A feedforward regulatory loop between HuR and the long noncoding RNA linc-MD1 controls early phases of myogenesis.

The muscle-specific long noncoding RNA linc-MD1 was shown to be expressed during early phases of muscle differentiation and to trigger the switch to later stages by acting as a sponge for miR-133 and miR-135. Notably, linc-MD1 is also the host transcript of miR-133b, and their biogenesis is mutually exclusive. Here, we describe that this alternative synthesis is controlled by the HuR protein, which favors linc-MD1 accumulation through its ability to bind linc-MD1 and repress Drosha cleavage. We show that HuR is under the repressive control of miR-133 and that the sponging activity of linc-MD1 consolidates HuR expression in a feedforward positive loop. Finally, we show that HuR also acts in the cytoplasm, reinforcing linc-MD1 sponge activity by cooperating for miRNA recruitment. An increase in miR-133 synthesis, mainly from the two unrelated miR-133a coding genomic loci, is likely to trigger the exit from this circuitry and progression to later differentiation stages.