GSE133212
Conditional Activation of NF-kB Inducing Kinase (NIK) in the Osteolineage Enhances both Basal and Loading-Induced Bone Formation
Studies from global loss-of-function mutants suggest that alternative NF-kB downstream of NF-kB inducing kinase (NIK) is a cell-intrinsic negative regulator of osteogenesis.  However, the interpretation of the osteoblast and/or osteocyte contribution to the bone phenotype is complicated by simultaneous osteoclast defects in these models.  Therefore, we turned to a transgenic mouse model to investigate the direct role of NIK in the osteolineage.  Osx-Cre;NT3 animals (NT3-Cre+), which bear a constitutively active NIK allele (NT3) driven by Osx-Cre, were compared to their Cre-negative, Control (Ctrl)  littermates.  NT3-Cre+ mice had elevated serum P1NP and CTX  levels.  Despite this high turnover state, microCT showed that constitutive activation of NIK resulted in a net increase in basal bone mass in both cortical and cancellous compartments.  Furthermore, NT3-Cre+ mice exhibited a greater anabolic response following mechanical loading compared to controls.  We next performed RNA-Seq on non-loaded and loaded tibiae to elucidate possible mechanisms underlying the increased bone anabolism seen in NT3-Cre+ mice.  Hierarchical clustering revealed two main transcriptional programs: one loading-responsive and the other NT3 transgene-driven.  Gene ontology (GO) analysis indicated a distinct upregulation of receptor, kinase and growth factor activities including Wnts, as well as a calcium-response signature in NT3-Cre+ limbs.  The promoters of these GO-term associated genes, including many known to be bone-anabolic, were highly enriched for multiple kB recognition elements (kB-RE) relative to the background frequency in the genome.  The loading response in NT3-Cre+ mice substantially overlapped (>90%) with Ctrl.  Surprisingly, control animals had 10-fold more differentially expressed genes (DEGs) in response to loading.  However, most top DEGs shared between genotypes had a high incidence of multiple kB-RE in their promoters.  Therefore, both transcriptional programs (loading-responsive and NT3 transgene-driven) are modulated by NF-kB.  Our studies uncover a previously unrecognized role for NF-kB in the promotion of both basal and mechanically-stimulated bone formation. Overall design: Differential gene expression in Ctrl vs NT3-Cre+ non-loaded and loaded limbs.  16 total samples.  n=4 each genotype (Ctrl or NT3-Cre+) as biological replicates.  Paired non-loaded and loaded samples for each mouse.