Painful neuromas: a potential role for a structural transmembrane protein, ankyrin G

Severe nerve injury induces the formation of a neuroma. Some neuromas cause excruciating pain. Overexpression of Na+ channels leads to hyperexcitability and painful phenomena. Ankyrin G, a multifunctional transmembrane protein of the axolemma, might be a key protein in neuroma formation because it binds Na+ channels in the initial segments of a regenerating axon and links with neuronal cell adhesion molecules. The authors wanted to determine if ankyrin G could be detected in neuroma, and if present, whether there would be differences in distribution between nonpainful neuromas, painful neuromas, and normal nerve.

First, frozen sections of nine nerve specimens obtained from six patients (six nonpainful neuromas, one painful neuroma, and two normal nerves) were immunocytochemically screened for ankyrin G by using confocal laser scanning microscopy. Second, specimens from 29 patients (seven painful neuromas, 15 nonpainful neuromas, and seven normal nerves) were examined using immunoblot analysis for their ankyrin G content. Western blot analysis detected ankyrin G, which was visualized by applying the enhanced chemiluminescence technique. Computerized densitometry was used to quantitate ankyrin G expression by comparing band intensities. Normal nerve served as control. Neurofilament was used as a marker for nerve tissue content. Ankyrin G could be detected and was found to be increased in neuromas. The mean band intensity values were 1838 for painful neuromas, 1166 for nonpainful neuromas, and 411 for normal nerves. In two cases the authors were able to compare specimens of painful neuroma and normal nerve from the same patient. The painful neuromas exhibited considerably higher levels of ankyrin G. Painful neuroma and normal nerve densitometry values were 499 and 165, respectively, for one patient, and 4254 and 821, respectively, for the other patient. Painful neuromas were also found to have higher neurofilament values than nonpainful neuromas.

Altered regulation of ankyrin G after nerve injury may lead to hyperexcitability and painful phenomena via clustering of Na+ channels. A propensity to overexpress ankyrin G after peripheral nerve trauma may turn out to be a factor in the development of painful neuromas and neuropathic pain. The relevant literature regarding the importance of ankyrin G for nerve regeneration and nerve membrane remodeling is reviewed.