IκB kinase-α acts in the epidermis to control skeletal and craniofacial morphogenesis

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Abstract
IκB kinase-α (IKK-α)1 exhibits protein-kinase-dependent and -independent functions. Its kinase activity is required for lymphoid organogenesis2 and mammary gland development3, whereas a kinase-independent activity is required for epidermal keratinocyte differentiation4. In addition to failed epidermal differentiation, IKK-α-deficient mice exhibit abnormal skeletal and craniofacial morphogenesis4,5,6. As similar defects are not exhibited by mice that experience systemic inhibition of NF-κB7, we postulated that the morphogenetic defects in IKK-α-deficient mice are not caused by reduced NF-κB activity but instead are due to failed epidermal differentiation that disrupts proper epidermal–mesodermal interactions. We tested this hypothesis by introducing an epidermal-specific Ikka (also known as Chuk) transgene into IKK-α-deficient mice. Mice lacking IKK-α in all cell types including bone and cartilage, but not in basal epidermal keratinocytes, exhibit normal epidermal differentiation and skeletal morphology. Thus, epidermal differentiation is required for proper morphogenesis of mesodermally derived skeletal elements. One way by which IKK-α controls skeletal and craniofacial morphogenesis is by repressing expression of fibroblast growth factor (FGF) family members, such as FGF8, whose expression is specifically elevated in the limb bud ectoderm of IKK-α-deficient mice.