Coincident enrichment of phosphorylated IκBα, activated IKK, and phosphorylated p65 in the axon initial segment of neurons

Phosphorylation of the inhibitory protein IκBα by the activated IκB kinase (IKK) is a crucial step in the activation of the transcription factor NF-κB. In neurons of the mammalian central nervous system, constitutive activation of NF-κB has been previously documented. The cellular compartments involved in this activation have not yet been fully identified. Here we document a striking enrichment of several molecules involved in NF-κB activation in the axon initial segment (AIS) of neurons: Phosphorylated-IκBα (pIκBα), activated IKK, and p65 phosphorylated at serine 536 were found to be enriched in the AIS in vivo as well as in vitro. Both, pIκBα and activated IKK, were associated with cytoskeletal components of the AIS. Activated IKK was associated with the membrane cytoskeleton, whereas pIκBα was sequestered to microtubules of the AIS. Colchicine-induced depolymerization of microtubules resulted in the loss of pIκBα in the AIS, demonstrating that the integrity of the axonal cytoskeleton is essential for the clustering of this NF-κB pathway component. These data provide the first evidence for a compartmentalized clustering of NF-κB pathway components in the AIS and implicate this neuronal compartment in the activation of NF-κB.