Expression Profiling and Regulation of Genes Related to Silkworm Posterior Silk Gland Development and Fibroin Synthesis

The posterior silk gland (PSG) is the most important suborgan responsible for the synthesis and secretion of silk core fibroin proteins in silkworm. Here, we performed genome-scale expression profiling analysis of silkworm PSG at the fourth molting (M4) and at day 1 (V1), day 3 (V3), day 5 (Vs), and wandering stage (W) of the fifth instar by microarray analysis with 22 987 probes. We found that the five genes of silk proteins secreted from PSG including fibroin heavy (H) and light (L) chains, P25, seroin 1, and seroin 2 basically showed obvious up-regulation at V3 which lasted to V5, while slight down-regulation at W. The expression of translation-related genes including ribosomal proteins and translation initiation factors generally remained stable from M4 to V5, whereas it showed clear down-regulation at W. Clustering analysis of the 643 significantly differentially expressed transcripts revealed that 43 of the important genes including seroin 1 and sugar transporter protein had co-expression patterns which were consistent with the rate changes of fibroin synthesis and PSG growth. Pathway analysis disclosed that the genes in different clusters might have co-regulations and and direct interactions. These genes were supposed to be involved in the fibroin synthesis and secretion. The differential expression of several hormone-related genes also suggested their functions on the regulation of PSG development and fibroin synthesis. 2D gel-based proteomics and phosphoproteomics profiling revealed that the phosphorylated proteins accounted for no more than one-sixth of the total proteins at each stage, which was much lower than the level in normal eukaryotic cells. Changes in the phosphorylation status and levels of several proteins such as actin-depolymerizing factor 1 and enolase might be deeply involved in fibroin secretion and tissue development. Shotgun proteomic profiling combined with label-free quantification analysis on the PSG at V3, V5, and W revealed that many small heat shock proteins (sHSP) were specially expressed at W, which was substantially consistent with the results from 2-DE analysis, and implied the close correlations of sHSP with the physiological states of PSG at W. A majority of significantly up-regulated proteins at V5 were related to ribosome pathway, which was different from the microarray results, implying that the translation-level regulation of ribosomal proteins might be critical for fibroin synthesis. In contrast, the ubiquitin-proteasome pathway related proteins appeared obviously up-regulated at W, suggesting that the programmed cell death process of PSG cells might be started before cocooning.