Abstract
Nematocysts, the stinging organelles of cnidarians, have remarkable mechanical properties. Hydra nematocyst capsules undergo volume changes of 50% during their explosive exocytosis and withstand osmotic pressures of beyond 100 bar. Recently, two novel protein components building up the nematocyst capsule wall in Hydra were identified. The cnidarian proline-rich protein 1 (CPP-1) characterized by a “rigid” polyproline motif and the elastic Cnidoin possessing a silk-like domain were shown to be part of the capsule structure via short cysteine-rich domains that spontaneously crosslink the proteins via disulfide bonds. In this study, recombinant Cnidoin and CPP-1 are expressed in E. coli and the elastic modulus of spontaneously crosslinked bulk proteins is compared with that of isolated nematocysts. For the fabrication of uniform protein nanofibers by electrospinning, the preparative conditions are systematically optimized. Both fibers remain stable even after rigorous washing and immersion into bulk water owing to the simultaneous crosslinking of cysteine-rich domains. This makes our nanofibers clearly different from other protein nanofibers that are not stable without chemical crosslinkers. Following the quantitative assessment of mechanical properties, the potential of Cnidoin and CPP-1 nanofibers is examined towards the maintenance of human mesenchymal stem cells.
Highlights
Nematocysts are harpoon-like organelles characteristic of the cnidarian phylum[1]
Our recent studies on minicollagen cysteine-rich domains (CRDs) suggested that the C-terminal CRD fold (C-CRD) is able to participate in several disulfide bonds, whereas the N-terminal CRD fold (N-CRD) is strictly monovalent[10]
It is plausible that cnidarian proline-rich protein 1 (CPP-1) possessing N-CRD type CRDs at both termini is less packed compared to Cnidoin that has C-CRD type folds in both C-terminal domains[7]
Summary
Nematocysts are harpoon-like organelles characteristic of the cnidarian phylum[1]. The development of Hydra nematocysts, which comprise four different types, occurs in the body column of the polyps in specialized cells, called nematocytes. In our proteome study of Hydra nematocysts, two new capsule proteins flanked by terminal CRDs have been identified; cnidarian proline-rich protein 1 (CPP-1) and Cnidoin (Fig. 1d)[7,16]. The combination of “rigid” CPP-1 and “elastic” Cnidoin seems to be a very promising strategy for the design of new biomaterials that are capable of forming stable structures via spontaneous crosslinking and realize outstanding toughness and flexibility as nematocyst capsules have. Liu et al have demonstrated that very thin (270 nm), chemically crosslinked gelatin nanofiber substrates can be used for the long-term culture of human pluripotent stem cells[34] They have shown that nanofiber substrates are advantageous over commonly used matrigel cultures to discriminate www.nature.com/scientificreports populations of different pluripotent stem cells, which can be attributed to the fact that the adhesion of stem cells to nanofibers is much weaker compared to the adhesion onto gel substrates[35]
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