Immunolocalization of DMP1 and sclerostin in the epiphyseal trabecule and diaphyseal cortical bone of osteoprotegerin deficient mice

Abstract

In order to define the osteocytic function in accelerated bone remodeling, we examined the distribution of the osteocytic lacunar-canalicular system (OLCS) and osteocyte-secreting moleculesdentin matrix protein (DMP) 1 and sclerostin-in the epiphyses and cortical bones of osteoprotegerin deficient (OPG −/− ) mice. Silver impregnation visualized a well-arranged OLCS in the wild-type epiphyses and cortical bone, whereas OPG −/− mice had an irregular OLCS in the epiphyses, but well-arranged canaliculi in the cortical bone. DMP1-positive osteocytes were evenly distributed throughout the wild-type epiphyses and cortical bone, as well as the OPG −/− cortical bone. However, OPG −/− epiphyses revealed weak DMP1-immunoreactivity. Thus, osteocytes appear to synthesize more DMP1 as the OLCS becomes regular. In contrast, sclerostin-immunoreactivity was significantly diminished in the OPG −/− epiphyses and cortical bone. In OPG −/− epiphyses and cortical bone, triple staining demonstrated few sclerostin-positive osteocytes in the periphery of a thick cell layer of alkaline phosphatase-positive osteoblasts and many tartrate resistant acid phosphatasepositive osteoclasts. Summarizing, the regular distribution of OLCS may affect DMP1 synthesis, while the cellular activities of osteoclasts and osteoblasts rather than the regularity of OLCS may ultimately influence sclerostin synthesis.Osteocytes are the most numerous cells in bone, being localized within their lacunae. The osteocytes are connected each other by means of their cytoplasmic processes interconnected through gap junctions (9, 23), which pass through narrow passage ways referred to as osteocytic canaliculi. Therefore, osteocytes build up functional syncytia, i.e., the osteocytic lacunar-canalicular system (OLCS) (1,7,15). To date, osteocytes and their canaliculi have been shown to sense the direction and strength of mechanical stress in bone and then, affect the communication among osteocytes and between osteocytes and osteoblasts (7,14,35). They may also regulate bone remodeling (16,22,27) and mineral metabolism (4,10,27). Osteocytes might control trafficking of minerals such as calcium and phosphate through their canaliculi, and might regulate osteoclastic and osteoblastic activities on the bone surface. These putative functions imply that OLCS feature a finely tuned arrangement, which may be altered by physical or chemical imbalances. Our previous work demonstrated that physiological bone remodeling would reconstruct the arrangement of OLCS more regularly; especially, the speed of bone deposition during remodeling would influence the regularity of OLCS (13, 31). The regularity of OLCS appears to be different in each region of

Immunolocalization of sclerostin synthesized by osteocytes in relation to bone remodeling in the interradicular septa of ovariectomized rats

“…Enzyme histochemistry for tartrate-resistant acid phosphatase (TRAP), and immunohistochemistry for sclerostin, DMP-1, ER, osteopontin and RANKL Tartrate-resistant acid phosphatase (TRAP) was detected as previously described [36]; briefly, For immunohistochemistry, dewaxed paraffin sections were used for detection of sclerostin, DMP-1 and osteopontin as previously reported [30,37]. Endogenous peroxidase inhibition was conducted for all sections, and that was the first step in staining unless otherwise stated.…”

“…Complex meshwork of cement lines broadly distributed in the OVX mesial region appeared to be in agreement with fragmented osteocytic lacunar-canalicular system, referred to as OLCS, in the corresponding region. Masuki et al demonstrated fragmented OLCS by stimulated bone remodeling in mice with osteoprotegerin-depletion, and markedly-reduced synthesis of sclerostin [30]. In our study, the estrogen deficiency induced intense reactivity of RANKL in the mesial region of the alveolar bone, which may provide a similar milieu for osteoclastogenesis as that reported by Masuki et al Therefore, it seems reasonable that enhanced RANKL by estrogen deficiency would stimulate bone remodeling in the deeper mesial region, resulting in the fragmented OLCS, which inhibits secretion of sclerostin in osteocytes.…”

“…RANK is a member of the membrane-associated tumor necrosis factor receptor family located on cell membranes of osteoclasts and their precursors [27,28], while RANKL is present on the cell membrane of osteoblast lineages [27][28][29]. Recently, Masuki et al [30] reported that osteocytic synthesis of sclerostin was markedly inhibited in long bones of mice lacking osteoprotegerin, a decoy receptor for RANKL inhibiting osteoclastogenesis [31], and therefore, it seems likely that osteocytes are influenced by estrogen deficiency-induced osteoporosis.…”

<b>Chronological immunolocalization of sclerostin and FGF23 in the mouse metaphyseal trabecular and cortical </b><b>bone </b>

“…Immature, primary trabeculae contained randomly-oriented OLCS, revealing an intense tissue nonspecific alkaline phosphatase (TNAP)-reactivity, a hallmark of osteoblastic activities (3,26), on their surface. In contrast, secondary trabeculae constructed relatively-regular OLCS, and mature cortical bone demonstrated geometrically well-arrangement of OLCS, featuring TNAP-positive flattened, bone-lining osteoblasts (11,23,37). Normal bone remodeling is well known to make bone structure better strengthened and arranged so as to be resist against the mechanical stress, and by means of bone remodeling, the distribution of OLCS becomes progressively more regular as the individual grows.…”

“…Normal bone remodeling is well known to make bone structure better strengthened and arranged so as to be resist against the mechanical stress, and by means of bone remodeling, the distribution of OLCS becomes progressively more regular as the individual grows. Therefore, the biological activities of osteocytes seem to take place in bone metabolism by means of specific molecules-sclerostin and FGF23 (11,23,37). However, histological features of metaphyseal trabeculae and cortical bone chronologically change the area comprised by the last quarter of the metaphyseal trabeculae and the endosteal surfaces of the cortical bone.…”