CD44 and the ERM Family in Bone Cells

Abstract

We describe here the immunohistochemical localization of CD44 (hyaluronate receptor) and the ezrin-radixin-moesin (ERM) family (actin binding proteins) in bone cells and discuss possible mechanisms for the organization of their cytoskeletons.
In the cells of osteoblast-lineage, active osteoblasts showed immunoreactivity to CD44 only on their cytoplasmic processes in the bone matrix. They showed degrees of immunoreactivity to radixin and moesin on the cytoplasmic side of their basolateral plasma membrane, but not in their cytoplasmic processes. On the other hand, osteocytes demonstrated intense immunolabeling to CD44, showing scarcely any immunoreactivity to the ERM family. In osteoclasts, intense immunoreactivity to CD44 was detected on their basolateral plasma membranes. The plasma membranes of the clear zone and the ruffled border were not immuno-labeled with CD44. As for the ERM family, the basolateral plasma membrane of the osteoclasts was stained with anti-moesin monoclonal antibody, but not with anti-ezrin or anti-radixin antibody. Thirty minutes after the administration of calcitonin, osteoclasts did not show either clear zones or ruffled borders. CD44 and moesin were colocalized all along their plasma membranes, including the region facing the bone surface.
These findings suggest that: 1) the CD44moesin-actin filament system is involved in the cytoskeletal organization of osteoclasts and their cell-polarity; and 2) other mechanisms, rather than the CD44 and the ERM family, may be involved in the cells of osteoblast-lineage.