Functional optimization of light-activatable Opto-GPCRs: Illuminating the importance of the proximal C-terminus in G-protein specificity

To activate a G protein-coupled receptor permanently with cell surface photodynamic action in the gastrointestinal tract

Different from reversible agonist-stimulated receptor activation, singlet oxygen oxidation activates permanently G protein-coupled receptor (GPCR) cholecystokinin 1 (CCK1R) in type II photodynamic action, with soluble photosensitizer dyes (sulphonated aluminum phthalocyanine, λmax 675 nm) or genetically encoded protein photosensitizers (KillerRed λmax 585 nm; mini singlet oxygen generator λmax 450 nm), together with a pulse of light (37 mW/cm2, 1-2 minutes). Three lines of evidence shed light on the mechanism of GPCR activated by singlet oxygen (GPCR-ABSO): (1) CCK1R is quantitatively converted from dimer to monomer; (2) Transmembrane domain 3, a pharmacophore for permanent photodynamic CCK1R activation, can be transplanted to non-susceptible M3 acetylcholine receptor; and (3) Larger size of disordered region in intracellular loop 3 correlates with higher sensitivity to photodynamic CCK1R activation. GPCR-ABSO will add to the arsenal of engineered designer GPCR such as receptors activated solely by synthetic ligands and designer receptors exclusively activated by designer drugs, but show some clear advantages: Enhanced selectivity (double selectivity of localized photosensitizer and light illumination), long-lasting activation with no need for repeated drug administration, antagonist-binding site remains intact when needed, ease to apply to multiple GPCR. This type of permanent photodynamic activation may be applied to functional proteins other than GPCR.

Engineering signalling pathways in mammalian cells

In mammalian cells, signalling pathways orchestrate cellular growth, differentiation and survival, as well as many other processes that are essential for the proper functioning of cells. Here we describe cutting-edge genetic-engineering technologies for the rewiring of signalling networks in mammalian cells. Specifically, we describe the recombination of native pathway components, cross-kingdom pathway transplantation, and the development of de novo signalling within cells and organelles. We also discuss how, by designing signalling pathways, mammalian cells can acquire new properties, such as the capacity for photosynthesis, the ability to detect cancer and senescent cell markers or to synthesize hormones or metabolites in response to chemical or physical stimuli. We also review the applications of mammalian cells in biocomputing. Technologies for engineering signalling pathways in mammalian cells are advancing basic cellular biology, biomedical research and drug discovery.

An overview of sphingosine-1-phosphate receptor 2: Structure, biological function, and small-molecule modulators

Over the past decade, there has been a notable increase in research on sphingosine-1-phosphate receptor 2 (S1PR2), which is a type of G-protein-coupled receptor. Upon activation by S1P or other ligands, S1PR2 initiates downstream signaling pathways such as phosphoinositide 3-kinase (PI3K), Mitogen-activated protein kinase (MAPK), Rho/Rho-associated coiled-coil containing kinases (ROCK), and others, contributing to the diverse biological functions of S1PR2 and playing a pivotal role in various physiological processes and disease progressions, such as multiple sclerosis, fibrosis, inflammation, and tumors. Due to the extensive biological functions of S1PR2, many S1PR2 modulators, including agonists and antagonists, have been developed and discovered by pharmaceutical companies (e.g., Novartis and Galapagos NV) and academic medicinal chemists for disease diagnosis and treatment. However, few reviews have been published that comprehensively overview the functions and regulators of S1PR2. Herein, we provide an in-depth review of the advances in the function of S1PR2 and its modulators. We first summarize the structure and biological function of S1PR2 and its pathological role in human diseases. We then focus on the discovery approach, design strategy, development process, and biomedical application of S1PR2 modulators. Additionally, we outline the major challenges and future directions in this field. Our comprehensive review will aid in the discovery and development of more effective and clinically applicable S1PR2 modulators.

A metabotropic glutamate receptor agonist enhances visual signal fidelity in a mouse model of retinitis pigmentosa

Many inherited retinal diseases target photoreceptors, which transduce light into a neural signal that is processed by the downstream visual system. As photoreceptors degenerate, physiological and morphological changes to retinal synapses and circuitry reduce sensitivity and increase noise, degrading visual signal fidelity. Here, we pharmacologically targeted the first synapse in the retina in an effort to reduce circuit noise without sacrificing visual sensitivity. We tested a strategy to partially replace the neurotransmitter lost when photoreceptors die with an agonist of receptors that ON bipolars cells use to detect glutamate released from photoreceptors. In rd10 mice, which express a photoreceptor mutation that causes retinitis pigmentosa (RP), we found that a low dose of the mGluR6 agonist l-2-amino-4-phosphonobutyric acid (L-AP4) reduced pathological noise induced by photoreceptor degeneration. After making in vivo electroretinogram recordings in rd10 mice to characterize the developmental time course of visual signal degeneration, we examined effects of L-AP4 on sensitivity and circuit noise by recording in vitro light-evoked responses from individual retinal ganglion cells (RGCs). L-AP4 decreased circuit noise evident in RGC recordings without significantly reducing response amplitudes, an effect that persisted over the entire time course of rod photoreceptor degeneration. Subsequent in vitro recordings from rod bipolar cells (RBCs) showed that RBCs are more depolarized in rd10 retinas, likely contributing to downstream circuit noise and reduced synaptic gain, both of which appear to be ameliorated by hyperpolarizing RBCs with L-AP4. These beneficial effects may reduce pathological circuit remodeling and preserve the efficacy of therapies designed to restore vision.

Molecular Property, Manipulation, and Potential Use of Opn5 and Its Homologs

Animal opsin is a G-protein coupled receptor (GPCR) and binds retinal as a chromophore to form a photopigment. The Opsin 5 (Opn5) group within the animal opsin family comprises a diverse array of related proteins, such as Opn5m, a protein conserved across all vertebrate lineages including mammals, and other members like Opn5L1 and Opn5L2 found in non-mammalian vertebrate genomes, and Opn6 found in non-therian vertebrate genomes, along with Opn5 homologs present in invertebrates. Although these proteins collectively constitute a single clade within the molecular phylogenetic tree of animal opsins, they exhibit markedly distinct molecular characteristics in areas such as retinal binding properties, photoreaction, and G-protein coupling specificity. Based on their molecular features, they are believed to play a significant role in physiological functions. However, our understanding of their precise physiological functions and molecular characteristics is still developing and only partially realized. Furthermore, their unique molecular characteristics of Opn5-related proteins suggest a high potential for their use as optogenetic tools through more specialized manipulations. This review intends to encapsulate our current understanding of Opn5, discuss potential manipulations of its molecular attributes, and delve into its prospective utility in the burgeoning field of animal opsin optogenetics.

Modulating signalling lifetime to optimise a prototypical animal opsin for optogenetic applications

Animal opsins are light activated G-protein-coupled receptors, capable of optogenetic control of G-protein signalling for research or therapeutic applications. Animal opsins offer excellent photosensitivity, but their temporal resolution can be limited by long photoresponse duration when expressed outside their native cellular environment. Here, we explore methods for addressing this limitation for a prototypical animal opsin (human rod opsin) in HEK293T cells. We find that the application of the canonical rhodopsin kinase (GRK1)/visual arrestin signal termination mechanism to this problem is complicated by a generalised suppressive effect of GRK1 expression. This attenuation can be overcome using phosphorylation-independent mutants of arrestin, especially when these are tethered to the opsin protein. We further show that point mutations targeting the Schiff base stability of the opsin can also reduce signalling lifetime. Finally, we apply one such mutation (E122Q) to improve the temporal fidelity of restored visual responses following ectopic opsin expression in the inner retina of a mouse model of retinal degeneration (rd1). Our results reveal that these two strategies (targeting either arrestin binding or Schiff-base hydrolysis) can produce more time-delimited opsin signalling under heterologous expression and establish the potential of this approach to improve optogenetic performance.

Cardiac optogenetics: shining light on signaling pathways

In the early 2000s, the field of neuroscience experienced a groundbreaking transformation with the advent of optogenetics. This innovative technique harnesses the properties of naturally occurring and genetically engineered rhodopsins to confer light sensitivity upon target cells. The remarkable spatiotemporal precision offered by optogenetics has provided researchers with unprecedented opportunities to dissect cellular physiology, leading to an entirely new level of investigation. Initially revolutionizing neuroscience, optogenetics quickly piqued the interest of the wider scientific community, and optogenetic applications were expanded to cardiovascular research. Over the past decade, researchers have employed various optical tools to observe, regulate, and steer the membrane potential of excitable cells in the heart. Despite these advancements, achieving control over specific signaling pathways within the heart has remained an elusive goal. Here, we review the optogenetic tools suitable to control cardiac signaling pathways with a focus on GPCR signaling, and delineate potential applications for studying these pathways, both in healthy and diseased hearts. By shedding light on these exciting developments, we hope to contribute to the ongoing progress in basic cardiac research to facilitate the discovery of novel therapeutic possibilities for treating cardiovascular pathologies.