Smart parallel automated cryo-electron tomography.

In situ cryo-electron tomography enables investigation of macromolecules in their native cellular environment. Samples have become more readily available owing to recent software and hardware advancements. Data collection, however, still requires an experienced operator and appreciable microscope time to carefully select targets for high-throughput tilt series acquisition. Here, we developed smart parallel automated cryo-electron tomography (SPACEtomo), a workflow using machine learning approaches to fully automate the entire cryo-electron tomography process, including lamella detection, biological feature segmentation, target selection and parallel tilt series acquisition, all without the need for human intervention. This degree of automation will be essential for obtaining statistically relevant datasets and high-resolution structures of macromolecules in their native context.

Cryo-EM Structure of the Human Amylin 1 Receptor in Complex with CGRP and Gs Protein.

Inhibition of calcitonin gene-related peptide (CGRP) or its cognate CGRP receptor (CGRPR) has arisen as a major breakthrough in the treatment of migraine. However, a second CGRP-responsive receptor exists, the amylin (Amy) 1 receptor (AMY1R), yet its involvement in the pathology of migraine is poorly understood. AMY1R and CGRPR are heterodimers consisting of receptor activity-modifying protein 1 (RAMP1) with the calcitonin receptor (CTR) and the calcitonin receptor-like receptor (CLR), respectively. Here, we present the structure of AMY1R in complex with CGRP and Gs protein and compare it with the reported structures of the AMY1R complex with rat amylin (rAmy) and the CGRPR in complex with CGRP. Despite similar protein backbones observed within the receptors and the N- and C-termini of the two peptides bound to the AMY1R complexes, they have distinct organization in the peptide midregions (the bypass motif) that is correlated with differences in the dynamics of the respective receptor extracellular domains. Moreover, divergent conformations of extracellular loop (ECL) 3, intracellular loop (ICL) 2, and ICL3 within the CTR and CLR protomers are evident when comparing the CGRP bound to the CGRPR and AMY1R, which influences the binding mode of CGRP. However, the conserved interactions made by the C-terminus of CGRP to the CGRPR and AMY1R are likely to account for cross-reactivity of nonpeptide CGRPR antagonists observed at AMY1R, which also extends to other clinically used CGRPR blockers, including antibodies.

Community recommendations on cryoEM data archiving and validation.

In January 2020, a workshop was held at EMBL-EBI (Hinxton, UK) to discuss data requirements for the deposition and validation of cryoEM structures, with a focus on single-particle analysis. The meeting was attended by 47 experts in data processing, model building and refinement, validation, and archiving of such structures. This report describes the workshop's motivation and history, the topics discussed, and the resulting consensus recommendations. Some challenges for future methods-development efforts in this area are also highlighted, as is the implementation to date of some of the recommendations.

Structural insight into selectivity of amylin and calcitonin receptor agonists.

Amylin receptors (AMYRs), heterodimers of the calcitonin receptor (CTR) and one of three receptor activity-modifying proteins, are promising obesity targets. A hallmark of AMYR activation by Amy is the formation of a 'bypass' secondary structural motif (residues S19-P25). This study explored potential tuning of peptide selectivity through modification to residues 19-22, resulting in a selective AMYR agonist, San385, as well as nonselective dual amylin and calcitonin receptor agonists (DACRAs), with San45 being an exemplar. We determined the structure and dynamics of San385-bound AMY3R, and San45 bound to AMY3R or CTR. San45, via its conjugated lipid at position 21, was anchored at the edge of the receptor bundle, enabling a stable, alternative binding mode when bound to the CTR, in addition to the bypass mode of binding to AMY3R. Targeted lipid modification may provide a single intervention strategy for design of long-acting, nonselective, Amy-based DACRAs with potential anti-obesity effects.

Community recommendations on cryoEM data archiving and validation: Outcomes of a wwPDB/EMDB workshop on cryoEM data management, deposition and validation.

In January 2020, a workshop was held at EMBL-EBI (Hinxton, UK) to discuss data requirements for deposition and validation of cryoEM structures, with a focus on single-particle analysis. The meeting was attended by 47 experts in data processing, model building and refinement, validation, and archiving of such structures. This report describes the workshop's motivation and history, the topics discussed, and consensus recommendations resulting from the workshop. Some challenges for future methods-development efforts in this area are also highlighted, as is the implementation to date of some of the recommendations.

Xanomeline displays concomitant orthosteric and allosteric binding modes at the M4 mAChR.

The M4 muscarinic acetylcholine receptor (M4 mAChR) has emerged as a drug target of high therapeutic interest due to its expression in regions of the brain involved in the regulation of psychosis, cognition, and addiction. The mAChR agonist, xanomeline, has provided significant improvement in the Positive and Negative Symptom Scale (PANSS) scores in a Phase II clinical trial for the treatment of patients suffering from schizophrenia. Here we report the active state cryo-EM structure of xanomeline bound to the human M4 mAChR in complex with the heterotrimeric Gi1 transducer protein. Unexpectedly, two molecules of xanomeline were found to concomitantly bind to the monomeric M4 mAChR, with one molecule bound in the orthosteric (acetylcholine-binding) site and a second molecule in an extracellular vestibular allosteric site. Molecular dynamic simulations supports the structural findings, and pharmacological validation confirmed that xanomeline acts as a dual orthosteric and allosteric ligand at the human M4 mAChR. These findings provide a basis for further understanding xanomeline's complex pharmacology and highlight the myriad of ways through which clinically relevant ligands can bind to and regulate GPCRs.

Pharmacological hallmarks of allostery at the M4 muscarinic receptor elucidated through structure and dynamics.

Allosteric modulation of G protein-coupled receptors (GPCRs) is a major paradigm in drug discovery. Despite decades of research, a molecular-level understanding of the general principles that govern the myriad pharmacological effects exerted by GPCR allosteric modulators remains limited. The M4 muscarinic acetylcholine receptor (M4 mAChR) is a validated and clinically relevant allosteric drug target for several major psychiatric and cognitive disorders. In this study, we rigorously quantified the affinity, efficacy, and magnitude of modulation of two different positive allosteric modulators, LY2033298 (LY298) and VU0467154 (VU154), combined with the endogenous agonist acetylcholine (ACh) or the high-affinity agonist iperoxo (Ipx), at the human M4 mAChR. By determining the cryo-electron microscopy structures of the M4 mAChR, bound to a cognate Gi1 protein and in complex with ACh, Ipx, LY298-Ipx, and VU154-Ipx, and applying molecular dynamics simulations, we determine key molecular mechanisms underlying allosteric pharmacology. In addition to delineating the contribution of spatially distinct binding sites on observed pharmacology, our findings also revealed a vital role for orthosteric and allosteric ligand-receptor-transducer complex stability, mediated by conformational dynamics between these sites, in the ultimate determination of affinity, efficacy, cooperativity, probe dependence, and species variability. There results provide a holistic framework for further GPCR mechanistic studies and can aid in the discovery and design of future allosteric drugs.

Parallel cryo electron tomography on in situ lamellae.

In situ cryo electron tomography of cryo focused ion beam milled samples has emerged in recent years as a powerful technique for structural studies of macromolecular complexes in their native cellular environment. However, the possibilities for recording tomographic tilt series in a high-throughput manner are limited, in part by the lamella-shaped samples. Here we utilize a geometrical sample model and optical image shift to record tens of tilt series in parallel, thereby saving time and gaining access to sample areas conventionally used for tracking specimen movement. The parallel cryo electron tomography (PACE-tomo) method achieves a throughput faster than 5 min per tilt series and allows for the collection of sample areas that were previously unreachable, thus maximizing the amount of data from each lamella. Performance testing with ribosomes in vitro and in situ on state-of-the-art and general-purpose microscopes demonstrated the high throughput and quality of PACE-tomo.

Understanding VPAC receptor family peptide binding and selectivity.

The vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating polypeptide (PACAP) receptors are key regulators of neurological processes. Despite recent structural data, a comprehensive understanding of peptide binding and selectivity among different subfamily receptors is lacking. Here, we determine structures of active, Gs-coupled, VIP-VPAC1R, PACAP27-VPAC1R, and PACAP27-PAC1R complexes. Cryo-EM structural analyses and molecular dynamics simulations (MDSs) reveal fewer stable interactions between VPAC1R and VIP than for PACAP27, more extensive dynamics of VIP interaction with extracellular loop 3, and receptor-dependent differences in interactions of conserved N-terminal peptide residues with the receptor core. MD of VIP modelled into PAC1R predicts more transient VIP-PAC1R interactions in the receptor core, compared to VIP-VPAC1R, which may underlie the selectivity of VIP for VPAC1R over PAC1R. Collectively, our work improves molecular understanding of peptide engagement with the PAC1R and VPAC1R that may benefit the development of novel selective agonists.

A structural basis for amylin receptor phenotype.

Amylin receptors (AMYRs) are heterodimers of the calcitonin (CT) receptor (CTR) and one of three receptor activity-modifying proteins (RAMPs), AMY1R, AMY2R, and AMY3R. Selective AMYR agonists and dual AMYR/CTR agonists are being developed as obesity treatments; however, the molecular basis for peptide binding and selectivity is unknown. We determined the structure and dynamics of active AMYRs with amylin, AMY1R with salmon CT (sCT), AMY2R with sCT or human CT (hCT), and CTR with amylin, sCT, or hCT. The conformation of amylin-bound complexes was similar for all AMYRs, constrained by the RAMP, and an ordered midpeptide motif that we call the bypass motif. The CT-bound AMYR complexes were distinct, overlapping the CT-bound CTR complexes. Our findings indicate that activation of AMYRs by CT-based peptides is distinct from their activation by amylin-based peptides. This has important implications for the development of AMYR therapeutics.

Dynamics of GLP-1R peptide agonist engagement are correlated with kinetics of G protein activation.

The glucagon-like peptide-1 receptor (GLP-1R) has broad physiological roles and is a validated target for treatment of metabolic disorders. Despite recent advances in GLP-1R structure elucidation, detailed mechanistic understanding of how different peptides generate profound differences in G protein-mediated signalling is still lacking. Here we combine cryo-electron microscopy, molecular dynamics simulations, receptor mutagenesis and pharmacological assays, to interrogate the mechanism and consequences of GLP-1R binding to four peptide agonists; glucagon-like peptide-1, oxyntomodulin, exendin-4 and exendin-P5. These data reveal that distinctions in peptide N-terminal interactions and dynamics with the GLP-1R transmembrane domain are reciprocally associated with differences in the allosteric coupling to G proteins. In particular, transient interactions with residues at the base of the binding cavity correlate with enhanced kinetics for G protein activation, providing a rationale for differences in G protein-mediated signalling efficacy from distinct agonists.

Structural and functional diversity among agonist-bound states of the GLP-1 receptor.

Recent advances in G-protein-coupled receptor (GPCR) structural elucidation have strengthened previous hypotheses that multidimensional signal propagation mediated by these receptors depends, in part, on their conformational mobility; however, the relationship between receptor function and static structures is inherently uncertain. Here, we examine the contribution of peptide agonist conformational plasticity to activation of the glucagon-like peptide 1 receptor (GLP-1R), an important clinical target. We use variants of the peptides GLP-1 and exendin-4 (Ex4) to explore the interplay between helical propensity near the agonist N terminus and the ability to bind to and activate the receptor. Cryo-EM analysis of a complex involving an Ex4 analog, the GLP-1R and Gs heterotrimer revealed two receptor conformers with distinct modes of peptide-receptor engagement. Our functional and structural data, along with molecular dynamics (MD) simulations, suggest that receptor conformational dynamics associated with flexibility of the peptide N-terminal activation domain may be a key determinant of agonist efficacy.

Positive allosteric mechanisms of adenosine A1 receptor-mediated analgesia.

The adenosine A1 receptor (A1R) is a promising therapeutic target for non-opioid analgesic agents to treat neuropathic pain1,2. However, development of analgesic orthosteric A1R agonists has failed because of a lack of sufficient on-target selectivity as well as off-tissue adverse effects3. Here we show that [2-amino-4-(3,5-bis(trifluoromethyl)phenyl)thiophen-3-yl)(4-chlorophenyl)methanone] (MIPS521), a positive allosteric modulator of the A1R, exhibits analgesic efficacy in rats in vivo through modulation of the increased levels of endogenous adenosine that occur in the spinal cord of rats with neuropathic pain. We also report the structure of the A1R co-bound to adenosine, MIPS521 and a Gi2 heterotrimer, revealing an extrahelical lipid-detergent-facing allosteric binding pocket that involves transmembrane helixes 1, 6 and 7. Molecular dynamics simulations and ligand kinetic binding experiments support a mechanism whereby MIPS521 stabilizes the adenosine-receptor-G protein complex. This study provides proof of concept for structure-based allosteric drug design of non-opioid analgesic agents that are specific to disease contexts.

Cryo-EM structure of the dual incretin receptor agonist, peptide-19, in complex with the glucagon-like peptide-1 receptor.

Dual agonists that can activate both the glucagon-like peptide-1 receptor (GLP-1R) and the gastric inhibitory polypeptide receptor (GIPR) have demonstrated high efficacy for the treatment of metabolic disease. Peptide-19 is a prototypical dual agonist that has high potency at both GLP-1R and GIPR but has a distinct signalling profile relative to the native peptides at the cognate receptors. In this study, we solved the structure of peptide-19 bound to the GLP-1R in complex with Gs protein, and compared the structure and dynamics of this complex to that of published structures of GLP-1R:Gs in complex with other receptor agonists. Unlike other peptide-bound receptor complexes, peptide-19:GLP-1R:Gs demonstrated a more open binding pocket where transmembrane domain (TM) 6, TM7 and the interconnecting extracellular loop 3 (ECL3) were located away from the peptide, with no interactions between peptide-19 and TM6/ECL3. Analysis of conformational variance of the complex revealed that peptide-19 was highly dynamic and underwent binding and unbinding motions facilitated by the more open TM binding pocket. Both the consensus structure of the GLP-1R complex with peptide-19 and the dynamics of this complex were distinct from previously described GLP-1R structures providing unique insights into the mode of GLP-1R activation by this dual agonist.

Routine sub-2.5 Å cryo-EM structure determination of GPCRs.

Cryo-electron microscopy (cryo-EM) of small membrane proteins, such as G protein-coupled receptors (GPCRs), remains challenging. Pushing the performance boundaries of the technique requires quantitative knowledge about the contribution of multiple factors. Here, we present an in-depth analysis and optimization of the main experimental parameters in cryo-EM. We combined actual structural studies with methods development to quantify the effects of the Volta phase plate, zero-loss energy filtering, objective lens aperture, defocus magnitude, total exposure, and grid type. By using this information to carefully maximize the experimental performance, it is now possible to routinely determine GPCR structures at resolutions better than 2.5 Å. The improved fidelity of such maps enables the building of better atomic models and will be crucial for the future expansion of cryo-EM into the structure-based drug design domain. The optimization guidelines given here are not limited to GPCRs and can be applied directly to other small proteins.

Structure and dynamics of semaglutide- and taspoglutide-bound GLP-1R-Gs complexes.

The glucagon-like peptide-1 receptor (GLP-1R) regulates insulin secretion, carbohydrate metabolism, and appetite and is an important target for treatment of type 2 diabetes and obesity. Multiple GLP-1R agonists have entered into clinical trials, with some, such as semaglutide, progressing to approval. Others, including taspoglutide, failed due to the high incidence of side effects or insufficient efficacy. GLP-1R agonists have a broad spectrum of signaling profiles, but molecular understanding is limited by a lack of structural information on how different agonists engage with the GLP-1R. Here, we report cryoelectron microscopy (cryo-EM) structures and cryo-EM 3D variability analysis of semaglutide- and taspoglutide-bound GLP-1R-Gs protein complexes. These reveal similar peptide interactions to GLP-1 but different motions within the receptor and bound peptides, providing insights into the molecular determinants of GLP-1R peptide engagement.

Structures of the human cholecystokinin 1 (CCK1) receptor bound to Gs and Gq mimetic proteins provide insight into mechanisms of G protein selectivity.

G protein-coupled receptors (GPCRs) are critical regulators of cellular function acting via heterotrimeric G proteins as their primary transducers with individual GPCRs capable of pleiotropic coupling to multiple G proteins. Structural features governing G protein selectivity and promiscuity are currently unclear. Here, we used cryo-electron microscopy (cryo-EM) to determine structures of the cholecystokinin (CCK) type 1 receptor (CCK1R) bound to the CCK peptide agonist, CCK-8 and 2 distinct transducer proteins, its primary transducer Gq, and the more weakly coupled Gs. As seen with other Gq/11-GPCR complexes, the Gq-α5 helix (αH5) bound to a relatively narrow pocket in the CCK1R core. Surprisingly, the backbone of the CCK1R and volume of the G protein binding pocket were essentially equivalent when Gs was bound, with the Gs αH5 displaying a conformation that arises from "unwinding" of the far carboxyl-terminal residues, compared to canonically Gs coupled receptors. Thus, integrated changes in the conformations of both the receptor and G protein are likely to play critical roles in the promiscuous coupling of individual GPCRs.

Cryo-EM performance testing of hardware and data acquisition strategies.

The increasing popularity and adoption rate of cryo-electron microscopy (cryo-EM) is evidenced by a growing number of new microscope installations around the world. The quality and reliability of the instruments improved dramatically in recent years, but site-specific issues or unnoticed problems during installation could undermine productivity. Newcomers to the field may also have limited experience and/or low confidence in the capabilities of the equipment or their own skills. Therefore, it is recommended to perform an initial test of the complete cryo-EM workflow with an 'easy' test sample, such as apoferritin, before starting work with real and challenging samples. Analogous test experiments are also recommended for the quantification of new data acquisition approaches or imaging hardware. Here, we present the results from our initial tests of a recently installed Krios G4 electron microscope equipped with two latest generation direct electron detector cameras-Gatan K3 and Falcon 4. Three beam-image shift-based data acquisition strategies were also tested. We detail the methodology and discuss the critical parameters and steps for performance testing. The two cameras performed equally, and the single- and multi-shot per-hole acquisition schemes produced comparable results. We also evaluated the effects of environmental factors and optical flaws on data quality. Our results reaffirmed the exceptional performance of the software aberration correction in Relion in dealing with severe coma aberration. We hope that this work will help cryo-EM teams in their testing and troubleshooting of hardware and data collection approaches.

Evolving cryo-EM structural approaches for GPCR drug discovery.

G protein-coupled receptors (GPCRs) are the largest class of cell surface drug targets. Advances in stabilization of GPCR:transducer complexes, together with improvements in cryoelectron microscopy (cryo-EM) have recently been applied to structure-assisted drug design for GPCR agonists. Nonetheless, limitations in the commercial application of these approaches, including the use of nanobody 35 (Nb35) to aid complex stabilization and the high cost of 300 kV imaging, have restricted broad application of cryo-EM in drug discovery. Here, using the PF 06882961-bound GLP-1R as exemplar, we validated the formation of stable complexes with a modified Gs protein in the absence of Nb35. In parallel, we compare 200 versus 300 kV image acquisition using a Falcon 4 or K3 direct electron detector. Moreover, the 200 kV Glacios-Falcon 4 yielded a 3.2 Å map with clear density for bound drug and multiple structurally ordered waters. Our work paves the way for broader commercial application of cryo-EM for GPCR drug discovery.

Structure and dynamics of the CGRP receptor in apo and peptide-bound forms.

G protein-coupled receptors (GPCRs) are key regulators of information transmission between cells and organs. Despite this, we have only a limited understanding of the behavior of GPCRs in the apo state and the conformational changes upon agonist binding that lead to G protein recruitment and activation. We expressed and purified unmodified apo and peptide-bound calcitonin gene-related peptide (CGRP) receptors from insect cells to determine their cryo-electron microscopy (cryo-EM) structures, and we complemented these with analysis of protein conformational dynamics using hydrogen-deuterium exchange mass spectrometry and three-dimensional variance analysis of the cryo-EM data. Together with our previously published structure of the active, Gs-bound CGRP receptor complex, our work provides insight into the mechanisms of class B1 GPCR activation.