In an approach to identify novel neuropeptides in Drosophila a team of researchers employed liquid chromatography and tandem mass spectrometry on extract from larvae (Baggerman et al., 2002, 2005). Bioinformatics showed that several of the identified peptides are derived from four novel precursors, designated neuropeptide-like precursors 1-4 (NPLP1-4) encoded by the genes CG3441, CG11051, CG13061 and CG15361 (Baggerman et al., 2002, 2005).
Of these, the NPLP1 has been most investigated. The NPLP1 precursor (CG3441) produces the four peptides IPNamide (NVGTLARDFQLPIPNamide), MTYamide (YIGSLARAGGLMTYamide), NAP peptide (SVAALAAQGLLNAP) and VQQ (NLGALKSSPVHGVQQ), which were confirmed by mass spectrometry (Baggerman et al., 2002). One more peptide could possibly be cleaved from the same precursor: NIATMARLQSAPSTHRDP (see Overend et al., 2012). The NPLP1 precursor is conserved in insects such as Apis mellifera (Hummon et al., 2006), Tribolium castaneum (Li et al., 2008), and Anopheles gambiae (Riehle et al., 2002). The receptor(s) of the NPLP1 peptides has not been identified, although it has been shown that one of the peptides, VQQ, activates the membrane receptor guanylate cyclase Gyc76c and stimulates cGMP production (Overend et al., 2012).
The NPLP2 precursor (CG11051) gives rise to the peptide TKAQGDFNEF, designated NEF peptide (Baggerman et al., 2002). A later mass spectrometry based study suggested that further peptides/fragments could be liberated from the precursor in Drosophila larvae that had been infected (Verleyen et al., 2006). The NPLP2 precursor has not been identified outside of Drosophila, except in the honeybee Apis mellifera (Hummon et al., 2006). No receptor has been identified.
The NPLP3 precursor (CG13061) can produce the peptides VVSVVPGAISHA (SHA peptide) and SVHGLGPVVIamide (VVIamide) (Baggerman et al., 2002). This precursor has only been identified in Drosophila species. No receptor has been identified.
The NPLP4 precursor (CG15361) can produce the peptide pQYYYGASPYAYSGGYYDSPYSY (YSYpeptide) (Baggerman et al., 2002). This precursor is not known outside Drosophila species. No receptor has been identified.
NPLP1-derived IPNamide has been localized to 28 neurons in the larval ventral nerve cord of the flies Neobellieria (Sarcophaga) bullata and Drosophila melanogaster (Verleyen et al., 2004, 2009). Also, some NPLP expressing neurons in the brain were detected. Some of the neurons of the ventral nerve cord may be neurosecretory cells that release peptides into the circulation (see Overend et al., 2012). In the larval and adult Drosophila brain IPNamide is localized to a subset of the clock neurons, the anterior dorsal DN1 neurons (DN1a) and in the larval brain the DN1 neurons express this peptide (Shafer et al., 2006).
The NPLP2 precursor transcript is broadly expressed in the adult fly: highly expressed in most tissue except midgut, renal tubules, trachea, ovary and testes (FlyAtlas). In the larva, the hindgut, trachea, fat body, salivary glands and carcass are enriched for nplp2 transcript (FlyAtlas). RNA sequencing analysis of wing imaginal discs in Drosophila identified nplp2 transcript in flies mutant in Pacman, a gene encoding an exoribonuclease (Jones et al., 2016).
The NPLP3 precursor transcript is enriched in the eye and carcass of adult Drosophila according to FlyAtlas.
According to FlyAtlas the NPLP4 precursor transcript is enriched in the eye, heart and hindgut of adult Drosophila; in the larva nplp4 is enriched in CNS, midgut, hindgut, salivary gland, carcass and especially trachea. It seems to be developmentally regulated.
The only function determined so far for any of the NPLP1-derived peptides is the action of VQQ peptide on the renal tubules of Drosophila (Overend et al., 2012). A likely role in circadian clock function and/or sleep is indicated by expression of IPNamide in a subset of clock neurons.
Upon infection (bacterial injection) of Drosophila larvae a number of peptides are released into the haemolymph. It was found that among these were peptides derived from the NPLP2 precursor (Verleyen et al., 2006). Thus, these peptides could be part of a humoral antimicrobial defence. The presence of nplp2 in imaginal discs, together with insulin-like peptide 8, could indicate a role in developmental growth control (Jones et al., 2016).
No functions are known for NPLP-3 and 4.
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