Intrinsic phototransduction persists in melanopsin-expressing ganglion cells lacking diacylglycerol-sensitive TRPC subunits

Abstract

In mammals, intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate various nonimage-forming photic responses such as circadian photoentrainment, pupillary light reflex, and pineal melatonin suppression. ipRGCs directly respond to environmental light by activation of the photopigment melanopsin followed by the opening of an unidentified cation-selective channel. Studies in heterologous expression systems and in the native retina have strongly implicated diacylglycerol-sensitive transient receptor potential channels containing TRPC3, TRPC6, and TRPC7 subunits in melanopsin-evoked depolarization. Here we show that melanopsin-evoked electrical responses largely persist in ipRGCs recorded from early postnatal (P6-P8) and adult (P22-P50) mice lacking expression of functional TRPC3, TRPC6, or TRPC7 subunits. Multielectrode array recordings performed at P6-P8 stages under conditions that prevent influences from rod/cone photoreceptors show comparable light sensitivity for the melanopsinevoked responses in these mutant mouse lines in comparison to wild-type mice. Patch-clamp recordings from adult mouse ipRGCs lacking TRPC3 or TRPC7 subunits show intrinsic lightevoked responses equivalent to those recorded in wild-type mice. Persistence of intrinsic lightevoked responses was also noted in ipRGCs lacking TRPC6 subunits, however of significantly smaller magnitudes. These results demonstrate that the melanopsin-evoked depolarization in ipRGCs is not mediated by either TRPC3, TRPC6, or TRPC7 channel subunits alone. They also suggest that the melanopsin signaling pathway includes TRPC6-containing heteromeric channels in mature retinas.

Intrinsically Photosensitive Retinal Ganglion Cells

“…The details of this cascade remain unclear, although there is evidence that it is G-protein dependent ) with G q a likely candidate and phospholipase C as the effector enzyme (Graham et al 2008). The current model suggests a heteromeric TRPC channel is the light-gated ion channel that may include TRPC6 and TRPC7 subunits (Hartwick et al 2007;Perez-Leighton et al 2011). The ion flux through this 2-APB-sensitive channel depolarizes (denoted with a lightning bolt in the membrane) the membrane potential, resulting in the activation of voltaged-gated Na+ channels (VGNCs; labeled 2).…”

“…Of the seven members of the subfamily of TRPC subunits that combine to form tetrameric channels (Hoffman et al 2002), TRPC3, TRPC6, and/or TRPC7 have all been implicated as the TRPC channel mediating the initial depolarization in ipRGCs Hartwick et al 2007;Sekaran et al 2007;Graham et al 2008). However, in vitro recordings from ipRGCs in mouse lines lacking expression of functional TRPC3, TRPC6 or TRPC7 subunits indicate that light-evoked depolarization persist largely unchanged (Perez-Leighton et al 2011). These results indicate that TRPC3, TRPC6, or TRPC7 homomeric channels do not mediate melanopsin-evoked depolarization in ipRGCs, but the possibility remains that these subunits may form heteromultimeric assemblies (Perez-Leighton et al 2011).…”

“…However, in vitro recordings from ipRGCs in mouse lines lacking expression of functional TRPC3, TRPC6 or TRPC7 subunits indicate that light-evoked depolarization persist largely unchanged (Perez-Leighton et al 2011). These results indicate that TRPC3, TRPC6, or TRPC7 homomeric channels do not mediate melanopsin-evoked depolarization in ipRGCs, but the possibility remains that these subunits may form heteromultimeric assemblies (Perez-Leighton et al 2011). The demonstration that ectopic expression of melanopsin in conventional RGCs confers intrinsic photosensitivity to these cells (Lin et al 2008) suggests that the channels gated by melanopsin may be widespread among ganglion cells.…”

Adaptation to steady light by intrinsically photosensitive retinal ganglion cells

“…Thus, Ca 2+ appears to enter the cell during the light response to initiate adaptation by accelerating the termination of phototransduction. Because our voltage-clamp experiments were performed at −80 mV (thus with voltage-gated Ca channels in the closed state), and ipRGC phototransduction appears independent of intracellular Ca 2+ stores (39,40), the Ca 2+ likely enters through the phototransduction channel, which consists of at least the transient receptor potential canonical (TRPC) channel isoforms 6 and 7 (i.e., TRPC6 and TRPC7) (28,(40)(41)(42). In physiological conditions, additional negative feedback may be mediated by Ca 2+ entry through voltage-gated Ca channels (39).…”

“…Because a common underlying adaptation process ultimately dictates both the flash and the step intensity-response relations, it is perhaps not surprising that, given the same Michaelis behavior by rods/cones and the ipRGCs, both cell types might also show the Weber-Fechner behavior. Still, rods/cones and ipRGCs have distinct phototransduction mechanisms, with the former involving a cGMP-mediated signaling pathway (20) and the latter a phospholipase-C-mediated pathway (28,(40)(41)(42). The cellular mechanism underlying light adaptation in rods and cones is now known to be elaborate, with consequences that happen to be describable (approximately) by the simple mathematical forms of the Michaelis and the Weber-Fechner relations.…”