miR-31-mediated local translation at the mitotic spindle is important for early development.

miR-31 is a highly conserved microRNA that plays critical roles in cell proliferation, migration, and differentiation. We discovered miR-31 and some of its validated targets are enriched on the mitotic spindle of the dividing sea urchin embryo and mammalian cells. Using the sea urchin embryo, we found that miR-31 inhibition led to developmental delay correlated with increased cytoskeleton and chromosomal defects. We identified miR-31 to directly suppress several actin remodeling transcripts, β-actin , Gelsolin , Rab35 and Fascin , which were localized to the mitotic spindle. miR-31 inhibition leads to increased newly translated Fascin at the spindles. Forced ectopic localization of Fascin transcripts to the cell membrane and translation led to significant developmental and chromosomal segregation defects, leading to our hypothesis that miR-31 regulates local translation at the mitotic spindle to ensure proper cell division. Furthermore, miR-31-mediated post-transcriptional regulation at the mitotic spindle may be an evolutionarily conserved regulatory paradigm of mitosis.