Nonselective cation channel activation during wound healing in the corneal endothelium

Making Sense of Electrical Stimulation: A Meta-analysis for Wound Healing

Electrical stimulation as a mode of external enhancement factor in wound healing has been explored widely. It has proven to have multidimensional effects in wound healing including antibacterial, galvanotaxis, growth factor secretion, proliferation, transdifferentiation, angiogenesis, etc. Despite such vast exploration, this modality has not yet been established as an accepted method for treatment. This article reviews and analyzes the approaches of using electrical stimulation to modulate wound healing and discusses the incoherence in approaches towards reporting the effect of stimulation on the healing process. The analysis starts by discussing various processes adapted in in vitro, in vivo, and clinical practices. Later it is focused on in vitro approaches directed to various stages of wound healing. Based on the analysis, a protocol is put forward for reporting in vitro works in such a way that the outcomes of the experiment are replicable and scalable in other setups. This work proposes a ground of unification for all the in vitro approaches in a more sensible manner, which can be further explored for translating in vitro approaches to complex tissue stimulation to establish electrical stimulation as a controlled clinical method for modulating wound healing.

In silico and in vitro analysis of cation-activated potassium channels in human corneal endothelial cells

The corneal endothelium is the inner cell monolayer involved in the maintenance of corneal transparence by the generation of homeostatic dehydration. The glycosaminoglycans of the corneal stroma develop a continuous swelling pressure that should be counteracted by the corneal endothelial cells through active transport mechanisms to move the water to the anterior chamber. Protein transporters for sodium (Na⁺), potassium (K⁺), chloride (Cl^-) and bicarbonate (HCO₃^-) are involved in this endothelial "pump function", however despite its physiological importance, the efflux mechanism is not completely understood. There is experimental evidence describing transendothelial diffusion of water in the absence of osmotic gradients. Therefore, it is important to get a deeper understanding of alternative models that drive the fluid transport across the endothelium such as the electrochemical gradients. Three transcriptomic datasets of the corneal endothelium were used in this study to analyze the expression of genes that encode proteins that participate in the transport and the reestablishment of the membrane potential across the semipermeable endothelium. Subsequently, the expression of the identified channels was validated in vitro both at mRNA and protein levels. The results of this study provide the first evidence of the expression of KCNN2, KCNN3 and KCNT2 genes in the corneal endothelium. Differences among the level of expression of KCNN2, KCNT2 and KCNN4 genes were found in a differentially expressed gene analysis of the dataset. Taken together these results underscore the potential importance of the ionic channels in the pathophysiology of corneal diseases. Moreover, we elucidate novel mechanisms that might be involved in the pivotal dehydrating function of the endothelium and in others physiologic functions of these cells using in silico pathways analysis.

The Electrical Response to Injury: Molecular Mechanisms and Wound Healing

Significance: Natural, endogenous electric fields (EFs) and currents arise spontaneously after wounding of many tissues, especially epithelia, and are necessary for normal healing. This wound electrical activity is a long-lasting and regulated response. Enhancing or inhibiting this electrical activity increases or decreases wound healing, respectively. Cells that are responsible for wound closure such as corneal epithelial cells or skin keratinocytes migrate directionally in EFs of physiological magnitude. However, the mechanisms of how the wound electrical response is initiated and regulated remain unclear. Recent Advances: Wound EFs and currents appear to arise by ion channel up-regulation and redistribution, which are perhaps triggered by an intracellular calcium wave or cell depolarization. We discuss the possibility of stimulation of wound healing via pharmacological enhancement of the wound electric signal by stimulation of ion pumping. Critical Issues: Chronic wounds are a major problem in the elderly and diabetic patient. Any strategy to stimulate wound healing in these patients is desirable. Applying electrical stimulation directly is problematic, but pharmacological enhancement of the wound signal may be a promising strategy. Future Directions: Understanding the molecular regulation of wound electric signals may reveal some fundamental mechanisms in wound healing. Manipulating fluxes of ions and electric currents at wounds might offer new approaches to achieve better wound healing and to heal chronic wounds.

The plasma membrane potential and the organization of the actin cytoskeleton of epithelial cells

The establishment and maintenance of the polarized epithelial phenotype require a characteristic organization of the cytoskeletal components. There are many cellular effectors involved in the regulation of the cytoskeleton of epithelial cells. Recently, modifications in the plasma membrane potential (PMP) have been suggested to participate in the modulation of the cytoskeletal organization of epithelia. Here, we review evidence showing that changes in the PMP of diverse epithelial cells promote characteristic modifications in the cytoskeletal organization, with a focus on the actin cytoskeleton. The molecular paths mediating these effects may include voltage-sensitive integral membrane proteins and/or peripheral proteins sensitive to surface potentials. The voltage dependence of the cytoskeletal organization seems to have implications in several physiological processes, including epithelial wound healing and apoptosis.

A possible role for membrane depolarization in epithelial wound healing

Linear narrow wounds produced on cultured bovine corneal endothelial monolayers heal by actin cable formation at the wound border and lamellar crawling of cells into the injured area. We report the novel finding that membrane potential depolarization occurs at the leading edge of wounds and gradually extends inward toward the neighboring cells. We have determined that the replacement of extracellular Na+by choline and the incorporation of phenamil, an inhibitor of the epithelial Na+channel (ENaC), provoke a decrease in the actin cable and depolarization areas and in the lamellar activity of the wound edges. To the contrary, extracellular Li+can successfully replace Na+in the determination of the depolarization and cytoskeletal responses. This finding supports the idea that membrane depolarization, not the increase in intracellular Na+concentration, is responsible for the formation of the actin cable, a result that is in agreement with previous evidence showing that nonspecific depolarization of the plasma membrane potential (PMP) of epithelial cells may promote characteristic cytoskeletal rearrangements per se (Chifflet S, Hernández JA, Grasso S, and Cirillo A. Exp Cell Res 282: 1–13, 2003). We suggest that spontaneous depolarization of the PMP of the cells at the wound borders determined by a rise in the ENaC activity of these cells constitutes an additional factor in the intermediate cellular processes leading to wound healing in some epithelia.

Regulation of a TRPM7-like current in rat brain microglia

Non-excitable cells use Ca2+ influx for essential functions but usually lack voltage-gated Ca2+ channels. The main routes of Ca2+ entry appear to be store-operated channels or Ca2+-permeable non-selective cation channels, of which the magnesium-inhibited cation (or magnesium-nucleotide-regulated metal cation) current has received considerable recent attention. This current appears to be produced by one of the recently cloned transient receptor potential (TRP) channels, TRPM7. In this study of rat microglia, we identified TRPM7 transcripts and a prevalent current with the hallmark biophysical and pharmacological features of TRPM7. This is the first identification of a TRPM7-like current in the brain. There is little known about how members of the TRPM sub-family normally become activated. Using whole-cell patch clamp recordings from rat microglia, we found that the TRPM7-like current activates spontaneously after break-in and that the current and its activation are inhibited by elevated intracellular Mg2+ but not affected by cell swelling or a wide range of intracellular Ca2+ concentrations. The TRPM7-like current in microglia appears to depend on tyrosine phosphorylation. It was inhibited by several tyrosine kinase inhibitors, including a peptide (Src 40-58) that was shown previously to inhibit Src actions, but not by inactive drugs or peptide analogues. The current did not depend on the cell activation state; i.e. it was the same in microglia recently removed from the brain or when cultured under a wide range of conditions that favor the resting or activated state. Because TRPM7 channels are permeable to Ca2+, this current may be important for microglia functions that depend on elevations in intracellular Ca2+.

Growth factor-like phospholipids generated after corneal injury

The present study provides evidence that growth factor-like glycerophosphate mediators of the lysophosphatidic acid (LPA) family are present in the aqueous humor and the lacrimal gland fluid of the rabbit eye. By use of a combination of HPLC, two-dimensional TLC, mass spectrometry, and the Xenopus oocyte bioassay, the LPA-like phospholipids LPA, cyclic PA, alkenyl-glycerophosphate (GP), lysophosphatidylserine, and phosphatidic acid were detected as physiological constituents of the fluids bathing the cornea. Corneal injury resulted in an increased production of some of these mediators. Alkenyl-GP, a novel member of the LPA family, has been identified in postinjury aqueous humor, establishing that it is generated endogenously. LPA and its homologues were found to be mitogenic in freshly dissociated keratocytes from uninjured corneas. There appears to be a link between the occurrence of LPA responsiveness in keratocytes activated by injury and the increase in LPA-like activity in aqueous humor. These data suggest that LPA and its homologues are involved in maintaining the integrity of the normal cornea and in promoting cellular regeneration of the injured cornea.