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Pryazhnikov E, Khiroug L. Sub-micromolar increase in [Ca(2+)](i) triggers delayed exocytosis of ATP in cultured astrocytes. Glia 2008; 56:38-49. [PMID: 17910050 DOI: 10.1002/glia.20590] [Citation(s) in RCA: 66] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
Astrocytes release a variety of transmitter molecules, which mediate communication between glial cells in the brain and modulate synaptic transmission. ATP is a major glia-derived transmitter, but the mechanisms and kinetics of ATP release from astrocytes remain largely unknown. Here, we combined epifluorescence and total internal reflection fluorescence microscopy to monitor individual quinacrine-loaded ATP-containing vesicles undergoing exocytosis in cultured astrocytes. In resting cells, vesicles exhibited three-dimensional motility, spontaneous docking and release at low rate. Extracellular ATP application induced a Ca(2+)-dependent increase in the rate of exocytosis, which persisted for several minutes. Using UV flash photolysis of caged Ca(2+), the threshold [Ca(2+)](i) for ATP exocytosis was found to be approximately 350 nM. Subthreshold [Ca(2+)](i) transients predominantly induced vesicle docking at plasma membrane without subsequent release. ATP exocytosis triggered either by purinergic stimulation or by Ca(2+) uncaging occurred after a substantial delay ranging from tens to hundreds of seconds, with only approximately 4% of release occurring during the first 30 s. The time course of the cargo release from vesicles had two peaks centered on <or=10 s and 60 s. These results demonstrate that: (1) [Ca(2+)](i) elevations in cultured astrocytes trigger docking and release of ATP-containing vesicles; (2) vesicle docking and release have different Ca(2+) thresholds; (3) ATP exocytosis is delayed by several minutes and highly asynchronous; (4) two populations of ATP-containing vesicles with distinct (fast and slow) time course of cargo release exist in cultured astrocytes.
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Affiliation(s)
- Evgeny Pryazhnikov
- Neuroscience Center, University of Helsinki, P.O. Box 56 (Viikinkaari 4), FIN-00014, Helsinki, Finland
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Pangršič T, Potokar M, Stenovec M, Kreft M, Fabbretti E, Nistri A, Pryazhnikov E, Khiroug L, Giniatullin R, Zorec R. Exocytotic release of ATP from cultured astrocytes. J Biol Chem 2007; 282:28749-28758. [PMID: 17627942 DOI: 10.1074/jbc.m700290200] [Citation(s) in RCA: 190] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
Astrocytes appear to communicate with each other as well as with neurons via ATP. However, the mechanisms of ATP release are controversial. To explore whether stimuli that increase [Ca(2+)](i) also trigger vesicular ATP release from astrocytes, we labeled ATP-containing vesicles with the fluorescent dye quinacrine, which exhibited a significant co-localization with atrial natriuretic peptide. The confocal microscopy study revealed that quinacrine-loaded vesicles displayed mainly non-directional spontaneous mobility with relatively short track lengths and small maximal displacements, whereas 4% of vesicles exhibited directional mobility. After ionomycin stimulation only non-directional vesicle mobility could be observed, indicating that an increase in [Ca(2+)](i) attenuated vesicle mobility. Total internal reflection fluorescence (TIRF) imaging in combination with epifluorescence showed that a high percentage of fluorescently labeled vesicles underwent fusion with the plasma membrane after stimulation with glutamate or ionomycin and that this event was Ca(2+)-dependent. This was confirmed by patch-clamp studies on HEK-293T cells transfected with P2X(3) receptor, used as sniffers for ATP release from astrocytes. Glutamate stimulation of astrocytes was followed by an increase in the incidence of small transient inward currents in sniffers, reminiscent of postsynaptic quantal events observed at synapses. Their incidence was highly dependent on extracellular Ca(2+). Collectively, these findings indicate that glutamate-stimulated ATP release from astrocytes was most likely exocytotic and that after stimulation the fraction of quinacrine-loaded vesicles, spontaneously exhibiting directional mobility, disappeared.
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Affiliation(s)
- Tina Pangršič
- Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, SI-1000 Ljubljana, Slovenia; Celica Biomedical Center, Proletarska cesta 4, SI-1000 Ljubljana, Slovenia
| | - Maja Potokar
- Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, SI-1000 Ljubljana, Slovenia; Celica Biomedical Center, Proletarska cesta 4, SI-1000 Ljubljana, Slovenia
| | - Matjaž Stenovec
- Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, SI-1000 Ljubljana, Slovenia; Celica Biomedical Center, Proletarska cesta 4, SI-1000 Ljubljana, Slovenia
| | - Marko Kreft
- Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, SI-1000 Ljubljana, Slovenia; Celica Biomedical Center, Proletarska cesta 4, SI-1000 Ljubljana, Slovenia
| | - Elsa Fabbretti
- Neurobiology Sector, International School for Advanced Studies (SISSA), Via Beirut 2-4, 34014 Trieste, Italy
| | - Andrea Nistri
- Neurobiology Sector, International School for Advanced Studies (SISSA), Via Beirut 2-4, 34014 Trieste, Italy
| | - Evgeny Pryazhnikov
- Neuroscience Center, University of Helsinki, PO Box 56 (Viikinkaari 4), FIN-00014 Helsinki, Finland
| | - Leonard Khiroug
- Neuroscience Center, University of Helsinki, PO Box 56 (Viikinkaari 4), FIN-00014 Helsinki, Finland
| | - Rashid Giniatullin
- Neurobiology Sector, International School for Advanced Studies (SISSA), Via Beirut 2-4, 34014 Trieste, Italy
| | - Robert Zorec
- Laboratory of Neuroendocrinology-Molecular Cell Physiology, Institute of Pathophysiology, Faculty of Medicine, University of Ljubljana, Zaloška 4, SI-1000 Ljubljana, Slovenia; Celica Biomedical Center, Proletarska cesta 4, SI-1000 Ljubljana, Slovenia.
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