Osteocytes communicate with osteoclast lineage cells via RANKL

Alveolar bone remodeling after tooth extraction in irradiated mandible: An experimental study with canine model

OBJECTIVES

The aim of the present study is to investigate the morphological and cellular changes in dental extraction socket that has been irradiated after the tooth extraction and to describe morphological characteristics of the osteocytes and osteocyte-lacunar-canalicular network (LCN) by scanning electron microscopy (SEM).

MATERIAL AND METHODS

Five beagle dogs aged 1-2 years were used in this study. One side of each mandible was irradiated in two sessions and the other side of mandible (non-irradiated) served as a control. The mandible bone blocks were processed by bulk staining en bloc in basic fuchsin and the specimens were embedded routinely in polymethyl methacrylate resin without preliminary decalcification. All blocks were subjected to micro-CT imaging, after that the specimens were prepared for light microscopy and SEM.

RESULTS

Alterations in bone macrostructure are minimal in irradiated bone, but the changes in LCN are clear. In the area of the tooth extraction socket, the connections of osteocytes to the vessels and to neighboring osteocytes were not observed both in irradiated and nonirradiated bone. However, osteoclasts were located in the bone surface entering inside to the bone between osteons. In the lamellar bone of lateral sides, a decrease in canalicular connections between osteocytes and periosteum was found in irradiated bone as compared to the non-irradiated side.

CONCLUSIONS

The novelty of the present study is that radiation disrupts osteocytes and their dendrites.

Osteocyte-directed bone demineralization along canaliculi

The mammalian skeleton stores calcium and phosphate ions in bone matrix. Osteocytes in osteocyte lacunae extend numerous dendrites into canaliculi less than a micron in diameter and which are distributed throughout bone matrix. Although osteoclasts are the primary bone-resorbing cells, osteocytes also reportedly dissolve hydroxyapatite at peri-lacunar bone matrix. However, robust three-dimensional evidence for peri-canalicular bone mineral dissolution has been lacking. Here we applied a previously reported Talbot-defocus multiscan tomography method for synchrotron X-ray microscopy and analyzed the degree of bone mineralization in mouse cortical bone around the lacuno-canalicular network, which is connected both to blood vessels and the peri- and endosteum. We detected cylindrical low mineral density regions spreading around canaliculi derived from a subset of osteocytes. Transmission electron microscopy revealed both intact and demineralized bone matrix around the canaliculus. Peri-canalicular low mineral density regions were also observed in osteopetrotic mice lacking osteoclasts, indicating that osteoclasts are dispensable for peri-canalicular demineralization. These data suggest demineralization can occur from within bone through the canalicular system, and that peri-canalicular demineralization occurs not uniformly but directed by individual osteocytes. Blockade of peri-canalicular demineralization may be a therapeutic strategy to increase bone mass and quality.

Effect of age on regulation of human osteoclast differentiation

Human skeletal aging is characterized as a gradual loss of bone mass due to an excess of bone resorption not balanced by new bone formation. Using human marrow cells, we tested the hypothesis that there is an age-dependent increase in osteoclastogenesis due to intrinsic changes in regulatory factors [macrophage-colony stimulating factor (M-CSF), receptor activator of NF-κB ligand (RANKL), and osteoprotegerin (OPG)] and their receptors [c-fms and RANK]. In bone marrow cells (BMCs), c-fms (r = 0.61, P = 0.006) and RANK expression (r = 0.59, P = 0.008) were increased with age (27-82 years, n = 19). In vitro generation of osteoclasts was increased with age (r = 0.89, P = 0.007). In enriched marrow stromal cells (MSCs), constitutive expression of RANKL was increased with age (r = 0.41, P = 0.049) and expression of OPG was inversely correlated with age (r = -0.43, P = 0.039). Accordingly, there was an age-related increase in RANKL/OPG (r = 0.56, P = 0.005). These data indicate an age-related increase in human osteoclastogenesis that is associated with an intrinsic increase in expression of c-fms and RANK in osteoclast progenitors, and, in the supporting MSCs, an increase in pro-osteoclastogenic RANKL expression and a decrease in anti-osteoclastogenic OPG. These findings support the hypothesis that human marrow cells and their products can contribute to skeletal aging by increasing the generation of bone-resorbing osteoclasts. These findings help to explain underlying molecular mechanisms of progressive bone loss with advancing age in humans.

RANKL subcellular trafficking and regulatory mechanisms in osteocytes

The receptor activator of the NF-κB ligand (RANKL) is the central player in the regulation of osteoclastogenesis, and the quantity of RANKL presented to osteoclast precursors is an important factor determining the magnitude of osteoclast formation. Because osteoblastic cells are thought to be a major source of RANKL, the regulatory mechanisms of RANKL subcellular trafficking have been studied in osteoblastic cells. However, recent reports showed that osteocytes are a major source of RANKL presentation to osteoclast precursors, prompting a need to reinvestigate RANKL subcellular trafficking in osteocytes. Investigation of molecular mechanisms in detail needs well-designed in vitro experimental systems. Thus, we developed a novel co-culture system of osteoclast precursors and osteocytes embedded in collagen gel. Experiments using this model revealed that osteocytic RANKL is provided as a membrane-bound form to osteoclast precursors through osteocyte dendritic processes and that the contribution of soluble RANKL to the osteoclastogenesis supported by osteocytes is minor. Moreover, the regulation of RANKL subcellular trafficking, such as OPG-mediated transport of newly synthesized RANKL molecules to lysosomal storage compartments, and the release of RANKL to the cell surface upon stimulation with RANK are confirmed to be functional in osteocytes. These results provide a novel understanding of the regulation of osteoclastogenesis.