As small RNAs exert profound effects on development and regulation of gene expression, their own expression must be similarly constrained to the right level in the right cell at the right time. A collaboration between the labs of Michael Rosenfeld and Witold Filipowicz reports that the RNA-binding protein KSRP (KH-type splicing regulatory protein) regulates the precursor processing of a subset of microRNAs. If this regulatory mechanism is disrupted, effects can be seen on proliferation, differentiation and apoptosis. Thus KSRP is involved in both mRNA turnover and in promoting mRNA expression. This work has uncovered a further level of complexity in the regulation of gene expression by mi-RNA. MicroRNAs (miRNAs) have a role in down-regulating gene expression, and the levels of specific miRNAs are important for correct embryonic development; however, the mechanisms by which this is regulated are unknown. The splicing regulatory protein, KSRP, is now found to regulate the precursor processing of a subset of miRNAs, and its disruption leads to effects on proliferation, differentiation and apoptosis. Consistent with the role of microRNAs (miRNAs) in down-regulating gene expression by reducing the translation and/or stability of target messenger RNAs1, the levels of specific miRNAs are important for correct embryonic development and have been linked to several forms of cancer2,3,4. However, the regulatory mechanisms by which primary miRNAs (pri-miRNAs) are processed first to precursor miRNAs (pre-miRNAs) and then to mature miRNAs by the multiprotein Drosha and Dicer complexes5,6,7,8, respectively, remain largely unknown. The KH-type splicing regulatory protein (KSRP, also known as KHSRP) interacts with single-strand AU-rich-element-containing mRNAs and is a key mediator of mRNA decay9,10. Here we show in mammalian cells that KSRP also serves as a component of both Drosha and Dicer complexes and regulates the biogenesis of a subset of miRNAs. KSRP binds with high affinity to the terminal loop of the target miRNA precursors and promotes their maturation. This mechanism is required for specific changes in target mRNA expression that affect specific biological programs, including proliferation, apoptosis and differentiation. These findings reveal an unexpected mechanism that links KSRP to the machinery regulating maturation of a cohort of miRNAs that, in addition to its role in promoting mRNA decay, independently serves to integrate specific regulatory programs of protein expression.

Regulated mRNA turnover is a highly important process, but its mechanism is poorly understood. Using interleukin-2 (IL-2) mRNA as a model, we described a role for the JNK-signaling pathway in stabilization of IL-2 mRNA during T-cell activation, acting via a JNK response element (JRE) in the 5′ untranslated region (UTR). We have now identified two major RNA-binding proteins, nucleolin and YB-1, that specifically bind to the JRE. Binding of both proteins is required for IL-2 mRNA stabilization induced by T-cell activation signals and for JNK-induced stabilization in a cell-free system that duplicates essential features of regulated mRNA decay. Nucleolin and YB-1 are required for formation of an IL-2 mRNP complex that responds to specific mRNA stabilizing signals.

The importance of post‐transcriptional mechanisms for the regulation of the homoeostasis of the immune system and the response to challenge by microorganisms is becoming increasingly appreciated. We investigated the contribution of microRNAs (miRNAs) to macrophage activation induced by lipopolysaccharide (LPS). We first observed that Dicer knockout in bone marrow‐derived macrophages (BMDMs) increases the LPS‐induced expression of some inflammation mediators. miRNA microarray analysis in BMDMs revealed that LPS significantly induces the expression of a single miRNA, miR‐155, and this induction depends on enhanced miR‐155 maturation from its precursors. The single‐strand RNA‐binding protein KHtype splicing regulatory protein (KSRP) binds to the terminal loop of miR‐155 precursors and promotes their maturation. Both inhibition of miR‐155 and KSRP knockdown enhance the LPS‐induced expression of select inflammation mediators, and the effect of KSRP knockdown is reverted by mature miR‐155. Our studies unveil the existence of an LPS‐dependent post‐transcriptional regulation of miR‐155 biogenesis. Once induced, miR‐155 finely tunes the expression of select inflammation mediators in response to LPS.—Ruggiero, T., Trabucchi, M., De Santa, F., Zupo, S., Harfe, B. D., McManus, M. T., Rosenfeld, M. G., Briata, P., Gherzi, R. LPS induces KH‐type splicing regulatory protein‐dependent processing of microRNA‐155 precursors in macrophages. FASEB J. 23, 2898–2908 (2009). www.fasebj.org