Regulatory giants join forces

Abstract

Animals regulate their crucial processes, such as developmental timing, through temporaland spatial-specific expression of regulatory factors. Aberrant expression of such factors can result in morphological defects and developmental retardation. Bethke and colleagues (Bethke et al., 2009) have described the interaction of two key pathways that are involved in animal developmental regulation: the steroidal hormone and microRNA-directed regulatory pathways. Through specific cross-talk, these systems regulate protein expression with exquisite responsiveness to the organisms’ surrounding environmental conditions. Both hormones and microRNAs play key roles in animal development (Bourguet et al., 2000; Mangelsdorf et al., 1995). Hormones are produced in response to stimuli that indicate whether environmental conditions — such as temperature and nutrient levels — are favourable for development (Rougvie, 2005). These hormones bind specific proteins, known as nuclear receptors, triggering developmental progression (Weinholds and Plasterk, 2005). In the absence of hormones the cell is arrested in development. A markedly different regulatory system from the hormone-receptor interaction, microRNAs are small RNA species that function by binding to sites on specific messenger RNA (mRNA) targets (Bartel, 2004). In animals, this interaction results in the attenuation of protein output from target mRNA. Previous studies have shown that the nematode, Caenorhabditis elegans, exhibits a similar aberrant developmental phenotype whether it is lacking a nuclear receptor (DAF-12) or microRNAs mir-48, mir-241 and mir-84 (collectively known as let-7 microRNAs) (Antebi et al., 2000; Abbott et al., 2005). This observation led Bethke and colleagues (Bethke et al., 2009) to probe the interaction between DAF-12 and the microRNA regulatory pathway. By fusing the promoter of one of the let-7 microRNAs with a reporter gene, the authors showed that DAF-12 with bound hormone specifically activates the expression of the reporter gene construct in cultured human cells. Removal of the let-7 microRNA promoter results in the loss of reporter gene expression. These results provide solid evidence of cross-talk between the microRNA and steroidal hormone regulatory pathways. Bethke et al. confirmed the interaction between the DAF-12 nuclear receptor and the let-7 microRNAs by generating C. elegans containing a let-7 microRNA promoter fused to the green fluorescent protein (GFP) gene. While GFP was broadly expressed in wild-type, fluorescence from GFP was diminished in mutants lacking DAF-12. The authors showed that there was direct regulation of the let-7 microRNAs by the steroidal hormone-DAF-12 interaction. Importantly, DAF-12 without a bound steroidal hormone was found to negatively regulate let-7 microRNAs. Hence the interaction of the nuclear receptor with the microRNA functions as a molecular switch: input of steroidal hormone determines whether the output is microRNA repression or activation (Fig. 1). As steroidal hormones are synthesised in response to environmental cues, let-7 microRNA regulation is therefore ultimately dependent on environmental conditions. A nuance to this model lies in the finding that let-7 promoter-GFP construct expression levels varied across Received November 22, 2009; accepted November 25, 2009