Graphene oxide, but not fullerenes, targets immunoproteasomes and suppresses antigen presentation by dendritic cells

Graphene oxide (GO) and C60 - or C60 -TRIS fullerenes, internalized by murine dendritic cells (DCs), differently affect their abilities to present antigens to T-cells. While C60 -fullerenes stimulate the ovalbumin-specific MHC class I-restricted T-cell response, GO impairs the stimulatory potential of DCs. In contrast to C60 -fullerenes, GO decreases the intracellular levels of LMP7 immunoproteasome subunits required for processing of protein antigens. This is important for the development of DC-based vaccines.

Oxidative stress, inflammatory biomarkers, and toxicity in mouse lung and liver after inhalation exposure to 100% biodiesel or petroleum diesel emissions

Over the past decade, soy biodiesel (BD) has become a first alternative energy source that is economically viable and meets requirements of the Clean Air Act. Due to lower mass emissions and reduced hazardous compounds compared to diesel combustion emissions (CE), BD exposure is proposed to produce fewer adverse health effects. However, considering the broad use of BD and its blends in different industries, this assertion needs to be supported and validated by mechanistic and toxicological data. Here, adverse effects were compared in lungs and liver of BALB/cJ mice after inhalation exposure (0, 50, 150, or 500 μg/m3; 4 h/d, 5 d/wk, for 4 wk) to CE from 100% biodiesel (B100) and diesel (D100). Compared to D100, B100 CE produced a significant accumulation of oxidatively modified proteins (carbonyls), an increase in 4-hydroxynonenal (4-HNE), a reduction of protein thiols, a depletion of antioxidant gluthatione (GSH), a dose-related rise in the levels of biomarkers of tissue damage (lactate dehydrogenase, LDH) in lungs, and inflammation (myeloperoxidase, MPO) in both lungs and liver. Significant differences in the levels of inflammatory cytokines interleukin (IL)-6, IL-10, IL-12p70, monocyte chemoattractant protein (MCP)-1, interferon (IFN) γ, and tumor necrosis factor (TNF)-α were detected in lungs and liver upon B100 and D100 CE exposures. Overall, the tissue damage, oxidative stress, inflammation, and cytokine response were more pronounced in mice exposed to BD CE. Further studies are required to understand what combustion products in BD CE accelerate oxidative and inflammatory responses.

Specificity of lipoprotein-associated phospholipase A(2) toward oxidized phosphatidylserines: liquid chromatography-electrospray ionization mass spectrometry characterization of products and computer modeling of interactions

Ca(2+)-independent lipoprotein-associated phospholipase A(2) (Lp-PLA(2)) is a member of the phospholipase A(2) superfamily with a distinguishing characteristic of high specificity for oxidatively modified sn-2 fatty acid residues in phospholipids that has been especially well characterized for peroxidized species of phosphatidylcholines (PC). The ability of Lp-PLA(2) to hydrolyze peroxidized species of phosphatidylserine (PS), acting as a recognition signal for clearance of apoptotic cells by professional phagocytes, as well as the products of the reaction has not been investigated. We performed liquid chromatography-electrospray ionization mass spectrometry-based structural characterization of oxygenated, hydrolyzed molecular species of PS-containing linoleic acid in either the sn-2 position (C(18:0)/C(18:2)) or in both sn-1 and sn-2 positions (C(18:2)/C(18:2)), formed in the cytochrome c- and H(2)O(2)-driven enzymatic oxidation reaction. Cytochrome c has been chosen as a catalyst of peroxidation reactions because of its likely involvement in PS oxidation in apoptotic cells. We found that Lp-PLA(2) catalyzed the hydrolysis of both nontruncated and truncated (oxidatively fragmented) species of oxidized PS species, albeit with different efficiencies, and performed detailed characterization of the major reaction products: oxygenated derivatives of linoleic acid as well as nonoxygenated and oxygenated species of lyso-PS. Among linoleic acid products, derivatives oxygenated at the C(9) position, including 9-hydroxyoctadecadienoic acid (9-HODE), a potent ligand of G protein-coupled receptor G2A, were the most abundant. Computer modeling of interactions of Lp-PLA(2) with different PS-oxidized species indicated that they are able to bind in the proximity (<5 Å) of Ser273 and His351 of the catalytic triad. For 9-hydroxy and 9-hydroperoxy derivatives of oxidized PS, the sn-2 ester bond was positioned very close (<3 Å) to the Ser273 residue, a nucleophile directly attacking the sn-2 bond, thus favoring the hydrolysis reaction. We suggest that oxidatively modified free fatty acids and lyso-PS species generated by Lp-PLA(2) may represent important signals facilitating and regulating the execution of apoptotic and phagocytosis programs essential for the control of inflammation.

Dual function of mitochondrial Nm23-H4 protein in phosphotransfer and intermembrane lipid transfer: a cardiolipin-dependent switch

The nucleoside diphosphate kinase Nm23-H4/NDPK-D forms symmetrical hexameric complexes in the mitochondrial intermembrane space with phosphotransfer activity using mitochondrial ATP to regenerate nucleoside triphosphates. We demonstrate the complex formation between Nm23-H4 and mitochondrial GTPase OPA1 in rat liver, suggesting its involvement in local and direct GTP delivery. Similar to OPA1, Nm23-H4 is further known to strongly bind in vitro to anionic phospholipids, mainly cardiolipin, and in vivo to the inner mitochondrial membrane. We show here that such protein-lipid complexes inhibit nucleoside diphosphate kinase activity but are necessary for another function of Nm23-H4, selective intermembrane lipid transfer. Mitochondrial lipid distribution was analyzed by liquid chromatography-mass spectrometry using HeLa cells expressing either wild-type Nm23-H4 or a membrane binding-deficient mutant at a site predicted based on molecular modeling to be crucial for cardiolipin binding and transfer mechanism. We found that wild type, but not the mutant enzyme, selectively increased the content of cardiolipin in the outer mitochondrial membrane, but the distribution of other more abundant phospholipids (e.g. phosphatidylcholine) remained unchanged. HeLa cells expressing the wild-type enzyme showed increased accumulation of Bax in mitochondria and were sensitized to rotenone-induced apoptosis as revealed by stimulated release of cytochrome c into the cytosol, elevated caspase 3/7 activity, and increased annexin V binding. Based on these data and molecular modeling, we propose that Nm23-H4 acts as a lipid-dependent mitochondrial switch with dual function in phosphotransfer serving local GTP supply and cardiolipin transfer for apoptotic signaling and putative other functions.

Adsorption of surfactant lipids by single-walled carbon nanotubes in mouse lung upon pharyngeal aspiration

The pulmonary route represents one of the most important portals of entry for nanoparticles into the body. However, the in vivo interactions of nanoparticles with biomolecules of the lung have not been sufficiently studied. Here, using an established mouse model of pharyngeal aspiration of single-walled carbon nanotubes (SWCNTs), we recovered SWCNTs from the bronchoalveolar lavage fluid (BALf), purified them from possible contamination with lung cells, and examined the composition of phospholipids adsorbed on SWCNTs by liquid chromatography mass spectrometry (LC-MS) analysis. We found that SWCNTs selectively adsorbed two types of the most abundant surfactant phospholipids: phosphatidylcholines (PC) and phosphatidylglycerols (PG). Molecular speciation of these phospholipids was also consistent with pulmonary surfactant. Quantitation of adsorbed lipids by LC-MS along with the structural assessments of phospholipid binding by atomic force microscopy and molecular modeling indicated that the phospholipids (∼108 molecules per SWCNT) formed an uninterrupted "coating" whereby the hydrophobic alkyl chains of the phospholipids were adsorbed onto the SWCNT with the polar head groups pointed away from the SWCNT into the aqueous phase. In addition, the presence of surfactant proteins A, B, and D on SWCNTs was determined by LC-MS. Finally, we demonstrated that the presence of this surfactant coating markedly enhanced the in vitro uptake of SWCNTs by macrophages. Taken together, this is the first demonstration of the in vivo adsorption of the surfactant lipids and proteins on SWCNTs in a physiologically relevant animal model.

Factoring-in agglomeration of carbon nanotubes and nanofibers for better prediction of their toxicity versus asbestos

Background

Carbon nanotubes (CNT) and carbon nanofibers (CNF) are allotropes of carbon featuring fibrous morphology. The dimensions and high aspect ratio of CNT and CNF have prompted the comparison with naturally occurring asbestos fibers which are known to be extremely pathogenic. While the toxicity and hazardous outcomes elicited by airborne exposure to single-walled CNT or asbestos have been widely reported, very limited data are currently available describing adverse effects of respirable CNF.

Results

Here, we assessed pulmonary inflammation, fibrosis, oxidative stress markers and systemic immune responses to respirable CNF in comparison to single-walled CNT (SWCNT) and asbestos. Pulmonary inflammatory and fibrogenic responses to CNF, SWCNT and asbestos varied depending upon the agglomeration state of the particles/fibers. Foci of granulomatous lesions and collagen deposition were associated with dense particle-like SWCNT agglomerates, while no granuloma formation was found following exposure to fiber-like CNF or asbestos. The average thickness of the alveolar connective tissue - a marker of interstitial fibrosis - was increased 28 days post SWCNT, CNF or asbestos exposure. Exposure to SWCNT, CNF or asbestos resulted in oxidative stress evidenced by accumulations of 4-HNE and carbonylated proteins in the lung tissues. Additionally, local inflammatory and fibrogenic responses were accompanied by modified systemic immunity, as documented by decreased proliferation of splenic T cells ex vivo on day 28 post exposure. The accuracies of assessments of effective surface area for asbestos, SWCNT and CNF (based on geometrical analysis of their agglomeration) versus estimates of mass dose and number of particles were compared as predictors of toxicological outcomes.

Conclusions

We provide evidence that effective surface area along with mass dose rather than specific surface area or particle number are significantly correlated with toxicological responses to carbonaceous fibrous nanoparticles. Therefore, they could be useful dose metrics for risk assessment and management.

The cytoplasmic rhodopsin-protein interface: potential for drug discovery

The mammalian dim-light photoreceptor rhodopsin is a prototypic G protein coupled receptor (GPCR), interacting with the G protein, transducin, rhodopsin kinase, and arrestin. All of these proteins interact with rhodopsin at its cytoplasmic surface. Structural and modeling studies have provided in-depth descriptions of the respective interfaces. Overlap and thus competition for binding surfaces is a major regulatory mechanism for signal processing. Recently, it was found that the same surface is also targeted by small molecules. These ligands can directly interfere with the binding and activation of the proteins of the signal transduction cascade, but they can also allosterically modulate the retinal ligand binding pocket. Because the pocket that is targeted contains residues that are highly conserved across Class A GPCRs, these findings imply that it may be possible to target multiple GPCRs with the same ligand(s). This is desirable for example in complex diseases such as cancer where multiple GPCRs participate in the disease networks.

Stimulation and clustering of cytochrome b5 reductase in caveolin-rich lipid microdomains is an early event in oxidative stress-mediated apoptosis of cerebellar granule neurons

The apoptosis of cerebellar granule neurons (CGN) induced by low potassium in the extracellular medium is a model of neuronal apoptosis where an overshot of reactive oxygen species (ROS) triggers the neuronal death. In this work, using dihydroethidium and L-012 as specific dyes for superoxide anion detection we show that this ROS overshot can be accounted by an increased release of superoxide anion to the extracellular medium. The amplitude and time course of the increase of superoxide anion observed early during apoptosis correlated with the increase of the content of soluble cytochrome b(5), a substrate of the NADH-dependent oxidase activity of the cytochrome b(5) reductase associated with lipid rafts in CGN. Western blotting and immunofluorescence microscopy approaches, including fluorescence energy transfer, pointed out an enhanced clustering of cytochrome b(5) reductase within caveolins-rich lipid rafts microdomains. Protein/protein docking analysis suggests that cytochrome b(5) reductase can form complexes with caveolins 1α, 1β and 2, playing electrostatic interactions a major role in this association. In conclusion, our results indicate that overstimulation of cytochrome b(5) reductase associated with lipid rafts can account for the overshot of plasma membrane-focalized superoxide anion production that triggers the entry of CGN in the irreversible phase of apoptosis. This article is part of a Special Issue entitled: Proteomics: The clinical link.

A mitochondria-targeted inhibitor of cytochrome c peroxidase mitigates radiation-induced death

The risk of radionuclide release in terrorist acts or exposure of healthy tissue during radiotherapy demand potent radioprotectants/radiomitigators. Ionizing radiation induces cell death by initiating the selective peroxidation of cardiolipin in mitochondria by the peroxidase activity of its complex with cytochrome c leading to release of haemoprotein into the cytosol and commitment to the apoptotic program. Here we design and synthesize mitochondria-targeted triphenylphosphonium-conjugated imidazole-substituted oleic and stearic acids that blocked peroxidase activity of cytochrome c/cardiolipin complex by specifically binding to its haem-iron. We show that both compounds inhibit pro-apoptotic oxidative events, suppress cyt c release, prevent cell death, and protect mice against lethal doses of irradiation. Significant radioprotective/radiomitigative effects of imidazole-substituted oleic acid are observed after pretreatment of mice from 1 h before through 24 h after the irradiation.

The enzymatic oxidation of graphene oxide

Two-dimensional graphitic carbon is a new material with many emerging applications, and studying its chemical properties is an important goal. Here, we reported a new phenomenon--the enzymatic oxidation of a single layer of graphitic carbon by horseradish peroxidase (HRP). In the presence of low concentrations of hydrogen peroxide (∼40 μM), HRP catalyzed the oxidation of graphene oxide, which resulted in the formation of holes on its basal plane. During the same period of analysis, HRP failed to oxidize chemically reduced graphene oxide (RGO). The enzymatic oxidation was characterized by Raman, ultraviolet-visible, electron paramagnetic resonance, Fourier transform infrared spectroscopy, transmission electron microscopy, atomic force microscopy, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and gas chromatography-mass spectrometry. Computational docking studies indicated that HRP was preferentially bound to the basal plane rather than the edge for both graphene oxide and RGO. Owing to the more dynamic nature of HRP on graphene oxide, the heme active site of HRP was in closer proximity to graphene oxide compared to RGO, thereby facilitating the oxidation of the basal plane of graphene oxide. We also studied the electronic properties of the reduced intermediate product, holey reduced graphene oxide (hRGO), using field-effect transistor (FET) measurements. While RGO exhibited a V-shaped transfer characteristic similar to a single layer of graphene that was attributed to its zero band gap, hRGO demonstrated a p-type semiconducting behavior with a positive shift in the Dirac points. This p-type behavior rendered hRGO, which can be conceptualized as interconnected graphene nanoribbons, as a potentially attractive material for FET sensors.

NMR-based screening of membrane protein ligands

Membrane proteins pose problems for the application of NMR-based ligand-screening methods because of the need to maintain the proteins in a membrane mimetic environment such as detergent micelles: they add to the molecular weight of the protein, increase the viscosity of the solution, interact with ligands non-specifically, overlap with protein signals, modulate protein dynamics and conformational exchange and compromise sensitivity by adding highly intense background signals. In this article, we discuss the special considerations arising from these problems when conducting NMR-based ligand-binding studies with membrane proteins. While the use of (13)C and (15)N isotopes is becoming increasingly feasible, (19)F and (1)H NMR-based approaches are currently the most widely explored. By using suitable NMR parameter selection schemes independent of or exploiting the presence of detergent, (1)H-based approaches require least effort in sample preparation because of the high sensitivity and natural abundance of (1)H in both, ligand and protein. On the other hand, the (19)F nucleus provides an ideal NMR probe because of its similarly high sensitivity to that of (1)H and the lack of natural (19)F background in biologic systems. Despite its potential, the use of NMR spectroscopy is highly underdeveloped in the area of drug discovery for membrane proteins.

Allosteric Modulation of G Protein Coupled Receptors by Cytoplasmic, Transmembrane and Extracellular Ligands

G protein coupled receptors (GPCRs) bind diverse classes of ligands, and depending on the receptor, these may bind in their transmembrane or the extracellular domains, demonstrating the principal ability of GPCRs to bind ligand in either domains. Most recently, it was also observed that small molecule ligands can bind in the cytoplasmic domain, and modulate binding and response to extracellular or transmembrane ligands. Thus, all three domains in GPCRs are potential sites for allosteric ligands, and whether a ligand is allosteric or orthosteric depends on the receptor. Here, we will review the evidence supporting the presence of putative binding pockets in all three domains of GPCRs and discuss possible pathways of communication between these pockets.

Ligand-independent antiapoptotic function of estrogen receptor-beta in lung cancer cells

Recent studies have demonstrated the presence of estrogen receptor (ER)beta in the mitochondria in various cell types and tissues, but the exact function of this localization remains unclear. In this study, we have examined the function of mitochondrial ERbeta in non-small-cell lung cancer (NSCLC) cells. Down-regulation of ERbeta by short hairpin RNA constructs sensitized NSCLC cells to various apoptosis-inducing agents such as cisplatin, taxol, and etoposide. The increased growth inhibition and induction of apoptosis in ERbeta-knockdown cells was observed irrespective of estrogen treatment, suggesting a ligand-independent role of ERbeta in regulating the intrinsic apoptotic pathway. Further, ERbeta from the mitochondrial fraction physically interacted with the proapoptotic protein Bad, in a ligand-independent manner. Glutathione-S-transferase pull-down assays and molecular modeling studies revealed that the DNA-binding domain and hinge region of ERbeta, and the BH3 domain of Bad were involved in these interactions. Further investigations revealed that ERbeta inhibited Bad function by disrupting Bad-Bcl-X(L) and Bad-Bcl-2 interactions. Reintroduction of ERbeta in the mitochondria of ERbeta knockdown cells reversed their sensitivity to cisplatin. Overall, our results demonstrate a ligand-independent role of ERbeta in regulating apoptosis, revealing a novel function for ERbeta in the mitochondria.

Human immunodeficiency virus type 1 Vpr: oligomerization is an essential feature for its incorporation into virus particles

HIV-1 Vpr, a nonstructural viral protein associated with virus particles, has a positive role in the efficient transport of PIC into the nucleus of non-dividing target cells and enhances virus replication in primary T cells. Vpr is a 96 amino acid protein and the structure by NMR shows three helical domains. Vpr has been shown to exist as dimers and higher order oligomers. Considering the multifunctional nature of Vpr, the contribution of distinct helical domains to the dimer/oligomer structure of Vpr and the relevance of this feature to its functions are not clear. To address this, we have utilized molecular modeling approaches to identify putative models of oligomerization. The predicted interface residues were subjected to site-directed mutagenesis and evaluated their role in intermolecular interaction and virion incorporation. The interaction between Vpr molecules was monitored by Bimolecular Fluorescence complementation (BiFC) method. The results show that Vpr forms oligomers in live cells and residues in helical domains play critical roles in oligomerization. Interestingly, Vpr molecules defective in oligomerization also fail to incorporate into the virus particles. Based on the data, we suggest that oligomerization of Vpr is essential for virion incorporation property and may also have a role in the events associated with virus infection.

Carbon nanotubes degraded by neutrophil myeloperoxidase induce less pulmonary inflammation

We have shown previously that single-walled carbon nanotubes can be catalytically biodegraded over several weeks by the plant-derived enzyme, horseradish peroxidase. However, whether peroxidase intermediates generated inside human cells or biofluids are involved in the biodegradation of carbon nanotubes has not been explored. Here, we show that hypochlorite and reactive radical intermediates of the human neutrophil enzyme myeloperoxidase catalyse the biodegradation of single-walled carbon nanotubes in vitro, in neutrophils and to a lesser degree in macrophages. Molecular modelling suggests that interactions of basic amino acids of the enzyme with the carboxyls on the carbon nanotubes position the nanotubes near the catalytic site. Importantly, the biodegraded nanotubes do not generate an inflammatory response when aspirated into the lungs of mice. Our findings suggest that the extent to which carbon nanotubes are biodegraded may be a major determinant of the scale and severity of the associated inflammatory responses in exposed individuals.

The importance of valine 114 in ligand binding in beta(2)-adrenergic receptor

G-protein coupled receptors (GPCRs) are transmembrane signaling molecules, with a majority of them performing important physiological roles. beta(2)-Adrenergic receptor (beta(2)-AR) is a well-studied GPCRs that mediates natural responses to the hormones adrenaline and noradrenaline. Analysis of the ligand-binding region of beta(2)-AR using the recently solved high-resolution crystal structures revealed a number of highly conserved amino acids that might be involved in ligand binding. However, detailed structure-function studies on some of these residues have not been performed, and their role in ligand binding remains to be elucidated. In this study, we have investigated the structural and functional role of a highly conserved residue valine 114, in hamster beta(2)-AR by site-directed mutagenesis. We replaced V114 in hamster beta(2)-AR with a number of amino acid residues carrying different functional groups. In addition to the complementary substitutions V114I and V114L, the V114C and V114E mutants also showed significant ligand binding and agonist dependent G-protein activation. However, the V114G, V114T, V114S, and V114W mutants failed to bind ligand in a specific manner. Molecular modeling studies were conducted to interpret these results in structural terms. We propose that the replacement of V114 influences not only the interaction of the ethanolamine side-chains but also the aryl-ring of the ligands tested. Results from this study show that the size and orientation of the hydrophobic residue at position V114 in beta(2)-AR affect binding of both agonists and antagonists, but it does not influence the receptor expression or folding.

Heterolytic reduction of fatty acid hydroperoxides by cytochrome c/cardiolipin complexes: antioxidant function in mitochondria

Cytochrome c (cyt c), a mitochondrial intermembrane electron shuttle between complexes III and IV, can, upon binding with an anionic phospholipid, cardiolipin (CL), act as a peroxidase that catalyzes cardiolipin oxidation. H(2)O(2) was considered as a source of oxidative equivalents for this reaction, which is essential for programmed cell death. Here we report that peroxidase cyt c/CL complexes can utilize free fatty acid hydroperoxides (FFA-OOH) at exceptionally high rates that are approximately 3 orders of magnitude higher than for H(2)O(2). Similarly, peroxidase activity of murine liver mitochondria was high with FFA-OOH. Using EPR spin trapping and LC-MS techniques, we have demonstrated that cyt c/CL complexes split FFA-OOH predominantly via a heterolytic mechanism, yielding hydroxy-fatty acids, whereas H(2)O(2) (and tert-butyl hydroperoxide, t-BuOOH) undergo homolytic cleavage. Computer simulations have revealed that Arg(38) and His(33) are important for the heterolytic mechanism at potential FFA-OOH binding sites of cyt c (but not for H(2)O(2) or t-BuOOH). Regulation of FFA-OOH metabolism may be an important function of cyt c that is associated with elimination of toxic FFA-OOH and synthesis of physiologically active hydroxy-fatty acids in mitochondria.

Mitochondrial targeting of electron scavenging antioxidants: Regulation of selective oxidation vs random chain reactions

Effective regulation of highly compartmentalized production of reactive oxygen species and peroxidation reactions in mitochondria requires targeting of small molecule antioxidants and antioxidant enzymes into the organelles. This review describes recently developed approaches to mitochondrial targeting of small biologically active molecules based on: (i) preferential accumulation in mitochondria because of their hydrophobicity and positive charge (hydrophobic cations), (ii) binding with high affinity to an intra-mitochondrial constituent, and (iii) metabolic conversions by specific mitochondrial enzymes to reveal an active entity. In addition, targeted delivery of antioxidant enzymes via expression of leader sequences directing the proteins into mitochondria is considered. Examples of successful antioxidant and anti-apoptotic protection based on the ability of targeted cargoes to inhibit cytochrome c-catalyzed peroxidation of a mitochondria-specific phospholipid cardiolipin, in vitro and in vivo are presented. Particular emphasis is placed on the employment of triphenylphosphonium- and hemi-gramicidin S-moieties as two effective vehicles for mitochondrial delivery of antioxidants.

Preferential binding of allosteric modulators to active and inactive conformational states of metabotropic glutamate receptors

Metabotropic glutamate receptors (mGluRs) are G protein coupled receptors that play important roles in synaptic plasticity and other neuro-physiological and pathological processes. Allosteric mGluR ligands are particularly promising drug targets because of their modulatory effects--enhancing or suppressing the response of mGluRs to glutamate. The mechanism by which this modulation occurs is not known. Here, we propose the hypothesis that positive and negative modulators will differentially stabilize the active and inactive conformations of the receptors, respectively. To test this hypothesis, we have generated computational models of the transmembrane regions of different mGluR subtypes in two different conformations. The inactive conformation was modeled using the crystal structure of the inactive, dark state of rhodopsin as template and the active conformation was created based on a recent model of the light-activated state of rhodopsin. Ligands for which the nature of their allosteric effects on mGluRs is experimentally known were docked to the modeled mGluR structures using ArgusLab and Autodock softwares. We find that the allosteric ligand binding pockets of mGluRs are overlapping with the retinal binding pocket of rhodopsin, and that ligands have strong preferences for the active and inactive states depending on their modulatory nature. In 8 out of 14 cases (57%), the negative modulators bound the inactive conformations with significant preference using both docking programs, and 6 out of 9 cases (67%), the positive modulators bound the active conformations. Considering results by the individual programs only, even higher correlations were observed: 12/14 (86%) and 8/9 (89%) for ArgusLab and 10/14 (71%) and 7/9 (78%) for AutoDock. These findings strongly support the hypothesis that mGluR allosteric modulation occurs via stabilization of different conformations analogous to those identified in rhodopsin where they are induced by photochemical isomerization of the retinal ligand--despite the extensive differences in sequences between mGluRs and rhodopsin.