Function and Evolution of a Mosquito Salivary Protein Family

Abstract

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.At least 14 orders or families of arthropods (containing over 400 different genera and more than 15,000 species) independently evolved to feed on vertebrate blood (1). To accomplish this task, these animals evolved sophisticated cocktails of salivary pharmacologic reagents that affect blood clotting, platelet aggregation, vascular contraction, host immunity, inflammation, and angiogenesis. With the development of transcriptome analysis, the salivary compositional diversity of several hematophagous arthropods is being revealed at a fast pace; however, the majority of these proteins have no known function (2).Among many different families of proteins unique to hematophagous arthropods, the D7 family has been recognized to be specifically expressed in the salivary glands of adult Diptera. These proteins are distant relatives of the odorant-binding protein superfamily, of which they are a distinct branch. In mosquitoes, two D7 subfamilies exist, the short family having molecular mass of 15-20 kDa and the long with 27-30 kDa, whereas sand flies appear to have only the long forms (3-5). According to a recent sialotranscriptome analysis, Anopheles gambiae, the main African malaria vector, has three long and five short D7 proteins in chromosome arm 3R (6), arranged in an inverted tandem repeat. In the closely related mosquito Anopheles stephensi, one short D7 protein, named hamadarin, has been expressed and shown to have anticlotting activity (7). No other function has been described for the other protein memb...

An insight into the sialotranscriptome of the non-blood feeding Toxorhynchites amboinensis mosquito

“…Also absent are members of the 62-kDa and 34-kDa families, 30-kDa antigen, basic 7.6-kDa peptide, and basic 3.8-kDa peptide. Many of these absent families were abundantly transcribed in AEdes and Culex transcriptomes (Arca et al, 2002;Calvo et al, 2006a;James et al, 1991;Valenzuela et al, 2002a), and their complete absence in over 1,000 transcripts is significant, indicating that these families may play a role in blood feeding. Expression of these proteins in hemostasis and inflammation bioassays may help to uncover their function.…”

Comparative genomics allows the discovery of cis -regulatory elements in mosquitoes

“…Although this trait is not unique to this group of arthropods, acquiring the necessary nutrients for egg development from ingested blood is a specialized adaptation in insects. There is debate on when and how hematophagy arose in mosquito evolution (44), but it is hypothesized that the occurrence of this trait in the larger clade is monophyletic (45,46). Most of the discovered mosquito motifs are associated with blood meal-regulated genes.…”

Identity and transfer of male reproductive gland proteins of the dengue vector mosquito, Aedes aegypti: Potential tools for control of female feeding and reproduction

“…albopictus gene, D7L2 , a member of the D7 family of proteins (long form). Prior to our study, members of this protein family have previously only been reported from the salivary glands of blood-feeding Diptera (Calvo et al, 2006), to our knowledge. In the salivary glands, D7 proteins are proposed to act as scavengers of biogenic amines to fight the defenses of their vertebrate hosts.…”

“…aegypti long form D7 protein and several short form An. gambiae D7 proteins bind serotonin, histamine, and norepinephrine in vitro (Calvo et al, 2006). AAEL009239, a predicted CRISP (discussed in section 3.5.D), shares highest sequence similarity with a gene expressed in the salivary glands of female Ae.…”