Meis homeodomain transcription factors control cell proliferation, cell fate specification and differentiation in development and disease. Previous studies have largely focused on Meis contribution to the development of non-neuronal tissues. By contrast, Meis function in the brain is not well understood. Here, we provide evidence for a dual role of the Meis family protein Meis2 in adult olfactory bulb (OB) neurogenesis. Meis2 is strongly expressed in neuroblasts of the subventricular zone (SVZ) and rostral migratory stream (RMS) and in some of the OB interneurons that are continuously replaced during adult life. Targeted manipulations with retroviral vectors expressing function-blocking forms or with small interfering RNAs demonstrated that Meis activity is cell-autonomously required for the acquisition of a general neuronal fate by SVZ-derived progenitors in vivo and in vitro. Additionally, Meis2 activity in the RMS is important for the generation of dopaminergic periglomerular neurons in the OB. Chromatin immunoprecipitation identified doublecortin and tyrosine hydroxylase as direct Meis targets in newly generated neurons and the OB, respectively. Furthermore, biochemical analyses revealed a previously unrecognized complex of Meis2 with Pax6 and Dlx2, two transcription factors involved in OB neurogenesis. The full proneurogenic activity of Pax6 in SVZ derived neural stem and progenitor cells requires the presence of Meis. Collectively, these results show that Meis2 cooperates with Pax6 in generic neurogenesis and dopaminergic fate specification in the adult SVZ-OB system. KEY WORDS: Pax6, TALE-homeodomain proteins, Adult neurogenesis, Subventricular zone, Mouse INTRODUCTIONThe subventricular zone (SVZ), located between the lateral ventricle and the striatum, and the subgranular zone (SGZ) of the dentate gyrus of the hippocampus are major stem cell niches in the adult

During eye development in D. melanogaster, the TALE-homeodomain protein Homothorax (Hth) is expressed by progenitor cells ahead of the neurogenic wave front, promotes rapid proliferation of these cells and is downregulated before cells exit the cell cycle and differentiate. Here, we present evidence that hth function is partially conserved in vertebrates. Retinal progenitor cells (RPCs) in chicks and mice express two Hth-related proteins, Meis1 and Meis2 (Mrg1), in species-specific temporal sequences. Meis1 marks RPCs throughout the period of neurogenesis in the retina, whereas Meis2 is specific for RPCs prior to the onset of retinal differentiation. Transfection of Meis-inactivating constructs impaired RPC proliferation and led to microphthalmia. RNAinterference-mediated knock-down of expression indicated that progenitor cells expressing Meis1 together with Meis2 proliferate more rapidly than cells expressing Meis1 alone. Transfection of Meis-inactivating constructs reduced the expression of cyclin D1 (Ccnd1) in the eye primordium and co-transfection of cyclin D1 partially rescued RPC proliferation. Collectively, these results suggest that (1) Meis1 and Meis2, similar to hth, maintain retinal progenitor cells in a rapidly proliferating state; (2) they control the expression of some ocular-determination genes and components of the cell cycle machinery; and (3) together with the speciesspecific differences in Meis1/Meis2 expression, combinatorial expression of Meis family proteins might be a candidate mechanism for the differential regulation of eye growth among vertebrate species.

The spatial-temporal expression and function of Drosophila melanogaster homothorax (hth) and its vertebrate homologue Meis2 in the eye anlage show a high degree of evolutionary conservation. Both are expressed by progenitor cells ahead of the neurogenic wave front, promote rapid proliferation of these cells, and are downregulated before cells exit the cell cycle and differentiate. Here, we show that downregulation of Meis2 accompanies, but is not required, for retinal differentiation. In addition, we provide evidence that the mechanisms that terminate expression of both genes differ. Hth expression in the fly eye imaginal disc is primarily repressed by the transforming growth factor beta family protein decapentaplegic, whereas Meis2 expression in the chick eye cup is terminated in response to sonic hedgehog signaling.