Suppression of Sclerostin Alleviates Radiation-Induced Bone Loss by Protecting Bone-Forming Cells and Their Progenitors Through Distinct Mechanisms

Abstract

Focal radiotherapy is frequently associated with skeletal damage within the radiation field. Our previous in vitro study showed that activation of Wnt/β-catenin pathway can overcome radiation-induced DNA damage and apoptosis of osteoblastic cells. Neutralization of circulating sclerostin with a monoclonal antibody (Scl-Ab) is an innovative approach for treating osteoporosis by enhancing Wnt/β-catenin signaling in bone. Together with the fact that focal radiation increases sclerostin amount in bone, we sought to determine whether weekly treatment with Scl-Ab would prevent focal radiotherapy-induced osteoporosis in mice. Micro-CT and histomorphometric analyses demonstrated that Scl-Ab blocked trabecular bone structural deterioration after radiation by partially preserving osteoblast number and activity. Consistently, trabecular bone in sclerostin null mice was resistant to radiation via the same mechanism. Scl-Ab accelerated DNA repair in osteoblasts after radiation by reducing the number of γ-H2AX foci, a DNA double-strand break marker, and increasing the amount of Ku70, a DNA repair protein, thus protecting osteoblasts from radiation-induced apoptosis. In osteocytes, apart from using similar DNA repair mechanism to rescue osteocyte apoptosis, Scl-Ab restored the osteocyte canaliculi structure that was otherwise damaged by radiation. Using a lineage tracing approach that labels all mesenchymal lineage cells in the endosteal bone marrow, we demonstrated that radiation damage to mesenchymal progenitors mainly involves shifting their fate to adipocytes and arresting their proliferation ability but not inducing apoptosis, which are different mechanisms from radiation damage to mature bone forming cells. Scl-Ab treatment partially blocked the lineage shift but had no effect on the loss of proliferation potential. Taken together, our studies provide proof-of-principle evidence for a novel use of Scl-Ab as a therapeutic treatment for radiation-induced osteoporosis and establish molecular and cellular mechanisms that support such treatment.

Periosteal Mesenchymal Progenitor Dysfunction and Extraskeletally-Derived Fibrosis Contribute to Atrophic Fracture Nonunion

“…The focal irradiator used in this study (SARRP) represents a great advantage over the whole‐bone irradiator by limiting the radiation field with mm‐scale accuracy. Our past experiments have demonstrated that SARRP radiation on one bone has no adverse effects on the contralateral bones . In the current study, we performed bilateral tibial fractures, and the contralateral nonirradiated tibias healed at a similar pace as tibias from mice that have never received SARRP radiation (data not shown).…”

“…This clinical relationship between radiation therapy, fracture, and healing provided the impetus for utilizing radiation as a model to induce and study atrophy during bone regeneration. Previously, we used a focal irradiator (Small Animal Radiation Research Platform [SARRP]), which can mimic clinical focal radiation therapy in rodents with submillimeter accuracy, to delineate the mechanism of radiation damage on metaphyseal trabecular bone and to investigate potential treatments for this devastating condition . Here, we extended our research to traumatic bone fracture and established a highly reproducible and clinically relevant mouse nonunion model.…”

Regulator of G Protein Signaling Protein 12 (Rgs12) Controls Mouse Osteoblast Differentiation via Calcium Channel/Oscillation and Gαi-ERK Signaling

“…A quantitative analysis of the femur morphology and microarchitecture was performed using a micro–computed tomography (μCT) system (Micro‐CT 35; Scanco Medical AG, Brüttisellen, Switzerland) as described by de Bakker and colleagues and Chandra and colleagues . Femurs from 12‐week‐old Osx Cre/+ ; Rgs12 +/+ , Rgs12 fl/fl and Osx Cre/+ ; Rgs12 fl/fl mice of either gender ( n = 3 per gender, n = 6 in total of each group) were fixed with 4% (wt/vol) paraformaldehyde (PFA) for 24 hours, rinsed with PBS, and scanned.…”

Sclerostin Antibody Treatment Stimulates Bone Formation to Normalize Bone Mass in Male Down Syndrome Mice

“…Using baseline body mass to minimize intragroup differences, 8‐week‐old male Ts(17 16 )65Dn (Ts65Dn) mice and wild‐type (WT) mice (Jackson Laboratory, Bar Harbor, ME, USA) were randomly assigned to treatment (SclAb) or control vehicle‐treated (Vehicle) ( n = 4–6/group each) for a total of four groups with all animals assigned and housed individually by genotype: group 1, WT (placebo); group 2, WT‐SclAb; group 3, Ts65 (placebo); group 4, Ts65‐SclAb. The groups received either vehicle (isotonic vehicle buffer) or SclAb (100 mg/kg/wk, both kindly provided by Dr Michaela Kniessel, Novartis, Basel, Switzerland) as weekly iv injections in the morning on days 0, 7, 14, and 21 as previously described before euthanization on day 28. Measurements of body weight occurred at the same time per week to allow calculation of SclAb dose.…”