Piecing together the mosaic of early mammalian development through microRNAs.

The microRNA (miRNA) pathway represents an integral component of the gene regulation circuitry that controls development. In recent years, the role of miRNAs in embryonic stem (ES) cells and mammalian embryogenesis has begun to be explored. A few dozens of miRNAs expressed in mammalian ES cells, either exclusively or nonexclusively, have been cloned. The overall role of miRNAs in ES cells and embryonic development has been assessed by examining the effect of knocking out Dicer, an RNase III enzyme required for miRNA and small interfering RNA biogenesis, as well as DGCR8, a nuclear protein specifically involved in miRNA biogenesis. In addition, the role of a cluster of miRNAs specifically expressed in ES cells, the miR-290-295 group, has been investigated by the knock-out approach. These analyses have revealed the crucial role of miRNAs in ES cell differentiation, lineage specification, and organogenesis, especially neurogenesis and cardiogenesis. Systematic investigation of the role of miRNAs in ES cells and embryos will allow us to find missing pieces of the mosaic of early development.