Insect homologs of the glycoproteins GPA2 and GPB5 were first identified and characterized in Drosophila melanogaster (Hsu et al., 2002; Sudo et al., 2005). These glycoproteins are also found throughout Bilateria (Jékely, 2013). Insect GPA2 and GPB5 each contain 10 highly conserved cysteine residues. These allow for the formation of a cysteine-knot structure which is characteristic of glycoprotein hormones (Paluzzi et al., 2014; Sudo et al., 2005). GPA2 and GPB5 form a heterodimer, which activates a leucine-rich repeat-containing GPCR (LGR); the first insect GPA2/GPB5 receptor was also deorphanized in D. melanogaster (Sudo et al., 2005).
Insect GPA2/GPB5 expression has so far only been localized in D. melanogaster (Sellami et al., 2011). In larvae, GPA2/GPB5 transcripts are expressed in four pairs of bilateral cells in the first four abdominal neuromeres. GPB5-GAL4 driven EGFP localization showed that these cells also persist in the adults. In addition, expression is also observed in a pair of cells in the pars intercerebralis and another pair of cells in the suboesophageal ganglion.
In D. melanogaster, genetic ablation of GPA2/GPB5 cells reduces survival to the adult stage (Sellami et al., 2011). In this study, it was hypothesized that GPA2/GPB5 may function as an anti-diuretic hormone in insects since its receptor is enriched in the hindgut and Malpighian tubules (tissues associated with ion and water homeostasis). In light of this, GPA2/GPB5 has recently been shown to regulate ion transport (specifically Na+ and K+) across A. aegypti hindgut (Paluzzi et al., 2014).
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