Structures of the Human PGD2 Receptor CRTH2 Reveal Novel Mechanisms for Ligand Recognition.

The signaling of prostaglandin D2 (PGD2) through G-protein-coupled receptor (GPCR) CRTH2 is a major pathway in type 2 inflammation. Compelling evidence suggests the therapeutic benefits of blocking CRTH2 signaling in many inflammatory disorders. Currently, a number of CRTH2 antagonists are under clinical investigation, and one compound, fevipiprant, has advanced to phase 3 clinical trials for asthma. Here, we present the crystal structures of human CRTH2 with two antagonists, fevipiprant and CAY10471. The structures, together with docking and ligand-binding data, reveal a semi-occluded pocket covered by a well-structured amino terminus and different binding modes of chemically diverse CRTH2 antagonists. Structural analysis suggests a ligand entry port and a binding process that is facilitated by opposite charge attraction for PGD2, which differs significantly from the binding pose and binding environment of lysophospholipids and endocannabinoids, revealing a new mechanism for lipid recognition by GPCRs.

Hepatic mitochondrial NAD + transporter SLC25A47 activates AMPKα mediating lipid metabolism and tumorigenesis.

BACKGROUND AIMS: SLC25A47 was initially identified as a mitochondrial HCC-downregulated carrier protein, but its physiological functions and transport substrates are unknown. We aimed to investigate the physiological role of SLC25A47 in hepatic metabolism. APPROACH RESULTS: In the treatment of hepatocytes with metformin, we found that metformin can transcriptionally activate the expression of Slc25a47 , which is required for AMP-activated protein kinase α (AMPKα) phosphorylation. Slc25a47 -deficient mice had increased hepatic lipid content, triglycerides, and cholesterol levels, and we found that Slc25a47 deficiency suppressed AMPKα phosphorylation and led to an increased accumulation of nuclear SREBPs, with elevated fatty acid and cholesterol biosynthetic activities. Conversely, when Slc25a47 was overexpressed in mouse liver, AMPKα was activated and resulted in the inhibition of lipogenesis. Moreover, using a diethylnitrosamine-induced mouse HCC model, we found that the deletion of Slc25a47 promoted HCC tumorigenesis and development through the activated mammalian target of rapamycin cascade. Employing homology modeling of SLC25A47 and virtual screening of the human metabolome database, we demonstrated that NAD + was an endogenous substrate for SLC25A47, and the activity of NAD + -dependent sirtuin 3 declined in Slc25a47 -deficient mice, followed by inactivation of AMPKα. CONCLUSIONS: Our findings reveal that SLC25A47, a hepatocyte-specific mitochondrial NAD + transporter, is one of the pharmacological targets of metformin and regulates lipid homeostasis through AMPKα, and may serve as a potential drug target for treating NAFLD and HCC.

Molecular basis for lipid recognition by the prostaglandin D2 receptor CRTH2.

Prostaglandin D2 (PGD2) signals through the G protein-coupled receptor (GPCR) CRTH2 to mediate various inflammatory responses. CRTH2 is the only member of the prostanoid receptor family that is phylogenetically distant from others, implying a nonconserved mechanism of lipid action on CRTH2. Here, we report a crystal structure of human CRTH2 bound to a PGD2 derivative, 15R-methyl-PGD2 (15mPGD2), by serial femtosecond crystallography. The structure revealed a "polar group in"-binding mode of 15mPGD2 contrasting the "polar group out"-binding mode of PGE2 in its receptor EP3. Structural comparison analysis suggested that these two lipid-binding modes, associated with distinct charge distributions of ligand-binding pockets, may apply to other lipid GPCRs. Molecular dynamics simulations together with mutagenesis studies also identified charged residues at the ligand entry port that function to capture lipid ligands of CRTH2 from the lipid bilayer. Together, our studies suggest critical roles of charge environment in lipid recognition by GPCRs.

A dual substrate-accessing mechanism of a major facilitator superfamily protein facilitates lysophospholipid flipping across the cell membrane.

Lysophospholipid transporter (LplT) is a member of the major facilitator superfamily present in many Gram-negative bacteria. LplT catalyzes flipping of lysophospholipids (LPLs) across the bacterial inner membrane, playing an important role in bacterial membrane homeostasis. We previously reported that LplT promotes both uptake of exogenous LPLs and intramembranous LPL flipping across the bilayer. To gain mechanistic insight into this dual LPL-flipping activity, here we implemented a combination of computational approaches and LPL transport analyses to study LPL binding of and translocation by LplT. Our results suggest that LplT translocates LPLs through an elongated cavity exhibiting an extremely asymmetric polarity. We found that two D(E)N motifs form a head group-binding site, in which the carboxylate group of Asp-30 is important for LPL head group recognition. Substitutions of residues in the head group-binding site disrupted both LPL uptake and flipping activities. However, alteration of hydrophobic residues on the interface between the N- and C-terminal domains impaired LPL flipping specifically, resulting in LPLs accumulation in the membrane, but LPL uptake remained active. These results suggest a dual substrate-accessing mechanism, in which LplT recruits LPLs to its substrate-binding site via two routes, either from its extracellular entry or through a membrane-embedded groove between transmembrane helices, and then moves them toward the inner membrane leaflet. This LPL-flipping mechanism is likely conserved in many bacterial species, and our findings illustrate how LplT adjusts the major facilitator superfamily translocation pathway to perform its versatile lipid homeostatic functions.

Calmidazolium Chloride and Its Complex with Serum Albumin Prevent Huntingtin Exon1 Aggregation.

Huntington's disease (HD) is a genetic disorder caused by a CAG expansion mutation in Huntingtin gene leading to polyglutamine (polyQ) expansion in the N-terminus side of Huntingtin (Httex1) protein. Neurodegeneration in HD is linked to aggregates formed by Httex1 bearing an expanded polyQ. Initiation and elongation steps of Httex1 aggregation are potential target steps for the discovery of therapeutic molecules for HD, which is currently untreatable. Here we report Httex1 aggregation inhibition by calmidazolium chloride (CLC) by acting on the initial aggregation event. Because it is hydrophobic, CLC was adsorbed to the vial surface and could not sustain an inhibition effect for a longer duration. The use of bovine serum albumin (BSA) prevented CLC adsorption by forming a BSA-CLC complex. This complex showed improved Httex1 aggregation inhibition by interacting with the aggregation initiator, the NT17 part of Httex1. Furthermore, biocompatible CLC-loaded BSA nanoparticles were made which reduced the polyQ aggregates in HD-150Q cells.

Unconventional N-H N Hydrogen Bonds Involving Proline Backbone Nitrogen in Protein Structures.

Contrary to DNA double-helical structures, hydrogen bonds (H-bonds) involving nitrogen as the acceptor are not common in protein structures. We systematically searched N-H…N H-bonds in two different sets of protein structures. Data set I consists of neutron diffraction and ultrahigh-resolution x-ray structures (0.9 Å resolution or better) and the hydrogen atom positions in these structures were determined experimentally. Data set II contains structures determined using x-ray diffraction (resolution ≤ 1.8 Å) and the positions of hydrogen atoms were generated using a computational method. We identified 114 and 14,347 potential N-H…N H-bonds from these two data sets, respectively, and 56-66% of these were of the Ni+1-Hi+1…Ni type, with Ni being the proline backbone nitrogen. To further understand the nature of such unusual contacts, we performed quantum chemical calculations on the model compound N-acetyl-L-proline-N-methylamide (Ace-Pro-NMe) with coordinates taken from the experimentally determined structures. A potential energy profile generated by varying the ψ dihedral angle in Ace-Pro-NMe indicates that the conformation with the N-H…N H-bond is the most stable. An analysis of H-bond-forming proline residues reveals that more than 30% of the proline carbonyl groups are also involved in n → π(∗) interactions with the carbonyl carbon of the preceding residue. Natural bond orbital analyses demonstrate that the strength of N-H…N H-bonds is less than half of that observed for a conventional H-bond. This study clearly establishes the H-bonding capability of proline nitrogen and its prevalence in protein structures. We found many proteins with multiple instances of H-bond-forming prolines. With more than 15% of all proline residues participating in N-H…N H-bonds, we suggest a new, to our knowledge, structural role for proline in providing stability to loops and capping regions of secondary structures in proteins.

Ablation of Proton/Glucose Exporter SLC45A2 Enhances Melanosomal Glycolysis to Inhibit Melanin Biosynthesis and Promote Melanoma Metastasis.

Sequence variation in SLC45A2 are responsible for oculocutaneous albinism type 4 in many species and are associated with melanoma susceptibility, but the molecular mechanism is unclear. In this study, we used Slc45a2-deficient melanocyte and mouse models to elucidate the roles of SLC45A2 in melanogenesis and melanoma metastasis. We found that the acidified cellular environment impairs the activity of key melanogenic enzyme tyrosinase in Slc45a2-deficient melanocytes. SLC45A2 is identified as a proton/glucose exporter in melanosomes, and its ablation increases the acidification of melanosomal pH through enhanced glycolysis. Intriguingly, 13C-glucose-labeled metabolic flux and biochemical assays show that melanosomes are active glucose-metabolizing organelles, indicating that elevated glycolysis mainly occurs in melanosomes owing to Slc45a2 deficiency. Moreover, Slc45a2 deficiency significantly upregulates the activities of glycolytic enzymes and phosphatidylinositol 3-kinase/protein kinase B signaling to promote glycolysis-dependent survival and metastasis of melanoma cells. Collectively, our study reveals that the proton/glucose exporter SLC45A2 mediates melanin synthesis and melanoma metastasis primarily by modulating melanosomal glucose metabolism.

The stability of R-spine defines RAF inhibitor resistance: A comprehensive analysis of oncogenic BRAF mutants with in-frame insertion of αC-ß4 loop.

Although targeting BRAF mutants with RAF inhibitors has achieved promising outcomes in cancer therapy, drug resistance remains a remarkable challenge, and underlying molecular mechanisms are not fully understood. Here, we characterized a previously unknown group of oncogenic BRAF mutants with in-frame insertions (LLRins506 or VLRins506) of αC-ß4 loop. Using structure modeling and molecular dynamics simulation, we found that these insertions formed a large hydrophobic network that stabilizes R-spine and thus triggers the catalytic activity of BRAF. Furthermore, these insertions disrupted BRAF dimer interface and impaired dimerization. Unlike BRAF(V600E), these BRAF mutants with low dimer affinity were strongly resistant to all RAF inhibitors in clinic or clinical trials, which arises from their stabilized R-spines. As predicted by molecular docking, the stabilized R-spines in other BRAF mutants also conferred drug resistance. Together, our data indicated that the stability of R-spine but not dimer affinity determines the RAF inhibitor resistance of oncogenic BRAF mutants.

SLC22A14 is a mitochondrial riboflavin transporter required for sperm oxidative phosphorylation and male fertility.

Ablation of Slc22a14 causes male infertility in mice, but the underlying mechanisms remain unknown. Here, we show that SLC22A14 is a riboflavin transporter localized at the inner mitochondrial membrane of the spermatozoa mid-piece and show by genetic, biochemical, multi-omic, and nutritional evidence that riboflavin transport deficiency suppresses the oxidative phosphorylation and reprograms spermatozoa energy metabolism by disrupting flavoenzyme functions. Specifically, we find that fatty acid ß-oxidation (FAO) is defective with significantly reduced levels of acyl-carnitines and metabolites from the TCA cycle (the citric acid cycle) but accumulated triglycerides and free fatty acids in Slc22a14 knockout spermatozoa. We demonstrate that Slc22a14-mediated FAO is essential for spermatozoa energy generation and motility. Furthermore, sperm from wild-type mice treated with a riboflavin-deficient diet mimics those in Slc22a14 knockout mice, confirming that an altered riboflavin level causes spermatozoa morphological and bioenergetic defects. Beyond substantially advancing our understanding of spermatozoa energy metabolism, our study provides an attractive target for the development of male contraceptives.

Submolecular probing of the complement C5a receptor-ligand binding reveals a cooperative two-site binding mechanism.

A current challenge to produce effective therapeutics is to accurately determine the location of the ligand-biding site and to characterize its properties. So far, the mechanisms underlying the functional activation of cell surface receptors by ligands with a complex binding mechanism remain poorly understood due to a lack of suitable nanoscopic methods to study them in their native environment. Here, we elucidated the ligand-binding mechanism of the human G protein-coupled C5a receptor (C5aR). We discovered for the first time a cooperativity between the two orthosteric binding sites. We found that the N-terminus C5aR serves as a kinetic trap, while the transmembrane domain acts as the functional site and both contributes to the overall high-affinity interaction. In particular, Asp282 plays a key role in ligand binding thermodynamics, as revealed by atomic force microscopy and steered molecular dynamics simulation. Our findings provide a new structural basis for the functional and mechanistic understanding of the GPCR family that binds large macromolecular ligands.

Orthosteric and allosteric action of the C5a receptor antagonists.

The C5a receptor (C5aR) is a G-protein-coupled receptor (GPCR) that can induce strong inflammatory response to the anaphylatoxin C5a. Targeting C5aR has emerged as a novel anti-inflammatory therapeutic method. However, developing potent C5aR antagonists as drugs has proven difficult. Here, we report two crystal structures of human C5aR in ternary complexes with the peptide antagonist PMX53 and a non-peptide antagonist, either avacopan or NDT9513727. The structures, together with other biophysical, computational docking and cell-based signaling data, reveal the orthosteric action of PMX53 and its effect of stabilizing the C5aR structure, as well as the allosteric action of chemically diverse non-peptide C5aR antagonists with different binding poses. Structural comparison analysis suggests the presence of similar allosteric sites in other GPCRs. We also discuss critical structural features of C5aR in activation, including a novel conformation of helix 8. On the basis of our results, we suggest novel strategies for developing C5aR-targeting drugs.