microRNAs may sharpen spatial expression patterns

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  • 22 February 2007
Abstract
microRNAs (miRNAs) form a ubiquitous class of small RNAs that silence target genes. Surprisingly, in many cases miRNA deletion has no dramatic phenotypic effect. Indeed, miRNAs acting in morphogenetic pathways may fine tune, rather than set up, their targets' expression patterns. Here, we construct a model in which the spatial expression pattern of a target gene, crudely established by a morphogen, is fine-tuned by a cognate miRNA. This is done by generating a sharp interface between domains of high and negligible target expression, thereby unambiguously defining the identity of each cell. The ability of miRNAs to move between cells is crucial to this sharpening. We examine the model in a context where the spatial transcription profiles of miRNA and target are mutually exclusive as, for example, is the case for some Hox genes in the early Drosophila embryo. We further consider an alternative scenario, where the two transcription profiles are approximately the same. We show that miRNAs can still sharpen the target expression pattern provided they become irreversibly localized to a cell before interacting with their targets. An experimental test of the proposed mechanism is suggested.

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