Target-directed miRNA degradation

microRNAs (miRNAs) are tiny RNAs that repress target mRNAs. While miRNA production has been extensively studied, less is known about miRNA degradation. One way to degrade miRNAs is through target-directed miRNA degradation (TDMD), a phenomenon first observed with synthetic and viral RNAs containing highly complementary binding sites to a miRNA. More recently, several endogenous RNAs that trigger TDMD have been described. For example, the Cyrano long noncoding RNA (lncRNA) induces destruction of the miR-7 miRNA in mice and in human cells, and this TDMD activity is mediated by a highly complementary site to miR-7. How does extensive complementarity invert the typical regulatory logic that occurs between a miRNA and its target? How common is TDMD and what are the biological roles for regulated miRNA degradation, especially in the brain? And why would an animal bother to make a miRNA if only to rapidly degrade it?