Diameter of the cell-to-cell junctional membrane channels as probed with neutral molecules

Metabolic cooperation between argininosuccinate synthetase and argininosuccinate lyase deficient human fibroblasts

Human fibroblasts deficient in either argininosuccinate synthetase or argininosuccinate lyase show low levels of incorporation of [14C]citrulline into protein. However, when these two cell types are co-cultured [14C]citrulline incorporation is restored to the levels found in control cultures. This metabolic cooperation is cell-density-dependent and does not occur by diffusion of argininosuccinate into the medium. Our results indicate that argininosuccinate passes between the two cell types via intercellular junctions, and this system provides a simple and accurately quantifiable model for the study of intercellular communication.

Correction of cell-cell communication defect by introduction of a protein kinase into mutant cells

The cell-to-cell permeability of the junctions of various cultured mammalian cell types depends on the concentration of intracellular cyclic AMP [( cAMP]i). The permeability rises when the cells are supplied with exogenous cyclic AMP or when their cyclic AMP synthesis is stimulated with choleragen or hormones; it falls when [cAMP]i is lowered by application of serum or due to increase in cell density. The rise and fall in permeability take several hours to develop (the rise is protein synthesis-dependent) and they occur concurrently with the rise and fall in the number of intramembrane particles of the gap junctions, which probably embody the cell-to-cell channels. Is this permeability regulation mediated by phosphorylating protein kinase? In many eukaryotes, the cyclic AMP receptor is a protein kinase consisting of a pair of regulatory subunits and a pair of catalytic subunits. The latter dissociate from the holoenzyme as the cyclic AMP binds to the regulatory subunits and, in this dissociated form, catalyse the phosphorylation of the target. The regulatory subunit occurs in two isoenzyme forms, I and II. The catalytic subunit seems invariant; subunits from different isoenzymes can substitute for each other. We show here that a mutant cell lacking the isoenzyme I is deficient in permeable junctions, and that this junctional defect is corrected when the mutant is supplied with exogenous catalytic subunit.

Cell-to-cell channels with two independently regulated gates in series: analysis of junctional conductance modulation by membrane potential, calcium, and pH

We study cell-to-cell channels, in cell pairs isolated from Chironomus salivary gland, by investigating the dependence of junctional conductance (gj) on membrane potentials (E1, E2), on Ca2+, and on H+, and we explore the interrelations among these dependencies; we use two separate voltage clamps to set the membrane potentials and to measure gj. We find gj to depend on membrane potentials whether or not a transjunctional potential is present. The pattern of gj dependence on membrane potentials suggests that each channel has two closure mechanisms (gates) in series. These gates pertain, respectively, to the two cell faces of the junction. By treating the steady-state gj as the resultant of two simultaneous but independent voltage-sensitive open/closed equilibria, one within each population of gates (i.e., one on either face of the junction), we develop a model to account for the steady-state gj vs. E relationship. Elevation of cytosolic Ca2+ or H+ at fixed E lowers gj, but at moderate concentrations of these ions this effect can be completely reversed by clamping to more negative E. Overall, the effect of a change in pCai or pHi takes the form of a parallel shift of the gj vs. E curve along the E axis, without change in slope. We conclude (1) that the patency of a cell-to-cell channel is determined by the states of patency of its two gates; (2) that the patency of the gates depends on membrane potentials (not on transjunctional potential), on pCai, and on pHi; (3) that pCai and pHi determine the position of the gj vs. E curve on the E axis; and (4) that neither Ca2+ nor H+ at moderate concentrations alters the voltage sensitivity of gj.

Effect of deuterium oxide on junctional membrane channel permeability

The effect of deuterium oxide on junctional membrane permeability to dichlorofluorescein was examined to determine the mode of transfer of the dye from one cell interior to another in the septate giant axon of earthworm. Dichlorofluorescein was shown to diffuse through the nexus passively and in a hydrated form. Additionally, evidence suggested an alteration of the cell-to-cell channel structure by deuterium/hydrogen exchange. Dichlorofluorescein was rendered impermeant at 6 degrees C in D2O and 4 degrees C in H2O. Action potentials, however, were capable of propagation from cell to cell at 4 degrees C in D2O and H2O. The results are consistent with a hydrophilic channel where solute molecules diffuse through the junction (nexus) in a hydrated form. The temperature blocks are presumably brought about by increasing hydration shells around solute and channel proteins with cooling until the solute is rendered too large to diffuse.

Alamethicin and related membrane channel forming polypeptides

Alamethicin and several related microbial polypeptides, which contain a high proportion of alpha-aminoisobutyric acid (Aib) residues, possess the ability to modify the permeability properties of phospholipid bilayer membranes. Alamethicin induces excitability phenomena in model membranes and has served as an excellent model for the study of voltage sensitive transmembrane channels. This review summarizes various aspects of the structural chemistry and membrane modifying properties of alamethicin and related Aib containing peptides. The presence of Aib residues in these sequences, constrains the polypeptides to 3(10) or alpha-helical conformations. Functional membrane channels are formed by aggregation of cylindrical peptide helices, which span the lipid bilayer, forming a scaffolding for an aqueous column across the membrane. After consideration of the available data on the conductance characteristics of alamethicin channels, a working hypothesis for a channel model is outlined. Channel aggregates in the lipid phase may be stabilized by intermolecular hydrogen bonding, involving a central glutamine residue and also by interactions between the macro-dipoles of proximate peptide helices. Fluctuations between different conductance states are rationalized by transitions between states of different aggregation and hence altered dimensions of the aqueous core or by changes in net dipole moment of the aggregate. Ion fluxes through the channel may also be affected by the electric field within the aqueous core.

Biological implications of gap junction structure, distribution and composition: a review

Traditionally, all gap junctions have been considered to be identical in structure and function throughout the animal kingdom. Functions ascribed to these membrane specializations have been fundamental and have not been thought to differ significantly with respect to their mechanism of action. More recent studies support the view, however, that structural and compositional diversity may reflect significant functional differences between gap junctions in different classes of tissue but no clear and definitive patterns have yet emerged. This review does not attempt to comprehensively analyze the totality of the vast gap junction and coupling literature but focuses instead upon those recent observations which raise new questions related to the biological activities of gap junctions in different tissues.

Cellular and paracellular resistances of the Necturus proximal tubule

Individual resistances of the apical cell membrane Ra, the basolateral cell membrane, Rbl, and the paracellular shunt, Rs, were determined in the Necturus proximal tubule using a set of three electrical parameters. Four electrical parameters were measured: the transepithelial resistance, (Rte), the apical and basolateral cell membrane resistance in parallel, (Rz free-flow tubules), the basolateral cell membrane resistance in oil-filled tubules, (Rz oil-filled), and the ratio of apical and basolateral cell membrane resistance (Ra/Rbl). Rte was determined from an analysis of the spatial decay of luminal voltage following luminal current injection. Rz free-flow and Rz oil-filled were measured by the analysis of the spatial decay of intracellular voltage deflections following cellular current injection in free flow and oil-filled tubules, respectively. Ra/Rbl was estimated from the ratio of voltage deflections across the apical and basolateral cell membranes following transepithelial current injection. In addition, the magnitude of cellular and luminal cable interactions was evaluated, by comparing the spatial decay of voltage deflections in the cell and in the lumen following intracellular current injection. The combined cell membrane resistance (Ra + Rbl) is between one to two orders of magnitude greater than the paracellular resistance. This result supports the view that the Necturus proximal tubule is a leaky epithelium.

Hormonal regulation of cell junction permeability: upregulation by catecholamine and prostaglandin E1

By cellular activation with hormones, we test the proposition (Loewenstein, W.R., Physiol. Rev. 61:829, 1981) that the permeability of cell junction is upregulated through elevation of the level of cyclic AMP. Cultured rat glioma C-6 cells, with beta-adrenergic receptors, and human lung WI-38 cells, with prostaglandin receptors, were exposed to catecholamine (isoproterenol) and prostaglandin E1, respectively, while their junctions were probed with microinjected fluorescent-labelled mono-, di-, and triglutamate. Junctional permeability, as indexed by the proportion of cell interfaces transferring the probes, rose after the hormones treatments. The increase in permeability took several hours to develop and was associated with an increase in the number of gap-junctional membrane with an increase in the number of gap-junctional membrane particles (freeze-fracture electron microscopy). Such interaction between hormonal and junctional intercellular communication may provide a mechanism for physiological regulation of junctional communication and (perhaps as part of that) for physiological coordination of responses of cells in organ and tissues to hormones.

Cell junction and cycle AMP: III. Promotion of junctional membrane permeability and junctional membrane particles in a junction-deficient cell type

The cyclic nucleotide effect on junction was studied in C1-1D cells, a mouse cancer cell type that fails to make permeable junctions in ordinary confluent culture. Upon administration of cyclic AMP, dibutyryl cyclic AMP, dibutyryl cyclic AMP plus caffeine (db-cAMP-caffeine), or cholera toxin (an adenylate cyclase activator), the cells acquired permeable junctions; they became electrically coupled and transferred fluorescent tracer molecules among each other - a transfer exhibiting the molecular size limit of permeation of normal cell-to-cell channels. The effect took several hours to develop. With the db-cAMP-caffeine treatment, junctional permeability emerged within two hours in one-fifth of the cell population, and within the next few hours in the entire population. This development was not prevented by the cytokinesis inhibitor cytochalasin B. Permeable junctions formed also in two other conditions where the cell-endogenous cyclic AMP level may be expected to increase: serum starvation and low cell density. After three weeks of starving, the cells of serum, a junctional permeability arose in confluent cultures, which on feeding with serum disappeared within two to three days. At low cell density, namely below confluency, the cells made permeable junctions, unstarved. In cultures of rather uniform density, the frequency of permeable junctions was inversely related to the average density, over the subconfluent range; at densities of about 1 X 10(4) cells/cm2, where the cells had few mutual contacts, 80% of the pairs presumed to be in contact were electrically coupled. In cultures with adjoining territories of high (confluent) and low cell density, there was coupling only in the last, and in this low-density state the cells were also capable of coupling with other mammalian cell types (mouse 3T3-BalbC and human Lesch-Nyhan cells).

Cell junction and cyclic AMP: II. Modulations of junctional membrane permeability, dependent on serum and cell density

Junctional molecular transfer (as indexed by the number of cell interfaces transferring fluorescent-labelled molecules) and concentration of endogenous cAMP were determined in mammalian cells in culture at varying serum concentration and cell density. In several cell types, on stepping the serum concentration from 10% (the concentration to which the cells had been adapted) to zero, the junctional transfer rose (reversibly) within 48 hr, as the endogenous cAMP concentration rose. The junctional transfer was inversely related to serum concentration over a range, most steeply so the transfer of large and charged molecules. One cell type showed no junctional change in response to serum; it showed also no endogenous cAMP change. Junctional transfer varied inversely with cell density over the range of 0.7-7 (10(4) cells/cm2) in 3T3 cells. In cultures seeded to various densities, or growing to various densities on their own, junctional transfer fell with rising density, and so did the endogenous cAMP concentration. Upon downstep from high density, junctional transfer was independent of cell density over the aforementioned range, and so was the endogenous cAMP concentration. These results, in conjunction with the effects of exogenous cAMP described in the preceding paper of this series, point to a cAMP-mediated junctional effect; a possible teleonomy for control of membrane junction is discussed.

Cell junction and cyclic AMP: 1. Upregulation of junctional membrane permeability and junctional membrane particles by administration of cyclic nucleotide or phosphodiesterase inhibitor

Mammalian cells in culture were exposed to cyclic AMP, dibutyryl cyclic AMP, the phosphodiesterase inhibitor caffeine, or a combination of the last two, while junctional molecular transfer was probed with the series of microinjected, fluorescent-labelled linear molecules Glu, Glu-Glu, Glu-Glu-Glu, and Leu-Leu-Leu-Glu-Glu. The junctional permeability for these molecules increased with each of the agents, most markedly with the dibutyryl cyclic AMP-caffeine combination, as the intracellular cyclic nucleotide concentration rose. The junctional permeability effect developed over several hours. When probed with molecules close to the limit of cell-to-cell channel permeation (the most sensitive setting), the effect was detectable both, as an increase in the (relative) junctional transit rate and as an increase in the number of transferring cell interfaces in the test populations. The number of transferring cell interfaces reached a maximum by 4 hr, when the junctional transit rate, hence the junctional permeability, was still rising. Nonjunctional membrane permeability for the probe molecules, as determined by intracellular fluorescence loss, was not significantly changed (nor was there significant nonjunctional cell-to-cell transfer of molecules before or after the treatments). The rise in junctional permeability was associated with an increase in the number of gap junctional membrane particles, as determined by freeze-fracture electron microscopy: the average size of the particle clusters increased, and the frequency of the clusters increased, particularly that of the smaller (and presumably newer) clusters. This effect was blocked by treatments with the protein synthesis inhibitors cycloheximide or puromycin. These agents caused particle diminution (diminution of cluster frequency but not of average cluster size), with or without cyclic nucleotide. The junctional effects may represent a cyclic AMP-promoted proliferation of cell-to-cell channels. Some physiological implications, in particular, implications for hormone-regulated tissues, are discussed.