Abstract
The principal Afrotropical malaria vector mosquito, Anopheles gambiae remains a significant threat to human health. In this anthropophagic species, females detect and respond to a range of human-derived volatile kairomones such as ammonia, lactic acid, and other carboxylic acids in their quest for blood meals. While the molecular underpinnings of mosquito olfaction and host seeking are becoming better understood, many questions remain unanswered. In this study, we have identified and characterized two candidate ammonium transporter genes, AgAmt and AgRh50 that are expressed in the mosquito antenna and may contribute to physiological and behavioral responses to ammonia, which is an important host kairomone for vector mosquitoes. AgAmt transcripts are highly enhanced in female antennae while a splice variant of AgRh50 appears to be antennal-specific. Functional expression of AgAmt in Xenopus laevis oocytes facilitates inward currents in response to both ammonium and methylammonium, while AgRh50 is able to partially complement a yeast ammonium transporter mutant strain, validating their conserved roles as ammonium transporters. We present evidence to suggest that both AgAmt and AgRh50 are in vivo ammonium transporters that are important for ammonia sensitivity in An. gambiae antennae, either by clearing ammonia from the sensillar lymph or by facilitating sensory neuron responses to environmental exposure. Accordingly, AgAmt and AgRh50 represent new and potentially important targets for the development of novel vector control strategies.
Highlights
Gaseous ammonia and its protonated ionic form, ammonium, are important molecules for life on earth
AgAmt and AgRh50 are clearly distinguishable as members of the two major ammonium transporter subfamilies (Figure 3) and share significant homologies with proteins encoded in several other insect genomes that are putative ammonium transporters (Figure 3, Table S1)
The AgRh50a transcript was more uniformly expressed with a lesser enhancement in heads (Table 1, Figure 4). These findings suggests partitioning of function between different isoforms of the resulting peptides, perhaps reflecting a more critical role for AgRh50a in ammonia clearance in the head and body and more specialized sensory roles for AgAmt and AgRh50b in the antennae
Summary
Gaseous ammonia and its protonated ionic form, ammonium, (collectively referred to as ammonium) are important molecules for life on earth. Cells have devised a number of mechanisms to deal with excess ammonia, including conjugation of amines to larger non-toxic compounds and secretion of ammonium in various forms [4,5,6]. The latter process requires the action of transmembrane proteins that increase the permeability of ammonium across cell membranes, and are classified as ammonium transporters [6,7]. Diverse mechanisms have evolved for the transport of ammonium and ammonium derivatives in cells These include the ammonium transporters (Amt) in bacteria and plants, the methylammonium/ammonium permeases (MEPs) in yeast, and the Rhesus (Rh) proteins in mammals [6,8,9]. A role for ammonium transporters in sensing environmental levels of ammonia has been described [10,11,12,13]
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