Novel Bioactive Peptides from Meretrix meretrix Protect Caenorhabditis elegans against Free Radical-Induced Oxidative Stress through the Stress Response Factor DAF-16/FOXO

Abstract

The hard clam <i>Meretrix meretrix</i>, which has been traditionally used as medicine and seafood, was used in this study to isolate antioxidant peptides. First, a peptide-rich extract was tested for its protective effect against paraquat-induced oxidative stress using the nematode model <i>Caenorhabditis elegans</i>. Then, three novel antioxidant peptides; MmP4 (LSDRLEETGGASS), MmP11 (KEGCREPETEKGHR) and MmP19 (IVTNWDDMEK), were identified and were found to increase the resistance of nematodes against paraquat. Circular dichroism spectroscopy revealed that MmP4 was predominantly in beta-sheet conformation, while MmP11 and MmP19 were primarily in random coil conformation. Using transgenic nematode models, the peptides were shown to promote nuclear translocation of the DAF-16/FOXO transcription factor, a pivotal regulator of stress response and lifespan, and induce the expression of superoxide dismutase 3 (SOD-3), an antioxidant enzyme. Analysis of DAF-16 target genes by real-time PCR reveals that <i>sod-3</i> was up-regulated by MmP4, MmP11 and MmP19 while <i>ctl-1</i> and <i>ctl-2</i> were also up-regulated by MmP4. Further examination of <i>daf-16</i> using RNA interference suggests that the peptide-increased resistance of <i>C. elegans</i> to oxidative stress was DAF-16 dependent. Taken together, these data demonstrate the antioxidant activity of <i>M. meretrix</i> peptides, which are associated with activation of the stress response factor DAF-16 and regulation of the antioxidant enzyme genes.