Regulation of miRNA processingContr

Regulation of miRNA processingControl of miRNA processing has emerged as another important mechanism in defining the spatiotemporal pattern of miRNA expression.Regulation of Drosha, Dicer and their double-stranded RBP partners. Drosha and Dicer generally operate in complexes with double-stranded RBP partners, such as DGCR8 and transactivation-responsive (TAR) RBP (TRBP) in mammals. Both the levels and activity of all of these proteins are subject to regulation that affects the accumulation of miRNAs. For example, DGCR8 has a stabilizing effect on Drosha through the interaction with its middle domain, whereas Drosha controls DGCR8 levels by cleaving hairpins present in the DGCR8 mRNA, thereby inducing its degradation13,14. Keeping the Drosha to DGCR8 ratio in check may be important, as a threefold excess of DGCR8 dramatically inhibits Drosha processing activity in vitro15. Interestingly, haem binding to DGCR8 promotes its dimerization and facilitates pri-miRNA processing16.Accumulation of Dicer is dependent on its partner TRBP, and a decrease in TRBP leads to Dicer destabilization and pre-miRNA processing defects17–19. In human carcinomas, mutations causing diminished TRPB expression impair Dicer function18. TRBP itself is stabilized through serine phosphorylation, catalysed by mitogen-activated protein kinase (MAPK)/extracellular regulated kinase (eRK)19. Cell growth and survival are elevated on TRBP phosphorylation, possibly as a consequence of upregulation of growth-stimulatory miRNAs and a decrease in let-7, a known suppressor of proliferation19. Notably, let-7 can target Dicer mRNA20, forming a negative feedback loop with the potential to broadly influence miRNA biogenesis, both physiologically and in cancer; other mechanisms can also contribute to the reported changes in Dicer and also Drosha levels in tumours21.Dicer is a large (~200 kDa) multi-domain protein, which is not only involved in the cleavage of pre-miRNAs but also participates in loading miRNAs into miRISC2. Dicer’s amino-terminal helicase domain may have an autoregulatory function as its removal stimulates catalytic activity of human Dicer, similarly as does the addition of TRBP22. There is also evidence to suggest that proteolysis may play a role in modulating Dicer activity 23,24or even changing its specificity towards DNA25