Melanopsin activates divergent phototransduction pathways in intrinsically photosensitive retinal ganglion cell subtypes
Curation statements for this article:-
Curated by eLife
eLife assessment
Retinal ganglion cells which are intrinsically photosensitive play important and emerging physiological roles. The mechanisms of phototransduction are still not well known and there exists a controversy regarding the ion channels responsible for the photocurrent. The authors of this manuscript present data that can contribute to understanding the actual ionic mechanisms in two of these cell types. This manuscript will be of general interest to biologists and neuroscientists and should help resolve a major issue in retinal physiology.
This article has been Reviewed by the following groups
Listed in
- Evaluated articles (eLife)
- Neuroscience (eLife)
Abstract
Melanopsin signaling within intrinsically photosensitive retinal ganglion cell (ipRGC) subtypes impacts a broad range of behaviors from circadian photoentrainment to conscious visual perception. Yet, how melanopsin phototransduction within M1-M6 ipRGC subtypes impacts cellular signaling to drive diverse behaviors is still largely unresolved. The identity of the phototransduction channels in each subtype is key to understanding this central question but has remained controversial. In this study, we resolve two opposing models of M4 phototransduction, demonstrating that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are dispensable for this process and providing support for a pathway involving melanopsin-dependent potassium channel closure and canonical transient receptor potential (TRPC) channel opening. Surprisingly, we find that HCN channels are likewise dispensable for M2 phototransduction, contradicting the current model. We instead show that M2 phototransduction requires TRPC channels in conjunction with T-type voltage-gated calcium channels, identifying a novel melanopsin phototransduction target. Collectively, this work resolves key discrepancies in our understanding of ipRGC phototransduction pathways in multiple subtypes and adds to mounting evidence that ipRGC subtypes employ diverse phototransduction cascades to fine-tune cellular responses for downstream behaviors.
Article activity feed
-
-
eLife assessment
Retinal ganglion cells which are intrinsically photosensitive play important and emerging physiological roles. The mechanisms of phototransduction are still not well known and there exists a controversy regarding the ion channels responsible for the photocurrent. The authors of this manuscript present data that can contribute to understanding the actual ionic mechanisms in two of these cell types. This manuscript will be of general interest to biologists and neuroscientists and should help resolve a major issue in retinal physiology.
-
Reviewer #1 (Public Review):
Retinal ganglion cells are diverse. In recent years it was recognized that several subtypes are intrinsically photoresponsive (ipRGCs). In earlier work, it was suggested that hyperpolarization-activated channels (HCN) were the main responsive element contributing to generating the photocurrents that activate signaling by these cells. Other groups, including the authors, have shown evidence that other ionic mechanisms might be in play.
In the current manuscript, the authors present a thorough and careful characterization of the electrophysiology of two types of ipRGCs, M2, and M4. Both pharmacological and genetic ablation of specific ion channels in mice were employed along with posthoc identification of cell types. The authors identify an important experimental problem with one of the drugs employed …
Reviewer #1 (Public Review):
Retinal ganglion cells are diverse. In recent years it was recognized that several subtypes are intrinsically photoresponsive (ipRGCs). In earlier work, it was suggested that hyperpolarization-activated channels (HCN) were the main responsive element contributing to generating the photocurrents that activate signaling by these cells. Other groups, including the authors, have shown evidence that other ionic mechanisms might be in play.
In the current manuscript, the authors present a thorough and careful characterization of the electrophysiology of two types of ipRGCs, M2, and M4. Both pharmacological and genetic ablation of specific ion channels in mice were employed along with posthoc identification of cell types. The authors identify an important experimental problem with one of the drugs employed previously to suggest the participation of HCN channels. This discovery leads the authors to suggest that in M4 ipRGCs, the depolarization induced by light is produced by activation of TRPC channels and inhibition of a leak of potassium channels. Importantly, prolonged application of the HCN channel blocker produced off-target (non-HCN related) effects that can explain previous results.
The authors go on to explore the responses of M2-type ganglion cells and also uncover the important participation of TRPC channels as well as a previously unrecognized role for T-type calcium channels. Since the authors also use pharmacological tools to uncover the participation of calcium channels in M2 cells, they make sure that the drugs employed do not produce off-target effects in cells where the ionic basis of the photocurrent is better established, namely the M1 type.
The author's evidence as a whole is convincing, and should be a major contribution to understanding the physiology of ipRGCs, but should be confirmed by other groups with different experimental approaches. -
Reviewer #2 (Public Review):
Results from these experiments confirm the role of TRP channels but raise serious doubts that HCN channels contribute to the light response, refuting the findings of an influential paper that appeared in Cell (Jiang et al., 2018). Instead, a major role for T-type voltage-gated Ca2+channels is suggested. Together, these results further clarify our understanding of intrinsic photosensitivity in ganglion cells. However, there are several technical issues that need to be clarified before the major claims of this paper are justified.
-
Reviewer #3 (Public Review):
This important body of work aims at identifying the divergent phototransduction pathways in different subtypes of melanopsin-expressing retinal ganglion cells. The authors use a combination of patch-clamp recordings of three subtypes of ipRGCs M1, M2, and M4, and their post hoc rigorous identification. The authors demonstrate that within their conditions of recordings and the choice of light stimulus recorded ipRGCs subtypes do not signal via HCN channels as previously proposed; and that M1 signal via TRPC channel, M2 signal via TRPC, or a newly identified T-Type Ca2+ channel. While the data seem to support the authors' claims that HCN channels are not involved in phototransduction pathways of ipRGCs here, the light stimulus used is different than in the previous study (Jing et al, 2018) which contradicts …
Reviewer #3 (Public Review):
This important body of work aims at identifying the divergent phototransduction pathways in different subtypes of melanopsin-expressing retinal ganglion cells. The authors use a combination of patch-clamp recordings of three subtypes of ipRGCs M1, M2, and M4, and their post hoc rigorous identification. The authors demonstrate that within their conditions of recordings and the choice of light stimulus recorded ipRGCs subtypes do not signal via HCN channels as previously proposed; and that M1 signal via TRPC channel, M2 signal via TRPC, or a newly identified T-Type Ca2+ channel. While the data seem to support the authors' claims that HCN channels are not involved in phototransduction pathways of ipRGCs here, the light stimulus used is different than in the previous study (Jing et al, 2018) which contradicts this claim. This opens up questions on whether this inconsistency originates in differences in light stimulus used in these studies or something else.
-
