STING-dependent interferon signatures restrict osteoclast differentiation and bone loss in mice
Stimulator of Interferon Genes (STING) is a key mediator of type-I interferon (IFN-I) signaling in response to a variety of stimuli, but the contribution of STING to homeostatic processes is not fully characterized. Previous studies showed ligand activation of STING limits osteoclast differentiation in vitro through the induction of IFN? and IFN-I Interferon Stimulated Genes (ISGs).  In a disease model (SAVI) driven by the V154M gain-of-function mutation in STING, fewer osteoclasts form from SAVI precursors in response to RANKL in an IFN-I-dependent manner. Due to the described role of STING-mediated regulation of osteoclastogenesis in activation settings, we sought to determine whether basal STING signaling contributes to bone homeostasis, an unexplored area. Using whole-body and myeloid-specific deficiency, we show that STING signaling prevents trabecular bone loss in mice over time and that myeloid-restricted STING activity is sufficient for this effect. STING-deficient osteoclast precursors differentiate with greater efficiency than wildtypes. RNA sequencing of wildtype and STING-deficient osteoclast precursor cells and differentiating osteoclasts reveals novel clusters of ISGs including a previously undescribed ISG set expressed in RANKL naïve precursors (tonic expression) and downregulated during differentiation. We identify a 50 gene tonic ISG signature that is STING-dependent and shapes osteoclast differentiation. From this list, we identify Interferon Stimulated Gene 15 (ISG15) as a tonic STING-regulated ISG that limits osteoclast formation.  Thus, STING is an important upstream regulator of tonic IFN-I signatures shaping the commitment to osteoclast fates, providing evidence for a nuanced and novel role for this pathway in bone homeostasis. Overall design: Bulk RNA sequencing of murine bone marrow derived osteoclast precursors at baseline (d0) and following RANKL stimulation for 2, 3 and 4 days differentiated in vitro.  Samples are from either WT or STING-deficient bone marrow.  Samples were isolated from three mice per genotype (triplicates).