Directed inter-domain motions enable the IsdH Staphylococcus aureus receptor to rapidly extract heme from human hemoglobin

Pathogenic Staphylococcus aureus actively acquires iron from human hemoglobin (Hb) using the IsdH surface receptor. Heme extraction is mediated by a tridomain unit within the receptor that contains its second (N2) and third (N3) NEAT domains joined by a helical linker domain. Extraction occurs within a dynamic complex, in which receptors engage each globin chain; the N2 domain tightly binds to Hb, while substantial inter-domain motions within the receptor enable its N3 domain to transiently distort the globin's heme pocket. Using molecular simulations, Markov modeling, and quantitative measurements of heme transfer kinetics, we show that directed inter-domain motions within the receptor play a critical role in the extraction process. The directionality of N3 domain motion and the rate of heme extraction is controlled by amino acids within a short, flexible inter-domain tether that connects the N2 and linker domains. In the wild-type receptor directed motions originating from the tether enable the N3 domain to populate configurations capable of distorting Hb's pocket, whereas mutant receptors containing altered tethers are less able to adopt these conformers and capture heme slowly via indirect processes in which Hb first releases heme into the solvent. Thus, our results show inter-domain motions within the IsdH receptor play a critical role in its ability to extract heme from Hb and highlight the importance of directed motions by the short, unstructured, amino acid sequence connecting the domains in controlling the directionality and magnitude of these functionally important motions.

Individual cells traffic the Vasopressin 2 Receptor to their cell surface with different success

G protein coupled receptors (GPCRs) translate the actions of hormones and neurotransmitters into intracellular signalling events. Mutations in GPCRs can prevent their correct expression and trafficking to the cell surface and cause disease. Single cell subcellular localisation measurements reveal that while some cells appear to traffic the majority of the vasopressin 2 receptor (V2R) molecules to the cell surface, others retain a greater number of receptors in the ER or have approximately equal distribution. Mutations in the V2R affect the proportion of cells able to send this GPCR to their cell surface but surprisingly they do not prevent all cells from correctly trafficking the mutant receptors. These findings reveal the potential for rescue of mutant receptor cell surface expression by pharmacological manipulation of the GPCR folding and trafficking machinery.

A Single Mutation in the Outer Lipid-Facing Helix of a Pentameric Ligand-Gated Ion Channel Affects Channel Function Through a Radially-Propagating Mechanism

Pentameric ligand-gated ion channels (pLGICs) mediate fast synaptic transmission and are crucial drug targets. Their gating mechanism is triggered by ligand binding in the extracellular domain that culminates in the opening of a hydrophobic gate in the transmembrane domain. This domain is made of four α-helices (M1 to M4). Recently the outer lipid-facing helix (M4) has been shown to be key to receptor function, however its role in channel opening is still poorly understood. It could act through its neighboring helices (M1/M3), or via the M4 tip interacting with the pivotal Cys-loop in the extracellular domain. Mutation of a single M4 tyrosine (Y441) to alanine renders one pLGIC—the 5-HT3A receptor—unable to function despite robust ligand binding. Using Y441A as a proxy for M4 function, we here predict likely paths of Y441 action using molecular dynamics, and test these predictions with functional assays of mutant receptors in HEK cells and Xenopus oocytes using fluorescent membrane potential sensitive dye and two-electrode voltage clamp respectively. We show that Y441 does not act via the M4 tip or Cys-loop, but instead connects radially through M1 to a residue near the ion channel hydrophobic gate on the pore-lining helix M2. This demonstrates the active role of the M4 helix in channel opening.

Identification of a Potent Antagonist of Smoothened in Hedgehog Signaling

Background Hedgehog signaling is essential to the regulation of embryonic development, tissue homeostasis, and stem cell self-renewal, making it a prime target for developing cancer therapeutics. Given the close link between aberrant Hedgehog signaling and cancers, many small molecular compounds have been developed to inhibit this pathway for treating cancer and several of such compounds have been approved by FDA (GDC-0449 and LDE-225). As we known, acquired drug resistance is the main obstacle of first generation of Smoothened inhibitors. Therefore, new Smoothened inhibitors that could inhibit wild-type and mutant receptors are essential for refractory cancers. ResultsWe established the Smoothened β arrestin 2 GFP high throughput screening platform based on the mechanistic discovery of Hedgehog signaling pathway, and discovered several active small molecules targeting Smoothened including 0025A 0025A. Here we show that 0025A can block the translocation of β arrestin 2 GFP to Smoothened , displace Bodipycyclopamine binding to wild type Smoothened or mutant Smoothened D473H and reduce the expression of Gli upon Hedgehog stimulation. In addition, we reveal that 0025A can effectively suppress hair follicle morphogenesis and hair growth in mice. Conclusions Thus, our results demonstrate that 0025A is a potent antagonist targeting Smoothened wild-type and mutant type and mutant receptors in the Hreceptors signaling pathway and may provide a new new therapy for refractory cancers.

Isoflurane Potentiation of GABAA Receptors Is Reduced but Not Eliminated by the β3(N265M) Mutation

Background: Mice carrying the GABAA receptor β3(N265M) point mutation, which renders receptors incorporating β3-subunits insensitive to many general anesthetics, have been used experimentally to link modulation of different receptor subtypes to distinct behavioral endpoints. Remarkably, however, the effect of the mutation on the susceptibility to modulation by isoflurane (a standard reference agent for inhalational vapors) has never been tested directly. Therefore, we compared the modulation by isoflurane of expressed α5β3(N265M)γ2L receptors with their wild type counterparts. Methods: Using whole-cell electrophysiological recording and rapid solution exchange techniques, we tested the effects of isoflurane at concentrations ranging from 80 μM to 320 μM on currents activated by 1 μM GABA. We measured drug modulation of wild-type α5β3γ2L GABAA receptors and their counterparts harboring the β3(N265M) mutation. Results: Currents elicited by GABA were enhanced two- to four-fold by isoflurane, in a concentration-dependent manner. Under the same conditions, receptors incorporating the β3(N265M) mutation were enhanced by approximately 1.5- to two-fold; i.e., modulation by isoflurane was attenuated by approximately one-half. Direct activation by isoflurane was also present in mutant receptors but also attenuated. Conclusions: In contrast to the complete insensitivity of β3(N265M) mutant receptors to etomidate and propofol, the mutation has only a partial effect on receptor modulation by isoflurane. Therefore, the persistence of isoflurane effects in mutant mice does not exclude a possible contribution of β3-GABAA receptors.

Suppression of hTAS2R16 Signaling by Umami Substances

Interaction between umami and bitter taste has long been observed in human sensory studies and in neural responses in animal models, however, the molecular mechanism for their action has not been delineated. Humans detect diverse bitter compounds using 25-30 members of the type 2 taste receptor (TAS2R) family of G protein-coupled receptor. In this study, we investigated the putative mechanism of antagonism by umami substances using HEK293T cells expressing hTAS2R16 and two known probenecid-insensitive mutant receptors, hTAS2R16 N96T and P44T. In wild type receptor, Glu-Glu, inosine monophosphate (IMP), and l-theanine behave as partial insurmountable antagonists, and monosodium glutamate (MSG) acts as a surmountable antagonist in comparison with probenecid as a full insurmountable antagonist. The synergism with IMP of umami substances still stands in the suppression of hTAS2R16 signaling. In mutagenesis analysis, we found that Glu-Glu, MSG, and l-theanine share at least one critical binding site on N96 and P44 with probenecid. These results provide the first evidence for a direct binding of umami substances to the hTAS2R16 through the probenecid binding pocket on the receptor, resulting in the suppression of bitterness.

Cadmium activates AMPA and NMDA receptors with M3 helix cysteine substitutions

AMPA and NMDA receptors are ligand-gated ion channels that depolarize postsynaptic neurons when activated by the neurotransmitter L-glutamate. Changes in the distribution and activity of these receptors underlie learning and memory, but excessive change is associated with an array of neurological disorders, including cognitive impairment, developmental delay, and epilepsy. All of the ionotropic glutamate receptors (iGluRs) exhibit similar tetrameric architecture, transmembrane topology, and basic framework for activation; conformational changes induced by extracellular agonist binding deform and splay open the inner helix bundle crossing that occludes ion flux through the channel. NMDA receptors require agonist binding to all four subunits, whereas AMPA and closely related kainate receptors can open with less than complete occupancy. In addition to conventional activation by agonist binding, we recently identified two locations along the inner helix of the GluK2 kainate receptor subunit where cysteine (Cys) substitution yields channels that are opened by exposure to cadmium ions, independent of agonist site occupancy. Here, we generate AMPA and NMDA receptor subunits with homologous Cys substitutions and demonstrate similar activation of the mutant receptors by Cd. Coexpression of the auxiliary subunit stargazin enhanced Cd potency for activation of Cys-substituted GluA1 and altered occlusion upon treatment with sulfhydryl-reactive MTS reagents. Mutant NMDA receptors displayed voltage-dependent Mg block of currents activated by agonist and/or Cd as well as asymmetry between Cd effects on Cys-substituted GluN1 versus GluN2 subunits. In addition, Cd activation of each Cys-substituted iGluR was inhibited by protons. These results, together with our earlier work on GluK2, reveal a novel mechanism shared among the three different iGluR subtypes for prying open the gate that controls ion entry into the pore.

SAT-LB122 Characterization and Functional Rescue of a Nephrogenic Diabetes Insipidus Causing S127F Substitution in V2 Vasopressin Receptor

Laura Szalai1, András Sziráki1, László Sándor Erdélyi1, András Balla1,2, László Hunyady1,2 1Department of Physiology, Semmelweis University, and 2MTA-SE Laboratory of Molecular Physiology, Budapest, Hungary The concentrating function of the kidney is important to maintain the water homeostasis of the body. It is regulated by the arginine-vasopressin system through the type 2 vasopressin receptor (V2R). Loss-of-function mutations of V2R in kidney can lead to nephrogenic diabetes insipidus (NDI) which results several symptoms such as polyuria, polydipsia, and hyposthenuria. In this study, we functionally characterized and investigated the potential rescue of a missense mutation (S127F) of the V2R. We monitored the cellular localization of the S127F mutant V2 receptor using HA-tagged receptors in confocal microscopy experiments. The S127F V2 receptor was detected only in the endoplasmic reticulum but not in the plasma membrane. Flow cytometry measurements revealed that only limited amount mutant receptor can be found on the cell surface compared to the wild type V2R. We also determined the cAMP signaling capability of the mutant receptor with BRET measurements. The S127F receptor was not able to increase the intracellular cAMP levels in response to vasopressin stimulation. Certain ER retention mutations can be rescued by pharmacological chaperones, which cause misfolded mutant receptors to present in the plasma membrane. We examined the effect of tolvaptan (a V2R antagonist) on the S127F V2 receptor. HEK293 cells were transiently transfected with the plasmid of the mutant receptor and after one day the cells were incubated for 18 hours with tolvaptan. After the pretreatment, the cells were exposed to vasopressin, and we were able to detect significant cAMP signal generation of the mutant receptor. We also checked whether the result after tolvaptan pretreatment was due to restored plasma membrane location of the receptor. We were able to demonstrate significant increase of the mutant receptors in the plasma membrane using flow cytometry. We also investigated the effect of MCF14 compound (a cell permeable high-affinity agonist for the V2R) on the mutant receptor and we found that the MCF14 was also capable to restore the cAMP signaling function of the receptor. This work was supported by the National Research, Development and Innovation Fund (NKFI K116954 and NVKP_16-1-2016-0039).

Role of the C-terminal tail in regulating Proteinase Activated Receptor 2 (PAR2) signalling

ABSTRACTThe C-terminal tail of G-protein-coupled receptors contain important regulatory sites that enable interaction with intracellular signalling effectors. Here we examine the relative contribution of the C-tail serine/threonine phosphorylation sites (Ser383-385, Ser387-Thr392) and the helix-8 palmitoylation site (Cys361) in signalling regulation downstream of the proteolytically-activated GPCR, PAR2. We examined Gαq/11-coupled calcium signalling, β-arrestin-1/-2 recruitment, and MAPK activation (p44/42 phosphorylation) by wild-type and mutant receptors expressed in a CRISPR/Cas9 PAR2-knockout HEK-293 cell background. We find that alanine substitution of the membrane proximal serine residues (Ser383-385Ala) had no effect on SLIGRL-NH2- or trypsin-stimulated β-arrestin recruitment. Alanine substitutions in the Ser387-Thr392 cluster resulted in a large (∼50%) decrease in β-arrestin-1/2 recruitment triggered by the activating peptide, SLIGRL-NH2, but was without effect on trypsin-activated β-arrestin-1/-2 recruitment. Additionally, we find that alanine substitution of the helix-8 cysteine residue (Cys361Ala) led to a (∼50%) decrease in β-arrestin-1/-2 recruitment in response to both trypsin and SLIGRL-NH2. We further show that Gαq/11inhibition with YM254890, inhibited ERK phosphorylation by PAR2 agonists, while genetic deletion of β-arrestin-1/-2 by CRISPR/Cas9 enhanced MAPK activation. Knockout of β-arrestins also enhanced Gαq/11-mediated calcium signalling. In line with these findings, C-tail serine/threonine and cysteine residue mutants that have decreased β-arrestin recruitment also showed enhanced ERK activation. Thus, our studies point to multiple mechanisms that regulate β-arrestin interaction with PAR2 to regulate receptor-mediated signalling.