Introduction
Our daily routines would be impossible without the wonders of electricity. This also applies to the cells in our bodies, each of which generates internal mini-currents to carry out its functions. It is less well known that cells in tissues (for example in our skin) can team up to form 'biological batteries' that can generate current upon wounding of the tissue. Zhao et al.1 now present evidence that this extracellular electric field is the driving force in wound healing, and they uncover signaling pathways that control this biological phenomenon.
All cells maintain a voltage gradient across their outer membrane (Fig. 1). Controlled changes in the ion permeability of the membrane lead to rapid alterations in voltage across the membrane, thereby generating action potentials. These trigger nerve cells to transmit signals, muscle cells to contract and gland cells to secrete hormones. Efforts from electrophysiologists and others have provided us with a deep understanding of the molecular details of these events.
Figure 1: Action potential in individual cells and injury potential in tissues.
(a) Individual cells maintain an electrical potential across the plasma membrane (Vm) as a result of the activity of membrane-bound ion channels. This results in a net negative charge on the inside of the cell relative to the outside. This resting membrane potential can be locally depolarized under the influence of cell stimuli, leading to an inward current (bottom). (b) Schematic representation of the generation of a transepithelial potential (VTEP) in human skin (individual cells in cornified layer and dermis are not shown). Selective, directional ion transport across the intact epithelium gives rise to a VTEP that can be measured directly across the epithelium (top; 70 mV in this case). Tight junctions between epithelial cells (not shown) create physical connections between cells, providing high electrical resistance to the epithelial sheet. Wounding of an epithelial sheet results in collapse of the VTEP at the wound (to 0 mV) without affecting the VTEP distally (70 mV). Na+ leaks out of the wound, resulting in an injury current toward the cut (thin arrows) and a lateral voltage gradient oriented parallel to the epithelial sheet (EF, electric field; thick arrows at bottom). The wound site is the cathode of the electric field (bottom). Figure is based on refs. 1 and 2.
Katie Ris
Full size image (49 KB)In contrast, the existence of steady, extracellular voltage gradients in tissues has received far less attention, even though their discovery in the nineteenth century predates that of cellular action potentials2. The potential difference across the layers of an epithelium (such as skin) derives from an asymmetric transport of ions across the layers. When the skin is cut, this transepithelial potential difference is short-circuited, inducing current to flow out of the lesion from underneath the wounded epithelium and giving rise to a steady electric field at the wound edge2 (Fig. 1).
Zhao et al. have now confirmed the presence of endogenous wound electric fields in a wide diversity of model systems, including organ cultures of stratified epithelia such as skin and cornea. These electric fields persist for several hours, with the flow of positive charge directed toward the wound center (Fig. 1).
The authors explored the ways electric fields control wound healing, first by using an assay in which a small area of a monolayer of epithelial cells is scratched. Over time, the neighboring cells then migrate and fill the damaged area. Application of an exogenous electric field, depending on polarity, led to faster wound closure or, notably, made cells move away from the wound. Similar responses were observed in the cornea whole-organ culture model. These new results support the notion that the electrical signal overrides other, coexisting closure-promoting signals such as the release of growth factors and loss of contact inhibition. Thus healing rates can be controlled by electrical cues.
An important question is, How does electrotaxis relate to chemotaxis, the movement of cells according to gradients of chemical cues in their environment? Much like chemotaxing cells3, electrotaxing cells change shape and become elongated and polarized. Could the electric field lead to a buildup of chemical gradients in the extracellular milieu that is then sensed by the cells via chemotactic receptors on their surfaces? This may not be a key controlling factor, as Zhao et al. found that electrotaxis was unaffected when fresh culture medium was flushed perpendicularly to the electrical vector. They also observed that a mutant strain of a slime mold that can no longer chemotax (owing to the absence of a G
subunit, a signaling protein linked to receptors for chemoattractants) retained the capacity to electrotax.
Despite these differences, the authors found that chemotaxis and electrotaxis share a dependence on similar intracellular signaling molecules, most notably the phosphoinositide 3-kinase (PI3K) enzymes. The lipids generated by these kinases in response to receptor stimulation are asymmetrically distributed in the membranes of chemotaxing cells4. Lipid phosphatases such as phosphatase and tensin homolog (PTEN) are key in establishing this intracellular lipid gradient. The authors found that PI3K-generated lipids become polarized inside cells under the influence of an electric field, to a degree similar to that observed in chemotaxing cells. They further present genetic and pharmacological evidence supporting a role for PI3K in electrotaxis: a small-molecule inhibitor of PI3K blocked electrotaxis, as did deletion of p110
, one of the isoforms of PI3K. Deletion of PTEN, a negative regulator of PI3K, enhanced electrotaxis.
Several important questions remain to be answered. Although key biochemical components controlling electrotaxis have now been identified, the ways in which these molecules become polarized in an electric field are not clear. For example, what determines the polarization of PI3K lipids in the electrotaxing cells? Could this simply be an electric field-induced asymmetric redistribution of cellular components, be it receptors, second messengers or signaling proteins? Evidence supporting this hypothesis has been presented1, 5, 6. It will also be interesting to apply the power of genetic screens in a variety of model systems to uncover additional genes involved in this phenomenon.
Last but not least, the p110 isoform of PI3K, found by Zhao et al.1 to be a key player in wound healing responses, is currently very actively pursued by the pharmaceutical industry as a new anti-inflammatory target7. It will therefore be of interest to test whether wounds heal slower in p110-null mice, and to test the effect of p110 pharmacological inhibitors on wound healing.
Poor wound healing (for example, in diabetes) is an important health-care issue, and hence new wound healing therapies are needed. Could the observations of Zhao et al. be used to improve wound healing? Previous attempts to apply electric fields to treat nonhealing skin wounds by insertion of metal electrodes directly into wound beds have had mixed success8, 9. The authors present evidence that there may be no need to resort to electrodes. Indeed, they find that manipulation of transepithelial ion transport in cornea wounds using agents known to alter ion transportation leads to changes in the transepithelial potential and results in augmented or decreased healing. Chemical intervention during wound healing with agents that affect ion transport may therefore be a more practical avenue for enhancing wound healing.
The findings of Zhao et al. are provocative and will most likely be met with some skepticism, but they will hopefully also stimulate researchers to have a closer look at electrotaxis. It will be critical for the authors to make their technological expertise widely available so that others may embark on these investigations, not only in the interest of basic science but also in the interest of human therapy, which could benefit tremendously.
