S. De Marchis

BDNF/ TrkB interaction regulates migration of SVZ precursor cells via PI3‐K and MAP‐K signalling pathways

Neuroblasts born in the subventricular zone (SVZ) migrate along the rostral migratory stream, reaching the olfactory bulb (OB) where they differentiate into local interneurons. Several extracellular factors have been suggested to control specific steps of this process. The brain‐derived neurotrophic factor (BDNF) has been demonstrated to promote morphological differentiation and survival of OB interneurons. Here we show that BDNF and its receptor TrkB are expressed in vivo throughout the migratory pathway, implying that BDNF might also mediate migratory signals. By using in vitro models we demonstrate that BDNF promotes migration of SVZ neuroblasts, acting both as inducer and attractant through TrkB activation. We show that BDNF induces cAMP response element‐binding protein (CREB) activation in migrating neuroblasts via phosphatidylinositol 3‐kinase (PI3‐K) and mitogen‐activated protein kinase (MAP‐K) signalling. Pharmacological blockade of these pathways on SVZ explants significantly reduces CREB activation and impairs neuronal migration. This study identifies a function of BDNF in the SVZ system, which involves multiple protein kinase pathways leading to neuroblast migration.

Expression of the secreted factors noggin and bone morphogenetic proteins in the subependymal layer and olfactory bulb of the adult mouse brain.

The antagonism between noggin and the bone morphogenetic proteins (BMPs) plays a key role during CNS morphogenesis and differentiation. Recent studies indicate that these secreted factors are also widely expressed in the postnatal and adult mammalian brain in areas characterized by different types of neural plasticity. In particular, significant levels of noggin and BMP expression have been described in the rodent olfactory system. In the mammalian forebrain, the olfactory bulb (OB) and associated subependymal layer (SEL) are documented as sites of adult neurogenesis. Here, using multiple approaches, including the analysis of noggin-LacZ heterozygous mice, we report the expression of noggin and two members of the BMP family, BMP4 and BMP7, in these regions of the adult mammalian forebrain. We observe that along the full extent of the SEL, from the lateral ventricle to the olfactory bulb, noggin and BMP4 and 7 are mainly associated with the astrocytic glial compartment. In the OB, BMP4 and 7 proteins remain primarily associated with the SEL while strong noggin expression was also found in cells located in different OB layers (i.e. granule, external plexiform, glomerular layers). Taken together our data lead us to hypothesize that within the SEL the antagonism between noggin and BMPs, both produced by the glial tubes, act through autocrine/paracrine inductive mechanisms to maintain a neurogenetic environment all the way from the lateral ventricle to the olfactory bulb. In the OB, their expression patterns suggest multiple regulatory roles on the unusual neural plasticity exhibited by this region.

Integration and sensory experience-dependent survival of newly-generated neurons in the accessory olfactory bulb of female mice

Newborn neurons generated by proliferative progenitors in the adult subventricular zone (SVZ) integrate into the olfactory bulb circuitry of mammals. Survival of these newly‐formed cells is regulated by the olfactory input. The presence of new neurons in the accessory olfactory bulb (AOB) has already been demonstrated in some mammalian species, albeit their neurochemical profile and functional integration into AOB circuits are still to be investigated. To unravel whether the mouse AOB represents a site of adult constitutive neurogenesis and whether this process can be modulated by extrinsic factors, we have used multiple in vivo approaches. These included fate mapping of bromodeoxyuridine‐labelled cells, lineage tracing of SVZ‐derived enhanced green fluorescent protein‐positive engrafted cells and neurogenesis quantification in the AOB, in both sexes, as well as in females alone after exposure to male‐soiled bedding or its derived volatiles. Here, we show that a subpopulation of SVZ‐derived neuroblasts acquires proper neurochemical profiles of mature AOB interneurons. Moreover, 3D reconstruction of long‐term survived engrafted neuroblasts in the AOB confirms these cells show features of fully integrated neurons. Finally, exposure to male‐soiled bedding, but not to its volatile compounds, significantly increases the number of new neurons in the AOB, but not in the main olfactory bulb of female mice. These data show SVZ‐derived neuroblasts differentiate into new functionally integrated neurons in the AOB of young and adult mice. Survival of these cells seems to be regulated by an experience‐specific mechanism mediated by pheromones.

GABAergic phenotypic differentiation of a subpopulation of subventricular derived migrating progenitors

Olfactory bulb interneurons are continuously generated throughout development and in adulthood. These neurons are born in the subventricular zone (SVZ) and migrate along the rostral migratory stream into the olfactory bulb where they differentiate into local interneurons. To investigate the differentiation of GABAergic interneurons of the olfactory bulb we used a transgenic mouse which expresses green fluorescent protein (GFP) under the control of the glutamic acid decarboxylase 65 kDa (GAD65) promoter. During development and in adulthood GFP was expressed by cells in the SVZ and along the entire length of its rostral extension including the distal portion within the olfactory bulb. The occurrence of GAD65 mRNA in these zones was confirmed by PCR analysis on microdissected regions along the pathway. Polysialic acid neural cell adhesion molecule, a marker of migrating neuroblasts in adults, was coexpressed by the majority of the GFP‐positive SVZ‐derived progenitor cells. Cell tracer injections into the SVZ indicated that ≈ 26% of migrating progenitor cells expressed GFP. These data show the early differentiation of migrating SVZ‐derived progenitors into a GAD65–GFP‐positive phenotype. These cells could represent a restricted lineage giving rise to GAD65‐positive GABAergic olfactory bulb interneurons.

Glutamatergic deafferentation of olfactory bulb modulates the expression of mGluR1a mRNA

Glutamate (Glu) released by olfactory nerve axons acts on postsynaptic ionotropic and metabotropic glutamate receptors expressed by principal neurones and interneurones of the olfactory bulb (OB). Using ZnSO4 lesioning of the rat olfactory mucosa and semiquantitative RT-PCR, we examined the effect of removal of the glutamatergic input to the OB on the expression of mGluR1a, mGluR1b and GluR1 mRNAs. Two days after lesioning, mGluR1a mRNA levels in OB increased by 45%. At this time, the expression of tyrosine hydroxylase (TH) mRNA, which is strictly dependent on olfactory nerve input, was still unchanged. In contrast, 16 days after lesioning, deafferented OB exhibited a decrease in both mGluR1a (-30%) and TH (-40%) mRNAs. GluR1 and mGluR1b mRNA levels were not affected at either time point. These results suggest that alterations in glutamatergic input to OB selectively modulate the expression of the mGluR1 splicing form possessing a longer C-terminal domain.

Adult neurogenesis and local neuronal progenitors in the striatum.

Mechanisms underlying neurogenesis in the subventricular-zone-olfactory-bulb system and dentate gyrus of the hippocampus are beginning to be delineated and show common regulative features. In both regions neurogenesis is attributable to progenitor cells whose progeny progressively matures to functional neurons under genetic and epigenetic influence. Persistence of endogenous neuronal progenitors and integration of new neurons in preexisting circuits provide an appealing model of study to develop therapy strategies for neurodegenerative diseases. Interestingly, comparative analysis in mammals indicates that low neurogenic activity is also present in regions classically considered nonneurogenic in both normal and pathological conditions. Neurogenesis in these regions can be due to progenitors derived from the subventricular germinal zone and/or local parenchymal progenitors. Although, in vivo, the origin, identity and putative function of parenchymal progenitors are still obscure, in vitro studies suggest that many regions of the adult central nervous system potentially contain multipotent parenchymal progenitors. The aim of this review is to delineate the common regulative features underlying adult neurogenesis in the main neurogenic regions and in the striatum focusing on our recent data concerning the existence of local parenchymal progenitors in the caudate nucleus of the adult rabbit.

Expression and localization of the calmodulin-binding protein neurogranin in the adult mouse olfactory bulb

Neurogranin (Ng) is a brain‐specific postsynaptic protein involved in activity‐dependent synaptic plasticity through modulation of Ca2+/calmodulin (CaM)‐dependent signal transduction in neurons. In this study, using biochemical and immunohistochemical approaches, we demonstrate Ng expression in the adult mouse olfactory bulb (OB), the first relay station in odor information processing. We show that Ng is principally associated with the granule cell layer (GCL), which is composed of granule cell inhibitory interneurons. This cell type is continuously renewed during adult life and plays a key role in OB circuits, integrating and modulating the activity of mitral/tufted cells. Our results indicate that Ng localizes in the soma and dendrites of a defined subpopulation of mature GABAergic granule cells, enriched in the deep portion of the GCL. Ng‐immunopositive cells largely coexpress the Ca+/CaM‐dependent kinase IV (CaMKIV), a downstream protein of CaM signaling cascade, whereas no colocalization was observed between Ng and the calcium‐binding protein calretinin. Finally, we demonstrate that adult neurogenesis contributes to the Ng‐expressing population, with more newly generated Ng‐positive cells integrated in the deep GCL. Together, these results provide a new specific neurochemical marker to identify a subpopulation of olfactory granule cells and suggest possible functional implications for Ng in OB plasticity mechanisms. J. Comp. Neurol. 517:683–694, 2009.

From chemical neuroanatomy to an understanding of the olfactory system.

The olfactory system of mammals is the appropriate model for studying several aspects of neuronal physiology spanning from the developmental stage to neural network remodelling in the adult brain. Both the morphological and physiological understanding of this system were strongly supported by classical histochemistry. It is emblematic the case of the Olfactory Marker Protein (OMP) staining, the first, powerful marker for fully differentiated olfactory receptor neurons and a key tool to investigate the dynamic relations between peripheral sensory epithelia and central relay regions given its presence within olfactory fibers reaching the olfactory bulb (OB). Similarly, the use of thymidine analogues was able to show neurogenesis in an adult mammalian brain far before modern virus labelling and lipophilic tracers based methods. Nowadays, a wealth of new histochemical techniques combining cell and molecular biology approaches is available, giving stance to move from the analysis of the chemically identified circuitries to functional research. The study of adult neurogenesis is indeed one of the best explanatory examples of this statement. After defining the cell types involved and the basic physiology of this phenomenon in the OB plasticity, we can now analyze the role of neurogenesis in well testable behaviours related to socio-chemical communication in rodents.

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. J. Comp. Neurol., 520:3055–3069, 2012.