Effects of Titanium Dioxide Nanoparticles on the Synthesis of Fibroin in Silkworm (Bombyx mori)

Silkworm (Bombyx mori) is an economically important insect, and its silk production capacity largely depends on its ability to synthesize fibroin. While breeding of B. mori varieties has been a key strategy to improve silk production, little improvement of B. mori silk production has been achieved to date. As a result, the development of sericulture economy has not progressed well, pointing to the need of new ways for improvement of B. mori silk production. Titanium dioxide nanoparticles (TiO₂ NPs), a food additive widely used for livestock, have been shown to promote animal growth and increase the protein synthesis in animals. However, no studies on effect of TiO₂ NPs on fibroin synthesis in B. mori have been available. In this study, the differential expression profiles of genes and proteins in the silk gland of B. mori fed without or with TiO₂ NPs (5 μg ml⁻¹) were analyzed and compared using digital gene expression (DGE), reverse transcription quantitative polymerase chain reaction (RT-qPCR), semi-qPCR, and Western blot analysis. The effects of TiO₂ NPs feeding on the activity of proteases in the midgut and the synthesis and transportation of amino acids in hemolymph were also investigated. DGE analyses showed that among a total of 4,741 genes detected, 306 genes were differentially expressed after the TiO₂ NPs feeding, of which 137 genes were upregulated whereas 169 genes were downregulated. 106 genes were shown to be involved in fibroin synthesis, of which 97 genes, including those encoding cuticular protein glycine-rich 10, serine protease inhibitor 28, aspartate aminotransferase, lysyl-tRNA synthetase, and splicing factor arginine/serine-rich 6, and silk gland factor-1 (SGF-1), were upregulated with the maximum induction of 8.52-folds, whereas nine genes, including those encoding aspartylglucosaminidase, the cathepsin L in Tribolium castaneum, and similar to SPRY domain-containing SOCS box protein 3, were downregulated with the maximum reduction of 8.11-folds. Transcription levels of nine genes were further verified by RT-qPCR, and the results were consistent with those with DGE. Transcription and expression levels of fibroin light chain (Fib-L) gene were increased after TiO₂ NPs feeding, indicating that TiO₂ NPs improves fibroin synthesis. Compared with that of control, the mean protease activity was increased by 56.67 % in the B. mori fed with TiO₂ NPs, and the transport of four key amino acids used for fibroin synthesis in hemolymph was also incr