Abundant secretory lipocalins displaying male and lactation-specific expression in adult hamster submandibular gland. cDNA cloning and sex hormone-regulated repression

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.

Male-specific submaxillary gland protein, a lipocalin allergen of the golden hamster, differs from the lipocalin allergens of Siberian and Roborovski dwarf hamsters

BACKGROUND

An increasing number of asthma cases upon exposure to hamsters and anaphylactic reactions following hamster bites are being reported, but the allergens responsible are still poorly characterized. In the Golden hamster, male-specific submaxillary gland protein (MSP), a lipocalin expressed in a sex- and tissue-specific manner in the submaxillary and lacrimal glands, is secreted in the saliva, tears and urine. The purpose of this study was to determine if MSP is an allergen, to identify IgE-reactive proteins of different hamster species and to analyse potential cross-reactivities.

METHODS

Fur extracts were prepared from four hamster species. Hamster-allergic patients were selected based on a history of positive IgE-test to hamster epithelium. The IgE-reactivity of patients' sera was investigated by means of immunoblot and ELISA. IgE-reactive proteins in fur extracts and the submaxillary gland were identified using anti-MSP antibodies, Edman sequencing or mass spectrometry. MSP was purified from Golden hamster and recombinant MSP was expressed in E. coli.

RESULTS

Four patients had IgE-antibodies against 20.5-kDa and 24-kDa proteins of Golden hamster fur extract, which were identified as MSP. IgE-reactive MSP-like proteins were detected in European hamster fur extract. Three patient sera showed IgE-reactive bands at 17-21 kDa in Siberian and Roborovski hamster fur extracts. These proteins were identified as two closely related lipocalins. Immunoblot inhibition experiments showed that they are cross-reactive and are different from MSP.

CONCLUSION

MSP lipocalin of the Golden hamster was identified as an allergen, and it is different from the cross-reactive lipocalin allergens of Siberian and Roborovski hamsters. Our findings highlight the need for specific tools for the in vitro and in vivo diagnosis of allergy to different hamster species.

Copious urinary excretion of a male Syrian hamster (Mesocricetus auratus) salivary gland protein after its endocrine-like release upon β-adrenergic stimulation

Salivary glands, although widely considered as typically exocrine, may also release specific proteins in an endocrine manner. However, endocrine release of salivary gland proteins is not generally acknowledged since the evidences are not easily demonstrable. Submandibular salivary glands (SMG) of male Syrian hamsters express male-specific secretory proteins (MSP; which are lipocalins) visible in SDS-PAGE of SMG extracts, as major bands and also detectable in immunoblots of whole-saliva and urine as low MSP crossreactions. We report here that MSP is localized in acinar cells of SMG and acute treatment with isoproterenol (IPR; non-specific β1/β2-adrenergic agonist) results in considerable release of MSP in SMG-saliva. Moreover, acute IPR treatment markedly depletes SMG-MSP in a dose- and time-dependent manner. However, MSP depleted from SMG, far exceeds that recovered in SMG-saliva. Blood, submandibular lymph nodes and kidney of IPR-treated males showed MSP crossreactions and SDS-PAGE of their urine revealed profuse MSP excretion; this was undetectable in IPR-treated-SMG-ablated males, confirming that a substantial amount of MSP depleted from SMG after IPR treatment enters circulation and is excreted in urine. Treatments with specific β1- or β2-adrenergic agonists also reduced SMG-MSP levels and resulted in copious urinary excretion of MSP. Co-treatments with specific β1/β2-blockers indicated that above effects of IPR, β1- and even β2-agonists are very likely mediated by β1-adrenoceptors. MSP's detection by SDS-PAGE in urine after β-agonist treatment is a compelling and easily demonstrable evidence of release into circulation of a salivary gland protein. The possible means (endocrine-like or otherwise) of MSP's release into circulation and significance of its presence in saliva, blood and urine of male hamsters are discussed.

Using mass spectrometry to detect buffalo salivary odorant-binding protein and its post-translational modifications

A large number of mammalian odorant-binding proteins, which are lipocalins, have been studied. These proteins participate in peri-receptor events by selecting and carrying odorant molecules. The present study aimed at identifying the buffalo salivary odorant-binding protein (sOBP), and to determine its post-translational modification using mass spectrometry. The buffalo salivary 21  kDa protein was initially separated adopting sodium dodecyl sulfate-polyacrylamide gel electrophoresis and it was identified as sOBP with high statistical reliability using liquid chromatography/tandem mass spectrometry (LC/MS/MS) and SEQUEST, for the first time. Further, the post-translationally modified peptides were screened adopting MS/MS. A total of four post-translational modifications, namely glycation at lysine-(59), hydroxylation at lysine-(134), ubiquitination at lysine-(121), and dihydroxylation in lysine-(108), were recorded. Moreover, these modifications have not been identified in buffalo salivary odorant-binding protein.

Cloning, overexpression, purification, crystallization and preliminary X-ray analysis of a female-specific lipocalin (FLP) expressed in the lacrimal glands of Syrian hamsters

Proteins belonging to the lipocalin superfamily are usually secretory proteins of molecular mass approximately 20 kDa with a hydrophobic pocket for the binding and transport of diverse small ligands. Various lipocalins have been associated with many biological processes, e.g. immunomodulation, odorant transport, pheromonal activity, retinoid transport, cancer-cell interactions etc. However, the exact functions of many lipocalins and the ligands bound by them are unclear. Previously, the cDNA of a 20 kDa lipocalin (FLP) which is female-specifically expressed in the lacrimal glands of Syrian (golden) hamsters and secreted in the tears of females has been identified and cloned. His-tagged recombinant FLP (rFLP) has now been cloned, overexpressed in Escherichia coli as a soluble protein and purified to homogeneity using Ni-affinity followed by size-exclusion chromatography. Purified rFLP was crystallized using the sitting-drop vapour-diffusion method. The crystals tested belonged to space group P2(1)2(1)2(1) and diffracted to beyond 1.86 A resolution. Solvent-content analysis indicated the presence of one monomer in the asymmetric unit.

Novel OBP genes similar to hamster Aphrodisin in the bank vole, Myodes glareolus

Background

Chemical communication in mammals involves globular lipocalins that protect and transport pheromones during their passage out of the body. Efficient communication via this protein - pheromone complex is essential for triggering multiple responses including aggression, mate choice, copulatory behaviour, and onset and synchronization of oestrus. The roles of lipocalins in communication were studied in many organisms and especially in mice (i.e. Mus musculus domesticus) which excrete Major Urinary Proteins (Mup) in excessive amounts in saliva and urine. Other mammals, however, often lack the genes for Mups or their expression is very low. Therefore, we aimed at characterization of candidate lipocalins in Myodes glareolus which are potentially linked to chemical communication. One of them is Aphrodisin which is a unique lipocalin that was previously described from hamster vaginal discharge and is known to carry pheromones stimulating copulatory behaviour in males.

Results

Here we show that Aphrodisin-like proteins exist in other species, belong to a group of Odorant Binding Proteins (Obp), and contrary to the expression of Aphrodisin only in hamster genital tract and parotid glands of females, we have detected these transcripts in both sexes of M. glareolus with the expression confirmed in various tissues including prostate, prepucial and salivary glands, liver and uterus. On the level of mRNA, we have detected three different gene variants. To assess their relevance for chemical communication we investigated the occurrence of particular proteins in saliva, urine and vaginal discharge. On the protein level we confirmed the presence of Obp2 and Obp3 in both saliva and urine. Appropriate bands in the range of 17-20 kDa from vaginal discharge were, however, beyond the MS detection limits.

Conclusion

Our results demonstrate that three novel Obps (Obp1, Obp2, and Obp3) are predominant lipocalins in Myodes urine and saliva. On the protein level we have detected further variants and thus we assume that similarly as Major Urinary Proteins in mice, these proteins may be important in chemical communication in this Cricetid rodent.

Histological and biochemical characterization of von Ebner's glands in the Syrian hamster; comparison with rat von Ebner's glands

We report here for the first time a morphological description and observations on some of the secretory proteins of the von Ebner's lingual salivary glands (VEG) of the Syrian hamster. Hamster VEG were macroscopically less distinct, but histologically similar to rat VEG. VEG extracts of hamster and rat were assayed for lipase, alpha-amylase and peroxidase activities. Unlike rat VEG, which is rich in lipase activity, hamster VEG extract had no detectable lipase activity and did not react with antibodies to either rat lingual lipase or human gastric lipase in Western blots. Immunohistochemical reactions with the anti-rat lingual lipase antibody were very weak in hamster VEG and strong in rat VEG. Moderate alpha-amylase enzyme activities and immunohistochemical reactions were demonstrated in both hamster and rat VEG. Peroxidase activity was negligible in the VEG, unlike the high activity in the submandibular glands of both species. An 18 kDa von Ebner's gland protein (VEGP), a member of the lipocalin superfamily of hydrophobic ligandbinding proteins, was abundant in rat VEG, but not detected in hamster VEG. Thus, hamster VEG differs from rat VEG in macroscopic appearance and the absence of lipase and VEGP. It is similar to rat VEG histologically and with regard to the presence of alpha-amylase and absence of peroxidase.

Sex differences in expression and differential regulation by androgen and estrogen of two odorant-binding tear lipocalins in lacrimal glands of immature hamsters

In adults of several mammalian species, lacrimal glands (LG) have sex differences but there is no report of any sexual dimorphism in LG of immatures. In LG and tears of adult hamsters, we found female-specific expression of two closely related odorant-/pheromone-binding lipocalins, FLP (female lacrimal protein) and MSP (male-specific protein; initially identified in salivary glands of males). Although, both androgens and estrogens markedly repress FLP and MSP in LG of adults, the expression of these lipocalins in females is due to their incomplete repression by endogenous estrogens. Here we report a marked sexual dimorphism in the expression of FLP and MSP in LG and tears of 20-day-old immature hamsters. The age-dependant expression of these lipocalins and effect of neonatal-gonadectomy and sex hormone treatments on their expression in immatures was investigated. FLP and MSP are detectable in LG at 10-day age in both sexes of hamster but by 20-day age levels of both lipocalins show sex differences wherein FLP is several fold higher in males and MSP is obliterated in males. Thereafter, FLP declines in male LG and is obliterated by 36-day age, resulting in female-specific expression of both LG lipocalins as seen in adults. In LG of 20-day-old immatures, FLP and MSP are insensitive to repression by androgen and estrogen, respectively, which was unlike the androgen/estrogen-repressed regulation of both lipocalins in adult LG. The estrogenic repression of FLP and androgenic repression of MSP in LG of immature hamsters could be prevented by treatment with tamoxifen and flutamide, respectively. Our studies indicate that (i) presence of gonads in immatures can have significant effects on LG lipocalins resulting in their sexually dimorphic expression, (ii) in immatures, unlike adults, the repressive effects of estrogen and androgen on LG lipocalins are selective for FLP and MSP, respectively, and (iii) these repressions are likely to be mediated by sex hormone receptors.

Male-specific protein (MSP): a new gene linked to sexual behavior and aggressiveness of tilapia males

MSP is a male-specific protein initially identified in the serum of sexually active Sarotherodon galilaeus males, and is shown herein to be present in the serum of sexually mature males, but not females, of three other tilapia species. Cloning of the MSP cDNA and analysis of its predicted amino-acid sequence revealed that it is an outlier lipocalin that contains a signal peptide in its N-terminal region. The abundance of highly homologous sequences found in fish and the monophyletic relationship to tetrapod Alpha-1-acid glycoprotein (AGP) places it as a clade XII lipocalin. MSP was shown to undergo major N-glycosylation, characteristic of many lipocalins. The expression pattern of MSP, as determined at both the RNA and protein levels, points to the liver, head kidney and testis as production tissues, and resembles a pattern typical of some hormones. We found that MSP is secreted in urine and seminal fluids, and is present in the skin mucus of socially dominant males. Moreover, we discovered a positive correlation between MSP levels in the serum and the dominance and aggressive behavior displayed by socially dominant males. Based on these data, we suggest that MSP is a novel male-specific lipocalin that may function in intra and inter-sex communication.

Estrogen and androgen repression of two female specific lacrimal lipocalins in hamster: Pituitary independent and sex hormone receptor mediated action

Sexual dimorphism in lacrimal gland (LG) gene expression is believed to be due to direct inductive effects of androgens mediated by androgen receptors (AR) but hypophysectomy dramatically curtails these inductive effects. Since, functional estrogen receptors (ER) could not be detected in LG, estrogen effects on LG are believed to be indirectly mediated by changes in levels of pituitary hormones. We found that two lipocalins expressed in female hamster LG display an unusual and marked repression by both androgens and estrogens, which could be detected both at the level of transcripts and proteins. Here, we investigate whether these repressions, (i) require presence of pituitary and (ii) are mediated by androgen and estrogen receptors. Pituitary-ablation but not gonadectomy reduced LG weights in hamster. However, both pituitary-ablation and gonadectomy induced abundant expression of the LG lipocalins, which were markedly repressed by androgen or estrogen treatment. AR- and ER-antagonists prevented these repressions and only ER-alpha- but not ER-beta-specific agonist could mimic the estrogenic repression. AR transcript and protein and ER-alpha transcript were also detected in hamster LG. Thus, pituitary factors are neither essential for the expression of these LG lipocalins nor for their estrogenic or androgenic repressions and these repressions are very likely mediated by functional ER and AR present in LG.

Urinary pheromones promote ERK/Akt phosphorylation, regeneration and survival of vomeronasal (V2R) neurons

The G protein-coupled pheromone receptor neurons (V1R and V2R) of the vomeronasal organ (VNO) are continually replaced throughout the lifetime of the mouse. Moreover, active signalling of V2Rs via the transient receptor potential 2(TRPC2) channel is necessary for regeneration of receptors, as the TRPC2 null mutant mouse showed a 75% reduction of V2Rs by the age of two months. Here we describe V2R mediated signalling in a neuronal line established from vomeronasal stem cells taken from postnatal female mice. Cells were immunoreactive for Galpha(o) and V2R, whereas V1R and Galpha(i) immunoreactivity could not be detected. Biological ligands (dilute urine and its protein fractions) were found to increase proliferation and survival of these neurons. Dilute mouse urine but not artificial urine also induced ERK, Akt and CREB signalling in a dose dependent way. The volatile fraction of male mouse urine alone was without effect while the fraction containing peptides (> 5 kDa) also stimulated ERK and Akt phosphorylation. The ERK, Akt and CREB phosphorylation response was sensitive to pertussis toxin, confirming the involvement of V2R linked Galpha(o). Dilute mouse urine or its high molecular weight protein fraction increased survival and proliferation of these neurons. Hence, urinary pheromones, which signal important social information via mature neurons, also promote survival and proliferation of their regenerating precursors. These data show that regenerating V2Rs respond to urine and the urinary peptides by activation of the Ras-ERK and PI3-Akt pathways, which appear to be important for vomeronasal neural survival and proliferation.

Purification, cloning and regulation of a novel acid-lipase-like protein of hamster expressed in lacrimal glands and tears during lactation

We report a novel 48-kDa tear acid-lipase-like protein (TALLP), which is markedly induced in lacrimal glands (LG) and secreted in tears of hamster dams during lactation. TALLP is undetectable in LG and tears of normal hamsters, but is also induced after gonadectomy in both sexes and this is prevented by androgen, estrogen or thyroid hormone treatment. These observations and the obliteration of TALLP upon cessation of lactation suggest that endogenous estrogens (in females) and androgens (in males) completely repress TALLP expression. Purified TALLP is monomeric, contains approximately 18% N-glycosylation and several pI isoforms. TALLP expression was tissue-specific and immunolocalized in LG acinar cells. The cDNA deduced amino-acid sequence of TALLP precursor (398 residue, containing a 19 residues signal-peptide) showed only 43-48% identity with all known mammalian acid-lipases, including even those of other rodents, suggesting that TALLP is a prototype of a new category, within the acid-lipase family. Surprisingly, although the catalytic triad residues and other sequence features important for lipolytic activity are conserved in TALLP, it has no detectable lipase activity. However, TALLP binds the polarity sensitive hydrophobic probe, 1-aminoanthracene (K(d)=12 microM). TALLP might have a unique substrate-specificity or a lipid-binding/carrier function in tears of hamster dams. This is the first report of an acid-lipase-like protein secreted in tears of any species. Since TALLP lacks the usual lipase activity, it can be an excellent model to understand better what other structural features in acid-lipases influence their catalytic activity.

cDNA cloning and regulation of two sex-hormone-repressed hamster tear lipocalins having homology with odorant/pheromone-binding proteins

A major 20-kDa protein is female-specifically expressed in exorbital lacrimal gland (LG) of hamsters and secreted in tears. Here, we identify this female-specific LG protein (FLP) as a lipocalin, having 85% protein sequence identity with male-specific submandibular salivary gland proteins (MSP) secreted in saliva and urine of male hamsters. MSP is also female-specifically expressed in LG and secreted in tears but FLP was undetectable in submandibular gland (SMG). FLP and MSP have similar sex-hormonal regulation in LG, which is different from regulation of MSP in SMG. Female-specific expression of FLP and MSP in LG is due to their incomplete repression by endogenous estrogens and gonadectomy in both sexes and lactation in females resulted in their marked induction, which was prevented by estrogen or androgen treatment. FLP and MSP show best sequence identity with odorant/pheromone-binding lipocalins (58-29%). Maximum identity (58%) is with rat odorant-binding protein (OBP) expressed in lateral nasal glands, followed by aphrodisin of hamster vaginal discharge (39%). Cognate transcript and a cross-reacting 20-kDa protein were detected in nasal glands of rat in both sexes but not in hamsters. Results suggest that two closely related lipocalin genes encode FLP and MSP, which are evolutionarily closer to rat OBP than to hamster aphrodisin and these have evolved different tissue-specificity and sex-hormonal regulation. Possible functions for FLP and MSP are suggested, considering their homology to odorant/pheromone-binding lipocalins, their presence in tears, saliva and urine as well as their sex-specific and lactation-induced expression.

Aphrodisin, an aphrodisiac lipocalin secreted in hamster vaginal secretions

Vertebrates communicate through pheromones, which favor biological regulations within each species. Aphrodisin, a protein belonging to the lipocalin superfamily, found in hamster vaginal secretions, is detected by the male accessory olfactory system and induces or facilitates its copulatory behavior. Although much is known about aphrodisin structure, the question of whether aphrodisin bears itself the pheromonal function or is simply a carrier for hydrophobic small pheromones has not been definitely solved. Arguments based on use of recombinant aphrodisin deprived of any natural ligand and its capability to convey hamster pheromonal compounds will be discussed, together with progresses concerning putative natural ligand(s).

Steroid hormone modulation of olfactory processing in the context of socio-sexual behaviors in rodents and humans

Primer pheromones and other chemosensory cues are important factors governing social interactions and reproductive physiology in many species of mammals. Responses to these chemosignals can vary substantially within and between individuals. This variability can stem, at least in part, from the modulating effects steroid and non-steroid hormones exert on olfactory processing. Such modulation frequently augments or facilitates the effects that prevailing social and environmental conditions have on the reproductive axis. The mechanisms underlying the hormonal regulation of responses to chemosensory cues are diverse. They are in part behavioral, achieved through the modulation of chemoinvestigative behaviors, and in part a product of the modulation of the intrinsic responsiveness of the main and accessory olfactory systems to conspecific, as well as other classes, of chemosignals. The behavioral and non-behavioral effects complement one another to ensure that mating and other reproductive processes are confined to reproductively favorable conditions.

Crystal structure of aphrodisin, a sex pheromone from female hamster

We have solved the crystal structure of aphrodisin, a pheromonal protein inducing a copulatory behaviour in male hamster, using MAD methods with selenium, at 1.63 A resolution. The monomeric protein belongs to the lipocalin family, and possesses a disulfide bridge in a loop between strands 2 and 3. This disulfide bridge is characteristic of a family of lipocalins mainly identified in rodents, and is analogous to the fifth disulfide bridge of the long neurotoxins, such as alpha cobratoxin. An elongated electron density was found inside the buried cavity, which might represent a serendipitous ligand of unknown origin. The analysis of the water accessible surfaces of the side-chains bordering the cavity indicates that Phe76 may be the door for the natural ligand to access the cavity. This residue defines the entry of the cavity as belonging to the consensus for lipocalins. The face bearing Phe76 might also serve for the interaction with the receptor.

Odorant and pheromone binding by aphrodisin, a hamster aphrodisiac protein

Aphrodisin is a soluble glycoprotein of hamster vaginal discharges, which stimulates male copulatory behavior. Natural aphrodisin was purified and its post-translational modifications characterized by MALDI-MS peptide mapping. To evaluate its ability to bind small volatile ligands, the aphrodisiac protein was expressed in the yeast Pichia pastoris as two major isoforms differing in their glycosylation degree, but close in conformation to the natural protein. Dimeric recombinant aphrodisins were equally able to efficiently bind odors (2-isobutyl-3-methoxypyrazine and methyl thiobutyrate) and a pheromone (dimethyl disulfide), suggesting that they could act as pheromone carriers instead of, or in addition to, direct vomeronasal neuron receptor activators.