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P. Barabas, T. Szikra, T. M. Bartholetti, W. B. Thoreson, D. Krizaj; Store-operated Calcium Entry Regulates Steady-state and Evoked Calcium Signals in Salamander Cones. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1013.
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Calcium stores are critical elements in intracellular signaling and synaptic transmission. Recent work in rod photoreceptors suggests that depletion of ER stores activates store-operated calcium entry (SOCE) through canonical TRPC1 channels. These channels were not expressed in cones. We analyzed properties of SOCE in cones, ascertained its role in tonic neurotransmission and investigated the pathway through which store depletion is communicated to the plasma membrane.
Calcium imaging was performed in dissociated cells and retinal slices from the salamander (Ambystoma tigrinum) retinas loaded with calcium indicator dyes. Immunofluorescence of isolated cells and slices was viewed on a confocal microscope. Paired recordings from cone-HC and cone-bipolar pairs were obtained from slices with the whole cell patch technique.
Irreversible inhibition of Ca release from ER stores caused a sustained increase in baseline calcium levels. This was potentiated by increasing the driving force for Ca entry and inhibited by lanthanide antagonists of Ca-permeable channels (La3+ and Gd3+; 1-10 uM). SOCE was blocked by removing extracellular source of Ca and potentiated by switching from Ca-free to mM Ca-containing saline. Store depletion-evoked Ca ‘overshoots’ were antagonized by uM concentrations of organic antagonists of SOCE, MRS-1845, SKF 96365 and 2-APB. MRS-1845 (15 uM) by itself had no effect on voltage-operated Ca currents and concentration levels in rods, cones or ganglion cells. However, a significant reduction in late EPSC was observed in cone-HC pairs during exposure to MRS-1845. Exposure to the diacylglycerol analog 1-oleoyl-2-acety-sn-glycerol (OAG) induced rapid [Ca2+]i elevations in a subset of cones. Immunohistochemistry revealed prominent STIM1 signals in cones.
We show that calcium stores in cones modulate both steady-state Ca signals in the cell body and neurotransmission at the synaptic terminal. This mechanism exhibits all hallmarks of SOCE, including potential activation by depletion-induced aggregation of STIM1 in subplasma membrane ER. Cone SOCE may involve TRPC3/6 or 7 family members of the TRPC family. While store-operated and/or TRP channels in cones may be distinct from those in rods, the properties of SOCE in cones suggests a fundamental role in light-adapted photopic signals.
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