Convergence of FPR-rs3-expressing neurons in the mouse accessory olfactory bulb

Clustering of vomeronasal receptor genes is required for transcriptional stability but not for choice

Rodents perceive pheromones via vomeronasal receptors encoded by highly evolutionarily dynamic Vr and Fpr gene superfamilies. We report here that high numbers of V1r pseudogenes are scattered in mammalian genomes, contrasting with the clustered organization of functional V1r and Fpr genes. We also found that V1r pseudogenes are more likely to be expressed when located in a functional V1r gene cluster than when isolated. To explore the potential regulatory role played by the association of functional vomeronasal receptor genes with their clusters, we dissociated the mouse Fpr-rs3 from its native cluster via transgenesis. Singular and specific transgenic Fpr-rs3 transcription was observed in young vomeronasal neurons but was only transient. Our study of natural and artificial dispersed gene duplications uncovers the existence of transcription-stabilizing elements not coupled to vomeronasal gene units but rather associated with vomeronasal gene clusters and thus explains the evolutionary conserved clustered organization of functional vomeronasal genes.

From immune to olfactory expression: neofunctionalization of formyl peptide receptors

Variations in gene expression patterns represent a powerful source of evolutionary innovation. In a rodent living about 70 million years ago, a genomic accident led an immune formyl peptide receptor (FPR) gene to hijack a vomeronasal receptor regulatory sequence. This gene shuffling event forced an immune pathogen sensor to transition into an olfactory chemoreceptor, which thus moved from sensing the internal world to probing the outside world. We here discuss the evolution of the FPR gene family, the events that led to their neofunctionalization in the vomeronasal organ and the functions of immune and vomeronasal FPRs.

Signal Detection and Coding in the Accessory Olfactory System

In many mammalian species, the accessory olfactory system plays a central role in guiding behavioral and physiological responses to social and reproductive interactions. Because of its relatively compact structure and its direct access to amygdalar and hypothalamic nuclei, the accessory olfactory pathway provides an ideal system to study sensory control of complex mammalian behavior. During the last several years, many studies employing molecular, behavioral, and physiological approaches have significantly expanded and enhanced our understanding of this system. The purpose of the current review is to integrate older and newer studies to present an updated and comprehensive picture of vomeronasal signaling and coding with an emphasis on early accessory olfactory system processing stages. These include vomeronasal sensory neurons in the vomeronasal organ, and the circuitry of the accessory olfactory bulb. Because the overwhelming majority of studies on accessory olfactory system function employ rodents, this review is largely focused on this phylogenetic order, and on mice in particular. Taken together, the emerging view from both older literature and more recent studies is that the molecular, cellular, and circuit properties of chemosensory signaling along the accessory olfactory pathway are in many ways unique. Yet, it has also become evident that, like the main olfactory system, the accessory olfactory system also has the capacity for adaptive learning, experience, and state-dependent plasticity. In addition to describing what is currently known about accessory olfactory system function and physiology, we highlight what we believe are important gaps in our knowledge, which thus define exciting directions for future investigation.

G-protein coupled receptors Mc4r and Drd1a can serve as surrogate odorant receptors in mouse olfactory sensory neurons

In the mouse, most mature olfactory sensory neurons (OSNs) express one allele of one gene from the repertoire of ~1100 odorant receptor (OR) genes, which encode G-protein coupled receptors (GPCRs). Axons of OSNs that express a given OR coalesce into homogeneous glomeruli, which reside at conserved positions in the olfactory bulb. ORs are intimately involved in ensuring the expression of one OR per OSN and the coalescence of OSN axons into glomeruli. But the mechanisms whereby ORs accomplish these diverse functions remain poorly understood. An experimental approach that has been informative is to substitute an OR genetically with another GPCR that is normally not expressed in OSNs, in order to determine in which aspects this GPCR can serve as surrogate OR in mouse OSNs. Thus far only the β2-adrenergic receptor (β2AR, Ardb2) has been shown to be able to serve as surrogate OR in OSNs; the β2AR could substitute for the M71 OR in all aspects examined. Can other non-olfactory GPCRs function equally well as surrogate ORs in OSNs? Here, we have generated and characterized two novel gene-targeted mouse strains in which the mouse melanocortin 4 receptor (Mc4r) or the mouse dopamine receptor D1 (Drd1a) is coexpressed with tauGFP in OSNs that express the OR locus M71. These alleles and strains are abbreviated as Mc4r → M71-GFP and Drd1a → M71-GFP. We detected strong Mc4r or Drd1a immunoreactivity in axons and dendritic knobs and cilia of OSNs that express Mc4r or Drd1a from the M71 locus. These OSNs responded physiologically to cognate agonists for Mc4r (Ro27-3225) or Drd1a (SKF81297), and not to the M71 ligand acetophenone. Axons of OSNs expressing Mc4r → M71-GFP coalesced into glomeruli. Axons of OSNs expressing Drd1a → M71-GFP converged onto restricted areas of the olfactory bulb but did not coalesce into glomeruli. Thus, OR functions in OSNs can be substituted by Mc4r or Drd1a, but not as well as by β2AR. We attribute the weak performance of Drd1a as surrogate OR to poor OSN maturation.

Recovery from Cogwheel Rigidity and Akinesia and Improvement in Vibration Sense and Olfactory Perception following Removal of an Epoxy-Oleic Acid DNA Adduct

The epoxy fatty acid cis-12,13-epoxy-oleic acid, which acts as a DNA adduct, may be generated during long-term storage of many seed oils, including those used in cooking, with frying oils and fried foods being a major source in the modern human diet. Removal of this epoxy fatty acid from the locus of the N-formyl peptide receptors was associated with recovery from cogwheel rigidity and akinesia as well as with improvement in vibration sense and olfactory perception.

Evolution of immune chemoreceptors into sensors of the outside world

Changes in gene expression patterns represent an essential source of evolutionary innovation. A striking case of neofunctionalization is the acquisition of neuronal specificity by immune formyl peptide receptors (Fprs). In mammals, Fprs are expressed by immune cells, where they detect pathogenic and inflammatory chemical cues. In rodents, these receptors are also expressed by sensory neurons of the vomeronasal organ, an olfactory structure mediating innate avoidance behaviors. Here we show that two gene shuffling events led to two independent acquisitions of neuronal specificity by Fprs. The first event targeted the promoter of a V1R receptor gene. This was followed some 30 million years later by a second genomic accident targeting the promoter of a V2R gene. Finally, we show that expression of a vomeronasal Fpr can reverse back to the immune system under inflammatory conditions via the production of an intergenic transcript linking neuronal and immune Fpr genes. Thus, three hijackings of regulatory elements are sufficient to explain all aspects of the complex expression patterns acquired by a receptor family that switched from sensing pathogens inside the organism to sensing the outside world through the nose.

Faecal bile acids are natural ligands of the mouse accessory olfactory system

The accessory olfactory system (AOS) guides behaviours that are important for survival and reproduction, but understanding of AOS function is limited by a lack of identified natural ligands. Here we report that mouse faeces are a robust source of AOS chemosignals and identify bile acids as a class of natural AOS ligands. Single-unit electrophysiological recordings from accessory olfactory bulb neurons in ex vivo preparations show that AOS neurons are strongly and selectively activated by peripheral stimulation with mouse faecal extracts. Faecal extracts contain several unconjugated bile acids that cause concentration-dependent neuronal activity in the AOS. Many AOS neurons respond selectively to bile acids that are variably excreted in male and female mouse faeces, and others respond to bile acids absent in mouse faeces. These results identify faeces as a natural source of AOS information, and suggest that bile acids may be mammalian pheromones and kairomones.

The accessory olfactory system (AOS) processes social chemosensory information and guides behaviors that are important for survival and reproduction in mammals. Here the authors report that mouse feces are a source of AOS neuronal activity and identify unconjugated bile acids in feces as a class of natural AOS ligands.

Chemosensory receptor specificity and regulation

The senses provide a means by which data on the physical and chemical properties of the environment may be collected and meaningfully interpreted. Sensation begins at the periphery, where a multitude of different sensory cell types are activated by environmental stimuli as different as photons and odorant molecules. Stimulus sensitivity is due to expression of different cell surface sensory receptors, and therefore the receptive field of each sense is defined by the aggregate of expressed receptors in each sensory tissue. Here, we review current understanding on patterns of expression and modes of regulation of sensory receptors.

Physiological characterization of formyl peptide receptor expressing cells in the mouse vomeronasal organ

The mouse vomeronasal organ (VNO) is a chemosensory structure that detects both hetero- and conspecific social cues. Based on largely monogenic expression of either type 1 or 2 vomeronasal receptors (V1Rs/V2Rs) or members of the formyl peptide receptor (FPR) family, the vomeronasal sensory epithelium harbors at least three neuronal subpopulations. While various neurophysiological properties of both V1R- and V2R-expressing neurons have been described using genetically engineered mouse models, the basic biophysical characteristics of the more recently identified FPR-expressing vomeronasal neurons have not been studied. Here, we employ a transgenic mouse strain that coexpresses an enhanced variant of yellow fluorescent protein together with FPR-rs3 allowing to identify and analyze FPR-rs3-expressing neurons in acute VNO tissue slices. Single neuron electrophysiological recordings allow comparative characterization of the biophysical properties inherent to a prototypical member of the FPR-expressing subpopulation of VNO neurons. In this study, we provide an in-depth analysis of both passive and active membrane properties, including detailed characterization of several types of voltage-activated conductances and action potential discharge patterns, in fluorescently labeled vs. unmarked vomeronasal neurons. Our results reveal striking similarities in the basic (electro) physiological architecture of both transgene-expressing and non-expressing neurons, confirming the suitability of this genetically engineered mouse model for future studies addressing more specialized issues in vomeronasal FPR neurobiology.

Anatomy, histochemistry, and immunohistochemistry of the olfactory subsystems in mice

The four regions of the murine nasal cavity featuring olfactory neurons were studied anatomically and by labeling with lectins and relevant antibodies with a view to establishing criteria for the identification of olfactory subsystems that are readily applicable to other mammals. In the main olfactory epithelium and the septal organ the olfactory sensory neurons (OSNs) are embedded in quasi-stratified columnar epithelium; vomeronasal OSNs are embedded in epithelium lining the medial interior wall of the vomeronasal duct and do not make contact with the mucosa of the main nasal cavity; and in Grüneberg's ganglion a small isolated population of OSNs lies adjacent to, but not within, the epithelium. With the exception of Grüneberg's ganglion, all the tissues expressing olfactory marker protein (OMP) (the above four nasal territories, the vomeronasal and main olfactory nerves, and the main and accessory olfactory bulbs) are also labeled by Lycopersicum esculentum agglutinin, while Ulex europaeus agglutinin I labels all and only tissues expressing G_αi2 (the apical sensory neurons of the vomeronasal organ, their axons, and their glomerular destinations in the anterior accessory olfactory bulb). These staining patterns of UEA-I and LEA may facilitate the characterization of olfactory anatomy in other species. A 710-section atlas of the anatomy of the murine nasal cavity has been made available on line.