Figure 3.

miR-9 regulates progenitor states in Vertebrates. (A) miR-9 is expressed in active neurogenic zones. Sagittal section through a zebrafish embryo at 48 h post fertilization, showing the expression of miR-9 as revealed by in situ hybridization (blue). miR-9 is expressed at the ventricular zone, and excluded from differentiated neurons expressing the protein HuC (magenta). Its expression is induced in neurogenic areas, where the expression of proneural genes such as neurogenin1 (neurog1) is detected (green). In contrast, miR-9 is excluded specifically from boundary regions, containing long-lasting neural progenitors, such as the MHB (big arrowhead) or rhombomere boundaries (small arrowheads). (B) Functional data suggest that miR-9 promotes the transition from a non-neurogenic progenitor, expressing high levels of Hes1, to a neurogenic progenitor, in which Hes1 levels oscillate. The miR-9 expressing neurogenic progenitor is in an ambivalent state, poised to respond to proliferation or differentiation cues. (C) Scheme representing negative feedback loops between miR-9 and its targets, some of which promoting proliferation (purple) and others promoting differentiation (green).