Visualization of the pH-dependent dynamic distribution of G2A in living cells

Chemoenzymatic labeling to visualize intercellular contacts using lipidated sortase A

Methods to label intercellular contact have attracted attention because of their potential in cell biological and medical applications for the analysis of intercellular communications. In this study, a simple and versatile method for chemoenzymatic labeling of intercellularly contacting cells is demonstrated using a cell-surface anchoring reagent of a poly(ethylene glycol)(PEG)-lipid conjugate. The surfaces of each cell in the cell pairs of interest were decorated with sortase A (SrtA) and triglycine peptide that were lipidated with PEG-lipid. In the mixture of the two cell populations, the triglycine-modified cells were enzymatically labeled with a fluorescent labeling reagent when in contact with SrtA-modified cells on a substrate. The selective labeling of the contacting cells was confirmed by confocal microscopy. The method is a promising tool for selective visualization of intercellularly contacting cells in cell mixtures for cell-cell communication analysis.

Lysophosphatidylcholine promotes intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 expression in human umbilical vein endothelial cells via an orphan G protein receptor 2-mediated signaling pathway

The oxLDL-based bioactive lipid lysophosphatidylcholine (LPC) is a key regulator of physiological processes including endothelial cell adhesion marker expression. This study explored the relationship between LPC and the human umbilical vein endothelial cell expression of intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1) with a particular focus on the regulation of the LPC-G2A-ICAM-1/VCAM-1 pathway in this context. We explored the LPC-inducible role of orphan G protein receptor 2 (G2A) in associated regulatory processes by using human kidney epithelial (HEK293) cells that had been transfected with pET-G2A, human umbilical vein endothelial cells (HUVECs) in which an shRNA was used to knock down G2A, and western blotting and qPCR assays that were used to confirm changes in gene expression. For in vivo studies, a rabbit model of atherosclerosis was established, with serum biochemistry and histological staining approaches being used to assess pathological outcomes in these animals. The treatment of both HEK293 cells and HUVECs with LPC promoted ICAM-1 and VCAM-1 upregulation, while incubation at a pH of 6.8 suppressed such LPC-induced adhesion marker expression. Knocking down G2A by shRNA and inhibiting NF-κB activity yielded opposite outcomes. The application of a Gi protein inhibitor had no impact on LPC-induced ICAM-1/VCAM-1 expression. Atherosclerotic model exhibited high circulating LDL and LPC levels as well as high aortic wall ICAM-1/VCAM-1 expression. Overall, these results suggested that the LPC-G2A-ICAM-1/VCAM-1 pathway may contribute to the atherogenic activity of oxLDL, with NF-κB antagonists representing potentially viable therapeutic tools for the treatment of cardiovascular disease.

Stepwise phosphorylation of leukotriene B4 receptor 1 defines cellular responses to leukotriene B4

Leukotriene B₄ (LTB₄) receptor type 1 (BLT1) is abundant in phagocytic and immune cells and plays crucial roles in various inflammatory diseases. BLT1 is phosphorylated at several serine and threonine residues upon stimulation with the inflammatory lipid LTB₄ Using Phos-tag gel electrophoresis to separate differentially phosphorylated forms of BLT1, we identified two distinct types of phosphorylation, basal and ligand-induced, in the carboxyl terminus of human BLT1. In the absence of LTB₄, the basal phosphorylation sites were modified to various degrees, giving rise to many different phosphorylated forms of BLT1. Different concentrations of LTB₄ induced distinct phosphorylation events, and these ligand-induced modifications facilitated additional phosphorylation events at the basal phosphorylation sites. Because neutrophils migrate toward inflammatory sites along a gradient of LTB₄, the degree of BLT1 phosphorylation likely increases in parallel with the increase in LTB₄ concentration as the cells migrate. At high concentrations of LTB₄, deficiencies in these two types of phosphorylation events impaired chemotaxis and β-hexosaminidase release, a proxy for degranulation, in Chinese hamster ovary (CHO-K1) and rat basophilic leukemia (RBL-2H3) cells, respectively. These results suggest that an LTB₄ gradient around inflammatory sites enhances BLT1 phosphorylation in a stepwise manner to facilitate the precise migration of phagocytic and immune cells and the initiation of local responses, including degranulation.

Real-time monitoring of pH-dependent intracellular trafficking of ovarian cancer G protein-coupled receptor 1 in living leukocytes

G-protein coupled receptors (GPCRs) are involved in many diseases and important biological phenomena; elucidating the mechanisms underlying regulation of their signal transduction potentially provides both novel targets for drug discovery and insight into living systems. A proton-sensing GPCR, ovarian cancer G protein-coupled receptor 1 (OGR1), has been reported to be related to acidosis and diseases that cause tissue acidification, but the mechanism of proton-induced activation of OGR1-mediated signal transduction in acidic conditions remains unclear. Here, pH-dependent intracellular trafficking of OGR1 was visualized in living leukocytes by a real-time fluorescence microscopic method based on sortase A-mediated pulse labeling of OGR1. OGR1 labeled on the cell surface with a small fluorescent dye was clearly observed to remain in the plasma membrane during incubation in mildly acidic medium (pH 6.6) and to be internalized to the intracellular compartments on changing the medium to slightly basic pH (7.7). Quantitative single-cell image analysis showed that most of the internalized OGR1s were then recycled to the plasma membrane for signal transduction if the extracellular pH was returned to the mildly acidic state. However, in a minor population of cells (40%), the internalized OGR1s were retained in endosomes or transported to lysosomes and degraded, leading to low efficiency of their recycling to the plasma membrane. Thus, the present live-cell monitoring strongly suggests that the signal transduction activity of OGR1 is regulated by pH-dependent internalization and recycling to the plasma membrane.

Deletion of Adipose Triglyceride Lipase Links Triacylglycerol Accumulation to a More-Aggressive Phenotype in A549 Lung Carcinoma Cells

Adipose triglyceride lipase (ATGL) catalyzes the rate limiting step in triacylglycerol breakdown in adipocytes but is expressed in most tissues. The enzyme was shown to be lost in many human tumors, and its loss may play a role in early stages of cancer development. Here, we report that loss of ATGL supports a more-aggressive cancer phenotype in a model system in which ATGL was deleted in A549 lung cancer cells by CRISPR/Cas9. We observed that loss of ATGL led to triacylglycerol accumulation in lipid droplets and higher levels of cellular phospholipid and bioactive lipid species (lyso- and ether-phospholipids). Label-free quantitative proteomics revealed elevated expression of the pro-oncogene SRC kinase in ATGL depleted cells, which was also found on mRNA level and confirmed on protein level by Western blot. Consistently, higher expression of phosphorylated (active) SRC (Y416 phospho-SRC) was observed in ATGL-KO cells. Cells depleted of ATGL migrated faster, which was dependent on SRC kinase activity. We propose that loss of ATGL may thus increase cancer aggressiveness by activation of pro-oncogenic signaling via SRC kinase and increased levels of bioactive lipids.

Microfluidic preparation of anchored cell membrane sheets for in vitro analyses and manipulation of the cytoplasmic face

Molecular networks on the cytoplasmic faces of cellular plasma membranes are critical research topics in biological sciences and medicinal chemistry. However, the selective permeability of the cell membrane restricts the researchers from accessing to the intact intracellular factors on the membrane from the outside. Here, a microfluidic method to prepare cell membrane sheets was developed as a promising tool for direct examination of the cytoplasmic faces of cell membranes. Mammalian cells immobilized on a poly(ethylene glycol)-lipid coated substrate were rapidly and efficiently fractured, with the sheer stress of laminar flow in microchannels, resulting in isolation of the bottom cell membrane sheets with exposed intact cytoplasmic faces. On these faces of the cell membrane sheets, both ligand-induced phosphorylation of receptor tyrosine kinases and selective enzymatic modification of a G-protein coupling receptor were directly observed. Thus, the present cell membrane sheet should serve as a unique platform for studies providing new insights into juxta-membrane molecular networks and drug discovery.

Quantitative image cytometry for analyzing intracellular trafficking of G protein-coupled receptors on a chemical-trapping single cell array

G protein-coupled receptors (GPCRs) are important targets in medical and pharmaceutical research fields, because they play key roles in a variety of biological processes. Recently, intracellular trafficking of GPCRs involving endosomal internalization and recycling to the plasma membrane has been studied as a regulation mechanism for GPCR activities. However, the absence of a quantitative single-cell analysis method has hampered conditional GPCR trafficking studies and the possibility of gaining significant insights into the mechanism of regulation of GPCR signaling. Here, we report a facile image cytometry method to analyze the trafficking of GPCRs. In this method, GPCR-expressing cells were arrayed with a photo-responsive cell-immobilizing reagent in a single-cell manner, and the tagged GPCR was visualized by pulse-labeling with a fluorescent dye through sortase-mediated peptide-tag ligation. We quantified the intracellular distribution changes of a pH-dependent GPCR, G2A, by time-course observation under mildly acidic and slightly basic pH conditions. The difference in pH-dependent G2A trafficking between individual cells was automatically detected by an image analysis custom software program, and simultaneously, the average distribution ratios were also determined for understanding the properties of G2A. The present method should be applicable for investigating the dynamic intracellular trafficking of a wide variety of GPCRs under various conditions in a high-throughput manner.

Biomolecular engineering for nanobio/bionanotechnology

Biomolecular engineering can be used to purposefully manipulate biomolecules, such as peptides, proteins, nucleic acids and lipids, within the framework of the relations among their structures, functions and properties, as well as their applicability to such areas as developing novel biomaterials, biosensing, bioimaging, and clinical diagnostics and therapeutics. Nanotechnology can also be used to design and tune the sizes, shapes, properties and functionality of nanomaterials. As such, there are considerable overlaps between nanotechnology and biomolecular engineering, in that both are concerned with the structure and behavior of materials on the nanometer scale or smaller. Therefore, in combination with nanotechnology, biomolecular engineering is expected to open up new fields of nanobio/bionanotechnology and to contribute to the development of novel nanobiomaterials, nanobiodevices and nanobiosystems. This review highlights recent studies using engineered biological molecules (e.g., oligonucleotides, peptides, proteins, enzymes, polysaccharides, lipids, biological cofactors and ligands) combined with functional nanomaterials in nanobio/bionanotechnology applications, including therapeutics, diagnostics, biosensing, bioanalysis and biocatalysts. Furthermore, this review focuses on five areas of recent advances in biomolecular engineering: (a) nucleic acid engineering, (b) gene engineering, (c) protein engineering, (d) chemical and enzymatic conjugation technologies, and (e) linker engineering. Precisely engineered nanobiomaterials, nanobiodevices and nanobiosystems are anticipated to emerge as next-generation platforms for bioelectronics, biosensors, biocatalysts, molecular imaging modalities, biological actuators, and biomedical applications.

Heteromerization of G2A and OGR1 enhances proton sensitivity and proton-induced calcium signals

Proton-sensing G-protein-coupled receptors (GPCRs; OGR1, GPR4, G2A, TDAG8), with full activation at pH 6.4 ∼ 6.8, are important to pH homeostasis, immune responses and acid-induced pain. Although G2A mediates the G13-Rho pathway in response to acid, whether G2A activates Gs, Gi or Gq proteins remains debated. In this study, we examined the response of this fluorescence protein-tagged OGR1 family to acid stimulation in HEK293T cells. G2A did not generate detectable intracellular calcium or cAMP signals or show apparent receptor redistribution with moderate acid (pH ≥ 6.0) stimulation but reduced cAMP accumulation under strong acid stimulation (pH ≤ 5.5). Surprisingly, coexpression of OGR1- and G2A-enhanced proton sensitivity and proton-induced calcium signals. This alteration is attributed to oligomerization of OGR1 and G2A. The oligomeric potential locates receptors at a specific site, which leads to enhanced proton-induced calcium signals through channels.