Calcium-induced uncoupling in communicating human lymphocytes

Gap junctions in the cardiovascular and immune systems

Gap junctions are clusters of intercellular channels directly connecting the cytoplasm of adjacent cells. These channels are formed by proteins named connexins and are present in all metazoan organisms where they serve diverse functions ranging from control of cell growth and differentiation to electric conduction in excitable tissues. In this overview we describe the presence of connexins in the cardiovascular and lympho-hematopoietic systems giving the reader a summary of the topics to be covered throughout this edition and a historical perspective of the discovery of gap junctions in the immune system.

Gap junctions in cells of the immune system: structure, regulation and possible functional roles

Gap junction channels are sites of cytoplasmic communication between contacting cells. In vertebrates, they consist of protein subunits denoted connexins (Cxs) which are encoded by a gene family. According to their Cx composition, gap junction channels show different gating and permeability properties that define which ions and small molecules permeate them. Differences in Cx primary sequences suggest that channels composed of different Cxs are regulated differentially by intracellular pathways under specific physiological conditions. Functional roles of gap junction channels could be defined by the relative importance of permeant substances, resulting in coordination of electrical and/or metabolic cellular responses. Cells of the native and specific immune systems establish transient homo- and heterocellular contacts at various steps of the immune response. Morphological and functional studies reported during the last three decades have revealed that many intercellular contacts between cells in the immune response present gap junctions or "gap junction-like" structures. Partial characterization of the molecular composition of some of these plasma membrane structures and regulatory mechanisms that control them have been published recently. Studies designed to elucidate their physiological roles suggest that they might permit coordination of cellular events which favor the effective and timely response of the immune system.

Functional gap junctions in thymic epithelial cells are formed by connexin 43

A multiparametric study was carried out to investigate the presence and possible role of communicating junctions in the thymus, particularly in the thymic epithelium, the major component of the thymic microenvironment. The presence of direct cell-cell communication mediated by gap junctions was demonstrated in human and murine thymic epithelial cells (TEC) by means of in situ and in vitro immunohistochemical labeling as well as in vitro fluorochrome injection and double whole-cell patch clamp experiments. Moreover, both immuno- and Northern blot studies revealed that the gap junction protein connexin 43 and its mRNA were present in TEC. Importantly, we showed that thymic endocrine activity, as ascertained by thymulin production, could be specifically down-modulated in vitro by a gap junction inhibitor, octanol. We also investigated the existence of gap junctions between TEC and thymocytes. In thymic nurse cells we were able to detect cell-cell communication, although only a minor percentage of epithelial/thymocyte pairs were coupled in a given moment. In contrast, intercellular communication was not detected between cultured phagocytic cells of the thymic reticulum and the respective rosetting thymocytes. We suggest that gap junctions formed by connexin 43 may represent a novel (and rather cell type-specific) pathway for intrathymic cellular communication, including TEC/TEC as well as possible TEC/thymocyte interactions.

Effect of cations and cation channel blockers on human natural killer cells

We studied the influence of various extracellular Ca++ and Mg++ concentrations on human natural killer (NK) cells and found that NK cell-mediated target cell lysis requires the presence of both divalent cations. The calcium channel blocker verapamil and the local anaesthetics procaine and lidocaine inhibited NK cell-mediated killing of K 562 target cells, whereas the selective K+ channel blocker tetraethylammonium and the selective Na+ channel blocker tetrodotoxin had no effect on NK activity. Our results demonstrate an important role of extracellular Ca++ and Mg++ for NK cell-mediated killing and suggest that a free transmembrane Ca++ passage is required for target cell lysis.

Effects of valinomycin on lymphocytes independent of potassium permeability

10(-7) M valinomycin affects human lymphocytes in the following manner: (1) it is non-toxic; (2) it inhibits mitogenesis; (3) it causes a reduction in cell ATP; and (4) it causes a marked increase in steady-state Na+ exchange. However, it has a minimal effect on cell ion (K+, Na+, Ca2+, Mg2+) contents and no effect whatever on K+ exchange. Neither the fast nor the slow fraction of steady-state K+ exchange is affected by 10(-7) M valinomycin. The various reported effects of valinomycin on lymphocyte functions cannot be assumed to be due to changes in plasma membrane K+ permeability. The mechanism of the increase in steady-state Na+ exchange, and whether or not it is related to inhibition of mitogenesis, are unsettled issues.

Electrophysiology of phagocytic membranes. III. Evidence for a calcium-dependent potassium permeability change during slow hyperpolarizations of activated macrophages

The roles of potassium and calcium in the slow hyperpolarizations of membranes of activated macrophages are investigated using standard intracellular electrical recording techniques. The amplitude of spontaneous slow hyperpolarizations decreases as a logarithmic function of the external potassium concentration in the culture medium. Similar dependence on the potassium gradient is observed when different levels of membrane potentials are imposed by constant current injection. The reversal potential for electrically evoked slow hyperpolarizations is -90 mV. A 10-fold increase in external potassium concentration causes a 60 mV shift of the reversal potential towards zero. Divalent cation ionophores (A23187 and X537A) can induce slow hyperpolarization responses in quiescent cells or permanent hyperpolarization in spontaneously active cells. The amplitude of the ionophore-induced hyperpolarizations is reduced by an increase in external potassium concentration in a manner consistent with data on slow hyperpolarization responses in the absence of ionophore. The calcium antagonist, verapamil, depresses the slow hyperpolarization responses at the concentration of 10(-5) M. It is suggested that the development of the hyperpolarizing response is due to a calcium-dependent potassium channel. The data support the assumption that spontaneous and artificially elicited slow hyperpolarization responses share a common calcium-dependent mechanism.

Lymphocyte membrane potential assessed with fluorescent probes

The membrane potential of mouse spleen lymphocytes has been assessed with two fluorescent probes. 3,3'-Dipropylthiadicarbocyanine (diS-C3-(5)) was used for most of the experiments. Solutions with high K+ concentrations depolarised the cells. Valinomycin, an inophore which adds a highly K+-selective permeability membranes, slightly hyperpolarised cells in standard (6 mM K+) solution, and in 145 mM K+ solution produced a slight additional depolarisation. These findings indicate a membrane whose permeability is relatively selective for K+. Very small changes in potential were seen when choline replaced Na+, or gluconate replaced Cl-, supporting the idea of K+ selectivity. The resting potential could be estimated from the K+ concentration gradient at which valinomycin did not change the potential-the "valinomycin null point" - and under the conditions used the resting potential was approx.-60 mV. B cell-enriched suspensions were prepared either from the spleens of nu/nu mice or by selective destruction of T cells in mixed cell populations. The membrane potential of these cells was similar to that estimated for the mixed cells. In solution with no added K+, diS-C3-(5) itself appeared to depolarise the lymphocytes, in a concentration dependent manner. With the 100 nM dye normally used, the membrane potential in K+-free solution was around -45 mV, and 500 nM dye almost completely depolarised the cells. In standard solution quinine depolarised the cells. Valinomycin could still depolarise these cells indicating that depolarisation had not been due to dissipation of the K+ gradient. Since in K+-free solution diS-C3-(5) blocks the Ca2+-activated K+ channels in human red blood cell ghosts and quinine also blocks this K+ channel it is suggested that the resting lymphocyte membrane may have a similar Ca2+-activated K+ permeability channel. Because of the above mentioned effect of diS-C3-(5) and other biological side effects, such as inhibition of B cell capping, a chemically distinct fluorescent probe of membrane potential, bis(1,3-diethylthiobarbiturate)-trimethineoxonol was used to support the diS-C3-(5) data. This new probe proved satisfactory except that it formed complexes with valinomycin, ruling out the use of this ionophore. Results with the oxonol on both mixed lymphocytes and B cell-enriched suspensions gave confirmation of the conclusions from diS-C3-(5) experiments and indicated that despite its biological side effects, diS-C3-(5) could still give valid assessment of membrane potential.

Calcium metabolism in intact isolated thymocytes

Isolated rat thymocytes incubated under proper metabolic conditions extrude Ca2+ previously taken up under metabolically unfavourable conditions. The extrusion can be supported by both respiratory and glycolytic energy but glycolysis seems to be more efficient for this purpose. La3+ (50--200 micron) and the ionophore A 23187 inhibit cell Ca2+ extrusion. Ruthenium Red (1--100 micron) does not influence cell Ca2+ extrusion while it inhibits the in situ mitochondrial cation uptake. All the results are consistent with a cell regulation model of Ca2+ content in which both plasma membrane and mitochondria co-operate, acting in opposite directions, in order to decrease cytosolic Ca2+ concentration. The possibility of Na+-Ca2+ hetero-exchange participation to cell Ca2+ homeostasis regulation is also discussed.

Cell-to-cell communication and myogenesis

Cell-to-cell communication was characterized in prefusion chick embryo myoblast cultures, and it was determined that the prefusion myoblasts can interact via gap junctions, ionic coupling, and metabolic coupling. The biological relevance of this communication was supported by the detection of gap junctions between myoblasts in embryonic muscle. Communication was also examined in fusion-arrested cultures to determine its potential relationship to fusion competency. In cultures that were fusion arrested by treatment with either 1.8 mM ethyleneglycolbis-(beta-aminoethyl ether)N,N'-tetraacetic acid (EGTA), 3.3 X 10(-6) M 5-bromodeoxyuridine (BUdR), or 1 microgram/ml cycloheximide (CHX), both gap junctions and ionic coupling were present. Therefore, it is possible to conclude that cell communication is not a sufficient property by itself, to generate fusion between myob-asts. The potential role of communication in myogenesis is discusssed with respect to these observations.