Highly biased agonism for GPCR ligands via nanobody tethering

Ligand-induced activation of G protein-coupled receptors (GPCRs) can initiate signaling through multiple distinct pathways with differing biological and physiological outcomes. There is intense interest in understanding how variation in GPCR ligand structure can be used to promote pathway selective signaling ("biased agonism") with the goal of promoting desirable responses and avoiding deleterious side effects. Here we present an approach in which a conventional peptide ligand for the type 1 parathyroid hormone receptor (PTHR1) is converted from an agonist which induces signaling through all relevant pathways to a compound that is highly selective for a single pathway. This is achieved not through variation in the core structure of the agonist, but rather by linking it to a nanobody tethering agent that binds with high affinity to a separate site on the receptor not involved in signal transduction. The resulting conjugate represents the most biased agonist of PTHR1 reported to date. This approach holds promise for facile generation of pathway selective ligands for other GPCRs.

vNARs as Neutralizing Intracellular Therapeutic Agents: Glioblastoma as a Target

Glioblastoma is the most prevalent and fatal form of primary brain tumors. New targeted therapeutic strategies for this type of tumor are imperative given the dire prognosis for glioblastoma patients and the poor results of current multimodal therapy. Previously reported drawbacks of antibody-based therapeutics include the inability to translocate across the blood-brain barrier and reach intracellular targets due to their molecular weight. These disadvantages translate into poor target neutralization and cancer maintenance. Unlike conventional antibodies, vNARs can permeate tissues and recognize conformational or cryptic epitopes due to their stability, CDR3 amino acid sequence, and smaller molecular weight. Thus, vNARs represent a potential antibody format to use as intrabodies or soluble immunocarriers. This review comprehensively summarizes key intracellular pathways in glioblastoma cells that induce proliferation, progression, and cancer survival to determine a new potential targeted glioblastoma therapy based on previously reported vNARs. The results seek to support the next application of vNARs as single-domain antibody drug-conjugated therapies, which could overcome the disadvantages of conventional monoclonal antibodies and provide an innovative approach for glioblastoma treatment.

The Neutrophil Dynamic Mass Redistribution Assay as a Medium throughput Primary Cell Screening Assay

In a typical G protein coupled receptor drug discovery campaign, an in vitro primary functional screening assay is often established in a recombinant system overexpressing the target of interest, which offers advantages with respect to overall throughput and robustness of compound testing. Subsequently, compounds are then progressed into more physiologically relevant but lower throughput ex vivo primary cell assays and finally in vivo studies. Here we describe a dynamic mass redistribution (DMR) assay that has been developed in a format suitable to support medium throughput drug screening in primary human neutrophils. Neutrophils are known to express both CXC chemokine receptor (CXCR) 1 and CXCR2 that are thought to play significant roles in various inflammatory disorders and cancer. Using multiple relevant chemokine ligands and a range of selective and nonselective small and large molecule antagonists that block CXCR1 and CXCR2 responses, we demonstrate distinct pharmacological profiles in neutrophil DMR from those observed in recombinant assays but predictive of activity in neutrophil chemotaxis and CD11b upregulation, a validated target engagement marker previously used in clinical studies of CXCR2 antagonists. The primary human neutrophil DMR cell system is highly reproducible, robust, and less prone to donor variability observed in CD11b and chemotaxis assays and thus provides a unique, more physiologically relevant, and higher throughput assay to support drug discovery and translation to early clinical trials. SIGNIFICANCE STATEMENT: Neutrophil dynamic mass redistribution assays provide a higher throughput screening assay to profile compounds in primary cells earlier in the screening cascade enabling a higher level of confidence in progressing the development of compounds toward the clinic. This is particularly important for chemokine receptors where redundancy contributes to a lack of correlation between recombinant screening assays and primary cells, with the coexpression of related receptors confounding results.

Biparatopic antibodies: therapeutic applications and prospects

Biparatopic antibodies (bpAbs) bind distinct, non-overlapping epitopes on an antigen. This unique binding mode enables new mechanisms of action beyond monospecific and bispecific antibodies (bsAbs) that can make bpAbs effective therapeutics for various indications, including oncology and infectious diseases. Biparatopic binding can lead to superior affinity and specificity, promote antagonism, lock target conformation, and result in higher-order target clustering. Such antibody-target complexes can elicit strong agonism, increase immune effector function, or result in rapid target downregulation and lysosomal trafficking. These are not only attractive properties for therapeutic antibodies but are increasingly being explored for other modalities such as antibody-drug conjugates, T-cell engagers and chimeric antigen receptors. Recent advances in bpAb engineering have enabled the construction of ever more sophisticated formats that are starting to show promise in the clinic.

Applications and challenges in designing VHH-based bispecific antibodies: leveraging machine learning solutions

The development of bispecific antibodies that bind at least two different targets relies on bringing together multiple binding domains with different binding properties and biophysical characteristics to produce a drug-like therapeutic. These building blocks play an important role in the overall quality of the molecule and can influence many important aspects from potency and specificity to stability and half-life. Single-domain antibodies, particularly camelid-derived variable heavy domain of heavy chain (VHH) antibodies, are becoming an increasingly popular choice for bispecific construction due to their single-domain modularity, favorable biophysical properties, and potential to work in multiple antibody formats. Here, we review the use of VHH domains as building blocks in the construction of multispecific antibodies and the challenges in creating optimized molecules. In addition to exploring traditional approaches to VHH development, we review the integration of machine learning techniques at various stages of the process. Specifically, the utilization of machine learning for structural prediction, lead identification, lead optimization, and humanization of VHH antibodies.

Structural basis of antibody inhibition and chemokine activation of the human CC chemokine receptor 8

The C-C motif chemokine receptor 8 (CCR8) is a class A G-protein coupled receptor that has emerged as a promising therapeutic target in cancer. Targeting CCR8 with an antibody has appeared to be an attractive therapeutic approach, but the molecular basis for chemokine-mediated activation and antibody-mediated inhibition of CCR8 are not fully elucidated. Here, we obtain an antagonist antibody against human CCR8 and determine structures of CCR8 in complex with either the antibody or the endogenous agonist ligand CCL1. Our studies reveal characteristic antibody features allowing recognition of the CCR8 extracellular loops and CCL1-CCR8 interaction modes that are distinct from other chemokine receptor - ligand pairs. Informed by these structural insights, we demonstrate that CCL1 follows a two-step, two-site binding sequence to CCR8 and that antibody-mediated inhibition of CCL1 signaling can occur by preventing the second binding event. Together, our results provide a detailed structural and mechanistic framework of CCR8 activation and inhibition that expands our molecular understanding of chemokine - receptor interactions and offers insight into the development of therapeutic antibodies targeting chemokine GPCRs.

Highly biased agonism for GPCR ligands via nanobody tethering

Ligand-induced activation of G protein-coupled receptors (GPCRs) can initiate signaling through multiple distinct pathways with differing biological and physiological outcomes. There is intense interest in understanding how variation in GPCR ligand structure can be used to promote pathway selective signaling ("biased agonism") with the goal of promoting desirable responses and avoiding deleterious side effects. Here we present a new approach in which a conventional peptide ligand for the type 1 parathyroid hormone receptor (PTHR1) is converted from an agonist which induces signaling through all relevant pathways to a compound that is highly selective for a single pathway. This is achieved not through variation in the core structure of the agonist, but rather by linking it to a nanobody tethering agent that binds with high affinity to a separate site on the receptor not involved in signal transduction. The resulting conjugate represents the most biased agonist of PTHR1 reported to date. This approach holds promise for facile generation of pathway selective ligands for other GPCRs.

Nanobody-based pannexin1 channel inhibitors reduce inflammation in acute liver injury

BACKGROUND

The opening of pannexin1 channels is considered as a key event in inflammation. Pannexin1 channel-mediated release of adenosine triphosphate triggers inflammasome signaling and activation of immune cells. By doing so, pannexin1 channels play an important role in several inflammatory diseases. Although pannexin1 channel inhibition could represent a novel clinical strategy for treatment of inflammatory disorders, therapeutic pannexin1 channel targeting is impeded by the lack of specific, potent and/or in vivo-applicable inhibitors. The goal of this study is to generate nanobody-based inhibitors of pannexin1 channels.

RESULTS

Pannexin1-targeting nanobodies were developed as potential new pannexin1 channel inhibitors. We identified 3 cross-reactive nanobodies that showed affinity for both murine and human pannexin1 proteins. Flow cytometry experiments revealed binding capacities in the nanomolar range. Moreover, the pannexin1-targeting nanobodies were found to block pannexin1 channel-mediated release of adenosine triphosphate. The pannexin1-targeting nanobodies were also demonstrated to display anti-inflammatory effects in vitro through reduction of interleukin 1 beta amounts. This anti-inflammatory outcome was reproduced in vivo using a human-relevant mouse model of acute liver disease relying on acetaminophen overdosing. More specifically, the pannexin1-targeting nanobodies lowered serum levels of inflammatory cytokines and diminished liver damage. These effects were linked with alteration of the expression of several NLRP3 inflammasome components.

CONCLUSIONS

This study introduced for the first time specific, potent and in vivo-applicable nanobody-based inhibitors of pannexin1 channels. As demonstrated for the case of liver disease, the pannexin1-targeting nanobodies hold great promise as anti-inflammatory agents, yet this should be further tested for extrahepatic inflammatory disorders. Moreover, the pannexin1-targeting nanobodies represent novel tools for fundamental research regarding the role of pannexin1 channels in pathological and physiological processes.

NanoTag Nanobody Tools for Drosophila In Vitro and In Vivo Studies

Nanobodies have emerged as powerful protein-binding tools to uncover protein functions. Using functionalized protein binders, proteins of interest can be visualized, degraded, delocalized, or post-translationally modified in vivo. We recently reported the use of two short peptide tags, 10-aa 127D01 and 14-aa VHH05, and their corresponding nanobodies, Nb127D01 and NbVHH05, for both in vitro and in vivo studies in Drosophila. Here, we provide detailed protocols for nanobody production and for visualization of proteins of interest in either fixed or live samples. In addition, we include protocols for endogenous protein tagging using CRISPR-mediated genome engineering. © 2022 Wiley Periodicals LLC. Basic Protocol 1: Nanobody production in S2 cells Basic Protocol 2: Nanobody expression and purification in bacterial cells Basic Protocol 3: Immunostaining with nanobodies Basic Protocol 4: Immunoblotting with nanobodies Basic Protocol 5: Immunoprecipitation with nanobodies prepared from S2 cells Basic Protocol 6: Immunoprecipitation with nanobodies prepared from bacteria Basic Protocol 7: NbVHH05 and Nb127D01 used as chromobodies Basic Protocol 8: NanoTag trap as a method to alter protein localization Support Protocol: CRISPR-mediated tagging of endogenous genes with NanoTags.

Characterization of a Nanobody-Epitope Tag Interaction and Its Application for Receptor Engineering

Peptide epitope tags offer a valuable means for detection and manipulation of protein targets for which high quality detection reagents are not available. Most commonly used epitope tags are bound by conventional, full-size antibodies (Abs). The complex architecture of Abs complicates their application in protein engineering and intracellular applications. To address these shortcomings, single domain antibodies (nanobodies, Nbs) that recognize short peptide epitopes have become increasingly prized. Here, we characterize the interaction between a Nb (Nb6E) and a 14-mer peptide epitope. We identify residues in the peptide epitope essential for high affinity binding. Using this information in combination with computational modeling we propose a mode of interaction between Nb6E and this epitope. We apply this nanobody-epitope pair to augment the potency of a ligand at an engineered adenosine A2A receptor. This characterization of the nanobody-epitope pair opens the door to diverse applications including mechanistic studies of the G protein-coupled receptor function.

Camelid Single-Domain Antibodies: Promises and Challenges as Lifesaving Treatments

Since the discovery of camelid heavy-chain antibodies in 1993, there has been tremendous excitement for these antibody domains (VHHs/sdAbs/nanobodies) as research tools, diagnostics, and therapeutics. Commercially, several patents were granted to pioneering research groups in Belgium and the Netherlands between 1996–2001. Ablynx was established in 2001 with the aim of exploring the therapeutic applications and development of nanobody drugs. Extensive efforts over two decades at Ablynx led to the first approved nanobody drug, caplacizumab (Cablivi) by the EMA and FDA (2018–2019) for the treatment of rare blood clotting disorders in adults with acquired thrombotic thrombocytopenic purpura (TPP). The relatively long development time between camelid sdAb discovery and their entry into the market reflects the novelty of the approach, together with intellectual property restrictions and freedom-to-operate issues. The approval of the first sdAb drug, together with the expiration of key patents, may open a new horizon for the emergence of camelid sdAbs as mainstream biotherapeutics in the years to come. It remains to be seen if nanobody-based drugs will be cheaper than traditional antibodies. In this review, I provide critical perspectives on camelid sdAbs and present the promises and challenges to their widespread adoption as diagnostic and therapeutic agents.

Protein visualization and manipulation in Drosophila through the use of epitope tags recognized by nanobodies

Expansion of the available repertoire of reagents for visualization and manipulation of proteins will help understand their function. Short epitope tags linked to proteins of interest and recognized by existing binders such as nanobodies facilitate protein studies by obviating the need to isolate new antibodies directed against them. Nanobodies have several advantages over conventional antibodies, as they can be expressed and used as tools for visualization and manipulation of proteins in vivo. Here, we characterize two short (<15aa) NanoTag epitopes, 127D01 and VHH05, and their corresponding high-affinity nanobodies. We demonstrate their use in Drosophila for in vivo protein detection and re-localization, direct and indirect immunofluorescence, immunoblotting, and immunoprecipitation. We further show that CRISPR-mediated gene targeting provides a straightforward approach to tagging endogenous proteins with the NanoTags. Single copies of the NanoTags, regardless of their location, suffice for detection. This versatile and validated toolbox of tags and nanobodies will serve as a resource for a wide array of applications, including functional studies in Drosophila and beyond.

Antibody drug conjugates in gastrointestinal cancer: From lab to clinical development

The antibody-drug conjugates (ADCs) are one the fastest growing biotherapeutics in oncology and are still in their infancy in gastrointestinal (GI) cancer for clinical applications to improve patient survival. The ADC based approach is developed with tumor specific antigen, antibody carrying cytotoxic agents to precisely target and deliver chemotherapeutics at the tumor site. To date, 11 ADCs have been approved by US-FDA, and more than 80 are in the clinical development phase for different oncological indications. However, The ADCs based therapies in GI cancers are still far from having high-efficient clinical outcomes. The limited success of these ADCs and lessons learned from the past are now being used to develop a newer generation of ADC against GI cancers. In this review, we did a comprehensive assessment of the key components of ADCs, including tumor marker, antibody, cytotoxic payload, and linkage strategy, with a focus on technical improvement and some future trends in the pipeline for clinical translation. The various preclinical and clinical ADCs used in gastrointestinal malignancies, their target, composition and bioconjugation, along with preclinical and clinical outcomes, are discussed. The emphasis is also given to new generation ADCs employing novel mAb, payload, linker, and bioconjugation methods are also included.

Nanobodies as sensors of GPCR activation and signaling

Nanobodies have emerged as useful tools to study G protein-coupled receptor (GPCR) structure, dynamic, and subcellular localization. Initially, several nanobodies have been developed as chaperones to facilitate GPCR crystallization. To explore their potential as biosensors to monitor receptor activation and dynamics, we here described protocols to characterize nanobody's interaction with GPCRs and their application as probes for protein identification and visualization on the cellular level. We also introduced a chimeric approach to enable a kappa-opioid receptor derived nanobody to bind to other GPCRs, including orphan GPCRs whose endogenous ligand or intracellular transducers are unknown. This approach provides a reporter assay to identify tool molecules to study the function of orphan GPCRs.

Targeting the latent human cytomegalovirus reservoir for T-cell-mediated killing with virus-specific nanobodies

Latent human cytomegalovirus (HCMV) infection is characterized by limited gene expression, making latent HCMV infections refractory to current treatments targeting viral replication. However, reactivation of latent HCMV in immunosuppressed solid organ and stem cell transplant patients often results in morbidity. Here, we report the killing of latently infected cells via a virus-specific nanobody (VUN100bv) that partially inhibits signaling of the viral receptor US28. VUN100bv reactivates immediate early gene expression in latently infected cells without inducing virus production. This allows recognition and killing of latently infected monocytes by autologous cytotoxic T lymphocytes from HCMV-seropositive individuals, which could serve as a therapy to reduce the HCMV latent reservoir of transplant patients.

Nanobodies as efficient drug-carriers: Progress and trends in chemotherapy

Nanobodies (Nb) have a promising future as a part of next generation chemodrug delivery systems. Nb, or VHH, are small (15 kDa) monomeric antibody fragments consisting of the antigen binding region of heavy chain antibodies. Heavy chain antibodies are naturally produced by camelids, however the structure of their VHH regions can be readily reproduced in industrial expression systems, such as bacteria or yeast. Due to their small size, high solubility, remarkable stability, manipulatable characteristics, excellent in vivo tissue penetration, conjugation advantages, and ease of production, Nb have many advantages when compared against their antibody precursors. In this review, we discuss the generation and selection of Nbs via phage display libraries for easy screening, and the conjugation techniques involved in creating target-specific nanocarriers. Furthermore, we provide a comprehensive overview of recent developments and perspectives in the field of Nb drug conjugates (NDCs) and Nb-based drug vehicles (NDv) with respect to antitumor therapeutics.

A Novel Small Molecular Antibody, HER2-Nanobody, Inhibits Tumor Proliferation in HER2-Positive Breast Cancer Cells In Vitro and In Vivo

Breast cancer is the most common malignant cancer in women worldwide, especially in developing countries. Herceptin is a monoclonal antibody with an antitumor effect in HER2-positive breast cancer. However, the large molecular weight of Herceptin limited its employment. In this study, we constructed and screened HER2-nanobody and verified its tumor-suppressive effect in HER2-positive breast cancer cells. HER2-nanobody was established, filtrated, purified, and was demonstrated to inhibit cell total number, viability, colony formation and mitosis, and promote cell apoptosis in HER2-positive breast cancer cells in vitro. Treated with HER2-nanobody, tumor growth was significantly inhibited by both intratumor injection and tail intravenous injection in vivo. The phosphorylation of ERK and AKT was restrained by HER2-nanobody in HER2-positive breast cancer cells. RAS-RAF-MAPK and PI3K-AKT-mTOR are two important pathways involved in HER2. It was credible for HER2-nanobody to play the tumor suppressive role by inhibiting the phosphorylation of ERK and AKT. Therefore, HER2-nanobody could be employed as a small molecular antibody to suppress HER2-positive breast cancer.

Therapeutic Antibodies Targeting Potassium Ion Channels

Monoclonal antibodies combine specificity and high affinity binding with excellent pharmacokinetic properties and are rapidly being developed for a wide range of drug targets including clinically important potassium ion channels. Nonetheless, while therapeutic antibodies come with great promise, K⁺ channels represent particularly difficult targets for biologics development for a variety of reasons that include their dynamic structures and relatively small extracellular loops, their high degree of sequence conservation (leading to immune tolerance), and their generally low-level expression in vivo. The process is made all the more difficult when large numbers of antibody candidates must be screened for a given target, or when lead candidates fail to cross-react with orthologous channels in animal disease models due to their highly selective binding properties. While the number of antibodies targeting potassium channels in preclinical or clinical development is still modest, significant advances in the areas of protein expression and antibody screening are converging to open the field to an avalanche of new drugs. Here, the opportunities and constraints associated with the discovery of antibodies against K⁺ channels are discussed, with an emphasis on novel technologies that are opening the field to exciting new possibilities for biologics development.

Selection of a picomolar antibody that targets CXCR2-mediated neutrophil activation and alleviates EAE symptoms

Receptors and their ligands are important therapeutic targets for about one third of marketed drugs. Here, we describe an epitope-guided approach for selection of antibodies that modulate cellular signaling of targeted receptors. We chose CXC chemokine receptor 2 (CXCR2) in the G-protein coupled receptor superfamily as receptor and a CXCR2 N-terminal peptide for antibody selection. We obtain a highly selective, tight-binding antibody from a 10¹¹-member antibody library using combinatorial enrichment. Structural and Hydrogen-Deuterium-Exchange mass spectrometry analyses demonstrate antibody interaction with an N-terminal region of CXCR2 that is part of the IL-8 epitope. The antibody strongly inhibits IL-8-induced and CXCR2-mediated neutrophil chemotaxis in vitro and alleviates hCXCR2-dependent experimental autoimmune encephalomyelitis symptoms in mice. As inappropriate neutrophil migration accompanies many diseases including inflammatory bowel disease, glomerulonephritis, allergic asthma, chronic obstructive pulmonary disease, and cancer, this antibody has potential for development as a therapeutic agent, akin to anti-TNF antibodies. However, an important difference here is that the antibody targets the chemokine receptor and competes with natural ligand, rather than targeting the ligand itself.

CXCR2 is central to neutrophil chemotaxis and hence to some inflammatory diseases. Here the authors demonstrate the value of an epitope-guided antibody panning method to develop a tight binding anti-hCXCR2 antibody, along with crystal structures of this antibody and antigen, that can block neutrophil chemotaxis and protect mice in an EAE model.

Detection of genome-edited and endogenously expressed G protein-coupled receptors

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors and major targets for FDA-approved drugs. The ability to quantify GPCR expression and ligand binding characteristics in different cell types and tissues is therefore important for drug discovery. The advent of genome editing along with developments in fluorescent ligand design offers exciting new possibilities to probe GPCRs in their native environment. This review provides an overview of the recent technical advances employed to study the localisation and ligand binding characteristics of genome-edited and endogenously expressed GPCRs.

Therapeutic depletion of CCR8+ tumor-infiltrating regulatory T cells elicits antitumor immunity and synergizes with anti-PD-1 therapy

BACKGROUND

Modulation and depletion strategies of regulatory T cells (Tregs) constitute valid approaches in antitumor immunotherapy but suffer from severe adverse effects due to their lack of selectivity for the tumor-infiltrating (ti-)Treg population, indicating the need for a ti-Treg specific biomarker.

METHODS

We employed single-cell RNA-sequencing in a mouse model of non-small cell lung carcinoma (NSCLC) to obtain a comprehensive overview of the tumor-infiltrating T-cell compartment, with a focus on ti-Treg subpopulations. These findings were validated by flow cytometric analysis of both mouse (LLC-OVA, MC38 and B16-OVA) and human (NSCLC and melanoma) tumor samples. We generated two CCR8-specific nanobodies (Nbs) that recognize distinct epitopes on the CCR8 extracellular domain. These Nbs were formulated as tetravalent Nb-Fc fusion proteins for optimal CCR8 binding and blocking, containing either an antibody-dependent cell-mediated cytotoxicity (ADCC)-deficient or an ADCC-prone Fc region. The therapeutic use of these Nb-Fc fusion proteins was evaluated, either as monotherapy or as combination therapy with anti-programmed cell death protein-1 (anti-PD-1), in both the LLC-OVA and MC38 mouse models.

RESULTS

We were able to discern two ti-Treg populations, one of which is characterized by the unique expression of Ccr8 in conjunction with Treg activation markers. Ccr8 is also expressed by dysfunctional CD4+ and CD8+ T cells, but the CCR8 protein was only prominent on the highly activated and strongly T-cell suppressive ti-Treg subpopulation of mouse and human tumors, with no major CCR8-positivity found on peripheral Tregs. CCR8 expression resulted from TCR-mediated Treg triggering in an NF-κB-dependent fashion, but was not essential for the recruitment, activation nor suppressive capacity of these cells. While treatment of tumor-bearing mice with a blocking ADCC-deficient Nb-Fc did not influence tumor growth, ADCC-prone Nb-Fc elicited antitumor immunity and reduced tumor growth in synergy with anti-PD-1 therapy. Importantly, ADCC-prone Nb-Fc specifically depleted ti-Tregs in a natural killer (NK) cell-dependent fashion without affecting peripheral Tregs.

CONCLUSIONS

Collectively, our findings highlight the efficacy and safety of targeting CCR8 for the depletion of tumor-promoting ti-Tregs in combination with anti-PD-1 therapy.

Therapeutic Nanobodies Targeting Cell Plasma Membrane Transport Proteins: A High-Risk/High-Gain Endeavor

Cell plasma membrane proteins are considered as gatekeepers of the cell and play a major role in regulating various processes. Transport proteins constitute a subclass of cell plasma membrane proteins enabling the exchange of molecules and ions between the extracellular environment and the cytosol. A plethora of human pathologies are associated with the altered expression or dysfunction of cell plasma membrane transport proteins, making them interesting therapeutic drug targets. However, the search for therapeutics is challenging, since many drug candidates targeting cell plasma membrane proteins fail in (pre)clinical testing due to inadequate selectivity, specificity, potency or stability. These latter characteristics are met by nanobodies, which potentially renders them eligible therapeutics targeting cell plasma membrane proteins. Therefore, a therapeutic nanobody-based strategy seems a valid approach to target and modulate the activity of cell plasma membrane transport proteins. This review paper focuses on methodologies to generate cell plasma membrane transport protein-targeting nanobodies, and the advantages and pitfalls while generating these small antibody-derivatives, and discusses several therapeutic nanobodies directed towards transmembrane proteins, including channels and pores, adenosine triphosphate-powered pumps and porters.

Expeditious Generation of Biparatopic Common Light Chain Antibodies via Chicken Immunization and Yeast Display Screening

Bispecific (BsAb) and biparatopic (BpAb) antibodies emerged as promising formats for therapeutic biologics exhibiting tailor-made functional properties. Over recent years, chicken-derived antibodies have gained traction for diagnostic and therapeutic applications due to their broad epitope coverage and convenience of library generation. Here we report the first generation of a biparatopic common light chain (cLC) chicken-derived antibody by an epitope binning-based screening approach using yeast surface display. The resulting monospecific antibodies target conformational epitopes on domain II or III of the epidermal growth factor receptor (EGFR) with lower double- or single-digit nanomolar affinities, respectively. Furthermore, the domain III targeting variant was shown to interfere with epidermal growth factor (EGF) binding. Utilizing the Knob-into-Hole technology (KiH), a biparatopic antibody with subnanomolar affinity was generated that facilitates clustering of soluble and cell-bound EGFR and displayed enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) compared to the parental antibodies. This strategy for generating cLC-based biparatopic antibodies from immunized chickens may pave the way for their further development in therapeutic settings.

Nanobodies as Versatile Tool for Multiscale Imaging Modalities

Molecular imaging is constantly growing in different areas of preclinical biomedical research. Several imaging methods have been developed and are continuously updated for both in vivo and in vitro applications, in order to increase the information about the structure, localization and function of molecules involved in physiology and disease. Along with these progresses, there is a continuous need for improving labeling strategies. In the last decades, the single domain antigen-binding fragments nanobodies (Nbs) emerged as important molecular imaging probes. Indeed, their small size (~15 kDa), high stability, affinity and modularity represent desirable features for imaging applications, providing higher tissue penetration, rapid targeting, increased spatial resolution and fast clearance. Accordingly, several Nb-based probes have been generated and applied to a variety of imaging modalities, ranging from in vivo and in vitro preclinical imaging to super-resolution microscopy. In this review, we will provide an overview of the state-of-the-art regarding the use of Nbs in several imaging modalities, underlining their extreme versatility and their enormous potential in targeting molecules and cells of interest in both preclinical and clinical studies.

Nanobodies in cancer

For treatment and diagnosis of cancer, antibodies have proven their value and now serve as a first line of therapy for certain cancers. A unique class of antibody fragments called nanobodies, derived from camelid heavy chain-only antibodies, are gaining increasing acceptance as diagnostic tools and are considered also as building blocks for chimeric antigen receptors as well as for targeted drug delivery. The small size of nanobodies (∼15 kDa), their stability, ease of manufacture and modification for diverse formats, short circulatory half-life, and high tissue penetration, coupled with excellent specificity and affinity, account for their attractiveness. Here we review applications of nanobodies in the sphere of tumor biology.

Monitoring Allosteric Interactions with CXCR4 Using NanoBiT Conjugated Nanobodies

Camelid single-domain antibody fragments (nanobodies) offer the specificity of an antibody in a single 15-kDa immunoglobulin domain. Their small size allows for easy genetic manipulation of the nanobody sequence to incorporate protein tags, facilitating their use as biochemical probes. The nanobody VUN400, which recognizes the second extracellular loop of the human CXCR4 chemokine receptor, was used as a probe to monitor specific CXCR4 conformations. VUN400 was fused via its C terminus to the 11-amino-acid HiBiT tag (VUN400-HiBiT) which complements LgBiT protein, forming a full-length functional NanoLuc luciferase. Here, complemented luminescence was used to detect VUN400-HiBiT binding to CXCR4 receptors expressed in living HEK293 cells. VUN400-HiBiT binding to CXCR4 could be prevented by orthosteric and allosteric ligands, allowing VUN400-HiBiT to be used as a probe to detect allosteric interactions with CXCR4. These data demonstrate that the high specificity offered by extracellular targeted nanobodies can be utilized to probe receptor pharmacology.

Targeting of G-protein coupled receptors in sepsis

The Third International Consensus Definitions (Sepsis-3) define sepsis as life-threatening multi-organ dysfunction caused by a dysregulated host response to infection. Sepsis can progress to septic shock-an even more lethal condition associated with profound circulatory, cellular and metabolic abnormalities. Septic shock remains a leading cause of death in intensive care units and carries a mortality of almost 25%. Despite significant advances in our understanding of the pathobiology of sepsis, therapeutic interventions have not translated into tangible differences in the overall outcome for patients. Clinical trials of antagonists of various pro-inflammatory mediators in sepsis have been largely unsuccessful in the past. Given the diverse physiologic roles played by G-protein coupled receptors (GPCR), modulation of GPCR signaling for the treatment of sepsis has also been explored. Traditional pharmacologic approaches have mainly focused on ligands targeting the extracellular domains of GPCR. However, novel techniques aimed at modulating GPCR intracellularly through aptamers, pepducins and intrabodies have opened a fresh avenue of therapeutic possibilities. In this review, we summarize the diverse roles played by various subfamilies of GPCR in the pathogenesis of sepsis and identify potential targets for pharmacotherapy through these novel approaches.

A review of antibody-based therapeutics targeting G protein-coupled receptors: an update

INTRODUCTION

G protein-coupled receptors (GPCRs) play key roles in many biological functions and are linked to many diseases across all therapeutic areas. As such, GPCRs represent a significant opportunity for antibody-based therapeutics.

AREAS COVERED

The structure of the major GPCR families is summarized in the context of choice of antigen source employed in the drug discovery process and receptor biology considerations which may impact on targeting strategies. An overview of the therapeutic GPCR-antibody target landscape and the diversity of current therapeutic programs is provided along with summary case studies for marketed antibody drugs or those in advanced clinical studies. Antibodies in early clinical studies and the emergence of next-generation modalities are also highlighted.

EXPERT OPINION

The GPCR-antibody pipeline has progressed significantly with a number of technical developments enabling the successful resolution of some of the challenges previously encountered and this has contributed to the growing interest in antibody-based therapeutics addressing this target class.

Generation of biparatopic antibody through two-step targeting of fragment antibodies on antigen using SpyTag and SpyCatcher

Biparatopic fragment antibodies can overcome deficiencies in avidity of conventional antibody fragments. Here, we describe a technology for generating biparatopic antibodies through two-step targeting using a pair of polypeptides, SpyTag and SpyCatcher, that spontaneously react to form a covalent bond between antibody fragments. In this method, two antibody fragments, each targeting different epitopes of the antigen, are fused to SpyTag and to SpyCatcher. When the two polypeptides are serially added to the antigen, their proximity on the antigen results in covalent bond formation and generation of a biparatopic antibody. We validated the system with purified recombinant antigen. Results in antigen-overexpressing cells were promising although further optimization will be required. Because this strategy results in high-affinity targeting with a bipartite molecule that has considerably lower molecular weight than an antibody, this technology is potentially useful for diverse applications.

Structure-guided discovery of a single-domain antibody agonist against human apelin receptor

Developing antibody agonists targeting the human apelin receptor (APJ) is a promising therapeutic approach for the treatment of chronic heart failure. Here, we report the structure-guided discovery of a single-domain antibody (sdAb) agonist JN241-9, based on the cocrystal structure of APJ with an sdAb antagonist JN241, the first cocrystal structure of a class A G protein-coupled receptor (GPCR) with a functional antibody. As revealed by the structure, JN241 binds to the extracellular side of APJ, makes critical contacts with the second extracellular loop, and inserts the CDR3 into the ligand-binding pocket. We converted JN241 into a full agonist JN241-9 by inserting a tyrosine into the CDR3. Modeling and molecular dynamics simulation shed light on JN241-9-stimulated receptor activation, providing structural insights for finding agonistic antibodies against class A GPCRs.

G Protein-Coupled Receptors in Asthma Therapy: Pharmacology and Drug Action

Asthma is a heterogeneous inflammatory disease of the airways that is associated with airway hyperresponsiveness and airflow limitation. Although asthma was once simply categorized as atopic or nonatopic, emerging analyses over the last few decades have revealed a variety of asthma endotypes that are attributed to numerous pathophysiological mechanisms. The classification of asthma by endotype is primarily routed in different profiles of airway inflammation that contribute to bronchoconstriction. Many asthma therapeutics target G protein-coupled receptors (GPCRs), which either enhance bronchodilation or prevent bronchoconstriction. Short-acting and long-acting β 2-agonists are widely used bronchodilators that signal through the activation of the β 2-adrenergic receptor. Short-acting and long-acting antagonists of muscarinic acetylcholine receptors are used to reduce bronchoconstriction by blocking the action of acetylcholine. Leukotriene antagonists that block the signaling of cysteinyl leukotriene receptor 1 are used as an add-on therapy to reduce bronchoconstriction and inflammation induced by cysteinyl leukotrienes. A number of GPCR-targeting asthma drug candidates are also in different stages of development. Among them, antagonists of prostaglandin D2 receptor 2 have advanced into phase III clinical trials. Others, including antagonists of the adenosine A2B receptor and the histamine H4 receptor, are in early stages of clinical investigation. In the past decade, significant research advancements in pharmacology, cell biology, structural biology, and molecular physiology have greatly deepened our understanding of the therapeutic roles of GPCRs in asthma and drug action on these GPCRs. This review summarizes our current understanding of GPCR signaling and pharmacology in the context of asthma treatment. SIGNIFICANCE STATEMENT: Although current treatment methods for asthma are effective for a majority of asthma patients, there are still a large number of patients with poorly controlled asthma who may experience asthma exacerbations. This review summarizes current asthma treatment methods and our understanding of signaling and pharmacology of G protein-coupled receptors (GPCRs) in asthma therapy, and discusses controversies regarding the use of GPCR drugs and new opportunities in developing GPCR-targeting therapeutics for the treatment of asthma.

Theranostics in immuno-oncology using nanobody derivatives

Targeted therapy and immunotherapy have become mainstream in cancer treatment. However, only patient subsets benefit from these expensive therapies, and often responses are short‐lived or coincide with side effects. A growing modality in precision oncology is the development of theranostics, as this enables patient selection, treatment and monitoring. In this approach, labeled compounds and an imaging technology are used to diagnose patients and select the best treatment option, whereas for therapy, related compounds are used to target cancer cells or the tumor stroma. In this context, nanobodies and nanobody-directed therapeutics have gained interest. This interest stems from their high antigen specificity, small size, ease of labeling and engineering, allowing specific imaging and design of therapies targeting antigens on tumor cells, immune cells as well as proteins in the tumor environment. This review provides a comprehensive overview on the state-of-the-art regarding the use of nanobodies as theranostics, and their importance in the emerging field of personalized medicine.

Antibodies Targeting Chemokine Receptors CXCR4 and ACKR3

Dysregulation of the chemokine system is implicated in a number of autoimmune and inflammatory diseases, as well as cancer. Modulation of chemokine receptor function is a very promising approach for therapeutic intervention. Despite interest from academic groups and pharmaceutical companies, there are currently few approved medicines targeting chemokine receptors. Monoclonal antibodies (mAbs) and antibody-based molecules have been successfully applied in the clinical therapy of cancer and represent a potential new class of therapeutics targeting chemokine receptors belonging to the class of G protein-coupled receptors (GPCRs). Besides conventional mAbs, single-domain antibodies and antibody scaffolds are also gaining attention as promising therapeutics. In this review, we provide an extensive overview of mAbs, single-domain antibodies, and other antibody fragments targeting CXCR4 and ACKR3, formerly referred to as CXCR7. We discuss their unique properties and advantages over small-molecule compounds, and also refer to the molecules in preclinical and clinical development. We focus on single-domain antibodies and scaffolds and their utilization in GPCR research. Additionally, structural analysis of antibody binding to CXCR4 is discussed. SIGNIFICANCE STATEMENT: Modulating the function of GPCRs, and particularly chemokine receptors, draws high interest. A comprehensive review is provided for monoclonal antibodies, antibody fragments, and variants directed at CXCR4 and ACKR3. Their advantageous functional properties, versatile applications as research tools, and use in the clinic are discussed.

Nanobodies: New avenues for imaging, stabilizing and modulating GPCRs

The family of G protein-coupled receptors (GPCRs) is the largest class of membrane proteins and an important drug target due to their role in many (patho)physiological processes. Besides small molecules, GPCRs can be targeted by biologicals including antibodies and antibody fragments. This review describes the use of antibodies and in particular antibody fragments from camelid-derived heavy chain-only antibodies (nanobodies/VHHs/sdAbs) for detecting, stabilizing, modulating and therapeutically targeting GPCRs. Altogether, it becomes increasingly clear that the small size, structure and protruding antigen-binding loops of nanobodies are favorable features for the development of selective and potent GPCRs-binding molecules. This makes them attractive tools to modulate GPCR activity but also as targeting modalities for GPCR-directed therapeutics. In addition, these antibody-fragments are important tools in the stabilization of particular conformations of these receptors. Lastly, nanobodies, in contrast to conventional antibodies, can also easily be expressed intracellularly which render nanobodies important tools for studying GPCR function. Hence, GPCR-targeting nanobodies are ideal modalities to image, stabilize and modulate GPCR function.

Bioluminescence Resonance Energy Transfer as a Method to Study Protein-Protein Interactions: Application to G Protein Coupled Receptor Biology

The bioluminescence resonance energy transfer (BRET) approach involves resonance energy transfer between a light-emitting enzyme and fluorescent acceptors. The major advantage of this technique over biochemical methods is that protein-protein interactions (PPI) can be monitored without disrupting the natural environment, frequently altered by detergents and membrane preparations. Thus, it is considered as one of the most versatile technique for studying molecular interactions in living cells at "physiological" expression levels. BRET analysis has been applied to study many transmembrane receptor classes including G-protein coupled receptors (GPCR). It is well established that these receptors may function as dimeric/oligomeric forms and interact with multiple effectors to transduce the signal. Therefore, they are considered as attractive targets to identify PPI modulators. In this review, we present an overview of the different BRET systems developed up to now and their relevance to identify inhibitors/modulators of protein⁻protein interaction. Then, we introduce the different classes of agents that have been recently developed to target PPI, and provide some examples illustrating the use of BRET-based assays to identify and characterize innovative PPI modulators in the field of GPCRs biology. Finally, we discuss the main advantages and the limits of BRET approach to characterize PPI modulators.

Current Approaches and Future Perspectives for Nanobodies in Stroke Diagnostic and Therapy

Antibody-based biologics are the corner stone of modern immunomodulatory therapy. Though highly effective in dampening systemic inflammatory processes, their large size and Fc-fragment mediated effects hamper crossing of the blood brain barrier (BBB). Nanobodies (Nbs) are single domain antibodies derived from llama or shark heavy-chain antibodies and represent a new generation of biologics. Due to their small size, they display excellent tissue penetration capacities and can be easily modified to adjust their vivo half-life for short-term diagnostic or long-term therapeutic purposes or to facilitate crossing of the BBB. Furthermore, owing to their characteristic binding mode, they are capable of antagonizing receptors involved in immune signaling and of neutralizing proinflammatory mediators, such as cytokines. These qualities combined make Nbs well-suited for down-modulating neuroinflammatory processes that occur in the context of brain ischemia. In this review, we summarize recent findings on Nbs in preclinical stroke models and how they can be used as diagnostic and therapeutic reagents. We further provide a perspective on the design of innovative Nb-based treatment protocols to complement and improve stroke therapy.

Targeting G protein-coupled receptor signaling at the G protein level with a selective nanobody inhibitor

G protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by mediating a GDP to GTP exchange in the Gα subunit. This leads to dissociation of the heterotrimer into Gα-GTP and Gβγ dimer. The Gα-GTP and Gβγ dimer each regulate a variety of downstream pathways to control various aspects of human physiology. Dysregulated Gβγ-signaling is a central element of various neurological and cancer-related anomalies. However, Gβγ also serves as a negative regulator of Gα that is essential for G protein inactivation, and thus has the potential for numerous side effects when targeted therapeutically. Here we report a llama-derived nanobody (Nb5) that binds tightly to the Gβγ dimer. Nb5 responds to all combinations of β-subtypes and γ-subtypes and competes with other Gβγ-regulatory proteins for a common binding site on the Gβγ dimer. Despite its inhibitory effect on Gβγ-mediated signaling, Nb5 has no effect on Gα_q-mediated and Gα_s-mediated signaling events in living cells.

The constitutive activity of the virally encoded chemokine receptor US28 accelerates glioblastoma growth

Glioblastoma (GBM) is the most aggressive and an incurable type of brain cancer. Human cytomegalovirus (HCMV) DNA and encoded proteins, including the chemokine receptor US28, have been detected in GBM tumors. US28 displays constitutive activity and is able to bind several human chemokines, leading to the activation of various proliferative and inflammatory signaling pathways. Here we show that HCMV, through the expression of US28, significantly enhanced the growth of 3D spheroids of U251− and neurospheres of primary glioblastoma cells. Moreover, US28 expression accelerated the growth of glioblastoma cells in an orthotopic intracranial GBM-model in mice. We developed highly potent and selective US28-targeting nanobodies, which bind to the extracellular domain of US28 and detect US28 in GBM tissue. The nanobodies inhibited chemokine binding and reduced the constitutive US28-mediated signaling with nanomolar potencies and significantly impaired HCMV/US28-mediated tumor growth in vitro and in vivo. This study emphasizes the oncomodulatory role of HCMV-encoded US28 and provides a potential therapeutic approach for HCMV-positive tumors using the nanobody technology.

Non-invasive assessment of murine PD-L1 levels in syngeneic tumor models by nuclear imaging with nanobody tracers

Blockade of the inhibitory PD-1/PD-L1 immune checkpoint axis is a promising cancer treatment. Nonetheless, a significant number of patients and malignancies do not respond to this therapy. To develop a screen for response to PD-1/PD-L1 inhibition, it is critical to develop a non-invasive tool to accurately assess dynamic immune checkpoint expression. Here we evaluated non-invasive SPECT/CT imaging of PD-L1 expression, in murine tumor models with varying PD-L1 expression, using high affinity PD-L1-specific nanobodies (Nbs). We generated and characterized 37 Nbs recognizing mouse PD-L1. Among those, four Nbs C3, C7, E2 and E4 were selected and evaluated for preclinical imaging of PD-L1 in syngeneic mice. We performed SPECT/CT imaging in wild type versus PD-L1 knock-out mice, using Technetium-99m (^99mTc) labeled Nbs. Nb C3 and E2 showed specific antigen binding and beneficial biodistribution. Through the use of CRISPR/Cas9 PD-L1 knock-out TC-1 lung epithelial cell lines, we demonstrate that SPECT/CT imaging using Nb C3 and E2 identifies PD-L1 expressing tumors, but not PD-L1 non-expressing tumors, thereby confirming the diagnostic potential of the selected Nbs. In conclusion, these data show that Nbs C3 and E2 can be used to non-invasively image PD-L1 levels in the tumor, with the strength of the signal correlating with PD-L1 levels. These findings warrant further research into the use of Nbs as a tool to image inhibitory signals in the tumor environment.

A cDNA Immunization Strategy to Generate Nanobodies against Membrane Proteins in Native Conformation

Nanobodies (Nbs) are soluble, versatile, single-domain binding modules derived from the VHH variable domain of heavy-chain antibodies naturally occurring in camelids. Nbs hold huge promise as novel therapeutic biologics. Membrane proteins are among the most interesting targets for therapeutic Nbs because they are accessible to systemically injected biologics. In order to be effective, therapeutic Nbs must recognize their target membrane protein in native conformation. However, raising Nbs against membrane proteins in native conformation can pose a formidable challenge since membrane proteins typically contain one or more hydrophobic transmembrane regions and, therefore, are difficult to purify in native conformation. Here, we describe a highly efficient genetic immunization strategy that circumvents these difficulties by driving expression of the target membrane protein in native conformation by cells of the immunized camelid. The strategy encompasses ballistic transfection of skin cells with cDNA expression plasmids encoding one or more orthologs of the membrane protein of interest and, optionally, other costimulatory proteins. The plasmid is coated onto 1 µm gold particles that are then injected into the shaved and depilated skin of the camelid. A gene gun delivers a helium pulse that accelerates the DNA-coated particles to a velocity sufficient to penetrate through multiple layers of cells in the skin. This results in the exposure of the extracellular domains of the membrane protein on the cell surface of transfected cells. Repeated immunization drives somatic hypermutation and affinity maturation of target-specific heavy-chain antibodies. The VHH/Nb coding region is PCR-amplified from B cells obtained from peripheral blood or a lymph node biopsy. Specific Nbs are selected by phage display or by screening of Nb-based heavy-chain antibodies expressed as secretory proteins in transfected HEK cells. Using this strategy, we have successfully generated agonistic and antagonistic Nbs against several cell surface ecto-enzymes and ligand-gated ion channels.

Camelid Single-Domain Antibodies: Historical Perspective and Future Outlook

Tremendous effort has been expended over the past two and a half decades to understand many aspects of camelid heavy chain antibodies, from their biology, evolution, and immunogenetics to their potential applications in various fields of research and medicine. In this article, I present a historical perspective on the development of camelid single-domain antibodies (sdAbs or V_HHs, also widely known as nanobodies) since their discovery and discuss the advantages and disadvantages of these unique molecules in various areas of research, industry, and medicine. Commercialization of camelid sdAbs exploded in 2001 with a flurry of patents issued to the Vrije Universiteit Brussel (VUB) and later taken on by the Vlaams Interuniversitair Instituut voor Biotechnologie (VIB) and, after 2002, the VIB-founded spin-off company, Ablynx. While entrepreneurial spirit has certainly catalyzed the exploration of nanobodies as marketable products, IP restrictions may be partially responsible for the relatively long time span between the discovery of these biomolecules and their entry into the pharmaceutical market. It is now anticipated that the first V_HH-based antibody drug, Caplacizumab, a bivalent anti-vWF antibody for treating rare blood clotting disorders, may be approved and commercialized in 2018 or shortly thereafter. This elusive first approval, along with the expiry of key patents, may substantially alter the scientific and biomedical landscape surrounding camelid sdAbs and pave the way for their emergence as mainstream biotherapeutics.

Opportunities for therapeutic antibodies directed at G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) are activated by a diverse range of ligands, from large proteins and proteases to small peptides, metabolites, neurotransmitters and ions. They are expressed on all cells in the body and have key roles in physiology and homeostasis. As such, GPCRs are one of the most important target classes for therapeutic drug discovery. The development of drugs targeting GPCRs has therapeutic value across a wide range of diseases, including cancer, immune and inflammatory disorders as well as neurological and metabolic diseases. The progress made by targeting GPCRs with antibody-based therapeutics, as well as technical hurdles to overcome, are presented and discussed in this Review. Antibody therapeutics targeting C-C chemokine receptor type 4 (CCR4), CCR5 and calcitonin gene-related peptide (CGRP) are used as illustrative clinical case studies.

Antibodies targeting G protein-coupled receptors: Recent advances and therapeutic challenges

Le STUDIUM conference was held November 24-25, 2016 in Tours, France as a satellite workshop of the 5^th meeting of the French GDR 3545 on "G Protein-Coupled Receptors (GPCRs) -From Physiology to Drugs," which was held in Tours during November 22-24, 2016. The conference gathered speakers from academia and industry considered to be world leaders in the molecular pharmacology and signaling of GPCRs, with a particular interest in the development of therapeutic GPCR antibodies (Abs). The main topics were new advances and challenges in the development of antibodies targeting GPCRs and their potential applications to the study of the structure and function of GPCRs, as well as their implication in physiology and pathophysiology. The conference included 2 sessions, with the first dedicated to the recent advances in methodological strategies used for GPCR immunization using thermo-stabilized and purified GPCRs, and the development of various formats of Abs such as monoclonal IgG, single-chain variable fragments and nanobodies (Nbs) by in vitro and in silico approaches. The second session focused on GPCR Nbs as a "hot" field of research on GPCRs. This session started with discussion of the pioneering Nbs developed against GPCRs and their application to structural studies, then transitioned to talks on original ex vivo and in vivo studies on GPCR-selective Nbs showing promising therapeutic applications of Nbs in important physiologic systems, such as the central nervous and the immune systems, as well as in cancer. The conference ended with the consensus that Abs and especially Nbs are opening a new era of research on GPCR structure, pharmacology and pathophysiology.

Nanobodies to Study G Protein-Coupled Receptor Structure and Function

Ligand-induced activation of G protein-coupled receptors (GPCRs) is a key mechanism permitting communication between cells and organs. Enormous progress has recently elucidated the structural and dynamic features of GPCR transmembrane signaling. Nanobodies, the recombinant antigen-binding fragments of camelid heavy-chain-only antibodies, have emerged as important research tools to lock GPCRs in particular conformational states. Active-state stabilizing nanobodies have elucidated several agonist-bound structures of hormone-activated GPCRs and have provided insight into the dynamic character of receptors. Nanobodies have also been used to stabilize transient GPCR transmembrane signaling complexes, yielding the first structural insights into GPCR signal transduction across the cellular membrane. Beyond their in vitro uses, nanobodies have served as conformational biosensors in living systems and have provided novel ways to modulate GPCR function. Here, we highlight several examples of how nanobodies have enabled the study of GPCR function and give insights into potential future uses of these important tools.