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.

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 Ca(2+)/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.

Developmental regulation of metabotropic glutamate receptor 1 splice variants in olfactory bulb mitral cells.

Alternative splicing of the metabotropic glutamate receptor 1 (mGluR1) receptor gene generates two major receptor isoforms, mGluR1a and mGluR1b, differing in intracellular function and distribution. However, little is known on the expression profiles of these variants during development. We examined the mRNA expression profile of mGluR1a/b in microdissected layers and acutely isolated mitral cells in the developing mouse olfactory bulb. This analysis showed that the two mGluR1 variants are differentially regulated within each bulb layer. During the first postnatal week, the mGluR1a isoform replaces GluR1b in the microdissected mitral cell layer (MCL) and in isolated identified mitral cells, coinciding with a developmental epoch of mitral cell dendritic reorganization. Although mGluR1a mRNA is expressed at high levels in both the adult external plexiform layer (EPL) and MCL, Western blotting analysis reveals a marked reduction of the mGluR1a protein in the MCL, where mitral cell bodies are located, and strong labeling in the EPL, which contains mitral cell dendrites. This suggests that there is increased dendritic trafficking efficiency of the receptor in adult. The temporal and spatial shift in mGluR1b/a expression suggests distinct roles of the mGluR1 isoforms, with mGluR1b potentially involved in the early mitral cell maturation and mGluR1a in dendritic and synapse function.

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 approximately 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.