Agonist efficiency from concentration-response curves: Structural implications and applications.

Agonists are evaluated by a concentration-response curve (CRC), with a midpoint (EC50) that indicates potency, a high-concentration asymptote that indicates efficacy, and a low-concentration asymptote that indicates constitutive activity. A third agonist attribute, efficiency (η), is the fraction of binding energy that is applied to the conformational change that activates the receptor. We show that η can be calculated from EC50 and the asymptotes of a CRC derived from either single-channel or whole-cell responses. For 20 agonists of skeletal muscle nicotinic receptors, the distribution of η-values is bimodal with population means at 51% (including acetylcholine, nornicotine, and dimethylphenylpiperazinium) and 40% (including epibatidine, varenicline, and cytisine). The value of η is related inversely to the size of the agonist's headgroup, with high- versus low-efficiency ligands having an average volume of 70 vs. 102 Å3. Most binding site mutations have only a small effect on acetylcholine efficiency, except for αY190A (35%), αW149A (60%), and those at αG153 (42%). If η is known, the EC50 and high-concentration asymptote can be calculated from each other. Hence, an entire CRC can be estimated from the response to a single agonist concentration, and efficacy can be estimated from EC50 of a CRC that has been normalized to 1. Given η, the level of constitutive activity can be estimated from a single CRC.

Revisiting autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) mutations in the nicotinic acetylcholine receptor reveal an increase in efficacy regardless of stochiometry.

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is a genetic form of epilepsy that is caused by mutations in several genes, including genes encoding for the α4 and ß2 subunits of the nicotinic acetylcholine (nACh) receptor. Pentameric α4ß2 nACh receptors are the most abundant nicotinic receptor in the mammalian brain and form two stoichiometries, the (α4)3(ß2)2 and (α4)2(ß2)3 receptors that differ in their physiological and pharmacological properties. The purpose of this study was to investigate how ADNFLE mutations ß2V287M, ß2V287L or α4T293I manifest themselves in different receptor stoichiometries. We expressed wild-type and mutant receptors in Xenopus oocytes and measured the response to ACh and other agonists at both receptor stoichiometries. For all three mutations, the efficacy of ACh at (α4)2(ß2)3 receptors was increased. At (α4)3(ß2)2 receptors, the efficacy of activation was increased both when two molecules of agonist, either ACh or the site-selective agonist sazetidine-A, were bound at the α4-ß2 interfaces, and when a third ACh molecule was bound at the α4-α4 site. Regardless of stoichiometry, the mutations increased the current elicited by low concentrations of ACh. Further, the smoking cessation agents, nicotine, varenicline and cytisine increased activation of mutant (α4)3(ß2)2 receptors, while only nicotine increased activation of mutant (α4)2(ß2)3 receptors. Chronic exposure of all agonists reduced ACh-activation levels at low and high ACh concentrations. From this, we concluded that mutations that cause ADNFLE manifest themselves in a change in efficacy regardless of the stoichiometry of the receptor.

Novel Approach for the Search for Chemical Scaffolds with Dual Activity with Acetylcholinesterase and the α7 Nicotinic Acetylcholine Receptor-A Perspective for the Treatment of Neurodegenerative Disorders.

Neurodegenerative disorders, including Alzheimer's disease, belong to the group of the most difficult and challenging conditions with very limited treatment options. Attempts to find new drugs in most cases fail at the clinical stage. New tactics to develop better drug candidates to manage these diseases are urgently needed. It is evident that better understanding of the neurodegeneration process is required and targeting multiple receptors may be essential. Herein, we present a novel approach, searching for dual active compounds interacting with acetylcholinesterase (AChE) and the α7 nicotinic acetylcholine receptor (nAChR) using computational chemistry methods including homology modelling and high throughput virtual screening. Activities of identified hits were evaluated at the two targets using the colorimetric method of Ellman and two-electrode voltage-clamp electrophysiology, respectively. Out of 87,250 compounds from a ZINC database of natural products and their derivatives, we identified two compounds, 8 and 9, with dual activity and balanced IC50 values of 10 and 5 µM at AChE, and 34 and 14 µM at α7 nAChR, respectively. This is the first report presenting successful use of virtual screening in finding compounds with dual mode of action inhibiting both the AChE enzyme and the α7 nAChR and shows that computational methods can be a valuable tool in the early lead discovery process.

Synthetic tolerance: three noncoding small RNAs, DsrA, ArcZ and RprA, acting supra-additively against acid stress.

Synthetic acid tolerance, especially during active cell growth, is a desirable phenotype for many biotechnological applications. Natively, acid resistance in Escherichia coli is largely a stationary-phase phenotype attributable to mechanisms mostly under the control of the stationary-phase sigma factor RpoS. We show that simultaneous overexpression of noncoding small RNAs (sRNAs), DsrA, RprA and ArcZ, which are translational RpoS activators, increased acid tolerance (based on a low-pH survival assay) supra-additively up to 8500-fold during active cell growth, and provided protection against carboxylic acid and oxidative stress. Overexpression of rpoS without its regulatory 5'-UTR resulted in inferior acid tolerance. The supra-additive effect of overexpressing the three sRNAs results from the impact their expression has on RpoS-protein levels, and the beneficial perturbation of the interconnected RpoS and H-NS networks, thus leading to superior tolerance during active growth. Unlike the overexpression of proteins, overexpression of sRNAs imposes hardly any metabolic burden on cells, and constitutes a more effective strain engineering strategy.

The effect of unliganded gating on agonist response in nicotinic receptors.

Understanding the agonist concentration-response curve (CRC) is the cornerstone in pharmacology. While CRC parameters, agonist potency (EC50) and efficacy (maximum response, Imax) are well-studied, the role of unliganded gating (minimum response, Imin) on CRC is often overlooked. This study explores the effect of unliganded gating on agonist response in muscle-type acetylcholine (ACh) receptors, focusing on the underexplored role of Imin in modulating EC50 and Imax. Three Gain-of-Function (GOF) mutations that increase, and two Loss-of-Function (LOF) mutations that decrease the unliganded gating equilibrium constant (L0) were studied using automated patch-clamp electrophysiology. GOF mutations enhanced agonist potency, whereas LOF mutations reduced it. The calculated CRC aligned well with empirical results, indicating that agonist CRC can be estimated from knowledge of L0. Reduction in agonist efficacy due to LOF mutations was calculated and subsequently validated using single-channel patch-clamp electrophysiology, a factor often obscured in normalized CRC. The study also evaluated the combined impact of mutations (L0) on CRC, confirming the predictive model. Further, no significant energetic coupling between distant residues (>15 Å) was found, indicating that the mutations' effects are localized and do not alter overall agonist affinity. These findings substantiate the role of unliganded gating in modulating agonist responses and establishes a predictive model for estimating CRC parameters from known changes in L0. The study highlights the importance of intrinsic activity in receptor theory.

CHRFAM7A diversifies human immune adaption through Ca2+ signalling and actin cytoskeleton reorganization.

BACKGROUND: Human restricted genes contribute to human specific traits in the immune system. CHRFAM7A, a uniquely human fusion gene, is a negative regulator of the α7 nicotinic acetylcholine receptor (α7 nAChR), the highest Ca2+ conductor of the ACh receptors implicated in innate immunity. Understanding the mechanism of how CHRFAM7A affects the immune system remains unexplored. METHODS: Two model systems are used, human induced pluripotent stem cells (iPSC) and human primary monocytes, to characterize α7 nAChR function, Ca2+ dynamics and decoders to elucidate the pathway from receptor to phenotype. FINDINGS: CHRFAM7A/α7 nAChR is identified as a hypomorphic receptor with mitigated Ca2+ influx and prolonged channel closed state. This shifts the Ca2+ reservoir from the extracellular space to the endoplasmic reticulum (ER) leading to Ca2+ dynamic changes. Ca2+ decoder small GTPase Rac1 is then activated, reorganizing the actin cytoskeleton. Observed actin mediated phenotypes include cellular adhesion, motility, phagocytosis and tissue mechanosensation. INTERPRETATION: CHRFAM7A introduces an additional, human specific, layer to Ca2+ regulation leading to an innate immune gain of function. Through the actin cytoskeleton it drives adaptation to the mechanical properties of the tissue environment leading to an ability to invade previously immune restricted niches. Human genetic diversity predicts profound translational significance as its understanding builds the foundation for successful treatments for infectious diseases, sepsis, and cancer metastasis. FUNDING: This work is supported in part by the Community Foundation for Greater Buffalo (Kinga Szigeti) and in part by NIH grant R01HL163168 (Yongho Bae).

Agonist efficiency links binding and gating in a nicotinic receptor.

Receptors signal by switching between resting (C) and active (O) shapes ('gating') under the influence of agonists. The receptor's maximum response depends on the difference in agonist binding energy, O minus C. In nicotinic receptors, efficiency (η) represents the fraction of agonist binding energy applied to a local rearrangement (an induced fit) that initiates gating. In this receptor, free energy changes in gating and binding can be interchanged by the conversion factor η. Efficiencies estimated from concentration-response curves (23 agonists, 53 mutations) sort into five discrete classes (%): 0.56 (17), 0.51(32), 0.45(13), 0.41(26), and 0.31(12), implying that there are 5 C versus O binding site structural pairs. Within each class efficacy and affinity are corelated linearly, but multiple classes hide this relationship. η unites agonist binding with receptor gating and calibrates one link in a chain of coupled domain rearrangements that comprises the allosteric transition of the protein.

CHRFAM7A: A human specific fusion gene, accounts for the translational gap for cholinergic strategies in Alzheimer's disease.

BACKGROUND: Cholinergic neuronal loss is one of the hallmarks of AD related neurodegeneration; however, preclinical promise of α7 nAChR drugs failed to translate into humans. CHRFAM7A, a uniquely human fusion gene, is a negative regulator of α7 nAChR and was unaccounted for in preclinical models. METHODS: Molecular methods: Function of CHRFAM7A alleles was studied in vitro in two disease relevant phenotypic readouts: electrophysiology and Aß uptake. Genome edited human induced pluripotent stem cells (iPSC) were used as a model system with the human context. Double blind pharmacogenetic study: We performed double-blind pharmacogenetic analysis on the effect of AChEI therapy based on CHRFAM7A carrier status in two paradigms: response to drug initiation and DMT effect. Mini Mental Status Examination (MMSE) was used as outcome measure. Change in MMSE score from baseline was compared by 2-tailed T-test. Longitudinal analysis of clinical outcome (MMSE) was performed using a fitted general linear model, based on an assumed autoregressive covariance structure. Model independent variables included age, sex, and medication regimen at the time of the first utilized outcome measure (AChEI alone or AChEI plus memantine), APOE4 carrier status (0, 1 or 2 alleles as categorical variables) and CHRFAM7A genotype. FINDINGS: The direct and inverted alleles have distinct phenotypes. Functional CHRFAM7A allele classifies the population as 25% non-carriers and 75% carriers. Induced pluripotent stem cell (iPSC) models α7 nAChR mediated Aß neurotoxicity. Pharmacological readout translates into both first exposure (p = 0.037) and disease modifying effect (p = 0.0048) in two double blind pharmacogenetic studies. INTERPRETATION: CHRFAM7A accounts for the translational gap in cholinergic strategies in AD. Clinical trials not accounting for this uniquely human genetic factor may have rejected drug candidates that would benefit 25% of AD. Reanalyses of the completed trials using this pharmacogenetic paradigm may identify effective therapy.

Gain-of-function GABRB3 variants identified in vigabatrin-hypersensitive epileptic encephalopathies.

Variants in the GABRB3 gene encoding the ß3-subunit of the γ-aminobutyric acid type A ( receptor are associated with various developmental and epileptic encephalopathies. Typically, these variants cause a loss-of-function molecular phenotype whereby γ-aminobutyric acid has reduced inhibitory effectiveness leading to seizures. Drugs that potentiate inhibitory GABAergic activity, such as nitrazepam, phenobarbital or vigabatrin, are expected to compensate for this and thereby reduce seizure frequency. However, vigabatrin, a drug that inhibits γ-aminobutyric acid transaminase to increase tonic γ-aminobutyric acid currents, has mixed success in treating seizures in patients with GABRB3 variants: some patients experience seizure cessation, but there is hypersensitivity in some patients associated with hypotonia, sedation and respiratory suppression. A GABRB3 variant that responds well to vigabatrin involves a truncation variant (p.Arg194*) resulting in a clear loss-of-function. We hypothesized that patients with a hypersensitive response to vigabatrin may exhibit a different γ-aminobutyric acid A receptor phenotype. To test this hypothesis, we evaluated the phenotype of de novo variants in GABRB3 (p.Glu77Lys and p.Thr287Ile) associated with patients who are clinically hypersensitive to vigabatrin. We introduced the GABRB3 p.Glu77Lys and p.Thr287Ile variants into a concatenated synaptic and extrasynaptic γ-aminobutyric acid A receptor construct, to resemble the γ-aminobutyric acid A receptor expression by a patient heterozygous for the GABRB3 variant. The mRNA of these constructs was injected into Xenopus oocytes and activation properties of each receptor measured by two-electrode voltage clamp electrophysiology. Results showed an atypical gain-of-function molecular phenotype in the GABRB3 p.Glu77Lys and p.Thr287Ile variants characterized by increased potency of γ-aminobutyric acid A without change to the estimated maximum open channel probability, deactivation kinetics or absolute currents. Modelling of the activation properties of the receptors indicated that either variant caused increased chloride flux in response to low concentrations of γ-aminobutyric acid that mediate tonic currents. We therefore propose that the hypersensitivity reaction to vigabatrin is a result of GABRB3 variants that exacerbate GABAergic tonic currents and caution is required when prescribing vigabatrin. In contrast, drug strategies increasing tonic currents in loss-of-function variants are likely to be a safe and effective therapy. This study demonstrates that functional genomics can explain beneficial and adverse anti-epileptic drug effects, and propose that vigabatrin should be considered in patients with clear loss-of-function GABRB3 variants.

Lower Tubulin Expression in Glioblastoma Stem Cells Attenuates Efficacy of Microtubule-Targeting Agents.

Sensitivity to microtubule-targeting agents (MTAs) varies among cancers and predicting the response of individual cancer patients to MTAs remains challenging. As microtubules possess vast molecular heterogeneity generated by tubulin isotypes and their post-translational modifications, we questioned whether this heterogeneity can impact MTA sensitivity. We investigated microtubule heterogeneity in 15 glioblastoma cell lines and measured sensitivity of orthogonal MTAs using a per-division growth rate inhibition method that corrects for the confounding effects of variable cell proliferation rates. We found that the tubulin profile is unique for each glioblastoma cell line and that the total α- and ß-tubulin levels impact on MTA sensitivity. The baseline levels of α- and ß-tubulin were up to 20% lower in cells that were not effectively killed by MTAs. We report that lower α/ß-tubulin expression is associated with lack of cell differentiation and increased expression of stemness markers. The dedifferentiated stem-like cells with low α/ß-tubulin levels survive MTAs treatment via reversible nonmutational dormancy. Our findings provide novel insights into the relationships between microtubules and MTAs and lay a foundation for better understanding of the sensitivity of cancer cells to MTAs.

Ligand Binding at the 4-4 Agonist-Binding Site of the 42 nAChR Triggers Receptor Activation through a Pre-Activated Conformational State.

The α4ß2 nicotinic acetylcholine receptor (nAChR) is the most abundant subtype in the brain and exists in two functional stoichiometries: (α4)3(ß2)2 and (α4)2(ß2)3. A distinct feature of the (α4)3(ß2)2 receptor is the biphasic activation response to the endogenous agonist acetylcholine, where it is activated with high potency and low efficacy when two α4-ß2 binding sites are occupied and with low potency/high efficacy when a third α4-α4 binding site is occupied. Further, exogenous ligands can bind to the third α4-α4 binding site and potentiate the activation of the receptor by ACh that is bound at the two α4-ß2 sites. We propose that perturbations of the recently described pre-activation step when a third binding site is occupied are a key driver of these distinct activation properties. To investigate this, we used a combination of simple linear kinetic models and voltage clamp electrophysiology to determine whether transitions into the pre-activated state were increased when three binding sites were occupied. We separated the binding at the two different sites with ligands selective for the α4-ß2 site (Sazetidine-A and TC-2559) and the α4-α4 site (NS9283) and identified that when a third binding site was occupied, changes in the concentration-response curves were best explained by an increase in transitions into a pre-activated state. We propose that perturbations of transitions into a pre-activated state are essential to explain the activation properties of the (α4)3(ß2)2 receptor by acetylcholine and other ligands. Considering the widespread clinical use of benzodiazepines, this discovery of a conserved mechanism that benzodiazepines and ACh potentiate receptor activation via a third binding site can be exploited to develop therapeutics with similar properties at other cys-loop receptors.

Tying up Nicotine: New Selective Competitive Antagonist of the Neuronal Nicotinic Acetylcholine Receptors.

Conformational restriction of the pyrrolidine nitrogen in nicotine by the introduction of an ethylene bridge provided a potent and selective antagonist of the α4ß2-subtype of the nicotinic acetylcholine receptors. Resolution by chiral SFC, pharmacological characterization of the two enantiomers, and determination of absolute configuration via enantioselective synthesis showed that the pharmacological activity resided almost exclusively in the (R)-enantiomer.

Engineered α4ß2 nicotinic acetylcholine receptors as models for measuring agonist binding and effect at the orthosteric low-affinity α4-α4 interface.

The nicotinic acetylcholine receptor α4ß2 is important for normal mammalian brain function and is known to express in two different stoichiometries, (α4)2(ß2)3 and (α4)3(ß2)2. While these are similar in many aspects, the (α4)3(ß2)2 stoichiometry differs by harboring a third orthosteric acetylcholine binding site located at the α4-α4 interface. Interestingly, the third binding site has, so far, only been documented using electrophysiological assays, actual binding affinities of nicotinic receptor ligands to this site are not known. The present study was therefore aimed at determining binding affinities of nicotinic ligands to the α4-α4 interface. Given that epibatidine shows large functional potency differences at α4-ß2 vs. α4-α4 interfaces, biphasic binding properties would be expected at (α4)3(ß2)2 receptors. However, standard saturation binding experiments with [(3)H]epibatidine did not reveal biphasic binding under the conditions utilized. Therefore, an engineered ß2 construct (ß2(HQT)), which converts the ß(-) face to resemble that of an α4(-) face, was utilized to create (α4)3(ß2(HQT))2 receptors harboring three α4-α4 interfaces. With this receptor, low affinity binding of epibatidine with a Kd of ∼5 nM was observed in sharp contrast to a Kd value of ∼10 pM observed for wild-type receptors. A strong correlation between binding affinities at the (α4)3(ß2(HQT))2 receptor and functional potencies at the wild-type receptor of a range of nicotinic ligands highlighted the validity of using the mutational approach. Finally, large differences in activities at α4-ß2 vs. α4-α4 interfaces were observed for structurally related agonists underscoring the need for establishing all binding parameters of compounds at α4ß2 receptors.

Presence of multiple binding sites on α9α10 nAChR receptors alludes to stoichiometric-dependent action of the α-conotoxin, Vc1.1.

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels involved in fast synaptic transmission. nAChRs are pentameric receptors formed from a combination of different or similar subunits to produce heteromeric or homomeric channels. The heteromeric, α9α10 nAChR subtype is well-known for its role in the auditory system, being expressed in cochlear hair cells. These nAChRs have also been shown to be involved in immune-modulation. Antagonists of α9α10 nAChRs, like the α-conotoxin Vc1.1, have analgesic effects in neuropathic pain. Unlike other nAChR subtypes there is no evidence that functional receptor stoichiometries of α9α10 exist. By using 2-electrode voltage clamp methods and maintaining a constant intracellular Ca(2+) concentration, we observed a biphasic activation curve for ACh that is dependent on receptor stoichiometry. Vc1.1, but not the α9α10 antagonists RgIA or atropine, inhibits ACh-evoked currents in a biphasic manner. Characteristics of the ACh and Vc1.1 activation and inhibition curves can be altered by varying the ratio of α9 and α10 mRNA injected into oocytes, changing the curves from biphasic to monophasic when an excess of α10 mRNA is used. These results highlight the difference in the pharmacological profiles of at least two different α9α10 nAChR stoichiometries, possibly (α9)3(α10)2 and (α9)2(α10)3. As a result, we infer that there is an additional binding site for ACh and Vc1.1 at the α9-α9 interface on the hypothesized (α9)3(α10)2 nAChR, in addition to the α10-α9 and or α9-α10 interfaces that are common to both stoichiometries. This study provides further evidence that receptor stoichiometry contributes another layer of complexity in understanding Cys-loop receptors.

Clostridia: the importance of their exceptional substrate and metabolite diversity for biofuel and biorefinery applications.

Clostridia are anaerobic Firmicutes producing a large array of metabolites by utilizing simple and complex carbohydrates, such as cellulose, as well as CO2/H2 or CO. Their exceptional substrate diversity is enhanced by their ability to produce a broad spectrum of chemicals that can be used as precursors to or directly as biofuels and industrial chemicals. Genetic and genomic tools are under intense development, and recent efforts to metabolically engineer clostridia demonstrate their potential for biofuel and biorefinery applications. Pathway engineering to combine established substrate-utilization programs, such as for cellulose, CO2/H2 or CO, with desirable metabolic programs could lead to modular design of strains suitable for many applications. Engineering complex phenotypes--aerotolerance, abolished sporulation, and tolerance to toxic chemicals--could lead to superior bioprocessing strains.

Small RNAs in the genus Clostridium.

The genus Clostridium includes major human pathogens and species important to cellulose degradation, the carbon cycle, and biotechnology. Small RNAs (sRNAs) are emerging as crucial regulatory molecules in all organisms, but they have not been investigated in clostridia. Research on sRNAs in clostridia is hindered by the absence of a systematic method to identify sRNA candidates, thus delegating clostridial sRNA research to a hit-and-miss process. Thus, we wanted to develop a method to identify potential sRNAs in the Clostridium genus to open up the field of sRNA research in clostridia. Using comparative genomics analyses combined with predictions of rho-independent terminators and promoters, we predicted sRNAs in 21 clostridial genomes: Clostridium acetobutylicum, C. beijerinckii, C. botulinum (eight strains), C. cellulolyticum, C. difficile, C. kluyveri (two strains), C. novyi, C. perfringens (three strains), C. phytofermentans, C. tetani, and C. thermocellum. Although more than one-third of predicted sRNAs have Shine-Dalgarno (SD) sequences, only one-sixth have a start codon downstream of SD sequences; thus, most of the predicted sRNAs are noncoding RNAs. Quantitative reverse transcription-PCR (Q-RT-PCR) and Northern analysis were employed to test the presence of a randomly chosen set of sRNAs in C. acetobutylicum and several C. botulinum strains, leading to the confirmation of a large fraction of the tested sRNAs. We identified a conserved, novel sRNA which, together with the downstream gene coding for an ATP-binding cassette (ABC) transporter gene, responds to the antibiotic clindamycin. The number of predicted sRNAs correlated with the physiological function of the species (high for pathogens, low for cellulolytic, and intermediate for solventogenic), but not with 16S rRNA-based phylogeny.

Resolving the TCA cycle and pentose-phosphate pathway of Clostridium acetobutylicum ATCC 824: Isotopomer analysis, in vitro activities and expression analysis.

Solventogenic clostridia are an important class of microorganisms that can produce various biofuels. One of the bottlenecks in engineering clostridia stems from the fact that central metabolic pathways remain poorly understood. Here, we utilized the power of (13) C-based isotopomer analysis to re-examine central metabolic pathways of Clostridium acetobutylicum ATCC 824. We demonstrate using [1,2-(13) C]glucose, MS analysis of intracellular metabolites, and enzymatic assays that C. acetobutylicum has a split TCA cycle where only Re-citrate synthase (CS) contributes to the production of α-ketoglutarate via citrate. Furthermore, we show that there is no carbon exchange between α-ketoglutarate and fumarate and that the oxidative pentose-phosphate pathway (oxPPP) is inactive. Dynamic gene expression analysis of the putative Re-CS gene (CAC0970), its operon, and all glycolysis, pentose-phosphate pathway, and TCA cycle genes identify genes and their degree of involvement in these core pathways that support the powerful primary metabolism of this industrial organism.