Sexual dimorphism in olfactory signaling

Do rodents smell with sound?

Chemosensation via olfaction is a critical process underlying social interactions in many different species. Past studies of olfaction in mammals often have focused on its mechanisms in isolation from other systems, limiting the generalizability of findings from olfactory research to perceptual processes in other modalities. Studies of chemical communication, in particular, have progressed independently of research on vocal behavior and acoustic communication. Those bioacousticians who have considered how sound production and reception might interact with olfaction often portray odors as cues to the kinds of vocalizations that might be functionally useful. In the olfaction literature, vocalizations are rarely mentioned. Here, we propose that ultrasonic vocalizations may affect what rodents smell by altering the deposition of inhaled particles and that rodents coordinate active sniffing with sound production specifically to enhance reception of pheromones. In this scenario, rodent vocalizations may contribute to a unique mode of active olfactory sensing, in addition to whatever roles they serve as social signals. Consideration of this hypothesis highlights the perceptual advantages that parallel coordination of multiple sensorimotor processes may provide to individuals exploring novel situations and environments, especially those involving dynamic social interactions.

Chemical communication and its role in sexual selection across Animalia

Sexual selection has been studied as a major evolutionary driver of animal diversity for roughly 50 years. Much evidence indicates that competition for mates favors elaborate signaling traits. However, this evidence comes primarily from a few taxa, leaving sexual selection as a salient evolutionary force across Animalia largely untested. Here, we reviewed the evidence for sexual selection on communication across all animal phyla, classes, and orders with emphasis on chemoreception, the only sense shared across lifeforms. An exhaustive literature review documented evidence for sexual selection on chemosensory traits in 10 of 34 animal phyla and indications of sexual selection on chemosensory traits in an additional 13 phyla. Potential targets of sexual selection include structures and processes involved in production, delivery, and detection of chemical signals. Our review suggests sexual selection plays a widespread role in the evolution of communication and highlights the need for research that better reflects animal diversity.

Pheromone Perception in Fish: Mechanisms and Modulation by Internal Status

Pheromones are chemical signals that facilitate communication between animals, and most animals use pheromones for reproduction and other forms of social behavior. The identification of key ligands and olfactory receptors used for pheromonal communication provides insight into the sensory processing of these important cues. An individual's responses to pheromones can be plastic, as physiological status modulates behavioral outputs. In this review, we outline the mechanisms for pheromone sensation and highlight physiological mechanisms that modify pheromone-guided behavior. We focus on hormones, which regulate pheromonal communication across vertebrates including fish, amphibians, and rodents. This regulation may occur in peripheral olfactory organs and the brain, but the mechanisms remain unclear. While this review centers on research in fish, we will discuss other systems to provide insight into how hormonal mechanisms function across taxa.

Variation in mouse chemical signals is genetically controlled and environmentally modulated

In most mammals and particularly in mice, chemical communication relies on the detection of ethologically relevant fitness-related cues from other individuals. In mice, urine is the primary source of these signals, so we employed proteomics and metabolomics to identify key components of chemical signalling. We show that there is a correspondence between urinary volatiles and proteins in the representation of genetic background, sex and environment in two house mouse subspecies Mus musculus musculus and M. m. domesticus. We found that environment has a strong influence upon proteomic and metabolomic variation and that volatile mixtures better represent males while females have surprisingly more sex-biased proteins. Using machine learning and combined-omics techniques, we identified mixtures of metabolites and proteins that are associated with biological features.

Cupid's quiver: Integrating sensory cues in rodent mating systems

In many animal species, males and females exploit different mating strategies, display sex-typical behaviors, and use distinct systems to recognize ethologically relevant cues. Mate selection thus requires mutual recognition across diverse social interactions based on distinct sensory signals. These sex differences in courtship and mating behaviors correspond to differences in sensory systems and downstream neural substrates engaged to recognize and respond to courtship signals. In many rodents, males tend to rely heavily on volatile olfactory and pheromone cues, while females appear to be guided more by a combination of these chemosensory signals with acoustic cues in the form of ultrasonic vocalizations. The mechanisms by which chemical and acoustic cues are integrated to control behavior are understudied in mating but are known to be important in the control of maternal behaviors. Socially monogamous species constitute a behaviorally distinct group of rodents. In these species, anatomic differences between males and females outside the nervous system are less prominent than in species with non-monogamous mating systems, and both sexes engage in more symmetric social behaviors and form attachments. Nevertheless, despite the apparent similarities in behaviors displayed by monogamous males and females, the circuitry supporting social, mating, and attachment behaviors in these species is increasingly thought to differ between the sexes. Sex differences in sensory modalities most important for mate recognition in across species are of particular interest and present a wealth of questions yet to be answered. Here, we discuss how distinct sensory cues may be integrated to drive social and attachment behaviors in rodents, and the differing roles of specific sensory systems in eliciting displays of behavior by females or males.

One for all and all for one: phenotype assortment and reproductive success in masculinized females

Homophily by morphological and behavioral traits has been described in several species of vertebrates, but its functional consequences remain poorly studied. Homophily by plurally breeding females may improve direct fitness by enhancing reproductive success. Female mammals may exhibit phenotypical masculinization due to exposure to androgens during early development, a condition that is associated with maternal performance during subsequent breeding. Our goal was to assess whether female composition (in terms of masculinization) of plurally breeding groups influences female fitness in a natural population of degus (Octodon degus). We assessed if plurally breeding female degus assort themselves by anogenital distance (AGD), an accurate measure of masculinization level. We also quantified if homophily by AGD phenotype affects female reproductive success and the reproductive output of the group. Plurally breeding groups typically included similarly masculinized (i.e., long AGD) females or similarly feminized (short AGD) females, indicating a strong degree of homophily. Females weaned more offspring in plurally breeding groups with more masculinized females. Additionally, standardized variance in the number of offspring weaned decreased in plurally breeding groups with mostly masculinized females, indicating greater reproductive equality in these groups. We conclude that female degus organize into homophilic social groups of similar AGD, and that social groups of masculinized females exhibit a higher reproductive success.

The molecular basis of OH-PCB estrogen receptor activation

Polychlorinated bisphenols (PCBs) continue to contaminate food chains globally where they concentrate in tissues and disrupt the endocrine systems of species throughout the ecosphere. Hydroxylated PCBs (OH-PCBs) are major PCB metabolites and high-affinity inhibitors of human estrogen sulfotransferase (SULT1E1), which sulfonates estrogens and thus prevents them from binding to and activating their receptors. OH-PCB inhibition of SULT1E1 is believed to contribute significantly to PCB-based endocrine disruption. Here, for the first time, the molecular basis of OH-PCB inhibition of SULT1E1 is revealed in a structure of SULT1E1 in complex with OH-PCB1 (4ʹ-OH-2,6-dichlorobiphenol) and its substrates, estradiol (E2), and PAP (3’-phosphoadenosine-5-phosphosulfate). OH-PCB1 prevents catalysis by intercalating between E2 and catalytic residues and establishes a new E2-binding site whose E2 affinity and positioning are greater than and competitive with those of the reactive-binding pocket. Such complexes have not been observed previously and offer a novel template for the design of high-affinity inhibitors. Mutating residues in direct contact with OH-PCB weaken its affinity without compromising the enzyme’s catalytic parameters. These OH-PCB resistant mutants were used in stable transfectant studies to demonstrate that OH-PCBs regulate estrogen receptors in cultured human cell lines by binding the OH-PCB binding pocket of SULT1E1.

Transcriptome Analysis of Zebrafish Olfactory Epithelium Reveal Sexual Differences in Odorant Detection

Animals have evolved a large number of olfactory receptor genes in their genome to detect numerous odorants in their surrounding environments. However, we still know little about whether males and females possess the same abilities to sense odorants, especially in fish. In this study, we used deep RNA sequencing to examine the difference of transcriptome between male and female zebrafish olfactory epithelia. We found that the olfactory transcriptomes between males and females are highly similar. We also found evidence of some genes showing differential expression or alternative splicing, which may be associated with odorant-sensing between sexes. Most chemosensory receptor genes showed evidence of expression in the zebrafish olfactory epithelium, with a higher expression level in males than in females. Taken together, our results provide a comprehensive catalog of the genes mediating olfactory perception and pheromone-evoked behavior in fishes.

Synchronous Infra-Slow Oscillations Organize Ensembles of Accessory Olfactory Bulb Projection Neurons into Distinct Microcircuits

The accessory olfactory system controls social and sexual behavior. In the mouse accessory olfactory bulb, the first central stage of information processing along the accessory olfactory pathway, projection neurons (mitral cells) display infra-slow oscillatory discharge with remarkable periodicity. The physiological mechanisms that underlie this default output state, however, remain controversial. Moreover, whether such rhythmic infra-slow activity patterns exist in awake behaving mice and whether such activity reflects the functional organization of the accessory olfactory bulb circuitry remain unclear. Here, we hypothesize that mitral cell ensembles form synchronized microcircuits that subdivide the accessory olfactory bulb into segregated functional clusters. We use a miniature microscope to image the Ca²⁺ dynamics within the apical dendritic compartments of large mitral cell ensembles in vivo. We show that infra-slow periodic patterns of concerted neural activity, indeed, reflect the idle state of accessory olfactory bulb output in awake male and female mice. Ca²⁺ activity profiles are distinct and glomerulus-specific. Confocal time-lapse imaging in acute slices reveals that groups of mitral cells assemble into microcircuits that exhibit correlated Ca²⁺ signals. Moreover, electrophysiological profiling of synaptic connectivity indicates functional coupling between mitral cells. Our results suggest that both intrinsically rhythmogenic neurons and neurons entrained by fast synaptic drive are key elements in organizing the accessory olfactory bulb into functional microcircuits, each characterized by a distinct default pattern of infra-slow rhythmicity.

SIGNIFICANCE STATEMENT Information processing in the accessory olfactory bulb (AOB) plays a central role in conspecific chemosensory communication. Surprisingly, many basic physiological principles that underlie neuronal signaling in the AOB remain elusive. Here, we show that AOB projection neurons (mitral cells) form parallel synchronized ensembles both in vitro and in vivo. Infra-slow synchronous oscillatory activity within AOB microcircuits thus adds a new dimension to chemosensory coding along the accessory olfactory pathway.

Genetic Perturbation of TIA1 Reveals a Physiological Role in Fear Memory

TIA1 is a prion-related RNA-binding protein whose capacity to form various types of intracellular aggregates has been implicated in neurodegenerative disease. However, its role in normal brain function is poorly understood. Here, we show that TIA1 bidirectionally modulates stress-dependent synaptic plasticity in the hippocampus, a brain region involved in fear memory and olfactory discrimination learning. At the behavioral level, conditioned odor avoidance is potentiated by TIA1 deletion, whereas overexpression of TIA1 in the ventral hippocampus inhibits both contextual fear memory and avoidance. However, the latter genetic manipulations have little impact on other hippocampus-dependent tasks. Transcriptional profiling indicates that TIA1 presides over a large network of immune system genes with modulatory roles in synaptic plasticity and long-term memory. Our results uncover a physiological and partly sex-dependent function for TIA1 in fear memory and may provide molecular insight into stress-related psychiatric conditions, such as post-traumatic stress disorder (PTSD) and anxiety.

Sensorimotor developmental factors influencing the performance of laboratory rodents on learning and memory

Behavioral studies in animal models have advanced our knowledge of brain function and the neural mechanisms of human diseases. Commonly used laboratory rodents, such as mice and rats, provide a useful tool for studying the behaviors and mechanisms associated with learning and memory processes which are cooperatively regulated by multiple underlying factors, including sensory and motor performance and emotional/defense innate components. Each of these factors shows unique ontogeny and governs the sustainment of behavioral performance in learning tasks, and thus, understanding the integrative processes of behavioral development are crucial in the accurate interpretation of the functional meaning of learning and memory behaviors expressed in commonly employed behavioral test paradigms. In this review, we will summarize the major findings in the developmental processes of rodent behavior on the basis of the emergence of fundamental components for sustaining learning and memory behaviors. Briefly, most sensory modalities (except for vision) and motor abilities are functional at the juvenile stage, in which several defensive components, including active and passive defensive strategies and risk assessment behavior, emerge. Sex differences are detectable from the juvenile stage through adulthood and are considerable factors that influence behavioral tests. The test paradigms addressed in this review include associative learning (with an emphasis on fear conditioning), spatial learning, and recognition. This basic background information will aid in accurately performing behavioral studies in laboratory rodents and will therefore contribute to reducing inappropriate interpretations of behavioral data and further advance research on learning and memory in rodent models.

Glia-mediated modulation of extracellular potassium concentration determines the sexually dimorphic output frequency of a model brainstem oscillator

Sexual dimorphism in behavior is widespread among animals, but the cellular mechanisms underlying neural control of this phenomenon are largely unknown. One behavior that has provided some important clues about how such sex differences might develop is the electric organ discharge of Apteronotus leptorhynchus. In this weakly electric fish, the mean discharge frequencies of males and females are 880 Hz and 740 Hz, respectively, with little overlap of the two frequency bands. The discharges are controlled, in a one-to-one fashion, by the neural oscillations of the pacemaker nucleus in the medulla oblongata. Experimental evidence has shown that the astrocytic syncytium associated with the neural network that generates these oscillations is significantly larger, and stronger coupled via gap junctions, in females than in males. In the present study, modeling of this network was performed to test the hypotheses that the sex-dependent differences in the structure and properties of the astrocytic syncytium mediate better buffering of extracellular potassium in females than in males, which in turn causes, via a lowering of the potassium equilibrium potential, a decrease in the oscillation frequency. Simulations of the neural activity of the pacemaker nucleus and its individual components demonstrated that under both spontaneous and induced conditions the oscillation frequency and the potassium equilibrium potential are strongly positively correlated. These simulations predict that sufficient separation of the electric organ discharge frequencies for establishment of the sexual dimorphism can be achieved by rather minor alterations in the concentration of the extracellular potassium concentration in the pacemaker nucleus.

Sex separation induces differences in the olfactory sensory receptor repertoires of male and female mice

Within the mammalian olfactory sensory epithelium, experience-dependent changes in the rate of neuronal turnover can alter the relative abundance of neurons expressing specific chemoreceptors. Here we investigate how the mouse olfactory sensory receptor repertoire changes as a function of exposure to odors emitted from members of the opposite sex, which are highly complex and sexually dimorphic. Upon housing mice either sex-separated or sex-combined until six months of age, we find that sex-separated mice exhibit significantly more numerous differentially expressed genes within their olfactory epithelia. A subset of these chemoreceptors exhibit altered expression frequencies following both sex-separation and olfactory deprivation. We show that several of these receptors detect either male- or female-specific odors. We conclude that the distinct odor experiences of sex-separated male and female mice induce sex-specific differences in the abundance of neurons that detect sexually dimorphic odors.

Isoform-specific therapeutic control of sulfonation in humans

The activities of hundreds, perhaps thousands, of metabolites are regulated by human cytosolic sulfotransferases (SULTs) - a 13-member family of disease relevant enzymes that catalyze transfer of the sulfuryl moiety (-SO₃) from PAPS (3'-phosphoadenosine 5'-phosphosulfonate) to the hydroxyls and amines of acceptors. SULTs harbor two independent allosteric sites, one of which, the focus of this work, binds non-steroidal anti-inflammatory drugs (NSAIDs). The structure of the first NSAID-binding site - that of SULT1A1 - was elucidated recently and homology modeling suggest that variants of the site are present in all SULT isoforms. The objective of the current study was to assess whether the NSAID-binding site can be used to regulate sulfuryl transfer in humans in an isoform specific manner. Mefenamic acid (Mef) is a potent (K_i 27 nM) NSAID-inhibitor of SULT1A1 - the predominant SULT isoform in small intestine and liver. Acetaminophen (APAP), a SULT1A1 specific substrate, is extensively sulfonated in humans. Dehydroepiandrosterone (DHEA) is specific for SULT2A1, which we show here is insensitive to Mef inhibition. APAP and DHEA sulfonates are readily quantified in urine and thus the effects of Mef on APAP and DHEA sulfonation could be studied non-invasively. Compounds were given orally in a single therapeutic dose to a healthy, adult male human with a typical APAP-metabolite profile. Mef profoundly decreased APAP sulfonation during first pass metabolism and substantially decreased systemic APAP sulfonation without influencing DHEA sulfonation; thus, it appears the NSAID site can be used to control sulfonation in humans in a SULT-isoform specific manner.

Neural circuits regulating sexual behaviors via the olfactory system in mice

Reproduction is essential for any animal species. Reproductive behaviors, or sexual behaviors, are largely shaped by external sensory cues exchanged during sexual interaction. In many animals, including rodents, olfactory cues play a critical role in regulating sexual behavior. What exactly these olfactory cues are and how they impact animal behavior have been a central question in the field. Over the past few decades, many studies have dedicated to identifying an active compound that elicits sexual behavior from crude olfactory components. The identified substance has served as a tool to dissect the sensory processing mechanisms in the olfactory systems. In addition, recent advances in genetic engineering, and optics and microscopic techniques have greatly expanded our knowledge of the neural mechanisms underlying the control of sexual behavior in mice. This review summarizes our current knowledge about how sexual behaviors are controlled by olfactory cues.

PDF-1 neuropeptide signaling regulates sexually dimorphic gene expression in shared sensory neurons of C. elegans

Sexually dimorphic behaviors are a feature common to species across the animal kingdom, however how such behaviors are generated from mostly sex-shared nervous systems is not well understood. Building on our previous work which described the sexually dimorphic expression of a neuroendocrine ligand, DAF-7, and its role in behavioral decision-making in C. elegans (Hilbert and Kim, 2017), we show here that sex-specific expression of daf-7 is regulated by another neuroendocrine ligand, Pigment Dispersing Factor (PDF-1), which has previously been implicated in regulating male-specific behavior (Barrios et al., 2012). Our analysis revealed that PDF-1 signaling acts sex- and cell-specifically in the ASJ neurons to regulate the expression of daf-7, and we show that differences in PDFR-1 receptor activity account for the sex-specific effects of this pathway. Our data suggest that modulation of the sex-shared nervous system by a cascade of neuroendocrine signals can shape sexually dimorphic behaviors.

Tissue-specific transcriptome analyses provide new insights into GPCR signalling in adult Schistosoma mansoni

Schistosomes are blood-dwelling trematodes with global impact on human and animal health. Because medical treatment is currently based on a single drug, praziquantel, there is urgent need for the development of alternative control strategies. The Schistosoma mansoni genome project provides a platform to study and connect the genetic repertoire of schistosomes to specific biological functions essential for successful parasitism. G protein-coupled receptors (GPCRs) form the largest superfamily of transmembrane receptors throughout the Eumetazoan phyla, including platyhelminths. Due to their involvement in diverse biological processes, their pharmacological importance, and proven druggability, GPCRs are promising targets for new anthelmintics. However, to identify candidate receptors, a more detailed understanding of the roles of GPCR signalling in schistosome biology is essential. An updated phylogenetic analysis of the S. mansoni GPCR genome (GPCRome) is presented, facilitated by updated genome data that allowed a more precise annotation of GPCRs. Additionally, we review the current knowledge on GPCR signalling in this parasite and provide new insights into the potential roles of GPCRs in schistosome reproduction based on the findings of a recent tissue-specific transcriptomic study in paired and unpaired S. mansoni. According to the current analysis, GPCRs contribute to gonad-specific functions but also to nongonad, pairing-dependent processes. The latter may regulate gonad-unrelated functions during the multifaceted male-female interaction. Finally, we compare the schistosome GPCRome to that of another parasitic trematode, Fasciola, and discuss the importance of GPCRs to basic and applied research. Phylogenetic analyses display GPCR diversity in free-living and parasitic platyhelminths and suggest diverse functions in schistosomes. Although their roles need to be substantiated by functional studies in the future, the data support the selection of GPCR candidates for basic and applied studies, invigorating the exploitation of this important receptor class for drug discovery against schistosomes but also other trematodes.

Controlling Sulfuryl-Transfer Biology

In humans, the cytosolic sulfotransferases (SULTs) catalyze regiospecific transfer of the sulfuryl moiety (-SO3) from 3'-phosphoadenosine 5'-phosphosulfate to thousands of metabolites, including numerous signaling small molecules, and thus regulates their activities and half-lives. Imbalances in the in vivo set points of these reactions leads to disease. Here, with the goal of controlling sulfonation in vivo, molecular ligand-recognition principles in the SULT and nuclear receptor families are integrated in creating a strategy that can prevent sulfonation of a compound without significantly altering its receptor affinity, or inhibiting SULTS. The strategy is validated by using it to control the sulfonation and estrogen receptor (ER) activating activity of raloxifene (a US Food and Drug Administration-approved selective estrogen receptor modulator) and its derivatives. Preventing sulfonation is shown to enhance ER-activation efficacy 10(4)-fold in studies using Ishikawa cells. The strategy offers the opportunity to control sulfuryl transfer on a compound-by-compound basis, to enhance the efficacy of sulfonated drugs, and to explore the biology of sulfuryl transfer with unprecedented precision.

Experience-Dependent Plasticity Drives Individual Differences in Pheromone-Sensing Neurons

Different individuals exhibit distinct behaviors, but studying the neuronal basis of individuality is a daunting challenge. Here, we considered this question in the vomeronasal organ, a pheromone-detecting epithelium containing hundreds of distinct neuronal types. Using light-sheet microscopy, we characterized in each animal the abundance of 17 physiologically defined types, altogether recording from half a million sensory neurons. Inter-animal differences were much larger than predicted by chance, and different physiological cell types showed distinct patterns of variability. One neuronal type was present in males and nearly absent in females. Surprisingly, this apparent sexual dimorphism was generated by plasticity, as exposure to female scents or single ligands led to both the elimination of this cell type and alterations in olfactory behavior. That an all-or-none apparent sex difference in neuronal types is controlled by experience-even in a sensory system devoted to "innate" behaviors-highlights the extraordinary role of "nurture" in neural individuality.

Tetrahydrobiopterin regulates monoamine neurotransmitter sulfonation

Monoamine neurotransmitters are among the hundreds of signaling small molecules whose target interactions are switched "on" and "off" via transfer of the sulfuryl-moiety (-SO3) from PAPS (3'-phosphoadenosine 5'-phosphosulfate) to the hydroxyls and amines of their scaffolds. These transfer reactions are catalyzed by a small family of broad-specificity enzymes-the human cytosolic sulfotransferases (SULTs). The first structure of a SULT allosteric-binding site (that of SULT1A1) has recently come to light. The site is conserved among SULT1 family members and is promiscuous-it binds catechins, a naturally occurring family of flavanols. Here, the catechin-binding site of SULT1A3, which sulfonates monoamine neurotransmitters, is modeled on that of 1A1 and used to screen in silico for endogenous metabolite 1A3 allosteres. Screening predicted a single high-affinity allostere, tetrahydrobiopterin (THB), an essential cofactor in monoamine neurotransmitter biosynthesis. THB is shown to bind and inhibit SULT1A3 with high affinity, 23 (±2) nM, and to bind weakly, if at all, to the four other major SULTs found in brain and liver. The structure of the THB-bound binding site is determined and confirms that THB binds the catechin site. A structural comparison of SULT1A3 with SULT1A1 (its immediate evolutionary progenitor) reveals how SULT1A3 acquired high affinity for THB and that the majority of residue changes needed to transform 1A1 into 1A3 are clustered at the allosteric and active sites. Finally, sequence records reveal that the coevolution of these sites played an essential role in the evolution of simian neurotransmitter metabolism.

Differences in peripheral sensory input to the olfactory bulb between male and female mice

Female mammals generally have a superior sense of smell than males, but the biological basis of this difference is unknown. Here, we demonstrate sexually dimorphic neural coding of odorants by olfactory sensory neurons (OSNs), primary sensory neurons that physically contact odor molecules in the nose and provide the initial sensory input to the brain’s olfactory bulb. We performed in vivo optical neurophysiology to visualize odorant-evoked OSN synaptic output into olfactory bub glomeruli in unmanipulated (gonad-intact) adult mice from both sexes, and found that in females odorant presentation evoked more rapid OSN signaling over a broader range of OSNs than in males. These spatiotemporal differences enhanced the contrast between the neural representations of chemically related odorants in females compared to males during stimulus presentation. Removing circulating sex hormones makes these signals slower and less discriminable in females, while in males they become faster and more discriminable, suggesting opposite roles for gonadal hormones in influencing male and female olfactory function. These results demonstrate that the famous sex difference in olfactory abilities likely originates in the primary sensory neurons, and suggest that hormonal modulation of the peripheral olfactory system could underlie differences in how males and females experience the olfactory world.

Sexually dimorphic control of gene expression in sensory neurons regulates decision-making behavior in C. elegans

Animal behavior is directed by the integration of sensory information from internal states and the environment. Neuroendocrine regulation of diverse behaviors of Caenorhabditis elegans is under the control of the DAF-7/TGF-β ligand that is secreted from sensory neurons. Here, we show that C. elegans males exhibit an altered, male-specific expression pattern of daf-7 in the ASJ sensory neuron pair with the onset of reproductive maturity, which functions to promote male-specific mate-searching behavior. Molecular genetic analysis of the switch-like regulation of daf-7 expression in the ASJ neuron pair reveals a hierarchy of regulation among multiple inputs—sex, age, nutritional status, and microbial environment—which function in the modulation of behavior. Our results suggest that regulation of gene expression in sensory neurons can function in the integration of a wide array of sensory information and facilitate decision-making behaviors in C. elegans.

DOI:http://dx.doi.org/10.7554/eLife.21166.001

For almost all species of animal, males and females will often behave differently in similar situations. Little is known about how these sex-specific differences are generated or, for example, how different the nervous system of a male is to that of a female. Moreover, it is also poorly understood how these underlying differences based on the biological sex of an animal are integrated with and influenced by its experiences and environment.

The roundworm Caenorhabditis elegans has two sexes, hermaphrodites and males. The male worms behave differently to the hermaphrodites in a number of situations. This means that these animals offer the opportunity to explore and understand sex-specific differences in behavior. It is also possible to analyze the underlying factors that contribute to behavior in C. elegans, because it has a relatively simple and well-defined nervous system.

Now, Hilbert and Kim show that a signal that influences how C. elegans explores in response to chemicals in its environment is expressed differently in male and hermaphrodite worms. The signal in question is molecule called DAF-7, which is released by several sensory neurons—nerve cells that are used for detecting cues from the environment. The sensory neurons that release DAF-7 are found in both sexes of C. elegans but the specific way that the male worms express this signal encourages them to search for mates. Hermaphrodites, on the other hand, do not need to search for mates because they can fertilize their own eggs.

Hilbert and Kim showed that the biological sex in combination with multiple other inputs – including the animal’s past diet and age – regulate how the DAF-7 signal is expressed in C. elegans. These inputs all converge onto a single pair of sensory neurons, which integrate the inputs and enable the worm to assess its current and past experiences and alter its behavior accordingly.

Moving forward the next challenge is to understand how information about both external environment and internal states, such as hunger, are communicated to and integrated by these sensory neurons. Decoding the signals behind this process may illuminate how biological sex and internal states influence behavior in other species of animals.

DOI:http://dx.doi.org/10.7554/eLife.21166.002

Interdependent Conductances Drive Infraslow Intrinsic Rhythmogenesis in a Subset of Accessory Olfactory Bulb Projection Neurons

The accessory olfactory system controls social and sexual behavior. However, key aspects of sensory signaling along the accessory olfactory pathway remain largely unknown. Here, we investigate patterns of spontaneous neuronal activity in mouse accessory olfactory bulb mitral cells, the direct neural link between vomeronasal sensory input and limbic output. Both in vitro and in vivo, we identify a subpopulation of mitral cells that exhibit slow stereotypical rhythmic discharge. In intrinsically rhythmogenic neurons, these periodic activity patterns are maintained in absence of fast synaptic drive. The physiological mechanism underlying mitral cell autorhythmicity involves cyclic activation of three interdependent ionic conductances: subthreshold persistent Na(+) current, R-type Ca(2+) current, and Ca(2+)-activated big conductance K(+) current. Together, the interplay of these distinct conductances triggers infraslow intrinsic oscillations with remarkable periodicity, a default output state likely to affect sensory processing in limbic circuits.

SIGNIFICANCE STATEMENT

We show for the first time that some rodent accessory olfactory bulb mitral cells-the direct link between vomeronasal sensory input and limbic output-are intrinsically rhythmogenic. Driven by ≥ 3 distinct interdependent ionic conductances, infraslow intrinsic oscillations show remarkable periodicity both in vitro and in vivo. As a novel default state, infraslow autorhythmicity is likely to affect limbic processing of pheromonal information.

Olfactory Receptors in Non-Chemosensory Organs: The Nervous System in Health and Disease

Olfactory receptors (ORs) and down-stream functional signaling molecules adenylyl cyclase 3 (AC3), olfactory G protein α subunit (Gαolf), OR transporters receptor transporter proteins 1 and 2 (RTP1 and RTP2), receptor expression enhancing protein 1 (REEP1), and UDP-glucuronosyltransferases (UGTs) are expressed in neurons of the human and murine central nervous system (CNS). In vitro studies have shown that these receptors react to external stimuli and therefore are equipped to be functional. However, ORs are not directly related to the detection of odors. Several molecules delivered from the blood, cerebrospinal fluid, neighboring local neurons and glial cells, distant cells through the extracellular space, and the cells’ own self-regulating internal homeostasis can be postulated as possible ligands. Moreover, a single neuron outside the olfactory epithelium expresses more than one receptor, and the mechanism of transcriptional regulation may be different in olfactory epithelia and brain neurons. OR gene expression is altered in several neurodegenerative diseases including Parkinson’s disease (PD), Alzheimer’s disease (AD), progressive supranuclear palsy (PSP) and sporadic Creutzfeldt-Jakob disease (sCJD) subtypes MM1 and VV2 with disease-, region- and subtype-specific patterns. Altered gene expression is also observed in the prefrontal cortex in schizophrenia with a major but not total influence of chlorpromazine treatment. Preliminary parallel observations have also shown the presence of taste receptors (TASRs), mainly of the bitter taste family, in the mammalian brain, whose function is not related to taste. TASRs in brain are also abnormally regulated in neurodegenerative diseases. These seminal observations point to the need for further studies on ORs and TASRs chemoreceptors in the mammalian brain.

Glia-derived neurons are required for sex-specific learning in C. elegans

Sex differences in behaviour extend to cognitive-like processes such as learning, but the underlying dimorphisms in neural circuit development and organization that generate these behavioural differences are largely unknown. Here we define at the single-cell level-from development, through neural circuit connectivity, to function-the neural basis of a sex-specific learning in the nematode Caenorhabditis elegans. We show that sexual conditioning, a form of associative learning, requires a pair of male-specific interneurons whose progenitors are fully differentiated glia. These neurons are generated during sexual maturation and incorporated into pre-exisiting sex-shared circuits to couple chemotactic responses to reproductive priorities. Our findings reveal a general role for glia as neural progenitors across metazoan taxa and demonstrate that the addition of sex-specific neuron types to brain circuits during sexual maturation is an important mechanism for the generation of sexually dimorphic plasticity in learning.

Cyclic Regulation of Sensory Perception by a Female Hormone Alters Behavior

Females may display dramatically different behavior depending on their state of ovulation. This is thought to occur through sex-specific hormones acting on behavioral centers in the brain. Whether incoming sensory activity also differs across the ovulation cycle to alter behavior has not been investigated. Here, we show that female mouse vomeronasal sensory neurons (VSNs) are temporarily and specifically rendered "blind" to a subset of male-emitted pheromone ligands during diestrus yet fully detect and respond to the same ligands during estrus. VSN silencing occurs through the action of the female sex-steroid progesterone. Not all VSNs are targeted for silencing; those detecting cat ligands remain continuously active irrespective of the estrous state. We identify the signaling components that account for the capacity of progesterone to target specific subsets of male-pheromone responsive neurons for inactivation. These findings indicate that internal physiology can selectively and directly modulate sensory input to produce state-specific behavior. PAPERCLIP.

Estrogen involvement in social behavior in rodents: Rapid and long-term actions

This article is part of a Special Issue ("Estradiol and cognition"). Estrogens have repeatedly been shown to influence a wide array of social behaviors, which in rodents are predominantly olfactory-mediated. Estrogens are involved in social behavior at multiple levels of processing, from the detection and integration of socially relevant olfactory information to more complex social behaviors, including social preferences, aggression and dominance, and learning and memory for social stimuli (e.g. social recognition and social learning). Three estrogen receptors (ERs), ERα, ERβ, and the G protein-coupled ER 1 (GPER1), differently affect these behaviors. Social recognition, territorial aggression, and sexual preferences and mate choice, all requiring the integration of socially related olfactory information, seem to primarily involve ERα, with ERβ playing a lesser, modulatory role. In contrast, social learning consistently responds differently to estrogen manipulations than other social behaviors. This suggests differential ER involvement in brain regions important for specific social behaviors, such as the ventromedial and medial preoptic nuclei of the hypothalamus in social preferences and aggression, the medial amygdala and hippocampus in social recognition, and the prefrontal cortex and hippocampus in social learning. While the long-term effects of ERα and ERβ on social behavior have been extensively investigated, our knowledge of the rapid, non-genomic, effects of estrogens is more limited and suggests that they may mediate some social behaviors (e.g. social learning) differently from long-term effects. Further research is required to compare ER involvement in regulating social behavior in male and female animals, and to further elucidate the roles of the more recently described G protein-coupled ERs, both the GPER1 and the Gq-mER.

Straightforward assay for quantification of social avoidance in Drosophila melanogaster

Drosophila melanogaster is an emerging model to study different aspects of social interactions. For example, flies avoid areas previously occupied by stressed conspecifics due to an odorant released during stress known as the Drosophila stress odorant (dSO). Through the use of the T-maze apparatus, one can quantify the avoidance of the dSO by responder flies in a very affordable and robust assay. Conditions necessary to obtain a strong performance are presented here. A stressful experience is necessary for the flies to emit dSO, as well as enough emitter flies to cause a robust avoidance response to the presence of dSO. Genetic background, but not their group size, strongly altered the avoidance of the dSO by the responder flies. Canton-S and Elwood display a higher performance in avoiding the dSO than Oregon and Samarkand strains. This behavioral assay will allow identification of mechanisms underlying this social behavior, and the assessment of the influence of genes and environmental conditions on both emission and avoidance of the dSO. Such an assay can be included in batteries of simple diagnostic tests used to identify social deficiencies of mutants or environmental conditions of interest.

The olfactory transcriptomes of mice

The olfactory (OR) and vomeronasal receptor (VR) repertoires are collectively encoded by 1700 genes and pseudogenes in the mouse genome. Most OR and VR genes were identified by comparative genomic techniques and therefore, in many of those cases, only their protein coding sequences are defined. Some also lack experimental support, due in part to the similarity between them and their monogenic, cell-specific expression in olfactory tissues. Here we use deep RNA sequencing, expression microarray and quantitative RT-PCR in both the vomeronasal organ and whole olfactory mucosa to quantify their full transcriptomes in multiple male and female mice. We find evidence of expression for all VR, and almost all OR genes that are annotated as functional in the reference genome, and use the data to generate over 1100 new, multi-exonic, significantly extended receptor gene annotations. We find that OR and VR genes are neither equally nor randomly expressed, but have reproducible distributions of abundance in both tissues. The olfactory transcriptomes are only minimally different between males and females, suggesting altered gene expression at the periphery is unlikely to underpin the striking sexual dimorphism in olfactory-mediated behavior. Finally, we present evidence that hundreds of novel, putatively protein-coding genes are expressed in these highly specialized olfactory tissues, and carry out a proof-of-principle validation. Taken together, these data provide a comprehensive, quantitative catalog of the genes that mediate olfactory perception and pheromone-evoked behavior at the periphery.

The sense of smell in mice involves the detection of odors and pheromones by many hundreds of olfactory and vomeronasal receptors. The genes that encode these receptors account for around 5% of the whole gene catalog, but they are poorly understood because they are very similar to each other, and are thought to be turned on randomly in only a small number of cells. Here we use multiple gene expression technologies to curate and measure the activity of all the genes involved in the detection of odors and find evidence of many new ones. We show that most genes encoding olfactory and vomeronasal receptors have complex, multi-exonic structures that generate different isoforms. We find that some receptors are consistently more abundant in the nose than others, which suggests they are not turned on randomly. This may explain why mice are particularly sensitive to some odors, but less attuned to others. We find that overall males and females differ very little in gene expression, despite having altered behavioral responses to the same odors. Thus diversity in receptor expression can explain differences in odor sensitivity, but does not appear to dictate whether sex pheromones are differentially detected by males or females.

Paradigms of sulfotransferase catalysis: the mechanism of SULT2A1

Human cytosolic sulfotransferases (SULTs) regulate the activities of thousands of signaling small molecules via transfer of the sulfuryl moiety (-SO3) from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to the hydroxyls and primary amines of acceptors. Sulfonation controls the affinities of ligands for their targets, and thereby regulates numerous receptors, which, in turn, regulate complex cellular responses. Despite their biological and medical relevance, basic SULT mechanism issues remain unresolved. To settle these issues, and to create an in-depth model of SULT catalysis, the complete kinetic mechanism of a representative member of the human SULT family, SULT2A1, was determined. The mechanism is composed of eight enzyme forms that interconvert via 22 rate constants, each of which was determined independently. The result is a complete quantitative description of the mechanism that accurately predicts complex enzymatic behavior. This is the first description of a SULT mechanism at this resolution, and it reveals numerous principles of SULT catalysis and resolves previously ambiguous issues. The structures and catalytic behaviors SULTs are highly conserved; hence, the mechanism presented here should prove paradigmatic for the family.

Expression and purification of mouse peptide ESP4 in Escherichia coli

Pheromones are species-specific chemical signals that regulate a wide range of social and sexual behaviors in many animals. In mice, the male-specific peptide ESP1 (exocrine gland-secreting peptide 1) is secreted into tear fluids and enhances female sexual receptive behavior. ESP1 belongs to the ESP family, a multigene family with 38 genes in mice. ESP1 shares the highest homology with ESP4. ESP1 is expressed in the extraorbital lacrimal gland, whereas ESP4 is expressed in some exocrine glands. Thus, ESP4 is expected to have a function that has not been elucidated yet. Large amounts of the purified ESP4 protein are required for structural and biochemical studies. Here we present an expression and purification scheme for the recombinant ESP4 protein. The N-terminally histidine-tagged ESP4 fusion protein was expressed in Escherichia coli as inclusion bodies, which were solubilized and purified by nickel affinity chromatography. The histidine tag was cleaved with thrombin and removed by a second nickel affinity chromatography step. The ESP4 protein was isolated with high purity by reversed-phase chromatography. For NMR analyses, we prepared a stable isotope-labeled ESP4 protein. Three repeated freeze-drying steps after the reversed-phase chromatography were required, to remove a volatile contaminating compound and to obtain an NMR spectrum with a homogeneous line shape. AMS-modification and far-UV CD spectroscopic analyses suggested that ESP4 has an intramolecular disulfide bridge and a helical structure, respectively. The present study provides a powerful tool for structural and biochemical studies of ESP4, leading toward the elucidation of the roles of the ESP family members.

A bidirectional circuit switch reroutes pheromone signals in male and female brains

The Drosophila sex pheromone cVA elicits different behaviors in males and females. First- and second-order olfactory neurons show identical pheromone responses, suggesting that sex genes differentially wire circuits deeper in the brain. Using in vivo whole-cell electrophysiology, we now show that two clusters of third-order olfactory neurons have dimorphic pheromone responses. One cluster responds in females; the other responds in males. These clusters are present in both sexes and share a common input pathway, but sex-specific wiring reroutes pheromone information. Regulating dendritic position, the fruitless transcription factor both connects the male-responsive cluster and disconnects the female-responsive cluster from pheromone input. Selective masculinization of third-order neurons transforms their morphology and pheromone responses, demonstrating that circuits can be functionally rewired by the cell-autonomous action of a switch gene. This bidirectional switch, analogous to an electrical changeover switch, provides a simple circuit logic to activate different behaviors in males and females.

Wake up and smell the conflict: odour signals in female competition

Odour signals used in competitive and aggressive interactions between males are well studied in the context of sexual selection. By contrast, relatively little is known about comparable signals used by females, despite current interest in the evolution of female ornaments and weaponry. Available evidence suggests that odour signals are important in competitive interactions between female mammals, with reductions or reversals of male-biased sexual dimorphism in signalling where female competition is intense. Scent marking is often associated with conflict between females over access to resources or reproductive opportunities. Female scent marks may therefore provide reliable signals of competitive ability that could be used both by competitors and potential mates. Consistent with this hypothesis, we report that aggressive behaviour of female house mice is correlated with the amount of major urinary protein (MUP) excreted in their urine, a polymorphic set of proteins that are used in scent mark signalling. Under semi-natural conditions, females with high MUP output are more likely to produce offspring sired by males that have high reproductive success, and less likely to produce offspring by multiple different sires, suggesting that females with strong MUP signals are monopolized by males of particularly high quality. We conclude that odour signals are worthy of more detailed investigation as mediators of female competition.

The genomic basis of vomeronasal-mediated behaviour

The vomeronasal organ (VNO) is a chemosensory subsystem found in the nose of most mammals. It is principally tasked with detecting pheromones and other chemical signals that initiate innate behavioural responses. The VNO expresses subfamilies of vomeronasal receptors (VRs) in a cell-specific manner: each sensory neuron expresses just one or two receptors and silences all the other receptor genes. VR genes vary greatly in number within mammalian genomes, from no functional genes in some primates to many hundreds in rodents. They bind semiochemicals, some of which are also encoded in gene families that are coexpanded in species with correspondingly large VR repertoires. Protein and peptide cues that activate the VNO tend to be expressed in exocrine tissues in sexually dimorphic, and sometimes individually variable, patterns. Few chemical ligand–VR–behaviour relationships have been fully elucidated to date, largely due to technical difficulties in working with large, homologous gene families with high sequence identity. However, analysis of mouse lines with mutations in genes involved in ligand–VR signal transduction has revealed that the VNO mediates a range of social behaviours, including male–male and maternal aggression, sexual attraction, lordosis, and selective pregnancy termination, as well as interspecific responses such as avoidance and defensive behaviours. The unusual logic of VR expression now offers an opportunity to map the specific neural circuits that drive these behaviours.

Functional genomics reveals dysregulation of cortical olfactory receptors in Parkinson disease: novel putative chemoreceptors in the human brain

Parkinson disease (PD) is no longer considered a complex motor disorder but rather a systemic disease with variable nonmotor deficits that may include impaired olfaction, depression, mood and sleep disorders, and altered cortical function. Increasing evidence indicates that multiple metabolic defects occur in regions outside the substantia nigra, including the cerebral cortex, even at premotor stages of the disease. We investigated changes in gene expression in the frontal cortex in PD patient brains using a transcriptomics approach. Functional genomics analysis indicated that cortical olfactory receptors (ORs) and taste receptors (TASRs) are altered in PD patients. Olfactory receptors OR2L13, OR1E1, OR2J3, OR52L1, and OR11H1 and taste receptors TAS2R5 and TAS2R50 were downregulated, but TAS2R10 and TAS2R13 were upregulated at premotor and parkinsonian stages in the frontal cortex area 8 in PD patient brains. Furthermore, we present novel evidence that, in addition to the ORs, obligate downstream components of OR function adenylyl cyclase 3 and olfactory G protein (Gαolf), OR transporters, receptor transporter proteins 1 and 2 and receptor expression enhancing protein 1, and OR xenobiotic removing UDP-glucuronosyltransferase 1 family polypeptide A6 are widely expressed in neurons of the cerebral cortex and other regions of the adult human brain. Together, these findings support the concept that ORs and TASRs in the cerebral cortex may have novel physiologic functions that are affected in PD patients.

miR-124 controls male reproductive success in Drosophila

Many aspects of social behavior are controlled by sex-specific pheromones. Gender-appropriate production of the sexually dimorphic transcription factors doublesex and fruitless controls sexual differentiation and sexual behavior. miR-124 mutant males exhibited increased male–male courtship and reduced reproductive success with females. Females showed a strong preference for wild-type males over miR-124 mutant males when given a choice of mates. These effects were traced to aberrant pheromone production. We identified the sex-specific splicing factor transformer as a functionally significant target of miR-124 in this context, suggesting a role for miR-124 in the control of male sexual differentiation and behavior, by limiting inappropriate expression of the female form of transformer. miR-124 is required to ensure fidelity of gender-appropriate pheromone production in males. Use of a microRNA provides a secondary means of controlling the cascade of sex-specific splicing events that controls sexual differentiation in Drosophila.

DOI:http://dx.doi.org/10.7554/eLife.00640.001

Like many animals, the fruit fly Drosophila uses pheromones to influence sexual behaviour, with males and females producing different versions of these chemicals. One of the pheromones produced by male flies, for example, is a chemical called 11-cis-vaccenyl-acetate (cVA), which is an aphrodisiac for female flies and an anti-aphrodisiac for males.

The production of the correct pheromones in each sex is genetically controlled using a process called splicing that allows a single gene to be expressed as two or more different proteins. A variety of proteins called splicing factors ensures that splicing results in the production of the correct pheromones for each sex. Sometimes, however, the process by which sex genes are expressed as proteins can be ‘leaky’, which results in the wrong proteins being produced for one or both sexes.

Small RNA molecules called microRNAs act in some genetic pathways to limit the leaky expression of genes, and a microRNA called miR-124 carries out this function in the developing brain Drosophila. Now, Weng et al. show that miR-124 also helps to regulate sex-specific splicing and thereby to control pheromone production and sexual behaviour.

Mutant male flies lacking miR-124 were less successful than wild-type males at mating with female flies, and were almost always rejected if a female fly was given a choice between a mutant male and a wild-type male. Moreover, both wild-type and mutant male flies were more likely to initiate courtship behaviour towards another male if it lacked miR-124 than if it did not.

The mutant male flies produced less cVA than wild-type males, but more of other pheromones called pentacosenes, which is consistent with the observed behaviour because cVA attracts females and repels males, whereas pentacosenes act as aphrodisiacs for male flies in large amounts. Weng et al. showed that these changes in the production of pheromones were caused by an increased expression of the female version of a splicing factor called transformer in the mutant males, but further work is needed to understand this process in detail.

DOI:http://dx.doi.org/10.7554/eLife.00640.002

Structure of the mouse sex peptide pheromone ESP1 reveals a molecular basis for specific binding to the class C G-protein-coupled vomeronasal receptor

Exocrine gland-secreting peptide 1 (ESP1) is a sex pheromone that is released in male mouse tear fluids and enhances female sexual receptive behavior. ESP1 is selectively recognized by a specific class C G-protein-coupled receptor (GPCR), V2Rp5, among the hundreds of receptors expressed in vomeronasal sensory neurons (VSNs). The specific sensing mechanism of the mammalian peptide pheromone by the class C GPCR remains to be elucidated. Here we identified the minimal functional region needed to retain VSN-stimulating activity in ESP1 and determined its three-dimensional structure, which adopts a helical fold stabilized by an intramolecular disulfide bridge with extensive charged patches. We then identified the amino acids involved in the activation of VSNs by a structure-based mutational analysis, revealing that the highly charged surface is crucial for the ESP1 activity. We also demonstrated that ESP1 specifically bound to an extracellular region of V2Rp5 by an in vitro pulldown assay. Based on homology modeling of V2Rp5 using the structure of the metabotropic glutamate receptor, we constructed a docking model of the ESP1-V2Rp5 complex in which the binding interface exhibited good electrostatic complementarity. These experimental results, supported by the molecular docking simulations, reveal that charge-charge interactions determine the specificity of ESP1 binding to V2Rp5 in the large extracellular region characteristic of class C GPCRs. The present study provides insights into the structural basis for the narrowly tuned sensing of mammalian peptide pheromones by class C GPCRs.

Testing the sulfotransferase molecular pore hypothesis

Human cytosolic sulfotransferases (SULTs) regulate the activities of hundreds of signaling metabolites via transfer of the sulfuryl moiety (-SO3) from activated sulfate (3'-phosphoadenosine 5'-phosphosulfate) to the hydroxyls and primary amines of xeno- and endobiotics. How SULTs select substrates from the scores of competing ligands present in a cytosolic milieu is an important issue in the field. Selectivity appears to be sterically controlled by a molecular pore that opens and closes in response to nucleotide binding. This point of view is fostered by structures showing nucleotide-dependent pore closure and the fact that nucleotide binding induces an isomerization that restricts access to the acceptor-binding pocket. Molecular dynamics models underscore the importance of pore isomerization in selectivity and predict that specific molecular linkages stabilize the closed pore in response to nucleotide binding. To test the pore model, these linkages were disrupted in SULT2A1 via mutagenesis, and the effects on selectivity were determined. The mutations uncoupled nucleotide binding from selectivity and produced enzymes that no longer discriminated between large and small substrates. The mutations did not affect the affinity or turnover of small substrates but resulted in a 183-fold gain in catalytic efficiently toward large substrates. Models predict that an 11-residue "flap" covering the acceptor-binding pocket can open and admit large substrates when nucleotide is bound; a mutant structure demonstrated that this is so. In summary, the model was shown to be a robust, accurate predictor of SULT structure and selectivity whose general features will likely apply to other members of the SULT family.

Two Drosophila DEG/ENaC channel subunits have distinct functions in gustatory neurons that activate male courtship

Trimeric sodium channels of the DEG/ENaC family have important roles in neurons, but the specific functions of different subunits present in heteromeric channels are poorly understood. We previously reported that the Drosophila DEG/ENaC subunit Ppk25 is essential in a small subset of gustatory neurons for activation of male courtship behavior, likely through detection of female pheromones. Here we show that, like mutations in ppk25, mutations in another Drosophila DEG/ENaC subunit gene, nope, specifically impair male courtship of females. nope regulatory sequences drive reporter gene expression in gustatory neurons of the labellum wings, and legs, including all gustatory neurons in which ppk25 function is required for male courtship of females. In addition, gustatory-specific knockdown of nope impairs male courtship. Further, the impaired courtship response of nope mutant males to females is rescued by targeted expression of nope in the subset of gustatory neurons in which ppk25 functions. However, nope and ppk25 have nonredundant functions, as targeted expression of ppk25 does not compensate for the lack of nope and vice versa. Moreover, Nope and Ppk25 form specific complexes when coexpressed in cultured cells. Together, these data indicate that the Nope and Ppk25 polypeptides have specific, nonredundant functions in a subset of gustatory neurons required for activation of male courtship in response to females, and suggest the hypothesis that Nope and Ppk25 function as subunits of a heteromeric DEG/ENaC channel required for gustatory detection of female pheromones.

Sensation in a single neuron pair represses male behavior in hermaphrodites

Pheromones elicit innate sex-specific mating behaviors in many species. We demonstrate that in C. elegans, male-specific sexual attraction behavior is programmed in both sexes but repressed in hermaphrodites. Repression requires a single sensory neuron pair, the ASIs. To repress attraction in adults, the ASIs must be present, active, and capable of sensing the environment during development. The ASIs release TGF-β, and ASI function can be bypassed by experimental activation of TGF-β signaling. Sexual attraction in derepressed hermaphrodites requires the same sensory neurons as in males. The sexual identity of both these sensory neurons and a distinct subset of interneurons must be male to relieve repression and release attraction. TGF-β may therefore act to change connections between sensory neurons and interneurons during development to engage repression. Thus, sensation in a single sensory neuron pair during development reprograms a common neural circuit from male to female behavior.

A nucleotide-gated molecular pore selects sulfotransferase substrates

Human SULT2A1 is one of two predominant sulfotransferases in liver and catalyzes transfer of the sulfuryl moiety (-SO(3)) from activated sulfate (PAPS, 3'-phosphoadenosine 5-phosphosulfate) to hundreds of acceptors (metabolites and xenobiotics). Sulfation recodes the biologic activity of acceptors by altering their receptor interactions. The molecular basis on which these enzymes select and sulfonate specific acceptors from complex mixtures of competitors in vivo is a long-standing issue in the SULT field. Raloxifene, a synthetic steroid used in the prevention of osteoporosis, and dehydroepiandrosterone (DHEA), a ubiquitous steroid precusor, are reported to be sulfated efficiently by SULT2A1 in vitro, yet unlike DHEA, raloxifene is not sulfated in vivo. This selectivity was explored in initial rate and equilibrium binding studies that demonstrate pronounced binding antisynergy (21-fold) between PAPS and raloxifene, but not DHEA. Analysis of crystal structures suggests that PAP binding restricts access to the acceptor-binding pocket by restructuring a nine-residue segment of the pocket edge that constricts the active site opening, or "pore", that sieves substrates on the basis of their geometries. In silico docking predicts that raloxifene, which is considerably larger than DHEA, can bind only to the unliganded (open) enzyme, whereas DHEA binds both the open and closed forms. The predictions of these structures with regard to substrate binding are tested using equilibrium and pre-steady-state ligand binding studies, and the results confirm that a nucleotide-driven isomerization controls access to the acceptor-binding pocket and plays an important role in substrate selection by SULT2A1 and possibly other sulfotransferases.

Management of Smell Dysfunction

Olfaction is an essential chemosensory system in the living world. Although less appreciated in humans, smell impairment significantly affects many aspects of quality of life. Smell disorders may be caused by an impaired nasal airway or by lesions in the olfactory system, leading to reduced or distorted smell perception. The most common causes of smell disorders are aging, upper respiratory tract infection, sinonasal disease, and head trauma. Recovery is rarely complete. Counseling is important in progressive or severe smell loss. In patients with distorted smell perception, antidepressant medication is sometimes necessary. Best response to treatment is achieved for nasal obstruction and sinonasal inflammatory disease. Treatment of olfactory impairment caused by sinonasal disease includes medication with topical and systemic steroids, or surgery for refractory cases. Although there are reports that surgical resection of olfactory neurons may lead to reinnervation and recovery of smell, adequate treatment of the smell loss remains an unmet need.

Cell death and sexual differentiation of behavior: worms, flies, and mammals

Sex differences in the nervous system are found throughout the animal kingdom. Here, we discuss three prominent genetic models: nematodes, fruit flies, and mice. In all three, differential cell death is central to sexual differentiation and shared molecular mechanisms have been identified. Our knowledge of the precise function of neural sex differences lags behind. One fruitful approach to the 'function' question is to contrast sexual differentiation in standard laboratory animals with differentiation in species exhibiting unique social and reproductive organizations. Advanced genetic strategies are also addressing this question in worms and flies, and may soon be applicable to vertebrates.