Trends in Biotechnology
ReviewSpider silks and their applications
Introduction
Silk has captured the imagination of humankind for millenia, largely due to the unrivaled visual and functional properties of silk fiber and the unique structures that have been generated by various silk-producing species in nature. These structures include amazing orb web structures spun to capture prey, cocoons to house developing offspring, adhesives used to anchor webs and fibrous tethers to capture flying prey. Despite significant interest in other silk sources, such as spider silk, silkworms have been the traditional source exploited. One reason that spider silk has lagged behind is that silkworms are fairly easy to domesticate, whereas spiders cannot be housed in high densities (Encyclopædia Britannica, retrieved December 27th, 2007, http://www.britannica.com/eb/article-6677). In addition, whereas one silkworm cocoon yields 600 to 900 m of fiber, only ∼137 m of fiber can be reeled from the ampullate gland of a spider and only ∼12 m of silk is found in a complete spider web [1]. Finally, many types of silk are utilized in web construction from a single spider. Therefore, the best option for moving the application of spider silks forward is the pursuit of biotechnological means of generating source material. Recent advances in genetic engineering have led to increased insight into silk proteins and the structural organization of spider-silk-encoding genes. With this knowledge, heterologous expression of spider-silk proteins in a range of host systems has been achieved, as well as the formation of new materials from recombinant-DNA-derived spider-silk proteins. These advances might make spider silk a viable choice in many new application areas, heretofore the domain of silkworm silk.
Section snippets
Chemistry, structure and function
The molecular structure of silk consists of regions of protein crystals separated by less organized protein chains. The primary structural modules give rise to diverse secondary structures that, in their turn, direct functions of different silks. As the most heavily studied secondary structure of silks, crystalline β-sheets contribute to the high tensile strength of silk fibers. Beta sheets form through natural physical crosslinking of amino acid sequences, which in spider and silkworm silk
Sources and cloning of spider silks
The spider silks that have been most extensively studied to date with recombinant DNA technology include those from N. clavipes (Major Spidroin dragline types I and II – MaSpI and MaSpII), Araneus diadematus (dragline ADF-3 and ADF-4) and N. clavipes flagelliform 14, 15, 16, 17, 18, 19, 20, 21, 22. Two main approaches have been used to obtain the gene encoding spider silks: (i) isolation of native spider silk amino acid sequences from peptide digests, followed by back-translation into the
Processing spider silks into biomaterials
One attractive application of spider silks is to emulate the diverse material functions of this family of proteins as a source of novel biomaterial designs. Insight into the assembly and processing of spider-silk proteins into various material forms has been a longstanding focus and has allowed the broadening of the field of applications for silks in general. Specifically, medical devices and tissue engineering applications are perhaps the most promising areas for the utilization of spider
Formation of novel biomaterials from recombinant spider-silk proteins
Silk proteins have been shown to solubilize in water, organic solvents and ionic liquids, indicating the versatile options available, and they can then be processed into new biomaterials, including fibers, films, gels, porous sponges and other related systems (Figure 3) 37, 42. The resulting structures and functions of the obtained materials are directly controlled by the content and distribution of crystalline β-sheets, a process that can be controlled by the mode of processing. A variety of
Functionalization of spider silk – novel composites
Surface functionalization is an important strategy for modifying the exterior of biomaterials to influence cell and tissue development, and this strategy has been applied to silk surface chemistry [42]. Surface modification usually targets carboxylic acid groups on the amino acids in the protein. Silkworm fibroin silk has been modified by coupling enzymes such as horseradish peroxidase, cell binding domains such as Arg-Gly-Asp (RGD) peptides and cell signaling factors such as parathyroid
Conclusions
The path ahead for spider-silk proteins remains challenging, mainly due to the inadequate supply of full length proteins that are required for a full exploration of the most interesting applications. Thus, currently, the more readily available silkworm silk is most widely applied in the biomedical area. Over the last decade there has been considerable progress in understanding the gene organization of spider silk. The cloning and expression techniques for spider silks have been improved, and
Acknowledgements
We thank our many collaborators, students and colleagues who have been instrumental in the studies described in part here. Grant support on silk-related systems is greatly appreciated and has included, at various times, the National Institutes of Health, the National Science Foundation and the Air Force Office of Scientific Research. Special thanks go to Andy Lovinger and Hugh Delong for their foresight and support.
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