Untangling Natural Webs
Thadomal Shahani Engineering College
Spiders produce a variety of silks with diverse properties. All spider species produce silk at some point in their lifetime. In the 1950s, spider silk, and in particular dragline silk, entered the focus of materials science owing to its outstanding mechanical properties. Studies have shown that spider silk outperform most other natural and synthetic fibers. As compared to other materials, spider silk has high mechanical stability, biocompatibility, smoothness, and thinness. Due to these unique characteristics, it has a wide range of applications in the textile and biomaterial industry.
It is surprising how spiders succeeded in converting food into natural glue, which on coming in contact with air turns into silk. Spider silk is incredibly tough and five times stronger than the steel of the same diameter.
It is almost as strong as Kevlar, which is one of the toughest man-made polymers. The material can stretch up to 40% of its length without breaking. The toughest dragline silk is produced by the Golden Orb-Weaving spider (Nephilia clavipes). But the territorial and cannibalistic nature of spiders prevents mass production of spider silk for commercial purposes. Moreover, the fiber obtained from the ampullate gland of a spider contributes to only 12 meters of silk. So, the best option for moving the application of spider silk forward is the development of new synthetic spider silks with enhanced functions and specific characteristics using genetic engineering.
Using modern biotechnology, scientists have put remarkable efforts into developing various cloning and production strategies specifically to design spider-silk-like genes for protein production. The epithelial cells of the major ampullate gland secrete ampullate spidroins – MaSpI and MaSpII, which provide all the mechanical and physiochemical properties. So far, two main approaches have been used to obtain the gene encoding spider silks. One of them includes isolating mRNAs from the silk producing glands and generation of cDNA from it. The other method includes isolation of native spider silk amino acid sequences from peptide digests, followed by reverse translation into the corresponding DNA sequence. Oligonucleotides are then chemically synthesized, which represent the sequence.
The recombinant DNA approach was then implemented which includes the following steps (i) Designing of genetic cassettes (ii) Digestion of the cloning vector (iii) Ligation of linker segment and silk monomer gene to a DNA vector (iv) Transformation of this recombinant DNA molecule into a host cell (v) protein expression and purification of the selected clones. Technologies like CRISPR- Cas9 and TALENs mediated gene editing can also to produce more extensible fibers in the future. CRISPR- Cas9 was used in the past, to successfully incorporate spider silk protein genes into the Bombyx mori genome. Similarly, TALENs were also used to produce spider silk from silkworms. Genetic fragments of a golden orb-web spider were introduced in silkworm genome by the use of molecular scissors. These processes were carried out in such a way that any sequence changes did not impact the production of other proteins.
Spider silk proteins have been produced in genetically modified bacteria (Escherichia coli) , yeasts (Pichia pastoris) , plants (tobacco, soybean, potato, Arabidopsis) , insects (silkworm larvae) and mammalian cells lines (BHT) and transgenic animals (mice, goats).
At present, this silk is feasible for a variety of biomedical processes, such as the use of silk-inorganic composites to regulate cell–material interfacial interactions, to control bonding to tissues and to modulate degradation profiles. Recently, it was also used in ligament tissue engineering for direct stem cell and ligament cell based reconstruction. Application in commodity high performance composite materials and tough materials, in general, will require more research and development of low-cost expression systems. Scaling up silk production, prevention of the formation of aggregates and conservation of mechanical properties of silk fibers are some factors that need to be considered for future development. Synthetic Spider silk seems to have a huge potential and impact not only on materials science and engineering but also on green chemistry and biomedicine.