Spatially restricted expression of regulators of G-protein signaling in primary olfactory neurons

Sensory Transduction in Photoreceptors and Olfactory Sensory Neurons: Common Features and Distinct Characteristics

The past decades have seen tremendous progress in our understanding of the function of photoreceptors and olfactory sensory neurons, uncovering the mechanisms that determine their properties and, ultimately, our ability to see and smell. This progress has been driven to a large degree by the powerful combination of physiological experimental tools and genetic manipulations, which has enabled us to identify the main molecular players in the transduction cascades of these sensory neurons, how their properties affect the detection and discrimination of stimuli, and how diseases affect our senses of vision and smell. This review summarizes some of the common and unique features of photoreceptors and olfactory sensory neurons that make these cells so exciting to study.

Stereochemical study of mouse muscone receptor MOR215-1 and vibrational theory based on statistical physics formalism

In the biosensor system, olfactory receptor sites could be activated by odorant molecules and then the biological interactions are converted into electrical signals by a signal transduction cascade that leads the toopening of ion channels, generating a current that leads into the cilia and depolarizes the membrane. The aim of this paper is to present a new investigation that allows determining the olfactory band using a monolayer adsorption with identical sites modeling which may also describe the static and the dynamic sensitivities through the expression of the olfactory response. Moreover, knowing the size of receptor site in olfactory sensory neurons provides valuable information about the relationship between molecular structure and biological activity. The determination of microreceptors and mesoreceptors is mostly carried out via physical adsorption and the radius is calculated using the Kelvin equation. The mean values of radius obtained from the maximum of the receptor size distributions peaks are 4 nm for ℓ-muscone and 6 nm for d-muscone.

RGS2 expression predicts amyloid-β sensitivity, MCI and Alzheimer's disease: genome-wide transcriptomic profiling and bioinformatics data mining

Alzheimer's disease (AD) is the most frequent cause of dementia. Misfolded protein pathological hallmarks of AD are brain deposits of amyloid-β (Aβ) plaques and phosphorylated tau neurofibrillary tangles. However, doubts about the role of Aβ in AD pathology have been raised as Aβ is a common component of extracellular brain deposits found, also by in vivo imaging, in non-demented aged individuals. It has been suggested that some individuals are more prone to Aβ neurotoxicity and hence more likely to develop AD when aging brains start accumulating Aβ plaques. Here, we applied genome-wide transcriptomic profiling of lymphoblastoid cells lines (LCLs) from healthy individuals and AD patients for identifying genes that predict sensitivity to Aβ. Real-time PCR validation identified 3.78-fold lower expression of RGS2 (regulator of G-protein signaling 2; P=0.0085) in LCLs from healthy individuals exhibiting high vs low Aβ sensitivity. Furthermore, RGS2 showed 3.3-fold lower expression (P=0.0008) in AD LCLs compared with controls. Notably, RGS2 expression in AD LCLs correlated with the patients' cognitive function. Lower RGS2 expression levels were also discovered in published expression data sets from postmortem AD brain tissues as well as in mild cognitive impairment and AD blood samples compared with controls. In conclusion, Aβ sensitivity phenotyping followed by transcriptomic profiling and published patient data mining identified reduced peripheral and brain expression levels of RGS2, a key regulator of G-protein-coupled receptor signaling and neuronal plasticity. RGS2 is suggested as a novel AD biomarker (alongside other genes) toward early AD detection and future disease modifying therapeutics.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

Activity-dependent and graded BACE1 expression in the olfactory epithelium is mediated by the retinoic acid metabolizing enzyme CYP26B1

It is well established that environmental influences play a key role in sculpting neuronal connectivity in the brain. One example is the olfactory sensory map of topographic axonal connectivity. While intrinsic odorant receptor signaling in olfactory sensory neurons (OSN) determines anterior-posterior counter gradients of the axonal guidance receptors Neuropilin-1 and Plexin-A1, little is known about stimulus-dependent gradients of protein expression, which correlates with the functional organization of the olfactory sensory map along its dorsomedial (DM)-ventrolateral (VL) axis. Deficiency of the Alzheimer's β-secretase BACE1, which is expressed in a DM(low)-VL(high) gradient, results in OSN axon targeting errors in a DM > VL and gene dose-dependent manner. We show that expression of BACE1 and the all-trans retinoic acid (RA)-degrading enzyme Cyp26B1 form DM-VL counter gradients in the olfactory epithelium. Analyses of mRNA and protein levels in OSNs after naris occlusion, in mice deficient in the olfactory cyclic nucleotide-gated channel and in relation to onset of respiration, show that BACE1 and Cyp26B1 expression in OSNs inversely depend on neuronal activity. Overexpression of a Cyp26B1 or presence of a dominant negative RA receptor transgene selectively in OSNs, inhibit BACE1 expression while leaving the DM(low)-VL(high) gradient of the axonal guidance protein Neuropilin-2 intact. We conclude that stimulus-dependent neuronal activity can control the expression of the RA catabolic enzyme Cyp26B1 and downstream genes such as BACE1. This result is pertinent to an understanding of the mechanisms by which a topographic pattern of connectivity is achieved and modified as a consequence of graded gene expression and sensory experience.

Single cell RT-PCR identification of odorant receptors expressed by olfactory neurons

Mammals have between 400 and 1,300 functional odorant receptor (OR) genes in their genomes. Each olfactory sensory neuron in the nose expresses only one single type of OR out of this vast repertoire. The OR expressed by an olfactory sensory neuron determines its functional activity and wiring to the olfactory bulb. Therefore, the identification of the OR type expressed by individual neurons contributes to the understanding of the mechanisms of odorant perception. Here we describe the protocol that combines single cell RT-PCR and degenerate PCR to identify the OR expressed by single olfactory neurons. In a primary PCR reaction, cDNAs corresponding to all mRNAs present in a single neuron are synthesized. In a secondary PCR reaction, the cDNAs corresponding to the OR expressed by this neuron are amplified using degenerate primers that match OR sequences.

Differential expression of RET receptor isoforms in the olfactory system

The glial cell line-derived neurotrophic factor (GDNF) family supports neurons by activating the tyrosine kinase receptor RET. The two main isoforms of RET, RET9 and RET51, differ in their carboxyl termini and have been implicated with distinct functions in the enteric and central nervous systems. Previously we reported the cellular localization of GDNF, neurturin and RET9 in the olfactory system [Maroldt H, Kaplinovsky T, Cunningham AM (2005) J Neurocytol 34:241-255]. In the current study, we examined immunohistochemical expression of RET9 and RET51 in neonatal and adult rat olfactory neuroepithelium (ON) and bulb to explore their potential functional roles. In the ON, RET9 was expressed by olfactory receptor neurons (ORNs) throughout the olfactory neuroepithelial sheet, whereas RET51 was restricted to ORNs situated in ventromedial and ventrolateral regions. Within these regions, RET51 was expressed by a subset of RET9-expressing ORNs. In olfactory bulb, RET9 expression was primarily on cell bodies, including olfactory ensheathing and periglomerular cells, and again, RET51 was expressed by a subset of RET9-expressing cells. RET51 was identified on axons in the olfactory nerve layer and glomerular neuropil, but only in the ventromedial and ventrolateral regions of the bulb. This regionalization correlated with the predicted axonal projection from expressing regions of the ON. RET51 was also expressed on dendrites in the external plexiform layer and glomerular neuropil. These results suggest RET9 may be the predominant functional isoform in the ON while RET51 plays a more selective role in a restricted region of the olfactory neuroepithelial sheet. In the bulb, RET9 is likely the main functional isoform while RET51 may be important in axonal and dendritic function/targeting.

Regional differences in olfactory epithelial homeostasis in the adult mouse

The olfactory sensory neurons in the nasal cavity of the adult mouse are organized into a few regions that differ in their molecular properties, as several classes of genes show regional expression. Most renowned is the fact that expression of each of hundreds of different odorant receptor genes is limited to one such region, or zone, of the olfactory neuroepithelial sheet. Zone differences are in place at birth, as exemplified here by the expression of neuronal progenitor marker Foxg1. We herein describe that an adult pattern showing regional differences in neurogenesis develops during the first few weeks of postnatal life which, e.g., is reflected in the temporal and regional regulation of the neuronal progenitor marker Ascl1. The most dorsomedial zone shows significantly fewer cells in S-phase in the adult but not in newborn mice by two different measures. Moreover, we show that there are regional differences in the relative differentiation, cell survival, and thickness of the olfactory epithelium. These findings are compatible with the view that zones are inherently distinct and that such differences contribute to generate regional differences in cellular homeostasis that in turn may modulate the capacity of a region to adjust to extrinsic influence.

Mechanisms of odorant receptor gene choice in Drosophila and vertebrates

Odorant receptors are encoded by extremely large and divergent families of genes. Each receptor is expressed in a small proportion of neurons in the olfactory organs, and each neuron in turn expresses just one odorant receptor gene. This fundamental property of the peripheral olfactory system is widely conserved across evolution, and observed in vertebrates, like mice, and invertebrates, like Drosophila, despite their olfactory receptor gene families being evolutionarily unrelated. Here we review the progress that has been made in these two systems to understand the intriguing and elusive question: how does a single neuron choose to express just one of many possible odorant receptors and exclude expression of all others?

Ric-8B interacts with G alpha olf and G gamma 13 and co-localizes with G alpha olf, G beta 1 and G gamma 13 in the cilia of olfactory sensory neurons

Olfactory sensory neurons are able to detect odorants with high sensitivity and specificity. We have demonstrated that Ric-8B, a guanine nucleotide exchange factor (GEF), interacts with Galphaolf and enhances odorant receptor signaling. Here we show that Ric-8B also interacts with Ggamma13, a divergent member of the Ggamma subunit family which has been implicated in taste signal transduction, and is abundantly expressed in the cilia of olfactory sensory neurons. We show that Gbeta1 is the predominant Gbeta subunit expressed in the olfactory sensory neurons. Ric-8B and Gbeta1, like Galphaolf and Ggamma13, are enriched in the cilia of olfactory sensory neurons. We also show that Ric-8B interacts with Galphaolf in a nucleotide dependent manner, consistent with the role as a GEF. Our results constitute the first example of a GEF protein that interacts with two different olfactory G protein subunits and further implicate Ric-8B as a regulator of odorant signal transduction.

WITHDRAWN: The Neural Cell Adhesion Molecule NCAM2/OCAM/RNCAM, a Close Relative to NCAM

Cell adhesion molecules (CAMs) constitute a large class of plasma membrane-anchored proteins that mediate attachment between neighboring cells and between cells and the surrounding extracellular matrix (ECM). However, CAMs are more than simple mediators of cell adhesion. The neural cell adhesion molecule (NCAM) is a well characterized, ubiquitously expressed CAM that is highly expressed in the nervous system. In addition to mediating cell adhesion, NCAM participates in a multitude of cellular events, including survival, migration, and differentiation of cells, outgrowth of neurites, and formation and plasticity of synapses. NCAM shares an overall sequence identity of approximately 44% with the neural cell adhesion molecule 2 (NCAM2), a protein also known as olfactory cell adhesion molecule (OCAM) and Rb-8 neural cell adhesion molecule (RNCAM), and the region-for-region sequence homology between the two proteins suggests that they are transcribed from paralogous genes. However, very little is known about the function of NCAM2, although it originally was described more than 20 years ago. In this review we summarize the known properties and functions of NCAM2 and describe some of the differences and similarities between NCAM and NCAM2.

Galphao/i-stimulated proteosomal degradation of RGS20: a mechanism for temporal integration of Gs and Gi pathways

The G(s) and G(i) pathways interact to control the levels of intracellular cAMP. Although coincident signaling through G(s) and G(i)-coupled receptors can attenuate G(s)-stimulated cAMP levels, it is not known if prior activation of the G(i) pathway can affect signaling by G(s)-coupled receptors. We have found that activated Galpha(o/i) interact with RGS20, a GTPase activating protein for members of the Galpha(omicron/i) family. Interaction between Galpha(o/i) and RGS20 results in decreased cellular levels of RGS20. This decrease was induced by activated Galpha(o) and Galpha(i2) but not by Galpha(q), Galpha(i1) or Galpha(i3.) The Galpha(o/i)-induced decrease in RGS20 can be blocked by proteasomal inhibitors lactacystin or MG132. Activated Galpha(o) stimulates the ubiquitination of RGS20. The serotonin-1A receptor that couples to G(o/i) reduces the levels of RGS20 and this effect is blocked by lactacystin, suggesting that G(o/i) promotes the degradation of RGS20. Expression of RGS20 attenuates the inhibition of beta-adrenergic receptor-induced cAMP levels mediated by the serotonin-1A receptor. Prior activation of the serotonin-1A receptor results in loss of the RGS20-mediated attenuation, and the loss of attenuation is blocked when lactacystin is included during the prior treatment. These observations suggest that G(o/i)-coupled receptors, by stimulating the degradation of RGS20, can regulate how subsequent activation of the G(s) and G(i) pathways controls cellular cAMP levels, thus allowing for signal integration.

Requirement for Slit-1 and Robo-2 in zonal segregation of olfactory sensory neuron axons in the main olfactory bulb

The formation of precise stereotypic connections in sensory systems is critical for the ability to detect and process signals from the environment. In the olfactory system, olfactory sensory neurons (OSNs) project axons to spatially defined glomeruli within the olfactory bulb (OB). A spatial relationship exists between the location of OSNs within the olfactory epithelium (OE) and their glomerular targets along the dorsoventral axis in the OB. The molecular mechanisms underlying the zonal segregation of OSN axons along the dorsoventral axis of the OB are poorly understood. Using robo-2(-/-) (roundabout) and slit-1(-/-) mice, we examined the role of the Slit family of axon guidance cues in the targeting of OSN axons during development. We show that a subset of OSN axons that normally project to the dorsal region of the OB mistarget and form glomeruli in the ventral region in robo-2(-/-) and slit-1(-/-) mice. In addition, we show that the Slit receptor, Robo-2, is expressed in OSNs in a high dorsomedial to low ventrolateral gradient across the OE and that Slit-1 and Slit-3 are expressed in the ventral region of the OB. These results indicate that the dorsal-to-ventral segregation of OSN axons are not solely defined by the location of OSNs within the OE but also relies on axon guidance cues.

Sniffing and spatiotemporal coding in olfaction

The act of sniffing increases the air velocity and changes the duration of airflow in the nose. It is not yet clear how these changes interact with the intrinsic timing within the olfactory bulb, but this is a matter of current research activity. An action of sniffing in generating a high velocity that alters the sorption of odorants onto the lining of the nasal cavity is expected from the established work on odorant properties and sorption in the frog nose. Recent work indicates that the receptor properties in the olfactory epithelium and olfactory bulb are correlated with the receptor gene expression zones. The responses in both the epithelium and the olfactory bulb are predictable to a considerable extent by the hydrophobicity of odorants. Furthermore, receptor expression in both rodent and salamander nose interacts with the shapes of the nasal cavity to place the receptor sensitivity to odorants in optimal places according to the aerodynamic properties of the nose.

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.

Differential requirements for semaphorin 3F and Slit-1 in axonal targeting, fasciculation, and segregation of olfactory sensory neuron projections

The formation of precise stereotypic connections in sensory systems is critical for defining accurate internal representations of the external world; however, the molecular mechanisms underlying the formation of sensory maps are poorly understood. Here, we examine the roles of two structurally unrelated repulsive guidance cues, semaphorin 3F (Sema3F) and Slit-1, in olfactory sensory axon fasciculation, targeting, and segregation. Using sema3F-/- mice, we show that Sema3F is critical for vomeronasal sensory neuron axonal fasciculation and for segregation of these sensory afferents from the main olfactory system; however, Sema3F plays only a minor role in targeting of apical vomeronasal neuron axons to the anterior accessory olfactory bulb (AOB). In addition, we show that Sema3F is required for lamina-specific targeting of olfactory sensory axons within the main olfactory system. In contrast to Sema3F, Slit-1 is dispensable for fasciculation of basal vomeronasal neuron axons but is critical for targeting these axons to the posterior AOB. These results reveal discrete and complementary roles for secreted semaphorins and slits in axonal targeting, fasciculation, and segregation of olfactory sensory neuron projections.

Zonal ablation of the olfactory sensory neuroepithelium of the mouse: effects on odorant detection

Olfactory sensory neurons that express a specific odorant receptor, out of a thousand different, are unevenly distributed within, but restricted to one of four zones of the neuroepithelial sheet in the nasal cavity in the mouse. This zonal restriction of neurons expressing the same odorant receptor may have consequences, e.g. in case of localized injury. We found that the chemical dichlobenil can produce specific and permanent ablation of neurons in odorant receptor expression zone 1, while a higher dichlobenil dose causes reversible toxicity in neighboring zones. In behavior tests, mice lacking part of the olfactory epithelium had an increased detection threshold concentration of two-four orders of magnitude for some odorants but not others, resembling the phenomenon of specific hyposmia. This indicates that the broad tuning properties of single odorant receptors and their large number cannot fully compensate for loss of the receptor(s) with the highest sensitivity for a particular odorant.

Zonal expression and activity of glutathione S-transferase enzymes in the mouse olfactory mucosa

The rodent olfactory mucosa is characterized by a mosaic of gene expression that is exhibited among various cell types. Olfactory sensitivity in these animals is conveyed through odorant receptor families that are distinctly expressed within various subsets of the olfactory neuron population. Receptor neurons that express a particular class of odorant receptors exhibit bilaterally symmetric zones, which generally define their location within the nasal cavity. Less characterized are zonal expression profiles of proteins among non-neuronal cell types of the olfactory mucosa. In this study, we survey the expression of three glutathione S-transferase (GST) isozymes (alpha, mu, and pi) in the mouse olfactory mucosa and characterize the zonal expression of the mu isozyme. Immunohistochemistry and Western blot analysis of the GST mu isozyme reveal that the lateral olfactory turbinates I, Ib, II, IIb, and III display a greater intensity of expression for GST mu, in comparison to the dorsal and septal regions of the mucosa. GST alpha and pi isozymes do not display any distinct zonal organization in olfactory tissue of the adult mouse. When the general substrate 1-chloro-2-4-dinitrobenzene (CDNB) was used to assess GST activity within the olfactory tissue, the lateral turbinate regions displayed a higher level of activity when compared to dorsal or septal regions. Analysis of GST mu expression in prenatal and early postnatal olfactory tissue also reveals a zonal expression of the isozyme. We relate the significance of these findings to metabolic topography and olfactory chemosensory function.

The worm's sense of smell

Animals sense their chemical environment using multiple chemosensory neuron types, each of which exhibits characteristic response properties. The chemosensory neurons of the nematode Caenorhabditis elegans provide an excellent system in which to explore the developmental mechanisms giving rise to this functional diversity. In this review, we discuss the principles underlying the patterning, generation, differentiation, and diversification of chemosensory neuron subtypes in C. elegans. Current knowledge of the molecular mechanisms underlying each of these individual steps is derived from work in different model organisms. It is essential to describe the complete developmental pathways in each organism to determine whether functional diversification in chemosensory systems is achieved via conserved or novel mechanisms. Such a complete description may be possible in C. elegans.

High basal expression and injury-induced down regulation of two regulator of G-protein signaling transcripts, RGS3 and RGS4 in primary sensory neurons

The regulators of G-protein signaling (RGS) proteins are a family of intracellular modulators of G-protein coupled receptor (GPCR) sensitivity. They act as GTPase accelerating proteins returning the Galpha subunit back to an inactive latent state. We find that RGS3 and RGS4 are constitutively expressed at high levels in C-fiber primary sensory neurons in the adult rat dorsal root ganglion (DRG) and transection of the sciatic nerve results in a substantial down regulation of these transcripts. RGS4 mRNA is expressed only in GDNF-responsive neurons and GDNF supports the expression of this transcript in primary DRG cultures. A PDZ domain containing subtype of RGS3 is the most abundant and regulated form of this protein within the DRG. Decreased levels of RGS3 and RGS4 in injured sensory neurons is likely to result in an increased GPCR sensitivity, and therefore contribute to alterations in cellular function seen after such lesions.

The aorta and heart differentially express RGS (regulators of G-protein signalling) proteins that selectively regulate sphingosine 1-phosphate, angiotensin II and endothelin-1 signalling

Normal cardiovascular development and physiology depend in part upon signalling through G-protein-coupled receptors (GPCRs), such as the angiotensin II type 1 (AT(1)) receptor, sphingosine 1-phosphate (S1P) receptors and endothelin-1 (ET-1) receptor. Since regulator of G-protein signalling (RGS) proteins function as GTPase-activating proteins for the G alpha subunit of heterotrimeric G-proteins, these proteins undoubtedly have functional roles in the cardiovascular system. In the present paper, we show that human aorta and heart differentially express RGS1, RGS2, RGS3S (short-form), RGS3L (long-form), PDZ-RGS3 (PDZ domain-containing) and RGS4. The aorta prominently expresses mRNAs for all these RGS proteins except PDZ-RGS3. Various stimuli that are critical for both cardiovascular development and function regulate dynamically the mRNA levels of several of these RGS proteins in primary human aortic smooth muscle cells. Both RGS1 and RGS3 inhibit signalling through the S1P(1) (formerly known as EDG-1), S1P(2) (formerly known as EDG-5) and S1P(3) (formerly known as EDG-3) receptors, whereas RGS2 and RGS4 selectively attenuate S1P(2)-and S1P(3)-receptor signalling respectively. All of the tested RGS proteins inhibit AT(1)-receptor signalling, whereas only RGS3 and, to a lesser extent, RGS4 inhibit ET(A)-receptor signalling. The conspicuous expression of RGS proteins in the cardiovascular system and their selective effects on relevant GPCR-signalling pathways provide additional evidence that they have functional roles in cardiovascular development and physiology.

Selective G protein beta gamma-subunit compositions mediate phospholipase C activation in the vomeronasal organ

Chemosensory neurons of the vomeronasal organ (VNO) are supposed to detect pheromones controlling social and reproductive behavior in most terrestrial vertebrates. Recent studies indicate that pheromone signaling in VNO neurons is mediated via phospholipase C (PLC) activation generating the two second messengers inositol-1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Since G alpha(i) and G alpha(o) predominantly expressed in VNO neurons are usually not involved in activating PLC, it was explored if PLC activation may be mediated by G beta gamma subunits. It was found that a scavenger for beta gamma dimers reduced the urine-induced IP3 formation in VNO preparations in a dose-dependent manner indicating a role for G beta gamma complexes. Towards an identification of the relevant G beta and G gamma subunit(s), PCR approaches as well as immunohistochemical experiments were performed. It was found that out of the five known G beta subtypes, only G beta2 was expressed in both G alpha(i) as well as G alpha(o) neurons. Experimental approaches focusing on the spatial expression profile of identified G gamma subtypes revealed that G gamma8-positive neurons are preferentially localized to the basal region of the vomeronasal epithelium, whereas G gamma2-reactive cells are restricted to the apical G alpha(i)-positive layer of the sensory epithelium. As IP3 formation induced upon stimulation with volatile urinary compounds was selectively blocked by G gamma2-specific antibodies whereas second messenger formation elicited upon stimulation with alpha2u globulin was inhibited by antibodies recognizing G gamma8, it is conceivable that PLC activation in the two populations of chemosensory VNO neurons is mediated by different G beta gamma complexes.

Functional characterization of the G protein regulator RGS13

The signaling cascades evoked by G protein-coupled receptors are a predominant mechanism of cellular communication. The regulators of G protein signaling (RGS) comprise a family of proteins that attenuate G protein-mediated signal transduction. Here we report the characterization of RGS13, the smallest member of the RGS family, which has been cloned from human lung. RGS13 has been found most abundantly in human tonsil, followed by thymus, lung, lymph node, and spleen. RGS13 is a GTPase-activating protein for Galpha(i) and Galpha(o) but not Galpha(s). RGS13 binds Galpha(q) in the presence of aluminum magnesium fluoride, suggesting that it bears GTPase-activating protein activity toward Galpha(q). RGS13 blocks MAPK activity induced by Galpha(i)- or Galpha(q)-coupled receptors. RGS13 also attenuates GTPase-deficient Galpha(q) (Galpha(q)QL) mediated cAMP response element activation but not transcription evoked by constitutively active Galpha(12) or Galpha(13). Surprisingly, RGS13 inhibits cAMP generation elicited by stimulation of the beta(2)-adrenergic receptor. These data suggest that RGS13 may regulate Galpha(i)-, Galpha(q)-, and Galpha(s)-coupled signaling cascades.

Multiple new and isolated families within the mouse superfamily of V1r vomeronasal receptors

Seven-transmembrane-domain proteins encoded by the vomeronasal receptor V1r and V2r gene superfamilies, and expressed by vomeronasal sensory neurons, are believed to be pheromone receptors in rodents. Four V1r gene families have been described in the mouse (V1ra, V1rb, V1rc and V3r). Here we have screened near-complete mouse genomic databases to obtain a first global draft of the mouse V1r repertoire, including 104 new V1r genes. It comprises eight new and extremely isolated families in addition to the four families previously identified. Members of these new families were expressed in vomeronasal sensory neurons. The genome-wide view revealed great sequence diversity within the V1r superfamily. Phylogenetic analyses suggested an ancient original radiation, followed by the isolation, divergence and expansion of families by extensive gene duplications and frequent gene loss. The isolated nature of these gene families probably reflects a specialization of different receptor classes in the detection of specific types of chemicals.

Mammalian chemosensory receptors

Chemosensory receptors are critical for the survival of many mammalian species, and their genes can comprise up to 1% of mammalian genomes. Odorant, taste, and vomeronasal receptors are being discovered and functionally characterized at a rapid pace which has been further accelerated by the availability of the human genome sequence. Five multigene families, consisting of >1,000 genes in the mouse, have been proposed to encode functional chemoreceptors. Although all of the chemoreceptor gene families encode G-protein coupled receptors, they are largely unrelated and uniquely specialized for the processing of different chemosensory modalities. Using members of the families as molecular probes, great insights are being gained into the different organizational strategies used by these sensory systems to encode information in both the periphery and the brain.

How smell develops

The mouse's sense of smell is built of approximately 1000 input channels. Each of these consists of a population of olfactory sensory neurons that express the same odorant receptor gene and project their axons to the same targets (glomeruli) in the olfactory bulb. A neuron must choose to express a singular receptor gene from a repertoire of approximately 1000 genes, and its axon must be wired to the corresponding glomerulus, from an array of approximately 1800 glomeruli. Genetic experiments have shown that the expressed odorant receptor specifies axonal choice of the innervated glomerulus, but it is not the only determinant. The mechanisms of odorant receptor gene choice and axonal wiring are central to the functional organization of the mammalian olfactory system. Although principles have emerged, our understanding of these processes is still limited.