Investigation of the electrical properties of microtubule ensembles under cell-like conditions
microtubules, bioelectricity, bionanowires, neuronal charge storage, impedance spectroscopy, cytoskeleton
Microtubules (MTs) are cylindrical polymers composed of the heterodimers of protein α, β- tubulin that play a variety of well-recognised intracellular roles, such as maintaining the shape and rigidity of the cell, aiding in positioning and stabilisation of the mitotic spindle for allowing chromosomal segregation, acting as ‘rails’ for macromolecular transport and forming cilia and flagella for cell movement. Since the tubulin dimer possesses a high negative electric charge of ~23e and a large intrinsic high dipole moment of approximately 1750 D [1,2], MTs have been implicated in electrically-mediated biological roles [3,4,5,6]. They have been modelled as nanowires capable of enhancing ionic transport [7,8], and simulated to receive and attenuate electrical oscillations [4,9,10,11]. In solution, MTs have been shown to align with applied electric fields [2,12,13,14,15,16]. Recently, MTs have also been modelled as the primary cellular targets for low-intensity (1–2 V), intermediate-frequency (100–300 kHz) electric fields termed TTFields (tumour-treating electric fields) that inhibit cancer cell proliferation, in particular glioma [17,18,19]. Indeed, MTs have been reported to decrease buffer solution resistance [12,13], leading to a conductance peak at frequencies close to the TTField regime . While these studies show that MTs are highly sensitive to external electric fields, answers to the questions ‘How do MTs effect a solution’s capacitance?’ and ‘What is the capacitance of a single MT?’ are still elusive and crucial to the determination of the dielectric properties of living cells. The tubulin concentration in mammalian cells varies in the micromolar range (~10–25 μM) [21,22]. In vitro, polymerizing tubulin at such high concentrations can lead to the formation of entangled networks, confounding quantification of the individual MT response to electric fields. Electro-rotation, di-electrophoresis and impedance spectroscopy are thus performed using low concentrations of tubulin, in the nanomolar regime, to enable robust observation of individual MTs.
Kalra, A. P., Patel, S. D., Bhuiyan, A. F., Preto, J., Scheuer, K. G., Rezania, V., . . . Tuszynski, J. A. (2020). Investigation of the electrical properties of microtubule ensembles under cell-like conditions, Nanomaterials, 10(2), 265. https://doi.org/10.3390/nano10020265
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