Cellular immunohistochemical localization of the matricellular protein myocilin in the intervertebral disc

Myocilin is a 55-57-kDa protein that is a member of the olfactomedin protein family. It is expressed in the cornea, sclera and trabecular network of the eye, myelinated peripheral nerves, heart, skeletal muscle, trachea and other tissues. Myocilin binds to a domain of fibronectin, type IV collagen and laminen in the trabecular meshwork of the eye, and its expression is influenced by transforming growth factor beta. Because these extracellular matrix components also are common in the intervertebral disc, the objective of our study was to determine whether the matricellular protein myocilin could be detected in the human or sand rat intervertebral disc using immunohistochemistry and to assess its localization. We investigated 16 specimens of human disc tissue and discs from six sand rats. Three human disc cell cultures grown in three-dimensional culture also were evaluated. Immunocytochemical annulus analysis showed the presence of myocilin within the disc cell cytoplasm in some, but not all, cells. Extracellular matrix in both the human and sand rat disc was negative for myocilin localization. Myocilin is believed to play a role in cell-cell adhesion and/or signaling. Myocilin may have such functions within the disc cell population in a manner similar to tenascin, SPARC and thrombospondin, which are other matricellular proteins recently shown to be present in the disc.