain to process complex information. The olfactory bulb is the first relay station of olfactory information in the vertebrate central nervous system. In the olfactory bulb, axons of the olfactory sensory neurons form synapses with dendrites of projection neurons that transmit the olfactory information to the olfactory cortex. Historically, the olfactory bulb projection neurons have been classified into two populations, mitral cells and tufted cells. The somata of these cells are distinctly segregated within the layers of the olfactory bulb; the mitral cells are located in the mitral cell layer while the tufted cells are found in the external plexiform layer. Although mitral and tufted cells share many morphological, biophysical, and molecular characteristics, they differ in soma size, projection patterns of their dendrites and axons, and odor responses. In addition, tufted cells are further subclassified based on the relative depth of their somata location in the external plexiform layer. Evidence suggests that different types of tufted cells have distinct cellular properties and play different roles in olfactory information processing. Therefore, mitral and different types of tufted cells are considered as starting points for parallel pathways of olfactory information processing in the brain. Moreover, recent studies suggest that mitral cells also consist of heterogeneous subpopulations with different cellular properties despite the fact that the mitral cell layer is a single-cell layer. In this review, we first compare the morphology of projection neurons in the olfactory bulb of different vertebrate species. Next, we explore the similarities and differences among subpopulations of projection neurons in the rodent olfactory bulb. We also discuss the timing of neurogenesis as a factor for the generation of projection neuron heterogeneity in the olfactory bulb. Knowledge about the subpopulations of olfactory bulb projection neurons will contribute to a better understanding of the complex olfactory information processing in higher brain regions.

Multifaceted actions of Zeb2 in postnatal neurogenesis from the ventricular-subventricular zone to the olfactory bulb

The transcription factor Zeb2 controls fate specification and subsequent differentiation and maturation of multiple cell types in various embryonic tissues. It binds many protein partners, including activated Smad proteins and the NuRD co-repressor complex. How Zeb2 subdomains support cell differentiation in various contexts has remained elusive. Here, we studied the role of Zeb2 and its domains in neurogenesis and neural differentiation in the young postnatal ventricular-subventricular zone (V-SVZ), in which neural stem cells generate olfactory bulb-destined interneurons. Conditional Zeb2 knockouts and separate acute loss- and gain-of-function approaches indicated that Zeb2 is essential for controlling apoptosis and neuronal differentiation of V-SVZ progenitors before and after birth, and we identified Sox6 as a potential downstream target gene of Zeb2. Zeb2 genetic inactivation impaired the differentiation potential of the V-SVZ niche in a cell-autonomous fashion. We also provide evidence that its normal function in the V-SVZ also involves non-autonomous mechanisms. Additionally, we demonstrate distinct roles for Zeb2 protein-binding domains, suggesting that Zeb2 partners co-determine neuronal output from the mouse V-SVZ in both quantitative and qualitative ways in early postnatal life.

Gradual wiring of olfactory input to amygdala feedback circuits

The amygdala facilitates odor driven behavioral responses by enhancing the saliency of olfactory signals. Before this processing, olfactory input is refined through the feedback provided by amygdala corticofugal projection (ACPs). Although the saliency of odor signals is subject to developmental changes, the stage at which this cortical feedback first occurs is not known. Using optogenetically-assisted intracellular recordings of the mouse cortical amygdala, we identified changes in the electrophysiological properties of ACPs at different developmental stages. These were consistent with a decrease in neuronal excitability and an increase in the amount of incoming accessory olfactory bulb (AOB) inputs, as confirmed by estimates of release probability, quantal size and contact number at the AOB-to-ACP synapse. Moreover, the proportion of ACPs activated in response to odors was dependent on the stage of development as revealed by c-Fos expression analysis. These results update standard accounts of how the amygdala processes social signals by emphasizing the occurrence of critical periods in the development of its sensory gating functions.

Cell Progeny in the Olfactory Bulb After Targeting Specific Progenitors with Different UbC-StarTrack Approaches

The large phenotypic variation in the olfactory bulb may be related to heterogeneity in the progenitor cells. Accordingly, the progeny of subventricular zone (SVZ) progenitor cells that are destined for the olfactory bulb is of particular interest, specifically as there are many facets of these progenitors and their molecular profiles remain unknown. Using modified StarTrack genetic tracing strategies, specific SVZ progenitor cells were targeted in E12 mice embryos, and the cell fate of these neural progenitors was determined in the adult olfactory bulb. This study defined the distribution and the phenotypic diversity of olfactory bulb interneurons from specific SVZ-progenitor cells, focusing on their spatial pallial origin, heterogeneity, and genetic profile.

Impact of Targeted Deletion of the Circadian Clock Gene Bmal1 in Excitatory Forebrain Neurons on Adult Neurogenesis and Olfactory Function

The circadian system is an endogenous timekeeping system that synchronizes physiology and behavior with the 24 h solar day. Mice with total deletion of the core circadian clock gene Bmal1 show circadian arrhythmicity, cognitive deficits, and accelerated age-dependent decline in adult neurogenesis as a consequence of increased oxidative stress. However, it is not yet known if the impaired adult neurogenesis is due to circadian disruption or to loss of the Bmal1 gene function. Therefore, we investigated oxidative stress and adult neurogenesis of the two principle neurogenic niches, the hippocampal subgranular zone and the subventricular zone in mice with a forebrain specific deletion of Bmal1 (Bmal1 fKO), which show regular circadian rhythmicity. Moreover, we analyzed the morphology of the olfactory bulb, as well as olfactory function in Bmal1 fKO mice. In Bmal1 fKO mice, oxidative stress was increased in subregions of the hippocampus and the olfactory bulb but not in the neurogenic niches. Consistently, adult neurogenesis was not affected in Bmal1 fKO mice. Although Reelin expression in the olfactory bulb was higher in Bmal1 fKO mice as compared to wildtype mice (Bmal1 WT), the olfactory function was not affected. Taken together, the targeted deletion of Bmal1 in mouse forebrain neurons is associated with a regional increase in oxidative stress and increased Reelin expression in the olfactory bulb but does not affect adult neurogenesis or olfactory function.

Lineage does not regulate the sensory synaptic input of projection neurons in the mouse olfactory bulb

Lineage regulates the synaptic connections between neurons in some regions of the invertebrate nervous system. In mammals, recent experiments suggest that cell lineage determines the connectivity of pyramidal neurons in the neocortex, but the functional relevance of this phenomenon and whether it occurs in other neuronal types remains controversial. We investigated whether lineage plays a role in the connectivity of mitral and tufted cells, the projection neurons in the mouse olfactory bulb. We used transgenic mice to sparsely label neuronal progenitors and observed that clonally related neurons receive synaptic input from olfactory sensory neurons expressing different olfactory receptors. These results indicate that lineage does not determine the connectivity between olfactory sensory neurons and olfactory bulb projection neurons.

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.

Primary dendrites of mitral cells synapse unto neighboring glomeruli independent of their odorant receptor identity

In the mouse olfactory bulb, neural map topography is largely established by axon-axon interactions of olfactory sensory neurons (OSNs). However, to make the map functional, the OSNs must make proper connections to second-order neurons, the mitral cells. How do the mitral-cell dendrites find their partner glomeruli for synapse formation with OSN axons? Here, we analyze dendrite connections of mitral cells in various mutant mice in which glomerular formation is perturbed. Our present results support the proximity model, whereby mitral cells tend to connect primary dendrites to the nearest neighboring glomeruli regardless of their odorant receptor identities. The physical location of glomeruli rather than the odorant-receptor specificity appears to play a key role in matching mitral cells with their partner OSN axons.

Olfactory ensheathing cells abutting the embryonic olfactory bulb express Frzb, whose deletion disrupts olfactory axon targeting

We and others previously showed that in mouse embryos lacking the transcription factor Sox10, olfactory ensheathing cell (OEC) differentiation is disrupted, resulting in defective olfactory axon targeting and fewer gonadotropin‐releasing hormone (GnRH) neurons entering the embryonic forebrain. The underlying mechanisms are unclear. Here, we report that OECs in the olfactory nerve layer express Frzb—encoding a secreted Wnt inhibitor with roles in axon targeting and basement membrane breakdown—from embryonic day (E)12.5, when GnRH neurons first enter the forebrain, until E16.5, the latest stage examined. The highest levels of Frzb expression are seen in OECs in the inner olfactory nerve layer, abutting the embryonic olfactory bulb. We find that Sox10 is required for Frzb expression in OECs, suggesting that loss of Frzb could explain the olfactory axon targeting and/or GnRH neuron migration defects seen in Sox10‐null mice. At E16.5, Frzb‐null embryos show significant reductions in both the volume of the olfactory nerve layer expressing the maturation marker Omp and the number of Omp‐positive olfactory receptor neurons in the olfactory epithelium. As Omp upregulation correlates with synapse formation, this suggests that Frzb deletion indeed disrupts olfactory axon targeting. In contrast, GnRH neuron entry into the forebrain is not significantly affected. Hence, loss of Frzb may contribute to the olfactory axon targeting phenotype, but not the GnRH neuron phenotype, of Sox10‐null mice. Overall, our results suggest that Frzb secreted from OECs in the olfactory nerve layer is important for olfactory axon targeting.

Frzb is expressed by olfactory ensheathing cells abutting the embryonic mouse olfactory bulb.

Frzb expression requires Sox10. Deletion of Frzb disrupts olfactory receptor neuron maturation, likely reflecting a defect in olfactory axon targeting.

Myelination of the developing lateral olfactory tract and anterior commissure

Both the lateral olfactory tract (LOT) and anterior limb of the anterior commissure (AC) carry olfactory information. The LOT forms the projection from the olfactory bulb to the ipsilateral olfactory cortices, while the AC carries odor information across the midline to the contralateral olfactory cortex and bulb. The LOT and AC differ on a number of dimensions, including early development and functional onset. The present work, examining their myelination in mice, reveals additional important differences. For example, the LOT initiates myelination 3-4 days earlier than the AC, evidenced by both an earlier increase in myelin basic protein staining seen with immunohistochemistry and an earlier appearance of myelinated fibers using electron microscopy. While both exhibit a period of rapid myelination, it occurs 4-5 days earlier in the LOT than the AC. The tracts also respond differently to early sensory restriction. Unilateral naris occlusion from the day after birth to postnatal day 30 had no consistent effects on the AC but resulted in significantly thinner myelin sheaths relative to axon caliber in the LOT. Finally, the two tracts differ structurally (the LOT contains larger, more densely packed axons with significantly thicker myelin sheaths resulting in a conduction velocity that is more than twice as fast as the AC). The findings indicate that these two large, accessible tracts provide an important means for studying brain maturation due to basic differences in both the timing of their maturation and general organization.

Septal contributions to olfactory bulb interneuron diversity in the embryonic mouse telencephalon: role of the homeobox gene Gsx2

Background

Olfactory bulb (OB) interneurons are known to represent diverse neuronal subtypes, which are thought to originate from a number of telencephalic regions including the embryonic dorsal lateral ganglionic eminence (dLGE) and septum. These cells migrate rostrally toward the OB, where they then radially migrate to populate different OB layers including the granule cell layer (GCL) and the outer glomerular layer (GL). Although previous studies have attempted to investigate regional contributions to OB interneuron diversity, few genetic tools have been used to address this question at embryonic time points when the earliest populations are specified.

Methods

In this study, we utilized Zic3-lacZ and Gsx2e-CIE transgenic mice as genetic fate-mapping tools to study OB interneuron contributions derived from septum and LGE, respectively. Moreover, to address the regional (i.e. septal) requirements of the homeobox gene Gsx2 for OB interneuron diversity, we conditionally inactivated Gsx2 in the septum, leaving it largely intact in the dLGE, by recombining the Gsx2 floxed allele using Olig2^Cre/+ mice.

Results

Our fate mapping studies demonstrated that the dLGE and septum gave rise to OB interneuron subtypes differently. Notably, the embryonic septum was found to give rise largely to the calretinin⁺ (CR⁺) GL subtype, while the dLGE was more diverse, generating all major GL subpopulations as well as many GCL interneurons. Moreover, Gsx2 conditional mutants (cKOs), with septum but not dLGE recombination, showed impaired generation of CR⁺ interneurons within the OB GL. These Gsx2 cKOs exhibited reduced proliferation within the septal subventricular zone (SVZ), which correlated well with the reduced number of CR⁺ interneurons observed.

Conclusions

Our findings indicate that the septum and LGE contribute differently to OB interneuron diversity. While the dLGE provides a wide range of OB interneuron subtypes, the septum is more restricted in its contribution to the CR⁺ subtype. Gsx2 is required in septal progenitors for the correct expansion of SVZ progenitors specified toward the CR⁺ subtype. Finally, the septum has been suggested to be the exclusive source of CR⁺ interneurons in postnatal studies. Our results here demonstrate that dLGE progenitors in the embryo also contribute to this OB neuronal subtype.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-017-0090-5) contains supplementary material, which is available to authorized users.

Nrp2 is sufficient to instruct circuit formation of mitral-cells to mediate odour-induced attractive social responses

Odour information induces various innate responses that are critical to the survival of the individual and for the species. An axon guidance molecule, Neuropilin 2 (Nrp2), is known to mediate targeting of olfactory sensory neurons (primary neurons), to the posteroventral main olfactory bulb (PV MOB) in mice. Here we report that Nrp2-positive (Nrp2⁺) mitral cells (MCs, second-order neurons) play crucial roles in transmitting attractive social signals from the PV MOB to the anterior part of medial amygdala (MeA). Semaphorin 3F, a repulsive ligand to Nrp2, regulates both migration of Nrp2⁺ MCs to the PV MOB and their axonal projection to the anterior MeA. In the MC-specific Nrp2 knockout mice, circuit formation of Nrp2⁺ MCs and odour-induced attractive social responses are impaired. In utero, electroporation demonstrates that activation of the Nrp2 gene in MCs is sufficient to instruct their circuit formation from the PV MOB to the anterior MeA.

Early Odorant Exposure Increases the Number of Mitral and Tufted Cells Associated with a Single Glomerulus

The highly specific organization of the olfactory bulb (OB) is well known, but the impact of early odorant experience on its circuit structure is unclear. Olfactory sensory neurons (OSNs) project axons from the olfactory epithelium to the OB, where they form spherical neuropil structures called glomeruli. These glomeruli and the postsynaptic targets of OSNs, including mitral and tufted cells (M/TCs) and juxtaglomerular cells, form glomerular modules, which represent the basic odor-coding units of the OB. Here, we labeled M/TCs within a single glomerular module of the mouse OB and show that odorant exposure that starts prenatally and continues through postnatal day 25 has a major impact on the structure of the glomerular module. We confirm that exposure increases the volume of the activated glomeruli and show that exposure increases M/TC number by >40% in a glomerulus-specific fashion. Given the role of M/TCs in OB output and in lateral inhibition, increasing the number of M/TCs connected to a single glomerulus may also increase the influence of that glomerulus on the OB network and on OB output. Our results show that early odorant exposure has a profound effect on OB connectivity and thus may affect odorant processing significantly.

SIGNIFICANCE STATEMENT

Experience shapes neural circuits in a variety of ways, most commonly by changing the strength of activated connections. Relatively little is known about how experience changes circuitry in the olfactory system. Here, we show that for a genetically identified glomerulus in the mouse olfactory bulb, early odorant exposure increases the number of associated mitral and tufted cells by 40% and 100%, respectively. Understanding the structural changes induced by early odorant experience can provide insight into how bulbar organization gives rise to efficient processing. We find that odorant experience increases the number of projection neurons associated with a single glomerulus significantly, a dramatic and long-lasting structural change that may have important functional implications.

Nectin spot: a novel type of nectin-mediated cell adhesion apparatus

Nectins are Ca(2+)-independent immunoglobulin (Ig) superfamily cell adhesion molecules constituting a family with four members, all of which have three Ig-like loops at their extracellular regions. Nectins play roles in the formation of a variety of cell-cell adhesion apparatuses. There are at least three types of nectin-mediated cell adhesions: afadin- and cadherin-dependent, afadin-dependent and cadherin-independent, and afadin- and cadherin-independent. In addition, nectins trans-interact with nectin-like molecules (Necls) with three Ig-like loops and other Ig-like molecules with one to three Ig-like loops. Furthermore, nectins and Necls cis-interact with membrane receptors and integrins, some of which are associated with the nectin-mediated cell adhesions, and play roles in the regulation of many cellular functions, such as cell polarization, movement, proliferation, differentiation, and survival, co-operatively with these cell surface proteins. The nectin-mediated cell adhesions are implicated in a variety of diseases, including genetic disorders, neural disorders, and cancers. Of the three types of nectin-mediated cell adhesions, the afadin- and cadherin-dependent apparatus has been most extensively investigated, but the examples of the third type of apparatus independent of afadin and cadherin are recently increasing and its morphological and functional properties have been well characterized. We review here recent advances in research on this type of nectin-mediated cell adhesion apparatus, which is named nectin spot.

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.

Distinct and Cooperative Functions for the Protocadherin-α, -β and -γ Clusters in Neuronal Survival and Axon Targeting

The clustered protocadherin (Pcdh) genes are divided into the Pcdhα, Pcdhβ, and Pcdhγ clusters. Gene-disruption analyses in mice have revealed the in vivo functions of the Pcdhα and Pcdhγ clusters. However, all Pcdh protein isoforms form combinatorial cis-hetero dimers and enter trans-homophilic interactions. Here we addressed distinct and cooperative functions in the Pcdh clusters by generating six cluster-deletion mutants (Δα, Δβ, Δγ, Δαβ, Δβγ, and Δαβγ) and comparing their phenotypes: Δα, Δβ, and Δαβ mutants were viable and fertile; Δγ mutants lived less than 12 h; and Δβγ and Δαβγ mutants died shortly after birth. The Pcdhα, Pcdhβ, and Pcdhγ clusters were individually and cooperatively important in olfactory-axon targeting and spinal-cord neuron survival. Neurodegeneration was most severe in Δαβγ mutants, indicating that Pcdhα and Pcdhβ function cooperatively for neuronal survival. The Pcdhα, Pcdhβ, and Pcdhγ clusters share roles in olfactory-axon targeting and neuronal survival, although to different degrees.

Role of Rb during Neurogenesis and Axonal Guidance in the Developing Olfactory System

The Retinoblastoma protein, Rb, was shown to regulate distinct aspects of neurogenesis in the embryonic and adult brain besides its primary role in cell cycle control. It is still unknown, however, whether Rb is required for tissue morphogenesis and the establishment of synaptic connections between adjacent tissues during development. We have investigated here the role of Rb during development of the olfactory system (OS), which heavily relies on reciprocal interactions between the olfactory epithelium (OE) and the olfactory bulb (OB). We show that mice carrying a telencephalic-specific deletion of Rb display several neurogenic defects in the OS during late development. In the OE, loss of Rb leads to ectopic proliferation of late-born progenitors (Tuj-1+), abnormal radial migration and terminal maturation of olfactory sensory neurons (OSNs). In the OB, deletion of Rb causes severe lamination defects with loss of clear boundaries between distinct layers. Importantly, starting around E15.5 when OB glomerulogenesis is initiated, many OSNs axons that project along the olfactory nerve layer (ONL) fail to properly innervate the nascent bulb, thus resulting in partial loss of connectivity between OE-OB and gradual neuronal degeneration in both tissues peaking at birth. This deficiency correlates with deregulated expressions of two key chemo-repellant molecules, Robo2/Slit1 and Nrp2/Sema3F that control the formation of dorsal-ventral topographic map of OSNs connections with OB glomeruli. This study highlights a critical requirement for Rb during neurogenesis and the establishment of proper synaptic connections inside the OS during development.

mSWI/SNF (BAF) Complexes Are Indispensable for the Neurogenesis and Development of Embryonic Olfactory Epithelium

Neurogenesis is a key developmental event through which neurons are generated from neural stem/progenitor cells. Chromatin remodeling BAF (mSWI/SNF) complexes have been reported to play essential roles in the neurogenesis of the central nervous system. However, whether BAF complexes are required for neuron generation in the olfactory system is unknown. Here, we identified onscBAF and ornBAF complexes, which are specifically present in olfactory neural stem cells (oNSCs) and olfactory receptor neurons (ORNs), respectively. We demonstrated that BAF155 subunit is highly expressed in both oNSCs and ORNs, whereas high expression of BAF170 subunit is observed only in ORNs. We report that conditional deletion of BAF155, a core subunit in both onscBAF and ornBAF complexes, causes impaired proliferation of oNSCs as well as defective maturation and axonogenesis of ORNs in the developing olfactory epithelium (OE), while the high expression of BAF170 is important for maturation of ORNs. Interestingly, in the absence of BAF complexes in BAF155/BAF170 double-conditional knockout mice (dcKO), OE is not specified. Mechanistically, BAF complex is required for normal activation of Pax6-dependent transcriptional activity in stem cells/progenitors of the OE. Our findings unveil a novel mechanism mediated by the mSWI/SNF complex in OE neurogenesis and development.

Knockout of Abi3bp in mice does not affect their olfactory function, mental state and NNK-induced lung tumorigenesis

Abi3bp was originally discovered as Abi3-Src homology 3 (SH3) binding protein and has been proved to have a broad expression profile in adult tissues. Although previous studies have indicated that Abi3bp may be associated with cancer suppression, cell senescence, dendritic refinement and mental disorder, most conclusions achieved were based on in vitro model or genome-wide association study. In this work, we constructed an Abi3bp-deficient mouse model and observed phenotypic changes. The generated Abi3bp-knockout mice are viable and fertile, develop normally and exhibit no significant differences in anxiety or depression-like behaviors, olfactory function and tumor incidence. These data suggest that the function of Abi3bp in in vitro models does not translate to a similar role in the intact animal. Its depletion may be compensated by other genes, which needs to be addressed in future studies.

Adult Olfactory Bulb Interneuron Phenotypes Identified by Targeting Embryonic and Postnatal Neural Progenitors

Neurons are generated during embryonic development and in adulthood, although adult neurogenesis is restricted to two main brain regions, the hippocampus and olfactory bulb. The subventricular zone (SVZ) of the lateral ventricles generates neural stem/progenitor cells that continually provide the olfactory bulb (OB) with new granule or periglomerular neurons, cells that arrive from the SVZ via the rostral migratory stream. The continued neurogenesis and the adequate integration of these newly generated interneurons is essential to maintain homeostasis in the olfactory bulb, where the differentiation of these cells into specific neural cell types is strongly influenced by temporal cues. Therefore, identifying the critical features that control the generation of adult OB interneurons at either pre- or post-natal stages is important to understand the dynamic contribution of neural stem cells. Here, we used in utero and neonatal SVZ electroporation along with a transposase-mediated stable integration plasmid, in order to track interneurons and glial lineages in the OB. These plasmids are valuable tools to study the development of OB interneurons from embryonic and post-natal SVZ progenitors. Accordingly, we examined the location and identity of the adult progeny of embryonic and post-natally transfected progenitors by examining neurochemical markers in the adult OB. These data reveal the different cell types in the olfactory bulb that are generated in function of age and different electroporation conditions.

Deletion of collapsin response mediator protein 4 results in abnormal layer thickness and elongation of mitral cell apical dendrites in the neonatal olfactory bulb

Collapsin response mediator protein 4 (CRMP4), a member of the CRMP family, is involved in the pathogenesis of neurodevelopmental disorders such as schizophrenia and autism. Here, we first compared layer thickness of the olfactory bulb between wild-type (WT) and CRMP4-knockout (KO) mice. The mitral cell layer (MCL) was significantly thinner, whereas the external plexiform layer (EPL) was significantly thicker in CRMP4-KO mice at postnatal day 0 (PD0) compared with WTs. However, differences in layer thickness disappeared by PD14. No apoptotic cells were found in the MCL, and the number of mitral cells (MCs) identified with a specific marker (i.e. Tbx21 antibody) did not change in CRMP4-KO neonates. However, DiI-tracing showed that the length of mitral cell apical dendrites was greater in CRMP4-KO neonates than in WTs. In addition, expression of CRMP4 mRNA in WT mice was most abundant in the MCL at PD0 and decreased afterward. These results suggest that CRMP4 contributes to dendritic elongation. Our in vitro studies showed that deletion or knockdown of CRMP4 resulted in enhanced growth of MAP2-positive neurites, whereas overexpression of CRMP4 reduced their growth, suggesting a new role for CRMP4 as a suppressor of dendritic elongation. Overall, our data suggest that disruption of CRMP4 produces a temporary alteration in EPL thickness, which is constituted mainly of mitral cell apical dendrites, through the enhanced growth of these dendrites.

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.

The Complete Genome Sequences, Unique Mutational Spectra, and Developmental Potency of Adult Neurons Revealed by Cloning

Somatic mutation in neurons is linked to neurologic disease and implicated in cell-type diversification. However, the origin, extent, and patterns of genomic mutation in neurons remain unknown. We established a nuclear transfer method to clonally amplify the genomes of neurons from adult mice for whole-genome sequencing. Comprehensive mutation detection and independent validation revealed that individual neurons harbor ∼100 unique mutations from all classes but lack recurrent rearrangements. Most neurons contain at least one gene-disrupting mutation and rare (0-2) mobile element insertions. The frequency and gene bias of neuronal mutations differ from other lineages, potentially due to novel mechanisms governing postmitotic mutation. Fertile mice were cloned from several neurons, establishing the compatibility of mutated adult neuronal genomes with reprogramming to pluripotency and development.

Bulk regional viral injection in neonatal mice enables structural and functional interrogation of defined neuronal populations throughout targeted brain areas

The ability to label and manipulate specific cell types is central to understanding the structure and function of neuronal circuits. Here, we have developed a simple, affordable strategy for labeling of genetically defined populations of neurons throughout a targeted brain region: Bulk Regional Viral Injection (BReVI). Our strategy involves a large volume adeno-associated virus (AAV) injection in the targeted brain region of neonatal Cre driver mice. Using the mouse olfactory bulb (OB) as a model system, we tested the ability of BReVI to broadly and selectively label tufted cells, one of the two principal neuron populations of the OB, in CCK-IRES-Cre mice. BReVI resulted in labeling of neurons throughout the injected OB, with no spatial bias toward the injection site and no evidence of damage. The specificity of BReVI labeling was strikingly similar to that seen previously using immunohistochemical staining for cholecystokinin (CCK), an established tufted cell marker. Hence, the CCK-IRES-Cre line in combination with BReVI can provide an important tool for targeting and manipulation of OB tufted cells. We also found robust Cre-dependent reporter expression within three days of BReVI, which enabled us to assess developmental changes in the number and laminar distribution of OB tufted cells during the first three postnatal weeks. Furthermore, we demonstrate that BReVI permits structural and functional imaging in vivo, and can be combined with transgenic strategies to facilitate multi-color labeling of neuronal circuit components. BReVI is broadly applicable to different Cre driver lines and can be used to regionally manipulate genetically defined populations of neurons in any accessible brain region.

Tag1 deficiency results in olfactory dysfunction through impaired migration of mitral cells

The olfactory system provides mammals with the abilities to investigate, communicate and interact with their environment. These functions are achieved through a finely organized circuit starting from the nasal cavity, passing through the olfactory bulb and ending in various cortical areas. We show that the absence of transient axonal glycoprotein-1 (Tag1)/contactin-2 (Cntn2) in mice results in a significant and selective defect in the number of the main projection neurons in the olfactory bulb, namely the mitral cells. A subpopulation of these projection neurons is reduced in Tag1-deficient mice as a result of impaired migration. We demonstrate that the detected alterations in the number of mitral cells are well correlated with diminished odor discrimination ability and social long-term memory formation. Reduced neuronal activation in the olfactory bulb and the corresponding olfactory cortex suggest that Tag1 is crucial for the olfactory circuit formation in mice. Our results underpin the significance of a numerical defect in the mitral cell layer in the processing and integration of odorant information and subsequently in animal behavior.

Postnatal development attunes olfactory bulb mitral cells to high-frequency signaling

Mitral cells (MCs) are a major class of principal neurons in the vertebrate olfactory bulb, conveying odor-evoked activity from the peripheral sensory neurons to olfactory cortex. Previous work has described the development of MC morphology and connectivity during the first few weeks of postnatal development. However, little is known about the postnatal development of MC intrinsic biophysical properties. To understand stimulus encoding in the developing olfactory bulb, we have therefore examined the development of MC intrinsic biophysical properties in acute slices from postnatal day (P)7-P35 mice. Across development, we observed systematic changes in passive membrane properties and action potential waveforms consistent with a developmental increase in sodium and potassium conductances. We further observed developmental decreases in hyperpolarization-evoked membrane potential sag and firing regularity, extending recent links between MC sag heterogeneity and firing patterns. We then applied a novel combination of statistical analyses to examine how the evolution of these intrinsic biophysical properties specifically influenced the representation of fluctuating stimuli by MCs. We found that immature MCs responded to frozen fluctuating stimuli with lower firing rates, lower spike-time reliability, and lower between-cell spike-time correlations than more mature MCs. Analysis of spike-triggered averages revealed that these changes in spike timing were driven by a developmental shift from broad integration of inputs to more selective detection of coincident inputs. Consistent with this shift, generalized linear model fits to MC firing responses demonstrated an enhanced encoding of high-frequency stimulus features by mature MCs.

The Origin, Development and Molecular Diversity of Rodent Olfactory Bulb Glutamatergic Neurons Distinguished by Expression of Transcription Factor NeuroD1

Production of olfactory bulb neurons occurs continuously in the rodent brain. Little is known, however, about cellular diversity in the glutamatergic neuron subpopulation. In the central nervous system, the basic helix-loop-helix transcription factor NeuroD1 (ND1) is commonly associated with glutamatergic neuron development. In this study, we utilized ND1 to identify the different subpopulations of olfactory bulb glutamategic neurons and their progenitors, both in the embryo and postnatally. Using knock-in mice, transgenic mice and retroviral transgene delivery, we demonstrate the existence of several different populations of glutamatergic olfactory bulb neurons, the progenitors of which are ND1⁺ and ND1^- lineage-restricted, and are temporally and regionally separated. We show that the first olfactory bulb glutamatergic neurons produced – the mitral cells – can be divided into molecularly diverse subpopulations. Our findings illustrate the complexity of neuronal diversity in the olfactory bulb and that seemingly homogenous neuronal populations can consist of multiple subpopulations with unique molecular signatures of transcription factors and expressing neuronal subtype-specific markers.

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.

Unraveling Cajal's view of the olfactory system

The olfactory system has a highly regular organization of interconnected synaptic circuits from the periphery. It is therefore an excellent model for understanding general principles about how the brain processes information. Cajal revealed the basic cell types and their interconnections at the end of the XIX century. Since his original descriptions, the observation and analysis of the olfactory system and its components represents a major topic in neuroscience studies, providing important insights into the neural mechanisms. In this review, we will highlight the importance of Cajal contributions and his legacy to the actual knowledge of the olfactory system.

Conditional ablation of Tbr2 results in abnormal development of the olfactory bulbs and subventricular zone-rostral migratory stream

BACKGROUND

Development of the olfactory bulb (OB) is a complex process that requires contributions from several progenitor cell niches to generate neuronal diversity. Previous studies showed that Tbr2 is expressed during the generation of glutamatergic OB neurons in rodents. However, relatively little is known about the role of Tbr2 in the developing OB or in the subventricular zone-rostral migratory stream (SVZ-RMS) germinal niche that gives rise to many OB neurons.

RESULTS

Here, we use conditional gene ablation strategies to knockout Tbr2 during embryonic mouse olfactory bulb morphogenesis, as well as during perinatal and adult neurogenesis from the SVZ-RMS niche, and describe the resulting phenotypes. We find that Tbr2 is important for the generation of mitral cells in the OB, and that the olfactory bulbs themselves are hypoplastic and disorganized in Tbr2 mutant mice. Furthermore, we show that the SVZ-RMS niche is expanded and disordered following loss of Tbr2, which leads to ectopic accumulation of neuroblasts in the RMS. Lastly, we show that adult glutamatergic neurogenesis from the SVZ is impaired by loss of Tbr2.

CONCLUSIONS

Tbr2 is essential for proper morphogenesis of the OB and SVZ-RMS, and is important for the generation of multiple lineages of glutamatergic olfactory bulb neurons.