Let-7 microRNA is a critical regulator in controlling the growth and function of silk gland in the silkworm

The silk gland is characterized by high protein synthesis. However, the molecular mechanisms controlling silk gland growth and silk protein synthesis remain undetermined. Here we demonstrated that CRISPR/Cas9-based knockdown of let-7 or the whole cluster promoted endoreduplication and enlargement of the silk gland, accompanied by changing silk yield, whereas transgenic overexpression of let-7 led to atrophy and degeneration of the silk gland. Mechanistically, let-7 controls cell growth in the silk gland through coordinating nutrient metabolism processes and energy signalling pathways. Transgenic overexpression of pyruvate carboxylase, a novel target of let-7, resulted in enlargement of the silk glands, which is consistent with the abnormal phenotype of the let-7 knockdown. Overall, our data reveal a previously unknown miRNA-mediated regulation of silk gland growth and physiology and shed light on involvement of let-7 as a critical stabilizer and booster in carbohydrate metabolism, which may have important implications for understanding of the molecular mechanism and physiological function of specialized organs in other species.丝腺以高效蛋白质合成能力为典型特征。然而，调控丝腺生长与丝蛋白合成的分子机制仍未明确。本研究证实，基于CRISPR/Cas9技术的let-7或其全基因簇敲低，可促进丝腺的核内复制与体积增大，并伴随蚕丝产量改变；而转基因过表达let-7则会导致丝腺萎缩与退化。机制分析表明，let-7通过协调营养代谢过程与能量信号通路，调控丝腺的细胞生长。丙酮酸羧化酶（pyruvate carboxylase）是let-7的全新靶基因，对其进行转基因过表达可导致丝腺体积增大，这与let-7敲低后的异常表型一致。综上，本研究数据揭示了一条此前未被报道的、由微小RNA（microRNA, miRNA）介导的丝腺生长与生理功能调控通路，并阐明了let-7作为碳水化合物代谢关键稳定因子与增强子的作用，这对于理解其他物种特化器官的分子机制与生理功能具有重要的参考价值。