Pharmacological characterisation of Ca2+ channels of the L-type in human peripheral blood lymphocytes

Dihydropyridine Receptors Are Selective Markers of Th2 Cells and Can Be Targeted to Prevent Th2-Dependent Immunopathological Disorders

Th1 cells that produce IFN-gamma are essential in the elimination of intracellular pathogens, and Th2 cells that synthetize IL-4 control the eradication of helminths. However, highly polarized Th1 or Th2 responses may be harmful and even lethal. Thus, the development of strategies to selectively down-modulate Th1 or Th2 responses is of therapeutic importance. Herein, we demonstrate that dihydropyridine receptors (DHPR) are expressed on Th2 and not on Th1 murine cells. By using selective agonists and antagonists of DHPR, we show that DHPR are involved in TCR-dependent calcium response in Th2 cells as well as in IL-4, IL-5, and IL-10 synthesis. Nicardipine, an inhibitor of DHPR, is beneficial in experimental models of Th2-dependent pathologies in rats. It strongly inhibits the Th2-mediated autoimmune glomerulonephritis induced by injecting Brown Norway (BN) rats with heavy metals. This drug also prevents the chronic graft vs host reaction induced by injecting CD4(+) T cells from BN rats into (LEW x BN)F(1) hybrids. By contrast, treatment with nicardipine has no effect on the Th1-dependent experimental autoimmune encephalomyelitis triggered in LEW rats immunized with myelin. These data indicate that 1) DHPR are a selective marker of Th2 cells, 2) these calcium channels contribute to calcium signaling in Th2 cells, and 3) blockers of these channels are beneficial in the treatment of Th2-mediated pathologies.

Non-voltage-gated L-type Ca2+ channels in human T cells: pharmacology and molecular characterization of the major alpha pore-forming and auxiliary beta-subunits

In T lymphocytes, engagement of the antigen receptor leads to a biphasic Ca2+ flux consisting of a mobilization of Ca2+ from intracellular stores followed by a lower but sustained elevation that is dependent on extracellular Ca2+. The prolonged Ca2+ flux is required for activation of transcription factors and for subsequent activation of the T cell. Ca2+ influx requires as yet unidentified Ca2+ channels, which potentially play a role in T cell activation. Here we present evidence that human T cells express a non-voltage-gated Ca2+ channel related to L-type voltage-gated Ca2+ channels. Drugs that block classical L-type channels inhibited the initial phase of the antigen receptor-induced Ca2+ flux and could also inhibit the sustained phase of the Ca2+ signal suggesting a role for the L-type Ca2+ channel in antigen receptor signaling. T cells expressed transcripts for the alpha(1) 1.2 and alpha(1) 1.3 pore-forming subunits of L-type voltage-gated Ca2+ channels and transcripts for all four known beta-subunits including several potential new splice variants. Jurkat T leukemia cells expressed a small amount of full-length alpha(1)1.2 protein but the dominant form was a truncated protein identical in size to a truncated alpha(1) 1.2 protein known to be expressed in B lymphocytes. They further expressed a truncated form of the alpha(1) 1.3 subunit and auxiliary beta1- and beta3-subunit proteins. Our data strongly suggest that functional but non-voltage-gated L-type Ca2+ channels are expressed at the plasma membrane in T cells and play a role in the antigen receptor-mediated Ca2+ flux in these cells.

Nicotinic receptor signaling in nonexcitable cells

The finding that neuronal nicotinic acetylcholine receptors (nAChRs) are present in non-neuronal cells both within and outside the nervous system raises some interesting issues. The mechanisms underlying receptor signaling and its downstream consequences in these cells remain to be elucidated. Factors controlling the release of acetylcholine and the extent of its diffusion are likely to be different for these cells than for traditional neuronal synapses. Recent advances on the physiologic functions of some of these cell types have provided a better insight into possible functional roles for nAChRs in nonexcitable cells. The presence of nAChRs on these cells also implies a broader scope for the actions of nicotine that needs to be considered from a clinical viewpoint. Revealing the potential physiologic roles for nAChRs on nonexcitable cells is likely to provide a more complete understanding of cholinergic signaling.

The effect of boromycin on the Ca2+ homeostasis

A boron-containing antibiotic, boromycin (BM), was found to influence the Ca2+ homeostasis in both excitable and non-excitable cells. In non-excitable cells (human erythrocytes and leucocytes) it inhibited the resting passive 45Ca2+ transport in 10(-6)-10(-5) mol/L concentrations. In human erythrocytes, the passive 15Ca2+ transport induced by the presence of 1 mmol/L NaVO3 was inhibited by boromycin (90% inhibition) as well. The inhibitory effect of BM on the NaVO3-induced passive 45Ca2+ transport was diminished in the presence of inhibitory concentrations of nifedipine (10 micromol/L -60% inhibition) or of those of K+o (75 mmol/L -20% inhibition). On the other hand, in rat brain synaptosomes, and rat cardiomyocytes, BM stimulated the passive 45Ca2+ transport in 'resting' cells at similar concentrations. In rat cardiomyocytes the stimulation was transient. The stimulatory effect on the passive 45Ca2+ transport in rat brain synaptosomes was accompanied with the increase of cytoplasmic Ca2+ concentration measured by means of the entrapped fluorescent Ca2+ chelator fura-2. The stimulatory effect of BM was diminished when synaptosomes were pre-treated with veratridine (10 micromol/L) which itself stimulated the passive 45Ca2+ transport. At saturating concentrations of veratridine, no stimulatory effect of BM was observed. These results could be explained by the indirect interaction of BM with both Ca2+ and Na+ transport systems via transmembrane ionic gradients of monovalent cations and could be useful in determining whether the cells belong to excitable, or non-excitable cells.

Phosphatidylinositol 3-kinase-dependent extracellular calcium influx is essential for CX(3)CR1-mediated activation of the mitogen-activated protein kinase cascade

Fractalkine, the first member of the CX(3)C chemokine family, induces leukocyte chemotaxis through activation of its high affinity receptor, CX(3)CR1. Like other chemokine receptors, CX(3)CR1 is coupled to a pertussis toxin-sensitive heterotrimeric G(i) protein, which is necessary for rapid rise in the concentration of intracellular calcium. Using a Chinese hamster ovary cell line stably transfected with the CX(3)CR1 receptor, we show that the source of calcium mobilized by fractalkine stimulation is the extracellular pool. Calcium influx is blocked by extracellular calcium chelators, as well as by divalent heavy metals such as Ni(2+), Co(2+), and Cd(2+) without affecting the integrity of intracellular stores. Remarkably, selective phosphoinositide 3-kinase (PI3K) inhibitors, wortmannin and LY294002, abolish the wave extracellular calcium, suggesting that an active PI3K is necessary for this event. The influx of extracellular calcium is in turn required to trigger the activation of the p42/44 mitogen-activated protein/extracellular signal-regulated kinase pathway, but is not necessary for other signals downstream to PI3K, such as phosphorylation of Akt. The potential role of this signaling cascade in fractalkine-mediated chemotaxis is discussed.