Mice Lacking the Extracellular Matrix Protein WARP Develop Normally but Have Compromised Peripheral Nerve Structure and Function

Uwe Hansen,John F Bateman,Shireen R Lamandé,Bent Brachvogel,Justin M Allen,Ursula Schlötzer-Schrehardt,Jamie Fitzgerald,Lynn Rowley,Laura Zamurs

doi:10.1074/jbc.m806968200

Abstract

WARP is a recently identified extracellular matrix molecule with restricted expression in permanent cartilages and a distinct subset of basement membranes in peripheral nerves, muscle, and the central nervous system vasculature. WARP interacts with perlecan, and we also demonstrate here that WARP binds type VI collagen, suggesting a function in bridging connective tissue structures. To understand the in vivo function of WARP, we generated a WARP-deficient mouse strain. WARP-null mice were healthy, viable, and fertile with no overt abnormalities. Motor function and behavioral testing demonstrated that WARP-null mice exhibited a significantly delayed response to acute painful stimulus and impaired fine motor coordination, although general motor function was not affected, suggesting compromised peripheral nerve function. Immunostaining of WARP-interacting ligands demonstrated that the collagen VI microfibrillar matrix was severely reduced and mislocalized in peripheral nerves of WARP-null mice. Further ultrastructural analysis revealed reduced fibrillar collagen deposition within the peripheral nerve extracellular matrix and abnormal partial fusing of adjacent Schwann cell basement membranes, suggesting an important function for WARP in stabilizing the association of the collagenous interstitial matrix with the Schwann cell basement membrane. In contrast, other WARP-deficient tissues such as articular cartilage, intervertebral discs, and skeletal muscle showed no detectable abnormalities, and basement membranes formed normally. Our data demonstrate that although WARP is not essential for basement membrane formation or musculoskeletal development, it has critical roles in the structure and function of peripheral nerves.

Highlights

WARP is a recently described member of the von Willebrand factor type A domain (VWA2 domain) superfamily of extracellular matrix (ECM) molecules, adhesion proteins, and cell surface receptors
The WARP protein is encoded by the Vwa1 gene and comprises a single N-terminal VWA domain containing a putative metal ion-dependent adhesion site (MIDAS) motif, two fibronectin type III repeats, and a unique C-terminal domain that contributes to WARP multimer formation [2, 3]
The collagen VI distribution in the outer perineurium layer of the nerve, where WARP is not expressed [5], was unaffected in the mutant mice (Fig. 4, E and F). These data demonstrate that the basement membranes surrounding myelinated axons in WARP-null peripheral nerves are intact, but the microfibrillar collagen VI matrix is disrupted in the endoneurium

Summary

Mice Deficient in the ECM Molecule WARP Have Nerve Defects

WARP deficiency on skeletal development and basement membrane formation. The homozygous null mice are viable, fertile, and do not exhibit overt abnormalities compared with wild type littermates. Neurological testing revealed that WARP-null mice exhibit a delayed response to acute painful stimulus and a disturbance in fine motor coordination, general motor function is not impaired Consistent with these findings, immunohistochemical analysis of peripheral nerves from WARP-null mice revealed that the collagen VI microfibrillar matrix was severely reduced and mislocalized compared with wild type mice. Electron microscopic examination of the sciatic nerve demonstrated a reduction in the collagen I ECM and the unusual partial fusing of the basement membranes of neighboring axons These data suggest an important role for WARP in organizing the peripheral nerve ECM and provides evidence for tissue-specific differences in the role of WARP in the assembly and/or integration of the ECM. Our studies provide further evidence for the critical role of ECM structure and organization in nerve function

EXPERIMENTAL PROCEDURES

RESULTS

Kinetic data for the interaction was obtained by injecting

DISCUSSION