Oxidized-LDL Induces Metabolic Dysfunction in Retinal Pigment Epithelial Cells.

Recently, mitochondrial dysfunction has gained attention as a causative factor in the pathogenesis and progression of age-related macular degeneration (AMD). Mitochondrial damage plays a key role in metabolism and disrupts the balance of intracellular metabolic pathways, such as oxidative phosphorylation (OXPHOS) and glycolysis. In this study, we focused on oxidized low-density lipoprotein (ox-LDL), a major constituent of drusen that accumulates in the retina of patients with AMD, and investigated whether it could be a causative factor for metabolic alterations in retinal pigment epithelial (RPE) cells. We found that prolonged exposure to ox-LDL induced changes in fatty acid ß-oxidation (FAO), OXPHOS, and glycolytic activity and increased the mitochondrial reactive oxygen species production in RPE cells. Notably, the effects on metabolic alterations varied with the concentration and duration of ox-LDL treatment. In addition, we addressed the limitations of using ARPE-19 cells for retinal disease research by highlighting their lower barrier function and FAO activity compared to those of induced pluripotent stem cell-derived RPE cells. Our findings can aid in the elucidation of mechanisms underlying the metabolic alterations in AMD.

Fluorescence detection of metabolic activity of the fatty acid beta oxidation pathway in living cells.

Detection of metabolic activity in living cells facilitates the understanding of the cell mechanism. Here, we report a fluorescent probe that can detect fatty acid beta oxidation (FAO) in living cells. This probe is metabolically degraded by the sequential enzyme reactions of FAO and can visualize the FAO activity with turn-on fluorescence.

Electron Microscopic Detection of Single Membrane Proteins by a Specific Chemical Labeling.

Electron microscopy (EM) is a technology that enables visualization of single proteins at a nanometer resolution. However, current protein analysis by EM mainly relies on immunolabeling with gold-particle-conjugated antibodies, which is compromised by large size of antibody, precluding precise detection of protein location in biological samples. Here, we develop a specific chemical labeling method for EM detection of proteins at single-molecular level. Rational design of α-helical peptide tag and probe structure provided a complementary reaction pair that enabled specific cysteine conjugation of the tag. The developed chemical labeling with gold-nanoparticle-conjugated probe showed significantly higher labeling efficiency and detectability of high-density clusters of tag-fused G protein-coupled receptors in freeze-fracture replicas compared with immunogold labeling. Furthermore, in ultrathin sections, the spatial resolution of the chemical labeling was significantly higher than that of antibody-mediated labeling. These results demonstrate substantial advantages of the chemical labeling approach for single protein visualization by EM.

Discovery of highly reactive peptide tag by ELISA-type screening for specific cysteine conjugation.

We report the discovery of a highly reactive peptide tag for the specific cysteine conjugation of proteins. Screening of cysteine-containing peptides using ELISA-type screening yielded a 19-amino acid tag (DCPPPDDAADDAADDAADD), named DCP3 tag, which enabled the rapid and selective labeling of the tag-fused protein with a synthetic zinc complex on the surface of living cells.

Reversible ratiometric detection of highly reactive hydropersulfides using a FRET-based dual emission fluorescent probe.

Hydropersulfide (R-SSH) is an important class of reactive sulfur species (RSS) involved in a variety of physiological processes in mammals. A fluorescent probe capable of real-time detection of hydropersulfide levels in living cells would be a versatile tool to elucidate its roles in cell signalling and redox homeostasis. In this paper, we report a ratiometric fluorescent probe for hydropersulfide sensing, based on a fluorescence resonance energy transfer (FRET) mechanism. This sensing mechanism involves a nucleophilic reaction of a hydropersulfide with the pyronine-unit of the probe, which modulates the intramolecular FRET efficiency to induce a dual-emission change. The reversible nature of this reaction allows us to detect increases and decreases of hydropersulfide levels in a real-time manner. The probe fluorometrically sensed highly reactive hydropersulfides, such as H2S2 and Cys-SSH, while the fluorescence response to biologically abundant cysteine and glutathione was negligible. Taking advantage of the reversible and selective sensing properties, this probe was successfully applied to the ratiometric imaging of concentration dynamics of endogenously produced hydropersulfides in living cells.

Peptide tag/probe pairs based on the coordination chemistry for protein labeling.

Protein-labeling methods serve as essential tools for analyzing functions of proteins of interest under complicated biological conditions such as in live cells. These labeling methods are useful not only to fluorescently visualize proteins of interest in biological systems but also to conduct protein and cell analyses by harnessing the unique functions of molecular probes. Among the various labeling methods available, an appropriate binding pair consisting of a short peptide and a de novo designed small molecular probe has attracted attention because of its wide utility and versatility. Interestingly, most peptide tag/probe pairs exploit metal-ligand coordination interactions as the main binding force responsible for their association. Herein, we provide an overview of the recent progress of these coordination-chemistry-based protein-labeling methods and their applications for fluorescence imaging and functional analysis of cellular proteins, while highlighting our originally developed labeling methods. These successful examples clearly exemplify the utility and versatility of metal coordination chemistry in protein functional analysis.

In-cell covalent labeling of reactive His-tag fused proteins.

A new method for in-cell protein labeling was developed. This method employed a binding-induced nucleophilic reaction between the Cys-appended His-tag and the Ni(II)-NTA containing an α-chloroacetamide. Using this method, not only labeling of His-tag fused proteins but also the detection of a protein-protein interaction was achieved inside living cells.

Selective covalent labeling of tag-fused GPCR proteins on live cell surface with a synthetic probe for their functional analysis.

Selective protein labeling with a small molecular probe is a versatile method for elucidating protein functions in living cells. In this paper, we report a covalent labeling method of tag-fused G-protein coupled receptor (GPCR) proteins expressing on cell surfaces utilizing small functional molecules. This method employs the selective and rapid reaction of a peptide tag and a molecular probe, which comprises the cysteine-containing short CA6D4x2 tag (CAAAAAADDDDGDDDD) and a tetranuclear Zn(II)-DpaTyr probe containing a reactive alpha-chloroacetyl moiety. The covalent labeling of tag-fused GPCRs such as bradykinin receptor (B2R) and acetylcholine receptor (m1AchR) selectively proceeded under physiological conditions during short incubation (10-30 min) with Zn(II)-DpaTyr probes bearing various functional groups. Labeling with fluorophore-appended Zn(II)-DpaTyr probes enabled visualization of the GPCRs on the surface of HEK293 cells by fluorescence. Labeling with the biotin-appended probe allowed introduction of a biotin unit into the GPCRs. This biotin label was utilized for fluorescence bioimaging studies and postlabeling blotting analysis of the labeled GPCRs by use of the specific biotin-streptavidin interaction. The utility of this labeling method was demonstrated in several function analyses of GPCRs, such as fluorescence visualization of the stimuli-responsive internalization of GPCRs and pH change in endosomes containing the internalized GPCRs.

FLAG-tag selective covalent protein labeling via a binding-induced acyl-transfer reaction.

A FLAG tag selective protein labeling method is newly developed in this study. Coupling of the selective binding between synthetic Ni-complex probe and FLAG tag with the acyl transfer reaction enables the site-selective covalent modification of FLAG peptide and FLAG-tag fused protein.