SCO-spondin: a new member of the thrombospondin family secreted by the subcommissural organ is a candidate in the modulation of neuronal aggregation

A number of cues are known to influence neuronal development including growth factors, cell-adhesion molecules, components of the extracellular matrix and guidance molecules. In this study, we present molecular and functional evidence that SCO-spondin, a novel relative of the thrombospondin family, could also be involved in neuronal development by modulating cell aggregative mechanisms. SCO-spondin corresponds to glycoproteins secreted by the subcommissural organ (SCO), an ependymal differentiation of the vertebrate brain located at the entrance to the Sylvian aqueduct. A cDNA clone of 2.6 kb, isolated from a bovine SCO cDNA library, was shown to be specifically and highly expressed in the bovine SCO by in situ hybridization and was subsequently sequenced. Analysis of the deduced amino acid sequence reveals the presence of four conserved domains known as thrombospondin (TSP) type I repeats. To account for the homology with thrombospondins and F-spondin, this secreted glycoprotein was called SCO-spondin. Two potent binding sites to glycosaminoglycan (BBXB) and to cytokine (TXWSXWS) are also found in the TSP type I repeats. The deduced amino acid sequence exhibits three other conserved domains called low density lipoprotein (LDL) receptor type A repeats. The possibility of SCO-spondin involvement in neuronal development as a component of the extracellular matrix is discussed regarding these molecular features. The idea of a modulation of cell-cell and/or cell-matrix interaction is further supported by the anti-aggregative effect observed on cultured neuronal cells of material solubilized from Reissner's fiber. That Reissner's fiber, the condensed secretory product of the SCO present along the whole spinal cord can be a potent morphogenetical structure is an important concept for the analysis of the molecular mechanisms leading to spinal cord differentiation.