44-kDal bone phosphoprotein (osteopontin) antigenicity at ectopic sites in newborn rats: kidney and nervous tissues

Abstract

Previous immunohistochemical data have shown that the 44-kDal bone phosphoprotein (44K BPP, also called sialoprotein I or oestopontin) recently isolated in our laboratory was synthesized by osteoblasts and osteocytes and was expressed early during differentiation of bone-forming cells. We report here the presence of 44K BPP antigenicity at certain ectopic sites, namely, the proximal-convoluted tubule of the kidney, neurons, sensory and secretory cells in the internal ear. To insure specificity and reproducibility, different immunohistochemical methods were used and affinity-purified antibodies against two separate preparations of pure 44K BPP were tested. In the cells of the proximal-convoluted tubule, 44K BPP immunoreactivity was observed within apical endocytotic vacuoles and within lysosomes. This staining thus correlates with the degradation of the 44K BPP epitope which we previously demonstrated to occur in serum. On the other hand, in the neurons of the acoustic ganglion and the sensory cells of the macula, 44K BPP immunoreactivity was associated with the Golgi apparatus indicating synthesis and secretion by these cells. The finding that the 44K BPP (or a structurally related molecule) is synthesized by neurons and neuroepithelial cells deserves further investigation with respect to a possible embryologic relationship between neuroectodermal cells and the precursors of some bone forming-cells of the skull.

Osteopontin overexpression is associated with arterial smooth muscle cell proliferation in vitro.

“…24 cDNA cloning and sequencing have shown that this gene encodes osteopontin (2ar, sialoprotein I, 44-kD bone phosphoprotein, and secreted phosphoprotein I), a noncollagenous sialoprotein that was first isolated from mineral bone extracellular matrix 21222526 but that has also been immunodetected in other tissues and fluids (e.g., kidney, uterus, plasma, and milk). 27 " 29 By Northern blot analysis, osteopontin mRNA was previously strongly detected in bone and the kidney, 23 aorta, and carotid artery. 22 The high steady-state level of osteopontin mRNA found in the aorta demonstrated that aortic tissue in conjunction with bone or the kidney was a relative specific site of osteopontin expression.…”

Two Distinct Osteoblast-Specific cis-Acting Elements Control Expression of a Mouse Osteocalcin Gene

“…The molecular effectors controlling differentiation in the osteoblastic lineage until the stage of a mineralizing cell have not been identified. Osteoblast differentiation is marked by the sequential activation of many genes encoding bone extracellular matrix proteins like alkaline phosphatase (3), collagens (49), proteoglycans (5), bone sialoprotein (16), osteopontin (30), and osteocalcin (39) and hormone receptors such as vitamin D 3 and parathormone receptors (40). Most of these genes are expressed in a differentiation-dependent manner in osteoblasts but also in other cell types.…”

Phosphorylation of the Proteins of the Extracellular Matrix of Mineralized Tissues By Casein Kinase-Like Activity

“…However, OPN is also pres-ent as a normal constituent of kidney, placenta, uterus, sensory, and neural cells of the inner ear, blood, and milk. Most importantly, it is secreted by many transformed and neoplastic cells (Craig et al, 1987;Yoon et al, 1987;Mark et al, 1988;Nomura et al, 1988;Senger et al, 1988Senger et al, , 1989Swanson et al, 1989). Because of its widespread occurrence, several other names have been coined: 44-kDa bone phosphoprotein (Butler, 1989), 2arl (Smith and Denhardt, 1987), and secreted phosphoprotein 1 (SPP-1) (Senger et al, 1988).…”