Hexapeptides from mammalian inhibitory hormone hunt activate and inactivate nematode reproduction

Increased reproduction (x3) of the entomopathogenic nematode Steinernema siamkayai occurred when exposed to one synthetic peptide (IEPVFT), while the fecundity of worms exposed to hexamer (KLKMNG) was reduced (x0.5). These hexamers were opposite ends of a 14 amino acid (aa) synthetic peptide KLKMNGKNIEPVFT (EPL030). The bioactivity of the hexamers is surprising it is a scrambled-sequence control of another peptide, MKPLTGKVKEFNNI (EPL001) which are bioinformatically obscure. EPL001 emerged from a physicochemical fractionation aimed at finding a postulated hormone that is reproductively related and tissue-mass reducing and has antiproliferative effects on human prostate cancer cells and rat bone marrow cells in vitro. Intracerebroventricular infusion of EPL001 in sheep was associated with elevated growth hormone in peripheral blood and reduced prolactin. The highly dissimilar EPL001 and EPL030 nonetheless have the foregoing biological effects in common in mammalian systems, while being divergently pro- and anti-fecundity respectively in the nematode Caenorhabditis elegans. Immunoprecipitation of EPL001 using an anti-EPL001 antibody suggests it encodes the sheep neuroendocrine prohormone secretogranin II (sSgII). Using bespoke bioinformatics with six sSgII residues deduced bioactivity to key aa: MKPLTGKVKEFNNI. Peptides more potent as cell inhibitors than EPL001 suggest a stereospecific bimodular tri-residue signature (i.e. simultaneous accessibility for binding of two specific trios of aa side chains, MKP & VFN). An evolutionarily conserved receptor is conceptualised as having dimeric binding sites, each with ligand-matching bimodular stereocentres. Sequence analysis and computational modelling suggest the activity of the control peptide EPL030 and its N- and C-terminal hexapeptide progeny is due the novel hormonal motif MKPVFN. KEY POINTS Synthetic hexapeptides have upregulated and downregulated nematode reproduction, following similar work in other nematode species and in fish and frogs. Peptides up to a 20mer have also been shown to inhibit the proliferation of human cancer and other mammalian cells in vitro and influence circulating levels of ovine pituitary hormones in vivo via intracerebroventricular peptide infusion. The amino acid sequence of the 14mer master peptide arose from a (rat) bioassay-guided fractionation using ovine materials, whose aim was to disclose an evolutionarily conserved inhibitory reproductive hormone involved in tissue mass determination. The bioactive peptides mimic the non-contiguous six-residue receptor-binding active face of a polypeptide hormone, which in mammals is a derivative of the secretory vesicle prohormone secretogranin II (SgII), taking the form of a novel, structually complex 70mer dubbed ‘SgII-70’ The ability has been gained to reinforce or block a default reproductive and tissue-building OFF signal, using peptide mimetics.