Characterization of the expression, function and signaling of the ovary ecdysteroidogenic hormone in the female yellow fever mosquito, Aedes aegypti
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The neuropeptide, ovary ecdysteriodogenic hormone (OEH), is released from medial neurosecretory cells (MNCs) in the brain of female Aedes aegypti, in response to blood feeding. Transcript and protein expression profile of OEH in different body parts at different life stages and in female tissues during egg maturation cycle indicated that brain OEH is main source of OEH present in other tissues. Besides head tissue, OEH transcript was found only in ovary at any point during egg maturation cycle. The basis of OEH isolation from head extracts was its stimulation to yolk deposition in blood-fed decapitated females in vivo and ecdysteroid production by ovaries in vitro. A truncated version of OEH peptide was shown to be bioactive in a previous study. In this study, we compared the bioactivity of both full length and truncated peptides and did not see any significant difference indicating the truncated version could be the mature peptide, processed from prepropeptide. Insulin-like peptides (ILPs), in particular ILP3, are also produced by cells in the same region of the female brain and have the same demonstrated bioactivity. This study in fact showed the colocalization of OEH and ILP3 in MNCs. The functional in vivo bioassays indicated redundant functions of OEH with ILP3 in stimulating midgut digestion, nutrient storage, yolk protein synthesis and yolk uptake by the oocytes. In in vitro tissue culture condition OEH could not stimulate those functions indicating OEH required some other signals or resources to exhibit its function, shown in vivo.The study further showed, as that of ILP3, the ecdysteroidogenic function of OEH could be modulated through amino acid sensing, target of rapamycin (TOR) pathway. The signaling study revealed that despite many overlapping functions, unlike ILP3, OEH neither bound with nor activated the mosquito insulin receptor (MIR). However, OEH activated the MIR downstream protein kinase, Akt indicating that there might be some cross-talk with ILP3 signaling. As expected, OEH activated the TOR downstream signaling proteins, ribosomal S6 kinase and 4EBP1. Finally, this research demonstrated that OEH mediated activation of signaling proteins differs in a tissue specific way.