Constitutive Activation of IKK2/NF-κB Impairs Osteogenesis and Skeletal Development

Pathologic conditions impair bone homeostasis. The transcription factor NF-kappa B regulates bone homeostasis and is central to bone pathologies. Whereas contribution of NF-kappa B to heightened osteoclast activity is well-documented, the mechanisms underlying NF-kappa B impact on chondrocytes and osteoblasts are scarce. In this study, we examined the effect of constitutively active IKK2 (IKK2ca) on chondrogenic and osteogenic differentiation. We show that retroviral IKK2ca but not GFP, IKK2WT, or the inactive IKK2 forms IKK2KM and IKK2SSAA, strongly suppressed osteogenesis and chondrogenesis, in vitro. In order to explore the effect of constitutive NF-kappa B activation on bone formation in vivo, we activated this pathway in a conditional fashion. Specifically, we crossed the R26StopIKK2ca mice with mice carrying the Col2-cre in order to express IKK2ca in osteoblasts and chondrocytes. Both chondrocytes and osteoblasts derived from Col2Cre/IKK2ca expressed IKK2ca. Mice were born alive yet died shortly thereafter. Histologically, newborn Col2Cre+/RosaIKK2ca heterozygotes (Cre+IKK2ca_w/f (het)) and homozygotes (Cre+IKK2ca_f/f (KI)) showed smaller skeleton, deformed vertebrate and reduced or missing digit ossification. The width of neural arches, as well as ossification in vertebral bodies of Cre+IKK2ca_w/f and Cre+IKK2ca_f/f, was reduced or diminished. H&E staining of proximal tibia from new born pups revealed that Cre+IKK2ca_f/f displayed disorganized hypertrophic zones within the smaller epiphysis. Micro-CT analysis indicated that 4-wk old Cre+IKK2ca_w/f has abnormal trabecular bone in proximal tibia compared to WT littermates. Mechanistically, ex-vivo experiments showed that expression of differentiation markers in calvarial osteoblasts derived from newborn IKK2ca knock-in mice was diminished compared to WT-derived cells. In situ hybridization studies demonstrated that the hypertrophic chondrocyte marker type-X collagen, the pre-hypertrophic chondrocyte markers Indian hedgehog and alkaline phosphatase, and the early markers Aggrecan and type-II collagen were reduced in Cre+IKK2ca_w/f and Cre+IKK2ca_f/f mice. Altogether, the in-vitro, in vivo and ex-vivo evidence suggest that IKK2ca perturbs osteoblast and chondrocyte maturation and impairs skeletal development.