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Zonal Organization of the Mouse Olfactory Systems

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Zonal Organization of the Mouse Olfactory Systems UMEÅ UNIVERSITY MEDICAL DISSERTATIONS NEW SERIES NO 911 ISSN 0346-6612 ISBN 91-7305-706-1 ZONAL ORGANIZATION OF THE MOUSE OLFACTORY SYSTEMS Fredrik Gussing Department of Molecular Biology Umeå University Umeå 2004 1 Cover picture: A coronal section of the mouse nasal cavity with zone-specific expression of the NQO1 gene (white signal) in the olfactory epithelium is shown. Copyright © 2004 by Fredrik Gussing ISBN 91-7305-706-1 Printed in Sweden by Solfjädern Offset AB, Umeå 2004 2 HOW DO I SMELL? 3 TABEL OF CONTENTS ABSTRACT 6 PAPERS IN THIS THESIS 7 ABBREVIATIONS 8 INTRODUCTION 9 The main olfactory system 10 Anatomy 10 The main olfactory epithelium 10 Regenerative capacity 11 The main olfactory bulb 12 The odorant receptors 13 Prereceptor events 13 Identification of the odorant receptors 13 Characteristics of odorant receptors 14 Spatial odorant receptor expression patterns 14 Signal transduction in olfactory sensory neurons 15 Downstream of the G-protein coupled receptor 15 Genomic characterization of odorant receptors 17 Functions of the odorant receptors 17 Odorant binding 17 Odorant receptor expression 18 The glomerular maps 19 Axonal convergence and neuronal specificity 19 Zonal organization of olfactory sensory neuronal projections 20 Medial and lateral maps 21 Neuropilins, ephs and ephrins 21 Olfactory bulb projections 21 The septal organ 22 The accessory olfactory system 23 Anatomy 23 The vomeronasal organ 23 Axonal projections 24 The accessory olfactory bulb 25 The vomeronasal receptors 25 Gene regulation of vomeronasal receptors 26 Receptor signaling in the vomeronasal neurons 27 Vomeronasal receptor - ligand interaction 28 Vomeronasal mediated behaviors 29 Aggression 29 Reproduction related behaviors 30 Genetic modifications in the VSNs 30 4 AIMS 32 RESULTS AND DISCUSSION 33 Overview 33 Patterns of gene expression in the primary olfactory neurons: correlation to receptor expression zones (Paper I and II) 33 Genes potentially involved in cell specification in the olfactory epithelium and in axonal guidance 33 Formation of odorant receptor zones by expression gradients 35 Identification of a new zonally restricted gene 35 Differences between the dorsomedial and ventrolateral zones in the olfactory epithelium 37 The zonal dichotomy of the vomeronasal epithelium (Paper III and IV) 37 Gαi2 protein in survival and function of apical vomeronasal neurons 38 Involvement of retinoic acid in the maintenance of basal vomeronasal neurons 39 Two mouse models to study zonal vomeronasal influences on behavior 41 Proposed function of the apical and basal zones 41 CONCLUDING REMARKS 44 ACKNOWLEDGEMENTS 45 REFERENCES 46 5 ABSTRACT Animals survey their environment for relevant odorous chemical compounds by means of the olfactory system. This system is in most vertebrates divided into a main and accessory olfactory system with two specialized neuroepithelia, the olfactory and the vomeronasal epithelium, respectively. The sensory neurons reside in these epithelia and together the neurons have an extraordinary sensitivity and are capable of detecting a vast number of different chemical molecules. After processing the chemical information, behavior may be altered. The information about a chemicals structure is deconstructed into a format that the brain may process. This is facilitated by organizing sensory neurons into a map and that the individual neuron responds only to one chemical feature. The sensory maps appear to have zones with different neuronal subpopulations. This thesis is addressing the fact that establishment, maintenance and function of these zones are unknown. We identify a gene (NQO1) to be selectively expressed in defined zone of the olfactory and the vomeronasal epithelia, respectively. NQO1-positive and negative axons segregate within the olfactory nerve and maintain a zonal organization when reaching olfactory bulb target neurons. These results indicate that one zone of both the accessory and the main olfactory projection maps is composed of sensory neurons specialized in reducing environmental and/or endogenously produced quinones via an NQO1-dependent mechanism. In addition, we have identified genes expressed in a graded manner that correlates with the dorsomedial-ventrolateral zonal organization of the olfactory epithelia. Considering the known functions of identified genes in establishment of cell specificity and precise axonal targeting, we suggest that zonal division of the primary olfactory systems is maintained, during continuous neurogenesis, as a consequence of topographic counter gradients of positional information. The vomeronasal sensory neurons (VSN) are organized into an apical and a basal zone. The zones differ in expression of e.g. chemosensory receptor families and Gα protein subunits (Gαi2 and Gαo). We have analyzed transgenic mice (OMP- dnRAR) in which the VSNs are unresponsive to the function of one of the genes identified herein (RALDH2). The phenotype observed suggests that endogenous produced retinoic acid is selectively required for postnatal survival of neurons in the Gαo-positive zone. Analyses of another mouse line target deleted in the Gαi2 gene (Gαi2 mutant) reveal a cellular phenotype that is opposite to that of OMP- dnRAR mice. Consequently in these mice, the apical Gαi2-positive zone is reduced whereas VSNs in the basal zone are not affected. Several social and reproductive behaviors are under the influence of the vomeronasal organ. We have analyzed some behavioral consequences of having deficient neurons that corresponds to either of the two zones. We propose that cues important for aggressive behavior are detected by apical vomeronasal zone, while cues detected by both apical and basal VSNs influence gender preference behavior. 6 PAPERS IN THIS THESIS This thesis is based upon the following articles and manuscript, which will be referred to in the text by their Roman numerals (I-IV). I Evidence for gradients of gene expression correlating with zonal topography of the olfactory sensory map. Norlin EM, Alenius M, Gussing F, Hägglund M, Vedin V, Bohm S. Mol Cell Neurosci. Sep;18(3):283-95. (2001) II NQO1 activity in the main and the accessory olfactory systems correlates with the zonal topography of projection maps. Gussing F, and Bohm S. Eur J Neurosci. 2004 May;19(9):2511-8. (2004) III Vomeronasal phenotype and behavioral alterations in Gαi2 mutant mice. Norlin EM, Gussing F, Berghard A. Curr Biol. Jul 15;13(14):1214-9. (2003) IV Inhibition of retinoid signaling in mature vomeronasal sensory neurons: postnatal degradation of a population of neurons and behavioral characterization. Gussing F, Hägglund M, Berghard A, Bohm S. Manuscript. (2004) Articles reprinted with permission from the publisher. 7 ABBREVIATIONS AC adenylyl cyclase AOB accessory olfactory bulb cAMP cyclic adenosine 3’,5’-monophosphate CNG cyclic nucleotide gated CNS central nervous system DAG diacylglycerol dnRAR dominant negative retinoic acid receptor IP3 inositol 1,4,5-triphosphate IRES internal ribosomal entry site MHC major histocompatibility complex MUP major urinary protein NADPH reduced nicotinamide adenine dinucleotide phosphate NQO1 NADPH:quinone oxidoreductase 1 O-MACS olfactory specific medium-chain acyl-CoA synthetase OB olfactory bulb OBP odorant binding protein OE olfactory epithelium OEC olfactory ensheathing cell OMP olfactory marker protein OR odorant receptor OSN olfactory sensory neuron P postnatal day PCR polymerase chain reaction PLC phospholipase C RA retinoic acid RALDH2 retinaldehyde dehyrogenase 2 RAR retinoic acid receptor RGS regulator of G-protein signaling RNCAM Rb8 neural cell adhesion molecule TRP2 transient receptor potential channel 2 VN vomeronasal VNX surgical removal of the vomeronsal organ VR vomeronasal receptor VSN vomeronasal sensory neuron Z1-4 odorant receptor expression zones 1-4 8 INTRODUCTION My Fifth Wonder is the olfactory receptor cell, located in the epithelial tissue high in the nose, sniffing the air for clues to the environment, the fragrance of friends, the smell of leaf smoke, breakfast, nighttime and bedtime, and a rose, even, it is said, the odor of sanctity.… If and when we reach an understanding of these cells and their functions, including the moods and whims under their governance, we will know a lot more about the mind than we do now, a world away. - Lewis Thomas (1983) In this thesis I give an introduction to the olfactory system, and the research progress in this area during the last decades. This covers both the main and the accessory olfactory system with focus on one of the model organisms, the mouse. The aims and questions that initiated the experimental part of this thesis work are presented before the results are summarized and discussed in relation to other findings in the field. Figure 1. Localization of the olfactory systems in mouse. A schematic representation of the mouse nasal cavity and brain. Three primary olfactory systems are illustrated, the accessory, the septal and the main olfactory system. Airflow (white arrows) passes the VN organ in the anterior part of the nose before reaching the SO and the OE. The nasal cavity and surface with turbinates that enlarge the receptive area, is lined with olfactory neuroepithelium. Axons of olfactory and VN sensory neurons project to their corresponding regions in the OB and AOB, respectively. 9 The main olfactory system Anatomy The main olfactory system detects airborne chemical molecules in our environment. When the air is inhaled through the nose of an animal it passes the nasal cavity. The
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