Effect of Isaxonine on Microtubule Dynamics

Nova Southeastern University, Davie, Florida, USA

Objective. This study was undertaken to determine if the neurotropic action of isaxonine is associated with an effect on tubulin-microtubule dynamics. Background. Isaxonine (Nerfactor) has been shown to accelerate peripheral nerve regeneration and successfully treat facial palsy and polyneuritis. Although the neurogenic activity of isaxonine has been suggested to be associated with stimulation of cellular microtubule assembly, no direct evidence for this mechanism has been presented. Methods. The effect of isaxonine on microtubule dynamics in the presence and absence of guanosine triphosphate (GTP) was examined. Results. In the absence of GTP, isaxonine (12.6 mM) was found to stimulate microtubule assembly equal to that of 1 mM GTP. The lowest concentration of isaxonine with measurable stimulatory activity on microtubule assembly was 2.1 mM. The effect of isaxonine (12.6 mM) on microtubule assembly was found to additive to that of GTP at optimal concentration (1.0 mM). In addition, isaxonine was shown to increase the rate of microtubule formation and stimulate microtubule formation at 4oC. Isaxonine was also shown to allosterically enhance [3H]-GTP binding to the exchangeable nucleotide binding site (E-site) of tubulin. Removal of the E-site nucleotide with alkaline phosphatase enhanced the microtubule stimulatory properties of isaxonine. Conclusion. Isaxonine is capable of stimulating microtubule assembly in the absence of GTP and stabilizing formed microtubules against cold-induced disassembly. Together with previous findings, these results further implicate that isaxonine’s action on the nerve fiber is at the microtubule level. Grants. This work was supported by NIH Grant RR03020 (RCMI) and NIH/MBRS/DRR Grant RR08111.