Immunohistochemical localization of guanine nucleotide-binding proteins in rat olfactory epithelium during development

Hyperlipidemic diet causes loss of olfactory sensory neurons, reduces olfactory discrimination, and disrupts odor-reversal learning

Currently, 65% of Americans are overweight, which leads to well-supported cardiovascular and cognitive declines. Little, however, is known concerning obesity's impact on sensory systems. Because olfaction is linked with ingestive behavior to guide food choice, its potential dysfunction during obesity could evoke a positive feedback loop to perpetuate poor ingestive behaviors. To determine the effect of chronic energy imbalance and reveal any structural or functional changes associated with obesity, we induced long-term, diet-induced obesity by challenging mice to high-fat diets: (1) in an obesity-prone (C57BL/6J) and obesity-resistant (Kv1.3(-/-)) line of mice, and compared this with (2) late-onset, genetic-induced obesity in MC4R(-/-) mice in which diabetes secondarily precipitates after disruption of the hypothalamic axis. We report marked loss of olfactory sensory neurons and their axonal projections after exposure to a fatty diet, with a concomitant reduction in electro-olfactogram amplitude. Loss of olfactory neurons and associated circuitry is linked to changes in neuronal proliferation and normal apoptotic cycles. Using a computer-controlled, liquid-based olfactometer, mice maintained on fatty diets learn reward-reinforced behaviors more slowly, have deficits in reversal learning demonstrating behavioral inflexibility, and exhibit reduced olfactory discrimination. When obese mice are removed from their high-fat diet to regain normal body weight and fasting glucose, olfactory dysfunctions are retained. We conclude that chronic energy imbalance therefore presents long-lasting structural and functional changes in the operation of the sensory system designed to encode external and internal chemical information and leads to altered olfactory- and reward-driven behaviors.

Deletion of voltage-gated channel affects glomerular refinement and odorant receptor expression in the mouse olfactory system

Olfactory sensory neurons (OSNs) expressing a specific odorant receptor (OR) gene send axonal projections to specific glomeruli, creating a stereotypic olfactory sensory map. Odorant receptor sequence, G-protein cAMP signaling, and axon guidance molecules have been shown to direct axons of OSNs toward central targets in the olfactory bulb (OB). Although the OR sequence may act as one determinant, our objective was to elucidate the extent by which voltage-dependent activity of postsynaptic projection neurons in the OB centrally influences peripheral development and target destination of OSNs. We bred OR-tagged transgenic mice to homozygosity with mice that had a gene-targeted deletion of the Shaker potassium ion channel (Kv1.3) to elucidate how activity modulates synaptic connections that formulate the sensory map. Here we report that the Kv1.3 ion channel, which is predominantly expressed in mitral cells and whose gene-targeted deletion causes a "super-smeller" phenotype, alters synaptic refinement of axonal projections from OSNs expressing P2, M72, and MOR28 ORs. Absence of Kv1.3 voltage-gated activity caused the formation of small, heterogeneous, and supernumerary glomeruli that failed to undergo neural pruning over development. These changes were accompanied by a significant decrease in the number of P2-, M72-, and MOR28-expressing OSNs, which contained an overexpression of OR protein and G-protein G(olf) in the cilia of the olfactory epithelium. These findings suggest that voltage-gated activity of projection neurons is essential to refine primary olfactory projections and that it regulates proper expression of the transduction machinery at the periphery.

From genotype to olfactory neuron phenotype: the role of the Olf-1-binding site

The highly organized pattern of gene expression leading to the determination of cellular phenotype derives from the interplay between genetic and epigenetic factors. This is mediated in part by distinctive DNA sequence motifs present in the regulatory regions of various genes and the transcription factors with which they interact. The phenotype of olfactory neurons is determined in part by the selective expression of novel isoforms of several genes involved in chemosensory transduction. To characterize the mechanisms determining olfactory neuron phenotype we have been studying the olfactory marker protein (OMP), the first olfactory-specific protein to be isolated and cloned. The temporal and spatial expression of OMP is regulated stringently and is highly restricted to mature olfactory neurons in all vertebrates from amphibians to humans. Identification of the specific elements responsible for regulating the expression of the OMP gene will elucidate the mechanisms leading to the determination of olfactory neuron phenotype. Using a combined in vivo (transgenic mice) and in vitro (electrophoretic mobility shift assays and DNase I footprinting) approach, we have identified and characterized a novel genomic motif that binds an olfactory tissue nuclear protein(s) that we designate Olf-1. We propose that Olf-1 is a novel olfactory-specific transacting factor responsible for directing the expression of genes containing the Olf-1 motif in olfactory neurons. Thus it may play a role in regulating the expression of genes associated with neuronal turnover and olfactory transduction.

Odorants selectively activate distinct G protein subtypes in olfactory cilia

Chemoelectrical signal transduction in olfactory neurons appears to involve intracellular reaction cascades mediated by heterotrimeric GTP-binding proteins. In this study attempts were made to identify the G protein subtype(s) in olfactory cilia that are activated by the primary (odorant) signal. Antibodies directed against the alpha subunits of distinct G protein subtypes interfered specifically with second messenger reponses elicited by defined subsets of odorants; odor-induced cAMP-formation was attenuated by Galphas antibodies, whereas Galphao antibodies blocked odor-induced inositol 1,4, 5-trisphosphate (IP3) formation. Activation-dependent photolabeling of Galpha subunits with [alpha-32P]GTP azidoanilide followed by immunoprecipitation using subtype-specific antibodies enabled identification of particular individual G protein subtypes that were activated upon stimulation of isolated olfactory cilia by chemically distinct odorants. For example odorants that elicited a cAMP response resulted in labeling of a Galphas-like protein, whereas odorants that elicited an IP3 response led to the labeling of a Galphao-like protein. Since odorant-induced IP3 formation was also blocked by Gbeta antibodies, activation of olfactory phospholipase C might be mediated by betagamma subunits of a Go-like G protein. These results indicate that different subsets of odorants selectively trigger distinct reaction cascades and provide evidence for dual transduction pathways in olfactory signaling.

Inhibition and enhancement of odorant-induced cAMP accumulation in rat olfactory cilia by antibodies directed against G alpha S/olf- and G alpha i-protein subunits

The odorant-induced accumulation of cAMP can be inhibited by antibodies directed against G alpha s/olf. In contrast, antibodies raised against G alpha i-subunits caused a strong enhancement of the odorant-induced cAMP accumulation. Western blotting and immunoelectron microscopy revealed the presence of both G alpha s/olf- and G alpha i-subunits in rat cilia preparations. The existence of both stimulatory and inhibitory odorant-induced regulation of adenylyl cyclase activity in olfactory cilia may indicate that an initial integration of different odorant stimuli begins at the level of primary reactions in the same effector enzyme.

Differential expression of manganese and copper-zinc superoxide dismutases in the olfactory and vomeronasal receptor neurons of rats during ontogeny

Superoxide dismutases (SODs) protect cells from damage by oxygen free radicals. Manganese (Mn) SOD is preferentially induced in terminally differentiating cells; induction of copper-zinc (CuZn) SOD is more closely associated with postnatal exposure to environmental sources of oxygen free radicals. The purpose of this study was to investigate ontogenetic changes in immunoreactivity for MnSOD and CuZnSOD relative to the expression of markers of neuronal and chemosensory differentiation in olfactory and vomeronasal receptor neurons (ORNs and VRNs, respectively), which mature with different time courses. Immunoreactivity for both SODs was detected in rat ORNs at embryonic day (E) 14, the earliest time point investigated, but not until E16 in vomeronasal neuroblasts. ORNs also expressed the neuronal marker protein gene product (PGP) 9.5 and the chemosensory cell marker olfactory marker protein (OMP) at E14; vomeronasal neuroblasts expressed PGP 9.5 at E16 but were not immunoreactive for OMP until postnatal day (P) 2. Immunoreactivity for MnSOD in ORNs and VRNs generally increased pre- and postnatally to a maximum at P11. Immunoreactivity for CuZnSOD did not increase markedly until after birth, reaching maximal levels at P11-P24. Within ORNs and VRNs, the most intense immunoreactivity was localized in the dendritic and supranuclear regions. The results indicate that in ORNs and VRNs, increases in MnSOD immunoreactivity during ontogeny parallel the ongoing differentiation and maturation of chemosensory receptor neurons; in contrast, the induction of immunoreactivity for CuZnSOD is associated with postnatal exposure to the ambient oxygen and xenobiotic environment.

G alpha 9/G alpha 11: immunolocalization in the olfactory epithelium of the rat (Rattus rattus) and the channel catfish (Ictalurus punctatus)

The immunohistochemical localization of G alpha 9/G alpha 11 was studied in the olfactory and respiratory epithelium of two representative vertebrates, the rat and the channel catfish. Localization in the rat was found at the apical surface of cells in the epithelium and within nerve tracts in the lamina propria. Immunostaining of neuronal cilia and supporting cell microvilli was confirmed by electron microscopy. Immunoreactivity on the ipsilateral neuroepithelium was abolished five weeks following unilateral bulbectomy. An emergence of patchy immunoreactivity was found, however, after fifteen weeks. In catfish, G alpha 9/G alpha 11 antigenicity was found at the apical surface of cells within the olfactory epithelium, at supranuclear regions within some cell bodies and in basal nerve tracts of the olfactory rosette. Immunoreactivity was removed with unilateral bulbectomy. Specific labelling in both rat and catfish was eliminated by preincubation of the G alpha 9/G alpha 11 antibodies with the cognate peptide. Proteins were extracted from olfactory tissues of both species and solubilized. Using western blotting, bands corresponding in apparent molecular weight to a 38,000 mol. wt protein were found. These data demonstrate the presence of G alpha 9/G alpha 11 in the olfactory tissues of these vertebrates and suggest a role in olfaction for this class of G-protein.

Freeze-fracture, deep-etch, and freeze-substitution studies of olfactory epithelia, with special emphasis on immunocytochemical variables

Freeze-fracturing and deep-etching are a well-suited set of methods to study membrane and cytoplasmic features. Various approaches are available. Possible variables include tissue preparation, fracturing only or fracturing followed by etching, modes and materials of replication, and various ways of combining freeze-fracturing and/or deep-etching with (immuno)cytochemistry. Freeze-substitution, in particular combined with embedding in methacrylate resins such as the Lowicryls, is becoming rather widely accepted for purposes of ultrastructural (immuno)cytochemistry. Most investigators active in this field agree that this combination yields superior results compared to (immuno)cytochemistry combined with more conventional means of thin section transmission electron microscopy. Yet relatively little information is available on the variations that can occur with different approaches of freeze-substitution immunocytochemistry. This review deals with some of the variations in freeze-fracturing, freeze-etching, and freeze-substitution as applied to olfactory epithelial structures and with the effectiveness of observations obtained by application of the above sets of methods in relating the special morphology of olfactory epithelial cellular structures with those obtained by other approaches. Indeed, the data obtained continue to provide an integral image in which that morphology can be related to the special biochemistry, cell and molecular biology, and electrophysiology of olfactory epithelial structures.

Cell migration from the olfactory placode and the ontogeny of the neuroendocrine compartments

The olfactory placode and its derivative, the olfactory pit, give rise to several different populations of migrating cells, which contribute to drive the organization of the prosencephalon, but also to form a part of the central neuroendocrine compartments. Some cell types are seemingly transient and can play a role in the establishment of the final connections. The understanding of the mechanisms involved in the migration and differentiation of these cell populations can give an insight on the interplay between peripheral structures and central nervous system and on the mechanisms of commitment, phenotype selection and control for neuroendocrine cells able to selectively "colonize" the brain.

Differential expression of vomeromodulin and odorant-binding protein, putative pheromone and odorant transporters, in the developing rat nasal chemosensory mucosae

Expression of the putative pheromone and odorant transporter, vomeromodulin, was characterized in developing rat nasal mucosae using in situ hybridization and immunocytochemistry. Initial expression of vomeromodulin mRNA and protein was detected at embryonic day (E)16 in the maxillary sinus component of the lateral nasal glands. The abundance of mRNA and protein in the lateral nasal glands increased with age and reached a peak at postnatal day (P)27. Also at P27, vomeromodulin mRNA and protein expression was initiated in vomeronasal glands and posterior glands of the nasal septum. Comparison of the developmental expression of odorant-binding protein, another carrier protein synthesized in the lateral nasal glands, with that of vomeromodulin demonstrated major differences. In contrast to vomeromodulin, odorant-binding protein was not detected until postnatal day 2 in the ventral component of the lateral nasal glands and anterior glands of the nasal septum. These results suggest that the expression of vomeromodulin and odorant-binding protein is developmentally and differentially regulated and confirms the suggestion that vomeromodulin may function in olfactory and vomeronasal perireceptor processes as a transporter for pheromones and odorants. In addition, the embryonic expression of vomeromodulin suggests its involvement in olfactory perireceptor processes in utero.

Developmental expression of G-proteins and adenylyl cyclase in peripheral olfactory systems. Light microscopic and freeze-substitution electron microscopic immunocytochemistry

Light microscopic immunohistochemistry coupled with freeze-substitution electron microscopic immunocytochemistry was used to localize alpha-subunits of G-proteins and type III adenylyl cyclase in developing rat olfactory epithelia. Some cilia immunoreacted with antibodies to GS alpha and type III adenylyl cyclase as early as prenatal day 15 (E15; E1 = sperm-positive), but immunolabelling with antibodies to Golf alpha was not observed until E16. From then on numbers of receptor cells with immunolabelled cilia increased for all three probes. Immunoreactivity for antibodies to the olfactory signal-transduction proteins tended to parallel cilium development, though Golf alpha lags somewhat behind. Newly formed cilia labelled along their lengths, whereas mature cilia labelled predominantly along their long distal parts. Dendritic knobs and ciliary necklaces showed little or no labelling. While at E22 most multiciliate cells immunolabelled with antibodies to Gs alpha, Golf alpha, and type III adenylyl cyclase, not all of these cells labelled with antibodies to olfactory marker protein. Olfactory axons immunoreacted more intensely than epithelial surface structures with antibodies to Gs alpha at E15; the reverse occurred by about E18. Immunoreactivity with antibodies to alpha-subunits of the G-proteins Go, Gq/G11, and Gi was also found as early as E15. Antibodies to Go alpha labelled receptor cell dendritic knobs and cilia during development only. Antibodies to Gi alpha labelled Bowman's glands, whereas those to Gq alpha/G11 alpha bound to receptor cell cilia and axons (primarily vomeronasal), and supporting cell microvilli. We propose that Gs is the predominant G protein in cilia of immature olfactory receptor cells, while Golf is predominant in cilia of mature cells. Axonal immunoreactivity for some G-protein antibodies suggests G-protein participation in processing of olfactory axon and/or axon terminal-bound signals.

Differential expression of alpha, mu, and pi classes of glutathione S-transferases in chemosensory mucosae of rats during development

The expression of three classes of glutathione S-transferases (GSTs), Alpha, Mu, and Pi was investigated in the nasal mucosae of rats during development using immunohistochemical methods. GST Alpha and Mu were first detected in the supranuclear region of sustentacular cells on embryonic days 16. The Bowman's glands expressed differential patterns of immunoreactivity during development, beginning at postnatal day (P) 2 and P6 for Alpha and Mu classes, respectively and being greatest at P11 for both. The acinar cells of vomeronasal glands in the vomeronasal organ expressed Alpha and Mu classes of GSTs from P11 onwards. In the septal organ of Masera, the supranuclear region of sustentacular cells expressed GSTs from P11 with little or no variation during development. In the respiratory mucosa, Alpha and Mu classes of GSTs were detected at the brush borders of ciliated cells and in the acinar cells of posterior septal glands, but not in anterior septal or respiratory glands located on the turbinates. Compared to olfactory mucosa, the changes in immunoreactivity for GSTs were less pronounced in the respiratory mucosa during development. Specific GST Pi immunoreactivity was not detected in the nasal mucosae at any stage of development studied. The occurrence of GSTs in the nasal mucosa, including olfactory, vomeronasal, septal, and respiratory epithelia, suggests that the GSTs are actively involved in the biotransformation of xenobiotics including odorants and pheromones, and may also participate in perireceptor processes such as odorant clearance. In addition, we have developed a working model describing the cellular localization of certain phase I (e.g., cytochrome P-450s) and phase II (e.g., GSTs, gamma-glutamyl transpeptidase) biotransformation enzymes in the olfactory mucosa and their proposed roles in xenobiotic metabolism.

Expression of olfactory receptor and transduction genes during rat development

The molecular components of olfactory reception and regulation are expressed in a tissue-specific manner. The functional attributes mediated by some of these proteins have been previously shown to display a well-defined developmental emergence during the last week of rat gestation. To gain a better understanding of the relations between chemosensory function and neuronal development, we studied the ontogeny of 7 olfactory-specific genes by quantitative PCR. Relative levels of expression during rat development were determined for each gene, starting at embryonic day 15 (E15) and ending at postnatal day 35 (P35). In addition, the level of expression of the different genes was quantified in juvenile rats. The onset of expression for olfactory receptors and the olfactory cation channel at embryonic day 19 (E19) coincides with the functional maturation of the sensory neurons. Olfactory G-protein and adenylyl cyclase are expressed earlier (approximately E16) while olfactory biotransformation enzymes appear later (E20-E21), just before birth. The sequence of developmental expression of olfactory receptor genes has possible implications to the establishment of neuronal connectivity in this sensory pathway.

Amitriptyline inhibits neurite outgrowth in chick cerebral neurons: a possible mechanism

Previous studies showed that amitriptyline (AMI), a tricyclic antidepressant, inhibited neurite outgrowth from chick embryonic cerebral explants and inhibited adenylyl cyclase activity in cerebral membrane preparations. In the present study, we have investigated the possibility that AMI may have additional effects on cellular metabolism and signal transduction that underlie AMI-mediated inhibition of neurite outgrowth. In vitro, AMI inhibited phospholipase C in a dose- and GTP-dependent manner in membranes from 8-day-old chick forebrain. Brain homogenates from 8-day-old chick embryos, treated in vivo for 6 days with AMI (20 micrograms/g/day), showed significant reductions in (1) phosphorylation of two polypeptides (49 and 105 kD), and (2) levels of three polypeptides (43, 53, and 92 kD). Western blots showed that the 43- and 53-kD polypeptides corresponded to actin and tubulin, respectively. Diolein and dilinolein, potent activators of protein kinase C, stimulated neurite outgrowth and reversed the inhibitory effects of AMI. Sphingosine, a protein kinase C inhibitor, significantly inhibited neurite outgrowth and eliminated the stimulatory effects of diolein and dilinolein on neurite outgrowth. These data suggest that AMI-mediated inhibition of neurite outgrowth involves multiple effects on cellular metabolism and signal transduction. A hypothesis consistent with our data is that AMI interferes in some manner with the action of G proteins in the signal transduction cascade.

Immunocytochemical localization of cell adhesion molecules in the developing and mature olfactory system

The localization of Ca+(+)-independent cell adhesion molecules (CAMs) in the developing and mature olfactory epithelium and bulb is reviewed. The CAMs included in this article are the neural cell adhesion molecule (N-CAM), the 180 kD component of N-CAM (N-CAM 180), the embryonic form of N-CAM (E-N-CAM), L1 glycoproteins, J1 glycoproteins, and the adhesion molecule on glia (AMOG). In addition, the expression of the L2-HNK-1 carbohydrate epitope, shared by N-CAM, L1, J1 and myelin-associated glycoprotein (MAG) in the adult olfactory epithelium and bulb has also been documented. For the localization of these molecules at the light and electron microscopic levels, immunocytochemical techniques were used and are described in detail. During development and organogenesis, the olfactory system exhibits a pattern of CAM expression similar to the general pattern described for the developing nervous system. In the adult olfactory system, however, a significant retention of CAMs characteristic for developmental and morphogenetic processes, such as E-N-CAM, AMOG, as well as the high molecular weight components of J1 glycoproteins, can be observed. The retention of these embryonic features are most likely associated with the cell turnover and high plasticity of this system. Moreover, the predominance of N-CAM 180 with respect to other components of N-CAM, as well as the absence of the L2/HNK-1 carbohydrate epitope, are also particular traits of the primary olfactory system which could be associated with its exceptional properties.

Ultrastructural localization of olfactory transduction components: the G protein subunit Golf alpha and type III adenylyl cyclase

Electron microscopy and postembedding immunocytochemistry on rapidly frozen, freeze-substituted specimens of rat olfactory epithelia were used to study the subcellular localization of the transduction proteins Golf alpha and type III adenylyl cyclase. Antibody binding sites for both of these proteins occur in the same receptor cell compartments, the distal segments of the olfactory cilia. These segments line the boundary between organism and external environment inside the olfactory part of the nasal cavity. Therefore, they are the receptor cell regions that most likely first encounter odorous compounds. The results presented here provide direct evidence to support the conclusion that the distal segments of the cilia contain the sites of the early events of olfactory transduction.

Expression of calmodulin mRNA in rat olfactory neuroepithelium

A calmodulin (CaM) cDNA was isolated by differential hybridization screening of a lambda gt10 library prepared from rat olfactory mucosa. This cDNA fragment, containing most of the open reading frame of the rat CaMI gene, was subcloned and used to characterize steady-state expression of CaM mRNA in rat olfactory neuroepithelium and bulb. Within the bulb mitral cells are the primary neuronal population expressing CaM mRNA. The major CaM mRNA expressed in the olfactory mucosa is 1.7 kb with smaller contributions from mRNAs of 4.0 and 1.4 kb. CaM mRNA was primarily associated with the olfactory neurons and, despite the cellular complexity of the tissue and the known involvement of CaM in diverse cellular processes, was only minimally evident in sustentacular cells, gland cells or respiratory epithelium. Following bulbectomy CaM mRNA declines in the olfactory neuroepithelium as does olfactory marker protein (OMP) mRNA. In contrast to the latter, CaM mRNA makes a partial recovery by one month after surgery. These results, coupled with those from in situ hybridization, indicate that CaM mRNA is expressed in both mature and immature olfactory neurons. The program regulating CaM gene expression in olfactory neurons is distinct from those controlling expression of B50/GAP43 in immature, or OMP in mature, neurons respectively.