Molecular cloning of a human neuronal nicotinic acetylcholine receptor beta 3-like subunit

Nicotinic receptor abnormalities of Alzheimer's disease: therapeutic implications

The neuronal nicotinic acetylcholine receptors (nAChRs) in the brain are important for functional processes, including cognitive and memory functions. The nAChRs acting as neuromodulators in communicative processes regulated by different neurotransmitters show a relatively high abundance in the human cortex, with a laminar distribution of the nAChRs of superhigh, high, and low affinity in the human cortex. The regional pattern of messenger RNA (mRNA) for various nAChR subtypes does not strictly follow the regional distribution of nAChR ligand-binding sites in the human brain. Consistent losses of nAChRs have been measured in vitro in autopsy brain tissue of Alzheimer's disease patients (AD), as well as in vivo by positron emission tomography (PET). Measurement of the protein content of nAChRs showed reduced levels of the alpha4, alpha3, and alpha7 nAChR subtypes. The finding that the alpha4 and alpha3 mRNA levels were not changed in AD brains suggests that the losses in high-affinity nicotinic-binding sites cannot be attributed to alterations at the transcriptional level of the alpha4 and alpha3 genes and that the causes have to be searched for at the translational and/or posttranslational level. The increased mRNA level of the alpha7 nAChR subtyep in the hippocampus indicates that subunit-specific changes in gene expression of the alpha7 nAChR might be associated with AD. The PET studies reveal deficits in nAChRs as an early phenomena in AD, stressing the importance of nAChRs as a potential target for drug intervention. PET ligands measuring the alpha4 nAChRs are under development. Studies of the influence of beta-amyloid on nAChRs in brain autopsy tissue from patients with the amyloid precursor protein 670/671 mutation have shown that there is no direct relationship between nAChR deficits and pathology. Treatment with cholinergic drugs in AD patients indicate improvement of the nAChRs in the brain, as visualized by PET. Further studies on neuroprotective mechanisms mediated via nAChR subtypes are exciting new avenues.

CHRNB2 is the second acetylcholine receptor subunit associated with autosomal dominant nocturnal frontal lobe epilepsy

Autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) is an uncommon, idiopathic partial epilepsy characterized by clusters of motor seizures occurring in sleep. We describe a mutation of the beta2 subunit of the nicotinic acetylcholine receptor, effecting a V287M substitution within the M2 domain. The mutation, in an evolutionary conserved region of CHRNB2, is associated with ADNFLE in a Scottish family. Functional receptors with the V287M mutation are highly expressed in Xenopus oocytes and characterized by an approximately 10-fold increase in acetylcholine sensitivity. CHRNB2 is a new gene for idiopathic epilepsy, the second acetylcholine receptor subunit implicated in ADNFLE.

Nicotinic acetylcholine receptors assembled from the alpha7 and beta3 subunits

Intracellular recordings were performed in voltage-clamped Xenopus oocytes upon injection with a mixture of cDNAs encoding the beta3 and mutant alpha7 (L247Talpha7) neuronal nicotinic acetylcholine receptor (nAChR) subunits. The expressed receptors maintained sensitivity to methyllycaconitine and to alpha-bungarotoxin but exhibited a functional profile strikingly different from that of the homomeric L247Talpha7 receptor. The heteromeric L247Talpha7beta3 nAChR had a lower apparent affinity and a faster rate of desensitization than L247Talpha7 nAChR, exhibited nonlinearity in the I-V relationship, and was inhibited by 5-hydroxytryptamine, much like wild type alpha7 (WTalpha7) nAChR. Single channel recordings in cell-attached mode revealed unitary events with a slope conductance of 19 picosiemens and a lifetime of 5 ms, both values being much smaller than those of the homomeric receptor channel. Upon injection with a mixture of WTalpha7 and beta3 cDNAs, clear evidence was obtained for the plasma membrane assembly of heteromeric nAChRs, although ACh could not activate these receptors. It is concluded that beta3, long believed to be an orphan subunit, readily co-assembles with other subunits to form heteromeric receptors, some of which may be negative regulators of cholinergic function.

Human neuronal nicotinic receptors

Nicotine is a very widely used drug of abuse, which exerts a number of neurovegetative, behavioural and psychological effects by interacting with neuronal nicotinic acetylcholine receptors (NAChRs). These receptors are distributed widely in human brain and ganglia, and form a family of ACh-gated ion channels of different subtypes, each of which has a specific pharmacology and physiology. As human NAChRs have been implicated in a number of human central nervous system disorders (including the neurodegenerative Alzheimer's disease, schizophrenia and epilepsy), they are suitable potential targets for rational drug therapy. Much of our current knowledge about the structure and function of NAChRs comes from studies carried out in other species, such as rodents and chicks, and information concerning human nicotinic receptors is still incomplete and scattered in the literature. Nevertheless, it is already evident that there are a number of differences in the anatomical distribution, physiology, pharmacology, and expression regulation of certain subtypes between the nicotinic systems of humans and other species. This review will attempt to survey the major achievements reached in the study of the structure and function of NAChRs by examining the molecular basis of their functional diversity viewed mainly from pharmacological and biochemical perspectives. It will also summarize our current knowledge concerning the structure and function of the NAChRs expressed by other species, and the newly discovered drugs used to classify their numerous subtypes. Finally, the role of NAChRs in behaviour and pathology will be considered.

Formation of oligomers containing the beta3 and beta4 subunits of the rat nicotinic receptor

The role of the beta3 and beta4 subunits of the nicotinic acetylcholine receptor in brain is still unclear. We investigated nicotinic receptor structure with antibodies directed against unique regions of the beta3 and beta4 subunits of the rat nicotinic acetylcholine receptor. Anti-beta4 detected a single band of 66 kDa in most regions of the brain that was strongest in striatum and cerebellum. The 60 kDa beta3 subunit was detected primarily in striatum and cerebellum, and faintly in hippocampus. Immunoprecipitation experiments established that the two subunits were coassembled in the cerebellum along with the beta2 subunit. Antibodies against the alpha4, beta2, beta3, and beta4 subunits immunoprecipitated approximately 75% of the bungarotoxin-insensitive nicotinic receptor from cerebellar extracts as determined by nicotine-dependent acetylcholine binding. Transfection of COS cells with cDNAs for these four subunits induced expression of a high affinity nicotinic receptor. Omission of only a single subunit from the transfection affected either the Bmax or the apparent KD of the receptor. Our data suggest that the beta3 subunit functions as a structural entity that links a relatively unstable alpha4beta2 heterodimer to a more stable alpha4beta4 heterodimer. The agonist-binding site formed by alpha4beta2 has a much greater affinity than does that formed by alpha4beta4. In this respect, nicotinic receptors that contain the beta3 subunit are structurally homologous to the muscle nicotinic receptor.

Comparative structure of human neuronal alpha 2-alpha 7 and beta 2-beta 4 nicotinic acetylcholine receptor subunits and functional expression of the alpha 2, alpha 3, alpha 4, alpha 7, beta 2, and beta 4 subunits

cDNA clones encoding human neuronal nicotinic acetylcholine receptor alpha 2, alpha 3, alpha 4, alpha 5, alpha 6, alpha 7, beta 2, beta 3, and beta 4 subunits were isolated from brainstem, hippocampus, prefrontal cortex, substantia nigra, thalamus, and IMR32 libraries. Human alpha 2 and alpha 6 and full-length beta 3 and beta 4 clones have not been previously reported. Deduced amino acid sequences of the alpha 2, alpha 6, beta 3, and beta 4 predicted mature peptides are 503 residues (56.9 kDa), 464 residues (53.7 kDa), 440 residues (50.8 kDa), and 477 residues (54.1 kDa), respectively. These sequences show 84 (alpha 2), 87 (alpha 6), 89 (beta 3), and 84% (beta 4) identity to the corresponding rat sequences. The amino termini of the human alpha 2 and beta 3 mature peptides contain 23 and six additional residues, respectively, compared to those of rat alpha 2 and beta 3. Recombinant receptors were expressed in Xenopus laevis oocytes injected with in vitro transcripts encoding either alpha 7 alone or alpha 2, alpha 3, or alpha 4 in pairwise combination with beta 2 or beta 4. Inward currents were elicited by the application of acetylcholine (1-100 microM) and other agonists; these responses were blocked 65-97% by application of 10 microM d-tubocurare, confirming functional expression of human nicotinic receptors.

Human nicotinic receptors--their role in aging and dementia

Multiple nicotinic receptors seem to exist in brain as revealed by neurophysiological, neurochemical, molecular and immunological studies. The mechanisms for their involvement in higher functions including learning and memory are still relatively unknown. The nicotinic receptor subtypes in human brain undergo changes during aging. Deficits of brain nicotinic receptors have been traced in neurodegenerative disorders as Alzheimer's disease and Parkinson's disease. Brain imaging studies in patients and neurochemical studies in autopsy brain tissue from Alzheimer patients reveal significant losses of the nicotinic receptors. New therapeutic compounds tried in Alzheimer's disease, aiming to increase cholinergic activity in the brain, act via the nicotinic receptors in brain. Augmentation of nicotinic receptor function in brain might be of importance for alleviating some of the cognitive impairments in Alzheimer's disease.