The extracellular matrix protein WARP is a novel component of a distinct subset of basement membranes.

WARP is a recently described member of the von Willebrand factor A domain superfamily of extracellular matrix proteins, and is encoded by the Vwa1 gene. We have previously shown that WARP is a multimeric component of the chondrocyte pericellular matrix in articular cartilage and intervertebral disc, where it interacts with the basement membrane heparan sulfate proteoglycan perlecan. However, the tissue-specific expression of WARP in non-cartilaginous tissues and its localization in the extracellular matrix of other perlecan-containing tissues have not been analyzed in detail. To visualize WARP-expressing cells, we generated a reporter gene knock-in mouse by targeted replacement of the Vwa1 gene with beta-galactosidase. Analysis of reporter gene expression and WARP protein localization by immunostaining demonstrates that WARP is a component of a limited number of distinct basement membranes. WARP is expressed in the vasculature of neural tissues and in basement membrane structures of the peripheral nervous system. Furthermore, WARP is also expressed in the apical ectodermal ridge of developing limb buds, and in skeletal and cardiac muscle. These findings are the first evidence for WARP expression in non-cartilaginous tissues, and the identification of WARP as a component of a limited range of specialized basement membranes provides further evidence for the heterogeneous composition of basement membranes between different tissues.