Topographic Mapping��The Olfactory System

Sensory systems must map accurate representations of the external world in the brain. Although the physical senses of touch and vision build topographic representations of the spatial coordinates of the body and the field of view, the chemical sense of olfaction maps discontinuous features of chemical space, comprising an extremely large number of possible odor stimuli. In both mammals and insects, olfactory circuits are wired according to the convergence of axons from sensory neurons expressing the same odorant receptor. Synapses are organized into distinctive spherical neuropils—the olfactory glomeruli—that connect sensory input with output neurons and local modulatory interneurons. Although there is a strong conservation of form in the olfactory maps of mammals and insects, they arise using divergent mechanisms. Olfactory glomeruli provide a unique solution to the problem of mapping discontinuous chemical space onto the brain.The olfactory system detects airborne organic and inorganic chemicals that originate from plant and animal metabolites and environmental and industrial sources. Odors mediate both innate and learned behaviors such as attraction and aversion, governing decisions to eat, mate, attack or flee from aggressors and predators. A remarkable feature of the olfactory system is the extraordinary diversity of possible odor molecules that exist. Although accurate measurements can be made of the detection range of visual wavelengths or auditory frequencies in humans, there are no reliable estimates of the number of odorous molecules that exist on earth or those that can be detected by a given animal species (Gilbert 2008). Nevertheless, the number of possible odorants is thought to be high, in the range of many thousands. Beyond odor detection lies the problem of odor discrimination: how can chemicals with slightly different physical properties be discriminated and associated with distinct odor percepts?Experiments investigating the molecular logic of olfaction over the last 20 years have led to major insights into how animals solve the problem of detecting and discriminating a vast number of different odor stimuli (Axel 1995). Four organizational principles have emerged from this work. First, large numbers of distinct odorant receptor (OR) genes are dedicated to olfaction: ∼1000 in mammals and ∼100 in insects. Second, each olfactory sensory neuron (OSN) typically expresses only a single receptor out of this large repertoire. Each OR has a distinct odor ligand profile, such that there are ∼1000 types of OSNs in the mouse and ∼100 types in the typical insect. Third, OSNs expressing the same receptor extend axons that converge on the same glomerulus in the brain, forming a map of ORs in the brain. Fourth, each odor is encoded in a combinatorial manner: One odor can activate multiple ORs, and each OR can respond to multiple odors (Malnic et al. 1999; Hallem and Carlson 2006). As a result, odor information is spatially encoded in the first relay station of the brain (reviewed in Mori et al. 2006). Strikingly, these four principles along with the anatomical organization of the olfactory system are quite similar in mammals and insects, suggesting a mechanism of convergent evolution (Hildebrand and Shepherd 1997).Recent studies in two genetic model organisms, the mouse and the fly, Drosophila melanogaster, have elucidated mechanisms governing how the topographic yet discrete olfactory map is established in the brain (see Luo and Flanagan 2007 for an excellent recent review). Although mice and flies have multiple olfactory subsystems and additional classes of chemosensory receptors (reviewed in Brennan and Zufall 2006; Touhara and Vosshall 2009), this article will focus on comparative mechanisms used to map ORs and OSNs from the main olfactory epithelium in the mouse and adult chemosensory structures in the fly to glomerular targets in the brain.     

Are Olfactory Receptors Really Olfactive?

Any living organism interacts with and responds specifically to environmental molecules by expressing specific olfactory receptors. In this paper, this specificity will be first examined in causal terms with particular emphasis on the mechanisms controlling olfactory gene expression, cell-to-cell interactions and odor-decoding processes. However, this type of explanation does not entirely justify the role olfactory receptors have played during evolution, since they are also expressed ectopically in different organs and/or tissues. Homologous olfactory genes have in fact been found in such diverse cells and/or organs as spermatozoa, testis and kidney where they are assumed to act as chemotactic sensors or renin modulators. To justify their functional diversity, homologous olfactory receptors are assumed to share the same basic role: that of conferring a self-identity to cells or tissues under varying environmental conditions. By adopting this standpoint, the functional attribution as olfactory or chemotactic sensors to these receptors should not be seen either as a cause conditioning receptor gene expression, or as a final effect resulting from genetically predetermined programs, but as a direct consequence of the environmental conditions olfactory receptor genes have explored during evolution. The association of odorant patterns with specific environmental or contextual situations makes their relationship semiotically triadic, due to the emergence of an interpretant capable of perceiving odorants as meaningful signs out of a noisy background. This perspective highlights the importance of odorant-receptor relationships as respect to the properties of the interacting partners. It is our contention that only when taken together can these different explanatory strategies provide a realistic account of how olfactory receptor genes have been structurally and functionally modified during evolution.     

Olfactory discrimination: when vision matters?

Many previous studies have attempted to investigate the effect of visual cues on olfactory perception in humans. The majority of this research has only looked at the modulatory effect of color, which has typically been explained in terms of multisensory perceptual interactions. However, such crossmodal effects may equally well relate to interactions taking place at a higher level of information processing as well. In fact, it is well-known that semantic knowledge can have a substantial effect on people's olfactory perception. In the present study, we therefore investigated the influence of visual cues, consisting of color patches and/or shapes, on people's olfactory discrimination performance. Participants had to make speeded odor discrimination responses (lemon vs. strawberry) while viewing a red or yellow color patch, an outline drawing of a strawberry or lemon, or a combination of these color and shape cues. Even though participants were instructed to ignore the visual stimuli, our results demonstrate that the accuracy of their odor discrimination responses was influenced by visual distractors. This result shows that both color and shape information are taken into account during speeded olfactory discrimination, even when such information is completely task irrelevant, hinting at the automaticity of such higher level visual-olfactory crossmodal interactions.     

In Utero and Lactational Lanthanum Exposure Induces Olfactory Dysfunction Associated with Downregulation of βIII-tubulin and Olfactory Marker Protein in Young Rats

We evaluated the role of βIII-tubulin in the morphology of olfactory receptor neuron (ORN) and olfactory dysfunction in offspring caused by prenatal and postnatal lanthanum exposure. Pregnant rats were exposed to 0.25% lanthanum chloride in drinking water from gestational day (GD) 7 until postnatal day 21. From postnatal day 23 until postnatal day 28, pups were examined with buried food pellet and olfactory maze test. The ultrastructural features of ORNs in the olfactory epithelium (OE) were observed by transmission electron microscope. The expression of βIII-tubulin and olfactory marker protein (OMP) in the tissue sections and homogenates of OE were, respectively, measured by immunodetection and western blot. Behavioral analysis of olfaction showed that lanthanum chloride exposure induced olfactory dysfunction. Offsprings exposed to lanthanum chloride showed enlarged ORN knobs and a decreased number of cilia. In addition, the levels of OMP and βIII-tubulin expression in lanthanum chloride exposure offsprings significantly decreased. Developmental lanthanum exposure could impair olfaction, and this deficit may be attributed to the downregulation of βIII-tubulin and OMP in the OE.     

Are Pheromones Detected Through the Main Olfactory Epithelium?

A major sensory organ for the detection of pheromones by animals is the vomeronasal organ (VNO). Although pheromones control the behaviors of various species, the effect of pheromones on human behavior has been controversial because the VNO is not functional in adults. However, recent genetic, biochemical, and electrophysiological data suggest that some pheromone-based behaviors, including male sexual behavior in mice, are mediated through the main olfactory epithelium (MOE) and are coupled to the type 3 adenylyl cyclase (AC3) and a cyclic nucleotide-gated (CNG) ion channel. These recent discoveries suggest the provocative hypothesis that human pheromones may signal through the MOE.     

Olfactory axon pathfinding: who is the pied piper?

Mammalian olfactory sensory neurons that express a particular odorant receptor (OR) project axons to the same few glomeruli in the olfactory bulb. In this issue of Neuron, Vassalli et al. use OR minigenes that coexpress histochemical markers and show that the determinants in the sensory neurons required to generate the stereotyped olfactory bulb map are the same as those needed for appropriate expression of the OR.     

Olfactory ensheathing glia and Schwann cells: two of a kind?

Prostatic carcinoma affects 1 in 11 men and targets bone with sclerotic metastases. The study of prostate carcinoma growth in bone has been hampered by the lack of suitable animal models. We have developed an in vivo model of prostate carcinoma growth in bone by inoculating three human prostate carcinoma cell lines (PC-3, DU-145, and LNCaP) into the tibia of congenitally athymic mice. Developing tumors were analyzed by radiographic, histologic, immunohistochemical, and in situ hybridization examination. Seven of the nine PC-3 inoculated mice and all (9/9) of the DU-145 inoculated mice developed tumors in the injected limb. In contrast, inoculation with LNCaP cells failed to produce tumors (0/9). Radiologically, the tumors had a mixed sclerotic/lytic appearance with extracortical extension. All the PC-3 tumors invaded the bone marrow cavity, cortical bone, and surrounding soft tissue. The DU-145 tumors were confined to the bone marrow cavity in 7/9 animals. CK18 and Ki67 localization identified the human tumor cells and their proliferative activity, respectively. The PC-3- and DU-145-induced tibial tumors expressed alpha(1)I procollagen and osteopontin mRNA, to varying degrees. All the tumors demonstrated an up-regulation of osteoclasts at the bone/tumor interface compared with the control limbs. Thus, this is a reliable and reproducible in vivo model of prostate carcinoma growth in bone enabling the study of the interactions that occur between prostate cancer cells and bone at an important part of the metastatic cascade, namely, growth and invasion at a distant site.     

Olfactory Deficits in an Alpha-Synuclein Fly Model of Parkinson’s Disease

Parkinson’s disease (PD) is the most common motor neurodegenerative disorder. Olfactory dysfunction is a prevalent feature of PD. It often precedes motor symptoms by several years and is used in assisting PD diagnosis. However, the cellular and molecular bases of olfactory dysfunction in PD are not known. The fruit fly Drosophila melanogaster, expressing human alpha-synuclein protein or its mutant, A30P, captures several hallmarks of PD and has been successfully used to model PD in numerous studies. First, we report olfactory deficits in fly expressing A30P (A30P), showing deficits in two out of three olfactory modalities, tested – olfactory acuity and odor discrimination. The remaining third modality is odor identification/naming. Second, oxidative stress is an important environmental risk factor of PD. We show that oxidative stress exacerbated the two affected olfactory modalities in younger A30P flies. Third, different olfactory receptor neurons are activated differentially by different odors in flies. In a separate experiment, we show that the odor discrimination deficit in A30P flies is general and not restricted to a specific class of chemical structure. Lastly, by restricting A30P expression to dopamine, serotonin or olfactory receptor neurons, we show that A30P expression in dopamine neurons is necessary for development of both acuity and discrimination deficits, while serotonin and olfactory receptor neurons appeared not involved. Our data demonstrate olfactory deficits in a synuclein fly PD model for exploring olfactory pathology and physiology, and for monitoring PD progression and treatment.     

Olfactory ensheathing cells - another miracle cure for spinal cord injury?

Several recent publications describe remarkably promising effects of transplanting olfactory ensheathing cells as a potential future method to repair human spinal cord injuries. But why were cells from the nose transplanted into the spinal cord? What are olfactory ensheathing cells, and how might they produce these beneficial effects? And more generally, what do we mean by spinal cord injury? To what extent can we compare repair in an animal to repair in a human?     

Olfactory epithelium progenitors: insights from transgenic mice and in vitro biology

The rodent olfactory epithelium (OE) is capable of prolonged neurogenesis, beginning at E10 in the embryo and continuing throughout adulthood. Significant progress has been made over the last 10 years in revealing the signals that drive induction, differentiation and survival of its Olfactory Receptor Neurons (ORNs). Our understanding of the identity of specific progenitors or precursors that respond to these signals is, however, less well developed, and the search is still on for the elusive, definitive multipotent neuro-glial OE "Stem cell". Here, we review several lines of evidence that support the existence of a heterogeneous population of neural and glial progenitors in the olfactory mucosa, and highlight the differences in the identity and activity of progenitors found in the embryonic and adult OE. In particular, we show how recent advances in mouse transgenesis, and in the development of in vitro assays of progenitor activity, have helped to demonstrate the existence of multiple classes of olfactory mucosa-based progenitors.     

Olfactory Ensheathing Cells Express α7 Integrin to Mediate Their Migration on Laminin

The unique glia located in the olfactory system, called olfactory ensheathing cells (OECs), are implicated as an attractive choice for transplantation therapy following spinal cord injury because of their pro-regenerative characteristics. Adult OECs are thought to improve functional recovery and regeneration after injury by secreting neurotrophic factors and making cell-to-cell contacts with regenerating processes, but the mechanisms are not well understood. We show first that α7 integrin, a laminin receptor, is highly expressed at the protein level by OECs throughout the olfactory system, i.e., in the olfactory mucosa, olfactory nerve, and olfactory nerve layer of the olfactory bulb. Then we asked if OECs use the α7 integrin receptor directly to promote neurite outgrowth on permissive and neutral substrates, in vitro. We co-cultured α7^+/+ and α7^lacZ/lacZ postnatal cerebral cortical neurons with α7^+/+ or α7^lacZ/lacZ OECs and found that genotype did not effect the ability of OECs to enhance neurite outgrowth by direct contact. Loss of α7 integrin did however significantly decrease the motility of adult OECs in transwell experiments. Twice as many α7^+/+ OECs migrated through laminin-coated transwells compared to α7^+/+ OECs on poly-L-lysine (PLL). This is in contrast to α7^lacZ/lacZ OECs, which showed no migratory preference for laminin substrate over PLL. These results demonstrate that OECs express α7 integrin, and that laminin and its α7 integrin receptor contribute to adult OEC migration in vitro and perhaps also in vivo.     

Olfactory learning and behaviour are ‘insulated’ against visual processing in larval Drosophila

We investigate the organization of behaviour across sensory modalities, using larval Drosophila melanogaster. We ask whether olfactory learning and behaviour are affected by visual processing. We find that: (1) Visual choice does not affect concomitant odour choice. (2) Visual context does not influence odour learning, nor do changes of visual context between training and test affect retrieval of odour memory. (3) Larvae cannot solve a biconditional discrimination task, despite generally permissive conditions. In this task, larvae are required to establish conditional associations: in light, one odour is rewarded and the other one is not, whereas in dark the opposite contingency is established. After such training, choice between the two odours is equal under light and dark testing conditions, suggesting that larvae do not establish odour memories specifically for one visual context only. Together, these data suggest that, in larval Drosophila, olfactory learning and behaviour are ‘insulated’ against visual processing.     

Ca2+ Extrusion by NCX Is Compromised in Olfactory Sensory Neurons of OMP?/? Mice

Background

The role of olfactory marker protein (OMP), a hallmark of mature olfactory sensory neurons (OSNs), has been poorly understood since its discovery. The electrophysiological and behavioral phenotypes of OMP knockout mice indicated that OMP influences olfactory signal transduction. However, the mechanism by which this occurs remained unknown.

Principal Findings

We used intact olfactory epithelium obtained from WT and OMP^−/− mice to monitor the Ca²⁺ dynamics induced by the activation of cyclic nucleotide-gated channels, voltage-operated Ca²⁺ channels, or Ca²⁺ stores in single dendritic knobs of OSNs. Our data suggested that OMP could act to modulate the Ca²⁺-homeostasis in these neurons by influencing the activity of the plasma membrane Na⁺/Ca²⁺-exchanger (NCX). Immunohistochemistry verifies colocalization of NCX1 and OMP in the cilia and knobs of OSNs. To test the role of NCX activity, we compared the kinetics of Ca²⁺ elevation by stimulating the reverse mode of NCX in both WT and OMP^−/− mice. The resulting Ca²⁺ responses indicate that OMP facilitates NCX activity and allows rapid Ca²⁺ extrusion from OSN knobs. To address the mechanism by which OMP influences NCX activity in OSNs we studied protein-peptide interactions in real-time using surface plasmon resonance technology. We demonstrate the direct interaction of the XIP regulatory-peptide of NCX with calmodulin (CaM).

Conclusions

Since CaM also binds to the Bex protein, an interacting protein partner of OMP, these observations strongly suggest that OMP can influence CaM efficacy and thus alters NCX activity by a series of protein-protein interactions.     

Evolution of the “OR37” Subfamily of Olfactory Receptors: A Cross-Species Comparison

Genes encoding the olfactory receptors of the “OR37” subfamily of the mouse are characterized by special features including a clustered expression pattern, assembly in two distinct gene clusters, and highly conserved putative promoter motifs. Mining the rat and dog databases revealed that these two species possess highly conserved clusters of OR37 genes at two syntenic genomic loci. In a prototherian mammal, the platypus (Ornithorhynchus anatinus), none of the characteristic OR37 genes were found. Examination of a metatherian mammal, the gray short-tailed opossum (Monodelphis domestica) revealed seven canonical OR37 genes, all phylogenetically related to cluster II genes and also organized similar to cluster II of eutherian species. In addition, their 5′ upstream regions comprised sequence motifs related to the putative regulatory sequences of cluster II genes. Typical cluster I OR37 genes were identified only in the eutherian mammals examined, including the evolutionary ancient anteater, wherein OR37 genes related to both clusters were present. Together, these results reveal novel information concerning the phylogenetic origin and important evolutionary steps of the mammalian-specific OR37 olfactory receptor family. [Reviewing Editor: Dr. Lauren Ancel Meyers] Reiner Hoppe and Thomas D. Lambert are contributed equally to this work.     

Brain Activity at 70–80 Hz Changes during Olfactory Stimulation Protocols in Drosophila

Oscillatory and synchronized activities in the mammalian brain have been correlated with the execution of complex cognitive tasks. Similar oscillations have been observed in local field potentials (LFPs) in flies, in this case correlated with different attentional states. To further test the significance of these oscillations we recorded LFPs from the brain of Drosophila melanogaster as it responded to the presentation of olfactory stimuli. We find that responses in the 70–80 Hz range increase during olfactory stimulation. Recurrent stimulation specifically decreased the power of LFPs in this frequency range. Delivery of electric shocks before olfactory stimulation modulated LFPs in the 70–80 Hz range by evoking a transient increase. These results suggest that these signals are a simple neuronal correlate of higher-order olfactory processing in flies.     

Chronic TNFα Exposure Induces Robust Proliferation of Olfactory Ensheathing Cells,but not Schwann Cells

TNFα is persistently elevated in many injury and disease conditions. Previous reports of cytotoxicity of TNFα for oligodendrocytes and their progenitors suggest that the poor endogenous remyelination in patients with traumatic injury or multiple sclerosis may be due in part to persistent inflammation. Understanding the effects of inflammatory cytokines on potential cell therapy candidates is therefore important for evaluating the feasibility of their use. In this study, we assessed the effects of long term exposure to TNFα on viability, proliferation, migration and TNFα receptor expression of cultured rat olfactory ensheathing cells (OECs) and Schwann cells (SCs). Although OECs and SCs transplanted into the CNS produce similar myelinating phenotypes, and might be expected to have similar therapeutic uses, we report that they have very different sensitivities to TNFα. OECs exhibited positive proliferative responses to TNFα over a much broader range of concentrations than SCs. Low TNFα concentrations increased proliferation and migration of both OECs and SCs, but SC number declined in the presence of 100 ng/ml or higher concentrations of TNFα. In contrast, OECs exhibited enhanced proliferation even at high TNFα concentrations (up to 1 µg/ml) and showed no evidence of TNF cytotoxicity even at 4 weeks post-treatment. Furthermore, while both OECs and SCs expressed TNFαR1 and TNFαR2, TNFα receptor levels were downregulated in OECs after exposure to100 ng/ml TNFα for 5–7 days, but were either elevated or unchanged in SCs. These results imply that OECs may be a more suitable cell therapy candidate if transplanted into areas with persistent inflammation.     

Changes in Olfactory Bulb Volume in Parkinson’s Disease: A Systematic Review and Meta-Analysis

Objective

The changes in olfactory bulb (OB) volume in Parkinson’s disease (PD) patients have not yet been comprehensively evaluated. The purpose of this meta-analysis was to explore whether the OB volume was significantly different between PD patients and healthy controls.

Methods

PubMed and Embase were searched up to March 6, 2015 with no language restrictions. Two independent reviewers screened eligible studies and extracted data on study characteristics and OB volume. Additionally, a systematic review and meta-analysis using a random-effects model were conducted. Publication bias was determined by using funnel plots and Begg’s and Egger’s tests. Subgroup analyses were performed to assess possible sources of heterogeneity.

Results

Six original case-control studies of 216 PD patients and 175 healthy controls were analyzed. The pooled weighted mean difference (WMD) in the OB volume between the PD patients and the healthy participants was -8.071 for the right OB and -10.124 for the left OB; these values indicated a significant difference among PD patients compared with healthy controls. In addition, a significant difference in the lateralized OB volume was observed in PD patients, with a pooled WMD of 1.618; these results indicated a larger right OB volume than left OB volume in PD patients. In contrast, no difference in the lateralized OB volume was found in healthy controls. No statistical evidence of publication bias among studies was found based on Egger’s or Begg’s tests. Sensitivity analyses revealed that the results were consistent and robust.

Conclusions

Overall, both the left and the right OB volume were significantly smaller in PD patients than in healthy controls. However, significant heterogeneity and an insufficient number of studies underscore the need for further observational research.