R-spondin, a novel gene with thrombospondin type 1 domain, was expressed in the dorsal neural tube and affected in Wnts mutants.

We identified a novel gene, which encodes a 265-amino-acid sequence with a thrombospondin (TSP) type 1 motif. Unlike the other secretory proteins of the TSP family, this gene encodes no apparent secretion cleavage site, but has a putative nuclear localization signal. Northern blot analysis showed transient expression in the central nervous system (CNS) during development. In situ hybridization showed its expression in the dorsal part of the neural tube on 10 and 12 dpc, especially in the boundary region between roof plate and neuroepithelium. This expression was enhanced in the rostral part. The signals were observed in other tissues such as truncal region neighboring forelimbs and mesenchymal tissues around the nasal cavity. We named this gene R-spondin (roof plate-specific spondin). Transfection of an epitope-tagged R-spondin into COS7 and 293 cells showed its localization in nuclei and medium, suggesting that R-spondin may become secretory or nuclear protein by some processing, while most of other proteins with TSP type 1 domain are secretory proteins. The expression of R-spondin was reduced in Wnt-1/3a double knockout mouse. R-spondin might be a novel marker of the boundary between the roof plate and neuroepithelium and may contribute to the development of dorsal neural tube under the regulation of Wnts.