The human neuromedulloblastoma cell line TE671 is shown by single-channel recordings to express nicotinic acetylcholine receptors (AChRs) that are blocked by alpha-bungarotoxin (alpha Bgt). These AChRs do not react with antisera to the alpha Bgt-binding protein of brain or with monoclonal antibodies (mAbs) to brain nicotinic AChRs that do not bind alpha Bgt. TE671 AChRs do react with autoantibodies to muscle AChRs from myasthenia gravis patients and with mAbs to muscle AChRs, including mAbs specific for extrajunctional AChRs. AChRs. AChRs purified from TE671 cells are composed of 4 kinds of subunits corresponding to those of muscle AChR. Sequences of cDNAs for the ACh-binding alpha subunit and the delta subunit of this AChR further identify it as muscle AChR. Expression of TE671 AChR can be up-regulated by nicotine and dexamethasone, and down-regulated by forskolin.
To establish the relationship between cholinergic ligand occupancy and channel open states, we recorded single-channel currents activated by different acetylcholine (ACh) concentrations from Torpedo californica ACh receptors reconstituted in lipid bilayers. Inspection of single-channel records shows that the frequency of occurrence of long openings increases with ACh concentration. Analysis of the probability distribution of open dwell times indicates that the ACh receptor channel has two distinct channel open states, short- and long-lived. The frequency of occurrence of the long openings over the short increased with ACh concentration, whereas the corresponding time constants were virtually unaltered. The extent of agonist occupancy at the ACh-binding sites in the purified cholinergic receptor appears, therefore, to correlate with an increased probability of the long-lived open state. These results are consistent with the notion that the two open-channel states arise from different extents of ligand occupancy at the receptor molecule.
The functional role of individual ACh receptor subunits in the mechanism of the nicotinic ACh receptor channel was examined using subunit-specific monoclonal antibodies (mAbs) as probes. Single-channel recordings from the Torpedo californica purified ACh receptor reconstituted in planar lipid bilayers were used as the assay to evaluate the influence of distinct mAbs on the ion conduction and gating characteristics of the ACh receptor channel. The mAbs that bind to the main immunogenic region on an extracellular domain of the alpha subunits do not perturb the open-channel conductance or lifetimes. A mAb that binds to extracellular domains of alpha and beta subunits and two mAbs that bind to the cytoplasmic surface of the beta and gamma subunits inhibit single-channel activity. Thus, mAbs with primary specificity for beta and gamma subunits affect channel gating. This approach may specify the functional roles of distinct structural domains in the ACh receptor molecule.
INTRODUCTION: The development of fully automated immunoassay platforms has improved the technical reliability of cerebrospinal fluid (CSF) biomarkers for Alzheimer's disease.
METHODS:We quantified Aβ1-42, Aβ1-40, tTau and pTau levels using the Lumipulse G System in 94 CSF samples from participants of the SPIN cohort with available 18 F-Florbetapir imaging. Amyloid scans were assessed visually and through automated quantification. We determined the cutoffs of CSF biomarkers that optimized their agreement with 18 F-Florbetapir PET and evaluated concordance between markers of the amyloid category.RESULTS: Aβ1-42, tTau and pTau (but not Aβ1-40) and the ratios with Aβ1-42 had good diagnostic agreement with 18 F-Florbetapir PET. As a marker of amyloid pathology, the Aβ1-42/Aβ1-40 ratio had higher agreement and better correlation with amyloid PET than Aβ1-42 alone.DISCUSSION: CSF biomarkers measured with the Lumipulse G System show good agreement with amyloid imaging. Combination of Aβ1-42 with Aβ1-40 increases the agreement between markers of amyloid pathology.
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