Class B1 GPCRs: Insights into Multi-Receptor Pharmacology for the Treatment of Metabolic Disease.

The secretin-like, class B1 sub-family of seven transmembrane-spanning G protein coupled receptors (GPCRs) consists of 15 members that coordinate important physiological processes. These receptors bind peptide ligands and utilize a distinct mechanism of activation that is driven by evolutionarily conserved structural features. For the class B1 receptors, the C-terminus of the cognate ligand is initially recognized by the receptor via a large N-terminal extracellular domain that forms a hydrophobic ligand binding groove. This binding enables the N-terminus of the ligand to engage deep into a large volume, open transmembrane pocket of the receptor. Importantly, the phylogenetic basis of this ligand-receptor activation mechanism has provided opportunities to engineer analogues of several class B1 ligands for therapeutic use. Among the most successful of these are drugs targeting the glucagon-like peptide-1 (GLP-1) receptor for the treatment of type 2 diabetes and obesity. Recently, multi-functional agonists possessing activity at the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor, such as tirzepatide, and others that also contain glucagon receptor activity, have been developed. In this article, we review members of the class B1 GPCR family with focus on receptors for GLP-1, GIP, and glucagon, including their signal transduction and receptor trafficking characteristics. The metabolic importance of these receptors is also highlighted, along with the benefit of poly-pharmacologic ligands. Further, key structural features and comparative analyses of high-resolution cryogenic electron microscopy structures for these receptors in active-state complex with either native ligands or multi-functional agonists are provided, supporting the pharmacological basis of such therapeutic agents.

Structural determinants of dual incretin receptor agonism by tirzepatide.

SignificanceTirzepatide is a dual agonist of the glucose-dependent insulinotropic polypeptide receptor (GIPR) and the glucagon-like peptide-1 receptor (GLP-1R), which are incretin receptors that regulate carbohydrate metabolism. This investigational agent has proven superior to selective GLP-1R agonists in clinical trials in subjects with type 2 diabetes mellitus. Intriguingly, although tirzepatide closely resembles native GIP in how it activates the GIPR, it differs markedly from GLP-1 in its activation of the GLP-1R, resulting in less agonist-induced receptor desensitization. We report how cryogenic electron microscopy and molecular dynamics simulations inform the structural basis for the unique pharmacology of tirzepatide. These studies reveal the extent to which fatty acid modification, combined with amino acid sequence, determines the mode of action of a multireceptor agonist.

Structure-based, multi-targeted drug discovery approach to eicosanoid inhibition: Dual inhibitors of mPGES-1 and 5-lipoxygenase activating protein (FLAP).

BACKGROUND: Due to the importance of both prostaglandins (PGs) and leukotrienes (LTs) as pro-inflammatory mediators, and the potential for eicosanoid shunting in the presence of pathway target inhibitors, we have investigated an approach to inhibiting the formation of both PGs and LTs as part of a multi-targeted drug discovery effort. METHODS: We generated ligand-protein X-ray crystal structures of known inhibitors of microsomal prostaglandin E2 synthase-1 (mPGES-1) and the 5-Lipoxygenase Activating Protein (FLAP), with their respective proteins, to understand the overlapping pharmacophores. We subsequently used molecular modeling and structure-based drug design (SBDD) to identify hybrid structures intended to inhibit both targets. RESULTS: This work enabled the preparation of compounds 4 and 5, which showed potent in vitro inhibition of both targets. SIGNIFICANCE: Our findings enhance the structural understanding of mPGES-1 and FLAP's unique ligand binding pockets and should accelerate the discovery of additional dual inhibitors for these two important integral membrane protein drug targets.

Synthetic protease-activated class B GPCRs.

G-protein coupled receptors (GPCRs) are the ligand detection machinery of a majority of extracellular signaling systems in metazoans. Novel chemical and biological tools to probe the structure-function relationships of GPCRs have impacted both basic and applied GPCR research. To better understand the structure-function of class B GPCRs, we generated receptor-ligand fusion chimeric proteins that can be activated by exogenous enzyme application. As a prototype, fusion proteins of the glucagon-like peptide-1 receptor (GLP-1R) with GLP-1(7-36) and exendin-4(1-39) peptides incorporating enterokinase-cleavable N-termini were generated. These receptors are predicted to generate fusion protein neo-epitopes upon proteolysis with enterokinase that are identical to the N-termini of GLP-1 agonists. This system was validated by measuring enterokinase-dependent GLP-1R mediated cAMP accumulation, and a structure-activity relationship for both linker length and peptide sequence was observed. Moreover, our results show this approach can be used in physiologically relevant cell systems, as GLP-1R-ligand chimeras were shown to induce glucose-dependent insulin secretion in insulinoma cells upon exposure to enterokinase. This approach suggests new strategies for understanding the structure-function of peptide-binding GPCRs.

Structural insights into probe-dependent positive allosterism of the GLP-1 receptor.

Drugs that promote the association of protein complexes are an emerging therapeutic strategy. We report discovery of a G protein-coupled receptor (GPCR) ligand that stabilizes an active state conformation by cooperatively binding both the receptor and orthosteric ligand, thereby acting as a 'molecular glue'. LSN3160440 is a positive allosteric modulator of the GLP-1R optimized to increase the affinity and efficacy of GLP-1(9-36), a proteolytic product of GLP-1(7-36). The compound enhances insulin secretion in a glucose-, ligand- and GLP-1R-dependent manner. Cryo-electron microscopy determined the structure of the GLP-1R bound to LSN3160440 in complex with GLP-1 and heterotrimeric Gs. The modulator binds high in the helical bundle at an interface between TM1 and TM2, allowing access to the peptide ligand. Pharmacological characterization showed strong probe dependence of LSN3160440 for GLP-1(9-36) versus oxyntomodulin that is driven by a single residue. Our findings expand protein-protein modulation drug discovery to uncompetitive, active state stabilizers for peptide hormone receptors.

Discovery and pharmacology of the covalent GLP-1 receptor (GLP-1R) allosteric modulator BETP: A novel tool to probe GLP-1R pharmacology.

The glucagon-like peptide-1 receptor (GLP-1R) is a significant therapeutic target for small molecule drug discovery given the therapeutic impact of peptide agonists in the diabetes sphere. We review the discovery and subsequent characterization of the small molecule GLP-1R allosteric modulator 4-(3-(Benzyloxy)phenyl)-2-(ethylsulfinyl)-6-(trifluoromethyl)pyrimidine (BETP). BETP is a covalent modulator of the GLP-1R, and we discuss the pharmacological implications and possible structural basis of this novel mode of action. We highlight the insights into class B G-protein coupled receptor pharmacology and biology provided by studies conducted with BETP. These include the descriptions of exquisite allosteric modulator probe dependence and biased signaling in vitro and in vivo. We conclude with an analysis of the utility of BETP as a chemical probe for the GLP-1R.

Structural Basis for ( S)-3,4-Dicarboxyphenylglycine (DCPG) As a Potent and Subtype Selective Agonist of the mGlu8 Receptor.

( S)-3,4-Dicarboxyphenylglycine (DCPG) was first reported in 2001 as a potent orthosteric agonist with high subtype selectivity for the mGlu8 receptor, but the structural basis for its high selectivity is not well understood. We have solved a cocrystal structure of recombinant human mGlu8 amino terminal domain (ATD) protein bound to ( S)-DCPG, which possesses the largest lobe opening angle observed to date among known agonist-bound mGlu ATD crystal structures. The binding conformation of ( S)-DCPG observed in the crystal structure is significantly different from that in the homology model built from an l-glutamate-bound rat mGlu1 ATD crystal structure, which has a smaller lobe opening angle. This highlights the importance of considering various lobe opening angles when modeling mGlu ATD-ligand complex. New homology models of other mGlu receptors based on the ( S)-DCPG-bound mGlu8 ATD crystal structure were explored to rationalize ( S)-DCPG's high mGlu8 receptor subtype selectivity.

Structural basis for GPR40 allosteric agonism and incretin stimulation.

Activation of free fatty acid receptor 1 (GPR40) by synthetic partial and full agonists occur via distinct allosteric sites. A crystal structure of GPR40-TAK-875 complex revealed the allosteric site for the partial agonist. Here we report the 2.76-Å crystal structure of human GPR40 in complex with a synthetic full agonist, compound 1, bound to the second allosteric site. Unlike TAK-875, which acts as a Gαq-coupled partial agonist, compound 1 is a dual Gαq and Gαs-coupled full agonist. compound 1 binds in the lipid-rich region of the receptor near intracellular loop 2 (ICL2), in which the stabilization of ICL2 by the ligand is likely the primary mechanism for the enhanced G protein activities. The endogenous free fatty acid (FFA), γ-linolenic acid, can be computationally modeled in this site. Both γ-linolenic acid and compound 1 exhibit positive cooperativity with TAK-875, suggesting that this site could also serve as a FFA binding site.

Determination of L-AP4-bound human mGlu8 receptor amino terminal domain structure and the molecular basis for L-AP4's group III mGlu receptor functional potency and selectivity.

L-2-Amino-4-phosphonobutyric acid (L-AP4) is a known potent and selective agonist for the Group III mGlu receptors. However, it does not show any selectivity among the individual group III mGlu subtypes. In order to understand the molecular basis for this group selectivity, we solved the first human mGlu8 amino terminal domain (ATD) crystal structures in complex with L-glu and L-AP4. In comparison with other published L-glu-bound mGlu ATD structures, we have observed L-glu binds in a significantly different manner in mGlu1. Furthermore, these new structures provided evidence that both the electronic and steric nature of the distal phosphate of L-AP4 contribute to its exquisite Group III functional agonist potency and selectivity.

3-heterocyclyl quinolone inhibitors of the HCV NS5B polymerase.

The discovery and optimization of a novel class of quinolone small-molecules that inhibit NS5B polymerase, a key enzyme of the HCV viral life-cycle, is described. Our research led to the replacement of a hydrolytically labile ester functionality with bio-isosteric heterocycles. An X-ray crystal structure of a key analog bound to NS5B facilitated the optimization of this series of compounds to afford increased activity against the target enzyme and in the cell-based replicon assay system.

Immunodominant T cell determinants of aquaporin-4, the autoantigen associated with neuromyelitis optica.

Autoantibodies that target the water channel aquaporin-4 (AQP4) in neuromyelitis optica (NMO) are IgG1, a T cell-dependent Ig subclass. However, a role for AQP4-specific T cells in this CNS inflammatory disease is not known. To evaluate their potential role in CNS autoimmunity, we have identified and characterized T cells that respond to AQP4 in C57BL/6 and SJL/J mice, two strains that are commonly studied in models of CNS inflammatory diseases. Mice were immunized with either overlapping peptides or intact hAQP4 protein encompassing the entire 323 amino acid sequence. T cell determinants identified from examination of the AQP4 peptide (p) library were located within AQP4 p21-40, p91-110, p101-120, p166-180, p231-250 and p261-280 in C57BL/6 mice, and within p11-30, p21-40, p101-120, p126-140 and p261-280 in SJL/J mice. AQP4-specific T cells were CD4+ and MHC II-restricted. In recall responses to immunization with intact AQP4, T cells responded primarily to p21-40, indicating this region contains the immunodominant T cell epitope(s) for both strains. AQP4 p21-40-primed T cells secreted both IFN-γ and IL-17. The core immunodominant AQP4 21-40 T cell determinant was mapped to residues 24-35 in C57BL/6 mice and 23-35 in SJL/J mice. Our identification of the AQP4 T cell determinants and characterization of its immunodominant determinant should permit investigators to evaluate the role of AQP4-specific T cells in vivo and to develop AQP4-targeted murine NMO models.

Function of human Rh based on structure of RhCG at 2.1 A.

In humans, NH(3) transport across cell membranes is facilitated by the Rh (rhesus) family of proteins. Human Rh C glycoprotein (RhCG) forms a trimeric complex that plays an essential role in ammonia excretion and renal pH regulation. The X-ray crystallographic structure of human RhCG, determined at 2.1 A resolution, reveals the mechanism of ammonia transport. Each monomer contains 12 transmembrane helices, one more than in the bacterial homologs. Reconstituted into proteoliposomes, RhCG conducts NH(3) to raise internal pH. Models of the erythrocyte Rh complex based on our RhCG structure suggest that the erythrocytic Rh complex is composed of stochastically assembled heterotrimers of RhAG, RhD, and RhCE.

A general protocol for the crystallization of membrane proteins for X-ray structural investigation.

Protein crystallography is used to generate atomic resolution structures of protein molecules. These structures provide information about biological function, mechanism and interaction of a protein with substrates or effectors including DNA, RNA, cofactors or other small molecules, ions and other proteins. This technique can be applied to membrane proteins resident in the membranes of cells. To accomplish this, membrane proteins first need to be either heterologously expressed or purified from a native source. The protein has to be extracted from the lipid membrane with a mild detergent and purified to a stable, homogeneous population that may then be crystallized. Protein crystals are then used for X-ray diffraction to yield atomic resolution structures of the desired membrane protein target. Below, we present a general protocol for the growth of diffraction quality membrane protein crystals. The process of protein crystallization is highly variable, and obtaining diffraction quality crystals can require weeks to months or even years in some cases.

Crystal structure of human aquaporin 4 at 1.8 A and its mechanism of conductance.

Aquaporin (AQP) 4 is the predominant water channel in the mammalian brain, abundantly expressed in the blood-brain and brain-cerebrospinal fluid interfaces of glial cells. Its function in cerebral water balance has implications in neuropathological disorders, including brain edema, stroke, and head injuries. The 1.8-A crystal structure reveals the molecular basis for the water selectivity of the channel. Unlike the case in the structures of water-selective AQPs AqpZ and AQP1, the asparagines of the 2 Asn-Pro-Ala motifs do not hydrogen bond to the same water molecule; instead, they bond to 2 different water molecules in the center of the channel. Molecular dynamics simulations were performed to ask how this observation bears on the proposed mechanisms for how AQPs remain totally insulating to any proton conductance while maintaining a single file of hydrogen bonded water molecules throughout the channel.

The crystal structure of E. coli rRNA pseudouridine synthase RluE.

Pseudouridine synthase RluE modifies U2457 in a stem of 23 S RNA in Escherichia coli. This modification is located in the peptidyl transferase center of the ribosome. We determined the crystal structures of the C-terminal, catalytic domain of E. coli RluE at 1.2 A resolution and of full-length RluE at 1.6 A resolution. The crystals of the full-length enzyme contain two molecules in the asymmetric unit and in both molecules the N-terminal domain is disordered. The protein has an active site cleft, conserved in all other pseudouridine synthases, that contains invariant Asp and Tyr residues implicated in catalysis. An electropositive surface patch that covers the active site cleft is just wide enough to accommodate an RNA stem. The RNA substrate stem can be docked to this surface such that the catalytic Asp is adjacent to the target base, and a conserved Arg is positioned to help flip the target base out of the stem into the enzyme active site. A flexible RluE specific loop lies close to the conserved region of the stem in the model, and may contribute to substrate specificity. The stem alone is not a good RluE substrate, suggesting RluE makes additional interactions with other regions in the ribosome.

Structure-guided design of peptide-based tryptase inhibitors.

Improved peptide-based inhibitors of human beta tryptase were discovered using information gleaned from tripeptide library screening and structure-guided design methods, including fragment screening. Our efforts sought to improve this class of inhibitors by replacing the traditional Lys or Arg P1 element. The optimized compounds display low nanomolar potency against the mast cell target and several hundred-fold selectivity with respect to serine protease off targets. Thus, replacement of Lys/Arg at P1 in a peptide-like scaffold does not need to be accompanied by a loss in target affinity.

Novel arylpyrazole compounds selectively modulate glucocorticoid receptor regulatory activity.

The activities of intracellular receptors are regulated by their cognate ligands. Here we show that a series of related arylpyrazole compounds, which specifically bind the glucocorticoid receptor (GR), selectively modulated GR-regulated biological functions in preadipocyte, pre-osteoblast, and lung epithelial cell lines. Indeed, when we monitored 17 endogenous GR target genes in one of these cell types, we found that distinct arylpyrazole compounds induced different expression patterns. We showed by chromatin immunoprecipitation that the arylpyrazole compounds regulated, in a gene-specific manner, either GR occupancy of the genomic glucocorticoid response element (GRE) or events after GR association, such as histone modification. Overall, our results establish that subtle differences in ligand chemistry can profoundly influence the transcriptional regulatory activity of GR, and that endogenous genes bearing natural GREs are especially sensitive detectors of these differences.

Dissecting and designing inhibitor selectivity determinants at the S1 site using an artificial Ala190 protease (Ala190 uPA).

A site-directed mutant of the serine protease urokinase-type plasminogen activator (uPA), was produced to assess the contribution of the Ser190 side-chain to the affinity and selectivity of lead uPA inhibitors in the absence of other differences present in comparisons of natural proteases. Crystallography and enzymology involving WT and Ala190 uPA were used to calculate free energy binding contributions of hydrogen bonds involving the Ser190 hydroxyl group (O(gamma)(Ser190)) responsible for the remarkable selectivity of 6-halo-5-amidinoindole and 6-halo-5-amidinobenzimidazole inhibitors toward uPA and against natural Ala190 protease anti-targets. Crystal structures of uPA complexes of novel, active site-directed arylguanidine and 2-aminobenzimidazole inhibitors of WT uPA, together with associated K(i) values for WT and Ala190 uPA, also indicate a significant role of Ser190 in the binding of these classes of uPA inhibitors. Structures and associated K(i) values for a lead inhibitor (CA-11) bound to uPA and to five other proteases, as well as for other leads bound to multiple proteases, help reveal the features responsible for the potency (K(i)=11nM) and selectivity of the remarkably small inhibitor, CA-11. The 6-fluoro-5-amidinobenzimidzole, CA-11, is more than 1000-fold selective against natural Ala190 protease anti-targets, and more than 100-fold selective against other Ser190 anti-targets.

Structural snapshots of human HDAC8 provide insights into the class I histone deacetylases.

Modulation of the acetylation state of histones plays a pivotal role in the regulation of gene expression. Histone deacetylases (HDACs) catalyze the removal of acetyl groups from lysines near the N termini of histones. This reaction promotes the condensation of chromatin, leading to repression of transcription. HDAC deregulation has been linked to several types of cancer, suggesting a potential use for HDAC inhibitors in oncology. Here we describe the first crystal structures of a human HDAC: the structures of human HDAC8 complexed with four structurally diverse hydroxamate inhibitors. This work sheds light on the catalytic mechanism of the HDACs, and on differences in substrate specificity across the HDAC family. The structure also suggests how phosphorylation of Ser39 affects HDAC8 activity.

The structure of the extracellular region of human hepsin reveals a serine protease domain and a novel scavenger receptor cysteine-rich (SRCR) domain.

Hepsin is an integral membrane protein that may participate in cell growth and in maintaining proper cell morphology and is overexpressed in a number of primary tumors. We have determined the 1.75 A resolution structure of the extracellular component of human hepsin. This structure includes a 255-residue trypsin-like serine protease domain and a 109-residue region that forms a novel, poorly conserved, scavenger receptor cysteine-rich (SRCR) domain. The two domains are associated with each other through a single disulfide bond and an extensive network of noncovalent interactions. The structure suggests how the extracellular region of hepsin may be positioned with respect to the plasma membrane.