Large-scale biophysics: ion flows and regeneration

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.

Animal cells connected by nanotubes can be electrically coupled through interposed gap-junction channels

Tunneling nanotubes (TNTs) are recently discovered conduits for a previously unrecognized form of cell-to-cell communication. These nanoscale, F-actin-containing membrane tubes connect cells over long distances and facilitate the intercellular exchange of small molecules and organelles. Using optical membrane-potential measurements combined with mechanical stimulation and whole-cell patch-clamp recording, we demonstrate that TNTs mediate the bidirectional spread of electrical signals between TNT-connected normal rat kidney cells over distances of 10 to 70 μm. Similar results were obtained for other cell types, suggesting that electrical coupling via TNTs may be a widespread characteristic of animal cells. Strength of electrical coupling depended on the length and number of TNT connections. Several lines of evidence implicate a role for gap junctions in this long-distance electrical coupling: punctate connexin 43 immunoreactivity was frequently detected at one end of TNTs, and electrical coupling was voltage-sensitive and inhibited by meclofenamic acid, a gap-junction blocker. Cell types lacking gap junctions did not show TNT-dependent electrical coupling, which suggests that TNT-mediated electrical signals are transmitted through gap junctions at a membrane interface between the TNT and one cell of the connected pair. Measurements of the fluorescent calcium indicator X-rhod-1 revealed that TNT-mediated depolarization elicited threshold-dependent, transient calcium signals in HEK293 cells. These signals were inhibited by the voltage-gated Ca(2+) channel blocker mibefradil, suggesting they were generated via influx of calcium through low voltage-gated Ca(2+) channels. Taken together, our data suggest a unique role for TNTs, whereby electrical synchronization between distant cells leads to activation of downstream target signaling.

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.

Ion channels at the nucleus: electrophysiology meets the genome

The nuclear envelope is increasingly viewed from an electrophysiological perspective by researchers interested in signal transduction pathways that influence gene transcription and other processes in the nucleus. Here, we describe evidence for ion channels and transporters in the nuclear membranes and for possible ion gating by the nuclear pores. We argue that a systems-level understanding of cellular regulation is likely to require the assimilation of nuclear electrophysiology into molecular and biochemical signaling pathways.

Complex life forms may arise from electrical processes

There is still not an appealing and testable model to explain how single-celled organisms, usually following fusion of male and female gametes, proceed to grow and evolve into multi-cellular, complexly differentiated systems, a particular species following virtually an invariant and unique growth pattern. An intrinsic electrical oscillator, resembling the cardiac pacemaker, may explain the process. Highly auto-correlated, it could live independently of ordinary thermodynamic processes which mandate increasing disorder, and could coordinate growth and differentiation of organ anlage.

Long-range neural and gap junction protein-mediated cues control polarity during planarian regeneration

Having the ability to coordinate the behavior of stem cells to induce regeneration of specific large-scale structures would have far-reaching consequences in the treatment of degenerative diseases, acute injury, and aging. Thus, identifying and learning to manipulate the sequential steps that determine the fate of new tissue within the overall morphogenetic program of the organism is fundamental. We identified novel early signals, mediated by the central nervous system and 3 innexin proteins, which determine the fate and axial polarity of regenerated tissue in planarians. Modulation of gap junction-dependent and neural signals specifically induces ectopic anterior regeneration blastemas in posterior and lateral wounds. These ectopic anterior blastemas differentiate new brains that establish permanent primary axes re-established during subsequent rounds of unperturbed regeneration. These data reveal powerful novel controls of pattern formation and suggest a constructive model linking nervous inputs and polarity determination in early stages of regeneration.

Electrical dimensions in cell science

Cells undergo a variety of physiological processes, including division, migration and differentiation, under the influence of endogenous electrical cues, which are generated physiologically and pathologically in the extracellular and sometimes intracellular spaces. These signals are transduced to regulate cell behaviours profoundly, both in vitro and in vivo. Bioelectricity influences cellular processes as fundamental as control of the cell cycle, cell proliferation, cancer-cell migration, electrical signalling in the adult brain, embryonic neuronal cell migration, axon outgrowth, spinal-cord repair, epithelial wound repair, tissue regeneration and establishment of left-right body asymmetry. In addition to direct effects on cells, electrical gradients interact with coexisting extracellular chemical gradients. Indeed, cells can integrate and respond to electrical and chemical cues in combination. This Commentary details how electrical signals control multiple cell behaviours and argues that study of the interplay between combined electrical and chemical gradients is underdeveloped yet necessary.

Particle tracking model of electrophoretic morphogen movement reveals stochastic dynamics of embryonic gradient

Some developmental events rely on an electrophoretic force to produce morphogenetic gradients. To quantitatively explore the dynamics of this process, we constructed a stochastic model of an early phase of left-right patterning: serotonin movement through the gap junction-coupled blastomeres of the Xenopus embryo. Particle-tracking simulations showed that a left-right gradient is formed rapidly, quickly reaching a final stable level. The voltage difference was critical for producing a morphogen gradient of the right steepness; gap junctional connectivity and morphogen mass determined the timing of the gradient. Endogenous electrophoresis drives approximately 50% of the particles across more than one cell width, and approximately 20% can travel across half the embryo. The stochastic behavior of the resulting gradients exhibited unexpected complexity among blastomeres' morphogen content, and showed how spatiotemporal variability within individual cells resulted in robust and consistent gradients across the embryonic left-right axis. Analysis of the distribution profile of gradient gain values made quantitative predictions about the conditions that result in the observed background level of laterality defects in unperturbed frog embryos. This work provides a general model that can be used to quantitatively analyze the unexpectedly complex dynamics of morphogens in a wide variety of systems.

Role of membrane potential in the regulation of cell proliferation and differentiation

Biophysical signaling, an integral regulator of long-term cell behavior in both excitable and non-excitable cell types, offers enormous potential for modulation of important cell functions. Of particular interest to current regenerative medicine efforts, we review several examples that support the functional role of transmembrane potential (V(mem)) in the regulation of proliferation and differentiation. Interestingly, distinct V(mem) controls are found in many cancer cell and precursor cell systems, which are known for their proliferative and differentiation capacities, respectively. Collectively, the data demonstrate that bioelectric properties can serve as markers for cell characterization and can control cell mitotic activity, cell cycle progression, and differentiation. The ability to control cell functions by modulating bioelectric properties such as V(mem) would be an invaluable tool for directing stem cell behavior toward therapeutic goals. Biophysical properties of stem cells have only recently begun to be studied and are thus in need of further characterization. Understanding the molecular and mechanistic basis of biophysical regulation will point the way toward novel ways to rationally direct cell functions, allowing us to capitalize upon the potential of biophysical signaling for regenerative medicine and tissue engineering.

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

Background

Investigation of the mechanisms of guided cell migration can contribute to our understanding of many crucial biological processes, such as development and regeneration. Endogenous and exogenous direct current electric fields (dcEF) are known to induce directional cell migration, however the initial cellular responses to electrical stimulation are poorly understood. Ion fluxes, besides regulating intracellular homeostasis, have been implicated in many biological events, including regeneration. Therefore understanding intracellular ion kinetics during EF-directed cell migration can provide useful information for development and regeneration.

Methodology/Principal Findings

We analyzed the initial events during migration of two osteogenic cell types, rat calvarial and human SaOS-2 cells, exposed to strong (10–15 V/cm) and weak (≤5 V/cm) dcEFs. Cell elongation and perpendicular orientation to the EF vector occurred in a time- and voltage-dependent manner. Calvarial osteoblasts migrated to the cathode as they formed new filopodia or lamellipodia and reorganized their cytoskeleton on the cathodal side. SaOS-2 cells showed similar responses except towards the anode. Strong dcEFs triggered a rapid increase in intracellular calcium levels, whereas a steady state level of intracellular calcium was observed in weaker fields. Interestingly, we found that dcEF-induced intracellular calcium elevation was initiated with a local rise on opposite sides in calvarial and SaOS-2 cells, which may explain their preferred directionality. In calcium-free conditions, dcEFs induced neither intracellular calcium elevation nor directed migration, indicating an important role for calcium ions. Blocking studies using cadmium chloride revealed that voltage-gated calcium channels (VGCCs) are involved in dcEF-induced intracellular calcium elevation.

Conclusion/Significance

Taken together, these data form a time scale of the morphological and physiological rearrangements underlying EF-guided migration of osteoblast-like cell types and reveal a requirement for calcium in these reactions. We show for the first time here that dcEFs trigger different patterns of intracellular calcium elevation and positional shifting in osteogenic cell types that migrate in opposite directions.

Regeneration: The origin of cancer or a possible cure?

A better understanding of the forces controlling cell growth will be essential for developing effective therapies in regenerative medicine and cancer. Historically, the literature has linked cancer and tissue regeneration-proposing regeneration as both the source of cancer and a method to inhibit tumorigenesis. This review discusses two powerful regeneration models, the vertebrate urodele amphibians and invertebrate planarians, in light of cancer regulation. Urodele limb and eye lens regeneration is described, as well as the planarian's emergence as a molecular and genetic model system in which recent insights begin to molecularly dissect cancer and regeneration in adult tissues.

Bioelectric mechanisms in regeneration: Unique aspects and future perspectives

Regenerative biology has focused largely on chemical factors and transcriptional networks. However, endogenous ion flows serve as key epigenetic regulators of cell behavior. Bioelectric signaling involves feedback loops, long-range communication, polarity, and information transfer over multiple size scales. Understanding the roles of endogenous voltage gradients, ion flows, and electric fields will contribute to the basic understanding of numerous morphogenetic processes and the means by which they can robustly restore pattern after perturbation. By learning to modulate the bioelectrical signals that control cell proliferation, migration, and differentiation, we gain a powerful set of new techniques with which to manipulate growth and patterning in biomedical contexts. This chapter reviews the unique properties of bioelectric signaling, surveys molecular strategies and reagents for its investigation, and discusses the opportunities made available for regenerative medicine.

A new tool for tissue engineers: ions as regulators of morphogenesis during development and regeneration

Currently, most of the research on how to encourage stem cells to replace missing tissues focuses on biochemical control, such as signaling by growth factors. In addition to basic questions, such as how are stem cells induced to differentiate into particular cell types, also inherent in those studies are practical questions about how to identify, grow, induce, and safely deliver stems cells to the proper target. At the Forsyth Center for Regenerative and Developmental Biology, we are examining a different set of signals, specifically bioelectric signals (the regulated movement of ions across membranes), including membrane voltage, pH, and gap junction activity and gating. We have found strong evidence that bioelectrical signals function at many critical, early points, both up- and downstream of transcriptional regulation, during the processes of normal morphogenesis and adult stem cell-based regeneration. Examples described include gap-junction-dependent regulation of stem cell identity in a flatworm, proton-flux-regulated establishment of left-right asymmetry in vertebrates, and proton-flux-initiated regeneration of a complex structure that includes spinal cord--the tadpole tail--in frogs.

Electrical fields in wound healing-An overriding signal that directs cell migration

Injury that disrupts an epithelial layer instantaneously generates endogenous electric fields (EFs), which were detected at human skin wounds over 150 years ago. Recent researches combining molecular, genetic and imaging techniques have provided significant insights into cellular and molecular responses to this "unconventional" signal. One unexpected finding is that the EFs play an overriding guidance role in directing cell migration in epithelial wound healing. In experimental models where other directional cues (e.g., contact inhibition release, population pressure etc.) are present, electric fields of physiological strength override them and direct cell migration. The electrotaxis or galvanotaxis is mediated by polarized activation of multiple signaling pathways that include PI3 kinases/Pten, membrane growth factor receptors and integrins. Genetic manipulation of PI3 kinase/Pten (Phosphoinositide 3-kinases/phosphatase and tensin homolog) and integrin beta4 demonstrated the importance of those molecules. The electric fields are therefore a fundamental signal that directs cell migration in wound healing. One of the most challenging question is: How do cells sense the very weak electric signals? Clinically, it is highly desirable to develop practical and reliable technologies for wound healing management exploiting the electric signaling.

Comparative aspects of animal regeneration

Most but not all phyla include examples of species that are able to regenerate large sections of the body plan. The mechanisms underlying regeneration on this scale are currently being studied in a variety of contexts in both vertebrates and invertebrates. Regeneration generally involves the formation of a wound epithelium after transection or injury, followed by the generation of regenerative progenitor cells and morphogenesis to give the regenerate. Common mechanisms may exist in relation to each of these aspects. For example, the initial proliferation of progenitor cells often depends on the nerve supply, whereas morphogenesis reflects the generation of positional disparity between adjacent cells-the principle of intercalation. These mechanisms are reviewed here across a range of contexts. We also consider the evolutionary origins of regeneration and how regeneration may relate to both agametic reproduction and to ontogeny.

Membrane potential controls adipogenic and osteogenic differentiation of mesenchymal stem cells

BACKGROUND

Control of stem cell behavior is a crucial aspect of developmental biology and regenerative medicine. While the functional role of electrophysiology in stem cell biology is poorly understood, it has become clear that endogenous ion flows represent a powerful set of signals by means of which cell proliferation, differentiation, and migration can be controlled in regeneration and embryonic morphogenesis.

METHODOLOGY/PRINCIPAL FINDINGS

We examined the membrane potential (V(mem)) changes exhibited by human mesenchymal stem cells (hMSCs) undergoing adipogenic (AD) and osteogenic (OS) differentiation, and uncovered a characteristic hyperpolarization of differentiated cells versus undifferentiated cells. Reversal of the progressive polarization via pharmacological modulation of transmembrane potential revealed that depolarization of hMSCs prevents differentiation. In contrast, treatment with hyperpolarizing reagents upregulated osteogenic markers.

CONCLUSIONS/SIGNIFICANCE

Taken together, these data suggest that the endogenous hyperpolarization is a functional determinant of hMSC differentiation and is a tractable control point for modulating stem cell function.

Modulation of potassium channel function confers a hyperproliferative invasive phenotype on embryonic stem cells

Ion transporters, and the resulting voltage gradients and electric fields, have been implicated in embryonic development and regeneration. These biophysical signals are key physiological aspects of the microenvironment that epigenetically regulate stem and tumor cell behavior. Here, we identify a previously unrecognized function for KCNQ1, a potassium channel known to be involved in human Romano-Ward and Jervell-Lange-Nielsen syndromes when mutated. Misexpression of its modulatory wild-type beta-subunit XKCNE1 in the Xenopus embryo resulted in a striking alteration of the behavior of one type of embryonic stem cell: the pigment cell lineage of the neural crest. Depolarization of embryonic cells by misexpression of KCNE1 non-cell-autonomously induced melanocytes to overproliferate, spread out, and become highly invasive of blood vessels, liver, gut, and neural tube, leading to a deeply hyperpigmented phenotype. This effect is mediated by the up-regulation of Sox10 and Slug genes, thus linking alterations in ion channel function to the control of migration, shape, and mitosis rates during embryonic morphogenesis. Taken together, these data identify a role for the KCNQ1 channel in regulating key cell behaviors and reveal the molecular identity of a biophysical switch, by means of which neoplastic-like properties can be conferred upon a specific embryonic stem cell subpopulation.

The mechanical transduction of physiological strength electric fields

In this article it is proposed that electric fields of physiological strength (approximately 100 V/m) are transduced by the mechanical torque they exert on glycoproteins. The resulting mechanical signal is then transmitted to the cytoskeleton and propagated throughout the cell interior. This mechanical coupling is analyzed for transmembrane glycoproteins, such as integrins and the glycocalyx, and for glycoproteins in the extracellular matrix of cartilage. The applied torque is opposed by viscous fluid drag and restoring forces exerted by adjacent molecules in the membrane or cartilage. The resulting system represents a damped, driven harmonic oscillator. The amplitude of oscillation is constant at low frequencies, but falls off rapidly in the range 1-1000 Hz. The transition frequency depends on parameters such as the viscosity of the surrounding fluid and the restoring force exerted by the surrounding structure. The amplitude increases as the fourth power of the length of the transmembrane glycoproteins and as the square of the applied field. This process may operate in concert with other transduction mechanisms, such as the opening of voltage-gated channels and electrodiffusion/osmosis for DC fields.

Live Imaging of Planarian Membrane Potential Using DiBAC4(3)

INTRODUCTIONThis protocol describes how to use the anionic membrane voltage-reporting dye DiBAC(4)(3) to generate images of cell membrane potential in live planarians. These images qualitatively reveal variations in time-averaged membrane potential across different regions of the organism. Changes in these images due to experimental treatments reveal how the particular treatment affects this physiological parameter. This method is a great improvement over standard electrophysiological techniques, which cannot be used to gain an understanding of the electrical properties of an entire worm or a regenerating fragment, due to small cell size and large cell number. When the proper controls are performed, this technique is a very powerful and simple way to gather physiologic data.

Electromagnetic effects - From cell biology to medicine

In this review we compile and discuss the published plethora of cell biological effects which are ascribed to electric fields (EF), magnetic fields (MF) and electromagnetic fields (EMF). In recent years, a change in paradigm took place concerning the endogenously produced static EF of cells and tissues. Here, modern molecular biology could link the action of ion transporters and ion channels to the "electric" action of cells and tissues. Also, sensing of these mainly EF could be demonstrated in studies of cell migration and wound healing. The triggers exerted by ion concentrations and concomitant electric field gradients have been traced along signaling cascades till gene expression changes in the nucleus. Far more enigmatic is the way of action of static MF which come in most cases from outside (e.g. earth magnetic field). All systems in an organism from the molecular to the organ level are more or less in motion. Thus, in living tissue we mostly find alternating fields as well as combination of EF and MF normally in the range of extremely low-frequency EMF. Because a bewildering array of model systems and clinical devices exits in the EMF field we concentrate on cell biological findings and look for basic principles in the EF, MF and EMF action. As an outlook for future research topics, this review tries to link areas of EF, MF and EMF research to thermodynamics and quantum physics, approaches that will produce novel insights into cell biology.

Tail regeneration in Xenopus laevis as a model for understanding tissue repair

Augmentation of regenerative ability is a powerful strategy being pursued for the biomedical management of traumatic injury, cancer, and degeneration. While considerable attention has been focused on embryonic stem cells, it is clear that much remains to be learned about how somatic cells may be controlled in the adult organism. The tadpole of the frog Xenopus laevis is a powerful model system within which fundamental mechanisms of regeneration are being addressed. The tadpole tail contains spinal cord, muscle, vasculature, and other terminally differentiated cell types and can fully regenerate itself through tissue renewal--a process that is most relevant to mammalian healing. Recent insight into this process has uncovered fascinating molecular details of how a complex appendage senses injury and rapidly repairs the necessary morphology. Here, we review what is known about the chemical and bioelectric signals underlying this process and draw analogies to evolutionarily conserved pathways in other patterning systems. The understanding of this process is not only of fundamental interest for the evolutionary and cell biology of morphogenesis, but will also generate information that is crucial to the development of regenerative therapies for human tissues and organs.

The alpha1 isoform of the Na+/K+ ATPase is up-regulated in dedifferentiated progenitor cells that mediate lens and retina regeneration in adult newts

Adult newts are able to regenerate their retina and lens after injury or complete removal through transdifferentiation of the pigmented epithelial tissues of the eye. This process needs to be tightly controlled, and several different mechanisms are likely to be recruited for this function. The Na(+)/K(+) ATPase is a transmembrane protein that establishes electrochemical gradients through the transport of Na(+) and K(+) and has been implicated in the modulation of key cellular processes such as cell division, migration and adhesion. Even though it is expressed in all cells, its isoform composition varies with cell type and is tightly controlled during development and regeneration. In the present study we characterize the expression pattern of Na(+)/K(+) ATPase alpha1 in the adult newt eye and during the process of lens and retina regeneration. We show that this isoform is up-regulated in undifferentiated cells during transdifferentiation. Such change in composition could be one of the mechanisms that newt cells utilize to modulate this process.

KCNQ1 and KCNE1 K+ channel components are involved in early left-right patterning in Xenopus laevis embryos

Several ion transporters have been implicated in left-right (LR) patterning. Here, we characterize a new component of the early bioelectrical circuit: the potassium channel KCNQ1 and its accessory subunit KCNE1. Having cloned the native Xenopus versions of both genes, we show that both are asymmetrically localized as maternal proteins during the first few cleavages of frog embryo development in a process dependent on microtubule and actin organization. Molecular loss-of-function using dominant negative constructs demonstrates that both gene products are required for normal LR asymmetry. We propose a model whereby these channels provide an exit path for K(+) ions brought in by the H(+),K(+)-ATPase. This physiological module thus allows the obligate but electroneutral H(+),K(+)-ATPase to generate an asymmetric voltage gradient on the left and right sides. Our data reveal a new, bioelectrical component of the mechanisms patterning a large-scale axis in vertebrate embryogenesis.

Pursuing a 'turning point' in growth cone research

Growth cones are highly motile structures found at the leading edge of developing and regenerating nerve processes. Their role in axonal pathfinding has been well established and many guidance cues that influence growth cone behavior have now been identified. Many studies are now providing insights into the transduction and integration of signals in the growth cone, though a full understanding of growth cone behavior still eludes us. This review focuses on recent studies adding to the growing body of literature on growth cone behavior, focusing particularly on the level of autonomy the growth cone possesses and the role of local protein synthesis.

The spark of life: electricity and regeneration

A recent paper shows that a proton pump, the vacuolar-type adenosine triphosphatase (V-ATPase), is necessary for Xenopus tadpole tail regeneration. The V-ATPase is synthesized in distal cells of the regeneration bud as an early response to amputation, and its role in regeneration is specifically to pump H+ out of cells. Although the downstream effects of proton movement are not known, they may be mediated through changes in membrane potential. This work could reopen a consideration of the role of electric fields in regeneration, an issue that has caused much controversy in the past.

What lies at the interface of regenerative medicine and developmental biology?

At a recent Keystone Symposium on `Developmental Biology and Tissue Engineering', new findings in areas ranging from stem cell differentiation,embryonic pattern formation and organ regeneration to engineered cell microenvironments, synthetic biomaterials and artificial tissue fabrication were described. Although these new advances were exciting, this symposium clarified that biologists and engineers often view the challenge of tissue formation from different, and sometimes conflicting, perspectives. These dichotomies raise questions regarding the definition of regenerative medicine,but offer the promise of exciting new interdisciplinary approaches to tissue and organ regeneration, if effective alliances can be established.