Life beyond cleavage: the case of Ago2 and hematopoiesis

Abstract

A recent article in The Economist (June 16, 2007) describes research on RNA as the 21st century’s Big Bang in biology. The “stars” of the RNA universe are small regulatory RNAs known as microRNAs (miRNAs). These ∼22-nt-long RNAs were discovered in 1993 (Lee et al. 1993; Wightman et al. 1993), but their role in biology remained obscure until 2001, when the Tuschl, Ambros, and Bartel laboratories identified hundreds of miRNAs in different organisms (Lagos-Quintana et al. 2001; Lau et al. 2001; Lee and Ambros 2001). Functional analyses of these miRNAs and their associated complexes subsequently led to the discovery of RNA-guided genesilencing pathways as important novel principles in controlling gene expression. miRNAs indeed regulate the expression of a large proportion of eukaryotic protein-coding genes at the post-transcriptional level by sequence-specific binding to the 3 untranslated region (3 UTR) of mRNAs (Lim et al. 2005). To date, RNA silencing is known to play an important role in diverse biological phenomena such as development, stem cell maintenance, cell proliferation, survival, differentiation, metabolism, and cancer (Kloosterman and Plasterk 2006). The primary transcripts of miRNA genes are mainly processed by the nuclear RNase III enzyme Drosha into stem–loop-structured miRNA precursors (pre-miRNAs) (Lee et al. 2003; Ruby et al. 2007), which are transported via the Exportin-5 receptor to the cytoplasm (Yi et al. 2003). The cytoplasmic RNase III enzyme Dicer cleaves the pre-miRNAs into double-stranded (ds) RNAs of ∼22 nt (Bernstein et al. 2001; Hutvagner et al. 2001). One strand of these dsRNA intermediates associates as a mature miRNA with a member of the Argonaute protein family to generate an active ribonucleoprotein complex known as RNA-Induced Silencing Complex (RISC). Within this effector complex, both the miRNA and the Argonaute protein fulfill specific roles. The singlestranded miRNA recognizes the target mRNA with full or partial base-pair complementarity. The Argonaute protein helps the miRNA in searching for its target mRNA and, upon recognition, can either cleave it or remain tethered to the mRNA to repress its translation and/or regulate its stability. While the biochemistry of the miRNA-guided cleavage reactions is well understood (Martinez and Tuschl 2004; Schwarz et al. 2004), it is still controversial to what extent RISC interferes with the initiation or elongation of translation or controls mRNA stability by sequestration into cytoplasmic processing bodies (P-bodies) (Kiriakidou et al. 2007; Peters and Meister 2007; Pillai et al. 2007). The Argonaute (Ago) proteins can be subdivided into Ago-like and Piwi-like subfamilies. The mammalian Piwi-like proteins are specifically expressed during spermatogenesis, where they bind a novel set of small RNAs termed Piwi-interacting RNAs (piRNAs) (Aravin et al. 2006; Seto et al. 2007). In contrast, the Ago proteins are broadly expressed in somatic cells, associate with miRNAs and are key actors in different RNA silencing pathways (Peters and Meister 2007; Tolia and Joshua-Tor 2007). The Ago gene family consists of four (AGO1– AGO4) and five (Ago1–Ago5) members in human and mouse, respectively (Peters and Meister 2007). However, only the Ago2 protein displays endonucleolytic or “Slicer” activity and can therefore execute miRNA-directed cleavage of target mRNA, provided that the basepairing between the Ago2-associated miRNA and the mRNA sequence is perfect (Liu et al. 2004; Meister et al. 2004). In case of partial complementarity, the Ago2 protein fails to cleave, but instead interferes with translation of the target mRNA via its translational repression activity. In addition to Ago2, other mammalian Ago proteins are also part of miRNA effector complexes that mediate translational inhibition of target mRNAs (Liu et al. 2004; Meister et al. 2004). Gene disruption in the mouse demonstrated that the Ago2 protein is essential for embryonic development (Liu et al. 2004). To study the function of Ago2 in adult hematopoiesis, O’Carroll et al. (2007) have used conditional gene inactivation to bypass the embryonic lethality and report in this issue of Genes & Development that Ago2 is a key regulator of B-lymphoid and erythroid development. Retroviral rescue experiments surprisingly revealed that the Slicer endonuclease activity, which is a unique and defining feature of Ago2, is dispensable for hematopoietic developCorresponding author. E-MAIL busslinger@imp.ac.at; FAX (43/1) 79730-223150. Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.1591407.