Targets of the nuclear factor I regulon involved in early and late development of postmitotic cerebellar granule neurons

Recent studies have shown that the nuclear factor I (NFI) family controls multiple stages of the postmitotic differentiation of cerebellar granule neurons (CGNs). Regulation of cell–cell signaling is an integral part of this NFI program, which involves expression of the cell adhesion molecules N cadherin and ephrin B1 throughout postmitotic CGN development. Here, we identify two additional downstream targets of NFI that are involved in extracellular CGN interactions. The cell adhesion molecule Tag‐1 is highly enriched in CGNs undergoing parallel fiber formation and is down‐regulated prior to onset of radial migration. We found that Tag‐1 expression was strongly reduced by NFI dominant repression in immature primary CGNs and in the cerebella of E18 Nfib‐null mice. Transient transfection and chromatin immunoprecipitation suggested that the Tag‐1 gene is directly regulated by NFI. Furthermore, functional, Nfi knockout and chromatin immunoprecipitation studies implicated Wnt7a as a direct target of NFI in maturing CGNs. Wnt7a is secreted by developing CGNs and is required for maturation of mossy fiber–CGN synaptic rosettes. Consistent with this, synapsin I was greatly reduced within the internal granule cell layer of P17 Nfia‐null mice. These findings indicated that NFI controls CGN postmitotic maturation through a combination of extracellular signaling molecules that operate either continuously to regulate multiple stages of development (N cadherin and ephrin B1) or primarily at early (Tag‐1) or late (Wnt7a) maturation steps. They also illustrate the importance of NFI as a critical link between cell‐intrinsic mechanisms and cell–cell interactions in the development of the mouse cerebellum. © 2009 Wiley‐Liss, Inc.