Detection of Olfactory Receptor I7 Self‐Assembled Multilayer Formation and Immobilization Using a Quartz Crystal Microbalance

Immobilization of Olfactory Receptors Carried by Nanosomes onto a Gold Sensor Surface

Mammalian olfactory receptors (ORs) constitute the largest family of G-protein-coupled receptors, with up to about 1000 different genes per species, each having specific odorant ligands. ORs could be used as sensing elements of highly specific and sensitive bioelectronic hybrid devices such as bioelectronic noses. After optimized immobilization onto the device, natural ORs provide molecular recognition of various odors with their intrinsic sensitivity, discrimination, and detection properties. However, the main difficulties are related to the low expression level of recombinant ORs, their stability and potential loss of activity. Such drawbacks can be successfully overcome in bioelectronic noses integrating nanosomes (nanometric membrane vesicles carrying ORs) that are stably immobilized through a specific antibody. The advantages of such a platform rely on the fact that ORs stay in the natural membrane environment, and thus preserve their full activity. Thanks to their small sizes, nanosomes offer potential for micro- and nano-scale sensor development. In this paper, we summarize the key elements regarding nanosomes production and manipulation and provide an example of their immobilization onto a gold sensor surface. Rat ORI7 is used as a representative OR that can be functionally expressed in Saccharomyces cerevisiae. The receptor was not purified but only nanosomes were prepared. Nanosomes were immobilized onto functionalized gold surface using the anti-I7 antibody. Utilization of the antibody provides enrichment of ORI7 on the sensor surface but also uniform and appropriate orientation of the receptors. These features are crucial in optimization of bioelectronic nose' analytical performances.

Applications and Advances in Bioelectronic Noses for Odour Sensing

A bioelectronic nose, an intelligent chemical sensor array system coupled with bio-receptors to identify gases and vapours, resembles mammalian olfaction by which many vertebrates can sniff out volatile organic compounds (VOCs) sensitively and specifically even at very low concentrations. Olfaction is undertaken by the olfactory system, which detects odorants that are inhaled through the nose where they come into contact with the olfactory epithelium containing olfactory receptors (ORs). Because of its ability to mimic biological olfaction, a bio-inspired electronic nose has been used to detect a variety of important compounds in complex environments. Recently, biosensor systems have been introduced that combine nanoelectronic technology and olfactory receptors themselves as a source of capturing elements for biosensing. In this article, we will present the latest advances in bioelectronic nose technology mimicking the olfactory system, including biological recognition elements, emerging detection systems, production and immobilization of sensing elements on sensor surface, and applications of bioelectronic noses. Furthermore, current research trends and future challenges in this field will be discussed.

Mimicking nature's noses: from receptor deorphaning to olfactory biosensing

The way in which organisms detect specific volatile compounds within their environment, and the associated neural processing which produces perception and subsequent behavioural responses, have been of interest to scientists for decades. Initially, most olfaction research was conducted using electrophysiological techniques on whole animals. However, the discovery of genes encoding the family of human olfactory receptors (ORs) paved the way for the development of a range of cellular assays, primarily used to deorphan ORs from mammals and insects. These assays have greatly advanced our knowledge of the molecular basis of olfaction, however, while there is currently good agreement on vertebrate and nematode olfactory signalling cascades, debate still surrounds the signalling mechanisms in insects. The inherent specificity and sensitivity of ORs makes them prime candidates as biological detectors of volatile ligands within biosensor devices, which have many potential applications. In the previous decade, researchers have investigated various technologies for transducing OR:ligand interactions into a readable format and thereby produce an olfactory biosensor (or bioelectronic nose) that maintains the discriminating power of the ORs in vivo. Here we review and compare the molecular mechanisms of olfaction in vertebrates and invertebrates, and also summarise the assay technologies utilising sub-tissue level sensing elements (cells and cell extracts), which have been applied to OR deorphanization and biosensor research. Although there are currently no commercial, "field-ready" olfactory biosensors of the kind discussed here, there have been several technological proof-of-concept studies suggesting that we will see their emergence within the next decade.

G-protein coupled receptor array technologies: site directed immobilisation of liposomes containing the H1-histamine or M2-muscarinic receptors

This paper describes a novel strategy to create a microarray of G-protein coupled receptors (GPCRs), an important group of membrane proteins both physiologically and pharmacologically. The H(1)-histamine receptor and the M(2)-muscarinic receptor were both used as model GPCRs in this study. The receptor proteins were embedded in liposomes created from the cellular membrane extracts of Spodoptera frugiperda (Sf9) insect cell culture line with its accompanying baculovirus protein insert used for overexpression of the receptors. Once captured onto a surface these liposomes provide a favourable lipidic environment for the integral membrane proteins. Site directed immobilisation of these liposomes was achieved by introduction of cholesterol-modified oligonucleotides (oligos). These oligo/cholesterol conjugates incorporate within the lipid bilayer and were captured by the complementary oligo strand exposed on the surface. Sequence specific immobilisation was demonstrated using a quartz crystal microbalance with dissipation (QCM-D). Confirmatory results were also obtained by monitoring fluorescent ligand binding to GPCRs captured on a spotted oligo microarray using Confocal Laser Scanning Microscopy and the Zepto-READER microarray imaging system. Sequence specific immobilisation of such biologically important membrane proteins could lead to the development of a heterogeneous self-sorting liposome array of GPCRs which would underpin a variety of future novel applications.

Gold Surface Functionalization and Patterning for Specific Immobilization of Olfactory Receptors Carried by Nanosomes

There is substantial interest in engineering solid supports to achieve functional immobilization of membrane receptors both for investigation of their biological function and for the development of novel biosensors. Three simple and practical strategies for immobilization of a human olfactory receptor carried by nanosomes are presented. The basis of the functionalization of solid gold surfaces is a self-assembled monolayer (SAM) containing biotinyl groups. Biotinyl groups are subsequently used to attach neutravidin and then biotinylated monoclonal antibody directed against the receptor to allow its specific grafting. Surface plasmon resonance technique is implemented for real-time monitoring of step-by-step surface functionalization and, in addition, for testing the functional response of immobilized olfactory receptors. We show that OR1740 is functional when immobilized via a tag attached to its C-terminus, but not via its N-terminus. Finally, we demonstrate that gold surfaces can be patterned by the SAMs tested using microcontact printing. AFM images of immobilized nanosomes onto a patterned surface suggest that small nanosomes flatten and fuse into larger vesicles but do not merge into a continuous layer. The whole study emphasizes the outstanding performances of the BAT/PEGAT SAM, which could be useful for developing on-a-chip sensor formats for membrane receptor investigations and use.