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

Silva H, P. de Carvalho R, Ventura ALM, Calaza KC, Silveira MS, Kubrusly RCC, de Melo Reis RA. The Healthy and Diseased Retina Seen through Neuron-Glia Interactions

The retina is the sensory tissue responsible for the first stages of visual processing, with a conserved anatomy and functional architecture among vertebrates. To date, retinal eye diseases, such as diabetic retinopathy, age-related macular degeneration, retinitis pigmentosa, glaucoma, and others, affect nearly 170 million people worldwide, resulting in vision loss and blindness. To tackle retinal disorders, the developing retina has been explored as a versatile model to study intercellular signaling, as it presents a broad neurochemical repertoire that has been approached in the last decades in terms of signaling and diseases. Retina, dissociated and arranged as typical cultures, as mixed or neuron- and glia-enriched, and/or organized as neurospheres and/or as organoids, are valuable to understand both neuronal and glial compartments, which have contributed to revealing roles and mechanisms between transmitter systems as well as antioxidants, trophic factors, and extracellular matrix proteins. Overall, contributions in understanding neurogenesis, tissue development, differentiation, connectivity, plasticity, and cell death are widely described. A complete access to the genome of several vertebrates, as well as the recent transcriptome at the single cell level at different stages of development, also anticipates future advances in providing cues to target blinding diseases or retinal dysfunctions.

Cell cycle regulation by ADP and IGF-1 in cultured late developing glia progenitors of the avian retina

In the avian retina, ADP induces the proliferation of late developing glia progenitors. Here, we show that in serum-containing retinal cell cultures, ADP-induced increase in [3H]-thymidine incorporation can be prevented by the IGF-1 receptor antagonists AG1024 and I-OMe-Tyrphostin AG 538, suggesting the participation of IGF-1 in ADP-mediated progenitor proliferation. In contrast, no increase in [3H]-thymidine incorporation is observed in retinal cultures treated only with IGF-1. Under serum starvation, while no increase in cell proliferation is detected in cultures treated only with ADP or IGF-1, a significant increase in [3H]-thymidine incorporation and number of PCNA expressing cells is observed in cultures treated concomitantly with ADP plus IGF-1, suggesting that both molecules are required to induce proliferation of retinal progenitors. In serum-starved cultures, although an increase in cell viability is detected by MTT assays in IGF-1-treated cultures, no significant increase in viability of [3H]-thymidine labeled progenitors is observed, suggesting that IGF-1 may contribute to survival of postmitotic cells in culture. While only ADP increases intracellular calcium, only IGF-1 induces the phosphorylation of Akt in the retinal cultures. IGF-1 through the PI3K/Akt pathway induces a significant increase in the transcription and expression of CDK1 with a decrease in phospho-histone H3 expression that is concomitant with an increase in the expression of cyclins D1 and E and CDK2. These findings suggest that IGF-1 stimulates CDK-1 mRNA and protein expression that enable progenitors to progress through the cell cycle. However, signaling of ADP in the presence IGF-I seems to be required for DNA synthesis.

Purinergic signaling in the retina: From development to disease

Retinal injuries and diseases are major causes of human disability involving vision impairment by the progressive and permanent loss of retinal neurons. During development, assembly of this tissue entails a successive and overlapping, signal-regulated engagement of complex events that include proliferation of progenitors, neurogenesis, cell death, neurochemical differentiation and synaptogenesis. During retinal damage, several of these events are re-activated with both protective and detrimental consequences. Purines and pyrimidines, along with their metabolites are emerging as important molecules regulating both retinal development and the tissue's responses to damage. The present review provides an overview of the purinergic signaling in the developing and injured retina. Recent findings on the presence of vesicular and channel-mediated ATP release by retinal and retinal pigment epithelial cells, adenosine synthesis and release, expression of receptors and intracellular signaling pathways activated by purinergic signaling in retinal cells are reported. The pathways by which purinergic receptors modulate retinal cell proliferation, migration and death of retinal cells during development and injury are summarized. The contribution of nucleotides to the self-repair of the injured zebrafish retina is also discussed.

Nucleotide P2Y13-stimulated phosphorylation of CREB is required for ADP-induced proliferation of late developing retinal glial progenitors in culture

Nucleotides stimulate phosphorylation of CREB to induce cell proliferation and survival in diverse cell types. We report here that ADP induces the phosphorylation of CREB in a time- and concentration-dependent manner in chick embryo retinal progenitors in culture. ADP-induced increase in phospho-CREB is mediated by P2 receptors as it is blocked by PPADS but not by the adenosine antagonists DPCPX or ZM241385. Incubation of the cultures with the CREB inhibitor KG-501 prevents ADP-induced incorporation of [³H]-thymidine, indicating that CREB is involved in retinal cell proliferation. No effect of this compound is observed on the viability of retinal progenitors. While no significant increase in CREB phosphorylation is observed with the P2Y1 receptor agonist MRS2365, ADP-induced phosphorylation of CREB is blocked by the P2Y13 receptor selective antagonist MRS2211, but not by MRS2179 or PSB0739, two antagonists of the P2Y1 and P2Y12 receptors, respectively, suggesting that ADP-induced CREB phosphorylation is mediated by P2Y13 receptors. ADP-induced increase in phospho-CREB is attenuated by the PI3K inhibitor LY294002 and completely prevented by the MEK inhibitor U0126, suggesting that at least ERK is involved in ADP-induced CREB phosphorylation. A pharmacological profile similar to the activation and inhibition of CREB phosphorylation is observed in the phosphorylation of ERK, suggesting that P2Y13 receptors mediate ADP induced ERK/CREB pathway in the cultures. While no increase in [³H]-thymidine incorporation is observed with the P2Y1 receptor agonist MRS2365, both MRS2179 and MRS2211 prevent ADP-mediated increase in [³H]-thymidine incorporation, but not progenitor's survival, suggesting that both P2Y1 and P2Y13 receptor subtypes are involved in ADP-induced cell proliferation. P2Y1 receptor-mediated increase in [Ca²⁺]_i is observed in glial cells only when cultures maintained for 9days are used. In glia from cultures cultivated for only 2days, no increase in [Ca²⁺]_i is detected with MRS2365 and no inhibition of ADP-mediated calcium response is observed with MRS2179. In contrast, MRS2211 attenuates ADP-mediated increase in [Ca²⁺]_i in glial cells from cultures at both stages, suggesting the presence of P2Y13 receptors coupled to calcium mobilization in proliferating retinal glial progenitors in culture.

Store-Operated Calcium Entry in Müller Glia Is Controlled by Synergistic Activation of TRPC and Orai Channels

The endoplasmic reticulum (ER) is at the epicenter of astrocyte Ca(2+) signaling. We sought to identify the molecular mechanism underlying store-operated calcium entry that replenishes ER stores in mouse Müller cells. Store depletion, induced through blockade of sequestration transporters in Ca(2+)-free saline, induced synergistic activation of canonical transient receptor potential 1 (TRPC1) and Orai channels. Store-operated TRPC1 channels were identified by their electrophysiological properties, pharmacological blockers, and ablation of the Trpc1 gene. Ca(2+) release-activated currents (ICRAC) were identified by ion permeability, voltage dependence, and sensitivity to selective Orai antagonists Synta66 and GSK7975A. Depletion-evoked calcium influx was initiated at the Müller end-foot and apical process, triggering centrifugal propagation of Ca(2+) waves into the cell body. EM analysis of the end-foot compartment showed high-density ER cisternae that shadow retinal ganglion cell (RGC) somata and axons, protoplasmic astrocytes, vascular endothelial cells, and ER-mitochondrial contacts at the vitreal surface of the end-foot. The mouse retina expresses transcripts encoding both Stim and all known Orai genes; Müller glia predominantly express stromal interacting molecule 1 (STIM1), whereas STIM2 is mainly confined to the outer plexiform and RGC layers. Elimination of TRPC1 facilitated Müller gliosis induced by the elevation of intraocular pressure, suggesting that TRPC channels might play a neuroprotective role during mechanical stress. By characterizing the properties of store-operated signaling pathways in Müller cells, these studies expand the current knowledge about the functional roles these cells play in retinal physiology and pathology while also providing further evidence for the complexity of calcium signaling mechanisms in CNS astroglia.

SIGNIFICANCE STATEMENT

Store-operated Ca(2+) signaling represents a major signaling pathway and source of cytosolic Ca(2+) in astrocytes. Here, we show that the store-operated response in Müller cells, radial glia that perform key structural, signaling, osmoregulatory, and mechanosensory functions within the retina, is mediated through synergistic activation of transient receptor potential and Orai channels. The end-foot disproportionately expresses the depletion sensor stromal interacting molecule 1, which contains an extraordinarily high density of endoplasmic reticulum cisternae that shadow neuronal, astrocytic, vascular, and axonal structures; interface with mitochondria; but also originate store-operated Ca(2+) entry-induced transcellular Ca(2+) waves that propagate glial excitation into the proximal retina. These results identify a molecular mechanism that underlies complex interactions between the plasma membrane and calcium stores, and contributes to astroglial function, regulation, and response to mechanical stress.

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.

ATP induces the death of developing avian retinal neurons in culture via activation of P2X7 and glutamate receptors

Previous data suggest that nucleotides are important mitogens in the developing retina. Here, the effect of ATP on the death of cultured chick embryo retina cells was investigated. In cultures obtained from retinas of 7-day-old chick embryos (E7) that were cultivated for 2 days (E7C2), both ATP and BzATP induced a ∼30 % decrease in cell viability that was time- and dose-dependent and that could be blocked by 0.2 mM oxidized ATP or 0.3 μM KN-62. An increase in cleaved caspase-3 levels and in the number of TUNEL-positive cells was observed when cultures were incubated with 3 mM ATP and immunolabeling for cleaved-caspase 3 was observed over neurons but not over glial cells. ATP-dependent cell death was developmentally regulated, the maximal levels being detected by E7C2-3. Nucleotides were able to increase neuronal ethidium bromide and sulforhodamine B uptake in mixed and purified neuronal cultures, an effect that was blocked by the antagonists Brilliant Blue G and oxidized ATP. In contrast, nucleotide-induced cell death was observed only in mixed cultures, but not in purified cultures of neurons or glia. ATP-induced neuronal death was blocked by the glutamatergic antagonists MK801 and DNQX and activation of P2X7 receptors by ATP decreased the uptake of [(3)H]-D-aspartate by cultured glial cells with a concomitant accumulation of it in the extracellular medium. These results suggest that ATP induces apoptosis of chick embryo retinal neurons in culture through activation of P2X7 and glutamate ionotropic receptors. Involvement of a P2X7 receptor-mediated inhibition of the glial uptake of glutamate is suggested.

Synchronization of Ca2+ oscillations: a capacitative (AC) electrical coupling model in neuroepithelium

Increases in intracellular [Ca(2+)] occur synchronously between cells in the neuroepithelium. If neuroepithelial cells were capable of generating action potentials synchronized by gap junctions (direct current electrical coupling), the influx of Ca(2+) through voltage-activated Ca(2+) channels would lead to a synchronous increase in intracellular [Ca(2+)]. However, no action potential is generated in neuroepithelial cells, and the [Ca(2+)] increase is instead produced by the release of Ca(2+) from intracellular Ca(2+) stores. Recently, synchronous fluctuations in the membrane potential of Ca(2+) stores were recorded using an organelle-specific voltage-sensitive dye. On the basis of these recordings, a capacitative [alternating current (AC)] electrical coupling model for the synchronization of voltage fluctuations of Ca(2+) store potential was proposed [Yamashita M (2006) FEBS Lett580, 4979-4983; Yamashita M (2008) FEBS J275, 4022-4032]. Ca(2+) efflux from the Ca(2+) store and K(+) counterinflux into the store cause alternating voltage changes across the store membrane, and the voltage fluctuation induces ACs. In cases where the store membrane is closely apposed to the plasma membrane and the cells are tightly packed, which is true of neuroepithelial cells, the voltage fluctuation of the store membrane is synchronized between the cells by the AC currents through the series capacitance of these membranes. This article provides a short review of the model and its relationship to the structural organization of the Ca(2+) store. This is followed by a discussion of how the mode of synchronization of [Ca(2+)] increase may change during central nervous system development and new molecular insights into the synchronicity of [Ca(2+)] increase.

Capacitative calcium entry and proliferation of human osteoblast-like MG-63 cells

Adult bone tissue is continuously being remodelled and bone mass is maintained by a balance between osteoclastic bone resorption and osteoblastic bone formation. Alteration of osteoblastic cell proliferation may account in part for lack of balance between these two processes in bone loss of osteoporosis. There is calcium (Ca2+) control in numerous cellular functions; however, involvement of capacitative Ca2+ entry (CCE) in proliferation of bone cells is less well investigated.

OBJECTIVES

The study described here was aimed to investigate roles of CCE in the proliferation of osteoblast-like MG-63 cells.

MATERIALS AND METHODS

Pharmacological characterizations of CCE were undertaken in parallel, with evaluation of the expression of transient receptor potential canonical (TRPC) channels and of cell proliferation.

RESULTS

Intracellular Ca2+ store depletion by thapsigargin induced CCE in MG-63 cells; this was characterized by a rapid transient increase of intracellular Ca2+ followed by significant CCE, induced by conditions that stimulated cell proliferation, namely serum and platelet-derived growth factor. Inhibitors of store-operated Ca2+ channels (2-APB and SKF-96365) prevented CCE, while voltage-dependent Ca2+ channel blockers had no effect. Expression of various TRPC channels was shown in the cells, some having been shown to be responsible for CCE. Voltage-dependent Ca2+ channel blockers had no effect on osteoblast proliferation while thapsigargin, 2-APB and SKF-96395, inhibited it. Cell cycle analysis showed that 2-APB and SKF-96395 lengthen the S and G2/M phases, which would account for the reduction in cell proliferation.

CONCLUSIONS

Our results indicate that CCE, likely attributed to the activation of TRPCs, might be the main route for Ca2+ influx involved in osteoblast proliferation.

Signal transduction pathways associated with ATP-induced proliferation of cell progenitors in the intact embryonic retina

ATP and ADP induce retinal cell proliferation through activation of PKC and extracellular signal-regulated kinases (ERKs). Here, we characterized the effect of purinergic agonists on the turnover of phosphoinositides and activation of ERKs during development of the chick embryo retina. When intact retinas were incubated with ATP, ADP or UTP, a dose-dependent accumulation of [(3)H]-phosphoinositides was observed (% of control, EC(50): 548+/-20.5%, 0.18 mM; 314+/-53.8%, 0.51 mM; 704+/-139.9%, 0.018 mM, respectively). Only the response promoted by ADP was completely inhibited by the P2 receptor antagonists, PPADS and suramin. All the responses decreased with the progression of retinal development. Western blot assays revealed that ATP, ADP and UTP stimulated the phosphorylation of ERKs in the chick embryo retina very early during development (% of control: 174+/-16; 199+/-16.4 and 206+/-37, respectively). The responses to ADP and UTP were transient and dose-dependent, showing EC(50) values of 0.12 mM and 0.009 mM. The response to ADP was inhibited by the antagonists PPADS and suramin and by U73122 and chelerythrine chloride, which block PLC and PKC, respectively. Conversely, chelerythrine chloride did not block the response induced by UTP. Immunohistochemical analysis revealed that ATP and ADP induced the phosphorylation of ERKs in cells of the neuroblastic layer of retinas from embryos at E8. Our data showed that ATP, ADP and UTP stimulate the turnover of InsPs and promoted the activation of ERKs in the chick embryo retina. ADP, through activation of P2Y(1) receptors, activated ERK pathway through PLC and PKC and UTP, via P2Y(4)-like receptors, induced the phosphorylation of ERKs through a pathway that did not involve PKC.

ATP-induced proliferation of developing retinal cells: regulation by factors released from postmitotic cells in culture

ATP is an important mitogen in the developing retina and its proliferative response decreases as chick retinal cells differentiate in culture. Both non-stimulated or ATP-induced proliferative response was abolished if cycling cells were cocultured with cells from older embryos or cultured with conditioned medium (CM) from postmitotic cells. The effect of CM was dose-dependent and reversible, as removal of CM from the cultures restored both basal and ATP-induced incorporation of [3H]-thymidine. The effect of CM was also dependent on the developmental stage of the retina used to prepare the medium. As tissues from older embryos were used, inhibition of the basal and ATP-induced proliferative response of the cells increased. Similar inhibition of ATP-induced increase in [3H]-thymidine incorporation was observed using CM from purified glial cultures. Neither ARL 67156, an ecto-ATPase inhibitor, prevented nor TGF-beta1 and TGF-beta2 mimicked the inhibitory effect of conditioned medium. Incubation of cells with CM or ATP for 24 h completely abolished the formation of [3H]-phosphoinositides induced by ATP. These effects were blocked by the P2 receptor antagonist PPADS and were not observed with dialysed CM, suggesting that agonist-dependent desensitization of P2 receptors occurred in cultures incubated with CM. However, removal of small molecules such as nucleotides by dialysis did not affect the decline in the proliferative activity induced by CM, suggesting that desensitization is not responsible for the conditioned medium-dependent cell cycle arrest of early developing retinal cells in culture. These results suggest that factors released from postmitotic cells induce the arrest of retinal cells in the mitotic state, a phenomenon that is concomitant with agonist-dependent P2 receptor desensitization.

Control of cell proliferation by neurotransmitters in the developing vertebrate retina

In the developing vertebrate retina, precise coordination of retinal progenitor cell proliferation and cell-cycle exit is essential for the formation of a functionally mature retina. Unregulated or disrupted cell proliferation may lead to dysplasia, retinal degeneration or retinoblastoma. Both cell-intrinsic and -extrinsic factors regulate the proliferation of progenitor cells during CNS development. There is now growing evidence that in the developing vertebrate retina, both slow and fast neurotransmitter systems modulate the proliferation of retinal progenitor cells. Classic neurotransmitters, such as GABA (gamma-amino butyric acid), glycine, glutamate, ACh (acetylcholine) and ATP (adenosine triphosphate) are released, via vesicular or non-vesicular mechanisms, into the immature retinal environment. Furthermore, these neurotransmitters signal through functional receptors even before synapses are formed. Recent evidence indicates that the activation of purinergic and muscarinic receptors may regulate the cell-cycle machinery and consequently the expansion of the retinal progenitor pool. Interestingly, GABA and glutamate appear to have opposing roles, inducing retinal progenitor cell-cycle exit. In this review, we present recent findings that begin to elucidate the roles of neurotransmitters as regulators of progenitor cell proliferation at early stages of retinal development. These studies also raise several new questions, including how these neurotransmitters regulate specific cell-cycle pathways and the mechanisms by which retinal progenitor cells integrate the signals from neurotransmitters and other exogenous factors during vertebrate retina development.

GABAB heterodimeric receptors promote Ca2+ influx via store-operated channels in rat cortical neurons and transfected Chinese hamster ovary cells

The GABAB receptors are generally considered to be classical Gi-coupled receptors that lack the ability to mobilize intracellular Ca2+ without the aid of promiscuous G proteins. Here, we report the ability of GABAB receptors to promote calcium influx into primary cultures of rat cortical neurons and transfected Chinese hamster ovary cells. Chinese hamster ovary cells were transfected with GABAB1(a) or GABAB1(b) subunits along with GABAB2 subunits. In experiments using the fluorometric imaging plate reader platform, GABA and selective agonists promoted increases in intracellular Ca2+ levels in transfected Chinese hamster ovary cells and cortical neurons with the expected order of potency. These effects were fully antagonized by selective GABAB receptor antagonists. To investigate the intracellular pathways responsible for mediating these effects we employed several pharmacological inhibitors. Pertussis toxin abolished GABAB mediated Ca2+ increases, as did the phospholipase Cbeta inhibitor U73122. Inhibitor 2-aminethoxydiphenyl borane acts as an antagonist at inositol 1,4,5-trisphosphate receptors and at store-operated channels. In all cell types, 2-aminethoxydiphenyl borane prevented Ca2+ mobilization. The selective store-operated channel inhibitor 1-[2-(4-methoxyphenyl)-2-[3-(4-methoxyphenyl)propoxy]ethyl-1H-imidazole hydrochloride prevented increases in intracellular Ca2+ levels as did performing the assays in Ca2+ free buffers. In conclusion, GABAB receptors expressed in Chinese hamster ovary cells and endogenously expressed in rat cortical neurons promote Ca2+ entry into the cell via the activation of store-operated channels, using a mechanism that is dependent on Gi/o heterotrimeric proteins and phospholipase Cbeta. These findings suggest that the neuronal effects mediated by GABAB receptors may, in part, rely on the receptor's ability to promote Ca2+ influx.

PACAP maintains cell cycling and inhibits apoptosis in chick neuroblasts

We previously reported that pituitary adenylate cyclase-activating polypeptide (PACAP) increased cAMP in neuroblast-enriched cultures from embryonic day 3.5 chick brain. Also, the neuroblasts expressed the mRNA, peptide, and receptor for PACAP. Here, we investigated downstream effects of increased cAMP by examining PACAP's role in regulating cell numbers during brain development. Using flow cytometry, we quantified proliferating cell nuclear antigen and DNA, and compared apoptotic cells and cells in cell cycle compartments under differing conditions. Untreated cultures showed high proliferative activity with little apoptosis. Addition of exogenous PACAP had no effect on this pattern. However, blocking endogenous PACAP with a receptor antagonist increased cell cycle exit, then increased apoptosis. We conclude that chick neuroblasts require production of PACAP to inhibit apoptosis and maintain full proliferative activity during early brain development.

Calcium signaling to nucleus via store-operated system during cell cycle in retinal neuroepithelium

Intracellular Ca(2+) is a regulatory signal for cell proliferation. To reveal Ca(2+) signal dynamics during cell cycle, we applied Ca(2+) fluorescence imaging to the neural retina of chick embryo, where the soma changes its position during the cell cycle. Purinoceptors were stimulated to cause Ca(2+) release from Ca(2+) stores, since the purinoceptor activation promotes DNA synthesis. Ca(2+) rises occurred in the nucleoplasm of cells at around S-phase. The soma of S-phase cell is located in the inner layer of the retinal neuroepithelium and issues an outer process, which extends to the ventricular surface. Fluorescent probes for endoplasmic reticulum (ER) showed that the ERs in the outer process and the nuclear envelope (NE) or peri-nuclear ER formed the Ca(2+) store. Depletion of the Ca(2+) store induced capacitative Ca(2+) entry (CCE), which caused Ca(2+) rises in the terminal of outer process and soma. The store-operated Ca(2+) signaling declined in M-phase cells and postmitotic cells (retinal ganglion cells (RGCs)) with the loss of the outer process. These results suggest that the Ca(2+) signaling to nucleus via the store-operated system including the ERs in the outer process is crucial for the cell cycle progression in the retinal neuroepithelium.

Expression of plasma membrane calcium pump isoform mRNAs in breast cancer cell lines

The plasma membrane Ca(2+) ATPase (PMCA) is an important regulator of free intracellular calcium, with dynamic regulation in the rat mammary gland during lactation. Recent studies suggest that Ca(2+) plays a role in cellular proliferation. To determine if PMCA expression is altered in tumorigenesis, we compared relative levels of PMCA1 mRNA. We found that the relative expression of PMCA1 mRNA is increased, by approximately 270% and 170%, in MCF-7 and MDA-MB-231 human breast cancer cell lines deprived of serum for 72 h, respectively, compared to the similarly treated MCF-10A human mammary gland epithelial cell line. Characterization of PMCA mRNA isoforms revealed that PMCA1b and PMCA4 mRNA are expressed in MCF-7, MDA-MB-231, SK-BR-3, ZR-75-1 and BT-483 breast cancer cell lines. We also detected PMCA2 mRNA expression in all the breast cancer cell lines examined. However, PMCA3 mRNA was only detected in BT-483 cells. Our results suggest that PMCA expression may be altered in breast cancer cell lines, suggesting altered Ca(2+) regulation in these cell lines. Our results also indicate that breast cancer cell lines can express mRNAs for a variety PMCA isoforms.

Purinergic and muscarinic modulation of the cell cycle and calcium signaling in the chick retinal ventricular zone

Spontaneous calcium transients occur in the ventricular zone of the chick retina and result from the endogenous release of neurotransmitters in the absence of action potentials. Calcium transients resulting from the activation of purinergic and muscarinic receptors occur in a mixed population of interphase and mitotic cells, whereas those produced by ionotropic GABA and glutamate receptors are mostly restricted to the interphase population, the GABA responses primarily coming from cells that express the neuronal marker TuJ-1. Muscarinic and purinergic receptors can act respectively as a brake and an accelerator on mitosis, whereas GABA and glutamate receptors are without effect. Our results suggest that the balance between muscarinic and purinergic activation acts to control the rate of retinal proliferation in early development.

ATP induces proliferation of retinal cells in culture via activation of PKC and extracellular signal-regulated kinase cascade

Both ATP and acetylcholine can induce the mobilization of intracellular calcium in the early developing chick embryo retina, a response that decreases during retinal development. In this study, the effects of these transmitters on the turnover of phosphoinositides and proliferation of developing retinal cells in culture were characterized. While ATP, UTP or carbachol were able to induce a >400% accumulation of phosphoinositides in retinal cell cultures, only ATP promoted a dose-dependent increase in [(3)H]-thymidine incorporation in cultured cells (EC(50)=8.6 microM), a response that was inhibited by the P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) (0.1 or 0.25 mM). ADP, but not UTP or adenosine, also stimulated the proliferation of retinal cells (EC(50)=5.8 microM), indicating that activation of P2Y1 receptors mediates the proliferative response of retinal cells to ATP. The mitogenic effect of ATP was completely prevented by the PKC inhibitor chelerythrine chloride (0.5 microM) and the phospholipase C (PLC) inhibitor U73122 (0.5 microM). PD 98059 (25 or 50 microM), an inhibitor of the activation of extracellular signal-regulated kinases (ERKs) also blocked the increase in [(3)H]-thymidine incorporation induced by ATP. Moreover, the effect of ATP was pronounced in cultures obtained from retinas at embryonic days 6-8, but not at day 9. Since Müller and bipolar cells are the predominant cell types that proliferate at these embryonic stages, our data suggest that ATP, through activation of P2Y1 receptors coupled to phospholipase C, PKC and MAP kinases, affects DNA synthesis in one or both of these cell types in culture.

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.

P2 Purinoceptor expression and functional changes of hypoxia-activated cultured rat retinal microglia

P(2) purinoceptors appear to modulate microglia function, but their role in hypoxic microglia has not been investigated. We examined in postnatal rat retinal microglia cultured under hypoxic (1% oxygen) condition, their P2 expression, proliferation and cytokine release in the presence or absence of the P2 receptor agonists and antagonists. Fura-2 fluorescence measurements of intracellular Ca(2+) rises to P2 receptor agonists and antagonists indicated that both P(2U) and P(2Z) were expressed in hypoxic microglia. Hypoxia induced BrdU incorporation and release of interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha) as well. The P(2U) agonist, UTP, maintained the BrdU incorporation, whereas the P(2Z) agonist, BzATP, suppressed it, but significantly enhanced IL-1beta and TNF-alpha release, suggesting that the P(2U) response may underlie the mitotic activity, and that of P(2Z), the IL-1beta and TNF-alpha release of hypoxia-activated microglia.