Fzr regulates silk gland growth by promoting endoreplication and protein synthesis in the silkworm

Silkworm silk gland cells undergo endoreplicating cycle and rapid growth during the larval period, and synthesize massive silk proteins for silk production. In this study, we demonstrated that a binary transgenic CRISPR/Cas9 approach-mediated Fzr mutation in silkworm posterior silk gland (PSG) cells caused an arrest of silk gland growth and a decrease in silk production. Mechanistically, PSG-specific Fzr mutation blocked endoreplication progression by inducing an expression dysregulation of several cyclin proteins and DNA replication-related regulators. Moreover, based on label-free quantitative proteome analysis, we showed in PSG cells that Fzr mutation-induced decrease in the levels of cyclin proteins and silk proteins was likely due to an inhibition of the ribosome biogenesis pathway associated with mRNA translation, and/or an enhance of the ubiquitin-mediated protein degradation pathway. Rbin-1 inhibitor-mediated blocking of ribosomal biogenesis pathway decreased DNA replication in PSG cells and silk production. Altogether, our results reveal that Fzr positively regulates PSG growth and silk production in silkworm by promoting endoreplication and protein synthesis in PSG cells.