Combinatorial coexpression of neural and immune multigene families in mouse vomeronasal sensory neurons

Neuroimmune pharmacology as a component of pharmacology in medical school curriculum

An introduction to the discipline of pharmacology is a standard part of the scientific foundation of medical school curricula. Neuroimmune pharmacology is a new subtopic that integrates fundamental concepts of neuroscience, immunology, infectious disease, and pharmacology. The integration of these areas is important to medical training in view of the growing concern over neurodegenerative diseases and cognitive disorders. This article introduces a submodule and concomitant syllabus for inclusion of neuroimmune pharmacology as a component of a pharmacology curriculum. The introductory lectures of neuroimmune pharmacology will concentrate on the role of the immune system in (1) schizophrenia and major depression; (2) neurodegenerative disorders; and (3) drug addiction. Emphasis will be placed on the competencies of critical thinking, problem solving, learning interest, and effectiveness of medical students. Problem-based learning and case study discussions will also be applied.

High-throughput microarray detection of vomeronasal receptor gene expression in rodents

We performed comprehensive data mining to explore the vomeronasal receptor (V1R and V2R) repertoires in mouse and rat using the mm5 and rn3 genome, respectively. This bioinformatic analysis was followed by investigation of gene expression using a custom designed high-density oligonucleotide array containing all of these receptors and other selected genes of interest. This array enabled us to detect the specific expression of V1R and V2Rs which were previously identified solely based on computational prediction from gene sequence data, thereby establishing that these genes are indeed part of the vomeronasal system, especially the V2Rs. One hundred sixty-eight V1Rs and 98 V2Rs were detected to be highly enriched in mouse vomeronasal organ (VNO), and 108 V1Rs and 87 V2Rs in rat VNO. We monitored the expression profile of mouse VR genes in other non-VNO tissues with the result that some VR genes were re-designated as VR-like genes based on their non-olfactory expression pattern. Temporal expression profiles for mouse VR genes were characterized and their patterns were classified, revealing the developmental dynamics of these so-called pheromone receptors. We found numerous patterns of temporal expression which indicate possible behavior-related functions. The uneven composition of VR genes in certain patterns suggests a functional differentiation between the two types of VR genes. We found the coherence between VR genes and transcription factors in terms of their temporal expression patterns. In situ hybridization experiments were performed to evaluate the cell number change over time for selected receptor genes.

Characterization of signaling function and expression of HLA class I molecules in medulloblastoma

Although known for the important function in the immune system, MHC class I molecules are increasingly ascribed an alternative role in modifying signal transduction. In medulloblastoma, HLA class I molecules are associated with poor prognosis, and can induce ERK1/2 activation upon engagement with ligands that bind to incompletely assembled complexes (so called open conformers). We here demonstrate that ERK1/2 activation in medulloblastoma can occur in the absence of endogenously synthesized β2m, formally excluding involvement of closed HLA class conformation. In addition, several experimental observations suggest that heterogeneity of HLA class I expression may be a reflection of the status of original cells before transformation, rather than a consequence of immune-based selection of HLA-loss mutants. These results contribute to our understanding of an immune system-independent role of HLA class I in the pathology of medulloblastoma, and cancer in general.

Uncx regulates proliferation of neural progenitor cells and neuronal survival in the olfactory epithelium

Uncx (Phd1, Chx4) is a paired homeobox transcription factor gene. It and its probable functional partners, Tle co-repressors, were expressed by neurally-fated basal progenitor cells and olfactory sensory neurons of the olfactory epithelium. Uncx expression was rare in olfactory epithelia of Ascl1(-/-) mice, but common in Neurog1(-/-) mice. In Uncx(-/-) mice olfactory progenitor cell proliferation, progenitor cell number, olfactory sensory neuron survival, and Umodl1 and Kcnc4 mRNAs were reduced. Evidence of sensory neuron activity and functional connections to the olfactory bulb argue that decreased neuronal survival was not due to loss of trophic support or activity-dependent mechanisms. These data suggest that UNCX acts downstream of neural determination factors to broadly control transcriptional mechanisms used by neural progenitor cells to specify neural phenotypes.

The male mouse pheromone ESP1 enhances female sexual receptive behaviour through a specific vomeronasal receptor

Various social behaviours in mice are regulated by chemical signals called pheromones that act through the vomeronasal system. Exocrine gland-secreting peptide 1 (ESP1) is a 7-kDa peptide that is released into male tear fluids and stimulates vomeronasal sensory neurons in female mice. Here, we describe the molecular and neural mechanisms that are involved in the decoding of ESP1 signals in the vomeronasal system, which leads to behavioural output in female mice. ESP1 is recognized by a specific vomeronasal receptor, V2Rp5, and the ligand-receptor interaction results in sex-specific signal transmission to the amygdaloid and hypothalamic nuclei via the accessory olfactory bulb. Consequently, ESP1 enhances female sexual receptive behaviour upon male mounting (lordosis), allowing successful copulation. In V2Rp5-deficient mice, ESP1 induces neither neural activation nor sexual behaviour. These findings show that ESP1 is a crucial male pheromone that regulates female reproductive behaviour through a specific receptor in the mouse vomeronasal system.

Chemical stress induces the unfolded protein response in olfactory sensory neurons

More than any other neuron, olfactory sensory neurons are exposed to environmental insults. Surprisingly, their only documented response to damaging stress is apoptosis and subsequent replacement by new neurons. However, they expressed unfolded protein response genes, a transcriptionally regulated defense mechanism activated by many types of insults. The unfolded protein response transcripts Xbp1, spliced Xbp1, Chop (Ddit3), and BiP (Hspa5) were decreased when external access of stressors was reduced by blocking a nostril (naris occlusion). These transcripts and Nrf2 (Nfe2l2) were increased by systemic application of tunicamycin or the selective olfactotoxic chemical methimazole. Methimazole's effects overcame naris occlusion, and the unfolded protein response was independent of odor-evoked neuronal activity. Chemical stress is therefore a major and chronic activator of the unfolded protein response in olfactory sensory neurons. Stress-dependent repression of the antiapoptotic gene Bcl2 was absent, however, suggesting a mechanism for disconnecting the UPR from apoptosis and tolerating a chronic unfolded protein response. Environmental stressors also affect both the sustentacular cells that support the neurons and the respiratory epithelia, because naris occlusion decreased expression of the xenobiotic chemical transformation enzyme Cyp2a5 in sustentacular cells, and both naris occlusion and methimazole altered the abundance of the antibacterial lectin Reg3g in respiratory epithelia.

The immune protein CD3zeta is required for normal development of neural circuits in the retina

Emerging evidence suggests that immune proteins regulate activity-dependent synapse formation in the central nervous system (CNS). Mice with mutations in class I major histocompatibility complex (MHCI) genes have incomplete eye-specific segregation of retinal ganglion cell (RGC) axon projections to the CNS. This effect has been attributed to causes that are nonretinal in origin. We show that a key component of MHCI receptor, CD3zeta, is expressed in RGCs. CD3zeta-deficient mice have reduced RGC dendritic motility, an increase in RGC dendritic density, and a selective defect of glutamate-receptor-mediated synaptic activity in the retina. Disrupted RGC synaptic activity and dendritic motility is associated with a failure of eye-specific segregation of RGC axon projections to the CNS. These results provide direct evidence of an unrecognized requirement for immune proteins in the developmental regulation of RGC synaptic wiring and indicate a possible retinal origin for the disruption of eye-specific segregation found in immune-deficient mice.

Immune proteins in brain development and synaptic plasticity

Many proteins first identified in the immune system are also expressed in the developing and adult nervous system. Unexpectedly, recent studies reveal that a number of these proteins, in addition to their immunological roles, are essential for the establishment, function, and modification of synaptic connections. These include proinflammatory cytokines (e.g., TNFalpha, IL-6), proteins of the innate immune system (e.g., complement C1q and C3, pentraxins, Dscam), members of the major histocompatibility complex class I (MHCI) family, and MHCI-binding immunoreceptors and their components (e.g., PIRB, Ly49, DAP12, CD3zeta). Understanding how these proteins function in neurons will clarify the molecular basis of fundamental events in brain development and plasticity and may add a new dimension to our understanding of neural-immune interactions in health and disease.

Subsystem organization of the mammalian sense of smell

The mammalian olfactory system senses an almost unlimited number of chemical stimuli and initiates a process of neural recognition that influences nearly every aspect of life. This review examines the organizational principles underlying the recognition of olfactory stimuli. The olfactory system is composed of a number of distinct subsystems that can be distinguished by the location of their sensory neurons in the nasal cavity, the receptors they use to detect chemosensory stimuli, the signaling mechanisms they employ to transduce those stimuli, and their axonal projections to specific regions of the olfactory forebrain. An integrative approach that includes gene targeting methods, optical and electrophysiological recording, and behavioral analysis has helped to elucidate the functional significance of this subsystem organization for the sense of smell.

Mammalian olfactory receptors

Perception of chemical stimuli from the environment is essential to most animals; accordingly, they are equipped with a complex olfactory system capable of receiving a nearly unlimited number of odorous substances and pheromones. This enormous task is accomplished by olfactory sensory neurons (OSNs) arranged in several chemosensory compartments in the nose. The sensitive and selective responsiveness of OSNs to odorous molecules and pheromones is based on distinct receptors in their chemosensory membrane; consequently, olfactory receptors play a key role for a reliable recognition and an accurate processing of chemosensory information. They are therefore considered as key elements for an understanding of the principles and mechanisms underlying the sense of smell. The repertoire of olfactory receptors in mammals encompasses hundreds of different receptor types which are highly diverse and expressed in distinct subcompartments of the nose. Accordingly, they are categorized into several receptor families, including odorant receptors (ORs), vomeronasal receptors (V1Rs and V2Rs), trace amine-associated receptors (TAARs), formyl peptide receptors (FPRs), and the membrane guanylyl cyclase GC-D. This large and complex receptor repertoire is the basis for the enormous chemosensory capacity of the olfactory system.

Diverse systems for pheromone perception: multiple receptor families in two olfactory systems

Traditionally, the olfactory epithelium is considered to recognize conventional odors, while the vomeronasal organ detects pheromones. However, recent advances suggest that vertebrate pheromones can also be detected by the olfactory epithelium. In the vomeronasal organ and the olfactory epithelium, structurally distinct multiple receptor families are expressed. In rodents, two of these receptor families, V1R and V2R, are expressed specifically in the vomeronasal organ and detect pheromones and pheromone candidates. A newly isolated trace amine-associated receptor detects some of the putative pheromones in the mouse olfactory epithelium. In addition, distinct second-messenger pathways and neural circuits are used for pheromone perception mediated by each receptor family. Furthermore, the function of these receptor families in these olfactory organs appears to differ among various vertebrate species. The systems for pheromone perception in vertebrates are far more complex than previously predicted.

Olfactory receptors: G protein-coupled receptors and beyond

Sensing the chemical environment is critical for all organisms. Diverse animals from insects to mammals utilize highly organized olfactory system to detect, encode, and process chemostimuli that may carry important information critical for health, survival, social interactions and reproduction. Therefore, for animals to properly interpret and react to their environment it is imperative that the olfactory system recognizes chemical stimuli with appropriate selectivity and sensitivity. Because olfactory receptor proteins play such an essential role in the specific recognition of diverse stimuli, understanding how they interact with and transduce their cognate ligands is a high priority. In the nearly two decades since the discovery that the mammalian odorant receptor gene family constitutes the largest group of G protein-coupled receptor (GPCR) genes, much attention has been focused on the roles of GPCRs in vertebrate and invertebrate olfaction. However, is has become clear that the 'family' of olfactory receptors is highly diverse, with roles for enzymes and ligand-gated ion channels as well as GPCRs in the primary detection of olfactory stimuli.

Normal adult climbing fiber monoinnervation of cerebellar Purkinje cells in mice lacking MHC class I molecules

Some immune system proteins have recently been implicated in the development and plasticity of neuronal connections. Notably, proteins of the major histocompatibility complex 1 (MHC class 1) have been shown to be involved in synaptic plasticity in the hippocampus and the development of projection patterns in the visual system. We examined the possible role for the MHC class 1 proteins in one well-characterized example of synaptic exuberance and subsequent refinement, the climbing fiber (CF) to Purkinje cell (PC) synapse. Cerebella from adult mice deficient for two MHC genes, H2-D1 and H2-K1, and for beta2-microglobulin gene were examined for evidence of deficient elimination of supernumerary CF synapses on their PCs. Electrophysiological and morphological evidence showed that, despite the absence of these MHC class 1 molecules, adult PCs in these transgenic mice are monoinnervated as in wild-type animals. These findings indicate that, at the level of restriction of afferent number at this synapse, functional MHC class 1 proteins are not required.

Cross-species comparison of metabolite profiles in chemosensory epithelia: an indication of metabolite roles in chemosensory cells

Comparative studies of chemosensory systems in vertebrates and invertebrates have greatly enhanced our understanding of anatomical and physiological constraints of chemical detection. Immunohistochemical comparisons of chemosensory systems are difficult to make across species due to limited cross-reactivity of mammalian-based antibodies. Immunostaining chemosensory tissues with glutaraldehyde-based antibodies generated against small metabolites in combination with hierarchical cluster analyses provide a novel approach for identifying and classifying cell types regardless of species. We used this "metabolite profiling" technique to determine whether metabolite profiles can be used to identify cell classes within and across different species including mouse, zebrafish, lobster and squid. Within a species, metabolite profiles for distinct cell classes were generally consistent. We found several metabolite-based cell classifications that mirrored function or receptor protein-based classifications. Although profiles of all six metabolites differed across species, we found that specific metabolites were associated with certain cell types. For example, elevated levels of glutathione were characteristic of nonsensory cells from vertebrates, suggesting an antioxidative role in non-neuronal cells in sensory tissues. Collectively, we found significantly different metabolite profiles for distinct cell populations in chemosensory tissue within all of the species studied. Based on their roles in other systems or cells, we discuss the roles of L-arginine, L-aspartate, L-glutamate, glycine, glutathione, and taurine within chemosensory epithelia.

Major histocompatibility complex (Mhc) class Ib gene duplications, organization and expression patterns in mouse strain C57BL/6

BACKGROUND

The mouse has more than 30 Major histocompatibility complex (Mhc) class Ib genes, most of which exist in the H2 region of chromosome 17 in distinct gene clusters. Although recent progress in Mhc research has revealed the unique roles of several Mhc class Ib genes in the immune and non-immune systems, the functions of many class Ib genes have still to be elucidated. To better understand the roles of class Ib molecules, we have characterized their gene duplication, organization and expression patterns within the H2 region of the mouse strain C57BL/6.

RESULTS

The genomic organization of the H2-Q, -T and -M regions was analyzed and 21 transcribed Mhc class Ib genes were identified within these regions. Dot-plot and phylogenetic analyses implied that the genes were generated by monogenic and/or multigenic duplicated events. To investigate the adult tissue, embryonic and placental expressions of these genes, we performed RT-PCR gene expression profiling using gene-specific primers. Both tissue-wide and tissue-specific gene expression patterns were obtained that suggest that the variations in the gene expression may depend on the genomic location of the duplicated genes as well as locus specific mechanisms. The genes located in the H2-T region at the centromeric end of the cluster were expressed more widely than those at the telomeric end, which showed tissue-restricted expression in spite of nucleotide sequence similarities among gene paralogs.

CONCLUSION

Duplicated Mhc class Ib genes located in the H2-Q, -T and -M regions are differentially expressed in a variety of developing and adult tissues. Our findings form the basis for further functional validation studies of the Mhc class Ib gene expression profiles in specific tissues, such as the brain. The duplicated gene expression results in combination with the genome analysis suggest the possibility of long-range regulation of H2-T gene expression and/or important, but as yet unidentified nucleotide changes in the promoter or enhancer regions of the genes. Since the Mhc genomic region has diversified among mouse strains, it should be a useful model region for comparative analyses of the relationships between duplicated gene organization, evolution and the regulation of expression patterns.

Expression of nonclassical class I major histocompatibility genes defines a tripartite organization of the mouse vomeronasal system

The epithelium of the mouse vomeronasal organ (VNO) consists of apical and basal layers of neuronal cell bodies. Vomeronasal sensory neurons (VSNs) with cell bodies in the basal layer express the G-protein subunit G alpha(o) and members of the V2R superfamily of vomeronasal receptor genes and project their axons to the posterior accessory bulb (AOB). V2R(+) VSNs also express particular patterns of a family of nine nonclassical class I major histocompatibility Mhc genes, the H2-Mv genes. The function of H2-Mv molecules remains unknown. H2-Mv molecules have been reported to be associated with V2R molecules and have been proposed to participate in pheromone detection. Here, we find that a substantial fraction of V2R(+) VSNs does not express these nine H2-Mv genes. The cell bodies of H2-Mv(+) and H2-Mv(-) VSNs reside in the lower and upper sublayers of the basal layer, respectively. This spatial segregation is maintained at the level of the AOB: H2-Mv(+) and H2-Mv(-) VSNs project their axons to the posterior and anterior subdomains of the posterior AOB, respectively. By generating a C-terminal green fluorescent protein fusion protein with M10.2 in gene-targeted mice, we observe subcellular localization of M10.2 not only in dendrites but also in axons of VSNs. Our results reveal a tripartite organization of the VNO and AOB, question the generality of the requirement of these nine H2-Mv molecules for V2R surface expression, and suggest that H2-Mvs can function in both dendrites and axons.

Searching for the ligands of odorant receptors

Through the sense of smell mammals can detect and discriminate between a large variety of odorants present in the surrounding environment. Odorants bind to a large repertoire of odorant receptors located in the cilia of olfactory sensory neurons of the nose. Each olfactory neuron expresses one single type of odorant receptor, and neurons expressing the same type of receptor project their axons to one or a few glomeruli in the olfactory bulb, creating a map of odorant receptor inputs. The information is then passed on to other regions of the brain, leading to odorant perception. To understand how the olfactory system discriminates between odorants, it is necessary to determine the odorant specificities of individual odorant receptors. These studies are complicated by the extremely large size of the odorant receptor family and by the poor functional expression of these receptors in heterologous cells. This article provides an overview of the methods that are currently being used to investigate odorant receptor-ligand interactions.

Structure and expression of MHC class Ib genes of the central M region in rat and mouse: M4, M5, and M6

The M region at the telomeric end of the murine major histocompatibility complex (MHC) contains class I genes that are highly conserved in rat and mouse. We have sequenced a cosmid clone of the LEW rat strain (RT1 haplotype) containing three class I genes, RT1.M6-1, RT1.M4, and RT1.M5. The sequences of allelic genes of the BN strain (RT1n haplotype) were obtained either from cDNAs or genomic clones. For the coding parts of the genes few differences were found between the two RT1 haplotypes. In LEW, however, only RT1.M5 and RT1.M6 have open reading frames; whereas in BN all three genes were intact. In line with the findings in BN, transcription was found for all three rat genes in several tissues from strain Sprague Dawley. Protein expression in transfectants could be demonstrated for RT1.M6-1 using the monoclonal antibody OX18. By sequencing of transcripts obtained by RT-PCR, a second, transcribed M6 gene, RT1.M6-2, was discovered, which maps next to RT1.M6-1 outside of the region covered by the cosmid. In addition, alternatively spliced forms for RT1.M5 and RT1.M6 were detected. Of the orthologous mouse genes, H2-M4, H2-M5, and H2-M6, only H2-M5 has an open reading frame. Other important differences between the corresponding parts of the M region of the two species are insertion of long LINE repeats, duplication of RT1.M6, and the inversion of RT1.M5 in the rat. This demonstrates substantial evolutionary dynamics in this region despite conservation of the class I gene sequences themselves.

Pheromone sensing in mice

Beginning with the neuroepithelium of the vomeronasal organ, the accessory olfactory system in rodents runs parallel to the main olfactory system and is specialized in the detection of pheromones. Only a small number of vomeronasal agonists carrying pheromonal information have been identified this far. These structurally diverse classes of chemicals include peptides secreted by exocrine glands and range from small volatile molecules to proteins and fragments thereof present in urine. Most pheromones activate both vomeronasal and main olfactory sensory neurons, making the identification of functionally relevant populations of sensory neurons difficult. Analyses of gene-targeted mice selectively affecting either vomeronasal or main olfactory signaling have attempted to elucidate the functional contribution of the different chemosensory epithelia to pheromone sensing in mice. These mouse models suggest that both the main and the accessory olfactory systems can converge and synergize to express the complex array of stereotyped behaviors and hormonal changes triggered by pheromones.

High levels of MeCP2 depress MHC class I expression in neuronal cells

BACKGROUND

The expression of MHC class I genes is repressed in mature neurons. The molecular basis of this regulation is poorly understood, but the genes are particularly rich in CpG islands. MeCP2 is a transcriptional repressor that binds to methylated CpG dinucleotides; mutations in this protein also cause the neurodevelopmental disease called Rett syndrome. Because MHC class I molecules play a role in neuronal connectivity, we hypothesised that MeCP2 might repress MHC class I expression in the CNS and that this might play a role in the pathology of Rett syndrome.

METHODOLOGY

We show here that transiently transfected cells expressing high levels of MeCP2 specifically downregulate cell-surface expression of MHC class I molecules in the neuronal cell line N2A and they prevent the induction of MHC class I expression in response to interferon in these cells, supporting our first hypothesis. Surprisingly, however, overexpression of the mutated forms of MeCP2 that cause Rett syndrome had a similar effect on MHC class I expression as the wild-type protein. Immunohistological analyses of brain slices from MECP2 knockout mice (the MeCP2(tm1.1Bird) strain) demonstrated a small but reproducible increase in MHC class I when compared to their wild type littermates, but we found no difference in MHC class I expression in primary cultures of mixed glial cells (mainly neurons and astrocytes) from the knockout and wild-type mice.

CONCLUSION

These data suggest that high levels of MeCP2, such as those found in mature neurons, may contribute to the repression of MHC expression, but we find no evidence that MeCP2 regulation of MHC class I is important for the pathogenesis of Rett syndrome.

Combinatorial co-expression of pheromone receptors, V2Rs

Basal neurons of the vomeronasal organ of the mouse express a superfamily of about 120 pheromone receptors, named V2Rs, that are grouped in four families, A, B, C, and D, according to sequence homology. Family-A, -B, and -D V2Rs are expressed as one receptor gene per cell, but we previously reported their co-expression with family-C V2Rs. Here, we show that basal neurons can be further grouped according to the combinatorial expression of different V2Rs. Altogether, these findings suggest that in each basal neuron a transcriptional program is active for expressing a combination of two compatible receptors and for excluding, at the same time, the expression of all other V2Rs. Further analyses revealed non-random combinations of co-expression between family-C V2Rs and genes of the class Ib major histocompatibility complex. Thus, each basal neuron of the vomeronasal organ represents a highly qualified sensory unit for detecting very specific combinations of pheromonal cues.

Mammalian pheromone sensing

The traditional distinction that the mammalian main olfactory system recognizes general odor molecules and the accessory (vomeronasal) system detects pheromones is no longer valid. The emerging picture is that both systems have considerable overlap in terms of the chemosignals they detect and the effects that they mediate. Recent investigations have discovered large families of pheromonal signals together with a rich variety of specific receptor systems and nasal detection pathways. Selective genetic targeting of these subsystems should help to unravel their biological role in pheromone-mediated behavioral responses.

Regulation of CNS synapses by neuronal MHC class I

Until recently, neurons in the healthy brain were considered immune-privileged because they did not appear to express MHC class I (MHCI). However, MHCI mRNA was found to be regulated by neural activity in the developing visual system and has been detected in other regions of the uninjured brain. Here we show that MHCI regulates aspects of synaptic function in response to activity. MHCI protein is colocalized postsynaptically with PSD-95 in dendrites of hippocampal neurons. In vitro, whole-cell recordings of hippocampal neurons from beta2m/TAP1 knockout (KO) mice, which have reduced MHCI surface levels, indicate a 40% increase in mini-EPSC (mEPSC) frequency. mEPSC frequency is also increased 100% in layer 4 cortical neurons. Similarly, in KO hippocampal cultures, there is a modest increase in the size of presynaptic boutons relative to WT, whereas postsynaptic parameters (PSD-95 puncta size and mEPSC amplitude) are normal. In EM of intact hippocampus, KO synapses show a corresponding increase in vesicles number. Finally, KO neurons in vitro fail to respond normally to TTX treatment by scaling up synaptic parameters. Together, these results suggest that postsynaptically localized MHCl acts in homeostatic regulation of synaptic function and morphology during development and in response to activity blockade. The results also imply that MHCI acts retrogradely across the synapse to translate activity into lasting change in structure.

Mammalian social odours: attraction and individual recognition

Mammalian social systems rely on signals passed between individuals conveying information including sex, reproductive status, individual identity, ownership, competitive ability and health status. Many of these signals take the form of complex mixtures of molecules sensed by chemosensory systems and have important influences on a variety of behaviours that are vital for reproductive success, such as parent-offspring attachment, mate choice and territorial marking. This article aims to review the nature of these chemosensory cues and the neural pathways mediating their physiological and behavioural effects. Despite the complexities of mammalian societies, there are instances where single molecules can act as classical pheromones attracting interest and approach behaviour. Chemosignals with relatively high volatility can be used to signal at a distance and are sensed by the main olfactory system. Most mammals also possess a vomeronasal system, which is specialized to detect relatively non-volatile chemosensory cues following direct contact. Single attractant molecules are sensed by highly specific receptors using a labelled line pathway. These act alongside more complex mixtures of signals that are required to signal individual identity. There are multiple sources of such individuality chemosignals, based on the highly polymorphic genes of the major histocompatibility complex (MHC) or lipocalins such as the mouse major urinary proteins. The individual profile of volatile components that make up an individual odour signature can be sensed by the main olfactory system, as the pattern of activity across an array of broadly tuned receptor types. In addition, the vomeronasal system can respond highly selectively to non-volatile peptide ligands associated with the MHC, acting at the V2r class of vomeronasal receptor. The ability to recognize individuals or their genetic relatedness plays an important role in mammalian social behaviour. Thus robust systems for olfactory learning and recognition of chemosensory individuality have evolved, often associated with major life events, such as mating, parturition or neonatal development. These forms of learning share common features, such as increased noradrenaline evoked by somatosensory stimulation, which results in neural changes at the level of the olfactory bulb. In the main olfactory bulb, these changes are likely to refine the pattern of activity in response to the learned odour, enhancing its discrimination from those of similar odours. In the accessory olfactory bulb, memory formation is hypothesized to involve a selective inhibition, which disrupts the transmission of the learned chemosignal from the mating male. Information from the main olfactory and vomeronasal systems is integrated at the level of the corticomedial amygdala, which forms the most important pathway by which social odours mediate their behavioural and physiological effects. Recent evidence suggests that this region may also play an important role in the learning and recognition of social chemosignals.

Differential impact of Lhx2 deficiency on expression of class I and class II odorant receptor genes in mouse

Odorant receptor (OR) genes can be classified into two types: fish-like class I OR genes and mammalian-specific class II OR genes. We have previously shown that Lhx2, a LIM-homeodomain protein, binds to the homeodomain site in the promoter region of mouse M71, a class II OR, and that a knockout mutation in Lhx2 precludes expression of all tested class II OR genes including M71. Here, we report that most class I OR genes, which are expressed in a dorsal region of the olfactory epithelium, are still expressed in Lhx2-deficient embryos. There are two exceptions: two class I OR genes, which are normally expressed in a more ventral region, are no longer expressed in Lhx2 mutant mice. Lhx2 is transcribed in olfactory sensory neurons irrespective of expression of class I or class II OR genes. Thus, a deficiency of Lhx2 has a differential impact on class I and class II OR gene expression.

Comparative genomic analysis identifies an evolutionary shift of vomeronasal receptor gene repertoires in the vertebrate transition from water to land

Two evolutionarily unrelated superfamilies of G-protein coupled receptors, V1Rs and V2Rs, bind pheromones and "ordinary" odorants to initiate vomeronasal chemical senses in vertebrates, which play important roles in many aspects of an organism's daily life such as mating, territoriality, and foraging. To study the macroevolution of vomeronasal sensitivity, we identified all V1R and V2R genes from the genome sequences of 11 vertebrates. Our analysis suggests the presence of multiple V1R and V2R genes in the common ancestor of teleost fish and tetrapods and reveals an exceptionally large among-species variation in the sizes of these gene repertoires. Interestingly, the ratio of the number of intact V1R genes to that of V2R genes increased by approximately 50-fold as land vertebrates evolved from aquatic vertebrates. A similar increase was found for the ratio of the number of class II odorant receptor (OR) genes to that of class I genes, but not in other vertebrate gene families. Because V1Rs and class II ORs have been suggested to bind to small airborne chemicals, whereas V2Rs and class I ORs recognize water-soluble molecules, these increases reflect a rare case of adaptation to terrestrial life at the gene family level. Several gene families known to function in concert with V2Rs in the mouse are absent outside rodents, indicating rapid changes of interactions between vomeronasal receptors and their molecular partners. Taken together, our results demonstrate the exceptional evolutionary fluidity of vomeronasal receptors, making them excellent targets for studying the molecular basis of physiological and behavioral diversity and adaptation.

Pheromonal communication in vertebrates

Recent insights have revolutionized our understanding of the importance of chemical signals in influencing vertebrate behaviour. Previously unknown families of pheromonal signals have been identified that are expanding the traditional definition of a pheromone. Although previously regarded as functioning independently, the main olfactory and vomeronasal systems have been found to have considerable overlap in terms of the chemosignals they detect and the effects that they mediate. Studies using gene-targeted mice have revealed an unexpected diversity of chemosensory systems and their underlying cellular and molecular mechanisms. Future developments could show how the functions of the different chemosensory systems are integrated to regulate innate and learned behavioural and physiological responses to pheromones.

Molecular biology of peptide pheromone production and reception in mice

Intraspecies communication via pheromones plays an important role in social and sexual behaviors, which are critical for survival and reproduction in many animal species. In mice, pheromonal signals are processed by the parallel action of two olfactory systems: the main olfactory system and the vomeronasal pathway. Pheromones are recognized by chemosensory receptors expressed in the main olfactory epithelium and by V1R- and V2R-type receptors expressed in the vomeronasal organ (VNO). Mice take advantage of the chemical properties of both types of pheromones (i.e., volatile/nonvolatile) to precisely control the spatial and temporal transmission of their individual signals. The recent discovery of the exocrine gland-secreting peptide (ESP) family, which appears to encode a VNO-specific ligand repertoire, should open a new avenue to understanding peptide pheromone-mediated communication via the vomeronasal pathway in mice. In this chapter, I will review the current knowledge on genetic and molecular aspects of peptide pheromones and their receptors, by focusing primarily on the mouse VNO system. It is also an intriguing aspect to discuss peptide pheromones in the context of the evolutionary importance of species-specific chemical communication.

TRPC2: molecular biology and functional importance

TRPC (canonical transient receptor potential) channels are the closest mammalian homologs of Drosophila TRP and TRP-like channels. TRPCs are rather nonselective Ca2+ permeable cation channels and affect cell functions through their ability to mediate Ca2+ entry into cells and their action to collapse the plasma membrane potentials. In neurons the latter function leads to action potentials. The mammalian genome codes for seven TRPCs of which TRPC2 is the largest with the most restricted pattern of expression and has several alternatively spliced variants. Expressed in model cells, TRPC2 mediates both receptor- and store depletion-triggered Ca2+ entry. TRPC2 is unique among TRPCs in that its complete gene has been lost from the Old World monkey and human genomes, in which its remnants constitute a pseudogene. Physiological roles for TRPC2 have been studied in mature sperm and the vomeronasal sensory system. In sperm, TRPC2 is activated by the sperm's interaction with the oocyte's zona pellucida, leading to entry of Ca2+ and activation of the acrosome reaction. In the vomeronasal sensory organ (VNO), TRPC2 was found to constitute the transduction channel activated through signaling cascade initiated by the interaction of pheromones with V1R and V2R G protein-coupled receptors on the dendrites of the sensory neurons. V1Rs and V2Rs, the latter working in conjunction with class I MHC molecules, activate G(i)- and G(o)-type G proteins which in turn trigger activation of TRPC2, initiating an axon potential that travels to the axonal terminals. The signal is then projected to the glomeruli of the auxiliary olfactory bulb from where it is carried first to the amygdala and then to higher cortical cognition centers. Immunocytochemistry and gene deletion studies have shown that (1) the V2R-G(o)-MHCIb-beta2m pathway mediates male aggressive behavior in response to pheromones; (2) the V1R-G(i2) pathway mediates mating partner recognition, and (3) these differences have an anatomical correlate in that these functional components are located in anatomically distinct compartments of the VNO. Interestingly, these anatomically segregated signaling pathways use a common transduction channel, TRPC2.

Molecular characterization of vomeronasal sensory neurons responding to a male-specific peptide in tear fluid: Sexual communication in mice

Pheromonal signals received by the vomeronasal organ (VNO) have been shown to elicit various behavioral and physiological responses that are typically stereotyped and preprogrammed. Recently, we found a novel male-specific peptide, named exocrine gland-secreting peptide 1 (ESP1), that is secreted in tear fluid and stimulates the VNO in mice. Excreted ESP1 appears to be transferred to the female VNO, where it induces c-Fos expression and elicits an electrical response in a small subset of vomeronasal sensory neurons (VSNs). We report here the identification of molecular components expressed in ESP1-stimulated VSNs by double-staining with c-Fos. We found that the c-Fos-induced cells were localized amongst the Gαo-expressing VSNs. Furthermore, the ESP1 signal was received by VSNs expressing a single type of vomeronasal receptor type 2 (V2Rp5). Finally, double in situ hybridization of the V2Rp5 and various members of the M1 and M10 families of major histocompatibility complex (MHC) class Ib molecules revealed that V2Rp5-expressing VSNs can express multiple MHC molecules. These results suggest that a V2R rather than MHC molecule is mainly responsible for the recognition of ESP1. The identification of the putative sex-pheromone ESP1 and its cognate receptor therefore will help clarify the molecular mechanisms underlying pheromone recognition in the mouse vomeronasal system.

Olfactory receptor gene repertoires in mammals

In mammals, olfaction is mediated by two distinct organs that are located in the nasal cavity: the main olfactory epithelium (MOE) that binds volatile odorants is responsible for the conscious perception of odors, and the vomeronasal organ (VNO) that binds pheromones is responsible for various behavioral and neuroendocrine responses between individuals of a same species. Odorants and pheromones bind to seven transmembrane domain G-protein-coupled receptors that permit signal transduction. These receptors are encoded by large multigene families that evolved in mammal species in function of specific olfactory needs.

Origin and evolution of the vertebrate vomeronasal system viewed through system-specific genes

Tetrapods have two distinct nasal chemosensory systems, the main olfactory system and the vomeronasal system (VNS). Defined by certain morphological components, the main olfactory system is present in all groups of vertebrates, while the VNS is found only in tetrapods. Previous attempts to identify a VNS precursor in teleost fish were limited by functional and morphological characters that could not clearly distinguish between homologous and analogous systems. In the past decade, several genes that specifically function in the VNS have been discovered. Here we first describe recent evolutionary studies of mammalian VNS-specific genes. We then review evidence showing the presence and tissue-specific expression of the VNS-specific genes in teleosts, as well as co-expression patterns of these genes in specific regions of the teleost olfactory epithelium. We propose that a VNS precursor exists in teleosts and that its evolutionary origin predated the separation between teleosts and tetrapods.

Expression of Toll-like receptor 3 in the human cerebellar cortex in rabies, herpes simplex encephalitis, and other neurological diseases

There is recent in vitro evidence that human neurons express the innate immune response receptor, Toll-like receptor-3 (TLR-3), and that expression is enhanced in viral infections. The authors examined the immunohistochemical expression of TLR-3 in the cerebellar cortex of postmortem human brains. Purkinje cells were found to express TLR-3 in all cases of rabies (4 of 4) and herpes simplex encephalitis (2 of 2) as well as in cases of amyotrophic lateral sclerosis (1 of 2), stroke (1 of 2), and Alzheimer's disease (3 of 3). In cases of viral infection, direct viral infection was not necessary for enhanced neuronal TLR-3 expression, suggesting that soluble factors likely play an important role in inducing TLR-3 expression. In addition to neurons, occasional Bergmann glia expressed TLR-3 in some cases. This study has provided evidence that human brain neurons can express TLR-3 in vivo and suggests that neurons may play an important role in initiating an inflammatory reaction in a variety of neurological diseases.

Olfactory receptors and signalling elements in the Grueneberg ganglion

The Grueneberg ganglion (GG) is a cluster of neurones present in the vestibule of the anterior nasal cavity. Although its function is still elusive, recent studies have shown that cells of the GG transcribe the gene encoding the olfactory marker protein (OMP) and project their axons to glomeruli of the olfactory bulb, suggesting that they may have a chemosensory function. Chemosensory responsiveness of olfactory neurones in the main olfactory epithelium (MOE) and the vomeronasal organ (VNO) is based on the expression of either odorant receptors or vomeronasal putative pheromone receptors. To scrutinize its presumptive olfactory nature, the GG was assessed for receptor expression by extensive RT-PCR analyses, leading to the identification of a distinct vomeronasal receptor which was expressed in the majority of OMP-positive GG neurones. Along with this receptor, these cells expressed the G proteins Go and Gi, both of which are also present in sensory neurones of the vomeronasal organ. Odorant receptors were expressed by very few cells during prenatal and perinatal stages; a similar number of cells expressed adenylyl cyclase type III and G(olf/s), characteristic signalling elements of the main olfactory system. The findings of the study support the notion that the GG is in fact a subunit of the complex olfactory system, comprising cells with either a VNO-like or a MOE-like phenotype. Moreover, expression of a vomeronasal receptor indicates that the GG might serve to detect pheromones.

MHC Class I-Like MILL Molecules Are β2-Microglobulin-Associated, GPI-Anchored Glycoproteins That Do Not Require TAP for Cell Surface Expression

MILL (MHC class I-like located near the leukocyte receptor complex) is a family of MHC class I-like molecules encoded outside the MHC, which displays the highest sequence similarity to human MICA/B molecules among known class I molecules. In the present study, we show that the two members of the mouse MILL family, MILL1 and MILL2, are GPI-anchored glycoproteins associated with beta2-microglobulin (beta2m) and that cell surface expression of MILL1 or MILL2 does not require functional TAP molecules. MILL1 and MILL2 molecules expressed in bacteria could be refolded in the presence of beta2m, without adding any peptides. Hence, neither MILL1 nor MILL2 is likely to be involved in the presentation of peptides. Immunohistochemical analysis revealed that MILL1 is expressed in a subpopulation of thymic medullary epithelial cells and a restricted region of inner root sheaths in hair follicles. The present study provides additional evidence that MILL is a class I family distinct from MICA/B.

Accessory chemosignaling mechanisms in primates

Accessory olfaction is defined as the chemoreceptive system that employs the vomeronasal complex (VNC) and its distinct central projections to the accessory olfactory bulb (AOB) and limbic/cortical systems. Comparisons of the structural and functional features of primate accessory olfaction can now be made at many levels. Advances in the understanding of molecular mechanisms of odorant transfer and detection, physiological analyses of signal processing, and appreciation of ontogenetic timetables have clarified the contribution of accessory chemoreception to the sensory map. Two principal functions dominate: the decoding of social information through the uptake of signals (often fluid-borne), and the provision of an essential pathway for the "migration" of presumptive neurocrine (GnRH) cells from the olfactory placode to the hypothalamus. VN "smelling" (vomerolfaction) is now seen to overlap with primary olfaction. Both systems detect signal compounds along the spectrum of volatility/molecular weight, and neither is an exclusive sensor. Both main and accessory chemoreception seem to require collaborative molecular devices to assist in odorant transfer (binding proteins) and (for the VNO) signal recognition (MHC1 proteins). Most adaptive-selective features of primate chemocommunication variously resemble those of other terrestrial mammals. VN function, along with its genome, has been maintained within the Strepsirrhines and tarsiers, reduced in Platyrrhines, and nearly extinguished at the Catarrhine up to hominin levels. It persists as an intriguing ancient sense that retains key features of past evolutionary events.

Genetic basis of olfactory communication in primates

The genes involved in olfactory communication in mammals via the vomeronasal system are summarized, and studies investigating these genes in primates are reviewed. Only five potentially functional vomeronasal receptor genes (V1RL s) have been found in humans, and only one of these (V1RL1) has been studied in other primates. V1RL1 has become a pseudogene repeatedly during primate evolution, but patterns of natural selection on primate V1RL genes demonstrate that this gene family diverged under natural selection throughout at least part of primate evolution. Evolution of the TRP2 gene, which encodes for an ion channel that is important in vomeronasal organ (VNO) signalling, strongly suggests that this signalling function was lost in ancestral Catarrhines. Overall, much work remains to be done to elucidate the repertoire of genes that are involved in pheromonal communication, particularly in Strepsirhines. Such studies promise unique insights into the evolution of this modality.

Differential expression of major histocompatibility complex class I molecules in the brain of a New World monkey, the common marmoset (Callithrix jacchus)

It has been supposed that central nervous neurons do not express MHC class I molecules. However, recent studies clearly demonstrated functional MHC class I expression in the rodent brain. In the present study, we have extended these studies and investigated the presence of MHC class I transcripts and proteins in the brain of a non-human primate species, the common marmoset monkey (Callithrix jacchus). Using in-situ hybridization, we found strong expression of MHC class I transcripts in neocortex, hippocampal formation, substantia nigra and nucleus ruber. In-situ hybridization with emulsion autoradiography demonstrated MHC class I mRNA in distinct pyramidal neurons of cortex and hippocampus, in granule neurons of the dentate gyrus, in dopaminergic neurons of substantia nigra and in motor neurons of nucleus ruber. Immunocytochemistry confirmed MHC class I protein expression in these neurons. Two monoclonal antibodies, MRC-Ox18 and HB115, reacted differentially with MHC class I proteins on neuronal and non-neuronal cells, respectively. Interestingly, in marmoset monkeys that were immunosuppressed with FK506 (tacrolimus), expression of neuronal MHC class I proteins, which could be detected with MRC-Ox18, was either very low (neocortex, nucleus ruber, substantia nigra) or absent (hippocampus). In contrast, class I expression in endothelial cells, which was detected by HB115, was not affected by immunosuppression. Our data show that selected neurons in the brain of a non-human primate express MHC class I molecules and that this expression can be modulated by immunosuppression.

MHC homologs in the nervous system — they haven’t lost their groove

Major histocompatibility complex (MHC) molecules have been implicated in a number of non-immune roles in the central nervous system, particularly in synaptic development and plasticity. The discovery of M10 (50% sequence identity to classical MHC molecules) proteins expressed in the vomeronasal organ adds to the list of non-traditional roles of MHC homologs. M10 molecules associate with the V2R class of vomeronasal receptors, a family of G-protein coupled receptors thought to function as pheromone receptors. Recent studies showing that classical MHC-binding peptides activate V2R-expressing neurons offer tempting clues that M10s might participate directly in the recognition of pheromone ligands, but M10 proteins do not bind to these peptides with significant affinity. Instead of presenting MHC-binding peptides, M10s might function as molecular chaperones to V2R receptors or more generally as modulators of neuronal function, as demonstrated elsewhere in the brain for classical MHC molecules.

The vomeronasal system in mice: from the nose to the hypothalamus- and back!

The vomeronasal pathway in rodents runs parallel to the main olfactory pathway and mediates responses to different classes of chemosensory stimuli. Both olfactory systems can converge and synergize to control reproductive behaviors and hormonal changes triggered by chemosensory cues. Novel experimental approaches expressing genetic transneuronal tracers in hypothalamic neurons regulating reproduction have set the stage to analyze how chemosensory inputs are integrated in the brain to elicit reproductive behaviors and hormonal changes, and how neuroendocrine status might modulate susceptibility to chemosensory cues.

Sex and gonadal steroid modulation of pheromone receptor gene expression in the mouse vomeronasal organ

Non-volatile chemosignals in rodents are detected by unique receptors in the vomeronasal organ of the accessory olfactory system. Although the vomeronasal organ has been implicated in the regulation of sexually dimorphic behavioral and neuroendocrine functions, the underlying cellular mechanisms are undetermined. In previous studies we showed that exposure to soiled male bedding augmented immediate early gene immunoreactivity in neurons of the basal zone of the vomeronasal organ, an effect that depended on gender and sex steroid expression. To determine whether this effect could be due to differences in vomeronasal organ receptor expression, we examined two representatives (VR1 and VR4) from different subfamilies of the V2R family of receptors that are expressed in the basal zone of the vomeronasal organ. Adult Swiss-Webster male and female mice were gonadectomized and implanted with capsules containing 17beta-estradiol, testosterone or neither steroid (control). Two weeks later vomeronasal organs were processed for in situ hybridization using probes from the N-terminal extracellular domains of VR1 and VR4. Expression of both VR1 and VR4 was significantly higher in males than in females. Estradiol, but not testosterone-treated, males had significantly lower levels of VR1 expression in the caudal vomeronasal organ compared with untreated gonadectomized males. In contrast, testosterone enhanced VR4 expression in males relative to similarly treated females. Despite these effects, we found no evidence that vomeronasal organ neurons express either androgen or estrogen receptors. These data show that expression of vomeronasal organ receptors in mice is sexually dimorphic and regulated by sex steroids. Thus, gonadal hormones may affect the response of vomeronasal organ neurons to chemosignals by altering levels of the receptors to which they bind.

Evolutionary dynamics of olfactory and other chemosensory receptor genes in vertebrates

The numbers of functional olfactory receptor (OR) genes in humans and mice are about 400 and 1,000 respectively. In both humans and mice, these genes exist as genomic clusters and are scattered over almost all chromosomes. The difference in the number of genes between the two species is apparently caused by massive inactivation of OR genes in the human lineage and a substantial increase of OR genes in the mouse lineage after the human-mouse divergence. Compared with mammals, fishes have a much smaller number of OR genes. However, the OR gene family in fishes is much more divergent than that in mammals. Fishes have many different groups of genes that are absent in mammals, suggesting that the mammalian OR gene family is characterized by the loss of many group genes that existed in the ancestor of vertebrates and the subsequent expansion of specific groups of genes. Therefore, this gene family apparently changed dynamically depending on the evolutionary lineage and evolved under the birth-and-death model of evolution. Study of the evolutionary changes of two gene families for vomeronasal receptors and two gene families for taste receptors, which are structurally similar, but remotely related to OR genes, showed that some of the gene families evolved in the same fashion as the OR gene family. It appears that the number and types of genes in chemosensory receptor gene families have evolved in response to environmental needs, but they are also affected by fortuitous factors.

MHC peptides and the sensory evaluation of genotype

Social interactions, such as finding and identifying a mate, often rely on the ability to sense molecular cues carrying information about genetic relationship and individuality. We summarize recent evidence for an unexpected mechanistic link between the immune and olfactory systems in enabling this identification process. In addition to their established role in the immune response, peptide ligands of major histocompatibility complex (MHC) molecules constitute a previously unknown family of social recognition signals detected by specific subsets of sensory neurons in the mammalian nose. This sensing of MHC peptides can be viewed as a form of functional genome analysis by the nose. Behavioral studies in mice and fish show that MHC peptides are accepted as olfactory cues that influence mate choice decisions and selective pregnancy failure. These findings provide a molecular mechanism by which an individual can sense the composition and compatibility of vital immune system molecules of a conspecific, with direct consequences for social behavior.

Growth and behavioral traits in Donaldson rainbow trout (Oncorhynchus mykiss) cosegregate with classical major histocompatibility complex (MHC) class I genotype

Although polymorphism in major histocompatibility complex (MHC) genes has been thought to confer populations with protection against widespread decimation by pathogens, this hypothesis cannot explain the type of large allelic diversity in classical MHC class I (Ia) in rainbow trout. Based on expression of Onmy-UBA (MHC class Ia) in trout neurons, we hypothesized that polymorphism in trout class Ia may contribute to polymorphism in behavioral traits. The present study examined whether polymorphism in Onmy-UBA was associated with behavioral variation in Donaldson rainbow trout (Oncorhynchus mykiss) using experiments on food competition, lure-catch, fright recovery, diel locomotor activity and activity characterized as dominance or aggression. These behavioral traits were investigated in fish having Onmy-UBA*401/*401 or *4901/*4901 homozygous, or Onmy-UBA*401/*4901 heterozygous genotypes (referred to as BB, FF and BF, respectively). The BB fish exhibited boldness, aggression, faster growth and crepuscular activity, while the FF fish showed little boldness, smaller body size, and diurnal activity with no aggressive behavior. The BF fish displayed traits intermediary to those of the BB and FF fish. These results are consistent with polymorphism in a single MHC class Ia locus driving variation in neural circuits, thereby creating behavioral variation in the trout. This is the first study in any animal to show a potential correlation between polymorphism in MHC class Ia genes with polymorphism of behavioral traits such as aggression.

Sex-specific peptides from exocrine glands stimulate mouse vomeronasal sensory neurons

In mammals, social and reproductive behaviours are modulated by pheromones, which are chemical signals that convey information about sex and strain. The vomeronasal organ, located at the base of the nasal septum, is responsible for mediating pheromone information in mice. Two classes of putative pheromone receptor gene families, V1R and V2R, are expressed by vomeronasal sensory neurons in mutually segregated epithelial zones of the vomeronasal organ. Although numerous studies have suggested that pheromones originate from urine, direct recordings of behaving mice have shown that neuronal firing in the vomeronasal system is modulated by physical contact with the facial area. Here we identify a male-specific 7-kDa peptide secreted from the extraorbital lacrimal gland. This peptide, which we named exocrine gland-secreting peptide 1 (ESP1), is encoded by a gene from a previously unrecognized large family clustered in proximity to the class I major histocompatibility complex (MHC) region. ESP1 is secreted from the eyes and is transferred to the female vomeronasal organ, where it stimulates V2R-expressing vomeronasal sensory neurons and elicits an electrical response. Our results indicate that mice respond to sex-specific peptides released from exocrine glands through the vomeronasal system during direct contact.

Protocols for two- and three-color fluorescent RNA in situ hybridization of the main and accessory olfactory epithelia in mouse

The main and accessory olfactory epithelia of the mouse are composed of many cell populations. Each sensory neuron is thought to express one allele of one of the approximately 1000 odorant or approximately 300 vomeronasal receptor genes. Sensory neurons die and are replaced by new neurons that differentiate from precursor cells throughout the lifetime of the individual. Neuronal replacement is asynchronous, resulting in the co-existence of cells at various stages of differentiation. Receptor gene diversity and ongoing neuronal differentiation produce complex mosaics of gene expression within these epithelia. Accurate description of gene expression patterns will facilitate the understanding of mechanisms of gene choice and differentiation. Here we report a detailed protocol for two- and three-color fluorescent RNA in situ hybridization (ISH) and its combination with immunohistochemistry, or detection of bromodeoxyuridine (BrdU)-incorporated DNA after labeling. The protocol is applied to cryosections of the main and accessory olfactory epithelia in mouse.