Involvement of P2 purinoceptors in the regulation of DNA synthesis in the neural retina of chick embryo

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.

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.

Injury-induced purinergic signalling molecules upregulate pluripotency gene expression and mitotic activity of progenitor cells in the zebrafish retina

Damage in fish activates retina repair that restores sight. The purinergic signalling system serves multiple homeostatic functions and has been implicated in cell cycle control of progenitor cells in the developing retina. We examined whether changes in the expression of purinergic molecules were instrumental in the proliferative phase after injury of adult zebrafish retinas with ouabain. P2RY1 messenger RNA (mRNA) increased early after injury and showed maximal levels at the time of peak progenitor cell proliferation. Extracellular nucleotides, mainly ADP, regulate P2RY1 transcriptional and protein expression. The injury-induced upregulation of P2RY1 is mediated by an autoregulated mechanism. After injury, the transcriptional expression of ecto-nucleotidases and ecto-ATPases also increased and ecto-ATPase activity inhibitors decreased Müller glia-derived progenitor cell amplification. Inhibition of P2RY1 endogenous activation prevented progenitor cell proliferation at two intervals after injury: one in which progenitor Müller glia mitotically activates and the second one in which Müller glia-derived progenitor cells amplify. ADPβS induced the expression of lin28a and ascl1a genes in mature regions of uninjured retinas. The expression of these genes, which regulate multipotent Müller glia reprogramming, was significantly inhibited by blocking the endogenous activation of P2RY1 early after injury. We consistently observed that the number of glial fibrillary acidic protein-BrdU-positive Müller cells after injury was larger in the absence than in the presence of the P2RY1 antagonist. Ecto-ATPase activity inhibitors or P2RY1-specific antagonists did not modify apoptotic cell death at the time of peak progenitor cell proliferation. The results suggested that ouabain injury upregulates specific purinergic signals which stimulates multipotent progenitor cell response.

Adenine Nucleotides Control Proliferation In Vivo of Rat Retinal Progenitors by P2Y1 Receptor

Previous studies demonstrated that exogenous ATP is able to regulate proliferation of retinal progenitor cells (RPCs) in vitro possibly via P2Y₁ receptor, a G protein-coupled receptor. Here, we evaluated the function of adenine nucleotides in vivo during retinal development of newborn rats. Intravitreal injection of apyrase, an enzyme that hydrolyzes nucleotides, reduced cell proliferation in retinas at postnatal day 2 (P2). This decrease was reversed when retinas were treated together with ATPγ-S or ADPβ-S, two hydrolysis-resistant analogs of ATP and ADP, respectively. During early postnatal days (P0 to P5), an increase in ectonucleotidase (E-NTPDase) activity was observed in the retina, suggesting a decrease in the availability of adenine nucleotides, coinciding with the end of proliferation. Interestingly, intravitreal injection of the E-NTPDase inhibitor ARL67156 increased proliferation by around 60 % at P5 rats. Furthermore, immunolabeling against P2Y₁ receptor was observed overall in retina layers from P2 rats, including proliferating Ki-67-positive cells in the neuroblastic layer (NBL), suggesting that this receptor could be responsible for the action of adenine nucleotides upon proliferation of RPCs. Accordingly, intravitreal injection of MRS2179, a selective antagonist of P2Y₁ receptors, reduced cell proliferation by approximately 20 % in P2 rats. Moreover, treatment with MRS 2179 caused an increase in p57^KIP2 and cyclin D1 expression, a reduction in cyclin E and Rb phosphorylated expression and in BrdU-positive cell number. These data suggest that the adenine nucleotides modulate the proliferation of rat RPCs via activation of P2Y₁ receptors regulating transition from G1 to S phase of the cell cycle.

The role of dinucleoside polyphosphates on the ocular surface and other eye structures

Dinucleoside polyphosphates comprises a group of dinucleotides formed by two nucleosides linked by a variable number of phosphates, abbreviated NpnN (where n represents the number of phosphates). These compounds are naturally occurring substances present in tears, aqueous humour and in the retina. As the consequence of their presence, these dinucleotides contribute to many ocular physiological processes. On the ocular surface, dinucleoside polyphosphates can stimulate tear secretion, mucin release from goblet cells and they help epithelial wound healing by accelerating cell migration rate. These dinucleotides can also stimulate the presence of proteins known to protect the ocular surface against microorganisms, such as lysozyme and lactoferrin. One of the latest discoveries is the ability of some dinucleotides to facilitate the paracellular way on the cornea, therefore allowing the delivery of compounds, such as antiglaucomatous ones, more easily within the eye. The compound Ap₄A has been described being abnormally elevated in patient's tears suffering of dry eye, Sjogren syndrome, congenital aniridia, or after refractive surgery, suggesting this molecule as biomarker for dry eye condition. At the intraocular level, some diadenosine polyphosphates are abnormally elevated in glaucoma patients, and this can be related to the stimulation of a P2Y₂ receptor that increases the chloride efflux and water movement in the ciliary epithelium. In the retina, the dinucleotide dCp₄U, has been proven to be useful to help in the recovery of retinal detachments. Altogether, dinucleoside polyphosphates are a group of compounds which present relevant physiological actions but which also can perform promising therapeutic benefits.

Purinergic signalling during development and ageing

Extracellular purines and pyrimidines play major roles during embryogenesis, organogenesis, postnatal development and ageing in vertebrates, including humans. Pluripotent stem cells can differentiate into three primary germ layers of the embryo but may also be involved in plasticity and repair of the adult brain. These cells express the molecular components necessary for purinergic signalling, and their developmental fates can be manipulated via this signalling pathway. Functional P1, P2Y and P2X receptor subtypes and ectonucleotidases are involved in the development of different organ systems, including heart, blood vessels, skeletal muscle, urinary bladder, central and peripheral neurons, retina, inner ear, gut, lung and vas deferens. The importance of purinergic signalling in the ageing process is suggested by changes in expression of A1 and A2 receptors in old rat brains and reduction of P2X receptor expression in ageing mouse brain. By contrast, in the periphery, increases in expression of P2X3 and P2X4 receptors are seen in bladder and pancreas.

NTPDase2 and the P2Y1 receptor are not required for mammalian eye formation

Eye formation in vertebrates is controlled by a conserved pattern of molecular networks. Homeobox transcription factors are crucially involved in the establishment and maintenance of the retina. A previous study of Massé et al. (Nature, 449: 1058-62, 2007) using morpholino knockdown identified the ectonucleotidase NTPDase2 and the P2Y1 receptor as essential elements for eye formation in embryos of the clawed frog Xenopus laevis. In order to investigate whether a similarly essential mechanism would be active in mammalian eye development, we analyzed mice KO for Entpd2 or P2ry1 as well as double KO for Entpd2/P2ry1. These mice developed normal eyes. In order to identify potential deficits in the molecular identity or in the arrangement of the cellular elements of the retina, we performed an immunohistological analysis using a variety of retinal markers. The analysis of single and double KO mice demonstrated that NTPDase2 and P2Y1 receptors are not required for murine eye formation, as previously shown for eye development in Xenopus laevis.

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.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

Release of ATP from avian Müller glia cells in culture

ATP can be released from neurons and act as a neuromodulator in the nervous system. Besides neurons, cortical astrocytes also are capable of releasing ATP from acidic vesicles in a Ca(2+)-dependent way. In the present work, we investigated the release of ATP from Müller glia cells of the chick embryo retina by examining quinacrine staining and by measuring the extracellular levels of ATP in purified Müller glia cultures. Our data revealed that glial cells could be labeled with quinacrine, a reaction that was prevented by incubation of the cells with 1μM bafilomycin A1 or 2μM Evans blue, potent inhibitors of vacuolar ATPases and of the vesicular nucleotide transporter, respectively. Either 50mM KCl or 1mM glutamate was able to decrease quinacrine staining of the cells, as well as to increase the levels of ATP in the extracellular medium by 77% and 89.5%, respectively, after a 5min incubation of the cells. Glutamate-induced rise in extracellular ATP could be mimicked by 100μM kainate (81.5%) but not by 100μM NMDA in medium without MgCl(2) but with 2mM glycine. However, both glutamate- and kainate-induced increase in extracellular ATP levels were blocked by 50μM of the glutamatergic antagonists DNQX and MK-801, suggesting the involvement of both NMDA and non-NMDA receptors. Extracellular ATP accumulation induced by glutamate was also blocked by incubation of the cells with 30μM BAPTA-AM or 1μM bafilomycin A1. These results suggest that glutamate, through activation of both NMDA and non-NMDA receptors, induces the release of ATP from retinal Müller cells through a calcium-dependent exocytotic mechanism.

Involvement of the PI3K/AKT pathway in ATP-induced proliferation of developing retinal cells in culture

ATP induces the proliferation of chick retinal cells in culture through the activation of P2Y1 receptors, PKC and MAP kinases. Together with MAP kinases, the PI3K/AKT pathway has also been implicated as an important mediator in proliferative events during development. Here we investigated the participation of the PI3K/AKT signal pathway on ATP-induced proliferation of chick embryo retinal cells in culture. When retinal cultures obtained from 7-day-old embryos were cultivated for 1 day and treated with ATP, a transient and dose-dependent phosphorylation of both ERK and AKT was observed, an effect that could be mimicked by 500 microM ADP and blocked by 100 microM PPADS, a P2 receptor antagonist. Maximal stimulation of both enzymes was obtained with 100 microM ATP in 5 min, decreasing thereafter. Activation of these pathways by ATP seemed to be independent, since LY294002 and U0126, inhibitors of PI3K and MEK, did not block the activation of ERK and AKT, respectively, although each compound blocked its respective target. Moreover, when the cultures were incubated with ATP in the presence of LY294002, a decreased incorporation of [(3)H]-thymidine was observed, as compared to cultures treated only with ATP, a decline that was also obtained by incubating the cells with ATP plus 0.5 microM API-59CJ-Ome, an inhibitor of AKT. No decrease in cell viability was observed with this concentration of API-59CJ-Ome. An increase in cyclin D1 expression, that could be inhibited by 10 microM LY 294002 or 20 microM U0126, was observed when cells were incubated with 500 microM ADP. No effect of PI3K and MEK inhibitors was observed in the expression of p27kip1 in the cultures. These results suggest that, besides the involvement of the MAP kinases pathway, ATP-induced cell cycling of late developing retinal progenitors in culture also involves the activation of the PI3K/AKT pathway.

Basal release of ATP: an autocrine-paracrine mechanism for cell regulation

Cells release adenosine triphosphate (ATP), which activates plasma membrane-localized P2X and P2Y receptors and thereby modulates cellular function in an autocrine or paracrine manner. Release of ATP and the subsequent activation of P2 receptors help establish the basal level of activation (sometimes termed "the set point") for signal transduction pathways and regulate a wide array of responses that include tissue blood flow, ion transport, cell volume regulation, neuronal signaling, and host-pathogen interactions. Basal release and autocrine or paracrine responses to ATP are multifunctional, evolutionarily conserved, and provide an economical means for the modulation of cell, tissue, and organismal biology.

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.

ATP controls cell cycle and induces proliferation in the mouse developing retina

Previous data suggest that nucleotides are important mitogens in the developing chick retina. Here, we extended the study on the mitogenic effect of ATP to newborn mouse retinal explants. Our results showed that P2Y(1) receptors were widely distributed in C57bl/6 mice retina and that the majority of PCNA positive cells co-localized with P2Y(1) receptor. To evaluate proliferation, retinal explants obtained from newborn mice were incubated with 0.5 microCi [(3)H]-thymidine or 3 microM BrDU 1h before the end of culture. Our data showed that ATP induced a dose-dependent increase in [(3)H]-thymidine incorporation, an effect that was mimicked by ADP but not by UTP and was blocked by the P2 antagonist PPADS in a dose-dependent manner. The increase in [(3)H]-thymidine incorporation induced by ATP was only observed in explants cultured for 3 days or less and was mimicked by the ectoapyrase inhibitor ARL 67156. It corresponded to an increase in the number of BrdU(+) cells in the neuroblastic layer (NL) of the tissue, suggesting that ATP, through activation of P2Y(1) receptors, induced proliferation of late developing progenitors in retinal explants of newborn mice. The increase in the number of BrdU(+) cells was observed across the whole NL when explants were incubated with ATP for 24h and no increase in the number of p-histone H3 labeled cells could be noticed at this time point. In longer incubations of 48h with ATP or 24h with ATP followed by a period of 24h in fresh medium, an increase in the number of BrdU(+) cells promoted by ATP was observed only in the middle and outer, but not in the inner NL. In these conditions, an increase in the number of p-histone H3 labeled cells was detected in the outer NL, suggesting that ATP induced cells to enter S and progress to G2 phase of the cell cycle in the first 24h period of incubation. ATP also induced an increase and a decrease in the expression of cyclin D1 and p27(kip1), respectively, in retinal progenitors of the NL. While the increase in the expression of cyclin D1 was observed when retinal explants were incubated for 3h or longer periods of time, the decrease in the expression of p27(kip1) was noticed only after 6h incubation with ATP. Both effects were blocked by the P2 receptor antagonist PPADS. These data suggest that ATP induces cell proliferation in retinal explants by inducing late developing progenitors to progress from G1 to S phase of cell cycle.

Cellular signaling and factors involved in Müller cell gliosis: neuroprotective and detrimental effects

Müller cells are active players in normal retinal function and in virtually all forms of retinal injury and disease. Reactive Müller cells protect the tissue from further damage and preserve tissue function by the release of antioxidants and neurotrophic factors, and may contribute to retinal regeneration by the generation of neural progenitor/stem cells. However, Müller cell gliosis can also contribute to neurodegeneration and impedes regenerative processes in the retinal tissue by the formation of glial scars. This article provides an overview of the neuroprotective and detrimental effects of Müller cell gliosis, with accounts on the cellular signal transduction mechanisms and factors which are implicated in Müller cell-mediated neuroprotection, immunomodulation, regulation of Müller cell proliferation, upregulation of intermediate filaments, glial scar formation, and the generation of neural progenitor/stem cells. A proper understanding of the signaling mechanisms implicated in gliotic alterations of Müller cells is essential for the development of efficient therapeutic strategies that increase the supportive/protective and decrease the destructive roles of gliosis.

Zebrafish mutations in gart and paics identify crucial roles for de novo purine synthesis in vertebrate pigmentation and ocular development

Although purines and purinergic signaling are crucial for numerous biochemical and cellular processes, their functions during vertebrate embryonic development have not been well characterized. We analyze two recessive zebrafish mutations that affect de novo purine synthesis, gart and paics. gart encodes phosphoribosylglycinamide formyltransferase, phosphoribosylglycinamide synthetase, phosphoribosylaminoimidazole synthetase, a trifunctional enzyme that catalyzes steps 2, 3 and 5 of inosine monophosphate (IMP) synthesis. paics encodes phosphoribosylaminoimidazole carboxylase, phosphoribosylaminoimidazole succinocarboxamide synthetase, a bifunctional enzyme that catalyzes steps 6 and 7 of this process. Zygotic gart and paics mutants have pigmentation defects in which xanthophore and iridophore pigmentation is almost completely absent, and melanin-derived pigmentation is significantly decreased, even though pigment cells are present in normal amounts and distributions. Zygotic gart and paics mutants are also microphthalmic, resulting from defects in cell cycle exit of proliferative retinoblasts within the developing eye. Maternal-zygotic and maternal-effect mutants demonstrate a crucial requirement for maternally derived gart and paics; these mutants show more severe developmental defects than their zygotic counterparts. Pigmentation and eye growth phenotypes in zygotic gart and paics mutants can be ascribed to separable biosynthetic pathways: pigmentation defects and microphthalmia result from deficiencies in a GTP synthesis pathway and an ATP synthesis pathway, respectively. In the absence of ATP pathway activity, S phase of proliferative retinoblasts is prolonged and cell cycle exit is compromised, which results in microphthalmia. These results demonstrate crucial maternal and zygotic requirements for de novo purine synthesis during vertebrate embryonic development, and identify independent functions for ATP and GTP pathways in mediating eye growth and pigmentation, respectively.

Opposing effects of P2X(7) and P2Y purine/pyrimidine-preferring receptors on proliferation of astrocytes induced by fibroblast growth factor-2: implications for CNS development, injury, and repair

Extracellular nucleotides play important trophic roles in development and central nervous system (CNS) injury, but the functions of distinct purinergic receptors and related signaling pathways have not been fully elucidated. In the present study we identified opposing effects of P2X and P2Y receptors on the ability of FGF2 to induce proliferation in primary cultures of rat cortical astrocytes. Low concentrations of ATP enhanced DNA synthesis induced by FGF2, whereas high concentrations inhibited FGF2-induced proliferation. Comparison of concentration-response experiments with ATP and 2',3'-O-(4-benzoyl)-benzoyl-ATP (BzATP) indicated that the inhibitory effect was mediated by P2X(7) receptors. Interestingly, activation of P2X(7) receptors led to a state of reversible growth arrest rather than cell death. Selectivity studies showed that proliferation evoked by epidermal growth factor and platelet-derived growth factor was also inhibited by P2X(7) receptors, but P2X(1) or P2X(3) receptors did not inhibit proliferation induced by FGF2. A marker of mitosis, phosphohistone-3, was reduced by BzATP and increased by UTP, suggesting that the enhancing effect of ATP on FGF2-induced proliferation was mediated by P2 purine/pyrimidine receptors. Phosphorylation of the growth arrest-related protein kinases p38/MAPK and SAPK/JNK was strongly increased by BzATP but only weakly affected by UTP. We conclude that P2Y purine/pyrimidine receptors enhance proliferation induced by FGF2 in astrocytes, whereas stimulation of P2X(7) receptors inhibits proliferation by shifting cells to a state of reversible growth arrest that may be mediated by protein kinase signaling. These trophic actions of P2X(7) and P2Y purine/pyrimidine receptors may contribute to the regulation of CNS development, adult neurogenesis, and the response of astrocytes to injury.

Nucleotides in ocular secretions: their role in ocular physiology

The eye is the sense organ that permits the detection of light owing to the existence of a sophisticated neuronal array, called the retina, which is responsive to photons. The correct functioning of this complex system requires the coordination of several intraocular structures that ultimately permit the perfect focusing of images on the neural retina. Light has to pass through different media: the tear, the cornea, aqueous humour, lens, and vitreous humour before it reaches the retina. Moreover, the composition and structure of some of these media can change due to several physiological mechanisms. Nucleotides are active components of the humours bathing relevant ocular structures. The tear contains nucleotides and dinucleotides that control the process of tearing, wound healing and protects of superficial infections. In the inner eye, the aqueous humour also presents a collection of mono and dinucleotides that affect pupil contraction, aqueous humour production and accommodation. Behind the lens and between this structure and the retina the vitreous humour can modify the physiology of the retinal cells, mostly the ganglion cells. By investigating the actions of nucleotides and dinucleotide present in the ocular humours we will be able not only to understand the functioning of the ocular structures but also to develop new pharmacological therapies for pathologies such as dry eye, glaucoma or retinal detachment.

Dynamic ATP signalling and neural development

Purinergic signalling plays a major role in the function of every organ including the brain. A growing body of evidence also suggests that purinergic signalling is important in the development of the retina, cochlea and neocortex. In these three contexts release of ATP through the spontaneous gating of connexin hemichannels in cells, respectively, of the retinal pigment epithelium, Köllicker's organ, and the radial glia triggers waves of intracellular Ca(2+) release. In the case of the developing retina and cortex, the released ATP acts to control proliferation of neuronal precursor cells, while in the cochlea it coordinates the spontaneous activity of adjacent hair cells to refine the tonotopic maps in the cochlear nucleus. Recently ATP-derived ADP signalling has been implicated at the very earliest stages of development, notably in triggering the gene expression necessary for formation of the eye. It is now timely to test the extent to which connexin hemichannel-mediated ATP release and accompanying Ca(2+) waves contribute to all stages of development.

Müller glia as an active compartment modulating nervous activity in the vertebrate retina: neurotransmitters and trophic factors

Müller cells represent the main type of glia present in the retina interacting with most, if not all neurons in this tissue. Müller cells have been claimed to function as optic fibers in the retina delivering light to photoreceptors with minimal distortion and low loss [Franze et al (2007) Proc Natl Acad Sci 104:8287-8292]. Most of the mediators found in the brain are also detected in the retinal tissue, and glia cells are active players in the synthesis, release, signaling and uptake of major mediators of synaptic function. Müller glia trophic factors may regulate many different aspects of neuronal circuitry during synaptogenesis, differentiation, neuroprotection and survival of photoreceptors, Retinal Ganglion Cells (RGCs) and other targets in the retina. Here we review the role of several transmitters and trophic factors that participate in the neuron-glia loop in the retina.

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.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

ATP stimulates mouse embryonic stem cell proliferation via protein kinase C, phosphatidylinositol 3-kinase/Akt, and mitogen-activated protein kinase signaling pathways

This study investigated the effect of ATP and its related signal cascades on the proliferation of mouse ESCs. ATP increased the level of [(3)H]thymidine/5-bromo-2'-deoxyuridine incorporation and the number of cells in both a time- and dose-dependent manner. AMP-CPP (a P2X(1) and P2X(3) agonist), ATP-gammaS (a P2Y agonist), and 2-methylthio-ATP (a P2X and P2Y agonist) stimulated [(3)H]thymidine incorporation. P2 purinoceptor antagonists (suramin, reactive blue 2) inhibited the ATP-induced increase in [(3)H]thymidine incorporation. Reverse transcription-polymerase chain reaction analysis revealed P2X(3), P2X(4), P2Y(1), and P2Y(2) expression in mouse ESCs. Adenylate cyclase inhibitor (SQ 22536), phospholipase C inhibitors (neomycin or U 73122), and protein kinase C (PKC) inhibitors (bisindolylmaleimide I or staurosporine) inhibited the ATP-induced increase in [(3)H]thymidine incorporation. ATP increased the level of intracellular cAMP and inositol phosphates. ATP translocated PKC alpha, delta, and zeta from the cytosol to the membrane compartment. ATP and its agonists increased [Ca(2+)](i). In addition, the ATP-induced increase in [(3)H]thymidine incorporation was completely inhibited by a combination of EGTA (extracellular Ca(2+) chelator) and 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-AM (intracellular Ca(2+) chelator). ATP phosphorylated Akt and p44/42 mitogen-activated protein kinases (MAPKs) in a time-dependent manner, and either suramin or reactive blue 2 (RB2) blocked the ATP-induced phosphorylation of Akt. Suramin, RB2, the phosphatidylinositol 3-kinase (PI3K) inhibitor (wortmannin), or the Akt inhibitor inhibited the phosphorylation of p44/42 MAPKs. The ATP-induced increase in [(3)H]thymidine incorporation was inhibited by wortmannin, the Akt inhibitor, and the MAPK kinase inhibitor (PD 98059). Suramin, RB2, PD 98059, and wortmannin blocked the ATP-induced increase in the cyclin D1, cyclin E, cyclin-dependent kinase (CDK) 2, and CDK4 levels. In conclusion, ATP stimulates mouse ESC proliferation through PKC, PI3K/Akt, and MAPKs via the P2 purinoceptors.

Effect of extracellular ATP on the human leukaemic cell line K562 and its multidrug counterpart

Extracellular ATP (ATPo) is capable of inducing different events on cells through receptor activation. The effect produced by ATPo was studied in the cell line K562 and its multidrug resistant (MDR) counterpart, Lucena 1. Lower ATPo concentrations (1 mM and 2.5 mM) led to high (3)H-thymidine incorporation but no increase in cell number. Similarly, the cell cycle profile indicated an increase of cells in S phase and a decrease in G1 and G2, suggesting that the cells did not duplicate their DNA content. Higher doses of ATP (5 mM and 10 mM), as well as UTP (5 mM) and the P2X(7) agonist BzATP, were cytotoxic. However, no expression of P2X(7) receptors could be detected by Western Blot nor were the cells permeabilised by ATP, suggesting that pore formation was not involved in cell death. Both ecto-ATPase and ecto-5'-nucleotidase activity could be demonstrated at the surfaces of K562 and Lucena 1 cells, the latter presenting a higher ecto-5'-nucleotidase activity. Adenosine induced cell death at lower concentrations (2.5 mM) on both cell lines. Furthermore, an increased number of dead cells could be observed when 5 mM Adenosine was used compared to the same concentrations of ATPo. It still remains to be elucidated the nature of the receptors involved in the induction of cell death in these cells.

Nucleotide signaling in nervous system development

The development of the nervous system requires complex series of cellular programming and intercellular communication events that lead from the early neural induction to the formation of a highly structured central and peripheral nervous system. Neurogenesis continuously takes place also in select regions of the adult mammalian brain. During the past years, a multiplicity of cellular control mechanisms has been identified, ranging from differential transcriptional mediators to inducers or inhibitors of cell specification or neurite outgrowth. While the identification of transcription factors typical for the stage-specific progression has been a topic of key interest for many years, less is known concerning the potential multiplicity of relevant intercellular signaling pathways and the fine tuning of epigenetic gene regulation. Nucleotide receptors can induce a multiplicity of cellular signaling pathways and are involved in multiple molecular interactions, thus opening the possibility of cross talk between several signaling pathways, including growth factors, cytokines, and extracellular matrix components. An increasing number of studies provides evidence for a role of nucleotide signaling in nervous system development. This includes progenitor cell proliferation, cell migration, neuronal and glial cellular interaction and differentiation, and synaptic network formation.

ATP Released via Gap Junction Hemichannels from the Pigment Epithelium Regulates Neural Retinal Progenitor Proliferation

The retinal pigment epithelium (RPE) plays an essential role in the normal development of the underlying neural retina, but the mechanisms by which this regulation occurs are largely unknown. Ca2+ transients, induced by the neurotransmitter ATP acting on purinergic receptors, both increase proliferation and stimulate DNA synthesis in neural retinal progenitor cells. Here, we show that the RPE regulates proliferation in the underlying neural retina by the release of a soluble factor and identify that factor as ATP. Further, we show that this ATP is released by efflux through gap junction connexin 43 hemichannels, the opening of which is evoked by spontaneous elevations of Ca2+ in trigger cells in the RPE. This release mechanism is localized within the RPE cells to the membranes facing the neural retina, a location ideally positioned to influence neural retinal development. ATP released from RPE hemichannels speeds both cell division and proliferation in the neural retina.

Calcium Waves Propagate through Radial Glial Cells and Modulate Proliferation in the Developing Neocortex

The majority of neurons in the adult neocortex are produced embryonically during a brief but intense period of neuronal proliferation. The radial glial cell, a transient embryonic cell type known for its crucial role in neuronal migration, has recently been shown to function as a neuronal progenitor cell and appears to produce most cortical pyramidal neurons. Radial glial cell modulation could thus affect neuron production, neuronal migration, and overall cortical architecture; however, signaling mechanisms among radial glia have not been studied directly. We demonstrate here that calcium waves propagate through radial glial cells in the proliferative cortical ventricular zone (VZ). Radial glial calcium waves occur spontaneously and require connexin hemichannels, P2Y1 ATP receptors, and intracellular IP3-mediated calcium release. Furthermore, we show that wave disruption decreases VZ proliferation during the peak of embryonic neurogenesis. Taken together, these results demonstrate a radial glial signaling mechanism that may regulate cortical neuronal production.

Ca2+ signalling and gap junction coupling within and between pigment epithelium and neural retina in the developing chick

Development of the neural retina is controlled in part by the adjacent retinal pigment epithelium (RPE). To understand better the mechanisms involved, we investigated calcium signalling and gap junctional coupling within and between the RPE and the neural retina in embryonic day (E) 5 chick. We show that the RPE and the ventricular zone (VZ) of the neural retina display spontaneous Ca(2+) transients. In the RPE, these often spread as waves between neighbouring cells. In the VZ, the frequency of both Ca(2+) transients and waves was lower than in RPE, but increased two-fold in its presence. Ca(2+) signals occasionally crossed the boundary between the RPE and VZ in either direction. In both tissues, the frequency of propagating Ca(2+) waves, but not of individual cell transients, was reduced by gap junction blockers. Use of the gap junction permeant tracer Neurobiotin showed that neural retina cells are coupled into clusters that span the thickness of the retina, and that RPE cells are both coupled together and to clusters of cells in the neural retina. Immunolabelling for Cx43 showed this gap junction protein is present at the junction between the RPE and VZ and thus could potentially mediate the coupling of the two tissues. Immunolabelling for beta-tubulin and vimentin showed that clusters of coupled cells in the neural retina comprised mainly progenitor cells. We conclude that gap junctions between progenitor cells, and between these cells and the RPE, may orchestrate retinal proliferation/differentiation, via the propagation of Ca(2+) or other signalling molecules.

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.

ATP-evoked calcium responses of radial glial (Müller) cells in the postnatal rabbit retina

Here we show that rabbit Müller cell differentiation from radial glial progenitor cells is accompanied by a decreasing capability to respond to specific stimuli (depolarization and extracellular adenosine 5'-triphosphate [ATP]) with an elevation of intracellular calcium. Intracellular free calcium was recorded in retinal wholemounts from young (postnatal days [P] 2 to 31) and adult rabbits. Images were taken from the nerve fiber/ganglion cell layers where the endfeet of radial glial/ Müller cells can be identified after selective uptake of calcium-sensitive dyes. The area of responding endfeet was determined as the percentage of the total area occupied by Müller cell endfeet, as an estimate of the percentage of responding cells. In response to depolarization (50 mM potassium), an increase of intracellular free calcium occurred in 19% of cells from young postnatal retinae (P2-31) but only in 2% from adults. This depolarization-induced calcium rise was caused both by a calcium influx from extracellular space and by an intracellular calcium release. The latter response was inhibited by the P2 receptor blocker pyridoxal phosphate 6-azophenyl-2',4'-disulfonic acid (PPADS), indicating that extracellular calcium-independent ATP release into the extracellular space occurs during retinal depolarization. When extracellular ATP (200 microM) was applied, calcium responses were recorded in 83% of cells from young postnatal retinae (P2-6); in the course of further development, both the percentage of responding cells (7% in retinae from adult rabbits) and the amplitude of the calcium responses decreased. It is concluded that during the differentiation of immature radial glia into mature Müller cells, stimulus-evoked intracellular calcium signaling mechanisms change.

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.

Spatiotemporal features of early neuronogenesis differ in wild-type and albino mouse retina

In albino mammals, lack of pigment in the retinal pigment epithelium is associated with retinal defects, including poor visual acuity from a photoreceptor deficit in the central retina and poor depth perception from a decrease in ipsilaterally projecting retinal fibers. Possible contributors to these abnormalities are reported delays in neuronogenesis (Ilia and Jeffery, 1996) and retinal maturation (Webster and Rowe, 1991). To further determine possible perturbations in neuronogenesis and/or differentiation, we used cell-specific markers and refined birth dating methods to examine these events during retinal ganglion cell (RGC) genesis in albino and pigmented mice from embryonic day 11 (E11) to E18. Our data indicate that relative to pigmented mice, more ganglion cells are born in the early stages of neuronogenesis in the albino retina, although the initiation of RGC genesis in the albino is unchanged. The cellular organization of the albino retina is perturbed as early as E12. In addition, cell cycle kinetics and output along the nasotemporal axis differ in retinas of albino and pigmented mice, both absolutely, with the temporal aspect of the retina expanded in albino, and relative to the position of the optic nerve head. Finally, blocking melanin synthesis in pigmented eyecups in culture leads to an increase in RGC differentiation, consistent with a role for melanin formation in regulating RGC neuronogenesis. These results point to spatiotemporal defects in neuronal production in the albino retina, which could perturb expression of genes that specify cell fate, number, and/or projection phenotype.

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.

Activation of P2Y receptors stimulates potassium and cation currents in acutely isolated human Müller (glial) cells

The ability of various neurotransmitters/neuroactive substances to induce fast, transient rises of Ca(2+)-activated K(+) currents (I(BK)) caused by release of Ca(2+) from intracellular stores was investigated in Müller glial cells of the human retina. Müller cells were enzymatically isolated from retinas of healthy donors or of patients with proliferative vitreoretinopathy, and the transmembrane ionic currents were recorded using the whole-cell and the cell-attached patch-clamp techniques. The results of the screening experiments indicate that human Müller cells express, in addition to GABA(A) and perhaps glutamatergic and cholinergic receptors, predominantly P2 receptors. ATP and other nucleotides exerted two effects on membrane currents: repetitive transient increases of the I(BK) amplitude and, in a subpopulation of cells investigated, the appearance of a transient cation conductance at negative potentials. ATP and UTP increased dose-dependently the I(BK) amplitude with half-maximal effects at 0.33 and 0.50 microM, respectively. Since several different P2 receptor agonists increased the I(BK), it is assumed that human Müller cells express a mixture of different types of P2Y receptors. In cell-attached patches, extracellular application of ATP or UTP transiently increased the open probability of single putative BK channels. The increase of I(BK) and the appearance of the cation conductance in whole-cell records were abolished when intracellular Ca(2+) was buffered by a high-EGTA pipette solution or when IP(3) was included in the pipette solution. The expression of agonist-evoked transient cation currents was found to be stronger in cells from patients as compared to cells from healthy donors. It is concluded that human Müller glial cells express P2Y receptors that, via IP(3) formation, cause intracellular Ca(2+) release. The increased intracellular Ca(2+) concentration stimulates the activity of BK channels and may induce opening of cation channels. Both the ATP-induced activity of BK channels and the increased expression of Ca(2+)-gated cation channels may be important in respect to proliferative Müller cell gliosis.

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.

Lysophosphatidic acid-induced Ca(2+) mobilization in the neural retina of chick embryo

Lysophosphatidic acid (LPA) plays various roles in the regulation of cell growth as a lipid mediator. We studied the effect of LPA on intracellular Ca(2+) concentration ([Ca2+]i) with Fura-2 in the neural retina of chick embryo during neurogenesis. Bath application of LPA (1-100 microM) to the embryonic day 3 (E3) chick retina caused an increase in [Ca2+](i) in a dose-dependent manner, with an EC(50) value of 9.2 microM. The Ca(2+) rise was also evoked in a Ca(2+)-free medium, suggesting that release of Ca(2+) from intracellular Ca(2+) stores (Ca(2+) mobilization) was induced by LPA. U-73122, a blocker of phospholipase C (PLC), inhibited the Ca(2+) rise to LPA. Pertussis toxin partially inhibited the Ca(2+) rise to LPA, indicating that G(i)/G(o) protein was at least partially involved in the LPA response. The developmental profile of the LPA response was studied from E3 to E13. The Ca(2+) rise to LPA declined drastically from E3 to E7, in parallel with decrease in mitotic activity of retinal progenitor cells. The signal transduction pathway and developmental profile of the Ca(2+) response to LPA were the same as those of the Ca(2+) response to adenosine triphosphate (ATP), which enhances the proliferation of retinal progenitor cells. The coapplication of LPA with ATP resulted in enhancement of Ca(2+) rise in the E3 chick retina. Our results show that LPA induces Ca(2+) mobilization in the embryonic chick retina during neurogenesis.

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.