Revealing and Attenuating the Electrostatic Properties of Tubulin and Its Polymers.

Tubulin is an electrostatically negative protein that forms cylindrical polymers termed microtubules, which are crucial for a variety of intracellular roles. Exploiting the electrostatic behavior of tubulin and microtubules within functional microfluidic and optoelectronic devices is limited due to the lack of understanding of tubulin behavior as a function of solvent composition. This work displays the tunability of tubulin surface charge using dimethyl sulfoxide (DMSO) for the first time. Increasing the DMSO volume fractions leads to the lowering of tubulin's negative surface charge, eventually causing it to become positive in solutions >80% DMSO. As determined by electrophoretic mobility measurements, this change in surface charge is directionally reversible, i.e., permitting control between -1.5 and + 0.2 cm2  (V s)-1 . When usually negative microtubules are exposed to these conditions, the positively charged tubulin forms tubulin sheets and aggregates, as revealed by an electrophoretic transport assay. Fluorescence-based experiments also indicate that tubulin sheets and aggregates colocalize with negatively charged g-C3 N4 sheets while microtubules do not, further verifying the presence of a positive surface charge. This study illustrates that tubulin and its polymers, in addition to being mechanically robust, are also electrically tunable.

Modeling effects of scar on patterns of dorsal column stimulation.

OBJECTIVE: The purpose of this study was to examine how scar formation may affect electrical current distribution in the spinal cord when using paddle leads placed in the epidural space during treatment with spinal cord stimulation. MATERIALS AND METHODS: A finite element model of the spinal cord was used to examine changes in stimulation using a guarded cathode configuration with and without scar. Additionally, two potential "compensatory" programming patterns were examined in order to understand how the three-dimensional electrical field may be affected by scar. Direct comparisons with prior studies in the literature and use of known anatomy of dorsal column fiber distributions also enabled a computational estimate of the number of fibers likely reaching threshold with each stimulus pattern. RESULTS: Notable potential and current distribution changes were found related to the modeled scar. Compensatory stimulation patterns (both in spatial and in amplitude dimensions) affect the fiber activation patterns in complex ways that may not be easily predetermined by a programming specialist. CONCLUSIONS: This study is one of the first to examine the effects of scar tissue on dorsal column stimulation and the only one using a detailed computational approach toward that end. It appears that different thickness and location of scar between electrode contacts and the dura may likely lead to a significant number and location of complex changes in the activated fibers. It is likely that a more complete assessment of scarring and its effect on the electrical environment of any given paddle lead would allow more accurate and predictable reprogramming of patients with commercially available systems in place.