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Rapid Release of Ca 2+ from Endoplasmic Reticulum Mediated by Na +/Ca 2+ Exchange. J Neurosci 2020; 40:3152-3164. [PMID: 32156830 DOI: 10.1523/jneurosci.2675-19.2020] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/08/2019] [Revised: 02/04/2020] [Accepted: 02/12/2020] [Indexed: 11/21/2022] Open
Abstract
Phototransduction in Drosophila is mediated by phospholipase C (PLC) and Ca2+-permeable TRP channels, but the function of endoplasmic reticulum (ER) Ca2+ stores in this important model for Ca2+ signaling remains obscure. We therefore expressed a low affinity Ca2+ indicator (ER-GCaMP6-150) in the ER, and measured its fluorescence both in dissociated ommatidia and in vivo from intact flies of both sexes. Blue excitation light induced a rapid (tau ∼0.8 s), PLC-dependent decrease in fluorescence, representing depletion of ER Ca2+ stores, followed by a slower decay, typically reaching ∼50% of initial dark-adapted levels, with significant depletion occurring under natural levels of illumination. The ER stores refilled in the dark within 100-200 s. Both rapid and slow store depletion were largely unaffected in InsP3 receptor mutants, but were much reduced in trp mutants. Strikingly, rapid (but not slow) depletion of ER stores was blocked by removing external Na+ and in mutants of the Na+/Ca2+ exchanger, CalX, which we immuno-localized to ER membranes in addition to its established localization in the plasma membrane. Conversely, overexpression of calx greatly enhanced rapid depletion. These results indicate that rapid store depletion is mediated by Na+/Ca2+ exchange across the ER membrane induced by Na+ influx via the light-sensitive channels. Although too slow to be involved in channel activation, this Na+/Ca2+ exchange-dependent release explains the decades-old observation of a light-induced rise in cytosolic Ca2+ in photoreceptors exposed to Ca2+-free solutions.SIGNIFICANCE STATEMENT Phototransduction in Drosophila is mediated by phospholipase C, which activates TRP cation channels by an unknown mechanism. Despite much speculation, it is unknown whether endoplasmic reticulum (ER) Ca2+ stores play any role. We therefore engineered flies expressing a genetically encoded Ca2+ indicator in the photoreceptor ER. Although NCX Na+/Ca2+ exchangers are classically believed to operate only at the plasma membrane, we demonstrate a rapid light-induced depletion of ER Ca2+ stores mediated by Na+/Ca2+ exchange across the ER membrane. This NCX-dependent release was too slow to be involved in channel activation, but explains the decades-old observation of a light-induced rise in cytosolic Ca2+ in photoreceptors bathed in Ca2+-free solutions.
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Liu CH, Bollepalli MK, Long SV, Asteriti S, Tan J, Brill JA, Hardie RC. Genetic dissection of the phosphoinositide cycle in Drosophila photoreceptors. J Cell Sci 2018; 131:jcs.214478. [PMID: 29567856 DOI: 10.1242/jcs.214478] [Citation(s) in RCA: 15] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2017] [Accepted: 03/16/2018] [Indexed: 11/20/2022] Open
Abstract
Phototransduction in Drosophila is mediated by phospholipase C-dependent hydrolysis of PIP2-, and is an important model for phosphoinositide signalling. Although generally assumed to operate by generic machinery conserved from yeast to mammals, some key elements of the phosphoinositide cycle have yet to be identified in Drosophila photoreceptors. Here, we used transgenic flies expressing fluorescently tagged probes (P4M and TbR332H), which allow in vivo quantitative measurements of PI4P and PIP2 dynamics in photoreceptors of intact living flies. Using mutants and RNA interference for candidate genes potentially involved in phosphoinositide turnover, we identified Drosophila PI4KIIIα (CG10260) as the PI4-kinase responsible for PI4P synthesis in the photoreceptor membrane. Our results also indicate that PI4KIIIα activity requires rbo (the Drosophila orthologue of Efr3) and CG8325 (orthologue of YPP1), both of which are implicated as scaffolding proteins necessary for PI4KIIIα activity in yeast and mammals. However, our evidence indicates that the recently reported central role of dPIP5K59B (CG3682) in PIP2 synthesis in the rhabdomeres should be re-evaluated; although PIP2 resynthesis was suppressed by RNAi directed against dPIP5K59B, little or no defect was detected in a reportedly null mutant (dPIP5K18 ).
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Affiliation(s)
- Che-Hsiung Liu
- Department of Physiology, Development and Neuroscience, Cambridge University, Downing St, Cambridge CB2 3EG, United Kingdom
| | - Murali K Bollepalli
- Department of Physiology, Development and Neuroscience, Cambridge University, Downing St, Cambridge CB2 3EG, United Kingdom
| | - Samuel V Long
- Department of Physiology, Development and Neuroscience, Cambridge University, Downing St, Cambridge CB2 3EG, United Kingdom
| | - Sabrina Asteriti
- Department of Physiology, Development and Neuroscience, Cambridge University, Downing St, Cambridge CB2 3EG, United Kingdom
| | - Julie Tan
- Program in Cell Biology, The Hospital for Sick Children, Peter Gilgan Centre for Research and Learning, 686 Bay Street, Room 15.9716, Toronto, ON M5G 0A4, Canada.,Department of Molecular Genetics, University of Toronto, Room 4396, Medical Sciences Building, 1 King's College Circle, Toronto, ON M5S 1A8, Canada
| | - Julie A Brill
- Program in Cell Biology, The Hospital for Sick Children, Peter Gilgan Centre for Research and Learning, 686 Bay Street, Room 15.9716, Toronto, ON M5G 0A4, Canada.,Department of Molecular Genetics, University of Toronto, Room 4396, Medical Sciences Building, 1 King's College Circle, Toronto, ON M5S 1A8, Canada
| | - Roger C Hardie
- Department of Physiology, Development and Neuroscience, Cambridge University, Downing St, Cambridge CB2 3EG, United Kingdom
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Katz B, Voolstra O, Tzadok H, Yasin B, Rhodes-Modrov E, Bartels JP, Strauch L, Huber A, Minke B. The latency of the light response is modulated by the phosphorylation state of Drosophila TRP at a specific site. Channels (Austin) 2017; 11:678-685. [PMID: 28762890 DOI: 10.1080/19336950.2017.1361073] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/19/2022] Open
Abstract
Drosophila photoreceptors respond to oscillating light of high frequency (∼100 Hz), while increasing the oscillating light intensity raises the maximally detected frequency. Recently, we reported that dephosphorylation of the light-activated TRP ion channel at S936 is a fast, graded, light-, and Ca2+-dependent process. We further found that this process affects the detection limit of high frequency oscillating light. Accordingly, transgenic Drosophila, which do not undergo phosphorylation at the S936-TRP site (trpS936A), revealed a short time-interval before following the high stimulus frequency (oscillation-lock response) in both dark- and light-adapted flies. In contrast, the trpS936D transgenic flies, which mimic constant phosphorylation, showed a long-time interval to oscillation-lock response in both dark- and light-adapted flies. Here we extend these findings by showing that dark-adapted trpS936A flies reveal light-induced current (LIC) with short latency relative to trpWT or trpS936D flies, indicating that the channels are a limiting factor of response kinetics. The results indicate that properties of the light-activated channels together with the dynamic light-dependent process of TRP phosphorylation at the S936 site determine response kinetics.
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Affiliation(s)
- Ben Katz
- a Department of Medical Neurobiology , Faculty of Medicine and the Edmond and Lily Safra Center for Brain Sciences (ELSC), Hebrew University , Jerusalem , Israel
| | - Olaf Voolstra
- b Department of Biosensorics , Institute of Physiology, University of Hohenheim , Stuttgart , Germany
| | - Hanan Tzadok
- a Department of Medical Neurobiology , Faculty of Medicine and the Edmond and Lily Safra Center for Brain Sciences (ELSC), Hebrew University , Jerusalem , Israel
| | - Bushra Yasin
- a Department of Medical Neurobiology , Faculty of Medicine and the Edmond and Lily Safra Center for Brain Sciences (ELSC), Hebrew University , Jerusalem , Israel
| | - Elisheva Rhodes-Modrov
- a Department of Medical Neurobiology , Faculty of Medicine and the Edmond and Lily Safra Center for Brain Sciences (ELSC), Hebrew University , Jerusalem , Israel
| | - Jonas-Peter Bartels
- b Department of Biosensorics , Institute of Physiology, University of Hohenheim , Stuttgart , Germany
| | - Lisa Strauch
- b Department of Biosensorics , Institute of Physiology, University of Hohenheim , Stuttgart , Germany
| | - Armin Huber
- b Department of Biosensorics , Institute of Physiology, University of Hohenheim , Stuttgart , Germany
| | - Baruch Minke
- a Department of Medical Neurobiology , Faculty of Medicine and the Edmond and Lily Safra Center for Brain Sciences (ELSC), Hebrew University , Jerusalem , Israel
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