Revealing and attenuating the electrostatic properties of tubulin and microtubules

Kalra, Aarat P.; Patel, Sahil D.; Winter, Philip; Wang, Hui; Carlson, Kris W.; Rezania, Vahid; Lewis, John; Meldrum, Al; Shankar, Karthik; Tuszynski, Jack A.

Revealing and attenuating the electrostatic properties of tubulin and microtubules

Author

Date

2020

Keywords

TTField (tumour-treating field) therapy, novel cancer-treatment, glioblastoma multiforme, tubulin, mitosis and macromolecular transport, microtubules, microelectrode fabrication, pH-sensitive fluorophore, Dynamic Light Scattering, ion-tubulin interactions

Abstract (summary)

Cancer treatment modalities such as chemotherapy and radiation therapy involve several side effects including an increased risk of getting cancer a second time. TTField (Tumour-treating field) therapy is a novel cancer-treatment modality that has attained U.S FDA approval for treatment of Glioblastoma Multiforme. TTFields are low intensity (1-2 V/cm), intermediate frequency (100-300 kHz) electric fields that have been shown to drastically lower tumor growth. While the only side-effects of TTField exposure are mild skin rashes, the exact mechanism of how TTFields act is not well understood. Models of TTFields posit that they target α, β- tubulin, a highly negatively charged (−50e), high dipole moment (∼1750 D) protein. Tubulin forms hollow cylindrical polymers termed microtubules, which form bundles that are crucial for mitosis and macromolecular transport. When exposed to TTFields, tubulin is expected to re-align and spatially relocate in response, inhibiting microtubule growth and interfering with mitosis. Using photolithography for microelectrode fabrication, we exposed unpolymerized tubulin containing solutions to AC electric fields and measured solution conductance. Interestingly, while we found that the presence of microtubules increased solution ionic conductance with a peak at TTField-like frequencies, the presence of unpolymerized tubulin reduced ionic conductance. Next, we used a parallel-plate electrode setup to compare the capacitance of solutions containing unpolymerized tubulin to those containing microtubules at identical physiological tubulin concentrations and ionic strengths. We found that while the presence of microtubules increased solution capacitance appreciably, the presence of unpolymerized tubulin did not. We are presently quantifying the chemical nature of the counterionic cloud around tubulin using a pH-sensitive fluorophore and Dynamic Light Scattering. Our results, in addition to displaying the significance of the tubulin polymerization state on the solution’ dielectric properties, also indicate that TTFields may target ion-tubulin interactions to inhibit tumour growth.

Publication Information

Kalra, A.P., Patel, S.D., Winter, P., Kumar, P., Wang, H., Rezania, V., . . . Tuszynski, J.A. (2020). Revealing and attenuating the electrostatic properties of tubulin and microtubules, Biophysical Journal, 118(3), 622a. https://doi.org/10.1016/j.bpj.2019.11.3355