Ankyrin G and voltage gated sodium channels colocalize in human neuroma--key proteins of membrane remodeling after axonal injury

We tested if ankyrin G could be detected in human neuroma, if it colocalized with site-specific peripheral nerve sodium channels that accumulate at axon tips of injured nerve, and if there are differences in the distribution of these proteins in non-painful neuroma and painful neuroma tissue vs. normal nerve. Frozen sections from one painful, six non-painful, and three normal nerves were immunocytochemically examined. A double labeling technique with highly specific antibodies against peripheral nerve type 1 (Na(v)1.7), and peripheral nerve type 3 (Na(v)1.8) sodium channels and anti-ankyrin G antibodies detected sodium channels and ankyrin G on the same section, using confocal laser scanning microscopy. Ankyrin G colocalized with both types of sodium channels. Neuroma specimens exhibited considerably larger immunofluorescence for both sodium channels and ankyrin G compared with normal nerve. The painful neuroma presented an even more pronounced immunolabeling in clusters. Findings support results from animal models that link ankyrin G with clustering of sodium channels at axon tips of unmyelinated, sprouting fibers. A common (repair-) mechanism that exists throughout the human nervous system for clustering sodium channels at a high density is assumed. A dysregulation in this membrane remodeling mechanism might be an initial step in a cascade that leads to a painful rather than a non-painful neuroma.