Defining nicotinic agonist binding surfaces through photoaffinity labeling

Nicotinic acetylcholine (ACh) receptor (nAChR) agonists are potential therapeutic agents for neurological dysfunction. In the present study, the homopentameric mollusk ACh binding protein (AChBP), used as a surrogate for the extracellular ligand-binding domain of the nAChR, was specifically derivatized by the highly potent agonist azidoepibatidine (AzEPI) prepared as a photoaffinity probe and radioligand. One EPI-nitrene photoactivated molecule was incorporated in each subunit interface binding site based on analysis of the intact derivatized protein. Tryptic fragments of the modified AChBP were analyzed by collision-induced dissociation and Edman sequencing of radiolabeled peptides. Each specific EPI-nitrene-modified site involved either Tyr195 of loop C on the principal or (+)-face or Met116 of loop E on the complementary or (-)-face. The two derivatization sites were observed in similar frequency, providing evidence of the reactivity of the azido/nitrene probe substituent and close proximity to both residues. [3H]AzEPI binds to the alpha4beta2 nAChR at a single high-affinity site and photoaffinity-labels only the alpha4 subunit, presumably modifying Tyr225 spatially corresponding to Tyr195 of AChBP. Phe137 of the beta2 nAChR subunit, equivalent to Met116 of AChBP, conceivably lacks sufficient reactivity with the nitrene generated from the probe. The present photoaffinity labeling in a physiologically relevant condition combined with the crystal structure of AChBP allows development of precise structural models for the AzEPI interactions with AChBP and alpha4beta2 nAChR. These findings enabled us to use AChBP as a structural surrogate to define the nAChR agonist site.     

Protein folding determinants: structural features determining alternative disulfide pairing in alpha- and chi/lambda-conotoxins

Alpha-conotoxins isolated from Conus venoms contain 11-19 residues and preferentially fold into the globular conformation that possesses a specific disulfide pairing pattern (C1-3, C2-4). We and others isolated a new family of chi-conotoxins (also called lambda conotoxins) with the conserved cysteine framework of alpha-conotoxins but with alternative disulfide pairing (C1-4, C2-3) resulting in the ribbon conformation. In both families, disulfide pairing and hence folding are important for their biological potency. By comparing the structural differences, we identified potential structural determinants responsible for the folding tendencies of these conotoxins. We examined the role of conserved proline in the first intercysteine loop and the conserved C-terminal amide on folding patterns of synthetic analogues of ImI conotoxin by comparing the isoforms with the regiospecifically synthesized conformers. Deamidation at the C-terminus and substitution of proline in the first intercysteine loop switch the folding pattern from the globular form of alpha-conotoxins to the ribbon form of chi/lambda-conotoxins. The findings are corroborated by reciprocal folding of CMrVIA chi/lambda-conotoxins. Substitution of Lys-6 from the first intercysteine loop of CMrVIA conotoxin with proline, as well as the inclusion of an amidated C-terminal shifted the folding preference of CMrVIA conotoxin from its native ribbon conformation toward the globular conformation. Binding assays of ImI conotoxin analogues with Aplysia and Bulinus acetylcholine binding protein indicate that both these substitutions and their consequent conformational change substantially impact the binding affinity of ImI conotoxin. These results strongly indicate that the first intercysteine loop proline and C-terminal amidation act as conformational switches in alpha- and chi/lambda-conotoxins.     

The invertebrate communities associated with a Chrysanthemum coronarium-invaded coastal sage scrub area in Southern California

The escape of ornamental plants is a main pathway of invasion into many ecosystems. Non-native plants can alter basal resources and abiotic factors leading to effects that ripple throughout an ecosystem. Invertebrates mediate these effects—responding quickly to abiotic and primary producer changes and, in turn, influencing other species. Invasions are of particular concern in the coastal sage scrub ecosystems of Southern California, where habitat loss and urban encroachment increase invasive species propagule sources and decrease native community resistance. The introduced annual Chrysanthemum coronarium (crown daisy) is a common invader with largely undocumented community-level effects. Our study tested the relationships between the invasive Chrysanthemum and a coastal scrub invertebrate community using a field study at the Tijuana River Estuary. We found similar or lower abundances and diversity of canopy fauna in the presence of Chrysanthemum. Community composition dramatically differed, however, in the presence Chrysanthemum, which was associated with higher abundances of dipterans, wasps and flower beetles, and lower abundances of hemipterans and thysanopterans than native shrubs. Differences in communities were consistent at the species- and order-levels, and were associated with the generally greater plant biomass and shadier conditions afforded by the natives. This study reveals that even a proportionally small amount of Chrysanthemum may shift the invertebrate community through alterations of abiotic properties and plant biomass. We recommend that Chrysanthemum be removed at the first sign of invasion or that spread is prevented since effects on the invertebrate community are dramatic and occur quickly.     

Maintenance treatment with esomeprazole following initial relief of non-steroidal anti-inflammatory drug-associated upper gastrointestinal symptoms: the NASA2 and SPACE2 studies

Non-steroidal anti-inflammatory drugs (NSAIDs), including selective cyclo-oxygenase-2 (COX-2) inhibitors, cause upper gastrointestinal (GI) symptoms that are relieved by treatment with esomeprazole. We assessed esomeprazole for maintaining long-term relief of such symptoms. Six hundred and ten patients with a chronic condition requiring anti-inflammatory therapy who achieved relief of NSAID-associated symptoms of pain, discomfort, or burning in the upper abdomen during two previous studies were enrolled and randomly assigned into two identical, multicentre, parallel-group, placebo-controlled studies of esomeprazole 20 mg or 40 mg treatment (NASA2 [Nexium Anti-inflammatory Symptom Amelioration] and SPACE2 [Symptom Prevention by Acid Control with Esomeprazole] studies; ClinicalTrials.gov identifiers NCT00241514 and NCT00241553, respectively) performed at various rheumatology, gastroenterology, and primary care clinics. Four hundred and twenty-six patients completed the 6-month treatment period. The primary measure was the proportion of patients with relapse of upper GI symptoms, recorded in daily diary cards, after 6 months. Relapse was defined as moderate-to-severe upper GI symptoms (a score of more than or equal to 3 on a 7-grade scale) for 3 days or more in any 7-day period. Esomeprazole was significantly more effective than placebo in maintaining relief of upper GI symptoms throughout 6 months of treatment. Life-table estimates (95% confidence intervals) of the proportion of patients with relapse at 6 months (pooled population) were placebo, 39.1% (32.2% to 46.0%); esomeprazole 20 mg, 29.3% (22.3% to 36.2%) (p = 0.006 versus placebo); and esomeprazole 40 mg, 26.1% (19.4% to 32.9%) (p = 0.001 versus placebo). Patients on either non-selective NSAIDs or selective COX-2 inhibitors appeared to benefit. The frequency of adverse events was similar in the three groups. Esomeprazole maintains relief of NSAID-associated upper GI symptoms in patients taking continuous NSAIDs, including selective COX-2 inhibitors.     

Androgens exacerbate motor asymmetry in male rats with unilateral 6-hydroxydopamine lesion

Parkinson's disease (PD) is a progressive neurodegenerative disorder characterized by dopamine neuron loss in the nigrostriatal pathway that shows greater incidence in men than women. The mechanisms underlying this gender bias remain elusive, although one possibility is that androgens may increase dopamine neuronal vulnerability to oxidative stress. Motor impairment can be modeled in rats receiving a unilateral injection of 6-hydroxydopamine (6-OHDA), a neurotoxin producing nigrostriatal degeneration. To investigate the role of androgens in PD, we compared young (2 months) and aged (24 months) male rats receiving gonadectomy (GDX) and their corresponding intact controls. One month after GDX, rats were unilaterally injected with 6-OHDA, and their motor impairment and asymmetry were assessed 2 weeks later using the cylinder test and the amphetamine-induced rotation test. Plasma samples were also collected to assess the concentration of testosterone and advanced oxidation protein products, a product of oxidative stress. GDX decreased lesion-induced asymmetry along with oxidative stress and increased amphetamine-induced rotations. These results show that GDX improves motor behaviors by decreasing motor asymmetry in 6-OHDA-treated rats, an effect that may be ascribed to increased release of striatal dopamine and decreased oxidative stress. Collectively, the data support the hypothesis that androgens may underlie the gender bias observed in PD.