Anti-diuretic action of a CAPA neuropeptide against a subset of diuretic hormones in the disease vector,Aedes aegypti.

The mosquito, Aedes aegypti, is a vector responsible for transmitting various pathogens to humans, and their prominence as chief vectors of human disease is largely due to their anthropophilic blood feeding behaviour. Larval stage mosquitoes must deal with the potential dilution of their haemolymph in fresh water, whereas the haematophagus A. aegypti female faces the challenge of excess ion and water intake after a blood meal. The excretory system, comprised of the Malpighian tubules (MTs) and hindgut, is strictly controlled by neuroendocrine factors, responsible for the control of diuresis across all developmental stages. The highly-studied insect MTs are influenced by a variety of diuretic hormones, and in some insects, anti-diuretic factors. In the present study, we investigated the effects of AedaeCAPA-1 on larval and adult female A. aegypti MTs stimulated with various diuretic factors including serotonin (5-HT), a corticotropin-related factor (CRF) diuretic peptide, a calcitonin-related diuretic hormone (DH31), and a kinin-related diuretic peptide. Overall, our findings establish that AedaeCAPA-1 specifically inhibits secretion of larval and adult MTs stimulated by 5-HT and DH31, whilst having no activity on MTs stimulated by other diuretic factors. Furthermore, although AedaeCAPA-1 acts as an anti-diuretic, it does not influence the relative proportions of cations transported by adult MTs, thus maintaining the kaliuretic activity of 5-HT and natriuretic activity of DH31. In addition, we tested the effects of the second messenger cGMP in adult MTs. Here, we established that cGMP displays similar effects as AedaeCAPA-1, strongly inhibiting 5-HT and DH31-stimulated fluid secretion, but with only minor effects on CRF-stimulated diuresis. Interestingly, although AedaeCAPA-1 has no inhibitory activity on kinin-stimulated fluid secretion, cGMP strongly inhibited fluid secretion by this diuretic hormone, which targets stellate cells specifically. Collectively, these results support that AedaeCAPA-1 inhibits select diuretic factors acting on the principal cells and this likely involves cGMP as a second messenger. Kinin-stimulated diuresis, which targets stellate cells, is also inhibited by cGMP suggesting that another anti-diuretic factor in addition to AedaeCAPA-1 exists and may utilize cGMP as a second messenger.