Role of inward rectifier potassium channels in salivary gland function and sugar feeding of the fruit fly, Drosophila melanogaster

Abstract

The arthropod salivary gland is of critical importance for horizontal transmission of pathogens, yet a detailed understanding of the ion conductance pathways responsible for saliva production and excretion is lacking. A superfamily of potassium ion channels, known as inward rectifying potassium (Kir) channels, is overexpressed in the Drosophila salivary gland by 32-fold when compared to the whole body mRNA transcripts. Therefore, we aimed to test the hypothesis that pharmacological and genetic depletion of salivary gland specific Kir channels alters the efficiency of the gland and reduced feeding capabilities using the fruit fly Drosophila melanogaster as a model organism that could predict similar effects in arthropod disease vectors. Exposure to VU041, a selective Kir channel blocker, reduced the volume of sucrose consumption by up to 3.2-fold and was found to be concentration-dependent with an EC of 68μM. Importantly, the inactive analog, VU937, was shown to not influence feeding, suggesting the reduction in feeding observed with VU041 is due to Kir channel inhibition. Next, we performed a salivary gland specific knockdown of Kir1 to assess the role of these channels specifically in the salivary gland. The genetically depleted fruit flies had a reduction in total volume ingested and an increase in the time spent feeding, both suggestive of a reduction in salivary gland function. Furthermore, a compensatory mechanism appears to be present at day 1 of RNAi-treated fruit flies, and is likely to be the Na-K-2Cl cotransporter and/or Na-K-ATPase pumps that serve to supplement the inward flow of K ions, which highlights the functional redundancy in control of ion flux in the salivary glands. These findings suggest that Kir channels likely provide, at least in part, a principal potassium conductance pathway in the Drosophila salivary gland that is required for sucrose feeding.

Chemical inhibition of Kir channels reduces salivary secretions and phloem feeding of the cotton aphid, Aphis gossypii (Glover)

“…The lack of poisoning symptomology after exposure to VU041 combined with the documented moderate inhibition potency of VU041 to aphid Kir channels suggests Kir channels may not be an acutely lethal target in aphids. Yet, although acute mortality is advantageous for aphid control, the lack of acute toxicity does not invalidate Kir channels as a novel target for aphicides because Kir channels have been shown to regulate insect systems that are likely to result in chronic mortality, such as reduced feeding efficacy …”

“…The realization that Kir channels constitute a critical conductance pathway in multiple arthropod systems 22,[24][25][26]34,49 has driven the discovery of multiple structurally diverse and specific inhibitors of insect Kir channels. 23,31,36 The growing pharmacological library targeting insect Kir channels has allowed for toxicological characterizations of insect Kir channels and data suggest these ion channels represent promising molecular targets for inducing acute lethality through the development of small-molecule inhibitors.…”

“…Inward rectifier potassium (Kir) channels conduct K + currents into cells at hyperpolarizing membrane potentials more readily than out of cells at depolarizing membrane potentials and have been found to play a critical role in multiple mammalian and arthropod physiological systems. In mammalian salivary glands, Kir channels drive anion efflux across the apical membrane of salivary exocrine acinar cells with minimum loss of K + from the cell as a means to support resting secretion .…”

“…Similarly, several recent lines of genetic and pharmacological evidence suggest Kir channels play important physiological roles in exocrine systems of dipteran insects as well. For instance, genetic depletion or pharmacological inhibition of Kir channels have been shown to reduce the fluid and ion secretion of dipteran Malpighian tubules and previous work has indicated Kir channels are critical for the salivary gland function and feeding of arthropods pests . The development of insect Kir channel pharmacology has enabled researchers to begin to characterize the physiological role of these channels in various arthropod tissue systems as well as determine the toxicological relevance to pest species.…”

“…For instance, genetic depletion or pharmacological inhibition of Kir channels have been shown to reduce the fluid and ion secretion of dipteran Malpighian tubules 24,[31][32][33][34] and previous work has indicated Kir channels are critical for the salivary gland function and feeding of arthropods pests. 25,26,35 The development of insect Kir channel pharmacology 23,34,36,37 has enabled researchers to begin to characterize the physiological role of these channels in various arthropod tissue systems as well as determine the toxicological relevance to pest species. For instance, the insect-specific Kir inhibitor termed VU041 was shown to inhibit A. glycines (soybean aphid) Kir1 and Kir2 currents in electrophysiological studies and topical application resulted in acute mortality, 38 indicating Kir channels are important for aphid survivorship.…”