iCLIP of the RNA-binding protein ARPP21

About half of all human and mouse miRNA genes are located within introns of protein-coding genes. Despite this, little is known about functional interactions between miRNAs and their host genes. The intronic miRNA miR-128 regulates neuronal excitability and controls dendrite outgrowth of projection neurons during development of the mouse cerebral cortex. Its host genes R3hdm1 and Arpp21 encode highly conserved, putative RNA-binding proteins. Here we use iCLIP to describe the RNA-binding activity of ARPP21, which recognizes uridine-rich sequences with exquisite sensitivity for 3'UTRs. Surprisingly, ARPP21 antagonizes miR-128 activity by co-regulating a subset of miR-128 target mRNAs enriched for neurodevelopmental functions. In contrast to miR-128, we show that ARPP21 acts as a positive post-transcriptional regulator, at least in part through interaction with the eukaryotic translation initiation complex eIF4F. This molecular antagonism is also reflected in inverse activities during dendritogenesis: miR-128 overexpression or knockdown of ARPP21 reduces dendritic complexity; ectopic ARPP21 leads to an increase. The regulatory interaction between ARPP21 and miR-128 is a unique example of convergent function by two products of a single gene.