First 3D Structure of Inactivated Cone Opsins Revealed in three different studies

Amazing stuff! I limit myself here to one of the three papers.

"... In a new study, ... two researchers ... have succeeded for the first time in determining the three-dimensional structure of human cone opsins in their dark state and showing how their molecular architecture enables their rapid activation by light. This provides important new insights into human vision and its evolution and may offer new starting points for the study of eye diseases that currently lack effective treatment. ..."

From the editor's summary and abstract:

"Editor’s summary

Human daytime vision relies on a trio of visual receptors called opsins, which are found in the cone cells in and around the central region of the retina.

The three opsins are tuned to long, medium, or short wavelengths of light, roughly corresponding to red, green, and blue, and mutations or other defects in cone cell function can lead to vision deficits.

Although the cell biology and biochemistry of color vision have been well studied, up to now, the molecular explanation for cone opsin spectral tuning and signaling kinetics has been limited by a lack of experimental structures.

Three papers in this issue now resolve this deficit.

Schmidt et al. determined structures of the dark state of the green and blue human cone opsins, which revealed important details of these receptors and provide a basis for a femtosecond-resolution spectroscopy study.

Ohashi et al. performed complementary structural, spectroscopic, and computational results with dark-state red and green cone opsins from macaques, which have color vision similar to humans.

Finally, Peng et al. studied all three human cone opsins in the presumed active state bound to a G protein and all-trans retinal.

The three papers together provide a clear picture of the features of these visual receptors that lead to different spectral properties, activation and inactivation kinetics, and recycling. ...

Structured Abstract

INTRODUCTION

High-acuity daylight vision relies on cone photoreceptors, specialized class A G protein–coupled receptors (GPCRs). Like other light-sensitive GPCRs, the three human cone opsins covalently bind vitamin A derivative 11-cis-retinal through a protonated Schiff base. Despite sharing the same chromophore, they detect distinct wavelengths of light and generate swift signaling responses at high repetition rates. Although cone opsins are central to human vision, and in contrast to the well-studied rod photoreceptor rhodopsin, the detailed molecular basis of these functional specializations remains elusive.

RATIONALE

To obtain structure-function relationships that extend the kinetic and mechanistic understanding of photopic vision, we solved cryo–electron microscopy (cryo-EM) structures of the two most evolutionarily and functionally divergent human cone opsins, short-wavelength-sensitive OPN1SW and medium-wavelength-sensitive OPN1MW, in their initial 11-cis-coupled state. We combined the structural data with multiple functional assays, hybrid quantum mechanics/molecular mechanics simulations, time-resolved spectroscopy, and multitaxon opsin sequence analysis.

RESULTS

Cryo-EM structures of cone opsins revealed receptor-specific activation mechanisms and distinct strategies for stabilizing the retinal Schiff base.

OPN1SW, representing the phylogenetically older vertebrate opsins, has a more constrained polar chromophore environment, which contributes to its blue-shifted maximum absorption wavelength (λmax), yet its stabilization is weaker than that of rhodopsin. The architecture of OPN1SW shows substantial divergences in the canonical GPCR microswitch networks, including the replacement of the toggle switch with Y6.48, a disrupted PIF triad, and the absence of a highly conserved sodium- or water-coordination site.

Collectively, these alterations favor a preactive conformation, also captured by cryo-EM. OPN1SW further uses W185ECL2 as a steric switch to transmit the retinal isomerization event across several helices through an extended aromatic network.

In contrast, OPN1MW contains a chloride ion within the chromophore-binding pocket that modulates wavelength sensitivity and influences the amplitude of G protein signaling. This chloride-binding site coevolved with a structural pathway on helix 2 that couples chromophore chemistry to canonical GPCR microswitches.

Both receptors have accessible binding pockets that allow rapid ligand hydrolysis and, consequently, fast retinal turnover.

Femtosecond transient-absorption spectroscopy resolved the photoisomerization cascade, supporting a model in which deprotonation and subsequent hydrolysis limit signal duration in cone opsins.

CONCLUSION

Our structural and mechanistic insights describe how distinctive chromophore environments and GPCR microswitch adaptations tune spectral sensitivity and signaling-state lifetimes in cone opsins. Conservation of central residues across short-wavelength-sensitive and medium-to-long-wavelength-sensitive opsins suggests shared mechanistic principles that shaped the evolution of daylight vision. Similar motifs in other GPCRs, including sensory receptors, inform the strategies for modulating receptor activation kinetics and signal duration."

First 3D Structure of Inactivated Cone Opsins Revealed | The Scientist "Scientists resolved the three-dimensional structures of light-sensitive cone opsins, offering new avenues for treating age-related vision loss."

New insights into human vision (original news release)

Experiment performed at ELI Enables New Insights into Human Vision (original news release) "The retina of the human eye contains six to seven million cone cells. These cells contain light-sensitive proteins known as cone opsins. They enable us to perceive our surroundings in detail in daylight. In a new study, researchers ... have now, for the first time, determined the three-dimensional molecular structure of human cone opsins in their dark state, that is, before they are activated by light."

Illuminating the molecular basis of human daylight vision (no public access)

Cone opsins use distinct chromophore-stabilization strategies to tune spectral sensitivity, activation kinetics, and retinal regeneration.

Gene therapy could help restore vision lost to degenerative disease

Good news!

"... The team found a way to activate 'dormant' cells in the retina to reprogram them into what are called induced neuron cells, which could then be converted into new light-sensing cells to restore lost vision. ...
In doing so, the team identified a pair of transcription factors called Ikzf1 and Ikzf4, which can be expressed in Müller cells to convert them into retinal neurons. These could in turn be coaxed to replace key light-sensing cells. Other studies have found promising results doing similar things with other transcription factors. ..."

"... Although various approaches such as gene therapy exist that offer hope of slowing or blocking the progression of photoreceptor cell loss, these techniques cannot restore lost cells and are therefore not useful for patients at the advanced stages of the disease. ...
In an approach that circumvents the need for transplantation, ... team found a way to reactivate dormant cells in the retina and transform them into neural-like cells that could ultimately be used to replace cells lost in retinal degeneration. ..."

From the significance and abstract:

"Significance

The sequential production of cell types during neural development is controlled by temporal identity transcription factors, and heterochronic expression of these factors in progenitors reprograms developmental potential and promotes the production of temporally inappropriate cell types. It remains unknown, however, whether temporal factors can reprogram terminally differentiated cells. Here, we report that the combined expression of early temporal identity factors Ikzf1 and Ikzf4, homologs of Drosophila hunchback (hb), can convert uninjured retinal glia into neuron-like cells. Furthermore, we show that Ikzf1/Ikzf4 can reprogram fibroblasts into induced neurons (iNs) by altering chromatin accessibility and enabling a neuronal gene expression program. This work uncovers the reprogramming ability of temporal identity factors, opening the door to cell therapy approaches for neurodegenerative diseases.

Abstract

Temporal identity factors are sufficient to reprogram developmental competence of neural progenitors and shift cell fate output, but whether they can also reprogram the identity of terminally differentiated cells is unknown. To address this question, we designed a conditional gene expression system that allows rapid screening of potential reprogramming factors in mouse retinal glial cells combined with genetic lineage tracing. Using this assay, we found that coexpression of the early temporal identity transcription factors Ikzf1 and Ikzf4 is sufficient to directly convert Müller glial (MG) cells into cells that translocate to the outer nuclear layer (ONL), where photoreceptor cells normally reside. We name these “induced ONL (iONL)” cells. Using genetic lineage tracing, histological, immunohistochemical, and single-cell transcriptome and multiome analyses, we show that expression of Ikzf1/4 in MG in vivo, without retinal injury, mostly generates iONL cells that share molecular characteristics with bipolar cells, although a fraction of them stain for Rxrg, a cone photoreceptor marker. Furthermore, we show that coexpression of Ikzf1 and Ikzf4 can reprogram mouse embryonic fibroblasts to induced neurons in culture by rapidly remodeling chromatin and activating a neuronal gene expression program. This work uncovers general neuronal reprogramming properties for temporal identity factors in terminally differentiated cells."

Gene therapy could help restore vision lost to degenerative disease Scientists in Canada have developed a new technique that may one day help restore some sight to patients with inherited vision impairment. The regenerative therapy works by expressing genes that convert dormant cells into new light-sensing cells in the retina to replace those lost to disease.

New hope for vision regeneration Researchers led by UdeM's Michel Cayouette have found a way to reactivate dormant cells in the retina and transform them to ultimately replace cells lost in retinal degeneration.

Direct neuronal reprogramming by temporal identity factors (open access)

Fig. 1 Ikzf1/4 expression induces morphological reprogramming of MG ex vivo