Nucleobindin is produced by bone cells and secreted into the osteoid, with a potential role as a modulator of matrix maturation.

Nucleobindin (Nuc), also known as CALNUC, is a Ca(2+)-binding protein, located in the nucleus, the Golgi apparatus and the endoplasmic reticulum (ER). The presence of a signal sequence in Nuc suggests secretion from the cell and it has been found in bone extracellular matrix. Within the present study, molecular biological and morphological methods were combined to evaluate the synthesis and distribution of Nuc in and around cells of rat metaphyseal and calvarial bone. Northern blot analysis and in situ hybridization of bone tissues confirmed that the protein was a product of bone cells. By electron microscopy, immunolabeling for Nuc was seen in osteoid of newly formed bone, on all surfaces facing the various bone cells and also in compact bone. Intracellularly, the gold particles were found in the rough ER of osteoblasts, which suggested synthesis of the protein by these cells. Compared to bone sialoprotein and osteopontin, Nuc demonstrated different localization pattern in bone trabeculae, with the majority of labeling restricted to nonmineralized osteoid. Moreover, the role of Nuc during the mineralization process was investigated in rat calvaria-derived primary osteoblasts grown under osteogenic conditions. Semiquantitative RT-PCR and Northern blot analysis showed Nuc expression to be low during cell proliferation, upregulated during differentiation and matrix maturation, but subsequently downregulated during mineralization. In summary, our data show that Nuc was synthesized by osteoblasts and osteocytes, and secreted into the osteoid, suggesting a role as a modulator of matrix maturation in the mineralization process in bone.