Bone resorption by osteoclasts is required for normal bone remodeling and reshaping of growing bones. Excessive resorption is an important pathologic feature of many diseases, including osteoporosis, arthritis, and periodontitis (Abu-Amer, 2005). On the other hand, deficient resorption leads to osteopetrosis which is characterized by increased bone mass and may lead to bone deformities or in severe cases to death (Blair and Athanasou, 2004; Del Fattore et al., 2006).
Recently, we identified a gene, nha-oc/NHA2, which is strongly up regulated during RANKL-induced osteoclast differentiation in vitro and in vivo. nha-oc/NHA2 encodes a novel cation/proton exchanger that is strongly expressed in osteoclasts. The purpose of this work was to further validate the restricted expression of nha-oc/NHA2 in osteoclasts by in situ hybridization. Our results showed that nha-oc is expressed predominantly in bone. In the head, expression was found in the supraoccipitale bone, calvarium, mandible, and maxilla. Furthermore, nha-oc positive cells co-express the osteoclast markers TRAP and cathepsin k, confirming nha-oc/NHA2 osteoclast localization. However, only a subset of cathepsin k-expressing cells is positive for nha-oc/NHA2, suggesting that nha-oc is expressed by terminally differentiated osteoclasts.