Topology of ligand binding sites on the nicotinic acetylcholine receptor

The nicotinic acetylcholine receptor (AChR) presents two very well differentiated domains for ligand binding that account for different cholinergic properties. In the hydrophilic extracellular region of both alpha subunits there exist the binding sites for agonists such as the neurotransmitter acetylcholine (ACh) and for competitive antagonists such as d-tubocurarine. Agonists trigger the channel opening upon binding while competitive antagonists compete for the former ones and inhibit its pharmacological action. Identification of all residues involved in recognition and binding of agonist and competitive antagonists is a primary objective in order to understand which structural components are related to the physiological function of the AChR. The picture for the localisation of the agonist/competitive antagonist binding sites is now clearer in the light of newer and better experimental evidence. These sites are mainly located on both alpha subunits in a pocket approximately 30-35 A above the surface membrane. Since both alpha subunits are sequentially identical, the observed high and low affinity for agonists on the receptor is conditioned by the interaction of the alpha subunit with the delta or the gamma chain, respectively. This relationship is opposite for curare-related drugs. This molecular interaction takes place probably at the interface formed by the different subunits. The principal component for the agonist/competitive antagonist binding sites involves several aromatic residues, in addition to the cysteine pair at 192-193, in three loops-forming binding domains (loops A-C). Other residues such as the negatively changed aspartates and glutamates (loop D), Thr or Tyr (loop E), and Trp (loop F) from non-alpha subunits were also found to form the complementary component of the agonist/competitive antagonist binding sites. Neurotoxins such as alpha-, kappa-bungarotoxin and several alpha-conotoxins seem to partially overlap with the agonist/competitive antagonist binding sites at multiple point of contacts. The alpha subunits also carry the binding site for certain acetylcholinesterase inhibitors such as eserine and for the neurotransmitter 5-hydroxytryptamine which activate the receptor without interacting with the classical agonist binding sites. The link between specific subunits by means of the binding of ACh molecules might play a pivotal role in the relative shift among receptor subunits. This conformational change would allow for the opening of the intrinsic receptor cation channel transducting the external chemical signal elicited by the agonist into membrane depolarisation. The ion flux activity can be inhibited by non-competitive inhibitors (NCIs). For this kind of drugs, a population of low-affinity binding sites has been found at the lipid-protein interface of the AChR. In addition, several high-affinity binding sites have been found to be located at different rings on the M2 transmembrane domain, namely luminal binding sites. In this regard, the serine ring is the locus for exogenous NCIs such as chlorpromazine, triphenylmethylphosphonium, the local anaesthetic QX-222, phencyclidine, and trifluoromethyliodophenyldiazirine. Trifluoromethyliodophenyldiazirine also binds to the valine ring, which is the postulated site for cembranoids. Additionally, the local anaesthetic meproadifen binding site seems to be located at the outer or extracellular ring. Interestingly, the M2 domain is also the locus for endogenous NCIs such as the neuropeptide substance P and the neurotransmitter 5-hydroxytryptamine. In contrast with this fact, experimental evidence supports the hypothesis for the existence of other NCI high-affinity binding sites located not at the channel lumen but at non-luminal binding domains. (ABSTRACT TRUNCATED)     

Efficacy and safety of 0.3% carbomer gel compared to placebo in patients with moderate-to-severe dry eye syndrome

PURPOSE: Carbomer gel is a water-soluble polymeric resin that has been reported to maintain the tear film in contact with the eye for an extended period. The efficacy and safety of this new artificial tear were assessed. METHODS: A multicenter, single-masked, randomized, placebo-controlled study was carried out on 123 patients with moderate-to-severe dry eyes. The placebo was a mannitol solution with benzalkonium chloride 0.008% as preservative. Patients were observed over an 8-week period, and subjective and objective changes analyzed, compared to a baseline of no therapy, after 1 to 7 days washout period from previous medication. RESULTS: All primary subjective symptoms decreased significantly in the carbomer gel-treated group compared to the placebo group (i.e., dryness, discomfort, and foreign body sensation). The carbomer gel also significantly improved the rose bengal staining score relative to placebo. When data for the primary subjective efficacy variables were stratified for disease severity, there was a statistically significant improvement from baseline by day 10 for severely affected patients and from day 42 for patients with moderate disease. Secondary subjective symptoms that improved significantly in the tear gel group compared to placebo were photophobia, erythema, tear breakup time, blurry-filmy, dry-sandy sensation, and physician impression. However, no significant improvements in the secondary subjective symptoms of tearing, itching, scaling, conjuctival discharge, palpebral conjunctival redness, bulbar conjuctival redness, conjunctival luster, relief of discomfort, ease of use, and overall acceptability were found in either group over the baseline score. In addition, neither carbomer gel nor placebo improved the baseline fluorescein staining score or the Schirmer test score. Two patients suffered local allergic reactions to the carbomer gel or its preservative, which settled on withdrawal of the medication. CONCLUSIONS: Carbomer gel was more efficacious than was placebo in improving a number of subjective and objective symptoms of moderate-to-severe dry eye syndrome. The results of this study indicate that carbomer gel was a safe as was the placebo.     

Effectiveness of mass neonatal BCG vaccination in the prevention of pulmonary tuberculosis: a case-control study in Nagpur, India

SETTING: Government Medical College, Nagpur, India, a tertiary care hospital. OBJECTIVE: To estimate the effectiveness of mass neonatal BCG vaccination in the prevention of pulmonary tuberculosis in Nagpur, India. DESIGN: A hospital-based pair-matched case-control study with a case of 1:3, including 144 cases of pulmonary tuberculosis and 432 controls. RESULTS: The overall vaccine effectiveness estimated in the present study was 60% (95% Confidence Interval [CI] 43%-72%). The protective effect was more in males in the age group 21-30 years. The prevented fraction was 39% (95% CI 24%-52%). CONCLUSION: The moderate effectiveness demonstrated in this study needs to be substantiated for other forms of tuberculosis by undertaking community-based case-control studies, before attempting to justify the use of mass neonatal BCG vaccination strategy as a part of the national programme.     

Identification and characterization of genes that interact with lin-12 in Caenorhabditis elegans

We identified and characterized 14 extragenic mutations that suppressed the dominant egg-laying defect of certain lin-12 gain-of-function mutations. These suppressors defined seven genes: sup-17, lag-2, sel-4, sel-5, sel-6, sel-7 and sel-8. Mutations in six of the genes are recessive suppressors, whereas the two mutations that define the seventh gene, lag-2, are semi-dominant suppressors. These suppressor mutations were able to suppress other lin-12 gain-of-function mutations. The suppressor mutations arose at a very low frequency per gene, 10-50 times below the typical loss-of-function mutation frequency. The suppressor mutations in sup-17 and lag-2 were shown to be rare non-null alleles, and we present evidence that null mutations in these two genes cause lethality. Temperature-shift studies for two suppressor genes, sup-17 and lag-2, suggest that both genes act at approximately the same time as lin-12 in specifying a cell fate. Suppressor alleles of six of these genes enhanced a temperature-sensitive loss-of-function allele of glp-1, a gene related to lin-12 in structure and function. Our analysis of these suppressors suggests that the majority of these genes are part of a shared lin-12/glp-1 signal transduction pathway, or act to regulate the expression or stability of lin-12 and glp-1.     

The growth and maturation of the National Cancer Data Base

BACKGROUND: The National Cancer Data Base (NCDB), a joint project of the Commission on Cancer of the American College of Surgeons and the American Cancer Society, is a cancer management and outcomes data base for health care organizations. It provides a comparative summary of patient care that is used by communities and participating hospitals for self-assessment. The most current (1994) data are described here. METHODS: Six calls for data have yielded a total of 4,580,000 cases for the years 1985-1994. A total of 1735 hospital cancer registries have each participated in at least one of the calls for data. RESULTS: Summing the last year's report from each of the 1227 hospitals that participated in 1994, the cases represent the equivalent of 57% of the estimated 1994 U.S. cancer cases. These data were received from all six regions of the country, including all 50 states. Ninety-seven percent of patients received all or part of their treatment at the reporting hospital. The four most common cancers are carcinomas of the breast (15.7%), lung (14.3%), prostate (13.1%), and colon (7.7%), and collectively they comprise a majority of new cases. CONCLUSIONS: The NCDB is a cancer management and outcomes data base for health care organizations that currently provides data on 57% of the estimated new cases in the U.S. Past data have been used extensively to assess patterns of care and outcomes.