Local calcium elevation and cell elongation initiate guided motility in electrically stimulated osteoblast-like cells

Persistent directional cell migration requires ion transport proteins as direction sensors and membrane potential differences in order to maintain directedness

BACKGROUND

Ion transport proteins generate small electric fields that can induce directional cell motility; however, little is known about their mechanisms that lead to directedness. We investigated Na, K-ATPase (NaKA) and Na+/H+ exchanger isoforms (NHE1 and 3) in SaOS-2 and Calvarial osteoblasts, which present anode- and cathode- directed motility, during electrotaxis.

RESULTS

Significant colocalizations of NaKA with vinculin and pNHE3 with ß-actin were observed to occur at the leading edges of cells. The directedness were attenuated when NaKA or NHE3 was inhibited, confirming their implication in directional sensing. Depending on the perceived direction, a divergent regulation in PIP2 levels as a function of NHE3 and NaKA levels was observed, suggesting that PIP2 may act as a spatiotemporal regulator of the cell membrane during electrotaxis. Moreover, at the same places where pNHE3 accumulates, bubble-shaped H+ clouds were observed, suggesting a physio-mechanical role for NHE3. The cell membrane becomes hyperpolarized at the front and depolarized at the back, which confirms NaKA activity at the leading edge.

CONCLUSION

We suggest a novel role for both NaKA and NHE3 that extends beyond ion translocation and conclude that they can act as directional sensors and Vmem as a regulatory cue which maintain the persistent direction in electrotaxis.

Changes in astrocyte shape induced by sublytic concentrations of the cholesterol-dependent cytolysin pneumolysin still require pore-forming capacity

Streptococcus pneumoniae is a common pathogen that causes various infections, such as sepsis and meningitis. A major pathogenic factor of S. pneumoniae is the cholesterol-dependent cytolysin, pneumolysin. It produces cell lysis at high concentrations and apoptosis at lower concentrations. We have shown that sublytic amounts of pneumolysin induce small GTPase-dependent actin cytoskeleton reorganization and microtubule stabilization in human neuroblastoma cells that are manifested by cell retraction and changes in cell shape. In this study, we utilized a live imaging approach to analyze the role of pneumolysin’s pore-forming capacity in the actin-dependent cell shape changes in primary astrocytes. After the initial challenge with the wild-type toxin, a permeabilized cell population was rapidly established within 20-40 minutes. After the initial rapid permeabilization, the size of the permeabilized population remained unchanged and reached a plateau. Thus, we analyzed the non-permeabilized (non-lytic) population, which demonstrated retraction and shape changes that were inhibited by actin depolymerization. Despite the non-lytic nature of pneumolysin treatment, the toxin’s lytic capacity remained critical for the initiation of cell shape changes. The non-lytic pneumolysin mutants W433F-pneumolysin and delta6-pneumolysin, which bind the cell membrane with affinities similar to that of the wild-type toxin, were not able to induce shape changes. The initiation of cell shape changes and cell retraction by the wild-type toxin were independent of calcium and sodium influx and membrane depolarization, which are known to occur following cellular challenge and suggested to result from the ion channel-like properties of the pneumolysin pores. Excluding the major pore-related phenomena as the initiation mechanism of cell shape changes, the existence of a more complex relationship between the pore-forming capacity of pneumolysin and the actin cytoskeleton reorganization is suggested.

Gene expression of human lung cancer cell line CL1-5 in response to a direct current electric field

BACKGROUND

Electrotaxis is the movement of adherent living cells in response to a direct current (dc) electric field (EF) of physiological strength. Highly metastatic human lung cancer cells, CL1-5, exhibit directional migration and orientation under dcEFs. To understand the transcriptional response of CL1-5 cells to a dcEF, microarray analysis was performed in this study.

METHODOLOGY/PRINCIPAL FINDINGS

A large electric-field chip (LEFC) was designed, fabricated, and used in this study. CL1-5 cells were treated with the EF strength of 0 mV/mm (the control group) and 300 mV/mm (the EF-treated group) for two hours. Signaling pathways involving the genes that expressed differently between the two groups were revealed. It was shown that the EF-regulated genes highly correlated to adherens junction, telomerase RNA component gene regulation, and tight junction. Some up-regulated genes such as ACVR1B and CTTN, and some down-regulated genes such as PTEN, are known to be positively and negatively correlated to cell migration, respectively. The protein-protein interactions of adherens junction-associated EF-regulated genes suggested that platelet-derived growth factor (PDGF) receptors and ephrin receptors may participate in sensing extracellular electrical stimuli. We further observed a high percentage of significantly regulated genes which encode cell membrane proteins, suggesting that dcEF may directly influence the activity of cell membrane proteins in signal transduction.

CONCLUSIONS/SIGNIFICANCE

In this study, some of the EF-regulated genes have been reported to be essential whereas others are novel for electrotaxis. Our result confirms that the regulation of gene expression is involved in the mechanism of electrotactic response.

Ion imaging during axolotl tail regeneration in vivo

Several studies have reported that endogenous ion currents are involved in a wide range of biological processes from single cell and tissue behavior to regeneration. Various methods are used to assess intracellular and local ion dynamics in biological systems, e.g., patch clamping and vibrating probes. Here, we introduce an approach to detect ion kinetics in vivo using a noninvasive method that can electrophysiologically characterize an entire experimental tissue region or organism. Ion-specific vital dyes have been successfully used for live imaging of intracellular ion dynamics in vitro. Here, we demonstrate that cellular pH, cell membrane potential, calcium, sodium and potassium can be monitored in vivo during tail regeneration in the axolotl (Ambystoma mexicanum) using ion-specific vital dyes. Thus, we suggest that ion-specific vital dyes can be a powerful tool to obtain electrophysiological data during crucial biological events in vivo.

DC microelectrode array for investigating the intracellular ion changes

Microelectrode arrays (MEAs) are extensively being used to study the electrical properties of cells. Most of the MEAs use metal electrodes which are in direct contact with the cells. When using DC currents, this leads to undesirable chemical influencing of the cell. Also, metal electrodes are unsuitable for the measuring of constant potentials. A new kind of MEA is developed which replaces the metal electrodes by electrolyte-filled microchannels with Ag/AgCl-electrodes at their ends. The surface of the DCMEA consists of a nanoporous membrane that acts as a homogenous cell substrate, thus avoiding any topographical guidance of the cells. It is adhered to a polydimethylsiloxane layer with four electrode channels embedded in it, using a novel plasma bonding method. A transparent polymer ground plate connects the channels to the silver electrodes as shown in Fig. 1. This MEA allows for the stimulation of the cells with stationary, non-homogenous electric fields, e.g. to simulate the electrical environment near wounds in vitro. It has been proposed in the literature that intracellular ions are involved during cell migration. The DCMEA can be used to simulate in vitro electric fields to investigate intracellular ion changes. By loading cells with ion specific fluorescence dyes, real-time ion kinetic changes can directly be carried out on DCMEA. These studies will be performed by using a time lapse video microscope. In this paper we present the detailed fabrication and testing of the new DCMEA. Results on intracellular ion flows will be presented using this DCMEA.