GPCR-targeting nanobodies: attractive research tools, diagnostics, and therapeutics

Antibody fragments targeting the extracellular domain of follicular stimulating hormone receptor for contraception in male dogs and cats

The increased LH levels resulting from the absence of negative feedback after castration has been linked to long-term health issues. A need exists for an alternative contraceptive agent that functions without interfering the LH pathways. This study aimed to develop antibody fragments against the follicular-stimulating hormone receptor (anti-FSHr) using phage-display technology and evaluate its effects on Sertoli cell functions. Phage clones against the extracellular domain of dog and cat FSHr selected from an antibody fragment phagemid library were analyzed for binding kinetics by surface plasmon resonance. Sertoli cells were isolated from testes of adult animals (five dogs and five cats). Efficacy test was performed by treating Sertoli cell cultures (SCCs) with anti-FSHr antibody fragments compared with untreated in triplicates. Expressions of androgen binding protein (ABP), inhibin subunit beta B (IHBB) and vascular endothelial growth factor A (VEGFA) mRNA in SCCs were quantified by RT-qPCR. The results demonstrated that the molecular weight of the purified dog and cat anti-FSHr antibody fragment was 25 kDa and 15 kDa, respectively. Based on protein molecular weight, the antibody fragment of dogs and cats was therefore, so-called single-chain variable fragments (scFv) and nanobody (nb), respectively. The binding affinity with dissociation constant (KD) was 2.32 × 10-7 M and 2.83 × 10-9 M for dog and cat anti-FSHr antibody fragments, respectively. The cross-binding kinetic interactions between the dog anti-FSHr scFv and the cat ECD of FSHr could not be fitted to the curves to determine the binding kinetics. However, the cross-binding affinity KD between the cat anti-FSHr nb and the dog ECD FSHr was 1.75 × 10-4 M. The mRNA expression of ABP, IHBB and VEGFA in SCCs was less (P < 0.05) in both dogs (12.26, 4.07 and 5.11 folds, respectively) and cats (39.53, 14.07 and 20.29 folds, respectively) treated with anti-FSHr antibody fragments, indicating the Sertoli cell functions were suppressed. In conclusion, this study demonstrated the establishment of species-specific antibody fragments against FSHr in SCCs for dogs and cats. The fragment proteins illustrate potential to be developed as non-surgical contraceptive agent targeting FSHr in companion animals.

The Prediction of DLL4 as a Prognostic Biomarker in Patients with Gastric Cancer Using Anti-DLL4 Nanobody

BACKGROUND

Angiogenesis and cancer metastasis depend on the DLL4/Notch signaling pathway. A new approach to treating angiogenesis could inhibit or block this pathway. In the present study, we investigated DLL4 expression as a biomarker capable of predicting survival outcomes in gastric cancer patients using a novel anti-DLL4 Nanobody.

PATIENTS AND METHODS

By using a recently developed anti-DLL4 Nanobody, the expression of DLL4 was evaluated in tissue samples from 135 gastric cancer patients. It was evaluated whether DLL4 expression is related to clinicopathological factors, overall survival (OS), and recurrence-free survival (RFS).

RESULTS

Sixty-five (48%) gastric cancer patients had a positive expression of DLL4 within the tumor tissue. Based on both the univariate and multivariate regression analyses, the expression of DLL4 was strongly associated with RFS (HR, 1.94; p = 0.008) and OS (HR, 2.06; p = 0.004). Moreover, the survival analysis demonstrated that DLL4 expression was a significant independent factor of unfavorable OS (HR, 2.7; p = 0.01) and RFS (HR, 2.3; p = 0.02) in gastric cancer patients.

CONCLUSION

DLL4 expression in gastric cancer patients may predict poor prognosis and survival. Furthermore, the current data demonstrate the potential of Nanobody for detecting DLL4, and it may lead to develop novel therapies and diagnostics for tumors.

State-of-the-art in engineering small molecule biosensors and their applications in metabolic engineering

Genetically encoded biosensors are crucial for enhancing our understanding of how molecules regulate biological systems. Small molecule biosensors, in particular, help us understand the interaction between chemicals and biological processes. They also accelerate metabolic engineering by increasing screening throughput and eliminating the need for sample preparation through traditional chemical analysis. Additionally, they offer significantly higher spatial and temporal resolution in cellular analyte measurements. In this review, we discuss recent progress in in vivo biosensors and control systems-biosensor-based controllers-for metabolic engineering. We also specifically explore protein-based biosensors that utilize less commonly exploited signaling mechanisms, such as protein stability and induced degradation, compared to more prevalent transcription factor and allosteric regulation mechanism. We propose that these lesser-used mechanisms will be significant for engineering eukaryotic systems and slower-growing prokaryotic systems where protein turnover may facilitate more rapid and reliable measurement and regulation of the current cellular state. Lastly, we emphasize the utilization of cutting-edge and state-of-the-art techniques in the development of protein-based biosensors, achieved through rational design, directed evolution, and collaborative approaches.

Computational Modeling and Characterization of Peptides Derived from Nanobody Complementary-Determining Region 2 (CDR2) Targeting Active-State Conformation of the β2-Adrenergic Receptor (β2AR)

This study assessed the suitability of the complementarity-determining region 2 (CDR2) of the nanobody (Nb) as a template for the derivation of nanobody-derived peptides (NDPs) targeting active-state β2-adrenergic receptor (β2AR) conformation. Sequences of conformationally selective Nbs favoring the agonist-occupied β2AR were initially analyzed by the informational spectrum method (ISM). The derived NDPs in complex with β2AR were subjected to protein-peptide docking, molecular dynamics (MD) simulations, and metadynamics-based free-energy binding calculations. Computational analyses identified a 25-amino-acid-long CDR2-NDP of Nb71, designated P4, which exhibited the following binding free-energy for the formation of the β2AR:P4 complex (ΔG = -6.8 ± 0.8 kcal/mol or a Ki = 16.5 μM at 310 K) and mapped the β2AR:P4 amino acid interaction network. In vitro characterization showed that P4 (i) can cross the plasma membrane, (ii) reduces the maximum isoproterenol-induced cAMP level by approximately 40% and the isoproterenol potency by up to 20-fold at micromolar concentration, (iii) has a very low affinity to interact with unstimulated β2AR in the cAMP assay, and (iv) cannot reduce the efficacy and potency of the isoproterenol-mediated β2AR/β-arrestin-2 interaction in the BRET2-based recruitment assay. In summary, the CDR2-NDP, P4, binds preferentially to agonist-activated β2AR and disrupts Gαs-mediated signaling.

The functional roles of chemokines and chemokine receptors in colorectal cancer progression

Colorectal cancer is a common malignancy with significant rates of morbidity and mortality. A number of factors, including the tumor microenvironment, chemokines, the inflammatory response, have an impact on the development of colorectal cancer. A critical component of the tumor microenvironment is chemokines. Various cell subsets are attracted to the tumor microenvironment through interactions with chemokine receptors. These cells have varying effects on the development of the tumor and the effectiveness of treatment. Additionally, chemokines can participate in inflammatory processes and have effects that are either pro- or anti-tumor. Chemokines can be exploited as targets for medication resistance and treatment in colorectal cancer. In this review, we discuss the expression of chemokines and chemokine receptors, and their relationship with immune cells in the tumor microenvironment. At the same time, we also collect and discuss the significance of chemokines and chemokine receptors in colorectal cancer progression, and their potential as molecular targets for CRC treatment.

Breaking barriers in antibody discovery: harnessing divergent species for accessing difficult and conserved drug targets

To exploit highly conserved and difficult drug targets, including multipass membrane proteins, monoclonal antibody discovery efforts increasingly rely on the advantages offered by divergent species such as rabbits, camelids, and chickens. Here, we provide an overview of antibody discovery technologies, analyze gaps in therapeutic antibodies that stem from the historic use of mice, and examine opportunities to exploit previously inaccessible targets through discovery now possible in alternate species. We summarize the clinical development of antibodies raised from divergent species, discussing how these animals enable robust immune responses against highly conserved binding sites and yield antibodies capable of penetrating functional pockets via long HCDR3 regions. We also discuss the value of pan-reactive molecules often produced by these hosts, and how these antibodies can be tested in accessible animal models, offering a faster path to clinical development.

Nanobody-based sensors reveal a high proportion of mGlu heterodimers in the brain

Membrane proteins, including ion channels, receptors and transporters, are often composed of multiple subunits and can form large complexes. Their specific composition in native tissues is difficult to determine and remains largely unknown. In this study, we developed a method for determining the subunit composition of endogenous cell surface protein complexes from isolated native tissues. Our method relies on nanobody-based sensors, which enable proximity detection between subunits in time-resolved Förster resonance energy transfer (FRET) measurements. Additionally, given conformation-specific nanobodies, the activation of these complexes can be recorded in native brain tissue. Applied to the metabotropic glutamate receptors in different brain regions, this approach revealed the clear existence of functional metabotropic glutamate (mGlu)2-mGlu4 heterodimers in addition to mGlu2 and mGlu4 homodimers. Strikingly, the mGlu4 subunits appear to be mainly heterodimers in the brain. Overall, these versatile biosensors can determine the presence and activity of endogenous membrane proteins in native tissues with high fidelity and convenience.

Development of anti DLL4 Nanobody fused to truncated form of Pseudomonas exotoxin: As a novel immunotoxin to inhibit of cell proliferation and neovascularization

Targeted tumor therapy is an attractive approach for cancer treatment. Delta-like ligand 4 (DLL4) is overexpressed in tumor vasculature and plays a pivotal role in tumor neovascular development and angiogenesis during tumor progression. Immunotoxins due to their superior cell-killing ability and the relative simplicity of their preparation, have great potential in the clinical treatment of cancer. The aim of this study was to develop a novel immunotoxin against DLL4 as a cell cytotoxic agent and angiogenesis maturation inhibitor. In present study, an immunotoxin, named DLL4Nb-PE, in which a Nanobody as targeting moiety fused to the Pseudomonas exotoxin A (PE) was constructed, expressed and assessed by SDS-PAGE, western blotting, ELISA and flowcytometry. The functional assessment was carried out via MTT, apoptosis and chicken chorioallantoic membrane (CAM) assays. It was demonstrated DLL4Nb-PE specifically binds to DLL4 and recognizes DLL4-expressing MKN cells. The cytotoxicity assays showed that this molecule could induce apoptosis and kill DLL4 positive MKN cells. In addition, it inhibited neovascularization in the chicken chorioallantoic membrane. Our findings indicate designed anti-DLL4 immunotoxin has valuable potential for application to the treatment of tumors with high DLL4 expression.

Accelerating GPCR Drug Discovery With Conformation-Stabilizing VHHs

The human genome encodes 850 G protein-coupled receptors (GPCRs), half of which are considered potential drug targets. GPCRs transduce extracellular stimuli into a plethora of vital physiological processes. Consequently, GPCRs are an attractive drug target class. This is underlined by the fact that approximately 40% of marketed drugs modulate GPCRs. Intriguingly 60% of non-olfactory GPCRs have no drugs or candidates in clinical development, highlighting the continued potential of GPCRs as drug targets. The discovery of small molecules targeting these GPCRs by conventional high throughput screening (HTS) campaigns is challenging. Although the definition of success varies per company, the success rate of HTS for GPCRs is low compared to other target families (Fujioka and Omori, 2012; Dragovich et al., 2022). Beyond this, GPCR structure determination can be difficult, which often precludes the application of structure-based drug design approaches to arising HTS hits. GPCR structural studies entail the resource-demanding purification of native receptors, which can be challenging as they are inherently unstable when extracted from the lipid matrix. Moreover, GPCRs are flexible molecules that adopt distinct conformations, some of which need to be stabilized if they are to be structurally resolved. The complexity of targeting distinct therapeutically relevant GPCR conformations during the early discovery stages contributes to the high attrition rates for GPCR drug discovery programs. Multiple strategies have been explored in an attempt to stabilize GPCRs in distinct conformations to better understand their pharmacology. This review will focus on the use of camelid-derived immunoglobulin single variable domains (VHHs) that stabilize disease-relevant pharmacological states (termed ConfoBodies by the authors) of GPCRs, as well as GPCR:signal transducer complexes, to accelerate drug discovery. These VHHs are powerful tools for supporting in vitro screening, deconvolution of complex GPCR pharmacology, and structural biology purposes. In order to demonstrate the potential impact of ConfoBodies on translational research, examples are presented of their role in active state screening campaigns and structure-informed rational design to identify de novo chemical space and, subsequently, how such matter can be elaborated into more potent and selective drug candidates with intended pharmacology.

[β-arrestins, their mechanisms of action and multiple roles in the biology of G protein-coupled receptors]

The stimulation of G protein-coupled receptors (GPCRs) induces biological responses to a wide range of extracellular cues. The heterotrimeric G proteins, which are recruited to the active conformation of GPCRs, lead to the generation of various diffusible second messengers. Only two other families of proteins exhibit the remarkable characteristic of recognizing and binding to the active conformation of most GPCRs: GPCR kinases (GRKs) and β-arrestins. These two families of proteins were initially identified as key players in the desensitization of G protein activation by GPCRs. Over the years, β-arrestins have been implicated in an increasing number of interactions with non-receptor proteins, expanding the range of cellular functions in which they are involved. It is now well established that β-arrestins, by scaffolding and recruiting protein complexes in an agonist-dependent manner, directly regulate the trafficking and signaling of GPCRs. Remarkable advances have been made in recent years which have made it possible i) to identify biased ligands capable, by stabilizing particular conformations of a growing number of GPCRs, of activating or blocking the action of β-arrestins independently of that of G proteins, some of these ligands holding great therapeutic interest; ii) to demonstrate β-arrestins' role in the compartmentalization of GPCR signaling within the cell, and iii) to understand the molecular details of their interaction with GPCRs and of their activation through structural and biophysical approaches.

Overview of follicle stimulating hormone and its receptors in reproduction and in stem cells and cancer stem cells

Follicle stimulating hormone (FSH) and its receptor (FSHR) have been reported to be responsible for several physiological functions and cancers. The responsiveness of stem cells and cancer stem cells towards the FSH-FSHR system make the function of FSH and its receptors more interesting in the context of cancer biology. This review is comprised of comprehensive information on FSH-FSHR signaling in normal physiology, gonadal stem cells, cancer cells, and potential options of utilizing FSH-FSHR system as an anti-cancer therapeutic target.

Atypical Roles of the Chemokine Receptor ACKR3/CXCR7 in Platelet Pathophysiology

The manifold actions of the pro-inflammatory and regenerative chemokine CXCL12/SDF-1α are executed through the canonical GProteinCoupledReceptor CXCR4, and the non-canonical ACKR3/CXCR7. Platelets express CXCR4, ACKR3/CXCR7, and are a vital source of CXCL12/SDF-1α themselves. In recent years, a regulatory impact of the CXCL12-CXCR4-CXCR7 axis on platelet biogenesis, i.e., megakaryopoiesis, thrombotic and thrombo-inflammatory actions have been revealed through experimental and clinical studies. Platelet surface expression of ACKR3/CXCR7 is significantly enhanced following myocardial infarction (MI) in acute coronary syndrome (ACS) patients, and is also associated with improved functional recovery and prognosis. The therapeutic implications of ACKR3/CXCR7 in myocardial regeneration and improved recovery following an ischemic episode, are well documented. Cardiomyocytes, cardiac-fibroblasts, endothelial lining of the blood vessels perfusing the heart, besides infiltrating platelets and monocytes, all express ACKR3/CXCR7. This review recapitulates ligand induced differential trafficking of platelet CXCR4-ACKR3/CXCR7 affecting their surface availability, and in regulating thrombo-inflammatory platelet functions and survival through CXCR4 or ACKR3/CXCR7. It emphasizes the pro-thrombotic influence of CXCL12/SDF-1α exerted through CXCR4, as opposed to the anti-thrombotic impact of ACKR3/CXCR7. Offering an innovative translational perspective, this review also discusses the advantages and challenges of utilizing ACKR3/CXCR7 as a potential anti-thrombotic strategy in platelet-associated cardiovascular disorders, particularly in coronary artery disease (CAD) patients post-MI.

Probing GPCR Dimerization Using Peptides

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins and the most common and extensively studied pharmacological target. Numerous studies over the last decade have confirmed that GPCRs do not only exist and function in their monomeric form but in fact, have the ability to form dimers or higher order oligomers with other GPCRs, as well as other classes of receptors. GPCR oligomers have become increasingly attractive to investigate as they have the ability to modulate the pharmacological responses of the receptors which in turn, could have important functional roles in diseases, such as cancer and several neurological & neuropsychiatric disorders. Despite the growing evidence in the field of GPCR oligomerisation, the lack of structural information, as well as targeting the 'undruggable' protein-protein interactions (PPIs) involved in these complexes, has presented difficulties. Outside the field of GPCRs, targeting PPIs has been widely studied, with a variety of techniques being investigated; from small-molecule inhibitors to disrupting peptides. In this review, we will demonstrate several physiologically relevant GPCR dimers and discuss an array of strategies and techniques that can be employed when targeting these complexes, as well as provide ideas for future development.

Optogenetic Activation of Intracellular Nanobodies

Intracellular antibody fragments such as nanobodies and scFvs are powerful tools for imaging and for modulating and neutralizing endogenous target proteins. Optogenetically activated intracellular antibodies (optobodies) constitute a light-inducible system to directly control intrabody activities in cells, with greater spatial and temporal resolution than intracellular antibodies alone. Here, we describe optogenetic and microscopic methods to activate optobodies in cells using a confocal microscope and an automated fluorescence microscope. In the protocol, we use the examples of an optobody targeting green fluorescent protein and an optobody that inhibits the endogenous gelsolin protein.

Nanobodies as versatile tools: A focus on targeted tumor therapy, tumor imaging and diagnostics

Monoclonal antibodies and vaccines have widely been studied for the immunotherapy of cancer, though their large size appears to limit their functionality in solid tumors, in large part due to unique properties of tumor microenvironment. Smaller formats of antibodies have been developed to throw such restrictions. These small format antibodies include antigen binding fragments, single-chain variable fragments, single variable domain of camelid antibody (so-called nanobody (Nb) or VHH). Since their serendipitous discovery, nanobodies have been studies at length in the fields of research, diagnostics and therapy. These antigen binding fragments, originating from camelid heavy-chain antibodies, possess unusual hallmarks in terms of (small) size, stability, solubility and specificity, hence allowing cost-effective production and sometimes out performing monoclonal antibodies. In addition, these small camelid heavy-chain antibodies are highly adaptable tools for cancer research as they enable specific modulation of targets, enzymatic and non-enzymatic proteins alike. Molecular imaging studies benefit from the rapid, homogeneous tumor accumulation of nanobodies and their fast blood clearance, permitting previously unattainable fast tumor visualization. Moreover, they are endowed with considerable therapeutic potential as inhibitors of receptor-ligand pairs and deliverers of drugs or drug-loaded nanoparticles towards tumors. In this review, we shed light on the current status of nanobodies in diagnosis and imaging of tumor and exploiting nanobodies revert immunosuppressive events, modulation of immune checkpoints, and as deliverers of drugs for targeted tumor therapy.

Anticancer opportunities at every stage of chemokine function

The chemokine system, comprising 48 chemokines and 23 receptors, is critically involved in several hallmarks of cancer. Yet, despite extensive efforts from the pharmaceutical sector, only two drugs aimed at this system are currently approved for clinical use against cancer. To date, numerous pharmacological approaches have been developed to successfully intervene at different stages of chemokine function: (i) chemokine availability; (ii) chemokine-glycosaminoglycan binding; and (iii) chemokine receptor binding. Many of these strategies have been tested in preclinical cancer models, and some have advanced to clinical trials as potential anticancer therapies. Here we will review the strategies and growing pharmacological toolbox for manipulating the chemokine system in cancer, and address novel methods poised for future (pre)clinical testing.

Radiomics, aptamers and nanobodies: New insights in cancer diagnostics and imaging

At present, cancer is a major health issue and the second leading cause of mortality worldwide. Researchers have been working hard on investigating not only improved therapeutics but also on early detection methods, both critical to increasing treatment efficacy and developing methods for disease prevention. Diagnosis of cancers at an early stage can promote timely medical intervention and effective treatment and will result in inhibiting tumor growth and development. Several advances have been made in the diagnostics and imagining technologies for early tumor detection and deciding an effective therapy these include radiomics, nanobodies, and aptamers. Here in this review, we summarize the main applications of radiomics, aptamers, and the use of nanobody-based probes for molecular imaging applications in diagnosis, treatment planning, and evaluations in the field of oncology to develop quantitative and personalized medicine. The preclinical data reported to date are quite promising, and it is predicted that nanobody-based molecular imaging agents will play an important role in the diagnosis and management of different cancer types in near future.

GPCRs: The most promiscuous druggable receptor of the mankind

All physiological events in living organisms originated as specific chemical/biochemical signals on the cell surface and transmitted into the cytoplasm. This signal is translated within milliseconds-hours to a specific and unique order required to maintain optimum performance and homeostasis of living organisms. Examples of daily biological functions include neuronal communication and neurotransmission in the process of learning and memory, secretion (hormones, sweat, and saliva), muscle contraction, cellular growth, differentiation and migration during wound healing, and immunity to fight infections. Among the different transducers for such life-dependent signals is the large family of G protein-coupled receptors (GPCRs). GPCRs constitute roughly 800 genes, corresponding to 2% of the human genome. While GPCRs control a plethora of pathophysiological disorders, only approximately one-third of GPCR families have been deorphanized and characterized. Recent drug data show that around 40% of the recommended drugs available in the market target mainly GPCRs. In this review, we presented how such system signals, either through G protein or via other players, independent of G protein, function within the biological system. We also discussed drugs in the market or clinical trials targeting mainly GPCRs in various diseases, including cancer.

Nanobody: a promising toolkit for molecular imaging and disease therapy

Nanobodies are the recombinant variable domains of heavy-chain-only antibodies, with many unique properties such as small size, excellent solubility, superior stability, quick clearance from blood, and deep tissue penetration. As a result, nanobodies have become a promising tool for the diagnosis and therapy of diseases. As imaging tracers, nanobodies allow an early acquisition of high-quality images, provide a comprehensive evaluation of the disease, and subsequently enable a personalized precision therapy. As therapeutic agents, nanobodies enable a targeted therapy by lesion-specific delivery of drugs and effector domains, thereby improving the specificity and efficacy of the therapy. Up to date, a wide variety of nanobodies have been developed for a broad range of molecular targets and have played a significant role in patients with a broad spectrum of diseases. In this review, we aim to outline the current state-of-the-art research on the nanobodies for medical applications and then discuss the challenges and strategies for their further clinical translation.

Structure- and sequence-based design of synthetic single-domain antibody libraries

Single-domain antibody fragments known as VHH have emerged in the pharmaceutical industry as useful biotherapeutics. These molecules, which are naturally produced by camelids, share the characteristics of high affinity and specificity with traditional human immunoglobulins, while consisting of only a single heavy chain. Currently, the most common method for generating VHH is via animal immunization, which can be costly and time-consuming. Here we describe the development of a synthetic VHH library for in vitro selection of single domain binders. We combine structure-based design and next-generation sequencing analysis to build a library with characteristics that closely mimic the natural repertoire. To validate the performance of our synthetic library, we isolated VHH against three model antigens (soluble mouse PD-1 ectodomain, amyloid-β peptide, and MrgX1 GPCR) of different sizes and characteristics. We were able to isolate diverse binders targeting different epitopes with high affinity (as high as 5 nM) against all three targets. We then show that anti-mPD-1 binders have functional activity in a receptor blocking assay.

Nanobodies: The Future of Antibody-Based Immune Therapeutics

Targeted therapy is a fast evolving treatment strategy to reduce unwanted damage to healthy tissues, while increasing efficacy and specificity. Driven by state-of-the-art technology, this therapeutic approach is especially true of cancer. Antibodies with their remarkable specificity have revolutionized therapeutic strategies for autoimmune conditions and cancer, particularly blood-borne cancers, but have severe limitations in treating solid tumors. This is mainly due to their large molecular size, low stability, tumor-tissue penetration difficulties, and pharmacokinetic properties. The tumor microenvironment, rich in immune-suppressing molecules is also a major barrier in targeting solid tumors by antibody-based drugs. Nanobodies have recently emerged as an alternative therapeutic agent to overcome some of the drawbacks of traditional antibody treatment. Nanobodies are the VHH domains found on the heavy-chain only antibodies of camelids and are the smallest naturally available antibody fragments with excellent antigen-binding specificity and affinity, equivalent to conventional antibodies but with molecular weights as low as 15 kDa. The compact size, high stability, enhanced hydrophilicity, particularly in framework regions, excellent epitope interactions with protruding CDR3 regions, and improved tissue penetration make nanobodies the next-generation therapeutics (Nano-BioDrugs). In this review, the authors discuss the interesting properties of nanobodies and their advantages over their conventional counterparts and provide insight into how nanobodies are being utilized as agonists and antagonists, bispecific constructs, and drug and enzyme-conjugates to combat the tumor microenvironment and treat disease.

The role of Eph receptors in cancer and how to target them: novel approaches in cancer treatment

INTRODUCTION

Erythropoietin-producing human hepatocellular (Eph) receptors are among the largest family of tyrosine kinases that are divided into two classes: EphA and EphB receptors. Over the past two decades, their role in cancer has become more evident.

AREAS COVERED

There is a need for new anticancer treatments and more insight in the emerging role of Eph receptors in cancer. Molecular mechanisms underlying the pro-tumorigenic effects of Eph receptors could be exploited for future therapeutic strategies. This review describes the variability in expression levels and different effects on oncogenic and tumor suppressive downstream signaling of Eph receptors in various cancer types, and the small molecules, antibodies and peptides that target these receptors.

EXPERT OPINION

The complexity of Eph signaling is a challenge for the definition of clear targets for cancer treatment. Nevertheless, numerous drugs that target EphA2 and EphB4 are currently in clinical trials. However, some Eph targeted drugs also inhibit other tyrosine kinases, so it is unclear to what extent the targeting of Eph receptors contributes to their efficacy. Future research is warranted for an improved understanding of the full network in which Eph receptors function. This will be critical for the improvement of the anticancer effects of drugs that target the Eph receptors.

FSH for the Treatment of Male Infertility

Follicle-stimulating hormone (FSH) supports spermatogenesis acting via its receptor (FSHR), which activates trophic effects in gonadal Sertoli cells. These pathways are targeted by hormonal drugs used for clinical treatment of infertile men, mainly belonging to sub-groups defined as hypogonadotropic hypogonadism or idiopathic infertility. While, in the first case, fertility may be efficiently restored by specific treatments, such as pulsatile gonadotropin releasing hormone (GnRH) or choriogonadotropin (hCG) alone or in combination with FSH, less is known about the efficacy of FSH in supporting the treatment of male idiopathic infertility. This review focuses on the role of FSH in the clinical approach to male reproduction, addressing the state-of-the-art from the little data available and discussing the pharmacological evidence. New compounds, such as allosteric ligands, dually active, chimeric gonadotropins and immunoglobulins, may represent interesting avenues for future personalized, pharmacological approaches to male infertility.

Prediction and targeting of GPCR oligomer interfaces

GPCR oligomerization has emerged as a hot topic in the GPCR field in the last years. Receptors that are part of these oligomers can influence each other's function, although it is not yet entirely understood how these interactions work. The existence of such a highly complex network of interactions between GPCRs generates the possibility of alternative targets for new therapeutic approaches. However, challenges still exist in the characterization of these complexes, especially at the interface level. Different experimental approaches, such as FRET or BRET, are usually combined to study GPCR oligomer interactions. Computational methods have been applied as a useful tool for retrieving information from GPCR sequences and the few X-ray-resolved oligomeric structures that are accessible, as well as for predicting new and trustworthy GPCR oligomeric interfaces. Machine-learning (ML) approaches have recently helped with some hindrances of other methods. By joining and evaluating multiple structure-, sequence- and co-evolution-based features on the same algorithm, it is possible to dilute the issues of particular structures and residues that arise from the experimental methodology into all-encompassing algorithms capable of accurately predict GPCR-GPCR interfaces. All these methods used as a single or a combined approach provide useful information about GPCR oligomerization and its role in GPCR function and dynamics. Altogether, we present experimental, computational and machine-learning methods used to study oligomers interfaces, as well as strategies that have been used to target these dynamic complexes.

Isolation and structural characterization of a Zn2+-bound single-domain antibody against NorC, a putative multidrug efflux transporter in bacteria

Single-chain antibodies from camelids have served as powerful tools ranging from diagnostics and therapeutics to crystallization chaperones meant to study protein structure and function. In this study, we isolated a single-chain antibody from an Indian dromedary camel (ICab) immunized against a bacterial 14TM helix transporter, NorC, from Staphylococcus aureus We identified this antibody in a yeast display screen built from mononuclear cells isolated from the immunized camel and purified the antibody from Escherichia coli after refolding it from inclusion bodies. The X-ray structure of the antibody at 2.15 Å resolution revealed a unique feature within its CDR3 loop, which harbors a Zn2+-binding site that substitutes for a loop-stabilizing disulfide bond. We performed mutagenesis to compromise the Zn2+-binding site and observed that this change severely hampered antibody stability and its ability to interact with the antigen. The lack of bound Zn2+ also made the CDR3 loop highly flexible, as observed in all-atom simulations. Using confocal imaging of NorC-expressing E. coli spheroplasts, we found that the ICab interacts with the extracellular surface of NorC. This suggests that the ICab could be a valuable tool for detecting methicillin-resistant S. aureus strains that express efflux transporters such as NorC in hospital and community settings.

Targeting G protein-coupled receptors in cancer therapy

As basic research into GPCR signaling and its association with disease has come into fruition, greater clarity has emerged with regards to how these receptors may be amenable to therapeutic intervention. As a diverse group of receptor proteins, which regulate a variety of intracellular signaling pathways, research in this area has been slow to yield tangible therapeutic agents for the treatment of a number of diseases including cancer. However, recently such research has gained momentum based on a series of studies that have sought to define GPCR proteins dynamics through the elucidation of their crystal structures. In this chapter, we define the approaches that have been adopted in developing better therapeutics directed against the specific parts of the receptor proteins, such as the extracellular and the intracellular domains, including the ligands and auxiliary proteins that bind them. Finally, we also briefly outline how GPCR-derived signaling transduction pathways hold great potential as additional targets.

Natural Killer Cell Hypo-responsiveness in Chronic Lymphocytic Leukemia can be Circumvented In Vitro by Adequate Activating Signaling

Chronic lymphocytic leukemia (CLL) is characterized by an acquired immune dysfunction, which may underlie the hampered efficacy of cellular immunotherapy. Most data on dampened immune responses in CLL come from studies investigating CLL and T cell interactions. Natural killer (NK) cells may be an attractive alternative source of effector cells in immunotherapy in CLL, provided that functionality is retained within the CLL micro-environment. Despite their important role in anti-tumor responses, NK cells are not extensively characterized in CLL. Here, we studied the expression of activating and inhibitory receptors on CLL-derived and healthy control (HC) NK cells, and their functional response towards several stimuli.

NK cells from CLL patients have an increased maturation stage, with an expansion of NKG2C⁺ NK cells in CMV seropositive individuals. The cytotoxicity receptor NKG2D is downregulated, and the killing capacity through this receptor was markedly reduced in CLL-derived NK cells. In contrast, activation via CD16 (FCγRIII) led to adequate activation and functional responses in CLL-derived NK cells. These findings indicate that NK cells in CLL are not intrinsically defect and still perform effector functions upon adequate activating signaling. Clinical relevance of this finding was shown by treatment with novel nanobody-Fc constructs, which induced cytotoxic responses in both CLL- and HC-derived NK cells via CD16. Our results show that NK cells, in contrast to the T cell compartment, retain their function within the CLL micro-environment, provided that they receive an adequate activating signal. These findings warrant future studies on NK cell mediated immunotherapeutic strategies in CLL.

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.

Half-life extension and non-human primate pharmacokinetic safety studies of i-body AD-114 targeting human CXCR4

Single domain antibodies that combine antigen specificity with high tissue penetration are an attractive alternative to conventional antibodies. However, rapid clearance from the bloodstream owing to their small size can be a limitation of therapeutic single domain antibodies. Here, we describe and evaluate the conjugation of a single domain i-body, AD-114, which targets CXCR4, to a panel of half-life extension technologies including a human serum albumin-binding peptide, linear and branched PEG, and PASylation (PA600). The conjugates were assessed in murine, rat and cynomolgus monkey pharmacokinetic studies and showed that the branched PEG was most effective at extending circulating half-life in mice; however, manufacturing limitations of PEGylated test material precluded scale-up and assessment in larger animals. PA600, by comparison, was amenable to scale-up and afforded considerable half-life improvements in mice, rats and cynomolgus monkeys. In mice, the circulating half-life of AD-114 was extended from 0.18 h to 7.77 h following conjugation to PA600, and in cynomolgus monkeys, the circulating half-life of AD-114-PA600 was 24.27 h. AD-114-PA600 was well tolerated in cynomolgus monkeys at dose rates up to 100 mg/kg with no mortalities or drug-related clinical signs.

Engineering of Nanobodies Recognizing the Human Chemokine Receptor CCR7

The chemokine receptor CCR7 plays a pivotal role in health and disease. In particular, CCR7 controls homing of antigen-bearing dendritic cells and T cells to lymph nodes, where adaptive immune responses are initiated. However, CCR7 also guides T cells to inflamed synovium and thereby contributes to rheumatoid arthritis and promotes cancer cell migration and metastasis formation. Nanobodies have recently emerged as versatile tools to study G-protein-coupled receptor functions and are being tested in diagnostics and therapeutics. In this study, we designed a strategy to engineer novel nanobodies recognizing human CCR7. We generated a nanobody library based on a solved crystal structure of the nanobody Nb80 recognizing the β2-adrenergic receptor (β2AR) and by specifically randomizing two segments within complementarity determining region 1 (CDR1) and CDR3 of Nb80 known to interact with β2AR. We fused the nanobody library to one half of split-YFP in order to identify individual nanobody clones interacting with CCR7 fused to the other half of split-YFP using bimolecular fluorescence complementation. We present three novel nanobodies, termed Nb1, Nb5, and Nb38, that recognize human CCR7 without interfering with G-protein-coupling and downstream signaling. Moreover, we were able to follow CCR7 trafficking upon CCL19 triggering using Nb1, Nb5, and Nb38.

CXCR4-targeting nanobodies differentially inhibit CXCR4 function and HIV entry

The chemokine receptor CXCR4 and its ligand CXCL12 contribute to a variety of human diseases, such as cancer. CXCR4 is also a major co-receptor facilitating HIV entry. Accordingly, CXCR4 is considered as an attractive therapeutic target. Drug side effects and poor pharmacokinetic properties have been major hurdles that have prevented the implementation of CXCR4-directed inhibitors in treatment regimes. We evaluated the activity of a new and promising class of biologics, namely CXCR4-targeting nanobodies, with the purpose of identifying nanobodies that would preferentially inhibit HIV infection, while minimally disturbing other CXCR4-related functions. All CXCR4-interacting nanobodies inhibited CXCL12 binding and receptor-mediated calcium mobilization with comparable relative potencies. Importantly, the anti-HIV-1 activity of the nanobodies did not always correlate with their ability to modulate CXCR4 signaling and function, indicating that the anti-HIV and anti-CXCR4 activity are not entirely overlapping and may be functionally separated. Three nanobodies with divergent activity profiles (VUN400, VUN401 and VUN402) were selected for in depth biological evaluation. While all three nanobodies demonstrated inhibitory activity against a wide range of HIV (X4) strains, VUN402 poorly blocked CXCL12-induced CXCR4 internalization, chemotaxis and changes in cell morphology. Each of these nanobodies recognized distinct, although partially overlapping epitopes on CXCR4, which might underlie their distinct activity profiles. Our results demonstrate the potential of CXCR4-targeting nanobody VUN402 as a novel lead and starting point for the development of a more potent and selective anti-HIV agent.

Expanding the Boundaries of Biotherapeutics with Bispecific Antibodies

Bispecific antibodies have moved from being an academic curiosity with therapeutic promise to reality, with two molecules being currently commercialized (Hemlibra® and Blincyto®) and many more in clinical trials. The success of bispecific antibodies is mainly due to the continuously growing number of mechanisms of actions (MOA) they enable that are not accessible to monoclonal antibodies. One of the earliest MOA of bispecific antibodies and currently the one with the largest number of clinical trials is the redirecting of the cytotoxic activity of T-cells for oncology applications, now extending its use in infective diseases. The use of bispecific antibodies for crossing the blood-brain barrier is another important application because of its potential to advance the therapeutic options for neurological diseases. Another noteworthy application due to its growing trend is enabling a more tissue-specific delivery or activity of antibodies. The different molecular solutions to the initial hurdles that limited the development of bispecific antibodies have led to the current diverse set of bispecific or multispecific antibody formats that can be grouped into three main categories: IgG-like formats, antibody fragment-based formats, or appended IgG formats. The expanded applications of bispecific antibodies come at the price of additional challenges for clinical development. The rising complexity in their structure may increase the risk of immunogenicity and the multiple antigen specificity complicates the selection of relevant species for safety assessment.

Structure of a Thyrotropin Receptor Monoclonal Antibody Variable Region Provides Insight into Potential Mechanisms for its Inverse Agonist Activity

BACKGROUND

The high constitutive, or ligand-independent, activity of the thyrotropin receptor (TSHR) is of clinical importance in some thyroid conditions, particularly well-differentiated thyroid carcinoma remnants following incomplete ablative therapy (surgery and radioiodine). Under these conditions, even total suppression of TSH by thyroid hormone administration does not fully reduce TSHR activity, a driver of thyrocyte growth.

METHODS

CS-17 is a murine monoclonal antibody that has inverse agonist activity in that it suppresses TSHR constitutive activity. This study crystallized the CS-17 Fab and determined its atomic structure at a resolution of 3.4 Å.

RESULTS

In silico docking of this structure to that of the TSHR extracellular domain was accomplished by targeting to TSHR residue tyrosine 195 (Y195) known to contribute to the CS-17 epitope. High affinity interaction between these two molecules, primarily by the CS-17 immunoglobulin heavy chain, was validated by energetic analysis (K_D of 8.7 × 10^-11 M), as well as by previously obtained data on a number of individual TSHR amino acids in three regions whose mutagenesis reduced CS-17 binding as detected by flow cytometry.

CONCLUSIONS

Structural insight at atomic resolution of a TSHR antibody with inverse agonist activity opens the way for the development of a molecule with therapeutic potential, particularly in thyroid carcinoma. For this purpose, CS-17 will require "humanization" by substitution of its constant region (Fc component). In addition, with its epitope defined, the CS-17 affinity can be increased further by mutagenesis of selected amino acids in its heavy- and light-chain complementarity determining regions.

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.

Intracellular Receptor Modulation: Novel Approach to Target GPCRs

Recent crystal structures of multiple G protein-coupled receptors (GPCRs) have revealed a highly conserved intracellular pocket that can be used to modulate these receptors from the inside. This novel intracellular site partially overlaps with the G protein and β-arrestin binding site, providing a new manner of pharmacological intervention. Here we provide an update of the architecture and function of the intracellular region of GPCRs, until now portrayed as the signaling domain. We review the available evidence on the presence of intracellular binding sites among chemokine receptors and other class A GPCRs, as well as different strategies to target it, including small molecules, pepducins, and nanobodies. Finally, the potential advantages of intracellular (allosteric) ligands over orthosteric ligands are also discussed.

Exploiting Nanobodies' Singular Traits

The unique class of heavy chain-only antibodies, present in Camelidae, can be shrunk to just the variable region of the heavy chain to yield VHHs, also called nanobodies. About one-tenth the size of their full-size counterparts, nanobodies can serve in applications similar to those for conventional antibodies, but they come with a number of signature advantages that find increasing application in biology. They not only function as crystallization chaperones but also can be expressed inside cells as such, or fused to other proteins to perturb the function of their targets, for example, by enforcing their localization or degradation. Their small size also affords advantages when applied in vivo, for example, in imaging applications. Here we review such applications, with particular emphasis on those areas where conventional antibodies would face a more challenging environment.

Development by Genetic Immunization of Monovalent Antibodies Against Human Vasoactive Intestinal Peptide Receptor 1 (VPAC1), New Innovative, and Versatile Tools to Study VPAC1 Receptor Function

Multi-membrane spanning proteins, such as G protein-coupled receptors (GPCRs) and ion channels, are extremely difficult to purify as native proteins. Consequently, the generation of antibodies that recognize the native conformation can be challenging. By combining genetic immunization, phage display, and biopanning, we identified a panel of monovalent antibodies (nanobodies) targeting the vasoactive intestinal peptide receptor 1 (VPAC1) receptor. The nine unique nanobodies that were classified into four different families based on their CDR3 amino acid sequence and length, were highly specific for the human receptor and bind VPAC1 with moderate affinity. They all recognize a similar epitope localized in the extracellular N-terminal domain of the receptor and distinct from the orthosteric binding site. In agreement with binding studies, which showed that the nanobodies did not interfere with VIP binding, all nanobodies were devoid of any functional properties. However, we observed that the binding of two nanobodies was slightly increased in the presence of VPAC1 agonists [vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating polypeptide-27 (PACAP-27)], but decreased in the presence of VPAC1 antagonist. As no evidence of allosteric activity was seen in VIP binding studies nor in functional assays, it is, therefore, possible that the two nanobodies may behave as very weak allosteric modulators of VPAC1, detectable only in some sensitive settings, but not in others. We demonstrated that the fluorescently labeled nanobodies detect VPAC1 on the surface of human leukocytes as efficiently as a reference mouse monoclonal antibody. We also developed a protocol allowing efficient detection of VPAC1 by immunohistochemistry in paraffin-embedded human gastrointestinal tissue sections. Thus, these nanobodies constitute new original tools to further investigate the role of VPAC1 in physiological and pathological conditions.

Allosteric nanobodies uncover a role of hippocampal mGlu2 receptor homodimers in contextual fear consolidation

Antibodies have enormous therapeutic and biotechnology potential. G protein-coupled receptors (GPCRs), the main targets in drug development, are of major interest in antibody development programs. Metabotropic glutamate receptors are dimeric GPCRs that can control synaptic activity in a multitude of ways. Here we identify llama nanobodies that specifically recognize mGlu2 receptors, among the eight subtypes of mGluR subunits. Among these nanobodies, DN10 and 13 are positive allosteric modulators (PAM) on homodimeric mGlu2, while DN10 displays also a significant partial agonist activity. DN10 and DN13 have no effect on mGlu2-3 and mGlu2-4 heterodimers. These PAMs enhance the inhibitory action of the orthosteric mGlu2/mGlu3 agonist, DCG-IV, at mossy fiber terminals in the CA3 region of hippocampal slices. DN13 also impairs contextual fear memory when injected in the CA3 region of hippocampal region. These data highlight the potential of developing antibodies with allosteric actions on GPCRs to better define their roles in vivo.

A monoclonal antibody raised against a thermo-stabilised β1-adrenoceptor interacts with extracellular loop 2 and acts as a negative allosteric modulator of a sub-set of β1-adrenoceptors expressed in stable cell lines

Recent interest has focused on antibodies that can discriminate between different receptor conformations. Here we have characterised the effect of a monoclonal antibody (mAb3), raised against a purified thermo-stabilised turkey β₁-adrenoceptor (β₁AR-m23 StaR), on β₁-ARs expressed in CHO-K1 or HEK 293 cells. Immunohistochemical and radioligand-binding studies demonstrated that mAb3 was able to bind to ECL2 of the tβ₁-AR, but not its human homologue. Specific binding of mAb3 to tβ₁-AR was inhibited by a peptide based on the turkey, but not the human, ECL2 sequence. Studies with [³H]-CGP 12177 demonstrated that mAb3 prevented the binding of orthosteric ligands to a subset (circa 40%) of turkey β₁-receptors expressed in both CHO K1 and HEK 293 cells. MAb3 significantly reduced the maximum specific binding capacity of [³H]-CGP-12177 without influencing its binding affinity. Substitution of ECL2 of tβ₁-AR with its human equivalent, or mutation of residues D186S, P187D, Q188E prevented the inhibition of [³H]-CGP 12177 binding by mAb3. MAb3 also elicited a negative allosteric effect on agonist-stimulated cAMP responses. The identity of the subset of turkey β₁-adrenoceptors influenced by mAb3 remains to be established but mAb3 should become an important tool to investigate the nature of β₁-AR conformational states and oligomeric complexes.

Going native: Direct high throughput screening of secreted full-length IgG antibodies against cell membrane proteins

Gel microdroplet – fluorescence activated cell sorting (GMD-FACS) is an innovative high throughput screening platform for recombinant protein libraries, and we show here that GMD-FACS can overcome many of the limitations associated with conventional screening methods for antibody libraries. For example, phage and cell surface display benefit from exceptionally high throughput, but generally require high quality, soluble antigen target and necessitate the use of anchored antibody fragments. In contrast, the GMD-FACS assay can screen for soluble, secreted, full-length IgGs at rates of several thousand clones per second, and the technique enables direct screening against membrane protein targets in their native cellular context. In proof-of-concept experiments, rare anti-EGFR antibody clones were efficiently enriched from a 10,000-fold excess of anti-CCR5 clones in just three days. Looking forward, GMD-FACS has the potential to contribute to antibody discovery and engineering for difficult targets, such as ion channels and G protein-coupled receptors.

CXCR4-Specific Nanobodies as Potential Therapeutics for WHIM syndrome

WHIM syndrome is a rare congenital immunodeficiency disease, named after its main clinical manifestations: warts, hypogammaglobulinemia, infections, and myelokathexis, which refers to abnormal accumulation of mature neutrophils in the bone marrow. The disease is primarily caused by C-terminal truncation mutations of the chemokine receptor CXCR4, giving these CXCR4-WHIM mutants a gain of function in response to their ligand CXCL12. Considering the broad functions of CXCR4 in maintaining leukocyte homeostasis, patients are panleukopenic and display altered immune responses, likely as a consequence of impairment in the differentiation and trafficking of leukocytes. Treatment of WHIM patients currently consists of symptom relief, leading to unsatisfactory clinical responses. As an alternative and potentially more effective approach, we tested the potency and efficacy of CXCR4-specific nanobodies on inhibiting CXCR4-WHIM mutants. Nanobodies are therapeutic proteins based on the smallest functional fragments of heavy chain antibodies. They combine the advantages of small-molecule drugs and antibody-based therapeutics due to their relative small size, high stability, and high affinity. We compared the potential of monovalent and bivalent CXCR4-specific nanobodies to inhibit CXCL12-induced CXCR4-WHIM-mediated signaling with the small-molecule clinical candidate AMD3100. The CXCR4-targeting nanobodies displace CXCL12 binding and bind CXCR4-wild type and CXCR4-WHIM (R334X/S338X) mutants and with (sub-) nanomolar affinities. The nanobodies' epitope was mapped to extracellular loop 2 of CXCR4, overlapping with the binding site of CXCL12. Monovalent, and in particular bivalent, nanobodies were more potent than AMD3100 in reducing CXCL12-mediated G protein activation. In addition, CXCR4-WHIM-dependent calcium flux and wound healing of human papillomavirus-immortalized cell lines in response to CXCL12 was effectively inhibited by the nanobodies. Based on these in vitro results, we conclude that CXCR4 nanobodies hold significant potential as alternative therapeutics for CXCR4-associated diseases such as WHIM syndrome.

β-arrestin signalling and bias in hormone-responsive GPCRs

G protein-coupled receptors (GPCRs) play crucial roles in the ability of target organs to respond to hormonal cues. GPCRs' activation mechanisms have long been considered as a two-state process connecting the agonist-bound receptor to heterotrimeric G proteins. This view is now challenged as mounting evidence point to GPCRs being connected to large arrays of transduction mechanisms involving heterotrimeric G proteins as well as other players. Amongst the G protein-independent transduction mechanisms, those elicited by β-arrestins upon their recruitment to the active receptors are by far the best characterized and apply to most GPCRs. These concepts, in conjunction with remarkable advances made in the field of GPCR structural biology and biophysics, have supported the notion of ligand-selective signalling also known as pharmacological bias. Interestingly, recent reports have opened intriguing prospects to the way β-arrestins control GPCR-mediated signalling in space and time within the cells. In the present paper, we review the existing evidence linking endocrine-related GPCRs to β-arrestin recruitement, signalling, pathophysiological implications and selective activation by biased ligands and/or receptor modifications. Emerging concepts surrounding β-arrestin-mediated transduction are discussed in the light of the peculiarities of endocrine systems.