IκB Kinase/Nuclear Factor κB-Dependent Insulin-Like Growth Factor 2 (Igf2) Expression Regulates Synapse Formation and Spine Maturation via Igf2 Receptor Signaling

Alterations of learning and memory in mice with deregulated neuron-specific nuclear factor kappa B (NF-kappa B) activity support the idea that plastic changes of synaptic contacts may depend at least in part on I kappa B kinase (IKK)/NF-kappa B-related synapse-to-nucleus signaling. There is, however, little information on the molecular requirements and mechanisms regulating this IKK/NF-kappa B-dependent synapse development and remodeling. Here, we report that the NF-kappa B inducing IKK kinase complex is localized at the postsynaptic density (PSD) and activated under basal conditions in the adult mouse brain. Using different models of conditional genetic inactivation of IKK2 function in mouse principal neurons, we show that IKK/NF-kappa B signaling is critically involved in synapse formation and spine maturation in the adult brain. IKK/NF-kappa B blockade in the forebrain of mutant animals is associated with reduced levels of mature spines and postsynaptic proteins PSD95, SAP97, GluA1, AMPAR-mediated basal synaptic transmission and a spatial learning impairment. Synaptic deficits can be restored in adult animals within 1 week by IKK/NF-kappa B reactivation, indicating a highly dynamic IKK/NF-kappa B-dependent regulation process. We further identified the insulin-like growth factor 2 gene (Igf2) as a novel IKK/NF-kappa B target. Exogenous Igf2 was able to restore synapse density and promoted spine maturation in IKK/NF-kappa B signaling-deficient neurons within 24 h. This process depends on Igf2/Igf2R-mediated MEK/ERK activation. Our findings illustrate a fundamental role of IKK/NF-kappa B-Igf2-Igf2R signaling in synapse formation and maturation in adult mice, thus providing an intriguing link between the molecular actions of IKK/NF-kappa B in neurons and the memory enhancement factor Igf2.