Potential Cellular Mechanisms For Electric Fields To Induce Wound Healing

Persistent, chronic skin wounds and ulcers pose an important medical problem,especially in the elderly or diabetic patients. Adult stem cells hold abilities to self-renewand differentiate and play an important role in tissue homeostasis, wound healing andregeneration. Migration of epidermal stem cells (EpSCs) into the center of a wound isfundamental to skin wound healing. To direct EpSCs to migrate into the center of a woundmay provide new approaches to promote wound healing. Endogenous electric fields (EFs)arise naturally at wounds with the EF vector directed towards the wound center. However,little is known about the effects of wound EFs on the biological behaviors of EpSCs or itseffect on skin wound healing.New blood vessel growth, named angiogenesis, plays an essential role in skin woundhealing process. Migration of endothelial progenitor cells (EPCs) is important in bloodvessel injury repair and angiogenesis. Being able to control migration and other behaviorsof EPCs may be of clinical significance in promoting vasculature repair and wound healing.We therefore investigated the effects of EFs on primary cultured human EPCs.Migration of large epithelial sheets is also fundamental to skin wound healing.Epithelial cell migration is mostly studied when cells are in isolation; the mechanisms ofmigration of epithelial sheets are therefore poorly understood. Large sheets of epithelialcells do not normally migrate which prevent detailed mechanistic investigation. We thusapplied small EFs, which mimic the naturally-occurring EFs, to induce directionalmigration of large epithelial sheets and study the mechanisms.In the current research, we first investigated whether modulation of endogenous EFscould affect skin wound healing rate. We then isolated, cultured, and characterized EpSCsand EPCs and studied electrotaxis of these tow types of stem cells. Next we investigatedhow EFs direct collective migration of large epithelial sheets and achieved some novelfindings about mechanisms of directional collective cell migration. The main findings are as follows:1. There are significant outward electric currents in a rat skin wound and modulationof endogenous wound-induced EFs promotes skin wound healing;2. Cultured EpSCs possess significant electrotaxis, in which assymmetric distributionof intracellular F-actin plays an essential role;3. Epidermal growth factor receptors (EGFRs) on the surface of EpSCs is one ofimportant molecules for the cells to sense electrical cues, and both extracellularsignal-regulated kinase1/2(ERK1/2) and phosphatidylinositol-3-kinase (PI3K)/proteinkinase B (AKT) pathways are responsible for EF-induced directional migration of EpSCsand asymmetric distribution of intracellular F-actin;4. Significant inward electric currents arise at sites of endothelium injury, with thecenter being anode;5. Human primary cultured EPCs respond to applied EFs through directional migrationto the anode, elongation and parallel orientation to the EF vector, while VEGFR-2and Srcsignaling were involved in EF-guided directional migration of EPCs.6. Large epithelial sheets, which do not normally move unless being wounded, migratedirectionally to the anode under small EFs;7. It is for the first time to our knowledge that EF-induced directional migration ofsheets of cells (collective electrotaxis) is significantly better than cells in isolation; this issurprising because cells in isolation normally migrate, while the whole cell sheets do not;8. E-cadherin plays an essential role in collective migration of epithelial sheets, whilegap junctions play a minimal role for migration of large cells sheets;9. Significant orientation of traction forces of the leading edge cells in an EF, whichare different from following cells, plays a leading role in migration of the whole cell sheets.