Bioinspired Squid Peptides─A Tale of Curiosity-Driven Research Leading to Unforeseen Biomedical Applications

The molecular design of many peptide-based materials originates from structural proteins identified in living organisms. Prominent examples that have garnered broad interdisciplinary research interest (chemistry, materials science, bioengineering, etc.) include elastin, silk, or mussel adhesive proteins. The critical first steps in this type of research are to identify a convenient model system of interest followed by sequencing the prevailing proteins from which these biological structures are assembled. In our laboratory, the main model systems for many years have been the hard biotools of cephalopods, particularly their parrot-like tough beak and their sucker ring teeth (SRT) embedded within the sucker cuptions that line the interior surfaces of their arms and tentacles. Unlike the majority of biological hard tissues, these structures are devoid of biominerals and consist of protein/polysaccharide biomolecular composites (the beak) or, in the case of SRT, are entirely made of proteins that are assembled by supramolecular interactions.In this Account, we chronicle our journey into the discovery of these intriguing biological materials. We initially focus on their excellent mechanical robustness followed by the identification and sequencing of the structural proteins from which they are built, using the latest "omics" techniques including next-generation sequencing and high-throughput proteomics. A common feature of these proteins is their modular architecture at the molecular level consisting of short peptide repeats. We describe the molecular design of these peptide building blocks, highlighting the consensus motifs identified to play a key role in biofabrication and in regulating the mechanical properties of the macroscopic biological material. Structure/property relationships unveiled through advanced spectroscopic and scattering techniques, including Raman, infrared, circular dichroism, and NMR spectroscopies as well as wide-angle and small-angle X-ray scattering, are also discussed.We then present recent developments in exploiting the discovered molecular designs to engineer peptides and their conjugates for promising biomedical applications. One example includes short peptide hydrogels that self-assemble entirely under aqueous conditions and simultaneously encapsulate large macromolecules during the gelation process. A second example involves peptide coacervate microdroplets produced by liquid-liquid phase separation. These microdroplets are capable of recruiting and delivering large macromolecular therapeutics (genes, mRNA, proteins, peptides, CRISPR/Cas 9 modalities, etc.) into mammalian cells, which introduces exciting prospects in cancer, gene, and immune therapies.This Account also serves as a testament to how curiosity-driven explorations, which may lack an obvious practical goal initially, can lead to discoveries with unexpected and promising translational potential.