Inward rectifier potassium (Kir) channels mediate salivary gland function and blood feeding in the lone star tick, Amblyomma americanum

Abstract

Background Tick feeding causes extreme morbidity and mortality to humans through transmission of pathogens and causes severe economic losses to the agricultural industry by reducing livestock yield. Salivary gland secretions are essential for tick feeding and thus, reducing or preventing saliva secretions into the vertebrate host is likely to reduce feeding and hinder pathogen life cycles. Unfortunately, the membrane physiology of tick salivary glands is underexplored and this gap in knowledge limits the development of novel therapeutics for inducing cessation of tick feeding. Methodology We studied the influence of inward rectifier potassium (Kir) channel subtypes to the functional capacity of the isolated tick salivary gland through the use of a modified Ramsay assay. The secreted saliva was subsequently used for quantification of the elemental composition of the secreted saliva after the glands were exposed to K + channel modulators as a measure of osmoregulatory capacity. Lastly, changes to blood feeding behavior and mortality were measured with the use of a membrane feeding system. Principal findings In this study, we characterized the fundamental role of Kir channel subtypes in tick salivary gland function and provide evidence that pharmacological inhibition of these ion channels reduces the secretory activity of the Amblyomma americanum salivary gland. The reduced secretory capacity of the salivary gland was directly correlated with a dramatic reduction of blood ingestion during feeding. Further, exposure to small-molecule modulators of Kir channel subtypes induced mortality to ticks that is likely resultant from an altered osmoregulatory capacity. Conclusions Our data contribute to understanding of tick salivary gland function and could guide future campaigns aiming to develop chemical or reverse vaccinology technologies to reduce the worldwide burden of tick feeding and tick-vectored pathogens.

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

“…To this point, we observed high mortality at 48‐ and 72‐h time points after foliar applications of VU041 to living cotton plants, but no acute mortality was observed after ingestion or contact exposure to VU041. Previous reports have shown Kir channels are critical for proper function of the nervous system and osmoregulatory capacities of diptera, thus mortality in these species after exposure to Kir inhibitors is likely due to altered neurological function, an inability to osmoregulate, or a combination of both. However, the lack of acute toxicity or signs of intoxication in A. gossypii after ingesting VU041 or after contact exposure, combined with the lack of phloem feeding, suggests the delayed mortality for this species is likely resultant of nutrient deficiencies stemming from the lack of phloem feeding (Fig.…”

“…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 …”

“…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.…”