New insight into the mechanism of in vivo fibroin self-assembly and secretion in the silkworm, Bombyx mori.

Fibroin of the silkworm consists of fibroin heavy chain (Fib-H) with hydrophobic intermediate repeats flanked by hydrophilic N and C terminal domains (NTD and CTD, respectively), fibroin light chain (Fib-L), and P25. However, the respective roles of each polypeptide in silk processing remain largely unknown. Here, a series of transgenic silkworms with different fusion gene expression cassettes were created in order to selectively express different fluorescent fusion proteins in silk glands. The roles of different components in silk processing were investigated via observing and analyzing the movement and distribution of these proteins in the silk gland and in cocoon silk. The data showed that hydrophilic NTDs were distributed on the surface of micelles, providing sufficient electrostatic repulsion to prevent premature crystallization of silk proteins. Hydrophilic CTD==Ls ("==" represents the disulfide bond) were located on the inner layer of micelles to control the solubility of large micelles. The results presented here elucidated the underlying mechanisms of silkworm silk processing in vivo. This is significant for the development of artificial spinning technology, novel silk biomaterials, and silk gland expression systems.

Strategies for Tuning the Biodegradation of Silk Fibroin-Based Materials for Tissue Engineering Applications.

The remarkable features of silk fibroin (SF) from the silkworm (Bombyx mori) have fueled its application as a candidate biomaterial for tissue regeneration and repair. For an ideal scaffold, the rate of degradation should be synchronized to match the rate of new tissue formation, and tuning this rate is essential, as diverse tissues differ in terms of regeneration period. In this Review, we discuss the factors influencing the degradability of SF, which can vary from days to several months, depending on the state of the raw material, the scaffold preparation process, morphological features, and host factors. This knowledge facilitates strategies for tuning the SF degradation rate, including manipulation of molecular weight, crystalline level, and cross-linking degree. Since these strategies have a great influence on the mechanical properties, the superiority of SF has to be sacrificed to satisfy the requirements for degradation rate. We further explore additional strategies, including the incorporation of degradation-promoting supplements such as blending with another polymer (e.g., gelatin) and the incorporation of enzyme-sensitive peptides. The information in this Review will likely aid scientists working with SF materials for the regeneration of diverse tissues.