The murine vasorin (Vasn) gene, initially known as Slit-like 2, encodes a transmembrane protein that shares structural similarities with the eponymous Slit proteins. However, whether it also shares functional similarities with these large secreted proteins remains to be elucidated. Here, we report expression of Vasn during embryonic and fetal development of the mouse using whole-mount in situ hybridization (WISH) and histochemical detection of β-galactosidase expressed from a targeted Vasn(lacZ) knock-in allele. Comparison of whole-mount staining patterns of both approaches showed identical expression domains, confirming that Vasn promoter-driven β-galactosidase expression faithfully reflects endogenous Vasn expression. Vasn is highly expressed in vascular smooth muscle cells (hence the name), a finding consistent with a previous report on its human homolog VASN, whose extracellular domain was shown to function as a TGF-β trap (Ikeda et al., 2004). Most striking, however, is Vasn's prominent expression in the developing skeletal system, starting as early as the first mesenchymal condensations appear. Moreover, distinct expression domains outside the bones, e.g., in the developing kidneys and lungs, suggest further roles for this gene in the mouse. Recently, it was shown that mitochondria-localized Vasn protects cells from TNFα- and hypoxia-induced apoptosis, and partial deletion of the Vasn coding sequence leads to increased sensitivity of hepatocytes to TNFα-induced apoptosis (Choksi et al., 2011). By providing a first comprehensive analysis of the Vasn expression pattern during mouse embryonic development, our study will help to further elucidate its biological functions.

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