Saliva of blood-sucking arthropods contains a complex and diverse mixture of antihemostatic, antiinflammatory, and immunomodulatory compounds. The D7 salivary family of proteins is abundantly expressed in blood-feeding Diptera and is distantly related to the odorant-binding protein superfamily. In mosquitoes, two subfamilies exist, the long and short D7 proteins. Ticks and kissing bugs evolved salivary lipocalins that act as efficient scavengers of biogenic amines, and a similar function was postulated for the D7 proteins. Accordingly, we expressed the five members of the small D7 family of the African malaria vector Anopheles gambiae and a D7 long form from Aedes aegypti and showed by isothermal microcalorimetry, a modified and very sensitive non-equilibrium chromatography/spectrum distortion method, and by smooth muscle bioassay that four of these five short D7 proteins and the D7 long form bind serotonin with high affinity, as well as histamine and norepinephrine. The nonbinding D7 protein is poorly expressed in the salivary glands and appears to be on the path to becoming a pseudogene. Scavenging of host amines would antagonize their vasoconstrictor, platelet-aggregating, and pain-inducing properties. It appears that counteracting biogenic amines is of strong adaptive value in the convergent evolution of arthropods to hematophagy. This adaptation has been solved independently in ticks, bugs, and mosquitoes by co-option of either member of the lipocalin or, as shown here, by the odorant-binding protein families.

Saliva of blood-sucking arthropods contains a cocktail of antihemostatic agents and immunomodulators that help blood feeding. Mosquitoes additionally feed on sugar meals and have specialized regions of their glands containing glycosidases and antimicrobials that might help control bacterial growth in the ingested meals. To expand our knowledge on the salivary cocktail of Ædes ægypti, a vector of dengue and yellow fevers, we analyzed a set of 4,232 expressed sequence tags from cDNA libraries of adult female mosquitoes. A nonredundant catalogue of 614 transcripts (573 of which are novel) is described, including 136 coding for proteins of a putative secretory nature. Additionally, a two-dimensional gel electrophoresis of salivary gland (SG) homogenates followed by tryptic digestion of selected protein bands and MS/MS analysis revealed the expression of 24 proteins. Analysis of tissue-specific transcription of a subset of these genes revealed at least 31 genes whose expression is specific or enriched in female SG, whereas 24 additional genes were expressed in female SG and in males but not in other female tissues. Most of the 55 proteins coded by these SG transcripts have no known function and represent high-priority candidates for expression and functional analysis as antihemostatic or antimicrobial agents. An unexpected finding is the occurrence of four protein families specific to SG that were probably a product of horizontal transfer from prokaryotic organisms to mosquitoes. Overall, this paper contributes to the novel identification of 573 new transcripts, or near 3% of the Æ. ægypti proteome assuming a 20,000-protein set, and to the best-described sialome of any blood-feeding insect.

CD300a is an immunoreceptor tyrosine-based inhibitory motif (ITIM) containing molecule that belongs to the CD300 family of paired activating/inhibitory receptors. It has been shown that its ligation inhibits activation signals on cells of both myeloid and lymphoid lineages. The ligands for CD300a have not been identified. Here, we show that a CD300a-Ig fusion protein specifically binds to apoptotic cells that are evolutionary apart, such as human and insect cells, suggesting that the ligand has to be conserved. Using surface plasmon resonance, ultracentrifugation, ELISA, and reporter cell assays, we identified phosphatidylethanolamine (PE) and phosphatidylserine (PS), 2 phospholipids that translocate to the outer leaflet of the plasma membrane of dead cells, as the ligands for CD300a. Mutational and structural modeling studies identified residues that are involved in the binding of CD300a to PE and PS and that form a cavity where the hydrophilic heads of PE and PS, can penetrate. CD300a down-regulates the uptake of apoptotic cells by macrophages and its ectopic expression in CD300a-negative cell lines also decreased the engulfment of dead cells. Collectively, our results indicate that PE and PS are ligands for CD300a, and that this interaction plays an important role in regulating the removal of dead cells.

The mosquito D7 salivary proteins are encoded by a multigene family related to the arthropod odorant-binding protein (OBP) superfamily. Forms having either one or two OBP domains are found in mosquito saliva. Four single-domain and one two-domain D7 proteins from Anopheles gambiae and Aedes aegypti (AeD7), respectively, were shown to bind biogenic amines with high affinity and with a stoichiometry of one ligand per protein molecule. Sequence comparisons indicated that only the C-terminal domain of AeD7 is homologous to the single-domain proteins from A. gambiae , suggesting that the N-terminal domain may bind a different class of ligands. Here, we describe the 3D structure of AeD7 and examine the ligand-binding characteristics of the N- and C-terminal domains. Isothermal titration calorimetry and ligand complex crystal structures show that the N-terminal domain binds cysteinyl leukotrienes (cysLTs) with high affinities (50–60 nM) whereas the C-terminal domain binds biogenic amines. The lipid chain of the cysLT binds in a hydrophobic pocket of the N-terminal domain, whereas binding of norepinephrine leads to an ordering of the C-terminal portion of the C-terminal domain into an α-helix that, along with rotations of Arg-176 and Glu-268 side chains, acts to bury the bound ligand.