Melanopsin triggers the release of internal calcium stores in response to light.

Melanopsin is the photopigment that confers photosensitivity upon intrinsically photosensitive retinal ganglion cells (ipRGCs). This subset of retinal ganglion cells comprises less than 2% of all RGCs in the mammalian retina. The paucity of melanopsin-positive cells has made studies on melanopsin signaling difficult to pursue in ipRGCs. To address this issue, we have established several cell lines consisting of a transformed human embryonic kidney cell line (HEK293) stably expressing human melanopsin. With these cell lines, we have investigated the intracellular rise in calcium triggered upon light activation of melanopsin. Our human melanopsin-expressing cells exhibit an irradiance-dependent increase in intracellular calcium. Control cells expressing human melanopsin, where the Schiff-base lysine has been mutated to alanine, show no responses to light. Chelating extracellular calcium has no effect on the light-induced increase in intracellular calcium suggesting that calcium is mobilized from intracellular stores. This involvement of intracellular stores has been confirmed through their depletion by thapsigargin, which inhibits a subsequent light-induced increase in intracellular calcium. Addition of the nonselective cation channel blocker lanthanum does not alter light-induced rises in intracellular calcium, further supporting that melanopsin triggers a release of internal calcium from internal stores. HEK293 cells stably expressing melanopsin have proven to be a useful tool to study melanopsin-initiated signaling.