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

Abstract

To assess the chronological participation of sclerostin and FGF23 in bone metabolism, this study tracked the immunolocalization of sclerostin and FGF23 in the metaphyses of murine long bones from embryonic day 18 (E18) through 1 day after birth, 1 week, 2 weeks, 4 weeks, 8 weeks, and 20 weeks of age. We have selected two regions in the metaphyseal trabeculae for assessing sclerostin and FGF23 localization: close to the chondro-osseous junction, i.e., bone modeling site even in the adult animals, and the trabecular region distant from the growth plate, where bone remodeling takes place. As a consequence, sclerostin-immunopositive osteocytes could not be observed in both close and distant trabecular regions early at the embryonic and young adult stages. However, osteocytes gradually started to express sclerostin in the distant region earlier than in the close region of the trabeculae. Immunoreactivity for FGF23 was observed mainly in osteoblasts in the early stages, but detectable in osteocytes in the later stages of growth in trabecular and cortical bones. Fgf23 was weakly expressed in the embryonic and neonatal stages, while the receptors, Fgfr1c and αKlotho were strongly expressed in femora. At the adult stages, Fgf23 expression became more intense while Fgfr1c and aKlotho were weakly expressed. These findings suggest that sclerostin is secreted by osteocytes in mature bone undergoing remodeling while FGF23 is synthesized by osteoblasts and osteocytes depending on the developmental/growth stage. In addition, it appears that FGF23 acts in an autocrine and paracrine fashion in fetal and neonatal bones.By establishing a communication network between osteocytes and osteoblasts, osteocytes are situated at the center of bone turnover and help determine how bones adjust to mechanical stress through bone remodeling. Osteocytes exist inside osteocytic lacunae and connect to neighboring osteocytes and osteoblasts on the bone surfaces via fine cytoplasmic processes that run through osteocytic canaliculi. Osteocytes interconnect their cytoplasmic processes via gap junctions (2,6,7,33), thereby building functional syncytia referred to as osteocytic lacunar-canalicular system (OLCS) (5,15,16,31). In the last few decades, many studies have reported on important osteocyte-derived factors, such as sclerostin (which regulates osteoblastic activities) and fibroblast growth factor 23 (FGF23, which affects the proximal renal tubules to reduce serum

Altered immunolocalization of FGF23 in murine femora metastasized with human breast carcinoma MDA-MB-231 cells

“…In this study, the micro-circumstance of the metastasized lesion might have been able to induce MDA-MB-231 to secrete FGF23. In our previous report, the embryonic and infant stages of bone expressed abundant FGFR1 and klotho, and FGF23 was produced mainly by osteoblastic cells rather than osteocytes in the embryonic stage [16]. However, as mice grow, FGF23synthesizing cells were seen to chronologically shift from osteoblastic cells to osteocytes.…”

“…All animal experiments were conducted under the Guidelines for Animal Experimentation of Hokkaido University (Approval No. [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31]. Human breast carcinoma MDA-MB-231 cells (American Type Culture Collection, Rockville, MD)…”

“…To evaluate the mRNAs encoding Fgf23, Fgfr1, Klotho, and Gapdh, total RNA was extracted from the femora of control mice, the femora of mice metastasized with MDA-MB-231 cells, the mammary glands of control mice, and mammary glands injected with MDA-MB-231 cells, as previously reported [16]. In brief, the metastasized femora were sagitally cut, and the bone-metastasized tumor nests inside the femora were confirmed under a binocular microscope and then extracted from the femora.…”

“…In a normal state, FGF23 is mainly synthesized by osteocytes in adults [15,16] and is circulated to reach the kidney.…”

Immunolocalization of Enzymes/Membrane Transporters Related to Bone Mineralization in the Metaphyses of the Long Bones of Parathyroid-Hormone-Administered Mice

“…However, recent studies have proposed an autocrine function of FGF23, which, at least in part, is related to bone mineralization. Osteoblasts/osteocytes express not only FGF23 but also the FGFR1c/αklotho receptor complex [ 27 , 28 ], consequently regulating bone mineralization in an autocrine manner [ 29 ]. In addition, PHEX has been postulated to bind the small integrin-binding ligand N-linked glycoprotein (SIBLING) family including osteopontin, matrix extracellular phosphoglycoprotein (MEPE), dentin matrix protein 1 (DMP1), dentin sialoprotein, and bone sialoprotein for the local regulation of mineralization in bone [ 30 , 31 , 32 , 33 , 34 , 35 ].…”