Focal denervation alters cellular phenotypes and survival in the rat olfactory bulb: a developmental analysis

Connectivity of Pathology: The Olfactory System as a Model for Network-Driven Mechanisms of Alzheimer's Disease Pathogenesis

The pathogenesis of Alzheimer’s disease (AD) has been postulated to preferentially impact specific neural networks in the brain. The olfactory system is a well-defined network that has been implicated in early stages of the disease, marked by impairment in olfaction and the presence of pathological hallmarks of the disease, even before clinical presentation. Discovering the cellular mechanisms involved in the connectivity of pathology will provide insight into potential targets for treatment. We review evidence from animal studies on sensory alteration through denervation or enrichment, which supports the notion of using the olfactory system to investigate the implications of connectivity and activity in the spread of pathology in AD.

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.

Sensory deafferentation transsynaptically alters neuronal GluR1 expression in the external plexiform layer of the adult mouse main olfactory bulb

Altered distribution of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunit GluR1 has been linked to stimulation-dependent changes in synaptic efficacy, including long-term potentiation and depression. The main olfactory bulb (OB) remains plastic throughout life; how GluR1 may be involved in this plasticity is unknown. We have previously shown that neonatal naris occlusion reduces numbers of interneuron cell bodies that are immunoreactive for GluR1 in the external plexiform layer (EPL) of the adult mouse OB. Here, we show that immunoreactivity of mouse EPL interneurons for GluR1 is also dramatically reduced following olfactory deafferentation in adulthood. We further show that expression of glutamic acid decarboxylase (GAD) 65, 1 of 2 GAD isoforms expressed by adult gamma-aminobutyric acidergic interneurons, is reduced, but to a much smaller extent, and that in double-labeled cells, immunoreactivity for the Ca(2+)-binding protein parvalbumin (PV) is also reduced. In addition, GluR1 expression is reduced in presumptive tufted cells and interneurons that are negative for GAD65 and PV. Consistent with previous reports, sensory deafferentation resulted in little neuronal degeneration in the adult EPL, indicating that these differences were not likely due to death of EPL neurons. Together, these results suggest that olfactory input regulates expression of the GluR1 AMPA receptor subunit by tufted cells that may in turn regulate GluR1 expression by interneurons within the OB EPL.

A leucine-rich repeat membrane protein, 5T4, is expressed by a subtype of granule cells with dendritic arbors in specific strata of the mouse olfactory bulb

Segregation of neuron-type-specific synaptic connections in different strata is a characteristic feature shared by the olfactory bulb (OB) and retina. In the mammalian OB, mitral cells form dendrodendritic synapses with granule cells (GCs) in the deep stratum of the external plexiform layer (EPL), whereas tufted cells form dendrodendritic synapses in the superficial stratum. In the search for membrane proteins with strata-specific expression patterns, we found that a leucine-rich repeat membrane protein (5T4 oncofetal trophoblast glycoprotein) was expressed selectively by a subset of superficial GCs. The somata of 5T4-positive GCs were localized in or near the mitral cell layer, and their apical dendrites ramified preferentially in the superficial stratum of the EPL, where tufted cell dendrites ramified. Strata-specific expression of 5T4 was found also in the retina: 5T4 was expressed selectively by rod-bipolar cells and a subset of amacrine cells whose dendrites ramified in a specific sublamina of the inner plexiform layer. During the perinatal and postnatal development of the OB, 5T4 expression paralleled in time the formation of dendrodendritic synapses in the EPL. Odor deprivation during the first postnatal month selectively reduced the thickness of the superficial stratum of the EPL and the number of 5T4-positive GCs. Because 5T4 is known to interact with actin cytoskeleton, these observations suggest that 5T4 is involved in the formation or maintenance of strata-specific dendritic ramification or synaptic connection of subsets of local interneurons.

Quantitative changes in calretinin immunostaining in the cochlear nuclei after unilateral cochlear removal in young ferrets

Neurons of the cochlear nuclei receive axosomatic endings from primary afferent fibers from the cochlea and have projections that diverge to form parallel ascending auditory pathways. These cells are characterized by neurochemical phenotypes such as levels of calretinin. To test whether or not early deafferentation results in changes in calretinin immunostaining in the cochlear nucleus, unilateral cochlear ablations were performed in ferrets soon after hearing onset (postnatal day [P]30-P40). Two months later, changes in calretinin immunostaining as well as cell size, volume, and synaptophysin immunostaining were assessed in the anteroventral (AVCN), posteroventral (PVCN), and dorsal cochlear nucleus (DCN). A decrease in calretinin immunostaining was evident ipsilaterally within the AVCN and PVCN but not in the DCN. Further analysis revealed a decrease both in the calretinin-immunostained neuropil and in the calretinin-immunostained area within AVCN and PVCN neurons. These declines were accompanied by significant ipsilateral decreases in volume as well as neuron area in the AVCN and PVCN compared with the contralateral cochlear nucleus and unoperated animals, but not compared with the DCN. In addition, there was a significant contralateral increase in calretinin-immunostained area within AVCN and PVCN neurons compared with control animals. Finally, a decrease in area of synaptophysin immunostaining in both the ipsilateral AVCN and PVCN without changes in the number of boutons was found. The present data demonstrate that unilateral cochlear ablation leads to 1) decreased immunostaining of the neuropil in the AVCN and PVCN ipsilaterally, 2) decreased calretinin immunostaining within AVCN and PVCN neurons ipsilaterally, 3) synaptogenesis in the AVCN and PVCN ipsilaterally, and 4) increased calretinin immunostaining within AVCN and PVCN neurons contralaterally.

Postnatal developmental expression of calbindin, calretinin and parvalbumin in mouse main olfactory bulb

The distribution of calbindin, calretinin and parvalbumin during the development of the mouse main olfactory bulb (MOB) was studied using immunohistochemistry techniques. The results are as follows: (1) calbindin-immunoreactive profiles were mainly located in the glomerular layer, and few large calbindin-immunoreactive cells were found in the subependymal layer of postnatal day 10 (P10) to postnatal day 40 (P40) mice; (2) no calbindin was detected in the mitral cell layer at any stage; (3) calretinin-immunoreactive profiles were present in all layers of the main olfactory bulb at all stages, especially in the olfactory nerve layer, glomerular layer and granule cell layer; (4) parvalbumin-immunoreactive profiles were mainly located in the external plexiform layer (except for P10 mice); (5) weakly stained parvalbumin-immunoreactive profiles were present in the glomerular layer at all stages; and (6) no parvalbumin was detected in the mitral cell layer at any stage.

Upregulation of calretinin immunostaining in the ferret inferior colliculus after cochlear ablation

In many systems, including ascending auditory pathways, calcium-binding proteins are markers of specific neuronal circuits. Previous studies suggest that calretinin immunostaining may be a specific marker for circuits in the inferior colliculus (IC) that code timing information. We undertook experiments to determine the changes in calretinin immunostaining in the IC that take place in response to cochlear ablation. Cochlear ablation was performed unilaterally in ferrets just after hearing onset. Animals survived for 2-3 months after ablation and brains were then processed for calretinin immunocytochemistry. The mean optical density and stained area of the calretinin immunopositive plexus in the IC were determined for five coronal sections through the right and left IC. In controls (n = 3), measurements of these parameters in the central nucleus of the IC showed symmetry between the two sides. In experimental animals (n = 8) the calretinin immunopositive plexus contralateral to the cochlear ablation was denser and larger than that in either the ipsilateral IC or in the IC of control animals. The calretinin plexus in the ipsilateral IC was slightly less dense and smaller than in controls but the differences did not reach statistical significance. IC volume measurements and synaptophysin immunostaining analysis in the central nucleus of the IC revealed no statistical differences between ablated and control animals or between the two sides in ablated animals. The significant increase in both mean optical density and immunostained area of the calretinin plexus in the IC contralateral to the cochlear ablation may reflect an upregulation in calretinin expression in the nuclei that contribute to this plexus.

Comparison of turnover in the olfactory organ of early juvenile stage and adult Caribbean spiny lobsters

Proliferation and turnover of neurons occurs in the olfactory systems of many animals. In this study, we examined developmental changes in turnover in the olfactory organ of the Caribbean spiny lobster Panulirus argus by examining two life-history stages-early juveniles and young adults. Turnover was compared using external morphology of the olfactory organ before and after molting to determine addition and loss of aesthetascs and other chemosensilla, and BrdU labeling to identify newly proliferated cells. The number of olfactory receptor neurons (ORNs) innervating each aesthetasc increased only slightly over development, but there was a net increase of olfactory sensory units (i.e. aesthetascs and their ORNs) at each molt. This increase was similar in early juveniles and young adults when expressed as absolute number of ORNs neurons but greater in early juveniles when expressed as a proportion of existing ORNs. The net increase in olfactory sensory units in early juveniles is due solely to addition, since virtually no aesthetascs are lost. In contrast, the net increase in olfactory sensory units in adults reflects addition of new units accompanied by considerable loss of old units. These developmental changes result in expansive enlargement of the olfactory organ without turnover in early juveniles, and a more modest growth combined with continuous turnover and replenishment of ORNs in adults.

Neonatal focal denervation of the rat olfactory bulb alters cell structure and survival: a Golgi, Nissl and confocal study

Contact between sensory axons and their targets is critical for the development and maintenance of normal neural circuits. Previous work indicates that the removal of afferent contact to the olfactory bulb affects bulb organization, neurophenotypic expression, and cell survival. The studies also suggested changes to the structure of individual cell types. The current work examines the effects of denervation on the morphology of mitral/tufted, periglomerular, and granule cells. Focal denervation drastically changed mitral/tufted cell structure but had only subtle effects on periglomerular and granule cells. Denervated mitral/tufted cells lacked apical tufts and, in most cases, a primary dendrite. In addition, the denervated cells had more secondary processes whose orientation with respect to the bulb surface was altered. Our results suggest that contact between olfactory axons and the bulb is necessary for cell maintenance and may be critical for the ability of mitral/tufted cells to achieve adult morphology

p73 is required for survival and maintenance of CNS neurons

Here, we show that the p53 family member, p73, is necessary for survival and long-term maintenance of CNS neurons, including postnatal cortical neurons. In p73-/- animals, cortical neuron number is normal at birth but decreases significantly by postnatal day 14 (P14)-P16 because of enhanced apoptosis. This decrease continues into adulthood, when p73-/- animals have approximately one-half as many cortical cells as their wild-type littermates. Cortical neurons express the DeltaNp73alpha protein, and overexpression of DeltaNp73 isoforms rescues cortical neurons from diverse apoptotic stimuli. Thus, DeltaNp73 isoforms are survival proteins in cortical neurons, and their deletion causes a gradual loss of cortical neurons in the weeks and months after birth. This decrease in CNS neuron number in p73-/- animals is not limited to the cortex; facial motor neuron number is decreased, and postnatal development of the olfactory bulb is greatly perturbed. These findings, together with our previous work showing that DeltaNp73 is essential for survival of peripheral sympathetic neurons (Pozniak et al., 2000), indicate that p73 isoforms are essential survival proteins in CNS as well as PNS neurons, and that they likely play a role not only during developmental cell death but also in the long-term maintenance of at least some adult neurons.