Osteocyte Apoptosis Is Induced by Weightlessness in Mice and Precedes Osteoclast Recruitment and Bone Loss

Abstract

Mechanical stimulation of cultured osteocytic cells attenuates their apoptosis. We report here that, conversely, reduced mechanical forces in the murine model of unloading by tail suspension increases the prevalence of osteocyte apoptosis, followed by bone resorption and loss of mineral and strength.Introduction: Mechanical loading is critical for the maintenance of bone mass; weightlessness, as with reduced physical activity in old age, bed rest, or space flight, invariably leads to bone loss. However, the cellular and molecular mechanisms responsible for these phenomena are poorly understood. Based on our earlier findings that physiologic levels of mechanical strain prevent apoptosis of osteocytic cells in vitro, we examined here whether, conversely, reduced mechanical forces increase the prevalence of osteocyte apoptosis in vivo and whether this event is linked to bone loss. Materials and Methods: Swiss Webster mice or OG2-11␤-hydroxysteroid dehydrogenase type 2 (OG2-11␤-HSD2) transgenic mice and wildtype littermates were tail-suspended or kept under ambulatory conditions. Static and dynamic histomorphometry and osteocyte and osteoblast apoptosis by in situ end-labeling (ISEL) were assessed in lumbar vertebra; spinal BMD was measured by DXA; and bone strength was measured by vertebral compression. Results: We show that within 3 days of tail suspension, mice exhibited an increased incidence of osteocyte apoptosis in both trabecular and cortical bone. This change was followed 2 weeks later by increased osteoclast number and cortical porosity, reduced trabecular and cortical width, and decreased spinal BMD and vertebral strength. Importantly, whereas in ambulatory animals, apoptotic osteocytes were randomly distributed, in unloaded mice, apoptotic osteocytes were preferentially sequestered in endosteal cortical bone-the site that was subsequently resorbed. The effect of unloading on osteocyte apoptosis and bone resorption was reproduced in transgenic mice in which osteocytes are refractory to glucocorticoid action, indicating that stressinduced hypercortisolemia cannot account for these effects. Conclusions: We conclude that diminished mechanical forces eliminate signals that maintain osteocyte viability, thereby leading to apoptosis. Dying osteocytes in turn become the beacons for osteoclast recruitment to the vicinity and the resulting increase in bone resorption and bone loss.

Suppression of Autophagy in Osteocytes Mimics Skeletal Aging

“…However, we did not detect any changes in osteocyte number or apoptosis in the conditional knock mice as measured by TUNEL staining, osteocyte density, or osteocyte-specific gene expression. Moreover, since osteocyte apoptosis has been associated with (40), or shown to cause (41-43), increased oste- oclast formation and bone resorption, an increase in osteocyte apoptosis would not explain the reduced osteoclast formation in mice lacking autophagy in osteocytes.…”

Decreased bone turnover with balanced resorption and formation prevent cortical bone loss during disuse (hibernation) in grizzly bears (Ursus arctos horribilis)

“…A possible mechanism for how PTH could explain the cortical bone remodeling observed in the current study is shown in Figure 6. Intact PTH (PTH 1-84), acting through the PTH1 receptor on osteocytes and osteoblasts, could prevent the increased apoptosis and decreased bone formation rates normally associated with disuse [3,52,53]. Prevention of osteocyte apoptosis would prevent increases in targeted remodeling.…”

Protection From Glucocorticoid-Induced Osteoporosis by Anti-Catabolic Signaling in the Absence of Sost/Sclerostin

“…Histomorphometric analysis was performed using OsteoMeasure high-resolution digital video system (OsteoMetrics, Decatur, GA, USA) interfaced to an Olympus BX51 fluorescence microscope (Olympus America Inc., Center Valley, PA, USA). (39) Osteoclasts were quantified on lumbar vertebra L 2 thin sections stained for tartrate-resistant acid phosphatase (TRAP) and counterstained with Toluidine Blue as published. (34,37) An osteoclast was defined as a TRAPpositive cell attached to the bone surface with more than one nucleus.…”