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

Phosphorylation of the inhibitory protein I kappa B alpha by the activated I kappa B kinase (IKK) is a crucial step in the activation of the transcription factor NF-kappa B. In neurons of the mammalian central nervous system, constitutive activation of NF-kappa B 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-kappa B activation in the axon initial segment (AIS) of neurons: Phosphorylated-I kappa B alpha (pI kappa B alpha), activated IKK, and p65 phosphorylated at serine 536 were found to be enriched in the AIS in vivo as well as in vitro. Both, pI kappa B alpha, and activated IKK, were associated with cytoskeletal components of the AIS. Activated IKK was associated with the membrane cytoskeleton, whereas pI kappa B alpha was sequestered to microtubules of the AIS. Colchicine-induced depolymerization of microtubules resulted in the loss of pI kappa B alpha in the AIS, demonstrating that the integrity of the axonal cytoskeleton is essential for the clustering of this NF-kappa B pathway component. These data provide the first evidence for a compartmentalized clustering of NF-kappa B pathway components in the AIS and implicate this neuronal compartment in the activation of NF-kappa B. (c) 2006 Elsevier Inc. All rights reserved.