Effects of sclerostin antibody on healing of a non‐critical size femoral bone defect

Abstract

Sclerostin is a glycoprotein secreted by osteocytes and inhibits osteoblastogenesis via inhibition of Wnt signaling. We hypothesized that sclerostin antibody (Scl-AbIII) would accelerate the healing of a murine femoral non-critical size bone defect model. A unilateral and unicortical 0.8 mm-sized drill hole was made in the proximal femoral shaft of adult female nude mice. One group of mice received subcutaneous injections of Scl-AbIII and a second group received vehicle only. Reporter MC3T3 osteoprogenitor cells were injected via the tail vein 3 days after surgery to monitor systemic trafficking of exogenous osteoprogenitors. Bioluminescence imaging (BLI), microcomputed tomography (microCT), micropositron emission tomography (microPET) and histological analysis were used to compare the bone healing responses to Scl-AbIII treatment. Bone mineral density (BMD) significantly increased at the defect site after week 1, and was significantly higher in the treatment compared with the control group at all time points. This finding was also confirmed on histological analysis by increased deposition of new woven bone. MicroPET scanning showed a trend for greater activity in the control group at day 21 compared with the Scl-AbIII group, indicating early bone maturation following treatment with Scl-AbIII. Whereas the BLI signals derived from the injected osteoprogenitor cells showed no differences between vehicle and Scl-AbIII treated groups, systemic migration of MC3T3 cells to the bone defect was clearly identified in both groups using immunohistochemistry. Keywords: bone defect healing; sclerostin antibody; Wnt signaling; animal model Facilitation of bone formation has been a long-standing goal of bone therapies. Over recent years it has been shown that the Wnt signaling pathway plays a central role in regulating the normal physiologic processes of bone turnover in response to mechanical loading. 1,2 Many soluble factors participate in regulating the activity of Wnt proteins, including sclerostin which acts as an inhibitor of Wnt signaling by binding to the Wnt co-receptor Lrp5/6. 3 Sclerostin is a glycoprotein expressed and secreted mainly by osteocytes, 4-6 and functions as a potent inhibitor of osteoblast activity. 2,3 Sclerostin knockout mice have been shown to have a high bone mass phenotype characterized by marked increases in BMD, bone volume, bone formation, and bone strength. 7 Antibodies directed against sclerostin have been shown to increase BMD, bone volume, and bone strength in ovariectomized rats 8 and primates, 7 to enhance bone healing, 9 and to reverse bone loss in a model of colitis. 10 Serum sclerostin levels were found to be significantly elevated in postmenopausal women compared with premenopausal women, with significant correlations between free estrogen levels and sclerostin, as well as between parathyroid hormone (PTH) and sclerostin. 11 Furthermore, men were noted to have higher serum sclerostin levels than women and in both sexes, serum sclerostin levels increased with age. 12 A blinde...

Endochondral fracture healing with external fixation in the Sost knockout mouse results in earlier fibrocartilage callus removal and increased bone volume fraction and strength

“…Sclerostin negatively regulates bone formation, and the development of Sclerostin inhibitory strategies provides a potential opportunity to enhance bone repair. Deficiency or antagonism of Sclerostin improves intramembranous ossification [22,[24][25][26][27][28][29]38], though the effect on endochondral ossification during fracture repair is not as well defined. We hypothesized that endochondral repair would be improved within the Sost −/− mouse in comparison to C57Bl/ 6J wild type controls, with earlier bony union, and resultant denser and stronger mineralized calluses.…”

“…In particular, the development and subsequent animal and human trials of anti-Sclerostin antibodies have provided evidence of stimulated bone formation and enhanced bone mass [17][18][19][20][21][22][23]. The anabolic effects of Sclerostin deficiency, via genetic knockout mice or anti-Sclerostin antibody treatment, have also been demonstrated in intramembranous bone healing studies [24][25][26][27][28][29]. These anabolic effects include increases in bone mineral density, bone volume and mechanical strength.…”

A Review of Osteocyte Function and the Emerging Importance of Sclerostin

“…At six weeks, the maximum failure load was increased by 68% 70 . Sclerostin antibody administered to rats with a critical-sized femoral defect and mice with a non-criticalsized femoral defect showed earlier healing, complete union, and physiologic maturation of the defect 71,72 . Similar conclusions can be drawn from the rat model of experimental periodontitis, in which sclerostin antibody treatment increased alveolar bone regeneration and filled substantial oral bone defects 73 .…”

Determining the Role of Sost and Sostdc1 During Fracture Healing

“…At later time points, this leads to higher mineral content in the abundantly formed new bone, thereby potentially increasing mechanical stability. Furthermore, early reports that successfully used Sost antibody treatment to promote fracture healing relied heavily on models that primarily healed by intramembranous bone formation [94,140,152,153]. This suggested that SostAb may be beneficial in problematic fracture healing that results from poor osteogenesis; however, Li et al (2011) [154] reported fracture repair in an endochondral model in Sost -/-mice, and Feng et al (2015) [155] reported similar results with Sost antibody treatment, suggesting that fracture repair may benefit from early intervention by reducing sclerostin levels, rather than only during osteogenesis.…”