Optical Mirroring of Electrical Signals from Non-Electrogenic Cells

Cell membrane potential is known to be an essential parameter to monitor the electrical activities of excitable cells, such as neurons and cardiac cells. However, recent data has revealed that it also plays a crucial role in non-excitable cells, such as epithelial cells. Electrical perturbations induce localized bioelectrical changes, which can trigger different cell responses, including migration, mitosis, and mutation.¹<br/>The directional migration of cells under an electric field is known as galvanotaxis, and represents a dominant mechanism in guiding the behavior of multiple cell populations in mammals, fishes, amphibians, and plants.^2,3 Wound healing is controlled by endogenous electric fields, generated by ion transport across the epithelium, which set a trans-epithelial potential difference (TCPD) of +40 mV in healthy conditions. At the wound, the TCPD falls to zero, but it is maintained at normal levels in its proximity (~ 0.5 mm) to drive epithelial cells to migrate and heal the wound.⁴ Bioelectrical cues are also involved in cellular mutation, responsible for the onset of multiple diseases. For example, cell electrical charges are the biophysical manifestation of metabolic patterns in cancer development and metastasis.⁵ Cancer cells exhibit the Warburg Effect, resulting in the secretion of lactate anions that produce a high concentration of negative surface charges.⁵ The membrane potential is also highly correlated with mitosis, DNA synthesis, cell cycle progression and overall cellular proliferation.⁶<br/>Understanding these mechanisms is the key to unravel precision diagnostic and therapeutic strategies. However, this dynamic information is currently hard to access because existing technologies for electrophysiological recording of resting cell membrane potentials are invasive: they require physical access to the intracellular compartments, which implies the disruption of the cell membrane (known as electroporation).⁷ This makes it challenging to perform passive measurements and prolonged recordings under varying conditions.<br/>A variety of alternative or complementary technologies based on optical methods were introduced in the past decades. However, the majority of them are designed to be implemented in electrogenic cells, which transport signals through action potentials (i.e., rapid polarization and depolarization of the cell membrane). Nevertheless, most of the cells in the human body do not fire action potentials, and their mechanisms of communication happen in a narrower signal range.<br/>In this work, we present a novel approach in electrophysiological recording, based on optical mirroring of bioelectric charges. The feasibility of this method was previously demonstrated for the high throughput recording of action potentials in human-derived cardiomyocytes.⁷ In this context, the technology was refined and implemented in epithelial cells.<br/>The nano-platform consisted of two chambers separated by a thin membrane. Pass-through electrodes enabled to mirror the cell membrane charge (top) all the way to the optical chamber (bottom), where dispersed fluorophores migrated to balance the charge, generating a dynamic fluorescent signal. Fluorescence was recorded with a camera, while exciting the fluorophores with incident light. Preliminary results on HEK cells yielded a fluorescence intensity increase of the 2.5% in presence of a cell on the microelectrode compared to reference microelectrodes. At present, this technology may be used to evaluate cell-substrate adhesion, monitor cell proliferation, and test novel biomaterials. In the near future, further investigations could allow extrapolating quantitative data on resting membrane potential in dynamic settings to perform cell/tissue studies on wound healing, cellular division, and cancer progression.<br/><br/>References:<br/>¹ Huang, Y. J. et al. Sci Rep 2016, 6, 21583.<br/>² Zhao, M. et al. Nature 2006, 442 (7101), 457-60.<br/>³ McCaig, C. D. et al. Physiol Rev 2005, 85 (3), 943-78.<br/>⁴ Song, B. et al. Proc Natl Acad Sci U S A 2002, 99 (21), 13577-82.<br/>⁵ Vieira, A. C. et al. PLoS One 2011, 6 (2), e17411.<br/>⁶ Kadir, L. et al. Front Physiol 2018, 9, 1661.<br/>⁷ Barbaglia, A. et al. Adv Mater 2021, 33 (7), e2004234