Structure and Implied Functions of Truncated B-Cell Receptor mRNAs in Early Embryo and Adult Mesenchymal Stem Cells: Cδ Replaces Cμ in μ Heavy Chain-Deficient Mice

Abstract

Stem cells exhibit a promiscuous gene expression pattern. We show herein that the early embryo and adult MSCs express B-cell receptor component mRNAs. To examine possible bearings of these genes on the expressing cells, we studied immunoglobulin μ chain-deficient mice. Pregnant μ chain-deficient females were found to produce a higher percentage of defective morulae compared with control females. Structure analysis indicated that the μ mRNA species found in embryos and in mesenchyme consist of the constant region of the μ heavy chain that encodes a recombinant 50-kDa protein. In situ hybridization localized the constant μ gene expression to loose mesenchymal tissues within the day-12.5 embryo proper and the yolk sac. In early embryo and in adult mesenchyme from μ-deficient mice, δ replaced μ chain, implying a possible requirement of these alternative molecules for embryo development and mesenchymal functions. Indeed, overexpression of the mesenchymal-truncated μ heavy chain in 293T cells resulted in specific subcellular localization and in G1 growth arrest. The lack of such occurrence following overexpression of a complete, rearranged form of μ chain suggests that the mesenchymal version of this mRNA may possess unique functions.