Lipocalins in Arthropod Chemical Communication

Chemosensory function of Varroa gnathosoma: transcriptomic and proteomic analyses

In this study, we evaluated the role of the gnathosoma (mouthparts) in chemosensing of the most devastating honey bee parasite, Varroa destructor mite. Through transcriptomic analysis, we compared the expression of putative chemosensory genes between the body parts containing the main chemosensory organs (the forelegs), gnathosoma and the rest of the body devoid of these two body parts. Furthermore, we checked the presence of chemosensory-related transcripts in the proteome of the gnathosoma. Our comparative transcriptomic analysis revealed the presence of 83 transcripts with known characteristic conserved domains belonging to eight chemosensory gene families in the three Varroa transcriptomes. Among these transcripts, 11 were significantly upregulated in the mite's forelegs, compared to 8 and 10 in the gnathosoma and body devoid of both organs, respectively. Whilst the gnathosoma and the forelegs share similar expression of some putative lipid carrier proteins, membrane-bound receptors, and associated proteins, they also differ in the expression profiles of some transcripts belonging to these protein families. This suggests two functional chemosensory organs that may differ in their chemosensory function according to specific characteristics of compounds they detect. Moreover, the higher expression of some chemosensory transcripts in the body devoid of forelegs and gnathosoma compared to the gnathosoma alone, may suggest the presence of additional function of these transcripts or alternatively presence of additional external or internal chemosensory organs. Insights into the functional annotation of a highly expressed gustatory receptor present in both organs using RNA interference (RNAi) are also revealed.

Tick salivary protein Cystatin: structure, anti-inflammation and molecular mechanism

Ticks are blood-sucking ectoparasites that secrete immunomodulatory substances in saliva to hosts during engorging. Cystatins, a tick salivary protein and natural inhibitor of Cathepsins, are attracting growing interest globally because of the immunosuppressive activities and the feasibility as an antigen for developing anti-tick vaccines. This review outlines the classification and the structure of tick Cystatins, and focuses on the anti-inflammatory effects and molecular mechanisms. Tick Cystatins can be divided into four families based on structures and cystatin 1 and cystatin 2 are the most abundant. They are injected into hosts during blood feeding and effectively mitigate the host inflammatory response. Mechanically, tick Cystatins exert anti-inflammatory properties through the inhibition of TLR-NF-κb, JAK-STAT and p38 MAPK signaling pathways. Further investigations are crucial to confirm the reduction of inflammation in other cell types like neutrophils and mast cells, and fully elucidate the underlying mechanism (like the structural mechanism) to make Cystatin a potential candidate for the development of novel anti-inflammation agents.

Chemosensory proteins as putative semiochemical carriers in the desert isopod Hemilepistus reaumurii

The order Isopoda contains both aquatic and terrestrial species, among which Hemilepistus reaumurii, which lives in arid environments and is the most adapted to terrestrial life. Olfaction has been deeply investigated in insects while it has received very limited attention in other arthropods, particularly in terrestrial crustaceans. In insects, soluble proteins belonging to two main families, Odorant Binding Proteins (OBPs) and Chemosensory Proteins (CSPs), are contained in the olfactory sensillar lymph and are suggested to act as carriers of hydrophobic semiochemicals to or from membrane-bound olfactory receptors. Other protein families, namely Nieman-Pick type 2 (NPC2) and Lipocalins (LCNs) have been also reported as putative odorant carriers in insects and other arthropod clades. In this study, we have sequenced and analysed the transcriptomes of antennae and of the first pair of legs of H. reaumurii focusing on soluble olfactory proteins. Interestingly, we have found 13 genes encoding CSPs, whose sequences differ from those of the other arthropod clades, including non-isopod crustaceans, for the presence of two additional cysteine residues, besides the four conserved ones. Binding assays on two of these proteins showed strong affinities for fatty acids and long-chain unsaturated esters and aldehydes, putative semiochemicals for this species.

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.

The diversity of lipocalin receptors

Lipocalins are important carriers of preferentially hydrophobic molecules, but they can also bind other ligands, like highly polar siderophores or intact proteins. Consequently, they are involved in a variety of physiological processes in many species. Since lipocalins are mainly extracellular proteins, they have to interact with cell receptors to exert their biological effects. In contrast to the large number of lipocalins identified in the last years, the number of receptors known is still limited. Nevertheless, some novel findings concerning the molecules involved in cellular uptake or signaling effects of lipocalins have been made recently. This review presents a detailed overview of the receptors identified so far. The methods used for isolation or identification are described and structural as well as functional information on these proteins is presented essentially in chronological order of their initial discovery.

The Odorant-Binding Proteins of the Spider Mite Tetranychus urticae

Spider mites are one of the major agricultural pests, feeding on a large variety of plants. As a contribution to understanding chemical communication in these arthropods, we have characterized a recently discovered class of odorant-binding proteins (OBPs) in Tetranychus urticae. As in other species of Chelicerata, the four OBPs of T. urticae contain six conserved cysteines paired in a pattern (C1-C6, C2-C3, C4-C5) differing from that of insect counterparts (C1-C3, C2-C5, C4-C6). Proteomic analysis uncovered a second family of OBPs, including twelve members that are likely to be unique to T. urticae. A three-dimensional model of TurtOBP1, built on the recent X-ray structure of Varroa destructor OBP1, shows protein folding different from that of insect OBPs, although with some common features. Ligand-binding experiments indicated some affinity to coniferyl aldehyde, but specific ligands may still need to be found among very large molecules, as suggested by the size of the binding pocket.