P2X7 receptor large pore signaling in avian Müller glial cells.

ATP is a pleiotropic molecule that promotes extra- and intracellular signaling to regulate numerous functions. This nucleotide activates purine and pyrimidine receptors at the plasma membrane, categorized as ionotropic P2X or G-protein-coupled receptor (GPCR) P2Y receptors. P2X are ligand-gated ion channel receptors, expressed in both retinal neurons and Müller cells leading to neuron-glia communication, calcium waves and neurovascular coupling. However, how P2X pore is formed upon ATP activation and how signaling pathways regulates the complex is still a matter of controversy. Here we studied the properties of the P2X7 receptor (P2X7R) using electrophysiology, single cell Ca2+ imaging, and dye uptake assay in purified avian Müller glia in culture. Our data show that ATP (or benzoyl-benzoyl ATP, BzATP) evoked large inward currents in patch-clamp studies while addition of P2X7R antagonist such as brilliant Blue G (BBG), abolished these currents. Ruthenium red (RU-2), a general transient receptor potential (TRP) inhibitor, reduced currents induced by ATP. Our data also point to the involvement of mitogen activated protein kinase (MAPK), phosphoinositide 3-kinase (PI3K), Ca2+-calmodulin kinase II (CAMKII), microtubules or protein kinase C (PKC) modulating ATP-induced ionic current in Müller cells. We show that ATP induced Ca2+ influx, partially inhibited by P2X7R antagonists (oxidized ATP or BBG), and totally inhibited by blockers of other pores such as transient receptor potential (TRPs) or connexin hemichannel. Additionally, MAPK, PKC, PI3K or CAMKII inhibitors also are involved in the modulation of intracellular calcium signaling. Finally, ATP induced 80-90% of dye uptake in Muller glia cells, while oxidized ATP (oATP), BBG or A740003 inhibited this effect. We conclude that large conductance channel and other P2XRs are not involved in the ATP-induced dye uptake, but signaling pathways such as MAPK, PI3-K, microtubules or PKC are involved in pore formation.

Visualizing Shifts on Neuron-Glia Circuit with the Calcium Imaging Technique.

Here, we report on selective in vitro models of circuits based on glia (astrocytes, oligodendrocytes, and microglia) and/or neurons from peripheral (dorsal root ganglia) and central tissues (cortex, subventricular zone, organoid) that are dynamically studied in terms of calcium shifts. The model chosen to illustrate the results is the retina, a simple tissue with complex cellular interactions. Calcium is a universal messenger involved in most of the important cellular roles. We explain in a step-by-step protocol how retinal neuron-glial cells in culture can be prepared and evaluated, envisioning calcium shifts. In this model, we differentiate neurons from glia based on their selective response to KCl and ATP. Calcium permeable receptors and channels are selectively expressed in different compartments. To analyze calcium responses, we use ratiometric fluorescent dies such as Fura-2. This probe quantifies free Ca2+ concentration based on Ca2+-free and Ca2+-bound forms, presenting two different peaks, founded on the fluorescence intensity perceived on two wavelengths.

Capsaicin inhibits lipopolysaccharide-induced adrenal steroidogenesis by raising intracellular calcium levels.

INTRODUCTION: Glucocorticoid release by adrenals has been described as significant to survive sepsis. The activation of transient receptor potential vanilloid type 1 (TRPV1) inhibited ACTH-induced glucocorticoid release by adrenal glands in vitro. OBJECTIVE: The aim of this study was to investigate if capsaicin, an activator of TRPV1, would prevent LPS-induced glucocorticoid production by adrenals. METHODS: Male Swiss-Webster mice were treated with capsaicin intraperitoneally (0.2 or 2 mg/kg) 30 min before LPS injection. All analyses were performed 2 h after the LPS stimulation, including plasma corticosterone and peritoneal IL-1ß and TNF-α levels. Furthermore, murine adrenocortical Y1 cells were used to assess the effects of capsaicin on LPS-induced corticosterone production in vitro. RESULTS: Capsaicin (2 mg/kg, i.p.) significantly reduced plasma corticosterone levels and adrenal hypertrophy induced by LPS without alter the levels of pro-steroidogenic cytokines IL-1ß and TNF-α in peritoneal cavity of mice, while the dose of 0.2 mg/kg of capsaicin did not interfere with adrenal steroidogenesis, attested by RIA and ELISA, respectively. Y1 cells express TRPV1, measured by immunofluorescence and western blot, and capsaicin decreased LPS-induced corticosterone production by these cells in vitro. Capsaicin also induces calcium mobilization in Y1 cells in vitro. CONCLUSIONS: These findings suggest that capsaicin inhibits corticosterone production induced by LPS by acting directly on adrenal cells producing glucocorticoids, in a mechanism probably associated with induction of a cytoplasmic calcium increase in these cells.

Glutathione-Induced Calcium Shifts in Chick Retinal Glial Cells.

Neuroglia interactions are essential for the nervous system and in the retina Müller cells interact with most of the neurons in a symbiotic manner. Glutathione (GSH) is a low-molecular weight compound that undertakes major antioxidant roles in neurons and glia, however, whether this compound could act as a signaling molecule in neurons and/or glia is currently unknown. Here we used embryonic avian retina to obtain mixed retinal cells or purified Müller glia cells in culture to evaluate calcium shifts induced by GSH. A dose response curve (0.1-10 mM) showed that 5-10 mM GSH, induced calcium shifts exclusively in glial cells (later labeled and identified as 2M6 positive cells), while neurons responded to 50 mM KCl (labeled as ßIII tubulin positive cells). BBG 100 nM, a P2X7 blocker, inhibited the effects of GSH on Müller glia. However, addition of DNQX 70 µM and MK-801 20 µM, non-NMDA and NMDA blockers, had no effect on GSH calcium induced shift. Oxidized glutathione (GSSG) at 5 mM failed to induce calcium mobilization in glia cells, indicating that the antioxidant and/or structural features of GSH are essential to promote elevations in cytoplasmic calcium levels. Indeed, a short GSH pulse (60s) protects Müller glia from oxidative damage after 30 min of incubation with 0.1% H2O2. Finally, GSH induced GABA release from chick embryonic retina, mixed neuron-glia or from Müller cell cultures, which were inhibited by BBG or in the absence of sodium. GSH also induced propidium iodide uptake in Müller cells in culture in a P2X7 receptor dependent manner. Our data suggest that GSH, in addition to antioxidant effects, could act signaling calcium shifts at the millimolar range particularly in Müller glia, and could regulate the release of GABA, with additional protective effects on retinal neuron-glial circuit.