Suppression of sost/sclerostin and dikkopf-1 augment intervertebral disc structure in mice
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Intervertebral disc (IVD) degeneration is a leading cause of low back pain, characterized by accelerated extracellular matrix breakdown and IVD height loss but there is no approved pharmacological therapeutic. Deletion of Wnt ligand competitor Lrp5 induces IVD degeneration, suggesting that Wnt signaling is essential for IVD homeostasis. Therefore, the IVD may respond to neutralization of Wnt ligand competitors sost(gene)/sclerostin(protein) and/or dikkopf-1 (dkk1). Anti-sclerostin antibody (scl-Ab) is an FDA-approved bone therapeutic that activates Wnt signaling. We (1) determined if pharmacological neutralization of sclerostin, dkk1 or their combination would stimulate Wnt signaling and augment IVD structure and (2) determined the prolonged adaptation of the IVD to global, persistent deletion of sost. Nine-week-old C57Bl/6J female mice (n = 6-7/grp) were subcutaneously injected 2x/wk for 5.5 wk with scl-Ab (25 mg/kg), dkk1-Ab (25 mg/kg), 3:1 scl-Ab/dkk1-Ab (18.75:6.25 mg/kg) or vehicle (veh). Separately, IVD of sost KO and wildtype (WT) mice (n = 8/grp) were harvested at 16 weeks of age. First, compared to vehicle, injection of scl-Ab, dkk1-Ab and 3:1 scl-Ab/dkk1-Ab similarly increased lumbar IVD height and β-catenin gene expression. Despite these similarities, only injection of scl-Ab alone strengthened IVD mechanical properties and decreased heat shock protein gene expressions. Genetically and compared to WT, sost KO enlarged IVD height, increased proteoglycan staining and imbibed IVD hydration. Notably, persistent deletion of sost was compensated by upregulation of dkk1, which consequently reduced the cell nuclear fraction for Wnt signaling co-transcription factor β-catenin in the IVD. Lastly, RNA-sequencing pathway analysis confirmed the compensatory suppression of Wnt signaling and revealed a reduction of cellular stress-related pathways. Together, suppression of sost/sclerostin or dkk1 each augmented IVD structure by stimulating Wnt signaling, but scl-Ab outperformed dkk1-Ab in strengthening the IVD. Ultimately, postmenopausal women prescribed scl-Ab injections to prevent vertebral fracture may also benefit from a restoration of IVD height and health.