Cellular and Molecular Mechanisms of Inflammation

Signal Transduction in Inflammatory Cells, Part A

Charles G. Cochrane, Michael A. Gimbrone

• Publisher: Academic Press
• Year: 2013
• Print ISBN: 9780121504038
• eBook ISBN: 9781483191539
• Topic: Medicine
• Subtopic: Internal Medicine & Diagnosis

Cellular and Molecular Mechanisms of Inflammation: Signal Transduction in Inflammatory Cells, Part A is a collection of papers that discusses the mechanisms of the transduction of signals linking stimulated receptors and cellular function. This book describes the pathways of signal transduction involved in stimulating functions of inflammatory cells connected with host defense and development of inflammatory injury. One paper notes the potential of using fluorescence methodology in analyzing ligand-receptor interactions in living systems during the natural abundance of cell surface receptors. Another paper discusses the structure and function of GTP-binding proteins in neutrophil signal transduction, particularly the role of oligomeric G proteins in signal transduction. One concern in signal transduction research is the physiological significance of the presence of multiple forms of proteins that can have identical functions. One paper reviews phosphatidylcholine breakdown and hormone action in the rat liver, focusing on G proteins and on inositol phospholipid breakdown. This book also discusses calcium translocation in signal transduction, as well as, a novel signal transduction pathway involving phosphatidylinositol 3-kinase. This book can prove beneficial for biochemists, micro-biologists, cellular researchers, and academicians involved in the study of cellular biology, physiology or oncology.

CHAPTER 1

Real-Time Analysis of Ligand–Receptor Dynamics and Binding Pocket Structure of the Formyl Peptide Receptor

Larry A. Sklar, Cancer Center, Cytometry, University of New Mexico School of Medicine, Albuquerque, New Mexico 87131; Los Alamos National Laboratory

Publisher Summary

This chapter provides an overview of the real-time analysis of ligand-receptor dynamics and the binding pocket structure of the formyl peptide receptor. Complementary spectrofluorometric and flow cytometric techniques have been developed to examine the interactions of formyl peptide ligands (L) with their cell surface receptors (R). These techniques have been used to study ligand-receptor and receptor-processing events in intact neutrophils and ternary complex interactions in permeabilized neutrophils and neutrophil membranes. Kinetic studies suggest that L binds to R at a diffusion-limited rate and that R undergoes rapid transitions involving three states prior to internalization. It is revealed through a spectroscopic analysis of the interaction between L and R that the binding pocket of R is large enough to contain not more than six amino acids and that a fluoresceinlabeled pentapeptide is quenched upon binding to R. The chapter also explains that cell activation involves a sequence of transient macromolecular assemblies, beginning with the binding of stimulatory ligands to cell surface receptors.

Introduction

We have developed complementary spectrofluorometric and flow cytometric techniques to examine the interactions of formyl peptide ligands (L) with their cell surface receptors (R). These general approaches have now evolved to the point that mechanistic studies based on kinetics of ligand binding and structural insight based on spectroscopic properties of probes in binding pockets may now be accessible to the scientific community. These techniques have been used to study ligand–receptor and receptor-processing events in intact neutrophils and ternary complex interactions in permeabilized neutrophils and neutrophil membranes. Kinetic studies suggest that L binds to R at a diffusion-limited rate and that R undergoes rapid transitions involving three states [LR, LRG (the ternary complex of L and R with the G protein), and a desensitized receptor “LRX,” which forms within seconds] prior to internalization. A model describing the kinetics of the activation events and based on the real-time methods suggests novel implications in signal transduction via ternary complex pathways. The methodology has also contributed to an understanding of the relationship between receptor occupancy and cell response, including quantitative insight into the details of amplification of intracellular signaling pathways.

Structural information about receptor binding pockets is also now beginning to emerge. We have learned through a spectroscopic analysis of the interaction between L and R that the binding pocket of R is large enough to contain no more than six amino acids and that a fluorescein-labeled pentapeptide is quenched upon binding to R. The quenching appears to be via a static mechanism involving protonation of the chromophore. An analysis of the primary sequence of the receptor suggests points of contact between L and R. We hypothesize that His-90 (putatively located in the extracellular loop connecting the second and third transmembrane domains) protonates L and quenches the probe.

We believe that a number of technological developments will continue to drive the applications and implications of real-time methods. For example, technology being developed at the National Flow Cytometry Resource at Los Alamos National Laboratory is expected to extend and generalize the analysis of receptor structure and dynamics to low-affinity peptide receptors of living biological systems. This technology is expected to be available to support a broad user community.

Birth of Real-Time Assays for Neutrophil Receptor Dynamics

It is now clear that cell activation involves a sequence of transient macromolecular assemblies, beginning with the binding of stimulatory ligands to cell surface receptors. Whereas receptor activities had classically been identified by physiological means, the introduction of radiolabeled ligands permitted a breakthrough in the ability to identify, characterize, and purify receptors. The early phases of our work to develop real-time spectrofluorometric and flow cytometric approaches to analyze neutrophil receptors had their origins in the late 1970s. These investigations were promoted by several important factors. First, it was beginning to be recognized that hormone receptors exhibited guanine nucleotide sensitivity in ligand binding (1). These observations indicated that ligand binding analysis was likely to play an important role in understanding signal transduction as well as being a tool for identifying the distribution of binding sites on various tissues and purifying the molecules. Second, there was at this time considerable interest in the idea that ligand or receptor internalization played a necessary role in signaling for some receptors (2). Niedel et al. (3) had just succeeded in preparing a rhodamine derivative of the newly identified chemoattractant formyl peptide and used fluorescence microscopy to visualize its rapid clustering and internalization. Finally, Zigmond and co-workers (4) had engaged upon the arduous task of attempting to unravel the connection between ligand-binding and cell responses for neutrophils chemotaxing in gradients. Their studies made obvious the need to take into account receptor expression or up-regulation, internalization, and recycling.

Our first attempt to understand the relationship between receptor occupancy and neutrophil response began in 1980. At that time a number of convenient assays were available to measure neutrophil responses continuously and in real time. These included recently introduced methods for detection of superoxide generation by cytochrome reduction and membrane depolarization by membrane potential-sensitive dyes. In an effort to understand how signaling might progress through a chemotactic process, we began to consider how cells would respond to a stimulus administered in a temporal gradient (5). That is, to mimic the temporal aspects of increasing stimulus concentration that a cell would encounter while chemotaxing up a spatial gradient, we delivered formyl peptide to cells in suspension, using an infusion pump to ramp the stimulus concentration. We found that the responses elongated in time, but eventually the rates of stimulus administration were so low that the cells failed to respond at all. Using very approximate arguments concerning the rates of ligand binding based on the studies by Zigmond et al. and noting the impact of the infusion on the responses, we suggested that low rates of occupancy were required to initiate responses and that chemotaxis might require as few as 1% of the receptors occupied per minute.

These initial observations prompted us to pursue a course toward understanding the relationship between ligand–receptor dynamics and cell response. A number of challenges immediately presented themselves. First, we were aware of a need to make early time, as well as time-dependent, measurements of binding at low levels of receptor occupancy. It was already clear that neutrophils had the capabilities of varying receptor expression in complicated ways (i.e., changing affinity during receptor desensitization and changing number through expression, internalization, and recycling). Investigators before us had already resorted to complex sampling protocols involving binding at one temperature, changing the temperature for the wash step, and changing again for analysis. We were uncertain about particular aspects of radioligand techniques, including the specificity of binding assays with hydrophobic ligands, the ultimate time resolution accessible with heterogeneous assays requiring wash steps, and the difficulty in performing time-dependent assays. As noted, when a number of processing events could proceed simultaneously, it was necessary to use temperature as a tool in trapping individual steps. The possibility of using fluorescent formyl peptides in real-time assays was strengthened for us by several reports of acetylcholine receptor dynamics with fluorescent ligands (6). Preparations of acetylcholine receptor suspended at micromolar concentration had permitted the analysis of agonists and antagonists with affinities comparable to or tighter than the receptor concentration. Would it be similarly possible to examine neutrophil receptors expressed at 50,000 per cell at cell densities up to 10 million per milliliter? In this case, receptor densities were expected to be no more than 1 nM!

Early Methods 1980–1985

The key to any real-time binding assay is the ability to discriminate free and bound ligand. Spectrally sensitive probes change absorbance or emission properties when ligands bind to receptors, resulting in wavelength shifts. Spectroscopic changes are also possible, such as a change in fluorescence depolarization resulting from altered rates of rotational relaxation of the free and bound ligand species, a change in intensity (i.e., quenching or enhancement), or a change in fluorescence lifetime. As we were soon to realize, single-cell detection (suggested by the success of the microscopic studies) also made flow cytometric detection reasonable. Not initially realizing the power of flow cytometry, we explored the feasibility of suspension methods in cuvettes. After synthesizing the fluoresceinated hexapeptide, we confronted two problems immediately: how to detect the signal from the ligand in the presence of overwhelming scatter signals from 10 million cells per milliliter and how to resolve the signals emanating from free and bound ligand molecules. Some years earlier I had uncovered a report by Walter Dandliker and colleagues (7) describing high-affinity antibodies to fluorescein which quenched the emission of fluorescein when fluorescein was bound to the antibody. As my co-workers were immunologists, I wondered whether it would be possible to use antibodies to physically discriminate the free and bound ligand. As luck would have it, Dandliker was in the same institution as I, and he willingly provided both advice and reagents. The advice was that, since the cell-bound ligand had been resolved from free ligand and light scatter through microscopy, there was no doubt that filters and monochromators could resolve scatter and fluorescent signal in the cell suspension. The reagent he had available was a preparation of high-affinity polyclonal antibody to fluorescein derived from hyperimmunized rabbits. The antibody proved essential in getting the work off the ground. It was a short time thereafter that we reported the use of the antibody to fluorescein in quenching the free fluoresceinated ligand in the presence of cell-bound ligand (8). Thus, it was possible to achieve a real-time analysis of ligand binding in cell suspensions with the cells and the receptors present at concentrations comparable to their natural abundance. These studies also used the antibody to interrupt cell stimulation during early phases of ligand binding. This was possible because the antibody bound free ligand rapidly, and the resulting complex bound poorly to the cell surface receptors. The results confirmed initial observations that responses were initiated by low levels of occupancy. Since we observed that individual responses required different levels of occupancy, these results began to sow the seeds for us of the concept of divergent pathways of cell response.

At this time flow cytometry was beginning to be used widely among immunologists, and an instrument was available in my institution for pilot studies of formyl peptide binding. The first experiment was designed to answer two questions: Could binding be detected on a single-cell basis, and what impact did washing the free ligand have on the measurement? Even with the instruments of that era, there was no trouble in detecting binding. To our surprise the cytometric signal was not remarkably different if cells labeled with 1 nM fluorescent ligand were washed free of the unbound ligand. Moreover, cells exposed to fluorescent ligand in the presence of excess nonfluorescent ligand remained essentially as dim as cells which had never seen the fluorescent ligand. These observations led to a realization that flow cytometry intrinsically discriminated between ligand which had accumulated on the cell surface from ligand dilute in the surrounding solution. The discrimination made possible continuous analysis of cells accumulating or releasing ligand in association or dissociation experiments. Since standards were not available to quantitate receptor numbers in flow cytometry according to the cell brightness, a calibration method was developed at the same time as these initial cytometric investigations (9). The...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Chapter 1: Real-Time Analysis of Ligand–Receptor Dynamics and Binding Pocket Structure of the Formyl Peptide Receptor
• Chapter 2: Structure and Function of GTP-Binding Proteins in Neutrophil Signal Transduction
• Chapter 3: Regulation of Phospholipase C Isozymes
• Chapter 4: Phosphatidylcholine Metabolism in Signal Transduction
• Chapter 5: Calcium Translocation in Signal Transduction
• Chapter 6: Inositol Phosphate Metabolism
• Chapter 7: Phosphatidylinositol 3-kinase: A Novel Signal Transduction Pathway?
• Index