UC San Diego

SOX2 is a pan-neural transcription factor expressed in neural precursor cells (NPCs) independently of their regional identity. It plays a crucial role in both neural development and in adult neurogenesis; however the molecular mechanisms underlying its function are poorly understood. Our previous data suggested that SOX2 controls LIN28, an inhibitor of let-7 miRNA biogenesis. We hypothesized that some of the pan-neural functions of SOX2 could be mediated by repression of let-7 miRNA activity. To identify miRNAs with pan-neural SOX2 dependency, I used NPCs with two different regional identities, dorsal and ventral. I modified a previously established human embryonic stem cell (hESC)-derivation protocol (which generated dorsal NPCs) by patterning NPCs with the Sonic hedgehog agonist purmorphamine, yieding ventral NPCs. Analysis of SOX2 targets in dorsal and ventral NPCs in addition to bioinformatics suggested that SOX2 represses the levels of let-7b and let-7i miRNA. Overexpression studies of let-7b and let-7i revealed that let-7b selectively suppresses NPC proliferation with no effect on neuronal differentiation, while let-7i abolishes neuronal differentiation without inhibiting NPC proliferation. These data suggest that the combined effect of let-7b and let-7i overexpression in NPCs photocopies the loss of SOX2 in these cells. Taken together, our results suggest that in our in vitro cultures, SOX2 suppresses the activity of let-7 miRNAs in NPCs. We propose that the function of let- 7 miRNA family downstream of SOX2 may be a general mechanism for controlling both NPC proliferation and neurogenesis in developmental and adult contexts