In vitro changes in capacitative Ca2+ entry in neuroblastoma X glioma NG108-15 cells

Neuronal Store-Operated Calcium Channels

The endoplasmic reticulum (ER) is the major intracellular calcium (Ca²⁺) storage compartment in eukaryotic cells. In most instances, the mobilization of Ca²⁺ from this store is followed by a delayed and sustained uptake of Ca²⁺ through Ca²⁺-permeable channels of the cell surface named store-operated Ca²⁺ channels (SOCCs). This gives rise to a store-operated Ca²⁺ entry (SOCE) that has been thoroughly investigated in electrically non-excitable cells where it is the principal regulated Ca²⁺ entry pathway. The existence of this Ca²⁺ route in neurons has long been a matter of debate. However, a growing body of experimental evidence indicates that the recruitment of Ca²⁺ from neuronal ER Ca²⁺ stores generates a SOCE. The present review summarizes the main studies supporting the presence of a depletion-dependent Ca²⁺ entry in neurons. It also addresses the question of the molecular composition of neuronal SOCCs, their expression, pharmacological properties, as well as their physiological relevance.

From neuroepithelial cells to neurons: changes in the physiological properties of neuroepithelial stem cells

The central nervous system, which includes the spinal cord, retina, and brain, is derived from the neural tube. The neural tube is formed of a sheet of cells called the neuroepithelium. During embryonic development, neuroepithelial cells function as neural stem cells: they renew themselves while undergoing interkinetic nuclear movements along the apico-basal axis during the cell cycle, and they produce postmitotic cells that function as newborn neurons. Neuroepithelial cells exhibit a robust increase in nucleoplasmic [Ca(2+)] in response to G protein-coupled receptor activation during S-phase when the nucleus is located in the basal region of the cell. This Ca(2+) rise is caused by the release of Ca(2+) from intracellular Ca(2+) stores, and the Ca(2+) release in turn activates Ca(2+) entry from the extracellular space, which is called capacitative (or store-operated) Ca(2+) entry. The Ca(2+) release and store-operated Ca(2+) entry are essential for DNA synthesis during S-phase. The activity of this store-operated Ca(2+) signaling system declines in parallel with the decreasing proliferative activity of neuroepithelial cells. When exiting the cell cycle, the cells lose the apical process where gap junctions are located. Following the loss of gap junction coupling, the postmitotic cells show a high input resistance, which allows them to be readily depolarized. The Ca(2+) response to the excitatory neurotransmitter glutamate appears and develops during neuronal differentiation. The glutamate-induced Ca(2+) rise increases transiently during natural cell death (apoptosis). The rise in Ca(2+) levels mediated by voltage-gated Ca(2+) channels also develops during neuronal differentiation. Thus, when neuroepithelial cells differentiate into neurons, a transition from a store-operated system to a voltage-operated system occurs in the main Ca(2+) signaling system. This transition may reflect a change in the mode of intercellular communication from a stored Ca(2+)-dependent mode to a plasma membrane potential-dependent mode.

Ion channel activities in neural stem cells of the neuroepithelium

During the embryonic development of the central nervous system, neuroepithelial cells act as neural stem cells. They undergo interkinetic nuclear movements along their apico-basal axis during the cell cycle. The neuroepithelial cell shows robust increases in the nucleoplasmic [Ca²⁺] in response to G protein-coupled receptor activation in S-phase, during which the nucleus is located in the basal region of the neuroepithelial cell. This response is caused by Ca²⁺ release from intracellular Ca²⁺ stores, which are comprised of the endoplasmic reticulum and the nuclear envelope. The Ca²⁺ release leads to the activation of Ca²⁺ entry from the extracellular space, which is called capacitative, or store-operated Ca²⁺ entry. These movements of Ca²⁺ are essential for DNA synthesis during S-phase. Spontaneous Ca²⁺ oscillations also occur synchronously across the cells. This synchronization is mediated by voltage fluctuations in the membrane potential of the nuclear envelope due to Ca²⁺ release and the counter movement of K⁺ ions; the voltage fluctuation induces alternating current (AC), which is transmitted via capacitative electrical coupling to the neighboring cells. The membrane potential across the plasma membrane is stabilized through gap junction coupling by lowering the input resistance. Thus, stored Ca²⁺ ions are a key player in the maintenance of the cellular activity of neuroepithelial cells.

Mechanistic and functional changes in Ca2+ entry after retinoic acid-induced differentiation of neuroblastoma cells

We have investigated effects of neuronal differentiation on hormone-induced Ca2+ entry. Fura-2 fluorescence measurements of undifferentiated SH-SY5Y neuroblastoma cells, stimulated with methacholine, revealed the presence of voltage-operated Ca2+-permeable, Mn2+-impermeable entry pathways, and at least two voltage-independent Ca2+- and Mn2+-permeable entry pathways, all of which apparently contribute to both peak and plateau phases of the Ca2+ signal. Similar experiments using 9-cis retinoic acid-differentiated cells, however, revealed voltage-operated Ca2+-permeable, Mn2+-impermeable channels, and, more significantly, the absence or down-regulation of the most predominant of the voltage-independent entry pathways. This down-regulated pathway is probably due to CCE (capacitative Ca2+ entry), since thapsigargin also stimulated Ca2+ and Mn2+ entry in undifferentiated but not differentiated cells. The Ca2+ entry components remaining in methacholine-stimulated differentiated cells contributed to only the plateau phase of the Ca2+ signal. We conclude that differentiation of SH-SY5Y cells results in a mechanistic and functional change in hormone-stimulated Ca2+ entry. In undifferentiated cells, voltage-operated Ca2+ channels, CCE and NCCE (non-CCE) pathways are present. Of the voltage-independent pathways, the predominant one appears to be CCE. These pathways contribute to both peak and plateau phases of the Ca2+ signal. In differentiated cells, CCE is either absent or down-regulated, whereas voltage-operated entry and NCCE remain active and contribute to only the plateau phase of the Ca2+ signal.

Suppression of EGF-induced cell proliferation by the blockade of Ca2+ mobilization and capacitative Ca2+ entry in mouse mammary epithelial cells

The role of intracellular Ca2+ stores and capacitative Ca2+ entry on EGF-induced cell proliferation was investigated in mouse mammary epithelial cells. We have previously demonstrated that EGF enhances Ca2+ mobilization (release of Ca2+ from intracellular Ca2+ stores) and capacitative Ca2+ entry correlated with cell proliferation in mouse mammary epithelial cells. To confirm their role on EGF-induced cell cycle progression, we studied the effects of 2,5-di-tert-butylhydroquinone (DBHQ), a reversible inhibitor of the Ca2+ pump of intracellular Ca2+ stores, and SK&F 96365, a blocker of capacitative Ca2+ entry, on mitotic activity induced by EGF. Mitotic activity was examined using an antibody to PCNA for immunocytochemistry. SK&F 96365 inhibited capacitative Ca2+ entry in a dose-dependent manner (I50: 1-5 microM). SK&F 96365 also inhibited EGF-induced cell proliferation in the same range of concentration (I50: 1-5 microM). DBHQ suppressed [Ca2+]i response to UTP and thus depleted completely Ca2+ stores at 5 microM. DBHQ also inhibited EGF-induced cell proliferation at an I50 value of approximately 10 microM. The removal of these inhibitors from the culture medium increased the reduced mitotic activity reversibly. Using a fluorescent assay of DNA binding of ethidium bromide, no dead cells were detected in any of the cultures. These results indicate that the inhibitory effects of SK&F 96365 and DBHQ on cell proliferation were due to the inhibition of capacitative Ca2+ entry and Ca2+ mobilization suggesting the importance of capacitative Ca2+ entry and Ca2+ mobilization in the control of EGF-induced cell cycle progression in mouse mammary epithelial cells.

EGF enhances Ca(2+) mobilization and capacitative Ca(2+) entry in mouse mammary epithelial cells

Effects of epidermal growth factor (EGF) on the intracellular Ca(2+) ([Ca(2+)](i)) responses to nucleotides, Ca(2+) release from thapsigargin-sensitive stores and capacitative Ca(2+) entry were investigated in cultured mouse mammary epithelial cells. EGF treatment induced proliferation of mammary epithelial cells. We checked for mitotic activity by immunocytochemistry with an anti-PCNA (proliferating cell nuclear antigen) antibody, which stains nuclei of the cells in S-phase of cell cycle. EGF treatment apparently increased the number of PCNA-stained cells compared to those treated with differentiating hormones (insulin, prolactin and cortisol) or without any hormone. Application of EGF did not induce any acute [Ca(2+)](i) response. EGF treatment for 1-2 days in culture, however, enhanced [Ca(2+)](i) responses including [Ca(2+)](i) increase by ATP, UTP and other nucelotides, Ca(2+) release from thapsigargin-sensitive stores, as well as capacitative Ca(2+) entry. Genistein, a tyrosine kinase inhibitor, prevented EGF-induced cell proliferation and the [Ca(2+) ](i) responses in a dose-dependent manner. These results indicate that EGF treatment enhances Ca(2+) mobilization and capacitative Ca(2+) entry, well correlated with cellular proliferation in mammary epithelial cells.

Ca2+ mobilization and capacitative Ca2+ entry regulate DNA synthesis in cultured chick retinal neuroepithelial cells

Release of Ca2+ from intracellular Ca2+ stores (Ca2+ mobilization) and capacitative Ca2+ entry have been shown to be inducible in neuroepithelial cells of the early embryonic chick retina. Both types of Ca2+ responses decline parallel with retinal progenitor cell proliferation. To investigate their potential role in the regulation of neuroepithelial cell proliferation, we studied the effects of 2,5-di-tert-butylhydroquinone (DBHQ), an inhibitor of the Ca2+ pump of intracellular Ca2+ stores, and of SK&F 96365, an inhibitor of capacitative Ca2+ entry, on DNA synthesis in retinal organ cultures from embryonic day 3 (E3) chicks and in dissociated cultures from E7 and E9 chick retinae. We demonstrate that both antagonists inhibit [3H]-thymidine incorporation in a dose-dependent manner without affecting cell viability or morphology. The inhibition of [3H]-thymidine incorporation by SK&F 96365 occurred in the same concentration range (IC50: approximately 4 microM) as the blockade of capacitative Ca2+ entry in the E3 retinal organ culture. At a concentration of 5 microM SK&F 96365. DNA synthesis was reduced by 71, 40 and 32% in the E3, E7 and E9 cultures, respectively. Application of DBHQ at concentrations which led to depletion of intracellular Ca2+ stores also inhibited [3H]-thymidine incorporation with IC50 values of 20-30 microM in the different cultures. Our results suggest the involvement of Ca2+ mobilization and capacitative Ca2+ entry in the regulation of DNA synthesis in the developing neural retina.

Voltage-sensitive Ca2+ channels, intracellular Ca2+ stores and Ca2+-release-activated Ca2+ channels contribute to the ATP-induced [Ca2+]i increase in differentiated neuroblastoma x glioma NG 108-15 cells

Activation of ionotropic P2X7 purinoreceptors in NG108-15 cells directly opens non-selective cation channels, leading to an increase in intracellular Ca2+ concentration ([Ca2+]i) and membrane depolarization and, hence, by indirect opening of voltage-stimulated Ca2+ channels (VSCC) to further increases of [Ca2+]i, whereas activation of the metabotropic P2Y receptor causes intracellular Ca2+ release. The quantitative contribution of Ca2+ entry and release to ATP-induced [Ca2+]i increase in differentiated NG108-15 cells is not known. Here we have investigated the Ca2+ influx and Ca2+ release components by studying [Ca2+]i in Fura-2-loaded cells and by using the following tools: nifedipine to block L-type VSCC, omega-conotoxin GVIa (omegaCT) to block N-type VSCC and thapsigargin to deplete intracellular Ca2+ stores. With 1.8 mM Ca2+ in the medium, ATP (600 microM) increased [Ca2+]i by 656 +/- 50 nM (n = 11). This response was reduced to 72% by nifedipine (50 microM), to 63% by omegaCT (1 microM), and to 31% by nifedipine and omegaCT in combination. Since nifedipine and omegaCT completely block VSCC in our model, the remaining 31% of [Ca2+]i increase could be caused by influx via P2X7-activated non-selective channels or by intracellular release mediated by P2Y receptors. When Ca2+-free medium was used to exclude Ca2+ influx, ATP (600 microM) increased [Ca2+]i by only 34 +/- 4 nM (n = 4), indicating that the majority of [Ca2+]i increase depends on Ca2+ influx. A similar rise by 37 +/- 4 nM (n = 4) was observed with the selective P2Y agonist UTP (150 microM). This small response was sensitive to thapsigargin and hence represents Ca2+ release. The remainder (i.e. total [Ca2+]i increase minus nifedipine-, omegaCT- and thapsigargin-sensitive [Ca2+]i increases) should, therefore, represent Ca2+ influx via P2X7 non-selective cation channels.