The prothoracicotropic hormone (PTTH) of Rhodnius prolixus (Hemiptera) is noggin-like: Molecular characterisation, functional analysis and evolutionary implications

Prothoracicotropic hormone (PTTH) is a central regulator of insect development that regulates the production of the steroid moulting hormones (ecdysteroids) from the prothoracic glands (PGs). Rhodnius PTTH was the first brain neurohormone discovered in any animal almost 100 years ago but has eluded identification and no homologue of Bombyx mori PTTH occurs in its genome. Here, we report Rhodnius PTTH is the first noggin-like PTTH found. It differs in important respects from known PTTHs and is the first PTTH from the Hemimetabola (Exopterygota) to be fully analysed. Recorded PTTHs are widespread in Holometabola but close to absent in hemimetabolous orders. We concluded Rhodnius PTTH likely differed substantially from the known ones. We identified one Rhodnius gene that coded a noggin-like protein (as defined by Molina et al., 2009) that had extensive similarities with known PTTHs but also had two additional cysteines. Sequence and structural analysis showed known PTTHs are closely related to noggin-like proteins, as both possess a growth factor cystine knot preceded by a potential cleavage site. The gene is significantly expressed only in the brain, in a few cells of the dorsal protocerebrum. We vector-expressed the sequence from the potential cleavage site to the C-terminus. This protein was strongly steroidogenic on PGs in vitro. An antiserum to the protein removed the steroidogenic protein released by the brain. RNAi performed on brains in vitro showed profound suppression of transcription of the gene and of production and release of PTTH and thus of ecdysteroid production by PGs. In vivo, the gene is expressed throughout development, in close synchrony with PTTH release, ecdysteroid production by PGs and the ecdysteroid titre. The Rhodnius PTTH monomer is 17kDa and immunoreactive to anti-PTTH of Bombyx mori (a holometabolan). Bombyx PTTH also mildly stimulated Rhodnius PGs. The two additional cysteines form a disulfide at the tip of finger 2, causing a loop of residues to protrude from the finger. A PTTH variant without this loop failed to stimulate PGs, showing the loop is essential for PTTH activity. It is considered that PTTHs of Holometabola evolved from a noggin-like protein in the ancestor of Holometabola and Hemiptera, c.400ma, explaining the absence of holometabolous-type PTTHs from hemimetabolous orders and the differences of Rhodnius PTTH from them. Noggin-like proteins studied from Hemiptera to Arachnida were homologous with Rhodnius PTTH and may be common as PTTHs or other hormones in lower insects.

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