FGF23 is mainly synthesized by osteocytes in the regularly distributed osteocytic lacunar canalicular system established after physiological bone remodeling

Abstract

This study aimed to evaluate whether the immunolocalization of fibroblast growth factor (FGF) 23 and dentin matrix protein 1 (DMP1) is associated with the spatial regularity of the osteocyte lacunar canalicular system(s) (OLCS). Femora of 12-weeks-old male ICR mice were fixed with 4% paraformaldehyde, decalcified with a 10% EDTA solution and then embedded in paraffin. We have devised a triple staining procedure that combines silver impregnation, alkaline phosphatase (ALPase) immunohistochemistry and tartrate-resistant acid phosphatase (TRAPase) enzyme histochemistry on a single paraffin section. This procedure permitted the visualization of ALPase-positive plump osteoblasts and several TRAPase-positive osteoclasts on those bone matrices featuring irregularly arranged OLCS, and of ALPase-positive bone lining cells on the bone matrix displaying the well-arranged OLCS. As observations proceeded from the metaphysis toward the diaphysis, the endosteal cortical bone displayed narrower bands of calcein labeling, accompanied by increased regularity of the OLCS. This implies that the speed of bone deposition during bone remodeling would affect the regularity of the OLCS. While DMP1 was evenly localized in all regions of the cortical bones, FGF23 was more abundantly localized in osteocytes of cortical bones with regularly arranged OLCS. In cortical bones, the endosteal area featuring regular OLCS exhibited more intense FGF23 immunoreaction when compared to the periosteal region, which tended to display irregular OLCS. In summary, FGF23 appears to be synthesized principally by osteocytes in the regularly distributed OLCS that have been established after bone remodeling.

Functional heterogeneity of osteocytes in FGF23 production: the possible involvement of DMP1 as a direct negative regulator

“…18 However, much larger numbers of FGF23-positive lacunae were observed in the cortical bone of the diaphysis, whereas DMP1 signals were detectable in most of the lacunae, as was observed in the trabecular bone ( Figure 1a). Notably, most of the FGF23-positive lacunae in this bone site tended to be predominantly located in the endosteal portion, not in the periosteal portion, thereby demarcating FGF23-positive and FGF23-negative areas in cortical bone, although DMP1-positive lacunae were detectable in the entire areas ( Figure 1a, left panels).…”

“…Along this line, the regulatory production of FGF23 in osteocytes is tightly associated with bone formation and maturation. On the basis of a series of immunohistochemistry studies, Ubaidas et al 18 proposed that the primary source of FGF23 is osteocytes established after physiological bone remodeling. This interpretation is consistent with our observations in the sense that hypermineralized mature lacunae possibly decrease their amount of DMP1 and become prone to enhancing the fgf23 expression in their encased osteocytes.…”

“…This may also explain the fact that a significantly lower number of osteocytes exhibit detectable levels of the FGF23 protein expression compared with that noted in trabecular bone, likely owing to the distinct bone turnover rate observed in these bone sites. 18 FGF23 production was mostly detectable in longitudinally arranged and spindle-shaped lacunae in lamellar bone (Figure 2). The formation of longitudinally arranged and spindleshaped osteocytes is highly associated with mechanical loading during postnatal bone development.…”

“…12 Although FGF23 mRNA is found in several tissues, this molecule is most abundantly expressed in bone, predominantly in osteocytes. [13][14][15][16][17][18] FGF23 essentially downregulates the serum levels of phosphate by inhibiting the absorption and reabsorption of phosphate from the gut and kidneys, respectively. In the kidneys, this hormone reduces the expression of sodium-phosphate transporters (type IIa and IIc) in the renal proximal tubular membrane, thereby downregulating phosphate reabsorption, thus resulting in the excretion of phosphate.…”

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

“…Silver impregnation was performed as previously described [39]. Dewaxed sections were soaked in a 1.5% Protargol-S solution diluted in borax-boric acid (pH 7.4) for 40 h at 37 o C.…”