Store‐operated channels regulate intracellular calcium in mammalian rods
Authors: Molnar Tünde , Barabas Peter , Birnbaumer Lutz , Punzo Claudio , Kefalov Vladimir , Križaj David 
Published: June 2012

Key points

Light closes cyclic nucleotide‐gated and voltage operated calcium channels in vertebrate rod photoreceptors, resulting in a decrease in the intracellular calcium concentration ([Ca2+]i). A protracted decrease in [Ca2+]i experienced under saturating illuminations is toxic for these cells.

Eukaryotic cells express voltage‐independent plasma membrane ion channels that protect against pathological [Ca2+]i decreases and can be activated by depletion of intracellular calcium stores. An invertebrate homologue of canonical transient receptor potential (TRPC) channels that have been implicated in store‐operated calcium entry (SOCE) in vertebrates is expressed in photoreceptors.

We show that mouse rods express a potent SOCE mechanism that gates cation entry which subsequently modulates activation of L‐type calcium channels. Furthermore, we show what the majority of the retinal Trpc1 signal is localized to rod photoreceptors.

We found, using knockout animal models, that neither TRPC1 nor TRPC3 channels contribute to SOCE in mouse rod perikarya, or regulate light‐evoked responses in the outer segment and the synaptic terminal, suggesting that the channels are receptor operated.

We conclude that mammalian rods express two new calcium signalling mechanisms associated with SOCE and TRPC1 signalling which modulate calcium homeostasis and may protect against prolonged [Ca2+]i decreases in saturating light.

Abstract  Exposure to daylight closes cyclic nucleotide‐gated (CNG) and voltage‐operated Ca2+‐permeable channels in mammalian rods. The consequent lowering of the cytosolic calcium concentration ([Ca2+]i), if protracted, can contribute to light‐induced damage and apoptosis in these cells. We here report that mouse rods are protected against prolonged lowering of [Ca2+]i by store‐operated Ca2+ entry (SOCE). Ca2+ stores were depleted in Ca2+‐free saline supplemented with the endoplasmic reticulum (ER) sequestration blocker cyclopiazonic acid. Store depletion elicited [Ca2+]i signals that exceeded baseline [Ca2+]i by 5.9 ± 0.7‐fold and were antagonized by an inhibitory cocktail containing 2‐APB, SKF 96365 and Gd3+. Cation influx through SOCE channels was sufficient to elicit a secondary activation of L‐type voltage‐operated Ca2+ entry. We also found that TRPC1, the type 1 canonical mammalian homologue of the Drosophila photoreceptor TRP channel, is predominantly expressed within the outer nuclear layer of the retina. Rod loss in Pde6brd1 (rd1), Chx10/Kip1−/−rd1 and Elovl4TG2 dystrophic models was associated with ∼70% reduction in Trpc1 mRNA content whereas Trpc1 mRNA levels in rodless cone‐full Nrl−/− retinas were decreased by ∼50%. Genetic ablation of TRPC1 channels, however, had no effect on SOCE, the sensitivity of the rod phototransduction cascade or synaptic transmission at rod and cone synapses. Thus, we localized two new mechanisms, SOCE and TRPC1, to mammalian rods and characterized the contribution of SOCE to Ca2+ homeostasis. By preventing the cytosolic [Ca2+]i from dropping too low under sustained saturating light conditions, these signalling pathways may protect Ca2+‐dependent mechanisms within the ER and the cytosol without affecting normal rod function.