Light-transduction in melanopsin-expressing photoreceptors of Amphioxus

摘要

Spatial vision in different organisms is mediated by 2 classes of photoreceptors: microvillar and ciliary. Recently, additional photosensitive cells implicated in nonvisual light-dependent functions have been identified in the mammalian retina. A previously unde-scribed photopigment, melanopsin, underlies these photore-sponses, and it has been proposed that its transduction mechanisms may be akin to the lipid-signaling scheme of invertebrate microvillar receptors, rather than the cyclic-nucleotide cascade of vertebrates. Melanopsin has an ancient origin in deuterostomia, and expresses in 2 morphologically distinct classes of cells in the neural tube of Amphioxus, the most basal extant chordate: pig-mented ocelli, and Joseph cells. However, to our knowledge, their physiology and alleged photosensitivity had never been investigated. We dissociated both types of cells, and conclusively demonstrated by patch-electrode recoding that they are primary photoreceptors; their receptor potential is depolarizing, accompanied by an increase in membrane conductance. The action spectrum peaks in the blue region, ≈470 nm, similar to the absorption of melanopsin in vitro. The light-dependent conductance rectifies inwardly; Na and Ca are differentially implicated in the 2 cell types. Fluorescence Ca imaging reveals that photostimulation rapidly mobilizes calcium from internal stores. Intracellular 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate severely impairs the photoresponse, indicating that light-evoked Ca elevation is an important event in photoexcitation. These observations support the notion that the lineage of microvillar photoreceptors and its associated light-signaling pathway also evolved in the chordates. Thus, Joseph cells and pigmented ocelli of the Amphioxus may represent a link between ancestral rhabdomeric-like light sensors present in prebilaterians and the circadian photoreceptors of higher vertebrates.