A UCI-led study could usher in a new paradigm for

image: Rhodopsin, the light sensor of the mammalian visual system, is embedded in the disc membranes of photoreceptor cells. The image above shows an electrospray mass spectrometer, in a dark, red-lit room, being used to generate ionized fragments of native disc membranes. Rhodopsin is activated by light while embedded in its disc membrane, causing it to interact with transducin, which then signals to downstream phosphodiesterase 6 (background). Following instantaneous flashes of light, synchronized with recordings on the mass spectrometer, the research team monitored the progress of the signal in real time through the disc’s native membranes. By capturing this signaling cascade, they documented the roles of lipids and ligands in rhodopsin signaling, highlighting new opportunities for drug target discovery in the native environment.
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Credit: UCI School of Medicine

Irvine, California, April 6, 2022 – In a new study at the University of California, Irvine, researchers have revealed the impact of native lipids on rhodopsin signaling and regeneration, potentially ushering in a new paradigm for drug discovery targeting coupled receptors to G proteins (GPCR).

GPCRs are cell surface receptors that respond to a variety of stimuli to activate signaling pathways across cell membranes. All GPCRs are membrane-bound and have rarely been studied in their native membrane environment. Recent advances have yielded atomic structures of key intermediates and roles for lipids in mediating signaling. However, capturing signaling events from a wild-type receptor in real time, across a native membrane to its downstream effectors, has remained elusive until now. These receptors represent by far the largest class of drug targets, and a large number of approved drugs modulate their functions.

In this new study published today in Nature, titled “Capturing a Rhodopsin Receptor Signaling Cascade Across a Native Membrane”, the researchers, using mass spectrometry, probed the archetypal class A GPCR, rhodopsin, directly in fragments native disc membranes. They monitored the real-time photoconversion of dark-adapted rhodopsin to opsin, delineating stepwise retinal isomerization and retinal-opsin adduct hydrolysis, further finding that the reaction is significantly slower. in its natural membrane environment than in artificial detergent micelles.

“Human diseases, ranging from cancer to cardiovascular disease to blindness, are all strongly impacted by the function of GPCRs. In addition to the quantitative analysis of the signaling function, this new technology has, for the first time, allowed the direct detection of new potential targets of therapeutic interest for the visual system, within native membranes. I am confident that similar work will be done on many other GPCR systems,” explained Krzysztof Palczewski, PhD, Donald Bren Professor of Ophthalmology at UCI School of Medicine and co-corresponding author.

Considering lipids ejected with rhodopsin from membrane fragments in the mass spectrometer, researchers were able to demonstrate that opsin can be regenerated in membranes through photoisomerized retinal-lipid conjugates, and obtained evidence for increased association rhodopsin with the long chain unsaturated phosphatidylcholine. during signal transduction.

The team also captured secondary steps in the signaling cascade following rhodopsin activation. Monitoring transducin (Gt) light activation and guanosine diphosphate (GDP) dissociation to generate an intermediate apo trimeric G protein, they observed Gta.GTP subunits interacting with phosphodiesterase 6 (PDE6), found in cone and rod photoreceptor cells, which hydrolyzes the second messenger molecule cyclic guanosine monophosphate (cGMP).

“By applying compounds targeting rhodopsin, we have shown how they stimulate or attenuate signaling via the rhodopsin-opsin and transducin signaling pathways,” Palczewski said. “Using instantaneous light flashes, synchronized with recordings on a mass spectrometer, we were able to capture the signaling cascade and demonstrate the roles of lipids and ligands in rhodopsin signaling. This work highlights new opportunities for drug discovery in native environments and may lead to a new way to study the pharmacology of membrane-bound receptors.

About UCI School of Medicine: Each year, the UCI School of Medicine trains more than 400 medical students and nearly 150 doctoral and master’s students. Over 700 residents and fellows are trained at the UCI Medical Center and affiliated institutions. The School of Medicine offers an MD; a dual MD/PhD medical scientist training program; and doctorates and master’s degrees in anatomy and neurobiology, biomedical sciences, genetic counseling, epidemiology, environmental health sciences, pathology, pharmacology, physiology and biophysics, and translational sciences. Medical students can also pursue MD/MBA, MD/MSc in Public Health, or MD/MSc through one of three mission-driven programs: Health Education to Advance Leaders in Integrative Medicine (HEAL-IM), Leadership Education to Advance Diversity-African, Black and Caribbean (LEAD-ABC) program, and Medical Education Program for the Latino Community (PRIME-LC). The UCI School of Medicine is accredited by the Liaison Committee on Medical Accreditation and ranks among the top 50 nationally for research. For more information, visit som.uci.edu.

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