Translocation of signalling proteins to the plasma membrane revealed by a new bioluminescent procedure

Cortisol Rapidly Facilitates Glucocorticoid Receptor Translocation to the Plasma Membrane in Primary Trout Hepatocytes

Glucocorticoids (GCs) stimulate rapid cell signalling by activating the membrane-anchored intracellular glucocorticoid receptor (GR). However, the recruitment of the GR to the plasma membrane to facilitate nongenomic signalling is far from clear. As cytosolic free calcium ([Ca²⁺]i) is involved in intracellular protein dynamics, we tested the hypothesis that acute elevation in cortisol levels rapidly stimulates GR translocation to the plasma membrane via a calcium-dependent process in rainbow trout (Oncorhynchus mykiss) hepatocytes. To test this, we monitored temporal changes in intracellular GR distribution in response to cortisol exposure. Immunofluorescence labelling showed that the GR was present in cytosolic and nuclear compartments in trout hepatocytes. However, upon cortisol exposure, the GR rapidly (within 5 min) formed punctate and colocalized with caveolin-1, suggesting plasma membrane localization of the receptor. This redistribution of the GR to the plasma membrane was transient and lasted for 30 min and was evident even upon exposure to cortisol-BSA, a membrane-impermeable analogue of the steroid. The rapid cortisol-mediated GR translocation to the plasma membrane involved F-actin polymerization and was completely abolished in the presence of either EGTA or Cpd5J-4, a calcium release-activated calcium (CRAC) channel blocker. Additionally, the modulation of the biophysical properties of the plasma membrane by cholesterol or methyl β-cyclodextrin, which led to changes in ([Ca²⁺]i) levels, modified GR translocation to the plasma membrane. Altogether, acute cortisol-mediated rise in ([Ca²⁺]i) levels rapidly stimulated the translocation of intracellular GR to the plasma membrane, and we propose this as a mechanism promoting the nongenomic action of the GR for hepatocyte stress resistance.

A Novel Loss of Function Melanocortin-4-Receptor Mutation (MC4R-F313Sfs*29) in Morbid Obesity

CONTEXT

Melanocortin receptor-4 (MC4R) gene mutations are associated with early-onset severe obesity, and the identification of potential pathological variants is crucial for the clinical management of patients with obesity.

OBJECTIVE

To explore whether and how a novel heterozygous MC4R variant (MC4R-F313Sfs*29), identified in a young boy (body mass index [BMI] 38.8 kg/m2) during a mutation analysis conducted in a cohort of patients with obesity, plays a determinant pathophysiological role in the obesity development.

DESIGN SETTING AND PATIENTS

The genetic screening was carried out in a total of 209 unrelated patients with obesity (BMI ≥ 35 kg/m2). Structural and functional characterization of the F313Sfs*29-mutated MC4R was performed using computational approaches and in vitro, using HEK293 cells transfected with genetically encoded biosensors for cAMP and Ca2+.

RESULTS

The F313Sfs*29 was the only variant identified. In vitro experiments showed that HEK293 cells transfected with the mutated form of MC4R did not increase intracellular cAMP or Ca2+ levels after stimulation with a specific agonist in comparison with HEK293 cells transfected with the wild type form of MC4R (∆R/R0 = -90% ± 8%; P < 0.001). In silico modeling showed that the F313Sfs*29 mutation causes a major reorganization in the cytosolic domain of MC4R, thus reducing the affinity of the putative GalphaS binding site.

CONCLUSIONS

The newly discovered F313Sfs*29 variant of MC4R may be involved in the impairment of α-MSH-induced cAMP and Ca2+ signaling, blunting intracellular G protein-mediated signal transduction. This alteration might have led to the dysregulation of satiety signaling, resulting in hyperphagia and early onset of obesity.

Optical approaches for single-cell and subcellular analysis of GPCR-G protein signaling

G protein-coupled receptors (GPCRs), G proteins, and their signaling associates are major signal transducers that control the majority of cellular signaling and regulate key biological functions including immune, neurological, cardiovascular, and metabolic processes. These pathways are targeted by over one-third of drugs on the market; however, the current understanding of their function is limited and primarily derived from cell-destructive approaches providing an ensemble of static, multi-cell information about the status and composition of molecules. Spatiotemporal behavior of molecules involved is crucial to understanding in vivo cell behaviors both in health and disease, and the advent of genetically encoded fluorescence proteins and small fluorophore-based biosensors has facilitated the mapping of dynamic signaling in cells with subcellular acuity. Since we and others have developed optogenetic methods to regulate GPCR-G protein signaling in single cells and subcellular regions using dedicated wavelengths, the desire to develop and adopt optogenetically amenable assays to measure signaling has motivated us to take a broader look at the available optical tools and approaches compatible with measuring single-cell and subcellular GPCR-G protein signaling. Here we review such key optical approaches enabling the examination of GPCR, G protein, secondary messenger, and downstream molecules such as kinase and lipid signaling in living cells. The methods reviewed employ both fluorescence and bioluminescence detection. We not only further elaborate the underlying principles of these sensors but also discuss the experimental criteria and limitations to be considered during their use in single-cell and subcellular signal mapping.

The emerging use of bioluminescence in medical research

Bioluminescence is the light produced by a living organism and is commonly emitted by sea life with Ca²⁺-regulated photoproteins being the most responsible for bioluminescence emission. Marine coelenterates provide important functions involved in essential purposes such as defense, feeding, and breeding. In this review, the main characteristics of marine photoproteins including aequorin, clytin, obelin, berovin, pholasin and symplectin from different marine organisms will be discussed. We will focused on the recent use of recombinant photoproteins in different biomedical research fields including the measurement of Ca²⁺ in different intracellular compartments of animal cells, as labels in the design and development of binding assays. This review will also outline how bioluminescent photoproteins have been used in a plethora of analytical methods including ultra-sensitive assays and in vivo imaging of cellular processes. Due to their unique properties including elective intracellular distribution, wide dynamic range, high signal-to-noise ratio and low Ca²⁺-buffering effect, recombinant photoproteins represent a promising future analytical tool in several in vitro and in vivo experiments.

Subcellular functions of proteins under fluorescence single-cell microscopy

A cell is a highly organized, dynamic, and intricate biological entity orchestrated by a myriad of proteins and their self-assemblies. Because a protein's actions depend on its coordination in both space and time, our curiosity about protein functions has extended from the test tube into the intracellular space of the cell. Accordingly, modern technological developments and advances in enzymology have been geared towards analyzing protein functions within intact single cells. We discuss here how fluorescence single-cell microscopy has been employed to identify subcellular locations of proteins, detect reversible protein-protein interactions, and measure protein activity and kinetics in living cells. Considering that fluorescence single-cell microscopy has been only recently recognized as a primary technique in enzymology, its potentials and outcomes in studying intracellular protein functions are projected to be immensely useful and enlightening. We anticipate that this review would inspire many investigators to study their proteins of interest beyond the conventional boundary of specific disciplines. This article is part of a Special Issue entitled: Physiological Enzymology and Protein Functions.

Enabling Aequorin for Biotechnology Applications Through Genetic Engineering

In recent years, luminescent proteins have been studied for their potential application in a variety of detection systems. Bioluminescent proteins, which do not require an external excitation source, are especially well-suited as reporters in analytical detection. The photoprotein aequorin is a bioluminescent protein that can be engineered for use as a molecular reporter under a wide range of conditions while maintaining its sensitivity. Herein, the characteristics of aequorin as well as the engineering and production of aequorin variants and their impact on signal detection in biological systems are presented. The structural features and activity of aequorin, its benefits as a label for sensing and applications in highly sensitive detection, as well as in gaining insight into biological processes are discussed. Among those, focus has been placed on the highly sensitive calcium detection in vivo, in vitro DNA and small molecule sensing, and development of in vivo imaging technologies. Graphical Abstract.

Methods to measure intracellular Ca(2+) fluxes with organelle-targeted aequorin-based probes

The photoprotein aequorin generates blue light upon binding of Ca(2+) ions. Together with its very low Ca(2+)-buffering capacity and the possibility to add specific targeting sequences, this property has rendered aequorin particularly suitable to monitor Ca(2+) concentrations in specific subcellular compartments. Recently, a new generation of genetically encoded Ca(2+) probes has been developed by fusing Ca(2+)-responsive elements with the green fluorescent protein (GFP). Aequorin has also been employed to this aim, resulting in an aequorin-GFP chimera with the Ca(2+) sensitivity of aequorin and the fluorescent properties of GFP. This setup has actually solved the major limitation of aequorin, for example, its poor ability to emit light, which rendered it inappropriate for the monitoring of Ca(2+) waves at the single-cell level by imaging. In spite of the numerous genetically encoded Ca(2+) indicators that are currently available, aequorin-based probes remain the method of election when an accurate quantification of Ca(2+) levels is required. Here, we describe currently available aequorin variants and their use for monitoring Ca(2+) waves in specific subcellular compartments. Among various applications, this method is relevant for the study of the alterations of Ca(2+) homeostasis that accompany oncogenesis, tumor progression, and response to therapy.

Hyaluronic acid induces activation of the κ-opioid receptor

INTRODUCTION

Nociceptive pain is one of the most common types of pain that originates from an injury involving nociceptors. Approximately 60% of the knee joint innervations are classified as nociceptive. The specific biological mechanism underlying the regulation of nociceptors is relevant for the treatment of symptoms affecting the knee joint. Intra-articular administration of exogenous hyaluronic acid (HA) in patients with osteoarthritis (OA) appears to be particularly effective in reducing pain and improving patient function.

METHODS

We performed an in vitro study conducted in CHO cells that expressed a panel of opioid receptors and in primary rat dorsal root ganglion (DRG) neurons to determine if HA induces the activation of opioid peptide receptors (OPr) using both aequorin and the fluorescent dye Fura-2/AM.

RESULTS

Selective agonists and antagonists for each OPr expressed on CHO cells were used to test the efficacy of our in vitro model followed by stimulation with HA. The results showed that HA induces stimulatory effects on the κ receptor (KOP). These effects of HA were also confirmed in rat DRG neurons, which express endogenously the OPr.

CONCLUSIONS

HA activates the KOP receptor in a concentration dependent manner, with a pEC(50) value of 7.57.