Figure 1: Scheme of microRNA (miRNA) biogenesis pathway and factors affecting miRNA processing activity of the Dicer complex. Cellular levels of functional miRNAs are established by a complex and balanced process that includes polymerase II (Pol II)-mediated transcription, pri-miRNA processing by Drosha/DGCR8 complex, pre-miRNA degradation by Translin/Trax and production of mature miRNAs by Dicer-PACT-TRBP-Ago2 complex. Caloric restriction, drugs (enoxacin) and neurotropic factors (BDNF) can stimulate Dicer activity, while cellular stress caused by reactive oxygen species (ROS), ultraviolet (UV) irradiation and hypoxia inhibit miRNA processing. Heat stress, viral infections and/or accumulation of misfolded proteins lead to formation of RNA-containing stress granules that sequester PACT and Ago2 and inhibit Dicer complex activity (Emde and Hornstein, 2014). BDNF: Brain derived neurotrophic factor; ORF: open reading frame; UTR: untranslated regions.