Long-distance electrical coupling via tunneling nanotubes

Tunneling nanotubes (TNTs) are nanoscaled, F-actin containing membrane tubes that connect cells over several cell diameters. They facilitate the intercellular exchange of diverse components ranging from small molecules to organelles and pathogens. In conjunction with recent findings that TNT-like structures exist in tissue, they are expected to have important implications in cell-to-cell communication. In this review we will focus on a new function of TNTs, namely the transfer of electrical signals between remote cells. This electrical coupling is not only determined by the biophysical properties of the TNT, but depends on the presence of connexons interposed at the membrane interface between TNT and the connected cell. Specific features of this coupling are compared to conventional gap junction communication. Finally, we will discuss possible down-stream signaling pathways of this electrical coupling in the recipient cells and their putative effects on different physiological activities. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.     

Neurotransmission: Chemical and electrical interneuron coupling

Recent studies have described the coupling between pairs of neocortical interneurons involving both electrical and chemical transmission; these new results may have important implications for the mechanisms underlying neuronal synchrony and rhythmic activity in the brain.     

Shaping bursting by electrical coupling and noise

Gap-junctional coupling is an important way of communication between neurons and other excitable cells. Strong electrical coupling synchronizes activity across cell ensembles. Surprisingly, in the presence of noise synchronous oscillations generated by an electrically coupled network may differ qualitatively from the oscillations produced by uncoupled individual cells forming the network. A prominent example of such behavior is the synchronized bursting in islets of Langerhans formed by pancreatic β-cells, which in isolation are known to exhibit irregular spiking (Sherman and Rinzel, Biophys J 54:411–425, 1988; Sherman and Rinzel, Biophys J 59:547–559, 1991). At the heart of this intriguing phenomenon lies denoising, a remarkable ability of electrical coupling to diminish the effects of noise acting on individual cells. In this paper, building on an earlier analysis of denoising in networks of integrate-and-fire neurons (Medvedev, Neural Comput 21 (11):3057–3078, 2009) and our recent study of spontaneous activity in a closely related model of the Locus Coeruleus network (Medvedev and Zhuravytska, The geometry of spontaneous spiking in neuronal networks, submitted, 2012), we derive quantitative estimates characterizing denoising in electrically coupled networks of conductance-based models of square wave bursting cells. Our analysis reveals the interplay of the intrinsic properties of the individual cells and network topology and their respective contributions to this important effect. In particular, we show that networks on graphs with large algebraic connectivity (Fiedler, Czech Math J 23(98):298–305, 1973) or small total effective resistance (Bollobas, Modern graph theory, Graduate Texts in Mathematics, vol. 184, Springer, New York, 1998) are better equipped for implementing denoising. As a by-product of the analysis of denoising, we analytically estimate the rate with which trajectories converge to the synchronization subspace and the stability of the latter to random perturbations. These estimates reveal the role of the network topology in synchronization. The analysis is complemented by numerical simulations of electrically coupled conductance-based networks. Taken together, these results explain the mechanisms underlying synchronization and denoising in an important class of biological models.     

Regulation of electrical coupling between Arabidopsis root hairs

Voltage clamp was used to measure the voltage dependence of cell-to-cell coupling via plasmodesmata between higher-plant cells (root hairs of Arabidopsis thaliana (L.) Heynh.). In addition, ionophoresis was used to introduce a variety of ions [Ca²⁺, inositol-trisphosphate, Li⁺, K⁺, Mg²⁺, ethylene glycol-bis(-aminoethyl ether)-N,N,N, N-tetraacetic acid (EGTA), 1,2-bis(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid (BAPTA), H⁺, and OH^–] to examine whether they regulate cell-to-cell coupling. Electrical coupling showed high variability in this single cell type at the same developmental stage; the coupling ratio ranged from near 0% to about 90% with a mean value of 32%. It was voltage independent for intracellular voltage gradients (transplasmodesmatal) of -163 to 212 mV. While Ca²⁺ closes the plasmodesmatal connections (at concentrations higher than those causing cessation of cytoplasmic streaming), inositol-trisphosphate and lithium are without effect. Apparently, inositol-trisphosphate may not cause increased cytosolic Ca²⁺ in root hairs. Alkalinization by OH ionophoresis caused a modest decline in cell-to-cell coupling, as did acidification by H⁺ ionophoresis (to an extent causing the cell to become flacid). Increases in cytosolic K⁺, Mg²⁺, and the calcium chelator BAPTA by ionophoresis had no effect on cell-to-cell coupling. The regulation (and lack thereof) reported here for plant plasmodesmata is quite similar to that of gap junctions.Abbreviations BAPTA 1,2-bis(2-aminophenoxy)ethane-N,N,N, N-tetraacetic acid     

On the mechanism of electrical coupling between cells of earlyXenopus embryos

Summary The mechanism of electrical coupling between cells of earlyXenopus embryos has been studied by examination of the nonjunctional membrane resistances and capacitances as a function of cleavage stage, the junctional and nonjunctional membrane resistances as functions of time during the first cleavage, and the electrical properties of the primitive blastocoel. The changes in membrane resitances and capacitances during the first two cleavages may be completely explained by the addition of new membrane, identical in specific resistance and capacitance to the original membrane, at a constant rate to furrows which are electrically connected to the perivitelline space. Microelectrode recording from the primitive blastocoel indicates that there is no electrical difference detectable between it and the perivitelline space. These results are discussed in the context of current theories of the mechanism of intercellular electrotonic coupling.     

Role of olivary electrical coupling in cerebellar motor learning

The level of electrotonic coupling in the inferior olive is extremely high, but its functional role in cerebellar motor control remains elusive. Here, we subjected mice that lack olivary coupling to paradigms that require learning-dependent timing. Cx36-deficient mice showed impaired timing of both locomotion and eye-blink responses that were conditioned to a tone. The latencies of their olivary spike activities in response to the unconditioned stimulus were significantly more variable than those in wild-types. Whole-cell recordings of olivary neurons in vivo showed that these differences in spike timing result at least in part from altered interactions with their subthreshold oscillations. These results, combined with analyses of olivary activities in computer simulations at both the cellular and systems level, suggest that electrotonic coupling among olivary neurons by gap junctions is essential for proper timing of their action potentials and thereby for learning-dependent timing in cerebellar motor control.     

Dye and electrical coupling of endothelial cells in situ

Electron microscopic studies show that endothelial cells of pig coronary arteries are linked by gap junctions. We investigated the dye and electrical coupling of these junctions in a strip of pig coronary artery in vitro. The membrane potential of two neighbouring (about 0.2 mm) endothelial cells were simultaneously recorded with two microelectrodes. The fluorescent dye lucifer yellow was microiontophoretically injected through one of the microelectrodes. The endothelial cells in situ were dye and electrically coupled. The dye coupling extended parallel to the longitudinal axis of the arteries. We conclude that an electrical message like the bradykinin and substance P hyperpolarizations of the endothelial cells can be conveyed electrotonically by the endothelium along the longitudinal axis of arteries.     

Effects of electrical coupling on the cone-horizontal cell circuit in the catfish retina

Based on experimental data, a model of the cone-horizontal cell (L-HC) circuit has been developed for the luminosity channel of the catfish retina and impulse responses of cones and L-HC's were replicated for various experimental conditions. Negative feedback from L-HC to the cone pedicle and increases in the dc levels of L-HC (H ⁰), that produce increases in the feedback gain, convert monophasic impulse responses to those that are biphasic, smaller and faster. Electrical coupling of cones and L-HC's lead to decremental spread of 2 radially outgoing waves with time courses of the coupled cones and L-HC's dependent on the spatial organization of the negative feedback circuit: however, the L-HC's impulse response on spreading outward shows an initial increase before decreasing. Interactions of the cone and L-HC waves were studied using Laplace transforms and the convolution theorem. The presence of a negative feedback circuit leads to deviations of the electrotonic decay from an exponential function. As a result of the dependency of the feedback gain on H ⁰, electrical coupling introduces non-linearities in the cone-L-HC circuit that are dependent on the mean illuminance level.     

Regulation of intermuscular electrical coupling by the Caenorhabditis elegans innexin inx-6

The innexins represent a highly conserved protein family, the members of which make up the structural components of gap junctions in invertebrates. We have isolated and characterized a Caenorhabditis elegans gene inx-6 that encodes a new member of the innexin family required for the electrical coupling of pharyngeal muscles. inx-6(rr5) mutants complete embryogenesis without detectable abnormalities at restrictive temperature but fail to initiate postembryonic development after hatching. inx-6 is expressed in the pharynx at all larval stages, and an INX-6::GFP fusion protein showed a punctate expression pattern characteristic of gap junction proteins localized to plasma membrane plaques. Video recording and electropharyngeograms revealed that in inx-6(rr5) mutants the anterior pharyngeal (procorpus) muscles were electrically coupled to a lesser degree than the posterior metacorpus muscles, which caused a premature relaxation in the anterior pharynx and interfered with feeding. Dye-coupling experiments indicate that the gap junctions that link the procorpus to the metacorpus are functionally compromised in inx-6(rr5) mutants. We also show that another C. elegans innexin, EAT-5, can partially substitute for INX-6 function in vivo, underscoring their likely analogous function.     

Cell-to-cell electrical coupling in seminiferous cords of mouse fetal testis

Experiments performed on mouse fetal testes in vitro using electrophysiological techniques and intracellular injections of the fluorescent tracer disodium fluorescein showed that junctional transfer occurs between cells inside the seminiferous cords. This suggests a possible metabolic cooperation between testis cells at an early stage of gonad development.     

Event-driven simulation of neural population synchronization facilitated by electrical coupling

Most neural communication and processing tasks are driven by spikes. This has enabled the application of the event-driven simulation schemes. However the simulation of spiking neural networks based on complex models that cannot be simplified to analytical expressions (requiring numerical calculation) is very time consuming. Here we describe briefly an event-driven simulation scheme that uses pre-calculated table-based neuron characterizations to avoid numerical calculations during a network simulation, allowing the simulation of large-scale neural systems. More concretely we explain how electrical coupling can be simulated efficiently within this computation scheme, reproducing synchronization processes observed in detailed simulations of neural populations.     

Frequency dependence of electrical coupling in Deiters' cells of the guinea pig cochlea

Immunolabeling with antibodies against connexins 26 and 30 showed that, in the guinea pig cochlea, supporting Deiters' cells are massively interconnected and form an orderly network within the organ of Corti. In paired patch-clamp recordings the coupling ratio (CR) of adjacent Deiters' cells at the apex of the cochlea (approximately 0.31) was 3-fold smaller than in isolated cell pairs due to shunting afforded by multicellular connectivity. With sinusoidal current stimuli the delay in signal propagation between adjacent cells increased with increasing frequency whereas the amplitude did not change significantly up to 200 Hz (corner frequency Fc approximately 220 Hz). Depolarizing voltage commands applied to an outer hair cell (OHC) elicited outward potassium currents in the OHC and inward currents in the abutting Deiters' cells, supplying direct evidence for potassium buffering in the organ of Corti. Computational analysis indicates that electrical signals injected into a Deiters' cell are transmitted across a network segment spanning 8 cell diameters. Thus electrical coupling in the organ of Corti is unlikely to influence the selectivity of frequency filtering performed mechanically by the mammalian cochlea.     

Pituitary adenylate cyclase-activating peptide enhances electrical coupling in the mouse adrenal medulla

Neuroendocrine adrenal medullary chromaffin cells receive synaptic excitation through the sympathetic splanchnic nerve to elicit catecholamine release into the circulation. Under basal sympathetic tone, splanchnic-released acetylcholine evokes chromaffin cells to fire action potentials, leading to synchronous phasic catecholamine release. Under elevated splanchnic firing, experienced under the sympathoadrenal stress response, chromaffin cells undergo desensitization to cholinergic excitation. Yet, stress evokes a persistent and elevated adrenal catecholamine release. This sustained stress-evoked release has been shown to depend on splanchnic release of a peptide transmitter, pituitary adenylate cyclase-activating peptide (PACAP). PACAP stimulates catecholamine release through a PKC-dependent pathway that is mechanistically independent of cholinergic excitation. Moreover, it has also been reported that shorter term phospho-regulation of existing gap junction channels acts to increase junctional conductance. In this study, we test if PACAP-mediated excitation upregulates cell-cell electrical coupling to enhance chromaffin cell excitability. We utilize electrophysiological recordings conducted in adrenal tissue slices to measure the effects of PACAP stimulation on cell coupling. We report that PACAP excitation increases electrical coupling and the spread of electrical excitation between adrenal chromaffin cells. Thus PACAP acts not only as a secretagogue but also evokes an electrical remodeling of the medulla, presumably to adapt to the organism's needs during acute sympathetic stress.     

Optical indications of electrical activity and excitation-contraction coupling in the early embryonic heart

Complete understanding of the ontogenesis and early development of electrical activity and its related contraction has been hampered by our inability to apply conventional electrophysiological techniques to the early embryonic heart. Direct intracellular measurement of electrical events in the early embryonic heart is impossible because the cells are too small and frail to be impaled with microelectrodes. Optical signals from voltage-sensitive dyes have provided a new and powerful tool for monitoring changes in membrane potential in a wide variety of living preparations. With this technique it is possible to make optical recordings from cells which are inaccessible to microelectrodes. An additional advantage of the optical method for recording membrane potential activity is that electrical activity can be monitored simultaneously from many sites in a preparation. Thus, applying a multiple-site optical recording method with a 100- or 144-element photodiode array and voltage-sensitive dyes, we have been able to monitor for the first time spontaneous electrical activity in pre-fused cardiac primordia in early chick embryos at the 6- and early 7-somite stages of development; we have been able to determine that the time of initiation of the heartbeat is the middle period of the 9-somite stage. In the rat embryonic heart, the onset of spontaneous electrical activity and contraction occurs at the 3-somite stage. This article describes ionic properties of the spontaneous action potential and genesis of excitation-contraction coupling in the early embryonic chick and rat hearts. In addition, an improved view of the ontogenetic sequence of spontaneous electrical activity and its implications for excitation-contraction coupling in the early embryonic heart are proposed and discussed.     

Effect of intracellular injection of cAMP on the electrical coupling of mammalian cardiac cells

The influence of cAMP on the electrical coupling of canine Purkinje fibers was investigated. It was found that the intracellular injection of the nucleotide enhances the cell-to-cell coupling appreciably. No change in the coupling coefficient (V2/V1) was found with the intracellular injection of 5-AMP. A slight decrease in input resistance (Vo/Io) was produced by cAMP injection and the time constant of the cell membrane (tau m) was also reduced. These findings indicate that the changes in intercellular coupling produced by cAMP were not related to an increase in resistance of the non-junctional membrane but to a decline in junctional resistance. The present results support the view that cAMP plays an important role in the modulation of junctional conductance in cardiac fibers.     

Dye and electrical coupling of endothelial cells in situ

Electron microscopic studies show that endothelial cells of pig coronary arteries are linked by gap junctions. We investigated the dye and electrical coupling of these junctions in a strip of pig coronary artery in vitro. The membrane potential of two neighbouring (about 0.2 mm) endothelial cells were simultaneously recorded with two microelectrodes. The fluorescent dye lucifer yellow was microiontophoretically injected through one of the microelectrodes. The endothelial cells in situ were dye and electrically coupled. The dye coupling extended parallel to the longitudinal axis of the arteries. We conclude that an electrical message like the bradykinin and substance P hyperpolarizations of the endothelial cells can be conveyed electrotonically by the endothelium along the longitudinal axis of arteries.     

Phytochrome control of electrical potentials and intercellular coupling in oat-coleoptile tissue

Summary Microelectrodes were used to demonstrate two electrical responses which occur in oat (Avena sativa L.) coleoptile parenchyma-cells during exposure to red light. The membrane potential of these cells depolarized 5–10 mV in several seconds in red light and repolarized more slowly in far-red light. By pulsing current through the cells, it was found that cellular coupling along the longitudinal axis of the coleoptile increased about 2-fold in red light, but that coupling along the lateral axis was not affected. The rapid changes in membrane potential are consistent with the idea of a membrane locale for early phytochrome action. The coupling experiments suggest that phytochrome may also affect plasmodesmata in this system.