Melanopsin activates divergent phototransduction pathways in ipRGC subtypes

Abstract

Melanopsin signaling within 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 model 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 in M2 phototransduction, TRPC channels act in conjunction with T-type voltage-gated calcium channels, 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.

Coexistence within one cell of microvillous and ciliary phototransductions across M1- through M6-IpRGCs

“…Our overall phototransduction work on ipRGCs ( 16 – 18 ), including the current data, share only limited agreement with that from another group ( 23 – 25 ). This other group agreed on the existence of the PLCβ4/TRPC pathway, but has not clearly acknowledged the existence of a cyclic-nucleotide-mediated phototransduction pathway ( 23 , 25 ).…”

Atypical retinal function in a mouse model of Fragile X syndrome

“…The melanopsin photocurrent is different from rod/cone evoked synaptic currents in that its duration is very prolonged, even when presented with a very short light pulse (Fig. S1) (Contreras et al 2023). We observed that sOn-α cells in Fmr1(-/y) retinas had slightly larger, more prolonged blue light evoked currents (Fig.…”

“…We identified sOn-α cells based on physiological and molecular features that are unique to this cell type (see Methods and Fig. S2) (Contreras et al 2023) and filled these cells with neurobiotin to visualize their dendrites (Fig. 1B-C).…”

Flp-recombinase mouse line for genetic manipulation of ipRGCs

“…The identities of M1, M2, and M3 ipRGCs were established using both dendritic stratification, soma size, and light responses. Cells with only OFF ramified dendrites were classified as M1 ipRGCs, cells with only ON stratified dendrites were identified as M2 ipRGCs, and bistratified cells were classified as M3 ipRGCs (M1: OFF; M2: ON; M3 ON/OFF) (Schmidt et al, 2008;Schmidt and Kofuji, 2009;Lee and Schmidt, 2018;Contreras et al, 2023). All cells identified as M2 were confirmed negative for SMI-32 immunolabeling, to ensure any cells with large somata were not M4 ipRGCs (data not shown) (Lee and Schmidt, 2018;Contreras et al, 2023).…”

“…For experiments measuring the intrinsic melanopsin response to a 50ms light stimulus (6.08 × 10 15 photons • cm -2 • s -1 ), cells were voltage clamped at -60 mV. We measured the maximum amplitude of the light response for each cell, using a similar analysis described in Contreras et al, 2023. Cells were smoothed using a 100ms sliding average.…”