Epithelia of the cornea, lens and retina contain a vast array of ion channels and pumps. Together they produce a polarized flow of ions in and out of cells, as well as across the epithelia. These naturally occurring ion fluxes are essential to the hydration and metabolism of the ocular tissues, especially for the avascular cornea and lens. The directional transport of ions generates electric fields and currents in those tissues. Applied electric fields affect migration, division and proliferation of ocular cells which are important in homeostasis and healing of the ocular tissues. Abnormalities in any of those aspects may underlie many ocular diseases, for example chronic corneal ulcers, posterior capsule opacity after cataract surgery, and retinopathies. Electric field-inducing cellular responses, termed electrical signaling here, therefore may be an unexpected yet powerful mechanism in regulating ocular cell behavior. Both endogenous electric fields and applied electric fields could be exploited to regulate ocular cells. We aim to briefly describe the physiology of the naturally occurring electrical activities in the corneal, lens, and retinal epithelia, to provide experimental evidence of the effects of electric fields on ocular cell behaviors, and to suggest possible clinical implications.

Migration of epidermal stem cells (EpSCs) into wounds may play an important role in wound healing. Endogenous electric fields (EFs) arise naturally at wounds. Consistent with previous reports, we measured outward electric currents at rat skin wounds using vibrating probes. Topical use of prostaglandin E2 significantly promoted wound healing. However, it is not known whether EpSCs respond to EFs. We first isolated and characterized EpSCs from rat skin. We then demonstrated that EpSCs isolated from the epidermis migrated directionally toward the cathode in EFs of 50-400 mV/mm. The directedness values increased in a dose- and time-dependent fashion. The migration speed of EpSCs was significantly increased in EFs. EFs induced asymmetric polymerization of intracellular F-actin and activation of the extracellular signal-regulated kinase 1/2 and phosphatidylinositol-3-kinase (PI3K)/protein kinase B pathways. Inhibition of epidermal growth factor receptor, extracellular signal-regulated kinase 1/2, or PI3K significantly inhibited the cathodal distribution of F-actin and the electrotactic response of EpSCs. These data for the first time show that EpSCs possess obvious electrotaxis, in which the epidermal growth factor receptor-mitogen activated protein kinase-PI3K pathways are involved. These data thus suggest a novel aspect of electric signaling in wound healing-to stimulate and guide migration of EpSCs and to regulate wound healing.

Endogenous electric fields (EFs) occur naturally in vivo and play a critical role during tissue/organ development and regeneration, including that of the central nervous system(1,2). These endogenous EFs are generated by cellular regulation of ionic transport combined with the electrical resistance of cells and tissues. It has been reported that applied EF treatment can promote functional repair of spinal cord injuries in animals and humans(3,4). In particular, EF-directed cell migration has been demonstrated in a wide variety of cell types(5,6), including neural progenitor cells (NPCs)(7,8). Application of direct current (DC) EFs is not a commonly available technique in most laboratories. We have described detailed protocols for the application of DC EFs to cell and tissue cultures previously(5,11). Here we present a video demonstration of standard methods based on a calculated field strength to set up 2D and 3D environments for NPCs, and to investigate cellular responses to EF stimulation in both single cell growth conditions in 2D, and the organotypic spinal cord slice in 3D. The spinal cordslice is an ideal recipient tissue for studying NPC ex vivo behaviours, post-transplantation, because the cytoarchitectonic tissue organization is well preserved within these cultures(9,10). Additionally, this ex vivo model also allows procedures that are not technically feasible to track cells in vivo using time-lapse recording at the single cell level. It is critically essential to evaluate cell behaviours in not only a 2D environment, but also in a 3D organotypic condition which mimicks the in vivo environment. This system will allow high-resolution imaging using cover glass-based dishes in tissue or organ culture with 3D tracking of single cell migration in vitro and ex vivo and can be an intermediate step before moving onto in vivo paradigms.

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