A novel type of Ca2+ channel in U-87 MG cells activated by anti-galactocerebroside

Voltage-dependent ionic conductances in the human malignant astrocytoma cell line U87-MG

Although the human malignant astrocytoma cell line U87-MG has been used in numerous studies, few findings are available on the properties of its membrane ion conductances. Characterization of the ion channels expressed in these cells will make it possible to study membrane ion conductance changes when a receptor is activated by its ligand. This will help to elucidate the functional properties of these receptors and their signal-transduction pathways in pathophysiological events. This work studied the voltage-dependent ionic conductances of U87-MG cells using the Whole-Cell Recording patch-clamp technique. Six types of voltage-dependent ionic currents were identified: (i) a TEA-, 4-AP- and CTX-sensitive Ca2+-dependent K+ current, (ii) a transient K+ current inhibited by 4-AP, (iii) an inwardly rectifying K+ current blocked by Ba2+ and 4-AP, (iv) a DIDS- and SITS-sensitive Cl- current, (v) a TTX-sensitive Na+ conductance and (vi) a L-type Ca2+ conductance activated by BayK-8644 and inhibited by Ni and the L-type Ca2+ channel inhibitor, nifedipine. In addition, electrical depolarizations elicited inward currents due to voltage-independent, Ca2+-dependent K+ influx against the electrochemical gradient, probably via an ouabain-sensitive Na+-K+ pump.

Co-activation of P2Y2 receptor and TRPV channel by ATP: implications for ATP induced pain

1. Extracellular ATP is recognized as a peripheral modulator of pain. Activation of ionotropic P2X receptors in sensory neurons has been implicated in induction of pain, whereas metabotropic P2Y receptors in potentiation of pain induced by chemical or physical stimuli via capsaicin sensitive TRPV1 channel. Here we report that P2Y2 receptor activation by ATP can activate the TRPV1 channel in absence of any other stimuli. 2. ATP-induced Ca2+ signaling was studied in Neuro2a cells. ATP evoked release of intracellular Ca2+ from ER and Ca2+ influx through a fast inactivating channel. The Ca2+ response was induced by P2Y receptor agonists in the order of potency ATP>or=UTP>or=ATPgammaS>ADP and was inhibited by suramin and PPADS. The P2X receptor agonist alpha beta methyl ATP was ineffective. 3. The Ca2+ influx was blocked by ruthenium red, an inhibitor of TRPV1 channel. Capsaicin, the most potent activator of the TRPV1 channel, evoked a fast inactivating Ca2+ transient suggesting the presence of endogenous TRPV1 channels in Neuro2a cells. NMS and PDBu, repressors of IP3 formation, drastically inhibited both the components of Ca2+ response. 4. Our data show co-activation of the P2Y2 receptor and capsaicin sensitive TRPV1 channel by ATP. Such functional interaction between endogenous P2Y2 receptor and TRPV1 channels could explain the ATP-induced pain.

Characterization of cholera toxin B subunit-induced Ca(2+) influx in neuroblastoma cells: evidence for a voltage-independent GM1 ganglioside-associated Ca(2+) channel

The role of endogenous GM1 ganglioside in neurite outgrowth has been studied in N18 and NG108-15 neuroblastoma cells with the GM1-specific ligand cholera toxin B subunit (Ctx B), which stimulates Ca(2+) influx together with neuritogenesis. Our primary goal has been to identify the nature of the calcium channel that is modulated by GM1. An L-type voltage-operated Ca(2+) channel (VOCC) was previously proposed as the mediator of this phenomenon. This investigation, employing fura-2 fluorescent measurements and specific channel blockers and other agents, revealed that GM1 modulates a hitherto unidentified Ca(2+) channel not of the L type. It was opened by Ctx B; was permeable to Ca(2+) and Ba(2+) but not Mn(2+); and was blocked by Ni(2+), Cd(2+), and La(3+). Although most dihydropyridines inhibited Ctx B-induced Ca(2+) influx as well as neurite outgrowth at higher concentrations, they and other VOCC blockers at normally employed concentrations failed to do so, suggesting uninvolvement of VOCC. In addition, Ca(2+) influx induced by Ctx B was not mediated by cGMP-dependent or G-protein-coupled nonselective cation channels, as demonstrated by the cGMP antagonist Rp-cGMPS or the G-protein/receptor uncoupling agent suramin, respectively. Finally, Ca(2+) influx was unlikely to be due to inhibition or reversal of Na(+)-Ca(2+) exchanger via Ctx B induction of Na(+) uptake, insofar as no effect was seen on blocking Na(+) channels, inhibiting Na(+)-K(+)-ATPase, or eliminating extracellular Na(+). The results suggest that this novel channel is gated by interaction with GM1, which, when associated with the channel and bound by appropriate ligand, promotes Ca(2+) influx. This in turn induces signaling for the onset of neuritogenesis.

Negative regulation of oligodendrocyte differentiation by galactosphingolipids

Galactocerebroside and sulfatide, major galactosphingolipid components of oligodendrocyte plasma membranes and myelin, are first expressed at a critical point, when progenitors cease to proliferate and commence terminal differentiation. We showed previously that an antibody to galactocerebroside/sulfatide arrested terminal differentiation, suggesting a role for these galactolipids in oligodendrocyte differentiation. We have now investigated the differentiation of oligodendrocytes (1) in response to other anti-galactolipid antibodies, showing that anti-sulfatide O4 but not anti-galactocerebroside O1 blocks terminal differentiation, perhaps by mimicking an endogenous ligand, and (2) in a transgenic mouse unable to synthesize these lipids because of mutation of the gene for ceramide galactosyltransferase, a key enzyme for galactosphingolipid synthesis. We find that galactosyltransferase mRNA expression begins at the late progenitor [pro-oligodendroblast (Pro-OL)] stage of the lineage and that the late progenitor marker pro-oligodendroblast antigen is not synthesized in the absence of galactosyltransferase. The principal outcome of the elimination of these galactolipids is a two- to threefold enhancement in the number of terminally differentiated oligodendrocytes both in culture and in vivo. Because the general pattern of differentiation and the level of progenitor proliferation and survival appear to be unaltered in the mutant cultures, we conclude that the increased number of oligodendrocytes is caused by an increased rate and probability of differentiation. In agreement with these two experimental approaches, we present a model in which galactosphingolipids (in particular galactocerebroside and/or sulfatide) act as sensors and/or transmitters of environmental information, interacting with endogenous ligands to function as negative regulators of oligodendrocyte differentiation, monitoring the timely progress of Pro-OLs into terminally differentiating, myelin-producing oligodendrocytes.