Characterization and expression patterns of let-7 microRNA in the silkworm (Bombyx mori)

Abstract

Backgroundlin-4 and let-7, the two founding members of heterochronic microRNA genes, are firstly confirmed in Caenorhabditis elegans to control the proper timing of developmental programs in a heterochronic pathway. let-7 has been thought to trigger the onset of adulthood across animal phyla. Ecdysone and Broad-Complex are required for the temporal expression of let-7 in Drosophila melanogaster. For a better understanding of the conservation and functions of let-7, we seek to explore how it is expressed in the silkworm (Bombyx mori).ResultsOne member of let-7 family has been identified in silkworm computationally and experimentally. All known members of this family share the same nucleotides at ten positions within the mature sequences. Sequence logo and phylogenetic tree show that they are not only conserved but diversify to some extent among some species. The bmo-let-7 was very lowly expressed in ova harvested from newborn unmated female adult and in individuals from the first molt to the early third instar, highly expressed after the third molt, and the most abundant expression was observed after mounting, particularly after pupation. The expression levels were higher at the end of each instar and at the beginning of each molt than at other periods, coinciding with the pulse of ecdysone and BR-C as a whole. Using cultured ovary cell line, BmN-SWU1, we examined the effect of altered ecdysone levels on bmo-let-7 expression. The expression was also detected in various tissues of day 3 of the fifth instar and of from day 7 of the fifth to pupa, suggesting a wide distributing pattern with various signal intensities.Conclusionbmo-let-7 is stage- and tissue-specifically expressed in the silkworm. Although no signals were detected during embryonic development and first larval instar stages, the expression of bmo-let-7 was observed from the first molt, suggesting that it might also function at early larval stage of the silkworm. The detailed expression profiles in the whole life cycle and cultured cell line of silkworm showed a clear association with ecdysone pulse and a variety of biological processes.

MicroRNA-dependent metamorphosis in hemimetabolan insects

“…The loss of Dicer-1 in D. melanogaster results in embryogenesis defects (7) and disruption of olfactory neuron morphogenesis (8). Moreover, developmental expression patterns of selected miRNAs in D. melanogaster (11,12) and B. mori (13,17) have revealed that a number of them are up-regulated in the transition from larvae to pupae, and of these let-7 is the most thoroughly studied (18,19). In the transition to adult of D. melanogaster, let-7 is required for neuromusculature remodeling (4), for proper timing in wing cell cycle, and for the maturation of neuromuscular junctions (3).…”

The role of let-7 in cell differentiation and cancer

“…Among the members, let-7a has identical sequence across various animal species from C. elegans to human. The increase in let-7 expression in late developmental stages has been reported in many organisms (Sempere et al 2002, Lancman et al 2005, Liu et al 2007, Wulczyn et al 2007. Previously, studies have shown the expression patterns of let-7 in vertebrates during development; however, the direct contribution of let-7 in development has not been demonstrated (Lancman et al 2005, Schulman et al 2005, Wulczyn et al 2007.…”

Characterization and profiling of MicroRNAs in posterior silk gland of the silkworm (Bombyx mori)

“…The bmolet-7 was stage-and tissue-specifically expressed in the silkworm. It was expressed in very low level from the first molt to the early third instar, expressed highly after the third molt, and the most abundant expression was observed after mounting, particularly after pupation (Liu et al 2007). Microarray analyses showed that bmo-let-7 expressed in all of the 10 tissues of fifth-instar day-3 larvae (head, body wall, ASG, PSG, midgut, fat body, ovary, testis, hemocyte and malpighian tubule) ).…”