Function and dynamics of the intrinsically disordered carboxyl terminus of β2 adrenergic receptor

Thermodynamic role of receptor phosphorylation barcode in cannabinoid receptor desensitization

The endocannabinoid signaling system is comprised of CB1 and CB2 G protein-coupled receptors (GPCRs). CB2 receptor subtype is predominantly expressed in the immune cells and signals through its transducer proteins (Gi protein and β-arrestin-2). Arrestins are signaling proteins that bind to many GPCRs after receptor phosphorylation to terminate G protein signaling (desensitization) and to initiate specific G protein-independent arrestin-mediated signaling pathways via a "phosphorylation barcode", that captures sequence patterns of phosphorylated Ser/Thr residues in the receptor's intracellular domains and can lead to different signaling effects. The structural basis for how arrestins and G proteins compete with the receptor for biased signaling and how different barcodes lead to different signaling profiles is not well understood as there is a lack of phosphorylated receptor structures in complex with arrestins. In this work, structural models of β-arrestin-2 were built in complex with the phosphorylated and unphosphorylated forms of the CB2 receptor. The complex structures were relaxed in the lipid bilayer environment with molecular dynamics (MD) simulations and analyzed structurally and thermodynamically. The β-arrestin-2 complex with the phosphorylated receptor was more stable than the non-phosphorylated one, highlighting the thermodynamic role of the receptor phosphorylation. It was also more stable than any of the G protein complexes with CB2 suggesting that phosphorylation signals receptor desensitization (end of G protein signaling) and arrest of the receptor by arrestins. These models are beginning to provide the thermodynamic landscape of CB2 signaling, which can help bias signaling towards therapeutically beneficial pathways in drug discovery applications.

Myosin VI drives arrestin-independent internalization and signaling of GPCRs

G protein-coupled receptor (GPCR) endocytosis is canonically associated with β-arrestins. Here, we delineate a β-arrestin-independent endocytic pathway driven by the cytoskeletal motor, myosin VI. Myosin VI engages GIPC, an adaptor protein that binds a PDZ sequence motif present at the C-terminus of several GPCRs. Using the D2 dopamine receptor (D2R) as a prototype, we find that myosin VI regulates receptor endocytosis, spatiotemporal localization, and signaling. We find that access to the D2R C-tail for myosin VI-driven internalization is controlled by an interaction between the C-tail and the third intracellular loop of the receptor. Agonist efficacy, co-factors, and GIPC expression modulate this interaction to tune agonist trafficking. Myosin VI is differentially regulated by distinct GPCR C-tails, suggesting a mechanism to shape spatiotemporal signaling profiles in different ligand and physiological contexts. Our biophysical and structural insights may advance orthogonal therapeutic strategies for targeting GPCRs through cytoskeletal motor proteins.

A Structural Mechanism for Noncanonical GPCR Signal Transduction in the Hedgehog Pathway

The Hedgehog (Hh) signaling pathway is fundamental to embryogenesis, tissue homeostasis, and cancer. Hh signals are transduced via an unusual mechanism: upon agonist-induced phosphorylation, the noncanonical G protein-coupled receptor SMOOTHENED (SMO) binds the catalytic subunit of protein kinase A (PKA-C) and physically blocks its enzymatic activity. By combining computational structural approaches with biochemical and functional studies, we show that SMO mimics strategies prevalent in canonical GPCR and PKA signaling complexes, despite little sequence or secondary structural homology. An intrinsically disordered region of SMO binds the PKA-C active site, resembling the PKA regulatory subunit (PKA-R) / PKA-C holoenzyme, while the SMO transmembrane domain binds a conserved PKA-C interaction hub, similar to other GPCR-effector complexes. In contrast with prevailing GPCR signal transduction models, phosphorylation of SMO promotes intramolecular electrostatic interactions that stabilize key structural elements within the SMO cytoplasmic domain, thereby remodeling it into a PKA-inhibiting conformation. Our work provides a structural mechanism for a central step in the Hh cascade and defines a paradigm for disordered GPCR domains to transmit signals intracellularly.

Mechanistic insights into G-protein coupling with an agonist-bound G-protein-coupled receptor

G-protein-coupled receptors (GPCRs) activate heterotrimeric G proteins by promoting guanine nucleotide exchange. Here, we investigate the coupling of G proteins with GPCRs and describe the events that ultimately lead to the ejection of GDP from its binding pocket in the Gα subunit, the rate-limiting step during G-protein activation. Using molecular dynamics simulations, we investigate the temporal progression of structural rearrangements of GDP-bound Gs protein (Gs·GDP; hereafter GsGDP) upon coupling to the β2-adrenergic receptor (β2AR) in atomic detail. The binding of GsGDP to the β2AR is followed by long-range allosteric effects that significantly reduce the energy needed for GDP release: the opening of α1-αF helices, the displacement of the αG helix and the opening of the α-helical domain. Signal propagation to the Gs occurs through an extended receptor interface, including a lysine-rich motif at the intracellular end of a kinked transmembrane helix 6, which was confirmed by site-directed mutagenesis and functional assays. From this β2AR-GsGDP intermediate, Gs undergoes an in-plane rotation along the receptor axis to approach the β2AR-Gsempty state. The simulations shed light on how the structural elements at the receptor-G-protein interface may interact to transmit the signal over 30 Å to the nucleotide-binding site. Our analysis extends the current limited view of nucleotide-free snapshots to include additional states and structural features responsible for signaling and G-protein coupling specificity.

Deep learning based local feature classification to automatically identify single molecule fluorescence events

Long-term single-molecule fluorescence measurements are widely used powerful tools to study the conformational dynamics of biomolecules in real time to further elucidate their conformational dynamics. Typically, thousands or even more single-molecule traces are analyzed to provide statistically meaningful information, which is labor-intensive and can introduce user bias. Recently, several deep-learning models have been developed to automatically classify single-molecule traces. In this study, we introduce DEBRIS (Deep lEarning Based fRagmentatIon approach for Single-molecule fluorescence event identification), a deep-learning model focusing on classifying local features and capable of automatically identifying steady fluorescence signals and dynamically emerging signals of different patterns. DEBRIS efficiently and accurately identifies both one-color and two-color single-molecule events, including their start and end points. By adjusting user-defined criteria, DEBRIS becomes the pioneer in using a deep learning model to accurately classify four different types of single-molecule fluorescence events using the same trained model, demonstrating its universality and ability to enrich the current toolbox.

Conformational dynamics underlying atypical chemokine receptor 3 activation

Atypical Chemokine Receptor 3 (ACKR3) belongs to the G protein-coupled receptor family but it does not signal through G proteins. The structural properties that govern the functional selectivity and the conformational dynamics of ACKR3 activation are poorly understood. Here, we combined hydrogen/deuterium exchange mass spectrometry, site-directed mutagenesis, and molecular dynamics simulations to examine the binding mode and mechanism of action of ACKR3 ligands of different efficacies. Our results show that activation or inhibition of ACKR3 is governed by intracellular conformational changes of helix 6, intracellular loop 2, and helix 7, while the DRY motif becomes protected during both processes. Moreover, we identified the binding sites and the allosteric modulation of ACKR3 upon β-arrestin 1 binding. In summary, this study highlights the structure-function relationship of small ligands, the binding mode of β-arrestin 1, the activation dynamics, and the atypical dynamic features in ACKR3 that may contribute to its inability to activate G proteins.

Using the sympathetic system, beta blockers and alpha-2 agonists, to address acute respiratory distress syndrome

Acute Respiratory Distress Syndrome (ARDS) manifests as an acute inflammatory lung injury characterized by persistent hypoxemia, featuring a swift onset, high mortality, and predominantly supportive care as the current therapeutic approach, while effective treatments remain an area of active investigation. Adrenergic receptors (AR) play a pivotal role as stress hormone receptors, extensively participating in various inflammatory processes by initiating downstream signaling pathways. Advancements in molecular biology and pharmacology continually unveil the physiological significance of distinct AR subtypes. Interventions targeting these subtypes have the potential to induce specific alterations in cellular and organismal functions, presenting a promising avenue as a therapeutic target for managing ARDS. This article elucidates the pathogenesis of ARDS and the basic structure and function of AR. It also explores the relationship between AR and ARDS from the perspective of different AR subtypes, aiming to provide new insights for the improvement of ARDS.

Control of G protein-coupled receptor function via membrane-interacting intrinsically disordered C-terminal domains

G protein-coupled receptors (GPCRs) control intracellular signaling cascades via agonist-dependent coupling to intracellular transducers including heterotrimeric G proteins, GPCR kinases (GRKs), and arrestins. In addition to their critical interactions with the transmembrane core of active GPCRs, all three classes of transducers have also been reported to interact with receptor C-terminal domains (CTDs). An underexplored aspect of GPCR CTDs is their possible role as lipid sensors given their proximity to the membrane. CTD-membrane interactions have the potential to control the accessibility of key regulatory CTD residues to downstream effectors and transducers. Here, we report that the CTDs of two closely related family C GPCRs, metabotropic glutamate receptor 2 (mGluR2) and mGluR3, bind to membranes and that this interaction can regulate receptor function. We first characterize CTD structure with NMR spectroscopy, revealing lipid composition-dependent modes of membrane binding. Using molecular dynamics simulations and structure-guided mutagenesis, we then identify key conserved residues and cancer-associated mutations that modulate CTD-membrane binding. Finally, we provide evidence that mGluR3 transducer coupling is controlled by CTD-membrane interactions in live cells, which may be subject to regulation by CTD phosphorylation and changes in membrane composition. This work reveals an additional mechanism of GPCR modulation, suggesting that CTD-membrane binding may be a general regulatory mode throughout the broad GPCR superfamily.

Single-Molecule Imaging of Integral Membrane Protein Dynamics and Function

Integral membrane proteins (IMPs) play central roles in cellular physiology and represent the majority of known drug targets. Single-molecule fluorescence and fluorescence resonance energy transfer (FRET) methods have recently emerged as valuable tools for investigating structure-function relationships in IMPs. This review focuses on the practical foundations required for examining polytopic IMP function using single-molecule FRET (smFRET) and provides an overview of the technical and conceptual frameworks emerging from this area of investigation. In this context, we highlight the utility of smFRET methods to reveal transient conformational states critical to IMP function and the use of smFRET data to guide structural and drug mechanism-of-action investigations. We also identify frontiers where progress is likely to be paramount to advancing the field.

Cellular lipids regulate the conformational ensembles of the disordered intracellular loop 3 in β2-adrenergic receptor

The intracellular loops of G protein-coupled receptors (GPCRs) have been shown to play a key role in G protein coupling and selectivity. We recently showed that the intrinsically disordered third intracellular loop (ICL3) of β2-adrenergic receptor is dynamic and equilibrates between open and closed conformations to regulate the G protein coupling. In this study, using the extensive molecular dynamics simulations in multi-lipid bilayer models, we show that the lipid phosphatidylinositol 4,5-bisphosphate (PIP2) stabilizes the active state of β2-adrenergic receptor by keeping ICL3 in an open conformation. This stabilization results in a tilt of the receptor within the membrane. Additionally, the ganglioside lipid, GM3 interacts with extracellular loops, impacting the ligand binding site allosterically. This demonstrates the active role of the chemistry of lipids in stabilizing specific GPCR conformations.

Generation of Comprehensive GPCR-Transducer-Deficient Cell Lines to Dissect the Complexity of GPCR Signaling

G-protein-coupled receptors (GPCRs) compose the largest family of transmembrane receptors and are targets of approximately one-third of Food and Drug Administration-approved drugs owing to their involvement in almost all physiologic processes. GPCR signaling occurs through the activation of heterotrimeric G-protein complexes and β-arrestins, both of which serve as transducers, resulting in distinct cellular responses. Despite seeming simple at first glance, accumulating evidence indicates that activation of either transducer is not a straightforward process as a stimulation of a single molecule has the potential to activate multiple signaling branches. The complexity of GPCR signaling arises from the aspects of G-protein-coupling selectivity, biased signaling, interpathway crosstalk, and variable molecular modifications generating these diverse signaling patterns. Numerous questions relative to these aspects of signaling remained unanswered until the recent development of CRISPR genome-editing technology. Such genome editing technology presents opportunities to chronically eliminate the expression of G-protein subunits, β-arrestins, G-protein-coupled receptor kinases (GRKs), and many other signaling nodes in the GPCR pathways at one's convenience. Here, we review the practicality of using CRISPR-derived knockout (KO) cells in the experimental contexts of unraveling the molecular details of GPCR signaling mechanisms. To mention a few, KO cells have revealed the contribution of β-arrestins in ERK activation, Gα protein selectivity, GRK-based regulation of GPCRs, and many more, hence validating its broad applicability in GPCR studies. SIGNIFICANCE STATEMENT: This review emphasizes the practical application of G-protein-coupled receptor (GPCR) transducer knockout (KO) cells in dissecting the intricate regulatory mechanisms of the GPCR signaling network. Currently available cell lines, along with accumulating KO cell lines in diverse cell types, offer valuable resources for systematically elucidating GPCR signaling regulation. Given the association of GPCR signaling with numerous diseases, uncovering the system-based signaling map is crucial for advancing the development of novel drugs targeting specific diseases.

The M3 Muscarinic Acetylcholine Receptor Can Signal through Multiple G Protein Families

The M3 muscarinic acetylcholine receptor (M3R) is a G protein-coupled receptor (GPCR) that regulates important physiologic processes, including vascular tone, bronchoconstriction, and insulin secretion. It is expressed on a wide variety of cell types, including pancreatic beta, smooth muscle, neuronal, and immune cells. Agonist binding to the M3R is thought to initiate intracellular signaling events primarily through the heterotrimeric G protein Gq. However, reports differ on the ability of M3R to couple to other G proteins beyond Gq. Using members from the four primary G protein families (Gq, Gi, Gs, and G13) in radioligand binding, GTP turnover experiments, and cellular signaling assays, including live cell G protein dissociation and second messenger assessment of cAMP and inositol trisphosphate, we show that other G protein families, particularly Gi and Gs, can also interact with the human M3R. We further show that these interactions are productive as assessed by amplification of classic second messenger signaling events. Our findings demonstrate that the M3R is more promiscuous with respect to G protein interactions than previously appreciated. SIGNIFICANCE STATEMENT: The study reveals that the human M3 muscarinic acetylcholine receptor (M3R), known for its pivotal roles in diverse physiological processes, not only activates intracellular signaling via Gq as previously known but also functionally interacts with other G protein families such as Gi and Gs, expanding our understanding of its versatility in mediating cellular responses. These findings signify a broader and more complex regulatory network governed by M3R and have implications for therapeutic targeting.

Therapeutic potentials of nonpeptidic V2R agonists for partial cNDI-causing V2R mutants

Loss-of-function mutations in the type 2 vasopressin receptor (V2R) are a major cause of congenital nephrogenic diabetes insipidus (cNDI). In the context of partial cNDI, the response to desmopressin (dDAVP) is partially, but not entirely, diminished. For those with the partial cNDI, restoration of V2R function would offer a prospective therapeutic approach. In this study, we revealed that OPC-51803 (OPC5) and its structurally related V2R agonists could functionally restore V2R mutants causing partial cNDI by inducing prolonged signal activation. The OPC5-related agonists exhibited functional selectivity by inducing signaling through the Gs-cAMP pathway while not recruiting β-arrestin1/2. We found that six cNDI-related V2R partial mutants (V882.53M, Y1283.41S, L1614.47P, T2736.37M, S3298.47R and S3338.51del) displayed varying degrees of plasma membrane expression levels and exhibited moderately impaired signaling function. Several OPC5-related agonists induced higher cAMP responses than AVP at V2R mutants after prolonged agonist stimulation, suggesting their potential effectiveness in compensating impaired V2R-mediated function. Furthermore, docking analysis revealed that the differential interaction of agonists with L3127.40 caused altered coordination of TM7, potentially contributing to the functional selectivity of signaling. These findings suggest that nonpeptide V2R agonists could hold promise as potential drug candidates for addressing partial cNDI.

Control of G protein-coupled receptor function via membrane-interacting intrinsically disordered C-terminal domains

G protein-coupled receptors (GPCRs) control intracellular signaling cascades via agonist-dependent coupling to intracellular transducers including heterotrimeric G proteins, GPCR kinases (GRKs), and arrestins. In addition to their critical interactions with the transmembrane core of active GPCRs, all three classes of transducers have also been reported to interact with receptor C-terminal domains (CTDs). An underexplored aspect of GPCR CTDs is their possible role as lipid sensors given their proximity to the membrane. CTD-membrane interactions have the potential to control the accessibility of key regulatory CTD residues to downstream effectors and transducers. Here we report that the CTDs of two closely related family C GPCRs, metabotropic glutamate receptor 2 (mGluR2) and mGluR3, bind to membranes and that this interaction can regulate receptor function. We first characterize CTD structure with NMR spectroscopy, revealing lipid composition-dependent modes of membrane binding. Using molecular dynamics simulations and structure-guided mutagenesis, we then identify key conserved residues and cancer-associated mutations that modulate CTD-membrane binding. Finally, we provide evidence that mGluR3 transducer coupling is controlled by CTD-membrane interactions in live cells, which may be subject to regulation by CTD phosphorylation and changes in membrane composition. This work reveals a novel mechanism of GPCR modulation, suggesting that CTD-membrane binding may be a general regulatory mode throughout the broad GPCR superfamily.

Ligand efficacy modulates conformational dynamics of the µ-opioid receptor

The µ-opioid receptor (µOR) is an important target for pain management1 and molecular understanding of drug action on µOR will facilitate the development of better therapeutics. Here we show, using double electron-electron resonance and single-molecule fluorescence resonance energy transfer, how ligand-specific conformational changes of µOR translate into a broad range of intrinsic efficacies at the transducer level. We identify several conformations of the cytoplasmic face of the receptor that interconvert on different timescales, including a pre-activated conformation that is capable of G-protein binding, and a fully activated conformation that markedly reduces GDP affinity within the ternary complex. Interaction of β-arrestin-1 with the μOR core binding site appears less specific and occurs with much lower affinity than binding of Gi.

Dynamics and interactions of intrinsically disordered proteins

Intrinsically disordered proteins (IDPs) are widespread in eukaryotes and participate in a variety of important cellular processes. Numerous studies using state-of-the-art experimental and theoretical methods have advanced our understanding of IDPs and revealed that disordered regions engage in a large repertoire of intra- and intermolecular interactions through their conformational dynamics, thereby regulating many intracellular functions in concert with folded domains. The mechanisms by which IDPs interact with their partners are diverse, depending on their conformational propensities, and include induced fit, conformational selection, and their mixtures. In addition, IDPs are implicated in many diseases, and progress has been made in designing inhibitors of IDP-mediated interactions. Here we review these recent advances with a focus on the dynamics and interactions of IDPs.

Role of G protein-coupled receptor kinases (GRKs) in β2 -adrenoceptor-mediated glucose uptake

Truncation of the C-terminal tail of the β2 -AR, transfection of βARKct or over-expression of a kinase-dead GRK mutant reduces isoprenaline-stimulated glucose uptake, indicating that GRK is important for this response. We explored whether phosphorylation of the β2 -AR by GRK2 has a role in glucose uptake or if this response is related to the role of GRK2 as a scaffolding protein. CHO-GLUT4myc cells expressing wild-type and mutant β2 -ARs were generated and receptor affinity for [3 H]-CGP12177A and density of binding sites determined together with the affinity of isoprenaline and BRL37344. Following receptor activation by β2 -AR agonists, cAMP accumulation, GLUT4 translocation, [3 H]-2-deoxyglucose uptake, and β2 -AR internalization were measured. Bioluminescence resonance energy transfer was used to investigate interactions between β2 -AR and β-arrestin2 or between β2 -AR and GRK2. Glucose uptake after siRNA knockdown or GRK inhibitors was measured in response to β2 -AR agonists. BRL37344 was a poor partial agonist for cAMP generation but displayed similar potency and efficacy to isoprenaline for glucose uptake and GLUT4 translocation. These responses to β2 -AR agonists occurred in CHO-GLUT4myc cells expressing β2 -ARs lacking GRK or GRK/PKA phosphorylation sites as well as in cells expressing the wild-type β2 -AR. However, β2 -ARs lacking phosphorylation sites failed to recruit β-arrestin2 and did not internalize. GRK2 knock-down or GRK2 inhibitors decreased isoprenaline-stimulated glucose uptake in rat L6 skeletal muscle cells. Thus, GRK phosphorylation of the β2 -AR is not associated with isoprenaline- or BRL37344-stimulated glucose uptake. However, GRKs acting as scaffold proteins are important for glucose uptake as GRK2 knock-down or GRK2 inhibition reduces isoprenaline-stimulated glucose uptake.

Cellular Lipids Regulate the Conformational Ensembles of the Disordered Intracellular Loop 3 in β2 Adrenergic Receptor

The structurally disordered intracellular loops (ICLs) of G protein-coupled receptors (GPCRs) play a critical role in G protein coupling. In our previous work, we used a combination of FRET-based and computational methodologies to show that the third intracellular loop (ICL3) modulates the activity and G protein coupling selectivity in GPCRs. In the current study, we have uncovered the role of several lipid components in modulating the conformational ensemble of ICL3 of the β2-adrenergic receptor (β2AR). Our findings indicate that phosphatidylinositol 4,5-bisphosphate (PIP2) in the inner leaflet of the membrane bilayer acts as a stabilizing anchor for ICL3, opening the intracellular cavity to facilitate G protein coupling. This interaction between PIP2 and ICL3 causes tilting of β2AR within the cellular membrane. Notably, this tilting of the receptor is supported by ganglioside GM3 stabilizing the extracellular loops on the outer leaflet of the bilayer, thereby exerting an allosteric effect on the orthosteric ligand binding pocket. Our results underscore the significance of lipids in modulating GPCR activity, proposing an allosteric mechanism that occurs through the receptor's orientation within the membrane.

Unraveling the Functional Significance of Unstructured Regions in G Protein-Coupled Receptors

Unstructured regions in functional proteins have gained attention in recent years due to advancements in informatics tools and biophysical methods. G protein-coupled receptors (GPCRs), a large family of cell surface receptors, contain unstructured regions in the form of the i3 loop and C-terminus. This review provides an overview of the functional significance of these regions in GPCRs. GPCRs transmit signals from the extracellular environment to the cell interior, regulating various physiological processes. The i3 loop, located between the fifth and sixth transmembrane helices, and the C-terminus, connected to the seventh transmembrane helix, are determinant of interactions with G proteins and with other intracellular partners such as arrestins. Recent studies demonstrate that the i3 loop and C-terminus play critical roles in allosterically regulating GPCR activation. They can act as autoregulators, adopting conformations that, by restricting G protein access, modulate receptor coupling specificity. The length and unstructured nature of the i3 loop and C-terminus provide unique advantages in GPCR interactions with intracellular protein partners. They act as "fishing lines", expanding the radius of interaction and enabling GPCRs to tether scaffolding proteins, thus facilitating receptor stability during cell membrane movements. Additionally, the i3 loop may be involved in domain swapping between GPCRs, generating novel receptor dimers with distinct binding and coupling characteristics. Overall, the i3 loop and C-terminus are now widely recognized as crucial elements in GPCR function and regulation. Understanding their functional roles enhances our comprehension of GPCR structure and signaling complexity and holds promise for advancements in receptor pharmacology and drug development.

The third intracellular loop of GPCRs: size matters

The third intracellular loop of G-protein-coupled receptors (GPCRs) shows remarkable diversity in sequence and overall length. Sadler and colleagues recently demonstrated that this domain acts as an 'autoregulator' of receptor activity and that its length contributes to receptor/G-protein coupling selectivity. These observations may prove useful for developing novel therapeutics.

Highly-Sensitive In Vitro Bioassays for FSH, TSH, PTH, Kp, and OT in Addition to LH in Mouse Leydig Tumor Cell

We demonstrate here that highly sensitive in vitro bioassays for FSH, TSH, and PTH can be set up in mouse Leydig Tumor Cells (mLTC), in addition to the normal LH/CG bioassay, after they were transfected with expression vectors encoding the corresponding Gs Protein-Coupled Receptors (GsPCR), such as FSHR, TSHR, or PTHR. Although the β2 adrenergic receptor is also a GsPCR, its expression in mLTC led to a significant but very low cAMP response compared to those observed with FSH, TSH, or PTH. Similarly, after transfection of the GiPCR MT1 melatonin receptor, we did not observe any inhibitory effect by melatonin of the LH or hCG stimulation. Interestingly, after transfection of mLTC with the human kisspeptin receptor (hKpR), which is a GqPCR, we observed a dose-dependent synergy of 10-12-10-7 M kisspeptin variants with a fixed concentration of 0.3 nM LH or hCG. Without any exogenous receptor transfection, a 2 h preincubation with OT or AVP led to a dose-dependent cAMP response to a fixed dose of LH or hCG. Therefore, highly sensitive in vitro bioassays for various hormones and other GPCR ligands can be set up in mLTC to measure circulating concentrations in only 3-10 µL of blood or other body fluids. Nevertheless, the development of an LHRKO mLTC cell line will be mandatory to obtain strict specificity for these bioassays to eliminate potential cross-reaction with LH or CG.