Biology subjects

Raicu, V.

Publications and source records attributed to Raicu, V..

8 recordsLinked to original sources

bioRxiv2026

Precise Alternation Between Image-Forming Sample Planes Enables Quantitative Monitoring of Receptor-Arrestin Interaction Dynamics at the Plasma Membrane of Live Cells

Investigations of G protein-coupled receptors (GPCRs) interactions with non-visual arrestins in living cells are essential to understanding the complex molecular mechanisms of GPCR-based signaling. Quantitative analysis of these interactions remains challenging in live cells, particularly when attempting to repeatedly image distinct cellular regions with high precision. Here, we describe the implementation of an optical imaging stabilization approach that integrates the recently developed Focal Readjustment for Enhanced Vertical Resolution (FREVR) technology into a multiphoton microscope, enabling high-precision alternation between image-forming sample planes with < 20-nanometer repeatability and stability over time. Using this setup, we monitored the dynamic recruitment of arrestin-2 (Arr2) to the plasma membrane of HEK-293 cells expressing muscarinic acetylcholine M2 receptors (M2R) by alternatively imaging distinct planes of interest, the basolateral membrane and a membrane cross-section. Following stimulation of M2R by agonist ligand, we observed a pronounced redistribution of cytoplasmic arrestin-2 toward the plasma membrane in both cellular cross-sections and at the basolateral membrane. This method enables direct comparison of receptor and arrestin dynamics across regions of individual cells with very high precision, eliminating the need for averaging over numerous cells in order to denoise biologically relevant signals, and thereby capturing physiological cell-to-cell variability.

biophysics↗

bioRxiv2025

Distinct Membrane Binding Properties of the Two Non-visual Arrestins

Membrane interactions play a crucial role in regulating arrestin activation and its binding to phosphorylated G protein-coupled receptors (GPCRs). Here, we utilize in vitro biophysical approaches and cell-based fluorescence intensity fluctuation analysis to systematically compare the membrane-binding properties of the two highly conserved arrestin subtypes, arrestin-2 and arrestin-3, under basal and stimulated conditions. Our findings reveal that arrestin-2 selectively engages the PI(4,5)P2-containing nanodiscs via its C-edge, whereas arrestin-3 primarily utilizes its finger loop to interact with negatively charged lipids. Notably, while the lipid bilayer alone does not activate arrestin, it synergistically enhances arrestin-2/3 activation in conjunction with a phosphorylated GPCR C-tail. Additionally, the spacing between receptor phosphorylation sites and the lipid bilayer modulates arrestin-membrane assembly. Live cell tracking further demonstrates that arrestin-2 and arrestin-3 exhibit distinct plasma membrane dissociation dynamics. These findings provide novel insights into the mechanisms governing arrestin activation and its functional interplay with membranes.

biochemistry↗

bioRxiv2024

Impact of photobleaching of fluorescent proteins on FRET measurements under two-photon excitation

Forster resonance energy transfer (FRET) is a widely used technique for nanoscale molecular distance measurements, which makes FRET ideal for studying protein interactions and quaternary structure of protein complexes. In this work, we were interested in how photobleaching of donor and acceptor molecules affects the FRET results under various excitation conditions. We conducted a systematic study, under two-photon excitation, of the effects of the excitation power and the choice of excitation wavelengths upon the measured FRET efficiencies of multiplex protein constructs, consisting of one donor and either one or two acceptors, using both the kinetic theory of FRET and numerical simulations under given excitation conditions. We found that under low excitation power and properly chosen excitation wavelengths the relationship between the FRET efficiency of a trimeric construct ADA agrees within 2% with the FRET efficiency computed (via the kinetic theory of FRET in the absence of photobleaching) from two dimeric constructs ADN and NDA. By contrast, at higher excitation powers the FRET efficiencies changed significantly, due to the photobleaching of both the donor (through direct excitation) and the acceptor (mostly through FRET-induced excitation). Based on these results and numerical simulations using a simple but powerful algorithm, we also provide guidelines for choosing appropriate experimental conditions for reliable FRET measurements in complexes of associating molecules of biological interest.

biophysics↗

bioRxiv2022

The efficacy of EphA2 tyrosine phosphorylation increases with EphA2 oligomer size

The receptor tyrosine kinase (RTK) EphA2 is expressed in epithelial and endothelial cells and controls the assembly of cell-cell junctions. EphA2 has also been implicated in many diseases, including cancer. Unlike most RTKs, which signal predominantly as dimers, EphA2 readily forms higher order oligomers upon ligand binding. Here we investigated if a correlation exists between EphA2 signaling properties and the size of the EphA2 oligomers induced by multiple ligands, including the widely used ephrinA1-Fc ligand, the soluble monomeric m-ephrinA1, and novel engineered peptide ligands. We used Fluorescence Intensity Fluctuation (FIF) spectrometry to characterize the EphA2 oligomer populations induced by the different ligands. Interestingly, we found that different monomeric and dimeric ligands induce EphA2 oligomers with widely different size distributions. Comparison of FIF brightness distribution parameters and EphA2 signaling parameters reveals that the efficacy of EphA2 phosphorylation on tyrosine 588, which is indicative of receptor activation, correlates with EphA2 mean oligomer size. However, other characteristics, such as the efficacy of AKT inhibition and ligand bias coefficients, appear to be independent of EphA2 oligomer size. This work highlights the utility of FIF in RTK signaling research and demonstrates a quantitative correlation between the architecture of EphA2 signaling complexes and signaling features.

biophysics↗

bioRxiv2020

In-cell detection of conformational sub-states of a GPCR quaternary structure: Modulation of sub-state probability by cognate ligand binding

ABSTRACTWhile the notion that G protein-coupled receptors (GPCRs) associate into homo- and hetero-oligomers has gained more recognition in recent years, a lack of consensus remains among researchers regarding the functional relevance of GPCR oligomerization. A relatively recent technique, Förster resonance energy transfer (FRET) spectrometry, allows for the determination of the oligomeric (or quaternary) structure of proteins in living cells via analysis of efficiency distributions of energy transferred from optically excited fluorescent tags acting as donors of energy to fluorescent tags acting as acceptors of energy and residing within the same oligomer. In this study, we significantly improved the resolution of the FRET-spectrometry approach to detect small differences between the interprotomeric distances among GPCR oligomers with subtle differences in quaternary structures. We then used this approach to study the conformational substates of oligomers of sterile 2 α-factor receptor (Ste2), a class D GPCR found in the yeast Saccharomyces cerevisiae of mating type a. Ste2 has previously been shown to form tetrameric oligomers at relatively low expression levels (between 11 and 140 molecules/μm2) in the absence of its cognate ligand, the α-factor pheromone. The significantly improved FRET spectrometry technique allowed us to detect multiple distinct quaternary conformational substates of Ste2 oligomers, and to assess how the α-factor ligand altered the proportion of such substates. The ability to determine quaternary structure substates of GPCRs provides exquisite means to elucidate functional relevance of GPCR oligomerization.Competing Interest StatementThe authors have declared no competing interest.View Full Text

biophysics↗

bioRxiv2019

Reply To: Molecular Brightness analysis of GPCR oligomerization in the presence of spatial heterogeneity

Annibale and Lohse have recently suggested a way1 in which the two-dimensional fluorescence intensity fluctuation (2D FIF) spectrometry2 may be further refined. Their main suggestion is to include a step in the analysis process where a case-by-case inspection of individual regions of interest of a membrane allows for selection of portions of the membrane which are "as homogenous as possible" and thereby exclude intensity spots potentially related to other sub-cellular structures. By incorporating that proposal into an objective and reproducible algorithm, here we show that 2D FIF has a built-in capability to automatically filter out such contributions, and that further removal of inhomogeneities does not alter the final results.

biophysics↗

bioRxiv2018

A general method to quantify ligand-driven oligomerization using single- or two-photon excitation microscopy

Current technologies for probing membrane protein association and stability in cells are either very laborious or lack the bandwidth needed for fully quantitative analysis. Here we introduce a platform, termed one- or two-dimensional fluorescence intensity fluctuation spectrometry, for determining the identity, abundance, and stability of oligomers of differing sizes. The sensitivity of this approach is demonstrated by using monomers and oligomers of known sizes in both solutions and cell membranes. The analysis was extended to uncover the oligomeric states and their stability for both the epidermal growth factor receptor, a receptor tyrosine kinase, and the G protein-coupled secretin receptor. In both cases, agonist ligand binding shifted the equilibrium from monomers or dimers to rather large oligomers. Our method can be used in conjunction with a variety of light-based microscopy techniques, is several orders of magnitude faster than current approaches, and is scalable for high-throughput analyses.

biophysics↗

bioRxiv2018

Ab initio derivation of the FRET equations resolves old puzzles and suggests measurement strategies

Quantitative FRET-based imaging methods rely on the determination of an apparent FRET efficiency (Eapp) as well as donor and acceptor concentrations, in order to uncover the identity and relative abundance of the oligomeric (or quaternary) structures of associating macromolecules. Theoretical work has provided \"upwards\" relationships between the experimentally determined Eapp distributions and the quaternary structure models that underlie them. By contrast, the body of work that predicates the \"downwards\" dependence of Eapp on directly measurable quantities (i.e., fluorescence emission of donors and acceptors) relies largely on plausibility arguments, one of which is the seemingly obvious assumption that the fraction of fluorescent molecules in the ground state pretty nearly equals the total concentration of molecules. In this work, we use the kinetic models of fluorescence in the presence and absence of FRET to rigorously derive useful relationships between Eapp and measurable fluorescence signals. Analysis of these relationships reveals a few anticipated surprises and some unexpected explanations for known experimental FRET puzzles, and it provides theoretical foundations for optimizing measurement strategies.

biophysics↗