Ca2+ Diffusion through Endoplasmic Reticulum Supports Elevated Intraterminal Ca2+ Levels Needed to Sustain Synaptic Release from Rods in Darkness.
In addition to vesicle release at synaptic ribbons, rod photoreceptors are capable of substantial slow release at non-ribbon release sites triggered by Ca(2+)-induced Ca(2+) release (CICR) from intracellular stores. To maintain CICR as rods remain depolarized in darkness, we hypothesized that Ca(2+) released into the cytoplasm from terminal endoplasmic reticulum (ER) can be replenished continuously by ions diffusing within the ER from the soma. We measured [Ca(2+)] changes in cytoplasm and ER of rods from Ambystoma tigrinum retina using various dyes. ER [Ca(2+)] changes were measured by loading ER with fluo-5N and then washing dye from the cytoplasm with a dye-free patch pipette solution. Small dye molecules diffused within ER between soma and terminal showing a single continuous ER compartment. Depolarization of rods to -40 mV depleted Ca(2+) from terminal ER, followed by a decline in somatic ER [Ca(2+)]. Local activation of ryanodine receptors in terminals with a spatially confined puff of ryanodine caused a decline in terminal ER [Ca(2+)], followed by a secondary decrease in somatic ER. Localized photolytic uncaging of Ca(2+) from o-nitrophenyl-EGTA in somatic ER caused an abrupt Ca(2+) increase in somatic ER, followed by a slower Ca(2+) increase in terminal ER. These data suggest that, during maintained depolarization, a soma-to-terminal [Ca(2+)] gradient develops within the ER that promotes diffusion of Ca(2+) ions to resupply intraterminal ER Ca(2+) stores and thus sustain CICR-mediated synaptic release. The ability of Ca(2+) to move freely through the ER may also promote bidirectional communication of Ca(2+) changes between soma and terminal.


CONCLUSIONS
Vertebrate rod and cone photoreceptors both release vesicles at synaptic ribbons, but rods also exhibit substantial slow release at non-ribbon sites triggered by Ca(2+)-induced Ca(2+) release (CICR). Blocking CICR inhibits >50% of release from rods in darkness. How do rods maintain sufficiently high [Ca(2+)] in terminal endoplasmic reticulum (ER) to support sustained CICR-driven synaptic transmission? We show that maintained depolarization creates a [Ca(2+)] gradient within the rod ER lumen that promotes soma-to-terminal diffusion of Ca(2+) to replenish intraterminal ER stores. This mechanism allows CICR-triggered synaptic release to be sustained indefinitely while rods remain depolarized in darkness. Free diffusion of Ca(2+) within the ER may also communicate synaptic Ca(2+) changes back to the soma to influence other critical cell processes.
Departments of Pharmacology and Experimental Neuroscience and Ophthalmology and Visual Sciences, Truhlsen Eye Institute, University of Nebraska Medical Center, Omaha, Nebraska 68198.
Minghui
http://orcid.org/0000-0002-0316-2289
M
Chen
Ophthalmology and Visual Sciences, Truhlsen Eye Institute, University of Nebraska Medical Center, Omaha, Nebraska 68198.
Matthew J
http://orcid.org/0000-0003-1615-4314
MJ
Van Hook
Departments of Pharmacology and Experimental Neuroscience and Ophthalmology and Visual Sciences, Truhlsen Eye Institute, University of Nebraska Medical Center, Omaha, Nebraska 68198 wbthores@unmc.edu.
Wallace B
http://orcid.org/0000-0001-7104-042X
WB
Thoreson
D000557:Ambystoma; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D003624:Darkness; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D005260:Female; D008297:Male; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D012433:Ryanodine / Q000494:pharmacology; D019837:Ryanodine Receptor Calcium Release Channel / Q000378:metabolism; D013569:Synapses / Q000187:drug effects / Q000502:physiology*; D009435:Synaptic Transmission / Q000187:drug effects / Q000502:physiology
D019837:Ryanodine Receptor Calcium Release Channel; D012433:Ryanodine; D002118:Calcium
calcium imaging; calcium-induced calcium release; endoplasmic reticulum; retina; rod photoreceptors; synaptic terminal