Binding Specificity of Native Odorant-Binding Protein Isoforms Is Driven by Phosphorylation and O-N-Acetylglucosaminylation in the Pig Sus scrofa

Variation of ewe olfactory secretome during a ram effect

Introduction

Under temperate latitudes, reproduction in Ovis aries displays a marked seasonality, governed by the photoperiod. In natural conditions, the transition between sexual rest and sexual activity in both sexes is induced by the decrease of day light. Meanwhile, specific odors emitted by a sexually active male are able to reactivate the gonadotropic axis of anovulatory ewes. This physiological effect is called "male effect", precisely ram effect in the ovine species. We have previously shown that the secreted proteins, namely Olfactory Binding Proteins (OBP), contained in the nasal mucus constitute the olfactory secretome (OS), the composition of which is determined by the status of oestrus cycle of females and differs between sexual rest and sexual activity periods. The objective of this study was to test the hypothesis that exposure to sexually active male can also modify the composition of ewes olfactory secretome during a male effect, as well as hormones produced by the reactivation of the oestrus cycle in sexual activity period under natural conditions.

Methods

We have set up a new non-invasive protocol of nasal mucus sampling and collected it from 12 ewes at different times during a ram effect. We analyzed the composition of their olfactory secretome by proteomics, mainly SDS-PAGE and MALDI-TOF mass spectrometry. As post-translational modifications of OBPs were a hallmark of ewes' sexual activity period, we were looking for glycosylation by western-blot and mass spectrometry.

Results

The efficiency of male effect was low in stimulated ewes as only 3 females displayed elevated progesterone levels in their blood. Besides, half of control ewes (non-stimulated ones) were cycled. We noticed a common OS profile in ewes in anoestrus, versus OS of cycled ones. A very clear and important result was the apparition of O-GlcNAcylation, previously detected only in sexual activity, after only 30 min of male introduction into the flock.

Discussion

This exploratory study paves the way for further experiments with larger flock to confirm and reinforce these results, and for eventually exploiting the nasal mucus as an indicator of females' receptivity to male odors.

Pheromones, binding proteins, and olfactory systems in the pig (Sus scrofa): An updated review

Pigs utilize multimodal communication for reproductive and other behaviors, and chemical communication is one of the key components. The success of reproduction relies on chemical communication favored by the steroid pheromones from boar saliva. These steroids were proven to be involved in advancing puberty in gilts (the boar effect) and in promoting estrus behaviors in gilts/sows, thereby helping to detect estrus and facilitating the timing of artificial insemination. The steroid pheromones bound with carrier proteins are evidenced in the mandibular (submandibular) salivary secretions of the boar. These salivary steroids bind with carrier proteins in the nasal mucus and vomeronasal organ (VNO) of the sows, eventually triggering a cascade of activities at the olfactory and endocrine levels. Besides steroid pheromones, pig appeasing pheromones (from mammary skin secretions of sows) have also been demonstrated to bind with carrier proteins in the nasal mucus and VNO of the piglets. Thus far, four different proteins have been identified and confirmed in the nasal mucus and VNO of pigs, including odorant binding proteins (OBPs), salivary lipocalin (SAL), pheromaxein, and Von Ebner's Gland Protein (VEGP). The critical roles of the chemosensory systems, main olfactory systems and VNO, have been comprehensively reported for pigs. This review summarizes the current knowledge on pheromones, their receptor proteins, and the olfactory systems of porcine species.

An odorant-binding protein in the elephant's trunk is finely tuned to sex pheromone (Z)-7-dodecenyl acetate

Chemical communication in elephants has been well studied at the chemical and behavioural levels. Pheromones have been identified in the Asian elephant (Elephas maximus), including (Z)-7-dodecenyl acetate and frontalin, and their specific effects on the sexual behaviour of elephants have been accurately documented. In contrast, our knowledge on the proteins mediating detection of pheromones in elephants remains poor and superficial, with only three annotated and reliable entries in sequence databases, two of them being odorant-binding proteins (OBPs), and the third a member of von Ebner's gland (VEG) proteins. Proteomic analysis of trunk wash extract from African elephant (Loxodonta africana) identified one of the OBPs (LafrOBP1) as the main component. We therefore expressed LafrOBP1 and its Asian elephant orthologue in yeast Pichia pastoris and found that both recombinant proteins, as well as the natural LafrOBP1 are tuned to (Z)-7-dodecenyl acetate, but have no affinity for frontalin. Both the natural and recombinant LafrOBP1 carry post-translational modifications such as O-glycosylation, phosphorylation and acetylation, but as these modifications affect only a very small amount of the protein, we cannot establish their potential effects on the ligand-binding properties of OBP1.

Odorant-binding proteins of mammals

Odorant-binding proteins (OBPs) of vertebrates belong to the lipocalin superfamily and perform a dual function: solubilizing and ferrying volatile pheromones to the olfactory receptors, and complexing the same molecules in specialized glands and assisting their release into the environment. Within vertebrates, to date they have been reported only in mammals, apart from two studies on amphibians. Based on the small number of OBPs expressed in each species, on their sites of production outside the olfactory area and their presence in biological fluids known to be pheromone carriers, such as urine, saliva and sexual secretions, we conclude that OBPs of mammals are specifically dedicated to pheromonal communication. This assumption is further supported by the observation that some OBPs present in biological secretions are endowed with their own pheromonal activity, adding renewed interest to these proteins. Another novel piece of evidence is the recent discovery that glycosylation and phosphorylation can modulate the binding activity of these proteins, improving their affinity to pheromones and narrowing their specificity. A comparison with insects and other arthropods shows a completely different scenario. While mammalian OBPs are specifically tuned to pheromones, those of insects, which are completely different in sequence and structure, include carriers for general odorants in addition to those dedicated to pheromones. Additionally, whereas mammals adopted a single family of carrier proteins for chemical communication, insects and other arthropods are endowed with several families of semiochemical-binding proteins. Here, we review the literature on the structural and functional properties of vertebrate OBPs, summarize the most interesting new findings and suggest possible exciting future developments.

Odorant-binding proteins in canine anal sac glands indicate an evolutionarily conserved role in mammalian chemical communication

Background

Chemical communication is an important aspect of the behavioural ecology of a wide range of mammals. In dogs and other carnivores, anal sac glands are thought to convey information to conspecifics by secreting a pallet of small volatile molecules produced by symbiotic bacteria. Because these glands are unique to carnivores, it is unclear how their secretions relate to those of other placental mammals that make use of different tissues and secretions for chemical communication. Here we analyse the anal sac glands of domestic dogs to verify the secretion of proteins and infer their evolutionary relationship to those involved in the chemical communication of non-carnivoran mammals.

Results

Proteomic analysis of anal sac gland secretions of 17 dogs revealed the consistently abundant presence of three related proteins. Homology searches against online databases indicate that these proteins are evolutionary related to ‘odorant binding proteins’ (OBPs) found in a wide range of mammalian secretions and known to contribute to chemical communication. Screening of the dog’s genome sequence show that the newly discovered OBPs are encoded by a single cluster of three genes in the pseudoautosomal region of the X-chromosome. Comparative genomic screening indicates that the same locus is shared by a wide range of placental mammals and that it originated at least before the radiation of extant placental orders. Phylogenetic analyses suggest a dynamic evolution of gene duplication and loss, resulting in large gene clusters in some placental taxa and recurrent loss of this locus in others. The homology of OBPs in canid anal sac glands and those found in other mammalian secretions implies that these proteins maintained a function in chemical communication throughout mammalian evolutionary history by multiple shifts in expression between secretory tissues involved in signal release and nasal mucosa involved in signal reception.

Conclusions

Our study elucidates a poorly understood part of the biology of a species that lives in close association with humans. In addition, it shows that the protein repertoire underlying chemical communication in mammals is more evolutionarily stable than the variation of involved glands and tissues would suggest.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-021-01910-w.

A new non-classical fold of varroa odorant-binding proteins reveals a wide open internal cavity

Odorant-binding proteins (OBPs), as they occur in insects, form a distinct class of proteins that apparently has no closely related representatives in other animals. However, ticks, mites, spiders and millipedes contain genes encoding proteins with sequence similarity to insect OBPs. In this work, we have explored the structure and function of such non-insect OBPs in the mite Varroa destructor, a major pest of honey bee. Varroa OBPs present six cysteines paired into three disulphide bridges, but with positions in the sequence and connections different from those of their insect counterparts. VdesOBP1 structure was determined in two closely related crystal forms and appears to be a monomer. Its structure assembles five α-helices linked by three disulphide bridges, one of them exhibiting a different connection as compared to their insect counterparts. Comparison with classical OBPs reveals that the second of the six α-helices is lacking in VdesOBP1. Ligand-binding experiments revealed molecules able to bind only specific OBPs with a moderate affinity, suggesting that either optimal ligands have still to be identified, or post-translational modifications present in the native proteins may be essential for modulating binding activity, or else these OBPs might represent a failed attempt in evolution and are not used by the mites.

Isolation, Identification, and Bioinformatic Analysis of Antibacterial Proteins and Peptides from Immunized Hemolymph of Red Palm Weevil Rhynchophorus ferrugineus

Red palm weevil (Rhynchophorus ferrugineus Olivier, 1791, Coleoptera: Curculionidae) is a destructive pest of palms, rapidly extending its native geographical range and causing large economic losses worldwide. The present work describes isolation, identification, and bioinformatic analysis of antibacterial proteins and peptides from the immunized hemolymph of this beetle. In total, 17 different bactericidal or bacteriostatic compounds were isolated via a series of high-pressure liquid chromatography steps, and their partial amino acid sequences were determined by N-terminal sequencing or by mass spectrometry. The bioinformatic analysis of the results facilitated identification and description of corresponding nucleotide coding sequences for each peptide and protein, based on the recently published R. ferrugineus transcriptome database. The identified compounds are represented by several well-known bactericidal factors: two peptides similar to defensins, one cecropin-A1-like peptide, and one attacin-B-like protein. Interestingly, we have also identified some unexpected compounds comprising five isoforms of pheromone-binding proteins as well as seven isoforms of odorant-binding proteins. The particular role of these factors in insect response to bacterial infection needs further investigation.

Stability of OBPs

Odorant binding proteins (OBPs) are small proteins, some of which bind odorants with high specificity. OBPs are relatively easy to produce and show a pronounced stability toward thermal and chemical denaturation. This high stability renders OBPs attractive candidates for the development of odorant detections systems. Unfortunately, binding of odorants is not easy to quantify due to lack of spectroscopic signals upon binding. Therefore, a possible approach to detect binding is to employ the shift in thermal or chemical stability upon ligand-protein interaction. Being a rather indirect approach, the experimental setup should be done with care. Here, the experimental results on stability of OBPs are summarized and issues which should be considered when performing stability experiments are discussed.

The Major Cat Allergen Fel d 1 Binds Steroid and Fatty Acid Semiochemicals: A Combined In Silico and In Vitro Study

The major cat allergen Fel d 1 is a tetrameric glycoprotein of the secretoglobin superfamily. Structural aspects and allergenic properties of this protein have been investigated, but its physiological function remains unclear. Fel d 1 is assumed to bind lipids and steroids like the mouse androgen-binding protein, which is involved in chemical communication, either as a semiochemical carrier or a semiochemical itself. This study focused on the binding activity of a recombinant model of Fel d 1 (rFel d 1) towards semiochemical analogs, i.e., fatty acids and steroids, using both in silico calculations and fluorescence measurements. In silico analyses were first adopted to model the interactions of potential ligands, which were then tested in binding assays using the fluorescent reporter N-phenyl-1-naphthylamine. Good ligands were fatty acids, such as the lauric, oleic, linoleic, and myristic fatty acids, as well as steroids like androstenone, pregnenolone, and progesterone, that were predicted by in silico molecular models to bind into the central and surface cavities of rFel d 1, respectively. The lowest dissociation constants were shown by lauric acid (2.6 µM) and androstenone (2.4 µM). The specific affinity of rFel d 1 to semiochemicals supports a function of the protein in cat's chemical communication, and highlights a putative role of secretoglobins in protein semiochemistry.

The Major Cat Allergen Fel d 1 Binds Steroid and Fatty Acid Semiochemicals: A Combined In Silico and In Vitro Study

The major cat allergen Fel d 1 is a tetrameric glycoprotein of the secretoglobin superfamily. Structural aspects and allergenic properties of this protein have been investigated, but its physiological function remains unclear. Fel d 1 is assumed to bind lipids and steroids like the mouse androgen-binding protein, which is involved in chemical communication, either as a semiochemical carrier or a semiochemical itself. This study focused on the binding activity of a recombinant model of Fel d 1 (rFel d 1) towards semiochemical analogs, i.e., fatty acids and steroids, using both in silico calculations and fluorescence measurements. In silico analyses were first adopted to model the interactions of potential ligands, which were then tested in binding assays using the fluorescent reporter N-phenyl-1-naphthylamine. Good ligands were fatty acids, such as the lauric, oleic, linoleic, and myristic fatty acids, as well as steroids like androstenone, pregnenolone, and progesterone, that were predicted by in silico molecular models to bind into the central and surface cavities of rFel d 1, respectively. The lowest dissociation constants were shown by lauric acid (2.6 µM) and androstenone (2.4 µM). The specific affinity of rFel d 1 to semiochemicals supports a function of the protein in cat’s chemical communication, and highlights a putative role of secretoglobins in protein semiochemistry.

Identification of potential chemosignals in the European water vole Arvicola terrestris

The water vole Arvicola terrestris is endemic to Europe where its outbreak generates severe economic losses for farmers. Our project aimed at characterising putative chemical signals used by this species, to develop new sustainable methods for population control that could also be used for this species protection in Great Britain. The water vole, as well as other rodents, uses specific urination sites as territorial and sex pheromone markers, still unidentified. Lateral scent glands and urine samples were collected from wild males and females caught in the field, at different periods of the year. Their volatile composition was analysed for each individual and not on pooled samples, revealing a specific profile of flank glands in October and a specific profile of urinary volatiles in July. The urinary protein content appeared more contrasted as males secrete higher levels of a lipocalin than females, whenever the trapping period. We named this protein arvicolin. Male and female liver transcript sequencing did not identify any expression of other odorant-binding protein sequence. This work demonstrates that even in absence of genome, identification of chemical signals from wild animals is possible and could be helpful in strategies of species control and protection.

The olfactory secretome varies according to season in female sheep and goat

Background

Small ungulates (sheep and goat) display a seasonal breeding, characterised by two successive periods, sexual activity (SA) and sexual rest (SR). Odours emitted by a sexually active male can reactivate the ovulatory cycle of anoestrus females. The plasticity of the olfactory system under these hormonal changes has never been explored at the peripheral level of odours reception. As it was shown in pig that the olfactory secretome (proteins secreted in the nasal mucus) could be modified under hormonal control, we monitored its composition in females of both species through several reproductive seasons, thanks to a non-invasive sampling of olfactory mucus. For this purpose, two-dimensional gel electrophoresis (2D-E), western-blot with specific antibodies, MALDI-TOF and high-resolution (nano-LC-MS/MS) mass spectrometry, RACE-PCR and molecular modelling were used.

Results

In both species the olfactory secretome is composed of isoforms of OBP-like proteins, generated by post-translational modifications, as phosphorylation, N-glycosylation and O-GlcNAcylation. Important changes were observed in the olfactory secretome between the sexual rest and the sexual activity periods, characterised in ewe by the specific expression of SAL-like proteins and the emergence of OBPs O-GlcNAcylation. In goat, the differences between SA and SR did not come from new proteins expression, but from different post-translational modifications, the main difference between the SA and SR secretome being the number of isoforms of each protein. Proteomics data are available via ProteomeXchange with identifier PXD014833.

Conclusion

Despite common behaviour, seasonal breeding, and genetic resources, the two species seem to adapt their olfactory equipment in SA by different modalities: the variation of olfactory secretome in ewe could correspond to a specialization to detect male odours only in SA, whereas in goat the stability of the olfactory secretome could indicate a constant capacity of odours detection suggesting that the hallmark of SA in goat might be the emission of specific odours by the sexually active male. In both species, the olfactory secretome is a phenotype reflecting the physiological status of females, and could be used by breeders to monitor their receptivity to the male effect.

Molecular complexity of the major urinary protein system of the Norway rat, Rattus norvegicus

Major urinary proteins (MUP) are the major component of the urinary protein fraction in house mice (Mus spp.) and rats (Rattus spp.). The structure, polymorphism and functions of these lipocalins have been well described in the western European house mouse (Mus musculus domesticus), clarifying their role in semiochemical communication. The complexity of these roles in the mouse raises the question of similar functions in other rodents, including the Norway rat, Rattus norvegicus. Norway rats express MUPs in urine but information about specific MUP isoform sequences and functions is limited. In this study, we present a detailed molecular characterization of the MUP proteoforms expressed in the urine of two laboratory strains, Wistar Han and Brown Norway, and wild caught animals, using a combination of manual gene annotation, intact protein mass spectrometry and bottom-up mass spectrometry-based proteomic approaches. Cluster analysis shows the existence of only 10 predicted mup genes. Further, detailed sequencing of the urinary MUP isoforms reveals a less complex pattern of primary sequence polymorphism in the rat than the mouse. However, unlike the mouse, rat MUPs exhibit added complexity in the form of post-translational modifications, including the phosphorylation of Ser4 in some isoforms, and exoproteolytic trimming of specific isoforms. Our results raise the possibility that urinary MUPs may have different roles in rat chemical communication than those they play in the house mouse. Shotgun proteomics data are available via ProteomExchange with identifier PXD013986.