Improving the odorant sensitivity of olfactory receptor-expressing yeast with accessory proteins

Design of an effective small expression tag to enhance GPCR production in E. coli-based cell-free and whole cell expression systems

G protein-coupled receptors (GPCRs) play crucial roles in sensory, immune, and tumor metastasis processes, making them valuable targets for pharmacological and sensing applications in various industries. However, most GPCRs have low production yields in Escherichia coli (E. coli) expression systems. To overcome this limitation, we introduced AT10 tag, an effective fusion tag that could significantly enhance expression levels of various GPCRs in E. coli and its derived cell-free protein synthesis (CFPS) system. This AT10 tag consisted of an A/T-rich gene sequence designed via optimization of translation initiation rate. It is translated into a short peptide sequence of 10 amino acids at the N-terminus of GPCRs. Additionally, effector proteins could be utilized to suppress cytotoxicity caused by membrane protein expression, further boosting GPCR production in E. coli. Enhanced expression of various GPCRs using this AT10 tag is a promising approach for large-scale production of functional GPCRs in E. coli-based CFPS and whole cell systems, enabling their potential utilization across a wide range of industrial applications.

Identification and Characterization of Proteins That Are Involved in RTP1S-Dependent Transport of Olfactory Receptors

Olfaction is mediated via olfactory receptors (ORs) that are expressed on the cilia membrane of olfactory sensory neurons in the olfactory epithelium. The functional expression of most ORs requires the assistance of receptor-transporting proteins (RTPs). We examined the interactome of RTP1S and OR via proximity biotinylation. Deubiquitinating protein VCIP135, the F-actin-capping protein sub-unit alpha-2, and insulin-like growth factor 2 mRNA-binding protein 2 were biotinylated via AirID fused with OR, RTP1S-AirID biotinylated heat shock protein A6 (HSPA6), and double-stranded RNA-binding protein Staufen homolog 2 (STAU2). Co-expression of HSPA6 partially enhanced the surface expression of Olfr544. The surface expression of Olfr544 increased by 50-80%. This effect was also observed when RTP1S was co-expressed. Almost identical results were obtained from the co-expression of STAU2. The interactions of HSPA6 and STAU2 with RTP1S were examined using a NanoBit assay. The results show that the RTP1S N-terminus interacted with the C-terminal domain of HSP6A and the N-terminal domain of STAU2. In contrast, OR did not significantly interact with STAU2 and HSPA6. Thus, HSP6A and STAU2 appear to be involved in the process of OR traffic through interaction with RTP1S.

Advances in the Production of Olfactory Receptors for Industrial Use

In biological olfactory systems, olfactory receptors (ORs) can recognize and discriminate between thousands of volatile organic compounds with very high sensitivity and specificity. The superior properties of ORs have led to the development of OR-based biosensors that have shown promising potential in many applications over the past two decades. In particular, newly designed technologies in gene synthesis, protein expression, solubilization, purification, and membrane mimetics for membrane proteins have greatly opened up the previously inaccessible industrial potential of ORs. In this review, gene design, expression and solubilization strategies, and purification and reconstitution methods available for modern industrial applications are examined, with a focus on ORs. The limitations of current OR production technology are also estimated, and future directions for further progress are suggested.

Olfactory Receptors as an Emerging Chemical Sensing Scaffold

Chemical biosensors are an increasingly ubiquitous part of our lives. Beyond enzyme-coupled assays, recent synthetic biology advances now allow us to hijack more complex biosensing systems to respond to difficult to detect analytes, such as chemical small molecules. Here, we briefly overview recent advances in the biosensing of small molecules, including nucleic acid aptamers, allosteric transcription factors, and two-component systems. We then look more closely at a recently developed chemical sensing system, G protein-coupled receptor (GPCR)-based sensors. Finally, we consider the chemical sensing capabilities of the largest GPCR subfamily, olfactory receptors (ORs). We examine ORs' role in nature, their potential as a biomedical target, and their ability to detect compounds not amenable for detection using other biological scaffolds. We conclude by evaluating the current challenges, opportunities, and future applications of GPCR- and OR-based sensors.

Key Amino Acids Alter Activity and Trafficking of a Well-conserved Olfactory Receptor

In this study, we elucidate factors that regulate the trafficking and activity of a well-conserved olfactory receptor (OR), Olfr558, and its human ortholog OR51E1. Results indicate that butyrate activates Olfr558/OR51E1 leading to the production of cAMP, and evokes Ca²⁺ influx. We also find Golf increases cAMP production induced by Olfr558/OR51E1 activation but does not affect trafficking. Given the 93% sequence identity between OR51E1 and Olfr558, it is surprising to note that OR51E1 has significantly more surface expression yet similar total protein expression. We find that replacing the Olfr558 N-terminus with that of OR51E1 significantly increases trafficking; in contrast, there is no change in surface expression conferred by the Olfr558 TM2, TM3, or TM4 domains. A previous analysis of human OR51E1 single nucleotide polymorphisms (SNPs) identified an A156T mutant primarily found in South Asia as the most abundant (albeit still rare). We find that the OR51E1 A156T mutant has reduced surface expression and cAMP production without a change in total protein expression. In sum, this study of a well-conserved olfactory receptor identifies both protein regions and specific amino acid residues that play key roles in protein trafficking, and also elucidates common effects of Golf on the regulation of both the human and murine OR.

Biohybrid sensor for odor detection

Biohybrid odorant sensors that directly integrate a biological olfactory system have been increasingly studied and are suggested to be the next generation of ultrasensitive sensors by taking advantage of the sensitivity and selectivity of living organisms. In this review, we provide a detailed description of the recent developments of biohybrid odorant sensors, especially considering the requisites for their perspective of on-site applications. We introduce the methodologies to effectively capture the biological signals from olfactory systems by readout devices, and describe the essential properties regarding the gaseous detection, stability, quality control, and portability. Moreover, we address the recent progress on multiple odorant recognition using multiple sensors as well as the current screening approaches for pairs of orphan receptors and ligands necessary for the extension of the currently available range of biohybrid sensors. Finally, we discuss our perspectives for the future for the development of practical odorant sensors.

Engineering G protein-coupled receptor signalling in yeast for biotechnological and medical purposes

G protein-coupled receptors (GPCRs) comprise the largest class of membrane proteins in the human genome, with a common denominator of seven-transmembrane domains largely conserved among eukaryotes. Yeast is naturally armoured with three different GPCRs for pheromone and sugar sensing, with the pheromone pathway being extensively hijacked for characterising heterologous GPCR signalling in a model eukaryote. This review focusses on functional GPCR studies performed in yeast and on the elucidated hotspots for engineering, and discusses both endogenous and heterologous GPCR signalling. Key emphasis will be devoted to studies describing important engineering parameters to consider for successful coupling of GPCRs to the yeast mating pathway. We also review the various means of applying yeast for studying GPCRs, including the use of yeast armed with heterologous GPCRs as a platform for (i) deorphanisation of orphan receptors, (ii) metabolic engineering of yeast for production of bioactive products and (iii) medical applications related to pathogen detection and drug discovery. Finally, this review summarises the current challenges related to expression of functional membrane-bound GPCRs in yeast and discusses the opportunities to continue capitalising on yeast as a model chassis for functional GPCR signalling studies.

A Pichia biosensor for high-throughput analyses of compounds that can influence mosquito behavior

Mosquitoes utilize their sense of smell to locate prey and feed on their blood. Repellents interfere with the biochemical cascades that detect odors. Consequently, repellants are highly effective and resource-efficient alternatives for controlling the spread of mosquito-borne illnesses. Unfortunately, the discovery of repellents is slow, laborious, and error-prone. To this end, we have taken a giant stride toward improving the speed and accuracy of repellant discovery by constructing a prototypical whole-cell biosensor for accurate detection of mosquito behavior-modifying compounds such as repellants. As a proof-of-concept, we genetically engineered Pichia pastoris to express the olfactory receptor co-receptor (Orco) of Anopheles gambiae mosquitoes. This transmembrane protein behaves like a cationic channel upon activation by stimulatory odorants. When the engineered Pichia cells are cultured in calcium-containing Hank's buffer, induction of the medium with a stimulatory odorant results in an influx of calcium ions into the cells, and the stimulatory effect is quantifiable using the calcium-sequestering fluorescent dye, fluo-4-acetoxymethyl ester. Moreover, the stimulatory effect can be titrated by adjusting either the concentration of calcium ions in the medium or the level of induction of the stimulatory odorant. Subsequent exposure of the activated Pichia cells to a repellant molecule inhibits the stimulatory effect and quenches the fluorescent signal, also in a titratable manner. Significantly, the modular architecture of the biosensor allows easy and efficient expansion of its detection range by co-expressing Orco with other olfactory receptors. The high-throughput assay is also compatible with robotic screening infrastructure, and our development represents a paradigm change for the discovery of mosquito repellants.

G protein-coupled receptors expressed and studied in yeast. The adenosine receptor as a prime example

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins with around 800 members in the human genome/proteome. Extracellular signals such as hormones and neurotransmitters regulate various biological processes via GPCRs, with GPCRs being the bodily target of 30-40% of current drugs on the market. Complete identification and understanding of GPCR functionality will provide opportunities for novel drug discovery. Yeast expresses three different endogenous GPCRs regulating pheromone and sugar sensing, with the pheromone pathway offering perspectives for the characterization of heterologous GPCR signaling. Moreover, yeast offers a ''null" background for studies on mammalian GPCRs, including GPCR activation and signaling, ligand identification, and characterization of disease-related mutations. This review focuses on modifications of the yeast pheromone signaling pathway for functional GPCR studies, and on opportunities and usage of the yeast system as a platform for human GPCR studies. Finally, this review discusses in some further detail studies of adenosine receptors heterologously expressed in yeast, and what Geoff Burnstock thought of this approach.

Membrane protein-based biosensors

This review highlights recent development of biosensors that use the functions of membrane proteins. Membrane proteins are essential components of biological membranes and have a central role in detection of various environmental stimuli such as olfaction and gustation. A number of studies have attempted for development of biosensors using the sensing property of these membrane proteins. Their specificity to target molecules is particularly attractive as it is significantly superior to that of traditional human-made sensors. In this review, we classified the membrane protein-based biosensors into two platforms: the lipid bilayer-based platform and the cell-based platform. On lipid bilayer platforms, the membrane proteins are embedded in a lipid bilayer that bridges between the protein and a sensor device. On cell-based platforms, the membrane proteins are expressed in a cultured cell, which is then integrated in a sensor device. For both platforms we introduce the fundamental information and the recent progress in the development of the biosensors, and remark on the outlook for practical biosensing applications.

The N-terminal region of RTP1S plays important roles in dimer formation and odorant receptor-trafficking

Receptor-transporting protein 1S (RTP1S) is an accessory protein that mediates the transport of mammalian odorant receptors (ORs) into the plasma membrane. Although most ORs fail to localize to the cell surface when expressed alone in nonolfactory cells, functional expression of ORs is achieved with the coexpression of RTP1S. However, the mechanism for RTP1S-mediated OR trafficking remains unclear. In this study, we attempted to reveal the mode of action and critical residues of RTP1S in OR trafficking. Experiments using N-terminal truncation and Ala substitution mutants of RTP1S demonstrated that four N-terminal amino acids have essential roles in OR trafficking. Additionally, using recombinant proteins and split luciferase assays in mammalian cells, we provided evidence for the dimer formation of RTP1S. Furthermore, we determined that the 2nd Cys residue is required for the efficient dimerization of RTP1S. Altogether, these findings provide insights into the mechanism for plasma membrane transport of ORs by RTP1S.

Engineering a Model Cell for Rational Tuning of GPCR Signaling

G protein-coupled receptor (GPCR) signaling is the primary method eukaryotes use to respond to specific cues in their environment. However, the relationship between stimulus and response for each GPCR is difficult to predict due to diversity in natural signal transduction architecture and expression. Using genome engineering in yeast, we constructed an insulated, modular GPCR signal transduction system to study how the response to stimuli can be predictably tuned using synthetic tools. We delineated the contributions of a minimal set of key components via computational and experimental refactoring, identifying simple design principles for rationally tuning the dose response. Using five different GPCRs, we demonstrate how this enables cells and consortia to be engineered to respond to desired concentrations of peptides, metabolites, and hormones relevant to human health. This work enables rational tuning of cell sensing while providing a framework to guide reprogramming of GPCR-based signaling in other systems.

Vapor detection and discrimination with a panel of odorant receptors

Olfactory systems have evolved the extraordinary capability to detect and discriminate volatile odorous molecules (odorants) in the environment. Fundamentally, this process relies on the interaction of odorants and their cognate olfactory receptors (ORs) encoded in the genome. Here, we conducted a cell-based screen using over 800 mouse ORs against seven odorants, resulting in the identification of a set of high-affinity and/or broadly-tuned ORs. We then test whether heterologously expressed ORs respond to odors presented in vapor phase by individually expressing 31 ORs to measure cAMP responses against vapor phase odor stimulation. Comparison of response profiles demonstrates this platform is capable of discriminating between structural analogs. Lastly, co-expression of carboxyl esterase Ces1d expressed in olfactory mucosa resulted in marked changes in activation of specific odorant-OR combinations. Altogether, these results establish a cell-based volatile odor detection and discrimination platform and form the basis for an OR-based volatile odor sensor.

Biomimetic “noses” have been proposed to replace trained animals for chemical detection. Here the authors select 31 mouse olfactory receptors (ORs), based on a large cell-based screen of >800 ORs against seven chemicals, to build an OR-based sensor able to discriminate structurally similar compounds.

Progress in the development of olfactory-based bioelectronic chemosensors

Artificial chemosensory devices have a wide range of applications in industry, security, and medicine. The development of these devices has been inspired by the speed, sensitivity, and selectivity by which the olfactory system in animals can probe the chemical nature of the environment. In this review, we examine how molecular and cellular components of natural olfactory systems have been incorporated into artificial chemosensors, or bioelectronic sensors. We focus on the biological material that has been combined with signal transduction systems to develop artificial chemosensory devices. The strengths and limitations of different biological chemosensory material at the heart of these devices, as well as the reported overall effectiveness of the different bioelectronic sensor designs, is examined. This review also discusses future directions and challenges for continuing to advance development of bioelectronic sensors.

New approaches in bioprocess-control: Consortium guidance by synthetic cell-cell communication based on fungal pheromones

Bioconversions in industrial processes are currently dominated by single-strain approaches. With the growing complexity of tasks to be carried out, microbial consortia become increasingly advantageous and eventually may outperform single-strain fermentations. Consortium approaches benefit from the combined metabolic capabilities of highly specialized strains and species, and the inherent division of labor reduces the metabolic burden for each strain while increasing product yields and reaction specificities. However, consortium-based designs still suffer from a lack of available tools to control the behavior and performance of the individual subpopulations and of the entire consortium. Here, we propose to implement novel control elements for microbial consortia based on artificial cell-cell communication via fungal mating pheromones. Coupling to the desired output is mediated by pheromone-responsive gene expression, thereby creating pheromone-dependent communication channels between different subpopulations of the consortia. We highlight the benefits of artificial communication to specifically target individual subpopulations of microbial consortia and to control e.g. their metabolic profile or proliferation rate in a predefined and customized manner. Due to the steadily increasing knowledge of sexual cycles of industrially relevant fungi, a growing number of strains and species can be integrated into pheromone-controlled sensor-actor systems, exploiting their unique metabolic properties for microbial consortia approaches.

Biomimetic Chemical Sensing by Fluorescence Signals Using a Virus-like Particle-Based Platform

The chemical receptors present in living organisms are promising tools for developing biomimetic chemical sensors. However, these receptors require lipid membranes for functioning under physiological conditions, which prevents their utilization in the production of cell-free in vitro chemical sensing systems. Here, we report the development of a cell-free biomimetic sensing platform using virus-like particles (VLPs) with intact ligand-gated Ca²⁺ channels and genetically encoded Ca²⁺ indicator (GECI). We observed that targeting GECI to the plasma membrane was essential for efficient loading GECI in the VLPs. Although the physiological Ca²⁺ concentration [Ca²⁺] maintained in the cells was low (∼10 nM), the concentration in the VLPs was high. This prevented the detection of the increase in [Ca²⁺] caused by binding of the ligand to the receptor. To address this problem, we employed Lyn-R-CEPIA1, which had low affinity for Ca²⁺, and a membrane targeting sequence. Thus, we succeeded in monitoring the activation of cyclic nucleotide-gated channels (CNG) on the VLPs by measuring the increase in fluorescence of Lyn-R-CEPIA1. Our VLP-based sensing system can act as a fundamental platform for all kinds of ligand-gated channels.

The mouse receptor transporting protein RTP1S and the fly SNMP1 support the functional expression of the Drosophila odorant coreceptor Orco in mammalian culture cells

BACKGROUND

Functional expression of vertebrate and insect odorant receptors (ORs) in mammalian culture cells is hampered by an incorrect trafficking of these proteins to the plasma membrane. Receptor transporting proteins (RTPs) have been found to enhance the activity of transfected mammalian ORs in several heterologous systems.

NEW METHODS

We co-transfected the Drosophila olfactory coreceptor (Orco) in HEK293 cells with a truncated form of the mouse RTP1 (RTP1S) or with the Drosophila sensory neuron membrane protein 1 (SNMP1), which is required for the detection of the pheromone cis-vaccenyl acetate and was shown to be apposed to Orco within the functional receptor unit.

RESULTS

Co-transfection of Orco with either of the two constructs led to an enhanced response to stimulations with the synthetic Orco agonist VUAA1, as compared to transfection with Orco alone.

COMPARISON WITH EXISTING METHODS

This method enhances the functional expression of Orco in HEK293 cells in comparison to conventional transfection with Orco alone and enables the use of a lower amount of Orco DNA for transfection.

CONCLUSION

Mammalian RTPs can enhance the expression of insect ORs. Moreover, the ability of SNMP1 to mimic the RTP1S effect may indicate possible new roles of this protein apart from being involved in pheromone detection. These results provide researchers with a fast and inexpensive way to optimize the functional expression of insect ORs in heterologous systems and open the search for insect proteins analogous to mammalian RTPs.

GPCR-Based Chemical Biosensors for Medium-Chain Fatty Acids

A key limitation to engineering microbes for chemical production is a reliance on low-throughput chromatography-based screens for chemical detection. While colorimetric chemicals are amenable to high-throughput screens, many value-added chemicals are not colorimetric and require sensors for high-throughput screening. Here, we use G-protein coupled receptors (GPCRs) known to bind medium-chain fatty acids in mammalian cells to rapidly construct chemical sensors in yeast. Medium-chain fatty acids are immediate precursors to the advanced biofuel fatty acid methyl esters, which can serve as a "drop-in" replacement for D2 diesel. One of the sensors detects even-chain C8-C12 fatty acids with a 13- to 17-fold increase in signal after activation, with linear ranges up to 250 μM. Introduction of a synthetic response unit alters both dynamic and linear range, improving the sensor response to decanoic acid to a 30-fold increase in signal after activation, with a linear range up to 500 μM. To our knowledge, this is the first report of a whole-cell medium-chain fatty acid biosensor, which we envision could be applied to the evolutionary engineering of fatty acid-producing microbes. Given the affinity of GPCRs for a wide range of chemicals, it should be possible to rapidly assemble new biosensors by simply swapping the GPCR sensing unit. These sensors should be amenable to a variety of applications that require different dynamic and linear ranges, by introducing different response units.