Reverse-phase chromatography isolation and MALDI mass spectrometry of the acetylcholine receptor subunits

Detection of alpha7 nicotinic acetylcholine receptors with the aid of antibodies and toxins

Neuronal nicotinic acetylcholine receptors (nAChRs) containing alpha7 subunit are well represented in the brain and some non-neuronal tissues, and their malfunctioning is associated with diverse pathologies. Therefore, detection and quantification of alpha7 nAChR are important tasks. The affinity-purified antibodies were prepared against the 1-23 and 179-190 fragments of the human and rat alpha7 nAChR extracellular domain. The specificity and selectivity of these alpha7 (1-23) and alpha7 (179-190) antibodies was tested by ELISA in model systems: the E. coli-expressed alpha7 subunit extracellular domain and the pituitary cell line GH(4)C(1) stably expressing human alpha7 nAChR. On the rat brain slices two antibodies and biotinylated alpha-cobratoxin specifically stained the hippocampus region known to be rich in alpha7 nAChR. Western blot analysis revealed that in the human thalamus membranes and in rat brain membranes, antibodies alpha7 (1-23) stained a single band of 62 kDa, while the alpha7 (179-190) antibodies stained a doublet of 53-54 kDa. The results obtained show that utilization of model systems and a combination of several antibodies with appropriately labeled toxins may provide better ways for detection of alpha7 nAChR.

Nicotinic receptors in the putamen of patients with dementia with Lewy bodies and Parkinson's disease: relation to changes in alpha-synuclein expression

A reduction in nicotinic receptor (nAChR) binding has previously been observed in putamen in Parkinson's disease (PD) and dementia with Lewy bodies (DLB). The present study demonstrates no concommitant reduction in the expression of alpha2-alpha7, beta2 and beta3 nAChR subunit proteins. Alphasynuclein, which can interfere with membrane protein function and is a key constituent of PD and DLB pathology, was increased (insoluble fraction) in both disorders, although nAChR binding loss did not correlate with alpha-synuclein expression within patient groups. The results point to a possible abnormality of striatal nicotinic receptor assembly in PD and DLB.

Mass spectrometry of nicotinic acetylcholine receptors and associated proteins as models for complex transmembrane proteins

Studies were conducted to optimize matrix-assisted laser desorption/ionization, time-of-flight mass spectrometry (MALDI TOF MS) in analyzing the composition of nicotinic acetylcholine receptors (nAChR) from Torpedo californica electric tissue in their membrane-bound, detergent-solubilized, and affinity-purified states. Mass spectra obtained from nAChR-rich membrane fractions gave reasonably good representations of protein compositions indicated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of those same samples. Efficiency of extraction of nAChR from membranes was not markedly different for most detergents, but quality and signal size of mass spectra were clearly influenced by detergent composition and concentration, protein concentration, and MALDI matrix composition. The best spectra, allowing detection and accurate size determinations for samples containing as little as 10 fmol of pure nAChR, were obtained for samples solubilized in Triton X-100 and assayed by use of a sinapinic acid matrix. Although informative spectra could be obtained for nAChR affinity purified on alpha-cobratoxin (Naja naja siamensis) columns and extracted using sinapinic acid, superior spectra with much higher signal:noise were obtained if extraction media contained Triton X-100 or sodium dodecyl sulfate. nAChR subunit masses determined were similar regardless of the membrane-associated, detergent-solubilized, or affinity-purified state of the preparation. These studies illustrate how masses can be determined for nAChR subunits and for other protein components in Torpedo membrane preparations, such as RAPsyn and Na(+)-K(+)-ATPase alpha and beta subunits. They also provide an underpinning for streamlined analysis of the composition of complex transmembrane proteins using MALDI TOF MS.

Physicochemical and immunological studies of the N-terminal domain of the Torpedo acetylcholine receptor alpha-subunit expressed in Escherichia coli

The nicotinic acetylcholine receptor (AChR) from the electric organ of Torpedo species is an oligomeric protein composed of alpha2 beta gamma delta subunits. Although much is known about its tertiary and quaternary structure, the conformation of the large extracellular domains of each of the subunits has not been analysed in detail. In order to obtain information about the spatial structure of the extracellular domain, we have expressed the N-terminal fragment 1-209 of the Torpedo californica AChR alpha-subunit in Escherichia coli. Two vectors coding for a (His)6 tag, either preceding or following the 1-209 sequence, were used and the recombinant proteins obtained (designated alpha1-209pET and alpha1-209pQE, respectively) were purified by affinity chromatography on a Ni2+-agarose column. The chemical structure of both proteins was verified by Edman degradation and mass spectrometry. The proteins were soluble in aqueous buffers but to make possible a comparison with the whole AChR or its isolated subunits, the recombinant proteins were analyzed both in aqueous solution and with the addition of detergents. The two proteins bound [125I]alpha-bungarotoxin with equal potency (KD approximately 130 nm, Bmax approximately 10 nmol.mg-1). Both were shown to interact with several monoclonal antibodies raised against purified Torpedo AChR. The circular dichroism (CD) spectra of the two proteins in aqueous solution revealed predominantly beta-structure (50-56%), the fraction of alpha-helices amounting to 32-35%. Nonionic (beta-octylglucoside) and zwitterionic (CHAPS) detergents did not appreciably change the CD spectra, while the addition of SDS or trifluoroethanol decreased the percentage of beta-structure or increased the alpha-helicity, respectively. The predominance of beta-structure is in accord with recent data on the N-terminal domain of the mouse muscle AChR alpha-subunit expressed in the mammalian cells [West et al. (1997) J. Biol. Chem. 272, 25 468]. Thus, expression in E. coli provides milligram amounts of the protein that retains several structural characteristics of the N-terminal domain of the Torpedo AChR alpha-subunit and appears to share with the latter a similar secondary structure. The expression of recombinant polypeptides representing functional domains of the AChR provides an essential first step towards a more detailed structural analysis.