Electric fields are novel determinants of human macrophage functions

Abstract

Macrophages are key cells in inflammation and repair, and their activity requires close regulation. The characterization of cues coordinating macrophage function has focused on biologic and soluble mediators, with little known about their responses to physical stimuli, such as the electrical fields that are generated naturally in injured tissue and which accelerate wound healing. To address this gap in understanding, we tested how properties of human monocyte-derived macrophages are regulated by applied electrical fields, similar in strengths to those established naturally. With the use of live-cell video microscopy, we show that macrophage migration is directed anodally by electrical fields as low as 5 mV/mm and is electrical field strength dependent, with effects peaking ∼300 mV/mm. Monocytes, as macrophage precursors, migrate in the opposite, cathodal direction. Strikingly, we show for the first time that electrical fields significantly enhance macrophage phagocytic uptake of a variety of targets, including carboxylate beads, apoptotic neutrophils, and the nominal opportunist pathogen Candida albicans, which engage different classes of surface receptors. These electrical field-induced functional changes are accompanied by clustering of phagocytic receptors, enhanced PI3K and ERK activation, mobilization of intracellular calcium, and actin polarization. Electrical fields also modulate cytokine production selectively and can augment some effects of conventional polarizing stimuli on cytokine secretion. Taken together, electrical signals have been identified as major contributors to the coordination and regulation of important human macrophage functions, including those essential for microbial clearance and healing. Our results open up a new area of research into effects of naturally occurring and clinically applied electrical fields in conditions where macrophage activity is critical.

A current affair: electrotherapy in wound healing

“…Under ES, vascular endothelial cells have also reported changes in cell elongation and orientation24–26 and upregulation of the levels of VEGF and IL-8 receptors 24. Recent studies have also tested the effects of ES on macrophages that exhibit enhanced phagocytic activity9 and platelets that display growth factors releases 80…”

“…The effect of electrical stimulation (ES) has been tested in vitro on different types of cells involved in wound healing, such as macrophages,9 fibroblasts,10–14 epidermal cells,15–20 bacteria,21–23 and endothelial cells24–26 that have demonstrated changes in cell migration, proliferation, and orientation, increase in proteins and DNA synthesis, and antibacterial effects. When applied on in vivo models27–40 and clinical stud ies,41–61 EST has shown positive effects on wound closure and healing rate.…”

Autophagy is required for the directed motility of keratinocytes driven by electric fields

“…After injury, keratinocytes in the local vicinity are instantly exposed to endogenous EFs, which serve as overriding and long-lasting directional signals (9,12,22,23,25). Mounting evidence indicates that electrical signals not only determine the migration direction of multiple cells but also increase migration speed, and keratinocytes are no exception (11)(12)(13)(14)(15). Although much is already known about how electrical cues are relayed to directional responses, studies focusing on the mechanisms underlying electrically enhanced motility are scarce.…”

“…To successfully re-establish a new epidermis, keratinocytes must acquire migratory capacity and become properly oriented. Endogenous wound electric fields (EFs), which result from disruption of the transepithelial potential, have been widely shown to induce directional migration, a phenomenon known as galvanotaxis or electrotaxis, in multiple types of cells during wound repair (9); these cells include fibroblasts (10,11), keratinocytes (12), macrophages (13), lymphocytes (14), and endothelial cells (15). EFs are generated immediately upon injury, with the wound center being electrically negative and serving as the cathode, whereas the undamaged regions surrounding the wound act as the anode (16,17).…”

Electrical Stimulation Directs Migration, Enhances and Orients Cell Division and Upregulates the Chemokine Receptors CXCR4 and CXCR2 in Endothelial Cells

“…When these tissues are wounded, these normal potentials give rise instantaneously to electrical fields at the wound edge which are in the order of 50–100 mV/mm [52-55]. EFs of this size have been shown to direct epithelial cell migration and division, enhance nerve sprouting and growth into the wound and stimulate macrophage directed invasion and phagocytic activity [3, 56, 57]. …”