Ras1CA overexpression in the posterior silk gland improves silk yield

Abstract

Sericulture has been greatly advanced by applying hybrid breeding techniques to the domesticated silkworm, Bombyx mori, but has reached a plateau during the last decades. For the first time, we report improved silk yield in a GAL4/UAS transgenic silkworm. Overexpression of the Ras1 CA oncogene specifically in the posterior silk gland improved fibroin production and silk yield by 60%, while increasing food consumption by only 20%. Ras activation by Ras1 CA overexpression in the posterior silk gland enhanced phosphorylation levels of Ras downstream effector proteins, up-regulated fibroin mRNA levels, increased total DNA content, and stimulated endoreplication. Moreover, Ras1 activation increased cell and nuclei sizes, enriched subcellular organelles related to protein synthesis, and stimulated ribosome biogenesis for mRNA translation. We conclude that Ras1 activation increases cell size and protein synthesis in the posterior silk gland, leading to silk yield improvement.

When inordinate tissue growth is beneficial: Improving silk production by increasing silk gland size

“…The increasing demand has driven notable efforts to improve silk production through hybrid breeding, a strategy that has been very successful during the past century [3], but silk production has reached the long-expected plateau, which is consubstantial to the empirical breeding approach. However, recent work by Sheng Li and associates [4] has helped to overcome this sericultural bottleneck.…”

“…Li's team [4] used the same Bombyx binary Gal4/UAS system to overexpress Ras1 CA oncogene in the posterior silk gland, and found that size of this gland region increased by almost 60% in transgenic silkworms with respect to controls, whereas silk production also increased by approximately 60%. As expected, food consumption also augmented, but only by 20%, thus resulting in an estimated 30% improvement in the conversion efficiency of food to silk.…”

“…As expected, food consumption also augmented, but only by 20%, thus resulting in an estimated 30% improvement in the conversion efficiency of food to silk. Subsequently, researchers embarked on an intensive study of the mechanisms underlying the observed phenotype ( Figure 1) [4]. They showed that RasI CA overexpression in the posterior silk gland of transgenic silkworm increased Ras activity and stimulated the phosphorylation of Raf and PI3K110, npg which in turn activated the Raf-MAPK and PI3K-Akt-TORC1-S6K/4EBP pathways.…”

“…In parallel, as a consequence of activation of the PI3K-Akt-TORC1-S6K/4EBP pathways, ribosome biogenesis and protein synthesis were enhanced. Finally, Ras activation up-regulated bHLH, a transcription factor that is specifically expressed in the silk gland and enhances fibroin gene expression, resulting in a higher fibroin production and silk yield (Figure 1) [4].…”

“…The weight contents of fibroin and sericin in total silk proteins are approximately 75% and 25%, respectively. The strategy planned by Sheng Li and his team [4] was to generate transgenic silkworms with increased Ras activity in the posterior silk gland in order to obtain silkworms with this part of the gland enlarged and thus able to produce more fibroin. Indeed, this sort of biotechnological brute-force approach gave the desired results.…”